# Removal of Methylene Blue from Aqueous Solution by Activated Carbon Prepared from Pea Shells (Pisum sativum)

Authors: Ünal Geçgel; Gülce Özcan; Gizem Çağla Gürpınar
Journal: Journal of Chemistry
 (2013)

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## Abstract

An activated carbon was prepared from pea shells and used for the removal of methylene blue (MB) from aqueous solutions. The influence of various factors such as adsorbent concentration, initial dye concentration, temperature, contact time, pH, and surfactant was studied. The experimental data were analyzed by the Langmuir and Freundlich models of adsorption. The adsorption isotherm was found to follow the Langmuir model. The monolayer sorption capacity of activated carbon prepared from pea shell for MB was found to be 246.91 mg g−1 at 25 ∘C. Two simplified kinetic models including pseudo-first-order and pseudo-second-order equation were selected to follow the adsorption processes. Kinetic studies showed that the adsorption followed pseudo-second-order kinetic model. Various thermodynamic parameters such as ΔH∘, ΔS∘, and ΔG∘ were evaluated. The results in this study indicated that activated carbon prepared from pea shell could be employed as an adsorbent for the removal of MB from aqueous solutions.


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## Body

## 1. Introduction

Wastewater from textiles, cosmetics, printing, dying, food processing, and paper-making industries is polluted by dyes. Discharge of these colored effluents presents a major environmental problem for developing countries because of their toxic and carcinogenic effects on living beings [1]. Therefore many methods such as activated carbon sorption, chemical coagulation, ion exchange, electrolysis, and biological treatments [2, 3], have been developed for removing dye pollutions from wastewater before being discharged into the environment. Of these methods, activated carbon sorption is highly effective for the removal of dyes and pigments as well as other organic and inorganic pollution [4–10].There are two basic processes to activate carbon materials, physical and chemical. The temperatures used in chemical activation are lower than that used in the physical activation process. As a result the development of a porous structure is better in the case of chemical activation method. Chemical activation can be accomplished in a single step by carrying out thermal decomposition of raw material with chemical reagents. Chemical activation processes have been carried out with acidic reagents, that is, ZnCl2 [11], H3PO4 [12], HCl [13], and H2SO4 [14] or with basic reagents KOH [14], K2CO3 [15], NaOH [12], and Na2CO3 [16].This leads to searches for new, cheap, and easily available resources of carbon as a feedstock for activated carbon. Many cheap, easily available agricultural and/or plant-based materials such as silk cotton hull, coconut tree sawdust, sago industry waste, banana pith, maize cob [17], rattan sawdust [18], jute fiber [19], pistachio shells [20], palm kernel shell [21], date pits [22], rice bran [23], coir pith [24], rice husk [12], mango seed kernel powder [25], rubberwood sawdust [26], plum kernels [27], rosewood sawdust [28], straw [29], coconut coir dust [30], palm fibre [31], pine cone [32], walnut shell, almond shell, hazelnut shell, apricot stones [33], coconut shells, groundnut shell, bamboo dust [34], deolied soya [35], and wheat husk [36] have been used as a source of the production of activated carbon for the removal of textile dye effluents.However there are no reports of obtaining active carbon from pea shells by chemical and physical activation methods [37]. Therefore, in this paper, the adsorption equilibrium and kinetics of methylene blue on activated carbon from pea shells were studied. The uptake of MB on activated carbon was examined as a function of temperature, initial dye concentration, adsorbent concentration, contact time, and surfactant. Adsorption isotherms and kinetic parameters were calculated and discussed. The thermodynamics of the adsorption were also evaluated.


## 2. Materials and Methods

### 2.1. Chemicals and Reagents

MB (basic blue 9, C.I. 52015; chemical formula, C16H18N3ClS, and molecular weight 319.85 g mol−1) supplied by Merck was used as an adsorbate and was not purified prior to use. A stock solution of MB dye was prepared 1000 mg L−1 by dissolving the required amount of dye powder in double distilled water. All working solutions of the desired concentrations were prepared by diluting the stock solution with distilled water. Zinc chloride (ZnCl2) and the anionic surfactant sodium dodecylsulfate (SDS) were obtained from Merck. The cationic surfactant cetyltrimethylammonium bromide (CTAB) and the nonionic surfactant polyoxyetheylene (10) cetyl ether (Brij-56) were obtained from Fluka. All the chemicals used were obtained as research-grade chemicals and were used without further purification.


### 2.2. Preparation of Activated Carbon

Pea shells were collected from local markets in Trakya region, Turkey. Pea shells were repeatedly washed with distilled water to remove dirt, dust, and other impurities. The washed shell materials were then dried in sunlight for two weeks. Chemical activation of the pea shells was carried out with ZnCl2 solution. 10 g of dried pea shells were well mixed, by stirring, with 100 mL of an aqueous solution that contained 20 g of ZnCl2. The mixture was heated at 80°C for 1 h. After heating, the slurry was kept in an oven at 100°C for 24 h. The resulting ZnCl2 treated sample was then subjected to carbonization and activation process in a programmable electrical furnace (Severn Furnaces Ltd.) at 500°C for 1 h. After activation, sample was allowed to cool and subsequently washed with 0.5 M HCl and then with deionized water several times. It was grounded and sieved through a 65-mesh. The resulting product was kept in a desiccator for further use.


