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Member Page
Read from Left to Right, Top to bottom, we have:
Brian David Laich, Indiana University of Pennsylvania, Spring 2012
Andrew Ryan Sibley, Bloomsburg University, Fall 2010
Jonathan Lee Demchak Jr, Lock Haven University, Spring 2011
Elizabeth Diane Dusack, Pennsylvania Highlands Community College, Spring 2011
Aleister Tanek Javas Mraz, Pennsylvania State University, Summer 2010
Grading Checklist
Group 6: Environmental Remediation Using Metal Nanoparticles
GRADING COMMENTS DELIVERABLE
A. Required for all groups
Title page with date and
group member signatures
Member page with pictures,
first and last name of each
group member
This grading checklist
Table of contents
Index of figures and graphs
Group statement on what
makes an effective group. A
few well written sentences
will be adequate.
Glossary with relative terms
defined. Minimum of 20
terms, at the end of the
paper
Proper spelling,
punctuation, and grammar
A discussion of key
concepts, technologies,
materials and process of
original article
Logical flow & overall
cohesiveness of paper
B. Grading of the following sections will be based on the outline provided
Introduction (Abstract)
Physical, Chemical,
Biological Descriptions
Creation Methods
Characterization Methods
Iron Particles
Contents
Metal nanoparticles for environmental remediation....................................................................................1
Member Page...............................................................................................................................................2
Grading Checklist........................................................................................................................................3
Contents.......................................................................................................................................................9
Index of Figures and Graphs......................................................................................................................10
Group Statement........................................................................................................................................11
Introduction...............................................................................................................................................12
Zero Valent Iron Nanoparticles.................................................................................................................18
Palladium Nanoparticles............................................................................................................................39
Magnesium Nanoparticles.........................................................................................................................53
Zinc Nanoparticles....................................................................................................................................60
Bibliography...............................................................................................................................................76
Index of Figures and Graphs
Figure Page
Number Figure Title Number
1 Image of Iron Nanoparticle (Martin, 2008)...............................................20
2 Pictograph representing Arsenic Adsorption (Ramos, 2009)....................26
3 Graphical depiction of Fe/M-2 nanoparticle synthesis (Xu, 2005)...........31
4 SEM image Fe/Pd BNPs (Xu, 2005).........................................................32
5A STEM image of agglomerated Fe/Pd nanoparticles (Xu, 2005)...............32
5B X-EDS image of Fe/Pd BNP (Xu, 2005)..................................................32
6 Reduction pathway of trichloroethene (Shao-ping, 2005)........................36
7 TEM image of Fe/Ag BNPs (Luo, 2010)..................................................37
8 Chart of tetrabromobisphenol A concentration (Luo, 2010).....................39
9 TEM image of CMC-stabilized Pd nanoparticles (He, 2009)...................45
10 TEM images of Pd nanoparticles (He, 2009)............................................47
11 Plot of concentration of TCE catalyzed by CMC-Pd (He, 2009)….........49
12 Au nanoparticles layered with Pd (Wong, 2009)......................................51
13 Comparison of the TOF of Pd, Pd/Au, and Pd/Al2O3 in presence
of chloride and sulfide ions (poisons) (Wong, 2009)................................54
14 SEM images of MgO (Nagappa, 2007).....................................................59
15 SEM images of Mg/Pd powder (Gardner, 2007).......................................60
16 Degradation of Arochlor 1260 over time (Gardner, 2007)........................61
17(a-c) SEM Images of Zinc nanoparticles (Geuger, 2009)..................................66
18 Zinc nanoparticle filtration mask (Hsu, 2005)...........................................68
Table 1 Cost comparison of Pd-Au nanoparticles and Pd-Al2O (Wong, 2009).....55
Table 2 CONCLUSION. ….................................................................................72
Group Statement
Building bonds of trust and respect right out of the gate was important to our team’s
development. In line with this, determining each individual’s strengths and weaknesses helped to
prepare the group for any academic challenge. A great strength that the group often drew upon
was the diversity of each member’s academic background. Jon and Liz were able to provide
biology, Aleister and Brian provided the physics and math, and Andrew added the chemistry to
the group. Another key element was our desire to refer to ourselves as G6, a name that helped us
set ourselves apart from others, allowing us to push harder and make fun of ourselves from time
to time. Born from this group mentality came our group motto, “Teamwork makes the dream
work!”
Introduction
There are many positive aspects for the use of metal nanoparticles for in-situ remediation
including the target of this work- ground water contaminate dehalogenation. Vinyl chloride
{VC}, trichloroethene {TCE}, polychlorinated biphenyls (PCBs), polychlorinated
naphthalenes {PCNs}, polychlorinated dibenzo-p-dioxins {Dioxins}, arsenic,
tetrabromobisphenol A {TBBPA} , and fluoride can all be removed from ground water using the
metal nanoparticles included in this work. Removal of these contaminants through the use of in-
situ remediation can cut costs of clean up by billions of dollars. Specifically, there are more than
1,200 hazardous waste sites in the U.S. which require immediate action and hundreds of other
hazardous waste sites needing to be addressed as well. It has been projected that clean up and
decontamination of these superfund sites could take up to 35 years, but through the use of in-situ
metal nanoparticles this time could be reduced to a mere 5 years (Burton, 2009).
Physics of NanoparticlesIt is common knowledge that nanoparticles interact very differently than the bulk forms
of those particles. This is largely due to the different surface interactions of the atoms of the
materials. When nanoparticles are formed, the surface area of the particles is more exposed than
the bulk materials because of the reduction in the volume. The reduction in volume is the
primary cause for the change in interactions, as it leads to the surface interactions of a particle to
dominate the reactions a particle undergoes. This is what causes levels of interactions for
nanoparticles to vary depending on the number of atoms that compose a particle.
In solid state physics, a maximum number of surface atoms that insure the minimum
volume of a particle is called the magic number. However, for iron based nanoparticles, this is
not actually a reflection of the best possible interactions for environmental remediation. The
primary ranges for constant high values of interaction of the nanoparticles falls within the range
of 18 to 23 atoms of Fe per nanoparticle (Bertolini, 2007). These particle ranges form clusters
that exhibit the highest reactivity for environmental remediation, even though the magic number
of these particles is at a higher level. This is because past this point multiple layers of particles
would develop that hinder in the reactions necessary for the complete dechlorination of
contaminated waters.
With iron nanoparticles in the correct range, interactions involving the donation of or
acceptance of free electrons help to cause the remediation. All of these interactions take place at
the surface of the particles and involve the use of iron as a reductant. In some rare cases, iron is
used as an oxidant, though for the scope of this paper that is limited to the remediation of
arsenic. Other metallic nanoparticles undergo similar reactions at the surface of the materials,
with bimetallic’s being a special case. Bimetallic’s stand apart from other materials, as the
additional metal surfaces create new catalytic reactions that are based upon the interaction of the
surface properties of the two materials.
The chemical interactions of the nanoparticles follow a simple pattern outlined in the
following section of the introduction. The important physical interaction that needs to be noted
for metallic nanoparticles is that the magic number does not necessarily apply in determining the
best form of the nanoparticles for environmental remediation.
Chemical Interactions and Particle Synthesis
In order to start the reactions which degrade contaminants, the metal nanoparticles need to
be reduced first. Each metal is reduced in different ways. For example, iron is often reduced by
a reaction that takes place with sodium borohydride. This reaction takes place in the presence of
water, oxidized iron, and borohydride. These three species react to form zero-valent iron, boric
acid ions, hydrogen molecules, and hydrogen ions. The sodium borohydride is generally added
to the solution in excess to accelerate the reaction.
Iron, as well as many other metals, can also be prepared as bimetallic nanoparticles
{BNPs}, some of which include iron/nickel, iron/palladium, and iron/silver. The synthesis of
these nanoparticles differs in procedure both by the particles' elemental composition and the
specific use for which the BNPs are being created. Fe/Ag BNPs can be synthesized easily by
reacting silver chloride in a solution of ethanol and henceforth adding iron powder to the
mixture, which is then often rinsed with hydrochloric acid after being placed in a shaker. The
Fe/Ag BNPs can then be removed simply via magnets. For the creation of Fe/Pd BNPs,
referred to as the modified polyol process, consists of the simultaneous thermal decomposition of
an iron-containing compound, such as iron pentacarbonyl or iron chloride, and reduction of
palladium acetylacetonate by 1,2-hexadecanediol in a diphenyl ether solution with oleic acid and
oleylamine (Wantanabe, 2006). In general, nanoparticles can then also be annealed in a vacuum
to enhance their structure and ordering on a substrate.
For the synthesis of palladium nanoparticles, palladium has often been reduced by
sodium borohydride, but it has also been found that ascorbic acid can be used as well (He,
2009). In this case, the ascorbic acid is reacted with palladium in the presence of heat to
accelerate the reaction. The palladium nanoparticle size relates to the temperature, such that a
higher temperature will yield a smaller particle. In addition, ascorbic acid is used due its being
both more environmentally friendly and generally safer to handle.
Magnesium oxide is, however, often produced in a totally different way. This reaction
takes place with a solution of magnesium nitride as an oxidizer and glycerin as a source of fuel
through which the reaction may take place. In this case, both species are placed in a dish and
most of the water is evaporated by the heating of the solution on a hot plate. A wet powder is
then formed and placed into a furnace to be heated even more. This powder actually combusts
and gives the appearance of being a black porous powder, which after being left in the furnace
for another 30 minutes turns into a white powder. This white powder is magnesium oxide
(Nagappa, 2007).
Magnesium {Mg} containing nanoparticles can be used in environmental remediation for
the removal of chlorinated contaminants via dehalogenation and for the removal of fluoride from
ground water. MgO nanoparticles synthesized through combustion are cost effective and range in
size from 12-23nm. These nanoparticles are characterized using powder X-ray
diffraction{PXRD}, scanning electron microscopy {SEM}, and transmission electron
microscopy {TEM}, and have been shown to successfully remove 97% of fluoride from tube
well water on its first go around while maintaining the ability to be regenerated and remove yet
another 76% of fluoride other contaminated systems. Magnesium palladium {Mg/Pd} BNPs
synthesized through iodine catalyzed reactions are often characterized by scanning electron
microscopy {SEM}. These nanoparticles can remove upwards of 90% of polychlorinated
biphenyls {PCBs}, polychlorinated naphthalenes {PCNs}, and polychlorinated dibenzo-p-
dioxins {Dioxins}, which are toxins commonly found in groundwater sites known to be
contaminated. (Harbrecht, 2001).
Biological Considerations of NanoparticlesMany different considerations are necessary with regards to the use of nanoparticles in
environmental remediation. In particular, the biological aspect is of primary importance due to
the fact that these endeavors are undertaken for the sole purpose of promoting the healthy
lifespans of a variety of lifeforms. Through one such study, it has been found that oxide
bimetallics are more effective than zero-valent metal nanoparticles in the reduction of
contaminants due to the fact that oxide-BNPs can diffuse further into a contamination zone than
their zero-valent counterparts. Moreover, oxide-BNPs have a higher reactivity to redox-
amenable environments than zero-valent nanoparticles (O'Donoghue, 1983). In particular, this
study looked at how an oxide coating on iron affected the outer-sphere complex and contaminant
interactions. The contaminants of concern are carbon tetrachloride {CT}, benzoquinone, by
trichloroethene, and other chlorinated aliphatic hydrocarbons (O'Donoghue, 1983). Through
electron transfer, CT is broken down into methane, carbon monoxide, or formate. This reduction
reduces the toxicity of the chemical but allows some of the remaining material to be transferred
into the air. This is easily removed from the environment through gas based treatments. This is
one aspect of how nanoparticles are used in the biological aspect of environmental remediation;
on the other hand, the use of catalysts can reduce the contaminates in the environment.
In the study it was found that the catalyst is used to induce dechlorination in the water,
which is important to reduce the pH in the water to maintain a pH of 7 (O'Donoghue, 1983).
When the catalyst is deposited on the cell wall inside the cytoplasm, the charged H radical is
changed and decreases the pH in the water system. Another study found this aspect of
interaction, but did not look at how to maintain the pH at 7. A different study used various doped
elements (i.e.- Ag, Ni, Pd) that were added to the contamination solution and over time were
observed to see the rate at which it achieved equilibrium pH and how well that was maintained
(Hyung, 2009). In this experiment, they placed pure water and zinc with and without
trichloroethene {TCE} into a solution at room temperature. The experiment was divided into two
sections : 50 minutes and 250 minutes of interaction and observation of the rate of change of pH.
It was found that with a buffer solution of Zn doped with Ag, Ni, and Pd in TCE, a rapid
resultant change in the pH was observed. It quickly increased to 7.5 in 50 minutes and in 250
minutes the pH reached equilibrium of 7.0 to 7.3 (Brinker, 1990). As a result, the dechlorination
in the water was induced and the water became safe to use in and outside of the lab.
Health ConcernsWith regards to the actual consequential health effects of nanoparticle exposure, it is known that
nanoparticles can enter our bodies via three main routes: skin, alimentary canal, and inhalation
thought the lungs. Once inside the body, nanoparticles, fibers specifically, can possibly become
lodged in the alveoli of the lungs, increasing the risk of developing lung cancer over time. Other
nanoparticles such as spheroidal nanoparticles can end up in the lungs as well, but these
nanoparticles can be evacuated as long as the particles don’t hinder the body’s ability to do so.
