Separation of Rare Earths by Ion-Imprinted Membranes: Comparison between Nonsolvent and Solvent Evaporation-Induced Phase Separation

Separation of adjacent rare earth ions by membranes is a highly challenging task because of the difficulty in structure control and selectivity improvement. The goal of this research is to determine which of the two different types of phase separation methods for preparing polymer separation membranes was more suitable for rare earth separation, including nonsolvent-induced phase separation (NIPS) and solvent evaporation-induced phase separation (SEIPS). The novelty of this separation membrane lay in the ion-imprinted polymers (IIPs) that were synthesized and mixed into the matrix to enhance selectivity. The membranes fabricated via NIPS and SEIPS were named N-IIMs and S-IIMs, respectively. Characterizations proved N-IIMs exhibited superior hydrophilicity and a looser structure with more pores inside, while S-IIMs showed a more compact, smooth, and thinner structure. S-IIMs with 20 wt % IIPs showed specific recognition and preferential permeation of Er(III) with β_(Er/Y) as high as 2.72. The enhanced selectivity was due to a template-directed "site matching" mechanism. The cycling performances of adsorption and permeation both retained more than 90% of their initial performances after five cycles, indicating good regeneration ability and potential for long-term application. The macroscale fabrication of this S-IIM and application into more complex rare earth mixed solutions would be a more prospective future development.