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Critical Reviews in Environmental Science and
Technology
ISSN: 1064-3389 (Print) 1547-6537 (Online) Journal homepage: http://www.tandfonline.com/loi/best20
A review on synthesis, characterization, and
applications of nano zero valent iron (nZVI) for
environmental remediation
Ritika Mukherjee, Rahul Kumar, Alok Sinha, Yangdup Lama & Amal Krishna
Saha
To cite this article: Ritika Mukherjee, Rahul Kumar, Alok Sinha, Yangdup Lama & Amal Krishna
Saha (2016) A review on synthesis, characterization, and applications of nano zero valent
iron (nZVI) for environmental remediation, Critical Reviews in Environmental Science and
Technology, 46:5, 443-466, DOI: 10.1080/10643389.2015.1103832
To link to this article: http://dx.doi.org/10.1080/10643389.2015.1103832
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Oct 2015.
Published online: 07 Oct 2015.
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A review on synthesis, characterization, and applications of
nano zero valent iron (nZVI) for environmental remediation
Ritika Mukherjee, Rahul Kumar, Alok Sinha, Yangdup Lama, and Amal Krishna Saha
Department of Environmental Science and Engineering, Indian School of Mines, Dhanbad, Jharkhand,
India
ABSTRACT
The recent spark in the interest for the usage of nano zero
valent iron (nZVI) as a remediation tool for contaminated land
and groundwater is mainly due to its higher reactivity in
comparison to micro ZVI, cost effectiveness, and potential to
treat a broad range of contaminants. The authors review the
recent developments and approaches made on synthesis on
nZVI, strucuture and characterization of nZVI, the challenges
faced in the transport of nZVI in the subsurface environment,
and the augmentation of the motility of nZVI. The effective use
of nZVI in remediating organic pollutants (halogenated organic
compounds, pharmaceutical waste, and azo dyes) and
inorganic pollutants (Ni
2C
,PO
4
3¡
,Co
2C
,Cu
2C
) carried out in
recent studies is discussed. The potential risks and limitations
of this emerging nanotechnology are also addressed.
KEYWORDS
Nano zero valent iron (nZVI);
remediation; groundwater
1. Introduction
The need for a sustainable and safe water supply is compelling developing coun-
tries to develop innovative and economical methods for purification and treatment
of water and wastewater. The multidisciplinary nano “boom” has led to the evolu-
tion of a wide array of novel technologies for both domestic and industrial applica-
tions, including improved drug delivery and new methods for the treatment of
contaminated water (Bystrzejewska-Piotrowska et al., 2009). Nanomaterials have
been shown to possess unique chemical, catalytic, electronic, magnetic, mechani-
cal, and optical properties owing to their small size (Jortner and Rao, 2002). As the
particle size decreases, proportion of atoms at the surface increases, raising its ten-
dency to adsorb, interact, and react with other atoms, molecules, and complexes to
achieve charge stabilization. The potential use of nanoparticles for the treatment of
contaminated groundwater has sparked a great deal of interest (Fu et al., 2014).
CONTACT Alok Sinha aloksinha11@yahoo.com Department of Environmental Science and Engineering,
Indian School of Mines, Dhanbad, Jharkhand, 826 004, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/best.
© 2016 Taylor & Francis Group, LLC
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY
2016, VOL. 46, NO. 5, 443–466
http://dx.doi.org/10.1080/106 43389.2015.1103832
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These particles can be used for in situ remediation of groundwater and can replace
costly technology such as permeable reactive barriers, currently in usage.
The key properties essential for the use of any engineered nanoparticle for in
situ remediation includes (a) high reactivity for contaminant removal, (b) suffi-
cient mobility within porous media, (c) sufficient reactive longevity, and (d) low
toxicity. However, it should be noted that the process involved is at a sensible cost
and competitive with other existing technology (Zhang, 2003; Crane and Scott,
2012). The use of nanometals, such as iron and bimetallic particles (O’Carroll,
2013), for subsurface remediation of halogenated organic compounds and heavy
metal contaminated sites has received significant attention in the past; however,
nanoscale zero valent iron is most commonly used. The adaptation of nano zero
valent iron (nZVI) provides several advantages compared to microscale ZVI,
which is used as reactive material in permeable reactive barriers, including (a) an
increase in the reductive degradation reaction rate, (b) a decrease of the reductant
dosage, (c) control over the risk of release of toxic intermediates, and (d) the gener-
ation of a nontoxic end product (Orth and Gillham, 1996; Wang and Zhang, 1997;
Liu et al., 2005; Li et al., 2006; Carroll et al., 2013).
Metallic or ZVI is a moderate reducing agent that can react with dissolved oxy-
gen and to some extent with water. This corrosion reaction can be accelerated or
inhibited depending on solution chemistry and this is put into productive use in
the treatment of hazardous and toxic materials as per the discovery of Gillham and
coworkers (Gillham and O’Hannesin, 1994; Orth and Gillham, 1996;O’Hannesin
and Gillham, 1998).
The practical applicability of ZVI lies in the fact that it can easily get oxidized to
C2 and C3 oxidation states and in the process reduce other organic as well as inor-
ganic impurities. Metallic iron easily acts as an electron donor:
Fe
0
! Fe
2 C
C 2e
¡
(1)
These electrons are in turn accepted by chlorinated hydrocarbons undergoing
reductive dechlorination (Vogel et al., 1987; Matheson and Tratnyek, 1994).
RCl C H
C
C 2e
¡
! RH C Cl
¡
(2)
From thermodynamic point of view coupling of reactions 1 and 2 are energeti-
cally favorable as the standard reduction potential (E
0
) of ZVI (Fe
2C
/Fe) is
–0.44 V, which is lower than many chlorinated organic and other hazardous metals
such as Pb, Cd, Ni, and Cr, thus are prone to reduction by ZVI (Li et al., 2006).
