Overevaluation of Electro-oxidation for Chemical Oxygen Demand Removal Using a Boron-Doped Diamond Anode: The Roles of Various Electrogenerated Oxychlorides and Countermeasure

Abstract

Electrochemical oxidation using a boron-doped diamond (BDD) anode has been extensively investigated in various wastewater treatments, where the electrogenerated oxychlorides (ClOx–, x = 1 to 4) from Cl– oxidation have been well illustrated. However, the false chemical oxygen demand (COD) reduction induced by ClOx– adulteration has been almost completely overlooked when evaluating the electrochemical wastewater treatment performance. In this study, it was revealed that all the ClOx– species except ClO4– served as strong oxidants in lieu of dichromate to degrade organic matter under digestion conditions during the COD test procedure. To eliminate the interference of ClOx– mixtures on the assessment of electrochemical COD removal performance, the modified sulfite-based reduction method was developed for the electrochemical case to remove ClOx– mixtures from the BDD-treated water samples before the COD test. It was demonstrated that the unadulterated COD removal performance of the BDD-based electrochemical system used for both synthetic and real wastewater treatments could be obtained after sulfite-based reduction of the electrogenerated ClOx– mixtures within 30 min in a sealed reaction chamber with the molar [sulfite]ini/[ClOx–]ini ratio of 1–3, where the reactants were isochronously depleted without producing oxidative radicals (SO4•– and •OH). Due to extreme lack of oxygen in the sealed ClOx–-sulfite system, radical propagation reactions were terminated and ClOx– constituents (ClO3–, ClO2–, and ClO–) were reduced by sulfite stoichiometrically via the oxygen transfer process with Cl– and SO42– as the final products. This new sulfite-based protocol could be regarded as a robust and adulteration-proof countermeasure for evaluating the real COD removal performance of an electrochemical system.