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NANOTECHNOLOGIES FOR WATER ENVIRONMENT

APPLICATIONS

SPONSORED BY Nanotechnology Task Committee of the Environmental Council

Environmental and Water Resources Institute (EWRI)

of the American Society of Civil Engineers

EDITED BY Tian C. Zhang

Rao Y. Surampalli Keith C. K. Lai

Zhiqiang Hu R. D. Tyagi

Irene M. C. Lo

Published by the American Society of Civil Engineers

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Library of Congress Cataloging-in-Publication Data Nanotechnologies for water environment applications / sponsored by Nanotechnology Task Committee of the Environmental Council, Environmental and Water Resources Institute (EWRI) of the American Society of Civil Engineers / edited by Tian C. Zhang [et al.]. p. cm. Includes bibliographical references and index. ISBN 978-0-7844-1030-1 1. Water--Purification--Technological innovations. 2. Water--Pollution--Prevention. 3. Nanotechnology. 4. Nanostructured materials--Environmental aspects. 5. Nanotechnology--Environmental aspects. I. Zhang, Tian C. II. Environmental and Water Resources Institute (U.S.). Nanotechnology Task Committee TD477.N36 2009 628--dc22 2009012962 American Society of Civil Engineers 1801 Alexander Bell Drive Reston, Virginia, 20191-4400 www.pubs.asce.org Any statements expressed in these materials are those of the individual authors and do not necessarily represent the views of ASCE, which takes no responsibility for any statement made herein. No reference made in this publication to any specific method, product, process, or service constitutes or implies an endorsement, recommendation, or warranty thereof by ASCE. The materials are for general information only and do not represent a standard of ASCE, nor are they intended as a reference in purchase specifications, contracts, regulations, statutes, or any other legal document. ASCE makes no representation or warranty of any kind, whether express or implied, concerning the accuracy, completeness, suitability, or utility of any information, apparatus, product, or process discussed in this publication, and assumes no liability therefore. This information should not be used without first securing competent advice with respect to its suitability for any general or specific application. Anyone utilizing this information assumes all liability arising from such use, including but not limited to infringement of any patent or patents. ASCE and American Society of Civil EngineersRegistered in U.S. Patent and Trademark Office. Photocopies and reprints. You can obtain instant permission to photocopy ASCE publications by using ASCEs online permission service (http://pubs.asce.org/permissions/requests/). Requests for 100 copies or more should be submitted to the Reprints Department, Publications Division, ASCE, (address above); email: permissions@asce.org. A reprint order form can be found at http://pubs.asce.org/support/reprints/. Copyright 2009 by the American Society of Civil Engineers. All Rights Reserved. ISBN 978-0-7844-1030-1 Manufactured in the United States of America.

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www.pubs.asce.orghttp://pubs.asce.org/permissions/requests/http://pubs.asce.org/support/reprints/

Preface

Nanomaterials have structures with exotic properties due to the interactions and physics between atoms. Built on the ability to control or manipulate at the atomic scale, nanotechnology involves research and technology development at the 0.1100 nm range. Nanotechnology enables a powerful new direction for our industries and human activities. Nanotechnology has the potential to enhance environmental quality and sustainability by improving detection/sensing techniques for environmental pollutants, controlling/removing environmental contaminations and development/utilization of new green engineering processes to reduce energy/resource consumption and generation of waste products. However, just like any new technology, nanotechnology may be abused and, thereafter, can harm the environment and sustainability.

The ASCEs Technical Committee on Hazardous, Toxic and Radioactive

Waste has identified nanotechnology as an important area for water environmental applications. This book brings together nanotechnology research and applications that contribute to enhanced protection of aquatic environments. It also addresses the problems and processes that might occur should nanotechnology be abused, including toxicology, interactions and risk management of nanomaterials in aquatic environments.

The book presents a discussion of fundamentals of nanomaterials and

nanosystems, various applications of nanotechnologies, behavior, and possible impacts of nanomaterials on human health and the environment. Chapter 1 is the introductory chapter which introduces historical development of nanotechnology, general applications and implications of nanotechnologies. Chapter 2 focuses on synthesis of nanomaterials concerning environmental applications. The major part of this book includes several chapters addressing applications of nanoparticles, including catalysts TiO2 nanoparticles for water purification (chapter 3), nanoparticles for treatment of chlorinated organic contaminants (chapter 4), removal of inorganic compounds such as arsenic (chapter 5) and heavy metal ions from aqueous environments (chapter 6). More applications and related research are introduced, including bimetallic nanoparticles (e.g., bimetallic Pd/Fe nanoparticles) for environmental remediation (chapter 7), challenges in groundwater remediation with iron nanoparticlesenhancement colloidal stability (chapter 8), iron-based magnetic nanoparticles for removal of heavy metals from industrial wastewater (chapter 9), nanoscale carbon materials for contaminant separation (chapter 10), nanoscale porous materials for water treatmentadvances and challenges (chapter 11), nanomembranes for water purification (chapter 12), fabrication and general applications of nano- or micro-sensors in environmental areas (chapter 13), and nanomaterials for environmental burden reduction, waste treatment and non-point source pollution

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control (chapter 14). The last part of the book consists of three chapters (1517). Chapter 15 addresses the fate and transport of nanomaterials in aquatic environments; chapter 16 discusses issues related to engineered nanomaterials as emerging contaminants in water; and chapter 17 presents environmental risks of nanomaterials and related management issues. This organization will help the readers readily find the information they are looking for.

We hope that this book will be of interest to scientists, engineers, government

officers, decision-makers, process managers, and practicing professionals. It will provide them an in-depth understanding of the fundamentals and environmental applications of nanotechnologies. The book also will serve as a reference for undergraduate and graduate students, as well as for practicing professionals.

The editors gratefully acknowledge the hard work and patience of all the

authors who have contributed to this book. The views or opinions expressed in each chapter of this book are those of the authors and should not be construed as opinions of the organizations they work for. Special thanks go to the faculty and staff members at the University of Nebraska-Lincoln: Dr. Yongfeng Lu for providing the cover pictures of nanomaterials developed in his laboratories; Ms. Arlys Blakey for her thoughtful comments and invaluable support during the development of this book; and Mr. Rui Ma for designing the cover of the book.

TCZ, RYS, KCL, ZH, RDT, IML

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Contributing Authors

Farhan Ahmad, Michigan State University, East Lansing, MI, USA Renbi Bai, National University of Singapore, Singapore

Achintya N. Bezbaruah, North Dakota State University, Fargo, ND, USA

Tung Xuan Bui, Gwangju Institute of Science and Technology, Gwangju, Korea

Guohua Chen, Hong Kong University of Science and Technology, Hong Kong, PRC

Bret J. Chisholm, North Dakota State University, Fargo, ND, USA

Heechul Choi, Gwangju Institute of Science and Technology, Gwangju, Korea

Syed. A. Hashsham, Michigan State University, East Lansing, MI, USA

Jiangyong Hu, National University of Singapore, Singapore

Jing Hu, Formulation Technologies, Ludwigshafen, Germany

Zhiqiang Hu, University of Missouri, Columbia, MO, USA

Chin Pao Huang, University of Delaware, DE, USA

Chuanyong Jing, Chinese Academy of Sciences, Beijing, PRC

Harjyoti Kalita, North Dakota State University, Fargo, ND, USA

Sushil Raj Kanel, Auburn University, Auburn, AL, USA

Sita Krajangpan, North Dakota State University, Fargo, ND, USA

Keith C.K. Lai, University of Texas, Austin, TX, USA

Lai Yoke Lee, National University of Singapore, Singapore

Minghua Li, University of Delaware, Newark, DE, USA

Zhihua Liang, University of Missouri, Columbia, MO, USA

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Hsing-Lung Lien, National University of Kaohsiung, Kaohsiung, Taiwan, China

