Systems and methods for degrading perfluoroalkyl and polyfluoroalkyl substances

Abstract

The invention provides systems and methods for degrading perfluoroalkyl and polyfluoroalkyl substances, or "PFAS" by electrocatalysis using a water oxidation electrocatalyst in a predominantly aqueous electrolyte containing high concentrations of hydroxide anions and high concentrations of alkali metal cations compared to standard electrolyte solutions. In some embodiments, the alkali metal cations are lithium cations.

Description

PATENT Atty Dkt No. UR32PC / 2-22051B SYSTEMS AND METHODS FOR DEGRADING PERFLUOROALKYL AND POLYFLUOROALKYL SUBSTANCES CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to, and the benefit of, U.S. Provisional Patent Application Nos.63 / 610,364, filed December 14, 2023, 63 / 626,016, filed January 28, 2024, and 63 / 566,843, filed March 18, 2024. The contents of each of these applications are incorporated herein by reference in their entirety. STATEMENT OF FEDERAL FUNDING

[0002] Not applicable. PARTIES TO JOINT RESEARCH AGREEMENT

[0003] Not applicable. BACKGROUND OF THE INVENTION

[0004] Perfluoroalkyl and polyfluoroalkyl substances, commonly referred to as “PFAS,” are a large group of synthetic, industrial chemicals. They are widely used in consumer, commercial, and industrial products because of their exceptional stability and nonstick, stain- repellent, and waterproof properties. They are also an important component of firefighting foams used in, for example, airport and military applications. PFAS are formed of chains of carbons bonded to fluorine atoms. As this chemical bond is one of the strongest known, such compounds are extremely resistant to degradation. As stated on the website of the National Institute of Environmental Health Sciences (“NIEHS”), a component of the National Institutes of Health: “these chemicals do not degrade in the environment. In fact, scientists are unable to estimate an environmental half-life for PFAS, which is the amount of time it takes 50% of the chemical to disappear.” See, https: / / www.niehs.nih.gov / health / topics / agents / pfc / index.cfm. For this reason, PFAS are sometimes called “forever chemicals.”

[0005] PFAS have been detected in air, surface waters, and soils and have been in humans and wildlife worldwide. In animal models, various PFAS were found to exhibit hepatotoxicity, developmental toxicity, and immunotoxicity. See, e.g., Lau, et al., ToxicolPATENT Sci.2007 Oct;99(2):366-94. doi: 10.1093 / toxsci / kfm128. A study of serum samples from a representative sample of the American population found PFAS to be present in more than 95% of participants. See, Kato, et al., Environ Sci Technol.2011 Oct 1;45(19):8037-45. doi: 10.1021 / es1043613. According to the website of the Agency for Toxic Substances and Disease Registry, a part of the U.S. Department of Health and Human Services, the two leading PFAS most Americans have in their blood are perfluorooctane sulfonic acid (“PFOS”) and perfluorooctanoic acid (“PFOA”). See, www.atsdr.cdc.gov / pfas / health- effects / us-population.html. PFOS and PFOA contamination of water inMinnesota required installation of a filtration PFOA in the drinking water below the current EPA health advisory level of 70 parts per trillion. Id.

[0006] The U.S. government recently launched a plan to combat PFAS pollution because effective PFAS remediation is urgently needed. New technologies are required for the destruction of PFAS without the generation of hazardous byproducts.

[0007] Current PFAS remediation efforts are mainly focused on containment at the source of use. But that does not solve issues with PFAS that are already in the environment. Emerging efforts of destructive PFAS removal have been unsuccessful to date, because of high cost, high energy requirements, low efficacy, and toxic byproducts. Systems and methods for degrading PFAS by electrocatalysis using a water oxidation electrocatalyst in an aqueous solution were recently described in co-owned International Patent Application No. PCT / US2023 / 017345, published as International Publication No. WO 2023 / 196259.

[0008] It would be desirable to have further improved systems and methods for degrading PFAS. Given the volumes of surface waters and groundwater contaminated with PFAS, it would further be desirable to reduce the cost and energy requirements needed to degrade PFAS. It would further be desirable to have additional means to reduce the levels of PFAS that are now found in the environment around airports, industrial sites, and military installations due to industrial processes and firefighting foams. Ideally, such means would be globally scalable, use nonprecious materials, work in aqueous media, have low energy consumption, and enable complete or almost complete defluorination of low concentrations of PFAS. Surprisingly, the present invention fulfills these and other needs.PATENT BRIEF SUMMARY OF INVENTION

[0009] In a first group of embodiments, the invention provides systems for degrading per- and poly-fluoroalkyl substances (“PFAS”), said systems comprising: (a) a first container, (b) an electrolyte solution disposed in said first container, which electrolyte solution comprises (1) 46 vol% or more water, and (2) a concentration of hydroxide anions of 1M or higher, and a concentration of alkali metal cations of 1M or higher, (c) one or more PFAS containing at least one covalent bond between a carbon atom and a fluorine atom, said one or more PFAS dissolved in said electrolyte solution, (d) a working electrode at least partially immersed in said electrolyte solution, (e) a water oxidation electrocatalyst immobilized on said working electrode and in contact with (1) said one or more PFAS compounds dissolved in said electrolyte solution and containing at least one covalent bond between a carbon atom and a fluorine atom, and (2) said electrolyte solution, (f) a counter electrode at least partially immersed in said electrolyte solution and electrically connected to said working electrode, and, (g) a source of electricity electrically connected to said system to provide an applied electric potential to said working electrode, wherein, when said water containing said one or more compounds is added to said electrolyte solution and an applied electric potential is applied to said working electrode, said applied electric potential causes said at least one covalent bond between a carbon atom and a fluorine atom in said PFAS to be broken, thereby degrading said PFAS. In some embodiments, the applied electric potential of element (g) is an anodic bias. In some embodiments, the applied electric potential of element (g) is a cathodic bias. In some embodiments, the applied electric potential is - 5 V to 5 V versus standard hydrogen electrode (“SHE”). In some embodiments, the applied electric potential is an anodic bias and is 0.5 V to 2 V versus SHE. In some embodiments, the applied electric potential is an anodic bias and is 1.0 V to 2 V versus SHE. In some embodiments, the applied electric potential is a cathodic bias and is -5 V to -0.5 V versus SHE. In some embodiments, the applied electric potential is a cathodic bias and is -2 V to -1.0 V versus SHE. In some embodiments, the electrolyte solution is 47 vol%, 48 vol%, 49 vol%, 50 vol% or more water. In some embodiments, the electrolyte solution is 60 vol% or more water. In some embodiments, the electrolyte solution is 70 vol% or more water. In some embodiments, the electrolyte solution is 80 vol% or more water. In some embodiments, the electrolyte solution is 90 vol% or more water. In some embodiments, the working electrode is carbon fiber paper. In some embodiments, the carbon fiber paper is hydrophilic. In some embodiments, the waterPATENT oxidation electrocatalyst is a metallic material, metal oxide, metal hydroxide, or metal oxy(hydroxide). In some embodiments, the water oxidation electrocatalyst is a nanostructured layered double hydroxide solid, nanostructured layered oxide solid, nanostructured layered oxy(hydroxide) solid, a perovskite, a polyoxometalate, or a metal- organic framework. In some embodiments, the water oxidation electrocatalyst is a metallic material, a nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid which contains an effective amount of one or more transition metals, a post-transition metal, or both a transition metal and a post-transition metal. In some embodiments, the one or more transition metals are first-row transition metals. In some embodiments, the one or more first- row transition metals are nickel and manganese or nickel and iron. In some embodiments, the post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin. In some embodiments, the nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid comprises nickel mixed with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal. In some embodiments, the nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid is comprised of a mix of nickel with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal is three parts nickel to one part of said transition metal, of said post-transition metal, or of both a transition metal and a post- transition metal. In some embodiments, the post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin. In some embodiments, the nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid is in the form of nanoparticles disposed on said working electrode. In some embodiments, the water oxidation electrocatalyst is [NiMn]-layered double hydroxide, oxide, or oxy(hydroxide). In some embodiments, the [NiMn]-layered double hydroxide, oxide, or oxy(hydroxide) is in the form of nanoparticles disposed on said working electrode. In some embodiments, the water oxidation electrocatalyst is [NiFe]-layered double hydroxide, oxide, or oxy(hydroxide). In some embodiments, the [NiFe]-layered double hydroxide, oxide, or oxy(hydroxide) is in the form of nanoparticles disposed on said working electrode. In some embodiments, the system further comprises (i) a source of ultraviolet light, which source of ultraviolet light is positioned to shine on said water oxidation electrocatalyst. In some embodiments, the ultraviolet light is deep ultraviolet light. In some embodiments, the source of electricity is a battery. In some embodiments, the system further comprises: (h) a reference electrode at least partially immersed in said predominantly aqueous solution and electrically connected to said working electrode and said counter electrode. In some embodiments, the reference electrodePATENT is a standard hydrogen electrode. In some embodiments, the reference electrode is Hg / HgO. In some embodiments, the PFAS is a perfluoroalkyl acid. In some embodiments, the PFAS is a perfluoroalkyl carboxylic acid. In some embodiments, the PFAS is perfluorooctanoic acid or perfluorooctanoate. In some embodiments, the PFAS is a perfluorosulfonic acid. In some embodiments, the perfluorosulfonic acid is perfluorooctanesulfonic acid or perfluorooctanesulfonate. In some embodiments, the PFAS is a GenX chemical. In some embodiments, the GenX chemical is hexafluoropropylene oxide dimer acid anion. In some embodiments, the PFAS is has an alkyl chain and said alkyl chain is four carbons in length. In some embodiments, the PFAS is perfluorobutane sulfonate or perfluorobutanoate. In some embodiments, the system further comprises a heater to heat said electrolyte solution above room temperature. In some embodiments, the system further comprises a source introducing air or oxygen into said electrolyte solution. In some embodiments, the electrolyte solution further comprises 54 vol% or less non-aqueous solvent. In some embodiments, the electrolyte solution is 60 vol% or more water and 40 vol% or less non-aqueous solvent. In some embodiments, the electrolyte solution is 90 vol% or more water and 10 vol% or less non- aqueous solvent. In some embodiments, the non-aqueous solvent is ethanol, methanol, 1- propanol, butanol, or acetonitrile. In some embodiments, the alkali metal cations are lithium cations. In some embodiments, the concentration of hydroxide anions is 2 M or higher. In some embodiments, the concentration of hydroxide anions is 4 M or higher. In some embodiments, the concentration of hydroxide anions is 6 M or higher. In some embodiments, the concentration of hydroxide anions is 8 M or higher. In some embodiments, the concentration of hydroxide anions is 9 M or higher. In some embodiments, the concentration of hydroxide anions is 2 M or higher and said concentration of alkali metal cations is 2 M or higher. In some embodiments, the concentration of hydroxide anions is 4 M or higher and the concentration of alkali metal cations is 4 M or higher. In some embodiments, the concentration of hydroxide anions is 6 M or higher and the concentration of alkali metal cations is 6 M or higher. In some embodiments, the concentration of hydroxide anions is 8 M or higher and the concentration of alkali metal cations is 8 M or higher. In some embodiments, the concentration of hydroxide anions is 9 M ± 0.5 M and the concentration of alkali metal cations is 9 M ± 0.5 M. In some embodiments, the concentration of hydroxide anions is 8 M and the concentration of alkali metal cations is 8 M. In some embodiments, the concentration of hydroxide anions is 9 M and the concentration of alkali metal cations is 9 M.PATENT

[0010] In another group of embodiments, the invention provides methods for degrading per- and poly-fluoroalkyl substances (PFAS). The methods comprise subjecting one or more PFAS to electrocatalysis on a water oxidation electrocatalyst, the water oxidation electrocatalyst being disposed on a working electrode, the electrode being disposed in an electrolyte solution comprising (1) 46 vol% or more of water and (2) a concentration of hydroxide anions of 1 M or higher and a concentration of alkali metal cations of 1 M or higher, wherein said PFAS is subjected to said electrolysis by providing a first applied electric potential of -5 V to 5 V versus standard hydrogen electrode to said working electrode for a first period of time, thereby breaking at least one carbon-fluorine bond in said PFAS, thereby degrading said PFAS, provided said applied electric potential is not an open circuit potential unless said open circuit potential follows at least one application of an anodic or a cathodic applied electric potential. In some embodiments, the concentration of hydroxide anions is 2 M or higher. In some embodiments, the concentration of hydroxide anions is 4 M or higher. In some embodiments, the concentration of hydroxide anions is 6 M or higher. In some embodiments, the concentration of hydroxide anions is 8 M or higher. In some embodiments, the concentration of hydroxide anions is 9 M or higher. In some embodiments, the concentration of hydroxide anions is 2 M or higher and said concentration of alkali metal cations is 2M or higher. In some embodiments, the concentration of hydroxide anions is 4 M or higher and said concentration of alkali metal cations is 4 M or higher. In some embodiments, the concentration of hydroxide anions is 6 M or higher and said concentration of alkali metal cations is 6 M or higher. In some embodiments, the concentration of hydroxide anions is 8 M or higher and said concentration of alkali metal cations is 8 M or higher. In some embodiments, the concentration of hydroxide anions is 8 M ± 0.5 M and said concentration of alkali metal cations is 8 M ± 0.5 M. In some embodiments, the concentration of hydroxide anions is 9 M ± 0.5 M and said concentration of alkali metal cations is 9 M ± 0.5 M. In some embodiments, the concentration of hydroxide anions is 2 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 2 M or higher. In some embodiments, the concentration of hydroxide anions is 4 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 4 M or higher. In some embodiments, the concentration of hydroxide anions is 6 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 6 M or higher. In some embodiments, the concentration of hydroxide anions is 8 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 8 M or higher. In somePATENT embodiments, the concentration of hydroxide anions is 8 M ± 0.5 M and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 8 M± 0.5 M. In some embodiments, the concentration of hydroxide anions is 9 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 9 M or higher. In some embodiments, the concentration of hydroxide anions is 9 M ± 0.5 M and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 9 M± 0.5 M . In some embodiments, the electrolyte solution is 60 vol% or more water. In some embodiments, the electrolyte solution is 70 vol% or more water. In some embodiments, the electrolyte solution is 80 vol% or more water. In some embodiments, the electrolyte solution is 90 vol% or more water. In some embodiments, the electrolyte solution further comprises a non-aqueous solvent. In some embodiments, the non-aqueous solvent contains a concentration of ions sufficient for ion conductivity. In some embodiments, the working electrode is hydrophilic carbon fiber paper. In some embodiments, the water oxidation electrocatalyst is a metal oxide solid, metal hydroxide solid, or metal oxy(hydroxide). In some embodiments, the water oxidation electrocatalyst is a metallic material, nanostructured layered double hydroxide solid, nanostructured layered oxide solid, nanostructured layered oxy(hydroxide) solid, a perovskite, a polyoxometalate, or a metal- organic framework. In some embodiments, the water oxidation electrocatalyst is a metallic material, nanostructured layered double hydroxide solid, nanostructured layered oxide solid, or nanostructured layered oxy(hydroxide) solid, in which one of the layers comprises an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal. In some embodiments, the transition metal is a first-row transition metal. In some embodiments, the first-row transition metal is manganese or iron. In some embodiments, the post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin. In some embodiments, the nanostructured layered double hydroxide solid, said nanostructured layered oxide solid, or said nanostructured layered oxy(hydroxide) solid, comprises nickel mixed with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal. In some embodiments, the nanostructured layered double hydroxide solid, said nanostructured layered oxide solid, or said nanostructured layered oxy(hydroxide) solid comprises nickel mixed with an effective amount of a transition metal. In some embodiments, the water oxidation electrocatalyst is [NiFe]-layered double hydroxide, [NiFe]-nanostructured layered oxide solid, or [NiFe]- nanostructured layered oxy(hydroxide) solid. In some embodiments, the [NiFe]-layered double hydroxide, [NiFe]-nanostructured layered oxide solid, or [NiFe]-nanostructuredPATENT layered oxy(hydroxide) solid is in the form of nanoparticles disposed on said working electrode. In some embodiments, the water oxidation electrocatalyst is [NiMn]-layered double hydroxide, [NiMn]-nanostructured layered oxide solid, or [NiMn]-nanostructured layered oxy(hydroxide) solid. In some embodiments, the [NiMn]-layered double hydroxide, [NiMn]-nanostructured layered oxide solid, or [NiMn]-nanostructured layered oxy(hydroxide) solid is in the form of nanoparticles disposed on said working electrode. In some embodiments, the first applied electric potential is an anodic bias. In some embodiments, the first applied electric potential is 0.5 V to 5 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is 1.0 V to 2 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is 1.50 V to 1.75 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is 1.6 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is a cathodic bias. In some embodiments, the first applied electric potential is -0.5 V to -5 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is -1.0 V to -2 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is -1.5 V to -1.75 V versus standard hydrogen electrode. In some embodiments, the first applied electric potential is -1.6 V versus standard hydrogen electrode. In some embodiments, the electrolyte solution is at room temperature. In some embodiments, the electrolyte solution is at a temperature above room temperature. In some embodiments, the electrolyte solution is at a temperature of 30 ˚C to 99 ˚C. In some embodiments, the electrolyte solution is at a temperature of 60 ˚C to 85 ˚C. In some embodiments, the electrolyte solution is at a temperature of 70 ˚C. In some embodiments, the electrocatalysis is conducted under pressure greater than ambient air pressure. In some embodiments, the method further comprises shining ultraviolet light on said water oxidation electrocatalyst. In some embodiments, the ultraviolet light is deep ultraviolet light. In some embodiments, the electrocatalysis is conducted by cycles consisting of a period of time in which said first applied electric potential at said first polarity is applied to said water oxidation electrocatalyst for a period of time, referred to as an “ON” period, followed by a period of time in which an open circuit potential is present, referred to as an “OFF” period, with each pair of an ON period and of an OFF period considered as one “cycle”. In some embodiments, the ON period is 1 minute ± 10 seconds. In some embodiments, the OFF period is 2 minutes ± 10 seconds to 20 minutes. In some embodiments, the OFF period is 2 minutes ± 10 seconds to 10 minutes. In some embodiments, the OFF period is 3 minutes to 7 minutes. In some embodiments, the OFFPATENT period is 5 minutes ± 30 seconds. In some embodiments, the ON period is 1 minute ± 10 seconds, and said OFF period is 2 minutes ± 10 seconds to 20 minutes. In some embodiments, the electrocatalysis is conducted for 20-150 cycles. In some embodiments, the electrocatalysis is conducted for 40-130 cycles. In some embodiments, the electrocatalysis is conducted for 120 cycles ±5 cycles. In some embodiments, (a) the first applied electric potential is provided at a first polarity for a first time period, followed by (b) providing an open circuit potential for a second time period, and further comprising step (a)’: applying a second applied electric potential at a second polarity, which second polarity is opposite that of said first polarity, for a third time period, wherein which step (a)’ is performed between steps (a) and (b). In some of these embodiments, the first time period is 30 seconds ± 15 seconds, said second time period is five minutes ± 30 seconds to 10 minutes ± 30 seconds, and said third time period is 0.5 seconds to 5 seconds. In some embodiments, the first time period is 30 seconds, said second time period is five minutes ± 30 seconds, and said third time period is 0.5 seconds to 4 seconds. In some embodiments, the first applied electric potential is 0.75 to 3.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is 1.5 to 2.0 V vs standard hydrogen electrode. In some embodiments, the second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is 0.75 to 3.0 V vs standard hydrogen electrode and said second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is 1.5 to 2.0 V vs standard hydrogen electrode and said second applied electric potential is -0.75 to - 2.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is -0.75 to -3.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is -1.5 to 2.0 V vs standard hydrogen electrode. In some embodiments, the second applied electric potential is 0.75 to 2.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is -0.75 to -3.0 V vs standard hydrogen electrode and said second applied electric potential is 0.75 to 2.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is -1.5 to -2.0 V vs standard hydrogen electrode and said second applied electric potential is 0.75 to 2.0 V vs standard hydrogen electrode. In some embodiments, the first applied electric potential is 1.5 to 2.0 V vs standard hydrogen electrode and is is applied for 30 seconds ± 10 seconds, said second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode and is applied for 0.5 to 4 seconds, and said second time period during which an open circuit potential is present is five minutes ± 30 seconds to 10 minutes ± 30 seconds. InPATENT some embodiments, the PFAS is a perfluoroalkyl acid. In some embodiments, the PFAS is a perfluoroalkyl carboxylic acid or perfluoroalkyl carboxylate. In some embodiments, the PFAS is perfluorooctanoic acid. In some embodiments, the PFAS is perfluorooctanesulfonic acid or perfluorooctanesulfonate. In some embodiments, the PFAS is hexafluoropropylene oxide dimer acid anion. In some embodiments, the method further comprises mineralizing said fluorine atoms dissociated from said PFAS by said electrocatalysis, the method comprising adding to said electrolyte solution cations that form water-insoluble fluorides, thereby mineralizing said fluorine atoms dissociated from said PFAS. In some embodiments, the cations that form water-insoluble fluorides are one or more of magnesium, calcium, barium, gallium, copper, zinc, zirconium, vanadium, chromium, and gold cations. In some embodiments, the cations that form water-insoluble fluorides are one or more of magnesium, calcium, copper, and zinc cations. In some embodiments, the cations that form water- insoluble fluorides are calcium cations. In some embodiments, the alkali metal cations are lithium cations. In some embodiments, the PFAS is present in said electrolyte solution at a concentration of 0.05 mM to 2 mM. In some embodiments, the PFAS is present in said electrolyte solution at a concentration of 0.05 mM to 1 mM. In some embodiments, the PFAS is present in said electrolyte solution at a concentration of 0.05 mM to 0.5 mM. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figs.1A-D. Fig.1A. Figure 1A is a graph showing the defluorination of 0.50 mM PFOS in 8.0 M aq LiOH as a function of number of pulsed electrolysis cycles (1 cycle = 1 min ON time at an applied potential Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential), [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode, with ultraviolet irradiation, no stirring. X axis: number of cycles. Y axis: fluoride present in the solution, in parts per million (“ppm”). The dashed line indicates 100% defluorination. Fig.1B. Fig.1B is a graph showing the defluorination of PFOS in 8.0 M aq LiOH electrolyte as a function of PFOS concentration; electrocatalysis conditions: 20 pulsed electrolysis cycles (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential), [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode, with ultraviolet irradiation, no stirring. X axis: Concentration of PFOS present in starting solution, in mM. Y axis: percent of defluorination of PFOS present in the starting solution. Fig.1C. Fig.1C is a graph showing the defluorination of 0.50 mM PFOS in 8.0 M aq LiOH as a function of applied potential at 10 (bottom row of boxes), 20 (middle row of boxes), or 60 (top row of boxes) pulsed electrolysis cycles (1 cycle = 1 min ON time at Eapp= +1.6 VRHE,PATENT followed by 5 min OFF time at open circuit potential), [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode, with ultraviolet irradiation, no stirring. The dashed line indicates 100% defluorination. X axis: Applied potential. Y axis: amount in ppm of fluoride released from PFOS present in the starting solution. Fig.1D. Fig.1D is a graph showing the estimated capital expense and operational electrical energy requirements of this work and existing PFAS destruction techniques, based on literature data [31-40], adapted from ref. [1]; BDD, boron-doped diamond. Figure 1D is from an open access article published under the terms and conditions of the Creative Commons Attribution (CC BY) license.

[0012] Figs.2A-E. Fig.2A. Fig.2A is a graph showing fluoride concentrations after 2 h of continuous chronoamperometry of 1.0 mM PFOS in solution as a function of the type of alkali cation. X axis: 8M aqueous solutions of, from left, LiOH, NaOH, KOH, and CsOH. Y axis: amount in ppm of fluoride released from PFOS present in the starting solution. Electrocatalysis conditions: 20 pulsed electrolysis cycles (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential), [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode, with ultraviolet irradiation, no stirring. Fig.2B. Fig.2B is a graph showing fluoride concentrations after 2 h of continuous chronoamperometry of 1.0 mM PFOS in solution as a function of the type of electrolyte anion. X axis: 8M aqueous solutions of LiOH or of LiClO4. Y axis: amount in ppm of fluoride released from PFOS present in the starting solution. Electrocatalysis conditions were as described for Fig.2A. Fig.2C. Fig.2C is a graph showing fluoride concentrations after 2 h of continuous chronoamperometry of 1.0 mM PFOS in solution as a function of alkali ion concentration, with the boxes and triangles showing the measured data and the lines showing the calculated linear fits for each alkali ion tested. Y axis: amount in ppm of fluoride released from PFOS present in the starting solution. X axis: Concentration of alkali ions, in Moles. Boxes: aqueous LiOH. Triangles: Aqueous KOH. Electrocatalysis conditions were as described for Fig.2A. Fig.2D. Fig.2D is a graph showing fluoride concentrations after 2 h of continuous chronoamperometry of 1.0 mM PFOS in solution as a function of electrolyte pH value, with the boxes and triangles showing the measured data and the lines showing calculated linear fits. Y axis: amount in ppm of fluoride released from PFOS present in the starting solution. X axis: pH of electrolyte solutions. Boxes: aqueous LiOH. Triangles: Aqueous KOH. Electrocatalysis conditions were as described for Fig.2A. Fig.2E. Fig.2E is a graph showing the defluorination yield in percent of 0.50 mM PFOS or of 0.50 mM PFOA in 8.0 M aq LiOH. Electrocatalysis conditions were as described for Fig.2A.PATENT

[0013] Figs.3A-C. Fig.3A. Fig.3A shows high-resolution XPS data and peak fits in the C 1s (y-axis magnified by a factor of 10), O 1s, F 1s, and S 2p core level regions of [NiFe]- layered double hydroxide on hydrophilic carbon fiber paper electrodes after 6 h in 8.0 M aq LiOH electrolyte with ultraviolet irradiation and no stirring, electrocatalyzed at 60 pulsed electrolysis cycles (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential), with 0.5 mM PFOS. Fig.3B. Fig.3B shows high-resolution XPS data and peak fits in the C 1s (y-axis magnified by a factor of 10), O 1s, F 1s, and S 2p core level regions of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes after 6 h in 8.0 M aq LiOH electrolyte with ultraviolet irradiation and no stirring, at open circuit potential, with 0.5 mM PFOS. Fig.3C. Fig.3C shows high-resolution XPS data and peak fits in the C 1s (y-axis magnified by a factor of 10), O 1s, F 1s, and S 2p core level regions of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes after 6 h in 8.0 M aq LiOH electrolyte with ultraviolet irradiation and no stirring, at open circuit potential without PFOS. The asterisks denote surface-adsorbed species.

[0014] Figs.4A-E. Fig.4A. Fig.4A is a graph showing the defluorination of 0.5 mM PFOS in 8.0 M aq LiOH after continuous 2 h chronoamperometry as a function of time elapsed before stirring the electrolyte at 800 rpm. Y axis: concentration of fluoride released from PFOS, in ppm. X axis: time elapsed before stirring for 5 minutes. Fig.4B. Fig.4B is a graph showing the defluorination of 0.5 mM PFOS in 8.0 M aq LiOH after continuous 2 h chronoamperometry as a function of stirring speed. Y axis: concentration of fluoride released from PFOS, in ppm. X axis: stirring speed, in rpm. Fig.4C. Fig.4C is a graph showing the defluorination of 0.5 mM PFOS in 8.0 M aq LiOH after 20 cycles of pulsed electrolysis (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential) as a function of stirring speed. Y axis: concentration of fluoride released from PFOS, in ppm. X axis: stirring speed, in rpm. The dashed lines in Figs.4A-4C indicate 100% defluorination. Fig.4D. Fig.4D is a graph showing the defluorination of 1.0 mM PFOS in 8.0 M aq LiOH after electrocatalysis as a function of the duration of ON time intervals, while keeping the total ON time constant; no stirring. Y axis: concentration of fluoride released from PFOS, in ppm. X axis: duration of electrocatalysis ON and OFF times. Fig.4E. Fig. 4E is a graph showing Defluorination of 1.0 mM PFOS in 8.0 M aq LiOH after electrocatalysis as a function of the duration of OFF time intervals; no stirring. Electrocatalysis conditions: [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode, applied potential Eapp= +1.6 VRHE,with ultraviolet irradiation.PATENT

[0015] Figs.5A-C. Fig.5A. Fig.5A shows survey and high-resolution XPS data and peak fits in the C 1s and Li 1s core level regions of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes after 6 h in 8.0 M aq LiOH electrolyte with ultraviolet irradiation and no stirring, electrocatalyzed at 60 pulsed electrolysis cycles (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential), with 0.5 mM PFOS. Fig.5B. Fig.5B shows survey and high-resolution XPS data and peak fits in the C 1s and Li 1s core level regions of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as stated for Fig.5A, but at open circuit potential, with 0.5 mM PFOS. Fig.5C. Fig.5C shows survey and high- resolution XPS data and peak fits in the C 1s and Li 1s core level regions of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as for Fig.5A, but at open circuit potential, without PFOS.

[0016] Figs.6A-F. Fig.6A. Fig.6A shows high-resolution XPS data in the Ni 2p core level region of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes after 6 h in 8.0 M aq LiOH electrolyte with ultraviolet irradiation and no stirring, electrocatalyzed at 60 pulsed electrolysis cycles (1 cycle = 1 min ON time at Eapp= +1.6 VRHE, followed by 5 min OFF time at open circuit potential), with 0.5 mM PFOS. Fig.6B. Fig.6B shows high- resolution XPS data in the Ni 2p core level region of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as for Fig.6A, but at open circuit potential, with 0.5 mM PFOS. Fig.6C. Fig.6C shows high-resolution XPS data in the Ni 2p core level region of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as for Fig.6A, but at open circuit potential, without PFOS. Fig.6D. Fig.6D shows high-resolution XPS data in the Fe 2p core level region of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as for Fig.6A. Fig.6E. Fig.6E shows high-resolution XPS data in the Fe 2p core level region of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as for Fig.6B. Fig.6F. Fig.6F shows high- resolution XPS data in the Fe 2p core level region of [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper electrodes under the same conditions as for Fig.6C.

