Functional Nanostructured Materials and Membranes for Water Treatment E-Book
160,99 €
Mehr erfahren.
- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Serie: Materials for Sustainable Energy and Development
- Sprache: Englisch
Membranes have emerged over the last 30 years as a viable water treatment technology. Earth's population is growing and the need for alternative ways to generate potable water is rising. The recent advent of nanotechnology opens the door to improving processes in membrane
technology, which is a promising step on the way to solving the earth's potable water problem. Current performance is enhanced and new concepts are possible by engineering on the nanoscale. This book presents key areas of nanotechnology such as fouling tolerant and robust membranes, enhanced destruction of pollutants and faster monitoring of water quality.
'Functional Nanostructured Materials and Membranes for Water Treatment' is part of the series on Materials for Sustainable Energy and Development edited by Prof. G.Q. Max Lu. The series covers advances in materials science and innovation for renewable energy, clean use of fossil energy, and greenhouse gas mitigation and associated environmental technologies.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 597
Veröffentlichungsjahr: 2013
Ähnliche
Table of Contents
Related Titles
Title Page
Copyright
Editorial Board
Foreword
Series Editor Preface
The Wiley Series on New Materials for Sustainable Energy and Development
Acknowledgments
About the Series Editor
About the Volume Editors
List of Contributors
Chapter 1: Target Areas for Nanotechnology Development for Water Treatment and Desalination
1.1 The Future of Water Treatment: Where Should We Target Our Efforts?
1.2 Practical Considerations for Nanotechnology Developers
1.3 The Water Treatment Market for New Nanotechnology
1.4 Purpose of This Book
1.5 Concluding Remarks
Chapter 2: Destruction of Organics in Water via Iron Nanoparticles
2.1 Introduction
2.2 Nanoparticles as Catalysts
2.3 Advanced Oxidation Processes
2.4 Nano Zero-Valent Iron (nZVI)
2.6 Summary
Chapter 3: Photocatalysis at Nanostructured Titania for Sensing Applications
3.1 Background
3.2 Fabrication of TiO2 Photoanodes
3.3 The Sensing Application of TiO2 Photocatalysis
3.4 The Sensing Application of TiO2 Photoelectrocatalysis
3.5 Photocatalytic Gas Sensing
3.6 Conclusions
Chapter 4: Mesoporous Materials for Water Treatment
4.1 Adsorption of Heavy Metal Ions
4.2 Adsorption of Anions
4.3 Adsorption of Organic Pollutants
4.4 Multifunctional Modification of Sorbents
4.5 Photocatalytic Degradation of Organic Pollutants
4.6 Conclusions and Outlook
Acknowledgments
Chapter 5: Membrane Surface Nanostructuring with Terminally Anchored Polymer Chains
5.1 Introduction
5.2 Membrane Fouling
5.3 Strategies for Mitigation of Membrane Fouling and Scaling
5.4 Membrane Surface Structuring via Graft Polymerization
5.5 Summary
Chapter 6: Recent Advances in Ion Exchange Membranes for Desalination Applications
6.1 Introduction
6.2 Fundamentals of IEMs and Their Transport Phenomena
6.3 Material Development
6.4 Future Perspectives of IEMs
6.5 Conclusions
Chapter 7: Thin Film Nanocomposite Membranes for Water Desalination
7.1 Introduction
7.2 Fabrication and Characterization of Inorganic Fillers
7.3 Fabrication and Characterization of TFC/TFN Membranes
7.4 Membrane Properties Tailored by the Addition of Fillers
7.5 Commercialization and Future Developments of TFN Membranes
7.6 Summary
Chapter 8: Application of Ceramic Membranes in the Treatment of Water
8.1 Introduction
8.2 Membrane Preparation
8.3 Clarification of Surface Water and Seawater Using Ceramic Membranes
8.4 Ceramic Membrane Application in the Microfiltration and Ultrafiltration of Wastewater
8.5 Conclusions and Prospects
Chapter 9: Functional Zeolitic Framework Membranes for Water Treatment and Desalination
9.1 Introduction
9.2 Preparation of Zeolite Membranes
9.3 Zeolite Membranes for Water Treatment
9.4 Conclusions and Future Perspectives
Acknowledgments
Chapter 10: Molecular Scale Modeling of Membrane Water Treatment Processes
10.1 Introduction
10.2 Molecular Simulations of Polymeric Membrane Materials for Water Treatment Applications
10.3 Molecular Simulation of Inorganic Desalination Membranes
10.4 Molecular Simulation of Membrane Fouling
Chapter 11: Conclusions: Some Potential Future Nanotechnologies for Water Treatment
11.1 Nanotubes
11.2 Graphene
11.3 Aquaporins
11.4 Metal–Organic, Zeolitic Imidazolate, and Polymer Organic Frameworks
11.5 Conclusions
Index
Related Titles
Lior, N.
Advances in Water Desalination
ISBN: 978-0-470-05459-8
García-Martínez, J., Serrano-Torregrosa, E. (eds.)
The Chemical Element
Chemistry's Contribution to Our Global Future
2011
ISBN: 978-3-527-32880-2
Peinemann, K.-V., Pereira Nunes, S. (eds.)
Membrane Technology
Volume 4: Membranes for Water Treatment
2010
ISBN: 978-3-527-31483-6
Li, N. N (ed.)
Advanced Membrane Technology and Applications
2008
ISBN: 978-0-471-73167-2
Kumar, C. S. S. R. (ed.)
Nanomaterials - Toxicity, Health and Environmental Issues
2006
ISBN: 978-3-527-31385-3
The Volume Editors
Prof. Dr. Mikel Duke
Victoria Univ., Inst. of
Sustainability+ Innovat., Blg. 4
Room 4.107, Hoppers Lane
Werribee, Victoria 3030
Australien
Prof. Dr. Dongyuan Zhao
Fudan University
Dept. of Chemistry
Handan Road 220
Shanghai 200233
Volksrep. China
Prof. Raphael Semiat
Technion - Israel Institute of
Technology, The Wolfson Chem.
Engineering Dept.
32000 Technion City, Haifa
Israel
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.: applied for
British Library Cataloguing-in-Publication Data
A catalogue record for this book is available from the British Library.
