Indian Journal of Chemical Technology
Vol. 18, May 2011, pp. 188-196
Non-conventional low-cost adsorbent from Euphorbia antiquorum L for the
removal of Direct Blue 53 from its aqueous solution
P Sivakumara* & P N Palanisamyb
a
Department of Chemistry, Arignar Anna Government Arts College, Namakkal 637 002, India
b
Department of Chemistry, Kongu Engineering College, Erode 638 052, India
Received 4 January 2010, accepted 21 February 2011
This paper reports the adsorption potential of activated carbon prepared from Euphorbia antiquorum L wood by H3PO4
impregnation in batch mode experiments. The effects of various process controlling parameters such as solution pH,
adsorbate concentration and temperature are analyzed. Maximum of 58.40 mg/g of dye adsorbed at a pH of 2.0 and the
adsorption decreases with increase in pH. Various kinetic and isotherm models are employed to analyze the feasibility and
mechanism of adsorption. The prepared adsorbent removed 145.45 mg/g of dye at an initial dye concentration of 100 mg/L
and the kinetics obeys pseudo second order. Freundlich isotherm fits exceptionally well with good r2 value. Thermodynamic
parameters like ∆G°, ∆H° and ∆S° are evaluated. The positive enthalpy proves that the adsorption process is endothermic in
nature. Experiments are also performed for recovery of dye loaded carbon through regeneration as a function of pH, the
maximum desorption occurs at a pH of 10 is only 19.29%. The prepared sorbent can be conveniently employed for the
colour removal as its adsorptive capacity is comparable with commercial adsorbents.
Keywords: Euphorbia antiquorum L, Activated carbon, Adsorption, Surface area, Desorption
More than 10,000 dyes have been widely used in
textile, paper, rubber, plastics, leather, cosmetic,
pharmaceutical, and food industries, which generated
huge volume of wastewater every year1. Some of the
ill effects of dye bearing wastewater are aesthetic
pollution of environment and also carcinogenicity due
to their degradation products. Several techniques like
electrochemical coagulation, reverse osmosis, nano
filtration and adsorption using activated materials are
used for the removal of dye from wastewater2.
Adsorption using activated carbon is widely favoured
because of their high adsorption capacities and
amphoteric properties which enables its adsorption of
both anionic and cationic dyes3,4.
Numerous adsorbents have been prepared from
agricultural by-products and waste plant materials like
canes from easy growing wood species5,6, agro
wastes7, eucalyptus bark8, babool wood9 and
bamboo10. Exploration of good low cost adsorbent
may contribute to the sustainability of the
environment and also offer promising benefits for the
commercial purpose.
The objective of the present work is to evaluate the
sorption of Direct Blue 53 from its aqueous solution
______________________
*Corresponding author (E-mail: shivagobi@yahoo.com)
onto Euphorbia antiquorum L wood activated carbon
prepared by H3PO4 impregnation method. The wood
cannot be used for any purpose, even for combustion
the wood burns with high smoke, this has tempted the
authors to use this wood for the preparation of an
activated carbon. To effectively apply this class of
sorbents, there is a need to understand the kinetics and
mechanism of interaction between the dye molecules
and the sorbent particle.
Materials and Methods
Chemical activation of the precursor
Euphorbia antiquorum L wood was used as raw
material for the preparation of activated carbon.
About the precursor, it is widespread throughout
peninsular India, it can be found growing up to an
altitude of 800 m, the vernacular name of the plant is
“Triangular Spurge”. One of the largest armed tree in
Euphorbias with an average height of 5-7 m, it has
been known to attain gigantic proportions if left
undisturbed. The odour of its latex is pungent and
lingering, it is easily propagated from seed or
vegetatively.