### 2.3. Adsorption Experiments

Adsorption equilibrium and kinetics were determined by the batch method. Adsorption isotherms were performed in a series of 250 mL conical flasks where dye solutions (100 mL) with different initial concentrations (100–350 mg L−1) at pH 6.5. Equal masses of 0.1 g of activated carbon were added to dye solutions. The flasks were placed in a thermostated water-bath shaker (Nuve ST 402) and agitation was provided at 150 rpm at different temperatures (25, 35, 45, and 55°C) for 200 min to ensure establishment of equilibrium. Activated carbon was separated by centrifugation at 3500 rpm (Hettich Rotofix 32A) for 10 min. Residual concentration of MB was determined spectrophotometrically (Rayleigh UV-1601) at 668 nm. Dilutions were made when absorbance exceeded 1.5. The percentage removal of dye from solution was calculated by the following:
(1)%Removal=(Co-Ct)Co×100,
where C0 and Ct (mg L−1) are the initial dye concentration and concentration at time t, respectively.The procedures of kinetic experiments were basically identical to those of equilibrium tests. The aqueous samples were taken at present time intervals, and the concentrations of dye were similarly measured. All the kinetic experiments were carried out for different dye concentrations (100–350 mg L−1) at 25°C. The amount of adsorption at time t, qt (mg g−1) was calculated by
(2)qt=(Co-Ct)VW.


## 2.1. Chemicals and Reagents

MB (basic blue 9, C.I. 52015; chemical formula, C16H18N3ClS, and molecular weight 319.85 g mol−1) supplied by Merck was used as an adsorbate and was not purified prior to use. A stock solution of MB dye was prepared 1000 mg L−1 by dissolving the required amount of dye powder in double distilled water. All working solutions of the desired concentrations were prepared by diluting the stock solution with distilled water. Zinc chloride (ZnCl2) and the anionic surfactant sodium dodecylsulfate (SDS) were obtained from Merck. The cationic surfactant cetyltrimethylammonium bromide (CTAB) and the nonionic surfactant polyoxyetheylene (10) cetyl ether (Brij-56) were obtained from Fluka. All the chemicals used were obtained as research-grade chemicals and were used without further purification.


## 2.2. Preparation of Activated Carbon

Pea shells were collected from local markets in Trakya region, Turkey. Pea shells were repeatedly washed with distilled water to remove dirt, dust, and other impurities. The washed shell materials were then dried in sunlight for two weeks. Chemical activation of the pea shells was carried out with ZnCl2 solution. 10 g of dried pea shells were well mixed, by stirring, with 100 mL of an aqueous solution that contained 20 g of ZnCl2. The mixture was heated at 80°C for 1 h. After heating, the slurry was kept in an oven at 100°C for 24 h. The resulting ZnCl2 treated sample was then subjected to carbonization and activation process in a programmable electrical furnace (Severn Furnaces Ltd.) at 500°C for 1 h. After activation, sample was allowed to cool and subsequently washed with 0.5 M HCl and then with deionized water several times. It was grounded and sieved through a 65-mesh. The resulting product was kept in a desiccator for further use.


## 2.3. Adsorption Experiments

Adsorption equilibrium and kinetics were determined by the batch method. Adsorption isotherms were performed in a series of 250 mL conical flasks where dye solutions (100 mL) with different initial concentrations (100–350 mg L−1) at pH 6.5. Equal masses of 0.1 g of activated carbon were added to dye solutions. The flasks were placed in a thermostated water-bath shaker (Nuve ST 402) and agitation was provided at 150 rpm at different temperatures (25, 35, 45, and 55°C) for 200 min to ensure establishment of equilibrium. Activated carbon was separated by centrifugation at 3500 rpm (Hettich Rotofix 32A) for 10 min. Residual concentration of MB was determined spectrophotometrically (Rayleigh UV-1601) at 668 nm. Dilutions were made when absorbance exceeded 1.5. The percentage removal of dye from solution was calculated by the following:
(1)%Removal=(Co-Ct)Co×100,
where C0 and Ct (mg L−1) are the initial dye concentration and concentration at time t, respectively.The procedures of kinetic experiments were basically identical to those of equilibrium tests. The aqueous samples were taken at present time intervals, and the concentrations of dye were similarly measured. All the kinetic experiments were carried out for different dye concentrations (100–350 mg L−1) at 25°C. The amount of adsorption at time t, qt (mg g−1) was calculated by
(2)qt=(Co-Ct)VW.