Bibliographic information published by the Deutsche Nationalbibliothek
The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form — by photoprinting, microfilm, or any other means — nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-527-32987-8
ePDF ISBN: 978-3-527-66849-6
ePub ISBN: 978-3-527-66848-9
mobi ISBN: 978-3-527-66847-2
oBook ISBN: 978-3-527-66850-2
Editorial Board
Members of the Advisory Board of the “Materials for Sustainable Energy and Development” Series
Foreword
It is with great pleasure to present to you this book on the “Functional Nanostructured Materials and Membranes for Water Treatment.” Nanomaterials are an emerging area in science that demonstrated achievements occurring in the last two decades. We can date its beginnings from the name, ‘nanotechnology’, which was originally coined by Norio Taniguchi, a professor from Tokyo University of Science, in 1974. The emergence of nanotechnology research appears to have come later with the advent of the scanning tunnelling microscope in 1981 and the discovery of fullerenes in 1985. So from that time, besides the concept, we acquired the capacity to identify and characterise nanostructured materials, and this marks the start of the field of science dedicated to the nanoscale.
Nanotechnology is basically the field of science involving the manipulation of matter or theory at the atomic or molecular scale. In doing this, we have unearthed in exciting new properties that can influence the wavelength of light, increase the efficiency of catalysts, selectively diffuse small molecules, and even allow particles to penetrate living cell membranes. It is interesting to note that the natural world has been based on nanostructured materials throughout evolution, and thus, mankind has just started to recognise the need for tools to explore properties of materials in 1 to 100 nm range. It is no wonder that governments around the world have invested billions of dollars specifically in nanotechnology. While industries can harness the different benefits of such materials for energy, foods, mining and, electronics, this book focuses on the advances for water treatment.
Water will always be an essential factor to our lives, and thus, there is an ongoing cause for researchers undertaking efforts to secure sustainable water sources and reduce water pollution. Water treatment is an area where I have spent much of my career, principally in the field of membrane science. During my work I have explored membrane transport properties and fouling mechanisms, which has contributed towards the underpinning science of the widespread membrane technology in the water industry. This is quite remarkable for a technology that was little heard of nearly half a century ago, and is recently one of the most widely adopted water treatment technologies. Over the years, membrane technology has involved working with nanostructured materials, although until now it has not been considered as a branch of nanotechnology. Now researchers are turning to nanotechnology to address the challenges to a sustainable water future, and in many cases this involves the marriage between nanotechnology and membrane technology. Many of the contributors to this book are membrane scientists and engineers who have the vision that membranes are an invaluable technology in water treatment and it can be enhanced by nanotechnology. Conventional membrane technology already includes a process of separating nano-dimensional molecules known as Nanofiltration. This technique is commercially applied, for example, to remove organic materials (e.g. natural organic matter or sugars) from salts. However, a recently successful development that deliberately combines membrane technology and nanotechnology is the inclusion of nanoparticles in desalination membranes. This work, coming out of the birthplace of reverse osmosis membranes, the University of California at Los Angeles in the USA, has now entered the market as a commercial desalination membrane. The inclusion of nanoparticles within the polymer structure of membranes has performance and practical benefits, that has also been explored in other forms of membranes, including ultra and microfiltration. While we see this activity rising rapidly into commercialisation, this book also presents work in catalysis, sensing, adsorption, membrane modification, ion exchange, inorganic membranes, and nanoscale modelling of membrane diffusion and interactions. Therefore, this book presents a comprehensive overview of the progress in nanotechnology to enhance membranes and other processes in water treatment. Whether you are an academic or working in industry, scientist or engineer, student or professional, this book will have relevance in your practice.
Professor Tony Fane
UNESCO Centre for Membrane Science and Technology
University of New South Wales, Sydney, Australia
and
Singapore Membrane Technology Centre
Nanyang Technological University, Singapore
Series Editor Preface
The Wiley Series on New Materials for Sustainable Energy and Development
Sustainable energy and development is attracting increasing attention from the scientific research communities and industries alike, with an international race to develop technologies for clean fossil energy, hydrogen and renewable energy as well as water reuse and recycling. According to the REN21 (Renewables Global Status Report 2012 p. 17) total investment in renewable energy reached $257 billion in 2011, up from $211 billion in 2010. The top countries for investment in 2011 were China, Germany, the United States, Italy, and Brazil. In addressing the challenging issues of energy security, oil price rise, and climate change, innovative materials are essential enablers.
In this context, there is a need for an authoritative source of information, presented in a systematic manner, on the latest scientific breakthroughs and knowledge advancement in materials science and engineering as they pertain to energy and the environment. The aim of the Wiley Series on New Materials for Sustainable Energy and Development is to serve the community in this respect. This has been an ambitious publication project on materials science for energy applications. Each volume of the series will include high-quality contributions from top international researchers, and is expected to become the standard reference for many years to come.
This book series covers advances in materials science and innovation for renewable energy, clean use of fossil energy, and greenhouse gas mitigation and associated environmental technologies. Current volumes in the series are:
In presenting this volume on Functional Nanostructured Materials and Membranes for Water Treatment, I would like to thank the authors and editors of this important book, for their tremendous effort and hard work in completing the manuscript in a timely manner. The quality of the chapters reflects well the caliber of the contributing authors to this book, and will no doubt be recognized and valued by readers.
Finally, I would like to thank the editorial board members. I am grateful to their excellent advice and help in terms of examining coverage of topics and suggesting authors, and evaluating book proposals.
I would also like to thank the editors from the publisher Wiley-VCH with whom I have worked since 2008, Dr Esther Levy, Dr Gudrun Walter, and Dr Bente Flier for their professional assistance and strong support during this project.
I hope you will find this book interesting, informative and valuable as a reference in your work. We will endeavour to bring to you further volumes in this series or update you on the future book plans in this growing field.
Brisbane, Australia
Gao Qing Max Lu
31 July 2012
Acknowledgments
Mikel Duke, Dongyuan Zhao and Raphael Semiat would like to thank all the authors for their hard work and commitment in producing their original contributions for this book. Also, we appreciate their responsiveness to our requirements in their ongoing comments and revision requests during the production phase of the chapters.
All contributions were peer reviewed, so we also extend a warm thanks to the reviewers for their time and effort in providing detailed and quality comments to the authors.
We would finally like to thank Esther Levy, Martin Graf and Claudia Nussbeck at the Wiley Editorial Office for their assistance, rapid response to questions, and enduring patience that enabled this book to be completed.
Mikel Duke wishes to thank the team at his group, the Institute for Sustainability and Innovation, Victoria University, for their patience and support in developing this book. He also acknowledges the funding agencies that provided research support related to the theme of this book including the Australian Research Council Linkage and Discovery Project schemes, the Victorian Smart Water Fund, the Australian Endeavour Awards, The Ian Potter Foundation, the National Centre of Excellence in Desalination Australia, the Australian Water Recycling Centre of Excellence, and the numerous industry and government partners. Max Lu's invitation to develop this book is greatly appreciated as well as the partnership of Dongyuan Zhao and Raphael Semiat in co-editing this book. Finally, Mikel wishes to thank Alicia and daughters, Eva and Lila, for their personal support.
