Studies on nanofiltration membrane formed by diisocyanate cross-linking of quaternized chitosan on poly(acrylonitrile) (PAN) support

A novel positively charged composite nanofiltration membrane is prepared by using PAN UF membrane as support layer, quaternized chitosan, i.e. called 2-hydroxypropyltrimethyl ammonium chloride chitosan (HACC) as active layer and diisocyanate as cross-linking reagent, respectively. Effects of membrane forming and testing variables on membrane properties are reported. The results indicate that a membrane from 2.0 wt% HACC casting solution, cured at 50 °C for 2 h, cross-linked at 50 °C for 9 h with hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI) (HDI/TDI (w/w) = 0.3 g/0.3 g) in 50 g ethanol, heat-treated at 50 °C for 20 min is found to have optimum performance. The membrane, called HACC/PAN composite NF membrane is positively charged, with a RMS roughness of 12.3 ± 0.52 × 103 nm. And the molecular-weight cut-off (MWCO) is approximately 560 Da (NF range) by measuring the rejections of glucose, sucrose and polyethylene glycols (PEG) (MW = 600–1000 Da) at a concentration of 1000 mg/L at 1.0 MPa, respectively. At 25 °C, the pure water permeability of this membrane is 10.59 kg h−1 m−2 MPa−1. The rejection to different salt solutions follows the decreasing order of MgCl2, CaCl2, NaCl, KCl, MgSO4, Na2SO4 and K2SO4, which is typical characteristics of positively charged NF membranes. Besides, the curve about the streaming potential illustrates the positively charged characteristics of this membrane, with a pressure osmotic coefficient of 7.73 mV MPa−1.