Nanocomposite Membranes for Water and Gas Separation 0128167106, 9780128167106

Table of contents :
Cover
Nanocomposite Membranes for Water and Gas Separation
Copyright
Contributors
1
Overview of membrane technology
Introduction
Transport phenomena in membranes
Water transport through polymeric membranes
Porous membranes
Dense membranes
Gas transport through polymeric membranes
Poiseuille flow (convective flow)
Knudsen diffusion
Molecular sieving
Solution diffusion
Membrane fabrication methods
Phase inversion
Nonsolvent-induced phase separation (NIPS)
Thermally induced phase separation (TIPS)
Evaporation-induced phase separation (EIPS)
Vapor-induced phase separation (VIPS)
Electrospinning
Stretching
Track-etching
Preparation methods for composite membranes
Dip coating
Interfacial (in situ) polymerization
Plasma polymerization
Nanocomposite membranes
Conventional nanocomposite membranes
Surface-located nanocomposite membranes
Thin-film nanocomposite (TFN)
TFC with nanocomposite substrate
Conclusion and future prospect
References
2
Recent progress in the development of nanocomposite membranes
Introduction
Membrane materials and processes
Nanofillers used in nanocomposite membranes
Nanoclays
Nanooxides
Fullerenes
Carbon nanotubes
Metallic nanoparticles
Silsesquioxanes
Boehmite
Types of polymer matrix nanocomposites
Polymer matrix nanocomposite processing techniques
Applications and recent developments of nanocomposite membranes
Nanocomposite membranes for gas separation
Nanocomposite membranes for pervaporation
Nanocomposite membranes for water treatment applications
Nanocomposites membranes for desalination
Nanocomposite membranes for PEM fuel cells
Advanced membranes for lithium-ion batteries (LIBs)
Nanocomposites membranes for biofuel recovery
Nanocomposite membranes in fuel cells
Nanocomposite membranes in the food industry
Nanocomposite membranes in healthcare applications
Concluding remarks
References
3
Synthesis route for the fabrication of nanocomposite membranes
Introduction
Material selection
Polymer
Solvent
Additives
Roles of nanofillers
Classification of nanofillers
Metal oxides
Carbon-based nanomaterials
MOFs
Zeolites
Fabrication of nanocomposite membranes
Phase inversion technique
Casting for flat sheet configuration
Spinning for hollow fiber configuration
Interfacial polymerization for fabrication of thin film nanocomposite membranes
Coating or deposition
Chemical grafting
Self-assembly
Layer-by-layer self-assembly
Modification of nanofillers
Conclusions
References
4
Transport phenomena through nanocomposite membranes
Introduction
Classification of nanocomposite membranes
Conventional nanocomposite membranes
Thin-film nanocomposite membranes
Thin-film composite with nanocomposite substrate membranes
Surface-located nanocomposite membranes
Transport phenomena in nanocomposite membranes
Mass transfer
Mass transfer mechanism through porous membranes
Mass transfer mechanism through nonporous membranes
Heat transfer
Conductive heat transfer
Convective heat transfer
Charge transfer
Electron conductivity
Proton conductivity
Conclusions
References
5
Application of functional single-element and double-element oxide nanoparticles for the development of
Introduction
Metal oxide nanomaterials: Types, properties, and synthesis strategies
Strategies for incorporating nanomaterials into the membrane matrix
Bulk/support modification
Blending or direct compounding
In situ preparation of nanomaterials in bulk
Surface/top layer modification
Coating/deposition
Grafting
In situ generation of nanomaterials
Functionalization and surface modification of metal oxides
Silane coupling agent
Carboxylic acid group
Organophosphorus
Ionic liquids
Polymer/copolymer
Polydopamine
Properties of polymer/single-element and double-element oxide nanocomposites
Mechanical properties
Physical/chemical properties
Physical
Chemical
Thermal properties
Conclusions
References
6
Development of advanced nanocomposite membranes by carbon-based nanomaterials (CNTs and GO)
Introduction
Nanocomposite membranes
Mixed matrix nanocomposite membranes
Thin-film nanocomposite membranes
Carbon nanomaterials
Carbon nanotubes
Graphene oxide
NCMs containing carbon nanomaterials
NCMs containing CNTs
NCMs containing GO and its derivations
Conclusion
References
7
Development of advanced nanocomposite membranes by molecular sieving nanomaterials (zeolite and MOF)
Zeolites
Preparation of zeolite membranes
Zeolitic crystalline membranes: Self-supported membranes
Self-supported zeolitic membranes prepared without supports
Self-supported zeolitic membranes prepared on temporary supports
Zeolite composite membranes: Zeolitic membranes on stable supports
Zeolite membranes on metallic supports
Zeolite membranes on ceramic supports
Zeolite-filled polymeric membranes
Application of zeolites membranes for separation
Gas separations
Liquid separations
Pervaporation
Water treatment
Metal-organic framework (MOF)
Preparation of MOFs membranes
In situ preparation
Growth on unmodified support
Growth on modified support
Secondary (seeded) growth
Microwave-assisted growth of ZIF membranes
Blending method
Other design strategies
MOFs membrane application
Liquid separation
Pervaporation
Organic solvent nanofiltration
Water treatment
Gas separation
Conclusions
References
Further reading
8
Development of nanocomposite membranes by electrospun nanofibrous materials
Introduction
History of electrospinning technique
An understanding into principle and working procedure of electrospinning
Electrospinning parameters
Different kinds of electrospinning techniques
Melt electrospinning
Gas jet electrospinning
Coaxial electrospinning
Emulsion electrospinning
Needleless electrospinning
Multispinneret electrospinning
Applications of electrospun membranes
Electrospun nanocomposite membranes
Nano clays
Metal oxides
Zeolites
Carbon nanotubes
Graphene
Graphene oxide
Conclusion
References
9
Development of nanocomposite membranes by biomimicking nanomaterials
Introduction
Biomaterials
Cell membrane
Fabrication of AQP-based desalination membrane
Fabrication of AQP-based PRL membranes
Fabrication of AQP-based TFN membranes
Future work
References
10
Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes
Introduction
Nanocomposite membranes
Nanofillers in pressure-driven membrane processes
Mineral nanofillers
Zeolites
Silica (SiO2)
Metal and metal oxide
Titanium dioxide (TiO2)
Silver (Ag)
Zinc oxide (ZnO)
Copper (Cu)
Carbon-based nanofillers
Carbon nanotubes (CNTs)
Graphene and graphene oxide (GO)
Nanomaterials in microfiltration and ultrafiltration membrane process
Nanomaterials in nanofiltration membrane process
Nanomaterials in reverse osmosis membrane process
Conclusion
References
Further reading
11
Prospects of nanocomposite membranes for water treatment by osmotic-driven membrane processes
Introduction
Transport Phenomena in Osmotic-driven membranes
Classification of osmotic processes
Water and solute permeability
Concentration polarization
External concentration polarization
Internal concentration polarization
Osmosis membrane development
Nanocomposite osmosis membranes
Carbon-based nanocomposite osmosis membranes
Mineral-based nanocomposite osmosis membranes
Metal- and metal oxide-based nanocomposite osmosis membranes
Biomimetic-based nanocomposite osmosis membranes
Concluding remarks
References
Further reading
12
Prospects of nanocomposite membranes for water treatment by membrane distillation
Introduction
Gas transport through a membrane in MD
Heat transfer through a membrane in MD
Various MD configurations
Essential parameters affecting the MD performance
Concentration in the feed solution
Temperature
Feed and permeate flow rates
MD configuration
Membrane properties
Liquid entry pressure (LEP)
Pore size distribution
Wettability and contact angle
Thickness
Mechanical and thermal stability
Fabrication and characterization of nanocomposite membranes
Materials
Methods of fabrication
Characterization
Nanocomposite membranes in MD
Dual membrane layer (hydrophilic-oleophobic)
Omniphobic membranes
Filler-based membranes
Conclusion
References
13
Prospects of nanocomposite membranes for water treatment by electrodriven membrane processes
Introduction
Membrane materials for electrodriven separations
Cation-exchange membranes (CEMs)
Cation-exchange materials based on sulfonic acid groups
Cation-exchange materials based on phosphonic acid groups
Anion-exchange membranes (AEMs)
Anion-exchange materials based on quaternary ammonium groups
Anion-exchange materials based on quaternary phosphonium groups
Bipolar membranes (BPMs)
Mixed matrix membranes (MMMs)
Functionalized membranes
Graphene oxide (GO)-based membranes
Metal-organic framework (MOF)-based membranes
Performance indices
Ion-exchange capacity
Water content
Surface electrical resistance
Surface zeta-potential
Tensile strength
Elongation at break
Swelling rate
Current-voltage curves
Ionic migration number
Selective separation of monovalent ions
Fouling reduction
Surface roughness and hydrophilic/hydrophobic property
Chemical properties
Membrane preparation methods
Polymer blending
Porous substrates filling method
In situ polymerization
Electrospinning method
Technology-driven modification method
Electrodriven
Radiation crosslinking
Applications
Electrodialysis
Conventional electrodialysis
Bipolar membranes used in electrodialysis
Continuous electrodeionization
Diffusion dialysis (DD)
Electrodialysis reversal (EDR)
Novel processes based on electrodriven membranes
Conclusion and outlook
References
14
Prospects of nanocomposite membranes for natural gas treatment
Introduction
Mixed matrix membranes
Thin-film nanocomposite membranes
Multilayered nanocomposite membrane
Conclusion and future prospects
References
Further reading
15
Prospects of nanocomposite membranes for nitrogen and oxygen enrichment
Introduction
Oxygen and nitrogen separation techniques
Pressure swing adsorption
Cryogenic distillation
Membrane technology
Membranes for O2/N2 separation: Transport phenomena and ideal membranes
Nanocomposite membranes in O2/N2 separation
Metal-organic framework (MOF)
Zeolite
Carbon molecular sieves
Current result of nanocomposite mixed matrix membrane in O2/N2 separation
Conclusion and future trend
References
16
Prospects of nanocomposite membranes for the recovery of hydrogen and production of syngas
Introduction
Gas separation membrane
Hydrogen separation membranes
Metallic membranes
Porous inorganic membranes
Zeolite membranes
Silica membranes
Carbon-based membranes
Metal-organic frameworks
Polymeric membranes
Mixed matrix membranes
Various types of nanocomposite membranes
Metal nanoparticles
Palladium
ZnO
MgO
TiO2
Inorganic nanoparticles
SiO2
Zeolites
Carbon-based fillers
MOFs
Conclusion
References
17
Prospects of nanocomposite membranes for gas separation by membrane contactors
Introduction
Major challenges of membrane contactors
Membrane wetting
Membrane fouling
Membrane degradation
Wetting prevention
Optimizing operating conditions
Optimizing membrane material
Composite membranes
Asymmetric membranes
Surface modification
Nanocomposite membranes
Selection of nanocomposite materials
Nanocomposite membranes for gas separations in membrane contactors
Current status and future perspectives
References
Further reading
18
Prospects of nanocomposite membranes in commercial scale
Introduction
Applications of nanocomposite membranes in commercial scale
Desalination
Fuel cell applications
Gas separation
Medical applications
Drug delivery
Blood purification: Dialysis
Cancer diagnosis and treatment
Diabetes diagnosis and treatment
Food packaging applications
Li-ion battery
Photocatalytic applications
Conclusion
References
19
Operational and environmental challenges of nanocomposite membranes
Introduction
Processing limitations of nanocomposites membranes
Long-term mechanical stability of nanocomposite membranes
Fouling
Factors influencing fouling
Modes of fouling
Membrane surface modification for fouling reduction
Hydrophobicity of membrane
Poor dispersion of NPs
Leaching of nanoparticles and membrane components
Economic factors
Environmental problems
Toxicity
References
Index
Back Cover

Citation preview

NANOCOMPOSITE MEMBRANES FOR WATER AND GAS SEPARATION

Micro and Nano Technologies

NANOCOMPOSITE MEMBRANES FOR WATER AND GAS SEPARATION

Edited by

MOHTADA SADRZADEH TORAJ MOHAMMADI

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-816710-6 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Simon Holt Editorial Project Manager: Emma Hayes Production Project Manager: Joy Christel Neumarin Honest Thangiah Cover Designer: Alan Studholme Typeset by SPi Global, India

Contributors

Hirra Ahmad Department of Chemistry, University of Agriculture, Faisalabad, Pakistan Saba Akram Department of Chemistry, University of Agriculture, Faisalabad, Pakistan Mohammad Amin Alaei Shahmirzadi Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran Mohamed Afizal Mohamed Amin Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai; Faculty of Engineering, Universiti Malaysia Sarawak, Kota Samarahan, Malaysia Parvin Arehjani Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran Asad Asad Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada Farhana Aziz Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia Dayang Norafizan Awang Chee Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai; Faculty of Resource Science and Technology, Universiti Malaysia Sarawak, Kota Samarahan, Malaysia Kok Chung Chong Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kajang, Malaysia Pei Sean Goh Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia Oindrila Gupta Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ, United States xi

xii

Contributors

Ahmad Fauzi Ismail Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia Juhana Jaafar Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia K. Juraij Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India Ali Kargari Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran Tayyaba Khalid Department of Chemistry, University of Agriculture, Faisalabad, Pakistan Samaneh Khanlari Center of Excellence for Membrane Science and Technology, Faculty of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran Behnam Khorshidi Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada Soon Onn Lai Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Kajang, Malaysia Woei Jye Lau Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, Malaysia Yanling Liu Department of Chemical Engineering, KU Leuven, Leuven, Belgium O. Manaf Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India Maliheh Mehrabian Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran Toraj Mohammadi Center of Excellence for Membrane Science and Technology, Faculty of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

Contributors

Atikah Mohd Nasir Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia Yasir Nawab Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan Sadaf Noamani Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada Emily Ortega Department of Chemical Engineering, KU Leuven, Leuven, Belgium Iqra Abdul Rasheed Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan Anum Rashid Department of Chemistry, University of Agriculture, Faisalabad, Pakistan Zulfiqar Ahmad Rehan Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan Abdul Rehman Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan C.R. Reshmi Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India Sagar Roy Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ, United States Mohtada Sadrzadeh Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada P. Sagitha Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India Dan Sameoto Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada Simin Shabani Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada

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Contributors

A. Sujith Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India Suja P. Sundaran Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India Asra Tariq Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan Ali R. Tehrani-Bagha Department of Chemical Engineering and Advanced Energy, American University of Beirut, Beirut, Lebanon Maryam Ahmadzadeh Tofighy Center of Excellence for Membrane Science and Technology, Faculty of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran Bart Van der Bruggen Department of Chemical Engineering, KU Leuven, Leuven, Belgium Nurul Widiastuti Department of Chemistry, Faculty of Science, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia Rika Wijiyanti Department of Chemistry, Faculty of Science, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia Muhammad Zahid Department of Chemistry, University of Agriculture, Faisalabad, Pakistan Yan Zhao Department of Chemical Engineering, KU Leuven, Leuven, Belgium A.K. Zulhairun Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia

CHAPTER 1

Overview of membrane technology Asad Asad, Dan Sameoto, Mohtada Sadrzadeh

Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada

1.1 Introduction Membrane separation processes are now well-established technologies in a wide range of applications, including biotechnology [1], pulp and paper [2], pharmaceutical [3], food processing [4,5], petroleum [6,7], and seawater desalination [8]. A membrane is a selective thin layer of a semipermeable material. By applying a potential gradient, such as pressure, temperature, electrical or concentration difference, as a driving force, the membrane separates undesired materials from a feed solution based on their sizes or affinity. In general, membranes are classified based on their material, their morphology, and their average pore size. Membrane materials can be organic and inorganic. Organic membranes, such as polysulfone (PSf ), polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), polyamide-imide (PAI), are the most common polymeric membranes for lab and industrial applications. On the other hand, the most common inorganic membranes are metallic, ceramic, and zeolite membranes. Although inorganic membranes, such as metals or ceramics, exhibit superior mechanical, chemical, and thermal properties compared to the polymeric ones, they are more expensive to fabricate and so not preferred [9]. In terms of cross-section morphology, membranes are divided into two types: anisotropic (asymmetric) and isotropic (symmetric) membranes as shown in Fig. 1.1. Anisotropic membranes include composite membranes, integrated asymmetric membranes and supported liquid membranes, while isotropic membranes consist of microporous membranes, nonporous dense membranes, and electrically charged membranes [10]. Regarding pore size, membranes can be categorized into dense and porous membranes. As can be observed in Table 1.1, porous membranes are used in microfiltration (MF), ultrafiltration (UF), and membrane distillation (MD) processes. In all other types of membrane processes, dense membranes are utilized. Table 1.1 shows the most common types of membrane separation processes. MF is employed to separate solutes with a diameter higher than 100 nm [11]. The MF membranes can separate sand, clays algae, and some bacteria. UF membranes are denser than MF membranes and can remove solutes with a diameter in the range of 10–100 nm.

Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00001-8

© 2020 Elsevier Inc. All rights reserved.

1

2

Nanocomposite membranes for water and gas separation

Fig. 1.1 Schematic diagrams of membranes with different morphologies. Table 1.1 Characteristics of various membrane processes Membrane process

F/P phase

Driving force

Membrane morphology

Separation principle

Microfiltration (MF)

L/L

ΔP

Porous 10
1–10 μm

Sieving mechanism

Ultrafiltration (UF)

L/L

ΔP

Porous 10
2–10
1 μm

Sieving mechanism

Nanofiltration (NF)

L/L

ΔP

Dense/porous 10
3–10
2 μm

Solutiondiffusion

Reverse osmosis (RO) Gas separation (GS)

L/L

ΔP

G/G

ΔP

Dense 10
4–10
3 μm Dense and porous

Pervaporation (PV) Electrodialysis (ED)

L/G

ΔP

Dense

L/L

ΔE

Dense IEM

Solutiondiffusion Solutiondiffusion and sieving mechanism Solutiondiffusion Donnan exclusion

Forward osmosis (FO) and dialysis

L/L

ΔC

Dense

Solutiondiffusion

Membrane Distillation (MD)

L/L

ΔT, ΔP

Porous 0.2–1.0 μm

Vaporliquid equilibrium

Application

Separation of macromolecular-tocellular-size particles (Bacteria/fat and some proteins) Separation of molecularto-macromolecular-size particles (all proteins) Separation of ionic-tomolecular-size particles (lactose) Separation of ions (all minerals) Separation of gases

Separation of liquid mixtures Separation of ions mostly in desalination of water Separation of neutral low-molecular-weight particles based on diffusion Removal of volatile components, concentration of solutions

Overview of membrane technology

UF membranes can separate all species removed by MF, as well as some viruses and humic organic materials. Nanofiltration (NF) membranes are denser than UF membranes and have a higher hydrodynamic resistance, which requires a stronger driving force (pressure) for filtration. NF membranes can separate solutes in the range of 1–10 nm. Reverse osmosis (RO) membranes are considered nonporous membranes, which can separate even very small, monovalent ions, such as Na+. Gas separation (GS) membranes can be porous and dense depending on the desired purification level. The most widely used GS applications [12, 13] are air separation (nitrogen or oxygen enrichment), hydrogen separation (e.g., the separation of H2 from N2, CO, and CH4 in petrochemical plants and refineries), and separation of CO2 from natural gas (gas sweetening). Pervaporation (PV) is a process used to separate more volatile components in liquid mixtures through a dense membrane. The driving force is a partial pressure difference generated by the application of a vacuum at the permeate side [14,15]. Due to the partial pressure difference at feed and permeate sides, the more volatile liquid vaporizes at a certain point within the membrane, then the vapor passes through the membrane and finally condenses at the permeate side. Membrane distillation (MD) is a thermally driven membrane separation process where vapor molecules transport from a feed stream to a permeate stream through a microporous hydrophobic membrane. In MD only water vapor passes through the membrane. Theoretically, a complete separation can be achieved. The major drawbacks of MD are low permeate flux and high susceptibility to flux decline over time due to both temperature and concentration polarization phenomena [16]. Electrodialysis (ED) is an electrically driven membrane separation process in which charged ions are separated from a feed solution through ion exchange membranes [17]. Ion exchange membranes are synthesized by attaching charged groups to the polymer backbone of the membrane material. Therefore, ions with the same charges are repelled, while ions with the opposite charge are free to permeate through. An anion exchange membrane, a membrane with fixed positive groups, allows only the permeation of negatively charged ions and a cation exchange membrane allows permeation of positively charged ions. In an ED process, cationic and anionic membranes are alternately arranged between an anode and a cathode plate. By applying an electrical potential, the ions migrate toward anode and cathode and the water is deionized [10,18,19]. Forward osmosis (FO) is an osmotically driven membrane separation process in which water molecules move from a feed solution (FS), with lower solute concentration, toward a higher concentration draw solution (DS) through a semipermeable membrane. The major advantage of FO over the other pressure-driven membranes such as RO is its low-energy consumption, as it relies on the natural osmosis phenomenon for separation, and creates less fouling. The main challenges of the FO process are the internal concentration polarization (ICP) inside the porous support as well as treatment, recovery, and

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Nanocomposite membranes for water and gas separation

Fig. 1.2 Schematic diagram of types of filtration modes: (A) cross-flow and (B) dead-end filtration.

recycling of the draw solution [20–23]. FO is not only limited to wastewater treatment and desalination but, with some minor tweaking, can also be used in other applications, such as power generation [24]. Membrane performance is generally measured by two parameters: flux and selectivity. The flux of a membrane can be expressed as the volume of a substance that permeates through a membrane per unit area over a period of time. The most common units used in the literature to describe the flux for water treatment applications are L m
2 h
1 (LMH) and gal ft.
2 day
1 (GFD). The common units for gas separation are Barrer [1 Barrer ¼ 10
10 (cm3(STP) cm)/(cm2 s cmHg)] and GPU [1 GPU ¼ 10
6 (cm3(STP))/(cm2 s cmHg)]. The selectivity of a membrane can be expressed by retention (R) when diluting aqueous mixtures consisting of solvent (mostly water) and solutes or the separation factor (α), which is mainly used for gas separation. There are two types of filtration modes: the crossflow and the dead-end filtration systems (Fig. 1.2). These types mainly depend on the direction of the feed streams with respect to the membrane surface and the permeate direction. In the crossflow filtration, the direction of the feed stream is tangential to the membrane surface and perpendicular to the permeate. The shear forces resulting from the interaction between the feed stream and the membrane surface help to remove some of the materials deposited on the membrane surface [25]. The dead-end mode is more prone to fouling as both the feed stream and the permeate are perpendicular to the membrane surface and the accumulation of the rejected particles increases on the membrane surface with time [26].

1.2 Transport phenomena in membranes The transport phenomenon across a membrane happens when a driving force, such as pressure, concentration, temperature, and electrical gradients, is applied to the system. In general, there are two models used to describe the permeation through membranes:

Overview of membrane technology

the pore-flow and the solution-diffusion models for transport in porous or dense membranes, respectively. In the following section, more details about these models related to water and gas separation are presented.

1.2.1 Water transport through polymeric membranes Based on their morphology, membranes for water separation can be categorized into porous and dense. 1.2.1.1 Porous membranes The separation of porous membranes is based on the sieving mechanism in which molecules larger than the membrane pores are rejected while smaller ones pass through. The pure water flux through a porous membrane can be described using Darcy’s law:
J ¼ A pf
pp ¼ AΔp, (1.1) where J is the volumetric flux, A is the hydraulic permeability constant, pf is the pressure at the feed side, pp is the pressure at the permeate side and Δp is the transmembrane pressure across the membrane. The hydraulic permeability constant, A, depends mainly on membrane morphology, including porosity, tortuosity, average pore size distribution, and feed characteristics, such as viscosity. Two approaches are commonly used to determine this constant, depending on the membrane’s internal structure. The HagenPoiseuille approach [26] assumes that the internal structures are a set of parallel cylindrical pores across the membrane with a length equal to the membrane thickness. Flux is then written as follows: J¼

ε r 2 ΔP , 8φτ δ

(1.2)

where ε is the membrane porosity, r is the average pore size, φ is the feed viscosity, τ is the tortuosity, and δ is the membrane thickness. The second approach, Kozeny-Carman [26], assumes the membrane structure as closed packed spheres: J¼

ε3 ΔP , 2 2 Kφ S ð1
εÞ δ

(1.3)

where ε is the membrane porosity, K is the Kozeny-Carman constant, which is a function of the tortuosity and the shape of pores, φ is the feed viscosity, and S is the internal surface area. 1.2.1.2 Dense membranes Molecules with sizes in the same order of magnitude of oxygen, nitrogen, and metal ions cannot be separated by porous membranes. Therefore, dense or nonporous membranes are used, such as NF and RO membranes. Although NF/RO membranes are considered

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Nanocomposite membranes for water and gas separation

nonporous, they do have pores at the molecular level, which allow transport of small molecules via diffusion. In NF/RO membranes, transport is affected by a combination of concentration polarization (CP) and fouling by organic matter, colloidal particles, and microorganisms. These parameters are interconnected and reduce the water permeate flux through membranes over time. Hoek and Elimelech were the first to propose a model to explain the effects of both colloidal fouling and concentration polarization on flux decline, which is called cake-enhanced concentration polarization (CECP) [27]. Based on their model, the total membrane resistance in NF/RO membranes consists of three major components: the hydrodynamic resistance of the membrane (the resistance of the membrane itself ); the resistance of the accumulated colloids fouling at the membrane surface (cake-layer fouling); and lastly the accumulation of ions on the membrane surface (concentration polarization). Eq. (1.4) shows the water flux permeation using the cake enhance concentration polarization model: J¼

Δp
Δπ , μðRm + Rc Þ

(1.4)

where J is the water flux permeated through the membrane, μ is the dynamic viscosity of water, Δπ, Rm, and Rc are the cake-enhanced osmotic pressure difference, hydrodynamic membrane resistance, and the cake layer resistance, and concentration polarization resistances, respectively. Hydrodynamic membrane resistance: The hydrodynamic membrane resistance (Rm) is the resistance of the membrane in the absence of any fouling materials. For pure water filtration, both Δπ and Rc will be removed from Eq. (1.4), and Rm is determined as follows: Rm ¼

Δp Jμ

(1.5)

Cake layer resistance: The cake layer resistance (Rc, 1/m) can be measured by the Kuwabara cell model [28]: Rc ¼

9AK Mc 9AK δc ð1
Ec Þ ¼ , 2 2a2 g∗ 2a g∗ ρp Am

(1.6)

where AK is the Kuwabara correction factor, which depends on neighbored particles in the cake layer. Mc (kg) is the mass of the deposited colloidal particles on the membrane surface measured by a simple mass balance, a is the particle diameter, g∗ is the electroosmotic effect of a swarm of charged colloidal particles, ρp is the density of the colloidal particles, Am is the membrane surface area, δc is the thickness of the cake layer and Ec is the porosity of the cake layer. Another way to measure the cake layer resistance (Rc) is to

Overview of membrane technology

determine permeate flux at a specific pressure, in the absence of ions (Δπ ¼0), and substituting in the rearranged form of Eq. (1.4) as follows: Rc ¼

Δp
Rm Jμ

(1.7)

Concentration polarization (CP): In the absence of fouling, the accumulation of ions on the membrane surface generates a transmembrane osmotic pressure (TOMP), which reduces the effective transmembrane pressure (TMP). The effect of TOMP is represented as the concentration polarization resistance (Δπ), which can be calculated from Eq. (1.8) by measuring the permeate flow rate across a membrane at a known TMP using salt water (Rc ¼ 0): Δπ ¼ Δp
JμRm

(1.8)

It must be noted that the cake layer intensifies the effect of concentration polarization and in the CECP model Δπ must be replaced with cake-enhanced osmotic pressure difference (Δπ∗). The derivation is provided elsewhere [29].

1.2.2 Gas transport through polymeric membranes Fig. 1.3 illustrates the permeation mechanisms of gases through membranes. There are three types of transport mechanisms proposed for porous membranes: Poiseuille flow; Knudsen diffusion; and molecular sieving. The use of any of these models depends on the membrane characteristics (e.g., porosity and average pore size) and mean free path of gas molecules. Capillary condensation and surface diffusion are other mechanisms in porous membranes. Capillary condensation is more common in experiments that include both condensable and noncondensable gases. In this mechanism, the adsorption of a condensable gas by the membrane results in blocking the permeation for the other type. In surface diffusion, penetrants that have a higher affinity to the membrane surface diffuse at a higher rate. In dense membranes, gas transport is governed by the solutiondiffusion mechanism. 1.2.2.1 Poiseuille flow (convective flow) Poiseuille flow happens when the mean free path of a gas molecule is smaller than the membrane pore radius (rp > 5 μ). The membrane’s average pore size is big enough to allow a convective flow of gas molecules in which gas molecules collide with each other and no separation happens. The mean free path of gas molecules is given as follows [30]:   3μ πRT 1=2 γ¼ , (1.9) 2p 2MW where μ is the gas viscosity, R is the universal constant, T is the temperature, p is the pressure, and MW is the gas molecular weight.

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Fig. 1.3 Mechanisms of gas permeation through porous and dense membranes: (A) convective flow, (B) Knudsen diffusion, (C) molecular sieving, and (D) solution-diffusion.

1.2.2.2 Knudsen diffusion Knudsen diffusion is more dominant when the mean free path of gas molecules is bigger than the membrane pore radius (5–10 nm). In this mechanism, a gas molecule has more interaction with the membrane surface than with other gas molecules. Therefore, the permeation rates of different gas molecules are independent of one another. Knudsen diffusion coefficient (Dk) is represented in the following equation [31]:     2rp 8RT 1=2 T 1=2 ¼ 97rp Dk ¼ MW 3 πMW

(1.10)

The maximum separation factor of two gas molecules can be estimated as the ratio between the molecular weights of these gases.

Overview of membrane technology

1.2.2.3 Molecular sieving ˚ [30]. In this Molecular sieving happens when membrane pores are in the range of 5–20 A mechanism, gas molecules pass through the membrane based on their size. Due to its high permselectivity property, this type of membrane has become more promising in gas separation. Zeolite and carbon membranes are typical membranes for gas separation through molecular sieving mechanism. 1.2.2.4 Solution diffusion There are three main steps for gas to transport through a dense membrane: (i) sorption of gas molecules from the feed side into the membrane surface; (ii) diffusion of gas through the membrane material; and (iii) desorption of gas molecules out of the membrane to the permeate side. Molecules smaller than the molecular pore of a membrane transport through diffusion while bigger molecules and condensable ones transport through by sorption phenomenon. Although there are some similarities between gas and liquids in transport through membranes, there are some distinct differences. For example, the affinity between liquids and polymers is generally much greater than between gases and polymers. Thus, the solubility of liquids in polymers is much higher than that of gases [26]. In some cases, the solubility is at such a high value that crosslinking is necessary to avoid polymer dissolution. Permeate gas flux through a dense membrane can be represented by the following equation: J ¼P

p2
p1 , l

(1.11)

where J is the steady-state gas flux, l is the thickness of the membrane, p2 is the partial pressure at the feed side (upstream side), p1 is the partial pressure at the permeate side (downstream side), and P is the permeability coefficient of a gas through a membrane, which is commonly expressed in Barrer. The permeability coefficient (P) of gases through a dense, nonporous, membrane can be defined by the following equation [26]: Permeability ðP Þ ¼ Solubility ðSÞ  Diffusivity ðDÞ

(1.12)

The solubility coefficient measures the amount of penetrant sorbed by a membrane. The diffusivity coefficient measures how fast penetrants can transport through a membrane. The separation of gas A over B (αA/B) in membranes is directly connected to the ideal separation of each of them individually and can be represented as the flux ratio for each gas under the same filtration conditions [32]: αA=B ¼

JA JB

(1.13)

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Since each flux is under the same filtration condition, they can also be written in terms of permeability as follows:    P A S A DA SA DA αA=B ¼ ¼ ¼ (1.14) P B S B DB SB DB From the previous equation, the division term of solubilities is called the selectivity solubility, while the other one is called the diffusivity selectivity. Gas solubility depends on the operating conditions, such as pressure and temperature, as well as gas condensability, and gas-polymer interaction. The solubility selectivity coefficient increases as the difference between the solubility parameters of gases increases. Generally, large gas molecules (heavier gases) are more condensable, and are therefore more soluble than smaller ones. At the same time, the diffusion coefficient of large gas molecules is lower than for smaller molecules. This means there is a tradeoff relationship between the solubility and diffusivity coefficients in which the overall gas separation depends on their relative magnitudes. Generally the diffusivity coefficient has a more dominant effect in glassy polymeric membranes since the separation depends on molecular-sieving (size separation). On the other hand, in rubbery polymeric membranes, the solubility coefficient is more dominant.

1.3 Membrane fabrication methods 1.3.1 Phase inversion Phase inversion is the most versatile method to prepare membranes in which different kinds of morphologies can be obtained for different applications [33]. All phase inversion membranes are based on precipitation of a polymer from an originally homogenous casting solution. The precipitation occurs through a demixing process where a polymer solution (made of polymer and solvent) is transformed from a liquid into a porous solid state due to an exchange between the solvent and a nonsolvent. The precipitation of polymer solution is governed by the thermodynamics and the kinetics of the phase inversion process, which subsequently affect the final morphology of prepared membranes. Thermodynamic and kinetic properties can be primarily controlled by changing the formulation of the casting solution, that is, the type and concentration of polymer, solvent, and additives [34]. Most available commercial membranes are prepared using the phase inversion method by one of the following techniques. 1.3.1.1 Nonsolvent-induced phase separation (NIPS) The procedure of membrane preparation by NIPS method is shown in Fig. 1.4. In this method, the precipitation of the polymer occurs due to the exchange of solvent from the polymer solution and the nonsolvent. After immersing the polymer film in the

Overview of membrane technology

Fig. 1.4 (A–D) Schematic diagram of the preparation process for NIPS membranes and (E–F) [7] SEM images of the internal structures and the skin layer of PES membranes.

nonsolvent bath, phase separation happens initially at the film interface. First, it starts by an abrupt change in the polymer chemical potential, causing a net movement of the polymer toward the film interface. This increases the polymer concentration at the interface until it becomes rigid and forms a skin layer, which prevents further transport of the nonsolvent into the cast film. The formation of the finger-type structure occurs in two stages: the initiation and the propagation. The initiation happens at points where the skin layer ruptures as a result of shrinkage stresses and syneresis. After that, the growth of fingers happens at the ruptured points and propagates toward the bottom of the polymer cast film [35,36]. Due to the continuous demixing process, the polymer solution is separated into a polymer-rich phase that creates the membrane structure and polymer-lean phase that forms the membrane pores. The internal structure is hierarchal with a dense skin layer at the solvent/nonsolvent interface. 1.3.1.2 Thermally induced phase separation (TIPS) The use of TIPS method to prepare microporous membranes was first introduced between 1980s and 1990s [37]. Most commercial MF membranes with isotropic cross-section morphology are made by this method [38]. TIPS relies on removing the thermal energy from a polymer solution to induce phase separation. The polymer solution is first prepared by dissolving a polymer in a solvent at an elevated temperature close to, or higher than, the glass transition temperature of the polymer (Fig. 1.5). The polymer precipitates due to a reduction in the solubility of the polymer by a decrease in solution temperature. Afterward, the solvent is extracted by either evaporation or freeze drying. The internal structures for this type of membranes are either porous honeycomb-like or spherulitic structures, depending on the polymer fraction in the casting solution [37]. One of the unique advantages of the TIPS method is the ability to fabricate membranes

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Fig. 1.5 (A–D) Schematic diagram of the preparation process for TIPS membranes and (E–F) [39, 40] SEM images for two types of membranes morphologies: cellular and spherulitic.

from polymers that are not soluble at room temperature. Moreover, the TIPS phase is binary in the absence of nonsolvent, which makes it easier to control the phase separation process as compared to the ternary NIPS systems. On the other hand, the TIPS method is more energy intensive than NIPS and has limited application range (e.g., not applicable for the fabrication of dense UF or NF membranes) [37]. Both TIPS and NIPS methods can be used to prepare self-supporting hollow fiber or capillary membranes. 1.3.1.3 Evaporation-induced phase separation (EIPS) In EIPS or dry cast method, membrane formation happens due to the evaporation of a volatile solvent from a mixture of polymer solution that contains volatile solvent, less volatile nonsolvent, and a polymer. With the continuous evaporation of solvent, the solubility of the polymer decreases, and the polymer solution separates into a polymer-rich phase and a polymer-lean phase. The polymer-rich solidification forms the solid matrix of the membrane, which surrounds the polymer-lean that is rich with solvent, with nonsolvent filling the pores. Finally, a porous membrane forms after extracting the nonsolvent from the membrane film [41–43] (Fig. 1.6). The membranes prepared by EIPS method are typically highly porous. However, the membrane morphology is significantly affected by the initial nonsolvent concentration and the rate of evaporation. At the low initial concentration of nonsolvent, delayed phase separation may lead to the formation of denser membranes. In addition, at higher drying rates, skinned membranes can be formed due to the high evaporation rate of solvent and nonsolvent, which is called trapping skinning [41].

Overview of membrane technology

Fig. 1.6 (A–D) Schematic diagram of the preparation process for EIPS membranes and (E–F) [42] SEM images of silicone rubber membranes prepared using EIPS method.

1.3.1.4 Vapor-induced phase separation (VIPS) This method was introduced for the first time by Zsigmondy and Bachman in 1918 [44]. In this method, a cast film consists of polymer and solvent is exposed to a vapor atmosphere of nonsolvent molecules (typically water). The precipitation of the polymer occurs due to the penetration of the vapor into the film, which eventually forms a symmetric porous membrane without a dense skin layer [44,45]. The thermodynamic properties of the casting solution in the NIPS and VIPS methods are almost similar, suggesting that the VIPS method should produce membranes with morphologies similar to those produced using the NIPS method. However, the difference between these two methods does not originate from the thermodynamic aspect and the kinetics of phase inversion plays a major role [46]. The main feature of the VIPS method is the slow and uniform diffusion rate of the vapor into the polymer solution resulting in uniform precipitation of the polymer without a sudden change in the polymer concentration [35]. Therefore, the resulting cross-section is symmetric without a skin layer, as shown in Fig. 1.7.

1.3.2 Electrospinning Electrospinning is an electrostatic fiber-formation technique that generates highly porous nanofibrous membranes from a variety of polymers. Electrospun nanofibrous membranes (ENMs) possess several attractive properties, such as interconnected open pores structure, high porosity, pore sizes ranging from several micrometers down to tens of nanometers, and high surface-area-to-volume ratio [47,48]. The formation mechanism of electrospun fibrous is governed by the uniaxial stretching of a viscoelastic solution. Fig. 1.8 shows a typical electrospinning setup, where a polymer solution is charged from a spinneret needle to a collector due to the application of high voltage difference. The applied voltage is

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Nanocomposite membranes for water and gas separation

Fig. 1.7 (A–D) Schematic diagram of the preparation process for VIPS membranes and (E–H) [46] cross-sectional SEM images of PSF membranes with different humidity and solvent concentrations.

Fig. 1.8 (A) Schematic of the electrospinning process, (B) [49] SEM image of the top surface and (C–D) [50] cross-sectional and top SEM images of ENMs.

Overview of membrane technology

typically >5 kV, which is enough to increase the repulsive force within the charged solution to overcome its surface tension, causing a jet to erupt from the tip of the spinneret [51]. The electrospinning parameters and the polymer solution properties can be varied to produce different ENM morphologies.

1.3.3 Stretching Stretching is a membrane fabrication technique developed in the 1970s, and commonly used to prepare UF, MF, and MD membranes (Fig. 1.9). This method is a solvent-free technique, where a polymer is first heated above its melting point, then extruded into a thin film followed by stretching to make it porous. Usually, stretched membranes are prepared in two consecutive steps: (i) cold stretching, which nucleates micropores in the film; and (ii) hot stretching, which controls their structure. Stretching technique is preferable for highly crystalline polymers, in which the crystalline areas provide strength and the amorphous regions provide flexibility to form porous structures [49].

1.3.4 Track-etching Track-etching consists of a two-step process. In the first step, a polymeric film is bombarded with energetic heavy ions, which damage the polymer chemical bonds and forming linear tracks across the film. The second step is the chemical etching process where the damaged tracks are chemically etched to produce hollow channels [53,54]. Track-etched membranes enable precisely control of pore size distribution. Moreover, pore density and pore size are independent parameters and can be controlled in a wide range allowing a simplified relation between water transport properties and the membrane morphology. The most commonly used materials for track-etched membranes are polyethylene naphthalate (PET), polypropylene (PP), and polycarbonate (PC) because of their high

(A)

(B)

1 µm

1 µm

Fig. 1.9 SEM images of the top surface of (A) polypropylene (PP) and (B) high-density polyethylene (HDPE) stretched membranes, prepared by 55% of cold stretching followed by 75% of hot stretching [52].

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Raw film Ion beam

Beamed film

Membranes Etching baths

Porous membrane

Damaged tracks

(A)

(B)

Fig. 1.10 Roll-to-roll track-etching manufacturing process [54]. Cross-sectional SEM images [53] of (A) PC track-etched membrane with cylindrical nonparallel pore channels; and (B) PP track-etched membrane with a tapered end toward the center.

mechanical strength and stability toward acids and organic solvents [49]. Fig. 1.10 shows a few samples of track-etched membranes prepared with different irradiation and chemical treatment parameters.

1.4 Preparation methods for composite membranes A breakthrough in the history of membrane technology was the development of composite membranes, which consist of a thin, dense layer supported by a porous sublayer. Each of these layers is prepared using different methods and materials allowing for flexibility in optimizing each layer independently. Moreover, the top layer, to be used as self-supporting membranes, can be prepared from materials such as elastomers that are difficult or impossible to prepare by conventional phase separation techniques. Using a support layer enables the fabrication of membranes with superior permeability, selectivity, mechanical strength, and chemical and thermal stabilities [11].

1.4.1 Dip coating Dip coating is a straightforward technique for synthesizing composite membranes with a very thin, dense layer supported by a porous sublayer. Fig. 1.11 illustrates the formation mechanism, where a porous substrate (flat sheet or hollow fiber) is immersed in a coating solution that contains a polymer or prepolymer solution. Next, the support layer is removed from the coating solution, resulting in an adhered layer of the coating solution

Overview of membrane technology

Fig. 1.11 Schematic diagram of a roll-to-roll dip-coating process.

on it. The membrane is then placed in an oven to ensure solvent evaporation and crosslinking of the coated polymer on the porous layer. The crosslinking ensures good attachment of the coated layer to the membrane surface since it has no mechanical or chemical stability on its own [11,56,57].

1.4.2 Interfacial (in situ) polymerization Interfacial polymerization (IP) is one of the most popular methods to prepare TFC membranes. In this method, a thin, selective layer, typically made of polyamide (PA), is formed on a microporous support an in situ polymerization reaction (Fig. 1.12). Although Morgan was the first to propose the use of IP to create a very thin PA layer on a porous substrate, his approach was not successfully deployed at industrial scales. In the early 1970s, Cadotte et al. successfully applied this technique to form a series of composite membranes with high performance [58,59]. In this method, two fast-reacting monomers (diamine and polyacyl chloride) are dispersed in two immiscible solutions, one of them is preferably water. First, a porous membrane is impregnated with an aqueous solution (e.g., water) containing the diamine monomers. After that, the impregnated membrane is brought into contact with an organic solution containing the acid chloride. Since both solutions are immiscible, the reaction takes place at the interface in sequential stages [60]. At first, a highly crosslinked layer forms at the interface, called the incipient layer, due to the extremely fast reaction between the monomers. This layer slows down the diffusion of the diamine monomers to the organic solution, shifting the IP reaction into a slowgrowth mode and resulting in a loose polymerized layer in the shape of ridges and valleys. Finally, the reaction stops once the PA layer prevents further diffusion of the diamine monomers into the organic layer [61].

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Fig. 1.12 (A) A schematic diagram shows the formation mechanism of polyamide layer on a PES support membrane using the interfacial polymerization (IP) reaction between MPD and TMC monomers. (B) SEM images of the top surface of the TFC membrane, the porous layer beneath it and the cross-sectional side of the TFC layer [57].

1.4.3 Plasma polymerization Plasma polymerization is a method to directly deposit a very thin, dense film on substrates. The plasma is formed by ionization of an organic monomer gas through an electrical discharge at high frequencies (Fig. 1.13). Plasma polymerization takes place in three stages: (i) the initiation stage, where atoms and free radicals are generated due to the collision of electrons and ions with the gas monomers; (ii) the propagation stage, which is the formation of the polymeric chain; and (iii) the termination process, which is necessary to close the polymer chain. Polymer films produced by plasma polymerization are highly branched and highly crosslinked [63]. Although plasma polymerization has been used to form composite membranes [64], it is primarily utilized as a surface modification method to alter the surface chemistry of membranes and to introduce favorable properties, such as increasing the hydrophilicity and antifouling properties of membrane surfaces [62].

Overview of membrane technology

Fig. 1.13 (A) schematic diagram of the plasma polymerization process and (B) a plasma reactor [62].

1.5 Nanocomposite membranes Although polymeric membranes are widely used for membrane separation processes, they suffer from three main challenges: (i) the tradeoff relation between permeability and selectivity; (ii) low thermal stability; and (iii) membrane fouling. All these challenges are associated with the material properties, not the method of membrane fabrication. Therefore, a new class of membranes was developed in the 1970s by combining nanomaterials within cellulose acetate (CA) membranes to improve the compaction resistance [65]. Later, this unique combination garnered signficiant attention as an alternative method to tune and enhance membrane performance [66]. For example, the most common polymers used in membrane applications, such as polyethersulfone (PES), polyvinylidene fluoride (PVDF), polypropylene (PP), polytetrafluoroethylene (PTFE), are hydrophobic. Therefore, incorporating hydrophilic nanomaterials into these membranes

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Fig. 1.14 Schematic of typical types of nanocomposite membranes.

changes their property from hydrophobic to hydrophilic [67,68]. Nanocomposite membranes are classified into four categories based on the location of the nanomaterials and the type of membrane: (1) conventional nanocomposite; (2) surface-located nanocomposite; (3) thin-film nanocomposite (TFN); and (4) thin-film composite (TFC) with nanocomposite substrate. A schematic diagram of the four types of nanocomposite membranes is shown in Fig. 1.14. Each one of these membranes has unique advantages over the others.

1.5.1 Conventional nanocomposite membranes In conventional nanocomposite membranes, the nanofillers are distributed within the membrane structures and influence the physiochemical properties of the bulk polymer (Fig. 1.15). The most widely used method to prepare this type of membrane is the phase separation technique, where nanomaterials are well dispersed within the polymer solution prior to phase inversion [68–72]. Nanomaterials can be presynthesized and then mixed within the polymer solution, or the fabrication process may include in situ synthesis of the nanomaterials within the polymer solution. The incorporation of nanomaterials within the polymer solution affects the thermodynamic and the kinetic properties

Overview of membrane technology

Fig. 1.15 Schematic diagram illustrating the process of synthesizing conventional nanocomposite membranes.

of the phase inversion process leading to a significant change in the final morphology of the membrane. For example, the addition of nanomaterials into the polymer solution enhances the thermodynamics of the system, which results in a speeding up the demixing rate between the solvent and the nonsolvent. The membrane morphology, in this case, favors finger-like structures. On the other hand, the addition of nanomaterials may also add kinetic hindrance to the system, which results in slowing down the demixing rate. The membrane morphology tends to go from finger-like to sponge-like structures. At low nanomaterial loading, the thermodynamics are more dominant as they increase the demixing process. However, at high nanomaterials loading, the kinetics are more dominant, which results in a slow demixing process [73].

1.5.2 Surface-located nanocomposite membranes The surface properties of membranes such as hydrophilicity, surface charge, roughness, and pore size have a massive influence on the membrane’s performance (Fig. 1.16). Therefore, modifying the membrane surface can significantly change its performance. Locating nanomaterials on the surface is an excellent technique to change the surface properties without affecting the membrane intrinsic structures. Nanomaterials are therefore very suitable to modify available commercial membranes. These types of membranes can be prepared using different methods, depending on the type of nanomaterials and the membrane material. The current methods include self-assembly [74,75], chemical grafting [76,77], electrostatic attraction [78], adsorption reduction [79], layer-by-layer assembly [80], and coating/deposition [81].

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Fig. 1.16 Schematic diagram of preparing surface-located nanocomposite membranes.

1.5.3 Thin-film nanocomposite (TFN) TFN membranes are a class of TFC membranes, which are fabricated by incorporating nanomaterials within the active layer to modify their properties (Fig. 1.17). The term TFN was presented for the first time in 2008 as a new concept of tailoring the performance of RO membranes for water treatment [82]. In general, the active layer of TFC membranes controls its performance in terms of permeability, selectivity, and fouling propensity. Therefore, introducing nanomaterials into this layer could alter its physiochemical properties such as hydrophilicity, surface charge, porosity, or even create water channels inside the active layer, which increases the permeability without a sacrifice in the selectivity [83]. The nanomaterials used in TFN membranes should have a size, functionality, internal structure that fit with the membrane polymer, as well as a suitable interfacial interaction with the membrane polymer [84,85]. The most common method to synthesize TFN membranes is in situ IP reactions between MPD dissolved in an aqueous phase and TMC in an organic phase where the nanomaterials are dispersed within either the aqueous or the organic solutions [85]. After the reaction is completed, the nanomaterials are entrapped within the active layer, which may provide water channels that accelerate water transport but not solutes, increase the hydrophilicity and decrease the fouling propensity [86].

Overview of membrane technology

Fig. 1.17 Schematic diagram illustrates the formation of TFN membranes.

1.5.4 TFC with nanocomposite substrate This class of membranes was mainly developed to reduce the compaction of membranes operating at high pressures. The fabrication of PA TFC membranes with nanocomposite sublayer is shown in Fig. 1.18. Nanomaterials within the microporous membrane support are thought to enhance its mechanical strength, which reduces the collapse of porous structures upon compaction. Results showed that TFC with nanocomposite support membranes have a higher initial permeability due to an increase in the hydrophilicity and porosity and exhibited less flux decline, due to better resistance to compaction compared with a regular one [87]. Recently, this concept was applied in the FO membranes to reduce the internal concentration polarization (ICP) in the porous layer, which results in an increase in the water flux through the membrane. The ICP significantly reduces the osmotic driving force and therefore lowers the permeate flux. It has been proven that an ideal TFC porous layer should be highly porous and hydrophilic with low tortuosity [88,89].

1.6 Conclusion and future prospect Membrane separation processes are currently the world’s leading technology for producing clean water with a contribution of 53% of the total world share [90,91]. Moreover, according to market reports, the global gas separation membrane market is expected grow substantially between 2017 and 2022, with a high expectation to reach USD 2.61 billion by 2022 at a compound annual growth rate (CAGR) of 7.2%. On the research scale, >50,000 research articles related to membrane technology have been published since the 1960s with about two-thirds published between 2008 and 2017, which indicates that membrane technology is growing research area [92]. This book chapter gives an

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Fig. 1.18 Schematic of the fabrication process of TFC with nanocomposite substrate.

overview of membrane technologies, including the common fabrication methods, types of membranes, main limitations and methods to overcome these limitations. The main challenges in membrane separation processes are (i) flux decline due to membrane fouling, and (ii) the tradeoff relationship between permeability and selectivity of a membrane. One of the common methods to overcome these challenges is incorporating nanomaterials within the membrane matrix, which significantly improves the physiochemical properties of membranes. Despite the significant improvements enabled by nanomaterials, there are still some major challenges in synthesizing nanocomposite membranes. These challenges are (i) poor dispersion of nanomaterials in the polymer solution (polymer and solvent), (ii) aggregation of nanomaterials within the membrane matrix which results in defects, (iii) low compatibility between nanomaterials and the membrane, and (iv) weak chemical interaction between nanomaterials and the membrane material, which may result in leakage of nanomaterials. More research is needed to develop long-term, stable nanocomposite membranes.

References [1] C. Charcosset, Membrane processes in biotechnology: an overview, Biotechnol. Adv. 24 (2006) 482–492. [2] M. Pizzichini, C. Russo, C. Di Meo, Purification of pulp and paper wastewater, with membrane technology, for water reuse in a closed loop, Desalination 178 (2005) 351–359. [3] J. Radjenovi
c, M. Petrovi
c, D. Barcelo´, Fate and distribution of pharmaceuticals in wastewater and sewage sludge of the conventional activated sludge (CAS) and advanced membrane bioreactor (MBR) treatment, Water Res. 43 (2009) 831–841. [4] M. Noronha, T. Britz, V. Mavrov, H. Janke, H. Chmiel, Treatment of spent process water from a fruit juice company for purposes of reuse: hybrid process concept and on-site test operation of a pilot plant, Desalination 143 (2002) 183–196.

Overview of membrane technology

[5] V. Mavrov, E. B
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CHAPTER 2

Recent progress in the development of nanocomposite membranes Oindrila Gupta, Sagar Roy

Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ, United States

2.1 Introduction Energy and the environment have always been crucial for social and economic development throughout the world. The supply and use of energy and natural resources must be sustainable and should cause minimal impact on the surrounding ecology. With the growing world’s population and the expansion of the industrial sector, the basic requirements for mankind have increased extensively and require the development of new and efficient technologies with minimal carbon footprint. Current conservation concerns include not only the limited supply of natural resources but also include disposal, recovery and recycling, and the social impact. Significant research and development efforts aim to use renewable energy in conjunction with green technologies while maintaining a very high process efficiency. Among these, membrane technologies and processes are gaining wide recognition as green technologies in various applications, including water and air purification, environmental remediation, green energy, as well as in the food, chemical, and pharmaceutical sectors. In a membrane separation process, the membrane, which is a permselective barrier that allows selective species to pass while retaining others, is the key component. The membrane itself determines the technological and economic efficiency of the overall process. Thus, improvements in membrane materials and membrane design significantly affect the separation performance. Recent advances in chemical synthesis techniques and fabrication processes open opportunities for the development of novel membrane materials that possess superior physical and chemical properties and enhanced separation efficiency. Furthermore, current innovations in nanotechnology, coupled with membrane processes, are resolving the technical challenges associated with standard separation and purification techniques. The unique properties of the nanostructured materials revolutionized the conventional acuity of separation methods, enabling new separation techniques that surpass existing achievements. Nanocomposite membranes (NCMs) are synthesized through the incorporation of nanomaterials (NMs) into the membrane matrix and offer superior performance in terms of both permeability and selectivity. Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00002-X

© 2020 Elsevier Inc. All rights reserved.

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These developments will eventually lead to commercially viable, stable modular systems. The newly developed NCMs are being studied thoroughly for major separation processes, such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), electrodialysis (ED), gas separation (GS), and pervaporation (PV). In conventional membrane-based separations, a “tradeoff” relationship between permeability and selectivity has been observed; a highly selective membrane with improved permeability is difficult to attain. It is possible that NCMs can resolve this paradox for more challenging real-time applications. This chapter discusses the recent progress in the development of novel NCMs and their potential applications in various fields, including water and air separation and purification.

2.2 Membrane materials and processes A membrane is a selective barrier that allows certain species to pass through it but stops others. Such species could be molecules, ions, or other small particles. Depending on the nature of the materials to separate or concentrate, various types of membranes and separation techniques have been developed. The membranes used in different application areas vary widely from each other in terms of their base materials, structures, function, and operating conditions. However, the membranes share various features that help to classify them in different categories, as given in Fig. 2.1. Polymeric membranes, a type of organic membranes, are very well known in the membrane separation industry market due to their excellent performance and low cost. A polymer has to have appropriate characteristics for the intended application, for example, a low binding affinity for separated molecules for biotechnology applications, and has Membrane classification

Origin

Biological

Morphology

Synthetic

Dense

Porous

Single material

Transport mechanism

Geometry

Hollow Tubular fiber

Hybrid Inorganic Organic Composite Symmetric Asymmetric Molecular seiving Glass Metallic Zeolite Ceramic Carbon

Membrane distillation

Pervaporation, Electrodialysis gas separation

Fig. 2.1 Classification of membranes.

Single material

Composite

Spiral wound

Flat sheet

Knudsen Solutiondiffusion diffusion

Liquid

Facilitated transport

Process

Reverse osmosis Nanofiltration Ultrafiltration Microfiltration

Recent progress in the development of nanocomposite membranes

to withstand the harsh cleaning conditions. The most common polymers in membrane synthesis are cellulose acetate, nitrocellulose, and cellulose esters (CA, CN, and CE), polysulfone (PS), polyethersulfone (PES), polyacrylonitrile (PAN), polyamide, polyimide, polyethylene and polypropylene (PE and PP), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylchloride (PVC), etc. The inorganic membranes, consisting of metals and its oxides, can be present in multilayer supporting structures, or as self-supporting structures. Compared to polymeric membranes, inorganic membranes can be more permeable, and can withstand more extreme conditions. To make them fully competitive, their cost price must be reduced by improvements in processing, the introduction of rapid fabrication methods, and possibly, incorporation in hybrid, polymeric structures. Ceramic membranes, a class of inorganic membranes, are produced from inorganic materials such as aluminum oxides, silicon carbide, and zirconium oxide. Ceramic membranes are very resistant to the action of aggressive media such as acids and strong solvents. They are chemically and thermally stable, and mechanically, and biologically inert. However, the inorganic membranes exhibit poor film-forming properties and brittleness, thus creating difficulty in fabrication of defect-free membranes. Recently, hybrid synthetic membranes have become increasingly important in industries requiring separation processes. The inherent advantages of both polymeric and inorganic materials make hybrid membranes suitable for particular separation applications. Depending on the nature of the inorganic materials, the hybrid membrane is often referred to as a nanocomposite membrane. Some of the nanocomposite membranes reported include inorganic nanoparticles such as TiO2, SiO2, Al2O3, Si, Ag, ZnO, ZrO2, Mg(OH)2, CaCO3, TiSiO4, and organic nanoparticles such as graphene oxide (GO), carbon nanotubes (CNTs), etc. and nanoparticle composites such as GO-SiO2, GO-TiO2, SiO2-TiO2, Ag-SiO2. A hybrid membrane contains multiphase, and multiple functionalities arise from the presence of polymer moiety and inorganic (nano) fillers. Membrane materials with enhanced performances may result from the interactions between the polymer and inorganic phases due to van der Waals’ force, π–π interaction, covalent, ionic, or hydrogen bond formation. These specific interface interactions offer more freedom in designing and customizing membranes to overcome the permeability-selectivity tradeoff effect, as well as improved mechanical, thermal, chemical stability for long-term operation. The advantages and disadvantages of the various types of membranes are discussed in Table 2.1. All of these membranes are distinguished from one another based on base materials, surface chemistry, bulk structure, morphology, and production method. Synthetic membranes utilized in a separation process can be of different geometry and of respective flow configuration. They can also be categorized based on their application and separation regime. The best-known synthetic membrane separation processes include water purification, RO, dehydrogenation of natural gas, and removal of cell particles by microfiltration (MF) and ultrafiltration (UF), removal of microorganisms from dairy products, and dialysis.

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Table 2.1 The advantages and disadvantages of the various types of membranes Membranes

Advantages

Disadvantages

Polymeric membranes

• • • •

• Low chemical and thermal

Simple synthesis and fabrication Low cost Enhanced mechanical stability Upscaling and variation in module form possible

stability

• Pore size cannot be controlled

• Follows the tradeoff between permeability and selectivity

Inorganic membranes

• Enhanced chemical, mechanical, and thermal stability

• Pore size can be controlled • Moderate the tradeoff between

• Fragile • Expensive • Difficult to scale up

permeability and selectivity Hybrid nanocomposite membranes

• Can operate at harsh conditions • Good mechanical and thermal stability • Lower requirement of energy • Surpasses the tradeoff between permeability and selectivity • Enhanced separation performance over native polymer membranes

• Brittle at high fraction of fillers in polymeric matrix

• Chemical and thermal stability depends on the polymeric matrix

Membranes and membrane processes are becoming highly important part of our daily life. Application areas include the generation of potable water from the sea, cleaning and concentration of industrial effluents and recovery of valuable constituents, separation of gases and vapors, food and packaging sectors, drug-delivery devices, etc. With the development of novel membrane materials and nanomaterials, advanced synthesis process and fabrication techniques, there is always a degree of randomness about such classifications as shown in Table 2.1, and new classes of membranes with enhanced separation performances can be evolved. Polymer nanocomposites fabricated using carbon black, pyrogenic silica, and diatomite as additives have existed for decades. Although their characterizations and the effect induced by the nanofillers were not fully understood yet, an understanding of the action of these fillers began with the synthesis of polyamide-6 with nanoclay fillers, as published by Usuki et al. in 1993 and Okada A. in 1995, from Toyota R&D. Very quickly, the variety of nanofillers available increased significantly. The different nanofillers used are nanoclays, nano oxides, CNTs, GOs, to name a few. Currently, the development of polymer nanocomposites is one of the most dynamic areas of progress. Nanofillers can significantly improve the different properties of the materials into which they are incorporated, such as optical, electrical, mechanical, thermal properties, or fire-retardant properties, sometimes in synergy with conventional fillers.

Recent progress in the development of nanocomposite membranes

2.3 Nanofillers used in nanocomposite membranes Polymer nanocomposites contain nanosized species that are distributed into the polymer matrix or a phase. Different multiphase solid materials are present in a nanocomposite where one of the phases has one, two, or three dimensions that is lower than 100 nm. Depending on its size and spatial distribution, the nanomaterials (NMs) can be classified as one-dimensional, two-dimensional, or three-dimensional. One-dimensional nanofillers are usually present in the form of plates, laminas, or shells. Two-dimensional nanofiller are nanotubes and nanofibers where the diameter is lower than 0.1 μm. Three-dimensional nanofillers are isodimensional such as nanometric silica beads. Nanofillers are incorporated in the polymer at rates from 1% to 10% by mass.

2.3.1 Nanoclays Nanoclays are nanoparticles containing layered mineral silicates. Depending on the chemical composition and nanoparticle morphology, nanoclays are organized into various groups such as montmorillonite, bentonite, kaolinite, hectorite, and halloysite. A typical schematic of nanoclay structure is shown in Fig. 2.2 [1]. Organically modified nanoclays known as organoclays are a class of hybrid organic-inorganic nanomaterials with possible application in polymer nanocomposites, as rheological modifiers, gas absorbents, and drug-delivery carriers. Nanoplate fillers can be natural or synthetic clays, as well as phosphates of transition metals. Clay-based nanocomposites have enhanced physical performances. The most widely used ones are phyllosilicates. They have a shell-shaped crystalline structure with nanometer-level thickness. Clays are generally categorized according to their crystalline structures and also to the quantity and position of the ions within the elementary mesh. The elementary mesh can be defined as the simplest atomic geometric pattern for duplicating the crystalline network, by reiterating itself open-endedly in the three directions.

Tetrahedral sheet Octahedral sheet

Si OH O Al, Mg

Tetrahedral sheet

Tetrahedral sheet

Exchangeable cationic species Water molecules

Fig. 2.2 The general structure of clay mineral. (Adapted from H.H. Murray, Structure and composition of the clay minerals and their physical and chemical properties, Dev. Clay Sci. 2 (2006) 7–31.)

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2.3.2 Nanooxides Titanium dioxide, a type of nano oxide in its rutile phase with a size that ranges between 200 and 300 nm, is commonly used in polymers as a white pigment. The Anastase form with photocatalytic properties is used as spherical particles with a diameter around 20 nm. It can also be converted by hydrothermal synthesis into titanium nanotubes. These nanotubes have an outer diameter of 10–20 nm, an inner diameter of 5–8 nm and a length of 1 μm. Alumina particles in sizes ranging from 20 nm to micrometric are used as fillers. They are made of spherical crystal particles of Al2O3. Nanoparticles of alumina are frequently used as inert fillers in polymers but show catalytic properties in certain specific conditions. Nano-antimony-Tin oxide (ATO) is another variety of metal oxide. It is a tetragonalshaped crystal particle with a diameter of about 15 nm and is frequently used as a flame retardant. Nanosilica belongs to a vast family of nanoparticles from various origins. The most commonly used is naturally occurring called diatomite. This filler originates from the skeleton of unicellular algae, the diatomea thus forming sedimentary layers. Though precipitated silica are used in polymers, it is not a nanoparticle, as its diameter is between 1 and 10 μm when micronized.

2.3.3 Fullerenes Naturally occurring molecules of carbon, in the form of hollow spheres, ellipsoids, or tubes are known as fullerenes. Fullerenes are a pure form of carbon molecules with a structure similar to that of graphite. Spherical fullerenes are called buckyballs (as shown in Fig. 2.3 [2]). Cylindrical fullerenes are called CNTs. Fullerenes were first produced by laser ablation of a graphite target in helium gas. However, a more recent synthesis method

Fig. 2.3 Buckyball fullerene with sixty carbon atoms. (Adapted from J. Nishinaga, Growth and Characterization of Fullerene/GaAs Interfaces and C60-Doped GaAs and AlGaAs Layers, in: Molecular Beam Epitaxy, Elsevier, 2018, pp. 533–550.)

Recent progress in the development of nanocomposite membranes

is laser vaporization. Fullerenes can also be synthesized by thermal decomposition of hydrocarbon, combustion of hydrocarbon, and thermal plasma pyrolysis of hydrocarbon.

2.3.4 Carbon nanotubes CNTs were discovered by Oberlin et al. and Endo et al. in 1976, without application, and then rediscovered by Iijima in 1991. They are characterized by a nanometer-level diameter and length. In general, three kinds of CNTs are synthesized and used for the development of nanocomposite membranes. Single-wall carbon nanotubes (SWCNT) present a diameter between 1 and 2 nm. Double-wall carbon nanotubes (DWCNT) have a diameter between 2 and 4 nm. Multiwall carbon nanotubes (MWCNT) have a diameter between 4 and 150 nm. Schematics of various types of CNTs are shown in Fig. 2.4 [3]. CNTs are produced in two ways. The first process is known as a catalytic chemical vapor decomposition process that occurs at medium temperatures (600–1000°C). The second production method is an electric discharge process operating under helium at high temperature (3000–4000°C). Both processes produce a mix between SWCNT, DWCNT, and MWCNT, with surface defaults and demonstrate vital catalytic residues. Another important parameter is the chirality of the CNT (i.e., the direction and deviation in the production process of CNT). When studied individually, the processes generate a mix of all possible chiralities. Depending on their purity and dispersion in the matrix, a wide range of new properties are available in nanotubes when used in nanocomposites.

2.3.5 Metallic nanoparticles Examples of some common metallic nanoparticles are nanosilver, nanozinc, and nanogold fillers. These particles have antibacterial properties on the surface due to their catalytic behavior, as well as electrical and magnetic properties.

Single-walled carbon nanotubes (SWNT)

Double-walled carbon nanotubes (DWNT)

Multi-walled carbon nanotubes (MWNT)

Fig. 2.4 Single and multiwalled carbon nanotubes. (Adapted from M. Khalil, B.M. Jan, C.W. Tong, M.A. Berawi, Advanced nanomaterials in oil and gas industry: design, application and challenges, Appl. Energy 191 (2017) 287–310.)

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Nanocomposite membranes for water and gas separation

R O Si O

R O

O

Si

O

R

O

Si

O

R

Si O R O

Si

R Si O Si

R

O Si O

R

Fig. 2.5 Molecular structure of polyhedral oligomeric silsesquioxane (POSS). (Adapted from H. Kavuncuoglu, H. Yalcin, M. Dogan, Production of polyhedral oligomeric silsesquioxane (POSS) containing low density polyethylene (LDPE) based nanocomposite films for minced beef packaging for extension of shelf life, LWT 108 (2019) 385–391.)

2.3.6 Silsesquioxanes It can be defined as a huge synthetic cage of macromolecules. Their chemical formula is R-SiO3/2 with R being alkyl or organofunctional groups. The 3-D symmetry and nanometer size make silsesquioxanes a very good building block for nanocomposites. An example of a silsesquioxane structure is given in Fig. 2.5 [4]. Some examples of Silsesquioxanes include polyhedral oligosilsesquioxanes (POSS), octasilsesquioxanes, and their polymeric derivatives. They are used as 3-D nanocomposites for the production of thin films, monoliths, and fiber-reinforced composites. However, the cost limits their use in polymers.

2.3.7 Boehmite It is an aluminum hydroxide orthorhombic dipyramidal crystal that can be used as a nanofiller in nanocomposites. It can be acquired naturally or by chemical production from saturated solutions. Its raw formula is γ-AlO(OH). A picture of boehmite nanorods is given in Fig. 2.6 [5].

AI O

AI

AI

O AI

OH

AI

AI O

OH

O O

O

O

O

AI

AI O

O

O

OH

AI

AI

AI O

OH

O O

O

O

O AI

O

O

AI

OH O

AI

O

O

OH

OH O

O

O

O

O

OH

OH

OH

OH

O

OH

OH

Fig. 2.6 Molecular structure of boehmite. (Adapted from V. Vatanpour, S.S. Madaeni, L. Rajabi, S. Zinadini, A.A. Derakhshan, Boehmite nanoparticles as a new nanofiller for preparation of antifouling mixed matrix membranes, J. Membr. Sci. 401 (2012) 132–143.)

Recent progress in the development of nanocomposite membranes

2.4 Types of polymer matrix nanocomposites The main component in polymer matrix nanocomposite is the polymer itself. Various kinds of polymers can be used in the preparation of polymer matrix nanocomposites. These polymers are named as: Thermoplastics; Thermosets; Elastomers. Thermoplastics become softer with the application of heat and become more unsolidified as heating progresses. Due to the absence of crosslinking, the curing process of thermoplastics is completely reversible. Thermoplastics can be remolded and recycled due to its ability to soften when heated and solidified when cooled. Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polybenzimidazole, acrylic, nylon, and Teflon are examples of thermoplastics. Thermosets, on the other hand, has crosslinking polymers that crosslink together during the curing process and form an irreversible chemical bond. They do not soften on heating but breakdown chemically at high temperature. Thermosets have improved mechanical properties, high chemical, and heat resistance properties. Thermosets are generally rigid and brittle compared to thermoplastics. Common examples of thermoset plastics and polymers include epoxy, silicone, polyurethane, and phenolic. Thermoset-based nanocomposites are generally most common nanocomposites and are used in many applications, but recently thermoplastic-based nanocomposites have attracted much of the research interest both in industry and academia. Properties mainly depend on the polymer structure, which in turn depends on the chemical composition of the polymer being used, surface morphology, and certain processing parameters. Thermoplastic and thermosets polymer respond differently to heat as a result of their difference in their molecular structure. An elastomer is a viscoelastic polymer having very weak intermolecular forces, low Young’s modulus, and high failure strain compared with other materials. The term, a portmanteau of elastic polymer, is often used interchangeably with rubber, although the latter is preferred when referring to vulcanizates. Each of the monomers, which link to form the polymer, is usually a compound of several elements among carbon, hydrogen, oxygen, and silicon. Elastomers are amorphous polymers maintained above their glass transition temperature (Tg), so that considerable molecular reconformation, without breaking of covalent bonds, is feasible. At ambient temperatures, such rubbers are thus relatively soft and deformable. Their primary uses are for seals, adhesives, and molded flexible parts. Application areas for different types of rubber are manifold and cover segments as diverse as tires, soles for shoes, and damping and insulating elements. The importance of these rubbers can be judged from the fact that global revenues are forecast to rise to US$56 billion in 2020. Natural rubber (NR), neoprene rubber, styrene-butadiene rubber (SBR), ethylene propylene diene rubber (EPDM), polybutadiene (BR), ethylene-vinyl acetate (EVA), silicone rubber are all examples of such elastomers.

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Nanocomposite membranes for water and gas separation

2.5 Polymer matrix nanocomposite processing techniques Polymer nanocomposites are fabricated by either chemical or mechanical processes. The main issue is the uniform and homogeneous dispersion of nanoparticles in the polymer matrix in polymer nanocomposite fabrication process. Because nanofillers tend to aggregate and form micron-sized filler clusters, nanoparticles’ dispersion in the polymer matrix is limited, weakening the properties of the nanocomposites. Efforts to disperse nanofillers uniformly and homogeneously in the polymer matrix with the help of chemical reactions, complicated polymerization reactions, or surface modification of filler materials are in progress. The polymer nanocomposites are fabricated mostly by the methods discussed: Intercalation method refers to the insertion or inclusion of polymer chains into the layered nanoplatelets structure. This process of intercalation can improve the bulk properties such as rigidity, contraction, and flammability. A top-down approach that requires surface modification of nanoplatelets for uniform dispersion of nanofillers in the polymer matrix is known as intercalation. Intercalated morphology develops when polymer chains diffuse into the spacing of a layered structure. The nanoplatelets can be homogeneously dispersed by the following two techniques: the chemical technique and the mechanical technique. The chemical technique is the in-situ polymerization method where nanoparticles are dispersed in a monomer followed by polymerization reactions. The nanofillers swell in a monomer solution, and polymer formation takes place between the intercalated sheets by polymerization technique. The mechanical technique involves the direct intercalation of polymer with the nanoplatelets through solution mixing. The polymer is dissolved in a cosolvent, and nanofillers swell in the solvent. The two solutions are then mixed together. The polymer chains in the solution intercalate into the nanofiller layers and displace the solvent. The melt intercalation method is broadly used in the industry. This method consists of mixing the nanofillers, such as clays, into the liquefied polymer matrix. A mixture of polymer and nanofibers are hardened either statically or under shear. The process is compatible with current industrial processes, such as extrusion and injection molding. Polymers not suitable for phase separation process can be used in this case. A similar technique is melt blending. In this process, polymer powder or pellets are melted that results in a viscous solution, and nanofillers are added to the polymer solution at a high shear rate along with diffusion at high temperatures. The end products’ shape can be made by compression and injection molding or fiber production technique. In situ, polymerization involves the swelling of the nanofillers in a monomer solution as the result of the low-molecular-weight monomer solution’s ability to pass easily in between layers causing swelling. The resulting mixture is then polymerized using radiation, heat, initiator diffusion, or by the organic initiator. The monomer is then polymerized between interlayers creating exfoliated or intercalated nanocomposites. A similar method is the in situ template production where the clay layers are fabricated

Recent progress in the development of nanocomposite membranes

in the presence of polymer chains. In this process, both polymer matrix and clay layers are dissolved in an aqueous solution, and a gel is usually refluxed at high temperature. The polymer chains are locked in inside the clay layers, and nucleation occurs at high temperatures on the polymer chains. The disadvantage of this technique is the decomposition of the polymers at high temperatures. The sol-gel method is a bottom-up approach, and it is based on a principle that is opposite to the methods discussed earlier. In this method, solid nanoparticles are dispersed in the monomer solution, forming a colloidal suspension of solid nanoparticles (sol), they form an interconnecting network between phases (gel) by polymerization reactions followed by the hydrolysis procedure. The polymer nanoparticle 3D network extends throughout the liquid. The polymer serves as a nucleating agent and promotes the growth of layered crystals. As the crystals grow, the polymer seeps between layers and a nanocomposite is formed. Direct mixing of polymer and nanofillers is a top-down nanocomposite fabrication approach based on the breakdown of agglomerated nanofillers during the mixing process. The method of mixing a polymer with nanofillers above the glass transition temperature of the polymer in the absence of solvents is called the melt compounding method. The solvent method/solution mixing involves mixing of polymer and nanofillers in a solution using solvents. The melt compounding method adds nanofibers to the polymer above the glass transition temperature. Here, shear stress is induced in the polymer melt by viscous drag, which is used to breakdown the nanofiller aggregates and promote homogeneous and uniform dispersion of the nanofiller in the polymer matrix. In 2010 Tanahashi et al. reported nanosized spherical particles of silica acting as the dispersed filler for the fabrication of various polymer nanocomposites. He studied the dispersion of silica particles into various polymers by melt-compounding each polymer with the prepared agglomerates [6].

