J. Environ. Eng. Manage., 19(3), 165-172 (2009)
165
THE ADSORPTION OF METHYLENE BLUE FROM AQUEOUS SOLUTION USING
WASTE AQUACULTURAL SHELL POWDERS
Wen-Tien Tsai,1,* Huei-Ru Chen,2 Kuan-Chi Kuo,2 Chia-Ying Lai,2
Tu-Cian Su,2 Yuan-Ming Chang1 and Jwu-Maw Yang3
1
Graduate Institute of Bioresources
National Pingtung University of Science and Technology
Pingtung 912, Taiwan
2
Department of Wood Science and Design
National Pingtung University of Science and Technology
Pingtung 912, Taiwan
3
Department of Pharmacy
Chia Nan University of Pharmacy and Science
Tainan 717, Taiwan
Key Words: Shellfish shell, methylene blue, adsorption, isotherm, kinetic modeling
ABSTRACT
Shellfish shell has become one of the most significant wastes in aquaculture. In this work, three
ground shells derived from oyster, hard clam and short-neck clam were used as mineral adsorbents
for removal of cationic dye (i.e., methylene blue) from aqueous solution at 25 °C and initial solution
pH of 7.0. The shell adsorbents possessed mesoporous structure according to their type IV isotherms
with the hysteresis loops corresponding to type H3, but they displayed poor pore properties. The
equilibrium adsorption capacity of methylene blue onto the oyster shell is significantly larger than
those onto the hard clam and short-neck clam shells, which are parallel to their pore properties. It
was further found that both of the Langmuir and Freundlich models were useful for describing the
equilibrium of adsorption system. The adsorption kinetics showed that the adsorption process can be
well described with a simple model, the pseudo-second order model. The parameters (amount of dye
adsorbed at equilibrium) of the model obtained in the present work were significantly in line with the
adsorption process parameter (i.e., initial dye concentration).
INTRODUCTION
Due to the increase of shellfish consumption in
the past decade, the derived shell from the aquacultural farming and/or commercial market has become
one of the most significant wastes in Taiwan. According to the statistic data on the aquacultural industry,
oyster (Crassostrea gigas) and clams (Meretrix lusoria and Paphia undulata) are the most predominant
in Taiwan, and 200 kt of waste shells per year at
least were thus produced [1]. Most waste oyster shells, however, were locally dumped into public
waters and lands without proper treatment, causing a
serious odor problem as a result of the remaining attached to the shellfish. This practice apparently was
not desirable in view of the environmental odor problems. On the other hand, the waste shell is primarilycomposed of calcium carbonate crystals laid down in
*Corresponding author
Email: [email protected]
a protein matrix [2]. Thus, its reuse or utilization has
attempted as a diversity of applications such as desulfurization sorbent [3,4], sludge conditioner [5], metal
ion adsorbent [6,7], construction material [8,9], eutrophication control [10,11], nutrition supplement [12],
and food additive [13,14].
Removal methods of organic pollutants in industrial discharges may be traditionally divided into three
main categories: physical, chemical, and biological
processes. Among them, physical adsorption is generally considered to be the most efficient method for
quickly lowering the concentration of dissolved organics in the wastewater. Also, activated carbon is the
most widely used adsorbent for removal of dyes from
the aqueous solution [15]. Despite the prolific use of
this adsorbent throughout the water/wastewater treatment and other industrial applications, the removal of
organic pollutants by activated carbon adsorption re-
J. Environ. Eng. Manage., 19(3), 165-172 (2009)
166
mains an expensive process because the adsorbent is
still expensive and has high regeneration cost after
exhausted. For these reasons, there is growing interest
in using low-cost alternatives to carbon adsorbent [1519]. In contrast with activated carbon, waste shellfish
shells are relatively inexpensive due to its accessibility
and abundance. In recent years, this mineral biomaterial has been tested as adsorbent, but the adsorbates
are heavy metal ions such as Pb and Cu [6,7].
Methylene blue, a basic dye, is a heterocyclic
aromatic chemical compound with a diversity of different fields, including the dyeing of silk, leather, paper, wool and cotton, and the production of ink and
copying paper, as well as the quality control test of
concrete and mortar [20]. On the other hand, it has
been used as an effective therapeutant and antibacterial to protect newly laid fish eggs from being infected
by fungus or bacteria in the aquacultural industry [21].
