Effect of Cu(II) on Mn(II) Oxidation by Free Chlorine to Form Mn Oxides at Drinking Water Conditions

The chemical oxidation of dissolved Mn(II) to Mn(III/IV) oxides (MnO_x) can lead to the accumulation of Mn deposits in drinking water distribution systems. However, Mn(II) oxidation by free chlorine is quite slow under mild conditions (e.g., pH 7.7 and 1.0 mg/L Cl₂). This study found a significant role for Cu(II) in Mn(II) oxidation under conditions relevant to the supply of chlorinated drinking water. At pH 7.7, dissolved Cu(II) accelerated Mn(II) oxidation more than 10 times with a dose of 20 μg/L. Solid characterization revealed that during Mn(II) oxidation, Cu(II) adsorbed to freshly formed MnO_x and produced Mn-Cu mixtures (denoted as MnO_x-Cu(II)). An autocatalytic model for the reaction kinetics suggested that the freshly formed MnO_x-Cu(II) had a much higher catalytic activity than that of pure MnO_x. Solid CuO also catalyzed Mn(II) oxidation, and kinetic modeling indicated that after an initial oxidation of Mn(II) facilitated by the CuO surface, the freshly formed MnO_x-Cu(II) on CuO surface played the dominant role in accelerating further Mn(II) oxidation. This study indicates a high potential for the formation of Mn oxides at locations in a drinking water distribution system or in premise plumbing where both Mn(II) and Cu(II) are available. It provides insights into the co-occurrence of other metals with Mn deposits that is frequently observed in distribution systems.