Synthesis, characterization and role of zero valent iron nanoparticle in removal of hexavalent chromium from chromium-spiked soil

1. 1 23
Journal of Nanoparticle Research
An Interdisciplinary Forum for
Nanoscale Science and Technology
ISSN 1388-0764
Volume 13
Number 9
J Nanopart Res (2011) 13:4063-4073
DOI 10.1007/s11051-011-0350-y
Synthesis, characterization and role of
zero-valent iron nanoparticle in removal
of hexavalent chromium from chromium-
spiked soil
Ritu Singh, Virendra Misra & Rana
Pratap Singh

2. 1 23
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3. RESEARCH PAPER
Synthesis, characterization and role of zero-valent iron
nanoparticle in removal of hexavalent chromium
from chromium-spiked soil
Ritu Singh • Virendra Misra • Rana Pratap Singh
Received: 14 October 2010 / Accepted: 16 March 2011 / Published online: 1 April 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Chromium is an important industrial
metal used in various products/processes. Remedia-
tion of Cr contaminated sites present both techno-
logical and economic challenges, as conventional
methods are often too expensive and difficult to
operate. In the present investigation, Zero-valent iron
(Fe0
) nanoparticles were synthesized, characterized,
and were tested for removal of Cr(VI) from the soil
spiked with Cr(VI). Fe0
nanoparticles were synthe-
sized by the reduction of ferric chloride with sodium
borohydride and were characterized by UV–Vis
(Ultra violet–Visible) and FTIR (Fourier transform
infrared) spectroscopy. The UV–Vis spectrum of Fe0
nanoparticles suspended in 0.8% Carboxymethyl
cellulose showed its absorption maxima at 235 nm.
The presence of one band at 3,421 cm-1
ascribed
to OH stretching vibration and the second at
1,641 cm-1
to OH bending vibration of surface-
adsorbed water indicates the formation of ferrioxyhy-
droxide (FeOOH) layer on Fe0
nanoparticles. The
mean crystalline dimension of Fe0
nanoparticles
calculated by XRD (X-ray diffraction) using Scherer
equation was 15.9 nm. Average size of Fe0
nanopar-
ticles calculated from TEM (Transmission electron
microscopy) images was found around 26 nm.
Dynamic Light Scattering (DLS) also showed
approximately the same size. Batch experiments
were performed using various concentration of Fe0
nanoparticles for reduction of soil spiked with
100 mg kg-1
Cr(VI). The reduction potential of Fe0
nanoparticles at a concentration of 0.27 g L-1
was
found to be 100% in 3 h. Reaction kinetics revealed a
pseudo-first order kinetics. Factors like pH, contact
time, stabilizer, and humic acid facilitates the reduc-
tion of Cr(VI).
Keywords Zero-valent iron nanoparticle Á
Characterization Á Remediation Á Contaminants Á
Reaction kinetics Á Humic acid Á Environment Á EHS
Introduction
Chromium compounds are used in various industries
(e.g., textile dying, tanneries, metallurgy, metal
electroplating, electronic, and wood preserving);
hence, large quantities of Cr have been discharged
into the environment due to improper disposal and
leakage (Kimbrough et al. 1999). Oxidation states of
R. Singh Á V. Misra (&)
Division of Ecotoxicology, Indian Institute of Toxicology
Research (Council of Scientific & Industrial Research),
Mahatma Gandhi Marg, Post Box 80, Lucknow 226 001,
UP, India
e-mail: virendra_misra2001@yahoo.co.in
R. Singh Á R. P. Singh
Department of Environmental Science, Babasaheb
Bhimrao Ambedkar University, Raebareli Road, Lucknow
226 025, UP, India
123
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DOI 10.1007/s11051-011-0350-y
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4. Cr range from -4 to ?6 (Cotton et al. 1999), but only
the ?3 and ?6 states are stable under most natural
environments. Cr(VI) is extremely mobile in the
environment and is toxic to humans, animals, plants,
and microorganisms (Cheryl and Susan 2000).
Because of its significant mobility in the subsurface
environment, the potential risk of ground water
contamination is high. Cr(III), on the other hand, is
less toxic, immobile, and readily precipitates as
Cr(OH)3 under alkaline or even slightly acidic
conditions (Puls et al. 1999). The compounds of
Cr(III) are reported to be 10–100 times less toxic than
those of Cr(VI) (Wei et al. 1993). According to its
toxicity, Cr was classified as a primary pollutant and
ranked as second among many toxic metals in the
environment for frequency of occurrence at Depart-
ment of Energy (DOE) sites (Sparks 1995).
Much research has focused on the remediation of
Cr(VI) and many treatment processes have been
developed. Physicochemical adsorption has been
researched for a longer time but the cost is high
and the Cr(VI) is just transferred instead of being
reduced (Bowman 2003). Bioremediation by the
strains of bacteria can effectively degrade Cr(VI)
and is economically favorable, but the presence of
bactericidal toxicants at many waste sites would
limit their growth and effectiveness (Chen and Hao
1998). Chemical reduction is known to remove
Cr(VI) rapidly and effectively using reducing agent
such as ferrous sulfate, sulfur dioxide, or sodium
bisulfate followed by precipitation as Cr(III) (Guha
and Bhargava 2005). One of the disadvantage of this
method is that they are expensive and complicated.
