The gradient superhydrophobic ZnO@PVDF nanofiber membrane with high-flux and anti-fouling for water-in-oil emulsion separation

Nanofiber membranes are widely used to separate water-in-oil emulsions. However, the membrane fouling from surfactant absorption reduces the permeation flux, while pressure-driven induces nanofiber slippage due to weak inter-fiber adhesion, leading to pore enlargement and a reduction in separation efficiency. To address these problems, this study developed gradient structure nanofiber composite membrane by in-situ growth ZnO nanowires on PVDF nanofibers through hydrothermal synthesis and chemical vapor deposition (CVD). The introduction of ZnO nanowires increases the surface roughness, endowing the membrane with superhydrophobicity, thus excellent the anti-fouling properties. Additionally, by virtue of the gradient growth ZnO nanowires, the pore size of membrane showed a trend from small to large. The top layer with small pore could efficiently collect micro-sized water droplets from the emulsions. The bottom PVDF nanofibers possessed large pore and high porosity, ensuring high permeation flux. As a result, the optimized membrane achieved a separation efficiency of 99.2 %, permeation flux of up to 7554 L m −2 h −1 for water-in-xylene emulsion under the driving pressure of 20 kPa. Besides, the obtained membrane exhibited favorable reusability with efficiency 91.1 % for water-in-diesel emulsion after 10 cycles. The work provides a way to construct gradient structured membrane for high efficiency and rapid water removal from oil products.