Published online by Cambridge University Press: 03 September 2012
The mechanism for the adsorption of uranyl onto alumina from aqueoussolution was studied experimentally and the data were modeled using a triplelayer surface complexation model. The experiments were carried out at lowuranium 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 dissociationconstants, pKal and pKa2, of the alumina surface weredetermined 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 theelectrolyte concentration. We therefore conclude that the uranium binds asan inner sphere complex. The results were modeled using the code FITEQL. Tworeactions of uranium with the surface were needed to fit the data, oneforming a uranyl complex with a single surface hydroxyl and the otherforming a bridged or bidentate complex reacting with two surface hydroxylsof the alumina. There was no evidence from these experiments of siteheterogeneity. The constants used for the reactions were based in part onpredictions made utilizing the Hard Soft Acid Base, HSAB, theory, relatingthe surface complexation constants to the hydrolysis of the sorbing metalion and the acid dissociation constants of the mineral oxide surface.