RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The sensitivity of metal ion adsorption on metal oxides for the degree of random surface heterogeneity (smeared out potential) using three electrolyte adsorption models is investigated. It is shown that the effect of random surface heterogeneity is hard to detect when the metal ion adsorption constants are adjusted together with the adjustment of K_q, i.e., a shift in the position of the energy distribution curve. This shift is necessary in order to keep the experimentally determined pristine point of zero charge at a fixed position. Some reasons for the low sensitivity are indicated. Cadmium adsorption isotherms for hematite and amorphous iron oxide in combination with Cd/H exchange data obtained at several pH values can be described satisfactorily with a simple homogeneous surface one- or two-pK basic Stern model. For both iron oxides the predominant surface species of cadmium is SO⁻CdOH⁺.