Synergistic effect of polymer-ZIF-8 and γ-CDs anchored polyamide membranes for enhanced Li+/Mg2+ separation

Metal-organic frameworks (MOFs) have received significant interest as ion-selective materials, due to the tunable pore size and diverse topological structures. However, given by the inferior interfacial compatibility, MOFs are difficult to uniformly disperse into polyamide membranes, which significantly hinders the development of MOFs-based polyamide nanofiltration membranes. Here, we proposed a strategy involving the use of polyethyleneimine (PEI) modified with ZIF-8 particles (P-ZIF-8) to eliminate interfacial problems and surface defects in polyamide membranes. Subsequently, configuring with γ-cyclodextrins (γ-CDs) to form a homogeneous graft solution, which were anchored to the polyamide layer surface by inducing a secondary interfacial polymerization (IP) reaction. The increased positive charge of the membranes improved the sieving performance of the monovalent and divalent ions. The composite nanofiltration membrane exhibited a uniform pore size of 0.55 nm and a water flux of 32 L‧m⁻²‧h⁻¹. The separation factor (S_Li⁺_/Mg²⁺) of the modified nanofiltration membrane (S_Li⁺_/Mg²⁺= 3.7) increased by 228 % compared to the PEI-TMC nanofiltration membrane (S_Li⁺_/Mg²⁺= 12.5). The composite membrane operated continuously for 48 h without deterioration in the separation performance. Density functional theory (DFT) was further elucidated the mechanism by which the introduction of P-ZIF-8 could effectively separate Li⁺/Mg²⁺. This work is of great significance for enhancing the synergy between MOFs and polyamide membranes, as well as for developing polymers-MOFs membranes to extract lithium resources from brines with complex compositions.