Implications of hydrogen peroxide on bromate depression during seawater ozonation.

ABSTRACT

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 (t1/2-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.