Dual-layer Composite Membranes Research Articles

Optimizing dual-layer composite membranes with functionalized CNT loadings using response surface methodology

ABSTRACT In this study, we optimized double-layer composite membranes comprising polyethersulfone (PES) and polyamide (PA) using response surface methodology (RSM). We assessed the impact of PES, piperazine (PIP), and trimesoyl chloride (TMC) concentrations, and interfacial polymerization (IP) reaction time on nanofiltration (NF) membrane performance, aiming to enhance pure water flux (PWF) and salt rejection. Optimal parameters for maximum PWF and salt rejection were determined: PES concentration at 14.0 wt%, PIP concentration at 1.5 w/v%, TMC concentration at 0.2 w/v%, and reaction time at 45 s. In addition, we improved membrane performance by incorporating functionalized multi-walled carbon nanotubes (f-MWCNTs) into the PES ultrafiltration substrate, enhancing hydrophilicity and porosity. The resulting PA NF membrane developed on a substrate containing 0.15 wt% f-MWCNT (TFNC) exhibited a PWF of 11.84 LMH/bar, surpassing the PWF of the same NF membrane on an unloaded substrate (TFC) by approximately 20%, while maintaining nearly identical salt rejection levels. Salt rejection followed the order: Na2SO4 > MgSO4 > NaCl, suggesting Donnan-exclusion mechanisms primarily governed the separation process due to repulsion forces between the negatively charged membrane surface and anions.

Graphene nanoplatelets based polyimide/Pebax dual-layer mixed matrix hollow fiber membranes for CO2/CH4 and He/N2 separations

Mixed matrix membranes are key in the preparation of stable, stretchy, durable and efficient membranes for gas separation applications. To this aim, the perfect filler nanomaterials’ dispersion is needed. At the same time the replacement of toxic solvents with green ones is also a priority in science, including the membrane technology. Herein, we present the development steps, their structural characteristics and the gas selectivity properties of graphene nanoplatelets (GNPs) based mixed matrix hollow fiber (HF) membranes. In specific, dual-layer composite polymeric hollow fiber membranes, for CO2/CH4 and He/N2 gas separations, were prepared by directly dip-coating a single selective layer on prepared porous HF supports without the use of any gutter layer. Asymmetric BTDA-TDI/MDI (P84) co-polyimide-based HFs that acted as porous supports, were fabricated via the dry-wet phase inversion process and the commercial poly(ether-block-amide) Pebax-1657 was chosen as a high CO2-selective separation layer material, dip-coated on the HF supports. The positive influence of the incorporation of GNPs as promising nanofiller in both, the polymeric matrix (support) and the selective layer, in parallel with the partial replacement of toxic NMP by GBL -a "green" solvent- in the support's spinning process, were all evaluated and confirmed. CO2/CH4 and He/N2 selectivities up to 82 and 20, respectively with CO2 and He permeances of 3 and 2.7 GPU, respectively were measured under real binary gas mixture conditions at 1.3 bar(a) transmembrane pressure at 298 K for dual-layer composite, with GNPs filler, HF (DL-HF) membranes.

PPy nanotubes-enabled in-situ heating nanofibrous composite membrane for solar-driven membrane distillation

