How Does Alkali Etching Work on the Polyamide Membrane to Obtain an m-Phenylenediamine-Based NF Membrane?

Fully aromatic polyamide (PA) membranes have been studied as nanofiltration (NF) membranes for decades because of their good chemical stability, but most of the membranes suffered from poor permeability and high rejection for monovalent ions. In this paper, an m-phenylenediamine (MPD)-based NF membrane was prepared by alkali activation on a reverse osmosis (RO) membrane. Characterization revealed the different etching paths on RO membranes fabricated by different concentrations of monomers of interfacial polymerization. The looser PA layer of RO membrane fabricated by lower-concentration monomers was easily invaded by NaOH molecules because of the higher molecular weight cutoff of 254 Da, which resulted in the transformation of separation grade from RO to NF. At the same time, the MPD-based NF membrane exhibited superior water permeability and pH resistance compared with a PIP-based membrane, which showed a potential to realize large-scale application in practical industry settings. It also provided a facile method for the preparation of a high-performance NF membrane with good chemical stability.