Waste waters containing different dyes are often discharged in aqueous effluents
from dyestuff manufacturing, dyeing and textile industries. The physical process
of dye removal from effluents may be divided into two main categories: adsorption
and membrane separation (Slokar and Le Marechal, 1998;
Vandevivere et al., 1998; Hao
et al., 2000). Adsorption is generally considered to be an effective
method for quickly lowering the concentration of dissolved dyes in an effluent.
In this regard, activated carbon has been extensively used for removal of color
resulting from the different classes of dyes and is now the most widely used
adsorbent for dyes. However, adsorption using activated carbon as adsorbent
is very expensive, the cost of regenerating the spent activated carbon is also
high (Girods et al., 2009). For this reason,
there is growing interest in utilizing biomass wastes/alternatives to activated
carbon as low-cost adsorbents (Gupta and Ali, 2002; Babel
and Kurniawan, 2003; Izadyar and Rahimi, 2007; Moazed,
2008; Gokturk and Kaluc, 2008; Lori
et al., 2008; Ali and Muhammad, 2008; Atmani
et al., 2009; Demiral and Gunduzoglu, 2010;
Nunes et al., 2009; Sze and
Cocoa (Theobroma cacao) is a plant species from the family of Sterculiaceae.
The evergreen tree growing crop, 6.0 to 8.0 m high in the tropical rainforest.
In Mexico, cocoa bean is used as stimulant and foodstuff. From Mexico, it spread
all over the world. Currently, Africa is the major producer of cocoa beans (53%
of the total world production) followed by Latin America (31.3%) and Asia (8.8%).
The five most important cocoa producing countries supply almost 75% of the total
production (Ivory Coast, 26.0%; Brazil, 22.9%; Ghana, 12%; Malaysia, 6.5%; Nigeria,
6.2%). In Nigeria, more than 1.5 million tons of Cocoa Pod Husks (CPH) are wasted
annually. Some of the CPH are used as organic fertilizer, soap making and animal
feed production, whereas in Malaysia, an estimated amount of 217600 pods and
9500 million tons of bean shells are available (Hertrampf
and Piedad-Pascual, 2003). Cocoa pod husks are produced after the removal
of the cocoa bean from the fruit. Basically, each ton of dry cocoa beans is
equivalent to about 10 tons of cocoa pod husks. Most of the husks are disposed
as waste which in turn resulted in landfill problem. This is because the use
of cocoa pod husks is limited to livestock feed due to theobromine content,
a significant cause of disease called inoculums (Serra and
Ventura, 1999). This cheap and abundant agricultural waste could be converted
to activated carbon. Conversion of CPH to activated carbon has two advantages.
First, the discarded agricultural waste is converted into useful and valuable
adsorbent. Secondly, wastewater treatment problem caused by textile industries
in Malaysia can be solved with the use of the adsorbent derived from CPH (Tan
et al., 2008).
This study is aimed at preparing and characterizing the activated carbon produced from cocoa pod husk, (CPHAC) investigating the adsorption of RBBR dye on CPHAC using batch process. Other factor studied includes; initial dye concentration, contact time, solution temperature, pH, adsorption isotherm, kinetics and thermodynamic parameters of the adsorption process.
MATERIALS AND METHODS
Preparation of CPHAC: CPH used for preparation of activated carbon was obtained from Cocoa Research and Development Centre, Cocoa Board Malaysia, Hilir Perak, Malaysia in 2010. CPH were firstly washed with water to remove dirt from it surface and subsequently dried at 105°C for 24 h to remove the moisture content. The dried CPH was ground into small pieces and sieved to the desired particle size of 1-2 mm and loaded in a stainless steel vertical tubular reactor placed in a tube furnace. Carbonization of the dried CPH was carried out at 500°C with heating rate of 10°C min-1 under purified nitrogen flown through at a flow rate of 150 cm3 min-1 for 1 h. In the second step, the char was activated under the same condition, but to a final temperature of 700°C for 2 h. Once the final temperature is reached, the nitrogen gas flow was switched to CO2 (200 cm3 min-1) to complete the activation process. The activated product was then cooled to room temperature under nitrogen flow and then washed with hot deionized water and hydrochloric acid (0.1 M) until the pH of the solution used for washing reached 6.5-7.0.
||SEM micrographs of (a) Char (magnification = 2000x) and (b)
CPHAC (magnification = 2000x)
Characterization of adsorbents: The precursor and the prepared activated carbon were characterized using various techniques in order to study their physical and chemical properties.
