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Kinetics of Ion Exchange on Clay Minerals and Soil: I. Evaluation of Methods¹

ABSTRACT

The effect of kinetic methodology on adsorption rate coefficients (k_a) and energies of activation for adsorption (E_aa) were investigated using five different techniques. These techniques were: miscible displacement, batch, static, stirred, and vortex batch. Kinetics of K adsorption were studied on Ca-saturated kaolinite, a Chester soil (fine-loamy, mixed mesic Typic Hapludults) and vermiculite. The k_a values were highest for kaolinite and lowest for vermiculite. The higher k_a values observed for kaolinite would be expected since a relatively pure kaolin would exhibit only easily accessible planar surface sites for K exchange. The k_a values for the Chester soil were a reflection of the intermediate properties of this soil between kaolinite and vermiculite. The type of kinetic technique greatly affected the time required for equilibrium to be attained in K adsorption. From greatest to least time, the order was: static > miscible displacement > batch > stirred > vortex batch. Both static and miscible displacement techniques yielded the lowest k_a and E_aa values for all three colloids. This is primarily because these methods represent systems where the influence of diffusion on the rate of K adsorption is at its maximum. Only for kaolinite did the k_a and E_aa values of both the stirred and batch techniques approximate, in magnitude, those of the vortex method. With the Chester soil and vermiculite, which contain appreciable interlayer sites, the stirred and batch techniques yielded k_a and E_aa values much lower than those for the vortex batch method. These results indicate that in colloids where more significant intraparticle diffusion occurs, the batch or stirring technique as used in this study is not effective in eliminating the influence of diffusion.

¹ Published as Miscellaneous Paper no. 1122 of the Delaware Agric. Exp. Stn. Contribution no. 191 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303.

² Former Research Assistant and Associate Professor of Soil Chemistry, respectively. The senior author is now Postdoctoral Fellow, Soil Sci. Dep., Univ. of Florida, Gainsville, FL 32611.

Received for publication October 30, 1985.

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