Electrochemical oxidation of organics in sulfate solutions on boron-doped diamond electrode: Multiple pathways for sulfate radical generation

This study scrutinized the roles of sulfate radicals (SO₄^[rad]−) and peroxydisulfate (PDS) formed from SO₄²⁻ in electrochemical organic oxidation on a boron-doped diamond (BDD) electrode. The substrate-specific performance of electrochemical oxidation using SO₄²⁻ as the electrolyte aligned with the reactivity of SO₄^[rad]− produced via radiolysis- or heat-induced PDS activation, but was distinct from the non-selective oxidation efficiency observed in an aqueous ClO₄⁻ solution. A comparison of the treatment efficiencies using different electrolytes (i.e., Cl⁻, SO₄²⁻, and ClO₄⁻) showed no pronounced enhancing effect of SO₄^[rad]− on the anodic oxidation of diverse organics (except perfluorooctanoate), which implied that direct electron transfer and hydroxyl radical-induced oxidation proceeded as complementary reaction routes. Repeated electrolytic oxidation caused substantial electrolyte exchange from Cl⁻ to ClO₄⁻, which retarded organic oxidation accompanied by ClO₄⁻ accumulation. Conversely, high-yield PDS production observed when SO₄²⁻ was used instead barely reduced treatment efficiency. Together with SO₄^[rad]− detection in the electron paramagnetic resonance spectrum, a correlation between 4-chlorophenol oxidation rate and the faradaic efficiency for SO₄²⁻ formation, monitored in PDS solutions while varying the cathode material, suggested cathodic PDS activation. The electrocatalytic performance was demonstrated to be further improved with anodically formed PDS activation through naturally occurring resistive heating or combination with UV photolysis as a post-treatment step.