Enhancing Ion Selectivity\nof Nanofiltration Membranes\nvia Heterogeneous Charge Distribution

Nanofiltration technology holds significant potential\nfor precisely\nseparating monovalent and multivalent ions, such as lithium (Li) and\nmagnesium (Mg) ions, during lithium extraction from salt lakes. This\nstudy bridges a crucial gap in understanding the impact of the membrane\nspatial charge distribution on ion-selective separation. We developed\ntwo types of mixed-charge membranes with similar pore sizes but distinct\nlongitudinal and horizontal distributions of oppositely charged domains.\nThe charge-mosaic membrane, synthesized and utilized for ion fractionation\nfor the first time, achieved an exceptional water permeance of 15.4\nLMH/bar and a Li/Mg selectivity of 108, outperforming the majority\nof published reports. Through comprehensive characterization, mathematical\nmodeling, and machine learning methods, we provide evidence that the\nspatial charge distribution dominantly determines ion selectivity.\nThe charge-mosaic structure excels by substantially promoting ion\nselectivity through locally enhanced Donnan effects while remaining\nunaffected by variations in feedwater concentration. Our findings\nnot only demonstrate the applicability of charge-mosaic membranes\nto precise nanofiltration but also have profound implications for\ntechnologies demanding advanced ion selectivity, including those in\nthe sustainable water treatment and energy storage industries.