CHARACTERIZATION OF HYDROUS URANYL SILICATE BY EXAFS

Extended X-ray absorption fine structure (EXAFS) analysis was performe d on uranyl orthosilicate, (UO2)(2)SiO4 . 2H(2)O, and uranium(VI) sorb ed onto silicic acid and silica gel. Uranyl orthosilicate was investig ated as a reference for EXAFS studies of similar but non-crystalline u ranium, oxygen, and silicon containing samples. Fitting the EXAFS spec trum yields the following distances for the first four coordination sh ells 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. Str uctural parameters of light elements such as oxygen and silicon at dis tances greater than 3.5 Angstrom could not he detected without a prior i knowledge of their presence. The EXAFS spectra of uranyl species sor bed at pH 4 onto silicic acid and silica gel are identical indicating similar uranyl coordination. The main characteristic of the surface sp ecies are two well-separated oxygen coordination shells in the equator ial uranyl plane at 2.27 and 2.50 Angstrom. The results of the EXAFS a nalysis favor the interpretation of the uranyl surface species as an i nner-sphere, mononuclear, bidentate complex.