Summary and outlook

The adsorption-based REEs separation has been explored for a long time, and the technology has evolved. For separating REEs from other cations with a different charge, cation exchangers may be recommended. However, for the selective separation of specific REEs, ligand-functionalized adsorbents promoting complexation are more suitable.
There remain strikingly large gaps in two directions with adsorption-based REE separation: 1) lack of industrial-level applications and setups, and 2) lack of fundamental level modeling and understanding of surface ligand behavior with the REE. Although there have been many REE separation studies at laboratory scale, there is a lack of examples of conversion to industrial level applications.
The challenges in SPE with functionalized adsorbents are the selection of selective ligands and predicting the behavior of these ligands once they are grafted onto a specific solid surface. No quantitative relationship has been developed between free-solution thermodynamics data and surface complexation constants for the functionalized adsorbents. Future research can employ quantum chemistry calculations to select ligands and predict the behavior after grafting. Understanding and modeling the ligand grafted on the solid surface and recognizing the difference in metal-ligand interaction on the surface against the metal-ligand interaction in the free solution can help better develop functionalized adsorbents.