There is fear that nanoparticles can also get into the blood stream and make their way to vital
organs as well as to the blood-brain barrier where they could possibly cross and enter the
cerebral spinal fluid {CSF}, which could produce a variety of negative consequences. No
conclusive evidence points in the negative direction for nanoparticles with regards to side effects
on humans, and although many proposed possible problems are brought up, studies on health
effects are inconclusive (Albrecht, 2006).
Zero Valent Iron Nanoparticles
Nanosized iron particles are used in a variety of industries currently, ranging from uses in
chemical catalysis to magnetic memory storage (Martin, 2008). However, the particles are
beginning to play an increasing role in environmental remediation due to their abilities to absorb
and neutralize certain organic toxins and compounds. Of significant importance are their abilities
to reduce chlorine and arsenic compounds, two common contaminants of drinking and well
waters (Ramos, 2009) (Zhang, 2003). They possess a specific structure that allows them to
perform chemical interactions with compounds at a much greater rate than bulky particles of iron
can achieve. Bulky particles of iron have been used in environmental remediation for years, often
in the form of a powder, or in siding for water plumbing (Tee, 2009).
Chemical industries use the nanoparticles as catalysts for growth of certain materials, as
well as for the cleavage of carbon-carbon bonds (Martin, 2008). Nanoparticles of iron are also
used in magnetic memory devices to shrink the dimension size and increase the storage capacity
of computer media (Martin, 2008). In its bulk form, iron is commonly used to create piping and
plumbing due to its interactions with destructive compounds. It is also a key component in
making steel. All of these other industries should be considered before iron is chosen to be used
as a nanoparticles remediation technique.
Iron is a very useful material though in environmental remediation. Iron has been used in
environmental remediation for many years, but the nanoparticles version of it offers several
distinct advantages over the larger powder versions. First, it has a surface area ratio that is
around 37 times larger than that of traditional iron powder. Traditional iron powder has a surface
area ratio of around 1 m2/g, whereas nanoparticles zero valent iron {nZVI} has a surface area
ratio of around 33.5 m2/g as measured through the nitrogen absorption method {NAM} (Wang,
1997). Secondly, the particles show a higher reactivity rate with halogenated organic compounds
{HOC} than would be predicted solely on the increased surface area (Wang, 1997). Finally, the
nanoparticles can be doped with various other materials, leading to even more reactive removal
of contaminants.
Figure 1. Picture of an iron nanoparticle, including oxide layer. Source: (Martin, 2008). The
core-shell structure of any particle is important for understanding the various interactions that
occur during remediation. The core is more often than not the reactive part of the particle, with
the shell serving as both a catalytic layer and porous transport for the different chemical
removals.
Several drawbacks though are noticeable. The first is that nZVI have a short reactivity
life if used without a dopant material (Zhang, 2003). The particles also quickly form an oxide
layer of various thickness upon creation, reducing the optimal reaction of the particles (Martin,
2008). Finally, the particles are able to not only undergo reduction reactions with certain
chemicals but also oxidize certain chemicals, leading to undesirable contamination that may be
worse than what was intended to be removed.
To create the nZVI mixtures of Sodium Borohydride (NaBH4) and ferric chloride (FeCl3) were
magnetically stirred at room temperature. During the mixing, the chemical reduction of ferric
iron by the borohydride produced the nZVI (Wang, 1997). During synthesis an oxide layer forms
around the particles of ferric iron which varies in thickness, as verified by X-ray
photospectrometry {XPS} and tunneling electron microscopy {TEM} (Martin, 2008). The oxide
layer is porous in nature, and is the main reason that reductions do not take place at their
optimum rate (Martin, 2008). Since the reactions have to filter through the pores of the oxide
layer, the maximum surface area is minimized around the pore sites. To insure a higher reactivity
of the nZVI, particles can be prepared in an acid bath to remove sections of the oxide layer
before doping (Tee, 2009). This can however, cause other problems such as excess hydrogen
generation. Hydrogen generation will occur at a much greater rate for acid treated nZVI under
anaerobic conditions. Generation of hydrogen will systematically lower the pH by increasing the
available hydronium ion concentration (Tee, 2009). To prevent this, different dopant layers can
be utilized.
Several different methods can be utilized to determine the surface structure and
composition of the nanoparticles. Those already named, such as XPS, TEM, and NAM will be
discussed, as well as the novel method of chemical oxidation with copper {COC}. XPS will be
able to generate information about the composition and structure of the nZVI. TEM will gather
information about the structural morphology of the particles, and NAM will allow for the porous
nature of the oxide layer to be observed. Finally, the COC method will allow for a more accurate
examination of the oxide layer thickness, and how it can be affected by doping processes
(Martin, 2008).
XPS generated information can be gathered through the use of several high-end devices,
but should focus on a broad low resolution range for shell composition and a high resolution scan
that can detect the 2p ranges. The broad scan should capture a range of voltages from 1 to 1000
eV, which will allow the scan to differentiate between the iron and oxide layers of the shell. The
results from this can help determine the composition of the shell, as well as its relative thickness.
The high resolution scan will allow for the mathematical modeling of the data to reveal the
nature of the iron as being either Fe3+ or Fe0 inside of the core (Martin, 2008). To prepare the
samples for XPS measurements, they should be placed on a conductive adhesive. Measurements
should cover a variety of angles, with special detail given to the magic angle. The magic angle is
the angle at which topographical information can be generated without any interference from the
cosine angle generated by Legendre polynomials, commonly found in higher level statistical
analysis. The higher level analysis is what will allow the XPS measurements to generate results
that can numerically approximate the thickness of the oxide shells (Martin, 2008). XPS will not
be able to generate a true measure of the thickness of the shell though, as it can only reveal the
amount of an element that is present.
TEM analysis can provide exact images of the core shell structure and thickness, but
suffers from inaccurate statistical analysis methods (Martin, 2008). It can make pictures that will
prove useful in surface description, but cannot provide true dimensions of the nZVI. To prepare
samples for TEM analysis, the particles can be placed in an ethanol suspension that is evaporated
onto a carbon film support. The carbon film support should be placed on a standard copper mesh
grid for TEM analysis and the scans should take place at ranges around 200kV (Martin, 2008).
The analysis illustrated that the crystal structure of the particles was body centered cubic {bcc}
and that the agglomerations of the structures in the suspension were around 50 – 150 nm in size
(Martin, 2008). The structures under analysis were sampled under bright and phase shift focus.
Bright field imaging provided information about the surface topology, while the phase shift
imaging allowed for statistical analysis for determination of oxide layer thickness.
Nitrogen absorption method or NAM is a method where nitrogen gas is passed through
porous materials in order to determine the size of the pores as well as their order and placement
on the surface. This is accomplished through mathematical modeling that is enabled thanks to the
BET theory. BET theory, so named for the scientists Stephen Brunauer, Paul Hugh Emmet, and
Edward Teller, helps to predict the physical adsorption of gas molecules onto a solid object. The
rate at which this occurs is determined by the equation that stems from this theory, and is based
on the much earlier Langmuir theory (Brunauer, 1938). Nitrogen gas is used due to the inert
nature of the gas, allowing only physisorption to take place that can lead to better deterministic
models of the surface area and topography. In the NAM, nitrogen gas is flowed over a sample
which is undergoing inspection. As the gas flows, certain amounts of it can be detected on the
surface of the samples in line with BET theory. These detected amounts can be used to
numerically approximate the pore size and locations present on the nanoparticles.
Chemical oxidation with copper {COC} allows for a very exact approximation of the
oxide layer thickness of the nZVI by creating a galvanic cell between copper ions and Fe0. This
was accomplished by first isolating the nZVI in a neutral environment that had undergone
oxygen purging through addition of nitrogen to a liquid storage tank. Then, cupric chloride salt
was added to the mixture of nZVI and reacted for one hour. The mixture then underwent analysis
by an atomic adsorption spectrometer. The results were then mathematically used to determine
the amount of oxide by comparing it to the amount of copper that was reduced. This is because
the reduction of Cu by Fe is based on surface area of the Fe present, which is known readily for
the nZVI. This amount is then compared to the mass of the particle to determine the oxide
thickness.
Two main accomplishments of nZVI in environmental remediation are the removal of HOC and
arsenic. Halogenated organic compounds are most often found in the form of chlorine
containing atoms. It is commonly found in brownfield or superfund areas, places where
industrial contamination has damaged the soil such that it must be purged of hazardous materials
before the land is again viable. There are over 1500 superfund areas in the United States alone
(Zhang, 2003). Arsenic is found in three forms, either As0, As(III) or As(V). As(III) is
commonly referred to as Arsenite and As(V) as Arsenate. It is a common poison in drinking
waters, effecting as much as 137 million people in over 70 countries, the US included (Ramos,
2009).
Figure 2. Pictograph representing arsenic adsorption. Source: (Ramos, 2009). On the oxide layer
of the shell, there were both oxidation and reduction reactions present. The diagram helps to
illustrate both the formation of the arsenic and the interaction of the oxide and core layers.
The mechanisms for decontamination of HOC and arsenic are based on the reduction of
the iron when it interacts with the different chemicals. These processes can be enhanced through
the use of catalysts and different preparation methods. A common HOC that is reduced through
the use of nZVI is trichloroethene {TCE}. When TCE is reacted with nZVI, the chlorine
molecules separate from the molecule and bond to the nZVI, causing an increase in the pH level
of the aqueous solution (Zhang, 2003).
One of the main problems that can occur with various dechlorination methods is
possible byproducts that may be just as damaging as the original substance. A major advantage
of the nZVI that was noted early on is that nanoscale particles are able to remove even the
byproducts of such reactions (Wang, 1997). Furthermore, they were found to achieve the
reactions in a shorter amount of time than commercial powders, achieving a rate that was nearly
ten times faster when doped accordingly (Wang, 1997). The particles also were able to react for
much longer periods of time, with one study showing the reactivity of nZVI being detectable up
to eight weeks after injection (Zhang, 2003). All of these were able to take place due to simple
interactions between the Fe0 and the chlorine atoms in solutions.
The reaction of arsenic is more complicated, and involves the interaction of the pH level
of the sample. Arsenate is found commonly in a range of pH levels from 2 to 12, while Arsenite
is usually only found in ranges below 9.2 pH. Both are found as compounds, with Arsenate
forming oxyanions such as H2AsO4 or HAsO4 and Arsenite forming the neutral compound
H3AsO3. Because of the strong interactions that nZVI has with HOC’s, it is expected that strong
interactions with the compounds of Arsenic should exist as well. The strong interactions that
take place however are not merely reduction reactions similar to the ones for HOC’s.
Both reduction and oxidation of As(III) were observed in a recent experiment (Ramos,
2009). In the experiment, a solution of nitrogen purged As(III) was treated with an amount of
nZVI. The solution had half of the mixture remain as As(III), while a third of the mixture was
reduced to As(0) and the rest was oxidized to As(V). The As(0) was found on the surface of the
nanoparticles, along the oxide shell, suggesting a strong interaction between the chemicals. This
was seen for concentrations of As(III) that match environmental conditions more closely than
prior lab designs (Ramos, 2009). The oxide layer was detected through XPS analysis and
revealed adsorption of the As(0) molecules on the shell of the nZVI. At concentrations between
50 and 100 mg/L, amounts of As(0) were detected on the surface of the shell, but at levels above
this there was little to no evidence of As(0) present on the shell. This does not stop this from
being a useful source for environmental remediation as the upper limit for safe drinking water is
10 micrograms/L, a level that is 5000 times smaller than the attempted levels (Ramos, 2009).
The reason for the lower levels of adsorption at higher levels is probably due to the higher pH
causing an increased rate of oxide formation on the surface of the shells. This would be caused
by the increased amount of ferric ion precipitate that would occur at the higher levels of As(III),
which in turn would lead to the formation of compounds that would increase the oxidation rate of
the nZVI (Ramos, 2009). This was supported by TEM and XPS analysis that indicated higher
thickness levels of oxide present on samples that were tested under 500 or 1000 mg/L of As(III)
as compared to those tested at 50 or 100 mg/L (Ramos, 2009). All of these reactions were
monitored over a 24 hour period and came to full completion during those times (Ramos, 2009).
The key thing to note from this study is that nZVI can both oxidize and reduce certain chemical
compounds, complicating the use of the particles in remediation efforts.
Bimetallic Iron Nanoparticles
In addition to nZVI, many bimetallics nanoparticles {BNPs} are also used in
environmental remediation. Bimetallic particles are generally structured using two elements
such that one acts as the core and the other its shell. A majority of the BNPs used in
environmental remediation involve iron as the core element with a secondary element added in
the hopes of improving upon the standard of effectiveness set by studies of nZVI particles.
However, this change in composition also heavily affects which and to what degree various
contaminants will react. While several elements have been coupled with iron in studies
performed since 2000, including palladium, platinum, silver, nickel, cobalt, and copper, only a
few of these are very common to environmental remediation (Zhang, 2003).
