Efficiency and mechanism of pollutant degradation and bromate inhibition by faceted CeO2 catalyzed ozonation: experimental and theoretical study

Abstract Reduction of potential carcinogenic bromate formation is a big challenge for the application of ozone in the treatment of Br−-containing wastewater. Three CeO2 nanocrystals individually with exposed (100), (110), and (111) facets were prepared and adopted as catalysts for the ozonation of Br—containing wastewater. It is found that faceted CeO2 greatly inhibited the bromate formation and simultaneously enhanced the pollutant removal. The catalytic activity follows the order of (100) > (110) > (111). In the presence of CeO2(100), there is 71.4±2.9% of sulfamethoxazole (SMZ) degradation (only 24.9±1.8% in ozone alone). Moreover, the generation of bromate was reduced by 29.6%, 38.8%, 52.4%, 66.4%, respectively, when the initial Br− concentration is 1.0, 2.0, 34.0, 65.0 mg L-1. Meanwhile, the antibacterial-active groups in most of the identified intermediates were destroyed and much lower biotoxicity of the treated wastewater is observed. The good reducibility and Lewis acidity of CeO2(100) favors the activation of ozone and the redox cycle of ceria. DFT calculations show that ozone facilely dissociates into surface O and O22- (O22-⇌•O2-) through interaction with the (100) facet. The experimental results demonstrate that •O2−, its derivatives (i.e. •OH and 1O2) and surface O are generated and get involved in SMZ degradation. It is noted that •O2− also functions as a key reductant to convert Ce4+ to Ce3+ which reduce HBrO/BrO− and BrO3− to Br− and therefore inhibit the formation of bromate. These results indicate that CeO2(100) catalyzed ozonation is a promising advanced oxidation process for the treatment of Br-containing wastewater, and crystal facet engineering is an efficient strategy to enhance the catalytic performance.