Thermodynamics of Zinc Adsorption on Calcite, Dolomite and Magnesite‐Type Minerals

Abstract

Abstract The adsorption of zinc chloride on calcite, dolomite and Casubstituted magnesite crystals from aqueous solution is treated theoretically; and a standard adsorption process is thereby rigorously defined. The thermodynamic characteristics Δ F 0 θ, Δ H 0 θ and Δ S 0 θ are determined for this process by means of adsorption isotherm data at various temperatures with crystals of measured surface area. It is found that about 10% of the adsorption sites probably available on calcite are occupied by zinc when the equilibrium Zn ++ aq concentration is 0.90 × 10 ‐6 M at 25.1° C. The Ca‐magnesite shows a somewhat greater affinity for zinc ion than calcite, while dolomite is intermediate. The temperature coefficients of Δ F 0 θ are opposite in sign for calcite and the other two minerals. The resulting endothermic heats of adsorption of Zn ++ aq on the dolomite and the Ca‐magnesite (Δ H 0 θ = 8.21 ± 2.4 and 22.7 ± 5.9 kcal/mole at about 27° C, respectively), as well as the corresponding very large positive entropies of adsorption (Δ S 0 θ = 56 ± 8 and 106 ± 20 cal/deg × mole at about 27° C, respectively) indicate that Zn ++ aq is dehydrated when adsorbed by these two minerals. The known compatability of Zn ++ with the MgCO 3 crystal lattice is suggested as the reason for the strong interaction of this ion with the dolomite and Ca‐magnesite, relative to calcite.