&EPA
United States
Environmental Protection
Agency
United States
Environmental Protection Agency
Office of Radiation and Indoor Air
Radiation Protection Division (6608J)
EPA 402-R-09-002
January 2009
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January 2009 EPA/402/R-09/002
Potential Nano-Enabled
Environmental
Applications for
Radionuclides
By
EnDyna, Inc.
McLean, VA 22102
EPA Contract No. 07-HQ-02407
Project Officer
Madeleine Nawar
Radiation Protection Division
U.S. Environmental Protection Agency
Washington, DC 20460
1 Office of Radiation and Indoor Air
I Radiation Protection Division
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Preface
"Potential Nano-Enabled Environmental Applications for Radionuclides" is an
informational document designed to familiarize interested parties with some of the
emerging nanotechnologies, and to recognize their potential environmental applications
and implications. Specifically, it is developed to assist in decision making for
incorporating nano-enabled technologies in mitigation of environmental contaminants
including radionuclides. The document represents a snapshot in time to elucidate some of
the base knowledge of nano-science which has evolved over the last 5-10 years. For the
purposes of this document, "nano-enabled technologies and/or processes" refer to
technologies which are enabled by a nano subsystem.
This document may be updated in the future, and if you have any comments on the
document or suggestions for incorporation in future updates, please contact: Ms
Madeleine Nawar, USEPA, ORIA, RPD, 1200 Pennsylvania Ave., NW (MC 6608J),
Washington DC 20460-0001, USA, Phone: 202-343-9229, Fax: 202-343-2306, Email:
nawar.madeleine@epa.gov
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Disclaimer
The opinions expressed within this report do not necessarily represent the views of the
U.S. Environmental Protection Agency (EPA). Mention of trade names or commercial
products does not constitute endorsement or recommendation for use. Similarly,
exclusions or absence of specific references is merely an indication that information
related to that entity was not readily available during the development of this
informational document.
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Acknowledgements
This report was developed by the Radiation Protection Division (RPD) of the U.S.
Environmental Protection Agency's (EPA's) Office of Radiation and Indoor Air (ORIA).
Ms. Madeleine Nawar served as the EPA Project Manager. Several individuals provided
valuable input on the content of this report throughout its development. Special
acknowledgement and appreciation is extended to RPD management and colleagues for
their support and encouragement.
A multi-disciplinary set of individuals provided peer review are:
• Jim Alwood, Chemical Control Division, Office of Pollution Prevention Toxic
Substances. U.S. EPA
• Terry Barton, Senior Environmental Scientist, Superfund and Technology
Liaison Program, U.S. EPA Region 6
• J. Michael Davis, Ph.D., Senior Science Advisor, National Center for
Environmental Assessment, Office of Research and Development, U.S. EPA
• Brendlyn Faison, Ph.D., Office of Science and Technology, U.S. EPA
• Richard Mattick, [in collaboration with U.S. EPA regional superfund staff],
Policy Analysis and Regulatory Management Staff, Office of Solid Waste and
Emergency Response, U.S. EPA
• Najm Shamim, Ph.D., Office of Pesticide Programs, Antimicrobials Division,
U.S. EPA
• Richard Wiggins, Ph.D., Senior Science Advisor; Research Planning and
Coordination Staff/National Health and Environmental Effects Research
Laboratory Office of Research and Development, U.S. EPA
• Ronald Wilhelm, Senior Scientist, RPD, ORIA, U.S. EPA
This document was prepared under Contract No. 07-HQ-02407, EnDyna, Inc., and under
the sponsorship of ORIA. Madeleine Nawar of EPA's RPD served as the Project Officer.
The primary authors were Dr. Smita Siddhanti and Dr. Ian Tasker, assisted by Daniel
Ruedy, of EnDyna, Inc. The EnDyna project team wishes to acknowledge the
contribution of the following individuals for their valuable comments, input, and
suggestions for this document's improvement.
• Rafaela Ferguson, RPD, ORIA, U.S. EPA
• Dr. Eric Nuttall, the University of New Mexico
• Dr. Igor Linkov, U.S. Army Corps of Engineers
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Table of Contents
Executive Summary vii
1.0 Introduction 1
1.1 Definition of Nanotechnology 1
1.2 Purpose of This Report 1
1.3 Background of Nanotechnology 2
1.4 Application of Nanotechnology 3
1.5 Risk Associated with Nanoparticles in the Environment 4
1.5.1 Nanomaterial Properties Associated with Risk 5
1.5.2 Possible Approaches to Nanotechnology Risk Assessment 6
1.5.3 Data Gaps and Limitations for Nanomaterials Risk Assessment 7
1.5.4 Approaches to Managing Nanotechnology Risk 9
1.6 Nanotechnology for Environmental Remediation 12
1.6.1 Current Applications 12
1.6.2 Awareness of Potential Environmental Benefits 13
1.6.3 EPA's Involvement in Nanotechnology 14
1.6.4 International Involvement in Nanotechnology 18
1.7 Nanotechnology for Environmental Remediation of Radionuclides 19
1.7.1 Technical Status 19
1.7.2 Rational Approach to Design in Nanotechnology 20
1.8 Structure of This Report and Summary of Examined Technologies 21
1.9 References 25
2.0 Nano-Enabled Remediation Technologies 30
2.1 Introduction 30
2.2 Zero-valent Iron Nanoparticles 32
2.2.1 Background 32
2.2.2 Description 32
2.2.3 Summary of Environmental Potential 37
2.2.4 References 37
2.3 Self-Assembled Monolayers on Mesoporous Supports 39
2.3.1 Description 39
2.3.2 Operational Considerations 43
2.3.3 Summary of Environmental Potential 43
2.3.4 References 44
2.4 Membranes: Nanofiltration and Affinity 46
2.4.1 Background 46
2.4.2 Nanofiltration Membranes 46
2.4.3 Summary of Environmental Potential 50
2.4.4 Electrospun Fibers 50
2.4.5 Surface Modified Membranes 52
2.4.6 References 55
2.5 Zeolites 58
2.5.1 Introduction 58
2.5.2 Description 58
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2.5.3 Potential Applications 59
2.5.4 Impacts, Hazards, Efficacy, and Limitations 62
2.5.5 Management of Zeolite Wastes 62
2.5.6 Summary of Environmental Potential 64
2.5.7 References 65
2.6 Other Nanoparticles 69
2.6.1 Nanodiamonds 69
2.6.2 Dendrimers 72
2.6.3 Argonne Supergel 74
2.6.4 Summary of Environmental Potential 75
2.6.5 References 75
2.7 Uranium Reduction by Bacteria 77
2.7.1 References 83
2.8 Carbon Nanotubes (Fullerenes) 85
2.8.1 Background 85
2.8.2 Description 85
2.8.3 Operational Considerations 89
2.8.4 Summary of Environmental Potential 89
2.8.5 References 90
3.0 Nano-Enabled Sensor Technologies 94
3.1 Introduction 94
3.1.1 Basics 94
3.1.2 Chemical Sensors 95
3.1.3 The Trend towards Miniaturization 96
3.1.4 Opportunities 97
3.1.5 References 98
3.2 "Lab-on-a-Chip" 99
3.2.1 Summary of Environmental Potential 102
3.2.2 References 102
3.3 Microcantilever Sensors 103
3.3.1 Summary of Environmental Potential 107
3.3.2 References 107
3.4 Spectroscopic Sensors 109
3.4.1 Probe Encapsulated by Biologically Localized Embedding Ill
3.4.2 Surface Plasmon Resonance 112
3.4.3 Summary of Environmental Potential 114
3.4.4 References 114
3.5 Nanowire Sensors 117
3.5.1 Background 117
3.5.2 Description 117
3.5.3 Summary of Environmental Potential 120
3.5.4 References 120
3.6 Nanobelts and Nanorods 122
3.6.1 Nanobelts 122
3.6.2 Nanorods 123
3.6.3 References 126
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4.0 Observations and Conclusions 128
Appendix A: Acronyms 131
Appendix B: Glossary 133
References for Figures 138
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Executive Summary
The remediation of radionuclides and heavy metals using current technology is generally
a costly and challenging proposition. Though funds for new technology development are
limited, the need for innovative technologies and transformational approaches continues
to be strong. In recent years, nanotechnology has risen to the forefront and the new
properties and enhanced reactivities offered by nanomaterials may offer a new, low-cost
paradigm to solving complex environmental and engineering problems. Many U.S.
federal agencies, including the U.S. Environmental Protection Agency (EPA), are
strongly supporting a wide range of nanotechnology research. Similarly, other countries
are also promoting research in this new field.
Environmental technologies frequently emerge as an offshoot of other technological
developments or scientific advances. They can also be further enabled by changes in
regulatory approach or stakeholder acceptance. Technologies such as real-time
contaminant measurements within the EPA Triad framework, bioremediation, permeable
reactive barriers, in-situ chemical oxidation, and enhanced attenuation have all been
added to the toolbox of environmental technologies over the past decade. During roughly
the same period, nanotechnology has evolved from an interesting (albeit arcane) area of
manufacturing science, to being heralded as a paradigm-shifting technological revolution
in the mold comparable to that of information technology or biotechnology. Coupled with
this broad evolution, there has been a considerable amount of interest in
nanotechnologies for a wide variety of applications in environmental remediation and
waste reduction. For example, zero-valent iron (ZVI) nanoparticles for the remediation of
chlorinated organics have effectively become a commercially-available technology. A
number of other technologies, such as nano-scale photocatalysts and improved
nanofiltration membranes, are also nearing commercialization.
Nanotechnologies applied to the remediation of radionuclides have been a slower area to
develop; this may well have been predictable since experience shows that the pathway for
development of radionuclide remediation technologies is generally burdened with
difficulties. However, it is important to realize that—although our conceptual awareness
of nanotechnology is relatively new—nanotechnologies have in fact been used safely and
effectively in the management of radioactive waste almost from the start of the nuclear
age; zeolites, now recognized as a nanotechnology due to their nanostructure, have been
used as ion exchangers to remove radioactive components from aqueous waste solutions
for half a century.
In this report, the current, early stages of development of nanotechnology applications for
remediation of radionuclides and heavy metals are divided into two areas: new
remediation methods and advancements in sensors. Though developments in these fields
still remain in their early stages, this report describes some of the more promising
remediation nanotechnologies and new sensors, and attempts to extrapolate general
developments in nanotechnologies to advances in radionuclide remediation and
monitoring. Some emerging examples already exist in both radionuclide remediation and
monitoring. For example, the "Lab-on-a-Chip" (LOG) device for analyzing solutions
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containing radionuclides and heavy metals is a promising development. Though a
recently-emerged technology, the LOG device is being actively investigated in the United
States and overseas for modification to radionuclide sensing, is advancing quickly, and is
already being applied to radionuclides and heavy metals. Examples of other new sensor
advances are reviewed and presented in this report.
There is considerable potential for nanotechnology to assist with the remediation of
radionuclide-contaminated sites. For example, ZVI nanoparticles, a reducing agent
already implemented for the remediation of chlorinated organics in groundwater, are an
excellent prospect for use as a reducing agent to precipitate uranium from contaminated
groundwater. ZVI nanoparticles are commercially available, and field tests for
remediation of heavy metals such as chromium (IV) have shown promise. Two
advantages of ZVI are the greater aerial distribution in saturated porous media and a high
degree of reactivity due to the greater surface area of these very fine particles. Another
example is the Self Assembled Monolayers on Mesoporous Supports (SAMMS)
technology, originally developed by the Department of Energy (DOE) primarily for
mercury but also with radionuclides in mind, which has recently had its first field
implementation.
There are strong scientific basis and solid research demonstrating that nanotechnologies
will make significant advances over a wide range of technological fields. Government
agencies around the world and the commercial sector now invest billions of dollars into
nanoscience and technology research and development. However, the amount of funding
environmental nanoscience research and development receives represents only a fraction
of this massive overall effort. Hence, environmental nanoscience has lagged significantly
behind other application and development areas in the nanoscience field. Because of
growing national and international interest in the environment, it is expected that
environmental nanoscience will grow.
This report presents a general introduction to nanotechnology, providing a broad context
to its narrower focus on nanotechnology for environmental remediation and waste
management. The introduction also addresses some risk considerations involving
nanotechnology, and discusses nanotechnology as applied to both environmental
remediation (in general) and the environmental remediation of radionuclides (in
particular). The body of the document consists of two main sections, one surveying
nanotechnologies for remediation, the other surveying nanotechnologies for sensors.
These two sections are divided into chapters which describe a nanotechnology or group
of related nanotechnologies, and include a brief summary of the environmental potential
of each.
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1.0 Introduction
1.1 Definition of Nanotechnology
Nanotechnology is the art and science of manipulating matter at the atomic or molecular
scale and holds the promise of providing significant improvements in technologies for
protecting the environment. While many definitions for nanotechnology exist, the U.S.
Environmental Protection Agency (EPA) uses the definition developed by the National
Nanotechnology Initiative (NNI), a U.S. Government research and development (R&D)
program established to coordinate multi-agency efforts in nanoscale science, engineering,
and technology. The NNI is comprised of 26 federal agencies, 13 of which have R&D
budgets in nanotechnology. The NNI (NNI 2007) requires nanotechnology to involve all
of the following:
1. Research and technology development at the atomic, molecular, or
macromolecular levels, in the length scale of approximately 1-100 nanometer
(nm) range in any direction;
2. Creating and using structures, devices, and systems that have novel properties and
functions as a result of their small and/or intermediate size; and
3. Ability to control or manipulate on the atomic scale.
Nanotechnology is thus the technology of the extremely small; one nm is defined as one
billionth of a meter. In comparison, 1 nm is one fifty-thousandth of the diameter of a
human hair, or, if a nanometer was scaled to the diameter of a child's marble, then a
meter would have to be scaled to the diameter of the Earth. Nanotechnology is often
regarded as being a product of the latter part of the twentieth century, a product of the
drive towards miniaturization led by the semiconductor industry. However, in a broader
sense, nanotechnology has been around, albeit unrealized as such, for a long time. Two
thousand years ago the ancient Greeks used a permanent hair-dying recipe that worked by
depositing 5 nm lead sulfide crystals inside hair. High-quality steel made in India before
the turn of the first millennium has been shown to contain—and owe its outstanding
properties to—carbide structures similar to modern carbon nanotubes. Medieval artists
colored stained glass using metal nanoparticles. The difference between these ancient
examples of "nanotechnology" and the current situation is the ability to understand—or at
least embark on a path towards understanding—the fundamental principles underlying
nanotechnological behavior, the ability to assess the current state of knowledge, and the
ability to systematically plan for the future based on that knowledge.
1.2 Purpose of This Report
The purpose of this report is to assist EPA in its exploration of the potential for using
nano-enabled technologies in the cleanup of radioactive contamination, and in decisions
to assist with the development of viable technologies in this area. For the purposes of this
report, "nano-enabled technologies" refers to technologies that are enabled by a nano
sub-system. This report will be used to identify and evaluate emerging applications and
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implications (both health and ecological) of nano-enabled technologies for the
remediation of sites contaminated with radionuclides.
1.3 Background of Nanotechnology
The history of nanotechnology is generally understood to have begun in December 1959
when physicist Richard Feynman gave a speech, "There's Plenty of Room at the Bottom"
(Feynman 1959), at an American Physical Society meeting at the California Institute of
Technology in which he identified the potential of nanotechnology. Feynman said it
should be possible to build machines small enough to manufacture objects with atomic
precision, and that if information could be written on an atomic scale, "all of the
information that man has carefully accumulated in all the books in the world can be
written ... in a cube of material one two-hundredths of an inch wide—about the size of
the smallest piece of dust visible to the human eye." He claimed that there were no
physical laws preventing such achievements, while noting that physical properties would
change in importance (e.g., gravity becoming less important), though surface phenomena
would begin to dominate behavior.
In 1974, Norio Taniguchi first used the word "nanotechnology" (Taniguchi 1974), in
regard to an ion sputter machine, to refer to "production technology to get the extra-high
accuracy and ultra-fine dimensions, i.e. the preciseness and fineness on the order of one
nanometer." In the 1980s, Eric Drexler authored the landmark book on nanotechnology,
"Engines of Creation" (Drexler 1986), in which the concept of molecular manufacturing
was introduced to the public at large. It is due to Drexler that much of the public's
imagination has been captured by the potential of nanotechnology and
nanomanufacturing. In 1985, fullerenes, or "buckyballs," were discovered (Kroto et al.
1985). By the 1990s, nanotechnology was advancing rapidly. In 1990, the first academic
nanotechnology journal was published, in 1993 the first Feynman Prize was awarded, and
by 2000 President Bill Clinton announced the U.S. National Nanotechnology Initiative
(NNI). NNI and other nanotechnology proponents now anticipate the development of
nano-enabled tools to help address many current challenges facing the United States and
the international community, including:
• Clean, secure, affordable energy;
• Stronger, lighter, more durable materials;
• Low-cost filters to provide clean drinking water;
• Medical devices and drugs to detect and treat diseases more effectively with fewer
side effects;
• Lighting that uses a fraction of the energy associated with conventional systems;
• Sensors to detect and identify harmful chemical and biological agents; and
• Techniques to clean up harmful chemicals in the environment.
This document focuses on the last of these challenges—techniques to clean up harmful
chemicals in the environmental. Nanotechnologies offer new and previously
unanticipated possibilities due to new properties and behaviors that occur at the nano
scale, which can be harnessed in a structured and planned manner. The growing ability to
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design and tailor technologies to specific ends indicates that problems that were once
regarded as impossible to solve can now be addressed in a rational manner. The size of
nanoparticles allows nanoscale behavior to be introduced into areas inaccessible to
conventional technologies. When material exists in the nanoparticulate form, it exhibits
unusual behavior that has made it the subject of great interest. This unusual behavior is
the result of two phenomena. First, the surface area-to-volume ratio of nanoparticles is
much greater than that of larger particles. Nanoparticles in the 10 nm range can have 50%
of their molecular structure exposed to the surface, a percentage that is millions of times
greater than that of typical powdered materials. Since chemical reactivity and catalytic
ability is directly related to surface area, chemical properties are greatly enhanced.
Second, at the nanoparticle scale, quantum behavior that is typically masked in larger
particles can be readily displayed.
1.4 Application of Nanotechnology
Descriptions of nanotechnology that characterize it purely in terms of the minute size of
the physical features with which it is concerned—assemblies between the size of an atom
and about 100 molecular diameters—make it sound as though nanotechnology is merely
using infinitely smaller parts than conventional engineering. However, working matters
are truly more complex; rearranging the atoms and molecules leads to new properties and
unusual behaviors. A transition is apparent between the fixed behavior of individual
atoms and molecules and the adjustable behavior of collectives. Many scientists are now
investigating the fundamental nature of nanotechnology in a wide spectrum of academic
fields—from the basic sciences to engineering. Much of known science (e.g., colloid
science, electronics, chemistry, physics, and genetics) will be applicable, but augmented
with exciting new breakthroughs.
The potential applications of nanotechnology range across a broad scale. For example, in
medical systems, it could be possible to improve the tissue compatibility of implants to
create scaffolds for tissue regeneration, or perhaps even to build artificial organs. Further,
new types of genetic therapies and anti-aging treatments could be possible.
Nanotechnology is currently used by leading businesses and industrial research
companies for a variety of technical and innovative applications. Examples include:
• ExxonMobil is using zeolites, minerals with pore sizes of less than 1 nm, as a
more efficient catalyst to break down or crack large hydrocarbon molecules to
form gasoline.
• IBM has added nanoscale layering to disk drives, thus exploiting the giant
magnetoresistive effect to attain highly dense data storage.
• Gilead Sciences is using nanotechnology in the form of lipid spheres, also known
as liposomes, which measure about 100 nm in diameter, to encase an anticancer
drug to treat the AIDS-related Kaposi's sarcoma.
• Carbon Nanotechnologies, a company co-founded by buckyball discoverer
Richard E. Smalley, is making carbon nanotubes more affordable by using a new
and more efficient manufacturing process.
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• Nanophase Technologies is utilizing nanocrystalline particles, incorporated into
other materials, to produce tough ceramics, transparent sun blocks, and catalysts
for environmental uses, among other applications.
Though vastly different in the outputs they produce, these companies all use
nanotechnology to develop more efficient, affordable, and, most recently,
environmentally-safe products.
1.5 Risk Associated with Nanoparticles in the Environment
As previously noted, the potential widespread application of nanomaterials in
environmental remediation is made possible by the miniaturization of materials down to
the nano-scale. However, this same enabling characteristic also influences risk by
changing the particles' potential for mobility, exposure, absorption, reactivity, and
toxicity. When a nanomaterial is used for environmental remediation, it is intentionally
introduced into the environment to exploit its unique properties. For example, nano-sized
colloidal iron nanoparticles can act as catalysts in redox reactions. Of particular concern
is the potential mobility of nanoparticles out of targeted sites or tissues, or whether
intentional or unintentional releases of highly mobile nano-particles into the environment
could be controlled (CRS 2008).
Nevertheless, nanomaterials can have side effects, and a risk assessment requires
knowledge of their distribution in the environment and food chain. Risk assessment is
required for understanding the nanoparticles' behavior to evaluate potential risks
associated with nanomaterial use for remediation. Side effects associated with the use of
nanotechnology, especially environmental risks associated with residual nanomaterials'
fate and transport in the environment, are not yet fully explored and understood.
Uncertainties of the nature and interaction of nanomaterials in the following areas add to
the complexity of risk concerns. These include: uncertainty in relationship between size,
surface area, and surface reactivity; and uncertainty in relationship of radionuclides and
nanomaterials. A clear understanding of the relationship between these parameters is still
evolving and is not yet clearly understood. Additionally, the relation between
radionuclides and nanomaterials is not yet determined.
Risk assessment is practiced by EPA and other federal agencies as a tool to evaluate risks
associated with chemicals and radionuclides in the environment. Risk assessment
approaches and procedures have been formulated by the National Academy of Sciences,
and have been subsequently tailored to specific applications by EPA and other federal
agencies. EPA risk assessment has four components: hazard identification, toxicity
assessment, exposure assessment, and risk characterization. Although the risk assessment
paradigm has been used successfully by the scientific community since the early 1980s,
its application to nanotechnology requires incorporating an uncertainty in basic
knowledge that is very large when compared to the uncertainty for other materials and
Pharmaceuticals. To do so requires an understanding of product life-cycle and the ability
to communicate effectively with personnel, stakeholders, and regulators.
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Although risk assessment/management of conventional chemicals is well established,
nanotechnology risk assessment is an emerging field; there is a growing body of
scientific literature dealing with potential associated risks, including:
• Effects of nano-particle introduction on soil chemistry /fertility /texture;
• Fate and transport (e.g., zero-valent iron (ZVI) or ZVI-uranium-contaminated
nano-particles);
• Exposure pathways and assessment (including environmental and occupational
exposures, both chronic and acute);
• Dose-response relationship and toxicity (including effects on organs such as
lungs, gills, liver, kidneys, and immune systems);
• Bioaccumulation and biomagnification (e.g., food chain); and
• Effects on humans as well as effects on ecosystems, ranging from impacts at
molecular (nano) scale through the microbial (microorganisms) and the meso
(small animals) to the macro (large animal communities).
The following sections provide a brief overview of: nanomaterial properties potentially
associated with risk (1.5.1), possible approaches to nanotechnology risk assessment
(1.5.2), data gaps and limitations for assessing nanomaterial risk (1.5.3), and an overview
of the proposed frameworks for risk management (1.5.4).
1.5.1 Nanomaterial Properties Associated with Risk
Characteristics of nanomaterials potentially leading to increased toxicity, risks and
associated modifying factors have been discussed in recent literature (Biswas and Wu
2005, Borm and Muller-Schulte 2006, Borm et al. 2006, Gwinn and Vallyathan 2006,
Kreyling et al. 2006, Medina et al. 2007, Nel et al. 2006, Oberdorster et al. 2007, Thomas
and Sayre 2005). In general, the following properties of nanomaterials are discussed as
potential risk drivers:
• Chemical composition is one of the key factors discussed. Nanomaterials may be
derived from bulk materials with known toxic properties. Moreover, in many
cases a given nanomaterial can be produced by different processes yielding
several derivatives of the same material. It is important to note that the chemical
properties of particles at the nanometer size can differ significantly from the
chemical properties of larger particles consisting of the same chemical
composition.
• Toxicity potential can be caused by either chemical toxicity based on chemical
composition or stress or stimuli caused by the surface, size, and/or shape of the
particle. Differentiation between these two toxic effects is not straightforward
(Brunner et al. 2006), but the size of the particle may indirectly dominate the
uptake of particles into cells (Limbach et al. 2005, Kreyling et al. 2006).
• Surface reactivity can increase the harm caused by nanomaterial in a cell and
reduce the potential for environmental degradation (Limbach et al. 2007).
Materials with active surfaces are deemed more harmful than the materials that lack
such surfaces. It should be noted that smaller particle size also means higher
surface reactivity because more of the atoms are in the surface of the particle.
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• Solubility and environmental mobility have been proven to greatly affect the risk
associated with nanomaterials (Brunner et al. 2006). Higher mobility results in
higher risk, and insoluble materials or materials with nanoparticles embedded in
a bulk material matrix may be less bioavailable.
• Agglomeration affects the toxicity as well. Particles that naturally agglomerate into
larger units can be interpreted as less toxic. Three agglomeration classes (low,
medium, and high) have been defined. In these, highest agglomeration
corresponds to lowest risk.
1.5.2 Possible Approaches to Nanotechnology Risk Assessment
Though multiple frameworks for nanomaterials risk assessment and risk management
have been proposed, none has been formally adopted as regulatory tool (Linkov et al.
2008). Two examples of the representative risk assessment frameworks are presented in
this section, while multiple risk assessment and risk management frameworks are
summarized in Section 1.5.5.
NanoRisk Framework
The NanoRisk Framework (www.NanoRiskFramework.com) was developed by DuPont
Corporation and the Environmental Defense Fund to address the environmental, health,
and safety risks of nanomaterials across all stages of product life cycle. It recommends
six distinct steps:
• Step 1. Describe Material and Application. Develop a general description of the
nanomaterial and its intended uses, based on information in the possession of the
developer or in the literature. The user also identifies analogous materials and
applications that may help fill data gaps in this and other steps.
• Step 2. Profile Lifecycle(s). The second step defines a process to develop three
sets of profiles—of the nanomaterial's properties, its inherent hazards, and its
associated exposures throughout the material's lifecycle. The properties profile
identifies and characterizes a nanomaterial's physical and chemical properties.
The hazard profile identifies and characterizes the nanomaterial's potential safety,
health, and environmental hazards.
• Step 3. Evaluate Risks. In this step, all the information generated in the profiles is
reviewed in order to identify and characterize the nature, magnitude, and
probability of risks presented by this particular nanomaterial and its anticipated
application. In so doing, the user considers gaps in the lifecycle profiles,
prioritizes those gaps, and determines how to address them—either by generating
data or by using, in place of such data," reasonable worst case" assumptions or
values.
• Step 4. Assess Risk Management. Here the user evaluates the available options
for managing the risks identified in Step 3 and recommends a course of action.
Options include engineering controls, protective equipment, risk communication,
and product or process modifications.
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• Step 5. Decide, Document and Act. In this step, appropriate to the product's
stage of development, the user consults with the appropriate review team and
decides whether or in what capacity to continue development and production.
Consistent with a transparent decision-making process, the user documents those
decisions and their rationale and shares appropriate information with the relevant
stakeholders, both internal and external.
• Step 6. Review and Adapt. Through regularly scheduled reviews as well as
triggered reviews, the user updates and re-executes the risk evaluation, ensures
that risk-management systems are working as expected, and adapts those systems
in the face of new information (e.g., regarding hazard data) or new conditions
(such as new or altered exposure patterns).
Through these six steps, the framework seeks to guide a process for risk evaluation and
management that is practical, comprehensive, transparent, and flexible.
Comprehensive Environmental Assessment Approach (Davis 2007):
The Comprehensive Environmental Assessment (CEA) approach combines life-cycle
perspective with the risk assessment paradigm. This systematic approach could guide
research strategy for assessing the risks of nanotechnology and avoid unintended
consequences (Davis 2007). The CEA approach begins with assessing product life cycle
stages including material production or extraction, manufacturing processes, distribution,
storage, use, and disposal (including recycling). At any given stage of the life cycle,
nanoscale substances and/or associated materials (e.g., manufacturing by-products) might
enter one or more of the environmental media—air, water, soil—and affect humans and
ecological receptors through multiple pathways, including food web. It is important to
identify nanomaterials in their primary form and, further, to consider the transport and
transformation processes they may undergo during the product life cycle.
To evaluate exposure in a comprehensive manner, it is important to consider, among
other things, both aggregate exposure across routes (inhalation, ingestion, and dermal
absorption) and cumulative exposure to multiple (primary and secondary) pollutants. The
CEA approach tends to focus on the range of exposure scenarios, including micro-
environmental and high-end exposures, not just "typical" or "average" exposure levels.
The human health and ecological hazards associated with respective contaminants can be
described qualitatively and, if possible, quantitatively.
±5.3 Data Gaps and Limitations for Nanomaterials Risk Assessment
Though the use of nanomaterials is increasing, understanding the environmental fate and
transport, toxicity, and potential human health and ecological risks associated with
nanomaterial use remains extremely limited. This is due to a variety of barriers, including
proprietary nature of information; and lack of standardization in nomenclature, metrics,
and materials (CRS 2008).
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Risk assessment requires information related to the following four components: hazard
identification, toxicity assessment, exposure assessment, and risk characterization. For
human health and environmental risk assessment of nanomaterials, the current knowledge
base contains data gaps. The literature has established these knowns (things we know that
we know) and unknowns (things we know that we don't know) in the nanotechnology
context for the four components (Davis 2007). Some of the data gaps for each of the risk
assessment components are described below.
• Predicting the environmental fate of nanomaterials: At present there are few
available studies on the environmental fate of nanomaterials but they are not
adequate to predict the fate of nanomaterials based on their fundamental
properties. Nanomaterials released to soil and ground water as part of remedial
activities can be strongly sorbed into soil due to their high surface areas, and
therefore become immobile. On the other hand, nanomaterials are small enough to
fit into the smaller spaces between soil particles, and therefore might travel farther
than larger particles before becoming trapped in the soil matrix. In addition to the
nanomaterial properties discussed above, types and properties of the soil and
water can affect nanomaterial mobility. Since nanoparticles are likely to be
transported as colloids, the mobility of mineral colloids in soils and sediments and
the availability of humic substances are important considerations. Additionally, in
the presence of various soils having different pH, and radionuclides,
nanomaterials may form different byproducts. The fate of nanomaterials in
aqueous environments is controlled by solubility or dispersability; interactions
between the nanomaterial and natural and anthropogenic chemicals in the system;
and biological and abiotic processes. Light-induced photoreactions are often
important in determining the environmental fate of chemical substances.
• Nanomaterial toxicity and bioaccumulation potential: Past literature provides some
evidence of toxicity and bioaccumulation potential, but the results are generally
inconclusive. Laboratory testing of nanomaterial uptake has shown a wide range of
potential affects on tested organisms, including bioaccumulation potential,
mortality, and biomarker-response. Nevertheless, the concentration of nanoparticles
used in this and other studies may far exceed concentrations that could potentially
result from those that would be associated with site remediation.
• Exposure estimation: When a nanomaterial is used for an environmental
application, exposure estimation might not be straightforward. Multiple variables
could influence nanomaterial exposure assessment, including the characterization
of the effectiveness of variations in biological reactivity, size, shape, and charge,
as well as factors that complicate the straightforward estimation of exposure such
as metabolism, excretion, and adduction to biological molecules. For example,
several studies on carbon nanotubes have shown that the toxicity and distribution
of nanoparticles is dependent upon the presence of functional groups, impurities,
fiber length, and aggregation status.
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• Risk Characterization: Given estimates of exposure and toxicity, the final step
involved in estimating the hazard of contaminant exposure is risk characterization
(i.e., the likelihood of adverse environmental impact at varying degrees of
exposure). Risk characterization must be developed separately for each
nanomaterial, or even for the same nanomaterials with different functions or at
different environmental life-cycle stages. Given the required effort, detailed risk
characterization may not be possible. Decision tools and databases could be
developed to allow the use of all available information, as well as proxy data for
making the best judgment on risk characterization.
±5.4 Approaches to Managing Nanotechnology Risk
As a field of science, nanotechnology is still young and uncertainty in basic knowledge
does not lend itself to fully informed and accurate risk assessments, but scientific
evidence does exist that suggest some nanoparticles may be hazardous. Some regulatory
and industry representatives caution that inadequate government oversight, or even a
perceived lack of oversight, could lead to consumer rejection of an entire range of
products incorporating nanotechnology. Such a reaction would significantly hinder
development and industry growth. To prevent a loss of consumer confidence, academic
researchers, policy analysts, and some nanotechnology entrepreneurs have been working
with federal agencies that are responsible for protecting the environment, workers, and
consumers (CRS 2008).
Assessment and management of the potential risks associated with nanotechnology is a
challenge with which U.S. agencies and countries in the European Union (EU) are
beginning to come to terms. Despite all the limitations, some progress in approaching
risk-based nanotechnology regulation is being made. In the EU for example, the
European Commission (EC) has adopted the opinion of the Scientific Committee on
Emerging and Newly Identified Health Risks (SCENIHR) on definitions for
nanotechnology. This advance takes one of the first steps towards risk assessment and
regulation: standardizing nomenclature, metrics, and materials. In addition, the EU is
currently conducting research to compare nanotechnology regulatory policies in the EU
with those in the United States. Similar to previous efforts to reach coordination on
regulation (including beef, chemicals, and genetically modified organism regulations),
those involved are hopeful that this research will improve transatlantic regulatory
cooperation of nanotechnology in the future (NanoReg News 2008).
A review of current nanomaterial risk management frameworks and related documents is
summarized in Table 1. Details of these can be found in the references provided.
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Table 1: Current nanomaterial risk frameworks 1
Agency
Focus
Citation
U.S. Environmental
Protection Agency
(EPA) White Paper
Comprehensive framework intended to
set forth current scientific knowledge
and its gaps related to possible
environmental benefits of
nanotechnology, as well as potential
risks from environmental exposure to
nanomaterials.
(US EPA 2007)
Federal Drug
Administration
(FDA)
Report intended to help assess questions
regarding the adequacy and application
of the FDA's regulatory authority to
nanomaterials, and to provide findings
and recommendations to the FDA
Commissioner.
(US FDA 2007)
Woodrow Wilson
Center
Paper intended to describe the
possibilities for government action to
deal with the adverse effects of
nanotechnology, and to provide
evidence relevant for determining what
needs to be done to manage
nanotechnology.
(Davies 2006)
Environmental
Defense (EDF)-
DuPont
Comprehensive framework for the
responsible development, production,
use, and end-of-life disposal of
nanomaterials, intended for use by
companies and other organizations.
