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Thermodynamics of Zinc Adsorption on Calcite, Dolomite and Magnesite-Type Minerals¹

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 F⁰, H⁰ and S⁰ 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 x 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 F⁰ 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 ( H⁰ = 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 ( S⁰ = 56 ± 8 and 106 ± 20 cal/deg x 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 MgCO₃ crystal lattice is suggested as the reason for the strong interaction of this ion with the dolomite and Ca-magnesite, relative to calcite.

Contribution from the Department of Agronomy, Utah State Agr. College, Logan, Utah. This work was supported in part by the U. S. Smelting, Mining and Refining Co., Salt Lake City, and in part by the U. S. Atomic Energy Commission under contract AT(11-1)80. This paper includes portions of a thesis submitted by the senior author in partial fulfillment of requirements for the Ph.D. degree at Utah State Agr. College. Presented before Div. II, Soil Science Society of America, Davis, Calif., Aug. 16, 1955.

² Formerly Research Assistant in Department of Agronomy. Utah State Agr. College, now Junior Chemist, Department of Soils and Plant Nutrition, Univ. of Calif., Davis, Calif.; and Associate Professor of Chemistry, Department of Chemistry, Utah State Agr. College, Logan, Utah, respectively.

Received for publication November 7, 1955.