Gradient cross-linked structure: Towards superior PVA nanofiltration membrane performance

Performance enhancement of the TFC membrane depends on the fine design of the functional layer. In this study, a gradient cross-linking strategy has been successfully applied to fabricate an ultra-thin gradient cross-linked PVA (GC-PVA) functional layer with the aid of surfactant on a polysulfone (PSF) support membrane. Such a strategy was accomplished by impregnating the crosslinker (Trimesic acid) on the porous support membrane prior to PVA coating. As confirmed by XPS and EDX studies, the GC-PVA membrane features a gradient crosslinked structure, where the bottom surface is densely cross-linked while the top is only slightly cross-linked. Overall, the lowest thickness achieved by the gradient-crosslinked ultrathin functional layer was 75.4 nm. This strategy significantly improved water flux compared with the conventional fully cross-linked PVA membranes while similar salt rejections can be maintained. In addition, the impact of key fabrication parameters, such as the concentration of PVA, surfactant, crosslinker, and the post-treatment temperature has been systematically studied. The optimized membrane had a significantly high water flux of 72.9 Lm(-2) h(-1) at 0.2 MPa and showed rejection rates of 38.2% and 90.7% towards NaCl, Na2SO4 aqueous solution (500 mg/L), respectively.