Positively charged membranes with fine-tuned nanopores for ultrafast and high-precision cation separation

Abstract

There is significant interest in high-performance positively charged membranes that enable the fast and high-precision separation of various cationic substances for purifying water and recovering precious metals. This study demonstrates that the ultrafast and precise separation of cations can be achieved using positively charged membranes with fine-controlled nanopores fabricated via a solvent-controlled swelling–diffusion (SD) method. Polyamide (PA) membranes are treated with a dimethyl sulfoxide/water mixture solvent containing a rationally designed, positively charged strong polyelectrolyte, followed by water washing. Solvent-induced PA activation and polyelectrolyte incorporation into PA cooperatively produce a highly-permeable and more uniform PA nanopore structure with a strong and stable positive surface charge. Consequently, the SD-modified membrane exhibits unprecedentedly high water permeance (∼22 L m⁻² h⁻¹ bar⁻¹) and high rejection (∼99.5% for MgCl₂, >90% for hazardous metals) and selectivity (∼23.4 for Li⁺/Mg²⁺) for divalent cations, significantly outperforming commercial and previously reported membranes. Our proposed strategy provides a commercially viable means for the fabrication of high-performance functional membranes and coatings and sheds light on their structure–performance relationship.