Supramolecular-Based Ion Separation Membranes for Direct Separation of Concentrated Mixed-Salt Solutions

Efficient ion separation from concentrated brines is critical for sustainable water resource utilization, zero-liquid discharge, and strategic ion recovery, yet it remains a formidable challenge for conventional ion-selective membranes (ISMs). Existing ISMs rely on weak ion–pore interactions, limiting their applicability to dilute solutions, whereas stronger binding designs often impose high diffusion barriers that suppress flux. Here, we report a supramolecular ISM in which 18-crown-6 (18C6) macrocycles are integrated into the one-dimensional nanochannels of covalent organic frameworks (COFs), forming a dual-channel architecture─supramolecular pathways for cations and macrocycle-separated free channels for anions. This design minimizes ion interference while enabling strong monovalent-ion recognition and rapid transport. As a result, the 18C6-COF membrane operates effectively in concentrated solutions, achieving high selectivity ( S K + /Mg 2+ = 254.7) and fast permeation ( P K + = 2403 mmol m –2 h –1 ), outperforming state-of-the-art ISMs. Experimental and simulation results show that monovalent cations migrate rapidly through the aligned 1D 18C6 channels via a knock-on-like process, even under strong ion–pore interactions, overcoming the traditional trade-off between binding affinity and transport kinetics. This study lays the groundwork for developing membranes with strong ion-channel interactions for high-concentration mixed-salt separation.