Removal of Cationic Dye (Methylene Blue) from Aqueous
Solution by Sludge Ash
Control/tracking Number: #488-AWMA-2005; Section #WM-3h
Chih-Huang Weng* and Yi-Fong Pan
Department of Civil and Ecological Engineering, I-Shou University, Da-Hsu Hsiang,
Kaohsiung 84008, Taiwan (*author for correspondence, email: chweng@isu.edu.tw)
ABSTRACT
Kinetics and equilibrium adsorption experiments were conducted to evaluate the adsorption
characteristics of a cationic dye (methylene blue, MB) onto bio-sludge ash. Results showed
that the ash could remove the dye effectively from aqueous solution. The adsorption rate was
fast and about 80% of absorbed-MB was removed in 10 min. The adsorption kinetics could
be expressed by the modified Freundlich equation and intra-particle diffusion model. It was
found that both the initial MB concentration and ionic strength could affect the rate of
adsorption. The effect of electrical double layer thickness on the adsorption kinetics was
discussed. The equilibrium adsorption data were correlated well to the non-linear multilayer
adsorption isotherm. The maximum adsorption capacities for MB were 7.3×10-6, 6.3×10-6,
and 5×10-6 mol/g, respectively, at temperature of 4oC, 14oC, and 24oC. Values of the firstlayer adsorption energy, Go, ranged from –6.62 to –7.65 kcal/mol, suggesting that the
adsorption could be considered as a physical process, which simultaneously enhanced by the
electrostatic effect. The multilayer adsorption energy, Go, ranging from –4.51 to –4.82
kcal/mol, suggesting that the adsorption was of the typical physical type. On the basis of the
monolayer dye adsorption capacity, the specific surface area of this ash sample was estimated
as 2.1–3.1 m2/g which is close to the value (3.7 m2/g) obtained via BET nitrogen gas
adsorption measurements.
INTRODUCTION
Adsorption techniques are widely used to remove certain types of pollutants in wastewater
treatment processes. With the selection of a proper adsorbent, the adsorption process can be a
promising technique for the removal of contaminants.1 Activated carbon has been widely used
as adsorbent for the removal of various pollutants as to its high adsorption capacity.
However, it is relatively high operation costs, problems of regeneration, and difficult to
separate from the wastewater after use. Therefore, a number of low-cost adsorbents have been
tries for treatment of wastewaters. The alternative adsorbents, such as sugar cane dust, perlite
(a glassy volcanic rock), bagasse pith, sludge, fly ash, red mud, bark, rice husk, hair, steel
plant slag coal, natural clay, Fuller’s earth, and many of them have been studied for removing
dyes from aqueous solutions.1-16 The adsorption process would provide an attractive
technology if the adsorbent is inexpensive and ready for use. Utilization of industrial solid
waste for the treatment of wastewater is a win-win strategy because it not only converts the
wastes into useful materials but it also alleviates the disposal problems. Furthermore, it is not
necessary to regenerate these cheap substitutes whereas regeneration of activated carbon is
essential. Such regeneration may result in additional effluent and the adsorbent may suffer a
considerable loss.
In this study, a sludge-ash sample obtained from a petroleum company in Kaohsiung, Taiwan
1
was studied for its potential use as an adsorbent for removal of a cationic dye (methylene
blue, MB) from aqueous solution. The ash is a waste of a fluidized bed incinerator combusted
with 90-95% bio-solid of a wastewater treatment and 5-10% sediments of petroleum storage
tanks at temperature 800-850oC for 2 seconds. While the annual production of the ash in this
company alone is about 2,000 tons, utilization of this waste has not yet been considered. This
paper reports the results for adsorption kinetics and equilibrium studies of MB onto sludgeash. Kinetic models including empirical modified Freundlich and intraparticle diffusion
equations were used to analyze the adsorption process and to describe the rate of adsorption.
The equilibrium isotherms were analyzed using a multilayer adsorption equation. Based on
the results of isotherm analysis, thermodynamic parameters for such systems and the specific
surface area of the ash were determined. Factors affecting adsorption, such as ionic strength,
surface loading, pH, and temperature, were evaluated. Results of this study will be useful for
future scale up of using this material as a low-cost adsorbent for the removal of cationic dyes.
MATERIALS AND METHODS
Ash Sample
Upon receiving the ash sample, it was dehydrated at 103oC overnight and then sieved to less
than 75 m in diameter using ASTM standard sieves. Before it was used in the adsorption
experiments, this sieved ash sample was washed thoroughly with distilled water. The washing
procedure is described as follows: 1. stirring a mixture containing 500 g ash and 1-L distilled
water for 10 min; 2. settling this mixture for 10 min; 3. analyzing conductivity of the
supernatant; 4. repeating steps 1 to 3 till the conductivity of the supernatant reaching a
constant value; 5. dehydrating the washed ash at 103oC overnight and stored in a deccator. It
was found that the washing cycle to reach a constant conductivity value was seven.
A scanning electron microscope (SEM) equipped with energy dispersive x-ray spectroscopy
(EDS) (Hitachi S2700) was used to characterize the washed ash for its basic constituents and
morphological information. The specific surface area and average pore radius of ash sample
was determined by BET-N2 surface area analyzer (Quantasorb, model QS-7, Quantachrom
Co., Greenvale, New York, USA). The zeta potential of the ash particulates was determined
instrumentally (Laser Zee 3.0, Pen Kem Inc., Bedford Hills, New York, USA). The pH value
of the ash was determined from a 1:1 w/w ratio of ash and 0.01 M CaCl2. Solid density of the
ash was determined followed the method described in ASTM D854.