Therefore, it is necessary to limit the potential negative impacts of aquacultural farm discharges on environmental quality, and also maintain high surface water quality through a recirculating aquaculture system
during cultivation of fish.
For the purpose of serving it as an effective decolorizing and detoxifying adsorbent, the local waste
shellfish shell was processed to increase its availability by mechanical grinding in the present study because the mineral bioresource used for the adsorption
of cationic dye in aqueous solutions has been scarcely
reported in the literature. Methylene blue was selected
as a model compound in this work mainly due to its
applications in aquaculture and in the measurement as
a means of determining the adsorption characterization of adsorbent [22]. Thus, the main objectives of
this work were to characterize the physical properties
of three common shellfish shells, to examine their adsorption behaviors of removing methylene blue from
aqueous solutions under the specified conditions, including the adsorption isotherms and kinetics, and to
evaluate the usefulness of common isotherm models
(i.e. Langmuir and Freundlich) and simple pseudosecond order model for the adsorption isotherms and
kinetics.
nants attached to the shells, and then dried in the sun
at least 2 h. The shells were further crushed and
ground by a rotary knife cutter to prepare powder particles. The shell powders were thus sieved to three different mesh number ranges of 80-100 (0.177-0.149
mm, average size = 0.163 mm), 100-200 (0.053-0.037
mm, average size = 0.112 mm), and ＞ 200 (＜ 0.074
mm) according to USA standard sieve designation.
The resulting shell powders were finally dried at 105
°C for 24 h, and stored in the desiccator prior to the
physical and adsorption characterizations.
2. Characterization of Shellfish Shell Powder
In order to elucidate the particle properties (e.g.,
surface morphology and particle size) of the resulting
shell powders, the powder texture was observed by the
scanning electron microscope (SEM). The surface
morphologies of the samples were examined using an
S-3000N (Hitachi High-technologies Co., Tokyo, Japan) SEM apparatus operated at a 15 kV accelerating
potential. Prior to the observation, the surface of the
sample was coated with a thin, electric conductive
gold film.
For the specific surface areas of shell powders,
they were obtained by measuring their nitrogen adsorption-desorption isotherms at -196 °C in a surface
area & porosity analyzer (ASAP 2020, Micromeritics
Co., Norcross, GA). The Brunauer-Emmett-Teller
(BET) and t-plot external surface areas were thus obtained based on BET and t-plot methods, respectively
[23]. The pore size distribution was calculated using
differential pore volume of Barrett-Joyner-Halenda
(BJH) adsorption-desorption [24]. The measurement
of the physical property was replicated twice.
In the measurement of true density (ρs), the contribution to the volume occupied by pores or internal
voids must be subtracted based on its definition. Because of the inertness and small molecule size (about
0.2 nm) of helium gas which enables it to penetrate
even the very small pores in the powder sample [23],
the ρs of the particulate sample was measured using a
helium displacement method with a pycnometer (AccuPyc 1340, Micromeritics Co., Norcross, GA).
METHODS
1. Materials
The cationic dye used as target adsorbate in the
present study is methylene blue (i.e., Basic Blue 9, or
C.I. 52015), which was purchased from Merck Co.
(USA). Its chemical formula and molecular weight are
C16H18N3SCl．4H2O and 320 g mol-1, respectively.