In addition, the removal of low levels of Cr is
limited.
Fe0
nanoparticles have long been used in the
electronic and chemical industries due to their
magnetic and catalytic properties. Now a days, use
of Fe0
nanoparticle is becoming an increasingly
popular method for treatment of hazardous and toxic
wastes and for remediation of contaminated soil and
ground water (Li et al. 2006; Li and Zhang 2006;
Lien et al. 2006). The large surface area of Fe0
nanoparticles further fosters-enhanced reactivity for
the transformation of the recalcitrant environmental
pollutants. Fe0
nanoparticle is a strong reducer and
it has been used to rapidly dehalogenate and degrade
a wide range of halogenated organic compounds
(Elliott et al. 2009; Hou et al. 2009; Tee et al. 2009;
Shih and Tai 2010; Wang et al. 2010; Singh et al.
2011), reduce nitro aromatic compounds (Agrawal
and Tratnyek 1996), degrade dye solutions (Cao
et al. 1999), and remove heavy metals (Buerge and
Hug 1999; Puls et al. 1999; Ponder et al. 2000;
Biterna et al. 2010). It is generally accepted that
nano Fe0
has a core–shell structure with a Fe0
core
surrounded by an oxide/hydroxide shell, which
grows thicker with the progress of iron oxidation
(Li and Zhang 2006, 2007). Martin et al. (2008)
determined the oxide layer thickness in core–shell
Fe0
nanoparticles by using high resolution transmis-
sion electron microscopy (HR-TEM) and high
resolution X-ray photoelectron spectroscopy (HR-
XPS) and the values were in the range of (2–4 nm)
and (2.3–2.8 nm) respectively. Recent innovations in
nanoparticle synthesis and production have resulted
in substantial cost reductions and increased avail-
ability of the Fe0
nanoparticles for larger scale field
applications (Xiao et al. 2009).
Materials and methods
Chemicals and solutions
Ferric chloride anhydrous (FeCl3), sodium borohy-
dride (NaBH4), and potassium dichromate (K2Cr2O7)
were obtained from CDH, India. 1,5-diphenylcarbaz-
ide (C13H14N4O) and Carboxymethyl cellulose
(CMC) were procured from S.D. Fine Chemicals
Ltd. India. Ethanol (C2H5OH) from Loba Chemie
Pvt. Ltd. India and acetone (CH3COCH3) was
purchased from Merck, India. Humic acid was
obtained from Aldrich Chemical Company, India.
All chemicals used were analytical reagent grade.
Collection of soil samples
Total 15 soil samples were collected from the Gheru
Campus of Indian Institute of Toxicology Research,
Lucknow following US EPA Standard Operating
Procedures for Soil sampling (US EPA 2000) in the
month of January 2010. After collection, all the
samples were air dried, passed through a 2 mm sieve,
packed in plastic bags and then stored in dark at 4 °C.
All the samples were free from chromium.
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5. Synthesis of Fe0
nanoparticles
Fe0
nanoparticles were synthesized by the reduction
of ferric chloride with sodium borohydride using the
method of Sun et al. (2006). In this, 1:1 volume ratio
of NaBH4 (0.2 M) and FeCl3 solution (0.05 M) were
vigorously mixed in a flask reactor for 30 min.
Excessive borohydride (0.2 M) was applied to accel-
erate the synthesis. A black precipitate of Fe0
was
obtained after addition of NaBH4 as per the following
Eq. 1 which is separated and stored under ethanol.
The reaction was carried out in an inert atmosphere.
For the preparation of fully dispersed and stabilized
Fe0
nanoparticles, 0.8% CMC was added to Fe0
nanoparticles and sonicated for homogenization.
4 Fe3þ
þ 3 BHÀ
4 þ 9H2O
! 4Fe0
þ 3H2BOÀ
3 þ 12 Hþ
þ 6H2 ð1Þ
Characterization of Fe0
nanoparticle
Fe0
nanoparticles prepared by the above method was
characterized with the help of multiple techniques
such as X-Ray Diffraction (XRD), Transmission
Electron Microscopy (TEM), Dynamic Light Scatter-
ing (DLS), UV–Vis Spectroscopy, and Fourier
Transform Infrared Spectroscopy (FTIR).
XRD analysis
XRD analysis of Fe0
nanoparticles was done with
(X-Pert Pro XRD system from PANalytical, Alme-
do, Netherlands) at 45 kV and 40 mA. It uses Cu Ka
radiation and graphite monochromator to produce
X-rays with wavelength of 1.54 A˚ . Fe0
nanoparticles
were placed in glass holder and scanned from 20° to
100°. Scanning rate was 2.5 min-1
. A broad peak
in XRD reveals existence of amorphous phase of
iron. The peak at 2h of 44.751° indicates the
presence of Fe0
nanoparticles. Particle size can
be presumed with the XRD by using Scherer
equation (2).
D ¼
0:9k
bcosh
ð2Þ
where D is the particle size in A˚ , k the wavelength of
Cu Ka radiation, i.e., 1.54 A˚ , b the full width at half
maximum (FWHM), and h is the angle obtained from
2h corresponding to maximum peak intensity. The
mean crystalline dimension of the Fe0
nanoparticle
was found to be 15.9 nm.