Iron has the ability to reduce and adsorb redox sensitive elements such as chro-
mium as well as dehalogenate the halogenated organic compounds as demon-
strated in laboratory as well as field studies at ambient temperatures (Liang et al.,
1996; Puls et al., 1999; Deng and Hu, 2001; Sinha and Bose, 2006; Kim et al., 2007;
Ludwig et al., 2007).
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Nanoscale iron particles are in a process to replace microiron particles and have
proven to be quite effective reductant and catalyst for a wide variety of common
environmental contaminants including chlorinated organic compounds and metal
ions (Lien and Zhang, 2001). Halogenated hydrocarbons such as chlorinated meth-
ane, ethane, ethylene, and benzene can be reduced to benign hydrocarbons by
nanoiron particles (Zhang, 2003) in comparison to the production of toxic daugh-
ter products such as vinyl chloride by microiron particles (Arnold and Roberts,
2000). A number of fi eld trials to measure the effectiveness of iron-based nanopar-
ticles have been developed (Elliott and Zhang, 2001; Zhang, 2003; Interstate Tech-
nology and Regulatory Council, 2005; Quinn et al., 2005). The nanoparticles are
injected as slurry into the subsurface environment, to remediate contaminated
groundwater plumes or source zones, avoiding trenching methods. The particles
are suspended in a hydrophobic fluid, as an emulsion, to prevent particle agglom-
eration and enhance reactivity and mobility. O’Hara et al. (2006) and Quinn et al.
(2005) reported substantial reductions of trichloroethylene (TCE) in soil (greater
than 80% reduction) and groundwater (60% to 100% reduction) during a field-
scale demonstration at Cape Canaveral Air Force Station (Florida), by injecting
emulsified nZVI particles. nZVI particles have also been used for the removal and
stabilization of metals or metalloid contaminants in groundwater such as arsenic
(Kanel et al., 2005, 2006) and removal of Cr(VI) from wastewater (Hu et al., 2004).
Here we review the synthesis, characterization, and applications of nZVI for reme-
diation of contaminated groundwater.
2. Synthesis of nZVI particles
Synthesis of iron nanoparticles can be done using two approaches: top-down and
bottom-up. In former approach large size materials are converted to nZVI with
the aid of mechanical and chemical processes such as milling, etching, and/or
machining (U.S. Environmental Protection Agency, 2010; Crane and Scott, 2012).
The latter approach is based on the “growth” of nanostructures atom-by-atom or
molecule-by-molecule via chemical synthesis, self-assembling, positional assem-
bling, and so on (Li et al., 2006). Three distinct synthetic schemes were utilized at
Lehigh University to prepare the nZVI. All involved the reduction and precipita-
tion of ZVI from aqueous iron salts using sodium borohydride as the reductant.
2.1 Synthesis of nZVI using ferric chloride (Type I nZVI)
In this synthesis, 0.25 M sodium borohydride is slowly added to 0.045 M ferric
chloride hexa-hydrate, in aqueous solution, in vigorously mixed conditions; such
that the volumes of both the borohydride and ferric salt solutions are approxi-
mately equal (i.e., 1:1 v/v). The mixing time generally kept is 1 hr. The resulting
nano-iron particles were then washed successively with a large excess of distilled
water, typically > 100 mL g
¡1
. The solid nanoparticle mass was recovered by vac-
uum filtration and washed with ethanol. If bimetallic particles were desired, the
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ethanol-wet nZVI mass is soaked in an ethanol solution containing approximately
1% palladium acetate. Figure 1a shows the SEM image of Type I nZVI particles
(Lien et al., 2006).
2.2 Synthesis of nZVI using ferrous sulfate (Type II and III nZVI)
This method was developed because of two concerns. First, ferric chloride salt is
highly acidic and hygroscopic, so safety was a major issue. Another concern was
the potential deleterious effects of excessive chloride levels from the nZVI matrix
in batch degradation tests where chlorinated hydrocarbons are the contaminant of
concern. Type II nZVI was prepared by pouring equal volumes of 0.50 M sodium
borohydride at 0.15 l min
¡1
into 0.28 M ferrous sulfate. The finished nanoparticles
were washed with distilled water (>100 mL g
¡1
), then by ethanol, purged with
nitrogen, and refrigerated in a sealed polyethylene container. The Type III nZVI
was also synthesized using the sulfate method. It was the latest generation of nZVI
with the average particle size of 50–70 nm. The moisture content of Type III nZVI
was lower (i.e., 20–30% in comparison to 40–60% for Types I and II; Lien et al.,
2006).
2.3 Synthesis using top-down approach
In recent years, Golder Associates Inc. has emerged as a market leader for large-
scale field deployment of nZVI. They produce nZVI in large quantities by top-
down approach by means of mechanical grinding of macroscale iron in planetary
ball mill systems. Though the method of production is very simple, the nanopar-
ticles so produced exhibit a very high surface energy and are thus prone to aggrega-
tion (Crane and Scott, 2012).
3. Structure of NZVI particles
The nZVI, produced by bottom-up approa ch, exhibit a typical core shell struc-
ture where the core consists of zero valent or metallic iron and a mixed valent
Figure 1. (a) SEM image of type I nZVI, (b) TEM of type II iron nanoparticles, and (c) TEM image of type
III iron nanoparticles (Lien et al., 2006). The scale bar represents 0.1 mm in (b) and (c) respectively.
© Sustainable Environment Research (SER). Reproduced by permission of Sustainable Environment
Research (SER). Permission to reuse must be obtained from the rightsholder.
