Sorption and desorption of uranium(VI) on silica: experimental and modeling studies

Sorption of U(VI) on silica was investigated as functions of contact time, pH, ionic strength, solid-to-liquid ratio (m/V) and U(VI) concentration by using a batch] experimental method. It was found that sorption kinetics is relatively rapid and 1h is sufficient to reach sorption steady state. The sorption of U(VI) increases from about 0 to 100% with pH over the range of 2.8 to 5.8. Sorption isotherms were collected at pH 4.3 and three ionic strengths (0.01, 0.1 and 0.5 mol/L NaCl). It was found that the sorption of U(VI) increases with decreasing ionic strength at pH 4.3 and relatively low U(VI) concentrations. The sorption of U(VI) with variable m/V at pH 4.2 and initial U(VI) concentration of 1.03 x 10(-4) mol/L indicated that the U(VI) distribution coefficient increases slightly with the increase of m/V over the range of 1 to 100 g/L. Description was performed by dilution of aqueous U(VI) concentration for desorption isotherms and by pH adjustment of the suspension for pH desorption edges. It was found that sorption/desorption is reversible with respect to aqueous pH values, whereas a hysteresis was found for desorption isotherms. All equilibrium data were fitted with a surface complexation model. A mononuclear inner-sphere surface complex SOUO21 and a polynuclear surface complex SO(UO2)(3) (OH)(5) were used to quantitatively interpret sorption of U(VI) on silica.