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

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Journal of Nanoparticle ResearchAn Interdisciplinary Forum forNanoscale Science and Technology ISSN 1388-0764Volume 13Number 9 J Nanopart Res (2011) 13:4063-4073DOI 10.1007/s11051-011-0350-y

Synthesis, characterization and role ofzero-valent iron nanoparticle in removalof hexavalent chromium from chromium-spiked soil

Ritu Singh, Virendra Misra & RanaPratap Singh

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RESEARCH PAPER

Synthesis, characterization and role of zero-valent ironnanoparticle in removal of hexavalent chromiumfrom chromium-spiked soil

Ritu Singh • Virendra Misra • Rana Pratap Singh

Received: 14 October 2010 / Accepted: 16 March 2011 / Published online: 1 April 2011

� 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 �Characterization � Remediation � Contaminants �Reaction kinetics � Humic acid � Environment � 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 � 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: [email protected]

R. Singh � R. P. Singh

Department of Environmental Science, Babasaheb

Bhimrao Ambedkar University, Raebareli Road, Lucknow

226 025, UP, India

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DOI 10.1007/s11051-011-0350-y

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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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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 BH�4 þ 9H2O

! 4Fe0 þ 3H2BO�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 0:9kbcosh

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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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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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 �= Cr VIð Þ½ �0¼ � 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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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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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 Cr2O�7 þ 7H2O ! 3Fe2þ

þ 2Cr OHð Þ3þ 8OH�ð4Þ

1�xð ÞFe3þ aqð Þ þ xð ÞCr3þ aqð Þ þ 3H2O

! CrxFe1�x OHð Þ3 sð Þ þ 3Hþ aqð Þ ð5Þ

1�xð ÞFe3þ aqð Þ þ xð ÞCr3þ aqð Þ þ 2H2O

! CrxFe1�x 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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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

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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.

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