Porous Membranes E-Book

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Veröffentlichungsjahr: 2025

Beschreibung

The book is essential for anyone seeking a deep understanding of porous membranes, as it offers valuable insights into manufacturing methods, innovative applications, and strategies for optimizing membrane design to meet critical project demands across various fields.

Porous Membranes: Breakthroughs in Manufacturing and Applications is a comprehensive guide to discovering the world of porous membranes and their applications. This volume gives a global perspective of basic concepts, featuring manufacturing approaches and potential applications where control of pore size and shape, and distribution can be decisive for the success of a membrane process. In-depth explanations elaborate on the key role assigned to a membrane’s pores in directing events that are crucial for the mandatory targets imposed by a project’s requirements. Further, discussions on how to manage and characterize materials from a molecular to macro scale to achieve highly defined architecture to enable high-performing separations are explored. Advances and innovation are central themes, providing useful solutions to current critical aspects and existing bottlenecks in the control of structural and chemical features of targeted membranes. This cross-disciplinary discussion opens new routes for membrane science in expanding fields, including water management, environmental remediation, recovery of targeted compounds, food, and health.

Readers will find this book:

• Introduces the strict relationship between extensively ordered porous membranes and enhanced productivity;
• Explores new approaches based on new membrane pore concepts;
• Emphasizes the feasibility and reliability of the proposed techniques within the context of a potential scale-up, analyzing critical issues and traits;
• Focuses on the role of porous membranes in some strategic membrane operations, providing clear evidence about the fundamental role of structure-separation properties for the success of membrane processes dedicated to natural resource management.

Audience

Researchers in chemistry, biology, biomedicine, materials science, textiles, and electronics who are involved with membranes and materials; technologists and product managers from industry, including those responsible for research and development, building prototypes and commercial devices, will find this book to be especially valuable.

Leseprobe

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Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])

Porous Membranes

Breakthroughs in Manufacturing and Applications

Edited by

Annarosa Gugliuzza

Research Institute on Membrane Technology, National Research Council of Italy

and

Wanqin Jin

Department of Chemical Engineering, Nanjing Tech, Jiangsu, University, China

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Library of Congress Cataloging-in-Publication Data

ISBN 978-1-394-30345-8

Front cover images courtesy of Annarosa GugliuzzaCover design by Russell Richardson

To my beloved parents

Preface

Membrane technology is envisioned as a practical pathway for implementing sustainable solutions to environmental and energy challenges, natural resource management, and forward-looking tools for social and economic policy planning. These solutions are not only suitable for community and industry development but also contribute to environmental health, thereby benefiting end-users. Membrane processes are recognized as pivotal in achieving specific objectives across various fields, engaging companies, research institutions, academic organizations, citizens, and public administrations. Implementing a membrane process can serve as the first step toward innovative problem-solving, which is increasingly vital in areas facing emergencies.

A new vision for completing successful projects using membrane technology must, however, include a clear understanding of how to design an archetype of membranes. Morphological and chemical features are essential for creating sophisticated architectures, as they play a critical role in controlling mechanisms and host-guest interfacial forces that facilitate or hinder the separation of molecular species.

One of the major characteristics of membranes is their porosity, including the concept of pore size. Membrane pores are typically regarded as micro- to sub-nanometer channels that allow free pathways for molecular species in liquid or gas states. Parameters such as size, density, distribution, distance, length, and tortuosity must be carefully considered to associate a discriminative ability with the pores. It is crucial to differentiate between geometrical and effective pores, larger and mean pore sizes, open and dead-end channels, and various pore shapes such as round, conical, and funnel-shaped. Precisely controlled pore morphology can enhance desired separation mechanisms, amplify process productivity and efficiency, and address critical events effectively both at the surface and within the bulk of membranes.

Reasonably, pores become key elements in reducing resistance to mass transfer, increasing heat flow, supporting suspended nanofilms, influencing the mechanical resistance of polymeric films, spreading or preventing wetting, assisting adsorption, or countering foulant adhesion. However, pores are not merely passive morphological features; they can be transformed into dynamic and intelligent gates in polymeric films.

This book uses accessible language to guide readers through the world of porous membranes, offering a narrative exploration of innovative strategies for fabricating and characterizing free pathways in interfaces with high levels of organization, structure, and dynamics. These pathways exhibit properties related to wetting, separation, adsorption, transport, sensing, and biocatalysis.

