Hydrophilic/hydrophobic nanofibres intercalated multilayer membrane with hierarchical structure for efficient oil/water separation

Developing a membrane integrating high permeating flux and high separation efficiency for the purification of oil-in-water emulsions is significant but still challenging due to the perm-selectivity “trade-off”. Herein, we design a nanofibrous membrane with special hydrophilic/hydrophobic intercalated multilayer structure to efficiently separate oil-in-water emulsions. The multilayer membrane is composed of alternatively deposited hydrophilic polyacrylonitrile (PAN)/ZnO nanoneedles composite fibers layers and hydrophobic poly(vinylidene fluoride-co-hexa-fluoropropylene) (PVDF-HEP) fibers layers, where vertically grown ZnO nanoneedles pierce through PVDF-HEP fibers layer and connect the whole membrane up and down. The special multilayer structure can exert the synergistic effect between size sieving mechanism of hydrophilic layers and coalescence mechanism of hydrophobic layers during the separation of oil-in-water emulsion, which overcomes the size mismatch between large pore sizes of membrane and small droplets of emulsion. With the mean pore size of 1.1 μm for the multilayer composite membrane, a high permeating flux up to 32,100 L m-2h−1 bar−1 with the oil content in filtrate as low as 4 mg L−1 for treating toluene-in-water emulsion (a mean droplet size of 650 nm) was achieved. The separation performance is obviously superior to the hydrophilic membrane without hydrophobic layers intercalated structure. Our work provides a new strategy for developing hydrophilic-hydrophobic composite membranes to alleviate the trade-off between permeating flux and separation efficiency in oil/water emulsion separation through rationally designing the membrane structure.