[0017] Figs.7A-C. Fig.7A. Fig.7A is a graph showing the defluorination of 0.5 mM PFOS in 8.0 M aq LiOH after 20 cycles of pulsed electrolysis (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential; no stirring) as a function of presence or absence of catalyst or ultraviolet irradiation. Y axis: concentration of fluoridePATENT released from PFOS, in ppm. X axis: conditions stated. Fig.7B. Fig.7B is a graph showing the defluorination of 0.5 mM PFOS in 8.0 M aq LiOH after 20 cycles of pulsed electrolysis (1 cycle = 1 min ON time at Eapp = +1.6 VRHE, followed by 5 min OFF time at open circuit potential; no stirring) as a function of applied potential, with ultraviolet irradiation. Y axis: concentration of fluoride released from PFOS, in ppm. X axis: left bar: applied potential, right bar: open circuit potential. Fig.7C. Fig.7C is a graph showing the defluorination of 0.5 mM PFOS in 8.0 M aq LiOH after 20 cycles of pulsed electrolysis (1 cycle = 1 min ON time at Eapp= +1.6 VRHE, followed by 5 min OFF time at open circuit potential; no stirring) as a function of the presence (left hand bar) or absence (right hand bar) of ultraviolet irradiation. Figs.7A-C: dashed line at top indicates 100% defluorination of PFOS.

[0018] Fig.8. Fig.8 presents, on the left side, SEM images of hydrophilic carbon fiber paper; scalebars are 20 µm. On the right hand side, Fig.8 presents histograms of (vertical black lines) diameters of the carbon fibers in the SEM images on the left, and (solid horizontally moving line) Gauss fits.

[0019] Figs.9A and B. Fig.9A. Fig.9A is a graph showing the defluorination of 0.5 mM hexafluoropropylene oxide dimer acid anion (HFPO-, sometimes referred to herein as “GenX”) in stagnant 9.0 M aqueous LiOH electrolyte as a function of pulsed electrolysis cycles (1 cycle = 30 s at EON = 1.6 VRHE, followed by 1 s at Erev = –1.0 VRHE, followed by 5 min at OCP), with deep UV light irradiation. Fig.9B. Fig.9B is a graph showing the defluorination in stagnant 9.0 M aqueous LiOH electrolyte as a function of HFPO- (“GenX”) concentration, using 120 pulsed electrocatalysis cycles, with deep UV light irradiation. Both Figs.9A and 9B: The dashed lines indicate 100% defluorination.

[0020] Figs.10A-D. Each of Figs.10A-D is a graph showing defluorination of 0.5 mM HFPO- in stagnant 9.0 M aqueous LiOH electrolyte, with deep UV light irradiation and 120 cycles of pulsed electrolysis, with varied pulse train sequences. Fig.10A. Fig.10A is a graph showing the effect of varying the Erev in pulse train sequences in which each of the 120 cycles consists of 30 s at EON= 1.6 VRHE, followed by 1 s at varied Erev, followed by 5 min at open circuit potential (“OCP”). Fig.10B. Fig.10B is a graph showing the effect of varying the EON.The pulse train sequence of each of the 120 cycles consists of: 30 s at the EONs shown in the graph, followed by 1 s at Erev= –1.0 VRHE, followed by 5 min at OCP. Fig.10C. Fig. 10C is a graph showing the effect of varying the time at Erev in each cycle. The pulse train sequence of each of the 120 cycles consists of: 30 s at EONat 1.6 VRHE, followed by timePATENT interval at Erevat –1.0 VRHE, followed by 5 min at OCP. Fig.10D. Fig.10D is a graph showing the effect of varying the time at OCP. The pulse train sequence of each of the 120 cycles consists of 1 cycle = 30 s at EON at 1.6 VRHE, followed by 1 s at Erev = –1.0 VRHE, followed by a time interval at OCP as set forth on the X axis.

[0021] Figs.11A and B. Fig.11A. Fig.11A is a graph showing the defluorination of 0.5 mM HFPO- in different molar solutions of stagnant aqueous LiOH electrolyte at 25℃, with deep UV light irradiation and 120 cycles of pulsed electrolysis (1 cycle = 30 s at EON= 1.6 VRHE, followed by 1 s at Erev = –1.0 VRHE, followed by 5 min at OCP). Fig.11B. Fig.11B is a graph showing the defluorination of 0.5 mM GenX under the same conditions as in Fig.11A, but with the electrocatalysis performed at 70℃. Both Figs.11A and 11B: The dashed lines indicate 100% defluorination.

[0022] Figs.12A-C. Fig.12A. Fig.12A is a graph showing the defluorination of PFOS in up to 6.0M aqueous [LiOH]x–[NaOH](1–x)electrolyte, with x = 0, 0.25, 0.5, or 0.75). Conditions: pulsed electrolysis (60 cycles, with 1 cycle = 1 min at 1.6 V vs RHE, followed by 5 min at open circuit potential, deep UV light irradiation) in stagnant electrolyte. The line is a linear fit. Error bars represent standard deviations of triplicate measurements. Fig.12B. Fig. 12B is a graph showing the defluorination of PFOS in up to 6.0M aqueous [LiOH]x– [NaOH](1–x) electrolyte, with x = 0, 0.25, 0.5, or 0.75). Conditions are otherwise the same as those stated for Fig.12A. The line is a linear fit. Error bars represent standard deviations of triplicate measurements. Fig.12C. Fig.12C is a graph showing the defluorination of PFOS in up to 6M aqueous [LiOH]x–[LiClO4](1–x) (x = 1, 0.25, 0.5, 0.75, 0) electrolyte. Conditions: pulsed electrolysis (60 cycles, with 1 cycle = 1 min at 1.6 V vs RHE, followed by 5 min at open circuit potential, deep UV light irradiation) in stagnant electrolyte. The lines is a power law fit. Error bars represent standard deviations of triplicate measurements.

[0023] Figs.13A-D. Fig.13A. Fig.13A is a graph showing integrated 2D-7Li–19F-HOESY- NMR cross peak volumes as a function of LiOH electrolyte. Error bars of NMR integrations are standard deviations of volume fits. Fig.13B. Fig.13B is a graph showing 2D-7Li–19F- HOESY-NMR data of of PFOS defluorination in 8.0 M aqueous LiOH. Fig.13C. Fig.13C is a graph showing integrated 2D-7Li–19F-HOESY-NMR cross peak volumes as a function of LiClO4electrolyte at 6M or of LiOH electrolyte at the molar concentrations stated. Error bars of NMR integrations are standard deviations of volume fits. Fig.13D. Fig.13D is a graph showing PFOS defluorination as a function of the LiOH concentration. Conditions: pulsedPATENT electrolysis (60 cycles, with 1 cycle = 1 min at 1.6 V vs RHE, followed by 5 min at open circuit potential, deep UV light irradiation) in stagnant electrolyte. Error bars represent standard deviations of triplicate measurements.

[0024] Fig.14. Fig.14 is a graph showing the defluorination of PFOS in 8.0 M aqueous LiOH as a function of the electrolyte temperature. Conditions: pulsed electrolysis (10 cycles, with 1 cycle = 1 min at 1.6 V vs RHE, followed by 5 min at open circuit potential, deep UV light irradiation) in stagnant electrolyte. The line is a linear fit. Error bars represent standard deviations of triplicate measurements.

[0025] Fig.15. Fig.15 is a schematic drawing of how Li–F ion pairing, combined with pulsed electrolysis and competitive anodic adsorption of OH– ions, facilitates the removal of fluoride from the anode surface. Legend: CFP, carbon fiber paper; Eapp, applied potential; EDL, electrochemical double layer; OCP, open circuit potential. Legend for “ball and stick” model of anionic PFOS: dark gray spheres = oxygen atoms, sphere with light shading = sulfur atom, spheres with slanted lines = carbon atoms, gray spheres with light centers = fluorine atoms. DETAILED DESCRIPTION Introduction

[0026] As set forth in the Background, perfluoroalkyl and polyfluoroalkyl substances, commonly referred to as “PFAS,” are synthetic chemicals that have been shown to have toxicity in animal models. Unfortunately, they are now widely distributed in the environment, and, based on a study of serum samples taken from individuals in a representative sample of the American public, most Americans carry one or more PFAS in their body. See, Kato, et al., Environ Sci Technol.2011 Oct 1;45(19):8037-45. doi: 10.1021 / es1043613. See also, Lau, et al., Toxicol Sci., 2007, 99(2):366-94. doi: 10.1093 / toxsci / kfm128. Methods of degrading PFAS would be highly desirable. PFAS are, however, extremely stable compounds and current technologies for degrading PFAS have been limited by a combination of high cost, high energy requirements, low efficacy, and the production of toxic byproducts.

[0027] The present inventors recently developed systems and methods for degrading PFAS by electrocatalysis using a water oxidation electrocatalyst in an aqueous solution. Those methods and systems are described in International Patent Application No. PCT / US2023 / 017345 (hereafter, the “‘345 PCT application”), published as InternationalPATENT Publication No. WO 2023 / 196259. The systems and methods described in the ‘345 PCT application allow PFAS to be degraded to low toxicity compounds at low energy cost, low capital expense, high efficiency, and provide the ability to do so under ambient conditions, and without the need for chemical oxidants.

[0028] Surprisingly. the present invention provides improvements to the systems and methods of the ‘345 PCT application that dramatically increase the rate at which PFAS can be degraded. Systems and methods employing these improvements are expected to further decrease the cost of degrading PFAS, and to further reduce the energy requirements to do so, both in the lab and in environmental settings, compared not only to the techniques known in the art prior to the ‘345 PCT application, but also to the systems and methods taught in the ‘345 PCT application. This combination of reduced cost and reduced energy needs increases the practicality of remediating PFAS in the environment and is expected to make the inventive systems and methods of this disclosure preferred for use by those wishing to remediate environmental contamination.

[0029] The ‘345 PCT application sets forth systems and methods using electrocatalysis to break down perfluoroalkyls using a metal-based water oxidation catalyst. The systems and methods of the present invention generally follow the teachings set forth in the ‘345 PCT application, but with improvements described below that dramatically improve the rate of degradation of PFAS. It is expected that persons interested in degrading PFAS can readily familiarize themselves with the teachings of the ‘345 PCT application regarding degrading PFAS by electrocatalysis using metal-based water oxidation catalysts. The discussion below will therefore focus on how to improve the degradation of PFAS by modifying the systems and methods taught in the ‘345 PCT application, although certains aspects will be mentioned here for completeness, clarity, or both.

[0030] For clarity, it is noted that the bond that makes PFAS so hard to degrade is the C−F bond. In the studies underlying the present disclosure, exemplar PFAS were used that were the only fluorine-containing components in the systems. Thus, in the studies underlying this disclosure, detection in the solution of any fluorine atoms (such as in the form of fluoride ions) in a solution containing a PFAS and subjected to electrocatalysis indicated that one or more C−F bonds in the PFAS in the solution were broken, resulting in the release of fluorine atoms from the PFAS. Any form of fluorine or fluoride released from the PFAS was then available to be reacted to form a less-toxic compound than the PFAS in which it wasPATENT originally present in the system. As used herein, the terms “defluorination” and “degrading” with respect to a PFAS refer to the breaking of C−F bonds in that PFAS that results in the release of fluorine atoms or ions from the PFAS into the surroundings of the PFAS, such as the electrolyte in which the PFAS is being subjected to electrocatalysis. As used herein, the terms “defluorination” and “degrading” with respect to PFAS are equivalent unless otherwise stated or required by context. Finally, it is noted that the phrase “per- or polyfluoroalkyl substances,” abbreviated as “PFAS,” refers to members of a group of compounds, and is plural. The studies reported below typically included only a single exemplar member of the group of PFAS compounds at a time to determine whether the systems and methods employed in a particular study did or did not result in the degradation of the particular PFAS compound that was used in that study. For convenience of reference, the term PFAS as used herein may refer to a single compound within the group of per- or polyfluoroalkyl substances, or may refer to two or more, unless otherwise specified or otherwise required by context.

[0031] In some embodiments, the PFAS is a fluoroether with six carbon atoms, sometimes referred to as a “GenX chemical.” Hexafluoropropylene oxide (HFPO) dimer acid and its ammonium salt are GenX chemicals, and the ammonium salt is sold under the trade name “GenX.” As reported below, pulsed electrolysis pulse trains with polarity reversal completely defluorinated the HFPO dimer acid anion, HFPO–, by UV light-assisted electrocatalysis at exemplar [NiFe]-(OH)2–hydrophilic carbon fiber paper anodes. Since all GenX chemicals are by definition fluoroethers with chains of six carbon atoms, it is believed that the systems and methods shown herein to completely defluorinate HFPO–will likewise be successful at completely defluorinating other GenX chemicals, as well as other PFAS that have six-carbon atom chains, but that are not fluoroethers, such as perfluorohexanesulfonic acid, which has a sulfonic acid functional group rather than an ether group. Some of the most common of the previous generation of PFAS compounds, such as perfluorooctanoic acid (“PFOA”) and perfluorooctanesulfonic acid (“PFOS”), have eight-carbon atom chains. For convenience of reference, PFAS with chains of carbon atoms longer than six will sometimes be referred to as “conventional” PFAS or “PFAS with more a carbon chain of more than six carbon atoms” to distinguish them from GenX chemicals or other more recently developed PFAS with chains of six carbon atoms.

[0032] In some embodiments, the methods discussed below use electrocatalysis in which a PFAS is subjected to cycles in which a first electric potential is applied to the system for aPATENT first period of time, followed by a period of time of open circuit potential. As discussed below, HFPO–required more cycles to completely defluorinate it than did the conventional PFAS compounds. As shown in Fig.11A, HFPO–also had to be subjected to electrocatalysis in 9.0 M LiOH to achieve complete defluorination at room temperature, although the molarity could be reduced to 8.0 M LiOH when the electrocatalysis was conducted at an elevated temperature (70 ℃), as shown in Fig.11B. In some embodiments, a second electric potential is applied for a period of time following application of the first electric potential, and before the period of time in which the open circuit potential is applied. The second applied electric potential is of reverse polarity to that of the first applied electric potential and is applied for a shorter period of time than that used for the first applied electric potential. PFAS and Degradation of PFAS

[0033] Studies underlying the present disclosure showed that the inventive systems and methods successfully degraded multiple PFAS tested as exemplars of the group. In some studies, the compound perfluorooctanoic acid (“PFOA,” a PFAS with an eight-carbon atom, or “C8” alkyl chain) was used, as it is a widely studied member of the PFAS group that has been used worldwide as an industrial surfactant and is also one of the most common PFAS compounds found in the blood serum of Americans studied to date.

[0034] At pHs above its pKa, perfluorooctanoic acid will instead be present in its anionic form, perfluorooctanoate. All of the PFAS tested to date have pKa values below 3. As most surface and ground waters have pH values closer to neutral pH than below pH 3, PFAS such as perfluorooctanoic acid will typically be present in the environment in their anionic form. Reference to the carboxylic acid form of PFOA, however, alerts the reader to the functional, or “head” group of the PFAS and assists in identifying structural features the PFAS has in common with other PFAS. Further, whether the PFAS is in its acid form or its anionic form does not change the components of the inventive systems by which the PFAS can be degraded or require changes to the steps of the inventive methods disclosed herein to degrade the PFAS. For convenience of reference, therefore, PFAS that have an acid form and an anionic form will generally be referred to herein by their acid form, with the understanding that they may be present in their anionic form in water to be remediated. For example, perfluorooctanesulfonic acid (“PFOS”), which is present in the anionic form perfluorooctanesulfonate depending on the pH of the solution in which it is present, will generally be referred to herein by its acid form (i.e., perfluorooctanesulfonic acid.PATENT PFOA has a carboxylic acid “head group” and a perfluorinated, n-octyl "tail group" and is therefore considered a perfluoroalkyl carboxylic acid. Given the structural similarity with other perfluoroalkyl carboxylic acids, the results oxidizing PFOA are expected to obtain with other perfluoroalkyl carboxylic acids, such as perfluorononanoic acid (“PFNA”) and perfluorodecanoic acid (“PFDA”), as well as with other perfluoroalkyl acids.

[0035] Further, since the C̶ F bond in PFOA is the same in PFAS more generally, which are characterized by their perfluoroalkyl composition, PFOA shares structural identity with other members of the PFAS group. Similar results were obtained using a second exemplar PFAS, perfluorooctanesulfonic acid, or “PFOS,” which bears a different functional head group than does PFOA. Further, studies revealed that an exemplar PFAS fluoroether with six carbon atoms in its alkyl chain (a “C6 PFAS”), hexafluoropropylene oxide (HFPO) dimer acid anion (HFPO-), sometimes referred to by its trade name, “GenX,” could be completely degraded by embodiments of the inventive electrocatalytic systems and methods, showing that the systems and methods are not limited to degrading PFAS having carbon chains of a particular length.

[0036] The hexafluoropropylene oxide dimer acid anion HFPO- structurally can be considered to consist of two three-carbon fluorocarbon branches adjacent to the ether moiety. This suggests that systems and method of the invention can defluorinate shorter-chain PFAS.

[0037] This expectation was confirmed by studies showing that the systems and methods initially used to degrade PFOA and PFOS also degrade PFAS with four-carbon chain PFAS (“C4 PFAS”). Studies were conducted using two exemplar C4 PFAS, perfluorobutane sulfonate (“PFBS”) and perfluorobutanoate (“PFBA”). Subjecting these two C4 PFAS to conditions that were sufficient to completely degrade PFOS resulted in >90% defluorination of each of PFAS. It is expected that increasing the number of cycles, increasing the molarity of the electrolyte, increasing the temperature at which the electrocatalysis is conducted, or combining two or more of these adjustments, would result in complete defluorination of PFBS and PFBA and, by extension, of other C4 PFAS. Degrading over 90% of a C4 PFAS present in a given sample, of course, is itself an almost complete remediation of the PFAS present in the sample.

[0038] As noted, the first PFAS subjected to embodiments of the inventive systems and methods were exemplar C8 PFAS. Longer carbon chains are more hydrophobic than shorter ones, and are thus less soluble in water and more prone to adsorption on the anodes of the electrocatalytic systems than are C8 PFAS. The studies to date defluorinating C4 PFAS andPATENT C8 PFAS did not evidence a substantial difference in ease of defluorination based on chain length. The results to date lead us to expect that PFAS with sulfonates (sulfonic acid) groups, PFAS with longer chain lengths will be degraded by the inventive systems and methods, as will PFAS with carboxylic acid groups (or their respective anions). PFAS with other structural motifs have not yet been subjected to electrocatalysis by the inventive systems and methods, but it is expected that most, if not all, will be susceptible to degradation by the inventive systems and methods. PFAS compounds with any particular structural motif of, and any particular PFAS of interest can, of course, be readily tested by the systems and methods set forth herein to determine whether its fluorine atoms can be dissociated by the water oxidation catalyst while in the electrocatalyst aqueous solution.

[0039] It is noted that PFOS was completely defluorinated after 80 cycles of pulsed electrocatalysis, while HFPO–required more cycles to be completely defluorinated. HFPO–was however, approximately 90% defluorinated after 80 cycles, as shown in Fig.9A. Figs. 11A and 11B show that the degradation of HFPO–was increased notably by increasing the molarity of Li+cations and OH- anions in the electrolyte, but that all of the molar concentrations tested degraded well over two-thirds of the HFPO–present in the system. Approximately 75% of PFOA was degraded after 2 hours of continous (non-pulsed) electrocatalysis, while more than 90% of PFOS was degraded after 2 hours under the same conditions. While more PFOS was degraded than was PFOA under the same conditions, the majority of each PFAS present in the system was degraded. Following the teachings of this disclosure, it is expected that the practitioner can adjust the electrocatalytic conditions (such as molarity of the Li+cations and OH- anions in the electrolyte, the temperature, the use of pulsed electrocatalysis and the number of cycles) to achieve degradation of some or all of PFAS present in a sample. Degradation of GenX and GenX Chemicals

[0040] Per the National Service Center for Environmental Publications (NSCEP) of the U.S. Environmental Protection Agency, “GenX” is a trade name for a technology developed to produce high-performance fluoropolymers that do not contain PFOA. A fact sheet on GenX from the NSCEP states that the major chemicals associated with the GenX technology are hexafluoropropylene oxide (HFPO) dimer acid and its ammonium salt. In water, both HFPO dimer acid and its ammonium salt dissociate to form the HFPO dimer acid anion HFPO–. As used herein, the term “GenX chemicals” refers to fluoroethers with six carbon atoms in theirPATENT alkyl chains. As noted above, hexafluoropropylene oxide (HFPO) dimer acid ammonium salt is sold under the trade name “GenX.” As used herein, however, the term “GenX,” used with reference to degradation of a specific PFAS compound, as opposed to the technology used to produce GenX chemicals, refers specifically to the HFPO dimer acid anion HFPO–.

[0041] GenX chemicals consist of fluoroethers with six carbon atoms in their alkyl chains, aiming at easier degradability and less toxicity than the longer-chain PFOA. See, Hughes, et al. Sci Total Environ, 2024, 908, article 168415; doi.org / 10.1016 / j.scitotenv.2023.168415. Nevertheless, GenX chemicals have contaminated oceans and the environment worldwide. They have become the predominant PFAS in many instances over time, as HFPO–may be harder to break down than PFOA. The higher chronic toxicity of HFPO–compared to its predecessor PFOA has been established, including by the U.S. Environmental Protection Agency. Human exposure to HFPO–leads to adverse health effects similar to those caused by PFOA. Therefore, remediating the presence of HFPO–in drinking water or environmental water is as important as the remediation of PFOA.

[0042] Although many studies exist on advanced reduction processes to degrade PFAS (see, e.g., Hughes, supra; Mousset and Doudrick, Curr. Opin. Electrochem., 2020, 22:221-227), oxidative PFAS defluorination is useful because efficient reductions require oxygen-free conditions. Oxygen exclusion is not practical on a large scale, especially in aqueous electrolyte in which the counter electrode reaction is water oxidation. The use of oxidative PFAS defluorination processes has the advantage of operating under ambient air conditions, eliminating the need for a membrane to separate the anodic and cathodic transformations. This simplifies the design of electrolyzers and enhances their cost-effectiveness.

[0043] Much reported research on HFPO–degradation has focused on laboratory-scale electrochemical systems with boron-doped diamond (BDD) electrodes because of their activity and stability. A recent technoeconomic analysis showed that BDD is not scalable for global PFAS destruction because of the high cost of the precious material. Sulfate-assisted HFPO–defluorination by at a ultrananocrystalline BDD anode on a niobium substrate achieved about 60% defluorination. Sulfate-radical-assisted HFPO–degradation at BDD anodes reached 71% defluorination. An electro-Fenton process, using a graphene-Ni-foam cathode paired with a BDD anode achieved 92.2% of HFPO–mineralization after 6 h of treatment. Olvera-Vargas et al., Chem. Eng. J, 2022, vol.430 article 132686; doi.org / 10.1016 / j.cej.2021.132686. Electroreduction was utilized to enhance the overallPATENT electro-degradation, achieving 92% defluorination of HFPO–by using a BDD anode and a gold cathode. (Hughes, supra.) A BDD-free UV / bisulfite system that utilized α-Fe2O3nanoparticles reached HFPO–defluorination of 88 and 57% under anaerobic and aerobic conditions, respectively. Electrooxidation by a commercial titanium suboxide reactive membrane anode defluorinated ca.40% of HFPO–. (Yang, et al., Environ. Res.2022, 204, 111995). Commercially viable systems for the destruction of PFAS, including HFPO–, however, would preferably not use auxiliary (radical forming) chemicals that are consumed, would preferably completely defluorinate PFAS molecules, and would preferably be solely comprised of nonprecious materials to enable widespread deployment.

[0044] The studies reported in the Examples, below, demonstrate that the systems and methods disclosed herein degraded (defluorinated) the exemplar GenX chemical HFPO dimer acid anion HFPO–. As all GenX chemicals, like the prototype GenX chemical HFPO dimer acid anion HFPO–, are fluoroethers with six carbon atoms in their alkyl chains and therefore share the same chemical backbone, it is expected that the systems and methods disclosed herein will degrade other GenX chemicals as they did HFPO–. Improving the Electrocatalytic Degradation of PFAS Using High Concentrations of OH- And Alkali-Metal Cations

[0045] We turn now to describing the multiple improvements in methods for degrading PFAS that dramatically improve the rate at which PFAS are degraded over even the systems and methods taught in the ‘345 PCT application.

[0046] The ‘345 PCT application reported studying the degradation of exemplar PFAS using an exemplar electrolyte, KOH at a concentration of 100 mM (which can also be stated as 0.1M), a concentration commonly used for electrocatalysis. Surprisingly, studies of the electrocatalytic degradation of an exemplar PFAS revealed that increasing the concentration of the salt present in the electrolytic solution by over an order of magnitude from the concentration commonly used resulted in dramatic increases in degradation of the PFAS. Studies underlying the present disclosure revealed that increasing the concentration of anions and cations in the electrolyte to 1M or more (an order of magnitude higher than the 0.1M concentration commonly used in electrocatalysis), notably increased the amount of PFAS degraded over the same interval of time. Further surprisingly, increasing the concentrations of electrolyte in the electrolyte solution 20, 40, 60 and 80 times the 0.1M concentration usually used in electrocatalysis increased the rate at which PFAS was degraded by multiplesPATENT compared to degradation of the PFAS at 0.1M. As shown in Fig.2C, the use of 4M KOH as the electrolyte more than quadrupled the amount of PFAS degraded compared to 0.1M KOH, while increasing the concentration of KOH to 8M (80 times the molar concentration used in the studies reported in the ‘345 PCT application, or almost two orders of magnitude higher than the 0.1M concentration used in standard electrocatalysis) resulted in approximately octupling the amount of PFAS degraded compared to the amount degraded over the same time period when the KOH was present at a concentration of 0.1M, as reflected by the concentration of fluoride in the solution following the electrocatalysis. (As noted above, PFAS was the only source of fluoride in the systems in the studies reported here; thus, an increase in the concentration of fluoride in the solution indicated an increase in the degradation of PFAS present in the system during the electrocatalysis.)

[0047] Even more surprisingly, the degradation of PFAS can be markedly increased by changing the cation used in the electrolyte solution, so long as a high concentration of hydroxide anions (OH-) is also present. Using LiOH as the electrolyte rather than the commonly-used electrolyte KOH resulted in dramatically higher rates of PFAS degradation compared to the degradation of the same PFAS in KOH as the electrolyte. Moreover, the degree of difference in the rate at which an exemplar PFAS was degraded in LiOH as the electrolyte compared to KOH as the electrolyte increase dramatically as the concentration of the respective bases increased. The degradation of PFAS when 8M LiOH was the electrolyte was approximately 1.75x that of the degradation of PFAS when 8M KOH electrolyte was used as the electrolyte, as shown in Fig.2C, and approximately double the rate of the degradation of PFAS in 8M KOH electrolyte, as shown in Fig.2A.

[0048] Further, a study comparing four different electrolytes, KOH, NaOH, CsOH, and LiOH, at the high molar concentration shown to dramatically increase PFAS degradation, showed that LiOH was suprisingly better than any of the other three electrolytes at promoting the degradation of PFAS by electrocatalysis. Fig.2A shows the results of four parallel studies, each of which subjected an exemplar PFAS to two hour of electrocatalysis in an electrolyte comprising an 8M concentration of one of the four different electrolytes just mentioned, in an exemplar system. KOH, NaOH, and CsOH each degraded approximately the same amount of PFAS during two hours of electrocatalysis. Using LiOH as the electrolyte in the same system, to degrade the same exemplar PFAS compound, under the same conditions, however, almost doubled the amount of PFAS degraded over the same time interval. These results show that LiOH is a surprisingly better electrolyte for degrading PFASPATENT than are other alkali-metal bases that might have been thought to be equivalents of one another, including the KOH that was the electrolyte used in the studies reported in the ‘345 PCT application.

[0049] It is believed that the increase in degradation rates seen with both KOH and LiOH at higher molarities is related to the high concentration of hydroxide anions present. As shown in Fig.2B, 6M lithium perchlorate did not serve as an electrolyte remotely comparable to 6M LiOH in allowing degradation of an exemplar PFAS, evidencing that the presence of Li+cations by themselves is not sufficient. Thus, the remarkable degradation of PFAS using a high concentration of LiOH shown in Fig.2B requires both the presence of a high concentration of Li+cations and a high concentration of hydroxide anions. The results of the studies herein suggest that a high concentration of OH- anions aids in increasing the rate at which C— F bonds get broken. The results shown in Fig.2C show that the degradation of PFAS improves as long as a 1M or higher concentration of hydroxide anions is present in the electrolyte solution. The results shown in Fig.2C show that high concentrations of OH- anions are preferred for conducting degradation of PFAS, and that the high concentration of OH- anions is preferably matched by a high concentration of alkali-metal cations, with Li+cations being especially preferred. As used herein, the term “high concentration” when referring to the concentration of OH- anions or of the cations of any alkali metal means that the concentration is 1 M, 1.5 M, 2 M, 2.5 M, 3 M, 3.5 M, 4 M, 4.5 M, 5 M, 5.5 M, 6 M, 6.5 M, 7 M, 7.5 M, or 8 M, with each successive molarity stated being preferred to any molarity stated before it.

[0050] As shown in Figs.11A, studies conducted with an exemplar PFAS fluoroether with six carbon atoms in its alkyl chain, hexafluoropropylene oxide (HFPO) dimer acid anion (HFPO-), which is sometimes referred to by its trade name, “GenX,” showed that, at room temperature, complete defluorination of HFPO- was not achieved when LiOH electrolyte was provided at 8 M, but was when the molarity was increased to 9 M. As practitioners will recognize, the upper concentration limit is given by the solubility of LiOH in water, which depends on the temperature. Water at higher temperature has a higher solubility for LiOH (and other salts). Thus, the practitioner can use concentrations of LiOH up to the solubility limit at any given temperature the practitioner chooses to use. The results shown in, for example, Figs.11A and Fig.11B, show that such higher concentrations will result in more complete degradation of a PFAS (up to, of course, complete defluorination) over any given interval of time.PATENT

[0051] Accordingly, in some embodiments, the molarity be higher than 8M, such as 8.5M or 9M. It is expected that increasing the concentration of their cations in the electrolyte solution up to the limit of the solubility of the salt provided to the solution will increase the rate at which PFAS present in the electrolyte solution will be degraded by electrolycatalysis in that electrolyte solution. The upper limit of molarity for some alkali metal cations is limited by the solubility of the salts containing them (in particular, CsOH) at any given temperature, and a solution containing as much of a salt as is soluble at a given temperature is referred to as a “saturated solution.” In some embodiments, the electrolyte is a saturated solution of LiOH. In some embodiments, the electrolyte is at room temperature and is a saturated solution of LiOH. In some embodiments, the electrolyte is at 70 ℃ and is a saturated solution of LiOH.

