Nandhakumar et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (1-9) July 2014
International Journal of
Research in Chemistry and Environment
Available online at: www.ijrce.org
ISSN 2248-9649
Research Paper
Adsorption of Hexavalent Chromium onto Microwave Assisted Zinc Chloride
Activated Carbon Prepared from Delonix regia Pods
1
Ramesh K., Rajappa A.2, *Nandhakumar V.3
Department of Chemistry, Arasu Engineering College Kumbakonam, INDIA
2
Department of Chemistry, Sri Manakula Vinayagar Engineering College Pondicherry, INDIA
3
Department of Chemistry, A.V.V.M Sri Pushpam College Poondi, INDIA
1
(Received 04th March 2014, Accepted 20th May 2014)
Abstract: An effective adsorbent was prepared from Delonix regia (Flame tree) pods and its various adsorption
characteristics were studied for removal of Chromium (VI) from aqueous solution. Optimized conditions for the
preparation of effective activated carbon were found to be microwave radiation power 850 W, radiation time 12
min, 60 % of ZnCl2 and impregnation time 24 hours. Batch mode adsorption experiments were carried out to assess
the effects of the system variables such as contact time, adsorbent dosage, pH, initial dye concentration and
temperature. Equilibrium was achieved in 80 min for all the studied initial concentrations. The equilibrium
adsorption data were analyzed with six isotherm models. Best fitting isotherm models were in the following order,
Langmuir = Sips > Freundlich > Tempkin > Harkins-Jura isotherm > Dubinin Raduskevich. The adsorption
kinetics was found to follow pseudo-second-order rate kinetic model, with the intra particle diffusion as the rate
determining step. Different thermodynamic parameters, like Gibb’s free energy (ΔG°), enthalpy (ΔH°) and entropy
(ΔS°) of the adsorption process have also been evaluated. Analysis of these values inferred that this adsorption was
endothermic, spontaneous and proceeded with increased randomness.
Keywords: Adsorption, ZnCl2 activated microwave carbon, Isotherms, Kinetics, pH effect, Chromium (VI)
© 2014 IJRCE. All rights reserved
produces brown woody seed pods merely a waste
material [7]. Recently, microwave energy has been widely
used in research and industrial processes [8].
Introduction
Industrial water pollution is a potential threat to
human health mainly because of the non-biodegradable,
hazardous heavy metals. Among these heavy metals
chromium is of considerable concern. Chromium
compounds are widely used in chemical manufacture, in
leather, textile and in other industries. The determination
of Chromium is important because of the contrasting
biological effect of its two common oxidation states,
chromium (III) and Chromium (VI). The former is an
essential metal, while the latter is toxic [1].
Compared with conventional heating techniques,
microwave heating has the following additional
advantages as follows: interior heating, higher heating
rates, selective heating, greater control of the heating
process, no direct contact between the heating source and
heated materials, and reduced equipment size and waste
[9-11]
. Hence microwave radiation is used to prepare
carbon from the plant material instead of conventional
heating methods.
The adsorption of Cr (VI) on various adsorbents
has been reported. They are bituminous coal [2],
sphagnum peat moss [3], coconut husks and palm pressed
fibers [4], sawdust [5], sugarcane bagasse and distillery
sludge [6]. In this present study, Microwave assisted zinc
chloride activated carbon (MWZAC) prepared from
Delonix regia (flame tree) pods is tried for the removal of
chromium (VI). Delonix regia belongs to royal Poinciana
or flamboyant, a member of the bean family which
Material and Methods
Preparation of Adsorbents: The air dried pods were cut
into small pieces and powdered in a pulveriser [7,12]. 20 g
of the powdered pods was mixed with 75 mL of ZnCl2
solution of desired concentration (20, 40 and 60 %). The
slurry was kept at room temperature for 24 hours, to
ensure the access of the ZnCl2 to the Delonix Regia pods.
