Clarifying the contributor of ammonia oxidation in chlorine-mediated electrochemical process

Chlorine-mediated electrochemical advanced oxidation (Cl-EAO) is a promising means of ammonia removal from wastewater. However, the role of active chlorine and chlorine radicals in the Cl-EAO process remains unclear. Carbonate ion (Na2CO3) is employed in this study to understand the main contributor of ammonia oxidation, and the EPR determination, free radical inhibition assay experiments, and probe experiments are used to clarify the role of chlorine radicals. The results of adding a low concentration of Na2CO3 (10 mM) to the Cl-EAO system show that active chlorine production is inhibited, but ammonia oxidation is promoted. Meanwhile, the results of adding a high concentration of Na2CO3 (50 mM) in the Cl-EAO system show that active chlorine levels increase, but ammonia oxidation rate is significantly inhibited. Obviously, the active chlorine production rate has little effect on ammonia oxidation, and the free radical detection and inhibition assays show that only 9.51% of oxidization is by active chlorine, proving that breakpoint chlorination oxidation is not the main pathway, while the chlorine radicals (Cl• and ClO•) are the main contributors, and the contribution of Cl• is comparable to ClO•. The chlorine radicals-contributing ammonia oxidation process follows zero-order kinetics, and ammonia oxidation process in this system is revisited based on the evidence from this study.