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Calcium-Magnesium Exchange Selectivity of Wyoming Montmorillonite in Chloride, Sulfate and Perchlorate Solutions

U.S. Salinity Lab., USDA-ARS, 4500 Glenwood Dr., Riverside, CA 92501

* Corresponding author.

ABSTRACT

The need to consider CaCl⁺ and MgCl⁺ complexes in Ca-Mg exchange reactions on montmorillonite was evaluated by examining cation selectivity in various anion backgrounds. The Ca-Mg exchange isotherms on SWy-1 Wyoming bentonite were determined in 0.0125 M SO₄, 0.025, 0.050, and 0.100 M Cl and 0.03 M perchlorate background. Calculated selectivity values for the reaction Ca²⁺ + MgX Mg²⁺ + CaX were K_v = 1.15, 1.11 and 1.10 for the SO₄, Cl and perchlorate media, respectively. There was no trend in K_v, with exchangeable Ca with any anion or with ionic strength in Cl media. The Ca preference in SO₄ and perchlorate media indicates that montmorillonite preference for Ca is not an artifact of complexation in Cl media. These K_v values are also consistent with earlier published Ca-Mg selectivity of the same clay in perchlorate media. The CaCl⁺ and MgCl⁺ complexes comprised an insignificant amount of the exchangeable cations.

M. F. Zahow is from Soil Salinity and Alkalinity Laboratory, Baccos, Alexandria, Egypt. We gratefully acknowledge funding provided M. F. Zahow by the Cooperative Arid-Lands Agric. Res. Program supported by U.S.-AID. Contract NEB-0170-A00-0047-00.

Received for publication January 4, 1988.

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