Dual-layer Membranes Research Articles

Dual-Layer MOF Composite Membranes with Tuned Interface Interaction for Postcombustion Carbon Dioxide Separation

Summary Metal-organic frameworks (MOFs) have immense potential as sorbents and inorganic membranes for CO2 separation. However, the trade-off between selectivity and adsorption capacity or permeability remains a challenge. Here, we present the fabrication of MOF core-shell particles and dual-layer membranes that show enhanced CO2/N2 separation performance compared with individual MOF materials. A highly selective MOF layer with narrow pore size is grown on the surface of a highly permeable MOF layer with large pore size. Chemical binding at the interface between the two MOFs makes it possible to form a continuous defect-free shell layer on the surface of the core or base layer. The two dual-layer MOF@MOF membranes, SIFSIX-3-Ni@SIFSIX-1-Cu and SIFSIX-3-Ni@HKUST-1, represent the first examples of pure inorganic membranes constructed from different MOFs with lattice mismatch. The CO2/N2 selectivity of the HKUST-1 and SIFSIX-1-Cu membranes is improved from 5.7 to 13.4 and 2.3 to 7.4, respectively, after the growth of a selective layer of SIFSIX-3-Ni.

Scaling mitigation in radio-frequency induction heated membrane distillation

Calcium sulfate (CaSO4) scaling is a limiting factor in membrane distillation as a result of concentration polarization and elevated temperatures. We assessed the influence of radio-frequency magnetic fields, used as a method of heating the membrane's surface, as part of a membrane distillation (RF-MD) process. The influence of RF heating on CaSO4 scaling is addressed in terms of distillate flux and crystal formation, and the results are compared to conventional MD systems. The RF-MD system included a thermally conducting dual-layer membrane containing a magnetic hydrophilic layer based on iron oxide-carbon nanotubes coated on a hydrophobic membrane. Heating the solution was mainly done directly at the membrane-water interface, and scaling was assessed at different feed flow velocities using feed solutions containing elevated concentrations of CaSO4 and NaCl and at ionic composition simulating RO brine. Results show only sporadic small CaSO4 crystals on the membrane surface following RF-MD, while high concentrations of small crystals were detected at the concentrate stream exiting the filtration module. Furthermore, the RF-MD system was able to produce enhanced distillate flux. The scaling mitigation mechanism is discussed in detail and hypothesized to be a result of the high-frequency movement and collision of the ions in the solution.

Theoretical guidance for fabricating higher flux hydrophobic/hydrophilic dual-layer membranes for direct contact membrane distillation

In this study, a new simultaneous heat and mass transfer theoretical model of the membrane distillation process operating under direct contact membrane distillation (DCMD) mode has been developed for the hydrophobic/hydrophilic dual-layer composite membranes. Using the proposed model, a criterion parameter (β) was derived to determine whether the flux will be increased for the prepared hydrophobic/hydrophilic composite membranes. A flux ratio (γDL/SLM) of dual-layer to single-layer membrane was also developed to predict the DCMD flux performance of the composite membranes. To validate the theoretical model, three sets of dual-layer hydrophobic/hydrophilic composite membranes and single-layer hydrophobic membranes with defined total thicknesses (240 μm, 260 μm, and 285 μm) and different thickness ratios (δ/δo=3.69, 2.48 and 2.19) were purposefully fabricated using electrospinning technique. Theoretical modelling results were compared with the experimental data in terms of DCMD flux ratio. The experimental data of flux ratios for the three sets of fabricated membranes at 60 °C, γDL/SLE, are 2.75, 1.97 and 1.80, respectively, which fit well with the model prediction values (γDL/SLM= 2.96, 2.18, and 1.97) within high agreement level at a relative standard deviation of 9.6%. Furthermore, the flux ratio of DL2 to SL2 membranes (dual-layer and single-layer membranes with 260 μm total thicknesses and δ/δo=2.48 thickness ratio) at different feed inlet temperatures still maintain a high degree of consistency with model values. Overall, the results show the experimental data agree well with the theoretical model prediction for the flux ratio within relative standard deviation of 10.1%, thereby confirming the validity of the proposed model. Additionally, a critical value of the thickness ratio (χ) was calculatedχ=1+1β, this value could provide useful guidance to design a highly efficient hydrophobic/hydrophilic dual-layer composite membrane for DCMD applications in the future work.