Iron-Palladium
As a bimetallic coupled with iron, palladium has been found to react with chlorinated
methanes, tetrachloroethene, trichloroethene, cis-dichloroethene, vinyl chloride, chlorophenols,
polychlorinated biphenyls, and chlorobenzenes (Yan, 2010). In general, Fe/Pd is more reactive
than other iron-based BNPs, and in treating chlorinated organic compounds, yields more
saturated products and less toxic intermediates (Yan, 2010). On the other hand, it is more costly
than many other elemental candidates for BNPs (Shao-ping, 2005).
The formation of this BNP can be done by mixing a solution of equal parts ethanol and
water with palladium chloride and nZVI, as mentioned in the introduction section “Nanoparticle
Synthesis”. This leads to 2-5 nm islands covering the outside of the nZVI with a weight percent
palladium of about 1.5 (Yan, 2010). Another method includes the synthesis of iron-palladium
BNP into a membrane matrix of polyacrylic acid and polyvinylidene fluoride (see Figure 3
below). The membrane matrix itself is created through the annealing of an aqueous solution of
polyacrylic acid, ethylene glycol, and iron sulfate on a support layer of hydrophillized
polyvinylidene. (Xu, 2005). The hydrophillization of the polyvinylidene layer was achieved
using hydroxyl propyl acrylate and tetra ethylene glycol deacrylate, and is important to creation
of the membrane matrix for the strong bonding of the layers, while the ethylene glycol is
activated as a partial cross-linking agent for the polyacrylic acid through a 3 hour annealing at
110°C (Xu, 2005). This partial cross-linking allows for the bonding of metal cations from the
iron sulfate to the membrane matrix, which will in turn be reduced by sodium borohydride to
form metal nanoparticles within the system. In this method, nZVI prepared in the membrane
matrix reacts through immersion with a solution of palladium acetate, whereby the palladium is
reduced and deposited on the surface of the iron nanoparticle (Xu, 2005).
Figure 3 – Graphical depiction of Fe/M¬2 nanoparticle synthesis in polyacrylic acid and
polyvinylidene fluoride membrane matrix. Source: (Xu, 20005).
It has been found that in this method an inversely proportional relationship exists between BNP
size and the molar ratio of polyacrylic acid to iron cation (Xu, 2005).
Various characterization methods of Fe/Pd BNPs are performed in order to discern a
variety of the material's properties. Scanning electron microscopy {SEM} may be used to
observe the BNPs (see Figure 4 below). X-ray energy dispersive spectroscopy may be used to
identify elemental composition (see Figure 5B below) and scanning transmission electron
microscopy techniques may be used to determine particle size and distribution (see Figure 5A
below).
Figure 4 – SEM image of the surface topography of Fe/Pd BNPs synthesized in a membrane
matrix of polyacrylic acid and polyvinylidene fluoride. Source: (Xu, 2005).
Figure 5A (-left) – Scanning transmission electron microscopy image of agglomerated Fe/Pd
nanoparticles.
Figure 5B (-right-) – X-ray energy dispersive spectroscopy image depicting elemental
composition of an agglomerate of Fe/Pd BNP. Source: (Xu, 2005).
While the foremost of these three images shows the structural orientation of the Fe/Pb BNPs in
membrane matrix, the image in Figure 5A demonstrates the relative size of the BNPs to be
approximately 50 nm in diameter, a useful detail for the determination of an object's surface area.
The image in Figure 5B shows the composition of these particles by element, whereby the high
ratio of the iron core to the palladium shell in the BNP is clear. This is found through the
emission of x-rays, which are characteristic to each particular element, when the electrons of the
atom's inner shell have been put into an excited state by either electrons or x-rays as sourced
from the SEM-tool.
One of the important contaminants for which Fe/Pd BNPs have been proven to be
effective in reduction is 2,2'-dichlorobiphenys, a highly toxic congener of polychlorinated
biphenyl. In studies done by Jian Xu at the University of Kentucky, 8.1 mg of 2,2'-
dichlorobiphenys per liter of 50/50 ethanol/water was fully degraded within a single hour by
Fe/Pd BNPs (Xu, 2005). The major byproduct of this reaction is biphenyl with 2-chlorobiphenyl
as an intermediate byproduct (Xu, 2005). In this reaction, the iron generates hydrogen via
corrosion reaction, which, in turn, undergoes catalysis through palladium to dechlorinate the 2,2'-
dichlorobiphenys molecule. Another major contaminant affected by Fe/Pd BNPs is
trichloroethene, which yields ethane and an oxidized iron layer as the consequences of reaction
(Yan, 2010). It has been found that Fe/Pd BNPs in water undergo drastic structural changes,
whereby the palladium becomes encompassed by the iron oxide layer, severely limiting its role
in remediation. In a study led by Weile Yan, in just 24 hours, due to this change in structure, the
reaction rate constant decreased from 5.7 per hour to about 0.96 per hour (Yan, 2010). Yan
suggests that further research into the specific cause of this phenomenon, whether it be method
of doping, degree of doping, the shape of nZVI, etc., be undertaken in order that modifications
and improvements might be made to the process.
Iron-Nickel
Another common bimetallic material used in environmental remediation is the core/shell
structure of iron/nickel. While effectively reactive with polychlorinated hydrocarbons, other
major targets for Fe/Ni BNPs include atrazine and p-chlorophenol due to the increased reactivity
of this BNP with these contaminants as compared to nZVI (Shao-ping, 2005). It has been
suggested and deduced that this be due to the reduction of these contaminants by an adsorbed
hydrogen atom on the nickel atoms of the BNP (Shao-ping, 2005). Researchers at Zhejiang
University make this claim through the observation of the linear sweep voltammetrical curves
of atrazine and p-chlorophenol using a model 273A potentiostat. This characterization tool
measures the fluctuation in potential between two electrodes caused by the oxidation or
reduction of chemicals within the system. In this observation, a reductive peak in the presence of
either atrazine or p-chlorophenol within a solution corresponds to the adsorption of hydrogen
atoms on a nickel electrode. As such, given only two alternatives for reduction within the
experiment (i.e.- nZVI or hydrogen atoms), the researchers conclude the effect to be caused
primarily by the hydrogen atoms.
One method of the creation of Fe/Ni BNPs includes the synthesis and adsorption of
nickel on an iron surface via the aforementioned polyacrylic acid/polyvinylidene fluoride
membrane matrix (see Figure 3 above) (Xu, 2005). In this case, nickel (II) sulfate was to be the
immersion solution used as the source of nickel for the synthesis of these BNPs. Utilizing this
method, Fe/Ni BNPs at average diameters of approximately 120nm have been found to be
formed. Alternatively, the simultaneous reduction of both metals of the Fe/Ni BNP will also
result in its creation (Xu, 2005).
Using atom absorption spectroscopy, scanning electron microscopy, and the N2-BET
method, researchers at Zhejiang University were able to identify the surface area, structure, and
quantity of the nickel "shell" on the iron surface (Shao-ping, 2005). Moreover, SEM images
revealed that the 120nm Fe/Ni BNPs were actually aggregates of Fe/Ni BNPs at approximately
20-30 nm in size (Shao-ping, 2005). Also through the SEM images, the researchers predicted
that by the appearance of the Fe/Ni BNPs having a spongier surface topography, it would hold
greater number of reactive sites than iron alone (Shao-ping, 2005). Using the N2-BET method
for the characterization of surface area, it was found that an increased doping of nickel on the
iron surface did indeed lead to a greater surface area, and so potentially a more highly reactive
nanoparticle than nZVI (Shao-ping, 2005). However, it was found that a maximum specific
surface area of 11.671 m2/g was achieved at a composition of approximately 2.96% Ni/97.04%
Fe, and that further deposition of nickel on the iron surface would lead to decreased specific
surface area (Shao-ping, 2005).
In a study done on the effectiveness of Fe/Ni BNPs on the reduction of trichloroethene by
the University of Kentucky in 2005, it has been found that whereas the pathway for the reduction
of trichloroethene to ethane using nZVI follows a series of four conversions, using Fe/Ni BNPs
allows for the direct reduction of trichloroethene to ethane through catalytic
hydrodechlorination (see Figure 6 below) (Shao-ping, 2005). In the prior case, the
dechlorination of trichloroethene takes place in series via dissociative electron transfer in the
following manner: trichloroethene => dichloroethene => vinyl chloride => ethylene => ethane.
In the latter, the reduction of trichloroethene is caused by hydrogen generated through the
corrosion reaction of iron in water and coupled with nickel as a catalyst. In addition, the nickel
coating helps to prevent iron oxidation (Shao-ping, 2005). In the same study, it was found that
Fe/Ni as a BNP was found to be 13x more effective than bulk Fe/Ni in the reduction of
trichloroethene. These exemplify just a few of the beneficial expectations for nickel's use as a
bimetallic nanoparticle in endeavors regarding environmental remediation.
Figure 6 – Reduction pathway of trichloroethene via nZVI vs. Fe/Pd BNPs. Source: (Shao-ping,
2005).
Iron-Silver
A third bimetallic nanoparticle consists of an iron core with a silver shell. Although far
less common, the use of silver in BNPs is effective with respect to chlorinated organics, such as
benzenes, and also has potential benefits in environmental remediation due to the well-
documented bactericidal properties of silver (Luo, 2010).
One method of synthesis of Fe/Ag BNPs used by researchers at Nanjing University
includes the deposition of silver on iron nanoparticles through the reduction of a silver chloride
solution in ethanol. After excessive stirring, a magnet is used to remove the BNPs, after which
they are washed consecutively in deoxygenated water, ethanol, and acetone, and dried for two
hours using nitrogen gas (Luo, 2010). This method generates aggregations of Fe/Ag BNPs found
to range from approximately 20nm to 100nm in diameter and in the form of long chains (see
Figure 7 below).
Figure 7 – Transmission electron microscopy image of Fe/Ag BNPs. Source: (Luo, 2010).
The characterization of these BNPs can be performed using the nitrogen absorption
method, X-ray diffraction spectroscopy, transmission electron microscopy, scanning electron
microscopy, X-ray fluorescence spectroscopy, and X-ray photo-electron spectroscopy. As in the
aforementioned study of Fe/Ni BNPs, the rough edges seen through the characterization of the
Fe/Ag BNPs creates a situation in which more potential reactive sites are available due to an
increased surface area of the BNP over either pure nZVI or Ag. Specifically, by percent
composition, the researchers at Nanjing University found that there was little structural
difference between iron and Fe/Ag BNPs containing only 1% silver, whereas at a composition of
14% silver, the qualitatively rough edges could be easily discerned, and that the average surface
area of the Fe/Ag BNPs is approximately 78 m2/g as contrasted with 51 m2/g in pure iron
powder (Luo, 2010). This difference in surface area is, once again, one of the primary
considerations in the determination of the predicted effectiveness of a nanomaterial's
dechlorination potential with regards to environmental remediation. Meanwhile, through X-ray
photo-electron spectroscopy techniques, it was also confirmed that a thin oxide layer existed on
the exposed nZVI surface (Luo, 2010). The effects of this oxide layer have been detailed in
previous sections on nZVI particles.
Focusing on the reduction of the brominated flame retardant, tetrabromobisphenol A, a
study performed by the researchers at Nanjing University coupled Fe/Ag BNPs with ultrasonic
cavitation in experiments in order to determine the effectiveness of the BNPs. Ultrasonic
cavitation was adopted as a method to keep the BNPs dispersed in solution in order to prevent
agglomeration and maintain a large surface area to volume ratio amongst nanoparticles. While
adopting ultrasonic cavitation alone sans either iron or silver nanoparticles as a control for the
experiment, the effectiveness of this method was proven over utilizing nZVI particles with
ultrasonic cavitation as well as over Fe/Ag BNPs without ultrasonic cavitation. In the Fe/Ag
BNPs method with ultrasonic cavitation, complete degradation of tetrabromobisphenol A
occurred in approximately 20-30 minutes, whereas in no other method after 60 minutes did the
concentration of tetrabromobisphenol A not begin to level out at varying concentrations from
about 40-100% of their initial states (see Figure 8 below) (Luo, 2010).
Figure 8 – Chart charting percent concentration of tetrabromobisphenol A in solution over a
period of time when exposed to a variety of decontaminates. Source: (Lou 2010).
In the same experiment, a pH-level of 7.9 was found to be optimum for the rate of
tetrabromobisphenol A degradation. (Luo, 2010) This is an important consideration in field
studies, where too high a pH-level may cause H2 bubbles and too low a pH-level may cause
hydroxide or carbonate passivation layers, both of which would act to impede
tetrabromobisphenol A degradation. (Luo, 2010)
In the reaction between tetrabromobisphenol A and Fe/Ag BNPs, the final byproduct
includes bisphenol A and mono-bromobisphenol A, and while the degradation of this highly toxic
contaminant is ultimately the goal of this research, many in-lab techniques (e.g.- ultrasonic
cavitation) may simply be unrealistic in field applications. However, a perspective should be
maintained which reflects the most important note in this research- that understanding the
process by which the degradation of a contaminant takes place will ultimately allow for the
development of more effective and efficient processes through this and further studies which
may be undertaken. Moreover, in order to truly understand some phenomenon, an experiment's
variables must be changed systematically, and while iron and its bimetallics are currently the
most well-studied metals in environmental remediation techniques, many other metals present
opportunity and potential in this same regard as well.
