This study uses column experiments to quantify the role of adsorption and precipitation in retarding Mn2+ mobilization in sandy aquifers under low-oxygen conditions. Flow-through columns were packed with pristine sandy sediment and were recharged by low-oxygen artificial solutions containing different Mn2+ concentrations. Outflow solutions from the column experiments were analyzed and simulated using a Mn2+ transport-reaction model. The results were used to demonstrate the importance of adsorption on Mn mobilization in three different aquifers.
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.