The Structure of U6+ Sorption Complexes on Vermiculite and Hydrobiotite

Eric A. Hudson1, †, Louis J. Terminello1, Brian E. Viani2, Melissa Denecke3, Tobias Reich3, Patrick G. Allen1, 4, Jerome J. Bucher4, David K. Shuh4 and Norman M. Edelstein4
1 Glenn T. Seaborg Institute for Transactinium Science, Lawrence Livermore National Laboratory, L-231, PO. Box 808, Livermore, California 94551, USA
2 Earth and Environmental Sciences Directorate, Lawrence Livermore National Laboratory, L-219, PO. Box 808, Livermore, California 94551, USA
3 Institut für Radiochemie, Forschungszentrum Rossendorf, Postfach 510119, D-01314 Dresden, Germany
4 Lawrence Berkeley National Laboratory, MS 70A-1150, 1 Cyclotron Rd., Berkeley, California 94720, USA
Present address: Lam Research Corp., CA-3, 4650 Cushing Pkwy., Fremont, California 94538, USA.

Abstract: The sorption of the uranyl oxo-cation (UO22+)at different types of binding sites on layer silicate mineral surfaces was investigated. Well-characterized samples of vermiculite and hydrobiotite were exposed to aqueous uranyl under conditions designed to promote surface sorption either at fixed charge ion-exchange sites or at amphoteric surface hydroxyl sites. The local structure of uranium in the sorption samples was directly measured using uranium L3-edge extended X-ray absorption fine structure (EXAFS). Polarized L1- and L3-edge X-ray absorption near-edge structure (XANES) measurements were used to characterize the orientation of uranyl groups in layered samples. X-ray diffraction (XRD) measurements of interlayer spacings were used to assess the effects of ion-exchange and dehydration upon the mineral structure. The most significant findings are: (1) Under conditions which greatly favor ion-exchange sorption mechanisms, uranyl retains a symmetric local structure suggestive of an outer-sphere complex, with a preferred orientation of the uranyl axis parallel to the mineral layers; (2) Upon dehydration, the ion-exchange complexes adopt a less symmetric structure, consistent with an inner-sphere complex, with less pronounced orientation of the uranyl axis; and (3) For conditions which favor sorption at surface hydroxyl sites, uranyl has a highly distorted equatorial shell, indicative of stronger equatorial ligation, and the detection of a neighboring U atom suggests the formation of surface precipitates and/or oligomeric complexes.

Key Words: Cation Exchange • EXAFS • Sorption • Surface Complex • Uranium • Uranyl • Vermiculite • XANES • XRD

Clays and Clay Minerals; August 1999 v. 47; no. 4; p. 439-457; DOI: 10.1346/CCMN.1999.0470406
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)