Indian Journal of Chemical Technology
Vol. 15, November 2008, pp. 533-540
Sorption of phenol from aqueous solution using activated carbon
prepared from Manilkara zapota seed
Kaushik Nath*, Mehul Thummar, Mahesh Vaghela & Pranab Jani
Department of Chemical Engineering, G H Patel College of Engineering & Technology, Vallabh Vidyanagar 388 120,
India
Email: nath_kaushik@hotmail.com
Received 6 May 2008; revised 12 September 2008
The potential of activated carbon prepared from the seeds of Manilkara zapota, an agricultural waste, was assessed for adsorptive
dephenolation from aqueous solution. ZnCl2 was used as the activating agent. Batch adsorption experiments were conducted to study the
effect of various physicochemical parameters such as initial concentration, dose of adsorbent, initial pH, and temperature. The
percentage removal of phenol was found to increase with the decrease in initial concentration of phenol. Maximum removal efficiency of
96% was achieved with 25 mg/L of initial phenol concentration at pH 4.0 and temperature 30oC. Equilibrium modelling by linearized
adsorption isotherms revealed that Freundlich isotherm could well represent the observed data for phenol adsorption on activated carbon
as compared to Langmuir isotherm. Thermodynamic studies revealed that the sorption of phenol by activated carbon is an endothermic
process, showing increase in sorption at higher temperature. Comparison of various kinetic models based on correlation coefficients
revealed that the pseudo second order model, an indication of chemisorption mechanism, fits better the experimental data than the
pseudo first order Lagergren model.
Keywords: Activated carbon, Adsorption, Phenol, Equilibrium, Kinetics
Phenols, often consisting of a variety of hydroxy benzenes or substituted
hydroxyl benzenes, are the most prevalent form of pollutant in the
chemical industry. Their presence in water bodies is noticed with bad
taste and odour1. In the presence of chlorine in drinking water, phenols
form chlorophenol, having a medicinal taste and which is quite
pronounced and objectionable2,3. Phenols are considered as priority
pollutants since they are harmful to organisms at low concentrations.
Ingestion of a small amount of phenol by human beings may cause
undesirable effect in the form of nausea, vomiting, skin irritation, and even
leads to capillary damage4. Phenols impart undesirable carbolic odour to
water even at extremely low concentration and are lethal to fish even at
concentrations of 5 to 25 mg/L5.
Dephenolation technologies include the classical solvent extraction
technique, adsorption on activated carbon, ion exchange, steam gas
stripping, photochemical conversion6 and emulsion membrane technique
using the principle of facilitated transport7. Klein and Lee8, have indicated
probable technologies for the treatment of wastewater containing phenol
and thiocyanate which include chlorination, ozonation, coagulation and
flocculation. The choice of treatment depends on effluent characteristics,
the economics involved and the standards set by government agencies.
Adsorption onto the surface of activated carbon and other adsorbents is
still by far the most widely used method for treating domestic and
industrial effluents9. Many investigators have studied the adsorptive
removal of phenolic compounds using various traditional as well as non
traditional adsorbents, e.g., palm seed coat activated carbon 10, silica gel
sludge11, bituminous shale12, low cost clay13, rice husk ash14, activated
bentonites15, iron-oxide coated sand16 and so on. Notwithstanding its
prolific use as an effective adsorbent, activated carbon remains as an
expensive material. In view of the tedious procedures for the preparation
and variable performance of carbon regeneration low cost potential and
single use adsorbents are desirable.
In practice, agrowaste materials have emerged as a better candidate for the
production of commercial activated carbons over the conventional raw
materials such as bituminous coal, peat and lignite. Utilization of
agricultural wastes is of great significance in India where more than 200
million tons of agricultural residues are generated annually17. The
utilization of these wastes provides additional employment and income to
marginal farmers and landless agricultural laborers, especially in
developing countries like India. Agricultural wastes, used for production
of activated carbons include nutshells 18, fruit stones19, Tamarind nut20, and
various other agricultural residues. Basically, there are two different
processes for the preparation of activated carbon: physical activation and
chemical activation.
