PVDF Membranes for Membrane Distillation - Controlling Pore Structure, Porosity, Hydrophobicity, and Mechanical Strength (original) (raw)
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Hydro Science & Marine Engineering, 2019
Although PVDF flat sheet membranes have been widely tested in MD, their synthesis and modifications currently require increased use of green and inexpensive materials. In this study, flat sheet PVDF membranes were synthesized using phase inversion and water as the pore former. Remarkably, the water added in the casting solution improved the membrane pore sizes; where the maximum pore size was 0.58 µm. Also, the incorporation of f-SiO2NPs in the membrane matrix considerably enhanced the membrane hydrophobicity. Specifically, the membrane contact angles increased from 96° to 153°. Additionally, other parameters investigated were mechanical strength and liquid entry pressure (LEP). The maximum recorded values were 2.26 MPa and 239 kPa, respectively. The modified membranes (i.e., using water as the pore former and f-SiO2NPs) were the most efficient, showing maximum salt rejection of 99.9% and water flux of 11.6 LMH; thus, indicating their capability to be used as efficient materials for...
2017
Development of highly porous flat sheet polyvinylidene fluoride (PVDF) membranes for membrane distillation Salim Amer Alsaery With the increase of population every year, fresh water scarcity has rapidly increased and it is reaching a risky level, particularly in Africa and the Middle East. Desalination of seawater is an essential process for fresh water generation. One of the methods for desalination is membrane distillation (MD). MD process separates an aqueous liquid feed across a porous hydrophobic membrane to produce pure water via evaporation. Polyvinlidene fluoride (PVDF) membranes reinforced with a polyester fabric were fabricated as potential candidates for MD. Non-solvent induced phase separation coupled with steam treatment was used to prepare the PVDF membranes. A portion of the prepared membrane was coated with Teflon (AF2400) to increase its hydrophobicity. In the first study, the fabricated membranes were characterized using scanning electron microscopy and other techn...
Membrane distillation (MD) is a promising separation technique capable of being used in the desalination of marine and brackish water. Poly(vinylidene fluoride) (PVDF) flat sheet nano-composite membranes were surface modified by coating with electro-spun PVDF nano-fibres to increase the surface hydrophobicity. For this purpose, the nano-composite membrane containing 7 wt.% super-hydrophobic SiO 2 nano-particles, which showed the highest flux in our previous work, was first subjected to pore size augmentation by increasing the concentration of the pore forming agent (Di-ionized water). Then, the prepared flat sheet membranes were subjected to nanofibres coating by electro-spinning. The uncoated and coated composite fabricated membranes were characterized using contact angle, liquid entry pressure of water, and scanning electron microscopy. The membranes were further tested for 6 h desalination by direct contact membrane distillation (DCMD) and vacuum membrane distillation (VMD), with a 3.5 wt.% synthetic NaCl aq as the feed. In DCMD the feed liquid and permeate side temperature were maintained at 27.5 C and 15 C, respectively. For VMD, the feed liquid temperature was 27 C and a vacuum of 94.8 kPa was applied on the permeate side. The maximum permeate flux achieved was 3.2 kg/m 2 .h for VMD and 6.5 kg/m 2 .h for DCMD. The salt rejection obtained was higher than 99.98%. The coated membranes showed a more stable flux than the uncoated membranes indicating that the double layered membranes have great potential in solving the pore wetting problem in MD.
Membranes
Here, we report the fabrication of a series of highly efficient polyvinylidene fluoride (PVDF) membranes via substrate morphology variations. A wide range of sandpaper grit sizes (150–1200) were utilized as casting substrates. The effect of the penetration of abrasive particles present on the sandpapers on the casted polymer solution was tuned, and the impact of these particles on porosity, surface wettability, liquid entry pressure and morphology were investigated. The membrane distillation performance of the developed membrane on sandpapers was evaluated for the desalination of highly saline water (70,000 ppm). Interestingly, the utilization of cheap and widely available sandpapers as a substrate for casting can not only help in tuning the MD performance, but also in producing highly efficient membranes with stable salt rejection (up to 100%) and a 210% increase in the permeate flux over 24 h. The findings in this study will help in delineating the role of substrate nature in cont...
