Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability [1 ed.] 012818485X, 9780128184851

Table of contents :
Cover
Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability
Copyright
List of Figures
Contributors
Preface
Part 1: Advanced water treatment applications of synthetic polymer membranes: Structure, preparation, and applications
1
Synthetic polymer-based membranes for treatment of oily wastewater
Introduction
Components in oily wastewater
Membrane technology for treatment of oily wastewater
Polymeric membranes for treatment of oily wastewater
Fluoropolymer membranes for treatment of oily wastewater
Sulfone-containing polymer membranes for treatment of oily wastewater
Mechanism of oil removal in membrane technology
Membranes with superwetting surfaces
Challenges and future perspectives
Conclusion
References
Further reading
2
Synthetic polymer-based membranes for desalination
Introduction
Cellulose acetate RO membranes
Thin-film-composite RO membranes
Procedure to prepare a thin-film-composite RO membrane
Microscopic characterization for membrane structure topology
Nanoscale characterization of internal membrane structure
Transport properties
Membrane module configuration
Module array in the RO process
Concluding remarks and future prospects
References
3
Synthetic polymer-based membranes for dye and pigment removal
Introduction
Polymeric membranes for dye and pigment removal
Nanofiltration
Membrane distillation
Ultrafiltration
Conclusions
References
4
Synthetic polymer-based membranes for photodegradation of organic hazardous materials
Conventional treatment for phenolic compound removal from wastewater
Chemical oxidation
Advanced oxidation processes
Photocatalysis
Semiconductors as photocatalysts
Perovskites
Synthetic polymeric membranes
Fouling mechanism
Photocatalytic membranes
Concluding remarks and future prospects
Acknowledgment
References
5
Synthetic polymer-based membranes for heavy metal removal
Introduction
Pressure-driven membranes for heavy metal removal
Low-pressure membranes
High-pressure membranes
Electrically driven membrane processes for chemical-free heavy metal ion removal
Deionization by ion-exchange membrane-based processes
Electrodialysis
Electrodeionization
Membrane capacitive deionization
Heavy metal recovery by MD
Concluding remarks
References
Further reading
6
Application of polymer-based membranes for nutrient removal and recovery in wastewater
Introduction
Development of pressure-driven membranes
RO/NF membranes
Membrane bioreactor
MBR membrane characteristics
MBR classification
MBR configurations
Development of osmotically driven membranes
Forward osmosis
Type of membrane
Development of thermally driven membranes
Membrane distillation
Type of membrane
Characteristic of membrane
Configurations of MD
Impact of operating conditions on DCMD performance
Temperature
Cross-flow velocity
Concentration
Hybrid processes and new membrane system trends
Future perspectives
Conclusion
References
Further reading
7
Synthetic polymer-based membranes for the removal of volatile organic compounds from water
Introduction
Pervaporation
PV performance
Membranes for PV
Silicone-based membrane
Nonsilicone membrane
Membrane distillation
Membranes for MD
Comparison of PV and MD
Conclusion and future remarks
References
8
Forward osmosis membranes for water purification
Main concept of forward osmosis
Applications of FO
Water purification
Industrial wastewater treatment
Surface water treatment
Membrane selection
Materials, dimensions, and background
Composite membranes
Hybrid processes
FO-RO
FO-MD
FO-NF and FO-UF
Industrial applications/large-scale FO installations
References
Part 2: Gas separation applications of synthetic polymer membranes: Structure, preparation, and applications
9
Synthetic polymer-based membranes for acidic gas removal
Introduction
Types of acid gases, sources, and impacts on the environment
Outline of acidic gas separation membrane techniques
Transport mechanism of polymer membranes
Design of acidic gas separation membranes
History of acidic gas polymer membranes
Acidic gas separation membranes of general polymers
Acidic gas separation membranes of composite type
Future developments
References
10
Synthetic polymer-based membranes for oxygen enrichment
Introduction
Membrane materials
Conventional polymeric materials
Homopolymers
Polymer blends
Copolymers
Nanoporous polymers
Polymers of intrinsic microporosity
Thermally rearranged polymers
Surface-modified membranes
Facilitated transport membranes
Organic/inorganic hybrid membranes
Mixed matrix membranes
Polymer magnetic membranes
Conclusion
References
11
Synthetic polymeric membranes for gas and vapor separations
Introduction
Membrane classification and fabrication
Membrane materials and structures
Membrane shapes and modules
Dense membranes
Fabrication of dense membranes
Solvent vaporization or dry phase inversion
Melt extrusion
Integrally asymmetric membranes
Synthesis methods: Phase inversion method
Case study
Thin-film-composite membranes
Synthesis methods of TFC
Case study
Mixed matrix membranes
Definitions and properties
Theoretical models
Mixed matrix membrane materials
Preparation methods of mixed matrix membranes
Solution blending
In situ polymerization
Sol-gel
Methods for avoiding nonideal interfacial defects
Membrane performance and characterization
Scanning electron microscopy
Transmission electron microscopy
Thermogravimetric analysis
Differential scanning calorimetry
Atomic force microscopy
Dynamic mechanical and thermal analysis
Fourier transform infrared
Positronium annihilation lifetime spectroscopy
Gas permeation tests
Constant pressure
Constant volume
Solubility measurement
Industrial applications
CO2 removal
Hydrogen recovery
Air separation
Air and gas dehydration
Separation of volatile organic compounds from N2
LPG recovery
Challenges
Plasticization
Aging
Acknowledgments
References
Further reading
12
Synthetic polymer-based membranes for hydrogen separation
Introduction
History of polymeric membranes for H2 separation
Mechanisms of gas transport in polymeric membranes
H2-selective membranes
Polymeric membrane characteristics for ideal H2 separation
Conclusion
References
Further reading
13
Polymeric composite membranes for gas separation: State-of-the-art 2D fillers
Introduction
Permeation mechanics through 2D polymeric composites
Theoretical modeling of 2D flakes in polymers for gas transport
Synthesis techniques
2D materials
Graphene and graphene oxides
Layered clays
Layered metal dichalcogenides
MOFs and zeolites
Other potential 2D fillers
Potential applications (analytical and industrial)
Conclusions
References
Further reading
Part 3: Energy Sustainability Applications of Synthetic Polymer Membranes: Structure, Preparation, and Applications
14
Synthetic polymer-based membranes for microbial fuel cells
Introduction
Renewable energies
Fuel cells
Microbial fuel cells
The mechanism of electron transfer in MFCs
The main influencer items on MFC performance
MFC applications
Energy conversion
Wastewater treatment
Biosensor
Biohydrogen production
The impact of membrane separators in MFCs
The main parameters for applying an ideal MFC membrane
Oxygen intrusion
Internal resistance (Ri)
pH splitting
Substrate crossover
Biofouling
Common membranes applied in MFCs
Ion-exchange membranes
Cation exchange membranes
Anion exchange membranes
Bipolar membranes
Membraneless MFCs
Nanocomposite polymer membranes
Different polymeric membranes
Polymer-polymer composites
Metal-polymer composites
Carbon-polymer composites
Salt bridge
Conclusion
References
15
Synthetic polymer-based membranes for direct methanol fuel cell (DMFC) applications
Introduction
Membrane structure
Layered membranes
Sandwiched membranes
Pore-filled membranes
Membrane fabrication
Membrane characterization
Ion exchange capacity
Water uptake
Swelling measurement
Hydrophilicity measurement
Proton conductivity measurement
Permeability measurement
Thermal stability
Mechanical stability
Morphology and elemental analysis study
Crystallinity of inorganic fillers
Potential structural changes in PEMs
DMFC single cell
Separation mechanism
Application of DMFC
Conclusion and future outlook
References
16
Polymeric composite membranes for anion exchange membrane fuel cells
Introduction
Anion exchange membrane fuel cell
Overview of anion exchange membranes for fuel cells
Organic-inorganic composite membranes
Nanofibrous and pore-filling electrolyte membranes
Summary and conclusions
References
17
Synthetic polymer-based membranes for lithium-ion batteries
Introduction
Membrane structure and characteristics for lithium-ion batteries
Membrane preparation techniques
Thermally induced phase separation
Nonsolvent induced phase separation
Particulate leaching
Replica molding
Freeze extraction
Electrospinning
Physical-chemical characterization of membranes
Thickness
Pore size and distribution
Porosity
Tortuosity
Permeability
Wettability
Mechanical strength
Thermal stability
Chemical stability
Electrical resistance
Electrochemical stability
Shutdown
Recent advances in separator membranes for Li-ion batteries
Conclusions
Acknowledgments
References
Further reading
18
Polymeric membranes for pressure-retarded osmosis
Introduction
Concept and development of PRO
Theories of PRO principles
Membranes for pressure-retarded osmosis
Fouling in the PRO membrane
Recent advancements in PRO
Future outlook and conclusion
Acknowledgment
References
Index
A
B
C
D
E
F
G
H
I
K
L
M
N
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P
Q
R
S
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V
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Z
Back Cover

Citation preview

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability Edited by

Ahmad Fauzi Ismail Wan Norharyati Wan Salleh Norhaniza Yusof

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: 978-0-12-818485-1 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Edward Payne Editorial Project Manager: Andrea Gallego Ortiz Production Project Manager: Poulouse Joseph Cover Designer: Mark Rogers Typeset by SPi Global, India

List of Figures Fig. 1.1 Fig. 1.2 Fig. 1.3 Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. Fig. Fig. Fig. Fig.

2.4 2.5 2.6 2.7 2.8

Fig. 2.9 Fig. 3.1 Fig. 3.2 Fig. 3.3 Fig. 3.4 Fig. 3.5 Fig. 3.6 Fig. 3.7 Fig. 3.8 Fig. 3.9 Fig. 4.1

Fig. Fig. Fig. Fig.

4.2 4.3 5.1 5.2

Fig. 5.3 Fig. 5.4 Fig. 5.5

General process of a pressure-driven membrane Schematic diagram of the filtration spectrum of related separation processes and separated components Demulsification phenomena of oil-in-water (O/W) emulsions using a membrane coalescence process Chemical structure of cellulose acetate Schematic representation of osmosis and reverse osmosis processes Chemical structure of polyamide formed by interfacial polymerization between MPD and TMC Schematic depiction of second-generation thin-film-composite membrane structure Process for preparation of a polymer support membrane Process for preparation of thin-film-composite RO membrane Scheme of polyamide deposition over the porous support membrane The transport process of salt and water flux through the RO membrane. (A) Defect-free RO membrane; (B) Imperfect RO membrane Module array connected in series within a single pressure vessel (A) and pressure vessels connected in parallel (B) (A) Chemical structure of mauve and (B) difference between dyes and pigments Chemical structure of reactive black 5 Schematic of the experimental setup Schematic of the NF membrane polyamide layer with and without the CNT layer (top) and the SEM cross-sectional image of the TFC membrane with the CNT interlayer (bottom) FESEM image of (A) cross-sectional image of the HF NF membrane and (B) the inner surface of the HF NF membrane after the dye concentration experiment Schematic illustration of the lab-scale FO-MD hybrid process (A) Dye removal efficacy and (B) flux of pristine and nanocomposite membranes Dye rejection properties of PAN-based membranes with different MWCOs Schematic experimental setup of a membrane photocatalytic reactor coupled with tight UF Schematic diagram of pollutant degradation by a photocatalyst. Although TiO2 is used as the model, the mechanism of the photocatalytic process for most metal oxide and semiconductor photocatalysts is the same Formation of a particle bridge at the neck of the membrane pores Schematic diagram of a mixed matrix membrane Schematic illustration of MEUF for heavy metal recovery Homogeneous versus heterogenous IEMs. Ionic pathways in (A) homogeneous and (B) heterogeneous IEMs, (C) permselectivity, and (D) areal resistances Schematic illustration of electrodialysis (ED). (A) Deionization process and (B) ED set up Electrodeionization (EDI). (A) Schematic illustration of deionization process and (B) stack construction Schematic illustration of membrane capacitive deionization (MCDI) process. (A) adsorption and (B) desorption/electrode regeneration

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List of Figures

Fig. 5.6 Fig. 5.7 Fig. 6.1 Fig. 6.2 Fig. 6.3 Fig. 6.4 Fig. 6.5 Fig. 6.6 Fig. 6.7 Fig. 6.8 Fig. 6.9 Fig. 6.10 Fig. 6.11 Fig. 7.1 Fig. 7.2

Fig. 7.3 Fig. 8.1 Fig. 8.2 Fig. 8.3 Fig. 9.1 Fig. 9.2 Fig. 9.3 Fig. 9.4 Fig. 9.5

Fig. 9.6 Fig. 9.7 Fig. 9.8 Fig. Fig. Fig. Fig. Fig.

10.1 10.2 10.3 10.4 10.5

Membrane distillation (MD). (A) Desalination process in MD, (B) direct contact MD, (C) sweeping gas MD, (D) vacuum MD, and (E) air gap MD Water contact angles on membrane surface with various wettability Pathway of (A) conventional nitrification-denitrification and (B) simultaneous nitrification-denitrification Schematic of the osmosis process Schematic of the reverse osmosis process Schematic of a cross-flow system Schematic of a dead-end system AnMBR configurations (A) external cross-flow and (B) submerged/immersed configurations The challenges of the FO process Schematic of a TFC membrane The characteristics of a TFN membrane Heat and mass transfer in the MD process Different types of MD configurations. (A) DCMD, (B) AGMD, (C) SGMD, and (D) VMD Pervaporation (PV) system Membrane distillation configurations: (A) direct contact membrane distillation (DCMD); (B) air gap membrane distillation (AGMD); (C) vacuum membrane distillation (VMD), and (D) sweep gas membrane distillation (SGMD) Schematic diagram of a vacuum membrane distillation (VMD) setup by Yao et al. Brief overview of FO Draw solute selection guide Membrane selection guide Concept of carbon capture and storage (CCS) Number of references whose keyword for each year is “Polymer membrane and acidic gas” Gas separation mechanisms in separation membranes Separation of mixtures of acidic and other gases using (A) acidic gas rejective membranes, and (B) acidic gas permselective membranes Chemical structures of polyethylene (PE), polydimethylsiloxane (PDMS), poly(1trimethylsilyl-1-propyne) (PTMSP), tetrafluoroethylene (TFE)/perfluoromethyl vinyl ether (PMVE) copolymer, teflon AF 2400, polycarbonate (PC), and polysulfone (PSF) Chemical structures of nylon 6, cellulose acetate (CA), poly (ether urethane urea), and fluorine-containing polyimides (6FDA-HAB) Chemical structures of polymers of intrinsic microporosity (PIM), thermally rearranged (TR) polymer, and 1-butyl-3-methylimidazolium (bmim)-based ionic liquids Relationship between carbon dioxide, nitrogen (A), and hydrogen sulfide (B) permeability of polymeric membranes Robeson’s upper bound correlation for O2/N2 separation Polymer-based membrane materials for O2/N2 separation Synthesis route and chemical structure of monomers for PIMs Thermal rearrangement mechanism, (A) TR-α polymer and (B) TR-β polymer Scheme for facilitated transport of gaseous molecules by a carrier (complex) through a membrane: (A) liquid membrane with a mobile carrier; (B) solid membrane with a fixed carrier

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Fig. Fig. Fig. Fig. Fig.

10.6 11.1 11.2 11.3 11.4

Fig. 11.5 Fig. 11.6 Fig. 11.7 Fig. 11.8 Fig. 11.9

Fig. 11.10 Fig. 11.11 Fig. 11.12

Fig. 11.13 Fig. 11.14 Fig. 11.15 Fig. 11.16

Fig. 11.17

Fig. 11.18 Fig. 11.19 Fig. 11.20

Fig. 11.21

Fig. 11.22

List of Figures

xvii

Comparison of O2/N2 separation performance of some polymeric membranes The structures of gas separation membranes Schematic of a polymeric flat sheet membrane Schematic of a polymeric hollow fiber membrane General types of modules used for gas separation processes: (A) plate-and-frame module, (B) spiral-wound module, and (C) shell and tube Solution-diffusion mechanism in a dense membrane Schematic of an integrally asymmetric membrane Preparation of integrally asymmetric flat sheet and hollow fiber membranes via phase inversion method Schematic of a thin-film-composite membrane (A) Schematic diagram of the procedure for the production of a PA membrane derived from MPD and TMC via IP (B) chemical reaction of MPD and TMC monomers to produce a PA oligomer and HCl Robeson graphs to compare different kinds of membranes for gas separation Schematics of polymer/inorganic filler mixed matrix membranes. (i) Symmetric flat dense mixed matrix membrane. (ii) Asymmetric hollow-fiber with a mixed matrix selective skin Illustration of different types of MMMs. (A) Polymer and inorganic phases connected by covalent bonds, and (B) polymer and inorganic phases connected by van der Waals force or hydrogen bonds Schematic of various organic-inorganic interface morphologies of MMMs Ideal MMM (left side), (A) interface void, and (B) rigidified polymer layer around the nanoparticle (right side) BCC structure considered for particle distribution in MMM Schematic illustration of expected morphologies of MMMs HNT/PEI across a dense selective skin layer; Case (I) ideal, Case (II) void (yellow-colored space surrounding the filler is the void), Case (III) rigidification (blue-colored space shows the rigidified region), Case (IV) blocking (black tips shows the blocked parts), and Case (V) blocking + void Comparison between the small pore size filler MMM morphology diagram (black arrows along with italic black words) proposed by Moore and Koros to the large pore size filler MMM morphology diagram proposed by Hashemifard et al. (colorful area along with red bold words) General procedures followed to produce asymmetric MMMs Cross-sectional SEM images of (A) PC/zeolite 4A (20%) and (B) PC/pNA (2%)/zeolite 4A (20%) SEM cross-section view of the polyacrylonitrile (PAN) support membrane. (A) Crosssection, (B) enlarged cross-section, (C) outer skin layer, (D) outer surface, (E) outer-inner interface, and (F) inner surface TEM images of poly(ether imide) nanocomposite membranes containing different weight fractions (10, 20, 30) of three fumed silica (TS610, TS530, TS720). At low fumed silica content, nanoparticles are well distributed in the polymer matrix TGA plot for pure poly(4-methyl, 2-pentyne) (PMP), fumed silica (FS), and polyoctatrimethyl silsesquixane (POSS) nanoparticles and nanocomposite membranes. POSS decomposed quickly while FS had a low weight loss at a specific temperature range

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Fig. 11.23 Fig. 11.24 Fig. 11.25 Fig. 11.26 Fig. 11.27 Fig. 11.28 Fig. 11.29 Fig. 11.30 Fig. 11.31 Fig. 11.32 Fig. 11.33 Fig. 11.34 Fig. 11.35 Fig. 12.1 Fig. 12.2 Fig. 12.3

Fig. 12.4 Fig. 12.5 Fig. 13.1

Fig. 13.2

Fig. 13.3

List of Figures

DSC plots of the pure polyurethane (PU) and polyether-based PU-silica nanocomposites AFM analysis for (A) polysulfone (PSf) substrate while (B), (C), and (D) are modified PSf membranes with different monomer concentrations for interfacial polymerization FTIR plot of pure silica, pure polyurethane (PU), and polyether-based PU-silica nanocomposites. In this plot, each peak is related to a specific bond Experimental set-up of gas permeation tests Experimental set-up of gas permeation test at constant volume Experimental set-up for measuring the solubility of pure gas in the membrane CO2 membrane separation plant from Newpoint Gas, LLC CO2/CH4 upper bound plot for new polymer materials Hydrogen recovery from ammonia purge stream by Prism membranes plant that installed in 1979 with capability of pure hydrogen recovery by about 90% O2/N2 upper bound plot for new polymer materials, TR polymers (⧫), PIMs (n), TBDA-SBI-P (▲) Image of the LPG recovery unit from off-gas installed by Membrane Technology and Research (MTR) CO2 permeance in an asymmetric PES/PI hollow fiber membrane as a function of fugacity with different compositions of the feed The introduction of polar-functionalized POSS nanoparticles to the PIM for CO2 separation Bed reactor facility for H2S and He schematic diagram Block diagram of various types of H2 separation membranes Main transport phenomenon in microporous structure. (A) Micrograph with surface diffusion domain, (B) micrograph with normal microporous structure, and (C) micrograph with blocking effect Mixed matrix membrane for H2/N2 performance reported from previous studies presented in Robeson line diagram Mixed matrix membrane for H2/CO2 performance reported from previous studies presented in Robeson line diagram Gas selectivity versus permeability showing the “Robeson” upper bound (solid line), highlighting the trade-off between permeability and selectivity. The improvement in this upper bound from 1991 to 2008 demonstrates the emergence of enhanced gas separation membranes Mechanisms of altering the permeation of composite membranes containing 2D fillers. (A) Functional groups altering solubility, (B) altering chain stacking and changing cross-linking density, (C) defects allowing some gas molecules to diffuse through the filler (Knudsen diffusion) while increasing the tortuosity of other gas molecules, (D) interfacial voids created at the interface of the two phases, (E) PCMR membrane providing catalytic conversion for selective gas removal, and (F) physisorption of gas molecules onto the surface of 2D flakes embedded within the composite membrane Composite synthesis techniques. (A) In situ polymerization that can result in three outcomes: (i) the layered material is encapsulated, but unperturbed by the polymer, (ii) the polymer intercalates the layered material, but the stack remains agglomerated, and (iii) the polymer intercalates and delaminates the layered material. (B) Melt mixing; and (C) solution blending

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List of Figures

Fig. 14.1 Fig. 14.2 Fig. 14.3 Fig. 14.4 Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.

14.5 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 15.9 15.10 15.11 15.12 15.13 15.14 15.15 15.16 15.17 15.18 15.19 15.20 15.21 16.1

Fig. 16.2 Fig. 16.3 Fig. 16.4 Fig. 16.5 Fig. 16.6

Fig. 16.7 Fig. 17.1 Fig. 17.2 Fig. 17.3

Schematic of different MFC configurations: (A) dual-chamber MFC, single-chamber MFC, (B) air-cathode, and (C) the cathode that is put in the anolyte Polarization curve Schematic of electron transfer mechanisms: (A) DET, (B) MET, and (C) nanowire Schematic of three various types of ion-exchange membranes: (A) CEM, (B) AEM, and (C) BPM Schematic of membrane-less MFC Typical electrically charged membrane for DMFC applications Asymmetric membrane Layered electrospun SPEEK/Cloisite 15A nanocomposite electrolyte membrane Sandwiched Nafion 211/PTRu + SiO2 + Nafion/Nafion 211 electrolyte membrane Sulfonated radiation grafted polystyrene pore-filled poly(vinylidene fluoride) membranes Different routes for membrane fabrications via phase inversion techniques Ternary phase diagram CA images for hydrophobic and hydrophilic surfaces Two-probe impedance cell EIS spectrum for untreated Nafion (R ¼ 0.7 Ω) and Nafion/GO (R ¼ 0.9 Ω) membranes Permeation cell Methanol permeation versus time TGA thermal curve for Nafion 212 (A) EDX image and (B) spectrum of PTFE-ZrP-PVA membrane XRD spectrum of Chitosan and Chitosan/E-MoS2 AFM images of (A) neat SPEES and (B) SPEES/cSMM membranes Schematic diagram of MEA construction I-V polarization curve for Nafion 115 at 1 M methanol concentration Chemical structures of (A) Nafion and (B) SPEEK polymer with repeating units of SO3  Ionic cluster domain of notable Nafion and methanol molecules Transport mechanisms inside cluster model of notable Nafion Schematic representation of the working principles of a typical alkaline anion exchange membrane fuel cell (AEMFC) and a proton exchange membrane fuel cell (PEMFC) Polarization curves of composite membranes at 60°C Hydroxide conductivity of hybrid membranes after treatment with 2 M KOH solution at 60°C (A) and 80°C (B) Ion conductivities of (A) Me-IL-TiO2, (B) Ethyl-IL-TiO2, and (C) HOEt-IL-TiO2 composite membranes Illustration of ionic pathways in a QPVA composite consisting of electrospun nanofibers A schematic of (A) crosslinked quaternized poly(vinybenzyl-divinylbenzene) bipolymer and (B) crosslinked quaternized poly(vinybenzyl-divinylbenzene-hexafluorobutyl methacrylate) terpolymer composite AAEMs. The bold black lines represent PE chains A schematic illustration of pore-filling anion exchange membranes. Anion exchange polymers are immobilized inside the pores of the porous substrate by grafting or cross-linking Schematic representation of the main constituents and the discharge process of lithium-ion battery systems Representation of the main steps in the thermally induced phase separation technique (TIPS) Illustration of the nonsolvent induced phase separation (NIPS) process

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Fig. 17.4 Fig. 17.5 Fig. 17.6 Fig. 17.7 Fig. 17.8 Fig. 17.9 Fig. 17.10 Fig. 17.11 Fig. 17.12 Fig. 17.13 Fig. 17.14 Fig. 17.15

Fig. 17.16

Fig. 17.17 Fig. 17.18 Fig. 17.19 Fig. 18.1 Fig. 18.2

Fig. 18.3 Fig. 18.4 Fig. 18.5 Fig. 18.6

Fig. 18.7

List of Figures

The schematic diagram of the solvent casting and particulate leaching technique The key stages of replica molding technique The schematic diagram of a freeze extraction process Schematic representation of the electrospinning technique Schematic representation of the large-scale preparation of PVDF-HFP separators Schematic representation of the preparation method and the proposed microstructure of the PVDF-HFP/LLZO composite separator Schematic representation of separators with different patterns—hexagons, lines, zig-zags, pillars, and conventional nonpatterned-designed to improve battery performance Schematic illustration for the: (A) preparation and (B) functional principle of PMIA@PVDF nanofiber separators Illustration for the preparation of a PI nanofibrous membrane separator Thermal shrinkage of a PP separator (left) and a PI membrane (right) over a temperature range from 150°C to 200°C (A) Surface SEM image of a PI matrix. (B) Cross-sectional SEM image of the API separator (inset is the surface image of the AlOOH coating layer) SEM images: (A) Celgard-PP separator, (B, C, and E) BNNT separators, and (D) low magnification image of BNNTs (inset shows digital photographs of Celgard PP and BNNT separators) (A) Photographs of graphene oxide (GO) dip coating depending on the polarity of the GO solution. (B) Schematic illustration of the wettability of the solvent with low surface energy on the conventional hydrophobic separator. (C) SEM image of the GO dip-coated separator. (D) Water contact angle measurement and (E) photograph of large-area wetting feature on the GO dip-coated separator Schematic illustration of the fabrication of MCS and an integrated cathode/MCS/anode assembly Schematic illustrations of (A) glass fiber (GF), (B) the MOF-GF composite separator (MOG), and (C) an enlarged view showing ion transport behavior in MOG Membrane preparation process of PEEK Initial PRO prototype proposed by Sidney Loeb (A) Due to the freshwater “lost” to the sea, this schematic diagram is referred to as the open loop. (B) Commonly referred to as an osmotic heat engine, this is a closed-loop PRO system schematic diagram The basic principles of (A) equilibrium (B) FO (ΔP ¼ 0), (C) RO (ΔP > Δπ), and (D) PRO (ΔP < Δπ) Schematic diagram of the salt concentration profile in PRO mode where the feed and draw solutions are in a crossflow (A) The polymerization product of MPD and TMC monomers, and (B) the product of BDSA and TMC Microscope images of: (A) nonwoven mesh and standard woven mesh. The wire diameter (μm) and opening size (μm) are as follows (B) (160, 250), (C) (50, 75), (D) (32, 45), (E) #(20, 20), and (F) (53  2, 7) Scanning electron microscopic (SEM) cross-sectional view of a TFC-PRO membrane

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List of Figures

Fig. 18.8 Fig. 18.9

Fig. 18.10

(A) External fouling in AL-FS orientation; (B) Both internal and external fouling in AL-DS orientation (A) Top finer fiber layer surface morphology before IP; (B) after IP surface morphology; (C) cross-sections of a nanofibrous TFC membrane FESEM image; (D) and (E) nanofiber layer enlarged image underneath the PA layer and top finer fiber layer, respectively (A) Diagram of backwash cleaning, (B) diagram of clean-in-place (CIP), (C) and the diagram of maintenance cleaning (MC)

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430 431

Contributors Farideh Abdollahi Sustainable Membrane Technology Research Group (SMTRG), Faculty of Petroleum, Gas and Petrochemical Engineering (FPGPE), Persian Gulf University (PGU), Bushehr, Iran Arif Aizat Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Mohammad Amin Alaei Shahmirzadi Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran Nur Hashimah Alias Membrane Technology Research Group, Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia Zahra Alihemati Sustainable Membrane Technology Research Group (SMTRG), Faculty of Petroleum, Gas and Petrochemical Engineering (FPGPE), Persian Gulf University (PGU), Bushehr, Iran Farhana Aziz Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Kyle J. Berean School of Electrical and Computer Engineering, RMIT University, Melbourne, Australia Weerapong Bootluck Center of Excellence in Membrane Science and Technology, Prince of Songkla University, Songkhla, Thailand C.M. Costa Centre of Physics; Centre of Chemistry, University of Minho, Braga, Portugal Pallabi Das CSIR—Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand, India Nur Atiqah Daub Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Suman Dutta Department of Chemical Engineering, Indian Institute of Technology (ISM) Dhanbad, Dhanbad, India

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Contributors

I G. Wenten Department of Chemical Engineering, Faculty of Industrial Technology; Research Center for Nanosciences and Nanotechnology, Bandung Institute of Technology, Bandung, Indonesia P.S. Goh Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Hasrinah Hasbullah Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Seyed Abdollatif Hashemifard Sustainable Membrane Technology Research Group (SMTRG), Faculty of Petroleum, Gas and Petrochemical Engineering (FPGPE), Persian Gulf University (PGU), Bushehr, Iran G.P. Syed Ibrahim Membrane Technology Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore, India Syarifah Nazirah Wan Ikhsan Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Noor Fauziyah Ishak Membrane Technology Research Group, Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia Arun M. Isloor Membrane Technology Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore, India Ahmad Fauzi Ismail Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Juhana Jaafar Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Hazlina Junoh Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Kourosh Kalantar-zadeh School of Chemical Engineering, University of New South Wales (UNSW), Kensington, Australia

Contributors

xxv

Ali Kargari Membrane Processes Research Laboratory (MPRL), Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran Dipak Khastgir Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India K. Khoiruddin Department of Chemical Engineering, Faculty of Industrial Technology, Bandung Institute of Technology, Bandung, Indonesia Watsa Khongnakorn Department of Civil Engineering, Faculty of Engineering; Center of Excellence in Membrane Science and Technology, Prince of Songkla University, Songkhla, Thailand Arash Khosravi Sustainable Membrane Technology Research Group (SMTRG), Faculty of Petroleum, Gas and Petrochemical Engineering (FPGPE), Persian Gulf University (PGU), Bushehr, Iran B. Lakshmi Department of Chemistry, Reva University, Bangalore, India S. Lanceros-M endez BCMaterials, Basque Center for Materials, Applications and Nanostructures, Leioa; IKERBASQUE, Basque Foundation for Science, Bilbao, Spain Woei Jye Lau Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, Johor, Malaysia Fauziah Marpani Integrated Separation Technology Research Group (i-STRonG), Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia P.M. Martins Centre of Physics; Institute of Science and Innovation for Bio-Sustainability (IB-S), University of Minho, Braga, Portugal I N. Widiasa Department of Chemical Engineering, Faculty of Engineering, Diponegoro University, Semarang, Indonesia Kazukiyo Nagai Department of Applied Chemistry, Meiji University, Kawasaki, Japan B.C. Ng Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Nik Abdul Hadi Md. Nordin Department of Chemical Engineering, Universiti Teknologi PETRONAS, Seri Iskandar, Perak, Malaysia

xxvi

Contributors

J. Nunes-Pereira Centre of Physics, University of Minho, Braga; Centre for Mechanical and Aerospace Science and Technologies (C-MAST-UBI), Universidade da Beira Interior, Covilha˜, Portugal Mohd Hafiz Dzarfan Othman Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Nur Hidayati Othman Membrane Technology Research Group, Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia Mostafa Rahimnejad Biofuel and Renewable Energy Research Center, Chemical Engineering Department, Babol Noshirvani University of Technology, Babol, Iran Mukhlis A. Rahman Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Paramita Ray Membrane Science and Separation Technology Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Bhavnagar, Gujarat, India Mohsen Rezaee Sustainable Membrane Technology Research Group (SMTRG), Faculty of Petroleum, Gas and Petrochemical Engineering (FPGPE), Persian Gulf University (PGU), Bushehr, Iran Wan Norharyati Wan Salleh Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Shuichi Sato Department of Electronic Engineering, Tokyo Denki University, Tokyo, Japan Norazlianie Sazali Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Norin Zamiah Kasim Shaari Membrane Technology Research Group, Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia Munawar Zaman Shahruddin Membrane Technology Research Group, Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia

Contributors

xxvii

S.I. Sharudin Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Krishna Kant Kumar Singh CSIR—Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand, India Puyam S. Singh Membrane Science and Separation Technology Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Bhavnagar, Gujarat, India Arezoo Tofighi Biofuel and Renewable Energy Research Center, Chemical Engineering Department, Babol Noshirvani University of Technology, Babol, Iran Leo Paul Vaurs Department of Civil Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla, Thailand Vijayalekshmi Vijayakumar Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Anita K. Wardani Department of Chemical Engineering, Faculty of Industrial Technology, Bandung Institute of Technology, Bandung, Indonesia Nursyazwani Yahya Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia Norhaniza Yusof Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia

Preface This book consists of 18 chapters with topics related to synthetic polymeric membranes for advanced water treatment, gas separation, and energy sustainability. The contributors come from Asian and European countries, including Malaysia, Japan, Indonesia, India, Iran, Thailand, Australia, and Portugal. The authors are experts in the development of synthetic polymeric membranes for various applications. In particular, this book gathers numerous promising synthetic polymeric membrane developments for improving and enhancing the current technologies used in water and wastewater treatment as well as purification, gas separation, and energy applications, including fuel cells. Those chapters emphasize the synthetic polymeric membrane fabrication techniques, the characterizations that suit the proposed applications, and the way forward for synthetic polymeric membranes for commercial use. This book has been separated based on three major topics: advanced water treatment, gas separation, and energy sustainability. The arrangement of the content of the book is as follows. In the first part, Chapters 1–8 cover synthetic polymer membranes for advanced water treatment applications. A thorough discussion on that topic is presented, including its membrane structure, preparation, and applications. This topic is covered by eight chapters: Synthetic Polymer-Based Membranes for Treatment of Oily Wastewater (Chapter 1), Synthetic Polymer-Based Membranes for Desalination (Chapter 2), Synthetic Polymer-Based Membranes for Dye and Pigment Removal (Chapter 3), Synthetic Polymer-Based Membranes for Photodegradation of Organic Hazardous Materials (Chapter 4), Synthetic Polymer-Based Membranes for Heavy Metal Removal (Chapter 5), Application of Polymer-Based Membranes for Nutrient Removal and Recovery in Wastewater (Chapter 6), Synthetic Polymer-Based Membranes for the Removal of Volatile Organic Compounds from Water (Chapter 7), and Forward Osmosis Membranes for Water Purification (Chapter 8). The second part of the book covers the use of synthetic polymeric membranes in gas separation applications. There are five chapters that covers this specialized topic: Synthetic Polymer-Based Membranes for Acidic Gas Removal (Chapter 9), Synthetic Polymer-Based Membranes for Oxygen Enrichment (Chapter 10), Synthetic Polymeric Membranes for Gas and Vapor Separations (Chapter 11), Synthetic Polymer-Based Membranes for Hydrogen Separation (Chapter 12), and Polymeric Composite Membranes for Gas Separation: State-of-the-Art 2D Fillers (Chapter 13). Due to the recent accelerated interest in membrane-based technology applications in energy sustainability, five important chapters will cover this interesting topic in Chapters 14–18. The five chapters that provide discussion on this topic are Synthetic Polymer-Based Membranes for Microbial Fuel Cells (Chapter 14), Synthetic Polymer-Based Membranes for Direct Methanol Fuel Cell (DMFC) Applications (Chapter 15), Polymeric Composite Membranes for Anion Exchange Membrane Fuel Cells (Chapter 16), Synthetic Polymer-Based Membranes for Lithium-Ion Batteries (Chapter 17), and Polymeric Membranes for Pressure-Retarded Osmosis (Chapter 18). The editors would like to highlight that apart from the growing number of research publications in membrane science and technology for solutions to environmental problems, the advantage of the membrane in a synthetic polymeric membrane is that it is foreseen as being a commercially viable material. With more than 500 pages, this book offers recent findings from research works from established researchers who specialize in synthetic polymeric membrane technologies that will definitely give great satisfaction to the readers. This book possesses its own uniqueness in that the chapters are contributed

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Preface

by established researchers from all around the globe based on their recent research findings. Thus, readers will find the most recent synthetic polymeric membrane materials that are appropriate for advanced water treatment, gas separation, and energy sustainability. This will also give a clear picture on the trend of advanced materials as the way forward for the mentioned applications. The editors would like to express their sincere thanks to all authors and coauthors for their kind support, encouragement, and understanding of the time taken for the book’s writing. Ahmad Fauzi Ismail Wan Norharyati Wan Salleh Norhaniza Yusof

CHAPTER

Synthetic polymer-based membranes for treatment of oily wastewater

1

Syarifah Nazirah Wan Ikhsan, Norhaniza Yusof, Ahmad Fauzi Ismail, Wan Norharyati Wan Salleh, Farhana Aziz, Juhana Jaafar, Hasrinah Hasbullah Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia

Chapter outline 1.1 Introduction ...................................................................................................................................3 1.2 Components in oily wastewater .......................................................................................................4 1.3 Membrane technology for treatment of oily wastewater .....................................................................5 1.4 Polymeric membranes for treatment of oily wastewater .....................................................................7 1.5 Fluoropolymer membranes for treatment of oily wastewater ..............................................................9 1.6 Sulfone-containing polymer membranes for treatment of oily wastewater ........................................ 10 1.7 Mechanism of oil removal in membrane technology ....................................................................... 13 1.8 Membranes with superwetting surfaces ........................................................................................ 14 1.9 Challenges and future perspectives .............................................................................................. 15 1.10 Conclusion .................................................................................................................................. 16 References .......................................................................................................................................... 17 Further reading .................................................................................................................................... 22

1.1 Introduction Technological advancement has increased the demands on energy usage, which is still majorly fueled by the oil and gas (O&G) industry. The development of this industry has raised concerns over the years due to the release of oily wastewater. The release of this detrimental wastewater has severely affected the environment and concurrently shifts the focus of more researchers toward its treatment [1]. The release of oily wastewater into the environment was not entirely the fault of the O&G industry alone; other commercialized industries such as the food and beverage industry, even at the residential level, have also played significant roles in the release of oily wastewater. As the emulsion of grease in water is one of the most difficult pollutants to remove, oily wastewater treatment is urgently needed in today’s field of environmental engineering problems [2]. The chemical contaminants in oil ultimately impact Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00001-0 # 2020 Elsevier Inc. All rights reserved.

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4

Chapter 1 Synthetic polymer-based membranes for oily wastewater treatment

the survival rates of the affected aquatic species by poisoning marine life and disrupting feeding while also causing chronic diseases, reproductive failure, and deformities [3]. Besides that, it also poses a threat to human health. It also can be manifested in air pollution through the atmosphere. By affecting the water resources, it indirectly affects crop production and destroys the natural landscape, probably due to the oil burner safety issue coalescence that has raised concerns, especially in environmental sustainability. The majority of conventional technologies used in treating this stubborn pollutant are neither entirely effective nor environmentally friendly. There are several techniques that are typically used for oily wastewater separation, including gravity or centrifugal force, electrostatic precipitation, cyclones, floatation, demulsification, heat treatment, adsorption, and membrane separation technologies [4]. The methods such as coagulation-flocculation make use of chemical additives that will be eventually released into the environment. As the era of green technology has taken its place in the globalized world, more study has been focused on treatment methods that can be both effective and ecologically friendly. Membrane separation technologies, in particular, are efficient and more effective in removing oil droplets from oil-water emulsions when compared to conventional methods [5]. This motivation has driven researchers to shift toward membrane technology, which offers better performance with customizable properties. A broad range of materials can be employed in the development of membranes, which makes it possible to reuse waste materials such as chitin from crustacean shells and even nanoparticles from animal bones. These kinds of materials can lead to the development of synthetic polymeric membranes for effective oily wastewater treatment with a fraction of the cost needed by conventional methods. A ceramic membrane, on the other hand, requires complex preparation and high-temperature sintering, which is unfavorable in terms of cost effectiveness. Therefore, this chapter will explore the use of synthetic polymer-based membranes for the treatment of oily wastewater in terms of their structure, preparation, and applications.

1.2 Components in oily wastewater In recent years, stringent legislation has been implemented regarding the discharge content of industrial wastewater. The new European standards require 200°C is excellent [57]. The membranes made of PS can be symmetric, asymmetric, or a combination of both, thereby offering the broadest range of membrane structures with high porosity. They are suitable for MF, UF, NF, or as a base support for composite membranes.

1.6 Sulfone-containing polymer membranes for treatment of oily wastewater

11

In terms of oil separation, hydrophilic membranes are the most effective as they can decrease the fouling occurrence and offer higher rejection efficiency due to their water-removing properties. Typically, improving the hydrophilicity of membranes requires complicated physical or chemical processes, for instance, surface modification, blending, and fabricating nanocomposite membranes. Alternatively, it can be easily realized by using hydrophilic dynamic membranes. A hydrophilic dynamic membrane is formed by in situ filtering a coating solution containing either inorganic or organic hydrophilic particles through a supporting membrane [58]. Salahi et al. [59] evaluated and compared the efficiency of five types of polymeric membranes to treat industrial oily wastewater: two MF membranes, PS (0.1 μm) and PS (0.2 μm), and three UF membranes, PAN (20 kDa), PAN (30 kDa), and PAN (100 kDa). They concluded that PAN (100 kDa) performed better than other membranes by removing 97.2% oil and grease content, 96.4% turbidity, 94.1% total suspended solids, and 31.6% total dissolved solids with a high permeation flux of 96.2 L m 2 h 1 and a 60% reduction in fouling resistance. Karakulski and Morawski [50] treated waste emulsions in a copper cable factory with a small size of oil droplets (0.1 and 0.4 μm for WIROL 5000 and DRAWLUB emulsions, respectively) using integrated UF and NF membranes. The UF pretreatment with the application of the PVDF membranes with an MWCO of 100 kDa effectively reduced the content of colloids, oil, and lubricants in the treated emulsions and allowed the complete removal of suspended solids. Therefore, the obtained UF permeates had SDI values below 5. Madaeni et al. [60] used microfiltration (MF-GRM) and ultrafiltration (UF-GRM) PSf membranes to treat synthetic oily wastewater with a 0.3% oil concentration obtained from the American Petroleum Institute in the Tehran refinery. Their results showed that the flux of MF-GRM for the synthetic feed during 2 h was high compared to the real feed, which contained solid particles and colloids. Jamshidi Gohari et al. [24] has successfully synthesized hydrophilic hydrous manganese dioxide nanoparticleincorporated mixed matrix PES membranes in which they have improve the membrane hydrophilicity and antifouling resistance against oil deposition and adsorption. There is significant improvement in water permeability and oil rejection when it is used to treat a synthetic oily solution containing 1000 ppm oil. A similar modification on a polymeric membrane was also demonstrated by Kumar et al. [61], in which they prepared the PSf-based mixed-matrix UF membranes by blending PSf with different polymer-grafted bentonite additives. The optimum loading of bentonite into the PSf membrane exhibited pure water flux as high as 650 m 2 L 1 and a separation performance of 98% oil rejection with reduced fouling-attenuated coefficients (0.50) over plain PSf and blended PSf/bentonite-8 membranes. A more recent study by Obaid et al. [62] also reported the excellent application of a PSf membrane to treat oily wastewater. Their study used PSf that was immersed in different NaOH concentrations to produce 64 modified membranes. These membranes were then characterized and the best-modified membrane was chosen to be investigated in the oil-water separation system. The best-modified membrane (M64) exhibited superior water flux of 11,865 and 14,016 LMH in addition to 99.9% oil rejection. More extensive examples on the use of sulfone-based membranes for the past 20 years are tabulated in Table 1.4.

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Chapter 1 Synthetic polymer-based membranes for oily wastewater treatment

Table 1.4 Examples of sulfone-based membranes employed for oily wastewater treatment for the past 20 years (1999–2019). Membrane

Preparation technique

Performance

Author

PSf-CNTs/GO

Fiber spinning

[63]

PES-HNT/HFO

Phase inversion

PES-TiO2/HMO

Phase inversion

PES/G-PANCMI

Dry wet spinning

PSf/NaOH

Electrospinning

PES-HMO

Phase inversion

PES

Phase inversion and corona air plasma modification –

PWP: 500 mL m 2 h 1 mmHg 1 R%: 100% PWP: 652 Lm2H R%: 99.7% PWP: 43.4 LMH R%: 98.61% PWP: 800 L m 2 h 1 R%: 98% PWP: 132 L m 2 h 1 bar 1 R%: 100% PWP: 573.2 L m 2 h 1 bar 1 R%: 100% PWP: 43.6 L m 2 h 1 R%: 99.6% PWP: 1300 and 300 L m 2 h 1 atm 1 R%: 98.3% and 98.2% PWP: 20 kg m 2 h 2 and 100 kg m 2 h 2 COD removal (%): 79% and 59% PWP: 332 L m 2 h 1 R%: 96.8% PWP: 82.98 L m 2 h 1 R%: 100% PWP: 120 L m 2 h 1 R%: 98.89% R%: 90% PWP: 0.253 g cm 2 min 1 R%: 99.9 R%: 99.9 R%: 99.9 PWP: 939 L m 2 h 1 bar 1 R%: 99.9 PWP: 760  105 L m 2 h 1 Pa 1 R%: 99.9 – PWP: 64.0  11.7 L m 2 h 1 R%: 99.9%

PSf and PES

PES-polyester PES-PP

–

PSf/NMP/PVP, PSf/NMP/PEG, PSf/ DMAc/PVP and PSf/DMAc/PEG Pluronic F127–PES

Phase inversion

PSf/nanosilica

Surface modification and phase inversion Phase inversion

PSf-PEG/PVP PSf-HEMA

Immersion precipitation Irradiation-induced coating

PES-Fe2O3 PES-PVA/PA PSf

Phase inversion Interfacial polymerization Phase inversion

PSf-PVP

Phase inversion

PSf PSf PES-N-vinyl-2-pyrrolidone (NVP)

Phase inversion Phase inversion Helium plasma treatment

[64] [65] [66] [62] [24] [67] [68]

[69]

[70] [71] [71a] [72] [73] [74] [75] [76] [77] [78] [79] [80]

1.7 Mechanism of oil removal in membrane technology

13

1.7 Mechanism of oil removal in membrane technology To unravel the mechanisms of organic fouling and chemical cleaning, it is critical to understand the foulant-membrane, foulant-foulant, and foulant-cleaning agent interactions at the molecular level. The separation of O/W emulsions employs preprocessing to convert the emulsion to free oil and water (demulsification) [80a]. Among the numerous demulsification techniques, membrane-based coalescers have attracted considerable attention because they are energy efficient, cost effective, and applicable to finely dispersed emulsions. This coalescing behavior is often related to the adhesion forces among the O/W emulsions and its interaction with their environment or outside surfaces, as illustrated in Fig. 1.3. The coalescence of oil droplets can be achieved through complexation and the subsequent formation of intermolecular bridges among organic foulant molecules [82]. In the treatment of oil-in-water emulsions, the porous membrane matrix can promote the coalescence of micron and submicron oil droplets into larger ones that can be easily separated by gravity. With a low droplet volume fraction, small droplets, and a large pore size, the oil droplets frequently passed through the membrane without attaching to its surface. After the steady coalescence of the droplets on the membrane surface at the permeation side, large oil droplets could be obtained [83]. With a higher droplet volume fraction, larger oil droplets, and smaller membrane pores, the oil droplets easily blocked the membrane pores and were thus forced away by the permeating flow, which yielded relatively small oil droplets. In membrane-based coalescence, a porous membrane acts as a coalescence-accelerating medium, whereas in most membrane separation processes, the oil droplets are rejected and only the water permeates the membrane. When an O/W emulsion is fed through a membrane, the oil droplets attach to the membrane surface and then coalesce during the permeation through the membrane pores. Because the oil droplets on the permeation-side surface of the membrane are far larger than those in the feed mixture, they can be easily separated from the water phase by an external force, such as a buoyancy force, as a result of the density difference between the oil and water. Pure oil droplets become charged in water by the preferential adsorption of hydroxyl ions at the oil surface. This natural charging process in combination with a weak van der Waals (VDW) attraction between oil droplets in water has led to the realization that fine, micron-sized oil droplets should Low concentration

Oil

Water

High concentration

Bridging droplet

FIG. 1.3 Demulsification phenomena of oil-in-water (O/W) emulsions using a membrane coalescence process [81].

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Chapter 1 Synthetic polymer-based membranes for oily wastewater treatment

be metastable in water [84]. However, these emulsions eventually phase separate due to the effect of gravity, in a similar manner to that observed when shaking a typical gassed mixture of an insoluble oil and water. Both the large droplets and fine droplets produced upon vigorous shaking readily coalesce into a separate oil phase when the oil and water have a different density, as is usually the case for common hydrocarbons.

1.8 Membranes with superwetting surfaces The wetting properties of particular materials are of importance in water and wastewater treatment due to the fact that they can determine how certain materials can be applied in treatment technology such as membrane filtration, coating separation, and many others. Specifically, for oil-water separation, the materials usually employed in this kind of application are preferred to be hydrophilic and oleophobic. The ability for materials to allow water to pass through while repelling oil can provide efficient separation without the use of energy and other treatments, which is very preferable as the treatment would be cost-efficient and easily operated. In particular, special wettable materials with distinct opposite affinities toward oil and water are believed to be promising materials for selective oil/water separation. Specifically, two kinds of special wettable materials are suitable for oil/water separation: hydrophobic and oleophilic materials and hydrophilic and oleophobic materials. The hydrophilicity of particular materials often depends on interfacial interaction in which it differs from hydrophobicity in terms of competition between the interfacial free energy of cohesion of the solid immersed in liquid (in this case, water). Usually, the hydrophilic surface is assumed to be fully wetted by water denoted with a contact angle of 0 degree. As is known, polymeric membranes are the most commonly used materials in wastewater treatment applications. However, they suffer from various drawbacks such as weak mechanical strength, weak antifouling capability, poor cycling performance, and many others. As a result, frequent cleaning and replacement of membranes may be required. Recent research has shown that by modifying novel filtration materials with special wettability, those drawbacks could be overcome [85]. Therefore, more exploration has been done focusing on the modification of a material’s hydrophilicity, particularly by deposited molecular structures and the modification of surface chemistry. Molecular modification or deposition of coatings is more common for inorganic substrates, whereas the modification of surface chemistry is broadly used in the case of polymeric materials [86]. Surface modification by either chemical reaction or physical absorption is an effective approach to impart hydrophilicity in particular materials. For example, surface grafting can be achieved by immobilizing hydrophilic polymer chains onto the materials (such as membranes or meshes). The grafting can be done via different ways such as incorporating a reactive group via introduction of an initiator site [87] or imposing the materials to low-temperature plasma [88], ultraviolet [89], gamma ray [90], or electron beam radiation [91]. As of late, extensive research has been done on the surface modification of filtration materials such as meshes and membranes. The effort to increase the hydrophilicity by means of modifying the surface properties has shown promising results, especially in the treatment of oily wastewater. As opposed to hydrophilicity, oleophobicity is the wetting property of certain materials that defines negative affinity toward oil or organic liquid. Oleophobicity is also often denoted as lipophobicity. Similar to water wetting, oil-wetting properties or the degree of oleophobicity can be determined

1.9 Challenges and future perspectives

15

through the contact angle by measuring how “spherical” a drop is that is sitting on the oleophobic surface. In hydrophobicity, water is used as the test fluid. However, in oleophobicity, other fluids are used, which are typically short-chain alkanes. One of the most common standard fluids for measurement is nhexadecane. The International Union of Pure and Applied Chemistry (IUPAC), uses the terms “oil” and “organic liquids” interchangeably. Organic liquids include anything that is liquid at room temperature and pressure and based on carbon. “Organic liquid” is a very wide definition, but it is certainly preferable to the dictionary definition of “a viscous liquid derived from petroleum,” which is not correct in this instance. A liquid does not have to be 100% oil to be oily. The surface energy of oleophobic materials is relatively low, as there is only one type of interaction between oil molecules: VDW forces. These are very weak forces that originate from instantaneous dipoles (analogous to an electrical charge) that are induced in atoms. Because VDW forces are so weak, the oil molecules are not strongly bonded to one another, meaning that the difference in energy between interactions at the surface and in the bulk is relatively small [92]. The oil surface tensions are approximately 20 mN m 1, depending on the oil. To be oleophobic, a surface must have a surface tension lower than 20 mN m 1. As the oleophobicity of the surface increases, the surface tension, although comparatively low, causes the liquid to pull itself away from the surface more and more into a sphere. A supplementary energy barrier between the Wenzel and the Cassie-Baxter caused by changing the liquid-vapor interface from concave to convex made it possible to stabilize the Cassie-Baxter interaction, which in turn made the surface be more oleophobic [92a]. To impart oleophobicity, one must either chemically modify a rough surface using a low surface energy or roughen a low energy surface. Additionally, increasing the surface-area-to-volume ratio can also impart oleophobicity. The important backbones to impart oleophobicity are surface energy, surface topography, and the type of liquid use to influence the wetting. Numerous studies have also noted that chemical modification aiming at lowering the surface energy can render certain materials as more oleophobic. Pan et al. [93] modified PDMS micropillar surfaces using FDTS and successfully synthesized superhydro-oleophobic micropillars. In their study, FDTS was blended into PDMS and the resultant modified PDMS recorded an oil contact angle of 115 degrees. The incorporation of FDTS has proven to smoothen the surface of PDMS, which in turn increases its oil repellency. Another study by Sun et al. [94] has also demonstrated imparting oleophobicity by lowering the surface energy. They coated commercial PTFE with dopamine to be used in a membrane distillation system. Their work has produced a smoother PTFE with lower binding energy as opposed to pristine PTFE. The modified membrane has also shown a threefold increase in the performance of oil-saline separation. The literature on various modifications done on surface energy and morphology toward imparting oleophobicity has been recorded extensively and more study has been vigorously done toward the use of this method.

1.9 Challenges and future perspectives Membrane filtration is a promising technology for separating oil droplets smaller than  10 μm [1]. Notably, the increasing interest in the employment of membrane filtration for the treatment of oily wastewater is evident in the significant increase in the number of publications over the last three decades (1988–2018).

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Chapter 1 Synthetic polymer-based membranes for oily wastewater treatment

The challenges associated with the membrane filtration of oily wastewater are centered on membrane fouling—a key obstacle that hinders the broader implementation of membrane technology for oil-water separation. Oil droplets, which can both deform and coalesce, are unique foulants that challenge the available knowledge on fouling. More studies on mechanistically understanding membrane fouling by oil are needed, particularly in view of the recent studies that have collectively shown oil to be distinctive from other particulate foulants. Unfortunately, membrane fouling is a major drawback that reduces productivity and increases the operational costs of membrane filtration. Many efforts have been dedicated to understanding the mechanisms of membrane fouling and to develop methods to effectively monitor and mitigate membrane fouling. Approaches to mitigating membrane fouling include fouling-resistant membranes, turbulence promoters, and membrane cleaning. For the novel membranes, on top on proving superiority in performance, practical issues such as the longer-term durability of the coats in the presence of cross-flow, the treatment of realistic feeds, and the uniformity of modification across large membrane areas have to be addressed as well. With respect to turbulence promoters, increasing the shear stress gives better membrane fouling mitigation on one hand, but may affect the membrane integrity on the other hand and thereby reduce rejection. As for membrane cleaning, the deformability of oil droplets and the propensity to form a contiguous film make total recovery more difficult, so effective membrane fouling mitigation is essential to prolong filtration and delay cleaning as much as possible. Additionally, the complexity of the oily wastewater component itself has posed challenges, especially in treatment using polymer membranes. Two main features of membrane processes should be emphasized in the development of new applications: (1) the modular character of the membrane technology, which allows envisaging its application in small-, medium-, and large-scale installations; and (2) the versatility in the synthesis of hybrid processes involving membrane processes with other operation units, either upstream as pretreatments or downstream as polishing stages for the recovery of valuable compounds from multicomponent mixtures. The complexities of the oily wastewater are usually dependent on the source of the wastewater itself, which narrows down the type of material combination that can be employed in the fabrication of the polymer membrane. This variety also affects the versatility of the developed membrane processes, in which only a certain type of process can be employed to a certain type of oily wastewater. For future reference, several parameters and possibilities should be taken into consideration toward the development of oily wastewater treatments utilizing polymeric membranes. The mechanism of oil coalescence should play a key role in determining the type of process that can be employed as well as the materials that will be used in the development of the membrane. The fact that a polymer membrane offers vast opportunities for modification and customization makes it possible to develop a more facile and versatile process that can be employed in different circumstances. However, more extensive and comprehensive research is needed to address the different factors contributing to the performance of the membrane itself as well as to decrease or fully avoid fouling as much as possible.

1.10 Conclusion The adverse impact of highly toxic oily wastewaters as well as the scarcity of water resources in many regions around the world have driven many researchers and scientists toward developing useful techniques and approaches to produce treated waters of a quality suitable for reuse and recycling. In

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addition to oil foulant, real-world oily wastewater contains various other kinds of foulants such as organic foulants, inorganic foulants, and biofilms. It is challenging to prevent fouling by these foulants at the same time. The property of the membrane (e.g., surface hydrophilicity and structure) and wastewater (e.g., composition and concentration), the configuration of the filtration module, and the operation conditions are all relevant to membrane fouling during filtration. Effective and reliable antifouling strategies still need to be explored, and some existing antifouling mechanisms need to be further clarified. Therefore, the development of a polymeric membrane has provided an extensive outlook toward greener and cost-effective techniques in solving the aforementioned challenges.

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Further reading [95] M.N. De Pinho, M. Minhalma, Introduction in Membrane Technologies, Separation of Functional Molecules in Food by Membrane Technology, Elsevier Inc, 2019https://doi.org/10.1016/B978-0-12-815056-6.00001-2. [96] S. De Gisi, M. Notarnicola, Industrial wastewater treatment, in: Encyclopedia of Sustainable Technologies, Elsevier, 2017https://doi.org/10.1016/B978-0-12-409548-9.10167-8.

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Synthetic polymer-based membranes for desalination

2

Puyam S. Singha, Paramita Raya, Ahmad Fauzi Ismailb Membrane Science and Separation Technology Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific and Industrial Research (CSIR), Bhavnagar, Gujarat, Indiaa Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysiab

Chapter outline 2.1 Introduction .................................................................................................................................. 23 2.2 Cellulose acetate RO membranes ................................................................................................... 24 2.3 Thin-film-composite RO membranes ................................................................................................ 25 2.3.1 Procedure to prepare a thin-film-composite RO membrane ............................................ 26 2.4 Microscopic characterization for membrane structure topology ........................................................ 30 2.5 Nanoscale characterization of internal membrane structure ............................................................. 31 2.6 Transport properties ...................................................................................................................... 31 2.7 Membrane module configuration .................................................................................................... 33 2.8 Module array in the RO process ..................................................................................................... 33 2.9 Concluding remarks and future prospects ....................................................................................... 34 References .......................................................................................................................................... 35

2.1 Introduction Desalination is the process of removing dissolved salts from saline water. Therefore, seawater or brackish water can be converted into potable water by desalination, which is becoming a necessary technique because of increasing water demand due to population growth, urbanization, and industrialization. Clean water is in great need to meet the growing demand for human intake as well as for agricultural and industrial uses. Water scarcity is becoming a global problem as billions of people are affected due to water problems. Millions of people die each year due to inadequate water supply, sanitation, and hygiene. There are only limited sources of potable water such as groundwater and surface freshwater from lakes, natural springs, and rivers. However, the availability of potable water can be largely increased by the desalination of seawater [1]. Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00002-2 # 2020 Elsevier Inc. All rights reserved.

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Chapter 2 Synthetic polymer-based membranes for desalination

Desalination by membrane processes is playing a major role in providing potable water, as the process is more economical than the other desalination processes of multistage flash distillation and multieffect distillation [2, 3]. The energy requirements for seawater membrane desalination as well as challenges and strategies to further reduce energy can be found elsewhere [4–7]. Besides, membrane processes such as reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) can reduce pollution from wastewater by removing undesired dissolved salts as well as a wide range of components, including suspended solids, pathogens, toxic metals, small organic compounds, and other harmful ions [8, 9].

2.2 Cellulose acetate RO membranes The main aim in developing a membrane desalination process to meet the demand for clean water was first initiated by Loeb and Sourirajan [10, 11] through preparation of a synthetic “reverse osmosis” membrane from cellulose acetate material by a phase inversion process. The chemical structure of the cellulose acetate is shown in Fig. 2.1. The preparation of the cellulose acetate membrane was done by the following steps. First, a desired amount of cellulose acetate was dissolved in a suitable solvent to form a casting dope. Second, the cellulose acetate solution was cast on a support. Finally, the cast solution film on the support was immersed in a coagulation solvent bath to undergo a phase inversion process resulting in the formation of a cellulose acetate film deposited on the porous support. The cellulose acetate film was then separated from the support and subjected to annealing in hot water, resulting in a cellulose acetate membrane of asymmetric pore structure comprising dense skin and gradient pores underneath. This is the appropriate membrane structure morphology for desalination in which the top skin layer acts as a selective layer whereas the bottom layer gives mechanical support. Thus, the cellulose acetate membrane is a semipermeable membrane that allows water containing low amounts of dissolved salt to permeate through, but retains water containing high amounts of dissolved salt under external pressure in excess of the osmotic pressure of the saline water solution. In this way, it is the reverse osmosis process of forcing potable water from brackish or seawater through the membrane by applying a pressure in excess of the

FIG. 2.1 Chemical structure of cellulose acetate. Own drawing.

2.3 Thin-film-composite RO membranes

25

FIG. 2.2 Schematic representation of osmosis and reverse osmosis processes. Own drawing, conceptualized from J. Kucera, Desalination, Scrivener Publishing Wiley, Massachusetts, 2014.

osmotic pressure. This is reverse to the natural process of osmosis in which solvent from a dilute solution moves toward the concentrated solution across the membrane [12]. The natural osmosis and reverse osmosis processes are schematically depicted in Fig. 2.2. The transport property of such a cellulose acetate membrane is described as the solution-diffusion model [13, 14]. Later, the cellulose acetate membrane was prepared in tubular form and became the first-generation commercial reverse osmosis membrane.

2.3 Thin-film-composite RO membranes The desirable characteristics of reverse osmosis membranes are high water permeability but high salt rejection efficiency; endurance for a long life that can tolerate a wide pH range of harsh acidic and alkaline environments; a low fouling tendency; and durable materials. Some of the limitations associated with cellulose acetate membranes are fouling due to biological attack, a narrow pH range of tolerance, and low flux. In order to overcome these limitations, another reverse osmosis membrane known as a thin-film-composite membrane was prepared in which the top selective layer is polyamide formed by interfacial polymerization by reaction between meta-phenylene diamine (MPD) and trimesoyl chloride (TMC) [15–23]. The chemical structure of the polyamide formed by interfacial polymerization is shown in Fig. 2.3. As shown in the reaction scheme of Fig. 2.3, the cross-linked polyamide may have a range of network structures with varied macromolecular structural units depending on the ratio of the linear polymer chain network having totally cross-linked networks of dCONHd linkages (“n” units) and

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Chapter 2 Synthetic polymer-based membranes for desalination

FIG. 2.3 Chemical structure of polyamide formed by interfacial polymerization between MPD and TMC. Own drawing.

pendant dCOOH groups (“m” units). The polyamide structure with more of the pendant COOH group shows a higher negative Zeta-potential. Further, the degree of cross-linking of the polyamide can be estimated by XPS analysis from the ratio of “n” and “m” units based on the atomic concentration of elements at the surface [24]. Reverse osmosis (RO) technology based on the thin-film-composite polyamide membrane is presently the most energy efficient of all the seawater desalination technologies [25–32]. The thin-film-composite membrane is comprised of three layers: the top, a selective layer of dense polyamide; the middle, a sublayer support of porous polymer; and the bottom, a nonwoven fabric, as depicted in Fig. 2.4. This membrane is the second-generation RO membrane. It has several advantages for desalination and other separation applications. It is mechanically robust and high strength for operation at high pressure. It exhibits high selectivity in terms of salt rejection efficiency and high productivity of potable water. It is also resistant in wide pH range (3–11) of operation.

2.3.1 Procedure to prepare a thin-film-composite RO membrane The preparation of the thin-film-composite membrane consists of two steps. In the first step, a polymer support membrane is prepared by a phase inversion process. Polymer support membrane preparation, as depicted in Fig. 2.5, is described below. Initially, a polymer casting solution was prepared by dissolving a desired amount of polymer such as polysulfone in a suitable solvent such as N,Ndimethylformamide (DMF). Subsequently, the polymer solution of a definite thickness was cast on nonwoven polyester, either by roller or knife blade, and finally the cast polymer solution film was immersed in a coagulating nonsolvent such as a water bath. The relative humidity of 10%–90% and the temperature of the casting chamber of 10–50°C can be varied to tune the membrane pores.

2.3 Thin-film-composite RO membranes

27

FIG. 2.4 Schematic depiction of second-generation thin-film-composite membrane structure. Own drawing.

FIG. 2.5 Process for preparation of a polymer support membrane. Own drawing.

Generally, polymer support membranes prepared by the phase inversion process are asymmetric in their nature. The phase inversion process itself is controlled by both the thermodynamic and the kinetic factors. The temperature and composition of the casting solution and nonsolvent bath, the substrate fabric wetting, the gate height of the casting blade, etc., all have great influence on membrane morphology and are crucial for the development of membranes with improved properties. The major

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Chapter 2 Synthetic polymer-based membranes for desalination

chemical and physical factors impacting the properties of the polymer support membranes obtained by the casting technique through the phase inversion process are listed below. 1. The nature of the backing layer of nonwoven polyester fabric, such as thickness, surface uniformity and smoothness, basic weight, permeability, porosity, etc. 2. The composition of the casting solution, the concentration of polymer in the solvent, the singlecomponent polymer in a single solvent, the multicomponent polymer blends in single solvents, the polymers in mixed solvents, and the polymers along with other soluble additives or insoluble additives such as silica, zeolite, etc., in solvent. 3. The overall thickness of the cast membrane over the support, top layer, and penetrated fraction inside the fabric layer and membrane surface roughness. 4. The casting speed, the contact time between the casting solution and the coagulating nonsolvent bath during the phase inversion process, and the residence time of the cast membrane in the coagulation bath. 5. The temperature, the moisture content (humidity) in the air of the casting chamber, and the residence time in the casting chamber. 6. The coagulating nonsolvent bath; the type of nonsolvent, minor solvent, or salt additives in the bath; and the temperature of the bath. In the phase inversion process, there is a diffusion exchange of the solvent and nonsolvent. This results in a thermodynamic instability of the polymer solution that in turn separates into solutions of polymerlean and polymer rich-phases consequent to the formation of a denser layer of smaller pores from the polymer-rich phase on top of a largely porous layer from the polymer-lean phase. Thus, this asymmetric porous structure is comprised of top smaller pores and bottom larger pores that are firmly controlled by the rate of diffusion exchange of the solvent and nonsolvent. It has been observed that an increase in the polymer solution concentration results in an enrichment of the polymer phase in the polymeric solution, which after phase inversion results in smaller pore morphology consequent to lesser flux. Second, the membranes prepared from a higher concentration of polysulfone are more hydrophobic as compared to membranes prepared from a lower concentration of polymers, which implies that a densely packed pore prohibited the penetration of water molecules. Membrane surface pores of nanometer-length scale (
20 nm pores) were achieved by a phase inversion process using a combination of increased polysulfone solution concentration and pore-former additives while a membrane polymeric support of high porosity (>100 nm) may be prepared from the casting dope of the lower polymer concentration. Such a polymer support membrane of different pore structure is influential in the formation of thin-film-composite membranes of different properties [33]. In the second step, the thin-film-composite membrane is prepared by the deposition of a dense polyamide layer over the polymer support membrane (PSM) using a coating process, as depicted in Fig. 2.6. The PSM is contacted with an aqueous solution of m-phenylenediamine (MPD) (the concentration of MPD in water may range from 0.1% to 4%) to allow absorption of the MPD solution in the PSM so that all the pores are filled with the MPD solution. Surface liquid from the support is drained off. The resultant MPD-absorbed support is then contacted with an organic (e.g., n-hexane) solution of trimesoyl chloride (TMC) (the concentration of the TMC in the solution may range from 0.05% to 0.2%) to allow interfacial polymerization between the MPD and TMC monomers, resulting in the formation of a thin polyamide layer over the support. Upon curing at about 60–80°C, it results in a thin-film-composite RO membrane.

2.3 Thin-film-composite RO membranes

29

FIG. 2.6 Process for preparation of thin-film-composite RO membrane. Own drawing.

The top polyamide layer is responsible for the selective separation process and therefore it mainly determines the stability and performance of the thin-film-composite membranes. Thus, the preparation conditions of the polyamide are vital in determining the properties of the final thin-film-composite membranes. The various factors impacting the properties of the thin-film-composite membrane are listed below. 1. The nature of the polymer support membrane such as thickness, surface pore size, bulk porosity, adherence to the backing fabric, average surface roughness, permeability, and cleanliness of the membrane surface, meaning free from foreign particles such as dust, grease, etc. 2. The absorption amount of aqueous MPD solution in the polymer support membrane; the concentration of MPD in the solution; additives such as glycerol, dimethylsulphoxide, aliphatic amine, surfactant, etc., in the aqueous solutions; the concentration of the additives; and the temperature of the MPD solution bath. 3. The concentration of TMC in the organic solution (n-hexane) and the temperature of the organic solution bath. 4. The interfacial polymerization reaction temperature and the time between TMC and MPD monomers to form a nascent polyamide over the support. 5. The curing of the nascent polyamide at a higher temperature for a desired period to form a mechanically robust thin film composite of polyamide and polymer support. 6. The removal of residual monomers and solvents by treating the composite membrane in citric acid and water. 7. The deposition of an antibacterial protective layer over the membrane by dip coating with aqueous glycerol solution containing surfactant and sodium metabisulfite. The formation of a polyamide layer over the porous support by the interfacial polymerization process is schematically shown in Fig. 2.7.

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Chapter 2 Synthetic polymer-based membranes for desalination

FIG. 2.7 Scheme of polyamide deposition over the porous support membrane. Own drawing.

The pore size of the support membrane is influential in the formation of the polyamide layer structure. The porous support of larger pores allows more penetration of polyamide material inside the support while the smaller pores of the support allow the formation of the polyamide layer mainly on the surface of the support [33].

2.4 Microscopic characterization for membrane structure topology Top-surface and cross-section SEM images of the samples revealed the microscopic morphology of the composite RO membrane. The polymer support membrane of the asymmetric porous structure due to smaller pores near the surface formed by nodular units and larger macrovoids underneath was identified by the SEM images [34, 35]. During the interfacial polymerization process over the porous support, the polyamide layer was initially penetrated inside the porous structure and then a top polyamide layer was deposited over the surface of the support. The top polyamide layer over the membrane support as well as the penetrated fraction of polyamide inside the nodular dendritic structure of the polysulfone membrane support was seen from the cross-section SEM images. The outermost top polyamide layer is a continuous film of rugged microstructure covering the porous support. The top polyamide layer is responsible for permeability and selectivity. The differences in surface roughness between the surfaces of porous support and the membrane can be clearly distinguishable by AFM observation [35–39]. Their differences in chemical structures were also observed by ATR-IR from the sample surfaces. Polyamide bands at 1660 cm1 due to amide I (C]O stretch), the 1547 cm1 due to amide II (CdN stretch), and the 1609 cm1 due to polyamide aromatic ring breathing along with the characteristic polysulfone bands were observed from the composite polyamide-polysulfone RO membrane. In this case, the IR penetration depth exceeded the polyamide layer thickness, revealing IR bands from

2.6 Transport properties

31

both the polyamide and polysulfone sublayers [40, 41]. The surface of the typical polysulfone support was comparatively smoother with an average surface roughness of only about 20 nm while the surface morphology of the RO sample showed a typical nodular morphology having an average surface roughness of about 120 nm. This implies that the minimum thickness of the defect-free, active polyamide layer of such an RO membrane should be over the value of 120 nm. Therefore, the surface roughness observed by AFM is an important parameter that can directly relate to the permeability of the membrane. Decreasing the roughness of the polyamide layer is the key to make a thinner polyamide layer over the support for high flux productivity. The roughness of the polyamide layer is found to be affected by interfacial polymerization, depending on the number of monomers and the types that participate in the reaction process at different reaction conditions and temperatures [42–45]. The polycondensation reaction between the MPD and TMC monomers is diffusion-limited growth process. And, the nature of polyamide network structure is controlled by the reaction rate. A large network structure and aggregation are obtained where the molecular addition is facilitated by absorbing the active nuclei on the slowly growing particles [46, 47].

2.5 Nanoscale characterization of internal membrane structure The small-angle scattering technique provided information on the internal features of the polyamide membranes. The fractal surfaces of the large structure of cross-linked polyamide comprising a close packing of globular structural units were observed using X-ray and neutron scattering at different length scales [34, 48]. Using synchrotron small-angle X-ray scattering, an average molecular spacing ˚ in the intrachain molecular packing was observed [49]. Based on these scattering of about 5.1–5.3 A data, it can be concluded that the polyamide structure is a tier structure of molecular packing, primary compacted chains, and clusters of primary units. Thus, the molecular spacing of the so-called “pores” of such membrane types is irregular and mostly noninterconnecting, as it arises from the spaces of the polymer intrachain segments constituting the polymer network [50–52]. These pores can also be estimated using PALS in terms of the average void radius from the infinite spherical potential well of the radius [53, 54].

2.6 Transport properties The transport properties of the RO membrane in terms of water and salt permeability are related to its active membrane structure of polyamide material deposited over the support [55]. The RO membrane pore is definitely larger than the kinetic size of the water molecule, and so the water transport can be explained on the basis of the pore flow model according to Eq. (2.1): Jw ¼

Qw  l A  ΔP

(2.1)

where Jw is the water permeability of the membrane, Qw is the water flow rate, l is the membrane thickness, A is the membrane area, and ΔP is the net operating pressure obtained by subtracting the osmotic pressure from the applied pressure differential across the membrane.

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Chapter 2 Synthetic polymer-based membranes for desalination

According to the pore flow model, a linear dependence of flux on the applied pressure is expected so that the flux per unit pressure is constant. This means that the water flow rate is directly proportional to the net operating pressure for a membrane type of definite thickness and area. The salt flux of the RO membrane may be constant in spite of the increasing pressure. Typically, salt flux is on the basis of the solution-diffusion model in which the salt permeation is by initially absorbing the hydrated salt in the hydrophilic membrane structure and subsequently diffusing out of the membrane barrier layer. The salt permeability of the membrane is defined in Eq. (2.2): Js ¼

Qs  l A  ΔC

(2.2)

where Js is the salt permeability of the membrane, Qs is the salt flow rate, l is the membrane thickness, A is the membrane area, and ΔC is the net concentration gradient across the membrane. The salt flow rate is directly proportional to the net concentration gradient across the membrane. A part of salt rejection by the membrane can occur due to repulsion between the negatively charged dCOOH groups at the membrane surface and the hydrated anions. Thus, the membrane rejects more multivalent anions than monovalent anions. The transport process is schematically shown in Fig. 2.8A. As shown in the schematic transport plots according to Eqs. (2.1), (2.2), the membrane exhibited an increase in water flow rate while the salt flow rate remained nearly constant with an increase in operating pressure, which subsequently resulted in the separation of salt from water.

FIG. 2.8 The transport process of salt and water flux through the RO membrane. (A) Defect-free RO membrane; (B) Imperfect RO membrane. Own drawing, conceptualized from S.M.J. Zaidi, F. Fadhillah, Z. Khan, A.F. Ismail, Salt and water transport in reverse osmosis thin film composite seawater desalination membranes, Desalination 368 (2015) 202–213; T.K. Sherwood, P.L.T. Brian, R.E. Fisher, Desalination by reverse osmosis, Ind. Eng. Chem. Fundam. 6 (1967) 2–12; L.E. Applegate, C.R. Antonson, The phenomenological characterization of DP-1 membranes, in: H.K. Lonsdale, H.E. Podall (Eds.), Reverse osmosis and membrane research, Plenum Press, New York, 1972.

2.8 Module array in the RO process

33

The membranes may exhibit same water permeability but different salt permeability depending on their hydrophilicity based on the chemical nature of the cross-linked network membrane. The hydrophilicity extent for the membrane is dependent on the presence of some pendant dCOOH groups in the cross-linked polyamide. A fully cross-linked polyamide with dCONHd linkages is less hydrophilic and exhibits a high salt rejection efficiency. Furthermore, the solution diffusion imperfection model may be operative in case of the increased salt permeation for a membrane type as a function of applied pressure [56, 57]. The contribution of pore flow transport is quite significant in the salt permeation rate of such a membrane type. This is schematically shown in Fig. 2.8B.

