Uranium(VI) sorption onto hematite surfaces
EMSL Project ID
2624
Abstract
Sorption processes on mineral surfaces are relevant for assessing the performance of deep geological nuclear waste deposit sites. Knowledge of basic sorption interactions on a molecular scale is prerequisite for understanding and modeling transport phenomena of actinide species in the environment. As iron phases are mineral phases with enhanced sorption properties, it is important to have detailed information on their interaction with actinides. We are interested in uranium(VI) sorption onto various mineral phases, including hematite. Of special interest is the identification of existing preferential surface sorption sites and the nature of the sorbed species. X-ray absorption fine structure (XAFS) spectroscopy is an element specific technique, which can be used for characterizing surface sorbed cation species. Bulk sorption studies on polycrystalline samples generally yield a mean, global view of the sorbed species. Differentiation between sorption sites on the various crystallographic planes present in a bulk sample is not possible. This can be done in experiments performed on single crystals, cut in specified orientations. By recording the XAFS spectrum of a cation sorbed single crystal at a grazing angle of incidence between the incoming synchrotron beam and the crystal surface (i.e., a GIXAFS experiment), the specific sorbed species on that plane can be identified and its structure characterized. Measuring the polarization dependence of the GIXAFS signal for the linear uranium(VI) or uranyl cation yields additional information of the orientation of the uranyl sorbed species on the surface. This facilitates identification of any preferential sorption sites. For example, the orientation of uranyl cations sorbed onto various a-Al2O3 crystallographic planes has been successfully identified from the polarization dependency of their U L3 edge GIXAFS spectra. To perform an analogous study on hematite surfaces (isostructural to a-Al2O3), large hematite crystals with various crystallographic surface planes must be available. Preparation of single crystal hematite surfaces with specified crystallographic planes is an experimental challenge. The oxygen-plasma-assisted molecular beam epitaxy (MBE) has been used successfully for growing epitaxial layers of hematite onto a-Al2O3. We wish to prepare epitaxially grown hematite with (110) and (100), using a-Al2O3 as substrate, with the oxygen-plasma-assisted MBE method. We intend to publish the results of the GIXAFS measurements with the hematite films in Physical Chemistry Chemical Physics (PCCP) coauthored with EMSL staff.
Project Details
Project type
Exploratory Research
Start Date
2003-04-08
End Date
2004-04-15
Status
Closed
Released Data Link
Team
Principal Investigator