Tuning composite nanofiltration membranes with -cyclodextrin for improved Mg²⁺/Li⁺ selectivity

As a new frontier method, nanofiltration (NF) membranes still face challenges in achieving a balance between selectivity and permeability, especially in the field of extracting lithium from brines. In the study, we addressed this issue by incorporating gamma-cyclodextrins (CDs) with amphipathic cavities onto the polyamide chains using interfacial polymerization (IP) reaction, employing polyethyleneimine (PEI) and trimesoyl chloride (TMC). The addition of gamma-cyclodextrin (gamma-CD) enhances the hydrophilicity and water flux of NF membrane on account of its remarkable structural properties. The outer surface of gamma-CD exhibits hydrophilic characteristics, while its inner cavity possesses hydrophobic properties. Benefiting from its water-attracting surface and water-repellent cavity, gamma-CD could enhance the overall water affinity and water permeability of the NF membrane. Furthermore, the dimensions of the gamma-CD cavity allowed for effective screening Mg2+ / Li+ to a certain extent. Notably, gamma-CD exhibited a superior capability to accommodate and encapsulate Mg2+ ions, indicating a higher binding affinity compared to Li+. Thus, Li+ was easier to pass through the NF membrane grafted by gamma-CDs than Mg2+. The optimized NF membrane demonstrated an enhanced magnesium-lithium separation coefficient of 23.61, 237 % better than that of the TFC-control membrane. This work introduces a novel approach for designing and constructing of high-permselectivity NF membranes for Mg2+ / Li+ separation utilization.