TEM
TEM images of Fe0
nanoparticles were recorded with
a HR-TEM (TEM, Tecnai 20 G-2) operated at
200 kV. Samples were prepared by depositing a
few droplets of dilute Fe0
nanoparticles solution on to
a carbon film. The average size was found to be
26.4 nm with standard deviation of 16.9 nm.
DLS
The mean hydrodynamic diameter of Fe0
nanoparti-
cles were determined by Nano Zetasizer ZS90 (ZEN
3690, Malvern Instruments, UK) using DLS (Internal
He–Ne laser, wavelength 633 nm, 25 °C). All the
tests were performed at a measurement angle of 90°
in duplicates. The refractive index of iron was set at
2.87 (Fatisson et al. 2010). All the samples were
sonicated for 30 min before analysis. The mean
hydrodynamic diameter was around 28.4 nm.
UV–Vis spectroscopy
The UV–Vis spectrum of Fe0
nanoparticles sus-
pended in 0.8% CMC was recorded using Thermo
spectronic GENESYS 10 UV scanning UV–Vis
spectrophotometer. The Fe0
nanoparticle showed its
absorption maxima at 235 nm.
FTIR spectroscopy
The FTIR spectra of the CMC stabilized Fe0
nano-
particles were recorded in the transmission mode at
room temperature using KBr pellet technique (1:20).
The KBr was dried in a dryer at 200 °C for 24 h, then
560 mg KBr was homogenized with sample and
ground afterward to fine powder with a mortar and
pestle. Shimadzu (Japan) infrared spectrophotometer
was used to determine the spectra of the sample
which was mixed with spectrally pure KBr and
pressed to form thin plates (radius 1 cm, thickness
0.1 cm), then were subjected to IR spectroscopic
analysis in the spectral range 500 and 4,000 cm-1
.
The band at 3,421 cm-1
was ascribed to OH stretch-
ing vibration and the one at 1,641 cm-1
to the OH
bending vibration of surface-adsorbed water.
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6. Calibration
Cr(VI) stock solutions (1,000 mg L-1
) were prepared
by dissolving 2.829 g of AR grade K2Cr2O7 in
1,000 mL of distilled deionized water; standard solu-
tions of desired concentration range (0.1–0.5 mg L-1
)
were prepared by diluting the stock solution.
Preparation of the spiked soil sample
100 lL of stock solution was taken by pipette and
spiked on to 1.0 g soil *100 mg kg-1
. The soil was
dried overnight, mixed properly, and then used for the
experiments.
Colorimetric method for analysis of Cr(VI)
(Method 7196 A-USEPA)
Cr(VI) was determined by oxidizing 1,5-diphenylc-
arbazide to 1,5 diphenylcarbazone which formed a
violet red colored complex with Cr(III). 1,5-diphenyl
carbazide solution was prepared by dissolving
250 mg diphenylcarbazide in 50 mL acetone.
1.0 mL of the extract to be tested was transferred to
10.0 mL volumetric flask. 200 lL of diphenylcar-
bazide solution was added to it and mixed properly.
Then five drops of 1 N HNO3 was added to maintain
its pH = 2 ± 0.5. The volume was made up to
10 mL with distilled water and allowed to stand for
5–10 min for full color development. For quantifica-
tion, an appropriate portion of it was transferred to
cuvette and the absorbance at 540 nm was measured
on spectrophotometer. Cr(VI) was analyzed in four
system, i.e., blank solution, stock solution, unspiked
soil, and spiked soil. All the systems were run in
parallel, in triplicates under similar experimental
conditions. Results are shown in Table 1.
Batch experiments
Batch experiments were carried out in the laboratory
to evaluate the efficacy of Fe0
nanoparticles for the
reduction of Cr(VI)-spiked soil. All individual exper-
iments were conducted in 15 mL glass vials. The
reaction was initiated by adding 10 mL of Fe0
nanoparticle suspension to 1 g of Cr(VI)-spiked soil
[Cr(VI) initial concentration = 100 mg kg-1
]. The
reaction mixture was allowed to react for 3 h with
continuous shaking. Then the solution was transferred
to centrifuge tubes and centrifuged at 5,000 rpm for
15 min. The supernatant obtained was filtered using
0.22 lm syringe driven micropore filter. Cr(VI) in the
filtrate was analyzed following diphenyl carbazide
complexation procedure 7196A of USEPA. No
change in pH was observed during the experiments.
Control tests with spiked soil were also carried out in
the absence of nanoparticles but otherwise under
identical conditions. All the experiments were per-
formed in triplicates. Effect of concentration, time,
stabilizer, humic acid, and pH on Cr(VI) reduction by
Fe0
nanoparticle was studied as follows.
Effect of Fe0
concentration
Experiments were carried out to determine concentra-
tion at which maximum reduction of Cr(VI) occurs.
Various concentration ranging from 0.01 to 0.30 g L-1
of Fe0
nanoparticles were added to Cr(VI)-spiked soil
[Initial Cr(VI) conc. = 100 mg kg-1
] and allowed to
react for 3 h. After that the reaction mixture was
extracted and analyzed for Cr(VI).
