Uranium at solid/water interfaces: Lessons to be learned from X-ray absorption spectroscopy

X-ray absorption spectroscopy is a versatile tool to investigate oxidation state and molecular structure or uranium in aqueous and solid phases and at their common interface. However, the structural analysis is often hampered by limited resolution and range. I will show recent advances to overcome these limitations, including improved XAFS data analysis approaches like Monte-Carlo and Landweber methods; coupling to XAFS-independent methods like DFT and surface complexation modeling as well as to other x-ray methods like HEXS which have a longer detection range. I will demonstrate the usefulness of these methods by showing recent results on uranyl sorption to mineral surfaces, with a focus on polynuclear and carbonate complexes; Fe(II)-driven interfacial redox processes of uranyl, also in comparison to other actinoides; colloid formation processes of U(IV) and its tetravalent actinoide neighbors.