Sorption and Speciation of Uranium on Silica Colloids

Abstract

Sorption behavior of uranium onto silica colloids is studied to derive surface complexation data. Several spectroscopic and structural methods, i.e., Time-resolved Laser-induced Fluorescence (TRLIF), X-ray Absorption Fine Structure (XAFS), and Small-angle X-ray Scattering (SAXS) are employed for characterization of colloid suspensions and surface-sorbed species. TRLIF and EXAFS allowed identification of two principal uranium species on the silica surface – bidentate coordinated uranyl complexes of the following stoichiometries (≡SiO)2UO2 and (≡SiO)2UO2OH−. No polynuclear species or surface precipitates are formed as shown by EXAFS and SAXS examination of suspensions at various silica/uranium ratios. However, when uranium coverage of the silica surface exceeds 7%, the colloid stability as determined by electrophoretic mobility increases. Potentially, this could affect colloid-facilitated transport.