The wood was cut into pieces of 2 to 3 cm size,
dried in sunlight for 10 days. The dried material
soaked in a boiling solution of 10% H3PO4 for 1 h and
SIVAKUMAR & PALANISAMY: NON-CONVENTIONAL ADSORBENT FOR REMOVAL OF DIRECT BLUE 53
kept at room temperature for 24 h. After 24 h the
wood material separated, air dried and carbonised in
muffle furnace at 400°C. The carbonised material was
powdered and activated in a muffle furnace at 800°C
for a period of 10 min. Then the material was washed
with plenty of water to remove residual acid until the
washings becomes neutral. The washed carbon was
dried, sieved to a particle size of 300 to 800 µm
(50 to 20 ASTM standard mesh) and stored in a tight
lid container for further adsorption studies. Physicochemical characters of the activated carbon was
studied as per the standard testing method11,12. The
BET surface area of the activated carbon was
measured at 77 K using N2 gas sorption analyzer
(Nova 1000, Quanta Chrome Corporation, USA). Inorder to determine the pore characteristics, the N2
adsorption-desorption isotherms was studied using
ASAP 2020 Gas sorption analyzer. Some important
physicochemical characteristics are given in Table 1.
Adsorbate
All the chemicals used are reagent grade. A Direct
dye (Direct Blue 53) having molecular formula
(Mol wt.: 960.82) with CI No.: 23860, λmax: 611 nm
(E. Merck, India) was chosen as the adsorbate. The
structure of the selected dye is shown in Fig. 1. The
dye content of the Direct Blue 53 taken for the
analysis is 85% and the same has been taken in to
consideration for the preparation of stock solution. A
stock solution containing 1000 mg of the dye per litre
was prepared by dissolving the dye in double distilled
water and was used to prepare the adsorbate solutions
by appropriate dilution as required.
Table 1  Physico-chemical characteristics of EAAC
S.No.
1
2
3
4
5
6
7
8
9
10
Properties
Values
pH
Moisture content, %
Bulk density, g/mL
Porosity, %
pHZPC
Surface area (BET), m2/g
Fixed carbon, %
Total pore volume (cm3/g)
Mesoo-pore volume (cm3/g)
Micro-pore volume (cm3/g)
6.90
7.56
0.48
55.32
6.82
918
57.94
0.4141
0.1778
0.2363
Fig. 1  Structure of Direct Blue 53
189
Adsorption studies
The adsorption experiments were carried out by
shaking 100 mg of adsorbent with 200 mL of dye
solution of 25 to 100 mg/L concentration at 200 rpm
in a temperature controlled orbital shaker. All the
kinetic and isotherm studies (except effect of pH)
were carried out at a pH of 8.22 (the natural pH of dye
in distilled water). Adsorption isotherm data were
obtained at 30, 40 and 50°C by shaking 100 mg of the
adsorbent with 200 mL of dye solution of various
concentrations by fixing the agitation time as
120 min. The mixture was withdrawn at specified
interval then centrifuged using electrical centrifuge
(universal make) at 5000 rpm for 10 min and
unadsorbed supernatant liquid was analyzed for the
residual dye concentration using Elico make Bio-UV
visible spectrometer (BL-198) at 611 nm. The amount
of dye adsorbed at onto carbon was calculated by the
following mass balance relationship:
qt =
( C0 − Ct ) × V
W
... (1)
where, Co and Ct are the initial and equilibrium liquidphase dye concentrations (mg/L) respectively, V is the
volume of dye the solution (L), and W is the weight of
adsorbent (g).
The effect of pH was studied by adjusting the pH
of the adsorptive solution by using 0.1 N NaOH and
0.1 N HCl. All the experiments conducted in
duplicate and mean of the two values are taken for
calculation. Maximum deviation was ± 4 %. In order
to simplify the discussion, hereafter in this paper
Direct Blue 53 will be designated as DB53 and the
Euphorbia antiquorum L activated carbon will be
designated as EAAC.