Dye
A water-soluble cationic dye, methylene blue (MB), C16H18ClN3S was purchased from
Riedel-de Haën Co. The structural formula of the dye is given in Figure 1. The aromatic
moiety of MB contains nitrogen and sulfur atoms. In the aromatic unit, dimethylamino groups
attach to it. The aromatic moiety is planar and the molecule is positively charged. The
dimensions of MB molecule are16.9Å for the length, 7.4 Å for the breadth, and 3.8 Å for the
thickness.17
2
Figure 1: Molecular structure of methylene blue
Kinetic adsorption experiments
Kinetic adsorption experiments were carried out to establish the effect of time on the
adsorption process and to identify the adsorption reaction rate. The experimental procedures
are described as follows: (1) Prepare a series of 1-L solution containing different MB
concentrations (1×10-5, 3.5×10-5, and 5×10-5 M ) with a constant strength of NaNO3 (2.5×10-3
or 3×10-2 M). (2) Measure initial pH and then add a given amount of the ash (1 g/L) into the
solution. (3). Agitate these solutions on a magnetic stirrer at 150 rpm for 5 h at 25oC. (4). At
the preset completion of time intervals, a 5 mL of solution was taken and immediately
filtrated through a 0.45 m membrane filter (supor-450, 25 mm, Gelman Sciences Ann Arbor,
MI) to collect the supernatant. (5) Determine the residual MB concentration in the
supernatant. The residual MB concentration in the supernatant was analyzed using a
spectrophotometer (HACH DR-2010, USA) at a wavelength of 615 nm. The amount of dye
adsorbed was determined as the difference in concentration between samples withdrawn at
two consecutive time intervals during the course of the adsorption experiments.
Effect of initial pH on equilibrium adsorption experiments
Batch method was used to obtain the dye adsorption characteristics as affected by solution pH
which was regarded as a principal factor in the analysis of the adsorption process. The
experimental procedures used as follows: (1) Prepare a 2-L solution with a constant strength
of NaNO3 (2.5×10-3 M) and a fixed MB concentration (5×10-5 M). (2) Distribute 100 mL of
solution to a series of 125-mL polyethylene bottles. (3) Adjust initial pH to cover a range
from 4 to 11 by either HNO3 or NaOH. (4) Add a given amount of the ash (2 g/L) into the
solution. (5) Shake these bottles on a reciprocal shaker at 150 excursions/min for 3 days at
25oC. It was confirmed that this reaction time was found to be adequate for reaching
equilibrium adsorption. (6) At the end of shaking, record the final pH of the mixed liquor. (7)
Filter the mixed liquor through a 0.45-µm filter paper (Gelman Sci.) to collect the supernatant.
(8) Determine the residual MB concentration in the supernatant. Control samples without
containing ash in the mixed suspension were also performed.
Equilibrium dye adsorption isotherm experiments
The batch method was also employed to study the influence of temperature on the dye
adsorption isotherms. The effect of temperature on the adsorption was investigated under
isothermal conditions at 4ºC, 14ºC and 24ºC by maintaining the mixtures in a water
circulation bath whose temperature varied to within ± 1ºC. The experimental procedures
employed for studying the effect of temperature on the dye adsorption isotherm were the same
as those described in the section above except that the pH of the mixed solution was adjusted
to 7.05 in step (3) and the temperature was controlled in step (5).
3
RESULTS AND DISCUSSION
Ash characterization
Table 1 shows the major properties of the ash. The pH value of the ash was 9.67 indicating
that it can be considered as an alkaline material. The washed ash had a BET-N2 specific
surface area of 3.70 m2/g with respect to the unwashed ash of 2.43 m2/g. Noted that the
average pore radius of the washed ash (6.04 nm) is much greater that the unwashed ash (3.54
nm). It appears that the cleaning process has enlarged the pore of the ash particle therefore
increased its specific surface area. A solid density of 2.74 g/cm3 was obtained indicating that
gravitation method can be used to separate the adsorbent.
The SEM micrograph of a typical ash sample at 2000x magnification depicted in Figure 2a
shows that the ash particles were mostly irregular in shape and porous. The results of EDS
analysis showed that the major elements of ash particles by weight are: O (36.61%), C
(13.28%), Si (18.23%), Al (6.17%), Fe (6.53%), Ca (5.42%), while the miner elements are:
Mg (1.75%), K (0.36%), V (0.61%) (Table 1). It was found that a trace amount of vanadium
(V) appeared in the ash, which is the common element existing in petroleum refinery residues.
Figure 3 shows the electrophoretic mobility measurements of ash particles in inert electrolyte
(NaNO3) after hydration for 1 day. The negative zeta potential was observed over pH 2.0 –
10.0, which indicated that the ash particles are negatively charged surfaces. Intuitively,
electrostatic interaction will be favorable for cationic ion (i.e., methylene blue) adsorption.
Based on the results shown in Figure 3, it can be realized that the zero point of charge (pHzpc)
should be less than 2.0. For pure silica oxide pHzpc value is 2.0.18 It is speculated that the
causes of lower pHzpc value of sludge ash particles is attributed to high silica oxide content
versus to that of alumina, ferric, and calcium oxides.
Table 1: Properties of washed sludge-ash.
Properties
Particle size
Ash pH
Solid density
Specific surface area
Specific surface
area*
Average pore radius
Average pore radius*
pHzpc
Value
< 75 m
9.67
2.74 g/cm3
3.70 m2/g
Elements
O
C
Si
Al
% by wt.
