Separation of trivalent actinides and lanthanides with sulfur donor ligands and extractants

Abstract

The separation of trivalent actinides and lanthanides in the processing of used nuclear fuel is challenging due to the similar sizes, charges, and redox properties of the two classes of metals. Typical separations procedures rely on ligands or extractants that can coordinate the metals with polarizable soft donor atoms such as nitrogen or sulfur. These soft donor atoms display a preference for coordinating the actinides which can be utilized to provide selectivity in this separation. Among the different ligands and extractants used for this separation, the sulfur containing dithiophosphinic acids have shown some of the highest reported selectivities, with separation factors of up to 100,000 for the separation of americium and europium. However, the extraction mechanism for the dithiophosphinic acids are not fully defined, especially for the transcurium actinides. The most commonly studied dithiophosphinic acid, bis(2,4,4-trimethylpentyl)dithiophosphinic acid (HC301), is known to extract metals as several different complexes dependent upon both the extracted metal and the conditions used. A more complete understanding this behavior will promote the optimization of HC301-based separation procedures and possibly the development of extractants with even greater selectivity. Additionally, HC301 is susceptible to degradation by radiolysis, oxidation, and hydrolysis which both reduces the amount of HC301 in the system and forms new species that can impact the separation. In this work, the current knowledge in the use of sulfur donating extractants, including the dithiophosphinic acids and HC301, for the separation of lanthanides and actinides is summarized and two new developments in the use of HC301 are reported. The first development is a novel method to quickly determine concentrations of HC301 by a colorimetric permanganometric titration to enable more effective monitoring of a potential separation process. The second project is a characterization of the extraction of the transplutonium actinides, Am-Es, by HC301 to determine how extensively these metals are extracted and what complexes are extracted. Additionally, the use of more degradation resistant and water-soluble sulfur donating ligands as an alternative to dithiophosphinic acids is examined. Overall, both HC301 and the aqueous sulfur donating ligands display selectivity for all the actinides tested. This selectivity seems to be greatly influenced by the formation of different complexes between the lanthanide and actinide series. The actinides tend to form complexes where the metal is more completely coordinated by sulfur and the metal-sulfur bonds are shorter whereas the lanthanides tend to have less coordination by sulfur and longer metal-sulfur bonds.