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Co-solvent assisted interfacial polymerization (CAIP) has been widely used to increase the water permeability of thin-film composite (TFC) reverse osmosis (RO) membranes. However, its outcomes are often poorly understood or unpredictable. To bridge the gap between conventional wisdom and the real effects of the co-solvent, we report—for the first time, to the best of our knowledge—empirical evidence in terms of the actual interfacial tension between two immiscible solutions used in CAIP. According to our results, dimethyl sulfoxide (DMSO), which is frequently used as a co-solvent, influences IP by interacting with trimesoyl chloride (TMC). The dipole-dipole interaction between DMSO and TMC was estimated to increase the TMC concentration at the interface and, thereby, the reaction rate. Due to the fast reaction, the diffusion barrier forms quickly, reducing the thickness and roughness of the active layer. The cross-linking degree was also determined to decrease due to the incomplete reaction that occurs when one of three acyl chloride groups interacts with Sδ+−Oδ− electrostatic dipoles of DMSO at the interface, as evidenced by the variation in unreacted acyl chloride groups in the active layer and by the nitrogen/oxygen ratio. Such morphological changes were consistent with the trend in the performances of the RO membranes prepared with different amounts of DMSO, and were used to interpret the possible transport phenomena. Image 1 • The interaction between DMSO and TMC was considered as a parameter dictating IP. • DMSO increased the TMC concentration at the interface during IP. • The increase in the TMC concentration led to a faster but incomplete IP reaction. • The faster and incomplete IP changed the morphological features of polyamide films. • A new mechanism was proposed to clarify the DMSO effect based on empirical evidence. [ABSTRACT FROM AUTHOR]

The photocatalytic membranes have been reported to treat textile dye effluents; however, it is challenging to maintain their long-term functionality due to the poor binding stability of nanoparticles on the membrane surface and the low stability of the membrane to UV radiation. To address this, PVDF photocatalytic hollow fiber membranes were prepared by coating with polydopamine (PDA) for 1–4 h, then with titanium dioxide nanoparticles (TiO 2) in the current study. SEM and EDX results showed that TiO 2 adhered firmly to the PDA-pretreated membrane after ultrasonic treatment. The PDA layer protected the membranes from UV irradiation. The PVDF-TiO 2 membrane without PDA pretreatment was destroyed after 24 h of UV irradiation, while after 9 days of continuous irradiation, the membranes treated with PDA for 2 h remained unchanged. Compared with the original membrane, applying PVDF membranes coated with PDA for 2 h and TiO 2 for 1 h increased the Reactive Red 239 rejection and flux recovery rates by 13% and 40%, respectively, achieving 60% COD removal. The permeate flux and dye rejection rate were stable after 5 cycles (20 h of operation). This study demonstrates that the obtained membranes can perform prolonged membrane filtration with high-performance durability for dye wastewater treatment. [Display omitted] • TiO 2 was strongly bonded to the PVDF membrane pretreated with PDA. • Introducing PDA-TiO 2 enhanced the stability of dye permeate flux and dye rejection. • The PVDF-PDA-TiO 2 membranes significantly improved the COD removal efficiency. • PVDF-PDA-2h-TiO 2 -1 h membrane showed excellent resistance to UV and reusability. • Self-protecting/cleaning, hydrophilicity, and durability of membrane all improved. [ABSTRACT FROM AUTHOR]

The focus of whey processing has been changed from waste treatment to the production of valuable products owing to the advancement of membrane technology. Producing high quality whey products at low energy consumption entails novel techniques for whey concentration. This study proposes forward osmosis (FO) as a potential approach to concentrate whey protein solutions, which is superior to the concentration via reverse osmosis (RO), mainly owing to the low hydraulic pressure during FO process. In particular, this work validated the feasibility of FO-based whey concentration using high-performance hollow fiber membranes that were fabricated in-house. The investigation focused on the effects of various operating conditions on the concentration performance. The experimental results reveal that optimal concentration efficiency would be achieved by appropriately choosing the cross-flow velocity, draw solution concentration and the maximum attainable concentration of whey protein solution. Especially, the flux decline behaviour as a function of the whey protein concentration in the feed solution indicated rapid formation of a polarized or gel foulant layer at the membrane surface. The proposed mechanism was supported by the investigation of physical cleaning with water, showing that the post-process cleaning after the concentration cycle was able to recover the membrane performance, whereas the intermediate cleaning during one concentration cycle had little impact on the concentration efficiency enhancement. This study has practical significance for the application of FO in the concentration of dairy streams and other liquid foods. [ABSTRACT FROM AUTHOR]

This study investigated the stability of the aquaporin-based biomimetic (ABM) membrane and its long-term reverse osmosis (RO) performance. A series of experiments showed that the ABM membranes exhibited good chemical stability after soaking in three commonly used chemical cleaning agents, namely ethylenediamine tetraacetic acid (EDTA), sodium hydroxide (NaOH) and citric acid, respectively. In addition, it was found that both water permeability (flux per unit applied pressure) and solute rejection of ABM membranes are sensitive to the temperature and applied pressure. In the long-term RO performance evaluation, a real RO feed water from water reclamation process was used as feed; this is the first reported application of real RO feed water to ABM membranes. The membranes were periodically cleaned and the recovery of water permeability of the ABM membranes was found to be greater than 90%. The quality of the water permeate exceeded the World Health Organisation standard for drinking water. Compared with a commercial RO membrane, the ABM membrane only needed half of the applied pressure to achieve the same water flux, suggesting the advantage and potential of ABM membranes to reduce energy consumption in practical applications. [ABSTRACT FROM AUTHOR]

The utilization of osmosis for engineered applications sparked off various emerging technologies relying on osmotically driven membrane processes (ODMPs). Represented by forward osmosis (FO) and pressure retarded osmosis (PRO), ODMPs show great promise to leverage the global water-energy nexus and have drawn considerable attention in recent years. However, their performance in practical applications is significantly affected by membrane fouling. Membrane fouling is a complex problem and is associated with the foulant deposition, concentration polarization and reverse solute diffusion (RSD) in ODMPs. The current paper provides a comprehensive review on membrane fouling in ODMPs with a focus on the elaboration of the factors and mechanisms governing the fouling behavior. Among those fouling factors and mechanisms, some are also applicable for pressure-driven membrane processes (e.g., reverse osmosis (RO) and nanofiltration (NF)), such as the effects of hydrodynamic conditions, feedwater composition, and membrane material and properties, and the cake-enhanced concentration polarization (CE-CP) mechanism. Others are unique for ODMPs, such as the effects of draw solution composition and membrane orientation, the internal concentration polarization (ICP) self-compensation effect, and the RSD-enhanced fouling. A general osmotic-resistance filtration model for ODMPs is presented in this paper to assist in the interpretation of the intrinsic interrelationships among those fouling factors and mechanisms. The impact and mechanisms of membrane fouling on contaminates removal are also reviewed briefly based on the limited existing literature on this topic. Finally, the available membrane fouling control strategies for ODMPs are summarized upon understanding the cause and effect of fouling. Based on the current review, future research prospects are proposed for further studying the membrane fouling in ODMPs. [ABSTRACT FROM AUTHOR]

The fabrication of new membrane that is able to produce stable high power density is essential for the development of pressure retarded osmosis (PRO) technology. In this work, thin film composite (TFC) polyetherimide membranes with three different substrate structures were fabricated and characterized. The PRO performance of the resultant membrane was evaluated through two pressure cycle experiments and stability tests. The primary objective of this work is to systematically study PRO membrane’s mechanical stability and mass transfer properties under high operating pressures, which determine the power density of the membrane in the PRO process. Experiments revealed that water permeability ( A p ) and salt permeability ( B p ) under different pressures varied. The A p and B p are better indicators for examining the variation of PRO membrane structure under pressure. Within the operating pressure range of 17.2 bar, the top polyamide layer of TFC PEI-2# membrane mainly experienced a reversible deformation. In the two pressure cycle tests, the water flux, specific salt flux and power density obtained in the upward and downward measurements in each cycle are close to each other. The first cycle and the second cycle also show excellent reproducibility. The membrane was able to maintain almost unchanged water flux and power density of 12.8 W/m 2 at 17.2 bar over the 10 h testing time, suggesting the membrane’s great potential to be used in practical application in the future. [ABSTRACT FROM AUTHOR]

The pressure retarded osmosis (PRO) process is a novel technology which generates green electrical energy via semi-permeable membranes. However, a major challenge in the PRO system is the lack of suitable membranes with satisfactory power density (i.e., the power output per unit membrane area). In this study, we have successfully fabricated a novel thin-film composite (TFC) PRO membrane consisting of a tiered structure of polyetherimide (PEI) nanofibrous support reinforced by functionalized multi-walled carbon nanotubes (f-CNTs) and an ultrathin polyamide-based selective top skin layer. The tiered support was made by a fine and a coarse PEI nanofiber layers. The thin finer 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. Our optimized membrane can endure a trans-membrane pressure up to 24 bar and generate a peak power density as high as 17.3 W/m 2 at 16.9 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution against deionized (DI) water. In addition, the long term PRO result shows that this membrane can generate a stable power density of 15.0±0.5 W/m 2 for a test period of 10 h. This demonstrates that our membrane holds great potential to be used in the PRO process. [ABSTRACT FROM AUTHOR]