2.6 Applications and recent developments of nanocomposite membranes Polymer nanocomposite membranes (PNMs) are advanced membranes that have discrete nanoparticles (NPs) in their polymer matrices. The addition of inorganic NPs to polymers improves mechanical toughness, optical activity, conductivity, and catalytic activity of the resultant composites significantly and has therefore attracted huge interest. The fabricated nanocomposite membranes have been used for different processes such as gas-gas, liquid-liquid, and liquid-solid separation. They can also be utilized for molecular separations, a varied field that has an impact on processes such as purification of biomolecules, environmental remediation, seawater desalination, and petroleum chemicals and production of fuels. Recent advances in nanotechnologies have increased appreciably the

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use of membrane technologies to improve water treatment. To increase flux and selectivity, reduce fouling, efforts have been made to modify membrane surfaces by using several chemical modification techniques, such as grafting hydrophilic compounds on the membrane. However, a satisfactory solution has yet to be achieved. Membrane-based separation and purifications are energy-efficient green technologies that are highly attractive due to their low cost. Despite this, their extensive use in gas separations has been limited due to the difficulty of achieving high selectivity, which produces high product purity and small operating costs, and good permeability, which in turn reduces membrane area and capital cost. Unfortunately, as the selectivity of conventional polymer membrane materials escalates, permeability decreases and vice versa. Efforts to overcome this critical limitation and merge the mechanical elasticity of polymers with the good selectivity characteristic of well-defined zeolite pores have led to the addition of micron-sized porous NPs, such as zeolite particles to organic polymers. In the 1990s, nanocomposite membranes were originally developed for gas separation processes, where polymers were filled with extremely selective zeolites to improve both permeability and selectivity. Owing to their excellent properties, nanocomposite membranes have also been tested for use in gas separation, sensor applications, proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells, lithium-ion battery, pervaporation (PV), organic solvent nanofiltration, and water treatment. Some of the major applications have been discussed in details here.

2.6.1 Nanocomposite membranes for gas separation In the last two decades, major improvements in the performance of polymeric membranes for gas separation have been made, and our understanding of the relationships among the structure, permeability, and selectivity of polymeric membranes has also been greatly advanced. Some polymeric membranes have already been used in the industry. For instance, a plant separating air into its constituent gases and producing pure nitrogen at nearly 24 t h
1 in Belgium by Praxair Co. began to operate in 1996. Membrane-based gas separation processes have become more attractive over traditional technologies in different gas separation and purification processes, such as natural gas treating, air separation, olefin/paraffin separation, and hydrogen purification. Gas molecules dissolve into the membranes and then transport through the membranes by means of diffusion in the free-volume pores. They offer lower capital and operating costs, lower energy requirements, and ease of operation. Despite these advantages and progress, polymeric membranes are still limited by the tradeoff trend between gas permeability and selectivity [7]. Modifications of the chemical structure of a polymer often lead to an enhancement in permeability at the cost of selectivity or vice versa. Polymer-inorganic nanocomposite materials, made of inorganic nanofillers disseminated at a nanometer level into a polymer matrix, have been investigated for gas separation and may provide a solution to the trade-

Recent progress in the development of nanocomposite membranes

off issue of polymeric membranes. These nanocomposite membranes can be obtained by sol-gel processes or by adding fillers, such as zeolites, silica, alumina, cyclodextrin, and molecular sieving carbons to a polymer matrix. Nanocomposite materials have flexibility and processability of polymers, and the selectivity and thermal stability of the inorganic fillers. Another important and practical method for improving the gas separation performance of polymeric membranes is the incorporation of inorganic materials like silica nanoparticles into the polymer matrix. The presence of nanoparticles in the polymer matrix enhances the mechanical strength and thermal stability of the polymer and increases the permeability of nanocomposite membranes. The permeability (P) of a gas through a membrane is proportional to the solubility (S) and diffusivity (D) of the gas in the membrane (P ¼ D  S). Adding inorganic nanofillers may affect the gas separation in two ways. First, the interaction between polymerchain segments and nanofillers may enhance gas diffusion by disrupting the polymerchain packing and increase the voids (free volumes) between the polymer chains. Second, the hydroxyl and other functional groups on the surface of the inorganic phase may interact with polar gases such as CO2 and SO2, improving the penetrants’ solubility in the nanocomposite membranes. Among all the nanocomposite membranes, polyimide/silica materials have received the most attention for the gas permeation studies. Polymer matrices integrated with highly crosslinked inorganic networks exhibit a wide range of multifunctional properties and have been studied extensively in recent years. The strong interaction between the polymer and the inorganic particles reduces segmental and subsegmental mobility and inhibits segmental packing of the polymer. Also, the interaction between the residual OH groups on the inorganic component and polar gases (such as CO2 and SO2), as well as the morphological changes can give rise to increased solubility associated with an increase of Henry’s contribution. Nanocomposite membranes with inorganic nanofillers embedded in a polymer matrix can provide economical, high-performance gas-separation membranes because the membrane is relatively easy to prepare and suitable for dispersing all kinds of inorganic materials in the organic matrix. Because the introduction of functional groups can improve the dispersion of fillers and change the chemical affinities of penetrants in nanocomposite membranes, modification of fillers and matrices has become an expanding field of research. Much work is still needed to develop polymer-inorganic nanocomposite membranes for gas separation. To determine the performance in separating the pure gases of CO2, CH4, and N2, supported mixed matrix membrane (SMMM) polyether block amide/nanoclay, based on polyacrylonitrile on nonwoven polyester (PAN/PE) was fabricated by the spincoating method, following optimization of fabrication conditions for single-layer mixed matrix membranes (SLMMMs). The permeability of CO2, the selectivity of CO2/N2, and the selectivity of CO2/CH4 of SMMM indicated an increase of about 364%, 18%, and 47.8% in comparison to the supported neat membrane [8].

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Nanocomposite membranes for water and gas separation

In developing hydrogen separation membranes, the design of oxide and nanocomposite materials with high mixed protonic-electronic conductivity such as lanthanide niobates and tungstates has been investigated. Nanocomposites with LaNb3O9 and Ni + Cu were prepared by ultrasonic dispersion and wet impregnation, then sintered using conventional thermal sintering and hot pressing. D2O exchange studies demonstrated very fast protonic transport in samples. The protonic conductivity values ranged between 10–4 Ω
1 cm
1 at 400°C but are sufficiently high for the practical application [9]. Nanocomposites with a high mixed ionic-electronic conductivity are promising materials for producing syngas from biofuels. Nanocomposites PrNi0.5Co0.5O3
δ +Ce0.9Y0.1O2
δ and Nd5.5WO11.25
δ + NiCu alloy have been used for these applications. For oxygen separation membranes, CH4 conversion was observed to be 50% with H2 content in the outlet feed being up to 25% at 900°C [10]. Membrane-based technologies are energy-efficient and environment-friendly processes for CO2 separation from large emission sources, for the reduction of CO2 levels in the atmosphere. Fillers like zeolite, carbon, and metal-organic framework (MOF) in MMMs fabrication and their CO2 permeability and CO2/CH4 and CO2/N2 selectivity have been investigated. The order of fillers for improved selectivity of CO2/CH4 and CO2/N2 is carbon > MOFs > zeolites. This is because of the interstitial channel and large surface area, which prefers CO2 over light gases. CO2 permeability is higher than CO2 selectivity during CO2 separation from flue gas in the postcombustion process, whereas both properties are important in CO2 separation from syngas [11]. High free-volume glassy polymers, such as polymers of intrinsic microporosity (PIMs) and polytrimethylsilylpropyne, have garnered attention as membrane materials due to their high permeability. However, loss of free volume over time, or aging, limits their applicability. The introduction of a secondary filler phase can reduce this aging; however cost or instability prohibits scale up for many fillers. A cheap, acid-tolerant, nanoparticulate hypercrosslinked polymer “sponge” as an alternative filler has been investigated. On adding the filler, permeability was enhanced and aging retarded [12]. By developing novel and easily implemented intelligent models, namely, least square support vector machine (CSA-LSSVM) and differential evolution-adaptive neurofuzzy inference system (DE-ANFIS), a recent study investigated the separation of a single gas by a novel membrane design comprising FS and POSS nanoparticles in a polymer matrix of PDMS. The developed LSSVM model is trained and optimized by using CSA and the minimization of a cost function of mean squared error (MSE). The results of the study revealed that the DE-ANFIS model can predict gas permeation more accurately and reliably than its counterpart. This suggests its potential as a precise and reliable ANFIS-based model for use in gas separation processes and the design of membrane technologies [13]. Another article [14] looked into two different types of polyhedral oligomeric silsesquioxanes (POSS) functionalized nanoparticles as additives for nanocomposite membranes

Recent progress in the development of nanocomposite membranes

for CO2 separation: one with amidine functionalization (Amidino POSS) and the second with amine and lactamide groups functionalization (Lactamide POSS). Composite membranes were produced by casting polyvinyl alcohol (PVA) layer, containing either amidine or lactamide functionalized POSS nanoparticles, on a polysulfone (PSf ) porous support. The interaction of the POSS nanoparticles with the porous support increased the crystallinity of the composite membranes, thereby playing an important role in the gas separation performance. The crystalline regions were affected by the conformation of the polymer chains, resulting in a decrease in the gas separation performance [15].

2.6.2 Nanocomposite membranes for pervaporation Recently, there has been considerable interest in organic/inorganic hybrid membranes composed of a polymer matrix and inorganic nanoparticles, which can be widely applied in the fields, including ultrafiltration and pervaporation. The inorganic moiety can improve membrane performance since it can enhance the chemical, mechanical, and thermal stability of hybrid membranes, tune the microstructure and hydrophilichydrophobic balance of hybrid membranes. Several kinds of inorganic nanoparticles, such as SiO2, Al2O3, Fe3O4, ZnO, ZrO2, CdS, and TiO2, have been introduced into polymer matrices to prepare polymer/inorganic nanoparticles membranes. Inorganic particles have been dispersed in the polymeric matrix for preparation of dense or porous composite membranes. The prepared membranes have adjustable physical properties, which are obtained by linking the properties of both organic polymers and inorganic dispersed particles. Owing to its low separation performance in pervaporation and gas separation, polyphenylene oxide (PPO) has also been used as a substrate for the preparation of membranes filled by silica and silane-modified silica nanoparticles. Examining the PV performance of filled PPO membranes with silica and silane-modified silica nanoparticles to separate methanol/MTBE mixtures has validated interesting results. Researchers have been interested in polymer-clay hybrid nanocomposites due to their improved mechanical and thermal properties, reduced thermal expansion, gas permeability, and minimum swelling. The extraordinary success with nylon 6-clay nanocomposites by Toyota researchers has been widely studied. Many studies in hybrid nanocomposite have used various polymer systems, including polyamide, polyimide, polyaniline, polyurethanes, polymethyl methacrylate, to efficiently enhance polymer properties. Wei showed the preparation method of polyimide-clay nanocomposite using mono-, di-, and tri-functional group swelling-agent-modified montmorillonite. Polarity between the clay surface and polymer can be reduced using alkylammonium for modifying clay and maleic anhydride for grafting onto the polyethylene backbone. The bigger the clay minerals, the more effectively the polyimide hybrid properties and gas permeability pathway are enhanced. Polymer nanocomposites are a mixed material comprising an organic polymer matrix in which nanoinorganic material are distributed. The

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inorganic materials increase the physical and mechanical properties of the polymer. The superior properties are apparently due to the nanoscale structure effects and the interaction between the organic and inorganic materials. Pervaporation methods provide more cost-effective substitutes for complex separation processes such as azeotropic mixtures, isomers, or heat-sensitive mixtures. Inorganic particles in nanocomposites include silica, zeolite, and metal oxide nanoparticle. There has been growing interest in discovering novel applications of porous carbons due to their ability to interact with molecules at their surfaces and also within the bulk of the material. Porous carbons are extensively used in gas separation, water purification, catalyst support, chromatography columns, natural gas storage, and electrode materials because of their outstanding hydrophobic nature, high surface area, and large pore volume, and chemical inertness, ideal mechanical and thermal stability. PDMScarbon molecular sieve (CMS), polyvinyl alcohol (PVA)-CMS, and PVA-graphite composite have been used for the removal of benzene from aqueous solution and separation of benzene/cyclohexane mixtures. CMS is one kind of 3D porous adsorbent similar to activated carbons, with homogeneous pore dimensions of the order of small molecular diameters, but the pores are easily blocked, which will lead to smaller permeability and selectivity. Graphite is composed of layers, which form the infinite extension of the polyacene structure [16]. It is a 2D structure with conjugated π bonds, which shows affinity toward aromatics. In addition, incorporation of graphite into the polymer matrix can effectively modify the packing of the polymer chain. Considering the superior properties such as high flexibility, low mass density, effective π–π stacking interaction between CNT and aromatic compounds, CNT with the 1D structure are considered to be an excellent substitute candidate for conventional nanofillers in the fabrication of nanocomposite pervaporation membranes. PVA, being polar and hydrophilic, is an ideal membrane material to separate benzene/cyclohexane mixtures due to the distinct preferential adsorption/solution of PVA toward benzene and cyclohexane since the solubility of benzene in water is one order of magnitude larger than that of cyclohexane. But because of the distinct difference of solubility parameters between benzene/cyclohexane and PVA, PVA often showed lower permeability to benzene and cyclohexane. The NCMs, fabricated with CNTs, are extensively studied in PV for alcohol dehydration. The PVA-multiwalled CNT membrane showed an increase in water flux with the loading of CNTs with unchanged separation factor up to 1 wt% [17]. The results showed that the addition of CNTs lessened the crystallinity in the pristine membrane and stimulated the micro-orientation that in due course reduced the free volume of PVA membrane matrix. Though the existence of CNTs reduces the free volume, the overall flux increases as the CNTs provide an interchanging quicker diffusion path to the permeating molecules. The MMMs, comprising CNTs and polymer polyelectrolyte complexes (PECs) as matrices, had a uniform dispersion of CNTs in the membrane

Recent progress in the development of nanocomposite membranes

matrix. In another study, multiwalled CNTs (MWCNTs) were functionalized with poly-3-hydroxybutyrate (PHB) and then aligned into the chitosan (CS) matrix [18]. The presence of PHB enhances the compatibility of the CNTs that assisted in a uniform distribution into the matrix. The NCMs displayed a comparatively high flux and selectivity for water when used for dehydration of 1,4-dioxane. The CS membrane was also modified with PVA-modified MWCNTs to be used in acetone dehydration. Additionally, the PVA-functionalized MWCNT was bulk aligned on the polyvinylidene fluoride (PVDF) membrane by a simple filtration method and then coated with CS to form a novel three-layer NCM [19]. The novel three-layer CS-thin PVA-MWCNT-PVDF NCM demonstrated a noteworthy improvement in water flux with only a minor decrease in separation factor (SF). In another work by Panahian et al., multilayer MMMs containing CNTs, PVA, PES, and polyester as an inorganic filler and selective top, intermediate and support layers, respectively, were fabricated for dehydration of ethanol-water mixtures [20]. The incorporation of functionalized MWCNTs by diisobutyryl peroxide into CS membrane has also been utilized for alcohol dehydration [21]. In pervaporation, a novel NCM, consisting of a crosslinkable 6FDA polyimide matrix and NH3 functionalized GO (i.e., NHGO) particles, has been studied for dehydration of alcohol. The membrane demonstrated a water permeability of 0.198 mg m
1 h
1 KPa
1 and a water-IPA molar selectivity of 6726, which was 35 times higher than that of the pristine-co-polyimide [22]. The Zwitterionic GO was incorporated into sodium alginate (SA) for improved water-alcohol separation [23]. The GO surface was also modified with methylnicotinamide chloride (MNA) for oil-water separation [24] that was shown to have improved interaction between the nanosheets and the sulfonated polyphenylenesulfone (sPPSU) polymer. The recently synthesized GO-framework (GOF) membranes influenced GO-aldehyde covalent bonds that assist in adjusting the microstructural properties of GOF. The alcohol dehydration performance of the GOF membrane was enhanced considerably in comparison to pristine GO membrane. Hua et al. utilized a zeolitic imidazolate framework (ZIF-90) NPs to fabricate NCMs with Matrimid® polymer for IPA dehydration [25]. NCMs, composed of phosphotungstic acid in SA for dehydration of alcohol by PV, have been studied [26]. The combination of nanosized SA zeolite materials into the PVA matrix displayed a significantly higher water flux and improved SF for the n-butanol-water system. Zeolites are microporous, crystalline alumino silicates that are known to absorb water molecules selectively within their pores when the pore size matches the size of the water molecules [27]. The membrane filled with zeolite NPs showed higher water flux and selectivity compared to the pristine polymer membranes [28]. Polybenzimidazole (PBI) and PBI/ZIF-8 NCMs for pervaporative dehydration of alcohols have been investigated [29]. The sorption and swelling studies demonstrated enhanced PV permeability of PBI/ZIF-8 NCMs, which could be due to the high fractional free volume (FFV) created by large cavities of ZIF-8 particles. Liu et al. fabricated

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Nanocomposite membranes for water and gas separation

SA-based MMMs via incorporation of two-dimensional ZIF-L nanosheets and zerodimensional ZIF-8 NPs for PV dehydration of ethanol [30]. The water flux and SFs for ZIF-L-/ZIF-8-filled membranes were obtained of 1218 g m
2 h
1/1840 and 879 g m
2 h
1/678, respectively. The suitable openings of ZIF-L nanosheets delivered the necessary molecular sieving effect and reduced water channels for faster transport of water molecules.

2.6.3 Nanocomposite membranes for water treatment applications Membrane filtration is an efficient tool for removing particulates or dissolved matters in secondary or tertiary water treatment. Membrane technology is rapidly growing. Improved permeability, selectivity, and antifouling efficiency offer cost- and energyeffective options for treating water for reuse or safe disposal. Membrane development research has recently focused on the addition of inorganic nanomaterials into polymeric materials, to increase membrane chemical reactivity and reduce fouling. There are two types of nanotechnology-enhanced membrane materials: the mixed matrix membrane (MMM) a mixture of nanomaterials (i.e., filler) in a polymer material (i.e., matrix) and a thin-film nanocomposite (TFN) the formation of a thin-film layer with fillers on a porous matrix. MMM and TFN present enhanced mechanical, physicochemical stability and reactivity compared with common membranes. MMM is used for the separation of large molecules, with an increased strength made by including agitated fillers in the matrix. The success of MMM depends on the interfacial quality between the fillers and the matrix. On the other hand, small molecules can be separated in the TFN membrane through nanofiltration (NF) or through RO membrane using solution-diffusion mechanisms. In recent years, nanocomposite membranes have greatly attracted the attention of scientists for water treatment applications such as wastewater treatment, water purification, removal of microorganisms, chemical compounds, and heavy metals. The incorporation of different nanofillers, such as CNTs, zinc oxide, GO, silver and copper nanoparticles, titanium dioxide, 2D materials into polymeric membranes has provided great advances, which include enhancing the hydrophilicity, suppressing the accumulation of pollutants and foulants, enhancing rejection efficiencies, and improving mechanical properties and thermal stabilities. Interfacial polymerization of TFN membranes was first developed by Jeong et al. TFNs are usually formed by embedding molecular sieve nanoparticles throughout a polyamide (PA) thin-film layer of an interfacially polymerized composite membrane. MCM-41 silica nanoparticles were also introduced into the TFN membrane for water purification showing an enhanced performance in comparison with the nonmodified membrane. PA-TiO2 nanocomposite membranes were also reported by Lee et al. and had a rejection value for MgSO4 of around 95% and a high water permeation flux at low pressure.

Recent progress in the development of nanocomposite membranes

Various other nanomaterials can be applied as a filler in nanocomposite membranes. Among the various nanomaterials studied, CNTs and nanosilver (nAg) particles have received a great deal of attention. CNTs have excellent mechanical, electrical, and thermal properties and partial antibacterial properties. CNTs can also alter the physicochemical properties of a membrane. CNT applications in polymer matrices have been partially impeded by their low solubility. Recent studies showed that surface modification, such as chemical or physical modifications like plasma treatment, can increase the dispersing properties of CNTs. Chemical modification, especially using acid, is one of the simplest and most inexpensive methods of enhancing the properties of CNTs. Nanosilver (nAg) particles are effective and well-known antibacterial agents. nAg can be incorporated into the membrane as a filler to deactivate microorganisms during water filtration, thus reducing membrane biofouling. The antimicrobial effects of nAg can be attributed to their capacity to disturb cell membrane functions, to interrupt electron transport chains, and to damage cell protein and DNA. Different oxidation states of silver have been tested in biopolymer composite membranes, and both MMM- and TFN-type of membranes have been generated through conventional methods. The presence of nAg in these membranes can largely enhance membrane antibiofouling properties but are often ineffective in improving membrane permeability and rejection ratio. Although progress has been made on the synthesis of nanocomposite membranes with various nanomaterials, reports of water treatment with membranes containing a combination of different nanomaterials are not widely studied. The successful incorporation of two nanomaterials (e.g., CNTs and nAg) is expected to produce membranes with enhanced permeability, selectivity, and reduced biofouling potential during filtration. Such a nanohybrid membrane would combine the unique properties of each component, potentially producing novel properties through component interaction. In line with the rapid expansion and application of material science and nanoscale engineering over the last three decades, water treatment technologies have experienced significant material-based advancements. More recently, treatment technologies incorporating engineered carbon nanomaterials, such as GO and fullerenes, have demonstrated superior and unique physical and chemical properties compared to traditional analogues. In particular, GO holds considerable potential for broad use in a variety of water treatment applications, including sorption, separation, antimicrobial, and catalysis. Recent progress in crumpling GO into 3D structures has made this endpoint even more attractive, as 3D-crumpled graphene gives rise to aggregation- and compression-resistant material properties, while maintaining the high specific surface area and electronic properties of the 2D flat material analogues. Traditional water treatment membranes manufactured from polymeric materials are designed either as a size-selective sieve or a dense physical barrier, permitting the transport of solutes based on size or differences in diffusion/deposition rates. Membrane design has typically been optimized to balance water permeability with separation specificity, including pathogens, molecules, and ionic retentates. In contrast to

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conventional, passive membrane technologies, reactive membranes are designed and engineered to promote simultaneous filtration and pathogen inactivation/pollutant destruction. Such strategies usually incorporate functional materials into/onto traditional membranes, including Ag and carbon nanomaterials for pathogen inactivation, and zero-valent iron, iron ions (Fe2+), and TiO2 for pollutant transformation and destruction. Engineered graphene and GO have demonstrated significant potential for ultrathin, ultrafast, and yet precise sieving membranes for gas and aqueous ions. Further, graphene/GO potential as a reaction platform, particularly for catalysis, has also been broadly described in numerous reports. Recent observations of intrinsic antimicrobial material properties further highlight graphene-based materials potential for antimicrobial/fouling-resistant water treatment membranes. Previously described flat GO membranes have shown 4–10 times higher flux than that of commercial nanofiltration membranes, while also demonstrating inactivation of 65% Escherichia coli (E. coli) after 1-h surface contact. A novel graphene-based nanofiltration membrane with selective laminar nanochannels has been developed by filtrating the magnesium-silicate-nanoparticles-modified reduced graphene oxide (MgSi-RGO) nanosheets on polyacrylonitrile (PAN) membrane [31]. The MgSi-RGO/PAN composite membrane showed improved selectivity for the separation of neutral solutes and dye solutes from water. Both the physical sieving and the electrostatic interaction are known to participate in the rejection of solutes. Owing to the serious water crisis worldwide, there is an urgent need for novel water treatment technologies that can provide a safe water supply in more energy-efficient, environmentally sustainable ways. The rapid progression of nanotechnology exhibits this possibility. New membranes can be designed using nanoparticles, nanowires, nanosheets, other nanostructured materials, and their composites with the goal of developing efficient water treatment techniques. Besides, the combination of nanomaterials with commercial membranes is also an effective way, not only to enhance the commercial membrane separation performance in permeability, selectivity, the structure robustness and antifouling, but also to render them with new properties, such as antibacterial and photodegradation. Hence, standardization is necessary to mitigate these issues and overcome the challenges.

2.6.4 Nanocomposites membranes for desalination In order to address the growing demand for fresh water due to economic and population growth, water treatment technologies, such as desalination, have been rapidly industrialized in an effort to protect water security. Electromembrane desalination processes, such as electrodialysis and membrane capacitive deionization, belong to a category of desalination technologies, which consist of the elimination of ions from ionic solutions by electrically charged membranes, ion exchange membranes. The challenges associated with ion exchange membranes have drawn the attention of many researchers, who have examined numerous approaches to improve their properties. The incorporation of

Recent progress in the development of nanocomposite membranes

nanomaterials is one of the popular approaches employed. The nanomaterials employed in ion exchange membrane fabrication include CNTs, graphene-based nanomaterials, silica, titanium (IV) oxide, aluminum oxide, zeolite, iron (II, III) oxide, zinc oxide, and silver. Remarkable developments have been accomplished in fabricating ultrapermeable and antifouling membranes. Much of the noteworthy progress is due to promising novel materials for desalination. Among them are aquaporin (AQP) proteins [32] and some carbon-based materials (CBMs) such as CNTs [33] and graphene-based materials [34]. These novel materials provide innovative dimensions for designing next-generation RO membranes. Although early RO membranes were of asymmetric cellulose acetate membranes, they have been replaced by thin-film composite (TFC) polyamide membranes [35]. Compared to the earlier membranes, TFC polyamide membranes show improved water permeability and salt rejection greater than 99.9% for certain SWRO membranes, lower operating temperature range (0–45°C), and better pH tolerance (1
11) [36]. Researchers have made various attempts to optimize the structure and chemistry of TFC polyamide membranes to achieve enhanced separation performance and antifouling properties. These approaches include changing the monomer types and concentration [37–40], modification of the membrane surface [41–43], and posttreatment [44,45]. CBMs, such as CNTs [46–48], nanoporous graphene (NPG) [49], and GO [50,51], are promising membrane materials owing to their excellent water transport properties. The rejection properties of these materials are dependent on their characteristic water channel dimensions, as well as chemical modifications (e.g., the presence of amine, carboxyl, and other groups) [52]. The water permeability of AQPs, CNTs, NPG, and GOF can assist in the preparation of ultrapermeable membranes. However, the major challenges of these membranes are their scalability, stability, and high cost that limits their usage in industry. They are also prone to defects in the rejection layer, which could unfavorably affect the solute rejection of the resulting membrane. Materials such as AQPs, CNTs, and GO can also be incorporated into a desalting matrix (e.g., polyamide thin-film layer) to form the membrane rejection layer. The polyamide matrix is used to preserve the integrity and salt rejection while AQPs, CNTs, and GO can be used as performance enhancers. A key benefit of the TFN methodology is its easy scale up. However, it is important to point out that the separation properties of TFN membranes are mostly limited by the material in the salt-rejecting matrix. Subsequently, only partial enhancement in permeability can be achieved. AQP-based TFN membranes can be fabricated in larger membrane areas and with enhanced membrane stability [53]. The dense polyamide membrane matrix not only performs adequate salt rejection but also acts as a protection shield to improve the membrane’s chemical and biological tolerance. In recent work, Baek and coworkers [54] reported ultrafiltration membranes with vertically aligned CNTs (VA-CNTs) in an

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epoxy. The resulting membranes, with CNT pore diameter of 4.8 nm, showed three times higher water permeability compared to a commercial UF membrane with the similar rejection of polyethylene oxide (PEO). Furthermore, the VA-CNT membranes exhibited a strong biocidal effect with almost 2 log scale bacteria reduction. Chan and coworkers [55] loaded zwitterionic CNTs onto a substrate using vacuum filtration to prepare high-performance RO membranes. The resultant CNTincorporated membranes demonstrated four times higher water permeability with almost identical NaCl rejection. The authors attributed the enhanced membrane performance to the ultra-fast transport properties of CNTs that were uniformly deposited in thin-film rejection layer. Recently, to fabricate TFN membranes, Xue and coworkers [47] functionalized MWCNTs with three different functional groups, namely, carboxyl (MWCNTCOOH), hydroxyl (MWCNT-OH), and amino groups (MWCNT-NH), followed by the incorporation the functionalized CNT into the piperazine aqueous solution. All the MWCNTs-incorporated membranes displayed enhanced pure water permeability and salt rejection. Additionally, owing to the adhesive strength between –NH2 and – COOH in the PA matrix, MWCNT-NH2-incorporated membranes showed superior salt rejection and stability over those of MWCNT-COOH-incorporated membranes. A pristine defect-free graphene monolayer is nonporous and hydrophobic and is impermeable to helium gas [56]. The creation of nanosized pores results in NPG that has exceptional molecular sieving properties. Water can easily pass through these subnanometer pores, while other larger molecular species are rejected on the basis of size exclusion. Cohen-Tanugi and coworkers [57] used MD simulations to investigate the water permeability and NaCl rejection of NPG. The separation performance of the NPG membrane is largely dependent on its pore size and chemistry. The authors also studied the mechanical stability of the NPG membrane, which demonstrated robust stability, enduring pressure of nearly 570 bar. Wang and coworkers [50] vacuum filtered a GO dispersion to form a GOF-based membrane on a PAN nanofibrous substrate. The resulting GOF membranes showed high rejection of dyes and moderate rejection of divalent ions with water permeability of approximately 2 L m
2 h
1 bar
1. The relatively low salt rejection usually found in GO-based PRL membranes may be attributed to the moderately wide channel dimen˚ for GO nanoplates when sion, as the typical interlayer spacing (d) is around 13.5 A enlarged in water [58]. GO-based TFN membranes can be formed by incorporating GO into a polyamide rejection layer via the interfacial polymerization. The incorporated GO nanoplates can increase membrane hydrophilicity and the interlayer channels between these nanoplates may have water channels in order to improve the membrane’s water permeability. Numerous studies have also found improved chemical resistance of GO-loaded TFN

Recent progress in the development of nanocomposite membranes

membranes, better chlorine resistance due to the hydrogen bonding between GO, polyamide that obstructs the first step of chlorination of the substitute of the amidic hydrogen with chlorine [59], as well as better resistance to strong oxidants and chemical agents because of the development of extensive amounts of polyester bonds [60]. GO can be dispersed in either aqueous M-phenylenediamine (MPD) solution [61] or organic TMC solution to develop the GO-based TFN membranes. Chae and coworkers disseminated the prepared GO nanoplates in an MPD aqueous solution, followed by interfacial polymerization with TMC. The subsequent GO-TFN membrane displayed an 80% enhancement in water permeability and considerably enhanced antibiofouling properties compared to those of the pristine TFC membrane. On the other hand, Yin and coworkers [62] prepared GO-based TFN membrane by dispersing the GO nanoplates in the TMC/hexane organic solution. With the increasing GO concentration, the water flux of the GO-TFN improved with a minor decrease in salt rejection for both NaCl and Na2SO4. Due to their extremely permeable nature, AQPs, CNTs, and GO-based TFNs can potentially outperform conventional TFN membranes. The salt-rejecting matrix (e.g., polyamide) maintains the high salt rejection required for desalination applications. Nevertheless, the ultimate permeability of a TFN membrane is largely determined by the matrix, with the loaded nanomaterials only acting as “enhancers.” In addition to the intrinsic rejection properties of AQPs, CNTs, and GO, the incorporation of these materials into the rejection layer may decrease its crosslinking degree [63] or even create defects in the rejection layer. Such issues need to be more adequately addressed in future studies. In addition, long-term leachability of the nanofillers from TFN membranes needs to be systematically studied. Due to the potential nanotoxicity of some nanomaterials (e.g., CNTs and graphene) [64], their leaching into the product water or to the environment can be an important concern that needs to be further addressed.

2.6.5 Nanocomposite membranes for PEM fuel cells Proton exchange membrane fuel cells (PEMFCs, also called polymer electrolyte fuel cells) provide advantages of clean and efficient energy conversion systems for automobiles, portable applications, and power generation. PEMFCs are well suited for a variety of applications by virtue of their efficiency, environment-friendly nature, and high power densities. In recent years, there have been extensive research efforts in the development of newer proton conducting membranes for higher-temperature proton-exchange membrane (PEM) fuel cell. Nafion, the conventional proton-conducting polymer electrolyte membrane, is expensive, mechanically unstable at temperatures above 100°C, and conductive only when soaked in water, which limits fuel cell operating temperatures to 80°C, which in turn results in lower fuel cell performance due to slower electrode kinetics and low CO tolerance. The operation of fuel cells at higher temperatures provides many

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system advantages such as smaller heat exchangers, and easier integration with reformers. Thus, the development of membranes, which are mechanically and chemically stable at higher temperatures (above 100°C), is an active area of research for producing economical fuel cells. The efforts to develop these higher temperature membranes include modification of the conventional host polymers via incorporation of various hygroscopic inorganic nanosized particles or by developing alternate new polymer systems. Considerable research efforts have been devoted to developing organic-inorganic composite membranes that will be able to operate at increased temperatures. Incorporation of hygroscopic inorganic nanomaterials, such as ZrO2, SiO2, TiO2, P2O5, and Zeolite nanoparticles and TiO2 nanotubes and nanowires, into the structure of the PFSA, has been shown to result in composite membranes with promising proton conductivity at high temperature and low relative humidity. Malhotra and Datta first proposed the incorporation of inorganic solid acids in the conventional polymeric ion-exchange membranes such as Nafion to serve the dual functions of improving water retention as well as providing additional acidic sites. They doped Nafion membranes with heteropolyacids, for example, phosphotungstic acid (PTA), and were able to show high fuel cell performance at lower RH and elevated temperature (120°C). Composite perfluorosulfonic membranes containing different types of inorganic fillers such as igroscopic oxides, surface modified oxides, zeolites, inorganic proton conductors show an increased conductivity with respect to the bare perfluorosulfonic membranes and allow operation up to about 150°C. Nafion can also be modified by the incorporation of hygroscopic oxides such as SiO2 and TiO2 to increase water uptake or inorganic solid acids such as ZrO2/SO4. Watanabe et al. modified Nafion PEMs by the incorporation of nanosized particles of SiO2, TiO2, Pt, Pt-SiO2, and Pt-TiO2 to decrease the humidification requirements of PEMs. When operated at 80°C under low humidification, PEMFC, the modified PEMs showed lower resistance than Nafion. This improvement was attributed to the suppression of H2 crossover by in situ Pt along with the subsequent sorption of the water produced on the incorporated oxides. Costamagna et al. incorporated zirconium phosphate into a Nafion membrane and the results obtained were similar. Zaidi et al. embedded heteropolyacids to various extents in sulfonated polyether ether ketone (S-PEEK). Other examples of polymer/inorganic composite membranes include Nafion®/Al2O3, Nafion®/ZrO2, Nafion®/ZrP, Nafion®/PTA, Nafion®/Zeolite, SPEK/ZrO2, SPEEK/ZrP, SPEK/(ZrO2/PTA), and PBI/(SiWA + SiO2). Recently, many different CBMs such as single-walled nanotubes, multiwalled nanotubes, stacked nanocups, and graphitic nanofibers have been used as electrocatalyst supports in fuel cells. The goal is to utilize these carbon nanostructures as highly conductive carbon scaffolds to attain large surface areas on which to disperse platinum nanoparticles. One issue that calls for new and improved carbon materials is the long-term stability of carbon supports under fuel cell operation. Durability tests have indicated that conventional carbon black supports undergo morphological changes and induce

Recent progress in the development of nanocomposite membranes

catalyst aggregation during extended use. CNTs have shown better stability than carbon black. The recent emergence of graphene nanoelectronics has opened a new avenue for utilizing 2-dimensional carbon material as a support in PEM fuel cells. Graphene is a single sheet of sp2-hybridized carbon that can be exfoliated from bulk graphite using mechanical cleavage, thermal exfoliation, and chemical functionalization. Though the chemical oxidation method is convenient to exfoliate graphene sheets via solution-based processes, it introduces functional groups such as carboxyl and epoxides. The presence of these functional groups makes it possible to suspend the individual GO sheets in both polar and nonpolar solvents.

2.6.6 Advanced membranes for lithium-ion batteries (LIBs) LIBs are strong competitors for fuel cells and are usually used in portable electronic devices and microelectronic devices with the advantage of a long life span. Membranes play a key role in both LIBs and PEMFCs by physically separating two different electrodes (anode and cathode) from electrical shorting and simultaneously serving as an electrolyte reservoir for ionic transport [65]. Ion-conducting polymer membranes might enhance lithium battery technology by replacing the liquid electrolyte currently in use and enabling the fabrication of flexible, compact, laminated solid-state structures free from leaks and available in various geometries. Commonly explored polymer electrolytes for these purposes are complexes of a lithium salt with a high-molecular-weight polymer such as PEO. But PEO tends to crystallize below 60 °C, whereas fast ion transport is a characteristic of the amorphous phase. So the conductivity of PEO-LiX electrolytes reaches about 10
4 S cm
1 at temperatures of 60–80 °C. In the case of LIBs, extensive studies have been done to improve their safety performance since the scale-up has promising application in electric vehicles. The electrolytes reacting with the active electrode materials are a primary challenge. As a result, polymer electrolyte is a good material of interest. Since 1973, a lot of studies on solid polymer electrolytes have been carried out such as blending, copolymerizing, crosslinking, and adding nanofillers to modify the polymer host to increase the ionic conductivity. Many kinds of nanoparticles, such as Al2O3, LiAlO2, MgO, SiO2, TiO2, ZrO2, double-layered hydroxide, molecule sieves, and organo-montmorillonite clays, can be directly filled into the polymer membrane to improve the mechanical strength as well as ionic conductivity. Polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP) is a copolymer consisting of crystalline vinylidene fluoride (VdF) and amorphous hexafluoropropylene (HFP) units. PVdF-HFP is very useful as the polymer matrix material in the polymer electrolyte of lithium rechargeable battery, owing to its exceptional chemical stability by the VdF unit, as well as due to the enhanced plasticity by HFP unit. In addition, the incorporation of inorganic nanoparticles to the PVdF-HFP results in improved mechanical strength and to the beneficial features of better lithium cation (Li+) transport

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properties, such as the improvement of electrochemical stability and interfacial compatibility with lithium electrode. Nanoparticles that have been used for these purposes are fumed silica (SiO2), nanocrystalline titania in both anatase and rutile form (TiO2), and alumina (Al2O3). These nanoparticles have been known to result in the enhancement of transport property by the suppression of Li+-F interactions (for Al2O3) and for substantial effects in a polymer matrix structure (for SiO2). TiO2 nanoparticles have also been used to improve transport properties, particularly for the lithium ionic conduction and cationic diffusivity in PEO-lithium ionic salt systems. Lewis acid-base interactions between the TiO2 surface and polymer chains or lithium salt anions are considered a favorable mechanism for the upsurge in ionic conductivity. The addition of nanosized ceramic fillers, such as Al2O3, BaTiO3, and SiO2, on the electrochemical properties of polymer electrolytes (PEs) based on PVdF-co-HFP is also a promising alternative. The incorporation of ceramic fillers leads to an improvement in ionic conductivity, ion transfer, and interfacial stability between lithium electrode-electrolyte. SiO2 in polyethylene glycol dimethyl ether results in an increase in elastic modulus and mechanical strength, which, due to the flexibility of the polymer chains, concurrently leads to an increase in ionic conductivity. Nanofillers can result in Lewis acid-base interactions between the polar surface group of the inorganic filler and electrolyte ionic species that produces added sites for ion migration, consequently refining ion mobility and ionic conductivity. Ceramic nanofillers enhance the mechanical properties of PEs. The incorporation of nanoclay into the host polymer matrix is an efficient method to reduce the ion pair formation. The presence of clay in the PEs could directly influences the mobility of cations while evading the mobility of counter anions. The positive charges on the surface of clay platelets act as Lewis acid centers and compete with Li+ ions to form complexes with the polymer. This results in structural alterations and advancement of Li+-conducting pathways and reduction of ionic coupling, which stimulates the lithium-salt dissociation. Polymer-clay nanocomposites also display higher thermal stability, dimensional stability, and chemical resistance and barrier properties by the addition of only 0.5–5 wt% nanoclay. In situ intercalation polymerization, solution intercalation, and polymer melt intercalation are the primary production pathways used for formulating polymer-clay nanocomposites. Using a liquid electrolyte of LiPF6 in carbonate solvent for gel polymer electrolyte that has been fabricated by nonsolvent induced phase separation (NIPS) method, polyacrylonitrile (PAN)/organic montmorillonite (OMMT) membranes can serve as a host. The PAN/OMMT membrane exhibits high liquid uptake (375.5%), enhanced dimensional stability under 150°C for 15 min, and excellent electrochemical stability up to 4.62 V. PAN/OMMT membrane can be applied for the sustainable safety lithium-ion battery can be realized [66]. MOFs have stimulated research interest in the field of electrochemical energy storage and conversion. The high porosity and versatile functionalities of MOF-related materials

Recent progress in the development of nanocomposite membranes

have been considered promising to stimulate the overall electrochemical performance. Nevertheless, the practical application of MOF-related materials in rechargeable batteries is delayed by many problems, such as the low-tap density and stability as well as high cost [67]. Since the initial works on MOF-177 as an anode88 and MIL-53 (Fe) as a cathode, MOFs have been extensively investigated as electrode materials for LIBs. Depending on their intrinsic properties, MOFs can store lithium ions based on intercalation/deintercalation or conversion reaction mechanisms. As cathodes in LIBs, MOFs are usually made of variable-valence metal ions and/or redox-active ligands. The early works are dedicated to a series of Fe-based MOFs with the Fe3+/Fe2+ redox couple as electroactive sites for lithium storage [68–70]. Another promising category is Prussian blue (PB) and its analogs (PBAs), which contains cyanide-bridged perovskite-type frameworks with big interstitial sites. Various PBAs with several components of alkali metals and transition metals have been fabricated and applied as cathodes in LIBs, such as K0.14Mn1.43[Fe(CN)6]6H2O, Rb0.7 Mn1.15[Fe(CN)6]2.5H2O, K0.1Ni[Fe(CN)6]7H2O [71]. The interaction between MOFs and guest molecules must be optimized for enhanced cycling stability. Another approach is to fabricate MOFs with redox-active metal centers and redox-active ligands. Other than quinone-type ligands, other ligands such as tricarboxytriphenyl amine [72] and tetrathiafulvalene tetracarboxylic acid [73] were also reported as electroactive ligands to construct MOF cathodes.

2.6.7 Nanocomposites membranes for biofuel recovery With the growing worldwide demand for renewable transport fuels, bioethanol has become an attractive alternative. It is typically used as blending agent with gasoline to enhance octane and reduce carbon monoxide and other smog-producing emissions. Besides ethanol, butanol has begun to draw public attention as a next-generation biofuel. Compared with ethanol, butanol has many distinct advantages, including greater combustion value, reduced volatility, and lower freezing point. Due to extreme product inhibition and butanol toxicity, with the current butanol fermentation process, the final concentration of butanol in fermentation broths is usually not high (less than 3 wt%). Distillation is a conventional, yet very energy-demanding, recovery choice for butanol. Consequently, alternate recovery methods that can reduce the recovery costs are the need of the hour, thus enhancing the biobutanol economics, such as liquid-liquid extraction, adsorption, gas stripping, steam stripping, and pervaporation. Widespread research found optimized hydrophobic pervaporation resources to get the most out of the separation performance of butanol-selective membranes in terms of separation factor, flux, and working stability. The most intensively studied materials include polydimethylsiloxane (PDMS), poly(1-trimethylsilyl-1-propyne) (PTMSP), and polyether block-amide (PEBA). PDMS is a combination of “organic-inorganic”

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elastomeric substance, often referred to as “silicone rubber,” displaying outstanding filmforming performance, stability at high temperatures, chemical and physiological inertness. Recently, silicate-filled PDMS membranes have been introduced by Te Hennepe, which established outstanding improvements on membrane performance as a result of the incorporation of zeolites. The formation of zeolite nanocrystals offers the chance to produce thin composite membranes. The easiest way to produce zeolite-polymer composite membrane is solution blending. Recognized approaches, such as organic functionalization of the fillers (e.g., fumed silica), targeting identical polarity of the particle surface groups, with the polymer medium, have been established for improved dispersion of the inorganic phase. The recently developed MOFs include metal centers connected by numerous organic linkers with adjustable surface areas and pore volumes. Compared with zeolites, the organic property of the framework may enable interfaces with the polymer that allow a good compatibility. As a result, integrating MOFs into polymeric matrices to formulate MMMs is supposed to be a simplistic way to increase the separation performance of polymeric membranes. MMMs, comprising porous particle fillers disseminated in a polymeric matrix, bring together the satisfactory features of polymers and particle fillers and have been shown to be favorable materials for PV separation. Commonly used inorganic particles in alcohol separations include silicalite, zeolite, silica, and carbonaceous particles. Nevertheless, these particle-filled membranes have some concerns, which limit their practical application. The primary concern is the incompatibility between the inorganic and polymeric phases, causing defects in the membranes. With such defects, the MMMs display little or no improvement in selectivity over the original polymer membranes. An additional challenge is that the separation properties of the MMMs normally follow distinct tradeoff relations: the higher the selectivity, the lower the permeability. Owing to their enhancement effects on metabolic reactions of these bioprocesses, the application of nanoparticles for biofuel production processes is increasing. Several nanomaterials, such as nanofibers, nanotubes, and metallic nanoparticles, have been reported in biofuel production processes [74]. Lin et al. [75] exploited the biohydrogen yield by adding Fe2O3 nanoparticles in dark fermentative biohydrogen production involving pure cultures of Enterobacter aerogenes in a starch medium. The biohydrogen yield increased from 164.5  2.29 mL H2/g starch to 192.4  1.14 mL H2/g starch when the nanoparticles concentration from 0 to 200 mg/L. Different nanoparticles have been measured in photosynthetic biohydrogenproducing processes. The photosynthetic activity of Chlorella vulgaris was maximized by the addition of optimum concentrations of silver nanoparticles and gold nanorods in batch processes [76]. TiO2 nanoparticles were also used to produce a maximum biohydrogen production rate of 1990 mL H2/L using photosynthetic bacterium Rhodobacter

Recent progress in the development of nanocomposite membranes

sphaeroides [77]. The rate of biohydrogen was enhanced by 50% in the presence of 60 μg/ mL TiO2 nanoparticles. Improvement of the biohydrogen-producing metabolic pathways, such as acetate and butyrate reactions, was observed by the addition of nanoparticles [78]. Photocatalytic hydrogen production comprises the splitting of water molecules into H2 and O2 using a lighting source, in the presence of a photocatalyst. Different semiconductor, nano-based materials have been used in photocatalytic hydrogen production. Due to its nontoxic properties, chemical stability, low cost, and high photocatalytic performance, titanium dioxide (TiO2) is reported to be the most promising photocatalyst [79]. Photocatalytic materials, such as cadmium sulfide nanofibers, were also reported for hydrogen production [80]. Other materials, such as TiO2-graphene nanocomposites, were also analyzed for the photocatalytic performance of hydrogen evolution from water splitting [81]. It was observed that TiO2-graphene nanocomposites possessed higher light absorption and charge separation efficiency than TiO2 nanocomposites alone.

2.6.8 Nanocomposite membranes in fuel cells Biofuel cells (BFCs) use biocatalysts, such as enzymes and microorganisms, for converting chemical energy into electrical energy. These BFCs are a new type of energy-conversion technique that is distinctive from conventional fuel cells, such as H2/O2 and methanol/ O2 fuel cells, in that they can function under reasonable conditions, such as in warm media and at ambient temperatures. Furthermore, if compared with the noble-metal catalysts that are used in traditional fuel cells, the biocatalysts used in the BFCs are more effective and discerning toward the biomass. As such, it is predicted that BFCs will be able to power bioelectronics in vivo, and that applications will be discovered in systems such as implantable biosensors or pacemakers that are used in the human body. It is known that CNTs have exceptional structural and electronic properties, and are being used in the growth of enzymatic BFCs. For example, CNTs have graphene sidewalls that are chemically inactive and extremely hydrophobic, which makes them good supports for the redox mediators that are usually engaged in the electron transfer of biocatalysts, such as enzymes and proteins, or for the transformation and oxidation of the NADH (nicotinamide adenine dinucleotide with hydrogen) cofactor when dehydrogenases are employed as the anode biocatalysts. The use of CNTs might fundamentally ease the direct electron transfer of the enzymes and proteins. On the other hand, CNTs have good conductivity (subject to the type of CNTs used) and a high surface-area-to-weight ratio, along with the ability to form a 3D matrix to be used for both enzyme immobilization and electrode reactions. Nonfluorinated aromatic polymer-based organic-inorganic nanocomposite PEMs provide certain advantages over perfluorinated polymeric membranes. To enhance the stability at elevated temperatures, aromatic hydrocarbon groups could be incorporated directly into the backbone of a hydrocarbon polymer. Various kinds of aromatic

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hydrocarbon-based polymers were developed for applications in fuel cells. Polyarylene polymers, such as polyethersulfones (PES), polyether ketones (PEK) with varying number of ether and ketone functionalities (PEEK, PEKK, PEKEKK), polyarylene ethers, and polyimide (PI), showed high thermal and chemical stability because of the presence of inflexible and bulky aromatic groups. Polyetheretherketones (Victrex PEEK) have excellent thermal oxidation resistance with a glass transition temperature of 143°C. Recently, polybenzimidazole (PBI), polybenzoxazole (PBO), and polybenzothiazole were also considered as potential candidates because of their long-time thermal stability at high temperatures. Du et al. [82] prepared composite membranes composed of sulfonated polyether ether ketone (SPEEK) and silica sulfuric acid (SSA), which was obtained by treating SiO2 nanoparticles with more volatile SO2Cl2. Kim and coworkers prepared sulfonated polyarylene ether sulfone/phosphotungstic acid modified with silica-containing functional group composite membrane. Composite PEM showed improved conductivity compared to the Nafion® membrane under low humidity conditions. Crosslinking of PVA with a multifunctional chemical compound has been widely used to decrease the solubility of the membranes. Yang studied PVA/hydrophilic MMt composite membrane applied on a direct methanol fuel cell. Thanganathan and coworkers studied hybrid nanocomposite membranes containing polyvinyl alcohol (PVA), phosphotungstic acid (PTA), 3-glycidyloxypropyltrimethoxysilane (GPTMS), 3-mercaptopropyltrimethoxysilane (MPTMS), and glutaraldehyde (GA). Dong prepared the sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK)/zirconium sulfophenyl phosphate (ZrSPP)/PTFE composite membranes with porous polytetrafluoroethylene (PTFE) membrane support. The proton conductivity of SPPESK/ZrSPP/PTFE composite membranes significantly increased at 120°C. Mixed-metal alkoxide systems have been attractive because of their potential properties and applications. In the case of combined metal alkoxides, the structure and morphology of the resulting network depended not only on the nature of the catalyst but also on the relative chemical reactivity of metal alkoxides. Metal-oxide-based inorganic fillers improved the mechanical properties, and also contributed to the blocking of fuel, such as methanol, by increasing the transport pathway tortuousness and improving the proton conductivity, which resulted in better cell performance.

2.6.9 Nanocomposite membranes in the food industry Nanocomposites have been considered an interesting route for developing new and advanced materials with enhanced performance properties. Renewable resource-based biopolymers, such as starch, cellulosic plastics, corn-derived plastics such as PLA, and polyhydroxyalkanoates need value-added applications to compete with existing fossil fuel-derived plastics.

Recent progress in the development of nanocomposite membranes

PHAs are biopolymers widely used to produce nanocomposites for food and packaging purposes. Packaging applications for nanocomposites that are expected to attract attention in the near future include antioxidant-releasing films, films containing color, lightabsorbing systems, antifogging and antisticking films, susceptors for microwave heating, gas permeable films, bioactive agents for controlled release, and insect repellant packages. Application of these exceptional materials can be extended by the accumulation of additional properties, such as antimicrobial or antioxidative functions, which occur through the development of nanocomposites formed with numerous types of nanoparticles such as nanoclays, silver nanoparticles, silver-zeolite, metal oxides, and functional biopolymers like chitosan. Bionanocomposite resources with such exceptional functional properties are likely to be used for the advancement of several ground-breaking foodpackaging technologies, such as active and intelligent packaging, high-barrier packaging, nanosensors, freshness indicator, self-cleaning, and nanocoating. Intelligent, or smart, packaging is proposed to screen and deliver statistics about the value of the packaged food or its enclosed environment to forecast or select the food’s shelf life. The intelligent/smart packaging may react to surroundings, alert a consumer to adulteration by pathogens, identify destructive chemicals or damaged products as a result of food deterioration, specify the quality of food, and begin self-healing. The control and progress of nanosized clay platelets made it probable for the formation of smart materials, by merging the wide type of properties offered by the clay with the functionality of organic components. The intelligent packaging application of nanocomposite is primarily based on the ability of the package to offer information about maintaining the product value such as package reliability (leak indicator), time-temperature records of the product, and determining the source of the packaged food products (nano barcodes or radio frequency identification (RFID)).

2.6.10 Nanocomposite membranes in healthcare applications Polymer nanocomposites can be successfully incorporated into a variety of fields of biomedical applications such as drug delivery, tissue engineering, gene therapy, biosensing, and bioimaging. The important role of nanocomposites in drug delivery is already recognized. The distribution of the drugs should be to the target site without adverse effects on healthy tissues, organs, or cells. Polymer nanocomposites have been reported to reform the diagnosis and treatment strategies for diverse diseases through targeted and controlled release [83]. The rational design of drug-loaded polymer nanocomposites may aid in overcoming the several physicochemical limitations that are associated with the solubility and stability of drugs in aqueous media [84,85]. The nanocomposites should possess highly efficient facile drug cargo encapsulation, provide protection against the premature release of drugs, and overcome several intra- and extracellular barriers in the human body [86].

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Cancer is among the leading clinical challenges of mortality worldwide and responsible for 8.8 million deaths as of 2015, according to the World Health Organization. The diagnosis of a tumor at its initial proliferative phase is very tedious, while the situation becomes critical in the case of asymptomatic tumors. There are diverse conventional detection techniques for cancer, including X-rays, MRIs, positron emission tomography, and computed tomography (CT) scans, as well as pap tests. These techniques can be used to detect morphological changes in malignant and benign neoplastic diseases. However, they are usually used in combination with other techniques such as autofluorescence bronchoscopy, single photon emission computed tomography (SPECT), biomolecular markers, confocal microendoscopy, endobronchial ultrasonography, and optical coherence tomography [87]. However, signs of increased metabolism are not sufficient indicators of a tumor. Thus, it becomes crucial to develop novel techniques for tumor detection at early stages. Over the past decade, the application of polymer nanocomposites has been expanded to cancer detection and treatment in terms of identifying tumor cells and selective irradiation of targeted tumor areas [88–90]. The use of magnetic nanoparticle-reinforced polymer matrix nanocomposites has been examined as an attractive platform for diverse therapies [91]. Unlike active targeting, magnetic targeting is a general magnetic-field-based targeting approach that makes use of physical interactions independent of the specific receptor expression [92]. Targeted drug delivery based on magnetism is feasible through the superparamagnetic susceptibility of magnetic nanoparticle-reinforced nanocomposites. For instance, polymer nanocomposites with fluoridated Ln3+-HA/Fe2O3 have been reported to obtain magnetic field stimuli-based cellular imaging of A549 cancer cells for cancer diagnosis [93]. The use of multifunctional magnetic polymer nanocomposites has thus been reported for such applications as MRI agents for in vivo tumor imaging [94] and synergetic cancer therapy involving magnetic fluid hyperthermia directed through MRI [95]. Recently, the involvement of nanotechnology in diabetes management using nanocomposites has been recognized in various respects. CNT-reinforced polymer nanocomposites-based sensors have been reported for glucose detection with a proven advancement in selectivity, sensitivity, linear concentration range, stability, responsiveness, and response time [96]. For diabetes management, a bioactive glass (BG)/patterned electrospun membrane (PEM) nanocomposite dressing has been developed for diabetic wounds, which are a remarkable medicinal challenge for extremely efficient therapies [97]. These results are promising for rapidly stimulating angiogenesis while enabling extremely efficient diabetic wound healing. Polymeric nanocomposites are also of great research interest in tissue engineering, especially due to their biocompatible nature and biodegradable kinetics. As such, the applicability of nanocomposites has been explored from various medical fields, including nerve tissue repair [98] and bone regeneration [99]. The production of polymer/metal nanocomposites on an industrial scale for economic development can provide numerous

Recent progress in the development of nanocomposite membranes

advantages for the community. However, standardization of the synthesis route and production process is necessary to fabricate high-quality products at large scale. In a recent study, incorporated organic-inorganic polyurethane (PU) nanocomposites were prepared for a better understanding of cell signaling and the effect of magnetite nanoparticles on cell proliferation and cell responses. The effects of the iron oxide nanoparticles (IONs) on the properties of polyurethane nanocomposites were investigated. According to the results, the magnetite polyurethane nanocomposites could be a potential choice for cell therapy and tissue engineering, especially nerve repair [100]. In recent years, the combination of natural polymers with nanoparticles has permitted the development of sophisticated and efficient bioinspired constructs. In this regard, the incorporation of bioactive glass nanoparticles (BGNPs) confers a bioactive nature to these constructs, which can then induce the formation of a bone-like apatite layer upon immersion in a physiological environment. It is expected that understanding the principles and the state-of-the-art of natural nanocomposites may lead to breakthroughs in many research areas, including tissue engineering and orthopedic devices [101]. Chitosan-based nanocomposite scaffolds are applied widely in medicine, in the area of drug delivery, tissue engineering, and wound healing. Due to their antimicrobial properties, chitosan matrices incorporated with nanometallic components are promising for wound dressings. Different combinations of Chitosan metal nanocomposites such as Chitosan/nAg, Chitosan/nAu, Chitosan/nCu, Chitosan/nZnO, and Chitosan/nTiO2 for improved healing or infection control has been discussed [102]. Chitosan-based nanocomposite films, hydrogels, or sponges have attracted attention from researches in wound management. Although few works report the antimicrobial properties of other metallic nanocomposites, the efficacy of nanocomposites could only be confirmed by evaluation in in vitro infectious and in vivo models. Silver nanoparticles (AgNPs) are metal-based nanoparticles and have gained considerable attention from researchers in wound-healing applications, owing to their physicochemical and biological properties. Silver nanoparticles and biopolymer-based biomaterials (AgNP-BMs) are noncytotoxic and safe for wound care patients. The unique intrinsic features of AgNP-BMs promote wound healing and effectively control the growth of microorganisms at the wound site. This strategy plays an important role in the treatment of both acute and chronic wounds. Advanced therapeutic approaches of AgNP-BMs and their potential role in wound-healing applications have been investigated [103]. AgNP-BMs are easy to apply to the wound site, as well as to be removed. The raw material and fabrication methodology is inexpensive due to the availability of the materials from natural sources. Ultimately they will fulfill clinician expectations and meet patient needs.

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2.7 Concluding remarks PNMs are impressive separation tools in chemical and pharmaceutical industries. Currently, thousands of membrane units are operating throughout the world, primarily to separate and purify the water, chemicals, food and beverages, and pharmaceuticals. It is estimated that the world demand for membranes will rise 8.5% annually to $26.3 billion in 2019. The development of improved membrane technology, in combination with nanomaterials, makes possible a green, sustainable future, with minimum waste generation around the world. The creation of superior PNMs and enhanced separation technologies can be utilized in food, chemical, and pharmaceutical industries. High selectivity, low energy consumption, moderate cost-to-performance ratio, and compact modular design have enforced the attention and utilization of PNM-based separations. However, despite the success of recent developments in membrane technology, several challenges remain unresolved. The technology is still suffering the flux-selectivity “tradeoff” relationship, and low permeation rate remains a critical issue hindering commercial applications. This issue will be resolved once the role of NP and its interfacial interactions with polymer matrices is understood. The NMs incorporated into the polymer matrix act as physical/chemical binders through the covalent and/or van der Waals forces of interaction with the polymer chains, which optimizes the reinforcing as well as separation characteristics of the membranes. The development of PNMs is one of the most potential areas for answering the practical challenges and constraints associated with separation and purification technologies. With the advancement of novel synthesis techniques, fabrication of nanostructured particles with unique properties becomes possible, which transform the conventional concepts of separation methodologies. This will result in enhanced separation and purification approaches to overcome the limitations and to exceed the present achievements. The newly synthesized nanoarchitectured PNMs have enabled the development of nextgeneration membranes, which are highly promising concerning the stability, permeation, and separation characteristics. These membranes will develop economical and ecofriendly separation and facilitate the development of modular systems that will work to enhanced existing technologies. The implementation of PNMs into sustainable separation strategies will unite and integrate the factors affecting the economy, society, and environment.

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CHAPTER 3

Synthesis route for the fabrication of nanocomposite membranes Atikah Mohd Nasir, Pei Sean Goh, Ahmad Fauzi Ismail

Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia

3.1 Introduction Membrane technology is one of the most effective means of wastewater treatment and gas separation. This is due to the characteristic of membranes itself that are equipped with billions of microscopic holes and pores that permit some constituents to pass through while preventing undesired constituents from passing [1]. Separation of gases is done to recover hydrogen from hydrocarbons, to remove carbon dioxide from natural gas, flue gas, or biogas. Polymeric membranes have been widely used for the separation of gases into their individual components. However, the performance of the polymeric membrane is restricted by the tradeoff limit between permeability and selectivity, which was introduced as Robeson’s upper bound by Robeson in 1991 [2]. The tradeoff means that highly permeable membranes commonly result in low pair selectivity and vice versa. Membrane technology is also favored for water separation such as drinking water treatment, brackish and seawater desalination, and industrial wastewater treatment. However, existing membrane technology for water separation that commonly employs polymeric membrane is also still restricted by their hydrophobicity and high fouling in nature. In the 1990s, the idea of incorporating inorganic material into organic polymeric membrane was introduced to overcome the constraints of the polymeric membrane in order to enhance the performance of the membrane in term of permeability and selectivity [3]. Nanocomposite membranes are made up of the dispersion of nanosized fillers such as metal oxide, carbon-based materials, zeolite, and metal-organic framework (MOFs) throughout the polymeric matrix. These membranes are new classes of advanced materials that offer significant potential in membrane-based technology especially for water and gas separation. The collaboration of nanotechnology and membrane technology contributed to the development of nanocomposite membranes that have been conceived as the dynamic and promising resolution for water and gas separation. To date, polymeric membranes have been incorporated with a wide range of inorganic nanofillers for various applications such as polyurethane (PU)/nickel oxide (NiO) for carbon dioxide gas separation [4], polysulfone (PSf )/hydrous ferric oxide (HFO) for Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00003-1

© 2020 Elsevier Inc. All rights reserved.

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lead removal [5], PA/titanate nanotube-carbon nanotube (TNT-CNT) for desalination of seawater [6] and polyvinylidene fluoride (PVDF)/titanate nanotube (TNT) for decoloration of palm oil mill effluent wastewater [7]. Nanocomposite membranes could enhance the properties, and stability of membranes and contribute to their unique functionalities such as adsorptive, photocatalytic, antibacterial, and oxidative properties. Properties of the polymeric membrane such as flux, mechanical strength, rejection, antifouling, permeability, and selectivity have been enhanced for water and gas purification. Fabrication of nanocomposite membranes mainly involves phase inversion, interfacial polymerization, coating-deposition, chemical grafting, self-assembly and layer-by-layer self-assembly, the first two being the most commonly used methods. The synthesis of nanocomposite membranes requires material selection, solution (dope or monomer solution) formulation, and fabrication technique. By introducing different kinds of nanofillers and additives, the structure and physicochemical properties of the membranes can be altered to meet the application of nanocomposite membrane either for water or gas separation. The membranes processes for water treatment are classified according to the size of the pores: (i) microfiltration (MF), (ii) ultrafiltration (UF), (iii) nanofiltration (NF), and reverse osmosis (RO) [8]. MF and UF are classified as low-pressure-driven membranes, which have a relatively large pore size compared to high-pressure-driven membranes such as NF and RO. The typical pore size range of MF is 0.1–10.0 μm, UF is 0.01–1.0 μm, NF is 0.001–0.01 μm, and RO less than 0.001 μm. RO membrane can be considered as nonporous membranes. MF and NF are mainly produced by the phase separation method, and remove constituents through physical separation, while NF and RO are generated by interfacial polymerization and primarily remove constituents via solution-diffusion mechanism [9]. Gas separation membranes are also nonporous and the mechanism of transport through theses membranes is solution-diffusion. These membranes are primarily produced by evaporative induced phase separation (EIPS) method.

3.2 Material selection Nanocomposite membranes are typically composed of polymer, nanofillers, and additives, such as pore former. The selection of materials is a crucial step in membrane fabrication and development as the characteristics of the materials can significantly affect the physicochemical properties of the resultant membranes. In this section, the materials that are typically used for the fabrication of phase inversion membranes and their effects on membrane properties are described.

3.2.1 Polymer Some commonly used polymers for nanocomposite membrane fabrication are PSf [10], polyethersulfone (PES) [11], PVDF [12], cellulose acetate (CA) [13], polyamide (PA) [14], and polydimethylsiloxane (PDMS) [15]. These polymers have been widely used

Synthesis route for the fabrication of nanocomposite membranes

for various applications, particularly for waste and gas treatment as they allow the formation of a wide range of pore size, are flexible, and relatively low cost. However, for water treatment applications, due to the nature of these polymers which are hydrophobic, the performance of these neat polymeric membranes is still restricted by low water permeability and a tendency to fouling.

3.2.2 Solvent Solvent plays a remarkable role in altering the morphological structure of membranes. Common solvents used for fabrication of phase inversion membranes include N-methyl-2-pyrrolidone (NMP) [16,17] and N,N-dimethylacetamide (DMAc) [18,19]. With the proper solvent selection, the desired membrane structure and, therefore, satisfactory membrane separation performance can be achieved. Solvents have various chemical and physical properties that induce different interactions with polymer chains and also result in different phase inversion processes during membrane fabrication. Therefore, membranes may have solvent-dependent morphologies and separation performances. The solubility parameter is one of the important criteria to be considered during solvent selection. Table 3.1 shows the Hansen solubility parameters of PSf and the applied solvents among the solvents such as NMP, DMAc, and tetrahydrofuran (THF) the solubility parameter of NMP is the most similar to that of PSf. In order to select the best solvent to dissolve PSf material, the total cohesion solubility (δt) parameter of the solvent should be almost the same as the δt of PSf. In this case, PSf shows a δt of 22.93, which is closer to the δt of NMP, which is 22.96 compared to the δt of THF, 19.46 [20]. δD is the dispersion cohesion (solubility) parameter, δH is the hydrogen bonding cohesion (solubility) parameter, δp is the polar cohesion (solubility) parameter, δt is the total cohesion (solubility) parameter, which is calculated as follows: δ2t ¼ δ2D + δ2p + δ2H.

3.2.3 Additives Depending on the application, nanocomposite membranes can be tailored to improve permeability and porosity. In order to enhance pore formation within the membrane structure, water-soluble polymers such as polyvinylpyrrolidone (PVP), polyethylene Table 3.1 Hansen solubility parameters of the applied solvents and PSf [20] Material

δD

δP

δH

δt

j δt,PSf 2δt,Solvent j

PSf NMP DMAc THF

19.7 18 16.8 16.8

8.3 12.3 11.5 5.7

8.3 7.2 10.2 8

22.93 22.96 22.77 19.46

0.00 0.03 0.16 3.47

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glycol (PEG), tetra ethylene glycol (TEG) have been commonly chosen as pore-forming additive [21]. These polymer additives are often used to accomplish the following tasks: increase the viscosity, introduce hydrophilicity, improve pore formation, and suppress macrovoid formation to create membranes with high pure water fluxes. Previous studies have revealed that, due to its excellent pore-forming ability and a wide range of molecular weight (i.e., 10, 24, 40, and 360 kDa), PVP is favored over other organic additives. Usually, the increase in membrane water flux with the addition of PVP is caused by the formation of finger-like or sponge-like structures in the membrane sublayer. It has been previously reported by researchers that PVP of higher molecular weight could lead to the suppression of macrovoids formation and result in denser skin layer and lower water flux [22]. Membranes with a dense structure and a thick top layer are preferable for gas separation [23]. Adding volatile solvents, such as THF, to solvent can delay the solvent-solvent demixing and result in a denser membrane structure with tiny drops of macrovoid. In this way, a substructure of large pore size can be obtained. Furthermore, THF was employed to form the selective layer of the nanocomposite membranes. The addition of nonsolvent additives such as ethanol and ethylene glycol has been widely employed to make the skin layer of the membrane denser [24].

3.2.4 Roles of nanofillers Inorganic nanoparticles have been used more than organic nanoparticles in the fabrication of nanocomposite membranes. Therefore, this chapter focuses on inorganic nanoparticles. During the last few decades, several solutions have been proposed to boost the performance of polymeric membranes for gas and water separation. One of them is impregnating polymeric membranes with inorganic nanofillers. Most recently, various inorganic fillers such as metal oxide [25], carbon-based nanoparticles [26], zeolite [27], and MOFs [28,29] have been incorporated into polymeric membranes to produce high-performance nanocomposite membranes. It is well known that the incorporation of nanofillers into the polymeric membranes enhances membrane hydrophilicity, mechanical strength, solute rejection, and antifouling properties for water separation application [30]. For gas separation application, inorganic nanofillers were added into polymeric membranes to surpass the Robeson upper bound and improve the of permeability and selectivity performance of the polymeric membrane [3]. Permselective membranes must have both high permeability and high selectivity in order to be competitive in gas separation applications. However, these two parameters often exhibit a tradeoff relationship. Recently, various high-performance nanocomposite membranes have been successfully fabricated by incorporating metal oxide [31], zeolite [32], MOFs [33,34], and carbon-based materials [35,36] for gas separation.

Synthesis route for the fabrication of nanocomposite membranes

3.2.5 Classification of nanofillers 3.2.5.1 Metal oxides Commonly, metal oxide nanoparticles are synthesized by physical and chemical approaches. Physical approaches included high-energy ball milling, inert gas condensation, plastic deformation, and ultrasound shot peening, while chemical approaches included microemulsion, oxidation coprecipitation, hydrolysis and thermal decomposition [37]. Oxidation coprecipitation has been widely used due to high yields, easily scalable, effective, faster, and low cost [38]. Metal oxide nanoparticles such as titanium dioxide [39], nickel oxide [4], manganese dioxide [17], zirconia [40], alumina [41] have been integrated into polymeric membranes to enhance their performance for water and gas separation. Metal oxide nanoparticles are easily synthesized and widely used as nanofillers in polymeric membranes due to their high surface area, excellent catalytic activity, hydrophilicity, chemical and thermal resistance, antibacterial and antifungal properties as well as their low cost [42]. Emadzadheh et al. studied the performance of PSf membrane impregnated by TiO2 for desalination [43]. The results show improvement in porosity, hydrophilicity, and pure water flux upon addition of TiO2. The authors found that as the loading of nanofiller increased, the pore size also increased, which mainly contributed to the improvement of water flux. Mondal et al. reported that incorporating nickel oxide nanoparticles into hollow fiber membranes increased membrane permeability almost twice compared to pure PSf membrane [44]. By the addition of 3 wt% of nickel oxide, the membrane showed improvement in porosity, hydrophilicity, and selectivity to the heavy metal ion in aqueous solution. Nickel oxide has also successfully been incorporated into PU for gas separation [4]. This PU/NiO nanocomposite membrane with NiO content of 1.0 wt%, showed 79.21% and 1.57% higher CO2/N2 permselectivity and CO2 permeability than pristine PU. Robeson’s upper bound was crossed by the permselectivity when nickel oxide nanoparticles were added to PU membranes. The high separation of PU/NIO membranes was due to the increase in permeability and permselectivity. 3.2.5.2 Carbon-based nanomaterials Carbon-based materials such as graphene and carbon nanotubes (CNTs) are easy to use, have a large surface area, and are chemically, mechanically, and thermally stable [45]. Therefore, they are promising nanofillers to enhance the performance of polymeric membranes [19,46]. CNTs have superior separation characteristics that improve membrane performance. Goh et al. reviewed the performance evaluation of CNTs in membrane separation processes and found that the smooth, hollow structure of the CNT could facilitate rapid mobility of liquid and gas molecules in the channel, thereby providing higher permeability than pristine polymer membranes [47]. CNTs have been proven as promising fillers in various membranes, due to their antifouling behavior, strength, disinfection properties, rejection, and permeability [46].

