Uranyl sorption onto alumina

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 (9x10(-11) - 5x10(-8) M) 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, pK(a1) and pK(a2) 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.