[0052] It is also noted that high molarity solutions of some of these bases become viscous; this can be adjusted by increasing the temperature of the electrolyte solution or by introducing more PFAS-contaminated water to be remediated to reduce the viscosity.

[0053] It is further believed that the results are not dependent on the Li+and hydroxide anions being provided to the electrolyte solution in the form of LiOH. Rather, it is expected that similar results will be obtained as long as both a 1 M or higher concentration of Li+and 1 M or higher concentration of OH- is present in the solution, such as 2 M concentrations of each, 4 M concentrations of each, 6 M concentrations of each, 8M concentrations of each, or 9 M concentrations of each. For example, the OH- anions can be contributed to the electrolyte solution from another base, while the Li+cations are contributed to the electrolyte solution from another compound containing lithium, so long as the other compound fully dissociates in water and the adduct of the two compounds is water soluble. Reaching High Concentrations of OH- anions and Alkali-Metal Cations

[0054] In many embodiments, water contaminated with PFAS, such as surface water from a lake or creek, or ground water, is already predominantly water and can be introduced directly into the system for performing electrocatalysis. It is expected that, in some embodiments, the PFAS-contaminated water to be remediated will contain no ions, or will contain some ions, but not in concentrations that provide the ionic strength necessary to serve as the electrolyte in an electrocatalysis cell to allow the PFAS to be degraded. It is contemplated that, in such embodiments, high concentrations of OH- anions and alkali-metal cations will be added to the water in amounts that will provide a 1M or higher concentration of OH- anions and alkali- metal cations in the resulting solution, which can then serve as an electrolyte forPATENT electrocatalyic degradation of PFAS in that solution. The OH- anions and alkali-metal cations can be provided either by adding a base such as KOH or LiOH already containing both the OH- anions and alkal metal cations, or by adding two or more compounds (for example, one that dissociates to provide the alkali-metal cations and one that dissociates to provide the OH- anions, in amounts that allow the desired high concentrations of the cations and anions to be achieved.

[0055] As noted elsewhere in this disclosure, electrocatalysis has been performed in the art for decades. It is assumed that persons of skill are well familiar with the extensive literature on electrocatalysis and are familiar with how to determine the concentration of ions present in a solution for use as an electrolyte. “Pulsed” Electrocatalysis of PFAS A. Subjecting PFAS to electrocatalysis in which periods of an applied potential are followed by periods in which no potential is applied

[0056] In another aspect, the present disclosure improves on the methods of the ‘345 PCT application by providing pulsing of the electrocatalytic reaction. Another group recently reported that subjecting compounds to cycles of electrolysis for 30 second periods, followed by a period during which no potential was applied to the system (sometimes referred to herein as “pulsed” electrolysis or “pulsed” electrocatalysis), resulted in more effective electroysis than subjecting the compounds to continuous electrocatalysis over the same period of time. Studies underlying the present disclosure showed that pulsed electrocatalysis of an exemplar PFAS resulted in better degradation of the PFAS than did continuous electrolysis in which a potential was applied for the same amount of time as potential was applied during the continuous electrolysis. For example, referring to Fig.4D, the first bar on the left shows the amount of fluoride released from an exemplar PFAs after an electrolysis in which a potential was applied for 20 minutes, while the third bar from the left shows that more fluoride was released from the same exemplar PFAS when a potential was applied 10 times for 2 minutes each (periods in which a potential was applied to the system are referred to as “ON” periods), with a 5 minute period during which no potential was applied (periods in which no potential is applied to the system are referred to as “OFF” periods) between each period in which the potential was applied.PATENT

[0057] The studies also found, however, that the pulsed electrolysis used by the group reporting it was not very effective for degrading PFAS. As shown in Fig.4D, the amount of fluoride released from an exemplar PFAS applying 30 second ON cycles 40 times (to provide 20 minutes of total electrocatalysis) is hard to distinguish from the amount of fluoride released from the same exemplar PFAS by 20 minutes of continuous electrocatalysis. Surprisingly, however, we found that by increasing the ON time to 1 minute increased the amount of PFAS released over 20 total minutes of electrocatalysis by some by some 30 – 40% compared to the 30 second ON cycles used by the other group, and that 2 minutes ON, 5 minute OFF cycles and 5 minutes ON, 5 minute OFF cycles both resulted in improved release of fluoride from PFAS compared to the 30 second ON cycles used by the other group. Further, as shown by Fig.4E, an OFF period of 5 minutes proved to provide substantially better release of fluoride from the exemplar PFAS than did 2 minutes OFF. A pair of ON and OFF periods is sometimes referred to herein as constituting one “cycle” of pulsed electrocatalysis.

[0058] In some embodiments, the invention provides for the electrocatalytic degradation of PFAS, cycling the electrocatalysis by providing an electric potential for a period (“ON” period) of approximately 1 minute (with “approximately” here meaning ±10 seconds, ±5 seconds, ±2.5 seconds, or ±1 second), or for exactly 1 minute, followed by an “OFF” period in which no potential is applied for 2 minutes ±10 seconds, or more, such as 2.25 minutes, 2.5 minutes, 2.75 minutes, 3 minutes, 3.25 minutes, 3.5 minutes, 3.75 minutes, 4 minutes, 4.25 minutes, 4.5 minutes, 4.75 minutes, 5 minutes, 5.25 minutes, 5.5 minutes, 5.75 minutes, 6 minutes, 6.25 minutes, 6.5 minutes, 6.75 minutes, 7 minutes, 7.25 minutes, 7.5 minutes, 7.75 minutes, 8 minutes, 8.25 minutes, 8.5 minutes, 8.75 minutes, 9 minutes, 9.25 minutes, 9.5 minutes, 9.75 minutes, 10 minutes, 10.25 minutes, 10.5 minutes, 10.75 minutes, 11 minutes, 11.25 minutes, 11.5 minutes, 11.75 minutes, 12 minutes, 13 minutes, 14 minutes, 15 minutes, 16 minutes, 17 minutes, 18 minutes 19 minutes, or 20 minutes. It is understood that longer “OFF” periods can be used. As such longer OFF periods do not result in more degradation of PFAS but do add time to the cycles, however, and thus reduce the amount of PFAS degraded over any particular period of time, it is not expected that they will prove popular. They could, however, be used if for some reason the practitioner wished to do so. In some embodiments, the methods comprise providing an applied electric potential of -1.6 V vs standard hydrogen electrode or of 1.6 V vs standard hydrogen electrode for 1 minute,PATENT followed by an OFF period of about 5 minutes to 10 minutes, with about in this sentence meaning ± 30 second of the times stated.

[0059] As further reported in Examples 1-5, and in Fig.1A, substantial degradation of an exemplar PFAS was achieved using 20 cycles of ON and OFF The studies reported in Examples 1-5 further show that more complete degradation of PFAS was achieved with 40, 60, or 80 cycles, with the degradation of the exemplar PFAS being tested complete at 80 cycles, and some reduction of the percent degraded below 40 cycles. As reported in the next subsection, however, hexafluoropropylene oxide (HFPO) dimer acid anion (HFPO-), however, proved more resistant to electrocatalytic degradation than did the conventional PFAS compounds degraded in the studies reported in Examples 1-5. As discussed in Examples 6 and 7, and as shown in Fig.9A, GenX required more than 90 cycles of pulsed electrocatalysis (1 cycle = 30 s at EON = 1.6 VRHE, followed by 1 s at Erev = –1.0 VRHE, followed by 5 min at open circuit potential, or “OCP”, with deep UV light irradiation at 9.0 M aqueous LiOH) to achieve complete defluorination.

[0060] In preferred embodiments, the PFAS is subjected to 40 or more cycles. In some embodiments the PFAS is subjected to 60 or more cycles. In some embodiments the PFAS is subjected to 80 or more cycles. In some embodiments, the PFAS is subjected to 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, or 120 cycles. While more than 120 cycles of pulsed electrocatalysis can be used if desired by the practitioner, it is expected that most if not all PFAS will be fully degraded by 120 cycles (particularly if the more efficient three step pulsed electrocatalysis process described in the next subsection is used).

[0061] As each cycle takes time, it is expected that practitioners will choose a number of cycles of electrocatalysis appropriate for their goal with respect to the material, such as ground or surface water, contaminated by PFAS. Where complete degradation is desired, a larger number of cycles, such as 120, will be performed. In other situations, such as ones in which there is a substantial body of PFAS-contaminated water to be remediated, the practitioner may choose to use the smallest number of cycles that will reduce the concentration of PFAS below a given threshold, such as the maximum concentration allowed by the EPA, rather than the number of cycles that would result in complete degradation ofPATENT PFAS present in the water. Choosing a smaller number of cycles allows more PFAS to be degraded per unit time. B. Adding a short period in which the polarity is reversed

[0062] As noted in the preceding section, while PFAS can be degraded by electrocatalysis in which a potential is continously applied, electrocatalysis in which PFAS are subjected to cycles in which a potential is first applied and then no potential is applied significantly improved the rate at which PFAS were degraded. An exemplar shorter-chain PFAS compound, hexafluoropropylene oxide (HFPO) dimer acid anion (HFPO-), however, proved more resistant to electrocatalytic degradation than did the longer-chain PFAS compounds degraded in the studies reported in Examples 1-5.

[0063] Studies underlying the present disclosure revealed that subjecting PFAS to cycles of electrolysis in which each cycle comprises (i) applying a potential for a period of time, (ii) reversing the direction of the potential for a second period of time, shorter than the first time period, and (iii) then applying no potential to the electrocatalytic system for a third period of time, surprisingly improved the degradation of an exemplar C6 PFAS compound, HFPO–. (For convenience of reference, each element of the sequence of (i), (ii), and (iii) just described will be referred to in the remainder of this subsection as a step. As noted in the preceding subsection, embodiments in which an electrocatalytic system is subjected to repetitive cycles of first applying a potential for a period of time followed by a period in which no potential is applied are sometimes referred to herein as “pulsed” electrocatalysis. The studies reported in Examples 6 and 7 reveal that the addition of a time interval at reversed polarity after C–F bond cleavage at anodic potential during pulsed electrolysis enhanced defluorination.

[0064] As noted above, for pulsed electrolysis in which the application of a first electric potential is followed directly by a period in which an open circuit potential is present, it was found that providing an applied electric potential for one minute was considerably more efficient in defluorinating PFAS than was the application of an applied electric potential for 30 seconds. Surprisingly, when a reverse potential pulse was applied in step (ii), it was discovered that the electric potential applied in step (i) could be shortened to 30 seconds and that a ON period longer than that did not further result in faster degradation of the PFAS being tested. Thus, the addition of step (ii) reduces the period of time for which the electric potential needs to be applied to achieve efficient degradation of PFAS, compared to thePATENT embodiments of pulse electrolysis discussed in the preceding subsection in which a reverse potential pulse is not applied.

[0065] As discussed in Examples 6 and 7, and as shown in Fig.9A, complete defluorination of HFPO- required more than 90 cycles of pulsed electrocatalysis (1 cycle = 30 s at EON = 1.6 VRHE, followed by 1 s at Erev= –1.0 VRHE, followed by 5 min at open circuit potential, or “OCP”, with deep UV light irradiation at 9.0 M aqueous LiOH). It is noted that the number of cycles in Fig.9A were increased in increments of 30. As only incomplete defluorination was obtained subjecting HFPO–to 90 cycles, but complete defluorination was obtained when HFPO–was subjected to 120 cycles, it can be assumed that complete defluorination can be achieved with a number of cycles between 90 and 120. As HFPO- is an exemplar of the compounds referred to as “GenX chemicals,” it is anticipated that other GenX chemicals will likewise require more than 90 cycles of pulsed electrocatalysis, but less than 120, to be completely defluorinated. As shown in Fig.10A, a reversed polarity of -1.0 (V vs RHE) or more improved obtaining complete defluorination of the exemplar GenX chemical HFPO–. As shown in Fig.10C, the duration of the reversed polarity need only be for an interval of one to three seconds. In some embodiments, the pulsed electrocatalysis of a PFAS is performed by the following steps: step (1) providing an applied electric potential of either 1.6 V vs RHE or of -1.6 V vs RHE for 30 seconds ± 10 seconds, step (2) providing an applied electric potential of 1 V at a polarity that is the reverse of the polarity applied in step (1) for 0.5 to 4 seconds, and, step (3) providing an open circuit potential for about 5 minutes to 10 minutes, with “about” here meaning ± 30 seconds, thereby performing one cycle, and providing 40 to 120 cycles, such as 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, or 120 times.

[0066] It is noted that the 30 second period in which the applied electric potential was provided in Example 6 is the period that was found in Examples 1-5 not to be efficient in the studies in which no reversed potential was applied to the electrocatalytic system. It is believed that adding a short period in which a reversed applied electric potential is provided allows the use of a shorter ON period in step (1).

[0067] Without wishing to be bound by theory, it is believed that fluorine ions released by electrocatalysis from the PFAS during the application of potential in step (i) adsorb to the anode of the electrocatalytic system and render it less accessible to molecules of the PFAS remaining in the electrolyte. Again without wishing to be bound by theory, it is believed thatPATENT the reversal of the potential in step (ii) causes the fluorine ions to come off the anode, and that the period in which an open circuit potential is applied to the electrocatalytic system in step (iii) allows the fluorine ions to diffuse into the electrolyte, as schematically depicted in Fig.15, opening up access to the anode to other molecules of PFAS in the electrolyte, thereby improving the overall rate of degradation of the PFAS compared to applying a continuous potential to the electrocatalytic system. Applied Potential

[0068] Suitable potentials for effecting electrocatalysis of PFAS are set forth in the ‘345 PCT application. As reported in Examples 1-5, and as shown in Fig.1C, studies underlying the present disclosure studied electrocatalysis of an exemplar PFAS using a potential (Eapp (V vs RHE)) of 1.2 to 2.0. The potential Eapp = +1.6 VRHE proved most efficient at degrading the exemplar PFAS, and is therefore a preferred embodiment for electrocatalysis of PFAS using LiOH as the electrolyte, particularly in performing pulsed electrocatalysis. The results in Fig. 1C, however, show that each of the other potentials tested also worked to degrade PFAS in the study. Accordingly, in some embodiments, the electrocatalysis of PFAS can be performed using a potential ranging from Eapp = +1.2 VRHE to Eapp = +2.0 VRHE. Further, the ‘345 PCT application shows that the electrocatalytic system works whether the bias is anodic or cathodic. Accordingly, it is expected that the electrocatalysis can also be used to degrade PFAS using a potential ranging from Eapp= -1.2 VRHEto Eapp= -2.0 VRHE. Aqueous electrolyte solutions

[0069] As noted in the ‘345 PCT application, the electrocatalysis is conducted with the PFAS compound in an electrolyte solution that is 46 vol% or higher water, preferably at least 50 vol% water, and more preferably, higher than 50 vol% water. The ‘345 PCT application As discussed further below, for compounds soluble in water, the electrocatalysis can be surprisingly be conducted in almost pure water, as long as enough ions are present or added to allow the water to serve as an electrolyte, and, of course, with the PFAS to be degraded present as the reactant to be dissociated in the course of being subjected to electrocatalysis.

[0070] As in the ‘345 PCT application, the systems and methods set forth herein allow the destruction of PFAS dissolved in the water by dissociating fluorine atoms bound to carbons in the PFAS, releasing the fluorine atoms into the electrolyte solution. The form in which the fluorines in the starting PFAS are dissociated from the PFAS and released into the electrolytePATENT is sometimes referred to herein as a derivative of the fluorines originally present in the PFAS. Without wishing to be bound by theory, it is believed that the fluorines dissociated from the PFAS are in the electrolyte in the form of fluoride. For convenience of reference, the fluorides (or whatever other form or derivative of the fluorines originally present in the PFAS are present in the electrolyte after the fluorine atom is dissociated from a carbon atom in the PFAS) will sometimes be referred to herein as “fluorine derivatives.” The fluorine derivatives released from PFAS can then precipitated out of solution and sequestered by contacting the fluorine derivative to form non-soluble compounds, as discussed in the ‘345 PCT application. It is generally expected that PFAS degraded by the systems and methods taught herein will have degradation products that are less toxic than the starting environmental pollutant, that will be less persistent in the environment than the starting environmental pollutant, or both.

[0071] As noted in the ‘345 PCT application, the discovery that PFAS can be degraded while present in an aqueous solution is particularly advantageous for the use of systems and methods of the invention in environmental remediation. As mentioned above, PFAS has contaminated groundwater or surface water around manufacturing sites in which PFAS was made or applied to products, and around airports or military facilities in which PFAS- containing firefighting foam have been released. In some embodiments, the inventive methods and systems allow providing PFAS-contaminated water directly to electrocatalysis without elaborate pretreatment (as noted below, however, the water might first be filtered to remove solids and any suspended particles might first be allowed to settle before commencing the electrocatalysis.) Thus, as discussed further below, it is contemplated that in some embodiments, water contaminated with PFAS will be provided directly to a container in which electrocatalysis will be conducted, and its ionic strength. In embodiments of the present invention, the ionic strength will be adjusted by adding LiOH to allow the contaminated water to serve as an electrolyte and then subjecting the PFAS in the resulting electrolytic solution to electrocatalysis using a water oxidation electrocatalyst.

[0072] The methods and systems of the ‘345 PCT application exploit the discovery that the electrocatalytic degradation of PFAS requires the use of an electrolyte solution with a percentage of water that is higher than 45 vol %. As in the ‘345 PCT application, in some embodiments, the percentage of water in the electrolytic solution used to degrade PFAS is 46 vol %, 47 vol %, 48 vol %, 49 vol %, 50%, or more, such as 55 vol %, 60 vol %, 65 vol %, 70 vol %, 75 vol %, 80 vol %, 85 vol %, 90 vol %, 95 vol %, 96 vol %, 97 vol %, 98 vol %,PATENT 99 vol %, 99.5 vol%, or higher, with a concentration of or above 50 vol% being preferred over any concentration below 50 vol %. In some embodiments of the present invention, the concentration of water in the electrolyte solutions is 50 vol % or more. In some embodiments, the concentration of water in the electrolyte solutions used in the present invention is 60 vol % or more. In some embodiments, the concentration of water in the electrolyte solutions used in the present invention is 70 vol % or more. In some embodiments of the present invention, the concentration of water in the electrolyte solutions is 75 vol % or more. While the percentages between 50 vol % and 95 vol % are set forth at 5 vol % intervals for conciseness, the concentrations of water above 50 vol % include any intermediate percentage between 50 vol % and 99.9 vol %, such as 51.25 vol %, 67.33 vol %, or 82.82 vol%.

[0073] For convenience of reference, a solution in which the concentration of water is sufficient to allow PFAS to be degraded in the solution to be subjected to electrocatalysis by a water oxidation electrocatalyst is sometimes referred to herein as a “first aqueous solution.” As the solution in which the concentration of water is sufficient to allow PFAS in the solution to be subjected to electrocatalysis by a water oxidation electrocatalyst will typically be close to at least 50 vol% water, and more typically will be one in which the percentage of water is higher than 50 vol%, such as 60 vol% water, 75 vol% water, 80 vol% water, or higher, the electrolyte solution in which the electrocatalysis is performed is sometimes referred to herein as a “predominantly aqueous solution.”

[0074] In one aspect, the invention relates to reducing the concentration of one or more PFAS compounds present in a water sample by removing and, preferably, mineralizing at least some of the fluorine atoms present in the PFAS compound or compounds. The reduction of concentration of PFAS is accomplished by a two-step process: (1) the one or more PFAS compounds in the electrolyte are placed in contact with a water oxidation electrocatalyst under an applied electric potential (e.g., an anodic or a cathodic bias) which results in breaking one or more C̶ F bonds in the one or more PFAS compounds, thereby dissociating fluorine atoms from their C̶ F bonds and releasing them into the electrolyte as fluorides and, (2) cations are added to the electrolyte to react with the fluorides to form a compound that is insoluble in the electrolyte.

[0075] The Hunter application is directed to degrading hydrocarbons by electrocatalysis in a solution of which 95 vol % or more is a non-aqueous solvent (e.g., a solution of which 95 vol % or more is an organic solvent). In contrast, and without wishing to be bound by theory, it isPATENT believed that conducting the electrocatalysis of PFAS in a solution that is 46 vol % or more water, and preferably 50 vol % or more water, is important to the ability of the inventive systems and methods to break the C̶ F bonds in the PFAS, thereby releasing the fluorine atoms from the PFAS. As persons of skill will appreciate, the C̶ F bond in a PFAS compound is one of the strongest known and is considerably stronger than the C̶ H bonds present in hydrocarbons; breaking a C̶ F bond requires different conditions than those sufficient to break a C̶ H bond. In preferred embodiments, the compounds subjected to electrocatalysis by the inventive systems and methods are not hydrocarbons.

[0076] Further without wishing to be bound by theory, it is believed in the art that electrocatalysis proceeds in part by the formation of catalytic intermediates. Without wishing to be bound by theory, it is believed that using an electrolyte solution that is 46 vol% or more water, and preferably 50 vol% or more water, enables proton-coupled electron transfer (“PCET”), thereby reducing the energy of the catalytic intermediates and smoothing out the PCET process, sometimes referred to in the art as “potential leveling.” Without wishing to be bound by theory, it is believed that potential leveling does not occur under the electrolytic conditions set forth in the Hunter application and that such conditions do not allow electrocatalytic degradation of a PFAS to occur.

[0077] Studies underlying the ‘345 PCT application subjecting an exemplar PFAS compound to electrocatalysis in solutions having water present at different concentrations revealed that the exemplar PFAS was not degraded when water was present at 45 vol%, but did occur when water was present at 50 vol%. Thus, electrocatalysis of PFAS should be performed using an electrolyte solution that is at least 46 vol% or more water, and preferably is 47 vol%, 48 vol%, 49 vol%, with each successive larger percentage being preferred to the smaller one to its left. In preferred embodiments, the electrolyte solution is 50 vol% or more water, and preferably has more water than non-aqueous solvent. Any particular electrolyte solution with any particular concentration of water, such as 64.75 vol% water or 73.22 vol% water, with the remainder made up of a non-aqueous solvent, such as ethanol, can be readily tested by simply preparing the solution and subjecting an exemplar PFAS compound to electrocatalysis following the teachings herein. If the PFAS is degraded by the electrolysis (as evidenced, for example, by the release of fluorine atoms or ions from a PFAS, that particular combination of water and non-aqueous solvent is suitable for use in degrading the PFAS.PATENT

[0078] It is understood that the contaminated water, such as water being drawn from a drainage basin on an industrial site, may contain solids, whether being drawn in with the water or suspended in it. In such cases, the practitioner may choose to filter the solids out of the contaminated water before subjecting the contaminated water to electrocatalysis. In some cases, contaminated water that is milky or that otherwise has particles suspended in it may be provided time for the suspended particles to settle out before the water, now bearing a reduced burden of particles, is subjected to electrocatalysis.

[0079] The ‘345 PCT application notes that the fact that the PFAS can be subjected to electrocatalysis without needing to be in a purified aqueous environment provides yet a further advantage, as PFAS-contaminated water from surface water or groundwater will typically also contain a variety of naturally-occurring organic compounds, such as those from living or decomposing vegetation and from organisms living in the water, as well as synthetic pesticides that have leached into the water and fertilizer that has run-off farms, lawns, or other land to which it was applied. It is anticipated that, in some embodiments, the PFAS- contaminated water will not have to be purified to remove such compounds before being subjected to electrocatalysis. Not having to remove such naturally-occurring organic compounds before electrocatalysis in these embodiments of the inventive methods and systems is expected to reduce the time and cost for remediating PFAS contamination, and to provide the inventive methods and systems with significant advantages compared to currently-available techniques. Water Oxidation Electrocatalysts

[0080] The water oxidation electrocatalysts contemplated for use in the inventive systems and methods are metal-based. Thus, as used herein, the term “water oxidation electrocatalyst” means a metallic material, including metal oxides, metal hydroxides, and metal oxy(hydroxide)s. Metallic materials can convert under anodic bias in aqueous solution into oxides, hydroxides, or oxy(hydroxides) and, under cathodic bias oxides, hydroxides, or oxy(hydroxides) can become metallic.

[0081] In some embodiments, for example, the metal-based water oxidation electrocatalyst may comprise one or more transition metals. In an embodiment, for example, the water oxidation electrocatalyst may comprise Ru. In some embodiments, the water oxidation electrocatalyst is a metal oxide or a metal hydroxide. In some embodiments, the water oxidation electrocatalyst is a metal oxide or metal hydroxide that comprises one or more earthPATENT abundant metals. In some embodiments, the water oxidation electrocatalyst is a metal oxide or metal hydroxide that comprises one or more metals selected from the group consisting of Ni, Fe, Co, Mn, Zn, Sc, V, Cr, Cu, or a lanthanide. In some embodiments, the water oxidation electrocatalyst is a layered solid. In some embodiments, the water oxidation electrocatalyst is a layered solid that is a layered double hydroxide, oxide, or oxy(hydroxide). In some embodiments, the water oxidation electrocatalyst is a layered double hydroxide, oxide, or oxy(hydroxide) solid that comprises a Ni hydroxide, oxide, or oxy(hydroxide), an Fe hydroxide, oxide, or oxy(hydroxide), or a Ni--Fe hydroxide, oxide, or oxy(hydroxide). In some embodiments, the water oxidation electrocatalyst is a layered double hydroxide, oxide, or oxy(hydroxide) solid that is nanostructured. In some embodiments, the water oxidation electrocatalyst is a layered double hydroxide, oxide, or oxy(hydroxide) solid that is generated via pulse laser ablation in liquid. In some embodiments, the water oxidation electrocatalyst is other than an organometallic catalyst. In some embodiments, the water oxidation electrocatalyst is a heterogeneous catalyst. In some embodiments, the water oxidation electrocatalyst is of non-precious metals. In some embodiments, the water oxidation electrocatalyst is a perovskite, a polyoxometalate, or a metal-organic framework. In an embodiment, for example, the water oxidation electrocatalyst is a solid, such as solid particles having physical dimensions less than or equal to 100 µm, or optionally less than or equal to 10 µm. In some embodiments, the water oxidation electrocatalyst is a nanostructured solid, such as a solid having nano-features with dimensions less than or equal to 1 µm, or optionally less than or equal to 200 nm. For example, the water oxidation electrocatalyst may comprise one or more transition metals. In another example, the water oxidation electrocatalyst is an inorganic catalyst. In another example, the water oxidation electrocatalyst is a catalyst material other than an organometallic catalyst. In some embodiments, the water oxidation electrocatalyst may be a nickel-iron layered double hydroxide, oxide, or oxy(hydroxide). In some embodiments, the water oxidation electrocatalyst may be a nickel-manganese layered double hydroxide, oxide, or oxy(hydroxide).

[0082] The ‘345 PCT application reported studies using several exemplar water oxidation electrocatalysts. The first exemplar water oxidation electrocatalyst was comprised of the metals nickel and iron (“NiFe”). NiFe water oxidation catalysts have been known for decades. In 2014 and 2016, one of the present inventors and her collaborators reported an improved version of a NiFe water oxidation electrocatalyst, using laser-made nanosheets of [NiFe]-layered double hydroxide. See, Hunter, et al., J. Am. Chem Soc., 2014,PATENT 136(38):13118-13121 (hereafter, “Hunter 2014”); Hunter, et al., Energy Environ Sci., 2016, 9(5):1734-1743. The [NiFe]-layered double hydroxide nanocatalyst made by pulsed laser in liquids synthesis outperforms analogous materials prepared by conventional methods. Forsythe, et al., Chem. Rev.2021, 121(13):7568-7637.

[0083] Intrinsically more active catalysts aid the efficient and complete abstraction of fluorine atoms from PFAS. Smaller catalysts provide a higher surface to area ratio, providing more active sites of the catalysis, turning over more material, and that are intrinsically more active. Accordingly, in some embodiments, the water oxidation catalyst is provided in the form of a NiFe nanocatalyst made by pulsed laser in liquids synthesis, or nanocatalysts of other metals, made as described in the papers referenced above.

[0084] A second exemplar water oxidation electrocatalyst tested was a nickel and manganese (“NiMn”)-layered double hydroxide. Like the first exemplar water oxidation electrocatalyst, the NiMn electrocatalyst was able to dissociate fluorines from PFAS in a predominantly aqueous electrolyte. The NiMn electrocatalyst was, however, surprisingly three times better than the NiFe-layered double hydroxide at degrading PFAS than was the NiFe electrocatalyst. In some preferred embodiments, the water oxidation catalyst is provided in the form of a NiMn nanocatalyst made by pulsed laser in liquids synthesis.

[0085] The ‘345 PCT application noted that, while layered double hydroxides were used in the studies described above, oxides and oxy(hydroxides) were expected to perform similarly, as all these materials turn into the oxy(hydroxide) under anodic electrocatalysis and all form materials that are similar under cathodic electrocatalysis. Thus, while layered double hydroxides are a preferred material for use as an electrocatalyst in some embodiments, it is contemplated that in other embodiments, metal oxides and oxy(hydroxides) can be used instead.

[0086] More generally, the water oxidation electrocatalyst can be nanostructured layered double hydroxide solid, an oxide solid, an oxy(hydroxide) solid, a perovskite, a polyoxometalate, or a metal-organic framework. In some embodiments, the electrocatalyst is a nanostructured layered double hydroxide solid, oxide solid, or oxy(hydroxide) solid which contains an effective amount of one or more transition metals, a post-transition metal, or both a transition metal and a post-transition metal. The one or more transition metals can be selected from the periodic table first-row transition metals. The post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin. In somePATENT embodiments, the nanostructured layered double hydroxide solid, oxide solid, or oxy(hydroxide) solid comprises a divalent metal mixed with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal. It is noted that any divalent metal can be used, but nickel is the best performing metal tested as of the writing of this disclosure.

