Electro-assisted functional nanofiltration membrane for enhancing Li+ enrichment and Li+/Mg2+ separation

Abstract

The electric field-assisted nanofiltration process exhibited considerable promise for the selective separation of monovalent and divalent ions, owing to the capability to precisely regulate ion migration behavior. In this study, polyethyleneimine (PEI)-modified ZIF-8 (P-ZIF-8) nanoparticles were incorporated into a polyamide matrix to fabricate a composite nanofiltration membrane with a surface enriched in positive charge density. Driven by the external electric field, the membrane effectively facilitated the dehydration of Li⁺, accelerated the mass transfer rate of monovalent ions, and provided an effective electric field compensation effect. At an operating pressure of 4 bar and a current of 8 mA, the membrane exhibited a Li⁺/Mg²⁺ separation factor of 272.9, with a Mg²⁺ rejection rate of 98.7 % and a Li⁺ permeability of 150 %, surpassing most previously reported performances. Moreover, the electric field-assisted strategy was successfully extended to other monovalent/divalent ion systems, the separation factors of 90.4 for Na⁺/Mg²⁺ and 18.8 for K⁺/Mg²⁺, respectively. Molecular dynamics (MD) simulations further elucidated that electric field-induced ion dehydration, coupled with enhanced counterion decoupling, synergistically facilitated the efficient separation of Li⁺ and Mg²⁺. Overall, this work presented a viable approach for achieving high Mg²⁺ rejection and Li ⁺ enrichment by coupling functionalized nanofiltration membranes with an electric field-assisted strategy, offering both theoretical insights and practical guidance for advancing electro-driven nanofiltration technologies in complex ion separation scenarios.