Formation and control of bromate in sulfate radical-based oxidation processes for the treatment of waters containing bromide: A critical review

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.