2.7 Membrane module configuration The most efficient module configuration of an RO membrane is the spiral wound membrane [58–61]. In this configuration, a membrane of definite length is folded at its central length to form two membrane sheets. The two membrane sheets are separated by a permeate spacer material in which the active side of the membrane layer faces the permeate spacer to form a leaf. The outside edges on three sides of the leaf assembly are sealed with the folded side left open to the permeate collector. The permeate collector is a plastic tube having perforated holes to collect the permeate from the membrane sheets. A feed spacer is added to the leaf assembly so that a number of these assemblies in which each membrane leaf is separated by a feed spacer are rolled spirally around the permeate tube. The spirally wound leaf assembly around the permeate tube is kept in tight condition with adhesive and insulating tape wrapped around it. Fiberglass-reinforced epoxy plastic is used for the outer wrapping to give rigidity and strength to the element for operation at higher pressure.

2.8 Module array in the RO process In the RO process, the feed stream is passed through the membrane module, breaking into reject stream and product stream. Therefore, the feed flow rate is the sum of the product flow rate from the permeate stream and the reject flow rate from the reject stream. The recovery (%) of the process system is calculated as 100  (product flow rate/feed flow rate) and the recovery of the system can be optimized by the design parameters of the RO process operating system. When connected the modules in series inside a single pressure tube in which the reject stream from the first module is passed to the feed of second module and the reject stream from the second module to the feed of third module, as shown in Fig. 2.9A, the recovery rate is increased. The total recovery of three modules connected in series is calculated as: Recovery ð%Þ ¼ R1 + ð1  R1 ÞR2 + ½1  ðR1 + R2 ÞR3

(2.3)

where R1, R2, and R3 are the recovery (%) of modules 1, 2, and 3, respectively. Therefore, more recovery can be obtained by connecting more module elements in series within the single pressure vessel. A number of such pressure vessels can be again connected in parallel or series in an RO desalination plant, depending on the membrane properties. The connection of the pressure vessel in parallel is to distribute the feed flow rate, as shown in Fig. 2.9B, thereby maintaining the feed/reject flow rate to provide only adequate flow with sufficient turbulence without damaging the element

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Chapter 2 Synthetic polymer-based membranes for desalination

FIG. 2.9 Module array connected in series within a single pressure vessel (A) and pressure vessels connected in parallel (B). Own drawing, conceptualized from J. Kucera, Desalination, Scrivener Publishing Wiley, Massachusetts, 2014.

structure, which could otherwise occur due to excessive flow or salt deposition. Thus, it is equally important to assess the condition of the membrane at different recovery rates of the process system because the increase in salt concentration in the reject stream at the higher recovery rate has an impact on the separating membrane material. Besides, the permeate salinity is higher because of higher salt flux as a result of a higher concentration gradient across the membrane at the higher recovery (%) rate.

2.9 Concluding remarks and future prospects Thin-film-composite membrane technology is the most successful and efficient technology for water desalination due to the production of a membrane with high water permeability but a rejection of >99% of all soluble ionic salts. Despite the overall high efficiency of the membrane technology, the energy consumption by the desalination process is high. For example, for desalination by reverse osmosis with a seawater feed of 35,000 ppm and 50% recovery, the thermodynamic minimum specific energy is 1.06 kWh m3 while single-stage operation increases the practical minimum specific energy to 1.56–2 kWh m3 if other parameters such as fouling are negligibly small. The energy required for the production of potable water in the RO seawater desalination plants is difficult to reduce further. The main inherent problem is associated with the membrane thickness of 150–200 nm, which is difficult to reduce further due to the chemical and physical conditions of membrane preparation. Ultrathin-film-composite membrane technology for low-energy desalination of seawater and highsaline groundwater at high recovery using different approaches should be attempted. These approaches include: (i) manipulation of the chemical structure of the active layer through interfacial reaction and

References

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the introduction of novel membrane material for enhancement of water permeability while maintaining >99% salt rejection efficiency; (ii) preparation of a novel supported graphene-sheet-based nanocomposite membrane; and (iii) integrated technology for lowering the desalination costs. An alternative membrane separation technology that uses less material and energy while permitting the recovery and reuse of process streams is certainly desirable. Besides, a major problem associated with RO seawater desalination membrane technology is reject water management. This is because disposal of the remaining water as reject water with a salinity about twice that of the feed seawater causes harmful effects to plant life when discharged to the land because most plants, except some salt-tolerant plants, cannot tolerate excess salinity. For a seawater desalination plant, because the plants are situated near the sea, the common practice is to discharge the reject back to the sea. The local effect on the marine environment due to the discharge of desalination plant reject is a concern raised by marine environmentalists and is a matter of research. In a brackish water desalination plant, the problem of reject management is more acute and reject water discharge remains the method of choice. RO reject contains a lot of chemicals such as chlorinating/dechlorinating agents, fluorides, calcium, chemicals for pH adjustment, etc. Treatment of the RO reject stream for agricultural or other nonpotable water reuse or discharging such RO reject in sewers and then treating it is very costly. A multiple effect evaporation method is also a very energy-intensive process to treat such an RO reject stream. An integrated zero discharge membrane process is required that uses multiple separation systems, including efficient membrane distillation technology that uses renewable solar energy or waste heat, forward osmosis, and an effective evaporation system to concentrate further in a small footprint. Furthermore, a boron-specific membrane or resin or a hybrid membrane-resin technology may be required to develop combined boron and excess salt removal from seawater because the best membrane only rejects
80% of boron. New research in wastewater treatment should be focused on treatment at the point of generation by appropriate membrane technology to reduce the complexity involved in wastewater treatment due to composition and volume. Various technologies for wastewater treatment can be further improved, such as (i) nanofiltration membranes for the separation of alkaline earth and heavy metal ions, (ii) superhydrophobic microfiltration membranes for the treatment of oily wastewater and oil spill clean-up, (iii) hybrid processes of chemical flocculation-reverse osmosis-nanofiltration-ultrafiltrationmembrane bioreactor for leather industry effluent, and (iv) a polymer-enhanced hollow fiber ultrafiltration membrane for the removal of heavy metals from wastewater and the removal of small organic molecules and pharmaceutical products from water, either by pervaporation or nanofiltration. Wastewater if treated at the point of generation can be expected to be reused and achieve zero discharge status.

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[29] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film composite membrane: a review, Desalination 287 (2011) 190–199. [30] L. Malaeb, G.M. Ayou, Reverse osmosis technology for water treatment: state of the art review, Desalination 267 (2011) 1–8. [31] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosis desalination: water sources, technology, and today’s challenges, Water Res. 43 (2009) 2317–2348. [32] N. Misdan, W.J. Lau, A.F. Ismail, Seawater reverse osmosis (SWRO) desalination by thin-film composite membrane: current development, challenges, and future prospects, Desalination 287 (2012) 228–237. [33] P.S. Singh, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, A. Prakash Rao, P.K. Ghosh, Probing the structural variations of thin film composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions, J. Membr. Sci. 278 (2006) 19. [34] P.S. Singh, V.K. Aswal, Compacted nanoscale blocks to build skin layers of reverse osmosis and nanofiltration membranes: a revelation from small-angle neutron scattering, J. Phys. Chem. C 111 (2007) 16219–16226. [35] P.S. Singh, P. Ray, A. Bhattacharya, N.K. Saha, A.V.R. Reddy, Techniques for characterization of polyamide thin film composite membranes, Desalination 282 (2011) 78–86. [36] S.-Y. Kwak, S.G. Jung, Y.S. Yoon, Structure–motion–performance relationship of flux-enhanced reverse osmosis (RO) membranes composed of aromatic polyamide thin films, Environ. Sci. Technol. 35 (2001) 4334–4340. [37] W.R. Bowen, T.A. Doneva, J.A.G. Stoton, The use of atomic force microscopy to quantify membrane surface electrical properties, Colloid and Surf. A 201 (2002) 73–83. [38] S.-Y. Kwak, D.W. Ihm, Use of atomic force microscopy and solid-state NMR spectroscopy to characterize structure–property–performance correlation in high-flux reverse osmosis (RO) membranes, J. Membr. Sci. 158 (1999) 143–153. [39] M. Hirose, H. Ito, Y. Kamiyana, Effect of skin layer surface structures on the flux behavior of RO membranes, J. Membr. Sci. 121 (1996) 209–215. [40] S. Belfer, J. Gilron, O. Kedem, Characterization of commercial RO and UF modified and fouled membranes by means of ATR/FTIR, Desalination 124 (1999) 175–180. [41] A.P. Rao, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, V.J. Shah, Structure–performance correlation of polyamide thin film composite membranes: effect of coating conditions on film formation, J. Membr. Sci. 211 (2003) 13–24. [42] B. Khorshidi, T. Thundat, B.A. Fleck, M. Sadrzadeh, A novel approach toward fabrication of high performance thin film composite polyamide membranes, Sci. Rep. 6 (2016). Article number: 22069. [43] S.A. Sundet, Production of composite membrane, US Patent 4,529,646 (1985). [44] M. Kurihara, Y. Fusaoka, T. Sasaki, R. Bairinji, T. Uemura, Development of cross-linked fully aromatic polyamide ultra-thin composite membranes for seawater desalination, Desalination 96 (1994) 133. [45] M. Kurihara, T. Uemura, Y. Nakagawa, T. Tonomura, The thin film composite low-pressure reverse osmosis membranes, Desalination 54 (1985) 75. [46] V.Z. Nikonov, V.M. Savinov, Polyamides, in: F. Millich, F.C.R. Carraher Jr. (Eds.), Interfacial Synthesis, vol. 2, Marcel Dekker, New York, 1977. [47] R. Sardar, J.S. Shumaker-Parry, Spectroscopic and microscopic investigation of gold nanoparticle formation: ligand and temperature effects on rate and particle size, J. Am. Chem. Soc. 133 (2011) 8179–8190. [48] S.A. Sundet, Morphology of the rejecting surface of aromatic polyamide membranes for desalination, J. Membr. Sci. 76 (1993) 175–183. [49] P.S. Singh, P. Ray, Z. Xie, M. Hoang, Synchrotron SAXS to probe cross-linked network of polyamide ‘reverse osmosis’ and ‘nanofiltration’ membranes, J. Membr. Sci. 421–422 (2012) 51–59.

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[50] T.D. Nguyen, K. Chan, T. Matsuura, S. Sourirajan, Effect of shrinkage on pore size and pore size distribution of different cellulosic reverse osmosis membranes, Ind. Eng. Chem. Prod. Res. Dev. 23 (1984) 501–508. [51] R.R. Sharma, R. Agrawal, S. Chellam, Temperature effects on sieving characteristics of thin-film composite nanofiltration membranes: pore size distributions and transport parameters, J. Membr. Sci. 223 (2003) 69–87. [52] R.R. Sharma, S. Chellam, Temperature effects on the morphology of porous thin film composite nanofiltration membranes, Environ. Sci. Technol. 39 (2005) 5022–5030. [53] V.K. Sharma, P.S. Singh, S. Gautam, P. Maheshwari, D. Dutta, R. Mukhopadhyay, Dynamics of water sorbed in reverse osmosis polyamide membrane, J. Membr. Sci. 326 (2009) 667–671. [54] V.K. Sharma, P.S. Singh, S. Gautam, S. Mitra, R. Mukhopadhyay, Diffusion of water in nanoporous NF polyamide membrane, Chem. Phys. Lett. 478 (2009) 56–60. [55] S.M.J. Zaidi, F. Fadhillah, Z. Khan, A.F. Ismail, Salt and water transport in reverse osmosis thin film composite seawater desalination membranes, Desalination 368 (2015) 202–213. [56] T.K. Sherwood, P.L.T. Brian, R.E. Fisher, Desalination by reverse osmosis, Ind. Eng. Chem. Fundam. 6 (1967) 2–12. [57] L.E. Applegate, C.R. Antonson, The phenomenological characterization of DP-1 membranes, in: H.K. Lonsdale, H.E. Podall (Eds.), Reverse osmosis and membrane research, Plenum Press, New York, 1972. [58] D.T. Bray, H.F. Menzel, Design study of reverse osmosis pilot plant, Office of Saline Water Research and Development Progress Report No. 176 (1966). [59] R.L. Riley, R.L. Fox, C.R. Lyons, C.E. Milstead, M.W. Seroy, M. Tagami, Spiral-wound poly(ether amide) thin film composite membrane systems, Desalination 19 (1976) 1113. [60] R.L. Riley, C.E. Milstead, A.L. Lloyd, M.W. Seroy, M. Tagami, Spiral wound thin-film composite membrane systems for brackish and sea water desalination by reverse osmosis, Desalination 23 (1977) 331. [61] R. Lesan, J. Tomaschke, Y. Kamiyama, T. Shintani, A comparison of different classes of spiral-wound membrane elements at low-concentration feeds, Ultrapure Water 7 (3) (1990) 18.

CHAPTER

Synthetic polymer-based membranes for dye and pigment removal

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G.P. Syed Ibrahima, Arun M. Isloora, B. Lakshmib Membrane Technology Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore, Indiaa Department of Chemistry, Reva University, Bangalore, Indiab

Chapter outline 3.1 Introduction .................................................................................................................................. 39 3.2 Polymeric membranes for dye and pigment removal ........................................................................ 41 3.2.1 Nanofiltration ............................................................................................................ 41 3.2.2 Membrane distillation ................................................................................................ 45 3.2.3 Ultrafiltration ............................................................................................................ 46 3.3 Conclusions .................................................................................................................................. 49 References .......................................................................................................................................... 50

3.1 Introduction Water is one of the most vital elements on Earth for humans, plants, and animals. However, the sudden booms in industrialization, population growth, and domestic and agricultural activities are continuously deteriorating the quality of water. Thus, water pollution has turned out to be a serious issue that hampers human and animal life. Uncontrolled anthropogenic activity such as the discharge of untreated wastewater into the water stream affects the surface water quality to a greater extent [1, 2]. Consequently, an increased amount of attention has been paid toward wastewater purification by governmental authorities, scientists, and academicians. Still, the situation is becoming worse as the discharge of untreated water into the water stream is increasing. The pollutants present in wastewater are highly detrimental to the environment and humans. Most of the pollutants are highly toxic and carcinogenic while producing many side effects in humans and aquatic organisms [3, 4]. Thus, it is necessary to develop a sustainable technology to mitigate pollutants in wastewater. The oldest synthetic pigments such as Han purple, Egyptian blue, and Han blue found in cave drawings indicate that colors have a direct or indirect psychological, economical, and anthropological impact on society. Perkin’s discovery of the first synthetic mauve dye (Fig. 3.1A) was pioneering to the modern synthetic dye manufacturing industries. In textile industries, dye molecules are mainly used to Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00003-4 # 2020 Elsevier Inc. All rights reserved.

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FIG. 3.1 (A) Chemical structure of mauve and (B) difference between dyes and pigments.

impart or alter the color of the cloth by absorption or reflection of visible light (400–700 nm), which offers different colors. In the pharmaceutical industries, food colors are used to make their products eye-catching and appetizing. Dyes and pigment can be distinguished based on their particle size, stability, and solubility. In general, dye molecules are finer, soluble in water, and not stable under UV irradiation; also, even at a dye concentration of 1.0 mg/L, they impart color to the water. In addition, dye molecules are nonbiodegradable, toxic, and recalcitrant. The existence of dyestuffs in the wastewater disturbs the penetration of sunlight and chemical oxygen demand (COD), which negatively affects the aquatic system [5–7]. However, pigments are stable under UV irradiation and insoluble in water. As these dye molecules are highly soluble in water, this forms a transparent solution, whereas the pigments form a dispersion due to their poor solubility (Fig. 3.1B). Based on nature, dyes can be classified into two types: anionic and cationic. The anionic dye molecules dissociate into negatively charged ions and the cationic dyes dissociate into positively charged ions in the aqueous solution. Concurrently, pigments also can be classified into organic and inorganic pigments. The organic and inorganic pigments such as phthalocyanine, quinacridone, triarylcarbonium, vegetable black, TiO2 white, iron blue, ultramarine blue, etc., are broadly used in industries. Among the emerging contaminants present in wastewater, dyes and pigments take prime importance. Industries such as plastics, paint, paper, textiles, leather, food, pharmaceutical, and cosmetics produce >0.7 million tons of dyestuffs every year [8, 9]. As these dye molecules consist of a more complex structure, it is very tough to degrade them by light, aerobic digestion, oxidizing agent, or heat. To treat this dye-stained wastewater, diverse methods such as coagulation, the photocatalytic process, adsorption, ozonation, plasma treatment, electrochemical treatment, etc., have been proposed. However, these methods are very time-consuming, use expensive catalysts, are difficult to scale up, and need secondary treatment. Hence, membrane technology comes into the picture for treating dye wastewater, as membrane technology is cost-effective, environmentally friendly, has low energy intake, and is easy to scale up [10, 11]. In this chapter, the recent progress on the use of polymeric membranes such as nanofiltration (NF), membrane distillation (MD), and ultrafiltration (UF) membranes for effective dye and pigment

3.2 Polymeric membranes for dye and pigment removal

41

removal is discussed. Finally, it concludes with a brief summary of the selection of an appropriate membrane for dye and pigment removal.

3.2 Polymeric membranes for dye and pigment removal Nowadays, dyes are being deployed in various industries such as paper, textile, plastic, rubber, etc. Currently, it is assessed that about 7  105 tons of dyes and pigment are produced annually and >10,000 different varieties of dyes and pigment are commercially available worldwide [12]. In the literature, methods such as coagulation, adsorption, biodegradation, advanced oxidation, and chemical and photochemical degradation have been reported to remove dyes from wastewater [13–16]. In adsorption, the adsorbents such as metal or metal oxide nanoparticles, activated charcoal, chitosan, etc., have been employed for dye removal. However, this method is time-consuming and not costeffective. To remove the insoluble dye molecules, coagulation is a suitable process. Nevertheless, this method requires an additional cost for treating the sludge as well as its disposal. Similarly, the dye molecules poison the organisms used in biodegradation, making degradation not efficient. In addition, as most of the synthetic dye molecules are stable enough to light, photochemical degradation is also expected to be less efficient. At the same time, the synthetic polymeric membranes such as NF, the adsorption membrane, and UF can be effectively employed to remove dye molecules from wastewater.

3.2.1 Nanofiltration Although reverse osmosis (RO) membranes exhibit a higher rejection ability toward dyes and pigments, the higher applied pressure and lowered permeate flux reduced its applicability for dye wastewater purification [17]. Consequently, NF membranes were used, beginning in 1990, and have demonstrated vital importance for recycling dye wastewater [18, 19]. NF membrane possess properties between RO and UF, therefore they could have advantages such as high permeate flux, reduced osmotic pressure difference, high retention toward divalent salts and organic molecules with molecular weight cut-off (MWCO) between 200 and 1000 Da with reduced applied pressure and maintenance [20]. As far as the NF is concerned, there are two main separation mechanisms proposed: Donnan exclusion and size exclusion. The Donnan exclusion works based on the electrostatic repulsion between the solute and NF membrane surface, whereas the size exclusion works based on the pore size of the NF membrane. Although the NF membranes exhibit improved permeate flux over RO membranes, direct treatment of dye wastewater with the NF membrane will lead to irreversible fouling [21]. Thus, flux decline owing to fouling and concentration polarization are the two major drawbacks of the NF membranes. Therefore, an engineered NF membrane with improved surface hydrophilicity, charge, and reduced surface roughness becomes the most important aspect in the preparation of NF membranes for dye wastewater recycling. In the recent literature, an enormous amount of effort has been undertaken to improve dye and pigment wastewater recycling. In dyeing industries, reactive dyes are often used to impart color to the cellulosic fabrics. However, as reactive dyes have low rates of fixation, around 50% of the dye molecules are discharged into the effluent, which leads to severe environmental problems. Yu et al. [22] proposed the separation of reactive black 5 (RB 5) (Fig. 3.2) from the dye/salt mixture using cellulose acetate (CA) asymmetric NF membranes and polyamide (PA) thin-film composite

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Chapter 3 Polymer-based membranes for dye and pigment removal

FIG. 3.2 Chemical structure of reactive black 5.

(TFC) NF membranes. The dye removal efficiency of the NF membrane was evaluated using a different concentration of RB 5 at an NaCl concentration of 1000 mg/L. It was observed that with an increase of dye concentration, the equilibrium permeate flux of the NF membranes was reduced. The reduced permeate flux was attributed to increased concentration polarization, osmotic pressure, and dye adsorption on the membrane surface. In addition, the dye removal performances of the PA TFC membranes and the CA asymmetric NF membranes were compared. The improved dye removal capacity of the PA TFC NF membranes was ascribed to the improved surface charge of the membranes. As RB 5 is negatively charged in solution, the negatively charged PA TFC membrane could exhibit higher removal efficiency via electrostatic repulsion. Thereby, the adsorption of dye molecules on the NF membrane surface could be avoided. Cellulose nanocrystal (CNC)-incorporated thin-film nanocomposite (TFN) membranes were reported by Bai et al. [23]. It was reported that the permeate flux and NaCl rejection were increased with an increase of CNC concentration. The TFN membrane exhibited improved efficiency of both anionic and cationic dyes. The improved dye removal of the TFN NF membrane was due to the decreased negative charge, smaller pore size, and enhanced surface hydrophilicity compared to the pristine TFC membrane. The schematic diagram of the experimental setup is represented in Fig. 3.3. Gong et al. [24] demonstrated the use of a polydopamine (PDA)-functionalized carbon nanotube (CNT) as an interlayer in the TFC NF membrane for methyl orange (MO) removal. The PA membranes were prepared with different thicknesses of the CNT interlayer. The as-formed CNT interlayer (100 nm) reduced the penetration of piperazine (PIP) into the polyethersulfone (PES) substrate pores. Thereby, the formation of the PA layer inside the PES substrate pore was avoided (Fig. 3.4), which resulted in the formation of a thin PA layer. Also, as the heat generated while reacting the PIP with trimesoyl chloride (TMC) was not dissipated into the dense and small pore-sized CNT interlayer, a high degree of cross-linked PA layer was obtained. The modified TFC membrane showed enhanced dye removal ability compared to the pristine TFC membrane. It was also mentioned that the removal of positively charged dyes such as methylene blue (MB) was 86.4%, whereas the negatively charged dyes such as methyl violet (MV) could be removed at >99.5%. As the modified TFC membrane was

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FIG. 3.3 Schematic of the experimental setup. Reproduced with permission from L. Bai, Y. Liu, A. Ding, N. Ren, G. Li, H. Liang, Fabrication and characterization of thin-film composite (TFC) nanofiltration membranes incorporated with cellulose nanocrystals (CNCs) for enhanced desalination performance and dye removal, Chem. Eng. J. 358 (2019) 1519–1528, https://doi.org/10.1016/j.cej.2018.10.147. Elsevier, Copyright 2019.

FIG. 3.4 Schematic of the NF membrane polyamide layer with and without the CNT layer (top) and the SEM cross-sectional image of the TFC membrane with the CNT interlayer (bottom). Reproduced with permission from G. Gong, P. Wang, Z. Zhou, Y. Hu, New insights into the role of an interlayer for the fabrication of highly selective and permeable thin-film composite nanofiltration membrane, ACS Appl. Mater. Interfaces 11(7) (2019) 7349–7356, https://doi.org/10.1021/acsami.8b18719. American Chemical Society, Copyright 2019.

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Chapter 3 Polymer-based membranes for dye and pigment removal

negatively charged, a greater amount of negatively charged dye molecules were rejected via electrostatic repulsion (Donnan effect). However, the lower rejection of positively charged dye molecules was due to the electrostatic attraction between the TFC membrane and the dye molecules. Positively charged NF membranes were prepared by Qi et al. [25]. The prepared NF membranes exhibited improved anionic and cationic dye removal capability. The industrial dyes such as Victoria blue B (99.2%), Neutral (98.2%), Tropaeolin O (98.3%), and Semixylenol orange (99%) were efficiently removed with enhanced permeability. The rejection mechanism for dye removal was proposed as size exclusion because the modified TFC NF membrane exhibited an MWCO of 210 Da. Tannic acid was covalently bonded to the TFC membrane surface to improve the anionic dye removal ability [26]. The as-prepared negatively charged TFC NF membrane demonstrated increased dye rejection with better permeability. As tannic acid consists of a negatively charged hydroxyl functional group, the surface negative charge was improved. Thereby, the anionic dyes were removed based on the electrostatic repulsion. Recently, Abadikhah et al. [27] reported the preparation of a multifunctional TFN NF membrane. The TFN membrane exhibited 98% rejection toward rose bengal with 0.2 wt% rGO@TiO2@Ag nanocomposite in the PA layer. However, a reduction in the dye rejection was observed with 0.4 wt% of nanocomposite in the PA layer, which was attributed to the formation of nonselective holes in the PA layer at a higher concentration of nanomaterials. After 3 h of contact time, the TFN membrane also demonstrated a 90% reduction in the number of viable bacteria such as Escherichia coli. In another recent report, an amino acid ionic liquid functionalized biomimetic TFC NF membrane was prepared via interfacial polymerization (IP) for pigment wastewater treatment [28]. The added amino acid ionic liquid acted as a humectant and improved the surface hydrophilicity and charge properties of the NF membrane. The modified membrane demonstrated improved pure water permeability (PWP) (63%) and pigment removal compared to the pristine TFC membrane. The use of hollow fiber (HF) NF membranes for dye removal has also been studied. Still, the commercially available NF membranes are flat sheet membranes. The increased surface-area-tovolume ratio and the support-free and high packing density nature of HF membranes compared to flat sheet membranes attracted much interest toward the fabrication of HF NF membranes for dye wastewater recycling. Although less focus has been paid to the preparation of HF NF membranes for dye wastewater purification, there is a wide scope for the performance improvement of HF NF membranes. In that respect, Wei et al. [29] reported the preparation of positively charged composite HF NF membranes via IP for dye wastewater purification. The dye removal efficiency of the HF NF membrane was performed with different pH and concentration. However, the dye removal efficacy decreased with an increase in the dye concentration. The decreased efficacy of the HF membrane owed to the increased concentration polarization and membrane fouling. The FESEM image of the inner surface of the HF NF membrane after the dye concentration experiment is depicted in Fig. 3.5. Gao et al. [30] developed the loose inner-selective PES HF NF membranes for dye removal. In this study, the effect of the addition of additives such as sulfonated polysulfone (PSF), polyvinylpyrrolidone (PVP), and polyethylene glycol (PEG) on the morphology of the HF membranes was also studied. The added additives reduced the pore size and finger-like macrovoids of the HF NF membranes. It was reported that the modified HF membrane rejected 96.64% of indigo carmine with improved water permeability and showed better performance at up to 76 h of continuous operation.

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FIG. 3.5 FESEM image of (A) cross-sectional image of the HF NF membrane and (B) the inner surface of the HF NF membrane after the dye concentration experiment. Reproduced with permission from X. Wei, S. Wang, Y. Shi, H. Xiang, J. Chen, Application of positively charged composite hollow-fiber nanofiltration membranes for dye purification, Ind. Eng. Chem. Res. 53(36) (2014) 14036–14045, https://doi.org/10.1021/ ie5017688. American Chemical Society, Copyright 2014.

3.2.2 Membrane distillation MD is one of the potential technologies for dye wastewater purification. It is a nonisothermal membrane-based separation process and is based on the diffusive and convective transportation of vapor across the hydrophobic membrane [31]. The hydrophobic polymeric membranes such as PVDF, polypropylene (PP), and polytetrafluoroethylene (PTFE) serve as barriers between the two nonisothermal solutions. The temperature difference between the feed and permeate generates the partial vapor pressure gradient, which acts as a driving force. The high mechanical, chemical, and thermal resistance of the ceramic membranes made it perform better than the polymeric membranes. However, due to the presence of a hydroxyl group on the ceramic membranes, they are inherently hydrophilic in nature. Thus, surface coating with hydrophobic material has become inevitable, which increases the fabrication cost. Therefore, polymeric membranes have remained popular for MD application. The potential of vacuum membrane distillation (VMD) for dye wastewater treatment was studied by Banat et al. [32]. MB was used as a model dye for estimating the VMD performance using a PP membrane module. The effect of feed temperature, dye concentration, and flow rate were investigated. The results indicated that the permeate flow rate was decreased with respect to time. However, the flux was increased with increased flow rate, which was attributed to the prevention of blocking of membrane pores by dye molecules. Thereby the concentration polarization was reduced to a greater extent at a high flow rate. Ge et al. [33] proposed polyelectrolyte supported a forward osmosis (FO)-MD hybrid process, and the hybrid process was successfully applied for the continuous treatment of dye wastewater. In this study, poly(acrylic acid) sodium (PAA-Na, 0.48 g/mL) was deployed as a draw solute to dehydrate the feed dye wastewater in FO and MD was used to reconcentrate the draw solution (PAA-Na). After 30 min, the steady state of dehydration of the acid orange 8 solution was reached via the FO-MD

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Chapter 3 Polymer-based membranes for dye and pigment removal

process. It was mentioned that the dye acid orange 8 concentration was increased and no electrolyte (PAA-Na) was leaked to the cold permeate side in MD with time. The obtained results confirmed the feasibility of the FO-MD hybrid process for dye wastewater treatment. Fig. 3.6 illustrates the FO-MD hybrid process. The methyl-functionalized mesoporous silica nanoparticles were incorporated on the PTFE/PVDF membrane surface to improve the direct contact membrane distillation (DCMD) performance [34]. The incorporated nanoparticles improved the flux as well as the low wetting properties. However, the increased concentration of nanoparticles decreased the DCMD performance due to the pore blockage. The modified membrane demonstrated 99% removal of CR dye from wastewater. Simultaneously, the dye rejection was decreased slowly with time due to the increased concentration of CR dye in the feed solution, which resulted in the concentration polarization. Another reason for the decreased dye removal efficiency was ascribed to the pore wetting due to fouling, which is one of the indispensable drawbacks in DCMD. Fig. 3.7 illustrates the pristine and nanocomposite membrane DCMD performance with time.

3.2.3 Ultrafiltration Dye wastewater recycling with UF membranes is getting more attention as it exhibits improved flux and high dye removal ability. Reactive dyes are the most predominantly used dye in the dyeing industries. However, due to the low fiber fixation property of reactive dyes, inorganic salts, for example, NaCl or Na2SO4, are added to improve the dye pickup capability of the cotton fabric [35]. Although NF membranes are a widely accepted technology for dye removal, they have an MWCO range of 200– 1000 Da; this presents high rejection toward dyes and divalent salts such as Na2SO4. The presence of inorganic salt in the dye wastewater makes it more difficult for biodegradation, complicates the treatment process [36], and increases the osmotic pressure of the solution, which would negatively affect the membrane flux [37]. In addition, the permeability of NF membranes is below 10 L/m2 h bar, which makes NF membranes less efficient for dye wastewater recycling [38, 39]. Similarly, drawbacks such

FIG. 3.6 Schematic illustration of the lab-scale FO-MD hybrid process. Reproduced with permission from Q. Ge, P. Wang, C. Wan, T.-S. Chung, Polyelectrolyte-promoted forward osmosis–membrane distillation (FO–MD) hybrid process for dye wastewater treatment, Environ. Sci. Technol. 46(11) (2012) 6236–6243. American Chemical Society, Copyright 2012.

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FIG. 3.7 (A) Dye removal efficacy and (B) flux of pristine and nanocomposite membranes. Reproduced with permission from N.P. Khumalo, L.N. Nthunya, E. De Canck, S. Derese, A.R. Verliefde, A.T. Kuvarega, … D.S. Dlamini, Congo red dye removal by direct membrane distillation using PVDF/PTFE membrane, Sep. Purif. Technol. 211 (2019) 578–586, https://doi.org/10.1016/j.seppur.2018.10.039. Elsevier, Copyright 2019.

as pore wetting, decrease of the temperature difference across the hydrophobic membrane, etc., limited the use of MD for dye wastewater treatment. Therefore, a membrane with a high permeability to inorganic salt is highly preferable for dye/salt separation in dye wastewater. The higher pore size of the UF membranes compared to NF membranes hampers the high rejection ability toward dye molecules. Hence, tight UF (t-UF) membranes are emerging as an effective alternative to recycle dye wastewater with improved rejection of dye molecules with high permeability to inorganic salts compared to the NF [40]. In that respect, the UH004 t-UF membrane performance was studied for the removal of both reactive and direct dyes [41]. The results indicated that the membrane demonstrated a rejection of >98.9% in the presence of 60 g/L Na2SO4 for all dyes studied. Thereby, the t-UF membrane presented a dye recovery of 97% after five diavolumes. Liu et al. [42] fabricated a positively charged t-UF membrane by peptide coupling between carboxylated cardo poly(arylene ether ketone)s (PAEK-COOH) and PEI. The as-prepared membranes exhibited an MWCO of 12.7 kDa and an isoelectric point at a pH of 7.4. The best performance membrane was obtained by reacting PEI with a molecular weight of 10,000 Da in the coagulation bath. The prepared membrane could reject 99.9% of congo red (CR) with a dye solution permeability of 80–84 L/m2 h bar in 100 h of long-standing operation. In addition, the improved dye rejection was ascribed to the asformed hydrophilic, denser, and thinner top skin layer. The fabrication of a t-UF membrane from different commercial UF membranes with optimized pore size was performed using PDA coating chemistry [43]. The porous PAN-based UF membrane pore size was reduced after treating with PDA. After 3 h of coating, the pore size was reduced to 2.75 nm and the MWCO was reduced from 100,000 to 11,670 Da. The coated PAN-based membrane evinced a pore size and MWCO of 2.1 nm and 6850 Da, respectively, after 10 h of PDA coating time. The PDA-coated membranes have not exhibited any decline in permeability as they had high permeability to inorganic salts such as NaCl or Na2SO4, which decreased the osmotic pressure of the feed solution. It is also reported that the UP5 membrane with an MWCO of 6050 Da demonstrated a 95.91% rejection of reactive orange 16 (RO16). Still, the PAN50 and UP5 UF membranes exhibited improved rejection toward direct red 80 (DR80) and DR23. Although these two membranes have high MWCOs, the

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Chapter 3 Polymer-based membranes for dye and pigment removal

increased rejection was attributed to the aggregation behavior of the dye molecules. In general, the direct dye molecules form an aggregate due to their hydrophobic interactions between the aromatic rings of neighboring dye molecules [44, 45]. Similarly, the reactive dyes also exhibit dye aggregation behavior; however, the degree of reactive dye aggregation is less compared to direct dyes. Fig. 3.8 represents the dye rejection properties of PAN base membranes with different MWCOs. In another study, sodium polyacrylate-modified PAN t-UF membrane was prepared for dye/salt separation [46]. The as-prepared t-UF membrane showed improved dye and salt rejection of >94% and < 2% with high permeance of >140 L/m2 h bar. In this method, ethanol was added into the PAN dope solution to induce the PAN aggregates via “incipient precipitation” of PAN. The formation of PAN aggregates of a few nanometers was confirmed by dynamic light scattering (DLS) analysis. The membrane prepared with 20% of ethanol demonstrated 98% of coomassie brilliant blue (CBB) removal with a permeance of 144 L/m2 h bar. The modified membrane also exhibited a 98.5% rejection of gold nanoparticles with an average diameter of 3.6 nm. Moreover, the higher antidye fouling property of the modified membrane was attributed to the formation of a stable and dense hydration layer on the membrane surface. The membrane also demonstrated >90% rejection, even after 20 cycles. Recently, a hybrid t-UF coupled with bipolar membrane electrodialysis (BMED) was proposed to treat dye wastewater [47]. A t-UF membrane with an MWCO of 5000 Da was prepared, which presented a rejection of >99.6% for both reactive and direct dyes. The t-UF membrane could exhibit higher rejection of both reactive and direct dyes owing to the formation of dye aggregation, which increased the size of the dye molecules. At the same time, the membrane also could transport >99.42% of both NaCl and Na2SO4. The hybrid membrane was used to concentrate reactive blue 194 from 997.9 to 7952.8 mg/L after eight diavolumes. Finally, BMED was performed to remove the trace amount of reactive blue 194 to get pure water. In another work, zinc oxide capped with a polyethylene glycol (ZnO-PEG) nanoparticle-embedded membrane photocatalytic reactor (MPR) coupled with a polypiperazine-amide (PPA) t-UF membrane was fabricated for industrial wastewater treatment [48]. In a typical method, 100 ppm of ZnO-PEG nanoparticles, 75% of the wastewater dilution, and pH 11 were optimized as operating conditions.

FIG. 3.8 Dye rejection properties of PAN-based membranes with different MWCOs. Reproduced with permission from M. Jiang, K. Ye, J. Deng, J. Lin, W. Ye, S. Zhao, B. Van der Bruggen, Conventional ultrafiltration as effective strategy for dye/salt fractionation in textile wastewater treatment, Environ. Sci. Technol. 52(18) (2018) 10698–10708. American Chemical Society, Copyright 2018.

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FIG. 3.9 Schematic experimental setup of a membrane photocatalytic reactor coupled with tight UF. Reproduced with permission from A.L. Desa, N.H.H. Hairom, L.Y. Ng, C.Y. Ng, M.K. Ahmad, A.W. Mohammad, Industrial textile wastewater treatment via membrane photocatalytic reactor (MPR) in the presence of ZnO-PEG nanoparticles and tight ultrafiltration, J. Water Process Eng. 31 (2019) 100872, https://doi.org/10.1016/j.jwpe.2019.100872. Elsevier, Copyright 2019.

The present photocatalyst degraded the dye molecules to a greater extent and avoided membrane fouling during UF. Furthermore, the quality of the treated water could fulfill all the criteria for discharge into the environment. The experimental setup for the MPR coupled with t-UF is presented in Fig. 3.9. Ye et al. [49] codeposited PDA/polyethylenimine (PEI) on the hydrolyzed polyacrylonitrile (HPAN) to fabricate t-UF for dye/salt separation. The fabricated t-UF membrane evinced an MWCO of 1700 Da and > 98.2% rejection for various reactive dyes with high permeation of inorganic salts (>97%). The air triggered PDA/PEI codeposition on the HPAN substrate for 30 h, yielding a t-UF membrane with an MWCO of 4190 Da. In this study, ammonium persulfate (APS) was used as an oxidant, which assisted in the quick codeposition of PDA/PEI on the HPAN substrate. Thus, APStriggered PDA/PEI codeposition for 2 h produced a membrane with an MWCO of 1260 Da and a pore size of 0.62 nm. However, the codeposition time of 0.25 h resulted in a pore size of 0.8 nm with an MWCO of 4770 Da. Thus, it clearly indicates that with increased PDA/PEI codeposition time, the t-UF membrane surface becomes tighter. Therefore, the above studies clearly indicate that the t-UF can be a potential candidate for dye/salt wastewater recycling.

3.3 Conclusions Synthetic polymeric membranes have been used for dye and pigment removal for many decades. The main drawback for the use of polymeric membranes is the tradeoff between permeability and rejection. Although NF and MD bestow better rejection for dye molecules, the reduced permeability in NF and pore wetting and the decrease of the temperature difference between the membrane in MD limited the usage for effective dye and pigment wastewater treatment. However, this can be easily overcome by using t-UF membranes. The improved permeability without compromise in the dye/pigment removal

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strengthens the usage of t-UF membranes. Membranes such as blends, nanocomposites, and hybrid tUF have been proposed in the literature to mitigate the drawbacks associated with t-UF. Still, the further improvement in the performance of t-UF makes these membranes potential candidates for dye and pigment removal.

References [1] G.P.S. Ibrahim, A.M. Isloor, E. Yuliwati, A.F. Ismail, Chapter 2—Carbon-based nanocomposite membranes for water and wastewater purification, in: W.-J. Lau, A.F. Ismail, A. Isloor, A. Al-Ahmed (Eds.), Advanced Nanomaterials for Membrane Synthesis and Its Applications, Elsevier, Amsterdam, 2019, pp. 23–44. [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marin˜as, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (7185) (2008) 301–310. [3] I. Ali, New generation adsorbents for water treatment, Chem. Rev. 112 (10) (2012) 5073–5091. [4] G.P.S. Ibrahim, A.M. Isloor, Inamuddin, A.M. Asiri, A.F. Ismail, R. Kumar, M.I. Ahamed, Performance intensification of the polysulfone ultrafiltration membrane by blending with copolymer encompassing novel derivative of poly(styrene-co-maleic anhydride) for heavy metal removal from wastewater, Chem. Eng. J. 353 (2018) 425–435, https://doi.org/10.1016/j.cej.2018.07.098. [5] A.M. Isloor, M.C. Nayak, Inamuddin, B. Prabhu, N. Ismail, A.F. Ismail, A.M. Asiri, Novel polyphenylsulfone (PPSU)/nano tin oxide (SnO2) mixed matrix ultrafiltration hollow fiber membranes: fabrication, characterization and toxic dyes removal from aqueous solutions, React. Funct. Polym. 139 (2019) 170–180, https://doi.org/10.1016/j.reactfunctpolym.2019.02.015. [6] V.R. Pereira, A.M. Isloor, U.K. Bhat, A. Ismail, A. Obaid, H.-K. Fun, Preparation and performance studies of polysulfone-sulfated nano-titania (S-TiO2) nanofiltration membranes for dye removal, RSC Adv. 5 (66) (2015) 53874–53885. [7] S.S. Shenvi, A.M. Isloor, A.F. Ismail, S.J. Shilton, A. Al Ahmed, Humic acid based biopolymeric membrane for effective removal of methylene blue and Rhodamine B, Ind. Eng. Chem. Res. 54 (18) (2015) 4965–4975, https://doi.org/10.1021/acs.iecr.5b00761. [8] M. Shahid, F. Mohammad, Recent advancements in natural dye applications: a review, J. Clean. Prod. 53 (2013) 310–331. [9] K. Yamjala, M.S. Nainar, N.R. Ramisetti, Methods for the analysis of azo dyes employed in food industry—a review, Food Chem. 192 (2016) 813–824. [10] M. Ben-Sasson, X. Lu, S. Nejati, H. Jaramillo, M. Elimelech, In situ surface functionalization of reverse osmosis membranes with biocidal copper nanoparticles, Desalination 388 (2016) 1–8. [11] G.M. Geise, H.S. Lee, D.J. Miller, B.D. Freeman, J.E. McGrath, D.R. Paul, Water purification by membranes: the role of polymer science, J. Polym. Sci. B Polym. Phys. 48 (15) (2010) 1685–1718. [12] V. Garg, R. Kumar, R. Gupta, Removal of malachite green dye from aqueous solution by adsorption using agro-industry waste: a case study of Prosopis cineraria, Dyes Pigments 62 (1) (2004) 1–10. [13] S.F. Azha, L. Sellaoui, E.H.E. Yunus, C.J. Yee, A. Bonilla-Petriciolet, A.B. Lamine, S. Ismail, Iron-modified composite adsorbent coating for azo dye removal and its regeneration by photo-Fenton process: synthesis, characterization and adsorption mechanism interpretation, Chem. Eng. J. 361 (2019) 31–40. [14] H. Bantawal, U.S. Shenoy, D.K. Bhat, Tuning the photocatalytic activity of SrTiO3 by varying the Sr/Ti ratio: unusual effect of viscosity of the synthesis medium, J. Phys. Chem. C 122 (34) (2018) 20027–20033. [15] R. Gobinath, A. Dharanya, P. Dinesh, G. Elango, S. Saravanan, Coagulation performance of activated neem leaf powder in treating low strength dye waste water, J. Water Pollut. Purif. Res. 2 (1) (2019) 26–31. [16] N. Klidi, D. Clematis, M.P. Carpanese, A. Gadri, S. Ammar, M. Panizza, Electrochemical oxidation of crystal violet using a BDD anode with a solid polymer electrolyte, Sep. Purif. Technol. 208 (2019) 178–183.

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[34] N.P. Khumalo, L.N. Nthunya, E. De Canck, S. Derese, A.R. Verliefde, A.T. Kuvarega, … D.S. Dlamini, Congo red dye removal by direct membrane distillation using PVDF/PTFE membrane, Sep. Purif. Technol. 211 (2019) 578–586, https://doi.org/10.1016/j.seppur.2018.10.039. [35] G.P.S. Ibrahim, A.M. Isloor, A. Moslehyani, A.F. Ismail, Bio-inspired, fouling resistant, tannic acid functionalized halloysite nanotube reinforced polysulfone loose nanofiltration hollow fiber membranes for efficient dye and salt separation, J. Water Process Eng. 20 (Supplement C) (2017) 138–148, https://doi.org/10.1016/j. jwpe.2017.09.015. [36] J. Luo, Y. Wan, Effect of highly concentrated salt on retention of organic solutes by nanofiltration polymeric membranes, J. Membr. Sci. 372 (1) (2011) 145–153, https://doi.org/10.1016/j.memsci.2011.01.066. [37] B. Van der Bruggen, B. Daems, D. Wilms, C. Vandecasteele, Mechanisms of retention and flux decline for the nanofiltration of dye baths from the textile industry, Sep. Purif. Technol. 22 (2001) 519–528. [38] G. Lai, W. Lau, P. Goh, A. Ismail, N. Yusof, Y. Tan, Graphene oxide incorporated thin film nanocomposite nanofiltration membrane for enhanced salt removal performance, Desalination 387 (2016) 14–24. [39] G. Lai, W. Lau, S. Gray, T. Matsuura, R.J. Gohari, M. Subramanian, … D. Emazadah, A practical approach to synthesize polyamide thin film nanocomposite (TFN) membranes with improved separation properties for water/wastewater treatment, J. Mater. Chem. A 4 (11) (2016) 4134–4144. [40] G.P.S. Ibrahim, A.M. Isloor, Inamuddin, A.M. Asiri, N. Ismail, A.F. Ismail, G.M. Ashraf, Novel, one-step synthesis of zwitterionic polymer nanoparticles via distillation-precipitation polymerization and its application for dye removal membrane, Sci. Rep. 7 (1) (2017) 15889, https://doi.org/10.1038/s41598-017-16131-9. [41] J. Lin, W. Ye, M.-C. Baltaru, Y.P. Tang, N.J. Bernstein, P. Gao, … B. Van der Bruggen, Tight ultrafiltration membranes for enhanced separation of dyes and Na2SO4 during textile wastewater treatment, J. Membr. Sci. 514 (2016) 217–228, https://doi.org/10.1016/j.memsci.2016.04.057. [42] C. Liu, H. Mao, J. Zheng, S. Zhang, In situ surface crosslinked tight ultrafiltration membrane prepared by one-step chemical reaction-involved phase inversion process between activated PAEK-COOH and PEI, J. Membr. Sci. 538 (2017) 58–67, https://doi.org/10.1016/j.memsci.2017.05.055. [43] M. Jiang, K. Ye, J. Deng, J. Lin, W. Ye, S. Zhao, B. Van der Bruggen, Conventional ultrafiltration as effective strategy for dye/salt fractionation in textile wastewater treatment, Environ. Sci. Technol. 52 (18) (2018) 10698–10708. [44] M. Ferus-Comelo, A.J. Greaves, An investigation into direct dye aggregation, Color. Technol. 118 (1) (2002) 15–19. [45] A. Navarro, F. Sanz, Dye aggregation in solution: study of C.I. direct red I, Dyes Pigm. 40 (2) (1999) 131–139, https://doi.org/10.1016/S0143-7208(98)00048-5. [46] G. Jiang, S. Zhang, Y. Zhu, S. Gao, H. Jin, L. Luo, … J. Jin, Hydrogel-embedded tight ultrafiltration membrane with superior anti-dye-fouling property for low-pressure driven molecule separation, J. Mater. Chem. A 6 (7) (2018) 2927–2934. [47] J. Lin, F. Lin, X. Chen, W. Ye, X. Li, H. Zeng, B. Van der Bruggen, Sustainable management of textile wastewater: a hybrid tight ultrafiltration/bipolar-membrane electrodialysis process for resource recovery and zero liquid discharge, Ind. Eng. Chem. Res. 58 (25) (2019) 11003–11012. [48] A.L. Desa, N.H.H. Hairom, L.Y. Ng, C.Y. Ng, M.K. Ahmad, A.W. Mohammad, Industrial textile wastewater treatment via membrane photocatalytic reactor (MPR) in the presence of ZnO-PEG nanoparticles and tight ultrafiltration, J. Water Process Eng. 31 (2019) 100872, https://doi.org/10.1016/j.jwpe.2019.100872. [49] W. Ye, K. Ye, F. Lin, H. Liu, M. Jiang, J. Wang, … J. Lin, Enhanced fractionation of dye/salt mixtures by tight ultrafiltration membranes via fast bio-inspired co-deposition for sustainable textile wastewater management, Chem. Eng. J. 379 (2020) 122321.

CHAPTER

Synthetic polymer-based membranes for photodegradation of organic hazardous materials

4

Nur Atiqah Daub, Farhana Aziz, Arif Aizat, Nursyazwani Yahya Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia

Chapter outline 4.1 Conventional treatment for phenolic compound removal from wastewater ......................................... 54 4.1.1 Chemical oxidation .................................................................................................... 56 4.2 Advanced oxidation processes ....................................................................................................... 57 4.2.1 Photocatalysis ........................................................................................................... 58 4.3 Semiconductors as photocatalysts ................................................................................................. 60 4.3.1 Perovskites ................................................................................................................ 60 4.4 Synthetic polymeric membranes ..................................................................................................... 61 4.5 Fouling mechanism ....................................................................................................................... 62 4.6 Photocatalytic membranes ............................................................................................................. 64 4.7 Concluding remarks and future prospects ....................................................................................... 66 Acknowledgment .................................................................................................................................. 67 References .......................................................................................................................................... 67

Hazardous wastes are wastes with properties that make them dangerous or potentially harmful to human health or the environment. Hazardous wastes can be liquids, solids, contained gases, or sludges. Many hazardous wastes appear to be more effectively and safely recycled, which also has great benefits including reducing the consumption of raw materials and the volume of waste materials that must be treated and disposed. Others wastes will be treated and disposed in landfills or incinerators. Recently, pharmaceutical wastes have gained increasing attention and become a major problem as they contain a lot of drugs, especially antibiotics, and it is necessary to remove pharmaceutical wastes before discharge [1]. The pharmaceutical and biotechnology industries are massive markets that generated almost $1 trillion around the world in 2013, which was expected to increase steadily at a rate of >5% within the next 5 years. These blossoming industries are very important in improving the human population’s quality

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00004-6 # 2020 Elsevier Inc. All rights reserved.

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of life and longevity. However, the high production rate of drugs and related substances leads to the high production of hazardous and chemically complex substances that have impacted nature. The huge amount of toxic chemicals, nutrients, and organic substituents used may lead to environmental pollution, especially pollution toward water bodies, including rivers, lakes, and oceans, if the wastewater from these industries is not treated properly and stringently. Studies carried out by Gadipelly et al. [2] and Moore et al. [3] showed that the presence of pharmaceutical products (PhPs) in water streams can cause horrendous effects toward aquatic organisms, such as feminization in fish and alligators and changing the behavior and migratory patterns of salmon. Pharmaceutical wastewater with high biological oxygen demand (BOD), chemical oxygen demand (COD), and pharmaceutically active compounds such as hormones, antibiotics, and complex compounds will not only harm aquatic organisms if released into the water stream without proper treatment, but they may also pose a serious threat toward the human population without access to clean drinking water. The health hazards of pharmaceutical byproducts in drinking water may lead to hormonal imbalance or disruption in the endocrine system due to the presence of endocrine-disrupting compounds (EDCs). It may also lead to fatalities as the interaction of pharmaceutical chemicals may have intrinsic biological activities with humans. Moreover, the development of antibiotic-resistant organisms (MROs) such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), and multidrug-resistant Mycobacterium tuberculosis (MDR-TB) due to the presence of pharmaceutical products can have a significant impact, especially in healthcare industries and researchers. One of the most abundantly used pharmaceutical products is phenol. Phenol is an aromatic organic compound and is appreciably soluble in water. It is used as a versatile precursor for different types of pharmaceutical drugs and herbicides. Moreover, the presence of phenol in wastewater is not just limited to pharmaceutical industries, as it can also be found in the effluent from petrochemical industries, petroleum refineries, resin manufacturing industries, paper mills, and dye synthesis plants [4]. Phenol possesses hazardous health effects that can be both acute and chronic. For instance, phenol exposure may cause irritation of the skin, eyes, and mucous membranes while long-term exposure to phenol may lead to irregular breathing, muscle weakness, tremors, coma, and respiratory arrest. Furthermore, the chronic effects cause by phenol exposure lead to anorexia, weight loss, and diarrhea while chronic exposure to phenols leads to irritation in the gastrointestinal and central nervous systems and the liver, kidney, and cardiovascular tissues in animals [5]. As phenol is highly toxic to humans and aquatic organisms within a concentration of 9–25 mg/L, the presence of this substance in wastewater cannot be tolerated, and thus must be degraded and removed efficiently [4].

4.1 Conventional treatment for phenolic compound removal from wastewater The high toxicity of phenol and its compounds toward humans and aquatic organisms, even at low concentrations, makes it vital to be efficiently removed from wastewater, whether it originates from industries or residential areas. Conventionally, phenol is treated or removed from wastewater via numerous techniques, and distillation technology is one of them. This technology may be either destructive or nondestructive, yet the latter one permitted the recovery of phenol and its derivatives [6]. The distillation process requires a great amount of energy to generate steam, and this azeotropic distillation

4.1 Conventional treatment for phenolic compound removal from wastewater

55

is capable of purifying water from phenol impurities based on the relative volatility of this compound [7]. A study carried out by Sklavos et al. [8] used a solar distillatory apparatus to treat and recover polyphenolic compounds from olive mill wastewater (OMW). With olive mill wastewater known to have a high organic load and multiple phenolic compounds with antioxidant properties, the simultaneous solar drying and distillation of it successfully recovered >50 types of phenolic compounds, including hydroxytyrosol and tyrosol. The use of insulation, on the other hand, resulted in a higher distillation temperature (up to 84.3°C and 78.5°C at air and sludge, respectively) and a shorter period for OMW dewatering while increasing the distillator’s performance by 26.1%. Apart from that, phenol can effectively be removed in wastewater by adsorption and extraction technologies, from trace concentrations to percent concentrations [5]. The most highly efficient and used material for adsorption is activated carbon. It is an effective adsorptive material for removing multiple organic and inorganic contaminants, even though it is expensive. In liquid-phase adsorption, the factors influencing the efficiency of activated carbon include the type of carbon, the carbon surface functionalities, the pH value of the coexisting liquid bulk phase, and the availability of oxygen [9]. Furthermore, multiple studies were carried out by pretreating the activated carbon such as by impregnation with nanoparticles, a combination with other adsorbents, and chemical modifications. The composites of activated carbon and chitosan are synthesized, and are the biosorbent for the adsorption of phenol and Cr(VI). Reaching 95% removal of both contaminants in concentrations of 50 mg/L for phenol and 200 mg/L for Cr(VI), the ratio of activated carbon to chitosan played a major role as the increasing amount of either adsorbent will significantly reduce the adsorption capability of the composites due to the overlapping of adsorption sites. The removal of phenol and its derivatives may also take place using membrane technologies. Known for their reliability and economic feasibility, membrane technologies consume less power to operate without affecting the quality of the effluent produced. These technologies are also well known to be easily scaled up with membrane modules [7]. Membrane technologies are stretched for various purposes, but the most important ones regarding phenol removal are membrane distillation pressure-driven membrane processes and extractive membrane bioreactors. Membrane distillation is a thermally driven membrane operation based on the difference of vapor pressure created across the microporous hydrophobic membrane. The production of pure distillate and concentrated feed only needs a low operating temperature of 50–80°C, and it can be further optimized using solar energy to heat the feed [10]. This promising nonisothermal membrane-based separation possesses characteristics unique to itself, such as the total rejection of specified contaminants and stable performance at high contaminant concentrations, apart from mild operating conditions. The application of this technology was reported by El-Abbassi et al. [11], who used a commercial flat-sheet polyetrafluoethylene (PTFE) membrane for a direct contact membrane distillation (DCMD) process in the treatment of olive mill wastewater (OMW). Suggesting that a microfiltration (MF) process be applied to significantly reduce the suspended solids of OMW and enhance the permeate flux of the DCMD process, they have successfully increased the concentration of hydroxytyrosol, the main phenolic compound in the OMW, by twofold (4.01–8.16 g/L) after 40 h. However, the membrane distillation process usually suffers from membrane fouling and pore wetting, making it almost impossible to be chosen as the main process for phenolic wastewater treatment in the near future. Reverse osmosis (RO) is a pressure-driven membrane technology process that is use to separate dissolved solids such as ions, mostly water-based solutions. This membrane-based demineralization technique allows some specific species such as water to selectively permeate through the

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semipermeable barriers of the membrane while retaining other unwanted dissolved species such as organic pollutants, inorganic salts, dyes, and hardness found in wastewater [7]. The removal and adsorption of phenol were studied using a polyamide thin-film-composite reverse osmosis membrane by Mnif et al. [12]. By focusing on the influence of different feed concentrations, ionic strength, transmembrane pressure, and phenol recovery, the retention of phenol by the RO membrane used may reach 80%, even though a low amount of adsorption at 23% was observed. Phenol retention was slightly improved by increasing the transmembrane pressure, lowering the feed concentration, and increasing the alkalinity of the phenol solution. However, they suggested that a combination of membrane and adsorption processes must be considered to reach maximum phenol removal without causing membrane fouling and salt deposition. An extractive membrane bioreactor (EMBR) is a system combining an aqueous-aqueous membrane process and a biological process for the synergistic removal and degradation of phenol by microorganisms. An EMBR set up by Ren et al. [13] for phenol-laden saline wastewater showed that the interdependent release of phenol and the separation of salt were achieved by a silicon rubber tube membrane. Achieving 136.9 mg/L/day of phenol removal and a successful separation of salts avoided the inhibition of microorganism dynamics collected from a municipal wastewater treatment facility’s aeration tank such as Proteobacteria, Bacteroidetes, Chloroflexi, and others, which gradually released an extracellular polymeric substance (EPS) for the absorption and degradation of phenol. The successful biodegradation of phenol via nitrification, denitrification, and other biological processes was majorly aided by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). In a separate study, Praveen and Loh [14] successfully developed a polypropylene hollow fibersupported liquid membrane bioreactor (HFSLMB) for the two-phase biodegradation of phenol by Pseudomonas putida. The simultaneous extraction and biodegradation allowed the exposure of cells to a small fraction of the total phenol, resulting in high specific growth rates. As a result, 4000 mg/L of phenol was biodegraded within 76 h and the long stability of the prepared HFSLMB can be achieved by a 5 h washing strategy every 100 h of usage, solely to reduce the formation of biofilm that may hinder the performance. Moreover, the new HFSLMB approach resulted in an easier scale-up design and continuous operation compared to the conventional two-phase biodegradation systems.

4.1.1 Chemical oxidation The use of chemical oxidants in the treatment of phenol wastewater aims to destructively remove or degrade the pollutant/contaminant without any intention to either reuse or extract them. With low usage of reagents and energy, mild operating conditions (temperature and pH), and within a low concentration (usually the parts per million range), the use of chemical oxidants is commonly favored, even though it is harmful toward the environment without proper posttreatment. Certain risks such as the formation of hazardous intermediate phenolic compounds must be accounted for so that further treatment can be taken to meet the stringent discharge standard. Chlorine, ferrate (Fe(VI)), and permanganate (Mn(VII)) are examples of commonly used chemical oxidants to treat phenol wastewater. The chlorination process of wastewater containing Bisphenol A (BPA) and nonylphenol as targeted compounds, among other compounds, was studied by Noutsopoulos et al. [15]. Endocrine-disrupting chemicals (EDCs) that are usually labeled as emerging pollutants which can cause adverse affects toward humans and other organisms should be remove. The variation of the aqueous pH showed that BPA presented high reactivity under all pH conditions, and this is due to the attack of chlorine to the

4.2 Advanced oxidation processes

57

phenolic ring, making them highly reactive upon deprotonation. They postulated that the effects occur because phenolic dOd is better at activating the aromatic ring toward a substitution reaction than dOH. Moreover, as the pH is higher than the pKa of the compounds, the anionic phenolate form predominates and the higher reactivity of the compounds is expected. However, the chlorination process results in the multiple formation of BPA byproducts, including 2-chloro BPA, dichloro-BPA, 2,20 ,6-trichloro-BPA, 2,20 ,6,60 -tetrachloro-BPA, and others, mostly due to the mechanism of reaction between phenolic compounds and sodium hypochlorite through the electrophilic attack of HOCl on the phenoxide ion. Conversely, the use of permanganate and ferrate is favored and widely studied due to their high reduction potentials. The ability of ferrate to oxidize various contaminants in a wide range of pH and the coagulation/flocculation properties exhibited by ferric hydroxide increase their efficiency in wastewater treatment. As for permanganate, it is favorable as it is relatively cheap and stable with easy-to-handle properties. Moreover, it is known not to form any chlorinated or brominated byproducts [16]. Obviously, the application of chemical oxidants gives rise to a complete solution to the pollutant problem, as the main aim is to mineralize pollutants to carbon dioxide, water, and inorganic substances, or at least transformation into harmless products. Based on this statement, researchers began to develop a special class of oxidation techniques, usually addressed as advanced oxidation processes (AOPs) [17].

4.2 Advanced oxidation processes AOPs can be generally defined as aqueous-phase oxidation methods based on highly reactive, unselective species such as (primarily, but not exclusively) hydroxyl radicals (OH•), leading to the destruction of the pollutants. Furthermore, OH radicals attack most parts of organic molecules with rate constants usually on the order of 106 to 109 M1 s1, making them an extraordinarily potent reactive species. The generation of hydroxyl radicals may be initiated through multiple routes, including primary oxidants (hydrogen peroxide, ozone, and wet air oxidation), energy sources (UV light, visible light, ultrasonic, microwave, and heat), and catalysts (semiconductor, zinc oxide, iron, etc.) [18]. The main examples of AOPs are the homogenous and heterogenous photocatalysis methods, either by irradiation through UV or visible light [2]. On the other hand, heterogenous photocatalysis marked the different phase of photocatalysts (solid) used compared to the reactants (aqueous) [19]. Ozone (O3) acts as powerful oxidizing agent that capable of decomposing in water to form reactive hydroxyl radicals, which are the stronger oxidizing agents than ozone itself. Indirect oxidation attack specific functional groups or organic contaminants through electrophilic mechanism. Ozone also possesses disinfector properties. However, several drawbacks such as a high O3 regeneration cost, low solubility in water, and a low oxidation rate toward stable organic compounds limit the application of this technology. Due to that, the coupling of ozone with hydrogen peroxide and/or UV radiation is usually done to increase its capability. The application of the Fenton reaction in the removal of phenolic compounds in wastewater has also been studied [20]. The heterogenous Fenton process involves the reaction of hydrogen peroxide with ferrous or ferric ions via a free radical chain reaction that forms hydroxyl radicals. Commonly used as a pretreatment in combination with other processes, especially biological treatment,

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Chapter 4 Photodegradation of organic hazardous materials

the Fenton process is usually postulated as the most economic AOP, mostly due to its simplicity and mild operational conditions.

4.2.1 Photocatalysis Photocatalysis is the photochemical transformation accelerated by the action of a catalyst, usually a semiconductor photocatalyst such as titanium dioxide (TiO2), zinc oxide (ZnO), tungsten oxide (WO3), silver phosphate (AgPO4), or cadmium sulfide (CdS) as the main photocatalyst. Best suited for effluents with high chemical oxygen demand (COD) and for complete transformation of highly refractory organic contaminants to reach the biological treatment level, the photocatalytic process usually obeys the Langmuir-Hinshelwood kinetic model, which reduces to pseudo-first or zero-order kinetics, depending on the operating conditions [2]. The photocatalytic process may happen homogenously or heterogeneously. Homogenous photocatalysis occurs as the reactants and the photocatalyst exist in the same phase, or in the case for wastewater treatment, both exist in the aqueous phase. Ozone as well as the Fenton and photo-Fenton processes are the prime examples for this process [19]. As for heterogenous photocatalysis, metal oxides and semiconductors are usually chosen due to their unique properties. The absence of electronic state continuum energy is replaced by the presence of a void region, or the band gap, which comprises the area from the top of the valance band to the bottom conduction band [21]. The mechanism for heterogenous photocatalysis is always explained almost the same regarding the type or structure of the catalyst itself, which is the band structure of electronic energy in photocatalysts. As illustrated in Fig. 4.1 using TiO2 as the model photocatalyst, the process is initiated

electron Migrate to surface e –

Conduction band

CO2 & H2O hv H2O ® H+ + OH–

TiO2

Bandgap energy

OH– + h+ ® OH• O2 + e– ® O2•–

Valence band h+

Organic compounds Migrate to surface hole

UV light irradiation FIG. 4.1 Schematic diagram of pollutant degradation by a photocatalyst. Although TiO2 is used as the model, the mechanism of the photocatalytic process for most metal oxide and semiconductor photocatalysts is the same [18].

4.2 Advanced oxidation processes

59

when a photon with energy, hv, equal or greater than the band-gap energy, Ebg, reaches the surface of the photocatalyst, resulting in the formation of the electron in the conduction band and positive holes in the valance band. Both the formed hole and the excited electron will then migrate to the surface of the catalyst, undergoing two main redox reactions. The electron will reduce the reactive metal or metal oxide found in the catalyst, and subsequently react with absorbed oxygen to form superoxide radical anions ðO2  Þ while the positive charge hole in the valance band will take part in the formation of hydroxyl radicals by the reaction with water molecules (H2O). The resulting radicals formed will then oxidize the organic pollutant to ideally form carbon dioxide and water, or at least transform the pollutant into its harmless intermediate form. Furthermore, both hydroxyl and superoxide radicals are known to be capable of microorganism inactivation [18, 22]. The efficacy of photocatalytic degradation usually relies on several aspects, including the initial concentration of the pollutant, the amount of light reaching the photocatalyst surface, the duration of light irradiation, and the surface area of the photocatalysts. Increasing the surface area will subsequently increase the photocatalytic activity of most photocatalysts, thus the use of nanosized catalysts instead of the bulk counterpart is commonly favored. The estimation of the photocatalyst band gap is crucial in order to determine the catalytic reaction’s potential [23]. Different ways have been suggested for estimation, such as the electrochemical measurements for an electrode prepared using the photocatalyst material that is later compared with the Mott-Schottky plot by Albery et al. [24]; and the photoabsorption spectrum, known as the Tauc plot or Scaife’s estimation [25]. Referring to the first method, this technique is used by estimating the CB bottom position from the flat-band potential by using potential-photocurrent curves or a Mott-Schottky plot as comparison. The VB-position, on the other hand, will be estimated from the CB-bottom position and the band gap because those differences correspond to the band gap of the photocatalyst. The Tauc plot differs from the previous technique as the nth power of the Kubelka-Munk function’s (F(R)) product and the photon energy (hv) of the diffuse-reflectance of a photocatalyst are plotted against wavelength. However, this seems to be commonly misunderstood as reports claimed that the transition mode of the band-to-band excitation (either direct, indirect, or others) could be determined through comparison of plots with different n values. As for the latter one, which is the Scaife’s estimation, this technique utilized electrochemical measurements to determine the band positions of the photocatalyst. The analysis of photocurrent-applied potential relations is done without using Mott-Schottky plots of capacitance and wavelength dependence. The general relationship between them was defined by Eq. (4.1): Vfb ðSHEÞ ¼ 2:94  Eg

(4.1)

where Vfb is the flat band potential and Eg is a band gap. The suggestion made by Scaife regarding this equation is that the position of the VB top is always constant, regardless of the kind of metal found in metal oxides, as long the metal oxides are without partly filled d-levels. This is due to the fact that most metal oxides consist of O 2p orbitals, and this has been recognized for visible light-sensitive photocatalysts. Nevertheless, there is claim in Scaife’s work that several semiconductors are not limited to metal oxides.

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Chapter 4 Photodegradation of organic hazardous materials

4.3 Semiconductors as photocatalysts In general understanding, the definition of photocatalysis, the photo-induced catalytic process, may refer to anything that catalyzes a reaction, whether the catalyst exists in a solid, aqueous, or gaseous state. The idea for using a semiconductor as a photocatalyst was sparked by the work of Honda and Fujishima in 1972 that was published in Nature [26]. Even though terms such as “photocatalyst,” “photocatalysis,” and “photocatalytic” were nowhere to be found in this research, the use of titanium dioxide (TiO2) or titania for the water-splitting process under light irradiation, or the electrochemical photolysis that produced hydrogen and oxygen, created a phenomenon called the “Honda-Fujishima Effect.” Since then, studies related to the use of a semiconductor as a light-driven catalyst have been heavily carried out, including in the field of wastewater treatment and management. TiO2 has three metastable phases known as rutile, anatase, and brookite. Among those, the anatase crystalline phase is favored in photocatalysis due to its optimal performance under UV irradiation [18]. However, problems arose regarding the use of TiO2 for wastewater treatment. This is the use of UV light (λ < 385 nm) as the irradiation source due to a large band gap (3.2 eV) to produce valence band holes (h+) and an excited conductance band electron (e), which eventually participates in the formation of hydroxyl radicals via reduction-oxidation processes. This may be hazardous toward a human operator as UV light may induce the formation of cancerous cells, and the large band gap requires a high amount of energy to activate the photocatalyst. These problems outweigh the advantages of TiO2 such as commercial availability, nontoxicity, chemical stability, and environmental friendliness (when it is not released toward the environment, and is efficiently retrieved after treatment for reuse or disposal). Due to that matter, researchers began to produce desirable TiO2 by modification such as doping, or alternatively found different semiconductors that can outperform it.

4.3.1 Perovskites Perovskite-type oxide photocatalysts have a general formula of ABO3, where the A position is typically occupied by a rare earth ion while the B position is occupied by a transitional metal ion with a charge of +2 or +3. The application of a perovskite-like structure expands in multiple disciplines, including electronic and magnetic materials, solid oxide fuel cells, gas sensors, optics, and catalytic photo-oxidation of pollutants in wastewater treatment [27]. Moreover, these oxides also encounter a promising interest including their stability in high temperature, aggressive medium, stabilization of transition metal ions in unusual oxidation states as well as high oxygen mobility in the perovskite structure [28]. One of the well-studied perovskite photocatalysts is lanthanum orthoferrite, LaFeO3. This p-type semiconductor possesses several important physical properties, including an orthorhombically distorted perovskite structure with G-type antiferromagnetic characteristics, a high Nell temperature (TN ¼ 467°C), and a colossal dielectric constant [29]. In the perspective for wastewater treatment, LaFeO3 is a considerable material to be used as a photocatalyst as it has an optimal band gap of eV that responds to a wide range of wavelengths in both the ultraviolet and visible light ranges. However, it is worth noting that the properties and catalytic activities of LaFeO3 strongly depend on several factors, including the preparation methods, the crystallinity, the particle size, the surface morphology, and the optical band gap [30]. According to Hao and Zhang [31], LaFeO3 synthesized by high-temperature solid reaction methods results in the formation of large particles with small specific surface areas, lowering the photocatalytic activity of the catalyst. In Table 4.1, the fabrication and synthesis via multiple different methods is listed.

4.4 Synthetic polymeric membranes

61

Table 4.1 Preparation methods of LaFeO3. Methods Low-temperature gel combustion

Polymer pyrolysis

Pechini method One-step hydrothermal Sol-gel rapid calcination Sol-gel autocombustion

One-step microwave assisted Impregnation process

Surface area Temp. (°C) Particle size (nm) (m2/g) 200 300 400 700 800 900 1000 900 280 700 500 600 700 800 900 300 500 600 700 800 900

23 32 36 34.8  1.5 47.2  1.5 65.7  2.4 74.0  2.8 – 41.0  0.1 29.04 24 28 65.7 79.1 104.1 54

20–50

27.02 19.94 12.95 –

5.6 67 – 25.8 22.55 20.04 8.5 5.8 17.2 – 129.6 101.6 85.4 34.5

Refs. Hao and Zhang [31]

Phokha et al. [32]

Taran et al. [28] Gomez-Cuaspud et al. [33] Li et al. [34] Parida et al. [35]

Tang et al. [36] Su et al. [37]

4.4 Synthetic polymeric membranes Membrane separation processes have increasingly become viable alternative methods for wastewater treatment [38]. The pressure-driven membrane processes mainly consist of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) [38]. Membranes are typically made of polymeric materials and inorganic (ceramic) materials. They are manufactured in a diversity of configurations such as hollow fiber, spiral, and tubular structures. Each type of configuration possesses varying degrees of separation. Polymer materials that are commonly used to prepare MF/UF membranes are polysulfone (PSf), polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), and cellulose acetate (CA) [38]. This polymeric membrane has some advantages such as high efficiency to remove particles, small size, low energy requirements, and inexpensive compared to ceramic-based membranes. It also has some disadvantages such as the inability to separate volatile compounds and the tendency to foul more quickly, which results in flux decline and rejection deterioration.

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Chapter 4 Photodegradation of organic hazardous materials

Membrane separation such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) can be used to separate different-sized materials. A considerable amount of experimental work and theoretical modeling studies over the past two decades have made possible the use of low-pressure-driven membranes for the MF of membrane pore size between 0.1 and 5 μm. UF with a membrane pore size 50 years ago and have shown an outstanding potential to reduce chemical hazards associated with conventional solvent extraction technologies [41]. Also, PIM applications are substantially stable and provide a safe method for the removal of phenol from dilute aqueous solutions. Cellulose triacetate (CTA) or polyvinyl chloride (PVC) play a role as the mechanical strength to the membrane; hence, they are more frequently used as the base polymer. CTA is a polar polymer with a number of acetyl and hydroxyl moieties that have the ability to form highly oriented hydrogen bonding, therefore giving the CTA a crystalline structure. CTA is formed by the acetylation of cellulose acetate and shows a good stability that exhibits no verification of diminished performance after weeks of use. Due to its strength, inertness, and compatibility, PVC is used as the polymer backbone in membranes with a variety of plasticizers. It is also a nonflammable, lightweight, durable polymer formed from a vinyl chloride monomer. The intermolecular interactions that influence C-Cl functional groups in PVC are nonspecific dispersion forces that result in an amorphous structure [42]. There are many types of extractants, including basic, acid, and chelating; neutral and solvating; and macrocyclic and macromolecular. Different extractants exhibit different transport efficiencies. In order to provide selective membrane permeability for targeted species, a specific carrier needs to be used. It is an important component that is responsible for the ion pair complex formation. Without extractant, the metal ion complex cannot occur. A plasticizer is commonly used to improve the compatibility of the PIM’s component and enhance transport efficiency. In a study conducted by Meng et al. [43], PIM was prepared by solvent evaporation containing PVC as the polymer matrix and N,N-di(1-methylheptyl) acetamide (N503) as a specific carrier. A recent study was also conducted by Benosmane et al. [40], in which they used a mixture of CTA and cellulose acetate (CA) with two calix [4] resorcinarenes as a carrier. The result showed that the removal of phenol with a mixture of CTA and CA has high analytical potential. However, due to their hydrophobic nature, they are susceptible to fouling. This limits their use in filtration processes and applications in wastewater treatment and the pharmaceutical industry while also shortening their working lifespan.

4.5 Fouling mechanism Membrane fouling is defined as the process in which solute or particles deposit onto the membrane surface or into membrane pores such that membrane performance deteriorates. The predominant fouling mechanisms observed with tangential flow microfiltration membranes are classified into three

4.5 Fouling mechanism

63

Bridge formation

Flow direction

FIG. 4.2 Formation of a particle bridge at the neck of the membrane pores [45].

stages of development: the build-up of a cake layer on the membrane surface, the blocking of membrane pores, and the adsorption of fouling material on the membrane surface or in the pore wall [44]. As flow tries to move across the membrane, micron-sized particles or suspended droplets tend to form bridge-like structures within the void spaces, as illustrated in Fig. 4.2, and are trapped within the porous matrix. If the flock of particles is larger than the pore diameter, they would deposit on the wall or be trapped within the neck of the pore, subsequently acting as a collector of more droplets or fine particles released from the porous medium [45]. The hydrodynamic drag due to the ongoing flow of the host medium may not be sufficient to entrain the foulants into the flow. Membrane fouling is affected by the surface hydrophilicity, where improving the hydrophilicity of the membrane can reduce membrane fouling to some extent. Therefore, different methods to hydrophilize the membrane surfaces have been investigated, such as blending a hydrophilic polymer with a hydrophobic polymer, grafting hydrophilic branches on hydrophobic polymer backbones, and the deposition of hydrophilic films on hydrophobic materials [46]. Membrane fouling is induced by complex physical and chemical interactions between various fouling constituents in the feed as well as between these constituents and the membrane surface. It is worth noting that membrane fouling and the characteristics are determined by the concentration of the major constituents, the feed water composition, the water chemistry, the membrane properties, the temperature mode operation, and the hydrodynamic characteristics of the feed. The overall membrane performance would be affected by any factors that could change the hydrodynamic characteristics of the membrane modules and the chemical characteristics of the feed waters. Membrane fouling can be classified as reversible fouling, which occurs due to the cake layer or concentration polarization of materials at the membrane rejection surface, and irreversible fouling, which occurs by chemisorption and pore plugging mechanisms. The application of a pressure-driven membrane process such as MF, NF, or UF as the photocatalytic membranes in PMRs with suspended photocatalysts leads to membrane fouling due to the presence of TiO2 particles. However, this problem might be overcome by the application of a submerged membrane.

