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A Critical Evaluation on the Use of Kinetics for Determining Thermodynamics of Ion Exchange in Soils¹

ABSTRACT

Thermodynamics of K-Ca exchange were compared using equilibrium and kinetic approaches in two Delaware soils. The classical Argersinger theory was employed for the equilibrium approach, while a kinetic approach was studied using miscible displacement, batch, and vigorously mixed batch techniques. These three techniques were used to determine how diffusion affects the comparison between thermodynamic parameters using kinetic and equilibrium approaches. Rate coefficients and energies of activation were profoundly affected by the type of kinetic technique employed and their magnitude was in the order: vigorously mixed batch > batch > miscible displacement. Energies of activation for adsorption (E_aa) in the two soils ranged from 7.42 kJ mol^–1 using the miscible displacement technique to 32.96 kJ mol^–1 with the vigorously mixed batch, while energies of activation for desorption (E_ad) ranged from 11.87 to 42.1 kJ mol^–1 for the two methods, respectively. The magnitude of the E_aa and E_ad values indicated pronounced diffusion effects in the miscible displacement and batch techniques, which were greatly reduced with the vigorously mixed batch method. Thermodynamic parameters (G°, H°, and S°) calculated using the equilibrium and kinetic approaches compared very well in trend, and gave the same inferences of ion behavior for the two soils studied. However, except for the vigorously mixed batch technique, the magnitude of the thermodynamic parameters for the two approaches compared poorly. For example, the G° values calculated using the equilibrium approach avg 4.67 kJ mol^–1 for the two soils, while they avg 4.92 kJ mol^–1 with the vigorously mixed batch, 2.12 kJ mol^–1 with the batch, and 2.12 kJ mol^–1 with the miscible displacement. The degree of comparison between the two approaches was directly related to the extent of diffusion controlled exchange. When the influence of diffusion was significantly reduced, as with the vigorously mixed batch technique, our data would indicate that a kinetic approach can be successfully used to gather thermodynamic information about a soil system.

¹ Published as misc. paper no. 1124 of the Delaware Agric. Exp. Stn. Contribution no. 173 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303.

² Graduate Research Assistant and Associate Professor of Soil Chemistry, respectively.

Received for publication September 3, 1985.

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