The discharge of dyes in the environment is worrying for both toxicological
and esthetical reasons (Metivier-Pignon et al., 2003).
Methylene blue (MB) is the most commonly used substance for dying cotton, wood
and silk. MB can cause eye burns which may be responsible for permanent injury
to the eyes of human and animals. On inhalation, it can give rise to short periods
of rapid or difficult breathing while ingestion through the mouth produces a
burning sensation and may cause nausea, vomiting, profuse sweating, mental confusion
and methemoglobinemia (Ghosh and Bhattacharyya, 2002).
Therefore, the treatment of effluent containing such dye is of interest due
to its harmful impacts on receiving waters.
Colour removal from industrial wastewaters by adsorption techniques has been
of growing importance due to the chemical and biological stability of dyestuffs
to conventional water treatment methods and the growing need for high quality
treatment (Basar, 2006). However, commercially available
activated carbons are still considered as expensive materials due to the use
of non-renewable and relatively expensive starting material such as coal, which
is unjustified in pollution control applications (Attia
et al., 2008). Therefore, in recent years, this has prompted a growing
research interest in the production of activated carbons from renewable and
cheaper precursors which are mainly industrial and agricultural by-products,
such as apricot shell (Karagozoglu et al., 2007),
male flower of coconut tree (Senthilkumaar et al.,
2006), jute fiber (Senthilkumaar et al., 2005),
rubber wood sawdust (Kalavathy et al., 2005),
corncob (Tseng et al., 2006), bamboo (Hameed
et al., 2007) and oil palm fibre (Tan et al.,
At present, Malaysia is one of the largest exporters of palm oil in the international
market. One of the significant problems in the palm fruit processing is managing
of the wastes generated during the processes. Approximately 15 million tons
of EFB waste is generated annually throughout Malaysia by palm oil mills (Rahman
et al., 2007). In practice this biomass is burnt in incinerator by
palm oil mills which creates environmental pollution problems in nearby localities
and also offers limited value to the industry. To make better use of this abundant
waste, it is proposed to convert it into activated carbon. Conversion of coconut
husk to activated carbon will serve a double purpose. First, unwanted agricultural
waste is converted to useful, value added adsorbents and second, the use of
agricultural by products represents a potential source of adsorbents which will
contribute to solving part of the wastewater treatment problem in Malaysia.
From our previous study, EFB-based activated carbon was found to be an effective
adsorbent for removing chlorophenol from aqueous solutions (Tan
et al., 2009).
The present study aims to evaluate the potentiality of EFB-based activated carbon to remove MB from aqueous solutions. The experimental data of the adsorption process were analyzed to study the adsorption isotherms, kinetics, thermodynamics and mechanism of MB on the EFB-based activated carbon. The feasibility of regenerating the spent activated carbon using ethanol desorption was then determined.
MATERIALS AND METHODS
Preparation of activated carbon: EFB used for preparation of activated
carbon in this study was obtained from a local palm oil mill. The activated
carbon preparation procedure was referred to our previous study (Tan
et al., 2008) where the pre-treated EFB was loaded in a stainless
steel vertical tubular reactor placed in a tube furnace and the carbonization
of the precursor was carried out by ramping the temperature from room temperature
to 700°C with heating rate of 10°C min-1 and hold for 2 h.
Throughout the carbonization process, purified nitrogen (99.995%) was flown
through at flow rate of 150 cm3 min-1. The activated carbon
was prepared using physiochemical activation method consisting of potassium
hydroxide (KOH) treatment followed by carbon dioxide (CO2) gasification
by applying the optimum operating conditions obtained from our earlier study
(Tan, 2008). The char produced from the carbonization
process was mixed with KOH pallets with KOH:char Impregnation Ratio (IR) of
2.9:1. The dried mixture was then activated under the same condition as carbonization,
but to a final temperature of 844°C. Once the final temperature was reached,
the nitrogen gas flow was switched to CO2 and activation was held
for 1.8 h. The activated product was then cooled to room temperature under nitrogen
flow and then washed with hot deionized water and 0.1 M HCl until the pH of
the washing solution reached 6-7.
Batch equilibrium and kinetic studies: MB supplied by Sigma-Aldrich (M) Sdn Bhd, Malaysia was used as an adsorbate and was not purified prior to use. Deionized water was used to prepare all the solutions and reagents. MB has a chemical formula of C16H18N3SCl, with molecular weight of 319.86 g mol-1.