36.61
13.28
18.23
6.17
2.43 m2/g
Fe
6.53
6.04 nm
3.54 nm
< 2.0
Ca
Mg
K
V
5.42
1.75
0.36
0.61
*Unwashed sample.
4
Figure 2: SEM image of sludge-ash. Magnification 2000x.
Figure 3: Zeta potential of sludge-ash as a function of pH.
0
zeta potential (mV)
-10
-20
-30
NaNO3
-40
1x10-3 M
5x10-3 M
5x10-2 M
-50
0
2
4
6
8
pH
5
10
12
Kinetic studies
Kinetic models
For the sake of convenient for practical uses two kinetic models, the modified Freundlich and
the intraparticle diffusion equations are used to describe the adsorption kinetics of MB onto
ash. The modified Freundlich equation was original developed by Kuo and Lotse 19:
q = kCot1/m
(1)
where k is the adsorption rate constant; q is the amount of MB adsorbed (mol/g) at time t; Co
is the initial MB concentration (mol/L); t is the reaction time (hr); k is the apparent rate
constant (L/g-min), and m is the Kuo-Lotse constant. The values of k and m were used
empirically to evaluate the effect of MB surface loading and ionic strength on the adsorption
process.
In a rapidly stirred batch reactor, the adsorbates are most probably transported from the bulk
of the solution in to the solid adsorbent through intra-particle diffusion/transport process,
which is often the rate limiting step in many adsorption processes.20-21 The possibility of
intraparticle diffusion as proposed by Furusawa and Smith 22 was used to identify the
diffusion mechanism:
q = kpt1/2 + C
(2)
where kp is the intraparticle diffusion rate constant (mol/min1/2/g); C is the intercept. The
intraparticle diffusion model has been used to describe many adsorption kinetics to identify
their adsorption mechanisms.7,13,16,23-24
Effects of initial MB concentration
Figure 4 shows the effects of reaction time on the amount of MB adsorbed by ash under
different initial MB concentrations. As shown, the adsorption increases with increasing initial
MB concentration. The amount of MB adsorbed increases from 3.2 to 6.0 mol/g by increasing
initial MB concentration from 1×10-5 to 5×10-5 mol/L under conditions of initial pH 7.2, 24oC
and ionic strength of 2.5×10-3 mol/L. The results show that a rapid increase in MB adsorption
occurred within 10 min with more than 80% of total MB adsorption completed in all cases.
An asymptotic trend with slow progress adsorption was found after approximately 30 min.
Generally, when adsorption involves a surface reaction process, the initial reaction is rapid.
Then, a slower adsorption reaction would be follow as the available adsorption site gradually
decreases.
6
Figure 4: Effects of initial MB concentration on the adsorption kinetics of MB onto sludgeash. Experimental conditions: Ash 1 g/L; NaNO3 2.5×10-3 M; initial pH 7.2; final pH 8.8;
24oC. Solid lines represent the best fit of Modified Freundlich equation.
7 10 -6
6 10 -6
q (mol/g)
5 10 -6
4 10 -6
3 10 -6
Methylene blue
2 10 -6
1x10-5 M
3.5x10-5 M
1 10 -6
5x10-5 M
0
0
50
100
150
200
250
300
350
t (min)
In Figure 4, the solid lines represent the best fit of Eq. (1) to the experimental data. Table 2
lists the corresponding model fitting parameters, i.e. k, m, and the correlation coefficient (r).
The value of r is high (all greater than 0.97) indicating that the data was well correlated to the
modified Freundlich equation. The adsorption rate was found to be inversely proportional to
the initial MB concentration. In other words, an increase in the surface loading led to a
decrease in the adsorption rate. Thus, when the surface loading was increased from 1×10-5 to
5×10-5 mol/g, the value of k decreased from 0.15 to 0.06 (L/g-min).
The intra-particle diffusion plots are given in Figure 5. Table 2 shows the corresponding
model fitting parameters. The values of qt were found to be linearly correlated with the value
t1/2 at different initial concentrations. The value of r is high (all greater than 0.94) indicating
the adsorption mechanism follows the intra-particle diffusion process. This confirms that the
adsorption of MB on the ash was a multi-step process, involving adsorption on the external
surface and diffusion in to the interior pore of the ash. The mechanism for the removal of MB
by adsorption may be assumed to involve the following steps25: 1. Migration of MB from bulk
of the solution to the surface of ash. 2. Diffusion of MB through the boundary layer to the
surface of ash. 3. Adsorption of dye at an active site on the surface of ash. 4. Intra-particle
diffusion of MB in to the interior pore structure of the ash particle.
The intra-paricle diffusion rate, kp, was in the range of 9.07×10-8 – 1.5 ×10-7 mol/min1/2-g. It
was found that the values of kp increased with increasing initial MB concentration. The
driving force of diffusion was very important for adsorption processes. Generally the driving
force changes with the adsorbate concentration in bulk solution. The increases of adsorbate
concentration result in an increase of the driving force, which will increase the diffusion rate
of MB.23 The values of intercept (C) give an idea about the thickness of boundary layer; i.e.,
the larger the intercept, the greater the boundary layer effect.24
7
Generally, if the adsorption steps are independent of one another, the plot of qt versus t1/2
usually shows two or more intercepting lines depending upon the exact mechanism, the first
intercepting line represents surface adsorption and the second one intraparticle diffusion.16 As
shown in Figure 6, the plots did not clearly exhibit more than two intercepting lines,
indicating the steps were indistinguishable. While the applicability of intra-particle diffusion
model has indicated that the MB adsorption onto ash is the rate-determining step, still it
would not give sufficient indication about which of the two steps (surface adsorption or intraparticle diffusion) was the limiting step. Ho26 pointed out that it is essential for the qt vs. t1/2
plots to go though the origin if the intra-particle diffusion is the sole rate limiting step. In this
study, because the values of C range 1.74×10-6 - 3.59×10-6 mol/g, it may be concluded that
surface adsorption and intra-particle diffusion were concurrently operating during the
adsorption of MB onto ash.
