Nanostructured Polymer Membranes, Volume 1: Processing and Characterization

By Visakh P. M.

This book is intended to serve as a "one-stop" reference resource for important research accomplishments in the area of nanostructured polymer membranes and their processing and characterizations. It will be a very valuable reference source for university and college faculties, professionals, post-doctoral research fellows, senior graduate students, and researchers from R&D laboratories working in the area of polymer nanobased membranes. The various chapters are contributed by prominent researchers from industry, academia and government/private research laboratories across the globe and comprise an up-to-date record on the major findings and observations in the field.

• Language: English
• Publisher: Wiley
• Release date: Nov 18, 2016
• ISBN: 9781118831748

Book preview

Preface

Many recent research accomplishments in the area of polymer nanocomposite membrane materials are summarized in this book, Nanostructured Polymer Membranes: Processing and Characterizations. State-of-the-art on membrane technology and chemistry and new challenges being faced in the field are discussed. Among the topics reviewed are characterization of membranes; current techniques for the processing and characterization of ceramic and inorganic polymer membranes; supramolecular membranes; organic membranes and polymers for removal of pollutants; membranes for CO2 separation; polymer nanomembranes; liquid membranes; separation technology based on ionic liquid membranes; membrane distillation; and alginate-based membranes and films.

This book is intended to serve as a one-stop reference resource for important research accomplishments in the area of nanostructured polymer membranes and their processing and characterizations. It will be a very valuable reference source for university and college faculties, professionals, post-doctoral research fellows, senior graduate students, and researchers from R&D laboratories working in the area of polymer nanobased membranes. The various chapters are contributed by prominent researchers from industry, academia and government/private research laboratories across the globe and comprise an up-to-date record on the major findings and observations in the field.

Chapter 1 provides an overview of the techniques and processes detailed in later chapters, along with the state of art, new challenges and opportunities in the field. In chapter 2, principles and fundamentals of membrane separation are presented in addition to a description of different types of membrane processes (pressure-driven membrane methods and liquid membranes) and the chemical and physical methods for membrane modification. Chapter 3 provides fundamental ideas about all types of characterization techniques for polymer-based nanomembranes such as FTIR, Raman spectroscopy, X-ray spectroscopy, electron spectroscopy, atomic force microscopy mass spectrometry and surface hydrophilicity. The next chapter mainly concentrates on the preparation, characterization and applications of ceramic and inorganic polymer membranes. Chapter 5 gives an overview of supramolecular membranes, primarily focusing on polymeric membranes and mixed matrix membranes for gas separation applications. Other topics discussed in this chapter are membranes synthesized from self-assembly, hydrogen bonding, π-π stacking of block copolymer systems, small molecules, and nanoparticles; and a summary of recent research into gas membrane separations.

Chapter 6 explains organic membranes and polymers to remove pollutants. It provides fundamental aspects of membranes as well as processes, including membranes as electro-ultrafiltration, ultrafiltration coupled to ultrasound, flotation coupled to microfiltration, liquid-phase polymer-based retention and liquid surfactant membrane. The next chapter is essential for tracking the progress in membrane development. It is a comprehensive review of recent studies in CO2 separation using different technologies, CO2 permeation properties, and breakthroughs and challenges in developing efficient CO2 separation membranes. Chapter 8 reports the state-of-the-art on fabrication methods of polymeric nanomembranes according to their specific needs and illustrates the most useful materials, employing mostly glassy and rubbery polymers. Often, to enhance membrane properties or to prevent undesired behavior, the fabrication is followed by different kinds of surface treatments. The authors discuss recent investigations on mechanical, thermal and gas transport properties of nanomembranes that frequently reveal a different behavior with respect to the polymeric membranes of greater thickness. The next chapter presents an introduction to liquid membrane separation techniques such as emulsion liquid membranes, immobilized liquid membranes, salts liquid membranes, hollow fiber contained liquid membranes, bulk hybrid liquid membranes and bulk aqueous hybrid liquid membranes. The author of this chapter also discusses the theory behind liquid membranes, along with their material design, preparation, performance and stability, and their applications in the separation and removal of metal cations from a range of diverse matrices, gas separation, etc.

