Laboratory study on the adsorption of Mn2+ on suspended and deposited amorphous Al(OH)3 in drinking water distribution systems

Abstract

Manganese (II) is commonly present in drinking water. This paper mainly focuses on the adsorption of manganese on suspended and deposited amorphous Al(OH)3 solids. The effects of water flow rate and water quality parameters, including solution pH and the concentrations of Mn2+, humic acid, and co-existing cations on adsorption were investigated. It was found that chemical adsorption mainly took place in drinking water with pHs above 7.5; suspended Al(OH)3 showed strong adsorption capacity for Mn2+. When the total Mn2+ input was 3 mg/L, 1.0 g solid could accumulate approximately 24.0 mg of Mn2+ at 15 °C. In drinking water with pHs below 7.5, because of H+ inhibition, active reaction sites on amorphous Al(OH)3 surface were much less. The adsorption of Mn2+ on Al(OH)3 changed gradually from chemical coordination to physical adsorption. In drinking water with high concentrations of Ca2+, Mg2+, Fe3+, and HA, the removal of Mn2+ was enhanced due to the effects of co-precipitation and adsorption. In solution with 1.0 mg/L HA, the residual concentration of Mn2+ was below 0.005 mg/L, much lower than the limit value required by the Chinese Standard for Drinking Water Quality. Unlike suspended Al(OH)3, deposited Al(OH)3 had a much lower adsorption capacity of 0.85 mg/g, and the variation in flow rate and major water quality parameters had little effect on it. Improved managements of water age, pipe flushing and mechanical cleaning were suggested to control residual Mn2+.