Interfacial electron transfer and hydroxyl radical pathways in sulfate-enhanced perfluorobutanoic acid degradation on boron-doped diamond.

Electrochemical oxidation on boron-doped diamond (BDD) anodes is effective for per- and polyfluoroalkyl substances (PFAS) removal, yet the role of supporting electrolytes in regulating PFAS transformation at the anodic interface remains unclear. Here, perfluorobutanoic acid (PFBA) was used as a representative short-chain PFAS to evaluate electrochemical degradation in BDD systems with five common electrolytes. Na2SO4 exhibited the highest PFBA removal and defluorination efficiencies, with an apparent rate constant 1.6 and 2.6 times those of the Na2S2O8 and NaNO3 systems, respectively. The Na2SO4 system also maintained degradation efficiencies above 96% across initial PFBA concentrations from 4.67 × 10-4 to 4.67 × 10-1 mM. Evidence indicates that sulfate enhancement is not primarily governed by sulfate-radical or persulfate-mediated pathways, but arises from interfacial regulation at the BDD anode. Specifically, sulfate regulates the electrode-solution interface, suppresses oxygen evolution, facilitates anodic direct electron transfer to initiate PFBA oxidation, and sustains effective •OH participation in the subsequent chain defluorination process. Non-target screening, targeted quantification, and in situ analysis support a decarboxylation-initiated pathway involving CF2O formation and stepwise defluorination. These findings provide mechanistic insight for electrolyte selection and interfacial design in electrochemical PFAS treatment systems.