Dongyuan Zhao would like to thank the NSF of China, the Fudan University, the China Ministry of Education, and the China Ministry of Science and Technology, who provided their support for research during the writing of the book.
Raphael Semiat would like to thank the team of the Rabin Desalination Laboratory at the Technion IIT for their help during this work.
About the Series Editor
Professor Max LuEditor, New Materials for Sustainable Energy and Development Series
Professor Lu's research expertise is in the areas of materials chemistry and nanotechnology. He is known for his work on nanoparticles and nanoporous materials for clean energy and environmental technologies. With over 500 journal publications in high-impact journals, including Nature, Journal of the American Chemical Society, Angewandte Chemie, and Advanced Materials, he is also coinventor of 20 international patents. Professor Lu is an Institute for Scientific Information (ISI) Highly Cited Author in Materials Science with over 17 500 citations (h-index of 63). He has received numerous prestigious awards nationally and internationally, including the Chinese Academy of Sciences International Cooperation Award (2011), the Orica Award, the RK Murphy Medal, the Le Fevre Prize, the ExxonMobil Award, the Chemeca Medal, the Top 100 Most Influential Engineers in Australia (2004, 2010, and 2012), and the Top 50 Most Influential Chinese in the World (2006). He won the Australian Research Council Federation Fellowship twice (2003 and 2008). He is an elected Fellow of the Australian Academy of Technological Sciences and Engineering (ATSE) and Fellow of Institution of Chemical Engineers (IChemE). He is editor and editorial board member of 12 major international journals including Journal of Colloid and Interface Science and Carbon.
Max Lu has been Deputy Vice-Chancellor and Vice-President (Research) since 2009. He previously held positions of acting Senior Deputy Vice-Chancellor (2012), acting Deputy Vice-Chancellor (Research), and Pro-Vice-Chancellor (Research Linkages) from October 2008 to June 2009. He was also the Foundation Director of the ARC Centre of Excellence for Functional Nanomaterials from 2003 to 2009.
Professor Lu had formerly served on many government committees and advisory groups including the Prime Minister's Science, Engineering and Innovation Council (2004, 2005, and 2009) and the ARC College of Experts (2002–2004). He is the past Chairman of the IChemE Australia Board and former Director of the Board of ATSE. His other previous board memberships include Uniseed Pty Ltd., ARC Nanotechnology Network, and Queensland China Council. He is currently Board member of the Australian Synchrotron, National eResearch Collaboration Tools and Resources, and Research Data Storage Infrastructure. He also holds a ministerial appointment as member of the National Emerging Technologies Forum.
About the Volume Editors
Professor Mikel Duke is the Principal Research Fellow of Membrane Science and Deputy Director of the Institute for Sustainability and Innovation at Victoria University, Australia. He has worked in membrane research for over 12 years and has 92 peer-reviewed publications in this field. His focus is on development of ceramic and polymeric membranes and their processes, specializing in molecular scale diffusion and optimizing functional material parameters. He is the recipient of an Australian Research Council Linkage International Fellowship and an Endeavour Executive Award and the founding chair of the Membrane Society of Australasia.
Professor Dongyuan Zhao is Cheung Kong Professor of the China Education Ministry, Vice Director of the Advanced Materials Laboratory at Fudan University and Visiting Professor at Monash University (Australia). He is an academician of the Chinese Academy of Sciences. With over 350 peer-reviewed papers earning>20 000 citations, he is the 65th Most-Cited Scientist in Chemistry (according to ISI). His research interests are in the synthesis of porous materials and their application in catalysis, separation, photonics, sorption, environmental decontamination, sensors, and so on.
Professor Raphael Semiat is the Yitzhak Rabin Memorial Chair in Science, Engineering and Management of Water Resources at Technion, Israel Institute of Technology. He has wide industrial experience in the research and development of chemical processes. His current interests and activities are centered on water technologies, including desalination, and chemical-environmental processes and use of nano particles for removal of organic matter and heavy metals from water. He has published more than 140 papers in scientific journals.
List of Contributors
Chapter 1
Target Areas for Nanotechnology Development for Water Treatment and Desalination
Mikel Duke, Raphael Semiat, and Dongyuan Zhao
It is of no surprise to many around the world that water is a priority research area: water is one of the most fundamental elements of our existence. We use water for drinking, cleaning, cooking, removing waste, recreation, manufacturing, cooling, and so on. These uses have criteria related to the quality for its intended purpose (e.g., drinking) and quality to minimize its harm to the environment when it is disposed after use. To meet these criteria, there is nearly always a treatment process that requires energy and chemicals. Our health and our environment are significant priorities. We have conflicting issues regarding energy usage and chemicals that lead to pollution and harmful by-products associated with their production, delivery, and disposal; hence, we need to minimize the usage of those resources while improving water quality and counter the risk of human illness or the damage to our ecosystems. This is therefore, the motivation for innovative technologies for water treatment: reduced energy and chemicals use. Of course this must be achieved at low cost.
1.1 The Future of Water Treatment: Where Should We Target Our Efforts?
Many research endeavors ranging from fundamental to applied, typically address specific issues and aim to make improvements based on relative measures. For example, we can report improvements to new types of materials to remove microcystin, or we can, for the first time, apply a commercial ceramic membrane to filter an industrial waste. But what in common drives such efforts, and where is all this heading in terms of the greater needs of the society for low energy, safe, and reliable water?
Technologies that are used for water treatment include adsorption, coagulation, reaction, heating, and filtration. These have the effect of removing or deactivating/converting unwanted elements, such as salt, organics, odors, microbes, suspended solids, and toxins. The choice of technologies and the required removal varies considerably around the world, driven by specific quality needs or regulatory requirements. Water treatment systems must
reliably provide water fit for its intended purpose (e.g., drinking water) and
collect contaminated water (by humans and/or industry) and remove harmful components before its release to the environment or reuse.
The connection between these points (i.e., closing the loop) increases as technologies become more efficient, reliable, and available. Ideally, we might like to take any water source, regardless of its contamination, and convert it directly to drinking quality water. This is known as direct potable reuse and has many incentives for a future with sustainable water [1]. This pathway to adopting such treated wastewaters for applications that include human contact (irrigation, drinking, washing, etc.) will involve not only efficient technologies but also evidence of their reliability and is thus another measure from an innovative solution.
