Thermodynamics of Potassium Exchange in Soil Using a Kinetics Approach
1981
Sparks, D. L. | Jardine, P. M.
Thermodynamics of potassium (K) exchange using a kinetics approach was investigated in Ca-saturated samples from the Ap and B21t horizons of a Matapeake soil from Delaware. Kinetics of adsorption and desorption were determined at temperatures of 0, 25, and 40°C on each soil horizon using a miscible displacement technique. Energies of activation for adsorption and for desorption (Eₐ and Ed, respectively) ranged from 3.83 to 5.52 kcal mol⁻¹. The Ed values were higher than the Eₐ values, indicating that more energy was needed to desorb K than to adsorb K. Thermodynamic and pseudother-modynamic parameters were determined using Gibbs' and Eyring's reaction rate theories. The free energy for K exchange (ΔG°) values were negative (ranging from 1,155 to 1,294 cal mol⁻¹) and increased with increasing temperature. The free energy of activation values were higher for K desorption (ΔGd‡) than for K adsorption (ΔGₐ‡), suggesting a greater free energy requirement to desorb K. The excellent agreement between ΔG° calculated from Gibbs' theory and from Eyring's reaction rate theory indicated that pure thermodynamic parameters could be calculated using a chemical kinetics approach. The enthaply (ΔH°) values were exothermic and indicated stronger binding of K⁺ ions in the B21t horizon than in the Ap horizon of the Matapeake soil. The latter was related to the difference in external surface-to-interlayer surface-charge ratio in the two horizons. The enthalpy of activation (ΔH‡) values in both horizons were higher for desorption (ΔH‡d), than for adsorption (ΔHₐ‡), suggesting the heat energy required to overcome the K desorption barrier was greater than for that of K adsorption.
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