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

Ozone treatment of Mn2+-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 Mn2+ 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 O-3 is consumed than stoichiometrically required indicating that higher manganese oxidations states can also be reduced by O-3 (e. g., MnO2+ + O-3 -> Mn2+ + 2O(2)). 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). Mn2+-containing waters are accompanied by Fe2+ that also induces MnO4- formation. Fe2+ reacts 650-times faster with O-3 than Mn2+, and the oxidation of Mn2+ 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 Fe2+ in these waters rather than due to complexation of Mn2+ by NOM or inorganics.