Structure-activity relationship approach toward the improved separation of actinides(III) over lanthanides(III) using amine-triamide ligands.

[Display omitted] • Two novel unsymmetrical NTAamide ligands were developed for the An3+/Ln3+ separation. • The unsymmetrical NTAamide ligands show significantly higher extractability than NTAamide(n -Oct). • The extractability can be improved by shortening the chain length of the substituent. • A neutral complexation extraction model was determined by solvent extraction and complexation study. The efficient separation of trivalent actinides over lanthanides is a major challenge for the partitioning and transmutation (P&T) strategy in spent nuclear fuel reprocessing due to their very similar properties. In this context, the present work reports three amine-triamide NTAamides (nitrilotriacetamides), to investigate the influence of ligand structure on the extraction of Am3+ over Eu3+. Among the three ligands, N , N -diethyl- N ′, N ′, N″ , N″ -tetraoctylnitrilotriacetamide shows higher extraction ability for Am3+ compared with that of N , N -dibutyl- N ′, N′ , N″ , N″ -tetraoctylnitrilotriacetamide and N,N,N′,N′,N″,N″ -hexaoctylnitrilotriacetamide (NTAamide(n -Oct)) in the examined acid range of 0.01–0.50 mol/L HNO 3. The structure–activity relationship study reveals that the two unsymmetrical NTAamides with varying alkyl substituted groups have higher extraction efficiency to Am3+ and relatively lower selectivity for Am3+ over Eu3+ than the symmetrical NTAamide(n -Oct) due to the steric hindrance effect. The complexation study by means of slope analysis, ESI-MS, and UV–vis titration suggests the formation of 1:2 metal/NTAamide extracted species. Meanwhile, the NTAamide is identified as a tetradentate ligand by FT-IR and XPS analyses. Furthermore, the analysis of Raman and TRLFS indicates that the NO 3 − and the H 2 O molecule do not participate in the inner coordination sphere of Eu3+. By combining the results of solvent extraction and complexation, a neutral complexation extraction model is proposed. Additionally, the stability constants (log β) and thermodynamic parameters (Δ H , Δ S , and Δ G) regarding the complexation of Eu3+ with NTAamides are also presented. [ABSTRACT FROM AUTHOR]