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Cation Selectivity in Sodium-Calcium, Sodium-Magnesium, and Calcium-Magnesium Exchange on Wyoming Bentonite at 298 K¹

ABSTRACT

Exchange isotherms were prepared for Na⁺ in Na⁺Ca²⁺ and Na⁺Mg²⁺ exchange reactions, and for Mg²⁺ in Na⁺Mg²⁺ exchange reactions, at 298 K on Wyoming bentonite suspended in a 0.05M perchlorate background. These isotherms were essentially congruent with the appropriate thermodynamic nonpreference exchange isotherms. It was concluded from this fact that there is essentially no difference in the affinity of montmorillonite clay for Ca²⁺ vs. Mg²⁺. However, exchange isotherms prepared for Na⁺ in Na(I) Ca(II) and Na(I) Mg(II) exchange, as well as published ones for Mg(II) in Ca(II) Mg(II) exchange on montmorillonite suspended in a 0.05M chloride background consistently indicate a slight preference on the clay for Ca(II) over Mg(II). This preference for Ca(II) is concluded to be the result of the formation of CaCl⁺ complexes, which are more stable thermodynamically and have a greater affinity for the clay than MgCl⁺ complexes.

¹ Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Professor of Soil Science, Research Associate IV, Visiting Postdoctoral Fellow, and Graduate Research Assistant, respectively.

Received for publication November 10, 1982. Accepted for publication April 19, 1983.

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