[0087] The ‘345 PCT application noted that a considerable amount of information is available in the art with regard to metals suitable for use as electrocatalysts, including Hunter, Gray, and Mueller, Chem. Rev., 116:14120−14136 (2016); DOI: 10.1021 / acs.chemrev.6b00398, the entirety of which is incorporated herein by reference. It is expected that, in view of the extensive teachings in the art, the person of skill can choose the particular materials they wish to employ as a water oxidation electrocatalyst in different embodiments of the inventive systems and methods.

[0088] In some embodiments, the water oxidation catalyst in the inventive systems and methods may be used with conventional electrodes. As mentioned above, however, nano- sized catalytic particles are more efficient at removing fluorine atoms from PFAS, and these nanoparticles are preferably positioned on an electrode with a high surface area. In preferred embodiments, therefore, nanoparticles of the electrocatalyst are preferably immobilized on a high surface area electrode support.

[0089] Typically, in embodiments in which an anodic bias is used, the water oxidation electrocatalyst is placed on the anode. In some preferred embodiments, hydrophilic carbon fiber paper (“CFP”) is the electrode support, as it is inexpensive, electrically conductive, chemically inert, nontoxic, robust, and scalable. As described in the Hunter application, however, the electrode on which the water oxidation electrocatalyst is placed (typically the working electrode) may be fluorine-doped tin oxide (“FTO”), indium tin oxide (ITO), an allotrope of carbon other than carbon fiber (e.g., graphite, glassy carbon, or pyrolytic carbon), a metal (e.g., Pt, Ti, Ni, Au), or any combination of these.

[0090] The water oxidation electrocatalyst may be drop cast from a solution onto the anode (or, in a system in which a cathodic bias is to be used, the cathode), and the drop-cast solution allowed to dry, thereby immobilizing the solid water oxidation electrocatalyst on the anode or cathode. Similarly, a suspension or a dispersion of the water oxidation electrocatalyst may be prepared and then drop-cast onto the anode or cathode. The water oxidation electrocatalyst may also be immobilized on the anode or cathode by a solution coating technique, a vaporPATENT deposition technique, or other techniques known in the art and appropriate to the selected water oxidation electrocatalyst, such as, but not limited to, doctor blading, dip coating, spin coating, electrophoretic deposition, pulsed laser ablation, pyrolysis, sputtering, thermal evaporation, and laser ablation. The water oxidation electrocatalyst may be provided on the electrode (e.g., the anode or cathode) at a selected loading density selected over the range of 0.01 µg / cm2to 10 g / cm2. An applied electric potential may be applied to the water oxidation electrocatalyst indirectly, that is, via applying the electric potential to the anode or to the cathode with which the water oxidation electrocatalyst is in electronic communication or on which the water oxidation electrocatalyst is immobilized.

[0091] As discussed further below, studies underlying the ‘345 PCT application discovered that the bias applied to the system can be a cathodic bias rather than the anodic bias usually used in electrocatalytic systems. Accordingly, either bias can be applied to the water oxidation electrocatalyst. As persons of skill will appreciate, under a cathodic bias, electrocatalysts that function as water oxidation electrocatalysts under an anodic bias are not oxidative under a cathodic bias and are therefore not water oxidation electrocatalysts, even though nothing about the compositions themselves, or anything about the electrocatalytic system has changed except the bias. The compositions described above as being water oxidation electrocatalysts are water oxidation electrocatalysts under anodic bias, but are also electrocatalysts in the electrocatalytic systems if a cathodic bias is applied instead. For convenience of reference, the term “water oxidation electrocatalyst” designates a composition that is a water oxidation electrocatalyst in the electrocatalytic system when an anodic bias is applied, even if a cathodic bias is applied instead. Anodic bias, SHE, and RHE

[0092] In studies reported in the ‘345 PCT application, an Ag / AgCl reference electrode was used, and a constant potential of +1.2 V versus Ag / AgCl was applied. As persons of skill are aware, a “constant potential” means that there is a constant potential applied, no matter if it is positive or negative. “Anodic bias” means that a positive constant potential is applied; as “bias” is by definition a fixed DC voltage. The pH of an aqueous solution describes its proton concentration; a pH of 0 means that there are 1 mol per liter of protons in water. In aqueous systems, a “normal hydrogen electrode,” or “NHE,” which is pH-dependent, can be used to measure potential. In non-aqueous systems, however, there is not enough water present for the pH scale is be useful. Accordingly, electrochemists typically use a “standard hydrogenPATENT electrode,” or “SHE,” rather than a “normal hydrogen electrode,” or “NHE,” and reference or convert measurements back to pH 0 aqueous conditions. Methods of correlating SHE measurements and NHE measurements are known in the art.

[0093] SHE measurements also assume specific standard conditions, in particular, that the electrolysis is conducted with air pressure equivalent to sea level and at a temperature of 25 ℃. The studies reported herein were conducted at an air pressure of approximately 340 feet above sea level, and at temperatures that were close to, but not necessarily exactly, 25 ℃. The potentials applied in the studies reported below are therefore stated as “Reversible Hydrogen Electrode,” or “RHE,” measurements. While the conditions under which the studies were conducted are not precisely those used for SHE measurements, and therefore would require small corrections if stated at the level of several decimal points, for all practical purposes, the potentials used in the studies can be stated as RHE without the need to state them instead as SHE.

[0094] For purposes of conducting electrocatalysis of PFAS with water oxidation electrocatalysts by the methods and systems of the invention, it is expected that a potential range of 0.5 V to 5 V versus SHE will work. In some embodiments, the potential range is 0.75 V to 3 V vs. SHE. In some embodiments, the potential range is 1.0 V to 2 V vs. SHE. In some embodiments, the potential range is 1.0 V to 1.7 V vs. SHE. In some embodiments, the potential is 1.25 V ±0.20 vs. SHE. In some embodiments, the potential is 1.25 V ±0.10 vs. SHE. Potentials that were used and shown to work in studies underlying the present disclosure are Eapp= + 1.2 to +2.0 VRHE. Cathodic Bias

[0095] The ‘345 PCT application reported that a practitioner will typically choose to use an anodic bias, for its slightly better results, but that either bias can be applied to the water oxidation electrocatalyst to subject a PFAS or another target compound to electrocatalysis. The applied electric potential can therefore be either negative or positive, with a range of values from -5 V to 5V versus standard hydrogen electrode (which, as noted in the preceding section, is abbreviated as “SHE”), provided that the potential is not an open circuit potential. In some embodiments, the potential range is -0.75 V to -3 V vs. SHE. In some embodiments, the potential range is -1.0 V to -2 V vs. SHE. In some embodiments, the potential range is - 1.0 V to -1.7 V vs. SHE. In some embodiments, the potential is -1.25 V ±0.20 vs. SHE. In some embodiments, the potential is -1.25 V ±0.10 vs. SHEPATENT Solutions and Electrolytes

[0096] As noted above, in some embodiments, the systems and methods of the invention subject PFAS to electrocatalysis in an electrolyte solution that is predominantly aqueous, which for purposes of this disclosure means that the solution is 46 vol% or more water, and is preferably 50 vol%, 60 vol%, 65 vol%, 70 vol%, 75 vol%, 80 vol%, 85 vol%, 90 vol%, 91 vol%, 92 vol%, 93 vol%, 94 vol%, 95 vol%, 96 vol%, 97 vol%, 98 vol%, 99 vol%, or more, water, with solutions that are 60 vol% or more water being preferred over those that are less than 60 vol% water. Solutions of liquids contaminated with PFAS, which solutions have a lower vol% of water than that desired by the practitioner, can of course, simply have more water added to them until they reach the percentage of water desired by the practitioner.

[0097] As explained in the ‘347 PCT application, electrolysis requires an electrical current to flow by the movement of ions through a solution towards electrodes partially or wholly immersed in the solution. As set forth in the Introduction, studies reported below showed that LiOH was a surprisingly better electrolyte for the degradation of PFAS than three other electrolytes tested. Thus, it is expected that systems and methods following the teachings of the present disclosure will employ LiOH to contribute at least some, preferably a majority, and still more preferably, all of the ions allowing electrocatalysis to occur.

[0098] The choice of LiOH to provide the necessary ions will guide in part the metals from which to select the particular water oxidation electrocatalyst to be used. The practitioner will not want to use electrocatalysts that are not stable in a base. It is expected that the metal water oxidation electrocatalysts are stable in a base.

[0099] As persons of skill will be aware, in solutions that are 50 vol% or more water, thereby lowering thepercentage of any non-aqueous solvent present, the practitioner will wish to check that the electroconductivity of the electrolytic solution is maintained. Some studies underlying the present disclosure tested the ability of the water oxidation electrocatalyst to dissociate fluorine from an exemplar PFAS in solutions in which the water content was 40 vol%, 45 vol%, or 50 vol%, with the remaining percentage of solution made up of an exemplar organic solvent, acetonitrile. It is known in the art that 100 mM of any strongly dissociating base, acid, or salt is sufficient to impart adequate electroconductivity. See, e.g., A.J. Bard, L.R. Faulkner, ELECTROCHEMICAL METHODS: FUNDAMENTALS AND APPLICATIONS, 2nd ed., Wiley (New York, 1980). Accordingly, use of LiOH is expected to provide sufficient electroconductivity.PATENT

[0100] As electrolysis has been conducted for almost two centuries, materials suitable for forming solutions for conducting electrolysis are well known and it is expected that practitioners are well familiar with the concentrations of ions for making the predominantly aqueous solution into an electrolyte suitable for use in electrocatalysis, and with how to measure those concentrations. It is also assumed that persons of skill are familiar with maintaining electroconductivity of the electrolyte solution and can readily do so using any particular combination of water and a non-aqueous solvent to allow the electrolyte solution to serve as an electrolyte permitting the electrocatalysis of PFAS having a single covalent bond between a carbon atom and a fluorine atom. For clarity, the term “single covalent bond” is used here to indicate that the bond being broken by the electrocatalysis is a single bond between the two atoms described, as opposed to there being a double bond between the two atoms, not that there is, for example, only one carbon-fluorine bond in the molecule to be degraded.

[0101] In laboratory settings, a compound to be tested for degradation by electrocatalysis is typically added to a container holding purified water, and LiOH is then added to create a solution with an ionic conductivity sufficient for the solution to serve as an electrolyte. Stated another way, the LiOH is added to provide an ion concentration enabling charge transport.

[0102] The ion concentration of the contaminated water is typically checked, either prior to being provided to the container or while in the container, to determine whether the concentration of ions is sufficient for the water (i.e., the solution of water contaminated by the pollutant) to serve as an electrolyte, that is, that it has sufficient ionic conductivity to serve as an electrolyte. If it does not, the concentration of ions can be adjusted by the practitioner by standard methods, such as adding LiOH to the water to increase the ion concentration to a level allowing the water to serve as an electrolyte. As noted elsewhere in this disclosure, electrocatalysis has been conducted in the art for over a century, and persons of skill are well familiar with how to measure ion concentrations and to determine whether the concentration of ions present in a particular solution is sufficient for it to serve as an electrolyte.

[0103] Depending on the particular embodiment chosen by the practitioner, the solution of water containing the pollutant may be, except for the pollutant and any added LiOH, almost pure water, or may have a percentage of non-aqueous solvent or solvents. For example, thePATENT solution may be 65% water and (ignoring the presence of the pollutant to be degraded and LiOH added to adjust the concentration of ions) 35% non-aqueous solvent, 70% water and (ignoring the presence of the pollutant to be degraded) 30% non-aqueous solvent, or 90% water and (ignoring the presence of the pollutant to be degraded) 10% non-aqueous solvent.

[0104] Some PFAS are soluble in water. Other PFAS may be less soluble in water, or be non-soluble, and will typically contaminate soil. Such non-water soluble PFAS will typically settle in river beds or lake bottoms, where they can enter the food chain or be stirred up by dredging or by ship activity. In some embodiments, non-water soluble PFAS that are soluble in organic solvents can be extracted from contaminated soil or mud scooped from river beds or the like and the organic solvent containing the extracted pollutant can then be used as the non-aqueous solvent in electrolyte solutions, such as those described elsewhere in this disclosure. Information about the degree to which most important PFAS are soluble in water, in polar protic solvents, or in aprotic solvents is known in the art, and it is expected that the practitioner can choose an appropriate non-aqueous solvent for the particular PFAS whose presence the practitioner wishes to reduce. As noted earlier, any particular solution of water and an organic solvent can be readily tested for its ability to degrade PFAS of interest in an electrocatalytic system as described in the Examples and applying an electric potential. Degradation of the PFAS of interest by the electrocatalytic system indicates that that particular solution is suitable for degrading the PFAS of interest.

[0105] As noted, in some embodiments, organic solvents such as alcohols or acetone may be used as a non-aqueous part of the electrolyte solution. The organic solvent may be recycled and reused by distilling the organic solvent from the electrolyte after all or most of the dissolved pollutant has been degraded. Conducting Electrocatalysis of PFAS and Mineralization of Released Fluorines or ionic forms thereof

[0106] It is noted that “electrolysis” is used in this disclosure in its usual sense to refer to the decomposition of a selected compound (such as the exemplar PFAS compounds used in the Examples) by the application of electric energy. As the electrolysis of PFAS in embodiments of the inventive methods and systems involves the use of a metal water oxidation electrocatalyst, the methods of the invention are sometimes referred to herein more specifically as “electrocatalysis.”PATENT

[0107] Electrocatalysis is a standard procedure and it is assumed that persons of skill are familiar with conventional materials and methods for conducting it. Accordingly, only some features of the procedure will be touched on here. As practitioners will appreciate, electrolysis is typically conducted in a container holding an electrolyte and, usually, an analyte or reactant of interest. In applications relevant to the present disclosure, the analyte or reactant of interest is a PFAS compound or two or more PFAS compounds. As described in, for example, “Overview of Reference Electrodes and Alternative Reference Electrodes” (Pine Research Instrumentation, Durham, NC, document no. DRK10053 (Rev001, 2016)), at least two electrodes are electrically connected by the electrolyte; one, sometimes referred to as the “working electrode,” which allows electron transfer to the analyte or reactant, and a counter electrode which maintains electroneutrality by allowing a reaction of the electric sign (i.e., positive or negative) opposite to that of the electron transfer to the analyte or reactant. The voltage is typically referred to as the potential difference between the working electrode and the counter electrode. As it is known that the potential difference between these two electrodes can change during the electrolysis for a variety of reasons, an additional counter electrode, referred to as a “reference electrode,” is typically included. The reference electrode has a potential that does not change, and allows accurate control of the potential of the working electrode. A variety of reference electrodes are known and it is expected that the person of skill can readily select a reference electrode for use in the electrocatalytic systems and methods taught herein.

[0108] The systems and methods of the present disclosure comprise a container in which an electrolyte electrically connects a working electrode, counter electrode, and, preferably, a reference electrode. A water oxidation electrocatalyst is disposed on the working electrode and is typically immobilized thereon. As used in standard practice, the “working electrode” is the one on which the desired reaction takes place.

[0109] As used herein, the terms “water oxidation catalyst” and “water oxidation electrocatalyst” mean the same thing.

[0110] As reported in the ‘345 PCT application, fluorine released from the PFAS dissolves in the predominantly aqueous solution, and is present as fluoride in the electrolytic solution. No fluorine gas bubbles were observed leaving the container during the studies reported in the ‘345 PCT application.PATENT

[0111] Once the electrocatalysis has been conducted for a time or to a point determined by the practitioner, fluoride released into the electrocatalysis solution can be precipitated to mineralize it in a form that is less toxic than the PFAS in which it was previously present. This can be accomplished by adding to the predominantly aqueous solution ions of atoms that will bind fluorine more avidly than hydrogens in the water bind fluorines that have dissolved into the predominantly aqueous solution, thereby forming a solution comprising hydrofluoric acid.

[0112] In some embodiments, the ions of such atoms are added by adding a second aqueous solution in which has been dissolved a compound that provides ions of the desired atoms. In studies reported in the ‘345 PCT application, a saturated aqueous CaCl2 solution was added to the aqueous solution containing fluorides stemming from fluorine released by electrocatalysis from an exemplar PFAS, resulting in the formation of fluorine-containing calcium solids. In other embodiments, a compound can be added directly to the first aqueous solution to add the desired cations. Calcium was used in the studies reported in the ‘345 PCT application as it is both inexpensive and abundant. Other cations that form water-insoluble fluorides, however, can be used to mineralize the fluorides. In some embodiments, the cations used are of the alkaline earth metals, Mg2+, Sr2+, and Ba2+. Beryllium is not suitable, both because it is toxic and because BeF2is very soluble in water and creation of BeF2would not result in mineralizing the fluorine. In some embodiments, the cations are of lithium, Li+. Other metal cations also form water-insoluble fluorides and thus can be used are zirconium, Zr4+, gallium, Ga3+, copper, Cu2+, zinc, Zn2+, vanadium, V3+, and chromium, Cr3+. The oxidation states of the metal cations are specified, as they matter for the water-solubility of the resulting fluorides.

[0113] In studies of electrocatalysis of an exemplar PFAS, PFOA set forth in the ‘345 Application, fluorine released from PFOA was precipitated by adding calcium chloride solution to the electrolyte, resulting in a white precipitate. As discussed in the ‘345 PCT application, analysis of the precipitate revealed that fluorine was present in the precipitate, evidencing the removal of fluorine from the exemplar PFAS, which was the only source of fluorine in the experimental set-up.PATENT Batch and flow systems for conducting electrocatalysis of PFAS using water oxidation catalysts

[0114] The container in which the electrolysis is conducted can be two cylinders electrically connected by an electrolyte, as in a Hoffman voltameter, but is more typically a tank, vat, tub, or other container that can hold the amount of predominantly aqueous solution which the practitioner wishes to use. (For ease of reference, these various forms in which the predominantly aqueous solution containing the PFAS to be degraded can be subjected to electrocatalysis will be referred to simply as a “container” unless reference to a specific type is required.) In some embodiments, the fluorine released from the PFAS is precipitated by adding a compound that will dissolve in the predominantly aqueous solution to provide atoms of the desired type to react with the fluorine in the solution (for convenience, such compounds will simply be referred to as a “precipitation compound”). In other embodiments, the fluorine released from the PFAS is precipitated by adding to the container a second solution, in which the compound just described has already been dissolved (for convenience, such compounds will simply be referred to as a “precipitation solution”). The resulting precipitate is understood to contain fluorine from the PFAS as a component and is sometimes referred to herein as the fluorine having been “mineralized,” or as “mineralized fluorine.”

[0115] Precipitation of the fluorine in the predominantly aqueous solution within the container may allow the precipitate to coat the water oxidation catalyst, particularly in embodiments in which the water oxidation catalyst is provided as nanoparticles immobilized on an electrode. Accordingly, in preferred embodiments of batch processes, after the electrocatalysis of the PFAS has reached a point (in time or in electrocatalysis of the PFAS), the electrodes are removed from the predominantly aqueous solution before adding a precipitation compound or precipitation solution to the predominantly aqueous solution. In some embodiments, the resulting mineralized fluorine is then removed from the container before subjecting more PFAS to electrocatalysis.

[0116] In another set of embodiments, the PFAS may be subjected to electrocatalysis in a flow process. Use of UV light to increase the rate of electrocatalysis

[0117] Conducting the electrocatalysis with ultraviolet (“UV”) light shining on the electrocatalyst improves the amount of fluorine released from PFAS per unit time. Lamps,PATENT bulbs, light-emitting diodes (“LEDs”), and flashlights that produce UV light are well known and scores are commercially available.

[0118] Conveniently, the UV light can be provided by positioning one or more sources of the UV wavelength chosen by the practitioner to shine on the water oxidation electrocatalyst. If the electrode on which the water oxidation electrocatalyst is disposed has sides and the water oxidation electrocatalyst is disposed on more than one side, the UV light is preferably positioned to shine on each side bearing the water oxidation electrocatalyst. In some embodiments, more than one source of UV light is provided so that the entire area of the electrode bearing the water oxidation electrocatalyst receives UV light.

[0119] In some studies reported in the ‘345 PCT application investigating whether the presence of UV light improved the electrocatalysis of PFAS employed light at a wavelength of 254 nm. Light of this wavelength was convenient for use, in part because the absorbance of UV light of this wavelength is used as a water quality test that provides a quick measurement of the organic matter in water. The measurement technique works by shining ultraviolet light at 254 nm through a quartz cell that contains a representative water sample. Lamps, bulbs, and flashlights providing light of this wavelength are also used in a number of devices commercially sold to disinfect laboratory equipment.

[0120] It is expected that UV light of the UV-C spectral band, which is considered to be from 100 to 300 nm, will be suitable to improve the electrocatalysis of PFAS, with UV light with a wavelength from 200 to 280 nm, considered to be “deep” UV light, being preferred. A comparison of electrocatalysis of PFAS using UV at 254 nm and electrocatalysis of PFAS at a wavelength of 365 nm showed that the 254 nm wavelength was much more effective. In some embodiments, UV light with a wavelength from 100 nm to 300 nm is used to improve the electrocatalysis of PFAS. In some embodiments, UV light with a wavelength from 200 nm to 280 nm is used to improve the electrocatalysis of PFAS. In some embodiments, UV light with a wavelength from 220 nm to 280 nm is used to improve the electrocatalysis of PFAS. In some embodiments, UV light with a wavelength of 254 nm ± 10 nm is used to improve the electrocatalysis of PFAS. Non-Aqueous Solvents

[0121] A variety of non-aqueous solvents (e.g., organic solvents), such as acetonitrile, dimethyl sulfoxide (or “DMSO”), and ethanol, are known in the art. In general, any non-PATENT aqueous solvent that is miscible in water should be able to serve as the organic phase in a predominantly aqueous solution comprising the electrolyte in the electrocatalytic methods and systems in embodiments of the invention. Any particular non-aqueous solvent can be readily tested for its suitability for use as the organic phase in any particular concentration in a predominantly aqueous solution comprising the electrolyte by conducting an assay using the conditions set forth in the Examples, with any adjustment to electroconductivity required due to the presence of the non-aqueous solvent, and determining if fluorines are dissociated from an exemplar PFAS during electrocatalysis in the presence of the particular concentration of the particular non-aqueous solvent being tested. Temperature and Pressure

[0122] The electrocatalytic degradation of PFAS can be conducted at any temperature and pressure, so long as the electrolyte is liquid. Since the electrolyte contains dissolved solute (for example, LiOH), the boiling point and the freezing point of the electrolyte will vary depending on the compound and the concentration of the compound in the electrolyte solution. For example, a 10% NaOH solution has a freezing point of -10 ℃, while a 50% NaOH solution has a freezing point of 12 ℃ and a boiling point of 135 ℃. A 10% KOH solution has a freezing point of -3 ℃ and a boiling point of 101 ℃, while a 45% KOH solution has a freezing point of -29 ℃ and a boiling point of 132 ℃. The freezing and boiling points for a solution of LiOH (or of combinations of compounds which together contribute Li+ and hydroxide anions to the electrolyte solution) at any particular concentration a practitioner may wish to use may be readily determined and is within the skill of the person of skill.

[0123] The methods of the invention can be practiced at temperatures below 0 ℃, if desired, so long as the particular electrolyte solution the practitioner chooses remains liquid at the temperature chosen. Similarly, if desired, the methods of the invention can be practiced at temperatures above 100 ℃; if the particular electrolyte solution the practitioner chooses to use boils at the temperature chosen, care should be taken to replenish it so that the level of electrolyte remains sufficient for the degradation to precede.

[0124] Similarly, the pressure under which the methods are practiced may be varied. First, the ambient pressure will vary depending on the elevation above or below sea level at which the electrocatalysis is perforned. Second, while it is contemplated that most electrocatalysis will be conducted under whatever ambient air pressure is present at the location at which thePATENT electrocatalysis is performed, some practitioners may choose to conduct the methods at a pressures other than the ambient pressure, usually by performing the electrocatalysis within a pressurized container. Pressurized containers permit the use of higher temperatures than do non-pressurized containers. The term “ambient air pressure” denotes the air pressure at whatever location the electrocatalytic degradation of PFAS is being performed.

[0125] Electrocatalytic degradation of PFAS can be conducted at temperatures below room temperature and even below 0 °C, so long as the electrolyte remains liquid at the temperature at which the electrocatalysis is conducted. It is well known, however, that chemical reactions usually proceed more quickly if they occur at higher temperatures than they do at a lower temperature. In some embodiments, therefore, the electrocatalysis is conducted using an electrolyte that has been heated above room temperature. In some embodiments, the electrolyte is heated to 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 99 °C, or 100 °C, or to a temperature between any two of the temperatures just listed. If desired, the temperature of the electrolyte can be raised to 100 °C or higher. In these embodiments, the container should be covered to avoid or reduce the loss of electrolyte, and the container and cover are preferably selected to be able handle both the heat and the pressure that will be produced by heating the electrolyte above its boiling point. Materials Forming the Electrode Supporting the Electrocatalyst

[0126] The electrocatalyst used to degrade PFAS is typically performed by depositing the electrocatalyst on hydrophilic carbon fiber paper, or “CFP” (all references to “CFP” in this disclosure relate to hydrophilic CFP unless otherwise stated). The electrocatalysis of an exemplar PFAS compound by an electrocatalyst disposed on the nickel mesh support was slightly less than, but within the same order of magnitude as, the electrocatalysis of the same exemplar PFAS by the same electrocatalytic material, but disposed on CFP, indicating that materials other than CFP that are electrically conductive can serve as a support for the electrocatalyst in some embodiments of the inventive systems and methods. The electrocatalyst is typically dispersed on the electrode. In some preferred embodiments, the electrically conductive supports for the electrocatalyst are made of one or more metals. In embodiments in which the electrocatalyst is provided in the form of nanoparticles or other small particles, the electrically conductive supports are preferably in the form of a mesh, which provides a larger surface area to which the nanoparticles or other small particles can adhere and, consequently, a larger surface area over which electrocatalysis of the compoundPATENT to be degraded can take place, thereby reducing the time needed to degrade any particular amount of the target compound. Stirring vs. Non-Stirring

[0127] A study revealed that electrocatalysis of the target compound proceeded more quickly if the electrolyte solution was not stirred. Without wishing to be bound by theory, it is surmised that not stirring the electrolyte allows the target compound dissolved in the electrolyte to stay in contact with the electrocatalyst longer than in electrolytes that are stirred and that this more prolonged contact increases the rate at which the target compound is degraded. Definitions

[0128] Units not otherwise defined herein are defined as the units are defined by International System of Units, abbreviated as “SI.”

[0129] "Reaction time" refers the time duration during which anodic bias is applied to the water oxidation electrocatalyst, or to a working electrode on which the electrocatalyst is disposed.

[0130] "Nanoparticles" refers to a material (e.g., water oxidation electrocatalyst) provided as solid particles with at least one size dimension in the range of 1 nm to 1 µm. Relevant examples of a size dimension include: length, width, diameter, area-based diameter, and volume-based diameter. The nanoparticle volume, area, weight, and area each may be an average property reflective of the nanoparticle size distribution. Interaction among nanoparticles may lead to aggregation of the nanoparticles into larger aggregates, or clusters of nanoparticles. As used herein, the term "nanoparticle" is not intended to include a cluster or aggregate of nanoparticles.

[0131] As used herein, the term “degrading,” when referring to the electrocatalysis of a PFAS refers to the breaking of at least one covalent bond in the PFAS and, in preferred embodiments, results in breaking the PFAS into two or more smaller compounds.

[0132] As used herein, the “first-row transition metals” are scandium (Sc), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn). In preferred embodiments, water oxidation electrocatalysts comprising two or more transition metals are predominantly nickel.PATENT EXAMPLES Example 1

[0133] This Example describes the materials and methods used in the studies described in the following Examples.

[0134] All chemicals were used as received. Deionized water was obtained from a Thermo Scientific Barnstead™ Smart2Pure™ Pro UV / UF 15 LPH Water Purification System (Thermo Fisher Scientific, Inc., Waltham, MA) and had a resistivity of ≥17.5 MΩ ∙ cm. All experiments were performed at room temperature and in ambient air. Glassware was cleaned before use with aqua regia, rinsed with copious amounts of water, and dried. Error bars are standard deviations of triplicate measurements. Data analysis and graphing were performed with Igor Pro 8.04 (WaveMetrics, Inc., Portland, OR) unless otherwise noted. Catalyst preparation

[0135] The process to prepare hydrophilic carbon fiber paper as previously described. (See, Wilsey, et al., Adv. Mater. Interfaces, 2023, 10(2):2201684; doi.org / 10.1002 / admi.202201684, hereafter, “Wilsey 2023 A”). Briefly, we selectively functionalized surfaces of as-purchased carbon fiber paper (Fuel Cell Store, Bryan, TX, AvCarb MGL190, 78% porosity) by sonication in 1 M aqueous sodium dodecyl sulfate solution, followed by electrooxidation in 0.1 M pH 8.7 aqueous KHCO3 electrolyte at +1.63 V vs. Ag / AgCl for 20 min. Electrodes had geometric dimensions of 3.0 cm (length) x 3.0 cm (width), resulting in a geometric electrode area Ageo of 9.0 cm2and a carbon surface area A∑ of 4200 cm2. This number was calculated by considering the reported specific surface area Svof MGL190 of 5.6 m2g–1, the thickness d and density ρ of MGL190 of 0.19 mm andg cm–3, respectively, given by the carbon fiber paper manufacturer, and using the equation A∑ = Ageo ∙ d ∙ ρ ∙ Sv.