Janus Nanofibrous Membranes for Desalination by Air Gap Membrane Distillation

The hydrophobic/hydrophilic dual-layer membrane can be considered as a membrane with two different surface properties. It is also named the Janus membrane by researchers for the mythical god with t...

Embedding topography enables fracture guidance in soft solids

The natural topographical microchannels in human skin have recently been shown to be capable of guiding propagating cracks. In this article we examine the ability to guide fracture by incorporating similar topographical features into both single, and dual layer elastomer membranes that exhibit uniform thickness. In single layer membranes, crack guidance is achieved by minimizing the nadir thickness of incorporated v-shaped channels, maximizing the release of localized strain energy. In dual layer membranes, crack guidance along embedded channels is achieved via interfacial delamination, which requires less energy to create a new surface than molecular debonding. In both membrane types, guided crack growth is only temporary. However, utilizing multiple embedded channels, non-contiguous crack control can be maintained at angles up to 45° from the mode I fracture condition. The ability to control and deflect fracture holds great potential for improving the robustness and lifespan of flexible electronics and stretchable sensors.

Fabrication of triple layer composite membrane and its application in membrane distillation (MD): Effect of hydrophobic-hydrophilic membrane structure on MD performance

Electrospinning is a notable technique to prepare nanofibrous membranes with remarkable features required for membrane distillation (MD). However, hydrophobic membrane fabricated by electrospinning alone could hardly ensure a high flux. In this study, triple layer membrane that is composed of a PVDF-PTFE hydrophobic layer, a PET support layer, and a chitosan-polyethylene oxide (CS-PEO) hydrophilic layer, was fabricated by electrospinning. Firstly, the PVDF-PTFE and CS-PEO layers were optimized, whose contact angles and thicknesses were 144.3°, 55 μm and 20°, 75 μm, respectively. Then, the triple layer membrane (TL-M), dual layer membrane without hydrophilic layer (DL-M) were fabricated and tested in a DCMD process using 3.5 wt% NaCl as the feed to illustrate the effect of hydrophilic layer. The triple layer composite membrane (average flux: 19 kg/m−2 h−1, rejection rate: 99.92%) showed better performance in comparison to the dual-layer membrane (average flux: 15 kg/m−2 h−1, rejection rate: 99.88%). In addition, the driving forces of TL-M (from 14.34 to 14.16 kPa) and DL-M (from 13.26 to 12.88 kPa) were calculated. Results suggested the incorporation of hydrophilic layer could increase the driving force, and largely enhance the membrane flux which could ensure the triple layer membrane possesses a good potential for DCMD application.

Comprehensive Study of Morphological Modification of Dual-Layer Hollow Fiber Membrane

The NiO–YSZ/LSCF–YSZ dual-layer hollow fiber membrane receives increasing attention which is due liable to quick changes in properties during co-extrusion followed by the co-sintering method. The effect of mixed bore liquid of water + ethanol + n-methyl-2-pyrrolidone and co-sintering temperature on physical and chemical characteristics of the obtained membrane was systematically explored in this study. Here, the characterization of the membrane was inspected using scanning electron microscope–energy-dispersive X-ray, three-point bending, mercury porosimetry, X-ray diffraction and nitrogen tightness tests as well as the oxygen permeation test. Results indicated that membrane prepared using 100% water as a bore liquid produced a sandwich structure. On the contrary, by employing 40:10:50% of water/ethanol/NMP as a mixed bore liquid and sintering for 8 h at 1300 °C, an open-channel structure in the microstructure gives a good porosity as tested using mercury porosimetry, mechanical properties and nitrogen permeability with no secondary phases. In addition, the co-sintering effect was examined by increasing temperature to 1400 and 1500 °C. However, an impurity phase of LaZr2O7 was formed which is due to the chemical reaction of La and Sr into YSZ structure. Oxygen permeate concentration through the dual-layer membrane was found to be slightly different as compared to the single-layer hollow fiber membrane. The results further suggest that the outer layer should be tighter and thinner to enhance the oxygen ion diffusion.

