Microbial biotransformation of aqueous film-forming foam derived polyfluoroalkyl substances

Fast analysis of short-chain and ultra-short-chain fluorinated organics in water by on-line extraction coupled to HPLC-HRMS

A rapid on-line solid-phase extraction liquid chromatography high-resolution mass spectrometry (on-line SPE-LC-HRMS) method was developed to analyze 11 ultra-short and short-chain PFAS in surface water. Analytical optimization involved screening 7 chromatographic columns and 5 on-line SPE columns, as well as evaluating SPE loading conditions, filters, sample acidification, chromatographic mobile phases, and SPE loading mobile phases. The optimized method was then applied to 44 river water samples collected in Eastern Canada, including sites near airports with fire-training areas. Among the 11 targeted PFAS, the most frequently detected were trifluoroacetic acid (TFA, 4.6-220 ng/L), perfluorobutanoic acid (PFBA, 0.85-33 ng/L), perfluoropentanoic acid (PFPeA, 1.2-2100 ng/L), trifluoromethane sulfonic acid (TMS, 0.01-4.3 ng/L), and perfluorobutane sulfonic acid (PFBS, 0.07-450 ng/L). Levels of C3-C5 perfluoroalkyl carboxylic acids (PFCAs), C2-C4 perfluoroalkyl sulfonates (PFSAs) and n:3 polyfluoroalkyl acids (n = 2,3; n:3 acids) were significantly higher in water bodies near fire-training area sites compared with rivers in urban areas. In contrast, TFA, TMS, and 1:3 acid were not significantly elevated, likely reflecting atmospheric deposition or other diffuse sources for these compounds. Nontarget and suspect screening analysis revealed an abundance of other ultra-short and short-chain PFAS in AFFF-impacted water bodies. Perfluoroalkyl sulfonamides (FASA, C2, C3, and C5), perfluoroalkyl sulfonamide propanoic acids (FASA-PrA, C1-C2) and n:3 acids (n = 1, 4, and 5) were detected for the first time in environmental surface waters.

Biotransformation of PFAA Precursors by Oxygenase-Expressing Bacteria in AFFF-Impacted Groundwater and in Pure-Compound Studies with 6:2 FTS and EtFOSE

Numerous US drinking water aquifers have been contaminated with per- and polyfluoroalkyl substances (PFAS) from fire-fighting and fire-training activities using aqueous film-forming foam (AFFF). These sites often contain other organic compounds, such as fuel hydrocarbons and methane, which may serve as primary substrates for cometabolic (i.e., nongrowth-linked) biotransformation reactions. This work investigates the abilities of AFFF site relevant bacteria (methanotrophs, propanotrophs, octane, pentane, isobutane, toluene, and ammonia oxidizers), known to express oxygenase enzymes when degrading their primary substrates, to biotransform perfluoroalkyl acid (PFAA) precursors to terminal PFAAs. Microcosms containing AFFF-impacted groundwater, 6:2 fluorotelomer sulfonate (6:2 FTS), or N-ethylperfluorooctane sulfonamidoethanol (EtFOSE) were inoculated with the aerobic cultures above and incubated for 4 and 8 weeks at 22 °C. Bottles were sacrificed, extracted, and subjected to target, nontarget, and suspect screening for PFAS. The PFAA precursors 6:2 FTS, N-sulfopropyldimethyl ammoniopropyl perfluorohexane sulfonamide (SPrAmPr-FHxSA), and EtFOSE transformed up to 99, 71, and 93%, respectively, and relevant daughter products, such as the 6:1 fluorotelomer ketone sulfonate (6:1 FTKS), were identified in quantities previously not observed, implicating oxygenase enzymes. This is the first report of a suite of site relevant PFAA precursors being transformed in AFFF-impacted groundwater by bacteria grown on substrates known to induce specific oxygenase enzymes. The data provide crucial insights into the microbial transformation of these compounds in the subsurface.

