Sodium-Calcium, Calcium-Potassium, and Potassium-Sodium Exchange Equilibria on a Montmorillonitic Soil

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We carried out experiments on Na-Ca, Ca-K, and K-Na exchange equilibria on a montmorillonitic soil to obtain ionic composition data of both exchanger and aqueous phases. In this experiment we used a chloride background ionic medium instead of a perchlorate ionic medium because the amounts of Ca adsorbed from the CaCl_2 and Ca(ClO_4)_2 solutions were nearly the same. The total adsorbed cationic charge (TC) for the Na-Ca exchange system was about 0.32 eq/kg and the value did not vary with the proportion of the adsorbed ions. On the other hand, the TC value for exchange systems involving K was about 0.3 eq/kg, indicating that about 0.02 eq/kg of K was not exchangeable to 1 M NH_4OAc that was used for extracting the adsorbed cations. The natural logarithms of thermodynamic equilibrium constants were 0.717, 2.764, and -1.550 for Na⇾Ca, Ca⇾K, and K⇾Na exchange reactions, respectively. There was only a small thermodynamic inconsistency among these values irrespective of the reduction in TC. The relationship between the excess Gibbs energy of mixing of the exchange reaction and mol fractions of the cations on the exchanger for each exchange system met a condition imposed by a subregular solution model.
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