The book reveals how porous membranes with tailored architectures and specific properties can be derived from traditional materials and flexibly applied in fields such as sustainable natural resource management, environmental remediation, food processing, waste treatment, advanced fuel cells, and the recovery of valuable products. It also explores the potential of novel membrane types for cleaner purification processes through numerous case studies, including water desalination, ion filtration, and the removal of pollutants such as pesticides, bacteria, pharmaceuticals, and heavy metals from aqueous streams, as well as hydrogen purification and CO2 capture.

This book covers a choice of topics and split into three sections, each dealing with aspects related to the role of pores in membranes and related fabrication, characterization and application.

Part I consists of two introductive chapters. Chapter 1 (Gugliuzza) is an introduction to basic pore concepts and transport mechanisms needed to drive a wide set of membrane operations. Chapter 2 (Gordano) examines from more traditional to advanced methods to estimate membrane porosity within a range of a few micron to nanometer values.

Part II comprises four chapters focused on achieving greener and more sustainable approaches for fabricating membranes with highly defined and dynamic pores. Chapter 3 (Liu) presents advanced grafting-based procedures to create responsive gates in membranes. The pore size can be adjusted in response to external stimuli, which subsequently affects selectivity. Size sieving and affinity effects are highlighted as powerful tools for smart pollutant removal. Case studies are examined alongside process-oriented design, stability, and scalable membrane production. Chapter 4 (Díaz) explores the use of click chemistry as a viable method for preparing anion exchange membranes with customizable functionalities for fuel cell applications. A straightforward and broadly applicable strategy for all functional groups is proposed, emphasizing the cost-effectiveness and scalability of the synthesis. Chapter 5 (Cardea) discusses the application of supercritical CO2 phase separation to generate porous membranes in a solvent-free manner, aligning with green and circular strategies. The effects of process parameters on membrane porosity are analyzed, and case studies are presented in specific application areas, including filtration, biomedical, pharmaceutical, and electronics. Chapter 6 (Gugliuzza) focuses on the fabrication of highly defined porous membranes for membrane contactor processes. A range of techniques is reviewed, from traditional phase inversion methods to innovative procedures capable of achieving lithographic precision in polymeric architectures. The chapter delves into the technological fundamentals of advanced membrane science, such as membrane contactors, while examining top-down and bottom-up fabrication approaches. The advantages and limitations of each process are analyzed, and structure-transport relationships are discussed, detailing the influence of individual morphological parameters on final membrane properties and transport performance.

Part III consists of four chapters dedicated to advances in separation membranes based on porous materials with a special focus on bio-purification, environmental remediation, recovery of value-added products, water desalination and gas separation. Chapter 7 (Cui) explores membrane bioreactor technology, which combines biological degradation with the complete physical retention of bacterial flocs and suspended particles. An overview of membrane fabrication with pore sizes ranging from 0.002 to 0.05 μm is provided, along with a classification of membrane bioreactors and their commercialization at scale. Chapter 8 (Donato) focuses on porous imprinted membranes for the recovery of target compounds and environmental remediation. After introducing the fundamental concepts and mechanisms of molecular imprinting for selective separations, the chapter examines the potential applications of molecularly imprinted membranes in fields such as agro-food, bioactive molecule recovery, water treatment, hazardous compound removal, and ion filtration. Chapter 9 (Gugliuzza) investigates the role of few-layered materials in porous membranes for water desalination. The discussion covers ion filtration and membrane distillation, emphasizing the creation of interlayered nanochannels capable of distinguishing between mono- and divalent ions. Additionally, the confinement of defective few-layer materials within porous membranes is analyzed for their ability to enhance transport properties. The chapter highlights successful water desalination processes for freshwater production and the recovery of high-quality salts through sorption-desorption mechanisms. Chapter 10 (Jin) discusses how gas flux is maximized through sub-nanometer channels in two-dimensional material membranes due to their atom-level thickness. In-plane nanopores and sub-nanometer interlayer channels in monolayer, laminar, and nanosheet-based mixed-matrix membranes are examined within the context of gas separation. The chapter covers fabrication, testing with various gas pairs for hydrogen purification, CO2 capture, and the purification of gaseous streams, all based on molecular sieving mechanisms. Future perspectives are explored, with a particular focus on graphene, its derivatives, and graphene oxide.

We are pleased to have edited this book, which documents advances in breakthrough manufacturing techniques for the fabrication of solvent-free, high-definition porous membranes and explores their potential applications in the efficient and sustainable processing of waste, water, gases, energy sources, food, and bio-products.

