Multi-component separation of small molecular/ionic pollutants with smart pH-gating membranes

Precise molecular separation through a membrane is critical in sustainable water purification applications. Routine separation membranes with inflexible molecular transmission channels are effective for the separation of two substances, which cannot meet the practical treatment requirements of multicomponent complex wastewater. We report a pH-gating membrane (PGM) formed by cross-linking a polyaniline/polyimide substrate with polyethyleneimine. PGMs exhibit tunable and reversible molecular sieving architectures under pH stimuli, which are attributed to the contraction and extension of polyaniline (PANI) chains during the transition between protonation and deprotonation. Based on this smart pH gating property, precise separation of the three-component mixture of Alcian Blue 8 GX, Orange II, and NaCl can be achieved with a single membrane at pH 8 and 2. The dye molecule-to-NaCl selectivity of this membrane reaches 942.1. It outperforms most state-of-art membranes, demonstrating promising applications for the selective separation and recovery of valuable products instead of harmful discharge.