THE SPECIFIC ADSORPTION OF DIVALENT Cd, Co, Cu, Pb, AND Zn ON GOETHITE

The specific adsorption of divalent Cd, Co, Cu, Pb, and Zn on goethite is measured as a function of pH. For each mole of heavy metal adsorbed approximately two moles of H+ ions are displaced from the interface. Using these results the heavy metal adsorption data are expressed as functions of the solution concentrations of both H+ and metal ions, and the interfacial reaction is described by the equation, ₂SH+M²⁺= S₂M+₂H⁺. The adsorption data are consistent with an electrochemical model of the simultaneous adsorption of H⁺ ions and divalent metal ions on to the oxide. The intrinsic affinities of the metal ions for the oxide surface increase in the order, Cd < Co < Zn < Pb < Cu. However, besides the affinity of the metal ion for the surface, the adsorption curves are considered to be influenced by surface charge, the adsorption density of the metal ions and their size. The analysis of the data in terms of H⁺ and M²⁺ ion adsorption is considered to be complementary to the hydrolysis model for heavy metal adsorption.