## 3. Results and Discussion

### 3.1. Effect of Contact Time and Initial Concentration of Dye

The adsorption data for the removal of MB versus contact time at different concentrations are presented in Figure1. The dye removal percentage increased with the increase in contact time and remained constant after an equilibrium time was reached. As shown in Figure 1, as the initial MB concentration increased from 100 to 350 mg L−1 the equilibrium percentage dye removal decreased from 99.65 to 70.71%. This may be attributed to the fact that at low concentrations the ratio of surface active sites to the total dye molecules in the solution is high and hence all dye molecules may interact with the activated carbon. The dynamic equilibrium was influenced by the initial concentration. The time taken to reach equilibrium was 40 and 100 minutes at lower concentrations of 100 and 150 mg L−1, respectively, and 180 minutes for higher concentrations (200, 250, 300, and 350 mg L−1).Figure 1
Effect of contact time and initial dye concentration on MB adsorption (concentration of adsorbent = 1 g L−1, temp. = 25°C, and pH = 6.85).


### 3.2. Effect of Adsorbent Concentration

The adsorption of methylene blue was investigated by taking the following values as constant: 250 mg L−1dye concentration, 6.85 value of pH, 180 minutes contact time, a temperature of 25°C, a stirring speed of 150 rpm, and different adsorbent concentrations (0.05–0.20 g 100 mL−1). As in Figure 2, it can be clearly seen that with increasing adsorbent concentration, removal dye percentage also increases. As the adsorbent concentration increases from 0.05 g to 0.20 g, the removal dye percentage increases from 33.58% to 99.41%. This is the result of increased surface area and active centers with increasing adsorbent concentration. As it can be seen in Figure 2, when the adsorbent concentration is 0.15 g and above, the change in suspended dye percentage is very low and the value reaches its maximum.Figure 2
Effect of adsorbent concentration on MB adsorption (C0 = 250 mg L−1, pH = 6.85, temp. = 25°C, and contact time = 180 min).


### 3.3. Effect of Solution pH

The effect of the initial pH value of the solution on the adsorption process was investigated in conditions of 250 mg L−1 of initial dye solution concentration, 0.1 g 100 mL−1 of adsorbent concentration, 180 min of contact time, a temperature of 25°C, and a 150 rpm stirring speed. The pH of the dye solution varied between 2.0 and 11.5, as it can be seen in Figure 3. The pHzpc (pH of zero net proton charge) is defined here as the pH value at which the net surface charge is equal to zero. The point of zero charge (pHzpc) of active carbon that was prepared from pea shells was found to be 5.86. At high pH values (pH>pHzpc), the surface of the adsorbent is charged negatively and the adsorption of dye molecules increases due to the electrostatic attraction between the surface and cationic dye molecules. At low pH values (pH<pHzpc), since the surface of the adsorbent is charged positively, the expected tendency is decreasing adsorption of dye due to the electrostatic repulsive between cationic dye molecules and the surface of the adsorbent. However, in this study, as can be seen in Figure 3, an increase in the percentage of removal dye was observed by both increasing and decreasing pH. At pH values of 2.0 and 11.5, the maximum percentage of dye removal was found to be 97.30%. At high pH values, the increase of the dye adsorption can be explained as the negative charging of the surface depending on the pHzpc value of the adsorbent. However, at low pH values, the reason for the dye adsorption value not decreasing can be explained as the replacement of the hydrogen ions on the surface of the adsorbent with the cationic dye molecules in the solution. The fact that, in an acidic medium the initial pH value of adsorption firstly increases and then reaches an initial value by decreasing can be accepted as an indication of the previous situation.Figure 3
Effect of pH on MB adsorption (Co = 250 mg L−1, concentration of adsorbent = 1 g L−1, temp. = 25°C, and contact time = 180 min).


### 3.4. Effect of Temperature

The effect of temperature on the adsorption of methylene blue at 0.1 g in 100 mL adsorbent concentration, 180 min of contact time, pH 6.85, and a 150 rpm stirring speed for the different dye concentrations (100–350 g L−1) seen in Figure 4 was investigated. As the temperature increased from 25°C to 55°C, the removal of dye percentage also increased. The adsorption capacity increases with temperature due to the increase of the rate of diffusion of the adsorbate molecules across the external boundary layer and the internal pores of the adsorbent particle, which decreases in case viscosity of the solution for highly concentrated suspensions. In addition, changing the temperature will change the equilibrium capacity of the adsorbent for a particular adsorbate [38, 39]. The maximum removal dye percentage at 55°C is found to be 99.85, 99.71, 98.32, 91.89, 84.84, and 76.91% for the dye concentrations of 100, 150, 200, 250, 300, and 350 mg L−1, respectively.Figure 4
Effect of temperature on MB adsorption (concentration of adsorbent = 1 g L−1, pH = 6.85, and contact time = 180 min).