Polyvinylidene fluoride (PVDF) electrospun nanofibrous membranes (ENMs), consolidated with a heatpress process and of various thicknesses were fabricated and tested in a DCMD cell at five different operating conditions. Membranes as thin as 27 μm were sufficiently robust for evaluation and gave a transmembrane flux as high as 60 L/h m 2 . An analytical model was created to estimate the optimal membrane thickness for DCMD operations. It is found that the value of optimal thickness increases with reduced heat transfer coefficients; decreased feed inlet temperature; increased membrane permeability; and increased salinity. Even for 10% NaCl the predicted optimum was estimated to be 13 μm which was too thin for experimental confirmation. Based upon this analysis but with due allowance for the variation of heat transfer coefficients with temperature dependent physical properties a single Matlab model was created to fit the five sets of data. With the introduction of a structural derivation factor, which reflected the experimentally determined variation of porosity and pore size with thickness, the model was found to fit the data very well. [ABSTRACT FROM AUTHOR]

Abstract: Due to the unique properties of amphiphilic Pluronic block copolymers, they have been blended with other polymers such as PVDF to prepare membranes. Considering the stability issue, the strong pore-forming ability of Pluronic F127, and the mechanical strength of the resultant membranes, Pluronic/LiCl mixed additive consisting of a very low concentration of Pluronic has been used in this study. The effects of spinning conditions on PVDF hollow fiber fabrication have also been studied. It is found that the use of merely 0.2wt% of Pluronic is sufficient to impose significant impacts on the morphology, filtration performance, and mechanical properties of the resultant fibers. Hollow fibers spun using higher coagulant temperature, higher take-up speed, and water as the bore fluid exhibit better mechanical properties and filtration performance. PVDF hollow fiber membranes with PWP of 1180L/m2 hMPa and MWCO of 5kDa were obtained when spun with optimized spinning conditions and using 3wt% of LiCl and 0.2wt% of Pluronic as the mixed additive. The fibers can withstand a pressure of 0.55MPa from the lumen. [Copyright &y& Elsevier]

In view of the proliferating applications involving membrane-based separation, the inevitable membrane fouling phenomenon is well-studied. However, the influence of foulant shape on membrane fouling remained incompletely understood. Therefore, the goal of this study was to investigate the fouling behaviors by three different shapes of latex particles (namely, peanut, pear and sphere) with similar mean diameters in the range of 3.5–3.8 μm. External and internal fouling were studied using polycarbonate track-etched (PCTE) membranes with nominal pore sizes of 2 and 8 μm, respectively. Flux data were taken, and fouling models, optical coherence tomography (OCT) and field-emission scanning electron microscope were employed. Regarding external membrane fouling, the sphere-shaped particles displayed the worst flux decline, which is tied to the greatest tendency to deposit on the membrane pores, as revealed by the network fouling model. As for internal fouling, the flux decline curves were more similar among the particle types. The fouling model shows the sphere-shaped particles gave the densest internal cake, which caused slightly steeper flux decline in the latter part of the filtration. Furthermore, OCT indicates that the highest fouling fractions at 7.5 μm below the membrane were exhibited for the peanut-shaped particles, which is due to the preferential vertical orientation resulting in deeper penetration. Results here highlight the non-negligible influence of foulant particle shape, which warrants further understanding and accounting for membrane fouling studies. [Display omitted] • Used OCT to reveal influence of foulant particle shape on membrane fouling. • Spheres were more prone to pore-blockage and thus gave worst flux decline. • Non-spherical particles gave less dense internal cakes due to rotational effects. • Peanut-shaped particles penetrated deeper due to preferential vertical orientation. • Particle shape affected flux decline more when external fouling was dominant. [ABSTRACT FROM AUTHOR]

In a gas–liquid membrane contactor, a larger pore size can result in a lower membrane mass transfer resistance. However, the membrane pore size is usually limited by the concern of pore wetting, e.g. a large pore size means a higher wetting tendency. As a breakthrough, this paper reported a porous polyetherimide (PEI) hollow fiber membrane with high surface porosity and large pore size to minimize the membrane mass transfer resistance by using a triple-orifice spinneret in the hollow fiber spinning process, and followed by a novel approach of fluorinated silica (fSiO2) nanoparticles (NPs) incorporation to make the membrane surface highly hydrophobic and chemical resistant to prevent the membrane from wetting caused by the large pore size on the membrane surface. The newly developed composite hollow fiber membranes showed the advancing contact angle value of 123.3°, receding contact angle value of 107.2°, and contact angle hysteresis of only 15.9°, indicating the high water resistant property. The composite membrane also exhibited a higher rigidity property compared with the original PEI substrate. The CO2 absorption flux of the composite membranes was investigated in both physical and chemical absorptions in a gas–liquid membrane contactor system. The membrane contactor showed a stable performance throughout the 60d long-term operation using a 2M sodium taurinate aqueous solution as the liquid absorbent. The highly hydrophobic composite hollow fiber membrane was able to outperform a conventional polymeric hydrophobic membrane in term of superior gas absorption flux and outstanding long-term stability, suggesting that the formation of organic–inorganic composite membranes is an effective way to enhance the feasibility of membrane contactor processes for practical applications. The results demonstrated the important role of membrane fabrication and modification techniques in facilitating the commercialization of membrane contactor technology. [ABSTRACT FROM AUTHOR]

Abstract: Aquaporin (AQP) based biomimetic membrane has attracted increasing attention in recent years because of its potential application for water purification and seawater desalination, attributed to the exceptionally high permeability and selectivity of AQPs. Despite its superior performance, AQP based membrane faces several challenges associated with the design and preparation of such membranes for practical application. In this current study, a novel and simple method was introduced to prepare an aquaporin Z (AqpZ) based biomimetic nanofiltration (NF) membrane with a relatively large membrane area of 28.26cm2. The proteoliposome, incorporated with AqpZ, was fully encapsulated into the selective layer through crosslinking of a polyelectrolyte with the membrane substrate made by poly(amide–imde) (PAI). The water flux of the AqpZ based membrane was around 50% higher than the mutant one. At optimal preparation conditions, the AqpZ based membrane could offer a water flux of 36.6Lm−2 h−1 with a MgCl2 rejection of 95% at 0.1MPa. Results from this study showed that AqpZ could maintain its activity even under harsh environmental conditions of thermal treatment at 343K for 2h. These results and findings may provide useful insights on developing next generation of biomimetic membranes. [Copyright &y& Elsevier]

Abstract: Pressure retarded osmosis (PRO) is a promising membrane process to harvest the vast amount of free energy from the mixing of fresh and salty waters without adverse environmental impacts. A competent membrane for PRO should possess the features of high water permeability to allow water permeation, excellent salt rejection to maintain the osmotic driving force, low structure parameter to minimize the internal concentration polarization (ICP), and robust strength to withstand the required hydraulic pressure. In this work, we have developed a novel hollow fiber membrane by balancing these competing factors based on our prior experience in the thin-film composite (TFC) membranes. The newly developed TFC hollow fiber membrane can achieve a power density of 20.9W/m2 at a pressure of 15bar, using synthetic seawater brine (1.0M NaCl) as the draw solution and synthetic river water (1mM NaCl) as the feed water, respectively. The simultaneous specific reverse salt flux was found to be 0.03mol/L, which is much lower than reported flat-sheet membranes, as the self-supported hollow fiber membrane can eliminate the deformation-enhanced reverse salt diffusion that is typical for flat-sheet membranes. However it was observed that the water permeability and structure parameter of the TFC hollow fiber membrane varied noticeably corresponding to the change in the hydraulic pressure imposed in the fiber lumen. Over a certain range (ΔP<15bar), such variations of increasing A value and decreasing S value proved to have positive impacts, as they facilitate the water permeation and reduce the ICP in the PRO process. [Copyright &y& Elsevier]

Abstract: This study aims at developing an effective method to modify hydrophobic polyvinylidene fluoride (PVDF) membranes for membrane distillation (MD) which is tolerant to salty feed solution containing mineral oil. A novel approach was explored to obtain a highly hydrophilic membrane surface via plasma induced grafting of polyethylene glycol (PEG) and subsequent TiO2 particles deposition onto the membrane surface. The mechanism of oil affinity onto a membrane surface was quantitatively investigated by the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory. A series of characterizations were conducted for the virgin and modified membranes. Membrane distillation tests for synthetic seawater containing 0.01wt% of mineral oil were also performed to investigate the influences of membrane hydrophobicity, membrane surface structure, pH value of the feed solution and hydrodynamic conditions on oil fouling behavior. The SEM and FTIR measurements confirmed the presence of PEG and TiO2 particles on the PVDF membrane substrate. The hydrophilically modified PVDF-PT membrane exhibits a positive interfacial free energy with oil in the feed, indicating the TiO2 deposition effectively improve the anti-fouling property of PVDF-P membrane and PVDF membrane. It was also found that the PVDF-PT membrane presents a stable water flux over 24h of operation without fouling and wetting due to combined effects of enhanced surface hydrophilicity and reduced membrane pore sizes. In addition, the pH value of the feed solution plays an important role in oil fouling. More severe fouling can be observed at the pH which made the membrane surface and oil droplets carry opposite charges. Moreover, a higher flow velocity enhances the shear force exerted on the oil droplets, thus leading to less oil present on the membrane surface and less oil fouling of the membrane. [Copyright &y& Elsevier]

