Multi method (STXM, μfT-IR, μxAFS) approach to identify uranium speciation in argillites from the Lodève Basin (France)

Abstract

Spatially resolved spectroscopic investigations with a nanometer to micrometerscale resolution (μ-XRF, μ-XAFS, STXM and μ-FTIR) focused on the U speciation in two clay-rich samples originating from the Autunian shales (Permian Lodève Basin; France) a natural uranium deposit associated with organic matter (bitumen). The goal of this study was to determine the U oxidation state in the samples and to ascertain if any correlation between the U distribution and that of other selected elements present in the clay (K, Ca, Ti, Fe, Zr and C) exists. Identification of uranium associated with organic matter and/or mineral phases (e.g. iron oxides, clay minerals) can potentially give insight into the mechanism of uranium immobilization in this sedimentary formation. μ-XRF and μ-XAFS measured in confocal geometry show that the U is present as U(IV) and its distribution is not correlated to areas with a high Fe content, but rather correlates with the distribution of lighter elements. U L3 μ-EXAFS recorded at areas with high U concentration reveals that U in these hot spots likely present as a nanocrystalline uraninite (UO2) - like phase. Clay fractions of the samples characterized by XRD are mainly composed of chlorite and illite. STXM carbon K-edge, potassium L-edge and iron L-edge investigations conducted on sulphur-embedded microtomes show two types of organic matter present, one positively correlated with the elements K and Fe and another of pure organic composition building fracture infill. The μ-FTIR measurements clearly reveal a direct association of organic matter with clay minerals. From combined results a tentative hypothesis of U immobilization is put forward. © 2009 IOP Publishing Ltd.