THE HEXAVALENT ACTINIDES: SOME NEW DIRECTIONS IN THE STUDY OF REDOX KINETICS AND COMPLEXATION THERMODYNAMICS.

摘要

The environmental behavior of the actinide elements will depend on their redox and complexation chemistries. Chapter I of this Dissertation briefly reviews actinide solution chemistry on the fundamental and applied (i.e. environmental) levels. Three projects involving the hexavalent actinides are then discussed.; Solvent extraction by thenoyltrifluoracetone was found suitable for investigation of tracer-level actinide redox reactions. This technique was employed in the study of reactions of Np(VI) with water-soluble carboxylates, phenols, and other oxygen-containing organic ligands. Results from this study are presented in Chapter II. Ligand properties necessary to effect reduction were identified as aromaticity and the presence of a hydroxy function. A rate equation for the reduction of Np(VI) by salicylate was derived which shows first order dependency on ligand and metal and inverse dependency on hydrogen ion concentration. Ligand substitution para to a phenolic group was found to slow ligand oxidation by Np(VI). Reasons for this effect are suggested.; Chapter III describes the development and characteristics of a coated-wire electrode specific for actinyl cations. The electrode was found to respond rapidly and reproducibly to 10('-5) - 10('-2) M U(VI) with a slope almost twice that predicted by the Nerst equation. Tri- and tetravalent class a cations did not interfere with uranyl detection by the electrode, but pentavalent interference was severe. Possible mechanisms for the function of the ion-sensitive membrane are suggested, and potential uses of the electrode in actinide redox studies are discussed.; The results of an investigation of the thermodynamics of Pu(VI) complexation by bicarbonate at pH 8 are presented in Chapter IV. From the previously determined 1:1 stability constant and from calorimetric data, the stability constant for formation of the 1:2 complex was estimated, and the enthalpy and entropy of formation of both complexes were determined. Based on thermodynamic cycles and comparison with literature data, PuO(,2)(OH)(HCO(,3)) and PuO(,2)(CO(,3))(,2)('2-) were proposed as structures for the 1:1 and 1:2 complexes, respectively.