Study On The Evolution Of Chloride And Oxygen Ions In The Process Of DSA Anodic Oxidation And Cathodic Reduction And The Effect On COD Masking

Shaped stable electrodes（DSA）have become the most widely used anode material in electrochemical water treatment because of their stable performance,low cost and good electrocatalytic performance.However,during the electrochemical oxidation of chlorinated wastewater using DSA electrodes,the Cl^-in the water is gradually oxidized to form large amounts of oxychlorides.While the formation of oxychlorides has received much attention in various electrochemical treatment systems,their interference with COD measurements has not received much attention.This can lead to a serious overestimation of electrochemical COD removal performance and create the illusion that effluents treated by electrocatalysis meet COD discharge standards.Therefore,the aim of this paper is to investigate the generation of oxychlorides in different DSA electrode systems and their interference with the determination of COD removal performance of different DSA electrode systems,and to control the generation of oxychlorides while achieving enhanced performance of the electrocatalytic system by constructing a cathode-anode synergistic electrocatalytic system.To this end,Ti/EBNTA/RuO₂-IrO₂-SnO₂-Sb₂O₅ anodes with high catalytic performance were prepared and three different cathode materials（Ti,stainless steel and Cu/Zn）were used to investigate the differences in the distribution of oxychlorides and the removal performance（phenol,COD and TOC）in the different cathode electrochemical oxidation systems.It was found that the removal of TOC was essentially the same for different cathode systems when the current density was 20-40m A cm^-2,while the removal of COD was consistent from high to low in order of Ti,stainless steel,Cu/Zn cathodes and the number of oxychlorides produced in correspondence.The experimental phenomenon that the above COD removal rates all returned to similar levels after the removal of oxychlorides（Cl O^-and Cl O₃^-）using sulphite reduction further demonstrated the interfering effect of oxychlorides generated during the electrocatalytic process on COD determination.Electrochemical analysis,electrochemical reduction experiments and tert-butyl alcohol bursting experiments were also used to propose a mechanism for the oxidation system using Cu/Zn cathodes to produce the lowest concentration of oxychlorides and the least interference with COD measurements,and the Cu/Zn cathodes were found to have the strongest reduction capacity for Cl O^-and Cl O₃^-and mainly through direct reduction.To address the shortcomings of the poor mass transfer performance of the plate electrode,this paper further develops a highly efficient penetrating three-dimensional DSA anode TFE/EBNTA/RTO based on a titanium filter membrane with titanium dioxide nanotubes as an intermediate layer,and prepares a Ti/EBNTA cathode by modifying the Ti cathode.Physical characterization by SEM and electrochemical analysis by LSV showed that the new three-dimensional anodes have a rich pore structure and high electrocatalytic activity（high oxygen precipitation potential and low chloride precipitation potential）.The change in operating mode further improves the mass transfer coefficient and the production rate of active species in the electrocatalytic system,thus increasing the efficiency of organic removal.It is worth noting that the increase in oxidation performance is accompanied by an increase in the production of oxychlorides and the masking effect of COD.However,when used with the Ti/EBNTA cathode,the production of Cl O^-and Cl O₃^-was reduced to 51%and 28%of the Ti cathode system and the error in COD determination was reduced to 7%.In addition,the use of Ti/EBNTA in the treatment of real wastewater also showed good control of Cl O^-（9.4 m M Cl O^-and 1.9 m M Cl O₃^-）and a low COD masking effect（6%）without compromising the oxidation performance.This is mainly since the generation of oxygen vacancies on the Ti/EBNTA cathode further shortens the reaction path for the direct reduction of oxychlorides at the electrode surface,reducing the energy supply required for the reduction of oxychlorides.At the same time,the Ti/EBNTA cathode exhibits better electrical conductivity,which enhances the electron transport properties and thus improves the adsorption of oxychlorides.Overall,this study suggests that electrocatalytic treatment of highly chlorinated wastewater can result in over-evaluation of COD removal performance and provides an idea and scientific basis for the construction of an oxidation system that combines high catalytic performance with realistic evaluation of COD removal performance.