Optimization of loose nanofiltration performance of a polybenzimidazole hollow fiber membrane: Air gap and post-crosslinking effects

Current loose nanofiltration membrane materials are not suitable for harsh dye/salt separation environments because they are not resistant to high temperatures, acids and bases, and organic solvents. In this work, a polybenzimidazole (PBI) hollow fiber loose nanofiltration membrane (HFLNM) was prepared by dry-wet spinning and used for dye and salt separation. The membrane showed good heat resistance (25–70 ℃), acid resistance (immerse in10 mol/L HCl solution for 100 h) and solvent resistance (immerse in EtOH, MeOH and IPA for 120 h, respectively). The effects of air gap distance on the membrane structure and properties as well as the effect of cross-linking using K2S2O8 on the solvent resistance of the membrane were investigated. By adjusting the air gap distance to 6 cm, the dense layer of the optimized membrane got thinner. With the help of double layer finger-like pore and strong negative charge, the membrane exhibited outstanding water flux (64.5 L·m−2·h−1), low NaCl salt rejection (∼ 7%), and excellent Congo Red (CR) dye rejection (100%) at the dye/salt mixture solution (100 mg/L Congo red, 1 g/L NaCl) and operating pressure of 0.1 MPa. Moreover, the crosslinked PBI membrane showed good stability after 72 h immersion in polar aprotic solvents (NMP, DMF, and DMAc). Hence, PBI HFLNM has a good application prospect in the field of dye/salt separation in harsh environments.