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Kinetics of Ion Exchange on Clay Minerals and Soil: II. Elucidation of Rate-limiting Steps¹

ABSTRACT

Kinetics of K⁺ adsorption were investigated on kaolinite, a Chester loam soil and vermiculite using static, stirred, and vortex batch techniques. The objective of this study was to elucidate the rate-limiting steps for K⁺ adsorption on the clay minerals and soil. We hypothesized that it is possible under laboratory conditions to set up a system in which the global rate is limited by mass transfer (under static conditions), in which only the intraparticle diffusion step is rate-limiting (stirred system), and a system in which the rates of film and intraparticle diffusion are both relatively rapid, presumably rendering the reaction step to be rate-controlling (vortex batch). We derived and assigned additive resistance relations to the three proposed experimental methods according to the above stated assumptions. Observed rate coefficients obtained from static, stirred and vortex batch systems were combined and used to calculate rate coefficients for film diffusion (k_f), intraparticle diffusion (k_I), and reaction kinetics (k_r) in a static system. Film diffusion and intraparticle diffusion rate coefficients were approximately the same in vermiculite, indicating that both steps were rate-determining. In kaolinite and the Chester soil, film diffusion was the rate-limiting step. These conclusions were also verified using the parabolic diffusion equation. The k_I values were on the avg 8.7 times as great as k_f values in the kaolinite system, indicating that intraparticle diffusion was not important for kaolinite. Higher energies of activation for adsorption (E_aa) were observed for the reaction step than for intraparticle diffusion or film diffusion steps. These findings were expected since chemical reactions are more sensitive to temperature changes than diffusion processes.

¹ Published with the approval of the Director of the Delaware Agric. Exp. Stn. as miscellaneous paper no. 1133. Contribution no. 193 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303.

² Graduate Research Assistant and Associate Professor of Soil Chemistry, respectively. Senior author is now Postdoctoral Research Fellow, Soil Science Dep., Univ. of Florida, Gainesville 32611.

Received for publication October 30, 1986.

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