Spatially resolved micro X-ray fluorescence investigation of a uranium-rich sediment

Micro X-ray fluorescence (μ-XRF) experiments in confocal geometry are made at the Fluoro-Topo-Beamline at ANKA to map Fe, As, U and other trace element distributions at various depths of a U-rich sediment sample. A new planar refractive X-ray lens (CRL) array developed and fabricated by Forschungszentrum Karlsruhe, Institute for Microstructure Technology, is used as primary focusing optic, providing a 5 x 2 μm2 (H x V) beam spot. Correlations extracted from elemental maps reveal that U is consistently in close association with As. However, areas rich in As almost invariably do not overlap U hot spots. As a direct result of the high lateral resolution using the CRL array, it is possible to discern an As-rich boundary layer surrounding Fe(II)-nodule surfaces in the sample. These results are consistent with a suggested mechanism of U immobilisation where mobile groundwater-dissolved U(VI) was reduced on arsenopyrite sites in the sediment, leading to precipitation of less-soluble U(IV). (orig.)