### 3.5. Effect of Surfactants

As surfactants are typically present in wastewater, an effect of their presence on dye adsorption was studied. The effect of surfactants on MB adsorption is shown in Figure5. From Figure 5, it is clearly seen that the percentage of dye removal in the absence of surfactant is 84.14% and the percentage of MB removal in the presence of anionic surfactant SDS increased to 99.38%, on the other hand cationic surfactant CTAB and nonionic surfactant Brij 56 decreased to 36% and 41.65%, respectively. This may be explained by the help of the electrostatic attraction (MB-SDS) and repulsion (MB-CTAB) and hydrophobic interactions (MB-Brij 56) between dye and surfactant molecules.Figure 5
Effect of surfactants on MB adsorption (C0 = 250 mg L−1, concentration of adsorbent = 1 g L−1, pH = 6.85, temp. = 25°C, and contact time = 180 min).


### 3.6. Adsorption Isotherms

The adsorption isotherm indicates how the adsorption molecules are distributed between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. An adsorption isotherm study was carried out on two well-known isotherms (Langmuir and Freundlich) at different temperatures. The Langmuir model assumes that the adsorptions occur at specific homogeneous sites on the adsorbent and is used successfully in many monolayer adsorption processes [40]. The Freundlich model endorses the heterogeneity of the surface and assumes that the adsorption occurs at sites with different energy levels of adsorption [41]. The applicability of the isotherm equation is compared by judging the correlation coefficients, R2.The linear form of Langmuir isotherm equation is given as(3)Ceqe=1Qob+CeQo,
where Ce is the equilibrium concentration of the dye solution (mg L−1), qe is the amount of dye adsorbed per unit mass of adsorbent (mg g−1), and Qo (mg g−1) and b (L mg−1) are the Langmuir constants related to the maximum adsorption capacity and the rate of adsorption, respectively. As seen in Figure 6 the plots of Ce/qe versus Ce for the adsorption of MB onto activated carbon at different temperatures give a straight line of slope (1/Qo) and intercept (1/Qob).Figure 6
Langmuir isotherm plots for MB adsorption at different temperatures.The essential characteristics of Langmuir isotherm can be expressed in terms of dimensionless equilibrium parameter (RL) [42]. The parameter is defined by
(4)RL=11+bCo,
where b is the Langmuir constant and Co is the highest dye concentration (mg L−1). The value of RL indicates the type of the isotherm to be either unfavorable (RL>1), linear (RL=1), favorable (0<RL<1), or irreversible (RL=0).The logarithmic form Freundlich equation is given by the following:(5)lnqe=lnKF+1nlnCe,
where KF (mg g−1) (Lmg-1)1/n and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity of the adsorbent, respectively. Their values were obtained from the intercepts (lnKF) and slope (1/n) of linear plots of lnqe versus lnCe at different temperatures (Figure 7). The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero [43]. A value of 1/n below one indicates a normal Langmuir isotherm while 1/n above one is indicative of cooperative adsorption [44].Figure 7
Freundlich isotherm plots for MB adsorption at different temperatures.Table1 shows the values of the parameters of the two isotherms and related correlation coefficients at different temperatures. The comparison of correlation coefficients (R2) of the linearized form of both equations indicates that the Langmuir model yields a better fit for the experimental equilibrium adsorption data than the Freundlich model. With the maximum adsorption capacity, Qo values increased with increasing temperature. Its maximum value was determined as 270 mg g−1 at 55°C. In the present study, the values of RL were in the range of 0-1. This again confirmed that the Langmuir isotherm was favorable for adsorption of MB on the activated carbon prepared from pea shells. Furthermore, from Table 1 it is apparent that the value of the Freundlich constant 1/n, obtained for the present systems indicates favorable adsorption, as it lies between 0 and 1.Table 1
The related parameters for the adsorption of MB on activated carbon prepared from pea shell at different temperatures.
Adsorption isotherms and its constants
Temperatures (°C)
25
35
45
55
Langmuir adsorption isotherms constants
Q
o (mg g−1)
246.91
261.10
269.54
270.00
b (L mg−1)
0.2978
0.4576
0.5908
0.7361
R2
0.9973
0.9992
0.9988
0.9990
RL
0.0095
0.0062
0.0048
0.0039
Freundlich adsorption isotherms constants
KF(mgg-1)(Lmg-1)1/n
119.32
129.45
142.72
150.60
1/n
0.1572
0.1635
0.1525
0.1423
R2
0.9909
0.9847
0.9787
0.9696Table2 lists the comparison of the maximum monolayer adsorption capacity of MB on various adsorbents. Compared with some data in the literature, Table 2 shows that the activated carbon studied in this work has a very large adsorption capacity.Table 2
Comparison of adsorption capacities of various adsorbents for MB.
Activated carbon source
Q
o (mg g−1)
Reference
Pea shell
246.91
This work
Walnut shells
3.53
33
Pistachio shells
129.0
20
Coir pith
5.8
24
Apricot stones
4.1
33
Almond shell
1.3
33
Hazelnut shell
8.82
33
Euphorbia rigida
114.45
45
Date pits
80.31
22
Mango seed kernel powder
142.86
25
Jute fiber
225.64
19
Coconut tree sawdust
4.70
17
Silk cotton hull
2.40
17
Sago waste
4.51
17
Maize cob
5.00
17
Banana pith
4.67
17
Date pits
123.10
22
Palm fibre
277.78
31
Bamboo dust
143.20
34
Coconut shell
277.90
34
Groundnut shell
164.90
34
Coconut coir dust
15.25
30
Rattan sawdust
294.12
18