Palladium Nanoparticles
Palladium {Pd} as nanoparticles can also be nanopowders or nanodots. They are black in
color, as a bulk material it is a silver color, and normally range from being 20-100 nm in
diameter. Applications for Palladium nanocrystals include numerous uses in catalysts,
electrocatalyst, catalytic converters, chemical synthesis, magnetic nanopowder (i.e.- Palladium
with ruthenium or rhenium nanoparticles in a copper pad surface), plastics, and nanofibers
(American Elements, 2001-2010). Pd is also in environmental remediation to remove
trichloroethene {TCE} from ground water.
One of the biggest uses for Pd nanoparticles is as a catalyst and electrocatalyst. Pd as a
nanoparticle has a lot of surface area to accept hydrogen. This is what makes it such a great
catalyst. As the Pd collects the hydrogen it either speeds up a reaction, acting as a catalyst, but
could also create electrical energy. Pd is being researched now in fuel cells this creation of
electrical energy. At Brown University some chemists use the Pd at the nano scale. They made
Pd nanoparticles that were 4.5 nm in diameter. They keep them from agglomerating by attaching
a weak binding amino ligand to the particle. The particle was then attached to a carbon platform
and then the ligand can be just “washed away”. The carbon platform then can create energy for
12 hours by just losing 16% of the surface area. Before, over a 12 hour period 64% of the
surface area was lost. The surface area is lost because the particles agglomerate together to
become more stable (Nanotechnology: The A to Z of Nanotechnology, 2009).
Catalytic converters also started to use, or companies started experimenting, on the
nanoscale. Pd is a common metal that is used in most catalytic converter. Catalytic converters
are used on cars, or any machine, to reduce the toxicity of the exhaust that was produced. Pd is
used as an oxidation catalyst in catalytic converters. The Pd produces carbon dioxide from the
carbon monoxide that is created from the engine. On the nanoscale, Mazda Motor Corporation
had developed a catalytic converter that uses 70-90% less metal. They use nanoparticles that are
less than 5 nm in diameter. The nanoparticles are “studded onto the surface of tiny ceramic
spheres”. These spheres are then “embedded into fixed positions”. The use of nanoparticles
allow for more surface area for the chemical reactions to take place and also uses less metal that
is expensive (Stafford, 2007).
Some of the other fields (i.e. magnetic nanoparticles, plastics, and nano fibers) are fairly
new. As such, there is still much research that is necessary before they can be adequately
compared. Pd though is being researched extensively for applications in environmental
remediation. Some techniques that are currently being used are costly and not very
environmentally friendly.
The pump-and-treat method is used in many sites of remediation today. This method is
still very expensive and improvements for this method are well needed. Pump-and- treat is a
method of carbon adsorption and air stripping. Carbon adsorption is when ground water is
pumped through a carbon containing unit and the contaminated organics adsorb to the high
surface area carbon. The water is then sent back to where ever it was needed before. The now
saturated carbon is replaced and not used again. It is usually disposed in landfills or incinerated
because it costs too much for it to be prepared for reuse. In air stripping the groundwater comes
in contact with air and the contaminated organics vaporize into the gas phase. This contaminated
air is vented or goes through carbon adsorption before venting. The expensive costs come from
replacing the carbon “filters” after saturation is reached. Also the disposing of the saturated
carbon is not environmentally friendly.
Catalysts such as palladium are being researched to replace singular carbon sheets in
filtration processes. Palladium in nanoparticle form was developed because it is safer then
carbon adsorption and air stripping. Pd acts as a heterogeneous catalyst, similar to most of the
Group VIIIB metals. Pd nanoparticles are used as a catalyst to remove TCE, and other
chlorinated organic compounds. The chlorinated compounds are removed from the
contaminated water by adsorbing to the Pd surface. At the same time, Hydrogen atoms adsorb to
the surface as well. The species then react and the chlorine then disassociates with the molecule
as the Hydrogen atoms bond to the molecule. When all the chlorines are removed an ethane
molecule (less toxic) is formed. The Pd nanoparticles are great for this reaction because the
metal catalyzes well and has a large surface area for chlorinated adsorption.
Palladium nanoparticles were usually produced by using borohydride as the reducing
agent and carboxymethyl cellulose {CMC} as a stabilizer. Borohydryde is a very toxic chemical
and there was an alternant process created that was greener and less toxic. This process uses
ascorbic acid as the reducing agent, which is more environmentally friendly and less toxic. This
process can be done at room temperature but it is not favored. At room temperature the size and
shape of the nanoparticles were not able to be controlled. The sizes ranged from 15 nm to more
than 100 nm. The shapes ranged from spheres to rods and aggregates of each (He, 2009).
The synthesis of Pd nanoparticles was experimented in a one step process. In this
experiment the temperatures were varied to see the difference temperature has on the synthesis in
the greener method. The temperatures studied were at 22, 50, 80, and 95 °C. The reaction
occurred under the following conditions: 1mL of 0.05 M Na2PdCl4 (i.e.- a palladium chlorinated
salt) was added to 250 mL of 0.15 wt % of CMC. The solution was then kept at the
experimented temperature while being magnetically stirred. About 3.5 mL of 0.05 M ascorbic
acid was added to the heated solution. The reaction was timed for 5 min. and then air cooled
back down to room temperature. The solution was kept under room temperature and stirring
conditions for 24 hours after the reaction had taken place. Pd nanoparticles utilize this synthesis
method to this date due to the phenomenal results that are generated by this process (He, 2009).
The reactions that had taken place revealed qualitative data before actual characterization
was utilized. At 95°C the solution turned a dark brown color right when the ascorbic acid was
added. This elucidated a reduction of the Pd2+ with the formation of the nanoparticles. The
reaction was much different at the lower temperature of 22°C. In about 2 minutes the solution
became a brown color similar to the higher temperature solution. Since it took a long time and
the color did not quite match, it was thought the particles were larger than the particles formed in
the 95°C reaction. On the other hand with the absence of CMC the Pd particles aggregated and
crashed out of solution with in one day. This shows the CMC is very important in keeping the
nanoparticles from being attracted to each other (He, 2009).
The Pd nanoparticles were characterized with a Zeiss EM 10 TEM at a voltage of 60 KV.
The samples were prepared by placing three droplets of the Pd nanoparticle solution onto a
copper grid. The copper grid was then air dried. The size distribution and shapes were
determined by this process. X-ray diffraction {XRD} was also taken of the samples with a
Rigaku Miniflex powder X-ray defractometer with Cu Kα radiation. The samples were
centrifuged to separate them from solution with ethanol as the antisolvent. Then the particles
were dried at 80°C in an oven (He, 2009).
After the characterizations were performed more data needed to be analyzed. The 95°C
synthesized nanoparticles under the TEM revealed that the particles were separated with no
agglomeration observable. There was “823 nanoparticles” with a “mean diameter of 3.6 nm was
estimated with a standard deviation {SD} of 0.5 nm (see Figure 9 below). The XRD on the
particles showed different planes all pointing to a face-centered cubic- {fcc-}lattice. The mean
diameter of these particles was determined to be 4.4 nm. The size difference “could result from
many factors such as sample preparation and the effect of the stabilizers as well as inherent
errors of these two methods” (He, 2009). The TEM image and the histogram of the nanoparticles
synthesized at 95°C can be seen below.
Figure 9: Representative TEM image of CMC-stabilized Pd nanoparticles synthesized at 95
°C in an aqueous system, and the corresponding particle size distribution histogram. The mean
diameter is 3.6nm for these nanoparticles. Source: (He, 2009).
The temperatures had seemed to have been a factor in producing the Pd nanoparticles.
The other particles from the different reactions, of different temperatures (22, 50, 80°C), were
characterized similarly to the 95°C nanoparticles. The particles synthesized at 22°C had a mean
diameter of 55 nm with a SD of 36 nm. The longer time for the color to change did indicate
larger particles. The particles synthesized at 50°C were 26 nm in diameter with a SD of 8.0 nm.
The color change in this reaction took only a few seconds. The particles synthesized at 80°C
were 6.5 nm in diameter with a SD of 1.9 nm. The color change was as immediate as the
reaction at 95°C. The data indicates that the mean size of the nanoparticles do indeed represent
the nanoparticle size. The hotter the reaction temperature, the smaller the particles will become
(He, 2009). The TEM images and histograms for 20, 50, and 80°C can be seen in Figure 10.
Figure 10: TEM images of Pd nanoparticles synthesized at (a) 22 °C, (b) 50 °C, and (c) 80
°C, along with the histograms of these Pd nanoparticles. The mean diameters found on the
histogram were 55nm ± 36nm, 26nm ± 8.0nm, and 6.5nm ± 1.9nm respectively. Source: (He,
2009).
The experiments were then taken further and the nanoparticles were compared to TCE
degradation. It was seen that the nanoparticles that were synthesized at 95°C had a 100%
degradation of TCE in 6 min (see Figure 11 below). The reaction took place in the presence of
H2 to ensure the catalytic ability of the Pd nanoparticles. The same reaction took place with the
other nanoparticles and the degradation of TCE decreased over the decreasing temperatures. The
degradation of TCE was 52% with the nanoparticles synthesized at 22°C, 70% with the
nanoparticles synthesized at 50°C, and 87% with the particles synthesized at 80°C (see Figure 11
below). These degradation results suggest that the nanoparticles synthesized at 95°C had the
optimum surface area volume ratio. This also showed that this ratio was the larger than all of the
other nanoparticles synthesized at different temperatures. In order for TCE to degrade through
the reaction with Pd nanoparticles the nanoparticles must have a predominance of surface area to
ensure enough activation sites for the reaction to take place. It was also pointed out that it was
not just TCE that was broken down. Vinyl chloride {VC} and dichloroethenes {DCEs} were not
detected after the same reaction (He, 2009).
Figure 11: Plot of concentration of TCE catalyzed
by CMC-Pd nanoparticles versus time. The
nanoparticles were synthesized at different
temperatures under ambient conditions and compared.
Source: (He, 2009).
The role of CMC is very important to the stabilization of the nanoparticles. The CMC
used had the molecular weight of 90,000 which indicates an understanding of a baseline level for
the CMC. The CMC is so large that when it reacts with the Pd nanoparticles it does not allow
the agglomeration during nucleation period. The CMC can be bonded through two types of
groups. It has both a carboxyl (COO-) and hydroxyl (OH-) groups. These groups allow the CMC
to form a monolayer on the Pd. The sterics between these large molecules allow for the Pd then
to stay apart as small nanoparticles (He, 2009).
As seen previously it was tested that Pd nanoparticles do have an impact on contaminated
water remediation. It was found that bimetallics have certain properties that will enhance the
way Pd nanoparticles act towards TCE and other chlorinated compounds.
Water does not only contain chlorinated molecules. There are many different
contaminants in water that are not needed but is less toxic than chlorinated compounds.
Contaminants could include sulfides that were found to poison or degrade the Pd nanoparticles.
Pd then was experimented with gold {Au} nanoparticles to reduce this degradation of the
catalytic ability of Pd in sulfide rich water. Au has been known to enhance the catalytic ability
of Pd in many remediation techniques. The way it does is still not fully understood and is being
researched. To make a bimetallic of Au and Pd was very hard. The former technique created the
BNP by depositing the Au and Pd on to a substrate (alumina, silicon, carbon) which was then put
through gas-phase heat treatment. This was problematic because the size of the particles could
not be readily controlled.
Another approach was taken knowing that Au was inert towards hydrodechlorination
{HDC} and Pd was chemically active towards HDC. This understanding allowed scientists to
believe that the nanoparticle should have a Au core and a Pd shell. In 2005 Au nanoparticles
were synthesized that possessed a 20nm diameter by the Turkevich-Frens method. This method
was also known as the citrate method. The Pd was then deposited by a Pd chloride salt and
ascorbic acid reducing agent (note: the use of the ascorbic acid was seen earlier). The Au-Pd
nanoparticle worked more effectively than the competitors (i.e.- Pd nanoparticles, Pd/Al2O3, and
Pd Black). The reaction rate constant for TCE HDC was more than 10 times Pd nanoparticles,
more than 70 times Pd/Al2O3, and more than 2000 times greater than the Pd Black. This was the
first evidence that the bimetallic Pd-Au was better than the competitors for catalyzing TCE HDC
at room temperature (Wong, 2009).
Using 20 nm Au particles was not realistic because of the cost of gold. To circumvent
this Au nanoparticles were processed down to 4 nm in diameter. The synthesized Pd was then
deposited onto the Au nanoparticle using Pd chloride salt and H2 gas reducing agent. Ascorbic
acid was not used because the H2 presented more reduction than ascorbic acid at this level. The
smaller 4 nm Au nanoparticles were more active than the larger 20 nm Au nanoparticles. The
smaller nanoparticles also were seen to have a larger coverage of Pd on the Au nanoparticles
compared to the larger nanoparticles. This is seen in Figure 12 by the rate constant. The percent
coverage was around twice as much. It was noticed that the smaller nanoparticles had around
70% Pd surface coverage. This was thought that this was the optimum coverage for TCE HDC
(Wong, 2009).
The Au-Pd nanoparticles were not able to be used directly in ground water remediation.
