Enhanced PFAS elimination by integrated nanobubble-electrochemical system

Abstract

Degrading PFAS includes generating nanobubbles with a gas in a liquid to produce a nanobubble liquid, combining the nanobubble liquid, the PFAS, and an electrolyte to yield an electrolyte composition, where the PFAS is in liquid form and migrates to a surface of the nanobubbles, and electrolyzing the electrolyte composition in an electrolytic cell including an anode and a cathode in electrical communication, thereby adsorbing the PFAS on a surface of the anode and electrochemically degrading the PFAS.

Description

Attorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d ENHANCED PFAS ELIMINATION BY INTEGRATED NANOBUBBLE- ELECTROCHEMICAL SYSTEM CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Patent Application No.63 / 648,642 filed on May 16, 2024, which is hereby incorporated by reference herein in its entirety. STATEMENT OF GOVERNMENT SUPPORT

[0002] This invention was made with government support under 1449500 awarded by the National Science Foundation. The government has certain rights in the invention. TECHNICAL FIELD

[0003] This invention relates to enhanced per- and polyfluoroalkyl substances (PFAS) elimination by integrated nanobubble-electrochemical system. BACKGROUND

[0004] Per- and polyfluoroalkyl substances (PFAS) are “forever chemicals” that are toxic and hard to degrade. Current regulations limit the presence of PFAS below a few parts per trillion concentrations in waters. SUMMARY

[0005] This disclosure describes the use of nanobubbles as a contaminant carrier in an electrochemical system, utilizing the stability of nanobubbles in solution and the hydrophobic nature between gases and PFAS. The presence of nanobubbles of different gases such as air, nitrogen (N2), argon (Ar), and oxygen (O2) can promote mass transfer toward an electrode surface due at least in part to the non-polar nature of long C-F chains present in PFAS. The combination of nanobubbles and electrolysis can reduce time and cost for electrochemical treatment due at least in part to the ability to address both low and high concentrations of PFAS at a current density below 100 mA cm-2.

[0006] In a first general aspect, degrading PFAS includes generating nanobubbles with a gas in a liquid to produce a nanobubble liquid; forming an electrolyte composition comprising the nanobubble liquid, the PFAS, and an electrolyte, where the PFAS is in liquid form and migratesAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d to a surface of the nanobubbles, and electrolyzing the electrolyte composition in an electrolytic cell including an anode and a cathode in electrical communication, thereby adsorbing the PFAS on a surface of the anode and electrochemically degrading the PFAS.

[0007] Implementations of the first general aspect can include one or more of the following features. Generating the nanobubbles can include providing the gas to a nanobubble generator including, for example, a static mixer, a nano-nozzle, or a membrane. In some cases, electrolyzing the electrolyte composition includes applying a current density up to 100 mA cm-2to the electrolyte composition. Electrolyzing the electrolyte composition can include circulating the electrolyte composition through the electrolytic cell. Suitable examples of the gas can include nitrogen, argon, oxygen, carbon dioxide, air, or any combination thereof. The electrolytic cell can include a boron-doped diamond anode and a stainless-steel cathode. In certain implementations, the electrolytic cell includes a platinum anode or a metal oxide anode. Suitable examples of the metal oxide anode include lead oxide, ruthenium dioxide, iridium dioxide, titanium dioxide, tin dioxide, or any combination thereof. The first general aspect can further include forming the electrolyte composition in a vessel upstream of the electrolytic cell. In some cases, the first general aspect further includes providing the electrolyte composition to the electrolytic cell. Electrolyzing the electrolyte composition can include recirculating the electrolyte composition through the electrolytic cell and the vessel.

[0008] Electrolyzing the electrolyte composition can include migrating the PFAS to the anode via the nanobubbles. In certain implementations, migrating the PFAS to the anode via the nanobubbles yields a hydrophobic layer including the PFAS on the surface of the anode. Electrochemically degrading the PFAS can yield fluoride ions and carbon dioxide. A diameter of the nanobubbles can be up to 1000 nm (e.g., at least 50 nm, up to 500 nm, or up to 300 nm). The liquid can be water. In some cases, a concentration of the nanobubbles in the electrolyte composition is in a range of 107nanobubbles per mL to 109nanobubbles per mL. Generating the nanobubbles and electrolyzing the electrolyte composition can occur at the same time (e.g., simultaneously). In certain cases, generating the nanobubbles occurs prior to electrolyzing the electrolyte composition.

