Method for in SITU PFAS waste treatment

Abstract

To capture PFAS in situ a liquid metal solution is placed in an aquifer by injection into at least one remediation well created in a PFAS contaminated plume. The injected metal solution is converted to solid mineral form and modified to increase its potential for sorption by PFAS. Over time PFAS is concentrated on the metal minerals. The process may be reversed, by modifying the metal mineral to release PFAS back to groundwater and it is immediately recovered to the ground surface through pumping wells. The recovered PFAS can then be destroyed by a separate appropriate technology.

Description

METHOD FOR IN SITU PFAS WASTE TREATMENTCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Serial No. 63 / 562,367 filed March 7, 2024, which is incorporated by reference in its entiretyBACKGROUND OF THE INVENTION

[0002] The present invention is directed to a structure for treating in ground contamination, and more particularly a method for trapping and treating by removal per - and polyfluoroalkyl substances (“PFAS”) from groundwater using in situ engineered mineral sorption.

[0003] As known in the art, PFAS is used for a wide range of consumer and industrial products, including firefighting foams. These foams have been used in numerous sites around the world in firefighting training areas, aqueous film forming foam storage areas, firefighting equipment areas, and emergency response sites among others.

[0004] The release of PFAS containing foams through firefighting training, equipment testing, incident response, storage or spillage has resulted in soil and groundwater contamination. At the same time PFAS compounds are being regulated to increasingly lower concentrations and are extremely persistent and mobile in the environment requiring remediation. PFAS is a known pollutant with health ramifications.

[0005] However, PFAS are extremely persistent and mobile in the environment. As result PFAS do not lend themselves to conventional remediation. Prior art remediation techniques utilize initial separation / concentration (waste minimization) prior to application of a contaminant destruction step, which can be accomplished through several demonstrated independent technologies. In situ treatment of this type of waste has been tried. One method of treatment is using colloidal activatedcarbon as a bonding agent in the contaminated soil. However, this approach is associated with high capital costs, high energy use, high operational costs, and relatively long operational time frames, and because of the nature of the bonds; difficulty in recovering sequestered PFAS in the future. Consequently this technique does not allow PFAS to ultimately be destroyed.

[0006] Accordingly, a structure and methodology to overcome the shortcomings of the prior art is desired.SUMMARY OF THE INVENTION

[0007] To capture PFAS in situ a metal is modified to increase its potential for sorption by PFAS. The modified metal is placed in liquid form. At least one remediation well is created in a PFAS contaminated plume. The liquid metal solution is injected into the wells, where it is then converted to a solid mineral form that is bonded to the aquifer materials. PFAS in the PFAS contaminated groundwater plume bonds to the precipitated metal mineral and becomes concentrated.

[0008] To dispose of the concentrated PFAS, the metal mineral that contains bonded, or sorbed PFAS is modified to reduce PFAS sorption and may result in the metal dissolving into liquid form. Both the liquid metal and PFAS are then recovered by pumping from a remediation well.

[0009] In one embodiment of the invention, the metal is one of Aluminum and Iron, or a combination of the two.

[0010] In another embodiment of the invention, the modification is the pH of the metal being changed.

[0011] In still another embodiment of the invention, the sorption and release of the metals is controlled by manipulation of the pH potential of the groundwater.

[0012] In yet another embodiment of the invention, the modification is the ionic strength of interaction of the liquid metal being changed to either bond with the PFAS or promote PFAS desorption. In still another embodiment of the invention, the modification is the ionic strength of the groundwater is changed to either bond with the PFAS or promote PFAS desorption

[0013] In yet a further embodiment of the invention the sorption and release of the metals is controlled by manipulation of the redox potential of the metals. In yet a still further embodiment of the invention the sorption and release of the metals is controlled by manipulation of the redox potential of the groundwater.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present disclosure will be better understood by reading the written description with reference to the accompanying drawing figures in which like reference numerals denote similar structure and refer to like elements throughout in which:

[0015] Fig. 1 is a flowchart showing the sorption process of PFAS into an oxidized metal;

[0016] Fig. 2 is a flowchart showing the adsorption process of PFAS from the oxidized metal;

[0017] Figs. 3 A-3D a series of steps for in situ treatment of PFAS in accordance with the invention.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Generally, in a first step 102, a metal is reduced and dissolved to be formed into a liquid solution. Remediation wells are formed and extend to a PFAS plume to be treated in a step 104. The liquid metals are injected into the PFAS plume in a step 106, utilizing the wells. The injected metal are then oxidized by injection of air or a liquid oxidant in a step 108, causing the precipitation of metal oxide minerals onto the aquifer material. The PFAS of the plume thensorb to the immobile solid metal oxide minerals, in effect concentrating and locking up the PFAS and preventing the PFAS from bonding with the water in the aquifer.

