Reverse osmosis 9780128114681, 0128114681, 9780128115398, 0128115394

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Reverse Osmosis

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Reverse Osmosis

Ahmad Fauzi Ismail Kailash Chandra Khulbe Takeshi Matsuura

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 # 2019 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: 978-0-12-811468-1 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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Contents Preface ............................................................................................................................... ix

CHAPTER 1 Introduction—Do RO Membranes Have Pores?.................................................. 1 1.1 Research Before 2000 ............................................................................. 3 1.1.1 Preferential Sorption-Capillary Flow (PS-CF) model .................. 3 1.2 RO Transport Mechanisms...................................................................... 5 1.3 Electron Microscopic Image, Evidence for the Absence of Pores?....... 6 1.4 RO Transport Models.............................................................................. 8 1.5 Support to the Pore Model? .................................................................. 15 1.6 Research after 2001............................................................................... 16 1.6.1 Positron Annihilation Spectroscopy (PALS) .............................. 16 1.6.2 Molecular Dynamics (MD) Simulation ...................................... 18 1.7 Conclusions and Future Directions ....................................................... 21 References..................................................................................................... 22

CHAPTER 2 RO Membrane Preparation ...................................................................................25

2.1 Preparation of Cellulose Acetate Membrane by Phase Inversion Technique .............................................................................. 25 2.2 Preparation of Ultrathin Membrane ...................................................... 28 2.3 Thin-Film Composite (TFC) Membrane .............................................. 32 2.4 Surface Modification of TFC Membrane ............................................. 39 2.4.1 Use of Hydrophilic Amine Monomer ....................................... 40 2.4.2 Additives to the Aqueous Phase................................................ 40 2.4.3 Change of Solvent for Organic Phase....................................... 41 2.4.4 Soaking ...................................................................................... 41 2.4.5 Post Surface Treatment by Aqueous Solutions......................... 41 2.4.6 Coating....................................................................................... 42 2.4.7 Plasma Treatment ...................................................................... 42 2.4.8 Grafting ...................................................................................... 42 2.4.9 Surface Modifying Macromolecules ......................................... 43 2.4.10 Surface Pattern Formation......................................................... 43 2.5 Thin-Film Nanocomposite (TFN) Membrane ...................................... 43 2.6 Biomimetic Membrane.......................................................................... 44 2.7 Inorganic Membrane ............................................................................. 46 2.8 Summary................................................................................................ 48 References..................................................................................................... 49

v

vi Contents

CHAPTER 3 RO Membrane Characterization ...........................................................................57

3.1 Characterization by Membrane Transport ............................................ 57 3.2 Characterization by Membrane Morphology........................................ 59 3.2.1 Scanning Electron Microscopy (SEM) ....................................... 59 3.2.2 Transmission Electron Microscopy (TEM) ................................ 65 3.2.3 Atomic Force Microscopy (AFM) .............................................. 66 3.2.4 Positron Annihilation Lifetime Spectroscopy (PALS) ............... 69 3.2.5 Neutron Scattering (NS) .............................................................. 70 3.2.6 Electron Paramagnetic Resonance (EPR) Spectroscopy ............ 71 3.2.7 Wide-Angle X-Ray Scattering (WAXS) and Small-Angle X-Ray Scattering (SAXS) ........................................................... 72 3.3 Characterization by Membrane Surface Chemistry.............................. 74 3.3.1 Fourier Transform Infrared Spectroscopy-Attenuated Total Reflection (FTIR-ATR) ..................................................... 74 3.3.2 Auger Electron Spectroscopy (AES) .......................................... 76 3.3.3 X-ray Photoelectron Spectroscopy (XPS)................................... 76 3.3.4 Energy Dispersive X-Ray Spectroscopy ..................................... 78 3.3.5 Raman Spectroscopy (RS)........................................................... 78 3.3.6 Scanning Transmission X-ray Microscopy (STXM) .................. 80 3.4 Other Characterization Techniques....................................................... 80 3.4.1 Nuclear Magnetic Resonance (NMR)......................................... 80 3.4.2 Photoacoustic Spectroscopy (PAS) ............................................. 81 3.4.3 Differential Scanning Calorimetry (DSC) .................................. 81 3.4.4 Thermogravimetric Analysis (TGA) ........................................... 81 3.4.5 Contact Angle Measurement ....................................................... 82 3.4.6 Zeta Potential Measurement........................................................ 83 3.4.7 Graft Density ............................................................................... 83 3.4.8 Tensile Strength Measurement.................................................... 84 3.5 Summary of RO Membrane Characterization Methods ....................... 85 References..................................................................................................... 86

CHAPTER 4 RO Membrane Transport ......................................................................................91

4.1 Solution-Diffusion Model ..................................................................... 91 4.1.1 Solvent Transport Equation......................................................... 93 4.1.2 Solute Transport Equation........................................................... 94 4.2 Solution-Diffusion-Imperfection Model ............................................... 95 4.3 Irreversible Thermodynamics................................................................ 95 4.4 Pore Flow Model by Gl€uckauf ............................................................ 97 4.5 Finely Porous Model ............................................................................ 99 4.6 Surface Force-Pore Flow Model ......................................................... 102

Contents vii

4.7 Molecular Dynamics Simulation......................................................... 104 4.7.1 Construction of Membrane........................................................ 105 4.7.2 Reverse Osmosis........................................................................ 106 4.7.3 Simulation Results..................................................................... 107 4.7.4 Solute Transport ........................................................................ 109 4.8 CNTs MD Simulation ......................................................................... 112 References................................................................................................... 115

CHAPTER 5 RO Membrane Module ....................................................................................... 117 5.1 Module Description ............................................................................. 117 5.1.1 Module Type.............................................................................. 117 5.1.2 Feed Spacer................................................................................ 122 5.1.3 Permeate Spacer ........................................................................ 123 5.1.4 Endcap ....................................................................................... 124 5.1.5 Larger Modules.......................................................................... 124 5.2 Studies on the Spacers......................................................................... 124 5.2.1 Computation and Experiments .................................................. 124 5.2.2 Module Observation .................................................................. 124 5.2.3 Module Imaging by Particle Image Velocimetry ..................... 126 5.2.4 Computational Fluid Dynamics................................................. 131 5.2.5 Nuclear Magnetic Resonance Imaging ..................................... 132 5.2.6 Small-Angle Neutron Scattering Imaging................................. 136 5.2.7 Fouling Monitoring by Ultrasonic Time-Domain Reflectometry............................................................................. 136 References................................................................................................... 139

CHAPTER 6 Hybrid System ..................................................................................................... 143 6.1 6.2 6.3 6.4 6.5

Reverse Osmosis-Evaporator .............................................................. 143 Microfiltration-RO............................................................................... 143 Ultrafiltration (UF)-RO ....................................................................... 144 Nanofiltration (NF)-RO....................................................................... 145 Forward Osmosis (FO)-RO ................................................................. 146 6.5.1 Seawater Desalination ............................................................... 146 6.5.2 Wastewater Treatment............................................................... 147 6.5.3 Simultaneous Wastewater and Seawater Treatment................. 149 6.6 Pressure-Retarded Osmosis (PRO)-RO............................................... 150 6.7 Pervaporation (PV)-RO ....................................................................... 152 6.8 RO-Reverse Electrodialysis, RO-Electrodialysis Reversal, and RO-Ion Exchange ......................................................................... 155 References................................................................................................... 158

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CHAPTER 7 RO Economics ...................................................................................................... 163 7.1 General Trend in the Water Production Cost ..................................... 163 7.2 Energy Consumption ........................................................................... 167 7.3 RO Process Economy.......................................................................... 170 7.4 Calculation Method of Water Cost ..................................................... 171 7.5 Case Study ........................................................................................... 176 7.6 Megaton Project of Japan.................................................................... 182 References................................................................................................... 184

CHAPTER 8 RO Membrane Fouling ....................................................................................... 189 8.1 Types of Fouling ................................................................................. 191 8.1.1 Colloidal Fouling ....................................................................... 192 8.1.2 Organic Fouling ......................................................................... 193 8.1.3 Scaling ....................................................................................... 194 8.1.4 Biofouling .................................................................................. 195 8.2 Prevention and Control of Membrane Fouling................................... 196 8.2.1 Modification of Membrane Surfaces ........................................ 198 8.2.2 Pretreatment of RO Feed and Other Methods .......................... 203 8.3 Fouling Prevention of Seawater.......................................................... 208 8.3.1 Seawater Characterization ......................................................... 208 8.3.2 Pretreatment of Seawater........................................................... 213 References................................................................................................... 217

CHAPTER 9 RO Applications ................................................................................................... 221 9.1 RO for the Production of Potable Water ............................................ 221 9.2 Industrial Use....................................................................................... 222 9.2.1 Ultrapure Water ......................................................................... 222 9.2.2 Agricultural Use ....................................................................... 225 9.2.3 Food Industry............................................................................. 229 9.2.4 Petroleum Industry .................................................................... 233 9.2.5 Mining Industry ......................................................................... 234 9.3 Space Applications .............................................................................. 242 9.4 Other Applications............................................................................... 244 9.4.1 Hydrogen Production................................................................. 244 References................................................................................................... 246

CHAPTER 10 Organic Solvent NF (OSN)................................................................................ 249 10.1 OSN Membranes .............................................................................257 10.2 OSN Membrane Applications .........................................................265 10.3 Organic Liquid RO..........................................................................274 References.................................................................................................275

Index ............................................................................................................................... 285

Preface Since the first cellulose acetate membrane for seawater desalination was announced more than half a century has passed. Even though reverse osmosis (RO) is considered as a well-established separation process, RO is still continuing its steady growth both in the commercial market and as a popular research topic. For example, according to the Membrane Technology 2011 prediction [Membrane Technology (2011) volume 2011, page 7], the annual sales of RO will grow to about 40 billion dollar by 2020, due to increasing demand for clean and processed water and mandatory government regulation. Remarkable progress has also been made in the area of membrane preparation and characterization. This book was written as a comprehensive review of progress in all aspects of RO. It should be emphasized that the book has not been written as an introduction to RO. Readers are expected to have a sufficient amount of background knowledge on RO and all the related subjects. For this reason, the recent progress in each chapter is summarized in tables with thorough description of only a few examples. Chapter 1 is reproduction of one of the latest articles published in Desalination. The central theme of the chapter is the membrane “pore” around which the progress during the past 50 years has been revolving, irrespective of whether the researcher is “for” or “against” the presence of pores at the top dense layer of the RO membrane. The article starts from the 1950s when the development of cellulose acetate membrane was launched on the basis of the preferential sorption-capillary flow (PS-CF) mechanism, which later went into direct confrontation with the sorption-diffusion (S-D) model in which pores are considered as imperfection of the membrane. It is further shown in this chapter how the advanced characterization instrument has revealed the heterogeneous structure of the top surface of the RO membrane and begun to measure its “pore size” and “pore size distribution.” The advanced transport theory based on molecular dynamics (MD) simulation also revealed the presence of the multimodal pore size distribution. In Chapter 2, it is told that many cellulosic materials were fabricated by the phase inversion technique and tested for their RO performances in the early stage of RO membrane development. But the cellulose acetate membrane was eventually replaced by the thin-film composite (TFC) polyamide membrane fabricated by in situ polymerization. Thus, TFC membrane developed by Cadotte in the 1980s is now dominating in the commercial market. Many research papers have been published to improve TFC membranes by changing the monomer combination for in situ polymerization. Surface modification of the TFC membrane has also been attempted to enhance the RO performance and to mitigate membrane fouling. The latest progress in RO membrane fabrication was made by the development of nanocomposite membranes (NCMs) in which hydrophilic nanoparticles are incorporated in the TFC membrane to enhance both

ix

x Preface

membrane performance (flux and selectivity) and fouling resistance. The report on the longawaited inorganic RO membranes still rarely appears in the literature. Chapter 3 is for membrane characterization. This is the area where the fastest progress has been observed by the use of advanced characterization techniques. The prices of the characterization equipment are reasonable nowadays and the characterization facility is available in many laboratories worldwide. Currently, characterization by SEM, TEM, AFM, XPS, EDX, FTIR, DSC, TGA, and by the measurement of contact angle and zeta potential is routine laboratory exercises. The publication has become difficult without reporting detailed characterization results, particularly when thin-film nanocomposites (TFNs) are developed. Although it is questionable how the characterization method has contributed to the improvement of membrane performance, it should be admitted that a deeper insight has been obtained for the chemical and physical structure of the RO membrane, which has undoubtedly contributed to advancement of material sciences. In this chapter, the principle of each characterization method is described with some examples of applications for the membrane characterization. Chapter 4 is for membrane transport. The equations for the solution diffusion model, the irreversible thermodynamic model, and the pore flow model are derived in this chapter. It is explained how the dual pore size distribution, including “network pore” and “aggregate pore” have emerged from the transport analysis based on the pore model. The presence of the multiple pore size distribution was later confirmed by the advanced membrane characterization techniques, as shown in Chapter 1. Recently, practically no report can be found in the literature on the derivation of simple transport equations by which RO flux and selectivity can be predicted. Instead, it is more fashionable to study the membrane transport based on molecular dynamic simulation (MDS). The studies on solvent (water) and solute transport by MDS were attempted for both TFC and TFN membranes. Chapter 5 is for membrane module. TFC RO membrane is installed mostly in the spiral wound module. The only hollow fiber module left in the commercial market is cellulose triacetate membrane of Toyobo. As for the progress of the spacer design, it was made inside the industry. Although the improved spacer should have contributed immensely to the reduction of concentration polarization and fouling while minimizing the pressure drop in the module, the results are seldom reported in the literature. The academic research has been mostly focused on advanced methods such as computational fluid dynamics, imaging by particle image velocimetry, and nuclear magnetic resonance. An attempt was made to monitor the real-time fouling by applying ultrasonic time-domain reflectometry at the canary cell. Chapter 6 is for the RO system. Even though many system designs have been made to minimize the water production cost by optimizing series/parallel combination of RO modules, the design and construction of the hybrid system is currently gaining importance. One of such examples is microfiltration (MF) or ultrafiltration (UF)/RO hybrid in which MF or UF is used for the pretreatment of the feed stream before entering into the RO module. The emerging membrane

Preface xi

processes such as forward osmosis (FO) and pressure-retarded osmosis (PRO) are now combined with RO to reduce the energy consumption and improve the product water quality. The treatment of highly concentrated RO brine by membrane distillation (MD) is also very important to increase the production of drinking water as much as possible from seawater while alleviating the environmental impact caused by the release of concentrated brine back into the ocean. Chapter 7 is for RO economics. In this chapter, it is described how the water production cost has decreased from early days of more than 2 US$/m3 to the current 0.5 US$/m3 by the progress of RO technology. The main contribution to the cost reduction was made by the dramatic increase of the market, the improvement in membrane performance, and especially the reduction in energy consumption by the use of advanced energy recovery system such as pressure exchanger. The breakdown of energy requirement was also attempted to discuss if the further advancement in the membrane performance is indeed required. Some examples of water production cost estimation were also shown in this chapter. Chapter 8 is for membrane fouling. This chapter was added since membrane fouling is considered as the main culprit to prevent the further applications of RO and other membrane processes. Since the fouling mitigation by the development of novel RO membranes or by the modification of the RO membrane surface was already discussed in Chapter 2, this chapter is more focused on the pretreatment of the feed stream into the RO module. The advancement was made mainly inside the industry based on the water chemistry and there are not many reports in the literature. In general, membrane fouling is classified into different categories and the appropriate prevention method should be considered for each category. Especially, more detailed description was made for seawater desalination. As it was shown in Chapter 9, RO applications are mainly in water treatment or separation of aqueous solutions. This has not changed since the onset of RO process, even though the amount of water production has increased enormously. Few new areas of RO applications were explored with a notable exception of RO applications in space. Since there are other good books available on RO applications, only few typical examples are shown in this chapter. Finally, Chapter 10 is for the treatment of organic solvents. Even though the separation of organic mixtures by RO has been the dream of membrane researchers since the onset of RO, this area remains practically unexplored due to the insufficient membrane selectivity between organic compounds and the poor durability of polymeric membranes in the organic environment. Durability can be improved significantly by the use of ceramic materials but desired selectivity can not necessarily be achieved. For this reason, current research in this field is mostly for the development of NF and its applications. In this chapter, the thorough description of organic solvent nanofiltration (OSN) was made, hoping that a breakthrough will be made in organic solvent reverse osmosis (OSR) in the nearest future.

xii Preface

The book was written for engineers, scientists, professors, graduate students as well as general readers in universities, research institutions, and industry who have some experiences and background in R&D of RO. It is therefore the authors’ wish to contribute to the further development of membrane science and technology in general and RO in particular by showing the future directions in the R&D of the field.

Ahmad Fauzi Ismail Kailash Chandra Khulbe Takeshi Matsuura

Chapter

1

Introduction—Do RO Membranes Have Pores? ACRONYM AFM CA DMSO FO MD MF MPD NF PA PALS PRO PS-CF RO SANS S-D SEM SWCNTs TFC TMC UF

atomic force microscopy cellulose acetate dimethyl sulfoxide forward osmosis molecular dynamics microfiltration metaphenylene diamine nanofiltration polyamide positron annihilation spectroscopy pressure retarded osmosis preferential sorption-capillary flow reverse osmosis small-angle neutron scattering model: solution-diffusion model scanning electron microscope single walled carbon nanotubes thin film composite trimesoyl chloride ultrafiltration

When one of the coauthors (TM) arrived at Dr. Sourirajan’s laboratory at the National Research Council of Canada in October 1968, Dr. Sourirajan gave him a manuscript of the book “Reverse Osmosis,” which was later published in 1970 [1]. He read the book eagerly and was especially fascinated by the chapter, where Sourirajan wrote how he had launched his reverse ☆

This chapter was taken from the paper “Progress in transport theory and characterization method of reverse osmosis (RO) membrane in the last 50 years, desalination, available online September 30, 2017”. Reverse Osmosis. https://doi.org/10.1016/B978-0-12-811468-1.00001-3 # 2019 Elsevier Inc. All rights reserved.

1

2 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

osmosis (RO) research. According to the book, the invention of the LoebSourirajan RO membrane for seawater desalination was made on the basis of the Preferential Sorption-Capillary Flow (PS-CF) model. As the name of the model implies, pores are required for the transport of water through the RO membrane. In the 1960s, the solution-diffusion model (S-D model) was presented by Lonsdale [2] and it soon became the mainstream of the RO transport model. Since Lonsdale regarded the pores as the defects of the nonporous semipermeable membrane, S-D model has been used for a long time to justify the nonexistence of pores in the perfect dense layer of the RO membrane. It should, however, be pointed out that the S-D model is neutral on this issue and does not say anything about the presence or the absence of pores. It might also be noteworthy to mention that many researchers seemed to believe the presence of pores in the RO membrane deep in their mind. One of the coauthors (TM) remembers the Gordon Conference held in the 1980s where many heated discussions were exchanged on the presence or the absence of pores in the RO membrane. In one of the conferences W. Pusch, Max Planck Institute, Germany, asked the conference participants if they were pore-philic or pore-phobic. To my surprise, more than half raised their hands to show that they were pore-philic. In Sourirajan’s laboratory, attempts were continued to interpret the experimental data based on the pore model, which eventually led to the concept of the bimodal pore size distribution comprising the “network pore” and “aggregate pore” [3, 4]. However, when the bimodal distribution was proposed in 1984, it was almost completely ignored by the membrane community, evidenced by very few citations the paper received. Computer did not count the number of citations those days but we could feel how unpopular the theory was. In the meantime, the membrane characterization techniques were making remarkable progress. In the 1960s and 1970s, the only characterization tool was scanning electron microscope (SEM) that did not allow the resolution below 10 nm when the polymeric membrane surface was investigated. Needless to say that it was impossible to observe the sub-nanometer pores at the membrane surface and, therefore, the top skin layer was generally thought to be dense and homogeneous. In 1994, a paper on the characterization of cellulose acetate (CA) RO membranes by small-angle neutron scattering (SANS) appeared all of a sudden. In the paper S. Krause, Rensselaer Polytechnic Institute, Troy, New York, concluded that SANS data could be explained by the bimodal distribution of pores in the dense skin layer of CA RO membrane.

1.1 Research Before 2000 3

As the industrial membrane fabrication method shifted from the phase inversion technique of CA membrane to thin-film composite (TFC) polyamide membrane, so did the membranes as the object of membrane characterization. Nowadays most of the characterization methods are applied to TFC polyamide membranes. Particularly, positron annihilation spectroscopy (PALS) gained popularity in the beginning of the millennium to characterize the synthetic polymeric membranes for various applications. In the PALS papers the term “free volumes” is often used instead of “pores.” But irrespective of the chosen term, the measured size is indicative of the channel through which the material transport occurs. Kwak’s group at Seoul National University characterized the polyamide TFC membrane by PALS and concluded that the bimodal pore size distribution was observed, assigning these distinctive groups of the pores to the “network” and “aggregate” pores. At almost the same time progress was also made in the membrane transport theory. Instead of interpreting the experimental data of membrane performance by a set of simple transport equations, it is more fashionable nowadays to use the molecular dynamics (MD), by which the structure of the polymeric membrane and the material transport through the membrane is simulated by using a set of computer software. It is particularly interesting to note that many of the MD simulation have resulted in the polymeric membrane structure with bi- or multimodal pore size distributions. Both the characterization and MD simulation, in which sophisticated instrument or computer software, respectively, is used, describe the membrane structure and the membrane transport very much in detail. However, they have not necessarily offered a clear guideline to improve the membrane performance. In this respect, a paper recently published by Araki et al. is interesting as it shows the disappearance of aggregate pores in the nanocomposite TFC membrane in which single-walled carbon nanotubes (SWCNTs) are incorporated. In this chapter the progress made in the understanding of the RO membrane structure and the RO membrane transport is reviewed in historical perspective.

1.1 RESEARCH BEFORE 2000 1.1.1 Preferential Sorption-Capillary Flow (PS-CF) model According to Sourirajan’s book, the following fundamental equation called the Gibbs adsorption isotherm [1] was the basis for the earliest development of RO membrane at the University of California Los Angeles (UCLA).

4 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

Table 1.1 The Thickness of Pure Water Layer at the Air/Sodium Chloride Solution Interface [5] NaCl Concentration (molality)

Pure Water Layer Thickness (nm)

0 0.747 1.603 2.435

0.56 0.38 0.34 0.24

Size of water molecule ¼ ca 0.1 nm.

Γ¼

1 ∂σ RT ∂ ln a

(1.1)

where Γ is the surface excess, R the universal gas constant, T the absolute temperature, σ the surface tension, and a is the activity. The equation predicts the presence of a very thin pure water layer at the surface of saline water. Table 1.1 summarizes the thickness of pure water layer at the air/aqueous sodium chloride interface calculated by Eq. (1.1), assuming a step function for the salt concentration vs the distance from the interface [5]. The table shows that the thickness ranges from 0.24 to 0.56 nm, depending on the concentration of sodium chloride. Prof. Yuster challenged for the first time to skim the pure surface water layer mechanically but failed. Believing in the presence of pure water layer at the interface, Sourirajan continued the challenge but by a different approach. Sourirajan tried to collect the pure water layer through a membrane under pressure applied on the saline water. Sourirajan’s attempt was rewarded by an immediate success. He was able to collect desalinated water as the permeate of the membrane! After the initial few attempts with cellulose and silicone-coated cellulose membranes, a commercial CA membrane from Sartorius was used, which resulted in a high salt rejection, enabling drinking water production from seawater. However, the flux of water was miserably low, with few drops of permeate collected in a day, and the membrane was thought to be practically useless. It is noteworthy that Reid and Breton obtained, quite independently, similar experimental results of seawater desalination by using a CA membrane at the University of Florida [6]. It took another 4 years for Loeb and Sourirajan to develop the CA membranes with fluxes of practical usefulness, which opened up the avenue to the novel membrane desalination process, called RO. According to the PS-CF model, desalination by RO membrane occurs in the following way: when salty water, for example, sodium chloride solution, is

1.2 RO Transport Mechanisms 5

Preferential sorption-capillary flow model Seawater Purewater layer

ti Membrane

Pore (r > ti) Pore (r = ti) n FIG. 1.1 PS-CF model.

in contact with the surface of a membrane, an interfacial pure water layer is formed at the solution/membrane interface. Assuming an analogy between the sodium chloride solution/air interface and the sodium chloride solution/ membrane interface, the thickness of the pure water layer, ti, should be as low as a fraction of nanometer. In the presence of a pore, the diameter of which is smaller than or equal to 2ti, the pure water layer will flow through the pore under the pressure applied on the sodium chloride solution and appear on the other side of the membrane as permeate (see Fig. 1.1). If the pore size is larger than 2ti, the sodium chloride solution will flow through the center of the pore, which leads to the leakage of sodium chloride into the permeate. On the basis of the above model, an appropriate chemical property of the membrane surface that allows the formation of the pure water layer at the membrane/solution interface, as well as the presence of the pores of appropriate sizes at the membrane surface, constitute the indispensable twin requirements for the desalination of salty water.

1.2 RO TRANSPORT MECHANISMS Several RO transport mechanisms were proposed, as discussed extensively by Merten [7–9] at almost the same time as the PS-CF model was presented. Let us now look into some of them. According to Reid and Breton [6, 10, 11], the mass transport through the CA membrane is caused by two mechanisms: (1) the molecules which can associate with the membrane through hydrogen bonding, that is, water, combine with the membrane and are transported through it by alignment-type diffusion; (2) those which cannot enter into hydrogen bonding with the membrane, that is, salts, are transported by hole-type diffusion. Consequently,

6 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

according to their mechanism, the presence of holes (pores) in the membrane contributes to the leakage of the salt through the membrane and hence should be avoided. The solution (sorption)-diffusion (S-D) mechanism, which was favored by Lonsdale et al. [2, 12–15], is currently the most broadly accepted mechanism. According to the S-D model, both water and salt are sorbed in the membrane at one side, diffuse through the membrane and desorbed at the other side. Both sorption and diffusion coefficients are the values unique for the membrane material when they are completely nonporous and perfect [2, 12–15]. Any imperfectness that arises due to the presence of pores will cause the leakage of the salt and should be avoided. Banks and Sharples [16–18] also considered that the mechanism of RO was one of diffusive flow through the pore-free layer on the membrane surface. According to Michaels et al. [19], water transport in RO membrane occurs by molecular diffusion through the polymer matrix, and solute transport by parallel mechanisms involving sorption, activated diffusion, and hydrodynamic flow. According to Sherwood et al. [20], water and solute cross the membrane by parallel processes of diffusion and pore flow. The RO process has also been interpreted in terms of nonequilibrium thermodynamics [21–23]. In all the RO transport models mentioned above, except for those based on nonequilibrium thermodynamics that do not require any specific transport mechanisms, pores are considered to be culprits who make the membrane imperfect and allow the leakage of the salt. Sourirajan’s PS-CF mechanism was therefore in direct collision against the ones that were generally accepted in the 1960s, and hence considered at best as controversial.

1.3 ELECTRON MICROSCOPIC IMAGE, EVIDENCE FOR THE ABSENCE OF PORES? In 1964 Riley took the first SEM picture of a freeze-dried RO membrane [24, 25] and observed the asymmetric structure of its cross section, that is, a thin dense layer that lies on top of a much thicker porous support layer. When a picture was taken from above the top surface, no pores smaller than 10 nm could be observed. Since it is the top dense layer that governs the mass transport of the membrane, Riley’s SEM picture was used to justify the requirement of “nonporous” and “perfect” membrane to enable the

1.3 Electron Microscopic Image, Evidence for the Absence of Pores? 7

“semipermeability” of the RO membrane. It is interesting to note that the discussions were made in 1960s based on the resolution of 10 nm that could be achieved for the polymeric membrane by SEM at that time. In 1970 Schultz and Asunmaa [26] took high-resolution SEM pictures that showed more details of an ultrathin dense CA membrane of thickness 60 nm. As shown in Fig. 1.2, circular unit cells with an average diameter of 18.8 nm were compacted in irregular manner at the membrane surface. Assuming that the spaces between the unit cells are filled with water and subtracting the thickness of the monolayer of immobilized water at the unit cell wall, the pore radius was calculated to be 1.85 nm. They also found a similar structure at the top surface of an asymmetric RO membrane. It is noteworthy that the pore size obtained by Schultz and Asunmaa was very large compared to those that were later obtained by many other methods. A more detailed analysis of the membrane surface was made by Paner et al. in 1973 [27]. Fig. 1.3 shows the cross-sectional structure of a polyamide

n FIG. 1.2 Surface SEM image of ultrathin cellulose acetate membrane. (From R.D. Schultz, S.K. Asunmaa,

Ordered water and the ultrastructure of the cellular plasma membrane, in: J.F. Danielli, A.C. Riddiford, M. Rosenberg (Eds.), Recent Progress in Surface Science. Academic Press, New York, N.Y., 1970, pp. 291–332.)

