Mn(II) Dissolution from the Surface of Manganese Oxide-Coated Filter Media

Abstract

Manganese oxides on the surface of filter media adsorb soluble manganese and organic matter in the water. Previous studies have suggested that surface coated manganese removes soluble manganese by adsorption and subsequent oxidation in the presence of oxidants at neutral pH's. However, the adsorbed Mn(II) and surface coated manganese oxide coatings can be dissolved from the surface of filter media under anoxic conditions and/or acidic conditions in the presence of adsorbed organics on the surface of the filter media. The first objective of this investigation was to determine what conditions might lead to dissolution of adsorbed Mn(II) and Mn02(s) from the surface of manganese-coated filter media. The second objective was to develop a better understanding of manganese dissolution by organics and solution pH in the absence and presence of free chlorine. The third objective was to recommend procedures to control the release of manganese from the filter media. Experiments were performed at pH 5.5 to 8.5 using TOC concentrations similar to those encountered in water treatment plants in the absence of and presence of free chlorine. Humic acid was used as the source of total organic carbon (TOC) in the feed water. The results demonstrated that dissolution of Mn(II) from the surface of manganese-coated filter media was a function of pH of the applied solution. Mn release was significant at pH values below 6.0, but was below 0.05 mg/l (the MCL for Mn) at neutral and alkaline pHs. TOC concentrations in the applied solution did not appear to influence the manganese dissolution rate significantly. Under acidic conditions, the application of free chlorine to the influent at a dosage sufficient to maintain a free chlorine residual controlled the release of manganese in the filter effluent. There was a potential for reduction of MnO2(s) and release of Mn^2+ to occur under acidic conditions unless chlorine was fed continuously.