Nitrogen adsorption-desorption measurements: Nitrogen adsorption-desorption measurement was carried out to determine the surface area, total pore volume and mesopore volume of the samples by using Volumetric adsorption apparatus (Micromeritics Model ASAP 2020, US). The sample to be tested was degassed for 4 h under vacuum at 120°C prior to analysis. The sample was then cooled in liquid nitrogen. The analysis was carried out at 77 K by allowing known volume of nitrogen gas in and out of the sample and measuring the equilibrium pressure. The surface area, total pore volume and mesopore volume analysis were performed by a software (Micropore version 2.46). Mesopore volume was calculated by subtracting total volume, obtained at a relative pressure of 0.99 from the micropore volume, which was obtained from t-plot equation.
Surface morphology (SEM): Scanning electron microscopy (Model VPFESEM
Supra 35VP) was used to study the surface morphology, surface textures and the
development of porosity of char and activated carbon prepared. Results obtained
are presented in Fig. 1a and b.
Proximate analysis (TGA): Thermo Gravimetric Analyzer (TGA) (Model Perkin Elmer TGA7, US) was used to analyze the moisture content, volatile matter, fixed carbon and ash contents in the precursor, char and the prepared activated carbon. Five gram of the sample was transferred into the platinum pan of the TGA analyzer. The furnace chamber was then raised and the sample was degassed for few min. Nitrogen gas was allowed to pass through the furnace to ensure an inert atmosphere and the sample was heated from ambient temperature to 110°C until a constant weight is obtained for the moisture content determination. The temperature was then increased to 850°C and held for 7 min. The temperature was then decreased to 800°C. The nitrogen gas was switch off while oxygen gas was allowed to burn the sample. Fixed carbon content was determined from the weight loss during the burning stage. Ash content and the remaining mass were determined at the end of the analysis.
|| FTIR spectrum of both char and CPHAC
Elemental analysis: Elemental analysis was performed to determine the elemental composition of organic matter of precursor, char and the prepared activated carbon using Perkin Elmer (Series II) elemental analyzer.
Fourier transform infrared (FTIR): Fourier transform infrared (FTIR) spectroscopic analysis was used to study the surface chemistry of the char and the prepared activated carbon using FTIR spectroscope (FTIR-2000, Perkin Elmer). The FTIR spectra (Fig. 2) give information about the characteristic functional groups on the surface of the char and the prepared activated carbon. The analysis was conducted by encapsulating the sample in potassium bromide pellets. The spectra were measured from 4000 to 400 cm-1.
Remazol Brilliant Blue Reactive (RBBR) dye: RBBR dye supplied by Sigma-Aldrich (M) Sdn Bhd, Malaysia was used as adsorbate. De-ionized water was used to prepare all reagents and solutions.
Batch equilibrium studies: Batch equilibrium tests were carried out for adsorption of RBBR dye on the CPHAC prepared. The effects of initial dye concentration, contact time, temperature and solution pH on the adsorption uptake were investigated. Sample solutions were withdrawn at equilibrium to determine the residual concentration. The solutions were filtered prior to analysis in order to minimize interference of carbon fines with the analysis. For equilibrium studies, the experiment was carried out for 24 h to ensure that equilibrium was reached. The linear Beer-Lambert relationship between absorbance and concentration i.e., a calibration curve was established by plotting the graph of absorbance versus concentration of the dye solution. The concentration of RBBR dye solution before and after adsorption were determined using a double UV-vis spectrophotometer (UV-1800 Shimadzu, Japan) at a maximum wavelength of 597 nm. The amount of RBBR dye adsorbed at equilibrium, qe (mg g-1) was calculated using Eq. 1.
where, Co and Ce (mg L-1) are the liquid-phase concentrations of RBBR dye at initial and at equilibrium, respectively. V (L) is the volume of the solution and W (g) is the mass of CPHAC used.
Effect of initial RBBR dye concentration and contact time: In order to study the effect of initial concentration and contact time on the RBBR dye uptake, 100 mL of RBBR dye solution with initial concentration of 20-100 mg L-1 were prepared in a series of 250 mL Erlenmeyer flasks. An equal mass of 0.50 g of the CPHAC was added into each flask covered with glass stopper and the flasks were then placed in an isothermal water bath shaker (Model Protech, Malaysia) at constant temperature (30°C), with rotation speed of 135 rpm, until equilibrium point was reached. In this case, the experiment was conducted without pH adjustment.