The nanoparticles existed in a water suspension of those particles. For these nanoparticles to be
useful they needed to be maintained into a solid matrix. If they were not, agglomeration of the
particles would occur and the reactivity would be lost. A system was generated to have the
contaminated water come into contact with the supported nanoparticles. A porous ceramic oxide
or polymetric resin was used to support the nanoparticles. The particles were attached through
covalent interactions with the ceramic oxide or resin. This setup was tested under water flow
and the nanoparticles held up against a 4.4mL per min flow. The contaminated water contained
about 15 ppm TCE and about 1.5 ppm H2. The H2 is needed to keep the pH at around 7 to 8 and
also allow for the catalytic reaction to take place for the nanoparticles. The nanoparticles seemed
to have still been useful over a four day period (Wong, 2009).
The turnover frequency {TOF} is the amount of TCE molecules broken down per Pd
atom per second. A larger TOF means better nanoparticles because it is degrading TCE in less
time and with less Pd atoms. As the amount of coverage by the Pd atoms on the Au nanoparticle
increased the TOF increased as well. After about 70% coverage the TOF decreased though. It
was hypothesized that this was due to the Pd atoms creating a second layer decreasing the
surface area for the TCE to react. Through this data it was determined that more than a 70% Pd
coverage on a Au nanoparticle it was not effective. The optimum bimetallic nanoparticle
between Au and Pd was 70% coverage by Pd atoms (Wong, 2009).
Figure 12: Au nanoparticles layered with Pd; 20 nm Au nanoparticles are the blue squares and 4 nm Au nanoparticles are the red triangles. The 4 nm particles have a higher rate constant at 70% of Pd. Source: (Wong, 2009).
The characterization of these bimetallic nanoparticles is very limited. With the use of a
TEM the Pd layer could not have been seen in surface characterization. The scientists are certain
that the Pd is only one atom thick on the Au nanoparticle because it could not be characterized
by the TEM. X-ray absorption spectroscopy {XAS} is a technique that can provide information
about atomic coordination numbers and interatomic distances. This was performed on the Au
nanoparticles that were covered 70% with Pd atoms. It was seen that the particles had a larger
diameter than the Au nanoparticles which showed support that the Pd formed a mono-layer on
the nanoparticles. The XAS also determined that the PD was coated on the outside of the Au
nanoparticle and was not in the core. This confirms that the particles are what the scientist set
out to synthesize, a Au rich core and a Pd coating on the outside of nanoparticles (Wong, 2009).
Palladium can also be used as a catalyst for environmental remediation with aluminum
oxide. There was no research on this pair because it was found to be economically unfavorable.
The Pd-Au nanoparticles were calculated to be three orders of magnitude less than the Pd-Al2O3.
This is because the Pd-Au nanoparticles cost about 20 times less than the Pd-Al2O3 (Wong,
2009).
There were no harmful environmental side effects found with the use of the Pd
nanoparticles and the Pd-Au nanoparticles. Some drawbacks of Pd nanoparticles though show
some concern. It was mentioned before that Pd nanoparticles were degraded by sulfides in the
contaminated water. Figure 13 shows the degradation of the Pd NP because of Sulfide and
chloride ions. The bimetallic Pd-Au solves this problem and in a cost effective way. There also
were studies done with sodium chloride and the effect it has on Pd, Pd-Al2O3, and Pd-Au
nanoparticles. The sodium chloride did not affect the Pd-Au nanoparticles to decompose TCE.
In the other studies the sodium chloride changed the reactivity of the Pd and Pd-Al2O3
nanoparticles towards TCE. Sodium chloride decreased the degradation of TCE by about 50%. These
studies show the Pd-Au nanoparticles have the upper hand on TCE HDC.
There were many advantages touched upon about the Pd-Au nanoparticles. Another
great advantage is that Pd and Pd-Au can be synthesized in an environmentally friendly and safe
way. Borohydryde was used before as a reducing agent, which was not safe to handle. The
reducing agent created pollution and very harmful byproducts. Ascorbic acid is used now, which
is a lot safer. Even though it takes heat to create the nanoparticles with ascorbic acid it is much
safer to use and it does not pose a threat to the environment.
Palladium is known to be a precious metal in the so called “platinum group”. Other
metals in this group include platinum, rhodium, ruthenium, iridium, and osmium. Palladium has
Figure 13: Pd’s TOF can be decreased by poisons. The most damaging ones are Chloride and sulfide ions. Pd-Au nanoparticles were found to have a high tolerance to the ions. The Pd-Au nanoparticles have about 30% surface coverage of Pd. Source: (Wong, 2009).
the lowest melting point in this group, raising it to an even more expensive level. On July 7,
2010 Pd was selling for $446 per ounce as a bulk material (KITCO, 2010). This could be very
costly but by using cheaper and raw chemicals in the making of the Pd nanoparticles the overall
cost can be reduced. The reaction only takes one batch chemistry reaction that can be completed
in 5 minutes, allowing for simultaneous generation of particles and doping.
Since Palladium is still in the research stage the cost for large scale application is not
known. There was an article that demonstrated that Pd-Al2O3 was used in a HDC reactor at a US
Superfund site. This lasted a year and was very successful. The reactor reduced TCE from
concentrations of about 3700 ppb to less than 1 ppb (Wong, 2009). This was back in 2000 when
the price of Pd was $27 per gram. This came out to be about $270 per kilogram of catalyst used.
The total amount of catalyst used was 52 kilograms. The point that the remediation was a
success is great, but the cost effectiveness was not cheap to commercialize. More studies are
taking place to reduce the amount of money it takes to use Pd as a catalyst for environmental
remediation.Table 1 shows the comparison between Pd-Al2O3 and Pd-Au costs in 2008. The
table shows the study’s findings on possibilities for cheaper Pd nanoparticles that can be used for
environmental remediation.
Table 1: Cost comparison of Pd-Au nanoparticles and Pd-Al2O3.Source: (Wong, 2009).
Magnesium Nanoparticles
MgO nanoparticles are used for dehalogenation of water; specifically they are used for
defluoridation of water. For years fluoride was touted as being good for dental health. It was put
into municipal water supplies and reached fairly high levels. But as with all things, too much of a
good thing was not a good thing. The World Health Organization {WHO} suggests 0.6ppm in
drinking water provides sufficient fluoride needed for bone and teeth growth, but intake of too
much fluoride can lead to a disease known as fluorosis. In many countries around the world,
around 62 million people are exposed to drinking water containing high concentrations of
fluoride and are in need of an effective technique for fluoride removal. Methods used for
defluoridation include: coagulation and precipitation of fluoride, silty clay and natural minerals
to remove the excess fluoride, soil sorbent and oxidative minerals, and ion exchange using
commonplace metal oxides such as magnesia and alumina. MgO nanoparticles are relatively new
in the realm of environmental remediation, but it has some sizeable contributions to the field.
Studies show that adsorbent MgO powder produced via the combustion method can remove
approximately 97% of fluoride from water. On top of this the MgO powder can be regenerated
and reused and still remove about 76% of fluoride from water. Most removal of fluoride occurs
within 10 minutes of introduction of the MgO powder to the water, a rate that is 6 times faster
than other modern methods (Nagappa, 2007).
MgO nanoparticles are used in applications outside of environmental remediation as well.
MgO nanoparticles are used in conjunction with Ag to develop medical devices which have
antimicrobial properties (Nechula, 2009). MgO nanoparticles are also used in solar cells where
they are applied as a coating over TiO2. Coating the TiO2 with MgO nanoparticles has been
shown to improve energy conversion by 45% (Suk Jung, 2005). It has also been noted that MgO
nanoparticles are capable of adsorbing and destroying organophosphorus particles which can be
mimics of warfare agents. MgO nanoparticles have also been documented to destroy
fluorocarbons (Albrecht, 2006).
MgO nanoparticles are commonly produced via decomposition of magnesium salts and
magnesium hydroxide. MgO nanoparticles produced this way are often quite large as compared
to nanoparticles prepared using other methods and are known to have a low surface area and
large grain size which is not desirable as a destructive absorbent used for dehalogenation.
Production of MgO nanoparticles that are desirable for dehalogenation of water can be produced
via combustion of magnesium nitrate and glycine. In this process magnesium nitrate in an
aqueous solution is combined with glycine in a petri dish. The magnesium nitrate acts as an
oxidizer and the glycine acts as a fuel. Water is evaporated from the mixture using a hot plate
and then the mixture is placed into a muffle furnace at around 400 °C. Inside the furnace the
mixture is further evaporated and eventually the residual powder undergoes smoldering
combustion producing a black product. This product turns white with an additional half hour in
the muffle furnace and is referred to as a non-carbonaceous powder. MgO can also be
synthesized via hydrothermal and sol-gel methods (Nagappa, 2007). The hydrothermal method
produces MgO using aqueous solutions at high temperatures and high vapor pressures. The sol-
gel method employs either metallic alkoxides or organometallics in solution which are then
polymerized to form a wet gel using low temperatures. Thermal annealing is then used to densify
the wet gel and produce polycrystals or a dry gel (Sol-gel Method, 2010).
MgO nanoparticles produced via the combustion method appear to be a top notch
adsorbent for the removal of fluoride from water. MgO nanoparticles synthesized though
combustion are known as “as made MgO” and have a face centered cubic structure. MgO is a
good adsorbent because it is porous, has a large surface area, and is of course very small ranging
from approximately 12-23nm. The smaller the size of the MgO nanoparticles the more efficient
they are at destructive sorption of contaminants. Referred to as MgO powder, the combustion
derived MgO nanoparticles can be produced in large quantities quite quickly and with a cost
effective price tag. In addition these nanoparticles can be regenerated using NaOH treatment and
still are capable of removing approximately 76% of fluoride present in water (Nagappa, 2007).
MgO nanoparticles are characterized using powder X-ray diffraction {PXRD}, scanning
electron microscopy {SEM}, and transmission electron microscopy {TEM} (Nagappa, 2007).
PXRD is commonly used to provide information pertaining to phase identification and can give
information about unit cell dimensions of crystalline structures. PXRD works by shooting
electrons at a target and when the electrons have enough energy they dislodge inner shell
electrons from the target. When these inner shell electrons are dislodged they produce
distinguishing X-ray spectra. These X-rays are collected and aimed back at the sample and as the
geometry of the incoming rays satisfies the Bragg Equation (i.e. - nλ= 2d sin(θ) ) constructive
interference causes a peak in intensity which can be picked up by a detector which processes the
signal and transfers the information to a screen (Dutrow, 2010). SEM is used to image MgO
nanoparticle surface topography. In SEM, a beam of electrons is shot at the sample and in the
case of MgO nanoparticles the secondary electrons bouncing off of the sample provide the signal
that the in-lens detector picks up on and transfers the information to the software which produces
the images seen on screen (see Figure 14). TEM involves shooting a beam of electrons at the
sample and when the beam hits the sample some of the beam is transmitted. This transmitted part
of the beam is focused using an objective lens to produce an image which then passes through
more lenses which enlarge the image. The image finally makes its way to a phosphor image
screen which is where the light is generated that the user can see (Transmission Electron
Microscope, 2010).
Figure 14: SEM images of as made MgO (a), used MgO (b), and regenerated MgO (c). Source:
(Nagappa, 2007).
Mg/Pd nanoparticles used in dehalogenation of water are synthesized via iodine catalyzed
reactions which appear to occur through the parameters of Ostwald ripening (Harbrecht, 2001).
The nanoparticles are imaged using scanning electron microscopy {SEM} and have a large
diameter of around 4µm. The particles topography and composition are studied using SEM. For
topographical images, lower energy secondary electrons provide the signal which is collected
with an in-lens detector; this detector transfers the information to the software which produces
the image. The composition was studied using higher energy backscattered electrons and a
backscattered electron {BSE} detector. Use of a BSE detector allows an intricate look into the
heart of Mg/Pd nanoparticles used in dehalogenation of water which tend to be composed of a
magnesium core with a palladium coating on the surface. Figure 15 (below) shows images of
Mg/Pd nanoparticles imaged using SEM. The top images were taken using the in-lens detector
for lower energy secondary electrons and the bottom images were taken using a BSE detector for
higher energy backscattered electrons. The lighter colored spots on the images are the palladium
while the large dark masses are the magnesium (Gardner, 2007).
Figure 15: SEM images of Mg/Pd powder. Source: (Gardner, 2007).
As mentioned above Mg/Pd nanoparticles are used for dehalogenation of water.
Specifically Mg/Pd nanoparticles are used to dechlorinate polychlorinated biphenyls {PCBs},
polychlorinated naphthalenes {PCNs}, and polychlorinated dibenzo-p-dioxins {Dioxins}.
These contaminants are found in sediments of waterways and are currently removed most
commonly by dredging. After dredging the sediment has to be treated before being returned. This
is an expensive and time consuming process, which can be improved upon by using Mg/Pd
nanoparticles to chemically degrade chlorinated organics via reductive dehalogenation in-situ.
