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 O3 is consumed than stoichiometrically required indicating that higher manganese oxidations states can also be reduced by O3 (e.g., MnO2 +  + O3→Mn2 + + 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). Mn2 + -containing waters are accompanied by Fe2 +  that also induces MnO4− formation. Fe2 +  reacts 650-times faster with O3 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.