Geochemical modeling considering oxidation rate of manganese and surface complexation to manganese precipitate for treatment of acid mine drainage

In Japan there are 97 abandoned mines that generate mine drainage and most of them are expected to need wastewater treatment for metal removal semi-permanently. In order to operate efficient and economically sustainable treatments, the mechanism involved in the removal of metal must be elucidated in detail for each mine. Conventional models represent the concentration of metals in wastewater based on chemical equilibrium calculations containing the surface complexation model to describe the adsorption onto ferrihydrite and aluminum hydroxide. However, the concentration of manganese and cadmium in weak alkaline area could not be predicted with satisfactory accuracy with the conventional methods because they do not take into account the oxidation/precipitation of manganese and the consequent surface complexation of cadmium onto the manganese precipitate. Therefore in this study, we developed a new model which includes the oxidation rate of Mn²⁺ and the surface complexation model of cadmium onto manganese precipitate. As for the oxidation rate of manganese, the rate constant was determined by experiments at different pH and/or manganese concentrations and by fitting the experimental results with a first order kinetic model.. The surface complexation model to describe the adsorption of cadmium onto manganese precipitate was determined by assessing the exchange capacity of manganese precipitate through experimental measurements of specific surface area by BET analysis and through site density data from literature. The equilibrium constant for the adsorption of cadmium onto manganese precipitate was determined by performing removal experiments on synthetic wastewater containing cadmium. Results showed that the model built in this study can reproduce accurately the concentration profiles of metals in actual neutralization treatments of acid mine drainage. Furthermore, the obtained results highlighted that taking into account the kinetics of manganese oxidation and the surface complexation model of cadmium is essential for an accurate modeling in weak alkaline area.