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Available online at www.jpsscientificpublications.com
Life Science Archives (LSA)
ISSN: 2454-1354
Volume – 1; Issue - 1; Year – 2015; Page: 6 - 12
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
Research Article
STUDIES ON THE REMOVAL OF Cr (VI) FROM WASTEWATER BY
STISHOVITE - TiO2 NANOCOMPOSITE
V. T. Priya1* and V. Venkateswaran
2
1Research scholar, Research and Development Centre, Bharathiyar university, Coimbatore, Tamil Nadu,
India. 2Principal, Sree Saraswathi Thyagaraja College, Pollachi, Tamil Nadu, India.
Abstract
The adsorption behavior of Cr(VI) from aqueous solution onto Stishovite-TiO2 nanocomposite was
investigated as a function of parameters such as initial metal concentration, contact time, pH
and temperature.
The Langmuir and Freundlich adsorption models were applied to describe the equilibrium isotherms. The
pseudo-second-order kinetic, Elovich, intraparticle diffusion models were used to describe the kinetic data
and rate constants were evaluated. The final result shows that the adsorption process under study could be
best described by the pseudo second order equation. The experimental data fitted well with both Langmuir
and Freundlich isotherms. Thermodynamic parameters reveals that the adsorption to be spontaneous and
endothermic.
Article History Received : 05.02.2015
Revised : 18.02.2015
Accepted : 24.02.2015
Keywords: Stishovite-TiO2 Nano composite, Cr (VI), Adsorption kinetics and Isotherm
1. Introduction
Pollution by heavy metals is caused by
many industrial wastewaters such as those let out
by metal plating units, mining operations, battery
manufacturing, paints and pigments and the glass
production industries. Heavy metals are not
biodegradable and their presence in streams and
lakes leads to biomagnifications in living
organisms, causing health problems in animals,
plants and human beings (Mehmet et al., 2006).
Several clay materials, either in their
natural form or in modified forms had been
utilised as adsorbents for water and wastewater
treatments because of their low cost, availability in
abundance in all parts of the world, high
* Corresponding author: V. T. Priya
Tel.: +91-9952215776
E-mail: drspriya2010@gmail.com
adsorption capacities high surface area they
provide and large ion exchange potentials.
Reports on the use of clays and polymers
are available in literature. Several clay minerals
like sodium montmorillionite (Abollino et al.,
2003), hydroxy apatite (Badillo-Almaraz and Ly,
2003), siderite, fugirite and magnetite (Johnson
and Sherman, 2008), rectorite (Zili Liu et al.,
2012), vermiculite (Machado et al., 2006) had
been effectively used as adsorbents. But, the
studies on the use of both of them in combination
in the form of a composite are not that much
available. Hence, it was thought worthwhile to
investigate the efficiency of Stishovite-TiO2
nanocomposite as an adsorbent in removing Cr
(VI) from aqueous solutions and the results are
summarized in the following sections.
V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 7
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
2. Methodology
2.1. Preparation of Stishovite-titanium-di-oxide
nanocomposite
Stishovite (3g) was allowed to swell in 15
ml of water-free alcohol and stirred for 2 hours at
25 °C to get a uniform suspension. At the same
time, the titanium dioxide (3 g) was dispersed into
water-free alcohol (15 ml). Then, the diluted
titanium dioxide was slowly added into the
suspension of Stishovite and stirred for a further 5
hours at 25 °C. Finally, 5 ml alcohol mixed with
0.2 ml deionized water was slowly added. The
stirring was continued for another 5 hours at 25 °C
and the resulting suspension was kept overnight in
a vacuum oven for 6 hours at 80°C.
2.2. Determination of Chromium (VI) (APHA
et al., 1998)
Hexavalent Chromium was determined
spectrophotometrically by diphenyl carbazide
method. To a series of standard solutions of Cr
(VI) (5.50 µg), 3 ml of 2 N sulphuric acid, 2 drops
of phosphoric acid and 0.5 ml of diphenyl
carbazide solution (0.5% in acetone) were added
and made up to 25 ml with doubly distilled water.
