Adsorption of benzoic acid onto high specific area activated carbon cloth

Journal of Colloid and Interface Science 284 (2005) 83–88
www.elsevier.com/locate/jcis

Adsorption of benzoic acid onto high specific area activated carbon cloth
Erol Ayranci ∗ , Numan Hoda, Edip Bayram
Chemistry Department, Akdeniz University, 07058 Antalya, Turkey
Received 20 August 2004; accepted 19 October 2004
Available online 18 December 2004

Abstract
The adsorption of benzoic acid from aqueous solution onto high area carbon cloth at different pH values has been studied. Over a period of
125 min the adsorption process was found to follow a first-order kinetics and the rate constants were determined for the adsorption of benzoic
acid at pH 2.0, 3.7, 5.3, 9.1, and 11.0. The extents of adsorption and the percentage coverage of carbon cloth surfaces were calculated at
125 min of adsorption. Adsorption isotherms at pH values of 2.0, 3.7, and 11.0 were derived at 25 ◦ C. Isotherm data were treated according
to Langmuir and Freundlich equations and the parameters of these equations were evaluated by regression analysis. The fit of experimental
isotherm data to both equations was good. It was found that both the adsorption rate and the extent of adsorption at 125 min were the
highest at pH 3.7 and decreased at higher or lower pH values. The types of interactions governing in the adsorption processes are discussed
considering the surface charge and the dissociation of benzoic acid at different pH values.
 2004 Elsevier Inc. All rights reserved.
Keywords: Adsorption; Benzoic acid; Carbon cloth; Surface charge

1. Introduction
Adsorption of organic molecules from aqueous solution
on activated carbon is a widely used method in raw and
wastewater treatments and in food, beverage, pharmaceutical, and chemical industries [1]. The adsorption capacity of
activated carbon is related to its surface area, pore structure,
and surface chemistry. The surface chemistry of activated
carbon is characterized by heteroatoms that compose the surface such as oxygen, nitrogen, hydrogen, sulfur, and phosphorous [2]. Those heteroatoms are in the form of functional
groups such as ketones, carboxyls, phenols, ethers, lactones,
or nitro groups and they have a significant effect on the
chemical character, acidity, and degree of hydrophobicity of

the carbon surface [3,4]. The characteristics of the adsorbate
also influence the adsorption process. These characteristics
are molecular size, solubility, pK, and the nature of adsorbate molecules. Ionic strength and pH of the medium affect
the adsorption process by controlling electrostatic interac* Corresponding author. Fax: +90-242-227-89-11.

E-mail address: (E. Ayranci).
0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2004.10.033

tions between the adsorbent and the adsorbate. The carbon
surface charge and the dissociation or protonation of the adsorbate are determined mainly by the pH of the solution. The
carbon surface charge will be positive when the pH is lower
than the pH at the point of zero charge of the surface (pHpzc )
and will be negative when pH is higher than pHpzc [5].
Benzoic acid constitutes a simple model for complex matrices that may be present in the aqueous phase. Therefore
there are many reports in the literature on the adsorption
of benzoic acid from the aqueous phase on various materials such as activated carbon, synthetic calcite, soil, metal
hydroxides, mineral surfaces, silica, calcite, dolomite, and
some metal oxides [6–14]. In recent literature the use of
high specific area carbon cloth appears to be an attractive
alternative for selection of the adsorbent. For example, studies on adsorption and electrosorption at high area carbon
cloth have been reported for various adsorbates such as inorganic S-containing anions [15], ethylxanthate and thiocyanate [16,17], phenol, phenoxide and chlorophenols [18],
some aromatic heterocyclic compounds [19], pyridine [20],
and some pesticides [21,22] in relation to wastewater purification.


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E. Ayranci et al. / Journal of Colloid and Interface Science 284 (2005) 83–88

The aim of the present study is to determine the kinetics and equilibrium states of adsorption of benzoic acid onto
high specific surface area activated carbon cloth. A systematic study is conducted in order to observe the influence of
pH on the adsorption process.

