Subnano-Confined Membrane Enables Efficient Brine Concentration and Salt Crystallization in Electrodialysis.

Zero liquid discharge (ZLD) enables water and salt recovery while preventing secondary pollution, yet it is often energy-intensive and costly. Electrodialysis (ED), a widely employed technology for brine concentration in ZLD, is still in progress to overcome its insufficient concentrating ability due to unrestricted water transport through ion-exchange membranes. In this study, a tailored subnano-confined membrane (SCM) was prepared using a commercially available nanofiltration membrane and an ion exchange polymer. The ED system equipped with the SCM achieved a high concentration difference of 185 g L-1 NaCl in a single-stage operation and further produced crystallized NaCl at the third stage, offering a much simplified and effective concentrating process. The SCM featured ultralow water permeance, and the hydration number of ions passing through it was substantially reduced, which contributed to its superior salt-concentrating ability. Further analysis recognized that the unique structure of SCM that comprised subnano channels was the major contributor to its strong ion dehydration effect. This research is expected to advance ZLD toward enhanced energy efficiency, streamlined processes, and cost-effectiveness.