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Chapter 4 Photodegradation of organic hazardous materials

4.6 Photocatalytic membranes The mixed matrix membrane (MMM), which is a popular research field, is a type of membrane formed by incorporating fillers in a synthetic polymer matrix, as illustrated in Fig. 4.3. Its preparation procedures mainly involve only blending and phase inversion. A mixed-matrix membrane is an alternative approach to the conventional polymer membrane. Inorganic fillers embedded together in the membrane act as the pathway for selective material [47]. Inorganic fillers are usually the porous type that provides superior separation properties. Inorganic filler will be added into the membrane dope solution prior to casting or spinning. Usually, there are three methods used to prepare the solution. The filler particle will be dispersed in the solvent before the addition of the polymer, or the addition of the polymer into the solvent until a homogeneous solution forms. Then, filler will be added, or the polymer and filler are dissolved in the solvent separately before mixing the filler solution into the polymer solution [48]. A typical mixed-matrix membrane contains a dispersion of nanoparticles in a bulk continuous polymer phase. With the incorporation of inorganic particles within the polymer matrix, the mixed-matrix membranes inherit some of the characteristics of inorganic particles, especially their superior separation performance [38]. Hamid et al. [49] studied a submerged system for oily wastewater treatment using mixed matrix membranes. The PVDF ultrafiltration membranes were prepared using poreforming and hydrophilic additives and focused on the effects of membrane morphology and the transport properties [38]. Mixed matrix membranes have demonstrated better impact in wastewater treatment. The hydrophilic nature of the metal oxides and the mechanical stiffness of the polymers combined to influence wastewater treatment. In addition, mixed matrix membranes were lagging in commercialization by their own defects. The symmetric dispersion of nanoparticles, the uncontrollable pore size, fouling, and the leaching of nanoparticles in a coagulation bath are major drawbacks in mixed matrix membrane [38]. A chemical modification method could be employed to improve the hydrophilicity of the membrane, but the main polymer molecules would change and the advantages of the polymer might be reduced. Compared to the chemical modification method, physical modifications such as mixing with other macromolecules are preferably used to enhance membrane hydrophilicity. A blending approach has been extensively utilized for polymeric membrane fabrication due to its facile preparation

FIG. 4.3 Schematic diagram of a mixed matrix membrane [38].

4.6 Photocatalytic membranes

65

procedure, versatility to incorporate desirable properties on the membrane, and its profound ability to simultaneously modify the membrane properties during the phase inversion process [38]. Among the physical modification methods, blending with inorganic material has garnered great interest owing to its stable performance and easy preparation. Inorganic nanoparticles that could be blended with polymer membranes include metal oxides, silica (SiO2), alumina (Al2O3), ferrous ferric oxide (Fe3O4), zinc oxide (ZnO), zirconium (ZrO2), and titanium dioxide (TiO2) [49]. With the aid of nanoparticles, this can reduce membrane fouling. Hamid et al. [49] studied a submerged system for oily wastewater treatment using mixed matrix membranes. The membrane showed an 82.5 L m2 h1 maximum flux and a 98.83% rejection of refinery wastewater at 1.95 wt% TiO2 concentration. Moreover, similar work has also been reported on the effect of additives in the same composite membrane on the performance. Similar findings were also reported by Ong et al. [50] in which they found that a PVDF-TiO2 membrane achieved the highest flux at 2 wt% of TiO2 concentration; further increasing this concentration up to 4 wt% would adversely affect the membrane performance owing to the TiO2 agglomeration on the membrane surface [50]. A photocatalytic membrane can be defined as a combination of photocatalyst and membrane whereby when a light is applied, the photodecomposition of the pollutants takes place on a surface of a membrane or within its pores. The use of a catalytic method together with the physical effects of UV/visible light radiation make it possible to substantially enhance the process of oxidizing destruction of an organic pollutant and, in a number of cases, bring it to complete mineralization. There are various types of photocatalysts often used in photocatalytic treatment. However, the titanium dioxide (TiO2) photocatalyst has been extensively used due to its high chemical stability, low cost, lack of toxicity, nonphotocorrosive nature, and high chemical stability; it has also been shown to be the most active catalyst. In addition, titania have always shown the best photocatalytic performances with maximum quantum yields. Polyoxomelate-based photocatalytic membranes are an interesting photocatalytic membrane that can be prepared by heterogenization in/on polymeric membranes of polyoxomelates decatungstate. Polyoxomelates (POMs) are polyanionic metal oxide clusters of early transition metals with significant application in oxidation reactions for wastewater treatment and fine chemistry. Decatungstate shows high photocatalytic activity for wastewater treatment as well as worthy properties. In certain cases, the photocatalytic efficiency of decatungstate was proved to be more effective than a well-established catalyst, TiO2, in the decontamination of wastewater using solar or UV irradiation. In addition, the application of decatungstate with respect to its low toxicity and low cost made this compound receive much attention in photocatalytic decomposition technology. According to Tzirakis et al. [51], decatungstate appeared to be a relatively more effective photocatalyst for various organic pollutants such as phenol. Lipophilic derivatives of decatungstate such as tetrabutilamonium salt (n-C4H9N)4W10O32, that also indicated as TBAW10 were used to avoid leaching out the photocatalyst from the membrane. The photocatalytic membranes were characterized by different properties that can be described by solid-state characterization techniques such as FT-IR and UV-vis spectroscopy. These techniques established that the structure and spectroscopic properties of the catalyst were preserved in the heterogenized form. Despite the remarkable effectiveness of the photocatalytic membrane in terms of removing organic materials, the fouling that is caused by the deposition of photocatalysts on the membrane with a consequent flux decline and light scattering still limits the membrane performance. Thus, the use of a photocatalytic membrane with PMR strongly improved the performance of the photocatalytic membrane in removing phenol [52]. The removal of phenol in water was successfully performed

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Chapter 4 Photodegradation of organic hazardous materials

by PVDF membrane containing TBAW10 which carried out in continuous flow-through photocatalytic membrane reactors (PMR) under emitting light from 310 nm to visible light of mercury vapor lamp. About 50% of the phenol degradation in both the homogenous and heterogeneous was converted [52]. Nevertheless, for a homogenous reaction, only 34% mineralization to CO2 and water was achieved due to various persistent intermediates. However, the phenol was completely converted to CO2 and water during photodegradation carried out by a PVDF catalytic membrane. PMRs are hybrid reactors in which photocatalysis is coupled with a membrane separator that serve as a simple barrier for the photocatalyst and also a selective barrier for the molecules to be degraded. PMRs are a very assuring method for solving the problems involved in separating photocatalyst products as well as byproducts of photodecomposition from the reaction mixture. PMRs can be divided into two groups: reactors with a catalyst suspended in the reaction mixture and reactors with a catalyst supported in/on the carriers such as glass, quartz, titanium metals, zeolites, stainless steel, pumice stone, etc. [7] Furthermore, most of the PMRs are hybrid systems coupling photocatalysis with pressure-driven membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The application of PMRs has many advantages with respect to conventional photoreactors such as control of the residence time in the reactor and continuous process with a simultaneous catalyst. Besides, additional operation such as coagulation-flocculation-sedimentation can be avoided in order to remove catalyst from solution. However, the operating parameters play a vital role in affecting the good performance of the PMRs with both suspended and immobilized photocatalysts. Additionally, according to previous studies by Mohammadi et al. [7], using Degussa-P25 and Riedel-De Haen as nanosized TiO2 photocatalysts along with an RO membrane separator has been proven for phenol removal. They demonstrated that within 1 h of continuous operation with a typical condition (initial phenol concentration 100 ppm and pH ¼ 5), a UV/TiO2/RO hybrid system could remove approximately 50% of phenol. Besides phenols and chlorophenols, PMRs with photocatalytic membranes have been applied for the removal of different organic compounds such as sodium dodecyl benzene sulfonate, dyes, herbicides, 4-nitrophenol, and many others. Mohammadi et al. [7] reported that PMRs with microfiltration/ultrafiltration membranes have been used for the removal of different pollutants such as BPA, chlorophenol, and 4-nitrophenol while PMRs with nanofiltration membranes have been used for the removal of 4-nitrophenol. Different materials and different ways can be used to prepare photocatalytic membranes for PMRs. A good contact between pollutants and photocatalysts in suspended PMRs allows achieving high efficiency due to a large active surface area, which makes the degradation effectiveness higher than when immobilized in/on the membrane.

4.7 Concluding remarks and future prospects There are various promising methods for the removal of organic hazardous wastes, including distillation technologies, adsorption and extraction technologies, membrane technologies, chemical oxidation, and AOPs. Membrane separation processes have increasingly become viable alternative methods for wastewater treatment, although their tendency to foul more quickly results in flux decline and rejection deterioration, limiting their applications. Photocatalytic membranes and photocatalytic membrane reactors (PMR) have recently emerged as promising technologies for the removal of hazardous waste from wastewater treatment. The synergistic effect of physical separation and chemical

References

67

oxidation stimulates contaminant removal and reduces membrane fouling. A better understanding of the overall performances is needed to overcome the limitations that contribute to fouling. Further investigations are essential to improve the removal of hazardous wastes.

Acknowledgment This work was supported by the Ministry of Higher Education (MOHE) Malaysia [FRGS Grant R. J130000.7851.5F007], [HICOE: R. J090301.7846.4 J190] and Universiti Teknologi Malaysia (GUP Tier 1; Grant No. Q.J130000.2546.18H39).

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[14] P. Praveen, K.C. Loh, Simultaneous extraction and biodegradation of phenol in a hollow fiber supported liquid membrane bioreactor, J. Membr. Sci. 430 (2013) 242–251. Available from: https://doi.org/10.1016/j. memsci.2012.12.021. [15] C. Noutsopoulos, E. Koumaki, D. Mamais, M.C. Nika, A.A. Bletsou, N.S. Thomaidis, Removal of endocrine disruptors and nonsteroidal anti-inflammatory drugs through wastewater chlorination: the effect of pH, total suspended solids and humic acids and identification of degradation by-products, Chemosphere 119 (2015) S109–S114. Available from: https://doi.org/10.1016/j.chemosphere.2014.04.107. [16] J.-Q. Jiang, Advances in the development and application of ferrate(VI) for water and wastewater treatment, J. Chem. Technol. Biotechnol. 89 (2) (2014) 165–177. [17] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Advanced oxidation processes (AOP) for water purification and recovery, Catal. Today 53 (1) (1999) 51–59. [18] S. Leong, A. Razmjou, K. Wang, K. Hapgood, X. Zhang, H. Wang, TiO2 based photocatalytic membranes: a review, J. Membr. Sci. 472 (2014) 167–184. [19] C.H. Wu, C.L. Chang, Decolorization of Reactive Red 2 by advanced oxidation processes: comparative studies of homogeneous and heterogeneous systems, J. Hazard. Mater. 128 (2–3) (2006) 265–272. [20] L. Xu, J. Wang, A heterogeneous Fenton-like system with nanoparticulate zero-valent iron for removal of 4chloro-3-methyl phenol, J. Hazard. Mater. 186 (1) (2011) 256–264. Available from: https://doi.org/10.1016/ j.jhazmat.2010.10.116. [21] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (3) (1995) 735–758. [22] H. Ling, K. Kim, Z. Liu, J. Shi, X. Zhu, J. Huang, Photocatalytic degradation of phenol in water on asprepared and surface modified TiO2 nanoparticles, Catal. Today 258 (2015) 96–102. Available from: https://doi.org/10.1016/j.cattod.2015.03.048. [23] B. Ohtani, Revisiting the fundamental physical chemistry in heterogeneous photocatalysis: its thermodynamics and kinetics, Phys. Chem. Chem. Phys. 16 (5) (2014) 1788–1797. [24] W.J. Albery, G.J. O’Shea, A.L. Smith, Interpretation and use of Mott-Schottky plots at the semiconductor/ electrolyte interface, J. Chem. Soc. Faraday Trans. 92 (20) (1996) 4083–4085. [25] D.E. Scaife, Oxide semiconductors in photoelectrochemical conversion of solar energy, Sol. Energy 25 (1) (1980) 41–54. [26] A. Fujishima, K. Honda, Electrochemical photolysis of water one and two-dimensional structure of poly (L-alanine) shown by specific heat measurements at low, Nature 238 (1972) 37–38. Available from: https://www.nature.com/articles/238037a0.pdf. [27] K. Peng, L. Fu, H. Yang, J. Ouyang, Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity, Sci. Rep. 6 (December 2015) (2016) 1–10. Available from: https://doi.org/10.1038/srep19723. [28] O.P. Taran, A.B. Ayusheev, O.L. Ogorodnikova, I.P. Prosvirin, L.A. Isupova, V.N. Parmon, Perovskite-like catalysts LaBO3 (B ¼ Cu, Fe, Mn, Co, Ni) for wet peroxide oxidation of phenol, Appl. Catal. B Environ. 180 (2016) 86–93. Available from: https://doi.org/10.1016/j.apcatb.2015.05.055. [29] X. Qi, M. Zhang, X. Zhang, Y. Gu, H. Zhu, W. Yang, et al., Compositional dependence of ferromagnetic and magnetoelectric effect properties in BaTiO3-BiFeO3-LaFeO3 solid solutions, RSC Adv. 7 (82) (2017) 51801–51806. [30] S. Thirumalairajan, K. Girija, N.Y. Hebalkar, D. Mangalaraj, C. Viswanathan, N. Ponpandian, Shape evolution of perovskite LaFeO3 nanostructures: a systematic investigation of growth mechanism, properties and morphology dependent photocatalytic activities, RSC Adv. 3 (20) (2013) 7549–7561. [31] X. Hao, Y. Zhang, Low temperature gel-combustion synthesis of porous nanostructure LaFeO3 with enhanced visible-light photocatalytic activity in reduction of Cr(VI), Mater. Lett. 197 (2017) 120–122. Available from: https://doi.org/10.1016/j.matlet.2017.03.133.

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[32] S. Phokha, S. Pinitsoontorn, S. Rujirawat, S. Maensiri, Polymer pyrolysis synthesis and magnetic properties of LaFeO3 nanoparticles, Phys. B Condens. Matter 476 (2015) 55–60. [33] J.A. Go´mez-Cuaspud, E. Vera-Lo´pez, J.B. Carda-Castello´, E. Barrachina-Albert, One-step hydrothermal synthesis of LaFeO3 perovskite for methane steam reforming, React. Kinet. Mech. Catal. 120 (1) (2017) 167–179. [34] L. Li, X. Wang, Y. Zhang, Enhanced visible light-responsive photocatalytic activity of LnFeO 3 (Ln ¼ La, Sm) nanoparticles by synergistic catalysis, Mater. Res. Bull. 50 (200) (2014) 18–22. [35] K.M. Parida, K.H. Reddy, S. Martha, D.P. Das, N. Biswal, Fabrication of nanocrystalline LaFeO3: an efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition, Int. J. Hydrog. Energy 35 (22) (2010) 12161–12168. [36] P. Tang, Y. Tong, H. Chen, F. Cao, G. Pan, Microwave-assisted synthesis of nanoparticulate perovskite LaFeO3 as a high active visible-light photocatalyst, Curr. Appl. Phys. 13 (2) (2013) 340–343. Available from: https://doi.org/10.1016/j.cap.2012.08.006. [37] H. Su, L. Jing, K. Shi, C. Yao, H. Fu, Synthesis of large surface area LaFeO3 nanoparticles by SBA-16 template method as high active visible photocatalysts, J. Nanopart. Res. 12 (3) (2010) 967–974. [38] M. Padaki, R. Surya Murali, M.S. Abdullah, N. Misdan, A. Moslehyani, M.A. Kassim, et al., Membrane technology enhancement in oil-water separation. A review, Desalination 357 (2015) 197–207. [39] W.L. Loh, T.T. Wan, V.K. Premanadhan, K.K. Naing, N.D. Tam, V.H. Perez, Experimental study of the separation of oil in water emulsions by tangential flow microfiltration process. Part 1: analysis of oil rejection efficiency and flux decline, J. Membr. Sci. Technol. 5 (1) (2014) 1–6. [40] N. Benosmane, B. Boutemeur, S.M. Hamdi, M. Hamdi, Removal of phenol from aqueous solution using polymer inclusion membrane based on mixture of CTA and CA, Appl. Water Sci. 8 (1) (2018) 1–6. Available from: https://doi.org/10.1007/s13201-018-0643-8. [41] N.S.W. Zulkefeli, S.K. Weng, N.S. Abdul Halim, Removal of heavy metals by polymer inclusion membranes, Curr. Pollut. Rep. 4 (2) (2018) 84–92. [42] M. O’Rourke, R.W. Cattrall, S.D. Kolev, I.D. Potter, The extraction and transport of organic molecules using polymer inclusion membranes, Solvent Extr. Res. Dev. Jpn. 16 (2009) 1–12. Available from: http://www.scopus.com/ inward/record.url?eid¼2-s2.0-69549103423&partnerID¼40&md5¼0c2271d9c3d649eebb9df6c44a2fa697. [43] X. Meng, C. Gao, L. Wang, X. Wang, W. Tang, H. Chen, Transport of phenol through polymer inclusion membrane with N,N-di(1-methylheptyl) acetamide as carriers from aqueous solution, J. Membr. Sci. 493 (2015) 615–621. Available from: https://doi.org/10.1016/j.memsci.2015.06.037. [44] W.L. Loh, T.T. Wan, V.K. Premanadhan, K.K. Naing, N.D. Tam, V.H. Perez, Experimental study of the separation of oil in water emulsions by tangential flow microfiltration process. Part 2: the use of ultrasound for in-situ controlling of the membrane fouling, J. Membr. Sci. Technol. 5 (1) (2014) 2–6. [45] P. Poesio, G. Ooms, Formation and ultrasonic removal of fouling particle structures in a natural porous material, J. Petrol. Sci. Eng. 45 (2004) 159–178. [46] Y. Lu, S. Hong, M.L. Li, Y.S. Li, Application of the Al2O3-PVDF nanocomposite tubular ultrafiltration (UF) membrane for oily wastewater treatment and its antifouling research, Sep. Purif. Technol. 66 (2009) 347–352. [47] Y. Zhang, F. Liu, Y. Lu, L. Zhao, L. Song, Investigation of phosphorylated TiO2–SiO2 particles/polysulfone composite membrane for wastewater treatment, Desalination 324 (2013) 118–126. [48] I.-S. Chang, C.-M. Chung, S.-H. Han, Treatment of oily wastewater by ultrafiltration and ozone, Desalination 133 (2001) 225–232. [49] N.A.A. Hamid, A.F. Ismail, T. Matsuura, A.W. Zularisam, W.J. Lau, E. Yuliwati, et al., Morphological and separation performance study of polysulfone/titanium dioxide (PSF/TiO2) ultra filtration membranes for humic acid removal, Desalination 273 (1) (2011) 85–92.

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[50] C.S. Ong, W.J. Lau, P.S. Goh, B.C. Ng, A.F. Ismail, Investigation of submerged membrane photocatalytic reactor (sMPR) operating parameters during oily wastewater treatment process, Desalination 353 (2014) 48–56. [51] M.D. Tzirakis, I.N. Lykakis, M. Orfanopoulos, Decatungstate as an efficient photocatalyst in organic chemistry, Chem. Soc. Rev. 38 (9) (2009) 2609–2621. [52] P. Argurio, E. Fontananova, R. Molinari, E. Drioli, Photocatalytic membranes in photocatalytic membrane reactors, Processes 6 (9) (2018) 162.

CHAPTER

Synthetic polymer-based membranes for heavy metal removal

5

I G. Wentena,b, K. Khoiruddina, Anita K. Wardania, I N. Widiasac Department of Chemical Engineering, Faculty of Industrial Technology, Bandung Institute of Technology, Bandung, Indonesiaa Research Center for Nanosciences and Nanotechnology, Bandung Institute of Technology, Bandung, Indonesiab Department of Chemical Engineering, Faculty of Engineering, Diponegoro University, Semarang, Indonesiac

Chapter outline 5.1 Introduction .................................................................................................................................. 71 5.2 Pressure-driven membranes for heavy metal removal ...................................................................... 72 5.2.1 Low-pressure membranes ........................................................................................... 72 5.2.2 High-pressure membranes .......................................................................................... 74 5.3 Electrically driven membrane processes for chemical-free heavy metal ion removal ......................... 78 5.3.1 Deionization by ion-exchange membrane-based processes ............................................. 78 5.3.2 Electrodialysis ........................................................................................................... 80 5.3.3 Electrodeionization .................................................................................................... 83 5.3.4 Membrane capacitive deionization .............................................................................. 85 5.4 Heavy metal recovery by MD .......................................................................................................... 88 5.5 Concluding remarks ...................................................................................................................... 90 References .......................................................................................................................................... 91 Further reading .................................................................................................................................. 101

5.1 Introduction Heavy metal removal is one of the notable processes to avoid environmental issues due to the direct or indirect discharge of industrial wastewater into the environment. Because those components are known to be toxic to the health of humans and other living organisms, wastewater containing heavy metals should be treated properly to meet the stringent discharge standards [1–3]. In addition, heavy metals are not biodegradable, thus discharging a low concentration of these components may lead to long-term accumulation. Therefore, heavy metal recovery may be a promising approach to address this issue. Membrane technology has been applied in various industrial sectors due to the advantages of a highquality product, the reduction of chemical consumption, a low operating cost, lower energy consumption, a lower footprint, and easy scale up [4–9]. Furthermore, a membrane is able to perform molecular Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00005-8 # 2020 Elsevier Inc. All rights reserved.

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separation, which provides the possibility of obtaining a high-purity product [10, 11]. This ability also allows one to simultaneously conduct water reuse and trace component recovery from industrial wastewater. Depending on the driving force of the separation, membrane processes are generally classified into pressure-driven, electrically driven, concentration-driven, and thermally driven categories [12–14]. Those membrane-based processes have been demonstrated by numerous studies for heavy metal removal from industrial wastewater. Most of the processes use membranes fabricated from polymeric materials, as they are easy to fabricate, simple to modify, and use low-cost materials. This chapter summarizes the latest developments in polymer-based membranes for heavy metal removal, including low- and high-pressure membranes, electrically driven membrane processes, and membrane distillation (MD).

5.2 Pressure-driven membranes for heavy metal removal 5.2.1 Low-pressure membranes An ultrafiltration (UF) membrane is a physical sieving process that removes solutes based on the membrane pore size (5–20 nm) and the molecular weight of the solutes (1–100 kDa) [15, 16]. The main advantage of UF lies in its low operating pressure. The low pressure of UF leads to minimal space requirements as well as a reduction in energy consumption and capital costs [17, 18]. Nevertheless, UF cannot be applied directly to recover heavy metals from wastewater due to its relatively large pore size. Therefore, the addition of complexing agents or membrane modification may be necessary. The addition of charged groups on the UF membrane can be an alternative to increase the recovery rate of heavy metals. The separation of metal ions by charged UF membranes is a result of the repulsion of the ions by the fixed charge groups on the membrane skin. This method aims to provide a high-water flux and high removal efficiency of heavy metals. Nonetheless, studies of charged UF for heavy metal removal are still limited. For improving the performance of the UF membrane in Cr6+ removal, Yao et al. [19] prepared two novel positively charged UF membranes, that is, a tertiary amine-based UF membrane (TA membrane) and a quaterized TA membrane. The first membrane was prepared by blending tertiary amine-containing block copolymer polymethyl methacrylate-b-dimethylamino-2ethyl methacrylate with polyvinylidene fluoride. The blended material was then fabricated to the TA membrane via a nonsolvent induced phase separation process. Meanwhile, the second membrane was synthesized by modifying the TA membrane via surface quaternization. The results showed that these two charged UF membranes were able to achieve 100% rejection of Cr6+. Micellar-enhanced ultrafiltration (MEUF) has been proposed as a promising strategy to overcome the limitations of UF in heavy metal removal. It has been utilized to remove various types of heavy metals from wastewater. The underlying principle of MEUF is to increase the molecular size of the metal ions by the addition of surfactants [17, 20]. The addition of a surfactant to the wastewater promotes the complexation of metal ions with the surfactant. The surfactant molecules will attach to each other, forming aggregates with a large molecular size that are called micelles. The metal ions tend to be adsorbed in the structure of the micelles by ionic or hydrophobic interaction [17, 21]. The large molecular size of the micelles makes them easily removed when passed through the UF membrane while unbound metal ions and surfactant monomers are able to penetrate through the UF membrane and come out in the permeate stream, as shown in Fig. 5.1.

5.2 Pressure-driven membranes for heavy metal removal

73

FIG. 5.1 Schematic illustration of MEUF for heavy metal recovery.

The surfactants for MEUF are mostly amphiphilic molecules that consist of a hydrophobic chain and a hydrophilic head group. In the application of heavy metal removal, an ionic surfactant with the opposite charge to the metal ion is required to form an ion-pair complex [21]. Anionic surfactants, including sodium dodecyl sulfate [22–26] and sodium dodecyl benzonate sulfonate [27, 28], have been widely used for cationic heavy metal removal. Meanwhile, cationic surfactants, including cetylperidinium chloride (CPC) [29, 30], cetyltrimethyl ammonium bromide [29, 31], and octadecylamine acetate [32], could be utilized to remove anionic heavy metals. Besides the type of surfactant, the concentration of the surfactant also greatly affects the MEUF effectiveness. When the surfactant concentration is below the critical micellar concentration (CMC), micelles are not formed and the surfactant remains present as its monomer that can easily penetrate through the UF membrane pores [33]. By increasing the concentration of surfactant up to the CMC, the micelle will be gradually formed and provide sites that are able to attach the metal ions. However, a further increase in surfactant concentration induces the breaking of micelles into smaller molecules, which causes a decline in metal removal efficiency. According to a study reported by Baek et al. [34], the chromate removal was increased with increasing the concentration of the surfactant CPC. It is in agreement with the study of Baek and Yang [35] that reported the increase of nitrate removal from 56% to 78% and to 89% as the molar ratio (nitrate:chromate:CPC) increased from 1:1:3 to 1:1:5 to 1:1:10, respectively. However, when the CPC concentration was too high, the chromate removal decreased due to the increase of Cl ions in the solution. The removal efficiency of the MEUF process is also influenced by the operating pressure, pH, and temperature of the solution. Several studies showed that heavy metal removal was increased by increasing the pressure due to the increase of the micelle aggregation layer on the membrane surface [36–38]. The metal removal was also increased by the increase of pH. Juang et al. [39] reported that the removal of cationic heavy metals, including Co2+, Cr3+, Cu2+, Mn2+, and Zn2+, increased more than 80% when

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Chapter 5 Polymeric membranes for heavy metal removal

the pH was increased from 2 to 12. A similar result was also obtained by Xu et al. [40] where the cadmium removal was increased from 83% to 99% by increasing the pH from 3 to 11. Meanwhile, heavy metal removal decreased as the temperature increased due to the demicellization process as the effect of the palisade layer disruption of the micelle [20]. When the temperature increased, surfactant ions started detaching from the micelle, resulting in the passage of more surfactant monomers to the permeate side [33]. The drawback of MEUF is the production of a secondary pollutant when the complexations of the metal-surfactant are not treated properly. Therefore, further separation of the metal ions from the micelle has become a concern in MEUF processes. Kim et al. [41] added nitric acid, sulfuric acid, and hydrochloric acid to the MEUF retentate and successfully recovered 95% of cadmium and copper. The addition of chelating agents can also be used to make a complexation with metals in the retentate solution. The chelating agent and metals are then separated from the solution by a further UF process. The previous study showed that recovery of 82.5%, 99.9%, and 100% of copper could be obtained by the addition of iminodiacetic acid, ethylenediaminetetraacetic (EDTA), and citric acid, respectively [41]. Several researchers have developed polymer-enhanced ultrafiltration (PEUF) as an alternative process to remove heavy metals from wastewater. The principal of PEUF is similar to MEUF. However, PEUF uses a water-soluble polymer to form a complexation with metal ions and forms a macromolecule with a high molecular weight. Polymers such as polyethyleneimine (PEI) [42–45], polyacrylic acid (PAA) [42, 43, 46], poly (acrylic acid-co-maleic acid) (PAM) [47], polyvinyl amine (PVAm) [48], and poly(ammonium acrylate) [49] have been utilized as complexing agents in PEUF to recover various types of heavy metals, as shown in Table 5.1. PEUF offers the advantages of high removal efficiency with high binding selectivity, thus producing concentrate with highly concentrated metal [3]. As reported by Qiu et al. [47], the application of a complexing agent of copolymer of maleic acid and acrylic acid in PEUF was able to remove more than 99% of Cu2+, Zn2+, Ni2+, and Mn2+. Meanwhile, 99% of Pb2+ and Fe3+ could be rejected by the addition of PVAm as the polymeric agent in PEUF [48]. In PEUF processes, the removal efficiency of heavy metals is not only affected by the type of metal and polymer, but also the polymer-to-metal ratio and the pH. Qiu et al. [47] indicated that increasing the ratio of polymer to metal was able to improve the removal efficiency of heavy metal ions. The rejection of 99.8%, 98.8%, 99.0%, and 99.6% for Cu2+, Zn2+, Ni2+, and Mn2+ could be obtained with the polymer/metal ratio at 6, 7, 7, and 6, respectively. Meanwhile, Barakat and Schmidt [54] reported that the efficiency of metal rejection was higher at neutral and higher pH than at lower pH. At higher pH, metal ions have higher binding with the polymeric ligands while at low pH, the affinity of the polymeric ligands toward the metal ions is weak. This is due to the presence of positive charges and the low stability of the complexion metal-polymer at low pH condition. When the pH is increased, the affinity and stability of the metal-polymer complexes would be increased.

5.2.2 High-pressure membranes Nanofiltration (NF) is a pressure-driven membrane that performs between UF and reverse osmosis (RO). It is mainly produced from synthetic polymers that are negatively charged on the surface, thus able to dissociate the heavy metals [55]. Therefore, the removal mechanism in NF combines the rejection of uncharged components by a sieving mechanism and electrical (Donnan) effects between the metal ions in solution and the membrane [18, 56]. NF offers the advantages of higher rejection of the multivalent metal ions than UF as well as higher water permeability and lower operating pressure

5.2 Pressure-driven membranes for heavy metal removal

75

Table 5.1 Performance of MEUF and PEUF in heavy metal removal. UF type

Membrane

Surfactant

MEUF

Polyamide

Sodium dodecylsulfate

MEUF

Polycarbonate

MEUF

Polyethersulfone

Sodium lauryl ether sulfate Cetylpyridinium chlorid

MEUF

Polyethersulfone

Sodium dodecylsulfate

MEUF

Polysulfone

Sodium dodecylsulfate

PEUF

Polyethersulfone

Carboxy methyl cellulose Poly(acrylic acid) Polyethyleneimine

Polyvinyl amine

PEUF

Polysulfone

PEUF

Polyvinyl alcohol

PEUF

Polyvinyl butyral

Poly(ammonium acrylate) Polyethyleneimine Polyethyleneimine

Poly (acrylic acidco-maleic acid)

Initial concentration (mg/L)

Removal efficiency (%)

Co Cr3+ Cs2+ Cu2+ Mn2+ Sr2+ Zn2+ Ni2+

50 50 50 50 50 50 50 0.056

99.8 99.7 83.3 99.2 99.8 99.9 98 98.6

Cr3+ Ni2+ As5+ Cd2+ Cu2+ Pb2+ Zn2+ Zn2+ Cd2+ Cu2+ Cr3+ Ni2+ Ni2+ Zn2+ Cu2+ Ni2+ Zn2+ Cu2+ Fe3+ Pb2+ Cd2+

– – 10 50 50 50 50 50 50 10 10 10 – – 50 – – 10 10 10 112.4

99 91 98 99 99 99 99 97 98 97.6 99.5 99.1 93.2 99.9 94 98.1 91.6 97 99 99 99

Cr3+ Cd2+ Cu2+ Pb2+ Cu2+ Mn2+ Ni2+ Zn2+

10 100 100 100 10 10 10 10

99 99.5 99.5 99.5 99.8 99.6 99 98.8

Metal ion 2+

Refs. [39]

[50] [35] [51] [52]

[23] [53] [54]

[43] [44] [43] [48]

[49] [45] [42]

[47]

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Chapter 5 Polymeric membranes for heavy metal removal

compared to RO [13, 57]. It has been considered as an energy-efficient process for the removal of heavy metals. There are several commercially available NF membranes that have been utilized to remove various types of heavy metals, as summarized in Table 5.2. Most of the commercial NF membranes are able to remove more than 90% of heavy metals. Merwe [70] divided rejection in NF into three distinctly different phenomena: (a) rejection of multivalent anions, (b) rejection of cations, and (c) rejection based on size. NF is able to remove multivalent
 anions such as sulfate SO4 2
to a high degree (95%–99%) [71] while the rejection of monovalent anions such as chloride ion (CI
) is typically only 5%–45% [72, 73]. This is due to the strong electrical repulsion between the negative charges of multivalent anions with the negative charge on the

Table 5.2 Performance of commercial NF membranes in the removal of heavy metals. Membrane

Pressure (MPa)

Metal ion

Initial concentration (mg/L)

Removal efficiency (%)

AFC30

2.0

Cd2+ Pb2+ Zn2+

0.03 0.61 0.50

94 97 88

AFC80 AFC80PCI BQ01 MPS-34 NE 404090 NF90

5.0 4.0 0.55 1.0 3.0

Pb2+ Cr5+ As5+ Cr6+ Pb2+

100 120 0.32 60 400

99.9 98.6 85 97 97.5

1.0 1.0

Cr6+ Cr6+ Ni2+

60 60 133

99.5 96.5 99

NF270

1.0

Cr6+ Ni2+

60 133

95 98.5

0.9

Cd2+ Co2+ Cu2+ Ni2+ Zn2+

325 438 421 439 414

68 96 100 93 84

2.0

Cr3+ Ni2+

5.3 1.3

99 94

3.04

Cd2+ Ni2+

250 250

82.69 98.94

2.0

Cd2+ Ni2+

50 50

97.26 98.90

0.3

Cr3+ Cu2+ Pb2+

0.69 0.23 0.03

100 99 93

0.29

Ni2+

2000

94

NF300

NTR-7250

Refs. [58] [58] [58] [59] [60] [61] [62] [62] [62] [63] [63] [63] [63] [64] [64] [64] [64] [64] [65] [65] [66] [66] [67] [67] [68] [68] [68] [69]

5.2 Pressure-driven membranes for heavy metal removal

77

membrane surface [74]. For cations, the rejection is high when the cations are associated with multivalent anions to maintain electroneutrality. For example, when sodium is associated with sulfate, it will be rejected to roughly the same degree as the sulfate ion [75]. On the other hand, uncharged dissolved materials and some positively charged ions can also be rejected if they are larger than the NF molecular weight cut off. NF can be manufactured by two preparation techniques, polymer phase inversion and interfacial polarization [75]. In polymer phase inversion, the resulting NF membrane is a homogeneous asymmetric membrane. It is commonly prepared from cellulose acetate and sulfonated polysulfone. Meanwhile, interfacial polarization forms a thin-film-composite layer on top of a porous substrate. The thin-film composite mainly was formed by cross-linked polyamide polymers that reacted to the carboxylic group. For the porous substrates, various polymers could be utilized, including polysulfone (PS) [76, 77], polyethersulfone (PES) [78, 79], polyphenylsulfone (PPSU) [80], polyvinyl alcohol (PVA) [81], and polyacrylonitrile (PAN) [82, 83]. The pore size and the presence of charged groups on the NF membrane surface play important roles in metal removal efficiency. An NF membrane with smaller pores and a highly charged surface has better performance in the removal of metal ions. Several studies have been conducted to develop an NF membrane with smaller pores and a highly charged surface. Zhu et al. [84] designed a small-pore NF with a dual layer hollow fiber membrane using polybenzimidazole (PBI) and PES/polyvinylpyrrolidone (PVP). The result showed that the dual-layer NF membrane was able to reject Mg2+ and Cd2+ with rejection rates of 98% and 95%, respectively. Meanwhile, Gao et al. [85] used chelating polymers from negatively charged functional groups such as PAM, PAA, and poly (dimethylamine-coepichlorohydrin-co-ethylenediamine) (PDMED) for modifying the positively charged PEI cross-linked P84 hollow fiber substrate. They successfully removed heavy metals such as Pb(NO3)2, CuSO4, NiCl2, CdCl2, ZnCl2, Na2Cr2O7, and Na2HASO4 with a rejection rate of about 98%. This is due to the ability of chelating polymers to change the membrane pore size and surface charge. In heavy metal removal by NF, pH also significantly affects the removal efficiency. At a neutral pH, the charged groups (i.e., the carboxylic and sulfonic groups) on the membrane surface are negatively charged [75]. When the pH decreases, the charged groups will be released from the NF surface, thus eliminating the electrical interaction between metal ions and the membrane. This is in agreement with the results from various studies [59, 86, 87] that showed a decrease of heavy metal rejection rates with rising pH. The change of pH also leads to the change of ion solubility, causing alteration of its removal rate. Bouranene et al. [88] investigated that the rejection of Pb2+ was higher than Co2+ at pH  5. The difference between the rejections of the two cations was increased as the pH increased. In addition, the operating parameters such as pressure, temperature, and cross-flow velocity also affect the NF performance. Gherasim and Mikulasek [59] and Ozaki et al. [58] reported that NF provides good separation at pressures of 10 bar or higher. Meanwhile, the increase of temperature and cross-flow velocity leads to the increase of NF membrane flux. Nonetheless, the rejection of NF membranes toward heavy metals is not affected by temperature. An RO membrane contains extremely small pores and is able to achieve a very high rejection of both monovalent and multivalent particles. Shenvi et al. [69] illustrated the separation mechanism of heavy metals in RO through three basic principles: (a) absorption of water from the feed solution by the membrane surface, (b) diffusion of water across the membrane due to the gradient of concentration, and (c) water molecules move down the gradient to the permeate side of the membrane. By those three principles, the water molecules desorb from the membrane and form a nearly pure solution on the

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Chapter 5 Polymeric membranes for heavy metal removal

permeate side while the heavy metals are retained and concentrated on the feed side. Several studies [89–92] have been conducted to investigate the performance of RO in the removal of heavy metals. The results showed that more than 99% removal efficiency of Cu2+, Ni2+, Zn2+, and As5+ could be obtained by RO at various operating pressures. Although RO for heavy metal removal has been studied, the high power consumption limits its applications [93].

5.3 Electrically driven membrane processes for chemical-free heavy metal ion removal 5.3.1 Deionization by ion-exchange membrane-based processes Electrically driven membrane processes, such as electrodialysis (ED), electrodeionization (EDI), and membrane capacitive deionization (MCDI), use a charged membrane for cation/anion separation and electrical potential difference as the driving force of ion transport. The membrane can be classified into cation- and anion-exchange membranes based on the fixed charged in the membrane matrix. Cation/ anion separation occurs due to a phenomenon known as Donnan exclusion. The cation-exchange membrane (CEM), which has a negative fixed charge, allows cations (counterions) to pass across the membrane but the membrane rejects anions (coions). On the other hand, anions will pass through the anion-exchange membrane (AEM) while cations are rejected due to the presence of a positive charge in the AEM matrix. As a result of this process, an electrically driven membrane process will produce deionized and concentrated streams simultaneously. Because the separation is driven by an electrical potential, chemical regeneration, as encountered in the conventional ion-exchange system, is no longer required. To achieve an effective separation, ion-exchange membranes (IEMs) with high conductivities and permselectivities are favored [94, 95]. A high conductivity or low resistance results in lower energy consumption, which is associated with the high ionic transfer and low overall stack/cell resistance [95–97]. Highly selective IEMs can attain a high separation factor and reduce the required membrane area, leading to a reduced stack/module cost. Otherwise, inadequate membrane selectivity will decrease the product quality and increase the separation stage for achieving a similar separation level. In general, there are two types of IEMs, that is, homogeneous and heterogeneous membranes, as shown in Fig. 5.2A and B [14, 94, 95, 98, 99]. Homogeneous and heterogeneous membranes are distinguished based on their structure. Homogeneous membranes are usually prepared from polymers that contain functional groups. Therefore, the functional sites are finely distributed in the membrane matrix. In contrast, heterogeneous IEMs are usually fabricated by incorporating a polymer that has functional groups into a noncharged polymer matrix [97]. Generally, heterogenous IEMs exhibit lower permselectivity and higher resistance than the homogenous membrane due to the uneven distribution of the functional groups. Moreover, voids or gaps between functional groups and the polymer matrix become nonselective regions, which allows the transport of coions and decreases the permselectivity of the membrane. A comparison of several homogeneous and heterogeneous IEM properties is shown in Fig. 5.2C and D. In addition to its lower separation properties, heterogeneity or the degree of conductive (functional sites) fraction in the membrane also affects the electrochemical behavior of electrically driven membrane processes, such as limiting the current density, the concentration polarization, the water dissociation reaction, and the electroconvection [95]. Limiting current density decreases with heterogeneity

5.3 Electrically driven membrane for heavy metal removal

79

FIG. 5.2 Homogeneous versus heterogenous IEMs. Ionic pathways in (A) homogeneous and (B) heterogeneous IEMs, (C) permselectivity, and (D) areal resistances. Reproduced with permission from A.N. Hakim, K. Khoiruddin, D. Ariono, I.G. Wenten, Ionic separation in electrodeionization system: mass transfer mechanism and factor affecting separation performance, Sep. Purif. Rev. (2019) 1–23, https://doi.org/10.1080/ 15422119.2019.1608562, Copyright Taylor and Francis.

while the others show opposite trends. Nevertheless, heterogenous IEMs also exhibit several advantages of high mechanical strength, good dimensional stability, and a simple fabrication procedure [100, 101]. High mechanical and dimensional stability will improve the stack/cell integrity while the simple fabricating procedure will reduce the membrane cost. Most available IEMs are polymeric membranes that usually have lower stability against an oxidative environment. It limits their applicability for the treatment of solutions having harsh conditions. Therefore, Dzyazko et al. [102, 103] employed an inorganic membrane in an EDI stack for chromate recovery from industrial wastewater. Results of the study reveal that the inorganic membrane showed good stability in an acidic chromate solution. However, the use of an inorganic membrane is still limited, which may be because of its high fabricating cost.

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It has been mentioned that ionic separation in IEM-based processes occurs due to Donnan exclusion. The Nernst-Planck equation is usually used to express the ion transport through IEMs, which can be represented by Eq. (5.1) [104]: Ji ¼ D i

dCi F dφ + vCi
Di zi Ci RT dx dx

(5.1)

where C represents the concentration of i, D is the diffusion coefficient of i, F is Faraday’s constant, R is the ideal gas constant, T is the temperature of the solution, v is the solution velocity, x is the length between the CEM and AEM (spacer thickness), z is the valence of component i, and φ is the electrical potential difference. Eq. (5.1) elucidates three transport mechanisms [4, 105]: (i) diffusion, (ii) electromigration, and (iii) convection. Ion migration across the IEM will follow two main steps: electromigration resulting in concentration polarization in the boundary layer followed by diffusion, which is driven by concentration difference [106]. The transport of counterions through IEM can be expressed by Eq. (5.2) [4]: zi J i ti ¼ X Ji

(5.2)

i

This equation shows the ratio of flux of i to the total flux of all ions. The transport number of counterions in the membrane (tm i ) and in the solution phases (ti) is then used to determine the permselectivity of the membrane (Ps). The relationship between Ps and transport number is expressed by [107]: Ps ¼

tm i
ti 1
ti

(5.3)

In the membrane, the transport number of counterions is typically 1, which indicates that the major fraction of the electric current is carried by counterions. In contrast, the transport number of counterand coions is almost similar in the solution, that is, 0.5. The difference of the counterion transport number results in the occurrence of the concentration polarization phenomenon [94]. This phenomenon generally decreases the performance of electromembrane processes due to increasing the water splitting rate in the diluate compartment and the precipitation reaction in the concentrate compartment [94]. To avoid the negative effects, the electromembrane process may be operated at a turbulence regime [108, 109].

5.3.2 Electrodialysis Fig. 5.3 illustrates ED cell configuration and the typical set up. In ED, CEM and AEM are arranged alternately between an electrode pair. The membranes are separated by a spacer for creating compartments or chambers. When electrolyte solutions are transferred into the compartments and potential difference is established on the electrode, ions are attracted toward the electrode with the opposite charge. Because the membranes perform a selective separation, deionization occurs in the diluate compartment while enrichment happens in the concentrate chamber. This process results in the formation of two streams with different ion concentrations, namely diluate and concentrated. During initial development, ED was commercialized for brackish water desalination as an RO alternative [94]. Unlike RO, which is limited by osmotic pressure, ED is capable of achieving a high water recovery. Also, ED can be used to attain a high concentration factor in the concentrate chamber. This ability is used for concentrating seawater during table salt production [94]. Furthermore, ED requires less complicated pretreatment due to its lower scaling tendency. In addition, the low operating

5.3 Electrically driven membrane for heavy metal removal

FIG. 5.3 Schematic illustration of electrodialysis (ED). (A) Deionization process and (B) ED set up.

81

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Chapter 5 Polymeric membranes for heavy metal removal

pressure makes the ED system simpler than RO. However, in desalination, ED is only competitive for low to moderate salinity because the energy consumption increases with ionic concentration. ED has been applied for heavy metal removal with satisfactory results [110, 113]. Some reported ED performances for heavy metal removal are tabulated in Table 5.3. Nataraj et al. [110] reported an investigation of a pilot-scale ED performance for Cr(VI) removal from a model solution with various initial chromium concentrations, that is, 10–50 mg/L. The ED showed a good performance by meeting the maximum allowable level of 0.1 mg/L for chromium. The highest removal efficiency was achieved at a lower initial concentration (10 mg/L). Abou-shady et al. [111] examined the performance of ED for Pb2+ and NO3
separation as a function of pH. They found that the separation was most effective at a pH between 3 and 5. At the pH range, energy consumption was estimated to be 1.25–1.50 Wh/L while the current efficiencies were 7.5%–35%. During operation, scaling was not observed because the solution was kept acidic to dissolve Pb2+. Gherasim et al. [113] investigated the performance of ED during Pb2+ removal from a model aqueous solution. According to their results, the optimum operating conditions were 10 V applied voltage, 70 L/h flow rate, and 25°C feed temperature. Under these conditions, a final concentration of 1–2 mg/L could be reached from the initial Pb concentration of 500–1000 mg/L. Furthermore, the concentration of Pb in the concentrate compartment was enriched fivefold, which gave the possibility of its reuse. Cifuentes et al. [114] confirmed the effectiveness of ED in Cu and Fe separation as well as water recovery from the working solution of copper electrowinning operations. Several works also developed IEMs to enhance the separation performance of ED during heavy metal removal. Caprarescu et al. [112] synthesized IEMs and employed the membrane in an ED cell for zinc removal from a synthetic effluent. The membrane was prepared by a phase inversion technique using 80% acrylic copolymer and 20% polyvinyl alcohol in a dimethyl sulfoxide mixture containing 5% ion-exchange resins. When the synthesized membrane with the highest ion-exchange capacity was used, the ED could obtain 80% zinc removal. The removal was achieved at 232.89 kW/m3 energy consumption and 8% current efficiency. Nemati et al. [116] synthesized heterogenous CEM from a mixture of poly(vinyl chloride) and 2-acrylamido-2-methylpropane sulfonic acid-based hydrogel (AMAH). The membrane was then characterized by a solution containing Na+ and Ba2+. The introduction of AMAH resulted in an increasing Ba2+ flux. When the membrane was used to treat Pb2+ and Ni2+

Table 5.3 ED performances in heavy metal removal. Heavy metals

Ci (mg/L)

Cf (mg/L)

Removal (%)

CE (%)

E (kWh/m3)

Refs.

Cr(VI) Pb2+ Pb2+ Pb2+ Pb2+ Pb2+ Zn Ni2+

10 800 1000 500 1000 50 6000 50

0.08 – – 1.82 1.00 – – –

99.2 – 62–84 100 100 99.9 80 96.9

– 7.5–35 – 82.80 72.40 – 8 –

3 1.25–1.50 – 0.16 0.36 – 232.89 –

[110] [111] [115] [113] [113] [116] [112] [116]

CE, current efficiency; Cf, final concentration; Ci, initial concentration; E, energy consumption.

5.3 Electrically driven membrane for heavy metal removal

83

solutions, the membrane could obtain remarkable removals of heavy metals with 99.9% and 96.9% removal efficiency, respectively. Jafari et al. [117] fabricated heterogeneous CEM by incorporating carboxy methyl cellulose-co-Fe3O4 nanoparticles into the PVC matrix. The membrane was used in an ED system and characterized for the treatment of Pb2+ solution. A membrane with the highest Pb2+ flux was successfully produced via the addition of 16% carboxy methyl cellulose and 2% Fe3O4 nanoparticles. Even though ED has demonstrated its effectiveness in treating heavy metal solutions, the current efficiency is still relatively low. Therefore, to achieve an almost complete heavy metal removal, ED will require higher energy consumption. This is because the ED stack resistance or conductivity depends on the solution conductivity. As the solution concentration decreases, the total resistance of the stack/cell increases. At a certain ionic concentration, a major portion of the electric current will be used to carry out water splitting and ionic transportation will no longer be efficient. This phenomenon occurs when the process reaches a limiting current density caused by concentration polarization [94]. Therefore, if the effluent concentration is expected to be very low, an additional posttreatment will be needed.

5.3.3 Electrodeionization It was demonstrated that ED could be used to remove heavy metals from a wastewater solution effectively. However, the removal efficiency decreases with decreasing ion concentration. As the ion concentration decreases, the overall stack resistance increases, leading to high energy consumption and low separation efficiency. To address this issue, EDI was developed by introducing ion-exchange resins into the conventional ED stack. A schematic of the EDI process and the stack configuration is depicted in Fig. 5.4. By combining ED and conventional ion-exchange processes, EDI allows achieving a deep deionization process without chemical regeneration [118]. The presence of ion-exchange resins keeps the conductivity of the diluate compartment high, even at a very low ion concentration [119]. Therefore, ionic flux and current efficiency remain high at high resistance solution. Accordingly, ion-exchange resins were considered as an ionic bridge that facilitates fast ionic transfer in EDI compartments [114]. Formerly, EDI was developed for producing ultrapure water [118, 120]. EDI was used to produce ultrapure water on a commercial scale, replacing the conventional mixed-bed ion exchange [121, 122]. The interesting features of EDI have attracted researchers to explore other potential applications. Similar to ED, EDI can produce two streams with different ion concentrations, allowing purification and concentrating applications. By performing a deep deionization process, EDI can be used to recover pure water and valuable components by achieving a high concentrating factor toward the components in the concentrate stream [120, 123, 124]. For instance, Souilah et al. [125] carried out a study to examine EDI performance in the treatment of electrolysis effluent containing 40 mg/L Zn, 6 mg/L Cu, and 4 mg/L Cd. The EDI stack was filled by mixed-bed ion-exchange resins. By employing the EDI, the conductivity of the effluent was reduced to 40 μS/cm. Simultaneously, the metal contents were enriched 100-fold in the concentrate compartment, which can be reused for the subsequent electrolysis process. Xing et al. [126] demonstrated the recovery of Cr(VI) solution from wastewater. The Cr(VI) concentration was successfully increased to 6300 mg/L from an initial 40–100 mg/L in the concentrate compartment. Moreover, the energy consumption was estimated to be 41–7.3 kWh/mol Cr. In addition to its ability to perform heavy metal ion enrichment, EDI was also able to separate ions from the

84

Chapter 5 Polymeric membranes for heavy metal removal

FIG. 5.4 Electrodeionization (EDI). (A) Schematic illustration of deionization process and (B) stack construction.

5.3 Electrically driven membrane for heavy metal removal

85

mixture, as reported by Lounis et al. [127] and by Taghdirian et al. [128]. The separation was facilitated by ion-exchange resins that were based on different ionic migrations. According to their studies, the final ratios of 3 and 155 were achieved for Mo/U and Ni2+/Co2+ mixtures, respectively. Semmens et al. [129] employed bench-scale EDI for the removal of copper sulfate from a plating rinse solution. The rinse solution would be reused in the process. They found that EDI with ionexchange resins only in the diluate compartment could produce the best effluent quality. Feng et al. [130] investigated the performance of EDI on the treatment of a synthetic wastewater solution containing copper. The EDI exhibited good separation performance by achieving >99.5% Cu2+ removal, so the Cu2+ concentration in the effluent was reduced to 0.23 mg/L. In addition, the copper was concentrated at five- to 14-fold in the concentrate stream. Arar et al. [131] examined the effect of operating parameters on EDI performance during Cu2+ ion removal from an aqueous solution. Obviously, the performance was influenced by initial concentration, flow rate, and sulfuric acid concentration in the electrode compartment. The EDI performance in Cu2+ ion removal was also theoretically investigated in a reported study [132]. The potential application of EDI in the removal of Ni2+ was also reported. Spoor et al. [133, 134] found that the formation of Ni(OH) in EDI compartments could be prevented by acidifying the feed solution. Dzyazko et al. [135, 136] also suggested a similar approach. They also found that the highest Ni2+ diffusion was observed for ion-exchange resins with the highest ion-exchange capacity. In the following works, Dzyazko et al. [137] compared organic and inorganic ion-exchange resins inserted in EDI cells for Ni2+ ion removal. They found that inorganic ion-exchange resins exhibited lower nickel transport than organic ion-exchange resins. The effect of ion-exchange resin size distribution and applied voltage on Ni2+ removal was investigated by Lu et al. [138–140]. The results showed that a narrow size distribution was apparated to be effective for Ni2+ removal. The EDI could remove more than 99.8% nickel and produced an effluent with >3 MΩ cm resistivity. EDI performances in other heavy metal removals with satisfactory results were also reported in the literature (see Table 5.4). For instance, CrO2 4
ions were removed with 99.8% removal efficiency [141] and more than 95% Pb2+ was removed in a study [142]. Despite the effective separation performance, heavy metal recovery by EDI was mostly conducted under lab-scale conditions. Further works are still needed for commercialization because commercially available EDI stacks are fabricated for ultrapure water production. In addition, the commercial EDI stacks are typically designed for a low ion concentration or conductivity.

5.3.4 Membrane capacitive deionization MCDI utilizes electrical potential difference and porous electrodes for driving the ionic transport and storing adsorbed ions, respectively [148]. Schematics of the adsorption and desorption processes of MCDI are illustrated in Fig. 5.5A and B. In adsorption, ions are attracted toward an electrode due to electrostatic force. The amount of adsorbed ion depends on the ionic capacity of the electrode. An electrode with a high ion capacity will store a large amount of ions. During the adsorption process, freshwater or desalinated water is produced. When electrodes reach the saturation point, regeneration should be performed. This can be conducted by simply reversing the electrode polarity. The desorption process produces an effluent containing a high concentration of ions. The introduction of IEMs in MCDI prevents coion adsorption, which generally occurs in the conventional capacitive deionization (CDI) [148–150]. Therefore, MCDI usually displays better ionic separation and energy efficiency than CDI, leading to higher desalination efficiency. Even though MCDI operation seems to be simpler than

86

Chapter 5 Polymeric membranes for heavy metal removal

Table 5.4 Performances of electrodeionization (EDI) in heavy metal removal. Component to be removed

Operating conditions

Results

Refs.

Uranium

C ¼ 25 Q ¼ 5–10 V ¼ 2.5–5.0 I ¼ 0.05–0.10 C ¼ 10–100

Removal >98%

[143]

Sr removal ¼ 97.6% Cs Removal ¼ 67.8% Product conductivity ¼ 40 μS/cm Concentrating factor >100.

[144]

CCr(VI) in diluate ¼ 0.09–0.49 CCr(VI) in concentrate ¼ 6300 CE ¼ 16.1–18.8% EC ¼ 4.1–7.3 kWh/mol Removal ¼100%

[126]

Cs and Sr Zn, Cu, Cd

Cr(VI)

CZn ¼ 40 CCu ¼ 6 CCd ¼ 4 i ¼ 400 C ¼ 40–100 I ¼ 0.2–0.3

Sr2+

Q ¼ 200 C ¼ 50 i ¼ 50 C ¼ 50

Arsenic

Am ¼ 48

Separation of Mo/U

CMo ¼ 40 CU ¼ 23.6 In chlorhydric acid medium I ¼ 0.10 Am ¼ 90.25 Ni/Co ¼ 3/1 (initial molar ratio) C ¼ 100

Cr(VI)

Separation of Ni2+/Co2+

[125]

[145]

CSr in product ¼ 0.0415 EC ¼ 7.66 kWh/m3 Removal ¼ 99% EC ¼ 0.8–1.5 kWh/m3 Factor of selectivity (Mo/U)¼3

[127]

Ni/Co ¼155 (final molar ratio)

[128]

[146] [147]

Am, membrane area (cm2); C, ion concentration (mg/L); CE, current efficiency; EC, energy consumption; i, current density (A/m2); I, current (A); Q, flow rate (mL/min); V, applied voltage (volt).

ED and EDI, a breakthrough may occur due to the saturation of the electrode, leading to inconsistent product quality. Commonly, MCDI is used for the desalination of brackish water. The simple operation and system, low-pressure operation, and lower energy consumption make MCDI an attractive alternative to the RO system. In addition, it is also possible to perform selective separation by selecting the suitable membranes and electrodes [151]. For instance, lithium ions could be separated from a mixture containing various ions by using an asymmetric hydrogenated manganese oxide (HMO)-activated electrode [152]. The electrode showed the following order of selectivity: Li+ ≫ Mg2+ > Ca2+ > K+ > Na+. According to the results, Lithium ion separation required an energy consumption of 23.3 Wh/g of lithium. More efficient lithium removal was obtained by Siekierka et al. [153]. The energy consumption was estimated to be 0.08 Wh/g when the MCDI was equipped with a lithium-manganese-titanium oxide (LMTO) electrode. Furthermore, the electrode had a lithium adsorption capacity of 35 mg/g of Li+ with a removal

5.3 Electrically driven membrane for heavy metal removal

87

FIG. 5.5 Schematic illustration of membrane capacitive deionization (MCDI) process. (A) adsorption and (B) desorption/ electrode regeneration.

efficiency of 60%. Shi et al. [154] employed MCDI for Mg2+/Li+ separation. Monovalent selective CEM was used to attain high separation efficiency. The study showed that 2.95 lithium selectivity could be obtained with 1.8 Wh/mol or 12.5 Wh/g energy consumption. Dong et al. [155] demonstrated Pb2+ separation from a Ca2+ and Mg2+ mixture. The separation was performed by an asymmetric MCDI cell. Their study suggested the use of MCDI with AEM only for effective Pb2+ separation. The performance of MCDI during Cr(VI) and F
removal was reported by Gaikwad and Balomajumder [156]. The MCDI was equipped by a homemade activated carbon electrode prepared from Limonia acidissima shells. The electrode showed a relatively good chromium adsorption capacity of 0.8086 mg/g. Moreover, when MCDI was operated at an initial chromium concentration of 10 mg/L, MCDI was able to achieve 92.2% chromium removal. Palladium recovery from catalyst solution wastewater by MCDI was reported in a study [157]. MCDI could remove up to 99.94% Pd from the wastewater with 1.42–1.52 Pd to ammonium ion selectivity. However, palladium could not be easily desorbed from the electrode, thus incomplete electrode regeneration was observed. It was considered as the effect of a high interaction between palladium ions and the electrode. To address this issue, a highly porous N-doped graphene-based

88

Chapter 5 Polymeric membranes for heavy metal removal

capacitive device was developed [158]. The MCDI was assessed to remove multiple heavy metals (Pb2+, Cd2+, Cu2+, Fe2+) with concentrations of 0.05–200 mg/L. The novel electrode enhanced the performance of MCDI, showing removal efficiencies of 90%–100%. The electrode also showed good regeneration cycles. MCDI is usually operated at a relatively low applied voltage (per cell) to avoid the extensive parasitic effects of Faradaic reactions [159]. The typical Faradaic reactions include anodic oxidation, cathodic reduction, and Faradaic ion storage [159]. The Faradaic reactions will decrease the efficiency of the MCDI process and reduce the integrity of the electrodes and membranes. This will limit the application of MCDI for the treatment of a high concentration heavy metal solution. Therefore, more works are needed to explore the feasibility of MCDI in heavy metal removal, especially for high concentrations.

5.4 Heavy metal recovery by MD MD utilizes temperature difference to drive the transport and a porous hydrophobic membrane as a selective barrier between the liquid feed phase and the vapor permeate phase (Fig. 5.6.). In general, water is vaporized in the liquid feed phase due to the high temperature. The water vapor is then transferred through membrane pores while the liquid phase is prevented from passing through due to the membrane hydrophobicity or lower wettability. Finally, the water is condensed in the permeate phase. There are four configurations for MD, that is, direct contact, sweeping gas, vacuum MD, and air gap [160, 161]. Each configuration is depicted in Figs. 5.6B–E. According to the mass transfer step, MD may be used to recover pure water from wastewater. In addition, MD is not limited by osmotic pressure, thus obtaining high water recovery is possible. These features make MD an interesting alternative for heavy metal

FIG. 5.6 Membrane distillation (MD). (A) Desalination process in MD, (B) direct contact MD, (C) sweeping gas MD, (D) vacuum MD, and (E) air gap MD.

5.4 Heavy metal recovery by MD

89

removal by performing both pure water and heavy metal recovery [162, 163]. Compared to other membrane processes, MD has several advantages such as complete rejection of nonvolatile compounds, low operating pressure, a less complex system, and independent to feed concentration [164, 165]. Several reported performances of MD for heavy metal removal are tabulated in Table 5.5. VMD was used by Ji [166] for removing a heavy metal solution containing 600–3000 mg/L Zn, 200–1000 mg/L Ni, and 400–2000 mg/L Cu. The performance of VMD as a function of pH, calcium, and EDTA was investigated. The vacuum MD was able to produce an effluent with 99% when used to filter a model solution containing Pb, Cd, Cr, Cu, and Ni. The membrane also showed a relatively high flux, >23 L/m2/h, even at high feed concentration (2500 mg/L). Theoretical and experimental investigations were also conducted by Attia et al. [168] to examine the performance of air gap MD in heavy metal removal. The results confirmed a 99% removal of heavy metals by a superhydrophobic electrospun membrane. Attia et al. [169] fabricated a superhydrophobic membrane from polyvinylidene fluoride via the electrospinning technique. The membrane was then used in air gap MD and the performance was compared to the pristine membrane. The superhydrophobic membrane could remove 99.4% lead, which was higher than that obtained by the pristine membrane, 72.8%. MD has several disadvantages that hinder its application, such as temperature polarization, wetting phenomenon, and low permeating flux [173]. MD membranes are mainly fabricated from polymeric materials such as polypropylene, polytetrafluoroethylene, and polyvinylidene fluoride [174, 175]. These materials are known to have high hydrophobicity. Hydrophobicity is usually characterized by the water contact angle of the water droplet on the surface. According to the contact angle, membrane wettability may be classified as hydrophilic and hydrophobic membranes, as illustrated in Fig. 5.7. Despite their high Table 5.5 MD performances for heavy metal removal. Heavy metals

MD configuration

Operating conditions

As

Air gap

As

Direct contact

Zn, Ni, Cu

Vacuum

Ni

Direct contact

Pb, Cd, Cr, Cu, and Ni

Air gap

T ¼ 85 C ¼ 0.24 T ¼ 30–61 C ¼ 1.2 T ¼ 57 CZn ¼ 600–3000 CNi ¼ 200–1000 CCu ¼ 400–2000 T ¼ 40–60 C ¼ 2.93 C ¼ 2500

Results

Refs.

R ¼ 99.8

[170]

R ¼ 100 J ¼ 49.8 Permeate conductivity 23

[167]

C, concentration (mg/L); J, water flux (kg/m2/h); R, removal (%); T, temperature (°C).

[166]

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Chapter 5 Polymeric membranes for heavy metal removal

FIG. 5.7 Water contact angles on membrane surface with various wettability.

hydrophobicity, membranes fabricated from those materials are still susceptible to wetting. In the wetting phenomenon, membrane pores may be partially or completely filled by liquid. Wetting decreases the membrane flux due to increasing the mass transfer resistance. The liquid phase filling the membrane pores may act as a bridge, which causes the contamination of the permeate phase [176, 177]. Recently, numerous efforts have been made to improve membrane hydrophobicity. A comprehensive discussion on superhydrophobic membrane preparation has been reviewed in the literature [6]. A superhydrophobic membrane may be fabricated by incorporating material with low surface energy, increasing membrane surface roughness in the micro/nanoscale, and improving the membrane fabrication process. Efforts at improving MD performance are also directed toward using nanoparticles for modifying the membrane [178].

5.5 Concluding remarks Heavy metals are known to be very toxic, and can pose detrimental effects to the environment and negatively impact human health if they discharged directly. To avoid any adverse impacts, industrial wastewater containing heavy metals should be properly handled to fulfill the discharge standard. It has been reported that membrane-based processes, including pressure-driven membrane, electrically driven membrane, and MD, can be used to remove heavy metals effectively. Furthermore, several membrane-based processes, such as ED, EDI, and MD, were able to achieve a high enrichment factor in the concentrate stream, which allows one to recover or reuse the heavy metals. However, to achieve the high enrichment factor, the feed solution should be kept at a lower pH. Therefore, the polymeric membrane used for heavy metal removal should be chemically stable. Despite their effectiveness, most of the heavy metal removals by membranes were still investigated in lab-scale tests. For future applications, membrane durability, scaling and fouling formation, and long-term performance need to be assessed at the pilot or commercial scale.

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Further reading [179] A.N. Hakim, K. Khoiruddin, D. Ariono, I.G. Wenten, Ionic separation in electrodeionization system: mass transfer mechanism and factor affecting separation performance, Sep. Purif. Rev. (2019) 1–23, https://doi. org/10.1080/15422119.2019.1608562.

CHAPTER

Application of polymer-based membranes for nutrient removal and recovery in wastewater

6

Watsa Khongnakorna,b, Leo Paul Vaursa, Weerapong Bootluckb, Woei Jye Lauc Department of Civil Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla, Thailanda Center of Excellence in Membrane Science and Technology, Prince of Songkla University, Songkhla, Thailandb Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Skudai, Johor, Malaysiac

Chapter outline 6.1 Introduction ................................................................................................................................ 103 6.2 Development of pressure-driven membranes ................................................................................. 107 6.2.1 RO/NF membranes .................................................................................................. 107 6.2.2 Membrane bioreactor ............................................................................................... 110 6.3 Development of osmotically driven membranes ............................................................................. 114 6.3.1 Forward osmosis ...................................................................................................... 114 6.4 Development of thermally driven membranes ................................................................................ 118 6.4.1 Membrane distillation .............................................................................................. 118 6.5 Hybrid processes and new membrane system trends ..................................................................... 123 6.6 Future perspectives ..................................................................................................................... 124 6.7 Conclusion .................................................................................................................................. 125 References ........................................................................................................................................ 125 Further reading .................................................................................................................................. 134

6.1 Introduction The World Health Organization (WHO) reported that approximately 2.1 billion people in the world are currently using nonsafe drinking water and that in 2025, half the world’s population will be living in water-stressed areas [1]. Although 71% of the Earth’s surface is covered by water, only an estimated 0.007% of the water is considered to be freshwater [2, 3]. Globally, the potable and nonpotable water demand is approximately 32 million m3/day, therefore water reuse is of importance to meet a continuously increasing water requirement [4]. Nutrient pollution is the major problem of aquatic ecosystems that has impacted many streams, rivers, lakes, bays, and coastal waters for the past several decades, Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00006-X # 2020 Elsevier Inc. All rights reserved.

103

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resulting in serious environmental and human health issues as well as negative impacts on the economy. In this chapter, the term nutrients will encompass nitrogen and phosphorous, which are the major pollutants in aquatic systems and also two of the main essential compounds required for effective fertilization. Nutrient pollution in groundwater can be harmful to humans due to the presence of nitrate (nitrogenbased compound) in drinking water, even at concentration as low as 50 mg/L (e.g., the blue baby syndrome). To control eutrophication, the Ministry of Ecology and Environment (MEE) of China has recommended a total phosphorus (TP) concentration limit of 0.1 mg/L for streams entering drinking water resources (such as lakes or reservoirs) (Class II and III) and 0.2 mg/L for other flowing waters (Class IV and V). Recovered nutrients could be reused in several urban applications such as green walls, parks, urban farming, or as fertilizers. Nitrogen and phosphorus fertilization is indispensable to sustain high agricultural yields, and currently, the production of nitrogen fertilizers accounts for 1%–2% of world energy consumption. Offsetting this energy consumption by reusing those nutrients from wastewater is desirable; however, their concentrations in wastewater often get diluted, making it difficult and energy-intensive to recover via conventional physical/chemical processes. Nutrients can be recovered from various sources of waste such as animal manure digestate [5], urine [6], digestate sludge effluent [7], landfill leachate [8, 9], municipal wastewater [10, 11], or municipal anaerobic digested sludge [12, 13]. Most of the nutrients in municipal wastewater come from urine, which accounts for 80%, 50%, and 70% of the nitrogen (N), phosphorus (P), and potassium (K) present in this water, respectively. The mass of nitrogen and phosphorus that can be recovered per cubic meter of domestic wastewater is approximately equal to 15, 25, and 8 g of organic nitrogen, ammonia, and phosphorus, respectively [14]. The advantages of nutrient recovery from wastewater are: (I) Increased water availability. Water free from nutrients will be available to be used for multiple applications (agricultural, domestic, etc.). (II) Reduction of wastewater treatment plant (WWTP) capacity. Conventional WWTPs require long periods for aeration in order to remove nutrients and therefore large areas due to the relatively low loading. Nutrient removal and recovery processes allow higher loading, which will therefore reduce plant capacity. (III) Better use of energy. If, at the full WWTP operation scale, the energy requirements for conventional processes (activated sludge) and nutrient recovery processes (in this example, a membrane bioreactor (MBR)) are similar [15], recovering nutrients represents a source of income while conventional processes are simply wasted energy with no benefit. (IV) Green production of fertilizers from wastewater. Current fertilizer production requires natural resources and a large quantity of energy (
8.7 Wh/g of N for ammonia production) [16]. Based on 500 mg/L of ammonia-containing wastewater, it was calculated that the energy requirement for ammonia recovery via reverse osmosis (RO) comprised between 1.18 and 1.86 Wh per gram of ammonia removed [17]. Considering that approximately 20% of the manufactured nitrogen and phosphorus is contained in domestic wastewater, nutrient recovery from wastewater will significantly reduce the overall energy consumption for fertilizer production [18]. (V) Low operating costs in WWTPs linked to pipe and reactor maintenance due to blockage by phosphorous (P) salts. Phosphorus can precipitate in WWTPs and form polyphosphate, which

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results in multiple operating problems and high maintenance costs [19]. Removing phosphorus can thus eradicate those issues and reduce costs. (VI) Decrease of the eutrophication problems in receiving waterways. (VII) Reduction of P scarcity [20]. Nitrogen removal from wastewater can be achieved by converting it to dinitrogen gas or by concentration followed by its recovery as fertilizer. In the past two decades, nitrogen has been removed by biological organisms such as microalgae [21], wetland plants [22], and biological nitrogen removal (BNR) processes [23]. Studies reported a maximum N removal rate of 60% by aqua plant species, which depended on the composition of the wastewater streams, the nature of the plants, or the design of the constructed wetlands [24]. The most popular process is the conventional BNR method, which has proved to be cost-effective with good performance. Nitrification/denitrification is the most energy-intensive (high oxygen consumption) process. Denitrifiers use nitrite or nitrate as their electron acceptor, and full nitrification requires 4.57 mg O2/mg N while partial nitritation/anammox has been proposed for low-energy operations, but they still require 3.43 mg O2/mg N. The key factor to enhance the nitrogen removal efficiency is the carbon-to-nitrogen ratio (C/N). Municipal wastewater is characterized by a low C/N ratio, which results in low nitrogen removal efficiency. An external carbon source (e.g., methanol, ethanol, glucose, or sodium acetate) must be added to provide electron donors for denitrification, which is a costly control process [23]. Simultaneous nitrification and denitrification (SND) could be an efficient way to treat low C/N wastewater in the same tank. Nitrogen is consumed for the production of poly-hydroxy-alkanoate (PHA) in aerobic conditions. Denitrification occurs with the uptake of PHA by anoxic microbes [25, 26]. The comparisons of a nitrification/denitrification pathway and SND are presented in Fig. 6.1. To elucidate the SND mechanism, they proposed that nitrified and denitrified bacteria were able to coexist at low levels of dissolved oxygen (DO). Nitrogen removal of 99.9% was reported at a low  COD/TN ratio (3.8:1) and DO level (0.2–0.5 mg/L). The total effluent NO 2 and NO3 concentrations

FIG. 6.1 Pathway of (A) conventional nitrification-denitrification and (B) simultaneous nitrification-denitrification. Adapted from H. Chai, Y. Xiang, R. Chen, Z. Shao, L. Gu, L. Li, Q. He, Enhanced simultaneous nitrification and denitrification in treating low carbon-to-nitrogen ratio wastewater: treatment performance and nitrogen removal pathway, Bioresour. Technol. 280 (2019) 51–58, https://doi.org/10.1016/j.biortech.2019.02.022.

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were both lower than 0.01 mg/L. Heterotrophic nitrification and aerobic denitrification were responsible for nitrogen removal. Ammonia stripping is another wastewater treatment process that uses spraying technologies. It promotes the transformation of NH+4 toward NH3. The released NH3 could be absorbed in a phosphoric acid solution, and then precipitated to form struvite after the addition of Mg/MgO. Struvite is a phosphate mineral with the following formula: MgNH4PO46H2O. Its formation requires stoichiometric amounts of ammonium, phosphate, and magnesium at alkaline pH, as described in the following equation [19]. Mg2 + + PO4 3 + NH4 + + 6H2 O ! MgNH4 PO4  6H2 O #

For the past 10 years, struvite precipitation has been considered the most significant and successful process for simultaneous N and P recovery. Potassium (K) can also be recovered via K struvite precipitation [20, 27]. Struvite can be used as a slow-release and long-acting fertilizer for nitrogen recycling [28]. The conditions for struvite precipitation are impacted by many factors such as the type of chemical added, the PO4:Mg molar ratio, the pH, etc. It is well known that phosphate and ammonium ions may undergo hydrolyzation at different pH values, which can affect, to a certain extent, struvite precipitation. Changes in pH could result in different species of phosphate and ammonium ions being formed [19]. Several chemical methods have been used for P removal, including precipitation, flocculation, and adsorption. Phosphorus removal by P-precipitation involves the addition of metal salts (iron (III), alumina (III)) to react with soluble phosphate and form solid precipitates. Then, they are removed by solid separation processes, including clarification and filtration. Chemical treatments have been applied on phosphorus-containing wastewater to produce struvite. Flocculation uses polymers such as alum, ferric chloride, etc., to destabilize colloidal particles, which forms aggregates [29]. Adsorption is the removal of dissolved P via a surface reaction on a solid material called the adsorbent [30]. Titanium mesostructured adsorbents are commonly used for phosphorous adsorption: they present excellent phosphorus adsorption ability with the Langmuir model and a pseudofirst/s-order kinetic model. In addition, the combination of different chemical processes, that is, Fe(III)/UV/NaOH, can be used to enhance the removal of phosphonate (in the form of Ca(II) complexes). The basic steps of this process include: Fe(III) replacement by the complex Ca(II) to form Fe(III)-phosphonate, UV-mediated cleavage of the CdN and CdP phosphonate bonds to form phosphate and other byproducts, and the final removal of TP through coprecipitation [31]. Biological treatment processes called enhanced biological phosphorus removal (EBPR) can remove P to a concentration lower than 0.5 mg P/L. The main element in EBPR implementation is the presence of an anaerobic tank (absence of nitrate and oxygen) prior to the aeration tank. Under these conditions, P is anaerobically released to produce energy, and then a group of heterotrophic bacteria, called polyphosphate-accumulating organisms, is selectively enriched within the activated sludge to remove a large amount of P [32]. These processes are usually ineffective, however, for organic phosphonate removal, which forms complex phosphonate compounds in the presence of flocculants. This results in an increased concentration of dissolved organic phosphorus in the wastewater effluent [33]. Moreover, EBPR processes are sensitive to several factors such as the type of organic compound, metal salts, nitrate, ammonium, and heavy metal content, which can inhibit the process [34]. Urine is another source of wastewater containing a large amount of nitrogenous compounds and phosphate that can be valorized as fertilizers. Ishii and Boyer [35] showed that by using a urine

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separation system, an efficiency of 80% and a struvite precipitation for nutrient recovery of 5.52 kg N/m3 and 0.447 kg P/m3 could be achieved. Chemical and biological techniques have been widely used for the treatment of nitrogen and phosphorous in wastewaters; however, these techniques are expensive, use high energy, are chemicalintensive, and require strict control of the operating conditions. Membrane technologies such as RO and nanofiltration (NF) represent a promising approach but their unsatisfactory urea and ammonia rejection, membrane fouling and scaling, and high operational costs are the main problems that need to be addressed. This chapter will focus on the different membrane processes for nutrient removal and recovery. First, pressure-driven membranes including NF/RO and MBRs will be discussed. The second part will focus on osmotically pressure-driven membranes. Third, emphasis will be placed on thermally driven technologies. Finally, emerging hybrid systems for nutrient removal and recovery will be addressed.