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Graphene oxide (GO) is a two-dimensional carbon nanomaterial obtained by exfoliating oxidized layers of graphite via chemical oxidation methods. GO has many functional groups, including carboxyl groups at the edges and hydroxyl and epoxy groups on the basic plane. These oxygen-containing functional groups give GO the potential to form organic links to the host polymer. One of the major hurdles that limit the application of carbon-based materials in large scale operation is the price of the materials. Furthermore, separation and regeneration of the carbon-based materials after gas and water treatment are costly and still remain a challenging task. Hence, the development of hybrid materials such as carbon-based nanocomposite membranes seems to be a promising approach to overcome those limitations. Wan Azelee et al. fabricated thin-film nanocomposite (TFN) membranes by incorporating acid-treated MWCNT-TNT into the PA layer of thin-film composite (TFC) membranes for desalination [6]. The results revealed that membranes containing 0.05% acid-treated MWCNT-TNT enhanced the water permeation approximately 57.45% over neat PA membrane. Furthermore, NaCl and Na2SO4 rejections were as high as 97.97% and 98.07%, respectively. Choi et al. fabricated a PSf/MWCNT nanocomposite membrane by phase inversion, utilizing NMP as a solvent and water as a coagulant [48]. As reported by Mukherjee et al., the addition of GO to PSf membrane has made the membrane highly permeable, hydrophilic, and charged [49]. The authors also found the loading of GO into PSf membrane at only 0.20 wt% efficiently adsorbed heavy metal ions such as Pb(II), Cu(II), Cd(II), and Cr(III) with maximum adsorption capacity of 79, 75, 68, and 154 mg/g, respectively. This finding indicated that GO-based nanocomposite membranes are promising solutions for water treatment. Karkooti et al. evaluated PES/GO nanocomposite membranes for the treatment of produced water samples rich with organic matter from Athabasca oil sands of Alberta [19]. The results illustrated that the PES/GO nanocomposite membranes had higher water flux and rejection of organic matter compared to bare PES membrane. Due to enhanced membrane surface properties by impregnation of GO within the membrane, it also suppressed the fouling tendency by 30% compared to bare PES membrane. 3.2.5.3 MOFs MOFs consist of metal-coordinated through organic ligands to form a porous crystal structure [50]. Because of the exceptionally large surface areas, the high porosity, and the nonappearance of shrouded volumes, MOFs have been widely used in applications such as gas decontamination, gas detachment, heterogeneous catalysis, drug delivery, and sensors [51]. Most recently, impregnating MOFs into a polymer matrix for separation applications experienced rapid growth [50]. Various nanocomposite membranes that incorporate MOF materials such as the Zeolitic Imidazole Framework (ZIF) series, Materials Institute Lavoisier (MIL) series, and Universitetet i Oslo (UiO) series are ideal nanofillers in nanocomposite membrane due to their excellent compatibility with various polymer matrices

Synthesis route for the fabrication of nanocomposite membranes

[14,52–55]. He et al. incorporated UiO-66 into a PES membrane in the form of thin film [56]. This membrane shows promising results with removal efficiency for As (96.5%) and Se (97.4%). However, this kind of membrane has very low water flux, only 11.5 L/m2 h. This could be due to the properties of MOFs, which is hydrophobic [57]. Wang et al. fabricated PDMS/ZIF-7 nanocomposite membrane for pervaporation recovery of butanol from aqueous solution [58]. Pervaporation analysis revealed that this nanocomposite membrane enhanced total flux (1689 g/m2 h) and achieved the highest separation factor (66) as compared to pure PDMS membranes, which have a flux of 1080 g/m2 h and a separation factor of 51. Improvement in flux and separation might have resulted from the enlarged free volume in the polymeric membrane due to the incorporation of superhydrophobic ZIF-7 nanoparticles. 3.2.5.4 Zeolites Zeolites are mineral-crystalline aluminosilicate and hydrated alkali and alkaline earth metals with a tridimensional lattice that are divided into two categories of natural zeolites (clinoptilolite, analcime, limonite, phillipsite, mordenite) and artificial zeolites. [59]. Coupling zeolites into polymeric membranes to be polymer-zeolite membranes made them more flexible, elastic, and cheaper than alumina-based zeolite membranes [60]. This is due to their structure of well-defined pores and a framework that allows significant movement of alkali and alkaline earth metals in order to counteract total negative charge between Si4+ and Al3+ ions. This makes them suitable for heavy metal remediation from water system [61]. For example, Yurekli et al. successfully embedded zeolite into PSf membrane in a flat sheet configuration to treat Ni(II) and Pb(II) in water systems [62]. The result shows PSf/zeolite had a very high adsorption capacity: 682 mg/g and 122 mg/g for Pb(II) and Ni(II) ions, respectively. The water permeability of the membrane is 57 L/m2 h bar. Gorgojo et al. fabricated a PSf/Nu-(62) zeolite nanocomposite membrane for gas separation [32]. A gas permeation test carried out for the 50/50 of (H2/CH4) binary mixture revealed a significant enhancement of hydrogen separation. The H2 permeability and H2/CH4 separation selectivity were 110 Barrer and 13 for neat PSf membranes. PSf/ Nu-(62) nanocomposite membranes were 36 Barrer and 398. The good performance of this nanocomposite membrane is partly due to the molecular sieve of Nu-(62) zeolite, which has two different types of eight-membered-ring channels with limiting dimen˚. sions of 2.4 and 3.2 A

3.3 Fabrication of nanocomposite membranes 3.3.1 Phase inversion technique Fabrication of nanocomposite membranes is mostly based on the phase inversion method in which nanofillers are dispersed in polymer solution prior to the phase inversion

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process, and can be prepared in either flat sheet or hollow fiber configurations. This type of membrane is mainly used in MF or UF processes due to its typical porous structure. The mixture of polymer, pore former, nanofillers, and solvent is called a dope solution. Dope solutions can be fabricated into membranes by phase inversion technique. The composition of dope solutions depends on the desired application of the membranes. Basically, nanocomposite membranes for water application employ 17–20 wt% polymer pellets, 0.5–10 wt% pore former, and 0.5%–30% nanofillers [63,64]. The composition of nanocomposite membranes for gas separation employs higher loading of polymer (25–30 wt%), higher loading of additives such as THF (15–30 wt%), an additional nonsolvent additives such as ethanol (5%) and equivalent loading of nanofillers [31,65]. 3.3.1.1 Casting for flat sheet configuration Casting polymeric dope solution requires glass plate, casting knife, coagulation bath, and washing bath. The casting technique will mold the nanocomposite polymeric dope solution into a flat sheet configuration [25]. Fig. 3.1 shows a schematic diagram of a casting flat sheet membrane set-up. Flat sheet nanocomposite membranes can be fabricated by pouring the polymeric dope solution slowly onto a smooth glass plate at room temperature to avoid the formation of air bubbles. Typically, the solution is cast using a glass rod to form a membrane with the desired thickness and evaporation time [66]. After casting, the flat sheet membrane and the glass plate were immersed in the first water coagulation bath. After the membrane was formed and peeled off the glass plate, it was transferred into a second coagulation bath that contains deionized water and washing bath. Finally, the membrane was dried at room temperature until the time of use.

Fig. 3.1 Schematic diagram of casting flat sheet membrane set-up.

Synthesis route for the fabrication of nanocomposite membranes

Fig. 3.2 Schematic diagram of spinning hollow fiber membrane set-up [67].

3.3.1.2 Spinning for hollow fiber configuration Spinning technique, which involves the continuous production of a single fiber or multiple fibers via extrusion through a spinneret and a solidification process, can convert a polymeric dope solution into a hollow fiber configuration. Fig. 3.2 shows the schematic diagram of a hollow fiber spinning set-up [67]. A spinneret is a main device containing a hollow needle with a larger outer-diameter channel that extrudes the polymer solution, and another channel (smaller inner diameter) that extrudes solvent (bore fluid). Extrusion of the solvent and polymer can be accomplished either through the use of a metering pump or gas extrusion. Spinning methods can be divided into wet and dry-wet spinning. In dry-wet spinning, a dry air gap larger than zero is applied between the end of the spinneret and the surface of coagulation bath. The presence of an air gap enabling stress relaxation of the polymer chain prevents or reduces defects [68,69]. In wet spinning, the polymeric dope solution is directly extruded into a water coagulation bath. The exchange between solvent within the dope and the coagulant (water in coagulation bath) causes phase inversion resulting in the removal of solvent, solidification, and precipitation of the hollow fiber membranes. However, wet spinning can lead to defects such as irregularities and voids [70].

3.3.2 Interfacial polymerization for fabrication of thin film nanocomposite membranes In the 1980s, Cadotte discovered that the method for fabricating composite membranes through interfacial crosslinking of an ultrathin selective layer on the top of a more porous substrate layer could enhance the performance of the membrane in terms of permeability

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and solute rejection [71]. Nanofiltration and reverse osmosis systems frequently employ thin-film nanocomposite (TFN) membranes that are fabricated by the combination of phase inversion and interfacial polymerization for separation application [72]. Recently, various nanofillers, such as metal oxide, carbon-based materials, and MOFs, were successfully integrated into TFN membranes [73]. TFN membranes consist of an ultrathin PA selective layer on the top of a porous support. An ultrathin PA layer was synthesized by an interfacial polymerization method, while the porous support was synthesized by the phase inversion method. Interfacial polymerization involves the use of two immiscible solvent phases, each containing one monomer. Firstly, diamine such as m-phenyl diamine (MPD) is dissolved in an aqueous phase, and secondly, an acyl chloride such as trimesoylchloride (TMC) is dissolved in an organic solvent, such as hexane. Nanofillers such as zeolitic imidazolate framework-8 (ZIF-8) were dispersed into the TMC/hexane solution [57]. Normally concentration of nanofillers within TMC/hexane solution is quite low compared to conventional nanocomposite membrane, which varies between 0.0025 and 0.40 wt% [74]. By mixing both mixtures of two different solvent phases, the diamine diffuses to the organic phase and reacts with the acyl chloride through a nucleophilic substitution to form a PA layer [74]. The support membrane was prepared by the same procedure as flat sheet casting, using the phase inversion technique [72]. Yin and Deng have illustrated the fabrication of TFN membranes by a combination of phase inversion and interfacial polymerization techniques, as shown in Fig. 3.3 [75]. To start the interfacial polymerization between the selective layer and the substrate layer, MPD aqueous solution was poured on the top surface of the substrate membrane to ensure the penetration of MPD solution into the pores

Fig. 3.3 Fabrication of TFN membranes by phase inversion method [75].

Synthesis route for the fabrication of nanocomposite membranes

Fig. 3.4 Interfacial polymerization reaction between MPD (containing NH2-MIL-125(Ti) MOF nanoparticles) and TMC to form a crosslinked structure on the inner side of PSf hollow fiber membrane [34].

of substrate membrane. Then, TMC/hexane solution with nanofillers was poured onto the substrate surface. The excess solutions were drained off from the substrate surface and rinsed with pure n-hexane. The TFN membranes then were dried at ambient conditions and kept in an oven. The resulting TFN membranes were stored in deionized water until further use. Even though TFN is commonly fabricated on flat sheet membranes, most recently Ingole et al. successfully fabricated TFN in a hollow fiber configuration by using cylindrical membrane modules containing five fibers to perform interfacial polymerization on the inner surface of hollow fiber membrane, as shown in Fig. 3.4 [34]. The TFN membranes were fabricated by impregnating small loads of (NH2-MIL-125(Ti) MOF nanoparticles into the PA layer on the inner side of a PSf hollow fiber substrate to enhance the water vapor transport from the mixture gas [34].

3.3.3 Coating or deposition Coating or deposition is a process to locate nanoparticles on the membrane substrate by dip-coating or filtration-deposition. For example, TiO2 nanoparticles were deposited onto various polymeric membranes, including PSf, PVDF, and polyaniline (PAN) surfaces, by dipping it in 1% TiO2 aqueous suspension for a 1 min before it is pressurized at 400 kPa for 2 h using compressed nitrogen gas [76]. On the other side, MOF nanoparticles such as ZIF-8 and ZIF-67 were successfully deposited on top of porous asymmetric polyimide P84® membrane [77]. These membranes were applied by two posttreatments: first dipping into dimethylformamide (DMF) solution; second, filtering the DMF for 10 min at a pressure of 20 bar right after organic solvent nanofiltration. The deposition of MOF nanoparticles by this method allows their controlled location in the TFN film.

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This coating/deposition method guarantees homogeneous coverage of the support with the filler and avoids the loss of reactants during the nanocomposite membrane washing in the IP process.

3.3.4 Chemical grafting One of the techniques to fabricate nanocomposite membranes is by chemical grafting. Chemical grafting of nanoparticles on the membrane surface guarantees a stronger attachment compared to van der Waals and electrostatic interactions. Furthermore, the formation of chemical bonding between nanoparticles and the membrane can decrease the potential for nanoparticles to release into the water stream leading to potential risks to human. Yin et al. successfully incorporated Ag nanoparticles onto the surface of a PA layer via covalent bonding (AgdS) with cysteamine (H2Nd(CH2)2dSH) as a bridging agent [78]. Fig. 3.5 shows the schematic diagram of the immobilization of Ag nanoparticles onto the surface of a PA layer of TFC membrane that used cysteamine to facilitate the chemical grafting process.

3.3.5 Self-assembly The self-assembly technique places nanoparticles onto the specific membrane surface that contains functional groups, such as carboxyl or sulfone groups through coordination and H-bonding interactions [79,80]. The membrane could be pretreated to introduce such functional groups prior to self-assembly process [81]. For example, a hydrophobic PVDF membrane was treated with poly(styrene-alt-maleic anhydride) (SMA) to enrich the membrane surface with the carboxylate group to form H-bonding with TiO2 nanoparticles [82]. The finding showed that self-assembled TiO2 nanoparticle on the PVDF/SMA membrane could enhance the membrane’s hydrophilicity and antifouling capability.

Fig. 3.5 Schematic diagram of immobilization of Ag nanoparticles onto the surface of PA layer of the membrane [78].

Synthesis route for the fabrication of nanocomposite membranes

3.3.6 Layer-by-layer self-assembly The layer-by-layer self-assembly technique was first introduced by Decher [83]. This method forms organic and inorganic films through the alternate deposition of nanoparticles with opposite electrical surface charge (electrostatic interaction), H-bonding groups, or chemical bonding from dilute solution. Mansouri et al. fabricated a thin film of PEI/TiO2 by sequential deposition of the negatively charged substrate in a cationic PEI and then in an anionic TiO2 solution, the process was repeated according to the desired number of nanoparticle layers [84]. For each layer, the substrate was dipped into the solution for certain time, washed with distilled water, and dried. Wang et al. successfully fabricated a polyelectrolyte complex (PEC)/GO nanocomposite membrane using polycation poly(ethyleneimine) (PEI) modified GO within polyanion polyacrylic acid (PAA) [85]. Recently, Zhang et al. successfully deposited a GO framework layer on a Torlon hollow fiber membrane via a layer-by-layer approach [86]. Fig. 3.6 shows the synthesis route of Torlon/GO nanocomposite membrane. The substrate layer was first crosslinked with hyperbranched polyethyleneimine (HPEI), then followed by the sequential deposition of the GO and ethylenediamine (EDA) and followed by an amine-enrichment modification by HPEI.

3.4 Modification of nanofillers Incorporation of inorganic nanoparticles to a polymer matrix can significantly improve thermal, mechanical, rheological, electrical, catalytic, fire retardancy, and optical properties. However, integrating nanofillers to polymeric membranes could lead to particle aggregation and form bulk particles due to Van der Waals forces, reducing the potentially active sites for heavy metal adsorption [37]. Hence, modification of the surface of inorganic nanofillers prior to the incorporation into the polymeric membranes is crucial. The modification could improve the interfacial interactions between both the polymeric matrix and inorganic nanofillers. There are three main methods to modify the surface of inorganic nanofillers: (1) surface absorption by chemical treatments; (2) surface grafting polymeric molecules (3) acid treatment [46,87]. Surface absorption involves reactions with small molecules such as silane coupling agents while grafting polymeric molecules involves the formation of covalent bonds to the hydroxyl groups already present on the inorganic nanofillers. Chemical treatment is commonly employed to enhance the degree of dispersion within the polymeric liquid. For example, to enhance hydrophobic interactions with the syndiotactic polypropylene matrix, an Al2O3 nanoparticle surface was treated with two different silane coupling agents, (3-chloropropyl) triethoxysilane and (octyl)triethoxysilane [88]. Then, the surface of TiO2 nanoparticles was modified by reaction with a γ-aminopropyltriethoxy silane coupling agent [89]. The γ-aminopropyltriethoxy is

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Fig. 3.6 Schematic diagram of the fabrication of Torlon/GO membrane via a layer-by-layer approach [86].

adsorbed on the surface of the TiO2 nanoparticles at its hydrophilic end and interacts with hydroxyl groups that already exist on the nanoparticle’s surface. The second approach for the modification of inorganic nanofillers is grafting synthetic polymers to the nanofiller surface. The main purpose of this modification is to improve the chemical functionality and modify the surface topology of the neat nanofillers and organic materials. As shown in Fig. 3.7, monomers that have low molecular weight can penetrate the aggregated nanoparticles and react with the activated sites on the

Fig. 3.7 Schematic diagram of (A) agglomerated nanoparticles in the polymeric matrix without grafting polymer; (B) nanoparticles further separated due to the presence of grafting polymer [87].

Synthesis route for the fabrication of nanocomposite membranes

nanoparticle surface, hindering the aggregation of nanoparticles. Moreover, the compatibility of the polymeric matrix and nanofillers was also significantly improved upon the functionalization [90]. A common strategy to improve the dispersion of CNTs is through treatment with strong acids such as H2SO4 and HNO3 to make them homogeneously dispersed in the organic solvent such as NMP before blending with polymeric membranes [48]. CNTs treated by acid could form carboxyl groups on the surface, then interact with the polar organic solvent, improving dispersion within the resultant membrane [6,91]. Other strategies include surface grafting, as in the preparation of hyperbranched polyamine-ester (HPAE)-MWNTs or polyacrylic acid (PAA)-MWNTs [46]. New functional groups can be introduced to zeolite materials through several processes of modification, improving substantially its activity and selectivity on the removal of several substances [92]. Many authors modified natural zeolite on environmental applications such as impregnation of Fe and Zr ions into the zeolite [93,94] magnetic Fe-Mn binary oxide-loaded zeolite [95]. The hydrophobicity of MOFs made them not suitable for water applications; hence, there is a need to increase the hydrophilicity of MOF. For example, Park et al. enhanced the hydrophilicity of copper benzene tricarboxylate [Cu3(BTC)2] by treating it with sulfuric acid. The acid-treated-[Cu3(BTC)2] incorporated into PSf membrane showed improved water flux from 175.0 L m2 h for neat PSf membrane to 245.8 L m2 h [96].

3.5 Conclusions Fabrication of nanocomposite membranes for water and gas separation is the main focus of current research due to the new characteristics and functions obtained from their synergistic effects. However, several limitations were encountered during the fabrication of nanocomposite membrane. First, the aggregation of nanoparticles within polymer matrices is a common problem that hindered nanoparticles from homogeneously distributing within the polymeric matrix. Second, nanoparticles can leach from the polymeric matrix, which could result in environmental pollution. This issue must be taken seriously during the fabrication process. In order to improve the dispersion process and prevent the leaching of nanoparticles, several solutions were suggested. These include modification of nanoparticles surface, dispersing nanoparticles within the solvent by sonication before addition of polymer pellet, optimize the loading of nanoparticles within a low range, and locate the nanoparticles in an ultrathin layer on the top of the substrate membranes. The compatibility between nanoparticles and their host polymer is essential to the stability of nanoparticles and optimal nanocomposite membrane performance. Further studies should be directed toward improving the fabrication of nanocomposite membranes, especially physicochemical modifications to enhance the compatibility of inorganic nanofillers and polymeric membranes. Other than that, new fabrication

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techniques need to be established to improve the performance for nanocomposite membranes for wider application. Recently, new fabrication technologies, such as computational-aided design and three-dimensional (3D) printing, have offered the development of nanocomposite membrane with desired designs and structures for various applications, opening new possibilities in material sciences and engineering. With more advances in this field, the nanocomposite membrane can serve as a practical alternative to resolve issues related to membrane separations.

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CHAPTER 4

Transport phenomena through nanocomposite membranes Samaneh Khanlari, Maryam Ahmadzadeh Tofighy, Toraj Mohammadi

Center of Excellence for Membrane Science and Technology, Faculty of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

4.1 Introduction A nanocomposite membrane is a membrane containing particles with at least one dimension smaller than 100 nm. The incorporation of nanoparticles into the polymer matrix helps to overcome some of the disadvantages of the pristine polymer membranes and to develop membranes with higher flexibility, better processability, improved mechanical and thermal properties [1]. These membranes can be used for gas-gas and liquidliquid separations as well as solid-liquid separation. Beside separation processes, many other applications are known for nanocomposite membranes, namely, direct methanol membrane fuel cells, proton exchange membrane fuel cells, lithium-ion batteries, in which nanocomposite membranes are used for purposes other than separation. Presence of nanoparticles in polymer-based membranes not only alters the structure, physical and chemical properties, including porosity, hydrophilicity, chemical, thermal and mechanical stability, but also demonstrates a brilliant ability to render antibacterial and photocatalytic property to the fabricated nanocomposite. Table 4.1 lists the nanoparticles frequently used in membrane studies and their effects on the performance of the membranes. As can be observed in this table, different characteristics, including hydrophilicity, porosity, mechanical properties, thermal properties, fouling resistance, and selectivity, can be tuned, modified, and optimized using appropriate nanoparticles, subject to well-selected loading and fabrication considerations. There are also instances that two or more nanoparticles are employed simultaneously in order to improve the aforementioned characteristics. Moreover, many of the nanoparticles can be modified with different functions, and this functionalization improves their functionality. Accordingly, one can categorize nanofiller incorporated membranes as follows: (i) Single nanoparticle membrane nanocomposite: This class of nanocomposite membrane is the first generation, the most well-known, and the most utilized among nanocomposite membranes as focused in Table 4.1.

Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00004-3

© 2020 Elsevier Inc. All rights reserved.

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Table 4.1 Selective list of different types of nanoparticles and their contribution to nanocomposite membranes performancea Nanoparticles classification

Effects on membrane performance

Ref.

0D

Addition of 0.5% TiO2 nanoparticles improved water permeability without remarkable reverse solute flux Silica content contributed linearly to Young’s modulus increasing in the polyester-based membrane Mitigated biofouling was achieved in a reverse osmosis seawater desalination Irreversible fouling reduced from 40% to 25% as a result of improved hydrophilicity and desirable surface topology Improved hydrophilicity and improved adsorptive heavy metal ions removal were observed by introducing goethite nanorods to the system Polyaniline-incorporated nanocomposite membranes demonstrated higher permeability and fouling resistance Improved permeability and fouling resistance were observed, when graphene nanosheets were employed in the form of the thin-film nanocomposites Improved irreversible fouling resistance, optimum mechanical and thermal properties, as well as increased porosity, were observed in nanocomposites containing 0.5% GO nanosheets At 0.2% loading of nanoclay, CO2 permeability and CO2/N2 and CO2/CH4 selectivity increased 364%, 18%, and 47.8%, respectively Incorporation of MOF in polymer membrane resulted in improved permeability and water selectivity Permeability and selectivity enhanced when using POF

[2]

TiO2 SiO2 Ag Activated carbon

1D

Goethite nanorods (inorganic) Polyaniline nanorods (organic)

2D

Graphene

Graphene oxide (GO)

Nanoclay 3Db

Metal-organic frameworks (MOFs) Porous organic frameworks (POFs) Zeolites

Improved CO2 capture in nanocomposites compared to pristine membranes was observed

[3] [4] [5]

[6]

[7]

[8]

[9]

[10]

[11] [12] [13]

a

There is another classification of nanomaterials, with the possibility of misconception with the one presented in this table, in which nanoparticles are classified as 1D, 2D, and 3D, based on the number of dimensions they have with nanosize. In that categorization nanospheres, nanorods, and nanosheets are 3D, 2D, and 1D nanoparticles, respectively [14]. b According to some references, incorporation of 3D nanoparticles in polymer matrices results in mixed matrix membranes rather than nanocomposite membranes [15].

(ii) Dual nanoparticle membrane nanocomposite: Although incorporation of a single choice of nanoparticles showed to be highly promising, dual nanoparticle nanocomposites, in which two different nanoparticles are dispersed to be used in one filtration system, also gives more versatility toward material modification and membrane performance. For example, one nanofiller can be added to topical layer to

Transport phenomena through nanocomposite membranes

Fig. 4.1 (A) TEM micrograph of TiO2@GO nanofiller. (B) Schematic illustration of mass transportation path through the nanocomposite topical layer in TiO2@GO/PES nanocomposite (Adapted from J. Wang, et al., Construction of TiO2@graphene oxide incorporated antifouling nanofiltration membrane with elevated filtration performance, J. Membr. Sci. 533 (2017) 279-288.)

modify topological aspects in order to improve selectivity, or mitigate fouling and the other one can be dispersed in the base membrane layer to namely improve other characteristics like mechanical properties, pore size, and diameter. In a study by Shon et al., a dual layer nanocomposite membrane was fabricated by means of GO on the topical active layer, and halloysite nanotube on the substrate layer for fouling resistance and high osmotic power density. The results showed that mechanical properties of the membrane are improved, owing to the nanocomposite substrate, and higher fouling resistance and ion selectivity are developed as a result of material modification on the upper layer [16]. (iii) Functionalized/modified nanoparticle nanocomposite: These nanoparticles, which are recently addressed as α@β indicating any α modification on β nanoparticle basis. For example, Fe3O4@SiO2 [17], TiO2@GO [18], are nanocomposites that the basis of SiO2 is modified with Fe3O4 nanoparticles. Fig. 4.1 illustrates a schematic of the use of modified nanoparticles in nanocomposite formation and mass transfer pathway through the nanocomposite topical layer.

4.2 Classification of nanocomposite membranes Nanocomposite membranes are categorized into four different categories, based on the membrane’s structure, and the place nanoparticles are located, as follows:

4.2.1 Conventional nanocomposite membranes In conventional nanocomposites, nanoparticles may be either inorganic metals and metal oxides (e.g., Ag, Cu, Se, TiO2, ZnO, Fe3O4, SiO2, ZrO2), or organic materials, including biobased species (e.g., GO, C60, CNTs). Any choice of two or even three is also possible. Nanocomposite membranes are mostly fabricated via phase inversion where

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Fig. 4.2 Schematic of different types of nanocomposite membranes. (Adapted from J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment, J. Membr. Sci. 479 (2015) 256–275.)

nanoparticles are previously dispersed in the casting solution. Their porous structure made them appropriate to be widely used in microfiltration and ultrafiltration (UF) processes [1]. According to different studies, nanoparticles incorporation in polymer-based membranes can positively alter mechanical and chemical properties of the membranes, namely, hydrophilicity, porosity, chemical stability, charge density, and thermal properties. This also can improve antibacterial and photocatalytic properties of the membranes (see Fig. 4.2).

4.2.2 Thin-film nanocomposite membranes Thin-film nanocomposites consist of an ultrathin barrier layer nanocomposite on the top and a more porous substrate underneath (see Fig. 4.2). Thin-film nanocomposite membrane was made by Cadotte in 1980 for the first time to desalinate seawater in a reverse osmosis filtration [19]. These membranes are widely used for seawater and brackish water desalination, heavy ions removal, water softening, pesticides and other organic micropollutants and byproducts of disinfections removal, as well as the elimination of endocrine, disrupting compounds, and pharmaceutically active compounds [1, 20].

4.2.3 Thin-film composite with nanocomposite substrate membranes Thin-film composites with nanocomposite substrate membranes were first developed in 2010 by Pendergast et al. to investigate the effects of nanofiller on compaction behavior of the membranes [21]. The nanocomposite substrate provides mechanical support to minimize the collapse of the porous structure as a result of compaction. Consequently, these membranes reveal higher permeability at the beginning and demonstrate less reduction in flux as a result of compaction when compared with the original thin film composite membranes [1] (see Fig. 4.2).

Transport phenomena through nanocomposite membranes

4.2.4 Surface-located nanocomposite membranes Beside membrane structure, some other factors, including porosity and thickness, hydrophilicity, pore size, charge density, and roughness, are considered as parameters with crucial impacts on membrane separation performance and antifouling characteristics. These are why that surface modification can significantly improve the efficiency of membranes without any changes in intrinsic properties of the modified membranes. Accordingly, the method of surface-located nanocomposite membrane is applicable to prefabricated membranes, namely, commercially available membranes. In this method, nanoparticles can be deposited directly on the surface of prefabricated membranes; hence, this method can be considered as post-treatment. There are several different methods by which surface-located nanocomposite membranes are fabricated, including self-assembly, coating/deposition, and chemical grafting [1, 22].

4.3 Transport phenomena in nanocomposite membranes Different aspects of transport phenomena are involved in membrane-based separation processes; among them, mass transfer and heat transfer gained more attraction.

4.3.1 Mass transfer Mass transfer through a membrane, either a nanocomposite or not, is a nonequilibrium process, and separation of different species is owing to differences in their transport rates. Different driving forces are involved in membrane separation processes, including pressure, concentration, temperature, and electrical potential. One thermodynamic function can include all these parameters in electrochemical potential. Beside mechanical sieving and selective sorption, diffusion is a primary mode of separation occurs in membranes, which is the basis for mass transfer [23]. Accordingly, a semiempirical equation can describe how a single component i is transported through a membrane: Ji ¼ D

dηi dx

(4.1)

i In this equation, which clearly reminds Fick’s first law, dη dx is the chemical potential gradient of component i, and D is the phenomenological diffusion coefficient [24]. Eq. (4.1) can be rewritten in a general form as Eq. (4.2), which presents Fick’s first law of diffusion:

∂C (4.2) ∂x
where J is the diffusion flux, C is the concentration, and ∂C ∂x demonstrates the concentration gradient while x is the position along which mass transfer occurs. In this expression, D is the diffusion coefficient or diffusivity that influences the diffusion process, J ¼ D

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where the membrane thickness plays an important role. Eq. (4.3) suggests a method by which D can be estimated. D¼

Fδm ρCeq

(4.3)

where F and δm are the permeation flux and the membrane thickness, respectively, while ρ and Ceq are the penetrant density and the solvent concentration at equilibrium on the feed side of the membrane. It’s worth mentioning that the most important assumption by which Eq. (4.3) is developed is that the concentration profile along the membrane thickness is linear. The other method by which the diffusion coefficient can be calculated is as follows: D1
¼ 2

δ2m 7:19 t1


(4.4) 2

This equation represents the average diffusion coefficient at the time t when the flux is half of the steady-state flux. This value is applicable for either neat or nanocomposite membranes. Two other important parameters are computable as diffusion characteristic time based on D and D1
. Formulations given in Eqs. (4.3) and (4.4) describe diffusion coefficients 2

at times when the permeation is experiencing steady-state conditions. Before that, when the permeation has not been steady yet, the following equation is applicable: τ¼ τ1
¼ 2

δ2m D

(4.5)

δ2m D1


(4.6)

2

For mass diffusion demonstration through nanocomposite membranes, initially the following assumptions are considered to simplify the problem [25]: (i) the system is composed of a simple fluid and a pseudo-one-component polymerbased nanocomposite with the complex interface; (ii) the two components, polymer and nanofiller, are completely miscible; (iii) temperature is kept constant throughout the system; (iv) the density of the overall system is not changing; (v) the whole system is mechanical in equilibrium; (vi) no bulk flow is considered (i.e., the mass flux is relatively small). Considering the aforementioned limits, one can extend all modeling proposed for singlecomponent membranes for nanocomposite membranes.

Transport phenomena through nanocomposite membranes

Governing equations for a one-dimensional diffusion mode mass transport process through nanocomposite membranes under small negligible deformation are given as follows [26]:



∂F ∂c ¼ ρ ∂x ∂t

 ∂c ∂m11 ∂A11 + E11 + Λ11 ∂x ∂x ∂x   ∂m11 F ∂m11 ∂ F ¼  m11 ρð1  c Þ ∂x  ∂x ρð1  c Þ ∂t      m11 m  λ G0 ð1  c Þ 1  c 2  ceq kB T 1 =K ∗   ∂A11 ∂ F A11  λA1111 cΓ 1 ¼ ∂x ρð1  c Þ ∂t F ¼ ρD

(4.7) (4.8)

(4.9) (4.10)

Eq. (4.7) represents the general expression of one-dimensional mass conservation. Eq. (4.8) is an extension of Fick’s first law of diffusion. It includes a convective flux due to the relaxation of the viscoelastic polymer (characterized by a conformation tensor m) and another convective flux due to the relaxation of the complex interfaces between nanoparticles and polymer matrix [26]. Table 4.2 lists some studies done investigating the effect of nanocomposite formation on mass transfer through nanocomposite membranes. As can be observed, the addition of Table 4.2 A summary of different phenomena observed as a result of nanoparticle incorporation in membrane matrices Nanofiller

Polymer

Note

Ref.

1

Nanoclay

Poly(dimethylsiloxane)

[26]

2

Titanium dioxide

Polybutadiene

3

Silica nanoparticle

Different polyimides/ polyester

4

Hydrous metal oxide

Polyacrylonitrile

Introducing nanoclay to the membrane decreased mass transfer coefficient (liquid purification) Permeability coefficient increased more than three times compared to the unfilled polymer (gas separation) Higher mass transfer rate (permeability) was observed (gas separation) Mass transfer rate experienced a minimum, both decreasing and increasing manners were observed (water treatment)

[27]

[3, 28]

[6]

Continued

97

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Nanocomposite membranes for water and gas separation

Table 4.2 A summary of different phenomena observed as a result of nanoparticle incorporation in membrane matrices—cont’d Nanofiller

Polymer

Note

Ref.

5

Carbon nanotube(single and multiwalled)

brominated poly(2,6diphenyl-1,4phenylene oxide)

[29]

6

Cellulose nanocrystals

Poly(ε-caprolactone)

7

Cellulose nanofiber

Chitosan

8

Cellulose whiskers

Xylan

9

ZnO nanoparticles

Poly(lactic acid)

The mass transfer rate of the nanocomposite membrane increased compared to that of the pristine membranes (gas separation) A reduced mass transfer rate was observed as a result of nanocellulose presence (water treatment) Cellulose nanofiber presence decreased mass transfer rate through the membrane (water vapor) Reduction in mass transfer property (water transmission) was observed Mass transfer decreased through nanocomposite membranes (water vapor)

[30]

[31]

[32]

[33]

nanofillers might increase or decrease mass transport rate/coefficient through the membranes. In case of decreasing mass transport by addition of nanofiller, which is more common, the findings could be due to the barrier property of impermeable nanoparticles (e.g., layered silica) exerts on the polymer membrane. In other words, nanoparticles act as mechanical obstacles increasing the diffusion path for the molecules, therefore decreasing mass transfer [26]. The same phenomena occur in polymer films and objects in order to increase barrier [34] and durability properties [35]. There are also instances that the presence of nanoparticles increases mass transport in membranes. This situation, which is not frequently seen except in gas separation membranes, could be because of increase in solubility parameter of the overall system by means of the addition of specific nanoparticles while permeability of the nanoparticles is negligible [27]. Beside solubility parameter, change in viscoelastic property could be another source for increasing mass transfer rate in nanocomposite membranes, especially when dealing with highly elastic nanoparticles and gas separation [28]. Different possible scenarios as a result of nanofiller incorporation in membranes are summarized in Fig. 4.3. As can be seen, both decreasing and increasing in mass transfer rate/coefficient are possible as a result of nanocomposite formation. Each of the potential cases is described here:

Transport phenomena through nanocomposite membranes

Fig. 4.3 Possible scenarios after nanoparticle incorporation in porous membranes and justifications.

Case I Addition of nanoparticles with higher elastic behavior compared to viscoelastic polymers improves elastic collision behavior between membrane pores/channels and the molecules passing through; hence, by saving more energy as a result of elastic collision, the molecules tend to pass more quickly, which renders higher mass flow/mass transfer rate in macroscopic scope. This phenomenon is valid for both gaseous and liquid molecules [28]. Case II Solubility parameter plays a crucial role in the mass transfer rate through membranes. For example, as a result of the addition of more hydrophilic nanoparticles to either hydrophobic or less hydrophilic membranes, the affinity between the resulting membranes and aqueous solutions is improved, and the membranes facilitate passage of the molecules, which means higher mass transfer rate in the membranes. This idea is applicable to both liquid and gaseous systems and the same idea is valid for the hydrophobic counterpart. It is addressed as thermodynamically controlled mass transfer [27]. Case III Introducing nanoparticles to polymeric matrices causes extra free volume through the membranes due to insufficient packing after mixing. Moreover, voids form at the interface of the polymer and the nanoparticles as a result of ineffective adhesion between the components [3]. Case IV Addition of specific nanoparticles to the polymer-based membranes results in remarkable increases in mass transfer through them by making a corridor-like pathway inside the membranes to facilitate passing of the selected molecules. For example, MOFs, which are 3D nanoparticles made of metal ions (or clusters) connected with organic ligands, are a class of new materials, which provide corridors inside the membrane when adding to polymer matrices [36]. Fig. 4.4 illustrates how the incorporation of MOFs in the membrane matrices can improve the mass transfer of the penetrating molecules

99

100

Nanocomposite membranes for water and gas separation O

CH3

S

O

O

O

CH3

Polysulfone (PSf) Sulfonated Polysulfone (sPSf) CH3

(A)

O HO3S

O S

O CH3

UiO-66

O

Nanofilm ~400 nm

PSf/sPSf UiO-66 nanoparticles

(B)

Osmotic pressure

7Å

Nanofilm

(C)

H2O

Na2SO4

H2O

Fig. 4.4 (A) Polymer matrix chemical structure and UiO-66 MOF used for membrane fabrication. (B) Schematic macroscopic (left) and microscopic (right) illustrations of polymer-based nanocomposites. (C) Mass transfer through the nanocomposite membrane. (Adapted from T.Y. Liu, et al., Metal–organic framework nanocomposite thin films with interfacial bindings and self-standing robustness for high water flux and enhanced ion selectivity, ACS Nano 12 (9) (2018) 9253–9265.)

through the membrane by making pathways within the membranes. The addition of 30% of embedded UiO-66 MOFs has led to an increase in osmotic water flux from 0 to 73 LMH [36], showing a remarkable contribution of MOFs in mass transfer facilitation. Case V The addition of various nanofillers to the polymer solutions increases the viscosity of the casting solutions to different extents. Regardless of the method by which the

Transport phenomena through nanocomposite membranes

membrane is fabricated, the higher viscosity means the lower porosity, which results in a lower mass transfer rate. For example, if phase inversion is the method of membrane fabrication, higher viscosity alters the morphology from finger-like to spongy, which elaborates mass transfer through the membrane. This phenomenon is addressed as thermodynamically controlled mass transfer [6]. It should be noted that in most of the real conditions, two or more cases are involved, and the resulting trends are different below and above specific loadings compared to below that point. It is due to the different mechanisms controlling mass transfer in membranes with different nanofiller loadings [6]. 4.3.1.1 Mass transfer mechanism through porous membranes There are different mechanisms by which diffusive mass transfer occurs through membranes. These mechanisms are: (i) When the membrane average pore diameter is much bigger than the average free path of the passed-by permeating molecules, the molecules barely may hit the pores walls; however, the collision between the molecules is more probable. This mechanism is called molecular diffusion. This mechanism of diffusive mass transfer is important in systems that have medium temperature and pressure [37]. (ii) On the other hand, when the membrane average pore diameter is much smaller than the average free path of the penetrating molecules, the collision between the molecules and the wall of the pores is dominant compared to the collision between the molecules. This mechanism is called Knudsen diffusion. This mode of diffusive mass transfer is common in high-temperature and high-pressure systems [37]. (iii) When the situation is not that extreme, both collisions between the molecules and the pores walls and the molecules are probable. This mechanism of diffusive mass transfer is called transition diffusion. This mode of diffusive mass transfer is dominant in low-pressure and low-temperature systems [37]. These considerations and different conditions are considered in Knudsen number (Kn) that indicates which mechanism is dominant in mass transfer through a membrane [38]: λ d kB T λ ¼ pffiffiffi 2πPσ 2 Kn ¼

(4.11) (4.12)

where d is the average diameter of the pores in the membrane, and λ is the average free path length of the molecules passing through the membrane at temperature T, σ is the collision diameter for the diffusive molecules, P is vapor pressure, and considering kB as Boltzmann constant. Antoine equation can be used to calculate vapor pressure (P) at temperature T:

101

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Nanocomposite membranes for water and gas separation B

P ¼ eA T + C

(4.13)

Table 4.3 provides a guideline to find the dominant diffusion mechanism and equations at different Kn numbers. As can be observed, for each Kn, different diffusion equations are governing at Knudsen diffusion and molecular diffusion. When transition diffusion is recognized, the diffusion constant can be calculated from those of molecular diffusion and Knudsen diffusion. The use of nanoparticles in membrane formation might change the diffusion mechanism remarkably as the pore size in the membranes is highly dependent on nanocomposite involvement in the membranes. Viscosity and balance of hydrophilicity/ hydrophobicity of casting solution and porosity of the resultant membrane are the key characteristics affected by nanoparticles presence, which dramatically change pore size in membranes [39–41]. Hence, the mechanism of diffusive mass transfer may change from molecular diffusion to transitional diffusion and Knudsen diffusion, or vice versa. Guerout-Elford-Ferry Equation can describe parameters involved in calculating average pore diameter (d), which is a crucial factor in diffusive mass transfer through membranes, and momentum transfer between membrane pores wall and passing molecules: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2:9  1:75εÞ  8δm μQ d¼ (4.14) ε  A  4P

Table 4.3 Mechanisms of mass transfer through porous membranes with porous media Kn value

Kn < 0.01

0.01 < Kn < 10

10 < Kn

Diffusion mode Diffusion equation

Molecular diffusion 1 ε Mw DP DM ¼ Pa τδm RT DP ¼ 1.89  105T2.072 Transition diffusion 1 1 1 ¼ + DT DM DKn Knudsen diffusion   εd 8RT 1=2 Mw DKn ¼ 3τ πMw RT δm ρm ε¼1 ρP ð2  εÞ2 τ¼ ε

Mechanism

Note

Moleculesmolecules collisions

Momentum transfer between molecules

Molecules collision with each other as well as pores walls Moleculespores walls collisions

Momentum transfer between molecules + momentum transfer between molecules and membrane pores Momentum transfer between molecules and membrane pores

Transport phenomena through nanocomposite membranes

where μ is the viscosity of water, Q is the volumetric rate of water permeated from the membrane, A is effective area of the membrane, and ΔP is operational pressure [41]. Membrane distillation is a well-known example for mass transfer through porous membranes in which mass transfer is exclusively limited to water. In this process, mass transfer includes two different mass transfers, one through the feed side boundary layer and the other one across the membrane [42]. In other words, first liquid water vaporizes at the feed side (liquid/vapor interface), then the vaporized water passes through the membrane porous structure, and finally, it condenses at the permeate side. The mass transfer driving force for the vapor molecules is vapor pressure difference, which is one of the two key factors involved in MD mass transfer, the other one is, of course, the porosity of the membrane. Accordingly, a general mass transport equation can be started [42]: J ¼ CΔP

(4.15)

Eq. (4.16) presents a linear relationship applicable across MD membranes to predict mass flux of water vapor (Jm) versus vapor pressure difference. Jm ¼ Cm ΔPv

kf Mw ðPvm1  Pvm2 Þ RTm

(4.16)

in which Cm is the MD coefficient, which is a function of temperature, pressure, and the membrane composition. Porosity, thickness, and pore size diameter are other parameters that are effective on Cm. Mw, kf, and Pvm1, and Pvm2 are molecular weight of water (kg/ kmol), mass transfer coefficient (m/s), and water vapor pressure (Pa) at the feed and the permeate membrane surface, respectively. The mass transfer coefficient kf, which covers the effect of nanoparticles incorporation in MD membrane, can be determined either experimentally or theoretically [43]. Nanoparticles incorporation is known as a promising method in MD process, which can improve both permeation flux and salt rejection, via porosity adjustment, to diminish two challenges of improving efficiency and reducing liquid formation inside the membrane pores, which have already limited applications of MD membranes [44, 45]. Overall porosity, pore size diameter, and hydrophilicity are three crucial factors affected by the presence of nanofillers in membranes [6]. Accordingly, Eq. (4.16), which is originally derived for mass transfer in neat membranes, is also applicable to nanocomposite membranes, knowing the fact that the effect of nanoparticles present in membranes alters different parameters affecting the MD coefficient. 4.3.1.2 Mass transfer mechanism through nonporous membranes Mass transfer mechanism through nonporous membranes entirely differs from that of porous membranes. This mechanism is based on molecular-level solution-diffusion in which mass transfer is achieved as a result of solubility and diffusivity of penetrating mass

103

104

Nanocomposite membranes for water and gas separation

[46]. Separation through nonporous membranes is mainly applicable in the separation of gaseous mixtures [47]; however, cases of liquid separations are also studied through them [48, 49]. Mass transfer across a nonporous membrane occurs in three different steps: Step 1: Penetrants dissolve into the membrane (feed side). Step 2: Penetrants diffuse through the membrane (thickness of the membrane). Step 3: Penetrants desorbed into the permeate stream (permeate side). Accordingly, any functions that are effective on any of the steps mentioned here can alter mass transfer through nonporous membranes, from which nanoparticles incorporation is of interest of this chapter. Nanoparticles can alter mass transfer by changing the solubility parameter in nonporous membranes. Fig. 4.5 summarizes different possible scenarios happening after nanoparticle incorporation in nonporous membranes. Although there are cases where adding nanoparticle increases mass transfer coefficient through nonporous nanocomposite membranes (Cases I, II, and III), it is usually exerting the drawback of poorer selectivity [50], so this approach would be a tradeoff between selectivity and flux, which is a familiar dilemma in separation processes. The rationale behind the similar cases presented in Fig. 4.5 and Fig. 4.3 is the same. The only scenario exclusively applicable to nonporous membranes is presented as Case IV (Fig. 4.5) in which presence of nanoparticles in the membrane structure acts as mechanical obstacles against molecules and the penetrating molecules/species have to traverse a longer path prior to leaving the membrane. Accordingly, the rate of mass transfer drops to different extent regarding the nanofiller loading in the system. This idea better describes gaseous systems [26]. It should be noted that not all types of nanoparticles with different geometries, namely, nanorods, nanospheres, and nanosheets, render the same effect on decreasing

Fig. 4.5 Possible scenarios after nanoparticle incorporation in nonporous membranes and justifications.

Transport phenomena through nanocomposite membranes

Fig. 4.6 Schematic illustration of the shortest possible path for molecules to pass (A) a neat polymer membrane and (B) a nanocomposite membrane with randomly dispersed impermeable nanosheets.

mass diffusion through nonporous at the same loading percent. Briefly, layered nanoparticles, which have only one dimension in nanorange, show a greater effect on decreasing mass transfer coefficient compared to nanorods and nanospheres [51] (see Fig. 4.6). In the aforementioned scenario (Case IV), the resulting output is subject to impermeability assumption for nanoparticles; however, there are situations where nanoparticles are also permeable, in which permeability of the nanocomposite membrane is a function of the permeability of both nanofiller and the polymer matrix. Maxwell model can be employed to describe the effect of these parameters [52]: Pr ¼

PM Pd + 2Pc  2φnanofiller ðPc  Pd Þ ¼ PC Pd + 2Pc + φnanofiller ðPc  Pd Þ

(4.17)

where Pr, PM, and PC are relative permeability of the nanocomposite membrane, the permeability of the nanocomposite membrane and permeability of the pristine membrane, respectively. Pd, Pc and φnanofiller also represent permeability of nanofiller (the discrete phase), polymer matrix (the continues phase), and the volume fraction of nanofiller, respectively [52].

4.3.2 Heat transfer Nanocomposite membranes are promising and innovative materials in heat transfer involved membrane processes. Depending on the effect of nanoparticles on either increasing or decreasing thermal conductivity of the resulting substance, different results are expected [53]. Different modes of heat transfer include conduction, convection, and radiation, from which conduction is mainly the case of study in membrane science. Eq. (4.18) shows heat transfer formula in solids. q ¼ mCp ΔT

(4.18)

105

106

Nanocomposite membranes for water and gas separation

In this equation, the amount of heat transferred, q, is a function of mass (m) of the object, the temperature difference (4T ), and specific heat capacity (Cp). Beside nanofillers loading percent, nanofillers dispersion is also important in conductivity of the synthesized nanocomposite membranes [54]. In case of asymmetric nanoparticles, conductive heat transfer in membranes is not only a function of nanoparticles presence in nanocomposite membranes, but also their orientation along heat transfer direction plays a crucial role [53, 54]. 4.3.2.1 Conductive heat transfer Conductive heat transfer is the most common heat transfer mode studied highly more by the researchers. Thermal conductivity coefficient of nanocomposites can be determined using Eq. (4.19): K ¼ αρCp

(4.19)

in which α is the thermal diffusivity of the nanocomposite (mm2/s), ρ is its density (g/cm3), and Cp is its specific heat (J/g K) [55]. Cp can be directly measured using differential scanning calorimetry (DSC) measurements, and in order to calculate α, Eq. (4.20) can be used: α¼

1:38 L 2 π 2 t1=2

(4.20)

Thermal diffusivity is measured via a laser flash method as first introduced by Parker et al. in 1961 [56]. In this method, laser-sourced heat pulses are irradiated on the front side of an L length square shape sample. The heat is transmitted through the sample thickness direction and is measured by an infrared camera. The time when half of the maximum temperature at the back side of the sample is achieved is called t1/2 to be used to calculate the thermal diffusivity (Eq. (4.20)). Table 4.4 lists nanoparticles used in nanocomposite membranes with increased thermal conductivity. As mentioned earlier, the direction of heat transfer is important while measuring heat diffusivity. This fact is more crucial when dealing with nanocomposites, especially where asymmetric nanofillers, for example, nanorods, nanosheets, and nanoplates, are involved. Methods of preparation and additional treatments can induce sorts of alignment/orientation in one preferred direction rather than the others, which may increase conductivity behavior in that specific direction. In one study, the effect of graphene nanoparticles loading on improving the thermal conductivity of a PVDF-based nanocomposite membrane was 212% compared to the neat membrane; however, after alignment of those nanoparticles, thermal conductivity increased 226% [53]. However, the majority of the works have focused on increasing the thermal conductivity (e.g., in heat exchanging membranes) or decreasing the thermal conductivity (e.g., membrane distillation) and may also be an asset in insulation purposes.

Transport phenomena through nanocomposite membranes

Table 4.4 Use of nanoparticles in polymer-based membranes for heat conductivity improvement Nanoparticle

Matrix

Note

Ref.

Graphene

Poly(vinylidene fluoride) PVDF

[53]

Graphene oxide and reduced graphene oxide Aluminum oxide

Polyacrylamide (PAAm)

Copper@reduced graphene oxide

PVDF

Improved heat conductivity of 212% compared to the neat membrane was observed Improved heat conductivity of 100–150% compared to the neat membrane was observed Improved heat conductivity >700% compared to the neat membrane was observed At copper content of 20%, the heat conductivity of the nanocomposite membrane was improved by 325%

Bisphenol A epoxy

[57]

[55]

[58]

4.3.2.2 Convective heat transfer Different modes of heat transfer are involved in membrane distillation, including: (i) At the feed side: Convective heat transfer from the surface at the feed side to the vapor-liquid interface at the membrane surface: Q1 ¼ h1 ðT1  T1m Þ,

(4.21) 2

where Q1 is the flux of heat transferred at the feed side (W/m ), and h1 is the convective heat transfer coefficient at the same side (W/m2 K). (ii) Through the membrane: Conduction and evaporation occur within the porous structure of the membrane [43].

qc ¼

qe ¼ Jm ΔHv

(4.22)

km ðT1m  T2m Þ δm

(4.23)

where qe is the heat flux by evaporation (W/m2), Jm is the mass flux of vapor (kg/m2 h), and Δ Hv is the water vapor latent heat of vaporization (kJ/kg). Also, qc is the heat flux by conduction (W/m2), km is the average thermal conductivity of membrane and vapor (W/mK), δm is the membrane thickness, and T1m and T2m are the temperature at the membrane surfaces in the feed side and permeate side, respectively (°C, K). Moreover, the heat transfer coefficient of the membrane can be computed as follows: hm ¼

ð1  εÞkg km ¼εK + δm δm

(4.24)

107

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Nanocomposite membranes for water and gas separation

As shown in Eq. (4.24), the heat transfer coefficient is presented as a function of the thermal conductivity constant of the membrane and thermal conductivity of the vapor that fills the membrane pores (K and kg, respectively) [43]. Presence of nanoparticles in the polymer matrix of the membranes alters different parameters, including the membrane porosity (ε), and the heat conductivity of the membrane; hence, the heat transfer coefficient may experience a remarkable change depending on the amount of involved nanoparticles. Eq. (4.25) represents how the membrane porosity can be computed based on the Smolder-Franklin equation [59] as: ρ ε¼1 m (4.25) ρp where the numerator and the denominator of the fraction reflect the density of the nanocomposite membrane and the density of the mixture of polymer and nanofiller by which the membrane is made [38] according to the mixture law as follow: ρmixture ¼ ρp φp + ρnanofiller φnanofiller

(4.26)

in which φ is the fraction of the component in the mixture; however, considering the low loading percent of the nanofiller, the change it makes in the mixture density is negligible. It is considering the whole conditions the same as what previously mentioned in part 4.1; all can be used for the nanocomposite systems, subject to calculating km for the nanoparticle-incorporated membranes.

4.3.3 Charge transfer Although mass transfer and heat transfer are the most common transport phenomena studied in membrane processes, there are other less studied transport phenomena including charge transfer [60–63], which is a crucial factor in conductivity of materials. What is meant when talking about charge transfer is both electron conductivity and proton conductivity. The difference in charge makes different materials appropriate for charge transfer in case of electron and proton conductivity. 4.3.3.1 Electron conductivity Conductive polymers are applicable in many versatile fields of membrane separation processes, including, but not limited to, salt rejection, protein separation, and mineral recovery [64]. Moreover, membranes with electrical conductivity provide a chlorine-free, nondestructive possibility to mitigate biofouling [65] by self-cleaning [66]. Although several conductive polymers, for example, Nafion [67], polyaniline [68], and polypyrrole [69], are used in membrane fabrication, they have several drawbacks, including high cost, not being soluble in common solvents, and having glass transition temperature higher than their degradation point [65]. Hence, nanotechnology can get involved reasonably by the addition of electrically conductive nanoparticles to ordinary membrane matrices in order to improve electron

Transport phenomena through nanocomposite membranes

transfer, that is, electroconductivity in membranes. Several nanoparticles can provide electron transfer capability in the membranes, subject to sufficient amount of loading percent and efficient distribution and dispersion. The effect of carbon nanotube (CNT) content on electron transfer performance of UF membrane made of poly (vinyl alcohol) improves up to 4000 S/m, which is the same order of magnitude as pure graphite [65] when 20 wt% of CNT is added. In other words, CNT presence in the membranes facilitates charge (electron) transfer noticeably, which can be used in mitigating fouling issue in the membranes. Among different methods employed for mitigation or removal of biofouling, periodic electrolysis is known as a fast and straightforward self-cleaning method, which requires high electrical conductivity [66]. 4.3.3.2 Proton conductivity Applications of membranes with the ability of proton conducting include, but not limited to, fuel cells [70], hydrogen recovery, pollution control, separation of biomoieties/biomaterials, drug controlled release, etc. These membranes, called electromembranes [23], can be fabricated from either intrinsically conductive polymers [71], for example, polyaniline and polypyrrole [23], with or without nanoparticles, or incorporation of electroconductive nanoparticles to regular membrane matrices [72]. In one study, positive charge transfer ability in a membrane to serve in a fuel cell increased from 0.0006 S/cm in the neat PVA-based membrane, to 0.0024 S/cm in the PVA-based nanocomposite membranes containing 7 wt% of GO [72]. The same study also showed that 7 wt% of functionalized sulfonated GO is more effective in increasing charge transfer compare to the pristine counterpart rendering charge conductivity of 0.041 S/cm. GO is recognized as one of the best nanofillers employed in fuel cells due to its high proton conductivity, intrinsic mechanical strength, and chemical and thermal stabilities [73].

4.4 Conclusions Although the neat membrane separation is considered as a rather well-known method of selective separation and efficient purification, incorporation of nanoparticles in this technology improves several aspects of the membranes some of which are categorized in mass, heat, and charge transfer category. Regarding the fact that separation processes are mainly based on transport phenomena, use of nanoparticles shows to be effective in tuning and modification of transport phenomena, both increasing and decreasing cases. Different mechanisms are involved in increasing or decreasing mass transfer in porous and nonporous nanocomposite membranes, among which, recently introduced MOFs gain attention as a potential for increasing mass transfer within the membranes. Metallic nanoparticles reveal the highest effect on increasing heat transfer in nanocomposite membranes, while carbon-based nanoparticles incorporation in polymer matrices improve charge conductivity of the membranes.

109

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[50] S.M. Momeni, M. Pakizeh, Preparation, characterization and gas permeation study of PSf/MgO nanocomposite membrane, Braz. J. Chem. Eng. 30 (2013) 589–597. [51] C. Wolf, et al., How the shape of fillers affects the barrier properties of polymer/non-porous particles nanocomposites: a review, J. Membr. Sci. 556 (2018) 393–418. [52] K.M. Gheimasi, et al., Prediction of CO2/CH4 permeability through Sigma-1–Matrimid®5218 MMMs using the Maxwell model, J. Membr. Sci. 466 (2014) 265–273. [53] H. Guo, et al., Thermal conductivity of graphene/poly(vinylidene fluoride) nanocomposite membrane, Mater. Des. 114 (2017) 355–363. [54] A. Li, C. Zhang, Y.-F. Zhang, Thermal conductivity of graphene-polymer composites: mechanisms, properties, and applications, Polymers 9 (2017) 437–453. [55] H.A. Poostforush, Superior thermal conductivity of transparent polymer nanocomposites with a crystallized alumina membrane, eXPRESS Polym. Lett. 8 (4) (2014) 293–299. [56] W.J. Parker, R.J. Jenkins, C.P. Butler, G.L. Abbott, Flash method of determining thermal diffusivity, heat capacity, and thermal conductivity, J. Appl. Phys. 32 (1961) 1679–1684. [57] R. Shemshadi, et al., A smart thermoregulatory nanocomposite membrane with improved thermal properties: simultaneous use of graphene family and micro-encapsulated phase change material, Text. Res. J. (2018)0040517517750644. [58] Y.L. Li Shuang, Z. Yan, Ultradispersed nanocomposites membrane of copper@reduced graphene oxide/PVDF for enhanced thermal conductivity, in: 21st International Conference on Composite Materials, Xi’an, 20–25th August 2017, (2017). [59] K. Smolders, A.C.M. Franken, Terminology for membrane distillation, Desalination 72 (3) (1989) 249–262. [60] A.C. Roberto Ambrosio, M.L. Mota, K. de la Torre, R. Torrealba, M. Moreno, H. Vazquez, J. Flores, I. Vivaldo, Polymeric nanocomposites membranes with high permittivity based on PVAZnO nanoparticles for potential applications in flexible electronics, Polymers 10 (1370) (2018) 1–17. [61] T. Araki, et al., Water diffusion mechanism in carbon nanotube and polyamide nanocomposite reverse osmosis membranes: a possible percolation-hopping mechanism, Phys. Rev. Appl. 9 (2) (2018)024018. [62] J.E. Weaver, et al., Investigating photoinduced charge transfer in carbon nanotube perylenequantum dot hybrid nanocomposites, ACS Nano 4 (11) (2010) 6883–6893. [63] L. Ahmadian-Alam, H. Mahdavi, A novel polysulfone-based ternary nanocomposite membrane consisting of metal-organic framework and silica nanoparticles: as proton exchange membrane for polymer electrolyte fuel cells, Renew. Energy 126 (2018) 630–639. [64] W.E. Price, C.O. Too, G.G. Wallace, D. Zhou, Development of membrane systems based on conducting polymers, Synth. Met. 102 (1999) 1338–1341. [65] C.F. de Lannoy, et al., A highly electrically conductive polymer–multiwalled carbon nanotube nanocomposite membrane, J. Membr. Sci. 415–416 (2012) 718–724. [66] B.S. Lalia, et al., Electrically conductive membranes based on carbon nanostructures for self-cleaning of biofouling, Desalination 360 (2015) 8–12. [67] S.P. A K Sahu, P. Sridhar, A.K. Shukla, Nafion and modified-Nafion membranes for polymer electrolyte fuel cells: an overview, Bull. Mater. Sci. 32 (3) (2009) 285–294. [68] L. Xu, et al., Stimuli responsive conductive polyaniline membrane: in-filtration electrical tuneability of flux and MWCO, J. Membr. Sci. 552 (2018) 153–166. [69] M. Zhou, et al., Electrochemical preparation of polypyrrole membranes and their application in ethanol-cyclohexane separation by pervaporation, J. Membr. Sci. 108 (1) (1995) 89–96. [70] E. Abouzari-Lotf, M. Etesami, M.M. Nasef, 18—Carbon-based nanocomposite proton exchange membranes for fuel cells, in: A.F. Ismail, P.S. Goh (Eds.), Carbon-Based Polymer Nanocomposites for Environmental and Energy Applications, Elsevier, 2018, , pp. 437–461. [71] W. Hu, S. Chen, Z. Yang, L. Liu, H. Wang, Flexible electrically conductive nanocomposite membrane based on bacterial cellulose and polyaniline, J. Phys. Chem. 115 (26) (2011) 8453–8457. [72] H. Beydaghi, M. Javanbakht, E. Kowsari, Synthesis and characterization of poly(vinyl alcohol)/ sulfonated graphene oxide nanocomposite membranes for use in proton exchange membrane fuel cells (PEMFCs), Ind. Eng. Chem. Res. 53 (43) (2014) 16621–16632. [73] Y.M. Kolsoum Pourzare, S. Farhadi, Advanced nanocomposite membranes for fuel cell applications: a comprehensive review, Biofuel Res. J. 12 (2016).

CHAPTER 5

Application of functional single-element and double-element oxide nanoparticles for the development of nanocomposite membranes Mohammad Amin Alaei Shahmirzadi, Ali Kargari

Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran

5.1 Introduction During the last decade, nanocomposite membranes have been at the forefront of research activities in the field of separation and energy as they find applications in the gas separation process, water and wastewater treatment, desalination, fuel cell, battery, etc. [1–4]. Among the nanomaterials used for preparation of nanocomposite membranes, metal oxides have received considerable attention due to the large diversity of compounds and unique features such as availability in various shapes and sizes, and capability to have different functionality [5–7]. The nanocomposite membranes synergistically combine the advantages of polymer matrix with the unique properties of the nanomaterials [8,9]. Interfacial interactions between the nanomaterials and the polymer matrix have great impacts on mechanical and thermal properties as well as performance of nanocomposite membranes [10,11]. The nanomaterials usually tend to be aggregated due to the high specific surface area and high surface energy, so it is difficult to uniformly distribute them within the polymer matrix [3,12,13]. The poor compatibility between nanomaterials and polymer not only adversely affected the desirable properties of nanocomposite membranes but also may undermine the intrinsic properties of the polymer matrix [5,14,15]. This drawback could be tackled by surface modification and changing the surface chemistry of nanomaterials and introduction of mutually interactive functional groups [16,17]. Surfaces of the metal oxide nanoparticles have been modified via functional groups and modifiers such as coupling agents, polymers/copolymers, and some additives [5,12,16]. The surface modification of nanomaterials via functional groups improves the interfacial compatibility between polymer and filler, in addition to introducing new functionality on their surface and/or within the bulk.

Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00005-5

© 2020 Elsevier Inc. All rights reserved.

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This chapter focuses on the recent scientific and technological developments related to the application of functional single-element and double-element oxide nanoparticles for the preparation of nanocomposite membranes. The surface modification of various metal oxides and the strategies for their incorporation into the polymer matrices will be discussed.

5.2 Metal oxide nanomaterials: Types, properties, and synthesis strategies Types: Metal oxide nanoparticles are attractive from a technological and scientific point of view and have become an area of intense scientific interest for a large variety of applications, including (opto) electronic materials, energy storage, fuel cells, catalysis, sensors, pigments, environmental remediation, and nanomedicine [18–20]. These types of nanoparticles can be classified into two main categories: (1) single-element metal oxide (e.g., TiO2, MgO, CuO, Fe2O3, Al2O3, SiO2, etc.), and (2) multielement oxide (e.g., zeolites, mixed-metal oxides, GO-metal oxides, etc.). Metal oxide nanomaterials have gained increasing interest because of their large diversity of compounds, availability in various shapes and sizes, and capability to have different functionality. Due to their designable properties, metal oxide nanomaterials are utilized for the separation process in both gas and liquid phases [7,21]. Fabrication of nanocomposite membranes using incorporation of metal oxide nanomaterials into the polymer matrix is known as one of their main applications in separation processes [22,23]. Properties: Metal oxide nanomaterials are considered to be innocuous, antibacterial, highly hydrophilic, and stable to UV irradiation for photocatalytic degradation of contaminants, making them suitable for water treatment and desalination [8,24]. For example, in semiconductor metal oxides, photocatalytic properties are created during the photocatalysis process, which provide superoxide radical anions and hydroxyl radicals in liquid phase for strong oxidizing and decomposition of organic compounds [25]. Furthermore, some metal oxide nanomaterials may possess reactive anchoring sites like hydroxyl, amine, and carboxyl, for further stability of nanomaterials in the polymer matrix and create new surface functionality [24,26]. Additionally, embedding metal oxide nanomaterials into the polymer matrix alters the transport properties of nanocomposite membranes. Water transport and solutes rejection can be improved due to higher affinity of metal oxide nanomaterials toward water and increased surface charge of nanocomposite membranes compared to pristine ones, respectively [8,27]. In the case of gas separation membranes, the incorporation of metal oxides into the polymer matrix normally applies similar gas transport behavior as that of impermeable nanomaterials like silica which is the result of disruption of chain packing, agglomeration of nanoparticles, and creation of more membrane fractional free volume [3,13]. In addition to the improvement in gas diffusivity, metal oxides tend to have more interaction

Application of functional single-element and double-element oxide

with polar gases like CO2, resulting in higher gas solubility [28]. Moreover, some functional groups like the silane coupling agent can be introduced to metal oxides to improve the affinity of filler with gases and the polymer-filler compatibility [29,30]. In addition, incorporation of metal oxide nanomaterials into the polymer matrix improves the thermal and mechanical stability in both porous and dense membranes. Synthesis strategies: The synthesis route has a great impact on the properties of nanoparticles such as the particles size distribution, morphology, purity, quantity, and quality [31]. Synthesis strategies can be classified based on several factors including the nature of synthesis (chemical or physical), media of synthesis, the sources of energy, etc. [32]. They are generally classified into: (1) bottom-up method [32]: refers to the synthesis of nanomaterials from bottom (atomic level), and can be carried out based on two main approaches, physical and chemical. (2) Top-down: Refers to size reduction and breaking up of bulk materials until they get reduced to nanoscale. The overall schematic classification of metal oxide nanomaterials synthesis routes has been shown in Fig. 5.1. The synthesis routes have already been extensively discussed in detail in the literature [32,33] and are not further detailed here. The intrinsic properties and structural effects of some of the metal oxide nanoparticles that have been used in the fabrication of the nanocomposite membrane are listed in Table 5.1. As mentioned in Table 5.1, metal oxide nanoparticles cause significant changes in morphology and crystallinity, properties (mechanical, thermal, and chemical), performance (permeability and selectivity), and functionality (antifouling, antibacterial, photocatalytic, and chlorine resistance) of nanocomposite membranes.

5.3 Strategies for incorporating nanomaterials into the membrane matrix Fabrication techniques and strategies for incorporating nanomaterials into the membrane matrix play vital roles for determining the membrane properties and performance. In general, nanocomposite membranes are prepared via two major strategies, including bulk/support modification and surface/top layer modification, as shown in Fig. 5.2. The nanomaterials can be embedded individually either in the surface/top layer or the bulk/support layer of the membrane or simultaneously as a combination of both. In bulk/support modification, the new functionalities of nanomaterials are distributed throughout the membrane, but in surface/top layer modification, the new functionalities are introduced to the membrane surface or active layer. Both strategies are widely employed in the literature [2,34] to achieve membranes with broad range properties and applications, but each of them possesses their inherent advantages and disadvantages. The selection of a suitable strategy depends on the various factors such as polymer phase, and application as well as type, size, and amount of nanomaterials, and consequently one that will give a nanocomposite membrane with better physicochemical properties and

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Coprecipitation

Sol-gel

Hydrothermal

Thermolysis Chemical/ solution

Microemulsion Electrodeposition Oxidation and reduction Biomimetic

Bottom up Combustion

Chemical vapor deposition

Evaporation

Vapor deposition

Physical vapor deposition

Sputtering

Physical/ aerosol

Spray pyrolysis

Atomic layer deposition

Molecular beam epitaxy

Lithography

Flame deposition

Synthesis/processing methods of metal oxide nanomaterials

Top down Milling

Fig. 5.1 Various synthesis methods employed for metal oxide nanomaterials. Table 5.1 The intrinsic properties of some of the metal oxide nanoparticles used for fabrication of nanocomposite membranes Nanoparticles

Property

TiO2

Photocatalytic activity for contaminants removal, multifunctional inorganic nanoparticle, antifouling property and hydrophilicity, good mechanical resistance, change in polymer chains rigidification and packing density, affinity with some gas species, low cost Mechanical and chemical stability, multifunctional inorganic nanoparticle, antifouling property and hydrophilicity, change in polymer chains rigidification and packing density, affinity with some gas species Thermal and chemical stability, suitable for high-temperature catalytic reactions and oily wastewater filtration, change in polymer chains rigidification and packing density, affinity with some gas species Mechanical stability, antifouling and antibacterial properties, change in polymer chains rigidification and packing density, affinity with some gas species Magnetic improvement of thermal stability of polymers Change in polymer chains rigidification and packing density, affinity with some gas species

SiO2 ZrO2 Al2O3 Fe3O4 MgO

Application of functional single-element and double-element oxide

Table 5.1 The intrinsic properties of some of the metal oxide nanoparticles used for fabrication of nanocomposite membranes—cont’d Nanoparticles

Property

CuO

Mechanical and physical stability, antimicrobial and antibiofouling properties, affinity with some gas species Catalytic, antibacterial and bactericide activities, multifunctional inorganic nanoparticle, mechanical and chemical stability, hydrophilicity, thermal and chemical stability, Change in polymer chains rigidification and packing density, low cost Mechanical and physical stability, high adsorption capacity, ion exchange property, sieving characteristic, change in polymer chains rigidification and packing density, high porosity

ZnO

Zeolites

Solution blending

Emulsion or suspension blending

Blending

Melt blending Bulk/support modification

Sol-gel process

In situ chemical reduction In situ Ion-exchange route

Fabrication techniques

In situ polymer reaction

Surface/top layer modification

Grafting

Layer-by-layer assembly

In situ

Self-assembly

Coating/ deposition

Adsorptionreduction

Fig. 5.2 Strategies for incorporating nanomaterials into polymeric membranes.

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performance [6,35]. Furthermore, the nanocomposite membranes can be prepared by traditional methods, modification of them, or combination with new approaches, which will be explained in the following sections.

5.3.1 Bulk/support modification 5.3.1.1 Blending or direct compounding Blending or direct compounding is known as a common, fast, simple, and easy-tocontrol method to synthesize both dense and porous nanocomposite membranes. The blending membranes are typically done by mixing additives (organic or inorganic) as a disperse phase in a polymer matrix as continuous phase. The main concern in this fabrication method is obtaining homogeneous dispersion of nanomaterials in dope solution and their uniform distribution in the membrane matrix because they strongly tend to be aggregated. To prevent the agglomeration, the surface of nanomaterials is modified and/ or some solubilizing or crosslinking agents are incorporated into the dope solution to enhance their consistency and interaction with the polymer matrix [36]. In this scenario, the nanomaterials are presynthesized independently prior to blending and then mixed by solvent and polymer (s) (ex situ synthesis). The mixing is usually performed by the solution, emulsion or suspension, and melt blending. Solution blending is more common compared to other ones, done in liquid phase, and suitable where both the polymer and the inorganic additives are dissolved or dispersed in the solution. Emulsion or suspension blending is quite a similar solution blending but has some differences. This method is employed where polymers are difficult to dissolve in common solvents, like TiO2/poly(methyl methacrylate) and SiO2/polystyrene systems. In melt blending, nanomaterials are homogenously dispersed in the polymer melt and then the nanocomposite membrane is prepared by extrusion [10,37]. 5.3.1.2 In situ preparation of nanomaterials in bulk This strategy is known as a molecular-level design method, providing the in situ generation of nanomaterials within the bulk of the polymer and/or on its surface. In situ preparation permits the one-pot synthesis of both polymer and nanomaterials, which exhibited several advantages over the blending or direct compounding of nanomaterials and the polymer. In situ generation of nanomaterials could improve the compatibility and interfacial interactions of the polymer-filler as well as filler dispersion due to molecularlevel mixing of the polymer, solvent and starting materials of nanomaterials (precursors) [2]. By proper selection of the polymer, solvent, and nanomaterial precursors and control of the formation reaction, some functional groups such as hydroxyl and amino can be introduced on the filler surface, which boosts membrane functionality and filler-polymer interaction and makes the membranes appropriate for a specific target [38]. The difficulties of this method can be related to compatibility between components of the casting solution (polymer, solvent, and nanomaterial precursors). An appropriate solvent can

Application of functional single-element and double-element oxide

solve the problem and provides a good solubility and uniform dispersion of both polymer and inorganic phase, thereby improving the interaction of all components of casting solution [39]. The in situ preparation of nanomaterials into the polymer matrix may occur in different stages of membrane fabrication based on solution components, target, and application: (1) preparation of the dope solution, (2) during the phase inversion or solidification of the dope solution, and (3) after the solidification of the dope solution [38]. In general, four synthesis routes have been used for achieving an effective nanocomposite membrane via the in situ method (Fig. 5.3), including sol-gel process, in situ reduction, ion-exchange method, and in situ polymer reaction [38]. Among them, the sol-gel process and ion-exchange method are taken into the consideration for synthesis of metal oxides nanoparticles on/within the membrane matrix. In the sol-gel process, the nanocomposite membranes are prepared via in situ synthesis of metal oxide nanoparticles using the well-known hydrolysis and polycondensation reactions of starting materials like metal salts or metal alkoxide [1]. In the second synthesis route (ion exchange method), metal oxide nanoparticles are synthesized through anion exchange (anions of precursors to hydroxyl). In the next step, the casting solution is prepared by in situ mixing of the nanoparticles sol and polymer solution [1,2,38].