Figure 6: Intra-particle diffusion plots at different initial concentrations.
7 10-6
6 10-6
qt (mol/g)
5 10-6
4 10-6
3 10-6
Methylene blue
2 10-6
1x10-5 M
3.5x10-5 M
5x10-5 M
1 10-6
0
0
5
10
15
20
t1/2 (min 1/2)
Table 2: Kinetic constants for the removal of MB adsorption by sludge ash.
Initial MB Surface
(mol/L) loading
(mol/g)
1×10-5
1×10-5
I
(mol/L)
Modified Freundlich
Intraparticle diffusion
k
Kp
(L/g-min)
m
r
C
1/2
(mol/min -g) (mol/g)
r
2.5×10-3 0.150
0.132 0.995 9.07×10-8
1.74×10-6 0.968
3.5×10-5 3.5×10-5
2.5×10-3 0.067
0.120 0.984 1.23×10-7
2.71×10-6 0.940
5×10-5
5×10-5
2.5×10-3 0.060
0.121 0.985 1.50×10-7
3.59×10-6 0.951
1×10-5
1×10-5
3×10-2
0.100
0.192 0.973 1.13×10-7
1.26×10-6 0.941
1×10-5
1×10-5
5×10-2
0.098
0.192 0.984 1.00×10-7
1.29×10-6 0.988
1×10-5
1×10-5
1×10-1
0.068
0.19
0.975 8.64×10-8
6.14×10-7 0.961
8
Effects of ionic strength
Ionic strength is one of the key factors affecting the electrical double layer (EDL) structure of
a hydrated particulate. Increasing ionic strength could lead to a decrease in the thickness of
the EDL, thereby resulting in a decrease in adsorption. The thickness of EDL (m), 1/, can be
determined from the relationship:
1 2F 2 I ×1000 - 0.5
=(
)
κ
εε o RT
(3)
where F is the Faraday constant (96500 C/mol), I is the ionic strength, R is molar gas constant
(8.314 J/(mol-K), T is the absolute temperature (K),  is the dielectric constant of water (78.5),
and o is the vacuum permittivity (8.854×10-12 C/V-m).
Figure 6 shows the influence of ionic strength on the MB adsorption kinetic. As shown, at the
beginning the MB was adsorbed by the exterior surface of ash, the adsorption rate was fast.
When the adsorption of the exterior surface reached saturation, MB entered into the ash
particle by the pore within the particle and was adsorbed by the interior surface of the particle.
The solid lines shown in Figure 6 represent the best fit of modified Freundlich equation to the
experimental data. Results show that higher ionic strength caused lower adsorption and the
adsorption rate (Table 2). Figure 7 shows the linearity of the plots demonstrated that intraparticle diffusion played a significant role in the uptake of MB by sludge ash. This confirms
that adsorption of MB onto ash was a multi-step process, involving adsorption on the external
surface and diffusion in to the interior of the pore. In Table 2, the values of C increased with
decreasing ionic strength indicating that the boundary layer effect become prominent at a
lower ionic strength condition. Noted that the plots shown in Figure 7 are nearly parallel to
one another with very similar slopes. As seen in Table 2, values of kp are about the same order
of magnitude (mean value of kp is 0.98 ×10-8 mol/min1/2-g). This may imply that the rate of
diffusion is not affected much by the changes of ionic strength.
Figure 8 depicts the linear relationships between rate constant k (based on the fitting results of
modified Freundlich equation) and the thickness of the EDL. As indicated by equation (3), an
increase in ionic strength would lead to decrease in 1/ and increase the amount of indifferent
ions approaching or possibly masking the negatively charged ash surface. Thus, the results
shown above can be attributed in part to competition between MB and Na ion for surface sites
as the ionic strength increased.
9
Figure 6: Effects of ionic strength on the adsorption kinetics of MB onto sludge-ash.
Experimental conditions: Ash 1 g/L; initial pH 7.2; final pH 8.8; MB 1×10-5 M; 24oC. Solid
lines represent the best fit of Modified Freundlich equation.
3.5 10-6
3 10-6
q (mol/g)
2.5 10-6
2 10-6
1.5 10-6
NaNO3
0.0025 M
0.03 M
0.05 M
0.1 M
1 10-6
5 10-7
0
0
50
100
150
200
250
300
350
t (min)
Figure 7: Intra-particle diffusion plots for the removal of MB by adsorption onto sludge-ash
at different ionic strengths.
3.5 10-6
3 10-6
qt (mol/g)
2.5 10-6
2 10-6
NaNO 3
1.5 10-6
0.0025 M
0.03 M
0.05 M
0.1 M
1 10-6
5 10-7
0
5
10
15
t1/2 (min 1/2)
10
20
Figure 8: A linear relationship between the adsorption rate constant and electrical double
layer thickness (1/).