Effect of stabilized Fe0
The effectiveness of CMC (0.8%) stabilized Fe0
nanoparticles (0.01–0.27 g L-1
) for reduction of
Cr(VI)-spiked soil in comparison to non-stabilized
Fe0
nanoparticles were tested in a series of batch
experiments.
Effect of contact time
Reduction efficacy of Cr(VI) by Fe0
nanoparticles
(0.27 g L-1
) were determined at various time inter-
vals (0, 30, 60, 90, 120, 150, and 180 min). At
different time intervals, reaction mixture were cen-
trifuged and filtered. The filtrate was analyzed for
Table 1 Analysis of Cr(VI) in soil samples (spiked and un-
spiked), blank, and stock solution
System Cr(VI) conc. (mg kg-1
)
Blank solution Not detected
Stock solution 99.9
Unspiked soil Not detected
Spiked soil 97 ± 2
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7. Cr(VI). Percent reduction was also studied in the
same experiment.
Effect of humic acid on removal efficiency
of Cr(VI) by Fe0
nanoparticles
To investigate the effect of humic acid on removal
efficiency of Cr(VI) by Fe0
nanoparticles, various
concentrations (0–100 mg kg-1
) of humic acid was
spiked in Cr(VI) loaded soil samples. These samples
were treated with 0.27 g L-1
Fe0
nanoparticles. At
predetermined time intervals (0, 30, 60, 90, 120, 150,
and 180 min) reaction mixture was centrifuged,
filtered, and analyzed for Cr(VI).
Effect of pH
To study the effect of pH on Cr(VI) reduction, the
Cr(VI)-spiked soil were treated with Fe0
nanoparti-
cles (0.27 g L-1
) suspension at an initial pH of 4, 6,
7, 9, and 11 (adjusted with 1 N HCl or 0.1 N NaOH)
and at a soil to solution ratio of 1 g:10 mL. Reaction
mixture was allowed to react for 2 h. Upon centri-
fuging for 15 min, supernatants were sampled and
analyzed for Cr(VI).
Removal rate of Cr(VI) present in soil
via reductive process using Fe0
nanoparticles:
a kinetic study
Kinetic studies concerning models for remediation of
soil containing Cr(VI) may be carried out via redox
and/or leaching processes. Reaction kinetics of
Cr(VI) (100 mg kg-1
) with Fe0
nanoparticles
(0.27 g L-1
) was studied in triplicates at pH 7.1.
The kinetic model of Cr(VI) reduction by Fe0
nanoparticles can be described using the pseudo-first
order kinetic equation (Franco et al. 2009).
ln Cr VIð Þ½ Š= Cr VIð Þ½ Š0¼ À kobst ð3Þ
where [Cr(VI)] and [Cr(VI)]0 are the instantaneous and
initial concentration of Cr(VI) in mg kg-1
, respec-
tively, ‘t’ is the remediation time (min), and ‘kobs’ is the
kinetic rate constant representing the over all removal
rate for remediation (min-1
). Analysis of the kinetic
data reveals that the overall removal rate of Cr(VI)
from soil follows a pseudo-first order kinetic model
with standard deviation ranging from 0.44 to 1.9.
Results and discussion
The UV–Vis spectrum of Fe0
nanoparticles in 0.8%
CMC is shown in (Fig. 1). The Fe0
nanoparticles
showed its absorption maxima at 235 nm. CMC
alone did not show any peak. This observation is
similar to that obtained by Morgada et al. (2009).
FTIR techniques provide information about vibra-
tional state of adsorbed molecule and hence the
nature of surface complexes. The band at 3,421 cm-1
was ascribed to OH stretching vibration and the one
at 1,641 cm-1
to the OH bending vibration of
surface-adsorbed water (Fig. 2) which suggests the
formation of ferrioxyhydroxide (FeOOH) layer on
Fe0
nanoparticles. The XRD analysis of Fe0
nano-
particles is shown in Fig. 3. The peak at 2h of
44.751° indicates the presence of Fe0
nanoparticles.
The mean crystalline dimension of the Fe0
nanopar-
ticle was found to be 15.9 nm when calculated by
Scherer equation. TEM image of Fe0
nanoparticles
(Fig. 4a) showed that the nanoparticles are mostly
spherical in shape forming chain like aggregates.
After the examination of more than 200 nanoparti-
cles, a particle size distribution was calculated, which
indicates that 90% of the particles were within the
range of 50 nm, although few large aggregates with
diameter around 100 nm were also observed
(Fig. 4b). The average size found was 26.4 nm and
standard deviation 16.9 nm. Particles size was further
determined by Zetasizer using DLS. The mean
hydrodynamic diameter was found to be 28.4 nm
(Fig. 5), approximately the same size which was
determined with TEM.
Various concentration of Fe0
nanoparticles ranging
from 0.01 to 0.30 g L-1
were tested for its reduction
Fig. 1 UV–Vis absorption spectra of Fe0
nanoparticles stabi-
lized with CMC (0.8%)
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8. potential on the soil spiked with 100 mg kg-1
Cr(VI). Results showed rapid increase in the reduc-
tion of Cr(VI) with the increase in the concentration
of Fe0
nanoparticles. For the complete reduction of
100 mg kg-1
Cr(VI)-spiked soil, 0.27 g L-1
of
non-stabilized Fe0
nanoparticle is required in 3 h.
