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Cation Exchange Equilibria in Soils Expressed as Cation-Ligand Complex Formation¹

ABSTRACT

A model for representing cation exchange reactions as part of the equilibrium reactions in a soil-water system is proposed. The exchange reaction in this model is considered to be a composite of two half complexation reactions. These complexation reactions are assumed to occur between the exchanging cations and the negatively charged surfaces considered as hypothetical free ligands. The model was added as a subroutine to the GEOCHEM program which calculates ionic equilibrium in soil water systems. Two formation constants, K'_Caf for the Ca-ligand complex and K'_Naf for the Na-ligand complex were estimated by assuming K'²_Naf/K'_Caf = K_VCa-Na = 1 in the range 1 > E_Ca > 0.7. A value of log K'_Caf = 2.5 was found to provide the best fit to the relationship between E_S = (E_Na/(CEC-E_Na)) vs. SAR given by the U.S. Salinity Laboratory Report (1954). The fitted value of K'_Caf was used for calculating the K-ligand formation constant; K'_Kt and the Mg-ligand formation constant, K'_Mgr, using the appropriate experimental Vanselow selectivity coefficient. The calculated formation constants corresponding to the cations in a given soil water system were used to predict the distribution of the cations in solution and on the solid phase by means of the GEO-CHEM program. Good agreement was found for the K-Ca and Ca-Mg exchange in soils when comparing experimental data to the ones predicted by the proposed model.

¹ Contribution from the Dep. of Soil & Environmental Sciences, Univ. of California, Riverside, CA 92521.

² Postdoctoral Research Associate, Assistant Professor, respectively. Address for senior author now Dep. Soil Science, TECHNION, Haifa 32000, Israel.

Received for publication July 23, 1984. Accepted for publication December 6, 1984.