5.3.2 Surface/top layer modification Besides the addition of nanomaterials within the polymer matrix, modification of the membrane surface/top layer by nanomaterials has been extensively studied for fabrication nanocomposite membranes [4,40]. The literature survey shows that this method is more efficient than embedding the nanomaterials into the polymer matrix [41,42]. The latter strategy has a much lower impact on the membrane’s intrinsic structures in comparison to the former one; hence, there is a suitable choice for modification of commercially available membranes. There are different methods to apply the nanomaterials for modification of membrane surface/top layer: grafting, in situ synthesis (e.g., layer-by-layer (LBL) assembly, self-assembly, adsorption-reduction), and coating/deposition. These methods can be employed individually or in combination with each other. 5.3.2.1 Coating/deposition Coating/deposition is widely used to form a layer(s) of nanomaterials onto the surface of the membranes through dip-coating or filtration-deposition. The interaction of the coated layer and membrane is not strong enough to make a stable functionality on the membrane surface and nanomaterials may also be released during the filtration process, which limits its practical use. 5.3.2.2 Grafting Grafting is a method wherein inorganic nanoparticles or monomers or some functional groups are introduced onto the membranes [43,44]. The grafting can be initiated through

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Fig. 5.3 Different in situ synthesis strategies for nanocomposite membranes preparation: (A) sol-gel process; (B) in situ chemical reduction; (C) ion-exchange route, and (D) in situ polymer reaction [38].

Application of functional single-element and double-element oxide

various methods such as photoinitiation and radiation, chemical initiation, plasma initiation, and enzymatic initiation [44]. The selection for an appropriate initiation method depends on the physicochemical structure of the polymer matrix and the desired characteristics. In the grafting method, nanomaterials are covalently bonded onto the membranes, thereby reducing the loss of nanomaterials and functionality depletion [43]. 5.3.2.3 In situ generation of nanomaterials Introduction of surface functionality via in the situ method can be performed using different techniques (e.g., adsorption-in situ reduction, self-assembly, and LBL assembly), individually or simultaneously. Among these techniques, adsorption-in situ reduction has been used more than others for in situ generation of metal nanoparticles onto the membrane [38]. Self-assembly is another strategy, which involves soaking the top layer of the porous membrane support in a dilute colloidal suspension of nanoparticles (especially for TiO2 nanoparticles) [43]. The formed layer of nanomaterials is attached onto the membrane by Coordination and H-bonding [2]. LBL assembly is a promising and simple technique, which involves the attachment of monolayer or multiple layers of nanomaterials in nanoscale on the support layer by chemical bonding and/or electrostatic attraction [45]. The applications of these techniques for modification of the membranes have been reviewed by several authors in detail [38,46,47].

5.4 Functionalization and surface modification of metal oxides Generally, most of the inorganic nanomaterials are highly polar, indicating poor compatibility with many of commercially available polymers such as poly arylene sulfones, PP, PVDF. Due to the high specific surface area and high surface energy, nanomaterials tend to be aggregated to reach an energy-favorable state in polymer/solvent/nanomaterials system, reducing their efficiency. The polymer/nanomaterial interfacial interactions can significantly affect the compatibility between polymer and nanomaterials, membrane formation process (phase separation for porous membrane and drying/evaporation for dense membrane), membrane morphology, leaching of the nanomaterials and their stability during the long-term operation, and dispersion within the polymer matrix. In addition to bulk modification, the polymer/nanomaterial interface will determine compatibility between the polymer and nanomaterials, dispersibility, and stability on the surface of the polymeric membrane. The poor polymer/nanomaterial interaction can be tackled by surface modification of nanomaterials and introduction of mutually interactive functional groups on their surface without changing the morphology and topology of the nanomaterials. Metal oxide nanoparticles can be generally modified by two approaches [16]: (1) physically, which is typically carried out with surfactants or macromolecules adsorbed on the surface of nanoparticles, and (2) chemically, which

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C O

OH

O O

O O

O O Si O

O Dopamine

Si Trimethoxy silane

O

O C

NH

O S

O O S C

Amine

Cysteine

Fig. 5.4 Common chemical moieties for the anchoring of polymers and functional groups at the surface of nanoparticles [51].

is typically carried out through covalent bonding of some functional group on the surface of nanoparticles. Some functional groups and modifiers, which include, but are not limited to, Silane coupling agents [30], amino groups [48], carboxylic and hydroxyl groups [49–51], sulfonic acid groups [52] as well as some additives like polyethylene glycol monomethyl ether (PEG) [53], ionic liquids (ILs) [54], polydopamine (PD) [55], and polyethylenimine (PEI) [53] have been employed in the functionalization and surface modification of nanomaterials. An overview of some viable surface functional groups is presented in Fig. 5.4. The functionalization and surface modification of nanomaterials could alter wettability, surface charge, and surface roughness in nanometer level enhancing the area exposed to feed side, affinity to certain molecules (improvement of selectivity), permeability, interfacial interface of polymer/filler, mechanical/thermal stability, etc. The main disadvantage of physical modification of metal oxide nanoparticles is that they are thermally and solvolytically unstable because of the weak interaction between nanoparticles and modifier [16]. Chemical modification is more stable and shows better polymer/nanoparticle compatibility. The recent advances in the surface modification of metal oxide nanoparticles and interface researches are summarized in the following sections to provide an insight for fabrication of nanocomposite membranes in both liquid and gas separation applications.

5.4.1 Silane coupling agent Silane coupling agents are known as the most prominent members of modifiers for nanomaterial modification, especially metal oxide nanoparticles. These compounds

Application of functional single-element and double-element oxide

are organofunctional with two different functional groups that react with organic and inorganic compounds, respectively, and act as an interface bonding agent [16,56] to introduce various functional groups, including alkyl, vinyl, amino, epoxy, thiol, chlorine methyl, phosphoric acid groups, etc. onto the surface of nanoparticles [11]. Silane coupling agents have the general formula (X–(CH2)n–Si–R3). The X denotes the organic functional group such as phenyl, vinyl, amino, epoxy, etc., and the R represents to hydrolyzable functional group such as methoxy, ethoxy, etc. [16,57]. The metal oxide nanoparticles typically possess the hydroxyl group (OH) on their surface. The trialkoxy groups of silane coupling agent are hydrolyzable and react with the hydroxyl groups on the surface of nanomaterials via the sol-gel reaction, while amino/functional groups interact with the polymer matrix [12,58]. The silane modification changes the surface properties of metal oxide nanoparticles from hydrophilic to hydrophobic and gives rise to the compatibility of filler surface (hydrophilic) with a polymeric matrix (hydrophobic) and provides better stability and uniformity of the inorganic filler in the polymer phase and elimination of the interfacial voids at the polymer/filler interface [59]. Among the silane coupling agents, Amino-organosilanes such as 3-aminopropyltrimethoxysilane (APTMS), 3-aminopropyltriethoxysilane (APTES), 3-aminopropylmethyldiethoxysilane (APMDES), N-b-(Aminoethyl)-c-aminopropyltrimethoxy silane (AEAPTMS), 3-(trimethoxysilyl)propylmethacrylate (TMOPMA), and aminopropyldimethylethoxysilane (APDMES) were extensively employed to modify the surface of inorganic nanomaterials. The selection of a suitable coupling agent strongly depends on the chemical structure of the polymer matrix. Gas separation: Surface modification using Amino-organosilanes effectively enhances the filler dispersion, polymer/filler compatibility, and membrane affinity toward CO2 molecules, which results in an increase in CO2 permeability and selectivity. Rezakazemi et al. [60,61] used poly(hydrogenmethylsilane) as a silane coupling agent to fabricate PDMS/ zeolite 4A nanocomposite membranes. Results showed that poly(hydrogenmethylsilane) caused the formation of a binding between PDMS chains and the filler surface. Pechar et al. evaluated the surface modification of ZSM-2 [62] and zeolite L [63] with aminopropyltriethoxysilane (APTES) as a silane coupling agent. Results indicate that a good compatibility was obtained between polymer (6FDA-6FpDA-DABA) and fillers, with superior changes in gas (He, CO2, CH4, O2, and N2) permeability due to partial blockage of the zeolite pores by aminopropyltriethoxysilane (APTES) and permeation of gas molecules through the polymer matrix instead of inorganic fillers. The utilization of common silane coupling agents like aminopropyltriethoxysilane (APTES) and aminopropyltrimethoxysilane (APTMS) with three hydrolyzable groups leads to the formation of a large number of coupling points on the filler surface, which blocks filler pores. To mitigate this negative impact, some alternative silane coupling agents with a lower number of hydrolyzable groups such as 3-aminopropyldiethoxymethyl silane (APDEMS) have been proposed [64,65]. Ebadi Amooghin et al. [30] modified the microsized NaY zeolite (FAU-type) particles via aminosilane grafting using 3-aminopropyl(diethoxy)methylsilane (APDEMS) as a

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Fig. 5.5 (A) The grafting reaction between APDEMS and zeolite surface, and (B) the reaction between Matrimid and the surface-modified zeolite [30].

silane coupling agent and then embedded them into the Matrimid® 5218 matrix to avoid the pore blockage, formation of interfacial voids at polymer/filler interface, and polymer chain rigidification. Fig. 5.5 indicates the silanization reaction between the coupling agent and filler surface as well as the possible interaction between modified filler and Matrimid® 5218 matrix. The silane coupling agent reacts with the hydroxyl groups of filler, the amino group of silane coupling agent reacts with the imide group of Matrimid® 5218 matrix, and

Application of functional single-element and double-element oxide

as a result a covalent bonding is formed between the polymer and filler, improving polymer/filler compatibility. The permeability of CO2 and CO2/CH4 selectivity of the Matrimid® 5218/modified zeolite membrane was improved by 16% and 57%, respectively, compared with the pristine Matrimid® 5218 membrane. In addition to the zeolites as double-element oxide nanoparticles, single-element oxide nanoparticles such as SiO2 [66,67] and TiO2 [68] nanoparticles have also been modified by silane coupling agents. Hassanajili et al. [69] studied the surface modification of SiO2 nanoparticles for incorporation in polyesterurethane matrix. The nanoparticles were functionalized with octylsilane and PDMS to introduce hydrophobicity on the surface of the nanoparticles. The modification of fillers with long hydrophobic chains diminished their agglomeration and provided uniform dispersion compared to unmodified ones in the polymer matrix. Data displayed that the unmodified SiO2/polyesterurethane membrane exhibited better performance for CO2/CH4 separation. The –OH groups on the surface of SiO2 nanoparticles caused relatively lower-resistance pathways and facilitated the CO2 permeation. Similar results were obtained in another research study by Duval et al. [70]. Different types of silane coupling agents, including-aminopropyltriethoxy silane (A-1100), N-β-(aminoethyl)γ-aminopropyltrimethoxysilane (A-1120), and styryl amine functional silane (Z6032), were introduced onto the surface of SiO2 nanoparticles. In the next step, the functionalized fillers were embedded into the polymer matrix including polysulfone, cellulose acetate, polyetherimide, poly(4-methyl-1-pentene) (TPX), poly(2,6-dimethyl-pphenylene oxide) (PPO), and polyimides. For all the filler/polymer combination, the interfacial voids between polymer and filler were reduced and selectivity slightly improved. Liquid separation: Surface organic-functionalization of inorganic nanoparticles using the silane-coupling agent has been recently applied to improve material surface property and particle distribution as well as avoid the leaching out of nanoparticles during the membrane fabrication and filtration process. Wu et al. [26] studied the surface modification of SiO2 nanoparticles and used polysiloxane to link the PEG molecules onto the surface of SiO2 nanoparticles. The modified nanoparticles were directly blended with the casting solution containing PVDF as polymer. The surface modification remarkably improved the stability of nanoparticles in the polymer matrix during the filtration and cleaning processes, enabling the practical application of nanocomposite membranes. In a similar study, a polyamide thin-film nanocomposite (TFN) membrane was fabricated using aminosilanized TiO2 nanoparticles as filler in the top layer [71]. N-[3-(Trimethoxysilyl)propyl]ethylenediamine (AAPTS) was used as a silane coupling agent and grafted onto the surface of nanoparticles (Fig. 5.6) to minimize the aggregation and improve the compatibility of the polymer matrix and TiO2 nanoparticles. With the addition of low amounts (i.e., 0.1 wt%) of modified nanoparticles into the polyamide layer, the water flux of TFN was almost

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Fig. 5.6 Schematic representation of PA TFN membrane fabricated by dispersing aminosilanized TiO2 nanoparticles in MPD aqueous solution [71].

doubled in comparison to the thin film composite (TFC) membrane. The in situ preparation and functionalization of inorganic metal oxide nanoparticles is also studied, which is a flexible and versatile method to further enhance the physicochemical properties and performance of nanocomposite membranes. Wu et al. [72] investigated

Application of functional single-element and double-element oxide

the in situ preparation and functionalization of SiO2-NH2 nanoparticles via TEOS as precursor and aminopropyltriethoxysilane (APTES) as silane coupling agent during nonsolvent-induced phase inversion. It was suggested that a reaction occurred between imide groups of poly(ether imide) as polymer matrix and amine groups of modified SiO2, which led to uniform distribution and better compatibility of the filler/polymer. The prepared membrane also exhibited good performance for separation of enzymes or proteins. The modification of other types of metal oxide nanoparticles such as zeolites [73,74], Al2O3 [75,76], etc. has also been studied in the literature.

5.4.2 Carboxylic acid group Carboxylic acid functionalization of metal oxide nanoparticles has been generally used to stabilize the nanoparticles. The introduction of carboxylic acid functional groups on the surface of metal oxide nanoparticles is usually carried out through coordination of a variety of binding (carboxylate) moieties to the metal atoms [77]. The carboxylate moiety acts as a ligand for vacant coordination sites of surface metal atoms [16]. The carboxylic acids can completely cover the surface of nanoparticles, but they often do not have a strong interaction with the surface of nanoparticles due to their relative weakness and can be easily detached [78]. Trejo and Frey [79] studied introducing the carboxylic acid-coated iron oxide nanoparticles on the surface of the Nylon 6 membrane by three approaches: (1) simultaneous electrospinning/electrospraying, (2) LBL assembly, and (3) chemical grafting. All the prepared membranes represent potentials for the treatment of polluted rivers because of the functionality and properties of carboxylic acid-coated iron oxide nanoparticles. The uniform dispersion of nanoparticles on the surface of the membranes via three different approaches followed this sequence: electrospinning/electrospraying > chemical grafting > LBL assembly. The membranes were washed using the solution with different pH values to evaluate the nanoparticles stability on the surface of membranes. Results indicated low leaching out and release of carboxylic acid-coated iron oxide nanoparticles during the filtration process for all the prepared membranes, which depended on pH of the solution and electrostatic and intermolecular forces. The carboxyl functionalized-TiO2 nanoparticles with photocatalytic activity were deposited onto the surface of TFC membrane using a self-assembly technique by two adsorption approaches [80]. The approaches were based on the interaction of TiO2 with two oxygen atoms of the carboxylate group or formation of hydrogen bonds between the hydroxyl group of TiO2 and carbonyl group. Deposition of nanomaterials on the surface of the membrane makes it more hydrophilic, which would improve the antifouling properties. Hojjati et al. [50] proposed a new method (RAFT polymerization) to synthesize TiO2/ poly acrylic acid, which was initiated from the surface of nanoparticles. 2-{[(Butylsulfanyl)carbonothioyl]sulfanyl}propanoic acid as bifunctional RAFT agent,

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was shown to be both a modifying agent for functionalization of nanoparticles and a living polymerization agent. Results indicated that polymerization of poly acrylic acid was grafted around the nanoparticles using the “graft-from” approach and TiO2 nanoparticles were uniformly dispersed into the polymer matrix. Xin et al. [81] investigated the incorporation of three types of functionalized SiO2 microspheres containing carboxyl acid, sulfonic acid, and pyridine groups using methacrylic acid, p-styrenesulfonate, and 4-vinylpyridine, respectively, to fabricate sulfonated poly ether ether ketone (SPEEK) mixed matrix membrane for CO2/CH4 separation (Fig. 5.7). Pyridine functionalized SiO2 mixed matrix membrane showed higher CO2 permeability than those functionalized with carboxyl, sulfonic, and unmodified SiO2 membranes, owing to the penetration of nanoparticles into the polymer matrix. Furthermore, the membranes prepared by functionalized SiO2 represent better CO2/ CH4 (N2) than unmodified SiO2 membranes. The pyridine functionalized SiO2 mixed matrix membrane enhanced the CO2/CH4 and CO2/N2 selectivity because of the strong affinity between CO2 molecules and acid-based sites.

Fig. 5.7 Preparation of three types of functionalized SiO2 microspheres: (A) carboxylic acid functionalized SiO2 microspheres (SiO2-C), (B) sulfonic acid functionalized SiO2 microspheres (SiO2S), and (C) pyridine functionalized SiO2 microspheres (SiO2-N) [81].

Application of functional single-element and double-element oxide

5.4.3 Organophosphorus Metal oxide nanoparticles can be functionalized with organophosphorus molecules, such as phosphonic acid, alkylphosphoric acid, and their salts to improve the electrochemical properties and polymer/filler compatibility [82]. These molecules can be adsorbed in three different modes: mono-, bi-, or tridentate; the tridentate mode possesses a more stable structure [16]. These organophosphorus molecules only react with hydroxyl functional groups of metal oxides and only cover the monolayer surface of the nanoparticles. For example, in the case of iron oxide nanoparticles, the phosphonic acid functional groups are more stable than carboxylic acid groups and show a higher grafting density [83]. Organophosphorus-functionalized metal oxides are typically used for fabrication of fuel cell nanocomposite membranes due to their improved electrochemical properties. Embedding conductive fillers such as metal oxide nanoparticles into the polymer matrix like Nafion® is one of the strategies most employed to enhance the performance of fuel cell membranes during conditions of changing humidity. As reported in the literature [82,84], incorporation of phosphonated functionalized metal oxides not only augments the proton conductivity of nanocomposite membranes, but also leads to uniform dispersion of nanoparticles into the polymer matrix, which notably upgrades the thermal and mechanical properties as well as conductivity of the nanocomposite membranes. Wang et al. [85] prepared nanocomposite membranes of Nafion as polymer matrix and phosphonic acid-functionalized SiO2 nanoparticles as filler by the sol-gel method. All the nanocomposite membranes delivered better dimensional stability, proton conductivity, and water retention than pristine Nafion membrane, due to the presence of both acidic groups and nanoparticles. Compared with the pristine Nafion, the nanocomposite membrane with phosphonic acid-functionalized SiO2 nanoparticles exhibited 80 mV higher at 0.8 A cm 2, 120°C, and 35% relative humidity. The performance of the nanocomposite membrane made with unmodified SiO2 nanoparticles was lower than the membrane containing phosphonic acid-functionalized SiO2.

5.4.4 Ionic liquids ILs are salts that are composed of bulky and asymmetric organic cations and inorganic or organic anions [86]. ILs have superior physicochemical properties, including low melting point (below 100°C), liquid state at ambient temperature, thermal and chemical stability, high gas solubility and great affinity to some gas species (e.g., CO2, Olefins, etc.), negligible volatility, nonflammability, and are environment friendly [87,88]. ILs have been referred to as “designer solvents” due to the variability of the anions and cations and their designable and tunable structure to adjust physicochemical properties [86,89]. These unique properties create new opportunities for ILs in the IL-based membranes. Common IL cations include imidazolium, pyridinium, ammonium, phosphonium, and

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pyrrolidinium. IL anions can be selected from a wide range of inorganic anions, simple halides, or small inorganic anions such as halides, polyatomic inorganics, and polyoxometallates [90,91]. The addition of ILs as the third component into the polymer/filler system enhances the permeability and selectivity of the nanocomposite membrane for gas separation, which was initially reported by Hudiono et al. [92,93]. Furthermore, ILs act like lubricants (wetting agent) and improve interfacial wetting between the organic (polymer) and inorganic (filler) phases, resulting in better compatibility in the polymer/filler/IL system [12,54,94]. Hudiono et al. [92] prepared a nanocomposite membrane containing polymerizable styrene-based IL, IL and SAPO-34 zeolite for CO2/N2 and CO2/CH4. The addition of IL into the system led to formation of a selective layer around the zeolite, enhanced the permeability, and improved the compatibility and interfacial interactions of polymerizable styrene-based IL and SAPO-34 zeolite. The ILs plasticized the polymerizable styrene-based IL, causing the better interaction of polymer chains and surface of the fillers. Flexibility of the three-component system (polymer/filler/IL) allows the polymer chains to better interact/adhere with the filler without the formation of voids around the filler. Hudiono et al. [93] also investigated varying the loading of IL and zeolite on permeability and selectivity of CO2/CH4 and CO2/N2 of the nanocomposite membrane. Results indicated that both permeability and selectivity were declined at low loading of ILs, which could be related to the fact that a low amount of ILs did not completely cover the surface of fillers and their agglomeration. At higher loading of ILs with a constant loading of filler, permeability enhanced due to higher gas diffusion. In another research study, only a small amount of acetate-based ILs were incorporated into the titanosilicate ETS-10/chitosan nanocomposite membrane [95]. The titanosilicate ETS-10 is built from orthogonal TiO6 octahedra and SiO2 tetrahedra. The addition of IL strongly increased the permeability and selectivity as well as flexibility of polymer chains. Hu et al. [96] studied the surface modification of filler (SAPO34 zeolite) by amine and IL for CO2/H2 and CO2/CH4. After surface modification with amine and IL, SAPO34 became smoother and the voids disappeared. Modification of filler with IL resulted in increment in CO2 permeability while it reduced CO2/H2 and CO2/CH4 selectivities. However, both permeability and selectivity improved after the filler was treated with amine and IL. The interaction between nanoparticles and ILs can be interpreted with the electrostatic interaction concept. The coverage of nanoparticles with some ILs develops an electric double layer, leading to a stronger electrostatic repulsion between the nanoparticles in the polymer phase (illustrated schematically in Fig. 5.8) and avoiding the agglomeration of nanoparticles [94,97]. The improved dispersion of nanoparticles allows higher gas permeability. In the case of a porous membrane for liquid separation processes, Shi et al. [98] prepared a PVDF/TiO2 nanocomposite microfiltration membrane using IL-modified TiO2 nanoparticles. The addition of IL-modified TiO2 nanoparticles with different loading

Application of functional single-element and double-element oxide

Fig. 5.8 Electrostatic stabilization of NPs by imidazolium-based ILs [97].

into the PVDF matrix changed the crystalline structure to spherical shape and sharply affected the crystallization temperature. Furthermore, IL-modified TiO2 had a great impact on porosity and pure water flux of membranes and improved their antifouling properties.

5.4.5 Polymer/copolymer Inorganic nanomaterials can also be modified by covalent grafting of low-molecularweight preformed polymer chains onto their surface including either the “grafting to” or “grafting from” approaches [99]. Generally, grafting is an extensively used method for modification of almost all types of filler due to: (1) polymer chains are linked onto the nanoparticles by robust covalent bonds [11], and (2) grafting is independent of the filler synthesis, providing a wide range of choices for the modifiers grafting reactions [11]. In the “grafting to” method, the previously synthesized macromolecular modifier reacts onto a desired substrate surface [16]. The “grafting-from” method proves more advantageous because polymer chains are grown from an initiator-terminated selfassembled monolayer, thereby decreasing steric hindrance of neighboring bonded chains

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“grafting to”

A Inorganic building block

IN

A

IN

IN IN

IN

IN

IN

IN IN

Polymerization: “grafting from”

Individual inorganic-organic core-shell particles

A: Anchoring group IN: Initiating group

Fig. 5.9 Schematic representation of “grafting to” or “grafting from” approaches [99].

[100]. The schematic representations of the “grafting to” or “grafting from” approaches are shown in Fig. 5.9. The most common polymers/copolymers grafted onto the surface of nanoparticles are polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), polyethylenimine (PEI), polymethacrylic acid (PMAA), poly(methyl methacrylate) (PMMA), polymers containing amine groups and their derivatives. PEG is a low-molecular-weight and linear polymer derived from ethylene oxide with repeated units of H-(O-CH2-CH2)n-OH. PEG is the most common polymer for modification of nanoparticles due to its advantages such as solubility in a number of solvents (water, apolar, and organic polar), high surface mobility, good biocompatibility, low toxicity and cost, and uncharged hydrophilic residues, which reduce the agglomeration of particles [16,101,102]. Akbari et al. [101] studied the pure and PEG grafted silica nanoparticles that were synthesized using the sol-gel method in the presence of PEG as a reactant and employed to prepare high-density polyethylene nanocomposite membranes via the TIPS method for filtration of Humic acid. Water flux and contact angle measurement showed that nanocomposite membranes became more hydrophilic membranes in the presence of PEG. Fouling analysis indicated that complete blocking and cake filtration occurred during filtration of Humic acid by the PEG-grafted silica nanoparticles/ high-density polyethylene nanocomposite membrane, both of which are a reversible fouling mechanism. Borandeh et al. [103] improved the gas (CO2, CH4, O2, and N2) separation efficiency and polymer/filler compatibility in poly(methyl methacrylate) nanocomposite membranes by grafting of methoxy poly(ethylene glycol) methacrylate on the TiO2 surface through the radical polymerization technique. The CO2 permeability increased from 2.75 Barrer to 32.48 Barrer and CO2/N2 selectivity increased from 36.7 to 56.9 for pristine poly(methyl methacrylate) and nanocomposite membrane containing 5 wt% PEG-modified TiO2, respectively. PEG-modification of nanoparticles also improved polymer/filler compatibility, and free volume fraction of the

Application of functional single-element and double-element oxide

poly(methyl methacrylate) matrix. Polymer brush functionalized SiO2 with poly(monomethoxy oligoethylene glycol methacrylate) was also investigated to be embedded in Nafion for fuel cell applications [104]. PVP is a water-soluble and nontoxic material commonly used as additive in membrane fabrication and enhances the membrane hydrophilicity. PVP may be washed out during the membrane formation and/or filtration process. PVP can be retained in the membrane matrix through grafting on the polymer chains or inorganic additives [105]. Song et al. [106] fabricated the ultrafiltration membrane from polysulfone as polymer matrix and PVP-grafted silica nanoparticles as additive. The reaction procedure for preparation of PVP-grafted silica nanoparticles is illustrated in Fig. 5.10. Polysulfone/ PVP-grafted silica nanoparticles showed higher water flux than the pristine polysulfone membrane (2.3 times higher) without any reduction in solute removal. The contact angle of nanocomposite membranes continuously decreased by increasing the PVP-grafted silica nanoparticles loading. The contact angle of Polysulfone/PVP decreased with increasing the immersion time in the coagulation bath due to washing out of embedded PVP, while the contact angle of Polysulfone/PVP-grafted silica nanoparticles nanocomposite membranes did not change with immersion time in the coagulation bath. In another research, magnetic Fe3O4@SiO2 nanoparticles were modified with grafting of PVP on their surface, and then incorporated in the polyethersulfone matrix [107]. The nanocomposite membrane showed higher thermal resistance, hydrophilicity, and fouling resistance in comparison to polyethersulfone and polyethersulfone/PVP membranes. Polyethylenimine (PEI), especially such a kind with low molecular weight, is known as a modifier with hydrophilic nature and long branched polymer chains. The presence of different types of amines (primary, secondary, and tertiary) in its chemical structure makes

Fig. 5.10 Reaction procedure for preparation of PVP-grafted silica nanoparticles [106].

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it chemically reactive [108]. The reactivity of PEI can improve the dispersibility of fillers into the polymer matrix, facilitate the interfacial interactions between polymer and filler, and act as carriers for CO2 separation [109]. Zargar et al. [108] modified the surface of SiO2 to fabricate the TFN membrane for desalination. The introduced functional groups onto the nanoparticles indicated promising chemical characteristics and better interfacial interaction and compatibility between the polymer and filler, which enhance the hydrophilicity, morphology, separation performance, and mechanical properties of the TFN membranes. The primary amine groups of PEI of nanoparticles and acyl chloride of the TMC monomer can react with each other to provide better interfacial interaction. The TFN membrane containing modified SiO2 with PEI showed up to 46% improvement in water flux and higher salt rejection. Poly(methyl methacrylate) (PMMA) and its derivatives (e.g., polyhydroxyethylmethacrylate (PHEMA)) have been extensively used in the literature [104,110–112] to graft on the surface of nanoparticles and facilitate their distribution in the membrane matrix. Zhu et al. [113] prepared a nanocomposite ultrafiltration membrane using poly (2-hydroxyethyl methacrylate) (PHEMA) grafted silica (SiO2) nanoparticles as inorganic additive and polyethersulfone as polymer matrix. The PHEMA was grafted onto the SiO2 via the RAFT polymerization method. The membrane characterization showed that modified nanoparticles migrate toward the top layer. Furthermore, the surface porosity, hydrophilicity, and antifouling ability improved. Tradeoff between permeability and selectivity was broken by the addition of modified SiO2, and the leaching out of nanoparticles during phase inversion and the long-term filtration process significantly reduced. In another research study, they also studied the modification of SiO2 nanoparticles via poly(2-dimethylamino ethyl methacrylate) [114]. Tumnantong et al. [115] synthesized poly(methyl methacrylate) (PMMA)-SiO2 nanoparticles via differential microemulsion polymerization. The modified fillers were uniformly dispersed in natural rubber as membrane matrix, notably improved the water selectivity during the pervaporation process, and enhanced the mechanical properties.

5.4.6 Polydopamine Polydopamine (PD) has been recently used as a good adhesion agent and/or coupling ligand in the fabrication of nanocomposite membranes. PD can attach to almost all types of organic and inorganic support via the oxidative self-polymerization of dopamine to form a controllable and durable film [12]. Shao et al. [116] prepared TiO2 nanoparticles/PVDF ultrafiltration nanocomposite membranes. Self-polymerization of dopamine was used to uniformly distribute the TiO2 nanoparticles on PVDF support and form a strong binding force between them. The possible mechanisms of deposition of TiO2 nanoparticles on the PD modified membranes and their performance are illustrated in Fig. 5.11. The characterization of prepared

Application of functional single-element and double-element oxide

Fig. 5.11 The possible mechanisms of binding TiO2 on the PD-modified membranes and their performance [116].

membranes after modification revealed that water flux, bovine serum albumin (BSA) rejection, and antifouling performance were improved. A free-radical method was also employed in the literature to fabricate self-cleaning and self-protected nanocomposite membranes [117]. A PD layer was formed onto the polysulfone support, and then amine-functionalized TiO2 were deposited onto the PD layer via amine-catechol adduct formation. The TiO2 can protect the polysulfone support with self-cleaning property. In comparison to the conventional membrane prepared by direct blending of additive (TiO2) and polysulfone, the TiO2/PD/polysulfone provided more contact area for TiO2 photocatalytic nanoparticles to degrade the pollutions and consequently improved the cleaning efficiency. The PD layer protects the polysulfone layer from being attacked by free radicals during UV irradiation. The TiO2/PD/polysulfone displayed long-term durability after 9 days operation, while the performance of TiO2/polysulfone declined after one-day operation. Wu et al. modified the surface of TiO2 nanoparticles via dopamine and PEI and then incorporated them in the SPEEK matrix. Young’s modulus values were increased after the addition of modified nanoparticles, which demonstrated that a strong interaction was formed between sulfonic groups of SPEEK and amino groups of modified filler. The amino group of the modified filler acts as active sites under humid atmosphere and

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facilitating CO2 diffusion in nanocomposite membranes. The nanocomposite containing PEBA as polymer matrix and modified TiO2 as additive were fabricated for CO2/N2 separation [118]. The TiO2 nanoparticles were modified and amine-functionalized via one-step reaction with dopamine and PEI. The amine-functionalized TiO2 nanoparticles could facilitate CO2 transfer and also improved CO2/N2 selectivity because of the reversible reaction that occurred between CO2 and amine groups.

5.5 Properties of polymer/single-element and double-element oxide nanocomposites The (functional) oxide nanoparticles are incorporated into the polymer matrix to result in new hybrid materials with desirable mechanical, physical/chemical, and thermal properties.

5.5.1 Mechanical properties A common reason for the addition of inorganic nanoparticles into the polymer matrix is to modify the mechanical properties such as tensile strength, flexural and Young’s modulus, stiffness, and hardness. The homogeneous dispersion of fillers improves the transfer of stress from organic phase (polymer) to inorganic phase (oxide nanoparticles) resulting in higher mechanical strength; however, nonhomogeneous dispersion of fillers leads to the formation of defective structure and stress convergence points in the polymer matrix, weakening the mechanical properties. In some systems, there is a threshold concentration for the addition of filler into the polymer matrix, at which distribution of fillers changes from homogeneous to nonhomogeneous and the modulus goes down. Functionalization of nanoparticles improves the compatibility between the fillers and polymer matrix, and the mechanical strength tends to increase. Incorporation of unmodified fillers causes poor interaction between the polymer and fillers and a decrease in mechanical strength.

5.5.2 Physical/chemical properties 5.5.2.1 Physical Uniform dispersion of oxide nanoparticles into the polymer matrix provides significant improvement in physical properties such as viscoelastic properties, crystallinity, density, conductivity, and structure and morphology. The nanocomposite membranes generally possess higher viscoelastic properties than the pristine membranes. The storage module of nanocomposite membranes depends on several factors, including filler/polymer interaction, loading of nanoparticles, and size of nanoparticles. The storage module is greater in systems with good filler-matrix interaction, higher loading of nanoparticles, and lower particle size. There may be some particular nanocomposite systems in which minor changes were observed in their crystallinity versus neat polymers, but the crystallinity of crystalline and

Application of functional single-element and double-element oxide

semicrystalline polymers are not greatly influenced by the addition of the oxide nanoparticles. The density or free volume of membranes is affected by the addition of nanoparticles into the polymer matrix and their measurement are common methods applied to evaluate the compatibility and interfacial adhesion between the polymer and nanoparticles. The theoretical density is calculated by additivity rules and then compared with the experimental value to evaluate the polymer/filler compatibility and reveal the clustering formation or the presence of interfacial voids at the polymer/filler interphase. The combination of oxide nanoparticles with conducting polymers can significantly improve the electrochemical conductivity of membranes for batteries and fuel cell application. Depending upon the nature, size, and functional groups of nanoparticles as well as synthesis strategy, electrochemical properties of the nanocomposite membrane can be controlled. Functional groups such as –NH2, –COOH, and –SO3 have been introduced onto the surface of nanoparticles to improve the applicability of conducting polymers. The effects of oxide nanoparticles and their functional groups on the structure and morphology of membranes have been evaluated in previous sections. 5.5.2.2 Chemical Oxide nanoparticles are able to give some properties such as (photo) catalytic, antibacterial, antifouling, chlorine resistance, and oxidative stability to nanocomposite membranes, which do not possess these properties in the pristine membranes. The new functionality of some of the metal oxide nanoparticles and their effects on chemical properties of nanocomposite membranes are listed in Table 5.1. The effects of oxide nanoparticles on the chemical properties of nanocomposite membranes are investigated in detail in our published paper [2] and the literature [6,43].

5.5.3 Thermal properties The thermal properties of polymeric membranes such as thermal stability and thermal conductivity can be improved by the embedding of single-element and double-element oxide nanoparticles into the polymer matrix. In addition to the intrinsic thermal properties of the polymer and the fillers, the thermal properties of the nanocomposite membranes depend on several factors such as shape and size of nanoparticles, functional groups and modifiers on their surface, nanoparticle networking in the polymer matrix and polymer/nanoparticle interactions, and polymer/nanoparticle system surface energy [5]. The addition of oxide nanoparticles mostly leads to the formation of interaction (e.g., hydrogen bond or other coordinate bonds) between the inorganic network (nanoparticles) and polymer chains, which confines the thermal motion of macromolecules and enhances their rigidity, thereby increasing the needed energy for polymer chain movement and breakage [30]. The presence of functional groups on the surface of oxide nanoparticles

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also improves the interfacial adhesion between polymer chains and fillers strengthen the rigidity of polymer chains.

5.6 Conclusions Incorporation of nanoparticles into the polymeric membranes has proven to be an effective way for improving the separation properties of the membranes. Nanocomposite membranes currently suffer from some particle/polymer and particle/particle interactions. These interactions are the bad adhesion of polymer to particle and agglomeration of the particles. Functionalization of the nanoparticles is a facile way for controlling these interactions. Moreover, the use of special functionalized particles could be a way to increase the selectivity of the membranes especially in gas separation applications. Several challenges have limited the application of functionalized nanoparticles in polymeric membranes: 1. Functionalization may improve the compatibility of the polymer and the nanoparticle but may negatively affect the separation properties, especially the selectivity. 2. In the case of porous nanoparticles like zeolites or MIFs, functionalization may affect the size of the pores or even blind them. 3. Functionalization may increase the adhesion of the particles via strong attraction between the functional groups. Altogether, application of functionalized nanoparticles in the polymeric membranes has been successful, but fabrication methods should be revisited.

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CHAPTER 6

Development of advanced nanocomposite membranes by carbon-based nanomaterials (CNTs and GO) Maryam Ahmadzadeh Tofighy, Samaneh Khanlari, Toraj Mohammadi

Center of Excellence for Membrane Science and Technology, Faculty of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

6.1 Introduction Due to increased pollution of water resources, providing safe and clean drinking water to humans is a challenge [1]. While more than 70% of the earth’s surface is covered with water, only 0.8% of the earth’s water is fresh, and explosive population increase and worldwide industrialization have caused serious water problems [2]. Millions of people around the world have no access to safe drinking water or proper sanitation [3]. Therefore, a clean water supply for human consumption, industries, energy production, and agriculture is one of the most important concerns of human society [4]. Because of the abundance of salt water and the relative scarcity of fresh water on the planet, in order to provide fresh water and protect human health as well as the environment, sea water desalination and polluted water treatment become critical issues [5]. In recent years, membrane technology has received a lot of attention due to its high efficiency, environment friendliness, convenience, selective separation, stability, and flexibility to be combined with other separation processes. Therefore, membrane technology is becoming a vital component of many industrial processes, such as medical, pharmacological, food, chemical, and biotechnological industries, as well as water and wastewater treatment [6–8]. Membrane technology is responsible for more than 53% of fresh water production worldwide [9]. Membranes are a thin sheet of natural or synthetic material that primarily function as a barrier, allowing selective transport of a specific component from one side of the membrane to the other. Separation of unwanted contamination species from clean water through suitable membranes is the principle behind the membrane filtration [10,11]. Membranes for use in water treatment need to have efficient selectivity or contaminant rejection, adequate water permeability, good resistance to fouling/scaling, and sufficient

Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00006-7

© 2020 Elsevier Inc. All rights reserved.

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mechanical integrity [5]. Over the past 20 years, much research has been done on the preparation of synthetic membranes that could be made using inorganic materials (ceramics) or organic materials (polymers) [6]. Unfortunately, these membranes are still too expensive, and development of novel techniques and materials are essential to address the tradeoff between selectivity and permeation flux [12]. To improve separation performance, researchers have developed a huge range of polymer nanocomposite membranes (NCMs) by incorporating inorganic nanofillers, such as carbon nanomaterials, metal organic framework (MOF), carbon molecular sieve (CMS), titanium oxide (TiO2), zeolite and silica into polymer matrices such as polyether sulfone (PES), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyvinyl chloride (PVC), polypropylene (PP), polyethylene (PE), cellulose acetate (CA), polyamide (PA), and chitosan (CS). With this technique, the disadvantages of one material can be compensated for with the advantages of other materials. This combination overcomes the drawbacks of processing the inherently brittle inorganic materials and enhances the physical properties, as well as separation performance, of the host polymeric membranes [12–14]. NCMs are divided into two main groups: mixed matrix nanocomposite membranes (MMNMs) and thin-film nanocomposite membranes (TFNMs). MMNMs are commonly fabricated via a single-step phase inversion technique, and nanofillers embedding into polymer matrix can improve both antifouling properties and permeation flux of the MMNMs. In these membranes, controlling the distribution of the nanofillers within the polymer matrix is essential [9,13]. TFNMs can be considered a molecular sieve constructed of a thin but dense layer (thickness < 200 nm) deposited on top of a porous support layer (thickness about 50 μm) for use in water desalination or water purification systems. The incorporation of nanomaterials into the dense, thin layer and/or support membrane can improve the TFNMs performance. Proper choice of nanofillers can significantly improve stability, rejection, and permeation flux of the prepared TFNMs [4,9]. TFNMs are commonly fabricated via an in situ/interfacial polymerization (IP) technique. The main advantage of TFNMs over MMNMs is that the skin and the support layers of TFNMs can be optimized discretely, which allows more versatile membranes to be designed [15,16]. Carbon nanomaterials with interesting properties have attracted the attention of many scientists around the world and prompted them to further develop polymer NCMs, with carbon nanotubes (CNTs) and GO the most extensively researched. Due to the strong tendency of carbon nanomaterials to agglomerate in polymer matrices, carbon nanostructure modification is needed to improve their dispersion in solvents, and improve the prepared NCMs properties [17,18]. In this chapter, current progress in preparation and characterization of polymeric NCMs containing carbon nanomaterials (CNTs and GO) for water treatment applications are reviewed.

Nanocomposite membranes containing CNTs and GO

6.2 Nanocomposite membranes 6.2.1 Mixed matrix nanocomposite membranes MMNMs are NCMs made of nanomaterials and polymer matrices [19]. The MMNMs have received great attention recently, due to the use of unique properties of nanomaterials and the processability of polymeric membranes [20]. MMNMs can overcome the tradeoff between selectivity and permeation flux and also improve resistance to membrane fouling during water treatment [21]. Generally, incorporating nanomaterials into polymer matrices can improve membranes properties such as fouling resistance, hydrophilicity, mechanical, chemical and thermal stability, and surface charge density [11,22]. The incorporation of nanomaterials into polymer matrices can introduce photocatalytic and antibacterial characteristics to the membranes. Fabrication methods for MMNMs are similar to regular polymer membrane fabrication techniques. Various nanomaterials can be used for MMNMs preparation, including organic nanomaterials, such as polymeric nanomaterials, inorganic nanomaterials, such as ceramics, metals or glass, and carbon nanomaterials, such as CNTs, GO, and their derivations [23]. The addition of nanomaterials provides additional diffusion paths and affects the morphology, roughness, porosity, and pore size of the membrane [24]. A number of membrane preparation techniques have been used to fabricate MMNMs, including phase inversion [23], stretching [25], track-etching [26], and electrospinning [27]. The choice of suitable techniques for the fabrication of MMNMs is limited by the selection of polymers, nanomaterials, and desired membrane structure. Phase inversion, a commonly used method for the MMNMs fabrication, is done by immersing a homogeneous polymer/nanofiller solution in a nonsolvent bath to convert a single phase into a two-phase system [24]. Among various phase inversion techniques, immersion precipitation is the most commonly used technique for the fabrication of MMNMs [26,28–30].

6.2.2 Thin-film nanocomposite membranes A thin-film composite (TFC) membrane consists of an active layer and a sublayer with porous structure. TFC membranes, as the primary type of nanofiltration (NF) and reverse osmosis (RO) membranes, have been widely used to desalination applications [31–33]. Also, TFC membranes have shown great potential to eliminate inorganic micropollutants such as pharmaceutically active compounds and pesticides [34]. In addition, TFC membranes have had the important role in forward osmosis (FO) process development for energy-saving desalination application [35]. Compared to the asymmetric membranes, TFC membranes exhibit superior advantages because of their higher water permeation flux and solute rejection, which are mainly due to the optimization ability of active layer and support layer [36]. Among various TFC membrane fabrication methods [37,38], in situ/interfacial polymerization (IP) is the most common technique and has been used

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primarily for synthesis of commercially available NF and RO membranes for water treatment and purification. Most of these membranes have a PA thin layer above the porous membrane support. The most commonly used active monomers employed to the fabrication of these membranes are trimesoyl chloride (TMC) and m-phenylenediamine (MPD) [6,39]. TFNMs are novel TFC membranes that have nanomaterials incorporated within the active thin layer and/or sublayer structures. These nanomaterials can significantly enhance stability, rejection, permeability, surface charge density, hydrophilicity, and fouling resistance of the prepared TFNMs [40].

6.3 Carbon nanomaterials 6.3.1 Carbon nanotubes CNTs were first developed in 1991 by Iijima [41]. CNTs exhibit extraordinary physical and chemical properties such as high mechanical stiffness, high specific surface area, high flexibility, high aspect ratio (length-to-diameter ratio), low mass density, frictionless surface, effective π-π stacking interaction with aromatic compounds and one-dimensional structure. They have therefore received great attention as an important member of carbon family and their various applications have been studied continuously. The CNT walls consist of a hexagonal lattice of carbon atoms of graphene sheets and their ends are usually capped by half-fullerene-like structures. The CNTs’ structure can be classified as single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs), as shown in Fig. 6.1 [43]. Current synthesis techniques of CNTs, including electric arc discharge [44], laser ablation [45], and chemical vapor deposition [46,47], are commercially used to produce large quantity of CNTs. In general, CNTs synthesis is a significant challenge, because their alignment (armchair, zig-zag, or chiral), dimensions (diameter and length), and the walls number should be controlled [48]. Since CNTs tend to form bundles and are chemically inert, it is difficult to achieve homogeneous CNTs distribution in the

Fig. 6.1 CNTs structure (A) SWCNTs, (B) DWCNTs, and (C) MWCNTs [42].

Nanocomposite membranes containing CNTs and GO

polymer matrices [49]. Uniform CNTs dispersion in the polymer matrices can significantly improve separation performance and is a critical issue in the fabrication of CNTs-polymer NCMs. Therefore, to reduce the van der Waals forces between CNTs and improve CNTs dispersion in the polymer matrices, CNTs functionalization with specific functional moieties is of great significance [50,51]. There are several approaches for CNTs functionalization, including defect functionalization (purification and oxidation), and covalent and noncovalent functionalization [43]. Noncovalent functionalization [52] is based on the intermolecular interactions between CNTs and other molecules, while covalent modification [53] is based on the covalent bonds breaking and the new covalent bonds forming. The tubular structure of CNTs provides stable nanochannels with smooth surface and variable size for mass transport. The smooth interior surfaces of CNTs offer lowresistance routes, allowing to preferential permeation for molecules. Therefore, polymer membranes filled with CNTs show higher fluid fluxes and can be used to overcome the limitations of homogeneous polymer membranes. The obtained high fluid fluxes are attributed to the inherent smoothness of the interior CNTs and the molecular ordering phenomena inside the CNTs nanochannels that is due to the minimal interaction of permeating molecules (water) with the CNT inner wall [54].

6.3.2 Graphene oxide Graphene and graphene derivatives, such as graphene oxide (GO), have also received increased attention in recent years, due to their numerous unique properties. Graphene is a two-dimensional monolayer of carbon atoms with sp2-hybridized arranged in a regular hexagonal pattern (honey-comb like) and is the basic structural unit of other carbon allotropes, such as graphite, fullerenes, and CNTs [55]. Graphene, with excellent electrical, mechanical, and thermal properties, has been considered as a inorganic nanofiller to improve polymer nanocomposite properties [42,56,57]. The predicted physicochemical properties of graphene-based nanocomposites depend on the distribution of the graphene layers inside the polymer matrix and their interfacial bonding. However, due to its incompatibility with organic polymers, pristine graphene does not form homogeneous composites with proper properties. To overcome this incompatibility, graphene oxidation has been proposed to introduce highly hydrophilic oxygen-containing functional groups onto the graphene surface [58]. GO is a modified form of graphene that consists of various oxygen-containing functional groups, such as hydroxyl and epoxy groups on the basal plane and carboxyl groups at the edges. These oxygen-containing functional groups can improve the compatibility of graphene with organic polymers [59]. As an important carbon nanofiller with many attractive properties such as superior mechanical stability, hydrophilic nature, two-dimensional structure, high flexibility,

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good miscibility with polymers and its negatively charged surface, GO can be incorporated with polymeric membranes via membrane surface functionalization or embedding into polymer matrices, and can improve permeation flux, surface hydrophilicity, mechanical strength, and fouling resistance of the prepared NCMs [60,61]. The oxygen-containing functional groups of GO can also provide suitable chemical reactant sites for a variety of further membrane modifications. Also, the inherited structural features of GO in NCMs can create relatively regular channels for water penetration. Therefore, GO is a preferred candidate for the fabrication of polymer NCMs to be used in water and wastewater treatment [62, 63].

6.4 NCMs containing carbon nanomaterials 6.4.1 NCMs containing CNTs Badawi et al. [9] prepared MWCNTs/CA NCMs for use in desalination via phase inversion. Strong acidic medium was used to functionalize MWCNTs in order to enhance the dispersion of MWCNTs within the polymer matrix. Their results demonstrated that both the number of membrane macrovoids and pore size decreased with increasing CNTs content, as shown in Fig. 6.2. Using a 1000-ppm NaCl solution as feed, the membranes with the lowest CNT content achieved 54% permeation flux improvement with a

Fig. 6.2 Macrovoids in CNTs/CA NCMs: (A) 0 wt% CNTs, (B) 0.0005 wt% CNTs, (C) 0.005 wt% CNTs, and (D) 0.01 wt% CNTs [9].

Nanocomposite membranes containing CNTs and GO

minimal decrease in salt retention (
6%). With further addition of CNTs, reduced permeation rates were observed, due to the reduced surface area and porosity. Khalid et al. [64] fabricated dodecylamine (DDA)-functionalized MWNTs/PSf (DDA-MWNTs/PSf) NCMs for desalination via phase inversion and studied the antifouling properties of the prepared membranes. Compatibility and interfacial adhesion between the PSf matrix and the inorganic nanotubes were enhanced by the long alkyl chains of DDA-functionalized MWNTs, as shown in Fig. 6.3. Surface hydrophilicity, roughness, and morphology of the fabricated NCMs, as a function of DDA-MWNTs loading, were studied. Compared with the neat PSf membrane, permeability and fouling resistance of the prepared NCMs were improved significantly. The NCMs prepared with 0.5 wt% DDA-MWNTs loading displayed the lowest total flux loss (29%) and the highest flux recovery (83%), with the reduced irreversible fouling resistance (17%). Zarrabi et al. [3] prepared modified TFNMs via IP between TMC and piperazine monomers and amine functionalized MWCNTs (NH2-MWCNTs) as a hydrophilic modifier for desalination. Their results demonstrated that, compared to the unmodified membrane, the membranes containing NH2-MWCNTs in the PA layer have a smoother surface and higher hydrophilicity and therefore exhibit better fouling resistance. The best separation performance, including 95.72% and 36.71% rejection of NaCl and Na2SO4 salts (for 2000 mg/L of NaCl and Na2SO4 solutions), was obtained for the membrane modified with 0.005 wt% NH2-MWCNTs. Azelee et al. [4] fabricated PA TFNMs incorporated with acid-treated MWCNTstitania nanotubes (TNTs) hybrid as nanofiller for desalination. The prepared hybrid nanofiller was introduced to the PA selective layer via IP of MPD and TMC monomers

DDA-MWCNTs

C O

N

(CH2)6

H Possible hydrogene bonding O S O

O

CH3 CH3

Polysulfone

O n

Fig. 6.3 Possible hydrogen bonding between the sulfonic group of PSf and the amide group of DDAMWNTs [64].

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over a porous commercial PSf UF support. Their obtained results demonstrated that the acid-treated MWCNTs-TNTs improve the surface properties (roughness, surface charge, and contact angle) of the membrane, and therefore water permeability increases significantly without decreasing the salt rejection. For the TFNM containing 0.05 wt% acid treated MWCNTs-TNTs, the highest water permeability of 0.74 LMHbar was achieved, which was 57.45% higher than that of the neat PA membrane. The Na2SO4 and NaCl rejection of this membrane was 98.07% and 97.97%, respectively; almost similar to those of the neat membrane. Takeuchi et al. [2] prepared RO membranes of MWCNTs-PA via IP between MWCNTs-dispersed MPD and TMC on a porous PSf substrate for desalination. The prepared MWCNTs-PA membrane showed 96.0% salt rejection and 0.38 g/m2 h permeation flux, at 0.7 MPa for 0.2% NaCl aqueous solution. Their results demonstrated that with decreasing salt concentration and increasing running pressure, the salt rejection increases. Due to the presence of MWCNTs, the MWCNTs-PA membranes show negative nature. As can be observed in Fig. 6.4, the isoelectric point (IEP) of the MWCNTs-PA membrane decreases with increasing MWCNTs content. Accordingly, it showed an ordered salt rejection performance for four salt solutions (Na2SO4 > MgSO4 > NaCl > MgCl2). Donnan model was used to explain these data in details. Farahani et al. [65] prepared PES NCMs comprising SiO2, cloisite 30B clay, TiO2, hydroxyl-functionalized MWCNTs (MWCNTs-OH), and carboxyl-functionalized MWCNTs (MWCNTs-COOH). Their results demonstrated that these nanoparticles could improve hydrophilicity and porosity and thus open the channels up for water transport enhancement. As compared with the neat PES membrane, the prepared NCMs showed better water permeation flux and antifouling properties due to their improved

4.5 4.0 Isoelectric point (IEP)

152

3.5 3.0 2.5 2.0 1.5 1.0 0

0.1

0.2

0.3

0.4

0.5

0.6

CNT content (wt.%) in dispersed solution

Fig. 6.4 IEP of MWCNTs-PA membranes as a function of MWCNTs content [2].

0.7

Nanocomposite membranes containing CNTs and GO

porosity and hydrophilicity. However, further loading of nanoparticles led to diminished fouling resistance and water permeation flux due to the possible agglomeration. Mulopo et al. [66] synthesized CNTs/PSf NCMs via phase inversion for membrane bioreactor (MBR) application. Their results demonstrated that adding CNTs to the PSf NCM increases permeation flux across the membranes, which may be attributed to the presence of O-H bonds that alter the membrane roughness and contact angle. Membrane modification with CNTs increases the membrane roughness from 13.93 nm to 62.39 nm. Zheng et al. [67] synthesized sulfonated MWCNTs (S-MWCNTs) with pendant alkyl sulfonic acid groups from hydroxyl-functionalized MWCNTs (MWCNTsOH), as shown in Fig. 6.5. Excellent dispersion of the prepared S-MWCNTs in water facilitated IP to prepare poly(piperazine amide) TFNMs. In order to obtain the optimum separation performance, water permeation flux and salt rejection of the NCMs were adjusted by optimizing the S-MWCNTs content. The TFN-0.01% membrane showed high water permeation flux of 13.2 LMHbar, 1.6 times more than the neat membrane, without compromising the salt rejection performance (96.8% Na2SO4 rejection). The TFN-0.01% membrane, with enhanced surface hydrophilicity also showed better antifouling ability to BSA, with 91.2% water flux recovery ratio compared to 82.0% for the neat membrane. Lee et al. [68] prepared CNTs/PSf nanocomposite supports with significantly improved surface porosity while maintaining both surface pores size and hydrophobicity. Such support surface characteristics led to the formation of a defect-free PA selective layer with large surface area, which resulted in permeation flux enhancement of the prepared NCMs (up to 35%). Ma et al. [69] fabricated CNT-loaded PA TNFMs using the electrospray-assisted IP method, shown in Fig. 6.6. In this method, amine and acyl chloride monomer solutions were electrosprayed into microdroplets to allow controlled growth of the PA rejection layer. Simultaneously, CNTs were dispersed uniformly under the electrical field action. The CNT loading in the PA rejection layer improved the membrane’s water permeation

OH

O O S

O

OH

O

SO3H

O

SO3H

O

SO3H

110∞C, 24 h, DMSO OH

MWCNT-OH

Fig. 6.5 Route for the SMWCNT synthesis [67].

SMWCNT

153

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Fig. 6.6 Schematic diagram of the electrospray-assisted IP method [69].

flux up to sixfold. This preparation method allowed simultaneous control of nanomaterial dispersion and rejection layer thickness. Vatanpour et al. [70] synthesized polypyrrole (PPy)-coated oxidized and raw MWCNT nanocomposites and used them for hydrophilic modification of PVDF UF membranes. Compared with the neat membrane, the prepared MMNMs exhibited higher hydrophilicity (smaller contact angles). The results demonstrated that pure water flux increases from 152.8 LMH (for the neat PVDF membrane) to 378.8 and 399.3 LMH for 0.1 and 1 wt% of PPy: raw and PPy: ox-MWCNT MMNMs, respectively. Also, the antifouling properties of the prepared MMNMs improved significantly. The rejection results revealed that increasing surface porosity and mean pore size of the MMNMs (due to the functionalized CNTs addition) do not affect the membrane’s BSA rejection efficiency. Vatanpour et al. [71] fabricated RO TFNMs via IP of MPD and TMC monomers modified with amine-functionalized MWCNTs (MWCNT-NH2) for desalination. Their results demonstrated that permeation flux and hydrophilicity of the modified membranes are improved without any salt rejection reduction. The best desalination and antifouling performance was obtained for the membrane containing 0.002 wt% MWCNT-NH2. Incorporation of MWCNT-NH2 into the PA layer of the prepared membranes improved their fouling resistance by increasing hydrophilicity and the negative charge density of the membrane surface, and reducing the membrane surface roughness. Mahdavi et al. [72] synthesized two nanocomposites by depositing hydrophilic PPy on raw or oxidized MWCNTs (PPy-raw or PPy-oxidized MWCNTs) via in situ oxidative polymerization of pyrrole and applied them to NF TFNMs fabrication for

Nanocomposite membranes containing CNTs and GO

Fig. 6.7 Preparation of PEG-functionalized CNTs [73].

desalination. Their results demonstrated that the PPy-oxidized MWCNTs NCMs reveal surprisingly high permeation flux where the PPy-raw MWCNTs NCMs exhibit improved permeation flux as well. The SEM images indicated that the PPy-raw MWCNTs NCMs have smooth surfaces and consequently show improved antifouling properties. Khalid et al. [73] synthesized polyethylene glycol (PEG)-functionalized CNTs (PEGCNTs), as shown in Fig. 6.7, and then fabricated (PEG-CNTs)/PSf NCMs via phase inversion for wastewater treatment by MBR. According to the obtained results, the optimum NCM, containing 0.25 wt% PEG-CNTs, exhibited approximately fourfold increase in pure water and protein solution permeabilities of 16.8 and 10.8 LMHbar, respectively. Moreover, the addition of PEG-CNTs to the PSf membranes reduced interaction between the membrane surface and the protein molecules, and improved the fouling resistance by 72.9%.

6.4.2 NCMs containing GO and its derivations Abdel-Karim et al. [74] incorporated reduced graphene oxide (rGO) nanoplatelets with different degrees of reduction (different oxygen content) as fillers to PVDF matrices for membrane distillation (MD) application. With the addition of fillers, changes in morphology of the prepared MMNMs were observed, due to the increased viscosity and hydrophilicity of the casting solutions. Their results demonstrated that the MMNM containing 0.5 wt% rGO with 58% reduction (optimum degree of reduction) exhibits an improved MD performance, with permeation flux of 7.0 LMH, which represents approximately 169% enhancement in comparison with the neat PVDF membrane, without compromising salt rejection (>99.99%). Wang et al. [75] synthesized an exfoliated hydrotalcite/GO (EHT/GO) hybrid nanosheet and uniformly dispersed it in polyethyleneimine (PEI) solution to be used as an aqueous phase for IP. The PA selective layers of the TNFMs were formed on the surface of PES substrate via IP of branched PEI and TMC as the aqueous phase and the organic phase monomers, respectively, for desalination. The results demonstrated that EHT/GO incorporation into the PA membrane can effectively enhance its water permeation flux (26 LMH at 0.8 MPa). The salt rejection of all the membranes followed

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the order of MgCl2 > NaCl > MgSO4 > Na2SO4, which confirms that the membranes were positively charged. The prepared EHT/GO membrane had 97.0% MgCl2 rejection, which exhibited its superior water-softening performance. Fathizadeh et al. [76] prepared nitrogen-doped GO quantum dots (N-GOQD) and then fabricated PA/N-GOQD TFNMs by IP for RO desalination. Their results demonstrated that with the addition of 0.02 wt/v% N-GOQD to a PA membrane, water permeation flux increases three times without compromising salt rejection. The result demonstrated that the improved membrane surface hydrophilicity, membrane surface area, and also the larger cavity introduction at the interface between PA matrix and N-GOQD enhance water permeability. Also, the thermal stability of the PA membrane was greatly improved due to the favorable chemical bonding between N-GOQD and the PA matrix. Kim et al. [77] prepared PA NCMs containing GOT (GO coated by tannic acid) in the active layer via IP of an organic solution containing TMC and an aqueous solution containing MPD and GOT. Compared to the PA membranes containing only tannic acid and/or GO and the PA membrane without any additives, the prepared PA membrane containing GOT exhibited significant enhancement of water permeation flux, antimicrobial properties, and chlorine resistance. This improvement may be attributed to the GOT’s advantageous properties, such as hydrophilicity, compatibility with the polymer matrix, barrier property, and oxidative stress capability. Kang et al. [78] fabricated PSf UF NCM using sulfonated GO (S-GO) as an additive. The results demonstrated that, compared to the neat PSf membrane, the addition of a small amount of S-GO (70%, and the flux remained at around 2 L m
2 h
1 throughout the test period. This indicated that the membrane was resistant to organic fouling and the chlorine stability studies also confirming the chemical stability of the zeolite membranes. The incorporation of zeolites nanoparticles into TFC RO membrane is also known to improve the water permeability of the membrane. However, some of the zeolites, for instance, NaA, might limit the application of the TFN for seawater desalination. Therefore, the chemically stable silicalite-1 nanoseolite was incorporated into the skin layer of the RO composite membrane via interfacial polymerization technique [43]. The membrane exhibited excellent chemical stability toward acid and multivalent cation, compared with the NaA-mixed membrane. Besides, silicalite-1 showed a better permeability than NaA, due to larger channel pores and a higher water diffusion rate in silicalite-1. Therefore, silicalite-1 mixed membrane has great potential in large-scale seawater desalination. A study to separate trivalent salts (FeCl3 and AlCl3) from aqueous solution using Faujasite (FAU) zeolite composite ultrafiltration on ceramic support prepared by facile uniaxial support showed a maximum rejection of 81% for FeCl3 and 75% for AlCl3 at an applied pressure of 276 kPA for feed concentration of 250 ppm [44]. Yurekli incorporated the NaX nanoparticles in PSf membrane to remove lead and nickel cations from synthetically prepared solutions [45]. The performance of the hybrid membrane was determined under dynamic conditions. From this study, the sorption capacity and water hydraulic permeability of the membrane can be improved by tuning the fabrication condition, for example, the evaporation period of the casted film and the NaX loading. The maximum sorption capacity of the hybrid membrane for the lead and nickel ions was measured as 682 and 122 mg g
1, respectively (60 min of filtration, 1 bar of transmembrane pressure). The coupling process suggested that membrane architecture

Development of advanced nanocomposite membranes

could be efficiently used for treating metal solutions with low concentrations and transmembrane pressures.

7.2 Metal-organic framework (MOF) Metal-organic frameworks (MOFs) are a new class of porous hybrid organic/inorganic materials, with zeolite-like features. MOFs are known to be stable up to temperatures above 200°C, and are characterized by very high surface areas, due to the porosity of the structures [46]. The MOF structures consist of metallic clusters and organic linkers; these combinations give many possible arrangements between metals and organic compounds to form particles with different textural, chemical properties, porosity, and flexibilities [47]. The nodes of the MOF structure are formed from metal ions, or metal ion clusters called secondary building units (SBUs), and organic ligands bridge the SBUs in regular, geometric patterns [48]. The wide diversity of SBUs and connectivity of the ligands allows for a variety of networks. For instance, a combination of Zn2+ with 1,4-benzene dicarboxylic acid (H2bdc) yields IRMOF-1. When the same metal is combined with different ligand (2-methylimidazole), ZIF-8 will form. The different MOFs structures are shown in Fig. 7.6.

7.2.1 Preparation of MOFs membranes 7.2.1.1 In situ preparation Growth on unmodified support The synthesis of ZIF-8 through in situ growth involves only one step; immersing the support in the growth solution without any preattached crystal on the surface of the

ZTF-1 PCN-12 bdc

MOF-200

MIL-101(Cr)

IRMOF-1 (MOF-5)

tpdc

Zn4O(CH3COO)6 IRMOF-16

MOF

HKUST-1 MOF-74 (Mg, Ni, Co)

MOF-5

(A)

Bio-MOF-11

(B)

MOF-177

MOF-180

MOF-200

Fig. 7.6 MOF structures with different metal and organic linkers. (A) represents the important MOFs reported for high gas storage properties, (B) ditopic carboxylate linkers with different lengths to produce a variety of MOFs materials with the same network topology. Adapted with permission from Y. Zhao, Z. Song, X. Li, Q. Sun, N. Cheng, S. Lawes, Metal organic frameworks for energy storage and conversion, Energy Storage Mater. 2 (2016) 35–62. doi:10.1016/j.ensm.2015.11.005.

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support. All the nucleation, growth, and intergrowth of crystals on the substrate occur during the same fabrication step [49]. However, Shah et al. [49] noted that the absence of strong interfacial bonding is a common challenge in the fabrication of MOF on the native substrate. This might be affected by the surface chemistry of the support, for example, the zeta potential and surface acidity [50]. Xu et al. [51] reported on the in situ growth of ZIF-8 on alumina hollow fiber membrane outer layer from a concentrated synthesis gel with a molar composition of 1 Zn2+: 8 Hmim: 75 methanol. The alumina hollow fiber membrane was heated at 80°C overnight before being placed in Teflonlined autoclave. The ZIF-8 synthesis solution was poured into the autoclave, and heating at a given temperature (25°C, 100°C, and 150°C) for 5 h. The ZIF layer is well grown on ceramic substrate, and its thickness is about 6 μm with some ZIF-8 crystals is likely to present in the pores of alumina support. The FIB-SEM image of the cross-section of ZIF-8 layer reveals that ZIF-8 layer also possesses many irregular-shaped microcavities.

Fig. 7.7 SEM images of ZIF-8 membranes supported on the ceramic hollow fiber prepared from the high-concentration synthesis gel at 100°C for 5 h: (A) surface, (B) cross-section created by FIB, (C) crosssection, and (D) elemental mapping image of the cross-section. Adapted with permission from G. Xu, J. Yao, K. Wang, L. He, P.A. Webley, C. sheng Chen, H. Wang, Preparation of ZIF-8 membranes supported on ceramic hollow fibers from a concentrated synthesis gel, J. Membr. Sci. 385–386 (2011) 187–193. doi:10.1016/j.memsci.2011.09.040.

Development of advanced nanocomposite membranes

Fig. 7.7 shows the SEM image of the prepared membrane. Apart from that, in situ growth of ZIF-8 using interfacial synthesis approach was reported by Li et al. [52]. In this approach, ZIF-8 membranes were synthesized on porous polyethersulfone supports based on a liquid-liquid interfacial coordination mechanism by firstly, saturating the substrate with the aqueous zinc nitrate, followed by pouring an octanol-based 2-methylimidazole solution on the surface. After a certain reaction time, a selective ZIF-8 layer was formed on the porous polymeric support. According to the cross-section images, the ZIF-8 layer only has a thickness of around 250–300 nm.

Growth on modified support As mentioned in the previous section, the ZIF-8 membrane on unmodified supports is prone to several problems, such as cracking. Therefore, a further modification of the support surface is sometimes crucial to develop a crack-free ZIF-8 membrane. McCarthy et al. [53] reported the synthesis of ZIF-8 membranes by modifying the support with ligand 2-methylimidazole solution through a thermal deposition method. The ligands bonded to the surface of the support through an activation process, forming a strong aluminum-nitrogen covalent bond at 200°C. The researchers also mentioned that the presence of sodium formate is also important to obtain continuous and thoroughly intergrown ZIF-8 membranes because the high pH value sparked the deprotonation of surface linkers, enhancing the quality of the membrane formed. For the poorly intergrown membrane, a secondary growth method was applied using the membrane as seeded support and regrown into a well-intergrown membrane in the presence of sodium formate. The researchers also found zinc chloride to be a better zinc source compared to zinc nitrate in forming a well-intergrown ZIF-8 membrane. However, no film was obtained after a synthesis time of 36 h using zinc chloride without the presence of sodium formate. In 2013, Zhang et al. reported on a scalable and straightforward method for preparing low-defect ZIF-8 tubular membranes, by modifying the substrate surface with an ultrathin ZnO layer, followed by surface activation with Hmim solution [54]. The ZnO and Hmim reacted, creating uniform nucleation sites on the surface of the ZnO layer. This is thought to be responsible for the good performance of the membrane in terms of its permeability and permselectivity. Moreover, the resulting excellent adhesion between the ZIF-8 and ZnO also prevented delamination. Wang et al. [55] reported on the in situ growth of ZIF-8 on porous alumina and zinc oxide hollow fiber membrane, with only 5–6 μm thickness with a crack-free ZIF membrane, which exhibits excellent hydrogen separation performance. The surface functionalization with Hmim is one of the vital aspects for successful ZIF growth. It is believed that the Hmim could react with ZnO to form a series of Hmim coordinated Zn2+ compounds, which are important precursors to promote the embryonic ZIF-8 nuclei.

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7.2.1.2 Secondary (seeded) growth In comparison with the in situ growth method, the secondary or seeded growth method involved the seeding of nuclei onto the support where the membrane would subsequently grow [56]. Algieri et al. also stated that their method is favorable due to the thinner and less defective membrane produced because each step involved can be independently manipulated. However, this method is relatively complex compared to the in situ growth method explained earlier. The seeding method can be done through various techniques, for instance, dip-coating [57,58], rubbing [59], reactive seeding [60], and immersion [61]. In 2012, Pan et al. synthesized the ZIF-8 membrane by dip-coating using a seeded hydrothermal growth on yttria-stabilized zirconia (YSZ) fiber support [58]. In the method, YSZ support was dipped in the seed suspension for 10 s, producing a 2.5 μm membrane. However, the overall coverage of the ZIF-8 particles was relatively low due to the blockage of the seeds inside the pores of the YSZ. Venna and Carreon [62] also reported on the synthesis of ZIF-8 membrane using secondary seeded growth bit with rubbing method and in situ crystallization. The seeding process was prepared using hydrothermal method, and the support was calcinated under 900°C for 30 min prior the rubbing process with dry ZIF-8 seed. The membrane was 5–9 μm thick. In 2016, Lai and his coworkers reported a preliminary study on the preparation of ZIF-8 membrane on α-alumina via in situ growth, as well as secondary growth using the rubbing and dip-coating methods [63]. From SEM images in the preliminary study, the researchers found uncovered porous support areas and the ZIF-8 grains formed on the support are not uniform. For the ZIF-8 membrane synthesized through the rubbing method, the crystal size was in the range of 1–4.7 μm, whereas the dip-coating method produced crystals with a size of 2.5–5 μm. These results imply that secondary growth can result in a poorly intergrown ZIF-8 layer if the seed layer is not formed properly. On the other hand, the in situ growth of ZIF-8 on the support membrane showed better formation. The continuous, small, and loosely packed ZIF-8 grains observed at 4 h synthesis time, and better coverage of ZIF-8 on alumina can be seen through SEM image. However, defects were observed when the synthesis time increased to 36 h. Nevertheless, in terms of size, the ZIF-8 grains obtained at 36 h are relatively uniform, ranging from 1.5 to 2.2 μm. Some researchers reported the use of sodium formate to further improve the ZIF-8 deposition on the support. For example, McCarthy et al. reported the effect of sodium formate in the heterogeneous nucleation and growth of ZIF crystals [53]. Shah et al. [64] also reported specifically on the role of sodium formate for producing well-intergrown continuous ZIF-8 membranes on unmodified support. It was found that the existence of sodium formate can enhance the heterogeneous nucleation of ZIF-8 crystals on alumina supports, besides being able to promote intergrowth of ZIF-8 crystals. It was confirmed that sodium formate reacts with zinc source to form zinc oxide layers on α-alumina supports, which in turn promote heterogeneous nucleation.

Development of advanced nanocomposite membranes

7.2.1.3 Microwave-assisted growth of ZIF membranes The remarkable advantages of microwave technology for chemical synthesis have provoked growing interest among researchers. Microwave technology may help to reduce synthesis time. It is claimed as green technology because it does not produce any hazardous material like gas fumes. Nor is it heated using an external energy source [65]. Kwon and coworkers [66] reported a rapid and simple microwave-assisted seeding technique for the synthesis of high-quality ZIF-8 membrane. The microwave seeding process involves three steps: first, saturating the porous support with a metal solution, then exposing the saturated support to ligand precursor, and lastly by rapid crystal formation under microwave irradiation. It is crucial to maintain a high concentration of both precursors on the support prior to microwave irradiation because it will maximize the heterogeneous crystal formation and at the same time, minimize the undesirable homogeneous crystal formation. Additionally, the rapid absorption of microwave energy by metal ions inside the supports further increases the local temperature inside the support, resulting in the rapid formation of ZIF-8 inside and on the surface of the supports. Recently, a study on the preparation of mixed metal, CoZnZIF-8, with various Co/Zn contents and mixed ligand ZIF-7-8 using microwave irradiation method was reported [67]. This study is the first example of a one-step synthesis of mixed metal, and ligand CoZn-ZIF-7-8 successfully obtained using microwave irradiation. Other than the short synthesis time, crystallization using microwave irradiation produced higher yield, reduced the amount of ligand and solvent, and eliminated the use of deprotonators when compared to conventional methods reported earlier.

7.2.1.4 Blending method Another approach to fabricate MOF membranes is a blending method to produce MOFbased MMMs. The blending method is more reproducible and requires no expensive supports. The membrane produced is more stable rather than bare MOF, especially long term, compared to the membrane prepared by the previously explained method. Two methods, substrate-based blending, and substrate-free blending, were developed to design MOF-based MMMs for liquid separation [68]. Substrate-based blending was widely applied to design thin MOF-based MMMs in liquid separation, especially for pervaporation. Some porous substrates, such as polymeric membranes and ceramic tubes, were used to support the obtained MMMs and improve their stability and strength. The conventional procedure of this method involves three steps: (1) mixing the MOFs and polymer into the solvent (MOF ink); (2) casting the mixed solution on the porous support by spincoating/dip-coating/flat membrane casting process; and (3) a curing or drying process to remove the casting solvent. With this procedure, thin MOF-based MMMs with a thickness of 0.3–20 mm are formed on the surface of a porous support.