Development of A Novel Corrugated Polyvinylidene difluoride Membrane via Improved Imprinting Technique for Membrane Distillation.

Membrane distillation (MD) is an attractive technology for desalination, mainly because its performance that is almost independent of feed solute concentration as opposed to the reverse osmosis process. However, its widespread application is still limited by the low water flux, low wetting resistance and high scaling vulnerability. This study focuses on addressing those limitations by developing a novel corrugated polyvinylidene difluoride (PVDF) membrane via an improved imprinting technique for MD. Corrugations on the membrane surface are designed to offer an effective surface area and at the same time act as a turbulence promoter to induce hydrodynamic by reducing temperature polarization. Results show that imprinting of spacer could help to induce surface corrugation. Pore defect could be minimized by employing a dual layer membrane. In short term run experiment, the corrugated membrane shows a flux of 23.1 Lm−2h−1 and a salt rejection of >99%, higher than the referenced flat membrane (flux of 18.0 Lm−2h−1 and similar rejection). The flux advantage can be ascribed by the larger effective surface area of the membrane coupled with larger pore size. The flux advantage could be maintained in the long-term operation of 50 h at a value of 8.6 Lm−2h−1. However, the flux performance slightly deteriorates over time mainly due to wetting and scaling. An attempt to overcome this limitation should be a focus of the future study, especially by exploring the role of cross-flow velocity in combination with the corrugated surface in inducing local mixing and enhancing system performance.

Electrospun Nanofiber Membranes Incorporating PDMS-Aerogel Superhydrophobic Coating with Enhanced Flux and Improved Antiwettability in Membrane Distillation.

Electrospun nanofiber membranes (ENMs) have garnered increasing interest due to their controllable nanofiber structure and high void volume fraction properties in membrane distillation (MD). However, MD technology still faces limitations mainly due to low permeate flux and membrane wetting for feeds containing low surface tension compounds. Perfluorinated superhydrophobic membranes could be an alternative, but it has negative environmental impacts. Therefore, other low surface energy materials such as silica aerogel and polydimethylsiloxane (PDMS) have great relevancy in ENMs fabrication. Herein, we have reported the high flux and nonwettability of ENMs fabricated by electrospraying aerogel/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) over electrospinning polyvinylidene fluoride- co-hexafluoropropylene (PVDF-HFP) membrane (E-PH). Among various concentrations of aerogel, the 30% aerogel (E-M3-A30) dual layer membrane achieved highest superhydrophobicity (∼170° water contact angle), liquid entry pressure (LEP) of 129.5 ± 3.4 kPa, short water droplet bouncing performance (11.6 ms), low surface energy (4.18 ± 0.27 mN m-1) and high surface roughness ( Ra: 5.04 μm) with re-entrant structure. It also demonstrated nonwetting MD performance over a continuous 7 days operation of saline water (3.5% of NaCl), high antiwetting with harsh saline water containing 0.5 mM sodium dodecyl sulfate (SDS, 28.9 mN m-1), synthetic algal organic matter (AOM).

Depletion of VOC in wastewater by vacuum membrane distillation using a dual-layer membrane: mechanism of mass transfer and selectivity

Mechanism of the organic selectivity and mass transfer resistance of the dual-layer membrane in vacuum membrane distillation.

High-efficiency water-selective membranes from the solution-diffusion synergy of calcium alginate layer and covalent organic framework (COF) layer

Covalent organic framework (COF) is an emergent class of ordered porous materials with high stability. The fabrication of thin and continuous COF membrane for small molecule separation remains challenging due to the relatively large pore size of COF materials and weak COF-substrate interfacial interaction. Herein, a two-dimensional COF layer was spin-coated on a porous substrate followed by constructing a bio-inspired calcium alginate (Alg-Ca) layer to obtain a dual-layer membrane with a thickness less than 100 nm. The Alg-Ca layer afforded a superhydrophilic surface to capture water molecules, and the porous COF layer rendered selective water channels. The dual-layer membrane synergistically strenthened the solution-diffusion mechanism through the manipulation of the membrane structure and transport process. The synergy of two layers endowed the membrane with superior water-selective performance with permeation flux of 3614 g/(m2h) and water content in permeate of 99.7 wt% for n-butanol dehydration. The fabrication of dual-layer membrane in this study may offer a new strategy for highly efficient separation of liquid small molecule mixtures.

