Biotransformation of Emerging Polyfluoroalkyl Substances Derived from Aqueous Film-Forming Foam (AFFF) under Nitrate-, Sulfate-, and Iron-Reducing Conditions

While the aerobic transformation of emerging per- and polyfluoroalkyl substances (PFAS) in aqueous film-forming foams (AFFF) has been extensively studied, their fate under anoxic/anaerobic conditions remains poorly characterized. This study investigated the stability and biotransformation of nine emerging polyfluoroalkyl compounds in a composite AFFF under nitrate-, sulfate-, and iron-reducing conditions. The biotransformation behavior of seven compounds under anoxic conditions is reported for the first time. After 300 days of incubation, 6:2 fluorotelomer sulfonate and three compounds containing betaine or quaternary ammonium groups exhibited high persistence across all three reducing conditions. In contrast, the other five compounds containing thioether or tertiary amine groups underwent significant biotransformation, which followed first-order kinetics. Their biotransformation rates followed a consistent hierarchy: nitrate-reducing > sulfate-reducing > iron-reducing conditions. No abiotic transformation was observed under anoxic conditions, in contrast to previous studies conducted under aerobic conditions. Based on the products identified by high-resolution mass spectrometry, comprehensive redox-dependent biotransformation pathways were proposed. Notably, no significant defluorination products were observed, particularly with the absence of short-chain perfluoroalkyl carboxylic acids. Redox-dependent shifts in microbial composition led to differential expression of functional enzymes, ultimately governing the varying biotransformation behavior of AFFF-derived PFAS. This study provides fundamental insights into the fate of emerging PFAS in anoxic subsurface environments.