6.2 Development of pressure-driven membranes 6.2.1 RO/NF membranes While osmosis is a process that naturally occurs where water flows through a semipermeable membrane from low concentration to high concentration (Fig. 6.2), RO/NF processes are the opposite. Pressure higher than the osmotic pressure is applied to RO/NF processes to drive water from high to low concentration (Fig. 6.3) through a semipermeable membrane to separate dissolved solids from liquid and therefore purify water.

Δp

High concentration

Low concentration

FIG. 6.2 Schematic of the osmosis process. Adapted from M. Qasim, M. Badrelzaman, N.N. Darwish, N.A. Darwish, N. Hilal, Reverse osmosis desalination: a state-of-the-art review, Desalination 459 (2019) 59–104, https://doi.org/10.1016/j.desal.2019.02.008.

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Pressure

High concentration

Low concentration

FIG. 6.3 Schematic of the reverse osmosis process. Adapted from M. Qasim, M. Badrelzaman, N.N. Darwish, N.A. Darwish, N. Hilal, Reverse osmosis desalination: a state-of-the-art review, Desalination 459 (2019) 59–104, https://doi.org/10.1016/j.desal.2019.02.008.

Osmotic pressure (π) is related to the colligative properties of the chemical solutions involved. For ideal dilute solutions obeying van’t Hoff, the osmotic pressure can be calculated as follows: π ¼ iCRT

Where π is expressed in atm, i is the van’t Hoff factor (dimensionless), C is the molar concentration of a nonpermeable solute in the feed solution (mol/L), R is the universal gas constant (0.08206 L atm/mol K), and T is the temperature of the feed solution (K). The van’t Hoff factor is a measure of the effect of a solute upon colligative properties. Nanofiltration (NF) is known as a low-pressure RO membrane-softening process. RO and NF can be distinguished by their operating pressure and pore size. RO processes require higher pressure than NF. RO operates at between 10 and 100 bar while NF operates at between 5 and 20 bar. Regarding the pore sizes, their range is 200–400 Da and they are lower than 125 Da for NF and RO, respectively [36]. RO/NF membranes can be classified by their membrane structure and pore shape into isotropic microporous, nonporous, dense, electrically charged, asymmetric, ceramic, and liquid membranes [37]. Most of the RO/NF membranes are made from cellulose acetate (CA) and thin film composites (TFC). CA is the oldest type of commercial membrane material. The advantages of CA membranes are their low cost and tolerance to chlorine, but they have several disadvantages such as low salt rejection, a narrow pH range (4–8), narrow temperature limits (0–35°C), high operating pressures (100–500 MPa), and low permeability, which involves high energy consumption. TFC membranes consist of a typically 0.2 mm thick barrier layer on top of a support layer. The advantages of these membranes are their high flux and low pressure, great chemical stability, high salt rejection, a wide range of pH (2–12), and a wide range of temperatures (0–40°C) [38]. As far as the author is concerned, however, they have not been used in a real scale yet. Two types of flow configurations are distinguished in membrane systems: cross-flow and dead-end feed configurations. In the cross-flow (also defined as tangential flow) configuration, the water feed is split into two streams, as shown in Fig. 6.4. Water passing through the membrane can be collected as pure water and it is called permeate. A high concentration of particles and dissolved solids is collected in the brine (also called concentrate or reject).

6.2 Development of pressure-driven membranes

Feed Qf = Feed flow rate Cf = Feed concentration

109

Brine

Membrane Permeate

Qb = Brine flow rate Cb = Brine concentration

Qp = Permeate flow rate Cp = Permeate concentration

FIG. 6.4 Schematic of a cross-flow system. Adapted from M.A. Abdel-Fatah, Nanofiltration systems and applications in wastewater treatment, Ain Shams Eng. J. (2018), https://doi.org/10.1016/j.asej.2018.08.001.

Feed

Membrane Permeate

FIG. 6.5 Schematic of a dead-end system. Adapted from M.A. Abdel-Fatah, Nanofiltration systems and applications in wastewater treatment, Ain Shams Eng. J. (2018), https://doi.org/10.1016/j.asej.2018.08.001.

The second flow configuration is the dead-end feed system in which feed water directly passes though the membrane. One disadvantage of this process is that foulant will accumulate on the membrane until backwash flushing is performed, as seen in Fig. 6.5 [37]. RO/NF is an economic and effective process for desalination. This technique has been used to treat solutions having salt concentrations from 500 to 30,000 mg/L in large-scale wastewater treatment, and seawater and groundwater plants. Maximum recoveries of approximately 60% can be obtained at the lab scale, producing goodquality water. Percent recovery can be calculated as: Percent recovery ¼

Qp  100 Qf

where Qp and Qf are the permeate and feed flow rate, respectively. Salt rejection, Rs (in %), is determined by measuring the permeate and the feed solution conductivity. The salt rejection, Rs, is calculated as: 

 Cp  100 Rs ¼ 1  Cf

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Chapter 6 Application of polymer-based membranes

where Cp and Cf (in mS/cm) are the conductivities of the permeate and feed solution, respectively. Note that this can also be calculated using total dissolved solid concentrations. Despite great RO/NF membrane performances, fouling is a real hindrance for all types of membranes. On membrane processes, fouling causes multiple problems including flux decline, increase of the energy demand, requirement for more maintenance, frequent chemical cleaning, and a shorter membrane lifespan. Generally, fouling can be classified into four major categories: 1. Organic fouling: dissolved or colloid organic materials are adsorbed on the membrane surface. 2. Colloidal (particulate) fouling: colloids and particulate matters accumulate on the surface membrane. 3. Scaling (inorganic fouling): ions are precipitating and form inorganic compounds on the membrane surface. 4. Biofouling (biological fouling): microorganisms cause agglomeration of extracellular materials on the membrane surface. To prevent fouling from occurring and disturbing membrane operations, preventive and mitigative actions must be undertaken. They could be hydrodynamic preventions that include the increase of crossflow velocity, air sparkling, or surface scouring. Fouling can also be controlled with an appropriate design and operation mode taking into account the type of membrane, the pretreatment process, the hydrodynamic conditions, and the cleaning process [39]. RO/NF membranes have not really been applied for nutrient removal and recovery; therefore, they will not be further developed in this chapter.

6.2.2 Membrane bioreactor MBR technology applied for wastewater treatment has been developed for more than 30 years and was first presented by Yamamoto et al. [40] in 1989. MBR technology provides an alternative solution to activated sludge. In comparison to activated sludge, MBR processes present the following advantages: higher effluent quality, higher loading capacity, low area requirement, longer solid retention time (SRT), lower sludge production, and the potential for simultaneous nitrification/denitrification in long SRTs [41, 42]. Fouling can occur and remains a major problem. An MBR is a process coupling a biological process and membrane filtration. The biological performances depend on the operating conditions such as organic loading rate (OLR), hydraulic retention time (HRT), SRT, food-tomicroorganism (F/M) ratio, etc. The filtration separation mechanism is carried by size sieving using microfiltration (MF) or/and ultrafiltration (UF) membranes. Membrane pore size is generally between 0.1 and 0.01 μm. The membrane filtration in the process depends on the membrane characteristics and properties.

6.2.2.1 MBR membrane characteristics Polymeric membranes are fabricated from hydrophilic polymers such as polypropylene (PP) [43], polyvinylidene fluoride (PVDF) [44, 45], polytetrafluoroethylene (PTFE) [46, 47], the polysulfone (PS)/ polyethersulfone (PES) family, CA [48, 49], and polyethylene (PE) [50]. Polymeric membranes are currently widely used because of the ease of manufacturing their pore sizes. Due to their hydrophobic nature, however, they tend to be easily fouled.

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The separation mechanism in MBR processes is size exclusion (or sieving). MF and UF membranes are typically used to guarantee the complete physical retention of bacterial flocs within the bioreactor. Bacteria floc size is between 50 and 150 μm, thus, the membrane pore size used in the MBR system usually ranges from 0.03 to 0.1 μm [45, 51]. Chang et al. [52] found that a large pore size or porosity (void volume of the membrane) could result in a high initial flux, even at low pressure. The pore size of the membranes can have an impact on membrane fouling, depending on the size of the particles in the wastewater feed stream. Composite membranes have been developed and produced from different material mixtures to increase the strength of the final product. Typically, surface modifications by plasma [53] coating, addition of nanoparticles (e.g., nanosilver [54], or TiO2 [55, 56]) on the active layer and other forms of support layers to increase the hydrophobicity of membranes deliver membranes tolerating higher flux with lower fouling risks.

6.2.2.2 MBR classification MBR processes can be classified according to the biological respiration, which differs in aerobic and anaerobic conditions. This chapter only focuses on anaerobic membrane bioreactors (AnMBRs) due to their wide application in nutrient removal and recovery. Conventional processes, that is, activated sludge for nitrogen removal, require a significant amount of energy, which represents 4% of the electricity demand in WWTP in the United States [14, 57]. The oxygen demand for conventional nitrification processes is equal to 4.57 g/g N oxidized or 45 MJ kg/N [58]. AnMBR or hybrid MBR processes require low energy for nutrient removal and recovery because of the anaerobic microbial biomass nutrient uptake [59–61].

6.2.2.3 MBR configurations AnMBR systems are implemented based on two configurations: external/side-stream and submerged/ immersed, as presented in Fig. 6.6. This impacts the geometry of the membrane (hollow fiber, flat sheet, or tubular) and the direction of the water flow through it (in-to-out or out-to-in). Most of the flat sheet modules are employed in submerged configuration and tubular modules are employed in side stream or external configurations. Tubular modules give higher flux than flat sheets. Currently, hollow fibers are being developed and operated at high flux due to their high packing density [62]. The performances of some MBR processes in treating ordinary pollutants (i.e., C, N, and P) are presented in Table 6.1. Many researchers have studied nutrient recovery from wastewater through AnMBR, as shown in Table 6.1 [68], and found that more than 90% ammonium removal and recovery could be achieved. The effects of some operating parameters such as feed pH, temperature, OLR, TSS, and wastewater chemical composition are among the main parameters to be assessed for optimal struvite precipitation in MBR processes [59, 60, 65]. Kim et al. [7] investigated the performance of a pilot MBR coupled with an electrocoagulation process for phosphorous removal (EPR process), which used electrolytic cells to coagulate and separate metals from the aqueous phase. Phosphorous ions released from the anaerobic settling tank were coagulated during an electrochemical reaction with aluminum ions discharged from aluminum plate electrodes in the EPR tank. Phosphate (PO3 4 P) and TP removal efficiencies by electrocoagulation were 89.2% and 79.9%, respectively. Even though it was thought that microorganisms could stably grow in neutral pH resulting from the EPR mechanism, AlPO4 crystal flocs in the reactor were adsorbed by extracellular polymeric substances, which could lead to fouling. In addition, Li et al.

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Biogas

Recirculation pump Anaerobic bioreactor

Wastewater

Permeate Retentate

Pump

Membrane module

Sludge waste

(A) Biogas

Permeate pump

Permeate

Anaerobic bioreactor Wastewater Membrane module

Gas sparging

Pump Sludge waste

(B) FIG. 6.6 AnMBR configurations (A) external cross-flow and (B) submerged/immersed configurations. Adapted from W. Wang, Q. Yang, S. Zheng, D. Wu, Anaerobic membrane bioreactor (AnMBR) for bamboo industry wastewater treatment, Bioresour. Technol. 149 (2013) 292–300, https://doi.org/10.1016/j.biortech.2013.09.068.

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Table 6.1 Comparison of AnMBR/MBR for nutrient recovery performances.

Wastewater

Type of membrane (materials)

Pore size

Operating condition

% removal % recovery

Type of fouling Adhesive iron oxides and viscous extracellular polymeric substances Solid concentration, cake layer

[63]

TP removal 79.9%

Extracellular polymeric substances/ AlPO4 crystal flocs

[7]

COD removal 88%–95% Ammonium removal 99%

High P/C ratio in EPS

COD removal 48% Total nitrogen removal 67%

Low membrane fluxes applied, no severe fouling High solid concentration, cake layer

[65]

N/A

[5]

N/A

[67]

Municipal wastewater

Flat-plate ceramic membrane

100 nm

HRT 12 h SRT 30 days

N removal 95% N recovery 50%

Municipal wastewater treatment plants Municipal wastewater

Flat sheet membrane

0.2 μm

N removal 85%

Flat-sheet PVDF Flat-sheet PES

0.08 μm

HRT of 3 days OLR 8.84 kg COD/m3 day HRT 2.71– 2.95 h DO 0.06– 0.17 mg/L ORP 300 to 100 mV OLR 0.4– 3.2 kg COD/ m3 day SRT 50 days Sludge wasting 0.1 L/day OLR 0.070– 0.185 kg COD/kg VSS/day

0.2 μm

Meatprocessing wastewater

Hollow fiber ultrafiltration membrane

0.04 μm

Winery wastewater

Hollow fiber membrane module

0.2 μm

Municipal wastewater treatment plant Piggery wastewater

Hollow fiber polyvinylidene fluoride (PVDF)

0.3 μm

SRT 3.5 days

NH4-N recovery 37.5%

Nylon sheet

20 μm

HRT 2 days

Winery wastewater

Kubota (United Kingdom) microfiltration membranes

0.4 μm

Flow rate ¼ 10 m3/ day;

NH4-N removal 95% TP removal 75% N recovery 20% P recovery 80% TN ¼ 54% TP ¼ 60%–65%

References

[64]

[66]

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Chapter 6 Application of polymer-based membranes

[63] developed novel acidogenic phosphorus recovery followed by chemical phosphorus removal using different Fe dosages coupled with an MBR for enhanced wastewater treatment. Fe-MBR and FeP-MBR enabled high P removal and P recovery (90% and 50%, respectively). When Fe(III) was dosed into the raw wastewater, hydrolyzation and coprecipitation happened rapidly with hydroxyl ions and orthophosphate to form hydrous ferric oxide and ferric phosphate (FePO4). Petta et al. [65] studied a system consisting of an upflow anaerobic sludge blanket reactor followed by an anoxic-aerobic ultrafiltration membrane bioreactor made of a hydrophilic nonionic 0.04 μm material and a posttreatment based on chemical precipitation with lime or adsorption on granular activated carbons. The nutrients in the digestate were removed by anoxic aerobic MBR. The maximum TN removal efficiency was close to 70%. The efficiency decreased when the pH went up to 9. The removal of phosphates and color was attributed to the chemical precipitation and adsorption processes, respectively, rather than the biological process. This indicates that the AnMBR-based hybrid or coupling with other systems could concentrate nutrients over a wide range of wastewater sources due to the tolerance to high OLRs and struvite formation. Fouling is a major disadvantage of the AnMBR process, though. Biopolymers were denotified as the predominant organic foulants in the MBR, which involved the formation of a cake on the surface.

6.3 Development of osmotically driven membranes 6.3.1 Forward osmosis Forward osmosis (FO) is a membrane separation technology for water reuse and desalination. FO uses the difference in osmotic pressures existing between each side of a semipermeable membrane. FO transports water through a selective permeable membrane from a low to a high concentration. The FO process can be applied for water distillation as well as other important applications such as food concentration, wastewater treatment, alcohol dehydration, power generation, etc. The advantage of FO is that it utilizes the natural existing driving force derived from the osmotic pressure gradient between the feed solution and the draw solution. As a consequence, the energy requirement is low, especially compared to RO (lower than 0.3 kWh/m3 compared to 1.2–1.5 kWh/m3) [8]. In addition, Fig. 6.7 shows the challenges of the FO process that focuses on: (1) Development of membrane materials with properties such as high hydrophilicity, high rejection, lower membrane fouling, good chemical resistance, and good mechanical strength. (2) Selection of new draw solutions for high osmotic pressure and draw solution recovery in FO processes. (3) Low energy and operating cost for full-scale FO applications. The development of new materials for FO membranes aims at increasing the water flux; reducing membrane fouling from the feed solution, also called external concentration polarization, and membrane fouling from the draw solution, also called internal concentration polarization (ICP); increasing the rejection; and improving the mechanical strength as well as the chemical resistance. The methods for preparing an FO membrane include the phase inversion and interfacial polymerization (IP) process. The FO membrane is a hydrophilic surface of any dense nonporous membrane. The commercialization of FO membranes started in the 1990s and was developed by Hydration Technology Inc. (HTI, Albany, Oregon, United States). There are many types of material polymers for FO membranes such as CA, cellulose triacetate (CTA), polybenzimidazole, polysulfone (PS), polyvinylpyrrolidone (PVP), polyamide (PA), CA, etc. [69–71].

6.3 Development of osmotically driven membranes

115

FIG. 6.7 The challenges of the FO process.

6.3.1.1 Type of membrane There are four types of membrane for FO applications: 1. Flat sheet membrane. Flat sheet membranes made of CTA were widely used in the first research stages of the FO membrane processes. The CTA membrane characteristics are high hydrophilicity, salt rejection ranging from 88% to 95%, and water permeate of 2.1–3.8 L/m h. The advantages of CTA membranes include a high hydrophilic membrane allowing high water flux, good mechanical strength, and low membrane fouling. Flat sheet FO membranes made of CTA have an embedded polyester mesh mechanical support. The thickness of the membrane is lower than 50 μm [70]. 2. Hollow fiber membrane. Hollow fiber FO membranes are fabricated using CTA. Wang et al. synthesized hollow fiber FO membranes with a pore size of 0.3–0.63 nm and a contact angle equal to 43 degrees. The water flux of the hollow fiber FO membranes was 1.34 L/m h, and the salt rejection was 97%–99% [72]. 3. TFC membrane. These membranes must be engineered by using the phase-inversion technique followed by IP of flat sheet membranes and hollow fiber membranes. Fig. 6.8 shows the schematic of a TFC-FO membrane having a selective layer pore size of 0.42 nm and salt rejection between 95% and 99.6%. The membrane layer is made from PES, PA, polysulfone (PS), and cellulose ester. Several reports used PA as a selective layer and added a support layer made from polysulfone. The advantages of the TFC-FO membrane are high water flux and high salt rejection [73–76]. 4. Thin film nanoparticle (TFN) membrane. Recently, new types of material membranes called TFN membranes have been designed. These membranes are prepared via IP. The selective layer is made

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Chapter 6 Application of polymer-based membranes

Polyamide active layer

Polyethersulfone (PES) substrate

FIG. 6.8 Schematic of a TFC membrane. Adapted from R.C. Ong, T.S. Chung, J.S. de Wit, B.J. Helmer, Novel cellulose ester substrates for high performance flat-sheet thin-film composite (TFC) forward osmosis (FO) membranes, J. Membr. Sci. 473 (2015) 63–71, https://doi.org/10.1016/j.memsci.2014.08.046.

from a PA layer while the support layer is made from material polymers such as polysulfone (PS) or PES and a mixed matrix such as zeolite NaA nanoparticle or titanium dioxide (TiO2) (Fig. 6.9). TFN membranes have a pore size of 0.74–0.79 nm, high salt rejection between 97.4% and 98.7%, and water permeability of 4.2  1012 m Pa1 s1 [77–79]. Table 6.2 shows the development of FO membranes using different materials and synthesis methods. The most commonly used methods to produce membranes are phase inversion and IP, which have, however, some disadvantages such as polarization concentration (ICP) and salt reverse flux. More efforts are still need to develop membranes with improved properties that are able to cope with these issues. In addition, new materials such as nanocomposites are under development. The application of FO membranes for nutrient recovery and removal have been studied since the 2010s. Xue et al. [87] found that the FO process could concentrate nutrients present in a municipal wastewater feed solution. It is easier to recover phosphate compared to ammonium ions because of the difference in hydrate radiuses, but phosphate ions more easily accumulate in the feed side due to their larger hydrated radius. Polyamide active layer

Polyethersulfone (PES) substrate Zeolite NaA nanoparticle,TiO2

FIG. 6.9 The characteristics of a TFN membrane. Adapted from D. Emadzadeh, W.J. Lau, M. Rahbari-Sisakht, H. Ilbeygi, D. Rana, T. Matsuura, A.F. Ismail, Synthesis, modification and optimization of titanate nanotubes-polyamide thin film nanocomposite (TFN) membrane for forward osmosis (FO) application, Chem. Eng. J. 281 (2015) 243–251, https://doi.org/10.1016/j.cej.2015.06.035.

6.3 Development of osmotically driven membranes

117

Table 6.2 Summary of the development of FO membranes. Year

Membranes

Materials

Preparation methods

References

2007 2008

Polybenzimidazole (PBI) Cellulose acetate

Dry-jet wet phase inversion Phase inversion and then annealing at 80–95°C Dry-jet wet phase inversion (i.e., coextrusion technology) Dry-jet wet spinning and IP

[80] [81]

2009

Hollow fiber NF Flat sheet cellulose acetate Dual-layer hollow fiber NF Hollow fiber

2010 2011 2011

Hollow fiber NF Nanoporous PES Flat sheet TFC PA

2012

TFC PA

2013

Thin-film inorganic (TFI)

2014

Thin-film nanocomposite (TFN) Tribore hollow fiber TFC Flat sheet TFC Flat sheet TFC PA

2010

2014 2015 2015

PBI-PES/PVP PES substrates, polyamide (PA) active layer Cellulose acetate PES cast on PET fabric PSf nanofiber support, PA active layer Super porous CNT nonwoven Bucky-paper (BP) support, PA active layer Stainless steel mesh (SSM) substrate, microporous silica xerogel active layer PSf-titanium dioxide (TiO2) nanocomposite substrate, PA active layer Matrimid 5218 polymer substrate, PA active layer PEG cellulose ester substrate Polyamide active layer

Dry-jet wet spinning Phase inversion Electrospinning and interfacial polymerization Plasma treatment of CNT BP support and IP

[81a] [81b] [82] [83] [84] [73]

Dip-coating and calcination for 4 h at 500°C in nitrogen followed by cooling to 25°C IP

[85]

Dry-jet wet spinning and IP

[86]

Phase inversion and IP Phase inversion and IP

[75] [74]

[78]

The pH of the feed solution may also influence nutrient recovery due to the diffusion of protons from the feed to the draw side. This could have the effect of increasing the pH of the feed solution. Ammonium ions, however, are more easily adsorbed on the membrane surface due to their positive surface charge, which causes electrostatic attractions while phosphate ions are pushed and then condensed in the feed side due to electrostatic repulsions. Ansari et al. [88] demonstrated 92% phosphate recovery from digested sludge by the FO process using seawater as a draw solution. The concentration process also provided ideal conditions for calcium phosphate precipitation. Phosphate and calcium concentrations coprecipitated due to the high rejection rate and pH elevation was observed. In particular, the concept of fertilizer-driven FO (FDFO) has been demonstrated to be a viable FO application to reclaim and reuse wastewater and impaired waters for agriculture applications such as fertilizer. Urine recovery by FO allowed the extraction of approximately 50% of the nitrogen and 40% of the phosphorous while 60% of the urine concentration was found with P-precipitation on the feed solution as struvite due to the reverse salt flux of Mg2+ [89]. The pH balance in the feed side and the type of draw solution have an impact on the performances

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and fouling of the FO membranes, which affects struvite formation. Further studies on FO membrane syntheses should focus on membrane materials and their abilities to adsorb the proton diffusion for phosphate and ammonia ions. More recently, the combination of osmotically and thermally driven membrane processes, including FO and membrane distillation (MD), has been investigated for enhancing the treatment and recovery of nutrients. Therefore, the following sections will focus on thermally driven membrane processes and hybrid processes.

6.4 Development of thermally driven membranes 6.4.1 Membrane distillation MD is a membrane process that utilizes porous and hydrophobic membranes. The vapor pressure difference between the feed and permeate side acts as the driving force and is the result of the feed and permeate side temperature difference. The vapor pressure across the membrane condenses on the permeate side. Hydrophobic membranes only admit water vapor across the membrane. Temperature should not exceed the feed solution boiling point, but must be high enough to generate vapor. The heat transfer and mass transfer in the bulk and membrane surface are presented in Fig. 6.10.

6.4.1.1 Type of membrane Hydrophobic and microporous membranes are applied in MD processes. In general, microporous hydrophobic membranes are made of different types of polymers such as PVDF, polytetrafluorethylene (PTFE), PE, and PP. Among them, PTFE membranes present the highest hydrophobicity and show outstanding thermal stability and chemical resistance. PTFE membranes are the most expensive, however. PVDF membranes exhibit good thermal and chemical resistance. There are two types of membrane modules used in MD, namely flat sheet and hollow fiber. Membrane Tbf

Tfm

Tpm

Tbp Mass transfer Heat transfer

Cbf

Cfm

FIG. 6.10 Heat and mass transfer in the MD process. Adapted from J. Zhang, N. Dow, M. Duke, E. Ostarcevic, S. Gray, Identification of material and physical features of membrane distillation membranes for high performance desalination, J. Membr. Sci. 349(1–2) (2010) 295–303, https://doi.org/10.1016/j. memsci.2009.11.056.

6.4 Development of thermally driven membranes

119

6.4.1.2 Characteristic of membrane Membrane thickness is an important characteristic in MD processes and ranges between 50 and 250 μm [90, 91]. To obtain high permeate flux, membranes should be as thin as possible. In contrast, to achieve better heat efficiency, the membrane should be thick. Lagana` et al. [92] suggested that the optimum membrane thickness should be in the range of 30–60 μm. Membranes with higher porosity offer greater surface area for evaporation and lower conductive heat loss. Generally speaking, the MD membrane porosity varies from 30% to 85%. Membranes used in MD have pore sizes varying from 0.1 to 1 μm [93]. The permeate flux increases with increasing pore size. In order to avoid pore wettability, however, a small pore size should be selected [94]. The membrane thermal conductivity should be small in order to reduce heat loss through the membrane from the feed side to the permeate side. The conductive heat loss is inversely proportional to the membrane thickness. Thermal conductivities generally vary in the range of 0.15–0.45 W/m K [90]. Contact angle is a common measure of the hydrophobic or hydrophilic behavior of a material. It is related to the pore wettability. When contact angle values are greater than 90 degrees, the material is considered hydrophobic; otherwise, the material is considered hydrophilic [93]. MD requires membranes to remain dry, which only allows water vapor to go through. Therefore, the limitation of MD is membrane wetting or liquid entry pressure (LEP). LEP is the minimum pressure needed to be applied onto a feed solution to ensure that the vapor gradient passes through the membrane without the pores getting wet. LEP is defined by the membrane characteristics. LEP is correlated to the liquid surface tension, the contact angle of the liquid on the membrane surface, and the shape and pore size of the membrane. Izquierdo-Gil et al. [95] revealed that LEP could be comprised of between 200 and 400 kPa for a membrane pore size of approximately 0.2 μm while it could be as low as 100 kPa for membranes with pore sizes of 0.45 μm. Kullab and Martin [96] indicated that fouling not only causes the pores to clog in MD membranes, which reduces the effective area of permeate flux, but it also leads to pressure drop. When the pressure drop exceeds the LEP, it could partially wet the membrane.

6.4.1.3 Configurations of MD MDs are classified into four different configurations (Fig. 6.11): (a) direct contact MD (DCMD), (b) vacuum MD (VMD), (c) air gap MD (AGMD), and (d) sweeping gas MD (SGMD). 1. DCMD is the process in which membranes are in direct contact with the liquid phase. This is the simplest configuration capable of producing a reasonably high flux. It is best suited for applications such as desalination and concentration of aqueous solutions (e.g., juice concentrates) [97–101]. 2. SGMD is the process in which some stripping gas is used as a carrier for the produced vapor. It is applied when volatiles must be removed from an aqueous solution [102–106]. 3. AGMD is the process in which an air gap is interposed between the membrane and a condensation surface. This configuration is the most energy efficient, but the flux obtained is generally low. AGMS is the most employed MD configuration [107], particularly when energy availability is low. 4. VMD is the process in which the permeate side is vapor or air under reduced pressure with, if needed, the permeate being condensed in a separate device. This configuration is useful when volatiles must be removed from an aqueous solution [108, 109]. DCMD is the most common MD configuration for nutrient removal and recovery, so this chapter will mostly focus on DCMD.

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Chapter 6 Application of polymer-based membranes

Membrane

Membrane

Aqueous solution Aqueous solution

Air gap

Cooling plate

Aqueous solution

(A)

(B) Membrane

Membrane

Sweeping gas

Vacuum

Aqueous solution

(C)

Aqueous solution

(D)

FIG. 6.11 Different types of MD configurations. (A) DCMD, (B) AGMD, (C) SGMD, and (D) VMD. Adapted from M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285(1–2) (2006) 4–29, https://doi.org/10.1016/j.memsci.2006.08.002.

6.4.1.4 Impact of operating conditions on DCMD performance The key parameters affecting the performances of DCMD or the final flux are:

Temperature Temperature is an important factor that affects the permeate flux. A higher temperature increases the transmembrane vapor pressure and therefore increases the permeate flux. It was found that the permeate flux increased by more than 2 L/m h for every 1°C increase in the feed solution [110]. The temperature, however, should be varied with precaution in the separating process in order to avoid denaturing, destroying, or impacting sample properties such as protein compounds.

Cross-flow velocity To reduce the temperature and concentration polarization effects, the feed and permeate flow rates must be increased. When the flow rate is increased, the temperature and nonvolatile solute concentration at the membrane surface becomes closer to the corresponding bulk temperature and bulk concentration. The flow rate should be investigated, however, in order to avoid membrane pore wetting [111].

Concentration The solution concentration is another factor that could have an effect on the permeate flux. Increasing the nonvolatile concentration in the feed solution results in the reduction of the permeate flux [111].

6.4 Development of thermally driven membranes

121

The applications of MD for nutrient removal and recovery have been investigated by many authors since 2006. At the experimental scale, several MD configurations have been tested [112–115]. The operating parameters impacting the performances of MD include pH, temperature, and cross-flow velocity in the turbulent regime. High pH in the feed solution can increase the amount of volatile ammonia compounds, so that the migration of ammonia to the permeate side is improved as well as its accumulation. When the pH of the feed solution exceeds 8.5, there are negligible changes in the amount of ammonia permeated [112–115]. MD processes use high temperature to increase the volatility of ammonia compounds that easily accumulate in the permeate side [115, 116]. A higher feed temperature may enhance the concentration polarization, however, which leads to the crystallization of soluble substances near or on the membrane surface that can accumulate and cause membrane fouling [117]. Increasing the feed flow rate could facilitate the recovery of volatile ammonia in the permeate due to a heat and mass transport increase [113]. Kim et al. [118] studied the removal of nutrients from the digestate after anaerobic digestion at different operating conditions to observe the fouling mechanisms. They noticed that the flux decline occurred more rapidly when the pH of the feed solution increased from 7 to 8.5 at which the ammonium ion can be transformed. More than 99% COD and TP rejection were achieved regardless of the distillation conditions and processing time. The rejection rate of total nitrogen was dominantly affected by the extent of the cake layer formed on the membrane surface and the subsequent pore blocking, rather than by the pH and temperature of the feed solution. That agrees with the work of Tun et al. [119], who attempted to determine the most appropriate membrane material and to find the optimal conditions (flow velocity of 31.75 cm/s, 40°C temperature difference, and basic pH) for the best dewatering performance of ammonia. They found that the total amount of transferred ammonia and acclimated water volume on the permeate side for all membranes tested increased as the temperature rose from 40°C to 70°C. The PTFE/PP (PTF045LD0A) membrane exhibited successful total ammonia nitrogen enrichment in the feed solution. This might be due to the lower membrane thickness and higher LEP value. The ratio of transferred ammonia, however, was low at high temperature because the fouled layer on the membrane blocked water molecules more easily than ammonia molecules due to the higher dipole moment of water. Khan and Nordberg [120] used an AGMD process made of PTFE flat sheet membranes supported by PP with a porosity of 80% and a thickness of 0.2 mm for concentrating nutrients and recovering process water from complex digestate reject water. There was no leak, wetting, or fouling of the membrane, but some permeate flux reduction was observed. They noticed, however, some phosphate mineral precipitation occurring on the membrane surface. Yan et al. [121] applied DCMD to treat anaerobic digestion effluent for both pollutant removal and nutrient concentration. The acidification of the anaerobic digestion effluent considerably increased the nutrient concentration; the recovery of ammonia and phosphate reached 47% and 31%, respectively. This confirms that high velocity, high temperature, and high pH affect the recovery efficiency, especially the effect of ammonia vapor. It can be concluded that the optimum temperature difference should be 40°C, the velocity should be the laminar regime, and the pH should be around 7.5. The operating parameters have an effect on the enrichment of nutrients, as does the membrane material. Khumalo et al. [122] studied urine removal by adding methyl-functionalized silica nanoparticles on a PVDF and PTFE membrane in a DCMD configuration. A higher concentration of PTFE showed higher removal efficiency due to the high dense porous spongy structure of the membrane cross-section. Table 6.3 summarizes some MD characteristics (of which most of them are in a DCMD configuration) and performances. PTFE membranes appeared to be the most appropriate for ammonia recovery as they have low thickness and pore size. Membrane pore size should be 0.22–0.45 μm. Membrane thickness is inversely proportional to the NH3 mass transfer.

Table 6.3 Summary of commercial MD process applications and operating conditions for nutrient removal and recovery. Type of membrane (materials)

Pore size (μm)

Hydrophobicity/ contact angle

Human urine

PTFE/PP PTFE/PP PVDF

0.45

138

Digestate produced from the anaerobic digestion of livestock wastewater Anaerobic digestion effluent

PVDF

0.22

118

PVDF

0.45

Digestate sludge reject water

PTFE

Human urine

Anaerobic digestate from food and dairy farm waste

Wastewater

Operation condition/flow rate

% removal % recovery

N concentration (mg/L)

P concentration (mg/L)

Type of fouling

References

Across flow velocity of 31.75 cm/s ΔTi ¼ 40°C 0.18 m/s ΔTi ¼ 40°C

N/A

7900 1.22  0.04 gN/L

1100

N/A

[119]

P, 99.4% removal N, 99.7% removal

741.8  22.9

31  33

Cake layer on the membrane surface

[118]

132

135 mL/min ΔTi ¼ 30°C

377  2.83

45.45  1.33

Crystallization /scaling on the surface

[121]

0.22

N/A

1870

N/A

Precipitation of phosphate minerals on the membrane surface

[120]

PVDF/PTFE/ fMSNs

N/A

115.5

Flow rate 1.8–2.3 L/min and 2 L/min for the hot and cold side ΔTi ¼ 36–40°C 0.15 m/s ΔTi ¼ 30°C

P, 37% recovery N, 41% recovery Removal efficiency of NH4-N exceeded 99%

95% ammonia removal

N/A

N/A

[122]

Polypropylene

0.59

N/A

Water, 75% recovery 70% recovery of ammonium and phosphate for liquid fertilizer

1200

18,000

Deposition of organic and crystallization of salt in the membrane pores Ammonium and organic volatile accumulation on membrane surface

8 cm/s ΔTi ¼ 40°C

[117]

6.5 Hybrid processes and new membrane system trends

123

6.5 Hybrid processes and new membrane system trends Hybrid membrane processes are the combination of different membrane processes operating simultaneously for nutrient recovery. The mechanisms and processes of membrane hybrids and their application will be presented in this section. Hybrid FO/MDs and hybrid MBR are the most commonly used technologies to recover nutrients. Volpin et al. [123] used an FO/MD hybrid system for dewatering and concentrating urine for nutrient recovery. The FO process was made of a flat sheet PA TFC while the MD process included both PVDF and PTFE membranes. MD was used as a pretreatment to increase the overall nitrogen rejection of urine wastewater and to prevent MD membrane wetting. Urea showed higher solute permeability and lower rejection compared to NH3 and NH4 due to the higher solute transport coefficient, lower molecular weight, and higher diffusivity over urea. Diffusivity induced urea and NH+4 accumulation in the draw solution, which had the consequence of increasing the NH3 vapor pressure. A high concentration of gaseous NH3 would significantly increase the nitrogen transport to the produced water. As discussed above, however, the thermal pressure-driven membrane is affected by the temperature, pH, and concentration of the feed. Membrane scaling occurred due to carbonate and phosphate precipitation on the membrane active layer with the formation of urine complex coprecipitant at pH above 8. Membrane thickness was found to be the dominant factor impacting the ammonia flux and was found to be proportional to the NH3 mass transfer [123]. Both Qiu and Ting [124] and Holloway et al. [125] worked on an osmotic membrane bioreactor consisting of an FO membrane and a biological process for ammonium recovery. Nutrients can be enriched within the bioreactor and then recovered by chemical precipitation. There is no need to add mineral salts as precipitators for the subsequent chemical nutrient recovery because Mg2+ and Ca2+ ions are also rejected and then accumulated within the bioreactor [126]. Good ammonia rejection was obtained by FO, and membrane fouling was reduced by microbe particles [127] and by cation adsorption with flocs, which prevented ammonia precipitation on the surface [125]. Even though AnMBRs were able to concentrate nutrients due to their tolerance to high OLRs, low fouling, and high P and N removal/recovery efficiency, active studies on hybrid AnMBRs are constantly being pursued. Hybrid complementarity between AnMBR and MD has been used for carbon, trace elements, and ammonium nitrogen removal [128, 129]. More than 90% removal was reported and the removal mechanism was described to be ammonium transportation through the MD membrane via ammonia evaporation. In addition, nutrient removal in hybrid MBRs was achieved by high retention time, pH, and temperature, but free ammonia present in the acidic condition reduces nutrient recovery as struvite [63]. Hou et al. [130] installed a microbial electrochemical unit into an anaerobic osmotic membrane bioreactor (AnOMBR) to form a microbial recovery cell (MRC)-AnOMBR system. Electricity derived from the MRC was used to extract nutrients and mineral salts from the AnOMBR bulk solution, which were then driven to a separated solution to be recovered. Overall, 65%, 45%, and more than 93% of phosphate, ammonium ions, and organics were recovered, respectively. Later, bioelectrochemical systems (BES) were developed for nutrient removal/recovery. BES is a cation-exchange membrane that uses electrodes (anode and cathode) separated in different chambers. Microbial fuel cells (MFC) and microbial electrolysis cells are the most studied BES. Zhang et al. [131] used a bioelectrochemical prototype system for simultaneous removal of nitrogen and phosphorus from synthetic wastewater. They found that the current generation played a central role in the removal

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Chapter 6 Application of polymer-based membranes

process, and applying a higher current generation supported by an external voltage source could improve the removal efficiency. Nitrogen was removed by direct current-driven ammonium migration, and phosphorus was removed via ion exchange (IE) with hydroxide ions generated in the cathode. Phosphorus removal and recovery in a BES require a high pH in the cathode to precipitate phosphorus compounds. The effect of ion transportation, pH, and initial ammonia concentration affects the BES performances as follows [6]: 1. Ions are transported according to the following order: NH4 +  Na+ > K+ > Ca2+ ≫ Mg2+. That affects the nutrient accumulation and recovery. 2. Na+ diffusion is possible even though there could be an Na+ unbalanced concentration between the anode compartment and the cathode compartment resulting from pH control. 3. The higher the ammonium concentration, the higher the transport of cations to the cathodes. Mohammed and Ismail [132] used a sequencing biocathodic MFC tubular type on a MFC, aerobic bioreactor, and anoxic bioreactor, the so-called MFC-AB-ANB system. It was continuously operated and presented more than 98% ammonia and COD removal with a clean renewable energy production of 165.22 mW/m2. In addition, this hybrid process presented low fouling due to the reduction of the extracellular polymeric substance-protein (EPSp)/extracellular polymeric substancepolysaccharide (EPSc) ratio and a decrease of the sludge viscosity caused by the increase of applied voltage [133]. Physiochemical processes coupled with AnMBR can increase the IE for nitrogen recovery [66]. Even though this system shows relatively low NH4-N removal performance (i.e., 37.5%) because of the selectivity of IE for Ca2+ and mg2+ over ammonium, it can achieve high energy recovery: 0.38kWh/m3 wastewater from the combination of the energy produced via anaerobic digestion of preconcentrated organics (0.26 kWh/m3 wastewater) and the energy saved via nitrogen reuse (0.12 kWh/m3 wastewater). Recent trends consist of combining adsorptive membranes and membrane capacitive deionization (MCDI). This novel technology is based on the adsorption and electric double layer (EDL) principle and capacitive charge storage [134]. The electrosorption mechanisms induced by this technology are based on the following principle: ions are absorbed and stored in diffuse double layers between the electrodes and the solution interface by electrostatic interactions after an electrical potential has been applied to the electrode. EDL is formed on the electrode surface and acts as a capacitor with properties depending on the solution concentration. MCDI cycle operations were performed to compare the effects of the optimized MCDI pretreatment on IE. Ammonium recovery of 63% was found for the whole MCDI + IE process. The membrane used in these processes needs special functional groups on the surfaces, including – COOH, –SO3H, and –NH2 groups posed or doped on adsorptive membranes that can adsorb contaminant ions through surface complexation or ion-exchange mechanisms [135].

6.6 Future perspectives Polymeric membranes are used for nutrient removal and recovery, and are still under constant R&D. Many membrane technologies were developed as alternatives to conventional processes: activated sludge, chemical precipitation, etc. Membrane technologies including pressure-driven

References

125

force, osmotically driven force, and thermally driven force were first developed and later the hybrid system was introduced. Adsorptive membranes and MCDI have recently attracted the attention of researchers. Investigations are currently been carried out to develop membranes combining filtration, IE, and adsorption properties using substances such as zeolite, clay, etc. The combination with ceramic materials (adsorbent) could help increase the membrane strength and adsorption capacity. The binding instability of these materials onto the polymer matrix, however, needs to be overcome.

6.7 Conclusion For many years, a tremendous amount of energy has been wasted to treat nitrogen and phosphorus contained in wastewater, neglecting the existing potential for recovering those essential agricultural compounds and offsetting the cost of producing fertilizers. The growing awareness of the negative effects of intensive agricultural practices, global warming, and increasing energy and water demands has influenced researchers and industries to look for alternative solutions such as struvite precipitation or membrane technologies, not only to remove those nutrients but also to recover and valorize them. Since their introduction in the past decade for nutrient removal and recovery, membrane performances have continuously improved. Several membrane technologies have been developed (MBR, FO, MD, hybrid membrane), and for each, several configurations and multiple materials with different properties such as pore size, hydrophilicity, thickness, etc., have been tested. Much effort has been invested to determine the optimum operating conditions leading to the maximum nitrogen and phosphorus recovery. Currently, the most effective and most used membrane technologies for this purpose are the MD processes. Nutrient removal can reach values higher than 90% while maximum recoveries are around 75%. Although the performances of membranes for nutrient removal and recovery are comparable to conventional processes, progress remains to be made to decrease fouling problems, maintenance and capital costs, energy consumption, cleaning, etc. Higher recovery could be achieved by improving membrane materials and properties, or by combining different technologies.

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[105] M. Khayet, P. Godino, J.I. Mengual, Nature of flow on sweeping gas membrane distillation, J. Membr. Sci. 170 (2) (2000) 243–255, https://doi.org/10.1016/S0376-7388(99)00369-5. [106] C.A. Rivier, M.C. Garcı´a-Payo, I.W. Marison, U. Von Stockar, Separation of binary mixtures by thermostatic sweeping gas membrane distillation: I. Theory and simulations, J. Membr. Sci. 201 (1–2) (2002) 1–6, https://doi.org/10.1016/S0376-7388(01)00648-2. [107] A.S. J€onsson, R. Wimmerstedt, A.C. Harrysson, Membrane distillation—a theoretical study of evaporation through microporous membranes, Desalination 56 (1985) 237–249, https://doi.org/10.1016/0011-9164(85) 85028-1. [108] S. Bandini, C. Gostoli, G.C. Sarti, Separation efficiency in vacuum membrane distillation, J. Membr. Sci. 73 (2–3) (1992) 217–229, https://doi.org/10.1016/0376-7388(92)80131-3. [109] G.C. Sarti, C. Gostoli, S. Bandini, Extraction of organic components from aqueous streams by vacuum membrane distillation, J. Membr. Sci. 80 (1) (1993) 21–33, https://doi.org/10.1016/0376-7388(93) 85129-K. [110] T.Y. Cath, D. Adams, A.E. Childress, Membrane contactor processes for wastewater reclamation in space: II. Combined direct osmosis, osmotic distillation, and membrane distillation for treatment of metabolic wastewater, J. Membr. Sci. 257 (1–2) (2005) 111–119, https://doi.org/10.1016/j.memsci.2004.07.039. [111] M. Khayet, T. Matsuura, Membrane Distillation: Principles and Applications, Elsevier, 2011. https://doi. org/10.1016/B978-0-12-815818-0.00003-5. [112] Z. Ding, L. Liu, Z. Li, R. Ma, Z. Yang, Experimental study of ammonia removal from water by membrane distillation (MD): the comparison of three configurations, J. Membr. Sci. 286 (1–2) (2006) 93–103, https:// doi.org/10.1016/j.memsci.2006.09.015. [113] M.S. El-Bourawi, M. Khayet, R. Ma, Z. Ding, Z. Li, X. Zhang, Application of vacuum membrane distillation for ammonia removal, J. Membr. Sci. 301 (1–2) (2007) 200–209, https://doi.org/10.1016/j. memsci.2007.06.021. [114] O. Thygesen, M.A. Hedegaard, A. Zarebska, C. Beleites, C. Krafft, Membrane fouling from ammonia recovery analyzed by ATR-FTIR imaging, Vib. Spectrosc. 72 (2014) 119–123, https://doi.org/10.1016/j. vibspec.2014.03.004. [115] A. Zarebska, D.R. Nieto, K.V. Christensen, B. Norddahl, Ammonia recovery from agricultural wastes by membrane distillation: fouling characterization and mechanism, Water Res. 56 (2014) 1–10, https://doi.org/ 10.1016/j.watres.2014.02.037. [116] Z.P. Zhao, L. Xu, X. Shang, K. Chen, Water regeneration from human urine by vacuum membrane distillation and analysis of membrane fouling characteristics, Sep. Purif. Technol. 118 (2013) 369–376, https:// doi.org/10.1016/j.seppur.2013.07.021. [117] U. Rao, R. Posmanik, L.E. Hatch, J.W. Tester, S.L. Walker, K.C. Barsanti, D. Jassby, Coupling hydrothermal liquefaction and membrane distillation to treat anaerobic digestate from food and dairy farm waste, Bioresour. Technol. 267 (2018) 408–415, https://doi.org/10.1016/j.biortech.2018.07.064. [118] S. Kim, D.W. Lee, J. Cho, Application of direct contact membrane distillation process to treat anaerobic digestate, J. Membr. Sci. 511 (2016) 20–28, https://doi.org/10.1016/j.memsci.2016.03.038. [119] L.L. Tun, D. Jeong, S. Jeong, K. Cho, S. Lee, H. Bae, Dewatering of source-separated human urine for nitrogen recovery by membrane distillation, J. Membr. Sci. 512 (2016) 13–20, https://doi.org/10.1016/j. memsci.2016.04.004. ˚ . Nordberg, Membrane distillation process for concentration of nutrients and water recovery from [120] E.U. Khan, A digestate reject water, Sep. Purif. Technol. 206 (2018) 90–98, https://doi.org/10.1016/j.seppur.2018.05.058. [121] Z. Yan, K. Liu, H. Yu, H. Liang, B. Xie, G. Li, F. Qu, B. van der Bruggen, Treatment of anaerobic digestion effluent using membrane distillation: effects of feed acidification on pollutant removal, nutrient concentration and membrane fouling, Desalination 449 (2019) 6–15, https://doi.org/10.1016/j. desal.2018.10.011.

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[122] N. Khumalo, L. Nthunya, S. Derese, M. Motsa, A. Verliefde, A. Kuvarega, B.B. Mamba, S. Mhlanga, D. S. Dlamini, Water recovery from hydrolysed human urine samples via direct contact membrane distillation using PVDF/PTFE membrane, Sep. Purif. Technol. 211 (2019) 610–617, https://doi.org/10.1016/j. seppur.2018.10.035. [123] F. Volpin, L. Chekli, S. Phuntsho, N. Ghaffour, J.S. Vrouwenvelder, H.K. Shon, Optimisation of a forward osmosis and membrane distillation hybrid system for the treatment of source-separated urine, Sep. Purif. Technol. 212 (2019) 368–375, https://doi.org/10.1016/j.seppur.2018.11.003. [124] G. Qiu, Y.P. Ting, Direct phosphorus recovery from municipal wastewater via osmotic membrane bioreactor (OMBR) for wastewater treatment, Bioresour. Technol. 170 (2014) 221–229, https://doi.org/10.1016/ j.biortech.2014.07.103. [125] R.W. Holloway, A.S. Wait, A.F. da Silva, J. Herron, M.D. Schutter, K. Lampi, T.Y. Cath, Long-term pilot scale investigation of novel hybrid ultrafiltration-osmotic membrane bioreactors, Desalination 363 (2015) 64–74, https://doi.org/10.1016/j.desal.2014.05.040. [126] T. Yan, Y. Ye, H. Ma, Y. Zhang, W. Guo, B. Du, Q. Wei, D. Wei, H.H. Ngo, A critical review on membrane hybrid system for nutrient recovery from wastewater, Chem. Eng. J. 348 (2018) 143–156, https://doi.org/ 10.1016/j.cej.2018.04.166. [127] G. Qiu, S. Zhang, D.S. Raghavan, S. Das, Y.P. Ting, The potential of hybrid forward osmosis membrane bioreactor (FOMBR) processes in achieving high throughput treatment of municipal wastewater with enhanced phosphorus recovery, Water Res. 105 (2016) 370–382, https://doi.org/10.1016/j.watres. 2016.09.017. [128] P. Jacob, P. Phungsai, K. Fukushi, C. Visvanathan, Direct contact membrane distillation for anaerobic effluent treatment, J. Membr. Sci. 475 (2015) 330–339, https://doi.org/10.1016/j.memsci. 2014.10.021. [129] X. Song, W. Luo, J. McDonald, S.J. Khan, F.I. Hai, W.E. Price, L.D. Nghiem, An anaerobic membrane bioreactor–membrane distillation hybrid system for energy recovery and water reuse: removal performance of organic carbon, nutrients, and trace organic contaminants, Sci. Total Environ. 628 (2018) 358–365, https://doi.org/10.1016/j.scitotenv.2018.02.057. [130] D. Hou, L. Lu, D. Sun, Z. Ge, X. Huang, T.Y. Cath, Z.J. Ren, Microbial electrochemical nutrient recovery in anaerobic osmotic membrane bioreactors, Water Res. 114 (2017) 181–188, https://doi.org/10.1016/j. watres.2017.02.034. [131] F. Zhang, J. Li, Z. He, A new method for nutrients removal and recovery from wastewater using a bioelectrochemical system, Bioresour. Technol. 166 (2014) 630–634, https://doi.org/10.1016/j. biortech.2014.05.105. [132] A.J. Mohammed, Z.Z. Ismail, Slaughterhouse wastewater biotreatment associated with bioelectricity generation and nitrogen recovery in hybrid system of microbial fuel cell with aerobic and anoxic bioreactors, Ecol. Eng. 125 (2018) 119–130, https://doi.org/10.1016/j.ecoleng.2018.10.010. [133] A. Ding, Q. Fan, R. Cheng, G. Sun, M. Zhang, D. Wu, Impacts of applied voltage on microbial electrolysis cell-anaerobic membrane bioreactor (MEC-AnMBR) and its membrane fouling mitigation mechanism, Chem. Eng. J. 333 (2018) 630–635, https://doi.org/10.1016/j.cej.2017.09.190. [134] H. Sakar, I. Celik, C. Balcik-Canbolat, B. Keskinler, A. Karagunduz, Ammonium removal and recovery from real digestate wastewater by a modified operational method of membrane capacitive deionization unit, J. Clean. Prod. 215 (2019) 1415–1423, https://doi.org/10.1016/j.jclepro. 2019.01.165. [135] M.R. Adam, M.H. Othman, R.A. Samah, M.H. Puteh, A.F. Ismail, A. Mustafa, M.A. Rahman, J. Jaafar, Current trends and future prospects of ammonia removal in wastewater: a comprehensive review on adsorptive membrane development, Sep. Purif. Technol. (2018), https://doi.org/10.1016/j. seppur.2018.12.030.

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Further reading [136] H. Chai, Y. Xiang, R. Chen, Z. Shao, L. Gu, L. Li, Q. He, Enhanced simultaneous nitrification and denitrification in treating low carbon-to-nitrogen ratio wastewater: treatment performance and nitrogen removal pathway, Bioresour. Technol. 280 (2019) 51–58, https://doi.org/10.1016/j.biortech.2019.02.022. [137] H. Chang, Q. Fu, N. Zhong, X. Yang, X. Quan, S. Li, J. Fu, C. Xiao, Microalgal lipids production and nutrients recovery from landfill leachate using membrane photobioreactor, Bioresour. Technol. 277 (2019) 18–26, https://doi.org/10.1016/j.biortech.2019.01.027. [138] M.E. Christiaens, K.M. Udert, J.B. Arends, S. Huysman, L. Vanhaecke, E. McAdam, K. Rabaey, Membrane stripping enables effective electrochemical ammonia recovery from urine while retaining microorganisms and micropollutants, Water Res. 150 (2019) 349–357, https://doi.org/10.1016/j.watres.2018.11.072. [139] X. Ma, Y. Li, H. Cao, F. Duan, C. Su, C. Lu, J. Chang, H. Ding, High-selectivity membrane absorption process for recovery of ammonia with electrospun hollow fiber membrane, Sep. Purif. Technol. 216 (2019) 136–146, https://doi.org/10.1016/j.seppur.2019.01.025. [140] A.L. Prieto, H. Futselaar, P.N. Lens, R. Bair, D.H. Yeh, Development and start up of a gas-lift anaerobic membrane bioreactor (Gl-AnMBR) for conversion of sewage to energy, water and nutrients, J. Membr. Sci. 441 (2013) 158–167, https://doi.org/10.1016/j.memsci.2013.02.016. [141] M. Qasim, M. Badrelzaman, N.N. Darwish, N.A. Darwish, N. Hilal, Reverse osmosis desalination: a stateof-the-art review, Desalination 459 (2019) 59–104, https://doi.org/10.1016/j.desal.2019.02.008. [142] L. Setiawan, R. Wang, S. Tan, L. Shi, A.G. Fane, Fabrication of poly (amide-imide)-polyethersulfone dual layer hollow fiber membranes applied in forward osmosis by combined polyelectrolyte cross-linking and depositions, Desalination 312 (2013) 99–106, https://doi.org/10.1016/j.desal.2012.10.032. [143] L. Shi, S. Xie, Z. Hu, G. Wu, L. Morrison, P. Croot, H. Hu, X. Zhan, Nutrient recovery from pig manure digestate using electrodialysis reversal: membrane fouling and feasibility of long-term operation, J. Membr. Sci. 573 (2019) 560–569, https://doi.org/10.1016/j.memsci.2018.12.037. [144] Z. Wang, H. Gong, Y. Zhang, P. Liang, K. Wang, Nitrogen recovery from low-strength wastewater by combined membrane capacitive deionization (MCDI) and ion exchange (IE) process, Chem. Eng. J. 316 (2017) 1–6, https://doi.org/10.1016/j.cej.2017.01.082. [145] N. Graham, Guidelines for Drinking-Water Quality, Addendum to Volume 1—Recommendations, World Health Organisation, Geneva, 1998. 36 pp. [146] Y. Ye, H.H. Ngo, W. Guo, Y. Liu, S.W. Chang, D.D. Nguyen, H. Liang, J. Wang, A critical review on ammonium recovery from wastewater for sustainable wastewater management, Bioresour. Technol. (2018), https://doi.org/10.1016/j.biortech.2018.07.111.

CHAPTER

Synthetic polymer-based membranes for the removal of volatile organic compounds from water

7

Nur Hidayati Othmana, Fauziah Marpanib, Nur Hashimah Aliasa, Munawar Zaman Shahruddina, Noor Fauziyah Ishaka, Norin Zamiah Kasim Shaaria Membrane Technology Research Group, Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysiaa Integrated Separation Technology Research Group (i-STRonG), Faculty of Chemical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysiab

Chapter outline 7.1 Introduction ................................................................................................................................ 135 7.2 Pervaporation ............................................................................................................................. 137 7.2.1 PV performance ....................................................................................................... 139 7.2.2 Membranes for PV ................................................................................................... 140 7.3 Membrane distillation .................................................................................................................. 145 7.3.1 Membranes for MD .................................................................................................. 148 7.4 Comparison of PV and MD ............................................................................................................ 150 7.5 Conclusion and future remarks ..................................................................................................... 151 References ........................................................................................................................................ 151

7.1 Introduction Water is an essential substance for humans and other living creatures. As the worldwide population grows, the ability to meet the rising demand for clean water is a challenge. In addition to that, the expansion of industrial activities, global warming, and climate change have become major causes contributing to clean water scarcity in many regions worldwide. One of the major contributors to water pollution is wastewater effluent. Owing to improper wastewater treatment facilities, effluents are often discharged into surface water sources. These pollutants will deteriorate the water quality of the receiving surface Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00007-1 # 2020 Elsevier Inc. All rights reserved.

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water body, making these sources unsuitable for drinking, irrigation, and aquatic life. Therefore, wastewater treatment and recycling systems are essential for fulfilling the increased demand for freshwater [1]. Many industrial wastewaters from petroleum refineries, chemical plants, and groundwater generate volatile organic compounds (VOCs), particularly chlorinated and aromatic hydrocarbons such as benzene, toluene, ethyl benzene, and xylene (BTEX). The discharge of effluents containing VOC contaminants to nearby water bodies is likely to pose serious health impacts, which often go unnoticed [2]. VOCs can be defined as materials that contain carbon atoms and have a high vapor pressure at room temperature [3]. Their high vapor pressure results from a low boiling point that causes large numbers of molecules to evaporate from the liquid or solid form of the compound, which is known as volatility. VOCs often create serious water and air pollution as most of them are toxic and can cause environmental problems. When VOCs are oxidized in the presence of nitrogen oxides, they lead to the formation of photochemical smog, which is harmful to humans, animals, and vegetation and might lead to ozone depletion and global warming. Apart from creating pollution, the emission of these solvents also results in a loss of potential resources and energy. Some of the VOCs are toxic and known to be carcinogenic, teratogenic, or mutagenic, which may cause short- and long-term adverse health effects. Therefore, it is legally compulsory to treat wastewater before discharging it into a water reservoir, such as a lake, ocean, or seawater. VOCs are released into water and air from both anthropogenic and natural emissions. Anthropogenic VOC emissions have been increasing dramatically due to the rapid development of chemical, petrochemical, and allied industrial processes [4]. Industrial waste streams usually contain both aromatic (e.g., benzene, toluene, xylene, etc.) and halogenated (e.g., chloro-ethylene, 2 chloromethane, chloro-ethane, chloroform, etc.) VOCs. Other sources of VOC release include municipal waste, agricultural operations, and vehicular emissions. VOCs of natural sources are produced from fossil fuel deposits, volcanoes, wetlands, oceans, and vegetation. Bacteria also occupy a much higher proportion of total VOC emissions [3]. With the continuous increase of VOC emissions and their harmful impact on human health and the environment, the Goteborg protocol that outlines stringent emission regulations has been proposed. It specifies that the reduction of VOC emissions by 2020 should be half the sum released in 2000 [5]. The stringent governmental regulations on industrial pollution have placed increased pressure on industries to analyze and assess various ways of disposing their waste streams containing VOCs. As a result, continuous efforts to develop effective VOC elimination techniques are of great and urgent significance. The VOC control techniques generally can be divided into recovery and destruction methods based on whether the VOCs can be recovered, as presented in Table 7.1. The recovery methods include distillation, carbon adsorption, air stripping, coagulation, chemical treatment, and membrane separation while the destruction techniques include incineration, photocatalytic oxidation, ozone catalytic oxidation, plasma catalysis, and biological degradation [6]. Destruction methods mainly convert VOCs into CO2 and H2O. This requires vast amounts of energy, owing to the high reaction temperature, while also producing some toxic byproducts such as NOx, O3, OH• radicals, and secondary organic aerosols [3]. Environmental engineers often face difficulty in making design decisions when selecting the proper treatment method, particularly for contaminated wastewater with a low VOC concentration. At present, different processes such as biological treatment, adsorption by activated carbon, and air stripping in packed columns are used at the industrial scale for VOC removal. However, the carbon adsorption process is only economical at a low VOC concentration and it requires frequent regeneration of the carbon bed, making this process unattractive for aqueous feed containing a high VOC concentration [7]. Air stripping also has a limited ability to remove VOCs, especially for low Henry’s constant compounds.

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Table 7.1 The available control techniques for VOC removal. Method Recovery

Destruction

Advantage

Disadvantage

Carbon adsorption

Produce high purity of permeate Suitable for low VOC concentration

Air stripping

Available in packed columns

Coagulation Chemical treatment Membrane separation

Simple technology Less operating time

Costly and consumes large amounts of energy (high operating temperature) Requires frequent regeneration of carbon bed and produces a large volume of byproducts Limited for low Henry’s constant compounds and requires a costly posttreatment Time-consuming process Ecotoxicological effect on the environment

Distillation

Incineration Photocatalytic oxidation Ozone catalytic oxidation Plasma catalysis Biological degradation

Low energy consumption, environmentally friendly, cost-effective Convert VOCs into CO2 and H2O

Prone to fouling and frequent maintenance

High energy consumption due to high reaction temperature Some toxic byproducts produced such as NOx, O3, OH• radicals, and secondary organic aerosols

Membrane technology has been considered an important unit in many industrial processes, particularly for water desalination, wastewater treatment, gas separation, and in the petrochemical industry [8]. The driving force of membrane separation is based on differences in several parameters such as molecule size, concentration, electric charge, solubility between various components in mixtures and membrane materials, and partial pressure [9]. Among the advantages of membrane usage are their proven selectivity and separation potential, low energy consumption, environmentally friendly nature, reliability for remote locations (portable), small footprint, simple operation, and cost-effectiveness as compared to conventional water treatment. In addition, the product stream can be reused to reach higher purity. Synthetic membrane separation processes using polymeric membranes have been widely used in various applications, particularly in water treatment plants [10].

7.2 Pervaporation Pervaporation (PV) is a separation process in which minor components of a liquid mixture are preferentially or selectively transported by partial vaporization through a dense nonporous or permselective (selectively permeable) membrane. This technology is suitable for separating compounds that

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have low water solubilities. PV has been widely investigated for various wastewater treatment applications such as liquid hydrocarbon separation [11–14] and dehydration of alcohol to intensify the esterification reaction [15–17]. In recent years, PV using dense membranes has emerged as a promising remediation method in the removal of VOCs from industrial wastewater or contaminated groundwater [18–21]. These compounds include petroleum-based solvents such as benzene and toluene and chlorinated solvents such as trichloroethylene [22]. The driving force of the PV process is the difference in chemical potential due to the concentration gradient between phases on the opposite side of the membrane (interfacial barrier) [23]. This is not similar as in the case of distillation where the driving force is based on a liquid-vapor balance and volatility. The separation is based on the molecular differences in sorption and the diffusion of various components in the supply feed, which is considered an interesting alternative for separating azeotropic mixes or mixes with similar boiling points [24]. In addition to this, PV has low energy consumption because only the permeating component must be converted in the gas phase [25]. Besides, PV could avoid contamination of the required flow as compared to azeotropic distillation where entrainers or extra components are used to eradicate the azeotrope. PV is believed to be a promising technology in treating dilute VOCs in either groundwater or aqueous effluent. To date, many of those efforts have been focused on developing new membrane materials for targeted VOC removal [16, 26] and pilot-scale PV trials [27]. Fig. 7.1 shows a typical PV system in which the separation of species occurs due to the difference in the feed pressure and a vacuum is applied on the downstream side. PV is the only membrane separation processes that allows a phase transition to occur. Thus, only a minimum amount of energy is required, and it can be supplied in the form of latent heat of vaporization. The PV process is typically carried out by placing a liquid stream, containing organic species, in contact with one side of a nonporous selective membrane while a vacuum is applied to the other side [28]. The liquid will be selectively absorbed and diffused across the polymeric membrane followed by evaporation at the permeate stream [29]. The permeate component is converted into the vapor phase due to the low (partial) vapor pressure on the permeate side. This low vapor pressure is normally achieved by placing a (slight) vacuum that

VOCs wastewater

PV membrane Heater Feed pump

Permeate vapor Vacuum/ (containing VOCs) sweepinggas/ temperature Condenser

Permeate liquid

FIG. 7.1 Pervaporation (PV) system.

Retentate liquid (free from VOCs)

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139

can be supplied by a vacuum pump or by a sweep gas on the permeate side of the membrane. In most cases, the permeate is recondensed. The PV process involves three steps: (i) Selective sorption in the membrane on the feed side. (ii) Selective diffusion through the membrane. (iii) Desorption in the gas phase on the permeate side. The membrane is selected such that it selectively permeates the minor components (VOCs) and retains the major component (aqueous). Because various components in the liquid feed have different affinities for the membrane and different diffusion rates through the membrane, a VOC compound at low feed concentration can have a significantly higher concentration in the vapor phase and will result in a highly concentrated liquid when condensed. There is also an alternative process known as vapor permeation, which is more suited if the feed supply from an earlier process step is in vapor form.

7.2.1 PV performance The performance of PV separation is characterized by two parameters: permeation flux and selectivity. The permeation flux represents the rate of permeation that can be achieved by the membrane and is expressed in terms of the amount of permeate collected (Q) per effective area (A) of the membrane through which the permeant passes per unit of operating time (t). J¼

Q At

(7.1)

The selectivity describes the degree of separation attained in PV. It can be measured by either the separation factor (α) or the enrichment factor (β). Co/Cw represents the concentration of organic and water species, respectively, and the superscripts of P and F designate the permeate and feed in the feed and permeate, respectively. ∝¼

½Co =Cw P ½Co =Cw F

(7.2)

Both J and α are not only based on the intrinsic properties of the membrane used, but also depend on the experimental conditions. A small change in the operating conditions or a slight modification in the membrane could change the results significantly, so it is very difficult to compare the PV results between sets of data obtained under different operating conditions. According to Baker et al. [30], membrane permeability (J), permeance (J/l), and selectivity (α) are considered the best ways to represent the PV data. The overall performance of the PV membrane is also often evaluated in terms of the permeation separation index (PSI), which can be expressed by Eq. (7.3). PSI ¼ Ji ð∝  1Þ

(7.3)

From Eq. (7.3), when α ¼ 1, it is clear that no separation occurs and zero PSI indicates either zero flux or zero separation.

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7.2.2 Membranes for PV PV has been successfully utilized in the dehydration of organic compounds, but still faces a big challenge for VOC removal from water due to the size. VOC molecules are usually much bigger than water, making it more difficult to diffuse across the membrane. The membrane also has the tendency to swell or dissolve in aqueous VOC solutions. In order to adapt this PV technology for VOC removal, significant efforts have been made in the development of a membrane with high separation performance, excellent physicochemical stability, and good solvent resistance. The membrane is considered the heart of PV, and a dense membrane is commonly used where diffusion of species takes place in the free volume present between the macromolecular chains of the polymeric membrane material [24]. In general, hydrophobic membranes are preferable for separating VOCs from dilute solutions because they favor the permeance of the less hydrophilic (organic) compound. Owing to its hydrophobic nature, the membrane only permits the VOC and rejects water molecules [31]. Besides easier separation, it also consumes less energy than separating water from aqueous solutions. Interaction between organic compounds and the membrane can be described by the solubility parameter theory. The solubility parameter can be calculated from three force components during the solubilization process: hydrogen bonding forces (δh), dispersion forces (δd), and polar forces (δp). By comparing the solubility parameters of organic compounds to those of the membrane material, an approximation of the affinity of a solute to the PV membrane can be made [32]. Until now, many PV membranes reported for VOC removal have been based on hydrophobic materials such as polyether-block-polyamide (PEBA), polydimethylsiloxane (PDMS), polyurethane (PU), and natural rubber. However, these conventional PV membranes show a typical “trade-off” relationship between permeability and selectivity, which means that a highly permeable membrane with excellent selectivity is rather difficult to attain. Although many efforts have been made to modify membrane surfaces to reduce fouling, for instance by using several chemical modification techniques such as grafting hydrophilic compounds on the membrane and combining various polymer materials, a satisfactory solution has not been achieved. A recent breakthrough in nanotechnology has widened the range of applications of membrane technologies to improve water treatment. Mixed matrix membranes (MMMs) are advanced membranes that include the dispersion of nanoparticles such as zeolites, silica, carbon molecular sieves (CMS), and graphene into the polymer matrices. They can be utilized for different membrane processes such as gas-gas, liquid-liquid, and liquid-solid separation.

7.2.2.1 Silicone-based membrane Membranes prepared from organophilic polymers are generally employed for the removal of VOCs from aqueous solution. Silicone-containing polymers generally exhibit good organophilicity properties. PDMS, also known as silicone rubber, has an alternating –O–Si–O– unit structure and is considered to be one of the stable materials for PV operation. PDMS membranes exhibit high selectivity and permeability toward organic substances because of their flexible structure [23]. They have been widely investigated for separating many organic-water mixtures such as alcohols, ketones, phenols, hydrocarbons, and chlorohydrocarbons, owing to their exceptional wide-ranging performance in permselectivity, stability, and production cost [19, 33, 34]. A molecular dynamics computer simulation was also conducted to examine important aspects of the PV process for the PDMS membrane [35]. Polytrimethylsilylpropyne (PTMSP) is a high free volume glassy polymer that has gained much attention for selective VOC separation from its aqueous solution, as it exhibits a higher separation factor and

7.2 Pervaporation

141

flux than the PDMS membrane. The PTMSP membrane is known to show preferential permeation of ethanol in the PV of ethanol-water mixtures and the permselectivity is related to the existence of the free volume and to the hydrophobicity of the membrane surface [36]. Claes et al. [37] has prepared a series of silica-filled poly(1-trimethylsilyl-1-propyne) (PTMSP) layers on top of ultrafiltration support membranes. They have used it in the pervaporative separation of an ethanol/water mixture. It was found that the reduction of the thickness of the separating PTMSP top layer and the addition of hydrophobic silica particles resulted in clear flux increases as compared to dense PTMSP membranes. Urugami et al. [38] has systematically investigated the removal of VOCs such as benzene and chloroform in aqueous solutions by using the poly(methylmethacrylate)-poly(dimethylsiloxane) (PMMAg-PDMS), poly(ethylmethacrylate)-PDMS (PEMA-g-PDMS), and poly(n-butylmethacrylate)-PDMS (PBMA-g-PDMS) graft copolymer PV membranes. The permeability and permselectivity for aqueous VOC solutions through their graft copolymer membranes were discussed in detail based on the membrane structure. It was found that the PMMA-g-PDMS and PEMA-g-PDMS membranes showed a good benzene and chloroform permselectivity, but their performances were highly dependent on DMS content. Investigations into the chemical and physical structure of these graft copolymer membranes revealed that their permselectivity was also significantly influenced by the microphase-separated structure [38]. Besides grafting, the addition of 1-allyl-3-butylimidazilium bis (trifluoromethane sulfonyl) imide ([ABIM]TFSI) hydrophobic ionic liquid into the poly(methyl methacrylate)-graft-poly (dimethylsiloxane) (PMMA-g-PDMS) membranes was also found to improve benzene selectivity [39]. The membrane showed high benzene permselectivity when applied for the removal of a trace amount of benzene (0.05 wt%) in water. A hydrophobic composite membrane with a cross-linked poly(dimethylsiloxane)-poly(methyl hydrogen siloxane) (PDMS-PMHS) selective layer was prepared and tested for PV separation of five organic solvent-water mixtures (5 vol% VOC ratio). The VOCs tested were ethanol, methanol, 1-butanol, acetone, and ethyl acetate (EA). The PV and swelling experiments revealed that both the 1-butanol and the EA solutions showed the highest affinity for the composite membrane. The order of increasing permeability and increasing selectivity obtained was methanol, ethanol, acetone, 1-butanol, and EA-water solution [40]. Hosseini et al. [41] prepared a hydrophobic composite membrane of PDMS-poly(methyl hydrogen siloxane) (PDMS-PMHS) for the solvent recovery of dimethylsulfoxide (DMSO) from its aqueous solution. It was found that the PV performance was dependent on the operating temperature, where a higher flux was obtained when the temperature increased from 25 to 70°C. A separation factor of 57 was achieved, even at a 10 wt% feed concentration. In order to improve the organic selectivity of the PDMS membrane, hydrophobic adsorbents such as a CMS, a mesoporous molecular sieve (MCM-41), and zeolite have been incorporated in the membrane. These membranes showed a higher separation factor as compared to the bare membranes [42–44]. It is believed that a capillary driving force is generated when an adsorbent is used in the membrane. The presence of these particles increased the permeation rate of the organic components through the membranes. Wang et al. [45] synthesized MCM-41@ZIF-8 hybrid particles and dispersed them into the PDMS matrix to form a composite membrane for an alcohol permselective PV process. The hydrophobicity and alcohol affinity of the MCM-41@ZIF-8/PDMS membrane was found to improve significantly due to its hierarchical structure. In addition, owing to the porosity of MCM41 and ZIF-8 particles, the flux of the hybrid membrane was significantly improved [45]. Lue et al. [46] prepared a series of 10-μm zeolite/PDMS MMMs for ethanol-water separation. It was found that the ethanol solubility and diffusivity increased as the zeolite loading increased. They compared the

142

Chapter 7 Synthetic polymer-based membranes

prepared membrane with other reported literature values and found that their membranes exhibited the highest ethanol permeability and the third-highest level of selectivity on the 10% ethanol solution among the PDMS-ZSM zeolite (60%) [38] and PDMS-silicalite (60%) membranes. Wang et al. [47] uniformly dispersed a zeolitic imidazolate framework (ZIF-7) into PDMS MMMs and used this for butanol recovery from aqueous solution. The PV performance revealed that the ZIF-7/ PDMS MMMs had better flux and the highest separation factor occurred at 20 wt% ZIF-7 loading when separating 1 wt% butanol aqueous solution at 60°C. The enhanced flux may result from the enlarged free volume in the polymer matrix caused by the incorporation of ZIF-7 nanoparticles. In addition, the superhydrophobic ZIF-7 pore channels and butanol preferential permeation may contribute to the improvement of the separation factor. The ZIF-7/PDMS membrane also remains intact during the 240 h continuous operation, which is the key to industrial applications for membrane-based separation processes. PDMS MMMs were developed using hollow spheres (HS) covered with a shell of silicalite-1 crystals [48]. The membrane permeability increased significantly because the hollow core of the HS allows a very fast flow of the permeating compound. Moreover, the zeolitic shell improves the ethanol selectivity through its specific pore structure and hydrophobicity while additional cross-linking of the HS with the PDMS matrix further increases the selectivity of the polymer matrix. For ethanol recovery, Huang et al. [49] fabricated polyphosphazene nanotube (PZSNTs)/PDMS nanocomposite membranes. The PZSNTs/PDMS membranes exhibited a higher separation factor for the water-ethanol mixture compared with PDMS membranes. The enhanced separation performance in the presence of PZSNT is mainly due to the strong affinity toward ethanol, compatibility with the polymer matrix, and reduced diffusion resistance. The nanotubes with smaller diameters showed a better flux and separation factor (SF) as the interface surface increased. As the PZSNT content increased, the selectivity increased and the permeation flux increased at first to a maximum value and then remained the same. The PV performances for VOC removal from water using PDMS-based membranes are listed in Table 7.2.

