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

Abstract

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 100day 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 (t 1/2 ): 6:2 FTAB (no obvious products were detected) ≫ 6:2 FTSA (t 1/2 ≈28.8 days) > 6:2 FTAA (t 1/2 ≈11.5 days) > 6:2 FTNO (t 1/2 ≈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.

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

“…This supports both past and recent findings of these intermediates and highlights the importance of further investigation into oxygenases as potential catalysts for precursor transformation. It is especially noteworthy in the case of 6:2 FTS that the major transformation product, 6:1 FTKS, has only been reported once before and as a minor intermediate (no more than 16.2 ± 5 mol % while our studies produced >80 mol %) . Additionally, for EtFOSE, both methane and ammonia oxidizing consortia added singlet oxygen to yield EtFOSAA, corroborating past findings and adding these two consortia to the list of bacteria able to carry out this transformation.…”

“…A similar compound class ( n :2 FTKS) was identified previously, but the structure was not elucidated . The elucidated structure of 6:1 FTKS was recently found in activated sludge microcosms transforming 6:2 FTS and identified as Ketone −6:2 FTSA . N:1 FTKS consists of n:2 FTS with a singlet oxygen added to an unfluorinated carbon (Section S16 Figure S16.1).…”

“…The production of PFHpA indicates cleavage of the alkyl sulfonate without shortening of the fluorinated chain, as both 6:2 FTS and PFHpA have 6 fluorinated carbons. There is debate on whether PFHpA observed in previous 6:2 FTS studies with activated sludge originated from longer chain PFAS or other sources. ,, However, PFHpA was recently shown to be the daughter product of 6:2 FTS in studies with Rhodococcus jostii RHA1 and was detected in soil microcosms and activated sludge. , PFHpA was the most abundant terminal transformation product measured in treatments with ENV425 and the AOC consortium, indicating this pathway may be as environmentally important at some sites as the more studied unsaturated acid pathway that proceeds with 6:2 fluorotelomer unsaturated acid (6:2 FTUA) as a key intermediate, terminating at PFHxA, PFPeA, and PFBA (Figure ). ,,− Our study suggests that production of ketone 6:1 FTKS by hydroxylation of the β carbon is followed by production of the terminal daughter, PFHpA, without defluorination (Figure ).…”