The adsorption of aqueous heavy metals on inorganic minerals

Two models have been used to explain the in-situ control of heavy metals: (1) solubility controls, where precipitation of a solid phase occurs under varying metal and ligand concentrations, and (2) surface chemical controls, where adsorption or exchange occurs at the solid/solution interface. Based on experiments presented in this paper, surface chemical controls can account for the removal of heavy metals, particularly Zn(II), from metal solutions which are undersaturated with respect to the hydroxide, oxide, or other controlling solid phases. Adsorption isotherms are presented for varying solution pHs, total metal ion concentrations, ionic strengths, and mineral substrates. The minerals chosen for illustration are SiO₂, TiO₂, FeOOH, Al₂O₃, MnO_x and HgS which range widely in surface acidity, electrical double layer properties, specific surface areas, and surface functional groups.