[0009] Each nanobubble has a gas-liquid interface with an area based on its diameter. Compared to micro- and macro-bubbles, nanobubbles present negligible buoyancy effects, improving their stability in solution. Nanobubbles of various gases can be gas-liquid massAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d transfer promoters. Nanobubble stability depends at least in part on storage conditions and solution components. Under mild conditions, nanobubbles can have a lifetime of days, weeks, months, and even years. Depending on the gas, nanobubbles can increase or decrease their stability due at least in part to interactions with ions and water molecules. Inert gases, such as N2, Ar, and air (e.g., 79 % N2 and 21% O2), can provide a higher concentration and stability than active gases, such as O2and CO2.This can be due at least in part to the interaction of active gases with water.

[0010] In a second general aspect, a system for degrading PFAS includes a nanobubble generator; a vessel fluidly configured to receive nanobubbles generated by the nanobubble generator and to contain an electrolyte composition comprising the nanobubbles, the PFAS, and an electrolyte; and an electrolytic cell configured to receive the electrolyte composition and degrade the PFAS.

[0011] In some cases, the system includes a recycle loop configured to return at least some of the electrolyte composition in the electrolytic cell to the vessel, a recycle loop configured to return at least some of the fluid in the vessel to the nanobubble generator, or both.

[0012] The details of one or more embodiments of the subject matter of this disclosure are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG.1A is a flow-chart showing operations in a process for degrading PFAS. FIG.1B shows nanobubbles concentration (x106particles mL-1) generated in deionized water using nanoparticle track analysis. FIG.1C shows a size distribution (nm) of bubbles generated in deionized water using nanoparticle track analysis.

[0014] FIG.2A shows electrochemical elimination of perfluorooctanoic acid (PFOA) in Na2SO4under different current densities. FIG.2B shows kinetics rate constant determination considering a pseudo first order reaction.

[0015] FIG.3A shows nanobubble concentration over time considering nanobubble generation with and without a simultaneous electrolysis system at 25 mA cm-2. FIG.3B shows nanobubble concentration over time considering nanobubble generation with and withoutAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d simultaneous electrolysis system at 50 mA cm-2. FIG.3C shows PFOA elimination over time considering the generation of nanobubbles with electrolysis at 25 mA cm-2. FIG.3D shows PFOA elimination over time considering the generation of nanobubbles with electrolysis at 50 mA cm-2.

[0016] FIG.4A shows the nanobubble diameter size distribution during generation before simultaneous nanobubbles. FIG.4B shows simultaneous nanobubble generation with electrolysis. FIG.4C shows generation of nanobubbles until a constant concentration was reached, followed by electrolysis.

[0017] FIG.5A shows a process flow diagram of a batch mode for PFOA elimination using the integrated nanobubbles and electrolysis system. FIG.5B shows a process flow diagram of a continuous mode for PFOA elimination using the integrated nanobubbles and electrolysis system.

[0018] FIG.6A shows PFOA elimination over time using different gases. FIG.6B shows the effect of air nanobubbles in the PFOA elimination at 5 mA cm-2. FIG.6C shows the effect of air nanobubbles in the PFOA elimination at 25 mA cm-2. FIG.6D shows the effect of air nanobubbles in the PFOA elimination at 50 mA cm-2.

[0019] FIG.7 shows the kinetic constant and the elimination percentage of the electrochemical degradation of PFOA, comparing results achieved by electrolysis and by the integrated nanobubbles and electrolysis process with different gases.

[0020] FIG.8A shows the PFOA removal percentage by electrolysis and by the integrated nanobubbles and electrolysis process at different current densities using air. FIG.8B shows the relationship between the kinetics constant and electrical energy per order at various current densities using air. DETAILED DESCRIPTION

[0021] This disclosure describes an integrated nanobubble and electrolysis method for the elimination of per- and poly-fluoroalkyl substances (PFAS). This method demonstrates perfluorooctanoic acid (PFOA) elimination (> 90%) at low and high concentrations ranges. The integrated nanobubble and electrolysis system can reduce time and energy consumption of electrolysis at least in part by increasing the mass transfer with nanobubbles. Without the use of additional chemicals, nanobubbles can increase elimination percentages for individual PFASAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d solutions and mixtures of them. In an electrochemical system, the electrical energy is typically 16.2 kWh m-3per order without nanobubbles and 9.5 kWh m-3per order at 25 mA cm-2with nanobubbles. Energy conservation and shorter treatment times are attainable for different operational conditions of applied current density. In some cases, the applied current density ranges from 5 mA cm-2to 100 mA cm-2(e.g., 25 mA cm-2). The electrochemical system can include a cathode and an anode. The anode can include materials with high overpotential of oxygen evolution. Suitable examples of anodes for the electrochemical system can include boron-doped diamond, Magneli phase electrodes, platinum, and lead iodide.

