Design and fabrication of nanofibrous thin film composite membranes by electrospinning for osmotically driven membrane processes
Date of Issue2015
School of Civil and Environmental Engineering
Singapore Membrane Technology Centre
Nowadays, high water demands due to fast global population growth and rapid economic development call for new technologies to provide clean water with less energy consumption. The serious environmental pollution caused by fossil fuel usage also require us to develop environmentally friendly technologies to harvest energy. As an emerging technology, osmotically driven membrane processes (ODMPs) including forward osmosis (FO) and pressure retarded osmosis (PRO) processes, are considered to be attractive technologies to treat waste water, recycle used water and harvest energy from salinity-gradients. However, FO and PRO technologies are still in the development stage. Developing osmotic membranes with both high water permeation and excellent ion rejection is a challenging task. The desired osmotic membrane may be a thin film composite (TFC) membrane consisting of an ultrathin polyamide layer with high water permeability and low solute permeability, and a highly porous support layer for low internal concentration polarization (ICP), which is one of the obstacles to improving FO/PRO membrane performance. In addition, high mechanical properties are essential for the membrane to withstand high hydraulic pressures in PRO processes. Fortunately, electrospun nanofibrous membranes have great potential to fulfil all the requirements of an ideal membrane support layer due to their open and porous structural features and ease with which nanomaterials can be incorporated to increase the mechanical properties of the nanofibers. Additionally, interfacial polymerization can be carried out on electrospun nanofibrous membrane surfaces to obtain an ultrathin polyamide selective layer. After incorporating nanomaterials in either the polyamide selective layer or the support layer, thin film nanocomposite (TFN) membranes may have great potential to exhibit excellent performance in ODMPs. The objectives of this research are to design and develop novel nanofiber-based composite membranes for ODMPs. Specifically, this thesis presents the development of novel nanofiber-supported polyamide TFC and TFN membranes with high water permeation, good selectivity, and excellent mechanical properties for FO and PRO processes. Electrospinning was utilized to fabricate a variety of highly porous nanofibrous substrates while interfacial polymerization was exploited to form the thin polyamide rejection layer on the substrate. In the beginning, we explored the feasibility of using electrospun polyvinylidene fluoride (PVDF) nanofibers as the substrate to make high-performance FO membranes. polyamide thin films were successfully formed via interfacial polymerization directly on two electrospun PVDF nanofiber substrates, S1 and S2, having different surface properties in terms of pore size and surface roughness. Experimental results revealed that a denser and less permeable polyamide layer was formed on the S1 substrate which possessed relatively smaller pore sizes (mean pore size: 0.32 ± 0.07 μm; maximum pore size: 0.50 ± 0.10 μm), while a looser polyamide layer with higher water permeability was attained on the S2 substrate with larger pore size (mean pore size: 0.40 ± 0.02 μm; maximum pore size: 0.59 ± 0.04 μm). The difference in polyamide layer structure was believed to be associated with the substrate structure, leading to different cross-linking degrees during the interfacial polymerization. The basic FO performance of resulting TFC membranes has been investigated. A water flux of 30.4 L/m2h was achieved when the active layer was oriented towards the 1.0 M NaCl draw solution while the ratio of reverse salt flux to water flux could be kept as low as 0.21 g/L. Following the success of the PVDF nanofiber composite FO membrane, some nanomaterials were proposed for incorporation in the nanofibrous substrates to provide more water channels and improve the substrate's hydrophilicity. We fabricated a novel TFN membrane consisting of a polyetherimide (PEI) nanofibrous substrate incorporating silica nanoparticles and an ultrathin polyamide skin layer. PEI was also chosen in this research as it is a versatile polymer with a wide range of applications due to its excellent thermal, mechanical and solvent-resistance properties. It was verified that the incorporation of silica nanoparticles assisted in reducing the structure parameter of the substrate in terms of porosity, increasing additional water channels and hydrophilicity, and improving membrane performance significantly in the FO process. To further enhance both the performance and mechanical properties of the composite membrane, we proposed and fabricated a novel TFN membrane consisting of a PEI nanofibrous support reinforced by functionalized multi-walled carbon nanotubes (f-CNTs) and an ultrathin polyamide rejection layer. In addition to the superior inter-connected porous structure of the nanofibrous support, the well-distributed f-CNTs in the nanofibers increased the substrate porosity by 18%, reduced the structure parameter by 30%, and significantly improved the substrate tensile strength by 53%. The high mechanical strength provided by the embedded f-CNTs enabled us to further increase the substrate porosity to reduce the ICP, which was demonstrated by the high performance of TFN-2 FO membrane with a structure parameter of 310 µm. Its water flux at 1.0 M NaCl draw solution was 61 and 33 L/m2h in the active layer-facing-DS (AL-DS) and active layer-facing-feed water (AL-FW) orientations, respectively. To the best of our knowledge, it is the first time that the CNTs incorporation increases both water flux and membrane mechanical strength. It is believed that this novel design strategy has great implications for PRO membrane fabrication. In the last part of this study, we have successfully fabricated a novel TFN membrane consisting of a tiered structure of f-CNTs-reinforced PEI nanofibrous support and an ultrathin polyamide-based selective skin layer for PRO process. The tiered support was made by fine and coarse PEI nanofiber layers. The fine fiber reinforced with well dispersed f-CNTs has been found to increase mechanical stability of the polyamide selective layer, allowing the support to withstand high hydraulic pressure in the PRO system. The optimized membrane can endure a trans-membrane pressure up to 24 bar and generate a peak power density as high as 17.3 W/m2 at 16.9 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution against de-ionized (DI) water. In addition, the long term PRO results show that this membrane can generate a stable power density of 15.0 ± 0.5 W/m2 for a test period of 10 hours. This demonstrates that the membrane holds great potential to be used in the PRO process. In conclusion, this thesis presents the design and development of novel nanofibrous TFC and TFN membranes based on studies of the fundamental mechanisms of interfacial polymerization on a variety of nanofibrous substrates, fabrication of highly porous nanofibrous TFN FO membrane by incorporation of silica nanoparticles, development of robust and highly porous nanofibrous TFN FO membrane by incorporation of f-CNTs, and the design of a PRO membrane with a tiered structure. This work contributes to the development of membrane fabrication technology and facilitates practical applications of nanofibrous membranes in ODMPs.
DRNTU::Engineering::Environmental engineering::Water treatment
DRNTU::Engineering::Materials::Mechanical strength of materials
DRNTU::Engineering::Materials::Mechanical strength of materials
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