Binary K-Ca exchange on a clay separated from a soil with dominate illite (mica) mineralogy: 2- Determination of selectivity Coefficients and thermodynamic parameters and effects on them of K-depletion

Document Type : Research Paper


1 University College of Agriculture and Natural Resources, University of Tehran

2 Associate Professor, Faculty of Agricultural Engineering and Technology, University of Tehran


Selectivity coefficients and thermodynamic parameters of K-Ca exchange in a clay separated from a soil with dominant illite (mica) mineralogy were studied at two different ionic strengths (0.3 and 0.002 M). The clay was depleted for periods of 0, 6, 25 and 150 hours. The values of the Kerr (KKer), the Vanselow (KV), the Gapon (KG), the Gaines-Thomas (KGT) and the Davies (KD) selectivity coefficients were increased with K saturation. In the range of EK≤0.7, KV was smaller than 1, suggested that the exchangers preferred K+ over Ca2+ in this range, however at high EK values, Ca2+ was preferentially selected. Rothmund-Kornfeld model was fitted well to exchange data. β in this equation was smaller than 0.66, demonstrating heterogeneity of exchange sites in the clays. The value of β decreased with K-depletion. Furthermore, β decreased with reduction in ionic strength from 0.3 to 0.002 M, indicating an increase in surface sites heterogeneity. Regular solution (RS) model was also well fitted to the data. Thermodynamic equilibrium constants (Kex) were smaller than 1 and the standard free energy of the exchange reactions (ΔG°ex) were positive in all clays. The values of Kex in different K-depleted clays were not significantly different, indicating that the depletion-collapse cycle of these illitic clays did not change their selectivity for K+ or Ca2+. The decrease in the ionic strength led to the increase in Kex and decrease in ΔG°ex, an indication of reduced tendency of clay for K+ relative to Ca2+. With increasing EK, Kex remained relatively constant. ƒK increased with ECa and ƒCa decreased. Comparison of ƒK and ƒCa calculated by the thermodynamic method and the regular solution model indicated that the values of ƒK in RS model were greater than of thermodynamic method.


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