Preparation of omniphobic PVDF membranes with silica nanoparticles for treating coking wastewater using direct contact membrane distillation: Electrostatic adsorption vs. chemical bonding

In order to reduce the wetting and fouling propensity of polyvinylidene fluoride (PVDF) membrane during coking wastewater treatment by membrane distillation, two kinds of omniphobic PVDF composite membranes were prepared by coating SiO2 nanoparticles on PVDF membrane surface via electrostatic adsorption or chemical bonding respectively, and subsequently fluorination. The coated SiO2 nanoparticles observed as amounts of nano-scale protrusions by scanning electron microscope constructed a re-entrant structure and enhanced membrane surface roughness. The omniphobicity of the modified membranes was confirmed by high contact angles (> 150°) for water, diiodomethane and ethylene glycol on the membrane surface. The omniphobic layer with SiO2 nanoparticles coated via chemical bonding was more stable than electrostatic adsorption. Both the omniphobic PVDF membranes exhibited stable and high rejections of the inorganic and organic contaminants, which were determined by conductivity, total organic carbon and three-dimensional excitation-emission fluorescence spectroscopy of the distillate. The modified omniphobic membranes also showed much less flux decline than the pristine PVDF membrane in direct contact membrane distillation tests (120 h), suggesting their excellent anti-fouling performance. This study indicates that coating silica nanoparticles via chemical bonding is an effective and promising method to fabricate omniphobic PVDF membrane for coking wastewater treatment.