Chemical activation process enjoys several advantages over the physical
activation process. One is the lower temperature in which the process is
accomplished. The other is that the global yield of the chemical activation
tends to be greater since burnoff char is not required. Among the
numerous dehydrating agents, zinc chloride in particular is the widely
used chemical agent in the preparation of activated carbon. Knowledge of
different variables during the activation process is very important in
developing the porosity of carbon sought for a given application.
Chemical activation by ZnCl2 improves the pore development in the
carbon structure, and because of the effect of chemicals, the yields of
carbon are usually high4.
The feasibility of activated carbon prepared from Manilkara zapota seeds,
as low cost sorbents, for dephenolation of wastewater has been assessed in
this study. Various parameters of adsorption are studied to find out the
optimum conditions for adsorption of phenol onto activated carbon thus
prepared.
Experimental Procedure
Preparation of activated carbon
Manilkara zapota seeds collected from local market were washed with
distilled water to remove the water-soluble adherent impurities. This was
followed by drying in an air oven at 80°C to get rid of the moisture and
other volatile impurities. The dried seeds were grounded and sieved to a
particle size range of 150-200 m. Chemical activation of the powdered
seeds was done with ZnCl2. In the zinc chloride activation an important
factor is the degree of impregnation. It is defined as the ratio of the weight
of active agent added to the weight of carbonizing material. Carbons were
prepared from 1.0 to 0.25 at 700 oC for 1 h. For carbonization, 10 g of
dried seeds were well mixed with 100 mL of a solution that contained 10 g
of ZnCl2. The mixing was performed at 50°C for 1 h. After mixing, the
slurry was subjected to vacuum-drying at 100°C for 24 h. The resulting
chemical loaded sample was placed in a crucible and heated to the final
carbonization on temperature under a nitrogen flow rate of 150 mL min -1
STP. The product was washed sequentially with 0.5 N HCl, hot water, and
finally cold distilled water to remove adhered impurities and then dried at
110°C. The experiments were performed using different chemical ratios
(100-500%) and carbonization temperatures (200-600oC). Weight loss of
the carbon sample was calculated on a chemical-free basis from the
differences of weight of product before and after washing4. The optimum
INDIAN J. CHEM. TECHNOL., NOVEMBER 2008
534
chemical ratio (mass basis), temperature and carbonization time were
found to be 200%, 500oC and 1 h respectively. The physicochemical
characteristics of the prepared activated carbon are presented in Table 1.
The relationship between the phenol uptake capacity qe (mg/g) of
adsorbent and the residual phenol concentration Ce (mg/L) at equilibrium
is given by Freundlich isotherm as:
Batch adsorption study
Table 1―Physicochemical characteristics of activated carbon prepared
from Manilkara zapota seeds
1
ln qe  ln k  ln Ce
n
Parameter
Values
0.78
Bulk density (g mL1)
Moisture (%)
4.31
Ash (%)
9.15
Solubility in water (%)
6.65
Solubility in 0.25 M HCl (%)
32.25
Iodine number
275
0.75
Surface area (m2g1)
Porosity (%)
15.5
Batch adsorption kinetics and equilibrium studies were carried out using
the bottle point isotherm technique by placing a known quantity of the
adsorbent in glass bottles containing 100 mL of an aqueous solution of
phenol with a predetermined concentration. Analytical grade phenol (SD
Fine Chemicals) was used to prepare a stock solution of phenol. The
bottles were placed over a magnetic stirrer and temperature was
maintained until equilibrium was attained. At the end of adsorption
process, the adsorbent particles were filtered out through Whatman No. 42
filter paper and the equilibrium concentration of phenol in the supernatant
was analyzed. For pH study, the pH of the solution was adjusted using
dilute NaOH and H2SO4. The adsorption amount was calculated as
follows:
q
v(c1  c 2 )
w
... (1)
where q is the adsorption amount (mg/g), w the weight (g) of the prepared
activated carbon, v the volume of solution, c1 and c2 are the concentrations
(mg/L) of phenol before and after adsorption respectively.
