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

Abstract

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.

Does KMnO4 preoxidation reduce the genotoxicity of disinfection by-products?

“…The MnO 2 present during chlorination included residual MnO 2 from KMnO 4 preoxidation and newly formed MnO 2 from Mn(II) chlorination. Based on the previous literature, the rate constant of the reaction of Tyr with chlorine (10 5 ) (Na and Olson, 2007;Armesto et al, 1993) was approximately 3e4 orders of magnitude higher than that of Tyr with MnO 2 (10 1 -10 2 ) (Altaf et al, 2009 (iii) Cl 2 consumption competition between Mn(II) and BP-4/ intermediates/residual Tyr Mn(II) can be oxidized by chlorine as previously shown (Allard et al, 2013;Hao et al, 1991). During chlorination, dissolved Mn(II) is initially oxidized to MnO 2 .…”

“…In the second step (chlorination), the added chlorine and MnO 2 continue to react with residual precursors/intermediates. The Mn(II) formed during preoxidation can be adsorbed onto the reactive sites of the precipitated MnO 2 and oxidized by chlorine to form MnO 2 (Allard et al, 2013;Hao et al, 1991). This autocatalytic reaction competes with other oxidation reactions of residual precursors/intermediates with MnO 2 and chlorine.…”

Preliminary Assessment of Ferrate Treatment of Metals in Acid Mine Drainage

“…For example, the estimate of k 2 by Goodwill et al (2016b) does not account for speciation differences between Mn +2 , and Mn(OH) +1 (aq) as a function of pH. In general, the kinetics of Mn oxidation with various oxidants decreases as pH decrease due to increased fractions of Mn +2 versus Mn(OH) +1 (pK = 10.6) (Morgan and Stumm, 1964; Knocke et al, 1987; Allard et al, 2013). It is also possible that Fe and Mn formed very small (<20 nm) particles that were operationally defined as dissolved.…”

Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water

“…Bromine oxidation rates are approximately 6 times of the chlorine oxidation rates with respect to Cr(OH) 3(s) (reaction 10 versus reaction 1). Prior studies demonstrated that HOBr is more electrophilic than HOCl and, therefore, more reactive toward electron-rich compounds including Mn(II), 53 ammonia, 76 phenols, 77,78 steroid estrogens, 79 and dissolved organic matter with amines and phenolic moieties. 80,81 In the redox system of this study, the reactive surface sites of Cr(III) solids serve as electron-rich centers to react with HOBr.…”

“…50,51 Wastewater discharges from energy and oil sectors in United States can increase bromide concentration by as much as 20 times in the future. 52 Although the occurrence of bromide raises concerns on metal release and disinfection by-products formation, 53,54 its impact on Cr release has not been explored. The objectives of this study were to investigate the kinetics and mechanisms of Cr(VI) formation via the oxidation of three model Cr(III) solids by chlorine in drinking water, with emphasis on the effects of pH and bromide, and to examine the formation of Cr intermediate species during the oxidative process.…”