Hong Ying Lin, University of Delaware, Newark, DE, USA

Irene M. C. Lo, Hong Kong University of Science and Technology, Hong Kong, PRC

Xiaoguang Meng, Stevens Institute of Technology, Hoboken, NJ, USA

Say Leong Ong, National University of Singapore, Singapore

Dhriti Nepal, Auburn University, Auburn, AL, USA

How Yong Ng, National University of Singapore, Singapore

Huy Quang Nguyen, University of Missouri, Columbia, MO, USA

Hosik Park, Gwangju Institute of Science and Technology, Gwangju, Korea

Guo-Bin Shan, INRS, Universite du Quebec, Quebec, QC, Canada

Junhong Shan, National University of Singapore, Singapore

Rao Y. Surampalli, U.S. Environmental Protection Agency, Kansas City, KS, USA

R.D. Tyagi, INRS, Universite du Quebec, Quebec, QC, Canada

K.H. Wee, National University of Singapore, Singapore

Paul Westerhoff, Arizona State University, Tempe, AZ, USA

Song Yan, INRS, Universite du Quebec, Quebec, QC, Canada

Tian C. Zhang, University of Nebraska-Lincoln, Omaha, NE, USA

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Contents

Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6

Introduction Background Past, Present, and Future Environmental Applications Implications of Nanotechnology and Research Needs Summary References

1 1 2 4 8 11 12

Chapter 2

2.1 2.2 2.3 2.4 2.5

Synthesis of Nanoparticles and One-Dimensional Nanomaterials Introduction Nanoparticles One-Dimensional Nanomaterials Conclusions References

14 14 12 28 32 33

Chapter 3

3.1 3.2 3.3 3.4 3.5 3.6 3.7

Nanotechnostructured Catalysts TiO2 Nanoparticles for Water Purification Background of TiO2 as a Semiconductor Photocatalyst Photocatalytic Mechanism, General Pathways, and Kinetics Intrinsic Photocatalytic Activity Reaction Variables Photocatalytic Degradation of Specific Waterborne Pollutants Conclusions References

43 44 46 50 58 71 75 78

Chapter 4

4.1 4.2 4.3 4.4 4.5

4.6 4.7

Nanoparticles for Treatment of Chlorinated Organic Contaminants Introduction Overview of Chlorinated Organic Solvents Biodegradation of Chlorinated Organic Solvents Nanoscale Zero-Valence Iron (NZVI) Application of Other Nanoscale Metallic Particles in Chlorinated Organic Compound Degradation Conclusions References

93 93 93 94 100 108 111 111

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Chapter 5 5.1 5.2 5.3 5.4 5.5

Nanoparticles for Treatment of Arsenic Introduction Environmental Chemistry of Arsenic Treatment of Arsenic Using Nanocrystalline TiO2 Treatment of Arsenic Using Nanoparticles Other Than TiO2 References

116116 117 119 132 133

Chapter 6

6.1 6.2 6.3 6.4 6.5

Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions Introduction Iron-Based Nanoparticles for Removal of Heavy Metal Ions Polymeric Nanoparticles for Removal of Heavy Metal Ions Conclusions References

137 137 142 148 153 154

Chapter 7 7.1 7.2 7.3 7.4 7.5

Bimetallic Nanoparticles Introduction Micro-Sized Bimetallic Particles Bimetallic Nanoparticles Conclusions References

159159 162 169 184 184

Chapter 8

8.1 8.2 8.3

8.4 8.5 8.6

Challenges in Groundwater Remediation with Iron Nanoparticles: Enabling Colloidal Stability Introduction Current Status of nZVI Surface Modification Surface Modification with Amphiphilic Polysiloxane Graft Copolymers Conclusions Acknowledgements References

191 191 192 198 206 206 206

Chapter 9

9.1 9.2

9.3 9.4 9.5 9.6 9.7

Iron-Based Magnetic Nanoparticles for Removal of Heavy Metals from Electroplating and Metal-Finishing Wastewater Introduction Applications of Magnetic Nanoparticles on Environmental Pollution Control Laboratory Production of Magnetic Nanoparticles Nanoparticles Characterizations Batch Kinetics and Equilibrium Adsorption Studies Conclusions References

213 213 215 218 223 235 263 264

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Chapter 10

10.1 10.2 10.3 10.4 10.5 10.6 10.7

Nanoscale Carbon Materials for Contaminant Separation Introduction Properties and Potential Applications Carbon Nanotubes and Fullerenes for Contaminant Separation Adsorptive Removal on Mesoporous Carbon Other Nanoscale Carbons Perspectives of Nanoscale Carbon References

269 269 274 277 297 300 301 302

Chapter 11

11.1 11.2 11.3 11.4 11.5 11.6 11.7

Nanoscale Porous Materials for Water Treatment: Advances and Challenges Introduction Nanoscale Porous Materials Fate/Transport of Nanoscale Porous Materials in Porous Media Discussion Conclusions and Perspectives Abbreviation References

312 312 313 349 351 352 353 354

Chapter 12 12.1 12.2 12.3 12.4 12.5 12.6

Nanomembranes Introduction Nanomembranes in Drinking Water Treatment Nanomembranes in Water Reclamation and Reuse Nanomembranes in Seawater Desalination Future of Nanomembranes in Water Beneficiation References

367367 368 385 396 403 403

Chapter 13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8

13.9

13.10 13.11 13.12 13.13 13.14

Nanosensors Introduction Characteristics of Sensors for the Water Industry Working Mechanisms and Types of Sensors Fabrication and Synthesis of Micro- and Nano-Scale Materials Detection Limit as Key Parameter for Pathogens in Water Labeling Approaches Signaling Methods Magnetic Particle Based Immunoassays for the Detection of Pathogens: Commercial Devices Label-Free Detection Sample Processing Market Trends Conclusions Acknowledgements References

412412 416 417 421 422 423 427 428 430 433 434 435 436 436

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Chapter 14

14.1 14.2 14.3 14.4 14.5 14.6

Nanomaterials for Environmental Burden Reduction, Waste Treatment, and Non-Point Source Pollution Control Introduction Environmental Burden Reduction Treatment of Industrial and Agricultural Wastes Nanomaterials for Non-Point Source Pollution Control Summary References

444 444 445 455 459 462 462

Chapter 15

15.1 15.2 15.3 15.4 15.5 15.6 15.7

Fate and Transport of Nanomaterials in Aquatic Environments Introduction Mass Balance Equations and Transport Processes Interphase Transfer Processes Transformation Processes NM-Induced Characteristics, Interactions and Behaviors Conclusions References

474 474 477 497 503 523 543 545

Chapter 16

16.1 16.2 16.3 16.4

16.5 16.6 16.7 16.8 16.9

Engineered Nanomaterials as Emerging Contaminants in Water Introduction What Are Emerging Contaminants Classification of Nanomaterials Sources, Detection and Fate of Engineered Nanomaterials in Aquatic Systems Stability of Nanomaterials in Aquatic Systems Examples of NM Fate in Engineered and Natural Systems Conclusions Acknowledgements References

558 558 559 561 562 570 572 580 581 582

Chapter 17 17.1 17.2 17.3 17.4 17.5 17.6

Environmental Risks of Nanomaterials Introduction Routes of NMs into the Water Environment Hazardous Effects of NMs on Human and Animal Health Risk Management Conclusions References

591591 592 595 600 609 610

Editor Biographies 619 Index 623

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1

CHAPTER 1

Introduction

Tian C. Zhang and Rao Y. Surampalli 1.1 Background

Nanotechnology involves research, development, control, and characterization of

materials or devices between the atomic and micrometer scales (usually at the 0.1100 nm range). Generally, fabrication/applications of nanomaterials (NMs, materials that contain nanoparticles, NPs) involve the following major steps: ultra-miniaturization/molecular manufacturing of NPs, functionlization of NMs, incorporating NMs into nanocomposites, and final product or application. NMs exhibit novel properties that differ from those of bulk materials due to the interactions and physics between atoms. In addition, properties of NMs can be manipulated via different synthesizing methods (either top-down or bottom-up methods) that dictate the shape and arrangement of atoms and molecules within the NMs. Because of their unique properties, NMs have been used in many different sectors for all kinds of purposes.

Currently, environmental applications of nanotechnologies provide new

opportunities for us to detect, control, and remediate environmental pollutions. However, as with any new technology or chemical substance, there is a potential for harm to human and natural ecosystems from nanotechnology. For these reasons, research related to environmental nanotechnology mainly focus on two major directions: (a) synthesizing new NMs (e.g., magnetic NMs or nanomembranes) or developing new techniques (e.g., nanosensors) for enhanced environmental protection; and (b) defining the problems and processes that might occur in the two ecosystems by evaluating the environmental and economic benefits/risks of new nanotechnologies.