0.16
k(L/g-min)
0.14
0.12
I=
3x10-2
I = 2.5x10-3 M
M
I = 5x10-2 M
0.1
y=0.069+0.014x; r=0.950
0.08
I = 1x10-1 M
0.06
0
1
2
3
4
5
6
Electrical double layer thickness, 1/(nm)
7
Equilibrium studies
Effect of initial pH
The effect of initial pH of MB solution on the amount of MB adsorbed was studied by
varying initial pH under different MB concentrations at 24oC. The results are shown in Figure
9. As seen, the removal efficiency of MB increased with decreasing initial MB concentration
and remained constant values over initial pH 4-11. Methylene blue is a basic dye. When the
solution pH is above pHzpc, the negative charged ash surface is favorable for the adsorption
MB dye. The changes of solution pH after adsorption are shown in Figure 10. It was found
that the final pHs approach to the ash pH 9.67 (Table 1) despite of the adsorption of cationic
MB+ ion with release of H+ ion from the active site of the ash surface.
11
Figure 9: Effect of pH on the removal of MB by sludge-ash. Experimental condition: Ash 2
g/L; MB 5×10-5 M; NaNO3 2.5×10-3 M; reaction time 72 hr; 24oC.
100
Methylene blue
1x10 -5 M
2.5x10 -5 M
5x10 -5 M
Removal (%)
80
60
40
20
0
4
5
6
7
8
9
10
11
12
pHi
Figure 10: Relationship between final pH (pHf) and initial pH (pHi). Experimental condition:
Ash 2 g/L; MB 5×10-5 M; NaNO3 2.5×10-3 M; reaction time 72 hr; 24oC.
12
Ash pH 9.67
10
pHf
8
6
MB = 5x10 -5 M
I = 2.5x10 -3 M
Ash = 2 g/L
24℃
4
2
0
4
5
6
7
8
9
10
11
12
pHi
Adsorption isotherms
Considering the adsorption of MB onto ash is a multilayer equilibrium type and is formulated
as:
First layer adsorption
S+M
 S-M;
K1
(3)
Second layer adsorption
S-M + M
 S-M2;
K2
Second layer adsorption
S-M2+ M
 S-M3;
K3
…………………………………………………..
nth layer adsorption
S-Mn-1 + M
 S-Mn;
Kn
12
where S represents the available ash surface sites (mol/g); M is the equilibrium MB
concentration S-Mn is the ash surface complex (mol/g); and Kn (L/mol) equilibrium
adsorption constant corresponding to the nth layer. It is expected that the adsorption affinity
of the first layer (i.e. K1) would be much greater than the subsequent layers (i.e., K2, K3,…..,
and Kn) because dye adsorption within the multilayer results from the attachment of the dye
to surface and to subsequent layers. Thus, it is likely that the adsorption affinity for
subsequent multilayers would be the same, i.e., K2 = K3 = .….= Kn. For the sake of
simplicity, the total multilayer adsorption density (, mol/g) may be expressed as:

m K1C e
(1  K 2 C e )[1  (K1  K 2 )C e ]
(4)
where m, is the maximum monolayer adsorption density (original ash site density, mol/g)
and Ce is the equilibrium MB concentration (mol/L).
Figure 11 shows the effect of temperature on the MB adsorption isotherms where the solid
lines correspond to the best fit of equation (4). The maximum monolayer adsorption density
(m) and the equilibrium constants (K1 and K2) together with the correlation coefficients (r)
are summarized in Table 3. From the latter values, it may be stated that the experimental data
were well correlated (r > 0.99) by the multilayer adsorption isotherm. The values of m are
7.3×10-6, 6.3 ×10-6, and 5 ×10-6 mol/g, respectively, for 4oC, 14oC, 24oC. As shown in Table 3,
the values of K2 remained relatively constant, suggesting the multilayer adsorption was not
greatly affected by changed in the temperature. The K1 values were much greater than those
of K1, indicating that the adsorption affinity of the first layer was much higher than that of
subsequent multilayers. The results indicated that the magnitude of adsorption was inversely
proportional to the solution temperature, suggesting that the adsorption reaction was
exothermic in nature. Similar exothermic results have also been reported for several related
dye adsorption studies.1,7,27
A comparison of MB adsorption capacities of various adsorbents is given in Table 4. It is seen
that our results (5 ×10-6 mol/g) are quit similar to the investigations of coal fly ash. Some of
the large adsorption capacities recoded were: 2.71×10-3 mol/g for bark 5, 4.1×10-4 for sepiolite
gels 28, 9.59×10-4 mol/g for coal 5, and 9.25×10-4 mol/g for Rice husk 5, 2.02×10-4 mol/g for
bagasse fly ash 29, and 6.16×10-5 mol/g for banana peel 14. The MB adsorption capacity of
sludge ash is smaller than other adsorbents. This may be attributed in part to the small specific
surface area (3.7 m2/g) of the sludge ash compared to that of low-cost adsorbents such as
banana peel or orange peel (20.6 -23.5 m2/g), fly ash treated with NaNO3 (27.6 m2/g),
sepiolite gels (340 m2/g), and bagasse fly ash (440 m2/g). Albeit sludge ash has relatively low
adsorption capacity, it can obtain cheaply in large quantity and it has high acid neutralizing
capacity. Because of its high alkaline nature, sludge ash may be as an attractive agent for the
treatment of acidic dye wastewater.
13
Figure 11: Effect of temperature on the isotherm for MB adsorption onto sludge-ash. Ash 2
g/L; NaNO3 2.5×10-3 M; initial pH 7.05; final pH 9.83; reaction time 72 hr. Solid lines
represent the best fit of the equation for multilayer adsorption.
2 10-5
4℃
14℃
24℃
q (mol/g)
1.5 10-5
1 10-5
5 10-6
0
0
5 10-5
0.0001
0.00015
0.0002
Ce (mol/L)
Table 3: Multilayer adsorption constants and specific surface area as determined by dye
adsorption.
