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

Abstract

The Co(II)/peroxymonosulfate (Co(II)/PMS) process, producing sulfate radicals (SO₄^•-), effectively removes organic pollutants in water, while producing a significant amount of bromate (BrO₃^-) in the presence of bromide (Br^-). This paper investigates the ammonia (NH₃) addition, chlorine-ammonia (Cl₂-NH₃) and ammonia-chlorine (NH₃-Cl₂) pretreatment strategies in controlling BrO₃^- formation in 20 min in the Co(II)/PMS process at pH 4.0. The addition of NH₃ retarded the BrO₃^- formation, but only at a reduction level of about 9.5% for NH₃ concentration of 50 μM, and was mainly attributed to the protonation of NH₃ at pH 4 (99.99% as NH₄⁺, did not react with HOBr). Both the Cl₂-NH₃ and NH₃-Cl₂ pretreatment strategies at HOCl and NH₃ dosages of 15 and 50 μM, respectively, reduced 95% or more of the overall BrO₃^- formation and retarded the BrO₃^- formation, with the NH₃-Cl₂ pretreatment strategy outperforming Cl₂-NH₃. The reduction of the BrO₃^- formation was mainly attributed to the formation of monochloramine (NH₂Cl) in both pretreatment strategies. NH₂Cl effectively outcompetes SO₄^•- to react with HOBr and forms NHBrCl, with the apparent reaction rate constant between NH₂Cl and HOBr more than 100 times faster than that between SO₄^•- and HOBr. However, the oxidation/degradation of NHBrCl in the Co(II)/PMS process reforms HOBr, and, although less in quantity, is oxidized to BrO₃^- at higher Co(II) and Br^- concentrations. Thus, the NH₃-Cl₂ and Cl₂-NH₃ pretreatment strategies inhibit the BrO₃^- formation more significantly at lower Co(II) and Br^- concentrations. In all cases, the generation of SO₄^•- in 20 min was not affected by the implementation of the three BrO₃^- pretreatment strategies.

Keywords: Acidic pH; Ammonia; Bromate; Co(II)/PMS; Monochloramine.