446 R. MUKHERJEE ET AL.
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oxide shell of Fe(II) and Fe(III) is formed as a result of oxidation of the core
shell (Fig ure 2). Thus this ZVI is actually a manufactured material not found
naturally such as Fe(II) and Fe(III). nZVI is fairly reactive in water and pos-
sess excellent elec tron donating properties, which makes it a versatile remedia-
tion material (Stumm and Morgan, 1996). It is the core that provides the
reducing power (the electron source) for the reactions whereas the she ll pro-
vides the site for chemical complex reactions (chemisorptions ) and electro-
static interactions. The core shell structure has important implications on the
chemical proper ties of nZVI. The defective and disordered nature of the oxide
shell renders it potentially more reactive compared to a simple passive oxide
layer formed on bulk iron material (Wang et al., 2009). The shell exhibits less
contrast compared to the dense interior core. When the nanoparticles agglom-
erate, they have a continuous oxide shell but the m etallic c ores are separated
by a thinner interfacial oxide layer (»1 nm). The oxid e layer is amorphous
and disorder ed, owing to the e xtremely small radii of the nanoparticles, which
hinders crystalline formation. Moreover the presence o f boron as precursor
also contribute to defective sites and the oxide layer (Carpenter et al., 2003;
Ponder et al., 2001).Theoxidelayerhassemiconductorpropertiesaswell
(Balko and Tra tnyek, 1998;Wang,2009) and so charge transfer is relatively
facile due to small thickness and pre sence of defecti ve sites and allow re duc-
tion of contaminants to occur. Apart from reductive dehalogenation of organic
compounds and inorganic contaminants, nZVI is capable of b eing applied to a
broader spectrum of contaminants amenable to red uction, surfa ce sorp tion,
precipitation, or indeed combinations of these (Yan et al., 2010). According to
thecoreshellmodel,themixedvalenceoxide shell is large ly insoluble under
neutral pH and thereby protects the core from rapid oxidation. Composition
Figure 2. The core-shell model of nZVI and schematic representations of the reaction mechanisms
for the removal of Hg(II), Ni(II), Zn(II), and H
2
S (Yan et al., 2010).
© Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the
rightsholder.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 447
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of oxide shell depends on the fabrication process and environmental condi-
tions (Li et al., 2006).
4. Characterization of NZVI
Morphological analysis of the iron nanoparticles prepared with the method of fer-
ric iron reduction by sodium borohydride have been done with the help of trans-
mission electron microscopy (TEM) and scanning electron microscopy (SEM; Sun
et al., 2006; Lin et al., 2010)(Figure 3). Preparation of the sample is done by
depositing 2–3 drops of dilute ethanol solution of the sample onto a carbon film
supported on a 300-mesh copper grid. This was followed by placing the sample in
a vacuum hood till ethanol evaporated completely (Sun et al., 2006; Yan et al.,
2010). The results obtained from TEM analysis has shown that the nanoparticles
appear agglomerated forming chain-like formations due to magnetic and electro-
static interactions. It can be clearly inferred from the image that a single particle
comprises a dense core surrounded by a thin shell exhibiting markedly less con-
trast than the interior core (Yan et al., 2010).
Figure 3. Micrographs of (a) a single particle and (b–d) aggregates of iron particles (Sun et al., 2006).
© Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the
rightsholder.
448 R. MUKHERJEE ET AL.
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The particle size and size distribution of nZVI is measured with an Acoustic Spec-
trometer that utilizes the sound pulses transmitted through a particle suspension to
measure the properties of suspended particles (Dukhin a nd Goetz, 2002). The instru-
ment is connected to a four-necked conical flask containing a suspension of iron nano-
particles in deionized water. The advantage of working with this instrument is that it is
flexible with no limitation on opaque materials (Dukhin and Goetz, 2002;Morrison
and Ross, 2002). The specific surface area of the nanoparticles is determined with the
classic BET method (the Brunauer–Emmett–Teller isotherm). The BET isotherm is the
basis for determining the extent of nitrogen adsorption on a given surface. Sample prep-
aration is done by predrying at room temperature (22 § 1
C) in a vacuum desiccator,
and degassing at 90
Cfor1handthenat200
Cfor4h(Sunetal.,2006;Hwangetal.,
2011). X-ray diffraction is used to investigate the material structure of nanoparticles
(Sun et al., 2006). It can be inferred from the diffraction pattern that the particles pres-
ent were mainly in the zero valent state and all iron are in closed phase cubic structure
(Chatterjee et al., 2010). The surface composition for iron nanoparticle can be deter-
mined by X-ray photoelectron spectroscopy (XPS) up to a depth of less than 10 nm.
The iron nanoparticle samples are prepared by drying in a small nitrogen-purged hood
at room temperature and packed into the sample cell directly, to prevent oxidation of
samples. X-ray absorption near edge structure (XANES) uses synchrotron radiation to
photoionize the core electrons of Fe atom and provides useful information on the
valenceofiron(Sunetal.,2006). The surface charge of iron nanoparticles is often char-
acterized by the zeta (z) potential, which is defined as the electric potential at the surface
of shear relative to that in the distant bulk medium. It is the major factor determining
the mobility of particles in an electrical field (Sun et al., 2006).
A detailed knowledge of the surface properties is essential for understanding the
reaction mechanisms, kinetics, and intermediate/product profiles. The transport,
distribution, and fate of nanoparticles in the environment also depend on these
surface properties. The average particle size of the chloride method nZVI varies
between 50 and 200 nm whereas for sulfate method nZVI size varies between 50
and 70 nm (Lien et al., 2006). The average BET surface area of both type of nZVI
is in the range of 35 § 3m
2
/g (Lien et al., 2006).
