Fabrication of omniphobic PVDF composite membrane with dual-scale hierarchical structure via chemical bonding for robust membrane distillation

In recent years, omniphobic membranes have drawn much attention due to the strong repellence to various liquids. In this study, a PVDF composite membrane with omniphobic characteristics was prepared by constructing dual-scale hierarchical structure and introducing long chemical chains with the low surface energy. The special architecture was derived from the immobilization of TiO2 @PDA@Cu composite nanoparticles on the surface via chemical bonding, and the fluorosilanization treatment with 1H,1H,2H,2H-perfluorodecyltriethoxysilane for membrane surfaces. A series of characterizations were implemented to explore composite nanoparticles' morphology and composition, as well as membrane morphology and surface analysis. In addition, antifouling and anti-wetting behaviors of pristine and composite membranes were investigated by successive direct contact membrane distillation (DCMD) experiments, in which different contaminations containing inorganic salts, organic matter, surfactants and mineral oil were chosen as the feed. The results displayed that composite nanoparticles were obtained through the chelating effect of polydopamine (PDA) and the dual-scale structure was realized with the assistance of trimesoyl chloride (TMC). The omniphobic membrane exhibited strong repulsive forces to various liquids, such as water, SDBS solution (0.4 mM), CTAB solution (0.4 mM), tween-20 solution (0.4 mM) and mineral oil emulsion (0.2% v/v) with the contact angles of 168.0 +/- 2.0 degrees, 157.1 +/- 1.5 degrees, 160.0 +/- 1.2 degrees, 158.3 +/- 1.5 degrees and 152.5 +/- 3.0 degrees, respectively. Furthermore, compared to pristine membrane, the omniphobic membrane presented superior stability, higher salt rejection rate and excellent anti-wetting and anti-fouling performances in the DCMD tests. Thus, the composite membrane with superhydrophobic and superoleophobic features possessed great application potential for high salinity wastewater treatment in membrane distillation.