Removal of sulfamethoxazole by electrochemically activated sulfate: Implications of chloride addition

Electrochemical oxidation is considered to be an attractive alternative to chemical oxidation for the treatment of polluted water. Given the of ability of boron-doped diamond (BDD) electrodes to generate hydroxyl radicals (OH), they are often selected for the degradation of persistent organic contaminants. Recently, BDD anodes have been demonstrated to form strong oxidants, sulfate radicals (SO₄^-), directly from sulfate ions. In this study, electrochemical activation of sulfate to SO₄^- at BDD anodes enhanced the removal of an antibiotic sulfamethoxazole (SMX). The rate of SMX oxidation was 6 times higher in sulfate anolyte compared to inert nitrate anolyte. Addition of chloride accelerated the disappearance of SMX in both anolytes due to electrochlorination. Yet, mineralization efficiency was decreased, particularly in Na₂SO₄ anolyte due to the scavenging of SO₄^- by Cl^-. Electrogenerated SO₄^- yielded nitroso- and nitro-derivatives, which were not observed in the absence of sulfate. The peak intensities of chlorinated TPs were three orders of magnitude lower in Na₂SO₄ than in NaNO₃ anolyte, suggesting that addition of sulfate may lower the formation of chlorinated organics. However, attention should be paid to the formation of inorganic byproducts, as the formation rates of toxic chlorate and in particular perchlorate were higher in Na₂SO₄ anolyte.