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INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 2, No 4, 2012
© Copyright 2010 All rights reserved Integrated Publishing Association
Research article ISSN 0976 – 4402
Received on March 2012 Published on May 2012 1962
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous
solution using starch – stabilized nanoscale zerovalent iron as adsorbent :
Equilibrium and kinetics Selvarani. M
1, Prema. P
2
1- Research Scholar, Post Graduate and Research Department of Zoology,
V. H. N. S. N. College, Virudhunagar - 626 001, Tamil Nadu, India
2- Assistant Professor in Zoology, Post Graduate and Research Department of Zoology,
V. H. N. S. N. College, Virudhunagar - 626 001, Tamil Nadu, India
doi:10.6088/ijes.00202030080
ABSTRACT
Groundwater remediation by nanoparticles has received increasing interests in recent years.
The present work was conducted to investigate the feasibility of using new class of stabilized
zerovalent iron (ZVI) nanoparticles for In situ reductive immobilization of Cr(VI) in water.
The nanoparticles were prepared by chemical reduction method using Ferrous sulfate
heptahydrate (FeSO4.7H2O) with sodium borohydride (NaBH4) as a reducer. This sample
was subjected to XRD for the determination of crystallinity and average particle size. The
size of the particle is 12.4 nm. The morphology of the nanoparticles was observed using
SEM. Batch experiments reported that the synthesized starch stabilized zerovalent iron
nanoparticles were able to rapidly reduce Cr(VI) in aqueous solution. The extend of Cr(VI)
reduction was increased with increasing concentration of iron nanoparticles (0.1 g/L – 0.4
g/L) and inversely with initial Cr(VI) concentration (10 mg/L – 25 mg/L) as well as with
decreasing pH (3 – 10). The equilibrium data obtained from the experimental results were
then fitted to the Freundlich and Langmuir isotherm models. Kinetic models were examined
with pseudo first order rate reaction. The Correlation coefficient between experimental
parameters and time shows that there is a strong positive correlation for Cr(VI) reduction.
This study indicates that the zerovalent iron nanoparticles, especially those which were
starch-stabilized can yield a high removal efficiency in the reduction of Cr(VI) contaminated
groundwater.
Keywords: Starch Stabilized Feo, XRD, SEM, Cr(VI) transformation.
1. Introduction
With the advancement of industrialization, agricultural and urban activities, the levels of
groundwater contamination have increased many folds in the last few decades. The
increasing contamination of groundwater by toxic metal ions is a significant environmental
hazard to drinking water supplies. Chromium (Cr), which is one of the most toxic and
important heavy metals commonly found in wastewater from industrial activities mainly
through tanning and electroplating industries (Sayari et al., 2005). In the United States, it is
the second most common inorganic contaminant in water after lead. Chromium, essentially
exists in two oxidation forms namely Cr(III) and Cr(VI). Over a narrow concentration range,
Cr(III) is proved to be biologically essential to mammals as it maintains an effective glucose,
lipid and protein metabolism, whereas Cr(VI) is reported to have toxic effect on humans and
it is considered to be genotoxic and carcinogenic in nature (Cheuhan and
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1963
Sankararamakrishnan, 2011). Major toxic effects of Cr(VI) are chronic ulcer, dermatitis,
corrosive reaction in nasal septum and local effects in lungs. Therefore, the hexavalent form
of Cr has been considered to be more hazardous due to its carcinogenic properties (Kobya,
2004). The permissible limit of Cr(VI) in industrial effluents is set at 0.5 mg/L by the
Ministry of Environment and Forests (MoEF), by Government of India (Panda et al., 2011).
The high toxicity of Cr(VI) is related to its ability to cross the cell membrane and its strong
oxidation properties (Girard and Hupert, 1996). As a result, removing Cr(VI) from industrial
wastewater has become necessary in order to avoid contamination of natural and raw water
used for public supply. Much research has focused on the remediation of Cr(VI) and many
treatment processes have been developed. Environmental chemists and material scientists
have always focused their attention on the development of cost-effective adsorbents which
could be exploited for the effective removal of Cr(VI) from aqueous solution. Recently,
nanomaterials have received considerable attention due to their small particle size, large and
controllable surface area, cost-economy, non-toxicity and ease of preparation. Reduction of
Cr(VI) to Cr(III) by powder or granular zerovalent iron (Feo) particles and non-stabilized or
agglomerated ZVI nanoparticles have been investigated in a number of laboratory and field
studies. Cr(VI) reduction by Feo appears to be one of the most promising chemical
technologies. Powell et al. (1995) suggested that the mechanism of Feo is a cyclic, multiple
step electrochemical corrosion process and confirmed that aluminosilicate minerals could
enhance the rate of Cr(VI) reduction. Cr(VI) removal capacity highly depended on reaction
time, solution pH, temperature, initial Cr(VI) concentration and material dosage (Shen et al.,
2012).
