JOURNAL OF ENVIRONMENTAL PROTECTION SCIENCE (2009), Vol. 3, pp. 111 – 116.
Equilibrium Isotherm Studies of Methylene Blue Adsorption onto Activated Carbon Prepared from Delonix regia Pods Yuh-Shan Ho,1* R. Malarvizhi,2 N. Sulochana2 1 2
Department of Biotechnology, College of Health Science, Asia University, Taichung 41354, Taiwan Department of Chemistry, National Institute of Technology, Trichy- 620 015, India
Abstract The adsorption capacity of Methylene blue from aqueous solution onto activated carbon prepared from Delonix regia pods (Flame tree pods) was investigated under various experimental conditions. Batch mode experiments were conducted to assess the potential of the above activated carbon for the removal of Methylene blue from aqueous solution. Equilibrium isotherm studies have been done by varying the following three parameters: initial concentration of Methylene blue dye solution, volume of the dye solution, and adsorbent dose on the uptake of dye from the solution. Non-linear analysis was used to compare the best-fitting isotherms. The equilibrium data obtained were fitted to Langmuir, Freundlich, and RedlichPeterson isotherm models. Keywords: Sorption, Isotherms, Methylene blue, Redlich-Peterson model, Non-linear analysis, Chi-square
JEPS (2009), Vol. 3, pp. 111 – 116 __________________________________________________________________________________ 1. Introduction Many treatment methods have been used to remove the dyes from wastewater. These can be divided into physical, chemical, and biological methods. Among the various methods, adsorption is an effective separation process for a wide variety of applications. It is now recognized as an effective and economical method for the removal of both organic and inorganic pollutants from wastewaters. The most widely used adsorbent is an activated carbon because of its high surface area due to the presence of micro and meso pores. A number of studies have also been performed using activated carbon prepared from agricultural wastes for the removal of dyes from aqueous solution. The waste materials include coconut tree flower and Jute fiber (1), oil palm fiber (2), palm kernel shell (3), corncob (4), Wood apple outer rind (5) and bagasse pith (6). To design a suitable reactor for wastewater treatment, knowledge about the equilibrium adsorption isotherm is of the utmost importance. Consequently, it is important to determine a suitable isotherm model to describe the mechanism of adsorption involved in the solid-liquid system under various conditions. Basic dyes are the brightest class of water soluble dyes used by the textile industries, and Methylene blue is one of the most frequently used dyes in all industries (7). *Author to whom all correspondence should be addressed Tel: 866 4 2332 3456 ext. 1797 Fax: 866 4 2330 5834 E-mail:
[email protected]
It dissociates into cation and chloride ion in aqueous solution (8). Presence of this dye in water leads to various health effects like eye burns, and irritation to the gastrointestinal tract with symptoms of nausea, vomiting and diarrhea. It may lead to methamoglobinemia, cyanosis and dyspnea, if inhaled directly. It also may cause irritations to the skin (9,10). In this study, we have used Delonix regia (flame tree) pods that belong to the royal Poinciana or flamboyant, a member of the bean family. It produces brown woody seed pods―merely a waste material with a hard outer rind containing high cellulose substance; this was selected as a raw material to produce the activated carbon. The carbon was prepared through carbonization and activation processes using concentrated sulphuric acid. The removal of Methylene blue by the above activated carbon was analyzed under the three different conditions (i.e., changing the initial concentration of the dye, changing the adsorbent dose, and changing the volume of the dye solution at room temperature). The suitable isotherm model that explains the adsorption process is given separately and the combined effect on the isotherm model is also discussed later. 2. Experimental 2.1. Preparation of adsorbent The activated carbon was prepared from the flame tree pods. The pods were cut into smaller pieces and soaked in concentrated H2SO4 at 1:1 ratio (weight of raw material/volume of acid) for 48 h and activated at 160°C for 6 h. The activated carbon was repeatedly washed with distilled water until the pH of the wash water became the pH of the distilled water (nearly 6). The carbon obtained 111
