Bromate formation from sulfate radical-based oxidation of aqueous bromide and its control

Abstract

Sulfate radical-based (SO₄^•-) advanced oxidation processes (AOPs), including the UV/persulfate (UV/PDS) and Co(II)/peroxymonosulfate (Co(II)/PMS) processes, effectively removed organic pollutants in water, but produced a significant amount of bromate (BrO₃^-), a probable human carcinogen, in the presence of bromide (Br^-). This thesis work evaluated the BrO₃^- formation kinetics in another SO₄^•--based AOPs, the UV/PMS process, and investigated the kinetics and explained the mechanisms of the ammonia (NH₃) addition, chlorine-ammonia (Cl₂-NH₃) and ammonia-chlorine (NH₃-Cl₂) pretreatment strategies in controlling the BrO₃^- formation in the UV/PMS, UV/PDS and Co(II)/PMS processes. The UV/PMS process shared the same two-step BrO₃^- formation scheme as the UV/PDS process, but formed much less BrO₃^- at similar oxidation potentials. The low BrO₃^- formation was mainly due to the fast Br^- but slow hypobromous acid/hypobromite (HOBr/OBr^-) oxidation rates. The BrO₃^- formation increased linearly with increasing PMS dosages. It also increased with increasing Br^- concentrations from 0.5 to 10 μM, but decreased at 20 μM. In addition, the BrO₃^- formation was suppressed by higher pH, natural organic matter concentrations and alkalinity. In the UV/PMS and UV/PDS processes, the NH₃ addition pretreatment strategy effectively inhibited the BrO₃^- formation at 12 and 60 μM, respectively, at 2 μM Br^- and 200 μM oxidant concentrations. The inhibition was mainly attributed to the quenching of HOBr/OBr^- by NH₃ to bromamines, and was thus more effective at higher NH₃ dosages. The NH₃ addition had similar or even better BrO₃^--formation inhibition efficiency than the Cl₂-NH₃ and NH₃-Cl₂ pretreatment strategies, mainly because the addition of HOCl transformed NH₃ to monochloramine (NH₂Cl) and bromochloramine (NHBrCl), which got degraded in the UV-based processes, and thus accelerated the NH₃ consumption rates. On the other hand, in the Co(II)/PMS process, the Cl₂-NH₃ and NH₃-Cl₂ pretreatment strategies retarded and inhibited the BrO₃^- formation more significantly than the NH₃ addition, mainly because of the formation of NH₂Cl in the former two processes and the protonation of NH₃ at pH 4 (99.99% as NH₄⁺, which did not react with HOBr) in the latter process. NH₂Cl effectively outcompeted SO₄^•- to react with HOBr and formed 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 reformed HOBr, which, although less in quantity, was oxidized to BrO₃^- at higher Co(II) and Br^- concentrations. In all cases, the generation of SO₄^•- was not affected by the execution of the three pretreatment strategies.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology