tions for trivalent cations is much greater than for divalent Gadolinium and yttrium show very similar adsorption behavcations. Following the same lines as in Ref.
(2), thirteen iors on silica. For the both trivalent cations, the adsorption different inner-sphere monodentate complexes of trivalent isotherms are insensitive to the ionic strength. The slope of the cations can be proposed (Table 1). The number of possible log -log adsorption isotherms at a constant pH is considerably outer-sphere monodentate complexes is also higher than for lower than 1, except for the range of very low equilibrium condivalent cations.
centrations. This behavior can be explained without invoking
Adsorption of gallium(III) and indium(III) on alumina surface heterogeneity considering the presence of a small conwas recently studied (3), and the effect of addition of various centration of anions, An 2 -, as an impurity. A hypothetical ternary complex SiOGdAn is very strong and this leads to en-amounts of NaNO 3 (up to 0.1 mol dm -3 ) on the course of hanced uptake at low initial concentrations. When [ Gd 3/ ] Σ· the uptake curves was found to be negligible. Considering [ An 2 -] , only a small fraction of Gd can be adsorbed as the high initial concentration of the trivalent cations (3 1 SiOGdAn and the percentage of uptake is low. α§ 1997 Academic 10 -3 mol dm -3 ), this result is not very surprising because Press the difference between the highest and the lowest concentra-Key Words: gadolinium; yttrium; silicon(IV) oxide; specific adtion of the counterions (nitrate) studied in Ref.
(3) was only sorption; triple-layer model; surface complexation; ternary surface one order of magnitude and even less for the data points at complex. pH below pH 2.
Table 3 in Ref.
(3) reports formation constants of four different surface complexes of Ga and analogous complexes of In.