Abstract: The use of additives in the polymer dope solutions is one of the most powerful strategies to control the pore structure of poly(vinylidene fluoride) (PVDF) membranes for various applications. In present work, the effects of amphiphilic pluronic block copolymer as an additive on PVDF membrane formation has been studied and a membrane formation mechanism involving pluronic has been proposed. The surface tension–concentration curve reveals that no micelles are formed in pluronic/N-methyl-2-pyrrolidone (NMP) solutions while the thermodynamics and kinetics are found to be similar for the systems containing pluronic and polyethylene glycol (PEG), respectively. The membranes prepared with pluronic as the additive exhibit larger surface pore, porosity, and macrovoids than that with PEG, indicating that pluronic has a stronger pore-forming ability. Instead of the commonly used concept of instantaneous or delayed demixing, it is believed that the stronger pore-forming ability of pluronic is caused by the hydrophobic–hydrophobic interaction between pluronic and PVDF that delays the solidification process. Since pluronic is not completely stable in PVDF membrane matrix, the membranes are soaked in 2-propanol (IPA) to purposely wash out pluronic. The loss of pluronic after IPA-treatment causes an increase in water permeation and a decrease in rejection, indicating that the presence of pluronic on the membrane pore surface can narrow down the pore size. By adding lithium chloride as a second additive, the growth of macrovoids is suppressed. Moreover, the resultant membranes exhibit both higher permeability and selectivity. [Copyright &y& Elsevier]

Hydrophobic surface can be produced by either lowering the surface free energy or enhancing the surface roughness or a combination of both. This study explored a novel method to form a highly hydrophobic organic–inorganic composite hollow fiber membrane by incorporating the fluorinated silica (fSiO2) inorganic layer on the polyetherimide (PEI) organic substrate. The formation of the composite membrane was examined and confirmed by ATR-FTIR, TGA and SEM. Ultrasonication test was conducted to examine the structural stability of the composite membrane. The experimental results revealed that the fSiO2 layer was tightly bonded to the PEI substrate through silane chemical reactions. The incorporation of the fSiO2 layer on the membrane surface could enhance the surface roughness and greatly reduce the surface free energy due to the hydrophobic compound perfluorodecyltriethoxysilane (PFTS). The hydrophobicity of the composite membrane surface was dramatically elevated, as evidenced by the dynamic contact angles increment, e.g. from original 66.7° to 124.8° for advancing values, and from 49.5° to 100.6° for receding values. In addition, it was observed the mechanical property of the composite membrane was better than some of the conventional polymeric membranes such as polyethersulfone (PES), and polyvinylidene fluoride (PVDF). The composite membrane was also not as brittle as the pure inorganic membrane. Highly hydrophobic membrane is the core element in gas–liquid membrane contactor and the newly developed PEI-fSiO2 composite hollow fiber membranes were intended for the use in such application. The CO2 absorption flux of the composite membranes was investigated in both physical and chemical absorptions in a gas–liquid membrane contactor system. Moreover, the membrane contactor showed a reasonably stable performance throughout the 31 days long-term operation using a 2M sodium taurinate aqueous solution as the liquid absorbent and pure CO2 as the feed gas. The chemical compatibility test indicated that after the long-term constant contact with the sodium taurinate, the hydrophobicity of the PEI-fSiO2 composite membrane still maintained and was much higher than that of original PEI substrate and conventional hydrophobic polymeric membranes such as PVDF. The incorporation of the fSiO2 inorganic layer not only offered high hydrophobicity, but also would protect the polymeric substrate from the attacks of chemical absorbents, making the membrane a longer lifespan. [ABSTRACT FROM AUTHOR]

Abstract: Recently membrane distillation (MD) has received intensive interests for a range of applications such as desalinations of seawater and brine. One of the major obstacles for MD application is the lack of an optimized MD membrane that can produce a high and stable flux in long-term operation. Two types of superhydrophobic PVDF nanofiber membranes, integrally-modified and surface-modified PVDF membranes, have been successfully fabricated by electro-spinning followed by surface modification, which includes dopamine surface activation, silver nanoparticle deposition and hydrophobic treatment. The modification is convenient because of mild reactions and wide applicability. These novel composite nanofiber membranes have been characterized by a series of measurements and benchmarked against commercial PVDF flat sheet membrane for MD application. The characterizations reveal that the modifications have altered the membrane surface morphology and topology and made the membrane superhydrophobic due to their hierarchical structures. Compared with unmodified membrane, the integrally-modified membrane (I-PVDF) can achieve a high and stable MD water flux of 31.6Lm−2 h−1 using a 3.5wt% NaCl as the feed solution while the feed and permeate temperatures were fixed at 333K and 293K, respectively. To the best of our knowledge, this result is superior to all other PVDF flat-sheet membranes tested under the same or similar conditions, which is believed to be attributed to the open-surface pore structure and thin thickness of the PVDF nanofiber membrane with the aid of electro-spinning. The superhydrophobic nature of the membrane surface brought by the integral modification on all nanofibers renders the membrane anti-wetting property while remaining high water flux. [Copyright &y& Elsevier]

Abstract: The main challenges for membranes used in membrane distillation (MD) are well-designed porous structure and hydrophobic surface property. Hydrophobic nanofiber membranes possess high hydrophobicity, high surface porosity and adjustable pore sizes and membrane thickness, which make them an attractive candidate as MD membrane. The current study aims to fabricate and optimize polyvinylidene fluoride (PVDF) nanofiber membranes for MD application. Scanning electron microscopy (SEM), capillary flow porometer, geniometer, a homemade setup for liquid entry pressure (LEP) measurement and direct contact MD (DCMD) setup were used to characterize the resultant nanofiber PVDF membranes. Polymer dope compositions, spinning parameters such as sprayer moving speed and chamber moisture were optimized to examine their effects on pore size distribution of the membranes. Inorganic additives were also added into the dope to improve the electro-spinability of diluted polymer dopes in order to further decrease membrane pore size. Meanwhile, a heat-press post-treatment was considered as a necessary step to improve fresh nanofiber membrane integrity, enhance water permeation flux and help prevent membrane pores from wetting in DCMD operation. The experiment confirmed that all the electrospun membranes exhibit a rough surface with high hydrophobicity (>135° water contact angle). The post-treated PVDF nanofiber membranes were able to present a steady water permeation flux of about 21kgm−2 h−1 throughout the entire testing period of 15h, using a 3.5wt% NaCl solution as the feed under the feed and permeate inlet temperatures of 323K and 293K, respectively. This result was better than these of commercial PVDF membranes and the nanofiber PVDF-clay membrane reported in literature, suggesting the competency of PVDF nanofiber membranes for DCMD applications. [Copyright &y& Elsevier]

Abstract: A novel dual-layer forward osmosis (FO) hollow fiber membrane has been designed and successfully fabricated by using a triple orifice spinneret. The fiber consists of two layers made from polyamide-imide (PAI) polymer for the outer layer and polyethersulfone (PES) polymer for the porous inner layer. Specifically, after obtaining asymmetric microporous PAI/PES dual-layer hollow fibers via non-solvent induced phase inversion, polyethyleneimine (PEI) polyelectrolyte modification on the outer PAI layer was applied to produce a nanofiltration (NF)-like thin layer, while the PES porous inner layer remained intact as PES is inert to PEI. The membrane morphology, structure and surface property were carefully tailored by adjusting polymer dope composition and spinning conditions. These membranes were subsequently characterized by a series of standard protocols in terms of membrane structure, permeability and salt rejection, and were utilized in FO process. It was found that the resultant dual-layer NF hollow fiber membrane can achieve pure water permeability of 15.9lm−2 h−1 bar−1 and a high rejection to divalent cations up to 89%. In FO process, the dual-layer hollow fiber exhibited a water flux of 27.5lm−2 h−1 in the orientation of active layer facing feed water by using 0.5MMgCl2 as draw solution and de-ionized (DI) water as feed at room temperature. The newly developed dual layer hollow fibers outperform all the single layer and dual-layer NF hollow fibers reported in the literature for FO applications. [Copyright &y& Elsevier]

