Increase in Cation Adsorption Induced by Surface Complexation of Sulfate on Andisols and Prediction by "Four-Plane Model"

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The "Four-plane model" is one of the surface complexation models developed for analyzing the electrostatic charge of synthetic oxides. This model which was applied to the B horizon of Andisols, was equilibrated with an electrolyte solution containing KCl, KNO_3, K_2SO_4 in the concentration range of 0.0033, 0.01, 0.033, 0.1 M, and pH range of 2 to 11. The amounts of NO_3^-, Cl^-, SO_4^<2-> and K^+ adsorbed were determined from the products remaining in the liquid phase. The increase in the pH value results in the increase of K^+ adsorption, and decrease of anion adsorption. The increase in the equilibrium concentration increased both K^+ and anion adsorption. The parameters of the model were determined by measurements and the iterative method. The model have a good agreement between calculated and measured values of cation and anion adsorption. Among the anion species, the amount of adsorbed anions was in the order of SO_4^<2->&Gi;Cl^->NO_3^-. The differences in the degree of adsorption were mainly associated with the differences in the equilibrium constants of surface complexation. The surface complexation of the anions produced a negative charge and increased cation adsorption. The degree of surface complexation was most significant in the K_2SO_4 solution, and the phenomenon was considered to be important not only for anion adsorption but also for cation adsorption on Andisols.
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