The exact mechanism by which dechlorination of PCBs, PCNs, and Dioxins by Mg/Pd
nanoparticles is not yet fully understood, but it occurs regardless. Mg/Pd nanoparticles work so
well in fact that they are capable of removing more than 90% of the targeted contaminants:
PCBs, PCNs, and Dioxins within a few minutes or hours depending on the specific contaminant
(see Figure 16 below). An understanding of the chemistry behind the amazing capabilities of
these nanoparticles could result in their use to dechlorinate contaminants in-situ, thus reducing
the need for dredging (Gardner, 2007).
Figure 16: The degradation of Arochlor 1260 (i.e. - PCB Congener) over time. Source: (Gardner,
2007).
Advantages of Mg/Pd nanoparticles used for dechlorination of polychlorinated organics
include that they can remove more that 90% of contaminants and that they can work in-situ
without the need to remove sediments containing the contaminants to treat them. Magnesium
prices are low, costing a few cents per ounce. Disadvantages include that the mechanisms behind
the phenomena are not completely understood and that palladium is expensive at around 450
dollars an ounce. A solution to these disadvantages would be the use of less expensive metals in
the production of bimetals which is being pursued (Gardner, 2007).
While uses of Mg/Pd nanoparticles outside of environmental remediation could not be
found, uses of Mg and Pd nanoparticles alone abound. Currently research is being conducted for
the use of magnesium nanoparticles in micro-electro-mechanical systems {MEMS} and
nanoelectromechanical systems {NEMS}, for use as bio nano materials for the development of
bio sensors, and other nanofabricated devices such as improving everything from textiles to
solar energy conversion as noted above. Mg nanoparticles are also used in quantum dot synthesis
and are known to increase the strength and decrease the weight of magnesium metal which is
used in transportation devices (e.g.- cars, airplanes, etc.), and for coatings, plastics, and many
other applications (Group, 2010).
Pd nanoparticle use covers a vast array of topics and fields. Pd nanoparticles are used in
conjunction with Mg nanoparticles for environmental dehalogenation; other uses include
hydrogen sensors which are capable of detecting hydrogen leaks generating hydrogen levels of
25ppm. These new hydrogen sensors are very sensitive because as hydrogen binds to palladium
creating palladium hydride, the palladium nanoparticles swell to the point that they come into
contact with their neighbors thus decreasing the overall resistance which can be detected and
used to determine hydrogen levels (Xiao, 2010). Palladium nanoparticles are used as catalysts,
for example in catalytic converters for cars. Palladium nanoparticles can be combined as a
bimetal and show superparamagnetic properties. Palladium nanoparticles are also used in
polymers to form active polymer membranes. For example palladium nanoparticles are used in
the membranes of electroactive polymers {EAPs} which basically act as a synthetic piezoelectric
material and are referred to as artificial muscles (Nazir, 2008).
Zinc Nanoparticles
In today’s society there have been many problems in the developing world that are tied to
environmental contamination. Some common pollutants are organic halides, energetic materials,
PCB, halogentaed aliphatic organochlorine pesticides, halogenated herbicides, nitroaromatics,
metals, and halogentaed organic solvents. This is a global issue as societies face pollutants that
are causing many illnesses despite their available uses in industrial applications. In recent years
many scientists have taken an interest in how metal nanoparticles can help clean up the
environment. In processes using metal nanoparticles, scientists aim to reduce any side effects
that may occur in the drinking water. Through various studies numerous metals have been used
to enhance the rate of environmental remediation. One of the key metals that made a huge
contribution to the efforts of remediation is Zinc. Researchers have looked at Zinc, Zinc Oxide,
and Zero-valent Zinc in nanoparticle suspensions. Research is focused on improving methods for
Zinc-based particle synthesis, characterization, and application. Different forms of particle
materials, side effects, advantages and disadvantages of this material vary depending on the
synthesis route chosen, as well as the application for which the particles are designed.
There are numerous synthesis routes for Zinc nanoparticles, many of which mimic
natural Zinc compounds. Due to the environmental prevalence of Zinc, researchers can add
different elements and create new materials that can be used for environmental remediation with
a lower risk of secondary contamination. In addition to the safe application of Zinc in certain
environmental settings, it has another distinct advantage as being a material that can readily
undergo either reduction or oxidation based upon the desired application. Through many studies
researchers have found that Zinc can be made through aqueous combination of sodium
hydroxide with zinc acetate dehydrate (Jun, 2009). Researchers have been using zinc
nanoparticles created through this method for dechlorination of water. Particles produced in this
manner have also been used to create different bimetallic particles using dopants such as Ag, Ni,
Pd. Different dopant combinations have been found to be more effective at stabilizing the pH of
the water samples at around 7.2 to 7.3 (Li, 1998). Researchers have tested these experiments in
and out side of the labs and have determined that Zn has a fast rate of contaminant removal in
both water and soil based trials.
Another process for creating Zn uses methods similar to the creation of non-oxide
semiconductor nanoparticles through pyrolysis of organic metallic precursors dissolved in
anhydrate solvents (Geiger, 2009). Researchers place the Zinc particles in high-elevated
temperatures in a closed environment in the presence of polymer stabilizer or capping agents
for this procedure (Geiger, 2009). Different capping agents cause important changes in the
amount that the material’s risk of cross-contamination is reduced. Different agents also create
stronger or weaker adhesions for the metals to dope to one another.
Although there are many processes to synthesize Zn, each process has different
mechanisms for helping the environment. Through comparison of different synthesis methods,
scientists have determined that the best form of Zn to use in remediation is doped Zinc particles
that are transported in liquid media. Non-doped zinc or particles where zinc is the dopant can
achieve similar results, but have been found to have more costly side effects on the environment.
Figure 17(a-c) (-below) shows images of Zn nanoparticles taken by a SEM microscope. Figure
17(a) presents Zn nanoparticles before dopant materials are added, 17(b) is an individual Zn
nanoparticle, and 17(c) is Zn when it was doped with Pd. Another example of a bimetallic
nanoparticle that researchers have taken an interest in for environmental remediation is zero-
valent zinc.
Figure 17(a-c): Images of Zinc nanoparticles under a SEM. Source: (Geiger, 2009).
Zero-valent Zinc is used in the industry aspect of environmental remediation. Zero-
valent Zinc is a powder that is prepared in an aqueous condition. (Guobin, 2009) This is used to
ease the doping process of the Zn, allowing for the easy stabilization of the pH for drinking water
sources. (Guobin, 2009) It was found that when doping Zn with Ag, Ni or Pd, the particles
generated could be used to purify water that has been contaminated with trichloroethene {TCE},
organic solvents, or degreasing agents. Zero-valent Zinc powder is used in industry to control
leaks in storage tanks, accidental spills and many other factors that demand neutralization due to
environmental concerns. Due to increased attention that metal nanoparticles have received in
recent years for industry applications, concern over the characterization of particles has been on
the rise.
Characterization methods abound for nanostructures, but nanoparticles present special
challenges due to their size and structures. In one study researchers looked at how the
characterized properties of Zn matched up with the properties of the bulk material. Zn is a wide-
band gap semiconductor, which has many ways to be produced or characterized, such as: sol-gel
process, hydrothermal process, and solution deposition (Hsu, 2005). Sol-gel process is a wet
chemical technique that is used for metal oxides. From a solution of particles suspended in a
liquid a gel substance is prepared that can be used to aid in the doping of Zn (Hsu, 2005). In this
process, the metal gradually moves towards the gel-like solution while the metal alkoxides
undergo hydrolysis. During this time, the material being created can be characterized using two
different methods. The first way to characterize Zn is when a base-catalyzed sol undergoes a
chemical reaction. During the reaction, the particles may grow to sufficient size and become
colloids (Hsu, 2005). As a result the Zn nanoparticle will undergo self-assembly mechanisms
that will prepare the metal nanoparticles for transport to sites of concern to reduce
contamination. If an acid-catalyzed sol was used, the intermolecular forces will have sufficient
strength to cause aggregation before the growth of the nanoparticles in the sol-gel network can
be completed. As a result, the growth of a more open network of low density polymers is created
and exhibits certain advantages with regards to the physical properties of the metal nanoaprticles.
Acid-catalyzed sols seem to be highly effective at removing contaminants generated by the glass
and ceramic industries (Hsu, 2005).
Hydrothermal synthesis is another process that creates unique particle characteristics.
During this synthesis method, the material is placed in a hot water bath under high pressure (i.e.-
“autoclave”), allowing for the synthesis of single crystals in the nanoparticle. In this experiment,
the temperature is maintained differently at the opposite ends of the growth chamber. The higher
temperature end of the chamber is used to dissolve the nutrients. The cooler end of the chamber
is used to cool the Zn nanoparticles and casue addition growth. This method is used to
theoretically model research for particle use in the environment with the control of water and the
soil contamination.
A final possible synthesis method is solution deposition. In solution deposition a flexible
plastic substrate {PES} is used in the creation of the particle films as it allows for easy use in
environmental remediation. A solution to promote adhesion and allow for lithographic processes
was first spin-coated onto the substrate. Two aluminum electrodes were formed on the substrate
by combining lithographic process with a deposition method. Before the deposition process, the
surface was treated with plasma-based ozone to promote the adhesion of the substrate and metal
electrodes. The photoresist used in the process was a positive resist as the bonds binding it to the
surface were weakened during following UV exposure. ZnO NPs were spin-coated onto the
substrate through the use of a liquid media transport. The filmed formed by this process was
subsequently annealed before Alumina layers were deposited. The alumina layers helped to
construct a filtration system that would be useful in reduction reactions (Hsu, 2005).To gain a
better understanding of this process, Figure 18 (-below) shows the experiment layers in a block
diagram form. As result of the characterization, researchers tests these method and look at rates
of the different materials.
Figure 18: Final product of solution deposition. Source: (Hsu, 2005). Represents a stacked layer
filter. ZnO NP are filtered through pores in the alumina layer and react with metal deposits in the
Aluminum layer.
Although there are many uses for ZnO nanoparticles some of the key aspects that
researchers had focused on include photochemical reaction activity and hydroxide conjugates
absorption. Hydroxide conjugates absorption helps to determine when to use certain ZnO BNP’s
and in the type of environmental setting that would be most appropriate. The most economic and
promising industrial process for pollutant treatments involving BNP’s is the reduction of global
atmospheric pollution and the purification of polluted water. Photocatalysis is an advanced
oxidation process {AOP} that can be used for the degradation of organic pollutions in a simple
and easy manner (Geiger, 2009). Researchers have used ZnO bimetallic materials in this process
to reduce specific contaminants in industrially used areas, such as brownfields or superfund
sites. Some contaminants that are removed during the process included the following:
Chlorinated phenols, Rhodamine dyes, Direct blue 53, Acid red, Ethyl violet and Methylene
blue. (Geiger, 2009) The reduction of contaminants at such sites is important in reducing cross
contamination between water-to-water, water to humans, and water to the soil. Hydroxide
conjugates absorption focuses on removing undesirable organic materials from the environment
in water and soil levels found below ground level. The best material to use in this aspect is
bimetallic materials (e.g.- ZnO) because it can remove such difficult materials as arsenate,
copper, trichloroethane (i.e.- a solvent of 2-methyle), Polychlorinated biphenyls, DCA and
mercury (Guobin, 2009)-(Geiger, 2009)-(Li, 1998). Researchers use this process because of its
simplicity, low cost, and effective removal of heavy metals (Geiger, 2009).
Although metals are good to remove contamination from the environment, there usually
are a few side effects that could occur. Some of these side effects are tendencies to cause copper
deficiency and hemolytic anemia in those subject to the drinking water that has been treated
(Porter, 2010). The reason why this occurs is by an excesses amount of copper in the form of
nanoparticles enters the water supply. As a result, nausea, vomiting and diarrhea can occur.
These side effects occur with low amounts of copper in the body and can be reduced over time
(Porter, 2010). If an excess amount of copper enters the body in the range of milligrams to grams
of copper hemolytic anemia can result (Porter, 2010). This can be reduced by removing the
contaminants from the water supply by using nanoparticles to reduce copper, or by using particle
combinations that do not involve copper.
Some advantages of Zn is its simple use, cheap production cost, effectiveness at
removing heavy metals and different contaminants. Some disadvantages of Zn include the need
to remove leftover particles in the area of exposure as well as various health effects. These
disadvantages can be overcome through the use of filtration systems within housing locations
that are near remediation sites. Research is also being done to address how to minimize the
amount of Zn that is used to reduce various contaminants. By doing this, the side effects might
be able to be reduced to a level that is acceptable for wide-scale usage.
Conclusion
Overall, environmental remediation is a topic whose scope goes far beyond the abilities
of a few journal articles, lectures, or courses. The endeavors of creating a sustainable
environment are not only in the best interests to the general lifeform, but they encompass entire
schools of thought ranging from chemistry, to physics, from biology, to economics, and more.
The major players in environmental remediation today include the contaminants themselves,
potential sources of decontamination (e.g.- metal nanoparticles, most popularly), the knowledge
and theories of scientific observation in both the past and present, and the group-mind of
humanity. While it is impossible for an individual or a small group of individuals to know it all,
the more humanity knows in general- that is, the more the bar of knowledge (and thus,
responsibility) is raised-, the more capable it becomes in dealing with the issues of its own
creation and its own potential demise.
