Study on the mechanism of cerium oxide catalytic ozonation for controlling the formation of bromate in drinking water

This study evaluated the formation of bromate (BrO3) in the catalytic ozonation with cerium oxide (CeO2) compared with single ozonation and several catalytic ozonation with metal oxides (i.e. magnesium oxide (MgO) and synthetic goethite (FeOOH)). The results showed that the least BrO3- was generated in the O-3/CeO2 system. Primary experiments have confirmed that both Br- and BrO3- could be hardly adsorbed by CeO2, and thus the inhibition of BrO3- in the O-3/CeO2 process was mainly ascribed to the effect of CeO2 on the ozone decomposition and subsequent hydroxyl radical (HO center dot) formation in the bulk solution. Firstly, the degradation of para-chloronitrobenzene (pCNB) was examined and the results showed that less pCNB was degraded by O-3/CeO2 than single ozonation, suggesting that HO center dot formation was inhibited in the O-3/CeO2 system. Furthermore, the effect of inorganic anions (i.e. sulfate (SO42-) and nitrate (NO3-)) on the systems was investigated. It was found that SO42- had a negative effect on the BrO3- inhibition in the O-3/CeO2 process, which was due to that SO42- inhibited the adsorption of O-3 and oxygen-containing species by CeO2 through competing the active sites of CeO2. Moreover, the inhibition of BrO3- formation in the catalytic ozonation with the CeO2 samples calcined at different temperatures was also studied. The results showed that the efficiency of inhibition decreased in the following sequence CeO2 (450 degrees C) > CeO2 (650 degrees C) > CeO2 (250 degrees C). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses on the CeO2 specimens showed that CeO2 (450 degrees C) had the highest Ce(IV) to Ce(III) ratio and the least lattice oxygen and adsorbed oxygen amount. Therefore, a new mechanism about the inhibition of BrO3- formation in the O-3/CeO2 system was proposed. Both O-3 molecules and some oxygen-containing intermediates from O-3 decomposition in solution will be adsorbed on the active sites of CeO2, and the less lattice and adsorbed oxygen also promote the adsorption of oxygen-containing species on the CeO2 surface. This will result in the inhibition of O-3 decomposition into HO center dot in solution and thus inhibition of BrO3- formation. This study improves our understanding of the O-3/CeO2 process for controlling BrO3- formation and also guides the practical application.