Microbial Dehalogenation of 3,5,6-Trichlorooctafluorohexanoic Acid under Different Reducing Conditions

Chlorinated polyfluoroalkyl substances (Cl-PFAS) have emerged as promising alternatives to legacy PFAS due to their enhanced microbial reactivity and improved environmental degradability. However, their transformation mechanisms under environmentally relevant reducing conditions remain poorly characterized. This study investigated the microbial dehalogenation of 3,5,6-trichlorooctafluorohexanoic acid (CTFE3), a representative Cl-PFAS, under nitrate-, sulfate-, iron-reducing, and methanogenic conditions. Microbial defluorination was observed across all reducing environments, with higher total defluorination efficiencies (∼60%) under nitrate- and sulfate-reducing conditions compared to iron-reducing and methanogenic conditions (∼30%) under the tested experimental conditions. Proposed biotransformation pathway analysis suggested that CTFE3 underwent more diverse and sequential hydrolytic dechlorination under nitrate- and sulfate-reducing conditions, which was associated with more extensive defluorination. Genes associated with hydrolytic dechlorination were consistently enriched under these conditions, but not in iron-reducing or methanogenic environments. Metagenomic binning further identified key taxa (e.g., Methyloversatilis discipulorum, Herbaspirillum seropedicae, Paracoccaceae, and Rhodobacteraceae-related bacteria) harboring both hydrolytic dechlorination and nitrate/sulfate-reduction genes, suggesting their involvement in CTFE3 hydrolytic dechlorination and subsequent defluorination. This study demonstrates that reducing conditions play an important role in shaping CTFE3 transformation patterns and highlight hydrolytic dechlorination as a viable pathway associated with extensive microbial defluorination, thereby offering insights for sustainable Cl-PFAS remediation.