In order to study the effect of MB initial concentration and contact time on the adsorption uptake, adsorption tests were performed in a set of 43 Erlenmeyer flasks (250 mL) where 100 mL of MB solutions with initial concentrations of 50-500 mg L-1 were placed in these flasks. Equal mass of 0.1 g of the prepared activated carbon with particle size of 200 μm was added to each flask and kept in an isothermal shaker of 120 rpm at 30°C for 30 h to reach equilibrium. The pH of the solutions was original without any pH adjustment. Similar procedures were followed for another two sets of Erlenmeyer flask containing the same initial dye concentrations and same activated carbons dosage, but were kept under 40 and 50°C for thermodynamic studies. Aqueous samples were taken from each of the MB solutions at preset time intervals using disposable filtered syringes and the concentrations were then analyzed. The concentrations of MB in the supernatant solution before and after adsorption were determined using a double beam UV-Visible spectrophotometer (UV-1601 Shimadzu, Japan) at 668 nm wavelength. The amount of adsorption at equilibrium, qe (mg g-1), was calculated by:
where C0 and Ce (mg L-1) are the liquid-phase concentrations of dye at initial and equilibrium, respectively. V is the volume of the solution (L) and W is the mass of dry adsorbent used (g).
The equilibrium data were then fitted using four different isotherm models, namely the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models.
For kinetic studies, the amount of adsorption at time t, qt (mg g-1), was calculated by:
where Ct (mg L-1) is the liquid-phase concentrations of dye at time, t.
The kinetic data were then fitted using the pseudo-first-order, pseudo-second-order and intraparticle diffusion models.
Regeneration of spent activated carbon: The feasibility of regenerating
the spent activated carbon saturated with MB was evaluated using ethanol desorption
technique (Tanthapanichakoon et al., 2005). Initially,
batch equilibrium tests were performed on the fresh activated carbons prepared
where 100 mL of MB solution with initial concentration of 400 mg L-1
were placed in 250 mL Erlenmeyer flasks 0.1 g of the fresh EFB-based activated
carbon was added into the flask and placed in an isothermal water bath shaker
at 30°C, with rotation speed of 120 rpm, agitated for 48 h until complete
equilibrium was attained. The solution pH was kept original without any pH adjustment.
The concentrations were similarly measured using UV-visible spectrophotometer
and the concentration of adsorbate adsorbed at equilibrium, Cad (mg
L-1) was calculated as the difference between the initial and equilibrium
The spent activated carbon was then separated from the solution and washed with deionized water to remove any unsorbed MB. The sample was then dried at 110°C in an oven and then added into Erlenmeyer flask containing 100 mL of 95 vol% ethanol for desorption of the MB. The flask was kept in the isothermal water bath shaker at the same temperature for the same time duration as the adsorption tests. After desorption, the concentrations of MB desorbed, Cde (mg L-1) was similarly measured using the UV-visible spectrophotometer. The percent desorption was calculated using Eq. 3:
RESULTS AND DISCUSSION
Effect of MB initial concentration and agitation time on adsorption equilibrium:
Figure 1 shows the adsorption uptake versus the adsorption
time at various initial MB concentrations at 30°C. It indicated that the
contact time needed for MB solutions with initial concentrations of 50-200 mg
L-1 to reach equilibrium was around 2 h. However, for MB solutions
with higher initial concentrations of 300-500 mg L-1, equilibrium
times of around 24 h were required. This observation could be explained by the
theory that in the process of dye adsorption, initially the dye molecules have
to first encounter the boundary layer effect and then diffuse from the boundary
layer film onto adsorbent surface and then finally, they have to diffuse into
the porous structure of the adsorbent (Senthilkumaar et
al., 2005). Therefore, MB solutions of higher initial concentrations
will take relatively longer contact time to attain equilibrium due to higher
amount of dye molecules (Fig. 1). The amount of MB adsorbed
on the activated carbon increases with time and at some point in time, it reaches
a constant value beyond which no more MB is further removed from the solution.
At this point, the amount of the dye desorbing from the activated carbon is
in a state of dynamic equilibrium with the amount of the dye being adsorbed
on the activated carbon.
||MB adsorption uptake versus adsorption time at various initial
concentrations at 30°C on EFB-based activated carbon
The amount of dye adsorbed at the equilibrium time reflects the maximum adsorption
capacity of the adsorbent under those operating conditions. In this study, the
adsorption capacity at equilibrium (qe) was found to increase from
41.11 to 384.57 mg g-1 with an increase in the initial dye concentrations
from 50 to 500 mg L-1. When the initial concentration increased,
the mass transfer driving force would become larger, hence resulting in higher
adsorption of MB.