[0019] In a preferred non limiting embodiment, the metal is one of aluminum and iron in oxide form.More preferably they are selected from the group in Table 1 below.Table 1. Metal oxides proposed for synthesis in the laboratory

[0020] Even more particularly, ferrihydrite and aluminum hydroxide are metal oxides with high reactivity towards charged compounds in water. Surface area is important for normalization and maximizing of PFAS sorption. Both of the above metals have high surface area with ferrihydrite showing a range of 250 to 1260 m2 / g.

[0021] PFAS sorption on ferrihydrite and aluminum hydroxide will occur as a result of several mechanisms. One mechanism is electrostatic attraction between the positively charged metal ion and negatively charged headgroup in PFAS. This mechanism facilitates the sorption of PFAS into ferrihydrite and aluminum hydroxide.

[0022] As will be discussed below, adsorption of PFAS to hematite, an iron oxide compound, has been shown to improve substantially with decrease of pH from 6 to 3 owing to improved electrostatic interactions. While increasing pH above the pH of hematite (7.7-7.8) resulted in significantly poor PFAS adsorption.

[0023] Another adsorption mechanism is hydrogen bonding. Long chain PFAS are adsorbed strongly in the presence of anions such as nitrate indicating that the mechanism of PFAS sorptionon ferrihydrite is not just an electrostatic interaction. The stronger sorption of long chain PFAS is due to hydrogen bonding between fluorine moieties present in PFAS and positive charged -OH21 / 2+or O3H12+. groups of ferrihydrite. This is because in PFOS, (a representative molecule of the large class of PFAS molecules), by way of nonlimiting example, the charge excess of -0.1 to -0.2 per fluorine atom creates enough charge density to make hydrogen bonding with ferrihydrite. Evan PFAS lacking charged headgroups under low pH conditions such as perfluoro octane sulfonamide (FOSA) is strongly sorbed to ferrihydrite; indicating the significance of hydrogen bonding.

[0024] Still another adsorption mechanism that results from adsorption of PFAS onto metal is ligand exchange where the PFAS anions replace the surface hydroxyl groups of metal hydrous oxides. Iron oxides can form inner surface complexes with the carboxylate group present in perfluoro carboxylic acids. The mechanism involves ligand exchange between carboxylic acid head group in PFCA and coordinated OH groups on the iron oxides.

[0025] As seen from the above, several factors working alone or together can affect PFAS sorption on ferrihydrite and aluminum hydroxide including pH, chain length, and functional groups of PFAS. Taking each in turn:

[0026] pH- The surface charge of ferrihydrite is affected by the pH of the soil. It has been noted that PFAS sorption of ferrihydrite is inversely proportional to pH. The point of zero charge (pzc) of ferrihydrite is estimated to be at a pH of about 8. 1. A lower pH will impart a net positive surface charge on the ferrihydrite thus improving the electrostatic attraction to anionic PFAS. Therefore, the lower the pH, the higher will be the net positive surface charge on ferrihydrite and better sorption of anionic PFAS.

[0027] One study showed that the potential zeta at pH values between 4 and 5 was reported to be > 27mV which decreased with increasing pH and reached nearly zero at pH 7.7 close to an estimated pzc value of 8.1. The highest percentage sorption of PFAS were observed as pH<5 and above. This pH poorsorption was observed for all PF AS. Similar results were obtained for aluminum oxides, such as alumina and boehmite, where increases in pH lead to a decreased PF AS adsorption. Lower pH has also been shown to improve PF AS sorption kinetics.

[0028] Turning to chain length, the length of the perfluoroalkyl chain in PFAS affects their degree of sorption to fenihydrite and aluminum hydroxide. Long chain PFAS exhibited better sorption compared to short chain PFAS at pH <5. In one study the percentage adsorption of long chain PFAS to fenihydrite at pH 4 ranged from 60% (perfluoro octanoic acid) to 100% (perfluoro decanoic acid) while the sorption of short-chain PFAS from 31% (Perfluoro pentanoic acid) to 43% (Perfluoro hexanoic acid). Therefore, it is generally expected that perfluorinated carbons of six or more will exhibit meaningful sorption to fenihydrite.

