Ligand-assisted elution chromatography for separation of lanthanides

Lanthanides (Ln's) are the major components of rare earth elements, which are critical components of many high-value products. The ions of adjacent Ln's have the same valence and very similar ionic radii. They cannot be separated using conventional adsorption or ion exchange processes. Current production of high-purity Ln's is based on multiple sequential and parallel solvent extraction processes, which require large amounts of toxic solvents and result in serious negative impact on the environment. In this study, a ligand-assisted elution chromatography process for the separation of Ln's was developed for the first time for titania, which is a robust and inexpensive inorganic sorbent. A selective ligand for Ln's, ethylenediaminetetraacetic acid (EDTA), was found to adsorb on the sorbent. The adsorbed EDTA became strong adsorption sites for the Ln's. Desorption of Ln's was driven by reversible reactions of Ln's with EDTA in the mobile phase. The overall sorbent selectivity for the reaction and adsorption process was approximately equal to the ratio of the sorbent selectivity to the ligand selectivity. The separation mechanisms were tested and verified using rate model simulations and experimental data for the separation of praseodymium (Pr), neodymium (Nd), and samarium (Sm). Simulations based on the model were used to design efficient linear gradient elution and stepwise elution processes. The purity and yield of all three Ln's were found to be above 95% in the designed processes. Stepwise elution can be implemented in a continuous process for increasing sorbent productivity and reducing costs for large-scale separation. Ligand assisted elution processes are much simpler and more environmentally friendly than the conventional solvent extraction processes.