Adsorption isotherms: The analysis of the adsorption equilibrium data
by fitting them into different isotherm models is an important step to find
the suitable model that can be used for design purposes (El-Guendi,
1991). Adsorption isotherm is basically important to describe how solutes
interact with adsorbents and is critical in optimizing the use of adsorbents.
Adsorption isotherm study was carried out on four isotherm models, namely the
Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models. The applicability
of the isotherm equation to describe the adsorption process was judged by the
correlation coefficients, R2 values. For the Langmuir isotherm, when
Ce/qe is plotted against Ce, a straight line
with slope of 1/Qo and intercept of 1/QoKL
is obtained. For the Freundlich isotherm, the plot of log qe versus
log Ce gives a straight line with slope of 1/n and intercept of log
KF. A plot of qe versus ln Ce for the Temkin
isotherm yields a linear line with slope of B and intercept of (B ln A). For
Dubinin-Radushkevich isotherm, a plot of ln qe versus ε2
enables the constants qs and E to be determined (Tan
et al., 2009).
Table 1 shows the four isotherm models used, together with
all the constants and R2 values obtained from each plot. The Langmuir
model yielded the best fit with R2 value equal or higher than 0.98,
as compared to the other three models. Conformation of the experimental data
into Langmuir isotherm equation indicated the homogeneous nature of EFB-based
activated carbon surface, i.e., each dye molecule/EFB-based carbon adsorption
had equal adsorption activation energy. The results also demonstrated the formation
of monolayer coverage of dye molecule at the outer surface of the activated
carbon. Similar observations were reported by the adsorption of MB on activated
carbons prepared from olive-seed waste residue (Stavropoulos
and Zabaniotou, 2005) and corncob (Tseng et al.,
2006). The activated carbon prepared in this work had relatively large adsorption
capacity (357.14- 416.67 mg g-1) if compared to some adsorbents reported
in the literature.
Adsorption kinetic studies: The rate constant of adsorption is determined
from the pseudo-first-order equation given by Langergren
and Svenska (1898) as:
||Langmuir, Freundlich, Temkin and Dubinin-Radushkevich Isotherm
model constants and
where k1 is the first-order adsorption rate constant (1/h).
The pseudo-second-order equation (Ho and McKay, 1999)
based on equilibrium adsorption is expressed as:
where, k2 (g mg-1 h) is the rate constant of second-order adsorption.
The constants and R2 values obtained from the two linear plots are
summarized in Table 2. It was found that for pseudo-first-order
equation, the R2 values were relatively small and the experimental
qe values did not agree with the calculated values, indicating that
the adsorption of MB on the activated carbon did not follow this model. However,
for pseudo-second-order model, there was a good agreement between the experimental
and the calculated qe values. Besides, the R2 values obtained
for this model were greater than 0.99 for all MB concentrations, indicating
the applicability of the second-order kinetic model to describe the adsorption
process of MB on the EFB-based activated carbon. Based on the higher R2
and the smaller deviation values between the experimental and calculated qe,
the pseudo-second-order model was more suitable to describe the adsorption kinetics
of MB on the EFB-based activated carbon. This suggested that the overall rate
of the adsorption process was controlled by chemisorption which involved valency
forces through sharing or exchange of electrons between the sorbent and sorbate
(Ho and McKay, 1999).
Adsorption mechanism: As the above kinetic models were not able to identify
the diffusion mechanism, thus intraparticle diffusion model based on the theory
proposed by Weber and Morris (1962) was tested. It is
an empirically found functional relationship, common to the most adsorption
processes, where uptake varies almost proportionally with t1/2 rather
than with the contact time t. According to this theory:
|| Pseudo-first-order and Pseudo-second-order kinetic model
parameters for MB adsorption on EFB-based activated carbon at 30°C
||Plots of intraparticle diffusion model for MB adsorption on
EFB-based activated carbon at 30°C
where, kpi (mg g-1 h1/2), the rate parameter
of stage I, is obtained from the slope of the straight line of qt
versus t1/2, as shown in Fig. 2. Ci,
the intercept of stage I, gives an idea about the thickness of boundary layer,
i.e., the larger the intercept, the greater the boundary layer effect. If intraparticle
diffusion occurs, then qt versus t1/2 will be linear and
if the plot passes through the origin, then the rate limiting process is only
due to the intraparticle diffusion. Otherwise, some other mechanism along with
intraparticle diffusion is also involved. As can be seen from Fig.
