Surface processes of Np on selected mineral oxides. Vibrational spectroscopic investigations
Final Report Abstract
Mineral oxyhydroxides play a decisive role during the reactive transport of dissolved contaminants in the environment. The capability of rocks and soils to retard aqueous heavy metal ions is determined by surface reactions at solid-liquid interfaces. They range from sorption processes to formation of new mineral phases with possible intermediate stages, such as oligomerization, colloid formation, clustering or precipitation. A comprehensive understanding of the chemical reactions and physical interactions between the dissolved heavy metal species and the mineral surfaces is essential for predicting the cycling of contaminants such as actinides and other radioactive contaminants in the bio- and geosphere. In the present work, the speciation of uranium(VI) and neptunium(V) at environmentally relevant mineral – water interfaces of oxides of alumina, iron, silica, and titania has been comprehensively investigated by the application of complementary spectroscopic techniques. Attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy is a very useful tool for the in situ and time-resolved identification of molecule species sorbed on mineral phases on a molecular level. In addition, it has been previously shown that aqueous solutions of actinides can be investigated even at concentrations in the micromolar range, allowing the measurement at increased pH conditions without the risk of formation of colloidal and solid phases. By the application of X-ray absorption spectroscopy, detailed information about prevailing redox states of the actinyl ions and interatomic distances between single U and Np atoms and surface functionalities of the surface complexes can be obtained. The formation of actinyl complexes on metal oxides was monitored under environmentally relevant conditions, namely broad pH range from 4 – 10, moderate ionic strengths (0.001 – 1 M), in the presence and absence of additional ligands, provoking the formation of ternary sorption complexes. For U(VI) different surface species were identified on TiO2 and Al2O3 as a function of surface coverage. For instance U(VI) inner- outer sphere complexes were identified on different TiO2 phases. In case of alumina the formation and transition of surface complexes from sorption of U(VI) monomers and oligomers to surface precipitation was observed and confirmed by a multi-spectroscopic approach. In contrast to U(VI), Np surface chemistry seems to me less complex, since the formation of only one stable Np(V) complex was found for a number of different mineral oxide phases. The multiplicity of IR spectroscopic experiments carried out within this study yields a profound collection of spectroscopic data which will be used as references for future investigations of more complex sorption systems in aqueous solution. Furthermore, from a methodological point of view, this study comprehensively extends the application of ATR FT-IR spectroscopic experiments to a wide range in the field of radioecology. The results obtained in this work contribute to a better understanding of the geochemical interactions of actinides, in particular U(VI) and Np(V), in the geo- and biosphere. Consequently, more reliable predictions of actinides migration which are essential for the safety assessment of nuclear waste repositories can be performed.
Publications
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Uranium sorption on various forms of titanium dioxide – influence of surface area, surface charge and impurities, Environmental Science and Technology 45 (2011) 5536-5542
M. J. Comarmond, T. E. Payne, J. J. Harrison, S. Thiruvoth, H. K. Wong, K. Müller, H. Foerstendorf
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Sorption of U(VI) at the TiO2 – water interface: An in situ vibrational spectroscopic study, Geochimica Cosmochimica Acta 76 (2012) 191-205
K. Müller, H. Foerstendorf, T. Meusel, V. Brendler, M. J. Comarmond, G. Lefèvre, T. E. Payne
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Spectroscopic identification of binary and ternary surface complexes of Np(V) on gibbsite, Environmental Science and Technology 47 (2013) 14418–14425
K. Gückel, A. Rossberg, K. Müller, V. Brendler, G. Bernhard, G. Foerstendorf, H.
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The surface reactions of U(VI) on γ-Al2O3. In situ spectroscopic evaluation of the transition from sorption complexation to surface precipitation, Chemical Geology 357 (2013) 75- 84
K. Müller, H. Foerstendorf, V. Brendler, A. Rossberg, K. Stolze, A. Gröschel
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Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study, Applied Geochemistry 49 (2014) 116-125
K. Schmeide, S. Gürtler, K. Müller, R. Steudtner, C. Joseph, F. Bok, V. Brendler
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In situ spectroscopic identification of neptunium(V) inner-sphere complexes on the hematite-water interface. Environmental Science and Technology, Environ. Sci. Technol., 2015, 49 (4), pp 2560–2567
K. Müller, A. Gröschel, A. Rossberg, F. Bok, C. Franzen, V. Brenldler, H. Foerstendorf