Effect of solution temperature: The effect of solution temperature on the RBBR dye adsorption process was examined by varying the adsorption temperature at 30, 45 and 60°C by adjusting the temperature controller of the water bath shaker, while other operating parameters such as volume of RBBR dye and rotation speed were kept constant. The concentration of RBBR dye was varied ranging from 20-100 mg L-1.
Effect of solution pH: The effect of solution pH on the RBBR dye adsorption process was studied by varying the initial pH of the solution from 3 to 11. The pH was adjusted by adding 0.1 M hydrochloric acid or 0.1 M sodium hydroxide and was measured using pH meter (Model Delta 320, Mettler Toledo, China). The RBBR dye initial concentration was fixed at 100 mg L-1 with adsorbent dosage of 0.5 g/100 mL. The solution temperature was maintained at 30°C. The RBBR dye percent removal was calculated using Eq. 2.
Batch kinetic studies: For the batch kinetic studies, the same procedure was followed, but the aqueous samples were taken at preset time intervals. The concentrations of RBBR dye were similarly measured. The RBBR dye uptake at any time, qt (mg g-1), was calculated using Eq. 3.
where, Ct (mg L-1) is the liquid-phase concentration of RBBR dye at any time, t (mins).
RESULTS AND DISCUSSION
Characterization of CPHAC
Surface area and pore size characteristics: Table 1
reports the BET surface area, Langmuir surface area, pore volume and mesopore
volume of the precursor, char and the CPHAC. The activated sample has a relatively
high BET surface area of 502.7 m2 g-1 and possesses a
high total pore volume of 0.504 cm3 g-1. The high BET
surface area and total pore volume of the CPHAC were due to the CO2
|| Properties of precursor char and CPHAC
The CO2 gas molecules diffuse into the char and activated samples
thereby increasing the CO2-carbon reaction leading to the development
of additional pores on the samples. CO2 gasification was found to
promote the formation of mesopores and enhance the surface area of activated
carbon (Tseng et al., 2006).
Surface morphology: Scanning Electron Microscopy (SEM) was used to characterize
the surface morphology of the char and the activated carbon samples. Figure
1a and b show the SEM micrographs of both the char and
activated sample, respectively. In Fig. 1a, the SEM micrograph
shows that the char surface was rough and the pores were not properly developed,
whereas in Fig. 1b, there are several pores formed on the
activated carbon surface. The carbonization process created gradual porosity
on the activated sample leading to the formation of additional pores on the
sample thereby increasing the ability of inorganic material removal (Suzuki
et al., 2007). The well-developed pores resulted to large surface
area and porous structure of the activated carbon (Hameed
and Daud, 2008). These pores allowed a good surface for RBBR dye to be trapped
and adsorbed into (Amin, 2008) thus, reflecting the
potential use of CPH waste as a precursor for activated carbon preparation.
Proximate analysis: The volatile matter content in the precursor was very high but low in ash content. However, the volatile matter content in the CPHAC was sharply reduced when compared to that of the precursor. This is because organic substances present in the sample became unstable at elevated temperature; the heat breaks the molecular bond and linkage. As a result, these substances volatilizes off both as gas and liquid products. The fixed carbon content was satisfactory enough in CPHAC, having a value of 75.4% compared to the precursor which has only 19.3%, indicating that the precursor was suitable in activated carbon conversion. The ash content in CPHAC was considered high, due to silica and cellulose. For moisture content, it left only 2% in CPHAC compared to that of the precursor having 3.2%. The higher fixed carbon and low moisture contents made CPHAC a very good adsorbent in the adsorption of RBBR dye (Table 2).
Elemental analysis: In this case, the total amount of carbon present
in the sample improved after pyrolysis when compared to the precursor. The heat
supplied during the carbonization process initiates the thermal degradation
process. Decomposition occurred since the volatile compounds could not maintain
its stability at high temperatures, the volatiles matter are completely removed
during carbonization leaving only the portion of stable carbon as residues.