(EDF-DuPont 2007)
Quebec Commission
Comprehensive discussion of the
scientific, legal, and ethical implications
of nanotechnology, intended to help
uphold the protection of health and the
environment, as well as respect for
many values such as dignity, liberty,
integrity, justice, transparency, and
democracy.
(QC 2006)
Royal Society
Comprehensive framework intended to
summarize current scientific knowledge
and applications of nanotechnology, and
(RS & RAE 2004)
1 Linkov, I., Satterstrom, K., (2008). "Nanomaterial Risk Assessment and Risk
Management: Review of Regulatory Frameworks." In: Linkov, I, Ferguson, E.,
Magar, V. (in press). "Real Time and Deliberative Decision Making: Application to
Risk Assessment for Non-chemical Stressors. Springer, Amsterdam.
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Agency
Focus
Citation
to identify possible health and safety,
environmental, ethical, and societal
implications or uncertainties.
Department for
Environment, Food,
and Rural Affairs
(DEFRA)
Trial Voluntary Reporting Scheme to
collect data from organizations in the
nanotechnology industry to help the
United Kingdom develop appropriate
controls for risks to the environment and
human health from nanomaterials.
(UK DEFRA 2006)
Responsible
NanoCode
Paper intended to highlight key issues
that emerged from a business workshop
on nanotechnology, including
development of a responsible
nanotechnology code.
(RNC 2006)
European
Commission
Scientific
Commission on
Emerging and Newly
Identified Health
Risks (SCENIHR)
Technical document intended to assess
the appropriateness of current risk
assessment methodologies for the risk
assessment of nanomaterials, and to
provide suggestions for improvements
to the methodologies.
(EC SCENIHR
2007)
European
Commission Action
Plan
Plan intended to help Europe build on
its strengths and advances to ensure that
nanotechnology research is carried out
with maximum impact and
responsibility, and that the resulting
knowledge is applied in products that
are useful, safe, and profitable.
(EC 2005)
International Risk
Governance Council
(IRGC) Policy Brief
Brief intended to assist policy makers in
developing the processes and
regulations to enable the development
and public acceptance of
nanotechnology.
(IRGC 2007)
IRGC White Paper 1
Comprehensive framework intended to
advance the development of an
integrated, holistic, and structured
approach for the investigation of risk
issues and the governance processes and
structures pertaining to them.
(IRGC 2005)
IRGC White Paper 2
Comprehensive framework which
applies general IRGC risk governance
framework to the field of
nanotechnology.
(IRGC 2006)
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In response to public concern for the protection of the environment and human health
from the potential hazards of nanomaterials, EPA has released a nanotechnology research
strategy to protect the environment (EPA 2008). In addition, EPA has also recently
completed the Basic Program phase of the EPA's Nanoscale Materials Stewardship
Program (NMSP), where some organizations are showing their support for a successful
beginning of a voluntary information collection program (NanoReg News 2008).
Currently, the main challenge facing EPA is minimizing nanotechnology's potential for
unintended, harmful consequences, while pursuing the positive aspects of nanomaterial
use, including potential of environmental remediation applications. In the Office of
Research and Development's (ORD) recently released Draft Nanomaterial Strategy (EPA
2008), EPA both builds on and is consistent with the NNI's previous reports, as well as its
own Nanotechnology White Paper (EPA 2007). In Draft Nanomaterial Strategy, EPA
gives special attention to its mission to protect the environment and its specific needs for
being able to regulate nanotechnology (NanoReg News 2008). It also identifies four
major areas for future research:
• environmental fate and transport,
• human health implications,
• risk assessment methodology, and
• risk mitigation strategies.
Initially, EPA's research will focus on nanoscale titanium dioxide, ZVI, nanosilver,
carbon nanotubes, and cerium oxide, with the intention that the resulting body of
knowledge can be extrapolated for classes of nanomaterials in the future, particularly as
nomenclature, metrics, and materials become standardized. Anticipated outcomes from
this research program are expected to be focused research products that address risk
assessment and risk management needs for nanomaterials in support of the various
environmental statutes for which EPA is responsible (NanoReg News 2008).
1.6 Nanotechnology for Environmental Remediation
±6.1 Current Applications
Recent advances in the design, production, and fundamental understanding of
nanomaterials have led to preliminary investigations of their use in the remediation of
chlorinated organics in the subsurface, and to a wide range of suggestions for their
potential use in the remediation of other environmental contaminants, including those at
radiologically-contaminated sites. The potential for innovative use is directly related to
the enhanced chemical activity resulting from the increased surface area and the
manifestation of quantum effects, yet it also takes advantage of a third feature of
nanoparticles, namely their ability to be transported to areas inaccessible to other
remediation approaches. For example, if we consider bioremediation, part of the promise
lies in the fact that microbes capable of destroying organic contaminants can directly
access subsurface pore structure that is too small for regular particles to access. However,
there is still much of the pore structure in the subsurface that is too small even for
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microbes. Nanoparticles are typically two orders of magnitude smaller than bacteria and
can thus be transported to an occluded contaminant that is hidden from direct bacterial
contact. Current applications, discussed in more detail in Sections 2 and 3 of this
document, include zeolites and nanoparticulate ZVI.
1.6.2 Awareness of Potential Environmental Benefits
Since nanotechnology and nanomaterials are comparatively new technologies, only
partial knowledge of the environmental fate, transport, effects, and risks associated with
them is available, and some of this information appears to be contradictory. Ultimately,
this knowledge will be crucial in deciding whether a technology involving nanomaterials
is an appropriate remedy. Concerns in this regard have been so strong that a report by the
Royal Society (RS) and the Royal Academy of Engineering (RAE) in the United
Kingdom has recommended "that the use of free (that is, not fixed in a matrix)
manufactured nanoparticles in environmental applications, such as remediation, should
be prohibited until appropriate research has been undertaken and it can be demonstrated
that the potential benefits outweigh the potential risks (RS/RAE 2004)." Thus, it is
important to conduct a comprehensive review and evaluation of both the effectiveness
and the potential impacts of available treatment options.
Within all of the discussion about nanotechnology opportunities, consideration of the
application of nanotechnology to environmental problems seems to have been
subordinated to consideration of risks of nanotechnology. Though the potential has been
recognized, often it is the risks posed to human health and the environment that have
dominated discussion of nanotechnology and the environment. The term "disruptive
technology" was introduced in 2003 (Uldrich and Newberry 2003) to describe a new
technology that is significantly cheaper (or performs better) than a current technology,
and will revolutionize worldwide markets by superseding the existing technology. The
industries on which nanotechnology will likely have a disruptive effect were analyzed
and at the high-end (i.e., largest impact) included:
• Healthcare
• Long-term care
• Electronics
• Telecom
• Packaging
• United States Chemical
• Plastics
• Apparel
• Pharmaceutical
• Semiconductor
At the low-end, the industries included:
• Cosmetics
• Chocolate
• Batteries
• Blue jeans
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• Khakis
• Fluorescent tagging
• Mouthwash
At the time of Uldrich and Newberry 2003, the environmental industry was not classified
on the list.
±6.3 EPA's Involvement in Nanotechnology
Fortunately, nanotechnology in the environmental industry is progressing, gradually
becoming more advanced and better recognized. EPA is obligated to protect human
health and safeguard the environment by better understanding and addressing potential
risks from exposure to nanoscale materials, but it is also interested in researching and
developing the possible benefits of nanotechnology. Since 2001, EPA has played a
leading role in funding research and setting research directions for developing
environmental applications of nanotechnology. Two recent examples demonstrate EPA's
commitment: the "Nanotechnology White Paper" and the "Workshop for
Nanotechnology on Site Remediation."
In February 2007, EPA published the "Nanotechnology White Paper" (EPA 2007) based
on the work of a cross-Agency workgroup created in December 2004 by EPA's Science
Policy Council. It describes key science issues EPA should consider to ensure that society
accrues the important environmental protection benefits that nanotechnology may offer.
The document notes that since 2001, EPA's Science To Achieve Results (STAR) grant
program has funded 36 research grants—totaling nearly $12 million—in the application
of nanotechnology to protect the environment, including the development of: 1) low-cost,
rapid, and simplified methods of removing toxic contaminants from water; 2) new
sensors that are more sensitive for measuring pollutants; 3) green manufacturing
nanomaterials; and 4) more efficient, selective catalysts. Additional projects have been
funded through the Small Business Innovation Research (SBIR) program, and there are
14 recent STAR program projects focused on studying the possible harmful effects, or
implications, of engineered nanomaterials.
This white paper also describes the benefits, risk assessment, and responsible
development of nanotechnology, and outlines the involvement in nanotechnology by
various EPA Offices including:
• Office of Pollution Prevention and Toxics (OPPT) — activities include reviewing
premanufacture notifications for a number of nanoscale materials that have been
received under the Toxics Substances Control Act, initiating a Nanoscale
Materials Stewardship Program to encourage submission and development of
information for nanoscale materials (see
http://www.epa.gov/oppt/nano/stewardship. htm), public outreach, and
international engagement on nanotechnology issues with the Organization of
Economic Cooperation and Development and International Organization for
Standardization (ISO).
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• Office of Air and Radiation/Office of Transportation and Air Quality
(OAR/OTAQ) — activities include reviewing an application for registration of a
diesel additive containing cerium oxide. Cerium oxide nanoparticles are being
marketed in Europe as on- and off-road diesel fuel additives to decrease
emissions, and some manufacturers claim fuel economy benefits.
• Office of Pesticide Programs (OPP) — activities include working with members
of the pesticide industry regarding licensing/registration requirements for
pesticide products that utilize nanotechnology.
• Office of Solid Waste and Emergency Response (OSWER) — activities include
investigating potential implications and applications of nanotechnology by such
means as: an October 2005 workshop on "Nanotechnology for Site Remediation"
organized with EPA's ORD and several other federal agencies; and a July 2006
symposium entitled, "Nanotechnology and OSWER: New Opportunities and
Challenges."
• Office of Enforcement and Compliance Assurance (OECA) — activities include
reviewing Agency information on nanotechnology (e.g., studies, research);
evaluating existing statutory and regulatory frameworks to determine the
enforcement issues associated with nanotechnology; evaluating the science issues
for regulation/enforcement that are associated with nanotechnology, and;
considering what information OECA's National Enforcement Investigations
Center (NEIC) may need to consider to support the Agency.
Importantly, EPA's white paper presents a set of key nanotechnology recommendations,
including:
• Environmental Applications Research. The Agency should continue to
undertake, collaborate on, and support research to better understand and apply
information regarding environmental applications of nanomaterials.
• Risk Assessment Research. To ensure that research best supports Agency
decision-making, EPA should conduct case studies to further identify unique risk
assessment considerations for nanomaterials. The Agency should also continue to
undertake, collaborate on, and support research to better understand and apply
information regarding nanomaterials':
o chemical and physical identification and characterization,
o environmental fate,
o environmental detection and analysis,
o potential releases and human exposures,
o human health effects assessment, and
o ecological effects assessment.
• Pollution Prevention, Stewardship, and Sustainability. The Agency should
engage resources and expertise to encourage, support, and develop approaches
that promote pollution prevention, sustainable resource use, and good product
stewardship in the production, use, and end-of-life management of nanomaterials.
Additionally, the Agency should draw on new, "next generation"
nanotechnologies to identify ways to support environmentally beneficial
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approaches such as green energy, green design, green chemistry, and green
manufacturing.
• Collaboration and Leadership. The Agency should continue and expand its
collaborations regarding nanomaterial applications and potential human health
and environmental implications.
• Intra-Agency Workgroup. The Agency should continue to convene a standing
intra-Agency group to foster information sharing on nanotechnology science and
policy issues.
• Training. The Agency should continue and expand its nanotechnology training
activities for scientists and managers.
Another recent example of EPA's involvement in using nanotechnology for better
environmental remediation is the October 2005 "Workshop on Nanotechnology for Site
Remediation" (EPA 2005). This workshop explored the place of nanotechnology among
existing remedial techniques, considered the overall state of the science, and examined
some case studies (primarily of nanoparticulate ZVI). Workshop participants also
discussed research needs and data gaps. Their findings may be summarized into four
broad categories (i.e., performance; toxicity; basic nanoscience; and fate and transport) as
follows:
Performance
• Better understanding of injection techniques and control of nanoparticle
movement in the subsurface; better ways to overcome other limitations on
material emplacement such as permeability changes over time with injections;
• Development of a set of performance-assisting tools such as validation methods,
modeling, scale-up equations, statistics, and QA/QC approaches;
• Development of performance metrics/standards of the nanoparticles, including
manufacturing, size, shape, storage, and property and reactivity standards for
nanoparticles;
• Development of performance assessment and performance prediction tools for the
contaminants being remediated;
• Development of in-situ sensors, real-time techniques, and analytical methods for
detecting nanoparticles;
• Better understanding of the effects of treatment trains (options and optimization
of combinations) and synergies of multiple contaminants "cocktail treatment";
• Need national test sites covering major geologic settings to permit rigorous field
demonstration; and
• Need for contingency ability to remove nanoparticles from wastewater and
drinking water.
Toxicity
• Recognizing gaps in data and in the understanding of toxicological effects on
humans, flora, and fauna and how low risk toxicity can be measured in the
environment;
• Recognizing lack of information on fate, stability, and potential for transformation
(for example heavy metal sequestration);
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• Understanding the difference between maximizing remedial effects while
minimizing toxicological problems;
• Integrating toxicological behavior into risk-based decision making; and
• Better understanding of macro-scale toxicity properties and whether it can be
extrapolated to nano-scale using knowledge of size, shape, chemistry.
Basic Nanoscience
• Discovering the existence of intrinsic nano-scale processes and mechanisms (such
as surface area versus electronic effects and bulk properties versus surface effect);
• Discovering the difference between extrapolating macro-scale and micro-scale to
nano-scale, or determining if there is there a unique nano-scale physico-chemistry
that cannot be identified at the macro-scale;
• Better understanding needed of agglomeration behavior to control both in-situ
reactivity and transport; and
• Recognizing the need for better information products and information synthesis.
Fate and Transport
• Major risks and uncertainties seen in fractured media and in the transport of
sequestered contaminants;
• Better monitoring and detection techniques are needed for fate and transport as
well as for performance studies; and
• Need much better information on site-to-site variability of fate and transport
rather than assuming that behavior identified at one site can be applied widely;
Investigation of fate and transport of nanoparticles for environmental applications is
important since an understanding in this area is critical to evaluating performance (the
need to know how far and in what form the nanoparticle will travel to perform its
function), toxicity (the risk of the nanoparticle traveling unanticipated distances in very
toxic forms), and cost (directly related to performance). Though the fate of the
nanoparticle will depend heavily on the characteristics and material of the nanoparticle,
recent work has indicated that transport may not be as grave a concern as once thought. It
has been noted that the common assumption that nanoparticles will be highly mobile in
porous media due to their tiny size in comparison with pore spaces, is an
oversimplification (Tratynek 2006). The mobility of nanoparticles in the saturated
subsurface is the product of the number of nanoparticle collisions with the porous
medium per unit transport distance (where collisions arise from Brownian diffusion,
interception and gravitational sedimentation) and the sticking coefficient. Calculations for
a typical subsurface using a range of sticking coefficients indicates that transport will
range from millimeters to a few tens of meters. These calculations seem to have been
borne out in field tests with iron nanoparticles used to remediate chlorinated organics.
In fact, an area of current interest is how to coat particles so that they will not stick and
will travel further to attain their remedial purpose. This indication of limited travel only
applies in porous media and would not be expected to hold in fractured geologic media or
in surface water. As a result, EPA is continuing investigations of fate and transport
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through research2 such as that on the environmental fate of single-walled carbon
nanotubes in the estuarine environment at the University of South Carolina. Similar work
on environmental fate and transport is also being conducted at a number of academic
centers, such as the Center for Biological and Environmental Nanotechnology (CBEN) at
Rice University, which aims to understand and manipulate the wet/dry interface between
nanosystems and biological systems, and the Nanomaterials in the Environment,
Agriculture, and Technology Organized Research Unit (NEAT-ORU) at the University of
California Davis, which emphasizes research on the interaction of materials and the
natural and man-made environment.
±6.4 International Involvement in Nanotechnology
The United States is not alone in exploring the benefits of nanotechnology for
environmental remediation and its implications on human health, welfare, and the
environment. Two recent reports from Europe help paint a picture of current thinking.
A 2007 report from the United Kingdom's Department for Environment, Food and Rural
Affairs (DEFRA) titled "Environmentally Beneficial Nanotechnologies: Barriers and
Opportunities" (Walsh 2007), was commissioned to provide an overview of the areas
where nanotechnology could have a beneficial environmental impact above current
technology and the barriers preventing its adoption. Five nanotechnological applications
were subject to detailed investigation: fuel additives, solar cells, the hydrogen economy,
batteries, and insulation.
A workshop on "Nanotechnologies for Environmental Remediation" (Rickerby and
Morrison 2007) was convened at the Joint Research Center in Ispra, Italy in 2007. This
workshop brought leading scientists together from across the EU to present their latest
work in environmental remediation and discuss issues including:
• The most effective nanotechnologies for pollution prevention or cleanup;
• The dependability and proximity to market of remediation techniques based on
nanotechnologies;
• The additional research needed to exploit the full potential of nanotechnology for
remediation;
• The most promising environmental nanotechnologies and those that should be
further explored;
• The potential risks of using nanotechnologies in remediation applications.
• Whether there is a need for targeted funding for infrastructure to support
nanotechnologies for environmental remediation; and
• The opportunities for setting up EU collaborative projects on environmental
nanotechnologies.
The issues of environmental remediation were considered to be quite substantial and
poorly addressed by conventional technologies. They include access to clean drinking
2 ORD STAR
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water; removal of airborne pollutants; and the cleanup of industrially-contaminated sites
(in particular ex-military sites). Although there have been a number of laboratory
demonstrations of nanotechnology applications for environmental cleanup, there remains
the issue of translating these into industrial-level processes. The workshop consisted of
two principal sessions: "Water Treatment and Purification," and "Air, Water and Soil
Decontamination." Participants discussed the following important issues:
• Photocatalytic treatment of water for degradation of pollutants and the destruction
of microorganisms;
• Titanium dioxide (TiO2) nanopowder immobilization on substrates to produce a
photogalvanic system with a photoanode for photodegradation of organic
pollutants;
• Photocatalytic inactivation of bacterial spores and photocatalytic disinfection of
water;
• Membrane nanofiltration for the treatment of process and wastewater;
• Colloidal and interface chemistry;
• Development of a system utilizing iron-precipitating bacteria to co-precipitate;
• Organic and inorganic pollutants, such as arsenic;
• Sol-gel synthesis of nanosized TiC>2 particles on polypropylene fibers for the
photocatalytic degradation of organic pollutants;
• Ultra nanocrystalline diamond used as an electrode for water treatment and
disinfection;
• Feasibility of a Lab-on-a-Chip (LOG) device for analyzing highly radioactive
solutions;
• Development of catalytic trap technology and advanced systems for particulate
matter;
• Incorporating TiC>2 in building materials or surface coatings to impart self-
cleaning and de-polluting properties; and
• A double skin sheet reactor (DSSR) for water purification.
1.7 Nanotechnology for Environmental Remediation of Radionuclides
1.7.1 Technical Status
Discussion in Section 1.6 has illustrated that research of radionuclide remediation is
lacking in the growing interest of environmental nanotechnology. The mention in Section
1.6.4 of investigating the feasibility of an LOG device for analyzing highly radioactive
solutions is an encouraging exception. In spite of this current situation, the potential for
nanotechnology to assist with the remediation of radionuclide-contaminated sites is
considerable. For example, zeolites, nanostructured materials that can also be made as
nanoparticles, already have a long history of use in the treatment of liquid radioactive
waste. ZVI nanoparticles, a reducing agent already implemented for the remediation of
chlorinated organics in groundwater, are an excellent prospect for use as a reducing agent
to precipitate uranium form contaminated groundwater. LOG is a recently emerged
sensing technology that is being actively investigated for modification to radionuclide
sensing. The Self-Absorbed Monolayers on Mesoporous Supports (SAMMS) technology,
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originally developed by DOE primarily for mercury, but also with radionuclides in mind,
has recently had its first field implementation.
In addition to these examples of current or near-term applications of nanotechnology to
radionuclides, it is clear from the analysis of the scientific and engineering principles that
other technologies can be developed, and it is clear from analysis of broader
governmental and industrial issues and initiatives that a rational approach to design of
nanomaterials and nanotechnologies is being pursued. Two examples, one technical
(rational design of organic ligands) and the other programmatic (Chemical Industry R&D
Roadmap for Nanomaterials by Design) demonstrate the potential of a rational design
approach.
± 7.2 Rational Approach to Design in Nanotechnology
The rational design of complexants or sequestrants in organic chemistry draws on
expertise from a number of areas of chemistry, including supramolecular chemistry,
molecular recognition, host-guest chemistry, molecular mechanics, and molecular
modeling. The conceptual origins go back over 100 years to the idea developed by Emil
Fischer, who proposed that enzyme-substrate interactions occur through a "lock and key"
mechanism. Subsequent research was stimulated by work such as the elucidation of DNA
structure, where non-bonding interactions were critical to understanding the three-
dimensional structure. As far back as the mid to late 1990s, government and industry
began shifting their focus in the area of separations from complexants and sequestrants to
separations platforms such as membranes. The U.S. chemical industry presents one of the
best examples of this structured approach. The U.S. chemical industry is the world's
largest, accounting for over 26% of global chemical production (over $450 billion per
year) (DOE 2007). The chemical industry worldwide is the largest component of the
nanotechnology industry; worldwide the nanotechnology industry is estimated at $130
billion (Cientifica 2007), of which the chemical industry accounts for approximately $69
billion (53%). With its enormous scientific and engineering basis (accounting for one out
of every four U.S. patents) and knowledge of varied product requirements, the chemical
industry is well positioned and motivated to explore a rational approach to
nanotechnology.
Currently, much nanomaterial development occurs through empirical or Edisonian R&D,
in which a nanomaterial structure with interesting properties is discovered, an application
is subsequently sought and development of a product retaining the nanoproperty is
attempted. This approach is hit or miss. As a result, a smaller-than-optimal number of
products are produced even though time-to-market can be rapid. Against this background,
in 2003, the Chemical Industry Vision2020 Technology Partnership (Vision2020) issued
its technology roadmap, "Chemical Industry R&D Roadmap for Nanomaterials by
Design: From Fundamentals to Function (Chemical Industry Vision2020 2003)."
Vision2020 is an industry-led partnership process among public and private sector
stakeholders in the chemical and allied industries. The chemical industry has recognized
both the significance of nanomaterials to its operations and markets and a solution-
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oriented approach to materials development, or "nanomaterials by design", is needed.
Conceptually, nanomaterials by design is the ability to employ scientific principles in
deliberately creating structures with nanoscale features (e.g., size, architecture) that
deliver unique capabilities for target applications and require detailed understanding of
science at the nanoscale. The general concept of rational design originated within the
chemical community after it found early applications in pharmaceutical design.
Vision2020's roadmap presented four major research areas for nanotechnology
development:
• Manufacturing and Processing—developing unit operations and robust scale-up
and scale-down methodologies for manufacturing, with emphasis on synthesis,
separation, purification, stabilization, and assembly.
• Characterization Tools—developing analytical tools for measuring and
characterizing nanomaterials both in scientific research and in numerous areas of
production.
• Fundamental Understanding and Synthesis—developing new paradigms for the
creation and controlled assembly of nanoscale building blocks, based on an
understanding of physics and chemistry at the nanoscale.
• Modeling and Simulation—developing computational tools to predict bulk
properties of materials that contain nanomaterials and can bridge between scales
from atoms, to self-assembly, to devices.
Once these developments are achieved, large numbers of diverse products could rapidly
enter global markets to solve long-standing problems and stimulate economic growth for
decades to come. A library of nanomaterials and synthesis techniques could be
established by 2020 for use by material producers and end-users worldwide, offering
diverse, high-quality nanomaterial building blocks with well-characterized compositions,
stable architectures, and predicted properties. Safe, reproducible, cost-effective, and
clearly-defined manufacturing and assembly methods would be available to incorporate
nanomaterials into systems and devices designed to perform specified functions, while
retaining nanoscale attributes. The roadmap suggests a twenty-year period for the
completion of this development.
1.8 Structure of This Report and Summary of Examined Technologies
The remainder of this report is organized into three sections listed below:
• Section 2: Nano-Enabled Remediation Technologies (each technology description
is followed by references specific to that technology)
• Section 3: Nano-Enabled Sensor Technologies (each technology description is
followed by references specific to that technology)
• Section 4: Observations and Conclusions
IB*
-------
Table 2 below provides a summary of the technologies examined in this document and
includes a reference to the section in which the technology is discussed, a brief summary
of the technology, an estimate of the maturity level for the technology and the main
medium in which the technology operates.
Table 2: Summary of Examined Nanotechnologies and Nanosensors
Technology
Zero-valent
Iron (ZVI)
Nanoparticles
Self-Absorbed
Monolayers on
Mesoporous
Supports
(SAMMS)
Nanofiltration
Membranes
Zeolites
Section
Section
2.2
Page 32
-L dg^t ~J Z,
Section
20
.3
Papp 39
-L cisit' j y
Section
2.4
T>Q(Tp
J^agC
46
Section
2.5
Patrp S8
A d ftt- -^ O
Summary
ZVI nanoparticles are a
nanotechnology modification
of the established ZVI
technology in which the ZVI
undergoes oxidation and
consequently is able to reduce
species, such as chlorinated
organics and higher valency
toxic metals (e.g., uranium and
chromium).
SAMMS is a separations
technology designed for the
removal of soluble species in
aqueous solution. It is likely to
be a strong competitor to
conventional ion exchangers,
solid-supported complexants,
or other solid sorbents. The
fundamental scientific
underpinnings make SAMMS
an extremely flexible
separations technology.
Nanofiltration and reverse
osmosis membranes are a
mature separation technology
designed for the removal of
paniculate and soluble species
in aqueous solution. They
occupy the part of the
spectrum of filtration
technologies where extremely
small particulates and soluble
species can be removed from
aqueous solution.
Zeolites are a well-established
ion exchange technology that
is used, among a wide range of
other, non-separatory
industrial applications, for the
treatment aqueous waste
streams containing radioactive
solutes. Zeolites are a
nanotechnology that had
Maturity
The use of macro-scale ZVI
in subsurface permeable
reactive barriers is a well-
established technology.
Nano-scale ZVI has already
undergone field
demonstrations.
Commercially available and
being used in the field for
mercury remediation, with
ongoing development for a
wide range of toxic and
radioactive metals.
Conventional nanofiltration
and reverse osmosis
membranes are well-
established. The use of
insights from recent
nanotechnology
developments for improved
membranes is well
underway and commercial
products are expected to be
available in the near future.
A mature, long-established
technology, widely used for
the treatment of radioactive
waste streams and under
ongoing development.
Media
Aqueous
Aqueous
Aqueous
Aqueous/
soils
-------
Technology
Other
Nanoparticles
Uranium
Reduction by
Bacteria
Carbon
nanotubes
(fullerenes)
Lab-on-a-Chip
(LOC)
Microcantilever
Sensors
Section
Section
2.6
Page
69
Section
2.7
Page 77
Section
2.8
"Pflpp
J^agC
85
Section
3.2
Papp 99
-L ci sit ^ y
Section
3.3
Page 103
-L Clgt _LV/^J
Summary
established uses predating the
general concept of
nanotechnology.
Nanodiamonds, dendrimers,
and the Argonne Supergel are
presented to demonstrate the
range of possibilities offered
by nanoparticles — particularly
the ease with which a
nanoparticle can be
incorporated into another
technology application and the
ability to engineer specific
properties into a nanoparticle
at the atomic level.
By using the appropriate
micro-organism and nutrient
addition, soluble uranium (VI)
can be converted in-situ to
much less soluble uranium
(IV) by enzymatic reduction.
A class of hollow, spherical, or
ellipsoidal molecules
composed entirely of carbon
atoms in a cage-like structure.
They have a range of potential
applications, from sensing
elements, to components of
advanced nano-composite
materials that may enable
better membranes for reverse
osmosis and nanofiltration.
LOC is a platform that can
integrate a number of
laboratory analytical functions,
such as separations and
analysis of components of a
mixture, on a single
microprocessor chip using
fluid volumes in the nanoliter
and lower range, thus allowing
realization of small, portable
equipment.
A microcantilever is an
extremely small beam
supported at one end and
capable of well-defined
bending and vibrational
Maturity
Various depending on
specific technology.
The technology concept is
mature and commercially
available.
Nanotubes are now
commercially available in
"tons-per-year" quantities.
Applications are in the
laboratory stage with future
development likely to speed
up now that availability has
been increased and cost
decreased.
The technology is
commercially available for
biological research.
The technology concept is
mature and commercially
available.
Media
Various
depen-
ding on
specific
techno-
logy.
Aqueous,
soil, sub-
surface
Aqueous
Aqueous
Aqueous
-------
Technology
Spectroscopic
Sensors
Nanowire
Sensors
Nanobelts and
Nanorods
Section
Section
3.4
Page 109
Section
3.5
Page
117
Section
3.6
Doo-p
rage
122
Summary
behavior that can be accurately
monitored. When the
microcantilever surface is
modified so as to be able to
bind a target species, the
changes in vibrational
behavior allow concentration
of the target species to be
determined.
Spectroscopic measurements,
particularly fluorescence
spectroscopy, are one of the
most sensitive conventional
detection technologies,
routinely going to the single
molecule level in the
laboratory. Nano-enabled
technologies promise to
expand the availability and
application of these
techniques. Probe
Encapsulated by Biologically
Localized Embedding
(PEBBLE), surface plasmon
resonance, nanobelts and
nanorods are discussed as
examples.
Nanowires are solid, rod-like
materials with diameters in the
5-100 nm range and are most
often made from metals or
semi-conducting metal oxides.
Their main application is
expected to be in sensors, with
the underlying phenomenon
that is exploited being the field
effect upon which field effect
transistors are based.
Nanobelts are a class of
nanostructure often viewed as
a type of nanowire. Nanobelts
form ribbon-like structures
with widths of 30-300 nm,
thicknesses of 10-30 nm, and
lengths in the millimeter
range. Nanorods are solid
nanostructures
morphologically similar to
nanowires but with aspect
ratios of approximately 3-to-5.
Maturity
Various, depending on
specific technology.
Early stage of development.
Under investigation for
various applications.
Media
Primarily
aqueous
Aqueous
Various
depen-
ding on
specific
techno-
logy.
-------
This report does not address technologies complementary to radionuclide remediation or
sensing. For example, if a subsurface plume were to contain both Resource Conservation
and Recovery Act (RCRA) organics and radionuclides as contaminants, the material was
brought to the surface, and a problematic mixed waste was produced; a nanotechnology
that could destroy the RCRA organic would accomplish much in assisting with the
management of the radionuclide. Though this type of consideration is beyond the scope
of this report, it represents yet another approach by which nanoparticles can assist in the
management of radionuclide problems.
In addition to the technologies that are presented in this report, there are a number of
other technologies or scientific concepts that over the short term, as nanotechnology
advances, may have remedial potential for radionuclides. The prediction of future
technology success is a task always fraught with risks, particularly as the time period of
the prediction grows. It is often pointed out that at the end of the nineteenth century many
scientists believed that all of the basic fundamental principles governing physics and
chemistry were known, and that few big discoveries left with only details remaining. The
difficulties in technical prediction still remain. Some of the choices for the following
sections are obvious. For instance, technologies such as ZVI nanoparticles, SAMMS, and
zeolites have already found commercial or near-commercial applications in the treatment
of radionuclides. Others are included since they are being actively researched for closely
similar applications. For instance, dendrimers are being examined for application in the
treatment of heavy metals and the transfer of understanding from heavy metals to
radioactive metals is usually fairly straightforward; it is very common in research to use
the lanthanides as models for the actinides since the non-radioactive lanthanides are much
easier to work with than the radioactive actinides and useful predictions and
extrapolations of behavior from one class to the other can be made. For the rest of the
technologies common factors employed in the analysis of technology potential were used.
For example, factors such as the existence of a standardized framework or platform
within which to work, the existence of a wealth of related knowledge and supporting
research, the technology offering a technically elegant or conceptually compelling
solution to a problem, or the robustness and flexibility of a technology are all positive
indicators of its likely success. The reader should of course understand that, particularly
in a rapidly developing field, there is always inherent uncertainty in making
extrapolations of likely technology success.
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2.0 Nano-Enabled Remediation Technologies
2.1 Introduction
In dealing with the remediation of environmental contamination or the closely-related
concern of managing wastes, options for treating non-radiological (though potentially
very hazardous) materials can be divided into two broad categories: separation of the
contaminant and transformation of the contaminant. For example, if an aqueous stream is
contaminated with trichloroethylene (TCE), the stream could be cleaned by passage
through a bed of activated charcoal to separate the TCE from the stream, or by passage
over a bed of ZVI, in which case the ZVI would transform the TCE. In the case of
radionuclide contamination, the situation is complicated by the fact that no amount of
transformation will ever remove the radiological characteristic of the waste—a uranium
atom will always remain a uranium atom no matter how it speciates chemically.
Transformations may be useful, but only to the extent that they assist in bringing about a
separation. For radiological contamination, the separations of interest may be the removal
of the radioactive species from its host matrix (i.e., an extraction), or it may involve
processes such as fixation or stabilization, in which the radioactive material is separated
from any mobilization and/or transport pathways so that the risk it poses is reduced or
eliminated altogether by preventing it from being made available to a receptor.
The remediation of radionuclide-contaminated sites has been problematic for decades.
Radionuclides typically remain a matter of concern even at concentrations that are orders
of magnitude lower than those for most non-radiological materials. Further, the
geological matrix can present considerable heterogeneities even over small distances and
is inherently difficult to characterize. The situation is further complicated by the timeline
for developing remedial technologies. In contrast to enabling technologies such as
sensors, remedial technologies are generally much more time-consuming and involve
considerably higher costs and risks. These problems are amplified when radionuclides are
involved. Though the novel chemical and physical properties offered by nanoparticles
may yield new approaches to remediation, the fundamental difficulties posed by
radionuclides will not disappear. This does not mean that nanoparticles offer little
promise for environmental remediation. On the contrary, the potential appears to be great,
but it is more likely to come in the areas of remediation of non-radiological contaminants,
pollution prevention through the development of greener manufacturing processes, better
waste treatment of non-radiological contaminants, and sensors than in the area of
remediation of radiological material.