A review of the occurrence and microbial transformation of per- and polyfluoroalkyl substances (PFAS) in aqueous film-forming foam (AFFF)-impacted environments

Aqueous film-forming foams (AFFFs) have been extensively used for extinguishing hydrocarbon-fuel fires at military sites, airports, and fire-training areas. Despite being a significant source of per- and polyfluoroalkyl substances (PFAS), our understanding of PFAS occurrence in AFFF formulations and AFFF-impacted environments is limited, as is the impact of microbial transformation on the environment fate of AFFF-derived PFAS. This literature review compiles PFAS concentrations in electrochemical fluorination (ECF)- and fluorotelomer (FT)-based AFFFs and provides an overview of PFAS occurrence in AFFF-impacted environments. Our analysis reveals that AFFF use is a predominant point source of PFAS contamination, including primary precursors (polyfluoroalkyl substances as AFFF components), secondary precursors (polyfluoroalkyl transformation products of primary precursors), and perfluoroalkyl acids (PFAAs). Moreover, there are discrepancies between PFAS concentration profiles in AFFFs and those measured in AFFF-impacted media. For example, primary precursors constitute 52.6 % and 99.5 % of PFAS mass in ECF- and FT-based AFFFs, respectively, whereas they represent only 0.7 % total mass in AFFF-impacted groundwater. Conversely, secondary precursors, which constitute <1 % of PFAS in AFFFs, represent 4.0-27.8 % of PFAS in AFFF-impacted environments. The observed differences in PFAS levels between AFFFs and environmental samples are likely due to in-situ biotransformation processes. Biotransformation rates and pathways reported for AFFF-derived primary and secondary precursors varied among different classes of precursors, consistent with the PFAS occurrence in AFFF-impacted environments. For example, readily biodegradable primary precursors, N-dimethyl ammonio propyl perfluoroalkane sulfonamide (AmPr-FASA) and n:2 fluorotelomer thioether amido sulfonate (n:2 FtTAoS), were rarely detected in AFFF-impacted environments. In contrast, key secondary precursors, perfluoroalkane sulfonamides (FASAs) and n:2 fluorotelomer sulfonate (n:2 FTS), were widely detected, which was attributed to their resistance to biotransformation. Key knowledge gaps and future research priorities are presented to better understand the occurrence, fate, and transport of AFFF-derived PFAS in the environment and to design more effective remediation strategies.

Research Needs Regarding the Vapor Intrusion Potential of Volatile Per- and Polyfluoroalkyl Substances

Certain per- or polyfluoroalkyl substances [e.g., fluorotelomer alcohols (FtOHs), perfluorooctane sulfonamides (FOSAs), and perfluorooctane sulfonamidoethanols (FOSEs)] have sufficient volatility to merit investigation of the vapor intrusion pathway on a site-specific basis, when they occur as subsurface contaminants in sufficient concentrations near occupied buildings. This perspective summarizes some of the evidence that these categories of per- or polyfluoroalkyl substances are volatile and offers specific research questions and objectives, for purposes of further assessing whether FtOHs, FOSAs, and/or FOSEs can pose indoor exposures via soil vapor intrusion and under what circumstances.

Hunting the missing fluorine in aqueous film-forming foams containing per- and polyfluoroalkyl substances

Since aqueous film-forming foams (AFFFs) are major sources of per- and polyfluoroalkyl substances (PFAS), understanding the quantity and type of PFAS present in AFFFs is crucial for assessing environmental risk and remediation. We characterized 25 foams from Canada and Europe, including two non-AFFFs and two fluorine-free AFFFs. We used liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS) to identify novel PFAS, as well as total oxidizable precursor assays (TOP) and total organofluorine (TOF) measurements for comparison. LC-HRMS showed that the two non-AFFF foams and two PFAS-free AFFFs contained little or no PFAS, confirmed by TOF measurement using combustion ion chromatography (CIC). The PFAS-containing AFFFs, however, spanned a wide concentration range of TOF (2200-45,000 mg F/L) and contained 22 new classes of polyfluoroalkyl substances not previously reported. As a result of identifying new compounds, LC-HRMS was fully able to capture the oxidizable precursors determined by TOP assay in all tested fluorotelomer (FT) AFFFs, while unknown compounds still constituted a significant fraction (19-53 mol%) in most electrochemical fluorination (ECF) AFFFs. A fluorine mass balance was achieved by comparing the amounts of compounds identified by LC-HRMS with those detected by CIC, although LC-HRMS overestimated TOF with a recovery of 127 ± 36%.