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Nandhakumar et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (1-8) July 2014
Table 1
Nomenclature
Ci , Ct and Ce
qe and qt
V
W
Qe
Ce
Q0
b
RL
C0
Kf and n
bT
aT
qm
B and A
qD
B
ε
E
R
T
k1
k2
t
h
kp
C
N
Kc
∆G°
∆S°
∆H°
C1 & T1
Nomenclature
Initial Concentration, at the time ‘t’ and at equilibrium respectively
Quantity adsorbed at the time ‘t’ and at equilibrium respectively
Volume of the Chromium solution in liter (L)
Mass of the adsorbent in gram (g)
Amount of solute adsorbed per unit weight of adsorbent (mg/g)
Equilibrium concentration of solute in the bulk solution (mg/L)
Adsorption efficiency
Adsorption energy
Separation factor
Initial concentration of the Chromium (VI) solution
The constants incorporating all factors affecting the adsorption capacity and intensity of
adsorption respectively
Tempkin constant related to heat of sorption (J/mg)
Equilibrium binding constant
Constant related to adsorption capacity (mg/g)
Isotherm constants
Theoretical saturation capacity (mg/g)
Constant related to the mean free energy
Polanyi potential
Mean free energy of adsorption
Gas Constant
Temperature (K)
Rate constant of adsorption (l/min)
Second-order constants
Time in minutes
Initial adsorption rate (mg/g min)
Intra-particle diffusion rate constant
Thickness of the boundary film
Number of data points
Equilibrium constant
Standard free energy
Entropy of adsorption
Enthalpy of adsorption
Initial Concentration and Temperature
Table 2
Physico-chemical characteristics of MWZAC
Properties
Values
pHzpc
Particle size, µm
Surface area (BET), m2/g
Pore volume, cm3/g
Pore size (Pore width), nm
Bulk density, g/mL
Fixed Carbon, %
Moisture content, %
7.01
53 - 90
586
0.3986
2.7174
0.52
71.11
4.36
Then the slurry was subjected to microwave
heating of pre- determined power (450, 600 and 850
watts) for pre- determined duration (8, 10 and 12
minutes).Thus the carbonized samples were washed with
0.5 M HCl followed with hot distilled water and cold
distilled water until the pH of the washings reach 7. Then
the carbon was filtered and dried at 423 K.
Adsorption of Delonix Regia pods carbon with
ZnCl2 generates more interspaces between carbon layers
to more surface area and micro porosity. The increase in
porosity with ZnCl2 activation suggests that the porosity
created by this reactant is due to spaces left by ZnCl2 after
the corresponding washing. ZnCl 2 activation causes
electrolytic action termed as swelling in the molecular
structure of cellulose, which leads to the breaking of
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Nandhakumar et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (1-8) July 2014
lateral bonds in the cellulose molecules resulting in
increased inter and intra voids [13, 14]. Totally 27 number
of carbons were prepared by varying preparation
parameters such as concentration of ZnCl2 solution,
Microwave heating watts power and radiation times [15].
The carbon showing maximum % removal was chosen
for further adsorption study.
flask was agitated using rotary shaker with 180 rpm for
pre-determined duration. Then 1 ml of aliquot was taken
from sample and diluted to 25 ml by double distilled
water, adsorbents were then separated by centrifugation
and concentration of the solution was determined by
diphenyl carbozide method. The percentage removal of
the Cr (VI) from the solution was calculated by the mass
balance relationship. To study the effect of pH were
brought to 2 to 10 by adding Con HCl and 6 N NaOH.
Physico-chemical characteristics of MWZAC:
Physico-chemical characteristics of MWZAC were
presented in the Table 2. Percentage of fixed carbon,
surface area, pHzpc and other values are reasonable to
function as a good adsorbent.
Diphenyl carbozide method: A 0.25% (W/V) solution
of diphenyl carbozide was prepared in 50% (V/V)
acetone. 1 mL of the sample solution was pipette out into
25 mL standard flasks. To this 1 mL of 6 N H 2SO4 was
added followed by 1 mL of diphenyl carbozide and the
total volume was made up to 25 mL using double
distilled water. Concentration of Cr(VI) was estimated
by the intensity of the reddish brown color developed due
to complex formation using Systronics Double Beam
UV-visible Spectrophotometer: 2202 at the wave length
of 540 nm [18, 19].