Analyses
The concentration of residual phenol in the sorption medium was
determined spectrophotometrically (Model Systronics 106). The
absorbance of the coloured complex of phenol with 4-amino antipyrine
was recorded at wave length, = 500 nm. The pH of the feed solution was
measured by a pH electrode (Systronics). All the results presented in this
work are the average of a minimum of duplicate experiments. Variation
estimates between two different sets of experimental data points were
carried out using the data analysis and technical graphics software
Microcal Origin 5.0 and the variation was within ±5%.
Langmuir and Freundlich isotherms
The Langmuir and Freundlich models are most widely used to describe the
experimental data of adsorption isotherms. In the present work both the
models were used to describe the relationship between the amount of
phenol adsorbed and its equilibrium concentration for activated carbon.
The basic assumption of the Langmuir isotherm model is the formation of
a monolayer of adsorbate on the outer surface of the adsorbent and no
further adsorption thereafter. The linear form of Langmuir model is
expressed as:
Ce
1 Ce


qe Qb Q
... (2)
where qe is the amount of adsorbate adsorbed per unit weight of adsorbent
(mg/g) and Ce is the equilibrium concentration of the adsorbate (mg/L).
The constants Q and b are Langmuir constants. The values of Q and b are
calculated from the intercept and slope of the plot of Ce/qe versus Ce.
… (3)
where the intercept ln k is a measure of adsorbent capacity and the slope
1/n is the sorption intensity. The Freundlich model assumes that the uptake
of any adsorbate occurs on a heterogeneous surface by multilayer
adsorption and that the amount of adsorbate adsorbed increases infinitely
with an increase in concentration. The values of k and n are calculated
from the intercept and slope of the plot of ln qe versus ln Ce respectively.
Results and Discussion
Effect of adsorbate concentration
Fig. 1―Effect of adsorbate concentration on the removal of phenol by
prepared activated carbon (Batch adsorption, temperature: 30oC, adsorbent
dose: 15 g/L, pH: 4.0)
Effect of initial concentration of phenol on adsorption by activated carbon
was studied at four different initial concentration (10, 25, 50 and 100
mg/L) at a fixed dose of adsorbent (10 g/L). Maximum contact time of
adsorption was 140 min. During the experiment pH (4.0) and temperature
(30oC) were also kept constant. The results of these experiments are
depicted in Fig. 1. The percentage removal of phenol was found to
increase with the decrease in initial concentration of phenol. Maximum
removal
efficiency
of
96%
was
achieved
with
25 mg/L of initial phenol concentration. However, with higher initial
concentration of phenol the percentage removal dropped significantly. For
an initial concentration of 50 and 100 mg/L removal efficiency was 83 and
68% respectively. This might be due to the lack of available active sites at
high concentration resulting in increased competition for the adsorption
sites and the adsorption process increasingly slows down. Similar
observation is also reported by the adsorption of eosin dye on activated
carbon by Purkait et. al.21.
Figure 1 also reveals that up to 100 mg/L of initial phenol concentration in
the feed, the maximum percentage removal was accomplished after a
contact time of 120 min and thereafter no appreciable change took place.
It indicates that for lower initial concentration of phenol in the feed, the
adsorption was very fast. Similar trends of the equilibrium contact time of
phenol adsorption are reported in case of silica gel sludge11 (contact time:
90 min) and activated bentonites15 (contact time: 140 min). The plot of
percentage removal of phenol versus contact time resulted in a non-linear
exponential curve indicating that the process of removal of phenol by
adsorption is first order with respect to time22.