Stability and Biotransformation of 6:2 Fluorotelomer Sulfonic Acid, Sulfonamide Amine Oxide, and Sulfonamide Alkylbetaine in Aerobic Sludge

The 6:2 fluorotelomer sulfonamide (6:2 FTSAm)-based compounds signify a prominent group of per- and polyfluoroalkyl substances (PFAS) widely used in contemporary aqueous film-forming foam (AFFF) formulations. Despite their widespread presence, the biotransformation behavior of these compounds in wastewater treatment plants remains uncertain. This study investigated the biotransformation of 6:2 FTSAm-based amine oxide (6:2 FTNO), alkylbetaine (6:2 FTAB), and 6:2 fluorotelomer sulfonic acid (6:2 FTSA) in aerobic sludge over a 100-day incubation period. The biotransformation of 6:2 fluorotelomer sulfonamide alkylamine (6:2 FTAA), a primary intermediate product of 6:2 FTNO, was indirectly assessed. Their stability was ranked based on the estimated half-lives (t1/2): 6:2 FTAB (no obvious products were detected) ≫ 6:2 FTSA (t1/2 ≈28.8 days) > 6:2 FTAA (t1/2 ≈11.5 days) > 6:2 FTNO (t1/2 ≈1.2 days). Seven transformation products of 6:2 FTSA and 15 products of 6:2 FTNO were identified through nontarget and suspect screening using high-resolution mass spectrometry. The transformation pathways of 6:2 FTNO and 6:2 FTSA in aerobic sludge were proposed. Interestingly, 6:2 FTSAm was hardly hydrolyzed to 6:2 FTSA and further biotransformed to perfluoroalkyl carboxylic acids (PFCAs). Furthermore, the novel pathways for the generation of perfluoroheptanoic acid (PFHpA) from 6:2 FTSA were revealed.

New PFASs Identified in AFFF Impacted Groundwater by Passive Sampling and Nontarget Analysis

Monitoring contamination from per- and polyfluoroalkyl substances (PFASs) in water systems impacted by aqueous film-forming foams (AFFFs) typically addresses a few known PFAS groups. Given the diversity of PFASs present in AFFFs, current analytical approaches do not comprehensively address the range of PFASs present in these systems. A suspect-screening and nontarget analysis (NTA) approach was developed and applied to identify novel PFASs in groundwater samples contaminated from historic AFFF use. A total of 88 PFASs were identified in both passive samplers and grab samples, and these were dominated by sulfonate derivatives and sulfonamide-derived precursors. Several ultrashort-chain (USC) PFASs (≤C3) were detected, 11 reported for the first time in Australian groundwater. Several transformation products were identified, including perfluoroalkane sulfonamides (FASAs) and perfluoroalkane sulfinates (PFASis). Two new PFASs were reported (((perfluorohexyl)sulfonyl)sulfamic acid; m/z 477.9068 and (E)-1,1,2,2,3,3,4,5,6,7,8,8,8-tridecafluorooct-6-ene-1-sulfonic acid; m/z 424.9482). This study highlights that several PFASs are overlooked using standard target analysis, and therefore, the potential risk from all PFASs present is likely to be underestimated.

