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 HO Production.

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 HO 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 HO 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 HO, resulting in considerable HO 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 Cl, O, ClO, HO, and Cl. In stage II, the gradual accumulation of HO consumed free chlorine, favoring the formation of O and HO. In stage III, excessive HO quenched the free radicals. Insights into these multistage mechanisms reveal that the rapid degradation of chlorinated byproducts by O and HO occurs in stage II of the ORR/EO-Cl system.