However, in case of CMC (0.8%) stabilized Fe0
nanoparticle only 0.09 g L-1
is needed for complete
reduction under similar condition showing greater
reactivity than non-stabilized Fe0
nanoparticle
(Fig. 6). Reduction efficacy of Cr(VI) by Fe0
nano-
particle (0.27 g L-1
) showed increase in reaction rate
with increase in contact time. Effect of humic acid on
removal efficiency of Cr(VI) by Fe0
nanoparticle is
illustrated in Fig. 7. The data showed that 100%
Cr(VI) reduction was achieved by 0.27 g L-1
Fe0
nanoparticle in 3 h in absence of humic acid. In
contrast, in the presence of 80 mg kg-1
humic acid,
same results were obtained in 1 h. This indicates that
humic acid with Fe0
nanoparticle assist the reduction
of Cr(VI). Leachability of Cr(VI) from spiked soil at
different pH for 2 h in the presence and absence of
Fe0
nanoparticles showed that leachability of Cr(VI)
increases with increase in pH. 85 to 96% leachability
Fig. 2 FTIR of stabilized
Fe0
nanoparticles
Fig. 3 XRD pattern of Fe0
nanoparticles
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9. was noticed between pH 4 and 11 in 2 h. However,
when Fe0
nanoparticles (0.27 g L-1
) are present, total
desorbed Cr(VI) was reduced to 10% over the pH range
of 4–11 indicating reduction of Cr(VI). Reaction
kinetics of Cr(VI) with Fe0
nanoparticle showed
pseudo-first order reaction. The average values of
‘kobs’ was found to be 5.25 9 10-3
min-1
(Fig. 8).
These observations indicate that the removal of Cr
by Fe0
nanoparticle is based on the transformation of
Cr(VI) to Cr(III). These observations are in agree-
ment with the observations of Powell et al. (1995)
and Powell and Puls (1997), who had given a
thorough evaluation of the Cr(VI) removal by Fe0
in systems of natural aquifer materials with varying
geochemistry and suggested that the mechanism of
Cr(VI) reduction by Fe0
is a cyclic, multiple reaction
electrochemical corrosion mechanism. Several work-
ers have investigated the feasibility of using a new
class of stabilized Fe0
nanoparticle for in situ
reductive immobilization of Cr(VI) in water and in
a sandy loam soil (Xu and Zhao 2007; Wu et al.
2009). Some researchers (Pratt et al. 1997; Eary and
Rai 1998; Singh and Singh 2003) believe that in Fe0
-
treatment systems, the removal mechanism of Cr(VI)
involve instantaneous adsorption of Cr(VI) on Fe0
surface where electron transfer takes place and
Cr(VI) is reduced to Cr(III) with oxidation of Fe0
to
Fe(III), and subsequently, Cr(III) precipitates as
Cr(III) hydroxides and/or mixed Fe(III)/Cr(III)(Oxy)
hydroxides as per the following equations:
3Fe0
þ Cr2OÀ
7 þ 7H2O ! 3Fe2þ
þ 2Cr OHð Þ3 þ 8OHÀ
ð4Þ
1Àxð ÞFe3þ
aqð Þ þ xð ÞCr3þ
aqð Þ þ 3H2O
! CrxFe1Àx OHð Þ3 sð Þ þ 3Hþ
aqð Þ ð5Þ
1Àxð ÞFe3þ
aqð Þ þ xð ÞCr3þ
aqð Þ þ 2H2O
! CrxFe1Àx OOHð ÞðsÞ þ 3Hþ
aqð Þ ð6Þ
where x vary from 0 to 1. The solubility of CrxFe1-
x(OH)3 is lower than that of Cr(OH)3. Alternatively,
Cr(III) may also precipitate in the form of CrxFe1-
x(OOH) (Cao and Zhang 2006).
Fe0
nanoparticles exhibit characteristics of both
iron oxy-hydroxides (i.e., as a sorbent) and metallic
iron (i.e., as reductant). For Cr(VI) removal, Fe0
Fig. 4 a TEM Image of Fe0
nanoparticles (Scale: 100 nm).
b Particle size distribution of Fe0
nanoparticles using TEM
Fig. 5 Particle size analysis of Fe0
nanoparticles by DLS
Fig. 6 Cr(VI) removal by stabilized and non-stabilized Fe0
nanoparticles; Initial Cr(VI) conc. = 100 mg kg-1
; Time = 3 h;
pH = 7.1
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10. nanoparticles mainly act as a reductant. The reduced
Cr(III) can be incorporated into the iron oxy-hydroxide
shell forming (CrxFe1-x)(OH)3 or CrxFe1-xOOH at the
surface. At high initial Cr concentration, this structure
may serve as a passive layer at the surface and hinder
further reduction of Cr(VI).
Li and Zhang (2007) explained the reduction of
metal cations on the basis of the standard oxidation–
reduction potential of the metal ions. The standard
electrode potential of Fe0
, Cr(VI), and Cr(III) is
(-0.41 V), (1.36 V), and (-0.74 V), respectively.