Desorption studies
Desorption studies as a function of pH were
conducted to analyze the possibility of reuse the
adsorbent for further adsorption and to make the
process more economical. After adsorption
experiments the dye loaded carbon washed gently
with double distilled water to remove any unadsorbed dye, if present. Desorption studies were
conducted using several such carbon samples. 500 mg
of the dye loaded carbon agitated above the
equilibration time with 50 mL of double distilled
water of various pH and the desorbed dye was
estimated as stated in the adsorption studies12,13.
190
INDIAN J. CHEM. TECHNOL., MAY 2011
Results and Discussion
Effect of agitation time and initial dye concentration on
adsorption
The amount of DB53 removed by EAAC increases
from 43.48 to 145.45 mg/g while increasing the initial
concentration from 25 to 100 mg/L (Fig. 2a). Rapid
uptake of dye noticed at the initial 20 min of contact
time (1.74 mg/g/min at 25 mg/L to 5.35 mg/g/min at
100 mg/L of initial dye concentration). Adsorption
rate decreases from 0.145 mg/g/min at 25 mg/L to
0.641 mg/g/min at 100 mg/L and reaches equilibrium
at 90 min. The initial rapid uptake is due to the
concentration gradient created by the vacant
adsorbent surface between adsorbate in solution and
adsorbate on the carbon. The increase in dye
concentration eases the resistance and makes more
contact between dye and solvent. Availability of dye
molecules in the vicinity of adsorbent also increased
while increasing the concentration, which results in
high uptake of dye at higher concentration14.
The results indicate that the adsorption reaches
equilibrium at 90 min, beyond which there is no
change in the adsorption capacity and therefore
120 min is fixed as equilibration time for isotherm
studies.
pH is increased, the number of negatively charged
sites increases and positively charged sites decreases.
Poor adsorption of DB53 at higher pH value is due to
the competition between the negatively charged
hydroxyl ions and anionic dye for the sorption sites16.
Effect of temperature on dye removal
Figure 2b represents the uptake of DB53 onto
EAAC at 30, 40 and 50°C keeping the initial dye
concentration at 50 mg/L. The equilibrium sorption
capacity of DB53 onto EAAC increases from
80.86 mg/g to 91.49 mg/g while increasing the
temperature from 30 to 50°C. The increase in uptake
with increase in temperature indicates the adsorption
of DB53 by EAAC is an endothermic process, the
same was conformed by the thermodynamic studies.
Fig. 2  Effect of agitation time on the adsorption DB53 onto
EAAC (a) initial Concentration Variation (b) temperature
variation
Effect of pH
Initial pH is one of the most important
environmental factors influencing the solution
chemistry of the dyes. Hydrolysis, complexation by
organic and/or inorganic ligands, redox reactions,
precipitation are strongly influenced by pH and, on
the other side, strongly influence the speciation and
adsorption availability of the dyes15.
The effect of pH for the adsorption of DB53 onto
EAAC over a pH range of 2 to 12 is represented in
Fig. 3. The uptake of DB53 decreased from
58.40 mg/g to 16.00 mg/g when the solution pH was
increased from 2 to 12. The maximum uptake of
DB53 by EAAC was obtained at pH 2.0. When the
Fig. 3  Effect of pH on the adsorption and desorption of DB53
onto EAAC
SIVAKUMAR & PALANISAMY: NON-CONVENTIONAL ADSORBENT FOR REMOVAL OF DIRECT BLUE 53
Kinetics of adsorption
Many kinetic models have been proposed to
elucidate the mechanism of solute adsorption. The
rate and mechanism of adsorption is controlled by
various factors like physical and/or chemical
properties of adsorbent as well as mass transfer
process. These kinetic models are useful for the
design and optimization of effluent-treatment process.
In order to investigate the mechanism of DB53
adsorption by EAAC the following four kinetic
models were considered.
Pseudo first-order kinetic model
The pseudo first-order kinetic model was proposed
by Lagergren17.
The integrated form of the model is
log(qe − qt ) = log qe −
k1
t
2.303
… (2)
where, qe is the amount of dye adsorbed at
equilibrium (mg/g), qt is the amount of dye adsorbed
at time t (mg/g), k1 is the first-order rate constant
(min -1) and t is time (min).
