Multistage Generation Mechanisms of Reactive Oxygen Species and Reactive Chlorine Species in a Synergistic System of Anodic Oxidation Coupled with in Situ Free Chlorine and H2O2 Production

Abstract

Electro-oxidation (EO) is an efficient approach to removing refractory organics in wastewater. However, the interference from chlorine ions (Cl^–) can generate reactive chlorine species (RCS), potentially leading to the production of undesirable chlorinated byproducts. A novel approach involving the cathodic oxygen reduction reaction (ORR) for in situ H₂O₂ production has emerged as a promising strategy to counteract this issue. This study systematically investigated the dynamics and transformation of RCS and reactive oxygen species (ROS) in an ORR/chloride-containing EO (EO-Cl) system, elucidating their respective roles in organic removal and chlorinated byproduct minimization. Distinct generation rates and patterns were observed for free chlorine and H₂O₂ in the ORR/EO-Cl system. The rapid generation of free chlorine at the anode quickly reached a dynamic equilibrium, which contrasted with the moderate, continuous cathodic production of H₂O₂, resulting in considerable H₂O₂ accumulation over time. This difference established kinetics-driven ROS and RCS formation and distribution, influencing the subsequent organic degradation process. Three distinct stages were identified in the degradation process. In stage I, free chlorine was the primary species, along with reactive species including Cl₂^•–, ¹O₂, ClO^•, HO^•, and Cl^•. In stage II, the gradual accumulation of H₂O₂ consumed free chlorine, favoring the formation of ¹O₂ and HO^•. In stage III, excessive H₂O₂ quenched the free radicals. Insights into these multistage mechanisms reveal that the rapid degradation of chlorinated byproducts by ¹O₂ and HO^• occurs in stage II of the ORR/EO-Cl system.