Adsorption of trivalent cations on silica

Gadolinium and yttrium show very similar adsorption behaviors on silica. For the both trivalent cations, the adsorption isotherms are insensitive to the ionic strength. The slope of the log-log adsorption isotherms at a constant pH is considerably lower than 1, except for the range of very low equilibrium concentrations. This behavior can be explained without invoking surface heterogeneity considering the presence of a small concentration of anions, An(2-), as an impurity. A hypothetical ternary complex SiOGdAn is very strong and this leads to enhanced uptake at low initial concentrations. When [Gd3+] much greater than [An(2-)], only a small fraction of Gd can be adsorbed as SiOGdAn and the percentage of uptake is low. (C) 1997 Academic Press.