Underestimated role of hydroxyl radicals for bromate formation in persulfate-based advanced oxidation processes

Wanqi Xu; Congcong Ni; Ning Deng; Xin Huang

doi:10.1016/j.envres.2024.118870

Underestimated role of hydroxyl radicals for bromate formation in persulfate-based advanced oxidation processes

Environ Res. 2024 Apr 3;252(Pt 3):118870. doi: 10.1016/j.envres.2024.118870. Online ahead of print.

Authors

Wanqi Xu¹, Congcong Ni², Ning Deng³, Xin Huang⁴

Affiliations

¹ School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, China. Electronic address: xwq@shu.edu.cn.
² School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, China. Electronic address: congcongni95@shu.edu.cn.
³ School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, China. Electronic address: dengning@shu.edu.cn.
⁴ School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, China. Electronic address: huangxin2008@shu.edu.cn.

PMID: 38579994
DOI: 10.1016/j.envres.2024.118870

Abstract

In persulfate-based advanced oxidation processes (PS-AOPs), sulfate radicals (SO₄^•-) have been recognized to play more important roles in inducing bromate (BrO₃^-) formation rather than hydroxyl radicals (HO^•) because of the stronger oxidation capacity of the former. However, this study reported an opposite result that HO^• indeed dominated the formation of bromate instead of SO₄^•-. Quenching experiments were coupled with electron paramagnetic resonance (EPR) detection and chemical probe identification to elucidate the contributions of each radical species. The comparison of different thermal activated persulfates (PDS and PMS) demonstrated that the significant higher bromate formation in HEAT/PMS ([BrO₃^-]/[Br^-]₀ = 0.8), as compared to HEAT/PDS ([BrO₃^-]/[Br^-]₀ = 0.2), was attributable to the higher concentration of HO^• radicals in HEAT/PMS. Similarly, the bromate formation in UV/PDS ([BrO₃^-]/[Br^-]₀ = 1.0), with a high concentration of HO^•, further underscored the dominant role of HO^•. As a result, we quantified that HO^• and SO₄^•- radicals accounted 66.7% and 33.3% for bromate formation. This controversial result can be reconciled by considering the critical intermediate, hypobromic acid/hypobromate (HOBr/BrO^-), involved in the transformation of Br^- to BrO₃^-. HO^• radicals have the chemical preference to induce the formation of HOBr/BrO^- intermediates (contributing ∼ 60%) relative to SO₄^•- radicals (contributing ∼ 40%). This study highlighted the dominant role of HO^• in the formation of bromate rather than SO₄^•- in PS-AOPs and potentially offered novel insights for reducing disinfection byproduct formation by controlling the radical species in AOPs.

Keywords: Bromate; Hydroxyl radicals; Hypobromic acid/Hypobromate; Persulfate-based advanced oxidation processes; Sulfate radicals.