Three shellfish shell samples, which were obtained
from commercial market and seafood restaurant in
Kaohsiung city (Taiwan), include oyster shell, hard
clam shell and short-neck clam shell. The samples
were first cleaned with tap water to remove fresh rem-
3. Adsorption Studies
3.1. Adsorption isotherm
Adsorption isotherms of methylene blue onto
three shellfish shell powders (average size = 0.112
mm) were determined by using the standard method
(D-3860) of the American Society for Testing and
Materials. In each experiment, the shell powders with
different particle masses (0.2-1.0 g) were poured into
flasks, which had fixed value (0.050 L) of solutions
containing methylene blue of 10 mg L-1. Adsorption
isotherm was carried out by shaking (at about 130 rpm)
a thermostatted shaker bath at temperature of 25 ± 0.1
Tsai et al.: Methylene Blue Adsorption onto Shell Powders
3.2. Adsorption Kinetics
All the experiments of adsorption kinetics were
carried out in an about 3 L stirred batch adsorption
apparatus with four baffles as similarly described in
our previous studies [25]. All the dye solutions were
prepared with de-ionized water. In the present study,
the adsorption uptake of the oyster shell powder was
investigated at the specified conditions (i.e., adsorption temperature = 25 °C, initial solution pH = 7.0 and
agitation rate = 400 rpm) under the controlled process
parameters including particle size (i.e., 0.163, 0.112,
and ＜ 0.074 mm) and initial dye concentration (0.55.0 mg L-1). The solution sample (about 15 cm3) was
taken at specified time up to 2 h and then filtrated with
MCE membrane. The analysis of dye concentration in
the filtrate solution was immediately measured with
UV/Visible spectrophotometer at 661 nm as described
above. It should be noted that the final pH of the suspensions at steady state was found to be about 10 for
all kinetic experiments. The amount (qt, mg g-1) of dye
adsorbed onto the shell powder was directly determined by the difference between the initial dye and
filtrate dye concentrations at any sampling time (t,
min). In order to evaluate the statistical significance of
data in the adsorption kinetics and to calculate the adsorption data with a pseudo-second order method, a
preliminary experiment was also repeated under identical conditions, showing that the reproducibility of
the measurements was within 8% in the experiments.
RESULTS AND DISCUSSION
1. Characterization of Shellfish Shell Powder
The data in Table 1 indicated the BET surface
areas, external surface areas, and ρs of shell powders.
Table 1. Physical characterizations of the shellfish shells
Ave. size
(mm)
Sample
BET surface
area (m2 g -1)
True density
(g cm-3)
2.49
2.92
4.08
1.82
0.99
2.36
2.65
2.61
2.84
2.80
Oyster shell
0.163
0.112
＜ 0.074
Hard clam shell
0.112
Short-necked clam shell
0.112
0.6
3 -1
Volume
(cm3/g
gSTP)
, STP)
Volumeadsorded
adsorbed (cm
°C and initial solution pH of 7.0 for all experiments.
The dye solutions were contacted for about 24 h,
which had been shown previously in excess of the
contact time to reach adsorption equilibrium. Thereafter, the solution sample (about 15 mL) was taken and
then filtrated with mixed cellulose esters (MCE)
membrane (Pore size: 0.45 μm). The dye concentrations in the filtrate solution were immediately measured with UV/Visible spectrophotometer (Model: U2900, Hitachi High-technologies Co., Tokyo, Japan)
at 661 nm, at which the maximum absorbency occurred. The amount of dye adsorbed was thus determined by the difference of the initial and equilibrium
liquid-phase concentration. In order to evaluate the
statistical significance of data in the adsorption isotherms and to evaluate the adsorption data for fitting
the common isotherm models (i.e., Langmuir and
Freundlich), a preliminary experiment was also repeated under identical conditions, showing that the reproducibility of the measurements is within 10% in
the experiments.
167
Adsorption
Desorption
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure
)
Relative
pressure(P/P
(P/P
0 0)
Fig. 1. Nitrogen adsorption-desorption isotherms of
oyster shell powder.
It can be found that physical properties between them
are slightly different according to the data on BET
surface area. With BET area sequence as follows: oyster shell ＞ hard clam shell ＞ short-neck clam shell.
On the other hand, the specific surface areas of sample
oyster shell increased as its particle size decreased
from 0.162 to ＜ 0.074 mm. It was well known that
any powder containing a large number of more fine
particles generally increases in specific surface area
[23]. From the N2 adsorption-desorption isotherms
(Fig. 1), the shell sample (i.e., oyster shell powder
with particle size ＜ 0.074 mm) displayed poor pore
properties towards probe molecule (i.e., nitrogen), and
also showed the presence of desorption hysteresis.