For metals with E0
far more positive than Fe0
nanoparticle, as in case of Cr(VI), the removal
mechanism is predominantly reduction and precipi-
tation. Reduction and precipitation of metal ions by
Fe0
nanoparticles depends on transport of the dis-
solved metal ions to the surface and electron transfer
(ET) to the metal ion. Potential ET pathways from the
surface to the sorbed ions/molecules may include:
(i) Direct Electron transfer (DET) from Fe0
through
defects such as pits or pinholes, where the oxide
layer is interpreted as a simple physical barrier.
(ii) Indirect Electron transfer (IET) from Fe0
through the oxide layer via the oxide conduc-
tion band, impurity bands, or localized bands.
After reduction, Cr(III) may be removed through the
precipitation or co-precipitation in terms of (CrxFe1-x)
(OH)3 or CrxFe1-xOOH as its E0
is (-0.74) which is
slightly more than E0
of Fe0
nanoparticle.
The natural organic matter (NOM) such as humic
acid plays an important role in Cr(VI) reduction by
virtue of functional groups such as quinones (Trat-
nyek et al. 2001). Humic acid is well known in
having a high binding affinity toward Fe2?
and Fe3?
(Tipping 2002), as well as having a strong tendency
to be adsorbed on to iron oxides surfaces (Gu et al.
1994). Our observation that humic acid with Fe0
nanoparticle assist the reduction of Cr(VI) can be
explained on the basis of adsorption phenomena. The
adsorption of humic acid inhibits iron corrosion,
thereby prolonging the lifetime of nano Fe0
. On the
other hand adsorbed humic acid can transfer electron
from inner Fe0
to Fe(III) to reduce Cr(VI) in solution.
The Fe(III) humic acid complexes formed on the
outer oxide layer or in solution can regenerate
reactive Fe(II) to reduce Cr(VI). This is supported
by the recent studies which have shown that in
addition to direct sorption on Fe0
surfaces, humic
Fig. 7 Effect of humic acid
(HA) and Fe0
nanoparticle
on Cr(VI) reduction;
Initial Cr(VI)
conc. = 100 mg kg-1
; Fe0
nanoparticle
conc. = 0.27 g L-1
;
Time = 3 h; pH = 7.1
(error bars represent the
standard deviations of
triplicates)
Fig. 8 Reaction kinetics of Cr(VI); Initial Cr(VI)
conc. = 100 mg kg-1
; Fe0
nanoparticle conc. = 0.27 g L-1
;
pH = 7.1
4070 J Nanopart Res (2011) 13:4063–4073
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11. acid complexation with dissolved iron released from
corrosion results in the formation of colloids and
aggregates in solution which may affect contaminant
removal (Tsang et al. 2009).
Besides NOM, minerals are also reported to influ-
ence the performance of Fe0
nanoparticles. Several
authors have reported chromate adsorption on mineral
surfaces such as ferric oxide, aluminum oxide, hema-
tite, goethite, magnetite, siderite, etc. (Eary and Rai
1989; Ilton and Veblen 1994; Patterson et al. 1997).
Buerge and Hug (1999) observed that the mineral
surfaces strongly affect the reduction rate of Cr(VI) by
regulating Fe(II) and Cr(VI) speciation. They further
observed that Fe(II) bearing minerals accelerate Cr(VI)
reduction in the following order a-FeOOH & c-FeO-
OH ) montmorillonite[kaolinite & SiO2 ) Al2O3.
The variation in the kinetics of Cr(VI) reduction by
Fe(II) bearing minerals mainly depends on their nature,
amount of Fe(II) adsorbed on the mineral surface and
the pH of the solution. Most of the researchers are of the
view that chromium sorption by hydrous iron oxides
takes place through adsorption, precipitation, and
coprecipitation via inner sphere surface complexation
(Charlet and Manceau 1992; Crawford et al. 1993).
However, there are a group of workers (Davis and
Leckie 1979; Zachara et al. 1987) who believes that
chromate adsorption on iron oxyhydroxide and kao-
linite occurs by outer sphere complexation. On the
basis of above studies it may be concluded that the soil
used in the present study which has high content of
minerals like quartz, mica, and ferromagnesium sili-
cates, etc. may also assist Fe0
nanoparticles in Cr(VI)
reduction. Further studies in this direction will help to
elucidate the detailed mechanism of the role of
minerals in Fe0
nanoparticles-mediated reactions.
Along with the toxicity of Fe0
nanoparticles, an
area of concern is the toxic implications of boron/
boron oxide nanoparticles (Liu et al. 2009; Strigul
et al. 2009) which may form during the synthesis of
Fe0
nanoparticles as by product. These studies
suggest that the toxicological aspects should also be
taken into consideration while doing environmental
remediation studies with Fe0
nanoparticles.
Conclusions
The study reveals that Fe0
nanoparticles play a key role
in Cr(VI) removal through reduction/immobilization
and also in reducing the toxicity due to Cr. Addition of
humic acid to Fe0
nanoparticles and factors like
concentration, pH and time of treatment facilitates
the process. The study further suggests that the
stabilized Fe0
nanoparticles may be used for in situ
reductive immobilization of Cr(VI) contaminated soils
or other Cr(VI)-laden solid wastes, which may lead to
an innovative remediation technology that is likely
more cost effective and less environmentally
disruptive.
