Uranyl nanocapsules are poly-peroxo oxometalates with the general formula [UO2(O2)1.5]nn- or [UO2(O2)(OH)]nn-; n has discrete values including 20, 24, 28, and 60. These are closed capsules with square, pentagonal and hexagonal faces. There are also related open capsules (crowns) and derivatives with other bridging polydentate ligands including pyrophosphate and oxalate. All contain alkali counterions inside and outside the capsules; and these dynamically exchange in both aqueous solution and crystalline lattices. To consider employment of these capsules in the many separations, extractions and sequestrations within the nuclear fuel cycle, we need to be able to manipulate them via solvent extraction, dissolution, precipitation, etc. Understanding the behavior of the counterions is key to these processes. In this presentation, I will talk generally about the importance of these counterions in aqueous and non-aqueous systems in defining speciation and solubility. Our major tools of characterization include small-angle X-ray scattering (SAXS) and multi-nuclear solution and solid-state NMR, which respectively provide detailed information and the capsule and the counterions.
We have developed a solvent extraction process that transfers the nanocapsules into organic solvent (i.e. kerosene) based on counterion exchange. For example long chain ammonium surfactants pull the capsules into the non-aqueous phase and the counteranions travel into the aqueous phase. We have demonstrated this entire process starting with simulated spent nuclear fuel, showing complete transfer of the uranium capsules, intact into the kerosene.
Once in the organic phase, the counterions become completely immobilized, different from their behavior in the aqueous phase. This provided opportunity to investigate coordination environments inside the capsule, of Li in particular. Combined computational and NMR studies suggest the surfactant cationic heads surround the capsules and completely block the lithium from exiting, giving rise to unprecedented behavior, including extremely rigid coordination of the alkalis, and rapid motion of the capsule within the inverse micelles.
Acknowledgements: This work was performed by the Materials Science of Actinides, an Energy Frontier Research Center funded by the Department of Energy, under award number DE-SC0001089.