Having provided this cautionary background, the research involved in preparing this
report indicates that nanotechnology is likely to provide some useful tools for the toolbox
of remediation technologies applicable to radionuclide-contaminated sites. For the past
two decades, it has been recognized that environmental contamination by radionuclides
constitutes one the most difficult environmental problems. The cost of remediation of
such sites may be enormous. Major decreases in anticipated costs do not seem to be likely
through incremental improvements to existing approaches and transformational
alternatives that have been actively sought. Since nanotechnology is still in its infancy,
-------
the possibility of transformational approaches arising should not be dismissed, even if
there is little indication at present regarding how this will come about.
Already, nanotechnology does offer some intriguing possibilities. One example is that
macroscale ZVI has been used successfully for years as a means of treating chlorinated
organics in groundwater via use of permeable reactive barriers (PRBs). Nanoscale ZVI is
currently being investigated for the same application, exploiting the fact that its
rheological properties allow it to be injected. ZVI has also been investigated as a
reductant to transform materials of concern, such as chromium and uranium, from their
higher valency, more soluble forms, to lower valency, and soluble forms possessing
lower risk. It seems likely that nanoscale ZVI may offer the same chemical reductant
properties, possibly even enhanced, with the flexibility of injectability. This conclusion
involves an extrapolation, but it is a fairly small one. A second example, and one that
does not involve an extrapolation, is zeolites—nanostructured, ion-exchanging
separations materials that have been used in radioactive material waste management for
half a century. A third example is the Self-Assembled Monolayers on Mesoporous
Supports (SAMMS) technology, which offers a new platform for separations agents. This
technology is currently being demonstrated for mercury removal and, in combination
with the fundamental understanding already available, the extrapolation to a technology
for the separation of radionuclides is small.
Separations may be the area where nanotechnology makes its biggest impact in
remediation. Carbon nanotubes can be incorporated into membranes and permit
extremely fast fluid flow; they are under active investigation as a desalination
technology. The combination of existing knowledge on ligand design—an area of
fundamental importance to separations science—with new nanostructures, such as
dendrimers, may offer greatly improved separations performance. The following sections
describe technologies or technology concepts that have either been demonstrated for
radionuclide treatment, such as zeolites, or that can be comfortably extrapolated to
radionuclides, such as ZVI. Since a certain degree of extrapolation is necessarily
involved, the survey in this report is not intended to be comprehensive, but rather seeks to
provide information on reasonable possibilities that may yield technologies in the near
future. The technologies examined in this report are:
• ZVI nanoparticles
• SAMMS
• Membranes: nanofiltration and affinity
• Zeolites
• Other nanoparticles
• Uranium reduction by bacteria
• Carbon nanotubes
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2.2 Zero-valent Iron Nanoparticles
2.2.1 Background
The massive surface area and unique properties of nanoparticles have led to much
research on their application to environmental remediation. Across the broad array of
environmental concerns, research ranges from the use of TiO2 nanoparticles for
photocatalytic treatment of nitrous oxides in plant emissions, to the use of naturally
occurring metal oxide nanoparticles for the treatment of organic contaminants in
groundwater. There are, however, few examples of commercial-scale technologies that
use nanoparticles for environmental remediation of non-radiological materials, and even
fewer for remediation of radionuclides. The closest example to a commercial technology
is the use of iron nanoparticles as a reductant for the remediation of chlorinated organics
in water; this is a direct modification of the use of iron filings or microscale iron powder
for the remediation of chlorinated organics in water. In addition, one company (Pars
Environmental Inc.) has successfully used iron nanoparticles at several chromium-
contaminated sites for reducing soluble and carcinogenic chromium in the +6 valence
state to insoluble and non-carcinogenic chromium in the +3 valence state.
In general, nanoiron particles can treat the following contaminates in a range of
geological settings:
• Contaminants:
o Halogenated aliphatics (PCE, TCE, 1,1,1 -TCA, 1,1,2,2-TeCA)
o Halogenated aromatics
o PCBs
o Halogenated herbicides and pesticides
o Nitroaromatics
o Metals (e.g., Cr6+, As)
• Geologic Conditions:
o Sand
o Silt
o Fractured rock
o Landfills
o Fill materials
o Sediments
Additionally, EPA is preparing a fact sheet on the use of nanotechnology for site
remediation and will include information about sites where the technology has been
tested.
2.2.2 Description
Iron nanoparticles represent the only field application of free-released nanoparticles for
environmental remediation. First suggested in 1996 and now the subject of a number of
-------
reviews (Kumar 2006, Zhang and Elliott 2006, Lo et al. 2006), the use of iron
nanoparticles is a variation on the use of ZVI as a PRB.
The non-nanoparticle ZVI PRB technology has been used to remediate contaminated
groundwater for almost 20 years (Senzaki and Kumagai 1988) and has been demonstrated
in the field and in the laboratory for a wide range of contaminants, including the
reduction of nitrates, bromates, chlorates, nitroaromatic compounds, chlorinated ethanes,
chlorinated methanes, and brominated or carbarylated pesticides, and the removal of
arsenic, lead, uranium, mercury, and hexavalent chromium (Nuxoll et al. 2003, Arnold
and Roberts 2000). Typically, iron filings or microscale iron powder have been used to
create a barrier a few meters wide, contained by gravel supporting beds. The chemistry is
well understood as the corrosion of iron based on the following reactions:
aq) + O2(aq) -> 2Fe2+(aq) + 2H2O(\)
4H+(aq) + O2(aq) -> 4FeYaqJ + 2H.2O([)
2H2O(\)
As ZVI is oxidized to ferrous and/or ferric iron, pH increases, hydrogen is evolved,
oxidizable materials are consumed, and the strong reducing conditions created are
favorable for the pathways (oxide-mediated electron transfer from the metal to the
chlorinated organic, reduction of the chlorinated organic by the ferrous iron and reduction
by evolved hydrogen) — leading to complete dechlorination. Eventually, ferric or ferrous
iron may precipitate as a solid or remain in solution, depending on the pH and redox
conditions. Mineral precipitates of carbonates, sulfides, and/or oxides may form coatings
on the reactive grains, inhibit the performance of the iron, and reduce the porosity and
permeability of the aquifer, but analysis of the evidence suggests that destruction of
chlorinated solvents can still continue to completion. Additionally, the generation of
strong reducing conditions and hydrogen gas foster anaerobic microbial growth and
increase natural biological degradation in the field (Henn and Waddill 2006).
The first field-scale application of conventional ZVI for groundwater remediation was at
the Canadian Forces Base in Borden, Ontario in 1991. A treatment zone was excavated,
isolated by sheet pile; filled with a mixture of granular iron and sand; and the sheet piles
were removed. Contaminated groundwater flowed through the treatment zone and the
chlorinated ethenes (perchloroethylene and trichloroethylene) were almost completely
removed. The approach is now a standard, and regulatory guidance for the general use of
PRBs (including those involving iron) is available (ITRC 1999a), together with design
guidance (ITRC 1999b), and recently learned lessons and new directions (ITRC 2005).
The concept of using nanoparticle iron to remediate chlorinated organics follows almost
immediately from the conventional ZVI treatment. The small size of the particles (-10
nm) would allow them to be injected into a geologic matrix and reach areas unavailable
even to the microbes (-1,000 nm) used in bioremediation. Since reactivity appears to be
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a direct function of total iron particle surface area, their performance is expected to be
superior to that of larger particles. Production of ZVI nanoparticles is relatively easy,
involving the reduction of ferric ion by sodium borohydride (Wang and Zhang 1997):
4Fe3+ + 3BH 4+ 9H2O -> 4Fe°j +3H2BO 3
12H+ + 6H2
A successful, pilot-scale study has already been performed (Henn and Waddill 2006)
using 300 pounds of palladium-catalyzed and polymer-coated nanoscale ZVI particle
suspension at the Naval Air Station in Jacksonville, Florida. The particle suspension was
injected via a gravity feed and recirculated through a source area containing chlorinated
volatile organic compounds (VOCs). Between 65 and 99% aqueous-phase VOC
concentration reduction occurred, due to abiotic degradation, within five weeks of the
injection and then yielded to slower biological degradation. Aqueous-phase VOC
concentrations were reduced up to 99 % and were near or below applicable regulatory
criteria. Though there are still considerable knowledge gaps, the technology appears to
have great potential.
Recently, Pars Environmental, Inc. has developed a ZVI particle which, over time,
encapsulates the metal contaminant; it is reported that this encapsulated layer
immobilizes the contaminant for up to 30 years. This stepwise process resulting in the
reduction of Cr(VI) to Cr(IV) and encapsulation of Cr(IV) in an onion-like skin around a
nano ZVI particle is illustrated in Figure 1.
1.ZVI
2. Adsorption of nano iron
3. Reduction of Cr(VI)
4. Formation of
Cr-Fe alloy
(hydroxide)
\
Figure 1. Encapsulation stabilization of the contaminant
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The key features of this process as illustrated for Cr(IV) are:
1. Nano Cr-Fe layer
a. Thickness - 10 nm
b. -40 atoms
2. 2/3 of the mass from Cr(VI)
a. Cr 0.667 Fe 0.333 OOH
3. Insoluble
4. Reducing power (FeO)
The same reducing ability of ZVI is suggested for the remediation of radionuclides and
heavy metals to bring about a reduction of the metal to a less soluble, lower oxidation
state. For chromium, a conventional ZVI PRB consisting of 450 tons of granular ZVI was
installed in 1996 at the United States Coast Guard Support Center in Elizabeth City,
North Carolina, and has been subject to considerable documentation (Puls et al. 1998,
Blowes et al. 1997, Blowes et al. 1999, FRTR 1998). The PRB was a continuous trench
approximately 46 m long, 7 m deep, and 0.6 m thick. Concentrations of Cr(VI) had been
as high as 5 mg/L, but decreased to non-detectable levels (<0.0025 mg/L) after
installation of the PRB. The removal mechanism for Cr(VI) was determined to be
precipitation as Cr(III) oxyhydroxide or co-precipitation with iron oxyhydroxide. Despite
this success, a similar ZVI PRB at the Haardkrom site electroplating facility in Kolding,
Denmark, was not effective, with speculation being that the TCE present may have
depleted the chromium-reducing ability of portions of the barrier (Roehl et al. 2005).
For uranium, a number of conventional ZVI barriers (Bronstein 2005) have been
demonstrated, including:
• Bodo Canyon Disposal Site, La Plata County, Colorado—a number of PRBs
(composed of ZVI, copper wool, and steel wool) were used to treat arsenic,
molybdenum, selenium, uranium, vanadium, and zinc. The barrier containing ZVI
operated from August 1999 until June 2004 (when flow ceased from the seep and
remediation was no longer needed). It maintained effluent uranium concentrations
of less than 0.01 mg/L, and was highly effective in treating contaminants.
• Cotter Corporation Uranium Mill, Canon City, Colorado—a ZVI PRB was used
to treat molybdenum and uranium. Though the barrier eventually failed for
molybdenum, uranium concentrations remained at less than 0.006 mg/L. It was
found that the ZVI was clogged by mineral precipitants. Modifications, including
a pretreatment zone composed of coarse gravel and ZVI, were suggested.
• Fry Canyon Site, Fry Canyon, Utah—a PRB of ZVI, amorphous ferric oxide
(AFO), and phosphate rock was used to treat uranium. The ZVI barrier has been
the most effective, removing 99.9% of uranium.
• Mecsek Ore Site, Pecs, Hungary—a PRB composed of ZVI and shredded cast
iron was used to treat uranium, and concentrations within the groundwater in
2003 were reduced to less than 1% of the influent value after passing through the
barrier.
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• Monticello Mill Tailings Site, Monticello, Utah—a ZVI PRB was used to treat
uranium, arsenic, manganese, molybdenum, selenium, vanadium, and nitrates.
Results show that the barrier was effective in treating the contaminants. Nearly all
of the uranium, arsenic, manganese, molybdenum, selenium, and vanadium were
removed from the groundwater, and nitrate, molybdenum, and manganese were
greatly reduced. Since 2000, contaminants have been reduced to below detectable
levels.
• Rocky Flats Environmental Technology Site (Solar Ponds Plume), Golden,
Colorado—a barrier composed of ZVI and wood chips was used to treat nitrate
and uranium. Remediation goals required a reduction of uranium from 20-28
pCi/L to 10 pCi/L. Surface stream samples below 10 pCi/L for uranium indicate
that the PRB is working properly.
• Y-12 Site, Oak Ridge, Tennessee—a barrier composed of ZVI and peat materials
is being used to treat uranium, technetium, and nitric acid. A funnel and gate
barrier failed due to leaking. A continuous trench was subsequently installed and
has shown that uranium and technetium concentrations have decreased, but
because of reactions with groundwater constituents, the lifespan of the ZVI wall
may be significantly shorter than expected.
Overall, these results strongly support the case for using conventional ZVI as an effective
reductant for radionuclides, such as uranium. Thus, the potential for iron nanoparticles is
considerable. Extensive research is ongoing, including studies on formulation of the iron
nanoparticles, delivery vehicles and methods of in-situ stabilization. A potential remedial
scheme using ZVI nanoparticles is shown in Figure 2.
Contaminant
Source
Dechlorination of Organic
Solvent (e.g., COj, C;C14)
Injection of Iron
Nanoparticles
Transformation of
Fertilizers (e.g., NO,-)
Detoxification, of Pesiticides
(e.g., Undone, DDT)
Immobilization of
Metals fc.g., Pb, Cr, As)
Figure 2. Potential remedial scheme using ZVI nanoparticles
Obviously, a considerable amount of further work must be performed to establish
baseline performance and to develop the knowledge base required to gain regulatory
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acceptability for the technology as applied to uranium. However, iron nanoparticles
remain a leading contender for application to the remediation of radionuclides.
2.2.3 Summary of Environmental Potential
ZVI nanoparticles are the first field application of free-released nanoparticles for
environmental remediation. The use of macro-scale ZVI in subsurface PRBs is a well-
established technology for the reduction of both chlorinated hydrocarbons and toxic
metals in contaminated ground water; ZVI nanoparticles use exactly the same chemistry,
but take advantage of the increased surface area and the rheological ability of
nanoparticles to flow in the subsurface and permeate crevices where contaminants may
reside. The technology is potentially applicable in most circumstances where macro-scale
ZVI would be employed. Where mixed contaminants, such as chlorinated organics and
higher valency toxic metals, are present, ZVI nanoparticles may be able to accomplish
the remediation of both types of material. The demonstrated ability of ZVI to encapsulate
a heavy metal contaminant (chromium) through a combination of adsorption and
reduction processes, and the ability of ZVI to act as a reducing agent offers significant
potential for the remediation of radioactive species such as uranium or plutonium where
the reduced form of the metal is of much lower solubility and can be effectively removed
from solution. Risk issues are not expected to be significant since data, to date, show that
the iron nanoparticles do not travel extensively, and nanoparticles of iron oxides (into
which nanoparticulate iron will eventually transform) are ubiquitous in groundwater.
2.2.4 References
Arnold, W.A., and A.L. Roberts. 2000. Pathways and kinetics of chlorinated ethylene and
chlorinated acetylene reaction with Fe(0) particles. Environmental Science &
Technology. 34: 1794-1805.
Blowes, D.W., CJ. Ptacek, J.L. Jambor. 1997. In situ remediation of Cr(VI)
contaminated groundwater using permeable reactive walls: Laboratory studies.
Environmental Science and Technology. 31 (12): 3348-3357.
Blowes, D.W., R.W. Puls, R.W. Gillham, CJ. Ptacek, T. Bennett, J.G. Bain, CJ. Hanton-
Fong, C J. Paul. 1999. An In situ Permeable Reactive Barrier for the
Treatment of Hexavalent Chromium and Trichloroethylene in Ground Water:
Volume 2 Performance Monitoring. EPA/600/R-99/095b. Cincinnati, OH: United States
Environmental Protection Agency.
Bronstein, Kate. 2005. Permeable Reactive Barriers for Inorganic and Radionuclide
Contamination. Prepared for U.S. EPA, Washington DC. Website. Accessed October
2007. http://clu-in.org/download/studentpapers/bronsteinprbpaper.pdf
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Federal Remediation Technologies Roundtable (FRTR). 1998. In Situ Permeable
Reactive Barrier for Treatment of Contaminated Groundwater at the U.S. Coast Guard
Support Center, Elizabeth City, North Carolina. Website. Accessed October 2007.
http://costperformance.org/profile.cfm?ID=287&CaseID=287.
Henn, K.W. and D.W. Waddill. 2006. Utilization of nanoscale zero-valent iron for source
remediation-A case study. Remediation Journal. 16(2): 57-77'.
Interstate Technology & Regulatory Council (ITRC). 1999a. Regulatory Guidance for
Permeable Reactive Barriers Designed to Remediate Chlorinated Solvents, 2nd Edition,
PBW-1. Washington, DC: ITRC Permeable Reactive Barriers Team.
ITRC. 1999b. Design Guidance for Application of Permeable Reactive Barriers for
Groundwater Remediation, PBW-2. Washington, DC: ITRC Permeable Reactive Barriers
Team.
ITRC. 2005. Permeable Reactive Barriers: Lessons Learned/New Directions, PRB-4.
Washington, DC: ITRC Permeable Reactive Barriers Team.
Kumar, C.S.S.R. 2006. Nanomaterials: Toxicity, Health and Environmental Issues.
Wiley-VCH, Germany.
Lo, I.M.-C., R.Y. Surampalli, and K.C.K. Lai (Eds.). 2006. Zero-Valent Iron Reactive
Materials for Hazardous Waste and Inorganics Removal. ASCE Publications, Reston,
VA.
Nuxoll, E.E., T. Shimotori, W.A. Arnold, E.L. Cussler. 2003. Iron Nanoparticles in
Reactive Environmental Barriers. Presentation at the AIChE Annual Meeting, November
20,2003.
Puls, R. W., D.W. Blowes, R.W. Gillham. 1998. "Emplacement verification and long-
term performance monitoring of a permeable reactive barrier at the USCG Support
Center, Elizabeth City, North Carolina." In Groundwater quality: remediation and
protection, Vol. 250. M. Herbert, and K. Kovar, eds. Amherst, Mass: International
Association of Healthy Soils Publication, 459-466.
Roehl, K.E., K. Czurda, T. Meggyes, F. Simon, D.I. Stewart. 2005. Long-term
Performance of Permeable Reactive Barriers. Elsevier, New York.
Senzaki, T. and Y. Kumagai. 1988. Treatment of 1,1,2,2-Tetrachloroethane with iron
powder. Kogyo Yosui. 357(1): 2-7.
Wang, C. and W. Zhang. 1997. Nanoscale metal particles for dechlorination of PCE and
PCBs. Environmental Science and Technology. 31(7), 2154-2156.
Zhang, W.-X. and D.W. Elliott. 2006. Applications of iron nanoparticles for groundwater
remediation; Remediation Journal. 16(2): 7-21.
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2.3 Self-Assembled Monolayers on Mesoporous Supports
SAMMS is an award-winning (PNNL 2007) hybrid of two active areas in materials
science—mesoporous materials and molecular self-assembly on the mesoporous material
surface—that combine to create a material capable of efficiently removing target species
from aqueous solutions and other liquids.
A highly developed, engineering-scale technology that is entering the market, SAMMS
reached the commercial threshold in 2006 for mercury remediation. The conceptual
developer, Pacific Northwest National Laboratory (PNNL) states that currently
demonstrated laboratory production capacity is approximately 5 kg. Battelle (the operator
of PNNL) and Mobil Oil Corporation (developer of the original mesoporous support,
now ExxonMobil) are establishing commercial production capacity for the manufacture
of these materials. Mobil has demonstrated the ability to produce mesoporous substrates
in batch quantities of one ton or more; scale-up of the functionalization process to these
production levels is underway. Costs for these materials are expected to fall into the
range of selective anion ion exchange resins. PNNL maintains a Web site devoted to
SAMMS. In addition to excellent descriptions of the technology (SAMMS 2006), the
Web site provides links to information on:
• current R&D activities;
• publications;
• general articles;
• lanthanide and actinide investigations; and
• Other forms (thiol, chelate, anion, and actinide) of SAMMS.
The following descriptions have drawn heavily from the information contained in the
PNNL Web site and its links.
2.3.1 Description
Mesoporous materials, one component of the SAMMS hybrid, are porous substances
with pore diameters in the range of 2-50 nm; materials below this size range are called
"microporous" (and usually include materials such as zeolites, aluminophosphates
(AlPOs), clathrasils, pillared and non-pillared clays) and above this range they are called
"macroporous". Mesoporous materials are thus within the 1-100 nm length scale that
typically defines nanotechnology. The term "mesoporous" can apply to any material with
pore size in the appropriate range, but in the case of the SAMMS technology, structured
mesoporous ceramics are usually involved. In 1992, researchers at Mobil, while
exploring novel microporous materials for new catalysts, synthesized a new type of
silica-based material, named the M41S family, which possessed uniform, mesoporous-
scale pores regularly distributed throughout the solid material. The synthesis used
cylindrical surfactant micelles to direct (create) the structure formation of the pores in the
sol-gel preparation process. The larger pores allow bulk solution to enter and easily
explore the interior's entire surface; this is in contrast to the behavior of the smaller pores
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typical of zeolites (typically 0.5-1 nm), where size exclusion is often a dominant effect.
Fryxell (SAMMS 2006) has described the structure as follows:
"To envision what mesoporous ceramic look like, think of a glass honeycomb
in which the holes in the honeycomb are very uniform and are only 60
Angstroms across (roughly twice the width of a double-helix strand of DNA),
and the walls are only 10 Angstrom thick (about the size of a "typical" amino
acid molecule). These parallel, open-ended pores result in a material that has
extremely high surface area, all of which is accessible to solution. "
SAMMS are created by attaching a monolayer of molecules to mesoporous ceramic
supports. The method uses linear molecules possessing different chemical functionalities
at each end. The chemical functionality at one end has an affinity for the mesoporous
surface, while the chemical functionality at the other end has an affinity for the target
species to be bound by the SAMMS material. A combination of intermolecular and
intramolecular forces drive the bi-functional molecules to assemble along the entire
length of the pore as a close-packed monolayer, completely covering the surface of the
mesoporous support, thus exposing a new chemical surface and interface to bulk solution.
Figure 3 (SAMMS 2006) presents a graphical representation of the process.
Self-assembled monolayer
SAMMS
•^
*j
Mesoporous support
Figure 3. Formation of SAMMS
Possessing physical characteristics similar to activated carbon, mesoporous materials
have very high surface area to mass ratios. SAMMS powder can have a specific surface
area of approximately 1000 m2/g. For comparison, the surface of a tennis court is
approximately 260 m2. Monolayer molecules fully cover the available surface and thus
present a large area for binding target species. The large surface area, combined with
large pore sizes that allow entry of solution, create a hybrid material with fast kinetics,
high material loading, and excellent selectivity. Both the properties of the molecules in
the monolayer and the pore size of the mesoporous support can be tailored for a specific
-------
application. The functional group facing the interior of the pore can be designed to
selectively bind targeted molecules while the pore size, monolayer length, and density
can be adjusted to give the material specific diffusive and kinetic properties.
SAMMS is developing in several engineered forms (such as beads, membranes, and
membrane cartridges) and can be delivered with a variety of chemically active
substances. In addition, the SAMMS technology is developing systematically in a
number of functional forms including:
Thiol-SAMMS, primarily targeting Hg, Ag, Au, Cu, Cd, and Pb;
Chelate-SAMMS, primarily targeting Cu, Ni, Co, and Zn;
Anion-SAMMS primarily targeting chromate, arsenate, selenite,
pertechnetate; and
Actinide-SAMMS primarily targeting Am, Pu, Th, Np, and U.
and
a. Thiol-SAMMS. Thiol-SAMMS was the first and is the most extensively
investigated functional form, having been specifically developed for the removal of
mercury from liquid media (Chen et al. 1999, Mattigod et al. 1997). Thiol-SAMMS
is also the first commercial application of the SAMMS technology
(ChemicalProcessing.com 2006). The Thiol-SAMMS functional form takes
advantage of the fact that the thiol functional group has a very strong affinity for
mercury (in fact "mercaptans", an alternate name for thiols, derives from the Latin
"met-curium captans" meaning mercury capturers). In Thiol-SAMMS, a monolayer
of mercaptopropyl siloxane coats the mesoporous surface with the thiol groups
pointing towards the pore center, presenting a thiol surface to the solution in the
lined pores. This surface is able to bind cationic, organic, metallic, and complexed
forms of mercury with great affinity (distribution coefficient, or Kd, ~1 x 108) and
rapid kinetics. A graphical representation is shown in Figure 4.
Figure 4. Thiol-SAMMS showing mercury atoms (blue) binding to sulfur atoms
(yellow) from thiol groups
b. Chelate-SAMMS. In Chelate-SAMMS (SES 2007, SAMMS 2006), chelating
functional groups (such as ethylenediamine, ethylenediamine triacetic acid, pyridine,
etc.) are attached to the surface of the mesoporous materials using a similar chemistry
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to that used for preparation of the thiol-SAMMS. The chelating group is tailored for
the targeted metal (e.g., ethylenediamine for copper and 1,2-hydroxypyridinone
(HOPO) for the actinides). Results are promising and research is continuing to
optimize the synthesis and metal loadings.
An intriguing aspect of Chelate-SAMMS, and one that demonstrates the versatility of
the technology, is through a synthesis modification, which is a functional form of
SAMMS that is able to bind cesium (Lin et al. 2001). The basis is that metal
ferrocyanides have been widely investigated as ion exchangers for cesium, with
potassium cobalt ferrocyanide having the advantages of high capacity and selectivity
for cesium, but the drawback of being a very fine powder that is difficult to coax into
an effective engineered form. In the SAMMS approach, mesoporous silica is
functionalized with an ethylenediamine terminated silane. Copper(II) ions are then
incorporated simply by stirring a copper(II) chloride solution with the functionalized
mesoporous support for a few hours, and refluxing in toluene to produce Cu-EDA-
SAMMS. This is converted to Cu-Ferrocyanide-EDA SAMMS simply by mixing Cu-
EDA-SAMMS with sodium ferrocyanide. The conclusion of the work was that:
"The synthesis of the cesium selective SAMMS is a simple, direct, and
convergent synthesis. These nanocomposite materials are easily made from
commercially available bulk materials and are highly selective for cesium from
various high-salt and acid solutions. The high surface area of the mesoporous
silica creates a high loading capacity for cesium sorption. The fast binding
kinetics and high loading capacity of SAMMS sorbent materials are due to the
rigidly open pore structure and rapid interfacial chemistry. The chemical
specificity results from the selectivity of the copper(II) ferrocyanide interface
within the nanoporous structure. This unique combination of properties makes
SAMMS-based methodology a strong candidate for the cleanup of cesium-
containing nuclear wastes and contaminatedgroundwater" (Lin et al. 2001).
c. Anion-SAMMS. Anion-SAMMS functional forms (SAMMS 2006, SES 2007) are
being investigated since there is a great need for effective anion binders (e.g., in the
cleanup of pertechnetate contamination at DOE sites). While organic ion exchange
resins based on quaternary ammonium ion exchange chemistry and inorganic
materials, such as alumina, are available, they are orders of magnitude less effective
than state-of-the-science cation exchangers. The SAMMS approach mimics the
strategy used for Cu-Ferrocyanide-EDA SAMMS by lining the pores of the
mesoporous material with cationic transition metals complexes specifically targeting
those complexes that would allow for a direct interaction between the cationic
receptor and the target anion. Research (Fryxell 2001) has shown that copper or
nickel ethylenediamine complexes immobilized on mesoporous silica are extremely
efficient ion binding materials for chromate, comparable in terms of loading to the
best cation exchangers. By exploiting variations in transition-metal complex
chemistry, the promise is held that effective ion exchangers for tetrahedral anions can
be developed. It is also worthy of note that Thiol-SAMMS capped with soft metals,
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such as silver or mercury, have shown promise as "getters" for radioiodine (Mattigod
et al. 2003a, Mattigod et al. 2003b).
d. Actinide-SAMMS. Actinide-SAMMS (Fryxell 2001, Lin et al. 2005, SAMMS
2006, SES 2007) is the least developed area of the SAMMS technology. There exists
a body of research on SAMMS applied to lanthanide separations (Fryxell 2001,
Fryxell 2004, Yantasee 2005, Lin et al. 2005), and parallels between lanthanide
chemistry and actinide chemistry are fairly well understood. In addition, the
theoretical basis has been explored (Fryxell 2001) and laboratory experiments suggest
that by examining potent actinide binding ligands from prior studies and synthesizing
functional forms such as amidophosphonic acid-SAMMS and hydroxypyridinone-
SAMMS, excellent actinide sorbents can be developed.
At the present time no studies have been conducted on the fate and transport of SAMMS
material.
2.3.2 Operational Considerations
PNNL staff assert that SAMMS is clearly superior to commercially-available sorbents,
with 99% of Thiol-SAMMS' mercury-adsorbing action taking place in the first five
minutes; tests at PNNL show that the material can remove 99.9% of mercury in a
simulated wastewater, reducing the mercury level to well below EPA discharge limits
(ChemicalProcessing.com 2006). Treatment costs appear to be an order of magnitude
lower than those of the best available alternative technologies.
From ChemicalProcessing.com: "...says Shas Mattigod, a staff scientist at PNNL. 'There
is no comparison.' Mattigod calls treatment costs an order of magnitude lower than those
of the best available alternative technologies. 'We estimate that it will cost about $200,
including material, analysis and labor, to treat similar volumes of this waste solution,' he
says. 'This would save $3,200 over the more-traditional disposal methods.'"
2.3.3 Summary of Environmental Potential
The SAMMS technology is one of the most promising nano-enabled environmental
technologies. It is probably the only nanotechnology so far that was both specifically
designed for, and has already found commercial application in, environmental
management. It is a technology used for separating aqueous solutions, but is more likely
to find application in sorbent units as opposed to being released into the environment to
perform its separately behavior. Technologically, it combines rational ligand design for
targeting species in solution with molecular self assembly on mesoporous support
structures. Rational ligand design is well understood; molecular self assembly and
mesoporous material behavior are firmly enough understood to allow commercial
application. Yet both are benefiting greatly from further developments, many coming
from various areas of nanotechnology research. The technology is extremely flexible,
allowing a wide variety of aqueous species to be targeted for removal. The SAMMS
material offers the benefit of high capacity, and also appears to be a strong candidate as a
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waste form that may be disposed of directly, thus significantly easing further processing
requirements.
2.3.4 References
ChemicalProcessing.com. "Nanosponge soaks up mercury". 2006. Website. Accessed
October, 2007. http://www.chemicalprocessing.com/industrynews/2006/052.html.
Chen, Xiaobing, X. Feng, J. Liu, G. Fryxell, M. Gong. 1999. Mercury Separation and
Immobilization using Self-Assembled Monolayers on Mesoporous Supports (SAMMS).
Separation Science and Technology. 34 (6-7): 1121-1132.
Fryxell, G.E. 2001. Actinide-Specific Interfacial Chemistry of Monolayer Coated
Mesoporous Ceramics Number 65370. FINAL REPORT U.S. Department of Energy
Environmental Management Science Program. Website. Accessed October 2007.
http://www.osti.gov/em52/2000projsum/65370.pdf.
Fryxell, GE, H. Wu, Y. Lin, W.J. Shaw, J.C. Birnbaum, J.C. Linehan, Z. Me, K.
Kemner, S. Kelly. 2004. Lanthanide selective sorbents: self-assembled monolayers on
mesoporous supports (SAMMS). Journal of Materials Chemistry. 14 (200): 3356-3363.
Lin, Y., G. Fryxell, H. Wu, M. Engelhard. 2001. Selective Sorption of Cesium Using
Self-Assembled Monolayers on Mesoporous Supports. Environmental Science
Technology. 35(19): 3962 -3966.
Lin, Y., S.K Fiskum, W. Yantasee, H. Wu, S.V. Mattigod, G.E. Fryxell, K.N. Raymond,
J.Xu. 2005. Incorporation of Hydroxypyridinone (HOPO) Ligands into Self-Assembled
Monolayers on Mesoporous Supports (SAMMS) for Selective Actinide Sequestration.
Environmental Science and Technology. 39:1332-1337.
Mattigod, S. V., Feng, X., Fryxell, G. E., Liu, J., Gong, M., Ghormley, C., Baskaran, S.,
Me, Z., Klasson, K. T. 1997. Mercury Separation from Concentrated Potassium
Iodide/Iodine Leachate Using Self-Assembled Mesoporous Mercaptan Support
(SAMMS) Technology. PNNL-11714. Prepared for the U.S. DOE, Washington DC.
Mattigod, S.V., R. Skaggs, G.E. Fryxell. 2003a. Removal of Heavy Metals from
Contaminated Waters Using Novel Nanoporous Adsorbent Materials. PNWD-SA-5955,
Battelle, Pacific Northwest Division, Richland, WA.
Mattigod, S. V., G. E. Fryxell, R. J. Serne, K. E. Parker, F. M. 2003b. Evaluation of
Novel Getters for Adsorption of Radioiodine from Groundwater and Waste Glass
Leachates. RadiochimicaActa. 91: 539-545.
Pacific Northwest National Laboratory (PNNL). R&D100 Awards. Website. Accessed
October, 2007. http://www.pnl.gov/about/rd 1 OOawards.asp.
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Self Assembled Monolayers on Mesoporous Supports (SAMMS). 2006. Website.
Accessed October, 2007. http://samms.pnl.gov.
Steward Environmental Solution (SES). 2007. Website. Accessed November 2007.
http://www.stewardsolutions.com.
Yantasee W., G.E. Fryxell, Y. Lin, H. Wu, K.N. Raymond, J. Xu. 2005.
Hydroxypyridinone (HOPO) Functionalized Self-Assembled Monolayers on Mesoporous
Supports (SAMMS) for Sequestering Rare Earth Cations. Journal of Nanoscience and
Nanotechnology. 5(4): 527-535.
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2.4 Membranes: Nanofiltration and Affinity
2.4.1 Background
The term "membrane" covers a wide range of processes, including those used for gas/gas,
gas/liquid, liquid/liquid, gas/solid, and liquid/solid separations. Membrane technology is
well-established and is used in many processes. Membrane production is a large-scale
operation. Membranes are used for the separation of radionuclides as well as other
contaminants. This section discusses three nanotechnologies that are often used in the
filtering processes and show great potential for applications in remediation.
1. Nanofiltration (and its sibling technologies: reverse osmosis, ultrafiltration, and
microfiltration), is a fully-developed, commercially-available membrane
technology with a large number of vendors. Nanofiltration relies on the ability of
membranes to discriminate between the physical size of particles or species in a
mixture or solution and is primarily used for water pre-treatment, treatment, and
purification (Van der Bruggen and Vandecasteele 2003, Rautenbach and Groeschl
1990, Rautenbach et al. 1997, Atkinson 2002, Costa and de Pinho 2006). There
are approximately 600 companies worldwide offering membrane systems.