2. Materials and methods
2.1. Materials
The carbon cloth used in the present work was obtained
from Spectra Corp. (MA) coded as Spectracarb 2225, having a specific area of 2500 m2 g−1 (measured using the Kr,
BET method by the manufacturer). Benzoic acid, hydrochloric acid, and sodium hydroxide were obtained from Merck
(Germany). Deionized water was used in the adsorption experiments.
2.2. Treatment of the carbon cloth
The carbon cloth material was found [15] to provide
spontaneously a small but significant quantity of ions into
the conductivity water used, probably due to its complex
structure originating from its somewhat unknown proprietary preparation procedure. A deionization cleaning procedure was therefore applied, as described previously to avoid
desorption of ions during the adsorption measurements [15].
In this procedure, a carbon cloth sample was placed in
a flow-through washing cup and eluted with 5 L of warm
(60 ◦ C) conductivity water in a kind of successive batch operation for 2 days with N2 bubbling in order to avoid possible
adsorption of CO2 that might have been dissolved in water.
The outflow water from each batch was tested conductometrically for completeness of the washing procedure. The
washed carbon cloth modules were then dried under vacuum
at 120 ◦ C and kept in a desiccator for further use. The carbon
cloth was cut to desired dimensions (about 0.5 × 1.5 cm) and
weighed accurately.
2.3. Adsorption cell
A specially designed cell was used to carry out the adsorption studies and simultaneously to perform in situ concentration measurements by means of UV absorption spectrophotometry. The cell (Fig. 1) was V shaped with one arm
containing the carbon cloth attached to a short Pt wire sealed
to a glass rod and the other arm containing a thin glass tube
through which N2 gas was passed for the purposes of mixing and eliminating any dissolved CO2 . The two arms were

connected to a glass joint leading to a vacuum pump at the
upper part of the V-shaped cell in order to provide the opportunity for initial outgassing of the carbon adsorbent, and the
cell and solution. A quartz spectrophotometer cuvette was
sealed to the bottom of the adsorption cell.

Fig. 1. Diagram of the adsorption cell.

2.4. Optical absorbance measurements
Adsorbate solutions were prepared by dissolving a fixed
amount of benzoic acid in water and adjusting the pH to
2.0, 3.7, 5.3, 9.1, and 11.0 by additions of 0.1 M HCl or
0.1 M NaOH and diluting to a final volume to keep the
benzoic acid concentration the same in all solutions. pH
values of solutions were measured with a pH meter (Jenway 3040 ion analyzer). Calibration curves were prepared
at each pH value to convert the absorbance data of kinetic
and equilibrium experiments to concentration data. A Shimadzu 160A UV/Vis spectrophotometer was used for optical
absorbance measurements. The absorbance measurements
were conducted in situ during the study of the kinetics of adsorption process as follows. In all experiments, the size and
the mass of the carbon cloth were kept as constant as possible (about 18.0 ± 0.1 mg). Its mass was accurately measured
and recorded each time for calculation of fractional coverage, θ , or the amount of adsorption per unit area, M, of the
carbon cloth. Carbon cloth pieces were prewetted by leaving
in water for 24 h before use. The idea of using prewetted carbon cloth originates from our previous findings that
prewetting enhances the adsorption process [15,16].
Carbon cloth was dipped into the adsorption cell initially
containing only water and vacuum was applied to remove
all air in the pores of the carbon cloth. Then wetted and degassed carbon cloth was removed from the cell for a short
time and water in the cell was replaced with a known volume of sample solution (20 mL). The sliding door of the
sample compartment of the spectrophotometer was left half
open and the quartz cuvette fixed at the bottom of the adsorption cell (which now contained the sample solution) was

E. Ayranci et al. / Journal of Colloid and Interface Science 284 (2005) 83–88

inserted into the front sample compartment. A Teflon tube
connected to the tip of a thin N2 -bubbling glass tube was
lowered from one arm of the adsorption cell down the UV
cell to a level just above the light path to provide effective mixing. Finally, the carbon cloth, which was removed
temporarily after wetting and degassing, was inserted from
the other arm of the adsorption cell into the solution. Then,
quickly, an opaque curtain was spread above the sample
compartment of the spectrophotometer, over the cell, to prevent interference from external light.
The program for monitoring the absorbance at the specific wavelength of maximum absorbance predetermined by
taking the whole spectrum of benzoic acid was then run on
the built-in microcomputer of the spectrophotometer. Absorbance data were recorded in programmed time intervals
of 1 min over a period of 125 min.
Absorbance data were converted into concentration data
using calibration relations predetermined at the wavelength
of interest for the corresponding pH of benzoic acid.
2.5. Determination of adsorption isotherms
The adsorption isotherms of benzoic acid on carbon cloth
were determined on the basis of batch analysis. Carboncloth pieces of varying masses were allowed to equilibrate
with benzoic acid solutions of known initial concentration
at 25 ◦ C for 48 h. Preliminary tests showed that the concentration of benzoic acid remained unchanged after 8–10 h of
contact with the carbon cloth. So, the allowed contact time
of 48 h ensures the equilibration. The initial concentration of
benzoic acid solutions was 1.96 × 10−4 M at pH 3.7 and 2.0
and 1.96 × 10−5 M at pH 11.0. The equilibrium concentrations of benzoic acid solutions were measured spectrophotometrically. The amount of benzoic acid adsorbed per unit
mass of carbon cloth, qe , was calculated by
V (c0 − ce )
,

