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 (O-center dot(2)-) 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 O-center dot(2)-, and photocatalytic degradation of methyl orange (a refractory dye, with O-center dot(2)- 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.