[0022] FIG.1A shows operations in process 100 to degrade PFAS. In 102, the nanobubbles are generated with a gas in a liquid to produce a nanobubble liquid. A diameter of the nanobubbles can be up to 1000 nm (e.g., between 50 nm and 1000 nm, between 50 nm and 500 nm, between 50 nm and 300 nm, or between 75 nm and 300 nm), and the liquid can be water. Examples of suitable gases typically include air, nitrogen, argon, oxygen, carbon dioxide, or any combination thereof. Generating the nanobubbles can include providing the gas to a nanobubble generator including, for example, a static mixer, a nano-nozzle, or a membrane.

[0023] In 104, the nanobubble liquid, the PFAS, and an electrolyte are combined to yield an electrolyte composition, where the PFAS is in liquid form and migrates to a surface of the nanobubbles. The nanobubble liquid, the PFAS, and the electrolyte can be combined in a vessel upstream of or in the electrolytic cell. A concentration of the nanobubbles in the electrolyte composition is typically in a range of 107nanobubbles per mL to 109nanobubbles per mL.

[0024] In 106, the electrolyte composition is electrolyzed in an electrolytic cell including an anode and a cathode in electrical communication. The PFAS is adsorbed on a surface of the anode and the PFAS is electrochemically degraded, which can yield fluoride ions and carbon dioxide. In some cases, the electrolytic cell includes a boron-doped diamond anode and a stainless-steel cathode. Suitable examples of the anode can include a platinum anode and a metal oxide anode (e.g., lead oxide, ruthenium dioxide, iridium dioxide, titanium dioxide, and tin oxide). Electrolyzing the electrolyte composition typically includes applying a current density of up to 100 mA cm-2(e.g., up to 25 mA cm-2or up to 50 mA cm-2) to the electrolyte composition. In some cases, electrolyzing the electrolyte composition includes circulating the electrolyte composition through the electrolytic cell. In certain implementations, electrolyzing the electrolyte composition includes recirculating the electrolyte composition through theAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d electrolytic cell and the vessel. Electrolyzing the electrolyte composition typically includes migrating the PFAS to the anode via the nanobubbles, which can yield a hydrophobic layer including the PFAS on the surface of the anode. In some cases, generating the nanobubbles and electrolyzing the electrolyte composition occur at the same time (e.g., simultaneously). In certain implementations, generating the nanobubbles occur prior to electrolyzing the electrolyte composition.

[0025] A system for degrading PFAS includes a nanobubble generator; a vessel configured to receive nanobubbles generated by the nanobubble generator and to contain an electrolyte composition comprising the nanobubbles, the PFAS, and an electrolyte; and an electrolytic cell configured to receive the electrolyte composition and degrade the PFAS. In some cases, the system includes a recycle loop configured to return at least some of the electrolyte composition in the electrolytic cell to the vessel, a recycle loop configured to return at least some of the fluid in the vessel to the nanobubble generator, or both. EXAMPLES

[0026] A pressurized gas stream was fed into a diffuser to form microbubbles. The microbubbles were dragged by a liquid stream to enter the nanobubble generator, where, under shearing force in a static mixer, the microbubbles were converted to nanobubbles. The nanobubble concentration for optimal conditions can be higher than 107nanobubbles per mL. Gas pressure and water flow rate were set to 68947.6 Pa (10 psi) and 120 L h-1,respectively. The nanobubble concentrations can be reached using different nanobubble generation methods and flow rate and gas pressure conditions. Air, O2, N2, and Ar nanobubbles were characterized in terms of concentration (particles mL-1) and diameter size (nm), as shown in FIGS.1B and 1C. FIG.1B shows the concentration of the nanobubbles (x108particles mL-1). FIG.1C shows the size distribution (nm) of nanobubbles generated in deionized water. Both measurements were performed using nanoparticle track analysis.

