Application of electrokinetic data to test the adsorption models

The effect of adsorption of Gd(III) on the electrokinetic potential of sili ca (Aerosil, 390 m(2)/g) was studied at various Gd(III) concentrations (ran ging from 10(-6) to 10(-2)mo/ dm(-3)) and at four different solid to liquid ratios (ranging from 0.05 to 8% of silica by weight). Up to some critical concentration of trivalent cations, their effect on the electrokinetic pote ntial of silica is negligible and the sign is negative over the entire stud ied pH range. This critical concentration increases when the solid to liqui d ratio increases. When Gd(III) concentration exceeds the critical value, t he magnitude of the negative electrokinetic potential of silica is reduced. This effect is substantial at pH 6 but it is rather insignificant when the pH is very high (pH > 8) or very low (pH < 4). When the Gd(III) concentrat ion is even higher, the sign of the electrokinetic potential is reversed to positive over certain pH range, which depends on the solid to Liquid ratio and Gd(m) concentration. The shape of experimental electrokinetic curves o f silica in the presence of trivalent cations often shows maximums and doub le isoelectric points, thus, it is very complex in comparison with the shap e of the percentage of uptake vs. pH curves. Therefore, a test of an adsorp tion model based on electrokinetic curves is much more demanding than a tes t based merely on uptake vs. pH curves. The parameters of surface complexat ion model (SCM) derived from analysis of a large set of uptake curves were used to predict the course of electrokinetic curves. The calculated and exp erimentally observed maximums and isoelectric points do not exactly match ( a difference up to one pH unit), but the model curves qualitatively reflect the trends observed in electokinetic experiments.