Modeling Manganese Oxidation with KMnO4 for Drinking Water Treatment

Manganese (Mn) in a drinking water distribution system can cause multiple aesthetic problems including discolored water and fouling or scaling of fixtures. Oxidation and solid-liquid separation processes are typically employed at a treatment plant to limit the concentration of Mn entering the distribution system. Potassium permanganate (KMnO4) is commonly used to oxidize the manganous ion (Mn+2) to manganese oxide (MnO2). In this study, a mechanistic model is applied to the oxidation of manganese at a treatment plant. Literature kinetic constants (determined with artificial water) are compared with the values obtained for the plant's natural water. The solution and surface phase oxidation rate constants determined with the natural water are two to six orders of magnitude less than those determined with the artificial water. The reduced oxidation rate in the natural water is attributed to the presence of dissolved organic matter, which can exert a competitive demand on the oxidant and interfere with the oxidation by complexing Mn+2. The development of an additional rate constant for the oxidation of dissolved organic matter improves the modeling results for KMnO4 concentration versus time, but only marginally explains the Mn+2 oxidation rate differences.