After few minutes the absorbance was measured
at 540 nm against a reagent blank. A calibration
graph with absorbance vs Chromium (VI)
concentration was prepared. The concentration in
the sample was determined using the calibration
graph.
2.3. Preparation of Chromium (VI) stock
solution (1000 ppm)
A stock solution of (1000 mg/L) of
Chromium (VI) was prepared by dissolving 5.658
g of dried potassium dichromate (K2Cr2O7.H2O) in
doubly distilled water and making up to 1000 ml.
2.4. Characterization of adsorbent
Physico-chemical characteristics of the
adsorbents were studied as per the standard testing
methods. The XRD pattern of Stishovite - TiO2
nanocomposite (Fig.1) showed characteristic
peaks at 28° confirming the presence of Stishovite
- TiO2 phase in the nanocomposite. The surface
morphology of the adsorbents was visualized via
scanning electron microscopy (SEM) (Fig.2). The
diameter of the composite range was 50 µm.
2.5. Batch adsorption experiments
Entire batch mode experiments were
carried out in the temperature range 301 K to 317
K by taking 50 ml of the respective metal solution
and known amount of the adsorbent in a 100 ml
conical flask. The flasks were agitated for pre
determined time intervals in a thermostat attached
with a shaker at the desired temperature. The
adsorbent and adsorbate were separated by
filtration. Studies on the effects of agitation time,
pH, initial metal concentration, adsorbent dose and
temperature were carried out by using known
amount of adsorbent and 50 ml of metal solution
of different concentrations. Metal solution (50 ml)
with different amounts of adsorbent was taken to
study the effect of adsorbent dosage on the
removal of metals.
Figure - 1: XRD analysis of Stishovite - TiO2
Composite
Figure – 2: SEM of Stishovite - TiO2
Nanocomposite
V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 8
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
3. Results and Discussion
3.1. Effect of agitation time and initial dye
concentration
The effect of initial metal concentration
and contact time for the removal of Cr (VI) is
shown in Fig.3. For this study 50ml of 10 to 40
mg/L of metal solution was agitated with 100mg
of adsorbent. The extent of removal of metal was
faster in initial stages, then showed decreasing
pattern and finally became constant showing the
attainment of equilibrium. The extent of removal
was found to be 86.58%. The curves obtained are
single and smooth, indicating monolayer coverage
on the adsorbent surface (Namasivayam and
Periasamy, 1993).
Figure - 3: Effect of initial dye concentration
3.2. Effect of adsorbent dosage
The effect of adsorbent dosage on the
metal removal was studied by keeping all other
experimental conditions constant except that of
adsorption dosage. The amount adsorbed per unit
mass of the adsorbent decreased with increase in
adsorbent concentration (Fig.4). The decrease in
unit adsorption with increasing dose of adsorbent
may basically be due to the fact that adsorption
sites remaining unsaturated during the adsorption
process.
3.3. Effect of pH
Adsorption experiments were carried out at
various pH values ranging from 6 to 11
maintaining the required pH by adding necessary
amount of dilute hydrochloric acid and sodium
hydroxide solutions. A pH meter calibrated with
4.0 and 9.0 buffers were used. The Figure - 5
indicates that maximum metal removal had
occurred in basic medium. It was observed that as
the pH increases the sorption capacity also
increases.
Figure - 4: Effect of adsorbent dosage
3.4. Effect of temperature
Temperature has an important effect on the
adsorption process. Fig.6 shows effect of different
temperatures on the removal of Cr (VI) by the
nanocomposite. The amount of metal adsorbed
increases with increasing temperature from 301K
to 317K indicating the adsorption process to be
endothermic. This may be due to the fact that as
the temperature increases, rate of diffusion of
adsorbate molecules across the external boundary
layer and internal pores of adsorbent particle
increase.
Figure – 5: Effect of pH
V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 9
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
Figure – 6: Effect of temperature
3.5. Adsorption isotherms
Langmuir and Freundlich isotherms were
used to determine the amount of metal adsorbed
and its equilibrium concentration.
