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

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 SO 4 , 0.025, 0.050, and 0.100 M Cl and 0.03 M perchlorate background. Calculated selectivity values for the reaction Ca 2+ + Mg X ⇌ Mg 2+ + Ca X were K v = 1.15, 1.11 and 1.10 for the SO 4 , 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 SO 4 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.