Hence, a linear trace is expected between the two
parameters, log(qe-qt) and t, provided the adsorption
follows first order kinetics. The values of k1 and qe can
be determined from the slope and intercept.
The results of a plot log(qe-qt) versus t at various
initial dye concentrations and temperatures (figure not
shown) are given in Table 2. The calculated
equilibrium sorption capacity qe(cal) and the qe(exp)
191
values have shown large deviation at various initial
dye concentrations as well as at various temperatures.
The Lagergren pseudo first order rate law failed to
explain the adsorption of DB53 by EAAC with poor
fit at various concentrations and temperatures. The
adsorption data for the whole range of temperatures
studied shows deviation from the straight line.
Pseudo second-order kinetics
The adsorption may also be described by pseudo
second-order kinetic model18 if the adsorption does
not follow the first-order kinetics. The linearized form
of the pseudo second-order model is
t
1
1
=
+ t
2
qt k2 qe
qe
… (3)
where, k2 is the second-order rate constant
(g/mg/min).
A plot of t/qt and t should give a linear relationship
if the adsorption follows second-order. The qe and k2
can be calculated from the slope and intercept of the
plot.
The pseudo second-order plot at various initial dye
concentrations and temperatures are given in Figs 4a
and 4b and results are given in Table 2. The
equilibrium sorption capacity qe(cal) and qe(exp) are
in close agreement at various concentrations as well at
various temperatures of study. From the results it is
clear that the initial sorption rate, h, and equilibrium
sorption capacity qe, increases with increasing the
temperature and initial dye concentration. The
Table 2  Calculated kinetic parameters for the adsorption of DB53 onto EAAC at various initial concentrations
Concentration (mg/L)
25
Pseudo first order kinetics
k1 (min -1)
qecal (mg/g)
qeexp.(mg/g)
r2
Pseudo second order kinetics
k2 × 10-4 (g/mg min)
h
qecal(mg/g)
r2
Elovich model
α (mg/g min)
β (g/mg)
r2
Intra particle diffusion model
kid (mg/g min)
r2
50
75
Temperature (°C)
100
30
40
50
0.0679
38.57
43.48
0.9950
0.0507
57.61
80.85
0.9821
0.0509
82.52
114.18
0.9907
0.0560
123.25
145.45
0.9703
0.0504
57.57
80.86
0.9360
0.0469
58.04
85.11
0.9216
0.0392
54.79
91.49
0.9703
41.00
8.23
45.87
0.9969
20.16
20.57
85.47
0.9964
14.17
22.08
120.48
0.9948
9.04
15.03
156.25
0.9979
20.16
20.57
85.47
0.9964
28.63
22.73
88.50
0.9972
25.54
14.73
94.34
0.9966
13.24
0.099
0.9807
23.58
0.054
0.9825
33.82
0.039
0.9837
37.46
0.029
0.9966
23.58
0.054
0.9825
231.62
0.092
0.9815
494.80
0.072
0.9512
2.44
0.9985
4.32
0.9537
6.59
0.9770
10.51
0.9681
4.32
0.9537
3.19
0.9890
4.12
0.9963
INDIAN J. CHEM. TECHNOL., MAY 2011
192
activation energy for chemisorption (g/mg). A plot of
qt versus ln t gives a linear trace (figure not shown)
with a slope of (1/β) and an intercept of 1/β ln (α β).
The initial sorption rate, α, increases from 13.24 to
37.46 mg/g/min while increasing the initial
concentration from 25 to 100 mg/L and 23.58 to
494.80 mg/g/min while increasing the temperature
from 30 to 50°C. Similar trend of rapid initial uptake
was observed experimentally. The other coefficient, β,
decreases with increasing the initial dye concentration
and increases with increase in temperature.
