Kinetics and thermodynamics of uranium(VI) sorption on hydrous silica

Abstract

Sorption of uranyl ion, UO22+, on SiO2·xH2O (silica gel) has been investigated as a function of time, amount of silica gel (0.10–0.60g), uranium concentration (5.00×10−5–1.00×−3M), ionic strength (0.20–1.40M NaClO4), pH (∼2.0–6.0), and temperature (283–333K). Based on kinetic data, the diffusion coefficient of UO22+ion was calculated to be 6.83(±0.22)×10−12m2s−1under particle diffusion-controlled conditions. The rate of sorption of uranyl ion on silica gel was determined to be 8.56(±0.50)×10−2min−1at 298K, pH 1.82±0.05 and 0.20M NaClO4. The uranyl sorption data on silica gel followed the Freundlich, Langmuir and Dubinin–Radushkevich (D–R) isotherms. Uranyl sorption (11.48±0.03 (0.20M NaClO4) to 34.81±0.20 (1.40M NaClO4) at pH 2.79±0.03 using 0.10g silica gel. The sorption decreased in the presence of 1.00×10−3M fluoride and oxalate ions; while the presence of carbonate anion showed negligible change in uranyl sorption profile in the pH range of 2 to 6 at 0.20M NaClO4. Temperature variation experiments showed that the sorption of uranium on silica gel had an endothermic enthalpy, which became more spontaneous at elevated temperatures.