[0136] Laser-made [NiFe]-layered double hydroxide nanocatalysts were obtained following synthesis details reported elsewhere (Hunter, et al., J. Am. Chem. Soc., 2014, 136:13118- 13121. In brief, we utilized an 8 ns pulse laser with a wavelength of 355 nm, generated by the third harmonic of a 10 Hz Q-switched Nd:YAG laser (Spectra-Physics Quanta-Ray LAB- 190), to irradiate a suspension of iron powder (Alfa, −200 mesh, 99+%) in a 10 mL solution of 3.0 M nickel nitrate (Alfa, 98%) in water. This process was carried out in a 30 mL glass beaker at room temperature in ambient air for a duration of 60 min, with each pulsePATENT delivering 90 mJ of energy. Following the laser synthesis, any unreacted iron powder was separated from the aqueous suspension of [NiFe]-layered double hydroxide nanoparticles using a strong magnet. The solid nanoparticulate powder was obtained through centrifugation and subjected to five water washes, followed by two washes with acetone (VWR, Radnor, PA), after which the nanopowder was dried under house vacuum. Subsequently, suspensions of nanoparticulate powder at a concentration of 2 mg mL–1in water were vortexed, drop-cast onto hydrophilic carbon fiber supporting electrodes, and dried under a heat lamp at 60°C in ambient air. Physical characterization

[0137] X-ray photoelectron spectra (XPS) were acquired using a Kratos Axis Ultra XPS instrument equipped with a monochromatized Al Kα radiation source. Samples were washed with copious amounts of water and dried in ambient air before pumping them into the XPS instrument. The instrument operated in high-power mode, with settings of 200 W and 15 kV, while maintaining a base chamber pressure of 3.0 × 10–8mbar. Samples were securely affixed to double-sided adhesive copper tape. For survey scans, the spectral range covered 0 to 1200 eV, with a step size of 1 eV, a dwell time of 200 ms, and an analyzer pass energy of 140 eV, averaged over 5 scans. Subsequently, high resolution core level region scans were conducted, focusing on specific binding energy ranges. These scans employed a 0.1 eV step size, an average dwell time of 260 ms, and an analyzer pass energy of 20 eV, averaged over 5 scans. Binding energies were calibrated against the C 1s peak arising from adventitious carbon, taken to have a binding energy of 284.8 eV. Following data acquisition, binding energies and quantitative peak areas were derived after Shirley background subtraction (see, Shirley, Phys. Rev. B, 1972, 5:4709-4714) and Gaussian / Lorentzian envelope peak fitting. Quantification of different components was accomplished using instrument-specific atomic sensitivity factors derived from standard materials. The XPS data analysis was performed using CasaXPS (Version 2.3.24).

[0138] Scanning electron microscopy (SEM) imaging was conducted using a JEOL JSM- 5900LV SEM instrument equipped with a thermionic tungsten electron gun, operated at 25 kV with a working distance of 10 mm. Hydrophilic carbon fiber paper was mounted on a 1- inch diameter aluminum SEM stubs (Ted Pella, Inc., Redding, CA) with carbon tape (Electron Microscopy Sciences, Hatfield, PA). Diameters of carbon fibers were deduced from SEM images using ImageJ software (Schneider, et al., Nature Methods, 2012, 9(7):671–675.PATENT doi:10.1038 / nmeth.2089), and the published GIFT fiber diameter quantification macro (Huling, et al., PLoS One, 2022, 17:e0275528). Electrocatalysis

[0139] Electrochemical investigations were performed in ambient air and at room temperature, using a standard Pyrex® three-electrode, single-compartment 50 mL electrochemical cell, equipped with a Teflon™ lid that remained above the liquid surface, and featuring a flat quartz window for ultraviolet light illumination (254 nm lamp, 40 mW cm–2). Anodes consisted of laser-made [NiFe]-layered double hydroxide nanocatalysts immobilized on hydrophilic carbon fiber paper (444 µg cm–2geo catalyst mass loading), and cathodes were neat hydrophilic carbon fiber paper. A Pt wire pseudo-reference electrode that was calibrated in each electrolyte against a hydrogen reference electrode (Gaskatel HydroFlex®) served as reference electrode. Platinum wire has been shown to be a suitable and stable reference electrode in various electrochemical systems. The anode was placed parallel to the cathode at a distance of 16 mm, secured by the Teflon™ lid, and that stack was centered in the Pyrex® cell, normal to the quartz window. Aluminum clips were used to electrically contact the working and counter electrodes to the leads of the potentiostat (BioLogic, Seyssinet-Pariset, France, 8-slot VSP3e potentiostat / galvanostat / EIS system). The data were not corrected for any uncompensated resistance losses. All potentials are reported vs the reversible hydrogen electrode (RHE). Chronoamperometry data were collected at anodic applied potentials of 1.2, 1.4, 1.6, 1.8, or 2.0 V vs RHE. In pulsed electrolysis, one cycle denotes one ON time at an applied potential of +1.6 V vs RHE followed by one OFF time at open circuit potential; ON times were 0.5, 1, 2, or 5 min, while OFF times were 2, 5, or 10 min. The cell was filled with 40 mL electrolyte, and the liquid was stirred at 400 or 800 rpm on a multi-position stir plate (G-Biosciences, St. Louis, MO, BT1016) or not stirred at all. Preparation of solutions

[0140] Aqueous (pH 14.0) 2.0, (pH 14.2) 4.0, (pH 14.8) 6.0 or (pH 14.9) 8.0 M LiOH was prepared by adding 48.7, 95.6, 143.4, or 191.6 g of lithium hydroxide monohydrate (Thermo Scientific, 98%), respectively, to a 1 L volumetric flask that was subsequently filled to the 1 L mark with water and mixed well until all solid was dissolved. Aqueous (pH 7.8) 6.0 M LiClO4was prepared by adding 63.8 g lithium perchlorate anhydrous (Thermo Scientific, 95%) to a 100 mL volumetric flask that was subsequently filled to the 100 mL mark with water and mixed well until all solid was dissolved. Aqueous (pH 14.9) 8.0 M NaOH wasPATENT prepared by adding 32.0 g of sodium hydroxide (Fisher Thermo, 98.9%) to a 100 mL volumetric flask that was subsequently filled to the 100 mL mark with water and mixed well until all solid was dissolved. Aqueous (pH 12.1) 0.01, (pH 13.0) 0.1, (pH 13.9) 1.0, (pH 14.1) 2.0, (pH 14.3) 4.0, (pH 14.6) 6.0, (pH 14.7) 7.0, or (pH 15.0) 8.0 M KOH was prepared by adding appropriate measured amounts of potassium hydroxide (Fisher Chemical, 85%) to a 100 mL volumetric flask that was subsequently filled to the 100 mL mark with water and mixed well until all solid was dissolved. Aqueous (pH 14.7) 8.0 M CsOH was prepared by adding 119.9 g of cesium hydroxide hydrate (Thermo Scientific, 99.9%) to a 100 mL volumetric flask that was subsequently filled to the 100 mL mark with water and mixed well until all solid was dissolved. Electrolytes were stored in plastic containers. PFOS solutions with concentrations of 0.05, 0.10, 0.25, 0.50, 1.00, or 2.00 mM were prepared by placing 1.07, 2.155.38, 10.76, 21.52, or 43.04 mg potassium perfluorooctanesulfonate (Synquest Laboratories, Alachua, FL, 95%) into a 50 mL Falcon® tube (Corning, Corning, NY) and adding 40 mL of aqueous electrolyte. The PFOS-containing solutions were subjected to ultrasonic agitation (JeKen Ultrasonic Cleaner Ltd., Dongguan City, CN, PS-40A digital ultrasonic bath, 240 W, 10 L) for 5 min followed by sitting undisturbed for 30 min, to ensure proper mixing and dissolution. An analog procedure was used to prepare 0.50 mM PFOA solution, in which 8.3 mg perfluorooctanoic acid (Beantown Chemical Corp., Hudson, NH, 95%) was added to a 50 mL Falcon® tube before electrolyte addition. Chemicals were weighed on a Sartorius A 120 S analytical balance (Sartorius Lab Instruments GmbH & Co. KG, Goettingen, Germany) or a Mettler Toledo AT201 (Mettler-Toledo, LLC, Columbus, OH) analytical balance.

[0141] A Mettler Toledo SevenExcellence pH / Ion / C / DO meter S975-K with a InLab® Expert Pro-ISM pH probe (Mettler Toledo) was used to measure the pH values of aqueous KOH and CsOH electrolytes. The pH values of aqueous NaOH and LiOH electrolytes were obtained by calculations, using the Pitzer equation within its validity range or assuming complete salt dissociation (Table 1).PATENT Table 1. Calculated pH values for concentrated aqueous LiOH an dNaOH solutions; γOHhydroxide ion activity. Aqueous electrolyte pH (Pitzer equation) pH (assumed γOH =1) 2.0 LiOH 13.99 14.30 4.0 LiOH 14.24 14.60 6.0 LiOH 14.40* 14.78 8.0 LiOH 14.51* 14.90 8.0 LiOH 15.21* 14.90 * Calculated pH value for aqueous electrolyte with an alkali hydroxide concentration beyond the accepted validity of the Pitzer equation. Quantification of fluoride

[0142] A fluoride ion selective electrode (Thermo Scientific Orion, 9609BNWP) together with a total ionic strength adjustment buffer (“TISAB”) solution was used to quantify fluoride ions in aqueous solutions. TISAB II was prepared as described by Cho et al., J. Electroanal. Chem., 2020, 858:113837 by sequentially adding 34.2 mL acetic acid (Mallinckrodt Chemicals, glacial), 34.8 g sodium chloride (Fisher Chemical, 100.2%), and 0.18 g trisodium citrate dihydrate (Alfa Aesar, a division of Thermo Fisher Scientific, 99%) to 300 mL water in a 500 mL beaker that was stirred until all chemicals were dissolved. Subsequently, the pH value of this aqueous solution was adjusted to 5.20 to 5.25, measured by a Mettler Toledo SevenExcellence pH / Ion / C / DO meter InLab® Expert Pro-ISM probe, by adding 5.0 M aq NaOH drop by drop while stirring the buffer solution. The 5.0 M aq NaOH solution was prepared by adding 20.0 g NaOH to a 100 mL volumetric flask that was subsequently filled to the 100 mL mark with water and mixed well until all solid was dissolved. This pH-adjusted solution is denoted as TISAB II. In strongly alkaline electrolytes, we used the following procedure that prevented hydroxide ion interference in fluoride ion selective electrode measurements and confirmed the accuracy of measured fluoride concentrations in strongly alkaline aqueous test solutions with added NaF (Sigma Aldrich, 99%) with known concentrations. The pH value of solutions of fluoride in strongly alkaline electrolytes was first lowered to approximately 5.5 by addition of potassium acetate buffer, following the recommendations of the manufacturer of the fluoride ion selective electrode. Fluoride concentrations in all aqueous electrolytes after electrocatalytic PFOS or PFOA defluorination were determined by adding 1 mL of solution to 9 mL of potassium acetatePATENT buffer in a 50 mL Falcon® tube and subsequently adding 10 mL of TISAB II solution. Potassium acetate buffer was prepared by combining 34.5 mL glacial acetic acid and 65.5 mL water in a 1 L beaker, in which the solution was stirred, and adjusting the pH value of the mixture to 5.0 by adding a solution of 20 g KOH in 20 mL of water drop by drop. The fluoride ion selective electrode was calibrated using fluoride standards with different concentrations, obtained from sequential dilutions by a factor of ten each with water from a commercially available fluoride standard (Thermo Scientific, 0.1 M fluoride), resulting in fluoride concentrations of 10, 1.0, 0.1, or 0.01 mM. Calibration was conducted in solutions that were prepared by adding 1 mL of each fluoride standard to 9 mL of potassium acetate buffer in a 50 mL Falcon® tube and adding 10 mL of TISAB II solution. Example 2

[0143] This Example presents the results of studies using the materials and methods described in Example 1.

[0144] We observed complete defluorination of 0.50 mM PFOS solution in aqueous 8.0 M LiOH electrolyte using 60 pulsed electrolysis cycles that each consisted of 1 min ON time at an applied potential of +1.6 V vs RHE, followed by 5 min OFF time at open circuit potential, electrocatalyzed by laser-made [NiFe]-layered double hydroxide nanosheets on a hydrophilic carbon fiber paper anode, with ultraviolet irradiation and without stirring (Fig.1A). Increasing the number of pulsed electrolysis cycles to 80 also completely defluorinated 0.50 mM PFOS in aqueous 8.0 M LiOH electrolyte, whereas decreasing the number of pulsed electrolysis cycles below 40 lowered the defluorination yield (Fig.1A). Nevertheless, we performed most pulsed electrolysis experiments at 20 cycles, which took 2 h and reached (92.3 ± 1.1) % defluorination yield with 0.50 mM PFOS, to enable more correlations.

[0145] Defluorination yields depended on the PFOS concentration. We obtained the highest defluorination yields with lower concentrations of PFOS in aqueous 8.0 M LiOH (Fig.1B). Electrocatalysis using pulsed electrolysis at 20 cycles defluorinated more than 93% of PFOS in solution with concentrations of ≤0.5 mM. Defluorination reached 99% with 0.05 mM PFOS in solution, which corresponds to 27 ppm and is relevant in the context of environmental pollution remediation. In contrast, defluorination yields of higher concentrations of PFOS of 1.00 or 2.00 mM were only 69.8 ± 1.1% or 23.3 ± 1.1 %, respectively (Fig.1B), suggesting that adsorption of PFOS at the anode or the concentration of generated oxidants was rate-limiting for C–F bond cleavage in PFOS. Adsorption of PFOSPATENT at graphitic carbon materials has been reported (Wilsey et al., Chem. Comm.2023, 59:11895- 11922, hereafter “Wilsey 2023 B”). PFAS that possess a negatively charged terminal group, such as the sulfonate group of PFOS, have been shown to adsorb at graphitic surfaces. (Brusseau, Water Res., 2019, 152:148-158). The hydrophobic fluorinated alkyl chains render PFOS electronegative while the hydrophilic polar sulfonate head group enables the formation of micelles, which act as surfactants in solution, facilitating improved adhesion to carbon adsorbates. Evidence from studies underlying this disclosure corroborates that the hydrophilic carbon fiber paper–[NiFe]-layered double hydroxide assemblies enabled PFOS adsorption and that defluorination occurred in the anode microenvironment.

[0146] The optimal applied potential for the defluorination of 0.50 mM PFOS in aqueous 8.0 M LiOH electrolyte was +1.6 V vs RHE, for 10, 20, or 60 pulsed electrolysis cycles (Fig. 1C). Therefore, we used an applied potential of +1.6 V vs RHE for this work. We rationalize this optimum applied potential by consideration of the thermodynamic potentials of the water–oxygen redox system at pH 14.9, which is the pH value we obtained for aqueous 8.0 M LiOH. We assumed Nernstian behavior between pH values of 14.9 and 14.0; the thermodynamic potentials E of the water–oxygen redox system are known at pH 14.0. Reactive species of the water–oxygen redox system are deprotonated at pH 14.9 because of their respective pKa values. At +1.6 V vs RHE at pH 14.9, alkaline water oxidation can directly form deprotonated hydrogen peroxide, HOO–, (EpH 14.9 = 0.81 V) through the two- proton–two-electron pathway, whereas the one-proton–one-electron water oxidation pathway to form O•–(EpH 14.9 = 1.72 V) is not accessible at that applied potential. Nevertheless, decomposition of HOO–by ultraviolet light can form O•–, and HOO– can be oxidized to OO•–(EpH 14.9 = 0.15 V), whose thermodynamic potential at pH 14.9 is only 0.60 V, which likely provides insufficient driving force for C–F cleavage in advanced oxidation processes. The four-proton–four-electron alkaline water oxidation pathway to form dioxygen is also accessible at +1.6 V vs RHE at pH 14.9, as a thermodynamic potential of 0.35 V is required.

[0147] Scaled up to the treatment of a 1 m3batch of polluted water, the process of this work requires 5.256 kWh of operational electrical energy at a capital expense of $0.10 million U.S. dollars (Fig.1D), which are both lower than those of existing techniques. For example, electrooxidation of 0.5 mM PFOS by boron-doped diamond electrodes (Fig.1D) took 20 h to reach only ~50% conversion in a rotating disk reactor, which is impractical on a large scale. Scale-up to treatment of a 1 m3batch of polluted water renders boron-doped diamondPATENT electrodes cost-prohibitive, as a working electrode of 15.2 m2of at a cost of $8.58 million and 180 kWh m–3electrical energy during operation are required.

[0148] Next, we examined how electrolyte conditions, i.e. type and concentration of alkali ions, presence or absence of high hydroxide ion concentrations, and type of PFAS substrate (PFOS or perfluorooctanoic acid, PFOA), correlated with defluorination yields, as set forth in Example 3. Example 3

[0149] This Example presents the results of studies of the electrocatalytic degradation of an exemplar PFAS in the presence of different electrolytes.

[0150] Lithium ions produced the highest PFOS defluorination yield compared to other alkali ions in electrolytes with 8.0 M alkali ion hydroxide concentrations. The defluorination yield in aqueous 8.0 M LiOH electrolyte was twice as high as in aqueous 8.0 M NaOH, KOH, or CsOH electrolytes. Defluorination in NaOH electrolyte was comparable to that of defluorination in KOH electrolyte, whereas defluorination in either NaOH and KOH as the electrolyte outperformed that in CsOH electrolyte by a factor of 1.1 (Fig.2A).

[0151] A few reports of lithium mediation of electrochemical transformations exist, albeit not in the context of PFAS defluorination, and particularly not in aqueous electrolytes. Lithium-supported electrochemical Birch reductions in nonaqueous media were utilized to synthesize pharmaceutically relevant building blocks (see, Peters et al., Science, 2019, 363:838-845). Lithium-mediated ammonia electrosynthesis from dinitrogen has been reported in nonaqueous electrolytes (see, e.g., Schwalbe, et al., ChemElectroChem, 2020, 7:1513). Electrocatalysis in aqueous 6.0 M LiOH vs aqueous 6.0 M LiClO4 electrolytes showed a factor of 12.8 less defluorination in the perchlorate compared to the hydroxide electrolyte; the solubility limit of LiClO4 in room-temperature water dictated the use of 6.0 M instead of 8.0 M solutions (Fig.2B).

[0152] Higher concentrations of aqueous base and concomitant pH values were beneficial for PFOS defluorination. Electrolyte concentration series of aqueous 2.0 to 8.0 M LiOH or aqueous 0.01 to 8.0 M KOH showed linear increases of defluorination yields as base concentrations increased (Fig.2C). The slope for LiOH was 17.5 ± 1.5 whereas that for KOH was 8.7 ± 0.3, ergo the LiOH slope was a factor of 2.0 steeper than the KOH slope. These results suggest that a high concentration of hydroxide ions was required for efficient PFOSPATENT defluorination, in addition to a high concentration of lithium ions. Plotting defluorination yields of these LiOH or KOH concentration series vs the electrolyte pH revealed that the LiOH slope was a factor of 1.8 steeper than the KOH slope (Fig.2D), corroborating that a high hydroxide ion concentration was needed for efficient PFOS defluorination.

[0153] We measured the pH values of strongly alkaline aqueous KOH and CsOH solutions with a potentiometric pH probe because electrolytes that contain alkali ions larger than sodium do not suffer from alkaline error. Alkaline error occurs when H+ions within the pH electrode are partly or completely replaced by small alkali ions, leading to a lower than real measured pH value. The determination of pH values of our aqueous LiOH and NaOH solutions that possessed high lithium or sodium ion concentrations was not trivial because of lithium and sodium ion error artifacts in potentiometric measurements. Therefore, in keeping with published reports, we calculated the pH values of our aqueous LiOH and NaOH solutions. Solution pH values are often estimated using models that have been devised to account for alkaline error. Several equations have been reported, with numerous corrections to calculate the values of activity coefficients for electrolyte solutions, such as the Debye– Hückel, Hegleson, Davies, and Pitzer equations. Considering our experimental parameters, we chose to use the Pitzer equation (Pitzer, J. Phys. Chem., 1973, 77:268-277) to calculate activity coefficients, as the Pitzer equation was reported to have increased validity at high electrolyte concentrations, compared to other methods. However, the accuracy of the Pitzer equation depends on the electrolyte of choice. We used aqueous 2.0, 4.0, 6.0, or 8.0 M LiOH or 8.0 M NaOH electrolytes in our work. The Pitzer equation has been reported to have a validity regime of at most 4.0 M for LiOH and 6.0 M for NaOH. Therefore, we additionally calculated the pH value of each electrolyte, assuming complete dissociation, corresponding to an activity coefficient of unity (Table 1). Based on concentration alone, the anticipated pH values for these high concentration electrolyte solutions were 14–15. We found discrepancies in the comparison of pH values resulting from the Pitzer model to those calculated based on an activity coefficient of unity. In the cases beyond the validity of the Pitzer equation (aqueous 6.0 or 8.0 M LiOH or 8.0 M NaOH) we, therefore, took the pH values calculated based on an activity coefficient of unity.

[0154] Defluorination of PFOS was 1.2 times more efficient than that of PFOA under otherwise identical conditions (Fig.2E), presumably because cleaved sulfonate headgroups may have remained in the electrode microenvironment, where they can form SO4•–radicals near the anode to assist defluorination in advanced oxidation processes. Several mechanismsPATENT have been proposed for electrochemical aqueous PFOS destruction. Reported electrooxidation pathways of perfluorinated sulfonic acids or sulfonates commence with an initial step involving a direct electron transfer to the anode, a process identified as rate- limiting based on density function theory calculations. Because the bond dissociation energy of the C–S bond (272 kJ mol–1) in PFOS is lower than that of the C–C bonds (346 kJ mol–1) or that of the C–F bonds (464.5–481.5 kJ mol–1), desulfonation occurs after the initial electron transfer, resulting in the formation of a•C8F17 or•C7F15 radical, depending on mechanism, from which CF2moieties are unzipped by advanced oxidation processes involving•OH or OO•–radicals to form shorter-chain perfluoroalkyl radicals, ultimately producing CO2and HF, which gives rise to fluoride in the electrolyte that can later be safely mineralized as calcium-containing solids.

[0155] X-ray photoelectron spectroscopy (XPS) data demonstrate the presence of adsorbed C–F2 and C–F3 moieties, adsorbed sulfate, and adsorbed deprotonated hydrogen peroxide at [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode surfaces after 6 h of pulsed electrocatalysis in aqueous 8.0 M LiOH electrolyte with 0.5 mM PFOS (Fig.3A). Analog data after immersion of the anode for 6 h in the aqueous 8.0 M LiOH electrolyte with 0.5 mM PFOS at open circuit potential show higher contents of adsorbed C–F2 and C–F3 but almost no adsorbed sulfate and zero adsorbed deprotonated hydrogen peroxide (Fig.3B). In contrast, no adsorbed C–F2 and C–F3 moieties, no adsorbed sulfate, and no adsorbed deprotonated hydrogen peroxide were detectable after 6 h at open circuit potential in aqueous 8.0 M LiOH electrolyte without PFOS (Fig.3C). Additional XPS data are in Fig.5 and Fig. 6. We quantified surface contents of individual species with respect to total surface carbon by peak fitting, accounting for relative sensitivity factors.

[0156] The elements carbon, oxygen, and minor amounts of nickel were present in the XPS data of working electrode surfaces at all three electrochemical conditions; notably, lithium was absent (Fig.3 and Fig.5). We were unable to detect iron in the Fe 2p core level region (Fig.6), since the laser-made [NiFe]-layered double hydroxide nanosheets had a nickel-to- iron ratio of 3.5:1 and the catalyst loading was only 4 mg on the high surface area hydrophilic carbon fiber paper with a carbon surface area of 4200 cm2, resulting in an average catalyst loading of 0.9 µg cm2and a weak Ni 2p signal. At open circuit potential without any PFOS present in the aqueous 8.0 M LiOH solution, fitting the C 1s and O 1s regions with peaks as described previously for hydrophilic carbon fiber paper (see, Wilsey 2023 A), matched the data well and did not require inclusion of any additional peaks (Fig.3C).PATENT

[0157] At open circuit potential with 0.5 mM PFOS present in the aqueous 8.0 M LiOH solution, we detected the elements fluorine and sulfur in XPS data, in addition to carbon, oxygen, and nickel (Fig.3B and Figs.6B and 6E). Fitting the C 1s signal required inclusion of two additional peaks with central binding energies of 292.2 and 294.5 eV, attributable to C–F2 and C–F3 moieties, respectively. Additionally, a strong F 1s peak was present with a central binding energy of 689.4 eV, which is consistent with the reported range of C–F bonds in PFOS (688–689 eV), thus providing evidence of adsorption of PFOS at the surface of hydrophilic carbon fiber paper–[NiFe]-layered double hydroxide assemblies. The weak S 2p signal exhibited a peak with a central binding energy of 168.4 eV, attributable to sulfate whose reported binding energy range is 168.3–169.4 eV. Likewise, matching the O 1s signal required an additional peak to keep the contents of the carbon and oxygen containing species consistent in both the C 1s and O 1s regions. This additional O 1s peak at a central binding energy of 532.2 eV is assignable to sulfate S=O moieties stemming from PFOS.

[0158] After PFOS electrocatalysis, which consisted of pulsed electrolysis at 60 cycles (1 cycle was 1 min ON time at an applied potential of +1.6 VRHE, followed by 5 min OFF time at open circuit potential), with 0.5 mM PFOS present in the aqueous 8.0 M LiOH solution, we observed in XPS data the elements carbon, oxygen, nickel, fluorine, and sulfur (Fig.3A and Figs.6A and 6D). The surface sulfate content was three times higher than that obtained at open circuit potential with 0.5 mM PFOS present in the aqueous 8.0 M LiOH solution, indicating that negatively charged sulfate head groups adsorbed preferentially when the surface of the hydrophilic carbon fiber paper–[NiFe]-layered double hydroxide assembly was positively charged. The S 2p core level region exhibited a peak at a central binding energy of 168.4 eV, attributable to sulfate. Likewise, an additional O 1s peak at a central binding energy of 532.2 eV was detected, attributable to sulfate S=O moieties from PFOS. Interestingly, we observed a drop in surface fluorine content after electrocatalysis, presumably due to increased defluorination; note that 60 cycles of pulsed electrolysis led to complete defluorination of PFOS in the entire electrolyte solution (Fig.1).

[0159] In the post-electrocatalysis XPS data, an additional broad feature was present in the O 1s region with a central binding energy of 538.7 eV. We did not detect this higher binding energy feature in electrodes after immersion at open circuit potential, with or without PFOS. Adsorbed water on carbon surfaces, including on hydrophilic carbon fiber paper, has a reported central binding energy range of 535.3–536.1 eV [12, 73, 74], which is too low to explain our signal. Reported binding energies of water in LiCl solutions are around 538.0 eVPATENT

[0072] , but we did not observe lithium in the XPS data (Fig.5), suggesting that the 538.7 eV feature was not due to adsorbed water. Dioxygen physisorbed on graphite was reported to have binding energy of 538.1 eV and a peak width of 1.2 eV

[0075] , which we included as the low binding energy peak into fitting the broad feature (Fig.3). Dioxygen could have originated from water oxidation at the anode; we note that we did not observe bubble formation, suggesting that the four-proton–four-electron alkaline water oxidation was not a dominant process in which charge was lost. Inclusion of a dioxygen peak into fitting the broad feature left a peak with a central binding energy of 539.5 eV and a width of 1.9 eV to match the data. We assigned this 539.5 eV peak to adsorbed deprotonated hydrogen peroxide that originated from the two-proton–two-electron water oxidation that is energetically accessible at high pH and an applied potential of +1.6 V vs RHE (see 3.1.). The O 1s XPS spectrum of hydrogen peroxide is known, with a central binding energy of the O 1s peak of 540.9 eV

[0076] . The pKa value of hydrogen peroxide is 11.6

[0077] , ergo hydrogen peroxide was deprotonated in aqueous 8.0 M LiOH solution. Our observed binding energy of 539.5 eV (Fig.3) was 1.4 eV less than the reported H2O2 binding energy of 540.9 eV

[0076] , presumably because surface adsorption and deprotonation shifted the O 1s electron binding energy to lower values. An O 1s shift of 1.4 eV to lower binding energy is reasonable, as estimated from O 1s binding energy values for water, which is the most closely related chemical to H2O2, and for which more XPS data are known. The O 1s central binding energies of liquid water on liquid water and water adsorbed on hydrophilic carbon fiber paper are 538 eV [76, 78, 79] and 535.3–536.1 eV

[0012] , respectively. We chose central binding energy values for liquid water because surface-adsorbed water molecules form liquid-like-structured nanoclusters above graphitic carbon basal planes [12, 80]. Ergo, the overall central binding energy shift was 1.9–2.7 eV, going from water to deprotonated water adsorbed on hydrophilic carbon fiber paper. This central binding energy shift to lower values for water explains the central binding energy shift of 1.4 eV to lower energy that we observed for deprotonated hydrogen peroxide adsorbed on hydrophilic carbon fiber paper, compared to reported XPS data of gaseous hydrogen peroxide

[0076] . As a result, our XPS data provide direct evidence for deprotonated hydrogen peroxide adsorbed on hydrophilic carbon fiber paper. Under an applied potential of +1.6 V vs RHE, the negatively charged HOO–species was electrostatically attracted to the positively charged anode, effectively trapping deprotonated hydrogen peroxide within the anode microenvironment. With ultraviolet irradiation, this adsorbed deprotonated hydrogen peroxide likely served as a source of reactive oxygen species, particularly O•–radical anions [1, 81]. The pKa value of the•OH radical is 11.8

[0082] ,PATENT ergo the hydroxyl radical was deprotonated in aqueous 8.0 M LiOH solution, so that ultraviolet light decomposition of deprotonated hydrogen peroxide produced negatively charged O•–radical anions that were electrostatically attracted to the positively charged anode, trapping these O•–radical anions within the electrode microenvironment. Therefore, PFOS defluorination occurred via advanced oxidation processes by interfacial reactive oxygen species O•–from HOO–within the anode microenvironment. Results of electrolyte agitation and pulsed electrolysis vs continuous chronoamperometry experiments, described in the following, corroborate our finding that PFOS defluorination happened indeed in the anode microenvironment and not in the bulk electrolyte. Example 4

[0160] This Example presents the results of studies on the effect of the electrode microenvironment on PFAS defluorination.

[0161] Zero stirring and use of pulsed electrolysis instead of continuous chronoamperometry resulted in the most efficient PFOS defluorination (Fig.4), indicating that a thick electrochemical boundary layer and long residence times of reactive intermediates in the anode microenvironment played a significant role in C–F bond cleavage. Electrolyte solutions that are not stirred are sometimes referred to herein as being “stagnant.”

[0162] More defluorination was observed in 2 h continuous chronoamperometry experiments when longer time intervals had elapsed before the electrolyte was stirred for 5 min (Fig.4A), suggesting that PFOS breakdown occurred near the anode surface after initial adsorption via the sulfonate headgroup; apparently, headgroup-less, shorter-chain PFOS breakdown products were unable to re-adsorb at the anode. In strongly alkaline aqueous electrolyte, PFOS was deprotonated, i.e. anionic and attracted to the positively charged anode, because the pKavalue of sulfonic acid protons of PFOS is –3.27

[0083] . We observed in XPS data surface sulfate at anodes (see above).

