Measured and Modeled Removal Mechanisms for Bench-Scale Constructed Wetlands Receiving Lead Mine Water

Based on a period of 505-2595 days of operating lab-scale wetlands (bioreactors) treating simulated lead mine drainage containing Pb, Zn, and sulfate, a model of flow and metals removal, both equilibria and kinetics, was developed using PHREEQC, a freely-available aqueous geochemistry model.To develop parameters for the model, the fundamental question: "Where does the metal go?" had to be answered.Although the model predicts sulfide formation, we observed limited metal sulfides based on extractive assays.Despite the presence of sulfide in pore water, our wetlands display significant uptake of metals from the water via adsorption instead of via sulfide precipitation.Our operating results have shown that plants do not have a noticeable impact on removal of metals in these wetlands beyond perhaps recharge of organic matter by decaying plants.We speculate that beyond know inaccuracies in extractive assays; the metals undergo very slow reactions in-situ which cause changes in the form of the metals from adsorbed to less soluble forms, presumably due to sulfides present from the biological sulfate reduction.Overall, metals removal in our wetlands appears to follow a two-step process: first adsorption to the sediment, then sulfide formation.