• A superhydrophobic photothermal composite membrane was demonstrated. • PDMS binder enhanced the structural stability and hydrophobicity of dual-layer membrane. • The PPy NTs/PVDF nanofibrous composite membrane showed outstanding solar-driven DCMD performance. Solar-driven membrane distillation is an emerging desalination technology with low energy consumption and efficient vapor collection, but the low vapor flux and limited efficiency hinders its practical application. Here, we demonstrated an innovative photothermal composite membrane by depositing polypyrrole nanotubes (PPy NTs) as functional coating layer onto an electrospun poly (vinylidene fluoride) (PVDF) nanofibrous support layer via vacuum filtration stabilized by adhesive polydimethylsiloxane (PDMS). According to the proportional relationship between the fibrous materials diameter and the pore size, the large-scale surface gaps between adjacent nanofibrous could be replaced by smaller pores from the PPy NTs layer on the PVDF substrate. The PDMS with low surface energy was used as an adhesive to in-situ solder the loose PPy NTs together as well as to enhance the adhesion between the PPy layer and the PVDF substrate to form an integrated and robust dual-layer composite membrane with superhydrophobicity, high porosity and controllable pore size. By exploring the optimal match of PPy loading and PDMS concentration, the solar-to-thermal performance and structural stability could be significantly improved. With 1 sun irradiation, the selected composite membrane thus showed a high-water vapor flux of 1.3 kg/(m 2 ·h) and a photothermal conversion efficiency of 81.6%, specially, the membranes maintained stable permeate conductivity during the 10 h solar-driven DCMD seawater desalination test. These results indicated that the photothermal dual-layer superhydrophobic composite membranes has promising potential for desalination with the help of sustainable energy sources such as sunlight.

Investigation on effect of ionic liquid on CO2 separation performance and properties of novel co-casted dual-layer PEBAX-ionic liquid/PES composite membrane

Dual-layer composite membranes composed of Pebax 1657, different 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) contents and Polyethersulfone (PES) were fabricated by co-casting method, to investigate CO2/light gases separation, due to the high affinity of CO2 to Pebax and the ionic liquid (IL). Gas permeation test, which was performed at 25 °C and pressure range of 4–8 bar, indicated remarkable increment in CO2, CH4 and N2 permeance through membranes, especially CO2, as well as ideal CO2/CH4 and CO2/N2 selectivities, with IL increment. At 6 bar, CO2 permeance increased 152.97% meanwhile, at 8 bar, CO2/CH4 and CO2/N2 selectivities increased 59.22% and 28.64%, respectively, in the membrane containing 20 wt. % IL, which is the optimum IL content. In addition, CO2 permeance and selectivity increased with pressure. FTIR, XRD, DSC, SEM, dynamic frequency sweep, contact angle and tensile test results revealed the formation of defect-free dense Pebax/IL top-layer on porous PES sub-layer. IL addition led to formation of new bonds between the IL and Pebax which cause changes in the chain packing and crystallinity of Pebax and fabrication of more amorphous top-layer and more porous sub-layer. These results are confirmed by decrement in the mechanical properties of Pebax/IL casting solutions and composite membranes, by IL increment.

A comprehensive overview of dual-layer composite membrane for air (O2/N2) separation

The separation of air (O2/N2) via a polymeric membrane has recently piqued the interest of academic researcher as well as the industrial sector. Because of its remarkable characteristics, the polymeric membrane has emerged as one of the innovative and fast growing technology. However, two major problems faced by membrane technology, which hinder its growth in the commercial sector are, 1): The trade-off between permeability and selectivity. 2): Maintaining physical and chemical structural stability in a long-term commercial scale process. Recent advancements in membrane material, structural, and process design have enabled the development of dual-layer composite (DLC) membranes. This concept combines the benefits of both thinner mixed matrix membranes (MMMs) based active layer and porous support substrate. Due to these properties, the membrane exhibits higher perm-selectivity with enhanced mechanical strength as compared to single layer polymeric membrane. This review article mainly focused on the developmental progress of DLC membrane throughout the years. In which membrane structural details, selection criteria, fabrication methodologies, application [e.g., air (O2/N2) separation] were critically reviewed. In addition, challenges arising in the DLC membrane production and future prospects for the development of these membranes were also thoroughly discussed in this literature. This creates a paradigm for future research in the commercial development of these membranes for the air (O2/N2) separation process, which can be utilized in both medical and industrial sectors. [Formula: see text]

Dual-layered composite nanofiber membrane with Cu-BTC-modified electrospun nanofibers and biopolymeric nanofibers for the removal of uremic toxins and its application in hemodialysis