was dried at 105 ± 1°C for nearly 2 h to remove the moisture. The above prepared carbon was named FTPC. 2.2. Carbon characterization The IR spectrum of the FTPC was recorded in the range of 4,000 cm-1 to 450 cm-1 using a KBr disk for reference (Figure 1). 100.0 90 80
3860.66
70 812.21
60
%T
780.72 745.54 614.91
50 40
Figure 2. SEM picture of activated carbon
30
of dye solution were carried out at room temperature. Based on the preliminary studies and for less electrical power consumption, the following experimental conditions were fixed for the following studies. Effect of initial dye concentration was studied using 100 mg of activated carbon and 50 ml of different concentrations of dye solution in the range of 20 to 100 mg/dm3 in the screw capped containers and shaken for 2 h after adjusting the pH of the solution to 7 ± 0.1. The required initial pH of the solution was adjusted by using various concentrations of HCl (0.1 N, 0.01 N, 0.001 N) and NaOH (0.1 N, 0. 01 N, 0.001 N). The influence of carbon dose on dye adsorption was determined by taking 50 ml of 50 mg/dm3 of MB dye solutions and shaking with varying amounts of adsorbents ranging from 0.025 to 0.20 g of FTPC after adjusting the solution to pH 7.0 ± 0.1, for 2 h at room temperature. Effect of volume of dye solution was carried by taking different volumes of 50 mg/dm3 dye solution (25 to 125 ml) using 100 mg of carbon for 2 h shaking after adjusting the pH of the solution to 7 ± 0.1. After equilibrium time, the activated carbon was separated by using filters and the absorbance of the clear liquid was analyzed using spectrophotometer at a wavelength of 665 nm. All of the experiments were carried out at 25°C.
20 3400.11
1032.80
10
1379.31
1624.09
1166.05
2853.87
0.0 4000.0
2925.02
3000
2000
1500
1000
450.0
cm-1
Figure 1. FTIR spectrum of activated carbon Due to low temperature carbonization (160°C) of the flame tree pods, some functional groups may be present on the surface of the carbon. A band at 2,922 is due to the aliphatic CH stretching of methyl group and a sharp band around 1,707, 1,613 is due to C=O and N-H groups respectively. Bands around 1,400, 1,350 and 665 are due to phenolic OH, C-N and -SO3H groups respectively. The BET surface analyzer was used to determine the surface area (4.08 m2/g) and the micropore area (6.7689 m2/g) of the carbon. The Scanning Electron Microscope was used to study the surface morphology of the carbon obtained from the flame tree pods (Figure 2). The pH of the carbon was found to be 4.1 due to the existence of functional groups like phenol, carboxylic acid, and lactones. 2.3. Experimental conditions All the chemicals used for this experiment are of analytical grade. The stock solution of 1,000 mg/dm3 Methylene blue was prepared using distilled water. Solutions of desired concentration were prepared by diluting the stock solution stepwise. A calibration graph of absorbance versus concentration was constructed using systronics photometer (model 104) at maximum wavelength of 665 nm. Batch mode experiments were conducted using 150 ml capacity closed containers using ORBITEK shaker at 300 rpm (maximum range that can be obtained from this instrument). For isotherm studies, effect of initial dye concentration, effect of carbon dose, and effect of volume
3. Results and Discussion 3.1. Method of experimental data analysis Ho reported that it is better to use the non-linear analysis method for analyzing the experimental data instead of linear regression analysis to compare the best-fitting isotherms (11). When using the non-linear method, there was no problem with transformations of non-linear isotherms to linear forms. In this study, a trial-and-error procedure, which is applicable to computer operation, was used to compare the best-fitting isotherms using an optimization routine to maximize the coefficient of
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determination between the experimental data and isotherms in the solver add-in in Microsoft Excel (12). The Chi-square test was used to compare the best-fitting isotherms to the experimental data (13). The equivalent mathematical statement was as follows:
χ =∑ 2
(q
e
− qe , m ) qe , m
2
(1)
where qe,m equilibrium capacity obtained by calculated from model (mg/g) and qe was the equilibrium capacity (mg/g) from the experimental data. If data from the model were similar to the experimental data, χ2 would be a small number and vice versa. 3.2. Adsorption isotherm models An adsorption isotherm is the relationship between the adsorbate in the liquid phase and the adsorbate adsorbed on the surface of the adsorbent at equilibrium at constant temperature. The equilibrium adsorption isotherm is very important to design the adsorption systems. For solidliquid systems, several isotherms are available. The Langmuir isotherm takes an assumption that the adsorption occurs at specific homogeneous sites within the adsorbent; the equation is (14,15) the following:
