The Structures of Polynuclear Th(IV) Hydrolysis Products
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0986-OO09-02
The Structures of Polynuclear Th(IV) Hydrolysis Products Richard E. Wilson, S. Skanthakumar, and L. Soderholm Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL, 60439
ABSTRACT The hydrolysis chemistry of the tetravalent actinides is discussed based on recent studies with thorium. The coupling of high energy x-ray scattering and single crystal diffraction has provided insight into the structures of the polynuclear complexes formed by hydrolytic reactions of the tetravalent actinides. The success of these experiments with thorium presents many opportunities for the elucidation of the structures and thermodynamic quantities describing the dissolved polynuclear complexes of the heavier actinides.
INTRODUCTION The tetravalent actinides (An4+) are among the hardest cations in the Periodic Table, an attribute arising from their high ionic charge and small ionic radii. As a result, they readily hydrolyze, even under acidic conditions. This propensity towards hydrolysis severely complicates attempts to realize a fundamental understanding of their solution chemistry because of the different solubilities of the An(OH)n(4-n)- species. In addition, hydrolyzed species tend to undergo condensation reactions to form oligomeric oxides or oxyhydroxides of variable size and solubility, and crytallinity. Despite this complicated chemistry, there exists a considerable volume of data regarding the formation, stability, speciation, and solubility of the tetravalent actinides. The wide scatter among these studies arises principally because of the difficulty in identifying and quantifying the species present in solution, their stoichiometries and structure[1]. Indeed even the structure of the tetravalent aqua ions is a matter of considerable debate. The disagreement in the thermodynamic data is most remarkable in the measurement of the solubility constants for amorphous and crystalline phases of the An(IV) oxides and hydroxides[2]. The variability in the measured values has been ascribed to the formation of colloidal species of arguable structure[1, 3-5]. Hardness of the An4+ ions increases with increasing atomic number. Thorium, the softest of the series, has no fewer than six different hydroxide complexes assigned to its speciation as a result of hydrolysis reactions. Moving across the period to the heavier actinides, U(IV), Np(IV), and Pu(IV), these discrete polynuclear species are absent in the known thermodynamic databases[2]. The absence of the discrete molecular species is particularly troublesome in light of the general tendency to use homologues such as Ce(IV), Ln(III), U(VI), and even Fe(III) to draw conclusions and inferences about the chemistry of the middle actinides. There is no doubt that interesting chemistry has come from these studies, however its relevance to tetravalent actinide chemistry is unclear. In light of the peculiar absence of the discrete polynuclear complexes of U(IV), Np(IV), and Pu(IV), we have undertaken a study of the structures of the hydrolysis reactions
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