5. Mobility and transport of nZVI in subsurface
The mobility of nanomaterials (NMs) in porous media is influenced by their ability
to attach to mineral surfaces to form aggregates. NMs that readily attach to the
mineral surfaces may be less mobile in ground water aquifers (Wiesner et al.,
2006); smaller NMs that can fit into the interlayer spaces between soil particles
may travel longer distances before becoming trapped in the soil matrix and soils
with high clay content tend to stabilize NMs and allow greater dispersal (U.S. Envi-
ronmental Protection Agency, 2008a). The surface chemistry and therefore the
mobility of NMs in porous media may be affected through the addition of surface
coatings. Although nZVI is widely used in site remediation, information is limited
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 449
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on its fate and transport in the environment. While increased mobility due to the
smaller size may allow for efficient remediation, there are insufficient data regard-
ing whether such NMs could migrate beyond the contaminated plume area and
persist in drinking water aquifers or surface water (U.S. Environmental Protection
Agency, 2008a). Issues affecting the fate and transport includes the contaminant
concentration, the nZVI synthesis process, particle agglomeration, age of the nZVI
particles, improper handling during application, particle density, the soil matrix,
ionic strength of the groundwater, hydraulic properties of the aquifer, depth to the
water table, presence of organic matter, and other geochemical properties, such as
pH, dissolved oxygen (DO), oxidation reduction potential, and concentrations of
competing oxidants (U.S. Environmental Protection Agency, 2008b). Aggregation
of bare nZVI particles decreases the surface area of the nZVI, which decreases the
mobility and reactivity, thereby limiting the radius of influence (He et al., 2007).
Several conditions causing the nZVI particles to agglomerate, includes the nZVI
particle concentration, the magnetism of the particles, size distribution, and zeta
(z) potential (Phenrat et al., 2007; Phenrat et al., 2009). Application of the nZVI
particles at too high of a concentration may increase the chances of particle
agglomeration (Saleh et al., 2007). Recent research shows that particles having con-
centrations of 30 mg/l or less are mobile regardless of the size distribution and
magnetic forces (Phenrat et al., 2009). Larger particles with higher ZVI content are
more magnetically attracted to each other and soil grains, which may also increase
the nZVI deposition potential. Smaller particles with low ZVI content travel far-
ther than those with high nZVI content because they are less likely to agglomerate
(Phenrat et al., 2009). Another condition that can cause agglomeration is the
z -potential of the particles. Particles with z-potential values greater than C30 mV
and less than –30 mV are considered stable, with maximum instability, or agglom-
eration, taking place at zero (Zhang and Elliott, 2006). Field-scale studies have
revealed 5–7-ft radius of influence of nZVI particles (ESTCP, 2010) and they can
travel up to a distance of 20 m in groundwater and can remain reactive for period
of 4–8 weeks (Lien et al., 2006).
5.1 Modification of nZVI
Magnetic attraction between nanoiron particles causes the rapid aggregation of
particles (Phenrat et al., 2007) so various organic coatings are used nowadays, such
as emulsions, polymers, and polyelectrolytes, that limit reactivity and increase the
mobility of nanoiron in the subsurface (Ponder et al., 2000). Bare and pure nZVI
are more prone to react with dissolved oxygen and oxygen-rich compounds so, to
prevent it from non target oxidation, it is essential to coat with surfactants or poly-
electrolytes (Krajangpan et al., 2008). Particles enhanced with certain polyelectro-
lytes have proven to be more mobile than bare nZVI even after aging. Particles
coated with certain polyelectrolytes (e.g., polyasarate, carboxymethyl cellulose
[CMC], polystyrene sulfonate) can remain mobile for at least eight months after
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the original injection depending on the hydrochemistry and geochemistry found at
a site (Kim et al., 2009). Selvarani and Prema (2012) used starch stabilized nano-
iron particles for Cr (VI) removal for better reactivity and mobility. The stabiliza-
tion of the nZVI particles can be enhanced by using various surface coatings. Some
of them are the following:
Hydrophilic biopolymers such as starch, guar gum, alginate, and aspartame
(He and Zhao, 2005; Tiraferri et al., 2008; Saleh et al., 2008; Tiraferri and
Sethi, 2009; Bezbaruah et al., 2009)
CMC (He et al., 2007)
Chitosan (Zhu et al., 2006; Geng et al., 2009)
Natural organic matter such as humic acid (Xie and Shang, 2005; Zhu et al.,
2008)
Polyelectrolytes such as polyacrylic acid, ion-exchange resins, and block
copolymers (Kanel and Choi, 2007; Zhao et al., 2008; Sirk et al., 2009)
Amphiphiles including various surfactants, which can be anionic, cationic, or
non-ionic (Hydutsky et al., 2007; Kanel et al., 2007; Zhu et al., 2008) and
block copolymers (Saleh et al., 2005; Saleh et al., 2007; Saleh et al., 2008)
Various oil-based microemulsions (Quinn et al., 2005)
Polyvinyl alcohol-co-vinyl acetate-co-itaconic acid, used during the synthesis of
the nZVI, leads to the formation of stable nZVI dispersion, which is stable for over
six months (Sun et al., 2007). Wu et al. (2005) used cellulose acetate to coat nZVI.
Effectiveness of various biodegradable dispersants was tested for nanoiron stability.
PAA (poly acrylic acid) was found to be best among various dispersants (Yang
et al., 2007). Allabaksh et al. (2010) synthesized stable nZVI using different chelat-
ing agents. They used ethylenediaminetetraacetic acid (EDTA), diethylenetriamine
pentacetic acid (DTPA), nitriloacetic acid (NTA), trans-1,2-diaminocyclohexane-
N,N,N,
0
N
0
-tetraacetic acid (CDTA), hydroxyethylenediaminetetraacetic acid
(HEDTA), triethylene tetraamine (TRTA), and N-cetyl-N,N,N-trimethyl ammo-
nium bromide (CTAB) chelating agents. The chelating effect was found to be the
best for EDTA, NTA and HEDTA, but the least for CDTA and CTAB. Another
study carried out by Krajangpan et al. (2008) was a landmark in the field of pro-
duction of stabilized iron nanoparticles. They synthesized a series of amphiphilic
polysiloxane graft copolymers (APGCs). Treatment of nZVI with APGCs was
assessed. The APGC possessing the highest concentration of carboxylic acid
anchoring groups provided the highest colloidal stability.