8 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

n FIG. 1.3 Cross-sectional image of polyamide-hydrazide membrane. (From M. Panar, H.H. Hoehn, R.R.

Hebert, The nature of asymmetry in reverse osmosis membranes. Macromolecules 6 (1973) 777–780.)

hydrazide RO membrane near the surface. The image shows that the surface is covered with a closest monolayer packing of micelles with diameters from 40 to 80 nm. Beneath the monolayer there is a porous layer where the spherical micelles are irregularly packed with void spaces of 7.5–10.0 nm. In the surface monolayer, the micelles are contacting with each other, compressed, deformed, and few void spaces exist between the micelles. Most interestingly, a similar structure was found at the surface of the polyamide hydrazide solution prior to its casting to a thin film. According to Kesting [28], “nodule,” which Schultz and Asunmaa observed as “micelle,” is the aggregate of sphere-shaped “individual macromolecules.” Further according to Kesting, the void spaces between the macromolecular spheres form the pores of the RO membrane, while the void spaces between the nodules correspond to those of ultrafiltration (UF) and microfiltration (MF) membranes. Therefore, even the SEM images were interpreted in two different ways. One group of researchers used the image as an evidence for the absence of pores [24, 25], while the other group used it as an evidence for the presence of pores at the RO membrane surface [26–28].

1.4 RO TRANSPORT MODELS The RO transport models were also made based on the absence or presence of the pores. The S-D model is considered as the most popular model that does not require the presence of any pores, as already mentioned. Briefly, the model comprises the following three steps for the transfer of both solvent (water) and the solute (salt) through the membrane: (1) absorption to the membrane, (2) diffusion through the membrane, and (3) desorption from

1.4 RO Transport Models 9

Membrane

Desorption

Absorption

Diffusion

n FIG. 1.4 S-D model.

the membrane (Fig. 1.4). The chemical potential gradient from the upstream side to the downstream side of the membrane is the driving force for the mass transfer. When the difference in hydrostatic pressure is greater than the difference in osmotic pressure between the upstream and downstream sides of the membrane, a chemical potential difference of water across the membrane drives water against natural direction of water flow by osmosis. Thus the water transport through the membrane is given by. NA ¼ LðΔp  Δπ Þ

(1.2)

where NA is the water flux through the membrane (subscript A denotes water), L the water permeability coefficient, Δp the transmembrane pressure difference, and Δπ is the osmotic pressure difference between the upstream and downstream sides of the membrane. L is given by L¼

DSV RTl

(1.3)

where D is the diffusivity of water in the membrane, S the water solubility in the membrane, V the partial molar volume of water, and l is the thickness of the skin layer of the membrane. Thus, among the three steps given in Fig. 1.4, steps (1) and (3) are represented by S and step (2) by D. Assuming that the passage of water and salt through the membrane are independent of each other, salt transport through the membrane occurs by the concentration difference between the upstream and the downstream side of the membrane as the driving force.

10 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

Then, the salt transport through the membrane is given by.
 NB ¼ B Cfs  Cp

(1.4)

where NB is the salt flux (subscript B denotes salt), B the salt transport parameter, and Cfs and Cp are the salt concentration at the membrane surface on the feed side and in the permeate, respectively. B is further given by. B¼

DB KB l

(1.5)

where DB is the diffusivity of salt through the membrane and KB is the partition coefficient of salt between the solution and the membrane. Thus, KB is a parameter representing steps (1) and (3) and DB step (2). Lonsdale has also shown how the permeation flux and selectivity can be calculated when L and B are given. (The details of the derivation of the equations are given in Chapter 4.) There are also a number of papers in which the RO transport is discussed assuming the presence of pore. One of those is the Gl€ uckauf model [29]. Suppose water phase of dielectric constant D (dimensionless) and the polymer phase of dielectric constant D0 are in contact with each other and there is a pore of radius r in the polymer phase. When an ion enters the pore, the potential of the ion steadily increases and it reaches a maximum value at uckel the mean distance of the ionic cloud, 1 =κ , according to the Debye-H€ model. When this distance is exceeded, an ion of the opposite charge will enter the pore, reducing the potential of the first ion due to the ion-pair formation. The work required to bring the particle to the distance of 1 =κ from the pore entrance was approximated by the work required to bring the ion into the cavity of spherical shape as shown in Fig. 1.5. The work, Δ W00 , is then given by ΔW 00 ¼

NZ 2 E2 ð1  αÞQ  8πD 8:854  1012 r + αbQ


(1.6)

where Q is D/D0 and α is the fraction of solid angles over the whole sphere, as shown in Fig. 1.5, which can be given by
1=2 α ¼ 1  1 + κ2 r2

(1.7)

and b is the ionic radius. The probability of finding the ion at this energy level is exp(Δ W00 /RT). Thus, the concentration in the pore is c2 exp(Δ W00 /RT). [c2 is the feed

1.4 RO Transport Models 11

Jα

α

s

1-α

R

s

J (1-α)

s1

Ion Membrane n FIG. 1.5 Gl€uckauf model.

concentration and the same as Cfi in Eq. (1.4).] Assuming that the concentration in the pore is equal to the permeate concentration, c3, ! NZ2 E2 ð1  αÞQ
 c3 ¼ c2 exp  8πD 8:854  1012 r + αbQ

(1.8)

Eq. (1.8) predicts that the permeate concentration c3, (1) increases with increasing ionic radius b, (2) increases with increasing value of α, that is, of c2 (κ increases with an increase in c2), (3) increases with increasing pore radius, r, (4) decreases with increasing value of Q, that is, D/D0 , and (5) decreases with increasing ionic charge Z, all of which are in agreement with the trends found in the RO experimental results. Another paper based on the pore model was published by Jonsson and Boesen in 1975 [30]. When Eq. (1.10) of their paper is slightly modified, the solute separation, f 0 , is given by the following equation:   uτδ exp D     f0 ¼ 1 b uτδ exp 1 1+ K D

(1.9)

where u is the water velocity in the pore, τ the tortuosity factor, δ the membrane thickness, and D is the diffusivity of solute in water. b and K are the friction parameter and the ratio of solute concentration between the inside

12 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

and outside of the pore, respectively. It is worth noting that both b and K are given as the function of the ratio (solute radius/pore radius). In other words, the above equation requires pore radius to be solved. Later Eq. (1.9) was further modified by considering the interaction force working between the solute and the pore wall and the following equation was derived: 8 2 > >

> : 0 1+

9 > > = ð R 7 expðuðr Þδ=DÞ 7  uðr Þrdr = u ð r Þrdr 5 bðr Þ > 0 > ; ½expðuðrÞδ=DÞ  1 3

eφðrÞ=RT

(1.10)

where r is the radial distance from the pore center and R is the pore radius. The analogy between Eqs. (1.9) and (1.10) is quite obvious. The difference is that u and b are now given as the function of r and the potential function φ(r) is introduced to express the interaction force working between the solute and the pore wall. Furthermore, the solute separation was given as an average of all pores when the pore size distribution was taken into account as follows: ð∞ f0 ¼1

1 c2

c3 ðr ÞuðrÞrdr dR 0  ðR Y ðRÞ uðr Þrdr dR

Y ðRÞ

∞ ð∞

∞

ð R

(1.11)

0

where c3 ¼ c2

1+

exp ðuðr Þδ=DÞ bðr Þ ½ exp ðuðr Þδ=DÞ  1

(1.12)

eφðrÞ=RT

and Y(R) is the Gaussian distribution function. Eq. (1.11) allows us to obtain the pore size distribution of a membrane by fitting the calculated to the experimental solute separation data. It was soon found that a bimodal Gaussian distribution made the best fit for the tested CA membranes (Fig. 1.6) [3, 31]. The bimodal pore size distribution is consistent with the existence of two kinds of pores at the membrane surface, namely the polymer network pores and the polymer aggregate pores; the former kind arises from the spaces between the polymer segments constituting the polymer network within each supermolecular polymer aggregate in the film casting solution, and the latter kind arises from the spaces between the neighboring such as supermolecular polymer aggregate themselves (4) (Fig. 1.7). Furthermore, it was found that the pore size changes by annealing of the CA membrane at high temperatures, as illustrated in Table 1.2.

1.4 RO Transport Models 13

30 77°C

25

67°C

30

15

Y (Rb) × 10–10, m–1

Y (Rb) × 10–10, m–1

Unshrunk

20

10

Unshrunk 20

10

5

0

0 20

77°C

30

40

67°C

50

60

70

80

10

Rb × 1010, m

n FIG. 1.6 Bimodal pore size distribution of cellulose acetate membrane. (From K. Chan, L. Tinghui, T.

Matsuura, S. Sourlrajan, Effect of shrinkage on pore size and pore size distribution of cellulose acetate reverse osmosis membranes. Ind. Eng. Chem. Prod. Res. Dev. 23 (1984) 124–133.)

Polymer segment

Polymer aggregate

Aggregate pore

Network pore

n FIG. 1.7 Network pore and aggregate pore.

Another model was proposed by Singh et al. in 1998 [32]. The model is based on the sieving mechanism, that is, the solute rejection is either 100% or 0% depending on whether the solute radius is larger or smaller than the pore radius, and the pore size distribution is given by the log-normal distribution function. Although the log-normal distribution had been employed in a

14 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

Table 1.2 Effect of Annealing on Pore Radii Radius of annealed membrane, ×1010 m Radius of unannealed membrane, ×1010 m

Annealed 67°C

Annealed at 77°C

70.9 69.9 68.9 67.9 66.9 65.9 64.9 63.9 62.9 61.9 60.9 59.9 58.9 57.9 50.9 40.9 30.9 20.9 10.9 9.9

58.3 56.9 49.9 47.7 44.9 42.9 40.2 36.0 25.4 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2 9.2

55.5 50.5 46.5 43.5 40.3 34.9 24.3 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4 8.4

The table shows that the pore of 70.9  1010 m of an unannealed membrane became 58.3 and 55.5  1010 m after annealing at 67 and 77°C, respectively, etc. From K. Chan, L. Tinghui, T. Matsuura, S. Sourlrajan, Effect of shrinkage on pore size and pore size distribution of cellulose acetate reverse osmosis membranes, Ind. Eng. Chem. Prod. Res. Dev. 23 (1984) 124–133.

number of publications before Singh, all of them were for UF membranes. Singh et al.’s work was very unique since the applicability of the log-normal distribution was extended to nanofiltration (NF) and RO membranes and also to the pore sizes measured by atomic force microscopy (AFM). According to their method a straight line is drawn through the selectivity vs solute diameter plots on a log-normal probability graph. From the straight line the mean pore diameter is obtained as the solute size that corresponds to 50% separation and the standard deviation from the ratio of the solute sizes corresponding to 84.13% and 50% separation (see Fig. 1.8). Singh et al. used ethylene glycol solute of molecular weight that goes down to as low as 1000 Da in their experiments, and obtained a pore diameter as low as 0.7 nm for the sulfonated polyphenylene oxide NF/RO membrane they have prepared. Later the method was applied to a number of RO membranes using the solute of molecular weight as low as that of sucrose (342 Da).

1.5 Support to the Pore Model? 15

99.9 99

70 50 30 10S r2 = 0.95 15S r2 = 0.94 12S r2 = 0.95 10S r2 = 0.96

10 1

Solute separation, %

90

0.1 10

1

100

Solute diameter (nm) n FIG. 1.8 Log-normal distribution.

1.5 SUPPORT TO THE PORE MODEL? The mainstream of RO transport was the S-D model throughout all those years and this situation remains unchanged until now. Nevertheless, a strong support of the pore theory came all of sudden from Krause’s group in Rensselaer Polytechnic Institute, Troy, New York, in 1994 by a paper on characterization of CA membrane by SANS [33, 34]. They prepared ultrathin CA membranes and transferred them to the top of a substrate layer after layer to make a multilayer (1000 layers) sample, which was then subjected to SANS studies. The scattered intensity I(Q) was obtained as a function of Q, which is called the scattering vector and defined as Q¼

    4π θ sin λ 2

(1.13)

where λ is the wavelength of incident radiation and θ is the scattering angle. Analyzing I(Q) vs Q plot obtained from the multilayer sample swollen in D2O, they found that the membrane pore sizes were typically 1–1.5, 10, 20, and 55 nm and concluded that in the dense layer of the CA membrane there were pores of different origins. Especially, two largest ones seemed to correspond to the sizes of the interstitial spaces between the nodules (shown as supermolecular aggregates in Fig. 1.7) and nodule aggregates. Further, dry asymmetric CA membranes with and without annealing were prepared and subjected to SANS studies. They found the following pore sizes for the unannealed and annealed membranes.

16 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

Unannealed membrane: 0.5, 6, and 13.5 nm. Annealed membrane: 0.3, 2, 6, and 13 nm. Interestingly, in the unannealed membrane pore sizes in a range between 3 and 6 nm were missing. In other words, the pore size increased abruptly from 3 to 6 nm. On the other hand, no pore sizes were missing in the annealed membrane. It should be reminded that Chan et al. also reported, by analyzing the solute separation data, that pore radii in a range between 1 and 5 nm (Fig. 1.6) are missing, while the missing range is much narrowed by annealing. The SANS was later used to measure the pore size of RO TFC membrane by Singh et al. [35]. The intensity of scattering was collected in the Q range ˚ 1, and a correlation peak was found in the range of of 0.018–0.030 A ˚ 0.0505–0.0555 A1. From the signal, the pore size of 11–12 nm was calculated, which was likely due to the orderly arrangement of the repeating units of aggregated polyamide chains.

1.6 RESEARCH AFTER 2001 1.6.1 Positron Annihilation Spectroscopy (PALS) A new characterization technique emerged in the begnning of the millenium when PALS was applied to RO membrane by Simazu et al. in 2001 [36]. As mentioned by the authors, PALS had already been used to charactrerize gas separation membranes before their work, but they were the first ones who employed PALS to the RO membrane. Asymmetric CA RO membranes were prepared and annealed at 75–90°C before the membranes were freeze dried. The PA TFC membrane was also prepared by in situ polymerization and the top thin layer was removed from the substrate and vauum dried. Using the PALS signals, they calculated the pore size by the equation  τpickoff ¼ 0:5 1 

  1 R 2πR + sin =2π R + ΔR R + ΔR

(1.14)

where τpickoff is o-Ps lifetime pick-off (ns), R the pore radius (nm, called vacancy radius by Shimazu et al.), and ΔR ¼0.166 nm. For the CA membrane of high NaCl separation, the shorter lifetime component of τ3 ¼2.06–2.16 ns was observed and the corresponding pore radii were 0.29–0.30 nm. They also observed longer lifetime component of 131–134 ns for the CA membrane of low NaCl separation but no pore sizes could be assigned to those signals. On the other hand, for the polyamide TFC membranes the pore size measurement resulted in 0.22–0.25 nm.

1.6 Research after 2001 17

Simazu et al.’s work was soon followed by the work of Kwak’s group who applied PALS for the measurement of TFC polyamide (PA) membranes [37]. They prepared the TFC membrane by in situ polymerization of trimesoyl chloride (TMC) in n-hexane and m-phenylenediamine (MPD) in aqueous phase, but added a small amount of dimethyl sulfoxide (DMSO) into the aqueous phase. The water flux increased significantly while NaCl rejection decreased slightly by the addition of DMSO. When the dried membranes were subjected to PALS, four well-defined peaks were exhibited and the third and fourth lifetime components, called τ3 and τ4, respectively, were used to calculate the pore sizes of the membranes. They concluded that there were two types of pores having radii of 0.21–0.24 nm from τ3 and 0.35–0.45 nm from τ4, corresponding to the network pore and the aggregate pore, respectively, by Sourirajan. As the amount of DMSO increases the aggregate pore’s share in the pore size distribution increases (Fig. 1.9). As forward osmosis (FO) and pressure retarded osmosis (PRO) are studied intensively for the next-generation membrane technology, PALS was applied to investigate the semipermeable membranes developed for FO and PRO.

Network pore HN O C

O HN C

O C

N C H

C

O

O

C N H

HO

O

C

C HN

O O

O

C

C

NH

NH

O C N H

HN C

O C

HN

C

Chain 2

O

H N

O

O

O C

C O

NH

C HN

NH O

Chain 1

O

NH C

H N

O C OH

C

O O

NH

C O

O

C HN

C

NH

O H N

C O

C

O

C

Network pore Aggregate pore n FIG. 1.9 Two pores of TFC membrane detected by PALS. (From S.H. Kim, S.-Y. Kwak, T. Suzuki, Positron annihilation spectroscopic evidence to demonstrate the flux-

enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane, Environ. Sci. Technol. 39 (2005) 1764–1770.)

18 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

Zhang et al. prepared asymmetric membranes from cellulose derivatives cast on different substrate surfaces such as those of Teflon, Glass Mica, and TiO2 [38]. Different substrates were used in an attempt to develop a dense skin layer at the bottom of the membrane that was in contact with the substrate when the polymer dope was cast. Doppler energy broadening spectra (DBES) using PALS coupled with a slow positron beam was employed to investigate the depth profile of pores at the bottom dense skin layer of the membrane. For the CA membrane cast on a glass plate, the pore radius, called free volume radius by the authors, was found to be 0.3 nm from the lifetime component of τ3. The pore size did not change very much by annealing at 90°C but the porosity, called free volume fraction by the authors, changed considerably. Chung’s group [39] further applied PALS method for the measurement of pore sizes in PRO membranes. They have fabricated CA hollow fibers by dry-jet–wet-spinning method, annealed at temperatures 70–90°C and freeze dried. Using the lifetime component τ3, they have concluded that the pore size increased slightly from 0.29 to 0.30 nm by annealing at 90°C together with the increase in the fractional volume. They interpreted the data using the bimodal pore size distribution comprising the void between intramolecular chain segments (the network pore) and the void between nodule aggregates (the aggregate pore). At 90°C the network pore size is reduced but at the same time the aggregate pore size increased by the stress generated by the intramolecular alignment. Since PALS detects only the aggregate pore, the pore size increases by annealing. The PALS method for the pore size measurement was also applied for the dense membranes prepared from various cellulose derivatives. Especially for CA, the pore size was 0.3 nm when the membrane was dry and 0.357 nm when it was wetted by ethanol, likely due to the enlargement of the pore by swelling [40].

1.6.2 Molecular Dynamics (MD) Simulation Recent progress in the RO transport was made mostly by MD simulation for the aromatic polyamide TFC membranes. Harder et al. [41] simulated the formation of cross-linked polyamide layer by progressive cross-linking on the basis of a heuristic distance criterion until the system interconnectivity reaches completion. They have concluded that there were two peaks in the pore size distribution at the radii 0.3 and 0.5 nm, which are close to 0.2 nm (network pore) and 0.4 nm (aggregate pore) obtained by the PALS measurement of Kwak et al. [37].

1.6 Research after 2001 19

Shen et al. [42] simulated the formation of cross-linked FT-30 TFC membrane using a heuristic method. In this approach, TMC and MPD monomers move about randomly in a computational box. When the COCl and NH2 come close to each other amide bond is formed as it occurs in the in situ polymerization. They have also concluded that the pore size distribution of the dry polyamide membrane consists of network pores 0.42–0.48 nm in diameter and aggregate pores typically 0.70–0.90 nm. When the membrane is hydrated, the smaller pores are more expanded while the larger pores makeup small percentage of pores. Ding et al. [43] made the MD simulation for polyamide RO membrane in its hydrated state. To construct the polyamide membrane, they have used a twostage approach. In the first stage several linear chains of polyamide are packed in the simulation box in a relatively relaxed manner. In the second stage, a number of MPD monomers were added to the free volumes of uncross-linked network. Then, the cross-linking is simulated by bridging the NH2 group of MPD and remaining carboxylic groups in the polymer chain. They have concluded that the average pore size (called cavity size) was 0.25 nm, which is close to the diameters of 0.51 and 0.53 nm obtained by the SANS method. Thus, all of the MD simulations resulted in the formation of pore size distribution in the polyamide membranes. Most importantly, in some works the presence of multiple pores such as network and aggregate pores was confirmed. It was suggested that the removal of the larger aggregate pores is necessary to increase the rejection of salts, boron, and small organic molecules. But none of the instrumental characterization and MD simulation could suggest how the aggregate pores can be removed from the polyamide TFC membrane, while it is known that annealing is effective for the CA membrane. In this respect an interesting work was published recently by Araki et al. [44] on the MD simulation of carbon nanotube/polyamide RO membranes. They have found strong orientation of MPD monomer near SWCNTs, which makes macromolecular PA more ordered near the SWCNT. They started computational polymerization of the plain PA membrane by placing a single TMC molecule in the reaction box as a seed and then added 265 MPD and 212 TMC molecules to let the condensation reaction take place step by step within the limit of the reaction box. Finally, unreacted carboxyl group was hydrated. The polymerization resulted in the ring formation of 4 and 6 units. For the polymerization in the presence of SWCNT, SWCNT covered with 10 MPD on the SWCNT surface was placed in the reaction box and then 226 MPD and 183 TMC were added. As a result, SWCNT acted as a seed to

20 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

0.12

0.12 Hydrated PA

(A)

0.1

Pore distoribution probability

Pore distoribution probability

Hydrated SWCNT+PA < r > = 2.31 Å 0.08 0.06 0.04 0.02 0

(B)

0.1

< r > = 2.42 Å 0.08 0.06 0.04 0.02 0

1

1.5

2 2.5 Pore radius [Å]

3

3.5

4

1

1.5

2 2.5 Pore radius [Å]

3

3.5

4

n FIG. 1.10 Pore size distributions with (A) and without (B) SWCNT, calculated by MD simulation. (From T. Araki, R. Cruz-Silva, S. Tejima, K. Takeuchi, T.

Hayashi, S. Inukai, T. Noguchi, A. Tanioka, T. Kawaguchi, M. Terrones, M. Endo, Molecular dynamics study of carbon nanotubes/polyamide reverse osmosis membranes: polymerization, structure, and hydration, ACS Appl. Mater. Interf. 7 (2015) 24566–24,575.)

develop more oriented macromolecular networks around SWCNT. For membrane hydration water molecules were placed on both sides of the membrane and allowed to go into the membrane until sorption equilibrium was reached. The pore size distributions generated by the simulation are shown in Fig. 1.10. It is interesting to note that when the membranes were in the dry state both SWCNT +PA and PA membranes had a single distribution with average pore size (radius) of 0.225 and 0.223 nm, respectively. But when the membrane was hydrated, the SWCNT+ PA membrane retained a single distribution with a slightly increased radius of 0.231 nm, while PA exhibited a bimodal distribution with peaks at 0.15 and 0.25 nm with an average pore radius of 0.242 nm. The density of SWCNT + PA was 1.32 g/cm3 which is much higher than that of PA (1.27 g/cm3). In other words, polymer densification took place around SWCNT. By analyzing the hydration process, diffusivities of 3.07  105 and 3.38  105 cm2/s, respectively, were obtained for SWCNT +PA and PA. Finally, 3 wt% NaCl solution was placed on one side of the membrane, while water was placed on the other side. Water density and salt density profiles are shown in Figs. 1.11A and B. In Fig. 1.11A water density in PA was slightly higher than SWCNT +PA because the former membrane was looser than the latter. As shown in Fig. 1.11B, the salt did not penetrate into SWCNT +PA due to the absence of the water cluster in the aggregate pores that were able to accommodate salt.

1.7 Conclusions and Future Directions 21

0.03

0.003

0.025

0.0025

Na in SWCNT+PA Cl in SWCNT+PA

Density (N/Å3)

Density (N/Å3)

Na in PA 0.02

Membrane

0.015 SWCNT

0.01 0.005

SWCNT+PA

0.002

Membrane

Cl in PA 0.0003 0.0002

0.0015

0.0001

SWCNT 0

0.001

0

5

10

15

20

0.0005

PA 0 0

(A)

10

20

30

Z direction (Å)

40

50

0

60

0

10

(B)

20

30

40

50

60

Z direction (Å)

n FIG. 1.11 Density profile of (A) water and (B) ions. (From T. Araki, R. Cruz-Silva, S. Tejima, K. Takeuchi, T. Hayashi, S. Inukai, T. Noguchi, A. Tanioka, T.

Kawaguchi, M. Terrones, M. Endo, Molecular dynamics study of carbon nanotubes/polyamide reverse osmosis membranes: polymerization, structure, and hydration, ACS Appl. Mater. Interf. 7 (2015) 24566–24575.)

Thus, the incorporation of SWCNT will increase the solute rejection with little sacrifice of the water permeability, which can be readily compensated by decreasing the membrane thickness since the mechanical strength of SWCNT +PA is higher than the PA. The simulation also suggests that SWCNT should remove boron and small organic molecules more effectively than PA due to the absence of the aggregate pore. This was confirmed by the work of Kurihara et al. who achieved boron rejection of above 96% when the pore radius, measured by PALS, was 0.27 nm [45].

1.7 CONCLUSIONS AND FUTURE DIRECTIONS The advanced characterization methods such as SANS and PALS showed that the top dense layer of the RO membrane was not uniform and homogeneous. It has a heterogeneous structure. Some of SANS and PALS investigations have also concluded that the data could be interpreted by assuming bimodal pore size distributions comprising network pore and aggregate pore, as proposed earlier by using molecular probes of various sizes in RO experiments. The advanced transport study by MD simulation also concluded that there are bimodal pore size distributions, the pore sizes of which correspond to those of network and aggregate pores.

22 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

It is likely that the material transport through the top skin layer of the RO membrane does not take place by molecular hopping from one site to the other in a uniformly distributed macromolecular segments but it occurs through channels of different origins. As the characterization technique becomes sophisticated, a day might arrive when the motion of the solute in the membrane can be directly observed. As for the membrane transport, we will be able to obtain more precise information on the membrane structure and the material transport through the membrane, which will enable to offer the firm guideline to improve the membrane performance.

REFERENCES [1] S. Sourirajan, Reverse Osmosis, Academic Press, Cambridge, MA, 1970. [2] H.K. Lonsdale, Chap 4, in: U. Merten (Ed.), Desalination by Reverse Osmosis, The MIT Press, Cambridge, MA, 1966. [3] K. Chan, L. Tinghui, T. Matsuura, S. Sourlrajan, Effect of shrinkage on pore size and pore size distribution of cellulose acetate reverse osmosis membranes, Ind. Eng. Chem. Prod. Res. Dev. 23 (1984) 124–133. [4] S. Sourirajan, Lectures on Reverse Osmosis, National Research Council of Canada, Ottawa, Canada, 1983. Lectures 4 and 6. [5] T. Matsuura, Synthetic Membranes and Membrane Separation Processes, CRC Press, Boca Raton, FA, 1993. [6] C.E. Reid, E.J. Breton, Water and ion flow across cellulosic membranes, J. Appl. Polym. Sci. 1 (1959) 133–143. [7] U. Merten, Flow relationships in reverse osmosis, Ind. Eng. Chem. Fundam. 2 (1963) 229–232. [8] U. Merten, Chap 2, in: U. Merten (Ed.), Desalination by Reverse Osmosis, The MIT Press, Cambridge, MA, 1966. [9] U. Merten, Desalination by pressure osmosis, Desalination 1 (1966) 297–310. [10] E.J. Breton, Jr. (1957). Water and ion flow through imperfect osmotic membranes, US. Dept. Interior, OSW, Res. Develop. Progr. Report (No. 16). [11] E.J. Breton Jr., C.E. Reid, AIChE Chem. Eng. Symp. Ser. 24 (1959) 171. [12] H.K. Lonsdale, U. Merten, R.L. Riley, Transport properties of cellulose acetate osmotic membranes, J. Appl. Polym. Sci. 9 (1965) 1341–1362. [13] H.K. Lonsdale, U. Merten, R.L. Riley, K.D. Vos, and J.C. Westmoreland (1964). Reverse Osmosis for Water Desalination. US. Dept. Interior, OSW, Res. Develop. Progr. Report, (No. 111). [14] H.K. Lonsdale, U. Merten, M. Tagami, Phenol transport in cellulose acetate membranes, J. Appl. Polyrn. Sci. 11 (1967) 1807–1820. [15] R.L. Riley, H.K. Lonsdale, C.R. Lyons, U. Merten, Preparation of ultrathin reverse osmosis membranes and the attainment of theoretical salt rejection, J. Appl. Polym. Sci. 11 (1967) 2143–2158. [16] W. Banks, A. Sharples, Studies on desalination by reverse osmosis, J. Appl. Chem. 16 (1966) 28–32.