### 3.7. Adsorption Kinetics

The kinetic adsorption data were processed to understand the dynamics of adsorption process in terms of the rate constant order. Thus, the kinetic of MB adsorption onto activated carbon prepared from pea shells was analyzed using pseudo-first-order and pseudo-second-order kinetic models.The rate constant of adsorption is determined from the pseudo-first-order equation given by Langergren and Svenska [45]:
(6)log(qe-qt)=logqe-k12.303t,
where qe and qt are the amounts of MB adsorbed (mg g−1) at equilibrium and at time t (min), respectively, and k1 (min−1) is the adsorption rate constant. A plot of log (qe-qt) versus t should give a linear relationship, from which k1 and qe can be determined from the slope and intercept of the plot, respectively (Figure 8).Figure 8
Pseudo-first-order adsorption kinetics of MB.The pseudo-second-order kinetic model [46] is expressed as
(7)tqt=1k2qe2+1qet,h=k2qe2,
where h is the initial adsorption rate (mg g−1min−1) and k2 (g mg−1min−1) is rate constant of second-order adsorption. If the second-order kinetics is applicable, then the plot of t/qt versus t should show a linear relationship. Values of k2 and equilibrium adsorption capacity qe were calculated from the intercept and slope of the plots t/qt versus t (Figure 9).Figure 9
Pseudo-second-order adsorption kinetics of MB.Table3 lists the results of the rate constant studies for different initial dye concentrations by the pseudo-first-order and second-order models. The correlation coefficient, R2 for the pseudo-second-order adsorption model has a high value, and its calculated equilibrium adsorption capacity, qe,cal, is consistent with experimental data, qe,exp. Hence it can be assumed that the adsorption of MB follows the pseudo-second-order kinetic model.Table 3
Pseudo-first-order and pseudo-second-order models for the adsorption on activated carbon prepared from pea shell.
Kinetic models and its parameters
Initial concentrations (mg/L)
100
150
200
250
300
350
q
e
,
exp (mg g−1)
99.65
148.73
190.66
217.20
227.85
247.49
Pseudo-first-order kinetic
qe,cal (mg g−1)
13.36
84.49
84.86
125.96
123.08
116.17
k1  (min−1)
0.049
0.043
0.024
0.025
0.022
0.022
R2
0.9638
0.9704
0.9958
0.9871
0.9889
0.9830
Pseudo-second-order kinetic
qe,cal  (mg g−1)
100.40
154.32
201.21
232.02
245.10
264.55
k2  (g mg−1 min−1)
0.00910
0.00110
0.00050
0.00030
0.00028
0.00029
h (mg g−1 min−1)
91.41
26.92
19.30
18.17
16.79
20.68
R2
0.9999
0.9998
0.9999
0.9998
0.9995
0.9993


### 3.8. Thermodynamic Studies

Thermodynamic parameters including Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated from the following:
(8)ΔG°=-RTlnK,lnK=ΔSoR-ΔHoRT,
where R is the universal gas constant (8.314 J mol−1 K−1), T is the absolute temperature (Kelvin), and K is the equilibrium constant. Since Langmuir adsorption isotherm is the best fit, the value of b was used in place of K in the calculations of all thermodynamic parameters.Equation (11) is known as the van’t Hoff equation, and it expresses a relationship between equilibrium constant and temperature. Plots of lnK versus 1/T should be a straight line as shown in Figure 10. The values of ΔH° and ΔS° were calculated from the slope and intercept of van’t Hoff plot, respectively. The thermodynamic parameters associated with the adsorption of MB onto activated carbon prepared from pea shells are listed in Table 4. Negative values of ΔG° show the feasibility and spontaneity of the adsorption process. According to the van’t Hoff equation, standard enthalpy and the entropy values in the range of 25–55°C were obtained as 14.57 k J mol−1 and 0.147 k J mol−1 K−1, respectively. The positive value of ΔH° indicates the endothermic nature of the adsorption process. From Table 1, the maximum adsorption capacity increased from 246.91 to 270.00 mg g−1 with an increase in solution temperature from 25 to 55°C. This further confirmed the endothermic nature of the adsorption process. The positive value of ΔS° shows the increasing randomness of the solid-solution and adsorption medium interface during the adsorption.Table 4
Values of thermodynamics parameters for MB adsorption on activated carbon prepared from pea shell.
Temperature (K)
Δ
G
° (k J mol−1)
Δ
H
° (k J mol−1)
Δ
S
° (k J mol−1 K−1)
R
2
298
−28.40
14.57
0.147
0.9891
308
−30.46
318
−32.12
328
−33.73Figure 10
Plot oflnK versus 1/T for the enthalpy change of the adsorption process.