High Efficiency Removal of Cytokines and HMGB-1 by Continuous Hemofiltration With a Dual Layered Polyethersulfone Membrane: An Ex Vivo Study.

Cytokines and high mobility group box chromosomal protein-1 (HMGB-1) play key roles in inflammatory conditions. While hemofiltration has been shown to remove cytokines, removal of cytokines and HMGB-1 by hemofiltration using a polyethersulfone membrane has not been reported. This study aimed to test the hypothesis that the polyethersulfone membrane will achieve higher removal performance for substances including inflammatory cytokines compared to other hemofilters, while retaining low albumin removal capacity. Subjects were eight healthy volunteers. We collected 400 mL each of blood samples into containers with heparin and added 30 mg of lipopolysaccharide to spike cytokines and HMGB-1. After incubation at 39ºC for 12 h, each blood sample was circulated through a hemofiltration circuit with a polyethersulfone hemofilter (2.1 m2 or 1.1 m2 ) at a filtration flow rate of 2 L/h. Measurement samples were collected from arterial, venous, and ultrafiltrate sampling points. Concentrations of cytokines (IL-1ß, IL-4, IL-6, IL-8, IL- 10, and tumor necrosis factor [TNF-a]), HMGB-1, and albumin were determined at each time point (1, 4, 8, 12, and 24h). High sieving coefficients (SCs) above 0.8 were obtained for all cytokines except for TNF-a as well as HMGB-1, whereas the SC for albumin was less than 0.04 with both hemofilters. The hemofilter with a larger membrane area achieved significantly higher clearances for TNF-a and HMGB-1, and slower decreases in SCs over time for IL-1ß, IL-6, IL-8, TNF-a, and albumin. Continuous hemofiltration with a polyethersulfone membrane achieved high efficiency removal of cytokines and HMGB-1, without excessive removal of albumin.

Thin mixed matrix and dual layer membranes containing metal-organic framework nanosheets and Polyactive™ for CO2 capture

Preparation methods are presented of thin dual layer membranes (DLM's) and mixed matrix membranes (MMM's) based on nanosheets of the Cu-BDC metal-organic framework (MOF, lateral size range 1–5 µm, thickness 15 nm) and commercially available poly(ethylene oxide)–poly(butylene terephthalate) (PEO–PBT) copolymer (Polyactive™) and their performances are compared in CO2/N2 separation. The MMMs and DLMs represent two extremes, on the one hand with well-mixed components and on the other hand completely segregated layers. Compared to the free-standing membranes, the thin PAN- and zirconia-alumina-supported MMMs showed significant enhancement in both permeance and selectivity. The support properties affect the obtained selective layer thickness and its resistance impacts the CO2/N2 selectivity. The permeance of thin DLM's is among the highest reported literature data of MOF based thin MMMs, but have a modest selectivity. Addition of the nanosheets in the thin MMMs improves the CO2/N2 selectivity of the already selective polymer further to 77. The nanosheets in the thin MMMs make a gutter layer on the PAN support superfluous. The small pore support ZrO2-alumina does not need a gutter layer.XRD analysis reveals that the spatial distribution of MOF nanosheets and polymer chains packing were responsible for differences in the permeation performance of the free-standing, thin dual layer and mixed matrix membranes.

Development of a dual-layered PVDF-HFP/cellulose membrane with dual wettability for desalination of oily wastewater

Meeting environmental discharge or reuse water qualities and freshwater demands requires the development of innovative processes like membrane distillation (MD) for treating oily wastewater. Desalinating oily wastewater through MD is, however, challenging because of immediate contamination and wetting of the membrane caused by oil or other low surface tension contaminants. In this work, a novel concept of stacking two membranes with different wettabilities to obtain a dual-layered membrane was developed. The hydrophilic top layer, composed of electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and regenerated cellulose, displayed superoleophobicity underwater and prevented oil passage to the hydrophobic PVDF-HFP bottom layer during MD process. When tested in direct contact MD of saline feed containing 1000 ppm of oil, the stacked dual-layered membrane yielded fresh water flux up to 12.8 kg m−2 h−1 with complete salt rejection.