Abstract: One of the major barriers that hinder the application of forward osmosis (FO) processes is the lack of optimized membranes that possess high water flux, low salt leakage and good anti-internal concentration polarization (ICP) and anti-fouling/anti-scaling properties during practical FO operations. In this study, novel composite FO hollow fiber membranes with an RO-like selective skin and an NF-like secondary selective skin were successfully developed for the first time. The fabrication procedures involved making ultrafiltration hollow fiber substrate using poly(amide–imide) (PAI) polymer material via phase inversion method, followed by interfacial polymerization (IP) and chemical modification to yield a polyamide RO-like inner skin and a positively charged NF-like outer skin, respectively. It was found that the sequence of conducting IP on the substrate inner surface prior to the chemical modification of the outer skin was preferred in order to achieve better performance for the resultant double-skinned hollow fibers. The newly developed FO hollow fibers exhibited high water permeability of 2.05L/m2 hbar and 85% rejection to NaCl at 1bar pressure as well as superior FO water flux of 41.3L/m2 h and low ratio of salt flux over water flux of 0.126g/L when using DI water and 2.0M NaCl as feed and draw solutions, respectively, in the active layer facing draw solution (AL-DS) orientation. Furthermore, the experimental results showed that the composite FO hollow fiber membrane was able to outperform single-skinned membrane when the feed contains divalent ions or the feed exhibits high scaling tendency to membrane. This suggested that the integration of RO- and NF-like two selective skins in FO membrane is an effective way to minimize the ICP effect, mitigate the membrane scaling and thus enhance the feasibility of FO processes for practical applications. [Copyright &y& Elsevier]

Abstract: A new type of forward osmosis (FO) hollow fiber membranes with an antifouling nanofiltration (NF)-like selective layer has been developed in this work. The fabrication involves a step of porous poly(amide-imide) (PAI) ultrafiltration (UF) hollow fiber spinning via phase inversion, followed by simple polyelectrolyte post-treatments using positively charged polyethyleneimine (PEI) for cross-linking and negatively charged polystyrene sulfonate sodium salt (PSS) for deposition. The membranes are subsequently characterized by a series of standard protocols and evaluated in FO process. The PAI FO membranes possess a pure water permeability of 3.7–4.3Lm−2 h−1 bar−1 and Na2SO4 rejections up to 85% at 1bar. In the FO process, the membranes can achieve water flux of 17 and 12Lm−2 h−1 for active layer facing draw solution (AL-facing-DS) and active layer facing feed water (AL-facing-FW) configurations, respectively, using a 2000ppm Na2SO4 aqueous solution as the feed and 0.5M Na2SO4 as the draw solution. When the feed contained 1000ppm bovine serum albumin (BSA) and 2000ppm Na2SO4, a steady water flux of 11Lm−2 h−1 can be maintained using 0.5M Na2SO4 as the draw solution at ambient temperature of 23°C. The approach of making FO membranes developed in the present study offers the advantages of simple fabrication process, tailorable selective layer and promising membrane performance for protein-contained wastewater treatment by FO process. [Copyright &y& Elsevier]

Abstract: For the first time, a specially designed pressure retarded osmosis (PRO) hollow fiber membrane has been successfully developed and applied in the PRO process to demonstrate its potential for power generation. The membrane fabrication method is similar to that used for making thin-film composite (TFC) forward osmosis hollow fiber membranes, but further optimization and improvement have led to a new type of TFC hollow fiber membranes with much greater mechanical strength in addition to its excellent separation property and high water flux. The TFC PRO hollow fiber membranes have a water permeability (A) of 9.22×10−12 m/(sPa), salt permeability (B) of 3.86×10−8 m/s and structural parameter (S) of 4.6×10−4 m. It can withstand hydrostatic pressure as high as 9bar with its relatively large dimension of 0.98mm lumen diameter. This PRO hollow fiber membrane is superior to all other PRO membranes reported in the open literature in terms of power density. A power density as high as 10.6W/m2 can be achieved using seawater brine (1.0M NaCl) and wastewater brine (40mM NaCl), which suggests that the newly developed PRO hollow fiber membrane has great potential to be applied in PRO processes to harvest salinity gradient energy. A higher pressure is preferred as it allows generation of higher power density (pressures of 12bar may be optimal for seawater as the high salinity stream), and this can be realized by reduced fiber dimension. Further optimization of the membrane structure will be performed. [Copyright &y& Elsevier]

Abstract: Five types of novel hollow fiber module configurations with structured-straight fibers, curly fibers, central-tubing for feeding, spacer-wrapped and spacer-knitted fibers, have been designed and constructed for the direct contact membrane distillation (DCMD) process. Their module performances were evaluated based on permeation flux experiments, fluid dynamics studies, and tracer-response tests for flow distribution as well as process heat transfer analysis. The novel designs showed flux enhancement from 53% to 92% compared to the conventional module, and the spacer-knitted module had the best performance. The fluxes of all the modified configurations, except the structured-straight module, were independent of the feed flow velocity, and the modules with undulating membrane surfaces (curly and spacer-knitted fibers) were able to achieve more than 300% flux improvement in the laminar flow regime. The improved performance was attributed to the improved fiber geometries or arrangements that can provide effective boundary layer surface renewal and more uniform flow distribution, confirmed by the sodium chloride tracer response measurements. The heat transfer analysis underscores the advantage of the module with curly fibers with the least temperature polarization effect (temperature polarization coefficient=0.81–0.65 at T m =303–333K), which is favorable for enhancing permeation flux. [Copyright &y& Elsevier]

Abstract: The outer surface of polyvinylidene-co-hexafluoropropylene (PVDF–HFP) asymmetric microporous hollow fiber membrane prepared by phase inversion process was activated by 10wt.% of NaOH solution, and subsequently modified with the mixture solution of α,ω-triethoxysilane terminated perfluoropolyether or Fluorolink®S 10 (FS10) and tetraethoxysilane (TEOS) in order to enhance the hydrophobicity of the membrane. The chemical and physical changes of the membranes were investigated by attenuated total reflection infrared spectroscopy FTIR-ATR and scanning electron microscope (SEM). The characteristics of the modified membrane were examined in terms of liquid entry pressure of water (LEPw), pore size and pore size distribution, contact angle and mechanical properties. The modified fibers were also used in the membrane contactor system to evaluate their performance for CO2 absorption. FTIR-ATR result showed the enrichment of fluorine and ethoxysilane on the modified PVDF–HFP membranes leading to higher LEPw and contact angle than those of the unmodified membrane by 36% and 33%, respectively. The contact angle increased from 95.5° to 127.8°. Compared to the unmodified PVDF–HFP hollow fiber, the cross-sectional morphology and mechanical properties of the modified membrane changed insignificantly, while the membrane mean pore size was reduced from 32.7nm to 25.2nm. The CO2 physical absorption performance of the modified fibers was better than the unmodified fibers. The membrane mass transfer coefficients of the unmodified and modified PVDF–HFP hollow fiber were 5.69×10−5 and 7.56×10−5 m/s, respectively, indicating that the modified membranes have good potential for application in the gas absorption process. [Copyright &y& Elsevier]

Abstract: Membrane wetting is a major concern for the application of membrane contactors. In order to develop a novel hydrophobic membrane that can be used in membrane contactors, a hydrophobic surface modification method for Torlon® polyamide-imide (PAI) membrane was explored, which has not been reported previously. Asymmetric microporous hollow fiber membranes were fabricated using PAI material followed by a simple surface modification using a silane coupling agent, 3-aminopropyltrimethoxysilane (APTMS), and a perfluro-compound (Fluorolink S10) to produce PAI hollow fiber membranes with a hydrophobic surface. The reactions between PAI and the modification reagents have been confirmed by experiments. Optimal modification conditions were investigated by varying the modification parameters, such as APTMS treatment temperature, APTMS concentration, APTMS treatment time, and Fluorolink S10 treatment time. The modified PAI membranes were, subsequently, characterized by a series of standard protocols in terms of membrane morphology, chemical structure, contact angle, porosity and critical water entry pressure. A preliminary study of using the modified membranes in a membrane contactor for CO2 absorption was also carried out. The newly developed PAI hollow fiber membranes showed much higher hydrophobicity, the contact angle was improved from 76° to 108–117° and the critical water entry pressure was improved from 0.7–0.8bar to 2.1–4.0bar. Moreover, the modified PAI hollow fiber membranes showed reasonably high CO2 flux by using water as a physical absorption liquid. The results indicate that the modified PAI membranes have great potential to be used in membrane contactor applications. [Copyright &y& Elsevier]