References 23

[17] W. Banks, A. Sharples, Studies on desalination by reverse osmosis. II. The relation between the fabrication procedure and the structure of cellulose acetate desalination membranes, J. Appl. Chem. 16 (1966) 94–99. [18] W. Banks, A. Sharples, Studies on desalination by reverse osmosis. III. Mechanism of solute rejection, J. Appl. Chem. 16 (1966) 153–158. [19] A.S. Michaels, H.J. Bixler, R.M. Hodges Jr., Kinetics of water and solute transport in cellulose acetate reverse osmosis desalination membranes, J. Colloid Sci. 20 (1965) 1034–1056. [20] T.K. Sherwood, P.L.T. Brian, R.E. Fisher, Desalination by reverse osmosis, Ind. Eng. Chem. Fundam. 6 (1967) 2–12. [21] J. Jagur-Grodzinski, O. Kedem, Transport coefficient and salt rejection in unchanged hyperfiltraion membranes, Desalination 1 (1966) 327–341. [22] J.S. Johnson Jr., L. Dresner, K.A. Kraus, in: K.S. Spiegler (Ed.), Principles of Desalination, Academic Press, New York, NY, 1966. Chap 8. [23] K.S. Spiegler, O. Kedem, Thermodynamics of hyperfiltration (reverse osmosis): Criteria for efficient membranes, Desalination 1 (1966) 311–326. [24] R.L. Riley, J.O. Gardner, U. Merten, Cellulose acetate membranes: Electron microscopy of structure, Science 143 (1964) 801–803. [25] R.L. Riley, U. Merten, J.O. Gardner, Replication electron microscopy of cellulose acetate osmotic membranes, Desalination 1 (1966) 30–34. [26] R.D. Schultz, S.K. Asunmaa, Ordered Water and the Ultrastructure of the Cellular Plasma Membrane, in: J.F. Danielli, A.C. Riddiford, M. Rosenberg (Eds.), Recent Progress in Surface Science, Academic Press, New York, NY, 1970, pp. 291–332. [27] M. Panar, H.H. Hoehn, R.R. Hebert, The nature of asymmetry in reverse osmosis membranes, Macromolecules 6 (1973) 777–780. [28] R.E. Kesting, The nature of pores in integrally skinned phase inversion membranes, in: T. Matsuura, S. Sourirajan (Eds.), Advances in Reverse Osmosis and Ultrafiltration, National Research Council of Canada, Ottawa, 1989, p. 3. [29] E. Gl€ uckauf, On the mechanism of osmotic desalting with porous membranes, in: Proceedings, First International Symposium on Water Desalination, 1, US. Department of the Interior, Office of Saline Water, Washington, DC, 1965, pp. 143–156. [30] G. Jonsson, C.E. Boesen, Water and solute transport through cellulose acetate reverse osmosis membranes, Desalination 17 (1975) 145–165. [31] 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. [32] S. Singh, K.C. Khulbe, T. Matsuura, P. Ramamurthy, Membrane characterization by solute transport and atomic force microscopy, J. Membr. Sci. 142 (1998) 111–127. [33] S. Kulkarnit, S. Krause, G.D. Wignall, B. Hammouda, Investigation of the pore structure and morphology of cellulose acetate membranes using small-angle neutron scattering. 1. Cellulose acetate active layer membranes, Macromolecules 27 (1994) 6777–6784. [34] S. Kulkarnit, S. Krause, Investigation of the pore structure and morphology of cellulose acetate membranes using small-angle neutron scattering. 2. Ultrafiltration and reverse-osmosis membranes, Macromolecules 27 (1994) 6785–6790.

24 CHAPTER 1 Introduction—Do RO Membranes Have Pores?

[35] P.S. Singh, A.P. Rao, P. Ray, A. Bhattacharya, K. Singh, N.K. Saha, A.V.R. Reddy, Techniques for characterization of polyamide thin film composite membranes, Desalination 282 (2011) 78–86. [36] A. Shimazu, K. Ikeda, T. Miyazaki, Y. Ito, Application of positron annihilation technique to reverse osmosis membrane materials, Radiation Phys. Chem. 58 (2000) 555–561. [37] S.H. Kim, S.-Y. Kwak, T. Suzuki, Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-filmcomposite (TFC) membrane, Environ. Sci. Technol. 39 (2005) 1764–1770. [38] S. Zhang, K.Y. Wang, T.-S. Chung, Y.S. Jean, H. Chen, Molecular design of the celluloseester-based forward osmosis membranes for desalination, Chem. Eng. Sci. 66 (2011) 2008–2018. [39] J. Su, S. Zhang, H. Chen, H. Chen, Y.C. Jean, T.-S. Chung, Effects of annealing on the microstructure and performance of cellulose acetate membranes for pressureretarded osmosis processes, J. Membr. Sci. 364 (2010) 344–353. [40] S.V. Satyanarayana, V.S. Subrahmanyam, H.C. Verma, A. Sharma, P. K. Bhattacharya, Application of positron annihilation: Study of pervaporation dense membranes, Polymer 47 (2006) 1300–1307. [41] E. Harder, D.E. Walters, Y.D. Bodnar, R.S. Faibish, B. Roux, Molecular dynamics study of a polymeric reverse osmosis membrane, J. Phys. Chem. B 113 (2009) 10177–10182. [42] M. Shen, S. Keten, M. Richard, R.M. Lueptow, Dynamics of water and solute transport in polymeric reverse osmosis membranes via molecular dynamics simulations, J. Membr. Sci. 506 (2016) 95–108. [43] M. Ding, A. Szymczyk, F. Goujon, A. Soldera, A. Ghoufi, Structure and dynamics of water confined in a polyamide reverse-osmosis membrane: A molecular-simulation study, J. Membr. Sci. 458 (2014) 236–244. [44] T. Araki, R. Cruz-Silva, S. Tejima, K. Takeuchi, T. Hayashi, S. Inukai, T. Noguchi, A. Tanioka, T. Kawaguchi, M. Terrones, M. Endo, Molecular dynamics study of carbon nanotubes/polyamide reverse osmosis membranes: Polymerization, structure, and hydration, ACS Appl. Mater. Interfaces 7 (2015) 24566–24575. [45] M. Kurihara, T. Sasaki, K. Nakatsuji, M. Kimura, M. Henmi, Low pressure SWRO membrane for desalination in the Mega-ton Water System, Desalination 368 (2015) 135–139.

Chapter

2

RO Membrane Preparation 2.1 PREPARATION OF CELLULOSE ACETATE MEMBRANE BY PHASE INVERSION TECHNIQUE Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the dry-wet phase inversion technique was used by Loeb and Sourirajan when they developed the first cellulose acetate membrane for seawater desalination [1]. Therefore, this method is often called the Loeb-Sourirajan method. According to the Loeb-Sourirajan method, a polymer solution is prepared by mixing polymer, solvent, and nonsolvent. The solution is then cast on an appropriate surface using a doctor blade to a thickness of about 250 μm. After partial evaporation of the solvent, the cast film is immersed in a coagulation bath that is filled with nonsolvent. Due to a sequence of two desolvation steps, that is, evaporation of solvent and solvent-nonsolvent exchange in the coagulation bath, solidification of polymer film takes place. It is desirable to choose a solvent of strong dissolving power with high volatility. During the first step of desolvation via solvent evaporation, a thin skin layer of solid polymer is formed instantly at the top of the cast film due to the loss of solvent. In the solvent-nonsolvent exchange process that follows, nonsolvent diffuses into, while solvent diffuses out of the polymer solution film through the thin solid layer. The change in the composition of the polymer solution film during the solvent-nonsolvent exchange process, often called a composition path, is illustrated schematically in a triangular diagram that involves polymer-solvent-nonsolvent (Fig. 2.1). At some moment, the content of solvent in the solution film becomes so low that the solvent no longer is able to hold polymer in one phase. Phase separation takes place at this moment, forming droplets of one liquid phase dispersed in the other continuous liquid phase. The moment of phase separation, and the size and the number of the dispersed droplets depend on the nature of solvent and nonsolvent and the polymer solution composition. The control of the number and the size of the droplets will eventually control the structure of the porous substrate [2]. The thin layer of solid polymer that Reverse Osmosis. https://doi.org/10.1016/B978-0-12-811468-1.00002-5 # 2019 Elsevier Inc. All rights reserved.

25

26 CHAPTER 2 RO Membrane Preparation

P

A

d a

b

B g C

N

S Tie line n FIG. 2.1 Composition path during the solvent-nonsolvent exchange process.

forms during the first evaporation step becomes the top skin layer, governing the selectivity and the flux of the membrane, while the porous structure that forms during the solvent-nonsolvent exchange step becomes the porous sublayer, providing the mechanical strength. Hence, the membrane obtained by the dry-wet phase inversion process is an integrally skinned asymmetric membrane. The top skin layer can also be made porous by lowering the polymer concentration in the casting solution and the solvent evaporation period, which is hereafter called porous skin layer. Ultrafiltration membranes have a porous skin layer. The asymmetric membranes can also be made in tubular form using a casting bob assembly and hollow fibers can be spun using a hollow fiber spinneret [3]. It should also be noted that there are two structures of the porous sublayer, the sponge-like structure and the finger-like structure. The sponge-like structure looks like a sponge where the pores are surrounded by the polymer walls that separate the pores. The walls are either closed or open, meaning that the pores are either isolated or connected to each other. The solution flux through the sponge-like structure is lower than the finger-like structure but the mechanical strength of the sponge-like structure is higher than the finger-like structure, which structure is desired depends on the preference between the flux and the mechanical strength. The formation of sponge-like and finger-like pores can be explained by using the composition path shown in Fig. 2.2. Note that during the

2.1 Preparation of Cellulose Acetate Membrane by Phase Inversion Technique 27

n FIG. 2.2 (A) Sponge-like structure and (B) finger-like structure of the porous sublayer.

solvent-nonsolvent exchange the composition of the cast polymer solution changes typically along line B, as the solvent moves out and the nonsolvent moves into the cast film. Point α is the composition at the start of the solvent nonsolvent exchange. Point β is on the bimodal line where the phase separation of the polymer solution into the polymer-rich phase (Δ) and the polymer-poor phase (•) at both ends of the straight line takes place, which is called the tie line. At point γ the polymer in the polymer-rich phase

28 CHAPTER 2 RO Membrane Preparation

solidifies as polymer gel, in which polymer-poor phase is entrapped. The structure of the solid polymer is set and no longer changes as the solvent-nonsolvent exchange continues to proceed. At point δ the solvent-nonsolvent exchange is completed, leaving the polymer-poor phase as pores. As already mentioned, phase separation occurs at β, but at this point both phases are liquid and mobile. The size of the polymer-poor phase dispersed in the polymer-rich phase depends on the time taken to move from β to γ. When the time taken is short, the size of the polymer-poor phase is small and dense polymer gel is formed at γ. When the time taken is long, the size of the polymer-poor phase grows and porous polymer gel is formed. The longer the time taken, the larger becomes the pore. The time to move from β to γ depends on the distance from the cast polymer film/nonsolvent (in the coagulation bath) interface. At the interface the time taken is very short and a thin dense layer is formed. The speed of solvent/nonsolvent exchange is slowed down by the formation of the dense layer and a sublayer with sponge-like pores is formed under the dense layer. As the distance from the interface increases the pore size increases. The formation of the finger-like structure is more complicated. While the composition of the thin dense layer at the surface moves from γ to δ, solvent outflux is more than nonsolvent influx (meaning the increase of the polymer content in the dense layer) and the dense layer shrinks, and sometimes breaks under the stress. The nonsolvent gushes into the sublayer resulting in the formation of the finger-like structure. In order to prevent the formation of finger-like structure, the stress should be relieved or the dense surface layer should be mechanically strengthened. Many cellulosic membranes were prepared according to the phase inversion method. Some of them are summarized in Table 2.1 [14].

2.2 PREPARATION OF ULTRATHIN MEMBRANE Recognizing that the key to the success for the fabrication of high-flux RO membrane depends on the thinness of the top layer, an attempt was made by Cadotte in 1970s to prepare ultrathin membranes from cellulosic materials [14]. According to their method, a thin layer of polymer solution was spread over the surface of water, resulting in an ultrathin layer with thicknesses of 140–500 nm [15]. Polymer is dissolved in a solvent, usually one of esters, ketones, or hydrocarbons (e.g., isobutyl acetate and cyclohexane). The polymer solution is

Table 2.1 Summary of Cellulosic Membranes Membrane Performance Operating Pressure (bar gauge)

Flux (L/ m2 h)

NaCl separation (%)

References

Polymer (22.1) Acetone (51.4) Glycerol (6.6) n-Propanol (19.9) Polymer (10) Acetone (20) Water (3) Propionamide (6) Polymer (10) Acetone (30) Water (5) Tartaric acid (2.5) Polymer (10) Acetone (22) Formamide (8) Maleic acid (2) Polymer (10) Acetone (15–20) Formamide or methanol (5–12) Maleic acid (1–5) Same as above

0.5

41

48.80

95.0

[4]

1.104

57

42.33

97.7

[5]

3.5 0.5

102 51

17.93 58.93

98.6 92.7

[6]

3.5

102

22.08

99.56

[7]

3.5

102

18.26

99.5

[8]

3.5

102

19.92 19.92 19.75 17.76 18.43 19.42 15.11 11.29 7.47

98.9 99.0 99.0 99.7 99.5 99.5 99.6 98.7 98.7

[8]

Polymer (20) Dioxane (55)

1.0 Seawater

57 102

23.74 17.26

99.0 99.46

[7]

Casting Solution Composition (wt Ratio)

CA

CA

CA

CA (ds ¼ 2.61)

CA (ds ¼ 2.61)

CA (ds ¼ 2.45 2.55 2.63 2.64 2.67 2.71 2.74 2.79 2.86)

(Continued)

2.2 Preparation of Ultrathin Membrane 29

NaCl Concentration (wt%)

Cellulosic Materiala

Membrane Performance Cellulosic Materiala

Casting Solution Composition (wt Ratio)

CA blend (ds 2.86:2.41 ¼ 2:3) CA, CTA blend (CA:CTA ¼ 1:1)

Acetone (35) Methanol (9) Maleic acid (3) Polymer (20) Dioxane (60) Acetone (35) Methanol (9) Maleic acid (4) – Triethylene glycol (1) Methyl carbitol (1) Polymer (30) Acetone (28) Formamide (42)

CA CA hollow fiber CA hollow fiber

CTA

CAP

CAP (ds ¼ 2.5)

CAB

Polymer (10) Acetic acid (12) Dioxane (75.5) Triacetin (25) Polymer (10) Acetone (20) Formamide (9) Maleic acid (2) Polymer (10) Acetone (15–20) Formamide or methanol (5–12) Maleic acid (1–5) Polymer (10) Acetone (20) Formamide (7) Maleic acid (4)

NaCl Concentration (wt%)

Operating Pressure (bar gauge)

Flux (L/ m2 h)

NaCl separation (%)

References

3.5

102

19.92

99.2

[6]

Seawater 0.5 3.5 0.3

102 41 41 17

34 41

99.54 96.8 96.1 76.0 86.0 88.0 98.0 99.0 97.4

[9] [10]

0.5

22.08 3.82 0.747 10.62 8.80 3.32 2.66 1.00 20.75

3.5

102

22.24

99.1

[6]

3.5

102

19.59

99.5

[8]

3.5

102

17.60

99.3

[6]

[11]

[12]

30 CHAPTER 2 RO Membrane Preparation

Table 2.1 Summary of Cellulosic Membranes—continued

CAB

CAB hollow fiber

CAM

CAM (ds ¼ 2.62)

CAM

CABen

EC

3.5

102

14.44

99.8

[8]

0.3

17

0.30–0.51

98.0

[11]

3.5

102

17.10

99.21

[6]

3.5 1.0 1.0

102 54.5 54.5

12.62 57.10 134.46

99.7 90.0 81.0

[8]

3.5

102

15.27

99.6

[7]

3.5

102

13.45

97.5

[6]

3.5

102

7.06

94.5

[12]

1.0

54.5

6.97

96.7

[13]

a CA (cellulose acetate), CTA (cellulose triacetate), CAP (cellulose acetate propionate), CAB (cellulose acetate butyrate), CAM (cellulose acetate methacrylate), CAben (cellulose acetate benzoate), EC (ethyl cellulose), ds (degree of substitution).

2.2 Preparation of Ultrathin Membrane 31

EC

Polymer (10) Acetone (15–20) Formamide or methanol (5–12) Maleic acid (1–5) Polymer (32) Acetone (39.17) Phosphoric acid triethyl ester (21.83) n-Propanol (5.23) Glycerol (1.77) Polymer (10) Acetone (20) Formamide (9) Maleic acid (3) Polymer (10) Acetone (15–20) Formamide or methanol (5–12) Maleic acid (1–5) Polymer (10) Acetone (16.7) Formamide (9) Maleic acid (3) Polymer (10) Acetone (30) Formamide (10) Maleic acid (5) Polymer (10) Methyl acetate (48) Methanol (12) Formamide (15) 70% perchloric acid (3) Polymer (22) Pyridine (35.1) Dioxane (23.4) Formamide (19.5)

32 CHAPTER 2 RO Membrane Preparation

then poured gently onto water and the solution spreads spontaneously over the surface. The solvent evaporates completely, leaving a thin polymeric membrane. The membrane thickness can be controlled by adjusting the solution viscosity (usually that of syrup) or by stretching the polymer film. The RO performance of ultrathin membrane was tested by placing it on top of a porous substrate membrane. The data obtained from the various cellulosic ultrathin membranes prepared by the above method are summarized in Table 2.2. From the table it can be observed that the performance of some of the ultrathin RO membranes is excellent, but the flux is low in most cases due to the dense polymeric layer.

2.3 THIN-FILM COMPOSITE (TFC) MEMBRANE This method, developed by Cadotte and coworkers of Film Tech in the 1970s, is currently most widely used to prepare high-performance reverse osmosis and nanofiltration membranes [16, 17]. A thin selective layer is deposited on top of a porous substrate membrane by interfacial in situ polycondensation. There are a number of modifications of this method primarily based on the choice of the monomers [18]. However, for the matter of simplicity, the polycondensation procedure is described by a pair of diamine and diacid chloride monomers [19]. A diamine solution in water (called aqueous phase) and a diacid chloride solution in hexane (called organic phase) are prepared. A porous substrate membrane is then dipped into the aqueous phase to fill the pores at the top of the porous substrate membrane with the diamine solution. The membrane is then immersed in the organic phase. Since water and hexane are not miscible, an interface is formed at the boundary of the two phases. Polycondensation of diamine and diacid chloride takes place at the interface, resulting in a very thin layer of polyamide. The preparation of composite membranes by the interfacial in situ polycondensation is schematically presented in Fig. 2.3. There are a number of combinations for the choice of polyfunctional amine and acid chloride monomers. Amine may be primary or secondary, aliphatic or aromatic. The aliphatic amines could be linear or cyclic, which includes ethylenediamine, polyethylenediamine, n-cyclohexanediamine, cyclohexanetriamine, and piperazine. Aromatic amines include phenylenediamine such as m-phenylenediamine (MPD), p-phenylenediamine (PPD), and triaminobenzene. Aromatic amines are preferable to aliphatic ones [20]. Using piperazine as a polyfunctional amine will produce nanofiltration TFC membranes [21] while using

Table 2.2 Performance of Ultrathin Membranesa [16] Functional Groups

Flux (L/m2 h)

NaCl Separation (%)

Cellulose triacetate Cellulose diacetate

OAc (3.0) OH(0.6) OAc (2.4) OH(0.9) OAc(2.1) OCOC2H5 (0.9) OAc (0.1) OCOC3H7 (0.72) OH (0.25) OAc (2.1)  OCOC7H15 (0.9) OAc (2.1) OCOC15H31 (0.9) OAc (2.1) OCOC6H5 (0.6) OAc (2.4) CH2COOCH3 (0.9) OAc (2.1) CH2COOC2H5 (0.9) OAc (2.1) OCOCH2OCH3 (0.9) OAc (2.1) OCOCH2OC2H5 (0.9) OAc (2.1) OCONHCH3 (0.9) OAc (2.1) OSO2CH3 (0.9) OAc (2.1) OSO2N(CH3)2 (0.9) OAc (2.1)

9.96 26.9

99.7 94.2

49.8

77

3.49

89.0

1.39

96.1

0.20

96.1

0.05

83.7

1.00

98.7

9.96

97.5

3.65

98.9

4.48

99.3

1.66

96.8

26.9

86.0

54.78

67.8

29.88

96.8

Cellulose diacetate Cellulose acetate propionate Cellulose acetate butyrate

Cellulose acetate octanoate Cellulose acetate palmitate Cellulose acetate benzoate Secondary cellulose acetate Secondary cellulose propionate Cellulose acetate methoxyacetate Cellulose acetate ethoxyacetate Cellulose acetate methyl carbamate Cellulose acetate methyl sulfonate Cellulose acetate dimethylamino sulfonate a

Operating pressure 57.74 bar, operating temperature 25°C, feed NaCl concentration 1000 ppm.

2.3 Thin-Film Composite (TFC) Membrane 33

Cellulosic Material

34 CHAPTER 2 RO Membrane Preparation

Diacid chloride solution

Diamine solution

(A)

(B)

(C)

(D)

n FIG. 2.3 Preparation of composite membrane: (A) porous membrane, (B) porous membrane immersed in aqueous diamine solution, (C) membrane surface contacted

with hexane-diacid chloride solution, and (D) formation of thin surface layer by interfacial polycondensation [19].

m-phenylenediamine and p-phenylenediamine will produce RO thin-film composite (TFC) membranes [22]. One problem that often occurs in the in situ polymerization is the formation of an ultrathin layer with a large number of defects, which results in poor membrane performance (very high flux with very low separation). To overcome this difficulty diacyl halides such as isophthaloyl chloride, terephthaloyl chloride, 2,6-pyridinedicarboxylic acid chloride, and phenylphosphoric dichloride should not be used alone without the addition of acyl halides with functionality greater than 2, like trimesoyl chloride (TMC) [23]. When TMC that has three COCl groups in an aromatic ring is mixed with phthaloyl chloride that has two COCl groups, cross-linking will form between two main chains. Furthermore, unreacted COCl will become – COOH upon contact with water and the membrane will become negatively charged. Monomers with reactive groups other than amine and acid chloride can also be used. Some combinations of polyfunctional amine and acid chloride recently tested are summarized in Table 2.3 [24]. Interfacial polymerization takes place in the organic phase side of the interface and not in the aqueous phase side, since the acid chloride has negligible solubility in water while the diamine is soluble in organic solvents [18, 39]. The diamine must continuously cross the water-hexane interface, diffuse through the polyamide film which has been formed, and then meet and react with the acid chloride in the organic phase side

Table 2.3 Newly Reported Monomers for Synthesis of Thin Film Composite Polyamide (PA) Membrane [24] Amine Source

Acid Chloride Source

References [25]

[26]

[27]

[28]

(Continued)

Table 2.3 Newly Reported Monomers for Synthesis of Thin Film Composite Polyamide (PA) Membrane [24]—continued Amine Source

Acid Chloride Source

References [29]

[30]

[31]

[32]

[33]

[34]

(Continued)

Table 2.3 Newly Reported Monomers for Synthesis of Thin Film Composite Polyamide (PA) Membrane [24]—continued Amine Source

Acid Chloride Source

References [35]

[36]

[37]

[38]

2.4 Surface Modification of TFC Membrane 39

to ensure the buildup of the thickness which is enough for the thin skin layer of the TFC membrane [18, 20, 40–43]. This means that polymerization reaction is strongly affected by the factors that influence the diffusion rate of the amine toward the organic phase, such as the nature of solvent, the reactant concentration, the phase miscibility, etc. [44–47]. It is better to use a large excess of diamine over acid chloride (typically about 20:1), in order to drive partitioning and diffusion of the diamine into the organic phase. Any factors that disturb the diffusivity and solubility of the diamine monomer in the organic phase affect not only the reaction rate but also the morphology and structure of the resulting polyamide film, which ultimately determines separation performance and interfacial properties [45, 46]. Recently, Kim et al. added dimethylsulfoxide (DMSO) to the aqueous phase, which produced TFC membranes with water fluxes up to fivefold higher than the TFC membrane prepared without DMSO while no considerable loss occurred in salt rejection. According to Kim, RO membranes are composed of two different types of pores. One is called network pores with radii in the range of 2.1–2.4 Ẳ, and the other called aggregate pores with radii in the range of 3.5–4.5 Ẳ. They have further concluded that the addition of DMSO to the aqueous phase increases the number and the size of network pores, thus leading to a large increase in flux (Fig. 2.4). One of the serious drawbacks of TFC PA membrane is that the membrane performance deteriorates significantly by exposure to chlorine since feed water is usually treated by disinfectant before it enters the RO module. Several attempts have been made to make the PA layer more chlorine resistant. One of such attempts was to replace amide with imide linkage [48]. Other works were to decrease the reactivity of nitrogen atom in the amide bond [49–51].

2.4 SURFACE MODIFICATION OF TFC MEMBRANE Since both the surface chemistry and the morphology of the membrane play a crucial role in determining the membrane performance, its enhancement has been attempted through the modification of the membrane surface [52, 53]. In fact, the latest researches of TFC membranes are mostly on the membrane surface modification as summarized below [54].

40 CHAPTER 2 RO Membrane Preparation

Network pore HN

C

O

O

C O

HN C

C

N H

O C

O C

O O

C

N H

HO

O C

C O

C HN

NH

NH

O

N H

C

O C

HN

HN C

C

Chain 2

O

H N

O

O

O

C

NH

C HN

C O

NH O

Chain 1

O

O C

NH C

C

H N OH O

O

Aggregate pore

C

NH

C O

O

C HN

C

NH

O H N

C O

C

O

Network pore

n FIG. 2.4 Schematic representation of possible molecular structure of network and aggregate pores in aromatic TFC membranes [44].

2.4.1 Use of Hydrophilic Amine Monomer Using polyvinyl alcohol (PVA)-based amine compound having a side chain amino group as the aqueous-phase monomer instead of MPD can produce high-flux TFC membranes for low-pressure applications [55].

2.4.2 Additives to the Aqueous Phase Addition of alcohols, ethers, sulfur-containing compounds, and monohydric aromatic compounds and more specifically DMSO in the aqueous phase can produce TFC membranes with an excellent performance. This was ascribed to the faster diffusion of amine due to an increase in aqueous-organic phase miscibility [44, 56–59]. Addition of sulfonated cyclodextrin increased negative charge density of the membrane surface, resulting in the enhancement of salt rejection and antifouling capacity [60]. Alkaline compounds neutralize the hydrochloric acid formed during the polycondensation reaction. Sodium hydroxide, sodium carbonate, and triethylamine can be used for this purpose. It is observed that the effect

2.4 Surface Modification of TFC Membrane 41

depends on the alkaline strength. Using different ammonium salt for the preparation of piperazine PA membrane, Xiang et al. concluded that ammonium salt of higher steric configuration resulted in better salt rejection [61].

2.4.3 Change of Solvent for Organic Phase The choice of organic solvent to prepare the organic phase is also important by affecting the solubility and diffusivity of amine in the organic phase. When the solvent was changed from hexane to toluene, the surface charge of the TFC membrane changed from neutral to positive [32, 62].

2.4.4 Soaking Soaking the freshly prepared TFC membranes in solutions containing various organic species, including glycerol, sodium lauryl sulfate (SLS), and the salt of triethylamine and camphorsulfonic acid (TEACSA), could increase the membrane flux in RO applications by 30%–70% [63]. The TFC physical properties (abrasion resistance) and flux stability could also be improved by the application of an aqueous solution composed of PVA and a buffer solution as a posttreatment step during the preparation of the TFC membranes [64, 65]. Use of DMF and DMSO as “activating solvents” helped removing the defects in the PA layer by compression generated by swelling of PA [66].