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An organophilic pervaporation membrane was fabricated on the inside surface of alumina capillary substrates by the solution-blending dip coating process, using ZIF-8 nanoparticles as filler and polymethylphenylsiloxane (PMPS) as polymer matrix [69]. After the heat-treatment process, the ZIF-8 nanoparticles were embedded in the PMPS phase homogeneously, with no interfacial voids. The top-layer thickness is about 2.5 mm, which may offer the possibility of achieving very high permeance. Substrate-free blending presents more flexible applications in designing MOF-based MMMs. Nonporous supports were utilized to ensure the complete delamination from the substrate to create free-standing MMMs [68]. For better mechanical strength and permeance, the thickness of the membrane is generally controlled between 18 and 30 mm for pervaporation application, and larger than 150 mm for water treatment purpose. The substrate-free blending method was utilized to prepare a MIL-53(Al) nanocomposite using poly(m-phenylene isophthalamide) (PMIA, polymer) and MIL-53(Al) particles in a mixture of N,N-dimethylacetamide, and LiCl [70]. The membranes were placed in a vacuum oven before being immersed in distilled water to induce phase inversion process. However, the agglomeration of nanoparticles occurred. To overcome this, the postsynthetic approach was applied for more excellent dispersion of nanoparticles. A new, chemically crosslinked membrane with the application of postsynthetic polymerization of the UiO-66-NH2 nanocrystals and polyurethane oligomer was developed [71]. The preparation of membrane involved the mixing of UiO-66-NH2 particles with isocyanate-terminated polyurethane oligomer in anhydrous chloroform and was fully dispersed by ultrasound. The reacted solution was poured into a PTFE dish and allowed to dry at room temperature prior to being heated in an air-circulating oven. The obtained, well-dispersed MMMs showed the ability to isolate hydrophilic dyes from water by means of different membrane affinities. 7.2.1.5 Other design strategies Apart from the aforementioned techniques, there are some other techniques reported for the preparation of MOF membranes. One of the techniques is the counterdiffusion method as reported by Kwon et al. [72]. This simple, yet highly versatile, method enabled the rapid preparation of well-intergrown ZIF-8 membranes on an alumina support with excellent microstructure. The synthesis method is basically the counterdiffusion concept in which a metal precursor solution is soaked in porous α-alumina supports followed by rapid solvothermal reaction in a ligand solution. This novel method showed a significant effect on healing defects of membranes as well as to reduce ligands and solvents consumption. Fig. 7.8 shows the schematic illustration for the reaction. As a measure to reduce the defect problems of ZIF-8 on the support surface, Jang et al. [73] reported on the new approach to control the diffusion rates of the two ZIF-8 precursors via the counterdiffusion method. The γ-Al2O3 layer, with a pore size of 5 nm, was layered on top of the α-Al2O3 disc (referred to as a γ-/α-Al2O3 disc) as a tuner to

Development of advanced nanocomposite membranes

Reaction zone 2+

Zn m-Im

Fig. 7.8 Schematic illustration of the membrane synthesis using the counterdiffusion-based in situ method: (1) a porous alumina support saturated with a metal precursor solution is placed in a ligand solution containing sodium formate, (2) the diffusion of metal ions and ligand molecules causes the formation of “reaction zone” at the interface (3) rapid heterogeneous nucleation/crystal growth in the vicinity at the interface leads to the continuous well-intergrown ZIF-8 membranes. Adapted with permission from H.T. Kwon, H.-K. Jeong, In situ synthesis of thin zeolitic–imidazolate framework ZIF-8 membranes exhibiting exceptionally high propylene/propane separation, J. Am. Chem. Soc. 135 (2013) 10763–10768. doi:10.1021/ja403849c.

decrease the diffusion rates. At the same time, γ-Al2O3 also serves as a protective layer to prevent thermal structural damage to the ZIF-8 membranes. It was found that the ZIF-8 grains in membrane ZIF-8_γα were predominantly formed inside the α-Al2O3 disc while filling 70% of the mesopores in the γ-Al2O3 layer. On the other hand, Li et al. [74] reported the synthesis of the ZIF-8 membrane using a solvothermal method on a porous alumina support. The support was initially immersed in 2-methylimidazole melting solution and then heated in a zinc nitrate aqueous solution through an infiltration method. A well-intergrown and continuous membrane was produced after 8 h at 120°C. The thickness of the membrane was about 12 μm, and the highest zinc concentration was found at the surface. Further, a novel pressure-assisted self-assembly (PASA) filtration technique was applied to prepare a MOF@GO membrane on a modified PAN support for pervaporation as shown in Fig. 7.9 [75]. The pretreated PAN support and the mixture of MOF and GO dispersion aqueous solution were installed in the membrane-preparation device. The PASA filtration process was operated at a constant pressure difference, DP ¼ 2.0 bar, which induced the formation of a MOF@GO membrane. These membranes exhibited competitive water permeation for ethyl acetate/water mixtures by pervaporation. Moreover, the procedures for both the synthesis of MOF and membrane preparation are environment friendly, as only water was used as a solvent. A nanosized MOF-intercalating approach like this may be extended to other laminated

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Fig. 7.9 Pressure-assisted self-assembly (PASA) filtration technique to prepare a MOF@GO membrane on a modified PAN. Adapted with permission from Y. Ying, D. Liu, W. Zhang, H. Huang, Q. Yang, C. Zhong, High-flux graphene oxide membranes intercalated by MOF with highly selective separation of aqueous organic solution, Appl. Mater. Interfaces (2016). doi:10.1021/acsami.6b14371.

membranes, providing valuable insights into designing and developing advanced membranes for effective separation of aqueous organic solutions through nanostructure manipulation of the nanomaterials. More recently, inspired by the solvothermal synthesis of MOFs, the rapid thermal deposition (RTD) method to design a well-interconnected ZIF-8 membrane on a modified macroporous stainless steel substrate was reported [76]. Hmim solution was added to the zinc acetate solution in a dropwise manner to prepare the precursor solution. The precursor solution was then spread on the substrate in a crucible to initiate the crystallization of ZIF-8 across the substrate. RTD has been demonstrated as a much more efficient route, yielding a high-quality MOF membrane within 1 h, as opposed to the 24 h required for solvothermal membranes.

7.2.2 MOFs membrane application 7.2.2.1 Liquid separation Pervaporation MOF membranes for pervaporation is a natural extension of the current industry standards, considering the chemical versatility and subsequent separation tunability that is more freely available in comparison with polymeric or zeolites membranes [48]. For example, in the separation of water from ethanol, a hydrophilic MOF, known as HKUST-1 (Cu3(btc)2), was used in a 40 wt% mixed-matrix membrane (MMM) to remove water from the system [77]. Adding HKUST-1 improved flux without sacrificing selectivity due to the selective permeability of water in the membrane. Fig. 7.10 shows some typical MOFs used in pervaporation MMMs. The applications of MOF-based membranes for pervaporation can be divided into four main categories: dehydration of organic solvents; removal of dilute organic compounds from aqueous streams; separation of organic-organic mixtures; and reversible reactions [68]. Due to the ease of design and modification of MOFs, along with the compatibility between MOFs and polymer matrix, MOF-based MMMs are superior in pervaporation compared to MMMs containing purely inorganic zeolites [37]. An integrated ZIF-71 membrane was prepared for the first time and used for pervaporation separation

Development of advanced nanocomposite membranes

Fig. 7.10 Structure of some typical MOFs used in pervaporation MMMs. Adapted with permission from Z. Jia, G. Wu, Metal-organic frameworks based mixed matrix membranes for pervaporation, Microporous Mesoporous Mater. 235 (2016) 151–159. doi:10.1016/j.micromeso.2016.08.008.

of alcohol (methanol and ethanol)-water and dimethyl carbonate (DMC)-methanol mix˚ ) is close to the window size of ZIF-71 tures [78]. The kinetic diameter of ethanol (4.53 A ˚ (nominal 4.8 A), which led to a relatively slower diffusion rate for ethanol molecules. In ˚ ) in the ZIF-71 contrast, the diffusion of methanol (with a kinetic diameter of 3.63 A channel is relatively fast, resulting in better separation performance. The ZIF-71 membrane shows good pervaporation performance, especially in DMC-methanol separation. The study also indicates that ZIF-71 pervaporation membranes demonstrate good performance not only for organics-water separation but for organics-organics systems as well. Apart from that, a continuously grown Ni2(L-asp)2bipy membrane was prepared on porous SiO2 discs by a seeding-secondary growth method, as shown in Fig. 7.11 [79]. The hydrophilic surface of the membrane was switched to hydrophobic by vapor deposition of PDMS, while the porosity was maintained even after the PDMS deposition. Pervaporation water/ethanol separation process was evaluated for both the PDMSdeposited membrane and the nondeposited PDMS membrane. Ni2(L-asp)2bipy membrane possesses a remarkably high flux of 27.6 kg m
2 h
1 and a separation factor of 73.6 for water/ethanol mixture with 50 wt% ethanol at the temperature of 30°C. The developed Ni2(L-asp)2bipy@PDMS membrane exhibits a lower separation factor due

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Wet-rubbing

Porous SiO2 disc

Seed layer Secondary growth

PDMS coating

Ni2(L-asp)2bipy@PDMS membrane

Ni2(L-asp)2bipy membrane

Fig. 7.11 Schematic illustration of the preparation process of Ni2(L-asp)2bipy and Ni2(L-asp)2bipy@PDMS membranes and water/ethanol separation on them. Adapted with permission from S. Wang, Z. Kang, B. Xu, L. Fan, G. Li, L. Wen, X. Xin, Z. Xiao, J. Pang, X. Du, D. Sun, Wettability switchable metal-organic framework membranes for pervaporation of water/ethanol mixtures, Inorg. Chem. Commun. 82 (2017) 64–67. doi:10.1016/j.inoche.2017.05.016.

to its hydrophobic surface, which may be applied to the membrane distillation for water/ethanol separation. The pervaporation studies using these two membranes provide insight into the effect of surface wettability on the bioethanol purification performance [79]. A novel MOF nanocomposite membrane, with high loading rate and excellent stability, has been developed to increase the economic and process efficiency of current biorefining processes, particularly for furfural (2-furancarboxaldehyde) refining process [80]. The membrane consists of a hierarchically ordered stainless-steel-mesh (HOSSM) that acts as a skeleton, while the ZIF-8 nanoparticle incorporated in the silicone rubber (PMPS) matrix create preferential pathways for furfural molecules by their ultrahigh adsorption selectivity. The HOSSM-ZIF-8-PMPS membrane shows very promising pervaporation and vapor permeation performance and excellent stability for the recovery and removal of furfural from dilute aqueous solution and biomass fermentation broth. A well-intergrown UiO-66 MOFs membrane fabricated via controlled solvothermal synthesis on prestructures yttria-stabilized cations was applied in organic dehydration using pervaporation [81]. The membranes provide a very high flux of up to ca. 6.0 kg m
2 h
1 and an excellent separation factor for separating water from i-butanol (next-generation biofuel), furfural (promising biochemical), and tetrahydrofuran (typical organic matter). It is comparable to the performance of commercial zeolite NaA membranes and also showed better stability compared to some commercial membranes, such as zeolite NaA membranes. Recently, MIL-60 membranes were prepared on

Development of advanced nanocomposite membranes

polydopamine (PDA)-modified α- Al2O3 disks to separate xylene isomers via pervaporation [82]. A well intergrown MIL-160 crystal with a thickness of about 25 μm was formed on the alumina support with no cracks or pinholes, suggesting a dense MIL160 membrane had formed on the PDA-modified α-Al2O3 disk. The separation performance of the MIL-160 membrane was tested by the pervaporation of single components of PX, OX, and MX, as well as the separation of equimolar binary PX/OX mixture using liquid feeds. Membranes prepared on the nonmodified support show almost no selectivity (1.17 for PX/OX, and 1.13 for PX/MX). For the MIL-160 membrane prepared on the PDA-modified α-Al2O3 disk, the PX flux (486 g
2 h
1) is much higher than the flux of OX (10 g
2 h
1) and MX (12 g
2 h
1), with ideal separation factors of PX/OX and PX/MX being 48.6 and 40.5, respectively, indicating that the MIL-160 membranes prepared on the PDA-modified Al2O3 disk display a high separation performance. The molecular sieving performance of the MIL-160 membrane was confirmed by the separation of equimolar PX/OX mixtures at 25–100°C by pervaporation. Comparing the PX flux in the mixture (467 g
2 h
1) with the flux of single component PX flux (486 g
2 h
1) at 75°C, only a slight reduction of the PX flux in the presence of OX is found, suggesting that the larger OX only slightly hinders the entrance of PX into the MIL-160 pore structure. For the 1:1 binary PX/OX mixture at 75°C, the mixture separation factor of PX/OX is 38.5, which is comparable to the PX/OX separation factor (40) of the zeolite MFI membrane by pervaporation. Organic solvent nanofiltration The good attributes of MOFs have attracted researchers to reveal their potential in other applications, including in OSN. The incorporation of [Cu3(BTC)2], MIL-47, MIL-53 (Al), and ZIF-8 as fillers in PDMS membranes had been studied to separate Rose Bengal from isopropanol [83]. However, because of the poor adhesion between the MOFs and PDMS, the fillers were not successfully introduced into the polymer. The adhesion was improved by modifying the MOF surface with trimethylsilyl groups. The MMM showed increased permeance but lower retention compared to the pristine membrane due to the effect of reduced polymer swelling and size exclusion of the filler. In 2013, Sorribas et al. prepared thin-film composite membranes containing MOF nanoparticles (ZIF-8, MIL-53(Al), NH2-MIL-53(Al), and MIL-101(Cr)) with the particle size of 50–150 nm in the PA layer via in situ polymerization on top of crosslinked polyimide porous support [84]. These membranes showed a dramatic increase in permeance when tested in MeOH/PS and THF/PS nanofiltration experiments compared to the same membranes with no MOFs, without sacrificing rejection. It was observed that there are direct trends between MeOH/PS permeance and the porosity of the MOFs added to the thin-film layer. The porosity of the MOFs provided preferential flow paths for the solvents, whereas the high rejection observed in this study was due to the excellent compatibility between PA and the organic moieties of the MOF particles. Mesoporous

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MIL-101(Cr) showed the best performance with flux increases from 1.5 to 3.9 for MeOH/PS m
1 h
1 bar
1 and 1.7 to 11.1 L m
1 h
1 bar
1 for THF/PS. The results proved that loading PA thin-film’s separating layer with small amounts of MOFs significantly improved the flux without sacrificing rejection. Nanoparticles of MOFs MIL-101(Cr), MIL-68(Al), and ZIF-11 with sizes of 70, 103, and 79 nm were also incorporated into an ultrathin PA layer from TFN membranes on top of polyimide P84® asymmetric support for OSN application [47]. Other important aspects that were also studied include the effect of a nonsolvent bath, chemical posttreatment, concentration of precursors of IP and the polymerization time, and the influence of different solvents (water, methanol, acetone, and THF) and solutes (Acridine Orange, Sunset Yellow, and Rose Bengal) on OSN. The results obtained showed that the hydrophilic character of the membranes had the most significant effect on the composite membrane’s performance. Maximum permeance of 6.2 L m
2 h
1 bar
1 and a rejection above 90% was obtained from the combination of ZIF-11 and posttreatment via filtration with dimethylformamide because better MOF-polymer interaction was achieved, probably due to the hydrophobic character of ZIF-11. In order to deal with selectivity issue, which is one of the main challenges problems in OSN, Campbell et al. [85] constructed membranes with uniform porous structure of HKUST-1 within the pores of polyimide membranes by using in situ growth method. Chemical modification was performed to improve the in situ growth membranes by introducing aryl carboxylic acid moieties (1,2,4-benzenetricarboxylic anhydride) to polyimide P86 ultrafiltration membranes, which then enabled coordination of HKUST-1 directly on the polymer. The results showed that chemically modified and HKUST-1 growth membrane exhibited better performance with a molecular weight cut-off of 794 g mol
1 and also high permeance [85]. Guo et al., prepared a defect-free TFN membrane by modifying the PA surface with long alkyl chains and in situ growth of zirconium-MOF (UiO-66-NH2) [86]. The modification of PA surface using dodecyl aldehyde improved the dispersibility of nanosized UiO-66-NH2 particles (20 nm) in hexane and enhanced the compatibility with the polymer phase, leading to nonselective defects in ultrathin MOF@PA TFN membrane. Methanol permeance was enhanced significantly after nanoparticle incorporation without compromising the tetracycline rejection. The novel TFN membrane prepared with organic phase solution containing 0.15% (w/v) modified UiO-66-NH2 showed remarkable methanol permeance of 20 L m
2 h
1 bar
1, with tetracycline rejection of 99%, suggesting promise for application in the pharmaceutical industry. Li et al. proposed another approach to incorporate MOF into polymeric membranes through an IP method inspired by traditional RO membrane fabrication [52]. Through the applied method, PES was first saturated with the aqueous zinc nitrate followed by 2-methylimidazole dissolved in octanol and left for certain reaction times for the ZIF-8 to form. The results indicated that higher concentrations and higher mole ratios

Development of advanced nanocomposite membranes

of 2-methylimidazole to zinc nitrate led to denser ZIF-8 membranes with fewer defects. However, due to the dense packing and smaller interparticle spaces, the changes also affected the permeance and rejection of the membrane. It is very clear that the higher 2-methylimidazole and zinc nitrate molar ratios resulted in much denser films with lower permeance. By changing the molar ratio of 2-methylimidazole to zinc nitrate from 1.9 to 15.9, ZIF-8/the permeance PES membranes varied from 5.6 to 37.5 L m
2 h
1 bar
1, with Rose Bengal rejections higher than 98%. A study involving the incorporation of ZIF-8 onto the surface of Graphene Oxide to form ZIF-8@GO composite determined the potential of the materials for OSN [87]. ZIF-8@GO nanoparticles were codeposited with polyethyleneimine (PEI) matrix on a tubular ceramic substrate through a vacuum-assisted assembly method. The method showed improved dispersion of ZIF-8 nanoparticles in a PEI matrix, as well as its compactness due to the tinplating effect of lamellar GO sheets and the transmembrane pressure. The OSN performance was evaluated on the basis of methanol permeance and retention of dye molecules. Methanol permeance increased when ZIF-8@GO laminates were embedded into the PEI layer. The retention remained higher than 99%. The excellent performance of the membrane may be due to the more well-defined pathways for solvent molecules provided by the good dispersion of ZIF- 8 nanoparticles in the PEI matrix. Water treatment A growing number of industrial applications use membrane-based separations. The emergence of MOFs has given new insight and might greatly improve the performance and the range of membrane separations that are possible [68]. There have been numerous studies on the preparation and promising performance of MOFs acting as continuous membranes or fillers in MMMs for nanofiltration, forward osmosis, ultrafiltration, and reverse osmosis application. The introduction of MOFs into membrane-based separations gives multifunctional effects for water treatment processes, for instance, catalytic degradation, heavy metal removal, and antibacterial properties. Since the size of an MOF’s pore window can range from 0.3 to 10 nm, depending on the composition, MOF membranes usually served as nanofiltration membranes. They compete with the RO membrane in term of performance. The use of MOF-based membranes in nanofiltration is mainly focused on dye removal. A novel LBL PA/ZIF-8 nanocomposite membrane was fabricated to overcome the aggregation of fillers, which lead to a significant decrease in membrane performance [88]. The multilayer membrane consists of a porous substrate, a ZIF-8 interlayer, and a PA coating layer, as shown in Fig. 7.11. An interlayer of ZIF-8 was grown on an ultrafiltration membrane through in situ growth, and the layer was coated with an ultrathin PA layer through interfacial polymerization (IP). The LBL PA/ZIF-8 showed better performance than the conventional PA/ZIF-8 TFN membrane because the in situ growth

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Nanocomposite membranes for water and gas separation

Interfacial polymerization

TFN-mZIF membrane

HPAN substrate

Nanoporous, hydrophilic PSS-ZIF-8

Enhanced water permeability High dye retention, goog Na2SO4/NaCl selectivity

HPAN support

Dye

Cl−1

H2O

SO42−

Fig. 7.12 Preparation of PSS-ZIF-8 membrane through IP and water permeability and selectivity of molecules. Adapted with permission from Z. Junyong, Q. Lijuan, U. Andrew, J. Hou, W. Jing, Zh. Yatao, L. Xin, Y. Shushan, L. Jian, T. Miaomiao, L. Jiuyang, B. Van Der Bruggen, Elevated performance of thin film nanocomposite membranes enabled by modified hydrophilic MOFs for nanofiltration, ACS Appl. Mater. Interfaces (2016). doi:10.1021/acsami.6b14412.

of ZIF-8 produces an interlayer with more particles and fewer aggregates. LBL PA/ZIF-8 performed better permeance and selectivity for dye removal with flux was up to 27.1 kg m
2 h
1, and the rejection reaches 99.8% compared to the pure PA membrane (flux of 11.2 kg m
2 h
1 and rejection of 99.6%). Apart from direct dyes, MOF-based membranes can also remove reactive dyes such as Reactive Blue 2 (RB2) and Reactive Black 5 (RB5). TFC and TFN ZIF-8 membranes display high retention for both dyes, which meets the prime separation requirement of NF membranes [89]. The preparation of membrane and molecular sieving through the membrane is shown in Fig. 7.12. Although lower dye retentions were measured than those for TFC (RB2: 99.42%, RB5: 99.92% compared to TFN-mZIF2 RB2: 99.12, RB5: 99.03), the water flux of TFN-mZIF2 was enhanced by 199.3% at 4 bar, which is promising for dye removal. A membrane derived from ZIF-8/PSS, based on a tubular alumina substrate through LBL self-assembly technique, was fabricated for nanofiltrating dyes from water [55]. The ZIF-8 particles were grown in situ into PSS layers to enhance compatibility and dispersion. In the study, there were outstanding nanofiltration properties toward methyl blue with the flux of 210 L m
2 h
1MPa
1 and the rejection of 98.6%. Additionally, the

Development of advanced nanocomposite membranes

membrane showed good pressure resistance because of the ceramic substance and inherent stability of ZIF-8. On the other hand, two different types of TFC membranes with ZIF-8 were prepared, one in PA with PSF support, fabricated through IP, the other one was in situ growth on PSF support with PA as top layer prepared through the LBL technique [90]. The membranes were tested for the common analgesic acetaminophen (or paracetamol MW 151 g mol
1). Due to the membrane’s defect-free characteristics, LBL membrane produced better performance with 55% acetaminophen retention and permeance equivalent to conventional PSF/PA membrane when compared to the membrane prepared through IP. A new metal-organic framework/graphene oxide composite (IRMOF-3/GO) with high adsorption capacity of copper (II) was prepared with maximal adsorption of 254.14 mg g
1 at pH 5.0 at room temperature [91]. Novel and highly efficient nanofiltration (NF) membranes can be readily fabricated via surface decoration of IRMOF-3/GO onto polydopamine (PDA)-coated polysulfone (PSF) substrate. The adsorption effect of IRMOF-3/GO and the enhancement of membrane caused the prepared NF membrane efficiently reject copper (II). The copper (II) rejection reached up to about 90% while maintaining a flux of about 31 L m
2 h
1 at the pressure of 0.7 MPa and pH 5.0. Furthermore, the membrane also maintains outstanding stability throughout a 2000-min NF testing period. Thus, the newly developed NF membrane shows promise for water-cleaning applications. Duan et al. prepared an advanced RO membrane by integrating MOFs within the PA layer to produce a MOF-based TFN structure for the desalination process [92]. The TFN membranes were characterized with RO tests fewer than 15.5 bar hydraulic pressure with 2000 ppm NaCl solution. ZIF-8 increased the water permeance up to 3.35  0.08 L m
2 h
1 bar
1 at 0.4% (w/v) loading, 162% higher than the pristine PA membrane. Further, a high NaCl rejection was maintained. This study experimentally verified the potential use of ZIF-8 in advanced TFN RO membranes. Furthermore, due to their unique structural components, which contain metal cores and organic ligands, MOFs have also exhibited remarkable antimicrobial properties. The antimicrobial property of the MOF-based membrane was also achieved by functionalizing a TFN membrane with ZIF/GO hybrid nanosheets [55]. The resultant hybrid nanosheets not only integrated the merits of both ZIF-8 and GO but also yielded a uniform dispersion of ZIF-8 onto GO nanosheets simultaneously, thus effectively eliminating the agglomeration of ZIF-8 in the active layer of membranes. ZIF-8/GO thin-film nanocomposites (TFN-ZG) membrane with typical water permeability (40.63 L m
2 h
1 MPa
1) allowed for efficient bivalent salts removal with the rejections of Na2SO4 and MgSO4 were 100% and 77%, respectively. Interestingly, the synthesized ZIF-8/GO nanocomposites had optimal antimicrobial activity (MIC: 128 μg mL
1), compared to ZIF-8 and GO separately, which sufficiently contributed to the excellent microbial activity

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Nanocomposite membranes for water and gas separation

Go was covered by

polyamide

TFN-GO membrane

(A)

Synergistic antimicrobial effect of ZIF-8 and GO Release of Zn2+ by ZIF-8

Dead cell ROS

Live cell

Plasma membrane disorganization

Cellular internalization

Oxidative stress by GO Depletion of cellular antioxidants

ZIF-8/GO hybrid

(B)

ROS

nanosheets

TFN-ZG membrane

Fig. 7.13 (A) Schematic diagrams for the antimicrobial mechanism of TFN-GO membranes and (B) TFN-ZG membranes. Adapted with permission from J. Wang, Y. Wang, Y. Zhang, A. Uliana, J. Zhu, J. Liu, B. Van Der Bruggen, Zeolitic Imidazolate framework/graphene oxide hybrid nanosheets functionalized thin film nanocomposite membrane for enhanced antimicrobial performance, ACS Appl. Mater. Interfaces 8 (2016) 25508–25519. doi:10.1021/acsami.6b06992.

of TFN-ZG (84.3%). The antimicrobial mechanisms of ZIF-8/GO hybrid nanosheets and TFN-ZG membranes were proposed, as shown in Fig. 7.13. ZIF-8/GO functionalized membrane with high antimicrobial activity, and salt retention makes it promising for water desalination, and it was suggested that ZIF-8-based crystal may offer a new pathway for the synthesis of multifunctional bactericidal. The potential antimicrobial mechanism of the ZIF-8/GO membrane is attributed to a synergistic effect involving graphene oxide and the gradual releasing of Zn2+ form ZIF-8. This research suggests a promising future for ZIF-8-based nanocomposites as multifunctional antimicrobial agents for versatile applications.

Development of advanced nanocomposite membranes

7.2.2.2 Gas separation Porous materials separate gases based on differences in diffusion coefficients or by a molecular sieving effect between the gases and porous material [48]. Many MOF membranes and MOF-polymer composite membranes have been explored for their potential application in gas separation. Among the many gases studied for separation by MOFs, the separation of CO2 from flue gas streams has become a major area of interest because of its relevance to climate change and the fact that many different MOFs have shown high selectivity for CO2 uptake [93]. For example, a high-quality isorecticular MOF-1 (IRMOF-1) membrane was prepared for the separation of CO2 from dry CO2-enriched CO2/CH4 and CO2/N2 mixture [94]. The membrane exhibits good separation for CO2/CH4 and CO2/N2 with separation factors of 328 and 410, respectively. The CO2 permeance reported for CO2/CH4 and CO2/CH4 is 2.55  10
7 and 2.06  10
7, respectively. The unique separation properties of IRMOF-1 membranes, such as “sharp molecular sieving” at high CO2 molar fraction and feed pressure and high CO2 permeance, could contribute to the production of high-purity CO2 from the lowgrade CO2 gas mixtures. Mixed matrix membranes (MMM) well-dispersed MOF fillers in polymer matrices were prepared via thermally induced oxidative crosslinking of the polymer matrix and the amorphized zeolitic imidazolate framework [95]. The resultant plasticization-resistant MMMs exhibited high selective separations for CO2/CH4 mixed gas feeds. The membrane could also serve as molecular sieves for other separation processes, such as pervaporation or OSN. Apart from CO2, another important gas is hydrogen (H2) because it could fulfill the world’s increasing energy requirements in an environment-friendly manner [96]. MOF-based membrane has shown potential for hydrogen separation [97]. The first MOF-based membrane reported consisted of an MOF-5 membrane fabricated using an in situ solvothermal method on top of alumina support [98]. The single gas permeation properties demonstrated that they follow Knudsen diffusion behavior. As the permeation rate of gases increases by a decrease in their molecular weight, H2, which has the lowest molecular weight, permeates faster than the heavier studied gases, like CH4, N2, CO2, and SF6. This phenomenon suggests that the pore-aperture size of MOF-5 is larger than all of the tested molecules in this case. Another MOF membrane, HKUST-1 MOF (also known as Cu3(btc)2 or MOF-199), was grown using the “twin copper source” solvothermal method [99]. ˚ , accessible HKUST-1 MOF has small cages structure with diameters of 13 and 10 A ˚ , respectively. High permeation flux of single through windows of ca. 11 and 9.3 A gas permeation test was expected due to the pore-aperture size of HKUST-1, which is greater than the kinetic diameter of the commonly studied gases. The permeation results also showed a permeation selectivity in favor of H2 with respect to other gases, such as H2/N2 ¼ 7, H2/CO2 ¼ 6.8, and H2/CH4 ¼ 5.9. On the other hand, another HKUST-1 membrane was prepared through LBL approach, firstly by seeding process

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followed by in situ solvothermal method. The single gas separation performances for this membrane were also evaluated, and the ideal selectivities of this membrane were 2.9, 3.7, and 5.1 for H2/CH4, H2/N2, and H2/CO2, respectively. These ideal selectivities indicate that the permeation behavior of this membrane follows the Knudsen diffusion. Another group of MOFs, which is known as ZIFs, also showed good performance for hydrogen separation. The unique properties of ZIFs, in terms of permanent porosity, pore-aperture size uniformity, and excellent chemical and thermal stability made them good candidates for their application as molecular sieve membranes [96]. Among all ZIFs, ˚ aperture size ZIF-8 is one of the most studied. It has an underlying topology with 3.4 A for the six-membered-ring as the sole entrance to the associated pores. Theoretically, this could separate H2 from the other larger associated gases. A ZIF-8 membrane for gas separation with selectivity for H2 was obtained via a novel microwave-assisted solvothermal process [100]. The membrane single-gas permeation results of this ZIF-8 membrane showed a higher H2/CH4 selectivity of 11.2, relative to MOF membranes reported earlier.

7.3 Conclusions The emergences of zeolites and MOFs have given new insights into the development of membrane technologies, particularly for liquid and gas separation. The promising characteristics of both materials in term of surface area, tunability, chemical, and structural design make them suitable candidates for membrane separation applications. Progressive research on the fabrication and performance of zeolites and MOFs membrane have led to a greater number of new methods to produce defect-free membranes for better performance. Despite the tremendous progress, some significant challenges remain with respect to the implementation of zeolites and MOF membranes for industrial application. The challenges include, but are not limited to, the demand for easy membrane development, inexpensive supports, and low-cost methods for large-scale fabrication. To meet the requirements of industrial demands, research efforts need to expand.

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Development of advanced nanocomposite membranes

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[82] W. Xiaocao, W. Wan, J. Jianwen, C. Jurgen, H. Aisheng, High-flux high-selectivity metal-organic framework MIL-160 membrane for xylene isomer separation by pervaporation, Angew. Chem. Int. Ed. Engl. (2018) https://doi.org/10.1002/anie.201807935. [83] S. Basu, M. Maes, A. Cano-Odena, L. Alaerts, D.E. De Vos, I.F.J. Vankelecom, Solvent resistant nanofiltration (SRNF) membranes based on metal-organic frameworks, J. Membr. Sci. 344 (2009) 190–198, https://doi.org/10.1016/j.memsci.2009.07.051. [84] S. Sorribas, P. Gorgojo, C. Tellez, J. Coronas, A.G. Livingston, High flux thin film nanocomposite membranes based on MOFs for organic solvent nanofiltration, J. Am. Chem. Soc. 135 (2013) 15201–15208, https://doi.org/10.1021/ja407665w. [85] J. Campbell, J.D.S. Burgal, G. Szekely, R.P. Davies, D.C. Braddock, A. Livingston, Hybrid polymer/ MOF membranes for organic solvent nanofiltration (OSN): chemical modification and the quest for perfection, J. Membr. Sci. 503 (2016) 166–176, https://doi.org/10.1016/j.memsci.2016.01.024. [86] X. Guo, D. Liu, T. Han, H. Huang, Q. Yang, C. Zhing, Preparation of thin film nanocomposite membranes with surface modified MOF for high flux organic solvent nanofiltration, AICHE J. 63 (2016) 1301–1312, https://doi.org/10.1002/aic. [87] H. Yang, N. Wang, L. Wang, H.X. Liu, Q.F. An, S. Ji, Vacuum-assisted assembly of ZIF-8@GO composite membranes on ceramic tube with enhanced organic solvent nanofiltration performance, J. Membr. Sci. 545 (2018) 158–166, https://doi.org/10.1016/j.memsci.2017.09.074. [88] L. Wang, M. Fang, J. Liu, J. He, J. Li, J. Lei, Layer-by-layer fabrication of high-performance polyamide/ZIF-8 nanocomposite membrane for nanofiltration applications, ACS Appl. Mater. Interfaces 7 (2015) 24082–24093, https://doi.org/10.1021/acsami.5b07128. [89] Z. Junyong, Q. Lijuan, U. Andrew, J. Hou, W. Jing, Z. Yatao, L. Xin, Y. Shushan, L. Jian, T. Miaomiao, L. Jiuyang, B. Van Der Bruggen, Elevated performance of thin film nanocomposite membranes enabled by modified hydrophilic MOFs for nanofiltration, ACS Appl. Mater. Interfaces (2016) https://doi.org/10.1021/acsami.6b14412. [90] S. Basu, M. Balakrishnan, Polyamide thin film composite membranes containing ZIF-8 for the separation of pharmaceutical compounds from aqueous streams, Sep. Purif. Technol. 179 (2017) 118–125, https://doi.org/10.1016/j.seppur.2017.01.061. [91] Z. Rao, K. Feng, B. Tang, P. Wu, Surface decoration of amino-functionalized metal-organic framework/graphene oxide composite onto polydopamine coated membrane substrate for highly efficient heavy metal removal, Appl. Mater. Interfaces (2016) 1–39, https://doi.org/10.1021/ acsami.6b15873. [92] J. Duan, Y. Pan, F. Pacheco, E. Litwiller, Z. Lai, I. Pinnau, High-performance polyamide thinfilm nanocomposite reverse osmosis membranes containing hydrophobic zeolitic imidazolate framework-8, J. Membr. Sci. 476 (2015) 303–310, https://doi.org/10.1016/j.memsci.2014.11.038. [93] K. Sumida, D.L. Rogow, J.A. Mason, T.M. Mcdonald, E.D. Bloch, Z.R. Herm, T. Bae, J.R. Long, Carbon dioxide capture in metal-organic frameworks, Chem. Rev. 112 (2012) 724–781, https://doi.org/10.1021/cr2003272. [94] R. Zabao, J.B. James, A. Kasik, Y.S. Lin, Metal-organic framework membrane process for high purity CO2 production, AICHE J. (2016) 1–6, https://doi.org/10.1002/aic. [95] K. Aylin, L.H. Wee, P. Martin, B. Sara, A.M. Johan, V. Ivo, F.J, highly selective gas separation membrane using in situ amorphised metal-organic frameworks, Energy Environ. Sci. 10 (2017) 2342, https://doi.org/10.1039/c7ee01872j. [96] O. Shekhah, V. Chernikova, Y. Belmabkhout, M. Eddaoudi, Metal-organic framework membranes: from fabrication to gas separation, Crystals 8 (2018) 412, https://doi.org/10.3390/cryst8110412. [97] T.M. Nenoff, MOF membranes put to the test, Nat. Publ. Group 7 (2015) 377–378, https://doi.org/ 10.1038/nchem.2218. [98] Y. Liu, Z. Ng, E.A. Khan, H. Jeong, C. Ching, Z. Lai, Microporous and mesoporous materials synthesis of continuous MOF-5 membranes on porous a -alumina substrates, Microporous Mesoporous Mater. 118 (2009) 296–301, https://doi.org/10.1016/j.micromeso.2008.08.054. [99] H. Guo, G. Zhu, I.J. Hewitt, S. Qiu, “Twin Copper Source” growth of metal-organic framework membrane: Cu3(BTC)2 with high permeability and selectivity for recycling H2, J. Am. Chem. Soc. 131 (2009) 1646–1647.

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Further reading [101] Y. Zhao, Z. Song, X. Li, Q. Sun, N. Cheng, S. Lawes, Metal organic frameworks for energy storage and conversion, Energy Storage Mater. 2 (2016) 35–62, https://doi.org/10.1016/j. ensm.2015.11.005. [102] J. Wang, Y. Wang, Y. Zhang, A. Uliana, J. Zhu, J. Liu, B. Van Der Bruggen, Zeolitic Imidazolate framework/graphene oxide hybrid nanosheets functionalized thin film nanocomposite membrane for enhanced antimicrobial performance, ACS Appl. Mater. Interfaces 8 (2016) 25508–25519, https:// doi.org/10.1021/acsami.6b06992.

CHAPTER 8

Development of nanocomposite membranes by electrospun nanofibrous materials P. Sagitha, C.R. Reshmi, O. Manaf, Suja P. Sundaran, K. Juraij, A. Sujith Material Research Laboratory, Department of Chemistry, National Institute of Technology, Calicut, India

8.1 Introduction As per the engineering point of view, electrospinning is considered as a kind of electrodeposition method through which fibrous membranes are developed from polymer melts or solutions [1]. It offers less expensive and rapid production of randomly oriented fibers on a well-equipped collector by the application of high electric field [2]. In early 1930s, electrospinning was known as a fiber fabrication technique. Later, by the effort of Renekers through his pioneer works on electrospinning, its ability to fabricate nanofibers became explored more [3]. The working principle of electrospinning includes a constant balance between electrostatic force and surface tension. The electrostatic force helps to produce thin fibers from the polymer solution and drive them to the counter electrode, and surface tension opposes this force by minimizing the solution surface [4]. Due to high efficiency, the simplicity of the process and ability to spin inorganic materials, electrospinning has made widespread attention [5]. The electrospun membranes possess many unique characteristics, such as high-surface-area-to-volume ratio, porosity, and flexibility in surface functionalization [6]. Although simple electrospun polymer membranes have many advantages, some of their characteristics, such as poor mechanical properties and limited functional groups, result in the application being limited to only some fields. We can augment their properties and functionalities by incorporating various nanomaterials in the electrospun membrane. The resulting composite nanofibers possess improved thermal, mechanical, and chemical stability, thus making it suitable for use in a much broader spectrum of fields. These composite nanofibrous membranes are used in biomedical, water filtration, photonic, catalytic, electronic, magnetic, energy storage, fuel cell, and solar cell applications [7]. Applications of the electrospun membrane are shown in Fig. 8.1. This chapter discusses the basic concept of electrospinning, different kinds of electrospinning methods, applications of the electrospun membrane, and development of polymer nanocomposite membranes. Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00008-0

© 2020 Elsevier Inc. All rights reserved.

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Fig. 8.1 Applications of electrospun membranes.

8.2 History of electrospinning technique An insight into the development of the electrospinning process by a historical point of view will be of interest. This section outlines the fascinating story of the invention of electrospinning from the year 1600 when William Gilbert explained about the electrostatic attraction of liquids. By the effort of Christian Friedrich Schonbein, fibrous cellulose was artificially prepared in 1846. Later in 1887, this process was demonstrated in a paper by Charles Vernon. The first patent in electrospinning was filed in 1900 by John Francis Cooley. John Zeleny explained the nature of liquid droplets at the metal capillary tip in 1914. John Zelenys concept was further used for mathematical modeling of liquid behavior under electrostatic forces. Almost 22 patents on electrospinning were awarded to Anton Formhals during 1931 and 1944. Anton Formals was the one who explained the principle of electrospinning process [8]. Rozenblum and Petryanov-Sokolov developed the first electrospun fibers in 1938. The remarkable invention on electrospinning was carried out in 1960 by Geoffrey Ingram Taylor. Taylor theoretically explained the cone developed from liquid droplet under the influence of electric forces, and later the cone was named as Taylor cone [9]. Reneker was the one who popularized the term electrospinning. From the beginning of 1990 itself, the technique of electrospinning started to gain more attention, and its demand is growing exponentially every year.

Development of nanocomposite membranes by electrospun nanofibrous materials

A detailed review on the historical evolution of the electrospinning process was given by Tucker et al. [10].

8.3 An understanding into principle and working procedure of electrospinning The working principle of electrospinning has close resemblance with electrospraying technique. In both electrospinning and electrospraying, conventional mechanical forces such as pneumatic and hydrostatic are replaced by electrostatic forces. Since electrostatic forces are used for drawing fibers from the polymer solution, the process of electrospinning is also known as electrohydrodynamic jetting [11]. A basic electrospinning instrument is composed of three major regions, which include the high-voltage supply system, syringe pump, and a collector. The schematic representation of basic electrospinning is shown in Fig. 8.2. A high-voltage supply system is used to generate electric forces necessary for charging the polymer solution. Usually, a voltage in the range of 10–30 kV is supplied and hence the name high-voltage supply system. A syringe pump is used to control the flow of viscous polymer solution mounted properly using a syringe equipped with a fine capillary or metallic needle. Collector is the one which continuously collects the produced fibers for molding them into thick fibrous membranes [12]. Initially, a homogeneous polymer solution with sufficient viscosity is loaded into a clean and dry syringe equipped with a metallic needle. The solution containing syringe was then mounted properly on the syringe pump. The distance between the needle tip and the collector has to be adjusted before applying the high voltage. Using the highvoltage supply system, the required voltage can be applied between the needle tip and grounded collector. The syringe pump drives the polymer solution toward the needle tip and forms droplets. Application of high voltage develops charges in the droplet surface, resulting in mutual charge repulsion, which opposes the surface tension. With a further increment in the intensity of the electric field, the hemispherical droplets elongated

Fig. 8.2 Schematic representation of basic electrospinning set up.

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and deformed into a cone shape known as the Taylor cone. Polymer jet was ejected out of the needle tip when the mutual charge repulsion could overcome the surface tension. At the time of flight between needle tip and collector, the solvent gets evaporated and is deposited in the form of a nanofibrous mat in the collector. Also, a bending instability was experienced on the moving jet, which wipes them back and forth. The bending instability causes stretching of the polymer jet, which in turn helps to reduce the fiber diameter significantly [13].

8.4 Electrospinning parameters The diameter and morphology of produced fibers are always affected by electrospinning parameters such as process, solution, and ambient parameters [14]. The process parameters include applied voltage, flow rate, and needle tip to collector distance. To form a stable Taylor cone, application of high voltage is required. Generally, a minimum voltage of 6 kV is essential for distorting the polymer droplets into Taylor cone. But, when the applied voltage is too high, higher charge density will be created in the polymer solution. This will cause fast ejection of the polymer solution from the needle tip producing small and less stable Taylor cone. To get a stable Taylor cone, a constant flow rate should be maintained. The drying of the ejected polymer solution and its stretching is related to the needle tip to collector distance, which is also an important process parameter. Insufficient distance will cause a reduction in time of flight and result in partially dried fibers in fused state. A detailed review about the effect of process parameters on electrospinning is given by Ghorani et al. [15] Concentration of polymer solution, the molecular weight of polymers, solvent evaporation rate, conductivity, viscosity, and surface tension are categorized under solution parameters. Usually, the ratio of solvent will be higher than the polymer in a particular electrospinning solution. Hence, the nature of the solvent is of prime importance. Fast evaporating solvents always fail in terms of electrospinnability due to clogging of the polymer solution in the needle tip. Similarly, high boiling point solvents will produce sticky fibers due to slow evaporation. Uyar et al. [16] demonstrated the effect of solvent parameters, especially the solvent conductivity on the morphology changes of electrospun polystyrene fibers. Schueren et al. [17] conducted a detailed study on the effect of both single and binary solvent system on the electrospinning of polycaprolactone for producing uniform bead free fibers in the nanometer range. The electrical and rheological properties of polymer solutions such as conductivity, surface tension, dielectric properties, and viscosity are highly affected by polymer molecular weight. In general, high-molecular-weight polymers are preferred since they can give the ample viscosity required for spinning. At very low viscosity no fiber formation will occur, and at very high viscosity it will be difficult to eject out the polymer solution out of the needle tip [18].

Development of nanocomposite membranes by electrospun nanofibrous materials

The ambient parameters include temperature, airflow, and atmosphere humidity [19]. Humidity less than 35% is always ideal for effective spinning process. Likewise, optimum temperature and air flow should be maintained throughout the process [20].

8.5 Different kinds of electrospinning techniques Varieties of electrospinning techniques such as melt spinning, emulsion spinning, coaxial spinning, and multiaxial spinning are known for extending their applications in a wide area of research [21]. A brief schematic representation of different kinds of electrospinning techniques is shown in Fig. 8.3.

8.5.1 Melt electrospinning In these decades, melt electrospinning process gained researchers’ attention because it overcomes the problem related to solvent evaporation in the conventional solution electrospinning. It is used for the production of synthetic nanofibrous membranes at elevated temperature [22]. Melt electrospinning consists of a higher spinning voltage, heating system with a sensor, and a metal collector (Fig. 8.3B). One electrode of the high voltage is

Fig. 8.3 Schematic representation of different variations of electrospinning techniques.

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connected to the collector and other to spinning melt. As the electrostatic repulsion overcomes the polymer melt surface tension, the discharged jet undergoes thinning in the electric force. During this process, the thermal energy of the ejected jet is lost to the surrounding environment and cooled fibers get deposited on the collector. The membrane obtained from melt electrospinning is used in textiles, sensors, filtration, wound dressing, and tissue engineering applications [23]. The first report regarding with melt electrospinning was given by Larrondo and Manley in 1981, for developing polypropylene fibers with 50 μm diameter. Later several polymer membranes are prepared using melt electrospinning, including polycaprolactone (PCL), polyamides, poly(lactic acid), polycarbonate (PC), poly(ethylene glycol), and polyethylene [24]. Like solution electrospinning, there are several factors, which influence the formation of fibers during melt electrospinning, which include, the molecular weight of the polymer, melt temperature, spinning voltage, working distance, and melt flow rate [25]. In case of solution electrospinning, use of a hazardous solvent is a major problem. Moreover, the solvent evaporation during solution spinning results in defects in the fiber surface. Compared to solution electrospinning, melt electrospinning is environment friendly, high yield and safer [26]. But the major limitation of melt electrospinning is its complex apparatus consisting of a heating system [27] and the diameter of the nanofibers produced by melt electrospinning is greater than that of solution electrospinning due to the higher viscosity and low conductivity of polymer melt. Several methods such as the addition of plasticizers or viscosity reducers, use of a gas-assisted method, and jet path temperature controlling are used to reduce the fiber diameter during melt electrospinning [25]. Asai et al. [28] compared the melt and solution electrospinning using poly(vinylidene fluoride) nanofibers. The group also investigated the piezoelectric properties of the fibrous membrane formed by varying the spinning parameters such as applied voltage and rotating speed of the collector. Electrospun parameters do not strongly influence the crystal structure of the fibers obtained from melt electrospinning. Researchers are interested in the melt electrospinning technique in biomedical field because of the absence of the toxic residual solvent. Lee et al. [29] carried out the melt electrospinning of poly (lactic acid) for bone tissue regeneration. They used gas-assisted meltelectrospinning to avoid quenching of the melted jet by ambient air. The cell viability and osteogenesis are better for fibers obtained by melt electrospinning than solution electrospinning. Cao et al. [24] prepared a carbon nanotube/polypropylene fibrous membrane by electrospinning. The membrane has some advantages such as being nontoxic, and showing good mechanical strength and electrical conductivity. So it can be used to produce tissue engineering scaffolds and electric devices. Karchin et al. [30] developed a biodegradable polyurethane fibrous membrane through melt electrospinning. The nontoxic scaffold having mechanical strength similar to native tissues can be used for the preparation of tissue engineering scaffold.

Development of nanocomposite membranes by electrospun nanofibrous materials

8.5.2 Gas jet electrospinning The fiber morphology of electrospun membranes can be tuned by varying the conductivity, viscosity, and surface tension of polymer solution. For that purpose, people are using different kinds of solvents or solvent mixtures for the same polymers. While using mixed solvent systems, one of the solvents should be highly volatile for the ease of drying. However, at low-humidity environment, the polymer droplet at the syringe tip dries quickly and makes it difficult for the formation of Taylor cone. To avoid this, a gas jet set up can be employed as represented in Fig. 8.3C. It facilitates the spinning in a high volatile solvent and increases productivity as per the required fiber characteristics. A typical example for gas jet electrospinning is given by Larsen et al. [31] They fabricated polylactic acid-based electrospun membranes using gas jet technique in highly volatile dichloromethane solvent system.

8.5.3 Coaxial electrospinning Coaxial electrospinning is a versatile technique to fabricate core/sheath fibers (Fig. 8.3D). When compared with basic electrospinning set up, the coaxial fibers have greater attention in biomedical applications. It facilitates the transportation of drug through a harsh environment and sustainable release into a targeted site. It can be achieved by tuning the thickness of the core/sheath phase. For biomedical scaffolds, biocompatible polymers have been employed as the sheath. Chang et al. [32] prepared hollow TiO2 nanofibers by coaxial electrospinning. Instead of dual axial mode, triaxial modes are also recently developed. Han and Steckl [33] reported triaxial electrospun fibers of PVP as the core and PCL as the intermediate/sheet for the release of two dyes Keyacid Uranine and Keyacid Blue.

8.5.4 Emulsion electrospinning Emulsion electrospun membrane gained attention due to low toxicity, biocompatibility, and biodegradability. It is considered as an alternative method for solution electrospinning (Fig. 8.3E). It can be widely used in biomedical, food, and other applications for developing membrane containing functional materials such as proteins, enzymes, flavonoids, etc. The electrospun solution may be water-in-oil (W/O) or an oil-in-water (O/W) emulsion. Emulsion electrospinning gives core-shell nanofibrous membrane using a single nozzle. Compared with the central part, evaporation of the solvent occurs more rapidly from the portion closer to the surface of the polymer jet. Thus viscosity increases more at the outer layer than the inner layer. Thus under the high-voltage electric field, the emulsion droplets undergo inward movement from the surface to center. In the axial direction of nanofiber, the droplet undergoes simultaneous condensation and stretching and results in core-shell nanofiber [34]. Nowadays, researchers are interested in the development of controlled drug-delivery polymer membrane. It provides reduced toxicity, reduced frequency of administration, and improved effectiveness of the drug.

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Electrospun nanofibers are used to prepare drug-delivery vehicles. In solution electrospinning, drug molecule is incorporated in a polymer solution by simple blending and results in nonuniform distribution of drug molecule in the membrane. Moreover, it causes burst release and leads to denaturation of biomolecules. Loss of bioactivity of the molecule in contact with organic solvent during solution electrospinning is also a severe problem [35]. Use of emulsion electrospinning is an alternative method for producing a controlled drug delivery vehicle. It is used for the preparation of core-shell nanofibers and can be used for the successful incorporation of biomolecules in the inner core of the fiber. During this process, one must dissolve the hydrophilic drug in water and hydrophobic polymer in solvent or vice versa. During stretching and solidification, oil phase evaporates, and hydrophilic biomolecules are encapsulated into the fibers. Emulsion electrospun fibrous membrane advantages such as burst release can be avoided, and controlled release of the drug can be achieved. This is because before entering the surrounding medium the bioactive molecules want to pass through core-shell matrix [36]. Basar et al. [36] prepared an antiinflammatory drug (ketoprofen)-incorporated PCL membrane through solution electrospinning and PCL/gelatin membrane by emulsion electrospinning. The membrane produced by emulsion electrospinning is nontoxic and completely suppresses burst release of the drug. Moreover, it gives a sustained and continuous release for more than 4 days. Moydeen et al. [37] developed a drug-delivery system where ciprofloxacin hydrochloride drug is incorporated in PVA/Dextran blend through emulsion electrospinning. The system gives a sustained drug release where the release is independent of drug concentration. In addition to the biomedical field, food ingredients such as carotenoids, vitamins, flavors, etc., are encapsulated in emulsion-based electrospun fibers. Thus during food production, functional ingredients are protected from harsh processing conditions. The bioactive molecules (vitamins and fatty acids), which are beneficial to human health, undergo degradation results that decreased the bioavailability of these substances. The bioactive component in the nanofibers produced by emulsion electrospinning has advantages such as controlled release properties, improved stability, and bioavailability. Moreover, the application of nanofibers produced by emulsion electrospinning also attracted in food packaging and enzyme immobilization.

8.5.5 Needleless electrospinning Syringeless electrospinning is a novel technique to produce nano or microfibers. As the name indicates, the technique does not require syringe to control the pumping of the polymer solution. Many variations of needleless spinning are also known. Typical schematic diagram of needleless electrospinning is shown in Fig. 8.3F. The main advantage of this technique over the syringe electrospinning is the high productivity by avoiding the

Development of nanocomposite membranes by electrospun nanofibrous materials

needle clogging. Whereas the disadvantage associated with this technique is the formation of nonuniform fiber diameter distribution. Few works are reported on needleless electrospinning for the fabrication of fibrous membranes [38].

8.5.6 Multispinneret electrospinning Even though electrospun fibers have distinguished properties, one of the most disadvantages associated with the electrospinning technique is the very low productivity. In a typical electrospinning set up, the feed flow rate and polymer concentrations are 2 mL/h and 0.2 g/mL respectively. It is expected that the maximum mass of fiber deposited on the collector after one hour is 0.4 g. In order to improve productivity, instead of a single syringe, a multisyringe set up can be employed, as shown in Fig. 8.3G, where multiple numbers of syringes can pump simultaneously at a time. Kim et al. [39] demonstrated the stability of the spun jet in multispinneret setup. Tijing et al. [40] conducted dual spinneret electrospinning set up for the fabrication of (Ag nanoparticle/polyethylene oxide)/PU hybrid electrospun membrane [3]. It is extensively used for the fabrication of polymer blends and composite. The composition of the components can be varied by varying the concentration as well as the ratio of the syringes. To avoid the interference of the electric field associated with the individual syringe, they must keep at least a minimum distance of 3 cm.

8.6 Applications of electrospun membranes Electrospun membranes with enough thickness and mechanical strength can be used for wide varieties of applications depending on the nature of polymer matrix and the filler type. They are most commonly applied in different industries and research areas such as biomedical, water filtration, sensing, electronics, and catalysis. Table 8.1 shows the list of applications of electrospun membrane. Electrospinning technique has been widely applied in tissue engineering due to the high similarity of developed fibers with the connective tissues, including extracellular matrix (ECM), bone, cartilage, ligament, and blood vessels. In addition, the morphology, porosity, interconnectivity, mechanical properties, and topology of the membranes can be easily tuned. Both natural and synthetic biocompatible polymers were employed for the fabrication of electrospinning tissue engineering scaffolds. Typically used synthetic polymers are polycaprolactone, polylactic acid, polyvinyl alcohol, and polyurethane. Most of these polymers have slow degradation profile. The functionalization of these polymers is essential to make them suitable for biomedical application. Electrospun membranes of natural polymers such as collagen, hyaluronic acid, gelatin, chitosan, silk, and even DNA were also applicable in various tissue engineering purposes.

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Table 8.1 List of applications of electrospun membranes. Applications of electrospun membranes

Refs.

Biomedical

Su et al. [41] Zhou et al. [42] Hatamzadeh et al. [43] Kadam et al. [44] Ray et al. [45] Ma et al. [46] Mondal and Sharma [47] Kadir et al. [48] Wang et al. [49] Xu et al. [50] Lian and Meng [51] Halder et al. [52] Dissanayake et al. [53] Liu et al. [54] Solarajan et al. [55] Subramanian et al. [56]

Filtration

Sensor

Catalytic

Electronic

Drug delivery Wound dressing Tissue engineering Air filtration Water filtration Oil-water separation Biosensor Gas sensor Optical sensor Enzyme catalysis Photocatalysis Organic conversions Solar cells Batteries Capacitors Conductive membranes

The high porosity and surface area to volume ratio of the electrospun membrane makes them a promising candidates for drug-delivery system. Numerous kinds of drugs, such as anticancer agents, antibacterial drugs, antibiotics and biological agents such as proteins, growth factors, ribonucleic acid, and deoxyribonucleic acid, can be easily loaded on electrospun nanofibers. Various strategies can be adapted to incorporate drug in electrospun membranes. This includes incorporation of drug during the preparation of the electrospun solution or by the coating of drug on the surface of the electrospun membrane. High specific surface area and a short diffusion passage length of nanofibers induce higher and controlled drug release rates compared to bulk materials [57,58]. Electrospun membranes are ideal materials for water purification purposes due to their tunable functionalities, wettability, and pore size distribution. Based on the pore size, electrospun membranes are categorized as microfilters and ultrafilters. The interconnected pores in electrospun membranes allow the selective passage of fluids and particles during the filtration process. Electrospun membranes were applicable for the removal of various water contaminants such as heavy metals, organic molecules, bacteria, virus, large-sized particles, and oil. Polycaprolactone, polyurethane, polyacrylonitrile, polysulfone, polyvinylchloride, etc., are the commonly used polymers for water purification applications [59]. Most of the hydrophobic polymers used in water purification suffer from the problem of fouling. The development of antifouling surface has attracted attention of various researchers. Hence, the use of nanocomposite membrane developed using hydrophilic functional additive is important in water purification.

Development of nanocomposite membranes by electrospun nanofibrous materials

Polymeric nanofibrous membranes have a porous structure with excellent pore connectivity and extremely high surface-to-volume ratio. These characteristics along with the functionalities and surface chemistry of the electrospun membranes make them good candidates for sensing applications [60]. Electrospun fibrous membranes are widely used for the chemical sensor, gas sensor, optical sensor, and biosensor applications [61]. The surface area of a catalyst is considered as an important factor in determining its catalytic activity. High-surface-area-to-volume ratio and porosity devoted to electrospun fibrous membranes make them as good candidates for catalytic applications when they are functionalized with specific catalytic groups. Bai et al. [62] fabricated polyacrylic acid/ polyvinyl alcohol (PAA/PVA) electrospun membranes impregnated with gold nanorods for catalytic reduction of 4-nitrophenol. It was observed that without fibrous catalysts, the reduction could not happen. In another work, Hu et al. [63] investigated the catalytic reduction of 4-nitrophenol by using Au/Ag bimetallic nanoparticle immobilized polyvinyl alcohol/polyethyleneimine (PVA/PEI) membranes. PEI amine functional group acts as a chelating agent to bind Au or Ag metal via electrostatic interaction. The membrane showed excellent catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol. Over the past few years, electrospun membranes with enhanced electrical properties have gained tremendous interest in various electronic applications. Electrospinning offers a new way to incorporate conductive fillers into the nonwoven mat. Several conductive fillers are known, which include graphene, carbon black, ionic liquids, and carbon nanotubes.

8.7 Electrospun nanocomposite membranes Composites are materials made from more than one component having different mechanical, physical, and chemical properties. These materials exhibit better properties than the individual components [64]. The fibers obtained by simply electrospinning of the polymers have limited application. Nowadays, the development and application of nanocomposites are regarded as one of the most progressive research areas. In the case of the electrospinning method, the nanocomposite fibrous materials are formed by incorporating functional fillers either during electrospinning or by the postmodification of the membrane. Nanocomposite membranes exhibit a wide range of potential applications that are determined by both polymer and functional additives. Various functional additives utilized so far include metal oxides, clay, zeolites, carbon nanotubes, graphene, and graphene oxide [65]. Different methods are used to fabricate composite electrospun membranes. Two common methods are (1) electrospun membranes were dipped into the nanofiller

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solution to adsorb nanoparticles on the membrane surface followed by a thermal or chemical treatment and (2) simple electrospinning after the incorporation of nanoparticles in electrospinning solution.

8.7.1 Nano clays Clays having sheet-like structure possess excellent thermal and mechanical stability. Clays mainly comprise phyllosilicates. Different types of clay include kaolinite, bentonite, montmorillonite/smectite, and sepiolite/palygorskite. The modification of nanofibrous membrane with clay having high surface area and aspect ratio could enhance its thermomechanical stability, permeation properties, swelling capacity, and rheological properties [66]. Due to the high flexibility, and low cost, clays and polymer/clay composites can be widely used as an adsorbent for the removal of contaminants from wastewater [67]. Clays are also important in biomedical applications because of the high exchange capacity, high-surface-area-to-volume ratio, and good adsorption capacity. Biocompatible polymer matrixes are modified with inorganic clay minerals to enhance the degradation properties and mechanical stability of the polymer matrix [68]. Nowadays, clay-polymer nanofibrous membranes gained much attention in various fields due to their hydrophilic character. Montmorillonite (MMT) is widely used due to its properties such as low cost, low toxicity, and good dispersion behavior. The adsorption capacity of electrospun nanofibers can be improved by the fabrication of nanocomposite of a polymer membrane with clay. Cai et al. [69] developed modified MMT functionalized electrospun cellulose acetate nanofibers. The developed composite nanofibrous materials showed higher thermal stability and good adsorption toward toxic Cr6+ ion from contaminated water as compared to the electrospun polymer membrane. Hosseini et al. [70] prepared an MMT-modified chitosan/poly(vinyl alcohol) (PVA) electrospun membrane. The modification with MMT enhanced the mechanical properties of chitosan/PVA blend membranes. Pure water flux of the composite membrane was greater than 1500 Lm 2 h 1. The composite membrane with 2 wt% of MMT exhibited 95% rejection toward Basic Blue 41 dye with good reusability. In the past decades, nanocomposites of polymer with layered silicate gained attention in the biomedical field, especially in drug release. Targeted and effective drug release is possible with clay-based nanocomposites [71]. The efficiency of clay can be improved by the development of composite with polymer nanofibrous membranes. Kumar et al. [72] studied the drug release behavior from nanoclay embedded electrospun poly (lactic acid) nanofiber for tumor treatment. Dexamethasone drug and organically modified montmorillonite nanoclay are incorporated into an electrospun solution for the formation of nanohybrid scaffold. Nanocomposite membrane showed a lower fiber diameter, higher thermal, and mechanical stability compared with the pure polymer membrane. Cell culture study confirmed the biocompatibility of the developed membranes. The beneficial

Development of nanocomposite membranes by electrospun nanofibrous materials

effects of the hybrid scaffold were confirmed by in vitro results of Melanoma tumor treatment in mice model with low side effects. Reshmi et al. [73] prepared a drug (gentamicin)-loaded montmorillonite/polycaprolactone nanocomposite membrane. This study opens a way to use this composite membrane for various biomedical applications such as sustained drug release, wound healing, pathogenic inhibition, and tissue-engineering applications.

8.7.2 Metal oxides Due to interconnected porosity and high-surface-area-to-volume ratio, metal oxidebased nanofibrous membranes are used for various applications such as supercapacitor, catalysis, gas sensing, biosensing, Li-ion battery for energy storing, electronic, wastewater treatment, tissue engineering, and biomedical devices. Metal oxide-based composite nanofibers are usually produced by coaxial, colloid-, melt-, and solution electrospinning. Several studies are reported about the uniform dispersion of various metal oxides in the polymer solution. Sekar et al. [74] studied the formation of Fe-doped ZnO nanoparticles functionalized PVA membrane. The developed membrane exhibited antibacterial activity against both Gram-positive and Gram-negative bacteria. Moreover, the membrane was biocompatible and could be used for various biomedical applications. Kim et al. [75] fabricated ZnO nanorods-coated PU nanofibers by postelectrospinning modifications for photocatalytic applications. Photocatalytic activity of ZnO nanoparticlescoated PU membranes were studied with the cationic dye methylene blue (MB). Results revealed that the catalytic reduction of MB enhanced significantly with the presence of ZnO-coated electrospun PU catalysts. Titania (TiO2) is another widely used inorganic nanomaterial having good chemical, thermal stability, and nontoxicity commonly used in photocatalysis. Nasar et al. [76] prepared reduced graphene oxide (rGO)/TiO2 composite nanofibers for photocatalytic degradation of methyl orange. Compared with commercial TiO2, the prepared membrane showed 6 times higher degradation efficiency. Abdal-hay et al. [77] prepared TiO2-modified polyvinyl acetate nanofibers by an air-jet spinning method. The developed composite membranes showed excellent photocatalytic activity against methylene blue after four cycles. This showed that compared with TiO2 nanoparticle, the developed membranes did not lose their photodegradation activity. Silica (SiO2) is another nanoparticle incorporated into electrospun nanofibers to take advantage of both nanoparticles and electrospun membrane. Compaction-resistant electrospun nanofibrous membranes with high adsorption capacity were fabricated by Hosseini et al. [78]. SiO2 is added to chitosan/PVA solution and the nanocomposite membranes showed higher average fiber diameter and roughness compared with Chitosan/PVA membrane. Moreover, the membranes exhibited high thermal stability and increased compaction resistance. The fabricated nanocomposite membranes were useful

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as antifouling dye-rejecting materials with excellent efficiency. Pirzada et al. [79] used sol-gel electrospinning to fabricate SiO2-based electrospun nanofibrous membranes. The synthesized nanocomposite membranes reported being useful in various fields such as tissue engineering, separation, and filtration. In addition to the metal oxide nanoparticles mentioned here, Fe3O4 [80] and vanadium-based oxides [81] have also been used to produce nanocomposite membranes.

8.7.3 Zeolites Zeolites are crystalline aluminosilicates having exchangeable cations and molecular dimension pores. These materials are mainly used in catalytic membrane reactors and in separation methods by coupling the advantages of zeolite and nanofibers. Bahi et al. [82] prepared a zeolite/lignin nanocomposite membrane, which exhibited excellent mechanical property, hydrophilicity, and water permeability. Polymer/zeolite composite can be used in artificial kidney applications by considering the beneficial properties such as nontoxicity and stability under physiological conditions of zeolite [83]. Polymer/zeolite composites are also used for the adsorption of heavy metal ions [84]. The composite membrane showed high adsorption due to the high surface area of nanofiber and exchangeable cations present in the zeolite.

8.7.4 Carbon nanotubes Carbon nanotubes (CNTs) are a promising carbonaceous filler in electrospinning due to their unique properties such as high conductivity, high mechanical strength, and oleophilicity. CNTs can be classified into single-walled and multiwalled carbon nanotubes. Single-walled carbon nanotubes are rolled single graphene layer, and multiwalled carbon nanotubes are made up of multiple rolled graphene layers [85]. Functionalization ability to the CNTs and their unique properties made them excellent candidates for the development of composite electrospun membranes for several applications. Major applications include electronics, oil/water separation, drug delivery, tissue engineering, artificial implants, and biomedical applications. Gu et al. [86] fabricated functionalized CNT composite membranes for efficient oil/water separation, and it was achieved through the deposition of functionalized CNT on poly (vinylidene fluoride) electrospun membranes. For the same application, Wu et al. [87] introduced functionally modified fluorinated CNT, which is doped into polystyrene electrospun membrane having oleophilicity and microsize void.

8.7.5 Graphene Among the allotropes of carbon, graphene is a two-dimensional sheet of sp2 hybridized carbon in atomic-scale thickness, and it is extended to honeycomb-like hexagonal lattice. While comparing with its thickness, it is 100 times stronger than steel. It has good

Development of nanocomposite membranes by electrospun nanofibrous materials

electrical/thermal conductivity, large theoretical surface area, high intrinsic mobility, high Young’s modulus, and it is considered as both oxidizing agent and a reducing agent. Due to the attractive properties, it is considered as one of the fillers for the fabrication of composite electrospun fibrous materials [88]. Yang et al. [89] fabricated conductive composite electrospun fibrous scaffold by using silk fibroin (SF) and graphene (G). They found that 3% G/SF composite membrane possessed improved electroactivity and mechanical property. Hence, they have used 3% G/SF for potential application of inducing electric field to cell culture.

8.7.6 Graphene oxide The chemical oxidation of graphite with strong oxidizing agent is a promising route toward the large-scale production of graphene oxide for commercial applications. Structurally it consists of many oxygen moieties, which make them soluble in the organic solvent. The major attraction toward the electrospinning is that while incorporating graphene oxide as the filler, it will enhance the electrical and mechanical properties of the spinning solution, even though it can act as an insulator to some extent due to the disturbance in the sp2 bonding network. It is mainly used for the production of graphene by chemical reduction strategy, and it can retain the electrical conductivity by recovering a honeycomb hexagonal structure. Ghobadi et al. prepared poly(vinyl alcohol)(PVA)/rGO composite electrospun fibers, showing the significant role of rGO in their morphology. Moreover, the introduction of rGO was responsible for the fine fiber alignment, tensile strength, and mechanical stiffness [90]. Zhu et al. [91] fabricated graphene oxide doped (2,2,6,6-tetramethylpiperidin-1-yl)oxyl functionalized cellulose nanofiber biohybrid for water purification. The composite hybrid exhibited a promising adsorption capacity toward Cu(II) ions. Moreover, GO imparted appreciable mechanical performance toward the so-called system. Sundaran et al. [92] prepared multifunctional polyurethane (PU)/GO composite electrospun membrane for dye adsorption from contaminated water. They have imparted antifouling and antibacterial properties by incorporating graphene oxide to their fibrous materials.

8.8 Conclusion Electrospinning has been considered as an emerging fiber fabricating technique with many variations for different kinds of applications. The fascinating properties of electrospun membranes such as a high-surface-area-to-volume ratio, porosity, mechanical strength, and flexibility for surface functionalization is the reason for its growing demand in academic and industrial research. Proper control over electrospinning parameters will produce fibrous membranes with desired properties. To date, many revolutions have happened to the basic structure of the electrospinning technique. Nano clay, metal oxides, zeolite, carbon nanotube, graphene, and graphene oxide have fascinating

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properties, which make them good filler for nanocomposite membranes. These fillers can easily impart many functional properties to the nanocomposite membranes. Hence, they are at the forefront of many applications such as tissue engineering, artificial implants, drug delivery, oil/water separation, catalysis, sensing, and electronic applications. They can also be subjected to either pre-electrospinning modifications or postelectrospinning modifications to form the nanocomposite membrane for different applications.

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CHAPTER 9

Development of nanocomposite membranes by biomimicking nanomaterials Simin Shabani, Behnam Khorshidi, Mohtada Sadrzadeh

Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada

9.1 Introduction With the growing world population and climate change, water scarcity will be one of the major challenges in the coming century [1, 2]. Membrane technology is a promising candidate to alleviate the water scarcity. However, the energy and cost efficiencies of these technologies must be improved for them to compete with the conventional chemical and physical water treatment processes such as coagulation, adsorption, and oxidation. The most widely used membrane water treatment technologies currently are reverse osmosis (RO) and nanofiltration (NF). In the RO process, hydraulic pressure is used to drive water from saline sources such as seawater, brackish groundwater, and industrial wastewaters. Most commercially available NF/RO membranes are made by thin-film composite (TFC) polyamide (PA) and its derivatives. The major challenges of TFC PA membranes have always been improving their water permeability, separation performance, and antifouling properties [3]. Typical water permeability of commercial TFC RO membranes are approximately 1–2 Lm
2 h
1 bar
1 (LMH/bar) for seawater RO (SWRO) and 2–8 LMH/bar for the brackish water RO (BWRO) [1, 2, 4]. Thus, improving RO/NF efficiency and antifouling properties are the main research areas in membrane science and technology. Most progress is associated with the development of novel materials for desalination [5]. Recently, there has been extensive interest in biomimetic membranes, which consist of biological or synthetic water channel within a lipid-like molecule or amphiphilic copolymer bilayer to form a composite active layer with superior functionalities and properties. The most notable biomimetic desalination membranes are protein-based membranes, specifically, aquaporin (AQP)-based membranes. Aquaporin (AQP1) was the earliest water channel protein discovered. It was discovered by Peter Agre in 1991, who won the 2013 Nobel Prize in Chemistry for this discovery. The osmotic water permeability for Aquaporin is as high as 6  10
14 to 11  10
14 cm3 sec
1 per channel Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00009-2

© 2020 Elsevier Inc. All rights reserved.

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[6, 7]. Another interesting property of AQP is its ability to reject small molecules like urea (or boric acid), which are hardly removed by conventional membranes [7]. Given that, researchers have looked to AQP-based biomimetic membranes to overcome the tradeoff between flux and rejection, i.e., producing membranes with both high water permeability and selectivity.

9.2 Biomaterials “Biomimetics” originates from the Greek words bios, meaning life, and mimesis, meaning imitator. Biomimetic materials refer to composite materials that have structures and functions similar to biological systems [8]. Biomimetic scientists and engineers have tried to solve problems using their understandings of nature in two ways. The most common way is to emulate natural material structure or functions. To this end, modern technology is implemented to design and fabricate artificial materials that mimic both the structural form and function of natural materials. With the advances in nanotechnology in recent years, these synthetic materials have been developed and characterized in the scale of molecules or atoms. The second aspect of biomimetic engineering design is imitating nature in the process design, which involves understanding and being inspired by natural phenomena and applying it in the engineering processes [9]. Traditionally, biomimetic materials include artificial materials that mimic biological systems by conventional methods; however, advances in technology enabled fabricating artificial materials utilizing biomolecules (e.g., proteins) and microbes (bacteria, fungi, viruses, archaea, protista, and symbionts). A desirable feature of biological processes is that they emit environment-friendly materials during the production process. Another desirable property of biological materials is their ability to grow without the need for final design specifications, using a recursive algorithm in their genetic code. This characteristic provides scientist with an opportunity to scale up nanoparticles into large structures with tailored properties. Table 9.1 shows the difference between biomaterials and traditional engineering materials regarding structure and processing. Biomimetic materials also provide outstanding properties including self-cleaning, drag reduction in fluid flow, energy conversion and conservation, high and reversible adhesion, high mechanical strength, self-assembly, and antireflection that can reduce costs compared to the conventional materials and processes. Biomimetic materials can be classified into three groups based on their properties: structural materials; functional materials; and biomimetic processes. Table 9.2 provides this classification with potential applications.