Effect of adsorbent dose
The effect of the adsorbent dose on removal of phenol was studied by
varying the dose of adsorbent from 5 to 40 g/L at fixed pH, temperature
and initial concentration (Fig. 2). It has been observed from
Fig. 2, that percentage removal of phenol increased with the increase in
the dose of adsorbent and maximum adsorption was recorded at adsorbent
NATH et al.: MANILKARA ZAPOTA SEED CARBON FOR SORPTION OF PHENOL
dose of 15 g/L beyond which no further increase was noticed. This could
be due to the increased availability of active adsorption sites and surface
area resulting from the conglomeration of the adsorbents especially at
higher adsorbent dose23. The plots of percentage removal of phenol versus
dose of adsorbent were found to be exponential for three different initial
concentration of phenol (Fig. 2) indicating that the amount of phenol
adsorbed varied in accordance with a fractional power term of the dose of
adsorbent [for example (dose)n, where n=fraction]. This suggests that the
adsorbed phenol either blocked the access to the initial pores or caused
particles to aggregate, thereby reducing the active site availability.
Effect of initial pH
Since the surface charge of an adsorbent could be modified by changing
the pH of the solution, pH plays an important role in the removal of
phenols by adsorption process. The extent of dephenolation by activated
carbon were studied at various pH values ranging from pH 2 to pH 8 with
constant optimum conditions. The results of the effect of pH on adsorption
of phenol are depicted in Fig. 3. The figure revealed that highest removal
efficiency was observed at a pH of 4.0 irrespective of the initial phenol
concentration. However, at higher pH values the percentage removal of
phenol decreased significantly. This behaviour can be explained in the
light of the types and ionic state of the functional groups on the surface of
the adsorbent as well as ionic chemistry of the solution. The ionic
fraction of phenolate ion ions can be estimated from the following
correlation24.
Fig. 2―Effect of adsorbent dosage on the removal of phenol by
adsorption onto the prepared activated carbon (Batch adsorption,
temperature: 30oC, pH: 4.0)
ions 
1
[1  10
( pka  pH )
]
… (4)
With the increase in pH of the solution ions increases. As a result phenol,
which is a weak acid (pKa =10), will be adsorbed to a lesser extent at
higher pH values due to the repulsive force prevailing at higher pH value.
Adsorption of phenol up to pH 4.0 suggests that the negatively charged
phenolate ions bind through electrostatic attraction to positively charged
functional groups on the surface of activated carbon, because at this pH
more functional groups carrying positive charge would be exposed 4. But at
pH above 4.0, it seems that the prepared activated carbon possesses more
functional groups carrying a net negative charge, which tends to repulse
the anions. Moreover, in the higher pH range phenols form salts, which
readily ionize leaving negative charge on the phenolic groups. Hence,
beyond pH 4.0, removal of less phenol took place and the rate of removal
reduced significantly. It merits mentioning here that above
pH 4.0 other mechanisms like physical adsorption may play an important
role in the adsorption of phenol and the exchange mechanism might be
affected. The results agree well with those obtained by Banat et al.24, and
Halouli et al.25, for the adsorption of phenols onto activated charcoal and
bentonite clay respectively. It is worthwhile to mention that the pH of the
solution was monitored before and after the adsorption and no noticeable
change in pH was observed.
Effect of temperature
The adsorption studies were carried out at three different temperatures, 30,
40 and 50oC while keeping the feed concentration and adsorbent dose (15
g/L) constant (Fig. 4). It was observed that with an increase in
temperature, adsorption capacity increased, thereby indicating the
adsorption as an endothermic process. Similar endothermic nature of
adsorption is reported in the adsorptive removal of chlorophenols from
water by bituminous shale12 and adsorption of phenol onto low cost clay13.
Maximum adsorption was recorded at 50 oC. The observed experimental
findings may be explained by the fact that with the increase in
temperature, the number of active sites available for adsorption must have
increased resulting in enhancement of percentage adsorption of phenol
onto activated carbon. Another possibility of decreasing the thickness of
the boundary layer surrounding the adsorbent with a rise in temperature
can also not be ruled out since this tends to reduce the mass transfer
resistance of adsorbate in the boundary layer26. Moreover, as diffusion is
an endothermic process, the increase in uptake of phenol may also be due
to an enhanced rate of intraparticle diffusion of sorbate. An increase in
pore diffusivity of adsorbate with temperature leads to increased removal
of phenol by its adsorption.
