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⁺/Mg²⁺ = 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.