X-ray absorption studies of uranium sorption on mineral substrates Page: 4 of 9
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X-RAY ABSORPTION STUDIES OF URANIUM
SORPTION ON MINERAL SUBSTRATES
ERIC A. HUDSON*, LOUIS J. TERMINELLO*, BRIAN E. VIANI*, TOBIAS REICH**,
JEROME J. BUCHER***, DAVID K. SHUH***, AND NORMAN M. EDELSTEIN***
* Lawrence Livermore National Laboratory, Livermore, CA 94551
** Forschungszentrum Rossendorf e. V., Institut fUr Radiochemie, Postfach 51 05 19,
D-01314 Dresden, Germany
*** Lawrence Berkeley Laboratory, Berkeley, CA 94720
Uranium L3-edge x-ray absorption spectra have been measured for uranium-mineral sorption
systems. An expansible layer silicate, vermiculite, was treated to obtain a collapsed and non-
expanding phase, thereby limiting access to the interior cation exchange sites. Samples were
prepared by exposing the finely powdered mineral, in the natural and modified form, to
aqueous solutions of uranyl chloride. EXAFS spectra of the encapsulated samples were
measured at the Stanford Synchrotron Radiation Laboratory. Results indicate that the uranyl
ion possesses a more symmetric local structure within the interlayer regions of vermiculite than
on the external surfaces.
Aqueous transport of actinides in the geosphere can be significantly retarded by sorption
and/or precipitation upon mineral surfaces. Despite the importance of this effect to
environmental remediation efforts and nuclear waste repository design, there is only limited
direct knowledge about the microscopic nature of the actinide-mineral interactions. X-ray
Absorption Spectroscopy (XAS) is one of the few techniques which can study the local
environment of elements present in low concentrations in a complex solid matrix, because it is
element-specific and probes only the short-range electronic and geometric structure. The
penetrating abilities of hard x-rays allow the in-situ study of encapsulated samples, which is
particularly desirable in the study of radioactive materials. In the experiments described here,
XAS was used to characterize uranium in contact with vermiculite, an expansible layer silicate
mineral. Vermiculite particles are characterized by =10 A Mg-Al-silicate layers which are
negatively charged due to isomorphous substitution. The negative charge is countered by
cations residing between the layers. These interlayer regions are normally hydrated and thus
accessible to aqueous-phase cations, permitting cation exchange. Results of our Extended X-
ray Absorption Fine Structure (EXAFS) measurements indicate that the uranyl ion possesses a
more symmetric local structure within the interlayer regions of vermiculite particles than on the
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Hudson, E. A.; Terminello, L. J. & Viani, B. E. X-ray absorption studies of uranium sorption on mineral substrates, article, November 1994; California. (digital.library.unt.edu/ark:/67531/metadc794554/m1/4/: accessed December 13, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.