Formation and control of bromate in sulfate radical-based oxidation processes for the treatment of waters containing bromide: A critical review
2020
Guan, Chaoting | Jiang, Jin | Pang, Suyan | Zhou, Yang | Gao, Yuan | Li, Juan | Wang, Zhen
Sulfate radical-based advanced oxidation processes (SR-AOPs) show a good prospect for effective elimination of organic contaminants in water due to the powerful oxidation capability and good adaptability of sulfate radical (SO₄•⁻). However, great concerns have been raised on occurrence of the carcinogenic byproduct bromate (BrO₃⁻) in SR-AOPs. The present article aims to provide a critical review on BrO₃⁻ formation during bromine (Br)-containing water oxidation by various SR-AOPs. Potential reaction mechanisms are elaborated, mainly involving the sequential oxidation of bromide (Br⁻) by SO₄•⁻ to Br-containing radicals (e.g., bromine atom (Br•)) and then to hypobromous acid/hypobromite (HOBr/OBr⁻), which acts as the requisite intermediate for BrO₃⁻ formation. Some key influencing factors on BrO₃⁻ formation are discussed. Particularly, dissolved organic matter (DOM) as a component ubiquitously present in aquatic environments shows a significant suppression effect on BrO₃⁻ formation, primarily attributed to the reduction of Br• by DOM to Br⁻. The reaction of Br• with DOM can hardly produce organic brominated byproducts, while their formation is mainly due to the bromination of HOBr/OBr⁻ generated through nonradical pathways such as the direct reaction of Br⁻ with oxidants (e.g., peroxymonosulfate (PMS)) or other reactive species derived from catalytic activators (e.g., Co(III) in the Co(II)/PMS process). The debromination of brominated pollutants during their oxidation by SO₄•⁻ results in the release of Br⁻, which, however, is not further transformed to BrO₃⁻ until coexisting organic matters are mineralized nearly completely. Furthermore, possible strategies for control of BrO₃⁻ formation in SR-AOPs as well as the future research needs are proposed.
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