Table 7.2 PV performances using PDMS-based membranes. Membrane type

VOCs

Cfeed

Top (°C)

Flux (g/m2 h)

Selectivity

References

PDMS flat sheet PDMS flat sheet PDMS hollow fiber PDMS flat sheet PDMS flat sheet VTES-silicalite-1/ PDMS 60 wt% VTES-silicalite1/PDMS PDMS-PMHS PDMS/silicate membrane ZIF-7/PDMS

Acetone Toluene Toluene Methanol Ethanol Methanol

45,000 ppm 92 ppm 500 ppm 0.3–3 wt% 0.3–3 wt% 10.51 wt%

50 30 32 30 30 65

750 10 44 370–560 520–900 5346

55 2320 4500 8 2 10

[50] [51] [52] [33] [33] [21]

Ethanol

5.3 wt%

50

140

30

[53]

DMSO Ethanol

10 wt% 5 wt%

70 30

565 110

57 37

[41] [54]

Butanol

1 wt%

60

1689

66

[47]

7.2 Pervaporation

143

7.2.2.2 Nonsilicone membrane The use of nonsilicone synthetic membranes for PV applications has also been reported in recent years. A polyetherimide block polymer (PEBA) is one of the most investigated materials used for the extraction of alcohol and hydrocarbons (aromatic). PEBA is a block copolymer of polyether that has a flexible backbone and a low glass transition temperature separated by polyamide blocks. This polymer was investigated for various separations of toluene, alcohol, dichloromethane, etc., from water [51, 55, 56]. Generally, membranes made from PEBA showed lower selectivity for alcohols but higher selectivity for aromatic compounds as compared to PDMS membranes. However, higher fluxes can be obtained during the PV of alcohols through PEBA membranes. Gross and Heintz [57] studied isotherms of aromatic compounds in organophilic PEBA membranes used in PV, and found that the sorption of phenol, aniline, 2-chlorophenol, 4-nitrophenol, 2,4dinitrophenol, 4,40 -isopropylidenediphenol (bisphenol A), and pyridine decreased with temperature. In addition, the number of free hydroxyl groups plays a certain role for the affinity of the aromatic compounds for the membrane material PEBA. Polyether block amide (PEBA) membranes were investigated for PV separation of isopropanol-water and ethyl butyrate-water mixtures [58]. It was shown that under the same operating conditions, the PV separation of the aqueous ethyl butyrate solution was more efficient than the separation of the aqueous isopropanol solutions. It was observed that an increase in temperature resulted in an increase in permeation flux and a reduction in the separation factor. Mujiburohman and Feng [59] utilized PEBA membranes for the separation of propyl propionate/water mixtures. It was observed that the permselectivity of the membrane for propyl propionate/water separation was mainly derived from its sorption selectivity due to the organophilicity of the membrane. The diffusivity of pure propyl propionate in the membrane was about 28 times higher than pure water diffusivity. A PEBA-2533 membrane has also been used for the separation of acetone-butanol-ethanol from dilute aqueous solutions by PV [60]. The separation of binary acetone-water, n-butanol-water, and ethanolwater mixtures by the membranes was carried out using a relatively thick (100 μm) membrane to evaluate the membrane permselectivity, which was found to be in the order of n-butanol > acetone > ethanol. The effects of the boundary layer became significant when a thinner membrane (30 μm) was used, especially for n-butanol separation. In addition, the separation of quaternary acetone-n-butanol-ethanol-water mixtures was also studied. The compatibility of polyether block amide (PEBA) and PU was investigated for phenol wastewater (0.1 wt% phenol aqueous solutions at 308.15 K). It was observed that the separation factor was 9.7 and the permeation flux was 84.1 g m2 h1 for 50 wt% PEBA/PU [61]. Tan et al. [62] used MCM-41 filled PEBA MMMs for the separation of n-butanol/water. The MMMs were observed to significantly improve the permeation flux and were almost equivalent to the n-butanol/water separation factor, which agrees well with the simulation results. Meanwhile, the incorporation of ZIF-8 particles into the PEBA-2533 matrix was also found to improve the membrane stability and enhance the phenol permselective PV due to the hydrophobic nature of ZIF-8. As compared to a pure PEBA-2533 membrane, the permeate flux of the ZIF-8-PEBA-2533 MMM with a 10 wt% loading amount increased from 846 to 1310 g m2 h1 while the separation factor increased from 39 to 53 at 70°C with an 8000 ppm feed of phenol [63]. The incorporation of ZSM-5 zeolite into PEBA membranes was also studied during pervaporative separation of n-butanol aqueous solution [64]. A 5% ZSM-5-PEBA membrane showed enhanced selectivity and flux due to the preferential adsorption and diffusion of n-butanol in the polymer matrix and zeolite channel. As the zeolite content reached 10%, zeolite agglomeration in the

144

Chapter 7 Synthetic polymer-based membranes

membranes was observed. The incorporation of ZSM-5 led to a decrease in the n-butanol flux activation energy of the composite membrane. ZSM-5/PEBA membranes were also used for the separation of EA from aqueous solution, where the membranes demonstrated high EA permselectivity with the increase of ZSM-5 loading. The best PV separation factor and total flux of a 10 wt% ZSM-5/PEBA membrane were 185.5 and 199.5 g2 h1, respectively, with a feed concentration of 5 wt% EA at 50°C [65]. Three-component MMMs based on PEBA were prepared by the incorporation of ZSM-5 zeolite nanoparticles and [Hmim] [PF6] ionic liquid for the separation of EA from aqueous solution using a PV process. The effect of ionic liquid concentration in the intermediate layer on the membrane separation performance was investigated. The results indicated that simultaneous loading of the ZSM-5 and [Hmim][PF6] in the PEBA matrix significantly improved the separation performance of the membrane, as the PSI of the prepared three-component MMM was 1.5 times the PSI of the neat PEBA membrane [66]. The incorporation of graphene oxide (GO) with ionic liquid N-octylpyridiniunm bis (trifluoromethyl) sulfonyl imide [OPY][Tf2N] (IL-GO) into a polyether block amide (PEBA) membrane has been investigated for the PV of butanol aqueous solutions [67]. The connecting ionic liquid to GO can effectively improve the membrane stability by preventing the loss of ionic liquid in the PV process. As ionic liquid [OPY][Tf2N] has good butanol affinity and hydrophobicity, the adsorption selectivity of butanol over water by the IL-modified GO increased by about four times compared with the pristine GO. Compared with a pristine PEBA membrane at 35°C, for 2.5 wt% butanol aqueous solutions, the separation factor and permeation flux of the MMM with 1 wt% content of IL-GO were increased by 31.5% and 18.2%, respectively [67]. The PV performances for VOC removal from water using PEBAbased membranes are listed in Table 7.3. Poly(vinylidene fluoride) (PVDF) is a semicrystalline polymer that has received great attention as a suitable membrane material for PV. It has good processability characteristics for membrane preparation as well as mechanical strength, thermal stability, chemical resistance, and hydrophobicity, which are very important for the actual application of PV membranes. Jian et al. [69] carried out an in-depth investigation on the use of integral asymmetric PVDF membranes for the extraction of a series of sparingly soluble nonpolar (benzene, toluene), moderately polar (chloroform), and polar (ethylacetate, acetone, ethanol, and dioxane) organic solutes in water. It was found that it can effectively separate polar organic feed components such as ethanol and acetone from water, but the separation factors were lower than nonpolar organics. Khayet and Matsura [70] also studied the use of PVDF flat sheet (FS) membranes for chloroform-water mixture separation. Sukitpaneenit and Chung [71] established the fundamental science

Table 7.3 PV performances using PEBA-based membranes. Membrane type

VOCs

Cfeed

Top (°C)

Flux (g/m2 h)

Selectivity

References

PEBA PEBA MCM-41/PEBA ZIF-8/PEBA-2533 ZSM-5/PEBA IL-GO/PEBA

Toluene EA n-Butanol Phenol EA Butanol

125 wt% 8.3 wt% 2.5 wt% 8000 ppm 5 wt% 2.5 wt%

30 30 35 70 50 35

12 260 500 1310 199.5 478.3

700 73 25 53 185 26.7

[51] [68] [62] [63] [65] [67]

7.3 Membrane distillation

145

and engineering of fabricating PVDF asymmetric hollow fiber (HF) membranes for ethanol-water separation and elucidated the complicated relationship among membrane morphology, pore size, pore size distribution, and separation. The PVDF asymmetric HF membranes demonstrate remarkable high fluxes of 3500–8800 g m2 h1 and reasonable ethanol-water separation factors of 5–8. It was found that the permeation flux was mainly controlled by the overall porosity and the contribution of large pore sizes of the membrane. The selectivity was observed to be greatly determined by membrane pore size and pore size distribution, which is consistent with the modified pore-flow model. Recently, many works have been focused on the modification of PVDF membranes for PV applications. The presence of the dispersed inorganic nanoparticles in the membrane structure can improve the membrane performance by enhancing mass transfer in the PV process as well as modifications on selectivity and mechanical properties. Khayet and Matsuura [72] utilized surface-modifying macromolecule (SMM)/PVDF membranes for dilute chloroform/water binary mixtures. The SMMs exhibit an amphipathic structure that consists of a main PU chain terminated with two low-polarity polymer chains (i.e., fluorine segments). The potential of polymers of intrinsic microporosity (PIM-1) for organophilic PV has been investigated for phenol/water mixtures, ethanol/water, butanol/water mixtures, and the removal of VOCs such as EA, dimethyl ether, and acetonitrile from wastewater [73]. Gao et al. [74] prepared PIM-1/PVDF thin-film-composite membranes for 1-butanol/water PV (5 wt%). At 65°C, flux up to 9000 g2 h1 with separation factors up to 18.5 were obtained. Poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) with low crystallinity was also investigated for the separation of EA from its aqueous solutions and the performances were significantly improved as compared to PVDF membranes [75]. Yongquan et al. [76] added 1-butyl-3-methyl-imidazolium-tetrafluoroborate [bmim]BF 4 ionic liquid into the PVDF-HFM membranes. It was found that the inclusion of [bmim]BF 4 in the PVDF-HFP matrix enhanced the diffusion coefficient of EA. The total permeation flux and separation factor of the membrane increased from 433.21 to 655.85 g m2 h1 and 75.75 to 143.88, respectively, as the ionic liquid content increased from 0 to 5 wt%. Carbon black (CB) was also added into the PVDF membranes and used for the removal of 2-propanol from the aqueous solution [77]. It was observed that the addition of CB filler resulted in the formation of membranes with denser structures; lower permeation flux and degree of swelling; and higher crystallinity, separation factor, contact angle, and PV separation index. Recently, silica (SiO2)/PDMS/PVDF composite membranes were prepared for phenol recovery from coal gasification wastewater. The phenol flux and separation factor were obtained as 6.55 g m2 h1 and 2.59, respectively. The results indicated that a separation factor based on 12 wt% SiO2/PDMS/PVDF composite membranes was about 2.5 times higher than that of the PDMS/PVDF membrane. Li et al. [78] tested a zeolite-5 (ZSM-5)/PDMS/PVDF HF composite membrane for phenol recycling from aqueous solutions. It was found that as the ZSM-5 concentration used was 40%, the separation factor and phenol permeability were 4.56 and 5.78 g m2 h1, respectively. The PV performances for VOC removal from water using PVDF-based membranes are listed in Table 7.4.

7.3 Membrane distillation Membrane distillation (MD) is an emerging thermally driven membrane separation process that has been applied extensively to desalination, food processing, and VOC removal from water. The driving forces of the MD process are the vapor pressure difference and the thermal gradient between both sides

146

Chapter 7 Synthetic polymer-based membranes

Table 7.4 PV performances using PVDF-based membranes. Membrane type

VOCs

Cfeed

Top (°C)

Flux (g/m2 h)

Selectivity

References

PVDF flat sheet PVDF hollow fiber PVDF hollow fiber PIM-1/PVDF

Benzene Benzene Ethanol 1Butanol EA EA

45,000 ppm 92 ppm 5 wt% 5 wt%

50 30 50 65

750 10 8800 9000

55 2320 8 18.5

[52] [69] [71] [74]

3 wt% 4 wt%

50 45

1900 655.85

175 143.88

[75] [76]

2Propanol Phenol Phenol

4 wt%

45

4180

61.34

[77]

0.0018 wt% 5000 ppm

50 50

3.9 5.78

3.9 4.56

[79] [78]

PVDF-HFP P(VDF-HFP)/[bmim] BF4 CB/PVDF SiO2/PDMS/PVDF 40 wt% ZSM-5/PDMS/ PVDF

of the membrane pores where the liquid/vapor equilibrium controls the process selectivity [80]. This is rather different from other membrane processes where commonly an absolute pressure difference, a concentration gradient, or an electrical potential gradient commonly drives mass transfer through a membrane. It is an evaporative process that uses hydrophobic porous membranes (microfiltration membranes) to physically separate the hot liquid feed from a cold liquid or gaseous phase. The MD membrane acts as a physical barrier between the two phases and the process selectivity is not really influenced by the membrane characteristics when the membrane has been correctly chosen. Polymeric microporous and hydrophobic membranes are widely used as MD membranes. The membranes are not wetted by aqueous mixtures and act only as support for the vapor-liquid interface, and they do not contribute in the separation performance [81]. Although MD and PV share some common characteristics such as phase change and external permeate condensation, significant differences exist between them. In PV, dense membranes are used instead of hydrophobic microporous ones. In PV, separation is determined by selective sorption and diffusion through the membrane, whereas in MD the separation is determined by the vapor-liquid equilibrium. There are four different modes of MD, depending on how the permeating vapor is recovered at the other side of the membrane: direct contact MD (DCMD), air gap MD (AGMD), sweeping gas MD (SGMD), and vacuum MD (VMD) (Fig. 7.2) [82]. DCMD allows the membrane to be in direct contact with liquid phases. This is the simplest configuration capable of producing reasonably high flux. It is best suited for applications such as desalination and concentration of aqueous solutions (e.g., juice concentrates) [82]. AGMD interposes an air gap between the membrane and a condensation surface and has the highest energy efficiency, although the flux obtained is generally low. The air gap configuration can be widely employed for most MD applications, particularly where energy availability is low. In VMD, the permeate side is vapor under reduced pressure and sometimes the permeate is condensed in a separate device. This configuration is useful when volatiles are being removed from an aqueous solution [83–86]. SGMD uses stripping gas as a carrier for the produced vapor. It is used when volatiles

7.3 Membrane distillation

Pore

Pore Permeate

(A)

DCMD

(B)

AGMD

Pore Pore Permeate-vapor

(C)

VMD

(D)

Sweeping gas

Vacuum

Feed

Membrane

Feed

Hot feed

Cold feed

Membrane Permeate

Feed

Membrane

Cooling plate Membrane

147

Permeate-vapor SGMD

FIG. 7.2 Membrane distillation configurations: (A) direct contact membrane distillation (DCMD); (B) air gap membrane distillation (AGMD); (C) vacuum membrane distillation (VMD), and (D) sweep gas membrane distillation (SGMD) [82].

are removed from an aqueous solution [87, 88]. Because water vapor condensation takes place inside the membrane module, the external condenser is not necessary for AGMD and DCMD configurations, as compared to SGMD and VMD. The VMD separation process is shown in Fig. 7.3. An aqueous feed solution is brought into direct contact with the upstream side of the porous hydrophobic membrane. By warming the feed solution, the water evaporates at the liquid-vapor (L-V) interfaces due to the heat of vaporization. The downstream side of the membrane is maintained under vacuum conditions. The hydrostatic pressure of the feed solution must not exceed the liquid entry pressure of water (LEPw). Owing to the hydrophobicity of the membrane, no liquid penetrates inside the pores and an L-V interface is formed at the membrane pore entrances [90]. Inside the pores, the hydrophobic characteristic of the pore wall are able to inhibit the vapor from condensation. Such a process configuration creates a driving force, that is, the vapor pressure difference between the upstream side and the downstream side of the membrane. The MD process is regarded as an energy-saving technology as it allows the integration of renewable energy sources such as solar energy with an MD module to heat the feed solution [91]. In comparison with conventional techniques, VMD has several advantages, including higher rejections of nonvolatile solutes and the possibility of working at relatively low evaporation temperatures, typically below 50–60°C, allowing low-value energy sources to supply the heat required for the evaporation. Besides, a vacuum is only used at the membrane unit, thus reducing the operation cost while the use of membrane modules makes the plant scale-up straightforward. With reference to the separation costs, VMD is also found to be economically comparable with other membrane alternatives such as PV [83]. However, there were two major factors hindering the development of MD technology. These barriers include the availability of membranes with suitable characteristics at a reasonable cost and the economics of the overall process are not favorable as compared to reverse osmosis.

Chapter 7 Synthetic polymer-based membranes

Temperature sensor

T

T

T

148

Heat exchanger

T Cooler Gear pump

Conductivity sensor

T

Flow meter

Hollow fiber membrane

Vacuum pump

Permeate tank

Conductivity sensor

Feed tank

T

Balance

Heater

Heat exchanger

FIG. 7.3 Schematic diagram of a vacuum membrane distillation (VMD) setup by Yao et al. [89].

7.3.1 Membranes for MD Generally, there are two types of membrane configurations used in MD: HF and FS membranes. HF membranes have relatively large specific surface areas as compared to FS but the flux of the HF module is typically low [46]. This low flux phenomenon is related to poor flow dynamics and the resultant high degree of temperature polarization. The HF configuration allows very high packing density. The feed is introduced into the shell side or into the lumen side of the HFs, and cooling fluid, sweeping gas, or negative pressure can be applied on the permeate side to form VMD, SGMD, or DCMD. Because of their large active area combined with a small footprint, HF modules have great potential in commercial applications [82]. MD uses a porous membrane that allows only water vapor to pass through. The process separation is not associated with the selective solubility of the organic components in the membrane material. Because the nonwettability of the membrane is the basic requirement for the process, a hydrophobic membrane can be used for the extraction of a broad variety of VOCs from aqueous mixtures. As the membrane is a contactor between two phases, nonvolatile solutes are 100% retained. The susceptibility to membrane wetting is evaluated based on the liquid entry pressure, which is defined as the pressure at which liquid penetrates through the membrane [92]. The development of VMD membranes still faces some constraints such as membrane pore wetting, high resistance of water vapor flow through the membrane due to the presence of trapped air in the pores, the temperature polarization phenomenon, and high conductive heat loss over the membrane [93–95]. Therefore, extensive research on VMD membranes should be carried out to move this technology forward. Recently, high-flux HF membranes with different features such as dual-layer hydrophilic-hydrophobic fibers [96],

7.3 Membrane distillation

149

triple-layer hydrophilic-hydrophobic fibers [97], deposition of thin electrospun fibers [98], and utilization of nanoparticles such as SiO2, TiO2, and CuO in the membrane matrix have been developed to improve the performance of VMD membranes [99–101]. The most common materials used for MD membranes are poly(tetrafluoroethylene) (PTFE), poly (vinylidenefluoride) (PVDF), and poly(propylene) (PP) with pore sizes ranging from 0.01 to 1 mm. Among these materials, PTFE has the highest hydrophobicity with good chemical and thermal stability and oxidation resistance. However, it also possesses the highest conductivity that can cause greater heat transfer through PTFE membranes. PP also exhibits good thermal and chemical resistance [33]. The utilization of commercial PP and PTFE membranes for VMD is limited due to their restricted pore size range and porosity as well as their symmetric structures. Alternatively, PVDF has become a new membrane material in recent years not only due to its good hydrophobicity, thermal resistance, and mechanical strength, but also due to the feasibility of forming asymmetric HF membranes via a phase inversion method [102]. The performances of commercial VMDs are listed in Table 7.5. Recently, new membrane materials such as carbon nanotubes, fluorinated copolymer materials, and surface-modified PES have been developed to make MD membranes with good mechanical strength and high hydrophobicity and porosity [92, 107–109]. As an example, 1.5 wt% of nSMM2 was implemented into a PES casting solution to obtain a VMD with high hydrophobicity (contact angle of 100 degrees) and a large mean pore size of 40 nm. PVDF HF membranes with an asymmetric structure have been prepared by the phase inversion method using dimethylacetamide (DMAc) as the solvent and LiCl-H2O as additives. The membranes are applied for VOC removal from water. Under optimal operating conditions, particularly the downstream vacuum level, feed temperature, and feed flow rate, the removal efficiency of trichloroethane from water was up to 97% [106]. Kong et al. [110] modified hydrophilic microporous cellulose nitrate membranes using plasma polymerization of octafluorocyclobutane and sandwiched it between two hydrophobic layers. The prepared MD membranes exhibited good performances similar to hydrophobic PVDF, PTFE or PP membranes. Surface modification of

Table 7.5 VMD performances using commercial-based membranes. Membrane type PTFE PTFE PTFE PTFE PTFE PP PP PVDF PVDF PVDF

VOC

Cfeed (%)

Top (°C)

Flux (g/m2 h)

Acetone EtAc Ethanol Isopropanol Benzene Benzene Ethanol Chloroform Ethanol Trichloroethane

5.0 3.8 5.0 5.0 0.1 0.1 5.0 0.1 5.0 0.062

25 32 35 25 50 50 30 25 30 40–60

15.4–29 0.6–21.6 6.8–20 4.7–12.3 0.03–34.4 0.03–34.4 8.9–9.5 0.64–16.1 13.5–15.1 0.002–0.3

Selectivity

Vacuum pressure (kPa)

References

7.1–2.1 38–5.1 8.8–5.3 15.3–6.1 7417–167 7417–167 7.1–7.9 72.5–7.9 5.6–6.7 1612–18

8.0–1.0 9.5–2 5.3–3.3 7.0–1.0 26.0–2.5 26.0–2.5 n.a. 1.7 n.a. 10.7

[103] [103] [84] [103] [83] [83] [104] [105] [104] [106]

150

Chapter 7 Synthetic polymer-based membranes

hydrophilic flat-sheet membranes of cellulose acetate and cellulose nitrate was also performed by radiation grafting polymerization and plasma polymerization into hydrophobic membranes, which were shown to possess good hydrophobicity required for MD process [106].

7.4 Comparison of PV and MD PV and MD are two promising membrane technology processes to remove VOCs from polluted wastewater. Table 7.6 presents a comparison of PV and MD in terms of a few characteristics. These processes have in common the fact that the permeates are in the vapor phase and the driving force is due to the chemical gradient (concentration for PV while pressure and thermal for MD). The main differences are related to the type of membrane used (porous for MD, but dense for PV), and the separation performances are therefore different. Furthermore, for the case of PV, the choice of appropriate membrane material is of fundamental importance for successful VOC removal. Meanwhile, in the case of MD, the membrane used acts only as a contactor for avoiding the contact between the phases and thus the selected membrane needs to have high stability of pore wetting properties (good hydrophobicity). Therefore, it was very hard to conclude which process is better in terms of VOC removal due to their different separation performances toward unspecific wastewater treatment industries. Mostly, the PV were used for low water solubile compounds in order to perform better separation and efficiency of COV removal. Table 7.6 The comparison of PV and MD.

Membrane type Separation performance Membrane driving force applied Membrane materials Mode

Advantage Drawback Wastewater treatment applications

PV

MD

Dense structure

Porous structure (microfiltration)

Selective sorption and diffusion through

Vapor liquid equilibrium

Concentration gradient

Vapor pressure difference and the thermal gradient

Polyether-block-polyamide (PEBA), polydimethylsiloxane (PDMS), polyurethane (PU), and poly(vinylidenefluoride) (PVDF) Vacuum applied in permeate side

Poly(tetrafluoroethylene) (PTFE), poly (vinylidenefluoride) (PVDF), and poly (propylene) (PP) Direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD), and vacuum membrane distillation (VMD) Low cost and up to 100% rejection of nonvolatile solutes Unstable pore wetting properties

Suitable for separating compounds that have low water solubilities and low energy consumption The performance depends on the membrane materials used Liquid hydrocarbon separation and dehydration of alcohol to intensify esterification reaction

Desalination and food processing

References

151

7.5 Conclusion and future remarks The contamination of ground, surface, and wastewater by VOCs is recognized as a major issue in many countries. The presence of VOCs in rivers, ponds, and water reservoirs will lead to serious detrimental effects to the natural environment and ecosystems. Also, many of the VOCs such as benzene and chlorinated hydrocarbons are known to be carcinogenic and dangerous to human health. Therefore, the development of efficient methods to remove VOCs from wastewater is important to both human health and environmental protection. Conventional separation technologies such as distillation, liquid-liquid extraction, carbon adsorption, and air stripping are not economically suitable for the removal of a trace amount of VOC from water, as they consume a large amount of energy (for distillation) and produce a large volume of byproducts (for carbon adsorption and liquid-liquid extraction) that requires costly posttreatment or generates extra pollution (for air stripping). In recent years, membrane separation processes such as PV and VMD have become of considerable interest for VOC removal from aqueous phases. The technologies have drawn utmost attention due to their high selectivity, low energy consumption, moderate cost-to-performance ratio, and compact modular design of the membrane system. PV is promising as many results reported in the literature indicate that VOCs can be selectively removed from the aqueous feed as almost pure liquid. Besides PV, several versions of MD, particularly VMD, have been tested for VOC removal from water. The energy consumption in the PV and VMD processes is much less than that in distillation processes due to a much lower operating temperature. Although VMD and PV share some common characteristics such as phase change and external permeate condensation, significant differences exist between them, particularly in membrane morphology. A dense membrane is used for PV while a porous membrane is utilized for VMD. The permeation flux and selectivity of VOCs are two important performance parameters for this technology to move forward. However, the flux and selectivity of a solute often negate each other, mainly due to the imperfection of membrane materials in discriminating different species in the solution. Therefore, there is still a need for a continuous effort in finding new membrane materials that are able to overcome the trade-off between high selectivity or high flux for specific VOC removal as well as advanced methods of membrane modification that are simple and cost-effective.

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ee, J. Zhang, J. de Li, M. Duke, J. Gomez, S. Gray, Advances in membrane distillation for water desalination and purification applications, Water (Switzerland) 5 (1) (2013) 94–196, https:// doi.org/10.3390/w5010094. [83] G.C. Sarti, C. Gostoli, S. Bandini, Extraction of organic components from aqueous streams by vacuum membrane distillation, J. Membr. Sci. 80 (1) (1993) 21–33, https://doi.org/10.1016/0376-7388(93)85129-K. [84] S. Bandini, C. Gostoli, G.C. Sarti, Separation efficiency in vacuum membrane distillation, J. Membr. Sci. 73 (2–3) (1992) 217–229, https://doi.org/10.1016/0376-7388(92)80131-3. [85] F.A. Banat, J. Simandl, Removal of benzene traces from contaminated water by vacuum membrane distillation, Chem. Eng. Sci. 51 (8) (1996) 1257–1265, https://doi.org/10.1016/0009-2509(95)00365-7. [86] A. Kujawska, J.K. Kujawski, M. Bryjak, M. Cichosz, W. Kujawski, Removal of volatile organic compounds from aqueous solutions applying thermally driven membrane processes. 2. Air gap membrane distillation, J. Membr. Sci. 499 (2016) 245–256, https://doi.org/10.1016/j.memsci.2015.10.047. [87] M. Khayet, C. Cojocaru, A. Baroudi, Modeling and optimization of sweeping gas membrane distillation, Desalination 287 (2012) 159–166, https://doi.org/10.1016/j.desal.2011.04.070. [88] R. Bagger-Jørgensen, A.S. Meyer, M. Pinelo, C. Varming, G. Jonsson, Recovery of volatile fruit juice aroma compounds by membrane technology: sweeping gas versus vacuum membrane distillation, Innov. Food Sci. Emerg. Technol. 12 (3) (2011) 388–397, https://doi.org/10.1016/j.ifset.2011.02.005. [89] M. Yao, Y.C. Woo, L.D. Tijing, J.S. Choi, H.K. Shon, Effects of volatile organic compounds on water recovery from produced water via vacuum membrane distillation, Desalination 12 (2018) 1259–1264, https://doi.org/10.1016/j.desal.2017.11.012. [90] C.-K. Chiam, A. Ibrahim, R. Sarbatly, Desalination in cross-flow vacuum membrane distillation under the negative membrane pressure difference, J. Appl. Sci. 308 (2014) 186–197, https://doi.org/10.3923/jas. 2014.1259.1264. [91] M.R. Qtaishat, F. Banat, Desalination by solar powered membrane distillation systems, Desalination 440 (2013) 146–155, https://doi.org/10.1016/j.desal.2012.01.021. [92] L. Eykens, K. De Sitter, C. Dotremont, L. Pinoy, B. Van der Bruggen, Membrane synthesis for membrane distillation: a review, Sep. Purif. Technol. 182 (2017) 36–51, https://doi.org/10.1016/j.seppur.2017.03.035. [93] E. Drioli, A. Ali, F. Macedonio, Membrane distillation: recent developments and perspectives, Desalination 356 (2015) 56–84, https://doi.org/10.1016/j.desal.2014.10.028.

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[94] M.A.E.R. Abu-Zeid, Y. Zhang, H. Dong, L. Zhang, H.L. Chen, L. Hou, A comprehensive review of vacuum membrane distillation technique, Desalination 356 (2015) 1–14, https://doi.org/10.1016/j.desal. 2014.10.033. [95] S. Bandini, G.C. Sarti, Heat and mass transport resistances in vacuum membrane distillation per drop, AICHE J. 45 (7) (1999) 1422–1433, https://doi.org/10.1002/aic.690450707. [96] S. Bonyadi, T.S. Chung, Flux enhancement in membrane distillation by fabrication of dual layer hydrophilic-hydrophobic hollow fiber membranes, J. Membr. Sci. 306 (1–2) (2007) 134–146, https://doi. org/10.1016/j.memsci.2007.08.034. [97] J.A. Prince, D. Rana, T. Matsuura, N. Ayyanar, T.S. Shanmugasundaram, G. Singh, Nanofiber based triple layer hydro-philic/-phobic membrane—a solution for pore wetting in membrane distillation, Sci. Rep. 4 (2014) 6949, https://doi.org/10.1038/srep06949. [98] M. Khayet, M.C. Garcı´a-Payo, L. Garcı´a-Ferna´ndez, J. Contreras-Martı´nez, Dual-layered electrospun nanofibrous membranes for membrane distillation, Desalination 426 (2018) 174–184, https://doi.org/10.1016/ j.desal.2017.10.036. [99] J.E. Efome, M. Baghbanzadeh, D. Rana, T. Matsuura, C.Q. Lan, Effects of superhydrophobic SiO2 nanoparticles on the performance of PVDF flat sheet membranes for vacuum membrane distillation, Desalination 373 (2015) 47–57, https://doi.org/10.1016/j.desal.2015.07.002. [100] M. Rezaei, W.M. Samhaber, Wetting behaviour of superhydrophobic membranes coated with nanoparticles in membrane distillation, Chem. Eng. Trans. 47 (2016) 373–378, https://doi.org/10.3303/CET 1647063. [101] M. Baghbanzadeh, D. Rana, T. Matsuura, C.Q. Lan, Effects of hydrophilic CuO nanoparticles on properties and performance of PVDF VMD membranes, Desalination 369 (2015) 75–84, https://doi.org/10.1016/j. desal.2015.04.032. [102] J.E. Efome, D. Rana, T. Matsuura, C.Q. Lan, Enhanced performance of PVDF nanocomposite membrane by nanofiber coating: a membrane for sustainable desalination through MD, Water Res. 89 (2016) 39–49, https://doi.org/10.1016/j.watres.2015.11.040. [103] S. Bandini, A. Saavedra, G.C. Sarti, Vacuum membrane distillation: experiments and modeling, AICHE J. 43 (2) (1997) 398–408, https://doi.org/10.1002/aic.690430213. [104] M.A. Izquierdo-Gil, G. Jonsson, Factors affecting flux and ethanol separation performance in vacuum membrane distillation (VMD), J. Membr. Sci. 214 (1) (2003) 113–130, https://doi.org/10.1016/S0376-7388(02) 00540-9. [105] M. Khayet, T. Matsuura, Preparation and characterization of polyvinylidene fluoride membranes for membrane distillation, Ind. Eng. Chem. Res. 40 (24) (2001) 5710–5718, https://doi.org/10.1021/ie010553y. [106] B. Wu, K. Li, W.K. Teo, Preparation and characterization of poly(vinylidene fluoride) hollow fiber membranes for vacuum membrane distillation, J. Appl. Polym. Sci. 106 (3) (2007) 1482–1495, https://doi.org/ 10.1002/app.26624. [107] K. Gethard, O. Sae-Khow, S. Mitra, Carbon nanotube enhanced membrane distillation for simultaneous generation of pure water and concentrating pharmaceutical waste, Sep. Purif. Technol. 90 (2012) 239–245, https://doi.org/10.1016/j.seppur.2012.02.042. [108] K. Ohta, I. Hayano, T. Okabe, T. Goto, S. Kimura, H. Ohya, Membrane distillation with fluoro-carbon membranes, Desalination 81 (1–3) (1991) 107–115, https://doi.org/10.1016/0011-9164(91)85049-Z. [109] D.E. Suk, T. Matsuura, H.B. Park, Y.M. Lee, Development of novel surface modified phase inversion membranes having hydrophobic surface-modifying macromolecule (nSMM) for vacuum membrane distillations, Desalination 261 (3) (2010) 300–312, https://doi.org/10.1016/j.desal.2010.06.058. [110] Y. Kong, X. Lin, Y. Wu, J. Chen, J. Xu, Plasma polymerization of octafluorocyclobutane and hydrophobic microporous composite membranes for membrane distillation, J. Appl. Polym. Sci. 46 (2) (1992) 191–199, https://doi.org/10.1002/app.1992.070460201.

CHAPTER

Forward osmosis membranes for water purification

8

Pallabi Dasa, Krishna Kant Kumar Singha, Suman Duttab CSIR—Central Institute of Mining and Fuel Research, Dhanbad, Jharkhand, Indiaa Department of Chemical Engineering, Indian Institute of Technology (ISM) Dhanbad, Dhanbad, Indiab

Chapter outline 8.1 Main concept of forward osmosis ................................................................................................. 159 8.2 Applications of FO ....................................................................................................................... 161 8.3 Water purification ....................................................................................................................... 161 8.3.1 Industrial wastewater treatment ................................................................................ 161 8.3.2 Surface water treatment ........................................................................................... 163 8.4 Membrane selection .................................................................................................................... 163 8.4.1 Materials, dimensions, and background ..................................................................... 163 8.4.2 Composite membranes ............................................................................................. 165 8.5 Hybrid processes ........................................................................................................................ 165 8.5.1 FO-RO .................................................................................................................... 165 8.5.2 FO-MD .................................................................................................................... 166 8.5.3 FO-NF and FO-UF .................................................................................................... 166 8.6 Industrial applications/large-scale FO installations ....................................................................... 167 8.7 References ................................................................................................................................. 168

8.1 Main concept of forward osmosis At the core of the process is diffusive transport, wherein the transport of molecules occurs in the direction of a concentration gradient (or concentration difference over a distance). A manifestation of this is osmosis, which involves selective diffusion through a semipermeable or differentially permeable membrane. When a semipermeable membrane (allowing only solvent to pass through) separates two liquids, there is a preferential solvent flow from the system having the higher solvent concentration to the system having the lower solvent concentration. This forms the crux of forward osmosis.

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00008-3 # 2020 Elsevier Inc. All rights reserved.

159

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Chapter 8 Forward osmosis membranes for water purification

Here, there are two solutions with different concentrations separated by a semipermeable membrane. One is a feed (with a higher solvent concentration/lower solute concentration) and the other is a draw solution (with lower solvent concentration/higher solute concentration). When these two solutions are separated by a semipermeable membrane, a concentration gradient is created across the membrane, under the influence of which solvent moves from the region of its higher concentration (feed side) to the region of its lower concentration (draw solution side). Thermodynamically, the different activity coefficients of the solutions linked to their concentrations creates a chemical potential difference between the solutions. Due to this chemical potential difference between the two solutions, the solvent transport aims to establish equilibrium with respect to the chemical potentials. Hypothetically, this solvent transport due to concentration gradient could be prevented by the application of a definite amount of pressure to the feed side to counterbalance the pull to achieve chemical potential equilibrium. This is the osmotic pressure. Osmotic pressure can be expressed quantitatively by the van’t Hoff law as OP ¼ icRT

i—no of ions c—concentration R—real gas constant T—temperature From this, it is clear that every solution can possibly exert some osmotic pressure due to its inherent concentration. Therefore, to prevent the solvent flow, a pressure numerically equal to the osmotic pressure difference of the two solutions needs to be exerted. Another way to picture this is that osmotic pressure aided flow is guided by the solute concentrations, that is, the highest solute concentration will exert the most osmotic pressure. However, the solvent flow will always take place along the chemical potential gradient or simply the concentration gradient path. Now, in the perspective of the process, the function of the draw solution is to artificially exert a high osmotic pressure, such that the solvent can move from the feed side to the draw solution side. This is the principal transport mechanism in forward osmosis; forward because mass transfer takes place along the concentration gradient. Ideally, it does not need any external driving force, but practically, to hasten the speed of the process, a small amount of pressure is applied. Fig. 8.1 provides a brief schematic about the crux of the process. Transport mechanism • Solution diffusion • Convective transport • Sieving

FIG. 8.1 Brief overview of FO.

Membrane structure • Asymmetric • Dense film • Active layer + support layer

Modules

• Plate and frame • Spiral wound • Hollow fiber

8.3 Water purification

161

8.2 Applications of FO The most widespread and industrially adopted application of forward osmosis is in water treatment. But apart from that, the process of forward osmosis possesses numerous applications in diverse fields ranging from fruit juice concentration and crystallization to energy production and drug delivery systems [1]. When the osmotic interface created between two solutions is through a hydrophobic semipermeable membrane, normal solvent flow is impeded, leading to a phenomenon where solvent evaporates under the dual forces of the concentration gradient and the hydrophobic semipermeable barrier. It is precisely this concept that has found application in the processes of crystallization, concentration, and controlled humidification or dehumidification without affecting the quality of the feedstock [2, 3]. The operational flexibility and precise control of the flow rates without applying any external driving force have also led to the development of osmotic pumps for medicinal applications, wherein the concentration difference is utilized to deliver a drug at a precise location or at controlled release rates [4]. In the domain of energy production, forward osmosis is increasingly being researched. A direct spinoff called pressure-retarded osmosis using the feed, membrane, and draw solution combination harnesses the osmotic flow in a fixed volume closed chamber. The solvent onslaught from the feed side builds up pressure, which in turn is used to drive a turbine for producing electricity [5]. In recent years, PRO has been used in dual production for simultaneous water purification and energy production [6]. For this chapter, however, we will focus on the efficacy of forward osmosis for wastewater treatment.

8.3 Water purification 8.3.1 Industrial wastewater treatment Forward osmosis is especially suited for industrial wastewater remediation and reclamation. The majority of industrial wastewaters such as steel plant effluents, pharmaceutical industry wastewater, and tannery effluents are complex matrices with a mixture of organic, inorganic, and toxic components. For these systems, the utility of FO lies in the fact that it promotes dewatering and aims to concentrate the loadings. This not only helps in recycling the water, but also facilitates the chances of value-added product recovery. It has been experimentally demonstrated that FO successfully removes a host of contaminants such as nitrates, sulfates, phosphates, hardness, high levels of suspended solids, CODs, and heavy metals from industrial effluents [7–10]. The operating principle of draw solution-induced dewatering makes it a broad spectrum, robust option for treating such complex streams. It also has successfully removed pharmaceutical and personal care products, organics such as phenolics, and petroleum residues [11, 12]. During the removal of these compounds, the membrane structure of forward osmosis (to be discussed later in this chapter) offers a unique advantage of reversible fouling and lower concentration polarization. For this reason, the flux decline is less and a sustainable flux can be produced, even with bench-scale FO systems. The advent of relatively fouling-free cross-flow FO modules aids greatly in reducing the concentration polarization encountered. In these systems, the feed flows transversely over the membrane surface. This tangential flow generates a sweeping action that drives away the solute buildup on the membrane surface. Membrane cells are like cassettes operating on the plate and frame

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Chapter 8 Forward osmosis membranes for water purification

design principle, with one or more cross-flow modules offering the advantage of easy coupling and decoupling as per the needs of the process. Another advantage of these flexible modular systems is that they can be easily integrated with the existing system options. So, industries that are interested in adopting FO technology don’t have to scrap all their existing infrastructure as it can be used in conjunction with the effluent treatment technologies that are in place. In fact, there are examples of integrated FO and conventional options such as activated sludge in treatment [13, 14]. However, because the process of forward osmosis produces a dilute draw solution, this process necessitates a secondary purification step to generate pure water and also the draw solutes. A number of draw solutes like simple inorganic compounds such as sodium chloride and magnesium sulfate, macromolecules such as glucose, chelating compounds such as polyelectrolytes, etc., have all been tested for exerting a higher osmotic potential than the feed solution. On the basis of the recovery options of the draw solutes, several methods have been developed such as thermally responsive hydrogels, magnetic salts, thermal draw solutes, etc. However, the most commonly used draw solutes include NaCl, MgSO4, etc., due to their low cost, easy recoverability, and wide availability. Fig. 8.2 provides a guide to the trade-offs that must be carefully considered prior to draw solution selection. Today, multinational companies from all over the world are investing in FO membrane production and large-scale FO installation to develop a more cost-effective yet efficient remediation process. Some of the industrial installations and membrane research companies are discussed in the case studies later in this chapter.

Osmotic pressure

Recovery option

Reverse salt diffusion

• Solvent flux is directly proportional to the osmotic pressure exerted. • It can be calculated from P = icRT

• Recovery option such as energy for thermal draw solutes, secondary filtration, etc.

• Monovalent draw solute, some organic compounds like ethanol, etc. are more prone to reverse salt diffusion due to their high moblity

• Divalent draw solutes especially calcium salts, sulfate salt tend to increase the membrane fouling

Fouling

Nature

Cost

FIG. 8.2 Draw solute selection guide.

• The selected draw solutes should be nonreactive, nonvolative and noncorossive

• Polyelectrolytes, high molecular weight draw solutes like protein, carbohydrates are expensive • Analysis of complex draw solutes is also relatively difficult

8.4 Membrane selection

163

8.3.2 Surface water treatment Forward osmosis also presents an attractive option for surface water remediation. The ability to selectively dewater the feed comes in handy for surface water treatment. The low concentration of contaminants present in the feed makes the administration of other modes of treatment such as adsorbent/ coagulant difficult. Here, FO membranes not only dewater such dilute streams through size exclusion, but also preferentially concentrate the feed due to its semipermeable nature. What is more interesting is that the process does not necessitate extreme conditions such as vacuum stripping or intense pressure such as reverse osmosis while guaranteeing a high separation efficiency at the same time. In surface water treatment, one of the toxic contaminants is arsenic, which severely affects millions of people in developing countries. FO is being researched as a viable option for treating arseniccontaining water. Studies have shown that by using simple draw solutes such as calcium chloride, traces of toxic contaminants such as arsenic and boron can be removed at a competitive recovery rate [15]. The use of polymer carbohydrates such as starch paste with amylase has been able to create the desired osmotic gradient, leading to up to 95% arsenic (V) rejection [16]. It has been experimentally demonstrated that the rejection of arsenic (five valency) in the presence of other solutes in the medium follows the order of humic acid > bicarbonate > nitrate > fluoride > sulfate > phosphate. It is also noteworthy that the presence of silicate decreases the rejection of arsenic by FO membranes [17]. For the removal of contaminants present in dilute concentrations, the effect of both internal and external concentration polarization on the flux is negligible. The process of forward osmosis, or rather pressure-assisted osmosis (PAO), has also proved to be efficacious for the removal of heavy metals such as cobalt, copper, and strontium [18]. This removal of heavy metals has also been extended to the treatment of hazardous wastewater from the nuclear industry, wherein the dewatering of feed waters contaminated with uranium, nucleotides, and other radioactive components harmful for exposure has been carried out by a dual channel bench-scale FO cell [19]. During the removal of simulated radioactive effluents, it was seen that with the hydrophobicity of the CTA membrane played a role in determining the rejection rate from the membrane while the hydrophobicity itself depended on the contact angle between the two surfaces. Even in mining industries where there is problem of acid mine drainage where streams containing a high load of heavy metals. Here, it is important to understand that FO membranes are unable to tolerate a pH as low as 2. Therefore, some preneutralization is necessary for remediation. But thereafter, FO has proved to be an efficacious option [20]. Besides the dissolved heavy metals, FO is also studied for the removal of difficult anions such as sulfates, nitrates, and phosphates (the latter two are notorious for the eutrophication of lakes) [21]. The removal range of FO also extends to the removal of dissolved organics such as phenolics, pharmaceutical and personal care products, and disinfection byproducts [22, 23]. Table 8.1 summarizes some of the cations and anions removed by the FO process.

8.4 Membrane selection 8.4.1 Materials, dimensions, and background In forward osmosis, primarily asymmetric membranes are used. These consist of a thin active layer (usually in the dense phase) at the top impregnated on a loose support layer. The dense top layer provides most of the selectivity with separation occurring due to its pore and chemical structure. Herein

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Chapter 8 Forward osmosis membranes for water purification

Table 8.1 Cations and anions that can be recovered in FO. S.R. no.

Contaminant

Removal efficiency

Flux

Membrane type

Feed

Draw solute

1.

Ammonium

89.3%

Polyacetate thin film

97.9%

0.32–1.37 LMH 9 L/day

Cobalt

9.93%– 99.96%

15.5–23.4 LMH

Magnesium chloride Seawater brine Sodium chloride

4.

Cadmium

78.87%

50 LMH

Thin-film composite with embedded polyester screen support (TFC-ES), cellulose triacetate with embedded polyester screen support (CTA-ES), cellulose triacetate on heat- or RF-weldable nonwoven support (CTA-NW) Cellulose acetate

Municipal wastewater Municipal wastewater Synthetic solution

2.

Phosphate

3.

Synthetic solution

5.

Mercury

95%

5 LMH

TFC FO

6.

Lead

99%

80 LMH

TFC

Hydrated magnesium sulfate Sodium chloride Sodium chloride

Cellulose acetate

Synthetic solution Synthetic solution

References [24] [10] [18]

[25]

[26] [27]

lies the fundamental difference from an RO membrane where a thick support layer is attached to the active layer of the membrane due to the large pressure it has to withstand. The fundamental nature of the FO membranes is different due to (a) a lower pressure requirement and (b) diffusivity of the draw solution required. In FO, the draw solution has to diffuse from the draw solution side through the support layer to the support layer-active layer surface while the solvent has to diffuse through the active layer and support layer and into the draw solution. Therefore, experiments using RO membranes for forward osmosis have suffered from severe concentration polarization due to increased mass transfer resistance in the feed and draw solution sides. The first generation of FO membranes used cellulose acetate and polyamide as the active layer substrates due to their selectivity and semipermeability, and polyester mesh as the support layer for providing mechanical support to the membranes. The dimensions of the active layer ranged from 0.1–1 μm while the thickness of the support layer ranged from 100 to 200 μm in thickness [28]. The development of thin film composites aimed at fusing the active layer and support layer marked a new generation in FO membrane research. In thin-film-composite (TFC) membranes, the active layer is polyamide deposited on top of a polyethersulfone or polysulfone layer, which is impregnated on a nonwoven fabric support sheet providing a layered structure. The flux obtained from the TFC membrane available in spiral and flat sheet structures is almost twice that from the previous membrane developments.

8.5 Hybrid processes

Membrane orientation

Mechanical strength

Structural parameter

FO mode:with active layer facing feed have yielded higher fluxes [2,3]

In case of performing PAO, the mechanical strength needs to be checked

Smaller the MTC, higher is the diffusivity and consequently higher mobility [1]

Solute permeability

Temperature and pH

Mass transfer coefficient

The pH and temperature range must be compatible with the feed and draw solutions

Smaller the MTC, higher is the diffusivity and consequently higher mobility [4]

The smaller the salt permeability values of the membrane, greater is the solvent pass selectivity

165

FIG. 8.3 Membrane selection guide.

There are several factors that are considered essential for membrane selection for a particular application, some of which have been summarized in Fig. 8.3:

8.4.2 Composite membranes Along with conventional polymeric membranes for reverse osmosis, in recent years technological developments toward ceramic forward osmosis membrane development have also begun. Until now, ceramic membranes with pore sizes in the nanofiltration range have not been developed for large-scale usage. Other areas studied widely are composite and mixed matrix membranes. The main concept includes tweaking the structure of an organic matrix by the incorporation of inorganic elements and vice-versa for inorganic matrices such as zeolites [29, 30]. Smart complexes with metal organic frameworks and metallic membrane-based chelate complexes are also being developed with higher selectivity in forward osmosis applications. Apart from selective transport, such membranes increase the probability of scale-up to meet future industrial demand [31]. Nanocomposites are also the newest addition to augment the performance efficiency of the forward osmosis membranes. Sandwiching the organic deposition with the distribution network also aides in improving the structural parameter property, with increased porosity and reduced tortuosity leading to sustained improvement of transmembrane throughput, phase selectivity, and overall flux from the process. The most common method of synthesizing such membranes is grafting inside the support layer. Membranes synthesized with graphene nanosheets grafted into a highly porous polymeric nonwoven support layer have shown reduced concentration polarization with a higher rejection of oily feedstocks [32].

8.5 Hybrid processes 8.5.1 FO-RO In the domain of salinity removal, FO can aid in reducing costs incurred in RO desalination. Most of the RO plants of the world are operating today on RO-based desalination, wherein the freshwater is

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Chapter 8 Forward osmosis membranes for water purification

extracted from saline water by applying pressure. This external pressure helps the freshwater to flow from the region of its lower concentration (brine solution) to the region of its higher concentration against the concentration gradient. Hence the name reverse osmosis; that is, flow occurs counter to the direction of normal osmosis. Though the process is heavily commercialized and enjoys a leading market share in both domestic and industrial sectors, there are two fundamental shortcomings. First, the movement against the concentration gradient needs high pressure (to the tune of 30–40 bars), which directly amounts to expenditure toward meeting the energy consumption. Second, as the process continues to remove freshwater from saline water, the salt concentration gradually rises. Beyond a fixed threshold value, the pump pressure is unable to cause freshwater flow against the concentration gradient. That is when the concentrated saline water is discarded. It is these two gray areas where FO can step in to reduce both energy consumption and water bypassing. The reject stream of the RO process consisting of concentrated brackish water could function as an effective draw solution for the FO process. An osmotic concentration gradient between the fresh feed and the concentrates from RO would facilitate more dewatering of the feed along with dilution of the concentrates, which can be then subjected to RO to recover freshwater. An integrated process with a looping RO circuit along with multipass FO can help cut down water wastage and operational costs currently suffered by the RO plants.

8.5.2 FO-MD Another interesting composite technology development is the merging of two novel separation processes integrating forward osmosis-membrane distillation for wastewater treatment. Membrane distillation is a technique that guarantees ultrahigh separation efficiencies. Here, the selective transport of vapor occurs across a hydrophobic membrane assembly. The feed is heated such that the membrane offers selective transport of the bulk solution in the vapor state only while impeding any liquid flow. On the permeate side, either a coolant is circulated or there is a sweep gas to condense the vapors, producing pure water/solvent. The contaminants present in the liquid state remain behind in the feed solution [33]. A hybrid process development involving the combination of FO and MD involves two different approaches: (i) pre-FO to further to decrease the energy consumption costs of MD when a lesser feed volume needs heating, and (ii) post-MD to recover the draw solutes. The second option is especially applicable to the platinum-based nanoparticle draw solutes that are being researched for switching polarity applications. The process has shown to be effective for the removal of toxic dye solutions, viruscontaminated solutions, and highly viscous solutions comprising macromolecular feedstocks [34, 35].

8.5.3 FO-NF and FO-UF One of the most widely researched and applied integrative processes with FO is downstream nanofiltration/ultrafiltration toward the production of pure water as well as for the recovery of draw solute. NF is used for ionic as well as organic draw solutes while ultrafiltration is used primarily for macromolecular draw solutes, which can be retained by the ultrafiltration membrane. The energy consumption of these membrane processes is also relatively lower due to their pore structure. Additionally, these

8.6 Industrial applications/large-scale FO installations

167

present a viable technology option and system designs operating in continuous and semicontinuous mode are already being scaled up. With nanofiltration, researchers have attained purity levels near drinking water levels, although it is important to note that nanofiltration is not suitable for the recovery of monovalent ionic draw solutes.

8.6 Industrial applications/large-scale FO installations FO presents an attractive solution to multidimensional problems encountered in water treatment systems. The successful laboratory-scale systems are slowly paving the way for pilot plant upscaling and thereby future full-fledged commercial-level implementation. Some of the first companies to set up research dedicated to forward osmosis and supplying FO membranes for research purposes are Oasys and HTI. Thereafter, Aquaporin has dominated in terms of market share in recent years. Stratkraft attempted commercialization through osmosis but failed to break even. Thereafter, Osmosis Energy UK has led the way in commercializing PRO using mainly hollow fiber FO membranes. Another area where FO can potentially revolutionize effluent treatment is the value-added products from the effluent stream with the underlying emphasis on waste to wealth. In a pilot-scale study on a spiral-wound FO module, Wang et al. demonstrated a 99.6% COD rejection with an equivalent amount of total phosphorus rejection (99%) with a flux of 6 LMH. The system also simultaneously aimed for ammonia reduction from effluent treatment [36]. In the domain of osmotic evaporation, there are several pilot-scale projects reported toward fruit juice concentration. In Australia, there are megaprojects and pilot and semicommercial plants toward this purpose [37]. There are also several studies reported toward large-scale cooling water blowdown remediation with the help of forward osmosis. Forward osmosis in the field of drug delivery systems is fairly commercialized. Osmotic pumps in diverse designs such as push-pull pumps, single-chamber, and multivariant delivery locations are already marketed by leading pharmaceutical companies such as OROS, ALZA, AZTEK, Johnson and Johnson, and the like [38]. The technology solution that possesses the potential to become a game changer in the field of water remediation and reclamation. FO has shown efficacy in the removal of heavy metals, dissolved organics, microbial content, salinity, and micropollutants. But there are several aspects on which research needs to progress simultaneously and concurrently to establish the process as a commercial-scale competitive option. First is the development of a lowcost downstream option that can produce water as per the output quality requirement as well as recover the draw solutes. Again, in the area of draw solute, selection has to be toward compounds that are low cost, can be produced on a large scale, and assure a high water flux. Another major issue is the production of large-scale FO membranes and their availability at a relatively feasible cost. There are several studies that have demonstrated novel FO membrane development that produces a high flux, but such studies are not backed up with a prototype level or semipilot scale studies. In fact, there is a scarcity of FO membranes for large-scale usage. Companies such as Aquaporin supply their membranes primarily for research options, and this is one of the biggest obstacles today in carrying out a pilot-scale FO study. These concerns along with issues such as reject management, if addressed, will help in making the technology available for daily usage.

168

Chapter 8 Forward osmosis membranes for water purification

References [1] P. Das, K.K.K. Singh, S. Dutta, Insight into emerging applications of forward osmosis systems, J. Ind. Eng. Chem. 72 (2019) 1–17. [2] P. Das, S. Dutta, K. Singh, S. Maity, Energy saving integrated membrane crystallization: a sustainable technology solution, Sep. Purif. Technol. 228 (2019) 115722. [3] K.L. Wang, S.H. Mccray, D.D. Newbold, E. Cussler, Hollow fiber air drying, J. Membr. Sci. 72 (1992) 231–244. [4] D.F. Stamatialis, B.J. Papenburg, M. Girones, S. Saiful, S.N. Bettahalli, S. Schmitmeier, et al., Medical applications of membranes: drug delivery, artificial organs and tissue engineering, J. Membr. Sci. 308 (2008) 1–34. [5] Y.C. Kim, Y. Kim, D. Oh, K.H. Lee, Experimental investigation of a spiral-wound pressure-retarded osmosis membrane module for osmotic power generation, Environ. Sci. Technol. 47 (2013) 2966–2973. [6] W. Akram, M.H. Sharqawy, Power generation with pressure retarded osmosis, in: ASME 2013 International Mechanical Engineering Congress and Exposition, Volume 6A: Energy, 2013. [7] High BOD and COD carpet dyeing wastewater recycled using forward osmosis, Membr. Technol. 2009 (2009) 8. [8] M. Li, X. Wang, C.J. Porter, W. Cheng, X. Zhang, L. Wang, et al., Concentration and recovery of dyes from textile wastewater using a self-standing, support-free forward osmosis membrane, Environ. Sci. Technol. 53 (2019) 3078–3086. [9] M. Qiu, C. He, Efficient removal of heavy metal ions by forward osmosis membrane with a polydopamine modified zeolitic imidazolate framework incorporated selective layer, J. Hazard. Mater. 367 (2019) 339–347. [10] G. Qiu, Y.-M. Law, S. Das, Y.-P. Ting, Direct and complete phosphorus recovery from municipal wastewater using a hybrid microfiltration-forward osmosis membrane bioreactor process with seawater brine as draw solution, Environ. Sci. Technol. 49 (2015) 6156–6163. [11] M. Xie, L.D. Nghiem, W.E. Price, M. Elimelech, Impact of organic and colloidal fouling on trace organic contaminant rejection by forward osmosis: role of initial permeate flux, Desalination 336 (2014) 146–152. [12] R.M. Abousnina, L.D. Nghiem, Removal of dissolved organics from produced water by forward osmosis, Desalin. Water Treat. 52 (2013) 570–579. [13] N.C. Nguyen, S.-S. Chen, H.-Y. Yang, N.T. Hau, Application of forward osmosis on dewatering of high nutrient sludge, Bioresour. Technol. 132 (2013) 224–229. [14] X. Zhang, Z. Ning, D.K. Wang, J.C.D.D. Costa, Processing municipal wastewaters by forward osmosis using CTA membrane, J. Membr. Sci. 468 (2014) 269–275. [15] X. Jin, Q. She, X. Ang, C.Y. Tang, Removal of boron and arsenic by forward osmosis membrane: influence of membrane orientation and organic fouling, J. Membr. Sci. 389 (2012) 182–187. [16] H. Yoon, J. Kim, J. Yoon, Forward osmosis as appropriate technology with starch-based draw agent, Desalin. Water Treat. 57 (2015) 10129–10135. [17] P. Mondal, A.T.K. Tran, B.V. Bruggen, Removal of As(V) from simulated groundwater using forward osmosis: effect of competing and coexisting solute, Desalination 348 (2014) 33–38. [18] X. Liu, J. Wu, C. Liu, J. Wang, Removal of cobalt ions from aqueous solution by forward osmosis, Sep. Purif. Technol. 177 (2017) 8–20. [19] X. Liu, J. Wu, J. Wang, Removal of nuclides and boric acid from simulated radioactive wastewater by forward osmosis, Prog. Nucl. Energy 114 (2019) 155–163. [20] B. Vital, J. Bartacek, J. Ortega-Bravo, D. Jeison, Treatment of acid mine drainage by forward osmosis: heavy metal rejection and reverse flux of draw solution constituents, Chem. Eng. J. 332 (2018) 85–91. [21] K. Xie, J. Song, L. Qiu, J. Wang, S. Zhang, X. Kang, et al., A new technology for removing nitrogen and phosphorus in saline wastewater by double membrane bioreactor, in: Proceedings of the 2017 6th International Conference on Energy and Environmental Protection (ICEEP), 2017.

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[22] J. Xu, T.N. Tran, H. Lin, N. Dai, Removal of disinfection byproducts in forward osmosis for wastewater recycling, J. Membr. Sci. 564 (2018) 352–360. [23] J. Li, Q. Liu, Y. Liu, J. Xie, Development of electro-active forward osmosis membranes to remove phenolic compounds and reject salts, Environ. Sci.: Water Res. Technol. 3 (2017) 139–146. [24] S. Jafarinejad, H. Park, H. Mayton, S.L. Walker, S.C. Jiang, Concentrating ammonium in wastewater by forward osmosis using a surface modified nanofiltration membrane, Environ. Sci.: Water Res. Technol. 5 (2019) 246–255. [25] S. Kamaruzaman, N.I.F. Aris, N. Yahaya, L.S. Hong, M.R. Razak, Removal of Cu (II) and Cd (II) ions from environmental water samples by using cellulose acetate membrane. J. Environ. Anal. Chem. 04 (2017) 220, https://doi.org/10.4172/2380-2391.1000220. [26] C.-Y. Wu, S.-S. Chen, D.-Z. Zhang, J. Kobayashi, Hg removal and the effects of coexisting metals in forward osmosis and membrane distillation, Water Sci. Technol. 75 (2017) 2622–2630. [27] Tamara Kawther Hussein Removal of lead, copper and nickel ions from wastewater by forward osmosis process . Printed in Iraq Second Engineering Scientific Conference College of Engineering—University of Diyala 16-17 December 2015, pp. 893–908 ISSN 1999-8716 [28] A. Tiraferri, N.Y. Yip, W.A. Phillip, J.D. Schiffman, M. Elimelech, Relating performance of thin-film composite forward osmosis membranes to support layer formation and structure, J. Membr. Sci. 367 (2011) 340–352. [29] B.H. Jeong, E.M.V. Hoek, Y.S. Yan, A. Subramani, X.F. Huang, G. Hurwitz, A.K. Ghosh, A. Jawor, Interfacial polymerization of thin film nanocomposites: a new concept for reverse osmosis membranes, J. Membr. Sci. 294 (2007) 1–7. [30] P. Aerts, A.R. Greenberg, R. Leysen, W.B. Krantz, V.E. Reinsch, P.A. Jacobs, The influence of filler concentration on the compaction and filtration properties of Zirfon (R)-composite ultrafiltration membranes, Sep. Purif. Technol. 22–23 (1–3) (2001) 663–669. [31] S. Chen, X. Lu, . Smart materials as forward osmosis draw solutes, Smart Materials for Advanced Environmental Applications Smart Materials Series. (2016) 19–50 (Chapter 2). [32] D. Qin, Z. Liu, D.D. Sun, X. Song, H. Bai, A new nanocomposite forward osmosis membrane customdesigned for treating shale gas wastewater, Sci. Rep. 5 (14530) (2015) 1–14. [33] N. Thomas, M.O. Mavukkandy, S. Loutatidou, H.A. Arafat, Membrane distillation research & implementation: lessons from the past five decades, Sep. Purif. Technol. 189 (2017) 108–127. [34] Q. Ge, P. Wang, C. Wan, T.-S. Chung, Polyelectrolyte-promoted forward osmosis–membrane distillation (FO–MD) hybrid process for dye wastewater treatment, Environ. Sci. Technol. 46 (2012) 6236–6243. [35] T.Y. Cath, V. Adams, A.E. Childress, Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement, J. Membr. Sci. 228 (2004) 5–16. [36] Z. Wang, J. Zheng, J. Tang, X. Wang, Z. Wu, A pilot-scale forward osmosis membrane system for concentrating low-strength municipal wastewater: performance and implications. Sci. Rep. 6 (2016) 21653, https:// doi.org/10.1038/srep21653. [37] A. Barbe, J. Bartley, A. Jacobs, R. Johnson, Retention of volatile organic flavour/fragrance components in the concentration of liquid foods by osmotic distillation, J. Membr. Sci. 145 (1998) 67–75. [38] H.-B. Lee, D.-H. Lee, B.-K. Kang, S.-Y. Jeung, G.-S. Khang, Evolution of the patent for osmotic drug delivery, J. Korean Pharm. Sci. 32 (2002) 241–258.

CHAPTER

Synthetic polymer-based membranes for acidic gas removal

9

Shuichi Satoa, Kazukiyo Nagaib a

Department of Electronic Engineering, Tokyo Denki University, Tokyo, Japan Department of Applied Chemistry, Meiji University, Kawasaki, Japanb

Chapter outline 9.1 9.2 9.3 9.4 9.5 9.6

Introduction ................................................................................................................................ 173 Types of acid gases, sources, and impacts on the environment ...................................................... 174 Outline of acidic gas separation membrane techniques ................................................................. 176 Transport mechanism of polymer membranes ................................................................................ 177 Design of acidic gas separation membranes ................................................................................. 179 History of acidic gas polymer membranes ..................................................................................... 181 9.6.1 Acidic gas separation membranes of general polymers ................................................ 181 9.6.2 Acidic gas separation membranes of composite type ................................................... 182 9.7 Future developments ................................................................................................................... 188 References ........................................................................................................................................ 188

9.1 Introduction Fossil fuels are a major energy resource in the world. In Japan, power production in the aftermath of the Great East Japan Earthquake has shifted from nuclear to thermal sources. However, the process of burning fossil fuels to generate electricity in thermal power plants produces carbon dioxide (CO2), which is a key greenhouse gas (GHG) and a major cause of global environmental issues. Reduction of CO2 emissions is thus considered an absolute necessity to prevent global warming from exacerbating. As shown in Fig. 9.1, Carbon dioxide capture and storage (CCS) is an excellent technology to reduce the amount of CO2 emissions while still continuing the use of fossil fuels [1]. This technology separates and recovers the CO2 generated at thermal power plants, which is then transported to suitable storage sites present underground or in the ocean at depths of about 1000 m. From the perspective of sustainable economic growth and global warming, it is one of the most suitable technologies to reduce

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00009-5 # 2020 Elsevier Inc. All rights reserved.

173

174

Chapter 9 Synthetic polymer-based membranes

FIG. 9.1 Concept of carbon capture and storage (CCS).

the greenhouse gas emissions that accompany the use of fossil fuels. Consequently, in Japan and many other countries, CCS technology has become a chief political policy over the last few years. In addition to CO2, other compounds such as sulfur oxides (SOX), nitrogen oxides (NOX), and hydrogen sulfide (H2S) are also emitted from the combustion of fossil fuels and oxidation reactions of nitrogen in automobile engines. Together, these atmospheric pollutants are classified as “acid gas,” responsible for acid rain and ocean acidification. There exist various technologies to separate and recover these acid gases, such as their chemical absorption using an amine compound [2], physical absorption using polyethylene glycol as the absorption liquid [3], and the pressure swing adsorption method using zeolite or activated carbon [4]. Among these, the “polymer” membrane separation technology is an efficient, compact, cost-effective, and energy-saving method for recovering CO2. In this chapter, the permeability and selectivity of acidic gases in polymer membranes as well as their sources and potential impacts on the environment will be discussed in detail.

9.2 Types of acid gases, sources, and impacts on the environment Acid gases primarily refer to CO2, SOX, NOX, H2S, or mixtures of natural gas containing significant amounts of these acidic gases. The sources of these and other impure gases are summarized in Table 9.1, wherein fossil fuels such as natural gas, coal, and oil are some of the common sources. These acidic gases are primarily emitted during the combustion of fossil fuels, such as from vehicular exhaust. Hence, these gases are primarily produced during the energy generation and consumption processes. The gases including benzene, toluene, and xylene (BTX) compounds recorded in the pollutant release and transfer register (PRTR), along with organic sulfur compounds such as mercaptans (RSH) and

9.2 Types of acid gases, sources, and impacts on the environment

175

Table 9.1 Acidic gas sources and components. Acidic gas source

Components

Liquefied natural gas (LNG) Coke oven gas Fuel combustion gas Automobile exhaust gas Steam reforming gas

C1
C6, CH, H2S, CO2, RSHa, COSb H2, CH4, N2, BTXc, CO, CO2, and O2 CO2, NOX, and SOX CO, CO2, CH, NOX, and PM H2, CO, CO2, and CH4

a

Mercaptan. Carbonyl sulfide. Benzene, toluene, and xylene.

b c

carbonyl sulfides (COS), are also contained. Many other kinds of gases are also produced during the vehicular combustion of fossil fuels. As summarized in Table 9.1, CO2 is mainly generated when the carbon present in hydrocarbonbased fuels (i.e., fossil fuels) combines with oxygen during the combustion process. On the other hand, SOX is generated when the inorganic and organic sulfur compounds contained in coal and petroleum are burned in power plants and industrial boilers. Nitrogen oxides (NOX) are formed when nitrogen and oxygen in the air combine at high temperatures in combustion furnaces and engines. Hence, the amount of NOX emissions from automobile exhausts and thermal power plants has been increasing. Finally, H2S is produced by hydrodesulfurization during the process of purification of natural gas as well as from sewage and refuse treatment plants due to the degradation of sulfur present in the sewage by anaerobic bacteria. Table 9.2 summarizes the adverse effects of these acid gases on the environment, wherein the primary impact is in the form of acid rain and subsequent acidification of the ecosystem and the overall environment. The emitted SOX and NOX get scattered in the atmosphere and react with the water (or moisture) and oxygen present in the clouds to form acidic compounds such as sulfuric or nitric acid, and precipitate as acid rain [5]. Acid rain causes many harmful effects, including the withering of foliage in forests, desertification due to soil contamination, chemical weathering of concrete and marble present in the exteriors of buildings, and rusting of metals. The acidic run-off finally flows into the rivers and oceans, thereby mixing with drinking water. The human health impacts of acid rain include irritation and infection of the eyes, throat, nose, and skin as well as various respiratory diseases. In recent years, ocean acidification has been identified as another serious problem of acid rain. This phenomenon is known to cause red tide and kills marine life. In addition to acid rain, ocean

Table 9.2 Effects of acidic gases on the environment. Area

Harm

Forest soil Building Human body Ocean

Dead trees, soil pollution Rusting of roof and wall Respiratory disease Red tide, ocean acidification

176

Chapter 9 Synthetic polymer-based membranes

acidification also results from the absorption of atmospheric CO2. As ocean acidification proceeds, it reduces the natural ability of the phytoplankton to absorb carbon dioxide [6], which in turn increases the proportion of carbon dioxide remaining in the atmosphere causing global warming [7]. Therefore, the environmental damage caused by acidic gases is enormous, and adequate mitigation measures against their excess emission need to be urgently adopted.

9.3 Outline of acidic gas separation membrane techniques One of the techniques for separating and recovering acid gases is the use of separation membranes, which is an efficient and energy-saving process compared to the chemical absorption, physical absorption, and adsorption methods. This technique separates only a specific substance from a mixture by permeating or blocking it by using the velocity difference of each component permeating the membrane, thereby removing the unnecessary substance. Although there are various inorganic and polymer separation membranes composed of inorganic or polymer materials, the latter type of membrane is the more cost-effective option. Fig. 9.2 shows the number of publications (including academic articles and patents) having the key words “polymer membranes” and “acidic gas.” These studies pertain to not only CO2, but to all other acidic gases as well. It can be observed that there has been an increasing trend in the number of studies focusing on this particular research area post-2010. Much progress can also be witnessed in the research and development of acidic gas polymer membranes. Given that increased CO2 emission is a global 50

Polymer membrane + acidic gas

References

40

30

20

10

2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 19 90 1989 1988 1987 1986 1985 1984 1983 1982 1980 1978 1976 1 97 5 1965

0

Year FIG. 9.2 Number of references whose keyword for each year is “Polymer membrane and acidic gas.”

9.4 Transport mechanism of polymer membranes

177

environmental concern, a vast majority of the studies are related to the separation and recovery of CO2, and the trend is further increasing. The first use of polymer separation membranes for the separation of methane (CH4) and CO2 from natural gas was reported from the United States [8, 9]. However, CO2, H2O, and H2S have better permeability to polymer membranes than CH4. Therefore, a cellulose acetate (CA) membrane, which is enriched in oxygen, was initially used, but later changed to a polyimide membrane having good durability. In addition, the removal of sulfurous acid coexisting in the natural gas was also taken into consideration. Likewise, recent studies pertaining to the CO2 separation project, in view of it being a global warming problem, have also been conducted in Japan, and it is believed that membranes with both high permeability and high selectivity are required for enhanced separation. In addition to CO2, separation techniques for SO2 have also attracted attention, owing to the problems of acid rain and ocean acidification. Research and development in both areas need to be actively conducted in the future.

9.4 Transport mechanism of polymer membranes The concept of permeation or separation of the acid gas molecules using polymer membranes is summarized in Fig. 9.3. Gas separation occurs due to the permeability of each gas in the permeation process. A porous membrane is a membrane with multiple pores or holes distributed throughout its structure, and the gas separation mechanism with this membrane is based on the Knudsen flow or the HagenPoiseuille flow (Case I). If the holes in the membrane are too large, the gas molecules pass through the hole. In this type of flow, the gas separation is inversely proportional to the ratio of the molecular weight of the target gas and its viscosity. The surface diffusion phenomenon occurs parallel to the Knudsen flow (Case II) [10]. Here, the cohesiveness of the gas molecules depends on the balance

Porous membrane

Porous membrane

Porous membrane

Porous membrane

Nonporous membrane

Knudsen flow

Surface diffusion

Capillary condensation

Molecular sieving

Solution-diffusion mechanism

FIG. 9.3 Gas separation mechanisms in separation membranes.

178

Chapter 9 Synthetic polymer-based membranes

of the surface diffusion and the Knudsen flow. The selectivity of the membrane to the acid gas is determined by the pore size independent of the polymer’s chemical structure. Capillary condensation and molecular sieving flow occur when the pore size is narrower and becomes similar to the size of the gas molecules (Cases III and IV) [11, 12], allowing only those molecules that are small to be selectively transported through the pores. In the solution-diffusion mechanism, the separation is achieved by passing the gas through nonporous membranes (composed of polymers) instead of porous membranes (Case V) [9]. The diameter of ˚ (0.2–0.5 nm), and the space formed between the molecules of the gas molecules should be about 2–5 A the polymer chains should be almost the same size as the gas molecules. Therefore, the deference between the molecular sieving and the solution-diffusion mechanism is not clear because the space was almost the same. If the position of the polymeric chains can be fixed as to that in the inorganic membrane, then the gaps between the polymer chains behave like through-holes and induce the molecular sieving effect. However, the polymer chains are thermally vibrated, causing random changes in their position in the membrane such that fixed holes cannot be formed. Therefore, gas separation polymeric membranes are based on the solution-diffusion mechanism, wherein the gas is first dissolved onto the surface of the membrane, and then diffuses through the intermolecular chain gaps. The performance of such membranes depends on the permeability and selectivity, where the permeability, P(A), of the gas component (A), is represented as the product of the solubility coefficient, S(A), and the diffusion coefficient, D(A): PðAÞ ¼ SðAÞ  DðAÞ

(9.1)

Permeation tests are often performed by using a single pure gas instead of a mixed gas to compare between different polymeric membrane materials. In the process, the parameters of each gas are systematically investigated. Thus, the ideal selectivity in terms of the ideal separation coefficient, α(A/B), can be represented as the ratio of the permeabilities of two pure gas components, P(A) of component A and P(B) of component B, as shown in Eq. (9.2): αðA=BÞ ¼

PðAÞ PðBÞ

(9.2)

Therefore, Eqs. (9.1), (9.2) can be combined as follows: αðA=BÞ ¼

    PðAÞ SðAÞ DðAÞ ¼  PðBÞ SðBÞ DðBÞ

(9.3)

where the solubility coefficient ratio (S(A)/S(B)) of the two gases is the solubility selectivity, and the diffusion coefficient ratio (D(A)/D(B)) is the diffusivity selectivity. Therefore, in order to increase the selectivity of the polymeric membrane to a particular gas, it is necessary to increase either the solubility selectivity or the diffusivity selectivity. The solubility selectivity of the gas in the polymeric membrane is related to the condensability of the penetrant gas molecules and an increase in condensability increases the solubility. On the other hand, diffusivity selectivity is dependent on the size of the gas molecules and the sieving capability of the polymeric membrane. Generally, diffusivity decreases as the size of the molecules increases. Thus, selectivity increases when the difference in gas condensability and molecular size becomes remarkable.

9.5 Design of acidic gas separation membranes

179

9.5 Design of acidic gas separation membranes As shown in Fig. 9.4, there are two choices when designing an acid gas separation membrane. One is to use an acidic gas rejective membrane (Fig. 9.4A), which allows other gases to pass preferentially without passing acid gases. Another is to use an acidic gas permselective membrane (Fig. 9.4B), which allows only the acidic gas to pass preferentially without passing other gases. In designing an acidic gas separation polymer membrane based on the solution-diffusion mechanism, considering the solubility and diffusivity of the acidic gases as compared to other gases under separation conditions is critical. Table 9.3 summarizes the various acidic gas properties and includes the impurities listed in Table 9.1 [13]. The critical volume (Vc) of the acidic gases, which is a measure of the 3D gas molecule size, was slightly higher than that of nitrogen and oxygen. No significant difference between the acidic and other gases was observed. Therefore, the diffusivity selectivity is considered to be not very high. However, the critical temperature (Tc) of the acidic gases, which is a measure of gas cohesiveness, was considerably higher than that of other gases. Therefore, the effective method for designing an acidic gas separation polymer membrane was to consider the difference in solubility selectivity. Table 9.4 lists the acidic gas selectivities in terms of solubility selectivity and diffusion selectivity based on Eq. (9.3). In the general polymer membrane, the solubility of acidic gases is much higher than that of other gases (S(acidic gas)≫ S(gas)), whereas the diffusivity of acidic gases is lower than that of other gases (D(acidic gas) < D(gas)). Therefore, the permeability of acidic gases is higher overall than that of other gases (P(acidic gas) > P(gas)). Based on the diffusivity selectivity separation method, D

(A)

(B) FIG. 9.4 Separation of mixtures of acidic and other gases using (A) acidic gas rejective membranes, and (B) acidic gas permselective membranes.

180

Chapter 9 Synthetic polymer-based membranes

Table 9.3 Physical properties of gases [13]. Gas

M (g/mol)

Vc (cm3/mol)

Tc (K)

H2 CH4 N2 C2H6 NO O2 H2S CO2 C3H8 NO2 SO2

2.02 16.0 28.0 30.1 30.01 32.0 34.08 44.0 44.1 46.01 64.03

65.0 98.6 90.1 146 57.7 73.4 98.6 94.1 200 167.8 122.2

33.3 191 126 305 180 155 373.2 304 370 431 430.8

Table 9.4 Design of acidic gas selective membranes.

General

Increase in diffusivity selectivity

Increase in solubility selectivity

SðAcidic gasÞ SðgasÞ

≫1

≫1

⋙1

DðAcidic gasÞ DðgasÞ

HF-NPSF > TMHF-NPSF.

10.2 Membrane materials

197

Among the synthesized copolymers, the TMHF-NPSF membrane possesses desirable permeability compared to the naphthalene polysulfone (NPSF) membrane without sacrificing selectivity [62].