[0029] Functional groups of PFAS, those with sulfonic acid head groups have been shown to have better sorption of alumina and other oxides of aluminum compared to carboxylic acid head groups. In a study that explored sorption of PFOA and PFOS onto boehmite and alumina, PFOS was shown to have a higher adsorptive capacity than PFOA.

[0030] Increasing the ionic strength introduces the cations which attach to the metal oxide surfaces and promote stronger electrostatic interaction with hydrophilic head groups. This promotes adsorption.

[0031] As a result of sorption as described above the PFAS is bound in the ground, in situ and does not leak into the aquifer. The bound PFAS may then be subsequently removed by inducing desorption and recovered through groundwater pumping. It can then be disposed of in conventional manner. As seen in Fig. 2, in a step 202 the adsorbed, bound, PFAS is removed from the site. In a step 204 the batched PFAS undergoes desorption to separate the PFAS. The separated PFAS is then remotely destroyed, away from the aquifer, in a step 206 as known in the art. Desorption techniques often require the reversal of the adsorption process. To recover the PFAS, the aquifer pH or redox is changed (by injecting andacid and / or strong reductant like sodium dithionite) which decreases the degree of PFAS sorption and may ultimately dissolved most or all of the metal minerals. It follows that the sorption and release of the metals is controlled by manipulation of the pH of the groundwater. Once the minerals are dissolved, both the metals and PFAS are now freely dissolved in groundwater and are recovered by pumping.

[0032] Treating the adsorbed PFAS to change the pH of one of the constituent elements causes a change in the surface charge. This creates a more negative surface, resulting in hydrophilic head groups detaching from metal oxide.

[0033] Change in the redox state also encourages desorption. Transforming redox-active metal oxides (such as iron minerals) to a reduced form ( dissolution of iron) resulting in the release of sorbed PFAS.

[0034] Lastly, lowering of the ionic strength causes the sorbed cations to be released, promoting PFAS desorption. Additionally, the presence of other anions, like phosphate and natural organic matter can lower the adsorption (promote desorption) of anionic PFAS on ferrihydrite by direct competition for sorption sites and lowering of positive surface charge. It was also found that the reason for a lower pH pzc value of 5.3 observed in naturally occurring ferrihydrite is due to the presence of adsorbed natural organic matter and phosphate.

[0035] Reference is now made to Fig. 3, in which the real-world application of method is described. In Fig. 3 A , at an aquifer 320, contaminated with a PFAS plume 314, remediation wells 310 are drilled. Engineered liquid metals 312 are injected into a treated PFAS zone to promote sorption. In step 3B. In a step 3C, the sorbed PFAS is recovered. The process can be repeated in a step 3 D.

[0036] Initial testing of the system has been promising as seen in table SI below:

[0037] It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method and in the construction set forth without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

[0038] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between.

Claims

CLAIMS1. A method for capturing PFAS in situ below ground, comprising the steps of: modifying a metal to increase the potential of the metal for sorption by PFAS; placing the modified metal in liquid form; creating at least one remediation well in liquid communication with a PFAS contaminated plume; injecting the liquid metal into the at least one remediation well, causing the liquid metal to bond with PFAS in the PFAS contaminated plume.

2. The method of claim 1, further comprising the step of removing the PFAS contaminated plume and changing at least one of the pH, redox and ionic strength of the PFAS contaminated plume.

3. The method of claim 1, wherein the metal is one of Iron oxide and Aluminum Oxide.

4. The method of claim 3, wherein the Iron oxide is one of ferrihydrite, goethite, hematite, and magnetite.

5. The method of claim 3, wherein the Aluminum oxide is gibbsite.

6. The method of claim 2, wherein the step of changing the pH includes changing the pH of the liquid metal.

7. The method of claim 6, wherein the step of changing the pH includes lowering the pH of the liquid metal.

8. The method of claim 2, wherein the step of changing redox of the PFAS contaminated plume includes changing the redox of the metal.

9. The method of claim 1, further comprising the step of modifying an ionic strength of interaction of the liquid metal.

10. The method of claim 2, wherein changing at least one of the pH, redox and ionic strength of the PFAS contaminated plume causes the liquid metal to one of bond with the PFAS or promote desorption of the PFAS. io

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