Preparation and characterization of a bi-layered nano-filtration membrane from a chitosan hydrogel and bacterial cellulose nanofiber for dye removal

A novel nanofiber composite nano-filtration membrane was prepared by combination of grafting, electrospinning and surface coating. First, carboxyl multi-walled carbon nanotubes were grafted onto bacterial cellulose molecular chains by solution reaction. The grafting products were characterized by TEM, FTIR, Raman, XPS and TGA measurements. Then, they were electrospun into nanofibers to be the supporting layer, and a coating of denser chitosan hydrogel thin film was used as the barrier layer. The morphology of the electrospun nanofibers and composite membrane were observed by SEM. The mechanical behavior of the composite membrane was investigated in dry and wet state. The filtration properties were evaluated in term of the pure water flux (PWF), dye rejection rate and anti-fouling properties. Results showed that the chitosan hydrogel coated electrospun nanofiber composite membrane had higher tensile strength and Young’s Modulus with the value of 11.75, 10.11 and 244, 211 MPa in dry and wet state, respectively. The PWF was increased from 52.1 to 140.7 L/m² h with the operation pressure increasing from 0.1 to 0.6 MPa. Direct Orange S, Procion Red mx-5B, Stilbene Yellow and Methylene Blue with molecular weight ranged from 300 to  800 g/mol were selected for dye removal test. The rejection rate could maintain over 90% for dyes with the molecular weight larger than 600 g/mol under 0.5 MPa. The chitosan hydrogel coated electrospun nanofiber composite membrane exhibited good anti-fouling properties for both oil and protein. These results indicate that these membranes are efficient for dye removal from wastewater, with high flux and rejection rate at high pressure.