Thermodynamics of Potassium-Calcium Exchange in Soils using Equilibrium and Kinetic Approaches

Thermodynamics of potassium exchange was investigated at different temperatures viz. 283, 298 and 313°K in Ca-saturated samples from Ap horizons (0–0.15 m) of red earth (fine loamy, mixed hyperthermic Typic Rhodhustalf), Karail (fine, mixed hyperthermic Entic Chromusterts) and alluvial (fine loamy, mixed hyperthermic Aeric Ochraqualf) soils of eastern Uttar Pradesh using equilibrium and kinetic approaches and a comparison was drawn between the two approaches. Conventional equivalent concept of Gaines and Thomas was used for equilibrium study. Kinetics of adsorption and desorption were determined using batch technique. The energy of activation for desorption values (E_d) were higher than that for adsorption values (E_a) for all the soils indicating that more energy was required to desorb K⁺ than to adsorb K⁺. The standard thermodynamic and pseudothermodynamic parameters were determined using Gibbs' and Eyring's reaction rate theory, respectively. The standard free energy (ΔG⁰) was negative and increased with increasing temperature. Enthalpy values (ΔH⁰) were negative indicating exothermicity and strong binding of K⁺ with soil sites. The free energy values of activation for K desorption (ΔG_d^#) were greater than that for K adsorption (ΔG_a^#) suggesting greater free energy requirement to desorb K+. Excellent agreement was observed between ΔG⁰ values calculated from Gibbs theory and Eyring's reaction rate theory. Thermodynamic parameters (ΔG₀, ΔH⁰ and ΔS⁰) using kinetic and equilibrium approaches compared very well in trend and gave similar inference on ion behaviour for the three soils studied. However, the magnitude of the thermodynamic parameters compared poorly. The degree of comparison between the two approaches seems to be a direct function of diffusion-eontrolled exchange which was evident from E_a and E_d values.