[0163] Likewise, we obtained higher defluorination yields with slower stirring speeds, with zero stirring being most beneficial, corroborating that a thick boundary layer was essential for efficient defluorination (Fig.4B, C). The short lifetimes of oxygen- and carbon-based radicals in aqueous electrolytes [1] that can be generated by the electrooxidations of this work dictate that these radicals reside within the electrode microenvironment. Zero or slower stirring speeds gave higher defluorination yields, irrespective whether continuousPATENT chronoamperometry (Fig.4B) or pulsed electrolysis was used (Fig.4C). Therefore, we used in the following zero stirring.

[0164] In pulsed electrolysis experiments, a constant applied potential is maintained for a brief period of seconds to minutes (called ON time), followed by a switch to open circuit potential for time durations on the order of minutes (called OFF time)

[0084] , to allow the boundary layer to re-establish equilibrium

[0085] ; this ON–OFF time cycle is repeated several times. Comparison of pulsed electrolysis with continuous chronoamperometry reveals insights into the importance of electrochemical and chemical transformations within the electrode microenvironment

[0085] . We found that the ON and OFF time durations mattered for defluorination in unstirred aqueous 8.0 M LiOH electrolyte (Fig.4D, E); we note that we maintained a constant total ON time of 20 min at an applied potential of +1.6 V vs RHE throughout these experiments to ensure comparability. Cycles consisting of 1 min ON time and 5 min OFF time produced the highest defluorination. Variation of the ON time to 2, 5, or 0.5 min generated less defluorination (Fig.4D). An ON time of less than 1 min apparently did not allow sufficient time for the defluorination reactions to proceed to completion. Since complete defluorination of all PFOS in the anode microenvironment was achieved within an ON time of 1 min, longer ON times did not improve defluorination, and overall defluorination yields were lower because the number of cycles was lower with longer ON times. Shortening the OFF time from 5 min to 2 min at 1 min ON time also significantly decreased defluorination, indicating insufficient re-equilibration after 2 min; a longer OFF time of 10 instead of 5 min did not enhance defluorination (Fig.4E), suggesting that the anode microenvironment was sufficiently re-equilibrated after 5 min.

[0165] Efficient PFOS defluorination required the presence of laser-made [NiFe]-layered double hydroxide catalyst nanosheets on hydrophilic carbon fiber paper and ultraviolet irradiation (Fig.7A). Without catalyst and ultraviolet light, PFOS defluorination was decreased by a factor of 24.0, compared to that in the presence of catalyst and ultraviolet light. An applied potential of +1.6 V vs RHE was needed to efficiently defluorinate PFOS; fluoride generation at open circuit potential for 2 h was negligible (Fig.7B). Detected fluoride after electrocatalysis without PFOS was also negligible (Fig.7C).

[0166] Our electrolyte agitation and pulsed electrolysis results together with our XPS data and finding that defluorination was more efficient at lower PFOS concentrations provide definite evidence that PFOS defluorination occurred within the anode microenvironment,PATENT likely via advanced oxidation processes involving short-lived O•–radical anions from ultraviolet light decomposition of HOO–species that were regenerated during electrocatalysis from water at the surface of laser-made [NiFe]-layered double hydroxide nanosheets on hydrophilic carbon fiber paper. Re-equilibration of this anode boundary layer during the OFF times of at least 5 min appeared to reset diffusional reaction conditions to initial conditions, and C–F bond cleavage occurred during the 1 min ON time intervals. We surmise that our PFOS defluorination process was efficient because the hydrophilic carbon fiber paper possessed a high carbon surface area of 4200 cm2, a high porosity of 78%

[0016] , and carbon fibers with a mean diameter of (6.8 ± 0.6) µm, determined from SEM images (Fig.8), leading to an anode surface at which electrolyte diffusion during the re-equilibration times was not significantly restricted by pore size, while simultaneously offering high surface area for PFOS adsorption and defluorination by adsorbed reactive oxygen species that were electrocatalytically regenerated by alkaline water oxidation in the anode microenvironment. Example 5

[0167] This Example discusses the results of studies reported in the preceding Examples.

[0168] We achieved complete PFOS defluorination in aqueous electrocatalysis with laser- made [NiFe]-layered double hydroxide nanocatalysts immobilized on hydrophilic carbon fiber paper anodes. All materials were nonprecious, as needed for globally scalable PFAS remediation technologies. Pulsed electrolysis of 0.50 or 0.05 mM PFOS in aqueous 8.0 M LiOH solution assisted by ultraviolet light irradiation enhanced electrical energy and capital expense efficiency. A PFOS concentration of 0.05 mM corresponds to 27 ppm, which is relevant for environmental water pollution remediation. High concentrations of lithium ions and high basicity were essential for efficient defluorination, which also worked for PFOA, albeit 1.2 times more efficiently for PFOS, presumably due to advanced oxidation processes by sulfate radicals near the anode that assisted defluorination. XPS data provided evidence for adsorbed C–F2 and C–F3 moieties, adsorbed sulfate, and adsorbed deprotonated hydrogen peroxide at [NiFe]-layered double hydroxide on hydrophilic carbon fiber paper anode surfaces post electrocatalysis.

[0169] A thick boundary layer was essential for efficient defluorination, and C–F bond cleavage occurred within the anode microenvironment via advanced oxidation processes that involved reactive oxygen species in the aqueous system. We found that slower stirring speeds improved defluorination yields, with zero stirring being most beneficial. Likewise, morePATENT defluorination was observed in 2 h continuous chronoamperometry experiments when longer time intervals had elapsed before the electrolyte was stirred for 5 min, suggesting that PFOS breakdown occurred near the anode surface after initial adsorption. Further, pulsed electrolysis, which allowed for boundary layer re-equilibration during each open circuit potential period of 5 min, enabled complete defluorination, whereas steady-state chronoamperometry with the same amount of charge passed led to a factor of 1.5 less defluorination. Electrolyte diffusion during the re-equilibration times was not significantly restricted by pore size at the anode surface that had a carbon porosity of 78% and a mean carbon fiber diameter of (6.8 ± 0.6) µm, while simultaneously offering a high carbon surface area of 468 cm2per geometric cm2, to enable PFOS defluorination by adsorbed reactive oxygen species that were electrocatalytically regenerated within the anode microenvironment. The studies of advanced aqueous electrocatalytic oxidation processes, particularly within anode microenvironments, reported herein provides more cost and energy efficient PFAS defluorination technologies than those previously available. Example 6

[0170] This Example discusses the use of the inventive systems and methods to defluorinate an exemplar GenX chemical, hexafluoropropylene oxide (HFPO) dimer acid anion, HFPO-.

[0171] As discussed above, perfluorooctanoic acid (PFOA) is a common PFAS chemical, which contains eight carbon atoms in its alkyl chain. The high chronic toxicity of PFOA prompted attempts to phase out the compound from facility emissions and product content. Per the National Service Center for Environmental Publications (NSCEP) of the U.S. Environmental Protection Agency, “GenX” is a trade name for a technology developed to produce high-performance fluoropolymers that do not contain PFOA. Per the NSCEP, the major chemicals associated with the GenX technology are hexafluoropropylene oxide (HFPO) dimer acid and its ammonium salt. In water, both HFPO dimer acid and its ammonium salt dissociate to form the HFPO dimer acid anion HFPO–. As used herein, the term “GenX chemicals” refers to fluoroethers with six carbon atoms in their alkyl chains. The term “GenX,” used with reference a specific PFAS compound (as opposed to the technology used to produce GenX chemicals), refers specifically to the HFPO dimer acid anion, HFPO–.

[0172] Exemplar anodes utilized in the defluorination of HFPO–consisted of pulsed laser in liquid synthesized surfactant-free [NiFe]-(OH)2nanosheets deposited on hydrophilic carbon fiber paper electrode supports. The resulting nanoparticles possessed layered doublePATENT hydroxide structure and nanosheet morphology, evident from X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. The XRD peaks exhibited broadening indicative of small crystallite size and the presence of stacking faults, including turbostratic disorder in the hydrotalcite-like structure, consistent with prior observations for laser- synthesized [NiFe]-(OH)2 nanosheets.

[0173] A rapid and environmentally friendly oxygenation process described in International Publication Number WO 2023 / 129686 was used to render carbon fiber paper hydrophilic, which is necessary for employing carbon fiber paper in an aqueous electrolyte. The carbon fiber paper exhibited a high surface area of 468 cm2per geometric cm2, which enhances electrocatalysis compared to a flat electrode support. The integration of laser-synthesized [NiFe]-(OH)2 nanocatalysts with hydrophilic carbon fiber paper resulted in a uniform distribution of nanocatalysts within the three-dimensional structure of the carbon fibers, effectively utilizing the high internal surface area of the electrode support. Energy-dispersive X-ray spectroscopy (EDX) analysis of the laser-made [NiFe]-(OH)2nanosheets showed a Ni:Fe ratio of 3:1. Only C, O, Fe, and Ni were detected in EDX spectra of the hydrophilic carbon fiber paper-supported [Ni0.75Fe0.25]-(OH)2nanosheets.

[0174] Using an exemplar electrocatalytic system, we observed complete defluorination of 0.5 mM HFPO–in aqueous 9.0 M LiOH electrolyte using 120 pulsed electrolysis cycles (Fig. 9a). The process was electrocatalyzed by [Ni0.75Fe0.25]-(OH)2nanosheets on hydrophilic carbon fiber paper anodes, with deep ultraviolet (UV) light irradiation and stagnant electrolyte. We recently showed, using an exemplar PFAS compound, PFOS, that a thick electrochemical double layer, achieved by not stirring the electrolyte, is important for efficient defluorination of PFAS in aqueous LiOH electrolytes. Each cycle in this embodiment consisted of a 30 second (“s”) interval with the electricity on (“EON”) = 1.6 VRHE, followed by a 1 s interval with the current reversed (“Erev”) = –1.0 VRHE, followed by a 5 min interval with the electricity off (“EOFF”) at open circuit potential (“OCP”). Increasing the number of cycles to 150 resulted in complete defluorination, although it required higher energy expenditure. Conversely, decreasing the number of cycles below 90 reduced the defluorination yield linearly. This suggests that C–F bond cleavage showed a first order dependence on the generated oxidant species (O•–).

[0175] The HFPO–concentration affected the defluorination efficiency, with lower HFPO–concentrations demonstrating better performance. Complete defluorination was achieved atPATENT concentrations ≤0.5 mM (Fig.9b). Conversely, defluorination yields were significantly lower at higher HFPO–concentrations, with 1.00 mM and 2.00 mM achieving only 69.0 ± 4.5 % and 39.0 ± 4.5 % defluorination, respectively. This suggests that either the adsorption of HFPO–on the anode or the concentration of generated oxidants was limiting for the cleavage of HFPO–C–F bonds. We have observed a similar concentration dependence of the defluorination of PFOS. This indicates that the chemical identity of the PFAS headgroup did not govern the PFAS adsorption at laser-made [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes, but that anode characteristics, i.e. available adsorbent sites and particularly the generation of oxidant species (O•–), are posing an upper limit to concentration for efficient PFAS defluorination. The equilibrium amount of adsorbed molecules increases with increasing solute concentration. See, e.g., Giles, et al., J. Colloid Interface Sci., 1974, 47:755- 765; Giles, et al., J. Colloid Interface Sci., 1974, 47:766-778; Radke and Prausnitz, AIChE J., 1972, 18:761-768. A higher number of adsorbed PFAS molecules contributes only to defluorination efficiency until all available adsorbent sites are saturated or all generated oxidant species are consumed within the anode microenvironment. The ability to defluorinate low PFAS concentrations is useful for practical applications. Nevertheless, the chemical identity of the PFAS headgroup and the PFAS structure affect the bond dissociation energy (BDE) of the PFAS C–F bonds, with the C–F BDE of GenX or PFOS ranging from 536.0 or 415.6 to 461.9 kJ mol–1, depending on the position of the C–F moieties within the molecules. See, Wilsey, et al., Chem. Comm., 2023, 59:11895-11922.

[0176] We engineered pulse train sequences for pulsed electrolysis, incorporating polarity reversal, to achieve complete defluorination of HFPO–through UV light-assisted electrocatalysis on [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes. Adding a short period of reversed polarity following the cleavage of the GenX C–F bonds at the anode enhanced the overall efficiency of HFPO–defluorination. To gain a deeper understanding of the process, applied potentials and time intervals at these potentials were systematically varied during pulsed electrolysis and quantified HFPO–defluorination. The results are shown in Figs.10A-D.

[0177] Complete defluorination of HFPO–required a reversed polarity potential Erev of at least –1.0 VRHE(Figure 10A). More negative potentials did not improve defluorination and lowered the energy efficiency, whereas an Erev of –0.36 VRHE (equal to OCP) and less negative Erev values led to inferior defluorination. Applied potentials that are less negative than OCP attract anions, including fluoride, produced by C–F bond cleavage. Studies fromPATENT the laboratory of the present inventors indicate that attraction of fluoride anions to the anode impedes release of the anions into the bulk electrolyte, fouling the anode and lowering the defluorination efficiency.

[0178] The improvement of using an Erev of –1.0 VRHE compared to Erev at OCP, which essentially eliminates the Erevpulse, was 3%. This improvement matters for deployment of PFAS defluorination technologies on a large scale. Pulse electrolysis can significantly optimize the electrode–electrolyte interface (see, Miličić, T., et al., Faraday Discuss., 2023, 246:179-197, hereafter, “Miličić 2023”), and improve the yield and selectivity of certain products compared to traditional steady-state electrolysis. (See, Liu, T., et al., J. Energy Comm., 2021, 59:69-82).

[0179] Defluorination required an applied potential of 1.6 VRHE to electrocatalytically regenerate the oxidant O•–within the anode microenvironment by alkaline water oxidation and deep UV light irradiation, to cleave the GenX C–F bonds. Using a 30 s EONpulse at OCP instead of 1.6 VRHE led to negligible defluorination (Fig.10B).

[0180] The least negative Erev value of –1.0 VRHE was chosen for energy conservation reasons. Variation of the time interval at Erevshowed a maximum at 1 s (Figure 10c). The purpose of the reversed polarity pulse was to repel fluoride ions, generated by C–F bond cleavage, from the exemplar [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anode. GenX has a pKa of 2.84, making it anionic in 9.0 M aqueous LiOH. Therefore, application of this reversed polarity pulse forms a tradeoff between fluoride repulsion and GenX adsorption at the anode. PFAS anions can adsorb at oxygenated carbon surfaces (see, e.g., Yu, H. et al., Sci. Total Environ.2023, 861, 160647), such as the hydrophilic carbon fiber paper of this work. The hydrophobic fluorinated alkyl chains of GenX confer an electronegative character, while the hydrophilic polar carboxylic head group enables formation of micelles. (Vakili, M., et al., Chem. Eng. J., 2021, 424, 130266. These micelles act as surfactants in solution, thereby enhancing the adhesion of HFPO–to carbon-based adsorbates. (Yu et al., supra). PFAS adsorption depends on the solution pH, with less sorption occurring on oxygenated carbon at higher pH values. (See, e.g., Riegel, M., et al., Environ Sci Europe, 2023, 35:12.) At reversed polarity pulse durations of less than 1 s, electrostatic fluoride repulsion appeared insufficient. Generated fluoride can block adsorption sites for HFPO–. In contrast, at longer pulse durations, the negative charges at the electrode impeded HFPO–adsorption by repelling the HFPO–anions.PATENT

[0181] The pulse train exhibited a time interval at OCP after the reversed polarity pulse, to re-establish equilibrium in the anode boundary layer. Defluorination increased as longer OCP time intervals were used (Fig.10d), indicating that at least 4 min were desirable for re- equilibration of the anode microenvironment.

[0182] Diffusion processes govern this re-equilibration in the stagnant electrolyte. The molecular diffusion flux is proportional to the temperature of the liquid. Therefore, HFPO–defluorination experiments was performed at elevated temperature. HFPO–defluorination yields at 70 ºC, as shown in Fig.11B, were (3 ± 2) % higher than those at room temperature, as shown in Fig.11A. Heating the electrolyte solution at industrial scales appears impractical but may be aided by utilization of waste heat. Complete HFPO–defluorination at room temperature required 9.0 M aqueous LiOH electrolyte, whereas complete HFPO–defluorination at 70 ºC was achieved in 8.0 M LiOH, balancing materials cost for LiOH with energy needs for heating the electrolyte.

[0183] The complete electrocatalytic defluorination of HFPO–was achieved in an advanced oxidation process, using UV-light-assisted pulsed electrolysis in aqueous LiOH electrolyte. Importantly, the systems and methods used do not need bisulfate or other auxiliary chemical agents that are consumed, ensuring scalability. Exemplar [Ni0.75Fe0.25]-(OH)2nanosheets on hydrophilic carbon fiber paper was used as anodes. The addition of a time interval at reversed polarity after C–F bond cleavage at anodic potential during pulsed electrolysis enhanced defluorination. The correlation between HFPO–defluorination and the systematic variation of applied potentials and time intervals during pulsed electrolysis highlights the importance of balancing the fluoride removal from the anode surface through electrostatic repulsion with facilitating HFPO–adsorption. Boundary layer re-equilibration by diffusion processes was equally as important. Applying an electrocatalysis pulse train sequence based on a mechanistic understanding of electrocatalytic PFAS defluorination in aqueous LiOH electrolytes enabled the complete defluorination of HFPO–. These findings underscore the effectiveness of pulsed electrocatalysis in breaking down persistent PFAS compounds, providing a scalable and eco-friendly solution for water treatment. This approach leverages nonprecious materials, making it a viable option for global PFAS remediation Example 7

[0184] This Example discusses the materials and methods used in the studies whose results are reported in Example 6.PATENT

[0185] All chemicals were used as received. Deionized water was obtained from a Thermo Scientific Barnstead Smart2Pure Pro UV / UF 15 LPH Water Purification System and had a resistivity of ≥17.5 MΩ • cm. All experiments were conducted at room temperature and under ambient air. Error bars represent the standard deviations from triplicate measurements. Data analysis and graphing were performed using Igor Pro 8.04 (Wavemetrics) unless otherwise noted. Catalyst Preparation

[0186] Hydrophilic carbon fiber paper preparation is described in Wilsey, M., et al., Adv. Mater. Interfaces, 2023, 10, 2201684. Briefly, the surfaces of the as-purchased carbon fiber paper (FuelCellStore, AvCarb MGL190, 78 % porosity) were oxygenated by sonication in 1.0 M aqueous sodium dodecyl sulfate solution, followed by electrooxidation in 0.1 M pH 8.7 aqueous KHCO3 electrolyte at 1.63 V vs. Ag / AgCl for 20 min. Electrodes had geometric dimensions of 3.0 cm (length) × 3.0 cm (width), resulting in a geometric electrode area Ageoof 9.0 cm2. Preparation of Solutions

[0187] Aqueous LiOH solutions with concentrations of 6.0, 7.0, 8.0, or 9.0 M were prepared by adding 14.4, 16.7, 19.1, or 21.5 g of lithium hydroxide monohydrate (Thermo Scientific, 98%), respectively, to a 100 mL volumetric flask, then filling to the 100 mL mark with deionized water and mixing until fully dissolved. The 0.05, 0.10, 0.25, 0.50, 1.00, or 2.00 mM HFPO–solutions were prepared by adding 0.69, 1.39, 3.47, 6.94, 13.89, or 27.77 mg undecafluoro-2-methyl-3-oxahexanoic acid (97%, Synquest Laboratories, Alachua, FL), respectively, into a 50 mL Falcon tube, followed by the addition of 40 mL of aqueous electrolyte. The HFPO–-containing solutions were ultrasonically agitated (PS-40A digital ultrasonic bath, 240 W, 10 L, Jeken Ultrasonic Cleaner Ltd, Dongguan City, Guangdong Province, China) for 5 min, then left undisturbed for 30 min to ensure proper mixing and dissolution. All chemicals were weighed using an A 120 S analytical balance (Sartorius Corp., Bohemia, NY) or an AT201 analytical balance (Mettler-Toledo, LLC, Columbus, OH). Physical Characterization

[0188] XRD data were collected using a Rigaku XtaLAB Synergy-S diffraction system (Rigaku Crp., Akishima, Tokyo, Japan) equipped with a HyPix-6000HE HPC detector atPATENT room temperature. A PhotonJet-S microfocus source generated CuKα radiation (λ = 1.54184 Å) at 50 kV, 1 mA. Two combination ω-φ “Gandolfi” scans were performed, each for 300 s: (1) ω from –62.00 to 31.00 degrees and φ rotated through 720 degrees, at θ = –42.127 and κ = 70.000 degrees; and (2) ω from –31.00 to 61.00 degrees and φ rotated through 720 degrees, at θ = 40.877 and κ = –70.00 degrees. The sample-to-detector distance was 34 mm. A light coating of viscous oil was used to secure the [Ni0.75Fe0.25]-(OH)2powder sample to a Nylon loop (0.1 mm ID).

[0189] SEM images of [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes were taken at UR-Nano, using a Zeiss Auriga scanning electron microscope, equipped with a Schottky field emission emitter, and operated at 20.00 kV with a working distance of 5.1 mm. EDX spectroscopy data were collected using a SEM-integrated EDAX Octane elect plus with silicon drift detector spectrometer. Double sided carbon tape was used to adhere the [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes to sample stubs. Electrocatalysis

[0190] Electrochemical investigations were conducted in ambient air at room temperature, using a standard Pyrex three-electrode, single-compartment 50 mL electrochemical cell. The cell was equipped with a Teflon lid positioned above the liquid surface and featured a flat quartz window for ultraviolet light illumination (254 nm lamp, 40 mW cm−2). The anodes consisted of laser-made [Ni0.75Fe0.25]-(OH)2 nanosheets immobilized on hydrophilic carbon fiber paper, with a catalyst mass loading of 444 µg cm–2geo. The cathodes were neat hydrophilic carbon fiber paper. The dry tab of each carbon fiber paper electrode was wrapped with ultrathin aluminum foil and inserted into an aluminum clip onto which the alligator clip of the potentiostat lead was clipped. A Pt wire pseudo-reference electrode, calibrated against a hydrogen reference electrode (Gaskatel HydroFlex®) in each electrolyte, served as reference electrode. Platinum wire has been shown to be a suitable and stable reference electrode in various electrochemical systems. (See, Kasem, K., and Jones, S., Platinum Metals Rev., 2008, 52:100-106. The anode was placed parallel to the cathode at a distance of 16 mm, secured by the Teflon lid, and centered in the Pyrex cell, perpendicular to the quartz window through which 40 mW cm–2254 nm light was supplied by a UV lamp. A BioLogic, 8-slot VSP3e potentiostat / galvanostat / EIS potentiostat was used. The data were not corrected for any uncompensated resistance losses. All potentials are reported vs the reversible hydrogen electrode (RHE). For the temperature dependent experiment, the cell, containing 40PATENT mL of electrolyte and covered with a Teflon lid, was placed on a multi-position hot plate stirrer (BT Labsystems, BT1016) and heated to 70 ºC. The electrolysis was started after heating for 1 h after which the temperature was stable. Quantification of Fluoride

[0191] The details of determining the fluoride concentration in the electrolyte after electrocatalysis are described briefly. In brief, a fluoride ion selective electrode (Thermo Scientific Orion, 9609BNWP) and a total ionic strength adjustment buffer (TISAB II) solution were used to quantify fluoride ions in aqueous solution. To prevent interference from hydroxide ions in fluoride ion selective electrode measurements, potassium acetate buffer was used to neutralize the hydroxide ions in strongly alkaline electrolytes. Fluoride concentrations in all aqueous electrolytes after electrocatalytic HFPO–defluorination were determined by adding 1 mL of solution to 9 mL of potassium acetate buffer in a 50 mL Falcon tube, followed by the addition of 10 mL of TISAB II solution. Example 8

[0192] This Example discusses the mechanistic role of LiOH in efficient, aqueous electrocatalytic PFOS defluorination.

[0193] Studies reported below demonstrate that synergistic effects of high lithium and high hydroxide ion concentrations for complete PFOS defluorination. Two-dimensional NMR data of electrolytes post pulsed electrolysis provide experimental evidence for Li–F ion pairing, which plays a crucial role in preventing anode fouling by produced fluoride, thus enabling sustained C–F bond cleavage. This Li–F ion pairing was increased at high pH, and elevated temperatures enhanced diffusion of Li–F ion pairs into the bulk electrolyte. High hydroxide ion concentrations additionally removed fluoride from the anode surface by competitive adsorption, corroborated by XPS data. Our findings provide quantitative mechanistic insights into the electrocatalytic defluorination process and offer a general route of enhancing the efficiency of anodic PFAS defluorination.

[0194] Advanced oxidation of PFAS in aqueous electrolyte requires reactive oxygen species (H2O2, HOO•–,•OH, O•–, O2•–), which can be produced directly at the anode or via deep UV- assisted decomposition of H2O2(or HOO–at high pH). Water oxidation yields O2, H2O2, or•OH; (see, e.g., Siahrostami, S., et al., J. Phys. Chem. Lett.2017, 8(6):1157-1160). H2O2 oxidation produces O2•–(see, Petlicki, J., and van de Ven, T., J. Chem. Soc., FaradayPATENT Trans.,1998, 94(18):2763-2767); and O2can be reduced to H2O2,•OH, HOO•–, or O2•–(or their deprotonated forms at high pH) (see, Kulkarni, A., et al., Chem. Rev., 2018, 118(5):2302-2312, DOI: 10.1021 / acs.chemrev.7b00488), which can oxidize PFAS. The cathodic aqueous reduction of dioxygen that stems from ambient air or anodic water oxidation can produce reactive oxygen species (see, Lu, Z., et al., Nat. Catal., 2018, 1 (2), 156-162. DOI: 10.1038 / s41929-017-0017-x) that are capable of C–F-bond cleavage, but the generation of cathodically produced reactive oxygen species through oxygen reduction is inherently limited by the solubility of oxygen in the electrolyte at ambient conditions and the oxygen reduction selectivity for hydrogen peroxide. (See, Siahrostami, S.. Chem Catal.2023, 3(3):100568, DOI: 10.1016 / j.checat.2023.100568. At high applied potentials, i.e. high overpotentials, at which operation is desirable because of enhanced oxygen reduction kinetics, the 4-electron–4-proton oxygen reduction to water outcompetes the desired 2- electron–2-proton pathway to hydrogen peroxide in aqueous media, limiting the amount of reactive oxygen species that can be generated cathodically.

[0195] The studies set forth above demonstrate the complete electrocatalytic defluorination of perfluorooctane sulfonate (PFOS) in 8.0 M LiOH, effective at PFOS concentrations as low as 27 ppm, using surfactant-free, laser-synthesized [NiFe]-(OH)2 nanosheets on hydrophilic carbon fiber paper anodes as anodes, to provide high surface area and avoid mechanistic complications by surfactants. Deep ultraviolet (UV) light-assisted defluorination was most efficient at 1.6 V vs RHE and occurred within the anode microenvironment, evident from pulsed electrolysis data, electrolyte agitation experiments, and surface composition data. X- ray photoelectron spectroscopy (XPS) data showed adsorbed C–F2, C–F3, sulfonate, O2, and HOO–, which forms the C–F bond cleaving oxidant O•–with deep UV light. High concentrations of lithium and hydroxide ions were essential for effective PFOS defluorination. The method set forth above employ only non-precious materials in an aqueous LiOH electrolyte, making it nearly one hundred times cheaper and more energy-efficient than using BDD electrodes.

[0196] Without wishing to be bound by theory, this Example sets forth mechanistic reasons why the high Li+and high OH–concentrations taught in this disclosue succeed in efficient anodic aqueous electrocatalytic PFOS defluorination. The defluorination process produces fluoride anions that cover the positively charged anode and deactivate it if these fluoride anions are not removed from the anode surface. Experimental evidence by two-dimensional7Li–19F-HOESY-NMR data shows that efficient Li–F ion pair formation occurred in aqueousPATENT LiOH electrolyte with high Li+and high OH–concentrations, ergo in conditions at which PFOS defluorination was efficient. Leveraging pulsed electrolysis, effective removal of generated fluoride anions from the anode surface was facilitated by Li–F ion pair diffusion, corroborated by temperature dependence data that showed more defluorination at elevated temperature. Additionally, XPS data confirmed that high OH–concentrations enabled interfacial F–removal by competitive adsorption. The mechanistic insights gained from these results enable scalable processes for efficient, cost-effective, and widely applicable technologies for PFAS destruction. Example 9

[0197] This Example sets forth and discusses the results of XPS studies of efficient, aqueous electrocatalytic PFOS defluorination in LiOH electrolyte.

[0198] The anodes used for PFOS defluorination in the studies discussed in Examples 8-10 consisted of laser-made [NiFe]-(OH)2 nanosheets on hydrophilic carbon fiber paper. Pulsed laser in liquids synthesis was used to prepare surfactant-free [NiFe]-(OH)2water oxidation nanocatalysts, following the protocol set forth in Hunter, B., et al., J. Am. Chem. Soc.2014, 136(38):13118-13121. DOI: 10.1021 / ja506087h, and Hunter, B., et al., Energy Environ. Sci. 2016, 9(5):1734-1743. DOI: 10.1039 / C6EE00377J. The laser method does not require surfactants for size control of resulting nanoparticles, offering precise control of surface chemistries of nanocatalysts. The produced material displayed layered double hydroxide structure and nanosheet morphology, evident from X-ray diffraction (XRD) and scanning electron microscopy (SEM) imaging data. Broadening was observed for all XRD peaks, attributable to the small crystallite size and the presence of stacking faults, including turbostratic disorder in the hydrotalcite-like structure, as previously reported for laser- synthesized [NiFe]-(OH)2 nanosheets. We used hydrophilic carbon fiber paper as electrode support to provide electrical contact to [NiFe]-(OH)2nanosheets. The high internal surface area of 468 cm2per geometric cm2of carbon fiber paper enhanced PFOS defluorination, compared to a flat electrode support. Carbon fiber paper was rendered hydrophilic by oxygenation, using the rapid, green chemistry process discussed in Wilsey 2023 A, supra. Briefly, the acid-free, transition-metal-free process involved sonication of carbon fiber paper in aqueous sodium dodecyl sulfate solution, followed by electrooxidation in aqueous KHCO3electrolyte. Laser-made [NiFe]-(OH)2 nanocatalysts were electrostatically integrated with hydrophilic carbon fiber paper, resulting in an even dispersion of nanocatalysts throughoutPATENT the three-dimensional structure of the carbon fibers, which effectively leveraged the high internal surface area of carbon fiber paper. Analysis by energy-dispersive X-ray spectroscopy (EDX) showed that the laser-made [NiFe]-(OH)2 nanosheets exhibited a Ni:Fe ratio of 3:1. EDX spectra of laser-made [Ni0.75Fe0.25]-(OH)2nanosheets on hydrophilic carbon fiber paper showed the presence of C, O, Fe, and Ni; no other elements were observed.