We believe this book can foster an interdisciplinary exchange of ideas and actions, contributing to the innovative design of high-performing porous membranes that provide rapid and practical solutions to critical global challenges. We hope you enjoy reading this work and find it valuable for your research endeavors.

Annarosa GugliuzzaWanqin JinMarch 2025

Acknowledgements

We acknowledge financial grant from “The Italian Ministry of Foreign Affairs and International Cooperation” within the framework of the Great Relevance International Project Italy (MAECI)-China (NSFC) 2018-2020 - New Materials, with particular reference to Two-dimensional Systems and Graphene (2DMEMPUR), MAE00691702020-06-26.

A special memory and thanks to Prof. Enrico Drioli who was a pioneer in the development of membrane processes for strategic technology areas, where he has left a permanent mark with his groundbreaking research.

PART IBASIC CONCEPTS ON POROUS MEMBRANES

1Porous Membranes: A Brief Introduction to Basics Concepts and Fields of Applications

Annarosa Gugliuzza

Research Institute on Membrane Technology-National Research Council (CNR-ITM), Rende, Italy

Abstract

Pores can be regarded as channels and sub-nanometer free pathways, in which transport can take place through. Porous membranes can be classified by pore size and subsequent capability to discriminate among molecular species according to various mechanisms. Transport mechanisms through porous media can be described according to mathematical equations, while structural elements can be regarded as critical issue for membrane productivity and efficiency. A classification is herein proposed for membranes used in pressure-driven membrane processes, including microfiltration, ultrafiltration, nanofiltration, and reverse osmosis, with a brief outline of dialysis and electrodialysis. Structure-property relationships are critically discussed for other advanced membrane operations, including membrane distillation, membrane crystallization, membrane condensation, and membrane emulsification. Finally, a consideration on free volume compared to free paths on an Å length scale is also provided for membranes worked in gas separation and pervaporation.

Keywords: Porous membranes, transport mechanism, microfiltration, ultrafiltration, nanofiltration, reverse osmosis, membrane contactors, gas separation and pervaporation

1.1 Introduction

Traditionally, a membrane is regarded as a perm-selective interface enabling mass, energy, charge, and signals transport under predefined driving forces [1]. More recently, the membrane has been regarded as an interactive and/or a dynamic and adaptable film; its permeability and selectivity can be controlled and readdressed under external triggers, if necessary [2]. Undoubtedly, morphological features decide transport and discriminating property of membranes. Pores, defined as free gaps through which molecular diffusion takes place, represent a decisive structural element for the occurrence of specific transport mechanisms. Of course, pore size, shape, and tortuosity generate the morphological environment wherein particles and various molecular species can be separated according to their size, geometry, molecular weight, as well as affinity and condensability [3]. The size and geometry of pores are the result of thermodynamic and kinetic events that control the formation of a membrane during i) phase separation or ii) imprinting action of pore formers (Figures 1.1a, b). During phase separation, regions poorer in polymer form pores in membranes in a more random way, leading to porous structures with low control of pore geometry and density distribution. When geometrically regular templates are assembled in predefined volumetric spaces, well-shaped and sized pore can be obtained throughout the membrane surface once removed. Sometimes, pores can be simply free gaps generated by the rearrangement of segment polymer chains, or they can consist of the intrinsic porosity of nanofillers, or more simply, tracks and voids shaped at organic/inorganic interfaces (Figure 1.1c). Further, the switching on/off of pores can be generated by self-assembly or stimulated arrangements of materials, enabling changes in size and shape (Figure 1.1d) [4].

A significant morphological aspect is the propagation of the pore length through membranes. Frequently, the length of the diffusion path is higher than the length of a straight capillary. In this case, a tortuosity factor is generated, which can compromise or affect molecular diffusion due to local restrictions in the pathways [5]. The active pore could be not placed on the membrane surface but rather confined within the bulk of the membranes. This stresses how the membrane morphology, as a whole, can affect its efficiency in regions that are part of the skin or sublayer of the film. Different morphological situations can usually occur in a membrane: a) pores connected to the surface and working as free channels; c) pores open on the surface but closed at the other end (dead-end); d) pores closed at both ends; and e) pores with restrictions in the middle path. So, a careful evaluation of the pore size, tortuosity, and free access throughout the membrane is always needed [6]. The number and density of pores in a membrane determine its overall porosity, expressed as the ratio of the volume of air embedded in the matrix to the volume of the solid film. This other morphological parameter is fundamental because it controls the degree of resistance to molecular transport. High porosity is strongly desired to increase the productivity of a membrane process. Highly porous films are, in fact, used as sublayers for tiny selective layers to provide good mechanical resistance without adding resistance to transport. On the other hand, too large a pore size can affect seriously the selectivity of the process.