Feo nanoparticels, due to their extremely high surface areas, can enchance the reduction
efficiencies remarkably. Typically, Feo nanoparticles are prepared by reducing Fe(II) or
Fe(III) in an aqueous solution using a strong reducing agent (NaBH4) appears most suitable
because of its minimal use of environmentally harmful solvents and chemicals. However,
because of their high surface area and high reactivity ZVI nanoparticles prepared using
traditional methods tend to either agglomerate rapidly or react quickly with the surrounding
media (eg: dissolved oxygen or water) resulting in rapid loss in soil mobility as well as
reactivity (He and Zhao, 2005). Because agglomerated ZVI nanoparticles are often in the
range of micron scale therefore they are not transportable or deliverable in soils. To prevent
particles from agglomeration, researchers have been trying to prepare more stable and
chemically more reactive Fe(0) nanoparticles using a dispersion as stabilizer. In this work, a
new class of Feo based nanoparticles using water-soluble starch as dispersant prepared by
borohydride reduction method, which were tested for their ability to reduce toxic hexavalent
chromium [Cr(VI)] ions form aqueous solution. Also, the influence of Cr(VI) and
nanoparticles concentration, variation in pH and temperature of solution on the efficacy of
Cr(VI) removal was evaluated. The experimental data were analyzed by fitting it into
Langmuir and Freundlich isotherm and pseudo first order kinetic models.
2. Materials and method
2.1 Materials
Ferrous sulfate heptahydrate (FeSO4.7H2O), Sodium borohydride (NaBH4), Starch and
Ethanol were purchased from Himedia (P) Ltd, Mumbai were used as starting materials
without further purification. Potassium dichromate (K2Cr2O7) was used as a model
contaminant. Milli-Q water was used throughout the experiment.
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1964
2.2 Methods
Preparation of starch-stabilized zerovalent iron nanoparticles
The preparation of starch-stabilized Fe0
nanoparticles was followed the methods described by
He and Zhao (2005). Starch serves as a stabilizer and dispersant that prevents the resultant
nanoparticles from agglomeration, thereby prolonging their reactivity and maintaining the
physical integrity. In brief, the preparation was carried out in a 250 ml conical flask attached
to a vacuum line. Before use, deionized (DI) water and the starch solution were purged with
purified N2 gas for 15 min to remove dissolved oxygen (DO). In a typical preparation, a
stock solution of 0.21 M FeSO4.7H2O was prepared right before use and then was added to
the starch solution through burette to yield the desired concentration of Fe and starch. Fe
concentration used in this study was 0.1 g L-1
and the corresponding starch concentration was
0.2% (w/v).
The Fe2+
ions were then reduced to Fe0 by adding a stoichiometric amount of NaBH4 aqueous
solution at a BH4-/Fe
2+ molar ratio of 2.0 to the mixture with magnetic stirring at 230 rpm
under ambient temperature. The ferrous iron was reduced to zero-valent iron according to the
following reaction:
Fe(H2O)62+
+2BH4−→Fe
0↓+2B(OH)3+7H2↑
The resultant black particles were separated from the solution by centrifugation at 4000 rpm
for 5 min and washed with N2 saturated deionized water and at least three times with
99% absolute ethanol. Finally, the synthesized starch stabilized Feo nanoparticles were dried
in an oven at 60oC.
2.3 Characterization of Synthesized Starch-Stabilized Fe0 Nanoparticles
2.3.1 X-ray Diffraction
The crystallographic analysis of the sample was performed by powder X-ray diffraction. The
X-ray diffraction patterns of synthesized starch-stabilized Fe0 nanoparticles were recorded
with an X‟pert PROPAN analytical instrument operated at 40 kV and a current of 30 mA
with Cu α radiation (λ=1.54060 Ao). A continuous scan mode was used to collect 2θ data
from 10.08o
to 79.93o. The diffraction intensities were compared with the standard JCPDS
files (No: 80 – 2186). Crystalline size of the nanoparticles was calculated from the line
broadening of X-ray diffraction peak according to the Debye-Scherer formula (Huang and
Tang, 2005).
