High-performance positively charged hollow fiber nanofiltration membranes fabricated via green approach towards polyethyleneimine layer assembly

In this study, high performance positively charged hyper-branched polyethyleneimine (PEI) nanofiltration (NF) layer was assembled successfully on negatively charged polyethersulfone/polyimide (PES/PI) hollow fiber ultrafiltration (UF) membranes under different conditions. In accordance with principles of green chemistry, glutaraldehyde (GA) as cross-linker and purely aqueous solutions were used as a less hazardous alternative compared to, e.g. trimesoyl chloride in an organic solvent. The effects of the surface roughness and charge of the substrate UF membranes, due to the presence of PI, and various fabrication conditions, such as pH of PEI aqueous solution, GA/PEI ratio and crosslinking reaction time, were investigated and discussed. Electron microscopy images revealed the successful assembly of the PEI NF layer at uniform coverage of the PES/PI UF membranes. It was found that the varied preparation conditions drastically affect the membrane surface hydrophilicity, surface zeta potential, permeation flux, and salt rejection. The membrane fabricated at optimum conditions had a molecular weight cut-off of <= 400 Da; steric hindrance and Donnan exclusion resulted to achieve salt rejections of 94.2% and 87.4% for MgCl2 and MgSO4, respectively. Moreover, fabricated membranes were tested through three cycles of six-hour filtrations and over 95% flux recovery after the filtration of salts via the backwashing process was recorded.