2. Electrospinning is a process utilized by the nanofiltration process, in which fibers
are stretched and elongated down to a diameter of about 10 nm. The modified
nanofibers that are produced are particularly useful in the filtration process as an
ultra-concentrated filter with a very large surface area. Studies have found that
electrospun nanofibers can capture metallic ions and are continually effective
through re-filtration.
3. Surface modified membrane is a term used for membranes with altered makeup
and configuration, though the basic properties of their underlying materials
remain intact.
With the global water market valued at over $300 billion, and water supplies becoming
an important international issue, nanofiltration is anticipated to have a very important role
in the future. In contrast to the types of filtration that rely on size discrimination, affinity
membranes use chemical recognition between the components of the membrane and
components of solution to effect separation. Affinity membranes and related technologies
are well-established and find their primary markets in the biomedical and biotechnology
industries.
2.4.2 Nanofiltration Membranes
Nanofiltration is one of a group of similar membrane processes (including reverse
osmosis, ultrafiltration, and microfiltration) used to separate components of a liquid
mixture. These four processes are best understood together and as a continuum in terms
of the size of particles that can be removed from a mixture.
-------
In the regular particle filtration of a liquid containing entrained particles, the liquid
mixture is forced (by gravity or applied pressure) through a filter medium that has pores
or passages of a size that allows the liquid and small particles to pass through, but
prevents passage of larger particles. A paper coffee filter is an example of a regular
particle filtration. The paper filter allows passage of the water containing dissolved or
extremely small material, but prevents passage of the larger coffee grounds. Smaller
pores or passages in the filter medium prevent larger particles from passing through with
the liquid. Figure 5 provides a graphical representation of the generalized filtration
process.
0o % m •
' n n 9 £> o
Figure 5. General representation of the filtration process
There are two basic types of filters: depth filters and membrane filters. Depth filters have
a significant physical depth and the particles to be retained are captured through out the
depth of the filter. Depth filters often have a labyrinthine three-dimensional structure,
with multiple channels and heavy branching so that there is a large pathway through
which the liquid must flow and by which the filter can retain particles. Depth filters have
the advantages of low cost, high throughput, large particle retention capacity, and the
ability to retain a variety of particle sizes. However, they can suffer from entrainment of
the filter medium, uncertainty regarding effective pore size, some uncertainty regarding
the overall integrity of the filter, and the risk of particles being mobilized when the
pressure differential across the filter is large.
The second type of filter is the membrane filter, in which depth is not considered
important. The membrane filter uses a relatively thin material with a well-defined
maximum pore size and the particle retaining effect takes place almost entirely at the
surface. Membranes offer the advantage of having well-defined effective pore sizes, can
be integrity tested more easily than depth filters, and can achieve more filtration of much
smaller particles. They tend to be more expensive than depth filters and usually cannot
achieve the throughput of a depth filter. Filtration technology has developed a well-
defined terminology that has been well addressed by commercial suppliers; the American
Standard Test Method (ASTM) has developed a standard (ASTM D6161-05) (ASTM
-------
2007) that addresses the terminology used for microfiltration, ultrafiltration,
nanofiltration, and reverse osmosis membrane processes.
Reverse osmosis, nanofiltration, ultrafiltration, and microfiltration are all membrane
processes able to remove small particles or soluble species. They all work on exactly the
same principle as regular particle filtration, but the distinguishing feature between them is
their effective pore size, and thus, the minimum size of particle that will be rejected by
the membrane; reverse osmosis membranes reject all but the very smallest species (small
soluble organic species that are not otherwise considered even to be "particles"), while
microfiltration allows considerably larger particles to pass through. Figure 6 provides a
graphic representation of the process characteristics of the four membrane processes, and
Figure 7 provides a graphic representation showing the size range and approximate
molecular weight range where each of the four membrane filtration technologies finds its
application.
Regular particle filtration will reject particles down to about the one micron (1,000 nm)
size range. If filtration to reject particles smaller than this limit is required, microfiltration
should first be considered. Microfiltration operates at the low end of familiar particle
filtration, being at the limit able to retain particles above the 100 nm size range. Its
separating abilities thus cease at the 100 nm limit usually associated with
nanotechnology. Microfiltration can remove most suspended solids and living material,
such as bacteria, but will not retain any type of dissolved solute or smaller biological
material, such as viruses. Ultrafiltration is effective only to the 10 nm size range,
allowing it to reject most proteins, viruses, and groundwater colloids. Nanofiltration is
usually used to remove material in the 0.5-10 nm range and will reject most of the larger
organic molecules, sugars, and multivalent ions, with only monovalent ions and water
being able to pass through. To remove the monovalent ions, reverse osmosis membranes
are required; these will reject almost all material, except water and simple organic species
(such as very short chain alcohols and acids).
Microfiltration
Ultrafiltration
Nanofiltration
Reverse Osmosis
Water Monovalent MuUsifsk'nt
ISM le>M
Water Mofiovalent MutUvalem Vir
ioiH loni
Figure 6. Membrane process characteristics
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100 2DD 1,000 20,000 100,000 500,000 IMP/ 5 MM
Note: 1 micron (micromotor) -4x10-5 inches - 1 x 104 Angstrom unite © 2004 - Koch Memftrane Systems
Figure 7. Size range for filtration separation processes
Benefits of water treatment nanofiltration include:
• Lower operating costs,
• Lower energy costs,
• Lower discharge and less wastewater than reverse osmosis,
• Reduction of total dissolved solids (TDS) content of slightly brackish water,
• Reduction of pesticides and VOCs (organic chemicals),
• Reduction of heavy metals,
• Reductions of nitrates and sulfates,
• Reduction color, tannins, and turbidity,
• Hard water softening,
• Being chemical-free (i.e., does not use salts or chemicals), and
• Water pH after nanofiltration is typically non aggressive.
For radionuclide treatment applications, nanofiltration and ultrafiltration have been
investigated as an ultra low-level analytical tool to separate actinides from other ionic
species in high-level radioactive waste solutions, and as a possible treatment option for
waste streams from the Los Alamos National Laboratory Plutonium Treatment Facility
(Smith 1993). In these applications, the nanofiltration and ultrafiltration membranes are
coupled with water-soluble chelating polymers (WSCP). WSCPs are polymers
engineered to contain both highly selective chelating functionalities to bind with targeted
metal ions, and solubilizing functionalities to allow the polymer to dissolve in water
(Smith et al. 1995). The polymer's overall size is large enough that it exceeds the
rejection limit for an ultrafiltration membrane. When the unchelated polymer is
introduced into a solution that contains the target ions for which the chelating groups
-------
were designed, the polymer binds with these target ions and nothing else. The chelated
polymer can then be separated from the solution (and all other ions in the solution) by
ultrafiltration; the chelated target ions can be separated from the polymer by adjusting the
solution chemistry; and the regenerated water-soluble chelating polymer can be recycled
(Jarvinen 1995). In these applications, ultrafiltration combined with WSCP has the
advantage of being aqueous-based (such as ion exchange resins), has a high throughput
and rapid kinetics (like two-phase liquid-liquid extraction systems), but does not have the
disadvantage of using organic solvent-based extractants.
WSCPs with molecular weights in the 100,000 range have been prepared based on
polyacrylic acid or polyethyleneimine with functional groups of phosphonic acid,
acylpyrazolone acid, and hydroxamic acid. In acidic nitrate and acidic chloride solutions,
these functional groups have a high affinity for actinides and a low affinity for alkali
metals and alkaline earths. The concentration of polymer is typically 1-2%, with the
ultrafiltration membrane typically having 10-100 |im pores (Gibson 1994, Smith 1993).
This technology has been tested at bench scale for the removal of heavy metal ions from
electroplating waste streams. Development continues (Moreno-Villoslada and Rivas
2002, Rivas et al. 2003, Rivas et al. 2006, Tomida et al. 1994, Kawano et al. 2002),
though dendrimers may compete as a chelating moiety in the future if their costs
decrease.
Summary of Environmental Potential
Nanofiltration is an established technology with numerous equipment developers,
manufacturers, and vendors. The major application is in reverse osmosis for desalination
and production of high purity water for specialized industrial uses, with small
applications for wastewater treatment. While current and past development has relied
predominantly on size exclusion effects, the promise of nanotechnology is that specific
chemical and physical behavior can be engineered into the materials, and that the slow
flow rates typical of reverse osmosis and nanofiltration can be overcome—allowing the
energy requirements and production costs for the production of pure water to be greatly
reduced. The general industrial uses of filtration technology should support developments
that will allow spin-off applications for environmental uses; a significant effort is being
made to apply nanotechnology developments of filtration technology to the production of
potable water in economically challenged arid regions. The primary environmental
applications are likely to be "end-of-pipe" and polishing uses. Nanofiltration combined
with water-soluble chelating polymers has been investigated for the removal of
radioactive species from aqueous waste streams (Smith 1993, Smith et al. 1995).
2.4.4 Electrospun Fibers
Electrospinning is a process for making nanofibers with diameters down to about 10 nm.
The technology produces the nanofibers from polymer solutions or melts, with the
extreme elongation and narrowing of the fiber occurring as a result of electrostatic
repulsion. Electrospinning has the characteristics of both the commercial electrostatic
spraying technique and the commercial spinning of fibers, each of which is a long
established technology in its own right. Electrostatic spraying is a coating technology,
-------
over a century old, involving atomizing a liquid from a spray gun and imparting an
electrical charge to the droplets by using an electrode placed at the end of a spray gun.
The electrode typically operates in the 30 to 140 kilovolt range with a current of 0 to 225
microamperes. The droplets are attracted toward a target that is either grounded or has a
positive charge, and produces a uniform coating of liquid (frequently paint). The
electrostatic spraying approach can also be used as a surface modification technique to
introduce nanoparticles onto fibers or other surfaces by spraying solutions that contain
only about one particle per droplet.
In comparison, electrospinning (first patented by Antonin Formhals in 1934) uses the
electrical charge to form a mass of fine polymer filaments. In electrospinning, a polymer
solution or melt is driven through a spray gun nozzle and forms a droplet at the tip. When
the voltage is applied to the nozzle, the droplet is stretched; if the viscosity of the material
is sufficiently high, the breakup encountered in electrostatic spraying does not occur.
Instead, a thin, charged liquid jet is formed. The jet elongates and is whipped
continuously by electrostatic repulsion, forcing it to follow a spiral path toward the
oppositely charged or grounded collector. The whipping action helps to elongate the jet
much further to a diameter on the order of tens of nanometers. Figure 8 illustrates the
basic process. Fibers are formed either by the melt cooling or the solvent evaporating
from the polymer solution. Nanoparticles can be mixed into, or produced directly in, the
polymer solution and spun with the fibers (Lee et al. 2005). Nanofibers produced in this
way have many potential applications, including high efficiency filter media, as
nanocomposite materials for water treatment membranes, catalysis, hydrogen storage, or
in biomedical applications (such as drug release carriers or artificial tissue).
Precursor
Pendant drop of polymer
Power supply
Jet initiation and
extension
Bending instability and
further elongation
Solidification of the jet
into fibers
Figure 8. Basic principle of electrospinning
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Investigations into the potential of electrospun fibers for water treatment have mainly
focused on their use in membranes, where the massive surface area (illustrated in Figure
9) offers special advantages. Ki et al. (Ki et al. 2007) examined membranes of a
nanofibrous blend of silk fibroin and wool keratose and found that it had an exceptional
performance for the adsorption of metal ions, that its adsorption capacity was maintained
after several desorption and re-adsorption cycles, and that it is very suitable for removing
and recovering heavy metal ions from water, potentially as an affinity membrane.
Ramakrishna et al. (Ramakrishna et al. 2006) reviewed electrospun nanofibers from the
perspective that their porous nature and large surface-to-volume ratio gives them the
potential for use in various applications where high porosity is desirable. They noted that
electrospinning has an advantage of comparatively low cost with relatively high
production rate, that the unique ability to produce nanofibers of different materials in
various fibrous assemblies, and that the ability to form porous fibers means that the
surface area of the fiber mesh can be increased tremendously. Since the fibers can be
surface functionalized, affinity membranes can easily be formed with the potential for
application in the removal of heavy metals from wastewater. They concluded that
electrospinning may well be one of the most significant nanotechnologies of the 21st
century.
Figure 9. An electrospun polysulphone membrane: (a) surface;
(b) cross-section; and (c) magnified cross-section images
Sang et al. (Sang et al. 2008) examined various filtration modes for using an electrospun
chloridized polyvinyl chloride nanofiber membrane (including static adsorption, direct
filtration, soil-addition filtration, diatomite-addition filtration, and micellar enhanced
filtration), and they concluded that the membrane, when used with micellar enhanced
filtration, can be used for the treatment of the groundwater containing heavy metals (such
as copper, lead, and cadmium) with high efficiency.
2.4.5 Surface Modified Membranes
Electron-beam-induced grafting is one of two methods in the category of ionizing
radiation—gamma radiation from cobalt-60 is the other. Electron-beam-induced
technology provides a pathway for customizing surfaces such as membranes; hence, the
commercial name eMembranes (developed by the Japan Environmental Purification
Research Institute). The technology allows membrane surface alternation without
-------
changing the basic properties of the underlying bulk materials. Electron-beam machines
are currently available and play a significant role in processing specialty polymeric
materials. For example, electron-beam-induced grafting has existed in some form for
decades; nuclear power plants have used ion exchange resins enhanced by electron-beam-
induced grafting to cleanse uranium from coolants. The future materials generated with
this technology is based on over 50 years of scientific research. The technology is
stimulating the development of new and promising membranes. Electron-beam-induced
grafting is being used by laboratories throughout the world to create surfaces, including
chemical resistance, wet ability, biocompatibility, antithrombo, dyability, and antistatic
properties. Grafting has also been used to produce ion exchange membranes for the
removal of heavy-metal for aqueous waste streams. An emerging application is biological
separations, but the basic concept also has targeted the removal of heavy metals from
industrial wastewaters. There is little doubt that many new and improved products will be
created to aid in the cleanup of radionuclides.
eMembrane technology is being extended to create specialized membranes for many
potential applications. The ability to create specialized nano-engineered membranes
opens up many envisioned possible future applications. An example of developing
applications is a new water remediation technology, which could remove—in one pass—
multiple contaminants such as viruses, radionuclides, heavy metals, and chlorinated
solvents. A membrane could be tailored for a specific cleanup application. This would
greatly reduce remediation costs and accelerate the cleanup process.
The following example of eMembrane development gives an indication of the broad and
varied possibilities for developing customized membranes and material surfaces, and
draws heavily from company provided literature. eMembrane's technology involves the
nano-grafting of polymer chains containing selective binding functionalities. This
technology can impart new and multiple polymeric material on existing materials and
membranes. At the heart of the technology is the technique of electron-beam induced
polymer grafting. Figure 10 provides a graphical representation of the technique showing
the attachment of polymeric material.
x>>>>x>> «.
nano-grafting -
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An electron beam irradiates a base membrane that can be made from any polymeric
material. Other radiation sources, such as gamma rays, can also be used for similar
radiation induced polymer grafting (Ramakrishna et al. 2005). The electron beam
irradiation generates active species, or radicals, on the surface of the polymer. The active
species are the sites at which other long polymer chains or brushes can be attached. The
polymer brushes may contain the binding functionalities at the time of attachment, or
they may be chemically functionalized after attachment has occurred. Electron beam
irradiation is seen as an advantageous method of surface modification since it can occur
after the basic polymer has been fabricated and does not affect the shape; by varying the
electron beam energy and other process characteristics, the depth and degree of
functionalization can be controlled.
eMembrane's electron-beam technology permits a density of polymer brush attachment
that is extraordinary—up to 10 trillion polymer brushes per square centimeter can be
attached giving a surface spacing of about 4 nm between each brush, a density far
exceeding any other technique. The brushes can range from 10 to 300 nm long and can
contain a variety of functionalities, allowing the production of highly tailored separation
material. For example, a microporous membrane with grafted functionalized polymer
brushes not only performs microfiltration (by molecular size cutoff), but the functional
groups on its brushes can also simultaneously capture and remove toxic metal ions,
soluble proteins, viruses, or cells from the filtrate. In effect, it has become an affinity
membrane (Ofsthun et al. 1999, Zou et al. 2001, Nasef 2004, Klein 1991). Materials in a
variety of shapes (e.g., film, hollow fiber, non-woven cloth, etc.) are grafted with
polymer brushes that extend off the surface of the starting material. Figure 11 provides a
graphical representation of the functionalized polymer surface with a variety of brushes
attached to facilitate a number of separations.
P O LYME R B RUS H
POLYMER SURFACE;
Figure 11. eMembrane-functionalized surface
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Environmentally-related applications currently under investigation by eMembrane, Inc.
include:
Ion
• Removal, collection of heavy metal ions from fluids,
• Ultra-pure water production
• Immobilized metal affinity membrane
Gas
• Removal of odors or toxic gases via neutralization
Biological
• Tools for proteomics
• Display of multilayer of various proteins for protein-protein interaction studies
• Immobilized enzymes for proteomics, biosensors, and bioreactors
• Large-scale biologies purification
Cell
• Immobilized cells for proteomics studies
• Removal of bacteria
• Bioreactors, artificial organs
Radiation cross-linking of naturally occurring polymers, such as polysaccharides, is being
investigated for the production of biodegradable hydrogel/nanogel. This new
nanomaterial consists of individual macromolecules that are internally cross-linked by
irradiation ionizing radiation.
Specifically modified nanogels are being studied for wastewater filtration. Gels
containing acid groups have to bind ions, including uranium and nickel, for use in the
removal of metals from aqueous media. These gels can be formulated and customized to
remove most contaminants found in water.
No fate and transport studies have been conducted, nor are anticipated, since this is a
process unit rather than a material. Because the technology is in the development state, no
operational or maintenance parameters have been determined, and no information on
impacts, hazards, efficacy, limitations, and waste management approaches is available.
One of the greatest, unknown concerns is what could happen if the fibers dislodge from
the surface and enter the drinking water supply.
2.4.6 References
ASTM International. 2007. Standard Terminology Used for Microfiltration,
Ultrafiltration, Nanofiltration and Reverse Osmosis Membrane Processes. Website.
Accessed October 2007.
http://www.astm.org/cgibin/SoftCart.exe/DATABASE.CART/REDLINE PAGES/D616
1 .htm?E+mystore.
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Atkinson, S. 2002. Nanofiltration concentrates coloured wastewater and produces potable
water. Membrane Technology. 2002(7): 11-12.
Costa, A.R. and M.N. de Pinho. 2006. Comparison of the performance of ultrafiltration
and nanofiltration in surface water treatment. Desalination. 199(1-3): 73-75.
Gibson, R.R., B.F. Smith, G.D. Jarvinen, T.W. Robinson, R.E. Barrans, K.V. Wilson.
1994. The Use of Water-Soluble Chelating Polymers as an Analytical Method for the
Separation of Actinides in Aqueous Systems. Presentation at the Eighteenth Annual
Actinide Separations Conference, Durango, CO, May 23-26, 1994.
Jarvinen, G.D., R.E. Barrans, N.C. Jr., Schroder, K.L. Wade, M.M. Jones, B.F. Smith,
J.L. Mills, G. Howard, H. Freiser, S. Muralidharan. 1995. Selective Extraction of
Tetravalent Actinides from Lanthanides with Dithiophosphonic Acids and
Tributylphosphate. In Separations of Elements. K.L. Nash and G.R. Choppin, eds.
Plenum Press, New York, 43-62.
Kawano, K., K. Hamaguchi, S. Masuda, T. Tomida. 2002. Binding Properties of a Water-
Soluble Chelating Polymer with Divalent Metal Ions Measured by Ultrafiltration. Poly(K-
acethylaminoacrylic acid). Industrial and Engineering Chemical Research. 41(20): 5079-
5084.
Ki, C.S., E.H. Gang, 1C Urn, Y.H. Park. 2007. Nanofibrous membrane of wool
keratose/silk fibroin blend for heavy metal ion adsorption. Journal of Membrane Science.
302(1-2): 20-26.
Klein, E. 1991. Affinity Membranes: Their Chemistry and Performance in Adsorptive
Separation Processes. Wiley-Interscience, Hoboken, NJ.
Lee, H.K., E.H. Jeong, C.K. Baek, J.H. Youk. 2005. One-step preparation of ultrafine
poly(acrylonitrile) fibers containing silver nanoparticles. Materials Letters. 59(23):2977-
2980.
Moreno-Villoslada I. and B.L. Rivas. 2002. Metal ion enrichment of a water-soluble
chelating polymer studied by ultrafiltration. Journal of Membrane Science. 208(1-2): 69-
73.
Nasef, M.M. 2004. Application of Electron Beam for Preparation of Membranes. Nippon
Genshiryoku Kenkyujo JAERI, Conf Journal. 37-54.
Ofsthun, N. J., P.J. Soltys, G.A. Kunas. 1999. Affinity membrane system and method of
using same. U.S. Patent No. 5,871,649.
Ramakrishna, S. W.-E. Teo, T.-C. Lim (Eds). 2005. Introduction to Electrospinning and
Nanofibers. World Scientific Publishing Company, Singapore.
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Ramakrishna, S., K. Fujihara, W.-E. Teo, T. Yong, Z. Ma, R. Ramaseshan. 2006.
Electrospun nanofibers: solving global issues. Materials Today. 9(3): 40-50.
Rautenbach R. and A. Groeschl. 1990. Separation Potential of Nanofiltration Membranes.
Desalination. 77(1-3): 73-84.
Rivas, B.L., E.D. Pereira, I. Moreno-Villoslada. 2003. Water-soluble polymer-metal ion
interactions. Progress in Polymer Science. 28(2): 173-208.
Rivas, B.L., S.A. Pooley, E.D. Pereira, A. Maureira. 2006. Water-Soluble
Polyelectrolytes with Metal Ion Removal Ability by Using the Liquid Phase Based
Retention Technique. World Polymer Congress-MACRO 2006. 245-246(1): 116-122.
Sang, Y., F. Li, Q. Gu, C. Liang, J. Chen. 2008. Heavy metal contaminated groundwater
treatment by a novel nanofiber membrane. Desalination. 223:349-360.
Smith B.F. 1993. Actinide separations for advanced processing of nuclear waste: Annual
Report 1993. Report LA-UR-93-4017, Los Alamos National Laboratory.
Smith, B.F., T.W. Robinson, J.W. Gohdes. 1995. Water-Soluble Polymers and
Composition Thereof. U.S. Patent DOE No. S-78, 350.
Tomida T., T. Inoue, K. Tsuchiya, S. Masuda. 1994. Concentration and/or removal of
metal ions using a water-soluble chelating polymer and a microporous hollow fiber
membrane. Industrial and Engineering Chemistry Research. 33:904-906.
Van der Bruggen B. and C. Vandecasteele. 2003. Removal of pollutants from surface
water and groundwater by nanofiltration: overview of possible applications in the
drinking water industry. Environmental Pollution. 122(3): 435-445.
Zou, H., Q. Lou, D. Zhou. 2001. Affinity membrane chromatography for the analysis and
purification of proteins. Journal of Biochemical and Biophysical Methods. 49(1-3):199-
240.
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2.5 Zeolites
2.5.1 Introduction
Zeolites are a well-established technology used in a range of processes and industries.
Zeolites are not new materials—they have been investigated for over two and a half
centuries, with stilbite and natrolite both being identified in the 1750s. Industrial
applications include catalysis in the petroleum industry (Venuto and Dekker 1979, Chen
et al. 1994), various uses in agriculture (St. Cloud 2007, BRZ Zeolite 2007, Zeolite
Australia 2007), horticulture (ZeoPro 2007), gas separations (Yang 1994, Kerry 2007,
Kanellopoulos 2000), domestic water treatment (McKetta 1999, Kawamura 2000, Faust
1998); and nuclear waste processing (Auerback et al. 2003, Choppin and Khankhasayev
1999). The value of zeolite catalysis to petroleum cracking is well in excess of $200
billion (MassNanoTech 2007). About 50 naturally occurring zeolites have been
identified; over 150 synthetic zeolites have been prepared and characterized; and further
thousands of combinations of framework and composition are available (MassNanoTech
2007, Baerlocher and McCusker 1996). Zeolites have long been used in the nuclear
industry (Auerback et al. 2003, Choppin and Khankhasayev 1999) owing to their
properties as ion exchangers. The planned siting of the United States' first deep geologic
radioactive waste repository at Yucca Mountain in Nevada, where design philosophy
called for both engineered and natural barriers (Ahn et al. 1981) to inhibit the transport of
any potentially leaking radionuclides, was influenced considerably by the local
abundance of the natural zeolites mordenite and clinoptilolite, both of which have large
cationic exchange capacities.
2.5.2 Description
Zeolites are crystalline aluminosilicates, compositionally similar to clay minerals, but
differing in their well-defined three-dimensional nano- and micro-porous structure.
Aluminum, silicon, and oxygen are arranged in a regular structure of [SiC^]" and [AlO/t]"
tetrahedral units that form a framework with small pores (also called tunnels, channels, or
cavities) of about 0.1-2 nm diameter running through the material. Figure 12 shows a
representation of a typical zeolite framework. It should be clearly noted that this is just
one of a large and growing number of types of zeolite framework. In 1970, the Atlas of
Zeolite Framework Types (Baerlocher and McCusker 1996) listed 27 known frameworks,
but by 2003, the number had grown to 145. The variety of size and shape available for
the pore structure is the source of zeolites' catalytic activity that is so important to the
petrochemical industry.
A second consequence of the framework being built from negatively charged units is that
it possesses a net negative charge that must be balanced by the presence of positively
charged cations. Most naturally occurring zeolites have the environmentally predominant
sodium ion as a loosely bound counter ion. These can be readily displaced by other ions
for which a particular framework has a much greater affinity, thus giving zeolites
significant ion exchange properties.
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Figure 12. A typical zeolite structure
It should be noted that the ion exchange and the pore size properties of zeolites are
partially linked. When the zeolite is in the sodium form (i.e., it has positively charged
sodium ions balancing the net negative charge on the aluminosilicate framework), the
sodium ions are associated with the tetrahedral aluminum or silicon atoms at the entrance
to the pores and, because of their finite size, they effectively reduce the diameter of the
pore opening slightly. If the sodium ions are replaced by potassium ions, which are larger
than the sodium ions, then the opening of the pore is effectively reduced even further.
This behavior permits a degree of control over the size of material that can enter the
pores.
Zeolites are usually aluminosilicates, but other tetrahedral atoms such as phosphorus,
gallium, germanium, boron, and beryllium can exist in the framework as well.
2.5.3 Potential Applications
General Applications. Zeolites have a wide range of commercial uses (InterSun 2007),
including:
Aquaculture
• Ammonia filtration in fish hatcheries
• Biofilter media
Agriculture
• Odor control
• Confined animal environmental control
• Livestock feed additives
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Horticulture
• Nurseries, greenhouses
• Floriculture
• Vegetables/herbs
• Foliage
• Tree and shrub transplanting
• Turf grass soil amendment
• Reclamation, revegetation, and landscaping
• Silviculture (forestry, tree plantations)
• Medium for hydroponic growing
Household Products
• Household odor control
• Pet odor control
Industrial Products
• Absorbents for oil and spills
• Gas separations
Environmental Applications. Although environmental applications of zeolites are small
compared with applications of their catalytic properties, considerable research and some
implementations have taken place including:
Radioactive Waste
• Site remediation/decontamination
Water Treatment
• Water filtration
• Heavy metal removal
• Swimming pools
Wastewater Treatment
• Ammonia removal in municipal sludge/wastewater
• Heavy metal removal
• Septic leach fields
Pasini (Pasini 1996) has described the state-of-the-art technology with regard to the use
of natural zeolites in the protection of the environment. He focuses on the possible cation
exchange procedures and principles that can be operated at an industrial level; the
removal of NH4+ from municipal and industrial wastewater; the possibilities for use of
natural zeolites for removal of heavy metals from water after laboratory experiments; and
how chemical and structural features make zeolites a powerful tool for the
decontamination of waters containing radionuclides. The compendium by Misaelides et
al. (1999) dealt with general environmental applications and contained much information
on the use of zeolites as radionuclide sorbents (Macasek 1999, Bish 1999, Rajec et al.
1999, Colella 1999), including consideration of the sorption and leaching properties of
the composites and complexes of natural microporous materials; investigation of natural
zeolites and nuclear waste management in the case of Yucca Mountain, Nevada; the
sorption of heavy metals and radionuclides on zeolites and clays; and environmental
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applications of natural zeolitic materials based on their ion exchange properties. As an
example of the depth to which these studies can go, Komarneni (Komarneni 1985)
investigated the use of one zeolite, phillipsite, in cesium decontamination and
immobilization, citing:
"The use of zeolites such as clinoptilolite in nuclear waste decontamination is a
common practice (IAEA 1972). Zeolites and zeolitic tufts have also been used to
decontaminate low and intermediate-level liquid nuclear wastes (Mercer and
Ames 1978) and to separate 37Cs from high-level radioactive defense wastes at
Hanford, Washington (Nelson and Mercer 1963, Brandt 1970, Buckingham
1970). Zeolitic ion ex-changers, such as lonsiv IE-95 (USNRC 1980) which
consists of a mixture of natural chabazite and erionite from Bowie, Arizona and
lonsiv IE-96 (which consists of chabazite) + Linde A-51 (Hofstetter and Hitz
1983) are currently used to clean up accident wastewater at the Three Mile
Island-Unit II reactor, Middletown, Pennsylvania. "
Radionuclide Applications. Zeolites are one of the few nanotechnologies that have been
investigated for environmental remediation purposes. Because of their ion exchange
properties, and the fact that they are a seemingly benign natural product that can bring
certain improvements (such as increasing the soil cation-exchange capacity and soil
moisture, improving hydraulic conductivity, increasing yields in acidified soils, and
reducing plant uptake of metal contaminants) to soil properties (Allen and Ming 1995),
zeolites have been examined for their ability to remediate heavy metals in soil (Weber et
al. 1984). Based on this work, Campbell and Davies (Campbell and Davies 1997)
performed an experimental investigation of plant uptake of cesium from soils amended
with clinoptilolite and calcium carbonate.
The origin of this work was the observation that radioactive cesium (137Cs) from the
Chernobyl accident of 1986 has unexpectedly remained in a bioavailable form in upland,
sheep-grazing soils of Great Britain. As a potential remedial measure, the zeolite
clinoptilolite was tested in a greenhouse pot experiment for its effectiveness in selectively
taking up cesium from two British soils: a lowland loam and an upland peat. Rye-grass
grown on 10% clinoptilolite-treated soils resulted in grass leaf tissue cesium
concentrations below 30 mg Cs kg"1 grass in all cases. Where no clinoptilolite had been
added, cesium in grass leaf-tissue reached 1,860 mg kg"1 in rye grown on peat and 150
mg kg"1 in rye grown on loam. In contrast, the addition of calcium carbonate to the Cs-
treated, clinoptilolite-free peat soil enhanced the grass concentration of Cs by
approximately five times, but this effect was not observed with the concentration of Cs in
grass grown from loam soils with the same treatments.
However, despite this apparent beneficial result of adding the zeolite, adverse side effects
were observed. Since the zeolite is in the sodium form, sodium ions are released and the
risk of sodium toxicity to plants increases as cation exchange proceeds. Further, since
clinoptilolite binds heavy metals in general, essential heavy metals (such as zinc) would
be markedly decreased by the application of zeolite, which in turn could result in
deficiency problems in animals. It was also noted that since grazing animals consume a
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considerable amount of soil in their diet, the consumption of radionuclide-laden zeolites
could itself bring risks.
In general, it has been noted that the main research behind the use of natural zeolites as a
remediation tool for contaminated soil has been conducted largely through laboratory and
greenhouse trials. There is very little evidence in the literature to support the long-term
use of natural zeolites in real remediation projects (Stead et al. 2000). It was also noted
that the future potential of using zeolites has not been fully appreciated, and that there is
an urgent need to undertake field trials and evaluate the in-situ efficiency for these
remediation purposes.
Since zeolites are natural materials and are mainly used in industrial processes, little
research is focused on their fate and transport, though an extensive volume of work exists
on their geological origin and behavior. Extensive data exists on operation and
maintenance parameters. As would be expected, specific details are highly dependent on
waste streams involved. Three references (IAEA 1967, IAEA 1984, IAEA 2002)
discussed below provide an excellent overview of the issues involved.
2.5.4 Impacts, Hazards, Efficacy, and Limitations
Zeolites are a bulk commodity. World production is on the order of 4 million tons per
year, with China producing and using about 2.5 million tons (primarily as a low-grade
additive to pozzolan cement); U.S. consumption is about 0.5 million tons. The primary
industrial use is as a petrochemical catalyst and the second largest use is as a detergent
builder. Thus, the use of zeolites in radionuclide remediation would be expected to have
little impact. Most zeolites, particularly those with current widespread uses, are regarded
as a safe material; they are currently being marketed as a health food and references to
their medicinal use date back thousands of years. Zeolites are also used as a feed additive
for cattle, pigs, chicken, and fish. It should be noted, however, that one zeolite, erionite,
is regarded as a carcinogen due to its fibrous nature and high iron content.
Regarding efficacy, though zeolites have had limited uses in environmental remediation
outside of their use in the nuclear industry as an ion exchanger for liquid radioactive
waste management, they are seen as having significant potential. Even the drawbacks
mentioned in the work of Campbell and Davies (Campbell and Davies 1997) (discussed
in Section 2.5.3) should be surmountable. To eliminate the sodium toxicity risk to soil,
the zeolite could be preconditioned into the ammonium form, which would likely lead to
plant growth improvements. Overcoming the concern of nutritionally important soil
nutrients binding together would require that the zeolite used (possibly synthetic) would
be designed to have a very high specificity for the target radionuclide and little else.
Alternatively, soil quality could easily be monitored and appropriate amendments made.
2.5.5 Management of Zeolite Wastes
Ion exchange in general is one of the most well-developed, common, and effective
treatment methods for liquid radioactive waste, and is widely used in the nuclear
industry. Zeolites are a large component of the inorganic ion exchangers used and an
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extensive amount of literature exists on various aspects of their treatment and disposal.
Among the best overviews of this technology base are the series of technical reports
issued by the International Atomic Energy Agency (IAEA) in 1967, 1984 and 2002
(IAEA 1967, IAEA 1984, IAEA 2002). The information below draws largely from the
most recent of these reports (IAEA 2002).
Prior to treatment for disposal, two pre-treatments—dewatering and size reduction—may
be needed. Dewatering is generally accomplished by pressure, vacuum filtration, or
centrifugation. If drying is needed, hot air is usually used as the drying medium with a
shallow bed of spent materials. Types of drying units include fluidized bed dryers,
vertical thin film dryers, and cone dryers.
Since inorganic ion exchange materials (such as zeolites) are generally resistant to
degradation by radiation or biological actions, they are treated by the use of direct
immobilization, or by high temperature processes (such as vitrification). The
immobilization matrices currently used include vitrification, cement, bitumen
encapsulation, polymer encapsulation, and disposal in high-integrity containers.
