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

Abstract

This study evaluated the formation of bromate (Br) 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 Br was generated in the O₃/CeO₂ system. Primary experiments have confirmed that both Br⁻ and Br could be hardly adsorbed by CeO₂, and thus the inhibition of Br in the O₃/CeO₂ process was mainly ascribed to the effect of CeO₂ on the ozone decomposition and subsequent hydroxyl radical () formation in the bulk solution. Firstly, the degradation of para-chloronitrobenzene (pCNB) was examined and the results showed that less pCNB was degraded by O₃/CeO₂ than single ozonation, suggesting that formation was inhibited in the O₃/CeO₂ system. Furthermore, the effect of inorganic anions (i.e. sulfate (S) and nitrate (N)) on the systems was investigated. It was found that S had a negative effect on the Br inhibition in the O₃/CeO₂ process, which was due to that S inhibited the adsorption of O₃ and oxygen-containing species by CeO₂ through competing the active sites of CeO₂. Moreover, the inhibition of Br 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°C) > CeO₂ (650°C) > CeO₂ (250°C). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses on the CeO₂ specimens showed that CeO₂ (450°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 Br formation in the O₃/CeO₂ system was proposed. Both O₃ molecules and some oxygen-containing intermediates from O₃ 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 O₃ decomposition into in solution and thus inhibition of Br formation. This study improves our understanding of the O₃/CeO₂ process for controlling Br formation and also guides the practical application.

Keywords:

• Cerium dioxide
• Ozonation
• Bromate
• Sulfate
• Calcination

Acknowledgments

This research was supported by Major Science and Technology Program for Water Pollution Control and Treatment (2009ZX07424-005-02). We gratefully acknowledge anonymous reviewers for the valuable comments and constructive suggestions and both Dr Yongze Liu and Dr Congwei Luo for fruitful discussion.