The argument for direct potable reuse is that it is far less energy and resource intensive than indirect potable reuse (i.e., holding the water in a large “diluting” body such as a reservoir before reusing). But under increasing economic, energy, and environmental pressures, we foresee that recent developments based on nanotechnology, which might take at least five years to come to market, are likely to become part of a future headed toward direct potable reuse systems. This might give some thought to how researchers might like to steer their work. For example, energy saving systems that produce high-quality water safely and reliably will be likely successes. The alternative to this is treating water that is “fit for purpose,” – it meets the minimum requirements for another purpose such as direct (nonpotable) reuse – for example, desalination of saline waste for reuse in an industrial boiler. Such opportunities require less public and government acceptance to engage (and are indeed already underway), but there must be a convenient user of this water to make fit-for-purpose treatment viable. So it appears that for technologies emerging in the next decade, we should aim to provide solutions that support the fit-for-purpose agenda, when the market for the water is known. Therefore, efforts that bring the costs and environmental impacts down to reliably deliver water for our direct consumption are most worthwhile.
In this book, we present nine chapters focused entirely on technology approaches to improve water quality. All of them essentially and ultimately aim to achieve fit-for-purpose, or even direct potable reuse, aligning with the future demands of the water industry. Specifically, the contributions have routes in nanotechnology respecting that the chemistry, materials, and thinking at this scale offers new opportunities for future water treatment. The technologies considered harness functions such as catalysis, sensing, diffusion, and adsorption.
1.2 Practical Considerations for Nanotechnology Developers
Any new nanotechnology that demonstrates virtue for water treatment must undergo a rigorous process to validate its full commercial and environmental potential. We have listed the following considerations/questions that should be determined/answered at the earliest phases of development to facilitate its success as a water treatment solution:
Nanoparticles cannot reach people, animals, or the environment.
No new hazardous by-products are inadvertently created.
Unwanted materials and by-products (if created) are completely removed or mineralized.
Is reaggregation of nanoparticles going to occur? Is this a problem?
Is the process that must be installed to harness the nanotechnology simple? Can untrained people use it? Is it expensive?
On top of these specific considerations, the work must also consider the broader implications:
Does the new treatment solve the problem or generate a new one?
What is the fate of the contaminants – can we completely destroy them? Recover? Or maybe they are returned to the environment? What is the cost?
We propose that the aforementioned points be considered in any future research and in turn publications to be considered when weighing up if their nanotechnology is on the right path to becoming a practical water treatment process. As you will see in the following contributions from the authors' areas of expertise, there has been a focus on the new technologies in working toward real needs in the water industry.
1.3 The Water Treatment Market for New Nanotechnology
Clearly, with the recent scientific developments in the last decade in the field of nanotechnology, these aspire to commercial use. But how big is this market expected to be? A report published in early 2011 by BCC Research [2] looked at the current market status of nanotechnology applied to water treatment and forecasts its growth. In 2010, they estimated the market (in US dollars) for nanostructured products used in water treatment to be $1.4 billion. By 2015, they expected this to grow to $2.2 billion. Interestingly, this was mostly confined to established products including membrane technology, which is categorized as nanobased (reverse osmosis, nanofiltration, and ultrafiltration). Of the nine original contributions to this book, six are based on membrane technology, aligning with the significance of membranes picked up by the market report. For emerging nanomaterials such as nanofiber fillers, carbon nanotubes, and nanoparticles alone the market estimate was $45 million in 2010, but is expected to grow rapidly to $112 million by 2015.
We have pointed out that researchers should target their efforts at achieving potable water quality, or at best a significant and defined fit-for-purpose application. At the same time, this should be done with demonstrated opportunities for cost reduction, energy savings, and chemical reduction while no other consequences emerge as a result. With these taken into account, successful nanotechnologies are expected by economists to experience growth in the sales in the next five years. Therefore, it seems safe to say that it is an exciting time for scientists and engineers to develop new technologies and theories as it is certain that the market will take them up in future. We are currently at the phase where science is demonstrating the concepts proposed, but there is still a lot of work ahead in terms of measuring the performance and economic potential in real application.
1.4 Purpose of This Book
There are a multitude of agendas for improving desalination and water recycling deployment, for example, demand management, public perception, “simple” solutions, and of course new technologies. None of these will solve our water issues alone, but in this book, we focus on new technologies and thinking that have been borne out by exploring at the nanoscale, which has emerged only in the last decade from fundamental level research. The chapters presented in this book cover the major areas where nanotechnology has shown promise in addressing the issues in water treatment that are understood by industry.
In Table 1.1, we broadly divide water treatment into the three categories: pollutant removal, detection/monitoring, and desalination. Many water treatment efforts can be defined under these categories for relatively simple purposes as shown. Some of the chapters spread across these categories (e.g., desalination and pollutant removal). It is interesting to point out that despite these broad and highly differing purposes, the concepts behind the technology share a lot in common, such as catalysis, adsorption, materials engineering, colloidal chemistry, and molecular diffusion. These are well-known scientific pillars of nanotechnology. Therefore, the purpose of this book is not only to demonstrate working nanotechnological solutions for major water treatments but also to highlight the common sciences and achievements that bring about such solutions.
Table 1.1 The purposes of each water treatment category aligned to the chapters presented in this book and the nanotechnology concepts applied
In the chapters, we show how nanotechnology leads us to develop new materials, improving existing technologies (e.g., membranes), and to enhance our understanding of complex processes (e.g., molecular simulations). Developing new materials from the bottom up offers new and exciting opportunities for efficiency improvements yet unseen by industry. Such materials are included in Chapters 2–4. Improving existing technologies is part of Chapters 5–9. For example, membrane technology has been successfully deployed at full scale for water filtration and desalination, but limitations are being realized through the ongoing issues related to fouling. Also, as current technologies such as membranes move into more challenging water treatment areas, these issues will become cost prohibitive. So the priority in research is to explore ways to enhance membrane life and performance by way of improved fouling tolerance and durability without compromising on the essential flux and selectivity features. We also broaden the thinking of nanotechnology beyond materials developing in Chapter 10, where nanodimension modeling gives fresh insight into molecular diffusion, interaction chemistries, and fouling mechanisms. This not only contributes new science but also offers new approaches to manage plant operation to gain better performance of our current membrane technologies.
1.5 Concluding Remarks
We have identified that fit-for-purpose, and ultimately direct potable reuse, should be on the minds of nanotechnology researchers when developing their technologies for likely uptake in no less than five years. The chapters in this book cover most of the global efforts underway to bring about these water treatment agendas. The authors of this book were identified when the book was conceived to provide expert contributions from their field, but we duly acknowledge that more nanotechnology research is being carried out beyond what has been published here particularly as the field is in a state of rapid growth with creative minds continually emerging with new ideas.