Intra particle diffusion model
In the batch mode adsorption process, initial
adsorption occurs on the surface of the adsorbent. In
addition, there is a possibility of the adsorbate to
diffuse into the interior pores of the adsorbent. Weber
and Morris22 suggested the following kinetic model to
investigate the adsorption is intra-particle diffusion or
not. According to this theory
qt = kd . t½
Fig. 4  Pseudo second order plot for the adsorption DB53 onto
EAAC (a) initial Concentration Variation (b) temperature variation
adsorption of DB53 by EAAC is explained well by
pseudo second-order kinetics with a very high
correlation coefficient. The rapid uptake of dye
indicates that the rate determining step could be
chemisorption in nature19.
Elovich model
The Elovich equation is mainly applicable for
chemisorption kinetics. The equation is often valid for
systems in which the adsorbing surface is
heterogeneous20.
The Elovich model is generally expressed as21:
dqt
= α e − β qt
dt
… (4)
Integrating this
conditions, gives
qt =
1
ln(αβ ) +
1
equation
for
the
boundary
ln t
… (5)
β
β
where, α is the initial adsorption rate (mg/g/min) and
β is related to the extent of surface coverage and the
… (6)
where, kd is the rate constant of the above model and
is analyzed by plotting qt versus t½.
A plot of qt versus t½ at various initial dye
concentrations and temperatures are shown in Fig. 5a
and 5b respectively and the results are given in
Table 2. The linear portion of the plot for widerange
of contact time between adsorbent and adsorbate does
not pass through the origin. This deviation from the
origin or near saturation may be done due to the
variation of mass transfer in the initial and final stages
of adsorption23,24. Such a deviation from the origin
indicates that pore diffusion is the only controlling
step24. From the Fig. 6 we could see that there are two
different regions before the equilibrium. The initial
pore diffusion due to external mass transfer
(r2 = 0.9968 to 0.9974) followed by the intra particle
diffusion (r2 is in the range 0.9537 to 0.9885). The
high correlation coefficient in the first region proves
that pore diffusion play a major role for the adsorption
DB53 on to EAAC.
Adsorption isotherm
The equilibrium existence of adsorbate between the
liquid and solid phase is well described by adsorption
isotherms. Experimental isotherm data collected at
different temperatures fit in Langmuir, Freundlich,
Tempkin and Dubinin-Raduskevich adsorption
isotherm models.
SIVAKUMAR & PALANISAMY: NON-CONVENTIONAL ADSORBENT FOR REMOVAL OF DIRECT BLUE 53
Langmuir model
The Langmuir model25 was originally developed to
describe the adsorption of gas on to solid surface. It
suggests the formation of monolayer adsorption and
also the surface is energetically homogeneous. The
Langmuir isotherm can be expressed as;
193
Q0 .bL .Ce
… (7)
(1 + bL .Ce )
Linear form of the rearranged Langmuir model is
qe =
Ce
C
1
=
+ e
qe Q0 .bL Q0
… (8)
where, Ce is the dye concentration in solution at
equilibrium (mg/L), Q0 is a constant related to
adsorption capacity (mg/g) and bL is Langmuir
constant related to energy of adsorption (L/mg).
The constants Q0 and bL can be calculated from the
slope and intercept of the plot of Ce/qe versus Ce. The
results of Langmuir plot (figure not shown) are given
in Table 3. Langmuir adsorption capacity varies from
208.33 to 192.31 mg/g for the range of temperatures
studied. The high adsorption capacity is due to high
surface area and porosity of the adsorbent. The
adsorption capacity is comparable with the results
reported in the literature.
The essential characteristics of Langmuir isotherm
can be expressed by dimensionless constant called
equilibrium parameter, RL26.
