Biotransformation of 6:2 Fluorotelomer Thioether Amido Sulfonate in Aqueous Film-Forming Foams under Nitrate-Reducing Conditions

Abstract

Despite the prevalence of nitrate reduction in groundwater, the biotransformation of per-and polyfluoroalkyl substances (PFAS) under nitrate-reducing conditions remains mostly unknown compared with aerobic or strong reducing conditions. We constructed microcosms under nitrate-reducing conditions to simulate the biotransformation occurring at groundwater sites impacted by aqueous film-forming foams (AFFFs). We investigated the biotransformation of 6:2 fluorotelomer thioether amido sulfonate (6:2 FtTAoS), a principal PFAS constituent of several AFFF formulations using both quantitative liquid chromatography−tandem mass spectrometry (LC-MS/MS) and qualitative high-resolution mass spectrometry analyses. Our results reveal that the biotransformation rates of 6:2 FtTAoS under nitratereducing conditions were about 10 times slower than under aerobic conditions, but about 2.7 times faster than under sulfate-reducing conditions. Although minimal production of 6:2 fluorotelomer sulfonate and the terminal perfluoroalkyl carboxylate, perfluorohexanoate was observed, fluorotelomer thioether and sulfinyl compounds were identified in the aqueous samples. Evidence for the formation of volatile PFAS was obtained by mass balance analysis using the total oxidizable precursor assay and detection of 6:2 fluorotelomer thiol by gas chromatography−mass spectrometry. Our results underscore the complexity of PFAS biotransformation and the interactions between redox conditions and microbial biotransformation activities, contributing to the better elucidation of PFAS environmental fate and impact.

Influence of microbial weathering on the partitioning of per- and polyfluoroalkyl substances (PFAS) in biosolids

“…This observation could be a result of limited precursor biotransformation in anaerobic conditions compared to that which occurs in aerobic conditions at WRRFs. [89][90][91] As demonstrated in Fig. 4, there is little to no change in total PFAA concentrations in days 1, 4, and 10 and clear increases in total PFAA concentrations in the day 32 and 91 samples.…”

“…This observation could be a result of limited precursor biotransformation in anaerobic conditions compared to that which occurs in aerobic conditions at WRRFs. 89–91…”

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

“…Among these chemicals, FOSA-based PFAS, including perfluorooctane sulfonamide betaine (PFOSB), quaternary ammonium salt (PFOSAmS), tertiary amine (PFOSAm), and amine oxide (PFOSNO), have been identified as precursors of PFOS in aerobic soil. − Notably, their stability was significantly influenced by the various terminal nitrogen-containing moieties. − Their half-lives can be ranked as follows: quaternary ammonium ( t 1/2 ≫ 180 days) ≈ carboxyl betaine ( t 1/2 = 675 days) ≫ tertiary amine ( t 1/2 = 47.5 days) > amine oxide ( t 1/2 = 15–24 days). − In addition, short-chain perfluorohexane sulfonamide (FHxSA)-based PFAS, e.g., perfluorohexane sulfonamide propyl tertiary amine (PFHxSAm), and quaternary amine (PFHxSAmS), were observed to be biotransformed to perfluorohexanesulfonate (PFHxS) in aerobic soil and lake sediment . 6:2 FtTAoS, an emerging FT-based PFAS found in AFFF, has been proven as a precursor of short-chain perfluoroalkyl carboxylic acids (PFCAs, C4–C6) in aerobic soil and nitrate-reducing soil . However, in sulfate-reducing soil, the dominant product of 6:2 FtTAoS was 6:2 fluorotelomer thioether propionate (6:2 FtTP), which did not undergo further reactions to produce PFCAs …”

“…27 6:2 FtTAoS, an emerging FT-based PFAS found in AFFF, has been proven as a precursor of short-chain perfluoroalkyl carboxylic acids (PFCAs, C4−C6) in aerobic soil 28 and nitrate-reducing soil. 29 However, in sulfate-reducing soil, the dominant product of 6:2 FtTAoS was 6:2 fluorotelomer thioether propionate (6:2 FtTP), which did not undergo further reactions to produce PFCAs. 30 The 6:2 fluorotelomer sulfonamide (6:2 FTSAm)-based compounds signify a prominent group of PFAS commonly used in contemporary AFFF formulations.…”

Impacts of Environmental and Engineered Processes on the PFAS Fingerprint of Fluorotelomer-Based AFFF

“…In contrast, fluorotelomer AFFF sites are less common and are poorly studied. Fluorotelomer AFFFs, which encompass all other brands of AFFF, , contain polyfluorinated precursors ,− that can break down to form a variety of perfluorinated carboxylic acids (PFCAs). − …”