Temperature-dependent adsorption of U(VI) onto γ-alumina

Abstract

Elevated temperature is expected at the deep geological repository (DGR) due to decay heat from spent nuclear fuel and positive geothermal gradient. The resulting elevated temperature would change the aqueous speciation and surface complexation of uranium which is the major component in spent nuclear fuel. Being a long-lived and radiotoxic element describing the uranium migration is of cardinal importance to demonstrate the long-term safety of DGR. A surface complexation model (SCM) describing the uranium adsorption on complex mineral systems would be an effective tool to predict uranium migration. To build such models information on dissolved and adsorbed species of uranium should be generated. Alumina can be considered a structural and functional analog to complex minerals like aluminosilicates which are omnipresent in the subsurface system. Therefore, the present study aimed to assess the temperature dependence on aqueous speciation and adsorption of U(VI) on alumina. Experimental temperatures were 25°C, 55°C and 70°C. Time-resolved laser fluorescence spectroscopy (TRLFS) was applied to assess the dissolved and adsorbed U(VI) species in the pH range 6.5-9.0. In terms of aqueous speciation, both hydrolysis species and tricarbonato coexist at 25°C. However, at elevated temperatures hydrolysis species dominated the system. In the case of adsorption, both uranyl hydrolysis species and tricarbonato species were found to be adsorbed at 25°C. However, at elevated temperatures, two hydrolysis species were adsorbed. This transition might have enhanced the U(VI) adsorption at elevated temperatures. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the adsorption of uranyl tricarbonato species at 25°C and electrophoretic mobility measurements suggest the mechanism to be inner-sphere coordination. Combining the present results with temperature-dependent adsorption studies on silica and aluminosilicates a reliable SCM can be proposed for the subsurface system to explain U(VI) migration.