Actinide Redox Speciation and Structure Analysis in Aqueous and Nonaqueous Solutions

Actinide redox chemistry plays a crucial role in nuclear technology like fuel reprocessing and nuclear waste disposal aspects including predicting of actinide mobility in the environment. Hydrolysis and complex formation of the early actinides are strongly related with their redox behavior in aqueous and nonaqueous solutions. We are interested in understanding the complexation of actinide compounds under controlled redox conditions in presence of inorganic oxo ligands and organic ligands with carboxylic groups.
This study is mainly focused on the use of EXAFS spectroscopy in combination with other supporting methods. The information from EXAFS spectroscopy is restricted to a pair distribution function providing coordination numbers and distances of the next neighbors. Two pathways were used to relate this information to the spatial arrangement of the ligands: (i) EXAFS was combined with DFT calculations which introduce constraints of molecular characteristics [1], and (ii) individual solution species were preserved in crystal structures and determined with single crystal diffraction. EXAFS was used subsequently to quantify differences or identify agreement between the coordination of complex structures in solution and solid state [2-4]. EXAFS is furthermore not very sensitive to differentiate between individual solution species, especially in not a single sample. This problem was solved in the following manner: (i) combination of EXAFS with other more species sensitive techniques, e.g. like UV-Vis spectroscopy [5] and (ii) by using sample series with broad variation of species distribution and subsequent application of statistical analysis techniques to separate the scattering contribution of individual solution species [6]. Finally, we investigated the correlation between the formal redox potential and the stability range of solution species [7].

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