CNT/PVP blend PVDF membranes for the removal of organic pollutants from simulated treated wastewater effluent

In this work, we established a simple and effective way of immobilizing CNT onto polyvinylidene fluoride (PVDF) membrane by casting dual-layer membranes. We explored two different methods of dual-layer casting: (i) layer by layer and (ii) simultaneous dual layer fabrication. The influence of these methods on the characteristics of the prepared membranes were investigated. The hydrophilicity of the top surface was ensured by blending appropriate quantity of polyvinyl pyrrolidone (PVP) into the polymer dope so that the feed water and its organic pollutant content have adequate contact with the membrane surface containing the CNT. Dynamic contact angle data revealed that CNT blend PVDF membranes were highly hydrophilic when compared to their pristine counterparts. Single layer and simultaneous cast double layer blend membranes showed bigger pores (∼10 μm), whereas layer-by-layer cast double layer blend membrane showed acceptable lower mean pore size (∼0.25 μm). Adsorption experiments were done using methylene blue (MB) as a model adsorbate. Composite membranes performed better in the removal of MB, while also showing high rate of filtration. This was achieved due to better membrane wettability, more open structure and higher concentration of CNT on the top surface of the blend membranes. An almost complete desorption of MB off the membrane surface was successfully accomplished using ethanol solution at a pH of 2, indicating the possible cyclic application of these membranes.

Antiwettability and Performance Stability of a Composite Hydrophobic/Hydrophilic Dual-Layer Membrane in Wastewater Treatment by Membrane Distillation

This study investigated the performance and applicability of a composite dual-layer polytetrafluoroethylene (PTFE) membrane coated with a dense hydrophilic polyurethane layer for treating wastewater containing surfactant and solvent in a direct contact membrane distillation process. The influences of operating temperature and water recovery on bare and composite PTFE membranes were studied using simulated wastewaters containing sodium chloride together with sodium dodecyl sulfate or ethanol. In contrast to the significant wetting of bare PTFE membrane due to the adsorption of wetting agent on membrane surface, the composite membrane exhibited satisfactory antiwettability and desalination stability. The mechanism of antiwettability of the composite membrane was related to the repellent effect of hydrophobic moiety of surfactant arrayed on the hydrophilic layer of the composite membrane, which prevented the wetting agent from reaching the hydrophobic layer of membrane. The results indicate that the dual-lay...

Fabrication of novel Janus membrane by nonsolvent thermally induced phase separation (NTIPS) for enhanced performance in membrane distillation

This study proposed to use the nonsolvent thermally induced phase separation (NTIPS) method to fabricate a novel Janus membrane for MD applications. The as-prepared dual-layer membrane consisted of a thin hydrophobic PVDF top-layer and a relatively thick hydrophilic PVDF-PVA sub-layer. By adopting a facile one-step co-casting technique and water soluble diluent ε-caprolactam (CPL), delamination-free dual-layer membrane was obtained. The SEM morphologies and FTIR crystalline analyses suggested the membrane formation mechanisms, where the hydrophobic top-layer was formed via NTIPS process, resulting in an ultra-thin dense skin with finger-like pores formed beneath; while the hydrophilic sub-layer was induced by TIPS, producing highly porous cellular structure with high degree pore interconnectivity. Combining the structural observation and MD performance results, suitable fabrication parameters were identified as a PVDF concentration of 15 wt% for the hydrophobic layer and coagulation temperature between 20 and 40 °C. The total membrane thickness was optimized as 100–150 µm, given the thickness of hydrophobic layer kept within an optimal range of 30–60 µm to ensure minimal mass transfer resistance. The Janus membrane exhibited stable salt rejection above 99.5% over continuous MD runs and superior permeation flux up to 165.3 kg m-2 h-1 at 80 °C, which was remarkably higher than reported MD membranes.

