Fabrication and characterization of novel asymmetric polyvinylidene fluoride (PVDF) membranes by the nonsolvent thermally induced phase separation (NTIPS) method for membrane distillation applications

Development of blend PEG-PES/NMP-DMF mixed matrix membrane for CO2/N2 separation

The carbon dioxide (CO2) separation technology has become a focus recently, and a developed example is the membrane technology. It is an alternative form of enhanced gas separation performance above the Robeson upper bound line resulting in the idea of mixed matrix membranes (MMMs). With attention given to membrane technologies, the MMMs were fabricated to have the most desirable gas separation performance. In this work, blend MMMs were synthesised by using two polymers, namely, poly(ether sulfone) (PES) and poly (ethylene glycol) (PEG). These polymers were dissolved in blend N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvents with the functionalised multi-walled carbon nanotubes (MWCNTs-F) fillers by using the mixing solution method. The embedding of the pristine MWCNTs and MWCNTs-F within the new synthesised MMM was then studied towards CO2/N2 separation. In addition, the optimisation of the loading of MWCNTs-F for blend MMM for CO2/N2 separation was also studied. The experimental results showed that the functionalised MWCNTs (MWCNTs-F) were a better choice at enhancing gas separation compared to the pristine MWCNTs (MWCNTs-P). Additionally, the effects of MWCNTs-F at loadings 0.01 to 0.05% were studied along with the polymer compositions for PES:PEG of 10:20, 20:20 and 30:10. Both these parameters of study affect the manner of gas separation performance in the blend MMMs. Overall, the best performing membrane showed a selectivity value of 1.01 + 0.05 for a blend MMM (MMM-0.03F) fabricated with 20 wt% of PES, 20 wt% of PEG and 0.03 wt% of MWCNTs-F. The MMM-0.03F was able to withstand a pressure of 2 bar, illustrating its mechanical strength and ability to be used in the post combustion carbon capture application industries where the flue gas pressure is at 1.01 bar.

Thermodynamics and kinetic analysis of membrane: Challenges and perspectives

The ultimate structure of the membrane is determined using two important effects: (i) thermodynamic effect and (ii) kinetic effect. Controlling the mechanism of kinetic and thermodynamic processes in phase separation is essential for enhancing membrane performance. However, the relationship between system parameters and the ultimate membrane morphology is still largely empirical. This review focuses on the fundamental ideas behind thermally induced phase separation (TIPS) and nonsolvent-induced phase separation (NIPS) methods, including both kinetic and thermodynamic elements. The thermodynamic approach to understanding phase separation and the effect of different interaction parameters on membrane morphology has been discussed in detail. Furthermore, this review explores the capabilities and limitations of different macroscopic transport models used for the last four decades to explore the phase inversion process. The application of molecular simulations and phase field to understand phase separation has also been briefly examined. Finally, it discusses the thermodynamic approach to understanding phase separation and the consequence of different interaction parameters on membrane morphology, as well as possible directions for artificial intelligence to fill the gaps in the literature. This review aims to provide comprehensive knowledge and motivation for future modeling work for membrane fabrication via new techniques such as nonsolvent-TIPS, complex-TIPS, non-solvent assisted TIPS, combined NIPS-TIPS method, and mixed solvent phase separation.

Effect of spraying polyvinyl alcohol solution on the surface of liquid film on the structure and antifouling properties of polyvinylidene membrane

Polyvinylidene fluoride (PVDF) membrane was prepared by a two-step method of surface gelation-immersion precipitation. The surface of the scraping solution film was sprayed with polyvinyl alcohol (PVA) aqueous solution to gel the film surface first, and then the liquid film was immersed in a coagulation bath for phase transformation to obtain the surface modified PVDF membrane. The effects of PVA solution with different mass fraction on the structure and properties of PVDF membrane were studied. The results showed that with the increase of spraying PVA mass fraction, the contact angle of the upper surface of PVDF membranes gradually decreased, the porosity and mean pore size of PVDF membranes gradually increased, and the crystallinity of PVDF membranes gradually decreased. The pores on upper surface of the membranes first increased and then decreased, and the finger-like macropore structure appeared inside the membranes, the water flux first increased and then decreased, and the rejection rate increased. Dynamic cycle filtration experiment of bovine serum protein (BSA) solution showed an increase in membrane flux recovery from 68.31 to 95.08% and a decrease in the irreversible fouling rate from 31.69 to 4.92%, indicating an improvement in the hydrophilicity and antifouling properties of the modified PVDF membrane.

