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-Î"[sulfite]/Î"[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.