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.