Bifurcated Fluorotelomer Carboxylic Acid Biotransformation: Enoyl-Hydration versus Hydroxylation Regulated by Redox Conditions

Abstract

Fluorotelomer carboxylic acids (FTCAs) are important precursors of perfluoroalkyl carboxylic acids (PFCAs) in wastewater and biosolids, yet their redox-dependent biotransformation remains poorly understood. We investigated 5:3 FTCA biotransformation in microcosms inoculated with activated sludge and anaerobic digester sludge under aerobic, nitrate-reducing, sulfate-reducing, and methanogenic conditions. After correction for abiotic losses, the pseudo-first-order removal rate was 0.074 d–1 under aerobic conditions, over an order of magnitude higher than under nitrate-reducing (0.0045 d–1) and methanogenic (0.0028 d–1) conditions; sulfate-reducing conditions showed minimal removal. Nontarget Nano-ESI-HRMS analysis further revealed ultrashort-chain PFCAs, including PFPrA, and previously unreported intermediates, including keto-5:3 FTCA and 3-F-4:3 FTCA. A bifurcated FTCA biotransformation framework was unveiled, initiated by Cα-centered hydroxylation and Cβ-centered enoyl-hydration. Hydroxylation promoted iterative CF2 moiety removal and progressive chain shortening, accounting for nearly 95% of measured products and 72.8% of fluoride release under aerobic conditions, whereas enoyl-hydration dominated (>60%) under nitrate reduction and favored PFCA formation. These findings provide quantitative and mechanistic evidence for competing FTCA biotransformation routes in wastewater microbiomes and highlight redox control as a critical lever for steering PFAS fate toward either PFCA accumulation or progressive defluorination.