Hydrogen is a crucial element in organic molecule bonding and in all organic
compounds. However, during pyrolysis process, the activation process does not
make much difference in the amount of hydrogen present (Oh
and Park, 2002) (Table 3).
|| Proximate content of precursor, char and CPHAC
|| Elemental analysis of precursor, char and CPHAC
|*Estimated by difference
Fourier transform infrared spectroscopy (FTIR): The FTIR spectra of both the char and CPHAC are shown in Fig. 2. OH stretching bonds were detected for both char and CPHAC at bandwidth around 3300-3500 cm-1. Besides, there are other peaks detected on the char spectrum, which are at the bandwidth of 2922, 1402 and 1118 cm-1 assigned to C-H stretching related to alkanes and alkyl groups, CH2 deformation in alkyl group and C-O-C stretch, respectively. Peaks at 1581 and 617 cm-1 on char spectrum indicated the presence of carboxylic acid and aliphatic C-H specie vibrations, respectively. On the other hand, the peaks appearing on CPHAC spectrum at 874 cm-1 reflects the existing C-H out of plane deformation groups. Some of the functional groups have disappeared in CPHAC, such as C-H stretch (2922 cm-1), carboxylic acid (1581 cm-1), C-O-C stretch (1118 cm-1) and aliphatic C-H species vibration (617 cm-1). These are as a result of high temperature in activation process that broke some intermolecular bonds.
Batch adsorption studies: Batch adsorption of RBBR dye on CPHAC was studied as a function of initial dye concentration, contact time, solution temperature and solution pH.
Effect of initial dye concentration and contact time on adsorption process:
Figure 3 shows the effect of initial dye concentration and
contact time on the adsorption of RBBR dye on CPHAC at 30°C. The plot showed
that the adsorption of RBBR dye increases with increasing time. The properties
of Remazal Black B Reactive Dye are shown in Table 4. The
adsorption was fast at initial stage of the contact period; thereafter it became
slow as it reaches equilibrium. This phenomenon was based on the fact that large
numbers of vacant surface sites were available for adsorption during the initial
stage (Tan et al., 2008; Bello
et al., 2008; Bello et al., 2010).
After a lapse of time, the remaining vacant surface sites became difficult to
occupy due to repulsive forces between the solute molecules on the solid and
bulk phases (Hameed and Ahmad, 2009). Equilibrium was
reached within 300 min at (20 mg L-1 to 60 mg L-1) and
350 min for (80 mg L-1 to 100 mg L-1) initial concentrations.
The amount of dye adsorbed at equilibrium time reflected the maximum adsorption
uptake of CPHAC. During the adsorption process, the dye molecules initially
have to encounter the boundary layer effect before diffusing from the boundary
layer film onto the adsorbent surface.
||Remazol Black B reactive dye adsorption uptake at different
initial concentrations and contact time on CPHAC (T=30°C; 0.5 g of CPHAC;
|| Properties of remazol black b reactive dye
Finally, the dye molecules have to diffuse into the porous structure of the
adsorbent (Senthilkumaar et al., 2005). This
theory explains the fact that due to the presence of large amount of dye molecules
it takes a relatively longer contact time to attain equilibrium at higher initial
concentrations. The plot shows that there was an increase in the adsorption
capacity at higher initial concentration of RBBR dye. This phenomenon was due
to the fact that the initial dye concentration provides an important driving
force to overcome the mass transfer resistance of the dye between the aqueous
and solid phases (Ozlem et al., 2009). The equilibrium
adsorption capacity (qe) for RBBR dye adsorption increased from 6.1-25.3
mg g-1 for the concentration increase from 20-100 mg L-1
at 30°C. As the initial dye concentration increased, the mass transfer driving
force becomes larger and this resulted in higher RBBR dye removal. The percentage
of RBBR dye removal decreased from 76.3 to 38.3%. Thus, the percentage of RBBR
dye removal was higher at low concentration due to the availability of unoccupied
binding sites on the CPHAC. At higher concentration, the binding sites on CPHAC
were almost completely occupied, thus leading to a decrease in the percent of
RBBR dye removal. Ozlem et al. (2009) showed
that the percentage of RBBR dye adsorption on cotton stalk and cotton hull decreased
from 54.9 to 10.8% and 79.6 to 16.7%, respectively with increasing concentration
from 25-300 mg L-1. The percentage of RBBR dye removal was strongly
affected by its initial concentration as shown in Table 5.
Effect of solution temperature: Figure 4 shows the
amount of RBBR dye adsorbed at equilibrium, qe (mg g-1)
on CPHAC as a function of the solution temperature at different initial concentrations
(20 to 100 mg L-1). RBBR dye adsorption uptake was found to increase
with increasing solution temperature from 30 to 60°C at all concentrations,
indicating the endothermic nature of the adsorption process.
|| Percentage removal of Remazol Black B Reactive Dye on CPHAC
|| Adsorption isotherm of Remazol Black B Reactive Dye on CPHAC
at different temperatures
Increasing the temperature was known to increase 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.
