Confocal [mu]–XRF and [mu]–XAFS Studies of an Uranium-Rich Sediment from a Nuclear Waste Disposal Natural Analogue Site

Abstract

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).

µ-X-ray fluorescence and µ-X-ray diffraction investigations of sediment from the Ruprechtov nuclear waste disposal natural analog site

“…The U distribution is localized to the left of the scanned area. As in our previous investigation [6], U is found in areas void of As and Fe. Likewise, scanned areas with high As and Fe Kα intensities appear as white pixels in the U Lα intensity distribution.…”

“…The present work is a continuation of previous micro-focused X-ray investigations on the same sediment [6][7][8]. Results from these previous studies allow us to formulate a hypothesis for one mechanism of U immobilization during diagenesis of the sediment involving arsenopyrite, AsFeS, as reducing agent.…”

“…A thin section of the same Ruprechtov bore core sample studied in [6] is obtained by cutting the sample, mounting it on a 25 × 45 mm 2 fused silica slide with an epoxy glue, and polishing it down to approximately a 20 µm thickness. The slide mount leads to substantial amorphous background in the XRD CCD images out to 2Θ of around 17°, which must be subtracted from the integrated 1D diffractograms.…”

“…The U(VI) migrates to a deeper, anoxic horizon with an underlying low-permeable kaolin layer and is reduced on arsenopyrite in the sediment, leading to precipitation of less-soluble U(IV). The arsenopyrite has been identified from its As K-edge X-ray absorption near edge structure (XANES) signature [6] and the observed linear dependence between Fe and As(0) distributions obtained from µ-XRF measurements on As-rich coatings on the surface of sediment framboidal Fe nodules [7]. The µ-XRF and µ-XRD investigations reported here aim at securing evidence of arsenopyrite in the sediment from its diffraction pattern.…”

Nature and reactivity of layered double hydroxides formed by coprecipitating Mg, Al and As(V): Effect of arsenic concentration, pH, and aging

“…Their As K edge energies were the same as the HNa 2 AsO 4 reference and differed from As(III) in the standard NaAsO 2 by about 3eV (Figure 2B), comparable to reported energy differences [31]. All the LDH samples and LDH references exhibited a pronounced shoulder similar to the HNa 2 AsO 4 spectrum but absent in the NaAsO 2 XANES ( Figure 2A).…”