Quaternary ammonium-functionalized γ-cyclodextrin tailored polyamide nanofiltration membranes for highly selective Li+/Mg2+ separation

Positively charged nanofiltration membranes effectively extract lithium resources from brine due to their excellent ion selectivity. However, current positively charged nanofiltration membranes generally suffer from low permeation flux and poor lithium ion transport efficiency. To address these issues, constructing nanoscale channels with precise ion sieving capability within nanofiltration membranes has become a recent research focus. In this study, a novel nanochannel material—quaternized γ-cyclodextrin (Q-γ-CD)—was synthesized. Q-γ-CD contains positively charged quaternary ammonium groups on its surface, which can enhance the positive charge density of the membrane surface, thereby improving the rejection of divalent cations (such as Mg 2+ ). Meanwhile, the cavity size and hydrophobic inner cavity of Q-γ-CD provide rapid transport pathways for lithium ions and water molecules. By introducing Q-γ-CD into an aqueous solution containing polyethyleneimine (PEI) and using trimesoyl chloride (TMC) as the organic-phase monomer, a high-performance PEI/Q-γ-CD/TMC nanofiltration membrane was fabricated. Under optimal formulation conditions, the fabricated nanofiltration membrane exhibited a thin separation layer (≤100 nm), a pore size of approximately 0.45 nm, and a high surface charge (isoelectric point,  ≥ 9). Thanks to the precise ion-sieving nanochannels provided by Q-γ-CD, the membrane’s pure water flux increased to 13.34 L·m −2 ·h −1 ·bar −1 , while the rejection of magnesium ions reached 97.8 %, and the Li + /Mg 2+ separation factor achieved a value of 18.5. This study presents a novel design strategy for positively charged nanofiltration membranes, offering theoretical support and practical guidance for achieving efficient magnesium-lithium separation.