Figure 1.1 Representation of mechanisms for pore generation in membrane: (a) regions poorer in polymer during phase separation; (b) imprinting action of organic/inorganic template; (c) free gaps formed at the organic/organic and organic/inorganic interfaces; and (d) self-assembly of materials with responsive properties.

Another important issue is that pore size, shape, and distribution can affect the final surface properties of membranes, producing singular textures able to amplify rugosity factors and subsequently enhance the repellence properties of the film [7]. Also, pores could be preferable sites for the adhesion of molecular compounds as a first step towards undesired clogging, polarization concentration, and fouling, which are responsible for flux decline and loss of performance.

Pore size resolves which kind of membrane operation can be implemented for each membrane-type. Usually, porous films are indicated for pressure-driven separation processes [8], including microfiltration (MF, 100 nm to 2 μm), ultrafiltration (UF, 2–100 nm), dialysis (2–5 nm), nanofiltration (NF 2–1 nm), and reverse osmosis (RO, < 1 nm). Advanced membrane contactor technologies such as membrane distillation (MD), membrane crystallization (MCr), membrane condensers (MCe), and membrane emulsifiers (ME) also require the use of porous membranes whose topography can decide the operation’s success.

Lastly, free volume may also be regarded as an extreme concept of pores; it represents the average free pathways distributed on the Ångström (Å) length scale in dense membranes, usually used for gas separation and pervaporation. It could be frozen or fluctuant as a function of the glassy or rubbery properties of the membrane. Hereafter, an outline of transport mechanisms and membranes for specific membrane operations is provided.

1.2 Overview on Pore Size Concept and Transport Mechanisms

Pore size is the key structural element that determines the type of transport across a membrane through mechanisms that discriminate among molecular size, affinity, and condensability. Passive, active, and assisted transport can be distinguished depending on morphology, chemistry of membranes, and nature of penetrants as well as working conditions. The most common transport mechanisms through porous media include Poiseuille flow, Knudsen diffusion, surface diffusion, capillary condensation, molecular sieving, and solution-diffusion (Figure 1.2).

Based on the discriminant ability of porous media, the mechanisms could be classified into the following categories: a) bulk flow; b) restricted diffusion; c) selective absorption; and d) solution-diffusion. A short mathematical description of each single transport mechanism is herein given.

1.2.1 Poiseuille Flow

Poiseuille flow is a viscous, unselective flux that takes place in macropores with sizes wider than 50 nm. The pore diameter is usually larger than the mean free path of the molecular penetrant. This kind of transport is well described by eq. 1.1:

(1.1)

where ε is the porosity; μ is the viscosity of the penetrant; η is the shape factor (assumed to be equal to the reciprocal tortuosity of the medium), r is the pore radius; pav is the mean pressure.

Figure 1.2 Schematic representation of classic transport mechanisms through porous membranes.

When the membrane has a densified skin, the occurrence of Poiseuille flow yields a clear indication of the presence of defects, cracks, or pinholes through the active layer of the film.

1.2.2 Knudsen Diffusion

Microporous membranes with pore sizes in the range of 2–50 nm make Knudsen diffusion predominant due to the collision of the molecules with the pore wall [9]. In this case, the mean free path of the molecules is greater than the pore size, while the selectivity is proportional to the ratio of the inverse square root of the molecular weights as hereafter described:

(1.2)

(1.3)

(1.4)

where Dk is diffusion coefficient and Jk is the Knudsen-type flux.

(1.5)

where αij is the selectivity between the species i and j, and Mj and Mi are the molecular masses of the components j and i, respectively.

1.2.3 Selective Surface Diffusion

Surface diffusion is a separation mechanism that exploits the different amounts of absorption of molecules—generally at the state of gas/vapor—along the pore walls forming the membrane. Non-absorbable or weakly absorbable penetrants can be separated from absorbable compounds. Generally, more condensable components in pores can exclude the passage of the smaller ones. This kind of diffusion in microporous films is well described by the Dubinin-Radushkevich isotherm:

(1.6)

where W is the volume of the species adsorbed in the micropores, W0 is the maximum of adsorbent per adsorbed mass, B is the structural parameter of the microporous membrane, and β is the affinity coefficient of the typical curve [10].

