N-donor extractantspreferentially extract Am(III) over Eu(III). However, most only work from weaklyacidic solutions. In contrast, BTPs (2,6-bis-(5,6-di-alkyl-1,2,4-triazin-3-yl)pyridines)developed at INE extract from up to 1 M nitric acid with high selectivity. However,their selectivity is not understood on a molecular level. This paper presentsthe current state of knowledge from our investigations. It is commonlyassumed that N-donor ligands bond more covalently to An(III) than they do toLn(III). However, an EXAFS study in organic solution did not reveal significantlydifferent metal ion-N bond lengths of the [Cm(BTP)3]3+ and[Eu(BTP)3]3+ complexes. On the other hand, further EXAFSinvestigations of U(III), Pu(III), and Am(III) complexes seem to confirm a bondlength contraction for the lighter actinides(III). Interestingly, theirextractability by BTP does not follow this trend. Rather, it roughly followsthat of the lanthanides(III), with the lightest ions being the least extractable.TRLFS titrations indicatesignificant differences in speciation: [Cm(BTP)3]3+ formsat a much lower BTP concentration as does [Eu(BTP)3]3+. Thestability constants differ by approx. three orders of magnitude. No (or only small)structural differences between actinides(III) and lanthanides(III) complexedwith BTP are found. The extractants' better thermodynamic affinity foractinides(III) vs. lanthanides(III) seems to be driving their selectivity inliquid-liquid extraction, as also insinuated by quantum chemical calculations.Whether the often-discussed higher degree of covalence in the An(III)�N bondvs. the Ln(III)�N bond is at selectivity's origin is not convincingly demonstratedso far.

Investigating
the An(III)/Ln(III) selectivity of N-donor extracting agents: The BTP case

239th National Meeting (2010)

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Investigating the An(III)/Ln(III) selectivity of N-donor extracting agents: The BTP case