Implications of hydrogen peroxide on bromate depression during seawater ozonation

The presence of hydrogen peroxide (H 2 O 2 ) in ozonation process can resist the formation of carcinogenic bromate (BrO 3 ¯) 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 H 2 O 2 on ozone (O 3 ) decomposition kinetics, bromide transformation and bromate depression, and the evolutions of BrO 3 ¯ under different scavengers were explored to speculate the primary bromate formation pathways. The results showed that the initial O 3 half-live period (t 1/2 -O 3 ) in seawater was only one-sixth of that in freshwater, and its attenuation rate increased analogously with the increase of H 2 O 2 concentration in both freshwater and seawater. The H 2 O 2 could promote the formation of BrO 3 ¯ via hydroxyl radical (•OH) based bromate pathways, nevertheless higher concentration of H 2 O 2 facilitated the reduction of HOBr/OBr¯ back to Br¯, resulting in 87.0% and 73.2% of BrO 3 ¯ retardment in freshwater and seawater, respectively. The suppression ratios of BrO 3 ¯ were up to 48.4% and 35.3% in freshwater with the addition of •OH and •O 2 ¯ 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 H 2 O 2 and •O 2 ¯ functioned as the key reductants for bromate depression. Based on these results, the Br¯ transformation mechanisms via O 3 , •OH, H 2 O 2 , and •O 2 ¯ reactions were speculated, and the feasibility of H 2 O 2 -ozonation was verified for the treatment of high Br¯-containing seawater.