Since the 1990s, studies in both directions have generated enough information.

It would be beneficial to overview these studies within the frame of applications and implications of nanotechnology. In 2007, the ASCEs Technical Committee on Hazardous, Toxic and Radioactive Waste identified nanotechnology for water

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environmental applications as an important area and started to bring together information generated on research and applications of nanotechnology in water environments. This book is the result of these efforts.

As an introduction to this book, we would like to introduce a broader picture of

nanotechnology with an emphasis on its applications and implications in water environments. This chapter starts with a brief overview of the nanotechnology realm, followed by an introduction of major applications of nanotechnologies in water environments. To round out the discussion, implications of nanotechnologies and research needs are described.

1.2 Past, Present, and Future Nanotechnology presents opportunities to create new and better products. The

realm of nanoscience, however, is not new. Chemists have been doing nanoscience for centuries; the medieval glass workers and Einstein (calculated the size of a sugar molecular as 1 nm) can be viewed as earlier nanoscientists. Nevertheless, modern vision of nanoscience starts in 1959 when Richard Feynman gave his speech Theres Plenty of Room at the Bottom, outlining the essentials of a nanotech capability and the prospects for atomic engineering. The word nanotechnology was used by Nario Taniguchi in 1974 to describe machining with tolerances of < 1 m. In 1985, Richard Smalley, together with Robert Curl and Sir Harold Kroto, discovered C60, the buckminsterfullerene (also known as the buckyball), symbolizing the dawn of The Coming Era of Nanotechnology.

Currently, nanotechnology is changing our world everywhere every day. In

general, there are four major steps in the NMs pipeline: fabrication and functionlization of NMs, incorporation into nanocomposites and final application. Overall, the roadmap of NMs involves four phases: basic research and development (R&D), applied R&D, production R&D (first applications), and mass production and incremental research (mass production). In many cases, these steps (or even phases) may combine or tangle together in order to produce a matrix NMs (Willems & van den Wildenberg, 2005). The remaining section describes briefly the pipeline and different phases involved in nanotechnology applications.

Fabrication of NMs can be achieved via (a) solid state methods (e.g., grinding,

milling, mechanical alloying techniques), (b) vapor methods (e.g., physical vapor deposition, PVD; chemical vapor deposition, CVD; vacuum evaporation on running liquids, VERL), (c) liquid-phase chemical synthesis methods (e.g., sol-gel approach, colloidal chemistry), (d) gas-phase chemical synthesis methods (flame pyrolysis, electro-explosion, laser ablation, plasma synthesis techniques), and (e) many other methods

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(e.g., microwaves techniques, combustion synthesis, delamination of layered materials, controlled crystallization from amorphous precursors). Functionalization (via coating and chemical modification) is an intermediate process that prepares NMs to be used for certain applications. In his book Engines of Creation: The Coming Era of Nanotechnology, K. Eric Drexler talks about the promises and dangers associated with engineering at a molecular scale. Functionalization is a step that allows us to use surface chemistry to engineer NMs so that unfavorable interaction between a biological entity and a NP can be eliminated. For example, functionalizing carbon nanotubes (CNTs) will change their surface chemistry and thereby their aggregation and deposition behavior. It has also been found that functionalized CNTs can be photo-active and can undergo chemical transformation when released in the environment. Therefore, functionalization is an important step for developing green nanotechnology in the pipeline of making NMs for reducing harmful impacts of emerging NPs to human health and environment. Nanocompositions (via melt compounding or during polymerization, blending and hot isostatic pressing, plasma spraying techniques, co-evaporation/co-depostion methods) incorporate NPs into polymeric nanocomposites, resulting in improved mechanical, electrical and optical properties, better barrier and flame retardant behavior, etc. Applications of nanotechnology include, but not limited to, (a) security (e.g., superior/ lightweight materials, advance computing, better sensors and sensor networks, powerful munitions), (b) healthcare/medical (e.g., faster/cheaper diagnostic equipment; novel drugs, targeted drug deliveries, biolabeling and detection, cancer treatments), (c) consumer products (e.g., sunscreens/cosmetics, anti-counterfeit devices, additives in paints, water- and stain-repellent textiles), (d) engineering (e.g., electronic products, cutting tool bits, molecular sieves, abrasion-resistant coatings, self-cleaning glass, lubricants and sealants/hydraulic additives), (e) transportation, (f) agriculture (controlled delivery of herbicides and pesticides), (g) resources (e.g., energy saving and utilization of renewable energy, dye-sensitized solar cells, fuel cell catalysts, increased efficiency of hydrogen generation from water) and environmental areas (e.g., water/wastewater treatment, environmental remediation, new biocides). Each of these aforementioned areas is only a tip of an iceberg of related applications.

Nanotechnology itself is evolving faster (Booker and Boysen, 2005; Willems & van den Wildenberg, 2005; USEPA, 2007). The NMs of the first generation (between 1985 and 2001) only have passive nanostructures, such as nano-structured coating and NMs (e.g., nano-metals, polymers, ceramics, catalysts, composites, NPs). The second generation is between 2001 and 2015 with active nanostructures being discovered/synthesized, such as amplifiers, transistors, targeted drugs and chemicals, longer-lasting nano-batteries (fuel cells, solar cells), low-power but high-density computer memory, adaptive structures, sensors and diagnostic nanoassays, high performance nanocomposites, ceramics, metals, and membranes. Mass production will be achieved for many NMs, such as NM-based solar cells, environmental/automotive catalysts, all kinds of improved electrodes and sensors, cutting tool bits, biological

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binding, nanoclay polymer composites, nanomembranes and photocatalyst for water purification, new dental composites, etc. The third generation is between 2015 and 2045 with various assembly techniques, networking being at the nanoscale and new architectures. 3-D nanosystems and molecular nanosystems will be realized during this period, which will revolutionize both human and natural ecosystems. For example, many engineered systems such as energy, transportation security systems may be totally different by then. Most of the Grand Challenges identified by the National Nanotechnology Initiative will be solved (NNI, 2000). While it seems that some of these are a long way off, these changes will come faster than what we believe. 1.3 Environmental Applications

Although NMs promise to revolutionize many of our industries, the near term

uses are in environmental remediation and green chemistry applications, which include (but not limited to): treatment and remediation of contaminated sites (e.g., soil, sediment), water and wastewater; nanocatalyst- or nanotechnology-enabled environmental benign nanomanufacturing and green process/engineering; energy and power; as well as environmental detection and monitoring with sensor/sensor networks. Chapters 4-15 cover the status of current knowledge of these applications. This section briefly outlines some of these applications and the recent development/trend.

It is difficult to track when NMs were first used for environmental applications.

The NP-based catalytic converter placed in the exhaust manifold of automobiles since the early 1970s may be viewed as the earliest success applications of nanocatalyst (Larsen, 2005). Some studies of synthesizing NPs for environmental remediation occurred in early 1990s (e.g., Dr. Klabunde synthesized several different nanocrystalline metal oxides during that time. See Glavee et al., 1995). Since 1996, zero-valent NPs (NZVI) have been used for environmental remediation of different sites contaminated by inorganic and organic pollutants. Applications of NZVI evolved into several different directions for research and applications. One is to develop different zero-valent metals such as zero-valent aluminum and bimetallic NPs. For example, bimetallic Fe/Al NPs can prevent the formation of a passive layer at the iron surface, and thus, maintain the reactivity of iron. Bimetallic Cu/Al NPs can dechlorinate dichloromethane that cant be degraded by bimetallic or conventional NZVI. Another direction is to develop magnetic Fe (or Fe related) NPs for easier separation after their use. These magnetic NPs can be used for sediment remediation and efficient drug delivery. The third direction is to modify the surface properties by coating the NZVI (or other NPs) with different chemicals with different functional groups. The surface-modified NPs may have much higher stability or can be used to target underground pollutants [e.g., non-aqueous phase liquids (NAPLs)] or to improve the traveling distance of NPs. Research on the second and third direction often tangle with each other and with other areas (e.g., making

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functionalized NMs) when researchers are seeking stability of these NMs/NPs. Finally, the remediation process related to NZVI has been evolved into the oxidative process driven by zero-valent iron in the presence of oxygen and a further developed system which is named ZEA (zero-valent iron, EDTA, Air). Although the mechanisms are still under investigation, it is highly possible that NZVI are capable of producing highly reactive and unselective hydroxyl radicals in the presence of oxygen. In addition, the process appear to continue to be an effective oxidant generator over longer time periods and over a range of pH (Joo and Cheng, 2006). Therefore, these processes can be widely used to treat pesticides, herbicides, and industrial chemicals as well as to purify contaminated water for different purposes.