D = kλ/ β Cosθ,
Where D is the thickness of the nanocrystal, „k‟ constant, „λ‟ wavelength of X-rays, „β‟ width
at half maxima of reflection at Bragg‟s angle 2θ, „θ‟ Bragg‟s angle.
2.3.2 Scanning Electron Microscopy
Surface morphology and the size distribution of the particles was investigated with the
Scanning Electron Microscope (SU 1510) operated at 15kV, magnification x350. The solid
samples were sprinkled on the adhesive carbon tape which is supported on a metallic disk.
The sample surface images were taken at different magnifications. The scale was about
100 µm which is equivalent to 57 mm for the synthesized starch-stabilized Fe0 nanoparticles.
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1965
2.4 Cr(VI) reduction studies
2.4.1 Preparation of Cr(VI) solution
Stock solution (1000 mg/L) of Cr(VI) was prepared by dissolving 2.829 g of K2Cr2O7 in
1000 ml of double distilled water. Experimental solutions of the desired concentrations were
obtained by successive dilutions and Cr(VI) was determined via 1,5-diphenylcarbazide
(DPC) colorimetric method by measuring the absorbance at a wavelength of 540 nm using
UV-Vis spectrophotometer. 0.1M NaOH or HCl was used for the adjustment of pH and
controlled by pH meter.
2.4.2 Batch experiments
The batch experiments for the reduction of Cr(VI) was performed in 250ml Erlenmeyer
flasks into which synthesized starch stabilized Feo nanoparticles were introduced, followed
by the addition of Cr2O72-
aqueous solution. The reaction solution was stirred at a speed of
500 rpm and periodically sampled by glass syringe. The samples were filtered immediately
through 0.2µm membrane filters and analyzed for Cr(VI). The absorbance of purple Cr(VI)-
diphenylcarbazide product developed after 10 min was measured at a wavelength of 540 nm
using UV-Vis spectrophotometer.
The effect of various parameters on the Cr(VI) reduction was studied. Starch stabilized Feo
nanoparticles concentration used in this study was 0.1 g to 0.4 g/L. The initial Cr(VI)
concentration was 10 mg to 25 mg/L, the initial pH was 3 to 10 and the initial temperature
was 15oC to 45
oC.
2.4.3 Reduction of Cr(VI) by starch stabilized Feo nanoparticles
According to Sethuraman and Balasubramanian (2010), the percentage of Cr(VI) removal
was calculated spectrophotometrically using the formula
Removal of Cr(VI)%
Where, Co and Ce represent initial and final concentration of Cr(VI).
The initial reduction rate of Cr(VI) can be described by pseudo first order reaction kinetics
expression (Ponder et al., 2000 and Liu et al., 2008).
The equilibrium adsorption capacity of Cr(VI) was calculated by
qe = (Co – Ce) X V/M
Where qe (mg/g) is equilibrium adsorption capacity, Co and Ce are initial and equilibrium
concentration (mg/L) of Cr(VI) respectively, V (L) is the volume of solution and M (g) is the
weight of the adsorbent.
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1966
3. Results and discussion
3.1 X-ray diffraction
The X-ray diffraction pattern shows that the synthesized starch-stabilized Feo
nanoparticles
are in amorphous stage and in tetragonal system. In the respective nanoparticles, the
intensive diffraction peaks were observed at a 2θ value of 44.94o from the lattice plane (311)
of face-centered cubic (fcc) Fe unequivocally indicates that the particles are made of pure
iron (Figure 1). Alidokht et al. (2011) reported that characteristic peak at 2θ value of 44.7o
indicates the crystalline nature of Feo nanoparticles. In the obtained spectrum, the Bragg‟s
peak position and their intensities were compared with the standard JCPDS files. The size of
the particles was found to be 12.4 nm.
Position [°2Theta]
20 30 40 50 60 70
Counts
0
100
200
A Fe
Figure 1: X-ray Diffraction Spectrum of Synthesized Starch Stabilized
Zero-Valent Iron Nanoparticles
3.2 Scanning Electron Microscopy
The scanning electron microscopy of synthesized starch-stabilized Fe0 nanoparticles shows
that the particles are hexagonal and spherical in nature (Figure 2).