10.2.1.4 Nanoporous polymers Polymers of intrinsic microporosity Polymers of intrinsic microporosity (PIMs) refer to a new class of polymers and have received a great deal of attention for membrane application. PIMs was first introduced by McKeown and Budd in 2004 [63,64]. PIMs may be synthesized from a wide range of monomers (Fig. 10.3), providing polymers with improved permeability and selectivity through designing the structure and chemical functionality of the polymer. PIMs are not only used as a polymer matrix for fabrication of mixed matrix membranes, but can also be blended with PIs and PEIs to enhance gas separation performance [65]. The presence of rigid polymer chains with a ladder-like backbone and interconnected pores (pore size < 2nm) leads to the formation of a large free volume (typically above 20% FFV) and microporosity, thus providing very high gas permeability [13,66]. For example, the permeability of gas species in PIM-1 is 100 times greater than conventional glassy polymers without sacrificing selectivity. PIMs exhibit thermal stability and good solubility in organic solvents, and are thereby suitable to be fabricated using the solution-casting method [13]. The synthesis route and chemical structure of monomers for PIM 1–10 are illustrated in Fig. 10.3. Budd et al. [32] synthesized PIM-1 and PIM-7 and evaluated the permeation properties of membranes formed from them. The O2 permeability and O2/N2 selectivity were 370 Barrer and 4.0 for PIM-1 and 190 Barrer and 4.5 for PIM-7, respectively. Staiger et al. [67] applied PIM-1 and PIM-7 membranes for O2/N2 separation and the prepared membranes exhibited extraordinary O2 permeability (786 Barrer) and no detrimental effects on selectivity (3.3). The maximum O2 permeability of PIM-1 reached 2270 Barrer. PIM membranes (TFMPSPIM1-4) comprising trifluoromethyl (-CF3) and phenylsulfone (-SO2C6H5) side groups were also synthesized via aromatic nucleophilic substitution polycondensation of 5,50 ,6,60 -tetrahydroxy-3,3,30 ,30 -tetramethylspirobisindane with tetrafluoroterephthalonitrile and a new monomer [68]. The presence of side chains on the polymer backbone acts favorably to increase the polymer solubility. The prepared membranes exhibit high O2/N2 selectivity (3.4–4.7) without losing O2 permeability (156-737 Barrer). Thin film composite (TFC) membranes were also fabricated from a series of PIMs (PIM-1, PIM1-CO1-50, PIM1-CO6-50, and their blend with PEI) as the top layer, in which their thickness ranged from 300 to 800 nm [69]. A modest O2/N2 selectivity was obtained at an O2 permeance of 85 GPU.

Thermally rearranged polymers Thermally rearranged (TR) polymers are another type of rigid and microporous polymer with extraordinary gas permeability and adequate selectivity for gas separation applications. TR polymers first proposed by Park et al. [70] possess high free volume elements and narrow cavity size distribution, derived from functionalized polyimides by a postthermal conversion process [71]. TR polymers are tunable and can be designed for specific gas species by controlling polymer structures and thermal reaction mechanisms [71]. The thermal rearrangement mechanism for the synthesis of TR polymers is shown in Fig. 10.4. However, the major concerns in the development of TR polymers are their brittleness and low mechanical strength. Park et al. [72] studied the synthesis and characterization of seven types of TR polymer membranes. Free volume and cavity size distribution were controlled by using different monomers and thermal

198

Chapter 10 Synthetic polymer-based membranes for oxygen enrichment

FIG. 10.3 Synthesis route and chemical structure of monomers for PIMs [65]. (Continued)

10.2 Membrane materials

FIG. 10.3, cont’d

199

200

Chapter 10 Synthetic polymer-based membranes for oxygen enrichment

FIG. 10.3, cont’d

10.2 Membrane materials

O

HX

201

Meta-position

X

350-450°C

N -CO2

N

O

Para-position

ortho-functional groups -XH : -OH, -SH, -NH2 N

(A)

O

S

N

N

Benzoxazole

H

O

N

C

N N N H

O

Benzothiazole

Pyrrolone

Benzimidazole

N 300-400°C -H2O

(B)

OH

O

FIG. 10.4 Thermal rearrangement mechanism, (A) TR-α polymer and (B) TR-β polymer [71].

treatment protocols to achieve excellent gas permeation properties. The O2 permeability and O2/N2 selectivity varied from 14–747 Barrer and 4.4–6.1, respectively. Calle et al. [73] proposed cross-linked TR poly(benzoxazole-co-imide) membranes for O2/N2 separation, which exhibit a synergistic effect of high permeability and high selectivity. Cross-linking leads to tightening and rigidifying of polymer chains, providing ultrafine micropores to effectively separate the gas species with high productivity. The O2 permeability and O2/N2 selectivity increased by 150% and 7% compared to the noncross-linked one. The reader is directed to recent reviews that have covered nanoporous polymers in detail [71,74,75].

10.2.1.5 Surface-modified membranes Cross-linking of the bulk or surface of the membranes can significantly influence the physical and chemical properties. Generally, cross-linking is induced to increase membrane durability in harsh conditions and achieve better gas permselectivity. Cross-linking modifications of the polymeric membrane material can be performed by several techniques, such as ultraviolet (UV) and ion beam radiation, thermal treatment, and chemical modification. Ion beam was irradiated (H+ irradiation) over a range of doses to modify the structure and surface properties of the PSf membrane [4]. The transfer of energy from the ions to the polymer chains results in considerable evolution in the chemical structure, microstructure, and transport properties of the prepared membranes. A simultaneous decrease in permeance and permselectivity was observed for the asymmetric membranes following H+ irradiation over a range of fluencies. For example, there is an

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85% decrease in the permeance of O2 and a 48% decrease in the O2/N2 selectivity following irradiation at 4  1014 H+/cm2. The sharp decrease in permeance can be due to compaction of the intermediate porous substrate to form a thick nonselective layer with Knudsen-type resistance. In contrast, modification of 6FDA-6FpDA by irradiation at similar doses resulted in simultaneous large increases in permeance and permselectivity relative to the virgin membrane [76]. The O2 permeability and O2/N2 selectivity of the modified membrane increased by 47% and 100% compared to the virgin one. The significant enhancement in permeability and selectivity can be related to the increment in FFV due to the polymer degradation and restriction of the polymer chain mobility. Sundell et al. [77] synthesized amorphous, high glass transition, cross-linkable poly(arylene ether sulfone) for gas purification membranes. The polymers include a moiety capable of several oxidation reactions and UV cross-linking. Two oxidation reactions of the polymers were performed, one by chemical treatment using Oxone and KBr and the other by elevated heat treatment in the air. The O2 permeability of one noncross-linked poly(arylene ether) was 2.8 Barrer, with an O2/N2 selectivity of 5.4. Following UV crosslinking, the O2 permeability decreased to 1.8 Barrer, and the O2/N2 selectivity increased to 6.2. UV irradiation was applied to modify the surface and gas-separation properties of PSf membranes [78]. After UV treatment, the permeation of O2 decreased from 3.4 to 2.3 m3 m2 h1 bar1, whereas that of N2 increased for all the pressures used from 1.7 m3 m2 h1 bar1 to about 3.4 m3 m2 h1 bar1. Moreover, the O2/N2 selectivity decreased from 7.8 to 1.5. Surface treatment of the polytrimethylsilylpropyne (PTMSP) membrane was made by the exposure of PTMSP to UV irradiation for a certain time [79]. The O2/N2 selectivity increased from 1.4 to 4 for pristine and surface-modified membranes, respectively. However, the O2 permeance dramatically decreased by approximately seven times. The literature survey indicates that the dose and duration of irradiation should be optimized. Long UV treatment [80] or the high dose of irradiation [70] leads to the formation of a nonselective layer, thereby both selectivity and permeability are reduced. Plasma treatment has been widely used to modify the membrane surface for O2/N2 separation [81–83]. The reader is directed to recent reviews that have evaluated the plasma treatment in detail [4]. Cross-linking through heat treatment has been studied extensively on different types of polymers. It was shown that thermal annealing at elevated temperatures for short periods of time can efficiently induce cross-linking in PASs. Many researchers reported their studies on heating below and above the glass transition temperature, Tg. Thermal annealing can uniformly activate the reactive groups throughout the membrane matrix and change its morphology. Shen and Lua [84] studied the effects of heat treatment on the gas permeation properties of the P84 Polyimide membrane. Data revealed that the P84 membrane annealed at 200°C showed higher O2/N2 selectivity than that at 80°C, but lower permeability for both O2 and N2. The permeability and selectivity of the membrane annealed at 80° C was 1.20 Barrer and 6.12, respectively, whereas the O2 permeability of the membrane annealed at 200°C decreased by 25% and its selectivity increased by 42%. This behavior could be attributed to densification and reduction of the excess free volume of the membranes. Polyimide Matrimid asymmetric hollow fiber membranes were also treated by thermal treatment at 150 and 250°C [85]. For example, the O2 permeability of the membrane annealed at 150°C increased by 492% (from 8.5 to 50.4 GPU) but selectivity reduced to approximately 1. These performances could be safely attributed to the cracks formed on the skin. Chemical cross-linking is a facile method and has been widely reported in the literature [86,87] because it can significantly alter the morphology and surface properties without material degradation. The chemistry of the polymer and cross-linker as well as the degree of sulfonation have a great impact

10.2 Membrane materials

203

on the final structure and permeation properties of the membranes. Therefore, the selection of a suitable cross-linker is a crucial factor in chemical cross-linking. There are two approaches to chemically crosslink the polymers: (1) the addition of a cross-linker into the polymer solution before membrane fabrication and (2) postmodification through immersion of the fabricated membrane in a cross-linker for a specified time. Recently, PIs have been extensively used as membrane material due to their productivity and selectivity. However, the low durability of virgin polyimide membranes is a big problem, which can be solved by cross-linking using diols and diamines as cross-linkers. Tin et al. [88] prepared dense membranes from Matrimid 5218, then modified them by immersing the membranes in pxylenediamine solution as a cross-linking agent. The chemical structure changed during the crosslinking process, which slightly improved the selectivity. The O2/N2 selectivity increased from 6.6 to 7.4 by increasing the immersion time from 0 to 21 days, respectively. The reader is directed to a recent review that has covered cross-linking of PIs in detail [86]. Shao et al. [89] investigated the cross-linking of poly(4-methyl-2-pentyne) (PMP) using 4,4-(hexafluoroisopropylidene) diphenyl azide (HFBAA) as the cross-linker. By increasing the cross-linker content from 0 to 3 wt.%, the O2 permeability decreased from 1490 to 990 Barrer, respectively, while the gas sorption remained constant. Compared to pure PMP, the O2/N2 selectivity increased by 43%.

10.2.2 Facilitated transport membranes In order to improve the productivity and selectivity (O2 over N2) of polymeric materials, facilitated transport has been proposed as a path to circumvent the permeability/selectivity trade-off. In facilitated transport membranes, a specific gas species is allowed to transport through the membrane matrix faster using mobile and/or fixed carrier molecules. Facilitated transport is a reaction-diffusion process, which is comprised of a chemical coupling reaction followed by diffusion. Some gas species can preferentially react with adsorbents containing transition metals at the feed/membrane interface to form a gas-carrier complex, then diffuse across the membrane matrix to eventually release the gas molecules at the permeate side. Typically, there are two types of facilitated transport of gaseous molecules by a carrier through the membrane: (1) liquid membrane with a mobile carrier and (2) solid membrane with a fixed carrier. These are schematically presented in Fig. 10.5.

FIG. 10.5 Scheme for facilitated transport of gaseous molecules by a carrier (complex) through a membrane: (A) liquid membrane with a mobile carrier; (B) solid membrane with a fixed carrier [3].

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Chapter 10 Synthetic polymer-based membranes for oxygen enrichment

Facilitated transport of O2 was first introduced by Tsumaki in 1938 using cobalt Schiff’s base O2 carrier [6]. Calvin et al. [90] discussed in detail the rapid and reversible chemical complexation of O2 with cobalt Schiff’s base. In particular, the incorporation of cobalt complexes such as cobalt porphyrin (CoP) and cobalt phthalocyanine (CoPc) into the polymer matrix leads to facilitated transport of O2 molecules over N2 [91]. The CoP complex tends to form a pocket resembling a cavity around the porphyrin plane and chemical bonding occurs between the O2 and metal complex rapidly and reversibly [3]. Shoji et al. [92] investigated the embedding of CoP in a Nafion membrane as a fixed carrier to facilitate O2 transport through the membrane. The O2 permeability through the membrane containing CoP was higher than the N2 permeability and increased with the content of CoP in the membrane. For instance, by increasing the concentration of CoP from 0 to 20 wt.%, the O2/N2 selectivity increased from 2.1 to 14, respectively. The incorporation of cobalt tetraazaporphyrin into poly(octyl methacrylate)-co-vinylimidazole was employed as an O2-facilitated transport membrane [93]. The O2 permeability and O2/N2 selectivity were 2.9 Barrer and 28, respectively. Nishide et al. [94] prepared the poly (vinylidene dichloride-co-vinylimidazole) membranes containing meso-tetrakis-(α, α, α, α-opivalamidophenyl) porphyrinatocobalt (CoP) as the fixed carrier to achieve high-performance membranes. An O2/N2 selectivity of greater than 100 was also reported at >20 wt.% filler loading. CoPcs are classified as macrocyclic complexes and are widely used in the synthesis of dyes and catalysts. The most important advantages of CoPcs are their facile synthesis and excellent stability [91]. The physicochemical properties of CoPcs can be tuned by the introduction of functional groups onto phthalocyanines. Kurdi and Tremblay [95] studied the incorporation of magnesium (II) phathalocyanine as an excellent additive in PEI as a membrane matrix to its performance for O2/N2 separation. The O2 permeability increased with the additive content but O2/N2 selectivity reduced. The effects of filler (CoPc) content on the performance of poly(octyl methacrylate-covinylimidazole) membranes was also evaluated [96]. Permeability of O2 and O2/N2 selectivity increased by 318% and 1300% with an increase in the carrier complex content from 20 to 40 wt.%. Nagar et al. [91] studied the effect of low loading of CoPc on the permeation properties of Peba1657 as the membrane matrix. The permeabilities of O2 and O2/N2 selectivity were enhanced from 0.06 to 1.2 GPU and from 2.9 to 8.5 with an increase in CoPc loading from 0 to 1 wt.%, respectively. Moreover, the molecular dynamic simulation showed that O2 transport through the membrane was preferentially facilitated due to the presence of the CoPc complex in the Peba1657 matrix.

10.2.3 Organic/inorganic hybrid membranes 10.2.3.1 Mixed matrix membranes MMMs can be fabricated by embedding inorganic materials into an organic polymer to (1) combine the desirable features of polymeric materials (such as good processibility and modularity) with those of inorganic materials (good mechanical/thermal stability) and (2) overcome permeability/selectivity trade-off. Several high-quality review papers have been published on the development and future prospectives of MMMs as an alternative option to the existing membrane [13,97–100]. There are several types of inorganic materials including graphene, zeolites, carbon nanotubes (CNTs), metals and metal oxides, metal organic frameworks (MOFs), covalent organic frameworks (COFs), zeolite imidazolate frameworks (ZIFs), carbon molecular sieves (CMS), etc. Compatibility between the organic and inorganic phases is the main concern in the preparation of MMMs [101,102]. The poor compatibility between nanomaterials and the polymer not only adversely affect the desirable properties of

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205

nanocomposite membranes, but may also undermine the intrinsic properties of the polymer matrix [103,104]. The surface modification of nanomaterials improves the compatibility of the organic/inorganic phases. Some functional groups and modifying agents have been used to improve the interfacial interactions between organic and inorganic phases [105,106]. These include but are not limited to silane coupling agents, amino groups, carboxylic and hydroxyl groups, and sulfonic acid groups as well as some additives such as polyethylene glycol monomethyl ether, ionic liquids, polydopamine, and poly(ethylenimine). Zeolites have been commonly used for the fabrication of MMMs owing to their thermal stability and excellent separation and permeation properties. Many types of zeolites with different structures, dimensions, and pore sizes (e.g., 4A, 5A, ZSM-5,13X, NaY, SAPO-34) have been synthesized, which significantly affected the sorption and diffusion of gas species. The literature survey reveals that zeolite ˚ , which can A is more suitable than other types for O2/N2 separation because of its pore size of 3.8 A accurately discriminate between the size and shape differences of sphero-cylindrical O2 and N2 molecules [107]. Wang et al. [108] prepared MMMs by incorporating zeolite 4A in the PSf matrix for O2/N2 separation. The O2 permeability and O2/N2 selectivity of the membrane with 25 wt.% filler loading increased by 38% and 30%, respectively, compared to the PSf membrane. The SEM micrograph also showed that no voids were observed between the filler and PSf, indicating that polymer chains completely interact with the zeolite A nanoparticles. Pechar et al. studied the amine functionalization of zeolite L [109] and ZSM-2 [110] with aminopropyl triethoxy silane (APTES) as the silane coupling agent, and their embedding into 6FDA-6FpDA-DABA as the polymer matrix. The FESEM images did not reveal the presence of voids between the polymer and the zeolite. Meanwhile, a superior change in the gas permeability of MMMs was observed compared to the pristine one, due to partial blockage of the zeolite pores by the silane coupling agent and permeation of the gas species through the polymer matrix instead of zeolite. CMSs can be produced from the pyrolysis of thermosetting polymers such as PI, phenolic resin, PAN, etc., and are able to effectively discriminate between gas species with a very similar size. CMS particles seem to be more compatible with glassy polymers, hence providing uniform dispersion of filler in the polymer matrix and good adhesion at the polymer/filler interface [107]. Vu et al. [111] studied the incorporation of CMS derived from the pyrolysis of the Matrimid precursor into two different polymers (Ultem 1000 and Matrimid 5218). The O2/N2 selectivity of the CMS-Ultem and CMSMatrimid MMMs increased by 8% and 20%, respectively. The O2 permeability of MMMs was also significantly enhanced compared to the pure Ultem 1000 and Matrimid 5218 membranes. In another study [112], CMS particles derived from the pyrolysis of PAN as the polymer precursor were embedded into the PSf (Udel P-1700) matrix. FESEM images showed that a good polymer-CMS adhesion was achieved in MMMs, even at high sieve loading (up to 35 wt.%). At 20 wt.% of CMS loading, the O2/N2 selectivity attained the highest value, which is 5.97 with the O2 permeability of 7.9617 Barrer. MOFs are organic/inorganic hybrid porous materials tailored by connecting metal complexes with organic linkers to obtain fascinating adjustable pore geometries and flexible frameworks [101]. MOFs possess various advantages compared to other porous inorganic additives: (1) Organic ligands are an intrinsic part of MOFs and allow additive particles to interact better with the polymer matrix and its functional group [113], (2) tunable cavities in terms of size, shape, and chemical functionalities, which can be adjusted by selecting suitable ligands during synthesis and postsynthetic functionalization processes [114], and (3) MOFs commonly offer a higher pore volume and a lower density than other fillers (e.g., zeolites), which provide greater impact for a given filler loading [114]. Zornoza et al. [115]

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Chapter 10 Synthetic polymer-based membranes for oxygen enrichment

prepared PSf MMMs using two different kinds of MOF (HKUST-1 and ZIF-8). The ZIF-8-PSf MMMs have the best performance results (O2/N2 selectivity ¼ 8.3 with 2.6 Barrer for O2 permeability) due to an increase in free volume together with a molecular separation effect because of the smaller pore size of ZIF-8. In another work [116], the effects of UiO-66 (Zr) and MIL-101 (Cr) as filler on the permeation and structural properties of the polyurethane ether base 1185 A-10 (Elastollan) were studied. The membrane containing UiO-66 (Zr) showed higher selectivity (5.5) compared to the pristine membrane (4.0) due to the significant increment (128%) in O2 permeability. For the MIL-101 (Cr) membrane, a slight increase in selectivity (4.2) and a significant change in O2 permeability (246%) were observed. The PICu3 (BTC)2 MOF hollow fiber membrane was prepared and used for gas separation [117]. By increasing the filler loading up to 3 wt.%, the permeability of both oxygen and nitrogen improved and reached 40 and 8 GPU, respectively. However, with a further increase in filler loading up to 6 wt.%, the oxygen and nitrogen permeability decreased to around 30 and 7 GPU, respectively. This result can be related to the pore blockage of MOF by polymer chains and rigidification of PI. The use of MIL-53 [118], CuBPY-HFS [119], and CuTPA [120] as filler is also reported in the literature. In recent years, CNTs have been extensively used as impressive filler in polymer composite preparation with diverse properties through the detailed control of the processing and fabrication procedure. Among single-walled CNTs (SWCNTs) or multiwalled CNTs (MWCNTs), MWCNTs are more desirable because of their relatively low cost and availability in larger quantities. Kim et al. [121] studied the incorporation of CNTs that were functionalized with long chain alkyl amines into the PSf matrix. Both the permeability and selectivity of the MMMs increased with increasing the filler loading compared to the PSf membrane. With the addition of 10 wt.% CNTs, the O2 permeability and O2/N2 selectivity increased from 0.84 to 1.23 Barrer and from 5.07 to 5.35, respectively. Peba1657 MMMs containing surface-modified MWCNTs with carboxylic and amine groups were applied for gas separation [122]. The amine-functionalized MMM showed 120% enhancement in the selectivity of O2/N2 compared to the neat membrane without a reduction in the O2 permeability. They argue that this result could be attributed to the homogeneous dispersion of the MWCNTs within the polymer matrix. Graphene, another type of sheet-like material, is known as a new category of carbon-based material and has excellent properties similar to CNTs. Graphene possesses nanopores that make it a very effective membrane material for the separation of gas species based on the molecular sieving mechanism [123]. Graphene-based materials are an inexpensive nanomaterial that could be employed as a suitable alternative for CNTs in the preparation of nanocomposites due to their mechanical, structural, and thermal properties. Reduced graphene oxide was successfully embedded into the PES matrix and its permeation and structural properties were studied [124]. It was found that the addition of filler led to a significant enhancement in gas permeability. The increment in permeability without a notable change in selectivity might be attributed to the poor compatibility between nanomaterials and PES that might have triggered the formation of the nonselective transport of gas molecules, which worsen the selectivity. Metal and metal oxide nanoparticles have gained increasing interest because of their large diversity of compounds, availability in various shapes and sizes, and capability to have different functionalities. Incorporation of metal oxides into the polymer matrix normally applies similar gas transport behavior as that of impermeable nanomaterials such as silica, which is the result of the disruption of chain packing, an agglomeration of nanoparticles, and the creation of more membrane FFV [107,125]. MMMs comprised of amine-functionalized TiO2 as filler and PSf as the polymer matrix were fabricated [126]. Functionalized TiO2 particles were not agglomerated and well dispersed throughout the PSf

10.2 Membrane materials

207

matrix. The amine group of filler has a stronger interaction with polar gas such as CO2 and O2 than nonpolar gas such as CH4 and N2. The O2 and N2 permeability decreased from 0.60 to 1.1 and 0.45 to 0.69, respectively, when the filler loading increased from 2 wt.% to 10 wt.%. The gas permeation properties of PDMS MMM containing modified silica (SiO2) nanoparticles were also investigated [127]. The surface of the SiO2 nanoparticles was modified using two coupling agents: dimethyloctyl silane (DMOS) and dimethylphenyl silane (DMPS). All prepared MMMs exhibited reduced O2 and N2 permeabilities and a modest increase in selectivities as compared to pure PDMS. Jiang et al. [128] evaluated the incorporation of magnesium oxide (MgO) particles into Matrimid 5218 to generate MMMs. The O2 permeability initially decreased with increasing the filler concentration. This is followed by a continuous increase with a further increase in the filler concentration. This could be due to the presence of a balance between the polymer chain rigidification and the intrinsic transport properties of MgO particles. The O2/N2 selectivity continuously decreased with increasing the MgO loading due to the nonselective pore structure of MgO.

10.2.3.2 Polymer magnetic membranes “Magnetic membranes,” as a new concept for O2/N2, were first introduced by Strzelewicz and Grzywna in 2007 [129] on the basis of the difference in magnetic properties of O2 and N2 when they were exposed to the magnetic field, that is, O2 is paramagnetic whereas N2 is diamagnetic in nature [6]. These substantial differences in response to the magnetic field can provide a chance to effectively separate the O2/N2 gas pair. This concept can be applied using two methods [4]: (1) small magnets are incorporated into the polymer matrix and/or (2) big magnets are placed on both sides of the permeation cell. Different types of magnetic praseodymium and neodymium microparticles have been used as filler to be embedded in various polymer matrices such as ethyl cellulose (EC) and PIs [4]. Rybak et al. [130] studied the addition of neodymium powder into EC solution, and then cast the membranes in an external magnetic field to magnetize the powder. The oxygen transport increased as magnetic field induction increased. Almost 56% of the oxygen enrichment was achieved by using a magnetic field induction. Velianti et al. [131] prepared the magnetic membrane using two common methods: (1) the addition of a superparamagnetic particle into the dope solution and (2) the deposition of a superparamagnetic particle on the surface of the polymeric membrane. Results indicated that the deposition of particles on the membrane surface provided better O2 separation performance with an increase in O2 permeance and the O2 mass transfer coefficient by about 580% and 11%, respectively. However, the selectivity is too low. The effects of the polymer matrix (EC and PPO) and different types of magnetic microparticles as filler (praseodymium and neodymium powder) were also studied [132,133]. It was observed that gas transport improved with the increase of membrane permeance, saturation magnetization, and magnetic particle filling. In order to track the recent developments of the permeability/selectivity trade-off in polymeric membranes, Fig. 10.6 was prepared by Himma and Wenten [5]. It is obvious that most surfacemodified membranes surpass the 1991 upper bound and some of them are above the 2008 upper bound. In addition, the permeability and selectivity of PIMs seems to be near the 2008 upper bound, or a few may lie above. Table 10.1 summarizes some different types of polymeric membranes developed for O2/N2 separation.

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Chapter 10 Synthetic polymer-based membranes for oxygen enrichment

100 1991 upper bound 2008 upper bound UV irradiation UV-ozone treatment Plasma treatment Plasma polymerization Chemical crosslinking Polyimide Polysulfone based PTMSP Rubbery polymers PPO

(a)

10

(b)

O2/N2 selectivity

(d) (d) (c) (h)

(k)

(f)

(h)

(m)

(i)

(c) (g)

(a) (i)

(e)

(l)

(n) (j)

(b) (f) (e) (k) (l)

(n)

(m) (j)

1 (g)

0.1 0.1

1

10

100

1000

10,000

O2 permeability (Barrer)

FIG. 10.6 Comparison of O2/N2 separation performance of some polymeric membranes [5].

Table 10.1 Some different types of polymeric membranes developed for O2/N2 separation. Membrane material

Additive or filler

O2 permeability (Barrer) or permeance (GPU)

O2/N2 selectivity

Ref.

21.6 GPU 40.9 Barrer 12 GPU 14.3 GPU 18.1 GPU 4.85 GPU 6.52 GPU 4.85 GPU 1.5 GPU 511 Barrer 8.15 Barrer

5.4 5.1 6.2 6.8 6.6 3.9 4.9 5.2 7.5 2.0 3.2

[34] [35] [36] [37] [39] [42] [42] [42] [41] [43] [49]

Homopolymers PSf PSf PES PES PI (Matrimid) PI (6FDA-BAPAF) PI (6FDA-DAP) PI (6FDA-DABA) PEI (Ultem) PDMS PU

– – – – – – – – – –

Table 10.1 Some different types of polymeric membranes developed for O2/N2 separation— cont’d Membrane material

O2 permeability (Barrer) or permeance (GPU)

O2/N2 selectivity

Ref.

– – – –

3.1 Barrer 121 GPU 3–36 GPU 3 Barrer 0.58 Barrer

5.9 2.1 9.9 4 5.3

[45] [46] [47] [49] [48]

– – –

15.2 Barrer 4.73 Barrer 2.18 Barrer

5.9 6.3 >6

[50] [50] [51]

– –

231 Barrer 167 Barrer

2.1 2.2

[53] [53]

– – – – – – – –

370 Barrer 190 Barrer 786 Barrer 3–5 m3 m2 h1 bar1 0.38 m3 m2 h1 bar1 14–747 Barrer 193 Barrer 1092 Barrer

4.0 4.5 3.3 2.3–3.1 4.3 4.4–6.1 3.8 3.8

[32] [32] [67] [69] [69] [72] [73] [73]

CoP cobalt tetraazaporphyrin CoPc CoPc

– 2.9 Barrer

14 28

[92] [93]

1.2 GPU 83.7 Barrer

8.5 56

[91] [96]

Zeolite 4A ZSM-5 CMS CMS ZIF-8 UiO-66 (Zr) MIL-101 (Cr) Cu3 (BTC)2 CNTs r-GO

1.8 Barrer 5.73 Barrer 24–435 Barrer 7.9 Barrer 2.6 Barrer 6.1 Barrer 9.7 Barrer 40 GPU 1.23 Barrer 2.8 GPU

7.7 4.7 8.7–13.3 5.9 8.3 5.5 4.2 5 5.3 1.12

[108] [110] [111] [112] [115] [116] [116] [117] [121] [124]

Additive or filler

Polymer blends PVP/EC (50/50) PES/PI/PI (50/25/25) PES/Matrimid 5218 (50/50) PU/PMMA (60/40) PU/PVAc/Pluronic (85/15/4) Copolymers 6FDA-TAB 6FDA-PMDA Poly(styrene-co-acrylonitrile)block-polystyrene PAI-g-PDMS PA-g-PDMS Nanoporous polymer PIM-1 PIM-7 PIM-1 PIM1-CO1-50 PIM1-CO6-50/PEI TR-1 XTR-PBOI-10 tPBO Facilitated transport Nafion Poly(octyl methacrylate)-covinylimidazole Peba1657 Poly(octyl methacrylatecovinylimidazole) Mixed matrix membranes PSf 6FDA-6FpDA-DABA Matrimid 5218 PSf PSf Elastollan Elastollan PI PSf PES

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Chapter 10 Synthetic polymer-based membranes for oxygen enrichment

10.3 Conclusion The synthesis of high-performance polymeric membranes for oxygen enrichment is a vibrant and productive field of research. Although several new materials have been synthesized in the laboratory, low selectivity and moderate permeability hamper their industrial use. However, some of the emerging materials such as nanoporous polymers (PIMs and TR polymers) and organic/inorganic hybrid membranes based on novel inorganic materials such as zeolite 4A, CSM, and MOFs seem to be promising. Therefore, they could be suitable options to replace conventional polymers in large-scale applications. In addition, applying surface engineering strategies can considerably improve the separation performance of existing polymeric membranes.

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CHAPTER

Synthetic polymeric membranes for gas and vapor separations

11

Seyed Abdollatif Hashemifard, Arash Khosravi, Farideh Abdollahi, Zahra Alihemati, Mohsen Rezaee Sustainable Membrane Technology Research Group (SMTRG), Faculty of Petroleum, Gas and Petrochemical Engineering (FPGPE), Persian Gulf University (PGU), Bushehr, Iran

Chapter outline 11.1 Introduction .............................................................................................................................. 218 11.2 Membrane classification and fabrication .................................................................................... 218 11.2.1 Membrane materials and structures ....................................................................... 218 11.2.2 Membrane shapes and modules ............................................................................ 219 11.2.3 Dense membranes ............................................................................................... 222 11.2.4 Integrally asymmetric membranes ......................................................................... 224 11.2.5 Thin-film-composite membranes ........................................................................... 226 11.3 Mixed matrix membranes ........................................................................................................... 229 11.3.1 Definitions and properties .................................................................................... 229 11.3.2 Theoretical models .............................................................................................. 231 11.3.3 Mixed matrix membrane materials ......................................................................... 238 11.3.4 Preparation methods of mixed matrix membranes ................................................... 239 11.3.5 Methods for avoiding nonideal interfacial defects ................................................... 241 11.4 Membrane performance and characterization ............................................................................. 242 11.4.1 Scanning electron microscopy ............................................................................. 243 11.4.2 Transmission electron microscopy ....................................................................... 244 11.4.3 Thermogravimetric analysis ................................................................................. 244 11.4.4 Differential scanning calorimetry ......................................................................... 246 11.4.5 Atomic force microscopy ..................................................................................... 246 11.4.6 Dynamic mechanical and thermal analysis ........................................................... 247 11.4.7 Fourier transform infrared ................................................................................... 248 11.4.8 Positronium annihilation lifetime spectroscopy ..................................................... 249 11.4.9 Gas permeation tests .......................................................................................... 249 11.4.10 Solubility measurement ...................................................................................... 252 11.5 Industrial applications ............................................................................................................... 253 11.5.1 CO2 removal ........................................................................................................ 253 11.5.2 Hydrogen recovery ............................................................................................... 255 11.5.3 Air separation ...................................................................................................... 256

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00011-3 # 2020 Elsevier Inc. All rights reserved.

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11.5.4 Air and gas dehydration ........................................................................................ 257 11.5.5 Separation of volatile organic compounds from N2 .................................................. 257 11.5.6 LPG recovery ....................................................................................................... 258 11.6 Challenges ................................................................................................................................ 260 11.6.1 Plasticization ...................................................................................................... 260 11.6.2 Aging .................................................................................................................. 263 Acknowledgments .............................................................................................................................. 264 References ........................................................................................................................................ 264 Further reading .................................................................................................................................. 272

11.1 Introduction The separation and purification of gas and vapor mixtures are important parts of traditional and modern industries. In the past decades, membrane-based gas separation systems have been studied widely in the literature. These systems were first used in 1980 for hydrogen separation on an industrial scale. Nowadays, membrane-based systems can successfully be coupled with different types of industrial separation processes such as pressure swing adsorption (PSA), cryogenic distillation, and absorption [1–3]. Principally, this type of separation is based on differences in the permeation rate of species passing through the membrane. Membrane gas separation technology has many advantages compared to conventional separation processes, including omitting the phase change during the process, lower energy consumption, lower operating temperature, lower investment costs, and being environmentally friendly [4]. Many studies have been performed on membranes gas and vapor separation. This technology has been used in industry for various aims such as separation of H2 in petrochemical and chemical industries, natural gas sweetening and conditioning, landfill gas recovery, flue gas separation, air separation, etc. [5–8]. The conventional polymeric membranes show trade-off limitations between permeability and selectivity shown in Robeson’s upper bound [9, 10]. In addition, there are challenges such as plasticization and physical aging [2, 11]. Therefore, to overcome the problems of polymeric membranes, their modification by the incorporation of fillers has been proposed. Recently, numerous studies have been carried out on nanocomposite membranes and mixed matrix membranes for gas and vapor separation [12, 13]. The focus of this chapter is mostly on the recent studies that have been performed on gas and vapor membrane separation while the basic principles of the membrane technology can be found elsewhere [3, 14, 15].

11.2 Membrane classification and fabrication 11.2.1 Membrane materials and structures The membranes used in gas separation processes are widely made of two types of materials: polymers [16–18] and inorganic materials such as ceramics [19] and metals [20]. The use of polymeric membranes has attracted a great deal of attention in the field of gas separation. These membranes have rather high selectivity but show low productivity because of their low free volume compared to porous materials. In addition, it is not possible for polymers to tolerate high temperatures and harsh chemical

11.2 Membrane classification and fabrication

219

Membrane structure for gas seperation

Symmetric membranes

Dense membranes

Asymmetric membranes

Composite membranes

Integrally asymmetric membranes

FIG. 11.1 The structures of gas separation membranes.

environments. Inorganic membranes have several advantages such as higher permeability and selectivity compared to polymeric membranes. In addition, inorganic membranes have good resistance against harsh chemical conditions and also sustain high pressures and temperatures. However, the main disadvantage of inorganic membranes is that selectivity could be regarded as a strong function of process conditions, especially temperature, pressure, and mole fraction of the condensable species in the feed [20]. The other disadvantages of inorganic membranes are their low membrane surface per module volume and brittleness properties [21]. Designing organic-inorganic hybrid membranes known as mixed matrix membranes (MMMs) can be considered a new approach to combine the prominent properties of both organic and inorganic materials [20] explained in the proceeding sections. The structural classification of gas separation membranes is shown in Fig. 11.1. Both symmetric and asymmetric membranes are used for gas separation processes. Each membrane structure along with the related fabrication method is described in Sections 11.2.3–11.2.5.

11.2.2 Membrane shapes and modules In addition to the different structures, membranes used in gas separation processes are geometrically different. Gas separation membranes can be made in three shapes: (i) flat sheets, (ii) tubes, and (iii) hollow fibers. The asymmetric flat sheet membrane is depicted in Fig. 11.2. A module is the building block of a membrane system [22]. Flat sheet membranes are used in the form of plate and frame or more effectively spiral wound modules [23]. Spiral-wound modules contain a number of membrane envelopes wound onto a central perforated collecting tube. Each membrane envelope is made of two membranes separated by a permeate spacer and sealed on three sides, but there is one open width-edge for the permeate flow [24].

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Chapter 11 Synthetic polymeric membranes

Dense layer

Porous layer

FIG. 11.2 Schematic of a polymeric flat sheet membrane.

Outer dense layer Inner dense layer Porous layer

FIG. 11.3 Schematic of a polymeric hollow fiber membrane.

Fig. 11.3 shows a hollow fiber membrane. Hollow fiber membranes are thin and fine capillaries. The outer diameter of hollow fiber membrane is generally between 1.0 and 1.5 mm. The inner diameters of tubular membranes are located generally within 5–25 mm [22]. Tubular and hollow fiber membranes are fixed in bundles and potted in parts, which resemble shell-and-tube heat exchangers. The tubular membranes are not superior to flat sheet membranes in terms of high pressure resistance and moreover have lower packing densities relative to flat sheet membranes [23]. Hollow fiber modules are also extensively used in gas separation processes [24] because of their high surface-area-to-volume ratio and self-supporting properties. These characteristics enable us to process a large volume of gas streams [25]. However, hollow fibers have disadvantages including the high pressure drop of the gas flow through the membrane lumen side, which is an important criterion in designing a membrane separation unit [23]. Fig. 11.4 illustrates the general types of membrane modules used for gas separation processes. Both the hollow fiber and the flat sheet membranes are commercially utilized. Selecting a specific membrane shape is dependent on various factors such as the nature of the polymer, the reproducibility of a given structure, the membrane separation and mechanical characteristics, the ease of formation of a certain configuration and structure, the nature of the gas mixture, and the economic aspects [23].

11.2 Membrane classification and fabrication

221

FIG. 11.4 General types of modules used for gas separation processes: (A) plate-and-frame module, (B) spiral-wound module, and (C) shell and tube.

222

Chapter 11 Synthetic polymeric membranes

11.2.3 Dense membranes The structure of a dense membrane is nonporous, uniform [23], and shows identical permeation properties throughout the cross-section at different parts [24]. The main mechanism of gas transport through a dense membrane is widely considered as solution-diffusion, which is generally a three-step phenomena. In the first step, the surface of the membrane adsorbs the gas molecules onto the upstream side. Next, the gas molecules diffuse across the polymeric matrix and finally, the gas molecules are desorbed from the other side [20]. Fig. 11.5 depicts the solution-diffusion mechanism schematically. Dense membranes are of high scientific value and are extensively used in the laboratory to study the polymer intrinsic properties [26]. Dense membranes with homogeneous structures may only be attractive to commercial sectors if they are produced as very thin films [23]. Dense or nonporous membranes can efficiently separate gases from each other; however, their transport rates are normally very low because of their high thickness. The solution-diffusion model is used to describe the transfer of gases and vapors through dense polymeric membranes, depicted in Fig. 11.5 [27]. In this model, the basic assumption is that there is not any porosity in the membrane selective layer, so different components are separated from each other due to their difference in solubility and/or diffusivity in the membranes. According to this model, the gas molecules are adsorbed by the membrane on the high-pressure side (feed side), then they diffuse through the membrane and desorb on the low-pressure side (permeate side) of the membrane [28]. Based on this model, gas permeability is defined as below [27]: PA ¼

JA l pA, 0  pA, 1

(11.1)

where PA is pure gas permeability, l is membrane thickness, pA,0 is the partial pressure of A on the feed side, and pA,1 is the partial pressure of A at the permeate side. The following equation is presented to calculate gas fluxes in dense membranes according to Fick’s law [27]. JA ¼ DA SA

FIG. 11.5 Solution-diffusion mechanism in a dense membrane.

pA, 0  pA, 1 l

(11.2)

11.2 Membrane classification and fabrication

223

DA is the diffusivity coefficient (a kinetic factor). SA is the solubility coefficient (a thermodynamic factor) of A compound in the membrane. By comparing Eqs. (11.1), (11.2), the permeability is defined as a function of solubility and permeability, leading to Eq. (11.3) [28, 29]. PA ¼ SA  DA

(11.3) 10

The permeability of the dense polymeric membrane is often expressed in Barrers (1 Barrer ¼ 10 cm3 2 1 1 (STP) cm cm s cmHg ). The membrane ideal selectivity is defined as a factor that shows the ability of the membrane to separate gas molecules, and it is defined by Eq. (11.4) [29]. αA=B ¼

PA SA DA ¼  PB SB DB

(11.4)

where SA/SB is called solubility selectivity and DA/DB is called diffusivity selectivity. Generally, in conventional glassy polymers, the ratio of the diffusion coefficients (DA/DB) is an effective factor on selectivity, which represents the size difference of the two molecules. In rubbery polymers, the ratio of the solubility coefficients (SA/SB) is an effective factor on selectivity, which represents the solubility of the two molecules in the membrane materials [28–30]. Free volume is another important factor that has a key role in the gas transport mechanism in the membrane. Free volume is void space between polymer chains. It is one of the most important structural factors affecting the transport of gas molecules in polymers [31]. The relationship between the diffusion coefficient and the free volume is determined by the Cohen-Turnbull model. This relation that is shown in Eq. (11.5) predicts that as the free volume increases, the diffusion coefficient increases by an exponential function [31–33].
 γν∗ D ¼ A exp  FFV

(11.5)

where D is the diffusion coefficient, A is a constant, ν* is a parameter related to the size of the gas molecule, γ is an overlap factor, and FFV is the fractional free volume of the polymer. Eq. (11.6) can be used to calculate FFV [31]. FFV ¼

V  V0 V

(11.6)

where V is the experimental specific volume of the polymer and V0 is the theoretical occupied volume of the polymer chains. Consequently, increasing the free volume will increase the diffusion coefficients [34].

11.2.3.1 Fabrication of dense membranes Polymer solutions or polymer melts are widely used to prepare dense symmetric membranes, as briefly described below [23].

Solvent vaporization or dry phase inversion In order to carry out the solvent vaporization, a polymer is dissolved in a volatile suitable solvent. First, a thin film of this solution is cast onto a glass plate. Then, the volatile solvent is allowed to be evaporated for a period of time in ambient temperature and pressure. Next, using some water, the thin film is released from the glass plate. Finally, the membrane is dried under vacuum to eliminate the residues of the solvent [35].

224

Chapter 11 Synthetic polymeric membranes

Melt extrusion It is also possible to prepare a dense membrane by means of the melt extrusion technique [26]. The fabrication mechanism involved a polymer melts which finally converts to a solid film [23]. Many polymers, best exemplified by polyethylene, polypropylene, and nylons, do not dissolve in normal solvents at room temperature, hence the membranes cannot be made by solution casting. To form a small piece of the film, a laboratory apparatus is normally used to press the polymer melt. The polymer is compressed between two heated plates. Typically, a pressure of 2000–5000 psi is applied for 1–5 min, at a plate temperature just below the melting point of the polymer [36].

11.2.4 Integrally asymmetric membranes Integrally asymmetric membranes are composed of a very thin skin layer on a porous support layer from the same material. These types of membranes are fabricated in both the hollow fiber and the flat sheet forms [24]. The porous support layer shows insignificant resistance against the gas flux. The ultrathin skin layer is responsible to the separation. This layer is very permeable because of its tiny thickness. Therefore, not only high-permeable polymers but also high-selective polymers have to be chosen for the sake of a particular gas separation process [37]. A single-step process is used to fabricate asymmetric membranes. During this process, the thin skin layer becomes an integral part of the microporous substrate. This makes the skin layer easier to be handled and protected. However, there is a major disadvantage because of the formation of pinholes and defects in the skin layer lowering the separation performance of the resultant membrane [26]. Fig. 11.6 shows a schematic of an integrally asymmetric membrane.

11.2.4.1 Synthesis methods: Phase inversion method Nowadays, there are numerous techniques to prepare asymmetric membranes. The most commonly utilized technique is the nonsolvent induced phase separation (NIPS) technique, which is one kind of phase inversion method. This method is known as the Loeb and Sourirajan technique. During this process, a homogeneous solution is cast onto a support in the form of a uniform film (flat sheet) or spun as a hollow fiber and further immersed in a nonsolvent bath. The homogeneous solution commonly Dense skin layer

Support layer

FIG. 11.6 Schematic of an integrally asymmetric membrane.

11.2 Membrane classification and fabrication

225

FIG. 11.7 Preparation of integrally asymmetric flat sheet and hollow fiber membranes via phase inversion method.

contains a polymer and a solvent. The nonsolvent is miscible with the solvent, but it is immiscible with the polymer. To form the membrane, solvent and nonsolvent are exchanged by means of different demixing mechanisms such as phase separation and precipitation of the polymer, that is, solidification. The most important determining factors in the structure formation of the resultant membrane are the composition and temperature of both the dope solution and the coagulation bath [38]. The fabrication of an integrally asymmetric flat sheet and a hollow fiber membrane via the phase inversion method are shown in Fig. 11.7. In addition, there are other techniques where the preparation of asymmetric membranes is performed using a dry phase inversion technique at room temperature. As an alternative to the coagulation bath, the polymer should first be dissolved in a mixture of less-volatile nonsolvent and volatile solvent. Then, the nonsolvent concentration increases as a result of the evaporation of the volatile solvent from the cast film. This increase leads to the phase inversion of the polymer solution in a polymer-poor and a polymer-rich phase. Further evaporation of the solvent results in the vitrification of the polymer and increases its concentration. It is not possible for a vitrified but still plasticized polymer to enter the phase separation process including a polymer-rich and a polymer-poor phase. Therefore, a dense skin will be obtained, provided that the evaporation is fast enough and the time scale of polymer vitrification at the air-cast film interface is shorter than the pore nucleation time scale. If film drying is still needed, the solvent has to be diffused slowly through the vitrified skin. Therefore, it is still possible for nucleation and pore growth to occur in the underlying film, which leads to the formation of a porous substructure and a dense skin [35]. This method is also known as precipitation by solvent evaporation. There is another method of fabrication in the case of the asymmetric gas separation membranes according to which there is no need for the solvent to evaporate during the formation process and no subsequent coating is also required to obtain the favorable separation properties. In order to prepare membranes, we need to contact a polymer solution with two nonsolvent baths in series. The first bath is required for a concentrated layer of polymer to result in at the interface. This could be compared with a prementioned evaporation step. The second bath is required for the actual coagulation (precipitation).

226

Chapter 11 Synthetic polymeric membranes

Selecting both nonsolvents is strongly dependent on the solvent type that is present in the polymer solution [38].

11.2.4.2 Case study One of the applications of asymmetric membranes is CO2 capture from flue gas. Climate change is an environmental concern because of the rapid increase in CO2 levels in the atmosphere. Membranes for gas separation processes have been efficiently commercialized for this purpose [39]. Table 11.1 summarizes the literature related to the application and performance of integrated asymmetric membranes for the gas separation process.

11.2.5 Thin-film-composite membranes Compared to the numerous techniques previously described for asymmetric membrane fabrication, thin-film-composite (TFC) membranes allow for the individual optimization of the properties of the dense active layer and the porous support layer to a greater extent. For each composite membrane, two processes need to be performed separately: support layer fabrication and active layer formation [44]. TFC membranes are composed of an asymmetric substrate membrane that is porous in nature and a coating layer that is applied to the top surface of the substrate membrane [45]. Indeed, using a composite structure consisting of a moderately inexpensive polymer material covered with a thin active layer of an expensive selective polymer decreases the overall membrane material cost as well [36]. Fig. 11.8 demonstrates a schematic of a TFC membrane. For fabricating high-performance TFC membranes, the properties of the porous supporting layer and the dense active layer should be taken into consideration [46]. The type of polymer, the polymer concentration, the type of solvent and nonsolvent, and the type of additive are the key factors in fabricating the porous supporting layer [47–51]. The effectiveness of the key factors in fabricating a dense active layer depends on the selected technique, which will be discussed in the next section.

Table 11.1 Typical studies conducted on the application of integrated asymmetric membranes for CO2 capture. Material

Structure

Application

Permeance (CO2)

Selectivity for CO2

Ref.

Polyether sulfone/ polyimide Poly(ether ether ketone) Polyimide Polyimide Polyimide

Hollow fiber Flat sheet Flat sheet Flat sheet Flat sheet

CO2/N2

60 GPU

40

[40]

CO2/N2 CO2/CH4 CO2/CH4 CO2/CH4

3.1 GPU 36.7 35 26.5

33 39 94.6 60

[35] [41] [42] [43]

11.2 Membrane classification and fabrication

227

Coated dense skin

Support layer

FIG. 11.8 Schematic of a thin-film-composite membrane.

11.2.5.1 Synthesis methods of TFC Similar to the production of integrally asymmetric membranes, the phase separation method can be applied to fabricate a TFC membrane porous supporting layer. There are three different major methods for fabricating an active layer in TFC membranes: interfacial polymerization (IP), coating, and surface modification [23]. The concept of IP has been recognized for more than 45 years since it was presented by Mogan in 1965 [52]. IP has been regarded as a method of preparing an active layer that is based on a polycondensation reaction between two monomers that are dissolved separately in immiscible solvents. One of these solvents, the aqueous polyamine solution, impregnates the substrate at the beginning. Then, the surface of the substrate contact with the inorganic solution contains carboxylic monomers. Subsequently, a quick formation of an ultrathin film, from several 10 nm to several μm thick, will occur at the interface. The film will remain attached to the porous supporting layer [53]. The commonly used reactive monomers are aliphatic/aromatic diamine such as piperazine (PIP), m-phenylenediamine (MPD), and p-phenylenediamine (PPD) in the aqueous phase and acid chloride monomers such as trimesoyl chloride (TMC) and isophthaloyl chloride (IPC) in the organic phase [52]. Fig. 11.9 shows a synthetic pathway employed to prepare a commercial MPD/TMC polyamide membrane. In another method for fabricating a TFC membrane, the active layer is formed by coating a selective layer on a microporous integrally asymmetric substrate support layer. Indeed, the coating method is an obvious way to make TFC membranes involving directly coating a porous substrate layer with a dilute polymer solution. Then, evaporating the solvent leaves a thin film of polymer on the surface of the substrate [23]. In this method, there might be two possibilities: (a) If the pores of the microporous supporting layer are very large, the resistance of gas flow is very low. In this case, the thin layer coated on top of the microporous supporting layer will govern both the flux and selectivity; (b) in another case, the supporting layer contains a defective integrally dense skin. In this case the supporting layer also controls the membrane performance via the solution-diffusion mechanism. The coated layer is fabricated to stop the

228

Chapter 11 Synthetic polymeric membranes

(A)

(B) FIG. 11.9 (A) Schematic diagram of the procedure for the production of a PA membrane derived from MPD and TMC via IP (B) chemical reaction of MPD and TMC monomers to produce a PA oligomer and HCl [54].

gas leakage by filling the defective pores. It is difficult to understand which mechanism is controlling the system. The dip coating method also can be used for fixing the defects in the membrane prepared by IP [39]. Making the thin active layer of TFC membranes by chemically reacting or electromagnetically irradiating membrane surfaces is a membrane surface modification method. In this method, inert gas plasma for cross-linking or in situ deposition of the polymeric materials to form a dense thin film on substrates is used. The plasma process for fabricating membranes for the gas separation process presents advantages, namely, high corrosion resistance, a highly cross-linked film, a flow-free polymer surface, and high throughput. However, plasma treatment can pose problems in scale-up [23].

11.2.5.2 Case study In 2012, Yuan et al. fabricated TFC membranes for CO2/N2 separation by the IP method from N-methyldiethanolamine (MEDA) and TMC on polydimethylsiloxane (PDMS) covering a polysulfone (PSF) support membrane. The interrelations between the formation conditions of the dense skin layer, the skin layer structure, and the final membrane separation properties were studied. The results revealed that membranes consist of thinner, more cross-linked, and less crystalline skin layer structures with higher CO2 permeance and good CO2/N2 selectivity. Such high-performance gas separation membranes were achieved by (1) decreasing MEDA solubility and increasing MEDA diffusivity in the organic solvent and (2) increasing MEDA concentration in the aqueous phase and reducing TMC

11.3 Mixed matrix membranes

229

concentration in the organic phase. In this research, CO2/N2 selectivity was about 87 and 64 for the two different fabricated membranes, respectively, and CO2 permeance was 1035 and 2905 GPU for the two different fabricated membranes, respectively [55].

11.3 Mixed matrix membranes 11.3.1 Definitions and properties In spite of economic and desired processing properties, the separation performance of polymeric membranes is limited by the polymer upper bounds introduced by Robeson (Fig. 11.10). Inorganic materials exhibit relatively high gas permeability and/or selectivity, but are exceedingly expensive for largescale manufacture as membranes for gas separation [56]. Because inorganic materials almost possess a sharp pore size distribution of precise shape and geometry, they can function as efficient molecular sieves to improve the diffusivity selectivity. Moreover, inorganic materials may offer specific chemical or physical functionality that in nature is not found in polymers, powering these materials to act as selective adsorbents to increase the solubility selectivity, which in turn also leads to improving the membrane separation performance [28]. The concept of a mixed matrix membrane (MMM) was proposed to surpass the above-stated limitations of both polymeric and inorganic membranes [56]. MMMs are formed by a uniform mixing of inorganic (disperse) and polymeric phases (continuous). Fig. 11.11 shows the schematic of an MMM, either a flat sheet or a hollow fiber [58]. According to the structure as shown in Fig. 11.12, MMM membranes can be categorized into two types with respect to the polymer-particle bonding:

Selectivity (αij)

(A) Continuous and dispersed phases bonded by covalent bonds. (B) Continuous and dispersed phases connected by the van der Waals force or hydrogen bonds [59].

Or ga n

ic

me

mb

ran

e

Permeability (Pi)

FIG. 11.10 Robeson graphs to compare different kinds of membranes for gas separation.

230

Chapter 11 Synthetic polymeric membranes

(i)

Dense structure Organic filler (ii)

Support layer

Dense skin

FIG. 11.11 Schematics of polymer/inorganic filler mixed matrix membranes. (i) Symmetric flat dense mixed matrix membrane. (ii) Asymmetric hollow-fiber with a mixed matrix selective skin [57].

Polymer Organic monomers or oligomers +

Copolymerization or condensation

Inorganic material precursors Nanoparticles

(A)

Polymer Organic polymers +

Solution blending

Inorganic nanoparticles (B)

Nanoparticles

FIG. 11.12 Illustration of different types of MMMs. (A) Polymer and inorganic phases connected by covalent bonds, and (B) polymer and inorganic phases connected by van der Waals force or hydrogen bonds [59].

11.3 Mixed matrix membranes

231

Table 11.2 Comparison of the separation properties of polymers and inorganics and the resultant MMMs [57]. Properties

Polymeric membrane

Inorganic membrane

Mixed matrix membrane

Cost Chemical and thermal stability Mechanical strength Compatibility to solvent Swelling Separation performance

Economical to fabricate Moderate

High fabrication cost High

Moderate High

Good Limited Frequently occurs Moderate

Poor Wide range Free of swelling Moderate/high

Handling

Robust

Brittle

Excellent Limited Free of swelling Exceed Robeson upper boundary Robust

According to the open literature, the majority of the MMMs exhibit enhanced permeability rather than selectivity at about a 2:1 ratio compared to a neat polymer membrane [58]. Table 11.2 illustrates the separation properties of typical polymers and inorganics and the resultant MMMs.

11.3.2 Theoretical models The prediction of gas permeability across an MMM is not an easy task due to the heterogeneity of phases in MMMs. Several mathematical models have been proposed to evaluate the gas permeability through MMMs [56]; mostly, the predictions are based on the permeability of the organic phase and the inorganic phase [60]. The models can be primarily categorized into the subsequent two models, one of which is used to estimate the permeation properties of an MMM when both the organic and inorganic phases are permeable and the other that is appropriate for an MMM composed of impermeable inorganic and permeable organic phases [61]. In a better classification, the most important models are classified into two major groups that are capable of predicting either a two-phase system (particle-polymer) or a three-phase one (particle-interfacial layer-polymer). The most widely used models are Maxwell, Bruggeman, Lewis-Nielsen, Pal, Chiew-Galandt, Bottcher, Higuchi, Felske, modified Maxwell, modified Felske, and modified Pal [62]. These models have been adopted from the present ones, either thermal or electrical conductivity models. Because of the close analogy between thermal or electrical conduction behavior and gas permeation through composite materials, the conductivity models are readily compatible to the permeability of gas species in MMMs [60]. The most widely employed model is the Maxwell model, which was primarily developed to predict the dielectric properties of composite materials [28]. The nanoscale morphology of MMM membranes strongly draws the transport properties of mixed phases. However, the interface between the polymer matrix and the dispersed phase occupies a very tiny volume fraction of less than 1010%; it is especially a significant indication of the whole transport property. Fig. 11.13 schematically shows the diagram of various morphologies of MMMs. Case 1 shows an MMM with ideal morphology that is in accordance with the original Maxwell Model prediction; Case 2 indicates the narrow gap between the polymer chains and the filler surface, which leads to the interface voids; Case 3 represents the chain rigidification of the polymer molecule neighborhood of the dispersed

232

Chapter 11 Synthetic polymeric membranes

Membrane

Case 1

Particle

Membrane

Case 2

Ideal morphology

Particle

Interfacial void

Membrane

Case 3

Particle

Rigidified polymer region

Membrane

Case 4

Particle

Pore blockage

FIG. 11.13 Schematic of various organic-inorganic interface morphologies of MMMs.

phase; and Case 4 shows a state in which the particle pores have been partially blocked by the polymer chains [63]. In the absence of an ideal contact between the surface of the nanoparticles and the polymeric matrix, there is a probability for the existence of interfacial gaps between the polymeric phase and the fillers, leading to an increase in the diffusion and permeability coefficients while the selectivity decreases significantly [13]. The connectivity of the interfacial gaps is a structural defect that reduces the performance of nanocomposite membranes. Moore and Koros have also observed these gaps in their studies [64]. On the other hand, if the repulsive interactions between the polymer and the nanoparticles are high or the stress is not uniform, it will be possible that a part of the polymer that is next to the nanoparticle becomes rigid, leading to a reduction in permeability [61]. These defects can be classified in three categories: interface voids, a rigidified polymer layer around the particles, and particle pore blockage. Interfacial voids reduce the selectivity and increase the permeability of MMMs. Gas molecules pass through the voids instead of nanoparticle pores and the polymer free volume. In the rigidified polymer layer, the mobility of polymeric chains around rigid particles is less than that of the bulk polymer, leading to a reduction in permeability and an increase in selectivity. The effect of pore blockage on the MMM comprising porous fillers is dependent on the degree of pore blockage as well as the molecular diameter of gases. Pore blockage significantly decreases the selectivity if the pore size of the

11.3 Mixed matrix membranes

Interface void around the particles

Mixed matrix membrane

(A) Inorganic filler Inorganic particles

233

Polymer matrix

Polymer matrix

Ideal morphology (A mixed matrix with no defects in the polymer-particle interface)

(B)

Rigidified polymer layer around the particles

FIG. 11.14 Ideal MMM (left side), (A) interface void, and (B) rigidified polymer layer around the nanoparticle (right side) [61].

porous filler before blockage is in the range of the molecular diameter of gases. When the original pore size of the fillers is larger than the molecular diameter of gases, pore blockage may increase the selectivity. In Fig. 11.14, the presence of these interface defects is shown around the nanoparticles. In summary, the quality of dispersion of the fillers in the MMM is very important. Generally, the agglomeration of the nanoparticles inside the polymeric matrix is intensified by growing the concentration of the inorganic fillers, leading to an increase in the number of interfacial and interstitial gaps [65–67]. For a small quantity of compatible well-dispersed fillers, the Maxwell model has been used successfully for predicting the separation properties of MMMs with ideal morphology. In order to attain the ideal morphology the inorganic materials such as CNT, zeolites, and MOFs have been used [28]. For predicting the incorporating permeability, the Maxwell model can be converted as follows: Pr ¼

1 + 2∅ðλd  1Þ=ðλd + 2Þ 1  ∅ðλd  1Þ=ðλd + 2Þ

(11.7)

where Pr is the permeability ratio of P/Pm, P is the permeability of MMM, Pm is the permeability of the continuous phase, ∅ is the volume fraction of the dispersed phase, recognized as loading, and λd is the permeability ratio of Pd/Pm where Pd is the permeability of the dispersed phase [60]. However, when the amount of the dispersed phase increases, nonidealities arises because of particle agglomeration and the appearance of interfacial defects between the organic and inorganic phases [28]. Bruggeman initially developed his model for predicting the dielectric constant of composites by the differential effective medium approach. Although the Bruggeman model is an upgrade over the Maxwell model, it has restrictions like that of the Maxwell model. The Lewis-Nielsen model was basically proposed for estimating the elastic modulus of composite materials. Pal originally proposed his model for the thermal conductivity of composites using the differential effective medium approach, taking into account the packing complexity of the dispersed phase. Later, Pal presented the modified Felske

234

Chapter 11 Synthetic polymeric membranes

model. With the same limitations as that of the modified Maxwell model, it is applicable only when the dispersed filler volume fractions is not that high. One can model the three-phase (continuous, dispersed, and interphase) MMM as a pseudo two-phase MMM with the continuous phase being one phase and the combined dispersed-interphase being the second phase. Models for these complex systems are based on the “nested applications” of the Maxwell or Lewis-Nielsen models. First, the model is employed to determine the permeability of the combined phase of dispersed and interphase phases. Then, the permeability of the whole MMM system is calculated by using the models for the round time to the permeability of the continuous and the combined phases. Also, there are other models in the literature such as Bottcher, Higuchi, HIM, and KJN. The KJN model was presented for predicting the gas permeability of MMMs containing tubular fillers [60, 68]. The permeation models for MMMs are summarized and reported, considering the number of phases, in Table 11.3. Hashemifard et al. developed a theoretical model based on a resistance modeling approach to predict MMM performance [70]. In order to develop the proposed model, the element of MMM is considered as a unit cell of body centered cubic (BCC) as shown in Fig. 11.15. The main parameters considered and discussed are dispersed filler loading, polymer matrix permeability, dispersed filler permeability, interphase permeability, and interphase thickness. The results generated from the proposed model have been verified using seven cases through the published experimental data. Excellent agreement has been obtained in most cases between the model results and the selected published values.

Table 11.3 Summary of the main existing permeation models for MMMs [60, 62, 68, 69]. Model

Phase

Equation

Maxwell

Two

Bruggeman

Two

1 + 2∅ðλd  1Þ=ðλd + 2Þ (11.8) 1  ∅ðλd  1Þ=ðλd + 2Þ
 1 λdm  1 ¼ ð1  ∅Þ1 (11.9) P3r λdm  Pr

Lewis-Nielson

Two

Pal

Two

Bottcher

Two

Chiew and Glandts Higuchi

Two

Kang-JonesNair (KJN)

Three

Three

Pr ¼

1 + 2∅ðλd  1Þ=ðλd + 2Þ (11.10) 1  ∅ψ ðλd  1Þ=ðλd + 2Þ 2
 3 Pd
1=3
∅m 1 6 7 Peff 6
Pc
7 ¼ 1  ∅ (11.11) 4 Pd Peff 5 Pc ∅m  Pc Pc

 Peff Pc 1 α+2 ¼ 3∅ðα  1Þ (11.12) Peff Pc   Peff ¼ 1 + 3β∅ + K∅2 + O ∅3 (11.13) Pc 3∅β Pr ¼ 1 + (11.14) 1  ∅β  KH ð1  ∅Þβ2 20 0 1 31 1

Pr ¼

Peff 6B ¼ 4@1  Pm

cos θ 1 C 7 C Pm B φA + @ Aφ5 1 1 Pf cos θ + sinθ cos θ + sinθ α α

(11.15)

11.3 Mixed matrix membranes

235

Table 11.3 Summary of the main existing permeation models for MMMs—cont’d Model

Phase

Felske

Three

Modified Maxwell

Three

Modified Pal

Three

Modified Felske

Three

HashemifardIsmailMatsuura (HIM)

Three

Equation

  P 2ð1  ∅Þ + ð1 + 2∅Þðβ=γ Þ ¼ (11.16) Pm ð2 + ∅Þ + ð1  ∅Þðβ=γ Þ "  # 2ð1  ∅Þ + ð1 + 2∅Þ Peff =Pm P   (11.17) ¼ Pr ¼ Pm ð2 + ∅Þ + ð1  ∅Þ Peff =Pm   2ð1  ∅s Þ + ð1 + 2∅s ÞðPd =Pi Þ (11.18) Peff ¼ ð2 + ∅Þ + ð1  ∅ÞðPd =Pi Þ 2 3

∅m Peff ∗ 1 6 λdl  1 7 7 ¼ 1  ∅s 36
 (11.19) 4 Peff ∗ 5 Pm ∅m λdl  Pi  
 λ  1 ∅z ∅m eff m ∗ ¼ 1 ðPr Þ1=3 (11.20) λeff ∗m  ðPr Þ ∅m Pr ¼

1 + 2∅ðβ  γ Þ=ðβ + 2γ Þ (11.21) 1  ∅ψ ðβ  γ Þ=ðβ + 2γ Þ " ! !#u 1 1  u   u   u  Pr ¼ 1 + ∅II  1 + ∅I 1 ∅iII λi  1 + 1 ∅dI λd  1 + ∅diI λi  1 + 1 (11.22)

Pr ¼

FIG. 11.15 BCC structure considered for particle distribution in MMM [60].

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Chapter 11 Synthetic polymeric membranes

Case II-B

Case II-A

Case II-C

Case V

Case I Case IV

Case III

FIG. 11.16 Schematic illustration of expected morphologies of MMMs HNT/PEI across a dense selective skin layer; Case (I) ideal, Case (II) void (yellow-colored space surrounding the filler is the void), Case (III) rigidification (blue-colored space shows the rigidified region), Case (IV) blocking (black tips shows the blocked parts), and Case (V) blocking + void [71].

A more detailed illustration of different types of interfacial phases was reported by Hashemifard et al. for MMMs comprising large pore size fillers. Five main morphologies including ideal (case I), void (case II), rigidified (case III), pore blocking (case IV), and agglomeration combined with pore blocking (case V) were discussed, as depicted in Fig. 11.16. Also the corresponding selectivitypermeability morphological diagram was proposed and compared with the morphological diagram proposed by Moore and Koros [64]. Case I represents an ideal morphology of MMMs. Case I is desired for gas separation. However, achieving an ideal morphology is very challenging and difficult to obtain. Case II exhibits a void morphology of MMMs. A poor adhesion between HNTs and the polymer leads to the formation of voids between HNTs and the polymer matrix, as shown. Case II can result in an increase in permeability but no significant change in selectivity at low HNT loading, as illustrated in Case II-A. However, as long as the voids do not form channels from one side of the skin layer to the other side, they should have no effect on membrane selectivity, which corresponds to a point for Case II-B. Case II-C exhibits the agglomeration combined with channeling morphology. The latter two morphologies result from poor particle distribution at moderate to high filler loading levels. Case II-B, and especially Case II-C, leads to a dramatic decrease in selectivity along with an increase in permeability. Case III represents a rigidified MMM morphology. In this case, polymer layers around the dispersed particles are rigidified. In comparison to Case I, Case III offers more desirable separation and transport properties because both permeability and selectivity will be increased. This represents a

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237

Selectivity

major difference between the fillers of large and small pore size, which has a significant influence on the transport and separation properties of MMMs. Sometimes, the polymer chain can even fill the pores at different degrees, which has been known as pore blocking morphology. This type of morphology corresponds to Case IV. Another case is a combination of pore blocking and channeling morphologies (Case V in Fig. 11.16). Sometimes, blocking may happen during the functionalization of HNTs or dope preparation stages. Again, poor adhesion between HNTs and the polymer matrix leads to void morphology [70]. In Fig. 11.17, a comparison is made between the morphological diagram proposed by Moore and Koros for MMMs including small pore size fillers with MMMs including the large pore size fillers proposed by Hashemifard et al. This figure reveals the intrinsic differences between small and large pore size fillers and their different influences on MMM separation and transport properties. From Fig. 11.17, it can be observed that the small pore size morphological diagram has rotated 45 degrees clockwise shifted to the large pore size morphological diagram. As a result, the region of ideal morphology for the small pore size fillers overlaps with the rigidified morphology of the large pore size fillers (Case III) and to a lesser extent with the blocking morphology (Case IV) as well. In comparison to Case I, Case III and part of Case IV offer more desirable separation and transport properties due to increases in both permeability and selectivity. This means that the rigidified morphology for the large pore size fillers plays the same role as the ideal morphology for the small pore size fillers.

Case IV (blockage) Rigidification

Case III (rigidification)

Partial blockage Ideal

Sever blockage Case V blockage +channeling)

Void

Case I (ideal)

Leaking

Case II (void)

Permeability FIG. 11.17 Comparison between the small pore size filler MMM morphology diagram (black arrows along with italic black words) proposed by Moore and Koros to the large pore size filler MMM morphology diagram proposed by Hashemifard et al. (colorful area along with red bold words) [71].

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Chapter 11 Synthetic polymeric membranes

11.3.3 Mixed matrix membrane materials The choice of the organic phase, that is, the polymer and the inorganic filler, is very important for fabricating an MMM. The selection of the polymer matrix affects the intrinsic separation properties of the produced membrane. The dispersed phase should be precisely determined by the various shapes and sizes and their compatibility with the polymer phase. The typical criteria involved are as follows: (a) (b) (c) (d)

Occurrence of average particle size within the nanoscale range. Particle dispersability in the continuous matrix. Compatibility of the filler particles with the polymer matrix. Physical and surface properties of both phases.

Koros et al. showed how to suitably choose the molecular sieve and polymer as the filler and matrix, respectively, and also studied new methods to develop a desired MMM with a well-strengthened interfacial zone hindering “nonselective voids.” Porous and nonporous particles are the two dominant categories of dispersed phases that have been widely utilized for MMM synthesis. A nanoparticle acts as a molecular sieve if it is microporous. It has the effect of separating the gas pairs according to the discrimination of their sizes or shapes. The final membrane is characterized by the prominent separation properties of the preferred component. When the pore size is considerably larger than the molecular size, the mechanism of the separation shifts toward the selective surface flow as well as adsorption. The nonporous dispersed phase leads to the reduction in the diffusion rate of the molecules with larger size, which reflects in the matrix tortuous pattern. The existence of nanoscale fillers can enhance the free volume by disrupting the polymer main chains, causing higher gas diffusivity enhancement [72]. From the open literature, various fillers— zeolites, clays, metal oxides, and silica nanoparticles (nonporous silica and mesoporous silica)—have been evaluated as fillers with different polymers to study their effect on the gas separation properties [58, 73]. Recently, along with the development of manmade organic and inorganic materials, a number of distinct materials have been introduced as promising fillers in MMMs. One can categorize them into four major classifications: (I) Microstructural design and changing the assembly of the present advanced fillers (e.g., metal organic framework (MOF), graphene oxide (GO), zeolitic imidazolate framework (ZIF), carbon nanotubes (CNTs), single-walled carbon nanotubes (SWCNTs), etc.). (II) Cosynthesis of advanced fillers (e.g., ZIF-8@GO). (III) Novel organometallic nanostructures (e.g., coordinated ligands, porous coordination polymers (PCPs), metal organic polyhedras (MOPs), ion-loaded macromolecules microporous, organic/ inorganic hybrids, etc.). (IV) Porous organic frameworks (POFs), for instance covalent organic frameworks (COFs), covalent triazine-based frameworks (CTFs), porous aromatic frameworks (PAFs), and conjugated microporous polymers (CMPs) [73]. When a nanoparticle is utilized as a porous dispersed phase in a polymer matrix, its structural and intrinsic properties such as pore size distribution, functional groups, and surface chemistry should be compatible with the organic matrix. For instance, activated carbon is appropriate for CO2/CH4 separation because it shows a considerably higher degree of solubility selectivity toward CO2 (as a polar

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239

gas) rather than for CH4 (as a nonpolar gas) while this filler is inappropriate for O2/N2 (e.g., air) separation. Hence, the dispersed phase must precisely stand for the size and shape differences of the gas molecules. Conversely, due to the different effects of the nonporous fillers on MMM separation capability with respect to the porous inorganic fillers, the interaction between the polymer main chain segments as well as the side functional groups and dispersed phase surface area must be considered. For example, the addition of silica nanoparticles to a polyimide matrix leads to disrupting the packing of the polymer chain and thus enhancing the gas pair permeation rates. In contrast, TiO2 nanoparticles in the polyimide matrix can enhance both CO2/CH4 and H2/CH4 selectivity because the interactions of H2 and CO2 with titania are more effective than the TiO2-CH4 ones [61]. The dispersed filler as well as the continuous matrix influence both the MMM separation performance and morphology. The glassy polymers with excellent selectivity are preferred to substantially permeable but weakly selective rubbery ones. Even though glassy polymers are preferred rather than rubbery polymers, owing to their rigid structures, poor adhesion between the glassy polymer matrix and the nanoparticle surface is the main challenge. Therefore, the weak filler-matrix interaction results in a void morphology that can exhibit poor separation properties [61]. Additionally, advanced polymer matrices, which were utilized in MMM fabrication, can be divided into three groups: (1) Polymers possessing high permeability such as polymers of intrinsic microporosity (PIMs). polyimides, thermally rearranged (TR) polymers, polyurethanes, and polyacetylenes. (2) Polymers with moderate permeability but high selectivity such as polyimide, polysulfone, cellulose acetate, etc. (3) Ionic liquid/poly ionic liquids with both high permeability and high selectivity [73].

11.3.4 Preparation methods of mixed matrix membranes MMMs are commonly synthesized in four major steps: (1) Preparation of membrane materials, that is, polymer, solvent, and inorganic filler, (2) Producing the dope solution by mixing the polymer, solvent, and filler, (3) Either casting or spinning the solution, and (4) Drying the flat sheet or hollow fiber membrane subsequently [74]. Fig. 11.18 shows the fabrication steps of a typical MMM. The substantial difference of properties between the organic matrix and inorganic fillers normally leads to undesired phenomena such as aggregation of the nanoparticles. Therefore, an MMM cannot be prepared simply by common procedures such as melt blending or normal mixing. Thus, the following methods have been proposed by different researchers to resolve the problem.

11.3.4.1 Solution blending Solution blending is known as an easy technique to fabricate MMMs. In this technique, a polymer is first dissolved in a solvent to prepare the dope solution, and then the inorganic filler is poured into the dope solution to be dispersed by mechanical stirring. The solution is cast to form the MMM by evaporating the solvent. The solution blending method is a simple technique to be performed and suitable for all kinds of inorganic fillers. As can be seen, the concentrations of the organic and inorganic ingredients are not too difficult to control. However, the aggregation of the inorganic fillers is still a severe problem in this technique.

240

Chapter 11 Synthetic polymeric membranes

Solvent

+ Nanoparticles

Dispersion

Polymer

Mixing

Dope solution Drying

Casting Flat sheet MMMs (cross section )

Spinning

Drying

Hollow fiber MMMs (cross section )

FIG. 11.18 General procedures followed to produce asymmetric MMMs.

11.3.4.2 In situ polymerization In this procedure, the filler particles are mixed properly with the polymer monomers, and then the polymerization of monomers is performed. In order to produce the initiating radicals (e.g., cations or anions) and then starting the polymerization, frequently some functional groups such as hydroxyl, carboxyl on the surface of inorganic particles can play role under high-energy radiation, plasma or other conditions. The starting point is located on the particle surface. For instance, TiO2 nanoparticle/ methacrylic acid was used to synthesize a poly(methacrylic acid) MMM under microwave radiation. In this method, a strong covalent bond between the filler functional groups and the polymer chains is formed. However, major improvements have been achieved in terms of particle dispersion, although the observations show a certain extent of particle aggregation in the final MMM.