2.4.5 Post Surface Treatment by Aqueous Solutions Several attempts have been made to increase membrane surface hydrophilicity by surface modification techniques, each having its own advantages and disadvantages. Hydrophilization by treating the membrane surface with water soluble solvent (acids, alcohols, and mixtures of acids, alcohols, and water) is one of the surface modification techniques, as mentioned earlier. This method increases the flux without changing the chemical structure but one of its disadvantages is that the water flux decreases with time because of the leaching of the hydrophilizing agent by water permeation [67]. Using a mixture of acid and alcohol in water for the surface treatment could improve the surface properties since acid and alcohol in water caused partial hydrolysis and skin modification, which produced a membrane with a higher flux and a higher rejection. It was suggested that the presence of hydrogen bonding on the membrane surface encourages the acid and water to react on these sites producing more charges [67, 68].

42 CHAPTER 2 RO Membrane Preparation

2.4.6 Coating Coating of natural materials like sericin made the membrane surface more hydrophilic, smooth, and negatively charged, resulting in enhanced salt rejection and fouling resistance [69]. Louie et al. [70] coated the surface of commercial TFC-RO membranes with a solution of polyether-polyamide (PEBAX 1657) to produce antifouling membranes. Atomic layer deposition (ALD) is a new attractive technique of TFC surface modification [71]. The number of ALD affected the membrane thickness, flux, and salt rejection.

2.4.7 Plasma Treatment Water permeability of PA-TFC-RO membranes could be increased using an oxygen plasma treatment by introducing carboxylic group which increased the hydrophilicity of the treated membrane. On the other hand, the chlorine resistance of the TFC-RO membrane could be enhanced using an argon plasma treatment which caused cross-linking to take place at the nitrogen sites on the membrane surface [72]. In addition, membrane surfaces anchored with hydrophilic polymers such as polymethacrylic acid and polyacrylamide exhibited super antibiofouling and surface cleaning properties.

2.4.8 Grafting A hydrophilic, charged TFC could be produced by using radical grafting of two monomers, methacrylic acid and poly(ethylene glycol) (PEG) methacrylate onto a commercial PA-TFC-RO membrane [73]. It was found that the use of amine containing ethylene glycol blocks enhanced the performance of the membrane and highly improved membrane water permeability by increasing the hydrophilicity [74]. The PEG and its derivatives have been used for surface modification. TFC membrane’s resistance to fouling could be improved by grafting PEG chains onto the TFC-RO membranes [75, 76]. Hydrophilization of the membrane surface by grafting zwitterionic compounds increased antifouling resistance of the membrane because the water was bound to the surface by strong ionic salvation [77–80]. Grafting of

2.5 Thin-Film Nanocomposite (TFN) Membrane 43

3-monomethylol-5,5-dimethylhydantoin (MDMH) made the membrane surface strongly antimicrobial and increased chorine tolerance.

2.4.9 Surface Modifying Macromolecules Another alternative and less common approach for the membrane surface modification is the introduction of an active additive. The basis of this technique is the idea that those additives can move toward the top film surface during membrane formation and alter membrane surface chemistry while keeping bulk properties unchanged. According to this method, only very small quantity of the additives is enough to change the surface chemistry of the membranes [81, 82]. Blending is a conventional technique used for membrane surface modification, and recently much attention has been given to utilize this technique, in which hydrophobic surface modifying macromolecules (SMMs) are blended to a base polymer for membrane surface modification [83]. Abu Tarboush et al. [84] improved the flux stability of PA-TFC-RO membrane by using hydrophilic SMMs formed simultaneously by in situ polymerization reaction when the polycondensation reaction takes place within the organic solvent of the TFC system.

2.4.10 Surface Pattern Formation A TFC membrane with a patterned surface was prepared by Maruf et al. using micro-processing technique [85]. It modified the local flow pattern and streamlines and produced local turbulence, affecting the foulant deposition [86].

2.5 THIN-FILM NANOCOMPOSITE (TFN) MEMBRANE The latest trend in the TFC-RO membrane is to incorporate nanoparticles in the in situ polymerized thin layer. The membranes prepared by this method is called thin-film nanocomposite (TFN) membranes and usually exhibited better separation performance and fouling resistance than the TFC membranes. Kwak et al. developed a new type of antifouling membranes as early as 2001 by incorporating TiO2 nanoparticles in the PA thin layer in order to reduce the loss of reverse osmosis permeability due to microbial fouling [87, 88]. But the turning point was 2007 when Hoek’s group of UCLA published a paper, in which they have reported that the water flux of TFC-RO membranes could be doubled without affecting the salt rejection by incorporating zeolite nanoparticles in the

44 CHAPTER 2 RO Membrane Preparation

thin layer of the TFC-RO membranes [89]. They have also commercialized their membranes via a company called Nanowater which was later absorbed by LG of Korea. Since then many nanoparticles have been tested as fillers of TFN membranes. In 2006 Holt et al. [90] reported the development of carbon nanotube membranes with pore sizes smaller than 2 nm by using a microelectro-mechanical systems (MEMS)compatible fabrication process. Carbon nanotubes, with diameters as small as a nanometer and with a smooth surface may offer a very unique molecular transport through their pores. In fact, there are several studies in recent years that suggested that the water transport through singlewalled carbon nanotubes (SWCN) would become much faster than the transport rate that the continuum hydrodynamic theory would predict. This was attributed by molecular dynamic (MD) simulation to the smoothness of the nanotube wall [91, 92]. Since Holt et al.’s report, there have been many attempts to incorporate the carbon nanotubes in TFC membrane to make use of CNTs’ unique feature of high permeability. Furthermore, the recent molecular dynamics simulation of RO membrane based on multilayer graphene [93] and the experimental proof that an effective separation membrane for desalination can be fabricated by etching nanosized pores in 2D graphene sheet [94] promoted the incorporation of graphene and graphene oxide into the thin PA layer of TFC membrane. Recent progress in the development of TFN membranes is summarized in Table 2.4.

2.6 BIOMIMETIC MEMBRANE Recently, aquaporin-based biomimetic membranes caught attention because of the intrinsically high water permeability and salt rejection of aquaporin. Commercialization of aquaporin incorporated TFC membrane is now being undertaken. Tang et al. [111] wrote a review of the properties of aquaporins, their preparation, and characterization. Very high permeability and salt rejection membranes can be obtained based on aquaporin protein function [112]. Kumar et al. [113], based on the measured water permeability of Aquaporin Z (AqpZ)-containing proteoliposomes, postulated that AqpZ-based biomimetic membranes can potentially achieve a membrane permeability as high as 167 μm/s bar (i.e., 601 L/m2 h bar), which is about two orders of magnitude more permeable compared to the existing commercially available seawater RO membranes [114].

2.6 Biomimetic Membrane 45

Table 2.4 Recent Development of TFN RO Membranes Fillers

Results

References

Water flux and the antibacterial fouling potential were increased Self-cleaning of the membrane surface was possible Hydrophilicity of the membrane surface was increased. Both flux and salt rejection showed maximum at filler loading of 0.005 wt%. The maximum rejection was 97% Salt rejection and water flux were 96% and 34 L/m2 h, respectively. Chlorine resistance was improved

[88]

Metal oxides and zeolite TiO2 nanoparticles TiO2 with UV radiation SiO2 nanoparticles

Hyper-branched aromatic polyamide-grafted silica (HBP-g-silica) Ag2O nanoparticles

Zeolite NaY zeolite

Halloysite nanotubes (HNTs) Alumino silicate single-walled nanotubes

Contact angle decreased. Flux increased from 26 to 40 L/m2 h and the salt rejection was 99%, with 0.003 wt% Ag2O nanoparticles Water flux could be doubled without affecting salt rejection Water flux increased from 0.95 to 1.78 m3/m2 day [23.3 to 43.7 gal/ft.2 day (gfd)] and high salt rejection of 98.8% was achieved Potentially improve the performance of TFC membrane during RO application Higher flux could be achieved while sustaining high salt rejection

[95] [96]

[97]

[98]

[89] [99]

[100] [101]

Carbon nanotubes (CNTs) and graphene oxide Multiwalled carbon nanotubes (MWCNTs) CNTs

CNTs. Acidified MWCNTs MWCNTs SWCNTs

Graphene oxide (GO) CNTs with acidic groups (CNTa), GO, and both CNTa and GO (CNTa-GO)

MWCNT modified by diisobutyryl peroxide

Flux, fouling resistance, and chlorine resistance were improved

[102]

Higher salt rejection [97.69% as compared with 96.19% (without CNTs)] and a near doubling of water flux [44 L/m2 day bar as compared with 26 L m2 day bar (without CNTs)] were achieved Water flux was significantly increased. Also, the durability and chemical resistance against NaCl solutions were increased Water flux was significantly improved. Membrane may have a potential application in separation of organic aqueous solution Membrane performance in terms of flow and antifouling was improved. Chlorine degradation was inhibited Molecular dynamic simulation (MDS) showed increase in the Na and Cl ion rejection and pore size distribution changed from mononodal to binodal Water flux, chlorine resistance, long-term durability, and mechanical properties were improved Mechanical strength, antimicrobial and antifouling properties, selectivity, water flux and thermal properties were significantly improved after incorporation of fillers. The best results were obtained by CNTa-GO MWCNT dispersion improved, Flux of 28 kg/m2 s and NaCl separation of 90% were achieved at 16 bar

[103]

[104] [105] [106] [107]

[108] [109]

[110]

46 CHAPTER 2 RO Membrane Preparation

Zhao et al. [115] successfully fabricated an aquaporin-based biomimetic membrane via interfacial polymerization method. It was noticed that the resulting membrane AMB-wild, with area greater than 200 cm2, had good mechanical stability when tested up to 10 bar under RO conditions. High water permeability (4.0 L/m2 h bar) and good NaCl rejection (around 97%) were observed at an applied pressure of 5 bar. Rejection was further improved at higher pressure. The membrane had superior separation performance compared to commercial RO membranes (BW30 and SW30HR), demonstrating the great potential of interfacially polymerized ABM membranes. Several design approaches have been pursued in facing the challenge of making the biomimetic membranes as stable, robust, scalable, and costeffective as their polymeric counterparts in the form of existing technologies such as RO membranes [114]. However, aquaporin membranes at present are on the laboratory scale, but production of these membranes could easily be established on an industrial scale (full-scale applications) in the near future.

2.7 INORGANIC MEMBRANE Inorganic membranes have the following attractive features compared with polymeric membranes. They are resistant to chlorine and other disinfectants and can withstand steam treatments making them less vulnerable to biofouling with a very long lifetime in water treatment. Unfortunately, the high fabrication cost and the poor processability (It is hard to make flexible flat sheet or hollow fibers.) prevent them from becoming as popular membrane material as organic polymers. Yet, inorganic membranes “deserve more attention” as they require much smaller foot print and the process becomes more energy efficient, which results in a life-cycle cost less than the existing membrane systems. Particularly, for the treatment of organic liquids, inorganic membranes seem more desirable, since swelling of the polymeric membranes causes deterioration of separation performances. There are different inorganic membranes but not many show RO membrane performance. Zeolites are capable of providing the required desalination properties while being potentially tolerant to feed waters which readily foul polymer membranes, and/or can withstand more cost-effective cleaning methods [116]. There are several methods for the preparation of zeolite membranes as follows [117]: (1) Embedded method: Barrer and James [118] were the first to fabricate zeolite membranes in which microcrystalline ion-exchanging zeolites

2.7 Inorganic Membrane 47

(2)

(3)

(4)

(5)

are bonded by inert polymeric fillers in such a way that the electrochemical behavior is determined by the crystals and by crystal contacts. The embedded method is not a good choice as in the final membrane there will be many defects and the performance is not reliable. In situ hydrothermal synthesis method: This is the most commonly used method. In this method a porous support is brought into direct contact with the synthesis solution or gel to allow the growth of a zeolite film on the surface of the support under hydrothermal conditions. The formation of the zeolite membrane under hydrothermal conditions involves the formation of the supersaturation region adjacent to the substrate surface, nucleation, aggregation, crystallization, and crystal growth. Seeding technique (secondary growth method): To enable better control of nucleation and crystal growth steps, seeding technique is better than other techniques. In this method, first a colloidal zeolite suspension of sub-μm-sized seed crystals is prepared. These crystals will be coated as a seed layer on the surface of the substrate. Hydrothermal synthesis is followed to grow zeolite film on the seed layer. The preparation of zeolite sol as crystal seeds is the key step in the secondary method. Microwave method: The microwave method for the preparation of zeolite membranes is similar to the conventional hydrothermal method except that the autoclave is placed in a microwave field. The synthesis time is reduced greatly. Direct gel crystallization (DGC) method was first proposed by Xu et al. [119], which is a novel method to synthesize zeolite membrane on porous supports. The zeolite ZSM-5 has been synthesized from amorphous aluminosilicate gels in a vapor of ethylenediamine, triethylamine, and water. The DGC has the benefit of better thickness control compared with liquid phase synthesis, since the amount of nutrient for growing zeolite is directly controlled by the amount of gel applied.

Some examples of zeolite membranes are illustrated in Table 2.5. Although the improvement of zeolite membranes has been remarkable during the past 10 years, their performance and economics are still no match for polymeric membranes. The zeolite membrane thickness is still at least three times higher than the current state-of-the-art polymeric RO membranes, causing higher resistance to water flux and due to this, inorganic membranes require at least 50 times higher membrane area than polymeric ones to achieve an equivalent production capacity.

48 CHAPTER 2 RO Membrane Preparation

Table 2.5 Studies on Reverse Osmosis by Inorganic Membranes Inorganic Material

Results

References

Thin film of Zeolite ZK-4

Molecular dynamic simulation showed promise of the membrane to be used in membrane-based separation of aqueous electrolyte solutions, as well as other similar systems Molecular dynamic simulation showed 100% rejection of salt ions. The permeability was about 2  109 m/Pa s when the membrane thickness was less than 3.5 nm Molecular dynamic simulation showed single layer of molybdenum disulfide will exhibit effective ion rejections while allowing high water flux NaCl separation as low as 22% but AlCl3 separation as high as 96% with pure water flux of 0.266 kg/m2 h at 3.4 MPa With 0.1 M NaCl feed solution, the membranes rejected 76% of Na+ ions while permitting a water flux of 0.112 kg/m2 h Water flux of 9  1013 m3/m2 s Pa with NaCl separation of 98% at 90°C was achieved. Excellent molecular sieving effect with 95% isopropanol separation was observed Both water flux and ion rejection increased considerably as aluminum ions were incorporated into the zeolite framework with a water flux increase from 0.112 to 1.129 kg/m2 h and ion rejection improvement from 90.6% to 92.9%

[120]

Hydrophilic FAU and hydrophobic MFI Molybdenum disulfide MFI type zeolite Hydrophobic MFI Microporous organosilica MFI-type zeolite

[121]

[122] [123] [124] [125]

[126]

2.8 SUMMARY Several attempts have been made to improve the performances of RO membranes, especially those of polyamide TFC membranes by incorporating nanosized fillers such as single-walled carbon nanotubes (SWCNT), multiwalled carbon nanotubes (MWCNT), graphene, graphene oxide, silica, zeolite, and others. Fouling mitigation, especially that induced by biofouling by grafting polymer chains to the membrane surface, coating highly hydrophilic layer, manipulation of surface charge, etc., have also been attempted with some successes. Inorganic membranes are more robust than polymeric membranes in an aggressive environment but their commercial applications are hampered by their high production cost. Yet, they seem to have a potential in the future. It is an enormous challenge to fabricate RO membranes with higher water flux, near complete rejection of dissolved species, low fouling propensity, tolerance to oxidants used in pretreatment for biofouling control and long durability, but progresses are being made day by day in these directions by the efforts of many researchers involved worldwide.

References 49

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Chapter

3

RO Membrane Characterization The characterization of RO membranes is important since this allows insight into the relationship between membrane chemistry, structure, and transport properties. The most widely used characterization method is the measurement of water flux and solute (usually NaCl) rejection; these can be easily measured and so give a quick indication of the suitability of the membrane for a particular application. However, the fluxes of water and solute provide only limited information about the characteristics and structure of the membrane and the role they play in water and solute transport. As a result, other characterization techniques are beginning to be employed in order to determine parameters such as pore size, barrier layer thickness, and membrane elemental composition. There are many common methods such as bubble point method, liquidliquid porosimetry, nitrogen adsorption/desorption, permporometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and contact angle measurement to characterize the membranes. Among these bubble point method, liquid/liquid porosimetry, and permporometry are applied for membranes whose pore sizes are as large as or larger than those of ultrafiltration and microfiltration and not for RO membranes. Now new techniques such as atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) are available to go deep into understanding of the structure, mechanism, morphology, etc., of the membrane, which can help to make desirable membranes. It should be noted that most of the characterization methods are routinely used nowadays. Therefore, only few typical examples are shown for each method in the following sections.

3.1 CHARACTERIZATION BY MEMBRANE TRANSPORT The RO permeation cell and the RO system are shown in Fig. 3.1. Their basic design has not changed since as early as 1960s although many electronic devices are nowadays attached for the data acquisition and storage. Reverse Osmosis. https://doi.org/10.1016/B978-0-12-811468-1.00003-7 # 2019 Elsevier Inc. All rights reserved.

57

58 CHAPTER 3 RO Membrane Characterization

Stainless steel porous plate

Feed out

Feed in

Product out

Feed solution inlet

N2 Gas under pressure

Pressure gauge

Surge tank

Purge valve Cell

Pump

Product outlet

Pressure regulator Feed solution outlet

n FIG. 3.1 Reverse osmosis cell and test system.

The permeation cell is made of stainless steel and consists of two detachable parts. The upper part is a high-pressure chamber. A wet membrane is mounted on a stainless steel porous plate embedded in the lower part of the cell such that the active layer of the asymmetric membrane faces the feed solution under high pressure. A wet Whatman filter paper is placed between the membrane and the porous plate to protect the membrane from abrasion. The feed solution is supplied to the feed chamber of the permeation cell by a pressure pump, while the permeate (product) solution is discharged from

3.2 Characterization by Membrane Morphology 59

the permeate (product) outlet, which is open to the atmosphere. The pressure of the solution is measured by a pressure gauge and the solution is released to the atmospheric pressure through a pressure regulator. Nitrogen gas under pressure by gas cylinder is used to load the dome of the pressure regulator and to provide the gas cushion in the surge tank. The design of a static cell is shown in Fig. 3.2. This cell is used for the batchwise operation when the quantity of the feed solution is limited. The cell is a stainless steel pressure vessel consisting of two detachable parts. The membrane is mounted on a stainless steel porous plate embedded in the lower part of the cell through which the permeate liquid is discharged into the atmosphere. The upper part of the cell holds the feed liquid under pressure. Compressed nitrogen is used to apply pressure on the feed liquid. The feed solution is kept well stirred during the permeation experiment, by means of a magnetic stirrer fitted in the cell about 0.3 cm above the membrane surface.

3.2 CHARACTERIZATION BY MEMBRANE MORPHOLOGY 3.2.1 Scanning Electron Microscopy (SEM) The SEM is used to study the morphology of a membrane by examining the top layers and cross sections in detail. It is a very common tool for the characterization of the membrane. In the SEM an electron beam, called primary electron, is focused on a sample. As a result of the interaction of the electron beam with the sample, the secondary electron is reflected. This signal can then be detected by an appropriate detector to obtain information about the surface topography and composition. The SEM was developed to overcome the resolution of light microscope which was limited by the wavelength of light. Electrons have much shorter wavelengths, enabling a much better resolution. The main components of the SEM include: (i) source of electrons, (ii) column down which electrons travel with electromagnetic lenses, (iii) electron detector, (iv) sample chamber, and (v) computer and display to view the images. Fig. 3.3 shows the schematic diagram of the SEM. The position of the electron beam on the sample is controlled by scan coils which are situated above the objective lens. These coils allow the beam to be scanned over the surface of the sample. This beam rastering or scanning, as the name of the microscope suggests, enables information about a defined area on the sample to be collected.

60 CHAPTER 3 RO Membrane Characterization

Static cell

A′ Magnetic stirrer

A SS porous plate View B

Membrane

Section AA′

View B

Pressure gauge

Pressure regulator

Static cell

Nitrogen tank under pressure

n FIG. 3.2 Design of the static cell.

Magnetic stirrer

3.2 Characterization by Membrane Morphology 61

Electron gun First condeser lens Objective aperture Second condeser lens

Objective aperture

Scan coils

Objective lens

Sample n FIG. 3.3 Simple schematic of the scanning electron microscope.

While atomic resolution cannot be provided, many SEMs have been known to achieve resolution below 1 nm. Typically, modern full-sized SEMs provide resolution between 1 and 20 nm whereas desktop systems can provide a resolution of 30 nm or more. Conductive coating can enhance the image contrast due to higher secondary electron yield. The resolution of SEM with field emission may go down to a range of 0.6–3 nm. However, in order to obtain such high resolution with polymer samples, preparation artifacts have to be minimized. One of the reasons for the artificial changes to the observed surface is the effect of impact energy induced during the conductive coating process. Usually, metals such as gold, palladium, chromium, platinum, and carbon and their mixtures are used for the conductive coating [1] by the following methods: (i) Magnetron sputter coating: This is the most popular way of applying a conductive layer on a nonconductive specimen. In this method, the sputter source (metal) and the sample are located in a common vacuum chamber. The pressure in the chamber is kept at about 10 Pa and a noble gas such as argon or xenon is introduced into the chamber. (ii) Ion beam sputtering coating: In this procedure the sample is placed under a much higher vacuum (8
103 Pa). An ion source generates a directed ion beam, which hits the target material, ejecting the atoms from the solid surface of the target material toward the specimen surface.

62 CHAPTER 3 RO Membrane Characterization

(iii) Penning sputter coating (PSC): It combines plasma generation and ejection of target material in one piece of equipment. From the source, a directed beam of neutral particles is emitted with energy comparable to the kinetic energy of an ion beam sputter coater. In addition, the emitted beam is directed through an electrical field, which filters out charged particles. Sample is maintained under high vacuum and must be kept in motion for a continuous coating. Using neutral particle source thin coatings are possible, even at room temperature. (iv) Electron beam evaporation: Evaporation through heating of the target material in a high vacuum is a reliable (especially in the TEM) and a widely used procedure. The directed beam of uncharged particles hits the sample with low kinetic energy. The same conditions concerning the geometry of the coating are valid in the ion beam and penning sputter procedures. Due to rapid development of computer hardware, image processing is now a useful technique to measure the morphological parameters quantitatively. Particularly, ImageJ software is widely used to measure the pore size and the pore size distribution [2–8]. How to prepare membrane sample without any artifacts is the problem for SEM and TEM studies. Careful drying is the first step to prepare a membrane sample, and in order to avoid collapse of the original structure, the freeze drying technique by using liquid nitrogen or critical-point drying method with carbon dioxide is usually employed. In order to observe cross sections by SEM the dried membrane is first fractured at liquid nitrogen temperature, and fixed perpendicularly to the sample holder. For TEM study, the dried sample is first embedded, if necessary, and then cut by a microtome. An example of the artifacts introduced in the process of conductive coating was shown by Schossig-Tiedemann and Paul [1]. When imaging sputtercoated polymer membranes, nodular structures can be observed on the membrane surface [9]. Fig. 3.4 shows the surface of an uncoated porous polyetherimide membrane studied by using a DSM 682 Gemini microscope with E ¼ E2 at 1 kV. The magnetron sputtered surfaces, represented in Figs. 3.5 and 3.6 were prepared at room temperature, and at liquid nitrogen temperature, respectively. The nodule size reduces with a decrease in the preparation temperature. It was concluded by Schossig-Tiedemann and Paul that for polymer samples with structural elements on a submicron scale, the surface structure can be significantly altered by energy impacts resulting from the preparation method. The recent developments in the area of conductive coating should suppress surface charging, minimize radiation

n FIG. 3.4 PEI-membrane, native specimen surface.

n FIG. 3.5 PEI-membrane, magnetron sputtered at room temperature.

64 CHAPTER 3 RO Membrane Characterization

n FIG. 3.6 PEI-membrane magnetron sputtered at 143 K.

damage, and increase electron emission from the surface. It was shown by them that the selection of a suitable coating procedure could lead to significantly improved results in the SEM images. Energy dispersive X-ray spectrometer (EDX) is used for elemental analysis of a sample often coupled with SEM. When the electron beam hits the sample, the electron at the inner shell of the sample atom is ejected and the electron at the outer shell fills the vacant hole. The difference in the high-energy inner shell and the low-energy outer shell is emitted in the form of X-ray. The number and the energy of the X-ray are measured by the energy dispersive spectrometer, giving information on the elemental composition of the sample (see also below EDX). The SEM equipped with EDS detector (EDAX) was used to image the surface and cross sections of freeze-dried membranes and to identify the elemental compositions and distribution of inorganic particles present on the surface of cake layer formed during fouling process in RO membranes [10].

3.2 Characterization by Membrane Morphology 65

Perreault et al. [11] studied the strong antimicrobial properties of thin-film composite polyamide membranes by a simple graphene oxide surface functionalization. Surface binding of graphene oxide was demonstrated by SEM and Raman spectroscopy (RS). The SEM can be applied to study membrane’s problems such as swelling, asymmetry, void size, and the morphological changes attendant upon hydrolysis [12].

3.2.2 Transmission Electron Microscopy (TEM) The TEM is a microscopic technique in which an image is formed when a beam of electrons is transmitted through an ultrathin specimen, interacting with the specimen as it passes through it; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or detected by a sensor such as a charge-coupled device (CCD) camera. The TEM specimens are required to be at most hundreds of nanometers thick. It is achieved by slicing the membrane sample using a microtome. Inukai et al. [13] synthesized high-performance reverse osmosis (RO) composite thin membrane using multiwalled carbon nanotubes (MWCNTs) and aromatic polyamide (PA) by interfacial polymerization. The microstructure of the MWCNTPA nanocomposite membrane was studied by using highresolution transmission electron microscopy (HRTEM). It was noticed that traces of PA were attached to the surface of MWCNT. It suggested that there was a good interaction between the monomers and the MWCNT walls, forming an ordered region of several nanometers thick, in agreement with the SEM observation. By conducting fast Fourier transformation (FFT) of TEM images it was shown that order of the polymer network (pattern) along the nanotubes surfaces might represent a unique aromatic PA structure when compared to the bulk PA. Kurihara et al. developed a novel low-pressure polyamide RO membrane via in situ interfacial polymerization in a project to enable production of 1,000,000 m3 of freshwater from seawater per day. The TEM image of the membrane in Fig. 3.7 shows the detail of the leaf-like structure at the surface of the TFC membrane. Each “leaf” consists of a thin layer with subnanometer pores which surrounds a void space. This structure provides an increased surface area, thus enhancing the water flux, while ensuring the high salt separation [14].

66 CHAPTER 3 RO Membrane Characterization

n FIG. 3.7 TEM image of the surface of Toray low-pressure RO membrane.

3.2.3 Atomic Force Microscopy (AFM) Nowadays AFM is a very common tool to study membrane surfaces. The AFM is used to find the roughness, nodule size, pore size, pore size distribution, and phases on the surface of RO membranes. In some cases roughness is directly proportional to flux. Surface property of RO membranes affecting membrane fouling includes both chemical and physical characteristics. It is believed that membranes with smoother surface are favorable for reducing membrane fouling caused by particulate and organic matters. The AFM consists of a cantilever which has a sharp tip (probe) with a radius of curvature in the nanometer range at its end to scan the membrane surface. When the tip is brought into proximity of a sample surface, a force, which depends on the distance between the tip and the sample, causes a deflection of the cantilever. The deflection is monitored by the laser beam reflected from the cantilever and kept constant by a feedback mechanism so that the force (and the distance) between the tip and the sample surface remain constant. When the tip is moved sideways it will follow the surface contours such as the trace B in Fig. 3.8. The feedback output equals the topology of the sample surface.

Cantilever tip

Front atom B A

n FIG. 3.8 Mechanism of AFM.

