The mechanism for the adsorption of uranyl onto alumina from aqueous
solution was studied experimentally and the data were modeled using a triple
layer surface complexation model. The experiments were carried out at low
uranium concentrations (9×10-11 - 5×10-8M) in a
CO2 free environment at varying electrolyte concentrations
(0.01 – 1 M) and pH (4.5 – 12). The first and second acid dissociation
constants, pKal and pKa2, of the alumina surface were
determined from potentiometric titrations to be 7.2 ± 0.6 and 11.2 ± 0.4,
respectively. The adsorption of uranium was found to be independent of the
electrolyte concentration. We therefore conclude that the uranium binds as
an inner sphere complex. The results were modeled using the code FITEQL. Two
reactions of uranium with the surface were needed to fit the data, one
forming a uranyl complex with a single surface hydroxyl and the other
forming a bridged or bidentate complex reacting with two surface hydroxyls
of the alumina. There was no evidence from these experiments of site
heterogeneity. The constants used for the reactions were based in part on
predictions made utilizing the Hard Soft Acid Base, HSAB, theory, relating
the surface complexation constants to the hydrolysis of the sorbing metal
ion and the acid dissociation constants of the mineral oxide surface.