Chapter 10 provides an overview of the recent progress in separation technology based on ionic liquid membranes; moreover, it covers issues relevant to this technology such as methods of preparation, mechanisms of transport, stability and fields of application. Chapter 11 on membrane distillation provides comprehensive coverage of both the fundamentals and recent developments associated with the application, process design, and membrane fabrication in this field. The final chapter provides a comprehensive overview of general properties, recent developments, and applications of alginate-based films and membranes. Sodium alginate is water-soluble, nontoxic, biocompatible, biodegradable, reproducible, and can yield coherent films or membranes upon casting or solvent evaporation.

In conclusion, the editors would like to express their sincere gratitude to all the contributors of this book, whose excellent support made the successful completion of this venture possible. We are grateful to them for the commitment and sincerity they showed towards their contributions. Without their enthusiasm and support, the compilation of a book would not have been possible. We would like to thank all the reviewers who have taken their valuable time to make critical comments on each chapter. We also thank the publisher, John Wiley and Sons Ltd. and Scrivener Publishing, for recognizing the demand for a book on the increasingly important area of Nanostructured Polymer Membranes Processing and Characterization and handling such a new project, which many other publishers have yet to address.

Visakh. P. M.

Olga Nazarenko

September 2016

Chapter 1

Processing and Characterizations: State-of-the-Art and New Challenges

Visakh. P. M.

Research Assistant, Department of Ecology and Basic Safety, Tomsk Polytechnic University, Tomsk, Russia

Corresponding author: [e-mail]

Abstract

A brief account of various topics concerning the processing and characterization of nanostructured polymer membranes is presented in this chapter. The different topics that are discussed include membrane technology and chemical characterization of membranes; ceramic and inorganic polymer membranes preparation, characterization and applications; supramolecular membranes synthesis and characterizations; organic membranes and polymers to remove pollutants; membranes for CO2 separation; polymer nanomembranes; liquid membranes; recent progress in separation technology based on ionic liquid membranes; membrane distillation; and preparation, characterization and applications of alginate-based membranes and films.

Keywords: Nanostructured polymer membranes, membrane processing, membrane characterizations, supramolecular membranes, organic membranes, liquid membranes, separation technology, ionic liquid membranes

1.1 Membrane: Technology and Chemistry

Membranes are used in a broad range of applications such as protein fractionation, purification of drugs, separation of gaseous mixtures, sample simplification in analytical procedures, production of ultrapure water and wastewater treatment, among others [1–5]. The membrane can be defined as a selective barrier that allows some species to permeate the barrier while retaining others. Membrane can be symmetric or asymmetric membrane according to their macroscopic configuration. Thus, asymmetric membranes consist of two layers; the top one is a very thin dense layer and is commonly called the skin layer or active layer and determines the permeation properties. In particular, separation methods directed by pressure can be categorized into four major membrane processes: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [6, 7]. Liquid membrane processes are commonly identified as three main configuration types: bulk liquid membrane, surfactant liquid membrane (or emulsion liquid membrane) and supported liquid membrane. Membranes can be referred to as porous and dense, and this first classification permits defining the two main mass transport models through the membranes. In membrane processes, the retained or rejected species accumulate near the membrane surface and as a consequence concentration polarization is produced.

According to the transport mechanisms, the separation methods by liquid membrane can be divided into six basic mechanisms of transport: simple transport, simple transport with chemical reaction in strip solution, facilitated transport, coupled counter-transport, coupled co-transport and active transport. The range of materials used for nanofiltration and reverse osmosis membranes is much smaller than that used for microfiltration and ultrafiltration, and is limited to polymers. Membrane material is required to be resistant to operation conditions and suitable for specific application. In many cases, additives are added to membrane phase during the fabrication to increase the permeability or reduce the fouling. Inorganic membranes have high selectivity and high permeability as well as thermal, chemical and mechanical stability but the cost of these are very high in comparison with polymer membranes. Organic and inorganic membranes can be modified for different applications by changes in the material chemical properties or by changes of pore size [8]. The above can be accomplished using methods such as chemical oxidation, incorporation of additives into the membrane matrix, plasma treatment, classical organic reactions, polymer grafting, interpenetrating polymer network, surfactant modification, self-assembly of the nanoparticles, among others [9].

Plasma surface treatment usually refers to a plasma reaction that either results in modification of the molecular structure of the surface, or atomic substitution. For example, simple inert gas [10], nitrogen, or oxygen plasmas have been used to increase the surface hydrophilicity of membranes [11], and ammonia plasmas have successfully yielded functionalized polysulfone membranes [12]. There are several potential advantages for the use of enzymes in membrane modification. Currently, the pressure-driven membrane processes are widely used in water treatment, biotechnology, food industry, medicine, and other fields [13].