Finally, we would like to make a mention of the current state of the world in which this book was written, which has given priority to water treatment research. At the time of writing, the world was undergoing major economic issues, specifically the Global Financial Crisis. This had a direct consequence to funding research that addresses our need for improving environmental and economic sustainability. Despite the uncertainty in global economies, climate change was recently accepted by politicians while society begins to witness never before weather activities such as severe and prolonged water scarcity and pollution. This is compounded by the rapid intensification of mining, resource extraction, and manufacturing that presents new water treatment challenges. So with the achievements in the fundamental science giving rise to nanotechnology in the last decade, now is an exciting time to drive emerging nanotechnology solutions to practically solve our most critical issues.
Whether you are an engineer or a scientist, a student, or working in industry or research, we hope this book serves your needs and gives you a comprehensive picture of the emerging nanotechnology and associated sciences now being applied to water treatment.
References
1 Khan, S.J. (2011) The case for direct potable reuse in Australia. Water – J. Aust. Water Assoc., 38 (4), 92–96.
2 BCC Research (2011) Nanotechnology in Water Treatment, BCC Research.
Chapter 2
Destruction of Organics in Water via Iron Nanoparticles
Hilla Shemer and Raphael Semiat
2.1 Introduction
Nanotechnology is expected to become a key component of future technologies. The following chapter focuses on degradation of organic pollutants by advanced oxidation processes (AOPs) catalyzed by nanoiron particles as well as nanoscale zero-valent iron (nZVI) and nZVI-based bimetallic nanoparticles (BNPs). Nanoparticles (NPs) have a large surface-to-volume ratio compared to other bulk materials, making them more reactive compared to their microsized counterparts. This characteristic also makes them excellent catalysts. Although particle size is an important consideration, many other factors such as geometry, composition, oxidation state, and chemical/physical environment can play a role in determining the nanoparticles' reactivity. Besides increased reactivity, nanoscale particles have the additional advantage of being relatively easily incorporated into support structures, potentially without significantly altering their reactivity. This broadens their potential applications.
Metallic nanoparticles are kinetically stable, because thermodynamics favors the formation of structures with a high surface/volume ratio, and consequently the bulk metal represents the lowest energy. In order to increase their stability, reactivity and/or mobility stabilizers are required to prevent the agglomeration of the nanoclusters by providing a steric and/or electrostatic barrier between particles. In addition, the stabilizers play a crucial role in controlling both the size and shape of the nanoparticles. Other possibilities to increase nanoparticles' stability, reactivity, and/or mobility include combination with other metals (such as bimetallic iron-based nanoparticles), support materials (such as activated carbon and zeolites), or embedding of the particles in organic membranes (i.e., emulsified nZVI).
Advances in nanoscience provide opportunities for developing nanoparticles with high activities for energetically challenging reactions, high selectivity to valuable products, and extended life times. The control of nanoparticle size and morphology represents a crucial goal to achieve in order to tune the physical properties of nanomaterials, which will enable full-scale implementation of environmental remediation.
2.2 Nanoparticles as Catalysts
A catalyst is a substance that increases the rate of a chemical reaction by reducing the required activation energy, yet is left unchanged by the reaction. Nanocatalysts represent the convergence of catalysts with nanotechnology, which is concerned with the synthesis and functions of materials at the nanoscale range (<100 nm) [1]. In a sense, all catalysis is in the nanoscale, as it involves chemical reactions at the nanoscale. However, in this chapter, a nanocatalyst is defined as a substance with catalytic properties that has nanoscale dimensions. An important feature of nanomaterials is that their surface properties can be very different from those shown by their macroscopic or bulk counterparts [2]. NPs have a large surface-to-volume ratio compared to other bulk materials. This characteristic makes them excellent catalysts. Although particle size is an important consideration, many other factors such as geometry, composition, oxidation state, and chemical/physical environment can play a role in determining NP reactivity. However, the exact relationship between these parameters and NP catalytic performance may be system dependent and is yet to be laid out for many nanoscale catalysts. Nevertheless, it is recognized that the enhanced catalytic activity and selectivity observed for small NPs is a combination of several factors acting in parallel. For instance, with decreasing NP size, the surface-to-volume ratio increases, resulting in a larger number of low-coordinated atoms available for interaction with chemical adsorbates. The distinct electronic properties of such sites are also expected to play a role in chemical reactivity, for example, by facilitating the dissociation of reactants or by stabilizing intermediate reaction species. Another parameter that plays a vital role in catalysis is the roughness of surface. Enhanced chemical activity has been observed for stepped surfaces as compared to smooth surfaces [3].
From a catalytic point of view, at the nanoscale, the reactivity of the active sites is influenced by the surrounding environment at a supramolecular level. This environment drives the local adsorption/desorption of reactants and products; for instance, local pH around the active site can be different from the bulk pH depending on the characteristics of the catalytic cavities. For this reason, adequate tuning of catalysts at the nanoscale level would allow enhancement of the catalytic properties. The nanosized catalytic materials are assembled at the microscale with the aim of obtaining optimal spatial distribution and catalyst composition in a final macrostructured catalyst (e.g., pellets, membranes, monoliths, and foams) [4]. A heterogeneous catalyst (i.e., the catalyst is in a different phase from the reactants) is characterized mainly by the relative amounts of different components (active species, physical and/or chemical promoters, and supports), shape and size, pore volume and distribution, and surface area. The nature of the active species is always the most important factor. Heterogeneous nanocatalysts are found both in colloidal and supported forms.
2.2.1 Colloidal Nanoparticles
Colloidal nanoparticles are used as catalyst in which the nanoparticles are finely dispersed in the aqueous solution. Colloidal nanoparticles tend to aggregate. The specific surface area of highly aggregated nanoparticles is likely to be very different from the specific surface area of dispersed nanoparticles. This is of importance mainly with respect to the determination of the reactive surface area and reactive sites on the particle surface. Aggregation of nanoparticles is difficult to avoid under environmental conditions. Therefore, in order to prevent aggregation, colloidal nanoparticle solutions are often stabilized. A good stabilizer is one that protects the nanoparticles during the catalytic process but does not passivate the surface of nanoparticles resulting in loss of nanoparticle catalytic activity (passivation is the formation of a nonreactive surface film). Accordingly, the preference of stabilizers should meet two challenges: (i) the development of methods for stabilizing nanoparticles by eliminating aggregation without blocking most of the active sites on the nanoparticle surfaces or otherwise reducing catalytic efficiency and (ii) controlling nanoparticle size, shape, and size distribution [5]. Some of the common stabilizers include polymers, block copolymers, dendrimers, surfactants, and other ligands. Transition-metal colloids are the most used nanocatalysts as a large number of atoms are present in the nanoparticle surface.
