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Thermodynamics of Potassium Exchange in Soil Using a Kinetics Approach¹

ABSTRACT

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 (E_a and E_d, respectively) ranged from 3.83 to 5.52 kcal mol^–1. The E_d values were higher than the E_a 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 (G°) 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 (G_d) than for K adsorption (G_a), suggesting a greater free energy requirement to desorb K. The excellent agreement between G° 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 (H°) 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 (H) values in both horizons were higher for desorption (H_d), than for adsorption (H_a), suggesting the heat energy required to overcome the K desorption barrier was greater than for that of K adsorption.

¹ Contribution from the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711. Published with the approval of the Director of the Delaware Agric. Exp. Stn. as Misc. Paper no. 940. Contribution no. 127 of the Dep. of Plant Science.

² Assistant Professor of Soil Chemistry and Undergraduate Student, respectively.

Received for publication March 27, 1981. Accepted for publication July 20, 1981.

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