One of the main problems arising from the operation of the membrane units is membrane fouling, which seriously hampers the applications of membrane technologies [14]. New membrane modification methods have been proposed, including the modification of membrane surfaces via microswelling for fouling control in drinking water [15], hydrogel surface modification of reverse osmosis membranes [16], modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow batteries [17], modification of ultrafiltration membranes via interpenetrating polymer networks for removal of boron from aqueous solution [18], among others.

1.2 Characterization of Membranes

Membrane morphology characterization is one of the indispensable components of the field of membrane research. Physical and chemical properties of membranes can be characterized with different laboratory techniques. Several microscopic techniques, both electronic as scanning and transmission electron microscopies, and atomic, as atomic force microscopy, have been used to analyze the pore structure and pore size distribution of the membrane. Microscopy methods, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) or atomic force microscopy (AFM), are the most direct methods to characterize the membrane pore structure. SEM can be used in various pore size characterization studies to visually inspect pore sizes and shapes. The AFM has proven itself to be a useful and versatile tool in the field of surface characterization. Porometry measurements can also give information about the pore size distribution (PSD) of membrane surface area [19].

Gas adsorption is one of the most popular methods and is generally used for the surface characterization and structural properties of porous materials, allowing the determination of their surface area, pore volume, pore size distribution and adsorption energy distribution of polymer membranes. One of the most promising methods is permporometry, where a mixture of non-condensable gas and condensable vapor is fed to a porous membrane and the permeation rate of non-condensable gas is measured [20]. Fourier transform infrared (FTIR) spectroscopy is widely used in structural characterization of membrane surfaces. With recent advances in the technology, the instrument has become simplified and some of the problems are reduced [21]. Raman spectroscopy technique usually employs a laser source and the scattered light and analyzes in terms of wavelength, intensity and polarization. Raman scattering is capable of detecting elastic vibrations of an entire nanoparticle, therefore Raman scattering is good for detecting nanoparticles on the membrane surface [22].

Energy-dispersive X-ray spectroscopy (EDS) analysis can be helpful for both membrane characterization and foulant characterization. For example, Sile-Yüksel et al. [23] used EDS analysis to determine the location of silver nanoparticles in different polymer membrane matrix. Corneal et al. [24] coated tubular ceramic membranes with manganase oxide nanoparticles. They examined the coating layer using SEM-EDS. With the help of EDS analysis they observed that the manganase oxide nanoparticles were not just successfully placed on the surface but also penetrated the membrane matrix. Soffer et al. [25] used EDS analysis to show colloidal iron fouling on ultrafiltration membrane surface. Long-term fouling of a reverse osmosis membrane was examined by Melián-Martel et al. [26]. The measuring method must be adapted according to charge places whether it is on the surface or inside the pores.

1.3 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications

Liquid electrolytes are liquid state electrolyte used to conduct the electricity. However, these conventional liquid electrolytes possess several disadvantages such as leakages of corrosive solvent and harmful gas, electrolytic degradation of electrolyte, formation of lithium dendrite growth, poor dimensional and mechanical stabilities, slow evaporation due to the gel state of polymer electrolyte, low safety performances, narrow potential window, poor interfacial stability and reduction in thermal, electrical and electrochemical stabilities as well [27]. Ionic liquids also offer some fascinating advantages, such as excellent chemical, thermal and electrochemical stabilities, high ionic conductivity due to high ion concentration, good oxidative stability and superior ion mobility as well as high cohesive energy density [28].

Krawiec et al. found that the particle size of filler is a vital parameter to govern the conductivity of the polymer electrolytes. They reported that the conductivity of nanosized Al2O3 added polymer electrolytes was higher about an order of magnitude that that of micrometer-sized Al2O3. High surface area to volume ratio of nanoparticles has become a driving force in the development of nanotechnology in various research fields, especially in materials science. The small particle size of the fillers can improve the homogeneity in the sample and its electrochemical properties [29]. The higher conductivity of nanoscale filler compared to micro-sized filler is also attributed to the rapid formation of the space charge region between the grains [30].

