Characterization of membrane distillation membranes prepared by phase inversion

In this paper, flat membrane distillation membranes have been successfully manufactured from PVDF/DMAc and PVDF/DMF blends by using phase inversion induced by an immersion precipitation technique. The structure of the membranes is asymmetric with a porous top layer and macrovoids, as assessed by SEM. The existence of MD fluxes in these membranes is established by performing various pure water flux experiments. A maximum in the MD fluxes for a particular value of the polymer content in the casting solution from which the membrane is manufactured has been observed. The dependence of the magnitude of the fluxes on the membrane thickness is also discussed and the influence of temperature polarization evaluated.     

Superhydrophobic modification of ceramic membranes for vacuum membrane distillation

The hydrophobically modified ceramic membranes have great potential for energy-efficient membrane distillation.In this work,flat-sheet ceramic membranes with a superhydrophobic surface were fabricated by grafting 1H,1H,2H,2H-perfluorooctyltrichlorosilane or 1H,1H,2H,2H-perfluorodecyltriethoxysilane and followed by ultraviolet irradiation.The surface water contact angle was improved from 46° of original ceramic membrane to 159°,which exhibited a stable and excellent superhydrophobic effect.The modified membranes showed a high flux of 27.28 kg· m-2.h-1 and simultaneously maintained an excellent retention rate of 99.99％,when used in vacuum membrane distillation process for treatment ofa 1 wt％ NaCl (75 °℃) aqueous solution.These results suggested that superhydrophobic modification of ceramic surface is a facile and cost-effective way to achieve higher membrane distillation performance.The superhydrophobically-modified ceramic membrane with an excellent desalination capacity would show considerable potential in practical membrane distillation utilizations.     

Innovative hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride (PVDF) composite membrane for vacuum membrane distillation

Though membrane distillation (MD) has gained more and more attention in the field of desalination, the wetting phenomenon was still a non-negligible problem. In this work, a method combined dip-coating and UV in situ polymerization for preparing hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride composite membranes. This composite membrane consisted of a top thin hydrophobic coating layer and hydrophilic substrate membrane. In terms of anti-wetting properties, contact angle and liquid entry pressure of all composite membranes (except for those based on 0.45 μm) exceeded 160° and 0.3 MPa, respectively. In particular, the desalination performance was tested in vacuum membrane distillation tests by feeding 3.5% (mass) saline solution (NaCl) at 60 ℃. The composite membranes with larger support pore size and lower PFPE content had higher membrane distillation flux. And for stability tests (testing the 0.22 μm membrane coated by 5% (mass) PFPE), the highest MD flux 29.08 kg·m^-2·h^-1 and stable salt rejection (over 99.99%) during the period. Except that, the effects of coating material concentration and pore sizes of substrate membrane were also investigated for surface morphology and topography, porosity, mechanical strength and pore size characteristics. This work provided a simple and effective alternative to prepare excellent hydrophobic composite membranes for MD applications.     

Robust super-hydrophobic inorganic nanoparticles modified layered structure Si2N2O membrane for membrane distillation

Robust super-hydrophobic ceramic membranes consisting of layered structure Si₂N₂O grains and organosilane-derived inorganic nanoparticles were successfully fabricated and employed for membrane distillation. First, phase inversion and sintering method were used to prepare porous Si₂N₂O membranes. The slurry composition and sintering temperature were optimized to obtain a pure phase Si₂N₂O membrane with high bending strength, tailored average pore size, and high permeability. Then, the Si₂N₂O membranes were modified with organosilane-derived inorganic nanoparticles through ammonolysis and pyrolysis reactions. Due to the micro and nano-hierarchical rough structures and the presence of -Si-CH₃ groups, the membranes showed super-hydrophobicity with a water contact angle of 152 ± 1°. Finally, the membranes were applied to desalinate seawater by sweeping gas membrane distillation. A stable water flux of 76 ± 0.9 L/(m² day) with a salt rejection of > 99% was recorded during 30 h distillation test at 75 °C, demonstrating the stability and durability of the membranes.     