Challenges, Opportunities and Future Directions of Membrane Technology for Natural Gas Purification: A Critical Review

Natural gas is an important and fast-growing energy resource in the world and its purification is important in order to reduce environmental hazards and to meet the required quality standards set down by notable pipeline transmission, as well as distribution companies. Therefore, membrane technology has received great attention as it is considered an attractive option for the purification of natural gas in order to remove impurities such as carbon dioxide (CO₂) and hydrogen sulphide (H₂S) to meet the usage and transportation requirements. It is also recognized as an appealing alternative to other natural gas purification technologies such as adsorption and cryogenic processes due to its low cost, low energy requirement, easy membrane fabrication process and less requirement for supervision. During the past few decades, membrane-based gas separation technology employing hollow fibers (HF) has emerged as a leading technology and underwent rapid growth. Moreover, hollow fiber (HF) membranes have many advantages including high specific surface area, fewer requirements for maintenance and pre-treatment. However, applications of hollow fiber membranes are sometimes restricted by problems related to their low tensile strength as they are likely to get damaged in high-pressure applications. In this context, braid reinforced hollow fiber membranes offer a solution to this problem and can enhance the mechanical strength and lifespan of hollow fiber membranes. The present review includes a discussion about different materials used to fabricate gas separation membranes such as inorganic, organic and mixed matrix membranes (MMM). This review also includes a discussion about braid reinforced hollow fiber (BRHF) membranes and their ability to be used in natural gas purification as they can tackle high feed pressure and aggressive feeds without getting damaged or broken. A BRHF membrane possesses high tensile strength as compared to a self-supported membrane and if there is good interfacial bonding between the braid and the separation layer, high tensile strength, i.e., upto 170Mpa can be achieved, and due to these factors, it is expected that BRHF membranes could give promising results when used for the purification of natural gas.

Synthesis of a biomimetic zwitterionic pentapolymer to fabricate high-performance PVDF membranes for efficient separation of oil-in-water nano-emulsions

Oily wastewater from industries has an adverse impact on the environment, human and aquatic life. Poly(vinylidene fluoride) (PVDF) membrane modified with a zwitterionic/hydrophobic pentapolymer (PP) with controlled pore size has been utilized to separate oil from water from their nano-emulsions. The PP has been synthesized in 91% yield via pentapolymerization of four different diallylamine salts [(CH2=CHCH2)2NH+(CH2)x A-], bearing CO2-, PO3H-, SO3-, (CH2)12NH2 pendants, and SO2 in a respective mol ratio of 25:36:25:14:100. Incorporating PP into PVDF has shown a substantially reduced membrane hydrophobicity; the contact angle decreased from 92.5° to 47.4°. The PP-PVDF membranes have demonstrated an excellent capability to deal with the high concentrations of nano-emulsions with a separation efficiency of greater than 97.5%. The flux recovery ratio (FRR) of PP-5 incorporated PVDF membrane was about 82%, which was substantially higher than the pristine PVDF.

A Facile Method to Control Pore Structure of PVDF/SiO2 Composite Membranes for Efficient Oil/Water Purification

