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.

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