Functionalized Nanofiber for Phase Separation in Oil-Water Multiphase Liquid: From Water Purification to Oil Recovery

Abstract

Due to global industrialization, the substantial oil-water multiphase liquid including oily wastewater and water-contaminated solvent, are discharged without proper treatment. It not only threatens the ecological environment and human health but also causes a huge waste of resources. However, conventional treatment methods have the limitation that can treat the oil-water mixture phase with high efficiency but not the oil-water emulsion phase. These emulsions containing lots of micro-size water droplets (in the oil phase) or oil droplets (in the water phase) are able to keep stable for a long time because of the addition of surfactants. It has been reported that advanced membrane technology has been considered a competitive method for cleaning up the oil-water emulsion due to its good separation performance, acceptable cost-efficiency, and facile operational conditions. Moreover, the electrospun nanofiber membrane attracts lots of attention due to a higher porosity than others, leading to a higher permeate flux. Nevertheless, easily fouling is one of the critical limitations that has resulted in unsatisfied oil-water separation performance. In this study, functionalized nanofibers with various wettabilities are developed to address the fouling issue during the processes of water purification and oil recovery. To begin with, the thesis scope and outline are highlighted in Chapter 1, and recent progress of relative research is generally introduced in Chapter 2. In Chapter 3, a functionalized nanofiber membrane with hydrophilic property is introduced to purify water from oil-in-water emulsion. An intriguing integration of the omni-directional protected porous membrane that combines a high porosity nanofiber membrane with a surface segregation mechanism is established for the first time. By applying polydimethylsiloxane (PDMS)-terminated triblock copolymer, the enrichment of the hydrophilic poly(ethylene oxide) (PEO) segment and the nonpolar PDMS segment on the surface of the nanofiber endowed the nanofiber membrane with underwater oleophobicity and low oil adhesionforce, exhibiting oil resistance as well as oil release property. An ultrahigh permeate flux of ~ 7115 L m−2 h−1 with a separation efficiency of ~ 97.88% is achieved under the driving force of gravity (~ 0.9 kPa). Moreover, the surface segregation nanofiber membrane shows excellent reusability and stable flux in a long-term test. With the assistance of stirring, the relatively stable permeate flux revealed this membrane could release the dramatic reduction in permeate flux that caused by oil fouling, suggesting it is the promising performances for the further particular application of oily wastewater. In Chapter 4, modified SiO2 nanofibers are sprayed coating on the surface of polyvinylidene fluoride (PVDF) nanofiber membrane to enhance the antifouling property for recovering oil from water-in-oil emulsion. In detail, we developed a nanofiber-based solution for the fabrication of a lotus leaf-like spray coating layer, with nonpenetrating properties, hierarchical micro/nanostructure, and cross-linked porous network. Such a biomimetic nanofiber-based coating, which has a controllable nanostructure by tailoring the concentration of tetraethyl orthosilicate (TEOS) and low surface energy originated from octadecyltriethoxysilane (OTS), achieved superhydrophobicity in air with a water contact angle of 161.4°, SA of 3.6°, and superhydrophobicity under various oils. Owing to these superwetting properties, the antifouling property of the PVDF nanofiber membrane was significantly enhanced by this biomimetic coating. The resultant membrane could successfully separate various water-in-oil emulsions with microscale water droplets and exhibited a completely recovered flux with a stable water rejection even after 20 cycles of recycling, which revealed the surprising durability and reusability. Chapter 5, as the final chapter of the dissertation, reported a Janus membrane with asymmetric wettability: two completely contrary wettabilities on each side. Such membrane can capture oil droplet from oil-contaminated water and spontaneously transport the oil in “one way”. Fluorinated SiO2 nanofibers are formed a superamphiphobic coating layer with oil contact angle of ~153.5°, on the PVDF nanofiber membrane surface. Due to the synthetic effect of the oil repellent force, liquid pressure and capillary force, the oil droplet gradually penetrate into the membrane from the superamphiphobic side in ~ 4 s, but cannot transport from the oleophilic side to superamphiphobic side. Such a Janus membrane possess an oil collect ability from the emulsion. To some extent, this process oil capture and unidirectional transfer can be considered as a promising method for both oil recovery and water purification in long-term operation because it avoids the formation of cake layer on the membrane surface. Ultimately, this study is aimed to provide an available, reliable, and facile fabrication of functionalized nanofiber with special wettability for phase separation in oil-water multiphase liquid. By modification of the surface chemical composition and construction of a biomimetic structure, fouling mitigation and effective phase separation are achieved. Suggested work in this thesis potentially can be applied for industrial oil-water phase separation.