The enhancement in the adsorption capacity might be due to the chemical interaction
between adsorbate and adsorbent, creating new adsorption sites hence increasing
the rate of intraparticle diffusion of adsorbate molecules into the pores of
the CPHAC at higher temperatures. The amount of RBBR dye removed at equilibrium
increased from 6.1 to 14.9 mg g-1 and 7.0 to 23.1 mg g-1,
respectively as temperature increases from 30 to 60°C (Fig.
Effect of solution pH: The pH of the solution was varied from pH 3 to
11. Initial concentration was set at 100 mg L-1 at 30°C. Plots
of the equilibrium adsorption capacity (qe) against pH of the solution
for RBBR dye adsorption was shown in Fig. 5. From the plot,
the influence of pH on the adsorption capacity was considered negligible. RBBR
dye adsorption was hardly affected by the pH variation. Similar trend was reported
by Vijayaraghavan et al. (2009) in a study on
the treatment of complex remazol dye effluent using sawdust-and coal-based activated
carbons. The decolorization of Remazol dyes was reported to be unaffected by
the solution pH.
Adsorption kinetics: Adsorption kinetics was investigated for better understanding of the dynamics of adsorption of RBBR dye onto CPHAC and to obtain predictive models that allow estimations of the amount adsorbed with respect to time.
The pseudo first order kinetic model: The pseudo first-order equation
generally expressed by Lagergren and Svenska (1898)
where, qe and qt are the adsorption capacities at equilibrium
and at time t, respectively (mg g-1). k1 is the rate constant
for pseudo-first order adsorption (min-1). A plot of ln (qe-qt)
against t at various concentrations and temperatures resulted in graphs with
negative slopes. k1 and qe are calculated from the slopes
and intercepts, respectively (Fig. 6).
||Effect of solution pH on the adsorption of Remazol Black B
Dye onto CPHAC (Co = 100 mg L-1; 0.5 g CPHAC; T =
||Pseudo-first order kinetic plot of the adsorption of Remazol
Black B Reactive Dye unto CPHAC at 30°C
Although, the correlation coefficients were high but comparison of the qe
calc. to the qe exp., the values do not agree. Results
are presented in Table 6a. Therefore, the adsorption of RBBR
dye onto CPHAC does not follow the pseudo-first order kinetics.
The pseudo second order kinetic model: The pseudo second order equation
expressed by Ho and McKay (1999) was used. Plots of
t/qt versus t gave linear graphs from which qe and k2
were estimated from the slopes and intercepts of the plot (Fig.
7) for temperatures 30-60°C. The correlation coefficients were as high
as 0.99 and there were good agreement between qe, cal. and qe,
exp data obtained. The results are presented in. Table
6. The good agreement shows that the pseudo- second order kinetic equation
fits the adsorption data well. Hence, the adsorption of RBBR dye onto CPHAC
follows the pseudo-second order kinetics (Table 6b).
Adsorption isotherms: The adsorption isotherm indicates how the adsorbed
molecules distribute themselves between the liquid phase and the solid phase
until the adsorption process reaches an equilibrium state.
||Pseudo-first-order and pseudo-second-order kinetic model parameters
for Remazol Black B Reactive Dye adsorption on CPHAC at 30°C
||Intraparticle diffusion model and Elovich kinetic models parameters
for Remazol Black B Reactive Dye adsorption on CPHAC at 30°C
||Pseudo-second order kinetic plot of the adsorption of Remazol
Black B Reactive Dye unto CPHAC at 30°C
This is basically important in describing how solutes interact with adsorbents
and is critical in optimizing the use of the adsorbent (Buckley,
1991; Carliell, 1993). The analysis of equilibrium
adsorption isotherm data were then carried out by fitting them to different
isotherm models so as to find a suitable model to describe the experimental
data. The adsorption isotherm studies were carried out using the following models.
