Characterization of hydrous uranyl silicate by EXAFS

Extended X-ray absorption fine structure (EXAFS) analysis was performed on uranyl orthosilicate, (UO2)(2)SiO4 . 2H(2)O, and uranium(VI) sorbed onto silicic acid and silica gel. Uranyl orthosilicate was investigated as a reference for EXAFS studies of similar but non-crystalline uranium, oxygen, and silicon containing samples. Fitting the EXAFS spectrum yields the following distances for the first four coordination shells of uranium: U-O-ax = 1.79 Angstrom, U-O-eq = 2.38 Angstrom, U-Si = 3.16 Angstrom, and U-U = 3.88 Angstrom. These values agree well with results from single-crystal X-ray diffraction (XRD) measurements. Structural parameters of light elements such as oxygen and silicon at distances greater than 3.5 Angstrom could not he detected without a priori knowledge of their presence. The EXAFS spectra of uranyl species sorbed at pH 4 onto silicic acid and silica gel are identical indicating similar uranyl coordination. The main characteristic of the surface species are two well-separated oxygen coordination shells in the equatorial uranyl plane at 2.27 and 2.50 Angstrom. The results of the EXAFS analysis favor the interpretation of the uranyl surface species as an inner-sphere, mononuclear, bidentate complex.