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

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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 Kv = 1.15, 1.11 and 1.10 for the SO4, Cl and perchlorate media, respectively. There was no trend in Kv, 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 Kv 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.
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Funding Information
  • United States Agency for International Development (NEB‐0170‐A00‐0047‐00)