Electrospun carbon-silica composite nanofibers for selective removal of oil and water
Tai, Ming Hang
Date of Issue2016
School of Civil and Environmental Engineering
There is an increasing demand for cost-effective and efficient oil spill remediation and purification technologies due to the rising public concern over the corresponding environmental impact and the increasingly stringent environmental regulations. This thesis explores the feasibility of carbon-based nanofibers and its derivatives as separation medium for mixture of oil and water. The pristine carbon nanofibers (CNFs) were prepared by a combination of electrospinning and thermal treatment. The electrospun nanofibers have average fiber diameter of 200-300 nm with carbon content up to ~ 80%. The microstructure of CNF is turbostratic with the carbon atomic sheets stacked together randomly. Despite non-crystalline structure, the CNFs could withstand a temperature as high as 500°C. With respect to its wettability property, CNF is hydrophobic and oleophilic. This makes it suitable for selective removal of oils and organic solvents from water. However, CNF suffers from low strain or toughness which restricts its applications. To reinforce CNFs, a flexible carbon-silica composite nanofibers were electrospun. The characterization results suggested that the electrospun composite nanofibers were constructed by carbon chains interpenetrated through a linear network of 3-dimensional SiO2. Thermogravimetric analysis indicated that the presence of insulating silica further improved the thermal resistance of the membrane. Additionally, the mechanical strength test showed that the membrane's toughness and flexibility could be enhanced if the concentration of SiO2 was maintained below 2.7 wt%. Thermal and chemical stability test showed that the membrane's wettability properties could be sustained at an elevated temperature up to 300 °C and no discernible change in wettability was observed under highly acidic and basic conditions. After surface-coating with silicone oil for 30 min, the composite membrane exhibited ultra-hydrophobic and superoleophilic properties with water and oil contact angles being 144.2 ± 1.2° and 0°, respectively. The enhanced flexibility and selective wetting property enable the membrane to serve as an effective substrate for separating free oil from water. Lab-scale oil-water separation test indicated that the membrane possessed excellent oil-water separation efficiency. In addition, its inherent property of high porosity allows oil-water separation to be performed in a gravity-driven process at high-flux. The advancement of compacting the material into a sponge using the same technology showed that the carbon-silica nanofibers were capable of adsorbing oil up to 140 times its own weight with sorption rate being solution viscosity dependent. The electrospun sponge has high porosity (99%) and is ultrahydrophobic and superoleophilic which are the essential characteristics for an efficient oil sorbent. Furthermore, it is lightweight and compressible. This makes the sponge regeneration and oil recovery feasible by using cyclic distillation and mechanical squeezing. The selective wetting property of carbon-silica nanofibers can be reversed by anchoring titanium dioxide (TiO2) nanosheets via solvothermal reaction. The hierarchical TiO2 micro/nanostructure that grows on the surface of CNFs renders the membrane superhydrophilic and underwater superoleophobic. Coupled with the characteristic of high porosity and micron-scale pore size, the membrane is capable of separating oil from water by gravity. Thermal analysis shows that the oil-water separation efficiency was greater than 99 %. Furthermore, the membrane has an oil breakthrough pressure up to 3.63 m. Stability test indicates that the membrane was stable in ultrasonic, thermal and extreme pH conditions. Without compromising the separation efficiency, a permeate flux of 400-700 L/m2-hr was achieved. The separation performance of carbon-silica nanofiber membrane was further evaluated by using a large amount of water-in-oil emulsion and under the condition of low pressure cross-flow microfiltration. Results show that the oil permeability of membrane is high without sacrificing its selectivity at low pressure. However, the membrane performance including emulsion rejection efficiency and flux decline is substantially influenced by the tangential flow rate and the applied pressure. The total resistance analysis revealed that the membrane might be fouled internally, externally or a combination of these under different operating condition. The contour plot suggests that the membrane performance is optimum when operated at high cross-flow velocity and relatively low applied pressure.