Permanganate formation in the reactions of ozone with Mn(II): a mechanistic study

Ozone treatment of Mn^2 +-containing waters is reported to lead to MnO₄⁻ formation. This complex reaction is investigated on the model level. In neutral solution, the main product is MnO₂ and only 10% MnO₄⁻ are formed even at high ozone excess. In acid solution, however, or in neutral solution when Mn^2 + is complexed by polyphosphate, oxalate, sulfate, bicarbonate or phosphate, the reaction proceeds to MnO₄⁻ in an up to 100% yield. In this reaction, three-times more O₃ is consumed than stoichiometrically required indicating that higher manganese oxidations states can also be reduced by O₃ (e.g., MnO^2 + + O₃→Mn^2 ++ 2O₂). In acid solution or in the presence of a complexing agent and with Mn(II) in excess, Mn(III) is formed by conproportionation of Mn(II) with Mn(IV). Mn^2 +-containing waters are accompanied by Fe^2 + that also induces MnO₄⁻ formation. Fe^2 + reacts 650-times faster with O₃ than Mn^2 +, and the oxidation of Mn^2 + by Fe(IV) to Mn(III) can circumvent MnO₂ formation. Mn(III) can then be oxidized to MnO₄⁻. The thermodynamics of the suggested reactions is discussed. The MnO₄⁻ formation in drinking water and wastewater is most likely due to the presence of Fe^2 + in these waters rather than due to complexation of Mn^2 + by NOM or inorganics.