Controlling bromate formation in the Co(II)/peroxymonosulfate process by ammonia, chlorine-ammonia and ammonia-chlorine pretreatment strategies.

The Co(II)/peroxymonosulfate (Co(II)/PMS) process, producing sulfate radicals (SO), effectively removes organic pollutants in water, while producing a significant amount of bromate (BrO) in the presence of bromide (Br). This paper investigates the ammonia (NH) addition, chlorine-ammonia (Cl-NH) and ammonia-chlorine (NH-Cl) pretreatment strategies in controlling BrO formation in 20 min in the Co(II)/PMS process at pH 4.0. The addition of NH retarded the BrO formation, but only at a reduction level of about 9.5% for NH concentration of 50 μM, and was mainly attributed to the protonation of NH at pH 4 (99.99% as NH, did not react with HOBr). Both the Cl-NH and NH-Cl pretreatment strategies at HOCl and NH dosages of 15 and 50 μM, respectively, reduced 95% or more of the overall BrO formation and retarded the BrO formation, with the NH-Cl pretreatment strategy outperforming Cl-NH. The reduction of the BrO formation was mainly attributed to the formation of monochloramine (NHCl) in both pretreatment strategies. NHCl effectively outcompetes SO to react with HOBr and forms NHBrCl, with the apparent reaction rate constant between NHCl and HOBr more than 100 times faster than that between SO and HOBr. However, the oxidation/degradation of NHBrCl in the Co(II)/PMS process reforms HOBr, and, although less in quantity, is oxidized to BrO at higher Co(II) and Br concentrations. Thus, the NH-Cl and Cl-NH pretreatment strategies inhibit the BrO formation more significantly at lower Co(II) and Br concentrations. In all cases, the generation of SO in 20 min was not affected by the implementation of the three BrO pretreatment strategies.