Acknowledgments Thanks are due to the Director, Indian
Institute of Toxicology Research, Lucknow, for his keen
interest in the preparation of this manuscript. The financial
support provided by CSIR Network Project (NWP-17) and
Uttar Pradesh Council of Science and Technology is also
acknowledged. This is IITR publication No. 2918.
References
Agrawal A, Tratnyek PG (1996) Reduction of nitro aromatic
compounds by zero-valent iron metal. Environ Sci Tech-
nol 30:153–160
Biterna M, Antonoglou L, Lazou E, Voutsa D (2010) Arsenite
removal from waters by zero-valent iron: batch and col-
umn tests. Chemosphere 78:7–12
Bowman RS (2003) Applications of surfactant-modified zeo-
lites to environmental remediation. Microporous Meso-
porous Mater 61:43–56
Buerge IJ, Hug SJ (1999) Influence of mineral surfaces on
chromium(VI) reduction by iron(II). Environ Sci Technol
33:4285–4291
Cao J, Zhang WX (2006) Stabilization of chromium ore pro-
cessing residue (COPR) with nanoscale iron particles.
J Hazard Mater 132:213–219
Cao J, Wei L, Huang Q, Wang L, Han S (1999) Reducing
degradation of azo dye by zero-valent iron in aqueous
solution. Chemosphere 38:565–571
Charlet L, Manceau AA (1992) X-Ray absorption spectro-
scopic study of the sorption of Cr(III) at the oxide-water
interface: II. Adsorption, coprecipitation, and surface
precipitation on hydrous ferric oxide. J Colloid Interface
Sci 148:443–458
Chen JM, Hao OJ (1998) Microbial chromium(VI) reduction.
Crit Rev Environ Sci Technol 28:219–251
Cheryl P, Susan MB (2000) Reflections on hexavalent chro-
mium: health hazards of an industrial heavyweight.
Environ Health Perspect 108:48–58
Cotton FA, Wilkinson G, Murillo CA, Bochmann M (1999)
Advanced inorganic chemistry. Wiley, New York
Crawford RJ, Harding IH, Mainwaring DE (1993) Adsorption and
coprecipitation of single heavy metal ions onto the hydrated
oxides of iron and chromium. Langmuir 9:3050–3056
Davis JA, Leckie JO (1979) Surface-ionization and complex-
ation at the oxide-water interface. 3. Adsorption of anions.
J Colloid Interface Sci 74:32–43
J Nanopart Res (2011) 13:4063–4073 4071
123
Author's personal copy

12. Eary LE, Rai D (1989) Kinetics of chromate reduction by
ferrous ions derived from hematite and biotite at 25 °C.
Am J Sci 289:180–213
Eary LE, Rai D (1998) Chromate removal from aqueous wastes
by reduction with ferrous ion. Environ Sci Technol
22:972–977
Elliott DW, Lien HL, Zhang WX (2009) Degradation of lin-
dane by zerovalent iron nanoparticles. J Environ Eng
135:317–324
Fatisson J, Ghoshal S, Tufenkji N (2010) Deposition of car-
boxymethyl cellulose-coated zerovalent iron nanoparticles
onto silica: roles of solution chemistry and organic mol-
ecules. Langmuir 26:12832–12840
Franco DV, Da Silva LM, Jardim WF (2009) Reduction of
hexavalent chromium in soil and ground water using zero-
valent iron under batch and semi-batch conditions. Water
Air Soil Pollut 197:49–60
Gu B, Schmiu J, Chen Z, Liang L, MaCarthy JF (1994)
Adsorption and desorption of natural organic matter on
iron oxide mechanisms and models. Environ Sci Technol
28:38–46
Guha S, Bhargava P (2005) Removal of chromium from syn-
thetic plating waste by zero-valent iron and sulfate-
reducing bacteria. Water Environ Res 77:411–416
Hou M, Wan H, Zhou Q, Liu X, Luo W, Fan Y (2009) The
dechlorination of pentachlorophenol by zerovalent iron in
presence of carboxylic acid. Bull Environ Contam Toxicol
82:137–144
Ilton ES, Veblen DR (1994) Chromium sorption by phlogopite
and biotite in acidic solutions at 25 °C: insights from
X-ray photoelectron spectroscopy and electron micros-
copy. Geochim Cosmochim Acta 58:2777–2788
Kimbrough DE, Cohen Y, Winer AM, Creelman L, Mabuni C
(1999) Critical assessment of chromium in the environ-
ment. Crit Rev Environ Sci Technol 29:1–46
Li XQ, Zhang WX (2006) Iron nanoparticles: the core-shell
structure and unique properties for Ni(II) sequestration.
Langmuir 22:4638–4642
Li XQ, Zhang WX (2007) Sequestration of metal cations with
zerovalent iron nanoparticles: a study with high resolution
X-ray photoelectron spectroscopy (HR-XPS). J Phys
Chem C 111:6939–6946
Li XQ, Elliott DW, Zhang WX (2006) Zero-valent iron
nanoparticles for abatement of environmental pollutants:
materials and engineering aspects. Crit Rev Solid State
Mater Sci 31:111–122
Lien HI, Elliott DW, San YP, Zhang WX (2006) Recent pro-
gress in zero-valent iron nanoparticles for groundwater
remediation. J Environ Eng Manag 16:371–380
Liu X, Wazne M, Christodoulatos C, Jasinkiewicz KL (2009)
Aggregation and deposition behavior of boron nano-
particles in porous media. J Colloid Interface Sci 330:
90–96
Martin JE, Herzing AA, Yan W, Li XQ, Koel BE, Kieley CJ,
Zhang WX (2008) Determination of the oxide layer
thickness in core-shell zerovalent iron nanoparticles.