Figure 2: Scanning Electron Mircograph of Synthesized Starch Stabilized
Zero-Valent Iron Nanoparticles
(311)
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1967
The micrograph shows that the synthesized particles did not appear as discrete particles but
form much larger dendritic flocs. The aggregation is due to the vandar waals forces and
magnetic interactions among the particles. This finding is very much closer to the earliest
report (Rahmani et al., 2011).
3.3 Cr(VI) reduction studies
3.3.1 Effect of initial Cr(VI) concentration on Cr(VI) reduction
The removal efficiency is defined as the fraction of Cr(VI) in the solution during the reaction
rate as a fraction of the initial concentration. To investigate the effect of initial Cr(VI)
concentration, the batch experiments were conducted at various concentration of Cr(VI) from
10 to 25 mg/L. Figure 3 explains that the removal efficiency increased inversely with the
initial Cr(VI) concentration.
Figure 3: Effect of initial Cr(VI) concentration on Cr(VI) removal efficiency by
Starch - stabilized Feo nanoparticles
The removal efficiency of Cr(VI) decreased from 96% to 63.6% with increasing the initial
Cr(VI) concentration. Similarly, the rate constant of Cr(VI) removal is decreased with
increasing initial Cr(VI) concentration. The decrease in the percentage removal of Cr(VI)
can be explained with the fact that the Feo nanoparticles had a limited active sites, which
would have become saturated above a certain circumstances (Kalpan and Gilmore, 2004).
kobs was obtained by plotting linear regression of ln normalized concentration of Cr(VI) vs
time. The kobs is ranging from 111 ± 7.5 x 10-3
min-1
to 46 ± 2.7 x 10-3
min-1
with increasing
initial Cr(VI) concentration (Table 1).
Table 1: Effect of initial Cr(VI) concentration on Cr(VI) reduction rate constants and half
lives for starch-stabilized Feo nanoparticles
Initial conc. of Cr(VI)
(mg/L)
kobs (min-1
) t1/2 obs (min) r2
10 111.0 ± 7.5 x 10-3
6.31 ± 0.42 0.986
15 38.0 ± 4.0 x 10-3
18.21 ± 1.70 0.995
20 46.0 ± 4.0 x 10-3
15.30 ± 1.46 0.982
25 46.0 ± 2.7 x 10-3
15.12 ± 0.91 0.984
Experimental Conditions: Conc. of Feo = 0.2 g/L, Temp = 28
oC, pH = 7, ω = 500 rpm.
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1968
In this study, it is also found that reduction of Cr(VI) is proceeded in two steps. During the
first few min of the experiment, decrease of Cr(VI) is much dramatic and the reaction can be
expressed with a pseudo-first-order kinetics. This indicates that the predominant mechanism
for the removal of Cr(VI) is most likely due to the adsorption to the Feo nanoparticles surface
rather than the reductive transformation (Geng et al., 2009). The reductions of Cr(VI) in other
Feo nanoparticles system have been reported as pseudo-first-order (Ponder et al., 2000; Xu
and Zhao, 2007). The two-step kinetics have been explained as reduction of Cr(VI) under the
existence of Feo followed by a faster physical or a chemical adsorptions (Deng et al., 1999;
Ponder et al., 2000; Kanel et al., 2005).
3.3.2 Effect of adsorbent concentration on Cr(VI) reduction
The effect of adsorbent concentration on the adsorption of Cr(VI) ions in aqueous solution
was examined by varying the adsorbent concentration from 0.1 g/L to 0.4 g/L is given in
Figure 4.
Figure 4: Effect of initial Fe
o concentration on Cr(VI) removal efficiency by
Starch - stabilized Feo nanoparticles
Removal efficiency increased from 31% to 95.5% with increasing concentration of adsorbent
dosage. Increase in adsorbent concentration generally increases the level of adsorption of
Cr(VI) ions because of an overall increase in surface of the adsorbent which in turn increase
the number of binding sites lead to the increase of Cr(VI) removal efficiency (Esposito et al.,
2001). It is believed that reduction reaction occurs on the iron surface. Similar findings have
also been reported by Lai keith (2008). The pseudo first order rate constants are summarized
in Table 2.