2, the linear lines of the second and third stages did not pass through
the origin and this deviation from the origin or near saturation might be due
to the difference in the mass transfer rate in the initial and final stages
of adsorption (Mohanty et al., 2005). It shows
that intraparticle diffusion was not the only rate limiting mechanism in the
Kinetic data as obtained by the batch method has been treated by the expressions
given by Boyd et al. (1947), which is in accordance
with the observations of Reichenberg (1953). The three
sequential steps in the adsorption are:
||Film diffusion, where adsorbate ions travel towards the external
surface of the adsorbent
||Particle diffusion, where adsorbate ions travel within the
pores of the adsorbent excluding a small amount of adsorption that occurs
on the exterior surface of the adsorbent
||Adsorption of the adsorbate ions on the interior surface of
The third step is considered to be very fast thus it cannot be treated as rate
limiting step. If external transport > internal transport, rate is governed
by particle diffusion.
||Boyd plots for MB adsorption on EFB-based activated carbon
If external transport < internal transport, rate is governed by film diffusion
and if external transport ≈ internal transport, the transport of adsorbate
ions to the boundary may not be possible at a significant rate thus, formation
of a liquid film surrounding the adsorbent particles takes place through the
proper concentration gradient (Mittal et al., 2008).
In order to predict the actual slow step involved in the adsorption process, the kinetic data were further analyzed using the Boyd model given by Eq. 7:
F represents the fraction of solute adsorbed at any time, t (h), as calculated using Eq. 8:
The calculated Bt values were plotted against time t (h), as shown
in Fig. 3. The linear lines for all MB initial concentrations
did no pass through the origin and the points were scattered. This indicated
that the adsorption of MB on the EFB-based activated carbon was mainly governed
by external mass transport where particle diffusion was the rate limiting step
(Kalavathy et al., 2005).
Adsorption thermodynamics: The value of changes in standard enthalpy (ΔHo) and standard entropy (ΔSo) were computed using the following equation:
||Thermodynamic parameters for adsorption of MB on EFB-based
where, R (8.314 J mol-1 K) is the universal gas constant, T (K) is the absolute solution temperature and Kd is the distribution coefficient.
The standard free energy (ΔGo) can be calculated using the relation below:
The values of ΔHo and ΔSo were calculated from
the slope and intercept of plot between ln Kd versus 1/T for MB initial
concentration of 500 mg L-1. The calculated values of ΔHo,
ΔSo and ΔGo are listed in Table
3. The positive value of ΔHo indicated the endothermic nature
of the adsorption interaction. The adsorption reaction for the endothermic processes
could be due to the increase in temperature increased the rate of diffusion
of the adsorbate molecules across the external boundary layer and in the internal
pores of the adsorbent particle, owing to the decrease in the viscosity of the
solution (Wang and Zhu, 2007). The positive value of
ΔSo showed the affinity of the EFB-based activated carbon for
MB and the increasing randomness at the solid-solution interface during the
adsorption process. The negative value of ΔGo indicated the
feasibility of the process and the spontaneous nature of the adsorption with
a high preference of MB on the activated carbon. The results showed that the
adsorption process was more favorable at higher temperature (50°C), due
to the endothermic nature of the adsorption system.
Regeneration of spent activated carbon: The feasibility of regenerating
the spent activated carbon saturated with MB was evaluated using ethanol desorption
technique. The MB desorption of 71% obtained in this work was considered reasonably
high. It was reported in the literature that by using ethanol desorption, the
regeneration efficiency of waste tires-based activated carbons for phenol and
Red 31 dye were, respectively 35-45 and 30-40 (Tanthapanichakoon
et al., 2005). Thus, ethanol desorption technique was shown to be
a promising way to regenerate the spent activated carbon prepared in this study
as 71% of the adsorption sites could be recovered from the regenerated activated
The present investigation showed that EFB-based activated carbon was a promising adsorbent for the removal of MB dye from aqueous solutions over a wide range of concentrations. Equilibrium data were fitted to Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms and the equilibrium data were best described by the Langmuir isotherm model, with maximum monolayer adsorption capacity of 357.14 to 416.67 mg g-1 with increase of solution temperature from 30 to 50°C. The adsorption kinetics was found to follow closely the pseudo-second-order kinetic model whereas the adsorption process was mainly governed by external mass transport where particle diffusion was the rate limiting step. The positive ΔHo value confirmed the endothermic nature of the adsorption interaction. Ethanol desorption technique was efficient in regenerating the spent activated carbon, giving MB desorption of 71%.
The authors acknowledge the research grant provided by University Science Malaysia under The Fundamental Research Grant Scheme that resulted in this study.