When compared with microparticles, nanoscale-based particles have higher reactivity because of their high specific area and more reactive surface sites. Their ability to remain in suspension enables injection of the nanoparticles into contaminated soils, sediments, and aquifers. Yet, owing to their tendency to form aggregates, it is difficult to maintain them in a suspension. Support of the nanoparticles may inhibit aggregation as well as increase their transport [6].
2.2.2 Supported Nanoparticles
According to the most common preparation procedures, the supported catalysts can be classified as follows [4]:
Supports where the catalyst is generated as a new solid phase (e.g., precipitation, sol–gel). The formation and precipitation of a crystalline solid occurs by supersaturation with physical (e.g., variations in temperature) or chemical (e.g., addition of bases or acids) perturbations of a solution, formation of stable small particles by nucleation and growth by agglomeration of those particles. The sol–gel method consists of the transformation of a solution into a hydrated solid precursor (hydrogel). This method has gained importance because of a better control of different properties such as texture and homogeneity.
Impregnated catalysts where the active phase is introduced or fixed on a preexisting solid (e.g., impregnation, ion exchange, adsorption, deposition–precipitation, and chemical- or physical-vapor deposition). Impregnation is based on the contact between the solid support and a certain volume of solution, which contains the precursor of the active phase. The method is called
wet impregnation
when an excess of solution is used and incipient wetness impregnation if the volume is equal or slightly less than the pore volume of the support. The ion exchange method is based on the replacement of ions on the surface of a support by electrostatic interactions. Adsorption consists in the controlled attraction of a precursor contained in an aqueous solution by charged sites on the support. In the deposition-precipitation method, slurries are formed and the precipitation occurs in interaction with a support surface by addition of an alkali solution. The most common method of preparing heterogeneous transition-metal nanocatalysts is by adsorption onto the support.
The prepared solids are separated from the mother liquor by filtration, decantation, and/or centrifugation, usually washed with distilled water (or specific organic solvents) for complete removal of impurities, and submitted to thermal treatments (e.g., drying and calcination-heating). After calcination, modification of the nature and/or structure of the phases stabilization of mechanical properties occurs. The substrates used as support for the nanocatalyst include silica, alumina, titanium dioxide, polymeric support, and carbon. The carbon support of choice is activated carbon, followed by carbon black and graphite [7]. The support materials stabilize metal catalysts against sintering at high reaction temperatures. Some support materials, especially reducible oxides, can also promote the activity and selectivity of active metal catalysts. Many innovations have been made in designing nanostructured support materials. One example is the use of carbon nanotube–inorganic oxide hybrid nanoparticles as a support for phase-transfer reactions (reactants and products are in different phases) to simplify the separation and purification processes. These hybrid nanoparticles are amphiphilic and stabilize water–oil emulsions. The metal catalysts, immobilized on the hybrid support, preferentially stay at the water–oil interface, where catalytic phase-transfer reactions happen. The emulsions are extremely stable and can be easily separated from the biphasic liquid by a simple filtration. The recycled hybrid nanoparticles can be reused without any special treatment [8].
2.3 Advanced Oxidation Processes
NPs can be coupled with AOPs to carry out degradation of organics. AOPs refer specifically to processes in which oxidation of organic contaminants occurs primarily through reactions with hydroxyl radicals. AOPs involve two stages of oxidation.
The reactive radicals are capable of decomposing a wide range of organic compounds. Depending on the structure of the organic compound in question, different reactions may occur including hydrogen atom abstraction, electrophilic addition, electronic transfer, and radical–radical interactions [9].
AOPs can be applied to fully or partially oxidize pollutants, usually using a combination of oxidants. In brief, Fenton process consists of hydrogen peroxide (H2O2) and iron as catalyst (H2O2/Fe2+(Fe3+)); ozonation is based on the strong oxidant properties of ozone (O3). Ozone is mostly used in conjunction with hydrogen peroxide and/or ultraviolet (UV) irradiation. Photocatalysis uses the photonic activation of the catalyst via light irradiation, producing reactive electron–holes (UV/TiO2). UV light combined with hydrogen peroxide (UV/H2O2) is based on photolysis of the peroxidic bond of hydrogen peroxide by absorption of the UV irradiation. The most efficient hydroxyl radical yields are obtained when shortwave UV wavelengths (200–280 nm) are used. In catalytic wet oxidation (CWO), the pollutant molecules are oxidized with pure oxygen or air at elevated temperatures (130–250 °C) and pressures (5–50 bar). Other AOPs include sonolysis, which consists of the ultrasonically induced acoustic cavitation and electrochemical oxidation, which produces large amounts of hydroxyl radicals directly in water. Each technology requires a different catalytic material as well as the optimization of their catalytic properties.
The efficiency of the various AOPs depends both on the rate of generation of the free radicals and on the extent of contact between the radicals and the organic compound. It is assumed that a combination of oxidants results in better degradation rates and efficiencies as compared to single-oxidant processes. This assumption relies on [10]
the similarity between the mechanisms of destruction (radical oxidation) of the different processes;
the enhancements in the rate of generation of free radicals using combined methods; and
the synergy among the methods to minimize the drawbacks of individual methods.
Catalyst loading, solution pH, and substrate concentration/nature are variables that should be considered among all the different catalytic-AOPs. Other parameters are specific for each process. Of importance are the oxygen partial pressure and the temperature in CWO, diffusion efficiency of gaseous O3 into the treated water in ozonation, the light intensity in photocatalysis, and the ratio between Fe and H2O2 in the Fenton process. An appropriate catalyst of AOPs is the one that shows the highest level of pollutant mineralization (to carbon dioxide and water) or pollutant degradation to less toxic/refractory compounds. It is important to note that the environmental risk posed by a contaminant is not necessarily decreased by its degradation. The oxidation by-products may be more toxic or lengthen/enhance the harmful effect of a contaminant. Catalyst deactivation must be taken into consideration as well, being normally due to sintering, poisoning of active sites, dissolution of active components to the liquid phase (leaching), or fouling of the catalyst surface as a result of deposition of reaction intermediates [4]. The fate of the residual oxidants (if exist) and by-products as well as the cost including energy consumption should also be considered. For example, the heating of 1 m3 of water to 10 °C consumes about 1 kg of fossil fuel. Augmentation of the pressure utilizes additional energy. Although energy recycle is often implemented, in processes such as CWO in which temperature and pressure are increased up to 250 °C and 50 bar, respectively, the issue of energy consumption should be properly considered.