Bottlenecks and recent improvement strategies of ceramic membranes in membrane distillation applications: A review

Ceramic membranes have received more attention currently from researchers in membrane distillation (MD) applications due to their outstanding properties. However, despite their superior mechanical, thermochemical stability, and resistance to harsh operating conditions, several bottlenecks still limit their applications in MD. Although there are several published articles on ceramic membranes in MD, the uniqueness of this review lies in the fact that it discusses the critical bottlenecks that significantly affect the performance of ceramic membranes in long-term operation and limit their scale-up to commercial MD applications. Furthermore, recent advances, strategies, and techniques to mitigate these limitations have also been discussed. A discussion on high ceramic membrane fabrication costs and mitigation strategies using alternative low-cost ceramic materials to erstwhile conventional ceramic materials has been presented. In addition, the inherent problems of the brittleness and wetting/fouling of ceramic membranes and recent advances in strengthening ceramic membranes and fouling/wetting control via the development of superhydrophobic and omniphobic ceramic surfaces in MD have been addressed. Consequently, technical bottlenecks that still exist despite recent development in ceramic membranes for MD applications have been highlighted and future research direction in developing robust ceramic membranes in MD applications has been elaborated.     

疏水石墨烯膜的制备及其用于膜蒸馏脱盐的研究进展

膜蒸馏技术由于理论截盐率高、操作条件温和及对盐浓度灵敏度低等优势,在脱盐领域中展现出巨大潜力。近年来,人们开始关注石墨烯材料在膜蒸馏脱盐领域中的应用。本文首先概述膜蒸馏技术的基本原理及常用膜材料;接着介绍石墨烯的疏水性质和疏水石墨烯膜的制备;再详细综述石墨烯混合基质膜、石墨烯复合膜及石墨烯纯膜这三类疏水石墨烯膜在膜蒸馏脱盐中的应用;最后总结疏水石墨烯用于膜蒸馏脱盐面临的主要挑战及未来研究方向。     

Development of polyethersulfone phase‐inversion membranes for membrane distillation using oleophobic coatings

Highly porous macrovoid‐free polyethersulfone membranes have been prepared using the phase‐inversion process with water as the non‐solvent. These membranes are of great interest for membrane distillation (MD) after application of a hydrophobic/oleophobic coating. The membrane structure was controlled by optimizing the process conditions and dope composition. Counter intuitively, increasing the polymer concentration favors the formation of larger surface pores under similar process conditions. A symmetric membrane is obtained when a sufficient amount of high‐molecular‐weight polyvinylpyrrolidone was added to the dope solution, which appears to play an important role in the structure formation process. The final membrane shows similar performance compared to commercial MD membranes. However, the membranes developed in this study show an oleophobic character, broadening the applications of MD. Moreover, the compressibility of these membranes is severely reduced compared to stretched membranes, which is expected to result in an improved MD performance at full scale. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45516.     

Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation

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/m² 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.     

Development of a vacuum membrane distillation unit operation: From experimental data to a simulation model

Due to the important role of thermal membrane separation processes for the chemical industry, simulation of those process steps is more and more relevant. In the simulation software AspenPlus© no unit operation for a membrane distillation step is available. Due to the lack of built in membrane distillation (MD) models, the aim of this work was to develop one for application in conceptual design on the AspenPlus^© platform. Therefore, this paper presents a user-customize one dimensional unit operation for vacuum MD on the basis of the dusty gas model. Binary butanol water mixtures of different concentrations served as feed streams. Experimental investigations on a hollow fibre and tubular polypropylene (PP) membrane module with a pore diameter of 0.2 μm resulted in raw data on transmembrane flux and selectivity. These experiments served to generate a component permeance data bank. On the basis of the results, a regression for the component permeance was performed. The implementation of the generated permeance functions in the programming code resulted in a unit operation in AspenPlus^© reproducing well the experimental work. Comparison of the model with the laboratory results show very good reliability for the different membranes investigated.     