The results indicated that the rate constant value rose from 9.7 ± 2.5 x 10-3
min-1
to
74 ± 4 x 10-3
min-1
when the Feo nanoparticles dosage was increased from 0.1 to 0.4 g/L. It
was reported that kobs are strongly dependent on the amount of Feo nanoparticles used and
linear variations of kobs with Feo
dose was observed for heterogeneous reactions for the
reduction of numerous contaminant by Feo (Ponder et al., 2000; Alowitz and Scherer, 2002).
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1969
Table 2: Effect of initial Feo concentration on Cr(VI) reduction rate constants and half lives
for starch-stabilized Feo nanoparticles
Initial conc. of Feo
(g/L)
kobs (min-1
) t1/2 obs (min) r2
0.1 9.7 ± 2.5 x 10-3
78.30 ± 27.25 0.993
0.2 46.0 ± 4.0 x 10-3
15.30 ± 1.46 0.982
0.3 57.0 ± 3.0 x 10-3
12.19 ± 0.63 0.994
0.4 74.0 ± 4.0 x 10-3
9.42 ± 0.52 0.996
Experimental Conditions: Conc. of (VI) = 20mg/L, Temp = 28 oC, pH = 7, ω = 500 rpm.
3.3.3 Effect of initial pH on Cr(VI) reduction
The influence of pH on Cr(VI) removal efficiency was studied by changing the initial pH
from 3 to 10. The relation between the initial pH of the solution and the percentage removal
efficiency of Cr(VI) is shown in Figure 5.
Figure 5: Effect of initial pH on Cr(VI) removal efficiency by
Starch - stabilized Feo nanoparticles
The removal efficiency increased significantly with decreasing pH. The percentage of Cr(VI)
decreased from 97% to 28.5% with increasing the initial pH. Under acidic condition, the
removal efficiency was rapid would accelerate the corrosion of Feo, thus enhancing Cr(VI)
reduction. It indicated that the Feo nanoparticles have a high reactivity at pH below 5. In
contrast, the plots for pH>8 show less rapid removal. This may be because of the formation
of mixed Fe and Cr oxyhydroxides at high pH values on the iron surfaces (Powell et al.,
1995; Lee et al., 2003). A decrease in kobs is noticed from 62 ± 11 x 10-3
min-1
at pH 3 to
16 ± 1.4 x 10-3
min-1
at pH 10 (Table 3).
Table 3: Effect of initial pH on Cr(VI) reduction rate constants and half lives for starch-
stabilized Feo nanoparticles
Initial pH kobs (min-1
) t1/2 obs (min) r2
3 62.0 ± 11.0 x 10-3
11.43 ± 1.93 0.984
5 39.0 ± 3.2 x 10-3
14.89 ± 1.47 0.989
8 26.0 ± 1.7 x 10-3
27.08 ± 1.80 0.992
10 16.0 ± 1.4 x 10-3
43.26 ± 3.65 0.979
Experimental Conditions: Conc. of (VI) = 20mg/L, Conc. of Feo = 0.2 g/L,
Per
cen
tag
e R
emo
val
eff
icie
ncy
of
[Cr(
VI)
]
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1970
Temp = 28 oC, ω = 500 rpm.
It shows that there is a dramatic change in the rate constant between the pH values. These
results demonstrate that the acidic conditions would accelerate the corrosion of Feo, thus
enhancing Cr(VI) reduction. Under certain circumstances, Feo donates the electrons either to
protons or to chromate anions and itself will be oxidized to Fe(III). The protons will be
reduced to hydrogen gas which in turn reduces Cr(VI) to cr(III) (Melitas et al., 2001). The
decrease in adsorption of Cr(VI) by increasing the pH is due to the competition between the
anions CrO42-
and OH- (Ponder et al., 2000).
3.3.4 Effect of temperature on Cr(VI) reduction
The relation between the temperature and Cr(VI) removal efficiency is depicted in Figure 6.
Figure 6: Effect of initial temperature on Cr(VI) removal efficiency by
Starch - stabilized Feo nanoparticles
It is noticed that as the temperature increases, the removal efficiency also increases. The
removal efficiency increased from 32.5% to 72% with increasing the initial temperature
from 15oC to 45
oC. The rate constant value (kobs) increased as the temperature increases
(Table 4), indicating that vibration rate of Cr(VI) increases at high temperature. Similar
findings have been reported by earlier studies (Wang et al., 2010).