Preparation of stock Solution: Potassium dichromate
(AR grade) was used as such. The Cr(VI) stock solution
was prepared by dissolving appropriate amount of
accurately weighed Cr (VI) in double distilled water to
a concentration of 1000mg/L. The experimental
solutions were prepared by proper dilution [16-17].
Adsorption experiments: The effect of parameters
such as initial concentration Cr (VI), adsorbent dose and
contact time was studied by batch mode technique
because of its simplicity. Pre-determined dose of the
adsorbent was taken in 250 mL iodine flask and 50 mL
and pre-determined concentration of the Chromium
solution was poured into the flask with pH of the solution
brought to 2 by adding Con. HCl. Then the content of the
Results and Discussion
Effect of contact time and initial concentration
on adsorption of Chromium onto the adsorbent:
The percentage of removal of Cr (VI) from aqueous
solution with respect to contact times and with different
initial concentrations was shown in Figure 1.
Table 3: Data Processing Tools
S. No.
1.
2.
Mass balance
relationships
Kinetic Models
& SSE %
Parameters
% of Removal
Quantity adsorbed at equilibrium, qe
Quantity adsorbed at the time t, qt
Pseudo First order kinetics
(Legergren equation)
Pseudo Second order kinetics
(Ho equation)
The initial adsorption rate h
Intra particle diffusion
(Weber–Morris equation)
Sum of error squares
Langmuir
3.
4.
Isotherms
Thermodynamic
Parameters
Formulae
(Ci - Ct)×V/Ci
(Ci - Ce) ×V/W
(Ci - Ct) × V/ W
log (qe-qt)=log qe - k1 /2.303 × t
t/qt=1
/
k2.qe2
+1/qe
t
h = k2qe2
qt = kpt1/2 + C
SSE (%) = √∑[(qe)exp-(qe)cal]2/ N
C e/Q e = 1/Q0b + Ce /Q0
Separation factor
Freundlich
Tempkin
Sips
Harkins – Jura
Dubinin – Raduskevich,
RL = 1 / (1+ bC0)
log Qe = log K f + 1/n log Ce
qe = RT/bT ln aT + RT/bT ln Ce
Ce 1/n /qe = 1/qm.b + 1/qm Ce1/n
1/qe² = [B/A]-[1/A] log Ce
ln qe = ln qD - Bε2
Polanyi potential
ε = RT ln (1+1/Ce)
Mean free energy of adsorption
Standard Free energy Change
Van’t Hoff equation
E = 1/ (2B) ½
∆G° =-RT ln Kc
ln Kc =∆S°/R - ∆H°/RT
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Nandhakumar et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (1-8) July 2014
Figure 1: Effect of Contact times and with different initial concentrations
Figure 2: First Order Kinetics
Figure 3: Second Order Kinetics
Figure 4: Intra Particle Diffusion
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Nandhakumar et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (1-8) July 2014
The adsorption process is characterized by a
rapid uptake of the adsorbate in the initial stages as
shown by the curves. The adsorption rate however
decreased marginally after the first ten minute and a
nearer constant after 80 minutes. The percentage of
removal increased with the increase in contact time.
However, the percentage of removal of Cr (VI) at
equilibrium decreased w i t h an increase of initial
concentration of the adsorbate. This is due to the
decrease in the ratio between available adsorption sites
and the concentration of solute in the solution [20 – 21]. It
is observed that the amount of solute adsorbed by the
adsorbent, increased with the increase of initial
concentrations of Cr (VI). Similar trend has been
reported in literature [22 – 24].
parameters calculated were given in the Table 4.
Between the first order and second order, second
order kinetic model seems to best describe the above
adsorption system as its R2 values were very close to
unity. Moreover, difference between qe (cal) and qe (exp)
values of second order is small when compared to first
order kinetic model. Statistically it is tested with the tool
Sum of error squares (SSE%)[25]. The ∆qe and SSE %
values were given in the Table 4, from which it was
concluded that second order kinetic model was more
appropriate rather than first order kinetic model.