[0199] XPS data revealed the presence of the elements carbon, oxygen, nickel, and iron at anode surfaces pre- and post- electrocatalysis, as expected for these integrated anodes. Pre- and post- electrocatalysis high-resolution C 1s spectra exhibited six peaks, an asymmetrical shape, and shake-up characteristics, consistent with reported graphitic carbon XPS data. The central binding energy of graphitic carbon ranged from 284.5 to 285.0 eV, consistent with reported values. Additionally, adventitious carbon was present, whose central binding energy was taken to be 284.8 eV. The remaining three peaks were attributable to carbon oxygenates, with C 1s binding energy ranges of 286.4 to 287.0 eV (C–O; hydroxyls, esters, and ethers), 287.4 to 288.0 eV (C=O; aldehydes, carbonyls, and ketones), and 288.7 to 289.2 eV (O– C=O; carboxyls and esters). Pre- and post- electrocatalysis O 1s data displayed two peaks with central binding energies of 531.6 to 532.3 eV, attributable to C=O functional groups, such as in carboxyls, aldehydes, carbonyls, esters, and ketones, and 533.0 to 533.7 eV, attributable to C–O species, such as in carboxyls, hydroxyls, and ethers, as observed previously. The carbon oxygenate C 1s peak fits were constrained to the observed carbon oxygenate O 1s atom percentages, using element-specific relative sensitivity factors resulting from photoemission cross-sections and analyzer transmission of photoelectrons. Additionally, an O 1s peak at a central binding energy of 530.6 eV was present, attributable to Ni(OH)2of the nanocatalyst. The corresponding Ni 2p core level region showed two Ni 2p3 / 2 and Ni 2p1 / 2 spin-orbit-split peaks with respective satellite peaks at central binding energies of 855.3 and 872.8 eV, respectively, indicative to Ni(OH)2. The Fe 2p core level regions exhibited the two Fe 2p3 / 2 and Fe 2p1 / 2 spin-orbit-split peaks at central binding energies of 711.8 and 723.1 eV, consistent with Fe(OH)2.

[0200] XPS data after electrocatalytic defluorination of 0.5 mM PFOS in 8.0 M LiOH exhibited additional peaks in the C 1s, O 1s, F 1s, and S 2p core level regions, attributable to adsorbed C–F2and C–F3moieties and adsorbed sulfonate, stemming from PFOS, as well as adsorbed deprotonated hydrogen peroxide. Atom percentages of fluorine and sulfur species were determined relative to total surface carbon by quantifying the signals in the F 1s, and S 2p core level regions, respectively. The strong F 1s peak had a central binding energy ofPATENT 689.4 eV, consistent with the reported range of C–F bonds in PFOS (688 − 689 eV), and an area that corresponded to 1.2 at% fluorine. The weak S 2p signal corresponded to 0.2 at% sulfur and showed two spin-orbit-split peaks with central binding energies of 168.8 and 170.3 eV, assignable to the S 2p3 / 2and S 2p1 / 2components of sulfonate S=O. The corresponding C 1s peaks of C–F3 and C–F2 species and O 1s peak of the sulfonate moiety appear in the same binding energy region as the stronger oxygenated carbon peaks. Nevertheless, we included these corresponding C–F3, C–F2, and sulfonate peaks to the overall C 1s and O 1s signal fits at the reported binding energies and with peak areas constrained to the atom percentages deduced from fitting the signals in the F 1s, and S 2p core level regions. The C 1s signals of the C–F2and C–F3moieties have reported binding energies of 292.2 and 294.5 eV, respectively. The O 1s peak of the PFOS sulfonate S=O moieties appears at a central binding energy of 532.2 eV. The deprotonated hydrogen peroxide, observed in the O 1s core level region as a peak with a central binding energy of 539.5 eV, originated from the two-proton– two-electron alkaline water oxidation to HOO–, which is energetically accessible at the conditions used here. This deprotonated hydrogen peroxide, together with deep UV light irradiation, generated the oxidant O•–, which defluorinated PFOS within the anode microenvironment. After complete PFOS defluorination in 8.0 M LiOH, no Li 1s signal was detectable. Example 10

[0201] This Example sets forth and discusses studies of the dependence of the identity and concentration of ions on PFOS defluorination.

[0202] Pulsed electrocatalysis experiments were conducted with exemplar [Ni0.75Fe0.25]- (OH)2–hydrophilic carbon fiber paper anodes and 0.5 mM PFOS in stagnant aqueous electrolyte at 1.6 V vs RHE, with deep UV light irradiation. Pulsed electrolysis consisted of 60 cycles, in which one cycle was comprised of a 1 min interval at +1.6 V vs RHE, followed by a 5 min interval at open circuit potential. We used aqueous 6.0 M [LiOH]x–[LiClO4](1–x) or 6.0 or 8.0 M [LiOH]x–[NaOH](1–x)(x = 1, 0.25, 0.5, 0.75, 0) electrolytes, to disentangle the effect of high Li+ and high OH– concentrations in the electrolyte on PFOS defluorination. The [LiOH]x–[LiClO4](1–x)electrolytes kept the Li+ concentration constant, while varying the OH–concentration, and spanned a pH range of 7.8 (pure LiClO4) to 14.8 (pure LiOH). The solubility limit of LiClO4 in water at standard conditions dictated use of 6.0 M solutions. The [LiOH]x–[NaOH](1–x)electrolytes kept the OH–concentration constant, while varying the Li+PATENT concentration, and had pH values of 14.8 (6.0 M [LiOH]x–[NaOH](1–x)) or 14.9 (8.0 M [LiOH]x–[NaOH](1–x)). We quantified fluoride concentrations using a fluoride ion selective electrode (DeMarco, R., et al., Electroanalysis, 2007, 19(19-20):1987-2001; DOI: 10.1002 / elan.200703916) combined with a total ionic strength adjustment buffer (“TISAB”) that sufficiently acidified the solution to prevent hydroxide ion interference, in accordance with the recommendations of the manufacturer of the fluoride ion selective electrode. This solution acidification by the TISAB additionally cleaved any (Li or Na)–F ion pairs that were stable in strongly alkaline electrolytes, to release all electrocatalytically generated fluoride ions into the analyte.

[0203] When the Li+concentration was varied at constant OH–concentration, it was found that the PFOS defluorination was most efficient at highest Li+concentration in the 6.0 or 8.0 M aqueous [LiOH]x–[NaOH](1–x) (x = 1, 0.25, 0.5, 0.75, 0) electrolytes. The defluorination exhibited a linear dependence on the Li+concentration (Figs.12A, B). Defluorination was higher in the 8.0 M than 6.0 M aqueous [LiOH]x–[NaOH](1–x)electrolyte. For varying the OH–concentration at constant Li+concentration, we found that the PFOS defluorination was most efficient at highest OH–concentration in the 6.0 M aqueous [LiOH]x–[LiClO4](1–x)(x = 1, 0.25, 0.5, 0.75, 0) electrolytes (Fig.12C).

[0204] Rate analysis of log–log plots of the data showed a first order dependence of fluoride production on the OH–concentration (slope: 1.14 ± 0.11), and less than first order dependence of obtained fluoride concentration on the Li+concentration. Interestingly, the slope of F–vs Li+concentration data was larger in 8.0 M than in 6.0 M aqueous [LiOH]x– [NaOH](1–x)electrolyte (0.82 ± 0.04 vs 0.67 ± 0.06), suggesting a cooperative effect of OH–and Li+ions. Example 11

[0205] This Example sets forth and discusses evidence for Li-F ion pairing.

[0206] We collected two-dimensional (7Li or 23Na)–19F-HOESY-NMR40 data of post electrocatalysis bulk electrolytes to quantify (Li or Na)–F ion pairing as a function of the Li+ or OH– ion concentration. Correlation of these 2D-NMR data to the observed PFOS defluorination allowed us to gain a quantitative mechanistic understanding of the role of Li–F pairing in PFOS defluorination efficiency.PATENT

[0207] In [LiOH]x–[NaOH](1–x)(x = 1, 0.25, 0.5, 0.75, 0) electrolytes with x ≥ 0.25, we observed7Li–19F cross peaks in7Li–19F-HOESY-NMR data (Fig.13B), providing evidence for Li–F ion pairing in electrolytes with 8.0 M OH–and at least 2.0 M Li+. Integration of the volumes of observed cross peaks allowed us to quantify Li–F ion pairing as a function of the chemical identity and concentration of ions in the electrolyte (Figs.13A and C). In [LiOH]x–[NaOH](1–x)(x = 1, 0.25, 0.5, 0.75, 0) electrolytes, increasing the Li+concentration correlated with increasing signal strength of the7Li–19F cross peak in a sigmoidal fashion (Fig.13A). At a constant OH–concentration of 8.0 M and Li+concentrations increasing from 2.0 to 6.0 M, we observed a 1.3 x increase of the signal strength of the7Li–19F cross peak. In contrast, increasing the Li+concentration from 6.0 to 8.0 M enhanced the signal strength of the7Li–19F cross peak by 2.1 x, corroborating the cooperative effect of OH–and Li+ions observed in the PFOS defluorination data (Fig.12). Comparable enthalpies of hydration for Li+and F–of –520 and –505 kJ mol–1, respectively, facilitate the formation of Li–F ion pairs in aqueous electrolytes containing Li+. (See, e.g., van der Vegt, N., et al., Chem. Rev.2016, 116(13):7626-7641; DOI: 10.1021 / acs.chemrev.5b00742. An increase in ion pairing enthalpies has been reported at elevated pH levels (e.g., Jin, E. et al., J. Mol. Biol.2000, 298(1): 95-110; DOI: 10.1006 / jmbi.2000.3639), aligning with our observation of a synergistic impact of OH–and Li+ions on PFOS defluorination. In aqueous LiOH electrolytes with concentrations that increased from 6.0 to 8.0 M, we observed a linear increase of the signal strength of the7Li–19F cross peak and a linear increase in PFOS defluorination from (69 ± 6.7) to (99 ± 6.7) % (Figs.13C, D). The7Li–19F cross peak showed 1.7 x more signal at 8.0 M than at 7.0 M LiOH (Fig.13C). In contrast, in aqueous 8.0 M NaOH, we did not observe a cross peak in23Na–19F-HOESY-NMR data, because Li–F ion pairing is stronger than Na–F ion pairing. (See, e.g., Marcus, Y. and Hefter, G., Chem. Rev. 2006, 106(11):4585-4621; DOI: 10.1021 / cr040087x.

[0208] We did not observe7Li–19F cross peaks in 2D-NMR data in either 6.0 M LiOH or LiClO4electrolyte, but defluorination was 22.3 x higher in 6.0 M LiOH than in 6.0 M LiClO4electrolyte. This suggests that a different mechanism than Li–F ion pairing contributed to efficient defluorination at high OH–concentration, namely competitive adsorption of OH–ions at the [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anode. Competition between aqueous OH−and F−ions for adsorbent sites has been reported on uncharged oxygenated graphitic carbon, with negligible F–adsorption at high pH. (See, Daifullah, A., et al., J. Hazard. Mater.2007, 147(1):633-643; DOI: https: / / doi.org / 10.1016 / j.jhazmat.2007.01.062).PATENT At 6.0 M OH–concentration, this competitive adsorption of OH–ions at the anode apparently inhibited adsorption of generated F–at the anode surface, thus limiting electrode fouling as PFOS defluorination proceeded. In contrast, the OH–concentration in 6.0 M pH 7.8 LiClO4 electrolyte was 6.3x10–7M, allowing detrimental anodic F–adsorption. Halides inhibit perchlorate adsorption on activated carbon. As a result, ClO4- ions are unable to completely displace the F–ions from the anode surface, aided by the larger anion volume of ClO4- of 0.082 nm3, compared to that of F–of 0.025 nm3.

[0209] XPS data of anodes after PFOS defluorination electrocatalysis in 6.0 M LiClO4 electrolyte provide evidence for the presence of adsorbed F–ions, whereas fluoride was absent at anodes after electrocatalysis in 6.0 M LiOH electrolyte. This supports our mechanistic proposal that competitive adsorption of OH–ions at the the [Ni0.75Fe0.25]-(OH)2– hydrophilic carbon fiber paper anode at high pH assisted PFOS defluorination by removal of produced fluoride from the anode. After PFOS defluorination electrocatalysis in 6.0 M LiClO4electrolyte, the F 1s core level region displayed a peak with a central binding energy of 686.1 eV, attributable to adsorbed fluoride, (see, e.g., Yamamoto, H., et al., Electrochemistry, 2021, 89(2):118-120; DOI: 10.5796 / electrochemistry.20-65148) in addition to the strong peak with a central binding energy of 689.4 eV, assignable to C–F bonds of PFOS. In contrast, we did not detect adsorbed fluoride in F 1s XPS data after PFOS defluorination electrocatalysis in 6.0 or 8.0 M LiOH electrolyte. The peak assigned to adsorbed fluoride amounted to 0.07 at% F–relative to total surface carbon. We included a corresponding adsorbed fluoride peak into the overall C 1s peak fitting of XPS data post electrocatalysis in 6.0 M LiClO4electrolyte, with an area constrained to this atom percentage and a central binding energy of 288.4 eV50. Additionally, after PFOS defluorination electrocatalysis in 6.0 M LiClO4 electrolyte, two spin-orbit-split peaks were observed in the Cl 2p core level region, with central binding energies of 208.7 and 210.4 eV, attributable to the Cl 2p3 / 2 and Cl 2p1 / 2 components of adsorbed perchlorate. We included the presence of ClO4–into the overall fit of the O 1s core level region as a peak with a central binding energy of 533.6 eV.

[0210] PFOS defluorination at [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes was (69 ± 6.7) or (3.1 ± 6.7) % efficient in 6.0 M LiOH or LiClO4electrolyte, respectively. In both cases, a Li 1s XPS signal was detectable. The Li 1s peak had a central binding energy of 56.9 eV, assignable to Li+. Peak area quantification revealed that 2.5 or 16.4 at% Li+relative to total surface carbon was present after PFOS defluorination in 6.0 M LiOH or LiClO4PATENT electrolytes, respectively. After complete PFOS defluorination in 8.0 M LiOH electrolyte, an analog Li 1s signal was absent. This suggests that high bulk electrolyte OH–concentrations effectively kept Li+and F–ions from the anode surface, likely by specific adsorption of OH–ions at the anode. While much is known about lithium ion intercalation into graphite in the context of lithium ion batteries with non-aqueous electrolytes, reports for aqueous systems are scarce and limited to halogen intercalation. Aqueous fluoride can intercalate into graphite. Here, fluoride, together with surface adsorbed perchlorate, as observed after PFOS defluorination in 6.0 M aqueous LiClO4electrolyte, apparently attracted Li+ to the anode surface, possibly for charge balance reasons. Example 12

[0211] This Example discusses PFOS defluorination data from studies at temperatures elevated above room temperature.

[0212] The studies discussed in Example 11 were conducted at room-temperature. Studies on PFOS defluorination were also conducted in 8.0 M aqueous LiOH electrolyte at three elevated temperatures, to elucidate the role of Li–F ion pair diffusion on the defluorination efficiency. For these temperature dependence experiments, we used only 10 instead of 60 pulsed electrolysis cycles, because we did not want to completely defluorinate PFOS at room temperature. We found that the PFOS defluorination efficiency increased linearly as a function of electrolyte temperature between 25 and 70˚C (Fig.14). We chose this temperature range such that the highest temperature was significantly below the boiling point of the electrolyte at standard conditions, which we measured to be (108 ± 1) ˚C, to prevent bubble formation. Bubbles would have given rise to electrolyte agitation, which would have added convection as a mass transport mechanism, thereby decreasing the thickness of the electrochemical double layer and complicating the overall analysis of the electrocatalytic process. We found in prior work that a thick electrochemical double layer, achieved by leaving the electrolyte stagnant, is imortant for efficient PFOS defluorination in 8.0 M aqueous LiOH electrolyte. Example 13

[0213] This Example discusses a mechanistic understanding of electrocatalytic aqueous LiOH- promoted defluorination of PFOS.PATENT

[0214] The data developed in the studies reported above provide a quantitative mechanistic understanding of why high Li+and high OH–concentrations are key for complete PFOS defluorination by electrocatalysis at laser-made [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes in aqueous LiOH electrolyte. PFOS is anionic at the pH values used here (≥7.8) because the pKa value of PFOS is –3.27. PFOS defluorination occurs in the anode microenvironment through an advanced oxidation process by the oxidant O•–that was electrocatalytically regenerated within the anode microenvironment by alkaline water oxidation and deep UV light irradiation. The cleavage of PFOS C–F bonds releases F–ions into the anode microenvironment, and those F–ions can specifically adsorb at the anode, thereby blocking surface sites that are needed as adsorbent sites for the adsorption of unreacted PFOS anions and active sites for water oxidation electrocatalysis. This specific adsorption of produced F–ions results in electrode fouling, which fundamentally limits PFOS defluorination. Yet, we observed complete PFOS defluorination by quantification of F–in the bulk electrolyte, suggesting that an effective mechanism existed for F–desorption from the anode surface and F–diffusion from the electrochemical double layer into the bulk electrolyte. The F–ion desorption and diffusion was efficient even though we kept the electrolyte stagnant, because a thick electrochemical double layer is beneficial for complete PFOS defluorination. Our results show that this F–removal from the anode occurred by Li–F ion pairing at high pH (Fig.13), combined with competitive anodic adsorption of OH–ions, and diffusion of these Li–F ion pairs into the bulk electrolyte (Fig.14), which prevented the re-adsorption the negatively charged F–ions at the positively charged anode. Leveraging pulsed electrolysis enabled F–removal from the anode during the time intervals at open circuit potential and the adsorption of unreacted PFOS anions during the next anodic potential interval. This way, C–F bond cleavage was sustained, without electrode fouling by produced fluoride (Fig.15).

[0215] The schematic in Fig.15 illustrates the complex electrode processes during PFOS defluorination in aqueous LiOH electrolyte. Alkaline water oxidation at laser-made [Ni0.75Fe0.25]-(OH)2–hydrophilic carbon fiber paper anodes, assisted by deep UV light irradiation, enables the production of O•–oxidants,3 which cleave PFOS C–F bonds, releasing fluoride. These F–ions compete with OH–ions for specific adsorption sites at the anode surface. This layer of specifically adsorbed anions attracts lithium cations from the electrolyte. Similar hydration enthalpies of Li+and F–enable Li–F ion pairing in Li+- containing aqueous electrolytes. Enhancement of ion pairing enthalpies occurs at higher pH,PATENT consistent with our finding of a cooperative effect of OH−and Li+ions in PFOS defluorination (Fig.12A-C). Additionally, the high pH value, and concomitant high OH–ion concentration, facilitates F–desorption from the anode, which has been attributed to the weakened electrostatic forces between the sorbent surface and F–at high pH. (See, e.g., Habuda-Stanić, M., et al., Materials, 2014, 7(9):6317-6366). Our observation of adsorbed fluoride in XPS data after PFOS defluorination only in 6.0 M LiClO4but not in 6.0 or 8.0 M LiOH corroborates that the high OH–ion concentrations of the LiOH electrolytes effectively displace fluoride from the anode–electrolyte interface. The Li–F ion pairing creates charge- neutral fluoride species, enabling fluoride desorption from the positively charged anode surface, and the Li–F ion pairs diffuse into the bulk electrolyte, effectively preventing anode fouling by produced F–ions. Additionally, pulsed electrolysis facilitated the Li–F ion pairing because switching the electrode from the anodic potential to open circuit potential removed the positive charges at the anode interface with the electrolyte, eliminating the electric-field- dependent electrostatic attraction of F–anions to the electrode, thus weakening the affinity between fluoride and the electrode, which in turn enables Li–F ion pairing.

[0216] In pulsed electrolysis (see, Miličić, T., et al., Faraday Discuss., 2023, 246:179-197), a constant potential (here 1.6 V vs RHE) is applied briefly (sec–min; ON time), followed by an interval at open circuit potential for minutes (OFF time), to allow the electrochemical double layer to re-establish equilibrium (see, Obasanjo, C., et al., Energy Fuels, 2023, 37(18):13601-13623; DOI: 10.1021 / acs.energyfuels.3c02152), this ON–OFF time cycle is repeated several times. Pulsed electrolysis (1 min ON time, 5 min OFF time) was important for efficient defluorination of PFOS in aqueous LiOH. The electrolyte composition governs the structure of the electrochemical double layer; hydrated Li+has a larger Stokes radius than hydrated Na+. (See, Waegele, M., et al., J. Chem. Phys.2019, 151(16):160902. DOI: 10.1063 / 1.5124878). Smaller hydrated cations enhance accumulation of cations in the outer Helmholtz plane of the electrochemical double layer, increasing the interfacial electric field, but the hydration enthalpies of Li+, Na+, and F–energetically favor Li–F over Na–F ion pairing.

[0217] Our observation that the PFOS defluorination efficiency increased linearly as a function of electrolyte temperature (Fig.14) corroborates that diffusion, particularly that of Li–F ion pairs, played a major role in the defluorination mechanism by transporting produced fluoride away from the anode surface. The molecular diffusion flux is proportional to the temperature of the liquid. Without any agitation or stirring, as was the case here, diffusion isPATENT the dominant mass transport process for charge-neutral species, such as the Li–F ion pairs of this work, both at anodic potential and at open circuit potential. In contrast, mass transport of ions occurs by diffusion only at open circuit potential, whereas a combination of diffusion and drift operate at applied potential. (See, e.g., Bard, A., and Faulkner, L., ELECTROCHEMICAL METHODS: FUNDAMENTALS AND APPLICATIONS; Wiley, 1980). Example 13

[0218] This Example discusses the conclusions from the studies set forth in Examples 8-12.

[0219] We prepared laser-made [Ni0.75Fe0.25]-(OH)2nanosheets immobilized on hydrophilic carbon fiber paper as anodes for aqueous PFOS defluorination. Complete photo-assisted electrocatalytic PFOS defluorination was achieved in aqueous 8.0 M LiOH electrolyte. Systematic variation of the Li+concentration at constant OH–concentration, together with variation of the OH–concentration at constant Li+concentration, allowed us to disentangle the roles of high Li+and high OH–concentrations on PFOS defluorination. We found that synergistic effects of high Li+and high OH–concentrations are essential for efficient PFOS defluorination.

[0220] Mechanistically, PFOS defluorination occurred in the anode microenvironment through an advanced oxidation process by the oxidant O•–that was electrocatalytically regenerated within the anode microenvironment by alkaline water oxidation and deep UV light irradiation. This process cleaves the C–F bonds of PFOS, releasing fluoride into the anode microenvironment. The produced F–ions can specifically adsorb at the anode and foul it, preventing subsequent adsorption of unreacted PFOS anions and blocking active sites for water oxidation electrocatalysis. Our results establish the mechanisms by which these generated F–ions are removed from the anode surface.

[0221] Two-dimensional7Li–19F-HOESY-NMR data demonstrate that efficient Li–F ion pair formation occurred in aqueous LiOH electrolyte with high Li+and high OH–concentrations, ergo in conditions at which PFOS defluorination was complete. Cooperative effects of Li+ and OH–ions enhanced Li–F ion pairing. High Li+ concentrations increased Li–F ion pairing. At high OH–concentrations, Li–F ion pairing was further increased, supporting our observation of a synergistic impact of OH–and Li+ions on PFOS defluorination. PFOS defluorination increased linearly as a function of temperature, indicating that diffusion played a role in PFOS defluorination. Pulsed electrolysis facilitated Li–F ion pair diffusion awayPATENT from the anode, by switching the electrode from the anodic potential to open circuit potential to disrupt the electrostatic attraction between F–anions and the electrode, enabling Li–F ion pairing in the anode microenvironment. Simultaneously, at high OH–concentrations, and thus high pH, OH–ions competitively adsorbed at the anode and displaced fluoride from the anode surface, evident from enhanced PFOS defluorination in neat aqueous LiOH, compared to mixed LiClO4and LiOH electrolytes. XPS data confirmed the inability of perchlorate ions to completely displace fluoride ions at the anode interface, verified by the detection of adsorbed fluoride in XPS data after electrocatalysis in neat (pH 7.8) 6.0 M LiClO4electrolyte, whereas fluoride was absent after electrocatalysis in high pH electrolytes.

[0222] Overall, our findings establish the mechanisms underlying the superior performance of aqueous LiOH electrolyte in PFOS defluorination, highlighting the complex interplay of Li+, F–, and OH–ions, and providing more efficient and sustainable PFAS defluorination technologies. Example 15

[0223] This Example sets forth the materials and methods used in the studies reported in Examples 8-14, above.

[0224] All chemicals were used as received. Deionized water with a resistivity of ≥17.5 MΩ· cm was sourced from a Thermo Scientific Barnstead Smart2Pure Pro UV / UF 15 LPH WaterPurification System. Experiments were conducted at room temperature and in ambient air. Glassware was cleaned with aqua regia, thoroughly rinsed, and dried before use. Error bars represent standard deviations of triplicate measurements. Data analysis and graphing were carried out using Igor Pro 8.04 (Wavemetrics), unless specified otherwise. Anode Preparation

[0225] Carbon fiber paper was made hydrophilic. Briefly, the process modified the surfaces of carbon fiber paper, purchased from FuelCellStore (AvCarb MGL190, with 78% porosity), via sonication in a 1.0 M aqueous solution of sodium dodecyl sulfate, followed by electrooxidation in a 0.1 M aqueous KHCO3electrolyte at pH 8.7 and +1.63 V vs. Ag / AgCl for a duration of 20 min. The electrodes used had dimensions of 3.0 cm (length) by 3.0 cm (width), resulting in a geometric electrode area of 9.0 cm2and a carbon surface area of 4200 cm2.PATENT

[0226] Laser-made [NiFe]-(OH)2nanosheets immobilized on the hydrophilic carbon fiber paper were used as anodes. Laser-made [NiFe]-(OH)2nanosheets were obtained by using 90 mJ, 8 ns pulses at 355 nm, produced by the third harmonic of a 10 Hz Q-switched Nd:YAG laser (Spectra-Physics Quanta-Ray LAB-190), to irradiate a suspension of iron powder (Alfa, −200 mesh, 99+%) in a 10 mL solution of 3.0 M nickel nitrate (Alfa, 98%) in water in a 30 mL glass beaker at room temperature in ambient air for a duration of 60 min. After the laser synthesis, any unreacted iron powder was separated from the aqueous suspension of produced [NiFe]-(OH)2nanoparticles using a strong magnet. The solid nanoparticulate powder was isolated through centrifugation, washed first five times with water, and then twice with acetone (VWR). The nanopowder was then dried under house vacuum. Following this, suspensions of the nanoparticulate powder at a concentration of 2 mg mL–1in water were vortexed, drop-cast onto hydrophilic carbon fiber supporting electrodes, and dried under a heat lamp at 60°C in ambient air. Electrocatalysis

[0227] Electrochemical tests were conducted in ambient air at room temperature using a standard Pyrex three-electrode, 50 mL electrochemical cell with a Teflon lid, which remained above the liquid surface, and a flat quartz window for UV light exposure (254 nm, 40 mW cm–2). The anodes were laser-made [NiFe]-(OH)2 nanosheets immobilized on hydrophilic carbon fiber paper (444 µg cm–2geo catalyst mass loading), and the cathodes were neat hydrophilic carbon fiber paper. A Pt wire pseudo-reference electrode, calibrated in each electrolyte against a hydrogen reference electrode (Gaskatel HydroFlex®), was used. Platinum wire has been shown to be a suitable and stable reference electrode in various electrochemical systems. The anode and cathode were positioned 16 mm apart and centered in the cell, with electrical connections made using aluminum clips and a potentiostat. Data are reported relative to the reversible hydrogen electrode (RHE), without correction for uncompensated resistance losses. Chronoamperometry data were collected at an anodic applied potential of 1.6 V vs RHE. In pulsed electrolysis, one cycle denotes one ON time of 1 min at an applied potential of 1.6 V vs RHE followed by one OFF time of 5 min at open circuit potential. The cell was filled with 40 mL electrolyte.

[0228] Aqueous electrolytes were prepared by adding the mass m (Table 2) of lithium hydroxide monohydrate (Thermo Scientific, 98%), sodium hydroxide (Fisher Thermo, 98.9%), or lithium perchlorate anhydrous (Thermo Scientific, 95%) to a 100 mL volumetricPATENT flask, filling the volumetric flask to its mark with water, and mixing liquid well until all solid was dissolved. Table 2. Preparation of aqueous electrolytes. Electrolyte xmLiOH (g) mNaOH (g)mLiClO4 (g) 1.0019.1 – –0.7514.3 8.0–8.0 M [LiOH]x–[NaOH](1-x)0.509.6 16.0–0.254.8 24.0–0.00– 32.0–1.0014.3 ––0.7510.8 6.0–6.0 M [LiOH]x–[NaOH](1-x)0.507.2 12.0–0.253.6 18.0–0.00– 24.0–1.0014.3––0.7510.8–16.06.0 M [LiOH]x– [LiClO4](1-x)0.507.2–31.90.253.6–47.90.00––63.8

[0229] A Mettler Toledo SevenExcellence pH / Ion / C / DO meter S975-K with a InLab Expert Pro-ISM pH probe was used to measure the pH values of aqueous 6.0 M LiClO4electrolyte. The pH values of the high-molarity aqueous NaOH and LiOH electrolytes used here cannot be measured because of alkaline error. Therefore, we were unable to measure the pH values of mixed-salt electrolytes. The pH values of aqueous 6.0 or 8.0 M LiOH and 8.0 M NaOH electrolytes were determined assuming complete salt dissociation.

