Construction of rough and porous surface of hydrophobic PTFE powder-embedded PVDF hollow fiber composite membrane for accelerated water mass transfer of membrane distillation
Graphical abstract
Introduction
Different from the pressure-driven membrane separation technologies such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), the efficient separation of thermally-driven membrane distillation (MD) is achieved by the vapor pressure difference on both sides of the hydrophobic porous separation membrane. Compared with the pressure-driven membrane separation processes, MD has outstanding advantages, such as mild operating conditions, theoretically 100% rejection rate, no limitation of osmotic pressure, and extensive heat sources including industrial waste heat, geothermal and solar energy [1]. MD has been applied in desalination, industrial wastewater treatment, food, pharmaceutical and biological fields [2], [3]. However, low permeate water flux and membrane pore wetting greatly restrict the wide development of MD technology. In addition to optimizing the MD process and membrane modules, the development of microporous separation membranes with strong hydrophobicity has become one of the most effective solutions.
The types of separation membranes used in MD process mainly include flat-sheet, hollow fiber, spiral-wound, tubular and capillary types. Among them, hollow fiber membrane with self-supporting structure possesses high packing density and low boundary layer resistance as well as the flexibility of its module design. In recent years, its application in the field of membrane distillation has gradually expanded [4]. The ideal separation membrane for MD process should have appropriate pore size, high hydrophobicity, low thermal conductivity and excellent chemical resistance to feed streams. In addition, for hollow fiber membrane, its special fabrication process also requires the good spinnability of separation membrane material. Nowadays, hydrophobic polymeric hollow fiber membrane materials for MD mainly include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and polypropylene (PP). With excellent membrane-forming and processing properties, PVDF can be dissolved in conventional organic solvents at room temperature and the microporous hollow fiber membrane for MD can be obtained by traditional phase separation methods such as nonsolvent induced phase separation (NIPS) [5]. PP has excellent solvent resistance and high crystallinity. PP hollow fiber membrane is prepared through the melt extrusion-stretching or thermally induced phase separation (TIPS), in which the PP polymer needs to be dissolved in a special solvent at high temperature. In addition to its excellent thermal and chemical stability, the hydrophobicity of PTFE is the strongest among the hydrophobic polymer hollow fiber membrane materials for MD. These comprehensive properties enable PTFE membrane to well inhibit the occurrence of membrane pore wetting. However, the characteristics of high solvent resistance and high melt viscosity of PTFE also make it difficult to obtain hydrophobic separation membrane by traditional membrane preparation technology such as NIPS and TIPS. At present, researchers have successfully prepared PTFE nanofiber membrane by electrospinning-calcination method [6], [7], [8], PTFE hollow fiber membrane by lubricant-melt stretching method [9] and PTFE/PVDF blend membrane by TIPS method [10], [11]. Most of these methods use polymers with low melting point such as polyvinyl alcohol (PVA) [7], [8], polythylene oxide (PEO) [6] and isoparl G [9] as binders, and then calcine them to obtain the fused PTFE powder nanofiber membrane, flat sheet membrane or hollow fiber membrane, so as to overcome the poor processability characteristic of PTFE. However, few reports are centered on the application of PTFE as superhydrophobic coating material on traditional separation membrane matrix (such as PVDF membrane).
In addition to these traditional MD hydrophobic polymeric materials for the preparation of hollow fiber membrane, the hydrophobic polymer sources can be expanded by the modification of the existing polymers through various polymer synthesis methods, such as various PVDF copolymers including PVDF-co-hexafluoropropylene (PVDF-HFP) [12], PVDF-co-chlorotrifluoroethylene (PVDF-CTFE) and PVDF-co-hexafluoropropylene (PVDF-FEP) [13] and fluorinated polyoxadiazole (f-POD) [14]. The development of these new hydrophobic materials is mainly based on improving the hydrophobic properties of PVDF hollow fiber membrane to further improve MD water flux and wetting resistance. Compared with the synthesis of new hydrophobic PVDF-based polymers based on membrane bulk structure modification, how to construct hydrophobic surface while maintaining the original PVDF membrane bulk structure has become a practical method. It is well known that the hydrophobic waxy material on the surface of lotus leaf as well as its rough double micro-nano structure determine its super hydrophobic characteristics. Therefore, except for the introduction of hydrophobic groups, particles and polymers on PVDF membrane surface to reduce the surface free energy of membrane materials, the changes of membrane surface microstructure and morphology will also have an important impact on the hydrophobicity of separation membrane. Researchers have roughened the surface of PVDF hollow fiber membrane through various surface roughening technologies, such as physical plasma irradiation [15], vacuum suction filtration coating nanoparticles [16], chemical deposition [17] and co-extrusion spinning [18]. In addition, crimping the surface morphology of hollow fiber membrane by heat treatment has also been proved to effectively enhance the mass transfer effect of MD [19]. A coating layer with rough surface would be formed on the surface of porous support membrane by vacuum suction filtration and chemical deposition methods. The coating layer not only increases the hydrophobicity of substrate surface, but also probably generates additional mass transfer resistance [20]. A special spinneret is necessary in the co-extrusion spinning technology and the composition of two-component spinning dope and the dual-layer spinning process need to be well adjusted and optimized [21]. Only a very thin layer is roughened on the substrate surface through plasma irradiation which requires special irradiation equipment. The pore structure on the membrane surface would be prone to change due to excessive bending during the crimping of hollow fiber membranes. Therefore, how to obtain a hydrophobic surface without increasing additional mass transfer resistance through a facile method has become one of the problems to be solved in the construction of hydrophobic membrane surface for MD. Compared with the preparation methods of the hydrophobic coating layer described above, a facile method of dilute solution coating uses the polymer dilute solution with a concentration of around 5 wt% to form a loose flocculent coating with macroporous structure by conventional phase inversion method. The coating of loose and macroporous hydrophobic layer can be expected to enhance membrane surface hydrophobicity and meanwhile accelerate the rapid transfer of water vapor generated on the membrane surface from the macropores in the coating layer to the permeate side without additional mass transfer resistance during MD process.
