Oxidation of Manganese(II) during Chlorination: Role of Bromide

The oxidation of dissolved manganese­(II) (Mn­(II)) during chlorination is a relatively slow process which may lead to residual Mn­(II) in treated drinking waters. Chemical Mn­(II) oxidation is autocatalytic and consists of a homogeneous and a heterogeneous process; the oxidation of Mn­(II) is mainly driven by the latter process. This study demonstrates that Mn­(II) oxidation during chlorination is enhanced in bromide-containing waters by the formation of reactive bromine species (e.g., HOBr, BrCl, Br₂O) from the oxidation of bromide by chlorine. During oxidation of Mn­(II) by chlorine in bromide-containing waters, bromide is recycled and acts as a catalyst. For a chlorine dose of 1 mg/L and a bromide level as low as 10 μg/L, the oxidation of Mn­(II) by reactive bromine species becomes the main pathway. It was demonstrated that the kinetics of the reaction are dominated by the adsorbed Mn­(OH)₂ species for both chlorine and bromine at circumneutral pH. Reactive bromine species such as Br₂O and BrCl significantly influence the rate of manganese oxidation and may even outweigh the reactivity of HOBr. Reaction orders in [HOBr]_tot were found to be 1.33 (±0.15) at pH 7.8 and increased to 1.97 (±0.17) at pH 8.2 consistent with an important contribution of Br₂O which is second order in [HOBr]_tot. These findings highlight the need to take bromide, and the subsequent reactive bromine species formed upon chlorination, into account to assess Mn­(II) removal during water treatment with chlorine