Langmuir isotherm model: Langmuir isotherm assumes monolayer adsorption
into a surface containing a finite number of adsorption sites which are uniform
for adsorption with no trans-migration of adsorbate in the plane of surface
(Langmuir, 1916). The linearized form of Langmuir adsorption
model is expressed as:
||Langmuir isotherm plot for Remazol Black B Reactive Dye adsorption
onto CPHAC at different temperatures
||Langmuir isotherm model parameters and correlation coefficients
for adsorption of Remazol Black B reactive dye at different temperatures
||Comparison of maximum monolayer adsorption of some dyes unto
where, Ce is the dye concentration in the solution at equilibrium
(mg L-1), qe is the dye concentration on the adsorbent
at equilibrium (mg g-1), qo is the monolayer adsorption
capacity of CPHAC (mg g-1) and b is the Langmuir biosorption constant
(L mg-1). A plot of Ce/qe versus Ce
gave a straight line with a slope 1/qo and an intercept of 1/qob
(Fig. 8). The R2 value of Langmuir isotherm (Table
7a and c) when compared with Freundlich isotherm indicates
that the adsorption of the RBBR dye onto CPHAC fits the Langmuir isotherm. Values
of qo are calculated and reported in Table 7a.
The value of qo obtained was compared with those from other adsorbents;
the values are reported in Table 7b. The qo values
obtained from this study showed that CPHAC is a good adsorbent for the removal
of RBBR dye from aqueous solutions.
To confirm the favorability of the process, the dimensionless equilibrium parameter (RL) defined by Eq. 5 was used:
||Freundlich, Temkin and Dubinin-Radushkevich isotherm model
parameters and correlation coefficients for adsorption of Remazol Black
B Reactive Dye onto CPHAC at different temperatures
||Freundlich isotherm plot for Remazol Black B Reactive Dye
adsorption onto CPHAC at different temperatures
where, Co is the highest initial dye concentration in solution,
is used to confirm the favorability of the adsorption process, that is (0<RL<1)
favorable, RL = 1 linear, RL = 0 irreversible or RL>1
unfavorable (Langmuir, 1916). The values of RL
reported in Table 7a obtained at various temperatures were
less than one, indicating that the adsorption of RBBR dye onto CPHAC is favorable.
Freundlich isotherm model: This assumes heterogeneous surface energies,
in which the energy term in Langmuir varies as a function of the surface coverage.
The linearized form of Freundlich model is represented by Freundlich
(1906) where, qe is the amount of RBBR dye adsorbed at equilibrium
(mg g-1), Ce is the equilibrium concentration of the adsorbate
(mg L-1); kf and n are constants incorporating the factors
affecting the adsorption capacity and the degree of non-linearity between the
solute concentration in the solution and the amount adsorbed at equilibrium,
respectively. Plots of log qe versus log Ce gave linear
graphs (Fig. 9) with lower values of R2 compared
to Langmuir model indicating that the adsorption data does not fits the Freundlich
isotherm model. The parameters are reported in Table 7c.
The values of kf and n from the graph are reported in Table
7c, it shows the heterogeneity of the material as well the possibility of
multilayer adsorption of RBBR dye through the percolation process and the values
of 1/n less than one indicates that the adsorption is favorable.
Temkin isotherm model: In order to consider the effect of the adsorbate
interaction on the adsorbent, Temkin isotherm model was tested on the experimental
data (Temkin and Pyzhev, 1940).
||Temkin isotherm plot for Remazol Black B Reactive Dye adsorption
onto CPHAC at different temperatures
where, AT and bT are constants, R is the gas constant
and T is the temperature. A plot of qe versus ln Ce gave
a straight-line graph (Fig. 10) where, bT and
AT values were calculated and presented in Table 7c.
AT increases as the temperature increases thereby inferring that
the adsorbate interaction with adsorbent increased with increasing temperature
hence, higher rate of sorption was observed as energy increases.
Dubinin-Radushkevich isotherm model, (D-R model): The D-R model (Dubinin
and Radushkevich, 1947) which does not, assumes a homogeneous surface or
a constant biosorption potential as the Langmuir model, is used to estimate
the characteristic porosity of the adsorbent and the apparent energy of adsorption.
It was also used to test the experimental data. It is written as
where, β is the free energy of sorption per mole of the sorbate as it migrates to the surface of the adsorbent from an infinite distance in the solution (mol2/J2), qo is the maximum adsorption capacity and ε is the Polanyi potential (J/mol) that can be written as:
A plot of ln qe versus ε2 gave a linear plot (Fig.
11) where, β and qo are obtained from the slopes and intercepts,
respectively. The values are presented in Table 7c. Similarly,
the β value obtained was then used to estimate the mean free energy of
adsorption (E). The results are presented in Table 7c.
