The separation of lanthanides and minor actinides is a difficult separation due to their similar ionic radii, hard acid nature, and shared preference for the trivalent oxidation state. The most aggressively pursued separation techniques employ solvent extraction and implement soft-donors (nitrogen or sulfur) that selectively interact with the trivalent actinides over the trivalent lanthanides. Heavily considered solvent extraction techniques for this separation use the soft, nitrogen donating aminopolycarboxylates as an actinide selective holdback or stripping reagent in an aqueous solution. Although nitrogen donor based separations have shown baseline success at separating lanthanides from actinides, it may be possible to improve upon the process by substituting softer, sulfur containing holdback reagents for the nitrogen containing holdback reagents. Research has been done with various dithiophosphinic acids that show high separation factors of up to approximately 100,000 when extracting actinides from lanthanide containing solutions. However, dithiophosphinic acids have limited radiolytic and hydrolytic stability. This has restricted the scale-up and development of processes centered on their use.<br/><br/>To improve on the condition sensitive organosulfur extractants, sulfur-based complexants in the aqueous phase have been considered here. Two reagents, 2,2’-thiodiacetic acid (TDA) and 2,5-thiophenedicarboxylic acid (TPA), were studied to assess their viability as aqueous holdback reagents. The formation constants of lanthanides and several transuranic actinides have been assessed with TDA and TPA by competitive extraction, potentiometric titration, isothermal titration calorimetry, and spectrophotometric titration. Throughout these experiments, formation constants were found to be higher for actinides than for lanthanides. The cause of this selectivity was further assessed by EXAFS and DFT studies. The different behavior observed between the lanthanides and actinides provides insight into fundamental actinide chemistry and may potentially prove to be the basis for an effective actinide-lanthanide separation.

Sulfur donating aqueous ligands for the separation of trivalent lanthanides and actinides

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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