Applying a Gouy–Chapman–Stern model for adsorption of organic cations to soils

An effective adsorption model was developed by S. Nir in 1978 for predicting adsorption to synthetic membranes and was also adapted by him afterwards to clay minerals. The main principles of the model are the solution of the electrostatic Gouy–Chapman equations while calculating the adsorbed amounts of the cations as the sum of those residing in the double-layer region and the cations that are chemically bound in a closed system. Thus, the equilibrium concentration in solution is influenced by the adsorption as the function of the amounts bound. The model was later developed and adapted for the adsorption of organic cations, and, in most cases, the fit of calculated results to measured adsorbed amounts of cations was very good, after calibrating the model for the chemical tested, with only two adjustable parameters for each chemical. In the current work, we tested the applicability of this model for the adsorption of a monovalent and a divalent organic cation by several soils, by considering the cation exchange capacities (CEC) and the specific surface areas (SSA) of the soils. For three out of four soils, a very good fit was obtained by using the required parameters for the calculations from previous studies of cation adsorption on clay minerals. The main advantage of the presented model over Langmuir and Freundlich adsorption isotherms is that no additional changes in the parameters were needed when the background concentration of salts in the suspensions was increased from 10 to 50 or 500 mM.