Solvent extraction studies of different diastereomers of modified diglycolamide ligands for An(III) and Ln(III) extraction

BackgroundSolvent extraction is currently the preferred technique for the separation of actinides (An) from fission products (FP), and especially lanthanides (Ln), as constituents of Spent Nuclear Fuel (SNF) or PUREX raffinate. For this purpose, the separation of minor actinides (MA), especially Am(III) from Cm(III), Eu(III) and other Ln(III) is the current challenge of interest. Nowadays, extractants only made by carbon, hydrogen, oxygen, and nitrogen (CHON principle) are preferred for the extraction of MA, because they can be completely incinerated. Diglycolamides (DGA) showed promising results as extractants for trivalent Ln and An, such as N,N,N’,N’-tetra-n-octyl diglycolamide (TODGA), which usually have a higher affinity for Cm(III) over Am(III). Wilden et al. reported a variation of the Am(III) selectivity over Cm(III) with differently orientated methyl substitutions (syn and anti) in the ligand backbone of Me2-TODGA (Wilden et al., 2019). Here, we study the selectivity and affinity of derivative extractants of TODGA and N,N,N’,N’-tetra-n-decyl diglycolamide (TDDGA) with dipropyl and ethyl-methyl substitutions in the molecule backbone, and their respective diastereomers.MethodsThe experiments were carried out at different nitric acid concentrations (0.8 – 9.8 mol L-1) containing all Ln(III) (except Pm) and other fission products. 241Am, 244Cm, 152Eu and 239Pu(IV) were added as radioactive tracers. The new extractants were tested as 0.1 mol L-1 solution in n-dodecane. The extraction was carried out at 22°C for 30 min. After centrifugation for 5 min., phases were separated by hand using fine-tip pipets. Organic and aqueous phase samples were measured using gamma and alpha spectrometry and ICP-MS.ResultsThe steric hindrance from the alkyl side chains gave generally low distribution ratios (D 10). The syn-ethyl-methyl substitutions for both analogues showed the highest D values for Am(III) and all metals (except Ru) by all of the tested ligands giving promising molecules for further studies. No third phase formation was observed for any of the ligands. The results of the extraction studies will be presented and discussed in the poster.ConclusionsAmong the tested ligands, syn-ethyl-methyl substitution for both diglycolamides (octyl and decyl side chains) gave the highest D values for all Ln and An. Nevertheless, the D values were much lower than for their unsubstituted parent molecules. Pu(IV) was extracted much better rising the attention for extraction of tetravalent ions. Further studies aim at increasing the D values by e.g., increasing the ligand concentration.ReferencesWilden, A., Kowalski, P. M., Klaß, L., Kraus, B., Kreft, F., Modolo, G., . . . Verboom, W. (2019). Unprecedented Inversion of Selectivity and Extraordinary Difference in the Complexation of Trivalent f-Elements by Diastereomers of a Methylated Diglycolamide. Chem. Eur. J., 25(21), 5507-5513. https://doi.org/10.1002/chem.20180616