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.