Abstract: The amphiphilic Pluronic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) have received much attention recently as both the surface modifier and pore former in membrane fabrication. In this study, a systematical study has been carried out to investigate the effect of Pluronics with different molecular architectures and contents as a pore-forming additive on the fabrication of polyethersulfone (PES) ultrafiltration (UF) hollow fibers, and to identify the most preferential features of Pluronic for making high performance membranes. The as-spun hollow fibers were characterized in terms of cross-sectional membrane morphology, membrane surface chemistry, mechanical properties, pure water permeation (PWP), molecular weight cut-off (MWCO), and pore size distribution. It was observed that the PWP and MWCO of the as-spun hollow fibers are dependent on the structure of additives. Among all the membranes spun with 5wt% additives, the hollow fibers spun using Pluronic F127 and F108 as the additives possess the highest PWP, the lowest MWCO, and the narrowest pore size distribution. It is possible that the PEO brush layer formed on the internal pore surface by Pluronic F127 and F108 might reduce the apparent pore size and hence improved the solute rejection of the resultant membranes. A comparison between Pluronic and poly(ethylene glycol) (PEG) as additives also confirmed the importance of the presence of PPO chain in Pluronic in the formation of high-performance membranes. When Pluronic F127 concentration was 10wt%, the as-spun hollow fiber exhibited the highest PWP of 113.8L/m2 hbar and the lowest MWCO of 9kDa. [Copyright &y& Elsevier]

Abstract: Hollow fiber membranes with a positively charged nanofiltration (NF)-like selective layer have been successfully fabricated by utilizing asymmetric microporous hollow fibers made of Torlon® polyamide–imide (PAI) material as the porous substrate followed by polyelectrolyte post-treatment using polyethyleneimine (PEI), which has not been reported previously. The reaction between PAI and PEI has been confirmed by experiments. The membrane structure and surface property are carefully tailored by adjusting polymer dope composition, spinning conditions and the post-treatment parameters. These membranes are, subsequently, characterized by a series of standard protocols in terms of membrane morphology, structure, permeability and salt rejection, and are applied in forward osmosis (FO) processes. The newly developed PAI hollow fiber membranes show a pure water permeability of 2.19–2.25L/m2 hbar and reasonable NaCl and MgCl2 rejections of 49% and 94% at 1bar, respectively. It is also found that in FO processes, when using 0.5M MgCl2 as a draw solution and DI water as the feed in the active layer facing feed water (AL-facing-FW) configuration at 23°C, the water fluxes of two PAI FO hollow fiber membranes are 8.36 and 9.74L/m2 h, respectively, and the J S /J V of the two membranes is smaller than 0.4g/L, which is lower than the data of 0.85g/L for commercial FO membrane from Hydration Technologies Inc. (HTI). Different from a neutral membrane, the positively charged FO membrane provides double electric repulsions to the salt transfer through the membrane in the AL-facing-FW configuration, leading to a reduction of salt penetration, while in the active layer facing draw solution (AL-facing-DS) configuration, the positive charges facilitate salt transportation. This study provides a simple and effective approach to make hollow fibers suitable for FO applications. [Copyright &y& Elsevier]

Abstract: Polyvinylidene fluoride-co-hexafluropropylene (PVDF-HFP) has received much attention recently as a promising membrane material for membrane contactor application. A systematic study has been carried out to investigate the effects of polyethylene glycol (PEG) with different molecular weights and different loadings as an additive on the fabrication of PVDF-HFP asymmetric microporous hollow fiber membranes. Moreover, the synergetic effects of coagulation temperature and the second additive (lithium chloride: LiCl) with PEG are also evaluated. Experiments revealed that the addition of PEG into the PVDF-HFP/NMP solution resulted in the system thermodynamically less stable in reaction with water, promoting rapid phase demixing in the phase inversion process. When the same 3wt% PEG was added into the dope solution, the dimension of finger-like macrovoids of the resultant membrane increased in parallel with the increase of PEG molecular weight from 200 to 600 and 6000kDa, and pure water permeability (PWP) also increased accordingly. An increase in PWP was also observed when PEG-200 loading in the dope solution was increased from 3 to 5 and 10wt%, corresponding to the morphology change of resultant membranes. As a synergetic effect of coagulation temperature with PEG, the finger-like pores occurred in the membrane at room temperature expanded to much larger macrovoids using 10°C water as the coagulant, and the big finger-like pores almost disappeared when the coagulation bath temperature was increased to 40°C because of delayed phase demixing. The big macrovoid size can also be suppressed by adding the second small molecule additive, LiCl, due to its strong interactions with NMP and PVDF-HFP to delay the dope precipitation. The irregular inner contour of the membrane can be eliminated by the increase of coagulation bath temperature to 40°C. The hollow fiber membrane made by a dope of PVDF-HFP/PEG-6000/LiCl/NMP (15/3/3/79 in weight) using 40°C water as the coagulant exhibited a high PWP of 117L/m2 hatm and reasonably good MWCO of 150kDa. An improvement has been made in the current work as compared to previous PVDF-HFP hollow fiber membranes reported in literatures. [Copyright &y& Elsevier]

Abstract: The performance of membrane distillation depends on both membrane and module characteristics. This paper describes strategies to improve the performance of hollow fiber membrane modules used in direct contact membrane distillation (DCMD). Three different types of hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membrane (unmodified, plasma modified and chemically modified) were used in this study of direct contact membrane distillation (DCMD). Compared to the unmodified PVDF hollow fiber membrane, both modified membranes showed greater hydrophobicity and mechanical strength, smaller maximum pore sizes and narrower pore size distributions, leading to more sustainable fluxes and higher water quality (distillate conductiviy<1μscm−1) over a one month test using synthetic seawater (3.5wt% sodium chloride solutions). Comparing the plasma and chemical modification the latter has marginally better performance and provides potentially more homogeneous modification. MD modules based on shell and tube configuration were tested to identify the effects of shell and lumen side flow rates, fiber length and packing density. The MD flux increased to an asymptotic value when shell-side Re f was larger than 2500, while the permeate/lumen side reached an asymptotic value at much lower Re p (>300). By comparing the performance of small and larger modules, it was found that it is important to utilize a higher shell-side Re in the operation to maintain a better mixing near the membrane surface in a larger module. Single fiber tests in combination with heat transfer analysis, verified that a critical fiber length existed that is the required length to assure sufficient driving force along the fiber to maintain adequate MD performance. In addition, for multi-fiber modules the overall MD coefficient decreased with increasing packing density, possibly due to flow maldistribution. This study shows that more hydrophobic membranes with a small maximum pore size and higher liquid entry pressure are attainable and favorable for MD applications. In order to enhance MD performance various factors need to be considered to optimize fluid dynamics and module configurations, such as fiber length, packing density and the effect of module diameter and flow rates. [Copyright &y& Elsevier]

Abstract: Forward osmosis (FO) has received intensive studies recently for a range of potential applications such as wastewater treatment, water purification and seawater desalination. One of the major challenges to be overcome is the lack of an optimized FO membrane that can produce a high water flux comparable to commercial RO membranes. Two types of thin-film composite FO hollow fibers with an ultra-thin polyamide-based RO-like skin layer (300–600nm) on either the outer surface (#A-FO) or inner surface (#B-FO) of a porous hollow fiber substrate have been successfully fabricated. These novel composite FO hollow fibers have been characterized by a series of standard protocols and benchmarked against commercially available FO flat sheet membranes and reported NF hollow fibers used for the FO process. The characterization reveals that the FO hollow fiber membranes possess a large lumen. The substrates are highly porous with a narrow pore size distribution. The active layers present excellent intrinsic separation properties with a hydrophilic rejection layer and good mechanical strength. The #B-FO hollow fiber membrane can achieve a high FO water flux of 32.2L/m2 h using a 0.5M NaCl draw solution in the active rejection layer facing draw solution (AL-facing-DS) configuration at 23°C. The corresponding salt flux is only 3.7g/m2 h. To the best of our knowledge, the performance of the #B-FO hollow fiber is superior to all FO membranes reported in the open literature. The current study suggests that the optimal FO membrane structure would possess a very small portion of sponge-like layer in a thin and highly porous substrate, which suggests a way for further improvement. [Copyright &y& Elsevier]

Abstract: A novel surface morphology of linear pore pattern on poly(vinylidene fluoride-co-hexafluoro propylene) (PVDF–HFP) flat-sheet porous membrane surfaces prepared by phase inversion technique has been observed. This interesting morphology was repeatedly captured from various casting systems, such as PVDF–HFP/N-methyl pyrrolidone(NMP)/glycol, PVDF–HFP/NMP/diethylene glycol, PVDF–HFP/NMP/glycerol and PVDF–HFP/NMP/glucose, with distilled water as a coagulant. The formation of the linear pore pattern was thought to be associated with the orientation of the macromolecular conglomerations in the casting solution, which occurred during the casting process. Thus, relevant experiments were designed and performed to support the hypothesis. The clearly observed linear pore pattern formed with less than 30s of exposure time before immersing into water bath became faded in 300s and completely disappeared in 600s of exposure time, which was in good accordance with the relaxation degree of orientated macromolecular conglomeration. In contrast, the membrane without experiencing the casting process showed a conventional surface morphology, which also demonstrated the effect of the casting process on the membrane surface morphology. In addition, a clearer image of the linear pore pattern was captured when lower concentration polymer dopes were used. In contrast, this phenomenon was invisible for the membrane formed by a higher concentration dope, which is in agreement with the reduction of orientation degree with increasing polymer concentration. Moreover, the systems containing additives with lower viscosity, such as glycerol, glycol, diethylene glycol and glucose, seem to be easier to form a linear pore pattern than the one containing distilled water, the latter produced a nearly elliptical pore pattern on the membrane surface instead. The investigation on this feature may provide more information for a complete description of the initiation and growth of the porous membrane. [Copyright &y& Elsevier]