## 3.1. Effect of Contact Time and Initial Concentration of Dye

The adsorption data for the removal of MB versus contact time at different concentrations are presented in Figure1. The dye removal percentage increased with the increase in contact time and remained constant after an equilibrium time was reached. As shown in Figure 1, as the initial MB concentration increased from 100 to 350 mg L−1 the equilibrium percentage dye removal decreased from 99.65 to 70.71%. This may be attributed to the fact that at low concentrations the ratio of surface active sites to the total dye molecules in the solution is high and hence all dye molecules may interact with the activated carbon. The dynamic equilibrium was influenced by the initial concentration. The time taken to reach equilibrium was 40 and 100 minutes at lower concentrations of 100 and 150 mg L−1, respectively, and 180 minutes for higher concentrations (200, 250, 300, and 350 mg L−1).Figure 1
Effect of contact time and initial dye concentration on MB adsorption (concentration of adsorbent = 1 g L−1, temp. = 25°C, and pH = 6.85).


## 3.2. Effect of Adsorbent Concentration

The adsorption of methylene blue was investigated by taking the following values as constant: 250 mg L−1dye concentration, 6.85 value of pH, 180 minutes contact time, a temperature of 25°C, a stirring speed of 150 rpm, and different adsorbent concentrations (0.05–0.20 g 100 mL−1). As in Figure 2, it can be clearly seen that with increasing adsorbent concentration, removal dye percentage also increases. As the adsorbent concentration increases from 0.05 g to 0.20 g, the removal dye percentage increases from 33.58% to 99.41%. This is the result of increased surface area and active centers with increasing adsorbent concentration. As it can be seen in Figure 2, when the adsorbent concentration is 0.15 g and above, the change in suspended dye percentage is very low and the value reaches its maximum.Figure 2
Effect of adsorbent concentration on MB adsorption (C0 = 250 mg L−1, pH = 6.85, temp. = 25°C, and contact time = 180 min).


## 3.3. Effect of Solution pH

The effect of the initial pH value of the solution on the adsorption process was investigated in conditions of 250 mg L−1 of initial dye solution concentration, 0.1 g 100 mL−1 of adsorbent concentration, 180 min of contact time, a temperature of 25°C, and a 150 rpm stirring speed. The pH of the dye solution varied between 2.0 and 11.5, as it can be seen in Figure 3. The pHzpc (pH of zero net proton charge) is defined here as the pH value at which the net surface charge is equal to zero. The point of zero charge (pHzpc) of active carbon that was prepared from pea shells was found to be 5.86. At high pH values (pH>pHzpc), the surface of the adsorbent is charged negatively and the adsorption of dye molecules increases due to the electrostatic attraction between the surface and cationic dye molecules. At low pH values (pH<pHzpc), since the surface of the adsorbent is charged positively, the expected tendency is decreasing adsorption of dye due to the electrostatic repulsive between cationic dye molecules and the surface of the adsorbent. However, in this study, as can be seen in Figure 3, an increase in the percentage of removal dye was observed by both increasing and decreasing pH. At pH values of 2.0 and 11.5, the maximum percentage of dye removal was found to be 97.30%. At high pH values, the increase of the dye adsorption can be explained as the negative charging of the surface depending on the pHzpc value of the adsorbent. However, at low pH values, the reason for the dye adsorption value not decreasing can be explained as the replacement of the hydrogen ions on the surface of the adsorbent with the cationic dye molecules in the solution. The fact that, in an acidic medium the initial pH value of adsorption firstly increases and then reaches an initial value by decreasing can be accepted as an indication of the previous situation.Figure 3
Effect of pH on MB adsorption (Co = 250 mg L−1, concentration of adsorbent = 1 g L−1, temp. = 25°C, and contact time = 180 min).


## 3.4. Effect of Temperature

The effect of temperature on the adsorption of methylene blue at 0.1 g in 100 mL adsorbent concentration, 180 min of contact time, pH 6.85, and a 150 rpm stirring speed for the different dye concentrations (100–350 g L−1) seen in Figure 4 was investigated. As the temperature increased from 25°C to 55°C, the removal of dye percentage also increased. The adsorption capacity increases with temperature due to the increase of the rate of diffusion of the adsorbate molecules across the external boundary layer and the internal pores of the adsorbent particle, which decreases in case viscosity of the solution for highly concentrated suspensions. In addition, changing the temperature will change the equilibrium capacity of the adsorbent for a particular adsorbate [38, 39]. The maximum removal dye percentage at 55°C is found to be 99.85, 99.71, 98.32, 91.89, 84.84, and 76.91% for the dye concentrations of 100, 150, 200, 250, 300, and 350 mg L−1, respectively.Figure 4
Effect of temperature on MB adsorption (concentration of adsorbent = 1 g L−1, pH = 6.85, and contact time = 180 min).


