Using Ionic Liquids to Extract
Neptunium—Pathways to Nuclear Recycling
Ionic liquids (ILs) are low-melting salts that have been proposed as electrolyte solutions for the separation of actinides during the reprocessing of used nuclear fuel. Functionalized ILs can address the low solubility of actinides by incorporating coordinating groups into the structure of the IL to promote complexation. In work published online June 24th (in print July 25th) in the chemistry journal Chemical Communications (ChemComm), a team lead by LANL scientists report on the unusual redox stability of neptunium in an ionic liquid.
In this novel series of spectrophotometric experiments, the team studied the behavior of neptunium in the ionic liquid betaine bistriflimide, [Hbet][Tf2N] at room temperature and 60 °C. They observed an unprecedented complex redox chemistry in neptunium, which showed up to three oxidation states (IV, V and VI) and up to six Np species existing simultaneously. Both redox reactions and coordination of betaine were observed for Np(IV), (V) and (VI). Their spectrophotometric study demonstrates for the first time the complex redox chemistry of an actinide element in an IL and illustrates the competitive nature of water and betaine coordination. The fact that all three valencies, Np(IV), (V), and (VI), exhibit unusual stability in [Hbet][Tf2N] the great potential for using ionic liquids to purify and separate actinides and fission products.
The enhanced stability of Np(VI) suggests the potential stabilities of other high-valent actinides, e.g. Pu(VI) and Am(VI), and an approach that could serve as the basis for novel separation technologies using higher oxidation states. A prominent example is the separation of americium from trivalent lanthanides and curium, the most difficult challenge in used nuclear fuel processing. Until now, the majority of investigations on the behavior and separation of actinides using ILs have been performed with uranium in its +VI oxidation state. Few studies have involved the light transuranium elements neptunium (Np), plutonium, or americium. Neptunium is a key constituent in used nuclear fuel and is the most problematic actinide for the long-term storage and disposal of nuclear waste in geological settings.
Reference: "Unusual Redox Stability of Neptunium in the Ionic Liquid [Hbet][Tf2N]," Chem. Commun., 2014, 50 (58), 7766 - 7769. Kristy Long, George Goff and Wolfgang Runde