Temperature
4oC
14oC
24oC
I
(mol/L)
2.5×10-3
2.5×10-3
2.5×10-3
m (mol/g)
7.3 ×10-6
6.3 ×10-6
5 ×10-6
K1
(L/mol)
1.68 ×105
3.16 ×105
4.29 ×105
14
K2
(L/mol)
3.60 ×103
3.56 ×103
3.53 ×103
r
0.992
0.990
0.980
SSA
(m2/g)
3.1
2.6
2.1
Table 4: Comparison of adsorption capacity of methylene blue on various adsorbents
Surface
area
(m2/g)
3.7
pHzpc
pH
Temp
(oC)
m
(mol/g)
K (L/mol)
< 2.0
9.83
24
5.0 ×10-6
4.29×105
NA
NA
NA
27
2.35×10-5
1.31×105
27.6
NA
5.2
30
2.2×10-5
4.56×106
Wang et al. 2005 15
15.6
21
NA
NA
5.2
5.2
30
30
1.4×10-5
7.8×10-6
NA
8.88×104
Wang et al. 2005 15
Wang et al. 2005 15
5.47
NA
NA
22
3.6×10-6
1.47×105
Janos et al. 2003 11
3.26
NA
NA
22
2.4×10-6
1.53×105
Janos et al. 2003 11
Banana peel
20.6-23.5
< 2.0
7.2
30
6.16×10-5
5.57×106
Clay
30
NA
NA
20
1.98×10-5
2.49×102
Orange peel
20.6-23.5
< 2.0
7.2
30
5.51×10-5
6.72×106
Neem sawdust
NA
7.2
NA
25
1.32×10-5
9.50×105
Annadurai et al.
2002 14
Gürses et al.
2004 12
Annadurai et al.
2002 14
Khattri and
Singh 2000b 7
Bagasse fly ash
440
5.8
8
Coal
NA
NA
NA
Hair
NA
NA
NA
Cotton waste
NA
NA
NA
Rice husk
NA
NA
NA
Bark
NA
NA
NA
Sepiolite gels
NA
340
NA
Adsorbent
Sludge ash
Neem leaf
powder
Fly ash
treated with
HNO3
Coal Fly ash
Red mud
Coal Fly ash
sample A
Coal Fly ash
sample B
Note: NA means not available. K is the Langmuir
by BET-N2 method.
Reference
This study
Bhattacharyya and
Sharma 2005 16
Gupta et al.
2000 29
McKay et al.
20
9.59×10-4
1.21×103
1999 5
McKay et al.
20
4.69×10-4
6.55×103
1999 5
McKay et al.
20
8.23×10-4
3.15×103
1999 5
McKay et al.
20
9.25×10-4
5.79×103
1999 5
McKay et al.
20
2.71×10-3
5.84×103
1999 5
Azanr et al.
25
4.1×10-4
NA
1992 28
adsorption constant. Specific surface area was all determined
30
2.02×10-4
6.84×104
In order to gain an insight into the mechanism involved in the adsorption process,
thermodynamic parameters for the present system were calculated. The changes in the
standard free energy (Go), enthalpy (Ho), and entropy (So) were calculated using the
following thermodynamic functions:
Go = -RTlnK
ln K 
o
(5)
o
S
H 1

R
R T
(6)
where R is universal gas constant (1.987 cal/mol-K); T is the absolute temperature (K)
Figure 12 shows the van’ Hoff plots of ln K versus 1/T. The values of Ho and So were
determined from the slopes and intercept, respectively. Table 5 lists the calculated
thermodynamic parameters. It is noted that all Go values listed in Table 5 are negative. This
15
suggests that the adsorption process is spontaneous with high preference of MB for ash. As
shown, the magnitude of free energy for first layer adsorption (Go1) is higher than that of the
subsequent multilayers (Go2). As indicated previously, the assumption of greater adsorption
energy for the first layer that for subsequent multilayers applied in deriving equation (4) was
justified.
The change of energy for physical adsorption are generally small than that of chemisorption.
The changes of energy for physical adsorption ranges from 0 to -4.7 kcal/mol, comparing to
that of chemical adsorption -19 to 95 kcal/mol 30. As shown in Table 5, values of (Go1) range
from -6.62 to -7.65 kcal/mol suggest that the adsorption process may be considered as
physical in nature being simultaneously enhanced by the electrostatic effect. Values of (Go2)
ranging from -4.51 to -4.82 kcal/mol suggest that the adsorption process was a typical
physical process. The values of Ho1 and Ho2 were 7.69 and -0.135 cal/mol, respectively.
The values of So1 and So2 were 51.8 and 15.8 cal/mol, respectively. The values of Ho1 and
So1 are positive suggest that the monolayer adsorption is spontaneous and endothermic. On
the other hand, the negative value of Ho2 and the positive value of So2 suggest that the
adsorption of subsequent multilayers is spontaneous and exothermic.
Figure 12: Relationship between ln K and 1/T.
14
13
12
ln K
11
10
K1
K2
y1 = 26.1-3870.3x1 ; R= 0.984
9
y2 = 7.9+67.7x2 ; R=0.999
8
7
6
0.0033
0.0034
0.0035
1/T (K-1 )
0.0036
0.0037
Table 5: Thermodynamic parameters for the adsorption of MB onto sludge-ash.