## 3.5. Effect of Surfactants

As surfactants are typically present in wastewater, an effect of their presence on dye adsorption was studied. The effect of surfactants on MB adsorption is shown in Figure5. From Figure 5, it is clearly seen that the percentage of dye removal in the absence of surfactant is 84.14% and the percentage of MB removal in the presence of anionic surfactant SDS increased to 99.38%, on the other hand cationic surfactant CTAB and nonionic surfactant Brij 56 decreased to 36% and 41.65%, respectively. This may be explained by the help of the electrostatic attraction (MB-SDS) and repulsion (MB-CTAB) and hydrophobic interactions (MB-Brij 56) between dye and surfactant molecules.Figure 5
Effect of surfactants on MB adsorption (C0 = 250 mg L−1, concentration of adsorbent = 1 g L−1, pH = 6.85, temp. = 25°C, and contact time = 180 min).


## 3.6. Adsorption Isotherms

The adsorption isotherm indicates how the adsorption molecules are distributed between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. An adsorption isotherm study was carried out on two well-known isotherms (Langmuir and Freundlich) at different temperatures. The Langmuir model assumes that the adsorptions occur at specific homogeneous sites on the adsorbent and is used successfully in many monolayer adsorption processes [40]. The Freundlich model endorses the heterogeneity of the surface and assumes that the adsorption occurs at sites with different energy levels of adsorption [41]. The applicability of the isotherm equation is compared by judging the correlation coefficients, R2.The linear form of Langmuir isotherm equation is given as(3)Ceqe=1Qob+CeQo,
where Ce is the equilibrium concentration of the dye solution (mg L−1), qe is the amount of dye adsorbed per unit mass of adsorbent (mg g−1), and Qo (mg g−1) and b (L mg−1) are the Langmuir constants related to the maximum adsorption capacity and the rate of adsorption, respectively. As seen in Figure 6 the plots of Ce/qe versus Ce for the adsorption of MB onto activated carbon at different temperatures give a straight line of slope (1/Qo) and intercept (1/Qob).Figure 6
Langmuir isotherm plots for MB adsorption at different temperatures.The essential characteristics of Langmuir isotherm can be expressed in terms of dimensionless equilibrium parameter (RL) [42]. The parameter is defined by
(4)RL=11+bCo,
where b is the Langmuir constant and Co is the highest dye concentration (mg L−1). The value of RL indicates the type of the isotherm to be either unfavorable (RL>1), linear (RL=1), favorable (0<RL<1), or irreversible (RL=0).The logarithmic form Freundlich equation is given by the following:(5)lnqe=lnKF+1nlnCe,
where KF (mg g−1) (Lmg-1)1/n and 1/n are Freundlich constants related to adsorption capacity and adsorption intensity of the adsorbent, respectively. Their values were obtained from the intercepts (lnKF) and slope (1/n) of linear plots of lnqe versus lnCe at different temperatures (Figure 7). The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero [43]. A value of 1/n below one indicates a normal Langmuir isotherm while 1/n above one is indicative of cooperative adsorption [44].Figure 7
Freundlich isotherm plots for MB adsorption at different temperatures.Table1 shows the values of the parameters of the two isotherms and related correlation coefficients at different temperatures. The comparison of correlation coefficients (R2) of the linearized form of both equations indicates that the Langmuir model yields a better fit for the experimental equilibrium adsorption data than the Freundlich model. With the maximum adsorption capacity, Qo values increased with increasing temperature. Its maximum value was determined as 270 mg g−1 at 55°C. In the present study, the values of RL were in the range of 0-1. This again confirmed that the Langmuir isotherm was favorable for adsorption of MB on the activated carbon prepared from pea shells. Furthermore, from Table 1 it is apparent that the value of the Freundlich constant 1/n, obtained for the present systems indicates favorable adsorption, as it lies between 0 and 1.Table 1
The related parameters for the adsorption of MB on activated carbon prepared from pea shell at different temperatures.
Adsorption isotherms and its constants
Temperatures (°C)
25
35
45
55
Langmuir adsorption isotherms constants
Q
o (mg g−1)
246.91
261.10
269.54
270.00
b (L mg−1)
0.2978
0.4576
0.5908
0.7361
R2
0.9973
0.9992
0.9988
0.9990
RL
0.0095
0.0062
0.0048
0.0039
Freundlich adsorption isotherms constants
KF(mgg-1)(Lmg-1)1/n
119.32
129.45
142.72
150.60
1/n
0.1572
0.1635
0.1525
0.1423
R2
0.9909
0.9847
0.9787
0.9696Table2 lists the comparison of the maximum monolayer adsorption capacity of MB on various adsorbents. Compared with some data in the literature, Table 2 shows that the activated carbon studied in this work has a very large adsorption capacity.Table 2
Comparison of adsorption capacities of various adsorbents for MB.
Activated carbon source
Q
o (mg g−1)
Reference
Pea shell
246.91
This work
Walnut shells
3.53
33
Pistachio shells
129.0
20
Coir pith
5.8
24
Apricot stones
4.1
33
Almond shell
1.3
33
Hazelnut shell
8.82
33
Euphorbia rigida
114.45
45
Date pits
80.31
22
Mango seed kernel powder
142.86
25
Jute fiber
225.64
19
Coconut tree sawdust
4.70
17
Silk cotton hull
2.40
17
Sago waste
4.51
17
Maize cob
5.00
17
Banana pith
4.67
17
Date pits
123.10
22
Palm fibre
277.78
31
Bamboo dust
143.20
34
Coconut shell
277.90
34
Groundnut shell
164.90
34
Coconut coir dust
15.25
30
Rattan sawdust
294.12
18


