Advanced oxidation of bromide-containing waters: Bromate formation mechanisms
Bromate formation in ozone-based advanced oxidation processes (AOPs) was investigated by laboratory experiments in combination with kinetic modeling. Oxidant concentrations were monitored during the experiments, which allows us to account for the relative contributions of ozone and OH radical pathways. It has been shown by gamma-irradiation of bromide-containing solutions in the pH range 6-8 that bromate can be formed by a pure OH radical mechanism and that hypobromous acid/hypobromite (HOBr/ OBr-) is a requisite intermediate in bromate formation. The presence of hydrogen peroxide (H2O2), as in H2O2-based AOPs, leads to a reduction of HOBr/OBr- and therefore becomes a key reaction for the control of bromate formation. The steady-state concentration of OH radicals in AOPs is usually not high enough to compensate for this reduction reaction. Therefore, in gamma-irradiation experiments, no bromate was formed in the presence of H2O2 because OH radicals were the only possible oxidants to further oxidize HOBr/OBr-. However, in ozone-based AOPs at pH 7, where ozone is present in combination with H2O2, bromate was still formed. This was attributed to the oxidation of Br-. by O-3, which was investigated at pH 7. Our experimental findings could be best explained by a corresponding second-order rate constant k = 1.5 x 10(8) M-1 s(-1).