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.