Quantitative modeling incorporating surface complexation for the treatment of acid mine drainage

Some of the many kinds of sulfide ore previously produced in Japan have generated acid mine drainage (AMD) by exposure to oxygen in groundwater or air after closure of the mines. AMD is typically treated by neutralization with lime (Ca(OH)₂) or calcium carbonate (CaCO₃). This process can remove many toxic elements from the AMD in a simple way, but the amount of neutralizer used is often decided based on experience because the removal mechanism of each element has not been quantified. Seasonal variation in the quality and quantity of AMD is considerable, and for many mines their AMD needs to be treated semi-permanently. Thus, more efficient and stable treatment is required in order to reduce chemical input and sludge generation. With a view to construction of a more efficient AMD treatment method and process, this study aims to develop a simulator that can represent water quality. To this end, a quantitative model that can represent changes in pH and the concentration of each element was constructed using a chemical equilibrium calculation that incorporates surface complexation by ferrihydrite and aluminum hydroxide, which have the potential to remove many kinds of toxic elements. For the surface complexation model of each hydroxide, the double layer model was used. By comparing the calculated results with experimental results obtained from neutralization of two kinds of actual AMD, it was confirmed that the quantitative model constructed in this study could successfully represent the changes in pH and residual concentration of each element.