11.3.4.3 Sol-gel The most widely utilized technique to fabricate MMMs is the sol-gel method. To produce an MMM by this method, organic monomers or polymers along with nanoparticle fillers are uniformly mixed in the solution. Then, the hydrolysis of the inorganic precursors happens and finally results in a welldispersed filler in the inorganic matrix. The major advantages of this technique are the moderate condition of the reaction generally at ambient temperature and pressure and the simply controllable concentrations of the reactants, for example, organic and inorganic components. Moreover, the ingredient of the organic and inorganic phases are dispersed very finely at the molecular or nanometer level that leads to a homogeneous MMM. For instance, Iwata et al. reported that by using the sol-gel method,

11.3 Mixed matrix membranes

241

an MMM of polyacrylonitrile (PAN) as the organic phase and tetraethoxysilane (TEOS) as the inorganic phase led to a good performance for air separation [59]. To ensure a membrane with a high gas flux, it is crucial to produce a thin membrane as much as possible. The most commonly and almost only applied technique to produce ultrathin membranes is the phase inversion method. However, the phase inversion method is still challenging to be employed for the mass production of hollow fiber MMMs. Typically, using a dry-jet/wet-quench spinning technique, the skin layer is formed via rapid evaporation of a volatile solvent during the air-gap time interval. Because of the rapid phase inversion, a highly porous structure is produced. To ensure the formation of a defect-free skin, a dope solution with high polymer concentration must be used. When we are dealing with MMMs, because the skin layer thickness and the particle sizes are comparable, the probability of defect formation increases as well. One simple method to control the skin defects in an MMM is to increase the skin layer thickness, which leads to a decline in the membrane flux. To overcome this challenge, using small size nanoparticles (20 nm range) has been suggested, which in turn may result in particle aggregation. A wellknown and commonly used technique to seal the microdefects is applying a silicone rubber coating on the skin layer. The high permeability of the silicone rubber layer can maintain both the selectivity and permeability of the original membrane. This technique was originally proposed by Henis and Tripodi and has been widely utilized by universities and industries, but it has its limitations [56].

11.3.5 Methods for avoiding nonideal interfacial defects Various interfacial nonidealities and different morphologies in MMMs essentially arise from organicinorganic incompatibility due to poor polymer-filler adhesion, polymer packing disruption, different thermal properties of continuous and noncontinuous phases, and repulsion force between the two different media. To inhibit the undesired nonideal morphologies, hence presenting a defect-free MMM, the subsequent approaches have been widely introduced by researchers: •

•

•

•

•

Casting at a temperature above the polymer glass transition temperature, or utilizing a polymer having enough low Tg (glass transition temperature) and producing the MMM near the Tg of the polymer matrix. Because it is practically very challenging to have nonvolatile solvents so that the boiling point is high enough not to exceed glassy polymers with high Tg, therefore maintaining the polymer chain mobility in the particle during MMM formation is almost impossible [61]. Annealing, heat treatment of the previously formed MMMs above the glass transition temperature, even though annealing the defective membranes does not necessarily cause any important enhancement in the resultant membrane morphology. Incorporation of a plasticizer into the dope solution, such as RDP Fyroflex, di-butyl phthalate, or 4-hydroxy benzophenone, to decrease the polymer Tg and thus maintain the flexibility of the polymer chain during membrane synthesis. Surface modification of particles applying coupling agents, aminopropyltriethoxy silane or APTES, 3-aminopropyl diethoxymethyl silane or APDEMS, styryl amine functional silane 3-aminopropyl dimethyl ethoxy silane, or APDMES, N-aminoethyl-aminopropyl trimethoxy silane are widely applied for modifying a coupling agent’s inorganic surface. Preparation of membranes using melt processing. This method is also utilized to keep polymer chain mobility throughout the membrane formation step. Clearly, melt processing is not a possible technique for industrial mass production.

242

•

•

• •

•

•

Chapter 11 Synthetic polymeric membranes

Using some copolymers such as polyimide siloxane. In this technique, rubbery segments (e.g., siloxane segments) enhance the interfacial organic-inorganic compatibility and thus almost a defect-free membrane is attained. Minimization of inorganic-solvent/nonsolvent interactions. In this approach, the filler particles are treated via vigorous surface modification and make an effort to substitute the hydroxyl groups with methyl groups at the particle surface. Using hydrophobic material for both inorganic and organic phases to control the interphase incompatibility. Modification of particles using a sizing technique. This is a rather simple technique. In this method, the physical deposition of the sizing chemical such as polyvinylpyrrolidone onto the surface of nanoparticles (e.g., CMS) is used. Priming method. Coat the surface of the filler particles with a dilute polymer solution before adding the rest of the polymer to the dope solution. Likely, this approach can decrease the stress at the polymer-filler interphase. Moreover, this approach leads a decrease in the agglomeration of nanoparticles, hence minimizing the defects in the interphase. Adding a low molecular-weight additive (LMWA) to the system composition, best exemplified by 2,4,6-triaminopyrimidine (TAP) and p-nitroaniline (pNA) [61]. Fig. 11.19 shows a SEM image of an MMM after using pNA as LMWA.

To summarize, among the above-mentioned methods for making a defect free MMM, utilizing silane coupling agents is the most promising and common technique for fabricating MMMs incorporating inorganic fillers such as zeolites. However, the other approaches are employed both for zeolites and for other fillers such as carbon molecular sieves, carbon nanotubes, clays, etc. It is noted that all the techniques may in some extent control the voids, but cannot remove them completely.

11.4 Membrane performance and characterization Over the past few decades, membranes have been used extensively for various gas and vapor separations in the industry. Therefore, the characterization of membranes plays an important role in improving their performance and developing membrane-based products. There are many tests to characterize the membranes fabricated for gas separation, some of which are discussed in this part. The common tests are scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which are used to study the morphology, pore structure, and distribution of nanoparticles and fillers in the membranes; thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC), which are both used to study the thermal stability and the acquiring of several polymer properties; atomic force microscopy (AFM), which is used to study the topology’ positronium annihilation lifetime spectroscopy (PALS), which is used for the evaluation of micropores in the dense layers; dynamic mechanical and thermal analysis (DMTA), which is used for the evaluation of mechanical strength and thermal stability; and Fourier transform infrared (FTIR), which is useful for acquiring the chemical formula. In addition to characterization techniques, it is important to evaluate the performance properties of the membranes. Permeability, diffusivity, and solubility are important parameters, and few methods are introduced in this part to obtain them.

11.4 Membrane performance and characterization

243

FIG. 11.19 Cross-sectional SEM images of (A) PC/zeolite 4A (20%) and (B) PC/pNA (2%)/zeolite 4A (20%) [75].

11.4.1 Scanning electron microscopy SEM analysis is used to characterize the surface features and evaluate the morphological changes such as the pore size, shape, and distribution of nanoparticles and fillers in the nanocomposites and the thickness of the membranes [76, 77]. This technique is commonly used to investigate the distribution of fillers in nanocomposite and mixed matrix polymeric membranes. SEM pictures can be taken from the surface of the samples to observe the distribution of nanoparticles or pores on the surface. In addition, the distribution of nanoparticles and the other morphological features inside the bulk of the sample can be seen via cross-sectional SEM pictures. In the case of cross-sectional observation of the membranes, the sample is recommended to be fractured in liquid nitrogen in order to preserve the reachable cross-sectional area. As an example, Fig. 11.20 shows a SEM cross-sectional view of the polyacrylonitrile (PAN) support membrane reported by Liang et al. [78]. The detailed description of the polymer can be found elsewhere. They observed from SEM images that a hollow fiber membrane had a fully porous inner surface but had a smooth and highly porous outer skin [79].

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Chapter 11 Synthetic polymeric membranes

FIG. 11.20 SEM cross-section view of the polyacrylonitrile (PAN) support membrane. (A) Cross-section, (B) enlarged crosssection, (C) outer skin layer, (D) outer surface, (E)outer-inner interface, and (F) inner surface. Reported by C.Z. Liang, T.-S. Chung, Robust thin film composite PDMS/PAN hollow fiber membranes for water vapor removal from humid air and gases, Sep. Purif. Technol. 202 (2018) 345–356.

11.4.2 Transmission electron microscopy TEM is a powerful tool to characterize the microscopic pore structure of membranes in a qualitative investigation in addition to technic distribution of nanoparticles in polymeric membranes. The resolution and magnification power of TEM are stronger than SEM. It is a microscopy technique in which a beam of electrons is transmitted through a sample to form an image. In the TEM images, the membrane polymer appears dark because the electrons are diffracted and the pores appear bright due to the lack of electron diffraction. In this method, if the specimens are too thin, larger pores would appear as long white areas without any polymer [80]. So, the thickness of the specimen is an important parameter in this method. As an example, Fig. 11.21 shows a TEM cross-sectional view of membrane samples. Sometimes, nanoparticles aggregate together and are not distributed well in nanocomposite membranes; then, in TEM images, the nanoparticle clusters are black and the white parts represent a good distribution of nanoparticles in membranes.

11.4.3 Thermogravimetric analysis TGA analysis is a method in which the mass of a sample is measured over time with changes in temperature in a specified trend. This measurement provides information about physical phenomena such as mass changes, temperature stability, oxidation/reduction behavior, decomposition, corrosion studies, and compositional analysis [81]. TGA is implemented in the presence of oxygen and also without

11.4 Membrane performance and characterization

(a)-10

(a)-20

245

(a)-30

TS610

0.2 µm

0.2 µm

0.2 µm

(b)-10

(b)-20

(b)-30

TS530

0.2 µm

0.2 µm

0.2 µm

(c)-10

(c)-20

(c)-30

TS720

0.2 µm

0.2 µm

0.2 µm

FIG. 11.21 TEM images of poly(ether imide) nanocomposite membranes containing different weight fractions (10, 20, 30) of three fumed silica (TS610, TS530, TS720). At low fumed silica content, nanoparticles are well distributed in the polymer matrix. Reported by S. Takahashi, D. Paul, Gas permeation in poly (ether imide) nanocomposite membranes based on surface-treated silica. Part 1: without chemical coupling to matrix, Polymer 47(21) (2006) 7519–7534.

ambient oxygen. As an example, Fig. 11.22 shows a TGA plot for pure and nanocomposites of poly (4-methyl, 2-pentyne) (PMP), the description of which is reported by Khosravi et al. [77]. The detailed description of the polymer and nanoparticles can be found elsewhere. They used TGA analysis to investigate the thermal stability of the nanocomposite membrane. Their results showed that thermal degradation for the membrane occurs at the single stage and polyoctatrimethyl silsesquixane (POSS) decomposed quickly while fumed silica (FS) had a low weight loss at a specific temperature range.

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Chapter 11 Synthetic polymeric membranes

100

Weight (%)

80

60

40 FS PMP POSS

20

0 0

100

200

300

400

500

600

700

800

Temperature (°C)

FIG. 11.22 TGA plot for pure poly(4-methyl, 2-pentyne) (PMP), fumed silica (FS), and polyoctatrimethyl silsesquixane (POSS) nanoparticles and nanocomposite membranes. POSS decomposed quickly while FS had a low weight loss at a specific temperature range.

Therefore, FS had better thermal stability effects in comparison to POSS. The temperature of degradation is obtained via the breakthrough point of the curves [77].

11.4.4 Differential scanning calorimetry DSC measures the amount of energy absorbed or released by a sample when it is exposed to a temperature change. This provides quantitative and qualitative data on endothermic and exothermic processes. Also, DSC analysis is applied to explore the effect of polymer blending on the glass transition temperature (Tg) of blend membranes. Fig. 11.23 shows a sample of DSC plots for pure polyurethane (PU) and polyether-based PU-silica nanocomposites to determine the effect of the silica nanoparticles on glass transition temperature and melting temperature [82]. DSC is an effective tool to characterize the physical properties of a polymer [83].

11.4.5 Atomic force microscopy This analysis can create surface topographic images in three dimensions in a nanoscale. The surface properties of pure membranes and nanocomposites are evaluated using this method. It can convert the data obtained from the analysis to the image and also calculate the average roughness parameter. The average roughness parameter is a major and important parameter in determining the topography of samples and is calculated by the following equation [84].

11.4 Membrane performance and characterization

247

Endothermic

PU PU-S5 PU-S10 PU-S20

–130

–30

70 170 Temperature (°C)

270

370

FIG. 11.23 DSC plots of the pure polyurethane (PU) and polyether-based PU-silica nanocomposites. Reported by M. Sadeghi, et al., Gas separation properties of polyether-based polyurethane–silica nanocomposite membranes, J. Membr. Sci. 376(1–2) (2011) 188–195.

Ra ¼

1 lx ly

Z 0

lx Z ly

jMðx, yÞj dxdy

(11.23)

0

In this equation, M(x, y) corresponds to the central plate surface and lx and ly correspond to the dimension of surface. Also, for the nanocomposite and mixed matrix membranes, the distribution of fillers on the surface of the membrane can be studied using the three-dimensional images obtained by this method. Fig. 11.24 shows AFM curves for (A) polysulfone (PSf) substrate while (B), (C), and (D) are modified PSf membranes.

11.4.6 Dynamic mechanical and thermal analysis DMTA is a technique used to study and analyze the mechanical and thermal features of the material. It is a technique to measure the viscoelastic properties of polymers. In this method, an oscillatory strain or stress usually enters the sample and analyzes its response to obtain the phase angle (δ) and deformation data. The results of DMTA are summarized in three parameters [85]: (1) Storage modulus (E0 ) (2) Loss modulus (E00 ) (3) Damping factor or tan δ This is a good way to measure the glass transition temperature (Tg). The glass transition temperature is the first drop point or midpoint of the storage modulus, or the first point of increase or midpoint of the loss modulus as a function of temperature [86].

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Chapter 11 Synthetic polymeric membranes

nm

nm 300

10

200 3

5 0

100

3

0

2

–5

2

–100 3

–200

μm

1

μm

–10

0

(A)

3

1

2

2 1

1

(B)

μm

0

nm

0

μm

0

nm

100

80 3

0

40 0

2

–100 3

1

–80

2

μm

μm

–40 2

3

1

2 1

1

0 0

(C)

3

μm

(D)

0

0

μm

FIG. 11.24 AFM analysis for (A) polysulfone (PSf) substrate while (B), (C), and (D) are modified PSf membranes with different monomer concentrations for interfacial polymerization. Reported by M.I. Baig, et al., Water vapor permeation behavior of interfacially polymerized polyamide thin film on hollow fiber membrane substrate, J. Taiwan Inst. Chem. Eng. 60 (2016) 623–635.

11.4.7 Fourier transform infrared One of the common methods for detecting polymers and finding information about the chemical structure of molecules and bonds and functional groups is the use of FTIR analysis [87, 88]. In this method, an IR wave is transmitted into the specimen and is absorbed by the molecular bonds in the sample structure, causing a drop in transmittance received by the detector. In other words, each particular bond causes a drop in transmittance at its specific wave length. These changes are recorded as plots. Each bond has a peak on the FTIR plots. As an example, Fig. 11.25 shows the FTIR spectra of the pure silica, pure polyurethane (PU), and polyether-based PU-silica nanocomposites, reported by Sadeghi et al. [82]. In this plot, each peak is related to a specific bond. For example, the Si-O-Si peak is around 1077 cm1 and is seen in all nanocomposite membranes.

11.4 Membrane performance and characterization

249

Silica

Transmittance (%)

PU-S20 PU-S10

PU

3900

3400

2900

2400

1900

1400

900

400

–1

Wavenumber (cm ) FIG. 11.25 FTIR plot of pure silica, pure polyurethane (PU), and polyether-based PU-silica nanocomposites. In this plot, each peak is related to a specific bond. Reported by M. Sadeghi, et al., Gas separation properties of polyether-based polyurethane–silica nanocomposite membranes, J. Membr. Sci. 376(1–2) (2011) 188–195.

11.4.8 Positronium annihilation lifetime spectroscopy PALS is a technique used to approximate the size and intensity of the dynamic and static free volume elements in the structure of pure or nanocomposite membranes. In this method, a positron is injected to the sample and forms a spin parallel bound state with an electron called o-positronium. Then, this is annihilated using another electron with a counterspin causing γ transmission. The lifetime of an o-positronium depends on the concentration of electrons in the surrounding media showing the volume of the free volume element. The intensity of the annihilation of O-Positronium also presents the distribution of available free volume elements [89, 90] (Table 11.4).

11.4.9 Gas permeation tests 11.4.9.1 Constant pressure One of the required tests for evaluating membrane performance is the gas permeation test at constant pressure. According to the constant pressure/variable-volume method, a specific high gas pressure is applied on one side of the membrane cell, and the permeated gas flows in the same direction with the

250

Chapter 11 Synthetic polymeric membranes

Table 11.4 Effects of the incorporation of fumed silica (FS) into poly(4-methyl, 2-pentyne) (PMP) nanocomposites [91]. OPs lifetime (ns)

˚) Average free volume element radius (A

System

τ3

τ4

r3

r4

PMP PMP containing 15 wt% FS PMP containing 30 wt% FS PMP containing 40 wt% FS

2.3 2.5 2.6 2.4

7.6 8.3 9 9.5

2.9 3.3 3.3 3.2

5.7 6.0 6.2 6.4

gas feed. After achieving steady-state conditions, the gas flow rate through the membrane is measured with a flowmeter device at the permeate side [92]. The following equation is used to determine the permeability coefficient [77, 91]. P¼

Nl p2  p1

(11.24)

where p2 is the feed pressure (cmHg), p1 is the permeate pressure (cmHg), l is the membrane thickness (cm), N is the steady-state gas flux through the membrane (cm3 (STP) cm2 s1), and the permeability coefficient (P) is reported in Barrer, where 1 Barrer ¼ 1010 cm3 (STP) cm cm2 s1 cmHg1. Fig. 11.26 shows a schematic of the system. P1

P2

P

Purge to outside

Gas regulator

Membrane module Gas cylinder Measuring flow rate

FIG. 11.26 Experimental set-up of gas permeation tests.

11.4 Membrane performance and characterization

251

11.4.9.2 Constant volume A constant volume/variable pressure method is used to measure the permeability of a gas through the membrane. Fig. 11.27 shows a schematic of the system. The gas enters the system from the reservoir and passes through the membrane cell. Then the gas is accumulated into a constant volume reservoir and the pressure change of permeating gases in the constant volume reservoir is recorded by a pressure transmitter in respect to the time. The following equation is used to calculate the gas permeability: P¼


 273:15  1010 VL dp   P0  76 dt 760AT 14:7

(11.25)

where P is the gas permeability across the membrane in Barrer (1 Barrer ¼ 1  1010 cm3 (STP) cm/cm2s cmHg), V is the constant volume vessel (cm3), L is the membrane thickness (cm), A is the membrane area (cm2), T is the experimental temperature (K), P0 is the feed pressure (psia), and (dp/dt) is the slope of pressure versus time [65]. The ideal selectivity can be calculated by Eq. (11.26). αA=B ¼

PA PB

(11.26)

where α is the selectivity of the membrane for gas A to gas B, PA is the permeability of gas A, and PB is the permeability of gas B. When a gas is added to one side of the membrane, the gas flow rate and gas concentration change with time. If the gas penetrates continuously along the membrane, and the time of permeation would be high enough, stable conditions will occur and the curve of the amount of gas permeating through the membrane tends to be a straight line against time, which has an intercept that commonly is called time-lag [93]. To calculate the diffusivity coefficient of each gas in a dense polymeric membrane, the time-lag method can be used. In this method, the diffusivity coefficient can be calculated by the following equation:

Vacuum pump

Oven Vent

PC

Feed stream

Membrane cell

PI Permeate stream

To GC Constant volume resevoir

Gas cylinder

Gas cylinder

FIG. 11.27 Experimental set-up of gas permeation test at constant volume.

252

Chapter 11 Synthetic polymeric membranes

D¼

L2 6θ

(11.27)

where D is the diffusivity coefficient (cm2/s), L is the membrane thickness (cm), and θ is the time-lag (s) [65, 93–95].

11.4.10 Solubility measurement To calculate the solubility of gases in a polymer, there are two experimental methods called permeation and sorption techniques. In the permeation experiment, the permeability and diffusivity are calculated first using a constant volume/variable pressure test, and then the gas solubility can be calculated with the following mathematical relation [96]: S¼

P D

(11.28)

where S is the solubility coefficient, P is the pure gas permeability, and D is the diffusivity. On the other hand, the sorption measurement technique is a direct method for measuring the solubility of gases [97]. Measuring the amount of pure gas sorption in the membranes is done using a system similar to that shown in Fig. 11.28. In this experiment, the amount of gas primarily in contact with the polymeric sample is determined. During the gas sorption process, the pressure of the module is reduced. When the system reaches the equilibrium (the sorption and discharging gas in the polymer is equaled), the reduction of module pressure stops. The difference between the primary and final amount of gas in the module is the amount of gas absorbed in the membrane. The concentration of absorbed gas in the polymer at a given pressure and temperature is calculated using the following equation: C¼

22,414 Vm ðPb  Pi Þ VP RT

(11.29)

Pressure sensitive transducer

TCS

Charge cell

Gas cylinder

Sample cell

Pump

FIG. 11.28 Experimental set-up for measuring the solubility of pure gas in the membrane.

11.5 Industrial applications

253

where C is the absorbed gas concentration in the polymer at equilibrium (cm3 (STP)/cm3), T is the absolute temperature entering the module (K), R is the gas constant (6236.56 cm3 cmHg/mol K), Pb is the pressure at the beginning of the sorption process (atm), Pi is the pressure at the end of the sorption process (atm), Vm is the module volume (cm3), and VP is the polymeric sample volume (cm3).

11.5 Industrial applications The membrane separation process as a high-efficiency technology can compete with common industrial processes such as cryogenic distillation, pressure swing adsorption (PSA), chemical absorption, etc. Membrane processes, in contrast to common processes for gas separation, occupy less space, consume less energy, have no moving parts, and are environmentally friendly [5, 98]. In the last few decades, this technology has grown and is used in various industrial processes. The most important industrial applications of membranes in gas separation are summarized in Table 11.5, but the applications are not limited to this table.

11.5.1 CO2 removal Natural gas is a mixture of several gases and its composition depends on the type of reservoir, but mainly consists of methane, ethane, carbon dioxide, hydrogen sulfide, heavier hydrocarbons, hydrogen, nitrogen, etc. To transfer this gas to the pipeline, it must be treated [99]. Therefore, the main purpose of gas sweetening is CO2/CH4 separation [100, 101]. Since 1980, membrane systems have been introduced to remove CO2 from natural gas as a rival to the amine absorption processes [102, 103]. In 2008, the membrane contribution to carbon dioxide removal was less than 5%, but since then, the use of these systems has multiplied and many studies have been done to improve their performance [102, 104]. Fig. 11.29 shows a CO2 membrane separation plant designed by Newpoint Gas, LLC. Another application of the membrane systems is in carbon capture (CO2/N2 separation) [105]. Nowadays, postcombustion is the most common commercial method for carbon capture [106]. However, postcombustion systems consume a lot of energy and often use chemical solvents such as amines, requiring a

Table 11.5 Common applications of membranes [5, 11, 31, 98]. Separation

Process

CO2/hydrocarbon CO2/N2 H2/N2 H2/CH4 H2/CO O2/N2 H2O/air VOCs/N2 LPG/hydrocarbons

Landfill gas upgrading, natural gas sweetening Power plant flue gas Ammonia purge gas Natural gas treatment, off-gas stream Syngas ratio adjustment Nitrogen generation, oxygen enrichment Air dehydration Polyolefin plant Natural gas treatment, off-gas stream

254

Chapter 11 Synthetic polymeric membranes

FIG. 11.29 CO2 membrane separation plant from Newpoint Gas, LLC. From www.newpointgas.com. Used with permission from Newpoint Gas, LLC.

103 2008 Upper bound

TR polymers

CO2/CH4 selectivity

PIMs Polyimides 2

10

101

1991 Upper bound

100 10–1

100

101

102

103

104

105

CO2 permeability (barrer) FIG. 11.30 CO2/CH4 upper bound plot for new polymer materials. Reported by D.F. Sanders, et al., Energy-efficient polymeric gas separation membranes for a sustainable future: a review, Polymer 54(18) (2013) 4729–4761.

recovery step. Therefore, several studies have been conducted on membrane systems as a substitute for the common methods, but they are not yet commercially used widely [107]. Ahmad et al. [108] synthesized MMMs with multiwalled carbon nanotubes (MWCNTs) for CO2/N2 separation. Their results showed that the membrane had a high permeance rate and high selectivity for CO2/N2. Fig. 11.30 shows a CO2/CH4 upper bound plot for new polymer materials, reported by Sanders et al. [31].

11.5 Industrial applications

255

11.5.2 Hydrogen recovery The first objective of membrane gas separation was the separation of hydrogen from the other gases [31]. Hydrogen is not found in its pure form in nature. It is produced in refineries according to the reaction of steam reforming. In this reaction, water reacts with methane and produces hydrogen and carbon monoxide, called syngas [109]. In the petroleum industry, hydrogen separation is performed for several processes such as syngas ratio adjustment (H2/CO), ammonia plants (H2/N2), off-gas stream, etc. [110, 111]. The ratio of H2 to CO in syngas is different depending on the composition of feed and the application of syngas in the other processes. The first company to present a patent for adjusting the H2:CO ratio was Permea (Monsanto), which introduced the first series of membranes called Prism membranes in 1977 [112]. Monsanto was also the first company to introduce and install a polysulfone hollow fiber system in 1979 for hydrogen recovery from the ammonia purge gas [113]. The system can recover hydrogen to about 95% of the ammonia purge gas [112]. An image of the hydrogen recovery unit with Prism membrane separators is depicted in Fig. 11.31. Another application of membrane gas separation systems is in the recovery of H2 from off-gas. Mivechian and Pakizeh [110] simulated various processes such as PSA, absorption, and polyimide membrane processes for H2 recovery from off-gas and eventually conducted an economic evaluation. Their results showed 95% hydrogen recovery for the membrane process while this amount was 94%

FIG. 11.31 Hydrogen recovery from ammonia purge stream by Prism membranes plant that installed in 1979 with capability of pure hydrogen recovery by about 90%. Used with permission from Air Products and Chemicals, Inc.

256

Chapter 11 Synthetic polymeric membranes

Table 11.6 H2 recovery membranes. Materials

PH2 (Barrer)

αH2/CO2

αH2/N2

αH2/CO

T (°C)

P (bar)

Ref.

Polyimide (BPDA-based) Poly(2,6-dimethylphenylene oxide) (PPO) Polydimethylsiloxane (PDMS) PBI + 2 wt% TBB PSf-Ac-zeolite 3A + 25 wt% zeolite PBI Matrimid/PBI (50/50 wt%) Polyetherimide

50 60

– –

83 31

50 –

60 22

– –

[36] [114]

1500 9.6 82 GPU

– 24 1.7

2.5 – –

– – –

35 150 25

1–15 14 12

[115] [116] [117]

0.6 13.06 26

3.75 6.05 –

125 181.38 71

– – 39

35 35 23

20 20 0.3–0.8

[118] [118] [119]

and 79% for absorption and PSA, respectively. Also, the purity of hydrogen was not significantly different among these processes. There are many polymers suitable for hydrogen recovery in the literature. Table 11.6 shows a comparison between the permeability and selectivity of some polymers for H2 recovery applications.

11.5.3 Air separation Air consists of 21% oxygen and 79% nitrogen. The common industrial processes for air separation are cryogenic distillation, PSA, and membrane processes [120–122]. Membrane systems are widely used for nitrogen generation. These systems can produce nitrogen with a purity of 95%–99% [3]. Low selectivity is one of the most important problems of membrane systems for air separation. As Baker has reported, increasing the oxygen/nitrogen selectivity from 8 to 12 (in the same permeation rate) will reduce the compressor size by about 20% [3, 31]. So, in recent decades, many studies have been done to improve the selectivity of these membranes. Single-stage membrane systems have been commercialized for nitrogen production. In these systems, the feed is divided into two streams after entering the membrane system. Oxygen and other fast gases in the air penetrate through the membrane and exit the system as permeate, and nitrogen is produced in retentate [11]. Generon (now part of MG) was the first company to introduce a membrane system for the separation of nitrogen from the air in the mid-1980s. Their system was made of poly(4-methyl-1-pentene) (TPX) membranes with the oxygen/ nitrogen selectivity about 4 [3]. Besides the generation of nitrogen, membrane systems are also used for oxygen enrichment. Enriched oxygen can be used in various processes such as the Claus process for the production of water and solid sulfide from hydrogen sulfide gas, in furnaces to reduce the NOx and other nitrogen compounds, etc. [98, 123]. Membrane systems, despite the extensive studies carried out on them, have not yet been commercialized for this process. Chemical instability and sealing problems have reduced their ability to compete with other methods, but this technology is in progress for this application [124, 125].

11.5 Industrial applications

257

10

O2/N2 selectivity

2008 Robeson upper bound

1 10

1000 O2 permeability (barrer)

FIG. 11.32 O2/N2 upper bound plot for new polymer materials, TR polymers (⧫) [31, 126, 127], PIMs (n) [29, 128], TBDASBI-P (▲) [129].

In recent decades, beneficial studies have been done to improve the O2/N2 separation performance. TR polymers, PIMs, and TBDA-SBI-P are the candidates for the polymer texture of the membranes. A comparison between the membranes reported in the literature is presented in Fig. 11.32.

11.5.4 Air and gas dehydration Removing water vapor in many gas streams such as natural gas, flue gas, and air is another application of the membrane systems. Water vapor in a natural gas stream can cause several problems such as hydrate formation. Water vapor in the air should be eliminated in air conditioning systems, the food industry, and space equipment [79, 130–135]. Membrane systems are more efficient than common methods for gas drying, such as desiccants and refrigeration systems [11, 79]. One of the membrane systems that is commercially available is the membrane called the Caltus dryer from the Air Products Company [136]. In the Caltus membrane dryer unit, the wet feed gas enters the hollow fiber membrane module from one side and the water vapor passes through the membrane and leaves the system in a countercurrent direction; eventually, the dehumidified retentate exits from another side. A comparison between the nanocomposite membranes for air and gas dehydration is presented in Table 11.7.

11.5.5 Separation of volatile organic compounds from N2 Volatile organic compounds (VOCs) are used in chemical and petrochemical industries. These compounds are valuable substances. Sometimes these compounds enter the atmosphere and in addition to many economic losses, they act as important air pollutant. Therefore, it is essential to separate and reuse them. One of the methods that is very seriously considered for this separation is the membrane system

258

Chapter 11 Synthetic polymeric membranes

Table 11.7 Common membranes for air and gas dehydration.

Materials

Water vapor permeance (GPU/Barrer)

Selectivity

Temperature (°C)

Ref.

PEI/PEBAX 1657 BTESO-Me-SiO2 PSf/MPD-TMC-Si NPs (10 nm) PSf/NaAlg ABn-NH TFN-3 PSf/polyamide—TiO2 PAN/regenerated cellulose Polyether imide (Ultem 1000)/Pebax 1657 PSf/MT-OH-TiO2

260 1800 2125 3.04 2809 1131 4000 1800 1396

274 H2O/N2 10 H2O/CO2 581 H2O/N2 54 H2O/C3H6 913 H2O/N2 548 H2O/N2 11,000 H2O/N2 1800 H2O/N2 510 H2O/N2

21 40 30 25 30 30 23 21 30

[132] [137] [138] [139] [140] [141] [78] [132] [142]

[143, 144]. Cha et al. [145] investigated the separation of VOCs from N2. They used a hollow fiber composite membrane with a polypropylene substrate and silicon skin. Their results showed that a small module consisting of 50 hollow fibers with a length of 25 cm could separate 97%–98% of VOCs from the feed stream with a flow rate of 60 cm3/min. Yoem et al. [146] studied the permeation performance of a VOCs/N2 mixture in the presence of polydimethylsiloxane (PDMS) membranes. Their results indicated that sorption processes are more effective than diffusion processes in the permeation of VOCs. Majumdar et al. [147] performed two experiments to remove VOCs from N2 and air in pilot and laboratory conditions. Their results showed that it was possible to remove VOCs up to 98% in the presence of nitrogen with a high (up to 95%) removal in the presence of air for designed systems.

11.5.6 LPG recovery Liquefied petroleum gas (LPG) contains propane (C3) and butane (C4). LPG is in the form of gas under normal pressure and temperature conditions, but under pressure of 200–900 kPa, its components become liquid [148, 149]. LPG can be recovered from natural gas streams, refinery-treated gas, flare gas, and fluid catalytic cracking (FCC) [149–151]. Bhupender et al. [152, 153] for the first time introduced the membrane process for LPG recovery. They used rubbery polymers to fabricate the membrane. Their results showed that membrane systems have a high performance for LPG recovery. Membrane Technology and Research (MTR) was the first company to introduce the VaporSep membrane system for LPG recovery. The system consists of two membranes. Hydrogen passes through the first membrane and, as a result, produces a pure stream of hydrogen, and LPG passes through the second membrane. These systems can be used in refinery fuel/ flare gas, PSA tail gas, etc. [154]. An image of the LPG recovery unit from off-gas is shown in Fig. 11.33.

11.5 Industrial applications

259

FIG. 11.33 Image of the LPG recovery unit from off-gas installed by Membrane Technology and Research (MTR). Used with permission from Membrane Technology and Research.

In the separation of heavier hydrocarbons such as LPG from natural gas, there are a few aspects that should be taken into account. From an engineering point of view, the pressure of the stream should remain, so the membrane unit should be designed to separate the heavier hydrocarbons in the permeate stream. As a result, the pressure of the lean methane stream is close to the feed because there is not a significant pressure drop for the retentate stream. Therefore, the membrane has to be selective to heavier hydrocarbons rather than the methane. These membranes are well known as reverse selective membranes [77]. Reverse selective membranes can separate gas molecules based on their solubility in membranes. In general, by increasing the molecular weight and critical temperature of the component, solubility is increased while the diffusion coefficient decreases. Polymers with flexible chains and very high free volume usually have poor sieving characteristics based on molecular size. Increasing the penetration pathway facilitates and accelerates the passage of molecules and increases the permeability in the membrane [34, 91]. In summary, reverse selective membranes can separate gas molecules based on their solubility selectivity [65, 155]. For reverse selectivity, the mechanism of separation must be competitive sorption and surface diffusion. Generally, polymers with high FFV are used to fabricate the reverse selective membranes. The heavier hydrocarbons that have a higher critical temperature are transported easier than light gases through these super permeable membranes. It is seen that the incorporation of fine dense nanoparticles such as fumed silica and titanium oxide can significantly enhance the selectivity and permeability coefficients [34, 66, 77, 156, 157]. PTMSP, PMP, and PDMS are the candidates for the polymer texture of the membranes. A comparison between the membranes reported in the literature is presented in Table 11.8.

260

Chapter 11 Synthetic polymeric membranes

Table 11.8 Comparison between the membranes reported in the literature for LPG recovery applications. Nanocomposite membrane

Performance

Operating condition

Ref.

PMP + 15 wt% FS

α(C3H8/CH4) ¼ 3.21, α(C3H8/N2) ¼ 6.63 P(C3H8) ¼ 4300 Barrer, not stable over time Mixed gas selectivity (C4H10/CH4) ¼ 18 P(C4H10) ¼ 10,350 Barrer, not stable over time Mixed gas selectivity (n-C4H10/CH4) ¼ 18 P(C4H10) ¼ 63,000 Barrer, not stable over time α(C3H8/CH4) ¼ 4.63, α(C3H8/N2) ¼ 9.84 P(C3H8) ¼ 7577 Barrer, stable over time

5 bar, 25°C

[91]

0.23 bar, 30°C

[158]

1.59 bar, 35°C

[159]

8 bar, 25°C

[77]

PMP + 20 wt% TiO2

PTMSP + 30 wt% FS

PMP + 3 wt% POSS + 20 wt% FS

11.6 Challenges 11.6.1 Plasticization Plasticization is a pressure-dependent phenomenon that occurs at high pressures [160, 161]. This phenomenon occurs when certain components are dissolved in the polymer matrix. Plasticization increases the segmental mobility and the free volume of the polymer, ultimately leading to the destruction of the polymer structure [160, 162, 163]. So, a major problem in high-pressure gas and vapor separations such as CO2/CH4, CO2/N2, and C4/C1 is the existence of condensable compounds such as CO2 and heavy hydrocarbons [8, 164]. Increasing the absorption of CO2 causes the swelling of the polymeric matrix, which increases the free volume of the polymer and enhances the CO2 permeability and diffusivity. Eventually, the membrane loses its selectivity [165]. Plasticization decreases the glass transition temperature (Tg). Membrane thickness is a factor affecting the intensity of the plasticization phenomenon [166]. Plasticization occurs more often by reducing the thickness of the membrane [8]. In permeability versus pressure curves, the permeability goes through a minimum. This is the minimum CO2 pressure necessary to induce plasticization, which is known as “plasticization pressure” [167–169]. Saberi et al. [167] presented a mathematical model for mixed gas permeation and also CO2-induced plasticization in glassy polymers. Their results showed that the presence of the second component in the feed led to a reduction of plasticization. This phenomenon reduced the permeability and diffusivity of CO2 by increasing the second component fraction in the feed. It was assumed that the diffusion coefficients for all components were exclusively a function of the plasticizing component. The partial immobilization model was employed to determine the fraction of mobile sorbed gases. The proposed model accurately predicted the permeation behavior of CO2 as the plasticizer and N2 in the presence of plasticization [167]. The final forms of the model for A as a plasticizer component and B as a normal component are given by Eqs. (11.30), (11.31), respectively, as follows: PA ¼

 
  
  DA0 FA C0HA bA FA C0HA bA exp βA kDA + fA2  exp βA kDA + fA2 βðfA2  fA1 Þ 1 + bA fA2 + bB fB2 1 + bA fA2 + bB fB2

(11.30)

11.6 Challenges

PB ¼

261




 FA C0HA bA FB C0HB bB FB C0HB bB DB0 kDB + fB2  kDB + fB1 exp βB fA2 kDA + ðfB2  fAB1 Þ 1 + bA fA2 + bB fB2 1 + bA fA2 + bB fB2 1 + bA fA2 + bB fB2 (11.31)

where kD is Henry’s law solubility coefficient (cm3 (STP)/cm3 polymer atm), C0 H is the hole saturation constant (cm3 (STP)/cm3 polymer), b is the hole affinity constant (atm1) that represents the ratio of the rate constants of gas adsorption and desorption in the microvoids, f is the fugacity (atm), and F is usually called the immobilization factor, which depends on the nature of the penetrant polymer system as well as the temperature. D0 is the diffusion coefficient of pure gas in the limit of dilute concentration, and β is an empirical constant that depends on the nature of the penetrant-polymer system, the temperature, and the membrane thickness, which is known as the plasticization parameter, indicating the penetrant plasticizing capability [167]. Fig. 11.34 shows the effect of feed composition on the CO2 permeances in asymmetric PES/PI hollow fiber membranes in a binary mixture of CO2/N2. This figure compares the experimental data of Visser et al. [170] with the predictions of the proposed model, calculated by the model, using the sorption parameters of CO2 and N2 depicted in Table 11.9. Considering pure CO2, permeance was enhanced with fugacity due to plasticization. This trend was due to the skin thickness of the asymmetric membranes. The plasticization pressure of the membranes decreases as the membrane thicknesses decrease [171]. The presence of N2 in the feed decreases the sorption of CO2 due to competitive sorption, and this decrease in solubility lowers the diffusivity of CO2 for a given fugacity and also suppresses plasticization, consequently reducing the CO2 permeance [172]. It is apparent that the model predictions

FIG. 11.34 CO2 permeance in an asymmetric PES/PI hollow fiber membrane as a function of fugacity with different compositions of the feed [167].

262

Chapter 11 Synthetic polymeric membranes

Table 11.9 Sorption parameters for CO2 and N2 in an asymmetric PES/PI hollow fiber membrane. Component

KD (cm3 (STP)/cm3 kPa)

C0 H (cm3 (STP)/cm3)

b (kPa21)

CO2 N2

0.73  102 0.13  102

32.31 3.19

1.7  103 2.3  103

From T. Visser, G. Koops, M. Wessling, On the subtle balance between competitive sorption and plasticization effects in asymmetric hollow fiber gas separation membranes, J. Membr. Sci. 252(1–2) (2005) 265–277.

showed a good agreement with respect to the experimental points, whereas the results without considering the plasticization did not match the experimental data [170]. The polymeric membranes used in gas separation are divided into two groups of glassy and rubbery. Their difference from each other is according to the range of the glass transition temperature. Room temperature is under the glass transition temperature for glassy polymers and is above the glass transition temperature of rubbery polymers [160]. In order to remove CO2, glassy polymers have been more attractive than rubbery polymers because of their higher selectivity [8, 162, 167]. Several methods have been reported to overcome plasticization in polymeric membranes. There are three common methods: (1) polymer blending, (2) cross-linking, and (3) thermal treatment [2, 8, 160]. In the first method, two or more miscible polymers with different ratios are mixed together and form a blend that has higher mechanical strength and thermal stability than the base polymers. Another way to overcome the CO2-induced plasticization is through cross-linking. In this method, cross-linking agents are used that can increase the selectivity of the membrane. The third common method is to use thermal treatment. In this method, with various temperature ranges, the polymer structure changes in such a way that it will ultimately become resistant to plasticization [2]. The plasticization effect in polymeric membranes and the methods used to overcome it have been studied by many researchers in the literature. Kapantaidakis el al. [8] investigated CO2 plasticization of polyethersulfone (PES)/polyimide (PI) blends for gas separation. It was shown that pure CO2 permeance increased with increasing pressure. Plasticization occurred, leading to a reduction in the ideal selectivity of the CO2/N2. However, plasticization was not shown in the case of the CO2/N2 mixture. Omidkhah et al. [7] prepared a novel cross-linked polyvinylalchohol (PVA)/formaldehyde (FA) membrane to be performed for CO2/CH4 separation in a pure gas experiment. Their results showed that pure CO2 and CH4 permeances decreased with increasing FA/PVA mass ratio in comparison with uncross-linked membranes, although the selectivity coefficients increased considerably. In another study, Shahid et al. [173] developed an MMM using different contents of metal organic frameworks (MOFs) and investigated the effects of different types of MOFs on CO2-induced plasticization in the MOF-MMM and the gas separation performance of the MMMs. Their results showed that the plasticization pressure increased to higher values and MOF-MMMs at high pressures had a reasonable selectivity for gas separation. Han el al. [174] presented a new method for the synthesis of thermally rearranged microporous polybenzimidazole (TR-TBI) membranes, and it was shown that the new membrane had a high performance for gas separation in the high temperatures. By the way, it seems that plasticization is also a subject of interest for researchers and experts working on membrane technology.

11.6 Challenges

263

11.6.2 Aging One of the challenges in the manipulation of membrane units in industrial applications is the ability to handle high industrial flowrates. As was discussed in the previous sections, a reverse selective membrane made of super permeable polymers is one of the most promising membranes in industrial applications [175]. These polymers have a high FFV and large permeability coefficients while their performance is not stable over time. For instance, Khosravi et al. showed that PMP nanocomposites lost their nitrogen permeability by 70% over 120 days [77]. This phenomenon occurs due to diminishing the nonequilibrium excess free volume of the polymeric matrix over time. The relaxation of polymeric chains, and collapsing the pathways and free volume lead to a decrease in the diffusion coefficients. Aging is self-retarding, meaning its intensity decreases by time. In other words, the membranes lose their permeability faster in the first periods of time. So, it is concluded that the driving force of aging is the FFV [77]. Decreasing the FFV leads to a reduction in the rate of aging over time. It is also reported that the rate of aging increases by decreasing the thickness of the membranes [176]. Generally, the incorporation of fillers in the polymeric matrix reduces the rate of aging due to the reduction of the polymer content in the sample. Kelman et al. showed that cross-linking the PTMSP nanocomposite membranes did not arrest the aging while the solvent resistance against many solvents was enhanced significantly [90, 176, 177]. Khosravi et al. revealed that the incorporation of nanospacers mitigated the relaxation of the polymeric chains, leading to the stability of the permeability coefficients in PMP nanocomposites, although the permeability coefficients were reduced in comparison to the pure polymeric samples [77]. As depicted in Fig. 11.35, Yong et al. showed that POSS nanoparticles have the capability to reduce the physical aging and plasticization effects in polymers of intrinsic microporosity (PIM) [178]. Lau et al. studied

Suppression of aging and plasticization by embedding nanoparticles Time = 0

Time = 0

FFV  Permeability 

FFV  Permeability 

Time >> 0

Time >> 0

FFV ¯¯

Permeability ¯¯

FFV ¯ Permeability ¯

Polymer chains Nanoparticles FFV Fractional free volume

FIG. 11.35 The introduction of polar-functionalized POSS nanoparticles to the PIM for CO2 separation [178].

264

Chapter 11 Synthetic polymeric membranes

the introduction of an ultraporous additive to the super glassy polymers holding the pathways open by stretching the polymer chains, leading to improvement of the stability [179]. In spite of the fact that there are leading studies in the literature providing the membrane society with clues to suppress the aging, this phenomenon is still a big challenge against the commercialization of membrane gas separation.

Acknowledgments First and foremost, praises and thanks to the God, the Almighty, for His showers of blessings throughout writing the book chapter to be completed successfully. First, I would like to express my deep and sincere gratitude to Professor Ahmad Fauzi bin Ismail for his invitation to contribute to this valuable book. Second, I am extremely grateful to my dear collogue Dr. Arash Khosravi for his close and valuable assistance during writing this chapter. Next, I would like to thank my postgraduate students Farideh Abdollahi, Zahra Alihemati, and Mohsen Rezaee for collecting the materials and editing the figures and tables as well as assisting in typing the manuscript. All the contributors discussed the results and contributed to the final form of the chapter. I would also like to thank them for their friendship, empathy, and great teamwork. Finally, I gratefully appreciate Andrea Gallego Ortiz from Elsevier for her kind assistance in answering our inquiries during the writing and submission process.

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[154] Y. Li, et al., Effects of novel silane modification of zeolite surface on polymer chain rigidification and partial pore blockage in polyethersulfone (PES)-zeolite A mixed matrix membranes, J. Membr. Sci. 275 (1–2) (2006) 17–28. [155] R.N. Pitman, et al., Next Generation Processes for NGL/LPG Recovery, Gas Processors Association, Tulsa, OK, 1998. [156] Z. He, I. Pinnau, A. Morisato, Nanostructured poly(4-methyl-2-pentyne)/silica hybrid membranes for gas separation, Desalination 146 (1) (2002) 11–15. [157] A. Trusov, et al., Gas/liquid membrane contactors based on disubstituted polyacetylene for CO2 absorption liquid regeneration at high pressure and temperature, J. Membr. Sci. 383 (1–2) (2011) 241–249. [158] W. Yave, et al., A novel poly(4-methyl-2-pentyne)/TiO2 hybrid nanocomposite membrane for natural gas conditioning: butane/methane separation, Macromol. Chem. Phys. 208 (22) (2007) 2412–2418. [159] S.D. Kelman, et al., The influence of crosslinking and fumed silica nanoparticles on mixed gas transport properties of poly[1-(trimethylsilyl)-1-propyne], Polymer 49 (13–14) (2008) 3029–3041. [160] M.S. Suleman, K.K. Lau, Y.F. Yeong, Plasticization and swelling in polymeric membranes in CO2 removal from natural gas, Chem. Eng. Technol. 39 (9) (2016) 1604–1616. [161] D. Knani, et al., Molecular modeling study of CO2 plasticization and sorption onto absorbable polyesters, Polym. Bull. 72 (6) (2015) 1467–1486. [162] M. Saberi, S. Hashemifard, A.A. Dadkhah, Modeling of CO2/CH4 gas mixture permeation and CO2 induced plasticization through an asymmetric cellulose acetate membrane, RSC Adv. 6 (20) (2016) 16561–16567. [163] R.A. Assink, Plasticization of poly (dimethyl siloxane) by high-pressure gases as studied by NMR relaxation, J. Polym. Sci. Polym. Phys. Ed. 12 (11) (1974) 2281–2290. [164] S. Kanehashi, et al., Effects of carbon dioxide-induced plasticization on the gas transport properties of glassy polyimide membranes, J. Membr. Sci. 298 (1–2) (2007) 147–155. [165] P. Pandey, R. Chauhan, A. Shrivastava, Carbon-dioxide-induced plasticization effects in solvent-cast polyethylene membranes, J. Appl. Polym. Sci. 83 (12) (2002) 2727–2731. [166] E. Sanders, Penetrant-induced plasticization and gas permeation in glassy polymers, J. Membr. Sci. 37 (1) (1988) 63–80. [167] M. Saberi, A. Dadkhah, S. Hashemifard, Modeling of simultaneous competitive mixed gas permeation and CO2 induced plasticization in glassy polymers, J. Membr. Sci. 499 (2016) 164–171. [168] A. Bos, et al., Suppression of CO2-plasticization by semiinterpenetrating polymer network formation, J. Polym. Sci. B Polym. Phys. 36 (9) (1998) 1547–1556. [169] J. Chiou, J.W. Barlow, D. Paul, Plasticization of glassy polymers by CO2, J. Appl. Polym. Sci. 30 (6) (1985) 2633–2642. [170] T. Visser, G. Koops, M. Wessling, On the subtle balance between competitive sorption and plasticization effects in asymmetric hollow fiber gas separation membranes, J. Membr. Sci. 252 (1–2) (2005) 265–277. [171] C.A. Scholes, et al., Plasticization of ultra-thin polysulfone membranes by carbon dioxide, J. Membr. Sci. 346 (1) (2010) 208–214. [172] M. Donohue, B. Minhas, S. Lee, Permeation behavior of carbon dioxide-methane mixtures in cellulose acetate membranes, J. Membr. Sci. 42 (3) (1989) 197–214. [173] S. Shahid, K. Nijmeijer, Performance and plasticization behavior of polymer–MOF membranes for gas separation at elevated pressures, J. Membr. Sci. 470 (2014) 166–177. [174] S.H. Han, et al., Highly gas permeable and microporous polybenzimidazole membrane by thermal rearrangement, J. Membr. Sci. 357 (1–2) (2010) 143–151.

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[175] C.H. Lau, et al., Reverse-selective polymeric membranes for gas separations, Prog. Polym. Sci. 38 (5) (2013) 740–766. [176] S.D. Kelman, et al., Crosslinking poly[1-(trimethylsilyl)-1-propyne] and its effect on physical stability, J. Membr. Sci. 320 (1–2) (2008) 123–134. [177] S.D. Kelman, et al., Crosslinking poly(1-trimethylsilyl-1-propyne) and its effect on solvent resistance and transport properties, Polymer 48 (23) (2007) 6881–6892. [178] W.F. Yong, et al., Suppression of aging and plasticization in highly permeable polymers, Polymer 77 (2015) 377–386. [179] C.H. Lau, et al., Ending aging in super glassy polymer membranes, Angew. Chem. Int. Ed. 53 (21) (2014) 5322–5326.

Further reading [180] S. Takahashi, D. Paul, Gas permeation in poly (ether imide) nanocomposite membranes based on surfacetreated silica. Part 1: without chemical coupling to matrix, Polymer 47 (21) (2006) 7519–7534.

CHAPTER

Synthetic polymer-based membranes for hydrogen separation

12

Norazlianie Sazali, Wan Norharyati Wan Salleh, Ahmad Fauzi Ismail Advanced Membrane Technology Research Centre (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, Johor, Malaysia

Chapter outline 12.1 Introduction .............................................................................................................................. 273 12.2 History of polymeric membranes for H2 separation ...................................................................... 274 12.2.1 Mechanisms of gas transport in polymeric membranes ............................................ 278 12.3 H2-selective membranes ............................................................................................................ 282 12.4 Polymeric membrane characteristics for ideal H2 separation ....................................................... 285 12.5 Conclusion ................................................................................................................................ 287 References ........................................................................................................................................ 288 Further reading .................................................................................................................................. 292

12.1 Introduction As early as the 1950s, membranes were applied for various gas separations, such as oxygen removal from air, H2 from petroleum refinery gas, and helium (He) from natural gas. This was approximately 25 years before commercial membranes with the ability to accomplish these separations were established. The earliest commercial gas separation membrane implementation was the recovery of H2 [1, 2]. H2/CO ratio modification in gas synthesis consisted of a mixture of H2 and carbon monoxide that was often obtained from steam formed by natural gas or the gasification of coal [3], H2 elimination from hydrocarbon [4], or H2 elimination from purge gases in the manufacturing of ammonia and other petrochemical processes [5]. Table 12.1 shows the distinction between CO2 and H2 in various properties related to the separation. Since the late 1980s, H2 has been typically separated with membranes, cryogenic systems, or pressure swing adsorption (PSA) [6]. PSA utilizes exceptional adsorbents such as zeolites to adsorb unwanted components at elevated pressures, hence refining the H2 gas [7]. Very low temperatures

Synthetic Polymeric Membranes for Advanced Water Treatment, Gas Separation, and Energy Sustainability https://doi.org/10.1016/B978-0-12-818485-1.00012-5 # 2020 Elsevier Inc. All rights reserved.

273

274

Chapter 12 Synthetic polymer-based membranes

Table 12.1 Distinction between CO2 and H2 in various properties relevant to separation.

Gas

Molecular weight

Kinetic diameter ˚) (A

Critical temperature (K)

Polarizability (10225 cm23)

Quadrupole moment (10227 esu21 cm21)

Others

CO2 H2

44 2

3.30 2.89

304.2 33.3

29.1 8.04

43.0 6.62

Acid Neutral

Quartz tube

Catalyst sample Thermocouple Condenser MicroGC

Electric furnace H2S in He Peristaltic pump Sampling probe Quartz wool

FIG. 12.1 Bed reactor facility for H2S and He schematic diagram. From K.R.G. Burra, G. Bassioni, A.K. Gupta, Catalytic transformation of H2S for H2 production, Int. J. Hydrogen Energy 43 (2018) 22852–22860.

are utilized for the cryogenic systems to critically select and condense unwanted components in purifying H2. Fig. 12.1 shows a schematic for H2S and He acting as the diluent gas [8]. Almost 80% of the current global energy demand is fulfilled by fossil fuels [5, 9]. Unlike fossil fuels, water is the only byproduct produced from the utilization of H2 as an energy source. H2 usage as a source of energy has huge potential in addressing energy security-related issues such as air pollution and climate change. Furthermore, H2 exists in abundance and possesses the highest energy content per unit of weight in comparison to commercial fuels [10]. Therefore, there is an increase in demand for H2 energy production.

12.2 History of polymeric membranes for H2 separation The usage of polymeric membranes in gas separation was first documented more than 180 years ago by Mitchell in his work on a mixture of carbon dioxide (CO2) and H2 [5]. In the 1970s, the first effective implementation of membrane gas separation focused on the removal of H2 from ammonia purge gas streams via polymeric membranes, and H2/CO ratio adjustment in the synthesis of gas [11]. A study by Wisniak et al. [12] concluded that Graham and coworkers discovered the vital step in comprehending permeation by proposing a solution-diffusion mechanism, in which permeated molecules were dissolved in the membrane’s upstream face before being transported across the membrane through a process similar to liquid diffusion. Fig. 12.2 shows a block diagram of various types of H2 separation membranes [13].

12.2 History of polymeric membranes for H2 separation

275

Mesoporous ceramic membrane Porous membrane

Polymeric membrane

Micro-porous ceramic membrane

Hydrogen separation membranes Dense metal membrane

Perovskite-type membrane Dense ceramic membrane

Proton conducting membrane

Nonperovskite-type membrane Cermet

FIG. 12.2 Block diagram of various types of H2 separation membranes. From M.R. Rahimpour, F. Samimi, A. Babapoor, T. Tohidian, S. Mohebi, Palladium membranes applications in reaction systems for hydrogen separation and purification: a review, Chem. Eng. Process. Process Intensif. 121 (2017) 24–49.

Membranes selecting H2 demand significantly less energy and are easier to manage. The parameters crucial for membrane processes are the temperature and the partial pressure of H2 at the inlet/outlet [14]. Unlike other separation techniques, membrane processes are suitable for small-scale and portable applications. Membrane processes can also be performed at various temperatures and pressures. One of the benefits of membrane separation is that it can be utilized in membrane reactors, which permit synchronous H2 generation and purification [2]. Additionally, H2 polymeric membranes can be employed at moderate temperatures within 350–450°C. This temperate range fits the prerequisite of hightemperature water-gas shift reactions in the steam regeneration of natural gas, the primary H2 manufacturing technique. In addition to that, H2 separation membranes can yield various level purity in concordance with the type of membrane separation. Membranes can be categorized into two essential groups: porous and dense [15]. Porous membranes can be created using polymers comprised of carbon, ceramic, and metals, such as polymeric membranes, ion-conductive ceramics, and dense metallic membranes, respectively, as shown in Table 12.2. A concise portrayal of these separation membranes is provided below. Dense polymeric membranes can be utilized to isolate H2 from gas mixtures at moderately lower temperature ranges around
110°C. The polymeric membranes are separated into two main classes: glassy and rubbery. Glassy membranes yield a comparatively higher selectivity with a lower H2 flux than the standard H2 permeability obtained from basic polymeric membranes. The polymeric membranes are typically favored for their low cost [16]. Nevertheless, the H2 permeability and selectivity of the polymeric membranes are much lower in comparison to the dense metallic membranes. The basic polymeric membranes are also resistant to impurities in the presence of H2S, HCl, and CO2 [17]. Subsequently, the polymeric membranes are less appealing in comparison to other dense membranes [18].

276

Chapter 12 Synthetic polymer-based membranes

Table 12.2 Concise portrayal of several separation membranes. Membrane types

Typical type Diffusion mechanism Driving force Operating temperature (°C) Permeability Typical selectivity Relative cost

Porous

Polymeric

Ceramic iron conducting

Dense metal

Silica, alumina, zeolites, carbon Size exclusion

Polyimide, cellulose acetate Solution diffusion

Polyimide, cellulose acetate Solution diffusion

Polyimide, cellulose acetate Solution diffusion

Pressure gradient
1000

Pressure gradient
110

Ionic gradient 700–1000

Pressure gradient 150–700

Moderate high Low moderate

Moderate high Moderate

Moderate Very high

Moderate Very high

Low

Low

Low

Low

Low selectivity material such as cellulose acetate is also utilized for polymer-based membranes in hydrogen gas separation. Breakthroughs in gas separation membrane material, which improve permeability and separation, include polyimide-based and brominated polysulfone-based commercial membranes. Currently, among polyimides, which are known to have low free volume, glassy polymer is the most widely used material for hydrogen separation via membranes [19]. Compared to cellulose acetate and polysulfone, aromatic polyimides are more suitable to be utilized at high temperatures due to their high glass transition temperature [20]. According to Takht Ravanchi et al. [21], the successful application of polyimide membranes in refineries for hydrogen recovery is due to the high stability and interesting separation factors (e.g., H2/N2 of ca 100–200) of the properties. Matrimid 5218, a polyimide-based material, showed a good H2/CH4 selectivity of 100 [22]. On the other hand, fluorinated polyimides exhibited a high H2 permeability with low selectivity, in contrast to nonfluorinated polyimides. A fluorinated polyimide, 6FDA-DDBT, showed a hydrogen permeability of 156 Barrer with only 80 H2/CH4 selectivity while BPDA-ODA, a nonfluorinated polyimide, showed an H2 permeability of 1.33 Barrer with a high H2/N2 selectivity of 370 [23]. Among the existing polyimide-based materials, diamino-modified polyimides are considered the most superior as they have an ideal H2/CO2 selectivity of 100, which exceeds the Robeson upper bound line [24]. Polybenzimidazole (PBI) has recently emerged as a prime candidate for low-cost PEMs due to its intrinsic physicochemical properties, for example, heat resistance and proton conductivity above 100° C without humidification. Polybenzimidazole has also been identified as a unique polymeric material for H2 separation, owing to its superior thermal stability and good intrinsic H2/CO2 selectivity under high-temperature environments [25]. In the late 1970s, DuPont pioneered the use of small-diameter hollow fiber membranes. Despite that, the yield of first-generation hollow fibers in fulfilling the gas separation economy was very small [26]. Monsanto overcame this issue by developing a multicomponent polysulfone hollow fiber membrane to recover H2 [27]. Limiting the dense and discerning regions of the fibers to a very small area greatly increased the transportation of the fibers. The implementation of these asymmetric membranes in recovering industrial-scale H2 from ammonia

12.2 History of polymeric membranes for H2 separation

277

purge gases was also successful. Then, Separex Corp established the Separex spiral-wound cellulose acetate membranes by targeting identical types of segregation using previous technology, natural gas dehydration, and purification [17]. The cellulose acetate membranes performed better than the hollow membranes because of their high resistance toward hydrocarbon impurities [28, 29]. By the mid-1980s, the implementation of membranes was broadened to include other applications such as recovering H2 from recycled refinery gas. Ube introduced polyimide membranes with the best heat- and solventresistance characteristics in Japan, and these membranes were first used in Seibu Oil’s Onoba City refinery. Membrane technology has shown remarkable progress since the development and utilization of synthetic polymeric membranes in the 1980s. The artificial polymeric membrane acts as a physical barrier between two phases while permitting differential transport to occur. The fabrication of different composite membranes was employed in several applications, mostly in gas separation for hydrogen purification. Based on interfacial polymerization techniques, Choi et al. [30] fabricated composite membranes with polyethersulfone (PES) substrate to investigate the separation of the H2/CO gas mixture. In their study, 1,3-cyclohexanebis methylamine (CHMA) was used as the aqueous phase monomer while trimesoyl chloride (TMC) was used as the organic phase monomer. Air was circulated in the oven to retain the operating temperature at a constant. A bubble flow meter was used to calculate the permeance and the retention flow rate of the gas. The recent increase in the popularity of the H2 economy has led to mounting attention toward H2 recovery to supply the increasing energy demand required by industrialized countries [31]. A proton exchange membrane (PEM) fuel cell [7, 32] that utilizes H2 as fuel to produce electrical energy from chemical energy is another appealing substitute to the combustion of fossil fuel for power generation. These fuel cells have several benefits such as having higher fuel efficiency compared to internal combustion engines; generating almost no HC, NOx, or CO; and having low CO2 levels. However, for these fuel cells to be broadly used, a good distribution of the H2 supply is needed. The solution-diffusion mechanism is unable to fully omit the trace contaminants that restrict the economic viability of polymeric materials for the production of ultrahigh pure H2 [13]. H2 is the most abundant component on Earth, and it can be extracted from biomass, water, or hydrocarbons such as natural gas or coal. H2 production can either be done via nuclear energy or electricity produced from sustainable resources such as solar, biomass, or wind [33]. The combustion of H2 only produces water, resulting in clean energy. Nevertheless, H2 production from hydrocarbons produces CO2, a greenhouse gas [34]. H2 is produced significantly globally (about 5 billion cubic meters annually) and is mainly utilized for the production of ammonia in the metallurgical and chemical industries (4%), methanol production (8%), oil refining (37%), and fertilizer (around 50%) [5, 35, 36]. A lot of effort has been made to improve the relevant technologies to support the H2 economy infrastructure with a huge emphasis on energy cost, environmental sustainability, and security (for both transport and stationary separations) [2, 37–39]. A dramatic increase in the global investment of H2 has been observed over the past few years, and is currently estimated to be several billion dollars. Previously, the Bush administration declared a $1.7 billion program specifically for the advancement of H2 technology, particularly for fuel cell vehicles [9]. By 2020, Japan intends to implement 4000 H2 filling stations [40]. Iceland aims for a thorough transition to H2 by 2030, which might be one of the most wellknown examples of the H2 economy [40]. Iceland plans to produce H2 via its own resources of geothermal and hydro fuel cells for adaptation in transportation (buses, cars, fishing boats, etc.) as well as

278

Chapter 12 Synthetic polymer-based membranes

stationary applications (businesses, homes). Currently, Hawaii is coordinating a study to evaluate the prospect of large-scale H2 usage, renewable energy, and fuel cells. Various technological issues related to H2 distribution and storage need to be resolved. Globally, there are several countries with vast gas and coal resources, which play an important role in the transition mentioned above [41, 42]. Substantial investment in new infrastructure such as storage facilities, fueling stations, and pipelines is also required for the main initiative of H2. The H2 economy encourages the potential of mixing energy while offering a cleaner environment [43–45].

12.2.1 Mechanisms of gas transport in polymeric membranes A membrane is known as a physical blockade that allows a selection of mass material to pass through, and is broadly utilized for purification and separation in various industries. Membranes can be categorized into inorganic, organic, and hybrids of inorganic/organic systems. An organic membrane consists of polymeric and biological constituents while an inorganic membrane can be further divided into a metallic membrane (dense phase) and a ceramic membrane (porous and nonporous). Table 12.3 shows the comparison of polymeric and inorganic membranes. A summary of definitions as well as basic concepts of membranes can be found in the report by IUPAC (International Union of Pure and Applied Chemistry) [10]. The cost, production, separation selectivity, membrane durability, and the stability of the mechanical integrity of operating conditions are significantly deliberated in all circumstances, and differ according to the implementation [46]. Nevertheless, the permeation rate and selectivity (or permeance) are the most elementary properties of a membrane. A higher selectivity leads to a higher efficiency of the process, with lower driving force (pressure ratio) to attain the separation, hence lowering the operation cost of the separation system. A higher flux requires a smaller membrane area, therefore lowering the system’s investment expenditure. Polymeric membranes can be grouped by two main characteristics: porous and nonporous, with various mechanisms that enable gas transportation across these membranes [47]. Diffusion in a porous membrane takes place through mechanisms that are highly reliant on the membrane’s

Table 12.3 The comparison of polymeric and inorganic membranes. Membrane

Advantages

Disadvantages

Current status

Inorganic

Long-term durability High thermal stability (>200°C) Chemical stability in wide pH High structural integrity Cheap

Brittle (Pd) Expensive

Small-scale applications Surface modifications to improve hydrothermal stability

Polymeric

Mass production (larger scale) Good quality control

Some have low hydrothermal stability Structurally weak, not stable, temp. limited Prone to denature and be contaminated (short life)

Wide applications in aqueous phase, and some gas separations

12.2 History of polymeric membranes for H2 separation

279

Surface diffusion domain

Knudsen diffusion domain

Pore surface Pore size

(A)

(B)

(C) Blocking effect

Surface diffusion only

FIG. 12.3 Main transport phenomenon in microporous structure. (A) Micrograph with surface diffusion domain, (B) micrograph with normal microporous structure, and (C) micrograph with blocking effect. From A. Caravella, P.F. Zito, A. Brunetti, E. Drioli, G. Barbieri, A novel modelling approach to surface and Knudsen multicomponent diffusion through NaY zeolite membranes, Microporous Mesoporous Mater. 235 (2016) 87–99.

morphology (i.e., pore size) as well as the diffusing molecule’s size: surface diffusion, Knudsen diffusion, molecular sieving, and capillary condensation. Fig. 12.3 shows the main transport phenomenon in the microporous structure [45]. In Knudsen diffusion, the pore size is selected so the diffusing species bumps into the pore walls more frequently in comparison with other diffusing molecules [48]. Surface diffusion enables molecules to be adsorbed onto the pore surface and be transported to a different site of lower concentration [49]. Capillary condensation takes place as the pore size and the penetrant’s interactions with the pore walls cause condensation in the pores, influencing the diffusion rate through the membrane. Meanwhile, the occurrence of molecular sieving is identified as the pore size being almost similar to the diffusing molecule’s size, hence needing activation energy to be conquered prior to diffusion. In thick nonporous membranes, transport across the membrane occurs through a solution-diffusion mechanism. Operated by the potential gradient of the chemical [50], penetrant molecules dissolve into the polymer membrane before diffusing across the membrane, as this mechanism commands permeation in polymeric gas separation membranes.

280

Chapter 12 Synthetic polymer-based membranes

Phase inversion and solution blending techniques were employed by Momeni and coworkers [51] to fabricate a nanocomposite membrane by the reinforcement of nanosized magnesium oxide (MgO) particles in the PSf matrix. PSf is classified as a glassy polymer because of its rigid structure and vacancy in chain mobility while MgO is one of the metal oxides used to strengthen and enhance the thermal resistance of membranes [37, 52]. MgO exhibits a good affinity for gaseous molecules and has the potential to be a metal oxide filler in nanocomposite gas separation membrane preparation [18]. For glassy polymers, gas permeation is determined by the gas diffusion ability of the polymer [37, 52]. The permeation of N2, H2, and CO2 in pure PSf and PSf/MgO nanocomposite membranes as well as their selectivity were investigated at an ambient temperature and pressure of 4  105 Pa [51]. When MgO particles were loaded into the membrane matrix, the permeability of H2 and N2 increased from 44.05  1016 and 0.93  1016 to 67.30  1016 and 2.00  1016 mol m/(m2 s Pa), respectively. The results demonstrated a similar trend to reports for various nonporous nanoparticle fillers reinforced in glassy polymers. Some properties that affect gas permeability through polymers include gas solubility, the molecular size of the gas, the void volume in the polymer, and also the chain mobility of the polymeric membrane. Next, the separation performance of membranes was calculated for selected gas pairs. The results for gas selectivity contradicted with gas permeability for MgO-filled membranes, where the ideal gas selectivity for the nanocomposites decreased for the two gas pairs of H2/N2, and H2/CO2. The H2/N2 gas pair decreased from 47.11 to 33.58 while H2/CO2 decreased from 1.71 to 1.42. The selectivity of gas is affected by the size of the gas molecules, in which smaller molecules tend to increase in ideal gas selectivity. To obtain the optimum process, cost, and performance of the membrane, fillers and additives need to be incorporated into the polymer matrix. Nanoscale fillers were more beneficial than microscopic fillers as the former had better properties of nanoparticles. Inorganic fillers such as silica, titania, and zinc oxide (ZnO) nanoparticles provide significant advantages in the preparation of advanced polymer/inorganic nanoparticle composite membranes. The combination of inorganic and organic components is widely used for the preparation of membrane-based hydrogen purification. Hybrid materials have complex morphology, structure, interaction, and functionality. Lastly, the utilization of ecofriendly and economical reinforcing fillers balances the process ability, performance, and cost-effectiveness. Table 12.4 shows several mixed matrix membranes (MMM) reported from previous studies. The fabrication of a thin MOF/polymer MMM on a tubular ceramic substrate was developed by Zhao and coworkers via a novel casting technique. The best H2/CO2 separation performance by the membrane was when MOF powders were loaded at 20 wt%, resulting in up to 53.1 selectivity. Their work produced the highest selectivity in comparison to the existing literature on MMMs, and exceeded the theoretical Robeson upper bound limit of gas separation by a pure polymeric membrane [58]. Perez et al. [59] investigated the gas permeance of H2 and CO2 when operated at high pressure and high temperature, and at different thicknesses of VTEC PI-1388 and 20 wt% (Al) NH2-MIL-53/VTEC PI-1388 MMMs. The operating condition was between 5–30 bar and 35–300°C. The MMM showed a higher permeability at about 70% compared to when using only VTEC PI-1388. The gas separation in both VTEC PI-1388 and MMM was dependent on the stage cut. At the stage cut of 0.05, the maximum separations of H2/CO2 in VTEC PI-1388 and MMM were 7.2 and 7.5, respectively. Novel FAU/Matrimid MMMs and NaX/Matrimid multilayer membranes supported by porous alumina plates were prepared and evaluated by Mundstock et al. [60] for H2/CO2 separation performance. In their work, they studied the quality of neat supported-NaX composite membranes when grown on layers

12.2 History of polymeric membranes for H2 separation

281

Table 12.4 Performance of mixed matrix membranes from previous studies. Performance 216

Permeability (×10 mol m/m2 s Pa)

Ideal selectivity

Samples

H2

CO2

N2

H2/N2

H2/CO2

References

6FDA-m-PDA 6FDA-2,4-DATr 6FDA-3,5-DBTF 6FDA-4BDAF 6FDA-3,30 -ODA 6FDA-3BDAF Bisphenol-F polysulfone Bisphenol-O polysulfone Dimethyl bisphenol-A polysulfone Dimethyl bisphenol-z polysulfone MOFs (NH2-CAU-1 and NH2-MIL-53) NH2-MIL-53/PI (VTEC) Zeolite FAU/Matrimid NH2-CAU-1/PMMA ZIF/Matrimid UiO-66/PI Cyclic olefin copolymer/GO ZIF-11/PBI Silica/PPO ZIF-11/Matrimid Cu3(BTC)2/PI

20.3 87.2 58.6 46 14 21 10 15 11 9.2 33 0.45 6 3.7 0.6 2.5 0.05 1.38 23 0.36 420

8.2 28.6 21.6 19 2.1 6.3 4.5 4.3 2.1 1.4 0.622 0.058 0.582 0.285 0.136 0.490 0.0012 0.197 6.38 0.08 15.11

0.36 1.31 1.17 0.98 0.10 0.24 0.2 0.2 0.09 0.06 – – – – – – – – – – –

56.39 66.56 50.09 46.94 140 87.5 50 75 122.22 153.33 – – – – – – – – – – –

2.476 3.049 2.713 2.42 6.67 3.33 2.22 3.49 5.238 6.57 53 7.7 10.3 13 4.4 5.1 40 7 3.6 4.4 27.8

[53] [53] [53] [54] [54] [54] [55] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68]

of 3-aminopropyltrietoxysilane (APTES) and polydopamine (PDA), two different presynthetic support modifications. The separation factors, α, for neat NaX, NaX/APTES, and NaX/PDA were 8.0, 8.9, and 10.3, respectively. The same group also suggested that adding ion-exchanged Na-FAU powders during MMM preparation could augment the separation performance by exchanging Na+ ions with higher ionic potentials such as CO2+, and thus, improve the separation factors (α ¼ 4.0 for NaX/Matrimid MMM and α ¼ 5.6 for CoX/Matrimid MMM) that lead to the enhancement of H2/CO2 separation. Cao and coworkers [61] introduced a thin compact MMM based on a CAU-1-NH2 and poly (methyl methacrylate) polymer, as these materials possess high hydrogen permeability with superior H2/CO2 selectivity. The highest H2/CO2 separation attained under these conditions was 13. Ordonez and groups [62] fabricated and tested MMMs with Zeolitic imidazolate frameworks, ZIF-8, up to 60 wt% as additives for gas separation. ZIF-8, which is made up by clusters of Zn2+ linked to imidazolate ligands, affects the permeability and selectivity of the membranes, depending on its loadings. When the loadings were 50 and 60 wt%, the permeability for all gases decreased while the ideal selectivity increased. This was due to the change in the gas transport mechanism where at lower loadings, it was driven by

282

Chapter 12 Synthetic polymer-based membranes

polymer, but at higher loadings, it shifted to ZIF-8 controlled gas transport [62, 63]. Various types of graphitic nanosheets were used for the fabrication of cyclic olefin copolymer (COC) composite membranes via melt processing [64]. The graphite nanosheets used in this study were graphite oxide (GO), octadecylamine (GO-ODA), and stearic acid modified graphite oxides (GO-SA). The permeability of CO2 and CH4 decreased when the amount of fillers increased for all series of membranes, except for hydrogen. Similar to the trend seen with CO2 and CH4, in series fabricated with GO and GO-ODA, H2 permeance decreased. However, in samples prepared with GO-SA, there was a positive correlation between the permeability of H2 and the amount of filler. A GO-SA-based membrane significantly improved the separation performance of H2/CO2 by 57.5% and 280% for H2/CH4 compared to the COC film [64]. Sa´nchez-Laı´nez et al. [65] studied the zeolitic imidazolate framework in two forms—nanosized (nZIF-11) and microsized (ZIF-11)—incorporated into polybenzimidazole (PBI)-based MMMs. An investigation for gas separation was carried out at temperature ranges between 70°C and 200°C for all membranes. Both nZIF-11 and ZIF-11 that were embedded into the PBI polymer matrix improved the H2 permeance and selectivity in comparison to the pure polymeric membrane in H2/CO2 separation. As the macro and nano ZIF-11 produced similar performances, it was suggested that an increase in temperature enhanced the gas permeability, independent of the particle size at low loadings ( > >
π π exp
exp F, b < D, b = k D 

ΔP
 Jw ¼ A > > B Jw S Jw > > :1 + ; exp
exp
k Jw D

(18.5)

This equation utilizes experimentally accessible parameters and incorporates the performance-limiting phenomena of ICP and ECP as well as salt leakage across the membrane. By substituting Eq. (18.4) into Eq. (18.3), the ideal power output is W ¼ AðΔπ
ΔPÞΔP

(18.6)

The theoretical value of maximum power density, Wmax can be achieved when the hydraulic pressure is equal to half the osmotic pressure gradient (ΔP ¼ Δπ/2). The equation can be rearranged as follows: Wmax ¼ A

Δπ 2 4

(18.7)

From the above Eq. (18.7), Wmax in the PRO system is directly proportional to the membrane water permeability A, and the square of the osmotic pressure difference. From this, it is suggested that the

18.4 Membranes for pressure-retarded osmosis

425

operating pressure for PRO processes is close to Δ π/2 and results in the maximum power output that can be produced. Normally, the desired operating pressure is approximately around 13–13.5 bar when river water and seawater are mixed together [1]. Nevertheless, it needs to be noted that in the real-world PRO processes, the real osmotic pressure gradient of the system is usually lower than the osmotic pressure gradient between freshwater and saline water. A study of the reverse salt diffusion was conducted by Touati and Tadeo based on a set up with various conditions [27]. The osmotic pressure differential was created by varying the concentration difference between the feed and draw solutions. It was reported that the higher the differential osmotic pressure, the higher the reverse salt flux. The higher cross-flow velocity was observed to result in higher reverse salt flux. The membrane orientation during PRO processes also affected the reverse salt flux, where the AL-DS orientation resulted in low reverse salt flux compared to AL-FS, thus rendering higher power density in the PRO system. The temperature of the draw and feed solutions also imposed significant effects on the PRO system. The higher the temperature of the solution, the higher the reverse salt flux due to the enlargement of pore size. In short, one of the vital components that needs to be considered before running the PRO system is the operating condition itself, so that the highest possible power density can be attained.