On the other hand, since the discovery of photovoltaic property of TiO2 in 1972,

this semiconductor material has been used widely in heterogeneous photocatalysis and photocatalytic purification of water and wastewater. The number of annual publications on TiO2 related to water purifications technology increased from ~100 in 1990 to > 900 in 2007. Although it is unknown when TiO2 was first used consciously as quantum-dots for treatment purposes, it is clear that photocatalytic purification of water and wastewater has great potential in becoming a major treatment process for disinfection and reduction of pollutants in water environments. In addition, doping with the appropriate dopant can enhance the photocatalytic efficiency and cause red shift in the band gap of TiO2, making it absorb in the visible range; this technique has been used to make NPs with more photoreactivity (Karn et al., 2005).

Sensors and sensor networks is another major application of nanotechnology.

Bio and/or chemo-sensors have been used for monitoring various environmental pollutants including pesticides, organic compounds, metals, and biological parameters. However, in the area of environmental monitoring, the application of bio- or chemo-sensors faces a number of significant obstacles. Ideally, a bio- and/or chemo-sensor should be small, provide a fast response time, be reversible, be capable of continuous measurements and be suitable for integration into other devices that allow quick remedial actions to be taken. Although simple in operation, a successful, direct, multi-analyte or even single-analyte micro-bio- and/or chemo-sensor is difficult to develop. The problems include difficulties encountered in microfabrication, bioreagent stability, and efficient generation of analyte signals resulting in incomplete or impossible regeneration of sensing surfaces. Fortunately, recent advances in nanotechnology, lab-on-a-chip (LOC) technique and microfluidics technique make it possible for us to make a new generation of micro- or nano-arrays for applications in various disciplines.

For example, currently, there is intense interest in using noble metal NPs (e.g., gold and silver) to make bio- and/or chemo-nanosensors because these noble metal NPs exhibit a strong absorption band that is not present in the spectrum of the bulk metal. This absorption band results when the incident photon frequency is resonant with the

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collective oscillation of the conduction electrons and is known as the localized surface plasmon resonance (LSPR). The LSPR induces electromagnetic fields surrounding the NPs and, thus, enhances the sensing signal by 8 - 14 orders of magnitude observed in surface-enhanced Raman scattering (SERS). With LSPR, SERS has transformed Raman spectroscopy from the least sensitive vibration spectroscopy to the only single-molecule spectroscopy, workable under ambient conditions, in aqueous media, and with the sensitivity sufficient for trace-level detection. Because LSPR- and SERS-based techniques only need simple, small, light, robust, and low-cost equipment, LSPR nanosensors have great potentials for combat, field-portable environmental or point-of-service medical diagnostic applications (Willets and van Duyne, 2007). To date, a variety of chemically and biologically relevant molecules can be detected by LSPR sensorsfrom biomarkers of Alzheimer's disease and anthrax to the direct detection of glucose and chemical-warfare agents (Willets and van Duyne, 2007). Considerable research on LSPR sensors or nanosensor arrays has been focused on (a) using the sensors (not arrays) to monitor the binding of molecules onto the surface-bound species of the sensors (e.g., antibody-antigen, DNA-DNA, and DNA-protein interactions), (b) sensitivity of NP shapes and structures to bulk refractive index changes. Recently, nano- or micro-line arrays fabricated with microfluidic channels formed in poly(dimethylsiloxane) (PDMS) and then attached to either glass or gold surfaces have been used in LSPR tests for detection of DNAs and RNAs (Lee et al, 2001), which opened the widow for developing multiplexed nanoarrays for multi-analyte detection.

It should be pointed out that nanotechnologies can be combined with existing

micro-technologies and/or micro-systems to have a significant impact on nearly all environmental branches (Fecht and Werner, 2004). For example, micromachined cantilevers-based sensors have a significant advantage in the absolute sensitivity achievable; novel coating methods are making these sensors more robust and reproducible. Since the invention of atomic force microscopy (AFM), AFM-based probes are used as the key component in teleoperated and automatic nanomanipulation systems, an emerging area enabling precise measurement and control of nanoscale phenomena. Nowadays, the potential to modify AFM cantilevers into different AFM probes has been demonstrated by attaching materials such as a FIRAT tip (for fast topographic imaging), and nanoscale electrodes (for biological activity). The interactions between a NM and a single microorganism can be evaluated directly if an AFM cantilever is coated or attached to the NM (Poggi et al., 2004a, 2004b; Torun et al., 2007). Furthermore, Georgia Tech researchers have created a nanoscale probe, the Scanning Mass Spectrometry (SMS) probe that can gently pull biomolecules (proteins, metabolites, and peptides) precisely at a specific point on the cell/tissue surface, ionize these biomolecules and produce dry ions suitable for analysis and then transport those ions to the mass spectrometer for identification. The probe does this dynamically (not statically), creating images similar to movies of cell biochemical activities with high spatial and temporal resolution. The SMS probe can be readily integrated with the

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Atomic Force Microscope (AFM) or other scanning probes, and can not only image biochemical activity but also monitor the changes in the cell/tissue topology during the imaging (ScienceDaily, 2005).

Applications of nanotechnologies bring about the solution to sustainability

issues. For example, lighter-weight NMs allow us to lighten our transportation products thereby burning less fuel, resulting in emitting fewer greenhouse gases. Substituting the clay-polypropylene nanocomposite materials or aluminum for steel in 1-years fleet of vehicles in the U.S. would result in an energy savings of 50-240 thousand tera joules, a reduction of 46 million tons of CO2 equivalents of greenhouse gases released, and a saving of 56 million tons of ore, and as much as 7 fewer occupational fatalities (Lloyd and Lave, 2003). As another example, buckypaper (made from tube-shaped carbon molecules 50,000 times thinner than a human hair) is envisioned as a wondrous new material for light, energy-efficient aircraft and automobiles, improved TV screens, more powerful computers, and many other products (Kaczor, 2008).

Environmental applications of NMs cover areas much broader than water

environments, such as reduction of environmental burden (the green process and engineering, process emission control, and desulfurization/denitrification of non-renewable energy sources, agriculture and food systems), reduction/treatment of industries and agricultures wastes (converting wastes into valuable products, groundwater remediation, adsorption and photocatalytic degradation, nanomembranes), and NPS pollution control. NMs, used as catalysts, adsorbents, membranes, and additives, show higher activities, capabilities, and superior properties due to their high specific areas and nano-sized effects. Thus, lower quantities of NMs can be used for reduction/treatment of environmental wastes with higher efficiencies and lower costs. For example, carbon nanotube (CNT) membranes have been made and tested recently for the transport of Ru(NH3)63+, multiple components of heavy hydrocarbons from petroleum, bacteria, water, ethanol, iso-propanol, hexane, and decane. The CNT membrane (pore diameter = ~7 nm, membrane thickness = 34126 m, density = 3.4 x 109/cm2) allows water flux to be 104-105 times the flux predicted by Hagen-Poiseuille (H-P) equation; the gas and water permeability of these CNT membranes are several orders of magnitude higher than those of commercial polycarbonate membranes (diameter = 15 nm), despite having pore sizes of an order of a magnitude smaller. Thus, CNT membranes allow us to design much more efficient treatment processes for drinking water treatment, desalinization, and wastewater treatment (e.g., secondary sedimentation tanks can be replaced with CNT membrane modules). Carbon nanotube and other nanocomposite materials, however, are currently very expensive; the challenge, therefore, is to improve their production yields and lower the costs of these materials for their use in large quantity. In some cases (e.g., emission control of the pollutants) alternative low-cost or non-toxic NMs (such as WCx) are used

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to replace precious metals (e.g., Pt, Pd, Ru, Rh, etc.) or toxic materials (e.g., ammonia). Currently, industry and governments have taken this firmly on board; funding for nanotechnologies has increased steadily. The European Union, for example, has decided to put 3.5 billion euros into nanotechnology research between 2007 and 2013, on top of private sector investment and national research budgets. The most frequently cited estimate is that the world market in nanotechnologies will amount to 1,000 billion dollars by 2015 (ETUC, 2008). 1.4 Implications of Nanotechnology and Research Needs

While nanotechnology may bring us endless benefits, applications of

nanotechnology may have some potential risks (issues) to the two ecosystems. The list of studies on the implications of nanotechnology can be very long. This book uses three chapters (16-18) to introduce the key issues relating to these implications.

