Metal nanoparticles for environmental remediation

Signatures

Date:

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

Standard in text citation

Reference requirements

Uniform labeling of figures

and tables

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

Iron Bimetallics

Palladium Compounds

Magnesium Compounds

Zinc Compounds

Conclusion (Details

comparisons)

Additional Comments:

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

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