Per- and polyfluoroalkyl substances and organofluorine in lakes and waterways of the northwestern Great Basin and Sierra Nevada

Per- and polyfluoroalkyl substances (PFAS) are anthropogenic chemicals that occur ubiquitously in the environment and have been linked to numerous adverse health effects in humans and aquatic organisms. Although numerous environmental monitoring studies have been conducted, only one has evaluated PFAS in surface waters of the northwestern Great Basin, which features unique topography that results in dozens of endorheic basins and terminal lakes with no natural outlet, where PFAS may accumulate. To close this knowledge gap, we evaluated the occurrence of PFAS in grab samples from 15 lakes (headwater and terminal lakes) and 10 rivers in the Great Basin located in Nevada and California of the United States. PFAS and organofluorine were quantified by liquid chromatography tandem mass spectroscopy (LC-MS/MS) and combustion ion chromatography, respectively. The highest concentrations of PFAS occurred in samples taken near sites with known or suspected prior aqueous film forming foam (AFFF) application (~20 to 4754 ng/L). Samples near wastewater treatment plants and in urban areas also tended to have PFAS concentrations greater than those measured in remote, less anthropogenically influenced areas (~2 to 15 ng/L, <3 ng/L respectively). In limited snapshot sampling events PFAS appeared to accumulate in terminal lakes to some extent; in-lake concentrations were two to five times greater than those of their inflows. Fluorotelomer sulfonates were present downstream of a known AFFF application area likely to have had fluorotelomer-based foams applied to it, and the concentrations decayed in a predictable manner, suggesting they may be used as an indicator of PFAS transport away from an AFFF source. In all but two samples, organofluorine concentrations were greater than the sum of targeted PFAS (on a F basis) (median of 0.6 % of organofluorine identified via LC-MS/MS), although there was considerable variability in organofluorine measured in replicate samples.

Predicting PFAS and Hydrophilic Trace Organic Contaminant Transport in Black Carbon-Amended Engineered Media Filters for Improved Stormwater Runoff Treatment

Improved stormwater treatment is needed to prevent toxic and mobile contaminant transport into receiving waters and allow beneficial use of stormwater runoff. In particular, safe capture of stormwater runoff to augment drinking water supplies is contingent upon removing dissolved trace organic contaminants (TrOCs) not captured by conventional stormwater control measures. This study builds upon a prior laboratory-based column study investigating biochar and regenerated activated carbon (RAC) amendment for removing hydrophilic trace organic contaminants (HiTrOCs) and poly- and perfluoroalkyl substances (PFASs) from stormwater runoff. A robust contaminant transport model framework incorporating time-dependent flow and influent concentration is developed and validated to predict HiTrOC and PFAS transport in biochar- and RAC-amended stormwater filters. Specifically, parameters fit using a sorption-retarded intraparticle pore diffusion transport model were validated using data further along the depth of the column and compared to equilibrium batch isotherms. The transport model and fitted parameters were then used to estimate the lifetime of a hypothetical stormwater filter in Seal Beach, CA, to be 35 ± 6 years for biochar- and 51 ± 17 years for RAC-amended filters, under ideal conditions with no filter clogging. This work offers insights on the kinetics of HiTrOC and PFAS transport within biochar and RAC filters and on the impact of filter design on contaminant removal performance and longevity.

Novel Per- and Polyfluoroalkyl Substances Discovered in Cattle Exposed to AFFF-Impacted Groundwater