2.3.1 Fenton-Like Reactions
Fenton-like reactions are defined as combination of hydrogen peroxide with ferric ions or other transition metals. The optimum pH for the homogeneous Fenton and Fenton-like processes ranges between 2 and 4. Yet, heterogeneous solid catalysts can mediate Fenton-like reactions over a wide range of pH values because Fe(III) species are immobilized within the structure and in the pore/interlayer space of the catalyst. As a result, the catalyst can maintain its ability to generate hydroxyl radicals from hydrogen peroxide, and iron hydroxide precipitation is prevented [11–13].
Solid nanocatalysts profile has been proposed to include [1]
high activity in terms of pollutant removal;
marginal leaching of active cations;
stability over a wide range of pH and temperature;
high hydrogen peroxide conversion with minimum decomposition; and
reasonable cost for practical applications.
In comparison with their microsized counterparts, nanoparticles show higher catalytic activity because of their large specific surface where catalytically active sites are exposed [13]. For example, Kwon et al. [14] evaluated two iron-oxide catalysts for the oxidation of carbon monoxide and methane at low temperatures. One of the materials (NANOCAT®) had an average particle size of 3 nm and a specific surface area of 250 m2 g−1, whereas the other material (Fe2O3PVS) had an average particle size of 300 nm and a surface area of 4 m2 g−1. Although both catalysts were effective, the nanocatalyst showed superior activity. Valdés-Solís et al. [15] have developed a new catalyst for the Fenton-like reaction using nanosized particles with a high surface area. These solid nanocatalysts showed high catalytic activity over a wide range of pH values (6–13) and H2O2 concentrations (0.005–3 M). Their high reactivity was attributed to the active sites located on the surface. As such, they have a low diffusional resistance, and are easily accessible, to the substrate molecules.
2.3.1.1 Iron Oxide as Heterogeneous Nanocatalyst
Nanoscale iron oxides were reported to be effective catalysts of the Fenton reaction. Zelmanov and Semiat [16] showed that the rate of degradation of ethylene glycol and phenol degradation by iron(III) oxide nanoparticles was up to 35 times higher than that of homogeneous photo-Fenton reaction.
Lim et al. [17] reported a highly active heterogeneous Fenton catalyst using iron-oxide nanoparticles immobilized to alumina coated mesoporous silica.
Fenton reactions can be catalyzed by dissolved iron from the iron oxide nanoparticles or by the oxide itself, which acts as a heterogeneous catalyst. Iron oxides in general are compounds with low to very low solubility. The driving force for iron oxide dissolution is the extent of undersaturation with respect to the oxide. Undersaturation is thus a requirement for dissolution as is supersaturation for precipitation. The factors that influence the rate of dissolution of iron oxides are the properties of the reaction system (temperature and UV light), the composition of the solution phase (pH, redox potential, concentration of acids, reductants, and complexing agents), and the properties of the oxide (specific surface area, stoichiometry, crystal chemistry, crystal habit, and the presence of defects or guest ions). Models that take all of these factors into account are not available. In general, only the specific surface area, the composition of the solution, and in some cases the tendency of ions in solution to form surface complexes are considered. Figure 2.1 shows schematically, the activation energies (Ea) of the various steps of iron oxide dissolution by protonation; as the detachment step is rate limiting, it is assigned the highest Ea[18].
Figure 2.1 Schematic representation of the consecutive steps of dissolution by protonation of iron oxides [18].
A mechanism of hydrogen peroxide decomposition on iron oxides surface (≡Fe(III); such as goethite, hematite, and ferrihydrite) was described by Kwan and Voelker [19], according to Equations 2.3, assuming that an Fe(III) site identical to that present in Equation 2.1 is regenerated by Equation 2.3.
2.1
2.2
2.3
The generation rate of hydroxyl radicals was found to be proportional to the product of [H2O2] and iron oxide surface area. This is consistent with a mechanism whose rate-limiting step involves H2O2 sorbed on the iron oxide surface. Hence, if Equation 2.2 is the rate-limiting step, then the rate of HO⋅ production will be proportional to the concentration of ≡Fe(III)H2O2.
Lin and Gurol [20] proposed a more detailed mechanism with its simplified form presented by Equations 2.4–2.8 (it should be noted that Equations 2.4 and 2.5 are unbalanced).
2.4
2.5
2.6
2.7
2.8
According to this mechanism, the reactions are initiated by the formation of a precursor surface complex of H2O2 with the oxide surface. (H2O2)s in Equation 2.4 represents the surface species of hydrogen peroxide, which might hold an inner- or an outer-sphere surface coordination. However, the progression of the reaction is expected to be through the inner-sphere formation directly with the surface metal centers. The surface complex may undergo a reversible electron transfer, which can be described as a ground-state electron transfer from ligand to metal within a surface complex (Equation 2.5). The electronically excited state can be deactivated through the dissociation of the peroxide radical (dissociation of the successor complex), as shown by Equation 2.6. The peroxide radical is a very active radical that can immediately react with other compounds. The reduced iron, being a reductant, can react with either H2O2 or oxygen (Equation 2.7). The peroxide and hydroxyl radicals produced during the reaction may react with Fe(III) and Fe(II) sites on the surface according to Equation 2.8.
In terms of the overall reaction, it was concluded that both heterogeneous and homogeneous oxidation of organic compounds occur simultaneously. While the production of active radicals, initiated by hydrogen peroxide, involves the reductive and nonreductive dissolution of iron oxide in heterogeneous processes, the iron ions (Fe2+, Fe3+, and complex iron species) react with hydrogen peroxide in the solution (i.e., homogeneous processes). It was further suggested that the intermediates derived from degradation of organic compounds (such as catechol and oxalic acid) promote the dissolution of iron by reductive and nonreductive pathways. Therefore, the reactivity of iron oxides in catalyzing organics degradation by hydrogen peroxide relates also to the tendency of iron to be dissolved by oxidation intermediates [21]. A schematic description of organic compound degradation by iron oxides immobilized onto a solid support is presented in Figure 2.2. Initially, hydrogen peroxide reacts with the iron oxide (≡Fe3+) to generate ≡Fe2+, hydroperoxyl, and superoxide anion radicals. Degradation of the organic compound is initiated by its reaction with hydroxyl radicals formed by the decomposition of hydrogen peroxide. The ≡Fe2+ is rapidly oxidized by excess hydrogen peroxide (Fenton reaction), and the iron ion in the solution may be dissolved or in complexes with organic intermediates [21].
Figure 2.2 Schematic description of organic compound degradation by iron oxides immobilized onto solid support [21].