Temperature
4oC
14oC
24oC
Go1
(kcal/mol)
-6.62
-7.22
-7.65
Go2
(kcal/mol)
-4.51
-4.66
-4.82
Ho1
(kcal/mol)
7.69
7.69
7.69
Ho2
(kcal/mol)
-0.135
-0.135
-0.135
So1
(cal/mol)
51.8
51.8
51.8
So2
(cal/mol)
15.8
15.8
15.8
Specific surface area determination
BET-N2 gas adsorption method has often been used for determination of SSA of a hydrous
solid. In this method, because SSA was determined in a dry-state environment, the obtained
SSA value does not properly represent the adsorption site of the hydrated adsorbent in
16
aqueous environment. As an alternative to the BET method, the adsorption of dyes from
aqueous solution has been used to determine the SSA of layered silicates 17,31, sludge 4,32, and
sludge ash 1. Assuming that the ash surface was homogeneous and completely covered by
dye molecules as the adsorption isotherm is established. Thus, the specific surface area (SSA,
m2/g) of the ash can then be related to the first layer adsorption density (m) as described in
equation (7):
SSA =m × N × A
(7)
where N is the Avogadro’s number (6.023×10-23 molecules/mol) and A is the apparent surface
area occupied by a monolayer of MB molecules. The MB molecules would be either
adsorbed in a flat or tilt orientation. On the basis of the results of x-ray photoelectron
spectroscopy and the synchrotron-based technique of near edge x-ray absorption fine structure
spectroscopy, Hähner 17 have shown that the MB molecules were tilted, with their largest face
inclined roughly at 65-70o with respect to the surface of mica. Accordingly, assuming that the
MB molecule facing the ash surface with a tile angle of  (Figure 13), the apparent MB area
(A) can be calculated as follow:
Apparent area = 16.9 Å× 3.8 Å  sin()
(8)
For the lowest and the highest tilt angle, 65 and 70o, the apparent area is 70.9 and 68.3 Å2,
respectively. In this study, an average value of apparent area 69.6 Å2 was used for SSA
determination of sludge ash. Table 3 list the calculated SSA of ash on the basis of m values
and equation (6). The SSA values determined by dye adsorption range from 2.1 to 3.1 m 2/g,
which is close to the value (3.7 m2/g) obtained via BET nitrogen gas adsorption
measurements.
Figure 13: Schematic drawing of MB molecule adsorb onto sludge ash. The shadow
represents the apparent area.
o
16.9 A
o
3.8 A

o
7.4 A
X
Sludge ash surface

17
X
CONCLUSIONS
The adsorption of MB onto sludge ash is favored at higher MB concentrations, lower ionic
strength, and low temperatures. The removal of methylene blue by adsorption on sludge ash
was found to be rapid at the initial period of contact time and then slows down with
increasing reaction time. The adsorption kinetic could be expressed by both modified
Freundlich equation and intra-particle diffusion model. The rate of adsorption decreases with
increasing initial MB concentration and ionic strength.
The equilibrium adsorption could be described by a non-linear multilayer adsorption
isotherm. The adsorption capacities for MB were 7.3×10-6, 6.3×10-6, and 5×10-6 mol/g,
respectively, at temperature of 4, 14, and 24oC. Thermodynamic parameters indicated that the
adsorption process was a spontaneous process. Values of the first-layer adsorption energy,
Go1, ranged from –6.62 to –7.65 kcal/mol, suggesting that the adsorption could be
considered as a physical process simultaneously enhanced by the electrostatic effect. The
multilayer adsorption energy, Go2, ranged from –4.51 to –4.82 kcal/mol, suggesting that the
adsorption was of the typical physical type. On the basis of the monolayer dye adsorption
capacity, the specific surface area of sludge ash was estimated as 2.1–3.1m2/g which is close
to the value (3.7 m2/g) obtained via BET nitrogen gas adsorption measurements.
This study showed that the sludge ash could be used as an adsorbent to remove MB dye from
aqueous solution. Albeit sludge ash has relatively low adsorption capacity, it can be obtained
cheaply in large quantity. The high alkaline nature of sludge ash may make it also as an
attractive neutralization agent for the treatment of acidic dye wastewater.
ACKNOWLEDGMENTS
The work was supported by National Science Council of Republic of China (Gand No. NSC
92-2815-C-214-001-E and NSC 93-2211-E-214-005). The assistance of Miss Su-Wha Chu in
conducting SEM/EDS analysis is greatly appreciated.
REFERENCES
1. Weng, C.H. “Adsorption Characteristics of New Coccine Dye on to Sludge Ash.”
Adsorption Science & Technology, 2002, 20(7), 669 – 681.
2. Gupta, G.S.; Prasad, G.; Singh, V.N. “Removal of chrome dye from aqueous solutions by
mixed adsorbents. fly ash and coal.” Water Research, 1990, 24(1), 45 – 50.
3. Ramakrishna, K.R.; Viraraghavan, T. “Dye removal using low cost adsorbents.” Water Sci.
Technology, 1997, 36, 189 – 196.
4. Wang, J.; Huang, C. P.; Allen, H. E.; Cha, D. K.; Kim, D. W. “Adsorption characteristics
of dye onto sludge particulates.” J. of Colloid and Interface Sci., 1998, 208, 518 – 528.
5. McKay, G.; Porter, J. F.; Prasad, G. R. “The removal of dye colours from aqueous
solutions by adsorption low-cost materials.” Water, Air, and Soil Pollution, 1999, 114,
423 – 438.
6. Khattri S. D.; Singh M. K. “Colour removal from dye wastewater using sugar cane dust as
an adsorbent.” Adsorption Science & Technology, 2000a, 17(4), 269 – 281.
7. Khattri, S. D.; Singh, M. K. “Colour removal from synthetic dye wastewater using a
bioadsorbent.” Water, Air, and Soil Pollution, 2000b, 120, 283 – 294.