9.3 Cell membrane The natural prototype for biomimetic membranes is a cell membrane, which is known as plasma membrane or a cytoplasmic membrane. It is a biological, semipermeable

Development of nanocomposite membranes by biomimicking nanomaterials

Table 9.1 Comparison between biomaterials and traditional engineering materials [9] Characteristics

Biomaterial

Traditional material

Chemical compositions Microstructure

Mostly earth-abundant elements (C, H, O, N, Ca, P, S, Si, etc.) Can be organized in different scale level (nano to macro), hierarchical structures Multifunctionality

A large variety of elements (Fe, Cr, Ni, Al, Si, C, N, O, etc.) Mostly microstructures at a single length scale Selection of materials according to the function Component replacement

Functions Failure prevention Formation/ fabrication Processing Design criteria Environmental impact

Capability of self-healing Growth by genetically guided/ self-assembly (approximate design) Ambient temperature, pressure and, neutral pH Modeling and remodeling, capability to changing environmental conditions Biodegradable

Fabrication from melts, powders, Involve high temperature and pressure, strong acid-base Secure design (consider large safety factor) Biodegradable/ nonbiodegradable

Table 9.2 Classification of biomimetic materials with their properties and applications[9] Material

Structural biomimetic material

Properties/ function

Biological prototypes

Biomimetic materials

Potential applications

Strength, toughness

Spider silk, abalone shell, bone

Strong and tough materials

Hardness, wear resistance

Enamel, DEJ

Tough ceramic composite

Impact

Horns, hoof

Adaptive (stiffness, shape)

Sea cucumber dermis, squid beak

Self-healing

Soft tissue, bones, plants

Friction and adhesion

Gecko, tree frogs, pitcher plant, shark skin

Damage-tolerant composites Adaptive nanocomposites with reversible stiffness change capability Self-healing composites, concretes Dry adhesive, low friction surface

Supertough CNT yarns, tough ceramic composites Cutting tools, wear-resistant coating Impact resistance Self-shaping, morphing structures

Self-healing composites, reads Surface friction control, drag Continued

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Table 9.2 Classification of biomimetic materials with their properties and applications—cont’d Material

Functional biomimetic materials

Properties/ function

Biological prototypes

Biomimetic materials

Potential applications

Stimuli responsive

Muscles, nastic action, sun-tracking plants Lotus leaf, gecko beef, pitcher plant, shark skin Beetle, butterfly, moth eye, feather Leaf, hydrogenase, enzyme, blood cells Proteinmediated mineralization

Artificial muscles, smart materials

Actuator control, sensing

Self-cleaning materials, antifouling coating

Self-cleaning and antifouling coating Monitoring, sensing, antireflectivity

Self-cleaning

Photonics

Catalysis

Biomimetic materials process

Biomineralization

Structural color material, antireflective materials Catalyst for oxygen, hydrogen evolution, oxygen evolution Processing at ambient temperature and neutral PH

Fuel cells, metal-air batteries, water splitting Formation of ceramic, metal, and nanoparticles

membrane that separates the inside of the cell from its environment and regulates the transport of multisized substances such as carbohydrates and amino acids into and out of the cell [10, 11]. All cell membranes contain a lipid bilayer, also called phospholipid bilayer, with embedded proteins, as shown in Fig. 9.1A [12, 13]. Each phospholipid molecule has a phosphate group on the head and two fatty acid chains as tails. The head group is negatively charged and hydrophilic, and the tails are nonpolar and hydrophobic (Fig. 9.1B). Hence, phospholipids are amphipathic molecules containing both hydrophilic and hydrophobic regions (Fig. 9.1C) [7, 12]. In addition to the lipid bilayer as the basis of the cell membrane, it consists of various proteins. The lipid bilayer can be considered the wall of the cell, and proteins serve as selective gates or sensors in this wall. There are numerous types of proteins, which are responsible for the transport of specific materials into or out of the cell. For example, some proteins let ions like Na+, K+ and sugar pass through them [7, 12]. There is a specific type of protein called aquaporins (AQPs) that only allows the transport of water molecules. AQP is a water channel that consists of six lipid, membrane-spanning domains, forming a narrow pore (about 2.3°) with hydrophobic amino acid residues

Development of nanocomposite membranes by biomimicking nanomaterials

Glycoprotein: protein with carbohydrate attached

Glycolipid: lipid with carbohydrate attached

Peripheral membrane protein (A)

Integral membrane protein

Phospholipid bilayer

Cholesterol

Channel protein

Hydrophilic head

Extracellular

Intracellular (B)

Hydrophobic tails

Phospholipid bilayer

Hydrophobic tail Hydrophilic head

R O O

P

O

Phosphate

O CH2

CH

O

O

C

OC

CH2

Glycerol

O

Saturated fatty acid Unsaturated fatty acid

(C)

Fig. 9.1 (A) Cell membrane consisting of proteins and phospholipid bilayer, (B) phospholipid bilayer, which comprises two layers of phospholipids, one layer exposed to the interior and another one exposed to the exterior of the cell, (C) Phospholipid molecule comprising a hydrophilic head and two hydrophobic chains [12].

[14]. The general structure of AQP1 and a schematic view of transport of water molecules in AQP1 are shown in Fig. 9.2. The transport mechanism in AQPs functions is as follows: (i) size exclusion at the narrowest part of the pore that rejects large ions compared with pore size; (ii) electrostatic repulsion that rejects the charged ions; and (iii) water dipole reorientation (proton exclusion) that facilitates a single-file transport of water molecules [14]. A very close analog of the cell membrane is proteoliposome, a lipid vesicle that contains a specific protein in the lipid membrane, as shown in Fig. 9.3 [7]. In synthesizing a vesicle, different lipid-like molecules, such as block copolymers and bolaamphiphiles, can be used as alternatives [7]. A vesicle that contains AQP with polymers is called proteopolymersome.

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Fig. 9.2 (A) General structure of AQP (AQP1 in human) [15] and (B) Schematic diagram of water molecules transport in AQP1 [13].

Fig. 9.3 Scheme of (A) lipid vesicle (liposome) showing phospholipid bilayer separates the interior of the cell from its surrounding [16] and (B) a lipid vesicle with incorporated protein (proteoliposome) [7].

9.4 Fabrication of AQP-based desalination membrane The primary components in the AQP-based membranes include AQP, an amphiphilic polymer (known as copolymer) or lipids that incorporates AQP, and a porous support. To synthesize these membranes, AqpZ, an AQP found in Escherichia coli cells is commonly used, because it is simple to harvest and express. Table 9.3 illustrates the material properties of AqpZ compared to the TFC PA [2]. Kumar and coworkers [17] were pioneers in synthesizing AQP-based biomimetic membranes, which was followed by a wave of studies by many researchers in the past decade. Generally, biomimetic membranes are classified into two groups: (i) AQP-based primary rejection layer (PRL) membrane in which AQPs are assembled in a lipid or polymeric bilayer on a porous substrate (Fig. 9.4A) and (ii) AQP-based thin-film

Development of nanocomposite membranes by biomimicking nanomaterials

Table 9.3 The material properties comparison between TFC PA and AqpZ [2] TFC PA

AqpZ

Material Transport mechanism

Crosslinked polymer Solution diffusion

Characteristic

Irregular pores in a random network the characteristic pore ˚ based on diameter of  4–5.8a A positron annihilation lifetime spectroscopy, possible heterogeneous pore distribution from some membranes 1–2 L m
2 h
1 bar
1 for SWRO  2–8 L m
2 h
1 bar
1 for BWRO  > 99% NaCl rejection (obtained from crossflow filtration tests) Prone to fouling No

Natural protein Size exclusion and charge repulsion Well-defined hour-glass shaped ˚ channel, pore size of  3 A

Separation properties

Antifouling properties Electrical conductance

 600 L m
2 h
1 bar
1; nearly 100% rejection (obtained from stopped-flow measurements of AQP-containing vesicles)

Not reported in the literature No

nanocomposite (TFN) membrane where vesicles containing AQP (proteoliposomes or proteo-polymersomes) are encapsulated in a dense polymer layer on a porous support layer (Fig. 9.4B) [2]. Table 9.4 provides the summary of the materials, testing conditions, and major outcomes of the reported biomimetic membranes in the literature.

Fig. 9.4 Schematic presentation of AQP laden membranes: (A) AQP-based PRL membrane where AQPs are embedded in a lipid or polymeric bilayer to form a selective layer. (B) AQP-based TFN membrane where vesicles containing AQP are immobilized on the thin-film selective layer [2].

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Testing conditions and membrane area (cm2)

Type

Test

Water flux (Lm22 h21 bar21)

PRL AqpZ-DOPCa

NF

3.6

RNaCl ¼ 20%

1 mM NaCl @ 1 bar, Area: 28.3

AqpZ-ABAb

NF

34.2

RNaCl ¼ 32.9%

200 ppm NaCl @ 5 bar, Area: 0.017

AqpZ-ABA

NF

16.1

RNaCl ¼ 45.1%

200 ppm NaCl @ 5 bar, Area: 0.2

AqpZ-DOPC/ DOTAPc AqpZ-ABA

NF

5.5

FO

AqpZ-DOPC/ DOTAP

FO/ NF

FO water flux ¼ 16.4 L m
2 h
1 FO water flux ¼ 32.1 L m
2 h
1 NF: 6.31

RNaCl ¼ 75% RMgCl2 ¼ 97% RNaCl ¼ 98.8%

TFN AqpZ-DOPC

FO salt flux ¼ 3.1 g m
2 h
1 NF: RMgCl2 ¼ 90%

500 ppm NaCl @ 4 bar, Area: 19.56 0.3 M sucrose as DS, 200 ppm NaCl as FS, Area: 0.096 2 M MgCl2 as DSd, DI water as FSe NF: 2000 ppm MgCl2 @ 4 bar Area: 36

RO

4

RNaCl ¼ 97% @ 5 bar

10 mM NaCl @ 5 bar, Area: > 200

AqpZ-DOPC

RO

8

RNaCl ¼ 97.5%

AqpZ-DOPC

RO

4.1

RNaCl ¼ 97.2%

500 mM NaCl @ 5 bar, Area: 34.2 10 mM NaCl @ 10 bar, Area: 42

Rejection

Major findings

Ref.

DOTAP coated NF 270, with both decreased water flux and RNaCl compared to virgin membranes Silanized GA substrate, high water permeability with low NaCl rejection, the amount of AqpZ has huge impact on membrane performance Gold-coated porous alumina substrate crosslinked with disulfide: high water permeability with less defects AQP containing lipid bilayers deposited on PSS/PEI/PAN substrate Gold- and cysteamine-coated polycarbonate with UV crosslinking

[18]

[19]

[24]

[22] [25]

AqpZ-DOPC/DOTAP coated on PDAmodified porous polysulfone substrate via amidation reaction to form covalent bonds

[23]

AqpZ containing vesicles incorporated in PA layer serving as protection layer via IP. Large membrane area can be obtained Vesicles embedded in PA rejection layer with superior water flux Vesicles embedded in PA rejection for a long-term stability test

[28]

[29] [27]

Nanocomposite membranes for water and gas separation

Table 9.4 Summary of AQP-based biomimetic membranes and their performance [2]

NF

6

RMgCl2 ¼ 96%

200 ppm MgCl2 @ 4 bar, Area: 0.785

NF

36.6

RMgCl2 ¼ 95%

AqpZ-ABA

NF/ FO

NF: 22.9 FO water flux ¼ 5.6 L m
2 h
1

RNaCl ¼ 61% RMgCl2 ¼ 75% FO: RNaCl ¼ 50.7%

AqpZ-ABA

FO

FO water flux ¼ 43.5 L m
2 h
1

FO salt flux ¼ 8.9 g m
2 h
1

100 ppm MgCl2 @ 1 bar, Area: 28.3 200 ppm salt @5 bar 0.3 M sucrose as DS and 200 ppm NaCl as FS Area: 0.196 0.5 M NaCl as DS, DI water as FS, Area 0.196

AqpZ-POPCg/ POPGh/ cholesterol

FO

FO water flux ¼ 21.8 L m
2 h
1

FO salt flux ¼ 2.4 g m
2 h
1

0.3 M sucrose as DS, 200 ppm MgCl2 as FS, Area 0.785

a

Vesicles embedded in PSS/PAA LBLf. Membranes with AqpZ showed higher water permeability compared to control PDA-coated vesicles incorporated in crosslinked PEI matrix AqpZ-vesicle-loaded membrane crosslinked by UV

Pressure-assisted sorption, further coated with cysteamine and crosslinked by polydopamine-histidine. The control membrane has FO water flux of 8.6 L m
2 h
1 and salt flux of 6.6 g m
2 h
1 Magnetic-assisted AQPs embedded membranes

[36]

[37] [38]

[35]

[39]

DOPC: 1,2-dioleoyl-sn-glycero-3-phosphocholine. ABA: methacrylate and functionalized poly(2-methyloxazolineb-dimethylsiloxane-b-2-methyloxazoline) PMOXA(1000)-b-PDMS(4000)-PMOXA(1000) triblock. c DOTAP: 1,2-dioleoyl-3-trimethylammonium-propane. d DS: draw solution. e FS: feed solution. f LBL: layer-by-layer deposition of polyacrylic acid (PAA) and polystyrene sulfonate (PSS). g POPC: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. h POPG: 1-palmitoyl-1-oleoyl-sn-glycero-3-phospho-(19-rac-glycerol). b

Development of nanocomposite membranes by biomimicking nanomaterials

AqpZ-POPC/ POPG/ cholesterol AqpZ-DOPC

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Nanocomposite membranes for water and gas separation

9.4.1 Fabrication of AQP-based PRL membranes In 2010, Kufman et al. [10] coated liposome (lipid vesicle without AQP) on a commercial NF membrane as a substrate to fabricate a biomimetic lipid bilayer membrane. In this study, the vesicle rupture approach was adopted to break vesicles into a plannar lipid bilayer. Li et al. [18] utilized vesicle fusion, facilitated by hydraulic pressure on a commercial NF membrane with positively charged spin-coated lipids. The spin-coated lipid layer decreased the overall permeability of the fabricated membrane and the presence of uncovered regions resulted in low salt rejection. Later, Zhong et al. [19] fabricated a biomimetic membrane with an ABA-block copolymer, including AqpZ on a cellulose acetate (CA) substrate, functionalized with methacrylate group using UV polymerization. The optimized membrane had water permeability of 34 Lm
2 h
1 bar
1 and NaCl rejection of 30% at AqpZ-to-polymer ratio of 1:50. Although the prepared membranes delivered superb water flux, the synthesis technique requires significant improvement to produce large-scale biomimetic membranes with high salt rejection capability. In recent years, further attempts have been made to minimize the defects and improve membrane permeation properties [20–22]. Ding et al. [23] made further modifications by forming a polydopamine (PDA) layer on the polysulfone (PSf ) substrate followed by an amidation reaction between carboxyl group of PDA and amino groups of AqpZ-DOPE bilayer in the presence of EDC/S-NHS as the catalyst (Fig. 9.5). The forward osmosis (FO) experiments showed maximum water flux of 19.2 LMH and minimum reverse salt flux of 3.2 g
2 h
1. Furthermore, in the case of blending a DOPE bilayer with a

Fig. 9.5 Schematic showing the biomimetic membrane modified by covalent bonding between amino groups in AqpZ-embedded in DOPE bilayer and carboxylic groups of PDA on polysulfone substrate [23].

Development of nanocomposite membranes by biomimicking nanomaterials

Fig. 9.6 Fabrication techniques for AQP-based PRL biomimetic membrane: (A) preparation of proteopolymersomes by mixing ABA triblock copolymers with AqpZ solubilized in n-Dodecyl-βD-maltopyranoside (DDM) detergent, (B) Functionalization of the PCTE substrate, (C) the rupture of proteopolymersomes on the substrate, and (D) the immobilization of proteopolymersomes through UV polymerization and covalent bonds between the methacrylate head groups of triblock copolymer and acrylate residues on the substrate [25].

positively charged DOTAP, a higher water flux of 23.1 LMH with a lower reverse salt flux of 3.1 g
2 h
1 was achieved. Duong et al. [24] used disulfide-functionalized, ABA-triblock copolymer on a goldcoated porous alumina substrate. Water flux of 16.1 LMHbar
1 and 45% salt rejection were obtained, which revealed the presence of some defects in the synthesized membranes. Wang et al. [25] synthesized an AQP-based PRL membrane via proteopolymersome (AqpZ with ABA triblock copolymer) rupture on a tracked-etched polycarbonate (PC) substrate coated with a layer of gold, as illustrated in Fig. 9.6. The membrane reported in this study showed promising results in an FO process: water flux of 16.4 Lm
2 h
1 and NaCl rejection of 98.8% for 1:100 AqpZ-ABA-triblock-copolymer ratio were obtained. The significant challenges in the fabrication of AQP-based PRL membranes are minimizing defects, improving water flux, enhancing mechanical robustness and, and scaling up for industrial applications.

9.4.2 Fabrication of AQP-based TFN membranes A significant drawback of PRL membranes, compared to the TFN membranes, is the fragile structure of the thin-film selective layer and the defects on the substrate formed during the bilayer formation process. To fabricate a mechanically robust and defect-free

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AQP-based membrane, entrapment of protein in a dense polymer layer (e.g., PA) is proposed, which results in high separation and preserves the original properties of proteins [2, 26, 27]. There are various methods for the immobilization of AQP-incorporating vesicles in the dense PA layer, such as UV polymerization, polymer crosslinking, interfacial polymerization, and surface imprinting. Furthermore, AQP-containing vesicles can be sealed by a polymer that protects them from environmental degradation [11]. The first attempt at synthesizing AQP-based TFN membranes was carried out by Zhao et al. [28], who introduced AQP-incorporated vesicles in a TFC membrane by using a modified interfacial polymerization method. They first soaked a PSf substrate in an aqueous mixture of m-phenylenediamine (MPD) and AqpZ-based proteoliposomes. Subsequently, the substrate was exposed to the trimesoyl chloride (TMC) in an organic phase to form a crosslinked PA layer with embedded proteoliposomes (Fig. 9.7.). This membrane was tested for filtration of 10 mM NaCl solution using a crossflow reverse osmosis (RO) setup. The results demonstrated that the synthesized membranes could withstand transmembranes pressures up to the 10 bar and were able to provide a water permeability and NaCl rejection as high as 4 Lm
2 h
1 bar
1 and 97%, respectively, under 5 bar pressure. Recently, Li et al. [29] utilized polyethersulfone (PES) hollow fiber substrate to prepare AqpZ-containing proteoliposomes hollow fiber biomimetic membranes (Fig. 9.8.). In this research, the ultrafiltration (UF) hollow-fiber substrate was first soaked in an aqueous MPD containing proteoliposomes, then it was exposed to the TMC solution. The RO tests provided a high water flux of 40 LMH at 5 bar and 97% salt rejection. To preserve proteoliposomes, Zhao et al. [30] coated an additional polymeric layer on AQP-based TFN membranes. This study showed that the lipid type in proteoliposomes and the ratio of protein to lipids had a significant effect on the permeation properties of

Fig. 9.7 Schematic diagram of the interfacial polymerization synthesis of AQP-based TFN: (A) conceptual model of the water transport mechanism in this membrane and (B) formation of TFN membrane by interfacial polymerization [28].

Development of nanocomposite membranes by biomimicking nanomaterials

AqpZ Interfacial polymerization

Lipid Polyamide layer

Proteoliposome

Fig. 9.8 Schematic presentation of the fabrication of AQP-based biomimetic membrane [29].

proteoliposomes. What’s more, the incorporation of cholesterol in 1,2-dioleoyl-snglycero-3-phosphocholine (DOPC) bilayer reduced the defects in the membrane and led to a higher water permeability. They attributed this result to the probable formation of ring-ring stacking between the rings of cholesterol and the aromatic residues on the AQP0 surface, which resulted in more stability of the biomimetic membrane. Since cholesterol has an inhibiting effect on the formation of pores and improves the bilayer properties, it was also used by other researchers [31, 32]. Sun et al. [33] prepared biomimetic membranes with AqpZ-incorporated vesicles on polyacrylonitrile (PAN) substrate coated with polydopamine (PDA). Then amine-functionalized proteoliposomes were stabilized on the surface using UV polymerization method. Then proteoliposomes were immobilized on the PDA layer by the amine-catechol bonding. In order to enhance membrane stability, further crosslinking was conducted by applying glutaraldehyde (GA) as a catalyst (Fig. 9.9). The water flux of membrane with AqpZ-to-lipid ratio of 1:100 increased by 65% compared to that of pristine membranes. Moreover, the NaCl and MgCl2 rejection was 66.2% and 88.1%, respectively. To overcome the shortcomings of interfacial polymerization method, such as an excessive amount of chemical usage, Li et al. [34] applied polymer crosslinking to fabricate a highly permselective AqpZ-based membrane. The AqpZ-incorporated

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Nanocomposite membranes for water and gas separation

Fig. 9.9 Schematic showing the immobilization of crosslinked proteoliposomes on a PDA-coated membrane [33]. PDA coating

Deposition

Proteoliposome

PAI membrane Aquaporin Z Branched PEI

Crosslinking

Fig. 9.10 Schematic presentation of AQP-based TFN membrane with the PDA functionalized proteoliposomes on a PA substrate and crosslinked with PEI [34].

liposomes were initially decorated with PDA, which is a biocompatible polymer to improve the affinity of proteoliposomes to the substrate via the formation of noncovalent and covalent bonds. Then, the functionalized proteoliposomes were immobilized on a poly (amide-imide) (PAI) substrate followed by crosslinking via a polyelectrolyte (Fig. 9.10.). The optimized membrane showed water flux of 36.6 Lm
2 h
1 with a rejection of 95% for MgCl2 at 0.1 MPa. Polymersomes prepared by ABA triblock copolymers were found to have a relatively higher stability compared to those of formed by phospholipids, which prevented vesicles from denaturing during fabrication and filtration processes. Inspired by this, Wang et al. [35] prepared proteopolymersomes, which was later crosslinked with disulfide anchors and immobilized on a gold-coated track-etched PC substrate via covalent bonding. Subsequently, the structure was stabilized with PDA-histidine (His) to seal the defects between the pores and surface of the vesicles. The resultant membrane was tested in

Development of nanocomposite membranes by biomimicking nanomaterials

Critical pressure

ABA block copolymer vesicle

(B) Cysteamine SAM

(C) Exposed region DDM

PDA-histidine coating

AqpZ Disulfide end group

(A)

(D)

Intruded region

Fig. 9.11 Schematic diagram of AQP-based TFN membrane: (A) formation of proteopolymersome from AQP Z and ABA triblock copolymer, (B) pressure-assisted proteopolymersome adsorption and immobilization on the gold-coated substrate, (C) self-assembled monolayer of cysteamine on the coated substrate through chemisorption, and (D) layer-by-layer coating with PDA-histidine (His) [35].

an FO process and showed a water flux of 17.6 LMH and salt rejection of 91.8%, using 6000 ppm NaCl solution as feed and 0.8 M sucrose as the draw solution (Fig. 9.11.). Table 9.3 provides a summary of studies that have been done in the past decade on AQP-based biomimetic membranes [2]. It is worth noting that, although the TFN membranes benefit from higher mechanical and chemical stability, their dense active layer reduces water permeability significantly. On the other hand, the PRL membranes typically provide higher water flux by taking full advantage of highly permeable aquaporin water channels.

9.5 Future work Inspiration from nature to synthesize high water-permeability membrane has led to the development of nanocomposite membranes by biomimicking materials. This field is rapidly advancing, and these membranes have a considerable potential to be utilized as alternatives for the current membranes. However, these membranes still face some technical

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challenges, which require more research for their successful deployment and commercialization. The major challenges are as follows: (1) Methods of expression and purification of membrane proteins, which are the hearts of these biomimetic membranes, are elaborate, time consuming, and costly. (2) During the fabrication and filtration process, when they are exposed to high saline feed solution like seawater or toxic materials in feed solution, these membranes are not stable or durable. (3) Since large quantities of AQP are needed for the fabrication of large-scale membranes, they are expensive. (4) Poor biological and hydrophobic compatibility of the host membrane with the aquaporin leads to the formation of defective membranes. (5) Due to their lower physicochemical stability, biomimetic membranes are difficult to clean using chemical and mechanical methods. Introducing a new technology is always fraught with challenges. Hence, the need to conduct more research and carry out extensive tests to overcome the problems mentioned here and move to widespread commercial production.

References [1] J.R. Werber, M. Elimelech, Permselectivity limits of biomimetic desalination membranes, Sci. Adv. 4 (2018), eaar8266. https://doi.org/10.1126/sciadv.aar8266. [2] Z. Yang, X.-H. Ma, C.Y. Tang, Recent development of novel membranes for desalination, Desalination 434 (2018) 37–59, https://doi.org/10.1016/J.DESAL.2017.11.046. [3] B. Khorshidi, T. Thundat, D. Pernitsky, M. Sadrzadeh, A parametric study on the synergistic impacts of chemical additives on permeation properties of thin film composite polyamide membrane, J. Membr. Sci. 535 (2017) 248–257, https://doi.org/10.1016/J.MEMSCI.2017.04.052. [4] K.P. Lee, T.C. Arnot, D. Mattia, A review of reverse osmosis membrane materials for desalination— Development to date and future potential, J. Membr. Sci. 370 (2011) 1–22, https://doi.org/10.1016/ J.MEMSCI.2010.12.036. [5] B. Khorshidi, I. Biswas, T. Ghosh, T. Thundat, M. Sadrzadeh, Robust fabrication of thin film polyamide-TiO2 nanocomposite membranes with enhanced thermal stability and anti-biofouling propensity, Sci. Rep. 8 (2018) 784, https://doi.org/10.1038/s41598-017-18724-w. [6] P. Agre, Aquaporin water channels, Biosci. Rep. 24 (2004) 127–163, https://doi.org/10.1007/ s10540-005-2577-2. [7] Y. Kaufman, V. Freger, Supported biomimetic membranes for pressure-driven water purification, in: On Biomimetics, InTech, 2011. https://doi.org/10.5772/19770. [8] D. Ruiz-Molina, F. Novio, C. Roscini, Bio- and Bioinspired Nanomaterials, Wiley, 2014. [9] Z. Xia, Biomimetic Principles and Design of Advanced Engineering Materials, n.d. [10] Y. Kaufman, A. Berman, V. Freger, Supported lipid bilayer membranes for water purification by reverse osmosis, Langmuir 26 (2010) 7388–7395, https://doi.org/10.1021/la904411b. [11] A. Giwa, S.W. Hasan, A. Yousuf, S. Chakraborty, D.J. Johnson, N. Hilal, Biomimetic membranes: A critical review of recent progress, Desalination 420 (2017) 403–424, https://doi.org/10.1016/ J.DESAL.2017.06.025. [12] J.G. Betts, P. DeSaix, E. Johnson, J.E. Johnson, O. Korol, D.H. Kruse, B. Poe, J.A. Wise, M. Womble, K.A. Young, Anatomy and Physiology, n.d. https://redshelf.com/book/18312/anatomyand-physiology-18312-9781947172043-j-gordon-betts-peter-desaix-eddie-johnson-jody-e-

Development of nanocomposite membranes by biomimicking nanomaterials

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

[29]

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[30] Y. Zhao, A. Vararattanavech, X. Li, C. HelixNielsen, T. Vissing, J. Torres, R. Wang, A.G. Fane, C.Y. Tang, Effects of Proteoliposome composition and draw solution types on separation performance of aquaporin-based Proteoliposomes: Implications for seawater desalination using aquaporin-based biomimetic membranes, Environ. Sci. Technol. (2013), 130111084054009. https://doi.org/10.1021/ es304306t. [31] Z. Chen, R.P. Rand, The influence of cholesterol on phospholipid membrane curvature and bending elasticity, Biophys. J. 73 (1997) 267–276, https://doi.org/10.1016/S0006-3495(97)78067-6. [32] H.G.L. Coster, The physics of cell membranes, J. Biol. Phys. 29 (2003) 363–399, https://doi.org/ 10.1023/A:1027362704125. [33] G. Sun, T.-S. Chung, K. Jeyaseelan, A. Armugam, Stabilization and immobilization of aquaporin reconstituted lipid vesicles for water purification, Colloids Surf. B 102 (2013) 466–471, https://doi. org/10.1016/J.COLSURFB.2012.08.009. [34] X. Li, R. Wang, F. Wicaksana, C. Tang, J. Torres, A.G. Fane, Preparation of high performance nanofiltration (NF) membranes incorporated with aquaporin Z, J. Membr. Sci. 450 (2014) 181–188, https://doi.org/10.1016/J.MEMSCI.2013.09.007. [35] H.L. Wang, T.-S. Chung, Y.W. Tong, K. Jeyaseelan, A. Armugam, H.H.P. Duong, F. Fu, H. Seah, J. Yang, M. Hong, Mechanically robust and highly permeable AquaporinZ biomimetic membranes, J. Membr. Sci. 434 (2013) 130–136, https://doi.org/10.1016/J.MEMSCI.2013.01.031. [36] G. Sun, T.-S. Chung, K. Jeyaseelan, A. Armugam, A layer-by-layer self-assembly approach to developing an aquaporin-embedded mixed matrix membrane, RSC Adv. 3 (2013) 473–481, https://doi. org/10.1039/C2RA21767H. [37] C.-G. Li, X. Qi, J. Guo, Dimensionality Reduction by Low-Rank Embedding, in: Springer, Berlin, Heidelberg, 2013: pp. 181–188. https://doi.org/10.1007/978-3-642-36669-7_23. [38] W. Xie, F. He, B. Wang, T.-S. Chung, K. Jeyaseelan, A. Armugam, Y.W. Tong, An aquaporin-based vesicle-embedded polymeric membrane for low energy water filtration, J. Mater. Chem. A 1 (2013) 7592, https://doi.org/10.1039/c3ta10731k. [39] G. Sun, T.-S. Chung, N. Chen, X. Lu, Q. Zhao, Highly permeable aquaporin-embedded biomimetic membranes featuring a magnetic-aided approach, RSC Adv. 3 (2013) 9178, https://doi.org/10.1039/ c3ra40608c.

CHAPTER 10

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes Zulfiqar Ahmad Rehana, Muhammad Zahidb, Saba Akramb, Anum Rashidb, Abdul Rehmana a Department of Polymer Engineering, National Textile University, Faisalabad, Pakistan Department of Chemistry, University of Agriculture, Faisalabad, Pakistan

b

10.1 Introduction Water is a basic need for the sustenance of life, and its purification and availability are currently a major concern. Growth in world’s population and drastic climate change has caused the current worldwide water situation. It is expected that the world’s population will increase to 6.3 billion by 2050, which will increase the need for agricultural products significantly. Industrialization also raises the consumption rate of water [1]. Hence, the demand for fresh water, particularly for drinking purposes and food production, is growing dramatically, and each year, 70% of worlds’ fresh water is accounted already for agricultural irrigation. Although the Earth is covered mostly by water, only a fraction of that water is accessible for human beings, and there are different regions in the world that still lack access to safe and pure water. Therefore, a proper strategy for resolving the water issue is essential. Membrane technology, due to its compelling economic and environmental advantages, is a reliable choice for water treatment. Membrane technology has been extensively used in water treatment processes such as municipal and industrial wastewater purification, seawater desalination, ultrapure water production, and water softening. Compared with conventional processes such as adsorption, oxidation, chemical coagulation, and cryogenic distillation, it offers various advantageous features like high product quality, less footprint, easy operation and control, along with less capital cost and easy maintenance [2]. Pressure-driven membrane processes use pressure difference as a driving force between the feed and permeate sides for solvent transport (usually water) through the membrane. Based on properties like charge, shape, and size, the dissolved components and particles are retained partially. Pressure-driven membrane processes, which are used in membrane technology for water treatment, are ultrafiltration (UF), microfiltration

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(MF), nanofiltration (NF), and reverse osmosis (RO). These processes are distinguished on the basis of pore size. MF, UF, NF, and RO membrane processes have been applied successfully for the removal of turbidity, colloidal particles, microorganisms, and dissolved organic matter (DOM). According to a global market report, RO is the most widely used membrane process, accounting for approximately 45%, where NF and UF combined account for 20%, and MF accounts for 30% of membrane technology market. The membranes employed for water treatment are mostly polymeric membranes. The major challenge of polymeric membranes is their susceptibility to fouling and biofouling. Additionally, polymeric membranes have less chemical and mechanical stability than inorganic membranes such as ceramic membranes. A new class of membranes, nanocomposite membranes, are promising solutions for such challenges [3]. Nanocomposite membranes are fabricated via a combination of nanomaterials and polymeric materials (matrix) to meet specific applications of water treatment by tuning their physiochemical properties and structure [2].

10.2 Nanocomposite membranes Nanocomposite membranes are a novel group of filtration materials composed of nanofillers within the matrix of polymers. Choosing materials for specific separation processes is a complex matter. In general, the important requisites are mechanical integrity at operational conditions and durability and separation and production efficiency. The general idea is to induce the functional properties of nanofillers to the polymer. For instance, metal oxide nanoparticles (e.g., TiO2 and Al2O3) increase the thermal and mechanical stability along with permeation flux of polymer membranes [4–7]. Similarly, incorporating zeolites improves the membrane’s hydrophilic character and in turn enhances its water permeability. Nanomaterials that are photocatalytic (e.g. TiO2 and bimetallic nanoparticles) and antimicrobial (e.g., CNTs and nanosilver) are used mainly to increase fouling resistance [8].

10.3 Nanofillers in pressure-driven membrane processes Among the latest applications of nanoparticles is their incorporation into polymeric membranes to enhance the performances of membranes like selectivity, permeability, hydrophilicity, and strength. The unique physical and chemical properties of nanomaterials have increased the interest of studying the synthesis of nanoparticles for specific magnetic, optical, catalytic, and electronic purposes. The incorporation of various nanofillers within polymer membranes provides several advantages like hydrophilicity enhancement, suppressing of foulants and pollutant accumulation, improvements in thermal and mechanical properties, and increase in removal efficiency of membranes.

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

The nanofillers commonly employed in nanocomposite membranes are metallic nanoparticles, such as titania, silica, ceramics, carbon nanotubes, carbon molecular sieves, metal-organic framework (MOF), and zeolites. Cellulose acetate (CA), polysulfone (PSf), polyethersulfone (PES) polyesters, polyphenylene oxide (PPO), polyetherimide (PEI), and polyimide (PI) were commonly utilized as polymer materials.

10.3.1 Mineral nanofillers Inorganic materials, such as zirconia, silica, zeolites, and alumina, were mostly utilized as fillers to improve membrane performance in terms of mechanical, chemical, and thermal stabilities, as well as permeation properties (permeate flux and rejection of contaminants). However, since the particle’s size is within the micrometer range, the resultant membrane application was limited to UF and MF, with pore sizes of ca. 0.01 and c. 0.1 μm, respectively. However, with the development of materials with at least one dimension in the nanometer range, the scope of inorganic materials was extended not only to NF but also to RO membranes. Most inorganic nanoparticles utilized in the modification of membranes are either metals or metallic oxides with some exceptions to NaA-type zeolites, hydrous manganese dioxide, CaCO3, and Mg(OH)2 reported in recent years [9]. Table 10.1 presents a summary of the application of inorganic nanomaterials (minerals, metals, and metal oxides) for the fabrication of MF, UF, NF, and RO nanocomposite membranes. 10.3.1.1 Zeolites Zeolites are crystalline porous aluminosilicates with definite channel structure and molecular dimensions. Various monovalent and divalent cations like Mg2+, K+, Ca2+, and Na+ can be exchanged readily in a contact solution. Zeolites have gained attention as potent membrane materials for both liquid and gas processes owing to their capability for molecular sieving that can separate molecules precisely based on size. Different classes of zeolite membranes have been fabricated for desalinations and water purification. These membranes exhibit few exceptional properties that can’t be met by conventional polymer membranes. Some of the properties consist of tunable pore sizes, low-fouling tendency, and chemical and thermal stability [9]. Dong et al. [26] reported a novel approach for the preparation of NF thin-film nanocomposite (TFN) membranes utilizing polysulfone (PS) support embedded with nanoparticles of zeolites. Such new membranes presented up to 49% nanoparticle coverage ratio, with 93.4% salt rejection and higher water permeability. The first commercial application of TFN membranes was accomplished by NanoH2O Inc., utilizing zeolites as nanofillers. It was reported that the developed membranes had twofold higher water flux than the conventional polyamide (PA) thin-film composite (TFC) membranes and the salt rejection was 99.7% [27].

239

Table 10.1 Role of inorganic nanofillers in nanocomposite membranes for water treatment.

Polymer

1

Silver (Ag)

PES

MF

2

SiO2

PVA

MF

3

TiO2

MF

4

Iron oxide

5

ZnO

PDA/ PEI PVC + CA PVDF

6

8

Iron oxide (FeO) Cerium oxide (CeO2) Copper (Cu)

9

Flux/ permeability

11,017 L/ m2 h bar 1711 L/m2 h

NF

5720  207 L/ m2 h 75 L/m2 h

MF

465 L/m2 h

PES

NF

36.85 L/m2 h

PA

NF

41.28 L/m2 h

PES

UF

249 L/m2 h

Ni

PVDF

UF

6.8 L/m2 h/ bar

10

TiO2, Al2O3, ZrO2

PES

UF

208.9 L/m2 h

11

SiO2-NH2

PSF

UF

3900 L/m2 h

12

CuO/ZnO

PES

UF

679.0 kg/m2 h

13

Zeolite

PSF

UF

–

14

Zinc oxide (ZnO) Titanium dioxide (TiO2) Silica, zeolite

PA

RO

48.9 L/m2 h

CA

RO

1.36 L/m2 h

PSF

RO

–

7

15

16

Performance

Reference

• High antibacterial activity

[10]

• • • •

[11]

• • • • • • • • • • • • • • • • • • • • • • • • • •

Higher antifouling property 1.69 MPa young modulus 8% direct red 23 rejection High flux recovery and rejection of BSA High water permeability High removal of lead More hydrophilicity High antifouling activity High water flux, more antifouling behavior, good salt rejection More hydrophilicity 99% salt rejection 86.3% Rejection ratio Good antibiofouling property 94% Flux recovery High antibacterial activity High permeance More water flux High fouling resistance More hydrophilicity More water permeation, more hydrophilicity, high protein rejection Improved water permeability More hydrophilicity 95% BSA rejection Improved permeability High selectivity Improved antifouling behavior 98.3% Salt rejection More biofouling resistance 94.5% Salt rejection More hydrophilicity

• High water flux • High mechanical stability

[12] [13] [14] [15] [16] [17] [18]

[19]

[20] [21]

[22]

[23] [24]

[25]

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Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

10.3.1.2 Silica (SiO2) Silica belongs to another conventional inorganic filler class, which has gained significant attention for the development of mixed matrix membranes (MMMs). It can be categorized into ordered mesoporous and nonporous silica. Silica fillers are introduced generally into polymer matrices via sol-gel reactions to create inorganic oxide particles at nanoscale within polymeric matrix [9]. Huang et al. [28] prepared PES nanocomposite membranes using mesoporous silica and observed increase in the pore size of sublayer and also improvement in pores interconnectivity between the bottom and sublayer. In particular, the mesoporous particles improved the membrane hydrophilicity displaying a 180.2 L/m2 h flux and 96.1% bovine serum albumin (BSA) rejection at a filler loading of 2 wt%. Kebria et al. [29] prepared polyetherimide (PEI) thin-film NF membrane loaded with SiO2 for the removal of dyes from organic and aqueous solutions. Two different terephthaloyl chloride (TPC) concentrations (0.1 and 0.5 wt%) and PEI were used for the interfacial polymerization in different SiO2 concentrations (0.1, 0.05, and 0.03 wt%). 13.3 L/m2 h higher flux and 100% rejection of cationic dye crystal violet were obtained using TPC (0.1 wt%) and SiO2 nanoparticles (0.1 wt%).

10.3.2 Metal and metal oxide The main advantage of using metal oxide nanoparticles is their low cost, facile synthesis method, and significant improvement in membrane hydrophilicity. Incorporation of these nanomaterials into the matrix would lead to enhanced permeability of the membrane without losing selectivity [30]. 10.3.2.1 Titanium dioxide (TiO2) Titanium dioxide (TiO2) is a photocatalytic particle with applications in different fields, as food color additive, disinfection agent, flavor enhancer, and for the decomposition of organic substances. Titanium dioxide is relatively cost-effective compared with other nanomaterials, is less toxic to humans, and displays high chemical and thermal stability. Moreover, due to the photocatalytic characteristics, TiO2 also exhibits water disinfection and antibiofouling properties. One of the great advantages of this nanomaterial is that it has an endless lifetime and remains unchanged during the degradation process of microorganisms. It is a promising candidate in membrane processes due to its high oxidation power and photo-induced hydrophilicity. The development of self-cleaning membranes can be a way to minimize the fouling problem and maintain water permeation through the membrane [31]. Shi et al. [32] demonstrated that the incorporation of titanium dioxide in the PVDF membrane improved the water flux, tensile strength, porosity, and hydrophilicity of the membrane. Bae et al. [33] fabricated antifouling nanocomposite membranes of TiO2/sulfonated PES. The self-assembly was done through interactions between sulfonic acid groups of

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sulfonated PES and TiO2 nanoparticles. TiO2-based nanocomposite membranes showed cake layer resistance of 33.27  1011 m 1, whereas polymeric membrane showed cake resistance of 58.7  1011 m 1. It indicates a significant decrease in membrane fouling using the nanocomposite membranes. Abedini et al. [34] fabricated TiO2-/CA-based membranes having different loading ranging from 5 to 25 wt%. The synthesis of nanoparticles was done in situ by sonochemical method. Experiments showed an increase in flux with increase in concentration of nanoparticles up to 20 wt%. The highest flux attained was 57.42 L/m2 h. Nevertheless, TiO2 nanoparticles undergo agglomeration at the top layer of membrane at 25 wt%, leading to the decrease in water flux. Mollahosseini and Rahimpour [35] coated PSf membranes with layer of TiO2 for enhancing its antifouling property. The prepared membrane was thicker and smoother with decreased tendency of fouling toward BSA. 10.3.2.2 Silver (Ag) Silver (Ag) is known as an antibacterial agent. Silver-based nanocomposite membranes offer antimicrobial properties, thus giving them the potential for various applications including water treatment [36]. Owing to potent biocidal properties of silver, it is the most explored antimicrobial agent for the development of nanocomposite membranes. Both nanoparticles and ions of silver have been studied for a wide range of water treatment processes, including the development of nanocomposite membranes [36]. In a study, Rehan et al. [37]. fabricated silver-based nanocomposite membranes. They reported probable interaction between SiO2 functional groups belonging to PES and Ag nanoparticles. This interaction resulted in prevention of Ag NP release in the permeate. Furthermore, the pure and Ag nanocomposite membranes were utilized for the filtration of urban wastewater (having plenty of bacteria) and seawater (deprived of bacteria). The combination of Ag nanoparticles with PES resulted in reduced flux (normalized flux J/J0) with decline from 71% to 64% in case of seawater and 74% to 60.32% in case of urban wastewater treatment. Antibiofouling and antibacterial activities were assessed against Escherichia coli and Pseudomonas aeruginosa. Increase in Ag nanoparticle concentration resulted in an enlarged inhibition zone, and Ag/PES nanocomposite membranes restricted the formation of biofilm. Vatanpour et al. [38] incorporated Ag nanoparticles with rGO into PES. The equilibrium contact angle decreased significantly by the addition of rGO/Ag. An increase in water flux from 220 to 429.8 kg/m2 h with loading of 0.2 wt% of rGO/Ag nanofillers was observed. rGO/Ag also induced an effective antifouling activity. The pristine PES film was prone to irretrievable fouling with fouling ratio of 51.7%, causing about 88% of total fouling. Adding 0.2 wt% of rGO/Ag nanofillers has decreased the ratio of irreversible fouling significantly to 32.8%. Biswas and Bandyopadhyaya [39] reported the surface modification of PES membranes utilizing silver nanoparticles. Sulfonation of PES membranes was done by

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

concentrated H2SO4 for generation of dSO3H groups on the membrane surface, which gave H+ and SO3 ions upon dissociation. By adding AgNO3, H+ ions were replaced with Ag+ ions, and the resulting Ag/PES nanocomposite membranes showed constant water flow rate of 3.45 L/h, owing to complete killing of E. coli. 10.3.2.3 Zinc oxide (ZnO) Zinc oxide (ZnO) is an inorganic nanoparticle with multifunctionalities like bactericide, antibacterial, and catalytic activity. Hong et al. [40] fabricated ZnO-based MF membranes by phase inversion method. The resultant PVDF membranes exhibited improved properties such as hydrophilicity, good BSA rejection, more water flux, and less surface roughness. In another study, Khan et al. [41] developed ZnO-based antibacterial cellulose acetate membranes. The hybrid membranes provided higher water flux and selectivity, as well as antibacterial activity. Researcher reported that ZnO in nanocomposite membranes displayed bactericidal mechanism in which highly reactive species containing oxygen (H2O2, O2, and OHd) are produced, triggering fatal destruction to the bacteria. The H2O2 generation was reported as a principal effect contributing to the bactericidal activity, which is the result of H2O2 penetration into cell walls. ZnO has an imperative role in the antibacterial activity due to its morphology and size. ZnO nanoparticles with higher surface area typically display high antibacterial activity owing to the generation of more reactive oxygen radicals and H2O2. Reactive oxygen species and H2O2 have thus inhibited bacterial growth through cell wall penetration. Balta et al. [42] fabricated PES/ZnO UF nanocomposite membranes with lower flux decline, higher potential for dye rejection, and better permeability in comparison with pure PES membrane, owing to increased hydrophilicity of ZnO-based membranes. Liang et al. [43] reported that the nanocomposite PVDF membranes filled with nanoparticles of ZnO displayed 100% recovery in water permeability after cleaning of membrane and thus showed a minimum irreversible fouling. 10.3.2.4 Copper (Cu) Copper metal and its compounds possess fungicidal and bactericidal effects against different algae, viruses, and microorganisms. The working mechanism of copper ions is not yet understood, but there are some hypotheses. For example, copper nanoparticles can interact with bacteria leading to the generation of reactive oxygen species, protein oxidation, lipid peroxidation, generation of superoxide ions, and DNA degradation [44]. In a recent study, copper oxide (CuO)-based UF membranes were fabricated to improve the antifouling properties of PES membranes. The resultant membranes possessed water flux of about 870 kg/m2 h and more hydrophilicity because of the presence of hydroxyl (OH) groups on the membrane surface [45].

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Ben-Sasson et al. [46] followed in situ procedure for biocidal Cu nanoparticles loading within polyamide composite thin-film membrane for reverse osmosis application. The membranes revealed a significant increase in water permeation after in situ modification with Cu nanoparticles such as 2.97 and 2.53 L/m2 h bar for Cu nanoparticles and pristine membranes, respectively. Conversely, there is slight decrease in salt rejection (98.31%) for modified membrane by Cu nanoparticles, while the number of bacteria attached decreased as compared with pure membrane. Zhang et al. [47] prepared Cubased nanocomposite membranes and obtained higher antibacterial efficiency, salt rejection, and water flux. Membrane was fabricated using glutaraldehyde as cross-linking agent, carboxylated Cu nanoparticles, and chitosan as polymer. As suggested by authors, their method provided cost-effective and simple way to improve both antifouling (protein) and antibacterial (long-lasting) performance.

10.3.3 Carbon-based nanofillers Table 10.2 presents a summary of the application of carbon-based nanomaterials for the fabrication of MF, UF, NF, and RO nanocomposite membranes. Carbon nanotubes (CNTs), graphene, and graphene oxide have been widely used for the development of water separation membranes. Nanocarbon materials have become very popular due to their outstanding physiochemical properties. Materials based on carbon are developed as nanoparticles, nanofibers, nanowires, nanotubes, nanorings, and fullerenes. Nanocarbon materials exhibit special advantages like high mass transfer properties, facile alteration of functionality, and induction of both hydrophobic and hydrophilic properties, as well as increased chemical stability [62]. 10.3.3.1 Carbon nanotubes (CNTs) CNTs are structurally composed of cylindrical sheets of graphite rolled up into tubelike structure with the nanometric diameter and lattice fencing appearance. Depending on the graphene shell layers, carbon nanotubes can be classified further into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs). The properties of CNTs are closely associated with their chirality (atomic arrangement), defects formation (morphology), and length and diameter of nanotubes. Due to their tunable features, the manipulation and rational design of CNT allow it to be used as catalyst, adsorbents, or membrane for wastewater treatment and desalination. Due to their ultrafast water flux, antimicrobial activity, high surface area, tunable pore size, electrical conductivity, and surface chemistry, CNTs have emerged as outstanding candidates in the field of membrane science, which has revolutionized the desalination and wastewater treatment processes. Generally, for water purification, CNTs can be configured into standalone CNT membranes or incorporated into a polymer matrix to overcome the flux/rejection trade-off observed in most of the conventional polymeric membranes. The special

Table 10.2 Role of carbon-based nanofillers in nanocomposite membranes for water treatment Polymer

Pressure-driven membrane process

1

CNT

PSF

MF

2

GO

PSF

MF

3

rGO/TiO2

PES

NF

45.0 kg/ m2 h

4

NH2-MWCNT

PA

NF

61.7 L/ m2 h

5

Carboxylated CNT

PES

NF

38.91 L/ m2 h

6

(rGO)/TiO2

PVDF

UF

7

Oxidized carbon nanotubes (OCNT)-GO

PVDF

UF

220 kg/ m2 h 203 L/ m2 h

8

GO-MWCNT

PVDF

NF

11.3 L/ m2 hbar

9

GO

PES

UF

340 L/ m2 h

• • •

10

Nanocarbon black

PSF

UF

307 L/ m2 h

• • • •

Water flux

Performance

Reference

0.6 L/ m2 hkP –

• More hydrophilicity

[48]

• • • • • • • • • •

[49]

• • • • • • • • •

Decrease in contact angle High dye rejection Good separation behavior Better permeability High dye rejection Good antifouling properties 95.72% rejection of NaCl salt High separation performance More hydrophilicity 660% higher water flux than PES membranes 87.25% Na2SO4 rejection Higher water permeability High antifouling properties More tensile strength Higher BSA rejection More hydrophilicity Higher water flux Higher rejection rate Good antifouling performance High rejection of humic acid Good antifouling properties Improvement of membrane reuseability High water flux Cost-effective More hydrophilicity 97.6% protein rejection

[50]

[51]

[52]

[53] [54]

[55]

[56]

[57]

Continued

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Nanomaterials

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

No.

246

No.

11

Nanomaterials

Activated carbon

Polymer

PES

Pressure-driven membrane process

UF

Water flux 2

9 L/m h

12

rGO/TiO2

PA

RO

51.3 L/ m2 h

13

MWCNT functionalized with H2SO4/HNO3

PA

RO

14

Graphene oxide (GO)

PA

RO

Up to 49 L/ m2 h –

Performance

Reference

• High hydrophilicity • Sulfate (95%) and Cu (97%)

[58]

• • • • • •

ions removal High tensile strength 99.4% salt rejection More hydrophilicity High water flux Rejection higher than 98% More hydrophilicity

• High rejection of Nnitrosodimethylamine (NDMA)

[59]

[60]

[61]

Nanocomposite membranes for water and gas separation

Table 10.2 Role of carbon-based nanofillers in nanocomposite membranes for water treatment—cont’d

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

interest for CNT membrane construction lies within the ability of dense and compact CNT networks to provide extraordinary water flux. Ideally, electrochemically active CNT-based membranes can remove proteins, salt, dyes, phenols, and viruses from water. Advanced nanotechnology takes control of CNT diameter that helps in the fabrication of carbon nanotube-coated RO membrane with high flux due to surface roughness and labile hydrophobicity [63]. Shen and coworkers [64] fabricated thin-film nanocomposite membranes with carbon nanotubes, which resulted in high water flux and increased the rejection rate of Na2SO4 about 99%. Zhang et al. [65] assessed polymer supported composite membranes of interlinked MWCNT-GO. These membranes were evaluated by treating the strontium-containing wastewater, which displayed higher flux values as compared with other NF membranes. Furthermore, these membranes rejected about 93% of EDTA-chelated Sr2+ from alkaline solution. Majeed et al. [66] prepared UF mixed matrix polyacylonitrile (PAN) membrane (MMNMs) through phase inversion process using MWCNTs. Based on MWCNT reinforcement properties, 36% resistance is shown by MMNMs toward membrane compaction leading to enhanced properties of transport in PAN membranes. Moreover, improvement in tensile strength of MMNMs was observed with MWCNTs addition in casting solution of PAN. Consequently, improved thermal/mechanical stability is provided by MWCNTs in MMNMs for applications at increased transmembrane pressures. In another study, Shawky et al. [67] fabricated composite membranes composed of PA polymer and MWCNT. It was observed that MWCNT incorporation into membrane enhanced both salt rejection efficiency and mechanical strength of nanocomposite membrane as compared with pristine PA membranes with slight decrease in permeation flux. 10.3.3.2 Graphene and graphene oxide (GO) Graphene is a promising material for water transport, due to its honeycomb lattice structure. It can be used as the backbone for a novel class of highly selective and permeable membrane materials for separation processes [44]. The use of graphene in RO membranes has made water desalination processes very successful. Nanographene was found to increase salt rejection from 33% to 100% [68]. The use of graphene in NF process for water purification allows effective elimination of organic dyes. For instance, graphenecoated PVDF showed high dye retentions (>99%). The graphene-based membranes showed different salt rejections with the performances from 20% to 60%, in the following order: Na2SO4 > NaCl > MgSO4 > MgCl2 [69]. Graphene oxide is obtained by oxidation of graphene. It possesses a hydrophilic nature, dissimilar to graphene that is mainly hydrophobic. GO can enhance the mechanical and thermal properties of polymeric membranes. Like most nanomaterials, the use of GO changes the nature of membranes from hydrophobic to hydrophilic profile, improving the performance permeability [44]. Chang et al. [70] reported the combined effect of

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PVP and GO on PVDF ultrafiltration membrane’s performance. The results predicted that the antifouling performance and hydrophilicity of the membranes were enhanced by adding PVP and GO. The authors found improved performance due to the establishment of hydrogen bonds between GO and PVP. Wu et al. [71] prepared PSf membrane with GO and SiO2 through silica nanoparticles deposition on graphene oxide nanosheets. The fabricated membrane showed promising antifouling behavior against protein (albumin) with 98% rate of rejection and excellent water flux.

10.4 Nanomaterials in microfiltration and ultrafiltration membrane process Microfiltration (MF) and ultrafiltration (UF) processes separate colloids and particles from aqueous media on the basis of particle capture or size exclusion. These processes are primarily used for the pretreatment of water, and thus, the risk of biofouling or fouling is higher than the posttreatment membrane processes such as RO. Microbial adhesion, adsorption of organics, and the deposition of particulates cause fouling, which results in chemical degradation of membrane materials, reduced permeate flux, higher transmembrane pressure, and a reduced membrane life span. Therefore, different nanomaterials are used to overcome these problems in MF and UF membranes [63]. There is always a typical trade-off relation between water permeability and selectivity of membranes. Incorporation of nanomaterials enables the improvement of water flux without a sacrifice or even reduction in removal efficiency of membranes. Using nanomaterials in a proper range can enhance the rejection efficiency of the nanocomposite membranes. The improvement in rejection and flux simultaneously is the ultimate goal of many research works in the literature [63]. Beside improvement in permeation properties, other functional properties of nanomaterials, such as antibacterial and antifouling properties, can also be endowed to the polymer membranes. Researchers fabricated PEI UF membranes by incorporating silver nanoparticles to improve the antifouling, permeation, and separation performance of the membrane. The fabricated nanocomposite membrane displayed high flux of about 97.2 L/m2 h, with high contaminant rejection. Additionally, these membranes exhibited high antibiofouling behavior against Gramnegative and Gram-positive bacteria [59]. Zhang et al. [72] recently cross-linked a GO composite with isophorone diisocyanate (IPDI) followed by coating on MF PVDF membrane. This procedure of cross-linking improved the removal of dyes (>96%) and heavy metal ions (40%–70% for Cr3+, Cu2+, Cd2+, and Pb2+) as compared with non-cross-linked PVDF-GO membranes. Demirel et al. [73] used Fe2O3 nanoparticles for the fabrication of polyvinyl chloride (PVC) UF nanocomposite membranes. The water flux increased from 522 to 782 L/m2 h with a high rejection of sodium alginate and 91.5% flux recovery ratio. Dong et al. [74] fabricated PVDF MF membranes by incorporating magnesium hydroxide nanoparticles.

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

Because of the presence of the number of hydroxyl group, these nanocomposite membranes exhibited high hydrophilicity and antifouling properties. Additionally, these nanoparticles also affected the porosity and permeability of the membranes. In another study, SiO2 nanoparticles were used to fabricate UF nanocomposite membranes. Water flux for membrane with 1 wt% of PVP-g-silica was found to be 2.3 times greater than neat PSf membrane. Moreover, increase in hydrophilicity was observed with increasing content of PVP-g-silica in PSF membrane. The PVP/PSF-g-silica membrane also possessed greater resistance toward fouling as demonstrated by fouling experiments utilizing nonionic surfactants with a little decrease in efficiency of salt rejection. Further, increase in membrane contact angle was observed with increase in SiO2 content [75]. Gul et al. [76] developed PES-CA UF membranes by incorporating the Ag nanoparticles. Further, Cu nanoparticles were also grown on the membrane surface. They comprehensively studied the membrane performance and properties. Their results revealed that the permeability of PES-CA and PES-CA-Ag2O membranes were 63.3 and 92.8 L/m2 h, respectively, while the PES-CA-Ag2O-Cu membranes showed permeability about 100 L/m2 h. The hybrid PES-CA-Ag2O and PES-CA-Ag2O-Cu membranes also possessed higher BSA rejection about 88.8% and 89.5% because of the increase in their hydrophilicity. Rahimpour et al. [77] employed PVDF/sulfonated PES blend UF membranes coated with TiO2 nanoparticles of different concentrations for water treatment. Although the synthesized nanocomposite membranes showed lower flux in comparison with PVDF membrane (noncoated), their antifouling property improved.

10.5 Nanomaterials in nanofiltration membrane process Nanofiltration (NF) membranes are used for water softening or removal of hardness (Ca2+ and Mg2+) form water. Surface charge and wettability are important surface properties of NF membranes. These properties can be manipulated by the incorporation of nanomaterials into the host matrices to obtain a high-performance membrane. Adding hydrophilic nanoparticles can reduce fouling of NF membranes due to increased surface hydrophilicity [24]. In one study, the modification of polyamide NF membranes was done by the incorporation of carboxylated MWCNTs. Thirty percent increase in water flux was observed with high hydrophilicity and antifouling properties. These membranes showed a high performance in terms of fouling resistance, permeability, and salt rejection [78]. Recently, Karimnezhad and coworkers [79] fabricated polyacrylonitrile NF membrane by incorporating two iron (Fe)-based nanoparticles, that is, goethite and maleate ferroxane. These iron-based nanoparticles showed high dye retention because of the strong repulsive forces between the functional groups of dye and the nanomaterials. In addition, these membranes exhibited more hydrophilicity, higher water permeability, and antifouling performance. Yang et al. [80] reported the enhanced antifouling and

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water flux of poly(m-phenyleneisophthalamide) NF membranes by the addition of GO. The nanocomposite membranes displayed minimum 90% dye retaining capacity. Madaeni et al. [81] synthesized a novel NF membrane of antifouling characteristics, consisting of PES and acid-oxidized multiwalled carbon nanotubes (MWCNTs). The salt rejection and water permeability of PES/MWCNT membrane increased because of development of large macrovoids in support layers of the membrane. It was reported that the surface segregation activities of MWCNTs has reduced fouling during filtration of BSA solution. They concluded that surface roughness plays an important part in antifouling performance. The negative charges of MWCNTs also helped in improving the salt rejection, following the Donnan exclusion mechanism. Zinadini et al. [82] prepared PES NF membranes by blending GO nanoplatelets within PES matrix. It is observed that rejection capability of PES/GO nanocomposite membranes was greater than pure PES membrane. Also, negative surface charge was induced by GO nanomaterial through entire range of pH. Lower contact angle, higher water flux, and better dye removal were reported for GO-blended PES membrane than unfilled membrane.

10.6 Nanomaterials in reverse osmosis membrane process Reverse osmosis (RO) is the most widely used industrial process for desalination dealing with 60% of the global load. Solute moves from low to high concentration, while an external pressure is applied opposite to the solvent’s natural flow due to osmotic pressure difference. The major obstacle in the growth of RO is again fouling, or biofouling, which could be prevented by pretreating the feed; modifying the membrane’s surface features; and optimizing the process condition, periodic cleaning, and module geometry. Typically, a more hydrophilic, smoother, and neutrally charged membrane is said to exhibit a higher fouling resistance [83, 84]. Usually, incorporating impermeable nanoparticles leads to a reduction in the membrane permeability as those particles can decrease the space available for the permeation through the polymer matrix. Consequently, the increase in membrane permeability is ascribed to defective structure formation, which lowers the rejection [63]. A number of nanomaterials are used for RO applications to enhance the properties of membranes such as Ag [85], cerium oxide (CeO2) [86], silica (SiO2) [87], and graphene oxide (GO) [58]. Wang and coworkers [86] developed cerium oxide-based PA reverse osmosis membrane, and their results revealed an increase in hydrophilicity from 85.4° to 65.7° and a 50% increase in water permeability without compromising the NaCl rejection. In other research, graphitic carbon nitride (g-C3N4) nanosheets were embedded in polyamide RO as hydrophilic modifiers. These nanocomposite membranes provided more water flux with high salt rejection with a maximum value about 99.7% [88]. Kazemimoghadam [89] prepared hydroxysodalite (HS) zeolite membrane for application in the RO process.

Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

It was seen that with increase in feed flow rate and temperature, there is substantial increase in permeate flux (4 L/m2 h) of zeolite HS membrane. Fathizadeh et al. [90] fabricated nanozeolite (NaX) PA membrane and modified top layer of RO PA membrane through surface coating. The synthesized membranes showed higher thermal stability and water permeability as compared with pristine PA membrane. Contact angle, roughness, and liquid-solid interfacial free energy increased for modified membrane, while a drastic decrease is observed in salt rejection. Zhao et al. [91] reported that 0.1 wt% MWCNT incorporation into the composite polyamide RO membranes doubled the water flux (28.05 L/m2 h), whereas rate of salt rejection (90%) maintained at similar rate. The MWCNT within composite matrix provided channels for increased transport of water with subsequent increase in water flux.

10.7 Conclusion Nanocomposite membranes play an important and innovative role in pressure-driven techniques to overcome the water scarcity issue. UF, MF, NF, and RO are powerful pressure-driven processes, which separate a wide range of components to obtain fresh water. New generation nanomaterials such as graphene oxide, carbon nanotubes, zeolites, silica, and other metal oxide proved as outstanding materials to improve membrane properties in terms of antifouling, antibacterial, hydrophilicity, and permeation while providing good mechanical and chemical stability. In fact, properties of nanomaterials depend on their unique size related to higher specific area, and this feature led to the development of membranes for more efficient water treatment and separation technology. In future, pressure-driven processes based on nanocomposite are predicted to continue to grow and rapidly lead to more improvements.

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Prospects of nanocomposite membranes for water treatment by pressure-driven membrane processes

[67] H.A. Shawky, S.R. Chae, S. Lin, M.R. Wiesner, Synthesis and characterization of a carbon nanotube/ polymer nanocomposite membrane for water treatment, Desalination 272 (1–3) (2011) 46–50. [68] G.C. Yang, Y.C. Chen, H.X. Yang, C.H. Yen, Performance and mechanisms for the removal of phthalates and pharmaceuticals from aqueous solution by graphene-containing ceramic composite tubular membrane coupled with the simultaneous electrocoagulation and electrofiltration process, Chemosphere 155 (2016) 274–282. [69] Y. Han, Z. Xu, C. Gao, Ultrathin graphene nanofiltration membrane for water purification, Adv. Funct. Mater. 23 (29) (2013) 3693–3700. [70] X. Chang, Z. Wang, S. Quan, Y. Xu, Z. Jiang, L. Shao, Exploring the synergetic effects of graphene oxide (GO) and polyvinylpyrrodione (PVP) on poly (vinylylidenefluoride)(PVDF) ultrafiltration membrane performance, Appl. Surf. Sci. 316 (2014) 537–548. [71] H. Wu, B. Tang, P. Wu, Development of novel SiO2-GO nanohybrid/polysulfone membrane with enhanced performance, J. Membr. Sci. 451 (2014) 94–102. [72] P. Zhang, J.L. Gong, G.-M. Zeng, C.H. Deng, H.C. Yang, H.Y. Liu, et al., Cross-linking to prepare composite graphene oxide-framework membranes with high-flux for dyes and heavy metal ions removal, Chem. Eng. J. 322 (2017) 657–666. [73] E. Demirel, B. Zhang, M. Papakyriakou, S. Xia, Y. Chen, Fe2O3 nanocomposite PVC membrane with enhanced properties and separation performance, J. Membr. Sci. 529 (2017) 170–184. [74] C. Dong, G. He, H. Li, R. Zhao, Y. Han, Y. Deng, Antifouling enhancement of poly (vinylidene fluoride) microfiltration membrane by adding Mg (OH)2 nanoparticles, J. Membr. Sci. 387 (2012) 40–47. [75] H. Song, C. Kim, Fabrication and properties of ultrafiltration membranes composed of polysulfone and poly (1-vinylpyrrolidone) grafted silica nanoparticles, J. Membr. Sci. 444 (2013) 318–326. [76] S. Gul, Z.A. Rehan, S.A. Khan, K. Akhtar, M.A. Khan, M. Khan, et al., Antibacterial PES-CA-Ag2O nanocomposite supported Cu nanoparticles membrane toward ultrafiltration, BSA rejection and reduction of nitrophenol, J. Mol. Liq. 230 (2017) 616–624. [77] A. Rahimpour, M. Jahanshahi, A. Mollahosseini, B. Rajaeian, Structural and performance properties of UV-assisted TiO2 deposited nano-composite PVDF/SPES membranes, Desalination 285 (2012) 31–38. [78] V. Vatanpour, M. Esmaeili, M. Safarpour, A. Ghadimi, J. Adabi, Synergistic effect of carboxylatedMWCNTs on the performance of acrylic acid UV-grafted polyamide nanofiltration membranes, React. Funct. Polym. 134 (2019) 74–84. [79] H. Karimnezhad, A.H. Navarchian, T.T. Gheinani, S. Zinadini, Incorporation of iron oxyhydroxide nanoparticles in polyacrylonitrile nanofiltration membrane for improving water permeability and antifouling property, React. Funct. Polym. 135 (2019) 77–93. [80] M. Yang, C. Zhao, S. Zhang, P. Li, D. Hou, Preparation of graphene oxide modified poly (m-phenylene isophthalamide) nanofiltration membrane with improved water flux and antifouling property, Appl. Surf. Sci. 394 (2017) 149–159. [81] V. Vatanpour, S.S. Madaeni, R. Moradian, S. Zinadini, B. Astinchap, Fabrication and characterization of novel antifouling nanofiltration membrane prepared from oxidized multiwalled carbon nanotube/ polyethersulfone nanocomposite, J. Membr. Sci. 375 (1–2) (2011) 284–294. [82] S. Zinadini, A.A. Zinatizadeh, M. Rahimi, V. Vatanpour, H. Zangeneh, Preparation of a novel antifouling mixed matrix PES membrane by embedding graphene oxide nanoplates, J. Membr. Sci. 453 (2014) 292–301. [83] M. Hayatbakhsh, M. Sadrzadeh, D. Pernitsky, S. Bhattacharjee, J. Hajinasiri, Treatment of an in situ oil sands produced water by polymeric membranes, Desalin. Water Treat. 57 (2016) 14869–14887. [84] F. Mohammadtabar, R.G. Pillai, B. Khorshidi, A. Hayatbakhsh, M. Sadrzadeh, Efficient treatment of oil sands produced water: process integration using ion exchange regeneration wastewater as a chemical coagulant, Sep. Purif. Technol. 221 (2019) 166–174, https://doi.org/10.1016/J. SEPPUR.2019.03.070. [85] U.M. Hirsch, N. Teuscher, M. R€ uhl, A. Heilmann, Plasma-enhanced magnetron sputtering of silver nanoparticles on reverse osmosis membranes for improved antifouling properties, Surf. Interfaces 16 (2019) 1–7.

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[86] Y. Wang, B. Gao, S. Li, B. Jin, Q. Yue, Z. Wang, Cerium oxide doped nanocomposite membranes for reverse osmosis desalination, Chemosphere 218 (2019) 974–983. [87] W. Yan, M. Shi, Z. Wang, Y. Zhou, L. Liu, S. Zhao, et al., Amino-modified hollow mesoporous silica nanospheres-incorporated reverse osmosis membrane with high performance, J. Membr. Sci. (2019). [88] S.S. Shahabi, N. Azizi, V. Vatanpour, Synthesis and characterization of novel g-C3N4 modified thin film nanocomposite reverse osmosis membranes to enhance desalination performance and fouling resistance, Sep. Purif. Technol. 215 (2019) 430–440. [89] M. Kazemimoghadam, New nanopore zeolite membranes for water treatment, Desalination 251 (1–3) (2010) 176–180. [90] M. Fathizadeh, A. Aroujalian, A. Raisi, Effect of added NaX nano-zeolite into polyamide as a top thin layer of membrane on water flux and salt rejection in a reverse osmosis process, J. Membr. Sci. 375 (1–2) (2011) 88–95. [91] H. Zhao, S. Qiu, L. Wu, L. Zhang, H. Chen, C. Gao, Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes, J. Membr. Sci. 450 (2014) 249–256.

Further reading [92] M.S.S.A. Saraswathi, D. Rana, S. Alwarappan, S. Gowrishankar, P. Vijayakumar, A. Nagendran, Polydopamine layered poly (ether imide) ultrafiltration membranes tailored with silver nanoparticles designed for better permeability, selectivity and antifouling, J. Ind. Eng. Chem. (2019).

CHAPTER 11

Prospects of nanocomposite membranes for water treatment by osmotic-driven membrane processes Behnam Khorshidi, Simin Shabani, Mohtada Sadrzadeh

Department of Mechanical Engineering, 10-367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada

11.1 Introduction The availability of clean water is decreasing every day, and our world is facing a global water crisis [1]. Approximately 18% of the world population lack access to safe potable water and 42% do not have access to adequate sanitation [2, 3]. Only about 2.8% of the water on earth is freshwater. However, most of this water consists of polar glaciers, which is not accessible [4]. The problem of clean water shortage will be aggravated by the rapid growth of population, climate change, and industrialization. Therefore, one of the biggest challenges of the current century is to meet the increasing demand for clean water. To address this problem, various water purification and wastewater treatment technologies have been adopted, such as distillation, oxidation, evaporation, reverse osmosis, nanofiltration, and ultrafiltration [5]. Since the majority of these technologies are energy intensive, the research for low-energy techniques to produce clean water has gained growing attention in recent years. The osmotic-driven process is one of the membrane-based technologies, which has shown great promise in water treatment and desalination processes. The phenomenon of osmosis, also called forward osmosis (FO), refers to the movement of fluid through a semipermeable membrane in response to a different concentration of solutes on either side of the membrane [6]. Osmotic processes have great potential to be used in various fields, particularly in wastewater treatment [7–10] and seawater desalination [11–14]. It can be effectively integrated with the other membrane technologies [15–20] and even can utilize renewable energy sources such as solar energy systems [21]. The prominent advantages of osmotic-driven processes over conventional pressure-driven membrane techniques are their capability to operate at low hydraulic pressure differences, lower operational costs, and less fouling of membranes [22–27]. In spite of the significant advantages, the application of the osmosis processes is limited by two significant challenges: (i) lack of highly permeable FO membranes, and (ii) concentration polarization within and at the surface of the membrane. Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00011-0

© 2020 Elsevier Inc. All rights reserved.

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11.2 Transport Phenomena in Osmotic-driven membranes 11.2.1 Classification of osmotic processes The physical phenomenon of the osmotic process can be defined as the transport of solvent (e.g., water) across a semipermeable membrane from a solution with the higher chemical potential to a solution with lower chemical potential. The chemical potential difference arises as a result of difference in either pressure, concentration, or temperature. In the field of water treatment, reverse osmosis (RO) is generally more familiar than osmosis where hydraulic pressure is applied to oppose and, exceed the osmotic pressure of an aqueous solution to produce purified water. FO and pressure-retarded osmosis (PRO) are two emerging osmotically driven membrane processes that have a potential for sustainable production of clean water and clean energy, respectively. These processes utilize the osmotic pressure difference of solution across a semipermeable membrane to draw water from a dilute solution (feed) to a highly concentrated salt solution (known as draw solution). Although FO and PRO follow similar physical principles, they are different from the application perspective. In PRO, a hydraulic pressure lower than the osmotic pressure difference is applied to pressurize draw solution. Then, the pressurized draw solution is sent to a hydroturbine to generate power. Fig. 11.1 shows the difference between FO, PRO, and RO processes according to their operating pressure.

11.2.2 Water and solute permeability The coupled models of classical solution-diffusion model and diffusion-convection are used to show the transport phenomena in semipermeable membranes. In pressure-driven

Fig. 11.1 Illustration of FO, PRO, and, RO processes. (A) In FO, no hydraulic pressure is applied, and water flows from the dilutive feed solution to the draw solution. (B) In PRO, the hydraulic pressure is applied on the draw side, which is less than the osmotic pressure difference across the membrane. (C) In RO, the hydraulic pressure applied to the feed solution is greater than the osmotic pressure difference across the membrane and water flows from feed to the permeate side of the membrane. (D) Classification of FO, PRO, and RO processes according to water flux versus applied pressure.

Prospects of nanocomposite membranes for water treatment

membrane processes, water flux (Jw) (ms
1) is directly proportional to the applied pressure (Δp) and the osmotic pressure difference (Δπ) of the two solution across the membrane. Jw ¼ AðΔp
σΔπ b Þ,

(11.1)

where A is water permeability constant (ms
1Pa
1) , which is an intrinsic characteristic of a membrane. Δ p and Δπ b are the applied pressure difference and the balk osmotic pressure difference across the membrane, respectively. σ is the reflection coefficient and has a value between 0 and 1. For the case of complete rejection, σ equals unity. Since in the FO process, no hydraulic pressure is applied, Eq. (11.1) can be expressed as:
 Fb Jw ¼ Aσ π Db (11.2) s
πs Fb where π Db s and π s are the bulk osmotic pressures of draw and feed solution, respectively. It is worth noting that the power generated by the PRO process is a parabolic function of Δp and water flux through the membrane.

w ¼ Jw Δp ¼ AðΔp
σΔπ b ÞΔp

(11.3)

As can be seen in Fig. 11.1, the maximum gross power that can be generated by PRO is at Δp = Δπ b/2. The salt flux (Js) is defined by following equation: Js ¼
BΔC

(11.4)

in which B is the salt permeability coefficient (ms
1) and ΔC is the concentration difference across the membrane active layer. The salt permeability coefficient is defined as follows: ð1
RÞJw R Cp R ¼1
, Cf

B¼

(11.5) (11.6)

where R is salt rejection and Cp and Cf are the salt concentration in the permeate and feed solutions, respectively. These equations predict the transport phenomena through semipermeable membrane only as a function of driving forces in the absences of concentration polarization, which may be valid only if the permeate flux is very low. Hence, for a complete description of mass transport in osmotic processes, the concentration polarization phenomena, which occur inside and outside of the membrane, need to be considered.