Vitrification has been widely evaluated for the immobilization of highly-active waste,
such as waste from the reprocessing of spent nuclear fuel, and has been evaluated for the
treatment of ion exchange resins (Jantzen et al. 1995, Cicero-Herman et al. 1998). The
excellent leach resistance property of the resulting glass waste form is the principal
advantage of vitrification. Vitrification processes are capital-intensive, and the melters
have a relatively short operational life (approximately 5 years). Vitrification processes
operate at temperatures ranging from 1100°C to 3000°C, depending on the waste
composition and glass forming additives used.
Cement immobilizing radioactive waste has been used in the nuclear industry and at
nuclear research centers for more than 40 years. Detailed descriptions of the process can
be found in references from the IAEA in 1993 and the Los Alamos National Laboratory
in 1997. Cement has many characteristics in its favor: it is readily available and widely
used in civil engineering, the raw material is inexpensive, and the processing equipment
can be based on conventional technology. The resulting waste forms are strong;
noncombustible and radiation resistant, have a high density (providing radiation
shielding), have a reasonable chemical stability, and have a moderate resistance to the
release of radionuclides. The high pH conditions typical for cement results in a low
solubility for many radionuclides by the formation of hydrolyzed species, carbonates,
etc., which provides a good resistance to leaching. The main disadvantage of the
cementation of spent ion exchange materials is that the final waste volume is high
compared with the initial volume, owing to the low waste loadings that are achievable.
The loadings can be increased by a pretreatment (such as grinding) of the spent ion
exchange materials before cementation, which improves the quality of the final cemented
products.
Bitumen is a generic term used to cover a wide range of high molecular weight
Hydrocarbons. Bitumen encapsulation is currently not used in the United States for
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disposal, although several bitumen varieties are commercially available overseas for the
immobilization of radioactive waste, including spent ion exchange resins. The main
characteristics that make bitumen suitable as a matrix material are:
• Its insolubility in water;
• Its high resistance to the diffusion of water;
• Its chemical inertness;
• Its plasticity and good rheological properties;
• Its good aging characteristics;
• Its high incorporation capacity, which leads to high volume reduction factors; and
• Its instant availability at a reasonable cost.
However, since it is an organic material, bitumen has the following disadvantages:
• It decreases in viscosity as a function of temperature, leading to a softening of the
matrix, which melts at temperatures of about 70°C;
• It is combustible, although not easily flammable (the flash point and flammability
temperatures are higher than 290°C to 350°C, depending on the type of bitumen);
• It has a lower stability against radiation than cement, especially under the higher
radiation fields often associated with spent ion exchange media; and
• It reacts with oxidizing materials such as sodium nitrate.
The immobilization of spent ion exchange resins in polymers is practiced at many
installations worldwide. Different types of polymers are used and further studies to
improve cost effectiveness, process simplicity, and product quality are being carried out
in many countries. Among the many polymers used are epoxy resins, polyesters,
polyethylene, polystyrene and copolymers, urea formaldehyde, polyurethane, phenol-
formaldehyde, and polystyrene.
2.5.6 Summary of Environmental Potential
Zeolites are a well-established technology with a variety of industrial uses ranging from
construction materials and detergent builders, to catalysts and separation agents. They are
one of the oldest separation technologies for the removal of radioactive components from
aqueous waste streams. The flexible tectonic structure and ability to be chemically
"tailored" to specific target species continues to stimulate their development. In addition
to their use as an "end-of-pipe" treatment for aqueous streams, zeolites are one of the few
materials offering the possibility of being an inexpensive amendment to soils
contaminated with radioactive species, since extremely high species selectivity and
binding strength can be designed into the material. Continued investigation of zeolites in
general is expected due to their catalytic properties; research in this area should support
further developments, potentially leading to environmental applications.
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2.5.7 References
Ahn, T.M., R. Dayal, RJ. Wilke. 1981. Evaluation of backfill as a barrier to radionuclide
migration in a high level waste repository. Technical Report BNL-NUREG-30107;
CONF-810499-2, Brookhaven National Laboratory. Website. Accessed November 2007.
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5158859.
Allen, E. R. and D. W. Ming. 1995. Recent Progress in the Use of Natural Zeolites in
Agronomy and Horticulture. Natural Zeolites '93, Occurrence, Properties, Use; D. W.
Ming and F. A. Mumpton, (Eds.); International Committee on Natural Zeolites.
Brockport, New York.
Auerback, S.M., K.M. Carrado, P.K. Dutta. 2003. Handbook of Zeolite Science and
Technology. Marcel Dekker, Inc., New York.
Baerlocher Ch. and L.B. McCusker. 1996. Database of Zeolite Structures. Website.
Accessed November 2007. http://www.iza-structure.org/databases/.
Bish, D.L. 1999. Natural Zeolites and Nuclear Waste Management; The Case of Yucca
Mountain, Nevada, USA. In Natural Microporous Materials in Environmental
Applications. P.Misaelides, F. Macasek, TJ. Pinnavaia, and C. Colella (Eds.). Kluwer
Academic Publishers, The Netherlands.
Brandt, H.L. 1970. B-plant recovery of cesium from Purex supernatant. Report ARH-
1639. United States Atomic Energy Commission, Hanford, Washington.
BRZ Zeolite. 2007. Website. Accessed October 2007.
http://www.zeolite.ca/agriculture.htm
Buckingham, J. S. 1970. Laboratory evaluation of zeolite material for removing
radioactive cesium from alkaline waste solutions. Report ARH-SA-49. United States
Atomic Energy Commission, Hanford, Washington.
Campbell, L.S., and B.E. Davies. 1997'. Plant andSoil. 189(l):65-74.
Chen, N.Y., T.F. Degnan, C.M. Smith. 1994. Molecular Transport and Reaction in
Zeolites: Design and Application of Shape Selective Catalysis. Wiley, VCH, Hoboken,
NJ.
Choppin, G.R. and M.K. Khankhasayev. 1999. Chemical Separation Technologies and
Related Methods of Nuclear Waste Management: Applications, Problems, and Research
Needs (NATO Science Partnership Sub-Series: 2). Springer, New York.
Cicero-Herman, C.A., P. Workman., K. Poole, D. Erich, J. Harden. 1998. Commercial
Ion Exchange Resin Vitrification in Borosilicate Glass. Westinghouse Savannah River
Co., Aiken, SC.
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Coleila, C. 1999. Environmental Applications of Natural Zeolitic Materials Based on
Their Ion Exchange Properties. In Natural Microporous Materials in Environmental
Applications. P.Misaelides, F. Macasek, TJ. Pinnavaia, and C. Colella (Eds.). Kluwer
Academic Publishers, The Netherlands.
Faust, S.D. 1998. Chemistry of Water Treatment. ISBN-10: 1575040115; ISBN-13: 978-
1575040110.
Hofstetter, K. J. and Hitz, C. G. 1983. The use of the submerged demineralizer system.
Separation Science Technology. 18:1747-1764.
International Atomic Energy Agency (IAEA). 1967. Operation and Control of Ion
Exchange Processes for Treatment of Radioactive Wastes, Technical Reports Series No.
78.
International Atomic Energy Agency (IAEA). 1972. Use of local minerals in the
treatment of radioactive wastes: Technical Report Series 136.
International Atomic Energy Agency (IAEA). 1984. Treatment of Spent Ion-exchange
Resins for Storage and Disposal, Technical Reports Series No. 254.
International Atomic Energy Agency (IAEA). 2002. Application of Ion Exchange
Processes for the Treatment of Radioactive Waste and Management of Spent
Ion Exchangers, Technical Reports Series No. 408.
InterSun. 2007. Zeolite Applications. Website. Accessed October 2007.
http://www.siberg.com/zeolite.htm.
Jantzen, C.M., D.K. Peeler, C.A. Cicero. 1995. Vitrification of Ion-exchange (IEX)
Resins: Advantages and Technical Challenges. Westinghouse Savannah River Co.,
Aiken, SC.
Kanellopoulos, N.K. (ed.). 2000. Recent Advances in Gas Separation by Microporous
Ceramic Membranes. Elsiever, New York.
Kawamura, S. 2000. Integrated Design and Operation of Water Treatment Facilities.
Wiley, Hoboken, NJ.
Kerry, F. G. 2007. Industrial Gas Handbook: Gas Separation and Purification. CRC
Press, Boca Raton, FL.
Komarneni, S. 1985. Philipsite In Cs Decontamination and Immobilization. Clays and
Clay Minerals. 33(2): 145-151.
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Los Alamos National Laboratory. 1997. Cement Waste-form Development for Ion-
exchange Resins at the Rocky Flats Plant. Rep. LA-13226.
Macasek, TJ. 1999. Sorption and Leaching Properties of the Composites and Complexes
of Natural Microporous Materials. In Natural Microporous Materials in Environmental
Applications. P. Misaelides, F. Macasek, TJ. Pinnavaia, and C. Colella (Eds.). Kluwer
Academic Publishers, The Netherlands.
MassNanoTech. 2007. University of Massachusetts Nanoscale Science and Engineering.
Website. Accessed October 2007.
http://www.umass.edu/massnanotech/faculty _auerbach.htm.
McKetta, JJ. 1999. Encyclopedia of Chemical Processing and Design: Volume 67 -
Water and Wastewater Treatment: Protective Coating Systems to Zeolite, 1st Edition.
CRC Press, Boca Raton, FL.
Mercer, B. W. and L.L. Ames. 1978. Zeolite ion exchange in radioactive and municipal
wastewater treatment. In Natural Zeolites: Occurrence, Properties, Use. L. B. Sand and
F. A. Mumpton (eds.). Pergamon Press, Elmsford, New York, 451-462.
Misaelides, P., F. Macasek, TJ. Pinnavaia, C. Colella (Eds.). 1999. Natural Microporous
Materials in Environmental Applications. Kluwer Academic Publishers, The Netherlands.
Nelson, J. L. and Mercer, B. W. 1963. Ion exchange separation of cesium from alkaline
waste supernatant solutions. United States Atomic Energy Commission, Hanford,
Washington.
Pasini, M. 1996. Natural zeolites as cation exchangers for environmental protection.
Mineralium Deposita. 31(6):563-575.
Rajec, P., F. Macasek, P.Misaelides. 1999. Sorption of Heavy Metals and Radionuclides
on Zeolites and Clays. In Natural Microporous Materials in Environmental Applications.
P. Misaelides, F. Macasek, T J. Pinnavaia, and C. Colella (Eds.). Kluwer Academic
Publishers, The Netherlands.
St. Cloud. 2007. Zeolite. Website. Accessed October, 2007.
http://www.stcloudmining.com/agriculture.html.
Stead, K., S.K. Ouki, N.Ward. 2000. Natural Zeolites—Remediation technology for the
21st Century. Eurosoil 2000, British Society of Soil Science, University of Reading,
September 4-6, 2000.
United States Nuclear Regulatory Commission (USNRC). 1980. Draft Programmatic
environment impact statement related to decontamination and disposal of radioactive
wastes resulting from March 1979 accident. Three Mile Island Nuclear Station, Unit 2:
U.S. Nuclear Regulatory Commission Report NUREG- 0683, Washington, DC.
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Venuto, P.B. and M. Dekker. 1979. Fluid Catalytic Cracking with Zeolite Catalysts.
ISBN-10: 0824777824; ISBN-13: 978-0824777821. Washington, ReportHW-76449, p.3.
Weber, M. A., K. A. Barbarick, and D. G. Westfall. 1984. Application of clinoptilolite to
soil amended with sewage sludge. Zeo-Agriculture: Use of Natural Zeolites in
Agriculture and Aquaculture. W. F. Pond and F. A. Mumpton (Eds.) Westview Press,
Boulder, CO.
Yang, R.T. 1994. Gas Separation by Adsorption Processes. World Scientific Publishing
Company, Imperial College Press, Singapore.
Zeolite Australia PTY LTD. 2007. Website. Accessed October 2007.
http://www.zeolite.com.au/products/agriculture.html.
ZeoPro. 2007. Website. Accessed October 2007. http://www.zeoponix.com/zeolite.htm.
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2.6 Other Nanoparticles
Though iron nanoparticles presented in Section 2.2 are the most fully developed type of
nanoparticle for environmental remediation, they are not the only type of nanoparticle
that has been suggested for environmental remediation. Some additional possibilities
include the use of TiO2 nanoparticles as photocatalysts for the destruction of organic
pollutants (Rajh et al. 2003), the separation of ionic solutes using nanoparticle-
crosslinked polymer hydrogels (Thomas et al. 2003), or the use of coated magnetic
nanoparticles in high-gradient magnetic separations (Moeser et al. 2004). The subsections
below present information on three possibilities: nanodiamonds, dendrimers, and
Argonne Supergel.
Nanodiamonds are the subject of much research due to their potential in electronic and
bio-imaging applications, and because they can be prepared directly, simply, and in
potentially large amounts. They are apparently biologically benign, and their ease of
surface functionalization and magnetic properties make them a potential separation
platform.
Dendrimers are a new class of polymer with tailorable properties, both at the surface and
in the interior of the particle. They have been of great interest in the area of separations,
and are potential complements to separation processes involving nanofiltration or
microfiltration.
Argonne Supergel is presented as a nano-enabled technology developed specifically for
radionuclide decontamination. These three examples (nanodiamonds, dendrimers, and
Supergel) are presented to provide a sense of the range of possibilities that nanoparticles
can offer for separation-based remediation.
2.6.1 Nanodiamonds
Nanodiamond is a term used for a group of diamond-related materials with nanoscale
dimensions, including diamond films and diamond nanoparticles. These are prepared by a
variety of methods, including high-pressure gas-phase nucleation and application of
shock waves to graphite (Dolmatov 2001). Within this group of diamond materials lies a
subgroup called detonation Nanodiamond (DND), or ultrananocrystalline diamond
(UNCD). These materials were discovered in Russia in 1963, and produced by the
detonation of oxygen deficient explosives, such as a 3-to-2 mixture of 2,4,6-
Trinitrotoluene (TNT) and Hexahydro-Trinitro-Triazine (RDX). Figure 13 shows a
Transmission Electron Microscope (TEM) image of DNDs.
DNDs were unknown in the West until recently, and have attracted much interest (Petrov
et al. 2006, Gruen et al. 2005) because of their unusually uniform shape and size
distribution and the fact that they can be produced in large quantities (Osawa 2003).
Before DND was widely known, Western industry produced microdiamonds by applying
an externally produced Shockwave to heated graphite at high pressures. This produced a
polycrystalline material with a wide size distribution and very few particles reaching
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down to the 10-20 nm size range. In contrast, DND has a fairly tight size distribution in
the 3-5 nm range, consisting of about 5,000 carbon atoms, and has a regular octahedral
shape (Figure 14).
Figure 13. TEM image of DND
Figure 14. Model of nanodiamonds demonstrating their regular octahedral shape
Nanodiamond production from a detonation produces a sooty product that is
approximately 50% DND. To obtain the nanodiamonds from this mixture, two challenges
must be overcome—the presence of graphitic impurities and the fact that the diamonds
tend to aggregate into clusters with average sizes of 30 |j,m, 3 |j,m, and 100-200 nm. The
aggregates can be broken up by a combination of ultrasound and high-speed zirconia
bead milling.
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Nanodiamonds possess interesting properties. Their surfaces have a tendency to change
from the regular diamond structure to a fullerene (buckyball) structure (Figure 15),
resulting in a material that appears to have a diamond within a fullerene cage and has
some of the physical properties of the nanodiamond combined with the rich chemistry of
the fullerene.
Figure 15. Structure with nanodiamond center and fullerene-like surface
The potential for surface chemical functionalization opens up a wide range of possible
applications for derivatized material. Even without chemical functionalization,
nanodiamonds have been suggested for a range of applications, including:
• Lapping and polishing applications
• All-rigid memory disk substrate
• Polycarbonate and CR-39 eyeglass lenses
• Miniature and precision ball bearings
• Optical and laser optical components
• Ceramics
• Precious stones
• Metallic mirrors and precision metal polishing
• Ferrite surface preparation
• Mechanical seal lapping
• Superhard and soft nanoabrasives
Other applications include:
• Surface germination for following growth of diamond-like films
• Ni-Diamond and Cr-Diamond electroplated hard coatings
• Molecular sieves
• Lubricant additive to engine oil
• Dry lubricants for metal industry (drawing of W-, Mo-, V-, Rh-wires)
• Reinforcing fillers for plastics and rubbers
• Chromatographic carriers
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A major area that is being investigated is the biomedical potential of nanodiamonds,
including their roles as drug carriers, implant coatings, and medical nanorobots.
Nanodiamonds are soluble in water, can be functionalized, and appear to be biologically
benign. Cytotoxicity research findings from the University of Dayton indicate that
nanodiamonds are biologically compatible materials across a range of sizes with regard to
a variety of cell types, with or without surface modifications (Schrand et al. 2007).
Nanodiamonds have also been suggested as sorbent materials (Dolmatov 2001, Gruen et
al. 2005), giving rise to the possibility of environmental remediation applications. Their
observed magnetic properties (Talapatra 2005) also open the possibility of their use in
magnetic separations. The radiation stability of the carbon also opens up the possibility as
a disposal waste form.
2.6.2 Dendrimers
A dendrimer is a highly and repetitively branched, three-dimensional polymer created by
a sequence of iterative chemical reactions starting from a central core. Each iteration is
known as a generation and has twice the complexity of the prior generation. The term
comes from dendron (the Greek word for tree), with the analogy being the branch-like
structure of the dendrimer. Dendrimers have been called the fourth major class of
polymeric architecture (after linear, cross-linked, and branched polymers), but unlike
other polymers where atom-by-atom control is not feasible and polydispersity (and the
variability it brings) is an inherent characteristic, dendrimers are characterized by
monodispersity (i.e., all dendrimer molecules are of a uniform and controllable size) and
well-defined properties. Figure 16 provides a graphical representation of a generalized
dendrimer structure.
generation
numbers
branching
points
termini
focal point'
(chemically addressable group)
DENDRIMER DENDRON
Figure 16. A generalized dendrimer structure
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Figure 16 allows some of the special chemical properties of dendrimers to be explained.
Starting at the core (GO) and moving through branching points Gl and G2, it can be seen
that as the dendrimer branches out in a predictable manner, large voids can exist within
the dendrimer internal structure. Both the size and the physical nature (e.g., hydrophobic
or hydrophilic characteristics) of these voids can be controlled during synthesis by
judicious selection of both the length and composition of the branch backbone. These
voids can be used to store material (metals, organic and inorganic molecules, and even
other nanoparticles), and together with the fact that dendrimers can easily move across
biological membranes, the controllable nature of the voids has led to suggested
applications of dendrimers as carriers of genetic material into cells (Kukowska-Latallo et
al. 1996), drug delivery agents (Lim and Simanek 2005), and diagnostic imaging agents
(Tomalia 2003). The voids can also be used to hold material undergoing reactions and
has led to investigations of dendrimers as nanoreactors (Chung and Rhee 2003).
A second feature that is illustrated in Figure 16 is the surface formed by the terminal
groups. As the number of generations of the dendrimer increases, the total external
surface area rises dramatically. If the terminal groups are designed to be binding moieties
for other species, then it is apparent that a dendrimer, compared to say a macroscopic
bead of ion exchange resin (which will have a much smaller surface area than an
equivalent amount of dendrimer), can potentially bind up a large amount of target
material. It is this aspect of dendrimers that is of prime concern for environmental
applications. Current expertise in dendrimer synthesis also allows for more than one type
of terminal group to be attached to the dendrimer, thus offering the possibility of
multifunctional molecules. Terminal groups can be attached to modify solubility, modify
binding capacity and specificity, and to allow further reaction with or attachment to other
surfaces or nanoparticles. It has been suggested that amphipathic dendrimers could be
synthesized with one half of the molecule (or one hemisphere) covered in hydrophobic
groups, and the other half containing hydrophilic groups.
Due to the degree of structural control that dendrimers make possible, and the fact that
their highly customizable properties should make them building blocks for other
nanomaterials, they have been the subject of much research, with the number of academic
publications approaching ten thousand. Commercial development has been slower than
once anticipated because of their high cost (on the order of $10 per milligram) and the
complexity of scaling-up production. Though these factors are themselves subject of
much research, dendrimers already have a market or near-market presence:
• Dade Behring, one of the world's largest medical diagnostic firms, is developing a
dendrimer-based, rapidly responding tool for detecting heart attacks and cardiac
damage.
• The U.S. Army Research Laboratory is developing a dendrimer-based anthrax
detection agent.
• Starpharma is developing the world's first dendrimer-based drug, Vivagel, to fight
sexually transmitted infections. It has been awarded $20 million by the National
Institutes of Health (NIH) to develop its HIV indication; given further awards to
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develop its genital herpes indication; and was granted Fast Track status by the
U.S. FDA in 2006 as a product for preventing HIV infection.
Dendrimers are currently under investigation as metal sequestering agents for waste
remediation technologies (Cohen et al. 2001), and for the removal of uranium from
aqueous streams (Diallo 2007). In the latter work, dendrimers may be compared with
chelating agents, which are widely used in uranium separation processes, such as solvent
extraction or ion exchange resins. Ion exchange resins with amino groups typically bind
on the order of 100 mg of uranium per gram of resin, while the work with
poly(amidoamine) and poly(propyleneimime) dendrimers, which contain nitrogen and
oxygen donors, can bind up to 2500 mg of uranium per gram of dendrimer without
reaching saturation in either acid or basic solutions. Further, the binding kinetics of the
dendrimers to uranium is very fast and reaches equilibrium in less than 20 minutes. Rapid
equilibration, high loading capacity, and selectivity mean that such dendrimers could thus
serve as high capacity and selective chelating agents for uranium. The dendrimer-
uranium complexes could be easily separated by ultrafiltration and then regenerated, thus
avoiding the need to add further reagents and simplifying the overall process.
2.6.3 Argonne Supergel
Argonne National Laboratory (ANL) has developed a system, called the "Supergel"
technique, to safely capture and dispose of radioactive elements in porous structures
outdoors (such as buildings and monuments), using a spray-on, super-absorbent gel and
engineered nanoparticles (ANL 2006). Porous structures are notoriously hard to clean. In
decommissioning and decontamination operations, it is common practice to demolish
contaminated structures or completely remove a significant surface layer rather than
attempt to remove radioactivity. ANL's Supergel technique preserves surfaces, which
means that monuments or buildings would not have to be defaced to remove radiation.
The Supergel was developed with funding from the Department of Homeland Security to
help fill a technology gap in preparedness for a terrorist attack with a "dirty bomb" or
other radioactive dispersal device, but it could also be used in more general
decontamination situations.
The Supergel technique uses commercially available equipment in a simple procedure.
First, a wetting agent and a super-absorbent gel are sprayed onto the contaminated
surface. The polymer gel used to absorb the radioactivity is similar to the absorbent
material found in disposable diapers. When exposed to water, the polymers form
something similar to a structural scaffold that allows the gel to absorb a large amount of
liquid. When sprayed on concrete, the wetting agent causes the bound radioactivity to re-
suspend in the concrete pores and the superabsorbent polymer gel then draws the liquid
out, along with the resuspended radioactivity. Inside the gel, the radioactive material
becomes fixed by engineered nanoparticles that also reside in the gel. After a period of
standing, the gel is vacuumed and recycled, leaving behind a relatively small amount of
radioactive waste for disposal.
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2.6.4 Summary of Environmental Potential
These three technologies—nanodiamonds, dendrimers, and Supergel—demonstrate the
broad potential of nanoparticles. Nanodiamonds, though relatively new, offer a wide
range of potential applications and can be produced in bulk at reasonable prices.
Dendrimers are a new class of polymer architecture that has caused much excitement due
to the potential of engineering properties at the molecular level into materials of uniform
and controllable size. The Argonne Supergel shows the ease with which nanoparticles
can be incorporated into other applications to address highly specific technology needs.
2.6.5 References
Argonne National Laboratory (ANL). 2006. 'Supergel' Systems Cleans Radioactively
Contaminated Structures. Website. Accessed October 2007.
http://www.anl.gov/National Securitv/docs/factsheet Supergel.pdf.
Chung, Y.-M., and H.-K. Rhee. 2003. Pt-Pd Bimetallic Nanoparticles Encapsulated in
Dendrimer Nanoreactor. Catalysis Letters. 85(3-4): 159-164.
Cohen, S.M., S. Petoud, K.N. Raymond. 2001. Synthesis and Metal Binding Properties of
Salicylate-, Catecholate-, and Hydroxypyridinonate-Functionalized Dendrimers.
Chemistry. 7(1): 272-279.
Dolmatov, V.Y. 2001. Detonation synthesis ultradispersed diamonds: properties and
applications. Russian Chemical Reviews. 70: 607.
Diallo, M.S. 2007. Dendrimer Based Chelating Agents and Separation Systems for
U(VI): Fundamental Investigations and Applications to In situ Leach Mining.
Presentation at the Global Uranium Symposium, Corpus Christi, TX, May 2007.
Gruen, D.M., A.O. Shenderova, A.Y. Vul (Eds.). 2005. Synthesis. Properties and
Applications of Ultrananocrystalline Diamond: Proceedings of the NATO Advanced
Research Workshop on Synthesis, Properties and Applications of Ultrananocrystalline
Diamond. St. Petersburg, Russia, 7-10 June 2004. Springer, New York.
Kukowska-Latallo, J.F., A.U. Bielinska, J. Johnson, R. Spindler, D.A. Tomalia, J.R.
Baker, Jr. 1996. Efficient transfer of genetic material into mammalian cells using
Starburst polyamidoamine dendrimers. National Academy of Science. 93(10): 4897-
4902.
Lim, J. and E.E. Simanek. 2005. Toward the Next-Generation Drug Delivery Vehicle:
Synthesis of a Dendrimer with Four Orthogonally Reactive Groups. Molecular
Pharmaceutics. 2 (4), 273 -277.
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Moeser, G.D., K.A. Roach, W.H. Green, T.A.Hatton, P.E. Laibinis. 2004. High-gradient
magnetic separation of coated magnetic nanoparticles. A.I.Ch/E. Journal. 50 (11): 2835-
2848.
Osawa, Eiji. 2003. Detonation Nanodiamond-Potential Material For FED. American
Physical Society, Annual APS March Meeting 2003, March 3-7, 2003.
Petrov, I, P. Detkov, J. Walch and O. Shenderova. 2006. Polydispersed Detonation
Nanodiamond and Approaches for its Fractioning. In Technical Proceedings of the 2006
NSTI Nanotechnology Conference and Trade Show, Volume 1. NSTI, Cambridge,
Massachusetts: 150-153.
Rajh, T., O. V. Makarova, M. C. Thurnauer, andD. Cropek. 2003. Surface Modification
of TiO2: a Route for Efficient Semiconductor Assisted Photocatalysis; Chapter 9 in
Synthesis, Functionalization and Surface Treatment of Nanoparticles. M.-I. Baraton
(Ed.), American Scientific Publishers, California: 147-171.
Schrand, A.M., H. Huang, C. Carlson, JJ. Schlager, E.O. Sawa, S.M. Hussain, L. Dai.
2007. Are Diamond Nanoparticles Cytotoxic? Journal of Physical Chemistry B. 111:2-7.
Thomas, P., B. Cipriano and S. Srinivasa Raghavan. 2003. Separation of Ionic Solutes
Using Nanoparticle-Crosslinked Polymer Hydrogels; Presentation at the American
Physical Society March Meeting, Denver, CO, March 6.
Tomalia, D. 2003. Dendrimers as multi-purpose nanodevices for oncology drug delivery
and diagnostic imaging. Nanomedicine: Nanotechnology, Biology and Medicine. 2(4):
309.
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2.7 Uranium Reduction by Bacteria
Bioreductive immobilization of uranium is a quasi nanotechnology. The precipitated
uranium and other minerals are in the form of nano size particles, whereas the bacteria
are about 1 |j,m. In this section, the process is described, equations are provided, and
photographs presented showing the nanoscale uraninite particles.
The large number of contaminated sites and volumes of contaminated groundwater and
soil call for innovative and economically attractive remediation technologies. To date,
pump-and-treat is the most widely used technology. Frequently, pump-and-treat has been
ineffective in permanently lowering contaminant concentrations in groundwater (Travis
and Doty 1990). A recent study by Quinton et al. (Quinton et al. 1997) showed that
groundwater cleanup technologies (such as pump and treat, permeable reactive barriers
with ZVI, and bio-barriers) are more expensive than in-situ bioremediation.
Microorganisms can reduce uranium indirectly by producing hydrogen sulfide (H2S) or
pure hydrogen (H^) in the course of other processes (abiotic reduction) or directly using
enzymes (enzymatic reduction). The first microorganisms identified to enzymatically
reduce U(VI) were the dissimilatory Fe(III)-reducing microorganisms, Geobacter
metallireducens and Shewanellaputrefaciem (Lovley et al. 1991). These microorganisms
used uranium as an electron acceptor, H2 or acetate as an electron donor to support
growth, and tolerated U(VI) concentrations as high as 8 mM. Several authors studied the
enzymatic reduction of U(VI) by various pure or mixed cultures of microorganisms,
including metal- and sulfate-reducing bacteria (a summary of previous work can be found
in Abdelouas et al. 1999a). These authors reviewed the literature on microbial reduction
of uranium and the significance of biogeochemical processes related to uranium mining,
tailings, and groundwater remediation. In Figure 17, the key reductive and oxidative
reactions are shown for this process; in Figure 18, a bacterium is shown surrounded by
nanosize uraninite particles.
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1/4O2(g) + H++e
1/5NO3- + 6/5H++e
1/2 MnO2+1/2 HCCV + 3/2H+ + e
1/2 UO2(CO3)22-
FeOOH(s) + HCCV + 2 H++ e
1/8SO42-+9/8H+ + e -
Oxidation of ethanol
1/5C2H5OH + 1/4H2O
-> 1/2 H2O
1/10N2(g) + 3/5H2O
1/2 MnCO3(s) + H2O
1/2 UO2(s) + HCO3-
FeCO3(s) + 2 H2O
1/8HS- + 1/2H2O
1/6CO2 + H++e
Figure 17. Reductive and oxidative reactions
Decreasing
redox intensity
Bacterium
r>-<
Figure 18. Microphotographs showing a bacterium surrounded with uraninite
nanoparticles
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The first demonstration of a feasible process for the in-situ immobilization of uranium as
a bioremediation strategy was conducted by a team of scientists from the University of
Massachusetts, PNNL, the University of Tennessee, and several other institutions
(Anderson 2003). The team conducted a two-month field study and demonstrated that by
adding acetate to the subsurface, they could stimulate the growth and proportion of
Geobacter species within the subsurface microbial community. At the same time, the
concentration of uranium in the ground water was greatly reduced.
The observation of bacteria able to immobilize uranium is an area of active study, both
from biochemical and nano-geological perspectives. Comparatively little is known about
either the bacteria or the processes they use; even questions over the long-term stability
of the immobilized uranium have yet to achieve a comprehensive answer. Recent
research at Virginia Polytechnic Institute (Virginia Tech 2006) with Shewanella
oneidensis MR-1 (one of the most common bacteria in the Earth) has shown that particle
size is important, with smaller nanoparticles of the iron (III) oxide hematitie showing a
lower rate of reduction than larger nanoparticles. Research on the same bacterium at
PNNL (Marshall 2006) has demonstrated that much of Shewanella's biochemistry of
immobilization occurs outside the cell, producing uniform, 5-nm particles of uraninite
trapped as strings of particles in a glue-like extracellular polymeric substance (Figure 19).
Figure 19. Uraninite nanoparticles trapped in an extracellular polymeric substance
exuded by Shewanella
A current five-year, $15-million U.S. Department of Energy (DOE) project led by Oak
Ridge National Laboratory (ORNL) (Edwards 2007) is trying to provide a further
understanding of the coupled microbiological and geochemical processes limiting
radionuclide bioremediation, and through an examination of terminal-electron accepting
processes involving geobacteraceae has shown that due to the stress imposed by low pH
on microbial metabolism, the terminal-electron accepting processes of acidic subsurface
sediment are inherently different from those of neutral pH environments and
-------
neutralization will be necessary to achieve sufficient metabolic rates for radionuclide
remediation.
In addition to direct bacterial processes for the reduction of uranium, indirect processes
may occur where bacteria or other microbes assist in the reduction processes by control
of the chemical environment. The dissimilatory iron reducing bacteria mentioned above
metabolically couple the oxidation of organic compounds with the reduction of Fe (III)
generating energy and Fe(II) complexes, the latter of which in turn lead to the production
of a range of Fe(II) minerals such as magnetite, siderite, vivianite, ferruginous smectite,
and green rust. In fact, the oxidation/reduction behavior of uranium, together with the
ability of micro-organisms to take advantage of such oxidation/reduction behavior, may
well play a significant role in the formation of uranium ore deposits (Dexter-Dyer 1984).
This behavior is being exploited in the concept of microbial mining of uranium, and may
be of importance to remediation. An example is "green rust", a class of iron (II)/iron (III)
hydroxide compounds having a pyroaurite-type structure consisting of alternating
positively charged hydroxide layers and hydrated anion layers. Green rusts are products
of both abiotic and microbially induced corrosion of iron, and occur in both microbially
mediated and abiotic reductive dissolution of ferric oxyhydroxides. Extended X-Ray
Absorption Fine Structure (EXAFS) studies have shown that uranyl ion can be reduced to
UO2 (U(IV)) by green rust (O'Loughlin 2003), with the uraninite forming nanoparticles
on the green rust crystal surface (Figure 20).
Uraninite nanopartieles^l
f
.si?
- -*>
:\
• .-
•>""
" - t', , 50 nm
w
ireen Rust particle
A
Figure 20. Uraninite nanoparticles on a green rust particle
The DOE has funded a project through its Environmental Management Science Program
(EMSP) to examine the processes underlying the potential use of dissimilatory metal-
reducing bacteria (DMRB) to create subsurface redox barriers for immobilizing uranium
-------
and other redox-sensitive metal/radionuclide contaminants (Roden 2005). The results of
these studies suggest that:
• the efficiency of dissolved U(VI) scavenging may be influenced by the kinetics of
enzymatic U(VI) reduction in systems with relative short fluid residence times;
• association of U(VI) with diverse surface sites in natural soils and sediments has
the potential to limit the rate and extent of microbial U(VI) reduction, and in turn
modulate the effectiveness of in-situ U(VI) bioremediation; and
• abiotic, ferrous iron (Fe(II))-driven U(VI) reduction is likely to be less efficient in
natural soils and sediments than would be inferred from studies with synthetic
Fe(III) oxides.
A key implication of these findings is that production of Fe(II)-enriched sediments during
one-time (or periodic) stimulation of DMRB activity is not likely to permit efficient,
long-term abiotic conversion of U(VI) to U(IV) in biogenic redox barriers designed to
prevent far-field subsurface U(VI) migration. Instead, ongoing DMRB activity will be
required to achieve maximal U(VI) reduction efficiency.