In general, there are two types of nanostructures: (a) fixed NPs (NMs with NPs

incorporated into a substance, material or device); and (b) free NPs (individual NPs of a substance). The immediate concern is with free NPs because of their mobility and their increased reactivity. However, fixed NPs also are of great concern, particularly when they can be released (and, thus, become free NPs) in the production lines or during the final disposal stage. The implications of nanotechnology cover all possible areas of both human and natural ecosystems. This section briefly covers some potential risks, such as health/safety, environmental and societal issues, together with some research needs on these issues.

In his 1986 book, Eric Drexler first mentioned gray goo, warning of self-

replicating nanotechnology running amuck and covering the earth. While most people downplayed this scenario, the concept brought nanotechnology to the attention of the public. In 2005, a survey was conducted about the step(s) in the value chain of NMs/NPs where people believe that a greater potential health, safety and environmental (HSE) hazard may occur; the results indicate that (a) during the manufacturing of NPs, the HSE hazard is 67.6%; (b) during the integration of NPs into a system is 40.5%; (c) during the recycling of the NMs is 21.6%; (d) during the normal life of the application is 13.5%; and (e) during the manufacturing of the final application is 8.1% (Willems & van den Wildenberg, 2005). While the survey results reflect the common perception of HSE risk, many dynamic processes are not considered by normal people. For example, the risks in manufacturing may be handled adequately through standard industry procedures, and therefore, may not even be a major concern. However, the low assessment of risk during the use of nanoproducts may reflect our limited knowledge of NMs (sometimes, people even dont know they are dealing with NPs). For example, titanium dioxide (TiO2) is everywhere, in toothpaste, paint, and other products. Its now being produced

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at 5 nm in size; the question is, do these relatively nontoxic materials become toxic on the nanoscale? There is some controversy surrounding sunscreens containing nano-sized particles of TiO2 or zinc oxide, which can absorb directly into the skin and can readily enter the bloodstream and other body tissues, and have not been proven safe. Therefore, the true issue associated with implications of NM applications is that the technology for creating NMs and the applications of them are far outpacing our knowledge and regulations about their potential toxicity.

Health and Safety Issues. Accumulated evidence indicates that the adsorption

of NPs via the lung, skin and gut can occur. However, a clear understanding of their distribution in the body (i.e., toxic kinetics) is not available, which requires (a) identification of potential target organs/tissues for toxicity assessment, (b) understanding of inter and intracellular transport and localization of NPs and their cellular toxicity. Clinical and experimental studies indicate that an ability to generate reactive oxygen species (ROS) and oxidative stress play a role in the ability of NPs to induce the toxic effects. However, inflammatory effects, genotoxicity, and physical piercing of membranes and other cell structures and molecules can all apply to NPs. The specific properties related to NMs behavior and toxic effects to living organisms are as yet poorly understood, such as the deposition, distribution, toxicity, pathogenicity and translocation potential and pathways for NPs within the host organism. For example, dermal uptake, penetration and toxicity of TiO2 NPs in the skin still are not clear.

Currently, research relating to the toxicology of NMs is focusing on discovering

(a) what controls the toxicity of representative NPs [TiO2, carbon black, C60, quantum dots, polystyrene particles, carbon nanotubes (single-walled and multi-walled), metal oxides (e.g., NZVI), and 100 nm PLGA (poly (D,L-lactic co-glycide) acid) NPs], (b) whether there are unknown factors, and (c) what the mechanisms are involved in. Both in vitro and in vivo methods are used to examine the toxicological properties of NPs. The methods will vary from particle to particle depending on mode of exposure and use. Moreover, frameworks allowing the extrapolation of in vitro results to natural systems or to assess the risks of forthcoming NMs based on previous knowledge are needed (DEFRA, 2006; Navarro et al., 2008).

Environmental Issues. Considerable studies have been conducted on the uptake,

toxicity and effects of NPs on groundwater, surface water, soil microorganisms, animals, and plants, especially in the context of remediation, water and wastewater treatment, and air pollution control. Most of these studies are conducted within the laboratories with bench-scale systems. Sufficient information is not available on evaluation of the whole life cycle of NMs, including their fabrication, storage/distribution, application/potential abuse, and disposal. The impact on human and natural ecosystems may be different at different stages of the life cycle.

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It is possible that the unique characteristics of NMs result in harmful interactions to the environment and biological systems. The in vivo effects of NPs to some ecological groups [e.g., invertebrates, vertebrates (e.g. fish) and plants] indicate that NPs can be highly toxic. For example, colloidal C60 fullerenes are taken up by the largemouth bass and accumulated into the fish brains. Eukaryotes (e.g., protists and metazoans) have highly developed processes for the cellular internalization of nanoscale (100 nm or less) and microscale (100100,000 nm) particles, namely endocytosis and phagocytosis, respectively. These processes are integral to key physiological functions such as intracellular digestion and cellular immunity. However, prokaryotes, like bacteria, may be largely protected against the uptake of many types of NMs, since they do not have mechanisms for the transport of NMs across their cell wall. It is worthy of noting that studies on the antimicrobial properties of NMs and their interactions with microorganisms are limited. It is not clear how microbial communities respond to NMs as a function of NM chemical and physical properties (such as composition and stability) and the environment in which the microbial community exists. The cytotoxic mechanisms of NMs on living organisms depend on the fate and transport of NMs upon their physical/chemical/biological interactions with cell materials. Currently, there are many unknowns concerning the fate and transport of NMs in the environment, including effects of different interphase transfer and transformation processes on the form, complexity, and the mechanisms of NM transport and removal in the environment. At present, fundamental research is focusing on (a) the influence of particle size, shape and number on basic aspects of ecotoxicology, mechanisms of action, dose response relationships (at all levels of biological organization) and toxicokinetic profiles (adsorption/uptake, distribution, metabolism and excretion), and (b) the extent to which fate, behavior and ecotoxicology of nanoparticles is governed by specific properties, common to some or all nanoparticles (DEFRA, 2006). Important research questions to be answered are related to (a) dose response relationships (e.g., are they affected by particles size, number or shape?), (b) interactions between NPs and other substances (e.g., do NMs affect the fate, behavior or exotoxicology of other substances in the environment?), (c) fate and transport of NMs (e.g., are NPs more persistent, bioaccumulative or toxic than those in bulk or dissolved form? What factors affect NMs agglomeration and other fate and behavior? Are any fate and transport effects of NMs generalizable to certain or all classes of particles?), (d) development of structure/activity relationships to predict fate and transport of NMs in the environment.

Societal Issues, considerable studies have been conducted on the ethical, legal,

and social implications (ELSI) of nanotechnology. Nanoethicists see nanotechnology as positively impacting our society in the future. For example, it is estimated that nanotechnology will create an additional 210 million jobs across the world by 2014 (ETUC, 2008). Many people believe that this technology has a tremendous long-term potential to completely revolutionize our society, resulting in better understanding of

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nature, a higher quality of life, and increased productivity in nearly all industrial branches. Others believe that nanotechnology is a technological tsunami that will result in significant social disruption, fundamentally altering the way people live. Nano skeptics believe that a nano-divide appears inevitable, which will develop first between the nano-poor (most of the worlds poorest countries) and the nano-enabled countries (Miller and Senjen, 2006). Moreover, nanotechnology may be used to develop chemical/biological weapons or for military purposes, which has the potential to destabilize international relationships via a nano-arms race. Obviously, it is too early to say which opinion is right. Considering the historical mistakes made by us (e.g., PCBs, asbestos), it would be wiser for us to handle NMs with great cautions.