Simple and effective corrugation of PVDF membranes for enhanced MBR performance
Journal of Membrane Science, 2015
Membrane fouling control in membrane bioreactors (MBRs) can be achieved by improving membrane properties. In this study, corrugated flat sheet polyvinylidenefluoride (PVDF) membranes were fabricated, characterized and tested in a lab-scale MBR for improved filterability and fouling resistance. A simple imprinting step was successfully developed and applied as part of the membrane preparation procedure, via phase inversion, to form corrugations on the membrane surface. The corrugation consisted of valleys-and-hills topography, which increased membrane effective surface area (A E ) by $ 50%. It also increased the membrane mean pore size (PS) as a result of changes in formation mechanism. Both higher A E and larger PS increased membrane permeability to about 5-6 times compared to the non-corrugated membrane, prepared under similar conditions. Surface corrugations reduced membrane fouling propensity as observed from the flux-stepping test and a lab-scale MBR operation, without affecting permeate quality.
Journal of Applied Polymer Science, 2018
In this study, poly(vinylidene fluoride) (PVDF) membranes were prepared using two different solvents with various polymer concentrations to investigate the predominant kinetic or thermodynamic aspects of membrane preparation in a phase separation process. For this purpose, dimethyl sulfoxide (DMSO) as a weak solvent and N-2-methylpyrrolidone (NMP) as a strong solvent were used with polymer concentrations between 8 and 15 wt %. Scanning electron microscopy and water content, contact angle, and pore size measurements were used to assess the factors affecting the physicochemical properties of the prepared membranes. The results showed that in the case of NMP, the membrane structure is mainly controlled by thermodynamic parameters, while when using DMSO, kinetic parameters are predominant. According to the results, the prepared PVDF-based membranes with DMSO exhibited a relatively denser top layer and less permeation compared to the NMP/PVDF membranes. The difference between the viscosities of the casting solutions with equal polymer concentrations in DMSO and NMP was considered to be the main effective factor in solvent/nonsolvent exchange, resulting in denser top layers in the DMSO/PVDF membranes.
Separation and Purification Technology, 2009
The mixture of inorganic salt LiCl and soluble polymer polyethylene glycol (PEG) 1500 as non-solvent additive was introduced to fabricate hydrophobic hollow fiber membrane of polyvinylidene fluoride (PVDF) by phase inversion process, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. Compared with other three membranes from PVDF/DMAc, PVDF/DMAc/LiCl and PVDF/DMAc/PEG 1500 dope solution, it can be observed obviously by scanning electron microscope (SEM) that the membrane spun from PVDF/DMAc/LiCl/PEG 1500 dope had longer finger-like cavities, ultra-thin skins, narrow pore size distribution and porous network sponge-like structure owing to the synergistic effect of LiCl and PEG 1500. Besides, the membrane also exhibited high porosity and good hydrophobicity. During the desalination process of 3.5 wt% sodium chloride solution through direct contact membrane distillation (DCMD), the permeate flux achieved 40.5 kg/m 2 h and the rejection of NaCl maintained 99.99% with the feed solution at 81.8 • C and the cold distillate water at 20.0 • C, this performance is comparable or even higher than most of the previous reports. Furthermore, a 200 h continuously desalination experiment showed that the membrane had stable permeate flux and solute rejection, indicating that the as-spun PVDF hollow fiber membrane may be of great potential to be utilized in the DCMD process.
Novel modified PVDF ultrafiltration flat-sheet membranes
Journal of Membrane Science, 2012
Polyvinylidene fluoride (PVDF) ultrafiltration membranes were modified using a new type of hydrophilic polyurethane additive, called L2MM. During phase inversion L2MM migrates to the membrane surface and functions as both a pore former and surface modifier. L2MM improved the surface hydrophilicity and significantly increased the PVDF membrane's pure water permeation (PWP) rate. PVDF membranes were modified with two L2MMs: L2MM(PEG-600) and L2MM(PEG-200). PWP tests, contact angle measurements, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to assess the impact of L2MM type, L2MM concentration and PVDF concentration on the performance and characteristics of modified PVDF membranes. XPS analysis and contact angle measurements indicated that the L2MM migrated to the membrane surface and the contact angle decreased by up to 19%. SEM and AFM images were used to investigate the relation between surface morphology and ultimate performance. The L2MMs had a significant effect on flux; increasing the L2MM concentration improved membrane PWP fluxes up to a maximum with further increases resulting in flux decreases. Both L2MM(PEG-200) and L2MM(PEG-600) increased the final PWP flux of modified PVDF membranes up to 6.5 times that of the control membranes. Furthermore, L2MM(PEG-200) increased the 100 kDa polyethylene oxide (PEO) separation from 88 to 96% compared to the control membrane.