qe =
qm K aCe (2) 1 + K aCe
where Ce is the equilibrium concentration (mg/dm3); qe is the amount of dye adsorbed (mg/g); qm is qe for a complete monolayer (mg/g); and Ka is adsorption equilibrium constant (dm3/mg). The Freundlich isotherm is an empirical equation employed to describe the heterogeneous system (16). The equation is given below:
q e = K F Ce 1/ n (3) where KF and 1/n are Freundlich isotherm constants. The Redlich-Peterson isotherm combines both the Langmuir and Freundlich isotherm equations and the mechanism of adsorption is hybrid and does not follow ideal monolayer adsorption (17). The equation is as follows:
qe =
ACe (4) g 1 + BCe
It has three isotherm constants, namely, A, B and g, which characterize the isotherm. In the above equation, constant g is the exponent, which lies between 0 and 1. If constant g is equal to one, the equation modifies to the Langmuir model. If constant g is equal to zero, then the equation changes to Henry’s law equation. In addition, Freundlich is
a special case of the Redlich-Peterson isotherm when constants A and B were much bigger than 1 (11). 3.3. Effect of initial concentration of the dye solution In general, isotherm studies for solid-liquid systems are carried out by changing adsorbate concentration and keeping other conditions such as adsorbent dose, volume of solution, adsorbent size, and solution pH as constant. They can also be carried out by changing any one of the following: adsorbent dose or adsorbate amount in the solution, which includes changing the initial adsorbate concentration of the solution and changing the volume of the adsorbate solution. The performance of adsorbents is usually gauged by their uptake. Adsorbents can be compared based on their respective maximum uptake values, qm, which can be calculated by fitting the Langmuir isotherm model to the actual experimental data if it fits. This approach is feasible if qm reaches a plateau. A high affinity between the adsorbent and adsorbate reflected in good uptake values at low concentrations, Cf, is desirable. This is characterized by a steep rise of the isotherm curve close to its origin. An adsorbent “better” at low concentrations may be “inferior” at higher ones, and vice versa. Therefore, it is necessary to compare the adsorbent capacity at “low” Cf and also at “high” Cf for the same pH of the solution (18). In general, a “good” adsorbent is one with a high qm, a steep initial adsorption isotherm slope, and low Langmuir constant values (19). In this study, Figure 3 shows a steep rise of the Langmuir isotherm curve―there is a strong affinity between activated carbon and Methylene blue. In addition, the effect of isotherm shape can also be used to predict whether an adsorption system is “favorable” or “unfavorable.” According to Hall et al. (20), the essential features of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter KR, which is defined by the following relationship:
KR =
1 1 + K a C0
(5)
where KR is a dimensionless separation factor, C0 is initial dye concentration (mg/dm3), and Ka is the Langmuir constant (dm3/mg). The parameter KR indicates the shape of the isotherm accordingly: Values of KR
Type of isotherm
KR > 1 KR = 1 0 < KR < 1 KR = 0
Unfavorable Linear Favorable Irreversible
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Figure 3. Fitted isotherm models for Methylene blue/activated carbon adsorption system by varying the initial dye concentration method.
Figure 4. Fitted isotherm models for Methylene blue/activated carbon adsorption system by varying the carbon dose method.
The values of KR for initial dye concentrations from 20 to 80 mg/dm3 were found to range from 0.0536 to 0.0139. The KR values indicate that adsorption is more favourable for the Methylene blue/activated carbon system. The experimental data with all three well known isotherm models are shown in Figure 3. In this adsorption system the g value becomes 1, which shows an overlap of the Redlich-Peterson isotherm on the Langmuir isotherm. The χ2 (0.301) value is also the same for both models. The Langmuir isotherm is a special case of the RedlichPeterson isotherm. The theoretical monolayer sorption saturation capacity, qm, was found to be 24.0 mg/g and the adsorption equilibrium constant, Ka, was found to be 0.957 dm3/mg.