The use of poly(vinylidene fluoride) (PVDF) microfiltration (MF) membranes to purify oily water has received much attention. However, it is challenging to obtain high-performance PVDF microfiltration membranes due to severe surface fouling and rapid decline of permeability. This study explored a new approach to fabricate high-performance PVDF/silica (SiO₂) composite membrane via the use of a polymer solution featuring lower critical solution temperature (LCST) characteristics and the non-solvent thermally induced phase separation method (NTIPS). Coupling with morphological observations, the membrane formation kinetics were analyzed in depth to understand the synergistic effect between the LCST solution properties and fabrication conditions in NTIPS. Utilizing such a synergistic effect, the transition from finger-like macrovoid pores to bi-continuous highly connected pores could be flexibly tuned by increasing the PVDF concentration and the weight ratio of SiO₂/PVDF in the dope solution and by raising the coagulation temperature to above the LCST of the solution. The filtration experiments with surfactant-stabilized oil-water emulsion showed that the permeation flux of the PVDF/SiO₂ composite membranes was higher than 318 L·m^-2·h^-1·bar^-1 and the rejection above 99.2%. It was also shown that the PVDF/SiO₂ composite membranes, especially those fabricated above the LCST, demonstrated better hydrophilicity, which resulted in significant enhancement in the anti-fouling properties for oil/water emulsion separation. Compared to the benchmark pure PVDF membrane in oily water purification, the optimal composite membrane T70 was demonstrated via the 3-cycle filtration experiments with a significantly improved flux recovery ratio (F_rr) and minimal reduced irreversible fouling (R_ir). Overall, with the developed method in this work, facile procedure to tune the membrane morphology and pore structure was demonstrated, resulting in high performance composite membranes suitable for oil/water emulsion separation.

A Prospective Concept on the Fabrication of Blend PES/PEG/DMF/NMP Mixed Matrix Membranes with Functionalised Carbon Nanotubes for CO2/N2 Separation

With an ever-increasing global population, the combustion of fossil fuels has risen immensely to meet the demand for electricity, resulting in significant increase in carbon dioxide (CO₂) emissions. In recent years, CO₂ separation technology, such as membrane technology, has become highly desirable. Fabricated mixed matrix membranes (MMMs) have the most desirable gas separation performances, as these membranes have the ability to overcome the trade-off limitations. In this paper, blended MMMs are reviewed along with two polymers, namely polyether sulfone (PES) and polyethylene glycol (PEG). Both polymers can efficiently separate CO₂ because of their chemical properties. In addition, blended N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF) solvents were also reviewed to understand the impact of blended MMMs’ morphology on separation of CO₂. However, the fabricated MMMs had challenges, such as filler agglomeration and void formation. To combat this, functionalised multi-walled carbon nanotube (MWCNTs-F) fillers were utilised to aid gas separation performance and polymer compatibility issues. Additionally, a summary of the different fabrication techniques was identified to further optimise the fabrication methodology. Thus, a blended MMM fabricated using PES, PEG, NMP, DMF and MWCNTs-F is believed to improve CO₂/nitrogen separation.

Formation of Polysulfone Hollow Fiber Membranes Using the Systems with Lower Critical Solution Temperature

This study deals with the investigation of the phase state of the polymer systems from polysulfone (PSF) with the addition of polyethylene glycol (PEG-400, Mn = 400 g·mol−1) and polyvinylpyrrolidone (PVP K-30, Mn = 40,000 g·mol−1) in N,N-dimethylacetamide (DMA), which feature lower critical solution temperatures (LCSTs). A fragment of the phase state diagram of the system PSF —PEG-400—PVP K-30—DMA was experimentally constructed in the following range of component concentrations: PSF 20–24 wt.%, PEG-400—35–38 wt.% and PVP—0–8 wt.%. It has been established that PVP addition substantially reduces the phase separation temperature down to 50–60 °C. Based on the obtained phase diagrams, a method for preparation of highly permeable hollow fiber membranes from PSF, which involves the processing of the dope solution at a temperature close to the LCST and the temperature of the bore fluid above the LCST, was proposed. Hollow fiber membranes with pure water flux of 1200 L·m−2·h−1 and a sponge-like macrovoid-free structure were obtained via LCST-thermally induced phase separation by free fall spinning technique.

