Studies on a novel nanofiltration membrane prepared by cross-linking of polyethyleneimine on polyacrylonitrile substrate

In this study, composite nanofiltration membranes with positive charges comprising of polyethyleneimine (PEI) active layer and polyacrylonitrile (PAN) substrate were prepared by cross-linking modification. PAN substrate was first hydrolyzed to create carboxyl groups with negative charges, followed by supramolecular assembly of PEI on the substrate surface. Afterwards, PAN/PEI membrane was impregnated in aqueous phase containing cross-linking agent to fabricate the PAN/PEIc membrane. While the hydrolyzed PAN substrate was negatively charged, the surface modification led to a charge reversal due to the skin layer of PEI. Surface property and morphology of the membrane were characterized by contact angle, scanning electron microscope (SEM) and atomic force microscope (AFM). Effects of cross-linking conditions (e.g. agent type, agent concentration and cross-linking temperature) and operating parameters (e.g. feed type, feed concentration and operating pressure) on the membrane performance were systematically investigated. The results showed that a PAN/PEIc membrane from PEI layer cross-linked with 12.0 wt% epichlorohydrin (ECH) at 65 °C for 15 h has an optimum NF performance (relatively high salt rejection and high permeation flux). This membrane so formed exhibited salt rejections of 92.82%, 69.76% and 61.31% for a feed MgCl2, MgSO4 and NaCl concentration of 2000 mg/L, respectively, and the corresponding permeation fluxes of 16.26, 16.03 and 17.88 L/(m2 h) at 1.0 MPa. The rejections to different salt solutions follow a decreasing order of MgCl2, MgSO4 and NaCl, which reveals typical characteristics of a positively charged NF membrane.