Fabrication of bioinspired super-hydrophilic nano-fibrous membrane for oil-water separation and directional water transportation

• Main Author: Li, Zhengtao
• Other Authors: Darren Sun Delai
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/137410
• Institution: Nanyang Technological University

Energy efficient water purification and transportation is the key to solve energy and water crisis throughout the world. Inspired by the natural special wetting surfaces, many super-hydrophobic materials were fabricated and studied for efficient oil-water separation and directional water movement. However, due to some limitations of hydrophobic materials, super-hydrophilic materials are more regarded as the candidate to overcome those shortcomings to achieve efficient oil-water separation and bulk water transportation. In this study, electrospinning of hydrophilic silk fibroin was first employed to fabricate the super-hydrophilic nano-fibrous materials. Electrospinning is able to create non-woven nanofibers, which allows the formation of novel materials with high porosity and rough surfaces to mimic the natural super-wetting structures. The as prepared membrane exhibited super-hydrophilic and underwater super-oleophobic. Excellent oil-water separation efficiency was achieved by this membrane. Another method to fabricate hydrophilic membrane was the surface modification of hydrophobic materials. Nano-sized metal oxide is able to introduce hydrophilicity of the surface and increase the surface roughness to amplify its hydrophilicity. In this study, electrospinning method and hydrothermal process were combined to fabricate the copper oxide (CuO)-modified polyvinylidene difluoride (PVDF) membrane. This approach successfully made the hydrophobic PVDF membrane become hydrophilic, which enabled the membrane to separate oil from water with excellent efficiency. By the similar synthesis process, zinc-oxide (ZnO) modified polysulfone (PSF) nano-fibrous membrane was also successfully fabricated. Due to the photo-sensitivity of ZnO, the membrane obtained the photo-responsive wetting behavior. After dark storage, the membrane was hydrophobic; after ultraviolet (UV) irradiation, it rendered hydrophilic. The oil-water separation test further showed that under UV irradiation this membrane performed efficient oil-water separation. Moreover, this membrane demonstrated self-cleaning property which prompted degradation of contaminants under UV irradiation. Subsequently, in order to overcome the fouling issues and noncontinuous operation of traditional gravity-driven dead-end approach, a facile membrane based set-up, mimicking the crossflow filtration of manta ray gill rakers, was fabricated by the aligned electrospinning of silk fibroin. In this process, as the oil-water mixture travels in a parallel manner across the super-hydrophilic membrane surface, water permeated through the membrane, while oil was rejected by the membrane and collected in the middle pipe. Experimental results showed that oil-water separation was achieved both efficiently and continuously. Moreover, materials with super-wetting behavior are also regarded as a great candidate for water transportation. Inspired by the bulk water transportation of trees, a novel nano-fibrous matrix was successfully fabricated in this study. Its unique structure and small fiber size is able to maintain the hydration and drives the movement of the water. At the same time, in order to achieve the continuous water movement, evaporation process, mimicking the water transportation process of tree, was utilized as the driving force for constant water movement. To conclude, different kinds of super-hydrophilic materials have been systematically investigated in this study. The experimental results presented showed their great performance in efficient oil-water separation, and more importantly the long-distance bulk water transportation which has the potential to overcome the hurdle of huge energy consumption in water transportation.