Based on the shape, it is characteristic of type IV isotherm according to the Brunaller, Deming, Deming
and Teller classification with the hysteresis loops corresponding to type H3, as recommended in the International Union of Pure and Applied Chemistry [26]. It
should be noted that the type IV isotherms are characteristic of the mesoporous materials due to the nitrogen condensation. Type H3 hysteresis loops are associated with porous solids having slit-shaped pores
with wide mouths [26]. Such pores occur because the
solid is mainly composed of plate-like particles or of a
layer structure that is typical of clay (i.e., calcite)
structure. This result was parallel to the observation of
pore size distribution in Fig. 2 based on the pore volumes of the BJH desorption branch in the measurement of nitrogen isotherms. It showed that the
168
J. Environ. Eng. Manage., 19(3), 165-172 (2009)
those of hard clam and short-neck clam shells (2.64 vs.
2.85 and 2.80 g cm-3, listed in Table 1), implying that
the former may contain trapped pores and/or closed
voids. This is also consistent with the measured values
of pore property seen in Table 1 because these pore
and voids may be opened as the powder size decreased, resulting in the increase in specific surface
area. From the data on ρs (Table 1), it further indicated
that these crystalline shells should be mainly composed of calcium carbonate in the form of calcite because its ρs is 2.71 g cm-3 [27].
The textural structure examinations of three
shellfish shell powders can be observed from the SEM
photographs with the magnifications of 300 and 1000.
From Fig. 3, it can be clearly seen that these samples
do not indicate well-defined pore structures, which
were in agreement with the results based on the N2 adsorption-desorption isotherms (Fig. 1). However, the
Fig. 2. Pore size distribution of oyster shell powder.
shell powders had a heterogeneous distribution of pore
widths with a range from 30 to 40 Å. On the other
hand, the ρs of oyster shell is significantly lower than
(a)
(b)
(c)
Fig. 3. SEM photographs (magnifications: Left, 300 × and Right, 1000×) of the shellfish shell powders for (a) oyster
shell, (b) hard clam shell, and (c) short-necked clam shell.
Tsai et al.: Methylene Blue Adsorption onto Shell Powders
Methylene blue (i.e., basic blue 9) was chosen as
adsorbate to evaluate the adsorpability and effectiveness of the resulting shell powders. To facilitate the
estimation of adsorption capacities at various liquidphase concentrations of methylene blue, the two wellknown equilibrium adsorption isotherm models
[28,29], the Langmuir and the Freundlich, were employed as follows:
Langmuir: qe = qmax
K L Ce
(1+ K L Ce)
Freundlich: qe = KF Ce1/n
(1)
(2)
In Eq. 1, Ce and qe are the concentration (mg L-1) and
amount (mg g-1) of methylene blue adsorbed at equilibrium, respectively, KL is the Langmuir constant for
measuring the intensity of the adsorption process (L
mg-1), and qmax is a constant related to the area occupied by a monolayer of absorbate, reflecting the adsorption capacity (mg g -1 ). In Eq. 2, K F is the
Freundlich constant relating to the extent of the adsorption. The slope 1/n, ranging between 0 and 1, is a
measure for the surface heterogeneity, or the degree of
non-linearity between dye concentration and adsorption. Table 2 shows the results of the Langmuir and
the Freundlich isotherm fits by using the adsorption
capacity data at 25 °C. From the data on the isotherms
(Fig. 4), the adsorption capacity of oyster shell is significantly larger than those of clam shells, as the oyster shell had larger specific surface area (Table 1). It
can be further seen that the Langmuir and Freundlich
models showed good fitting results based on the high
correlation coefficients (R2). However, the adsorption
isotherms revealed the shellfish shell adsorbents can
only uptake limited amount in relatively low concentration of the basic dye in aqueous solutions, which
was indicative of their poor pore properties as described above. By contrast, the isotherm qe of eggshell
Oyster shell
Hard clam shell
Short-neck clam shell
0.040
0.035
0.030
e
2. Adsorption Isotherms of Methylene Blue onto
Shellfish Shells
0.045
qeq(mg
g-1)
(mg/g)
oyster shell displayed a much rougher and irregular
surface structure, but the clam shells showed smoother
surfaces. As a result, the former may have a larger
specific surface area than those of the latter as listed in
Table 1.
