Thermodynamics of Potassium Exchange Reaction in Entisol and Vertisol Using a Kinetic Approach by Miscible Displacement Technique

Thermodynamics of Polilssium (K) exchange using a kinetic approach was investigated in Ca-saturated samples from Entisol and Vertisol. Kinetics of adsorption and desorption were determined at 10, 20 and 42°C using a miscible displacement technique. Energies of activation for adsorption (E_a) are 0.975 and 1.345 k cal mol⁻¹ and for desorption (E_d) are 2.965 and 3.103 k cal mol⁻¹ in Entisol and Vertisol, respectively. The E_d values were greater than E_a values, indicating that more energy was required to desorb K than to adsorb it. The free energy for K exchange (ΔG⁰) values 1.038 to 0.966 in Entisol and 0.881 to 0.808 K cal mol⁻¹ in Vertisol were negative and increased with increasing temperature. The free energy of activation values were higher for K desorption (ΔG_d⁺⁺) than for K adsorption (ΔG_a⁺⁺) indicating a greater free energy requirement to desorb K. The excellent agreement between ΔG⁰ calculated from Gibb's theory and from Eyring's reaction rate theory indicated that pure thermodynamic parameters could be calculated using a chemical kinetic approach. The enthalpy (ΔH⁰) values were exothermic and indicated stronger binding of K⁺ ions in Vertisol than that of Entisol. The enthalpy of activation (ΔH⁺⁺) values in these soils were higher for desorption (ΔH_d⁺⁺) than for adsorption (ΔH_a⁺⁺) suggesting that the heat energy needed to overcome the K desorption barrier was greater than for that of K adsorption.