Mica plays a role in reducing resin costs, enhancing processability and dissipating heat in exothermic thermosetting reaction. Other particulate fillers, such as graphite, carbon black, and aluminium flakes, are used to reduce mold shrinkage or to minimize the electrostatic charging. Electrochemical devices, especially batteries, show a wide range of electrical and electronic applications. These devices can not only be applied in portable electronic and personal communication devices, such as laptops, mobile phones, MP3 players, and PDAs, but also in hybrid electrical vehicles (EVs) and start–light–ignition (SLI), which serves as a traction power source for electricity [31]. The properties of the final alumina depend on the crystalline structure, morphology and microstructure of the polymorph. Therefore, many attempts have been studied with respect to their transformation mechanisms, changes in porosity, specific surface area, surface structure, chemical reactivity and the defect crystal structure of polymorph [32].

1.4 Supramolecular Membranes: Synthesis and Characterizations

Supramolecular chemistry has typically been focused within the inorganic field, with our understanding of porous silicas [33] leading to breakthroughs in electrochemical energy storage. New approaches have been designed and investigated to improve the membranes performance; this involves the incorporation of porous composite materials. Metal-organic frameworks (MOFs) are a class of supramolecular coordination polymers that have emerged in the literature over two decades ago, when they could be identified by single-crystal X-ray crystallography [34–38].

The MOF structures are obtained by a self-assembling process starting from metal ions that assemble together with linker molecules. MOFs are successfully synthesized from solvothermal reactions with metal and organic building blocks which are dissolved in organic solvents and heated up to 130 °C. In addition to the conventional heating used for solvothermal reactions, MOFs can be synthesized using electrochemistry, mechanochemistry and ultrasonic methods. Because MOFs can reversibly absorb carbon dioxide gas, they are promising materials for the selective capture of carbon from the atmosphere and flue gas. The large quadrupole moment of carbon dioxide molecules causes them to interact with the framework, increasing the uptake of the gas over other inert adsorbents such as zeolites. Polycrystalline thin films are made from direct synthesis where a bare substrate is used with the appropriate mother growth solution for the given MOF, heat treated as required for solvothermal synthesis. The method involves the metal and organic linker crossing a porous membrane and crystallizing at the interface [39].

Zeolites are widely used in industry for water purification, adsorbents, catalysts and gas separations. They are naturally found but can also be synthesized to incorporate a range of small inorganic and organic species. Supramolecular chemistry describes chemical systems comprising a number of assembled molecular subunits or components arranged in spatial organizations using noncovalent bonding like hydrogen bonding, metal coordination, and hydrophobicity. The first part of this chapter will focus on supramolecular chemistry concepts in polymeric membranes, followed by a short discussion on how metal coordination and host-guest chemistry play important roles in mixed-matrix membranes. Membranes fabricated via supramolecular chemistry are rarely reported for gas separations, and are more common for liquid separation or purification and filtration membranes. Polytrimethylsilylpropyne (PTMSP) membranes operate as size-selective membranes. Meanwhile, when PTMSP membranes are used to isolate hydrocarbons from mixtures containing condensable hydrocarbon vapors and permanent gases, these membranes operate in the reverse-selective mode [40]. Despite its unique property of high hydrocarbon/gas selectivity and permeability, PTMSP has apparently found no industrial applications. This is due in part to PTMSP being highly soluble in liquid hydrocarbons [41, 42].

Additive incorporation into polymer matrices remains one of the most common ways in which supramolecular chemistry is observed in membranes. For example, Merkel et al. reported that the incorporation of nonporous fumed silica nanoparticles into a PTMSP polymer matrix enhanced gas permeability [43]. Schmidt et al. used a bottom-up approach to form supramolecular nanofibers inside a scaffold to prepare stable polymer-microfiber/supramolecular-nanofiber composites for filter applications [44]. Upon solvent evaporation, and filtration over commercial microfiltration syringes, three-dimensional supramolecular networks were formed within cellulose acetate membranes that are suitable for inexpensive and fast water separations.

1.5 Organic Membranes and Polymers to Remove Pollutants

A membrane is a thin planar structure or interphase that separates two phases and permits mass transfer between the phases. Membranes can be classified into two main groups: (1) biological membranes and (2) artificial or synthetic membranes. The polymer membranes are the main type of membranes in the market because polymeric materials are easier to process and less expensive [45, 46].

