EXAFS Investigations of U(IV) species in aqueous solutions with a newly developed spectro-electrochemical cell

Actinides with their large number of oxidation states are susceptible to redox conditions, forming different aqueous complexes which may greatly differ by solubility and mobility. These complexes are often difficult to investigate due to their thermodynamic metastability. Therefore, we developed a new spectro-electrochemical cell which allows to study the structure and speciation of aqueous actinide complexes in situ by X-ray absorption spectroscopy, while applying and maintaining a constant potential.
The safety regulations for handling of radioactive materials at the ESRF prohibit any gas release from samples containing radionuclides. Therefore the electrochemical cell is air-tight and the applied reduction process prevents a water decomposition. The electrochemical cell comprises a double confinement. A well-defined anodic reaction is applied by using Ag as anode in equilibrium with hardly soluble AgCl. Formal cathodic potentials Ecs of the U(VI)/U(IV) couple were verified by cyclic voltammetry. The potential Ecs+E was then kept constant for the subsequent reduction process of U(VI) to U(IV).
The coordination of the U(VI) species before reduction and the electrochemically stabilized U(IV) ions in presence of different ligand systems has been studied by U LIII-edge EXAFS spectroscopy. In non-complexing perchloric acid the U(IV) aquo ion was obtained with 9 spherically arranged atoms in a distance of 2.41 Å. In concentrated chloride solutions we observed between 3 to 9 M [Cl] the complexes U(H2O)8Cl3+, U(H2O)6-7Cl22+ and U(H2O)5Cl3+ with U-O distances of 2.41 Å and U-Cl distances of 2.71 Å [1]. In presence of sulfuric acid, the coordination with SO42- seems to change from bidentate with U(VI) to monodentate fashion with U(IV) at low pH. We observed in presence of formic acid tetranuclear U(IV) species with U-U distances of 3.85 Å. Investigations in carbonate solutions are in progress.
[1] C. Hennig, J. Tutschku, A. Rossberg, G. Bernhard, A.C. Scheinost, Comparative EXAFS investigation of U(VI) and U(IV) aquo chloro complexes in solution using a newly developed spectro-electrochemical cell, Inorg. Chem., submitted