The prospect of synthesis of PES/PEG blend membranes using blend NMP/DMF for CO2/N2 separation

Carbon dioxide (CO2) emissions have been the root cause for anthropogenic climate change. Decarbonisation strategies, particularly carbon capture and storage (CCS) are crucial for mitigating the risk of global warming. Among all current CO2 separation technologies, membrane separation has the biggest potential for CCS as it is inexpensive, highly efficient, and simple to operate. Polymeric membranes are the preferred choice for the gas separation industry due to simpler methods of fabrication and lower costs compared to inorganic or mixed matrix membranes (MMMs). However, plasticisation and upper-bound trade-off between selectivity and permeability has limited the gas separation performance of polymeric membranes. Recently, researchers have found that the blending of glassy and rubbery polymers can effectively minimise trade-off between selectivity and permeability. Glassy poly(ethersulfone) (PES) and rubbery poly(ethylene) glycol (PEG) are polymers that are known to have a high affinity towards CO2. In this paper, PEG and PES are reviewed as potential polymer blend that can yield a final membrane with high CO2 permeance and CO2/nitrogen (N2) selectivity. Gas separation properties can be enhanced by using different solvents in the phase-inversion process. N-Methyl-2-Pyrrolidone (NMP) and Dimethylformamide (DMF) are common industrial solvents used for membrane fabrication. Both NMP and DMF are reviewed as prospective solvent blend that can improve the morphology and separation properties of PES/PEG blend membranes due to their effects on the membrane structure which increases permeation as well as selectivity. Thus, a PES/PEG blend polymeric membrane fabricated using NMP and DMF solvents is believed to be a major prospect for CO2/N2 gas separation.

A Mini Review on Antiwetting Studies in Membrane Distillation for Textile Wastewater Treatment

The textile industry is an important contributor to the growth of the global economy. However, a huge quantity of wastewater is generated as a by-product during textile manufacturing, which hinders the ongoing development of textile industry in terms of environmental sustainability. Membrane distillation (MD), which is driven by thermal-induced vapor pressure difference, is being considered as an emerging economically viable technology to treat the textile wastewater for water reuse. So far, massive efforts have been put into new membrane material developments and modifications of the membrane surface. However, membrane wetting, direct feed solution transport through membrane pores leading to the failure of separation, remains as one of the main challenges for the success and potential commercialization of this separation process as textile wastewater contains membrane wetting inducing surfactants. Herein, this review presents current progress on the MD process for textile wastewater treatment with particular focuses on the fundamentals of membrane wetting, types of membranes applied as well as the fabrication or modification of membranes for anti-wetting properties. This article aims at providing insights in membrane design to enhance the MD separation performance towards commercial application of textile wastewater treatment.

Novel poly(arylene ether ketone)/poly(ethylene glycol)-grafted poly(arylene ether ketone) composite microporous polymer electrolyte for electrical double-layer capacitors with efficient ionic transport

Polymer electrolytes have attracted considerable research interest due to their advantages of shape control, excellent safety, and flexibility.

Impacts of Sulfuric Acid on the Stability and Separation Performance of Polymeric PVDF-Based Membranes at Mild and High Concentrations: An Experimental Study

This study investigated the effects of an aqueous acidic solution at typical concentrations on polymeric polyvinylidene fluoride (PVDF)-based membranes. Flat-sheet PVDF-based membranes were completely embedded in sulfuric acid at varying concentrations. The effect of the acid concentration after a prolonged exposure time on the chemical, mechanical and physical properties of the membrane were checked via FE-SEM, EDX (Energy-Dispersive Spectrometer), FTIR, XRD, tensile strength, zeta potential, contact angle, porosity, pure water flux measurement and visual observation. The result reveals prompt initiation of reaction between the PVDF membrane and sulfuric acid, even at a mild concentration. As the exposure time extends with increasing concentration, the change in chemical and mechanical properties become more pronounced, especially in the morphology, although this was not really noticeable in either the crystalline phase or the functional group analyses. The ultimate mechanical strength decreased from 46.18 ± 0.65 to 32.39 ± 0.22 MPa, while the hydrophilicity was enhanced due to enlargement of the pores. The flux at the highest concentration and exposure period increased by 2.3 times that of the neat membrane, while the BSA (Bovine Serum Albumin) rejection dropped by 55%. Similar to in an alkaline environment, the stability and performance of the PVDF-based membrane analyzed in this study manifested general deterioration.

