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COMPARISON OF KINETIC EQUATIONS TO DESCRIBE POTASSIUM‐CALCIUM EXCHANGE IN PURE AND IN MIXED SYSTEMS

We used first-order, Elovich, parabolic diffusion, and zero-order equations to describe the kinetics of K-Ca exchange in kaolinite, montmorillonite, vermiculite and soils of the Atlantic Coastal Plain Region. The first-order equation was the best of the various kinetic equations studied to describe the reaction rate of K adsorption in the clay minerals and soils, as evidenced by the highest simple correlation coefficients (r) and the lowest values of the standard error of the estimate (SE). The parabolic diffusion law described K adsorption best on vermiculite and soils dominated by vermiculitic clay minerals. The parabolic diffusion law did not describe K adsorption well on kaolinite and soils high in kaolinite. These differences were related to the kinds of binding sites present in the pure and mixed systems. The Elovich and zero-order equations did not satisfactorily describe K adsorption in the soils and clays. Apparent potassium adsorption rate coefficients (ka/) were 0.23, 0.58, and 2.65 h-1 for vermiculite, montmorillonite, and kaolinite, respectively. The ka/ values for the soils ranged from 0.84 to 1.86 h-1, and their magnitude was related to the type and quantity of clay minerals present.