Improvement of porous polyvinylidene fluoride-co-hexafluropropylene hollow fiber membranes for sweeping gas membrane distillation of ethylene glycol solution

Porous polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) hollow fiber membranes were fabricated through a wet spinning process. In order to improve the membrane structure, composition of the polymer solution was adjusted by studying ternary phase diagrams of polymer/solvent/non-solvent. The prepared membranes were used for sweeping gas membrane distillation (SGMD) of 20 wt% ethylene glycol (EG) aqueous solution. The membranes were characterized by different tests such as N₂ permeation, overall porosity, critical water entry pressure (CEP_w), water contact angle and collapsing pressure. From FESEM examination, addition of 3 wt% glycerol in the PVDF-HFP solution, produced membranes with smaller finger-likes cavities, higher surface porosity and smaller pore sizes. Increasing the polymer concentration up to 21 wt% resulted in a dense spongy structure which could significantly reduce the N₂ permeance. The membrane prepared by 3 wt% glycerol and 17 wt% polymer demonstrated an improved structure with mean pore size of 18 nm and a high surface porosity of 872 m⁻¹. CEP_w of 350 kPa and overall porosity of 84% were also obtained for the improved membrane. Collapsing pressure of the membranes relatively improved by increasing the polymer concentration. From the SGMD test, the developed membrane represented a maximum permeate flux of 28 kg·m⁻²·h⁻¹ which is almost 19% higher than the flux of plain membrane. During 120 h of a long-term SGMD operation, a gradual flux reduction of 30% was noticed. In addition, EG rejection reduced from 100% to around 99.5% during 120 h of the operation.     

Preparation of PVC/PVP composite polymer membranes via phase inversion process for water treatment purposes

In this work, new composite membranes were successfully prepared via phase inversion technique using polyvinyl chloride(PVC) and polyvinylpyrrolidone(PVP) as polymers and tetrahydrofuran(THF) and N-methyl-2-pyrrolidone(NMP) as solvents. The prepared membranes have been characterized by scanning electron microscope(SEM), and fourier transforms infrared spectroscopy(FTIR). The scanning electron microscope results prove that the prepared membranes are smooth and their pores are distributed throughout the whole surface and bulk body of the membrane without any visible cracks. The stress–strain mechanical test showed an excellent mechanical behavior enhanced by the presence of PVP in the prepared membranes. The membranes performance results showed that the salt rejection reached 98% with a high flux. This, in turn, makes the prepared membranes can be applied for sea and brackish water treatment through membrane distillation technology.     

Exploring the spinning and operations of multibore hollow fiber membranes for vacuum membrane distillation

Hollow fiber membranes with a multibore configuration have demonstrated their advantages with high mechanical strength, easy module fabrication, and excellent stability for membrane distillation (MD). In this work, the microstructure of multibore fibers was optimized for vacuum MD (VMD). A microstructure consisting of a tight liquid contact surface and a fully porous cross‐section is proposed and fabricated to maximize the wetting resistance and VMD desalination performance. The new membrane exhibited a high VMD flux of 71.8 L m^?2 h^?1 with a 78°C model seawater feed. Investigations were also carried to examine various effects of VMD operational conditions on desalination performance. The 7‐bore membrane showed higher flux and superior thermal efficiency under the VMD configuration than the direct contact MD configuration. Different from the traditional single‐bore hollow fiber, the VMD flux of multibore membrane at the lumen‐side feed configuration was higher than that of the shell‐side feed due to the additional evaporation surface of multibore geometry. © 2013 American Institute of Chemical Engineers AIChE J, 60: 1078–1090, 2014     

Preparation and characterization of novel poly(vinylidene fluoride) membranes using self-assembled dibenzylidene sorbitol for membrane distillation

We proposed a method for the preparation of novel poly(vinylidene fluoride) (PVDF) membranes with self-assembled 1,3:2,4-di(3,4-dimethylbenzylidene) sorbitol (DMDBS). The vapor-induced phase separation (VIPS) method with a dry/wet process was used to produce DMDBS/PVDF composite membranes. The resulting membranes exhibited the coexistence of PVDF cellular pores and crystalline particles. The DMDBS molecules self-assembled into nanofibril structures, and a large number of nanofibrils were found on the surfaces and in the cross-sections of the prepared membranes. The DMDBS nanofibril networks in the PVDF matrix acted as reinforcing materials that enhance the hardness and stiffness of the membranes. Moreover, because of the entangled DMDBS networks, a greater strain was required to induce sample failure; therefore, the ductility of the membranes increased with increasing amounts of DMDBS. In addition, in a membrane distillation process, our composite membranes exhibited a good permeate flux that was comparable to that of commercial PVDF membranes.     