(1)
m
where V is the volume of benzoic acid solution in L, c0
and ce are the initial and equilibrium concentrations, respectively, of the benzoic acid solutions in mmol L−1 and m is
the mass of the carbon cloth in grams. Equation (1) gives qe
in millimoles benzoic acid adsorbed per gram carbon cloth.
qe =

85

which this difference is zero, i.e., initial and final pH are the
same, was determined to be the pHpzc value. The pHpzc value
of the carbon cloth used in this study was found to be 7.4.

3. Results and discussion
3.1. Absorption characteristics and calibration data for
benzoic acid at different pH
In Table 1, absorption characteristics and calibration data
for benzoic acid are given at different pH. Absorption maximum gradually shifts from 230 nm at pH 2.0 to 224 nm
at pH 5.3 and remains the same up to pH 11.0 while the
molar absorptivity varies between 10,800 and 8000 in this
pH range. Calibration data were evaluated according to
Lambert-Beer law by the method of least-squares analysis
with excellent correlations as indicated by the regression coefficients given in the last column of Table 1.
3.2. Kinetics and extents of adsorption of benzoic acid at
different pH
The initial concentrations of benzoic acid and the amount
of carbon cloth used were kept constant for kinetic studies
of the adsorption process at different pH in order to make a
comparative study. The initial concentration of benzoic acid

was 1.96 × 10−4 M and the amount of carbon cloth module
was 18.0 ± 0.1 mg. The decrease in concentration of benzoic acid with time as it is adsorbed onto carbon cloth at
different pH values is shown in Fig. 2. The natural pH of
1.96 × 10−4 M benzoic acid was 3.7. Solution of benzoic
acid at pH 2.0 was prepared by adding required amounts of
0.1 M HCl and solutions of benzoic acid at pH 5.3, 9.1, and
11.0 were prepared by adding required amounts of 0.1 M
NaOH while monitoring from the pH meter. It is seen from
Fig. 2 that the extent of adsorption during the period of
125 min is the highest at the natural pH of 3.7 and decreases
at higher or lower pH values.
A more quantitative comparison can be made in the extent
of adsorption of benzoic acid at different pH by introducing
two related terms: the amount of adsorbate adsorbed per unit
area of the carbon cloth, M, given by Eq. (2) and the percent-

2.6. Determination of pHpzc of carbon cloth
The batch equilibrium method described by Babiˇc et al.
[23] for the determination of pH at the point of zero charge
was used. Carbon cloth samples (0.08 g) were shaken in Erlenmeyer flasks for 24 h with 50 ml of 0.1 M NaNO3 at
different initial pH values, which were adjusted by adding
NaOH or HNO3 solutions. At the end of contact period, the
H+ and OH− ion concentrations were measured with the pH
meter. Then the amounts of OH− and H+ ions adsorbed were
calculated by subtracting the last measured concentrations of
H+ and OH− ions from the initial concentrations. The pH at

Table 1
Spectrophotometric and calibration parameters for benzoic acid at different
pH values

pH

λmax (nm)

ε (au cm−1 M−1 )a

rb

2.0
3.7
5.3
9.1
11.0

230
228
224
224
224

10,800
9200
8150
8000
8100

0.9982
0.9999
0.9994
1

0.9999

a ε is the molar absorptivity and au stands for absorbance unit.
b r is the correlation coefficient for fit of data to Lambert-Beer’s law.