11.2.3 Concentration polarization Concentration polarization appears when the concentration of retained solutes near the membrane surface becomes considerably higher compared to that of the bulk feed.

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Fig. 11.2 Schematic diagrams of external and internal concentration polarization. (A) Dilutive and concentrative ECP and, dilutive ICP in FO membrane. (B) Dilutive and concentrative ECP and, concentrative ICP in PRO membrane.

There are two types of concentration polarization in FO and RO: external concentration polarization (ECP) due to the accumulation of solutes at the membrane-solution interface, and internal concentration polarization (ICP) due to the diffusion draw solutes inside the support layer. Concentration polarization can also be dilutive or concentrative according to the membrane orientation, as well as the location that the concentration polarization happens. Fig. 11.2A and B depict the concentration polarization profile in FO and PRO, respectively.

11.2.4 External concentration polarization ECP occurs at both sides of the membrane and may be concentrative or dilutive depending on the membrane orientation. Concentrative ECP is due to the accumulation of retained solutes near the membrane surface, which leads to a higher concentrated solution compared to the bulk of feed solution. The diffusion of water into draw solution resulting in more dilutive draw solution at the membrane surface, which is referred to as the dilutive ECP. ECP causes the increase of osmotic pressure on the feed side and thus, reduces the net driving force. However, the detrimental effect of ECP can be alleviated by increasing flow velocity and turbulence at the membrane surface.

Prospects of nanocomposite membranes for water treatment

According to the boundary-layer film theory, the concentrative and dilutive ECP in FO and PRO modes can be predicted as [28, 29]:   π As Jw ¼ exp For FO mode (11.7) Fb πs kF   π ss Jw For PRO mode (11.8) ¼ exp
π Db kD s where k is the mass transport coefficient in the flow channel, which is related to the Sherwood number (Sh): k¼

ShD , dh

(11.9)

where D is the diffusion coefficient of solute in the draw and feed side of the membrane and dh is the hydraulic diameter of the flow channel. It worth noting that the ratio of osmotic pressure at the membrane surface to the bulk solution is assumed to be equal to that of concentrations based on the van’t Hoff equation. Sherwood number regarding the flow regime in a rectangular channel can be calculated from the following equations:   dh 0:33 Sh ¼ 1:85 ReSC For laminar flow Re  2100 (11.10) L Sh ¼ 0:04Re0:75 Sc 0:33 For Turbulent flow Re > 2100

(11.11)

where Re and Sc are Reynold number and Schmidt number, respectively, and L is the length of the flow channel.

11.2.5 Internal concentration polarization ICP phenomenon occurs in the support layer of membrane as a result of the permeation of water molecules into it, diluting the draw solution. Similar to ECP, there are two types of ICP depending on the membrane orientation: dilutive and concentrative ICP. The effect of ICP can be incorporated in the transport equation by [30, 31]:   1 B + Aπ ss
Jw K¼ ln For PRO mode (11.12) Jw B + Aπ Fb s   1 B + Aπ Db s For FO mode (11.13) ln K¼ Jw B + Jw + Aπ Fb s where K is the solute resistivity to salt transport in the porous substrate, which is a measure of the ICP severity. It can be defined as a function of the structural parameter (S) and the diffusion coefficient (D): K¼

S tτ ¼ D Dε

(11.14)

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In Eq. (11.13), τ and εeff are tortuosity and effective porosity of the support layer, respectively. By assuming that the salt permeability is negligible (B ¼ 0), Eqs. (11.12), (11.13) can be rearranged, and exponential terms in the resulting equations show concentrative and dilutive ICP modulus for FO and PRO mode, respectively [28, 29]: π AS s ¼ exp ð
Jw K Þ For FO mode π Db s

(11.15)

π AS s ¼ exp ðJw K ÞFor PRO mode π Fb s

(11.16)

By incorporating the correction factors in Eqs. (11.2), (11.7), (11.15), the water flux equation for FO mode becomes:    Jw Db Fb Jw ¼ A π s exp ð
Jw K Þ
π s exp (11.17) kF Similarly, incorporating Eqs. (11.2), (11.8), (11.16) leads to the following equation for PRO mode:     Jw Db Fb Jw ¼ A π s exp
(11.18)
π s exp ðJw K Þ kD These equations assume that there is no ECP at the porous substrate side, so the osmotic pressure on the substrate interface is equal to that of the bulk solution. Moreover, derivation of ECP and ICP modulus is under the assumption of high salt rejection, or σ ¼ 1. Considering ECP on both sides of the membranes and ICP in the substrate, the water flux in FO and PRO modes can be evaluated by [32]:      9 8 1 S Jw > > Db Fb > >
π exp < π exp
Jw k + D = k D D F       For FO mode Jw ¼ A (11.19) > > B Jw 1 S > > :1 + ;
exp
Jw exp + Jw k D DD kF      9 8 Jw 1 S > > Db Fb > >
π exp
Jw + < π exp
= k F DF kD        For PRO mode Jw ¼ A > > B 1 S Jw > > :1 + ; exp Jw +
exp
Jw kF DF kD

(11.20)

where DF and DD are the diffusion coefficient of salts in the feed and the draw solutions, respectively.

Prospects of nanocomposite membranes for water treatment

11.3 Osmosis membrane development The early generation of osmosis membranes was synthesized using cellulose acetate (CA) and cellulose triacetate (CTA) polymers via the phase inversion technique. These membranes had a single asymmetric structure with a dense skin layer above a porous sublayer. The cellulosic membranes offer several favorable characteristics to osmosis membranes. The materials are cost efficient and easily processable, have high mechanical strength, are intrinsically hydrophilic, and show fouling resistance, particularly toward organic compounds. However, the cellulose-based osmosis membranes are limited to narrow operational conditions, such as slightly acidic pH and moderate temperatures to maintain their operational integrity. These restrictions have limited the application of cellulosic osmosis membranes to be used in industrial water treatment with harsh wastewater streams [22]. The new generation of osmosis membranes is prepared in thin-film composite (TFC) structure. A TFC membrane has two main constituent layers. The first is porous support, which is typically prepaid by phase inversion technique using a dope solution of polysulfone (PSf ), polyethersulfone (PES), or polyacrylonitrile (PAN). The second layer is a thin active layer (100–200 nm), which is made of polyamide (PA) via interfacial polymerization (IP) reaction over the support surface [33–35]. The IP reaction takes place between an amine (e.g., m-phenylenediamine (MPD)) and acyl chloride (e.g., trimesoyl chloride (TMC)) monomers [36–38]. The TFC membranes are also used in RO and NF processes with quite similar manufacturing procedure. The main modification of TFC FO compared to TFC RO is the adjustment of the support structure. The support of osmosis membranes is thinner, more porous, less torturous, and more hydrophilic than the support of the TFC RO/NF membranes. TFC membranes offer several advantages over asymmetric cellulose-based membranes by delivering larger water permeability, lower salt permeability, and surviving a wider range of operating temperature and pH. Regarding that, the current market of osmosis membranes is dominated by TFC PA membranes. However, the preparation of the TFC membranes is more expensive than the CA or CTA-based membranes due to fabrication difficulties to control the properties of the thin polyamide layer during the polymerization reaction.

11.4 Nanocomposite osmosis membranes Along with the notable advancement of nanotechnology, multifunctional nanomaterials have been incorporated as potent additives in membrane material development. The general goals are to improve the membrane permeability and selectivity, tune the hydrophilicity and chemical functionality, control the surface topography, and enhance the antifouling capability of the nanocomposite membranes [39–43]. The nanofillers have been incorporated in membrane structure in five general ways: (i) bulk incorporation in the top rejecting layer, (ii) bulk incorporation in support layer, (iii) surface coating

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of the top rejecting layer, (iv) incorporation as interlayer between the top layer and substrate, (v) employment as free-standing top rejecting or support layer. So far, carbon nanomaterial, metal and metal oxide nanoparticle, and biomaterials are utilized to fabricate nanocomposite membranes for the FO application.

11.4.1 Carbon-based nanocomposite osmosis membranes Carbon-based nanofillers have attracted great attention as additives in membrane structure due to their superb mechanical, thermal, and electrical properties. The family of carbon nanomaterials that are utilized in osmosis membrane development mainly includes carbon nanotube (CNT), graphene oxide (GO), and reduced graphene oxide (rGO). CNTs are made of graphene sheets in the tubular structure. Depending on the number of the graphene sheets, the CNTs are classified into single-walled carbon nanotubes (SWCNTs) or multiwalled carbon nanotubes (MWCNTs). The large aspect ratio, unique cylindrical structure, and tunable surface chemistry have made the CNTs highly favorable to improve the perm-selectivity of the nanocomposite membranes. An example of the incorporation of surface functionalized CNTs to the polymer matrix is shown in Fig. 11.3, where the nanofillers were first carboxylated, then blended with the polyetherimide (PEI) nanofiber support [44]. It was reported that the mechanical strength and water permeability of the modified membranes were improved by the addition of functionalized CNTs. Table 11.1 presents a summary of the scientific endeavors that used carbon nanofillers to improve the performance of the osmosis membranes. Promoted water permeability, improved structural parameter, and enhanced antifouling propensity are the main characteristics, which are reported for the membranes modified by carbon-based nanofillers.

11.4.2 Mineral-based nanocomposite osmosis membranes Mineral nanomaterials have attracted great interest due to their abundant supply, low price, hydrophilic surface chemistry, and versatile structure. A wide range of minerals such as silica, zeolite, and natural clays with diverse morphologies such as solid, mesoporous, and tubular structures have been utilized in the structure of the osmosis membranes. These materials are incorporated into either substrate or the dense active layer of the composite membrane. Fig. 11.4 demonstrates an example of the incorporation of mesoporous Si nanoparticles in the PAN nanofiber [79]. It was observed that the addition of Si nanoparticles improved the specific surface area and the water uptake capability of the membranes. The modified membranes showed higher water permeability with improved salt rejection compared to unmodified membranes. Table 11.2 presents the reports that investigated the impact of mineral-based nanomaterials on the performance of the osmosis membranes. In general, higher permeability, lower structural parameter and ICP, as well as lower fouling propensity are reported for the osmosis membranes modified by mineral nanofillers.

Prospects of nanocomposite membranes for water treatment

f-CNTs COOH COOH HOOC

Normalization Intensity (a.u.)

MWCNTs f-CNTs MWCNTs

D band G band

IG/ID=0.59•0.01

IG/ID=0.66•0.07

COOH HOOC

1000

1500

2000

–1

Raman shift (cm )

Fig. 11.3 Surface functionalization of MWCNTs and bulk incorporation in PEI nanofiber mat [44].

11.4.3 Metal- and metal oxide–based nanocomposite osmosis membranes Metals and metal oxide nanomaterials such as TiO2, Ag, Fe3O4, and ZnO have been widely used as additives for membrane development owing to their versatile surface functionalities, high thermal and mechanical stability, and photocatalytic activity. Among these nanomaterials, TiO2 has attracted more attention for membrane modification. TiO2 nanomaterials are frequently used as either nanoparticles or nanotubes. Fig. 11.5 illustrates the microscopic image of TiO2 nanotubes along with an example of surface functionalization. The aminated TiO2 nanotubes were dispersed in amine-aqueous

265

266

Nanofiller

Support layer material (structure)

GO

PAN

GO (nanosheet, laminate)

GO

PSf (flat sheet)

PSf (flat sheet)

Active layer material

GOPAH

PA

PA

Modified layer

Modification technique

Operation configuration

Bulk of the active layer

LbL assembly of GO-PAH over the substrate

AL-FS, AL-DS

Surface of active layer

LbL, hybrid (H) grafting of GO/ poly L-Lysine (PLL) on PA surface

AL-FS

Addition of GO into PSf dope solution

AL-FS

Bulk of support layer

Performance summary

Reference

• Improved water

[45]

• •

• • • • •

GO

GO

PAN (flat sheet)

PSf (flat sheet)

PA

PA

Bulk of active layer

Surface of active layer

Dispersion of GO in MPDaqueous solution followed by IP reaction Coating of polydopamine induced graphene oxide (GO-PDA) to PA surface

AL-FS

• •

AL-FS

flux High rejection for sugary solution and low ionic strength Promoted hydrophilicity and smoothness of the membrane’s surface Lower biofouling and reverse solute flux Lower flux decline Improved water permeability Lower reverse solute flux Lower structural parameter, higher hydrophilicity Improved hydrophilicity and water flux Lower fouling propensity

• Enhanced water flux and selectivity • Improved bactericidal properties

[46]

[47]

[48]

[49]

Nanocomposite membranes for water and gas separation

Table 11.1 Summary of the studies on nanocomposite membranes made by carbon-based nanomaterials

GO

PA

Surface of active layer

GO

PSf (flat sheet)

PA

Bulk of active layer

GO

Nylon (flat sheet)

NIPAMMBA

Bulk of active layer

GO

PSf (flat sheet)

PA

Bulk of support layer

Coating of poly(tannic acid)
graphene oxide over PA layer Dispersion of PVP-coated GO in MPDaqueous solution Entwining GO in active layer using free radical polymerization

AL-FS

Addition of GO into PSf dope solution

AL-FS, AL-DS

AL-FS, AL-DS

AL-FS

• Formation of smooth, and hydrophilic coating • Improved filtration • Lower biofouling • Higher water flux and lower reverse solute flux • Enhanced surface hydrophilicity • Highly crosslinked GO/poly (NIPAM-MBA) network • High water permeability and salt rejection • Improved mechanical stability and chlorine tolerance • Dual-layered support with a porous sublayer and a dense top skin layer

[50]

[51]

[52]

[53]

• Improved water permeability and ion selectivity • Lower structural parameter and specific reverse salt flux Continued

Prospects of nanocomposite membranes for water treatment

PSf (flat sheet)

267

268

Nanofiller

GO

Support layer material (structure)

PVDF (nanofiber)

Active layer material

PA

Modified layer

Modification technique

Operation configuration

Performance summary

Reference

Bulk of support layer

Addition of GO into PVDF dope solution

AL-FS

• Reduced structural

[54]

• •

GO

GO

PES

PA

PSf (flat sheet)

PA

GO

PSf (flat sheet)

PA

rGO

MCE (flat sheet)

rGOPDA

Bulk of active layer

Bulk of active layer

Bulk of support and active layer Bulk of active layer

Incorporation of GO in amineaqueous solution followed by IP reaction Dispersion of GO in MPDaqueous solution followed by IP reaction Addition of GO into PSf dope solution and MPD-aqueous solution Deposition of rGO over MCE using vacuum filtration followed by PDA dip coating

AL-FS, AL-DS

• •

AL-FS

AL-FS

AL-FS

parameter and ICP Higher water permeability Improved flexibility and mechanical strength Improved water flux Higher reverse solute flux

• Improved surface hydrophilicity • Enhanced water flux with reduced reverse salt diffusion • Improved porosity and hydrophilicity • Greater antibiofouling activity

• Improved the hydrophilicity • Accelerated water permeation

[55]

[56]

[57]

[58]

Nanocomposite membranes for water and gas separation

Table 11.1 Summary of the studies on nanocomposite membranes made by carbon-based nanomaterials—cont’d

rGO (laminate)

PES (flat sheet)

rGO

Bulk of active layer

Deposition of rGO over PES substrate

AL-FS

• Improved antifoul•

CNT (multiwalled)

CNT (multiwalled)

PAI (hollow fiber)

Selfsupporting CNT

PES

CNT (multiwalled)

PSf

PA

PA

PA

Bulk of active layer

Bulk of support layer

Bulk of support

Bulk of active layer

Deposition of carboxylated CNTs over PAI followed by chemical treatment with PEI Formation of self-supporting CNTs using vacuum filtration followed by plasma treatment Blending of CNTs with PSf dope solution

AL-FS

Dispersion of aminefunctionalized CNTs in MPDaqueous solution followed by IP reaction

AL-FS, AL-DS

• • •

AL-FS

•

•

AL-FS

• Enhanced permeation performance • Improved mechanical property • Improved surface hydrophilicity • Enhanced water permeability with acceptable percentage of salt rejection

[59]

[60]

[61]

[62]

[63]

Continued

Prospects of nanocomposite membranes for water treatment

CNT (multiwalled)

CNTPEI

ing ability under electric field Higher flux recovery ration Formation of positively charged active layer Enhancement in water permeability Similar rate of solute reverse flux Higher support hydrophilicity, porosity, and water uptake Superior permeation performance

269

Table 11.1 Summary of the studies on nanocomposite membranes made by carbon-based nanomaterials—cont’d

PSf

CNT (multiwalled)

PEI (nanofiber)

Active layer material

PDATMC

PA

Modified layer

Modification technique

Operation configuration

Performance summary

Reference

Bulk of active layer

Dispersion of CNTs in DA-aqueous solution followed by IP reaction

AL-FS

• Formation of

[64]

Dispersion of carboxylated CNTs in PEI dope solution

AL-FS, AL-DS

Bulk of support layer

• • • • •

• CNT (multiwalled)

PEI (nanofiber)

PA

Bulk of support layer

Dispersion of carboxylated CNTs in PEI dope solution

AL-DS

•

• • CNT (multiwalled)

PES

PA

Bulk of support layer

Dispersion of functionalized CNTs in PES dope solution

AL-DS

• • •

double-skinned active layer Improved separation performance Enhanced antifouling propensity Improved connectivity and porosity of support Lower structural parameter and ICP Enhanced tensile modules and mechanical strength Higher permeation performance Formation of a tiered support by fine and coarse nanofiber layers Increased mechanical stability Higher performance with stable power generation Increased porosity and hydrophilicity of support Enhanced water permeability by active layer etching Promoted power density

[44]

[65]

[66]

Nanocomposite membranes for water and gas separation

CNT

270

Nanofiller

Support layer material (structure)

CNT

CNT

PSf

PVDF

PA

PA

Bulk of support and active layer

Interlayer

Dispersion of CNTs into PSf dope solution and MPDaqueous solution

AL-FS

Coating CNTs on support layer by vacuum filtration

AL-FS

• Increased porosity • • •

• CNT (multiwalled)

PSf

PA

PA

Bulk of support

Bulk of active layer

Dispersion of functionalized CNTs into PSf dope solution

AL-FS

Dispersion of sulfonated CNTs into PSf dope solution

AL-FS

• • •

• CNT (multiwalled)

CNT (hollow fiber)

PA

Bulk of support

Preparation of CNT hollow fibers using wetspinning method

AL-FS

• • •

[67]

[68]

[69]

[70]

[71]

271

Continued

Prospects of nanocomposite membranes for water treatment

CNT

PES

with lower structural parameter Enhanced antifouling capability Tradeoff effect between water flux and salt rejection Optimized space structure underneath the active layer Improved separation performance Improved support hydrophilicity and water flux Enhanced antifouling ability and flux recovery Formation of smoother and denser active layer with higher hydrophilicity Higher water flux with lower reverse salt flux Enhanced porosity, electroconductivity Lower structural parameter and ICP Promoted organic and microbial antifouling udder electric field

272

Nanofiller

CNT (singlewalled)

CNF

CNF

Support layer material (structure)

Active layer material

Modified layer

Modification technique

Operation configuration

Performance summary

Reference

PES

PA

Interlayer

Spray coating of PDA-coated CNT for support

AL-FS

• Improved effective

[72]

Bulk of support layer

Blending of carboxylated CNF in CTA casting solution

AL-FS, AL-DS

Bulk of support layer

Blending of aminefunctionalized CNF in CTA casting solution

AL-FS

CTA

CTA

CTA

CTA

• • • • • •

GO-CNT

Nylon

PA

Interlayer

Coating of GO and MWCNTs on support layer by vacuum filtration

AL-FS, AL-DS

• •

surface area of the active layer Greater water flux with similar salt rejection Enhanced surface hydrophilicity Improved water flux with lower solute diffusion Improved hydrophilicity and porosity Higher water flux with lower salt passage Lower structural parameter Promoted water flux with lower reverse salt passage Formation of thinner active layer with nano water channels

[73]

[74]

[75]

Nanocomposite membranes for water and gas separation

Table 11.1 Summary of the studies on nanocomposite membranes made by carbon-based nanomaterials—cont’d

GCD

Fullerene

Fullerenol

PES

PSf

PSf

PA

PA

PA

Bulk of active layer

Bulk of active layer

AL-FS

• Improved surface • •

AL-FS, AL-DS

• •

AL-FS

hydrophilicity Increased water permeation Enhanced antimicrobial activity Enhanced the water flux Formation of active layer with lower cross-link density

• Improved water flux • Lower fouling propensity

[76]

[77]

[78]

Prospects of nanocomposite membranes for water treatment

The bulk of the active layer

Dispersion of GCDs in MPDaqueous solution followed by IP reaction Dispersion of C60@PAF900 in TMC-hexane solution followed by IP reaction Dispersion of C60(OH)n in MPD-aqueous solution followed by IP reaction

273

274 Nanocomposite membranes for water and gas separation

Fig. 11.4 (A) Schematic presentation of a single mesoporous silica NP, clusters of Si NPs and their possible incorporation in the nanofiber mat; (B) TEM image of as-received mesoporous Si NPs; (C) and (D) FESEM images of Si-embedded PAN nanofibers [79].

Table 11.2 Summary of studies on nanocomposite membranes made by mineral-based nanomaterials

Nanofiller

Support material (structure)

Active layer material

Zeolite

PSf

PA

Modified layer

Modification technique

Bulk of active layer

Dispersion of NaY zeolite NPs in TMC-hexane solution

Operation configuration

Performance summary

Reference

AL-FS

• Improved

[80]

• Zeolite

PSf

PA

Bulk of support layer

Blending the zeolite NPs in PSf dope solution

AL-FS, AL-DS

• •

Zeolite

PES

PA

Blending the zeolite (clinoptilolite) NPs in PSf dope solution

AL-FS

•

• Zeolite

PAN

PAAPEI

Bulk of active layer

Incorporation of zeolite NPs between PAA-PEI LbL assembly

AL-FS, AL-DS

•

Silica

PSf

PA

Surface of active layer

Dip coating of PA TFC into an aminefunctionalized silica NPs suspension

AL-FS, AL-DS

• •

•

[81]

[82]

[83]

[84, 85]

Prospects of nanocomposite membranes for water treatment

Bulk of support layer

275

water permeability Lower salt rejection Enhanced water permeability Lower structural parameter and ICP Increased surface hydrophilicity and porosity Improved water flux Improved water permeability with comparable salt rejection Improved permeation performance Lower adhesion forces to organic foulant Reduced organic fouling

Continued

276

Nanofiller

Support material (structure)

Active layer material

Silica

PSf

PA

Modified layer

Modification technique

Bulk of active layer

Dispersion of silica NPs on MPD solution followed by IP reaction

Operation configuration

Performance summary

Reference

AL-FS, AL-DS

• Enhanced sur-

[86]

• Silica

PSf

PA

Bulk of support layer

Blending of silica NPs with PSf dope solution and double-blade co-casting

AL-FS, AL-DS

•

• Silica

PAN (nanofiber)

PA

Bulk of support layer

Blending of silica NPs with PAN dope solution

AL-FS, AL-DS

•

•

Silica

PES

PA

Bulk of support layer

Blending of silica NPs with PES dope solution followed by hydrofluoric acid etching

AL-FS

•

•

face hydrophilicity Higher permeability and salt rejection Improved water flux with low reverse salt/ water flux. Reduced structural parameter Enhanced specific surface area and water uptake Improved water permeability and salt rejection Improved water permeability and surface hydrophilicity Reduced ICP in support

[87]

[79]

[88]

Nanocomposite membranes for water and gas separation

Table 11.2 Summary of studies on nanocomposite membranes made by mineral-based nanomaterials—cont’d

Silica

PEI (nanofiber)

PA

Bulk of support layer

Blending of silica NPs with PEI dope solution

AL-FS, AL-DS

• Enlarged sur• •

PES

PA

Bulk of active layer

Dispersion of POMcoated silica NPs in MPD solution followed by IP reaction

AL-FS, AL-DS

•

MMt

PVA/ MMt

CH/ PAAc

Bulk of support and active layer

Dispersion of surface modified montmorillonite clay in PVA solution.

AL-FS, AL-DS

• • •

Boehmite

CTA/CA

CTA/ CA

Bulk of support layer

Blending of Boehmite NPs in CTA dope solution

AL-FS, AL-DS

•

•

[89]

[90]

[91]

[92]

Prospects of nanocomposite membranes for water treatment

Silica

face pore size and porosity Improved water flux Lower structural parameter and ICP Enhanced water flux with comparable salt rejection Increased support hydrophilicity Improved water flux Enhanced separation in AL-DS orientation Increased support porosity, lower structural parameter Enhanced water flux with comparable salt rejection, lower ICP

Continued

277

278

Table 11.2 Summary of studies on nanocomposite membranes made by mineral-based nanomaterials—cont’d

Zn2GeO4 (nanowire)

CaCO3

Active layer material

PES

PA

PSf

PA

Modified layer

Bulk of support layer

Bulk of support layer

Operation configuration

Performance summary

Reference

Blending of zinc germanium oxide nanowires in PES dope solution

AL-FS

• Increased

[93]

Blending of calcium carbonate NPs in PSf dope solution followed by hydrochloric acid etching

AL-FS, AL-DS

Modification technique

• • •

•

LDH

PSf

PA

Bulk of support layer

Blending of LDH NPs in PSf dope solution.

AL-FS, AL-DS

•

•

•

•

water flux in RO Lower water flux in FO Higher ICP More porous support with lower structural parameter Improved water flux with lower ICP Improved porosity and surface hydrophilicity Enhanced mechanical strength and thermal stability Promoted water flux with lower salt rejection Lower structural parameter and ICP

[94]

[95]

Nanocomposite membranes for water and gas separation

Nanofiller

Support material (structure)

HNT

PSf

PA

Bulk of active layer

Dispersion of halloysite nanotube in TMCcyclohexane solution prior to IP reaction

AL-FS, AL-DS

• Improved

• HNT

PSf

PA

Bulk of support layer

Blending of HNT in PSf dope solution.

AL-FS, AL-DS

• •

INT

PSf

PA

Bulk of support layer

Blending of INT in PSf dope solution.

AL-FS

•

• • Hap

CA

CA

Bulk of support layer

Blending of plasma treatment Hap NTs in CA dope solution.

AL-FS

• •

[96]

[97]

[98]

[99]

Prospects of nanocomposite membranes for water treatment

•

water flux with comparable salt rejection Enhanced fouling resistance Improved support hydrophilicity Decreased structural parameter and ICP Enhanced permeation performance Elevated support porosity and hydrophilicity Lower structural parameter and ICP Improved water flux and salt rejection Improved water permeability Comparable sat rejection

279

280

Nanocomposite membranes for water and gas separation

Fig. 11.5 Surface functionalized TNT used in support layer of TFC membranes [100].

solution before IP reaction [100]. In general, the composite membranes modified with the metal and metal oxide nanomaterials have shown promoted water permeability, higher mechanical strength, and improved antifouling resistance. Table 11.3 presents a summary of the reported research on nanocomposite membrane development using metal and metal oxide nanomaterials.

11.4.4 Biomimetic-based nanocomposite osmosis membranes Aquaporins (Aqps) are integral membrane proteins with embedded pores for the transport of water molecules through biological cells. Owing to their large water permeability with exceptional impermeability toward charging species, Aqps have received great interest as functional water channels for the development of biomimetic membranes for desalination and water recovery. The main focus of the research efforts has been to (i) develop innovate technique for robust incorporation and immobilization of the Aqps over the surface or in the bulk of the host polymeric membrane and (ii) optimize the membrane synthesis routes to maximize the incorporation capacity of Aqps in the membrane structure. Fig. 11.6 illustrates a schematic view and FESEM images of AqpZ-embedded block-copolymer film, which is covalently bonded to a gold-coated polycarbonate support. Table 11.4 provides a summary of studies to fabricate high-performance osmosis membranes by utilizing Aqps nanomaterials. The general synthesis routes have been

Table 11.3 Summary of the performance of the nanocomposite membranes modified by metal and metal oxide nanofillers

Nanofiller

Support material (structure)

Active layer material

TiO2 (NP)

PSf

PA

Modified layer

Modification technique

Operation configuration

Performance summary

Bulk of support layer

Blending of TiO2 NPs in PSf dope solution

AL-FS, AL-DS

• Improved sup•

• TiO2 (NT)

PSf

PA

PA

Bulk of active layer

Surface of active layer

Dispersion if aminefunctionalized TiO2 NTs in MPDaqueous solution prior to IP reaction

AL-FS, AL-DS

Dip coating of PDAtreated TFC in TiO2 NPs suspension

AL-FS

•

• •

•

•

[101, 102]

[100]

[103]

Continued

Prospects of nanocomposite membranes for water treatment

TiO2 (NP)

PSf

port porosity and hydrophilicity Enhanced water flux with a slight reduction in salt rejection Improved organic antifouling Improved water permeability with comparable salt rejection Enhanced surface wettability Higher wettability with lower roughness and surface potential Slight increase of rejection percentage of trace pharmaceuticals Enhanced antifouling ability

Reference

281

282

Nanofiller

Support material (structure)

Active layer material

TiO2 (NP)

PSf

PA

TiO2 (nanoporous particles)

TiO2 (NP)

PSf

PA

Commercial TFC FO (PSf-PA)

Modified layer

Modification technique

Operation configuration

Performance summary

Reference

Bulk of active layer

Dispersion if aminefunctionalized TiO2 NPs in MPDaqueous solution prior to IP reaction

AL-FS, AL-DS

• Improved wetta-

[104]

Dispersion if aminefunctionalized TiO2 NPs in TMChexane solution prior to IP reaction

AL-FS, AL-DS

Spray coating of TiO2 NPs on the surface of support of commercial membrane

AL-DS

Bulk of active layer

Surface of support layer

• •

•

• • •

TiO2 (NP)

PSf

PA

Bulk of support layer

Blending of PHEMA-grafted TiO2 NPs in PSf dope solution

FO (AL-FS, AL-DS)

• •

bility with lower surface roughness Improved water permeability Enhanced water flux with slight reduction in rejection Improved antifouling ability with high flux recovery Improved support hydrophilicity Enhanced water flux in AL-DS mode Increased organic antifouling ability Enhanced support hydrophilicity Increased water flux with lower ICP

[105]

[106]

[107]

Nanocomposite membranes for water and gas separation

Table 11.3 Summary of the performance of the nanocomposite membranes modified by metal and metal oxide nanofillers—cont’d

TiO2 (NP)

CTA/CA

CTA/ CA

Bulk of support layer

Blending of TiO2 NPs in CTA/CA casting solution

FO (AL-FS)

• Formation of

• TiO2 (NP)

PSf (nanofiber)

PA

Bulk of support layer

Blending of TiO2 NPs in PSf dope solution

FO (AL-FS)

•

•

Commercial TFC FO membranes (CTA, Aqp)

Surface of active layer

Grafting of TiO2 NPs to active surface using monomer chain

FO (AL-FS)

•

•

Ag (NP)

PAN

LbL of PAHPSS

Bulk of active layer

Dispersion of Ag NPs in PSS solution followed by LbL assembly

FO (AL-FS, AL-DS)

• •

[108]

[109]

[110]

[111]

Prospects of nanocomposite membranes for water treatment

TiO2 (NP)

smoother and more hydrophilic support Improved desalination performance Increased porosity, hydrophilicity, surface pore size Improved water flux at optimum NP concentration Improved wettability of CTA membrane, negative impact of wettability of Aqp membrane Enhanced water flux and fouling resistance of membranes Enhanced antimicrobial activity Improved permeation performance at optimum NPs concentration

Continued

283

284

Nanofiller

Support material (structure)

Active layer material

Ag (NP)

PAN

LbL of PAH/ PSS bilayers PA

Ag/AgCl (NP)

PES-PAA

Ag (NP)

Commercial TFC FO membrane (PSf-PA)

Ag (NP)

PES

PA

Modified layer

Modification technique

Operation configuration

Performance summary

Reference

Bulk of support layer

Blending of Ag NPs in PAN dope solution

FO (AL-DS)

• Higher water

[112]

Surface and bulk of the support layer

Alternative soaking process using AgNO3-aqueous and NaCl solutions followed by photolysis Dip coating of PDAcoated TFC membrane in silver nitrate aqueous solution

FO (AL-FS, AL-DS)

flux • Promoted antibiofouling ability • Improved antimicrobial performance • Enhanced water flux

FO (AL-DS)

• Lower water flux

In situ formation by a coating of zwitterion-modified PA layer with AgNO3/NaBH4 solution

FO (AL-FS)

Surface of support and active layer Surface of active layer

[113]

[114]

and reverse salt flux • Enhanced antibiofouling ability

• Higher water flux and hydrophilicity • Improved antifouling ability

[115]

Nanocomposite membranes for water and gas separation

Table 11.3 Summary of the performance of the nanocomposite membranes modified by metal and metal oxide nanofillers—cont’d

Ag/Pt

Fe3O4

PSf

PA

CTA-PA

Bulk of support layer

Bulk of support layer

Aminefunctionalized nanocellulose was decorated with Ag and Pt NPs and then blended with PSf dope solution Blending of magnetic NPs with CTA dope solution

FO (AL-FS)

• Improved water

FO (AL-FS, AL-DS)

• Improved water •

Fe3O4/ZnO

PES

PA

Blending of magnetite/zinc oxide NPs with PES dope solution as well as MPD-aqueous solution

FO (AL-FS)

• • •

flux and rejection Lower structural parameter and ICP Higher hydrophilicity Lower structural and ICP Improved water flux with lower flux reduction

[117]

[118] Prospects of nanocomposite membranes for water treatment

Bulk of active and support layer

[116]

flux and salt rejection

285

Fig. 11.6 Schematic view of design and synthesis of AqpZ-embedded vesicular membrane. (A) Reconstitution of AqpZ into vesicles generated from ABA block copolymer blends (DDM stands for dodecyl-β-d-maltoside). (B) Pressure-assisted adsorption and immobilization of vesicles on the surface of gold-coated polycarbonate (PCTE) support. (C) Self-assembly of a monolayer of cysteamine on the gold-coated surface through chemisorptions. The uncovered gold surface is functionalized with a self-assembled monolayer of cysteamine. (D) Layer-by-layer coating of polydopamine (PDA) and histidine (His) over the vesicular membrane. FESEM images of (E) gold-coated PCTE membrane containing a monolayer of self-assembled cysteamine. (F) Polycarbonate membrane with three-cycle coating of PDA-His on the top of chemisorbed cysteamine. (G) PCTE membrane having immobilized vesicle with coating of PDA-His. (H) Higher magnification of (G) [119].

Table 11.4 Summary of osmosis membrane modification using Aqp-based nanomaterials

Nanofiller

Support material (structure)

Active layer material

AqpZ

PCTE

AqpZembedded ABA vesicles

Apps

PAN

CA

LbL of PAH/ PAA/PSS

Selfassemble AqpZvesicle

Bulk of active layer

Bulk of active layer

Surface of the active layer

Operation configuration

Performance summary

Reference

Immobilizing AqpZembedded vesicles followed by covalently binding using disulfide groups, and stabilizing using LbL PDA- histidine coating Aqp-embedded proteoliposomes, containing magnetic NPs, were immobilized in the active layer under magnetic field

FO (AL-FS, AL-DS)

• Robust

[119]

Aqpz incorporation into vesicle followed by in situ imprinting polymerization over CA substrate

FO (AL-FS)

Modification technique

• FO (AL-FS)

•

•

•

•

immobilization of AqpZ on a support surface Improved water flux Higher incorporation capacity of Aqp Enhanced water flux with a slight decline in salt rejection Improved mechanical strength and stability Higher water permeability

[120]

[121]

Continued

Prospects of nanocomposite membranes for water treatment

Aqp

Modified layer

287

288

Nanofiller

Aqp

Aqp

Support material (structure)

Active layer material

PES (Hollow fiber)

PA

PEI (Hollow fiber)

PA

Modified layer

Modification technique

Operation configuration

Performance summary

Reference

• Improved

[122]

water flux and salt rejection • Superior performance compared to commercial membranes • Lower structural parameter • Improved water permeability with lower salt permeability

[123]

Bulk of active layer

Dispersion of Aqpincorporated proteoliposomes in MPD solution

FO (AL-DS)

Bulk of active layer

Aqp in lipid Vesicle

FO (AL-FS, AL-DS)

Nanocomposite membranes for water and gas separation

Table 11.4 Summary of osmosis membrane modification using Aqp-based nanomaterials—cont’d

Prospects of nanocomposite membranes for water treatment

the encapsulation of Aqps into vesicles or block-copolymer followed by immobilization over the support surface. To form an integrally skinned rejecting layer, the vesicles are connected by chemical crosslinking or by a coating of polyelectrolytes using label assembly. The reported results show an improved water permeability of Aqp-based biomimetic membranes compared to unmodified membranes.

11.5 Concluding remarks Over the last decade, utilization of nanomaterials with diverse internal structures and tuned surface functionalities in polymeric membranes has shown great promise for the development of high-performance osmosis membranes. The general idea for the development of nanocomposite membranes has been inducing the thermal, electrical, hydrophilic, and antibacterial properties of nanomaterials to the pristine membrane. For example, TiO2 nanoparticles were found to effectively generate highly oxidizing hydroxyl radicals, which readily attack and decompose organic contaminants in water [43]. The surface charge of the nanocomposite membranes prepared by conductive metal and metal oxide nanoparticles, like indium tin oxide (ITO), can be tuned by applying an external electrical field to prevent adsorption of foulants based on electrostatic repulsion [40]. Adding rod-shaped or tubal nanomaterials, such as CNT and TiO2 nanotubes, with a high ratio of length to diameter was reported to improve the thermal stability of membranes more than spherical nanoparticles [124]. GO nanosheets and nanoribbons offered an exciting opportunity to integrate all these desirable properties [125]. However, the incorporation of nanomaterials to a polymer film to fabricate a robust nanocomposite membrane may not induce the desired functionality to the polymer and even deteriorate its permeation properties. The major challenge is the severe aggregation of these nanomaterials, which forms nonselective voids in the polymer matrix and reduces the selectivity of membrane significantly. This problem becomes even more severe when nanomaterials with a higher aspect ratio, such as GO and CNT, are added to the polymer matrix. Although the developed nanocomposite membranes for the FO process in the literature have exhibited enhanced permselectivity, surface wettability, lower ICP, and improved antifouling tendency, the development of robust and defect-free nanocomposite osmosis membranes is still an open research area. By systematic investigation of surface properties of nanomaterials, and careful optimization of the synthesis routes, novel and integral nanocomposite membranes can be designed for osmotic-driven applications.

References [1] S.C. Anisfeld, Water Resources, Island Press, Washington, DC, 2010. [2] M. Schaefer, Water technologies and the environment: ramping up by scaling down, Technol. Soc. 30 (2008) 415–422, https://doi.org/10.1016/j.techsoc.2008.04.007.

289

290

Nanocomposite membranes for water and gas separation

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[107] W. Kuang, Z. Liu, G. Kang, D. Liu, M. Zhou, Y. Cao, Thin film composite forward osmosis membranes with poly(2-hydroxyethyl methacrylate) grafted nano-TiO2 as additive in substrate, J. Appl. Polym. Sci. 133 (2016) 1–10, https://doi.org/10.1002/app.43719. [108] X.J. Wang, X.L. Shi, S.F. Hou, J.H. Yang, K.L. Zhou, Z.P. Wang, L.G. Wang, Synthesis and characterization of CTA/CA-based forward osmosis membranes with hydrophilic Nano-titanium dioxide, Mater. Sci. Forum 867 (2016) 127–131, https://doi.org/10.4028/www.scientific.net/ MSF.867.127. [109] C. Zhang, M. Huang, L. Meng, B. Li, T. Cai, Electrospun polysulfone (PSf )/titanium dioxide (TiO2) nanocomposite fibers as substrates to prepare thin film forward osmosis membranes, J. Chem. Technol. Biotechnol. 92 (2017) 2090–2097, https://doi.org/10.1002/jctb.5204. [110] W. Xue, K.K.K. Sint, C. Ratanatamskul, P. Praserthdam, K. Yamamoto, Binding TiO2 nanoparticles to forward osmosis membranes: via MEMO-PMMA-Br monomer chains for enhanced filtration and antifouling performance, RSC Adv. 8 (2018) 19024–19033, https://doi.org/10.1039/c8ra03613f. [111] X. Liu, S. Qi, Y. Li, L. Yang, B. Cao, C.Y. Tang, Synthesis and characterization of novel antibacterial silver nanocomposite nanofiltration and forward osmosis membranes based on layer-by-layer assembly, Water Res. 47 (2013) 3081–3092, https://doi.org/10.1016/j.watres.2013.03.018. [112] X. Liu, L.X. Foo, Y. Li, J.Y. Lee, B. Cao, C.Y. Tang, Fabrication and characterization of nanocomposite pressure retarded osmosis (PRO) membranes with excellent anti-biofouling property and enhanced water permeability, Desalination 389 (2016) 137–148, https://doi.org/10.1016/j. desal.2016.01.037. [113] Q. Liu, J. Li, Z. Zhou, G. Qiu, J. Xie, J.Y. Lee, Dual-functional coating of forward osmosis membranes for hydrophilization and antimicrobial resistance, Adv. Mater. Interfaces 3 (2016) 1–8, https:// doi.org/10.1002/admi.201500599. [114] Z. Liu, Y. Hu, Sustainable antibiofouling properties of thin film composite forward osmosis membrane with rechargeable silver nanoparticles loading, ACS Appl. Mater. Interfaces 8 (2016) 21666–21673, https://doi.org/10.1021/acsami.6b06727. [115] M. Qiu, C. He, Novel zwitterion-silver nanocomposite modified thin-film composite forward osmosis membrane with simultaneous improved water flux and biofouling resistance property, Appl. Surf. Sci. 455 (2018) 492–501, https://doi.org/10.1016/J.APSUSC.2018.06.020. [116] P. Cruz-Tato, E.O. Ortiz-Quiles, K. Vega-Figueroa, L. Santiago-Martoral, M. Flynn, L.M. Dı´azVa´zquez, E. Nicolau, Metalized nanocellulose composites as a feasible material for membrane supports: design and applications for water treatment, Environ. Sci. Technol. 51 (2017) 4585–4595, https://doi.org/10.1021/acs.est.6b05955. [117] X.-Y. Chi, P.-Y. Zhang, X.-J. Guo, Z.-L. Xu, Interforce initiated by magnetic nanoparticles for reducing internal concentration polarization in CTA forward osmosis membrane, J. Appl. Polym. Sci. 134 (2017) https://doi.org/10.1002/app.44852. [118] R. Ramezani Darabi, M. Jahanshahi, M. Peyravi, A support assisted by photocatalytic Fe3O4/ZnO nanocomposite for thin-film forward osmosis membrane, Chem. Eng. Res. Des. 133 (2018) 11–25, https://doi.org/10.1016/J.CHERD.2018.02.029. [119] H.L. Wang, T.S. Chung, Y.W. Tong, K. Jeyaseelan, A. Armugam, H.H. P. Duong, F. Fu, H. Seah, J. Yang, M. Hong, Mechanically robust and highly permeable Aquaporin Z biomimetic membranes, J. Membr. Sci. 434 (2013) 130–136, https://doi.org/10.1016/j. memsci.2013.01.031. [120] G. Sun, T.S. Chung, N. Chen, X. Lu, Q. Zhao, Highly permeable aquaporin-embedded biomimetic membranes featuring a magnetic-aided approach, RSC Adv. 3 (2013) 9178–9184, https://doi.org/ 10.1039/c3ra40608c. [121] W. Xie, F. He, B. Wang, T.-S.S. Chung, K. Jeyaseelan, A. Armugam, Y.W. Tong, An aquaporinbased vesicle-embedded polymeric membrane for low energy water filtration, J. Mater. Chem. A 1 (2013) 7592–7600, https://doi.org/10.1039/c3ta10731k. [122] X. Li, S. Chou, R. Wang, L. Shi, W. Fang, G. Chaitra, C.Y. Tang, J. Torres, X. Hu, A. G. Fane, Nature gives the best solution for desalination: aquaporin-based hollow fiber composite membrane with superior performance, J. Membr. Sci. 494 (2015) 68–77, https://doi.org/ 10.1016/j.memsci.2015.07.040.

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[123] X. Li, C.H. Loh, R. Wang, W. Widjajanti, J. Torres, Fabrication of a robust high-performance FO membrane by optimizing substrate structure and incorporating aquaporin into selective layer, J. Membr. Sci. 525 (2017) 257–268, https://doi.org/10.1016/j.memsci.2016.10.051. [124] Y. Zhang, P. Liu, Polysulfone (PSF) composite membrane with micro-reaction locations (MRLs) made by doping sulfated TiO2 deposited on SiO2 nanotubes (STSNs) for cleaning wastewater, J. Membr. Sci. 493 (2015) 275–284, https://doi.org/10.1016/j.memsci.2015.06.011. [125] A. Karkooti, A.Z. Yazdi, P. Chen, M. McGregor, N. Nazemifard, M. Sadrzadeh, Development of advanced nanocomposite membranes using graphene nanoribbons and nanosheets for water treatment, J. Memb, Sci, 2018 https://doi.org/10.1016/j.memsci.2018.04.034.

Further reading [126] J.W. Krumpfer, T. Schuster, M. Klapper, K. M€ ullen, Make it nano-Keep it nano, Nano Today 8 (2013) 417–438, https://doi.org/10.1016/j.nantod.2013.07.006. [127] M.M. Pendergast, E.M.V. Hoek, A review of water treatment membrane nanotechnologies, Energy Environ. Sci. 4 (2011) 1946, https://doi.org/10.1039/c0ee00541j.

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CHAPTER 12

Prospects of nanocomposite membranes for water treatment by membrane distillation Sadaf Noamania, Mohtada Sadrzadehb, Ali R. Tehrani-Baghac a

Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada Department of Mechanical Engineering, 10–367 Donadeo Innovation Center for Engineering, Advanced Water Research Lab (AWRL), University of Alberta, Edmonton, AB, Canada c Department of Chemical Engineering and Advanced Energy, American University of Beirut, Beirut, Lebanon b

12.1 Introduction Membrane distillation (MD) is a nonisothermal separation process that works at relatively low feed temperature (50–80°C) and low working pressure. In the MD process, a hydrophobic porous membrane separates the hot liquid feed from the cold fluid permeate (Fig. 12.1). The driving force of separation in MD is a partial vapor pressure gradient resulting from the temperature difference across the membrane. There is a simultaneous transfer of mass (J) and heat (Q) through the membrane in the MD process. Hot feed fluid (typically salty water) comes in contact with a hydrophobic porous membrane at relatively low pressures close to atmospheric pressure. The hydrophobic membrane with small pores does not let hot fluid pass through the pores unless the liquid’s entry pressure is above a certain threshold. This will be explained further in Section 12.3.5. As a result of a partial vapor pressure gradient, the liquid-vapor moves through the pores and condenses to liquid again on the other side of the membrane (cold side) [1, 2]. Researchers in academia and industry found the MD process very attractive for several reasons: (a) it has a wide range of applications, as can be seen in Table 12.1; (b) it has potential to be used for water desalination with salt rejection rate as high as 100%; (c) it can be performed under atmospheric pressure; and (d) it has a relatively simple design compared with that of reverse osmosis.

12.1.1 Gas transport through a membrane in MD In MD, the hot feed solution is in direct contact with a hydrophobic porous membrane. Eq. (12.1) shows the mass transport of a single component, i, through the membrane:
 (12.1) Ji ¼ Bi Δpi ¼ Bi pi, m  pi, p where Bi is the membrane permeability, pi, m is the partial pressure of the component i at the liquid feed/membrane interface, and pi, p is the partial pressure of the component i at Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00012-2

© 2020 Elsevier Inc. All rights reserved.

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Fig. 12.1 Scheme showing the concept of MD for liquid purification. Liquid-vapor passes through a hydrophobic porous membrane and condenses on the other side of the membrane. Q and J represent the mass and heat transfer, respectively. Table 12.1 Applications of MD process Applications

Notes

References

Desalination for production of high-purity water Water purification

Complete removal of nonvolatile salts such as NaCl, KCl, LiBr Removal of water-soluble polymers such as glucose, sucrose, fructose To make concentrated juice, milk Breaking azeotropic mixtures, recovery of volatile aromas Treating wastewater containing textiles dyes, oils, heavy metals, radioactive materials

[3–6]

Food product concentration Separation of binary mixtures Wastewater treatment

[7, 8] [9–13] [14–16] [17, 18]

the permeate side. The partial pressure is directly proportional to temperature and, as a result, the mass transfer increases by increasing the temperature difference on both sides of the membrane in the MD process. The mean free path of gas molecules through pores (λi) can be calculated using Eq. (12.2). If the pores are relatively small (d < 0.1λi), the diffusive or Knudsen flow (Eq. 12.3) is mainly responsible for their mass transfer through the pores because the gas molecules collide more frequently with the pore wall than to each other [19, 20]. But, if the pores are relatively large (d > 100λi), gas molecules are transferred mainly by viscose or Poiseuille flow (Eq. 12.4) mainly via molecule-molecule collisions. When the pores are in the range of (0.1λi < d < 100λi), a combination of these two equations can be used (Eq. 12.5). By knowing the pore size distribution of the membrane, one can theoretically approximate the mass transfer flux of gas molecules across the membrane (Eq. 12.6).

Prospects of nanocomposite membranes for water treatment

kB T λi ¼ pffiffiffi 2π pm ðσ i Þ2 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi NK  dK3 π Knudsen ¼ Ji  Δpi dK < 0:1λi τt 18Mi RT NV  dV4 πp   Δpi dV > 100λi τt 128RT μ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  NT  dT3 π π p  dT Transition ¼ +  Δpi 0:1λi < dT < 100λi Ji τ  t  RT 128μ 18Mi RT JiViscose ¼

JiTotal ¼ JiKnudsen + JiTransition + JiViscose

(12.2) (12.3) (12.4) (12.5) (12.6)

where kB is the Boltzmann constant, pm and T are the mean pressure and temperature within the membrane pores, respectively, σ i is the collision diameter of gas molecules, τ is the pore tortuosity, t is the membrane thickness, Mi is the molecular weight of the component i, R is the gas constant, p is the average pressure in the membrane pores, μ is the viscosity of specie i, NK, NT, and NV are the number of pores corresponding to the pore sizes dK, dT, and dV, respectively.

12.1.2 Heat transfer through a membrane in MD The total heat transfer (Qtotal) in MD consists of the heat transfer through the feed boundary layer (Qf), the heat transfer through the membrane (Qm), and the heat transfer through the permeate layer (Qp), as can be seen in Fig. 12.2. The heat transfer through the membrane occurs either via conduction (QC) or heat of vaporization (QV). The following equations can be used for estimation of heat transfer through the membrane based on the boundary conditions. Qf ¼ hf  ðTb, f  Tm, f Þ 
 km
QC ¼  Tm, f  Tm, p ¼ hm  Tm, f  Tm, p t
 QV ¼ Jitotal  ΔHvap ¼ hV  Tm, f  Tm, p
 Qp ¼ hp  Tm, p  Tb, p

(12.7) (12.8) (12.9) (12.10)

where hf, hm, and hp are the heat transfer coefficients of feed, membrane, and permeate, respectively. km is the thermal conductivity of the membrane, t is the membrane thickness, Jtotal is the mass transfer flux, △Hvap is the enthalpy of vaporization, or the heat of i evaporation, Tm,f is the feed-membrane interface temperature, Tm,p is the permeatemembrane interface temperature, and Tb,f and Tb,p are the bulk feed (hot side) and permeate (cold side) temperatures, respectively.

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Fig. 12.2 Heat transfer resistance model in the MD process. Tm,f is the feed-membrane interface temperature, Tm,p is the permeate-membrane interface temperature, and Tb,f and Tb,p are the bulk feed (hot side) and permeate (cold side) temperatures, respectively.

Eq. (12.11) is obtained under steady-state conditions. Thus, the total heat flux (Qtotal) can be derived as a function of the total mass flux (Eq. 12.12). The total heat transfer increases by increasing the temperature difference between the hot feed and the cold permeate, as well as the total mass transfer. At steady-state conditions : Qf ¼ QC + QV ¼ Qp 2 31 61 1 17 6 7 Qtotal ¼ 6 + + 7 total 6hf Ji  ΔHvap hp 7 6 7
 hm + 6 7 Tm, f  Tm, p


  Tb, f  Tb, p

(12.11)

(12.12)

12.2 Various MD configurations The first and second MD patent applications were filed in 1963 [21] and 1967 [22], respectively. The first scientific journal paper on MD was published in 1967 [23]. The concept of using a hydrophobic porous membrane for water desalination was proposed in these documents. The number of scientific documents, such as journal/conference/review papers and book chapters, has increased almost exponentially since then. The reader can get more information about the historical perspective of MD by reading the following references [1, 24–26]. There are four commonly used configurations in the MD setup: (a) direct contact membrane distillation (DCMD) [27]; (b) air gap membrane distillation (AGMD) [28]; (c) sweeping gas membrane distillation (SGMD) [29]; (d) vacuum membrane distillation (VMD) [30]. These four configurations are shown in Fig. (12.3). DCMD is commonly used for desalination with excellent permeate flux and a complete salt rejection rate as high as 100%. However, it has a relatively high heat loss due to the direct contact of the cold permeate with the membrane surface (Eq. 12.12). The AGMD configuration

Prospects of nanocomposite membranes for water treatment

Fig. 12.3 Four different configurations of Membrane Distillation (MD) [2]. (A) DCMD: The membrane is in direct contact with the hot feed side (e.g., seawater) and the cold permeate side (e.g., pure water); (B) AGMD: an air gap between the membrane and the cold side acts as a condenser to turn vapor into liquid; (C) SGMD: a cold inert gas sweeps the transferred vapor out of the MD module. A condenser separates the vapor from the sweeping gas and turns it into liquid (product); (D) VMD: the vapor is vacuumed out of the air gap. A condenser turns the vapor into liquid (product).

was an attempt to solve this problem, but the air gap acts as a barrier against mass transfer and reduces the efficiency of the system. In the VMD configuration, air can be removed from the membrane pores by vacuum, which reduces the permeate pressure below the equilibrium vapor pressure and reduces the heat loss due to conduction. This configuration has the highest permeate flux and energy consumption under the same experimental conditions. The advantages and disadvantages of these four configurations have been summarized in Table 12.2 [2, 31–33].

12.3 Essential parameters affecting the MD performance MD is one of the promising candidates to solve the global freshwater crisis via desalination of seawater [34]. MD has some advantages over pressure-driven filtration methods such as reverse osmosis (RO) due to the independence of the feed solution salinity, high quality of produced pure water, and operating at atmospheric pressure. Additionally, it does not need thermal energy as high as conventional thermal technologies, and other sources of heating such as solar irradiation and geothermal can be used to reduce the total cost of

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Table 12.2 MD configuration and their advantages and disadvantages [2, 31–33] MD configuration

Advantages

Disadvantages

▪ High flux ▪ Improved mass transfer ▪ Negligible conductive heat loss ▪ Relatively high flux ▪ Simple design

▪ ▪ ▪ ▪ ▪ ▪

SGMD

▪ Better mass transfer than AGMD ▪ Less heat loss

▪ ▪

AGMD

▪ Flux close to that of DCMD ▪ Internal heat recovery ▪ Less heat loss due to conduction

VMD

DCMD

▪ ▪

Higher risk of membrane wetting Limited heat recovery High energy consumption High sensitivity to foulants High conduction heat loss Cold feed cannot be used as a coolant Addition cost for sweeping gas External condenser with a large volume Low sensitivity to foulants Air gap is limiting the mass transfer

separation. Permeate flux and salt rejection (%) are obtained from Eqs. (12.13) and (12.14), respectively [35]. m Permeation flux ¼ (12.13) At   Cp  100 (12.14) Rejection ð%Þ ¼ 1  Cf where m is the amount of water collected, t is the time of separation, A is the surface area of membrane in contact with hot feed solution, and Cp and Cf are the salt concentration (g/L) in permeate and feed solutions, respectively. The most critical parameters affecting MD performance and efficiency are as follows.

12.3.1 Concentration in the feed solution The salt concentration in the feed solution affects the permeation flux. Increasing the salt concentration leads to a drop in vapor pressure and increases the concentration polarization, which reduces the transmembrane vapor pressure and, as a result, the permeate flux [34, 36, 37].

12.3.2 Temperature As the temperature difference (Tb,f–Tb,p) increases, the water flux becomes higher due to the increase in vapor pressure (Eq. 12.1) [34, 38]. It is suggested that, even with a small

Prospects of nanocomposite membranes for water treatment

temperature difference, the feed temperature should be increased because it causes an increase in heat/mass transfer [39]. Fig. 12.4 shows the effect of temperature on the permeate flux in the MD process [40]. Increasing the temperature on the hot side (Tb,f) enhances the permeate flux noticeably, which also increases the cost of the MD process. The temperature difference should be optimized for obtaining the best efficiency. The temperature polarization effect, which is an intrinsic phenomenon, happens when water molecules evaporate at the interface of the membrane (Fig. 12.2). As a result, the membrane interface temperature at the feed side (Tm, f) would be lower than the bulk feed temperature (Tb, f), and the membrane interface temperature at the permeate side (Tm, p) would be higher than the bulk permeate temperature (Tb, p). This reduces the permeate flux because the actual temperature difference (Tm, f  Tm, p) decreases, and is a driving force for the heat and mass transfer (Eq. 12.12). The temperature polarization coefficient (TPC) is calculated using Eq. (12.15) [2]. Any deviation from unity reduces the performance of the MD process.

Fig. 12.4 Effects of temperature and feed flow rate on permeate flux in the DCMD process for seawater ([NaCl] ¼ 43 g/L) desalination. The coolant flow rate and temperature were 3.65 L/min and 25 °C, respectively. Adapted with permission from A. Khalifa, H. Ahmad, M. Antar, T. Laoui, M. Khayet, Experimental and theoretical investigations on water desalination using direct contact membrane distillation, Desalination, 404 (2017) 22–34.

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TPC ¼

The actual driving force ðinterfacialÞ Tm, f  Tm, p ¼ The theoretical driving force ðbulkÞ Tb, f  Tb, p

(12.15)

12.3.3 Feed and permeate flow rates Increasing the feed and permeate flow rates can lead to vapor permeation enhancement and higher permeation flux. However, pore wetting and leakage can occur at very high feed and permeate flow rates, which results in lower salt rejection percentages [2, 34]. Fig. 12.4 also shows the effect of feed flow rate on the performance of the MD system. Increasing the feed flow rate increases the permeate flux rate slightly. It should also be noted that the chance of pore wetting increases by increasing the feed flow rate, and it needs optimization.

12.3.4 MD configuration The MD configuration (Fig. 12.3) has a significant effect on separation performance. The permeate flux for different configurations is roughly in this order: VMD> DCMD> SGMD >AGMD. However, DCMD is the most popular configuration in research studies due to its relatively high performance (Table 12.2), simplicity, and ease of operation [2, 34, 41].

12.3.5 Membrane properties Membrane properties such as pore size, liquid entry pressure (LEP), wettability, porosity, thickness, and thermal stability are the most important parameters on the MD performance. 12.3.5.1 Liquid entry pressure (LEP) LEP is the maximum allowable working pressure for the MD process. The LEP can be estimated for a porous membrane using Eq. (12.16) [34, 42]: 4γcosθ β (12.16) D where γ is the surface tension of the feed liquid, θ is the liquid–membrane contact angle, D is the largest membrane pore diameter, and β is a geometric parameter determined by the pore structure (i.e., shape factor). If the pressure of the inlet feed flow is higher than the LEP, it can wet the membrane and pass through the pores. The LEP reduces at higher temperatures and flow rates. Thus, there is a higher chance of pore wetting at elevated temperatures and flow rates [2]. LEP ¼

12.3.5.2 Pore size distribution Membranes with large pores have higher permeation flux, as can be seen in Equations 12.1–12.6. However, a membrane with larger pores has a lower LEP and a

Prospects of nanocomposite membranes for water treatment

higher chance of pore wetting and leaking during the process. Therefore, the pores should be maximized to a certain limit to enhance the mass transfer and prevent the pore wetting. Researchers have typically used porous hydrophobic membranes with the average pore size in the range of 0.2–1 μm for the MD process [2, 34, 35, 42]. 12.3.5.3 Wettability and contact angle The MD process is typically used for separation of aqueous solutions. The surface tension of water is around 72.8 mN/m and as can be understood from Eq. (12.16), hydrophobic porous membranes should be used for this process [43]. The presence of surface active materials and oils can reduce the surface tension of the water in contact with the membrane, which will negatively affect the performance of the MD process due to pore wetting. As we will discuss in Section 12.5, one of the reasons for the addition of nanomaterials to membranes is to overcome this problem. 12.3.5.4 Thickness A thinner membrane has a higher permeate flux (Section 12.1.1), but this leads to lower thermal conductance and higher heat loss through the membrane (Section 12.1.2). Moreover, there is a chance of pore wetting and liquid condensation within a thick membrane that deteriorates the performance of the system. Therefore, the thickness of the membrane should be optimized [2, 34, 42]. 12.3.5.5 Mechanical and thermal stability The MD membrane should be robust, durable, and stable. Mechanical and thermal stabilities during the MD’s harsh conditions are crucial, since other parameters such as porosity, pore size, LEP can be negatively affected at elevated temperatures [34, 44]. The thermal conductivity of the membrane should also be very low in order to reduce heat loss by conduction (Eq. 12.8).

12.4 Fabrication and characterization of nanocomposite membranes 12.4.1 Materials Membrane properties are affected by many factors, primarily the material and fabrication method. Ceramic and polymeric membranes can be used in the MD process. However, polymeric membranes are much more common because they have acceptable mechanical and thermal stabilities, and are lighter, inexpensive, and easy to process [42]. The most common types of synthetic organic polymers, which have been commonly used in research, are polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polypropylene, polyethylene, polystyrene, polycarbonate, and polyurethane [42]. These are all hydrophobic polymers, and incorporation of other nanomaterials (e.g., carbon nanotubes, graphene, silica, titanium, zirconium oxide nanoparticles) in the membranes

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can positively affect their properties and improve the MD permeate flux and performance. This topic will be further discussed in Section 12.5.

12.4.2 Methods of fabrication There are various methods for the fabrication of hydrophobic membranes: sintering, stretching, phase inversion, and electrospinning [35, 45, 46]. Most of the nanocomposite membranes used in the MD process are fabricated by phase inversion and electrospinning techniques [35]. In the phase inversion method, a viscous polymer solution is cast to make a film on a support layer (e.g., plate, fabric, etc.) with a controlled speed and thickness. The casted film on the support is immersed in a coagulation bath containing a nonsolvent for the polymer (typically water). The polymer is solidified into a porous membrane due to the solvent exchange. The phase inversion method is a very simple, reliable, and costeffective method for membrane fabrication. This method can be used for the production of polymeric/nanocomposite membranes in a large industrial scale (Fig. 12.5). Electrospinning is another common method for the fabrication of hydrophobic membranes. In electrospinning, a polymer solution can be converted into nanofibers by applying a high voltage between the conductive nozzle and the collector. The micro/nanofibers, produced via a dry spinning method and solvent evaporation, can be collected on a substrate to make a porous membrane with the desired thickness [47]. The resulting electrospun membrane has very high porosity (>70%), interconnected open pore structure, and high roughness. Therefore, this method is used for the fabrication of hydrophobic porous membranes for the MD application [35, 48, 49].

Fig. 12.5 Scheme showing a continuous casting and phase inversion method for the production of membranes. A viscose polymer solution is cast on the surface of a support layer (e.g., nonwoven). The cast membrane enters a coagulation bath (containing a nonsolvent for the polymer). As a result of solvent exchange and phase inversion, a porous membrane is formed. Nanocomposite membranes can be fabricated by dispersing the nanoparticles in the starting polymer solution.

Prospects of nanocomposite membranes for water treatment

Fig. 12.6 Scheme showing a needleless electrospinning machine, which can continuously cover a support layer. High voltage is required to overcome the viscose and surface tension forces of the polymer solution. The polymeric solution is stretched toward the grounded collector and solidifies into several micro/nanofibers after fast solvent evaporation. A series of nozzles or wires can be used instead of the rotating cylinder for injecting the polymer solution. Nanocomposite membranes can be fabricated by dispersing the nanoparticles in the starting polymer solution [2].

Fig. 12.6 presents a needleless electrospinning setup schematically [2]. Despite the simple electrospinning setup, there are many parameters involved that should be optimized to achieve a suitable MD membrane. The type of polymer and solvent, polymer concentration, high voltage, the distance between the spinneret and the collector, temperature, humidity, feed and collecting rates are among the most critical and influential parameters [50–52]. PTFE porous membranes, commonly used in the MD process, are produced with a method called biaxial heat stretching. PTFE films are heated and quickly stretched to produce millions of pores in the structure [27, 53]. The porosity and the pore size distribution of the membrane can be controlled by the applied temperature and the stretching ratio [2, 53–55].

12.4.3 Characterization Some of the most important methods for characterization of membranes are presented briefly here. The thickness of membranes can be measured by a precision thickness gauge. The density of the membrane (ρM) can then be calculated by dividing the weight of the membrane by its volume. The porosity, or air volume fraction, of a membrane can be calculated from the following equation assuming that the membrane is composed of a known polymer with the density (ρP) and neglecting the density of air:

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ρM ¼ ρAir  VAir + ρP  ð1  VAir Þ   ρM Porosity ð%Þ ¼ VAir  100 ¼ 1   100 ρP

(12.17)

where VAir is the air volume fraction of the layer, and ρM and ρP are the density of the layer and the polymer, respectively. If the layer is composed of more than one polymer or nanoparticles, the average density of the mixture should be used in Eq. (12.17). The mechanical properties of membranes such as tensile strength, tensile strain, and Young’s modulus can be obtained using an electromechanical tensile testing machine according to the standard testing methods [2]. There are various methods to measure the pore size distribution in membranes, including thermoporometry, permporometry, mercury porosimetry, gas-liquid porosimetry, and liquid-liquid porosimetry [2, 45, 56]. The surface properties of nanocomposite membranes can be investigated by several techniques such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM micrographs can be used for investigating the surface morphology, measuring the size of the pores on the surface and the thickness of membranes. AFM can also be used to investigate the surface topography of membranes to provide a reliable 3D image of the surface with high resolution [2, 45, 56]. Researchers have used various techniques for measuring roughness such as atomic force microscopy [57], 3D profilometry [58], laser scanning microscopy [59], or contact mode scanning probe microscopy analysis [60]. The wettability of membranes can be investigated using an optical tensiometer or contact angle measurement. The same method can be used for measuring the surface free energy of the membrane [2].

12.5 Nanocomposite membranes in MD Many recent studies have sought to improve the performance of the MD process. Some of these studies have been summarized in Table 12.3. In the majority of these papers, phase inversion and electrospinning methods have been employed for the fabrication of nanocomposite membranes. Commercially available porous membranes have also been used in some of these studies as a substrate or a supporting layer. These commercial membranes (CM) have been coated/treated with chemical vapor deposition, spraying, casting, electrospinning, and wet chemical treatment. These research works can be categorized into three groups:

12.5.1 Dual membrane layer (hydrophilic-oleophobic) Hydrophilic nanoparticles or polymers can be added to one side of a hydrophobic porous membrane to enhance its hydrophilicity. This approach improves the surface water wettability and enhances the DCMD permeate flux (Table 12.3, #1–3) [61–63]. Such membranes have much lower sensitivity to oil contamination in the feed side

Table 12.3 Comparison of water desalination results using various nanocomposite membranes in the MD process

#

1

Type of membrane and method of preparation

M1: PVDF-HFP M2:M1 + coated cellulose layer

Feed solution

Permeate flux (kg/ m2/h)

70–20 ¼ 50

[NaCl] ¼ 35 g/L + 1 g/L mineral oil

M1: 15 (failed) M2: 12

DCMD

Salt rejection M1: 98% decaying M2: 100%

[61] 2019

60–20 ¼ 40

[NaCl] ¼ 35 g/L + 0.01% mineral oil

CM: 4 (failed) M1: 5.5 M2: 6.3

DCMD

Salt rejection CM: 98% decaying M1: 99% M2: 99%

[62] 2013

Characteristicsa

Thot– Tcold 5 ΔT (°C)

M1: D ¼ 380 nm M2: D ¼320 nm

MD configuration

Method: electrospinning + coating CM: D ¼ 100– 550 nm, t ¼ 125 μm M1: 150–270 nm M2: 110–330 nm

Ref. Year

Notes

2

CM: PVDF M1: PVDF + PEG M2: M1 + TiO2 NPs method: plasma grafting and deposition

3

CM: D ¼ 480 nm, CM: PTFE M1: PTFE coated with ES PVA t ¼ 185 μm, and silica NPs and crosslinked WCA ¼ 135° with glutaraldehyde OCAUW ¼ 36° Method: Electrospinning + M1: D¼410 nm sol-gel t ¼ 348 μm, WCA ¼ 21° OCAUW ¼ 156°

53–20 ¼ 33

[NaCl] ¼ 35 g/L + 1 g/L crude oil

CM: 19.5 (decaying) M1: 17.5

DCMD

Salt rejection CM: 93% (decaying) M1: 99%

[63] 2018

4

CM: PTFE M1: silanated glass fiber membrane Method: 5 step silanation and growth of Silica NPs

CM: D ¼ 450 nm, t ¼ 200–270 μm

60–20 ¼ 40

[NaCl] ¼ 1 M + [SDS]¼ 0.1 mM

CM: 32 M1: 16

DCMD

Salt rejection CM: 90% decaying M1: 99%

[64] 2014

5

CM: PVDF M1: PVDF + silica NPs + perfluorodecyltrichlorosilane (FDTS) Method: coating and grafting

CM: D ¼ 450 nm, t ¼ 125 μm t increment ¼ 3–5 μm

60–20 ¼ 40

[NaCl] ¼ 1 M + [SDS]¼ 0.1 mM

CM: >40 (failed) M1: 12

DCMD

Salt rejection CM: 80% decaying M1: 99%

[65] 2016

Continued

Table 12.3 Comparison of water desalination results using various nanocomposite membranes in the MD process—cont’d

#

Type of membrane and method of preparation

Characteristics

CM: D¼460 nm, t ¼ 100 μm M1: D ¼ 560 nm, t ¼ 83 μm M2: D ¼ 420 nm, t ¼ 87 μm M3: D ¼ 470 nm, t ¼ 91 μm

Feed solution

Permeate flux (kg/ m2/h)

60–20 ¼ 40

[NaCl] ¼ 35 g/L + [SDS]¼ 0.3 mM

CM: 23 M1: 36 M2: 33 M3: 33

DCMD

Salt rejection CM: 60% M1: 90% M2: 97% M4: 100%

[66] 2019

60–20 ¼ 40

[NaCl] ¼ 70 g/L + [SDS]¼ 1 mM

CM: 40 (decaying) M1: 45

DCMD

Salt rejection CM: 97% (decaying) M1: 99%

[67] 2018

50–25 ¼ 25

35 g/L aquoes NaCl solution

M1: 1.2 M2: 2.7 M3: 4.5

VMD

[68] Permeate 2019 flux enhancement M2: >100% M3: >270%

CM: 42 M1: 45

DCMD

Thot– Tcold 5 ΔT (°C)

6

CM: commercial PVDF M1: PVDF-HFP M2: M1 + PDMS (3%) M3: M2 + silica aerogel (30%) Method: Electrospinning + electrospraying

7

CM: commercial PTFE M1: CM coated with graphene Method: CVD coating

8

M1: PVDF M2: PVDF + MWCNT (2%) M3: PVDF + MWCNT + SiO2 NPs Method: Phase inversion

M1: D ¼ 60– 140 nm, t ¼ 92 μm M2: D ¼ 49– 114 nm, t ¼ 85 μm M3: D ¼ 43– 130 nm, t ¼ 113 μm ε ¼67–86% WCA ¼ 81–85°

CM: PTFE M1: PVDF-HFP + activated carbon (1.5 w.t.%)

CM: D ¼ 345 nm, 60–15 ¼ 45 t ¼ 120 μm, ε ¼70% M1: D ¼ 787 nm, t ¼ 200 μm, ε¼ 90%

9

Method: Electrospinning

ε ¼77–89% CM: D ¼ 400 nm, t ¼ 120 μm graphene layer: t ¼ 1.7 nm

[NaCl] ¼ 35 g/L

MD configuration

2.3 kPa

Notes

Salt rejection > 99%

Ref. Year

[69] 2018

10 M1: PVDF M2: PVDF + grafted ZnO NPs with PS Method: phase inversion

M1:: D 220 nm ε ¼ 38% M2: D ¼720 nm ε ¼ 75%

70–22 ¼ 48

[NaCl] ¼ 30 g/L

M1: 5 M2: 15

DCMD

Salt rejection > 99%

[70] 2018

11 M1: PVDF M2: PVDF + CaCO3 NPs Method: phase inversion

M1: D ¼ 113 nm, t ¼ 219 μm, ε ¼ 55%, WCA ¼ 72° M2: D ¼ 196 nm, t ¼ 221 μm, ε ¼64%, WCA ¼ 81°

53–20 ¼ 33

[NaCl] ¼ 35 g/L

M1: 12.4 M2: 14.6

DCMD

Salt rejection M1: decaying M2: >99% stable

[71] 2014

12 M1: PVDF M2: PVDF + 5 wt% MOF Method: Electrospining

M1: D ¼ 480 nm, t ¼ 47 μm, ε ¼ 61%, WCA ¼ 133° M2: D ¼ 630 nm, t ¼ 80 μm, ε ¼66%, WCA ¼ 138°

48–16 ¼ 32

[NaCl] ¼ 35 g/L

M1: 1.86 M2: 2.87

DCMD

Salt rejection M1: decaying M2: >99% stable

[72] 2018

a

D: average pore size diameter; WCA: water contact angle; OCAUW: oil contact angle under water; t: thickness; ε: porosity.

