Focus Area 2 – Molecular Science
Focus Area Lead by Eric Schelter
The goals of Focus Area 2(a) – Advanced Separations arise directly from the overall goals of the CAST, namely, 1) to develop first principles, predictive electronic structure approaches for chemical selectivity during processing. And 2) to develop new synthetic methodologies and provide a detailed understanding of electronic structure, from both experimental and computational perspectives, of new separations schemes for treating tank waste. Focus Area 2(b) personnel work closely and synergistically with other CAST members, especially in Focus Area 2(c) (Advanced Characterization Methods) and Focus Area 2(c) (Computational Studies of Actinides in Solution and of Waste Forms for Anions). New organic compounds and some metal complexes are synthesized and validated using experiment and theory (Penn, Purdue, FIU, FSU, and LANL) and the knowledge gained is used to develop hypotheses for testing with transuranic materials at designated institutions (LANL, FSU).
Discriminating between constituent cations in complex mixtures derived from dissolved nuclear material, which includes fission products, and neutron-capture products such as transuranic elements like neptunium, plutonium, americium, and, curium remains a challenge to recycling this material. This is especially difficult given the similarities in atomic radii, binding affinities, and charges for most metal cations in the solution. Of particular concern is americium which is most stable in the trivalent state (Am(III)) in acidic solutions like most lanthanides, and late actinides making it difficult to selectively remove. It is responsible for the majority of the long-term heat generated from the waste, and its selective removal is of paramount importance.
The most common oxidation state for the lanthanides is +3 in aqueous solutions. The actinides however are different, partially due to the shielding of the outer 5f electrons being relatively weaker than that of the 4f electrons in the lanthanides, resulting in slightly more diffuse valence electron orbitals. This difference is manifest in the actinides where high oxidation states (+4 to +6) are typical for the early actinides, while the divalent actinides are most common for the late actinides. The divide between these two actinide includes americium, which exhibits some interesting redox properties that can be exploited.
Soft Donor Ligands
Given the actinides’ more diffuse valence electron orbitals, they will have a slight preference for softer ligands than the lanthanides. Current investigations are exploring the development of tripodal ligands with soft sulfur-donor sites, and/or pyrazole groups to selectively bind actinides like americium. These ligands are ideal for caustic side separations, including those found in legacy tank waste.
Actinide Redox and Photoredox Tuning
All actinides in aqueous acidic conditions are strong enough Lewis acids in the penta- and hexavalent states that they readily form linear dioxo cations through hydrolysis of water. This includes UO22+, NpO22+, PuO22+, and AmO22+. This has created a need to develop chemical separations schemes which utilize a stabilized Am(VI) complex where AmO22+ is more easily distinguished from the lanthanides in solution.
Previous work has demonstrated that coordination of electron-donating ligands to f-block metal cations results in thermodynamic stabilization of higher oxidation states, e.g. a decrease in the M(IV/III) redox couple. We are applying these approaches to new directions in actinide separations, with a particular interest in americium. Information garnered about metals stabilized in certain oxidation states is also fundamental to the science of wasteforms. Our working hypothesis here is that the use of tailored ligand frameworks will stabilize the higher oxidation states, which will be evaluated using experiment and DFT. These fundamental observations are being leveraged into datasets for use by team members and members of the community in the development of new redox-based separations systems.
Electochemical oxidation of Am(III) to Am(IV) in potassium carbonate solution from D. E. Hobart, et al. (1982) Radiochimica Acta, 31,139. Image Source: http://periodic.lanl.gov/95.shtml
Actinide-Ligand Multiple Bonds and their Electronic Structures
Due to the complexity of the mixtures in spent nuclear fuel, in terms of chemical identity and diversity, transformation and segregation of the constituents is particularly challenging. Robust actinyl species, e.g. UO22+, are difficult to chemically transform and isolate due to their strongly-bound, trans-oxo ligands. Linear coordination geometries not only dominate the bonding of soluble actinyl species, but extend to related imido analogues of the uranyl and neptunyl ions, uranium(VI) mono(oxo) species, and uranium(V) dioxo derivatives, all of which serve as models for components at various points in the fuel cycle. In this segment of Thrust 3 we are synthesizing and characterizing actinides species with more than two multiply bound ligands. Addition of strong pi donors serves as a general strategy to weaken existing multiple bonds, gives an opportunity to understand actinide electronic structure, and provides operationally simple models for chemically challenging species in the nuclear fuel cycle. In each case, syntheses are paired with spectroscopic and structural characterization and complemented by computational studies for integration with other Thrust areas. We expect the proposed work is transformative, by demonstrating a general method for weakening and transforming An–element multiple bonds. Thus, the constituents in spent nuclear fuels could be more easily resolved when combined with current extraction methods.