86

E. Ayranci et al. / Journal of Colloid and Interface Science 284 (2005) 83–88

Fig. 2. Adsorption behaviors of benzoic acid from aqueous solutions at
different pH values onto the carbon cloth. The initial concentration is
1.96 × 10−4 M. The mass of the carbon cloth is 18.0 ± 0.1 mg.

age coverage at the carbon cloth surface, θ , given by Eq. (3),
M = (c0 − ct )V /2500m,
θ = (c0 − ct )V NA × 100

(2)
(4 × 10

19

× 2500m),

carbon cloth calculated from Eqs. (2) and (3) at 125 min
of adsorption are given in Table 2 at different pH values.
The extent of adsorption of benzoic acid according to the
θ and M values at 125 min decreases in the pH order of
3.7 > 2.0 > 5.3 > 9.1 > 11.0.

For the kinetic investigation of adsorption of benzoic acid
onto carbon cloth, the concentration versus time data were
treated according first-order law by plotting ln(c0 /ct ) as a
function of time. The linearity of such plots supports the validity of the assumption of first-order law and furthermore
the slopes of the lines provide the first-order rate constant
(k) for the adsorption process. Linear regression analysis of
the data provided the rate constants given in the fifth column
of Table 2 for the adsorption of benzoic acid at different pH
values. The last column of Table 2 shows the regression coefficients, r. The closeness of r to 1 supports the idea that
the adsorption process follows the first-order kinetics. When
the rate constants are examined, it can clearly be seen that
the adsorption rate is the highest at the natural pH of 3.7 and
very slow at pH values greater than 5.3. It should be noted
that the rate constant also decreases from a natural pH of 3.7
to a more acidic pH of 2.0. The interactions between benzoic acid and the carbon surface, which may explain these
kinetic results, will be discussed in Section 3.4.

(3)

where c0 and ct are the concentrations of the solutions at
the beginning and at a specific time during the adsorption
process, respectively. V is the volume of the solution, m
the mass of the carbon cloth module, and NA Avogadro’s
number. The calculations are based on the known specific
surface area of 2500 m2 g−1 for the carbon cloth provided
by the manufacturer, corresponding to an approximate value
of 4 × 1019 carbon sites/m2 of the surface determined by
the atomic radius of carbon but dependent on the actually
unknown geometry of surface carbon-atom packing. The
θ and M values for the adsorption of benzoic acid onto

3.3. Adsorption isotherms of benzoic acid at different pH
The equilibrium adsorption isotherms for benzoic acid
at different pH values were derived at 25 ◦ C. The qe data
versus equilibrium concentration (ce ) were treated according to well-known isotherm equations of Langmuir and Freundlich. The linear forms of Langmuir and Freundlich equations are given in Eqs. (4) and (5), respectively,
1
ce
ce
=
+
,
qe bqm qm
ln qe = ln K + (1/n) ln ce ,

(4)
(5)

Table 2
Parameters for the extent of adsorption (M, θ) and for the kinetics of adsorption (k, r) of benzoic acid at various pH values
pH

c0
(mol L−1 )

M
(10−8 mol (m2 C cloth)−1 )

θ

k

(min−1 )

r

2.0
3.7
5.3
9.1
11.0

1.96 × 10−4
1.96 × 10−4
1.96 × 10−4
1.96 × 10−4
1.96 × 10−4

6.68
7.40
3.69
3.04
2.00

0.101
0.111
0.056
0.046
0.030

0.0108
0.0143

0.0038
0.0031
0.0017

0.9964
0.9954
0.9925
0.9773
0.9598

Table 3
The parameters of Langmuir and Freundlich isotherm equations for the adsorption of benzoic acid at three pH
pH

2.0
3.7
11.0

Langmuir

Freundlich

qm
(mmol g−1 )

b
(L mmol−1 )

r

K
(mmol g−1 )(mmol L−1 )1/n

1/n

r

0.93
1.38
0.07

181.7
40.76
108.2

0.9970
0.9863
0.9804

2.70
6.24
0.38

0.35
0.59
0.53

0.9807
0.9887
0.9804

E. Ayranci et al. / Journal of Colloid and Interface Science 284 (2005) 83–88

87

Fig. 3. The fit of experimental adsorption data (Q) to Langmuir (---) and
Freundlich (—) models for benzoic acid at pH 2.0.

