Implications of hydrogen peroxide on bromate depression during seawater ozonation

The presence of hydrogen peroxide (H₂O₂) in ozonation process can resist the formation of carcinogenic bromate (BrO₃¯) efficiently, and the bromate depression is closely related with background water qualities, especially in high bromide-containing seawater. In this study, the freshwater and seawater were selected to investigate the effects of H₂O₂ on ozone (O₃) decomposition kinetics, bromide transformation and bromate depression, and the evolutions of BrO₃¯ under different scavengers were explored to speculate the primary bromate formation pathways. The results showed that the initial O₃ half-live period (t_1/2-O₃) in seawater was only one-sixth of that in freshwater, and its attenuation rate increased analogously with the increase of H₂O₂ concentration in both freshwater and seawater. The H₂O₂ could promote the formation of BrO₃¯ via hydroxyl radical (•OH) based bromate pathways, nevertheless higher concentration of H₂O₂ facilitated the reduction of HOBr/OBr¯ back to Br¯, resulting in 87.0% and 73.2% of BrO₃¯ retardment in freshwater and seawater, respectively. The suppression ratios of BrO₃¯ were up to 48.4% and 35.3% in freshwater with the addition of •OH and •O₂¯ scavengers, and the corresponding depressions in seawater decreased to 35.3% and 12.7%, indicating that •OH was dominant on bromate formation when the concentration of residual ozone was adequate to generate some bromine intermediates, meanwhile H₂O₂ and •O₂¯ functioned as the key reductants for bromate depression. Based on these results, the Br¯ transformation mechanisms via O₃, •OH, H₂O₂, and •O₂¯ reactions were speculated, and the feasibility of H₂O₂-ozonation was verified for the treatment of high Br¯-containing seawater.