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

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journal contribution
posted on 14.01.2020, 18:36 by Guiwei Li, Weiyi Pan, Lili Zhang, Ziqiao Wang, Baoyou Shi, Daniel E. Giammar
The chemical oxidation of dissolved Mn­(II) to Mn­(III/IV) oxides (MnOx) 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 MnOx and produced Mn–Cu mixtures (denoted as MnOx–Cu­(II)). An autocatalytic model for the reaction kinetics suggested that the freshly formed MnOx–Cu­(II) had a much higher catalytic activity than that of pure MnOx. 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 MnOx–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.