The mobility of nZVI is influenced by the chemistry of the aqueous phase (e.g.,
ionic strength, pH, natural organic matter [NOM]; Bian et al., 2011). Research
shows that the NOM enhances the mobility of nZVI. Even at lower concentration
of natural organic matter (2 mg/l), the mobility of nanoiron particles was higher
than no-NOM conditions. The sorption of natural organic matter on nZVI causes
reduction in sticking coefficient and hence accelerating the mobility (Johnson
et al., 2009). Also it has been found that bactericidal activity of nZVI toward gram-
negative Escherichia coli and gram-positive Bacillus subtilis can be mitigated in the
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 451
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presence of NOM (Chen et al., 2011). The pre-existence of NOM in transport
medium apparently has scavenging effect on attachment (i.e., sticking) of nZVI
aggregates to the immobile surface of transport medium (Giasuddin et al., 2007;
Zhang et al., 2011). Organic matter (humic acid [HA]) also has negative effects on
nZVI reactivity, as it introduces morphological changes to carbonate green rust
(Hwang and Shin, 2013). Dong and Lo (2013) investigated the influence of humic
acid (HA) on the surface-modified nZVI (SM-nZVI) and found HA to have pro-
found implication for nZVI stability. HA reacts with the SM-nZVI (PAA modified)
and complexes with SM-nZVI to enhance the electrostatic repulsion effect (i.e., it
increased the stability of the particles). On the other hand HA forms interparticle
bridging linkage with Tween-20 and starch modified nZVI, which induces agglom-
eration of nZVI. This suggests that the presence of NOM and HA in ground water
should be taken into consideration in choosing the proper SM-nZVI for particular
application (Dong & Lo, 2013).
6. Application of nZVI for remediation of organic compounds
Use of nZVI particles in advanced oxidation processes (AOPs) such as the Fenton
Process have shown an advantage over conventional method which requires
around 40–80 ppm of ferrous ion in the solution and this value is above the stand-
ards. In addition, the application of the homogeneous AOPs to large water flow
rate may produce large amount of sludge in the final neutralization step (Iurascu
et al., 2009). To avoid these disadvantages recently nZVI was used to degrade wide
range of organics at much lower operational cost (Pradhan and Gogate, 2010). In a
recent study the performance of nZVI in different AOPs such as F (Fenton),
electro Fenton (EF), and photo electro Fenton (PEF) processes on the degradation
of phenol was studied. The optimum dosage of nZVI and H
2
O
2
were found to be
0.5 g/l and 400 mg/l, respectively, and the complete removal of phenol was
observed within 30 min at these conditions at an initial pH and initial phenol con-
centration of 6.2 and 200 mg/l, respectively, and the removal process followed
pseudo first-order kinetics (Babuponnusami and Muthukumar, 2012). In an
another study the t8rinitroglycerin (TNG) C
3
H
5
(ONO
2
)
3
, a versatile chemical
widely used in the manufacture of dynamite, was degraded by using nZVI sup-
ported on the surface of an inert polymer such as nanostructured silica SBA-15
(Santa Barbara Amorphous No. 15). Both nZVI and nZVI/SBA-15 degraded TNG
(100%) in water to eventually produce glycerol and ammonium. The reaction fol-
lowed pseudo first-order kinetics and was faster with nZVI/SBA-15 (k
1
0.83
min
¡1
) than with ZVINs (k
1
0.228 min
¡1
; Saad, 2010).
6.1 Remediation of halogenated organic compounds
Chlorinated nitroaromatic compounds are widespread in the environment due to
the manufacturing and processing of a variety of industrial products, such as phar-
maceuticals, explosives, dyes, and pesticides (Lin et al., 2010; Liu et al., 2011) They
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are of interest to environmental scientists as they are known or suspected to be
human carcinogens, mutagens or toxins (Lin et al., 2010; Shen et al., 2008). nZVI
and palladium-doped nZVI immobilized on alginate bed were shown to degrade
TCE. The removal efficiency of TCE was > 99.8% when 50 g/l of Fe/Pd-alginate
(3.7 g Fe/l) was introduced to the aqueous solution. The removal of TCE by Fe/Pd-
alginate followed pseudo first-order kinetics and the major final products were
identified as ethane and butane. At low TCE concentration (4.4 mg/l) and excess
nZVI (»1.9 g/l), final reaction products were mainly composed of 80% of ethane
and »20% of C
3
–C
6
molecules. When limited amount of nZVI (»0.36 g/l) was
added to high TCE concentration (290 mg/l), even-numbered saturated alkanes
(C
2
,C
4
,C
6
) and other saturated and unsaturated hydrocarbons were detected
(Hojeong, 2010). Although vinyl chloride and cis-DCE were detected as intermedi-
ates, during reaction of nZVI with TCE, they were at a very low level and disap-
peared rapidly (Liu et al 2005). Following the model for TCE dechlorination by
nZVI presented by Liu et al. (2007) and a model for tetrachloroethylene (PCE)
dechlorination by surface modified nZVI (MRNIP2) presented by Kim (2009)it
was assumed that the reaction pathways could be described by reductive beta elim-
ination of PCE to acetylene followed by transformation of acetylene to ethene and
ethane. The use of nZVI to remediate dichlorophenyltrichloroethane (DDT) from
contaminated water has been studied by Poursaberi et al. (2012). Mixing an aque-
ous solution of 3 mg l
¡1
DDT with 30 mg l
¡1
Fe
0
resulted in 99.2% loss of DDT
within 4 hr. GC/MS analysis confirmed the formation of completely dechlorinated
hydrocarbon skeleton of DDT, namely diphenylethane (DPE), as the end product
in this treatment; thereby implying the removal of all five chlorine atoms (three
alkyl and two aryl) of DDT. It was concluded that Nanoscale Fe
0
was successful at
dechlorinating DDT, 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane (DDD) and 1-
chloro-2,2-bis (p-chlorophenyl)-ethane (DDMS). The use of nZVI with hydrogen