169
0.025
0.020
0.015
0.010
0.005
0.000
0.0
0.1
0.2
0.3
0.4
(mg/L)
CCe e(mg
L-1)
Fig. 4. Plots of the adsorption isotherm data at 25 °C for
methylene blue onto oyster shell, hard clam, and
short-necked
clam
powders
(adsorption
conditions: adsorbent particle size ≒ 0.112 mm,
initial solution concentration = 10 mg L-1,
agitation rate ≒ 130 rpm, and initial solution pH
= 7.0).
(BET surface area = 1.02 m2 g-1) for removal of methylene blue under the “same” conditions is only 0.8
mg g -1 [30]. In the present work the adsorption capacity of short-necked clam shell particle (BET surface
area = 0.99 m2 g-1) could intake the methylene blue of
0.89 mg g-1 in the aqueous solutions.
3. Adsorption Kinetics of Methylene Blue onto
Oyster Shell
The removal of methylene blue from the aqueous
solution onto the shellfish shell adsorbent was found
to be rapid at the initial period of contact time, and
then to become slow and even stagnate with the adsorption in progress, implying that an ion-exchange or
coulombic interaction between the particle surface and
methylene blue cation. A simple kinetic analysis of
adsorption, pseudo-second order equation, was thus
used to fit experimental data in the present work [31].
Its linear form was commonly expressed as follows.
t
1
1
=
+ t
qt (k qe2 ) qe
(3)
where k is the rate constant of pseudo-second-order
adsorption (g mg -1 min -1), qe and qt are the amount
Table 2. Isotherm parameters in the Langmuir and the Freundlich models for adsorption of methylene blue onto shellfish
shell powders at 25 °C
Powder sample
Oyster shell
Hard clam shell
Short-neck clam shell
qmax
(mg g-1)
0.084
0.060
0.893
Langmuir model
KL
(L mg-1)
3.05
3.43
0.10
2
R
0.964
0.929
0.976
Freundlich model
KF
n
(mg g-1 L1/n mg-1/n)
(-)
0.11
1.34
0.086
0.99
0.076
1.43
R2
0.982
0.925
0.975
170
J. Environ. Eng. Manage., 19(3), 165-172 (2009)
Table 3. Pseudo-second order parameters for the
adsorption of methylene blue onto the oyster
shell powder at various adsorbent particle sizes
and initial dye concentrations
Ct / C0
Process parameter
k
qe
(g mg-1min-1) (mg g-1)
R2
†
Ct / C0
Average particle size (mm)
0.163
0.90
0.26 0.98
0.112
7.57
0.23 0.999
2.99
0.35 1.000
＜ 0.074
‡
Initial concentration (mg L -1)
0.5
15.32
0.15 1.000
1.0
2.99
0.35 1.000
1.5
0.73
0.52 0.999
2.5
0.94
0.69 0.999
5.0
0.95
1.10 0.999
Note: General dsorption condition: adsorbent mass = 5.0 g,
agitation rate = 400 rpm, pH = 7.0, and temperature = 25
°C. †Initial concentration = 1.0 mg dm-3, ‡Adsorbent size
≤ 0.074 mm.
Time (min)
Fig. 5. Plots of adsorbed methylene blue amount versus
time onto the oyster shell powder with (a)
different sizes (adsorption conditions: adsorbent
mass = 1.0 g, agitation rate = 400 rpm, initial
solution pH = 7.0, and temperature = 25 °C;
Symbols: experimental data, full lines: calculated
from pseudo-second order kinetics model), and
(b) various dye concentrations (adsorption
conditions: initial solution pH = 7.0, adsorbent
mass = 1.0 g, agitation rate = 400 rpm, and
temperature = 25 °C; Symbols: experimental data,
full lines: calculated from pseudo-second order
kinetics model).
(mg g -1) of dye adsorbed at equilibrium and time t,
respectively. Due to the higher equilibrium adsorption
capacity of oyster shell than those of hard clam and
short-neck clam shells, the effects of oyster shell particle size and initial methylene blue concentration on
the adsorption rate have been carried out at the specified conditions.
