Fundamental studies of actinyl cation-cation interactions in mixed-solvent media
Burn, Adam Gerard
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The actinide cations ranging from uranium to americium are unique from other metal cations in that they form nearly linear dioxo cations at or above the pentavalent oxidation state. While the overall formal charge of the actinyl cations is positive, there are regions of partial negative charge associated with the oxygen atoms. Actinyl cation-cation interactions were first observed spectrophotometrically between NpO2+ and UO22+ in acidic, high ionic strength solution conditions, similar to the solution conditions in used nuclear fuel reprocessing streams. The weak interaction between the two cations is thought to occur between a pentavalent actinyl cation donor and hexavalent actinyl or highly charged metal cation acceptor. It is known that ion-pairing reactions occur more readily in solutions with diminished water content and lower dielectric constants than dilute aqueous media. The basic solution and inorganic chemistry governing actinyl cation-cation interactions was studied with electrochemical, spectrophotometric, and computational methods. Uranyl electrochemical studies in non-aqueous aprotic polar organic media demonstrated fast UO2+ disproportionation and uranium adsorption on the working electrode surface, depending on the supporting electrolyte used. The electrochemical studies did not reveal any indications of UO22+·UO2+ interactions. Neptunyl cation-cation interactions were spectrophotometrically observed in mixed aqueous-polar organic solvent solutions with highly charged metal cations. Small metal cations with similar hard/soft acid characteristics to the NpO2+ cation in mixed aqueous-aprotic polar organic media produced strong complexes. Computational density functional theory studies modeled the geometric and electronic changes in actinyl cation-trivalent metal cation structures. Similar geometric changes in the actinyl cation-trivalent metal cation structures were observed as a function of the coordinating trivalent metal cation ionic radius. The changes in the electron density in the trivalent metal cation and bridging oxygen of the actinyl cation in the modeled structures were dependent on the valence orbital type and radial extension of the trivalent metal cation. It is concluded that both the solution conditions and metal cation identity play crucial roles for influencing the free energy of complexation in actinyl cation-cation interactions.