where qe is the amount of solute adsorbed per unit mass of
adsorbent (mmol g−1 ), ce is the equilibrium concentration of
solute (mmol L−1 ), qm is the amount of solute adsorbed per
unit mass of adsorbent required for monolayer coverage of
the surface (mmol g−1 ), b is a constant related to the heat
of adsorption (L mmol−1 ), K ((mmol g−1 )(L mmol−1 )1/n )
and 1/n are constants which can be related to adsorption
capacity and the strength of adsorption or to the surface heterogeneity, respectively. The parameters of these equations
obtained by linear regression analysis of the experimentally
derived data are given in Table 3 together with regression
coefficients, r.
The fits of experimental data for benzoic acid at pH 2.0,
3.7, and 11.0 to Langmuir and Freundlich models are shown
in Figs. 3, 4, and 5, respectively. Visual observation from
these figures and the regression coefficients, r, given in the
forth and seventh columns of Table 3 show that the experimental adsorption data fit almost equally well to both
isotherm models of Langmuir and Freundlich. The adsorption capacity of carbon cloth for benzoic acid is the highest
at pH 3.7 as indicated by the qm parameter of the Langmuir model and the K parameter of the Freundlich model
(Table 3). This result is in agreement with the conclusion
reached upon the kinetic treatment discussed in the previous
section.

According to the classification of Giles et al. [24] all the
isotherms at the three pH values are of L type. A characteristic of L-type isotherms is that there is no strong competition
between the solvent and the adsorbate to occupy the adsorbent surface sites. Observation of L-type isotherms also
implies that the adsorbate molecule is not vertically oriented
at the surface [24].
3.4. Interactions of benzoic acid with carbon surface
It is known that at pH values lower than pHpzc the surface is positively charged [2,5,6,25,26]. Since pHpzc for the

Fig. 4. The fit of experimental adsorption data (Q) to Langmuir (---) and
Freundlich (—) models for benzoic acid at pH 3.7.

Fig. 5. The fit of experimental adsorption data (Q) to Langmuir (---) and
Freundlich (—) models for benzoic acid at pH 11.0.

carbon cloth is 7.4, the carbon surface is positively charged
below this pH value. On the other hand the natural pH of
1.96 × 10−4 M benzoic acid solution is 3.7 at which benzoic acid is ∼80% in anionic form of benzoate (pKa : 4.2).
Therefore at this pH the main interaction between the carbon surface and the adsorbate is expected to be electrostatic
attraction in nature. This explains the greatest extents of adsorption observed at this pH (Fig. 2, Table 2). At pH 2.0
the calculations show that benzoic acid is mainly in neutral
molecular form with only ∼0.6% dissociation. Although the
surface is more positively charged at this pH than that at
pH 3.7, because of the loss of negative charge on the adsorbate there is a slight decrease in the extents of adsorption


88

E. Ayranci et al. / Journal of Colloid and Interface Science 284 (2005) 83–88

(Fig. 2, Table 2). The types of interactions between the surface and the adsorbate at this pH (2.0) are expected to be

mainly dispersion interactions between π electrons of benzoic acid and π electrons in basal plane of the carbon cloth
in addition to some residual electrostatic interactions. At
pH 5.3, although the benzoic acid is almost completely in
anionic benzoate form, due to the decrease in surface positive charge compared to that at pH 2.0 or pH 3.7, there is a
decrease in the extents of adsorption (Fig. 2). Both electrostatic and dispersion forces are expected to be effective for
the interaction between adsorbate and carbon surface at this
pH (5.3). At pH 9.1 and 11.0 (which are greater than pHpzc )
the carbon surface is negatively charged and benzoic acid is
in anionic form. Electrostatic repulsion between the adsorbate and the carbon surface must be operative at these pH
values leading to the observation of the lowest extents of adsorption (Fig. 2, Table 2). Very small amounts of adsorption
observed at these pH values may result from π –π dispersion
interactions.

4. Conclusions
Adsorption of benzoic acid onto high specific area carbon cloth was found to follow first-order kinetics at different pH values. Experimental adsorption isotherm data at
25 ◦ C fitted well to both Langmuir and Freundlich equations. The rate and extent of adsorption were found to be
the highest at pH 3.7, which corresponds to the natural pH
of 1.96 × 10−4 M benzoic acid, in the pH range from 2.0 to
11.0. The very small rates and extents of adsorption observed
at pH 9.0 and 11.0 were attributed to increased electrostatic
repulsions between benzoate anion and the negative surface
charge at these pH values.

Acknowledgments
The authors thank the Scientific Research Projects Unit
of Akdeniz University for the support of this work through
project 2003.01.0300.009, the Spectra Corp. (MA), for pro-

viding the activated carbon cloth, and the Central Laboratory
Unit of Faculty of Agriculture of Akdeniz University for the

use of their facilities.

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