RL = 1/ (1+bL.C0)
… (9)
where, bL is the Langmuir constant and C0 is the
initial concentration (mg/L). The value of RL indicates
the nature of the adsorption process as given below
Fig. 5  Intra particle diffusion plot for the adsorption of DB53
onto EAAC (a) initial Concentration Variation and (b)
temperature variation
Fig. 6  Freundlich isotherm plot for the adsorption DB53 onto
EAAC at various temperatures
Table 3  Results of various isotherm plots for the adsorption
of DB53 onto EAAC
Temperature, °C
30
40
50
Langmuir isotherm
Q0 (mg/g)
208.33
200.0
192.31
bL (L/mg)
0.0085
0.0092
0.0098
kL
1.768
1.830
1.883
r2
0.9945
0.9545
0.9404
Freundlich isotherm
n
1.72
1.92
1.94
kf (mg1-1/n L1/n g-1)
20.58
27.99
32.13
r2
0.9956
0.9921
0.9912
Tempkin isotherm
bT (J/mg)
52.73
50.62
52.13
aT(L/g)
0.351
0.431
0.738
r2
0.9118
0.9878
0.9948
Dubinin-Raduskevich
isotherm
qD (mg/g)
1.00
1.00
1.00
B × 10-7(mol2/J2)
4.00
4.00
4.00
E (kJ/mol)
1.581
1.590
1.616
r2
0.9087
0.9518
0.9614
INDIAN J. CHEM. TECHNOL., MAY 2011
194
RL> 1 Unfavourable
RL = 1 Linear
0 < RL < 1 Favourable
RL = 0 Irreversible
 RT 
 ln(aT .C e )
qe = 
 bT 
… (12)
The linear form of Tempkin equation is
The RL value ranges between 0.405 and 0.9218 for
the range of temperatures studied indicate that the
adsorption of DB53 by EAAC is favourable.
qe =
RT
RT
ln aT +
ln C e
bT
bT
… (13)
Freundlich model27
It is a most popular model for a single solute
system, based on the distribution of solute between
the solid phase and aqueous phase at equilibrium28.
The Freundlich equation is expressed as;
q e = k f C e1 / n
… (10)
where, Kf is the measure of adsorption capacity and n
is the adsorption intensity
Linear form of Freundlich equation is
log q e = log k f +
1
log C e
n
where, bT is the Tempkin constant related to heat of
sorption (J/mg) and aT the Tempkin isotherm constant
(L/g),
A plot of qe versus ln Ce (figure not shown) is used
to determine the constants bT and aT. However, the
model failed to explain the adsorption isotherm
compared to other isotherms due to its poor fit for the
experimental data.
Dubinin-Radushkevich isotherm
The isotherm proposed by Dubinin32 has the
following form
… (11)
A plot of log qe versus log Ce (Fig. 6) gives a linear
trace with a slope of 1/n and intercept of log kf. When
1/n is >1.0, the change in adsorbed concentration is
greater than the change in the solute concentration.
The Freundlich model is suitable for use with
heterogeneous surface but can describe the adsorption
data over a restricted range only. It is often found that
when the Freundlich equation is fitted to data at
higher and intermediate concentrations, since the
Freundlich equation does not approach Henry’s Law
of ideal dilute solutions.
The Freundlich parameters kf and n are given in
Table 3. The Freundlich constant, kf increases with
increase in temperature. The value of n is greater than
1.0 indicating the adsorption DB53 onto EAAC is
favourable. Freundlich model is more appropriate to
explain the nature of adsorption with correlation
coefficient of 0.9912 to 0.9956 rather Langmuir
model shows poor fit (r2 = 0.9404 to 0.9945).
Tempkin isotherm
The Tempkin29 isotherm assumes that the fall in the
heat of adsorption is linear rather than logarithmic as
stated in Freundlich expression30. The heat of sorption
of all the molecules in the layer would decrease
linearly with coverage due to sorbate/sorbent
interactions31. The Tempkin isotherm is applied in the
following form
qe = qD .e − Bε
2
… (14)
Linear form of Dubinin-Radushkevich isotherm is
ln qe = ln qD – Bε2
… (15)
where, qD is the theoretical saturation capacity (mg/g),
B is a constant related to the mean free energy of
adsorption per mole of the adsorbate (mol2/J2) and ε is
polanyi potential which is related to the equilibrium
as follows;
ε = RT ln(1+1/Ce)
… (16)
where, R is the Universal gas constant
(8.314 J/mol/K) and Ce is the equilibrium dye
concentration of adsorbate in solution (mg/L).