The leaching of per- and polyfluoroalkyl substances (PFASs) from Australian firefighting training grounds has resulted in extensive contamination of groundwater and nearby farmlands. Humans, farm animals, and wildlife in these areas may have been exposed to complex mixtures of PFASs from aqueous film-forming foams (AFFFs). This study aimed to identify PFAS classes in pooled whole blood (n = 4) and serum (n = 4) from cattle exposed to AFFF-impacted groundwater and potentially discover new PFASs in blood. Thirty PFASs were identified at various levels of confidence (levels 1a-5a), including three novel compounds: (i) perfluorohexanesulfonamido 2-hydroxypropanoic acid (FHxSA-HOPrA), (ii) methyl((perfluorohexyl)sulfonyl)sulfuramidous acid, and (iii) methyl((perfluorooctyl)sulfonyl)sulfuramidous acid, belonging to two different classes. Biotransformation intermediate, perfluorohexanesulfonamido propanoic acid (FHxSA-PrA), hitherto unreported in biological samples, was detected in both whole blood and serum. Furthermore, perfluoroalkyl sulfonamides, including perfluoropropane sulfonamide (FPrSA), perfluorobutane sulfonamide (FBSA), and perfluorohexane sulfonamide (FHxSA) were predominantly detected in whole blood, suggesting that these accumulate in the cell fraction of blood. The suspect screening revealed several fluoroalkyl chain-substituted PFAS. The results suggest that targeting only the major PFASs in the plasma or serum of AFFF-exposed mammals likely underestimates the toxicological risks associated with exposure. Future studies of AFFF-exposed populations should include whole-blood analysis with high-resolution mass spectrometry to understand the true extent of PFAS exposure.

Hydroxyl Radical Transformations of Perfluoroalkyl Acid (PFAA) Precursors in Aqueous Film Forming Foams (AFFFs)

Historical releases of aqueous film forming foam (AFFF) are significant sources of poly- and perfluoroalkyl substances (PFASs), including perfluoroalkyl acids (PFAAs) and their precursors, to the environment. While several studies have focused on microbial biotransformation of polyfluorinated precursors to PFAAs, the role of abiotic transformations at AFFF-impacted sites is less clear. Herein, we use photochemically generated hydroxyl radical to demonstrate that environmentally relevant concentrations of hydroxyl radical (^•OH) can play a significant role in these transformations. High-resolution mass spectrometry (HRMS) was used to perform targeted analysis, suspect screening, and nontargeted analyses, which were used to identify the major products of AFFF-derived PFASs as perfluorocarboxylic acids, though several potentially semi-stable intermediates were also observed. Using competition kinetics in a UV/H₂O₂ system, hydroxyl radical rate constants (k_OH) for 24 AFFF-derived polyfluoroalkyl precursors were measured to be 0.28 to 3.4 × 10⁹ M^-1 s^-1. Differences in k_OH were observed for compounds with differing headgroups and perfluoroalkyl chain lengths. Also, differences in k_OH measured for the only relevant precursor standard available, n-[3-propyl]tridecafluorohexanesulphonamide (AmPr-FHxSA), as compared to AmPr-FHxSA present in AFFF suggest that intermolecular associations in the AFFF matrix may affect k_OH. Considering environmentally relevant [^•OH]_ss, polyfluoroalkyl precursors are expected to exhibit half-lives of ∼8 days in sunlit surface waters and possibly as short as ∼2 h during oxygenation of Fe(II)-rich subsurface systems.

Centurial Persistence of Forever Chemicals at Military Fire Training Sites

Drinking water contamination by per- and polyfluoroalkyl substances (PFAS) is widespread near more than 300 United States (U.S.) military bases that used aqueous film-forming foams (AFFF) for fire training and firefighting activities. Much of the PFAS at these sites consist of precursors that can transform into terminal compounds of known health concern but are omitted from standard analytical methods. Here, we estimate the expected duration and contribution of precursor biotransformation to groundwater PFAS contamination at an AFFF-contaminated military base on Cape Cod, Massachusetts, United States, by optimizing a geochemical box model using measured PFAS concentrations from a multidecadal time series of groundwater and a soil survey in the source zone. A toolbox of analytical techniques used to reconstruct the mass budget of PFAS showed that precursors accounted for 46 ± 8% of the extractable organofluorine (a proxy for total PFAS) across years. Terminal PFAS still exceed regulatory limits by 2000-fold decades after AFFF use ceased. Measurements and numerical modeling show that sulfonamido precursors are retained in the vadose zone and their slow biotransformation into perfluoroalkyl sulfonates (half-life > 66 yr) sustains groundwater concentrations of perfluorobutane sulfonate (PFBS) and perfluorohexane sulfonate (PFHxS). The estimated PFAS reservoir in the vadose zone and modeled flux into groundwater suggest PFAS contamination above regulatory guidelines will persist for centuries without remediation.