[0027] The liquid stream containing PFAS and nanobubbles was circulated through an electrochemical reactor. The electrochemical cell was operated in galvanostatic mode (constant current density), evaluating current densities from 5 mA cm-2to 100 mA cm-2. The electrochemical configuration involved a boron-doped diamond anode and a stainless steel cathode with a geometrical area of 25 cm2and a liquid flow rate of 40 L h-1. The electrochemicalAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d elimination of perfluorooctanoic acid (PFOA) employing this system without nanobubble generation is shown in FIGS.2A and 2B. FIG.2A shows electrochemical elimination of 100 μg L-1PFOA in 0.05 mol L-1Na2SO4under current densities of 5 mA cm-2, 10 mA cm-2, 25 mA cm-2, 50 mA cm-2, 75 mA cm-2, and 100 mA cm-2. FIG.2B shows a kinetics rate constant determination considering a pseudo first order reaction.

[0028] The nanobubble effect during the electrochemical elimination is shown in FIGS.3A to 3D. FIG.3A shows nanobubble concentration over time considering nanobubble generation with and without a simultaneous electrolysis system at 25 mA cm-2. FIG.3B shows nanobubble concentration over time considering nanobubble generation with and without a simultaneous electrolysis system at 50 mA cm-2. FIG.3C shows PFOA elimination over time considering the generation of nanobubbles and application of nanobubbles with electrolysis at 25 mA cm-2. FIG. 3D shows PFOA elimination over time considering the generation of nanobubbles and application of nanobubbles with electrolysis at 50 mA cm-2. Any single-compartment and / or divided electrochemical cell can be used regardless of the electrode material to be evaluated, electrolyte composition, and electrode configurations. The integrated nanobubble and electrolysis process can operate in batch mode and continuous mode.

[0029] In a batch mode, the liquid in a container was recirculated independently for nanobubble generation and electrolysis. The operational configurations using the batch mode can include generating nanobubbles until a set or constant concentration is reached. In some cases, the electrolysis was turned on while the nanobubble generation continued. A distribution of nanobubble diameters before and after simultaneous nanobubble generation and electrolysis is shown in FIG.4A.

[0030] The nanobubble generator and the electrolysis can operate simultaneously from the beginning of the process. A distribution of nanobubble diameters under simultaneous nanobubble generation with electrolysis is shown in FIG.4B. In some cases, the nanobubbles were generated until a set or constant concentration was reached and then the electrolysis was turned on without gas input. A distribution of nanobubble diameters under these conditions is shown in FIG.4C.

[0031] In a continuous mode, the liquid flow went through each process step once. Storage tanks and series and / or parallel circuits can be used for each step to improve process efficiency. FIG.5A is a process flow diagram of the integrated nanobubble and electrolysis treatment for PFAS, operating in batch mode. FIG.5B is a process flow diagram of the integrated nanobubbleAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d and electrolysis treatment for PFAS, operating in the continuous mode. Table 1 includes a description of parts for each system. Table 1. Description of system parts

[0032] FIGS. 6A-6D show the integrated nanobubble and electrolysis system with the addition of gases such as air, N2, Ar, and O2to improve the elimination of PFAS. The use of gases demonstrated improved electrochemical performance without addition of other chemicals. FIG. 6A shows PFOA elimination over time using different gases, including air, N2, O2, and Ar. FIG. 6B shows the effect of air nanobubbles on PFOA elimination at a current density of 5 mA cm-2. FIG. 6C shows the effect of air nanobubbles on PFOA elimination at a current density of 25 mA cm-2. FIG. 6D shows the effect of air nanobubbles on PFOA elimination at a current density of 50 mA cm-2.

[0033] FIG. 7 shows the kinetic constants and PFOA elimination percentages attained through electrolysis or the integrated nanobubbles and electrolysis process utilizing different gases. In this assessment, the initial PFOA concentration was 100 µg L-1, the current density was 25 mA cm-2, and the electrolyte was 0.05M Na2SO4in a volume of 1 L.

[0034] FIG. 8A shows the PFOA removal percentage achieved through electrolysis or the integrated nanobubbles and electrolysis process with air, utilizing different current densities. FIG. 8B shows a relationship between the kinetics constant and electrical energy per order at various current densities using air. In this assessment, the initial PFOA concentration was 100 µg L-1, the current density was 25 mA cm-2, and the electrolyte was 0.05M Na2SO4 in a volume of 1 L.

[0035] The integration of nanobubbles and electrolysis process for the electrochemical degradation of PFAS increased the kinetic constants of the electrochemical degradation of PFAS, when compared to the values obtained using electrolysis alone at the same current density. TheAttorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d kinetic constants showed improvement at low concentrations of PFAS (e.g., < 0.1 ppm), suggesting less energy consumption due at least in part to the improved mass transfer. No additional catalyst or chemical compounds were used in the process, which can provide a sustainable route to increase the efficiency of the electrochemical degradation of PFAS.

