Anode-dependent selectivity and formation mechanisms of inorganic chlorinated byproducts in electrochemical advanced...

Anode materials significantly affect Cl-species evolution in electrochemical advanced oxidation process (EAOP) for wastewater treatment. This study investigated the formation of inorganic chlorinated byproducts and the oxidation mechanisms using four anode materials: Ir-Ta@Ti and Ru-Ir-Sn@Ti as active anodes, and PbO₂ and boron-doped diamond (BDD) as non-active anodes. The results showed that active chlorine was the dominant byproduct in the Ir-Ta@Ti and Ru-Ir-Sn@Ti systems, whereas ClO₃^- and ClO₄^- were predominantly formed in the PbO₂ and BDD systems, respectively. These byproducts accounted for approximately 92 %, 98 %, 67 %, and 89 % of the initial Cl^- concentration of 607 mg/L at 120 min with a current density of 40 mA/cm². Kinetic rate constants for each chlorinated byproduct were provided. The quenching test demonstrated direct electron transfer was the primary oxidation pathway in the Ir-Ta@Ti and Ru-Ir-Sn@Ti systems, responsible for producing ClO^-as the primary chlorinated byproduct from Cl^-. In contrast, the PbO₂ system facilitated ClO₃^- formation primarily through a multi-electron transfer from Cl^- to ClO₂^-, followed by further oxidation to ClO₃^-. In the BDD system, indirect oxidation played a dominant role in generating ClO₄^-. Notably, despite substantial ^•OH concentration being detected in PbO₂ system, ClO₄^- was barely formed, likely due to no ClO₃^- absorbed on the surface to produce ClO₃^•. The preferred chlorinated byproducts and the corresponding oxidation mechanism for each anode are summarized. It suggests selecting appropriate anodes based on their oxidizing capacity and chlorinated byproduct. This study provides insight into controlling the production of undesirable chlorinated byproducts in EAOP.