亲水/疏水复合膜强化膜蒸馏深度处理工业废水的研究进展

工业废水中通常含有多种疏水性有机污染物及表面活性剂,传统疏水微孔膜用于膜蒸馏处理工业废水的过程中,这些污染物容易沉积在膜表面引发膜污染和膜润湿,导致膜蒸馏过程的低效甚至失败。亲水/疏水复合膜是一种表层亲水而底层疏水的非对称膜材料,可通过在膜表面形成水合层减缓污染物的吸附累积,同时保留疏水基底膜对污染物的高截留率,用于膜蒸馏过程可有效强化其处理复杂工业废水的效果。本文概述了构筑亲水/疏水复合膜的仿生学原理与表面润湿理论,介绍了复合膜常用的制备方法,重点分析了多种亲水材料改性制备的复合膜用于膜蒸馏深度处理工业废水的强化效果及强化机制,认为复合膜表面形成的亲水层可有效抑制工业废水中疏水性污染物与膜表面的疏水-疏水相互作用,减轻膜污染及膜润湿倾向,提高污染物截留效率,而氧化石墨烯等亲水物质可加速水分子通过,提升膜蒸馏产水通量。最后指出未来亲水/疏水复合膜的发展可以通过建立污染物在复合膜中的传递模型,进一步探究复合膜对工业废水处理过程的强化机制,通过优化调控复合膜结构,提升复合膜对工业废水中多种污染物的截留率和抗污染性能,实现膜蒸馏抗污染性、截留率和产水通量的同步提升,并通过开展中试研究验证复合膜用于工业废水深度处理的经济性和长期稳定性。     

Effect of surface modifying macromolecules stoichiometric ratio on composite hydrophobic/hydrophilic membranes characteristics and performance in direct contact membrane distillation

The stoichiometric ratio for the synthesis components of hydrophobic new surface modifying macromolecules (nSMM) was altered systematically to produce three different types of nSMMs, which are called hereafter nSMM1, nSMM2, and nSMM3. The newly synthesized SMMs were characterized for fluorine content, average molecular weight, and glass transition temperature. The results showed that fluorine content decreased with increasing the ratio of α,ω‐aminopropyl poly(dimethyl siloxane) to 4,4′‐methylene bis(phenyl isocyanate). The synthesized nSMMs were blended into hydrophilic polyetherimide (PEI) host polymer to form porous hydrophobic/hydrophilic composite membranes by the phase inversion method. The prepared membranes were characterized by the contact angle measurement, X‐ray photoelectron spectroscopy, gas permeation test, measurement of liquid entry pressure of water, and scanning electron microscopy. Finally, these membranes were tested for desalination by direct contact membrane distillation and the results were compared with those of commercial polytetraflouroethylene membrane. The effects of the nSMM type on the membrane morphology were identified, which enabled us to link the membrane morphology to the membrane performance. It was found that the nSMM2/PEI membrane yielded the best performance among the tested membranes. In particular, it should be emphasized that the above membrane was superior to the commercial one. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

A novel method of surface modification on thin-film composite reverse osmosis membrane by grafting poly(ethylene glycol)

A novel method of surface modification by grafting hydrophilic poly(ethylene glycol) (PEG) chains onto the surface of a thin-film composite (TFC) polyamide reverse osmosis (RO) membrane was performed. Aminopolyethylene glycol monomethylether (MPEG-NH₂) was used as grafting monomer. The membranes were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The changes in chemical composition and morphology of the membranes' surface indicated the successful grafting process. Furthermore, a preliminary experiment confirmed that the grafting of PEG chains improved membrane antifouling property.     

