Facile modification of polypropylene hollow fiber microfiltration membranes for nanofiltration

Modification of polypropylene (PP) hollow fiber microfiltration membranes for nanofiltration through dip-coating sodium carboxymethyl cellulose (CMCNa) skin layer followed by cross-linking with AlCl3 was investigated in this study. Modification parameters were studied to optimize the membrane performance and the obtained modified membranes were characterized in terms of surface and permeation properties. It was found that the surface property and solute separation capability of the PP hollow fiber membrane were significantly improved by coating CMCNa surface layer, and the optimized modified membrane took on a composite structure with a hydrophilic selective skin layer having an isoelectric point of pH 3.7. The composite layer was negatively charged at neutral pH and was relatively looser in acidic pH compared with that in basic or neutral. Permeation tests revealed that the desired modified membrane possessed a molecular weight cut-off (MWCO) of 760 Da and a high pure water permeability of 10.8 l/m2 h bar, and exhibited rejections to salts following the order of MgCl2 < CaCl2 < MgSO4 < NaCl < KCl < Na2SO4 at neutral pH. Additionally, the modified membrane could efficiently remove anionic dyes from saline aqueous with good long-term performance stability. Retentions to Congo red and Methyl blue were 99.8% and 99.6%, respectively, under neutral pH and 3.0 bar.

► PP hollow fibers were modified for NF through coating CMCNa followed by cross-linking with AlCl3.
► The modified PP hollow fiber took on a composite structure and has a MWCO of about 760 Da.
► Rejection order of the modified PP membrane is MgCl2 < CaCl2 < MgSO4 < NaCl < KCl < Na2SO4 at neutral pH.
► Retentions to Congo red and Methyl blue of the modified membrane are 99.8% and 99.6%, respectively.
► The modified PP membrane possessed good performance stability in filtration anionic dye solutions.