Positively charged thin-film composite hollow fiber nanofiltration membrane for the removal of cationic dyes through submerged filtration

• Positively charged hollow fiber NF membranes were successfully fabricated.
• The NF membrane had a MWCO of about 650 Da and PWP of 8.6 l/m2 h bar.
• Salt rejections follow the order of CaCl2 > MgCl2 > NaCl > MgSO4 > Na2SO4.
• The NF membrane could remove cationic dyes through submerged filtration.
• The NF membrane possessed good long-term performance stability.

Positively charged thin-film composite hollow fiber nanofiltration membranes were developed by the dip-coating method using polypropylene hollow fiber microfiltration membrane as support, polyvinyl alcohol (PVA) and polyquaternium-10 (the reaction product of hydroxyethyl cellulose with a trimethylammonium substituted epoxide, PQ-10) as coating materials, and glutaraldehyde as cross-linking agent. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), measurements of streaming potential and contact angle were employed to characterize the obtained membranes. The permeation properties were estimated through pressurized cross-flow permeation tests and the dye removal performance were evaluated through submerged filtration tests using cationic dye aqueous solutions. It was found that the salt rejection order of the membrane was CaCl2 > MgCl2 > NaCl > MgSO4 > Na2SO4 at neutral pH. The desired membrane with a molecular weight cut-off of about 650 Da had a pure water flux of 25.7 l/m2 h and a rejection of 92.8% to 500 mg/l CaCl2 aqueous solution at 3.0 bar. The membrane could effectively remove cationic dyes from aqueous solution with good long-term performance stability through submerged filtration. The observed rejections to Brilliant green, Victoria blue B and Crystal violet were 99.8, 99.8 and 99.2%, respectively, under the trans-membrane pressure of 0.7 bar.