Plot drawn between mass of Cr (VI) adsorbed
per unit mass of adsorbent at the time t (qt) versus t1/2 is
presented in Figure 4. The linear plots are attributed to
the macro pore diffusion which is the accessible sites of
adsorption. This is attributed to the instantaneous
utilization of the most readily available adsorbing sites on
the adsorbent surface. The values of kp obtained from the
slopes of straight lines are listed in Table 4 [17].
Kinetic models: The adsorption kinetics shows the
evolution of the adsorption capacity through time and it is
necessary to identify the types of adsorption mechanism
in a given system. Plots of different kinetic models
applied were given in the Figure 2 & 3 and the kinetic
Table 4
Kinetic parameters for the removal of Chromium VI by MWZAC
First Order Kinetics
C1
(ppm)
10
15
20
T1
(K)
305
315
325
305
315
325
305
315
325
Intra Particle
Diffusion
Second Order Kinetics
k1
(min-1)
qe(cal)
(mg/g)
qe(exp)
(mg/g)
R
0.032
0.051
0.122
0.039
0.055
0.138
0.039
0.60
0.119
10.12
10.99
14.49
16.11
16.48
22.69
19.99
20.28
25.70
19.75
20.25
20.75
23.50
24.50
25.5
26.50
28.00
29.50
0.803
0.800
0.909
0.843
0.918
0.992
0.918
0.951
0.984
2
SSE
%
k2 ×10-3
(g/mg.
min)
qe(cal)
(mg/g)
h
R2
SSE
%
kp
(mg/g.
min)
R2
3.21
3.08
2.08
2.46
2.67
0.93
2.17
2.57
1.26
12.50
18.44
38.10
06.80
11.27
22.21
05.00
09.10
16.20
20.41
20.83
21.28
25.00
25.64
26.71
28.52
29.41
30.30
05.21
08.00
17.25
04.25
07.41
15.84
04.07
07.87
14.87
0.996
0.998
0.999
0.995
0.998
0.999
0.994
0.998
0.999
0.22
0.19
0.17
0.50
0.38
0.40
0.67
0.47
0.26
0.939
0.939
1.013
1.400
1.713
1.677
2.192
2.192
2.156
0.990
0.990
0.979
0.973
0.977
0.878
0.990
0.990
0.932
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Figure 5: Isotherms
Table 5
Isotherm parameters for removal of Chromium VI by MWZAC
Langmuir Isotherm
Temperature
(K)
Q0
(mg/g)
305
315
325
29.41
32.25
33.33
b
(L/mg)
10
ppm
Freundlich Isotherm
RL
15
ppm
20
ppm
0.872
0.10
0.07
0.05
0.838
0.857
Sips Isotherm
b
qm
(L/mg)
(mg/g)
0.775
29.41
0.762
32.25
0.769
33.33
Dubinin Raduskevich
R2
n
0.996
0.995
0.994
5.18
4.81
4.55
kf
(mg1-1/n.L1/n.g-1
R2
17.02
17.62
18.32
Temkin Isotherm
bT
aT
(g/mg)
(L/g)
60.27
40.39
70.64
30.49
79.67
25.84
Harkins – Jura Isotherm
0.994
0.993
0.993
Temperature
(K)
305
315
325
5.18
4.81
4.55
Temperature
(K)
qD
(mg/g)
B×10-4 (mol2/J2)
E
(kJ/mol)
R2
Concentration
(ppm)
A
B
R2
305
315
325
25.77
27.09
28.48
0.30
0.20
0.20
0.129
0.158
0.158
0.907
0.903
0.899
10
15
20
38.46
09.90
15.62
1.62
0.29
0.39
0.998
0.808
0.997
n
Isotherm studies: The existence of equilibrium
between the liquid and solid phase is well described by
adsorption isotherms. Equilibrium data collected at
different temperatures were fitted in Langmuir,
Freundlich, Tempkin, sips, Harkins - jura, and DubininRaduskevich adsorption isotherm models [26]. These
isotherms are depicted in Figure 5.