The adsorption uptakes of the oyster shell powder with three different sizes under the initial dye concentration of 1.0 mg L-1, adsorption temperature of 25
°C, initial solution pH of 7.0, adsorbent dosage of 2.5
g L -1, and agitation speed of 400 rpm was investigated.
Figure 5a showed the ratio (Ct /C0) of residual dye
concentration to initial dye concentration at the average particle sizes of 0.163, 0.112, and ＜ 0.074 mm as
a function of contact time. As obtained by the fittings
of the pseudo-second order equation with high R2
(Table 3), it was clear that the oyster shell adsorbent
with the smallest particle size (i.e.,＜ 0.074 mm) exhibited the higher adsorption potential as compared to
those by hard clam shell and short-neck clam shell.
This result was parallel to the previous results regarding their surface areas (Table 1). Thus, the oyster shell
powder with particle size of ＜ 0.074 mm was further
used to study the adsorption characteristics at 25 °C
under various initial dye concentrations.
The fitting data on the effect of initial methylene
blue concentration on the adsorption rate of the cationic dye at adsorbent mass of 5.0 g (adsorbent dosage
of 2.5 g L -1), adsorbent size of ＜ 0.074 mm, initial solution pH of 7.0, adsorption temperature of 25 °C, and
agitation speed of 400 rpm are also presented in Table
3. As the initial dye concentration increased from 0.5
to 5.0 mg L -1, the fitted qe of methylene blue onto the
calcite-based adsorbent changed from 0.15 to 1.09 mg
g -1, indicating that the initial concentration provided a
powerful driving force to overcome the mass transfer
resistance between the aqueous and solid phases. It
was further noted from Fig. 5b that the curves of dimensionless dye concentration vs. time were smooth
and continuous to saturation adsorption at various initial concentrations of methylene blue on the shell particles. Clearly, the kinetic modeling of the dye adsorption onto the shell adsorbent well followed the
pseudo-second-order rate model with high R2 for all
the system in the present work. From the results in
Table 3, it was also found that the variations of k values seemed to have a decreasing trend with the increased initial dye concentration, which is consistent
with those by the previous study [32].
Tsai et al.: Methylene Blue Adsorption onto Shell Powders
CONCLUSIONS
Three shellfish shells have been used as adsorbents for the removal of methylene blue from aqueous
phase at 25 °C. The following conclusions can be
drawn:
● The specific surface area of the resulting powders
sequenced as follows: oyster shell ＞ hard clam shell
＞ short-neck clam shell. On the other hand, the
specific surface areas of sample oyster shell increased as the particle size decreased from 0.162 to
＜ 0.074 mm.
● A hysteresis loop, which is corresponding to type
H3, can be found from the nitrogen adsorptiondesorption isotherms of the oyster shell sample, indicating that it could be characteristic of the
mesoporous solid associated with wide mouth slitshaped pores.
● The adsorption capacity of methylene blue onto the
oyster shell at equilibrium is significantly larger
than those onto the hard clam and short-neck clam
shells. However, the adsorption isotherms revealed
the shellfish shell adsorbents can only uptake limited amount in relatively low concentration of the
basic dye in aqueous solutions.
● The adsorption kinetics of methylene blue onto the
oyster shell can be well described by a simple
pseudo-second order model. The kinetic parameters
thus obtained from the fittings of the model are dependent on adsorbent particle size and initial dye
concentration.
3.
4.
5.
6.
7.
8.
9.
10.
OMENCLATURE
Ce
C0
Ct
k
KF
KL
n
qe
qmax
qt
t
Concentration of dye at equilibrium, mg L-1
Concentration of dye at initial time, mg L-1
Concentration of dye at time t, mg L-1
Rate constant of pseudo-second-order
model, g mg-1 min-1
Constant of the Freundlich model, mg1-1/n
g-1 L1/n
Constant of the Langmuir model, L mg-1
Constant of the Freundlich model, Amount of dye adsorbed at equilibrium,
mg g-1
Amount of dye adsorbed onto complete
monolayer sites, mg g-1
Amount of dye adsorbed at time t, mg g-1
Time, min
11.
12.
13.
14.
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: December 17, 2008
Revision Received: February 25, 2009
and Accepted: February 28, 2009