Mobilization and retardation of reduced manganese in sandy aquifers: Column experiments, modeling and implications

The experimental data showed that adsorption of Mn2+ on the column solids was the main control on the Mn2+ breakthrough behavior. Nevertheless, up to 20% of the Mn that entered the experimental columns was precipitated as MnCO3. The rate of MnCO3 precipitation (kp = 0.04 h−1) was found to be ∼3 orders of magnitude slower than the rate of Mn2+ adsorption (ka = 10-200 h−1). Given the slow mineral-precipitation kinetics, the water flow rate is critical in determining the potential of MnCO3 precipitation in immobilizing Mn within an aquifer. A scale-up of the Mn retardation time found in the column experiments to natural aquifer conditions, suggests that adsorption is responsible for the prolonged retardation of Mn2+ observed in three different sandy aquifers. An important practical conclusion of this study is that the environmental response to perturbation in the hydro-geochemical regime of an aquifer might be delayed for several decades.