R2
0.996
0.995
0.994
R2
0.987
0.984
0.982
Langmuir isotherm well describes the present system
that is the existence of identical adsorption site. R2
value of Dubinin-Raduskevich isotherm is very low. In
Dubinin-Raduskevich isotherm, the very low value of
the constant ‘B’ related to the mean free energy of
adsorption per mole of the adsorbate has no significance
to decide whether the adsorption is physical or chemical
in nature. Results of various isotherms are presented in
Table 5.
R2 values of these isotherm plots reveal that
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Nandhakumar et al. Int. J. Res. Chem. Environ. Vol. 4 Issue 3 (1-8) July 2014
Table 6
Thermodynamic parameters for removal of Chromium (VI) by MWZAC
Concentration
(ppm)
10
15
20
Thermodynamic Parameters and their results
Temperature
ΔG°
ΔH°
kd
(K)
kJ/mol
kJ/mol
305
09.40
-5.68
315
10.65
-6.19
10.73
325
12.20
-6.76
305
04.70
-3.92
315
05.32
-4.38
08.65
325
05.79
-4.74
305
02.81
-2.62
315
03.18
-3.03
10.04
325
03.59
-3.45
ΔS°
kJ/mol
53.81
41.27
41.53
Figure 6: Effect of pH
forms such as HCrO4-, Cr2O72- and CrO4- in aqueous
solution and stability of these forms is dependent on the
pH of the system. The active form of Cr (VI) adsorbed is
HCrO4-. This form is stable at only lower pH range which
leads to high removal of chromium. But the concentration
of this form decreases with the increase of pH [27-29].
Effect of temperature: Increase of temperature
increased the percent removal. The change in standard
free energy, enthalpy and entropy of adsorption were
calculated. Thermodynamic parameters like ΔH°, ΔS°
and ΔG° were determined using Van’t Hoff’s plot, which
are given in Table 6 Negative standard free energy of
adsorption indicates that the adsorption process is
spontaneous in nature. The positive ∆H° values infer the
endothermic nature of adsorption, which was confirmed
by the experimental data i.e., adsorption capacity
increased with the increase of temperature, as shown in
the Table 6. Since ΔH° values are small the bonding
between Chromium (VI) and MWZAC surface should be
very weak. Positive value of ∆S° suggests that the
adsorption proceeds with increased randomness [18].
Conclusion
Microwave assisted zinc chloride activated
carbon (MWZAC) was prepared from Delonix regia
(Flame tree) pods found to have good capacity of
adsorption. Experimental data indicated that MWZAC
was effective in removing Chromium (VI) from aqueous
solution. Equilibrium adsorption was achieved in about
80 minutes for the dosage of 20 mg/50 mL of solution at
room temperature of 305 K for the initial concentration of
Chromium (VI) solutions ranging from 10 to 20 mg/L.
Kinetic studies revealed that the process of adsorption
follows pseudo second order kinetics. Langmuir and Sips
isotherm represents the equilibrium adsorption data well
when compared to other isotherms studied. The
separation factor RL values indicated that the adsorption
was favourable. Thermodynamic study revealed that the
adsorption system was spontaneous, endothermic with
increased randomness.
Effect of pH: Figure 6 shows the effect of initial pH of
the solution on the removal of Chromium (VI). The pH of
the solution is an important variable which controls the
adsorption. Hence, the influence of pH on the adsorption
of Chromium (VI) ions onto activated carbon was
examined in the pH range of 2 to 10. The adsorption
capacities of Chromium (VI) ions onto MWZAC
increased significantly, with decreasing pH value and the
maximum removal was attained at pH (2.0) [6]. At lower
pH, the Cr (VI) removal efficiency was higher at higher
pH the removal reduced considerably 1-2.The reason for
the high removal percent of Cr (VI) at lower pH range
was explained as below. The Cr (V) exists in different
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