Overlooked Role of Sulfur-Centered Radicals During Bromate Reduction by Sulfite

In this work, the kinetics and mechanisms of the reductive removal of BrO₃^- by sulfite in air atmosphere were determined. BrO₃^- could be effectively reduced by sulfite at pH_ini 3.0-6.0, and the reduction rate of BrO₃^- increased with decreasing pH. The coexisting organic contaminants with electron-rich moieties could be degraded, accompanied with BrO₃^- reduction by sulfite. The reaction stoichiometries of -Δ[sulfite]/Δ[bromate] were determined to be 3.33 and 15.63 in the absence and presence of O₂, respectively. Many lines of evidence verified that the main reactions in the BrO₃^-/sulfite system in air atmosphere included the reduction of BrO₃^- to HOBr and its further reduction to Br^-, as well as the oxidation of H₂SO₃ by BrO₃^- to form SO₃^·- and its further transformation to SO₄^·-. Moreover, SO₄^·- rather than HOBr was determined to be the major active oxidant in the BrO₃^-/sulfite system. SO₃^·- played a key role in the over-stoichiometric sulfite consumption because of its rapid reaction with dissolved oxygen. However, the formed SO₃^·- was further oxidized by BrO₃^- in the N₂ atmosphere. BrO₃^- reduction by sulfite is an alternative for controlling BrO₃^- in water treatment because it was effective in real water at pH_ini ≤ 6.0.