The study of dissimilatory metal reducing bacteria (DMRB, the general class of which
dissimilatory iron reducing bacteria are part) has recently led to some interesting
nanoscience discoveries that may offer new directions for nanotechnology. While most
biological oxidation-reduction reactions take place in the liquid phase using water soluble
species, dissimilatory reductions require a process where the electron acceptor is a solid
phase material. Understanding the details of this process has been the topic of much
research, with the focus largely being on c-type (monomeric) cyctochromes (heme
proteins generally bound in cell membranes) which are known to perform electron
transport. In 2005, Derek Lovley, who discovered Geobacter in 1987, published research
showing that conductive structures (known as pili or "microbial nanowires") only a few
nanometers wide but microns long, are produced by Geobacter, are electrically
conductive, and are indicated as being involved in electron transport (Reguera 2005).
Subsequently, an international group (Gorby 2006) showed that other bacteria can be
induced to produce nanowires (as small as 10 nm in diameter, but can reach hundreds of
microns in length) when kept in an oxygen-starved state, are electrically conductive, and
that this behavior is not limited to DMRB but might be a common bacterial strategy for
efficient electron transfer and energy distribution. When in a community, the bacterial
nanowires can cross and touch, and may allow for sharing of electrons among a network
of bacteria. Figure 21 illustrates the bacterial nanowire reaching across organisms, and
Figure 22 depicts a close-up image of a nanowire. The nanowires have been of great
interest as a potential production and supply method of nanowires for other applications
(such as sensors, nano-electronic components), as the basis of a possible remedial
treatment approach, or as a component of microbial fuel cells.
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Figure 21. Scanning electron microscope image of
Shewanetta ddeinensis strain MR-1
Figure 22. Scanning Tunneling Microscope (STM) images of isolated nanowires
from wild-type MR-1, with lateral diameter of 100 nm and a topographic height of
between 5 and 10 nm. Arrows indicate the location of a nanowire and a step on the
graphite
-------
In summary, the literature on microbially mediated reduction of U(VI) showed that U(VI)
can be reduced to U(IV) by enzymatic activity of microorganisms, including:
• metal- and sulfate-reducing bacteria;
• U(VI) can be reduced either by pure cultures or by mixed indigenous cultures;
• U(IV) precipitates as uraninite (UO2);
• complexation of U(VI) with organic and inorganic ligands can inhibit its
reduction by microorganisms; and
• complexation of U(IV) may inhibit its precipitation.
Uranium reduction by bacteria is an example of bioremediation. As its advocates point
out, bioremediation may be regarded as the oldest environmental technology of all,
having existed virtually as long as the human species. Prior to 1989, bioremediation as a
formal environmental technology was not widely known; it had a small number of
advocates, though its claims were backed by considerable laboratory and academic work.
The March 1989 Exxon Valdez disaster in Alaska and the subsequent use of
bioremediation using naturally occurring marine organisms together with added nutrients,
opened the way for bioremediation to become an established environmental remediation
option. Economically, bioremediation usually offers great overall cost savings compared
to competing technologies. In addition, risks tend to be smaller since contaminants are
not transferred from one medium to another for processing, and there is no waste
transportation involved. On the other hand, the bioremediation process may take much
longer than chemical or physical treatment alternatives, requiring ongoing monitoring to
ensure that progress is being made. When geological conditions are suitable and
sufficient time is available, bacterial reduction of uranium is likely to be an attractive
remedial alternative.
2.7.1 References
Abdelouas, A., W. Lutze, H. E. Nuttall. 1999a. Chapter 9: Uranium Contamination in the
Subsurface: Characterization and Remediation. In Uranium: Mineralogy, Geochemistry,
and Environment. P. C. Burns and R. Finch, (Eds.). Reviews in Mineralogy. 38: 433-473.
Anderson, R.T., H.A. Vrionis, I. Ortiz-Bernad, .T. Resch, .E. Long, R. Dayvault, K.
Karp, S. Marutzky, D.R. Metzler, A. Peacock, D.C. White, M.Lowe, D.R. Lovley. 2003.
Stimulated in situ removal of U(VI) from groundwater of a uranium-contaminated
aquifer. Applied and Environmental Microbiology. 69(10): 5884-5891.
Dexter-Dyer, B., M. Kretzschmar, W.E. Krumbein.1894. Possible microbial pathways in
the formation of Precambrian ore deposits. Journal of the Geological Society. 141 (2):
251-262.
Edwards L., K. Kiisel, H. Drake, I.E. Kostka. 2007. Electron flow in acidic subsurface
sediments co-contaminated with nitrate and uranium. Geochimica et Cosmochimica Ada.
71(3): 643-654.
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Gorby, Y.A., S. Yanina, J.S. McLean, K.M. Rosso, D.Moyles, A. Dohnalkova, TJ.
Beveridge, IS. Chang, B.H. Kim, K.S. Kim, D.E. Culley, S.B. Reed, M.F. Romine, D.A.
Saffarini|, E.A. Hill, L. Shi, D.A. Elias, D.W. Kennedy, G. Pinchuk, K. Watanabe, S.
Ishii, B. Logan, K.H. Nealson, J.K. Fredrickson. 2006. Electrically conductive bacterial
nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms.
Proceedings of the National Academy of Science. 103(30): 11358-63.
Lovley D.R., E.J.P. Phillips, Y. Gorby, E. Landa. 1991. Microbial reduction of uranium.
Nature. 350: 413-416.
O'Loughlin, E.J., S.D. Kelly, R.E. Cook, R. Csencsits, K.M. Kemner. 2003. Reduction
of uranium(VI) by mixed iron(II)/iron(III) hydroxide (green rust): formation of UO2
Nanoparticles. Environmental Science and Technology. 37: 721-727.
Quinton, G. E., R. J. Buchanan, D. E. Ellis, S. H. Shoemaker. 1997. A Method to
Compare Groundwater Cleanup Technologies. Remediation. 7-16.
Marshall, M.J., A.S. Beliaev, A.C. Dohnalkova, D.W. Kennedy, L. Shi, Z. Wang, M.I.
Boyanov, B. Lai, K.M. Kemner, J.S. McLean, S.B. Reed, D.E. Culley, V.L. Bailey, C.J.
Simonson, D.A. Saffarini, M.F. Romine, J.M. Zachara, J.K. Fredrickson. 2006. c-type
cytochrome-dependent formation of U(IV) nanoparticles by Shewanella oneidensis.
Biology. 4(8): 1324-1333.
Reguera, G., K.D. McCarthy, T. Mehta, J.S. Nicoll, M.T. Tuominen, D.R. Lovley. 2005.
Extracellular electron transfer via microbial nanowires. Nature. 435: 1098-1101.
Roden, E.E., and M.O. Barnett. 2002. Reductive immobilization of U(VI) in Fe(III)
oxide-reducing subsurface sediments: Analysis of coupled microbial-geochemical
processes in experimental reactive transport systems. Final Scientific/Technical Report—
EMSP 73914, U.S. Department of Energy, Washington, DC.
Talapatra, S., P.G. Ganesan, T. Kim, R. Vajtai, M. Huang, M. Shima, G. Ramanath, D.
Srivastava, S.C. Deevi , P.M. Ajayan. 2005. Irradiation-Induced Magnetism in Carbon
Nanostructure^1. Physical Review Letters. 95: 097201.
Travis, C. C. and C. B. Doty. 1990. Can Contaminated Aquifers at Superfund Sites be
Remediated? Environmental Science Technology. 24: 1464-1466.
Virginia Tech (2006, September 21). Particle Size Matters To Bacteria Ability To
Immobilize Heavy Metals. ScienceDaily. Website. Accessed December 2007.
http://www.sciencedaily.com/releases/2006/09/060915203321.htm.
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2.8 Carbon Nanotubes (Fullerenes)
2.8.1 Background
After long being available only in research-level quantities, commercial-scale C-60
fullerene production at the "tons-per-year" level is now available. The price of larger
fullerenes is still high and quantities available are still small. Commercial-scale
production at affordable prices is the target of intense research; over 1,500 patents have
been filed on various potential production technologies. No radionuclide remediation
technology is currently available or apparently under consideration, though the concept
has been discussed. Carbon nanotubes and fullerenes are of special interest since they
have been subject of much research both as components in remediations systems (either
as part of nano-composite membranes or as functionalized separation platforms), and as
sensors (primarily through a field effect transition mechanism). They are also one of the
icons of the nanotechnology age.
2.8.2 Description
Fullerenes (first discovered in 1985 by Robert Curl, Harold Kroto, and Richard Smalley),
are a class of hollow, spherical, or ellipsoid molecules—composed entirely of carbon
atoms—in a cage-like structure composed of pentagonal and hexagonal faces. They were
named after the architect Richard Buckminster Fuller due to their similarity to his
geodesic dome design, and are often referred to as "buckyballs". Fullerenes were the
seventh allotropic form of carbon to be discovered (together with the two forms of
diamond, the two forms of graphite, chaoit, and carbon (IV)). Their discovery led to Curl,
Kroto, and Smalley receiving the Nobel Prize for Chemistry in 1996. Figure 23 provides
a graphical representation of the 60-carbon atom containing C-60 fullerene, and Figure
24 illustrates a graphical representation of the 540-carbon atom containing C-540
fullerene.
Figure 23. C-60 fullerene
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Figure 24. C-540 fullerene
An infinite number of spherical fullerenes are believed to be able to exist. Known forms
include C-60, C-70, C-76, C-84, C-240, and C-540. All fullerenes consist of 12
pentagonal faces and a varying number of hexagonal faces. In general, for a fullerene C-n
there will be 12 pentagonal faces and half of n minus 10 (n/2-10) hexagonal faces; thus,
the C-60 fullerene has 12 pentagonal faces and 20 hexagonal faces.
Fullerenes have a rich and complex chemistry (Stevens 1994, Kadish and Roff 2000,
Taylor 1995, Andreoni 2000, Hirsch et al. 2005) that led to the publication of over 15,000
academic papers within 15 years of their discovery. C-60 behaves like an electron
deficient alkene, reacts readily with electron rich species, and participates in many
reactions (including oxidations, reductions, nucleophilic additions, electrophilic
additions, Diels-Alder reactions, and Friedel-Craft alkylations). This rich chemistry
permits a wide range of fullerene functionalization and opens the way to designing
functionalized fullerene for specific properties and purposes.
A large number of applications have been suggested for basic fullerenes and their
functionalized derivatives. Potential applications include organic photovoltaics, polymer
electronics, antioxidants, biopharmaceuticals, antibacterials, HIV inhibition, catalysts,
water purification, MRI agents, optical devices, scanning tunneling microscopy, and
atomic force microscopy (Nano-C 2006, Tang 2005, Da Ros et al. 2001).
Fullerenes have also been the subject of many studies related to radioactive materials.
They are being extensively investigated as carrier species for medical radionuclides in
cancer therapy (Saha et al. 2006, Dagani 2002, Braun 1999, Medical News Today 2005).
It has been observed that in the combustion of coal, which contains small amounts of
uranium, nanocrystals of the mineral uranitite are encased in fullerene-type cages. This
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potentially provides an unanticipated pathway for radiation exposure (Utsunomiya et al.
2002). They have also been explored as a technology for radioactive waste management.
DOE has determined that there are three major isotopes contributing to public radiation
dose as a consequence of radionuclide releases—iodine-129, technetium-99, and
neptunium-237—and fullerenes have been investigated as a sorbent for iodine (Schmett
2002). It is also worth noting that a structure closely related to fullerenes, the carbon
nanohorn (Figure 25), has been suggested as a possibility for radioactive waste disposal.
Figure 25. The carbon nanohorn
Though a vast amount of research has already been performed on basic carbon fullerenes,
the real potential may lie in the fact that the work performed to date may only represent
the tip of the iceberg. Many structures related to the basic fullerenes and the carbon
nanohorn have been discovered. For example, the carbon nanocone (Figure 26) and
"NanoBuds" (Figure 27) are relatively new materials developed by the Finnish company
Canatu Oy (Canatu 2004, Nanowerk 2007) by combining carbon nanotubes and
fullerenes. The resulting nanobuds possess properties of both materials (e.g., the
electrical conductivity of carbon nanotubes with the chemical flexibility of fullerenes).
Fullerene composites (Ltaief et al. 2006, Calleja et al. 1996, Barrera et al. 1994, Brabec et
al. 1998, Eklund and Rao 2000, Prassides 2004) and hybrid materials, such as a fullerene-
dendrimer-mesoporous silica hybrid (Nierengarten et al. 2004), have been described.
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Figure 26. The carbon nanocone
Energy, eV
£*,V ft^^?
c'" IP'
^ -6.66
§| -6.76
fc -6.S6
C -7'06
fc -7.16
C, -7,26
t -7,36
fc -7.46
C -7.56
• -7.66
Figure 27. Carbon nanobuds
Further, there exists a potentially enormous class of "inorganic fullerenes". First
described in 1992 (Materials Science Resource 2005), over 50 different types of
inorganic fullerenes, and nanotubes have been reported in the technical and scientific
literature, and include molybdenum sulfide, molybdenum selenide, tungsten sulfide,
tungsten selenide, transition-metal chalcogenides, transition-metal oxides, transition-
metal halides, in addition to mixed-phase, metal-doped, boron-based, silicon-based, and
pure metal nanotubes (Sano et al. 2003, Lvayen et al. 2007, Fu et al. 2005, Parilla et al.
2004, Xia et al. 2004, Remskar et al. 2001, Parilla et al. 1999, Halford 2005), and "onion-
like" fullerenes consisting of cages nested within other cages like Russian dolls (Cabio'h
et al. 2005, Golberg et al. 1999).
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2.8.3 Operational Considerations
As one of the longest known and most intensely investigated type of nanoparticle,
fullerenes have been the subject of much study in terms of fate and transport. However,
there is little consensus, other than the need for more research (Zepp and Westerhoff
2007, Wenger 2007, EIMS 2006, Handy and Owen 2006, Boxall 2007, Baalusha 2007,
SETAC 2007, Drobne 2007). Recently, the results on investigations into the role of C-60
nanoparticles in relation to toxicity and bioaccumulation of xenobiotic organic
compounds in Daphnia magna were presented (Johansen et al. 2007). The hypothesis
was that C-60 nanoparticles may act as carriers of xenobiotic organic chemicals
(nanovectors), but mixed results were observed and are detailed below.
• There was an increase in algal toxicity of atrazine with C-60 present; no changes
in toxicity of methylparathion towards algae and daphnia.
• There was a decrease in the toxicity of pentachlorophenol (PCP) towards algae
and daphnia after addition to C-60 suspensions.
• There was an increase in the toxicity of phenanthrene towards algae and daphnia
in C-60 suspensions since the sorbed phenanthrene is bioavailable.
• The uptake and excretion rates of phenanthrene and PCP were not significantly
affected by addition to C-60 suspensions.
Also, results on investigations into the effects of C-60 fullerene nanoparticles on soil
bacteria and protozoa (Johansen et al. 2007) showed that fullerenes seem to have no, or
only moderate, effects on the soil microbial community (regarding the number and
viability of bacteria and protozoa). The genetic diversity of bacteria and protozoa seems
to be altered slightly, but the mechanism behind the diversity changing effect is unclear
(possible direct toxic effects on some of the microorganisms or indirect effects by
sorption of nutrient or inhibiting factors in the soil). The conclusion was that since
various fullerenes are very recalcitrant, and their production is expected to increase to
very large quantities, it is important that their fate and ecotoxicology in complex
environmental matrices be evaluated thoroughly in regards to transport, degradation,
toxicology, and interactions with xenobiotics, and that there is a need for standardized
methods for exposure of nanomaterials to organisms in vitro and in more complex
systems.
2.8.4 Summary of Environmental Potential
Carbon nanotubes, and closely related materials, such as carbon nanohorns and carbon
nanobuds, combine unique properties per se (such as greatly enhanced flow rates of water
through the inside of the tubes over what would be expected from current theories) with
the broad potential that results from a rich and complex chemistry. The range of
applications is similarly wide, including both the development of advanced separately
processes (such as nanofiltration and reverse osmosis) and the development of nano-
enabled sensors with the carbon nanotube acting as the sensing element. Carbon
nanotubes have already been investigated for application to DOE's three major problem
isotopes (iodine-129, technetium-99 and neptunium-237) contributing to public radiation
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dose. Carbon nanotubes are one of the longest known and most widely investigated type
of nanoparticle, with research including hybrid products formed with other nano-
structured materials (such as dendrimers). Given the extremely broad range of
possibilities and the level of research interest, the likelihood of environmental
technologies resulting from carbon nanotubes is assessed as very high.
2.8.5 References
Andreoni, W. (Ed.). 2000. The Physics of Fullerene-Based and Fullerene-Related
Materials. Klewer Academic Publishers, The Netherlands.
Baalusha, M. 2007. The existing toxicity test with a terrestrial isopod Porcellio scaber is a
satisfactory starting point for assessing effects of nanoparticles. Presented at the SET AC
Europe Annual Meeting. May 18, 2007.
Accessed at http://se.setac.org/files/setac-eu-0121-2007.pdf.
Barrera, E.V., J. Sims, D.L. Callahan, V. Provenzano, J. Milliken, R.L. Holtz. 1994.
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3.0 Nano-Enabled Sensor Technologies
3.1 Introduction
3.1.1 Basics
Nanosensors are chemical sensors possessing a nanoscale sensing element. A chemical
sensor is a device capable of providing quantitative or semi-quantitative information on a
chemical species (or analyte) through calibration, and then brought into direct contact
with the species in its environment. According to the definition given by the International
Union of Pure and Applied Chemistry (IUPAC), a chemical sensor is:
"A device that transforms chemical information, ranging from the
concentration of a specific sample component to total composition
analysis, into an analytically useful signal. The chemical information,
mentioned above, may originate from a chemical reaction of the analyte or
from a physical property of the system investigated. Chemical sensors
contain two basic functional units: a receptor and a transducer part. Some
sensors may include a separator which is, for example a membrane
(IUPAC 1997)."
The receptor part of a sensor is defined by IUPAC as:
"The chemical information is transformed in it into a form of energy,
which maybe measured by the transducer. The receptor part maybe based
upon various principles: physical, chemical or biochemical (IUPAC
1997)."
The transducer part of a sensor is defined by IUPAC as a:
"Device capable of transforming energy carrying the chemical information about
the sample into a useful analytical signal (IUPAC 1997)."
In addition, a sensor may include an output system, which processes the transducer
output into a useable form and relays it to the outside world. In contrast to a sensor, a
chemical analysis (or assay) method or system requires many more processing steps than
simply bringing a device into contact with the analyte, and frequently involves the use of
additional reagents. It is important that the receptor and transducer parts of a sensor are
closely integrated. As an example of a receptor and transducer combination, imagine a
very small, reed-like beam with a reflective surface, capable of vibration that can be
measured by a laser. In its basic state, the beam will vibrate with one frequency. If part of
the surface of the beam is covered with a ligand that will bind highly selectively with a
target analyte, then in the absence of the analyte, it will vibrate with a second, different
frequency; in the presence of the analyte, it will vibrate with a third, yet again different
frequency that is a function of the analyte concentration. In this scheme the ligand is the
receptor and the beam/laser system is the transducer; the sensor operates by optical
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transduction of a chemical binding event. Receptor events may be transduced by
electrical, electrochemical, piezoelectrical, optical, magnetic thermal, mass sensitive, or
other means.
3.1.2 Chemical Sensors
Chemical sensors are used in many settings, including:
• Medical and health care arena for clinical diagnosis, drug screening, etc.;
• Pharmaceutical development, biotechnology, microbiology, bacteriology,
virology, genomics, and proteomics research;
• A vast range of industrial process control;
• Industrial effluent and pollution control;
• Safety;
• Environmental monitoring; and
• Defense and security (chemical and biological weapons, explosives, and narcotics
detection).
The United States chemical sensor market is valued at approximately $3 billion per year,
of which between one half and two thirds is for medical diagnostics, and much of the
remainder is for gas sensors. The market is growing; this is a result of past sensor
development research and, in turn, is contributing to current and future research for even
better sensors. The characteristics of an ideal chemical sensor include:
• Inexpensive—advantageous on its own merits and allows use of multiple
detectors in arrays;
• Low operating costs;
• Robust;
• Reliable;
• Reversible;
• Continuously useable;
• High specificity;
• High selectivity;
• High sensitivity;
• High accuracy;
• Repeatability;
• Fast speed of response;
• Broad dynamic range;
• Insensitivity to (or ability to compensate for) interference by factors such as
temperature, pH, ionic strength, electrical and magnetic fields, etc.;
• Small size for in-situ (biological or geological use); and
• Minimal perturbation of the sample.
Nanotechnology offers the promise of providing nanosensors capable of achieving many
of these ideal characteristics, particularly those associated with speed, selectivity,
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sensitivity, and size reduction. As a consequence, nanosensor development is one of the
most active areas in the whole of nanotechnology. There are a number of major drivers
for this activity. The first is miniaturization of sensors, which has been a pronounced
trend in sensor development for the past 50 years. Some advantages of miniaturization
are obvious. Smaller size:
• leads to less material used in fabrication, enabling mass production contributing to
lower cost and broadening market availability.
• leads to reductions in weight and power consumption, which dramatically
increase the versatility and range of options for use, and
• permits reduction of sample size and decrease in any reagent consumption.
3.1.3 The Trend towards Miniaturization
All of the preceding factors have encouraged the trend towards microsensors over the
past decades, but the move to the nanoscale invites factors beyond the simple
continuation of miniaturization.
• First, the new properties that are realized at the nanoscale and that have been
partly discussed in earlier sections can be used in either the receptor or transducer
parts of the sensor.
• A second nanoscale factor is that there are some inherent benefits in working
directly at the molecular level where the sensing phenomena take place.
A study that directly asked the question of whether smaller is better for sensors
(Kopelman and Dourado 1996) examined optical sensors and formalized the specific
advantages of having nanoscale dimension sensors. In most instances, there is an explicit
functional dependence of optode characteristics on the sensor radius (r), with the absolute
detection limit decreasing with the cube of the radius (r3), and the response time
decreasing with the square of the radius (r2). Other features that improve, as sensors get
smaller, include sample volume, sensitivity, invasiveness, spatial resolution, dissipation
of heat in sensor and/or sample, and materials cost. A third nanoscale factor is that
fabrication advances in the semiconductor and related industries, together with the
coming of control of fluids on the microscale, has allowed the integration of many
laboratory processing steps into a single device, and given rise to the concept and
implementation of the LOG discussed in Section 3.2.
The development of the LOG is directly related to a second major driver of nanosensor
development—the analytical needs of the biotechnological and biomedical industries.
Across many areas of biologically-related research, including the enormous areas of
pharmaceutical and proteomic research, there is a need for cellular-level and massive-
throughput analytical and sensing capabilities. The large-scale, low-cost manufacturing
potential for nanosensors, and the greatly reduced reagent demands associated with
sample preparation in their use, make them extremely attractive tools. In the medical
field, early detection and diagnosis can greatly reduce the cost of patient care associated
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with advanced stages of many diseases. Similarly, point-of-care sample analysis and real-
time diagnosis can provide major savings for treatment of less serious conditions.
An additional issue, not so much a driver for sensor development, but rather an enabling
aspect in comparison with the development of remedial technologies, is the comparative
ease of sensor development. Past experience, particularly in federal agencies with
environmental responsibilities (such as EPA, DOE, and Department of Defense (DoD)),
indicates that, generally, the time to maturity for a sensor development project (i.e., the
duration of the entire effort from initial idea, through proof-of-concept, scientific
development, engineering development, prototyping, demonstration, and deployment) is
considerably shorter and less complex than the time to maturity for a remedial technology
development project. Further, in terms of market penetration, or the transition from first
being commercially available to being widely deployed, the timeline is shorter and the
regulatory hurdles are much lower. Since the risks associated with sensors are much
smaller than with remedial technologies and the development costs are smaller, even
when allowance is made for the shorter development time, environmental sensors are a
much more attractive investment than remedial technologies. This situation is likely to
apply to nanotechnologies, as well as to conventional technologies.
3.1.4 Opportunities
In the environmental area (in general) and the remediation of radionuclides (in
particular), there is a marked need for sensors with:
• lower fixed and operating costs;
• better performance in terms of sensitivity and specificity; and
• more versatility in terms of portability and field operability.
Control of remedial processes, contaminant detection, compliance monitoring, and
environmental decision-making should all benefit from sensors with molecular level
detection and improved overall performance. Current methods are costly, time-intensive,
and limited (Sandia 2005). The use of in-situ or field operable sensors eliminates risks
and costs associated with sample collection, handling, custody, transport, and storage.
DOE's Savannah River Site requires manual collection of nearly 40,000 groundwater
samples per year, with a cost of between $100 and $1,000 per sample for off-site analysis
(Looney and Falta 2000).
It is anticipated that nanotechnology will have a bigger impact in providing sensors for
radionuclide remediation than it will in providing remedial technologies, due to:
• the well-structured basic understanding of sensor technology development;
• the shorter, less expensive timelines for sensor development; and
• the existing level of activity surrounding nanoscale sensor development.
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The number of development efforts for radionuclide sensors is still very small, but this is
because most efforts are being put into the medical and biotechnology areas, where there
is a much larger potential market. Given our level of understanding of how receptors are
integral to sensors and our level of understanding of how to design receptors for
radionuclide species, it is a small extrapolation from the current, biologically oriented
state-of-the-science to a future state-of-the-science, where radionuclide sensors can be
easily realized. The following sections describe sensor technologies, or technology
concepts, that can be comfortably extrapolated to radionuclides. As with Section 2 on
remedial technologies, since extrapolation is involved, the survey cannot be
comprehensive but rather seeks to provide information on reasonable possibilities that
may yield sensor technologies in the near future.
3.1.5 References
IUPAC. 1997. Compendium of Analytical Nomenclature—Definitive Rules 1997, 3rd
Edition. Chapter 7, Section 4. Prepared for Publication by Janos Inczedy, Tamas Lengyel,
and Allan M. Ure (Eds.). International Union of Pure and Applied Chemistry, Research
Triangle Park, North Carolina, USA. Website. Accessed November 2007.
http://www.iupac.org/publications/analytical_compendium/Cha07sec4.pdf
Kopelman, R. and S. Dourado. 1996. Is smaller better? - Scaling of characteristics with
size of fiber-optic chemical and biochemical sensors. Proceedings in Society of Photo-
Optical Instrumentation Engineers (SPIE). 2836, 2-11.
Sandia. 2005. Micro-Chemical Sensors for In situ Monitoring and Characterization of
Volatile Contaminants. Website. Accessed December 2007.
http://www.sandia.gov/sensor/MainPage.htmtfDescription.
Looney, B.B. and R.W. Falta (eds.). 2000. Vadose Zone Science and Technology
Solutions. Battelle Press, Columbus Ohio.
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3.2 "Lab-on-a-Chip"
A Lab-on-a-Chip (LOG) device, also known as a micro-total-analytical system
(microTAS) or microfluidics device, is a device that can integrate miniaturized laboratory
functions (such as separation and analysis of components of a mixture) on a single
microprocessor chip using extremely small fluid volumes on the order of nanoliters to
picoliters. From a technology categorization perspective, LOCs can be viewed as a subset
of microelectromechanical systems (MEMS) and combine miniaturized or novel sensing
systems, fluid flow control concepts from microfluidics, and the suite of fabrication
techniques (such as material deposition, material removal, surface patterning, and
electrical property modification) used by the semiconductor industry.
Currently, the main commercial applications of LOCs are in the medical and
biotechnological fields, where it is anticipated that developments so far are the heralds of
a technological revolution. In the same way that miniaturization changed computers from
machines of limited capabilities occupying large rooms to small and easily portable yet
powerful technology of today, over a period of a few decades, medical, biotechnological,
and chemical analysis is expected to move from room-sized laboratories to microchip-
based devices housed in hand-held or small portable readout consoles. Figure 28 shows
an example of an LOG device that was tested on the International Space Station in 2007.
Figure 28: LOC device tested on the International Space Station in 2007
At the heart of LOC devices are "chips", ranging in size from a fingernail to a credit card,
fabricated using processes adapted from the printed circuit industry such as lithography,
chemical etching, and laser machining. Figure 29 illustrates an impression of the size of
the chip. Figure 30 provides a functional diagram of LOCs.
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Figure 29. A comparison of the size of LOCs
Sons or
Healer
EC-Elution Cham be
R5- Waste
R7- DEP Fluid Carrier
R6- PCR Reagents
PCR
R1 Sample
R2- Lysi» Buffer &
Magnetic Scads
R3- Washing Buffer
R4- Resuspension Buffer
ieleclmphorelic
Sorting Electrode
Arrays
PCR Product Detectors
Electromagnet
LC- Lyais Chamber
R: Reservoir
P: Micropump
Figure 30. Functional diagram of LOCs
In a manner similar to the production of printed circuit boards using techniques such as
embossing and molding, microstructures (such as channels for liquid flow and pits for
mixing and reactions) are made on the chip by depositing layers of material on top of one
another on a surface, then patterning and selectively removing material to form a feature.
A flat top surface or lid is attached to enclose the channels or mixing pits, and reagents
can be driven around the system by pneumatic, electromotive, or capillary systems.
The LOG was first conceived by Michael Widner at Ciba-Geigy (now Novartis) in the
1980s, described conceptually in 1990 (Manz et al. 1990) with a groundbreaking work
being published in 1992 (Harrison et al. 1992). Further development occurred as a new
area of discovery—microfluidics—was developed in the 1990s. Microfluidics is an
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interdisciplinary field dealing with the behavior and control of extremely small volumes
of fluids and the design of systems that use these small volumes. Though most commonly
encountered in ink-jet printers, the vast majority of microfluidics applications have been
in biotechnology research, and some experts even regard it as a branch of biotechnology.
In some ways, Microfluidics parallels nanotechnology in that the behavior of fluids at the
microscale can differ substantially from the behavior at the macroscale; phenomena such
as surface tension, heat conduction, and fluidic resistance start to become important, and
issues such as evaporation, absence of turbulent flow, and the threat posed by presence of
air bubbles are critical to system design.
Initially, much of the impetus for continued development of LOCs came from the Human
Genome Project, a 13-year project coordinated by DOE and the National Institutes of
Health (NIH) that began in 1990 and was completed in 2003. Currently, much of the
impetus for the continued development of LOCs comes from the desire for point-of-care
medical diagnostics, whether in the doctor's office, on a spacecraft, or other remote
location. Additionally, development research is driven by the continued need for
miniaturization, both to reduce the costs and the environmental impacts of research
(green analytical chemistry). The LOG concept, already significant, is still considered to
be in its infancy. Development research continues in many areas. In the area of
fabrication materials, LOCs constructed using soft lithography techniques, rather than
silicon microchip fabrication processes, are being investigated. Soft lithography is an
alternative to silicon-based micromachining that uses replica molding of nontraditional
elastomeric materials to fabricate stamps and microfluidic channels. In an extension to the
soft lithography approach, multilayer soft lithography, with which devices consisting of
multiple layers may be fabricated from soft materials, is being used to build active
microfluidic systems containing on/off valves, switching valves, and pumps entirely out
of elastomer. The softness of these materials allows the device areas to be reduced by
more than two orders of magnitude compared with silicon-based devices. The other
advantages of soft lithography (such as rapid prototyping, ease of fabrication, and
biocompatibility) are retained (Unger et al. 2000).
Environmental LOCs are also being investigated. An environmental LOG project is being
funded by EPA with objectives to create a novel, nanomaterial-based submersible
microfluidic device, exploiting unique properties of metal nanoparticles and carbon
nanotubes for rapidly, continuously, and economically monitoring different classes of
priority pollutants. The project also seeks to understand the relationship between the
physical and chemical properties of these nanomaterials and their observed behavior. The
challenge addressed is to help transform the LOG concept to an effective environmental
monitoring system, and involves the examination of nanoparticle and nanotube materials
for the separation and detection processes, respectively (Wang 2007). In addition, the
NIH is supporting the development of a point detection disposable LOG with built-in
mercury precursor electrodes for heavy metal detection (Ahn 2006).
Within these development efforts, it is also recognized that for novel and innovative
technologies, even those that have an established market presence, close communication
between developers and future users is essential. For example, in the United Kingdom,
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research collaboration between five leading universities in the healthcare technology
assessment arena and a group of industrial partners—Multidisciplinary Assessment of
Technology Centre for Healthcare (MATCH) —is conducting a survey of LOG point-of-
care device manufacturers. Point-of-care in this context is defined as "analytical testing
performed outside the central pathology laboratory using a device or devices that can be
easily transported to the vicinity of the patient" (MATCH 2006). The aim is to assess the
value of LOG for the diagnosis of cardiac-related problems using case studies, and to
develop methods to shorten the time and decrease the costs of LOG development.
3.2.1 Summary of Environmental Potential
The LOG has great potential for addressing environmental needs. The technology
platform is mature and well-established, and as other nano-enabled sensing technologies
are developed, integration into the LOG should be facile. The twin features of rapid
sample throughput and field portability should make the LOG a valuable tool in filed
operations, particularly in circumstances such as the EPA Triad approach, where real-
time monitoring is required to guide the progress of remedial work.
3.2.2 References
Ahn, C.H. 2006. A Point Detection Disposable Lab-on-a-Chip With Built-in Mercury
Precursor Electrodes For Heavy Metal Detection. Website. Accessed December 2007.
www.biomems.uc.edu/sponsors/_index.html.
Harrison, D.J., A. Manz, Z. Fan, H. Liidi, H. M. Widmer. 1992. Capillary Electrophoresis
and Sample Injection Systems Integrated on a Planar Glass Chip. Analytical Chemistry.
64:1926-1932.
Manz, A., N. Graber, H.M. Widmer. 1990. Miniaturized total chemical analysis systems:
A novel concept for chemical sensing. Sensors and Actuators B: Chemical. 1(1-6): 244-
248.
Multidisciplinary Assessment of Technology Centre for Healthcare (MATCH). 2006.
Industrial Survey on Cardiac Point-of-care. Website. Accessed December 2007.
http://www.match.ac.uk/POCT industrial survev/index.jsp.
Unger, M.A, H.-P. Chou, T. Thorsen, A. Scherer, S.R. Quake. 2000. Monolithic
Microfabricated Valves and Pumps by Multilayer Soft Lithography. Science. 288(5463):
113-116.
Wang, J. 2007. Nanomaterial-Based Microchip Assays for Continuous Environmental
Monitoring. Grant number RD - 83090002-0. US E.P.A, Washington D.C.
http://vosemite.epa.gov/oarm/igms egfnsf/fca67bald90470b585256fb6006df291/0a3daa
8868al9a7c85256f90002109b9!OpenDocumentwww.nibib.nih.gov/.../1112April2006%
20POCT/FINAL%20book%20for%20NIBIB after.pdf
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3.3 Microcantilever Sensors
Microcantilever sensors are a technology which may develop into sensing systems for
radionuclides, or is at least an example showing the potential and emergence of a new
generation of highly sophisticated but flexible sensors. From a mechanical engineering
perspective, a cantilever is simply a beam supported at one end and capable of defined
bending and vibrational behavior. A microcantilever is simply a very small cantilever, the
properties of which can be understood from basic engineering principles.
