Thermodynamic Evaluation of Cation Exchange Equilibria in Binary and Ternary Systems on a Sandy Clay Loam Soil

Cation exchange equilibria involving cation pairs, viz., Ca²⁺-Na⁺, Mg²⁺-Na⁺ and Ca²⁺Mg²⁺, in varying cationic ratios at two electrolyte concentrations (10 and 100 m equiv. VI) were studied on a sandy clay loam soil from Narrogin, Western Australia. In heterovalent systems, there existed a preferential adsorption for divalent cations (Ca²⁺ and/or Mg²⁺) vis-a-vis monovalent cation (Na⁺), regardless of solution concentration. This is evident from the position and shape of the Na⁺ isotherms as well as from the sign and magnitude of free energy change (Δ Gr⁻¹) of exchange reactions. In homovalent systems, the position of the Mg²⁺ isotherm indicated a preferential adsorption of Ca²⁺. The smaller values of Gapon selectivity coefficient (KG) at different degrees of Na⁺ saturation also confirm the selectivity for divalent cations, irrespective of solution concentration. The surface activity coefficients (f_i), calculated for both heterovalent binary and ternary systems further confirm weaker Na-binding on the soil exchanger over a range of ESP values. The disproportionately higher values of f_Na corresponding to lower Na⁺ saturation is discussed in the light of the ‘tactoid model’. The preferential adsorption of Ca²⁺ and Mg²⁺vis-a-vis Na⁺ for the soil exchange sites was greater when they competed separately with Na⁺, than when all three cations competed together. At comparable fraction on the soil exchanger (N_Ca = N_Mg = 0.17 or 0.33), the values of fwere larger than those of f_Ca. This infers a competitive edge to Ca²⁺ over Mg²⁺ for the soil exchange sites. This is most evident at low proportions of these cations on the soil exchanger. The predictive equations developed in this study conform to the recognized exchange properties for all three cations. It is concluded that, in general, the behaviour of the ternary exchange system can be predicted with a reasonable accuracy from data for the relevant binary systems.