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 center dot -) 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 center dot - 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-, SO4 (2)(-), and ClO4-) showed no pronounced enhancing effect of SO4 center dot - 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 center dot - 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.