Table 4: Effect of initial temperature on Cr(VI) reduction rate constants and half lives for
starch-stabilized Feo nanoparticles
Initial temp
(oC)
kobs (min-1
) t1/2 obs (min) r2
15 13.0 ± 2.0 x 10-3
55.36 ± 9.34 0.989
25 17.0 ± 2.0 x 10-3
41.0 ± 4.59 0.990
35 26.0 ± 2.0 x 10-3
27.05 ± 3.52 0.995
45 76.0 ± 6.0 x 10-3
9.19 ± 0.82 0.989
Time (min)
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1971
Experimental Conditions: Conc. of (VI) = 20mg/L, Conc. of Feo = 0.2 g/L,
pH = 7, ω = 500 rpm.
Adsorption isotherms
To study the adsorption efficiency of Cr(VI) at the surface of the adsorbent, an attempt was
made to test for the Langmuir and Freundlich isotherm models on the obtained experimental
data.
Freundlich isotherm
The mathematical expression for the non-linear form of the Freundlich isotherm model
(Choy et al., 1999) can be given as
qe = KfCe1/n
This equation is frequently used in the linear form by taking the logarithm of both sides
log qe = log Kf + Ce
A plot of log qe versus log Ce gives a straight line (Figure 7).
0.2 0.4 0.6 0.8 1.0
1.6
1.8
2.0
2.2
2.4
Lo
g q
e
Log ce
Figure 7: Freundlich isotherm model for Cr(VI) adsorption by
Starch - stabilized Feo nanoparticles
Kf and n are the isotherm constants. The values of Kf and n along with the linear regression
co-efficient (r2) for the present experimental data have been obtained and are shown in
Table 5.
Langmuir isotherm
The non-linear form of the Langmuir isotherm model for monolayer adsorption is expressed
by equation (Gupta and Babu, 2009).
The equation can be rearranged the following linear form
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1972
The binding constant (qm) and the adsorbent capacity (Kq) are estimated by plotting Ce/qe
against Ce (Figure 8).
0.2 0.4 0.6 0.8
0.2
0.3
0.4
0.5
0.6
0.7
c e / q
e
ce
Figure 8: Langmuir isotherm model for Cr(VI) adsorption by
Starch - stabilized Feo nanoparticles
The experimental values of qm and Kq along with the linear regression co-efficient (r2) are
given in Table 5. The comparison of correlation coefficients (r2) indicates that both the
adsorption isotherm models match satisfactorily with the experimental data obtained from the
present study.
Table 5: Adsorption isotherm constants for Cr(VI) reduction by Starch-stabilized Feo
nanoparticles
Langmuir constants Freundlich constants
Qm Ka R2
Kf n R2
0.544 0.181 0.999 0.999 1.398 1.0
4. Conclusion
In this study, starch stabilized Feo nanoparticles has been successfully prepared by chemical
reduction method and its removal efficiency was evaluated using potassium dichromate
(K2Cr2O) as a model contaminant. The results obtained from the experimental conditions
support the conclusion that the concentration of starch stabilized Feo nanoparticles had
significant effect on the reduction rate of Cr(VI). The pH of the reaction mixture has a strong
effect on the Cr(VI) reduction efficiency with increasing initial pH as well as with decreasing
initial Cr(VI) concentration . The reduction rate is expressed by pseudo first order reaction
rate equation. Equilibrium data fitted well with both Langmuir and Freundlich adsorption
isotherm models. Thus the results from the current research provides compelling evidence
that the starch –stabilized Feo nanoparticles may be used for in situ reductive efficacy of
Cr(VI) contaminated water and soils, which may lead to an innovative remediation
technology is likely to be more cost effective and less environmentally disruptive.
Acknowledgement
The authors are very grateful for the financial support provided by Ministry of Science &
Technology, DST, Govt. of India for INSPIRE program (Dy.No.100/IFD/10706/) under
Assured Opportunity for Research Carrier (AORC).
Removal of toxic metal Hexavalent Chromium [cr(vi)] from aqueous solution using starch – stabilized
nanoscale zerovalent iron as adsorbent : Equilibrium and kinetics
Selvarani. M, Prema. P
International Journal of Environmental Sciences Volume 2 No.4, 2012 1973
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