Langmuir 24:4329–4334
Morgada ME, Levy IK, Salomone V, Farias SS, Lopez G,
Litter MI (2009) Arsenic(V) removal with nanoparticulate
zerovalent iron: effect of UV light and humic acids. Catal
Today 143:261–268
Patterson RR, Fendorf S, Fendorf M (1997) Reduction of
hexavalent chromium by amorphous iron sulfide. Environ
Sci Technol 31:2039–2044
Ponder SM, Darab JG, Mallouk TE (2000) Remediation of
Cr(VI) and Pb(II) aqueous solutions using supported,
nano-scale zero-valent iron. Environ Sci Technol
34:2564–2569
Powell RM, Puls RW (1997) Proton generation by dissolution
of intrinsic or augmented aluminosilicate minerals for in
situ contaminant remediation by zerovalence-state iron.
Environ Sci Technol 31:2244–2251
Powell RM, Puls RW, Hightower SK, Sabatini DA (1995)
Coupled iron corrosion and chromate reduction: mecha-
nism of subsurface remediation. Environ Sci Technol
29:1913–1922
Pratt AR, Blowes DW, Ptacek CJ (1997) Products of chromate
reduction on proposed subsurface remediation material.
Environ Sci Technol 31:2492–2498
Puls RW, Paul CJ, Powell RM (1999) The application of in situ
permeable reactive (Zero-valent iron) barrier technology
for the remediation of chromate contaminated ground
water: a field test. Appl Geochem 14:989–1000
Shih YH, Tai YT (2010) Reaction of decabrominated diphenyl
ether by zerovalent iron nanoparticles. Chemosphere
78:1200–1206
Singh IB, Singh DR (2003) Effect of pH on Cr–Fe interaction
during Cr(VI) removal by metallic iron. Environ Sci
Technol 24:1041–1047
Singh R, Singh A, Misra V, Singh RP (2011) Degradation of
Lindane contaminated soil using zero-valent iron nano-
particles. J Biomed Nanotechnol 7:177–178
Sparks DL (1995) Environmental soil chemistry. Academic
Press, San Diego
Strigul N, Vaccari L, Galdun C, Wazne M, Liu X, Christod-
oulatos C, Jasinkiewicz KL (2009) Acute toxicity of
boron, titanium dioxide, and aluminium nanoparticles to
Daphnia magna and Vibrio fischeri. Desalination 248:
771–782
Sun YP, Li XQ, Cao J, Zhang WX, Wang HP (2006) Char-
acterization of zero-valent iron nanoparticles. Adv Colloid
Interface Sci 120:47–56
Tee YH, Bachas L, Bhattacharyya D (2009) Degradation of
trichloethylene by iron-based bimetallic nanoparticles.
J Phys Chem C 113:9454–9464
Tipping E (2002) Cation binding by humic substances. Cam-
bridge University Press, Cambridge
Tratnyek PG, Scherer MM, Deng B, Hu S (2001) Effect of
natural organic matter, anthropogenic surfactants, and
model quinines on the reduction of contaminants by zero-
valent iron. Water Res 35:4435–4443
Tsang DCW, Graham NJD, Lo IMC (2009) Humic acid
aggregation in zerovalent iron systems and its effect on
trichloroethylene removal. Chemosphere 75:1338–1343
U.S. EPA Environmental Response Team (2000) Standard
Operating Procedures SOP-2012, Soil sampling
Wang Z, Huang W, Peng P, Fennell DE (2010) Rapid trans-
formation of 1,2,3,4-TCDD by Pd/Fe catalysts. Chemo-
sphere 78:147–151
Wei C, German S, Basak S, Rajeshwar K (1993) Reduction of
hexavalent chromium in aqueous solutions by polypyr-
role. J Electrochem Soc 140:60–62
4072 J Nanopart Res (2011) 13:4063–4073
123
Author's personal copy

13. Wu Y, Zhang J, Tong Y, Xu X (2009) Chromium(VI) reduc-
tion on aqueous solutions by Fe3O4-stabilized FeO
nanoparticles. J Hazard Mater 172:1640–1645
Xiao S, Shen M, Guo R, Wang S, Shi X (2009) Immobilization
of zerovalent iron nanoparticles into electrospun polymer
nanofibers: synthesis, characterization, and potential
environmental applications. J Phys Chem C 113:18062–
18068
Xu Y, Zhao D (2007) Reductive immobilization of chromate in
water and soil using stabilized iron nano-particles. Water
Res 41:2101–2108
Zachara JM, Girvin DC, Schmidt RL, Resch CT (1987)
Chromate adsorption on amorphous iron oxyhydroxide in
the presence of major groundwater ions. Environ Sci
Technol 21:589–594
J Nanopart Res (2011) 13:4063–4073 4073
123
Author's personal copy