Developing a Fundamental Basis for the Characterization, Separation, and Disposal of Plutonium and Other Actinides in High Level Radioactive Waste: The Effect of Temperature and Electrolyte Concentrations on Actinide Speciation
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For the hexavalent actinides, research has been completed to date using the uranyl cation (U022') as an analog for hexavalent PuO2 2. As with the work on the trivalent systems, we have used a combination of indirect and direct methods to obtain stability constants. Potentiometric titrations and calorimetry have been used to estimate stability constants and enthalpies and entropies of complexation for the 1:1, 1:2, and 1:3 uranyl:acetate complexes, as reported in Table 2. Stepwise, the thermodynamic parameters of the third complex significantly differ from the first two complexes in two respects: 1) the third step is exothermic while the first two steps are endothermic; 2) the entropy of the third step is much smaller than the first two steps. Such difference could be well explained with the structural information obtained by EXAFS. As shown in Figure IA, the third acetate coordinates to uranyl in a monodentate fashion. This means that the third step of complexation does not cause as much dehydration and disorder as the first two steps. Figure 1B shows that, at higher pH, the hydroxyl group could compete with acetate for uranyl coordination. Again, these data indicate that, for the hexavalent actinide systems, the effect of temperature on coordination could be underestimated from ambient temperature thermodynamic data
Table 2: Thermodynamic data for the complexation of the uranyl cation with acetate as a function of temperature (The data at 250C marked with * are from the literature).
Clark, Sue B.; Rao, Linfeng & Wood, Scott.Developing a Fundamental Basis for the Characterization, Separation, and Disposal of Plutonium and Other Actinides in High Level Radioactive Waste: The Effect of Temperature and Electrolyte Concentrations on Actinide Speciation,
report,
June 1, 2000;
Pullman, Washington.
(https://digital.library.unt.edu/ark:/67531/metadc787895/m1/3/:
accessed April 25, 2024),
University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu;
crediting UNT Libraries Government Documents Department.