3.5. Effect of volume change of dye solution The effect of volume of the dye solution changes the isotherm model (Figure 5). Though the effect of volume change of dye solution looks similar to the effect of initial concentration of the dye solution, the adsorption system follows the Redlich-Peterson isotherm model, which is the combination of both Langmuir and Freundlich isotherms. The Redlich-Peterson curve falls exactly between the other two models. The χ2 value for the Redlich-Peterson model is very low when compared to the other two models, which shows the hybrid nature of the model (Table 1). At lower Ce values it follows the Freundlich isotherm model, and at higher Ce values it follows the Langmuir isotherm model.
3.4. Effect of carbon dose The effect of carbon dose for the uptake of Methylene blue by activated carbon was found to increase by increasing the adsorbent dose, due to the increase of the activated site available for adsorption. In this adsorption system, Redlich-Peterson constants A and B were much bigger than 1 and the g value from Redlich-Peterson was 0.841, which shows an overlap of the Redlich-Peterson isotherm on the Freundlich isotherm (Figure 4). The Freundlich isotherm is a special case of the Redlich-Peterson isotherm when constants A and B were much bigger than 1. The Methylene blue/activated carbon (FTPC) adsorption system is described by both Freundlich and RedlichPeterson isotherms. The reason for this may be that the experimental data were not covered by the whole isotherm range if we used only the Freundlich model.
3.6. Combination of all the three effects The average of the experimental data obtained from the above three effects showed that the Methylene blue/FTPC system is following the Redlich-Peterson isotherm (Figure 6). It is reasonable that the relationship between equilibrium concentration, Ce, and adsorption capacity, qe, were found to be described by the Redlich-Peterson isotherm. The isotherm of FTPC/MB was not affected by the experimental conditions such as changing initial dye concentration, carbon dose, or volume of the dye solution. It is important to have experimental data, which should cover the whole isotherm range. The advantage of using the Redlich-Peterson isotherm is that the isotherm contains three parameters and incorporates the features of both Langmuir and Freundlich isotherms.
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pods carbon adsorption system very well. Moreover, to have experimental data covering the whole isotherm range may reduce errors on the isotherm study.
Figure 5. Fitted isotherm models for Methylene blue/activated carbon adsorption system by varying the volume of the dye solution method. Table 1. Fitted parameters of Methylene blue/activated carbon adsorption system Isotherm
Parameters Units
Langmuir
qm
Freundlich
M
V
C+M+V
24.0 21.7
25.1 23.3
dm /mg 0.957 2.18
0.643 1.24
χ2
0.301 1.01
0.504 3.76
1/n
0.176 0.159
0.276 0.192
Ka
KF
mg/g
C
3
dm3/g
χ2 Redlich-Peterson G
13.4 14.0
10.7 13.1
1.67 0.425
0.294 3.31
1.000 0.841 3
0.821 0.882
5. References 1. Senthilkumaar S, Kalaamani P, Porkodi K, Varadarajan PR, Subburaam CV. (2006) Adsorption of dissolved Reactive red dye from aqueous phase onto activated carbon prepared from agricultural waste. Bioresour. Technol.;97:1618-1625. 2. Tan IAW, Hameed BH, Ahmad AL. (2007) Equilibrium and kinetic studies on basic dye adsorption by oil palm fibre activated carbon. Chem. Eng. J.; 127:111-119.
8
B
dm /mg 0.957 1.14×10 2.29 3.63
A
dm3/g
χ2
Figure 6. Combined three isotherm methods for Methylene blue/activated carbon adsorption system
23.0 1.59×109 33.7 59.6 0.301 0.425
0.151 2.88
C, method by changing initial concentration of dye solution; M, method by changing activated carbon dose; V, method by changing volume of dye solution; C+M+V, for all experimental data 4. Conclusion Activated carbon prepared from flame tree pods can be used as an adsorbent for the removal of Methylene blue from aqueous solution. Non-linear analysis with trial-anderror procedure was successfully applied for comparing the best-fitting of three isotherms. Combination of experimental data obtained by changing initial dye concentration, carbon dose, and volume of the dye solution followed the Redlich-Peterson isotherm model. This model could be used to describe the Methylene blue/flame tree
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