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Biotransformation of Persistent Groundwater Contaminants: Trichloroethene and Poly- and Perfluoroalkyl Substances (PFASs)


Trichloroethene (TCE) is one of the most commonly detected groundwater contaminants in the United States and has been characterized by the U.S. Environmental Protection Agency as carcinogenic to humans. Past TCE storage and disposal procedures following use in dry-cleaning operations and metal degreasing has resulted in numerous contaminated sites where TCE and chlorinated transformation products, such as dichloroethene (DCE) and vinyl chloride (VC), are detected in soil, groundwater, and air. Poly- and perfluoroalkyl substances (PFASs) are key constituents of aqueous film-forming foams (AFFFs) and are responsible for the surface tension reduction properties that facilitate rapid foam spreading over ignited liquid fuels. Various PFASs have been detected in the soil and groundwater of AFFF-impacted sites, while certain PFASs, such as the eight-carbon homologs perfluorooctanoic acid (PFOA) and perfluorosulfonic acid (PFOS), have been linked to adverse human health effects.

The use of AFFF to extinguish chlorinated solvent-fueled fires has led to the co-contamination of TCE and PFASs at sites where foam wastewater and fuel were allowed to infiltrate the subsurface. Historically, groundwater and soil remediation at these sites was optimized for achieving TCE reductive dechlorination to ethene. However, due to recent increases in activities for measuring and characterizing PFAS contamination, particularly in groundwater and soils beneath firefighter training sites, greater attention is being paid to the fate and transformation of PFASs, as well as their effects on TCE-dechlorinating microbial communities. The in situ biotransformation of AFFF-derived PFAS compounds and the effects of AFFF and PFAS transformation products on TCE bioremediation must be understood.

The biotransformation of a principle PFAS compound used in multiple AFFF formulations, fluorotelomer thioether amido sulfonate (FtTAoS), was investigated under aerobic conditions in soil microcosms amended with AFFF. The aerobic biotransformation pathways for 4:2, 6:2, and 8:2 FtTAoS were determined by direct LC-MS/MS quantification of intermediate and end products and through the characterization of previously unidentified intermediate products with high resolution MS measurements. FtTAoS was biotransformed under aerobic conditions to a fluorotelomer sulfonate, two fluorotelomer carboxylic acids, and a suite of perfluorinated carboxylic acids. The detection of two intermediate compounds, corresponding to singly- and doubly-oxygenated species of FtTAoS, suggest that the first two reactions in the biotransformation pathways are sequential oxygen additions to the thioether group. This is likely followed by a third oxygenation and cleavage between the resulting sulfonate and the amidosulfonate group to form a fluorotelomer sulfonate. The perfluorinated carboxylic acids appear to be the end products of FtTAoS biotransformation and were persistent in live microcosms. An oxidative assay employing PFAS oxidation by hydroxyl radical was applied to microcosm samples to indirectly quantify the total concentration of polyfluorinated compounds present during FtTAoS biotransformation for closure of the mass balance. The assay produced a full mass recovery of PFAS oxidation products before and after complete FtTAoS biotransformation even though only 10% (mol/mol) of the initial amended FtTAoS was accounted for by directly-measured PFASs.

The effects of AFFF and various PFASs on anaerobic TCE dechlorination were investigated in a Dehalococcoides (Dhc)-containing microbial community that dechlorinated TCE to ethene. When AFFF formulations from three different manufacturers: 3M, National Foam, and Ansul were amended to the cuture's growth medium as the sole carbon and energy source, varying yet sufficient quantities of hydrogen and acetate were produced to support dechlorination during all three foam-amendments. However, TCE dechlorination only occurred under 3M AFFF amendment, while no dechlorination was observed under National Foam and Ansul AFFF amendments. All PFAS compounds were persistent in the anaerobic communities and did not transform biologically or abiotically. The degradation of diethylene glycol butyl ether (DGBE), the primary glycol ether solvent in most AFFFs, produced less hydrogen and acetate when amended alone than in AFFF-amended microcosms, suggesting that smaller quantities of other organic carbon substances in the foams, such as hydrocarbon surfactants, may be more easily fermentable. Amendment of 16 mg/L 6:2 fluorotelomer sulfonamido betaine (6:2 FtSaB) slowed TCE dechlorination while 32 mg/L FtSaB completely inhibited dechlorination, suggesting dechlorination did not occur in the National Foam AFFF-amended experiment due to the presence of its most abundant PFAS. In cultures amended with perfluroalkyl acids, 110 mg/L total perfluorosulfonic acids did not inhibit TCE dechlorination, while 110 mg/L total perfluorocarboxylic and perfluorosulfonic acids did, suggesting that inhibition is dependent on PFAS structure as well as concentration.

Carbon stable isotope analysis is a frequently employed tool used to confirm and quantify in situ bioremediation of PCE and TCE to ethene. The impact of growth condition on the carbon isotope fractionation of TCE by Dhc was investigated by quantifying fractionation while Dhc was grown in pure and co-cultures as well as in mixed communities. Enrichment factors were not significantly affected by changes in any of the tested growth conditions for the pure cultures, co-cultures or the mixed communities, indicating that despite a variety of temperature, nutrient, and co-factor-limiting conditions, carbon isotope fractionations remain consistent for given Dhc cultures. However, the fractionation factors for the pure and co-cultures were outside the range of those quantified for the mixed communities, indicating that the fractionation may be strain-dependant.

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