Predicting cation exchange equilibria in soil containing three heterovalent cations

Theoretical cation exchange equilibria relationships in a soil for a ternary system containing Na + , Ca 2+ and Mg 2+ are presented. The solid phase activity coefficients of cations for the three binary systems (i. e., Na + – Ca 2+ , Na + – Mg 2+ and Ca 2+ – Mg 2+ ) were used to calculate the Wilson parameters. Adsorbed phase activity coefficients of Na + , Ca 2+ and Mg 2+ corresponding to their equivalent fractions on the adsorbed phase of A horizon of a Camborthid for the ternary system (i. e., Na + – Ca 2+ – Mg 2+ ) were evaluated, using these Wilson parameters. Using the values of activity coefficients of adsorbed Na + , Ca 2+ and Mg 2+ in a ternary soil system and of thermodynamic equilibrium constants for the binary systems, the multiple linear regression relationships between the equivalent fractions of Na + , Ca 2+ and Mg 2+ on the adsorbed phase and their mole fractions in the equilibrium solution were obtained. The amounts of adsorbed phase cations so predicted showed a fair agreement with those observed experimentally in a ternary system.