Programs have been implemented of public dialogue and social research on ELSI

of nanotechnology. The two broad areas have dominated discussions: the possible toxicity of NMs and issues of public engagement and democratization of science. One important issue is that consumers should have the right to know what is in a product because, in many cases, manufacturers have published no information on tests done on nanotechnology products and their health hazards, or have not labeled consumer products as containing NMs (ETUC, 2008). For this reason, the ETUC (2008) demands full compliance with no data, no market principle, that is, to refuse to register chemicals for which manufacturers fail to supply the data required to ensure the manufacture, marketing and use of their nanometer forms that has no harmful effects for human health and the environment at all stages of their life cycle (ETUC, 2008). Obviously, implementation of this principle means considerable resources need to be injected. Compared with other areas, however, research funding in the ELSI area is small. Therefore, the ETUC calls for at least 15% of the research budgets related to nanotechnology to be earmarked for health and environmental aspects and to require all research projects to include health and safety aspects as a compulsory part of their reporting (ETUC, 2008).

A truly precautionary approach to compliance with no data, no market

principle is to develop new regulations with respect to the ELSI of nanotechnology. Although regulatory bodies (e.g., USEPA, FDA) have started dealing with the NMs potential risks, NMs remain effectively unregulated, that is, if the materials have already been approved in bulk form, the corresponding NMs are not subject to any special regulation regarding production, handling or labeling. This situation needs to be changed; NMs need to be regulated as new chemicals.

1.5 Summary

Described as the engine of the next industrial revolution, nanotechnologies have a far-reaching development and application potential. The real transformative

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power of nanotechnology lies in its capacity to act as a platform technology, enabling breakthroughs in various fields. Particularly, nanometer-scale structures and functions integrated with new advanced micro-systems and micro-technologies will have a significant impact on the development of new products and production technologies for nearly all industrial branches. However, nanotechnologies also raise many concerns about their potential risks to both human and natural ecosystems; unleashing NMs and nanoproducts prematurely could unintentionally lead to new health and environmental hazards. There is an urgent need for research on the ethical, legal, and social implications of nanotechnology and nanoproducts.

1.6 References Booker, R., and Boysen, E. (2005). Nanotechnology for Dummies. Wiley Publishing,

Inc. Hoboken, NJ. DEFRA (Department for Environment, Food, and Rural Affairs) (2006). Characterising

the Potential Risks Posed by Engineered Nanoparticles. UK Government Research a progress report, DEFRA, London, 2006.

ETUC (European Trade Union Confederation) (2008). ETUC Resolution on Nanotechnologies and Nanomaterials. Resolution adopted by the ETUC Executive Committee in their meeting held in Brussels on 24-25 June 2008.

Fecht, H.-J., and Werner, M. (ed.) (2004). The Nano-Micro Interface: Bridging the Nicro and Nano. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Glavee, G.N., Klabunde, K.J., Sorensen, C.M., and Hadjipanayis, G.C. (1995). Chemistry of borohydride reduction of iron (II) and Iron (III) ions in aqueous and nonaqueous media. Formation of nanoscale Fe, FeB, and Fe2B powders. Inorg. Chem., 34, 28-35.

Joo, S.H., and Cheng, I.F. (2006). Nanotechnology for Environmental Remediation, Springer, Inc., New York, pp1-165.

Kaczor, B. (2008). Future planes, cars may be made of buckypaper. The Associated Press, Oct. 19, available at (accessed 10/21/2008).

Karn, B., Masciangioli, T., Zhang, W.-X., Colvin, V., and Alivisatos, P. (eds.) (2005). Nanotechnology and the Environment Applications and Implications, American Chemical Society (ACS) symposium series 890, ACS, Washington, DC.

Larsen, S.C. (2005). Nanocatalysts for environmental technology. In Nanotechnology and the Environment Applications and Implications by Karn, B., Masciangioli, T., Zhang, W.-X., Colvin, V., and Alivisatos, P. (eds.), Chapter 36, ACS symposium series 890, ACS, Washington, DC.

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http://ap.google.com/article/

Lee, H. J., Coodrick, T. T., and Corn, R. M. (2001). SPR imaging measurements of 1-D and 2-D DNA microarrays created from microfluidic channels on gold thin films. Anal. Chem., 73(22), 5525-5531.

Lloyd, S.M., and Lave, L.B. (2003). Life cycle economic and environmental implications of using nanocomposites in automobiles. Environ. Sci. Technol., 37, 3458-3466.

Miller, G., and Senjen, R. (2006). The disruptive social impacts of nanotechnologyissue summary. Available at (accessed Oct. 2008).

Navarro, E., Baun, A., Behra, R., Haartmann, N.B., Filser, J., Miao, A.-J., Quigg, A., Santschi, P.H., and Sigg, L. (2008). Environmental behavior and ecotoxicity of engineered nanoparticles to algae, plants, and fungi. Ecotoxicology, 17, 372-386.

NNI (National Nanotechnology Initiative) (2000). National Nanotechnology InitiativeThe initiative and its Implementation Plan. National Science and Technology Council, Committee on Technology, Subcommittee on Nanoscale Science, Engineering and Technology, July 2000.

Poggi, M.A., Bottomley, L.A., and Lillehei, P.T. (2004a). Measuring the adhesion forces between alkanethiol-modified AFM cantilevers and single walled carbon nanotubes. Nano Letters, 4, 61-64.

Poggi, M.A., Boyles, J.S., Bottomley, L.A., McFarland, A.W., Colton, J.S., Nguyen, C.V., Stevens, R.M., and Lillehei, P.T. (2004b). Measuring the compression of a carbon nanospring. Nano Letters, 4, 1009-1016.

ScienceDaily (2005). New chem-bio sensors offer simultaneous monitoring. ScienceDaily, June 30, 2005.

Torun, H., Sarangapani, K.K., and Degertekina, F.L. (2007). Spring constant tuning of active atomic force microscope probes using electrostatic spring softening effect. Applied Physics Letters, 91, 253113.

Willems & van den Wildenberg (W&W) (2005). Roadmap Report on Nanoparticles. A project co-funded by the 6th Framework Programme of the EC, Barcelona, Spain, November 2005.

Willets, K.A., and van Duyne, R.P. (2007). Localized surface Plasmon resonance spectroscopy and sensing. Annu Rev. Chem., 58, 267-297.

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http://nano.foe.org.au

CHAPTER 2

Synthesis of Nanoparticles and One-Dimensional

Nanomaterials

Hosik Park and Heechul Choi 2.1 Introduction

Nanomaterials can be defined as materials that have an average phase or grain size of less than 100 nm. Nanomaterials exhibit novel properties which can significantly differ from those of bulk materials due to their unique physicochemical (i.e., size, shape) and surface (i.e., reactivity, conductivity) properties (Seigel, 1993). Also, nanomaterials themselves have different properties depending on how nanomaterials are synthesized and how their atoms and molecules are ordered. For example, metal oxide nanoparticles including semiconductor nanoparticles, which were synthesized using different methods and under different experimental conditions (i.e., temperature, reaction time), have different physicochemical and surface properties (Chan et al., 2002, Jung et al., 2007). Recently, numerous approaches based on the application of these properties have been developed and applied to the synthesis of nanomaterials. This implies that inherent properties pertaining to chemical reactivity or physical compaction play an essential role in nanomaterials synthesis.

In this chapter, several common and unique techniques for nanoparticles and one-dimensional nanomaterials synthesis will be introduced. 2.2 Nanoparticles

Primarily, synthesis techniques for nanoparticles can be divided into top-down approaches and bottom-up approaches. Top-down approaches typically start with a suitable bulk material and then break the bulk material into smaller pieces. Ball-milling or attrition and pattern formation are common methods of a top-down approach. For

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example, an attrition method can break the bulk material into nanomaterials ranging from tens to several hundreds of nanometers in diameter. However, broad size distribution, varied shape or geometry, and impurities from the milling medium are factors of concern for products made using this method. For this reason, bottom-up approaches for nanomaterial synthesis are becoming more popular, as this rapidly growing research area has great potential for use in the creation of technologically advanced and useful materials. A bottom-up approach directly forms nanomaterials from different kinds of precursors via mainly a chemical reaction. For example, homogeneous nucleation from a liquid or vapor, or heterogeneous nucleation on a substrate, are concepts for synthesizing nanomaterials using a bottom-up approach. There are several techniques related to bottom-up approaches. These techniques can be divided into thermodynamic approaches, equilibrium approaches, and kinetic approaches. For thermodynamic approaches, generation of supersaturation, nucleation, and subsequent growth are the primary procedures for nanomaterial synthesis. In kinetic approaches, the amount of precursors required for efficient nanomaterial growth or the space required for the reaction are the main limiting factors. Hence, synthesis of nanoparticles introduced in this section will follow bottom-up approaches. 2.2.1 Sol-Gel Process

The sol-gel process is a wet chemical process that results in the formation of either inorganic or organic-inorganic nanomaterials from a liquid phase (Livage et al., 1988). The sol-gel process is especially applicable for the synthesis of oxide nanomaterials (Cousin and Ross, 1990).

