Abstract
Micro-X-ray fluorescence and micro-X-ray absorption spectroscopy (μ-XRF/XAS) have been used to determine the elemental distribution and the coordination environment of U(VI) and Np(V,VI) in intact hardened cement paste (HCP) samples at the atomic level. Micro-XRF maps reveal a heterogeneous distribution of the actinides. Micro-XAS measurements on a few U(VI) hot spots show that two main U(VI) species contribute to the spectra: an U(VI) species with a chemical environment similar to that in U(VI) loaded HCP and calcium silicate hydrates (C-S-H) sorption samples and an U(VI) species with a chemical environment similar to that of U(VI) in calcium uranate. Micro-XANES indicates presence of both Np(IV,V) oxidation states in Np(V) loaded intact HCP, presumably caused by beam damage. Earlier studies have shown that U(VI) is taken up into the interlayer of C-S-H phases, and recently it was suggested that the same mechanism controls Np(IV,V) uptake by these phases. The present study is complementary to earlier ones carried out in our research group and demonstrates that information on the coordination environment of actinides is essential with the aim of developing a mechanistic understanding of actinide interaction with cementitious materials. The micro-scale studies show that actinide uptake by cementitious materials has to be interpreted in terms of solid solution formation rather than surface binding (ion exchange, surface complexes).
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Acknowledgements
Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program (P41PR001209). The SLS (Villigen PSI, Switzerland) and ESRF (Grenoble, France) are acknowledged for the provision of beam time. The authors thank the beamline scientists of the following facilities for their technical support: MicroXAS at SLS, BL 2-3 at SSRL and BM20 (ROBL) at ESRF. Thanks are extended to Dr. M. Marques-Fernandes and D. Kunz for assistance during measuring campaigns. Prof F.P. Glasser is gratefully acknowledged for provision of the uranium reference materials. Partial financial support was provided by the “ACTINET” project [sixth EC framework programme (FP)], by the European Project “RECOSY” (seventh EC FP) and by the National Cooperative for the Disposal of Radioactive Waste (Nagra), Switzerland. X.G. acknowledges the Generalitat de Catalunya for his post-doctoral grant (“Beatriu de Pinós” program). Portions of this research were carried out in the framework of a Marie Curie Fellowship with a grant to N.M. financed by the EC (Contract No. FP6-044811).
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Wieland, E., Dähn, R., Gaona, X., Macé, N., Tits, J. (2013). Micro- and Macroscopic Investigations of Actinide Binding in Cementitious Materials. In: Bart, F., Cau-di-Coumes, C., Frizon, F., Lorente, S. (eds) Cement-Based Materials for Nuclear Waste Storage. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-3445-0_9
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