An important question regarding the H2O2/nFe oxides system is whether the oxidation/reduction reactions on the oxide surface can transform the iron particles. This transformation may lead to substantial changes in the surface characteristics of the mineral, resulting in a different kinetic behavior and decomposition rate for H2O2 (i.e., each iron oxide type have different surface reactivity). In addition, iron oxide dissolution may reduce the total concentration of iron, changing the mechanism of the reaction from being surface to solution based. This matter was studied by Lin and Gurol [20] who found that the reactivity of iron oxide did not vary over several weeks of operation. These results suggest that H2O2 did not affect the surface reactivity of the iron oxide as well as its structure. This was further confirmed by images of the iron oxide surface taken with an electron scanning microscope prior and following exposure to H2O2, revealing no significant change in the iron oxide surface structure.
2.3.2 Photo-Fenton Reactions
The photo-Fenton reaction is rather similar to the Fenton one with the addition of UV irradiation. Its effectiveness is attributed to the photolysis of Fe3+ in acidic media yielding Fe2+ in conjunction with the reaction between Fe2+ and H2O2 to yield hydroxyl radicals (Fenton's reaction). Different iron-containing catalysts can be used for this process. First, bulk catalysts containing iron, such as hematite, goethite, or magnetite, may be considered. A different approach is the incorporation of iron into different supports, being zeolites, polymers, or carbon. The use of heterogeneous solid supports catalysts is considered particularly beneficial as very often complete mineralization of the organic pollutant can be reached, along with easy separation of the catalysts from the treated wastewater, not causing secondary metal ion pollution [22]. Besides iron, other transition metals, such as copper, can also catalyze the photo-Fenton reaction.
Of significant importance in determining the performance of the photo-Fenton reaction is the type of UV lamp used and its power. It is established that an increase in the UV radiation intensity results in an enhanced catalytic activity. When the UV intensity increases, a faster photoreduction of Fe3+ to Fe2+ is obtained resulting in a higher regeneration rate of Fe2+. Accordingly, more hydroxyl radicals are formed resulting in higher mineralization [22].
Feng et al. [23] suggested a simple mechanism of immobilized nanoiron particles catalysis of photo-Fenton reaction in which R is the organic compound and R* are the reaction intermediates
2.9
2.10
2.11
The chain of reactions is initiated by the photoreduction of Fe(III) to Fe(II) under UV irradiation. Then, the Fe(II) accelerates the decomposition of H2O2 in solution, generating highly oxidative hydroxyl radicals, whereas it is oxidized by H2O2 into Fe(III) (Fenton's reaction). The generated hydroxyl radicals attack the adsorbed organic molecules, giving rise to reaction intermediates. Finally, the reaction intermediates are mineralized into CO2 and H2O. The role of the adsorption of the organic molecules onto the nanoparticles and/or solid support surface is still not clear, although some researchers claim that strong adsorption of the organic reactant seems to be a necessary condition for the reaction to occur. Thus, extremely poor degradation is expected for organic compounds exhibiting no significant chemisorption [22].
2.3.3 Nanocatalytic Wet Oxidation
CWO is a reaction involving an organic compound in water and oxygen over a catalyst, either homogeneous or heterogeneous, at elevated temperatures and pressures. As in the Fenton reaction, heterogeneous catalysis is considered more efficient owing to the stability of solid catalysts compared to homogeneous or colloidal ones. In addition, a homogeneous catalyst requires a subsequent separation step, which is not required when a heterogeneous catalyst is used.
Platinum and ruthenium metals and cerium, titanium, manganese, and iron oxides deposited on zeolites, alumosilicates, ceria, alumina, and different types of carbon have been employed as catalysts in CWO reactions. Yet, the catalysts based on the platinum group metals demonstrated the higher activities. The major drawback of all these catalysts is their unstable character due to metal leaching [24].
Heterogeneous wet oxidation nanocatalysts should display the following characteristics:
high oxidation rates;
marginal leaching of active metal;
stability at acidic pH and high temperatures;
nonselectivity.
In CWO, an increase in temperature leads to a consequent increase in the oxidation rate. Possible effects of oxygen partial pressure and catalyst loading in a CWO reaction is presented in Figure 2.3, as a hypothetical behavior of the total organic carbon (TOC) conversion in a generalized reaction system at constant temperature. In the absence of a catalyst and increased oxygen partial pressure, the TOC conversion is low for nondegradable organics. Without oxygen and in the presence of a catalyst, some conversion can be achieved depending on the nature of the catalytic material. Increasing both oxygen and catalyst loading, a plateau of maximum efficiency can be observed in a region where changes in the corresponding values do not affect the global conversion. Moreover, for higher oxygen partial pressures, decrease in conversion is represented as well, in order to account for the possible occurrence of over-oxidation by the catalyst. At a constant oxygen partial pressure, decrease in conversion with higher amounts of catalyst represents the eventual acceleration of the termination step mechanism by the catalyst [4].
Figure 2.3 Conversion of TOC as a function of catalyst loading and oxygen partial pressure (PO2), in CWO[4].
Nanocatalyst-based catalytic wet air oxidation (CWAO) has been studied for the removal of phenolic compounds. The nanocatalysts included suspended iron nanoparticles and colloidal nanoclusters of SnO2 deposited on the surface of silica nanosphere. The degree of phenol removal with these catalysts did not exceed 70%, while leaching and the potential application for reuse of the recovered catalysts were not considered. But more importantly, these systems are considered unstable and tend to coagulate during the catalytic reactions, leading to catalyst deactivation [24]. Melero et al. [25] reported a nanocomposite of crystalline Fe2O3 and CuO particles with mesostructured SBA-15 silica as an active catalyst for wet peroxide oxidation processes. They found that the presence of copper prevents the leaching of iron species and increased TOC degradation. Recently, Botas et al. [26] prepared iron-containing catalysts on mesostructured SBA-15 silica and nonordered mesoporous silica for the oxidation of phenol aqueous solution in a catalytic fixed bed reactor in the presence of hydrogen peroxide. Sulman et al. [24] demonstrated that Pt-containing nanoparticles (size of 2.1–2.3 nm), synthesized in the pores of hyper-cross-linked polystyrene, displayed very different catalytic properties, in phenol CWAO, based on the amount of the incorporated Pt species (Pt(0), Pt(II), and Pt(IV)). With observed shielding of the catalytic sites when the Pt load was too high. Chemical oxygen demand (COD) removal ranged from 54 to 94%, depending on the Pt content, at catalyst concentration of 5.15 × 10−3 mol(Pt) l−1; phenol concentration of 0.44 mol l−1; temperature of 95 °C; pressure of 0.1 MPa; reaction time of 5 h; and oxygen flow rate of 0.018 m3 h−1.
