Aminal‐Linked Covalent Organic Framework Membranes Achieve Superior Ion Selectivity

Abstract

High‐salinity wastewater treatment is perceived as a global water resource recycling challenge that must be addressed to achieve zero discharge. Monovalent/divalent salt separation using membrane technology provides a promising strategy for sulfate removal from chlor‐alkali brine. However, existing desalination membranes often show low water permeance and insufficient ion selectivity. Herein, an aminal‐linked covalent organic framework (COF) membrane featuring a regular long‐range pore size of 7 Å and achieving superior ion selectivity is reported, in which a uniform COF layer with subnanosized channels is assembled by the chemical splicing of 1,4‐phthalaldehyde (TPA)‐piperazine (PZ) COF through an amidation reaction with trimesoyl chloride (TMC). The chemically spliced TPA‐PZ (sTPA‐PZ) membrane maintains an inherent pore structure and exhibits a water permeance of 13.1 L m⁻² h⁻¹ bar⁻¹, a Na₂SO₄ rejection of 99.1%, and a Cl⁻/SO₄²⁻ separation factor of 66 for mixed‐salt separation, which outperforms all state‐of‐the‐art COF‐based membranes reported. Furthermore, the single‐stage treatment of NaCl/Na₂SO₄ mixed‐salt separation achieves a high NaCl purity of above 95% and a recovery rate of ≈60%, offering great potential for industrial application in monovalent/divalent salt separation and wastewater resource utilization. Therefore, the aminal‐linked COF membrane developed in this work provides a new research avenue for designing smart/advanced membrane materials for angstrom‐scale separations.