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

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.