3.6. Langmuir isotherm
In linear form the Langmuir model
(Langmuir, 1916) is usually expressed as,
Where Ce is equilibrium concentration of
metal (mg/L), qe is the amount of metal adsorbed
at equilibrium (mg/g), Q0 and b are the Langmuir
constants correlated to adsorption capacity and
rate of adsorption, respectively. A linear plot of
Ce/qe vs Ce is shown in Fig.7. The values of Q0 and
b were calculated from the slope and intercept of
the plots and the values are given in Table 1.
These values indicate that the maximum
monolayer adsorption capacity of Stishovite-TiO2
nanocomposite was 12.33 mg/g. The crucial
features of the Langmuir isotherm was examined
by the dimensionless constant separation term
(RL) to determine high affinity adsorption.
RL was calculated as follows:
RL = 1/ (1+bCo), Where Co is initial metal
concentration (mg /L).
The nature of adsorption if, RL > 1
Unfavourable, RL = 1 Linear, RL = 0 Irreversible,
0 < RL < 1 Favourable. In the present study, the
RL values were less than one in the concentration
range studied, which shows that the adsorption
process was favourable.
Table – 1: The values of Langmuir constantQ0
and b in addition to RL constantQ0 and b in
addition to RL
Figure - 7: Langmuir isotherm
3.7. Freundlich isotherm (Freundlich, 1906)
The Freundlich isotherm can be
represented in its logarithmic form as,
)2(........................................log1
loglog efe Cn
Kq
Where Kf and n are Freundlich constants
representing adsorption capacity and intensity of
the adsorbent respectively. The plot of log qe vs
Conc. of
metal
(mg/L)
Chromium (Stishovite - TiO2
nanocomposite)
RL b Q0(mg/g) R
2
20 0.093
- -
- 40 0.049 - - -
60 0.033 0.486 12.330 0.9694
80 0.025 - - -
100 0.020 - - -
120 0.017 - - -
)1(............................................1
00 Q
C
bQq
C e
e
e
V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 10
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
log Ce shown in Fig.8 indicates that the adsorption
of Cr (VI) fit into the Freundlich isotherm. The
Freundlich constants (Kf and 1/n) are given in
Table - 2. The value of 1/n was less than one
which indicates a favorable adsorption (Bell,
1998).
Table – 2: The values of Freundlich constants
Kf and n
Metal
Kf
[mg1-1/n
L1/n
g-1
]
n
(mg/g) R
2
Chromium
with Stishovite
–TiO2
nanocomposite
PFR
8.831 1.729 0.9927
3.8. Kinetics of adsorption
In order to investigate the mechanism of
adsorption of Cr (VI) by the nanocomposite the
pseudo first order, pseudo second order and
Elovich models were considered.
The experimental data does not fit with pseudo
first order kinetic model.
Figure - 8: Freundlich isotherm
3.9. Pseudo second order kinetics
The pseudo second order chemisorptions
kinetic rate equation was expressed as (Ho et al.,
1999 and Ozacar, 2003).
)3.(.......................................2 Eqqqkdt
dqte
t
Here qe and qt were the adsorption capacity at
equilibrium and at time, t, respectively (mg/g) and
k2, the pseudo-second order rate constant
(g/mg/min). On integrating the Eq.3, for the
boundary conditions t=0 to t=t and qt = qe
)4.(.......................................
112 Eqtk
qqq ete
which is the integrated form of pseudo – second
order reaction. Eq.4 can be rearranged to obtain
)5.(.....................................11
2
2
Eqtqqkq
t
eet
Compared to Eq.3 and Eq.4 had an
advantage that k2 and qe can be obtained from the
intercepts and slope of the plot of (t/ qt) vs t and
there was no need to know any parameter
beforehand. The linearity of the plots (Fig.9)
clearly indicated that the adsorption process
followed pseudo second order kinetics.
Figure – 9: Pseudo second order kinetics
3.10. Elovich kinetic model (Low, 1960)
The Elovich equation is mainly applicable
for chemisorption processes involving
heterogeneous adsorbing surfaces. The Elovich
model in its integrated form can be
)6.(...........................).........ln(1
ln1
Eqabb
tb
qt
Where ‘a’ is the initial adsorption rate
(mg/g min) and ‘b’ is related to the extent of
V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 11
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
surface coverage and the activation energy for
chemisorptions (g/mg). A plot (Fig.10) of qt vs ln
t is a straight line, as expected, with a slope of 1/b
and an intercept log 1/b ln (ab) with good
correlation coefficients confirming the
chemisorptive nature of adsorption.