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(i.e., high oil-water contact angle underwater), which is a significant enhancement for fouling reduction. The pristine membrane fails in the MD process when the salty water contains oil, while the nanocomposite membrane can perform with a very high salt rejection rate. Their higher permeate flux rates can be explained by higher porosity and slightly larger pores [73]. Adding silica nanoparticles to the surface of PTFE commercially available membranes can increase the surface properties (i.e., lower WCA and higher OCA underwater). A robust oil-fouling-resistant membrane can be obtained by this approach [63, 74]. The pore wetting of the PTFE membrane can also be prevented by adding a condensed, thin layer of hydrophilic polyurethane layer [75].

12.5.2 Omniphobic membranes Hydrophobic MD membranes are prone to wetting by low surface tension liquids, like surfactant solutions. This vulnerability limits their use in treating industrial wastewater from, for example, the production of shale gas. The membrane can be grafted by silica or zinc oxide nanoparticles and coated with fluoroalkyl silane (perfluorodecyltrichlorosilane) to lower the membrane surface energy. As a result, an omniphobic membrane with high permeate flux and salt rejection (%) is obtained. Such nanocomposite membranes can perform well even in the presence of surfactants and oil (Table 12.3, #4–6) [64–66]. Several other research papers can be found using the same concept for fabricating an omniphobic membrane for the MD process [76–81].

12.5.3 Filler-based membranes The addition of nanoparticles to the membrane matrix can increase its pore diameter, narrow the distribution of pore size, and enhance the membrane porosity. The membrane surface roughness can also increase, which results in a higher WCA. It should be noted that the presence of excess nanoparticles in the membrane matrix can also deteriorate these properties. Therefore, the concentration of a filler should be wisely optimized [71]. Researchers have used a range of fillers (e.g., activated carbon, graphene, graphene oxide, carbon nanotubes, metal organic frame, ZnO, and CaCO3) to enhance the properties of membranes in the MD process (Table 12.3, #7–12) [67–72]. A PTFE membrane (D ¼ 0.2 μm, thickness ¼ 35 μm), cast with a mixture of PVDF and graphene oxide, was found to have a higher MD flux compared to that of the pristine membrane. The permeate flux increased at least 30% at various temperature differences. This was explained by better interaction of water vapor with polar groups of GO, activated diffusion of water vapor, and reduced temperature polarization due to the presence of conductive GO on the surface of the membrane [82]. Graphene has a 2D planar structure with a hydrophobic nature, which makes it attractive to use in the fabrication of hydrophobic membranes for the MD process.

Prospects of nanocomposite membranes for water treatment

Moreover, it is used to enhance mechanical, thermal, electrical, and functional properties of membranes [83–85]. Adding graphene oxide or graphene to a membrane (up to a certain level) can enhance its performance in the MD process by increasing the average pore size of the membrane [86, 87]. The optimized filler concentration has been in the range of 0.3–0.5 wt% [43, 88]. A PTFE membrane coated with a thin layer of graphene (1.7 nm) via CVD process can enhance the permeate flux of the membrane with very high salt rejection (>99%) even in the presence of 1 mM surfactant. The overlapping size is between 0.3 and 0.7 nm, and the nanochannels of mismatched overlapping grain boundaries do not let surfactant and oil molecules pass through [67]. Incorporating activated carbon (AC) in the membrane enhanced the permeate flux in the MD process. This was explained by fast transport along the AC surface, higher effective surface area, and rapid diffusion via adsorption/desorption on AC protrusions [69]. Carbon nanotubes (CNTs), with a one-dimensional cylindrical nanostructure and excellent thermal conductivity, can also enhance the overall water vapor transport by selective adsorption of water vapor and repelling the salty water in the MD process [89–92]. CNTs are hydrophobic and can be functionalized to improve their intermolecular interactions with the polymeric matrix. Their cylindrical nanostructure enables outstanding water transport and enhances the permeation flux in the MD process [93–95].

12.6 Conclusion Researchers in academia and industry found the MD process very attractive for several reasons: (a) it has a wide range of applications (Table 12.1); (b) it has high potential to be used for water desalination with salt rejection rate as high as 100% (Table 12.3); (c) it can be performed under atmospheric pressure; and (d) it has a relatively simple design (Fig. 12.3). Nevertheless, there are also some shortcomings, such as membrane wetting and fouling, which have prevented its application in large industrial scale. Several steps must be taken to overcome these shortcomings and improve the efficiency of the MD process: (1) develop long-life, reliable, thermally stable and low-cost membranes that are resistant against pore wetting and fouling due to the presence of oil and surfactant contaminations; (2) reduce the heat loss from the system and seek other sources of heating such as solar irradiation and geothermal to reduce the cost of separation; and (3) enhance the design of MD module to maximize the mass transfer and obtain a steady high permeate flow. Fabricating nanocomposite membranes is an attempt to develop fouling-resistant membranes with higher permeate flux. These membranes can be categorized into three groups of dual-layer (hydrophilic-oleophobic), omniphobic, and filler-based membranes (Section 12.5). The presence of a hydrophilic or omnophobic layer on the surface of a membrane enhances its resistance against oil and surfactant contamination in the feed

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solution. The presence of fillers (e.g., graphene, carbon nanotubes, silica nanoparticles, etc.) can enhance the permeate flux of the membrane by increasing the porosity and pore size of the resulting membranes.

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Prospects of nanocomposite membranes for water treatment

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CHAPTER 13

Prospects of nanocomposite membranes for water treatment by electrodriven membrane processes Yan Zhao, Yanling Liu, Emily Ortega, Bart Van der Bruggen Department of Chemical Engineering, KU Leuven, Leuven, Belgium

13.1 Introduction In recent years, environmental pollution, water and mineral resource scarcity, and human health have become critical global challenges. As a result of the depletion of mineral reserves, environmental pollution and water shortage researchers have begun to explore the extraction of abundant resources of ions or freshwater resources from salt water. Electrodriven membranes are the heart of electrodialysis (ED), electrodialysis reversal (EDR), and diffusion dialysis (DD), which selectively transfer ions under application of an electric field. These technologies have been applied on a large scale for over 60 years [1]. Electrodriven membranes allow for anion/cation separation without water flux. This kind of nanocomposite membrane consists of polymeric films bearing covalently bound ionic fixed charges, which enable the transport of oppositely charged electrolyte ions (counterions) through the membrane structure. At the same time, due to the electrostatic repulsion force, ions with the same charge sign (coions) are retained in the feed compartment [2]. These electrodriven membranes are applied in an electric field. For example, electrodriven membranes are used in ED in a system based on four main parts (electrodriven membranes, electrode chamber, diluted chamber, and concentrated chamber), as shown in Fig. 13.1. Cations/anions are selectively transported from one compartment (the diluted compartment) to another compartment (the concentrated compartment) through electrodriven membranes under the driving force of an electrochemical potential gradient, as shown in Fig. 13.2. In the application of an electric field force, the separation of anions and cations is based on Donnan equilibrium, chemical potential equilibrium, and the electric double layer [3]. Electrodriven membranes, also called ion-exchange membranes, are classified into anion-exchange membranes and cation-exchange membranes depending on the type of ionic groups attached to the membrane matrix. Because there are no osmotic pressure limitations, electrodriven membranes can be used to achieve a higher concentration of

Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00013-4

© 2020 Elsevier Inc. All rights reserved.

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Fig. 13.1 Schematic principles of electrodriven membranes in ED.

Fig. 13.2 Cations/anions are selectively transported from cation/anion-exchange membranes.

salts or a more pure solvent compared with other nanocomposite membranes, such as those applied in nanofiltration and reverse osmosis [4].

13.2 Membrane materials for electrodriven separations Electrodriven membranes or ion-exchange membranes (IEMs) can be considered ion-exchange resins in film form. The membrane properties determine, to a very large extent, the usefulness of an electrodriven process in certain applications. IEMs are typically composed of hydrophobic substrates, immobilized ionic groups, and movable counterions. Depending on the charge of their ionic groups, IEMs are broadly classified into cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs) [5]. The ionic groups will dissociate after sufficient penetration by water molecules, releasing cations or anions for the transfer of corresponding ions. Additionally, bipolar membranes

Prospects of nanocomposite membranes for water treatment

(BPMs), mixed matrix membranes (MMMs), and some functionalized membranes have been recently reported, which opens up the opportunity to further promote the electrodriven membrane processes [6].

13.2.1 Cation-exchange membranes (CEMs) Cation-exchange membranes contain negatively charged groups fixed to a polymer matrix, preferentially permeable to cations but rejecting anions. Typical cation-exchange groups are sulfonic acids, phosphonic acids, carboxylic acids, and phenolic hydroxide groups, among which the former two are more widely used [6–8]. 13.2.1.1 Cation-exchange materials based on sulfonic acid groups The sulfonic acid group is the most commonly used cation-exchange group in fuel cell applications and other electrodriven membrane processes, due to its high acidic strength (low pKa) and high proton conductivity. Among the sulfonic acid groups, the most common are polystyrene sulfonic acids and perfluoroalkyl sulfonic acids. The polystyrene sulfonic acids include crosslinked and noncrosslinked poly(α,β,β-trifluorostyrene sulfonic acid), introduced by Ballard (known as Ballard Advanced Material, generation 3) (Fig. 13.3). Although crosslinked polystyrene sulfonic acid has high proton conductivity, due to the tertiary carbon in the polymer backbone, it lacks chemical stability. In comparison, noncrosslinked poly(α,β,β-trifluorostyrene sulfonic acid) has the advantage of better chemical stability and a higher solubility in common dipolar aprotic solvents. Perfluoralkyl sulfonic acids (PFSAs) are known under the trade name Nafion from DuPont or other companies such as Asahi Glass (Flemion), Asahi Kasei (Aciplex), FuMa

Fig. 13.3 General structure of (A) crosslinked polystyrene sulfonic acid (m
15 mol%) and (B) noncrosslinked poly(α,β,β-trifluorostyrene sulfonic acid) with n ¼ 0.2–0.4 and m ¼ 0.8–0.6; R ¼ halogen, CF ¼ CF2, CnF2n+1, OX (X ¼ CnH2n+1, CnF2n+1, Aryl).

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Table 13.1 Structure of various PFSAs Trade name of PFSAs

Nafion Flemion Aciplex Fumion F Aquivion

Structure of sulfonated comonomer

m 1; n ¼ 2 m ¼ 0, 1; n ¼ 1–5 m ¼ 0–2; n ¼ 2–5 m 1; n ¼ 2 m ¼ 0; n ¼ 2

Tech (Fumion F), and Solvay (Aquivion) (Table 13.1). The major differences between the various PFSAs are the length and number of the sulfonic-acid-bearing side chains, which determine the equivalent weight (EW) or ion-exchange capacity (IEC) of the material. PFSA materials are widely used in chlor-alkali electrolysis and fuel cell applications due to their high chemical stability and high proton conductivity under highly humidified conditions. However, due to the complex synthesis and safety measures, these monomers and polymers are still expensive to produce. In general, cation-exchange materials can be synthesized in two ways: postsulfonation of an existing polymer, or the use of sulfonated monomers. Both approaches have advantages and disadvantages. Postsulfonation is easy to conduct but bears the risk of undesired side reactions, like degradation or crosslinking. In comparison, the use of sulfonated monomers allows control of the sulfonation site, as well as the number and distribution of sulfonic acid groups along the polymer chain. However, currently there are only a limited number of commercially available monomers (Fig. 13.4). A compromise

Fig. 13.4 Examples of currently commercially available sulfonated aromatic monomers.

Prospects of nanocomposite membranes for water treatment

between these two methods is the incorporation of monomers, such as hydroquinone, phenylhydroquinone, 2,5-diphenylhydroquinone, 9,90 -bis(4-hydroxyphenyl)-fluorene, which can be selectively sulfonated into the polymer backbone. The aromatic polymers that can be functionalized include poly(arylene ether sulfon)s, poly(arylene ether ketone)s, poly(phenylene oxide)s, poly(p-phenylene)s, polyimides, polyaramides, poly(benzimidazole)s, poly(arylene thio ether), nitrile group bearing poly(arylene ether), and poly(phenylene sulfone)s. The sulfonation of an aromatic molecule can be an electrophilic substitution reaction in which electron-rich aromatic sites are substituted preferentially. An existing polymer can also be sulfonated by the metalation (lithiation) route, where the sulfonic acid group is introduced to electron-poor sites of aromatic rings, reducing the risk of desulfonation and oxidative degradation. The cheapest and most popular approach to sulfonate aromatic polymers to treat them with concentrated sulfuric acid, which acts as a sulfonating agent and solvent. Commonly used sulfonating agents are listed in Table 13.2. 13.2.1.2 Cation-exchange materials based on phosphonic acid groups Phosphonic acid groups are also widely used as cation-exchange groups. Due to their higher selectivity, the phosphonic acid-based materials are mainly applied for the removal of heavy metal ions from water resources, especially in highly acidic environments. Phosphonic acids have also attracted attention in fuel cell applications because of their higher thermal stability compared to sulfonic acids. Commercial phosphonic acid-based materials are known under the trade names Purolite S940 (Purolite International), Lewatit TP260 (Lanxess), Amberlite IRC747 (Dow Chemical), or Duolite C467 (Dow Chemical). The phosphonic acid group is generally linked to the polymer backbone via an amino group, which is consequently called aminophosphonic acid. These materials can be prepared from an aminofunctionalized polymer, an aldehyde or ketone Table 13.2 Commonly used sulfonating agents Sulfonating agents

Reactivity

Electrophilic substitution

Chlorosulfonic acid Fuming sulfuric acid (oleum) Concentrated sulfuric acid Sulfurtrioxide/triethylphosphate (TEP) Trimethylsilylsulfonylchloride

High High Medium to high Medium to high Medium

Lithiation route

BuLi + SO2 or SO3 BuLi + sultones, halogeno-alkylsulfonates

High High

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Fig. 13.5 General reaction route of Karabachnik-Fields reaction (R1, R2, R3 ¼ H and/or alkyl and/or aryl; R4 ¼ alkyl or aryl). PCl3/AlCl3

Friedel-Crafts P(OAlkyl)3 Arbuzov Na-P(O)(OAlkyl)2

Michaelis-Becker

Fig. 13.6 Synthetic routes of alkyl or aryl phosphonates from halogenated educts (R ¼ alkyl, perfluoroalkyl, phenyl).

functionalized polymer, or a chloromethylated polymer via Karabachnik-Fields reaction (Fig. 13.5). Compared to sulfonation, phosphonation of organic compounds or polymers is more complicated, especially when the phosphonic acid group will link directly to an aromatic ring. Phosphonic-acid-based materials can be prepared starting from halogenated educts, arylhalides, polysulfones, polyethylenes, polypentafluorostyrenes, etc. The synthetic routes of alkyl or aryl phosphonates from halogenated educts by using different reactions are shown in Fig. 13.6.

13.2.2 Anion-exchange membranes (AEMs) Anion-exchange membranes contain positively charged groups fixed to a polymer matrix, which reject cations but allow the passage of anions [6, 9]. Some anion-exchange groups that have been used for the synthesis of AEMs are presented in Fig. 13.7. 13.2.2.1 Anion-exchange materials based on quaternary ammonium groups Quaternary ammonium groups are commonly used as anion-exchange materials because of the facile synthesis procedure and higher chemical stability relative to other AEMs. The quaternary ammonium groups are obtained by treating a polymer precursor bearing benzyl halide groups with trimethylamine (TMA). For example, the benzyl chloride function can be introduced into a polymer (e.g., polystyrene) either by using vinylbenzyl chloride as monomer or by chloromethylation of an existing polymer with the use of

Prospects of nanocomposite membranes for water treatment

R N+

+ N R

N

N

+ N R N

N

R N+

R5 N R4 +

N

R1 N

R3

+ R N

+N R2

R2

N

N R1

R3

P +

N N

N

MeO OMe

OMe R

R

NH 2+

Ru N N

OMe

O

N

N

R1

MeO

MeO

R O

N

R

OMe

N

N

N

+ P

MeO

+ P

N+ R2

OMe R

+N R

N

S+

Co +

Fig. 13.7 Examples of available anion-exchange groups.

catalysts (Fig. 13.8). For the polymers containing methyl groups in the backbone, the bromination of methyl groups is available as a synthetic route toward anion-exchange materials. The quaternary ammonium groups can also be obtained from many other polymers, including epichlorohydrin or epoxy-based polymers, poly(vinylalcohol), poly(acrylate)s or poly(olefine)s, perfluorinated polymers, chitosan. The main drawback of ammonium salt-based anion-exchange materials is their inadequate stability at elevated temperatures in highly alkaline environments, which limits their development and commercialization. Among the various anion-exchange groups available on an industrial scale, the benzyl trimethylammonium group (Type-I SBA) has the highest stability under thermal or alkaline conditions. Bulky substituents at the nitrogen and/or electron donating groups or unfavorable leaving groups might increase the alkaline stability of anion-exchange materials. In addition, degradation of the ionexchange group and the polymer main chain might occur leading to a loss of strong

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+n

m

+o

n

+o

CH2Cl Polymerization

Polymerization CICH2 – O – CH3+SnCl4 or CH2O+TMS–Cl+SnCl4

CH2Cl

n

m

o

n

o

NHR2+R-CI or NR3 +

–

CH2NR3 Cl

m

n

o

Fig. 13.8 Synthetic routes of polystyrene based anion-exchange materials with quaternary ammonium groups (TMS-Cl refers to trimethylsilyl chloride).

ion-exchange sites or even the overall ion-exchange capacity, which also needs to be taken into account. 13.2.2.2 Anion-exchange materials based on quaternary phosphonium groups Due to their antimicrobial properties compared to those of ammonium groups, quaternary phosphonium groups are promising candidates for the preparation of anionexchange materials. AEMs based on phosphonium groups can be simply synthesized by reacting polymer precursors containing benzyl halide with either trimethylphosphine or triphenylphosphine. However, the application of these materials is limited by their relatively low chemical stability in alkaline media. Phosphorus in quaternary phosphonium salts is in the oxidation state (III) and can be rapidly oxidized to phosphorus (V), yielding phosphinoxides and a hydrocarbon.

13.2.3 Bipolar membranes (BPMs) A bipolar membrane is a composite membrane composed of one cation-exchange layer (CEL), one anion-exchange layer (AEL), and an interfacial layer (IL) in between. Owing

Prospects of nanocomposite membranes for water treatment

to their ability to generate protons and hydroxide ions from water molecules under direct current polarization, BPMs enable many novelties in industrial and environmental applications of electrodriven membrane processes. When a potential is applied across a BPM, a large electric field is generated in the IL, in which the water molecules become sufficiently polarized and consequently dissociated. The accelerated water dissociation in BPMs, which is seven orders of magnitude higher than in free solution, is attributed to the protonation and deprotonation reaction between water molecules and fixed charge groups according to the so-called reversible proton transfer reaction mechanism [2, 6]. There are various methods to prepare BPMs, such as pasting a CEL and an AEL together by adhering with heat and pressure or by using adhesives, layer-by-layer casting CEL and AEL materials, respectively, functionalizing the two membrane sides to give cation and anion selectivity. The IL is the key element to facilitate water dissociation; thus, the chemical composition and structure of this region need to be elaborately designed to improve the BPM’s performance. One effective strategy is to incorporate catalysts into the IL, which can provide alternative paths with low effective activation energy for water dissociation. Materials with great catalytic activity include weak acids and bases, such as amino groups, pyridine, carboxylic acids, and phenolic and phosphoric acid groups, with an equilibrium constant close to the water dissociation reaction constant (pKa ¼ 7). Heavy metal ion complexes, such as ruthenium trichloride, chromic nitrate, indium sulfate, hydrated zirconium oxide, and hydrophilic macromolecules, such as polyethylene glycol, polyvinyl alcohol, starburst dendrimers, as well as inorganic substances, like graphene oxide, are also promising catalysts for enhancing water dissociation in the IL. Although high-performance, commercial BPMs have been available, improving their stability under extreme conditions, such as high temperature and overlimiting current density, remains a challenge for further industrial-scale applications and development.

13.2.4 Mixed matrix membranes (MMMs) Mixed matrix membranes (MMMs), also known as inorganic-organic hybrid membranes, are increasingly favored over pure organic polymers or inorganic materials. In MMMs, organic polymers are used as a continuous phase offering structural flexibility, processing ability, and tunable electronic properties. Inorganic particles, such as metal oxides, metal phosphates, graphene oxide, and carbon nanotube, are incorporated into polymer matrices to enhance thermal and mechanical stability and achieve desired electrochemical properties. The MMMs can combine the advantages of both organic and inorganic materials and result in new characteristics from the interactions between the two components. During preparation, inorganic particles can either be dispersed in polymer matrices through mechanical doping, or generated in situ, and linked to a polymer backbone through chemical bonding. Preparation methods include sol-gel process,

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blending, intercalation, in situ polymerization, and molecular self-assembly. Of these, the sol-gel process is the most popular because it is easy to achieve the fine distribution of inorganic particles. Although MMMs are more advantageous than the polymeric IEMs in terms of desired properties such as mechanical and thermal stabilities, achieving homogeneity on a molecular scale still remains challenging for the development of MMMs [10].

13.2.5 Functionalized membranes The rapid development of novel materials brings about many opportunities for gaining new, functionalized, high-performance IEMs. Among these materials, graphene oxide and metal-organic frameworks have shown great potential in membrane preparation and are, therefore, briefly introduced as follows [11]. 13.2.5.1 Graphene oxide (GO)-based membranes Due to the excellent electrical, mechanical, and thermal properties, GO is an attractive material for preparing IEMs, particularly proton-exchange membranes (PEMs) for fuel cell applications [11, 12]. GO can be used to prepare a freestanding film or is, more commonly, dispersed in a suitable polymer matrix to obtain a GO-based nanohybrid PEM. Owing to its large surface area and the presence of hydrophilic functional groups, the incorporation of GO provides a ready environment for proton conduction and enhances the water retention ability. GO has excellent compatibility with many polymers for preparing IEMs such as Nafion due to their strong interfacial attraction [13]. GO can modify both the backbone and the side chains of Nafion, leading to enhanced thermal and mechanical properties. Moreover, the properties of GO can be further improved by chemical functionalization, which is also beneficial for promoting the uniform dispersion of GO in different organic solvents. The functionalized GO materials used to prepare nanohybrid PEMs include sulfonated GO, polyoxometalate-coupled GO, triazole functionalized GO, silica-grafted sulfonated GO, polydopamine-modified GO [14]. 13.2.5.2 Metal-organic framework (MOF)-based membranes Metal-organic frameworks (MOFs) have gained interest in membrane preparation due to their characteristics such as high surface area, multifunctionality and chemical/thermal stability. MOFs with ion-exchange/sorption properties are considered next-generation ion-exchange materials, which combine a highly ordered porous structure and a large variety of binding groups.

13.3 Performance indices The performance indices of electrodriven membranes are multifaceted. Due to the special application of these kinds of nanocomposite membranes, they should be

Prospects of nanocomposite membranes for water treatment

comprehensively evaluated and analyzed according to the electrochemical properties, chemical properties, and physical and mechanical properties of the membranes. For traditional electrodriven membranes, such as anion-exchange membranes and cationexchange membranes, the following common performance indices should be provided.

13.3.1 Ion-exchange capacity The ion-exchange capacity (IEC), which refers to the number of ions exchanged per unit volume or mass of materials, is the key index for electrodriven membranes and their capacity for ion exchange. Generally, the IEC is proportionate to the number of charged groups of electrodriven membranes. The ion-exchange capacities of membranes are measured by using the titration method. For anion-exchange membranes, the IEC is measured by acid-base titration. Membrane samples should be dried at 80°C under vacuum until the weight remains constant. The dry AEM samples are then accurately weighed and converted to Cl in 0.1 M NaCl for 48 h. Next, these membranes should be washed with pure water ensuring that there is no absorbed NaCl on the surface of the membrane samples. Then the membranes should be immersed in 0.05 M Na2SO4 for 48 h. The solution is titrated using 0.01 M AgNO3, and K2CrO4 is chosen and used as an indicator. The IEC of membrane samples is calculated from the measured amounts of exchanged Cl and the IEC is calculated by using the following equation: IEC ðAEMÞ ¼

CAg ∙VAg , mDry

where CAg is the concentration of AgNO3, VAg is the volume of consumed AgNO3, and mDry is the weight of the dry membrane samples. For cation-exchange membranes, the IEC is also measured by acid-base titration. The dry membrane samples are typically soaked in 1 M HCl, thoroughly rinsed with pure water, and then soaked in 2 M NaCl for 48 h. The IEC was determined by titration of NaCl solution with 0.01 M NaOH. The final IEC values were calculated by taking into account the titration of blank 2 M NaCl solution. The IEC is calculated using the following equation: IEC ðCEMÞ ¼

CNaOH ∙VNaOH , mDry

where CNaOH is the concentration of NaOH, VNaOH is the volume of consumed NaOH, and mDry is the weight of the dry membrane samples.

13.3.2 Water content Water content (WC) for nanocomposite membranes is the ratio of the mass of water in the membrane to the quality of the dry membrane. WC is calculated by measuring the change

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of weight from when the membrane samples are hydrated. Prior to the measurement, the absolute dryness of nanocomposite membranes is attained by drying the samples in a vacuum oven at 60°C for over 24 h. After that, the nanocomposite membranes are immersed in water and the change of weight is measured. WC is calculated as mWet  mDry  rylc, WC ð%Þ ¼ mDry where mWet and mDry are the weight of wet and dry membrane samples, respectively.

13.3.3 Surface electrical resistance Surface electrical resistance is one of the most important characteristics for electrodriven membranes; it is equivalent to the ratio of current density to the potential gradient in parallel membranes. The lower the surface electrical resistance, the faster the ionic migration in the membrane, and the stronger the ionic conductivity in the electrodriven membranes. A desalination process, as shown in Fig. 13.9, measures the membrane surface electrical resistance. It is calculated according to the following equation:

Fig. 13.9 The membrane surface electric resistance measurement device.

Prospects of nanocomposite membranes for water treatment

UcU0  S, I where Rn is the surface electrical resistance of the membrane samples and expressed in Ω cm2, U is the voltage of the membrane, and U0 is the voltage of the blank expressed in V, I is the constant current through the membrane, and S is the membrane effective area. Rn ¼

13.3.4 Surface ζ-potential Surface ζ-potential is a parameter that describes the interaction of the electrical surface charges with their surroundings. Usually, the pH value of the environment is one of the most important factors determining the membrane surface ζ-potential. As a result of the charge overcompensation at each step, the inversion of membrane surface charge is produced by sequential adsorption. Membrane surface ζ-potential values are measured on the basis of streaming potential measurements.

13.3.5 Tensile strength The tensile strength (TS) of the membrane is one of the most important mechanical properties and is measured with a tensile strength instrument.

13.3.6 Elongation at break Elongation at break (EB) is also used to evaluate the mechanical strength of membranes.

13.3.7 Swelling rate The swelling rate (SR) of electrodriven membranes is calculated by measuring the change of length from before to after hydration. Prior to measurement, the absolute dryness of the membrane is attained by drying the samples in a vacuum oven at 60°C for over 24 h. The SR is calculated as S R ð %Þ ¼

LWet  LDry  ryal, LDry

where LWet and LDry are the length of wet and dry of nanocomposite membranes, respectively.

13.3.8 Current-voltage curves The current-voltage curves are chosen to provide insight into limiting currents and show the electrochemical behavior of electrodriven membranes in ED processes. Typically, the current-voltage curves show the electrodriven membranes over a wide range of currents, and display three different regions The first region is the Ohmic region, which occurs at low current densities. In this region, the potential drop across the membrane is directly

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proportional to the applied current. When the current density increases and approaches its diffusion-limited value (Ilim), the speed of ion transport through the membrane also increases. Thus, the concentration of ions at the dilute side, near the electrodriven membranes, rapidly causes an increase in resistance and forms a smaller slope. The second region is called the “plateau” region. In this region, further increasing the current density results in fewer significant changes in the membrane voltage drop. However, when the current density is increased to a certain threshold, it forms an advanced stage of concentration polarization in the solution, which may destabilize the boundary layer. This region is known as the overlimiting region. Compared to the diffusive supply out of the bulk liquid through the diffusion boundary layer, the concentration of counter-ions in the dilute side of the anion-exchange membrane decreased significantly, as a result of the more rapid transport through the membrane. In this region, the current density values increase again with the membrane voltage drop. This is shown in Fig. 13.10. A cell containing specific input and output devices may be used to examine fouled membranes in a 0.02 M NaCl solution. This cell allows for laminar flow within the channels, and the value of the limiting current can be calculated using the Lev^eque equation:  2  FDC h V 1 =3 theor ilim ¼ 1:47 , hðT1  t1 Þ LD where F is the Faraday number, D is the electrolyte diffusion coefficient, C is the input concentration of the electrolyte, L is the desalting channel length, h is the membrane distance, V is the linear flow velocity of the model solution, and T1 and t1 are the effective transport number of counter-ions in the membrane and their transport number in solution, respectively.

13.3.9 Ionic migration number A membrane’s ionic migration number is the ratio of ions migrating across the membrane to the total number of ions. It can be expressed in terms of the electric charge of ion migration. For the ideal electrodriven membrane, the migration number of counterions is 1, and the migration number of eponymous ions is 0. There are two methods to obtain I (A/cm2)

Ohmic region

Plateau region

Ilim

Overlimiting region U (V)

Fig. 13.10 The current-voltage curve of ion-exchange membrane.

Prospects of nanocomposite membranes for water treatment

the membrane’s ionic migration number. The first is the membrane potential method in which the membrane potential is measured in two similar electrolytes with different concentrations, and the membrane potential is used to calculate the migration number. Another method is to measure the transport number of films in the ED chamber under the external direct current electric field [15]. Generally, the number of counterion migration should be higher than 0.9.

13.3.10 Selective separation of monovalent ions Recently, membranes are of special utility for those applications in which monovalent anions/cations are the product of interest to be separated from mixtures with multivalent ions [16, 17]. Large-scale, industrial application of this type of nanocomposite membrane has been realized. Usually, the main mechanisms of transport and retention for ionexchange membranes with selective separation of ions are the electrostatic repulsive force between the fixed charges of the membrane surface and ions in solution (Fig. 13.11), the sieving effect related to the membrane structure and hydrated ionic diameter (Fig. 13.12), and the mobility of different ions (Fig. 13.13) [18–20]. The separation efficiency (SEP), which reflects the separation of two different ions, is used to illustrate the selective separation of monovalent ions from the mixed solution. The SEP between components I (monovalent ion) and II (multivalent ion) can be calculated as [20].

-

-

F(electrostatic repulsion)

-

-

_ _ _ _

F(electric _ field) _

- -- - -

_ _ _

+ + + + + + + +

+ + + + + + + +

+

F = F(electric field) - F(electrostatic repulsion) Monovalent anion

-

Divalent anion

Fig. 13.11 The electrostatic repulsion force effect for selective separation of monovalent anions. Since F (monovalent) > 0, monovalent anions will pass through the membrane; if F (divalent) < 0, divalent anions will not cross the membrane.

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-

-

-

-

Φ

-

D

- - - -

+ + + + + + + +

+ + + + + + + +

-

Smaller hydrated anion

+

Larger hydrated anion

Fig. 13.12 The sieving effect of the membrane. If the hydrated ion diameter (in practice, the total ionic hydration energy, hydrogen bond energy, and both the hydrogen bond energy and electric field force will contribute to the resulting hydrated ion diameter) is smaller than the membrane pore size, the anion will pass through; otherwise, the anion will not cross the membrane.

-

-

-

- - - - Monovalentanion

+ + + + + + + +

+ + + + + + + +

-

+

Multivalent anion

Fig. 13.13 The mobility of different ions. Many factors contribute to the mobility of ions, such as the binding affinity to the fixed ion-exchange sites, pH (which affects the properties of the ion-exchange membrane) and the specific ion channel. For example, if there is an ion with a strong binding affinity to the membrane ion-exchange sites, it will facilitate ion exchange through the membrane under the electric field.

Prospects of nanocomposite membranes for water treatment



   cII ðtÞ c I ðt Þ  cII ð0Þ cI ð0Þ SEP ðt Þ ¼  0Þ Þ, ð1YPcII ðtÞÞ=ðcII ð0Þ ÞÞ + ð1Kn cI ðtÞÞ=ðcI ð0Þ ÞÞ

(13.2)

where cII(0) and cI(0) are the initial concentrations of multivalent and monovalent ions, respectively, and cII(t) and cI(t) are the concentrations of multivalent and monovalent ions at time t. When the SEP is below 0, it indicates poor selectivity in separation of monovalent ions. When the SEP is above 0, it suggests that the higher the separation efficiency parameter, the better the selective separation of ions. The permselectivity of the membranes between components I (monovalent ion) and II (divalent ion), PIII is calculated by PIII ¼

JI =JII , cI =cII

where JI and JII are the flux of these ions through the membrane and are expressed in mol/ m2 s. When the permselectivity value is higher than 1, it suggests that the membrane has monovalent selectivity. Conversely, when the permselectivity is lower than 1, the membrane has no monovalent selectivity. The flux of ions (Ji) is calculated according to [19]. dc i dt , Ji ¼ A where V is the volume of compartment solution and A is the active area of the membrane. V9

13.3.11 Fouling reduction Fouling is the phenomenon of undesirable attachment of organic or inorganic substances to the surface or interior of the membrane material. This phenomenon is one of the key problems for modern chemical, agricultural, food, pharmaceutical processing, and water treatment. The classifications of ion-exchange membrane fouling include colloidal fouling (Fig. 13.14), organic fouling (Figs. 13.15 and 13.16), scaling (Fig. 13.17), and biofouling (Fig. 13.18) [16, 21–23].

13.3.12 Surface roughness and hydrophilic/hydrophobic property The Wenzel state is a creative concept of “roughness factor” for the limitation of Young’s model, as shown in Fig. 13.19. In this concept, the actual contact area for a water droplet and the membrane surface is larger than the contact area of the apparent geometry. The hydrophilic nature of the multilayer causes the surface to be more hydrophilic in geometry [19, 20]. In this theory, the variation of the surface hydrophilicity is related to the roughness factor, r, and expressed by

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Stern

Solid

Diffusion layer

Fig. 13.14 Model of positively charged colloidal particle.

(A)

(B)

Fig. 13.15 (A) Organic fouling materials usually possess a negative charge and easily coat the surface of an AEM; (B) The electrostatic forces of repulsion modified on the AEM surface for reduction of negatively charged original materials fouling in ED.

(A)

(B)

Fig. 13.16 (A) Organic fouling materials usually coat the hydrophobic membrane surface. (B) The hydrophilic materials are modified on the AEM surface for reduction of hydrophobic original materials fouling in ED.

Prospects of nanocomposite membranes for water treatment

Fig. 13.17 Schematic of a three-chamber microbial desalination cell for simultaneous substrate removal (anode), desalination (middle chamber), and energy production. 5

1

2

3

4

5

Fig. 13.18 Biofilm lifecycle. Stages in the development and dispersion of biofilm are shown proceeding from right to left. b

d

a

Noncomposite

Composite

Wenzel state

Cassie-Baxter state

Fig. 13.19 “Wenzel state” and “Cassie-Baxter state” for Young’s model.

r¼

actual area on membrane surface : apparent geometry area

The apparent contact angle, cos θa, is calculated by cos θa ¼

r ðγ mg  γ mw Þ : γ mw

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According to this theory, once the AEM surface is covered with hydrophilic materials, increasing the surface roughness, the hydrophilicity of the membrane surface will increase.

13.3.13 Chemical properties Chemical properties, such as acid resistance, alkali resistance, solvent resistance, temperature resistance, antioxidant properties, and antiirradiation properties, have become main topics in the field of electrodriven membranes.

13.4 Membrane preparation methods In addition to the use of materials with desired properties, suitable preparation methods are also important to achieve high performance in IEMs. The conventional process to obtain an IEM involves dissolving the selected materials in strongly polar solvents, casting the solution onto a leveling plate and evaporating the solvent. This approach is easy and widely used in the synthesis of IEMs. In the meantime, several novel preparation methods have been developed to tailor the properties and to improve the performance of IEMs. These include polymer blending, porous substrate filling, in situ polymerization, and electrospinning, as well as some post treatment/modification approaches [4, 5, 9, 18, 19, 24–30].

13.4.1 Polymer blending Polymer blending can rectify the shortcomings of a single component and combine the favorable characteristics of each component. By controlling the composition of two or more kinds of polymers, properties such as selectivity, ion conductivity, and chemical stability can be enhanced, while cost and water-swelling rates can be reduced. However, the compatibility of different components needs to be considered because excessive interfaces might lead to poor mechanical properties of the blending IEMs. Polymer blending strategies can be classified into four categories: fluorinated and nonfluorinated polymer blending; and functional and nonfunctional polymer blending. Fluorinated polymers are chemically stable and have excellent membrane forming abilities. However, their applications may be limited by high cost and excessive hydrophobicity. In comparison, nonfluorinated materials are relatively cheap but are insufficiently stable. Blending these two kinds of materials is a convenient and efficient approach to improve performance in the resulting IEMs. Ion functionalized polymers can also enhance ionic conductivities by constructing water channels, while excessive ionic groups may result in mechanical failure. Additionally, some nonfunctional and hydrophobic polymers can be adopted to restrict water swelling and improve stabilities of IEMs. The common materials used for blending include polytetrafluoroethylene,

Prospects of nanocomposite membranes for water treatment

polyvinylidene, polystyrene and its copolymers, poly(phenyl oxide), poly(ether sulfone), polyvinyl alcohol, poly(ether ether ketone), polybenzimidazole, polyaniline.

13.4.2 Porous substrates filling method Porous substrate filling is an effective method to prepare IEMs with low swelling rate and high selectivity. In this method, an appropriate porous substrate is the key element. In order to restrict the expansion of soft electrolyte polymers in pores, it should be chemically inert and mechanically stable. Both polymers and inorganic materials can be applied as substrates to prepare pore-filling IEMs. Some available polymeric substrates include porous polyaniline, high-density polyethylene, polypropylene, poly(ether sulfone), and polyimide. The pores in these materials are constructed by track etching or phase inversion methods. The small size and uniform distribution of pores in inorganic materials, like porous alumina as substrates, allow for higher selectivity of IEMs, compared to polymer substrates. In preparation of IEMs, the selected ionomers are generally poured on the substrate surface. The electrolytes flow into the pores and then an IEM can be obtained when the solvents are completely evaporated (Fig. 13.20). Relatively concentrated solutions of sufficient viscosity are favorable to retain the polymers inside the pores. This process may need to be repeated several times to ensure the formation of a defect-free membrane. In addition, pore-filling IEMs can also be prepared by directly immersing porous substrates into ionized polymers, which is called the pore-soaking technique. Both pore-filling and pore-soaking methods have been well developed and widely used, offering diverse possibilities for fabricating IEMs based on different substrates and functional polymers. However, the leaking of ionic materials and property decline need to be prevented during the long-term operation of these IEMs.

13.4.3 In situ polymerization The modification of existing polymers or direct polymerization of functionalized monomers consumes a large amount of organic solvents during both the reaction and membrane formation processes, which can pose toxicity risks to the environment. In this Ionic polymers

Inner pores

Ionic moieties

Pouring or immersed

Porous substrate

Pore filling ion exchange membranes

Fig. 13.20 Schematic illustrations of the preparation of IEMs with pore-filling or pore-soaking method.

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regard, a solvent-free strategy called in situ polymerization has been developed as a more environment-friendly method for preparing IEMs. During the process of in situ polymerization, organic solvents are replaced by liquid monomers, which can be fully incorporated into the resulting membrane. To begin, a poly(phenyl oxide) or brominated poly (2,6-dimethyl-1,4-phenylene oxide) is dissolved in liquid monomers, from which an uncharged membrane is formed by in situ polymerization. A postmodification reaction, such as the sulfonation reaction, is then performed to introduce the sulfonate anions to obtain the resulting CEM.

13.4.4 Electrospinning method The electrospinning method, can produce electrospun nanofibers with nanoscale diameters in preparation of high-performance IEMs for various applications. The electrospun nanofibers possess outstanding characteristics, including high porosity, large specific surface area, three-dimensional networks, fully interconnected pores, as well as high tensile modulus. Despite these advantages, the electrospinning method has only been applied in a laboratory setting; its full potential remains to be determined. For example, composite nanofibrous mats with an average diameter of 100 nm were prepared by electrospinning the mixed solutions of quaternized poly(phenyl oxide) and SiO2, which were subsequently treated with hot pressing to obtain the nanofiber AEMs. It was found that the separation efficiency of the resulting AEM for acid and metal ions was seven times higher than that of the bulk AEMs having a similar IEC.

13.4.5 Technology-driven modification method 13.4.5.1 Electrodriven Electrodeposition is a well-established technology for coating materials using an applied voltage in electrolytic cells, which can rapidly assemble ions, polymers, and colloids in much less time compared to immersive assembly. The layer-by-layer (LbL) electrodeposition method is a well-developed technique for modifying IEMs. In this method, polycations and polyanions can be alternately deposited on the pristine membrane surface as a functional layer for selective ion transport. The coating of polyelectrolyte multilayers can reinforce the membrane selectivity of monovalent ions over multivalent ions. During the electrodriven process, the transport of monovalent ions through the modified membrane is facilitated, while the transport of multivalent ions is restricted by both the enhanced electrostatic repulsion and size exclusion. Some polyelectrolytes adopted to modify commercial IEMs are presented in Fig. 13.21. 13.4.5.2 Radiation crosslinking Radiation-induced graft polymerization is a well-known technique to obtain modified materials with tailored properties for various applications. By using high-energy radiation (e.g., g-rays, electron beam, swift heavy ions), radicals can be induced on the polymer

Prospects of nanocomposite membranes for water treatment

Polycation:

Poly(ethyleneimine) (PEI)

Poly (diallyldimethylammonium chloride) (PDDA)

2-hydro-xypropyltrimethyl ammonium chloride chitosan (HACC)

Poly(allylamine hydrochloride) (PAH)

(A) Polyanion:

Poly(sodium 4-styrene sulfonate)

Poly (methacrylic acid) (PMAA)

(B) Fig. 13.21 Examples of polyelectrolytes used for modification of IEMs.

backbone, from which the polymerization between the irradiated polymer and another monomer can be initiated. This method offers the possibility of combining two highly incompatible polymers to obtain desired properties, and the degree of grafting can be closely controlled by appropriate selection of irradiation conditions. The graft polymerization can be conducted by a simultaneous or preirradiation method. Simultaneous irradiation is the simplest irradiation method, in which a polymer backbone is irradiated in the presence of selected monomers, while preirradiation involves a combination of the fore irradiation of the polymer backbone and its subsequent contact with monomers. Radiation-induced graft polymerization has been widely used to prepare many kinds of membranes, of which ion-exchange membranes are a major class. This is because it is easier to introduce ionic groups than using conventional methods. Both functional monomers and nonfunctional ones can be used to prepare radiation-grafted ionexchange membranes. Grafting of functional monomers such as acrylic acid and

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Nanocomposite membranes for water and gas separation

methacrylic acid can directly incorporate ionic groups to the polymer backbone, while grafting of nonfunctional monomers such as styrene, N-vinylpyridine, and vinylbenzylchloride will result in a neutral copolymer film, which needs to be further activated by post chemical treatment.

13.5 Applications Electrodriven membranes are core elements in applications such as electrodialysis, capacitive deionization, electrodialysis reversal, and other electrodriven membrane related processes. These applications have become very important in industry. In brackish water desalination, electrodriven membrane separation processes have been replaced more and more by pressure-driven membranes processes. However, in other applications, such as the production of high-quality industrial process water or the treatment of certain industrial effluents, electrodialysis, especially in combination with bipolar membranes or with ion-exchange resins, has become increasingly relevant [31–36].

13.5.1 Electrodialysis Electrodialysis (ED) is an electrochemical separation process that has been applied on an industrial scale for over 60 years. In traditional engineering, this method has been used to produce pure water from brackish water, industrial waste, seawater, and separation of solutions containing organic ionic species. Additionally, ED may be used to obtain important ion sources as well as to remove unnecessary ions. Due to their numerous ion functional groups and their ability to selectively transport ionic species while blocking neutral ones, IEMs are generally applied in separation and transport technologies. They are indispensable components in some traditional processes, such as diffusion dialysis (DD), electrodialysis (ED), and bipolar membrane electrodialysis (BMED). More recently, IEMs have been extended to novel applications associated with energy conversion and production, including reverse electrodialysis (RED), fuel cells (FC), and redox flow batteries (RFB) [37, 38]. 13.5.1.1 Conventional electrodialysis Conventional electrodialysis uses an assembly of alternatively stacked anion and cationexchange membranes to separate ions or produce pure water. Fig. 13.22 shows a series of anion- and cation-exchange membranes arranged in an alternating pattern between two electrodes. In this process, ions from the diluted compartment pass through anion/cation-exchange membranes to the concentrated compartment under the electric field. In an industrial-sized ED stack, hundreds of cell pairs are arranged between the electrodes. Different constructions of spacers and stacks, such as sheet flow or tortuous path flow stack designs, are applied. The concept of a sheet flow stack is shown in Fig. 13.23, and the design of a sheet flow and a tortuous path flow spacer design are illustrated in Fig. 13.24.

Prospects of nanocomposite membranes for water treatment

Diluate solution Concentrated solution

AEM

Anode

+ +

-

Electrode rinse

+ + + + + + + + + +

-

··· -

CEM

+ + -

AEM

+ -

-

+

- + + -

+ + + + + + + + + +

CEM

CEM

+ + -

+ -

···

+

+ + -

+ Cathode

+

Electrode rinse

Feed

Repeating unit AEM: anion exchange membrane; CEM: cation exchange membrane

Fig. 13.22 Schematic diagram illustrating the principle of electrodialysis. Electrode cell

Ion exchange membranes

Concentrate Diluate

Electrode rinse solution

Feed solution Feed solution Electrode rinse solution Electrode Spacers

Fig. 13.23 Schematic drawing illustrating the construction of a sheet flow stack design.

In the conventional ED process, the degree of desalination that can be achieved by passing the feed solution through a stack is a function of the solution concentration, the applied current density, and the time the solution is resident in the stack. In the feed and bleed operating mode, the brine and the product concentration can be determined

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Concentrate or diluate outlet

Concentrate or diluate outlet

Feed inlet

(A)

(B)

Feed inlet

Fig. 13.24 Schematic drawing illustrating (A) the design of a sheet flow and (B) a tortuous path flow spacer.

independently, and very high recovery rates or brine concentrations can be obtained. In conventional ED, several indices should be measured. 1. Concentration polarization and limiting current density This is the electrochemical behavior of electrodriven membranes in ED separation process. In practical applications, the operation of current should be kept under the limiting current density, shown in Fig. 13.25. In general, the limiting current density is experimentally determined and described as a function of the feed flow velocity in the ED stack by ilim ¼ aub FCsd :

I (A/cm2)

I lim

Electro convection and water spliting leads to overlimiting current and decreasing resistance

U (V) Current density is not limited Ion depletion at the membrane and resistance is determined surface in the diluate cell due to by Ohm’s law concentration polarization which limits the current

Fig. 13.25 Schematic drawing of a current versus voltage curve of an electrodialysis stack operated at constant solution flow velocities and feed and diluate concentrations indicating the limiting and the overlimiting current density.

Prospects of nanocomposite membranes for water treatment

Here, Cds is the concentration of the solution in the diluate cell, u is the linear flow velocity of the solution through the cells parallel to the membrane surface, F is the Faraday constant. Finally, a, b, and d are characteristic constants for a given stack design and must be determined experimentally. 2. The total energy required in electrodialysis for the actual desalination process is given by the current passing through the electrodialysis stack multiplied by the total voltage drop encountered between the electrodes. 13.5.1.2 Bipolar membranes used in electrodialysis With the development of electrodriven membranes, conventional electrodialysis can be combined with a new electrodriven membrane called a bipolar membrane. These bipolar membranes are utilized to produce acids and bases from corresponding salts. As shown in Fig. 13.26, the cation/anion-exchange membranes are installed together with bipolar membranes in alternating series in an electrodialysis stack. In this process, a typical repeating unit of an electrodialysis stack with bipolar membranes is composed of three cells: two monopolar membranes (anion/cation-exchange membranes) and a bipolar membrane. The cell between the monopolar membranes contains a salt solution, and the two cells between the monopolar and the bipolar membranes contain a base and an acid solution [37, 39]. Since bipolar membranes became available as commercial products, a large number of applications have been identified and studied extensively on a laboratory or pilot scale. However, in spite of the obvious technical and economic advantages of the technology, large-scale industrial plants are still quite rare. The main reluctance to use bipolar membrane electrodialysis originates from the shortcomings of currently available bipolar and monopolar membranes, which result in short, useful membrane life, poor current utilization, and high product contamination. Nevertheless, there are several smaller-scale Cation exchange layer

Anion exchange layer

OH– OH–

+ + + + + + + + + +

Anode

-

H2O

H2O Cathode

H+

H+

Fig. 13.26 Schematic diagram illustrating the structure of bipolar membrane.

347

Nanocomposite membranes for water and gas separation Base

Acid

Base

Acid

Repeating unit

Salt solution

···

OH–

+

+ + + + + + + + + +

+

+

-

+ Anode

-

AEM

H+

-

+ + + + + + + + + +

CEM

-

-

+ - + OH – + - + -

BM + + + + + + + + + +

BM

CEM

-

348

AEM

H+

··· -

+ + + + + + + + + +

Cathode

-

Salt solution

Salt solution AEM: anion exchange membrane; CEM: cation exchange membrane; BM: bipolar exchange membrane

Fig. 13.27 Schematic drawing illustrating the principle of electrodialytic production of acids and bases from the corresponding salts with bipolar membranes.

applications in the chemical process industry, biotechnology, food processing, and wastewater treatment. Generally, the structure and the function of a bipolar membrane is composed of a cation- and an anion-exchange layer with a 4- to 5-nm transition layer arranged between two electrodes (Fig. 13.27). 13.5.1.3 Continuous electrodeionization Continuous electrodeionization is similar to conventional electrodialysis. However, the cells that contain the diluate stream are filled with ion-exchange resins. The conductivity in these cells is substantially increased, and highly deionized water can be obtained as a product. There are some variations in the basic design of a continuous electrodeionization unit as far as the distribution of the ion-exchange resin is concerned. In some cases, the diluate cell is filled with a mixed-bed ion-exchange resin; in other cases, the cation- and anionexchange resins are placed in series in the cells. More recently, bipolar membranes have been incorporated into the process. The process design and hardware components needed in continuous electrodeionization are similar to those used in conventional electrodialysis. The main difference is the stack construction; in a continuous electrodeionization stack, the diluate cells and sometimes the concentrate cells are filled with an ion-exchange resin.

Prospects of nanocomposite membranes for water treatment

13.5.2 Diffusion dialysis (DD) In addition to the processes discussed so far, there are two more electrodialysis-related processes in which the driving force is not an externally applied electrical potential but a concentration gradient. The processes are referred to as diffusion dialysis and Donnan dialysis. Both processes are based on the electroneutrality requirement, which postulates that, on a macroscopic scale, there are no excess positive or negative charges. Diffusion dialysis uses anion- or cation-exchange membranes to separate acids and bases from mixtures with salts. A feed solution containing a mixture of NaCl and HCl is pumped through alternating cells, while water is fed in countercurrent flow through the other cells of a membrane stack. Protons and anions will penetrate the anion-exchange membranes while the salt cations are rejected. The net result is the removal of acids from a mixture with salts. Similarly, bases can be removed from salt solutions using a stack containing only cation-exchange membranes. Donnan dialysis can be used to exchange ions between two solutions separated by an ion-exchange membrane. The process has gained only limited practical relevance so far.

13.5.3 Electrodialysis reversal (EDR) Electrodialysis reversal (EDR) has been widely employed to produce fresh water from seawater, industrial wastewater, and underground water by desalting it to a desired level [40–44]. EDR employs ion-exchange membranes (IEMs) with an electrical potential difference as the driving force such that ions in solution migrate through the membranes (cation- and anion-exchange) in an electric field. To avoid the concentration polarization phenomenon, periodically reversing the polarity of the applied electrical current reverses the ion flow direction, and simultaneously exchanges the product and waste stream flows. Concentration polarization and fouling interfere on the ion transport across IEMs, resulting in the deterioration of the produced water quality, that is, lowering the EDR productivity. Therefore, switching the applied electric polarity is proposed as a method to minimize these phenomena on the membrane surface and improve the process performance. The production of energy by mixing sea water with river water through ionexchange membranes is a process referred to as reverse electrodialysis. This process, illustrated in Fig. 13.28, provides a clean and sustainable energy source. The design of a stack to be used in reverse electrodialysis is similar to the stack used in electrodialysis. The main difference is that the cells arranged in parallel between the electrodes are rinsed in alternating series by sea and river water. The ions in the seawater, Na+ and Cl ions, permeate from the sea water into the river water through the corresponding ion-exchange membrane and produce brackish water due to their electrochemical potential gradient creating an electrical current between the cathode and anode.

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Nanocomposite membranes for water and gas separation

River water Brackish water

Cathode

e+

- + -

-

+ + + + + + + + + +

-

+

- + -

-

+ + + + + + + + + +

-

+

+ + - + + + + + + - + + + -

-

Anode

e-

Brackish water Sea water

Fig. 13.28 Schematic drawing illustrating the concept of reverse electrodialysis used to generate electrical energy by mixing river and sea water.

The maximum amount of energy that can be recovered is the Gibbs free energy of mixing fresh water and seawater, which is given as follows: ΔGm ¼ Gb  ðGc  Gd Þ: Here ΔGm is the Gibbs free energy of mixing, Gb is the Gibbs free energy of the mixture, or the brackish water, Gc is the Gibbs free energy of the concentrate, or the sea water, and Gd is the Gibbs free energy of the diluate, the river water.

13.5.4 Novel processes based on electrodriven membranes Conventional membrane-based ion-exchange processes include electrodialysis, diffusional dialysis, and Donnan dialysis, which are used today in a large variety of applications from water desalination and waste water treatment to chemical reactors. Detailed descriptions of those applications can be found in other books and reviews. For example, a comprehensive analysis on these applications has been made by Strathmann in his recent book and reproduced here [45, 46]. In addition to traditional ion-exchange membrane-based processes, numerous novel ion-exchange membranes and processes have been studied and developed in the past

Prospects of nanocomposite membranes for water treatment

decades on a laboratory and on a pilot plant scale. New applications in the biomedical, food, and energy resource industries have also been identified. Some of these applications have gained increasing attention as efficient techniques in clean energy production and wastewater treatment. Another interesting application of ion-exchange membranes is the so-called electrochemical-ion exchange. In this process, an ion-exchange membrane is bonded directly to the surface of a metal mesh electrode. By applying a suitable current, ion exchange can be enhanced leaving, in some favorable cases, only parts per billion of metal ions in the effluent. The regeneration of the resin can be simply attained by reversing the current. Electrodialysis with a bipolar membrane (BMED) provided an update of conventional electrodialysis (ED). Up to now, substantial efforts have been made to use this new technology for clean production in aqueous systems and occasionally in nonaqueous systems. In aqueous systems, BMED is used in conventional chemical or biochemical production or resources recovery, such as producing inorganic acids/bases from the corresponding salts, recovery/production of organics from fermentation broth, and similar sources. Additionally, BMED is utilized in the food industry in purification or separation processes, such as the inhabitation of polyphenol oxidase in apple juice (the enzyme responsible for the enzymatic browning of cloudy juice) and separation of soybean proteins from other components without denaturing them, in order to produce protein isolates.

13.6 Conclusion and outlook The development of electrodriven membranes and related application processes may be capable of solving diverse problems. In the last decade, the development of new membrane materials has gained the advantage of an interdisciplinary approach. These approaches integrate recent advances in the field of material science focusing on the introduction of ionogenic groups into existing polymer backbones rather than the preparation of the polymeric material itself. Ion-exchange materials are important high-performance polymeric materials used in water treatment applications as well as in separation processes in food and pharmaceutical industries. Beyond these applications, ion-exchange materials are important in the field of alternative energy production, storage, and conversion. Efficiency is the main driving force for the development of new ion-exchange materials with improved properties. However, it should be noted that improving the preparation of electrodriven membranes is the most crucial topic for further study. In some applications, ion-exchange membrane processes provide higher-quality products or are more environment friendly and may therefore be used in spite of an initial cost disadvantage. Increasing costs of raw materials and environmental awareness have increased the application of ion-exchange membrane separation processes especially in highly industrialized and densely populated countries.

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Despite ongoing research in electrodialysis and related processes, there is a need for further work addressing both fundamental problems and application-oriented issues. One of the most urgent problems is the development of ion-exchange membranes with higher permselectivity, lower electrical resistance, better chemical and thermal stability, and lower overall cost. Additionally, there is a substantial amount of research and development needed for the evaluation and optimization of electrodialysis and related processes in new applications in the chemical process industry, biotechnology, and energy conversion. Other problems will require added fundamental research activities, such as a further understanding of electroconvection, the water dissociation in bipolar membranes, and the effect of ion-exchange membrane structures on membrane transport properties.

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Prospects of nanocomposite membranes for water treatment

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CHAPTER 14

Prospects of nanocomposite membranes for natural gas treatment A.K. Zulhairuna, Rika Wijiyantib, Nurul Widiastutib, Pei Sean Goha, Ahmad Fauzi Ismaila a Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, Skudai, Malaysia b Department of Chemistry, Faculty of Science, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia

14.1 Introduction The oil and gas industry is one of the largest sectors in the world in terms of economic value. The demand increases as global economies and infrastructure continue to rely heavily on fossil fuels despite the rise in the renewable energy. It is estimated that the world consumption of natural gas will reach over 110 trillion standard cubic feet per annum and is projected to rise over 200 trillion standard cubic feet per annum by 2040 [1]. Raw natural gas commonly consists of 30%–90% CH4, with various other gases, as well as light and heavy hydrocarbons. Despite this growing demand, newly discovered gas fields are poor in quality while most of existing conventional gas fields are declining [2]. Some of these low-quality gas reserves were discovered decades ago but due to the lack of economically feasible purification technologies, they have been left undeveloped [3]. However, with increasing natural gas demands, more attention has been paid to developing these lowquality gas reserves. For natural gas to reach commercial specifications, the typical treatment and processing stream includes inlet separation, sweetening, mercury removal, dehydration, natural gas liquids recovery, and, finally, compression for transportation. Based on the US pipeline specifications, the minimum gross calorific value of the natural gas should be 950 BTU/SCF, with maximum CO2 content of 2%, 4 ppm of H2S, 4% of inert gas, 7.0 lbs./MMCF water content, and maximum hydrocarbon dew point of 10°C at operating pressure [3]. This task will become more challenging for upgrading the low-quality unconventional natural gas fields since the gas requires a series of more complex procedures to produce sales gas as per market specifications. This in turn drives the development of novel technologies that are able to withstand high impurity content while sustaining economic viability. Among the potential technologies identified for acid gas removal from low-quality natural gas are adsorption process utilizing either liquid or solid desiccant, cryogenic distillation, direct conversion by chemical reactions, and membrane separation. To date, the

Nanocomposite Membranes for Water and Gas Separation https://doi.org/10.1016/B978-0-12-816710-6.00014-6

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Nanocomposite membranes for water and gas separation

most widely applied sweetening method is amine absorption, where alkaline amine solution is used to absorb the acidic gases in a high-pressure column. Nevertheless, it was shown that the operating costs for the current absorption-based methods would be uneconomical for treating high volumes of highly concentrated acid gases since the cost rises proportionately to the amount of acid gases in the feed stream [4]. On the other hand, this situation seems to favor membrane systems where the acid gas concentration in the feed affects only the capital cost of membrane modules, while the operating cost will be less significant due to the simplicity of the plant operation, which requires minimal supervision. Membrane technology has been applied in the treatment of natural gas since the 1980s for the removal of carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen (N2), helium (He), hydrogen (H2), natural gas liquids (NGL) and liquefied petroleum gas (LPG). Key requirements of a membrane for competitive and practical application in natural gas purification processes can be generalized into three criteria: (1) high permeability and selectivity; (2) mechanically, thermally, and chemically robust; (3) economically competitive [3, 5]. These requirements are certainly imperative and exert a pressing challenge for membrane technology to be highly desirable for natural gas treatment. Polymer has been used as the main material for membrane fabrications since more than a century ago. In comparison to other viable membrane materials such as ceramics and metals, polymeric materials possess good processability, ease of handling, and are very cost effective. Despite its benefits, the principal membrane requirements of permeability and selectivity are limited by a tradeoff relationship where a membrane of high permeability often exhibits low selectivity, and vice versa [6, 7]. The inverse relationship between permeability and selectivity creates the dilemma among membrane developers during the material selection stage. Therefore, this factor has been the motivation of most membrane research and development—developing a new material or improving the commonly used materials to surpass Robeson’s upper bound. The second most important feature of polymeric membranes is that they need to be chemically and physically stable through long exposures to high temperatures and pressures of the acid gas streams. Under these conditions, common modes of membrane failures are due to plasticization, swelling, aging, compaction, and surface damage. CO2 and H2S are the major contaminants in crude natural gas and the strongest plasticization agent. A membrane is presumed to be plasticized when the permeability is found to be increased as the operation progresses with time while the selectivity started to drop. Plasticization occurs due to an increase in its chain flexibility causing unselective diffusion. The degree of plasticization varies with polymeric material, morphology, thickness, temperature, pressure, and feed composition. Polymers with a high glass transition temperature have better resistance to plasticization. Moisture can cause membrane swelling while BTEX and heavy hydrocarbons (C6+) cause the formation of film on the membrane surface. The addition of corrosion inhibitors and some other additives to the pipeline risk membrane integrity. Over prolonged operation, the performance of polymeric membranes

Prospects of nanocomposite membranes

gradually degrades due to aging. Polymer matrix comprises free volume, which intermittently changes due to the thermal motion of the polymer segments required to reorganize themselves [8]. Upon aging, the polymer chain mobility decreases, reducing the free volume. This results in the reduction of permeability over the course of operation. Treating high-pressure gas streams imparts another challenge on membrane system. The preferred membrane configuration for industrial gas separation application is in hollow fiber format with diameters ranging from 0.1 to 0.3 mm, or larger. High feed pressures (>60 bar) could result in compaction and physical damage to the porous support [9]. The pore collapse will result in increasing mass transfer resistance across the membrane. Furthermore, if the fiber collapses, the fiber lumen will be closed flat, stopping the flow of permeated gas from the shell side. Although better performance and efficiency can be attained through innovative pretreatment trains, module designs, and process optimization [10–14], it should be noted that the aforementioned modes of membrane failure are mostly irreversible and should be taken into consideration during the membrane development stage. Over the last decades, nanocomposite membranes were being developed to surpass Robeson’s upper bound, and to resist plasticization, swelling, aging, as well as compaction. In this chapter, various synthesis techniques and structures of nanocomposite membranes and evaluations of their potential applications in natural gas treatment are reviewed. Nanocomposite membranes are membranes composed of more than two materials, where at least one of the components is in nanoscale, particularly nanoparticles. They are classified into different categories according to their morphology with three broad classes: (1) mixed matrix membranes (MMMs); (2) interfacial-polymerized thin-film nanocomposite membranes; (3) multilayered membranes. Furthermore, the performances of various nanocomposite membranes are deliberated in detail. The prospect of using nanocomposite membranes for natural gas treatment, in particular for CO2 removal, is discussed.

14.2 Mixed matrix membranes Commercially available polymeric membranes are relatively cheap due to high manufacturability, but offers relatively lower separation capabilities compared to inorganic membranes. Despite high permeability as well as selectivity, thermally and chemically robust, inorganic membranes require high capital due to expensive precursors and intricate fabrication processes. The other downside is that they are intrinsically brittle, and commonly tailored in modules with low surface-to-volume ratio. The combination of the organic polymer with inorganic particles yields mixed matrix membranes (MMMs) with the advantages of both polymeric and inorganic membranes: high selectivity of the dispersed fillers and easy processability of the polymers. In addition, they may offer enhanced physical, thermal, and mechanical properties to withstand the aggressive and harsh natural gas environments. MMMs are also a potential means to overcome the performance upper

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boundary on Robeson’s diagram. This approach is not limited to only improving the performance of gas separation membranes, but covers almost all membrane-related applications, such as pervaporation [15, 16], fuel cells [17, 18], photocatalytic membrane bioreactors [19, 20], and water purification applications [21, 22]. Even when this approach was first reported in 1973, the research did not aim to apply nanocomposite films to gas separation applications. At the time, Paul and Kemp [23] were only interested in the diffusion time lag of polymer films containing adsorptive fillers. Owing to the ease of processability of polymer material, the polymer matrix, as the continuous phase of the membrane, can be fabricated as a symmetric or asymmetric structure, flat sheet, or hollow fiber. Gas separation membranes should be dense and as thin as possible, preferably in the order of hundreds nanometer or less to provide high selectivity and permeability. The gas component is transported across the polymer matrix via solution-diffusion mechanism. Fillers alter the permeation properties of the matrix depending on their porosity, size, geometry, surface chemistry, and how much filler being added to the matrix. The nanoparticles as the dispersed phase can be either organic or inorganic, or both for metal organic frameworks (MOFs). Ideally, fillers of specific pores could be exploited to provide molecular sieving function to the matrix, while nonporous fillers could enhance the gas permeation properties by altering the polymer chain packing, increasing the free volume or producing nanogaps in the vicinity of the filler surface [24–26]. In addition, active or adsorptive fillers could act as carriers to a certain gas component, endowing a facilitated transport function. For instance, bipyridine-based UiO-67 MOF contains Lewis basic sites that may serve as CO2 carriers, consequently boosting the CO2 permeability as well as selectivity [27]. Therefore, nanofillers play an essential part in tailoring MMMs with superior permeability and selectivity [28]. A plethora of fillers have been tested in search for the best polymer-filler combinations. Among the commonly used fillers are zeolite, silica, metal oxides, carbons (e.g., carbon molecular sieves (CMS), carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene and graphene oxide), clays, and MOFs. Table 14.1 summarizes some of the latest examples of MMMs reported in the literature. Zeolites, SiO2, and TiO2 are particles conventionally used as fillers, which have been reported to improve the gas separation efficiency of MMMs. Besides improving the separation performance, other properties such as mechanical, thermal, and chemical properties were reported to be enhanced. For instance, Jusoh et al. [43] reported that the incorporation of only 1 wt% zeolite into T/6FDA-durene matrix increased the plasticization resistance up to 20 bar in comparison to pristine 6FDA-durene, which plasticized not >5 bar. This may be a result of the strong molecular interactions between zeolite particles and the polymer matrix, which may restrict the mobility of the polymer chain causing resistance to CO2-induced plasticization. However, most attempts utilizing these conventional fillers only resulted in marginal improvements or still suffered with the

Table 14.1 Different polymer/filler MMMs performances for CO2 separation

P (bar)

5

Asymmetric

5

CO2/CH4 selectivity

CO2/N2 selectivity

References

3.75

25

7.2 GPU

20.4

18.8

[29]

Asymmetric

3.5

30

2 GPU

–

39.6

[30]

5

Asymmetric

3.5

RT

4.9 GPU

–

35.2

[31]

30

Dense

10

25

17 Barrer

24

24

[32]

20

Dense

4.4

35

19.7 Barrer

17.9

17.6

[33]

0.4 30

AsymmetricHF Dense

3 2

25 RT

120 GPU 20.5 Barrer

25.9 19.6

28.8 27.5

[34] [35]

20

Dense

4

35

18.2 Barrer

23.6

23.6

[36]

30 1

Dense Asymmetric

10 5

25 25

42.6 21

45.6 –

[37] [38]

MOF UiO-67

10

Dense

5

30

16.6 Barrer 18.7 Barrer 75 GPU 26 Barrerm

75m

–

[27]

ZMOF Sodalite ZIF-8

20 27

Dense Dense

4 3

35 25

[39] [40]

15 14

Dense Dense

5 2

25 35

43.4m 92.3 92.6m 36m 31m

– –

MOF ZIF-8 MOF UiO-66

13.8 Barrerm 11 Barrer 10.9 Barrerm 81.2 Barrerm 1912 Barrerm

– –

[41] [42]

PSF

Zeolite SAPO-34 (modified with [emim] [TF2N]) Zeolite SAPO-34 (modified with [bmim][Ac]) Zeolite SAPO-34 (modified with [bmim][BF4]) Zeolite 4A (modified with [APTMS][Ac]) Nonporous fumed silica TiO2 nanotube Mesoporous silica MCM-41 Mesoporous silica MCM-48 Bio-MOF 1 Cloisite 15A Clay

Prospects of nanocomposite membranes

T (°C)

Filler

P-84 polyimide 6FDA-bisP 6FDA-DAM

Structure

PCO2 (Barrera/ GPUb)

Polymer

Matrimid 5218

wt% loading (optimum)

Continued

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360

Polymer

Filler

6FDA-durene PEI PU

Zeolite-T Organic MMT Clay Zeolite SAPO-34 NiO Epoxy Zeolite NaX Fumed silica (APTESfunctionalized) Zeolite NaX MOF ZIF-8 MOF-801 SiO2 Cloisite 30B Clay CNT-nanofluid GO-nanofluid Dopamine/PEG hollow capsule ZnO (modified with oleic acid) N-dope porous carbon

PEBAX 1657

PEBAX 1074 PIM

wt% loading (optimum)

Structure

P (bar)

T (°C)

PCO2 (Barrer/ GPU)

CO2/CH4 selectivity

CO2/N2 selectivity

References

1 2 20 5 20 2 15

Dense AsymmetricHF Dense Dense Dense Dense Dense

3.5 4 12 1 10 3 10

30 RT 25 30 25 25 25

843.6 Barrer 130 GPU 28.7 Barrer 321 Barrer 134 Barrer 32.3 Barrer 135 Barrer

19.1 18.4 25.6 21.8 16.4 – 28

– – 58.6 67.7 60.1 98 –

[43] [44] [45] [46] [47] [48] [49]

20 20 7.5 10 0.2 30 20 1

Dense Dense Dense Dense Dense Dense Dense Dense

4 4 3 4 14 2 2 2

25 25 20 25 RT 25 25 25

50.7 Barrer 112.7 Barrer 23 GPU 73.7 Barrer 245 Barrer 225 Barrer 170 Barrer 510 Barrer

– – – – 62 – – –

107.1 108.2 240 81.8 185 60 59 84.6

[26] [26] [50] [26] [51] [52] [52] [53]

8

Dense

10

25

152.3 Barrer

13.5

62.2

[24]

5

Dense

2

35

40,544

–

12.4

[54]

Permeability and selectivity data are for single gas, unless otherwise specified. m Permeability and selectivity for 50:50 CO2/CH4 mixed gas. Membrane structure is either dense or asymmetric flat sheet film, unless otherwise specified. HF denotes the membrane is asymmetric hollow fiber. Acronyms: PSF, polysulfone; PU, polyurethane; PEI, polyetherimide; PEBAX, polyether-block-amide; PIM, polymer of intrinsic microporosity; MMT, montmorillonite; IL, ionic liquid; [APTMS][Ac], 3-(trimethoxysilyl)propan-1-aminium acetate; RT, room temperature. a Permeability in Barrer, 1 Barrer ¼ 1010 cm3/(cm cm2 s1 cmHg). b Permeance in GPU, 1 GPU ¼ 1  106 cm3 (STP)/(cm2 s cmHg).

Nanocomposite membranes for water and gas separation

Table 14.1 Different polymer/filler MMMs performances for CO2 separation—cont’d

Prospects of nanocomposite membranes

“tradeoff” changes in permeability and selectivity, where only one property is successfully improved [55]. Further modifications or treatments on the filler are often necessary to obtain MMMs with superior gas separation performance or at least defect-free membranes. Early studies in MMMs revealed that polymer and inorganic filler blends are not always compatible, which give rise to a number of possible nonideal polymer-filler interface morphologies [56, 57]. Moore and Koros [57] classified five cases of nonideal MMM morphologies, as portrayed in Fig. 14.1. Case 0 represents the ideal integration between the continuous and dispersed phase, yielding MMM with better permeability and selectivity. Briefly, Case I represents matrix rigidification (decrease permeability, increase selectivity), Case II is “sieve-in-a-cage” morphology (increase permeability, no change or slightly reduce selectivity), Case III is for interfacial voids or defects (increase permeability, severe loss in selectivity), Cases IV and V are severe and mild pore clogging, respectively (decrease permeability, no change or slightly reduce selectivity). In order to combat these nonideal interfacial morphologies, several strategies have been proposed. The introduction of the dispersed phase should be more methodical. For instance, a certain period of priming is required to acclimate the fillers before adding them to a concentrated polymer solution. Better adhesion can be promoted between both phases. Small amounts of polymer are added to the filler solution before further addition of the bulk polymer. According to Vu et al. [58], the polymer used for priming Case 0

Case III Polymer

Polymer

Silane tethers Sieve 40

Zeolite 4A

Sieve

30 Case I

Case IV

Sieve

“Rigidified”polymer layer Case II

O2/N2 selectivity

Polymer

“Clogged” sieves

20

Case I 10 9 8 7

Case 0

Case V Case II Case IV

Case V Polymer

6

Neat ultem

5 0.1

Case III 1

O2 permeability, Barrers

5 mm 21 vol% Zeolite 4A in Udel* (Solvay Advanced Polymers; Alpharetta, GA)

Sieve

Reduced permeability region within sieve surface

Fig. 14.1 Nonideal polymer-filler interface morphologies for gas separation MMMs [57].

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may be from the same polymer or different from the main matrix polymer. For the case where the main polymer matrix has poor interaction with the dispersed filler, other polymers having good interaction with filler surface as well as good miscibility with the main matrix can be employed as the priming agent [59]. This approach may also help to minimize the agglomeration of inorganic particles at high loadings. Sonication protocol has also proven vital to obtain defect-free MMMs [41]. Priming and sonication procedures have been universally adopted in the preparation of MMMs, due to their simplicity [58, 60, 61]. Besides priming and sonication, filler particle size helps in achieving ideal dispersion throughout the matrix. Smaller particle size preferably