[0036] Although this disclosure contains many specific embodiment details, these should not be construed as limitations on the scope of the subject matter or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in this disclosure in the context of separate embodiments can also be implemented, in combination, in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments, separately, or in any suitable sub-combination. Moreover, although previously described features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.

[0037] Particular embodiments of the subject matter have been described. Other embodiments, alterations, and permutations of the described embodiments are within the scope of the following claims as will be apparent to those skilled in the art. While operations are depicted in the drawings or claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed (some operations may be considered optional), to achieve desirable results.

[0038] Accordingly, the previously described example embodiments do not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure.

Claims

Attorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d WHAT IS CLAIMED IS:

1. A method for degrading per- and polyfluoroalkyl substances (PFAS), the method comprising: generating nanobubbles with a gas in a liquid to produce a nanobubble liquid; forming an electrolyte composition comprising the nanobubble liquid, the PFAS, and an electrolyte to yield, wherein the PFAS is in liquid form and migrates to a surface of the nanobubbles; and electrolyzing the electrolyte composition in an electrolytic cell comprising an anode and a cathode in electrical communication, thereby adsorbing the PFAS on a surface of the anode and electrochemically degrading the PFAS.

2. The method of claim 1, wherein electrolyzing the electrolyte composition comprises applying a current density up to 100 mA cm-2to the electrolyte composition.

3. The method of claim 1, wherein electrolyzing the electrolyte composition comprises circulating the electrolyte composition through the electrolytic cell.

4. The method of claim 1, wherein the gas comprises nitrogen, argon, oxygen, carbon dioxide, or any combination thereof.

5. The method of claim 4, wherein gas comprises air.

6. The method of claim 1, wherein the electrolytic cell comprises a boron-doped diamond anode and a stainless-steel cathode.

7. The method of claim 1, wherein the electrolytic cell comprises a platinum anode.

8. The method of claim 1, wherein the electrolytic cell comprises a metal oxide anode.Attorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d 9. The method of claim 8, wherein the metal oxide anode comprises lead oxide, ruthenium dioxide, iridium dioxide, titanium dioxide, tin dioxide, or any combination thereof.

10. The method of claim 1, wherein forming the electrolyte composition occurs in a vessel upstream of the electrolytic cell.

11. The method of claim 10, further comprising providing the electrolyte composition to the electrolytic cell.

12. The method of claim 10, wherein electrolyzing the electrolyte composition comprises recirculating the electrolyte composition through the electrolytic cell and the vessel.

13. The method of claim 1, wherein electrolyzing the electrolyte composition comprises migrating the PFAS to the anode via the nanobubbles.

14. The method of claim 1, wherein migrating the PFAS to the anode via the nanobubbles yields a hydrophobic layer comprising the PFAS on the surface of the anode.

15. The method of claim 1, wherein electrochemically degrading the PFAS yields fluoride ions and carbon dioxide.

16. The method of claim 1, wherein a diameter of each of the nanobubbles is up to 1000 nm.

17. The method of claim 1, wherein the liquid comprises water.

18. The method of claim 1, wherein a concentration of the nanobubbles in the electrolyte composition is in a range of 107nanobubbles per mL to 109nanobubbles per mL.

19. The method of claim 1, wherein generating the nanobubbles and electrolyzing the electrolyte composition occurs simultaneously.Attorney Docket No.: 22193-0385WO1 / M24-280P^-WO1-d 20. The method of claim 1, wherein generating the nanobubbles occurs prior to electrolyzing the electrolyte composition.

21. The method of claim 1, wherein generating the nanobubbles comprises providing the gas to a nanobubble generator.

22. The method of claim 21, wherein the nanobubble generator comprises a static mixer, a nano-nozzle, or a membrane.

23. A system for degrading PFAS, the system comprising: a nanobubble generator; a vessel fluidly configured to receive nanobubbles generated by the nanobubble generator and to contain an electrolyte composition comprising the nanobubbles, the PFAS, and an electrolyte; and an electrolytic cell configured to receive the electrolyte composition and degrade the PFAS.

24. The system of claim 23, further comprising a recycle loop configured to return at least some of the electrolyte composition in the electrolytic cell to the vessel.

25. The system of claim 23, further comprising a recycle loop configured to return at least some of the fluid in the vessel to the nanobubble generator.

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