peroxide to treat pentachlorophenol contaminated soil offered new and effective
solution for treatment. By adding 5% of calcium carbonate for 40 h, the decay rate
of pentachlorophenol in Chengchun (Cf) series soil increased from 37% to 81%
and that in Pianchen (Pc) series increased from 41% to 75%. This treatment, add-
ing calcium carbonate in pentachlorophenol contaminated soil, was a new and rev-
olutionary method and served as the reference for on-site application (Liao et al.,
2007). Dehalogentaion of hexachlorobenzene (HCB) and polychlorinatedbiphenyl
(PCB) by nZVI showed sequential dehalogenation trend (Shih et al., 2011; Lowry
and Johnson, 2004). The end product of HCB reductive dehalogenation was 1,2-
DCB. In another study the reductive dechlorination of TCE sorbed in two model
soils (a potting soil and Smith Farm soil) using CMC-stabilized Fe-Pd bimetallic
nanoparticles was reported. The nanoparticles could effectively degrade TCE but
the soil organic matter (around 8.2 %) in the potting soil inhibited the degradation
kinetics. Four prototype surfactants were tested for their effects on TCE desorption
and degradation rates, including two anionic surfactants known as SDS (sodium
dodecyl sulfate) and SDBS (sodium dodecyl benzene sulfonate), a cationic
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 453
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surfactant hexadecyltrimethylammonium (HDTMA) bromide, and a nonionic sur-
factant Tween-80. All four surfactants were observed to enhance TCE desorption
at concentrations below or above the critical micelle concentration with the
anionic surfactant SDS being most effective (Zhang et al., 2011).
6.2 Remediation of pharmaceuticals
The interactions of tetracycline (TC) with nZVI modified by polyvinylpyrrolidone
(PVP-K30) was investigated using batch experiments as a function of reactant con-
centration, pH, temperature, and competitive anions. Degradation of TC was
strongly dependent on pH and temperature. The presence of silicate and phos-
phate strongly inhibited the removal of TC, whereas acetate and sulfate only
caused slight inhibition. LC–MS analysis of the treated solution showed that the
degradation products from TC resulted from the removal of functional groups
from the TC ring. The degradation products were detected both in the treated solu-
tion and on the surface of PVP-nZVI after 4-hr interaction, indicating that PVP-
nZVI could adsorb both TC and its degradation products (Chen et al., 2011). In
another study using ZVI, complete removal of amoxicillin (AMX) and ampicillin
(AMP) upon contact with Fe
0
and nFe
0
was obtained. The removal process was
attributed to three different mechanisms: (a) a rapid rupture of the b-lactam ring
(reduction), (b) an adsorption of AMX and AMP onto iron corrosion products,
and (c) sequestration of AMX and AMP in the matrix of precipitating iron hydrox-
ides (coprecipitation with iron corrosion products; Ghauch, 2009). Metronidazole
(MNZ) solution at 80 mg l
¡1
was rapidly removed by nZVI within 5 min, at initial
solution pH 5.60 and nZVI dose of 0.1 g l
¡1
. The removal process of MNZ fol-
lowed a pseudo-first order kinetics model. The removal rate in the nZVI/air pro-
cess was slightly higher than that in the nZVI/N
2
process emphasizing that oxygen
was a key factor for the removal of MNZ. MNZ removal efficiency by nZVI was
about 49 times higher than that by commercial iron powder under the same dos-
ages. The MNZ removal followed two processes: adsorption and degradation
(Fang et al., 2011). The pickling waste liquor discharged from steel industry was
utilized in preparation of nano zero valent metal (nZVM). nZVM was used to
degrade antibiotic metronidazole. The surface area–normalized rate coefficient
(k
SA
) for nZVM (0.254 l min
¡1
m
¡2
) was 375.2 times larger than that for commer-
cial iron powder (6.67£10¡4 l min
¡1
m
¡2
; Fang et al., 2010).
6.3 Remediation of azo dyes
Modern azo dyes are very resistant in natural environment; even their biological
degradation is not very easy. Zero valent iron is known to reduce the azo bond
and their products can be treated biologically. Fan et al. (2009) found that rapid
decolorization of azo dye methyl orange occurred by nZVI particles. Decoloriza-
tion was found to be very rapid, being completed in only 10 min. The products
were analyzed by GC-MS and they were sulfanilic acid, N,N-dimethyl-p-
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phenylenediamine and N-methyl-p-phenylenediamine. Satapanajaru et al. (2011)
studied decolorization of Reactive Black 5 (RB5) and Reactive Red 198 (RR198)
using nZVI technology. Lower pH, pitting corrosion and bimetallic system (add-
ing Pd and TiO
2
/light on the surface of n ZVI) enhanced the reduction efficiency.
The removal efficiency of both RB5 and RR198 was found to be greater using n
ZVI supported on sand, silica and biological sludge. Cation exchange resin sup-
ported nZVI was used by Shu et al. (2010) for decoloration of Acid Blue 113 azo
dye solution. The removal efficiency of AB 113 concentration and TOC was almost
100% and 12.6% within 30 min with 50.8 mg g
¡1
nZVI load, 20 gl
¡1
of resin load
and initial dye concentration of 100 mg l
¡1
. The color removal efficiency followed
modified pseudo first-order reaction. Lin et al. (2008) studied effective removal of
AB24 dye and factors affecting the rate of removal using nZVI. The reaction fol-
lowed pseudo first-order kinetics. They showed that at pH < 6, the removal of dye
was mainly due to reduction while at pH > 6 the adsorption phenomena domi-
nates. The orange (II) decolorization by n ZVI supported on bentonite was studied
by Xi et al. (2011). The Fe0
¡
clay material showed better reactivity toward the dye
than pure nZVI. In both the cases, the amount of dye reduced was comparable
that is, initial concentration of 96.5 mg l
¡1
of orange (II) reduced to 3 mg/l. Zhao
et al. (2008) found the rapid decolorization of water-soluble azo dye by nZVI
immobilized on cation exchange resin.
