Assessment of the impact of chloride on the formation of chlorinated by-products in the presence and absence of electrochemically activated sulfate

Abstract

Boron-doped diamond anodes are widely used for electrochemical oxidation of contaminants, due to their excellent capability to form hydroxyl radicals as well as inorganic radicals such as chloride, phosphate, and sulfate radicals, electrogenerated when their corresponding anions are present in the influent water. Addition of sulfate increases the electrooxidation rates of organic pollutants at boron-doped diamond (BDD) anode. However, the effect of chloride on the electrooxidation is considered to be less straightforward. The anodic oxidation of chloride yields chlorine and hypochlorous acid/hypochlorite, besides chloride radicals, and may form toxic chlorine oxyanions at higher applied potentials. The present study investigates such implications of chloride ions to the electrooxidation of a natural organic matter surrogate, resorcinol, in the presence of sulfate. The presence of sulfate yielded up to eightfold enhancement in the removal of resorcinol and total organic carbon (TOC) at both acidic and neutral pH. Chloride addition to Na2SO4 inhibited the resorcinol oxidation and mineralization, whereas it enhanced resorcinol oxidation in NaNO3. However, sulfate inhibited the electrogeneration of chlorinated organics by ∼2–4 folds compared to nitrate at low chloride concentrations (i.e., [SO42−]:[Cl−]>1). Although increasing the pH from pH 2 to pH 7 yielded a two times lower amount of chlorinated organics, electrolysis in Na2SO4 anolyte enhanced the generation of undesired chlorate and perchlorate at both acidic and neutral pH.