In view of the super hydrophobic characteristics of PTFE and its difficulty in spinning process, PTFE powder was directly incorporated into PVDF dilute solution in this study. A loose and porous hydrophobic PTFE powder-embedded PVDF coating layer would be formed on the surface of the as-spun PVDF hollow fiber support membrane by nonsolvent induced phase separation (NIPS) method. Compared with the previous hydrophobic modification methods of PVDF membrane and the preparation methods of PTFE membrane described above, the novelty of this study is mainly reflected in three aspects: 1. Instead of using PTFE water emulsion in electrospinning process, PTFE powder is directly used as the hydrophobic coating material on conventional PVDF separation membrane. 2. The hydrophobic coating formed is rough and macroporous, avoiding additional mass transfer resistance caused by the deposition or vacuum filtration coating methods; 3. The dilute solution coating method is facile without special equipment for physical irradiation or high energy consumption of calcination method. Membrane morphology, surface chemical compositions, hydrophobicity and wetting resistance were investigated by scanning electron microscope (SEM), Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR), dynamic water contact angle (WCA) and liquid entry pressure (LEP) analysis. Membrane separation performance including desalination and dyes removal properties was evaluated by VMD experiment. Finally, the MD separation mechanism of inorganic salt, anionic dye (Congo red, CR) and) and cationic dye (methylene blue, MB) by PTFE powder-embedded HFC membrane was proposed.
Section snippets
Materials
Poly(vinylidene fluoride) (PVDF, FR-904) powder was obtained by Shanghai 3F New Materials Co., Ltd., China and dried in a vacuum oven at 80 °C for 12 h before use. Polytetrafluoroethylene (PTFE) powder was obtained by Shanghai JINGCHUN Biochemical Technology Co., Ltd (China). Polyethylene glycol (PEG, Mw = 6, 000), N,N-dimethyl acetamide (DMAc) were supplied by Tianjin Kemiou reagent Co., Ltd. (China). Congo red (CR) and methylene blue (MB) were purchased from Tianjin FENGCHUAN Chemical Reagent
Surface chemical composition of different hollow fiber membranes
Fig. 3 presents the FTIR spectra of different hollow fiber membranes. Three typical absorption bands at 839, 1071, 1166 cm−1 in the FTIR spectrum of PVDF membrane correspond to the stretching vibration of CF group in PVDF molecules [27]. Another sharp peak at 1400 cm−1 which is assigned to the CH2 group in PVDF also emerges in the spectrum of PVDF membrane as shown in Fig. 3(a). Two obvious peaks at 1154 and 1224 cm−1 which are attributed to the stretching vibration of CF2 group appear in the
Conclusions
A rough and porous PTFE powder-embedded coating layer was successfully formed on PVDF hollow fiber support membrane by the dilute solution coating method. The improvement of MD water flux of composite membrane is attributed to the enhancement of mass transfer efficiency and heat transfer efficiency across composite membrane. The contribution of the PTFE powder-embedded coating layer is mainly based on the enhanced membrane surface hydrophobicity, the formation of a thinner boundary layer and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the funding for the Project supported by the Research and Cultivation Fund Project of Henan University of Engineering (PYXM202013), Project of Youth Talent Promotion in Henan Province (No. 2021HYTP029), Henan Science and Technology Research Plan Project (No. 222102240101) and National College Students Innovation and Entrepreneurship Training Program ( No. 202111517001).
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