Effective blockage of chloride ion quenching and chlorinated by-product generation in photocatalytic wastewater treatment

Photocatalytic degradation of pollutants in high salinity wastewater usually shows extremely low activities and produces highly toxic by-products, often related to the presence of excess chloride ion (Cl−). Herein, we report for the first time that involvement of Cl− (quenching active species and generating chlorinated by-products) could be effectively blocked during photocatalytic processes. Based on a comprehensive investigation of its mechanism, we found that Cl− could quench superoxide radicals (O2−) through a cyclic indirect quenching model with holes (h+) and hydroxyl radicals (OH) quenching as "initiators". Thus, scavenging h+ and OH could successfully block the chain reactions between Cl− and O2−, and photocatalytic degradation of methyl orange (a refractory dye, with O2− as dominant attacking species) could be enhanced by nearly 50 times, even when Cl− content was up to 10 wt%. More importantly, both HPLC-MS analyses and DFT calculation validated that, by blocking its quenching effect, Cl− could be successfully excluded from the pollutant degradation processes, thus preventing the generation of toxic chlorinated by-products. This work provides new insights into control of chlorinated by-products and proposes feasible strategies to extend photocatalytic technology in high salinity wastewater.