8. Dogan, M.; Alkan, M.; Onganer, Y. “Adsorption of methylene blue from aqueous solution
onto perlite.” Water, Air, and Soil Pollution, 2000, 120, 229 – 248.
18
9. Weng, C. H.; Chang, E. E.; Chiang, P. C. “Characteristics of new coccine dye adsorption
onto digested sludge particulates.” Water Sci. Tech., 2001, 44(10), 279 – 284.
10. Atun, G.; Hisarli, G.; Sheldrick, W.S.; Muhler, M. “Adsorptive removal of methylene
blue from colored effluents on fuller’s earth.” J. of Colloid and Interface Sci., 2003, 261,
32-39.
11. Janoš, P.; Buchtová, H.; Rýznarová, M. “Sorption of dyes from aqueous solutions onto fly
ash.” Water Research, 2003, 37, 4938 – 4944.
12. Gürses, A.; Karaça, S.; Dogar, C.; Bayrak, R.; Acikyildiz, M.; Yalçin, M. “Determination
of adsorptive properties of clay/water system: methylene blue sorption.” J. of Colloid and
Interface Sci., 2004, 269, 310-314.
13. Dogan, M.; Alkan, M.; Türkyilmaz, A.; Özdemir, Y., “Kinetics and mechanism of
removal of methylene blue by adsorption onto perlite.” J. of Hazardous Materials, 2004,
109, 141– 148.
14. Annadurai G.; Juang, R.S.; Lee, D.J. “Use of cellulose-based wastes for adsorption of
dyes from aqueous solution.” J. of Hazardous Materials, 2002, B92, 263 – 274.
15. Wang, S.; Boyjoo, Y.; Choueib, A.; Zhu, Z.H. “Removal of dyes from aqueous solution
using fly ash and red mud.” Water Research, 2005, 39, 129 – 138
16. Bhattacharyya, K.; Sharma, A. (2005) “Kinetic and thermodynamics of methylene blue
adsorption on Neem (Azadirachta indica) leaf powder. Dye and Pigments.” 65, 51 – 59.
17. Hähner, G.; Marti, A.; Spencer, N.D. “Orientation and electronic structure of methylene
blue on mica: A near edge x-ray absorption fine structure spectroscopy study.” J. Chem.
Phys., 1996, 104(19), 7749 – 7755.
18. Stumm, W.; Morgan, J. J. Aquatic chemistry. Wiley, New York, 1995.
19. Kuo, S.; Lotse, E. G. “Kinetics of phosphate adsorption and desorption by hematite and
gibbsite.” Soil Sci. Soc. Am. J., 1973, 116, 400 – 406.
20. Weber, Jr. W.J. Physico-chemical processes for water quality control. Ch. 5, New York,
Wiley Interscience, 1972, pp. 207 – 227.
21. McKay G. “The adsorption of dyestuff from aqueous solution using activated carbon:
analytical solution for batch adsorption based on external mass transfer and pore
diffusion.” Chem Eng J., 1983, 27, 187 –207.
22. Furusawa, T.; Smith, J.M. “Intraparticle mass transport in slurries by dynamic adsorption
studies.” J. AIChE, 1974, 20(1), 88 – 93.
23. Karaca, S.; Gürses, A.; Ejder, M.; Acikyildiz, M. “Kinetic modeling of liquid-phase
adsorption of phosphate on dolomite.” J. of Colloid and Interface Science, 2004, 277,
257 – 263.
24. Kannan, N.; Sundaram, M.M. “Kinetics and mechanism of removal of methylene blue by
adsorption on various carbons-a comparative study.” Dyes and Pigments, 2001, 51, 25 –
40.
25. Mathews, A.P.; Weber, W.J. “Effects of external mass transfer and iner-particle diffusion
on adsorption.” AIChE Symp. Ser., 1976, 73, 91 – 98.
26. Ho, Y.S. “Removal of copper ions from aqueous solution by tree fern.” Water Research,
2003, 37, 2323 – 2330.
27. Gupta G. S.; Prasad F.; Panday K. K.; Singh V.N. “Removal of chrome dye from aqueous
slutions by fly ash.” Water Air Soil Pollut., 1985, 37(12), 13 – 24.
28. Azanr, A.J.; Ruiz-Hitzky, C.E.; Lopez-Arbeloa, I.; Lopez-Arbeloa, F.; Santarn, J.; Alvarez,
A. “Adsorption of methylene blue on sepiolite gels: spectroscopic rhelogical studies.”
Clay Minerals, 1992, 27, 101 – 108.
29. Gupta, V.; Mohan, D.; Sharma, S.; Sharma, M. “Removal of basic dyes (Rhodamine B
and methylene blue) from aqueous solution using bagasse fly ash.” Separation Science
and Technology, 2000, 35(13), 2097 – 2113.
19
30. Jaycok, M.J.; Parfitt, G.D. Chemistry of interfaces, Ellis Horwood Limited, New York,
1981.
31. Shelden, R. A.; Caseri, W.R.; Suter, U.W. “Ion exchange on muscovite mica with
ultrahigh specific surface area.” J. of Colloid and Interface Science, 1993, 157, 318 – 327.
32. Wakeman, R. J.; Sorensen, B. L. “Filtration characterisation and specific surface area
measurement of activated sludge by rhodamine B adsorption.” Water Research, 1996,
30(1), 115 – 121.
KEY WORDS
Adsorption, Dye, Methylene blue, Sludge, Ash, Kinetics, Equilibrium
20