The separation of various components of a mixture is related directly to their relative transport rate within the membrane, which is determined by their diffusivity and solubility in the membrane material. Ultrafiltration membranes are used in electrodialysis pretreatment, electrophoretic paint, cheese whey treatment, juice clarification recovery of textile sizing agents, separation of oil/water emulsion, water treatment, and reverse osmosis pretreatment. The permeate is the portion of the fluid that has passed through the membrane and the retentate, or concentrate, is the portion containing the constituents that have been rejected by the membrane [47–49].

For a membrane separation method to be denominated as a hybrid separation method, changes beyond the simple incorporation of a different configuration or the simple change in a sequence in a separation line should be incorporated. The main disadvantage of flotation lies in the fact that the removal efficiency may be reduced if some of the undesired substances are not sufficiently hydrophobic, thus remaining in the bulk dispersion or solution [50]. Consequently, in the flotation coupled with microfiltration, the solid particles are partially removed by flotation, while clean water is obtained from the membrane module. Fewer solid particles remaining in the dispersion are then deposited on the membrane’s surface, resulting in decreased membrane fouling [51, 52]. Fouling has a direct impact on operating costs because a large part of the energy consumption is required to overcome fouling resistance and for periodic cleaning operations [53]. Geckeler et al. carried out the first experimental advances and analytical applications related with this technique [54–56]. Later, many research groups worked on the evaluation and description of retention properties of different water-soluble polymers (WSPs) for environmental and analytical applications [57–65].

One of the most promising techniques used is the application of separation methods based on the membrane process [66, 67]. Membrane filtration easily allows this separation by means of the method known as the LPR technique [68–75]. Among these methods, the membranes are the most promising for the enrichment of several ions from solution and their separation, especially where very low arsenic is required. The value of retention in the system with regenerated cellulose membrane is different than that reported in the literature where poly(ethersulfone) membrane was used as a filter [76]. At the present time, new research is being directed at improving the properties of the membranes, which are being modified to be an active component during the separation process. Thus, modification of the membrane is not only directed at permeability or antifouling properties.

1.6 Membranes for CO2 Separation

Numerous methods have been used for the separation of CO2. These methods include adsorption with porous solids (e.g., activated carbon and zeolites), amine absorption cryogenic separation and membrane-based separations [77, 78]. Adsorption technology is also being used for CO2 separation using different types of adsorbents. The low energy consumption and environment friendly nature was the main reason and focus of a large number of research studies on membrane technology [79]. Membranes (which generally consist of a semipermeable, thin, polymeric film) allow selective and specific permeation of some molecules while retaining others [80, 81].

Permeability and selectivity are the two main criteria that must be achieved in a good membrane. Membrane systems give reductions of over 70% in size and about 66% in weight compared to conventional separation columns [82]. The highpurity CO2 separation may require numerous membranes with different characteristics, due to their limited ability to achieve high degrees of gas separation [83]. Gas separation membranes use the differences in partial pressure as their driving force for separation [84]. One component dissolves into the membrane, diffusing through the membrane before passing to the other side in the final stage [85].

Most of the research studies in membrane gas separation have been carried out on nonporous (dense) polymeric membranes. These membranes play an important role in gas separation. Different types of polymers have been used to develop dense polymeric membranes for CO2 separation from different gases, including polyimides, polysulfones, cellulose, and polycarbonates. Polyamides are one of the most extensively investigated polymeric materials for membrane gas separation since they possess very high CO2 permeability, mainly those incorporating the group 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane (6FDA). Polyimides have become widely used membrane materials for gas and vapor separation due to their excellent thermal, chemical and mechanical stability in addition to their high gas separation. Polysulfone (PSF) is considered to be one of the most widely studied polymeric membrane material for CO2 separation for several gas streams [86, 87].

Gas permeation properties of PSF blends have been extensively investigated because of their low cost, chemical stability and mechanical strength [88]. Mixed-matrix membranes (MMMs), or composite membranes, are well-known in polymeric membranes development for gas separation. These membranes incorporate an inorganic material in the form of micro- or nanoparticles, hence combining the ease of polymer processing with the efficient gas permeation of a molecular sieve [89, 90]. Incorporating an inorganic material into a polymeric membrane can serve as a molecular sieve that enhances the gas permeance through the membrane or as a barrier that reduces the gas permeability [91].

