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SOIL CHEMISTRY

A New Approach to Estimate Ion Distribution between the Exchanger and Solution Phases

^a Dep. of Environmental Science, Univ. of California, Riverside, CA 92521
^b College of Resources and Environment, Southwest Univ., Chongqing, 400716, P.R. China

^* Corresponding author (hli22002{at}yahoo.com.cn).

Currently there are two approaches to treat ion exchange equilibrium between the exchanger phase and solution phase: the chemical reaction equilibrium and the Donnan equilibrium. Nevertheless, theoretical calculation of the ion distribution between these two phases remains impossible using these two approaches because of the difficulty in obtaining the activity coefficient of an ion in the exchanger phase for a solution mixture containing two or more types of electrolytes. Our analysis of ion diffusion in an electric field showed that the exchange process of electrostatic adsorption can be treated as a diffusion process. A new equilibrium equation was obtained by treating ion exchange as a diffusion process. The new equation can be used to predict the ion distribution between the exchanger and solution phases without knowing the activity coefficients of the adsorbed ions. The new approach was tested and verified using published experimental data for a Na–Ca-illite system. The experimental data and the theoretical predictions matched very well. The new ion distribution equation can provide another way to calculate ion exchange equilibrium for a well-dispersed suspension of dilute bulk solution containing two types of electrolytes.

Abbreviations: DDL, diffuse double layer