Density Functional Theory Calculations Decipher Complex Reaction Pathways of 6:2 Fluorotelomer Sulfonate to Perfluoroalkyl Carboxylates Initiated by Hydroxyl Radical

6:2
Fluorotelomer sulfonate (6:2 FTSA) is a ubiquitous environmental
contaminant belonging to the family of per- and polyfluoroalkyl substances.
Previous studies showed that hydroxyl radical (•OH) efficiently transforms 6:2 FTSA into perfluoroalkyl carboxylates
(PFCAs) of different chain lengths (C2–C7), yet the reaction
mechanisms were not elucidated. This study used density functional
theory (DFT) calculations to map the entire reaction path of 6:2 FTSA
initiated by •OH and experimentally verified the
theoretical results. Optimal reaction pathways were obtained by comparing
the rate constants calculated from the transition-state theory. We
found that 6:2 FTSA was first transformed to C7 PFCA and C6F13•; C6F13• was then further reacted to C2–C6 PFCAs. The
parallel addition of •OH and O2 to CnF2n+1• was essential to producing C2–C6 PFCAs. The critical step
is the generation of alkoxyl radicals, which withdraw electrons from
the adjacent C–C groups to result in chain cleavage. The validity
of the calculated optimal reaction pathways was further confirmed
by the consistency with our experimental data in the aspects of O2 involvement, identified intermediates, and the final PFCA
profile. This study provides valuable insight into the transformation
of polyfluoroalkyl substances containing aliphatic carbons in •OH-based oxidation processes.