Comparison between dual-layer (superhydrophobic–hydrophobic) and single superhydrophobic layer electrospun membranes for heavy metal recovery by air-gap membrane distillation

A novel approach to enhance membrane performance using electrospinning fabrication technique for recovery of heavy metals using air-gap membrane distillation is described. Accordingly, a comprehensive study was accomplished to fabricate a unique electrospun dual-layer membrane (ESD) with an upper superhydrophobic layer and hydrophobic electrospun support layer and compare with a superhydrophobic electrospun single layer membrane (ESS). Superhydrophobic alumina nanoparticles (Al2O3) were embedded in a low polymer concentration of polyvinylidene fluoride (PVDF) to produce superhydrophobic ESS and top layer of ESD using electrospinning technique, while a mat with different concentrations of PVDF were used as hydrophobic electrospun support layer. In this study, dual layer membranes were fabricated in two sets. In the first set, layer thickness was varied by changing the spinning volume of the top and support layer with maintain total spinning volume, while in the second set the fibre diameter of the support layer was varied by changing the polymer concentration. Moreover, the electrospun membranes were characterized in terms of membrane performance such as: permeate flux, heavy metal rejection and energy consumption; wettability performance such as liquid entry pressure (LEP), and water contact angle (WCA); membrane structure such as mean with maximum pore size and porosity; membrane integrity such as mechanical and thermal integrity. The heavy metal rejection was >99% for all single and dual layer membranes when filtering artificial wastewater (Pb, Cd, Cr, Cu, Ni). When compared with single layer electrospun membrane made from spinning 16 ml PVDF, dual layer membrane made from the same spinning volume exhibited some improvement, such as higher permeate flux above 23 l/m2∙h (LMH) when filtering 2500 ppm concentration heavy metal feed water. Additionally, both sets of dual layer membrane demonstrated better mechanical performance and slight reduction of LEP compared with single layer electrospun membrane.

Fabrication of a novel octadecylamine functionalized graphene oxide/PVDF dual-layer flat sheet membrane for desalination via air gap membrane distillation

In the present research, a novel dual-layer membrane was prepared by coating a very smooth layer of octadecylamine functionalized graphene oxide (GO-ODA) on the surface of polyvinylidene fluoride membrane. The GO and GO-ODA were characterized by XRD, TGA and FE-SEM and pure PVDF (unmodified) and dual-layer (modified) membranes were also characterized by contact angle, pore size distribution, liquid entry pressure (LEP), FT-IR, SEM, AFM, and ATR-FTIR. Modified membranes in comparison with unmodified membrane showed a superior performance in terms of surface roughness, hydrophobicity, water flux and NaCl rejection. In air gap membrane distillation (AGMD) experiments using a 3.5wt% NaCl solution as feed at 80°C, unmodified, M1 (low GO-ODA loading), and M2 (high GO-ODA loading) modified membranes showed water fluxes of 18.2, 13.8, and 16.7kg/m2·h and salt rejections of 88.5%, 96.3%, and 98.3%, respectively. This improvement in membrane performance was attributed to the existence of GO-ODA on the top layer of modified membranes that formed interconnected nano-channels with high surface area for high NaCl rejection and fast flow of water. Furthermore, GO-ODA with high hydrophobicity and low thermal conductivity on the surface of modified membranes contributed to reduce pore wetting, temperature polarization and heat diffusion across the membrane.

Moisture Transport: Tailoring Differential Moisture Transfer Performance of Nonwoven/Polyacrylonitrile‐SiO2 Nanofiber Composite Membranes (Adv. Mater. Interfaces 15/2017)

Xianfeng Wang, Bin Ding, and co-authors design a nonwoven/polyacrylonitrile-SiO2 nanofiber dual-layer membrane with high wettability gradient in article number 1700062. The developed fibrous membrane offers an excellent moisture management performance with an outstanding one-way transport capacity (1413%), excellent overall moisture management capacity (0.99), and a superior breathability (water vapor transfer rate of 12.73 Kg m−2 d−1), indicating a potential candidate for faster sweat release applications (e.g. sportswear, footwear and workwear).