Surface Complexation Modeling of Cd on Mn(III) Oxyhydroxide (γ-MnOOH) for Neutralizing Model of Acid Mine Drainage

Acid mine drainage (AMD) releasing from abandoned mines in Japan sometimes contains a high level of manganese (Mn) over the effluent standard (10 mg dm–3) with cadmium (Cd) and zinc. Cadmium is normally precipitate as hydroxides at pH 9–10, while it can be removed at a lower pH range of approximately 8–9 by surface complexation with metal hydroxides such as ferrihydrite and gibbsite. In this study, we examined the removal performance of Cd by manganite (γ-MnOOH) which is trivalent Mn oxide by an adsorption experiment, and found the Cd concentration decreased over pH 9.0 below the effluent standard (0.03 mg dm–3), which the surface complex equilibrium constant was Log K = –4.0. A neutralizing experiment of the AMD containing Mn (39 mg dm–3) and Cd (0.077 mg dm–3) from the X mine showed the Cd was removed at pH 8–10, and a chemical equilibrium calculation revealed the surface complexation on γ-MnOOH was the main removal mechanism. Our results therefore suggest the Mn oxidation and surface complexation with the product is the controlling factor Cd removability during the neutralization of AMD containing Mn.