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Radiation sensitivity of natural organic matter: Clay mineral association effects in the Callovo-Oxfordian argillite

Abstract : Clay-rich low-organic carbon formations (e.g., Callovo-Oxfordian argillite in France and Opalinus Clay in Switzerland) are considered as host rocks for radioactive waste disposal. The clay-organic carbon has a strong impact on the chemical stability of the clays. For this reason, the nature of the clay-organic carbon, the release of hydrophilic organic compounds, namely, humic (HA) and fulvic acids (FA) and the radiation sensitivity of the undisturbed host rock organics was investigated. The clay sample originates from Oxfordian argillite (447 m depth, borehole EST 104). HA and FA were extracted following the standard International Humic Substance Society (IHSS) isolation procedure. Synchrotron based (C-, K-, Ca-, O- and Fe-edge XANES) scanning transmission X-ray microscopy (STXM) and FT-IR microspectroscopy was used to identify under high spatial resolution the distribution of clay-organic matter with different functionality using principal component and cluster analysis. The results show that in this old (Jurassic) geological formation, small parts of the organic inventory (1–5%) keeps the structure/functionality and can be mobilized as hydrophilic humic substance type material (HA and FA). Target spectra analysis shows best correlation for isolated humic acids with organics found in smectite-rich regions, whereas the extractable FA has better spectral similarities with the illite mixed layer minerals (MLM) regions. After radiation of 1.7 GGy under helium atmosphere the same rock sample area was investigated for radiation damage. Radiation damage in the smectite and illite–MLM associated organic matter is comparably low with 20–30% total oxygen mass loss and 13–18% total carbon mass loss. A critical dose dc of 2.5 GGy and a optical density after infinite radiation (OD∝) of 54% was calculated under room temperature conditions. C(1s) XANES show a clear increase in CC bonds especially in the illite–MLM associated organics. This results suggests a combination of the formation of CC bond due to crosslinking via polymerization and mass loss due to bond breaking (scissioning) in the main chain or in side groups of the organic macromolecules upon irradiation.
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Soumis le : mardi 31 août 2010 - 15:42:21
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Thorsten Schäfer, P. Michel, Francis Claret, T. Beetz, S. Wirick, et al.. Radiation sensitivity of natural organic matter: Clay mineral association effects in the Callovo-Oxfordian argillite. Journal of Electron Spectroscopy and Related Phenomena, Elsevier, 2009, 170 (1-3), p. 49-56. ⟨10.1016/j.elspec.2008.05.007⟩. ⟨hal-00512777⟩



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