Foam fractionation for removal of per- and polyfluoroalkyl substances: Towards closing the mass balance

Foam fractionation has recently attracted attention as a low-cost and environmentally benign treatment technology for water contaminated with per- and polyfluoroalkyl substances (PFAS). However, data on the mass balance over the foam fractionation process are scarce and when available, gaps in the mass balance are often identified. This study verified the high treatment efficiency of a pilot-scale foam fractionation system for removal of PFAS from industrial water contaminated with aqueous film-forming foam. ΣPFAS removal reached up to 84 % and the removal of perfluorooctane sulfonic acid (PFOS) up to 97 %, but the short-chain perfluorobutanoic acid (PFBA) was only removed with a mean efficiency of 1.5 %. In general, mobile short-chain PFAS were removed less efficiently when the perfluorocarbon chain length was below six for carboxylic acids and below five for sulfonic acids. Fluctuations in treatment efficiency due to natural variations in the chemistry of the influent water were minor, confirming the robustness of the technology, but significant positive correlations between PFAS removal and influent metal concentration and conductivity were observed. Over all experiments, the mass balance closure did not differ significantly from 100 %. Nonetheless, PFAS sorption to the walls of the reactor was measured, as well as high PFAS emissions by the air exiting the reactor. PFAS emissions in aerosols correlated positively with mass balance closure. The elevated aerial PFAS concentrations measured in the experimental facility have implications for worker safety and prevention of PFAS-emissions to the atmosphere, and demonstrate the importance of installing appropriate filters on the air outlet of foam fractionation systems.

Nitrifying Microorganisms Linked to Biotransformation of Perfluoroalkyl Sulfonamido Precursors from Legacy Aqueous Film-Forming Foams

Drinking water supplies across the United States have been contaminated by firefighting and fire-training activities that use aqueous film-forming foams (AFFF) containing per- and polyfluoroalkyl substances (PFAS). Much of the AFFF is manufactured using electrochemical fluorination by 3M. Precursors with six perfluorinated carbons (C6) and non-fluorinated amine substituents make up approximately one-third of the PFAS in 3M AFFF. C6 precursors can be transformed through nitrification (microbial oxidation) of amine moieties into perfluorohexane sulfonate (PFHxS), a compound of regulatory concern. Here, we report biotransformation of the most abundant C6 sulfonamido precursors in 3M AFFF with available commercial standards (FHxSA, PFHxSAm, and PFHxSAmS) in microcosms representative of the groundwater/surface water boundary. Results show rapid (<1 day) biosorption to living cells by precursors but slow biotransformation into PFHxS (1-100 pM day^-1). The transformation pathway includes one or two nitrification steps and is supported by the detection of key intermediates using high-resolution mass spectrometry. Increasing nitrate concentrations and total abundance of nitrifying taxa occur in parallel with precursor biotransformation. Together, these data provide multiple lines of evidence supporting microbially limited biotransformation of C6 sulfonamido precursors involving ammonia-oxidizing archaea (Nitrososphaeria) and nitrite-oxidizing bacteria (Nitrospina). Further elucidation of interrelationships between precursor biotransformation and nitrogen cycling in ecosystems would help inform site remediation efforts.

Leaching of Perfluoroalkyl Acids during Unsaturated Zone Flushing at a Field Site Impacted with Aqueous Film Forming Foam

