Kinetic and mechanistic aspects of selenite oxidation by chlorine, bromine, monochloramine, ozone, permanganate, and hydrogen peroxide

Selenium (mainly in the forms of selenite (Se(IV)) and selenate (Se(VI)) is a regulated drinking water contaminant, but there is little information on the kinetics and mechanisms of Se(IV) oxidation during water treatment. Species-specific and apparent second-order rate constants for the oxidation of Se(IV) at pH 7.0 were determined in buffered solutions and they decrease in the order bromine (5.8 ± 0.3 × 103 M−1 s−1) > ozone (O3, 513.4 ± 10.0 M−1 s−1) > chlorine (61.0 ± 3.6 M−1 s−1) > permanganate (2.1 ± 0.1 M−1 s−1), monochloramine (NH2Cl, (1.3 ± 0.1) × 10−3 M−1 s−1), and hydrogen peroxide (H2O2, (2.3 ± 0.1) × 10−5 M−1 s−1). The reaction stoichiometries for the reactions of Se(IV) with bromine, O3, chlorine, NH2Cl, and H2O2 are 1:1. For Mn(VII), the stoichiometries varied with pH and were 5:2, 3:2, and 1:2 for acidic, neutral, and alkaline conditions, respectively. Based on the reaction orders and stoichiometries, the corresponding Se(IV) oxidation mechanisms for various oxidants are discussed. The role of bromide for Se(IV) oxidation was also investigated during chlorination and ozonation of Se(IV)-containing water. During chlorination, bromide-catalysis enhances the rate of the oxidation of Se(IV) to Se(VI) from 50% to nearly 90% with bromide concentrations of 50 μg L−1 and 200 μg L−1, respectively, at pH 7.0 and a chlorine dose of 2.0 mg L−1 (within 15 min). During ozonation, bromide had no effect on Se(IV) oxidation. Based on the determined second order rate constants, the oxidation of Se(IV) by chlorine and ozone were successfully predicted in a natural water by a kinetic model. The second order rate constants for the same oxidants were also investigated and/or evaluated for other related anions, such as arsenite (As(III)) and sulfite (S(IV)). They decreased in the order S(IV) > As(III) > Se(IV).