Fabrication of a sandwiched silicalite-1 membrane in a 2D confined space for enhanced alcohol/water separation

A dense zeolite layer with a thickness of approximately 500 nm was demonstrated by a confined-space strategy in a sandwiched mode of (SiO2)/(silicalite-1)/(SiO2). The gel-free secondary growth methodology bypasses the post-calcination step, avoiding excess energy consumption and possible film damage. Significantly enhanced pervaporation separation was observed with separation factors of 136 and 113, and fluxes of 2.3 and 2.2 kg m-2 h-1 for ethanol/n-butanol aqueous solutions, respectively. In addition, the membrane stability was confirmed by the 14 day pervaporation test.

Preparation and characterization of PVDF/CaCO3 composite membranes etched by hydrochloric acid

This study aimed to improve the pore size, porosity, and hydrophobicity of polyvinylidene fluoride (PVDF) membranes for desalination by vacuum membrane distillation (VMD). New membranes were prepared via etching PVDF/calcium carbonate (CaCO₃) composite membranes using hydrochloric acid (HCl), depending on the chemical reaction of CaCO₃ and HCl. Etched membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), contact angle (CA), atomic force microscope (AFM), and scanning electron microscopy (SEM). The results showed that CaCO₃ of composite membranes was completely reacted by 1.5 mol/L HCl after composite membranes had been etched 90 min. The crystallinity of etched membranes was the same as that of PVDF/CaCO₃ composite membranes, and no new functional groups appeared in etched membranes, which indicated that etched membranes had good chemical stability. The surface roughness increased and led to the increase of contact angle, which means the hydrophobicity of etched membranes was enhanced. As a result, the increment of permeation flux had been improved in a VMD process. It was found that the maximum flux of etched membrane was enhanced and up to 1.65 times of composite membrane when the concentration of sodium chloride (NaCl) solution was 5.0 wt%, and the maximum flux reached up to 30.9 kg m^-2 h^-1.

Adsorptive nanocomposite membranes for heavy metal remediation: Recent progresses and challenges

Heavy metal contamination in aqueous system has attracted global attention due to the toxicity and carcinogenicity effects towards living bodies. Among available removal techniques, adsorptive removal by nanosized materials such as metal oxide, metal organic frameworks, zeolite and carbon-based materials has attracted much attention due to the large active surface area, large number of functional groups, high chemical and thermal stability which led to outstanding adsorption performance. However, the usage of nanosized materials is restricted by the difficulty in separating the spent adsorbent from aqueous solution. The shift towards the use of adsorptive composite membrane for heavy metal ions removal has attracted much attention due to the synergistic properties of adsorption and filtration approaches in a same chamber. Thus, this review critically discusses the development of nanoadsorbents and adsorptive nanocomposite membranes for heavy metal removal over the last decade. The adsorption mechanism of heavy metal ions by the advanced nanoadsorbents is also discussed using kinetic and isotherm models. The challenges and future prospect of adsorptive membrane technology for heavy metal removal is presented at the end of this review.

A review of membrane development in membrane distillation for emulsified industrial or shale gas wastewater treatments with feed containing hybrid impurities

Investigations on membrane materials for membrane distillation (MD) and its applications have been ongoing since the 1990s. However, a lack of materials that produce robustly stable and up-to-the-mark membranes for MD for different industrial applications remains an ongoing problem. This paper provides an overview of materials developed for MD applications. Although key aspects of published articles reviewed in this paper pertain to MD membranes synthesized for desalination, future MD can also be applied to organic wastewater containing surfactants with inorganic compounds, either with the help of hybrid treatment processes or with customized membrane materials. Many industrial discharges produce effluents at a very high temperature, which is an available driving force for MD. However, there remains a lack of cost-effective membrane materials. Amphiphobic and omniphobic membranes have recently been developed for treating emulsified and shale gas produced water, but the problem of organic fouling and pore wetting remains a major challenge, especially when NaCl and other inorganic impurities are present, which further deteriorate separation performance. Therefore, further advancements in materials are required for the treatment of emulsified industrial wastewater containing surfactants, salts, and for oil or shale gas wastewater for its commercialized reuse. Integrated MD systems, however, may represent a major change in shale gas wastewater and emulsified wastewater that are difficult to treat.

