LAUSR

Abstract The ammonia-chlorine (NH3-Cl2) and chlorine-ammonia (Cl2-NH3) pretreatment strategies are commonly used for controlling bromate formation during the ozonation of bromide-containing waters. In this study, the roles of bromine-containing haloamines in ozonation after both strategies are identified at different pHs and initial bromide dosages. Bromide is primarily masked as bromochloramine (NHBrCl) and monobromamine (NH2Br) in the NH3-Cl2 and Cl2-NH3 strategies, respectively, and partially masked as dibromamine (NHBr2) with similar concentrations (p > 0.05) in both strategies. NHBrCl and NHBr2 are 4 times more stable than NH2Br in the presence of ozone, while they are all recalcitrant to HO , and thus make the NH3-Cl2 strategy control the bromate formation better than the Cl2-NH3 strategy. At pH 6, the Cl2-NH3 strategy reduces 15–36% of the bromate formation comparing with those at pHs 7 and 8, because more NH2Br is transformed to stable NHBr2, while the NH3-Cl2 strategy is less pH-dependent, because the NHBrCl concentrations remain the same at different pHs (p > 0.05). At initial bromide concentrations of 6.25–18.75 µM, the Cl2-NH3 strategy shows no difference in controlling the bromate formation (p > 0.05). However, ~2.5-time more bromate is formed at the bromide concentration of 18.75 µM comparing with those at 6.25 and 12.5 µM, in the NH3-Cl2 strategy, because the 6-min NH3-Cl2 pretreatment is insufficient to mask all bromide at high concentrations as NHBrCl. This study demonstrates that the masking of larger amount of bromide as NHBrCl and NHBr2 in ozonation after either strategy is essential in controlling the bromate formation.