1.2.4 Molecular Sieving

Molecular sieving takes place in pores of 3.0–5.2 Å [11]. Bigger molecules are stopped, while the diffusion of the smallest penetrants is allowed. The activation energy (Ea) plays a determinant role in the separation process and consists of the amount of energy necessary to promote the diffusion of one molecule through the narrow pathways. In this case, the flux can be expressed as:

(1.7)

where JMS is the molecular sieving flux and Ea,MS is the molecular sieving activation energy.

1.2.5 Solution-Diffusion Transport

This mechanism takes place in densified films where transient free gaps of the order of Å—free volume—allow penetrants to diffuse [12]. This mechanism can be diffusion-dependent and/or solution-dependent according to eq. 1.8:

(1.8)

where P is the permeability coefficient, D is the diffusion coefficient, and S is the solubility coefficient.

For single gases, the selectivity is given by:

(1.9)

If diffusion is the discriminating factor, the separation depends on the molecular size of the penetrants and subsequently on their diffusivity; in the case of solubility predominance, the species with higher affinity to the matrix will pass faster [13]. This kind of mechanism controls the transport through membranes equipping gas separation (GS), pervaporation (PV), and reverse osmosis (RO) plants.

In these kinds of processes, molecules are adsorbed onto the membrane face coming in contact with the upstream, diffused through the membrane, and are then desorbed on the downstream face of the membrane [13]. In the case of RO operation [14, 15], eq. 1.8 can be rewritten as follows:

(1.10)

where JAw is the water flux, Δp is the transmembrane pressure, Δπ is the difference in osmotic pressure between feed and permeate, and L is a constant related to membrane physical parameters described as:

(1.11)

where D is the water diffusion coefficient, S is the water solubility coefficient, V is the molar volume of water, R is the ideal gas constant, T is the room temperature, and l is the membrane thickness.

The osmotic pressure plays a key role in the overall RO process because it is necessary to cause water to leave the saline solution and move towards pure water (permeate) through the membrane. In the case of complete dissociation of salt ions, the osmotic pressure is termed as:

(1.12)

where π is the osmotic pressure, C is the concentration of salt ions expressed as the ratio between number of ions per gram of water and specific volume of water, and T is the solution temperature. While the diffusion of water through the membrane depends on the pressure, the flux of salt is concentration dependent:

(1.13)

where Js is the flux of salt through the membrane, B is the permeability constant of salt, and Cf and Cp are the concentration of salt at feed and permeate side, respectively. The constant B can be described as:

(1.14)

where Ds is the diffusion coefficient of salt, Ks is the partition coefficient of salt, and l is the membrane thickness.

The salt rejection can be calculated from:

(1.15)

It is relevant to observe the relationship between water and salt fluxes:

(1.16)

where Cwp and Csp are the concentration of water and salt in the permeate.

Rearranging eqs. 1.10 and 1.13, the rejection can be described as follows:

(1.17)

In this way, it becomes evident how the salt selectivity can be predicted from the evaluation of experimental conditions and membrane properties.

1.2.6 Mixed Transport Mechanisms

Membranes exhibit somewhat complex structures compared to simplified theoretical models, so multiple transport mechanisms frequently occur during a specific separation. Wider and sub-nanometer channels can coexist within the same matrix causing the occurrence of different molecular diffusion types [16]. In the case of gases, molecular sieving and adsorption diffusion could characterize the molecular separation through the same porous membrane at low temperatures. This mixed transport can be described by eq. 1.18:

(1.18)

where JAD/MS is the ratio of the adsorption diffusion and molecular sieving fluxes, while Ea,MS and Ea,ADS are the activated molecular sieving and adsorption diffusion energies.

Mixed molecular sieving and Knudsen diffusion may occur at higher temperatures. In this case, the transport can be described according to eq. 1.19:

(1.19)

An increase in temperature could cause a loss of selectivity shifting the transport toward a Knudsen-type diffusion:

(1.20)

(1.21)

(1.22)

Summarizing, the main mechanisms of transport through porous membranes include the concept of bulk flow, selective absorption, restricted diffusion, and solution-diffusion.

1.2.7 Active and Assisted Transport

In a more modern vision, the membrane is designed to be an interactive interface allowing one to direct, accelerate, or yield on-demand mass, energy, charge, or signal transport according to cooperative mechanisms. In recent years, there has been a larger interest in composite and responsive materials and manufacturing techniques, which allow materials with complementary functions to be allocated in predefined volumetric spaces. Like a piece of a jigsaw, each compound/moiety plays a role thereby leading to new functions and performance not otherwise achievable working individually. This prerogative amplifies the pore action in membranes, increasing the productivity, but at the same time enhancing the discernment ability of the membrane making the separation much more selective.

