Role of Manganese in Accelerating the Oxidation of Pb(II) Carbonate Solids to Pb(IV) Oxide at Drinking Water Conditions

Pb­(II) carbonate solids are corrosion products that form on the inner surfaces of lead service lines (LSLs) and can be oxidized by free chlorine to form Pb­(IV) oxide (PbO2). The formation of PbO2 can maintain low dissolved lead concentrations in drinking water, but PbO2 can dissolve if a free chlorine residual is not maintained. Experiments demonstrated that the oxidation of Pb­(II) carbonate by free chlorine was faster with manganese (Mn). Without Mn­(II), the oxidation of Pb­(II) carbonate was an autocatalytic process. With Mn­(II), the overall oxidation rate was 2 orders of magnitude faster than without Mn­(II). X-ray diffraction and free chlorine consumption profiles indicated that δ-MnO2 was formed within several minutes of the reaction of Mn­(II) with free chlorine, and δ-MnO2 catalyzed the oxidation of Pb­(II) carbonate by free chlorine. Free chlorine consumption profiles for Pb­(II) carbonate with and without Mn­(II) were interpreted based on the kinetics and stoichiometry of the underlying chemical reactions. These findings highlight the importance of Mn in accelerating the formation of PbO2 in water with Pb­(II) carbonate solids and free chlorine, and it may help explain why PbO2 is observed on LSLs of some but not all water systems that use free chlorine.