3.2 Characterization by Membrane Morphology 67

When the tip is in direct contact with the sample surface, it is called “direct contact mode.” But this mode is no longer popular since the force between the cantilever and the surface may cause damage to the surface of soft materials such as polymer. Hence, the cantilever is kept some distance away from the surface, which is called “noncontact mode.” Currently, another method called “tapping mode” is very popular. In this mode a fast oscillating probe is used for surface imaging and a short, intermittent sample contact prevents the development of inelastic surface deformation. The vertical separation between the probe tip and the surface is rapidly oscillated such that the probe taps the surface lightly. The discontinuous contact eliminates any lateral forces exerted on a surface by the scanning tip. The surface topology is usually expressed as roughness. Three roughness parameters; that is, the mean roughness (Ra), the mean square of the Z data (Rq), and the mean difference in height between the five highest peaks and the five lowest values (Rz) are used to represent the surface roughness. The roughness parameters depend on the curvature and the size of the TM-AFM tip, as well as on the treatment of the captured surface data (plane-fitting, flattening, filtering, etc.). Therefore, the roughness parameters should not be considered as absolute roughness value. The mean roughness is the mean value of the surface relative to the center plane for which the volumes enclosed by the images above and below this plane are equal, and is given by Ra ¼

1 Lx Ly

ð Lx ð Ly 0

jf ðx, yÞjdxdy

(3.1)

0

where f (x, y) is the surface relative to the center plane and Lx and Ly are the dimensions of the surfaces. The root mean square of the Z values (Rq) is the standard variation of the Z values within the given area and is calculated by Rq ¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X 2 Zi  Zavg N

(3.2)

where Zi is the height of ith pixel, Zavg the average of Zi in the given area, and N is the number of points within the given area. The size of the pore at the membrane surface is determined from the crosssectional profiles of the data along a reference line. An example of the measurements of pore diameters is shown in Fig. 3.9. The bright sites represent the unit of polymer structure called nodules and the dark sites are interstitial domains between nodules, which are often considered as “pores.” For each

20.0

68 CHAPTER 3 RO Membrane Characterization

Section analysis

nm

B

A

C

−20.0

0

L RMS lc Ra (lc) Rmax Rz Rz Cnt 0

0.25

0.75

0.50 μm

1.00 Horiz distance (L) Vert distance Angle Horiz distance Vert distance Angel Horiz distance Vert distance Angle Spectral period Spectral freq Spectral RMS amp

Spectrum

Top 65 C, Ra = 5,348 nm

141.72 nm 2.658 nm DC 2.113 nm 7.656 nm 7.656 nm 2

DC

141.72 nm 2.652 nm 1.072 dey 114.17 nm 0.466 nm 0.234 deg 78.735 nm 1.601 nm 1.165 deg DC 0 Hz 0.046 nm

Min

n FIG. 3.9 Section analysis of a TM-AFM image: a vertical displacement of the top surface of the dense PPO-TCE membrane (membrane was prepared by casting PPO

solution in trichloroethylene). A, B, C show the pair of cursors of each measurement.

pair of cursors (pointers), the horizontal and vertical distances are as shown in the figure. The diameter of the “pores,” that is, maximum width of the cross section of the dark site, can be measured with the help of a pair of cursors, as indicated in Fig. 3.9. By measuring the diameters of a large number of dark sites (at least 25), average pore size is obtained. However, these “pores” are not necessarily open ended. Fang and Duranceau [15] studied aromatic polyamide TFC RO membranes and a cellulose acetate (CA) NF membrane and correlated their flux decline rate with the surface roughness measured by AFM. The study showed that the higher productivity decline rates associated with polyamide RO membranes as compared to that of a CA NF membrane was due to the inherent ridge-and-valley morphology of the active layer of the TFC membrane. The unique polyamide active layer morphology was directly related to the surface roughness.

3.2 Characterization by Membrane Morphology 69

3.2.4 Positron Annihilation Lifetime Spectroscopy (PALS) Positron annihilation lifetime spectroscopy (PALS) technique can provide an unprecedented level of insight to our understanding of the internal structure of the active skin layer of RO membranes. It is capable of determining the free volume (FV) and hole properties directly at the atomic and nanoscale. Study of FV in polymer system is of great interest because the size and concentration of its elements (holes) affect numerous transport, mechanical, and other physiochemical properties of polymers. The foundations of this technique for probing polymers were based in particular on Walker-Brandt-Berko’s FV model [16]. According to this model, positronium (Ps) (bound atomic system which consists of an electron and a positron) tends to be localized or trapped before its annihilation in FV or, in other words, in areas with reduced electron density. Accordingly, annihilation characteristics (lifetimes and intensities of longer lifetime components of annihilation radiation) give information on concentrations, sizes, and distribution of FV [17, 18]. Positron annihilation lifetime spectra can be analyzed in terms of three lifetime components [19], viz., para-positronium (p-Ps) annihilation, τ1; free positron and positron-molecular species annihilation, τ2; and orthopositronium (o-Ps) annihilation, τ3. While τ1 and τ2 are of the order of few 100 ps, τ3 is of the order of nanoseconds. Each lifetime has intensity corresponding to the fraction of annihilations taking place with the respective lifetimes. The parameters τ3 and I3, corresponding to decay of o-Ps, provide the size specific for FVs and pores. By using the results of o-Ps lifetime, the mean FV hole radius can be calculated by following equation [19, 20]: τ3 ¼

   1 R 1 2πR 1 1 + sin 2 R0 2π R0

(3.3)

where τ3 (o-Ps life-time) and R (hole radius) are expressed in ns (nanosec˚ ), respectively, Ro is equal to R + Δ R, where Δ R is onds) and
1010 m (A ˚ )]. a fitted empirical electron layer thickness [¼ 1.66
1010 m (1.66 A The cavity volumes can be calculated from Vh ¼ 4лR3/3. Further, the fractional FV f may be estimated from the following empirical relation [21]: f ¼ CV F l3

(3.4)

where VF is FV, l3 the relative intensity of o-Ps lifetime component, and C is the scaling factor; C can be determined from the variation in FV with

70 CHAPTER 3 RO Membrane Characterization

temperature. However, in the absence of such data, it may be typically assigned a value of 1.0 [22], in that case the values of f obtained will be proportional to the actual FV fraction. ˚ ) and small scale of probe Because Ps has a relatively small size (1.59 A lifetime (ns), PALS is very sensitive in measuring small holes and FV ˚ and at a time of molecular motion from 1010 s in a size range of 1–20 A and longer. The positrons and Ps are localized in preexisting holes and FVs in polymers. Therefore, the measurements depend on the function of the temperature, pressure, degree of crystallinity, and time of aging [23]. Kim et al. [24] for the first time showed that the thin films of cross-linked aromatic polyamide RO membranes are composed of two types of pores; ˚ were detected from the τ3 lifethat is, pores with radii of about 2.1–2.4 A ˚ time component and those with 3.5–4.5 A radii from τ4 component. They have identified the former pores as the network pore and the latter as aggregate pore, and this study was applied to explain the flux-enhancement mechanism in thin-film composite (TFC) membranes.

3.2.5 Neutron Scattering (NS) Neutron diffraction or elastic neutron scattering (NS) is the application of NS for the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of the structure of the material. The technique is similar to X-ray diffraction but due to their different scattering properties, neutrons and X-rays provide complementary information. As well, since neutrons are electrically neutral, they penetrate matter more deeply than electrically charged particles of comparable kinetic energy; therefore they are valuable probes of bulk properties. Small-angle NS is in many respects very similar to small-angle X-ray scattering (SAXS); both techniques are jointly referred to as small-angle scattering (SAS). Advantages of SANS over SAXS are its sensitivity to light elements, the possibility of isotope labeling, and the strong scattering by magnetic moments. There are numerous SANS instruments available worldwide at Neutron Facilities such as research reactors or spallation sources. NS is routinely used in modern science to understand material properties on the atomic scale. The technique of SANS is used for studying the structure of a ˚ . For the polymeric membrane material on length scale of 10–1000 A research, SANS is a powerful technique to understand the polymer chain nanostructure as well as the pore structure characteristics of the membrane in order to improve the membrane performance.

3.2 Characterization by Membrane Morphology 71

Singh and Aswal [25] studied the typical polyamide TFC RO membrane by SANS. It was observed that membrane was composed of nanoscale building blocks. Dahdal et al. [26] used SANS to understand the mechanism of fouling of RO membranes, mainly biofouling and scaling by calcium phosphate.

3.2.6 Electron Paramagnetic Resonance (EPR) Spectroscopy The EPR, also called electron spin resonance (ESR), is a technique used to study chemical species with unpaired electrons. When an atomic or molecular system with unpaired electrons is subjected to a magnetic field, the electronic energy levels of the atom or molecule will split into different levels. The magnitude of the splitting depends on the strength of the applied magnetic field. The atom or molecule can be excited from one split level to another in the presence of an external radiation of frequency corresponding to the frequency obtained from the difference in energy between the split levels. Such an excitation is called a magnetic resonance absorption. The atom or molecule under investigation may have been in different environments in the actual sample. The magnetic resonance frequency will hence be influenced by the local environment of the atom or molecule. The ESR technique is therefore, a probe for a detailed identification of the various atomic and molecular systems and their environments and all associated parameters. Unlike nuclear magnetic resonance (NMR) spectra, where absorption is recorded directly, ESR spectrometers plot the first derivatives of the absorption curve [27]. It can be seen from quantum mechanics that every electron acts as a magnetic dipole. If it is placed in a static magnetic field, H, it can have only two possible orientations, with or against the field. In thermal equilibrium the greater the number of electrons occupy the lower the energy levels according to the Boltzmann statistics. In the presence of an electromagnetic variation field, the electrons in the lower energy levels can absorb photons of energy, hν (h is the Planck constant and ν the frequency) and thus are excited to a higher energy level which corresponds to the energy absorbed. The absorption energy can be observed as a function of external applied field value by means of high-frequency technique. The relation between the field value and the measuring frequency can be described quantitatively as follows: hν ¼ g βH

(3.5)

where g is the spectroscopic splitting factor or Lande’s constant and β is the Bohr constant. The g value of a free electron is 2.0035. The magnetic

72 CHAPTER 3 RO Membrane Characterization

interactions between the electron spins and nuclear spins cause the ESR spectrum to comprise of a number of lines rather than a single line. The arrangement of the resulting group of lines in the ESR spectrum is called the hyperfine structure of the spectrum. A stable radical can also be introduced into polymeric material. The radical, so introduced, is often called a spin label or a spin probe. It is invariably a nitroxide radical, which exhibits a three-line hyperfine structure. The peak shape and splitting depend on the radical’s environments. The nitroxide label is a monitor of motion. The shape of the ESR signal depends also on the orientation of the magnetic field relative to the axis of the radical. Thus, the spin label method is useful to study the environment of radicals at a molecular level. Khulbe et al. [28] studied the structure of the skin layer of asymmetric CA RO membranes with TEMPO [(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl] as a spin probe. It was observed that the mobility of TEMPO in the asymmetric membrane shrunk at 90°C was the same as TEMPO in a dense homogeneous membrane prepared from the same casting solution. Thus, the authors reported the following conclusions: (i) The pore sizes of the asymmetric membranes are larger when they were shrunk at lower temperatures. (ii) The space in the polymer network (the origin of the network pore) in the dense film was smaller when no swelling agent is added to the casting solution. (iii) The space in the polymer network in the dense film was smaller when the membrane was dry. Khulbe et al. [29] reported the EPR study on the structure and transport of asymmetric aromatic polyamide membranes for RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like CA membranes. A comparison was made with CA RO membrane. It was suggested that the EPR technique can be used to study the structure of RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.

3.2.7 Wide-Angle X-Ray Scattering (WAXS) and Small-Angle X-Ray Scattering (SAXS) Knowledge of crystalline morphology of polymeric RO membranes is not as important as that of the membranes for gas separation since polymeric membranes are highly swollen when they are in contact with water. But when a

3.2 Characterization by Membrane Morphology 73

linear polymer such as cellulose triacetate is used as the membrane material, increase in crystallinity may cause deterioration of the membrane performance. As well when inorganic compounds or metal organic frameworks (MOFs) are incorporated as fillers of the thin-film nanocomposite (TFN) membranes, the crystallinity of the filler is often examined by X-ray diffraction analysis. There are two kinds of this method: SAXS and wide-angle X-ray scattering (WAXS). The SAXS is a SAS technique where the elastic scattering of X-rays (wavelength 0.1 … 0.2 nm) by a sample which has inhomogeneities in the nm range is recorded at very low angles (typically 0.1–10°). The technique for WAXS is the same as SAXS. Only the distance from the sample to the detector is shorter and thus diffraction maxima at larger angles are observed. Depending on the measurement instrument used it is possible to do WAXS and SAXS in a single run (small- and wide-angle scattering, SWAXS). The WAXS is often used to determine the crystalline structure of polymers. The sample is scanned in a wide-angle X-ray goniometer, and the scattering intensity is plotted as a function of the 2θ angle. When X-rays are directed into solids, they will scatter in predictable patterns based on the internal structure of the solid. A crystalline solid consists of regularly spaced atoms (electrons) that can be described by imaginary planes. The distance between these planes is called the d-spacing. The intensity of the d-space pattern is directly proportional to the number of electrons (atoms) that are found in the imaginary planes. Every crystalline solid will have a unique pattern of d-spacing (known as the powder pattern), which is a “finger print” for that solid. In fact, solids with the same chemical composition but different phases can be identified by their pattern of d-spacing. The crystalline phase is also detected in polymer. In general, the crystalline phase may be regarded as impermeable, so that molecular diffusion in a semicrystalline polymer membrane is substantially lower than the more amorphous membrane because of the reduced space available for diffusion and the winding path around the crystallites. Permeation sites may be of either amorphous material or interstices between crystallites. Most polymeric materials used for RO membranes are of low or no crystallinity. Tamlin [30] successfully used grazing incidence small-angle X-ray scattering and wide-angle X-ray scattering (GISAXS and GIWAXS) to study the morphology of polyamide TFC RO membrane. Cruz-Silva et al. [31] made a comprehensive study of the chemical and physical effects of carbon nanotubes on the fully cross-linked polyamide network. The microstructure of the nanocomposite membrane was studied by SAXS and WAXS, HRTEM, and molecular dynamics.

74 CHAPTER 3 RO Membrane Characterization

3.3 CHARACTERIZATION BY MEMBRANE SURFACE CHEMISTRY 3.3.1 Fourier Transform Infrared SpectroscopyAttenuated Total Reflection (FTIR-ATR) Infrared (IR) spectroscopy is the subset of spectroscopy that deals with the IR region of the electromagnetic spectrum. It covers a wide range of techniques, the most common being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify compounds or investigate sample composition. A common laboratory instrument that uses this technique is an IR spectrophotometer. Fig. 3.10 shows schematic diagram of the IR spectroscopy. A beam of IR light is produced and split into two separate beams. One passes through the sample, the other passes through a reference which is often the substance the sample is dissolved in. Both the beams are reflected back toward a detector, however, first they pass through a splitter which quickly alternates the two beams before entering the detector. The two signals are then compared and a printout is obtained. A reference is used for two reasons: (1) This prevents fluctuations in the output of the source affecting the data. (2) This allows the effects of the solvent to be canceled out. (The reference is usually a pure form of the solvent the sample is in.) The IR spectrum of a sample is recorded by passing a beam of IR light through the sample. Examination of the transmitted light shows how much

Sample

IR source

Splitter Detector

Ref.

n FIG. 3.10 Typical FTIR apparatus.

Processor

Printout

3.3 Characterization by Membrane Surface Chemistry 75

energy was absorbed at each wavelength. This can be done with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all wavelengths at once. From this, a transmittance or absorbance spectrum can be produced, showing IR wavelengths at which the sample absorbs. Analysis of these absorption characteristics shows details about the molecular structure of the sample. This technique works almost exclusively on samples with covalent bonds. The modern instruments are FTIR instruments. The FTIR spectroscopy is a powerful tool for identifying types of chemical bonds in a molecule by producing an IR absorption spectrum that is like a molecular fingerprint. Thus, FTIR spectroscopy is a measurement technique for collecting IR spectra. Instead of recording the amount of energy absorbed when the frequency of the IR light is varied (monochromator), the IR light is guided through an interferometer. After passing through the sample, the measured signal is the interferogram. Performing a Fourier transform on this signal data results in a spectrum identical to that from conventional (dispersive) IR. The FTIR is perhaps the most powerful tool for identifying types of chemical bonds (functional groups). The wavelength of the light absorbed is characteristic of the chemical bond as can be seen in the attached IR spectroscopy correlation (Fig. 3.11). Today’s FTIR instruments are computerized, which makes them faster and more sensitive than the older dispersive instruments. The FTIR spectroscopy has been employed for analyzing microbial aggregates on membrane surfaces and can provide information about the chemical nature of the fouling layer. It allows one to distinguish the different kinds of fouling on the same membrane but cannot provide information about biofilm thickness [32]. In Fourier transform infrared spectroscopy-attenuated total reflection (FTIR-ATR), IR light reflects multiple times at the sample-crystal interface while traveling through the crystal that is placed under the surface (Fig. 3.12). The reflection forms evanescent wave that penetrates into the sample to the depth of 0.5–2 μm. Therefore, FTIR-ATR is often used for the analysis of the membrane surface. C C 4000

N

H

O

H 3200

X-H Attached to heteroatoms

4000

n FIG. 3.11 Infrared spectroscopy correlation.

2800

N C

Triples 2,380 CO2

O C

2300 2100

C-H

3000

C

C N

1800

C 1500

Doubles

2000

Finger print Singles

1460, 1380 nujol

cm-1

1000

76 CHAPTER 3 RO Membrane Characterization

n FIG. 3.12 Image of FTIR ATR (from FTIR-ATR Wikipedia).

Poly(arylene ether sulfone)/modified silica nanocomposite RO membrane, synthesized by Kim et al. was characterized by FTIR spectra, SEM, and thermogravimetric analysis (TGA) [33]. The study showed that in hyperbranched aromatic polyamide-grafted silica (HBP-g-silica), amine groups were grafted onto the silica surface, and amide bonds were successfully conjugated between a PES and an HBP-g-silica.

3.3.2 Auger Electron Spectroscopy (AES) In Auger electron spectroscopy (AES) the sample is bombarded with a highenergy electron beam. This causes the removal of core electron and as a consequence the upper level electron falls to the lower level and the third electron, called Auger electron, is excited by the energy released. The energy of the Auger electron is characteristic of the elements. Thus the elements of the sample and the chemical status of the sample are identified from the obtained spectrum. The depth of analysis is a few monolayers. Therefore, the information is obtained for the upmost surface layers. Polymeric membranes can be damaged due to bombardment with high-energy electron beams. This problem can be avoided when AES is performed with XPS [34]. Since the surface is damaged by AES analysis due to the surface ionization, AES is not used very much for membrane characterization [35].

3.3.3 X-ray Photoelectron Spectroscopy (XPS) The XPS, sometimes called electron spectroscopy for chemical analysis (ESCA), is used to study elemental compositions of composite RO membranes near the surface. This technique supplied verification of the polymer chemical structures expected from the interfacial polymerization reactions that formed the membranes. When irradiated by a monoenergetic beam of X-photons, atoms of the solid emit electrons. By measuring the energy of emitted electrons, EK, the binding energy EB is obtained by subtracting the EK from the energy of the

3.3 Characterization by Membrane Surface Chemistry 77

incident X-photon. The binding energy of the core electron is specific to an atom. Therefore, the identification and quantification of atoms become possible. A schematic illustration of the angle resolved-XPS (AR-XPS) is shown in Fig. 3.13: the angle (θ) between the normal to the sample surface and the electron trajectory into the detector is defined as the take-off angle (TOA). The effective sampling depth, z, can be calculated by z ¼ 3λ cosθ

(3.6)

where λ is the effective mean path for electrons to escape the surface. Using λ ¼ 2.1 nm, z ¼ 6.3 nm at θ ¼ 0°, and 3.15 nm at θ ¼ 60°. According to this principle, the identification and quantification of atoms in few nm depth from the top surface are possible by XPS [37]. This method is widely used to provide quantitative and qualitative chemical information of the top 1–20 nm of surface. Generally, the samples are analyzed at a series of TOAs (measured from the surface sample to the X-ray lens) to determine whether a compositional gradient exists near the surface. Since membrane surface modification is currently a popular research topic, XPS is frequently used to determine the surface composition of membrane. For example, migration of fluorine-containing surface-modifying macromolecules (SMMs) to the top surface of the polyethersulfone (PES) membrane during membrane casting was confirmed by determining the surface concentration of fluorine by XPS [36, 38, 39]. The XPS results further showed that the orientation of the SMM was such that the fluorine tails were present at the surface. Chitosan membranes after oxygen plasma treatment were characterized by an angle-resolved X-ray photoelectron spectrometer (ARXPS) to study the spatial orientation of surface chemical group [40].

n FIG. 3.13 Schematic diagram showing the setup of AR-XPS used at different take off angles [36].

78 CHAPTER 3 RO Membrane Characterization

Wagner et al. [41] modified the polyamide RO [XLE, polyamide thin-filmcomposite membranes manufactured by Dow Water & Process Solutions (Edina, MN) extra low energy] by grafting poly(ethylene glycol) (PEG) diglycidyl ether (PEGDE) to their top surfaces from aqueous solution to improve fouling resistance. The XPS was used to characterize the surface elemental content of modified and unmodified membranes. The XPS indicated the qualitative evidence of the presence of PEGDE on the membrane surface and the thickness of PEGDE layer. Further, XPS results were qualitatively consistent with the ATR-FTIR results.

3.3.4 Energy Dispersive X-Ray Spectroscopy Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS), sometimes called energy dispersive X-ray analysis (EDXA) or energy dispersive X-ray microanalysis (EDXMA), is an analytical technique used for the elemental analysis or chemical characterization of a sample. The EDS analysis can be used to determine the elemental composition of individual points or to map out the lateral distribution of elements from the imaged area. It can also be used to obtain compositional information on quasi-bulk specimens (low SEM magnification, high accelerating voltage) or on specific particles, morphologies, or isolated areas on filters or within deposits. The EDS has an analytical capability that can be coupled with several applications including SEM, TEM, and scanning transmission electron microscopy (STEM). Beverly et al. [42] demonstrated that EDS, combined with XPS, FTIR, and SEM, is a valuable diagnostic tool for the analysis of polymeric RO membrane failure and provides valuable information to aid the manufacturers in designing better membranes for RO.

3.3.5 Raman Spectroscopy (RS) The IR and Raman spectra of a molecule complement each other and information on the complete vibrational spectra of a molecule often requires both IR and Raman vibration. In RS, light scattered by the molecules contains frequencies other than that of incident monochromatic light. It is the differences between the frequencies of scattered light and the frequency of the incident light that correspond to the normal vibrational frequencies of the molecules. When a molecule absorbs radiations its energy increases in proportions to the photon. E ¼ hv ¼ hc=λ

(3.7)

where c is the velocity of light; h the Planck constant; λ the wavelength of the radiation; and ν is the frequency. The increased energy may be at the level of the electronic, vibrational, or rotational energy of the molecule.

3.3 Characterization by Membrane Surface Chemistry 79

Interaction between the molecular units and their surroundings can be readily detected by perturbation in the Raman spectra. An important area of research in polymer involves how the physical and mechanical properties of a polymer are influenced by molecular orientation induced by drawing. Polymers are varied between amorphous and crystalline state. The crystallinity content depends on the molecular weight of the polymer. Low-molecular weight polymer will have a high content of crystallites in comparison with high-molecular weight polymer. Various morphologies are possible between a completely crystalline and completely amorphous conformation. The formation of a crystalline region depends on the time allowed for the polymer to crystallize from the solution. Crystalline polymers have a number of morphological features that can be studied by RS. Polymeric membranes are processed in a variety of ways (casting of solutions by doctor blade or spinning, etc.), and thus it is fairly common that some orientation, either in plane or out of plane, will be induced. The morphology of the membrane, including the orientation of polymer chain and the degree of crystallinity of the polymer in the membrane can be studied by RS. It also enables monitoring of the morphology change during membrane formation. The morphology-membrane transport relationship will help to reveal the mechanism of mass transport of the membrane. The RS may also contribute to establish the transport of permeating molecules, either in gas or liquid form. However, so far no work has been done to correlate RS and permeation properties of membrane. The RS studies on polymeric membrane appear to hold a great potential in the future for many different applications. Study of crystalline structure of polymers will add to the existing knowledge of intramolecular forces in crystals and their effect on stable polymer structure. The use of vibration spectroscopy will help to understand the relationship between the structure and transport properties, which could have an important impact on designing membranes for specific separation problems. The RS study of isotropic polymer membranes can be related to the permeability by using FV as an intermediary property. However, no such work has been carried out so far. Kim et al. [43] studied spatial distribution of carbon nanotubes (CNTs) in the polyamide membranes, which were used for RO. Raman spectroscopic mapping has been utilized to visualize spatially the distribution of CNTs or other nanomaterials in other matrix. Cui et al. [44] showed that surface-enhanced Raman spectroscopy (SERS) could be used as a new and versatile tool for examining the fouling of protein on polyvinylidene fluoride (PVDF) membranes. The fouled area can be visualized by a combination of Raman mapping and silver staining.

80 CHAPTER 3 RO Membrane Characterization

3.3.6 Scanning Transmission X-ray Microscopy (STXM) This can be used for examining hydrated biofilms (in RO fouling) due to the ability of soft X-rays to penetrate water. Lawrence et al. [45] have used scanning transmission X-ray microscopy (STXM), confocal laser scanning microscopy (CLSM), and TEM to map the distribution of macromolecular subcomponents (e.g., polysaccharides, proteins, lipids, and nucleic acids) in a biofilm and demonstrated that this combination of multi-microscopy analysis can be used to create a detailed correlative map of biofilm structure and composition. Thus, it can help to understand the chemistry of fouling.

3.4 OTHER CHARACTERIZATION TECHNIQUES 3.4.1 Nuclear Magnetic Resonance (NMR) Similar to the unpaired electron in EPR, nuclei of certain atoms at the lower energy level can absorb the energy to be excited to the higher energy level, when external radiation of frequency corresponds to the frequency obtained from the difference between the higher and lower energy levels. Hydrogen is often used to generate detectable radio frequency signal. This principle is used in radiology to produce a 3-D medical imaging, which is called magnetic resonance imaging (MRI). A similar technique is used to produce the 3-D image of RO module without dissecting the module. Graft von der Schulenburg et al. [46] demonstrated the application of NMR to a spiral wound RO membrane module to understand the key design and operational parameters influencing biofilm fouling in RO membrane module. From the NMR data they were able to quantify an effective membrane surface area. From the NMR data they studied the extraction of (i) the spatial biofilm distribution in the membrane module, (ii) the velocity field and its evolution with biofouling, and (iii) displacement of propagators, which are distributions of molecular displacement of a passive tracer (e.g., water) in the membrane. Fridjonsson et al. [47] demonstrated that the use of Earth’s field (EF) NMR can provide early nondestructive detection of active biofouling of a commercial spiral wound RO membrane module. Solid-state NMR (SSNMR) spectroscopy is a kind of NMR spectroscopy characterized by the presence of anisotropic (directionally dependent) interactions. It is widely applicable for the investigation of noncrystalline or amorphous materials including polymers and polymeric membranes [48–50]. One area of solid-state NMR spectroscopy that has proven fruitful with regard to the investigation of membranes is 2H NMR spectroscopy.

3.4 Other Characterization Techniques 81

Even though the solid-state NMR is not as popular as other characterization methods, it can provide a large amount of information about local structure around selected atoms/nuclei and can be extensively used in the studies of new inorganic, organic and hybrid materials [51].

3.4.2 Photoacoustic Spectroscopy (PAS) Photoacoustic spectroscopy (PAS) is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. The absorbed energy from the light causes local heating and through thermal expansion a pressure wave or sound. A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths of the light. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solids, liquids, and gases [52]. The major advantage of PAS is that it is suitable for highly absorbing samples. Flemming [53] suggested that PAS can be used for monitoring the biofilm formation on the membrane during RO process. Schmid et al. [54] used PAS as a new biofilm-monitoring technique. The PAS combines features of optical spectroscopy and ultrasonic tomography and allows a depth-resolved analysis of optically and acoustically inhomogeneous media.

3.4.3 Differential Scanning Calorimetry (DSC) During a change in temperature, DSC measures a heat quantity, which is radiated or absorbed excessively by the sample on the basis of a temperature difference between the sample and the reference material. The DSC is particularly useful to identify the glass transition temperature of polymer where the polymer phase changes from glassy to amorphous or vice versa. The phase separation of polymer blend is also detected by the presence of multiple glass transition temperatures. The melting point can also be measured by DSC. The presence of water with various melting points was also confirmed by using DSC [55].

