Charged sub-nanochannels in Chitosan-Cu composite membranes for highly selective hydroxide transport and efficient alkali recovery

Alkali recovery from industrial wastewater is crucial for resource sustainability, yet conventional electrodialysis technology suffers from a trade-off between hydroxide transfer and ion selectivity, alongside poor alkaline stability. Herein, we fabricated a high-performance chitosan-based composite membrane (CS-Cu) featuring charged sub-nanochannels constructed via a facile coordination strategy. The coordination between Cu 2+ and the amino/hydroxyl groups of chitosan transforms the polymer matrix into a short-range ordered structure, creating charged and confined transport pathways. Electrodialysis results demonstrate that the CS-Cu composite membranes achieve an exceptional OH − / WO 4 2 − ${\mathrm{WO}}_4^{2-}$ selectivity of 227.43, significantly outperforming most reported polymer membranes. This superior selectivity is attributed to a synergistic mechanism involving enhanced electrostatic attraction from the charged short-range nanochannels walls, size sieving, and the Grotthuss mechanism within the nano-confined hydrogen-bonding network. Transition state theory analysis further revealed that the energy barrier for OH − transport is approximately 2.44 times lower than that for WO 4 2 − ${\mathrm{WO}}_4^{2-}$ , confirming the kinetic advantage of hydroxide permeation. Furthermore, the composite membranes exhibit good operational stability during cycling tests. This work provides a novel strategy for designing ion exchange membranes with high-precision alkali separation for industrial alkali recovery.