A New Type Of PVA Nanofiltration Membrane Based On Gradient Cross-linked Structure

A thin film composite membrane is composed of support membrane a porous support membrane and a dense functional layer which exhibits a specific solvent permeability and solute rejection.In this study,a gradient-crosslinking strategy has been successfully applied to fabricate an ultra-thin PVA composite membrane with the aid of sodium dodecyl sulfate（SDS）.First,the support membrane was coated with a crosslinking agent and SDS.Second,the impregnated support membrane was further coated with a of poly（vinyl alcohol）（PVA）solution.As confirmed by the XPS studies,this coating strategy resulted in the formation of a gradient-crosslinked structure,where the bottom layer facing the support membrane is densely crosslinked while the surface PVA layer is only slightly crosslinked.This configuration significantly improved water flux compared with the fully crosslinked PVA membranes.Based on the strategy,a series of PVA nanofiltration composite membranes were prepared by varying key fabrication parameters,such as concentrations of PVA,surfactant,crosslinking agent solutions and different post-treatment temperatures.Using benzaldehyde as crosslinking agent:the membrane fabricated at optimized conditions had rejection rates of 19.7%,72.7%and 96.8%towards 500 ppm of NaCl,Na₂SO₄,and semi-xylenol orange,respectively,and the water flux was 142.5 L/（m²·h）measured at 0.8 MPa.Using a low PVA concentration of 0.05%,an ultra-thin functional layer with a thickness of 52.9 nm could be achieved and with a extremely small surface roughness of 5.82 nm.Using benzaldehyde as crosslinking agent:The retention rate of NaCl and Na₂SO₄ were 38.2%and 90.7%,respectively,and the water flux of 0.2MPa was 72.9L/（m²·h）.When concentration of PVA was 0.1%,the functional layer can reach 62.8nm thickness and the surface roughness was 9.34nm.