3.4.4 Thermogravimetric Analysis (TGA) By measuring the weight loss vs temperature TGA can provide information on the amount of absorbed water in the polymeric membrane, the polymer decomposition temperature, and the decomposition weight loss. It is particularly important to know the thermal stability of polymeric membranes.

82 CHAPTER 3 RO Membrane Characterization

Kim et al. [33] synthesized composite RO membranes from sulfonated poly(arylene ether sulfone) containing amino groups (aPES) and HBP-gsilica with the aim of enhancing chlorination resistance and improving membrane performance. With the TGA analysis of HBP-g-silica it was observed that amine groups were grafted onto the silica surface, and amide bonds were successfully conjugated between aPES and HBP-g-silica. Mohan and Kullova´ [56] characterized the high-performance thin-film composite polyamide membranes by TGA for inorganic solute separation which were prepared by the interfacial polymerization of trimesoyl chloride (TMC) with diethylenetriamine, 1,3-cyclohexanebis(methylamine), 2,3-diaminopyridine (DAP), m-phenylenediamine (MPD), piperazine (PIP), or a mixture of MPD and PIP/DAP, on the surface of a reinforced microporous PES membrane support.

3.4.5 Contact Angle Measurement The contact angle measurement is performed to determine the surface hydrophilicity (wettability by water) of RO membrane with the intention of predicting membrane performance or fouling potential. The contact angle is the angle between the tangential to the liquid surface and the solid surface at a point where a liquid-vapor interface meets a solid surface (see Fig. 3.14). Usually water is used as liquid for the measurement of contact angle at the RO membrane surface. It quantifies the wettability of a solid surface by the liquid via the Young equation. γ SG  γ SL  γ LG cosθC ¼ 0

(3.8)

where θC is the contact angle, γ the interfacial tension, and the subscripts SG, SL, and LG represent the interface between solid and vapor, solid and liquid, and liquid and vapor, respectively (see Fig. 3.14). The measurement is done by sessile drop method using a goniometer. Contact angle of membrane is basically decided by the building material of the membrane. However, in practice contact angle hysteresis is observed, ranging from the so-called advancing (maximal) contact angle to the gLG qC gSL n FIG. 3.14 Contact angle.

gSG

3.4 Other Characterization Techniques 83

receding (minimal) contact angle. As well it may widely vary depending on chemical additives, solvents, manufacturing method, process condition, surface roughness, etc.

3.4.6 Zeta Potential Measurement Zeta potential of the membrane is determined by measuring streaming potential. Streaming potential is an electrical potential that originates when an electrolyte solution is driven through a porous material or through a channel with charged walls. To measure the streaming potential of RO membrane an electrolyte solution of known pH is driven through a channel formed between two RO membranes whose active layers are facing the solution. Two electrodes are placed at both ends of the channel and the potential is measured at different pressures. Zeta potential is an important tool to measure the electrical charge of the membrane surface. The zeta potential, ζ, is then calculated by the following equation: ζ¼

dU η
κB
dp εε0

(3.9)

where U is streaming potential, p the pressure difference, ε and ε0 are relative permittivity of liquid and permittivity of vacuum, respectively, and kB is conductivity of an electrolyte. The zeta potential of the membrane often changes from positive to negative as the pH of the solution is increased, affecting the deposition of charged particles (fouling) on the membrane surface. Wagner et al. [57] measured the zeta potential of an RO commercial membrane grafted with PEGDE by using an Anton Paar Sur PASS Electrokinetic Analyzer and associated software (Anton Paar USA, Ashland, VA). Two membrane samples separated by a spacer were loaded into the clamping cell, creating a channel for the electrolyte flow. A 10-mM NaCl solution was used as the background electrolyte. Streaming potential was measured as a function of feed pH, and the Fairbrother-Mastin approximation was used for calculating zeta potential from streaming potential.

3.4.7 Graft Density RO membranes can be modified by grafting the active layer on the surface. Wagner et al. [57] measured the density of PEGDE on the membrane surface by using a Rubotherm Magnetic Suspension Balance (Rubotherm GmbH, Bochum, Germany). A diagram of the apparatus is shown in Fig. 3.15.

84 CHAPTER 3 RO Membrane Characterization

Balance

Electromagnet

Permanent magnet Sensor core and coil Measuring load decoupling

Sample pail

n FIG. 3.15 Schematic of the magnetic suspension balance used to characterize PEGDE grafting density.

3.4.8 Tensile Strength Measurement Tensile strength is defined as the ability of a material to resist a force that tends to pull it apart. It is usually expressed as the measure of the largest force that can be applied in this way before the material breaks apart. It is an important property of an RO membrane as it usually operates at high pressure. Tensile test is carried out using any standard testing equipment and the standard test method for the tensile properties of plastics, which are specified in ASTM D638-10. When a material is subjected to a tensile pull force, it undergoes elongation until it breaks. The results can be seen in the form of a curve showing how the material reacted to the forces being applied. The point of interest is the percent elongation of the material and the stress at the point of ultimate failure where the materials (membrane) break or snap. ASTM D882 is commonly used for testing the tensile strength of polymer films. A schematic general diagram is illustrated in Fig. 3.16. Ginga and Sitaraman [58] discussed a new fracture testing technique that can be used to determine the tensile strength of low-strength thin films. This technique uses finite element analysis to extract the tensile strength from the experimental data.

3.5 Summary of RO Membrane Characterization Methods 85

Force measurement Grips for holding specimen firmly

Fixed head

Test specimen

Fixed head

Thickness 1/8” Constant rate of motion n FIG. 3.16 Schematic diagram (main) for measuring the tensile strength of films.

3.5 SUMMARY OF RO MEMBRANE CHARACTERIZATION METHODS Table 3.1 presents the summary of recently developed RO membrane characterization methods. Table 3.1 Summary of RO Membrane Characterization Methods Characterization Method Transport RO experiment Membrane morphology Scanning electron microscope (SEM) Transmission electron microscope (TEM) Atomic force microscope (AFM) Positron annihilation lifetime spectroscopy (PALS) Small-angle neutron scattering (SANS) Electron paramagnetic spectroscopy (EPR)

What Can Be Studied by the Method To obtain the water flux and the solute rejection To have images of surface and cross-section. To evaluate pore sizes and pore size distribution at the membrane surface To study the structure of an untrathin layer. To know if the filler is intercalated or exfoliated. To know the polymer surrounding the filler To have 3D images of the surface. To obtain the roughness parameters. To obtain the size of pores and nodules To obtain the pore (free volume) size and its distribution To obtain the pore size and pore size distribution. To study the mechanism of fouling and biofoulin To study the environment of the spin probe

(Continued)

86 CHAPTER 3 RO Membrane Characterization

Table 3.1 Summary of RO Membrane Characterization Methods Continued Characterization Method

What Can Be Studied by the Method

Wide-angle X ray scattering (WAXS)

To obtain crystalline structure of nanoparticles. To obtain information on the amorphous and crystalline structure of the polymeric membrane. To measure d-spacing of the polymeric membrane To study inhomogeneities in nm range

Small-angle X ray scattering (SAXS) Membrane surface chemistry Fourier transform infrared spectrometry (FTIR)-attenuated total reflection (ATR)

Auger electron spectroscopy (AES) X-ray photoelectron spectroscopy (XPS) Raman spectroscopy (RS) Other techniques Nuclear magnetic resonance (NMR) Photoacoustic spectroscopy (PAS) Differential scanning calorimetry (DSC)

Thermogravimetric analysis (TGA) Contact angle measurement Zeta potential measurement Graft density Tensile strength measurement

To know the functional groups in the polymeric membrane. To confirm the presence of nanoparticles in the polymeric membrane. To confirm the surface modification of membranes and nanoparticles. To know about the interaction between the nanoparticles and the membrane polymer matrix To make elemental analysis of the membrane surface To make elemental analysis of the membrane surface. To know the degree of cross-linking of PA layer in TFC membrane A complementary tool to FTIR. To know the distribution of CTNs in the TNC membrane. To know the protein fouling To know the biofouling distribution in the membrane module. To know the velocity field of the feed solution in the module To monitor the biofouling To measure the glass transition temperature and melting point of polymer. To study the phase separation of polymer blend. To know the presence of various kinds of water in the membrane To know the amount of absorbed water in the membrane. To know the high-temperature resistance of polymeric membrane To know the hydrophilic (hydrophobic) properties of the membrane surface To know the surface charge of the membrane To know the graft density of the surface coating To know the mechanical strength of the membrane

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90 CHAPTER 3 RO Membrane Characterization

[54] T. Schmid, C. Helmbrecht, U. Panne, C. Haisch, R. Neissner, Process analysis of biofilms by photoacoustic spectroscopy, Anal. Bioanal. Chem. 375 (2003) 1124–1129. [55] Y. Taniguchi, S. Horigome, The states of water in cellulose acetate membranes, J. Appl. Polym. Sci. 19 (1975) 2743–2748. [56] D.J. Mohan, L. Kullova´, A study on the relationship between preparation condition and properties/performance of polyamide TFC membrane by IR, DSC, TGA, and SEM techniques, Desal. Water Treatment 55 (2013) 586–596. [57] E.M. Van Wagner, B.D. Freeman, M.M. Sharma, A. Michael, M.A. Hickner, S.J. Altman, Polyamide desalination membrane characterization and surface modification to enhance fouling resistance, (2010)Sandia Report SAND2010-5540, Printed August 2010. [58] N.J. Ginga, S.K. Sitaram, New method to measure tensile strength of low modulus thin films, Int. J. Fract. 170 (2011) 199–206.

Chapter

4

RO Membrane Transport 4.1 SOLUTION-DIFFUSION MODEL Solution-diffusion model is currently the most popular to interpret the data on flux and salt rejection obtained by the reverse osmosis (RO) experiments. An excellent summary of the applicability of this model for the various membrane transport phenomena is given in the classic book of Crank and Park [1]. Lonsdale applied this model for RO transport [2] soon after RO emerged as a novel process for seawater desalination. The most general description of the mass transport across the membrane is based on an equation derived in the irreversible thermodynamics. Ji ¼ Lii Xi +

X Lij Xj

(4.1)

where Ji is the flux of the component i, Lii, and Lij are phenomenological coefficients, Xi and Xj are the forces under which the mass transfer takes place. The first term of the equation shows that the flux of ith component is caused by the force Xi, that is acting on ith component. The second term shows the relation between the flux of ith component and the forces acting on the components other than ith component. Lii is always positive and according to Onsager’s reciprocal relationship, Lij ¼ Lji

(4.2)

Furthermore, the thermodynamic conditions require that Lii Ljj  L2ij  0

(4.3)

The most appropriate choice of the force X for the molecular diffusion through the membrane under isothermal conditions without external forces is the chemical potential gradient and Xi ¼ rμi

(4.4)

where μi is the chemical potential of ith component. The negative sign was applied since the flux occurs from the higher (feed side) to the lower chemical potential (permeate side).

Reverse Osmosis. https://doi.org/10.1016/B978-0-12-811468-1.00004-9 # 2019 Elsevier Inc. All rights reserved.

91

92 CHAPTER 4 RO Membrane Transport

Then the flux is Ji ¼ Lii rμi

(4.5)

In Eq. (4.5), the second term of Eq. (4.1) was omitted, which means the flow of the ith component is decoupled from the flow of other components. In RO, both pressure and concentration gradient are considered as the driving force for the mass transport. Then, the chemical potential gradient is given by rμi ¼ RT r ln aim + νi rp

(4.6)

where aim (mol/m3) and νi are (m3/mol) the activity in the membrane and the partial molar volume of the ith component, respectively. When RO is a binary system of solute (A) and solvent (B) (it should be noted that subscripts A and B are used sometimes for water and solute, respectively, and sometimes for solute and water, respectively, depending on the original paper. The authors apologize for confusions caused by the exchange of the symbols and the subscripts), Eq. (4.6) is written for B as rμB ¼ RT r lnaBm + νB rp

(4.7)

Note that rμB is the chemical potential gradient inside the membrane. Integrating Eq. (4.7) form the feed (high pressure) to the permeate (low pressure) side,   RT ΔμB ¼ νB Δ lnaBm + Δp νB

(4.8)

Note the symbol Δ means (permeate side—feed side). Assuming that the thermodynamic equilibrium is established between membrane and solution phases, aBm ¼ aB, where aB is activity in the solution phase. Since the osmotic pressure π (Pa) is given by π¼

RT lnaB νB

(4.9)

From Eq. (4.8) ΔμB ¼ νB ðΔp  Δπ Þ

(4.10)

Note that Δ π is based on the osmotic pressure in the solution phase. As for the solute A, it is assumed that the solution is dilute and the activity coefficient remains constant.

4.1 Solution-Diffusion Model 93

Then, Eq. (4.6) is written as rμA ¼ RTr lnaAm + νA rp

(4.11)

rμA ¼ RTr lncAm + νA rp

(4.12)

where aAm, cAm, and νA are activity of the solute in the membrane (mol/m3), concentration of the solute in the membrane (mol/m3), and the molar volume of the solute (m3/mol), respectively. Again, integrating from the feed to the permeate side of the membrane, ΔμA ¼ RTΔ lncAm + νA Δp

(4.13)

The second term of Eq. (4.13) can be ignored when cAm on the permeate side is ethanol > methanol, which agrees with macroscale RO experiments. However, MD simulation does not allow one to calculate the solute rejection. It should also be noted that according to the MD simulation sodium chloride did not pass through the membrane. It did not reach even half way of the membrane cross section.

110 CHAPTER 4 RO Membrane Transport

30

30

20

20 4.14 ns

0

10 y (Å)

x (Å)

10

0.

03

−10

ns

−20

−10 Methanol

−20 Methanol

−30 −30 −20 −10

0

10

20

−30 −30 −20 −10

30

z (Å)

(A)

0

10

20

30

z (Å)

(B)

30

30 A

20

20

10

0.75 ns

B

10 5.04 ns

B

0 −10

y (Å)

x (Å)

4.14 ns

0

−10

0.75 ns

−20 −30 −30 −20 −10

−20

2-Propanol 0

10

20

A

−30 −30 −20 −10

30

z (Å)

(C)

5.04 ns

0

2-Propanol 0

10

20

30

z (Å)

(D)

n FIG. 4.10 Trajectory of alcohol molecule at 150 MPa (A, B) methanol and (C, D) isopropanol [9].

8 40 6

Free volume

30

4

20

2

10 Urea 0

Free volume (%)

Solute molecules/1000 water molecules

10 Methanol

50

2.0

2.2

Ethanol 2.4 2.6 Radius (Å)

2-Propanol 2.8

0 3.0

n FIG. 4.11 Number of solute molecules permeated through the membrane per 1000 of water molecules (solid symbols) based on spherical shape (closed symbols),

based on minimum cross-sectional area [9].

4.7 Molecular Dynamics Simulation 111

This work typically shows the advantages and disadvantages of nonlinear dynamic simulation (NLDS) as compared with the conventional transport models. 1. In general, NLDS looks a powerful tool to demonstrate the phenomena that take place when water and solute are transported through the membrane at a molecular level. But currently, the simulated results are far from reality and are not very useful for the purpose of membrane design. 2. While conventional transport models can represent the effects of the operational variables on the RO performance such as flux and salt rejection, NLDS is far more complicated and the results are sometimes against the trends observed by macroscale RO experiments. For example, 2.1 The feed pressure of 150 MPa is unrealistic. 2.2 The reliable separation data cannot be obtained. 2.3 The method of polycondensation and cross-linking applied in MD is different from the interfacial polycondensation currently used to fabricate the TFC membrane. 2.4 Starting from the same concentration of monomers, very different water fluxes are obtained depending on the initial distribution of monomers in the space. This is against our experience that the monomer concentrations are one of the important factors to govern the membrane structure and performance. 2.5 The permeation rate is much affected in the presence of solute even when the feed pressure is as high as 150 MPa. Considering the osmotic pressure that is substantially lower than the above operating pressure, permeation rate should not depend on the presence of the solute. 2.6 Even when the interfacial polymerization is started from the same monomer concentration, different free volumes are obtained depending on the initial monomer distribution, which is difficult to accept. 2.7 The solutes are trapped in large pores and its permeation is slowed down. This is an analogy to the gas transport in which gas molecules are trapped in Langmuir adsorption site. It sounds as if the presence of large pores increases the solute rejection. This is against the conventional transport model in which the presence of larger pores increases the flux and decreases the salt rejection. 2.8 The vibration of membrane atoms related to thermal vibrations and collisions with water molecules has a significant effect on the free volume of polymeric RO membrane. The flexibility of the soft bond (dihedral, Van der Waals, and electrostatic bonds) in the polymeric

112 CHAPTER 4 RO Membrane Transport

RO membranes allows many “gates” to be transiently opened for water transport via “dynamic membrane structure.” This statement is based on the concept that the transport of water (and solute) takes place by the hopping of molecules from one place to the other through the gate transiently opened by the thermal motions of macromolecular segments. A question arises if the polymers are mobile enough, particularly when they are hydrated. The presence of less mobile bound water is known in the polymeric membrane. They may restrict the motion of polymers in the membrane.

4.8 CNTs MD SIMULATION Since Iijima identified carbon nanotubes (CNTs) in 1991 [11], CNTs have been investigated in various fields and become extremely desirable for a wide range of applications. CNTs, with diameters as small as a nanometer and with a smooth surface may offer a very unique molecular transport through their pores. In fact, there are several studies in recent years that suggest that the water transport through single-walled CNTs (SWCNTs) would become much faster than the transport rate that the continuum hydrodynamic theory would predict. This was attributed by MD simulation to the smoothness of the nanotube wall [12, 13]. More specifically enhancement of water permeance over the HagenPoiseuille value is by a factor of 560–8400. This effect is ascribed to the significant occurrence of slip and hence, velocity of water at the pore wall is not zero. The occurrence of the slip is due to the very smooth pore wall and little affinity of water to the hydrophobic nanotubes. Even though the continuum concept of viscous flow may not be applicable through such a narrow pore of 1.6 nm diameter, yet the above mentioned large enhancement factor demonstrates that water transport is exceptionally fast and the concept of viscous flow may lose its meaning completely to discuss on the transport in the micropore. For example, efficient water transport is well documented for thin zeolite. In a membrane with pore diameter of 0.4 < Φp < 0.5nm, the water molecules just fit inside the pore and apparently have significant mobility with respect to the pore wall. A representative data for this case are the separation of water from 10% water/ethanol mixture by pervaporation. According to Kondo et al. [14], for a 10-μm-thick membrane on a porous multiple support water flux of 0.13 mol/m2 s was obtained with the separation factor of 47,000 at 120°C (zeolite NaA membrane). This shows the significant effect of the slip enhancement for the water molecule. According to Holt et al. [15], for the 2-μm-thick membrane double-walled carbon nanotube (DWCNT), the enhanced pure water flux was7–39 mol/m2 s at Δp ¼ 105 Pa and 120°C.

4.8 CNTs MD Simulation 113

Verweij et al. [16] have also quoted the reports of Manjumder et al. [17] and Holt et al. [15] on the fast water transport through the CNT membranes at room temperature. These experimental data corroborate the earlier results from the MD simulation done by Hummer et al. [12]. There are several questions however. Why does water wet the membrane first of all? A configuration was created in the Hummer et al.’s MD simulation in which water flow through the CNTs is driven by the osmotic pressure difference. Their study showed that the water flow was limited by the particle entry and exit events, and the length of the pore had hardly any effect. For the (6,6) SWCNT studied by Hummer et al., the pore diameter, 0.8 nm, is so narrow that only a single water molecule can be inserted, forming a single-file water chain. Verweij also quoted the statement of Truskett et al. [18] that the hydrophobic confining wall reduces the average number of favorable fluid–fluid interactions per molecule, which means it disrupts the hydrogen-bonding pattern in the fluid. Inside the SWCNT two hydrogen bonds are lost per molecule compared to bulk liquid water, costing it approximately +0.4 eV in binding energy. The van der Waals interaction between the water and the CNT wall which may partially recuperate the loss is approximately 0.17 eV. But still the binding energy of the water in CNT is ca 0.23 eV higher than the bulk water as the binary energy distribution depicted in Fig. 4.12(A) shows, which makes the insertion of water into the CNT pore improbable. However, Verweij et al. further argues that the contribution of the high energy water molecule is greater in the bulk water due to bulk water’s broader distribution. The presence of these high energy water molecules plays a disproportionately large role in the statistical mechanics equation for the chemical potential calculation, and eventually the chemical potential in the nanotube becomes by 0.04 eV less than that of the bulk water. Thus, water molecule can enter into the narrow pore. The water molecules confined in the (6,6) SWCNT may still have considerable entropy at 300 K, since the water molecules can rotate about the nanotube axis to which direction the hydrogen bonds are aligned. The water chains generated in the nanotube can no longer be considered as water of the ordinary sense. In the Poiseuille flow with a nonslip boundary condition at the pore wall, the dissipation rate comes entirely from the transverse momentum transfer inside the liquid, proportional to the bulk liquid viscosity. However, for the water in the nanotube the dissipation rate originates entirely from water chain/pore wall friction, instead of water–water momentum transfer. Hummer et al. [12] noted that the wall friction seems exceedingly small as in the case of gas diffusion. Indeed, graphite is an industrial-grade solid–solid lubricant. Verweij et al. describe the movement of water in the nanotube by the single-file diffusion

114 CHAPTER 4 RO Membrane Transport

0.4

Pbind (u)

0.3 Nanotube 0.2

0.1 Bulk 0 −30

(A)

−20 −10 Water binding energy u/(kcal mol−1)

Langevin force

(B)

The “freight train mechanism”

n FIG. 4.12 (A) Distribution of the water binding energy in bulk liquid water and inside the nanotube at 300 K and (B) illustration of the single-file diffusion of water

inside a SWCNT [16].

as illustrated in Fig. 4.12(B) by a chain of freight cars on a rail, even though the water chain inside the nanotube is not truly “solid.” The freight cars are pushed to both ends by Langevin thermal forces, which average to pressures. The cars do not necessarily form one long contiguous train from one end to the other but break up and recombine into new trains inside the tube any time. Abnormality in the structure and dynamics of water inside the sub-2 nm CNTs are often reported. To interpret the fast water movement, there is no physical basis to apply the Poiseuille flow equation which uses the bulk viscosity in solving the equation. One way of modifying the Poiseuille flow model is to incorporate the slip length Rslip, the ratio of the translational velocity at the wall, vx, to the characteristic velocity gradient in the transverse direction, ∂yvz, that is, Rslip ¼ vz/(∂yvz). Based on their experimental data, Holt et al. found that they must use a slip length of 100 nm, which is 100 times as large as the pore diameter. This means that the large velocity drop occurs just at the first layer of water at the pore wall. This is just another way of saying that the thin layer of water moves as a solid at the nanotube

References 115

wall. The key assertion of the Poiseuille flow is that water–water momentum transfer is a main component of the total dissipation. It is likely that this defining characteristic has broken down for the small CNTs. The waterCNT friction has become of paramount importance. Majaumder et al. [17] showed by MD simulation applied to MWNTs with pore diameter of 7 nm that the Poiseuille law may work well if the friction between the wall and the fluid is so large that no slip occurs. But this was not the case from the experimental data. They have reported the slip lengths that are 104 times greater than the pore diameter. Verweij says that water does not flow but translates as shown in Fig. 4.12(B).

REFERENCES [1] J. Crank, G.S. Park, Diffusion in Polymers, Academic Press, London, 1968. [2] H.K. Lonsdale, in: U. Merten (Ed.), Desalination by Reverse Osmosis, The MIT Press, Cambridge, MA, 1966. Chap 4. [3] T.K. Sherwood, P.L.T. Brian, R.E. Fisher, Desalination by reverse osmosis, Ind. Eng. Chem. Fundam. 6 (1967) 2–12. [4] O. Kedem, A. Katchalsky, Thermodynamic analysis of the permeability of biological membranes to nonelectrolytes, Biochem. Biophis. Acta 27 (1958) 229–246. [5] K.S. Spiegler, O. Kedem, Thermodynamics of hyperfiltration (reverse osmosis): Criteria for efficient membranes, Desalination 1 (1966) 311–326. [6] E. Gl€ uckauf, On the mechanism of osmotic desalting with porous membranes, in: Proceedings, First International Symposium on Water Desalination, 1 US. Department of the Interior, Office of Saline Water, Washington, DC, 1965, , pp. 143–156. [7] G. Jonsson, C.E. Boesen, Water and solute transport through cellulose acetate reverse osmosis membranes, Desalination 17 (1975) 145–165. [8] K. Chan, L. Tinghui, T. Matsuura, S. Sourirajan, Effect of shrinkage on pore size and pore size distribution of cellulose acetate reverse osmosis membranes, Ind. Eng. Chem. Prod. Res. Dev. 23 (1984) 124–133. [9] M. Shen, S. Keten, M. Richard, R.M. Lueptow, Dynamics of water and solute transport in polymeric reverse osmosis membranes via molecular dynamics simulations, J. Membr. Sci. 506 (2016) 95–108. [10] S.-Y. Kwak, S.G. Jung, S.H. Kim, Structure-motion-performance relationship of flux-enhanced reverse osmosis (RO) membranes composed of aromatic polyamide thin films, Environ. Sci. Technol. 35 (2001) 4334–4340. [11] S. Iijima, Helical microtubes of graphitic carbon, Nature 354 (1991) 56. [12] G. Hummer, J.C. Rasaiah, J.P. Noworyta, Water conduction through the hydrophobic channel of a carbon nanotube, Nature 414 (2001) 188–190. [13] A. Karla, S. Garde, G. Hummer, Osmotic water transport through carbon nanotube membranes, Natl. Acad. Sci. U.S.A. 100 (2003) 10175–10180. [14] M. Kondo, M. Komori, H. Kita, K. Okamoto, Tubular type pervaporation module with zeolite NaA membrane, J. Membr. Sci. 133 (1997) 133–141.

116 CHAPTER 4 RO Membrane Transport

[15] J.K. Holt, H.G. Park, Y. Wang, M. Stadermann, A.B. Artyukhin, C.P. Grigoropoulos, A. Noy, O. Bakajin, Fast mass transport through sub-2-nanometer carbon nanotubes, Science 312 (2006) 1034–1037. [16] H. Verweij, M.C. Schillo, J. Li, Fast mass transport through carbon nanotube membranes, Small 3 (2007) 1996–2004. [17] M. Majumdar, N. Chopra, R. Abdrews, B.J. Hinds, Nanoscale hydrodynamics: Enhanced flow in carbon nanotubes, Nature 438 (2005) 44. [18] T.M. Truskett, P.G. Debenedetti, S. Torquato, Thermodynamic implications of confinement for a waterlike fluid, J. Chem. Phys. 114 (2001) 2401–2418.

Chapter

5

RO Membrane Module 5.1 MODULE DESCRIPTION 5.1.1 Module Type There are typically two modules for reverse osmosis (RO): hollow fiber [1] and spiral wound.

5.1.1.1 Hollow Fiber Module Hollow fiber membrane module is featured by its surface/volume ratio that is orders of magnitude larger than the spiral-wound module, which makes the production rate (amount of desalinated water produced by a membrane) extremely large resulting in much smaller foot prints. Despite its great advantage Toyobo’s Hollosep is currently the only RO hollow fiber module, which is based on cellulose triacetate, especially after the withdrawal of Dupont’s Permasep module from the Middle East market. Even though the Dupont’s hollow fiber module no longer exists in the market, the principle of the module structure and construction is well described in their technical bulletin. Hollow fiber membranes are fibers of 0.1–1 mm diameter with a hollow space inside (Fig. 5.1). The feed is supplied either inside or outside of the hollow fiber and the permeate passes through the fiber wall to the other side of the fiber. The fiber wall has the structure of asymmetric membrane, the active skin layer facing the feed solution. A bundle of hollow fibers is mounted in a pressure vessel, and the open ends of U-shaped fibers are potted to the head plate (Fig. 5.2). In a typical example of Dupont’s permeator, the feed solution is distributed from the central distribution tube and flows radially through the hollow fiber bundle (Fig. 5.3).

5.1.1.2 Spiral-Wound Module Even though there is a long history for the spiral-wound module since the early development of RO, the basic structure of the spiral-wound module has not at all changed. As illustrated in Fig. 5.4, a permeate spacer is sandwiched between two membranes, the porous support side facing the spacer. Reverse Osmosis. https://doi.org/10.1016/B978-0-12-811468-1.00005-0 # 2019 Elsevier Inc. All rights reserved.