A Review on Porous Polymeric Membrane Preparation. Part I: Production Techniques with Polysulfone and Poly (Vinylidene Fluoride)

Porous polymeric membranes have emerged as the core technology in the field of separation. But some challenges remain for several methods used for membrane fabrication, suggesting the need for a critical review of the literature. We present here an overview on porous polymeric membrane preparation and characterization for two commonly used polymers: polysulfone and poly (vinylidene fluoride). Five different methods for membrane fabrication are introduced: non-solvent induced phase separation, vapor-induced phase separation, electrospinning, track etching and sintering. The key factors of each method are discussed, including the solvent and non-solvent system type and composition, the polymer solution composition and concentration, the processing parameters, and the ambient conditions. To evaluate these methods, a brief description on membrane characterization is given related to morphology and performance. One objective of this review is to present the basics for selecting an appropriate method and membrane fabrication systems with appropriate processing conditions to produce membranes with the desired morphology, performance and stability, as well as to select the best methods to determine these properties.

One-step tailoring surface roughness and surface chemistry to prepare superhydrophobic polyvinylidene fluoride (PVDF) membranes for enhanced membrane distillation performances

Superhydrophobic polyvinylidene fluoride (PVDF) membrane is a promising material for membrane distillation. Existing approaches for preparing superhydrophobic PVDF membrane often involve separate manipulation of surface roughness and surface chemistry. Here we report a one-step approach to simultaneously manipulate both the surface roughness and surface chemistry of PVDF nanofibrous membranes for enhanced direct-contact membrane distillation (DCMD) performances. The manipulation was realized in a unique solvent-thermal treatment process, during which a treatment solution containing alcohols was involved. We demonstrate that by using different chain-length alcohols in the treatment solvent, surface roughness can be promoted by creating nanofin structures on the PVDF nanofibers using an alcohol which has moderate affinity with PVDF. Meanwhile, surface chemistry can be tuned by adjusting the fraction distribution of crystal phases (nonpolar α phase and polar β phase) in the membrane using different alcohols. PVDF membranes with different surface wettabilities were used to evaluate the effects of surface roughness and surface energy on the DCMD performances. Combining both low surface energy and multi-scale surface roughness, pentanol-treated PVDF membrane achieved best anti-water property (water contact angle of 164.1° and sliding angle of 8.1°), and exhibited superior water flux and enhanced anti-wetting ability to low-surface-tension feed in the DCMD application.

Waterproof breathable layers - A review

Waterproof breathable layers (WPBLs) can be classified into two large groups of hydrophilic nonporous and hydrophobic porous layers. These layers (e.g., fabrics, films, membranes, and meshes) can be produced by various continuous and non-continuous processes such as coating, laminating, film stretching, casting, etc. The most common methods for production, characterization, and testing of WPBLs are presented and discussed in light of recent publications. The materials with high level of waterproofness and breathability are often used in outerwear for winter sports, sailing apparel, raincoats, military/police jackets, backpacks, tents, cargo raps, footwear and etc. WPBLs can also be used for other specialized applications such as membrane distillation, oil-water filtration, and wound dressing. These applications are discussed by presenting several good examples. The main challenge in the production of these layers is to compromise between waterproofness and breathability with opposing nature. The related research gaps, challenges, and future outlook are highlighted to shed more light on the topic.

Fabrication of a biconnected structure PVB porous heddle via thermally induced phase separation

Herein, a porous heddle of poly(vinyl butyral) (PVB) was successfully prepared by thermally induced phase separation with PEG400. A phase diagram of PVB was presented, and the effects of various parameters, such as polymer concentration, extrusion temperature, quenching temperature and take-up speed, on the morphology and properties of the PVB porous heddle were investigated. The pore size and porosity of the heddle increase as the extrusion temperature increases. Furthermore, upon increasing the quenching temperature during the TIPS process, the pore size and mechanical properties decrease, whereas porosity increases. In addition, due to the substantially unchanged crystallinity of the PVB heddle, the tensile strength increases since porosity decreases with the increasing take-up speed. The porosity of the prepared PVB porous heddle reached up to 74.63% when the PVB concentration, the quenching temperature and the extrusion temperature were 20 wt%, 0 °C and 170 °C, respectively. Thus, this porous heddle exhibiting a biconnected structure and significant mechanical properties is promising in the field of porous carrier materials.