While several studies have focused on perfluoroalkyl acid (PFAA) leaching from soils, field studies evaluating the relationship between PFAA mass removal and porewater concentrations as the PFAA source becomes depleted are lacking. Herein, in situ water flushing was performed at a site historically impacted with AFFF to accelerate the leaching of PFAAs from unsaturated soils in a highly characterized field test cell. Porous cup suction lysimeters were used to assess the changes in PFAA porewater concentrations as a function of PFAA mass removal from the unsaturated soils, where flushing was intermittently paused to determine ambient PFAA porewater concentrations. Results showed that the fractional decreases in PFAA porewater concentrations during flushing exceeded the fractional decrease in PFAA mass removal from the soil. PFOS porewater concentrations decrease by 76% (with negligible rebound) compared to only a 7.4% decrease in overall PFOS mass removed from the unsaturated zone. Overall, the results observed herein suggest that, when considering soil impacts to groundwater, less stringent soil cleanup criteria than those that consider an equivalent relationship between mass removal and mass discharge may be appropriate. In addition, remedial approaches that remove only a modest fraction of the PFAA soil mass may be protective of underlying groundwater, particularly for perfluorinated sulfonates with at least six carbons.

Neutral Per- and Polyfluoroalkyl Substances, Butyl Carbitol, and Organic Corrosion Inhibitors in Aqueous Film-Forming Foams: Implications for Vapor Intrusion and the Environment

Per- and polyfluoroalkyl substances (PFAS), butyl carbitol, and corrosion inhibitors are components of aqueous film-forming foams (AFFFs). Volatile (neutral) fluorotelomerization (FT)- and electrochemical fluorination (ECF)-based PFAS, butyl carbitol, and organic corrosion inhibitors were quantified in 39 military specification (MilSpec), non-MilSpec, and alcohol resistant-AFFF concentrates (undiluted) from 1974 to 2010. Fluorotelomer alcohols were found only in FT-based AFFFs and N-methyl- and N-ethyl-perfluoroalkyl sulfonamides, and sulfonamido ethanols were found only in ECF-based AFFFs. Neutral PFAS and benzotriazole, 4-methylbenzotriazole, and 5-methybenzotriazole occurred at mg/L levels in the AFFFs, while butyl carbitol occurred at g/L levels. Neutral PFAS concentrations in indoor air due to vapor intrusion of a nearby undiluted AFFF release are estimated to be anywhere from 2 to >10 orders of magnitude higher than documented background indoor air concentrations. Estimated butyl carbitol and organic corrosion inhibitor concentrations were lower than and comparable to indoor concentrations recently measured, respectively. The wide range of neutral PFAS concentrations and Henry's law constants indicate that field, soil-gas measurements are needed to validate the estimations. Co-discharged butyl carbitol likely contributes to oxygen depletion in AFFF-impacted aquifers and may hinder the natural PFAS aerobic biotransformation. Organic corrosion inhibitors in AFFFs indicate that these are another source of corrosion inhibitors in the environment.

Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil

Hydrothermal alkaline treatment (HALT) can effectively degrade per- and polyfluoroalkyl substances (PFASs) present in aqueous film-forming foam (AFFF). However, information is lacking regarding the treatment of PFASs in actual groundwater and soil from AFFF-impacted sites, especially for complex soil matrices. Given the lack of studies on direct soil treatment for PFAS destruction, we herein applied HALT to two groundwater samples and three soil samples from AFFF-impacted sites and characterized the destruction of PFASs using high-resolution mass spectrometry. Results showed that the 148 PFASs identified in all collected field samples, including 10 cationic, 98 anionic, and 40 zwitterionic PFASs, were mostly degraded to nondetectable levels within 90 min when treated with 5 M NaOH at 350 °C. The near-complete defluorination, as evidenced by fluoride release measurements, confirmed the complete destruction of PFASs. While many structures, including perfluoroalkyl carboxylic acids and polyfluorinated substances, were readily degraded, perfluoroalkyl sulfonates (PFSAs, C_nF_2n+1-SO₃^-), most notably with short chain lengths (n = 3-5), were more recalcitrant. Rates of PFSA destruction in groundwater samples were similar to those measured in laboratory water solutions, but reactions in soil were slow, presumably due to base-neutralizing properties of the soil. Further, the degradation of PFASs in groundwaters and soils was found to be a function of reaction temperature, NaOH concentration, and reaction time. These findings have important implications for the remediation of AFFF-impacted sites.