The thermodynamic parameters of the adsorption were determined using
the following equations:
G o  H o  TS o
... (5)
q 
S o
 H o
log  e  

 Ce  2.303 R 2.303 RT
Fig. 3―Effect of pH on the removal of phenol by adsorption onto the
prepared activated carbon (Batch adsorption, temperature: 30oC, adsorbent
dose: 15 g/L)
535
… (6)
INDIAN J. CHEM. TECHNOL., NOVEMBER 2008
536
The mechanism of adsorption depends on the physical and/or chemical
characteristics of the adsorbent as well as on the mass transfer process.
The kinetics of adsorption is important from the point of view that it
controls the process efficiency28. Various researchers have used several
kinetic models, where the adsorption has been treated as a first order29
pseudo first order30 and pseudo second order31 process. In the present
study, adsorption of phenol on activated carbon prepared from Manilkara
zapota seeds has been described by first and pseudo second order models
and also by a diffusion model.
Lagergren model
Lagergren proposed a pseudo-first order kinetic model. The integral form
of the model is
log( qe  q)  log qe 
Fig. 4―Effect of temperature on the removal of phenol by adsorption onto
the prepared activated carbon (Batch adsorption, adsorbent dose: 15 g/L,
pH: 4.0)
Table 2―Thermodynamic parameters for the adsorption of phenol on the
prepared activated carbon
Concentration ΔHo
of phenol
(kJ mol-1)
(mg/L)
-ΔSo
(kJ mol-1
K-1)
ΔGo
(kJ mol-1)
303 K
313 K
323 K
Kad
t
2.303
… (7)
where q is the amount of phenol sorbed (mg/g) at time t (min), qe is the
amount of phenol adsorbed at equilibrium (mg/g) and Kad is the
equilibrium rate constant of pseudo-first order adsorption (min-1). The plot
of log (qe–qt) versus t gave a straight line for the first order adsorption
kinetics (not shown).
Pseudo –second-order model
The expression for the pseudo-second order rate equation is given by
1
1
t


qt k2 qe 2 qe
… (8)
10
20.15
0.007
11.13
8.12
7.46
25
21.13
0.006
11.04
8.56
7.95
50
17.12
0.005
12.12
8.78
7.56
100
19.25
0.042
11.56
6.69
7.41
where qe is the maximum amount of phenol adsorbed per unit mass of the
activated carbon (mg/g), Ce is the equilibrium concentration (mg/L), R is
the gas constant (8.314 J mol/K) and T is temperature in Kelvin. It is
worth mentioning here that the experimental data considered for
calculation of thermodynamic parameters (ΔGo, ΔSo and ΔHo) are in the
linear range of equilibrium adsorption isotherm. Hence, Eq. (3) can be
used with the experimental data to evaluate entropy of adsorption ΔSo and
enthalpy of adsorption ΔHo from a plot of log (qe/Ce) versus 1/T. The
value of Gibbs free energy is then calculated from Eq. (5). The values of
these thermodynamic parameters, for four initial feed concentrations viz.
10, 25, 50 and 100 mg/L of phenol are given in Table 2. The negative
value of ΔSo suggests that there is little reduction of randomness as a result
of probable change in internal structure of activated carbon during the
adsorption of phenol27.
Adsorption isotherms
Adsorption data of phenol on activated carbon at three different
temperature viz., 30, 40, and 50oC were fitted with both the Langmuir and
Freundlich isotherms. The coefficients of these two isotherm models have
been shown in Table 3. The data provide information on the maximum
amount of activated carbon required to adsorb a particular mass of phenol
under specified system conditions. The values of n were 4.67, 3.34 and
3.32 at three different temperatures viz. 50, 40 and 30oC respectively
(Table 3). It is known that when the n value is greater than 1.0, conditions
are favourable to adsorption. Hence it indicates that adsorption of phenol
onto activated carbon agrees well with the Freundlich isotherm model
(Fig. 5). Correlation coefficients are also calculated by fitting the
experimental adsorption equilibrium data for the phenol-activated carbon
system, using both the isotherms (Table 3). It was found that the
adsorption isotherm for the phenol-activated carbon system could be
represented better by the Freundlich isotherm.
