Per- and polyfluoroalkyl substances (PFAS) are active ingredients in aqueous film-forming foam (AFFF), a type of firefighting formulation used for decades on military bases and at airports. Because of repeated impact events, a diverse suite of PFAS have migrated into groundwater aquifers around the country, endangering drinking water for millions of people. Remediation of PFAS has become a priority in much of the world, as several of the most common PFAS are toxic and bioaccumulative. The physical and chemical properties of PFAS make them difficult to remove from groundwater using traditional treatment technologies. Designing treatment systems for AFFF-impacted sites is further complicated by the frequent presence of co-occurring chemicals and the fact that PFAS composition can vary significantly according to AFFF manufacturer and production year.
This project was focused on the following objectives:
The hypothesis of this study was that in situ remediation of AFFF with co-occurring chemicals can be achieved by sequentially applying ISCO based on persulfate chemistry in combination with bioremediation strategies. The project team hypothesized that the initial treatment of PFAS and co-occurring chemicals with heat-activated persulfate ISCO could remove chemical inhibitors of aquifer microorganisms and that subsequent downstream microbial activity for in situ mixed chemicals degradation will be stimulated. In addition, bioremediation of organics in AFFF and potential co-occurring chemicals prior to ISCO can decrease the amount of oxidants needed for chemical removal.
Objective 1: ISCO on AFFF and co-occurring chemicals
The first goal was to establish if precursor and perfluoroalkyl carboxylic acid (PFCA) destruction was possible at lower persulfate activation temperatures. This approach could potentially lower remediation cost and enhance potential for downstream bioremediation processes, thereby broadening applications of persulfate ISCO. The results indicated that >90% removal of PFCA and precursors was achieved within about eight hours without aquifer solids present at 85°C.
Objective 2: Heat-activated persulfate oxidation (HAPO) impacts on microorganisms
After HAPO treatment and subsequent pH neutralization, high levels of sulfate will remain in the aquifer, which will likely affect downgradient microbial communities and bioremediation efforts. To examine this, the project team studied the effects of HAPO on enrichment cultures from AFFF-impacted sites, by first exposing an enrichment culture, which had been enriched to degrade benzene, toluene, ethylbenzene, and xylene (BTEX) components, to increasing concentrations of sodium sulfate, as this would be a primary salt remaining after HAPO had been conducted. With all concentrations of sulfate (10, 50, 100, and 200 mM), BTEX was degraded similarly to the no-salt control.
Objective 3: Bioremediation of AFFF and co-occurring chemicals before ISCO
The purpose of this task was to identify microcosms capable of consuming organics in 3M AFFF. The project team compared the consumption of dissolved organic carbon in electrochemical fluorination-based AFFFs and fluorotelomer-based AFFFs. Generally, in the tests, organics in fluorotelomer AFFFs degraded much more readily than in 3M AFFF.
Objective 4: Additional findings
This project measured a range of detected PFOS concentrations in two high salinity matrices. The data suggested that high concentrations of monovalent salts may be of higher concern for accurate quantification of PFOS in sampling saline waters, while high concentrations of divalent salts may have a greater effect on PFOS interfacial behavior and sorption to soil in the vadose zone.
Low temperature HAPO can be an effective PFAS remediation strategy at AFFF source zones, with some key limitations. Repeated injections of persulfate are likely needed for best transformation. Different microbial communities impacted by available carbon sources will affect n-dimethylammoniopropyl perfluorohexane sulfonamide (AmPr-FHxSA) biotransformation yields and product composition.
At sites with multiple chemicals, high concentrations of AFFF may inhibit anaerobic chlorinated solvent bioremediation, while AFFF and potentially perfluorohexane sulfonamide and AmPr-FHxSA can negatively impact aerobic BTEX degradation. In source zones where AFFF concentrations are the highest, transformation of co-occurring chemicals by microbial communities may be inhibited, while zones with lower AFFF concentrations or individual PFAS species will be less likely to interfere with bioremediation processes.
For the additional findings on salt effects on PFOS behavior in aqueous solution, PFSAs were not transformed, destroyed, or removed during HAPO, though high salt concentrations can affect PFSA behavior in aqueous solutions. The observed effects of salt addition suggest that salt causes PFOS to 'salt out' or form aggregates in solution, causing a decrease in detected PFOS concentrations. (Project Completion - 2022)
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Steffens, S.D., E.K. Cook, D.L. Sedlak, and L. Alvarez-Cohen. 2021. Under-reporting Potential of Perfluorooctanesulfonic acid (PFOS) in High Ionic Strength Conditions. Environmental Science & Technology Letters, 8(12):1032-1037. doi.org/10.1021/acs.estlett.1c00762.