Towards synergistic and deep defluorination of PFAS: Mechanistic insights into multiple reductive species and future perspectives.

PFAS are highly persistent because of the exceptional stability of their C-F bonds. ARPs have attracted increasing attention for PFAS destruction owing to the generation of reductive species such as eaq-, RHS, and CO2•-. This critical review summarizes their generation and characterization and comparatively examines their roles in PFAS defluorination, with emphasis on species-dependent reactivity and responses to reaction conditions. Mechanistically, eaq- preferentially undergoes electron attachment at molecular regions with greater electron-accepting ability, which may facilitate subsequent C-F activation and H/F exchange. Initial bond activation by free •H is closely related to the local radical susceptibility of PFAS, with decarboxylation favored for PFCA but much lower reactivity observed for PFSA. Surface-bound H* is generally influenced by PFAS adsorption configuration, with parallel adsorption potentially favoring H/F exchange by improving the accessibility of PFAS bonds to H*. In contrast, CO2•- may preferentially participate in PFAS degradation through radical addition reactions. These differences suggest that multiple reductive species may contribute to PFAS defluorination by either acting at different reactive regions of parent PFAS or participating sequentially in the transformation of fluorinated intermediates through H/F exchange or radical addition. Such complementary participation may help sustain C-F bond cleavage and promote deep defluorination. Available evidence for the combined participation of eaq-with RHS or CO2•- is critically evaluated, and future priorities are proposed for clarifying species-specific contributions and synergistic PFAS defluorination. This review provides a mechanistic basis for designing multi-species reductive systems.