The microcantilever was first developed in 1986 (Binnig et al. 1986) for use in Atomic
Force Microscopy (AFM), the premier tool for nanoscale imaging and measuring. In
AFM, an extremely sharp microscale tip, (with a tip radius of a few nanometers)
connected to the end of a microcantilever (up to 10 nm thick, about 500 nm wide and
about 2,500 nm long, and fabricated from Si or SisN/t), is positioned extremely close to
the surface of a sample and the sample, is then moved beneath the tip. Interactions
between the surface of the sample and the end of the tip arising from atomic forces (such
as Van der Waals force, electrostatic force, magnetic force, or capillary force) attract or
repel the tip (depending on mode of operation) and bend the microcantilever. Reflecting a
laser beam off the cantilever and monitoring the beam's deflection with photodiode
arrays measures the amount of bending. A graphical representation of the AFM sensing
components is provided in Figure 31, and a Scanning Electron Micrograph of a
microcantilever biosensor used for DNA detection is shown in Figure 32.
Sample Surface
Cantilever & Tip
PZT Scanner
Figure 31. Principle of operation of the atomic force microscope
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Mag= 1.63KX
EHT = 13.00kV
WD= 35mm
Signal A = MPSE Date :27 Oct 2005
Photo No. = 2180 Time :22:28:07
Figure 32. Scanning electron micrograph of fabricated microcantilever biosensor
used for DNA detection
The microcantilever is the simplest MEMS device. Early work soon demonstrated the
versatility of the device in AFMs, with the microcantilever able to perform in air, liquid,
or vacuum and across a range of temperatures. It also demonstrated the extreme
sensitivity of the AFM to environmental effects and impurities, and the need to control
these in making accurate measurements. However, studies on overcoming these issues
also showed that the sensitivity of AFM to these environmental factors could be turned
around, allowing the AFM to be a sensor for these same factors. For example, a
microcantilever fabricated from silicon and coated with an aluminum surface for
reflection of the laser beam can act as a "bimetallic strip" and respond to temperature
changes. If a thermal event takes place on the cantilever surface, the silicon and
aluminum expand to different extents and the cantilever bends, allowing the cantilever to
act as a calorimeter of near-ultimate sensitivity. Similarly, if the microcantilever is given
an absorbent coating that can attract water and is allowed to vibrate in dry air, it will have
a natural frequency of vibration; changes in humidity change the mass of the
microcantilever, and thus change the frequency of vibration, allowing it to act as a
humidity sensor. From this, it is a short step to the concept of coating the microcantilever
with a chemical functionality that binds selectively with a target analyte. In the absence
of the analyte, the microcantilever vibrates with one frequency, while in the presence of
the analyte, binding with the microcantilever coating occurs and vibration takes place at a
frequency directly related to the degree of analyte binding—hence concentration.
A related sensing mode is that the surface of the microcantilever can be coated with a
layer of material that contains the chemical functionality able to bind selectively with a
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target analyte. If this layer expands or contracts as the analyte is bound, then bending of
the microcantilever occurs in a manner analogous to that of the bimetallic strip used for
thermal sensing. The degree of bending is directly related to analyte concentration. This
behavior was the basis of a microcantilever radionuclide sensor development project at
ORNL. As described in Section 2.3 on SAMMS, if a head group at the end of a long
organic chain molecule can interact strongly with a substrate, then a closely packed
molecular monolayer of the chain molecules aligned with the chains pointing outwards
from the substrate surface can form spontaneously when the two are brought into contact.
The tail end of the organic chain can be functionalized prior to, or after, self-assembly
using chemical functionalities selective for a targeted species. Thiol (S-H) head groups
and a gold substrate form an excellent pairing for this type of behavior, and the self
assembly of alkanethiols has been observed to produce surface stress in the gold substrate
(Bergeretal. 1997).
Silicon and closely-related materials have been the main materials of construction for
microcantilevers. Silicon has been recognized as an outstanding mechanical material for
over a quarter of a century (Petersen 1982). However, silicon is not the only fabrication
material under investigation. Chemical sensing with micromolded plastic cantilevers and
production issues have been explored (McFarland and Colton 2005).
In addition to microcantilevers based on silicon, plastics; and related materials,
nanostructures (such as single-walled carbon nanotubes (SWCNT)) have been explored
for gas-sensing applications (Hsu 2007). The approach uses the fact that SWCNTs are
capable of interacting with the gaseous species—either directly through surface
adsorbtion, or indirectly by using a polymer analyte coated on its surface—and the higher
surface-to-bulk ratio available with a nanostructure leads to higher sensitivity and shorter
response time. As with other microcantilevers, the effect of bound or adsorbed species is
manifested either as a change in resonant frequency (which can be detected by using a
Wheatstone bridge circuit), or as an increased surface stress (which can be detected by
measuring the change in the capacitance value through comparison with a specific
reference capacitor). Hsu's work involved successful simulation, fabrication, and
manipulation of the SWCNT; development and simulation of a capacitive sensing circuit
layout; and consideration of packaging and integration issues, including use of a
"nanoglue" developed at Rensselaer Polytechnic Institute, based on the processing of a
self-assembled molecular monolayer and capability of bonding completely dissimilar
materials.
Among the drivers for further development was the widespread need for portable, real-
time, in-situ chemical, physical, and radiological sensors in a variety of applications
including the characterization and monitoring of mixed waste, ground water,
contaminated soil, and process streams. Microcantilever-based sensors were recognized
as a potential solution for this need. They also provide excellent sensitivity for important
metal ions in solution such as Hg2+, CrC>42", Sr2+, and TcO4". The ability of the
microcantilever to detect cesium (Thundat et al. 1999, Ji et al. 2000a) (though
irreversibility problems were observed) and chromate (Ji et al. 2000b) was demonstrated;
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concentrations below the parts per billion level were obtained with exceptional selectivity
even in the presence of other interferences.
Fundamental microcantilever research at ORNL showed that adsorption-induced changes
in the spring constant of a cantilever, leading to errors in the calculation of adsorbed mass
from shifts in resonance frequency (Thundat 2002). Simultaneous measurement of
resonance frequency and adsorption-induced bending was shown to allow the change in
spring constant to be determined. A silicon microcantilever with gold coating on one side
was found to respond selectively and sensitively to Hg(II) ions in solution, and while
modification of the Si surface with a silane reagent did not change the response to Hg,
modification of the gold surface with octanethiol greatly retarded the rate of deflection,
indicating the Hg(II) is reacting with the gold surface. The surface charge on the gold-
solution interface is postulated to reduce Hg(II) to a surface amalgam. Modification of
the gold surface with a monolayer 1,6-hexanedithiol makes the surface sensitive and
selective for (CH3)Hg+ adsorption-induced deflection. Na+, K+, Pb2+, Zn2+, Cd2+, Ca2+,
and Ni2+ in solution do not interfere with the response of the microcantilever to Hg. Gold
coated cantilevers with chemically modified surfaces respond sensitively to Ca2+ ions at a
concentration of 10"9 M. The sorption of a monolayer of 2-(4-mercaptophenoxy)-N, N-
diethyl-acetamide, as well as the agent bis (11-mercaptoundecyl) phosphate were shown
to detect Ca2+ ions, although the former was more selective. A self-assembled monolayer
of L-cysteine on a cantilever coated with gold on one side was shown to be effective for
the detection of a concentration of 10"10 M Cu2+. Both the Ca2+ and the Cu2+ were
relatively free from interference by each other and Na+, K+, Pb2+, Zn2+, Cd2+, and Ni2 in
solution (Thundat 2002).
Subsequent work showed that electrochemically-active metal ions (Cu, Cr, Hg and Pb)
could be detected by the novel approach of using a cantilever as a working electrode
since electrodeposition of electro-active metal ions on cantilever surface results in
cantilever bending. Together with the observation that that the cantilever bending is
extremely sensitive to electrochemical current in the solution, this has led to the
development of a technique where the cantilever serves as a reference/counter electrode
for electrochemical reactions occurring on another working electrode (Thundat et al.
2006), with work continuing towards the development of field-deployable, miniature
sensors with extremely high sensitivity, exceptional selectivity, and the ability to be
integrated into a wireless communication system that will allow real-time data to be
provided on concentration and speciation of multiple contaminants and their variation
with time.
Work has also continued on microcantilevers functionalized with metal-binding moieties.
Gold-coated sides of silicon nitride microcantilevers functionalized with the metal-
binding protein AgNt84-6 have been demonstrated to be sensors for the detection of
heavy metal ions, such as Hg2+ and Zn2+ (Cherian et al. 2003). On exposure to HgCb and
ZnCb solutions, the microcantilevers underwent bending corresponding to an expanding
gold side, while exposure to MnCb solution did not result in a similar bending, indicating
a weak or lacking interaction of Mn2+ ions with the AgNt84-6 protein. The
microcantilever bending data were consistent with data from electrophoresis that showed
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protein interaction with Zn2+ ions but not with Mn2+ ions, demonstrating that
microcantilever bending can be used to discriminate between metal ions that bind and do
not bind to AgNt84-6 protein in real time.
3.3.1 Summary of Environmental Potential
Microcantilever sensors are one of the few nano-sensor technologies that have already
been investigated with the detection of radioactive species in mind. The technology is
very flexible; all that is needed for detection of a species is a coating with a chemical
functionality capable of binding the target species. The technology is well-established,
reliable, and sensitive—its origin in the AFM makes it a gateway technology for the
nanotechnology age—and is easily integrated into the LOG platform. The versatility of
the technique is further expanded by the fact that many materials, including innovative
nanomaterials such as carbon nanotubes, may be usable as the cantilever.
3.3.2 References
Berger, R., E. Delamarche, H.P. Lang, Ch. Gerber, J.K. Gimzewski, E. Meyer, H.-J.
Giintherodt. 1997. Surface stress in the self-assembly of alkanethiols on gold. Science.
276: 2021-2024.
Binnig, G., C.F. Quate, C. Gerber. 1986. Physics Review Letters. 56: 930-933.
Cherian, S., R. K. Gupta, B. C. Mullin and T. Thundat. 2003. Detection of heavy metal
ions using protein-functionalized microcantilever sensors. Biosensors andBioelectronics.
19(15)411-416.
Hsu. J.C. 2007. Fabrication of Single Walled Carbon Nanotube (SW-CNT) Cantilevers
for Chemical Sensing. Thesis for Master of Science in Electrical Engineering (etd-
11082007-103811), Louisiana State University, May 2007.
Ji, H.-F., E. Finot, R. Dabestani, T. Thundat, G. M. Brown, P. F. Britt. 2000a. A Novel
Self-Assembled Monolayer (SAM) Coated Microcantilever for Low Level Cesium
Detection. Chemical Communications. 457-458.
Ji, H-F., T. G. Thundat, R. Dabestani, G. M. Brown, P. F. Britt, P. V. Bonnesen. 2000b.
Ultrasensitive I
73: 1572-1576.
Ultrasensitive Detection of CrC>42" Using a Microcantilever Sensor. Analytical Chemistry.
McFarland, A.W. and J.S. Colton. 2005. Chemical sensing with micromolded plastic
microcantilevers. Journal ofMicroelectromechanicalSystems. 14: 1375-85.
Petersen, K. E. 1982. Silicon as a mechanical material. Proceedings of IEEE. 70: 420-
457.
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Thundat, T., E. Finot, H-F. Ji, R. Dabestani, P. F. Britt, P. V. Bonnesen, G. M. Brown, R.
J. Warmack. 1999. Highly Selective Microcantilever Sensor for Cesium Ion Detection.
Proceedings of Electrochemical Society. 99(123)314-319.
Thundat, T.G. 2002. Microsensors for In situ Chemical, Physical, and Radiological
Characterization of Mixed Waste. EMSP-73 808-2002. U.S. DOE Environmental
Management Science Program Report, Washington DC.
Thundat, T.G., Z. Hu, G.M. Brown, B. Gu. 2006. Microcantilever Sensors for In situ
Subsurface Characterization. 2006 ERSD Annual Report. Oak Ridge National
Laboratory, Tennessee.
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3.4 Spectroscopic Sensors
Spectroscopy is the scientific study of the absorption, emission, or scattering of
electromagnetic radiation by atoms, molecules, ions, solids, liquids, or gases. The
underlying and unifying phenomenon behind all types of spectroscopy is that the
interaction of electromagnetic radiation with any type of matter can cause transitions
between quantized energy levels (often, though not always, electronic energy levels) of
the atoms or molecules. Since so many of these transitions can occur and can be
influenced and modified by other phenomena, spectroscopy provides the potential for
enhanced chemical analysis and sensing. The list of spectroscopic techniques and sub-
techniques is large and continually increasing. A partial sampling of these techniques
includes:
Absorption spectroscopy
Atomic absorption spectroscopy
Atomic emission spectroscopy
Atomic fluorescence spectroscopy
Attenuated total reflectance spectroscopy
Auger electron spectroscopy
Cavity-ringdown laser absorption spectroscopy
Electron paramagnetic spectroscopy
Electron spectroscopy
Electron spin resonance spectroscopy
Extended x-ray absorption fine structure spectroscopy
Fluorescence spectroscopy
Fluorescence correlation spectroscopy
Gamma-ray spectroscopy
Image correlation spectroscopy
Infrared spectroscopy
Intracavity-absorption spectroscopy
Laser spectroscopy
Laser-induced fluorescence
Mass spectrometry
Mossbauer spectroscopy
Nuclear magnetic resonance spectroscopy
Multiplex or frequency-modulated spectroscopy
Raman spectroscopy
Resonance-ionization spectroscopy
Tunable diode laser absorption spectroscopy
Surface-enhanced raman spectroscopy
UV-vis absorption spectroscopy
X-Ray spectroscopy
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Phenomena based on nanoscale effects are being examined in and by many of these types
of spectroscopy. Some examples include:
• Nanosensors for nitric oxide using cytochromes have been examined and show a
fast, reversible, and linear response up to 1 mM nitric oxide with a detection limit
of 20 nM (Barker et al. 1998).
• A fluorescence nanosensor for Cu2+ ions using surface functionalization of silica
particles with trialkoxysilane derivatized ligand and fluorescent dye has been
investigated (Brasola et al. 2003).
• PEBBLE nanosensors, in the 20 to 200 nm size range, have been made for zinc
detection using two fluorescent dyes (one sensitive to zinc and the other as a
reference) localized in a polymer matrix by a microemulsion (Sumner et al. 2002).
• A simple colorimetric technique for the detection of small concentrations of lead,
cadmium, and mercury) using reversible chelation/aggregation process for
functionalized gold nanoparticles has been developed (Kim et al. 2001).
• A distributed Bragg reflector (a high-quality reflector used in waveguides and
formed from multiple alternating layers of materials with varying refractive
index) optical sensing element for organic solvents has been developed using
stacks of Teflon®-like and gold nanoparticles. Absorption of organic vapors
causes swelling of the composite and affects the reflectivity (Convertino et al.
2004).
• The first nanometer-scale anion sensing fluorescent spherical nanosensors have
been developed (Brasuel et al. 2003).
• A rapid and sensitive fluorescence immunoassay has been developed for the
simultaneous detection and identification of multiple harmful microorganisms
using dye-doped silica nanoparticle-antibody conjugates (Zhao et al. 2004).
• The fabrication of submicron optical-fiber fluorescent sensors and particle-based
fluorescent nanosensors has been reviewed, and the functional characteristics of
miniaturized fluorescent sensors and their applications for quantitative
measurement of intracellular analytes have been discussed (Lu and Resenzweig
2004).
• A nanoscale fluoroimmunoassay for the herbicide atrazine in an LOG has been
developed using lanthanide oxide nanoparticle labels (Koivunen et al. 2004).
As these examinations indicate, most spectroscopic sensing methods detect the binding of
a target species to a receptor by incorporating a fluorescent moiety into the receptor and
then examining the fluorescence behavior. Fluorescence spectroscopy analyzes the
fluorescence of a material, where fluorescence is a non-thermally originating
luminescence phenomenon in which a molecule or other species absorbs a photon at one
wavelength (usually in the visible range), and then re-emits another photon with a longer
wavelength (usually in the ultraviolet range), with the difference in energy between the
two photons becoming thermal energy through vibrational relaxation. Fluorescence can
now reach down to the single molecule detection level and is such a powerful technique
that, over the past decade, it has become the dominant tool in biotechnology and medical
imaging (Geddes 2005). The following two examples demonstrate nanotechnologies that
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incorporate the fluorescence spectroscopy tool, PEBBLE and Surface Plasmon
Resonance.
3.4.1 Probe Encapsulated by Biologically Localized Embedding
A nanotechnology that incorporates the fluorescence tool is PEBBLE. PEBBLE
nanosensors are sub-micron sized optical sensors designed specifically for minimally
invasive analyte monitoring in viable, single cells. PEBBLE is a general term that
describes a family of matrices and nano-fabrication techniques used to miniaturize many
existing optode technologies. The main classes of PEBBLE nanosensors are based on
matrices of polyacrylamide hydrogel, sol gel silica, and cross-linked decyl methacrylate.
These matrices have been used to fabricate sensors for H4", Ca2+, K+, Na+, Mg2+, Zn2+, Cl",
NCV, O2, NO, and glucose that range from 30 to 600 nm in size. A host of delivery
techniques have been used to successfully deliver PEBBLE nanosensors into mouse cells
(Monson et al. 2003, Park et al. 2003, Xu et al. 2001, Buck et al. 2004, Xu et al. 2002,
Clark et al. 1999, Sumner et al. 2002). The PEBBLE nanosensor format offers the twin
benefits of protecting fluorescent indicator dyes from interferents and allowing
combination of multiple dyes, ionophores, and other components to create complex
sensing schemes. It is the multifunctionality that is the main advantage of PEBBLE
nanosensors for biological research. A conceptual representation is shown in Figure 33.
Figure 33. Conceptual representation of the multifunctionality of PEBBLE
nanosensors
PEBBLE nanosensors were specifically developed for biological work since they have
the advantages of a small size that permits intra-cellular biological measurements to be
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made without causing cell mortality; a polymeric matrix able to contain a combination of
indicator dyes, reference dyes, and catalysts (including some that might normally be toxic
to the cell); fast response time; and the option of being calibrated by either ex-vivo or ex-
vitro means. The use of both indicator dyes and reference dyes enables ratiometric
fluorescent detection, which can correct for variations in excitation source intensity and
minimizes the effects of photobleaching on quantitative measurements.
3.4.2 Surface P/asmon Resonance
While PEBBLE nanosensors offer a platform on which existing spectroscopic techniques
can be mounted, a spectroscopy that offers great potential for chemical sensing and has
achieved a much deeper understanding as a result of nanotechnology (specifically
through the development of the new field of nanoparticle optics) is surface plasmon
resonance (SPR) spectroscopy.
Comprehension of SPR first requires an understanding of the plasmon phenomenon. In
physics, photons are regarded as the particle equivalent of quantized electromagnetic
waves. From solid-state physics, it is also commonly understood that the electrons in a
metallic crystal can be successfully modeled as a quasi-ideal gas, or plasma.. Oscillations
in the charge density of this quasi-ideal gas can occur, and when waves are quantized as
standing waves then the particle equivalent is known as a plasmon. Surface plasmons are
plasmons confined to the surface of a material or the interface of plasma-containing
material with some other material. They propagate parallel to the material surface and are
evanescent waves that decay exponentially with distance from that surface; they are thus
very sensitive to any changes at the surface. Plasmons can be excited by coupling with
incident light to give the phenomenon of surface plasmon resonance.
Gold and silver are the classic metals for supporting surface plasmons, but metals such as
copper, chromium, and titanium also support the phenomenon. When surface plasmon
resonance is achieved on macroscale or mesoscale surfaces using very thin metal films,
the plasmon wave can propagate across the surface for a distance of tens or hundreds of
thousands of nanometers, but decays quickly as it moves outward from the surface. This
is known as propagating surface plasmon resonance. Surface plasmon resonance can also
be achieved on the nanoscale using metal nanoparticles, even though they are smaller
than the wavelength of the light involved. In this case the plasmon wave oscillates locally
around the nanoparticle. This manifestation of the phenomenon is called localized surface
plasmon resonance (LSPR). A comprehensive review of LSPR spectroscopy and sensing
is available (Willets and Van Duyne 2007).
The primary consequences of LSPR resonance include (1) localized electromagnetic field
enhancement that is responsible for the intense signals observed in surface-enhanced
spectroscopies, and (2) selective photon absorption and scattering (collectively called
extinction), which can be easily monitored using UV-vis spectroscopy (Haes and Van
Duyne 2002). Regarding the extinction behavior, theoretical considerations of the optical
properties of nanoparticles (Haes et al. 2004) indicates that there are at least four different
nanoparticle-based sensing mechanisms for transducing chemical-binding events into
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optical signals: (1) resonant Rayleigh scattering from nanoparticle labels (analogous to
fluorescent dye labels); (2) nanoparticle aggregation behavior; (3) charge-transfer
interactions at nanoparticle surfaces; and (4) local refractive index changes. The plasmon
resonance phenomenon thus provides a variety of approaches for measuring its behavior
that can be exploited in the development of sensors.
Changes to the surface have major effects on this resonance. It has been observed that
solutions of certain nanoparticles (e.g., gold) can give rise to intense colors; as far back as
the medieval period, artists working in stained glass used this effect (see Figure 34). The
origin of this effect, which is not manifested in the bulk material, is the absorption of
energy in the ultra-violet region of the spectrum as a result of surface plasmon resonance.
Changes to the surface of the gold particles (e.g., the binding of DNA or other bio
material) have noticeable and easily measurable effects on the frequency of the surface
plasmon resonance, and can be used as a sensing mechanism.
Figure 34. The colors in medieval stained glass are the result of surface plasmon
resonance
The most acclaimed use of LSPR spectroscopy using nanoparticles has been the detection
of a biomarker for Alzheimer's disease from synthetic and clinical samples (Haes and
Van Duyne 2004, Haes et al. 2005). It has also been shown to detect small molecules,
such as camphor (Zhao et al. 2006b). Further, with the appropriate recognition moiety,
LSPR should be easily modifiable to detect metals and radionuclides. In fact, the
feasibility of developing an SPR spectroscopy-based sensor, a technique very closely
related, for a radioactive material (pertechnetate) has already been demonstrated
(Anderson 2000). A previously developed fiber optic SPR sensor used for gas phase dew
point determination was modified to liquid-phase sensing by placing a coating of
polyethylene glycol (PEG, which has been used in aqueous biphasic extraction for
removal of pertechnetate from Hanford and Oak Ridge tank waste) on an SPR system
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consisting of a sapphire hemisphere coated with a thin gold film. The SPR sensor
generated high-resolution, low-noise SPR spectra suitable for high sensitivity sensing of
analyte adsorption onto the sensor surface, and was able to discriminate very small
changes in refractive index and show a unique response to 10 ppm perrhenate in NaOH
solution.
3.4.3 Summary of Environmental Potential
As a class, spectroscopic techniques (such as PEBBLEs, surface plasmon resonance, or
sensing technologies based on nanobelts and nanorods) offer the possibility of detection
capabilities down to the single molecule level. Sensing technologies for characterization
and monitoring uses are of extreme importance in the environmental field—both for
remediation and for process monitoring for pollution prevention applications. The
flexibility that can be imparted to nano-enabled sensors means that a wide variety of
species can be detected at the lowest levels, and creates a potentially large opportunity for
these and similar technologies.
3.4.4 References
Anderson, B.B. 2000. Feasibility Study for the Development of a Surface Plasmon
Resonance spectroscopy-based Sensor for the BNFL-Hanford. BNF-003-98-0308. U.S.
Department of Energy Savannah River Site, Technical Report.
Barker, S.L.R., R. Kopelman, M.A. Cusanovich. 1998. Fiber-Optic Nitric Oxide-
Selective Biosensors and Nanosensors. Analytical Chemistry. 70(5): 971-976.
Brasola E., F. Mancin, E. Rampazzo, P. Tecilla, U. Tonellato. 2003. A Fluorescence
Nanosensor for Cu2+ on Silica Particles. Chemical Communications. 24: 3026-3027.
Brasuel, M.G., TJ. Miller, R. Kopelman, M.A. Philbert. 2003. Liquid Polymer Nano-
PEBBLEs for Cl- Analysis and Biological Applications. Analyst. 128(10): 1262-1267.
Buck, S.M., Y.E. Koo, E.J Park, H. Xu, M.A. Philbert, M. Brasuel, R. Kopelman. 2004.
Optochemical nanosensor PEBBLEs: photonic explorers for bioanalysis with biologically
localized embedding. Current Opinion Chemical Biology. 8(5):540-546.
Clark, H.A., R. Kopelman, R. Tjalkens, M.A. Philbert. 1999. Optical nanosensors for
chemical analysis inside single living cells. 2. Sensors for pH and calcium and the
intracellular application of PEBBLE sensors. Analytical Chemistry. 71(21): 4837-4843.
Convertino, A., A. Capobianchi, A. Valentini, E.N.M. Cirillo. 2004. High Reflectivity
Bragg Reflectors Based on a Gold Nanoparticle/Teflon-Like Composite Material as a
New Approach to the Organic Solvent Detection. Sensors and Actuators B: Chemical.
100(1-2): 212-215.
Geddes, C.D. 2005. Advanced Concepts in Fluorescence Sensing. Springer, New York.
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Haes, A. J. and R. P. Van Duyne. 2002. A Highly Sensitive and Selective Surface-
Enhanced Nanobiosensor. Materials Research Society Symposium Proceedings. 723:
O3.1.1-O3.1.6.
Haes, AJ. and R. P. Van Duyne. 2004. A Unified View of Propagating and Localized
Surface Plasmon Resonance Biosensors. Analytical and Bioanalytical Chemistry. 379:
920-930.
Haes, A. J., L. Chang, W.L. Klein, R. P. Van Duyne. 2005. Detection of a biomarker for
Alzheimer's disease from synthetic and clinical samples using a nanoscale optical
biosensor. Journal of the American Chemical Society. 127:2264-2271.
Kim, Y., R.C. Johnson, J.T. Hupp. Gold Nanoparticle-Based Sensing of
"Spectroscopically Silent" Heavy Metal Ions. Nano Letters. 1(4): 165-167.
Koivunen, M.E., M.E., SJ. Gee, I.M. Kennedy, B.D. Hammock. 2004. Nanoscale
Fluoroimmunoassays with Lanthanide Oxide Nanoparticles: 'Lab-on-a-Chip'.
Presentation (Abstract ANYL 194) at the 227th ACS National Meeting, Anaheim, CA.
Lu, J. and Z. Rosenzweig. 2000. Nanoscale Fluorescent Sensors for Intracellular
Analysis. Journal of Analytical Chemistry. 366(6-7): 569-575.
Monson, E., M. Brasuel, M. Philbert, R. Kopelman. 2003. PEBBLE Nanosensors for in
vitro Bioanalysis. In Biomedical Photonics Handbook. T. Vo-Dinh (Ed.) CRC Press,
Boca Raton, Florida.
Park, E.J., M. Brasuel, C. Behrend, M.A. Philbert, R. Kopelman. 2003. Ratiometric
optical PEBBLE nanosensors for real-time magnesium ion concentrations inside viable
cells. Analytical Chemistry. 75(15): 3784-91.
Sumner, J.P., J.W.Aylott, E. Monson, R. Kopelman. 2002. A fluorescent PEBBLE
nanosensor for intracellular free zinc. The Analyst. 127(1): 11-16.
Willets, K.A. and R.P. Van Duyne. 2007. Localized Surface Plasmon Resonance
Spectroscopy and Sensing. Annual Review of Physical Chemistry. 58:267-297.
Xu, H., J.W. Aylott, R. Kopelman, TJ. Miller, M.A. Philbert. 2001. A real-time
ratiometric method for the determination of molecular oxygen inside living cells using
sol-gel-based spherical optical nanosensors with applications to rat C6 glioma. Analytical
Chemistry. 73(17): 4124-4133.
Xu, H., J.W.Aylott, R. Kopelman. 2002. Fluorescent nano-PEBBLE sensors designed for
intracellular glucose imaging. The Analyst. 127(11): 1471-1477.
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Zhao, X., L.R. Billiard, SJ. Mechery, Y. Wang, S. Jin, W. Tan. 2004. A rapid bioassay
for single bacterial cell quantitation using bioconjugated nanoparticles. Proceedings of
the National Academy of Sciences USA. 101(42): 15027-15032
Zhao, Y.-P., S.-H. Li, S.B. Chaney, S. Shanmukh, J.-G. Fan, R.A. Dluhy, W. Kisaalita.
2006. Designing Nanostructures for Sensor Applications. Journal of Electronic
Materials. 35(5): 846.
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3.5 Nanowire Sensors
3.5.1 Background
Nanowires are solid, rod-like materials with diameters in the 5-100 nm range, and are
most often made from metals or semiconducting metal oxides. Nanowires are different
from molecular wires (also sometimes referred to as "molecular nanowires"), which are
molecular entities with diameters typically less than 3 nm and repeating subunits. DNA is
the premier example of a molecular nanowire, with the repeating subunits being the
constituent nucleotides; inorganic examples include Li2Mo6Se6 and MoeSg-xIx. Research
on nanowires is often conducted along with parallel research on carbon nanotubes.
Though these materials are different, the sensing mechanisms and underlying behavior
are usually very similar. However, it should be noted that there are significant differences
in processing and scale-up potential; though similar devices may be made from these
nanostructures, this does not mean that similar commercial products will eventually be
available.
3.5.2 Description
Nanowire sensors have attracted much attention for two reasons. First, their large surface
area to volume ratio promises high sensitivity. Second, the size of the nanostructures is
similar to the size of species being sensed, thus the nanostructures make good candidate
transducers for producing the signals that are then read and recorded by conventional
instruments. The underlying phenomenon exploited in using nanowires is the field effect
on which field effect transistors (FETs) are based. The wire acts as the channel from
source to drain for the FET. If functional groups attached to the nanowire can act as a
receptor to bind with a target species (particularly a biological entity that possesses a
charge), then the charge on the surface of the nanowire changes. Since this can influence
electronic behavior into the depth of the nanowire, a gating effect occurs that can be used
in sensing. Figure 35 provides a representation of a nanowire configured as an FET.
Gate Layer
{e.g., p-Doped Si)
Nanowire
Figure 35. Nanowire in a field effect transistor (FET) configuration
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Since they were discovered, silicone nanowires have primarily been investigated for
sensing applications in biological systems (Cui et al. 2001). This is because the field
effects on a nanowire functionalized with receptors are considerably larger with charged
biological species than with ions. Nanowires have been viewed as one of
nanotechnology's most promising current products for biomedical research (Hood et al.
2004). Techniques for generating various types of important nanowires, nanorods,
nanobelts, and nanotubes, synthetic strategies, research activities unique properties (e.g.,
thermal, mechanical, electronic, optoelectronic, optical, nonlinear optical, and field
emission), and methods for assembling nanostructures into functional devices have been
well reviewed (Xia et al. 2003). A comprehensive review of current research activities on
chemical sensors based on nanotubes, nanorods, nanobelts, and nanowires, focusing on
experimental principle, design of sensing devices, and sensing mechanism has recently
been published (Huang and Choi 2007). Other sensing properties of silicon nanowires
have been explored, such as gas sensing, where it has been observed that upon exposure
to ammonia gas and water vapor, the electrical resistance of the HF-etched relative to
non-etched silicon nanowires sample is found to dramatically decrease—even at room
temperature (Zhou et al. 2003).
These materials have recently sparked considerable interest in nanoelectronics, in
composite nanomaterials, and as conducting polymer nanowire sensors. Sensors and
actuators assembled with conductive polymers nanowires are claimed to have superior
responding characteristics to their conventional counterparts (Liu et al. 2004,
Ramanathan et al. 2004, Alam et al. 2005). Conjugated polymers are organic
macromolecules which consist at least of one backbone chain of alternating double and
single bonds. This electronic structure allows a pi (TC) orbital system, modifiable by
intermolecular interactions and other functionalization, to extend along the entire length
of the backbone, bestowing one-dimensional conductivity on the polymer. Sensors and
sensor arrays based on conjugated polymers and carbon nanotubes have been
investigated. The four basic electrical transduction modes—conductometry (monitoring
the conductivity changes), potentiometric (monitoring the open circuit potential at zero
current), amperometry (monitoring the change in current while the potential is kept
constant), and voltammetric (monitoring the change in current while varying the applied
potential)—have already been investigated (Dai et al. 2002). Heavy metal-ion sensing for
drinking water analysis (Cu2+ and Ni2+ at parts per trillion range) have been demonstrated
using a conducting polymer nanojunction array. Each nanojunction is formed by bridging
a pair of nanoelectrodes separated with a small gap with electrodeposited, peptide-
modified polyanilines and signal transduction sensing mechanism using the change in
conductance due to polymer conformational changes induced by the metal-ion chelating
peptide (Aguilar et al. 2005). Dielectrophoretically assembled poly (3,4-
ethylenedioxythiophene)/ poly (styrenesulfonate) have been investigated as sensors for
acetone, methanol, and ethanol (Dai et al. 2002).
Conducting polymer nanowire sensors have even been formed directly in place in
microfluidics devices (Wang et al. 2006). Such an approach has a number of advantages,
including:
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4.
the monomeric precursor polymerizes directly on the electrode surface, producing
high-quality ohmic contacts;
addressability is inherent to this method because nanowires can be grown across
individual electrode junctions;
the introduction and delivery of small amounts of precursor monomers and
analytes are highly controllable and enable the rapid exchange of nanoliter-level
solutions on the same;
the turbulence-free environment within a microchannel helps the formation of
well-defined conducting polymer nanowires during the electropolymerization
process; and
once the nanowires are grown, the entire device is ready for use, without the
necessity of any postfabrication processing. Figure 36 provides a representation of
an actual device.
Working Electrodes
'
OuifHit i1 Reference Elecuotle
(C)
Figure 36. Conducting polymer nanowire sensor formed directly in microfluidics
device showing (a) actual view of fabricated device, (b) optical micrograph of
microfluidics device and (c) schematic with polyaniline and polypyrrole
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Various types of nanowire have been studied for environmental applications other than
sensors. For example, natural nanowires in electricigens (microorganisms able to
completely oxidize organic compounds to carbon dioxide with the sole electron acceptor
being an electrode) (Reguera et al. 2005, Reguera et al. 2006, Reguera et al. 2007); ZVI,
iron-nickel, and iron-palladium nanowires have been studied as an alternative to iron
nanoparticles in the remediation of chlorinated organics (Yoo et al. 2007); and
regenerable gold nanowires have been studied as sensors for mercury (Keebaugh et al.
