Dechlorination Triggers Defluorination: Novel Defluorination Pathways of Chlorinated Polyfluoroalkyl Substances (Cl-PFAS) by Nanoscale Zerovalent Iron

Chlorinated polyfluoroalkyl substances (Cl-PFAS) are increasingly detected in contaminated environments, and the relative ease of C-Cl cleavage offers opportunities for defluorination. Herein, we demonstrated, for the first time, the dechlorination of Cl-PFAS by nanoscale zerovalent iron (nZVI), and we revealed that such dechlorination can induce indirect defluorination when Cl is substituted at the middle of the perfluorocarbon chain. The carbanion (-^⊖CF-CF₂-), generated via two successive single-electron transfer steps, is the key intermediate enabling defluorination through β-elimination of F^- to form alkenyl products (-CF═CF-). The extent of defluorination was governed by the competitive reactivity with proton abstraction of carbanion to form hydrogenolysis products (-HCF-CF₂-). Through this novel pathway, an indirect defluorination of up to 25.3% was achieved for the investigated polychlorofluorocarboxylic acids within 120 h using nZVI. The reactivity for dechlorination at varying positions and subsequent β-elimination of F^- can be theoretically described using dissociative electron transfer and transition state theories, respectively. The lack of defluorination in ωCl-PFAS was attributed to the much higher activation energy required for β-elimination from the carbanion (^⊖CF₂-CF₂-) compared with proton abstraction. The alignment of theoretical calculations with experimental results provides a theoretical framework to predict dechlorination and defluorination reactivity of Cl-PFAS, facilitating the design of rapidly degradable alternatives.