Guo and coworkers prepared polysulfone-based mixed-matrix membranes (MMMs) incorporated with amine-functionalized titanium-based metal organic framework. One of the growing processes in the development of membrane science is the supported ionic liquid membranes (SILMs) technology. Due to their special properties, e.g., high thermal and chemical stability and low vapor pressure, ILs have become an ideal alternative to conventional organic solvents in a wide range of chemical applications at lab scale, such as separation and purification and chemical catalysis [92–97].

1.7 Polymer Nanomembranes

To cite some of the most recent examples of polymeric nanostructured nanomembranes, researchers have found that they may act as molecular sieves [98] or humidity sensors in optical microcavity [99]; others have found large applicability in advanced biomedical applications [100]. Watanabe et al. found that polystyrene (PS) is not a satisfactory material to build robust nanomembrane with big size because it is not tough enough [101], thus suggesting that PS is not a suitable material for nanomembranes. A semiconducting polythiophene derivative like poly(3-thiophene methyl acetate) (P3TMA) has been blended with poly(tetramethylenesuccinate) (PE44) [102] or with thermoplastic polyurethane (TPU) [103] in order to fabricate robust biodegradable nanomembranes for tissue engineering.

Poly(diallyldimethyl-ammonium chloride) (PDMADMAC) and poly(styrenesulfonate) (PSS) [104] have been used to control the gas permeability of polymeric nanomembranes. Thanks to its biocompatibility and biodegradability, poly(lactic acid) (PLA) in the form of nanosheets, has been proposed as a physical barrier against burn wound infection [105], for sealing operations in surgery [106], and for cell adhesion [46, 107]. Polysaccharide nanomembrane with a thickness of 75 nm is suitable for repairing a visceral pleural defect without any loss of respiratory functions of the lung [108], and the same polymer is used together with a poly(vinylacetate) (PVAc) to sandwich tetracycline antibiotics against bacterial infection, to form an antibiotic-loaded nanosheet [109].

Mixed-matrix membranes (MMMs) consist of a mixture of rubbery or glassy polymer with inorganic materials like zeolites [110], carbon molecular sieves [111], or nanoparticles [112]. Yin et al. [113] summarized the scientific and technological advances in developing nanocomposite membranes for water treatment, including ultrathin films. Recent studies have revealed that the incorporation of small amounts of a dense monolayer of planar graphene oxide in polyelectrolyte nanomembranes significantly enhances their mechanical properties [114]. The surface of the substrate has to be ideally flat to fabricate membranes with uniform thickness, in particular where it is necessary to transfer it from a soft support as in [115]. Freestanding nanoscale membranes with outstanding mechanical characteristics, made with polyelectrolyte multilayers (PEM) with a central interlayer containing gold nanoparticle, have been built combining spin coating with layer-by-layer (LbL) assembly [116]; graphene oxide layers have also been incorporated into the same previous multilayers fabricated in LbL assembly via Langmuir-Blodgett (LB) deposition. Evans et al. [117] synthesized conducting polymer nanocomposite film in a vacuum chamber oven following vapor phase polymerization (VPP) technique, and Fabretto et al. [118] investigated the influence of the VPP parameters on the dynamics of the polymerization process for use in large-scale electrochromic devices. Angelova et al. [119] presented a modular scheme to efficiently fabricate carbon nanomembranes utilizing a three-step procedure: deposition of self-assembled monolayer of polyaromatic molecules on solid substrate followed by electron irradiation to induce two-dimensional crosslinking, and transfer on the final support.

We should point out that the depth of surface plasma modification ranges from a few microns down to a few nanometers [120] depending on plasma energy. Recently, great attention has been devoted to the performance of polymeric nanomembranes with micro- or nanoporous structure. The LbL technique allows the fabrication of conjugated microporous nanomembranes with tunable selectivity and permeability. Zhou et al. targeted their attention on developing a thin composite polymer membrane for CO2 separation with high stability with respect to aging and plasticization [121]. In addition, reaction conditions, surface area, and surface morphology are parameters that can affect the membranes performance [122, 123]. The properties of the surfaces of thin film influence antifouling behavior, in particular their hydrophilicity, roughness, and electric charge density. Nanostructured nanomembrane of polyaniline (PANI) has proven its suitability for pH sensing, both doped with HCl alternated with poly(vinyl sulfonic acid) in a LbL assembly, and synthetized in the presence of water-soluble polyvinyl alcohol (PVA) [124, 125].