The values of E were found to be between the ranges 0.408-2.236 kJ mol-1
over the range of temperatures used in this study. Since E<8 kJ mol-1,
it suggests that the adsorption mechanism is physical in nature (Helfferich,
1962; Arivoli and Thenkuzhali, 2008; Dang
et al., 2009).
||Dubinin-Radushkevich isotherm plot for Remazol Black B Reactive
Dye adsorption onto CPHAC at different temperatures
Thermodynamic studies: There are three thermodynamic parameters that must be considered to characterize the adsorption process; they are: the enthalpy (ΔH°), Gibbs free energy (ΔG°) and entropy (ΔS°) due to transfer of one mole of solute from the solution onto solid-liquid interface. The values of ΔH° and ΔS° can be obtained from the following equation:
where, R (8.314 J/mol/K) is the universal gas constant, T (°C) is the absolute solution temperature and KL (L mg-1) is the Langmuir isotherm constant i.e., b. The values of ΔH° and ΔS° can be calculated, respectively from the slope and intercept of the Vant Hoff plot of ln KL versus 1/T (Fig. 12). ΔG° can then be calculated using the relation below:
In order to evaluate the activation energy of adsorption representing the minimum energy that reactants must have for the reaction to proceed, Arrhenius equation was applied. It is expressed as:
where, k2 (g mg-1 min) is the rate constant obtained from the pseudo second-order kinetic model, Ea (kJ mol-1) is the Arrhenius activation energy of adsorption and A is the Arrhenius factor. When ln k2 was plotted against 1/T, a straight line with slope of -Ea/R was obtained (Fig. 13).
The calculated values of ΔH°, ΔS° and ΔG° for adsorption
of RBBR dye on CPHAC are reported in Table 8. The important
thermodynamic function ΔH° is very useful whenever a differential change
occurs in the system. The positive ΔH° value obtained indicates that
the adsorption process was endothermic in nature. Endothermic adsorption process
could be due to the increased rate of diffusion of adsorbate molecules across
the external boundary layer and internal pores of adsorbents as a result of
increase in temperature (Wang and Zhu, 2007).
||Plots of ln k2 versus 1/T for adsorption of Remazol
Black B Reactive Dye onto CPHAC at different temperatures
||Plots of ln k2 versus 1/T for adsorption of Remazol
Black B Reactive Dye onto CPHAC at different temperatures
|| Thermodynamic parameters for the adsorption of Remazol Black
b Reactive Dye onto CPHAC at different temperatures
The positive value of standard entropy change, ΔS°, shows increased
randomness at the solid/solution interface occurring in the adsorption process
reflecting the affinity of the adsorbent toward the RBBR dye molecules. The
overall free energy change, ΔG° for RBBR dye adsorption was negative
showing that the adsorption process is spontaneous. Besides, Ea value
obtained was 34.6 kJ mol-1, a value lower than 40 kJ mol-1,
indicating that the rate-limiting step in the Remazol Black B Dye adsorption
process is physically controlled. This fact further confirmed that the adsorption
of RBBR dye onto CPHAC is physisorption in nature.
The present investigation showed that CPHAC is an effective adsorbent for removal of RBBR dye from aqueous solution. Adsorption of RBBR dye was found to increase with increase in contact time, RBBR dye initial concentration and solution temperature. The Langmuir isotherm model and the pseudo-second order kinetic model were found to fit the adsorption data well. The effect of pH on the adsorption process was negligible. The intraparticle diffusion model was found to be more favorable compared to Elovich model in the adsorption process. In addition to this, the positive values of ΔH° indicated that the adsorption process was endothermic in nature. Ea value of 34.6 kJmol-1 was obtained, showing that the rate-limiting step in the RBBR dye adsorption process is physically controlled.
The authors gratefully acknowledge the financial support received in the form of research grants (PJ/KIMIA 814003 and PJ/AWAM 814021) from Universiti Sains Malaysia that resulted in this article. The one year Post Doctoral Fellowship jointly awarded by USM-TWAS to Dr. Olugbenga Solomon BELLO (FR Number: 3240223483 in Year 2009) of the Department of P/A Chemistry Ladoke Akintola University of Technology, P.M.B 4000, Ogbomoso, Oyo State. Nigeria and the 12 month study leave granted him by his host institution to honor this fellowship are equally acknowledged.