The sol-gel process uses inorganic or metal-organic precursors. Molecular precursors are induced to undergo hydrolysis and condensation in solution to form bridging hydroxyl [M-p(OH)-M] or oxo (M-O-M) bonds. The most commonly used precursors are metal alkoxides [M(OR)n], where R is an alkyl group, inorganic salt, or organic salt. Otherwise, organic or aqueous solvents may be used to dissolve precursors, and catalysts are then added to promote hydrolysis and condensation reactions (Gesser and Goswami, 1989; Chandler et al., 1993; Brinker and Hurd, 1994). The overall reaction can be represented by following steps: Hydrolysis:

MOR + H2O MOH + ROH (Eq. 2.1) Condensation:

MOR+ HOM MOM + ROH (alcoxolation) (Eq. 2.2) MOH+ HOM MOM + H2O (oxolation) (Eq. 2.3)

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MOH2 + MOR M(OH)M + ROH (olation) (Eq. 2.4) where M denotes a metal or Si, and R is an alkyl group. Both forms of reactions are catalyzed by acids and bases; however, they may also proceed under neutral conditions. The rate of hydrolysis and gelation are strongly pH dependent. Table 2.1 lists common alkoxides for sol-gel processing.

Table 2.1 Common alkoxides for sol-gel processing.

Name Formula Formula Weight (g) Density at

20 C (g/cm3) TEOS Si(OC2H5)4 208 0.936

Trimethyl Borate B(OCH3)3 104 0.915

Aluminum Sec-Butoxide Al(OC4H9)3 246 0.967

Titanium Isopropoxide Ti(OC3H7)4 284 0.955

Zirconium Isopropoxide Zr(OC3H7)4 327 1.05

Normally, the alkoxide is dissolved in its parent alcohol and hydrolyzed by the

addition of water, in the case of more electronegative metals and metalloids, an acid or base catalyst is prerequisite. Through the hydrolysis process, alkoxide groups are replaced with a reactive M-OH hydroxo group, and subsequent condensation reactions involving the M-OH hydroxo group produce oligomers or polymers composed of M-O-M or M-(OH)-M bonds by alcoxolation, oxolation and olation. Alcoxolation and oxolation reaction form a bridging oxo group through the elimination of an alcohol molecules and water molecules, respectably. In case of olation reaction, bridging hydroxo groups are formed through the elimination of solvent molecules.

Both hydrolysis and condensation process lead to formation of a gel. Then, these are undergone the drying process and optionally subjected to thermal treatment. Depending on the drying process, two types of gels can be generated; aerogels (dried supercritically) and xerogels (dried without supercritical fluid). In general, aerogels have lower densities, higher porosities, and higher surface areas than xerogels, but both materials contain an architecture that includes interconnected particles and pores with nanoscopic dimensions (Hench and West, 1990).

Through careful control of sol preparation and the processing, various oxide nanoparticles have been synthesized, such as Fe2O3, SnO2 Al2O3, TiO2 and ZnO (Bruni et al., 1999; Manorama et al., 1999; Joo, et al., 2005; Shojaie-Bahaabad and Taheri-Nassaj, 2008). For example, Shojaie-Bahaabad and Taheri-Nassaj (2008) synthesized -

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Al2O3 nanoparticles by using AlCl36H2O and Al powder as raw materials. Aluminum chloride hexahydrate may be hydrolyzed to produce the sol:

AlCl3 + 3H2O Al(OH)3 + 3HCl (Eq. 2.5) In addition, through the reaction between Al powder and HCl aluminum chloride and hydrogen gas are produced:

2Al + 6HCl 22AlCl3 + 3H2 (Eq. 2.6) Therefore, Al can be used as a source of AlCl3 to produce the sol containing Al(OH)3 nanoparticles. Finally, the hydroxides groups produced aggregate together to form the gel. The obtained gel was dried and then ground and calcined in a furnace at different temperatures. The gel calcined at 1100 C resulted in the formation of crystalline -Al2O3 nanoparticles. It had a particle size distribution ranging from 32 to 100 nm after heat treatment at 1100 C. This material can be used as the catalyst supports and high temperature applications. 2.2.2 Forced Hydrolysis

A forced hydrolysis method is a simple method that can be used to synthesize uniformly-sized metal oxide nanoparticles based on a metal salt solution (Hu et al., 1998). A forced hydrolysis method can also be used to reference the process or ability of many hydrated metal ions, especially polyvalent metal cations, that readily deprotonate in aqueous solutions at elevated temperatures. This characteristic can be used to advantage in the preparation of colloidal particles from such materials. Since the hydrolyzed products of these metal ions are intermediates used to precipitate corresponding hydroxides, it is possible to prepare samples with a very narrow particle size distribution simply by heating the metal salt solution (Blesa et al., 1985). At an elevated temperature, the hydrolysis reaction should be faster and generate a respectively larger number of nuclei. This in turn should lead to the formation of smaller particles. In forced hydrolysis procedures, the pH and nature of the anions, solvents, and precursors all play a dominant role in nanomaterial synthesis.

This principle was demonstrated by the formation of monodisperse silica spheres

by Stber et al. (1968). The procedure for silica sphere fabrication is as follows (Stber et al., 1968):

Si(OR)4 + (alcohol + NH3 + H2O) 50 nm2 m SiO2 (Eq. 2.7)

Here, ammonia was reacted as a catalyst to form spherical silica particles. In basic conditions, three-dimensional structures are formed by a condensation reaction, instead

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of the linear polymeric chain that occurs in acidic conditions. An alcoholic solvent was added to control the reaction rate and particle size. In Stbers experiment, the reaction rate was found to be faster with methanol than n-butanol. In addition, particle sizes obtained under comparable conditions were smallest in methanol and largest in n-butanol; however, the addition of more alcohol led to a wide variance in size distribution. Similarly, different ligand sizes in the precursors affected the reaction rate and particle size, as smaller ligands brought about a faster reaction rate and smaller particle size. Also, temperature was found to be an important factor in terms of reaction rate and generation of small nuclei. A high temperature favors a fast hydrolysis reaction rate and results in high supersaturation, which in turn leads to the formation of larger number of small nuclei.

In addition, through this method various kinds of metal oxide nanoparticles such as -Fe2O3, ZrO2 and TiO2 can be synthesized (Hu et al., 1998; Wei et al., 1999; Wang et al., 2008). This method uses inexpensive starting chemicals, that is, inorganic metal salts (i.e., FeCl3, ZrOCl2xH2O, and Ti(SO4)2). For example, Wang et al. (2008) used FeCl3 and HCl solution to synthesize -Fe2O3 nanoparticles. This solution was heated to 100 C by 8 C/min in a water bath. In this process, aging time affected the size and morphology of -Fe2O3. With an increase in the aging time, irregularly shaped particles transformed into bar-shaped nanoparticles. A long aging time would permit the occurrence of Oswalt ripening to further narrow the size distribution. 2.2.3 Microemulsion Method

The microemulsion method has been used to synthesize nanoparticles by precipitation (Schwuger et al., 1995). Microemulsion acts as an interesting alternative reaction medium for the production of nanoparticles. For example, by adding a reducing agent into the microemulsion system or by mixing with nanodroplets, these microemulsions can be filled with different reactants thereby enabling the synthesis of metallic or metal oxide nanoparticles. Boutonnet et al. (1982) first reported the synthesis of monodisperse metal nanoparticles by microemulsion method in the early 1980s (Boutonnet et al., 1982).