Abstract: The rheological properties of polymer dope solutions influence the membrane structure and performance. In the present work, the effect of shear rate on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF–HFP) asymmetric microporous hollow fiber membranes has been studied in terms of membrane morphology, pure water flux, filtration rejection, mechanic strength and the performance of the membrane contactors made by as-spun hollow fibers for CO2 capture. The PVDF–HFP copolymer possesses strong viscoelasticity, which is believed to be associated with the occurrence of an irregular morphology in the PVDF–HFP hollow fibers made at a high dope extrusion rate. For the first time, the viscoelastic Mach number was applied to elaborate the deformation of the inner contour of the membranes by means of characterizing the delayed die swell in the process of hollow fiber fabrication. Based on the Mach number, several batches of PVDF–HFP dope solutions with an appropriate viscosity were adopted for hollow fiber fabrication at given dope extrusion rates. The effects of the shear rate on the permeation flux and molecular weight cutoff (MWCO) of the membranes made by Batch III-2L.5T dope solution (2wt.% LiCl and 0.5% Tween 80 as the additive) presented a V shape curve; while for the polymer dope of Batch V-2P9G (2wt.% PVP and 9wt.% glycerol as the additive), the effect of the shear rate on the resultant membranes showed a different pattern: increasing the shear rate resulted in smaller MWCOs, enhanced water permeation flux and higher elongation strength. These behaviors have been discussed thoroughly based on the shear-induced macromolecular chain orientation and close packing, variation of the porosity with the shear rate and shear-thinning rheological property of the dope solution under a high shear stress. [Copyright &y& Elsevier]

Abstract: The effects of two typical additives, lithium chloride (LiCl) and glycerol, on the fabrication of poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes were investigated in terms of membrane morphology, structure, permeation performance, hydrophobicity and mechanical property. The addition of the additives into the dope solution altered the morphology and structure of the resultant membranes, which was believed to be associated with the change of the thermodynamic and kinetic properties of the system in the phase inversion process. The membranes with improved pure water permeability (PWP) from 6.908×10−5 (7) to 4.835×10−4 (49), 3.256×10−4 (33) and 5.003×10−4 L/hm2 Pa (51L/hm2 atm) were obtained when 2wt.% LiCl, 4wt.% LiCl and 10wt.% glycerol was used as the additive, respectively. Among these, the membrane made from the dope containing 4wt.% LiCl possessed the highest retention capability of 40kDa molecular weight cut-off (MWCO). The LiCl or glycerol addition into the polymer dope also made the membranes exhibit a narrow pore size distribution. Moreover, the membrane hydrophobicity was affected less by LiCl and glycerol than by poly(vinyl pyrrolidone) (PVP). The addition of 4wt.% LiCl into the dope could form the membrane with four times stronger strain strength than the poly(vinylidene fluoride) (PVDF) commercial membrane while keeping similar rigidity. [Copyright &y& Elsevier]

Abstract: A systematic simulation has been carried out in membrane contactors to study CO2 capture by water and diethanolamine (DEA) aqueous solutions from a CO2/N2 mixture under the wetted and the non-wetted operation modes. Two types of microporous hollow fiber membrane modules made of polypropylene (PP) and polyvinylidene fluoride (PVDF) hollow fibers were used to conduct CO2 absorption experiments. The corresponding experimental data were used to verify simulated results. Experimental study showed that the membrane wetting could not be completely avoided especially in the case of chemical absorption. Both experimental and theoretical study disclosed that the membrane wetting would result in a significant drop of CO2 flux. The simulation results further revealed that for the physical absorption of CO2 by water, the proportion of membrane phase resistance in the overall mass transfer resistance increased from less than 5 to about 90% when the operation mode was shifted from non-wetted to wetted. As for the chemical absorption, analysis on the mass transfer resistance revealed that the ratio of the membrane resistance increased sharply from 10 to 70% when only 10% membrane length was wetted. With the introduction of an extra resistance caused by the membrane wetting, the mass transfer in the wetted membrane phase finally became the rate-controlling step. It was not as effective to enhance the CO2 flux by increasing the inlet gas velocity or the liquid velocity as in the non-wetted mode of operation. [Copyright &y& Elsevier]

Abstract: In the present work, poly(vinylidene fluoride-co-hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes were successfully fabricated using nonsolvent-induced phase inversion method which has not been reported previously. Analyses of thermodynamic characteristics of the PVDF-HFP/NMP/water system without/with PVP as an additive and the rheological property of the spinning dopes were carried out as a first step. Subsequently, the effects of spinning conditions (wet spinning, air gap, bore fluid composition), dope concentration and PVP addition on the resultant hollow fiber membranes were studied in terms of membrane morphology, structure, pure water flux and molecular weight cut-off (MWCO) as well as their hydrophobicity and mechanical properties. It was found that the wet spinning process produced hollow fibers with irregular inner contour under present experimental conditions. The strong viscoelastic property of PVDF-HFP material was thought to be associated with this phenomenon. A partial release of accumulated back stress in the nascent fibers might have caused the macromolecules to move inward in the transverse direction. Increasing the air gap could eliminate this inner contour deformation, as the outer surface of the nascent fiber was not vitrified prior to entering the water bath, thus allowing the orientated macromolecules to be relaxed. In addition, the nascent fiber is generally expected to experience an elongational stretch over the distance of the air gap, which might help to smooth the inner surface. Membranes made from the PVDF-HFP/NMP dopes were found to have small pure water fluxes no matter what the polymer concentration, the bore fluid composition and the air gap were used in the spinning processes. The pure water flux can be increased by adding PVP into the dope solution, up to 5wt.% PVP in the dope can promote the formation of macrovoids in the membrane. When the PVP content was increased to 10wt.%, the macrovoids were suppressed, but the membrane still presented a higher porosity, which was reflected from the significant increase in the pure water flux from 60 to 107L/(hm2 atm). In addition, PDVF-HFP hollow fiber membranes were more hydrophobic than those membranes made from pure PVDF, the former also exhibited pronounced ductile behavior when compared with the latter. [Copyright &y& Elsevier]

Abstract: In this paper, poly(vinylidene fluoride) (PVDF) hollow fiber membranes were prepared to make membrane contactors for CO2 capture. The hollow fiber membranes were spun with two different dope solutions at different shear rates in order to understand the influences of the rheological characteristics of the dope solution on the membrane structure and the system performance for CO2 adsorption. The membrane''s pore size distribution was characterized using the log normal distribution with the measurement of the rejection characteristics of dextran through the membrane. Lab-scale membrane contractors were finally prepared using resultant PVDF hollow fibers to conduct preliminary study on CO2 absorption. With the increase of the shear rate, the water permeation flux was increased and then maintained nearly the same level. The MWCO and CO2 absorption flux of the hollow fiber membranes spun from 17wt.% PVDF dope at different shear rates were almost the same. But for the hollow fibers spun from 20wt.% PVDF dope, increasing the shear rate in the spinning process resulted in bigger geometric mean diameters, larger MWCO and higher CO2 absorption flux. Compared with the hollow fiber membranes spun with high polymer concentration (20wt.%), the hollow fibers spun from low PVDF concentration (17wt.%) had a wider pore size distribution and bigger pores, which dominated the MWCO of hollow fiber membranes and presented a significant influence on the membrane performance. [Copyright &y& Elsevier]

Abstract: In order to gain a better understanding of CO2 absorption in a hollow fiber membrane contactor, theoretical simulations have been performed to describe CO2 capture by distilled water and aqueous diethanolamine (DEA) solutions. The studies were focused on gas phase behaviors by taking the distributions of gas concentration and gas velocity along the flowing direction into account. The corresponding experiments were also carried out in a Celgard MiniModule to verify the simulated results. In the case of physical absorption, both simulation and experimental results indicate that CO2 flux increases with the liquid velocity, while the inlet gas velocity has no significant effect on CO2 flux. The mass transfer resistance mainly exists on the liquid side. In the case of chemical absorption, the CO2 flux is significantly influenced by the inlet gas velocity while the liquid velocity has a limited effect. The analyses of CO2 concentration profiles in the gas and liquid phases as well as DEA concentration profile in the liquid phase reveal that CO2 concentration in the gas phase serves as the rate-determining factor for the reaction between CO2 and DEA. The significant CO2 loss in the gas phase results in an interesting DEA concentration profile near the liquid–gas interface. There exists an effective module length L eff, which suggests that increasing the module length is not a useful approach to enhance CO2 absorption when the module length is longer than L eff. [Copyright &y& Elsevier]