Adsorption kinetics
Fig. 5―Freundlich isotherm model for the adsorption of phenol on to the
prepared activated carbon (Adsorbent dose 15 g/L, initial phenol
concentration: 25 mg/L)
NATH et al.: MANILKARA ZAPOTA SEED CARBON FOR SORPTION OF PHENOL
537
adsorbate does not pass through the origin. Such deviation from the origin
or near saturation can be attributed to the difference in rate of mass
transfer in the initial and final stages of adsorption. Further, this deviation
also indicates that the pore diffusion is the only rate controlling step. A
perusal of Fig. 7 reveals that there are two distinct regions – the initial
pore diffusion due to external mass transfer effects followed by the
interparticle diffusion. The observations agree well with those obtained by
Allen et al.33, during the adsorption of basic dye on to sphagnum peat.
Conclusion
Fig. 6―Pseudo second order kinetics for the adsorption of phenol onto the
prepared activated carbon (Initial phenol concentration, 10, 25, 50 and
100 mg/L)
Table 3―Langmuir and Freundlich adsorption isotherm constants for
adsorption of phenol on prepared activated carbon
Temperature Langmuir constant
(oC)
Q
b (L/mg)
R2
Freundlich constants
k
n
R2
50
4.45
3.22×10-3
0.97
63.23 4.67 0.98
40
4.93
3.51×10-3
0.965 60.55 3.34 0.98
30
5.12
3.97×10-3
0.97
57.12 3.32 0.99
In Eq. (8), k2 (g/mg min) is the rate constant for the pseudo second-order
adsorption kinetics. The slope of the plot (t/qt) versus t (Fig. 6) gives the
value of qe and from the intercept k2 can be calculated.
Diffusion model
The intraparticle diffusion model is based on the theory proposed by
Weber and Morris32. According to this theory
q  kd t
… (9)
where kd is the rate constant of intraparticle diffusion (mg g-1min-1/2). The
applicability of this model can be examined by the linear plot of q versus
t1/2. The pore diffusion coefficient for the intraparticle transport of phenol
was calculated assuming spherical geometry of the sorbent using
following equation
t 1/ 2 
0.03ro
D
2
… (10)
where ro is the radius of the sorbent,
D
the pore diffusion coefficient
(cm2 s-1) and t1/2 is the time for half sorption. The value of D was found
out to be 3.39 ×1010 cm2s-1. The applicability of the above three models
can
be
examined
by
each
linear
plot
of
log (qe–qt) versus t (not shown), (t/qt) versus t, and q versus t1/2
respectively (Figs 6 and 7). To quantify the applicability of each model,
the correlation coefficient R2, was calculated from these plots. The
linearity of these plots indicates the applicability of the three models.
However, the correlation coefficient R2 showed that the pseudo second
order model, an indication of chemisorption mechanism, fits better the
experimental data (R2 > 0.997) than the pseudo first order Lagergren
model (R2, 0.956 –0.973). The interparticle diffusion had also some role in
the adsorption of phenol by activated carbon (Fig. 7). The linear portion
of the plot for a wide range of contact time between adsorbent and
Fig. 7―Kinetics of phenol removal according to the intraparticle diffusion
model (Initial phenol concentration, 10, 25, 50 and 100 mg/L)
Under the experimental conditions investigated, the best conditions for the
production of high surface area activated carbon from Manikara zapota by
chemical activation are chemical ratio (activating agent/precursor) of
200%, carbonization time of 1 h, and carbonization temperature of 500°C.