18.4 Membranes for pressure-retarded osmosis As stated before, to obtain the highest possible power density that is economically feasible, the PRO membrane must demonstrate the desired features of FO and RO membranes. Much significant progress has been made in PRO technology via plant- and lab-scale modeling as well as high-performance membrane design. From the discussions earlier, in order to ensure that the power density produced is commercially appealing, a high-performance PRO membrane should demonstrate a very good performance in these three criteria: (i) highly hydrophilic, (ii) good antifouling performance, and (iii) high mechanical strength. There are two types of membranes that can be used for PRO application: hollow fiber and flat sheet, both of which have been well studied for PRO application. These two configurations do not possess critical differences, but the Statkraft osmotic power plant at Tofte in Norway was operated using a flat sheet membrane. It has been suggested that the flat sheet membrane allows both the feed and draw solutions to have maximum surface contact with the membrane as opposed to hollow fiber membranes. TFC is typically made from a thin polyamide layer deposited on top of a nonwoven support sheet porous layer. The most common method in developing a PA TFC-PRO membrane is through interfacial polymerization (IP). The IP of TFC membranes was first introduced and developed by Cadotte and represented a breakthrough for RO applications in membrane performance [28]. This technique is based on the reaction between two monomers on polycondensation. One is a two-functional amine and the other is a tri-functional acid chloride. At first, the active surface of the support layer is soaked with aqueous amine solution. After the surface dries and is contacted with an organic phase containing a chloride acid monomer such as trimesoyl chloride (TMC), an ultrathin layer, also known as an active layer, is quickly formed at the interface of the two phases and attached to the support. The reaction takes place at the organic side of the interface, owing to the poor solubility of the acid chloride in water and the good solubility of the amine in organic solvents.

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Chapter 18 Polymeric membranes for pressure-retarded osmosis

Among the commonly used active monomers and chloride acids to form the functional PA layer of the membranes are m-phenylenediamine (MPD) and TMC. This polyamide exhibits extraordinary water flux and salt rejection properties [4]. Another amine monomer that can be used for the production of TFC PA membranes is p-phenylenediamine (PPD). In comparison to the TMC-MPD PA layer, the TMC-PPD results in a lower flux and higher salt rejection due to lower molecular chain mobility in the membrane [29]. According to Baron˜a et al., the addition of 2,20 -benzidinedisulfonic acid (BDSA) into MPD resulted in a more hydrophilic surface, and the flux in PRO increased as a function of BDSA concentration [30]. Fig. 18.5 shows the interfacial polymerization reaction of monomers used for TFC membrane preparation. The polyamide consists of A and B polymer repeating units. Fig 18.5A shows the polymerization product of MPD and TMC monomers while Fig. 18.5B shows the product of BDSA and TMC where the percentage of salt rejection increased with BDSA concentration. Another alteration to polyamides was done by Shintani et al., by increasing the degree of methyl-substitution of diamines [31]. However, this led to the decreased in salt rejection and increase to chlorine resistance. This research was conducted to study the usefulness of positron annihilation lifetime spectroscopy method and molecular dynamic simulation in predicting the salt rejection property of RO membranes. The phrase “TFN membrane” was first introduced by Hoek and his team in early 2007, in their published pioneering research work [32]. TFN is a recent type of composite membrane prepared via an interfacial polymerization process by embedding the nano-sized materials or particles in the membrane. A wide range of nano-sized materials such as carbon nanotubes (CNT), zeolites, and others have been explored to fabricate the high-performance TFC membrane. All these nanoparticles have extraordinary structural properties and surface functionalities such as enhanced hydrophilicity, water flux, antimicrobial functionalities, mechanical properties, and osmotic power. Thus, by incorporating these nanoparticles in the TFC membrane, an improved performance of the TFN membrane can possibly be fabricated [21]. Various experiments and theoretical studies have been conducted to verify that CNTs can provide fast water transport. Hence, the CNT has been proposed as a prospective nanomaterial that can enhance the water flux in the TFC membrane [21]. On the other hand, Idarraga-Mora et al. stated that the use of a polyester woven mesh with a variable opening size will also affect the performance of the PRO membrane [33]. From Fig. 18.6, the microscope images show big differences for different woven meshes under the same magnification. From the experimental results, the smaller the opening size, the higher the burst pressure of the membrane. In contrast, the water flux decreased due to larger contact between the membrane surface and the wire of the polyester woven mesh. It was then concluded that the burst pressure of the membrane has a minimum impact on the PRO, but the optimum operating pressure of the PRO has a significant impact on achieving higher power density. However, it is also important to optimize the opening size of the polyester woven mesh so that the PRO membrane can be sustained at high pressure and optimum power density can be achieved [33]. As mentioned earlier, both flat sheet and hollow fiber membranes can be used in PRO applications. Due to the high operational pressure applied on the membrane, there will be some deformation on the membrane structure and morphology. A cross-sectional micrograph of a TFC-PRO membrane is shown in Fig. 18.7 [34]. It is clearly shown that the entire support layer thickness demonstrates a large fingerlike macrovoid span. After operation under the PRO system at 15 bar pressure for 120 min, damage occurs at the cross-section structure of the referential PAN support. A reduction of 22.9% of the membrane thickness and a partially collapsed porous structure can lead to a dramatically decrease in membrane pure water permeability [34]. On the other hand, the hollow fiber membrane for PRO

18.4 Membranes for pressure-retarded osmosis

O

O

O

427

OH S O

CI

CI

H2N O

NH2

H2N CI

NH2 S

O

HO

TMC

MPD

O

BDSA

Interfacial polymerization

O

O

NH

NH

OH

O

n–1

linear part

NH

NH

O

NH

NH

O

O

m–1

crosslinked part

(A)

OH O

OH O

S

S

NH O

NH O

O O

O

O

NH O NH

S

O

OH

NH O

O

S

S

OH

n

linear part

O

O

OH

(B)

O

OH O

O

S OH

crosslinked part

NH

m

FIG. 18.5 (A) The polymerization product of MPD and TMC monomers, and (B) the product of BDSA and TMC [30].

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Chapter 18 Polymeric membranes for pressure-retarded osmosis

FIG. 18.6 Microscope images of: (A) nonwoven mesh and standard woven mesh. The wire diameter (μm) and opening size (μm) are as follows (B) (160, 250), (C) (50, 75), (D) (32, 45), (E) #(20, 20), and (F) (53  2, 7) [33].

FIG. 18.7 Scanning electron microscopic (SEM) cross-sectional view of a TFC-PRO membrane [34].

applications also faces the same problem of deformation after undergoing PRO operation. According to Wan et al., calcium chloride has been found to increase the tensile strength and burst pressures of the membranes, resulting in higher power density [35]. This shows that the membrane for the PRO system must sustain a very high pressure to work perfectly, even after the deformation of the membrane. These problems need to be addressed to reduce the cost of membrane replacement for the PRO system.

18.5 Recent advancements in PRO

429

FIG. 18.8 (A) External fouling in AL-FS orientation; (B) Both internal and external fouling in AL-DS orientation [36].

18.4.1 Fouling in the PRO membrane Membrane technology represented by PRO shows a good advancement in power generation. However, membrane fouling is what hinders the practical application of PRO. The cause of membrane fouling is the deposition of colloids or organic macromolecules on the membrane. Such a problem not only has a bad impact on the water flux, but it also affects the power density and shortens the membrane life. Basically, membrane fouling may occur at any location on the membrane. It can be divided into external fouling and internal fouling. External fouling can be explained as the foulant that comes from the FS deposited on the membrane active layer surface (AL) and forms a deposited layer, as illustrated in Fig. 18.8A. Internal fouling shows a more complex deposition, as shown in Fig. 18.8B. Membrane fouling is the major issue that reduces the service life and the separation performance of membranes. These issues need to be addressed with the growth of enhanced antifouling membranes for the impactful operation of the membranes. In general, the more hydrophilic the membrane surface, the better its antifouling performance [37]. Thus, the modification of the PRO membrane needs to be done with an association of hydrophilic materials, as this is the main approach in achieving improved antifouling PRO membranes. Once the hydrophilicity of the membrane has been improved, it will reduce the flow resistance of the membrane and improve the permeate penetration through membranes. Other than that, scaling problems often occur during PRO processes [38]. Zhang et al. found that gypsum scaling is affected by the composition of both the feed and draw solutions, the applied hydraulic pressure, the water flux level, and the membrane orientation. A PRO membrane with AL-DS orientation can be adversely affected by the ICP-induced gypsum clogging in the membrane support layer [38].

18.5 Recent advancements in PRO In the study conducted by Son et al. [39], a TFN membrane with CNT embedded within the support layer has been fabricated. It was found to be more advantageous compared to that with CNT embedded in the active layer. First, hydrogen bonding interactions between the sulfonic groups of PES and the carboxylic groups of CNT were stabilized within the polymer matrix [40]. Second, the polyamide active layer of the membrane was not affected and remained unchanged. Finally, the TFN membrane can be easily scaled up using a conventional membrane fabrication method [39]. As a result, the PRO performance of the TFN membrane exhibited a power density of 1.0–1.6 W m
2. On the other hand, Tian et al. [41] fabricated a new PRO membrane with a novel tiered nanofibrous support membrane for the PRO process. The multiwalled CNTs were purified and functionalized. This functionalized CNT was

430

Chapter 18 Polymeric membranes for pressure-retarded osmosis

incorporated in the substrate by mixing polyetherimide (PEI) polymer in the well-dispersed solution of functionalized CNT in the N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP) solvent. The focus of this study was to analyze the effect of the incorporation of functionalized CNT. Three types of membranes were prepared, where both the top and bottom layers were fabricated without functionalized CNT; this was named the T1 membrane. As for the T2 membrane, the bottom layer of the support was incorporated with 0.3 wt% of functionalized CNT. In the T3 membrane, both the top and bottom layers were incorporated with functionalized CNT. Resulting from these variations of membranes, the T3 membrane showed an increasing trend in mechanical strength as the finer fiber part of the membrane was tested at 24 bar pressure. The mechanical strength of the fabricated support was inversely proportional to its porosity [42]. However, the results from this experiment showed differently. Despite the pore size of the membrane decreasing, the total porosity of both membranes increased accordingly with the increasing of the mechanical strength. Fig. 18.9A and B shows the drastic decrease in the contact angle on the top finer fiber before and after interfacial polymerization. The resulting power density of membrane T3 give value of 17.3 W m
2 at 16.9 bar pressure higher than T1 as they can sustain higher operating pressure during PRO process [41]. This membrane also shows exceptional salt rejection despite its high water flux. In order to increase the life span of the membrane, it needs to be cleaned and well maintained. Lee and coworkers tested three approaches in membrane cleaning and made a comparison on how well the cleaning did in maintaining PRO membrane performance. The illustration of the cleaning process is shown in Fig. 18.10. First, a simple backwash process with 8 bar operating pressure of water flow was performed. The foulant dislodge was drained and the performance of the membrane after cleaning

FIG. 18.9 (A) Top finer fiber layer surface morphology before IP; (B) after IP surface morphology; (C) cross-sections of a nanofibrous TFC membrane FESEM image; (D) and (E) nanofiber layer enlarged image underneath the PA layer and top finer fiber layer, respectively [41].

18.5 Recent advancements in PRO

431

FIG. 18.10 (A) Diagram of backwash cleaning, (B) diagram of clean-in-place (CIP), (C) and the diagram of maintenance cleaning (MC) [43].

was found to increase up to 42%. Another cleaning method used was the clean-in-place (CIP) method, where the membrane module was washed using tap water on both sides under 8 bar pressure from the lumen side and 2.5 bar pressure at the shell side. The CIP technique managed to increase the membrane performance up to 57%. It is worth mentioning that no chemical was used in either the backwash or CIP cleaning processes. Maintenance cleaning (MC) was used for the optimal cleaning process of the membrane with an 80% increase of membrane performance after cleaning. The process is similar to the CIP technique, but at the shell side, the tap water flow was replaced by 200 ppm of hydrochloric acid (HCL) or 200 ppm of sodium hypochlorite (NaOCl) [43]. The use of nanomaterials in the membrane has been widely studied, as the presence of appropriately chosen nanomaterials can enhance the performance of the membrane. The highest power density that has been recorded was 34.2 W m
2 by using a novel TFC hollow fiber membrane incorporated with carbon quantum dots (CQDs) [44]. In this study, CQDs were incorporated into the polyamide selective layers. Despite showing high water flux, the reverse salt flux diffusion remained unchanged compared to the control [44]. Another material that has been embedded within the support layer is the silver nanoparticle (AgNP). According to Liu et al., the embedding of AgNPs in the polyacrylonitrile (PAN) support improved the hydrophilicity and antifouling properties of the PRO membrane. The TFN incorporated with 0.02 wt% of AgNPs showed an approximately 25% increment in water permeability when tested in a lab-scaled PRO system; the water flux was enhanced by 88.2% [45]. Zhang et al. grafted a PEI substrate membrane with aminosilane of 3-aminopropyltrimethoxysilane (APTMS).

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Chapter 18 Polymeric membranes for pressure-retarded osmosis

The findings on this study show that aminosilane was able to offer enhanced hydrophilicity to both the active and support layers of the PRO membranes by a straightforward grafting procedure. The grafting of aminosilane was also able to decrease the pore blocking tendency and act as a cross-linker to maintain the mechanical strength of the PRO membranes [46]. Another recent advancement for the PRO system is use with the RO system in an RO-PRO hybrid system. Seawater desalination or water purification through the RO system requires loads of energy and in the long term, this makes the cost of the RO process high. Through the hybridization of the RO-PRO system, the energy created via PRO can be used to help lower the energy consumption by the energy supplier [47]. Through the advancement and the continued study of PRO membrane development, it is possible for the RO plant to only use the energy produced by the PRO system. Ho et al. suggested that the membrane properties of both the PRO and RO membranes need to be improved to increase the performance of the membrane so that the RO-PRO hybrid system can be effective [48].

18.6 Future outlook and conclusion Membrane modification is one of the most promising ways to improve water flux, hence the power density of PRO. Currently, many types of nanomaterials, biomimetic molecules, and zwitterionic polymers have been potentially used to enhance the physicochemical properties of the membrane. For instance, a zwitterionic polymer, which is characterized with equal anion and cation groups on the molecular chains, is highly hydrophilic, so it can resist nonspecific protein adsorption, bacterial adhesion, and biofilm formation [49]. However, the development of zwitterionic applications is still at the lab scale and various difficulties are faced in industrial applications [49]. In terms of real applications of PRO, the membrane must be configured in the plant module due to its operating condition being different compared to laboratory-scale PRO cells [1]. Therefore, detailed design criteria are needed for PRO membrane modules such as flow pattern, module configuration, and the membrane spacer. Another essential key for PRO is the fouling of the membrane during the operation. The cause of the fouling problem can be linked to the quality of the feed and draw solutions. Other than that, bad pretreatment of the water stream can also be linked to fouling of the membrane while the other two areas are membrane characteristics and membrane module design. More R&D is needed to further investigate antifouling membranes and membrane cleaning using large membrane modules to further understand PRO membrane applications [1]. In conclusion, despite of the enormous amount of studies that have been conducted on the potential and issues of PRO in desalination and power generation, more research is still needed to provide future insights and information into this technology. The PRO membrane design and optimization are the essential keys to the successful operation of PRO at the commercial scale. It is believed that in the near future, with the continuing research on PRO membranes, perhaps PRO will represent a membrane technology that shows great promise to address the global challenges in water and energy demand.

Acknowledgment The authors would like to acknowledge the financial support provided by Universiti Teknologi Malaysia under Research University Grant (13H85) and the Ministry of Education under HICoE Grant (4J182).

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Index Note: Page numbers followed by f indicate figures, and t indicate tables.

A Acidic gas adverse environmental effects, 175, 175t membrane separation 1-butyl-3-methylimidazolium (BMIM)-based ionic liquids, 186, 186f carbon dioxide separation membranes, 186–187 cellulose acetate (CA) membrane, 177 composite type, 182–188 design, 179–181, 180t diffusion selectivity, 179–180 fluorine-containing polymer membranes, 186–187 ocean acidification, 188 permeability, 180, 182t plasticization and physical aging, 180–181 polymer membranes, 176–177, 176f, 181–188 polymers of intrinsic microporosity (PIM), 184 solubility selectivity, 179–180 sulfurous acid removal, 177 techniques, 176–177 thermally rearranged (TR) polymer, 184–186 transport mechanism, 177–178 ocean acidification, 175–176 sources and components, 174–175, 175t Acid rain, 175 2-Acrylamido-2-methylpropane sulfonic acid-based hydrogel (AMAH), 82–83 Adsorption membrane capacitive deionization (MCD), 85–86, 87f phenolic compound removal, 55 Advanced oxidation process (AOP), 57–59 AEM. See Anion exchange membrane (AEM) AEMFCs. See Anion exchange membrane fuel cells (AEMFCs) AFM. See Atomic force microscopy (AFM) Air gap membrane distillation (AGMD), 88f, 119, 120f, 146–147, 147f Air stripping, 136 Alkyltinane phenoxy-phosphazene membranes, 181 AMAH. See 2-Acrylamido-2-methylpropane sulfonic acid-based hydrogel (AMAH) Amino acid ionic liquid functionalized biomimetic thin-film nanocomposite (TFN) nanofiltration membranes, 44 Aminopropyl triethoxy silane (APTES), 205 Ammonia stripping, 106 Ammonium persulfate (APS), 49 Anaerobic digestion, 121 Anaerobic membrane bioreactors (AnMBRs), 111, 112f, 113t

configurations, 111, 112f fouling, 111–114 Anion exchange membrane (AEM), 78, 321, 322f, 366 Anion exchange membrane fuel cells (AEMFCs) advantages, 365 anion exchange membrane, 366–368, 378t anode gas diffusion layer (GDL), 366 anodic reaction, 366–367 cathode reaction, 367 humidified hydrogen feeding, 366–367 membrane electrode assembly (MEA), 366 nanofibrous and pore-filling electrolyte membranes cross-linked polyethylene (CLPE) substrate, 376 electrospinning, 373–374 ion exchange membranes, 376–377 quaternized poly(vinybenzyl-divinylbenzene) bipolymer, 375f quaternized poly(vinybenzyl-divinylbenzenehexafluorobutyl methacrylate) terpolymer composite, 375f quaternized polyvinyl alcohol (QPVA) nanofibers, 373–374, 374f radiation-induced emulsion grafting, 374 (vinylbenzyl)trimethyl ammonium chloride (VBTAC), 375–376 organic-inorganic composite membranes alkaline composite polymer electrolyte, 368 carbon nanotube (CNT) functionalization, 373 graphene, 371–372 hydroxide conductivity, 369f imidazolium-based ionic liquids (ILs), 368–369 ion conductivities, 369–371, 370f layered double hydroxides (LDH), 371 mechanical properties, 371t organic anion conductive filler, 373 physicochemical stabilities, 369–371 polarization curves, 369f polyhedral oligomeric silsesquioxane (POSS), 372–373 poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) membranes, 368 quaternary ammonium-modified LDH (QA-LDH) surface, 373 quaternized poly(vinylalcohol)/quaternized chitosan composite membranes, 373 synthesized silica, 368 working principles, 366, 366f Anionic surfactants, 73

437

438

Index

AnMBRs. See Anaerobic membrane bioreactors (AnMBRs) Annealing, 241 Anthropogenic volatile organic compounds (VOCs) emissions, 136 AOP. See Advanced oxidation process (AOP) APS. See Ammonium persulfate (APS) APTES. See Aminopropyl triethoxy silane (APTES) Arsenic rejection, 163 Asymmetric gas separation membranes, 225–226 Atmospheric pollutants, 174 Atomic force microscopy (AFM), 246–247, 325–326, 353 Azeotropic distillation, 138

B

2,20 -Benzidinedisulfonic acid (BDSA), 426, 427f BES. See Bioelectrochemical systems (BES) Bharadwaj’s model, 297–298 Bioelectrochemical systems (BES), 123–124 Biofouling, 110, 319 Biofuels, 310–311 Biological nitrogen removal (BNR) process, 105 Biosensor, 315 Bipolar membrane (BPM), 321–322, 322f Bipolar membrane electrodialysis (BMED), 48 2,2-Bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA), 194–195 Bisphenol A (BPA), 56–57 BMED. See Bipolar membrane electrodialysis (BMED) BMIM-based ionic liquids. See 1-Butyl-3-methylimidazolium (BMIM)-based ionic liquids BNR process. See Biological nitrogen removal (BNR) process Boron-specific membrane, 35 BPM. See Bipolar membrane (BPM) Bragg’s equation, 351–352 Bruggeman model, 233–234 1-Butyl-3-methylimidazolium (BMIM)-based ionic liquids, 186, 186f

C CA. See Contact angle (CA) CA membrane. See Cellulose acetate (CA) membrane Capacitive deionization (CDI), 85–86 Capillary condensation, 278–279 Carbon adsorption process, 136 Carbon capture and storage (CCS), 173–174, 174f Carbon dioxide (CO2) reduction, 173 carbon capture and storage (CCS), 173–174, 174f carbon dioxide production, 174f polymer separation membranes, 177 Carbon molecular sieves (CMS), 205 Carbon nanotubes (CNTs), 206, 426 Carbon-polymer composites, 327

Carbon quantum dots (CQDs), 431–432 Carbon-to-nitrogen ratio (C/N), 105 Carboxylated cardo poly(arylene ether ketone)s (PAEK-COOH), 47 Casting technique, 27–28 Cation exchange membrane (CEM), 78, 320–321, 322f, 338 Cationic surfactants, 73 CBB removal. See Coomassie brilliant blue (CBB) removal CCS. See Carbon capture and storage (CCS) CDI. See Capacitive deionization (CDI) CE. See Coulombic efficiency (CE) Cellulose acetate (CA) membrane, 177, 181, 185f, 276 asymmetric nanofiltration (NF) membranes, 41–42 reverse osmosis membrane chemical structure, 24, 24f limitations, 25 preparation, 24–25 separation, 24–25 transport property, 25 Cellulose nanocrystal (CNC)-incorporated thin-film nanocomposite (TFN) membranes, 43f Cellulose triacetate (CTA), 62, 115 CEM. See Cation exchange membrane (CEM) Ceramic membrane, 4, 62 Charge surface modifying macromolecules (cSMM), 353 Chemical cross-linking, 202–203 Chemical flocculation-reverse osmosis-nanofiltrationultrafiltration-membrane bioreactor, 35 Chlorination process, 56–57 CHMA. See 1,3-Cyclohexane bis methylamine (CHMA) CIP method. See Clean-in-place (CIP) method Clean-in-place (CIP) method, 430–431, 431f CLPE. See Cross-linked polyethylene (CLPE) CMC. See Critical micellar concentration (CMC) CMS. See Carbon molecular sieves (CMS) C/N. See Carbon-to-nitrogen ratio (C/N) CNTs. See Carbon nanotubes (CNTs) Coagulation-flocculation technique, 4 Cobalt phthalocyanine (CoPc) complexes, 204 Cobalt porphyrin (CoP) complexes, 204 Cohen-Turnbull model, 223 Colloidal fouling, 110 Constant pressure, 249–250 Constant volume, 251–252 Contact angle (CA), 346, 347f Conventional polymeric membranes copolymers, 196–197 glassy polymers, 193 homopolymers, 193–195 hydrogen separation, 282–284 nanoporous polymers polymers of intrinsic microporosity (PIMs), 197

Index

surface-modified membranes, 201–203 thermally rearranged (TR) polymers, 197–201 polymer blends, 195–196 rubbery polymers, 193 Coomassie brilliant blue (CBB) removal, 48 CoPc complexes. See Cobalt phthalocyanine (CoPc) complexes CoP complexes. See Cobalt porphyrin (CoP) complexes Coulombic efficiency (CE), 316 CQDs. See Carbon quantum dots (CQDs) Critical micellar concentration (CMC), 73 Cross-linked polyethylene (CLPE), 375–376 Cross-linking modifications, 201–203 cSMM. See Charge surface modifying macromolecules (cSMM) CTA. See Cellulose triacetate (CTA) 1,3-Cyclohexane bis methylamine (CHMA), 277

D DCMD. See Direct contact membrane distillation (DCMD) Dead-end feed system, 109, 109f Decatungstate, 65–66 Demulsification phenomena, 13, 13f Dense polymeric membrane gas and vapor separation fractional free volume (FFV), 222 free volume, 223 gas fluxes, 222 melt extrusion technique, 224 membrane ideal selectivity, 223 permeability, 222–223 solution-diffusion mechanism, 222, 222f solvent vaporization, 223 structure, 222 hydrogen separation, 275, 276t Desalination electrodialysis, 80–82 membrane capacitive deionization (MCD), 86–87 reverse osmosis membrane cellulose acetate material, 24–25 characteristics, 25 internal membrane structure, 31 membrane structure topology, 30–31 module array, 33–34 module configuration, 33 reject water management, 35 thin-film-composite membrane (see Thin-film-composite (TFC) membranes) transport properties, 31–33 seawater, 23 thin-film-composite membrane technology, 34–35 DET. See Direct electron transfer (DET)

439

Diffusion, 278–279 Diffusion coefficient, 178, 223 N,N00 -Dimethylaminoethyl methacrylate-divinylbenzene (DMAEMA-DVB) copolymers, 376 Dimethylsulfoxide (DMSO) recovery, 141 Direct contact membrane distillation (DCMD), 46, 47f, 55, 88f, 119, 120f, 146–147, 147f ammonia recovery, 121 concentration, 120–122 cross-flow velocity, 120 feed temperature, 121 operating parameters, 121, 122t Direct electron transfer (DET), 312–314, 313f Direct methanol fuel cell (DMFC) advantageous characteristics, 338 anode reaction, 338 application, 357–358 cathode reaction, 338 cation exchange membrane (CEM), 338 electrically charged membrane, 338f inorganic fillers inclusion, 339 membrane characterization atomic force microscopy (AFM), 353 crystallinity, 351–352 hydrophilicity measurement, 346 ion exchange capacity (IEC), 345 mechanical stability, 350–351 morphology and elemental analysis study, 351 permeability measurement, 348–349 proton conductivity measurement, 347–348 single cell, 353–355 swelling measurement, 346 thermal stability, 349–350 water uptake test, 345 membrane fabrication, 339–340 phase inversion technique, 343, 343f ternary phase diagram, 343–344, 344f membrane structure asymmetric membrane, 340, 340f dense membrane, 340 layered membrane, 341 membrane’s thickness, 340 pore-filled membrane, 342–343 porous membrane, 340 sandwiched membrane, 341–342, 342f methanol, 338 proton transport mechanism, 339–340 separation mechanism, 356–357 sulfonation process, 339 synthetic polymers, 339 Dissolution test, 345 Distillation. See Membrane distillation

440

Index

DMAEMA-DVB copolymers. See N,N00 -Dimethylaminoethyl methacrylate-divinylbenzene (DMAEMA-DVB) copolymers DMFC. See Direct methanol fuel cell (DMFC) DMSO recovery. See Dimethylsulfoxide (DMSO) recovery DMTA. See Dynamic mechanical and thermal analysis (DMTA) Donnan exclusion, 80 Dry phase inversion, 223 Dual-layer electrospun membranes, 89 Dye and pigment removal anionic dyes, 40 cationic dyes, 40 inorganic pigments, 40 membrane technology, 40 organic pigments, 40 pharmaceutical industries, 39–40 polymeric membranes adsorbents, 41 membrane distillation (MD), 45–46 nanofiltration, 41–44 synthetic dye molecules, 41 ultrafiltration, 46–49 synthetic pigment and dye, 39–40 textile industries, 39–40 Dynamic mechanical and thermal analysis (DMTA), 247

E ECP. See External concentration polarization (ECP) EDCs. See Endocrine-disrupting chemicals (EDCs) EDI. See Electrodeionization (EDI) EDL principle. See Electric double layer (EDL) principle EDX. See Energy dispersive X-ray spectrometer (EDX) EIS. See Electrochemical impedance spectroscopy (EIS) Electric double layer (EDL) principle, 124 Electricity generation, 310–311 Electrochemical impedance spectroscopy (EIS), 317, 395–396 Electrodeionization (EDI) copper sulfate removal, 85 Cr(VI) recovery, 83–85 deionization process, 83, 84f heavy metal removal, 83–85 nickel removal, 85 separation performance, 86t stack construction, 83, 84f ultrapure water production, 83–85 Electrodialysis, heavy metal removal cell configuration and set up, 80, 81f Cr(VI) removal, 82 deionization, 80 desalination, 80–82 effluent concentration, 83 ion-exchange membranes (IEMs), 82–83

Pb2+ removal, 82 performances, 82, 82t solution and ionic concentration, 83 Electrospinning lithium ion batteries, 390–391, 391f, 392t poly(phthalazinone ether sulfone ketone) (PPESK) fibrous membranes, 402 EMBR. See Extractive membrane bioreactor (EMBR) E-MoS. See Exfoliated molybdenum disulfide (E-MoS) Endocrine-disrupting chemicals (EDCs), 56–57 Energy dispersive X-ray spectrometer (EDX), 351, 352f Energy recovery device (ERD), 422–423 Enhanced biological phosphorus removal (EBPR), 106 ERD. See Energy recovery device (ERD) Exfoliated molybdenum disulfide (E-MoS), 352 External concentration polarization (ECP), 422 Extractive membrane bioreactor (EMBR), 56

F Facilitated transport membranes cobalt phthalocyanine (CoPc) complexes, 204 cobalt porphyrin (CoP) complexes, 204 gaseous molecules, 203f oxygen transport, 204 reaction-diffusion process, 203 Faradaic reactions, 88 FC. See Fuel cell (FC) 6FDA-HAB. See 4,4-Hexafluoroisopropylidenediphtalic anhydride and 3,30 -dihydroxy-4,40 -diamino-biphenyl (6FDA-HAB) FDFO. See Fertilizer-driven forward osmosis (FDFO) Felske model, 233–234 Fenton process, 57–58 Fertilizer-driven forward osmosis (FDFO), 117–118 Fertilizer production, 104 FESEM. See Field emission scanning electron microscopy (FESEM) Fiberglass-reinforced epoxy plastic, 33 Field emission scanning electron microscopy (FESEM), 351 Flat sheet membrane, 115 Fluorinated polyimides, 181, 185f, 276 FO-MD hybrid process. See Forward osmosis-membrane distillation (FO-MD) hybrid process FO membrane. See Forward osmosis (FO) membrane FO-NF. See Forward osmosis-nanofiltration (FO-NF) FO/RO membrane. See Forward osmosis/reverse osmosis (FO/RO) membrane Forward osmosis (FO) membrane, 114–118, 420–422 advantages, 114, 161–162 applications, 161 cations and anions recovery, 164t cellulose acetate and polyamide, 164

Index

composite membranes, 165 diffusive transport, 159 drug delivery systems, 167 fundamental nature, 163–164 hybrid process forward osmosis-membrane distillation (FO-MD), 166 forward osmosis-nanofiltration (FO-NF), 166–167 forward osmosis/reverse osmosis (FO/RO) membrane, 165–166 forward osmosis-ultrafiltration (FO-UF), 166–167 industrial applications, 167 industrial wastewater treatment draw solute selection guide, 162, 162f fouling-free cross-flow forward osmosis modules, 161–162 solution-induced dewatering, 161 large-scale usage, 167 membrane selection, 163–165, 165f nutrient recovery, 114–118 ammonium recovery, 116–117 challenges, 114, 115f development, 117t fertilizer-driven forward osmosis (FDFO), 117–118 flat sheet membrane, 115 material polymers, 114 phosphate recovery, 116–117 osmotic pressure, 114, 160 preparation, 114 process, 160, 160f semipermeable membrane, 159–160 surface water treatment, 163 thin-film-composite (TFC) membranes, 164 Forward osmosis-membrane distillation (FO-MD) hybrid process, 45–46, 46f, 123, 166 Forward osmosis-nanofiltration (FO-NF), 166–167 Forward osmosis/reverse osmosis (FO/RO) membrane, 165–166 Forward osmosis-ultrafiltration (FO-UF), 166–167 Fossil fuels, 173, 274, 417 FO-UF. See Forward osmosis-ultrafiltration (FO-UF) Fouling. See Membrane fouling Fourier transform infrared (FTIR) analysis, 248, 249f Freeze extraction, 389–390, 390f FTIR analysis. See Fourier transform infrared (FTIR) analysis Fuel cell (FC), 311. See also Direct methanol fuel cell (DMFC); Microbial fuel cell (MFC) membrane Fumasep, 320

G Gas and vapor separation membrane atomic force microscopy (ATM) analysis, 246–247, 248f carbon dioxide capture, 226, 226t classification and fabrication

441

flat sheet membranes, 219, 220f hollow fiber membrane, 220, 220f materials and structures, 218–219, 219f spiral-wound modules, 220f dense membranes fractional free volume (FFV), 222 free volume, 223 gas fluxes, 222 melt extrusion technique, 224 membrane ideal selectivity, 223 permeability, 222–223 solution-diffusion mechanism, 222, 222f solvent vaporization, 223 structure, 222 differential scanning calorimetric (DSC) analysis, 246 dynamic mechanical and thermal analysis (DMTA), 247 Fourier transform infrared (FTIR) analysis, 248 gas permeation tests, 249–252, 251f industrial applications, 253t air and gas dehydration, 257, 258t air separation, 256–257 carbon dioxide removal, 253–254, 254f hydrogen recovery, 255–256, 255f liquefied petroleum gas (LPG) recovery, 258–259 volatile organic compounds (VOCs) separation, 257–258 integrally asymmetric membranes, 224–226, 224f mixed matrix membrane (see Mixed matrix membrane (MMM)) modules, 221f phase inversion method, 224–226, 225f plasticization effect, 260–262, 261f positronium annihilation lifetime spectroscopy (PALS), 249 scanning electron microscopy (SEM), 243, 244f shapes, 219–221 solubility measurement, 252–253, 252f thermogravimetric analysis (TGA) analysis, 244–246, 246f thin-film-composite (TFC) membranes asymmetric substrate membrane, 226 coating method, 227–228 interfacial polymerization (IP), 227 N-methyldiethanolamine (MEDA), 228–229 reactive monomers, 227 surface modification method, 228 transmission electron microscopy (TEM), 244, 245f Gas diffusion layer (GDL), 366 Gas permeation tests, 249–252, 250f Gas transport mechanisms diffusion, 278–279 inorganic membrane, 278 Knudsen diffusion, 278–279 microporous structure, 279f mixed matrix membranes (MMM), 280–282, 281t, 283f

442

Index

Gas transport mechanisms (Continued) molecular sieving, 278–279 nanoscale fillers, 280 organic membrane, 278 phase inversion and solution blending techniques, 280 surface diffusion, 278–279 zeolitic imidazolate framework (ZIF), 282 GDL. See Gas diffusion layer (GDL) Glassy membranes, 275 Glassy polymers, 193 Graphene, 206, 299, 301, 371–372 Graphene oxide (GO), 301, 327, 405, 405f Gurley value, 394

H Hagen-Poiseuille flow, 177–178 Hazardous waste removal pharmaceutical wastes, 53–54 phenolic compound (see Phenolic compound removal) recycling benefits, 53 Heavy metal removal electrically driven membrane processes anion-exchange membrane (AEM), 78 cation-exchange membrane (CEM), 78 electrodeionization, 83–85 electrodialysis, 80–83, 82t ion-exchange membranes (IEMs), 78 membrane capacitive deionization (MCD), 85–88 industrial wastewater discharge, 71 membrane distillation (MD), 88–90, 88f molecular separation, 71–72 pressure-driven membrane high-pressure membrane, 74–78, 76t low-pressure membrane, 72–74 ultrafiltration (UF) membrane, 72 Heterogenous photocatalysis, 58–59 4,4-Hexafluoroisopropylidenediphtalic anhydride and 3,30 dihydroxy-4,40 -diamino-biphenyl (6FDA-HAB), 181 Hollow fiber (HF) membrane, 220, 220f forward osmosis (FO), 115 nanofiltration, 44, 45f Homogenous photocatalysis, 58 Homopolymers, 193–195 Honda-Fujishima effect, 60 HPAN. See Hydrolyzed polyacrylonitrile (HPAN) Hydrogen separation bed reactor facility, 274f vs. CO2 separation, 274t polymeric membranes artificial polymeric membrane, 277 cellulose acetate, 276 characteristics, 285–287

conventional polymeric membranes, 282–284 cross-linkable polymers, 284, 287–288 dense polymeric membranes, 275 fabricated composite membranes, 277 fluorinated polyimide, 276 gas transport mechanisms, 278–282 gravimetric sorption measurements, 287 vs. inorganic membranes, 278t polybenzimidazole (PBI), 276–277 polyimides, 276 porous membranes, 275 proton exchange membrane (PEM) fuel cell, 277 selectivity and permeability, 286–287 separation capability, 286 solution-diffusion, 284 solution-diffusion mechanism, 274 types, 285t pressure swing adsorption (PSA), 273–274 supercritical gases, 285 Hydrogen sulfide (H2S), 175 Hydrolyzed polyacrylonitrile (HPAN), 49 Hydrophobic polymeric membranes, 45

I ICP. See Internal concentration polarization (ICP) IEC. See Ion exchange capacity (IEC) IEMs. See Ion-exchange membranes (IEMs) IL-GO. See Ionic liquid-functionalized graphene oxide (IL-GO) Imidazolium-based ionic liquids, 368–369, 371–372 Imidazolium-functionalized octaphenyl polyhedral oligomeric silsesquioxane (Im-OPOSS), 372–373 Imidazolium functionalized poly (2,6-dimethyl-1,4-phenylene oxide) (ImPPO), 371–372 Imidazolium-functionalized poly(ether ether ketone) (ImPEEK), 372–373 Im-OPOSS. See Imidazolium-functionalized octaphenyl polyhedral oligomeric silsesquioxane (Im-OPOSS) Im-PEEK. See Imidazolium-functionalized poly(ether ether ketone) (Im-PEEK) ImPPO. See Imidazolium functionalized poly (2,6-dimethyl1,4-phenylene oxide) (ImPPO) Inorganic fillers inclusion, 339 Inorganic fouling, 110 Inorganic membranes, 278t Interfacial polarization, 77 Interfacial polymerization (IP), 30–31, 44, 425 Internal concentration polarization (ICP), 422 Internal resistance, 317 Ion beam irradiation, 201–202 Ion exchange capacity (IEC) anion exchange membrane fuel cells (AEMFCs), 367–368 direct methanol fuel cell (DMFC), 345

Index

Ion-exchange membranes (IEMs), 82–83 counterions transport, 80 electromigration, 80 heterogeneous, 78–79, 79f high conductivities, 78 homogeneous, 78–79, 79f inorganic membrane, 79 Nernst-Planck equation, 80 permselectivities, 78, 80 transport mechanisms, 80 Ionic liquid-functionalized graphene oxide (IL-GO), 371–372 Ionic liquid-immobilized nanofiller, 377 IP. See Interfacial polymerization (IP)

K Knudsen diffusion, 278–279, 295 Knudsen flow, 177–178 Kubelka-Munk function, 59

L Lanthanum orthoferrite (LaFeO3), 60, 61t Layered clays, 301–302 Layered double hydroxides (LDH), 371 LEP. See Liquid entry pressure (LEP) LEPw. See Liquid entry pressure of water Lewis-Nielsen model, 233–234 Liquefied petroleum gas (LPG) recovery, 258–259, 259f, 260t Liquid entry pressure (LEP), 119 Liquid entry pressure of water (LEPw), 147 Lithium ion batteries characteristics, 384 charging process, 384 constituents and discharge process, 384, 384f membrane preparation techniques electrospinning, 390–391 freeze extraction, 389–390, 390f nonsolvent induced phase separation (NIPS), 386–388 particulate leaching, 388, 388f replica molding, 389, 389f thermally induced phase separation, 386 membrane structure and characteristics, 385–386 polymer electrolyte separators, 385 polymer membrane characterization chemical stability, 395 electrical resistance, 395–396 electrochemical stability, 396 mechanical strength, 394–395 permeability, 394 pore size and distribution, 392–393 porosity, 393 shut down, 396 thermal stability, 395 thickness, 392

443

tortuosity, 393–394 wettability, 394 separator membranes active-oxide-incorporated separator, 396 blend separators, 402 BNNT separator, 404, 404f cross-linked membranes, 396 glass fiber (GF) separator, 406, 407f graphene oxide (GO), 405, 405f indications, 397–398t Li/Li symmetrical cells, 399–400 metal-organic framework (MOF), 406, 407f multifunctional separator, 402–404 organosoluble polyimide (PI), 402, 403f poly (acrylonitrile) (PAN) nanofibrous membranes, 407 poly (ether ether ketone) (PEEK) membranes, 406–407 poly(ethylene) (PE) separators, 405 polyimide (PI) nanofibrous membrane separators, 402, 402f poly(phthalazinone ether sulfone ketone) (PPESK) fibrous membranes, 402 poly(p-phenylene terephthalamide) (PPTA), 406 poly-m-phenylene isophthalamide@polyvinylidene fluoride (PMIA@PVDF) nanofiber separators, 400, 401f polyolefin (PP) separators, 405 poly(vinylidene fluoride)-co-hexafluoropropylene (PVDFHFP) separators, 396, 399f poly(vinylidene fluoride)-hexafluoropropylene (PVDFHFP/LLZO) composite separators, 399, 399f poly(vinylidene fluoride-co-trifluoroethylene) (PVDFTrFE) separators, 400 poly(vinylidene fluoride)/tetraethyl orthosilicate silane (PVDF/TEOS) composite separator, 400 separator-electrode interface, 400 zig-zag surface micropatterning, 400 solvents, 385 Lithium-manganese-titanium oxide (LMTO) electrode, 86–87 LMWA. See Low molecular-weight additive (LMWA) Loeb and Sourirajan technique, 224–225 Low molecular-weight additive (LMWA), 242 Low-pressure ultrafiltration membrane, heavy metal removal chromium removal, 72 micellar-enhanced ultrafiltration (MEUF), 72, 75t polymer-enhanced ultrafiltration (PEUF), 74, 75t quaterized tertiary amine-based ultrafiltration membrane, 72 tertiary amine-based ultrafiltration membrane, 72 LPG recovery. See Liquefied petroleum gas (LPG) recovery

M MacMullin number, 394 Magnesium oxide (MgO) particles, 206–207, 280

444

Index

Magnetic membranes big magnets, 207 neodymium powder addition, 207 permeability/selectivity trade-off, 207 small magnets, 207 Manganese dioxide nanoparticle-incorporated mixed matrix polyethersulfone membranes, 11 Mauve dye, 39–40, 40f Maxwell model, 297 MBR. See Membrane bioreactor (MBR) MCD. See Membrane capacitive deionization (MCD) MD. See Membrane distillation (MD) MEDA. See N-methyldiethanolamine (MEDA) Mediated electron transfer (MET), 312–314, 313f Melt extrusion technique, 224 Melt mixing, 298t, 299 Melt processing, 241 Membrane-based coalescers, 13, 13f Membrane-based demineralization technique, 55–56 Membrane bioreactor (MBR) advantages, 110 nutrient recovery electrocoagulation process for phosphorous removal (EPR process), 111–114 external cross-flow, 111, 112f filtration separation mechanism, 110 fouling, 110 membrane characteristics, 110–111 solid retention time (SRT), 110 submerged/immersed configurations, 112f Membrane capacitive deionization (MCD) adsorption, 85–86, 87f brackish water desalination, 86–87 capacitive deionization (CDI), 85–86 chromium removal, 86–87 desorption/electrode regeneration, 87f Faradaic reactions, 88 lithium ion separation, 86–87 palladium recovery, 87–88 Membrane distillation (MD) advantages, 88–90 configurations, 119, 120f contact angle, 119 direct contact membrane distillation (DCMD), 46, 47f dual-layer electrospun membranes, 89 dye wastewater purification, 45–46 heat and mass transfer, 118, 118f hydrophobicity, 89–90 liquid entry pressure (LEP), 119 membrane thickness, 119 microporous membranes, 118 nutrient recovery, 118–122

performances, 89, 89t permeate flux, 119 phenolic compound removal, 54–55 superhydrophobic membrane, 89 thermal conductivity, 119 vacuum membrane distillation (VMD), 45 vapor pressure, 118 volatile organic compounds (VOCs) removal air gap membrane distillation (AGMD), 146–147 direct contact membrane distillation (DCMD), 146–147 flat sheet (FS) membranes, 148 hollow fiber (HF) membranes, 148–149 hydrophilic microporous cellulose nitrate membranes, 149–150 materials, 149 microporous and hydrophobic membranes, 145–146 vs. pervaporation (PV), 150, 150t sweeping gas distillation (SGMD), 146–147 vacuum membrane distillation (VMD), 146–147, 148f vapor pressure difference, 145–146 wetting phenomenon, 89–90 Membrane electrode assembly (MEA), 353–354, 355f, 366 Membrane fouling biofouling, 319 colloidal fouling, 110 inorganic fouling, 110 nanofiltration (NF), 110 organic fouling, 110 phenolic compound removal, 62–63 pressure-retarded osmosis (PRO), 418, 429, 432 Membraneless microbial fuel cell (MFC) vs. ion-exchange membrane microbial fuel cell, 323 proton transfer, 323 schematics, 323f zero internal resistance, 323 Membrane separation acidic gas (see Acidic gas) phenolic compound removal cellulose triacetate (CTA), 62 cellulose triacetate (CTA) and cellulose acetate (CA) mixture, 62 ceramic membranes, 62 extractants, 62 microfiltration (MF), 62 polymer-inclusion membranes (PIM), 62 polymer materials, 61 polyvinyl chloride (PVC), 62 ultrafiltration (UF), 62 volatile organic compounds (VOCs) removal, 137 Mesoporous molecular sieve (MCM-41), 141–142 MET. See Mediated electron transfer (MET) Metal dichalcogenides, 302

Index

Metal organic frameworks (MOFs), 205–206, 302 Metal-polymer composites, 326–327 Meta-phenylene diamine (MPD), 25 Methyl violet (MV) removal, 42–44 MEUF. See Micellar-enhanced ultrafiltration (MEUF) MF. See Microfiltration (MF) MFC membrane. See Microbial fuel cell (MFC) membrane Micellar-enhanced ultrafiltration (MEUF) chelating agents, 74 chromate removal, 73 drawback, 74 nitrate removal, 73 performance, 75t principle, 72 removal efficiency, 73–74 surfactants, 73 Micelles, 72 Microbial fuel cell (MFC) membrane anode chamber, 311–312 biofouling, 319 biohydrogen production, 315 biosensor, 315 cathode chamber, 311–312, 320 configurations, 312f dual-chamber, 311, 312f electrical energy production, 311 electron transfer mechanism, 312–314, 313f energy conversion, 314 influencer items, 314 internal resistance, 317 ion-exchange membrane anion exchange membrane (AEM), 321 bipolar membrane (BPM), 321–322 cation exchange membrane (CEM), 320–321 membrane separators, 315–316 Nafion, 310 nonrenewable energy sources, 310 oxygen intrusion, 316–317 pH splitting, 317–318 polarization curve, 312, 313f polymeric membrane carbon-polymer composites, 327 features, 310 metal-polymer composites, 326–327 Nafion-containing and Nafion-alternative membranes, 324–325 nanocomposite, 324 polymer-polymer composites, 325–326 salt bridge (SB), 328 separators, 320 single-chamber, 311, 312f substrate crossover, 318–319 wastewater treatment, 314–315

445

Microbial recovery cell (MRC)-anaerobic osmotic membrane bioreactor (AnOMBR) system, 123 Microfiltration (MF), 111 phenolic compound removal, 62 process, 55 Mixed matrix membrane (MMM), 64, 64f air separation carbon molecular sieves (CMS), 205 carbon nanotubes (CNTs), 206 graphene, 206 inorganic materials, 204–205 metal and metal oxide nanoparticles, 206–207 metal organic frameworks (MOFs), 205–206 zeolites, 205 gas and vapor separation membrane asymmetric mixed matrix membranes, 240f body centered cubic (BCC) structure, 234 Bruggeman model, 233–234 criteria involved, 238 definitions and properties, 229–231, 230f, 231t Felske model, 233–234 fillers, 238 interfacial gaps, 232–233 interfacial voids, 232–233 Lewis-Nielsen model, 233–234 Maxwell model, 231–233 nanoparticles, 238–239 nonideal interfacial defects, 241–242 organic-inorganic interface morphologies, 232f Pal model, 233–234 polymer matrices, 239 pore blockage, 232–233 porous and nonporous particles, 238 rigidified polymer layer, 232–233 selectivity-permeability morphological diagram, 236–237 in situ polymerization, 240 sol-gel method, 240–241 solution blending, 239 hazardous waste removal acidic gas separation, 182–183 chemical modification method, 64–65 inorganic fillers, 64 nanoparticle dispersion, 64 oily wastewater treatment, 65 physical modifications, 64–65 PVDF ultrafiltration membranes, 64 hydrogen separation, 280–282, 281t, 283f volatile organic compounds (VOCs) removal, 140 Modulus elasticity, 350–351 MOFs. See Metal organic frameworks (MOFs) Molybdenum disulfide (MoS2), 302 Molybdenum sulfide (MoS2) nanomaterial, 372 Mott-Schottky plot, 59

446

Index

MPD. See Meta-phenylene diamine (MPD); m-phenylenediamine (MPD) m-phenylenediamine (MPD), 28, 426 Multiwalled carbon nanotube (MWCNT), 206, 327 MWCNT. See Multiwalled carbon nanotube (MWCNT)

N Nafion, 339 alternative membrane, 324–325 biofouling, 319 cation exchange membrane (CEM), 320 chemical structures, 356f efficiency, 315–316 electrochemical impedance spectroscopy (EIS), 348f Nafion 117/graphene oxide (GO), 347–348 Nafion 117 membrane, 317, 325–326 oxygen intrusion, 316 oxygen leakage, 316 pH splitting, 318 poly(vinylidene fluoride)/Nafion polymeric membrane, 325–326 power density, 326 thermogravimetric analyzer (TGA) thermal curve, 349–350, 350f Nanofillers, 377 Nanofiltration (NF) membrane, 35 Donnan exclusion, 41 dye and pigment removal amino acid ionic liquid functionalized biomimetic thin-film nanocomposite (TFN) nanofiltration membranes, 44 cellulose acetate (CA) asymmetric nanofiltration (NF) membranes, 41–42 cellulose nanocrystal (CNC)-incorporated thin-film nanocomposite (TFN) membranes, 42, 43f hollow fiber (HF) nanofiltration membranes, 44, 45f methylene blue (MB) removal, 42–44 multifunctional TFN nanofiltration membrane, 44 polydopamine (PDA)-functionalized carbon nanotube (CNT), 42–44 positively charged nanofiltration (NF) membranes, 44 reactive black 5, 41–42, 42f reactive dyes, 41 heavy metal removal commercial nanofiltration membranes, 76t interfacial polarization, 77 mechanism, 74–76 pH, 77 polymer phase inversion, 77 pore size, 77 porous substrates, 77 rejection phenomena, 76–77

nutrient recovery cellulose acetate (CA) membranes, 108 cross-flow system, 108, 109f dead-end feed system, 109, 109f desalination, 109 fouling, 110 maximum recoveries, 109 salt rejection, 109 permeate flux, 41 size exclusion, 41 Nanowire mechanism, 312–314, 313f Naphthalene polysulfone (NPSF) membrane, 196–197 Naphthenic acids, 4–5 Natural volatile organic compounds (VOCs) emissions, 136 Nernst-Planck equation, 80 NF membrane. See Nanofiltration (NF) membrane NIPS. See Nonsolvent induced phase separation (NIPS) Nitrogen oxides (NOX), 175 N-methyldiethanolamine (MEDA), 228–229 Nonisothermal membrane-based separation, 55 Nonsilicone membrane polyetherimide block polymer (PEBA) membrane, 143 acetone-butanol-ethanol separation, 143 graphene oxide (GO) with ionic liquid N-octylpyridiniunm bis (trifluoromethyl) sulfonyl imide [OPY][Tf2N] (IL-GO) incorporation, 144 mesoporous molecular sieve, 143 organophilic, 143 pervaporation (PV) performances, 144t propyl propionate/water separation, 143 three-component mixed matrix membranes (MMMs), 144 zeolitic imidazolate framework (ZIF-8), 143 ZSM-5 zeolite incorporation, 143–144 poly(vinylidene fluoride) (PVDF) membrane carbon black (CB) addition, 145 flat sheet (FS) membranes, 144–145 hollow fiber (HF) membranes, 144–145 pervaporation (PV) performances, 146t polar organic feed components separation, 144–145 poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP), 145 polymers of intrinsic microporosity (PIM-1), 145 silica/poly (dimethylsiloxane)/poly(vinylidene fluoride) (SiO2/PDMS/PVDF) composite membranes, 145 surface-modifying macromolecule (SMM), 145 Nonsolvent induced phase separation (NIPS), 224–225, 386–388, 387f NPSF membrane. See Naphthalene polysulfone (NPSF) membrane Nutrient pollution, 104 Nutrient recovery advantages, 104–105

Index

ammonia stripping, 106 bioelectrochemical systems (BES), 123–124 biological nitrogen removal (BNR) processes, 105 electric double layer (EDL) principle, 124 hybrid membrane processes, 123 nitrification/denitrification, 105, 105f nitrogen removal, 105–106 osmotically driven membranes, 114–118 phosphorus removal, 104–106 potassium (K) recovery, 106 pressure-driven membranes membrane bioreactor, 110–114 reverse osmosis/nanofiltration membranes, 107–110 thermally driven membrane process, 118–122 Nylon 6, 185f

O Ocean acidification, 175–176, 188 Octadecylamine-functionalized graphene oxide sheets, 371–372 Oil-in-water (O/W) emulsion, 5 Oily wastewater treatment chemical contaminants, 3–4 coagulation-flocculation, 4 commercialized industries, 3–4 components, 4–5 emulsions, 5–6 grease emulsion, 3–4 industrial wastes, 4 membrane separation technologies, 4 advantages, 6–7 challenges, 16 coalescence-accelerating medium, 13 demulsification phenomena, 13 droplet volume fraction, 13 limitations, 6–7 materials, 4 membrane fouling, 16 oil removal mechanism, 13–14 porous material, 6 pressure-driven membrane process, 6 superwetting surfaces, 14–15 van der Waals (VDW) attraction, 13–14 mixed matrix membrane (MMM), 65 oil and gas (O&G) industry, 3–4 oil and grease concentrations, 5, 6t petroleum wastewater, 4–5 pollutants, 4, 5t techniques, 4 Oleophobicity, 14–15 Olive mill wastewater (OMW), 54–55 Organic fouling, 110 Organic liquids, 15

447

Organic pollutants, 4 ORR. See Oxygen reduction reaction (ORR) Osmosis, 25f, 107f, 417–418. See also Forward osmosis (FO) membrane; Reverse osmosis (RO) membrane Osmotic pressure, 108, 160, 421 O/W emulsion. See Oil-in-water (O/W) emulsion Oxygen diffusion, 318–319 Oxygen enrichment membrane-based air separation benefits, 191 conventional polymeric materials, 193–203 copolymers, 196–197 facilitated transport membranes, 203–204 magnetic membranes, 207–209 membrane materials, 192–209 organic/inorganic hybrid membranes, 204–209 polymer blends, 195–196 polymeric membrane, 192, 208–209t synthetic polymers, 191–192 Oxygen/nitrogen (O2/N2) separation. See Oxygen enrichment Oxygen reduction reaction (ORR), 317–318 Ozone (O3), 57

P PAEK-COOH. See Carboxylated cardo poly(arylene ether ketone)s (PAEK-COOH) PALS. See Positron annihilation lifetime spectroscopy (PALS) PAN. See Polyacrylonitrile (PAN) PAO. See Pressure-assisted osmosis (PAO) Particulate leaching technique, 388, 388f PA TFC NF membranes. See Polyamide thin-film composite nanofiltration (PA TFC NF) membranes PBI. See Polybenzimidazole (PBI) PBI/IL-GO nanocomposite membranes. See Polybenzimidazole/1-(3-aminopropyl)-3methylimidazolium salt bromine ionic liquidfunctionalized graphene oxide (PBI/IL-GO) nanocomposite membranes PC. See Polycarbonate (PC) PDMS. See Polydimethylsiloxane (PDMS) PDMS-PMHS. See Poly (dimethylsiloxane)-poly(methyl hydrogen siloxane) (PDMS-PMHS) PEBA membrane. See Polyetherimide block polymer (PEBA) membrane PEMFC. See Polymer exchange membrane fuel cell (PEMFC); Proton exchange membrane fuel cell (PEMFC) Permeation cell, 348f Permeation separation index (PSI), 139 Perovskite-type oxide photocatalysts, 60 Pervaporation (PV) dense membranes, 137–138

448

Index

Pervaporation (PV) (Continued) energy consumption, 151 vs. membrane distillation (MD), 150 molecular differences, 138 performance, 139 permeation flux, 139 permeation separation index (PSI), 139 selectivity, 139 volatile organic compounds (VOCs) removal hydrophobic membranes, 140 mixed matrix membranes (MMMs), 140 nonsilicone membrane, 143–145 silicone-based membrane, 140–142 solubility parameter, 140 PES. See Polyethersulfone (PES) Petroleum wastewater, 4–5 PEUF. See Polymer-enhanced ultrafiltration (PEUF) Pharmaceutical products (PhPs), 53–54 Pharmaceutical wastes, 53–54 Phase inversion technique, 27–28, 339–340, 343, 343f Phenol health effects, 54 toxicity, 54 Phenolic compound removal activated carbon and chitosan, 55 adsorption, 55 advanced oxidation process (AOP), 57–59 chemical oxidation, 56–57 distillation technology, 54–55 membrane technologies distillation, 55 extractive membrane bioreactor (EMBR), 56 fouling mechanism, 62–63 membrane separation processes, 61–62 photocatalytic membrane, 64–66 polypropylene hollow fiber-supported liquid membrane bioreactor (HFSLMB), 56 reverse osmosis (RO) process, 55–56 photocatalysis degradation efficacy, 59 heterogenous photocatalysis, 58–59 homogenous photocatalysis, 58 perovskites, 60 photocatalyst band gap, 59 pollutant degradation, 58f semiconductor photocatalyst, 58, 60 Photocatalytic membrane, phenolic compound removal mixed matrix membrane (MMM) (see Mixed matrix membrane (MMM)) photocatalytic membrane reactors (PMR), 65–66 polyoxomelate-based photocatalytic membranes, 65–66 titanium dioxide (TiO2) photocatalyst, 65

Photocatalytic membrane reactors (PMR), 65–66 PhPs. See Pharmaceutical products (PhPs) pH splitting, 317–318 PI. See Polyimide (PI) PIM. See Polymer-inclusion membranes (PIM) PIM-1/PVDF. See Polymers of intrinsic microporosity/poly (vinylidene fluoride) membrane (PIM-1/PVDF) Plasticization effect, 260–262, 261f PMR. See Photocatalytic membrane reactors (PMR) Poly(1-trimethylsilyl-1-propyne) (PTMSP), 184f Polyacrylonitrile (PAN), 240–241 scanning electron microscopy (SEM), 243 ultrafiltration (t-UF) membrane, 47–48 Polyamide membranes internal structure characterization, 31 molecular spacing, 31 small-angle scattering technique, 31 Polyamide-polysulfone reverse osmosis (RO) membrane, 30–31 Polyamide thin-film composite nanofiltration (PA TFC NF) membranes, 41–42 Polybenzimidazole (PBI), 276–277, 282–284 Polybenzimidazole/1-(3-aminopropyl)-3-methylimidazolium salt bromine ionic liquid-functionalized graphene oxide (PBI/IL-GO) nanocomposite membranes, 371–372 Polycarbonate (PC), 184f Polydimethylsiloxane (PDMS), 140–141, 184f, 302 air separation, 196, 206–207 replica molding technique, 389 Polydimethylsiloxane-polymethyl hydrogen siloxane (PDMSPMHS), 141 Polydopamine (PDA)-functionalized carbon nanotube (CNT), 42–44 Polyether ether ketone (PEEK), 325–326 Polyetherimide block polymer (PEBA) membrane, 140, 143 acetone-butanol-ethanol separation, 143 graphene oxide (GO) with ionic liquid N-octylpyridiniunm bis (trifluoromethyl) sulfonyl imide [OPY][Tf2N] (ILGO) incorporation, 144 mesoporous molecular sieve, 143 organophilic, 143 pervaporation (PV) performances, 144t propyl propionate/water separation, 143 three-component mixed matrix membranes (MMMs), 144 zeolitic imidazolate framework (ZIF-8), 143 ZSM-5 zeolite incorporation, 143–144 Polyethersulfone (PES), 193–194, 277 Polyethylene glycol (ZnO-PEG) nanoparticle-embedded membrane photocatalytic reactor (MPR), 48–49 Polyhedral oligomeric silsesquioxane (POSS), 372–373 Polyimide (PI), 276, 282–284, 402, 402f Polyimide Matrimid asymmetric hollow fiber membranes, 202 Poly(ether ether ketone) (PEEK) membranes, 406–407, 407f

Index

Polymer blends, 195–196 Polymer-enhanced ultrafiltration (PEUF), 35, 74, 75t Polymer exchange membrane fuel cell (PEMFC), 338 Polymeric catalytic membrane reactor (PCMR), 295 Polymer-inclusion membranes (PIM), 62 Polymer phase inversion, 77, 82–83 Polymer-polymer composites, 325–326 Polymers of intrinsic microporosity (PIM) acidic gas separation, 184, 186f air separation, 197, 198–200f Polymers of intrinsic microporosity/poly(vinylidene fluoride) membrane (PIM-1/PVDF), 145 Polymer support membrane (PSM), 26–28, 27f Poly(methylmethacrylate)-poly(dimethylsiloxane) (PMMA-gPDMS), 141 Polyolefin (PP) separators, 405 Polyoxomelate-based photocatalytic membranes, 65–66 Poly phenyl sulfone (PPSU), 193–194 Polyphosphazene nanotube (PZSNTs)/polydimethylsiloxane (PDMS) nanocomposite, 142 Polypiperazine-amide (PPA) tight ultrafiltration (t-UF) membranes, 48–49 Polypropylene hollow fiber-supported liquid membrane bioreactor (HFSLMB), 56 Polypropylene-polyethylene-polypropylene (PP-PE-PP), 376–377 Polypyrrolone copolymers, 196 Poly(vinylidene fluoride-cotrifluoroethylene) (PVDF-TrFE) separators, 400 Polysiloxane membrane, 195 Polystyrene (PS) and polyphenylene oxide (PPO) blending, 195 Poly(styrene-b-isobutylene-b-styrene)/graphene oxide composite anion exchange membrane, 371–372 Polysulfone (PSF), 184f, 325–326 air separation, 193–194 copolymers, 196–197 mixed-matrix ultrafiltration (UF) membrane, 11 Polytetrafluorethylene (PTFE) membrane, 9, 15, 55, 118, 149 ammonia recovery, 121 urine removal, 121 Polytetrafluoroethylene-zirconium phosphate polyvinyl alcohol (PTFE-ZrP-PVA) membrane, 351, 352f Polytrimethylsilylpropyne (PTMSP) membrane, 140–141, 202 Polyurethane (PU), 140 Polyvinyl alcohol (PVA), 327 Polyvinyl chloride (PVC), 62 Polyvinylidene fluoride-co-hexafluoropropene (PVDF-HFP), 145, 396, 399f Polyvinylidene fluoride (PVDF) membrane, 9, 9–10t, 118, 149–150, 343 carbon black (CB) addition, 145 flat sheet (FS) membranes, 144–145

449

hollow fiber (HF) membranes, 144–145 pervaporation (PV) performances, 146t polar organic feed components separation, 144–145 poly(vinylidene fluoride-co-hexafluoropropene) (PVDFHFP), 145 polymers of intrinsic microporosity (PIM-1), 145 silica/poly (dimethylsiloxane)/poly(vinylidene fluoride) (SiO2/PDMS/PVDF) composite membranes, 145 surface-modifying macromolecule (SMM), 145 urine removal, 121 Poly(vinylidene fluoride)/tetraethyl orthosilicate silane (PVDF/ TEOS) composite separator, 400 Poly(4-vinylpyridine) (PVP) with ethylcellulose (EC) blending, 195 Pore-filled membrane, 342–343 Pore flow model, 31 Porogen, 388 Porous membranes, 275, 276t Positron annihilation lifetime spectroscopy (PALS), 249, 250t, 287 POSS. See Polyhedral oligomeric silsesquioxane (POSS) PP-PE-PP. See Polypropylene-polyethylene-polypropylene (PPPE-PP) PPSU. See Poly phenyl sulfone (PPSU) Preneutralization, 163 Pressure-assisted osmosis (PAO), 163 Pressure-driven membrane process, 6, 7f Pressure-retarded osmosis (PRO) advantages, 417–418 chemical potential, 421 closed-loop system, 419–420, 420f energy recovery device (ERD), 422–423 external concentration polarization (ECP), 422 feed and draw solutions, 418 forward osmosis (FO) mode, 420–422 high-water flux, 418 internal concentration polarization (ICP), 422 maximum power density, 424–425 membranes carbon nanotubes (CNT), 426, 429–430 carbon quantum dots (CQDs), 431–432 clean-in-place (CIP) method, 430–431, 431f fouling, 418, 429 modification, 432 m-phenylenediamine (MPD), 426 performance, 425 polyester woven mesh, 426, 428f reverse osmosis-pressure-retarded osmosis (RO-PRO) hybrid system, 432 thin film composite (TFC), 425–428 thin film nanocomposite (TFN), 426 trimesoyl chloride (TMC), 425

450

Index

Pressure-retarded osmosis (PRO) (Continued) open-loop systems, 419–420, 420f osmotic phenomenon, 421 performance, 418 power densities, 424t power output, 424 prototype plant system, 417–419 reverse osmosis (RO), 420–421 reverse salt diffusion, 425 salinity gradient, 419 salt concentration profile, 422, 423f spontaneous flow of water, 422 water flux equation, 423–424 Pressure swing adsorption (PSA), 273–274 Priming method, 242 PRO. See Pressure-retarded osmosis (PRO) Proton conductivity measurement, 347–348 Proton exchange membrane fuel cell (PEMFC), 277, 365 proton exchange membrane (PEM), 366 working principles, 366f PSA. See Pressure swing adsorption (PSA) PSF. See Polysulfone (PSF) PSf, 280 PSI. See Permeation separation index (PSI) PSM. See Polymer support membrane (PSM) PTFE membrane. See Polytetrafluorethylene (PTFE) membrane PTMSP. See Poly(1-trimethylsilyl-1-propyne) (PTMSP) PTMSP membrane. See Polytrimethylsilylpropyne (PTMSP) membrane PU. See Polyurethane (PU) Pure water permeability (PWP), 44 PV. See Pervaporation (PV) PVA. See Polyvinyl alcohol (PVA) PVC. See Polyvinyl chloride (PVC) PVDF membrane. See Polyvinylidene fluoride (PVDF) membrane PVDF/TEOS composite separator. See Poly(vinylidene fluoride)/tetraethyl orthosilicate silane (PVDF/TEOS) composite separator PVDF-TrFE separators. See Poly(vinylidene fluoridecotrifluoroethylene) (PVDF-TrFE) separators

Q QPSU membrane. See Quaternary polysulfone (QPSU) membrane QPVA nanofibers. See Quaternized polyvinyl alcohol (QPVA) nanofibers Quaternary polysulfone (QPSU) membrane, 368 Quaternized polyvinyl alcohol (QPVA) nanofibers, 373–374, 374f

R Renewable energies, 310–311, 417–418 Replica molding technique, 389, 389f Reverse osmosis (RO) membrane, 25f, 80–82, 107–110, 108f, 420–422 cellulose acetate material, 24–25 characteristics, 25 defect-free, 32f dyes and pigments rejection, 41 feed flow rate, 33 heavy metal removal, 77–78 hybrid membrane-resin technology, 35 imperfect, 32f, 33 integrated zero discharge membrane process, 35 internal membrane structure, 31 microscopic morphology, 30–31 module array, 33–34, 34f module configuration, 33 phenolic compound removal, 55–56 recovery, 33–34 reject water management, 35 salt and water flux, 32f salt permeability, 32 seawater feed, 34–35 thermodynamic minimum specific energy, 34–35 thin-film-composite reverse osmosis (RO) membrane, 25–30 transport properties, 31–33 water permeability, 31 Reverse osmosis-pressure-retarded osmosis (RO-PRO) hybrid system, 432 Reverse salt flux diffusion, 431–432 RO membrane. See Reverse osmosis (RO) membrane RO-PRO hybrid system. See Reverse osmosis-pressure-retarded osmosis (RO-PRO) hybrid system Rubbery polymers, 193

S Salinity gradient, 417–418 Salt bridge (SB), 328 SB. See Salt bridge (SB) Scaife’s estimation, 59 Seawater desalination membrane technology reverse osmosis (RO) membrane (see Reverse osmosis (RO) membrane) ultrathin-film-composite membrane technology, 34–35 Seawater reverse osmosis (SWRO) process, 420–421 Semiconductor photocatalyst, 58, 60 Semixylenol orange, 44 Separation factor, 139 SGMD. See Sweeping gas membrane distillation (SGMD) Silica/poly (dimethylsiloxane)/poly(vinylidene fluoride) (SiO2/ PDMS/PVDF) composite membranes, 145

Index

Silicone-based membrane graft copolymer membrane, 141 hydrophobic composite membrane, 141 polydimethylsiloxane (PDMS) membranes, 140–141 mesoporous molecular sieve and zeolite incorporation, 141–142 pervaporation (PV) performances, 142t polyphosphazene nanotube (PZSNTs)/ polydimethylsiloxane (PDMS) nanocomposite, 142 zeolitic imidazolate framework (ZIF-7), 142 polytrimethylsilylpropyne (PTMSP) membranes, 140–141 Silicotungstic acid (STA), 327 Simultaneous nitrification and denitrification (SND), 105–106, 105f Single-walled carbon nanotubes (SWCNTs), 206 SND. See Simultaneous nitrification and denitrification (SND) Solar distillatory apparatus, 54–55 Sol-gel method, 240–241 Solubility coefficient, 178 Solution blending, 239, 299 Solution-diffusion mechanism, 178, 274, 277 Solution-diffusion model, 33, 222 Solvent casting, 388f Solvent vaporization, 223 Sour water, 4–5 SPEEK. See Sulfonated poly ether ether ketone (SPEEK) SPEES membrane. See Sulfonated poly(phenylene ether ether sulfone) (SPEES) membrane STA. See Silicotungstic acid (STA) Struvite, 106 Substrate crossover, 318–319 Sulfonated poly(ether ether sulfone) (SPEEK)/Cloisite15A nanocomposite membrane, 341, 341f Sulfonated poly ether ether ketone (SPEEK), 318–319 Sulfonated poly(phenylene ether ether sulfone) (SPEES) membrane, 353, 354f Sulfonated poly sulfone (SPSU), 345 Sulfonated poly sulfone-functional polymer brush-modified graphene oxide (SPSU-FPGO), 345 Sulfone-containing polymer membrane, oily wastewater treatment, 10–12, 12t hydrophilic membranes, 11 microfiltration (MF-GRM) and ultrafiltration (UF-GRM) PSf membranes, 11 polyacrylonitrile (PAN), 11 polysulfone (PS) membrane, 10 polysulfone (PSf) mixed-matrix ultrafiltration (UF) membrane, 11 polyvinylidene fluoride (PVDF) membrane, 11 Sulfurous acid removal, 177 Sulfur oxides, 175 Superhydrophobic membrane, 35, 89–90

451

Superwetting surfaces hydrophilicity, 14 hydrophobicity, 14 oleophobicity, 14–15 organic liquids, 15 polydimethylsiloxane (PDMS) micropillar surfaces, 15 polytetrafluoroethylene (PTFE) membrane, 15 surface grafting, 14 surface modification, 14 Surface grafting, 14 Surface-modified membranes, 201–203 Surface-modifying macromolecule (SMM), 145 SWCNTs. See Single-walled carbon nanotubes (SWCNTs) Sweeping gas membrane distillation (SGMD), 88f, 119, 120f, 146–147, 147f SWRO process. See Seawater reverse osmosis (SWRO) process

T Tannic acid, 44 Tauc plot, 59 Teflon AF 2400, 184f TEM. See Transmission electron microscopy (TEM) Tensile stretch, 350–351 Tensile test, 350–351 Tetrafluoroethylene (TFE)/perfluoromethyl vinyl ether (PMVE) copolymer, 184f Tetramethylethylenediamine (TMEDA)-modified polyhedral oligomeric silsesquioxane (POSS), 372–373 TFC membranes. See Thin-film-composite (TFC) membranes TFN membrane. See Thin film nanoparticle (TFN) membrane Thermal annealing, 202 Thermal energy, 310–311 Thermally induced phase separation (TIPS), 386, 387f Thermally rearranged (TR) polymers, 184–186, 197–201 Thermal mechanical analysis (TMA), 395 Thin-film-composite (TFC) membranes, 115, 116f, 425–426 gas and vapor separation, 227–229 nanofiltration membranes amino acid ionic liquid functionalized biomimetic, 44 cellulose nanocrystal (CNC)-incorporated, 42 multifunctional, 44 polyamide thin-film composite nanofiltration (PA TFC NF) membranes, 41–42 polydopamine (PDA)-functionalized carbon nanotube (CNT), 42–44 tannic acid, 44 reverse osmosis factors impacting, 29 layers, 26 membrane surface pores, 28 m-phenylenediamine (MPD), 28 phase inversion process, 28

452

Index

Thin-film-composite (TFC) membranes (Continued) polyamide deposition, 29, 30f polyamide structure, 25–26, 26f polymer support membrane, 26–28, 27f Thin film nanoparticle (TFN) membrane, 115, 116f Tight ultrafiltration (t-UF) membrane fabrication, 47–48 hybrid, 48 membrane photocatalytic reactor (MPR), 48–49, 49f peptide coupling, 47 polyacrylonitrile (PAN) based ultrafiltration (t-UF) membrane, 47–48, 48f polydopamine (PDA)/polyethylenimine (PEI) codeposition, 49 polypiperazine-amide (PPA), 48–49 UH004 tight ultrafiltration (t-UF) membranes, 47 TIPS. See Thermally induced phase separation (TIPS) Titanium dioxide (TiO2), 58–60, 65, 326–327 TMA. See Thermal mechanical analysis (TMA) TMC. See Trimesoyl chloride (TMC) Tortuosity, 393–394 Transmission electron microscopy (TEM), 245f Trimesoyl chloride (TMC), 25, 277 Tropaeolin O, 44 TR polymers. See Thermally rearranged (TR) polymers t-UF membraner. See Tight ultrafiltration (t-UF) membrane Two-dimensional (2D) filler-polymer composites applications, 303–304 gas transport Bharadwaj’s model, 297–298 composite membranes, 297–298 diffusivity, 297 impermeable 2D particles, 297 Maxwell model, 297 Neilson’s model, 297–298 solubility, 297 graphene, 299 graphene oxide, 301 layered clays, 301–302 melt mixing, 298t, 299 metal dichalcogenides, 302 MOFs and zeolites, 302 permeation mechanics, 295–297, 296f selectivity and permeability parameters, 294–295, 294f in situ polymerization, 298t, 299 solution blending, 298t, 299 synthesis techniques, 298–299, 298t, 300f 2D filler materials, 294 Two-probe impedance cell, 347f

U Ultrafiltration (UF) membrane, 111 advantages, 72

dye and pigment removal inorganic salt, 46–47 peptide coupling, 47 tight ultrafiltration (t-UF) membrane (see Tight ultrafiltration (t-UF) membrane) heavy metal removal, 72–74 phenolic compound removal, 62 Ultraviolet/titanium dioxide/reverse osmosis (UV/TiO2/RO) hybrid system, 66

V Vacuum membrane distillation (VMD), 45, 88f, 119, 120f, 146–147, 147f advantages, 147 barriers, 147 energy consumption, 151 liquid-vapor (L-V) interfaces, 147 performances, 149t separation process, 147 setup, 148f van’t Hoff equation, 421 Vapor permeation, 139 Victoria blue B, 44 VMD. See Vacuum membrane distillation (VMD) Volatile organic compounds (VOCs) removal air stripping, 136 anthropogenic emissions, 136 carbon adsorption process, 136 control techniques, 137t destruction methods, 136 dimethylsulfoxide (DMSO) recovery, 141 high vapor pressure, 136 industrial wastewaters, 136 membrane distillation (MD) (see Membrane distillation (MD)) membrane separation, 137 natural emissions, 136 pervaporation (PV) (see Pervaporation (PV)) phase transition, 138–139 photochemical smog, 136 recovery methods, 136 separation, 257–258

W Wastewater treatment plant (WWTP), 104 Water-in-oil (W/O) emulsion, 5 Water pollution, 135–136 pollutants, 39 uncontrolled anthropogenic activity, 39 Water scarcity, 23 Water uptake test, 345

Index

Wetting phenomenon, 89–90 WWTP. See Wastewater treatment plant (WWTP)

X X-ray diffraction analysis (XRD), 352, 353f

Z Zeolites, 141–142, 302 Zeolitic imidazolate framework (ZIF), 142, 282 Zirfon, 320

453