Abstract: Preparation of poly(vinylidene fluoride-co-hexafluoro propylene) (F2.6) flat-sheet asymmetric porous membrane has been studied for the first time. Factors affecting F2.6 membrane pore structure and permeate performance, such as macromolecule pore formers (polyethylene glycol-400, 1000, 1540, 2000 and 6000), the small molecule former (glycerol), swelling agent (trimethyl phosphate) in casting solution, precipitating bath component and temperature, exposure time and ambient humidity, were investigated in detail. Average pore radius and porosity were used to characterize F2.6 membrane structure, and respectively, determined by ultrafiltration and gravimetric method for the wet membrane. Morphology of the resultant membranes was observed by scanning electronic microscopy (SEM). Final test on permeate performance of F2.6 porous membrane was carried out by a direct contact membrane distillation (DCMD) setup. The experimental F2.6 membrane exhibits a higher distilled flux than PVDF membrane under the same operational situations. The determination of contact angle to distilled water also reveals higher hydrophobic nature than that of PVDF membrane. [Copyright &y& Elsevier]

Abstract: Membrane surface modification is an attractive approach to reduce membrane fouling by foulants such as natural organic matters (NOM). A novel technique which combines controlled deposition by electrophoresis of charged moieties, with UV grafting on the membrane surface has been applied to modify poly(ether sulfone) membranes with several different monomers. The monomers included three strong polyelectrolytes [methacrylic acid (MA), acrylic acid (AA) and 2-acryamido glycolic acid (AAG)] and two weak polyelectrolytes [(2-hydroxyethyl)-methacrylate (HEMA) and N-vinylformamide (NVF)]. The molecular weight cut-off, pure water flux, wettability, surface charge and surface morphology were measured for modified and unmodified membranes. The filtration and fouling performance was tested and compared using surface water from a Singapore reservoir. It was found that the modified membrane surfaces exhibited more hydrophilic and negative charged features after the electrophoresis-UV grafting treatment. The MA and AAG grafting reduced the MWCO of the PES membranes to improve NOM retention, and decreased the permeability to less than half of the original. The reason for this was the formation of a thin layer on the membrane surface which reduced the porosity and pore size of the treated membranes. In contrast, the HEMA grafted membrane showed increased MWCO and permeability as a result of the monomer dissolution which enlarged the membrane pore sizes. Membrane characterization by FE-SEM observation confirmed these two mechanisms. With electrophoresis assistance, the modified membrane exhibited more homogeneous surface properties than that without electrophoresis. The modified PES membranes also exhibited lower fouling tendency than the unmodified membranes when filtering NOM due to their hydrophilic and negative charge character after modification. Although the MA modified commercial PW membrane had the same MWCO and pure water flux as the unmodified commercial PT membrane, it had a much lower NOM fouling tendency. [Copyright &y& Elsevier]

Abstract: Mixed-conducting ceramic membranes for air separation were studied experimentally and theoretically. Dense tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) membranes were successfully prepared by the plastic extrusion method. Oxygen permeation through the membranes was measured at different oxygen partial pressures in the feed stream (0.1925 × 105 to 1.01325 × 105Pa (0.19–1.0atm)) and different temperatures, between 700°C and 900°C. The effects of air flow rate and sweeping helium flow rate on the oxygen permeation were also investigated. A mathematic model has been developed to simulate the process of air separation in the BSCF membrane permeator for four cases, each under co-current or cross-flow patterns with purge or vacuum operation. The calculated results from the cross-flow pattern with purge operation are in good agreement with experimental data. Moreover, parametric study under co-current flow with vacuum operation reveals that the air flow rate should be sufficiently high to fully utilize the separation capacity of the membrane. Higher vacuum levels and smaller inner diameter of the membrane tubes are essential to achieve higher separation efficiencies. The theoretical solutions also indicate that there is an optimal shell side pressure and optimal tube length, under which the oxygen productivity reaches its maximum with a fixed air flow rate. [Copyright &y& Elsevier]

We have studied the rheological characteristics, membrane formation and CO2-induced plasticization phenomenon of almost defect-free copoly(4,4′-diphenylene oxide/1,5-naphthalene-2,2′-bis(3,4-dicarboxylphenyl) hexafluoropropane diimide) (6FDA-ODA/NDA) hollow fiber membranes. Experimental results indicate that the dope rheology changes from a Newtonian to non-Newtonian fluid with elasticity when increasing shear rate. The asymmetric 6FDA-ODA/NDA hollow fiber membranes exhibit plasticization if the feed pressure is greater than 75 psi. Chemical cross-linking modifications with p-xylenediamine were conducted at ambient temperature in order to enhance anti-plasticization characteristics of 6FDA-ODA/NDA hollow fiber membranes. Chemical modified membranes show a significant reduction in CO2-induced plasticization at least up 550 psi (37.4 atm or 37.9 × 106 Pa) if the immersion time is great 1.5 min. However, deteriorations in permeance and permselectivity are clearly observed if the immerse time is greater than 5 min. FTIR spectra confirm that the 6FDA-ODA/NDA polyimide can be chemically cross-linked with p-xylenediamine, while wide-angle X-ray diffraction (WAXD) spectra imply the d-space of 6FDA-ODA/NDA membranes slightly decreases with an increase in cross-linking time. SEM pictures show that the outer selective skin as well as substructure becomes denser and thicker after chemical modification. The slight decrease in d-space may account for the permselectivity increase for the cross-linked membranes if the immersion time is short. However, the benefit of d-space reduction is offset by a denser and more compact selective skin and substructure when the immersion is prolonged. [Copyright &y& Elsevier]

The transport of olefin and paraffin namely ethane, ethylene, propane and propylene in aromatic poly(1,5-naphthalene-2,2′-bis(3,4-phthalic) hexafluoropropane) diimide (6FDA-1,5-NDA) dense membranes was investigated. The gas permeability coefficients were measured at pressures from 2.5 to 16 atm for the C2 hydrocarbon gases and pressures up to 8.4 atm for C3 systems at 35 °C. This membrane exhibits permeabilities of 0.15, 0.87, 0.023 and 0.24 Barrer with respect to pure ethane, ethylene, propane and propylene, and shows an ideal selectivity of 5.8 for the separation of ethylene/ethane, 10 for propylene/propane, 7.6 for nitrogen/ethane and 50 for nitrogen/propane. The olefins showed a preferred permeability to paraffins and discussion were drawn to the permeability, diffusivity and solubility coefficients. The activation energies of permeation, diffusion and solution were also reported and the effect of temperature on the permeation properties was discussed for the pure gas permeability data obtained from 30 to 50 °C. The plasticisation effect was also found for propane and propylene, respectively, although it was neither detected in the saturated nor unsaturated C2 hydrocarbons at pressures up to 16 atm. [Copyright &y& Elsevier]

We report that 6FDA-2,6-DAT polyimide can be used to fabricate hollow fiber membranes with excellent performances for CO2/CH4 separation. In order to simplify the hollow fiber fabrication process and verify the feasibility of 6FDA-2,6-DAT hollow fiber membranes for CO2/CH4 separation, a new one-polymer and one-solvent spinning system (6FDA-2,6-DAT/N-methyl-pyrrolidone (NMP)) with much simpler processing conditions has been developed and the separation performance of newly developed 6FDA-2,6-DAT hollow fiber membranes has been further studied under the pure and mixed gas systems.Experimental results reveal that 6FDA-2,6-DAT asymmetric composite hollow fiber membranes have a strong tendency to be plasticized by CO2 and suffer severely physical aging with an initial CO2 permeance of 300 GPU drifting to 76 GPU at the steady state. However, the 6FDA-2,6-DAT asymmetric composite hollow fibers still present impressive ultimate stabilized performance with a CO2/CH4 selectivity of 40 and a CO2 permeance of 59 GPU under mixed gas tests. These results manifest that 6FDA-2,6-DAT polyimide is one of promising membrane material candidates for CO2/CH4 separation application. [Copyright &y& Elsevier]

Membrane fouling persists to be among key obstacles in liquid membrane-based separation processes, and thereby the underlying understanding has actively been pursued. The two gaps attempted to be addressed here are namely, (i) internal fouling, which has limited studies due to limitations of instruments available; and (ii) fouling by mixtures of oppositely charged particles, as an extension to the studies on effect of surface charge of pure foulants on fouling. With the sub-micron pure constituents (namely, negatively charged carboxylated latex particles and positively charged aminated latex particles) as benchmarks, mixtures of three different compositions of the two particle types were investigated. The membrane was polycarbonate track-etched (PCTE) membrane. Surprisingly, the mixtures gave worse flux decline than that by purely positively charged aminated latex, which contradicts electrostatic considerations. The fouling model indicates that the mixtures exhibited higher pore blockage parameters (α) than either of the pure constituents, which agree with the flux decline trends. On the other hand, the mixtures gave internal cake resistance parameters (R c /R m) in between that of the pure constituents, which disagree with the fouling trends. Furthermore, optical coherence tomography (OCT) analysis indicates that the mixtures had taller buildups on the membrane as well as heterogenous internal cakes. Therefore, the steepest flux declines for the mixtures seem to be better correlated with the extensive external fouling as manifested in the taller buildups on the membrane pores, rather than internal fouling since the internal cakes were more porous. The contradictory extensive external and mitigated internal fouling phenomena are unexpected based on electrostatic considerations, and thereby highlight that fouling resulting from mixtures may not be readily extrapolated from understanding based on pure particle feeds. Image 1 • Steeper flux decline by mixtures of oppositely charged constituents than pure ones. • Mixtures gave greater pore blockage than constituents, agreeing with flux trends. • Mixtures gave intermediate internal-cake resistance, deviating from flux trends. • OCT trends affirm taller external cake and porous internal cake by mixtures. • Contradictory extensive external fouling and mitigated internal fouling by mixtures. [ABSTRACT FROM AUTHOR]

