Bromate minimization during ozonation: Mechanistic considerations

Bromate formation during ozonation of bromide-containing natural waters is somewhat inversely connected to the ozone characteristics: an initial fast increase followed by a slower formation rate. During the initial phase most ly OH radical reactions contribute to bromate formation, whereas in the sec ondary phase both ozone and OH radicals are important. To minimize bromate formation several control options are presented: ammonia addition, pH depre ssion, OH radical scavenging, and scavenging or reduction of hypobromous ac id (HOBr) by organic compounds. Only the two first options are applicable i n drinking water treatment. By both methods a similar effect of a bromate r eduction of approximately 50% can be achieved. However, bromate formation d uring the initial phase of the ozonation cannot be influenced by either met hod. Ammonia (NH3) efficiently scavenges HOBr to NH2Br. However, this react ion is reversible which leads to higher required NH3 concentrations than ex pected. The rate constant k(NH2Br) for the hydrolysis of NH2Br by OH- to NH 3 and OBr- was found to be 7.5.10(6) M-1 s(-1). pH depression shifts the HO Br/OBr- equilibrium to HOBr and also affects the ozone chemistry. The effec t on ozone chemistry was found to be more important for bromate formation. For a given ozone exposure, the OH radical exposure decreases with decreasi ng pH. Therefore, for pH depression the overall oxidation capacity for a ce rtain ozone exposure decreases which in turn leads to a smaller bromate for mation.