Preparation of omniphobic PVDF membrane with hierarchical structure for treating saline oily wastewater using direct contact membrane distillation

Thermally driven membrane distillation process, for desalination of a wide spectrum of water, has attracted both scientific and industrial attention for decades. However, membrane fouling by contaminants in the feed stream was of great concern. Mitigating membrane fouling requires insights from surface and material science. We reported an omniphobic polyvinylidene fluoride (PVDF) membrane with hierarchical structure which was created by spray coating of the nano/microspheres onto a commercial PVDF porous substrate. The multiscale microspheres were prepared based on electrostatic interactions between silica nanoparticles (SiNPs) and polystyrene (PS) microsphere. A chemical binding agent, polymer 3-methacryloxypropyltrimethoxysilane (PA174) synthesized via free radical polymerization, was utilized to enhance the adhesion of the particles to the membrane support, leading to a robust structure. The final fluorination step using 1H, 1H, 2H, 2H-perfluorodecyltrimethoxysilane (17-FAS) attributed the membrane omniphobicity. The water/hexadecane contact angle of the present membrane surface was 176 degrees/138.4 degrees, and the water sliding angle was 7 degrees. Challenged with a hexadecane emulsion feed solution stabilized with the SDS surfactant in membrane distillation, the membrane showed a very stable flux and decrease in conductivity on the permeate side in contrast to decreased flux and increase in permeate conductivity for other benchmark membranes. This performance indicates that omniphobic membranes with hierarchical morphology are promising in addressing membrane wetting and fouling issues in the DCMD processes.