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

Abstract

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 HNO3. 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 NO3− and the H2O 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 (logβ) and thermodynamic parameters (ΔH, ΔS, and ΔG) regarding the complexation of Eu3+ with NTAamides are also presented.