Poly(vinylidene fluoride-co-hexafluoro propylene) membranes prepared via thermally induced phase separation and application in direct contact membrane distillation

A non-toxic and environmentally safe diluent, acetyl tributyl citrate, was employed to prepare poly(vinylidene fluoride)-co-hexafluoropropylene membranes via thermally induced phase separation. Effects of the polymer concentration on the phase diagram, membrane morphology, hydrophobicity, pore size, porosity and mechanical properties (tensile stress and elongation at break) were investigated. The results showed that the pore size and porosity tended to decrease with increasing polymer concentration, whereas the contact angle, liquid entry pressure and mechanical properties showed the opposite trend. In direct contact membrane distillation operation with 3.5 wt-% sodium chloride solution as the feed solution, the prepared membranes performed high salt rejection (>99.9%). Furthermore, the prepared membranes retained excellent performance in long-term stability tests regarding the permeate flux and salt rejection.     

A novel carrier of radionuclide based on surface modified poly(lactide-co-glycolide) nanofibrous membrane

Electrospun nanofibrous membrane is an approved drug carrier. However, the radionuclide carrier used an electrospun membrane is rare. In this study, Poly(lactide-co-glycolide)(PLGA) nanofibrous nonwovens were prepared through electrospinning technology, and then surface modification of the nonwoven was performed to stably conjugate the radioisotope with the fibrous membrane. A novel PLGA nanofibrous nonwoven conjugated with radioactive yttrium ⁹⁰Y for tumor internal radiotherapy was prepared for the first time. Evaluation of the stability of the radioisotope indicated that the leakage of ⁹⁰Y from the PLGA membranes can be neglected after 24 h incubation in saline. The retention of ⁹⁰Y on the PLGA membrane was 75% when five half lives of ⁹⁰Y expired and the vast majority of radioactive decay had occurred. This labeled nanofibrous membrane function as a novel radio-medical appliance with excellent surface hydrophilic and mechanical properties that can be directly implanted into the lesions not only to locally kill the cancerous cells but also to play the anti-adhesion role at where surgical procedures have been made to remove the tumor tissue.     

Integrated forward osmosis–membrane distillation (FO–MD) hybrid system for the concentration of protein solutions

For the first time, an integrated forward osmosis–membrane distillation (FO–MD) hybrid system has been demonstrated for the concentration of protein solutions, specifically a bovine serum albumin (BSA) solution. A hydrophilic polybenzimidazole (PBI) nanofiltration hollow fiber membrane and a hydrophobic polyvinylidene fluoride-polytetrafluoroethylene (PVDF-PTFE) hollow fiber membrane were fabricated and employed in the FO and MD processes, respectively. A concentrated NaCl solution was employed as the draw solute to dehydrate proteins in FO, while distillate water is a by-product during the re-concentration of diluted NaCl draw solution in MD. To determine suitable operating conditions for the hybrid system, independent characterizations were carried out for both FO and MD processes using different NaCl concentrations as draw solutes in FO and different feed temperatures in MD. It was found that the integrated system is stable in continuous operation when the dehydration rate across the FO membrane is the same as the water vapor rate across the MD membrane. Simple mathematical models consistent with the experimental results were also developed for the FO and the FO–MD hybrid systems. The newly developed FO–MD hybrid system is promising for the concentration of pharmaceuticals/protein solutions in the foreseeable future.     

Anti‐biofouling anion exchange membrane using surface modified quaternized poly(ether imide) for microbial fuel cells

This article deals with a novel way of improving the anti‐biofouling potential of an anion exchange membrane (AEM) by surface modification with ethanol amine (AEOH), a low cost material without affecting the chemical structure and morphology of Quaternized Poly(ether imide) ( QPEI), the host membrane. The anti‐biofouling potential of the AEM was evaluated using bacteria anti‐adhesion test, hydrophilicity, surface roughness, water uptake, and the AEOH modification time. The data reveal that power density in all MFCs attain the highest in the sixth batch and thereafter declined albeit in a varying rate as expected measuring the least for QPEI‐30. Periodical measurement of internal resistance and protein content on the membrane surfaces were found to be the least for QPEI‐30 when compared with others. A reduced biofouling with improved anti‐biofouling property is attributed to the enhanced hydrophilicity due to surface modification. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44432.