In order to prepare a kind of hemodialysis membrane with both adsorption and dialysis abilities, we prepared a novel dual-layer composite nanofiber membrane by combining two different kinds of composite electrospun nanofibers. One layer of the membrane contains specially prepared polydopamine/polyacrylonitrile (PDA/PAN) composite nanofibers modified by self-assembly of Cu-BTC on the surface of the nanofibers, which exhibit good adsorption capacity for urea. The second layer of the membrane contains biopolymers of chitosan/sericin (CS/SS) composite nanofibers, which show a good adsorption capacity for creatinine. The obtained dual-layer nanofiber membranes possess excellent blood compatibility. The maximum adsorption capacity of the dual-layer composite nanofiber membrane for creatinine is 100.50 mg/g, and the maximum adsorption capacity for urea can reach as high as 152.44 mg/g. In addition, a multi-layer flat-plate dialyzer was designed and manufactured for dialysis process. The prepared nanofiber membranes were used as a hemodialysis membrane for simulated dialysis tests. After 4 h of dialysis, the retention rate of bovine serum albumin (BSA) of the composite nanofiber membrane is 83.9%, the clearance rates on creatinine and urea are 82.3% and 92.8%, respectively. The prepared novel dual-layer membranes have the capacity to be utilized as a new adsorbent for hemodialysis applications.

Thickness Effect on Permeance of CO2/CH4 Gases in CA Coated PVDF Composite Membranes

The aim of this work is to study the effect of membrane thickness with respect to gas permeance and tensile strength. The influence of membrane thickness on gas permeation has received little attention to date. Single layer flat sheet membrane with average thickness of 25 μm and dual layer composite membranes with variable thickness of skin layer were fabricated by coating CA/ PEG selective layer on the polyvinylidene fluoride porous support. Permeation experiments were performed with CO2 and CH4 which revealed that permeance of CO2 was pronounced compared to CH4. Highest permeance of 0.87 gas permeation unit (GPU) was obtained at 4 bar with 19.4 μm skin layer. Fourier transform infra-red spectroscopy was used to study the existence of different functional groups in the membranes. Strength of the membranes was analyzed using tensile testing machine. Scanning electron microscopy was used to study the surface structure and morphology. It was found that by reducing the membrane thickness, the permeance of CO2 and CH4 increased without compromising on membrane strength.

Fabrication of CA/PEG/MWCNTs composite membranes for enhanced CO<SUB align="right">2/CH<SUB align="right">4 separation

Gas separation process through membrane technology has greatly improved greenhouse effect. Membrane technology is considered important for gas separation. The aim of this work was to study the effect of membrane thickness with respect to gas permeance and tensile strength. Single layer flat sheet membrane with thickness of 45 μm and dual layer composite membranes with variable thickness were fabricated by coating cellulose acetate/poly-ethylene glycol/multi-walled carbon nanotubes selective layer on the polyvinyl alcohol (PVA) porous support. Fourier transform infrared (FT-IR) spectroscopy was utilised to study the existence of different organic groups in the membranes. The crystal structures of MWCNTs in the membranes were characterised by X-ray diffraction (XRD). Tensile testing machine was used to analyse the maximum strength of the prepared membranes. Scanning electron microscopy (SEM) was used to study the surface structure and morphology of the composite membranes. The reduction in the thickness of the selective layer up to 8 μm resulted in a maximum CO2 permeance of 0.92 GPU. It was concluded that by reducing the membrane thickness the gas permeance increases.

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.

Fabrication of composite membrane with adsorption property and its application to the removal of endocrine disrupting compounds during filtration process

A novel dual-layer composite membrane, which has integrated filtration and adsorption, was prepared from polyvinylidene fluoride (PVDF) membrane and functionalized polypropylene (PP) non-woven fabric (or PP-g-SA-HEA) support. Stearyl acrylate (SA) and hydroxyethyl acrylate (HEA) were first grafted onto the original PP non-woven fabric using ultraviolet (UV) irradiation graft polymerization. The resulting functionalized PP non-woven fabric was then coated with PVDF membrane via phase inversion. The characterization showed that the surface density and wettability of PP-g-SA-HEA varied with total amount and proportion of the grafted SA and HEA. At appropriate degree of grafting, the composite membrane formed a clear dual layers structure without any delamination and had enhanced mechanical properties. The dual-layer composite membrane was applied in filtration to remove low-concentration endocrine disrupting compounds (EDCs) from water. The results showed that the membrane was able to effectively remove bisphenol A (BPA) and other EDCs by adsorbing onto the PP-g-SA-HEA support layer. Compared to the single layer PVDF membrane, the filtration performance of the dual-layer composite membrane was not affected. In addition, the used membranes could be simply recovered by washing with mixed deionized water–ethanol solution.

