Engineering PVDF omniphobic membranes with flower-like micro-nano structures for robust membrane distillation

Abstract

Conventional PVDF membranes are easily wetted and contaminated in membrane distillation (MD) process, especially when used in separating high-salinity water and oil-contained saline water. To address these issues, an omniphobic membrane was fabricated in this work. ZnO micro-flowers with a rough structure were successfully deposited onto the PVDF membrane with the assistance of polydopamine assistance. After undergoing a fluorination process, the omniphobic membranes were prepared by the synergistic effect of surface microstructure and free energy. The effects of varying Zn(NO₃)₂ concentrations and deposition times on morphology, pore size and separation efficiency were comprehensively examined. It was demonstrated that at a Zn(NO₃)₂ concentration of 50 mM and a deposition time of 0.5 h, the resulting PVDF composite membrane exhibited high liquid repellency towards all the test liquids, such as water, ethanol solution, paraffin oil, and etc. Furthermore, the membrane consistently exhibited impressive salt rejection behavior (99.99%) and maintained an average flux (15.54 kg·m⁻²·h⁻¹) when exposed to high-salinity solution (up to 20 wt%-NaCl). Meanwhile, during treating an oil-contained saline water emulsion with the oil content of 200 ppm, the membrane also displayed high resistance to wetting and fouling during the continuously 24 h tests. The membrane displayed a rejection rate of 99.99 % with an average vapor flux of 18.60 kg·m⁻²·h⁻¹. In summary, this omniphobic membranes demonstrated a great potential in MD for practical challenging wastewater treatment.