Effect of substrate morphology on characteristics of layer-by-layer self-assembly nanofiltration membrane for micropollutants removal

Layer-by-layer (LBL) hollow fiber nanofiltration (NF) membranes have emerged as a promising technology for the removal of micropollutants (MPs). Previous research has been largely focused on polyelectrolyte pairs and coating parameters. This paper studied the substrate morphology, a critical factor often being overlooked. Poly (allylamine hydrochloride) /poly (styrene sulfonic acid) sodium salt (PAH/PSS) coating was utilized to evaluate the impact of the substrate morphology. Two polyethersulfone substrates were selected: Substrate 1# with dense skins at both lumen/shell and Substrate 2# with skin at the lumen and an open shell surface. All LBL NF membranes showed molecular weight cut-off ranging from 180 to 223 Da and MgCl2 rejection around 94 %. For LBL membrane 1#, with coatings on both shell/lumen sides, the outer coating was defective due to scratches. For LBL membrane 2# with a coating at the lumen, significant PAH overcompensation was observed, because of the open inner and outer surfaces comparing to LBL membrane 1#. Higher rejection to positively charged MPs was resulted from Donnan effect, outperforming steric exclusion. The backwash stability of both membranes was excellent and independent of the substrate structure. This work provides a basis for substrate selection for fabricating LBL NF membranes.