[0230] Electrolytes were stored in plastic containers. PFOS solutions with 0.5 mM concentration were prepared by placing 10.76 mg of potassium perfluorooctanesulfonate (Synquest Laboratories, 95%) in a 50 mL Falcon tube and adding 40 mL of aqueous electrolyte. The PFOS-containing solution was subjected to ultrasonic agitation (Jeken PS- 40A digital ultrasonic bath, 240 W, 10 L) for 5 min followed by sitting undisturbed for 30PATENT min, to ensure proper mixing and dissolution. Chemicals were weighed on a Sartorius A 120 S analytical or a Mettler Toledo AT201 analytical balance. A Mettler Toledo SevenExcellence pH / Ion / C / DO meter S975-K with a InLab Expert Pro-ISM pH probe was used to measure the pH value of the aqueous LiClO4electrolyte. The pH values of aqueous 6.0 M NaOH and LiOH electrolytes were obtained by calculations. A fluoride ion selective electrode (Thermo Scientific Orion, 9609BNWP) together with a total ionic strength adjustment buffer solution was used to quantify fluoride ions in aqueous solutions. Physical Characterization

[0231] XPS data were obtained using a Kratos Axis Ultra XPS instrument with a monochromatized Al Kα source. Samples were washed with water and dried before affixing them to double-sided adhesive copper tape. The instrument operated at 200 W and 15 kV with a base pressure of 3.0 × 10–8mbar. Survey scans covered 0–1200 eV with a step size of 1 eV, dwell time of 200 ms, and analyzer pass energy of 140 eV, averaged over 5 scans. High-resolution core level scans followed with a 0.1 eV step size, 260 ms dwell time, and 20 eV pass energy, averaged over 5 scans. Binding energies were calibrated against the C 1s peak at 284.8 eV.28 Data analysis using CasaXPS (Version 2.3.24) to obtain binding energies and quantitative peak areas included Shirley background subtraction, Gaussian / Lorentzian envelope peak fitting, and quantification using instrument-specific atomic sensitivity factors derived from standard materials. Peak area quantifications gave surface elemental content relative to total surface carbon content, with an 8% relative error, derived from the standard deviation of triplicate measurements.

[0232] SEM imaging of [NiFe]-(OH)2–hydrophilic carbon fiber paper anodes pre- and post- electrocatalysis were collected at UR-Nano, using a Zeiss Auriga scanning electron microscope, equipped with a Schottky field emission emitter, and operated at 20.00 kV with a working distance of 5.1 mm. EDX spectroscopy was performed on electrodes using a SEM- integrated EDAX Octane elect plus with silicon drift detector (SDD) spectrometer.

[0233] XRD data were collected at room temperature (298 K) using a Rigaku XtaLAB Synergy-S diffraction system equipped with a HyPix-6000HE HPC detector. Powder samples were affixed to a Nylon loop (0.1 mm ID) with a light coating of viscous oil. CuKα radiation (λ = 1.54184 Å) was generated by a PhotonJet-S microfocus source at 50 kV, 1 mA. Two combination ω-φ “Gandolfi” scans were performed, each for 300 s: 1) ω from -62.00 to 31.00 degrees and φ rotated through 720 degrees, at θ = -42.127 and κ = 70.000 degrees; 2) ω fromPATENT -31.00 to 61.00 degrees and φ rotated through 720 degrees, at θ = 40.877 and κ = -70.00 degrees. The sample-to-detector distance was 34 mm. Two-Dimensional (7Li or23Na) –19F-HOESY-NMR

[0234] Two-dimensional NMR data were acquired on a 500 MHz JEOL NMR spectrometer. Each sample contained 750 µL of the respective electrolyte. All NMR experiments were performed at 25 °C. The collected spectra were analyzed using Delta version 6.2.0 and processed in Igor version 8.04. The 2D 7Li–19F-HOESY-NMR experiments were performed to measure the7Li–19F NOE correlations. The fluorine axis sweep width was set to 5885 Hz and with 375 increments giving a resolution of 15.7 Hz. The lithium axis sweep width was set to 1943 Hz with 100 increments giving a resolution of 19.4 Hz. A total of 64 scans were needed to identify the ion interactions, which resulted in a data acquisition time of 7.5 hours per sample. Likewise, 2D23Na–19F-HOESY-NMR data were collected to measure the23Na–19F NOE correlations. The fluorine axis sweep width was set to 5885 Hz and with 410 increments giving a resolution of 15.7 Hz. The sodium axis sweep width was set to 1323 Hz with 256 increments giving a resolution of 13.2 Hz. A total of 64 scans were used, which resulted in a data acquisition time of 7.5 hours per sample. Boiling Point Determination

[0235] The boiling point of 8.0 M aqueous LiOH was determined using a Thiele tube (150 mm length, 25 mm diameter, Eisco), which was securely clamped to a ring stand and filled with clear mineral oil (heavy, Fisher Chemical) up to a level of 1 cm above the apex of the top triangular arm of the Thiele apparatus. A thermometer (–10 to 260 ˚C, Durac Plus, SP Bel-Art) was carefully inserted into a one-holed rubber stopper that fit the Thiele tube and that had a slit down one side. A Durham tube (6 mm × 30 mm, JRLGD) was affixed to the side of the thermometer using a small rubber band so that bottom of the Durham tube was at the same height as the bottom of the thermometer. One mL of aqueous 8.0 M LiOH solution was filled into the Durham tube, and a capillary tube (Drummond Scientific) was placed into the LiOH solution. The rubber stopper and thermometer assembly were positioned in the Thiele tube such that the aqueous LiOH sample was positioned centered in height inside the Thiele tube. The apparatus was gently heated using a Bunsen burner (H-5890, Humboldt) with a flame height maintained at approximately 4 cm, utilizing a back-and-forth motion along the arm of the Thiele tube. The heating rate was maintained at 10 ˚C min–1until the temperature reached 90 ˚C. Subsequently, the heating rate was reduced to 5 ˚C min–1until thePATENT observation of a continuous stream of bubbles from the capillary tube within the aqueous 8.0 M LiOH solution, at which point the Bunsen burner was removed. The Thiele tube was allowed to cool at room temperature, and the temperature was recorded when the bubble at the bottom of the capillary disappeared, indicative of the boiling point. REFERENCES FOR EXAMPLES 1-5 [1] M.K. Wilsey, T. Taseska, Z. Meng, W. Yu, A.M. Müller, Advanced Electrocatalytic Redox Processes for Environmental Remediation of Halogenated Organic Water Pollutants, Chem. Commun., 59 (2023) 11895-11922. [2] J.W. Washington, T.M. Jenkins, Abiotic hydrolysis of fluorotelomer-based polymers as a source of perfluorocarboxylates at the global scale, Environ. Sci. Technol., 49 (2015) 14129- 14135. [3] J. Garnett, C. Halsall, H. Winton, H. Joerss, R. Mulvaney, R. Ebinghaus, M. Frey, A. Jones, A. Leeson, P. Wynn, Increasing Accumulation of Perfluorocarboxylate Contaminants Revealed in an Antarctic Firn Core (1958–2017), Environ. Sci. Technol., 56 (2022) 11246- 11255. [4] U. Unep, Report of the Conference of the Parties of the Stockholm Convention on Persistent Organic Pollutants on the work of its fourth meeting, United Nations environment programme: Stockholm convention on persistent organic pollutants. Geneva, (2009) 112. [5] S.H. Korzeniowski, R.C. Buck, R.M. Newkold, A.E. kassmi, E. Laganis, Y. Matsuoka, B. Dinelli, S. Beauchet, F. Adamsky, K. Weilandt, A critical review of the application of polymer of low concern regulatory criteria to fluoropolymers II: fluoroplastics and fluoroelastomers, Integrated Environmental Assessment and Management, 19 (2023) 326- 354. [6] A.R. Bock, B.E. Laird, PFAS Regulations: Past and Present and Their Impact on Fluoropolymers, in: B. Améduri (Ed.) Perfluoroalkyl Substances, The Royal Society of Chemistry, 2022, pp.1-21. [7] G. Nouri, Y. Noorollahi, H. Yousefi, Solar assisted ground source heat pump systems – A review, Applied Thermal Engineering, 163 (2019) 114351.PATENT [8] B. Trang, Y. Li, X.-S. Xue, M. Ateia, K. Houk, W.R. Dichtel, Low-temperature mineralization of perfluorocarboxylic acids, Science, 377 (2022) 839-845. [9] B.M. Hunter, J.D. Blakemore, M. Deimund, H.B. Gray, J.R. Winkler, A.M. Müller, Highly Active Mixed-Metal Nanosheet Water Oxidation Catalysts Made by Pulsed-Laser Ablation in Liquids, J. Am. Chem. Soc., 136 (2014) 13118-13121.

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[0236] t is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes.

Claims

PATENT CLAIMS 1. A system for degrading per- and poly-fluoroalkyl substances (“PFAS”), said system comprising: (a) a first container, (b) an electrolyte solution disposed in said first container, which electrolyte solution comprises (1) 46 vol% or more water, and (2) a concentration of hydroxide anions of 1M or higher, and a concentration of alkali metal cations of 1M or higher, (c) one or more PFAS containing at least one covalent bond between a carbon atom and a fluorine atom, said one or more PFAS dissolved in said electrolyte solution, (d) a working electrode at least partially immersed in said electrolyte solution, (e) a water oxidation electrocatalyst immobilized on said working electrode and in contact with (1) said one or more PFAS compounds dissolved in said electrolyte solution and containing at least one covalent bond between a carbon atom and a fluorine atom, and (2) said electrolyte solution, (f) a counter electrode at least partially immersed in said electrolyte solution and electrically connected to said working electrode, and, (g) a source of electricity electrically connected to said system to provide an applied electric potential to said working electrode, wherein, when said water containing said one or more compounds is added to said electrolyte solution and an applied electric potential is applied to said working electrode, said applied electric potential causes said at least one covalent bond between a carbon atom and a fluorine atom in said PFAS to be broken, thereby degrading said PFAS.

2. The system of claim 1, wherein said applied electric potential of element (g) is an anodic bias.

3. The system of claim 1, wherein said applied electric potential of element (g) is a cathodic bias.

4. The system of claim 1, wherein said applied electric potential is - 5 V to 5 V versus standard hydrogen electrode (“SHE”).

5. The system of claim 1, wherein said applied electric potential is an anodic bias and is 0.5 V to 2 V versus SHE.PATENT 6. The system of claim 1, wherein said applied electric potential is an anodic bias and is 1.0 V to 2 V versus SHE.

7. The system of claim 1, wherein said applied electric potential is a cathodic bias and is -5 V to -0.5 V versus SHE.

8. The system of claim 1, wherein said applied electric potential is a cathodic bias and is -2 V to -1.0 V versus SHE.

9. The system of claim 1, wherein said electrolyte solution is 47 vol%, 48 vol%, 49 vol%, 50 vol% or more water.

10. The system of claim 1, wherein said electrolyte solution is 60 vol% or more water.

11. The system of claim 1, wherein said electrolyte solution is 70 vol% or more water.

12. The system of claim 1, wherein said electrolyte solution is 80 vol% or more water.

13. The system of claim 1, wherein said electrolyte solution is 90 vol% or more water.

14. The system of claim 1, wherein said working electrode is carbon fiber paper.

15. The system of claim 14, wherein said carbon fiber paper is hydrophilic.

16. The system of claim 1, wherein said water oxidation electrocatalyst is a metallic material, metal oxide, metal hydroxide, or metal oxy(hydroxide).

17. The system of claim 1, wherein said water oxidation electrocatalyst is a nanostructured layered double hydroxide solid, nanostructured layered oxide solid, nanostructured layered oxy(hydroxide) solid, a perovskite, a polyoxometalate, or a metal- organic framework.

18. The system of claim 17, wherein said water oxidation electrocatalyst is a metallic material, a nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid which contains an effective amount of one or more transition metals, a post-transition metal, or both a transition metal and a post-transition metal.

19. The system of claim 18, wherein said one or more transition metals are first-row transition metals.PATENT 20. The system of claim 19, wherein said one or more first-row transition metals are nickel and manganese or nickel and iron.

21. The system of claim 18, wherein said post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin.

22. The system of claim 17, wherein said nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid comprises nickel mixed with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal.

23. The system of claim 22, wherein said nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid is comprised of a mix of nickel with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal is three parts nickel to one part of said transition metal, of said post-transition metal, or of both a transition metal and a post-transition metal.

24. The system of claim 22, wherein said post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin.

25. The system of claim 22, wherein said nanostructured layered double hydroxide, oxide, or oxy(hydroxide) solid is in the form of nanoparticles disposed on said working electrode.

26. The system of claim 1, wherein said water oxidation electrocatalyst is [NiMn]-layered double hydroxide, oxide, or oxy(hydroxide).

27. The system of claim 26, wherein said [NiMn]-layered double hydroxide, oxide, or oxy(hydroxide) is in the form of nanoparticles disposed on said working electrode.

28. The system of claim 1, wherein said water oxidation electrocatalyst is [NiFe]-layered double hydroxide, oxide, or oxy(hydroxide).

29. The system of claim 28, wherein said [NiFe]-layered double hydroxide, oxide, or oxy(hydroxide) is in the form of nanoparticles disposed on said working electrode.

30. The system of claim 1, further comprising (i) a source of ultraviolet light, which source of ultraviolet light is positioned to shine on said water oxidation electrocatalyst.

31. The system of claim 1, wherein said source of electricity is a battery.PATENT 32. The system of claim 1, further comprising (h) a reference electrode at least partially immersed in said predominantly aqueous solution and electrically connected to said working electrode and said counter electrode.

33. The system of claim 30, wherein said reference electrode is a standard hydrogen electrode.

34. The system of claim 32, wherein said reference electrode is Hg / HgO.

35. The system of claim 1, wherein said PFAS is a perfluoroalkyl acid.

36. The system of claim 1, wherein said PFAS is a perfluoroalkyl carboxylic acid.

37. The system of claim 1, wherein said PFAS is perfluorooctanoic acid or perfluorooctanoate.

38. The system of claim 1, wherein said PFAS is a perfluorosulfonic acid.

39. The system of claim 38, wherein said perfluorosulfonic acid is perfluorooctanesulfonic acid or perfluorooctanesulfonate.

40. The system of claim 1, wherein said PFAS is a GenX chemical.

41. The system of claim 39, wherein said GenX chemical is hexafluoropropylene oxide dimer acid anion.

42. The system of claim 1, wherein said PFAS is has an alkyl chain and said alkyl chain is four carbons in length.

43. The system of claim 42, wherein said PFAS is perfluorobutane sulfonate, or perfluorobutanoate.

44. The system of claim 1, said system further comprising a heater to heat said electrolyte solution above room temperature.

45. The system of claim 1, said system further comprising a source introducing air or oxygen into said electrolyte solution.

46. The system of claim 1, wherein said electrolyte solution further comprises 54 vol% or less non-aqueous solvent.PATENT 47. The system of claim 46, wherein said electrolyte solution is 60 vol% or more water and 40 vol% or less non-aqueous solvent.

48. The system of claim 46, wherein said electrolyte solution is 90 vol% or more water and 10 vol% or less non-aqueous solvent.

49. The system of claim 46, wherein said non-aqueous solvent is ethanol, methanol, 1- propanol, butanol, or acetonitrile.

50. The system of claim 1, wherein said alkali metal cations are lithium cations.

51. The system of claim 1, wherein said concentration of hydroxide anions is 2 M or higher.

52. The system of claim 1, wherein said concentration of hydroxide anions is 4 M or higher.

53. The system of claim 1, wherein said concentration of hydroxide anions is 6 M or higher.

54. The system of claim 1, wherein said concentration of hydroxide anions is 8 M or higher.

55. The system of claim 1, wherein said concentration of hydroxide anions is 9 M or higher.

56. The system of claim 1, wherein said concentration of hydroxide anions is 2 M or higher and said concentration of alkali metal cations is 2 M or higher.

57. The system of claim 1, wherein said concentration of hydroxide anions is 4 M or higher and said concentration of alkali metal cations is 4 M or higher.

58. The system of claim 1, wherein said concentration of hydroxide anions is 6 M or higher and said concentration of alkali metal cations is 6 M or higher.

59. The system of claim 1, wherein said concentration of hydroxide anions is 8 M or higher and said concentration of alkali metal cations is 8 M or higher.

60. The system of claim 1, wherein said concentration of hydroxide anions is 9 M ± 0.5 M and said concentration of alkali metal cations is 9 M ± 0.5 M.PATENT 61. The system of claim 1, wherein said concentration of hydroxide anions is 8 M and said concentration of alkali metal cations is 8 M.

62. The system of claim 1, wherein said concentration of hydroxide anions is 9 M and said concentration of alkali metal cations is 9 M.

63. A method for degrading per- and poly-fluoroalkyl substances (PFAS), said method comprising subjecting said PFAS to electrocatalysis on a water oxidation electrocatalyst, said water oxidation electrocatalyst being disposed on a working electrode, said electrode being disposed in an electrolyte solution comprising (1) 46 vol% or more of water and (2) a concentration of hydroxide anions of 1 M or higher and a concentration of alkali metal cations of 1 M or higher, wherein said PFAS is subjected to said electrolysis by providing a first applied electric potential of -5 V to 5 V versus standard hydrogen electrode to said working electrode for a first period of time, thereby breaking at least one carbon-fluorine bond in said PFAS, thereby degrading said PFAS, provided said applied electric potential is not an open circuit potential unless said open circuit potential follows at least one application of an anodic or a cathodic applied electric potential.

64. The method of claim 63, wherein said concentration of hydroxide anions is 2 M or higher.

65. The method of claim 63, wherein said concentration of hydroxide anions is 4 M or higher.

66. The method of claim 63, wherein said concentration of hydroxide anions is 6 M or higher.

67. The method of claim 63, wherein said concentration of hydroxide anions is 8 M or higher.

68. The method of claim 63, wherein said concentration of hydroxide anions is 9 M or higher.

69. The method of claim 63, wherein said concentration of hydroxide anions is 2 M or higher and said concentration of alkali metal cations is 2M or higher.PATENT 70. The method of claim 63, wherein said concentration of hydroxide anions is 4 M or higher and said concentration of alkali metal cations is 4 M or higher.

71. The method of claim 63, wherein said concentration of hydroxide anions is 6 M or higher and said concentration of alkali metal cations is 6 M or higher.

72. The method of claim 63, wherein said concentration of hydroxide anions is 8 M or higher and said concentration of alkali metal cations is 8 M or higher.

73. The method of claim 63, wherein said concentration of hydroxide anions is 8 M ± 0.5 M and said concentration of alkali metal cations is 8 M ± 0.5 M.

74. The method of claim 63, wherein said concentration of hydroxide anions is 9 M ± 0.5 M and said concentration of alkali metal cations is 9 M ± 0.5 M.

75. The method of claim 63, wherein said concentration of hydroxide anions is 2 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 2 M or higher.

76. The method of claim 63, wherein said concentration of hydroxide anions is 4 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 4 M or higher.

77. The method of claim 63, wherein said concentration of hydroxide anions is 6 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 6 M or higher.

78. The method of claim 63, wherein said concentration of hydroxide anions is 8 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 8 M or higher.

79. The method of claim 63, wherein said concentration of hydroxide anions is 8 M ± 0.5 M and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 8 M± 0.5 M.

80. The method of claim 63, wherein said concentration of hydroxide anions is 9 M or higher and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 9 M or higher.PATENT 81. The method of claim 63, wherein said concentration of hydroxide anions is 9 M ± 0.5 M and said alkali metal cations are lithium cations, which lithium cations are present in a concentration of 9 M± 0.5 M .

82. The method of claim 63, wherein said electrolyte solution is 60 vol% or more water.

83. The method of claim 63, wherein said electrolyte solution is 70 vol% or more water.

84. The method of claim 63, wherein said electrolyte solution is 80 vol% or more water.

85. The method of claim 63, wherein said electrolyte solution is 90 vol% or more water.

86. The method of claim 63, wherein said electrolyte solution further comprises a non- aqueous solvent.

87. The method of claim 63, wherein said non-aqueous solvent contains a concentration of ions sufficient for ion conductivity.

88. The method of claim 63, wherein said working electrode is hydrophilic carbon fiber paper.

89. The method of claim 63, wherein said water oxidation electrocatalyst is a metal oxide solid, metal hydroxide solid, or metal oxy(hydroxide).

90. The method of claim 63, wherein said water oxidation electrocatalyst is a metallic material, nanostructured layered double hydroxide solid, nanostructured layered oxide solid, nanostructured layered oxy(hydroxide) solid, a perovskite, a polyoxometalate, or a metal- organic framework.

91. The method of claim 63, wherein said water oxidation electrocatalyst is a metallic material, nanostructured layered double hydroxide solid, nanostructured layered oxide solid, or nanostructured layered oxy(hydroxide) solid, in which one of the layers comprises an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal.

92. The method of claim 91, wherein said transition metal is a first-row transition metal.

93. The method of claim 92, wherein said first-row transition metal is manganese or iron.PATENT 94. The method of claim 91, wherein said post-transition metal is selected from the group consisting of bismuth, gallium, indium, and tin.

95. The method of claim 91, wherein said nanostructured layered double hydroxide solid, said nanostructured layered oxide solid, or said nanostructured layered oxy(hydroxide) solid, comprises nickel mixed with an effective amount of a transition metal, a post-transition metal, or both a transition metal and a post-transition metal.

96. The method of claim 95, wherein said nanostructured layered double hydroxide solid, said nanostructured layered oxide solid, or said nanostructured layered oxy(hydroxide) solid comprises nickel mixed with an effective amount of a transition metal.

97. The method of claim 96, wherein said water oxidation electrocatalyst is [NiFe]- layered double hydroxide, [NiFe]-nanostructured layered oxide solid, or [NiFe]- nanostructured layered oxy(hydroxide) solid.

98. The method of claim 96, wherein said [NiFe]-layered double hydroxide, [NiFe]- nanostructured layered oxide solid, or [NiFe]-nanostructured layered oxy(hydroxide) solid is in the form of nanoparticles disposed on said working electrode.

99. The method of claim 96, wherein said water oxidation electrocatalyst is [NiMn]- layered double hydroxide, [NiMn]-nanostructured layered oxide solid, or [NiMn]- nanostructured layered oxy(hydroxide) solid.

100. The method of claim 99, wherein said [NiMn]-layered double hydroxide, [NiMn]- nanostructured layered oxide solid, or [NiMn]-nanostructured layered oxy(hydroxide) solid is in the form of nanoparticles disposed on said working electrode.

101. The method of claim 63, wherein said first applied electric potential is an anodic bias.

102. The system of claim 63, wherein said first applied electric potential is 0.5 V to 5 V versus standard hydrogen electrode.

103. The method of claim 63, wherein said first applied electric potential is 1.0 V to 2 V versus standard hydrogen electrode.

104. The method of claim 63, wherein said first applied electric potential is 1.50 V to 1.75 V versus standard hydrogen electrode.PATENT 105. The method of claim 63, wherein said first applied electric potential is 1.6 V versus standard hydrogen electrode.

106. The method of claim 63, wherein said first applied electric potential is a cathodic bias.

107. The system of claim 63, wherein said first applied electric potential is -0.5 V to -5 V versus standard hydrogen electrode.

108. The method of claim 63, wherein said first applied electric potential is -1.0 V to -2 V versus standard hydrogen electrode.

109. The method of claim 63, wherein said first applied electric potential is -1.5 V to -1.75 V versus standard hydrogen electrode.

110. The method of claim 63, wherein said first applied electric potential is -1.6 V versus standard hydrogen electrode.

111. The method of claim 63, wherein said electrolyte solution is at room temperature.

112. The method of claim 63, wherein said electrolyte solution is at a temperature above room temperature.

113. The method of claim 112, wherein said electrolyte solution is at a temperature of 30 ˚C to 99 ˚C.

114. The method of claim 112, wherein said electrolyte solution is at a temperature of 60 ˚C to 85 ˚C.

115. The method of claim 112, wherein said electrolyte solution is at a temperature of 70 ˚C.

116. The method of claim 63, wherein said electrocatalysis is conducted under pressure greater than ambient air pressure.

117. The method of claim 63, further comprising shining ultraviolet light on said water oxidation electrocatalyst.

118. The method of claim 117, further wherein said ultraviolet light is deep ultraviolet light.PATENT 119. The method of claim 63, wherein said electrocatalysis is conducted by cycles consisting of a period of time in which said first applied electric potential at said first polarity is applied to said water oxidation electrocatalyst for a period of time, referred to as an “ON” period, followed by a period of time in which an open circuit potential is present, referred to as an “OFF” period, with each pair of an ON period and of an OFF period considered as one “cycle”.

120. The method of claim 119, wherein said ON period is 1 minute ± 10 seconds.

121. The method of claim 119, wherein said OFF period is 2 minutes ± 10 seconds to 20 minutes.

122. The method of claim 119, wherein said OFF period is 2 minutes ± 10 seconds to 10 minutes.

123. The method of claim 119, wherein said OFF period is 3 minutes to 7 minutes.

124. The method of claim 119, wherein said OFF period is 5 minutes ± 30 seconds.

125. The method of claim 119, wherein said ON period is 1 minute ± 10 seconds, and said OFF period is 2 minutes ± 10 seconds to 20 minutes.

126. The method of claim 125, wherein said electrocatalysis is conducted for 20-150 cycles.

127. The method of claim 125, wherein said electrocatalysis is conducted for 40-130 cycles.

128. The method of claim 125, wherein said electrocatalysis is conducted for 120 cycles ±5 cycles.

129. The method of claim 63, wherein (a) said first applied electric potential is provided at a first polarity for a first time period, followed by (b) providing an open circuit potential for a second time period, and further comprising step (a)’: applying a second applied electric potential at a second polarity, which second polarity is opposite that of said first polarity, for a third time period, wherein which step (a)’ is performed between steps (a) and (b).PATENT 130. The method of claim 129, wherein said first time period is 30 seconds ± 15 seconds, said second time period is five minutes ± 30 seconds to 10 minutes ± 30 seconds, and said third time period is 0.5 seconds to 5 seconds.

131. The method of claim 129, wherein said first time period is 30 seconds, said second time period is five minutes ± 30 seconds, and said third time period is 0.5 seconds to 4 seconds.

132. The method of claim 129, wherein said first applied electric potential is 0.75 to 3.0 V vs standard hydrogen electrode.

133. The method of claim 129, wherein said first applied electric potential is 1.5 to 2.0 V vs standard hydrogen electrode.

134. The method of claim 133, wherein said second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode.

135. The method of claim 129, wherein said first applied electric potential is 0.75 to 3.0 V vs standard hydrogen electrode and said second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode.

136. The method of claim 129, wherein said first applied electric potential is 1.5 to 2.0 V vs standard hydrogen electrode and said second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode.

137. The method of claim 129, wherein said first applied electric potential is -0.75 to -3.0 V vs standard hydrogen electrode.

138. The method of claim 129, wherein said first applied electric potential is -1.5 to 2.0 V vs standard hydrogen electrode.

139. The method of claim 137, wherein said second applied electric potential is 0.75 to 2.0 V vs standard hydrogen electrode.

140. The method of claim 129, wherein said first applied electric potential is -0.75 to -3.0 V vs standard hydrogen electrode and said second applied electric potential is 0.75 to 2.0 V vs standard hydrogen electrode.PATENT 141. The method of claim 129, wherein said first applied electric potential is -1.5 to -2.0 V vs standard hydrogen electrode and said second applied electric potential is 0.75 to 2.0 V vs standard hydrogen electrode.

142. The method of claim 129, wherein said first applied electric potential is 1.5 to 2.0 V vs standard hydrogen electrode and is is applied for 30 seconds ± 10 seconds, said second applied electric potential is -0.75 to -2.0 V vs standard hydrogen electrode and is applied for 0.5 to 4 seconds, and said second time period during which an open circuit potential is present is five minutes ± 30 seconds to 10 minutes ± 30 seconds.

143. The method of claim 129, wherein said PFAS is a perfluoroalkyl acid.

144. The method of claim 129, wherein said PFAS is a perfluoroalkyl carboxylic acid or perfluoroalkyl carboxylate.

145. The method of claim 129, wherein said PFAS is perfluorooctanoic acid.

146. The method of claim 129, wherein said PFAS is perfluorooctanesulfonic acid or perfluorooctanesulfonate.

147. The method of claim 129, wherein said PFAS is hexafluoropropylene oxide dimer acid anion.

148. The method of claim 63, further comprising mineralizing said fluorine atoms dissociated from said PFAS by said electrocatalysis, said method comprising adding to said electrolyte cations that form water-insoluble fluorides, thereby mineralizing said fluorine atoms dissociated from said PFAS.

149. The method of claim 148, wherein said cations that form water-insoluble fluorides are one or more of magnesium, calcium, barium, gallium, copper, zinc, zirconium, vanadium, chromium, and gold cations.

150. The method of claim 148, wherein said cations that form water-insoluble fluorides are one or more of magnesium, calcium, copper, and zinc cations.

151. The method of claim 148, wherein said cations that form water-insoluble fluorides are calcium cations.

152. The method of claim 129, wherein said alkali metal cations are lithium cations.PATENT 153. The method of claim 148, further comprising mineralizing said fluorine atoms dissociated from said PFAS by said electrocatalysis, said method comprising adding to said electrolyte cations that form water-insoluble fluorides, thereby mineralizing said fluorine atoms dissociated from said PFAS.

154. The method of claim 153, wherein said cations that form water-insoluble fluorides are one or more of magnesium, calcium, barium, gallium, copper, zinc, zirconium, vanadium, chromium, and gold cations.

155. The method of claim 153, wherein said cations that form water-insoluble fluorides are one or more of magnesium, calcium, copper, and zinc cations.

156. The method of claim 153, wherein said cations that form water-insoluble fluorides are calcium cations.

157. The method of claim 63, wherein said PFAS is present in said electrolyte solution at a concentration of 0.05 mM to 2 mM.

158. The method of claim 63, wherein said PFAS is present in said electrolyte solution at a concentration of 0.05 mM to 1 mM.

159. The method of claim 63, wherein said PFAS is present in said electrolyte solution at a concentration of 0.05 mM to 0.5 mM.

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