117

118 CHAPTER 5 RO Membrane Module

Epoxy nub

Shell

Brine Feed

Product

Sample

Hollow fiber membrane

Epoxy tube sheet

Porous support block

n FIG. 5.1 Design of Du Pont’s hollow fiber assembly [2]. A single hollow fiber

Epoxy resin

Permeate (open to atmospheric pressure)

x

dx

L n FIG. 5.2 Schematic diagram of single hollow fiber [2].

Hollow fiber

Porous disk

Feed inlet Permeate outlet Feed distributor End sheet Concentrate outlet Feed flow direction

n FIG. 5.3 Schematic diagram of hollow fiber module [2].

Epoxy resin

5.1 Module Description 119

Feed spacer

Central tube

Permeate Permeate spacer Membrane

n FIG. 5.4 Basic structure of the spiral-wound module.

Permeate spacer

Membrane

Feed spacer

n FIG. 5.5 Multileaves spiral-wound module [2].

Three edges of the membranes are sealed with glue to form a membrane envelope, the open end connected to a percolated central tube. The membrane leaf so produced is wound spirally around the central tube together with a feed spacer. In order to make the leaf length shorter, several membrane leaves are wound simultaneously as shown in Fig. 5.5. Usually 2–6 leaves are used in a 4-in. module and 4–30 leaves in an 8-in. module. A polypropylene or a polyethylene net of 0.2–2.0 mm thickness is used for the feed spacer, whereas polyester cloth, 0.2–1.0 mm thick, hardened with melamine or epoxy resin is used for the permeate spacer. The feed solution flows through the feed spacer, parallel to the central tube, whereas permeate flows through the permeate spacer, spirally, perpendicular to the feed flow direction and is collected by the central tube. The structure of Osmonics (GE) module is illustrated in Figs. 5.6 and 5.7. The spiral-wound module is featured by 1. High-pressure durability 2. Compactness 3. Minimum membrane contamination

120 CHAPTER 5 RO Membrane Module

4. Minimum concentration polarization 5. Minimum pressure drop in the permeate channel. The module diameter is 4–8 in. but a 16-in. module is now available. The development of RO module is well summarized in the 2010 paper of Johnson and Busch [3]. According to Johnson and Busch, Dow Chemical developed cellulose triacetate membrane in hollow fiber configuration in 1971 (DOWEX) and Toyobo is still manufacturing cellulose triacetate hollow fiber modules for seawater desalination (Hollosep). DuPont also fabricated desalination membrane module in the hollow fiber configuration based on aromatic polyamide in 1969 (Permasep B-9 and B-10). These modules were leading in the Middle East market until Dupont withdrew from the market in 2001. The major disadvantages of Dupont hollow fiber module were: (1) high fouling and scaling tendency due to the narrow space between hollow fibers and due to the presence of the dead zone, (2) the foulant is difficult to remove due to the low cross-flow velocity, and (3) the limited pH range where

Pressure vessel assembly with clamps and end caps

Antitelescoping device

Outer cover Complete sepralator

Interconnector Permeate tube “O” ring

Concentrate seal

Membrane backing Outer cover

Mesh spacer Permeate carrier Membrane Adhesive bond

n FIG. 5.6 Osmonics spiral-wound module [2].

Sepralator cut-away shows layer and the relationship to feed and concentrate flow

5.1 Module Description 121

Crossection end view

Feed and Concentrate flow arrows. The left arrow indicates the feed solution with less solute and the right arrow the more concentrated solute.

Pressure on the membrane. Denotes equal pressure from the solution on all of the membrane.

Crossection side view

Permeate flow arrow indicates flow of pure water that passes through the membrane and spirals through the permeate carrier to the permeate tube

n FIG. 5.7 Another Osmonics spiral-wound module [2].

the membrane works safely, which lead to high pretreatment cost. Recognizing the shortcomings of cellulose triacetate and its hollow fiber configuration, Dow purchased thin film composite (TFC) polyamide membrane of Film Tech cooperation in 1985. The TFC membrane was in the spiral-wound configuration and the market shifted gradually from hollow fiber to spiralwound module since then. Seawater plants in Agip Gela (Italy), Agragua Gran Canaria (Spain), and Galilah (United Arab Emirates) [4–6] were the good examples of the spiral-wound module. In the paper of 2010 Johnson and Busch said that the capacity of the 8-in. module had doubled whereas the salt passage became three times less during the past 20 years. The progress is well summarized in Fig. 5.8 taken from their paper. The development of 16-in. diameter module made the membrane surface 4.3 times as large as that of the 8-in. module. Furthermore, the increase in the operating pressure from the earlier 1000–1200 psig allowed the product recovery of 60%. The increased salt passage with an increase in the product recovery was compensated by the improvement in the salt rejection.

122 CHAPTER 5 RO Membrane Module

99.9

2008: FILMTEC SW30XHR-400i

2009: FILMTEC SW30HRLE-440i

99.8 99.7

Rejection (%)

99.6

2003: FILMTEC SW30HRLE-440i

1996: FILMTEC SW30HR-380

2009: FILMTEC SW30XLE-440i

2009: FILMTEC SW30XLE-440i

2004: FILMTEC 2008: FILMTEC SW30XLE-400 SW30ULE-400i

99.5 99.4 99.3

1985: FILMTEC SW30HR-8040

99.2 99.1 99 4000

1985: FILMTEC SW30-8040

5000

1996: FILMTEC SW30-380

6000

7000

8000

9000

10000

11000

12000

13000

Flow (gp d) n FIG. 5.8 Progress in spiral-wound module performance. (From J. Johnson, M. Busch, Engineering aspects of reverse osmosis module design. Desalin. Water Treat.

15 (2010) 236–248.)

5.1.2 Feed Spacer The most popular feed spacer is the polypropylene net of thickness 0.6–0.9 mm as shown in Fig. 5.9. The feed spacer has two functions, one is to maintain a space between two membranes through which feed solution can flow, and the other is to promote mixing of the feed solution to reduce the concentration polarization. In the case of sodium chloride solute, the typical concentration polarization in the spiral-wound module is kept in the range of 1.05–1.15, meaning the concentration near the membrane is 5%–15% higher than the bulk feed. However, the efforts to reduce the concentration polarization by good mixing are compromised by the pressure drop at the feed channel. The feed spacer improvement by changing the spacer shape has been proposed [7–10]. It is also possible that the well-mixed feed by the feed spacer mitigates the scale formation, biofouling, and particulate fouling [11, 12], but this aspect has not yet been thoroughly studied. The spacer containing antibacterial materials such as silver and copper have been proposed [13, 14] to reduce biofouling.

5.1 Module Description 123

n FIG. 5.9 A typical feed spacer [3].

5.1.3 Permeate Spacer Permeate spacer provides a channel that allows the permeate to flow from the membrane to the central collection tube. In the commercial module the use of woven polyester fabric is most popular. Pressure drop by the permeate space has the following two serious effects. The first one is the decrease in the transmembrane pressure difference, which leads to the decrease in flux. The second is the occurrence of the local distribution of transmembrane pressure difference. Near the permeate collection tube the transmembrane pressure difference is high and the severe concentration polarization may take place, while at the end of the membrane leaf the transmembrane pressure is the lowest causing the underutilization of the membrane. Thus, the leaf length should be reduced to increase the module efficiency.

124 CHAPTER 5 RO Membrane Module

5.1.4 Endcap The role of the endcap is as follows: (1) it prevents the telescoping (relative axial movement) of the leaves and (2) the endcap holds a brine seal that prevents the bypassing of the feed solution through the space between the outer surface of the spirally wound membrane leaf and the inner surface of the pressure vessel.

5.1.5 Larger Modules The modules larger than the 8-in. diameter have been available for some time since it is known that the capital cost can be reduced significantly by increasing the size of the module. The market acceptance of the larger module has been slower however by several reasons. The larger module was thought to be fouled more easily than the smaller module, a perception which was disproved by the data from the Bedok plant in Singapore, showing that the fouling tendency was almost equal for both the small and large modules [15]. As well the large size and heavyweight of the module remained an obstacle for the use of 16-in. module. Nevertheless, the large modules have been installed in many pilot and commercial RO plants. After all, the technological advance enabled a significant increase in module performance, that is, 4-in. diameter of 1970 provided 250 L/h with salt separation of 98.5%, while 16-in. diameter can provide 4000–8000 L/h with salt separation of 99.7%.

5.2 STUDIES ON THE SPACERS 5.2.1 Computation and Experiments Since the properties of the spacers such as spacer thickness, spacer filament cross section, spacing between consecutive filaments, and the angle between two filaments affect the flow and concentration pattern in the membrane channels, numerous studies have been made to know the effects of those spacer parameters. The results are summarized in Table 5.1.

5.2.2 Module Observation The hydrodynamic conditions in the membrane module can be visibly observed by constructing a transparent module. Huang et al. constructed a high-pressure optical RO module to observe directly the bacterial deposition patterns and rates under RO operations [28]. The module could tolerate the pressure of 1200 psi and allowed the transmission of visible and ultraviolet light. Two kinds of commercial RO membranes were tested with and without spacers.

5.2 Studies on the Spacers 125

Table 5.1 Summary of the Studies on Spacer Properties Module

Spacer

Calculation or Experiment

Conclusions

Reference

Calculation and experiment

Hydrodynamic conditions are the same for both systems. Simulation can be done with spacer filled flat sheet

[16]

Feed spacer thickness: 0.0508, 0.0711, 0.1168 cm Ladder-type feed spacer

Experiment

As thickness decreases turbulence decreases

[17]

Calculation and experiment

[18]

Rectangular cell

Various commercial spacers

Calculation by CFD

Rectangular cell with a viewing slot

Various commercial spacers

Calculation and UF experiment

Computation limited to one small domain assuming local mass transfer coefficient becomes periodic

Commercial net spacers

Calculation by CFD

Concentration polarization (CP) changes locally, being well correlated to the flow pattern. Increase in Re number alone cannot decrease CP. Computation and experiment agree Filament diameter, filament diameter/distance between filaments, flow angle are important parameters for the design of the spacer. Good agreement with Costa’s experiment. For low flow rate: voidage and hydraulic angle should be 40%, 50–120°, respectively. For high flow rate: voidage and hydraulic angle should be 60%–70%, 70–90°, respectively Optimal spacer geometry is channel height/spacer thickness ¼ 4, flow attack angle ¼ 30°, angle between filaments ¼ 120° Good agreement with experiment in literature Increase of feed channel pressure drop caused by biofilm formation can be reduced by using thicker and/or modified feed spacer geometry and/or a lower flow rate. Numerical solution in agreement with experimental data

Spiral-wound module and spacer filled flat spiral-wound (Hydranautics, model 25 M 100AL) channel

Rectangular cell installed with CNDF501 Separem

Spiral-wound module

Calculation of the effect of spacer on biofouling

[19]

[20]

[21]

[22]

Continued

126 CHAPTER 5 RO Membrane Module

Table 5.1 Summary of the Studies on Spacer Properties Continued Module

Spiral-wound module

Spiral-wound module

Spacer

Calculation or Experiment

Conclusions

Reference

Ladder-type spacer

Calculation of fluid flow patterns through different spacer configurations, visualized using ANSYS FLUENT by varying the dimensionless filament spacing

[23]

3D printing of feed spacer

Strategy for developing, characterizing, and testing of feed spacers by numerical modeling, three-dimensional (3D) printing of feed spacers and experimental membrane fouling simulator (MFS) studies

Diamond and ladder space orientation

Calculation and experiment

Oval-shaped spacer vs circular spacer

2D calculation

Synergistic effect between pulsatile flow and spacer filament

2D calculation

When L (ratio of top or bottom filament spacing and channel height) < 3, the mass transfer coefficients on the feed and permeate channel are nearly equal. When L > 4, the mass transport at the top surface exhibits sharp decline, suggesting high fouling propensity. L ¼ 4 is the optimal value Good agreement between model calculation and experiment. The 3D printed spacer had (i) a lower pressure drop during hydrodynamic testing, (ii) a lower pressure drop increase in time with the same accumulated biomass amount A 3D numerical model combining fluid flow with a Lagrangian approach for particle trajectory calculations could describe very well the in-situ observations on particle deposition by membrane fouling simulator (MFS) Oval-shaped spacer tilted at 20° is superior to circular spacer in terms of pressure drop, mass transfer coefficient, and fouling Flow pulsations at the optimal frequency cause vortex shedding at Reynolds numbers below 350, while above 500 would be required without pulsatile flow

[24]

[25]

[26]

[27]

5.2.3 Module Imaging by Particle Image Velocimetry As repeatedly mentioned, concentration polarization and fouling cause deterioration of membrane performance in short or long term and both depend on the velocity profile in the membrane module. Particle imaging is

5.2 Studies on the Spacers 127

Top plate

Spacer

Width Middle plate

Top

Fl Len ow gt di h re ct io n

Outlet

Gas inlet Liquid inlet

Bottom Bottom plate n FIG. 5.10 Schematic illustration of flow cell (length 15 cm, width 5 cm, channel height depends on the

spacer thickness) [30].

a technology that allows the visualization of the fluid flow without disturbing the flow. This is a technique for particle measurements using digital imaging, one of the techniques by the broader term particle size analysis [29]. Willems et al. used this technique to monitor the fluid flows in the spacerfilled channels [30] by constructing a flow cell illustrated in Fig. 5.10. The cell stood vertically and the liquid entered through 1 mm holes (2 mm apart) approximately 5 mm from the bottom of the channel, and exited via a similar arrangement 5 mm from the top of the channel. The gas entered from a single 1 mm hole located 25 mm from the channel bottom when two phase experiments were made. The spacers were Conwed Plastics whose dimensions are given in Table 5.2. Tracer particles (50 μm polyamide to represent whole cell or 10 μm hollow glass to represent intermediate and single cell level) were added to the liquid entering the cell and illuminated by double-pulsed laser sheet. With the aid of digital camera the particle displacement within a time interval could be

128 CHAPTER 5 RO Membrane Module

Table 5.2 Spacer Specification Thickness (mm)

Filament Angle (°)

Filament Spacing (mm)

Porosity

Material

A B C

0.50 1.17 1.00

90 90 90

2.5 3.63 2.08

0.92 0.87 0.81

Nylon PP PP

−15

0.25

−15

0.25

−20

0.225

−20

0.225

−25

0.2

−25

0.2

0.175

−30

0.175

0.15 mm

−40

0.125

−45

−35 −40

0.1

0.1 −50

0.075

0.075 −55

−55 0.05

0.05 −60

−60

(A)

10

20

30 mm

40

50

0.125

−45

−50

−65

0.15 mm

−35

Avg V m/s

−30

velocity m/s

Name

0.025

−65

(B)

10

20

30

40

0.025

50

mm

n FIG. 5.11 Average (A) and instantaneous (B) flow-velocity profile for spacer B [30].

monitored and the velocity profile could be generated. The results are shown in Figs. 5.11–5.13. Fig. 5.11A shows that the average flow velocity of 0.1 m/s is uniformly distributed in the cell. Fig. 5.11B shows the instantaneous velocity profile within a much shorter time frame. From the figure both flow direction and velocity are not uniform indicating the effect of the spacer. As mentioned, Fig. 5.11A is the time-averaged velocity profile in the cell. It should however be noted that the velocity profiles are different at different depths in the cell. Therefore, Fig. 5.11A represents the velocity values averaged in terms of both time and depth. Fig. 5.12 shows the average velocity profile at different depths, that is, Fig. 5.12A shows that the flow is in \\\ direction at the bottom of the cell, while at the top the flow is in /// direction (Fig. 5.12C) and in the center the direction is the mixture of both (Fig. 5.12B).

5.2 Studies on the Spacers 129

5.0

5

4.5 4.0

4

3.5 3.0

3

2.5 2.0

0.5 0.0

1

Position mm

1.0

2 Position mm

1.5

0

−0.5 −1.0

−1

−1.5 −2.0

−2

−2.5

−1

(A)

−3.0 −3.5 0

1 2 Position mm

3

4

5

−3 −1

0

1 2 3 Position mm 5 1.4

(B)

4

5

1.3

4

1.2 1.1

3

0

Position mm

1

0.9 0.8 0.7 0.6

Avg V m/s

1.0 2

0.5 −1 −2 −3 −1

(C)

0

1

2 3 Position mm

4

0.4 0.3 0.2 0.1

5

n FIG. 5.12 The effect of the depth in the channel on the average flow-velocity profile (A) 0.4 mm below the center, (B) the center, and (C) 0.4 mm above the center

(average flow velocity 0.5 m/s, spacer C) [30].

130 CHAPTER 5 RO Membrane Module

D2245

B

D

B D

A

C

A

C

Spacer C

D3345

Spacer B n FIG. 5.13 Comparison of the calculated (left) and the experimental (right) flow profile (At ¾ of the channel height, filament spacer/channel height ¼ 2 for D2245 and spacer C and ¼ 3 for D3345 and spacer B) [30].

The experimentally observed results by PIV were further compared with those obtained by Shakaib et al. [31, 32] using computational fluid dynamics (CFD). As shown in Fig. 5.13, the agreement between the flow patterns predicted by simulation (left) and the experimental observation (right) is excellent.

5.2 Studies on the Spacers 131

Table 5.3 Summary of Investigations by PIV Module

Spacer

Conclusions

References

Rectangular flow cell, 2D image between two membranes

Flexible rhombus like structure, filament 0.37 mm diameter, 2.4 mm length, longer and shorter diagonal 3.5 and 2.5 mm, respectively Nonwoven commercial PP

Flow direction change occurs near the spacer rod due to the obstacle that the flow has to bypass

[33]

There are high-velocity and lowvelocity zones inside each mesh. The particle velocity is low in low-velocity zone, where biofouling occurs

[34]

Rectangular cell without any permeation through membrane (TrisepAMC1)

Willems et al. also applied PIV for the liquid–gas two-phase flow in the module. Some other studies by PIV are summarized in Table 5.3.

5.2.4 Computational Fluid Dynamics Three-dimensional CFD study was carried out for spacer-obstructed feed channels using finite volume package (FLUENT) by Shakaib et al. [31]. In the method, the governing equations were the continuity and three momentum equations. The fluid used was water that was incompressible and isothermal. No permeation and no slip conditions were further assumed. Two spacer geometries, diamond spacer and parallel spacer, were considered. (a) In diamond spacer (Fig. 5.14A) θ2 is a variable. θ2 ¼ 0 means, upper filaments with diameter d1 are parallel to the flow (x) direction and the lower filaments with diameter d2 are transverse to the flow direction. (b) In parallel spacer (Fig. 5.14B), there is no θ2. Parallel filaments are parallel to the x-direction and the cross filaments are parallel to the z-direction. Both filaments are at the same height. The dimensionless parameters were set as D1 ¼

d1 d2 l1 l2 , D2 ¼ , L1 ¼ , L2 ¼ hch hch hch hch

where hch is the channel height. An example of the calculation results is given in Fig. 5.15 for the diamond spacer, with θ2 ¼ 0 and D1 ¼ D2 ¼ 0.5. When the distance between the axial filaments are large (L1 ¼ 6, Fig. 5.15B) the velocity contours are similar on the two planes parallel to the x-direction and the high-velocity region (yellow color) stretches to the z-direction covering the entire range between the two parallel filaments. On the other hand, when the distance between the axial filaments are small (L1 ¼ 2, Fig. 5.15A), the highest velocity

132 CHAPTER 5 RO Membrane Module

Membrane surface (top wall) Flow θ1 Direction

d2

d1

l1

hch = d1+d2

θ2

Y

l2

Fluid flow

Membrane surface (bottom wall)

X Membrane channel with diamond spacer

Z

(A) Membrane surface (top wall)

d2 l2 hch = d1

Flow Direction Y

l1

X

Membrane surface (bottom wall)

Fluid flow Membrane channel with parallel spacer

Z

(B)

n FIG. 5.14 Schematics and computational domains for (A) diamond spacer and (B) parallel spacer [31].

(brown color) is much greater than that of L1 ¼ 6 (Fig. 5.15B, shown by yellow color) but restricted to the region directly above the parallel filament. The highest velocity region is very short to the z-direction from the parallel filament. In both Fig. 5.15A and B there are regions of negative velocity (dark blue color) at the bottom and behind the transverse filaments. This means that the flow of a fraction of liquid is reversed and after hitting the filament it is recirculated. Shakaib et al. have also made the simulation for the parallel spacer. As mentioned earlier, their simulation results were confirmed by particle image velocimetry (PIV) experiments.

5.2.5 Nuclear Magnetic Resonance Imaging Magnetic resonance imaging is a method that enables the in situ nondestructive observation of the inside of the RO or nanofiltration module. There were several attempts to use this method to investigate the fouling accumulation and velocity distribution profiles in the module. Vrouwenvelder et al. conducted a biofouling study in the laboratory constructed spacer membrane fouling simulator (S-MFS) along with the full-scale commercial module and test-rig investigations [11].

5.2 Studies on the Spacers 133

3.19e-01

2.29e-01 Y X Z

(A)

1.39e-01

4.90e-02

-4.12e-02 (m/s) Y X Z

(B)

n FIG. 5.15 x-Velocity contours (A) L1 ¼ 2, L2 ¼ 3; (B) L1 ¼ 6, L2 ¼ 3 [31].

The S-MFS was made of PVC and had a feed spacer channel dimensions of 4 cm (width)  16 cm (length) and 0.77 mm channel height. A spacer was sandwiched between two membranes and placed in the feed channel. Both the membrane and spacer were taken from the unused spiral-wound nanofiltration element (TRIsep TS 80). A biodegradable compound (sodium acetate 1.00 mg carbon/L) was added to the feed water to accelerate the biofilm formation. The S-MFS was fitted to an nuclear magnetic resonance

134 CHAPTER 5 RO Membrane Module

2D images

Flow

Inlet

Outlet

0

2

3

4

Time (days) n FIG. 5.16 In situ NMR 2D images in time. (The flow direction is from top to bottom. The image resolution is 98 μm/pixel) [11].

(NMR) radiofrequency coil of 2.5 cm inner diameter and the special biofilm distribution and velocity field were monitored using 1H NMR for 4 days. The two-dimensional (2D) NMR image is shown in Fig. 5.16. On day 0, the spacer is clearly seen. As the day passed the biofilm grew on the tracer junctions and branches. Especially, the biofilm growth started preferentially at the spacer junction near the S-MFS inlet. The 2D velocity profile is given in Fig. 5.17. On day 0, the flow was smooth and uniform in the S-MFS. The dark blue diamond is on the spacer junctions, meaning there was little flow at the spacer junctions. The flow was faster (brown color) at the spacer branches and the space between the branches. The velocity is very high at the S-MFS inlet. On day 2 the biofilm formation at the spacer junctions near the inlet (two-circled regions in Fig. 5.17-2) caused the distortion of flow field. The image is brighter near the inlet and darker near the outlet. The heterogeneity in the flow profile increases as the day advances and the flow is polarized in large stagnant regions (dark) and fast flowing regions (bright). On the last day, only a narrow high speed region is observed. It was also observed that the feed flow channel pressure drop increased 95%, 155%, and 680% for days 2, 3, and 4, respectively, compared with the day 0 pressure drop. Table 5.4 is the summary of module investigation by NMR.

5.2 Studies on the Spacers 135

Flow

Inlet

Velocity (m/s)

0.005

−0.0015

Outlet 0

2

3

4

Time (days) n FIG. 5.17 2D velocity profiles in time. (The flow direction is from top to bottom. The image resolution is 210 μm/pixel. Velocity increases from dark to bright.) [11].

Table 5.4 Summary of Other Module Investigation by NMR Module

Spacer

Conclusion

Reference

RO module with 2.5 in diameter (Hydranautics, ESPA1-2540) and rectangular cell

Commercial

NMR spectroscopy enables to obtain information on special biofilm distribution, velocity field and distribution of water in the module. Effective membrane area can be obtained after biofouling Two MFSs were fouled with the nutrient solution, then subsequently cleaned using combinations of sodium dodecyl sulfate (SDS) and sodium hydroxide (NaOH) and observed using MRI The module was biofouled with biodegradable nutrients. The Earth’s field (EF) NMR showed that fouling near the membrane module entrance significantly distorted the flow field through the whole membrane module. The total NMR signal is shown to be suitable for nondestructive early biofouling detection 3D NMR images can elucidate the interaction of hydrodynamics and mass transport on biofilm accumulation in membrane systems. Insights on the effect of cross flow velocity, flow stagnation, and feed spacer presence can be obtained, and in situ information on biofilm structure, thickness, and spatial distribution can be quantitatively assessed

[35]

Membrane fouling simulator (MFS) developed by the group

Commercial 61-mm diameter spiralwound RO membrane module (Dow FILMTEC Model XLE-2521)

Spiral-wound RO membrane module (Dow FILMTEC XLE-2521)

[36]

[37]

[38]

136 CHAPTER 5 RO Membrane Module

5.2.6 Small-Angle Neutron Scattering Imaging As shown in the Chapter 1, small-angle neutron scattering (SANS) was used to characterize polymeric membranes, especially to reveal their porous structure. Recently, SANS has also been used to study the membrane biofouling and scaling in the membrane module. Schwahn et al. made the real-time studies of scaling and biofouling on TFC membranes in RO and nanofiltration water treatment using in situ SANS [39]. A flow cell high-pressure SANS found 0.5 μm large cavities and ˚ diameter large rod-like cavities inside the nonwoven polyester 300 A and microporous polysulfone layer, respectively. In situ desalination experiments were carried out in cross-flow mode at an applied pressure of 6 bars and feed flow velocity of 0.2 cm/s. The scattering cross-section times sample thickness (μt ¼ Σt  DS) derived from the transmission coefficient showed an overall enhancement due to newly formed scattering centers, which was accompanied by a reduced membrane permeability measured. This observation is supported by enhanced scattering of the membrane due to μm large domains of mass fractal structure. Addition of BSA to the feed after desalination of 30 h enhanced the permeability which was accompanied by an about 50% decline of μm large scattering centers. Dahdal et al. used SANS technique to study the scaling by calcium phosphate at the biofouled RO membranes of the wastewater desalination plant [40]. Bovine serum albumin (BSA) coated on citrate-capped gold nanoparticles (BSA-GNPs) were added to a simulated wastewater effluent (SSE). Gold and citrate were homogeneously distributed in the nanoparticles, ˚ due to stabilization by the citrate GNPs, whose size became as small as 30 A groups. The gold volume fraction was of the order of 1% in GNPs. As for BSA-GNPs, on average two BSA monomers were grafted at 2.4 GNPs. When BSA-GNPs was added to SSE, mineralization occurred immediately forming stable composite particles of the order of 0.2 μm diameter with a mineral volume fraction between 50% and 80%. It was suggested that mineralization of total calcium phosphate and partial (5%–10%) calcium carbonate took place in the presence of BSA-GNPs. Schwahn et al. also used SANS to study the accumulation of GdCl3 during desalination by RO membrane [41].

5.2.7 Fouling Monitoring by Ultrasonic Time-Domain Reflectometry Another example of noninvasive monitoring of the RO module fouling is ultrasonic time-domain reflectometry (UTDR) [42]. This method is based on their earlier reports on the application of UTDR for fouling detection

5.2 Studies on the Spacers 137

Transducer T

Top

A

A′

F

A′

A

B

Amplitude (V)

T B

Fouling layer Membrane Support Bottom plate

(A)

Arrival time (μs)

(B)

n FIG. 5.18 (A) Path of ultrasound wave, (B) amplitude response versus time of interest (A: transducer-membrane surface transducer, A0 : transducer-foulant layer

surface transducer, B: transducer-bottom plate transducer) [42].

