Unraveling the Extraction and Complexation Trends of Rare-Earth Elements with a Nitrilotriacetamide Ligand
Abstract
The nitrilotriacetamide n-octyl derivative ligand (NTAamide(n-Oct)) is promising for minor actinide/lanthanide separation, yet periodic trends in its extraction and complexation across the lanthanide series remained unclear. This study systematically investigated its behavior with trivalent rare-earth ions (Ln3+, Y3+, and Sc3+) using solvent extraction, 1H NMR titration, UV–vis spectrophotometric titration, microcalorimetric titration, single-crystal X-ray diffractometry, ESI-MS, and FTIR. Key findings reveal opposing ionic radius dependencies. Solvent extraction showed that extraction efficiency negatively correlates with the ionic radius, decreasing with the ionic radius. Slope analysis indicated Sc3+ forms 1:2 metal–ligand extracted complexes, while other RE3+ ions form 1:1 complexes. Conversely, homogeneous and solid-state studies uniformly identified 1:2 stoichiometry for all RE3+ complexes, with complexation strength positively dependent on the ionic radius, increasing as the ionic radius decreases. Overall, the extraction and complexation ability trends of NTAamide(n-Oct) toward RE3+ exhibit opposing tendencies. Combined with the results of solvent extraction and complexation studies, the compositions of the complexes formed by NTAamide(n-Oct) with Sc3+ or other RE3+ are [ScL2](NO3)3 and [REL2][RE(NO3)6] (L = NTAamide(n-Oct)), respectively. Furthermore, an ion-pair extraction model is also proposed and discussed.