A plot of ln qe versus ε2 gives a linear trace
(figure not shown) and the constants qD and
B calculated from the slope and intercept respectively.
The mean free energy of adsorption E calculated from
B using the following equation
E = 1/ (2B)½
… (17)
Based on this energy of activation one can predict
whether an adsorption is physisorption or
chemisorption. If the energy of activation is
<8 kJ/mol, the adsorption is physisorption and if the
energy of activation is 8-16 kJ/mol, the adsorption is
SIVAKUMAR & PALANISAMY: NON-CONVENTIONAL ADSORBENT FOR REMOVAL OF DIRECT BLUE 53
195
suggested good affinity of the dye towards the
adsorbent and the adsorption is spontaneous in
nature33.
Negative values of ∆G° indicate that the adsorption
process is favourable and spontaneous in nature. ∆G°
decreased from -1.437 kJ/mol to 1.701 kJ/mol with
increase in temperature from 30 to 50°C.
Desorption studies
Desorption increases with increase of pH from 2 to
10 and there is no considerable change in desorption
above pH 10. As shown in Fig. 3, the maximum
percent of desorption observed at pH 10 was 19.20%.
Fig. 7  Van’t Hoff plot for the adsorption DB53 on to EAAC at
30, 40 and 50°C
Table 4  Thermodynamical parameters for the adsorption
DB53 onto EAAC
Temperature,
°C
∆H°,
kJ/mol
∆S°,
J/K/mol
∆G°,
kJ/mol
30
40
50
2.555
13.716
-1.437
-1.569
-1.701
chemisorption in nature27. The activation energy of
adsorption is found to be 1.581 kJ/mol indicates that
the adsorption is physisorption in nature.
Thermodynamics of adsorption
Thermodynamic parameters like ∆H°, ∆S° and
∆G° were measured based on Van’t Hoff’s plot.
∆S O ∆H O 1
ln bL =
−
R
R T
… (18)
where, bL is the Langmuir equilibrium constant, ∆H°
and ∆S° are the standard enthalpy and entropy
changes of adsorption respectively.
The values of ∆H° and ∆S° are calculated from the
slopes and intercepts of the linear plot of ln bL versus
1/T. The free energy of specific adsorption ∆G°
(kJ/mol) is calculated from the following expression
∆G° = ∆H° – T∆S°
… (19)
Van’t Hoff plot for the adsorption process is given
in Fig. 7 and the values are presented in Table 4.
The mean adsorption enthalpy ∆H° was found to
be 2.555 kJ/mol, indicates that the bonding between
DB53 and EAAC surface is very weak. The positive
enthalpy proves that the adsorption process is
endothermic in nature. Positive values of ∆S°
Conclusions
The present study revealed that activated carbon
prepared from Euphorbia antiquorum L wood by
H3PO4 impregnation can be employed as potential
adsorbent for the removal of direct dyes such as
Direct Blue 53 from its aqueous solution. Rapid
uptake of dye noticed at the initial 20 min of contact
time and the rate decreases thereafter and reaches
equilibrium at 90 min. The adsorption follows pseudo
second-order kinetics. The intra particle diffusion
study reveals that the removal of dye occurs through a
pore diffusion process. Freundlich model is more
appropriate to explain the nature of adsorption with
high correlation coefficient rather Langmuir model
shows poor fit. Thermodynamic parameters accounts
for the feasibility of the process at various
temperatures of study. The maximum percent of
desorption observed at pH 10 was 19.20%.
Acknowledgements
The authors gratefully acknowledge the financial
support granted by the University Grants Commission
(UGC), New Delhi, under the Major Research Project
Scheme to carry out this study.
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