Sorption of U(VI) and phosphate on γ-alumina: Binary and ternary sorption systems

The sorption of U(VI) and phosphate on γ-alumina was investigated in binary (phosphate/γ-alumina; U(VI)/γ-alumina) and ternary (U(VI)/phosphate/γ-alumina) systems as functions of contact time, pH, ionic strength, solid-to-liquid ratio and U(VI) and/or phosphate concentrations by using a batch experimental method. It was found that the sorption of phosphate on γ-alumina increases with pH from 2.5 to 5.2 and then decreases with pH from 5.2 to 9.4. The sorption of phosphate on γ-alumina is insensitive to ionic strength. On the other hand, the sorption of U(VI) on γ-alumina increases with increasing pH over the range of 4–6. The sorption of U(VI) on γ-alumina increases slightly with decreasing ionic strength. In the ternary sorption system, it was found that the presence of phosphate increases the sorption of U(VI), whereas the presence of U(VI) has little effect on the sorption of phosphate. The sorption of U(VI) and phosphate in binary and ternary systems were interpreted in terms of surface complexation models. The effects of γ-alumina dissolution and CO2 in the sorption systems were considered in modeling calculations. Four surface complexes of phosphate, XOHAlHPO4+, XH2PO4, XHPO4− and XPO42−, and two surface complexes of U(VI), XOUO2+ and XOUO2(OH)2−, were respectively used to reproduce the sorption of phosphate in phosphate/γ-alumina system and U(VI) sorption in U(VI)/γ-alumina system. The co-sorption of U(VI) and phosphate in the ternary sorption system was interpreted by a model which combines the surface complexation models for the binary sorption systems together in addition to considering the formation of a ternary surface complex, XOUO2HPO4−.