Surface processes of Np on selected mineral oxides. Vibrational spectroscopic investigations

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The structures of different interface complexes of neptunium (Np(V)) at mineral oxides are to be identified on a molecular level. The interface complexes will be studied under specific and environmentally relevant conditions by variation of selective parameters, such as pH or ionic strength of the solvent, and by the promotion of different surface complexation pathways. The structures of intermediate oligomeric, colloidal or precipitated molecule complexes were not determined so far, although there is scientific agreement about their occurrence. As major innovation of this project, the possible sorption processes, namely the development of monomeric and oligomeric surface complexes, and the formation of surface precipitates or surface colloids will be spectroscopically investigated. This in turn will allow a generalization of reaction patterns of chemically similar but experimentally difficult to access actinyl systems, e.g., Pa or Pu. In addition, the results will supply well-established quasithermodynamic sorption models with adequate approaches for competing processes such as surface precipitation. The processes will be mainly identified and characterized in aqueous solution by in situ ATR FT-IR spectroscopy. Because the characteristic frequencies of the vibrational modes of distinct functional groups, i.e. the neptunyl ion and the ubiquitous inorganic ligand such as the carbonate anion, are generally well separated in the infrared spectra, this technique will provide comprehensive molecular information of the interface processes. Complementary spectroscopic techniques, e.g. XAS and TRLFS, and microscopic methods, e.g. AFM and CLSM will also be applied.