Fabrication of hierarchical poly (vinylidene fluoride) micro/nano-composite membrane with anti-fouling property for membrane distillation

Hierarchical poly (vinylidene fluoride) (PVDF) micro/nano-composite membranes were fabricated by the immersion-deposition method. As a practical surface modification way, the immersion-deposition was explored to obtain superhydrophobic membranes via coating SiO2 nanoparticles onto PVDF membrane surface, following by fluoroalkylsilane coupling agents 1H, 1H, 2H, 2H - perfluorooctyltrichlorosilane (PFOTS) grafted on the silica. As a binder, PVDF was applied to immobilize silica nanoparticles in the process of modification. A variety of techniques were performed to examine the effects of surface modification on surface morphology, chemical composition and wettability of derived membranes, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and contact angle goniometry. The surface modification endowed the membrane surface superhydrophobicity, deriving from the hierarchical structures with PVDF microspheres and nano-fluorinated silica particles deposited on the membrane surface. The results showed that the maximum value of membrane surface contact angle was up to 161.5 degrees. The anti-fouling properties of the pristine and modified membranes were investigated by direct contact membrane distillation (DCMD) experiments with mixed solution including sodium chloride, calcium chloride and humic acid. The DCMD tests displayed that permeate flux of the modified membrane just had little decrease with the salt rejection rate above 99.99% during the long-term experiment, whereas permeate conductivity of the pristine membrane sharply increased and the permeate flux decreased gradually. The modified membrane presented excellent anti-fouling performances.