Controlled nucleation and growth of metal clusters occurs in the interior of surfactant aggregates. In this process, an ionic salt (i.e., Fe(BF4)2 or anhydrous FeC13) is dissolved in the hydrophilic interior of the micelles, while the surrounding continuous hydrophobic oil limits nucleation and growth to the micelle interior volume (Wilcoxon and Provencio, 1999). The addition of a co-surfactant (i.e., an alcohol) in the reaction system plays a role in controlling the interfacial tension. From this, microemulsions are spontaneously synthesized without the need for significant mechanical agitation. Through the ion-dipole interactions with the polar co-surfactant, the surfactant forms spherical aggregates in which the polar (ionic) ends of the surfactant molecules orient

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towards the center. The co-surfactants act as an electronegative spacer that minimizes repulsions between the positively charged surfactant heads.

In the microemulsion process, selection of the surfactants plays a very important

role in nanoparticle synthesis. To this end, a variety of surfactants can be used in the microemulsion process, such as cetyltrimethylammonium bromide (CTAB) (Zhang et al., 2006), poly (oxyethylene)5 nonyl phenol ether (NP5), poly (oxyethylene)9 nonyl phenol ether (NP9) (Fang et al., 1997), Sodium bis(2-ethylhexyl)sulphosuccinate (AOT), and pentaethylene glycol dodecyl ether (PEDGE) (Eriksson et al., 2004). There are various nanoparticles synthesized by using different precursors and surfactants. Some examples are listed in Table 2.2. Table 2.2 Examples of nanoparticles synthesized by microemulsion method.

Nanoparticle Type

Particle Size (nm)

Metal Precursor Surfactant Reference

Ag < 50 Ag(NO3) AOT Maillard et al. (2002)

Ni 4.2 NiCl2 CTAB Chen and Wu (2000)

NiO < 100 NiCl2 TritonX-100 Han et al. (2004)

Pd/Pt 5 PdCl2, H2PtCl6 AOT Wu et al. (2001)

Pt/TiO2 - H2PtCl2 PEDGE Kizling et al. (1996)

Fe/SiO2 4.813 Fe(NO3)3 NP5 Hayashi et al. (2002)

It should be noted that the surfactant should be chemically inert with respect to

all other components of the microemulsion; CTAB that is stable against mild oxidizers is one example of a surfactant in a microemulsion system. The counter ions of ionic surfactants should affect the precursor in the reaction system. In a reaction involving Ag+, for instance, the dissociated Br- ions of CTAB would cause the immediate precipitation of AgBr.

There are other aspects that also need to be taken into consideration for particle

synthesis in microemulsions. These factors include the mass percentage of the aqueous phase in the microemulsion, the average concentration of the reacting species in the aqueous domain, the water-surfactant ratio and structure and properties of the solubilizing water, and the dynamic behavior of the microemulsion (Cushing et al., 2004). These factors affect synthesis properties, such as product size, particle size distribution, agglomerate size, and phase of the final product. 2.2.4 Vapor Phase Reaction

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Nanoparticles can also be synthesized by vapor phase reactions. In general, this

synthesis procedure is conducted at elevated temperatures and under a vacuum; a vacuum is needed to maintain a low concentration of growth species to promote subsequent diffusion-controlled growth. Owing to the vapor pressure characteristics of compounds such as TiO2, a much higher concentration of droplets was obtained using the same equipment (Visca and Matijevic, 1979).

In the vapor phase synthesis of nanoparticles, conditions are maintained such that

the vapor phase mixture is kept thermodynamically unstable; conditions that include the presence of a supersaturated vapor (Swihart, 2003). If the conditions include high supersaturation, and the reaction kinetics permit, vapors rapidly nucleate and form a large number of extremely small particles. A decrease in temperature leads to a more rapid decrease in the equilibrium vapor pressure and a relatively higher level of supersaturation (Flagan and Lunden, 1995). After the formation of nucleation, the remaining supersaturation is reduced by either condensation or reaction between the vapor phase molecules and the resulting particles. The particles then grow by Brownian coagulation (Granqvist and Buhrman, 1976), and product particles are generally collected by thermophoretic deposition.

There are several factors that determine the characterization of the synthesized nanoparticles. These factors include temperature, flow rate of the carrier gas, precursor concentration, operation pressure, and the growth time; the size of the particles can be altered by changing the temperature, the flow rate of the carrier gas, and the precursor nuclei concentration (Maisels et al., 2000; Ohno, 2002; Wegner et al., 2002; Kim et al., 2004; Simchi et al., 2007). In addition, the rate of production of nanoparticles can be dramatically increased when the synthesis reactor is operated at a higher pressure, with correspondingly shorter growth times. Short growth times are also achieved by rapid cooling; neck formation in the agglomerate particles that do form is diminished by starting the growth process at a high initial temperature (Flagan and Lunden, 1995).

Despite the aforementioned challenges, various nanoparticles can be synthesized by vapor phase reactions. Nepijko et al. (2000) synthesized silver nanoaprticles of 23 nm in diameter by the gas aggregation technique. Another example is the Au nanoparticles. Au nanoparticles have been grown on various oxide substrates such as iron oxide (Haruta, 1997) and -alumina (Grisel and Nieuwenhuys, 2001).

MnO nanoparticles (Chang et al., 2005) can be synthesized by using a vapor phase reaction method. MnCl2 powders and several silicon substrates were placed on the quartz boat. Temperature and pressure were maintained at 778 C and 0.05 MPa, respectably, at a constantly-mixed gas flow (Ar2:H2) in a horizontal furnace. The particles formed are found to be composed of nearly round shape with diameters ranging

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from 20 to 40 nm. Here, hydrogen played an important role for selecting a particular product during the growing process. The final product was not Mn3O4 but MnO due to the decrease in the oxygen partial pressure by the reaction with hydrogen. However, with the absence of hydrogen, only Mn3O4 was formed. 2.2.5 (Flame) Aerosol Synthesis

Aerosol processes are commonly used in the large-scale commercial production of ultrafine particles (dp < 100 nm) and materials such as titania and silica (Ulrich and Rieh, 1982; Hartmann et al., 1989; Ahonen et al., 2001; Huisman et al. 2003; Backman et al., 2004). This use is due to the ease of formation of metal oxides from inexpensive water-soluble precursors (Pruss et al., 2000). A variety of chemical precursors have been used, including metal salts such as TiCl4 to synthesize TiO2, and SiCl4 to synthesize SiO2, metal alkoxides (Visca and Matijevic, 1979; Huisman et al., 2003).

Material properties, the aerosol volume concentration (volume of particles per

unit volume of gas), the resident time, and process temperature all play a key role in the system (Ulrich and Rieh, 1982; Backman et al., 2004; Jiang et al., 2007). These are significant factors that have the potential to determine coalescence or sintering, as well as product morphology and crystallinity.

Precursor materials are injected into the burner as a gas, droplets, or solid particles. Usually, liquid or solid precursors rapidly evaporate as they are exposed to high flame temperatures. Then, condensable molecules produced by either physical or chemical processes self-nucleate to form particles. After a high temperature step, the aerosol stream slowly cools to a lower temperature, allowing the particles to collect. Subsequent collision and coalescence leads to the formation of larger particles. Sometimes, aggregates are physically held together by bonds of varying strength, and these can then combine to form aggregates held together by necks formed as a result of sintering. These agglomerates can be relatively easily separated into their aggregate components.

The collision/coalescence mechanism for particle formation is based on a series of steps assumed to proceed as follows (Bandyopadhyaya et al., 2004): 1) A chemical (or physical) process converts the aerosol precursor to condensable

molecules. 2) The condensable molecules self-nucleate to form a cloud of stable nuclei. 3) The stable nuclei initially coalesce to form larger particles. 4) Coalescence ceases or slows significantly, leading to the formation of

agglomerate structures. 5) Coalescence and neck formation may continue for particles within the

agglomerate structures.

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These processes may occur simultaneously. The flame reactor is the most widely

used reactor in the fabrication of inorganic particles by the aerosol method. In the reaction chamber, the aerosol precursor and oxygen are mixed with each other and then burned; inert gases and fuels such as hydrogen or methane may also be present. The reaction stoichiometry for a liquid phase reaction can be described by the following equations.

2H2 + O2 2H2O (Eq. 2.8) SiCl4 + 2H2O SiO2 + 4HCl (Eq. 2.9)

Since the reaction occurs with water vapor, the process is referred to as flame hydrolysis. The gas coming out of the furnace contains silica particles, gaseous hydrochloric acid, hydrogen, and a small amount of chlorine. The agglomerates are collected in cyclone separators, which may be followed by a bag filter. This flame process