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 pHini 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 pHini ≤ 6.0.