In Situ Actinide X-ray Absorption Spectroelectrochemistry
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In Situ Actinide X-ray Absorption Spectroelectrochemistry Mark R. Antonio, Ming-Hsi Chiang, Clayton W. Williams, and L. Soderholm Chemistry Division, Argonne National Laboratory, Argonne, IL 60439-4831, U.S.A. ABSTRACT By using electrochemical techniques in simultaneous combination with EXAFS, we can stabilize a targeted charge state of a solution complex and probe the coordination of selected actinide (An) ions. In addition, we can acquire sufficient XANES over a range of controlled electrochemical potentials, in the vicinity of a redox couple, to perform a Nernst analysis. The prospect of this technique to provide here-to-fore unattainable information is demonstrated through examples of Np, Pu, and Bk speciation in solution. INTRODUCTION As a solvent and coordinating ligand, water provides the benchmark system upon which to construct a comprehensive picture of the redox speciation–coordination as a function of oxidation state–of the actinide elements. Toward this end, compilations of standard electrode potentials, such as those found in potential-pH (Pourbaix) diagrams, are heavily used to assess the complex equilibrium behavior of the uranium and the trans-U aquo ions. Yet, the original diagrams of M. Pourbaix[1] do not provide metrical information and, moreover, provide little information about ligands other than H2O, OH-, and O2-. For redox reactions of actinide (An) ions in aqueous media, the determination of thermodynamic formal potentials and coordination environments is important from a fundamental perspective in electroanalytical chemistry as well as practical perspectives concerning An mobility and reactivity in the geosphere. Whereas there are several techniques for determining formal potentials, they are limited to studies of simple systems with one redox active species, as generalized in Eq. (1). Ann+ + xe- D An(n-x)+, for x < n
(1)
We have made pioneering advances in the field of actinide X-ray absorption spectroelectrochemistry by demonstrating the use of purpose-built equipment[2] and new methodology[3] for in situ XANES and EXAFS. This technique involves the maintenance of strict electrochemical control during data acquisition at a synchrotron facility. The objective is to determine An formal potentials and An coordination and hydration behaviors of species that are not stable at rest potential. This is all done in solution as a function of targeted electrochemical potentials that are used to obtain either the single, valence-pure species (i.e., Ann+ and An(n-x)+) or mixtures of them, which are produced and stabilized solely by electrolytic means. Whereas the equipment and methods of X-ray absorption spectroelectrochemistry have evolved since the first description of its use in 1984,[4] none of the myriad of multipurpose cell designs[5-7] were directly suitable to achieve the application-specific containment and rigorous safety standards necessary for in situ experiments with
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radionuclides. This led us to the design of a purpose-built system that was originally introduced in 1
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