Table 2: Comparison of Metal Nanoparticles DiscussedMaterials for
synthesisCost for catalyst
Contaminants removed
Problems
Fe Sodium Borohydride/ FeCl3
Can lower costs more than traditional methods
Arsenic
Chlorinated compounds
Arsenic can be oxidized and/or reduced
Fe/Pd Zero valent iron/ PdCl/ Ethanol/ Water and other methods
More costly than other elemental candidates
Chlorinated compounds
Iron oxide can form, limiting the reactivity
Fe/Ni Polyacrylic acid/polyvinylidene fluoride membrane matrix
NA Atrazine
Chlorinated compounds
Aggregates easily
Fe/Ag Iron nanoparticles/ AgCl in ethanol
NA Chlorinated Compounds
NA
Tetrabromobisphenol A
MgO MgNO3/ Glycinein combustion
Low cost Fluoride NA
Mg/Pd Iodine catalyst NA Chlorinated compounds
Understanding of remediation
Pd Ascorbic Acid and Heat
$27/gram Chlorinated compounds
Chloride and sulfide ions act as poisons
Pd/Au Au nanoparticles/ PdCl/ H2
$142/gram Chlorinated compounds
Need to be used as a resin in a porous material as a filter
Zn Zinc particles in high-elevated temperatures in the presence of polymer stabilizer or capping agents
NA Nitroaromatics,
Halogenated compounds
NA
The metal NPs discussed previously are compared in the above table. Many different
metal and bimetal NPs used in environmental remediation were researched. Synthesis is carried
out using different processes for each NP. For example Fe is the base metal for all Fe bimetallic
NPs, and each bimetallic NP has its own synthesis process. Mg NPs paired with other elements
are used in environmental remediation. Synthesis of Mg/Pd NPs is very different from MgO
NPs. MgO NPs are formed by combustion. Mg/Pd NPs are formed by an iodine catalyst. Pd and
Pd/Au NPs are synthesized in very similar ways. The only difference is ascorbic acid is replaced
by H2 as the reducing agent of Pd. There are many bimetallic Zn NPs, Zn nanoparticles are
synthesized by heating Zn in the presence of polymer stabilizers or capping agents.
The reason for forming metallic nanoparticles is to reduce the cost for environmental
remediation. The NPs mentioned above improve environmental remediation or greatly reduce
the cost of remediation projects that may be in progress today. Fe NPs greatly reduce the costs
of the original methods of environmental remediation. Fe/Pd NPs are expensive, but are more
efficient than Fe alone. MgO NPs are easily synthesized in large quantities and are very pure
when synthesized via the combustion method. Market values of Pd are currently lower than past
years; this in turn means the cost of production of Pd containing NPs is lower. Pd/Au NPs are
efficient environmental remediation agents and are more cost effective than Pd/Al2O3.
The metal NPs remove an assortment of contaminants. Fe and Fe bimetallic NPs remove
arsenic, chlorinated compounds, tetrabromobisphenol A, and Atrazine from water. Mg
containing NPs (MgO and Mg/Pd) remove fluoride and chlorinated compounds. Pd and Pd/Au
NPs only remove chlorinated compounds but work considerably well compared to other metal
NPs. Zn removes nitroaromatics and halogenated compounds.
Several problems are associated with metal NP use for environmental remediation. Fe
can reduce the arsenic, but arsenic can also be oxidized by the Fe. Fe in the Fe/Pd NPs can
oxidize which limits the reaction. Fe/Ni NPs agglomerate together which reduces the surface
area. The Mg/Pd NPs react and remove contaminants but the exact mechanism for removal of
PCNs and PCDDs is not fully understood. Pd NPs can remove chlorinated compounds, but if
there are chloride or sulfide ions in the solution the Pd NP does not perform to its optimum
potential. Pd/Au NPs are also great for removing chlorinated compounds, but cannot be added
directly to the solution. Pd/Au NPs need to be on a porous material to be stable and perform.
Many different metal NPs are being synthesized for use in environmental remediation. Metal
NPs are the future of environmental remediation because of their low cost, high efficiency, and
in-situ applications. Combinations of metals as NPs can be used to remove many contaminants
that could be toxic to humans. New metal NPs will be synthesized to remove other toxins in the
near future.
Numerous contaminants exist which threaten both human endeavors and the well-being
of countless lifeforms within the enivronment of our planet. These include, but are in no way
limited to, polychlorinated biphenyls, polychlorinated napthalenes, vinyl chloride,
polychlorinated dibenzo-p-dioxins, trichloroethene, fluoride, arsenic, and tetrabromobisphenol
A. A working knowledge-base of the nature of these substances is important not only for
environmental considerations, but also for the effectively safe production of materials in the
industrial sector.
PCBs are most commonly known in the U.S. by their trade names. The most common
PCB mixture used in the U.S. was marketed under the name Aroclor. There are many PCB
congeners each of which is slightly different. PCBs were produced from 1929 to 1979 when the
production of PCBs was banned. PCBs properties: chemical stability, resistance, high boiling
point, and non-flammability led to their use in many products before the 1979 ban. PCB
applications include: plastics, caulking, floor finish, carbonless copy paper, oil based paint,
adhesives used in tape, insulating materials, transformers, and capacitors.
Conclusive evidence has been presented for animal carcinogenicity caused by PCBs as
well as other health effects on a plethora of organs and organ systems. It has been suggested by
studies that PCBs are probable human carcinogens and cause non-carcinogenic health effects in
humans as well (Basic - PCBs, 2009). As the many uses of PCBs in their day, listed above, may
suggest there are a lot of these contaminants in the environment, especially in the sediments of
waterways and even in fish themselves. The EPA has issued advisories because fish have been
found to contain high enough concentrations of PCBs that they could have adverse effects on
human health if consumed too often (Bigler, 1999). The combination of negative effects of PCBs
on life and their abundance in the environment present a situation where in-situ treatment of this
contaminant with metal nanoparticles could make a world of difference in the world within the
next few years.
Polychlorinated naphthalenes {PCNs} were used in plastics and rubbers, in wood
preservatives, in lubricants, and as dielectrics for coatings and in capacitors. PCNs were
produced for over eighty years before the Toxic Substances Control Act slowed their
production. Currently only a handful of company’s produce PCNs worldwide, but PCN leftovers
are a widespread contaminant in the environment. PCN exposure has been linked to increased
risks of liver disease while chronic exposure to PCNs is suggested to cause cancer though current
evidence is inconclusive (Minoru, 2000). PCNs are also found in sediments of waterways and
can be removed through the use of metal nanoparticles, thus relieving the problems caused by
this contaminant.
Vinyl chloride {VC} is used in the production of products for several major industries
including: rubber, glass, and paper. VC is also used in the production of electrical wire
insulation, piping, and medical supplies. The EPA set a maximum contaminant level {MCL} of
2 ppb for VC. VC enters water via two main routes; it either leaches out of polyvinyl chloride
{PVC} pipes or is introduced from plastic factories. Effects of consumption of water containing
VC levels above the MCL include increased risk of carcinogenesis. Current methods for
removal of VC include packed tower aeration which can be improved upon through the use of in-
situ metal nanoparticles (Basic – VC, 2010).
Polychlorinated dibenzo-p-dioxins {Dioxins}-or-{PCDDs} are often consumed in fish.
The EPA lists 210 related chemical compounds and has issued 59 advisories for fish
consumption. PCDDs enter the environment through combustion and incineration of products
containing PCDDs, bleaching of pulp in paper mills, and in other chlorinated organic chemicals.
Studies suggest that human exposure to PCDDs cause liver damage, induce carcinogenesis, and
could possibly be mutagenic, but evidence is inconclusive. Severe acne and skin rashes have
been documented when humans are exposed to high concentrations of PBDDs. In-situ
remediation of these contaminants using metal nanoparticles is looking bright for the future
(Bigler, 1999).
Trichloroethene {TCE} has been used around the world for various applications
including: solvents for organic materials, decaffeination of coffee, and preparation of pure
ethanol. The EPA has standards of a maximum contamination level {MCL} of 5 ppb. TCE is
suggested to be a likely carcinogen to humans as well as cause other health effects targeting the
nervous, immune, and endocrine systems as well as induce liver and kidney pathogenesis.
Production of TCE has been reduced and current remediation methods include use of bacteria
which can degrade TCE in-situ. Pitfalls of bacteria use for TCE remediation include that the pH
has to be within habitable limits for the bacteria and that they are relatively slow in degrading
these contaminants. The use of metal nanoparticles could accomplish the task of remediation at a
much faster rate than the bacteria (Trichloroethene, 2010).
Fluoride is added to drinking water to promote dental health and the World Health
Organization {WHO} guideline of 1.5mg/L is not to be exceeded. Over 200 districts in India
have been found to have excess fluoride in the ground water. As previously mentioned excessive
fluoride consumption can lead to a disease known as fluorosis which can affect teeth and bones.
MgO nanoparticles can remove excess fluoride form ground water in-situ and save time and
money as well as pain and suffering (Nagappa, 2007).
Arsenic is found in natural rock formations, and can be released through wood
preservatives, farming compounds and mining run-off primarily. The WHO has established a
minimum drinking level of 10 ppb. Excess arsenic exposure can result in cancer of the skin,
lung, kidney and urinary bladder, as well as hyperkeratosis and pigmentation changes. Increased
lung and kidney cancer risks are observed above 5 ppb, and so may still occur even if water is at
the safe drinking level. Current methods of removal are based around home water system
filtration, and are rather costly. Most of the cost is due to the arsenic being found in ground
water, meaning that any remediation efforts take place inside ground well pumps. Ground water
is a perfect target for nZVI can for remediation as they can be released at the higher sources of
arsenic contamination through hydraulic slurries. Nanoparticles also offer a lower cost of
treatment than house-based remediation efforts, and can accomplish much lower levels of
arsenic concentrations at faster rates (Fact, 2001) (Ramos 2009).
Tetrabromobisphenol A is a man made polymer that is used primarily in printing circuit
board resins. It finds other uses in high impact plastic systems, and thermoplastic devices. There
are currently no established minimum exposure levels for TBBPA as it is not directly
threatening. However, upon combination with certain chemicals in the environment, it can form
harmful halogenated organic compounds. Current removal methods are based upon filtration and
sewage treatment, as well as marine treatment of contaminated run-off water from industry sites.
This can be better managed through the use of metallic nanoparticles, which can increase the rate
at which TBBPA is removed from the environment.
Nanoparticles for environmental remediation are a continuing source of major research.
Metal nanoparticles are still not fully understood, and as such there remain several areas that
research can be focused towards. The key areas are better creation methods of nanoparticles in
industrial quantities, continuing development in characterization methods, development of
quantitative techniques for measuring chemical surface interactions of nanoparticles, and
theoretical studies focused upon what particles absorb and why they absorb certain things more
readily (Grassian, 2008).
Better creation methods for nanoparticles consist of a new focus toward modifying
systems used for low pressure chemical vapor deposition {LPCVD} in the semiconductor
industry for the creation of support-based deposition of the particles in ordered, collectible
patterns (Grassian, 2008). This would allow for larger quantities of the particles to be prepared,
and could allow for more uniform size control for particle creation. The drawback of larger
processing time would be remedied by the batch preparation that LPCVD can allow for, and the
ordered placing of the nanoparticles upon proper support structures would allow for easy
placement within current remediation technologies. The only drawback that would be present is a
higher risk of oxidization of the particles during removal from the LPCVD reactor (Grassian,
2008).
New characterization techniques promise to yield better results for the determination of
oxide layer thickness and core composition of nanoparticles. Characterization techniques also
promise to yield better in situ modeling and design. A technique that could accomplish both
would involve the use of a scanning mobility particle sizer {SMPS}. A SMPS utilizes two basic
systems, a differential mobility analyzer and a condensation particle counter. The SMPS can
analyze only gaseous substances, so solid or liquid suspensions of nanoparticles would need to
become aerosols before analysis could continue. The gaseous particles are then charged and
undergo a process of analysis similar to the operation of a residual gas analyzer. The differential
mobility analyzer allows only certain particle diameters to pass through to the condensation
particle counter. The condensation particle counter merely records the number and type of
particle that has passed through the channel. The device can measure particles from 2 nm to 700
nm in size, fitting nicely into the desired ranges of environmental remediation. Furthermore, the
device would allow for theoretical modeling of the flow of particles in different media which
could allow for easier development of in situ characterization techniques (Grassian, 2008).
Beyond the better in situ developments, the SMPS can be used to determine oxide layer
thickness, particle agglomeration, and other engineered layer thicknesses. This is through the use
of mathematical modeling to determine the equivalent spherical shape of particles that take on
amorphous forms. The amorphous forms are then related mathematically to the volume of a
similar sized sphere that can then be modeled in such a way to determine the thickness of any
layer surrounding the shell, the level of agglomeration, and the rate of change in particle size
(Grassian, 2008).