• Current status of Journal. • Accomplishments. • Future plan. [ABSTRACT FROM AUTHOR]

Interfacial polymerization is an effective technique to synthesize high performance polyamide thin film membranes. However, it is still very challenging to apply this technique on ceramic hollow fibres or tubular membranes, especially when the substrate pore size is in the microfiltration range. In this study, we demonstrated that thin polyamide layer can be synthesized directly on microfiltration ceramic tubular membranes with a surface pore size of 0.1–0.2 μm via interfacial polymerization without an intermediate layer. A thin polyamide layer with a thickness 30–40 nm was coated on the inner surface of the ceramic substrate by circulating the monomers of branched polyethyleneimine (PEI), piperazine (PIP), and trimesoyl chloride (TMC) through the membrane lumen. The thin film layer showed good integration with the ceramic substrate and could withstand high pressure of at least 10 bar. The mechanical property of the polyamide layer was examined using AFM and the modulus was measured. The thin film composite membranes demonstrated excellent nanofiltration performance with a pure water permeability of 16–18 LMH bar−1 and a molecular weight cut-off of ∼250 Da. The membranes also showed good salt rejections (>90%) to CaCl 2 , MgCl 2 and MgSO 4 , and still maintained high rejections of MgCl 2 and sucrose at elevated temperature of 80 °C. The use of highly inert ceramic substrates has enabled the application of polyamide membranes under more challenging conditions. Image 1 • PEI-based polyamide thin film was synthesized on microfiltration ceramic tubular membranes without an intermediate layer. • The membranes demonstrated excellent nanofiltration performance with a PWP of 16–18 LMH bar−1 and a MWCO of ∼250 Da. • The membranes also showed good divalent salt rejections for CaCl 2 , MgCl 2 and MgSO 4. • The rejections of MgCl 2 and sucrose remained high at elevated temperature of 80 °C. • The mechanical property of the polyamide film was studied using the AFM pinpoint mode and the modulus was measured. [ABSTRACT FROM AUTHOR]

This report demonstrates the fabrication of a composite hollow fiber membrane that efficiently recovers dissolved methane (dCH 4) from anaerobic wastewater treatment effluents via membrane contactor (MC) process. We first fabricated highly porous hollow fiber membranes (∼85% porosity) using polyvinylidene difluoride (PVDF). Then, we coated the whole structure (bulk), or lumen side of PVDF supports with polydimethylsiloxane (PDMS), a hydrophobic polymer possessing high gas permeability. The adjustment of the concentration of PDMS precursor mix could effectively serve for tuning the performance characteristics of MC membranes and helped suppress membrane wetting, a major performance-reducing factor. Despite their denser skin layers, the lumen-modified membranes demonstrated superior dCH 4 recovery flux. Mass transfer analyses have confirmed the importance of preserving bulk porosity while coating the membrane surfaces. The long-term performance tests performed using actual anaerobic effluents suggested that the PDMS coating did not accelerate fouling, such that the lumen-modified MC membranes showed an almost steady flux for 8 days with an anaerobic membrane bioreactor effluent. A high-strength effluent, obtained from an up-flow anaerobic sludge blanket reactor, induced faster fouling. However, we observed only a ∼20% flux decline in a 10-day operation. Thus, we propose that the designed PDMS-PVDF composite membranes are promising for industrial practices. Image 1 • We prepared PDMS-coated PVDF porous hollow fiber membranes for membrane contacting. • Both lumen- and bulk-modified PVDF membranes were successful in methane recovery. • Lumen-modified PDMS-PVDF membrane outperformed commercial PP and PDMS membranes. • Lumen-modified PVDF achieved long-term performance with actual anaerobic effluents. [ABSTRACT FROM AUTHOR]

Tailoring the hydrophobic properties of membranes is an important requirement in gas-liquid membrane contactor processes because the membrane can be wetted by liquid. This paper describes the hydrophobic modification of the chemical surface of a polyvinylidene fluoride hollow-fiber membrane using a commercial perfluoropolyether, Fluorolink S10. The contact angle of the modified membrane was influenced by the modification parameters, and hence parameter optimization was required to obtain a highly hydrophobic membrane. The sodium hydroxide concentration (pH 8, 10 and 12), dehydrofluorination time (30, 60 and 90 min), and chemical solution grafting time (30, 60 and 90 min) were identified as the control parameters of this modification and were optimized using the Taguchi approach. The optimized membrane was then applied to recover methane from anaerobic membrane bioreactor effluent and benchmarked with a commercial polypropylene membrane manufactured for degasification of water. The contact angle of the polyvinylidene fluoride membrane was improved by 32% after the surface modification (under the optimal modification condition indicated by the Taguchi method). The membrane mass transfer coefficient was measured as 1.53 × 10 −4 m/s. Our membrane showed better methane recovery performance than the commercial polypropylene membrane and a very stable flux without pore wetting during 10 days of operation with synthetic effluent (made by saturating a 60:40 methane/carbon dioxide mixture into tap water). In addition, the membrane prepared in this study could be operated without significant fouling (7% flux drop during 8 days of operation) with real anaerobic membrane bioreactor effluent. [ABSTRACT FROM AUTHOR]

To separate oil/water mixtures and emlusions, superwetting membranes have been developed via synergy between surface chemistry and topography. However, challenges remain as most of them exhibit unsatisfactory performance, limited strength and durability. Herein, we report a novel membrane with switchable super-wettability for oil and water. This novel composite membrane was developed by electrospinning a hierarchically porous polyvinylidene fluoride (PVDF)-silica composite nano/micro-beaded top layer and a PVDF nanofibrous intermediate layer on a non-woven support. The surface of the composite membrane was modified to exhibit high surface free energy. The membrane is in-air superamphiphilic, underwater superoleophobic and under-oil superhydrophobic. It can treat various types of oil/water mixtures, from simple layered solutions to emulsions (including surfactant-free or surfactant-stabilized oil-in-water and water-in-oil emulsions) without external driving force or under an ultralow pressure (0.1 bar) in a cross-flow filtration process. It shows a flux up to 2000 L/m 2 h and high separation efficiency (> 99.99% in terms of water and oil purities in the permeation) in the cross-flow filtration process. The membrane also exhibits an excellent robustness under harsh conditions, including strong acidic or alkaline solutions, hot water and petroleum. In addition, it presents remarkable antifouling and easy-cleaning properties, which were demonstrated in a 50-h continuous operation. [ABSTRACT FROM AUTHOR]

Pressure-retarded osmosis (PRO) has attracted worldwide attention for its potential applications in renewable osmotic energy harvesting, low energy seawater desalination, and industrial waste brine disposal. However, membrane fouling is one of the major issues limiting PRO performance with the use of a high-salinity draw solution and a low-value impaired feedwater. This study systematically investigated membrane fouling in the PRO process using a real wastewater retentate (a waste byproduct from wastewater reclamation plants) as the PRO feed. Organic fouling (e.g., by humic substances) and inorganic fouling (e.g., by calcium phosphate) were identified to be the major types of fouling, where the latter dominated the overall water-flux decline. The internal PRO membrane fouling was exacerbated by internal concentration polarization (ICP) and intermolecular interactions between the organic macromolecules and the scaling precursor ions (e.g., Ca 2+ ). A limiting flux that was independent of the initial flux (i.e., applied pressure) was observed during the PRO fouling and was explained by a novel PRO limiting flux model. A membrane with a smaller structural parameter could achieve a higher limiting flux in PRO. A simple and effective pressure-assisted osmotic backwashing protocol was developed to clean the internally fouled membrane that could restore over 92% of the water flux within a short cleaning period (15 mins). The PRO performance could also be significantly improved by decreasing the feedwater pH that mitigated alkaline scaling. Finally, performing the PRO membrane fouling test in the active-layer-facing-feed-solution (AL-FS) orientation revealed that the integral cellulose triacetate (CTA) membrane exhibited strong mechanical stability and anti-fouling tendency whereas the thin-film composite (TFC) membrane was not mechanically stable in this orientation. This study for the first time identified that an integral membrane instead of a TFC membrane is an excellent candidate for PRO operation in the AL-FS mode for fouling control. [ABSTRACT FROM AUTHOR]