Hydrophilic surface coating on hydrophobic PTFE membrane for robust anti-oil-fouling membrane distillation

The hydrophobic membranes used in membrane distillation (MD) are inherently prone to fouling by hydrophobic contaminants due to the long-range hydrophobic-hydrophobic interaction, which limits the application of this promising technology. In this study, a novel dual-layer composite membrane was fabricated through eletrospinning a hydrophilic poly(vinyl alcohol) (PVA) coating on a commercial hydrophobic polytetrafluoroethylene (PTFE) membrane and then cross-linking with glutaraldehyde for robust anti-oil-fouling MD. The prepared composite membrane showed an asymmetric wettability, the coating surface was underwater oleophobic with underwater oil contact angle of 148.7° and the PTFE substrate surface was hydrophobic with in-air water contact angle of 134.5°. The test results of the adhesive force between an oil droplet and membrane surface exhibited that the fabricated membrane had a desirable surface to resist oil adhesion. Direct contact MD experiments were conducted using a saline emulsion with 1000 mg/L crude oil to compare the performance of the modified membrane and the pristine PTFE membrane. The results demonstrated that the modified membrane can present stable performance with robust resistance to oil-fouling compared to the pristine PTFE membrane. It is believed that the fabricated composite membrane has great potential to be applied in MD process with high concentration of hydrophobic organics.

A novel dual-layer composite membrane with underwater-superoleophobic/hydrophobic asymmetric wettability for robust oil-fouling resistance in membrane distillation desalination

In this study, we developed a new type of composite membrane to mitigate oil fouling in membrane distillation (MD). The composite membrane consists of a polytetrafluoroethylene (PTFE) hydrophobic substrate and a hydrophilic poly(vinyl alcohol)/silica nanoparticles (PVA-Si) hybrid fibrous coating prepared via sol-gel and electrospinning. We characterized the pristine PTFE membrane and the modified membrane using contact angle measurements and tensiometer-based oil probe force spectroscopy. While the hydrophilic coating presented excellent oil fouling resistance, the glutaraldehyde cross-linking treatment augmented the in-air hydrophilicity and underwater superoleophobicity of the electrospun coating surface. The cross-linking treatment also prevented the formation of PVA-Si hydrogels for the electrospun hybrid fibers, which made availability to utilize the water-soluble polymer to modify hydrophobic membrane for anti-oil-fouling MD. By comparing the performance of the composite membrane and the pristine PTFE membrane in MD desalination experiments using a saline emulsion with 1000mg/L crude oil, it showed that the fabricated composite membrane was significantly more resistant to oil-fouling compared to the PTFE membrane. The results from this study suggest that underwater superoleophobic coating can effectively mitigate oil fouling in MD, and that the fabricated composite membrane can enable MD to desalinate hypersaline wastewater with high concentrations of hydrophobic contaminants.

Facile Fabrication of Composite Membranes with Dual Thermo- and pH-Responsive Characteristics.