Today, there is great attention towards the integration of organic and inorganic nanofillers and macromolecules with the intent to assist the transport of mass [17–19], electrical [20, 21], ionic charge [22, 23], as well as to stimulate release [24, 25], local rearrangement [26, 27] self-healing [28, 29], and self-cleaning events [30, 31].

1.3 Porous Membranes for Membrane Processes

Processes that use porous membranes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), membrane distillation (MD), membrane crystallization (MCr), membrane condensation (MCe), and membrane emulsification (ME). For all these processes, the structural elements of the membrane become key issues [15]. Currently, the market does not provide membranes whose structural features are close to those considered ideal for some processes such as MD, MCr, MCe, and ME, which are part of advanced and sustainable membrane contactor technology [32]. The performance of each process strongly depends on structure-property relationships, so controlled pore size and pore distribution, along with the degree of porosity and thickness, become a cause for high-quality separation. Well-patterned surfaces are envisaged to amplify point by point desired events with great benefits for the productivity and/or selectivity of separation, as well as high pore density is expected to reduce the resistance to transport [33]. Low thicknesses increase productivity but affect the mechanical stability [33] and efficiency of the film [34]. Controlled textures can enhance anti-wetting/anti-adhesion properties so that fouling and/or scaling can be limited [35].

The design of new nanostructured membranes becomes necessary to meet the requirement of developing sustainable membrane technologies on a larger scale. Based on mechanism types, each membrane process requires membranes with specific pore sizes to promote desired molecular separation (Figure 1.3).

Figure 1.3 Scheme of membrane processes working with porous membranes for selective separations.

MF uses membranes with pore sizes ranging from 10-1–101 μm; UF membranes exhibit pore sizes of 10-2–10-1 μm, while RO equips membranes with pores within the range of 10-4–10-3 μm. NF membranes fall in the range between UF and RO (10-3–10-2 μm). Gas separation (GS) and pervaporation (PV) are in the region of Å where the free volume concept becomes expression of free ways through which the penetrant diffuses [15].

1.3.1 Microfiltration Membranes

MF operations find large applications in the clarification and stabilization of the beverage industry, filtration of fluids, sterilization of food and pharmaceutical industrial chains, as well as the removal of suspended solids (Figure 1.4) [36].

When applying low pressures, macromolecules, suspended particles, and fats can be retained easily from waste. Hydrophilic membranes are usually preferred for the filtration of aqueous solutions, while hydrophobic membranes are proposed for applications (MD/MCr) where water is separated as a vapor rather than a liquid. Also, microporous membranes can be used as support for a liquid membrane solution, which usually consists of an organic phase containing carriers and/or modifiers for the stabilization of the phases coming in contact or the facilitated transport of selected molecular species [37].

Figure 1.4 Representative scheme of pressure-driven MF process: prefiltration of wastewater.

The fabrication techniques for MF membranes include traditional phase inversion, stretching, track-etching, and electrospinning. Depending on the selected manufacturing procedures, different materials can be used from traditional polysulfone (PS), poly(ether sulfone) (PES), polyacrylonitrile (PAN), cellulose acetate (CA) to polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE, Teflon), and poly (vinylidene fluoride) (PVDF).

1.3.2 Ultrafiltration Membranes

UF processes are preferred for the concentration or pretreatment of various chemical streams, food and beverage clarification, emulsion separation, pretreatment of water in integrated membrane processes, etc. [38]. UF is a low-pressure-driven process, which bases the separation on size-exclusion mechanisms, retaining molecular species from 2000 to 500000 Da. However, UF offers poor decontamination for potentially toxic elements making the process more highly susceptible to clogging and fouling (Figure 1.5).

The prerogatives of these membranes are mechanical, thermal, and chemical stability. For this reason, the materials used for membrane fabrication are traditional poly (ether ether ketone) (PEEK), polyacrylonitrile (PAN), polysulfone (PS), poly(ether sulfone), cellulose acetate (CA), polyimide (PI), poly(etherimide) (PEI), poly(vinylidenefluoride) (PVDF). In this case, phase inversion is the preferred manufacturing technique. Recently, zeolites, metal–organic frameworks (MOFs), and graphene oxide (GO), carbon-based, and other composite-based nanomaterials have also been incorporated into polymers to remove harmful elements from water through better controlled fouling phenomena.

