Micro-focused synchrotron radiation techniques to investigate actinide elements in geological samples are becoming an increasingly used tool in nuclear waste disposal research. In this article, results using mu-focus techniques are presented from a bore core section of a U-rich tertiary sediment collected from Ruprechtov, Czech Republic, a natural analog to nuclear waste repository scenarios in deep geological formations. Different methods are applied to obtain various, complementary information. Elemental and element chemical state distributions are obtained from micro-XRF measurements, oxidation states of As determined from micro-XANES, and the crystalline structure of selected regions are studied by means of micro-XRD. We find that preparation of the thin section created an As oxidation state artifact; it apparently changed the As valence in some regions of the sample. Results support our previously proposed hypothesis of the mechanism for U-enrichment in the sediment. AsFeS coating on framboid Fe nodules in the sediment reduced mobile groundwater-dissolved U(VI) to less-soluble U(IV), thereby immobilizing the uranium in the sediment.

Investigations by micrometer-scale X-ray fluorescence and X-ray absorption fine structure (micro-XRF and micro-XAFS) recorded in a confocal geometry on a bore core section of a uranium-rich tertiary sediment are performed in order to assess mechanisms leading to immobilization of the uranium during diagenesis. Results show uranium to be present as a tetravalent phosphate and that U(IV) is associated with As(V). Arsenic present is either As(V) or As(O); we found no evidence for As(III). The As(O) is observed to be intimately associated with the surface of Fe(II) nodules and likely arsenopyrite. A hypothesis for the mechanism of uranium immobilization is proposed, where arsenopyrite acted as reductant of groundwater-dissolved U(VI), leading to precipitation of less soluble U(IV) and thereby forming As(V).

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Our model considers the contaminants as dissolved in the aqueous phase and sorbed to the sediment or sorbed to mobile and immobile colloids. All sorption processes are assumed to be reversible and are modeled as a kinetic-Langmuir reaction. The Europium Migration ExperimentThe column experiments with •52Eu(II!) in a humic-rich groundwater were performed by Klotz and Lazik [1996]. Batch experiments with the same groundwater sediment system were conducted to determine the s0rption .distribution coefficient Rs More than 98% of europium in the injected solution is bound to colloidal humic particles, and the predominant humic colloid size is in the range from 1 to 100 nm.

We perform spatially resolved X-ray fluorescence and absorption fine structure investigations with a micrometer-scale resolution (µ-XRF and µ-XAFS) recorded in a confocal geometry on a bore core section from the uranium-rich tertiary sediment [1]. The aim of this study is to assess mechanisms leading to immobilization of the uranium during diagenesis of the uranium enriched horizon. In order to probe micro-volumes below the sample surface, a confocal irradiation-detection geometry is employed. For this purpose, two half-lenses are used; one is used to focus the primary beam and another collimating half-lens in front of the detector, perpendicular to the first lens [2]. By scanning sample areas at different depths, stacks of tomographic cross sections may be easily recorded. A polycapillary half-lens as well as planar compound refractive lenses are used to focus the beam to a 20 µm diameter spot and to an ellipsoid spot with about 2.5 µm vertical and 5 µm horizontal dimensions, respectively. By varying the energy of the beam at a constant sample position, XAFS spectra are recorded. Both the near edge XAFS (XANES) region and the extended XAFS (EXAFS) energy region are investigated. U L3 µ-XANES and µ-EXAFS results show uranium to be present as a tetravalent, poorly crystalline phosphate (or sulfate), possibly ningyoite (Fig. 1A&1B). The U L3 EXAFS is measured in a sample area bordering a lignite inclusion but does not resemble that expected for U ligation with organic material [3]. This suggests that lignite may have not been involved in U immobilization. Stacks of images at different depths reveal uranium to be near both framboidal Fe(II) nodules and to arsenic (Fig. 2). The arsenic present is either As(V) or As(0) (Fig. 3); we find no evidence for As(III). As(0) is observed to be intimately associated with the surface of Fe(II) nodules (Fig. 4) and likely arsenopyrite. The U(IV) is associated with As(V). That As(V) is found in addition to As(0), even in volumes probed below the surface, combined with the observation that the As(V) is associated with U hot spots allows to propose that arsenic was involved in the U(VI) immobilization during diagenesis. We propose a hypothesis for the mechanism of uranium immobilization, where arsenopyrite acted as reductant of ground water dissolved U(VI) leading to precipitation of less soluble U(IV) and thereby forming As(V).

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