Polyamide membranes with nanoscale ordered structures for fast permeation and highly selective ion-ion separation.

Fast permeation and effective solute-solute separation provide the opportunities for sustainable water treatment, but they are hindered by ineffective membranes. We present here the construction of a nanofiltration membrane with fast permeation, high rejection, and precise Cl - /SO 4 2- separation by spatial and temporal control of interfacial polymerization via graphitic carbon nitride (g-C 3 N 4 ). The g-C 3 N 4 nanosheet binds preferentially with piperazine and tiles the water-hexane interface as revealed by molecular dynamics studies, thus lowering the diffusion rate of PIP by one order of magnitude and restricting its diffusion pathways towards the hexane phase. As a result, membranes with nanoscale ordered hollow structure are created. Transport mechanism across the structure is clarified using computational fluid dynamics simulation. Increased surface area, lower thickness, and a hollow ordered structure are identified as the key contributors to the water permeance of 105 L m 2 ·h -1 ·bar -1 with a Na 2 SO 4 rejection of 99.4% and a Cl - /SO 4 2- selectivity of 130, which is superior to state-of-the-art NF membranes. Our approach for tuning the membrane microstructure enables the development of ultra-permeability and excellent selectivity for ion-ion separation, water purification, desalination, and organics removal.