2007).
3.5.3 Summary of Environmental Potential
Sensors based on nanowires are at an early stage of development. Their large surface
area-to-volume ratio promises high sensitivity. Functionalization of the nanowire will
allow a variety of species to be sensed.
3.5.4 References
Aguilar, A.D., E. S. Forzani, X. Li, N. Tao, L.A. Nagahara, I. Amlani, R. Tsui. 2005.
Chemical sensors using peptide-functionalized conducting polymer nanojunction arrays.
Applied Physics Letters. 87: 193108.
Cui, Y., Q. Wei, H. Park, and C. M. Lieber. 2001. Science. 293: 1289.
Dai, L., P. Soundarrajan, T. Kim. 2002. Sensors and sensor arrays based on conjugated
polymers and carbon nanotubes. Pure Applied Chemistry. 74(9): 1753-1772.
Hood, L., J.R. Heath, M.E. Phelps, B. Lin. 2004. Systems biology and new technologies
enable predictive and preventative medicine. Science. 306: 640-643.
Huang, XJ. and Y.-K. Choi. 2007. Chemical sensors based on nanostructured materials.
Sensors and Actuators B: Chemical. 122 (2): 659-671.
Keebaugh, S., WJ. Nam, SJ. Fonash. 2007. Manufacturable, Highly Responsive
Nanowire Mercury Sensors. Presentation at Nanotech 2007 Conference, Santa Clara, CA,
May 21,20070
Liu, H.Q., J. Kameola, D.A Czaplewski, H.G. Craighead. 2004. Polymeric nanowire
chemical sensor. Nanotechnology Letters. 4: 671-675.
Reguera, G., K. D. McCarthy, T. Mehta, J. S. Nicoll, M. T. Tuominen, D. R. Lovley.
2005. Extracellular electron transfer via microbial nanowires. Nature. 435:1098-1101.
Reguera, G., K. P. Nevin, J. S. Nicoll, S. F. Covalla, T. L. Woodard, D. R. Lovley. 2006.
Biofilm and Nanowire Production Leads to Increased Current in Geobacter
sulfurreducens Fuel Cells. Applied and Environmental Microbiology. 72(11): 7345-7348.
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Reguera, G., R.B. Pollina, IS. Nicoll, D.R. Lovley. 2007. Possible Nonconductive Role
of Geobacter sulfurreducens Pilus Nanowires in Biofilm Formation. Journal of
Bacteriology. 189: 2125-2127.
Wang, I, Y. L. Bunimovich, G. Sui, S. Savvas, Y. Guo, J. R. Heath, H.-R. Tseng. 2006.
Electrochemical Fabrication of Conducting Polymer Nanowires in an Integrated
Microfluidic System. Chemical Communications. 3075-3077.
Xia, Y., P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan. 2003.
One-dimensional nanostructures: Synthesis, characterization, and applications. Advanced
Materials. 15(5): 353-389.
Yoo, B.-Y., S.C. Hernandez, B. Koo, Y. Rheem, N.V. Myung. 2007. Electrochemically
fabricated zero-valent iron, iron-nickel, and iron-palladium nanowires for environmental
remediation applications. Water Science and Technology. 55: 149-156.
Zhou, X.T., J. Q. Hu, C. P. Li, D. D. D. Ma, C. S. Lee, S. T. Lee. 2003. Silicon nanowires
as chemical sensors. Chemical Physics Letters. 369(1-2): 220-224.
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3.6 Nanobelts and Nanorods
3.6.1 Nanobelts
Nanobelts are a class of nanostructure often viewed as a type of nanowire, usually made
from semiconducting metal oxides (such as SnC>2, ZnO, In2O3 or CdO, or selenides such
as CdSe). Nanobelts form ribbon-like structures with widths of 30-300 nm, thicknesses of
10-30 nm, and lengths in the millimeter range. They are chemically-pure and structurally-
uniform single crystals, possessing rectangular cross sections, clean edges, and smooth
surfaces.
Figure 37. SnO2 nanobelts
Much research has been conducted on the use of nanobelts as gas sensors. Current gas
sensors typically use metal oxides (such as SnO2) as the sensing element, and operate by
measuring changes in electrical conductance of the surface as it undergoes reduction or
oxidation reactions with the gas. Though the exact mechanism is not fully understood, it
appears that at high temperatures in the absence of oxygen, free electrons move easily
through the oxide and across the boundaries between crystal grains. When oxygen is
present, it is adsorbed on the oxide surfaces and at the grain boundaries, and (due to its
affinity for electrons) it removes free electrons from the underlying material, creating an
electron-depleted region and a potential barrier at the grain boundaries. This is
manifested as an increase in resistance. In contrast, when a reducing gas is present, it too
is adsorbed on the oxide surfaces and at the grain boundaries, where it can react with
oxygen, and thus lower the oxygen-generated potential barrier. This is manifested as a
reduction in resistance, allowing the sensor to act as a variable resistor dependent on gas
concentration.
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By virtue of their unique geometry, nanobelts have enormous surface areas per unit mass
and most of the material is at the surface. When gases are adsorbed onto the nanobelt
surface, the electron depletion or accumulation behavior becomes pronounced (as is the
effect on the current flowing along the nanobelt) allowing them to act in a fashion similar
to FETs. Further, their small size makes for greatly reduced power consumption over
macroscale sensors. Research (Fields et al. 2006, Comini et al. 2002) has shown this
behavior to be the basis for effective nanobelt gas sensors, and similar sensors have been
made for nerve gas detection (Yu et al. 2005), ethanol sensing (Wan et al. 2004, Xue et
al. 2005), and hydrogen sensing (Wang et al. 2005).
Nanobelts possess essentially similar properties to nanowires; they can exhibit FET
behavior and have functionalizable surfaces, thus making sensor development strategies
for nanowires also available to nanobelts.
3.6.2 Nanorods
Nanorods are solid nanostructures morphologically similar to nanowires but with aspect
ratios of approximately 3-to-5. They are formed from a variety of materials including
metals, semiconducting oxides, diamonds (aggregated diamond nanorods produced from
fullerenes are the hardest material so far discovered), and organic materials. Figure 38
provides an image of ZnO nanorods grown from aqueous solution.
Figure 38. ZnO nanorods grown in aqueous solution
Nanorods are produced by a number of techniques, including: a vapor-liquid-solid
approach; mechanical alloying; direct chemical synthesis using ligands for shape control;
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plasma arc discharge; laser ablation; and catalytic decomposition (Ayala-Sistos et al.
2005, He et al. 2002, Chopra et al. 1995, Goldberg et al. 1996, Goldberg et al. 2002,
Hamilton et al. 1993). They are under considerable investigation for applications such as
gas sensors; video displays; computer components; nanoelectronic and nano-
optoelectronic components; MEMS devices; and solar energy conversion. One of the
factors in their investigation is that both the size and shape can be controlled by use of
additives during synthesis so that scaling—as well as basic properties—can be studied
(Garcia and Semancik 2007), and flexibility is available for component integration
(Benkstein et al. 2006). By functionalizing the nanorods, amphiphilic entities that can
self-assemble and form convex curvature can be created—a capability of importance in
nanoelectronic and biomedical applications (Park et al. 2004, Chen et al. 2007). Figure 39
provides a graphic representation of a potential structure.
Figure 39. Self-assembly of gold-polymer nanorods into a curved structure
The particular morphology of nanorods also leads to properties that may have unusual,
niche applications. For example, the light emitted from or scattered off of gold nanorods
is strongly polarized along the rod length axis, an excellent property for an ideal
orientation probe (Kou et al. 2006).
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Nanorods have recently been subject of a newly demonstrated technique for the
fabrication of nanostructures (Zhao et al. 2006). An ideal nanostructure fabrication
method would have four desirable features: (1) the ability to control the size, aspect ratio,
and shape of the nanostructures; (2) the ability to grow the desired nanostructure at low
temperature and onto a particular substrate geometry (e.g., flat, cylindrical, or tapered);
(3) the ability to fabricate metallic and dielectric nanostructures with multilayer
structures; and (4) the ability to seamlessly integrate the fabrication process with other
conventional microfabrication techniques. The following four general approaches have
been employed to date:
• nanolithography-based methods which use advanced lithographic techniques,
such as electron beam lithography, x-ray lithography, and proximal probe
lithography with deposition and plasma etching processes;
• solution-based approaches which use very complicated, controlled wet chemical
reactions to synthesize nanostructures and require a detailed understanding of the
chemical reaction and crystal growth mechanisms;
• vapor-based methods, such as the vapor-liquid-solid method used since the 1960s,
which usually require higher temperatures and a specific catalyst for each
structure; and
• template-based methods using host nanoporous materials as forms.
None of these fabrication methods are desirable features of an ideal method of fabricating
nanostructured substrates, but Zhao et al. have investigated a novel nanostructure
fabrication technique in the production of nanorods called glancing angle deposition
(GLAD). GLAD is a physical vapor deposition technique in which the substrate is rotated
in the polar and azimuthal directions by two stepper motors programmed by a computer.
The experimental results have demonstrated that the GLAD technique offers several
strategic advantages compared to other nanofabrication techniques—most particularly
that the structures of the nanorods can be well designed by computer programming, a
feature that cannot be achieved by any other fabrication technique. Control of this type,
by whatever method is important since a novel, tapered form of nanorod, known as
"nanorice", has been shown to be the most sensitive surface plasmon resonance
nanosensor yet devised (Wang et al. 2006, Srivastava and Lee 2006).
Exploratory work on the use of nanorods for sensors is well underway. A biosensor for
determination of heavy metals based on hydrothermally-grown ZnO nanorod/nanotube
and metal-binding peptides has been presented. The ZnO acts as an FET and heavy metal
binding with a peptide causes an electrical signal change, which can be measured and
correlated to the concentration of heavy metals (Jia et al. 2007). A gold nanorod sensor
for mercury is able to determine mercury in tap water samples at the parts-per-trillion
level has also been developed. The selectivity and sensitivity result from the
amalgamation of mercury and gold, and the entire sensing procedure takes less than 10
minutes, with no sample separation and/or sample pre-concentration requirements. The
only step prior to mercury determination consists of mixing the water sample with a gold
nanorod solution in sodium borohydride. The limit of detection (6.6xlO"13 g.L"1) shows
excellent potential for monitoring ultra-low levels of mercury in water samples
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(Hernandez et al. 2007). In work on the control of homogeneity in shape, size, and the
organization of gold nanorods, the possibility of a copper sensor has been suggested. The
basis of the idea is the strong dependence of surface plasmon resonance peaks on the size
of gold nanorods, and the detection mechanism is based on the color change due to
plasmon-plasmon interaction between adjacent gold nanorods. The gold nanorods were
successfully functionalized with glutathione, and the addition of Cu2+ ions considerably
improves the assembly, opening the possibility of making a copper sensor (Afshar 2007).
3.6.3 References
Afshar, R. 2007. Gold nanorods: Synthesis and self assembly. Master Project Micro- and
Nanotechnology, University of Neuchatel, Switzerland, February 2007.
Ayala-Sistos, J., G. Rosas, R. Esparza, R. Perez. 2005. BN Nanorod Production Using
Mechanical Alloying. Advances in Technology of Materials and Materials Processing.
7(2): 175-180.
Benkstein, K.D., C. J. Martinez, G. Li, D. C. Meier, C., B. Montgomery, S. Semancik.
2006. Integration of Nanostructured Materials with MEMS Microhotplate Platforms to
Enhance Chemical Sensor Performance. Journal ofNanoparticle Research. 8(6): 809-
822.
Chen, T., Z. Zhang, S. C. Glotzer. 2007. A precise packing sequence for self-assembled
convex structures. Proceedings of the National Academy of Sciences. 104: 717-722.
Chopra, N. G., R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S. G. Louis, A.
Zettl. 1995. Boron Nitride Nanotubes. Science. 269: 966-967.
Comini, E., G. Faglia, G. Sberveglieri, Z. Pan, Z.L. Wang. 2002. Stable and highly
sensitive gas sensors based on semiconducting oxide nanobelts. Applied Physics Letters.
81:1869.
Fields, L.L., J. P. Zheng, Y. Cheng, P. Xiong. 2006. Room-temperature low-power
hydrogen sensor based on a single tin dioxide nanobelt. Applied Physics Letters. 88:
263102.
Garcia, S.P. and S. Semancik. 2007. Controlling the Morphology of Zinc Oxide
Nanorods Crystallized from Aqueous Solutions: The Effect of Crystal Growth Modifiers
on Aspect Ratio. Chemistry of Materials. 19(16), 4016-4022.
Goldberg, D., Y. Bando, M. Eremets, K. Takemura, H. Yusa. 1996. Nanotubes in boron
nitride laser heated at high pressure. Applied Physics Letters. 69: 2045-2047.
Goldberg, D., Y. Bando, M. Mitote, K. Kurashima, T. Sato, N. Grobert, M. Reyes, H.
Terrones, M. Terrones. 2002. Preparation of aligned multi-walled BN and B/C/N
nanotubular arrays and their characterization using HRTEM, EELS and energy-filtered
TEM. Physica. B. 323: 60-66.
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Hamilton, E. J. M., S. E. Dolan, C. M. Mann, H. O. Colijn, C. A. MacDonald, S. G.
Shore. 1993. Preparation of Amorphous Boron-Nitride and its Conversion to a
Turbostratic Tubular Form. Science. 260:659-663.
He, H., T.H. Wu, C.L. Hsin, K.M. Li, LJ. Chen, Y.L. Chueh, L. J. Chou, Z.L. Wang.
2002. Beaklike SnO(2) Nanorods with Strong Photoluminescent and Field-Emission
Properties. Small. 2(1), 116-120.
Hernandez, F.E., M. Rexand and A.D. Campiglia. 2007. Pushing the Limits of Mercury
Sensors with Gold Nanorods. Presentation atNanotech 2007 Conference, Santa Clara,
California, May 22, 2007.
Jia, W., E.T. Reitz, Y. Lei. 2007. Biosensor for heavy metals using hydrothermally grown
ZnO nanorods and metal binding peptides. Presentation at Nanotech 2007 Conference,
Santa Clara, California, May 22, 2007.
Kou, S.Z.X., C.-K. Tsung, M.H. Yeung, Q. Shi, G.D. Stucky, L. Sun, J. Wang, C. Yan.
2006. Growth of Gold Nanorods and Bipyramids Using CTEAB Surfactant. Journal of
Physical Chemistry B. 110: 16377-16383.
Park, S., J.-H. Lim, S.-W. Chung, C.A. Mirkin. 2004. Self-Assembly of Mesoscopic
Metal-Polymer Amphiphiles. Science. 303(5656): 348-351.
Srivastava, D. and I. Lee. 2006. Nanorice and Nanospears from Polymer Nanospheres.
Advanced Materials. 18: 2471-2475.
Wan, Q., H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, C. L. Lin. 2004. Fabrication
and ethanol sensing characteristics of ZnO nanowire gas sensors. Applied Physics Letters.
84: 3654-3656.
Wang, H. T., B. S. Kang, F. Ren, L. C. Tien, P. W. Sadik, D. P. Norton, S. J. Pearton, J.
Lin. 2005. Hydrogen-selective sensing at room temperature with ZnO nanorods; Applied
Physics Letters. 86: 243503.
Wang, H., D. W. Brandl, F. Le, P. Nordlander, N. J. Halas. 2006. Nanorice: a hybrid
nanostructure. Nano Letters. 6: 827.
Xue, X. Y., Y. J. Chen, Y. G. Wang, T. H. Wang. 2005. Synthesis and ethanol sensing
properties ofZnSnO3 nanowires. Applied Physics Letters. 86: 233101.
Yu, C., Qing Hao, S. Saha, L. Shi, Xiangyang Kong, Z. L. Wang. 2005. Integration of
metal oxide nanobelts with microsystems for nerve agent detection; Applied Physics
Letters. 86:063101.
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4.0 Observations and Conclusions
This document has synthesized a review of information on existing and potential
applications of nano-enabled technologies for remediation and sensing of environmental
pollutants, including radionuclides at contaminated sites. Since most environmental
nanotechnologies are just emerging and cover a very broad spectrum of potential
applications, the approach used in this document is to first present nanotechnologies that
are currently being field tested followed by those that are still in the research and/or
development stage. A wide range of promising nano-enabled technologies have been
presented and discussed to the extent feasible.
It is important to note that the majority of mature (tested and tried) nanotechnologies are
dominant in the electronics and biomedical fields. Which nanotechnologies will actually
migrate into environmental sensor (and possibly remediation) is challenging to determine
at this time with significant accuracy, but comments were made in the document on
extrapolation of nanotechnologies across scientific fields where appropriate. In addition
to gathering the information presented here, the process of searching and examining the
technical literature allowed some broad observations to be made. These collective
findings may be summarized as follows:
1. Nano-enabled technologies may be classified into two broad categories:
remediation and sensing technologies. Remediation includes detecting,
sequestration, and destruction of materials that constitute a threat to pubic health,
welfare, and the environment. Sensing technologies, with enhanced sensors using
nanomatrials, are used for detecting the presence of pollutants and could be used
in monitoring releases for regulatory compliance.
2. The classification in (1) above is most useful in presenting the overall state-of-
the-science if: a) a reasonable extrapolation is allowed from related behavior (e.g.,
it is accepted as reasonable that the success of a technology in the remediation of
heavy metals signals the technology's expected success for remediation of
radionuclides), and b) the concept of environmental remediation is expanded to
include radioactive waste management and radiological decontamination c)
sensing of foreign substance or element may also serve as a prerequisite for
furthering the development of environmental contaminant detection.
3. In general, our understanding of nanotechnology risk is still developing. More
information is needed on both the appropriate framework and on the basic
information to be used within that framework. Additionally, the relationship
between radionuclides and nanotechnology risk is even more elementary. A
growing body of information and knowledge of the fate and transport of various
nanomaterials should aid in the assessment of potential risks and mitigation of
hazards to public health and the environment posed by nanotechnologies
applications.
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4. The physical and chemical properties (i.e., high surface area, high reactivity, easy
dispersability, rapid diffusion, etc.) of nanomaterials provide the unique
properties/capabilities for various environmental nanotech applications. The term
"nanomaterial" is very broad and includes nanoparticles (based on overall size or
gross dimensions), nanostructured materials (which may be microscale,
mesoscale, or even macroscale particles, but still have nanoscale structural
features, such as the channels in zeolites), and nanocomposite materials.
5. A number of nano-enabled remediation technologies have either been field tested,
trial demonstrated, or are close to and/or awaiting, full-scale implementation.
These include ZVI nanoparticles (ZVI microparticles have already been used to
remediate uranium contamination of ground water), zeolites (nanostructured
materials that have been used in radioactive waste treatment for half a century and
have been investigated for the remediation of radioactive cesium in soils, and are
now available as nanoparticles), and a radiological decontamination gel
(developed at ANL and employing a nanoparticulate radionuclide sorbent). As
more nanotechnologies are applied in the field, performance data to include cost,
operation and maintenance should be collected real-time to aid in further
evaluation, and validating the sustainability of the environmental
nanotechnologies applied to radionuclides.
6. Based on analysis of the underlying technical and and scientific framework of a
number of available nano-enabled remedial technologies, it is anticipated that
nano-enabled technologies will have significant impact in the sensor development
area. This is due to well-established fundamentals of sensor behavior that allow
distinct advantages to be realized when the sensing events are measured at the
molecular level. Despite the great potential for nano-enabled sensors, there are
still some major hurdles to overcome in terms of reliable large volume production
of nanoscale sensing elements, effective nanoscale manipulation, and integration
of the nanoscale sensing elements with the external world.
7. The electronics/biotechnological/biomedical driver for nano-enabled subsystem
development has a two-fold benefit. It helps the cause of radionuclide site
remediation by maintaining a development impetus, the benefit of which can be
taken by those concerned with radionuclides. In doing so, this driver also puts
limits on its own utility to radionuclides since biosensing has become the
dominant paradigm and many developers are unaware of the needs of the
environmental community.
8. The issue of low visibility of environmental needs to the nano-enabled sensing
community could easily be addressed by a straightforward communications effort,
using avenues such as the Vision 2020 technology partnership or the MATCH
Programme in the United Kingdom.
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As can be ascertained from this report, nanotechnology is strongly supported by many
federal agencies, though relatively little support by comparison is available for
environmental applications. In addition, the market for environmental applications is
perceived limited in compared to other fields such as biomedical and/or electronics to
name a few. This may be a barrier to the rapid development and deployment of
nanotechnologies for environmental applications.
As discussed, there are many possible risk concerns associated with nanotechnology.
Further, in the case of nanoparticle in situ applications there are risk concerns voiced in
the literature and there is no regulatory guidance at this time. Risk concerns are a
potential barrier to the acceptance and widespread deployment of nanoparticle based
environmental technologies. Further, barriers may include the relatively limited market in
environmental nanotechnologies. It seems most likely that nanotechnology-based sensors
will be advanced further and will be more widely accepted, with nanotechnologies for
remediation developing more gradually.
As explained earlier, the purpose of this document is to provide an overview of the
potential applications of nano-enabled technologies and subsystems to radionuclides in
the environment, and to further an investigation of case studies to evaluate and validate
the sustainability of various applications as they emerge.
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Appendix A
Acronyms
AFM Atomic Force Microscopy
AFO Amorphous Ferric Oxide
ANL Argonne National Laboratory
AlPOs Aluminophosphates
ASTM American Standard Test Method
CBEN Center for Biological and Environmental Nanotechnology
DEFRA Department for Environment, Food, and Rural Affairs
DMRB Dissimilatory Metal-Reducing Bacteria
DND Detonation Nanodiamonds
DoD U.S. Department of Defense
DOE U.S. Department of Energy
DSSR Double Skin Sheet Reactor
EC European Commission
EDA Ethylenediamine
EOF Environmental Defense Fund
EMSP Environmental Management Science Program
EPA U.S. Environmental Protection Agency
EU European Union
EXAFS Extended X-Ray Absorption Fine Structure
FDA Federal Drug Administration
FET Field Effect Transistor
GLAD Glancing Angle Deposition
HOPO Hydroxypyridinone
IAEA International Atomic Energy Agency
IRGC International Risk Governance Council
IR Infrared
ITRC Interstate Technology & Regulatory Council
ITRS International Technology Roadmap for Semiconductors
ISO International Organization for Standardization
IUPAC International Union of Pure and Applied Chemistry
LOG Laboratory-on-a-Chip
LSPR Localized Surface Plasmon Resonance
MATCH Multidisciplinary Assessment of Technology Centre for Healthcare
MCDA multi-criteria decision analysis
MEMS Microelectromechanical Systems
microTAS Micro-Total Analytical System
NEAT-ORU Nanomaterials in the Environment, Agriculture, and Technology Organized Research
Unit
NEIC National Enforcement Investigations Center
NIH National Institutes of Health
nm nanometer
NMSP Nanoscale Materials Stewardship Program
NNI National Nanotechnology Initiative
NODE Nanowire-based One-Dimensional Electronics
OAR/OTAQ Office of Air and Radiation/Office of Transportation and Air Quality
OECA Office of Enforcement and Compliance Assurance
OPP Office of Pesticide Programs
OPPT Office of Pollution Prevention and Toxics
ORD Office of Research and Development
ORNL Oak Ridge National Laboratory
OSWER Office of Solid Waste and Emergency Response
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PCB Polychlorinated Biphenyls
PCE Perchloroethylene
PEBBLE Probe Encapsulated By Biologically Localized Embedding
PEG Polyethylene Glycol
PNNL Pacific Northwest National Laboratories
PRB Permeable Reactive Barrier
R&D Research and Development
RCRA Resource Conservation and Recovery Act
RDX Hexahydro-Trinitro-Triazine
RO Reverse Osmosis
RS/RAE Royal Society/Royal Academy of Engineering
SAMMS Serf-Assembled Monolayers on Mesoporous Supports
SBIR Small Business Innovation Research
SCENIHR Scientific Committee on Emerging and Newly Identified Health Risks
SES Steward Environmental Solutions
SET AC Society of Environmental Toxicology and Chemistry
SPR Surface Plasmon Resonance
STAR Science To Achieve Results
STM Scanning Tunneling Microscope
SWCNT Single-Walled Carbon Nanotubes
TCA Trichloroethane
TCE Trichloroethylene
TDS Total Dissolved Solids
TNT 2,4,6-Trinitrotoluene
UNCD Ultrananocrystalline diamonds
USNRC U.S. Nuclear Regulatory Commission
VOC Volatile Organic Compound
WSCP Water-Soluble Chelating Polymers
ZVI Zero-Valent Iron
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Appendix B
Glossary
Array: An arrangement of sensing elements in repeating or non-repeating units that are
arranged for increased sensitivity or selectivity.
Assembler: A general-purpose device for molecular manufacturing capable of guiding
chemical reactions by positioning molecules.
Atomic force microscope (AFM): An instrument able to image surfaces to molecular
accuracy by mechanically probing their surface contours. A kind of proximal probe.
Buckyball/C-60: see Fullerenes, of which "buckyballs" is a subset. The term
"buckyball" refers only to the spherical fullerenes and is derived from the word
"Buckminsterfullerene," which is a geodesic dome/soccer ball-shaped C-60 molecule. C-
60 was the first buckyball to be discovered and remains the most common and easy to
produce.
Cantilever: A beam supported at one end and capable of defined bending and vibrational
behavior.
Catalyst: A substance, usually used in small amounts relative to the reactants, that
modifies and increases the rate of a reaction without being consumed or changed in the
process.
Chemical Sensor: A device capable of providing quantitative or semi-quantitative
information on a chemical species (or analyte) through calibration, and then brought into
direct contact with the species in its environment.
Conjugated Polymers: Organic macromolecules that consist at least of one backbone
chain of alternating double and single bonds.
Dendrimers: Artificially-engineered or manufactured molecules built up from branched
unites called monomers. Technically, a dendrimer is a branched polymer, which is a large
molecule comprised of many smaller ones linked together.
Depth Filters: A type of filter in which the filter medium has a significant physical depth
and the particles to be retained are captured throughout the depth of the filter.
Disruptive Technology: Introduced in 2003, this term is used to describe a new
technology that is significantly cheaper (or performs better) than a current technology.
Electron beam lithography: Lithographic patterning using an electron beam, usually to
induce a change in solubility in polymer films. The resulting patterns can be subsequently
transferred to other metallic, semiconductor, or insulating films.
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Electrospinning: A process for making nanofibers with diameters down to about 10 nm
using an electrical charge to elongate and narrow fibers formed by an added melt.
Enabling science and technologies: Areas of research relevant to a particular goal, such
as nanotechnology.
Engineered/manufactured nanomaterials: Nanosized materials that are purposefully
made. These are in contrast to incidental and naturally-occurring nanosized materials.
Engineering/manufacturing may be done through certain chemical and/or physical
processes to create materials with specific properties. There are both "bottom-up"
processes (such as self-assembly) that create nanoscale materials from atoms and
molecules, as well as "top-down" processes (such as milling) that create nanoscale
materials from their macro-scale counterparts. Nanoscale materials that have macro-scale
counterparts frequently display different or enhanced properties compared to the macro-
scale form.
Exploratory engineering: Design and analysis of systems that are theoretically possible
but cannot be built yet, owing to limitations in available tools.
Exposure assessment: The determination or estimation (qualitative or quantitative) of
the magnitude, frequency, duration, route, and extent (number of people) of exposure to a
chemical, material, or microorganism.
Fullerenes: Pure carbon, cage-like molecules composed of at least 20 atoms of carbon.
The word "fullerene" is derived from the word "Buckminsterfullerene," which refers
specifically to the C-60 molecule and is named after Buckminster Fuller, an architect who
described and made famous the geodesic dome. C-60 and C-70 are the most common and
easy to produce fullerenes.
Green Rust: A class of Iron II/Iron III hydroxide compounds having a pyroaurite-type
structure consisting of alternating positively charged hydroxide layers and hydrated anion
layers.
Incidental nanosized materials: Nanomaterials that are the byproducts of human
activity, such as combustion, welding, or grinding.
Lab-on-a-Chip (LOC): Also known as a micro-total-analytical system (microTAS), or
microfluidics device, is a device that can integrate miniaturized laboratory functions,
such as separation and analysis of components of a mixture, on a single microprocessor
chip using extremely small fluid volumes on the order of nanoliters to picoliters.
Limited assembler: Assembler capable of making only certain products; faster, more
efficient, and less liable to abuse than a general-purpose assembler.
Macroporous: Mesoporous materials with a pore diameter range greater than 50 nm.
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Membrane filter: A filter that uses relatively thin material with a well-defined maximum
pore size, with particle retaining effects taking place almost entirely at the surface.
Mesoporous materials: Porous substances with pore diameters in the range of 2-50 nm.
Microcantilever: A very small cantilever, the properties of which can be understood
from basic engineering principles.
Microfluidics: An interdisciplinary field dealing with the behavior and control of
extremely small volumes of fluids and the design of systems that use these small
volumes.
Microporous: Mesoporous materials with a pore diameter range less than 2 nm.
Molecular manufacturing: Manufacturing using molecular machinery, giving molecule-
by-molecule control of products and by-products via positional chemical synthesis.
Molecular nanotechnology: Thorough, inexpensive control of the structure of matter
based on molecule-by-molecule control of products and byproducts; the products and
processes of molecular manufacturing, including molecular machinery.
Molecular recognition: A chemical term referring to processes in which molecules
adhere in a highly specific way, forming a larger structure; an enabling technology for
nanotechnology.
Nano-: A prefix meaning one billionth (1/1,000,000,000).
Nanobelts: A class of nanostructure often viewed as a type of nanowire, usually made
from semiconducting metal oxides, such as SnO2, ZnO, In2O3 or CdO, or selenides such
as CdSe. Nanobelts form ribbon-like structures with widths of 30-300 nm, thicknesses of
10-30 nm, and lengths in the millimeter range.
Nanoelectronics: Electronics on a nanometer scale, whether by current techniques or
nanotechnology; includes both molecular electronic and nanoscale devices resembling
today's semiconductor devices.
Nanomanufacturing: Same as molecular manufacturing.
Nanometer: One billionth of a meter.
Nanoparticle: Free standing nanosized material, consisting of between tens to thousands
of atoms.
Nanorods: Solid nanostructures morphologically similar to nanowires but with aspect
ratios of about 3-to-5. They are formed from a variety of materials including metals,
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semiconducting oxides, diamonds (aggregated diamond nanorods produced from
fullerenes are the hardest material so far discovered), and organic materials.
Nanoscale: Having dimensions measured in nanometers.
Nanoscience: The interdisciplinary field of science devoted to the advancement of
nanotechnology.
Nanosensors: Chemical sensors possessing a nanoscale sensing element.
Nanostructures: Structures at the nanoscale; that is, structures of an intermediate size
between molecular and microscopic (micrometer-sized) structures.
Nanotechnology: Research and technology development at the atomic, molecular or
macromolecular levels, in the length scale of approximately 1-100 nm range; creating and
using structures, devices, and systems that have novel properties and functions because of
their small and/or intermediate size; and the ability to control or manipulate on the atomic
scale.
Nanotube: Tubular structure, carbon and non-carbon based, with dimensions in the
nanometer regime.
Nanowire: High aspect ratio structures with nanometer diameters that can be filled
(nanorods) or hollow (nanotubes).
PEBBLE (Probe Encapsulated by Biologically Localized Embedding): sub-micron
sized optical sensors designed specifically for minimally invasive analyte monitoring in
viable, single cells.
Point-of-Care: Analytical testing performed outside the central pathology laboratory
using a device or devices that can be easily transported to the vicinity of the patient.
Quantum dot: A closely packed semiconductor crystal comprised of hundreds or
thousands of atoms, and whose size is on the order of a few nanometers to a few hundred
nanometers. Changing the size of quantum dots changes their optical properties.
Scanning Tunneling Microscope (STM): An instrument able to image conducting
surfaces to atomic accuracy; has been used to pin molecules to a surface.
Sealed assembler lab: A general-purpose assembler system in a container permitting
only energy and information to be exchanged with the environment.
Self-Assembled Monolayers on Mesoporous Supports (SAMMS): Nanoporous
ceramic materials that have been developed to remove contaminants from environmental
media.
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Self-assembly: The ability of objects to assemble themselves into an orderly structure.
Routinely seen in living cells, this is a property that nanotechnology may extend to
inanimate matter.
Self-replication: The ability of an entity such as a living cell to make a copy of itself.
Spectroscopy: The scientific study of the absorption, emission, or scattering of
electromagnetic radiation by atoms, molecules, ions, solids, liquids, or gases.
Sticking Coefficient: A standard term in surface chemistry and surface physics. It is the
ratio of the number of adsorbate molecules that adsorb, or "stick", to a surface, to the total
number of molecules that impinge upon that surface during the same period of time. A
value of 1.00 means all impinging molecules stick, while a value of 0.00 means none
stick. The concept is used in investigations of nanoparticle mobility in the subsurface.
Superlattice: Nanomaterials composed of thin crystal layers. The properties (thickness,
composition) of these layers repeat periodically.
Surface Modified Membranes: Using technology to change the surface of the
membrane, but not the underlying bulk material.
Water-Soluble Chelating Polymers (WSCP): Polymers engineered to contain both
highly elective chelating functionalities to bind with targeted metal ions, and solubilizing
functionalities to allow the polymer to dissolve in water.
Zeolite: A mineral with a pore size of less than 1 nm.
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Appendix C
References for Figures
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Figure 3 accessed at http://samms.pnl.gov/sammstech summary.pdf
Figure 4 accessed at http://sammsadsorbents.com/page/what-is-samms
Figure 5 accessed at
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g/4llpx-Filtration diagram.svg.png
Figure 6 and 7 accessed at http://www.kochmembrane.com/sep nf.html
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Figure 16 accessed at http://upload.wikimedia.Org/wikipedia/en/e/e8/Graphs.jpg
Figure 18 created by Dr. Eric Nuttall, University of New Mexico
Figure 19 accessed at http://www.pnl.gov/news/release.asp?id=175
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Figure 20 accessed at http://www.mesg.anl.gov/
Figure 21 accessed at http://www.whatsnextnetwork.com/technology/index.php/2006/07/
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Figure 24 accessed at
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Figure 31 accessed at http://www.answers.com/topic/atomic-force-microscope
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detector.html
Figure 33 accessed at http://nano.cancer.gov/news_center/monthly_feature_2005_dec.asp
Figure 34 accessed at http://picasaweb.google.com/lh/photo/OSEZilSwhpJgPv3-qpUfxg
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Figure 38 accessed at http://www.phy.bris.ac.uk/groups/electron_microscopy/index.html
Figure 39 accessed at http://nanotechnologytoday.blogspot.com/2007/ll/gold-nanorods-
shed-light-on-new.html
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