[43–45]. The principle of UTDR is as follows. The ultrasonic wave travels the distance between the transducer—reflecting interface—transducer and gives a signal at the arrival time. For example, signal A appears on Fig. 5.18B that corresponds the time required for traveling the distance transducer-membrane surface-transducer (Fig. 5.18A). When the membrane is covered by the foulant, the interface shifts to A0 , and so the signal from A to A0 (dotted line on Fig. 5.18B). Thus we can monitor the foulant film thickness since the time t, is related to the distance, 2Δd by t ¼ 2Δd=c

where c is the velocity of sound in water. Sim et al. constructed a canary cell, which is supposed to simulate faithfully the fouling of the spiral-wound seawater desalination membrane (SWDM). A commercial SWDM module (DOW TW30-2540) and the specially designed canary cell were run simultaneously using the feed solution coming from the same feed reservoir. The canary cell was comprised of a rectangular flow cell of 31 cm length and 6 cm width. The permeate and feed spacers and the membrane were taken from the commercial module to be installed in the canary cell. A 5 MHz transducer (Olympus Model Videoscan V-109RM) was externally mounted on the canary cell. A solution containing typically 2 g/L of NaCl and 0.8 g/L of colloidal silica (nominal diameter 20 nm) were used as the feed and the permeate flux was maintained at 35 L/m2 h, while the

138 CHAPTER 5 RO Membrane Module

transmembrane pressure drop was increased with time as the foulant layer thickness increased. The flow rate was constant at 0.17 m/s and the corresponding Reynolds number was 138. Fig. 5.19(i) shows the amplitude response versus time. A and B correspond to A and B of Fig. 5.18, respectively. Fig. 5.19(ii) is an expanded view of the 0.08 (B) 0.06 0h

Amplitude (V)

0.04 0.02

Foul 4 h

(A)

Clean 2 h

0

19.70 −0.02

19.80

19.90

20.00

20.10

20.20

20.30

−0.04 −0.06

(i)

Time (ms) 0.03 19.796

19.801 19.803

0.02

0.01 Amplitude (V)

0h 0 19.65

19.70

19.75

19.80

19.85

19.90

19.95

Foul 4 h

−0.01 −0.02 −0.03 −0.04

(ii)

Time (ms)

n FIG. 5.19 Experimental amplitude versus traveling time (19.803 corresponds to 0 h fouling, 19.796 corresponds to after 4 h fouling, 19.801 corresponds to after 2 h of

flushing with NaCl solution without colloidal silica) [42].

References 139

response A. There is a small shift in the arrival time from 0-h fouling (black, 19.803 μs) to 4 h fouling (blue 19.796 μs). After applying membrane flashing, the arrival time partially come back to 19.801 μs (red). The 0.007 μs decrease from 0 to 4 h fouling corresponds to the formation of 5.25  0.38 μm foulant layer, and 0.05 μs increase by the 2 h flashing that followed corresponds to the removal of 3.00  0.38 μm foulant layer, leaving 2.25 μm. It was further confirmed that the canary cell is capable of monitoring the state of fouling quantitatively and allows in situ, real time, nondestructive observations that reflect the fouling conditions of the commercial SWDM module.

REFERENCES [1] A. Kumano, Advances in hollow-fiber reverse-osmosis membrane modules in seawater desalination, in: N. Lior (Ed.), Advances in Water Desalination, 1, Wiley, Hoboken, New Jersey, 2012, pp. 309–376. [2] T. Matsuura, Synthetic Membranes and Membrane Separation Processes, CRC Press, Boca Raton, FL, 1993. [3] J. Johnson, M. Busch, Engineering aspects of reverse osmosis module design, Desalin. Water Treat. 15 (2010) 236–248. [4] A. Gorenflo, J.A. Redondo, F. Reverberi, Basic options and two case studies for retrofitting hollow fiber elements by spiral wound. RO technology, Desalination 178 (2005) 247–260. [5] F. Reverberi, A. Gorenflo, Three year operational experience of a spiral-wound SWRO system with a high fouling potential feed water, Desalination 203 (2007) 100–106. [6] A. Gorenflo, P. Sehn, The 13,500 m3/d plant in Galilah (UAE): Experiences and performance within the first 12 months of Operation, Deutsche Meerwasserentsalzung (DME) Conference Seawater Desalination in United Arabic Emirates, Berlin, (2006). [7] J. Schwinge, D.E. Wiley, A.G. Fane, Novel spacer design improves observed flux, J. Membr. Sci. 229 (2004) 53–61. [8] F. Li, W. Meindersma, A.B. de Haan, T. Reith, Novel spacers for mass transfer enhancement in membrane separations, J. Membr. Sci. 253 (2005) 1–12. [9] G. Guillen, E.M.W. Hoek, Modeling the impacts of feed spacer geometry on reverse osmosis and nanofiltration processes, Chem. Eng. J. 149 (2009) 221–231. [10] S.K. Karode, A. Kumar, Feed spacers for filtration membrane modules, (2006). US Patent 6989097. [11] J.S. Vrouwenvelder, D.A. Graf von der Schulenburg, J.C. Kruithof, M.L. Johns, M.C. M. van Loosdrecht, Biofouling of spiral-wound nanofiltration and reverse osmosis membranes: A feed spacer problem, Water Res. 43 (2009) 583–594. [12] P.R. Neal, H. Li, A.G. Fane, D.E. Wiley, The effect of filament orientation on critical flux and particle deposition in spacer-filled channels, J. Membr. Sci. 214 (2003) 165–178.

140 CHAPTER 5 RO Membrane Module

[13] H.-L. Yang, J.C.-T. Lin, C. Huang, Application of nanosilver surface modification to RO membrane and spacer for mitigating biofouling in seawater desalination, Water Res. 43 (2009) 3777–3786. [14] R. Hausman, T. Gullinkala, E. Escobar, Development of low biofouling polypropylene feedspacers for reverse osmosis, J. Appl. Polym. Sci. 114 (2009) 3068–3073. [15] R.S. Ong, J.E. Johnson, E. Zhao, Sixteen-inch reverse osmosis module performance in water reuse, in: Proc. SIWW Water Convention 2009, IWA, Singapore, 2009, p. 1098. [16] G. Schock, A. Miquel, Mass transfer and pressure loss in spiral wound modules, Desalination 64 (1987) 339–352. [17] S.S. Sablanl, M.F.A. Goosen, R. Al-Belushi, V. Gerardosb, Influence of spacer thickness on permeate flux in spiral-wound seawater reverse osmosis systems, Desalination 146 (2002) 225–230. [18] V. Geraldes, V. Semia˜o, M.N. Pinho, Hydrodynamics and concentration polarization in NF/RO spiral-wound modules with ladder type spacers, Desalination 157 (2003) 395–402. [19] S.K. Karode, A. Kumar, Flow visualization through spacer filled channels by computational fluid dynamics I. Pressure drop and shear rate calculations for flat sheet geometry, J. Membr. Sci. 193 (2001) 69–84. [20] A.R. Da Costa, A.G. Fane, D.E. Wiley, Spacer characterization and pressure drop modeling in spacer-filled channels for ultrafiltration, J. Membr. Sci. 87 (1994) 79–98. [21] F. Li, W. Meindersma, A.B. de Haan, T. Reith, Optimization of commercial net spacers in spiral wound membrane modules, J. Membr. Sci. 208 (2002) 289–302. [22] S.S. Bucs, A.I. Radu, V. Lavric, J.S. Vrouwenvelder, C. Picioreanu, Effect of different commercial feed spacers on biofouling of reverse osmosis membrane systems: A numerical study, Desalination 343 (2014) 26–37. [23] A. Saeed, R. Vuthaluru, H.B. Vuthaluru, Impact of feed spacer filament spacing on mass transport and fouling propensities of RO membrane surfaces, Chem. Eng. Commun. 202 (5) (2015). [24] A. Siddiqui, N. Farhat, S.S. Bucs, R.V. Linares, C. Picioreanu, J.C. Kruithof, M.C. M. van Loosdrecht, J. Kidwell, J.S. Vrouwenvelder, Development and characterization of 3D-printed feed spacers for spiral wound membrane systems, Water Res. 91 (2016) 55–67. [25] A.I. Radu, M.S.H. van Steen, J.S. Vrouwenvelder, M.C.M. van Loosdrecht, C. Picioreanu, Spacer geometry and particle deposition in spiral wound membrane feed channels, Water Res. 64 (2014) 160–176. [26] M. Amokrane, D. Sadaoui, M. Dudeck, C.P. Koutsou, New spacer designs for the performance improvement of the zigzag spacer configuration in spiral-wound membrane modules, Desalin. Water Treat. 57 (2015) 5266–5274. ´ lvarez-Sa´nchez, Synergies between pulsatile flow and [27] G.A. Fimbres-Weihs, J. A spacer filaments in reverse osmosis modules, in: A. Maciel-Cerda (Ed.), Membranes, Springer, Cham, Switzerland, 2017. [28] X. Huang, G.R. Guillen, E.M.V. Hoek, A new high-pressure optical membrane module for direct observation of seawater RO membrane fouling and cleaning, J. Membr. Sci. 364 (2010) 149–156. [29] Imaging particle analysis. Available from: URL: https://en.wikipedia.org/wiki/ Imaging_particle_analysis.

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[30] P. Willems, N.G. Deen, A.J.B. Kemperman, R.G.H. Lammertink, M. Wessling, M. van Sint Annaland, J.A.M. Kuipers, W.G.J. van der Meer, Use of particle imaging velocimetry to measure liquid velocity profiles in liquid and liquid/gas flows through spacer filled channels, J. Membr. Sci. 362 (2010) 143–153. [31] M. Shakaib, S.M.F. Hasani, M. Mahmood, Study on the effects of spacer geometry in membrane feed channels using three-dimensional computational flow modeling, J. Membr. Sci. 297 (2007) 74–89. [32] M. Shakaib, S.M.F. Hasani, M. Mahmood, CFD modeling for flow and mass. transfer in spacer-obstructed membrane feed channels, J. Membr. Sci. 326 (2009) 270–284. [33] M. Gimmelshtein, R. Semia, Investigation of flow next to membrane walls, J. Membr. Sci. 264 (2005) 137–150. [34] A.H. Haidari, S.G.J. Heijman, W.G.J. van der Meer, Visualization of hydraulic conditions inside the feed channel of Reverse Osmosis: A practical comparison of velocity between empty and spacer-filled channel, Water Res. 106 (2016) 232–241. [35] D.A. Graf von der Schulenburg, J.S. Vrouwenvelder, S.A. Creber, M.C.M. van Loosdrecht, M.L. Johns, Nuclear magnetic resonance microscopy studies of membrane biofouling, J. Membr. Sci. 323 (2008) 37–44. [36] S.A. Creber, J.S. Vrouwenvelder, M.C.M. van Loosdrecht, M.L. Johns, Chemical cleaning of biofouling in reverse osmosis membranes evaluated using magnetic resonance imaging, J. Membr. Sci. 362 (2010) 202–210. [37] E. Fridjonsson, S. Vogt, J.S. Vrouwenvelder, M. Johns, Early non-destructive biofouling detection in spiral wound RO membranes using a mobile Earth’s field NMR, J. Membr. Sci. 489 (2015) 227–236. [38] R.V. Linares, L. Fortunato, N.M. Farhat, S.S. Bucs, M. Staal, E.O. Fridjonsson, Minireview: novel non-destructive in situ biofilm characterization techniques in membrane systems, Desalin. Water Treatment 57 (48–49) (2016). [39] D. Schwahn, H. Feilbach, T. Starc, V. Pipich, R. Kasher, Y. Oren, Design and test of a reverse osmosis pressure cell for in-situ small-angle neutron scattering studies, Desalination 405 (2017) 40–50. [40] N.Y. Dahdal, V. Pipich, H. Rapaport, Y. Oren, R. Kasher, D. Schwahn, Small-angle neutron scattering studies of Mineralization on BSA Coated Citrate Capped Gold Nanoparticles used as a model surface for membrane scaling in RO wastewater desalination, Langmuir 30 (2014) 15,072–15,082. [41] D. Schwahn, V. Pipich, R. Kasher, Y. Oren, Accumulation of GdCl3 in the feed of a reverse osmosis system during desalination as determined by neutron absorption, J. Phys. Conf. Ser. 746 (1) (2017). [42] S.T.V. Sim, W.B. Krantz, T.H. Chong, A.G. Fane, Online monitor for the reverse osmosis spiral wound module—Development of the canary cell, Desalination 368 (2015) 48–59. [43] S.T.V. Sim, T.H. Chong, W.B. Krantz, A.G. Fane, Monitoring of colloidal fouling and its associated metastability using Ultrasonic Time Domain Reflectometry, J. Membr. Sci. 401–402 (2012) 241–253. [44] S.T.V. Sim, S.R. Suwarno, T.H. Chong, W.B. Krantz, A.G. Fane, Monitoring membrane biofouling via ultrasonic time-domain reflectometry enhanced by silica dosing, J. Membr. Sci. 428 (2013) 24–37. [45] A.H. Taheri, S.T.V. Sim, T.H. Chong, W.B. Krantz, A.G. Fane, Development of new techniques to predict reverse osmosis fouling, J. Membr. Sci. 450 (2014) 1–11.

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Chapter

6

Hybrid System Recently, the number of reports on the hybrid system in which RO and other processes are combined has increased. In this chapter some examples are shown. They were taken mainly from the review articles written by Ang et al. [1] and Suk and Matsuura [2].

6.1 REVERSE OSMOSIS-EVAPORATOR Gienger and Ray [3] attempted to reduce the energy needed to concentrate corn step water. Concentrating 6 wt% of solid to 50 wt% with a minimum energy was aimed at. Fig. 6.1 shows the reverse osmosis (RO)-evaporator hybrid system by which they have successfully reduced the energy requirement to one-third of the evaporator alone.

6.2 MICROFILTRATION-RO Municipal wastewater could not be processed to a satisfactory level by the conventional wastewater treatment for the reuse of water in various applications, especially when the reduction of TDS was required. The product quality is much better when wastewater is treated by RO than by the conventional treatment system. However, extensive pretreatment is required for RO to be in its maximum operation. Ghayeni et al. [4, 5] used hollow fiber microfiltration (MF) for the pretreatment of municipal wastewater. The flux was maintained below “critical flux” by applying low-pressure operation. As well the permeation of polio virus was minimized at low feed pressure. When the feed swage water contained a large amount of bacteria, passage of bacteria of mall sizes was observed. As for RO membranes, degree of biofouling depended on the membrane. The most hydrophilic membrane exhibited the least biofouling. MF pretreatment for SWRO was attempted as early as in 1997 [6]. A comparison between MF and the conventional pretreatment system revealed that the quality of MF permeate was far better than the conventional treatment. The cost analysis has also shown that MF is more economical than the conventional system. Reverse Osmosis. https://doi.org/10.1016/B978-0-12-811468-1.00006-2 # 2019 Elsevier Inc. All rights reserved.

143

144 CHAPTER 6 Hybrid System

RO membranes

*MVR evaporator

Fresh steep water

Retentate

(6 wt% solids)

(15 wt% solids)

Vapor compressor

High pressure pump Concentrate (50 wt% solids)

*MVR: mechanical vapor-recompression

Permeate

Product water

n FIG. 6.1 Hybrid RO-evaporator system [2].

Combined coagulation-dual media filtration-MF was used for the pretreatment of seawater for desalination by RO. Different MW ranges of organic matters could be removed by this system [7]. Ozone treatment was applied before seawater entered MF. It was found that ozone treatment significantly reduced fouling of the hydrophobic MF membrane [8]. Application of chlorination before MF also reduced the biofouling of the MF membrane significantly [9]. The MF membrane can remove bacteria but bacteria eventually regrow in the MF permeate using the chlorinated organic compounds as the food. Combined coagulation-adsorption-submerged MF enabled to remove 70% of dissolved organic carbon (DOC) from seawater with dosage of a small amount of flocculant and adsorbent [9, 10].

6.3 ULTRAFILTRATION (UF)-RO Ultrafiltration (UF) membranes have found wider applications for the pretreatment of seawater for RO desalination than MF due to its ability to remove suspended organics, silt pathogens, and viruses. Particularly, MF cannot remove viruses as stated above [11]. The summary of UF-RO hybrid systems is given in Table 6.1. Table 6.2 shows the cost of UF-RO hybrid system in comparison to the conventional pretreatment-RO system [26]. The specific investment costs of UF pretreatment will increase because of the investment of UF system but it is offset by a decrease in the desalination system, resulting in almost the same total specific investment cost. Operational costs are in general less

6.4 Nanofiltration (NF)-RO 145

Table 6.1 Summary of UF-RO Hybrid Systems Plant

Pretreatment System

Effect Observed

References

SWRO pilot plant in Singapore ONDEO Services of Gibraltar

MF and UF

UF was better than MF

[12, 13]

Coagulation-UF

[14]

Ashdod plant on the Mediterranean

Coagulation-UF system

Kindasa SWRO plant

Ferric chloride coagulation-UF

Yuhuan, Zhejiang province (China)

Polyferric sulfate-UF

SDI was reduced from 13 to 25 to 0.5. Conventional system could not reach 60% with 30% cost reduction Demonstration plant construction based on the above work Removal of colloidal matters and inorganic scale matters was possible 96.3% TOC was removed with 0.06–0.36 mg/L TOC in the filtrate. Gradual membrane fouling was observed Water production cost of 0.92 $/m3 with recovery factor of 76.2% Achievement of water recovery 92.8% by applying MCr on NF and RO retentate It was possible to remove hardness, turbidity, microorgsnisms, and to reduce chemical and energy consumption. Water production cost was reduced 30% Hybrid was the best with rejections of salinity 78.65, TDS 76.52, EC 76.42, Cl 63.95, and Na 70.91% Energy consumption for FDFO-NF was 1.08 kWh/ m3, which is 13.6% less energy than an MF-RO and 21% less than UF-RO

[31]

Umm Lujj, Saudi Arabia

(DFSMF)-NF NF UF-NF

NF for RO-MD NF-RO-Membrane Crystallization (MCr) NF for RO-MD

Desalination household scale plant (Luna Water 100 GPD) Treatment of mine impaired water

NF, RO, and NF-RO

Fertilizer drawn FO (FDFONF) is compared with MF-RO and UF-RO

reduced, leading to lower RO pressure. Another benefit of FO is that the rejection efficiency of the trace organic compounds (TrOCs) is higher than that of RO. Thus, TrOCs can be removed more effectively by the FO-RO hybrid system [42–44] due to the reverse salt flux. The rejection efficiency of boron is also higher for FO [45]. Table 6.4 shows the summary of hybrid FO-RO system for seawater desalination based on Chekli et al.’s work [50].

6.5.2 Wastewater Treatment FO-RO hybrid system can also be used for the wastewater treatment. Some of the recent examples are shown in Table 6.5.

[32] [33] [34]

[35, 36] [37, 38] [39]

[40]

[41]

148 CHAPTER 6 Hybrid System

Table 6.4 Summary of Hybrid FO-RO System for Seawater Desalination FO System

System Detail

Membrane

Draw solution

FO-RO

Glucose draw solution (DS) is diluted by seawater at FO and diluted glucose solution is subjected to RO to recover water. Secondary waste water is supplied to FO to dilute Red Sea water, which is then subjected to RO. Secondary wastewater is supplied to FO to dilute seawater, which is then subjected to RO to obtain product water. RO brine goes to second FO to be diluted before discharge. Wastewater supplied to FO to dilute seawater, which is then subjected to RO.

–

CTA

FO-RO

FO-ROFO

Pressure assisted FO (PAFO)RO

RO

Effect

References

Glucose

Lowpressure reverse osmosis (LPRO)

Low osmotic pressure of glucose, high internal concentration polarization (ICP)

[46]

Red Sea water

LPRO

Energy requirement 50% of SWRO (1.5 kWh/m3)

[47]

SW30 2540 Dow Filmtec

Wide range of organic compounds can be removed by FO

[48]

Simulation Pressure assisted FO (PAFO) at 6 bar further reduces the water production cost. System operation is stabilized

[49]

CTA

Table 6.5 Summary of Hybrid FO-RO System for Wastewater Treatment FO System

System Detail

Membrane

Draw solution

RO

MBRFO-RO

Water is drawn from MBR to DS in FO. RO produces water while DS is reconcentrated.

CTA

NaCl

SW30HR Dow Chemical

MBRFO-RO

Water is drawn from MBR to DS in FO. RO produces water while DS is reconcentrated.

CTA

Na-formate, acetate, propionate, Mg-acetate

SW30HR Dow Chemical

Effect

References NH+4 -N

were TOC and removed 99 and 98%, respectively. Substantially less backwashing was required than conventional BMR. Reverse salt fluxes of some solutes were lower than inorganic salts and organic anions are biodegradable.

[51]

[52]

6.5 Forward Osmosis (FO)-RO 149

6.5.3 Simultaneous Wastewater and Seawater Treatment Dilution of seawater alleviates the high pressure requirement of RO. Hence, wastewater and seawater can be provided into FO as the feed and DS, respectively, to dilute the seawater [53]. Then, the diluted seawater is subjected to RO to produce clean potable water. The system is shown in Fig. 6.2. This system was already used in desalination of Red Sea water [47]. Experiments have also been made for the FO-RO system where three SWRO modules were connected in series and combined with FO [54]. In these FO-RO processes, DS flow is once flow configuration and its regeneration and recycle are not required, offering the true benefit of FO as a low-energy process. As a whole this dilution of seawater by wastewater is not direct mixing of two streams. Therefore, fouling of RO membrane does not occur by wastewater components. Furthermore, there are two barriers (FO and RO) for the wastewater contaminants to enter to the clean potable water, which ensures the safe reuse of wastewater. The simultaneous treatment of waste and seawater is still in an early stage. One drawback of FO-RO hybrid system is rather the low FO membrane flux. To increase water recovery and to reduce the amount of brine discharge, Altaee and Hilal proposed NF-FO-BWRO process [55] (Fig. 6.3). The brackish water is first treated by NF producing clean water at relatively low pressure. The NF concentrate is then subjected to FO to further extract water in DS and discharged. The DS coming from FO goes into BWRO to produce clean water and also to reconcentrate DS. The reconcentrated DS is recycled back to FO. The product clean water from NF and BWRO can also

Concentrated wastewater

Wastewater Seawater intake

FO

RO Permeate

Pre-treatment

Brine n FIG. 6.2 Simultaneous wastewater and seawater treatment.

150 CHAPTER 6 Hybrid System

Disposal NF

FO

BWRO

Brackish water DS

Product water

Product water n FIG. 6.3 Schematics of NF-FO-BWRO system.

be combined to adjust the TDS of the product water [55]. The simulation showed that 90% of product water recovery is possible, 75% of which comes from NF. The simulation also showed that the recovery rate depends on the DS concentration. The higher the DS concentration, the higher becomes the recovery rate. Kim et al. evaluated various options for the full-scale modular configuration of FO process for osmotic dilution of seawater using wastewater for simultaneous desalination and water reuse through the FO-RO hybrid system. They found that the main limiting criteria for module operation were to maintain the feed wastewater pressure higher than the DS (seawater) pressure throughout the housing module for safe operation without affecting membrane integrity. To satisfy this requirement they have proposed a two-stage FO configuration with multiple housings (in parallel) in the second stage [56] (see Fig. 6.4). Ali et al. investigated a FO-RO hybrid process to treat seawater and impaired water from Ezz Steel plant in Alexandria, Egypt, simultaneously. A pilot-scale system with 4040 FO-FS module made up of several flat-sheet cellulose triacetate (CTA) with embedded polyester FO membranes (Hydration Technologies, Albany, OR) and Filmtec, spiral wound, type BW 30-40-40, TFC (polyamide composite) was utilized for the study. The results indicated that the salinity of seawater reduced from 35,000 to 13,000 mg/L after 3 h using FO system, while after 6 h it approached 10,000 mg/L. FO/RO system was tested on continuous operation for 15 h and it was demonstrated that no pollutant was detected neither in DS nor in RO permeate after the end of operating time [57].

6.6 PRESSURE-RETARDED OSMOSIS (PRO)-RO Pressure-retarded osmosis (PRO) is a device to generate power using osmosis. The principle is schematically shown in Fig. 6.5.

6.6 Pressure-Retarded Osmosis (PRO)-RO 151

Forward osmosis Concentrated FS

Wastewater

Fresh water Seawater Reverse osmosis Concentrated DS Two-stage FO modular configuration

SW

E1

E2

E3

E1

E2

E3

E4

E1

E2

E3

E4

E4

WW

Diluted SW P

n FIG. 6.4 Two-stage FO configuration proposed by Kim et al. [56].

When pure water and brine (salt solution) is placed on both sides of a semipermeable membrane, and a pressure ΔP (ΔP < Δπ) is applied on the brine side, water flows through the membrane from pure water side to brine side against the hydraulic pressure ΔP. Thus, power equal to (flow rate of water  ΔP) can be generated. The power is recovered by driving a turbine or by using a pressure exchanger. There are two advantages of coupling SWRO and PRO, where the concentrated brine is supplied to the PRO as DS, while solution of low salinity, for example, wastewater is supplied to PRO as the feed solution. One is that the power generation of PRO is enhanced due to the higher osmotic pressure of concentrated brine than seawater and the other is that the environmental problem is alleviated since the concentrated brine is diluted before it is discharged to into the ocean. Although there are many different ways to combine RO and PRO, they can be classified in two groups. In one the high pressure of DS is transferred to the RO feed via pressure exchanger and in the other the high-pressure DS spins turbine to generate electricity. The specific energy required for water production is thus reduced (Figs. 6.6–6.8).

Concentrated WW

152 CHAPTER 6 Hybrid System

DP

Pure water

Brine J

0 < DP < DH n FIG. 6.5 Principle of pressure-retarded osmosis.

There are a number of simulation studies for the RO-PRO hybrid system but only few experimental works have been done using either a small lab-scale equipment or a large demonstration plant, as summarized in Table 6.6, which was made based on the work of Kim et al. [65].

6.7 PERVAPORATION (PV)-RO Phenol is produced by various processes such as phenolic-resin production, pulp and paper industry, petrochemical refining and production, and coking. Phenol is very toxic and when even a small amount of phenol is combined with chlorine the taste of the drinking water is severely deteriorated. Therefore, Environmental Protection Agency (EPA) currently sets the upper limit

n FIG. 6.6 RO-PRO system in Japan [58].

6.7 Pervaporation (PV)-RO 153

Seawater

Pretreatment

To produced water tank

RO

Pressure exchanger Pretreatment

MD

Turbine Draw side Wastewater effluent

Pretreatment

Feed side PRO membrane module

n FIG. 6.7 PRO-MD-RO system in Korea [59].

Seawater basin

PX

Drain RO

Fresh water n FIG. 6.8 RO-PRO system of Achilli et al. [60].

To valuable resource recovery process

PRO

Fresh water

To effluent tank

154 CHAPTER 6 Hybrid System

Table 6.6 Some Experimental Results of PRO-RO Hybrid System PRO System RO-PRO

ROMDPRO

RO-PRO

RO-PRO

System Detail RO brine goes to DS side and pretreated wastewater goes to feed side of PRO (Fig. 6.6). RO brine goes to MD to be further concentrated. MD brine goes to the DS side and pretreated wastewater goes to the feed side of PRO (Fig. 6.7). RO brine goes to DS, filtrated tap water goes to the feed side of PRO (Fig. 6.8). High pressure of DS is transferred to seawater inlet. Same as above

Membrane CTA hollow fiber (Toyobo)

Draw Solution

RO

References 2

RO brine

7.7 W/m was obtained at 2.5 MPa

[58]

RO and MD water production capacity of 1000 m3/day and 400 m3/day, respectively, was achieved with power density of 5 W/m2

[59]

4040 PRO module (Oasys Water)

RO brine

SW30–2540 (Dow Film Tec)

Power density of 1.1–2.3 W/m2 was obtained

[60]

CTA membrane (HTI)

RO brine

SW30–4040 (Dow Film Tec)

Simulation based on the experimental data obtained from RO and PRO subsystem. Net specific power consumption for water production is 1.2 kWh/m3 at 50% RO recovery, 40% less than RO standalone Economic evaluation of RO-PRO hybrid system using model equations 13.5 W/m2 membrane power density. On top of 20% energy reduction by low-pressure RO membrane and RED further 10% energy saving was possible

[61]

RO-PRO

RO-PRO

Effect

10-in hollow fiber module

RO brine

Toray lowpressure RO

[62]

[63]

6.8 RO-Reverse Electrodialysis, RO-Electrodialysis Reversal, and RO-Ion Exchange 155

Table 6.6 Some Experimental Results of PRO-RO Hybrid System Continued PRO System RO-PRO

System Detail

Membrane

Draw Solution

RO

Pilot and demonstration plants of 20 m3/d and 240 m3/d PRO treatment capacity. Thin-film composite spiral-wound PRO membrane modules (8 in)

of phenol concentration in discharge waters at 14 ppm. Ray et al. [66] investigated on the pervaporation (PV)-RO hybrid system and compared its economics with the PV stand-alone system. As can be seen in Fig. 6.9A 100,000 gal/day of wastewater with 500 ppm of phenol is treated by PV, whereby the concentration of phenol is reduced to 14 ppm in the 95,500 gal/day of product water. The studied PV-RO hybrid system (Fig. 6.9B) is featured by a small PV unit (membrane area