Hierarchical Membrane for Photoelectrostatically Gated Monovalent-Divalent Ion Separation.

Monovalent-divalent ion separation remains a significant challenge for the selective recovery of ions from aqueous solutions. Here, we report a hierarchical membrane composed of a poly(ionic liquid) photo-grafted onto a carbon nitride thin film supported on porous anodic aluminum oxide. Under concentration gradient-driven natural diffusion, the membrane establishes a baseline level of monovalent-divalent selectivity. However, upon application of light and bias as external stimuli, the hierarchical membrane enables light-assisted modulation of ion transport, resulting in a significant increase in monovalent-divalent selectivity by over 200 times for Li+/Mg2+ and over 500 times for K+/Mg2+. Ion selectivity arises from hydration-dependent mobility and electrostatic confinement within charged nanochannels. This work establishes a simple method to prepare hierarchical membranes for dynamically regulating ion transport and provides design principles for advanced monovalent-divalent ion separation.