Facile fabrication of novel functional membranes with excellent dual thermo- and pH-responsive characteristics has been achieved by simply designing dual-layer composite membranes. pH-Responsive poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers and polystyrene blended with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) nanogels are respectively used to construct the top layer and bottom layer of composite membranes. The stretching/coiling conformation changes of the P4VP chains around the pKa (∼3.5-4.5) provide the composite membranes with extraordinary pH-responsive characteristics, and the volume phase transitions of PNIPAM nanogels at the pore/matrix interfaces in the bottom layer around the volume phase transition temperature (VPTT, ∼33 °C) provide the composite membranes with great thermoresponsive characteristics. The microstructures, permeability performances, and dual stimuli-responsive characteristics can be well tuned by adjusting the content of PNIPAM nanogels and the thickness of the PS-b-P4VP top layer. The water fluxes of the composite membranes can be changed in order of magnitude by changing the environment temperature and pH, and the dual thermo- and pH-responsive permeation performances of the composite membranes are satisfactorily reversible and reproducible. The membrane fabrication strategy in this work provides valuable guidance for further development of dual stimuli-responsive membranes or even multi stimuli-responsive membranes.

Fabrication of hydrophobic/hydrophilic composite hollow fibers for DCMD: Influence of dope formulation and external coagulant

Hydrophobic/hydrophilic dual-layer composite membranes were prepared for DCMD in this research. The polymers for constructing the main framework in both layers were PVDF. PVA was blended with PVDF to enhance hydrophilicity of inner layer. The effect of outer layer dope formulation and external coagulant on membrane structures and separation properties was investigated. It was found that fibers with the less PVA in the inner layer generally exhibited higher and more stable salt rejections, regardless of the conditions of the outer layer formation. For fibers with the higher PVA content in the inner layer, however, the variation of both flux and rejection was significant with changing outer layer fabrication conditions; i.e. the fibers prepared with no additive in its outer layer and using water as external coagulant led to negative flux and null rejection. Adding ethanol into the outer layer dope or replacing water with water/ethanol in external coagulant resulted in remarkably better membrane performance. The difference was attributed to the larger macrovoids developed in the membrane of poorer performance. In addition, the membrane with more hydrophilic inner layer could possess higher flux than those with the same thickness but a more hydrophobic inner structure.

Highly porous PVDF hollow fiber membranes for VMD application by applying a simultaneous co-extrusion spinning process

Highly porous polyvinylidene fluoride (PVDF) hollow fiber membranes are of great interest for membrane contactor applications such as sea water desalination by membrane distillation in order to enhance the flux of the process. For the first time in this paper, porous hydrophobic PVDF asymmetric hollow fiber membranes with a high outer surface porosity for use in vacuum membrane distillation (VMD) were successfully fabricated by applying a simultaneous co-extrusion spinning process through the non-solvent induced phase separation (NIPS) method. In this approach, five kinds of dual-layer composite membranes were prepared by employing different inner and outer polymer dopes. Single-layer membrane was also fabricated through the conventional dry-jet wet phase inversion method. Results revealed that the introduction of the simultaneous co-extrusion spinning process greatly increased the outer surface porosity of the dual-layer composite membranes. It was also observed that the VMD flux of the dual-layer membranes spun through the simultaneous co-extrusion spinning process was 54% higher than the single-layer membrane fabricated by the conventional dry-jet wet phase inversion method. A VMD flux as high as 61.8kg/(m2h) at 70°C was obtained for the novel dual-layer composite membranes.

New insights into fabrication of hydrophobic/hydrophilic composite hollow fibers for direct contact membrane distillation

Dual-layer composite membrane is a new design for direct contact membrane distillation (DCMD) with membrane performance potentially superior to that of single layer porous membranes. Using dry–wet phase inversion technology, novel dual-layer hollow fiber membranes were fabricated in current research. The outer layer was made from polyvinylidene fluoride (PVDF) with polyvinylpyrolidone (PVP) or glycerol as non-solvent additive, while the inner layer consists of PVDF and polyvinyl alcohol (PVA) blend. The effect of the nonsolvent additive type in the outer layer and that of PVA/PVDF blending ratio in the inner layer on the morphological, mechanical and separation characteristics of the composite membranes was investigated. Membrane performance was further correlated to the physicochemical and morphological characteristics of the membranes. In particular, a thorough investigation of pore wetting in DCMD was attempted for the first time in this work, observing the cross-sectional distribution of EDX chlorine signals as an indication of the penetration of sodium chloride solution into the pore from the feed.