In situ speciation of U(IV) and U(VI) aqueous complexes with a newly developed spectro-electrochemical cell

Natural aquatic and terrestrial environments exert large variations in redox state due to oxygen diffusion on one hand and microbial processes on the other hand. Actinides with their large number of oxidation states are especially susceptible to these redox changes, forming a large number of 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 (XAS), while applying and maintaining a constant potential.

While in-situ investigations by combining electrochemical cells with XAS are not new, the prevention of any gas-releasing electrode reaction due to the special safety regulations for actinides required a new electrochemical approach. We used an Ag metal electrode as anode, where Ag+ is released and precipitates as AgCl. The cell itself consists of a double confinement following the safety regulations for the use of radioactive materials at the ESRF. The cell body and the windows are machined from one piece of chemically resistant material, which is closed by air-tight cover plates.

First U L3-edge XAS spectra have been obtained from aqueous solutions of U(VI) and U(IV) in the presence of different ligand systems. The reduction of U(VI) to U(IV) was performed by applying a constant potential for several hours. Independent UV/vis measurements of the solutions were performed in order to verify that uranium was completely reduced. A quantitative analysis of the uranium redox species during the reduction procedure was performed by iterative transformation factor analysis of the XANES spectra. Finally, EXAFS measurements were used to solve the structure of the uranium redox species.