Fabrication and characterization of novel composite nanofiltration membranes based on zwitterionic O-carboxymethyl chitosan

• Zwitterionic NF membrane was prepared by O-carboxymethyl chitosan.
• RMS values decreased with the increase of the casting solution concentration.
• Salt rejection of the optimal NF membrane accorded with Na2SO4 > NaCl > MgSO4 > MgCl2.
• MWCO of the optimal NF membrane is about 689 Da.
• The optimal NF membrane exhibited good stability.

A novel composite nanofiltration membrane was prepared from O-carboxymethyl chitosan (O-CMC) and epoxy chloropropane (ECH) through coating and chemical cross-linking reaction on the polyacrylonitrile (PAN) supporting layer. O-CMC was synthesized from chitosan and monochloroacetic acid. Chemical structures and compositions of O-CMC and NF membranes were characterized by infrared spectroscopy, and morphological structures were characterized by atomic force microscopy and field emission scanning electron microscopy. Permeation performance of NF membranes was examined with a cross-flow flat NF membrane device. Preparing conditions of NF membranes were optimized by studying the change of salt rejection and permeation flux, and the optimum conditions were determined as follows: casting solution concentration 3 wt.%, ECH concentration; 2 wt.%, cross-linking temperature; 50 °C and cross-linking time; 12 h. Furthermore, properties of the optimal NF membrane were tested in different operating conditions. At 0.5 MPa and 25 °C, its salt rejection and permeation flux to 1000 mg·L− 1 Na2SO4 aqueous solution were 94.36% and 16.78 L · m− 2·h− 1, respectively. Rejections of the membrane to different inorganic electrolyte solutions (1000 mg·L− 1) accorded with the following order: Na2SO4 > NaCl > MgSO4 > MgCl2. In addition, the MWCO was about 689 Da, and the zeta potentials were negative in experimental pH range. At last, good durability and stability were exhibited by the means of long-term operation during 2 weeks.