Figure 1.5 Representative scheme of pressure-driven UF: removal of proteins and viruses from waste.

Selective diffusion is also the mechanism that controls the separation of smaller molecules from larger compounds or dissolved substances from colloidal particles. This kind of process is well known as dialysis and takes place under a gradient of concentration. Usually, the molecules are neutral, but in the case of electrolytes or charged membranes, Donnan effects take place as a subsequence of the unequal distribution of ions. The Donnan membrane principle inspires various engineered processes and materials to achieve better sustainability [39].

1.3.3 Nanofiltration Membranes

NF membranes find applications in water softening, water concentration, product recovery, and bivalent ion removal (Figure 1.6). This membrane process allows for the removal of impurities, sediments, and hazardous substances like arsenic and bivalent ions [40]. UF membranes usually exhibit an ionic charge, which is generated during the fabrication of thin layers on porous supports by interfacial polycondensation. So, their preferred use is for bivalent ion removal. Typical materials selected for NF membranes are polymers with carboxylic and sulfonic acids, which dissociate to become negatively charged.

Figure 1.6 Representative scheme of pressure-driven NF: selective removal of bivalent ions.

A particular family of positively or negatively charged membranes is classified as membranes for electrodialysis (ED), dedicated to selectively removing ionic species. Anion exchange membranes exhibit a positive charge and are used to remove cations only, while cationic exchange membranes are negatively charged and allow the diffusion of cations only. Generally, functional polymers with quaternary ammonium salts are used for anionic membranes, while dissociated sulfonic or carboxylic acid moieties are used for cationic membranes. This class of membranes is used for the desalination of brackish water, wastewater treatment, concentration of RO brines, and the treatment of chemical, drug, and beverage streams. Currently, major limitations for the scale up of this technology are represented by fouling, which affects the durability of NF purification, selectivity, and cost-benefit ratio for the environment. New types of highly defined and patterned membranes have demonstrated extreme promise for enhancing productivity-efficiency trade-offs [41].

1.3.4 Reverse Osmosis Membranes

RO plays a major role in water desalination and waste water treatment processes, as well as in the recovery of by-products from various chemical streams (Figure 1.7) [15, 42].

Figure 1.7 Representative scheme of pressure-driven RO process: production of freshwater from hypersaline streams.

In this kind of process, membranes with sub-nanometer pore sizes are chosen for molecular separation according to mechanisms that also depend on the intrinsic structure of the membrane. RO membranes are generally prepared by phase inversion or by interfacial in situ polymerization. They can be formed from unique or composite materials and can have asymmetric or layered structures. Among the various polymers, cellulose acetate is the most used, together with aromatic polyamides. In both cases, (bio) fouling and scaling represent a great limitation, causing flux decline over time. The use of disinfectants, anti-scaling agents, and other pre-treatment steps are proposed to contrast the adhesion of pollutants on the membrane surface. Despite that, effectiveness and durability of the process appears to be somewhat compromised with time. So, a good chance could be to optimize materials and manufacturing technologies as a practical route for giving longer stability and efficiency to the membrane process. However, RO membranes continue to dominate the membrane market for water desalination mainly, with a further increase of 8–10% by 2026.

1.3.5 Membrane Contactors Processes

This branch of membrane technology encompasses advanced and promising ecofriendly processes such as MD, MCr, MCe, and ME (Figure 1.8) [43]. Currently, there are no available membranes with structural and chemical features suitable for implementing these membrane operations on an industrial size. So, traditional PVDF, PP, and PTFE membranes are adapted according to the basics of the specific technology.

MD and MCr processes are targeted to recover mainly freshwater and minerals from hypersaline streams in the field of water management [19, 44]. These technologies exploit the ability of water or other volatile compounds to change from a liquid to a vapor state, diffuse through membrane pores as vapors, and then move again to a liquid state on the permeate side (Figure 1.8a). A difference in temperature applied across the membrane is one of the most used driving forces in these processes.

In this context, the ability of the membrane to prevent liquid intrusion inside the pores is regarded as a critical issue because efficiency and durability of the separations depend on the stability of the liquid-vapor-liquid phases. For this reason, hydrophobic commercial MF and UF membranes, i.e., PVDF, PP, and PET, are typically used. However, the lack of particular morphological and chemical features makes these traditional membranes unideal for this kind of membrane contactor process.