The percentage removal of phenol was found to increase with the decrease
in initial concentration of phenol. Maximum removal efficiency of 96%
was achieved with 25 mg/L of initial phenol concentration at pH 4.0 and
temperature 30oC. Freundlich isotherm was found to be the most suitable
for phenol adsorption on activated carbon as compared to Langmuir
isotherm. The pseudo second order model, an indication of chemisorption
mechanism, fits better the experimental data than the pseudo first order
Lagergren model However, the interparticle diffusion also plays some role
in the adsorption of phenol by activated carbon.
References
Patterson J W, Wastewater Treatment Technology (Ann Arbor Science: Ann Arbor,
MI), 1997.
Zumriye A & Yener J, Waste Management, 21 (2001) 695.
Costa E, Calleja G & Marjuan L, Adsorption Sci Technol, 5(3) (1988) 213.
Mohanty K, Jha M, Meikap B C & Biswas M N, Ind Eng Chem Res, 44 (2005)
4128.
Brown V M, Jordan D H M & Tiller B A, Water Res, 1 (1967) 587.
Ollis D F, Pelizzetti F & Serpone N, Phosolysis: Fundamentals and Application,
edited by N Serpone & E Pelizzetti (Wiley, New York): 603 (1989).
Cahn R P & Li N M, Sep Sci, 9 (1974) 505.
Klein J A & Lee D D, Biotechnol Bioeng Symp, 8 (1978) 379.
Fornwalt H J & Hutchins R A, Chem Eng, 73 (1966) 179.
Rengaraj S, Seuny-Hyeon M & Sivabalan R, Waste Management, 22 (2002) 543.
Idris A & Saed K, The Environmentalist, 23(4) (2003) 329.
Tutem E, Apa R & Unal C F, Water Res, 32(8) (1998) 2315.
Nayak P S & Singh B K, Deasalination, 207 (2007) 71.
Verma V K & Mishra A K, Indian J Chem Technol, 15 (2008) 140.
Al-Asheh S, Banat F & Abu-Aitah L, Sep Purif Technol, 33 (2003) 1.
Singh D K, Srivastava B & Yadav P, Indian J Chem Technol, 9 (2002)
285.
Reddy M C S, J Sci Ind Res, 65 (2006) 443.
Ahmadpour A & Do D D, Carbon, 35 (1997) 1723.
Lussier M G, Shull J C & Miller D J, Carbon, 32 (1994) 1493.
Ramadevi A & Srinivasan K, Indian. J Chem Technol, 12 (2005) 407.
538
INDIAN J. CHEM. TECHNOL., NOVEMBER 2008
Purkait M K, Dasgupta S & De S, J Environ Management, 76(2) (2005) 135.
Kannan N & Srinivasan T, Indian J Environ Protec, 18 (1997) 194.
Ucer A., Uyanik A & Aygun S F, Sep Purif Technol, 47(3) (2006) 113.
Banat F A, Al-Bashir B, Al-Asheh S & Hayajneh O, Environ Pollut, 107 (2000)
391.
Halouli K A & Drawish N M, Sep Sci Technol, 30 (1995) 3313.
Coskun R, Soykan C & Sacak M, Sep Purif Technol, 49(2) (2006) 107.
Manju G N, Raji C & Aniruddhan T S, Water Res, 32 (1998) 3062.
Jain A K, Gupta V K & Bhatnagar A, Sep Sci Technol, 38 (2003) 463.
Haribabu E H, Upadhya Y D & Upadhyay S N, Intian J Environ Studies, 43 (1993)
169.
Pandey K K, Prasad G & Singh V N, Water Res, 19 (1985) 869.
Tutem E, Apek R & Unal C F, Water Res, 32 (1998) 2315.
Ho Y S, Ng J C Y & McKay G, Sep Sci Technol, 36 (2001) 241.
Weber W J & Morris J C, J Sant Eng Div ASCE, 89 (1963) 31.
Allen S J, Mckay G & Khader K Y H, Environ Pollut, 56 (1989) 39.