Figure - 10: Elovich kinetic model
3.11. Thermodynamic of Adsorption
Thermodynamic parameters like ΔH0 and
ΔS0 were evaluated using Van’t Hoff’s equation
lnKc = ΔS0/R – ΔH
0/R ……………….Eq. (7)
Where KC is the Langmuir equilibrium
constant, ΔH0
(25.921 kJ/mol) and ΔS0
(3.118
kJ/mol), are the standard enthalpy and entropy
changes of adsorption respectively and their
values are calculated from the slopes and
intercepts respectively of the linear plot of ln Kc
vs 1/T. The free energy changes for the adsorption
process ΔG0 (kJ/mol) (7.805 – 8.22 at 301 K – 317
K) are derived using the relation
ΔG0 = ΔH
0 - T ΔS
0 …………….........Eq. (8)
Negative free energy change and positive
entropy change of adsorption indicate that the
adsorption process is favourable and spontaneous
in nature. The endothermic nature of adsorption is
confirmed by the positive value of ΔH0.
3.12. Desorption studies
Desorption studies with acetic acid
revealed that the regeneration of adsorbent was
not satisfactory, which confirms the chemisorptive
nature of adsorption.
4. References
1) Abollino, O., Aceto, M., Malandrino, M.,
Sarzanini, C and Mentasti, E., 2003.
Adsorption of heavy metals on Na-
montmorillonite. Effect of pH and organic
substances, Water Res., 37, 1619-1627.
2) APHA., AWWA., WPCF., 1998. Standard
methods for the examination of water and
wastewater, American Public Health Ass., 18
Ed., Washington DC.
3) Badillo-Almaraz, V.E and Ly, J., 2003.
Calcium sorption on hydroxy apatite in
aqueous solutions. Reversible and
nonreversible components, J. Colloid Interface
Sci., 258, 27-32.
4) Bell, T. R. K., 1998. Mass Transfer
Operations, 10th Ed., McGraw-Hill, New
York.
5) Freundlich, H.M.F., 1906. Uber die adsorption
in losungen. Zeitschriftsfur physialische
Chemie., 57, 385-471.
6) Ho, Y.S., McKay, G., 1999. Pseudo-second
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Biochem., 34, 451-465.
7) Johnson, J and Sherman, D.H., 2008. Sorption
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(fougerite) and magnetite, implications for
arsenic release in anoxic ground waters, Chem.
Geol., 255, 173-181.
8) Langmuir, I., 1916. The constitution and
fundamental properties of solids and liquids,
J. Am. Chem. Soc., 38, 2221-2295.
9) Low, M.J.D., 1960. Kinetic modeling of liquid
phase adsorption of reactive dyes and metal
ions on chitosan, Chem. Rev., 60, 267-312.
10) Machado, L.C.R., Torchia, C.B and Lago,
R.M., 2006. Floating photocatalysts based on
TiO2 supported on high surface area exfoliated
vermiculite for water contamination,
Catal. Commun., I, 538-541.
11) Mehmet, E.A., Sukru, D., Celalettin, O. and
Mustafa, K., 2006. Heavy metal adsorption by
modified oak sawdust: Thermodynamics and
kinetics. Journal of Hazardous Materials.
12) Namasivayam, C and Periasamy, K., 1993.
Bicarbonate treated peanut hull carbon for Hg
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V. T. Priya /Life Science Archives (LSA), Volume – 1, Issue – 1, Page – 6 to 12, 2015 12
© 2015 Published by JPS Scientific Publications Ltd. All rights reserved
13) Ozacar, M., 2003. Equilibrium and kinetic
modeling of adsorption of phosphorous on
calcined alunite, Adsorption 9, 125-132.
14) Zili Liu., FengPeng and Xiaoguo Liu., 2012.
Adsorption of heavy metals by sodium
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