Implications of hydrogen peroxide on bromate depression during seawater ozonation.

The presence of hydrogen peroxide (HO) in ozonation process can resist the formation of carcinogenic bromate (BrO¯) 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 HO on ozone (O) decomposition kinetics, bromide transformation and bromate depression, and the evolutions of BrO¯ under different scavengers were explored to speculate the primary bromate formation pathways. The results showed that the initial O half-live period (t-O) in seawater was only one-sixth of that in freshwater, and its attenuation rate increased analogously with the increase of HO concentration in both freshwater and seawater. The HO could promote the formation of BrO¯ via hydroxyl radical (•OH) based bromate pathways, nevertheless higher concentration of HO facilitated the reduction of HOBr/OBr¯ back to Br¯, resulting in 87.0% and 73.2% of BrO¯ retardment in freshwater and seawater, respectively. The suppression ratios of BrO¯ were up to 48.4% and 35.3% in freshwater with the addition of •OH and •O¯ 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 HO and •O¯ functioned as the key reductants for bromate depression. Based on these results, the Br¯ transformation mechanisms via O, •OH, HO, and •O¯ reactions were speculated, and the feasibility of HO-ozonation was verified for the treatment of high Br¯-containing seawater.