7. Remediation of inorganic contaminants
The adsorption of Ni
2C
by iron nanoparticles was studied by Nazlı (2008).
The n ZVI had shown high potential toward uptake of N i
2C
ion. Li and
Zhang (2006) also studied Ni (II) sequestration us ing iron nanoparticles. They
suggested that nZVI particles act as sorbent as well as reductant for Ni (II)
removal from water. In the reactor having 100 mg/l of Ni(II) and nZVI con-
centration of 5 g /l, the complete removal was achieved within 3 h. NZVI has
also been used in phosphate removal. Batch study conducted by Almeelbi and
Bezbaruah (2012)confirmed that when different concentrations of phosphates
(1,5,and10mgPO
4
3¡
-P/l), reacted with 400 mg nZVI/l, 88–95% phosphate
was removed in first 10 min, and 96–100% removal was ac hieved after
30 min. The maximum phosphate recovery of »78% was found to be at pH
12 within 30 min. The mechanism of phosphate remova l was determined to
be simultaneous adsorption and chemical precipitation (Wu et al., 2013;Wen
et al., 2014). The XRD patterns of the nZVI after the reactions indicated the
formation of crystalline vivianite (Fe
3
(PO
4
)
2
8H
2
O; Wu et al., 2 013). The
adsorption of PO
4
3¡
on nZVI followed both Langmuir and Freundlich model
and the maximum adsorption capacity was 245.65 mg/g (Wen et al., 2014).
Wu et al. (2013) and Wen et al . (2014) reported that the removal of phos-
phate decreased with an increasing pH due to the isoelectric point (IEP) of
nZVI, which contradicts the results by Almeelbi and Bezbaruah (2012). The
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 455
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phosphate removal efficiency increased with increasing dosage of nanoscale
iron particles, while it decreased with increasing initial phosphate concentra-
tion (Wu et al., 2013 ). The nZVI synthesized in a CMC–water solution
enhanced phosphate removal compared to nZVI synthesized in an ethanol-
water solution (Wu et al., 2013). The phosphate removal efficiency by nZVI
was considerably higher than micro ZVI (Almeelbi and Bezb aruah, 2012 ;Wu
et al., 2013). Hence, nZVI may be successfully used for remediation of eutro-
phic waters. Uzum et al. (2008) used nZVI particles with size 20–80 nm for
fast uptake of aqueous Co
2C
ions. This uptake wa s directly proportional to
increase in pH. Uzum et al. (2009) sho wed tha t kaolinite red uced th e aggre-
gartion of nZVI and showed higher uptake of Cu
2C
compared to Co
2C
.The
main mechanism for removal of Co
2C
was suggested to be adsorption and
precipitation while for Cu
2C
, it w as just a redox mecha nism. nZVI has also
been used as an adsorbent for removing cadmium from water ( Boparai et al.,
2011). The adsorption kinetic s followed pseudo second- order model and the
process can be better described by Langmuir isotherm compared to Freundlich
isotherm. nZVI are used for removal of both arsenic (III) and a rsenic (V)
from contaminated water (Kanel et al., 2005; Konstantina et al., 2007;Bang
et al., 2 005). 99.9% of arsenic (III) was found to be removed within 10 m in
by nZVI dosage of 1 g/l and pH 7. The efficiency of removal decreased with
increase in pH and arsenic concentration. In another study performed by Tan-
boonchuy et al. (2011) conclu ded that the favorab le cond ition for Arsenic (V)
removal can be acidic condition and high dissolved oxygen. Zhu et al. (2009)
studied removal of arsenate and arsenite by adsorption to nZVI supported on
activated carbon. The common m etal cation such as Ca
2C
,Mg
2C
was found to
increase arsenate adsorption and Fe
2C
lowering the arsenite adsorption. The
removal rate of both arsenate and arsenite was r eported to decrease in the
presence of phosphate and silicate while the effect of humic acid was almost
negligible. Morgada et al. (2009) studied the effect of UV light and humic
acid on arsenic (V) removal using nZVI. The presence of humic acid
decreased the removal rate by 50% while UV doubled the removal rate.
Though, humic acid improved the reaction rate in the presence of light.
Nitrate removal from water was studied by Kassaee et al. (2011) using iron
nanoparticles produced by arc discharge and reduction method. The nanopar-
ticles fabricated by arc discharge method are l arger and pure, have high er dis-
persity, and have higher efficiency in comparison with the nanoparticles
synthesized by reduction method. Low pH and high ionic strength of the solu-
tion increased nitrate removal. In another study performed by Hsu et al.
