Green Twist to Solvent Extraction and Separation of Metallic Species with Asymmetric Elfin Chelating Ligands

This competitive solvent extraction study of various metal species (27) in the periodic table was conducted by using a series of asymmetric ligands, tested for the first time, composed of N-heterocyclic receptors with variable ring size and a β-dicarbonyl fragment with the aim of providing a snapshot of their efficacy. We evaluated the effect of heteroatoms in the substituent on solvent extraction performance and the selectivity of this type of chelating extractant with equivalent binding sites but an unsymmetric backbone due to differing coordination environments. The influence of the chemical nature of the organic liquid phase on the solvent extraction process was also discussed: ionic liquids ([C1Cnim+][Tf2N−]) or typical organic diluents. Fe3+ can be extracted with all ligands only in IL medium (100%). The separation factors (SFs) between adjacent metals of 4f-series and iron towards other s-, p, d- and f-block metal ions have been assessed from a circular economy perspective: to extend the product lifecycle and regenerate natural resources. The SF values (Fe/Lns) were below ca. 6 for CHCl3, while the SFs for adjacent 4f-series ions were generally close to 1.1, with higher selectivity observed for selected ion pairs. In addition, the competitive solvent extraction process of 12 refractory metals as well as 8 platinum group metals was investigated with the synthesized ligands. The diluent n-heptane (a structurally symmetric alkane) outperformed isooctane (an asymmetric branched alkane) and chloroform (a polar asymmetric solvent) for refractory metal recovery with a maximum efficiency (100%). Furthermore, EPR, Raman and DTA-TG-MS spectroscopy analyses were used to study the extracted d- and f-chelate complexes in the ionic liquid media: Gd3+, Cu2+ and Fe3+.