Characterization of f-element complexes with soft-donor ligands for selective Americium separation

In recent years, much effort has been invested in recycling of minor actinides from PUREX raffinate solutions. Hydrophilic N-donor ligands such as tetrasodium-3,3',3'',3'''-([2,2'-bipyridine]-6,6'-diylbis(1,2,4-triazine-3,5,6-triyl))tetrabenzenesulfonate (SO3-Ph-BTBP, Figure 1) were used for selective separation of Am(III) from Cm(III) and other trivalent fission lanthanides by smart combination with diglycolamide ligands as extractants such as N,N,N',N'-tetra-n-octyl diglycolamide (TODGA, Figure 1).1,2 In this fundamental oriented work, the combination of SO3-Ph-BTBP with TODGA and its mono- and dimethylated derivates Me-TODGA and Me2-TODGA (see Figure 1) was investigated for their extraction behavior in nitric acid solution. All systems showed a high selectivity of An(III) over Ln(III), and Am(III) over Cm(III). Separation factors decreased in the order TODGA > Me-TODGA > Me2-TODGA. Moreover, a significant drop in distribution ratios of heavier lanthanides (Tb-Lu) was observed in all three extraction systems. This effect was further elucidated by speciation studies of the formed complexes with several lanthanides using UV/Vis spectroscopy. The formation of 1:2 complexes (metal-to-ligand ratio) with SO3-Ph-BTBP was confirmed for the studied lanthanides and stability constants log β of 7.7 ± 0.8 for Nd(III) at 10-3 mol L-1 HNO3 and 6.1 ± 0.4 for Ho(III) at 3 mol L-1 HNO3 were determined. No complexation was observed for Nd(III) at 3 mol L-1 HNO3. Therefore, the formation of complexes with a protonated form of SO3-Ph-BTBP seems to be possible with Ho(III), but not with Nd(III). The results of the solvent extraction experiments and speciation studies will be presented and discussed.AcknowledgementsFunding for this research was provided by the German Federal Ministry of Education and Research through the f-Char project under grant number 02NUK059D.References1. C. Wagner et al., Solvent Extr. Ion Exch., 2016. 34 (2): p. 103-113.2. A. Geist and P. J. Panak, Solvent Extr. Ion Exch., 2021. 39 (2): p. 128-151