(2011) revealed that the best nitrate remova l by nZVI was achieved at around
neutral pH. At initia l pH of 10, the end product of reduction was mainly
dominated by ammonium ions but at initial pH of 6 and 8, other nitrogen
containing species are also formed besides ammonium and nitrite. Ziajahromi
et al. (2012) reported that the r eduction of nitrate b y nZVI inc reased with
456 R. MUKHERJEE ET AL.
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dosage of nZVI and the rate of reduction decreased with increase in pH. nZVI
supported by polystyrene resins reduce the particle size and hence increases
the reduction of nit rate by nZVI (Jiang et al., 2011 ). Reduction of nitrate by
nZVI prepared from hydr ogen reduction of natural goethite (nZVI-N), hydro-
thermal goethite (nZVI-H) and ordinary ZVI (OZVI) was studied by Liu
et al. (2012). The reactivity toward nitrate removal followed the trend of
nZVI-N> nZVI-H> OZVI. Ryu et al. (2011) studied the reduction of high
concentration of nitrate and concluded that aggregation and presence of cata-
lyst both affect the reduction rate but aggregation effect is more pronounced
than catalytic effect. Decreasing the precursor (FeCl
3
.6H
2
O) concentr ation,
during the manufacture of nZVI, reduced the particle size and enhanced the
reactivity of nZVI toward nitrat e r eduction (Liou et al., 2006). The nZVI-
mediated microbial reduction was found to b e more effective for nitrate
removal as nZVI acts as source of electron for biological nitrogen reduction
(Shin and Cha, 2008). Olegario et al. (2010) studied the reduction of soluble
selenium (VI) to insoluble selenium (-II) by nano iron sus pensions, at a reac-
tion r ate four times higher than Fe
0
powder. Uranium (VI) was shown to
reduce to U(IV) by iron nanoparticles, for 48 hrs, af ter which it reoxidized to
U(VI), as Fe(II) oxidized to Fe(III) (Dickinson and Scott, 2010). Treatment of
acid mine drainage (AMD) with nZVI lead to decrease in concentration of
various contaminants such as Al, U, V, Cr, Cu, Ni, Cd, Zn, and As) due to
precipitation of reduced cations and copre cipitation w ith ir on oxy-hydroxides
(Klimkova et al., 2011 ). Nanoparticles of iron are able to reduce dimethyl sul-
fide at neutral and acidic pH to produce methane and iron sulfide. The reac-
tion does not occur at basic pH (Calderon et al., 20 12). Removal of Pb(II)
from aqueous solution by using PAA stabilized iron nanoparticles (Esfahani
et al., 20 14), kaolin-supporte d nZVI (Zhang et al., 2010), and zeolite-nZVI
composite (Kim et al., 2013) was also studied. Singh et al. (2011) used v aried
doses of nZVI on Cr(VI) containing soil samples for remediation of Cr(VI).
The Cr (VI) removal efficiency increased with decreasing initial pH and the
removal efficiency was four ti mes higher compared to ir on powder (Nhung
and Thuong, 2008). nZVI–Fe
3
O
4
nanocomposites, prepared by an in situ
reduction method, were successfully used fo r chromium (VI) removal in aque-
ous environment (Lv et al., 2012). Bentonite-supported nZVI decreased nZVI
aggregation and thus increased Cr (VI) removal efficiency (Shi et al., 2 011).
CMC-stabilized iron nanoparticles (CMC-Fe
0
) resulted in 100% degradation of
Cr(VI) in comparison to 50% reduction pr oceeded with unstabilize d Fe
0
nano-
particles (Madhavi et al., 201 2).
8. Environmental impacts of nZVI
In addition to concerns regarding the reactivity and mobility of nZVI there is little
literature regarding the risks that this technology may pose on human and
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 457
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ecological health (Tratnyek and Johnson, 2006). Studies have demonstrated that
the toxicity effects of nZVI are limited compared to other nanoparticles (Reijnders,
2006). The ecological impacts of nanoparticles on the environment can be summa-
rized as described subsequently.
8.1 Toxic effect to mammalian nerve cells
Different forms of nZVI (i.e., fresh, aged, and surface modified) are differentially
toxic to the rodent nerve cells (Phenrat et al., 2009). Fresh nZVI produced mor-
phological evidence of mitochondrial swelling and apoptosis. The results revealed
that partial or complete oxidation of nZVI reduces its redox activity, agglomera-
tion, sedimentation rate, and toxicity to mammalian cells. Surface modification of
nZVI also reduces its toxicity. In the presence of a polymeric coating, toxicity
effects are very limited or even absent (Li et al., 2010). All forms of nZVI aggre-
gated in soil and water in presence of a high concentration of calcium ions and
thus addition of calcium salts may help in reducing the toxicity of groundwater
due to nZVI (Keller et al., 2012).
8.2 Toxic effect to aquatic life
The effects of nZVI on medaka fish (Oryzias latipes) and their embryos were inves-
tigated recently. A dose- and time-dependent decrease in superoxide dismutase
(SOD) and malondialdehyde (MDA) activities was observed in the embryos. A sig-
nificant decrease of SOD and glutathione (GSH) activity was observed in liver and
brain samples taken from the adults, but as the exposure time increased, the adults
appeared to recover from the exposure by adjusting the levels of antioxidant
enzymes (Li et al., 2009).
8.3 Toxic effect to microorganisms
nZVI is found to be capable of removing viruses (e.g., ; X174 and MS-2) from
water by inactivating them and/or irreversibly adsorbing the viruses to the iron
(You et al., 2005). A study done by Lee et al. (2008) says that nZVI particles exhib-
ited a bactericidal effect on Escherichia coli that was not observed with other types
of iron-based compounds, such as iron oxide nanoparticles, microscale ZVI, and
Fe
3C
ions. nZVI coated with humic acid showed minor toxicity to E. coli (Li et al.,
2010).
9. Conclusions
The insight into the different methods of synthesis of nZVI, its structure and char-
acterization and its transport phenomenon establishes its role in the environmental
remediation. Aggregation of nZVI has been reported to be the major drawback for
its application. The modification of nZVI in order to overcome the challenges
faced in the transport of nZVI through the soil has been advocated by many
458 R. MUKHERJEE ET AL.
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researches. With more research work focusing on developing proper techniques to
improve the nZVI motility the function of nZVI can be enhanced. Hence, the
application of nZVI as a remediation tool appears to be more promising than con-
ventional ZVI (microscale) or other in situ remediation methods. However, con-
tinued research effort toward modifying this technology is required to minimize
the unexpected adverse environmental impact or potential risks.
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