In order to make use of this new information, better transportation and monitoring
methods must be developed for field analysis. A large advantage of this will be data that can be
modeled to determine the particle reactivity and possible reasons as to selectivity of certain
particle configurations. Two delivery methods were recently attempted in field tests involving
Fe-Pd nanoparticles. The tests were run using carbon and polyacrylic acid as support structures
for the particles as they were transferred into the slurry. Carbon structures were created using
XC-72 Vulcan that was reacted with diazonium salt. The salt was obtained from a reaction with
sulfonic acid (Grassian, 2008). Polyacrylic acid was used as supplied. In the tests there were no
precautions taken to reduce possible oxidization. The two trials were used in a soil sample and
were compared against each other and against non-supported nZVI for remediation affects and
rates. In the tests it was found that both supports worked well for transport of the nZVI except
for the case of low-clay soils. This is due to the clay in the soils acting as possible nanosupports
much like the other materials naturally. Despite this problem, other methods were discussed to
overcome this drawback, including the use of hydrofracturing to deliver the particles to direct
spots if low clay levels were a serious concern (Schrick, 2004). Another important result that has
been obtained is the optimum size for particle transport for environmental remediation based on
this study. The study was able to determine that the optimal size for transport of these specific
particles was a range of four to five hundred nanometers in equivalent diameters for the particles
(Schrick, 2004).
Metallic nanoparticles are a small part of the new field of nanoscience, and the promise
of environmental remediation is but one area that can be improved through the use of this tiny
technology. Nonetheless, it is a field that offers great promise for improving the lives of every
member of every society. As such, research in these and other areas are vital for the proper
application of this technology to the problems of the world at large.
Glossary
Arsenic – element with the symbol As that has an atomic number 33 and an atomic mass of 74.9 g/mol
Bragg Equation (nλ= 2dsin(θ) - gives the angles for coherent and incoherent wave scattering from an incident surface
Brown Field – environmentally contaminated piece of industrial property
Carcinogenesis – generation of cancer, development of new cancer
Carcinogenicity – the level to which a material is carcinogenic
Catalytic Hydrodechlorination – reduction of a chemical, usually a contaminant containing chlorine in water in the presence of a catalyst
Cerebral Spinal Fluid {CSF} – bodily fluid that surrounds the brain and spinal cord providing buoyancy, protection, chemical stability, and prevention of cerebral hypoxia
Congener – one of many variants of a chemical structure whose chemical makeup is the same
Dehalogenation – removing chlorine from organic contaminants
Destructive Sorption – the chemical transformation of a molecule when it absorps or adsorps to a nanoparticle
Fluoride – the reduced form of fluorine
Fluorosis – disease caused by excessive fluoride injection during the developing years of life
Galvanic Cell – battery capable of producing electrical current based on chemical exchange of electrons
Hydrolysis- a reaction in which water molecules are split into hydrogen and oxygen ions
In-Situ – in the field, real time data
Linear Sweep Voltammetrical Curve – a curve representing the difference in potential between two electrodes, the current of the first electrode is continuously measured over time while the second electrode acts as a reference for the system and any reduction or oxidation reactions taking place within the system will be recorded as spikes or fluctuations on the curve
Muffle Furnace – a furnace radiantly heated by an external heating chamber
Pathogenesis – development of a disease
Photocatalytic activity- a substance's reaction to ultraviolet light, such that an exciton is produced
Polychlorinated Biphenyls {PCBs} – an organic compound composed of one or several chlorine atoms bonded to a biphenyl
Polychlorinated Dibenzo-p-Dioxins {Dioxins} – environmental contaminant composed of two benzene rings bonded to one another by two oxygen bridges with chlorine atoms attached to the structure
Polychlorinated Naphthalenes {PCNs} – environmental contaminant composed of two benzene rings bonded to one another with chlorine atoms attached
Polycondensation reactions – a series of reactions which create a polymer chain
Polymer stabilizer – a substance added to a polymer to prevent its degradation
Potentiostat – electronic device which automatically controls the voltage and current of a multi-electrode system in which electrochemical reactions take place. One of these electrodes acts as a reference for the system while the second acts as the working electrode over which current can be measured
Pyrolysis- the chemical decomposition of organic materials by heat in the absence of oxygen
Redox-amenable – describing a substance which is likely to undergo a redox reaction
Solvothermal method- used to grow single crystals in an autoclave
Superfund Sites – toxic waste site that is in such need of attention that it has been placed on a national list of “things to do”
Tetrabromobisphenol A {TBBPA} – flame retardant composed of bisphenol A with four bromine atoms attached that can end up in the environment as a contaminant
Toxic Substances Control Act- U.S. law which gives the EPA authority to set restrictions on chemical substances
Trichloroethene {TCE} – compound composed of two carbons double bonded to one another with three chlorine atoms and one hydrogen atom attached, TCE is used as an industrial solvent
Vinyl Chloride {VC} – an organochloride molecule composed of two carbons double bonded together with three hydrogen atoms and one chlorine atom attached, this chemical is used to produce polymer polyvinyl chloride {PVC} and is toxic, carcinogenic, and flammable
BibliographyAlbrecht, M. A. (2006). Green Chemistry and the Health Implications of Nanoparticles. The royal Society of Chemistry , 417-432.
Anemia Definition. (2000, December 9). Retrieved July 8, 2010, from Medicine Net: <http://www.medterms.com/script/main/art.asp?articlekey=15491>
"Basic Information about Vinyl Chloride in Drinking Water." US Environmental Protection Agency. 5 Mar. 2010. Web. 10 July 2010. <http://www.epa.gov/safewater/contaminants/basicinformation/vinyl-chloride.html>.
"Basic Information| Polychlorinated Biphenyls {PCBs}| Wastes | US EPA." US Environmental Protection Agency. 24 Mar. 2009. Web. 10 July 2010. <http://www.epa.gov/epawaste/hazard/tsd/pcbs/pubs/about.htm>.
Bertolini, J. C. (2007). Nanomaterials and Nanochemistry. New York: Springer.
Bigler, Jeffrey. (1999). "Polychlorinated Biphenyls {PCBs} Update: Impact on Fish." United States Environmental Protection Agency, 1-7.
Brinker, C. J. (1990). Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing. Academic Press .
Brunauer, Stephen. (1938). Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc, 309-319.
Burton, A. (2009). Hit or Miss?: Benefits and Risks of Using Nanoparticles for in situ Remediation. Enviromental Health Perspectives .
Dutrow, B. L. (2009, November 20). X-ray Powder Deffecation. Retrieved July 8, 2010, from Geochemical Instumentation and Analysis: <http://serc.carleton.edu/research_education/geochemsheets/techniques/XRD.html>
Fact Sheet: Drinking Water Standards for Arsenic, January 2001, July 10, 2010, from <http://www.epa.gov/safewater/arsenic/regulations_factsheet.html>
Gardner, K. H. (2007). Dechlorination of Polychlorinated Biphenyls, Naphthalenes. Journal of Environmental Science and Health, 685-695.
Geiger, C. (2009). In Environmental Application of Nanoscale Reactive Metal Particles. ACS Symposium Series, 1-20.
Grassian, Vicki H. (2008) When Size Really Matters: Size Dependent Properties and Surface Chemistry of Metal and Metal Oxide Nanoparticles in Gas and Liquid Phase Environments. Journal of Physical Chemistry. pp. 18303-18313.
Guobin, S. (2009). Applications of Nanomaterials in Environment Science and Engineering. Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management , 110-119.
Harbrecht, B. (2001). Structure and Thermal Stability of the New Intermetallics MgPd2, MgPd3, and Mg3Pd5 and the Kinetics of the Iodine-catalyzed Formation of MgPd2. Journal of Solid State Chemistry , 113-120.
He, F. (2009). One-Step 'Green' Synthesis of Pd Nanoparticles of Controlled Size and thier Catalytic Activity for Trichloroethene Hydodechlorination. Ind. Eng. Chem Res. , 6550-6557.
Hsu, C.C; Wu, N.L. (2005). Synthesis and photocatalyic of Zn/ZnO2 composite. Journal of Photochemistry and Photobiology A: Chemistry 172, pp. 269-274.
Hyung Jun, J. (2009). Flexible TFTs based on Solution-Processed ZnO Nanoparticles. Nanotechnology , 6.
Jun, J.H.; Park, Byoungjun; Cho, Kyoungah; and Kim, Sangsig. (2009) Flexible TFTs based on solution-processed ZnO nanoparticles. Nanotechnology 20 (6pp).
KITCO. (2010). Retrieved July 6, 2010, from 24-hour Spot Chart- Palladium: http://www.kitco.com/charts/livepalladium.html
Laboratoire De Physique Des Lasers, Atomes Et Molecules. (n.d.). Retrieved July 7, 2010, from Sol-gel Method: http://www-phlam.univlille1.fr/pub/f/themas/phot/materiaux/Methode sol eng.htm
Li, W. (1998). Ultrafine Zinc and Nickel, Palladium, Silver Coated Zinc Particles Used for Reductive Dehalogenation of Chlorinated Ethylenes in Aqueous Solution. Croatica Chemica Acta , 853-872.
Luo, S. (2010). Reductive degradation of Tetrabromobisphenol A Over Iron-Silver Bimetallic Nanoparticles Under Ultrasound Radiation. Chemosphere , 672-678.
Magnesium Nanoparticles. (2001-2010). Retrieved July 7, 2010, from American Elements: http://www.americanelements.com/mgnp.html#research
Martin, J. E. (2008). Determination of the Oxide Layer Thickness in Core−Shell Zerovalent Iron Nanoparticles. Langmuir , 4329–4334.
Minoru, Omura, Masuda Yoshito, and Hirata Miyuki. (2000). "Onset of Spermatogenesis Is Accelerated by Gestational Administration of." Environmental Health Perspectives 108.6, 539-44.
Nagappa, B. (2007). Mesoporous Nanocrystalline Magnesium Oxide. Microporous and Mesoporous Materials , 212-218.
Nanotechnology: The A to Z of Nanotechnology. (2009, March 19). Retrieved July 6, 2010, from Palladium Nanoparticles Creating More Efficient Fuel Cell Catalysts: http://www.azonano.com/news.asp?newsID=10490
Nazir, R. (2008). Superparamagnetic bimetallic iron–palladium nanoalloy: synthesis and characterization. Nanotechnology .
Nechula, B. S. (2009). Enrichment of Anodic MgO Layers with Ag Nanoparticles. Mournal of Materials Science , 339-345.
O'Donoghue, M. (1983). A guide to Man-made Gemstones , 40-44.
Palladium Nanoparticles. (2001-2010). Retrieved July 6, 2010, from American Elements: <http://www.americanelements.com/pdnp.html>
Porter, R.S, and. Kaplan, J.L.; Copper. Merck manual online. 7/11/10. <http://www.merck.com/mmpe/sec01/ch005/ch005c.html>
Ramos, M. A. (2009). Simultaneous Oxidation and Reduction of Arsenic by Zero-Valent Iron Nanoparticles: Understanding the Significance of the Core−Shell Structure. The Journal of Physical Chemisty , 14591–14594.
Shao-ping, T. (2005). Rapid Dechlorination of Chlorinated Organic Compounds by Nickel/Iron Bimetallic System in Water. Journal of Zhejiang University Science , 627-631.
Schrick, Bettina. (2004) Delivery Vehicles for Zero Valent Metal Nanoparticles in Soil and Groundwater. Chemistry of Materials. pp. 2187-2193.
Stafford, N. (2007, October 10). RSC: Advancing the Chemical Sciences. Retrieved July 2010, 2010, from Catalytic Converters Go Nano: http://www.rsc.org/chemistryworld/News/2007/October/10100701.asp
Suk Jung, H. (2005). Preparation of Nanoporous MgO-Coated TiO2 Nanoparticles and their Application to the Electrode of Dye-Sensitized Solar Cells. Langmuir , 10332-10335.
Tee, Y.-H. (2009). Degradation of trichloroethene by Iron-Based Bimetallic Nanoparticles. Journal of Physical Chemistry , 9454–9464.
Transmission Electron Microscope {TEM}. (n.d.). Retrieved July 8, 2010, from <http://www.unl.edu/CMRAcfem/temoptic.htm>
"Trichloroethene Health Risk Assessment: Synthesis and Characterization." U.S. EPA ColdFusion Server. 10 Apr. 2010. Web. 10 July 2010. <http://cfpub.epa.gov/si/si_public_record_report.cfm?dirEntryId=23249>.
Wang, C.-B. (1997). Synthesizing Nanoscale Iron Particles for Rapid and Complete Dechlorination of TCE and PCBs. Environmental Science and Technology , 2154–2156.
Wantanabe, K. (2006). Transform to L10 Structure in FePd nanoparticles synthesized by Modified Polyol Progress. Science and Technology of Advanced Materials , 145-149.
Wong, M. S. (2009). Cleaner Water Using Bimetallic Nanoparticle Catalysts. J Chem Technol Biotechnol , 158-166.
Xiao, Z. (2005, May 25). Hydrogen Sensor Using Palladium Nanoparticles. Retrieved July 9, 2010, from http://www.understandingnano.com/nanoparticle-palladium-hydrogen-sensor.html
Xu, J. (2005). Membrane-based Bimetallic Nanoparticle for Environmental Remediation: Synthesis and Reactive Properties. Environmental Progress , 358-336.
Yan, W. (2010). Structural Evolution of Pd-Doped Nanoscale Zero-Valent Iron {nZVI} in Aqueous Media and Implications for Particle Aging and Reactivity. Environmental Science and Technology , 4288-4294.
Zhang, W.-X. (2003). Nanoscale Iron Particles for Environmental Remediation: An Overview. Journal of Nanoparticle Research , 323-332.