1.8 Liquid Membranes

Liquid membrane processes are those involving a selective liquid membrane phase in which simultaneous extraction/stripping occurs. Separation is achieved by permeation of solute through this liquid phase from a feed phase to a receiving phase. the liquid membrane serves a dual purpose of permitting selective transfer of one or more components through it from external phase to internal droplets and vice versa, and preventing mixing of external and internal phases. Ever since ELM was invented by Li [126] in 1968, the use of this method for the hydrometallurgical recovery of heavy metals has drawn the attention of many investigators. Frankenfeld and Li [127], Martin and Davies [128] and Kitagawa et al. [129] were among the earliest investigators to report the extraction of metal ions.

The capture and separation of CO2 using facilitated transport membranes has shown particular promise as a potential substitute for the existing process due to its high diffusivity coefficient in comparison with polymer membranes and the selective permeation of CO2 by solubility selectivity [130]. To create a supported liquid membrane, a liquid material capable of reversibly bonding with the gas intended for separation, or a material that can perform such a role, is dissolved and the solution is loaded into the pores of a porous support [131]. Hydrophobic microporous hollow fiber membranes with high porosity can be formed by a simple and convenient method of nonsolvent-induced phase inversion separation (NIPS) process, and membrane structure and morphology can also be optimized by adjusting various preparation conditions if a suitable solvent can be found for the selected polymer. Therefore, the thermal stability of the membrane material decides the membrane performance and the economy of the operation under high temperatures. For such applications, fluorinated polymers are good candidates due to their high hydrophobicity and chemical stability [132–134].

Liquid membrane technology has great potential for the removal of heavy metals from aqueous dilute solutions. Liquid membrane is a layer of an organic solvent separating two aqueous solutions. The liquid membranes can be classified into different types: bulk, emulsion, supported, and hollow fiber liquid membranes. Bulk liquid membranes consist of an aqueous feed and stripping phases, separated by a water-immiscible liquid membrane. They demonstrate stable transport properties. A strongly basic membrane can exchange anions or react with un-dissociated acid molecules [135], whereas a strongly acidic membrane can lower the dissociation of acids and enhance the sorption (extraction) of the weaker one [136].

Liquid membranes provide greater selectivity and permeability than the solid ion-exchange membranes. Data on electrodialysis of liquid membranes are rather low in comparison with membrane extraction. One of the first works in this field was conducted by Purin [137], who used electrodialysis through bulk liquid membranes to concentrate rhenium from industrial solutions.

A liquid membrane is a layer of an organic solvent separating two aqueous solutions. Compounds promoting the transport of substances from one aqueous solution to another may be dissolved in the organic phase. Based on this point, liquid membrane is gradually applied in gas separations, which has been intensively studied in the separation process. Generally, liquid membranes with and without supports can be differentiated. The process of liquid membrane in gas separation is mainly explained by the dissolve-diffusion mass transfer mechanism [138].

1.9 Recent Progress in Separation Technology Based on Ionic Liquid Membranes

Liquid membrane processes are based on membrane separation and liquid-liquid extraction (LLX) in a single step. The main principle of extraction processes lies in the use of a fixed or mobile reagent solution phase, which is often immiscible with water, placed between liquid or gaseous feeding and stripping phases. These are only some of the unique properties that make ionic liquids an alternative to organic solvents in many chemical separation and purification processes [139, 140].

The selection of adequate chemical structures and the high viscosity typical of ionic liquids allow the solubility of membranes to be reduced, avoiding losses of liquid and contributing to their stability. The application of liquid membranes has been widely investigated in separation processes, mainly for the separation of organic compound and metal ions. Heavy metals, such as chromium (Cr), have also been extracted from wastewater by using a double ionic liquids-based ELM. 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim+][NTf2–], was used to stabilize the membrane phase and tri-n-octylmethylammonium chloride was used as carrier. They achieved an extraction yield of chromium of approximately 97% and confirmed the capacity of [bmim+][NTf2–] to stabilize the membrane [141]. Emulsion liquid membrane is a promising technology for many separation processes, especially for heavy metals removal, wastewater treatment and separation of gases. However, the success of its application depends on several key factors, such as the adequate selection of emulsification method and emulsion formulation, factors that largely determine the stability of these membranes. The immobilization of ILs can increase efficiency, facilitate recycling and widen the range of applications of these compounds with unique chemical and physical properties. Ionic liquid can be immobilized as liquid phase in membrane materials for their application in separation processes. Another important application of these types of membranes is the separation of biobutanol, which has great potential as biofuel. This is produced during the fermentation of alcohols and ketones from biomass. Much progress has been made to improve the long-term stability of immobilized ionic liquid membranes. The most important application is the use of novel supporting materials [142] or the gelation of ionic liquids [143].

