Rare earth europium recovery using selective metal-organic framework incorporated mixed-matrix membrane

Rare-earth elements (REEs) play a crucial role in state-of-the-art technologies and sustainable energy generation. However, conventional production methods of REE often instigate detrimental impacts on environment. Hence, the development of efficient and sustainable hydrometallurgical methods for REE recovery from complex solution has become a crucial research focus. This study investigates a mixed-matrix membrane composed of a highly europium selective metal-organic framework-based adsorbent, Cr-MIL-PMIDA, embedded in sulfonated poly(ether ketone) (SPEK) polymer membrane matrix to preferentially concentrate europium (Eu³⁺) ions in the presence of other competing cations. The activated membrane notably reduced ionic conductivity for Eu³⁺ compared to other multivalent ions. Membrane extraction experiments further confirmed the selective behavior, demonstrating slower diffusion for Eu³⁺ compared to Mg²⁺ and Zn²⁺ cations. Especially, at pH 5, Mg²⁺ and Zn²⁺ recovery was greater than 30%, whereas Eu³⁺ recovery remained lower than 4%. We propose that the strong chemical affinity between the phosphate group and Eu³⁺ help partition of the Eu³⁺ ions in the membrane phase and inhibit the diffusion and further partitioning of the Eu³⁺ ion from bulk solution. Furthermore, we demonstrate the stability of the composite membrane and the embedded MOF particles in aqueous solution for up to 12 days without degradation, attributing it to the robust chemical stability of the MOF structure.