Structural order, modular surface properties represent a real bottleneck in achieving a competitive threshold in productivity and efficiency [18, 45]. High interfacial area, anti-wetting properties, and low thermal conductivity are other prerogatives for making the processes competitive on an industrial size. Omniphobic membranes, as well as double hydrophobic/hydrophilic membranes, have also been proposed to yield faster productivity [46, 47]; however, the design of targetable membranes continues to be an important challenge for the development of membrane MC/MCr on an industrial size.

Figure 1.8 Schemes of (a) MD/MCr; (b) MCe; and (c) ME.

Hydrophobic membranes with specific structural features are in high demand for MCe [48]. This technology is usually proposed for the dehydration of flue gas (Figure 1.8b). In this case, water and other condensable gases are retained, while non-condensable gases are diffused through membranes and collected on the permeate side.

Without neglecting the role of interfacial forces and wall shear stress, membranes for ME processes also need to be equipped with pores having suitable geometry, size, and distance for the production of stable and uniform micro and nanoemulsions (Figure 1.8c). Tendentially, droplet sizes range between 2 and 10 times the nominal membrane pore diameter [49]. As is known, the choice of materials and surface texture also determines the final hydrophilic or hydrophobic properties of the membrane surfaces, which are crucial for the uniform dispersion of droplets in a continuous phase. Actually, a major limitation to the scale up of this technology is due to the low levels of production of the dispersed phase through membranes for submicron droplets.

1.3.6 Gas Separation and Pervaporation Membranes

Gas separation and pervaporation membranes exhibit dense skin layers, which can be supported by symmetric or asymmetric porous sublayers, isotropic or anisotropic composition, as well as properties of glassy or rubbery materials [15]. However, if we would consider the concept of free volume similar to that of pores on Å scale, gas separation membranes could be classified as films wherein free gaps are frozen or fluctuant inside the matrix, depending on glassy or rubbery features of materials used (Figure 1.9).

Intrinsic structural and chemical features of materials and membranes, as well as the properties of gas and vapors, determine the kind of separation mechanism, ranging from Knudsen diffusion and capillary sorption [51, 52] to adsorption-diffusion-desorption [53, 54]. In the second case, a predominance of either a solution-dependent mechanism or diffusion-dependent mechanism can be prevalent, depending on the combined properties of the membrane and molecular species to be separated (Figures 1.9b, c).

Without going into the discussion of more complex aspects of the processes, we like to highlight the use of these membranes in gas separation processes of condensable from incondensable gases, air purification, CO2 capture, biogas processing, dehydration of gaseous and liquid streams, hydrogen recovery, as well as organic/organic separation, azeotrope separation, and the removal of organics from water through a process well-known as pervaporation (PV). These processes exploit different condensability and diffusivity of molecules through dense matrices (Figure 1.9d) [55, 56]. Various manufacturing approaches can be used to realize membranes for gas and vapor separation, including phase separation, multilayered deposition, polycondensation, etc. In each case, the most important aspect is to prevent the formation of holes and defects in the active skin layer to prevent selective properties of membranes from deterioration. On the other hand, tiny defect-free films should be desirable to reduce the resistance to transport, so that highly productive separation can be implemented. With this purpose, macroporous membranes are frequently used as sublayers to yield mechanical resistance without affecting transport resistance. A permeable gutter layer is placed between the sublayer and the selective thin film to make it compatible and defect-free.

Figure 1.9 Representative schemes of sorption-diffusion-desorption gas and vapor separation: (a) free gaps on Å scale; (b) controlled sorption mechanism in GS; (c) controlled diffusion mechanism in GS; (d) separation of azeotropes by PV. Figure 1.9a is adapted from and reprinted from [50] (2023) with permission from MDPI.

Conclusions

This chapter offers an outline of pore concept in membranes, exploring their role in transport, surface properties, and interfacial phenomena across different length scales. Morphological aspects of membranes are discussed concerning transport mechanisms and effects on permeability-selectivity trade-off. Mathematical descriptions of fluxes through membranes are provided along with an examination of the most suitable membranes for specific pressure-driven processes, membrane contactor operations, and gas and vapor separation. Structure-property relationships are assessed concerning with productivity and efficiency targets.

Acknowledgment

We acknowledge the financial grant from ‘the Italian Ministry of Foreign Affairs and International Cooperation’ within the framework of the Great Relevance International Project Italy (MAECI)-China (NSFC) 2018-2020 - New Materials, with particular reference to Two-dimensional systems and Graphene (2DMEMPUR), MAE00694722021-05-20.

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