In the work which focused on gelled supported ionic liquid membranes, the membrane obtained had good mechanical stability and maintained similar gas transport properties to the original ionic liquid. This work demonstrated that gelation is a simple and promising method to control the mechanical properties of ionic liquids-based membranes without sacrificing separation performance.

1.10 Membrane Distillation

Membrane distillation has potential for use in water desalination, feed solution degassing, treatment of industrial effluents, purification of pharmaceuticals, processing of foods, and removal of organic compounds, heavy metals from aqueous solutions [144–148] and radioactive wastes [149], and concentrating diluted nonvolatile acids such as sulfuric acid and phosphoric acid [150]. Direct contact membrane distillation is the simplest membrane distillation (MD) configuration. The membrane is in direct contact with the liquid phase and has the ability of producing a high flux. Hot feed is in direct contact with the hot side of the membrane surface and vapor molecules pass through the membrane toward the permeate side and condensation takes place inside the module. In this arrangement, pressure is maintained below the equilibrium vapor pressure to improve mass transfer. Vacuum membrane distillation (VMD) is beneficial for removing volatiles from an aqueous solution [151, 152]. Vacuum multi-effect membrane distillation (VMEMD) combines the advantages of multi-effect and vacuum concepts to make a multistage setup integrated into a compact plate and frame module. This configuration has been successfully commercialized [153]. Hollow fiber membranes have larger specific surface area than flat sheet ones, but they typically have low flux due to their poor flow dynamics and high degree of temperature polarization [154, 155]. The use of nanomaterials in developing the MD membranes is considered to be a new concept and only a few studies have been reported in the literature in that direction. New materials such as carbon nanotubes and fluorinated copolymers have been recently developed for membrane fabrication [156]. For membrane fabrication, different methods such as sintering, stretching, and phase inversion are mostly used. Sintering is used to prepare polytetrafluoroethylene (PTFE) membranes.

1.11 Alginate-based Films and Membranes: Preparation, Characterization and Applications

Since sodium alginate membranes are hydrophilic and soluble matrices, the crosslinking process with polyvalent cations has been used to improve their water barrier properties, mechanical resistance, cohesiveness and rigidity [157, 158]. It is well known that the formation of biopolymer films requires the addition of plasticizers to overcome their brittleness and improve their processing behavior [159–161]. For sodium alginate membranes, the addition of plasticizer enhances flexibility, decreases brittleness, as well as avoids shrinking during handling and storage [162–164]. Compared with pure alginate or alginic acid membranes, the mechanical properties of the blend films were significantly improved by introducing cellulose or regenerated cellulose. Bacterial cellulose (BC), a natural biopolymer synthesized in abundance by different strains of bacteria, displays high water content, high wet strength and chemical purity [165, 166].

Sodium alginate membrane is a polyelectrolyte with negative charges on it. Chitosan is the N-deacetylated derivative of chitin, a cationic polysaccharide composed of d-glucosamine and N-acetyl d-glucosamine residues with 1,4-linkages [167, 168]. The self-adhered (SA)/PVA-based IPN membranes extended drug release up to 24 h, while SA and PVA membranes discharged the drug quickly. Polyethylene glycol (PEG) is a biocompatible polymer with excellent biocompatibility and nontoxicity. Immiscible SA/PEG blend films with enhanced thermal stability have been developed [169]. Several SA-based composite films have been developed by adding reinforcements (fillers) of cellulose origin to enhance their performance and applicability [170–172]. Recently, Thu et al. developed an alginate-based bilayer hydrocolloid slow-release wound dressing film composed of an upper layer impregnated with a model drug (ibuprofen) and a drug-free lower layer that acts as a rate-controlling membrane [173]. Sodium alginate membrane-based films can be proposed as a material for drug delivery system because of their ability to enhance the efficacy and paracellular transport, as well as prolong the release time of drugs [174–177].

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