## 3.7. Adsorption Kinetics

The kinetic adsorption data were processed to understand the dynamics of adsorption process in terms of the rate constant order. Thus, the kinetic of MB adsorption onto activated carbon prepared from pea shells was analyzed using pseudo-first-order and pseudo-second-order kinetic models.The rate constant of adsorption is determined from the pseudo-first-order equation given by Langergren and Svenska [45]:
(6)log(qe-qt)=logqe-k12.303t,
where qe and qt are the amounts of MB adsorbed (mg g−1) at equilibrium and at time t (min), respectively, and k1 (min−1) is the adsorption rate constant. A plot of log (qe-qt) versus t should give a linear relationship, from which k1 and qe can be determined from the slope and intercept of the plot, respectively (Figure 8).Figure 8
Pseudo-first-order adsorption kinetics of MB.The pseudo-second-order kinetic model [46] is expressed as
(7)tqt=1k2qe2+1qet,h=k2qe2,
where h is the initial adsorption rate (mg g−1min−1) and k2 (g mg−1min−1) is rate constant of second-order adsorption. If the second-order kinetics is applicable, then the plot of t/qt versus t should show a linear relationship. Values of k2 and equilibrium adsorption capacity qe were calculated from the intercept and slope of the plots t/qt versus t (Figure 9).Figure 9
Pseudo-second-order adsorption kinetics of MB.Table3 lists the results of the rate constant studies for different initial dye concentrations by the pseudo-first-order and second-order models. The correlation coefficient, R2 for the pseudo-second-order adsorption model has a high value, and its calculated equilibrium adsorption capacity, qe,cal, is consistent with experimental data, qe,exp. Hence it can be assumed that the adsorption of MB follows the pseudo-second-order kinetic model.Table 3
Pseudo-first-order and pseudo-second-order models for the adsorption on activated carbon prepared from pea shell.
Kinetic models and its parameters
Initial concentrations (mg/L)
100
150
200
250
300
350
q
e
,
exp (mg g−1)
99.65
148.73
190.66
217.20
227.85
247.49
Pseudo-first-order kinetic
qe,cal (mg g−1)
13.36
84.49
84.86
125.96
123.08
116.17
k1  (min−1)
0.049
0.043
0.024
0.025
0.022
0.022
R2
0.9638
0.9704
0.9958
0.9871
0.9889
0.9830
Pseudo-second-order kinetic
qe,cal  (mg g−1)
100.40
154.32
201.21
232.02
245.10
264.55
k2  (g mg−1 min−1)
0.00910
0.00110
0.00050
0.00030
0.00028
0.00029
h (mg g−1 min−1)
91.41
26.92
19.30
18.17
16.79
20.68
R2
0.9999
0.9998
0.9999
0.9998
0.9995
0.9993


## 3.8. Thermodynamic Studies

Thermodynamic parameters including Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were calculated from the following:
(8)ΔG°=-RTlnK,lnK=ΔSoR-ΔHoRT,
where R is the universal gas constant (8.314 J mol−1 K−1), T is the absolute temperature (Kelvin), and K is the equilibrium constant. Since Langmuir adsorption isotherm is the best fit, the value of b was used in place of K in the calculations of all thermodynamic parameters.Equation (11) is known as the van’t Hoff equation, and it expresses a relationship between equilibrium constant and temperature. Plots of lnK versus 1/T should be a straight line as shown in Figure 10. The values of ΔH° and ΔS° were calculated from the slope and intercept of van’t Hoff plot, respectively. The thermodynamic parameters associated with the adsorption of MB onto activated carbon prepared from pea shells are listed in Table 4. Negative values of ΔG° show the feasibility and spontaneity of the adsorption process. According to the van’t Hoff equation, standard enthalpy and the entropy values in the range of 25–55°C were obtained as 14.57 k J mol−1 and 0.147 k J mol−1 K−1, respectively. The positive value of ΔH° indicates the endothermic nature of the adsorption process. From Table 1, the maximum adsorption capacity increased from 246.91 to 270.00 mg g−1 with an increase in solution temperature from 25 to 55°C. This further confirmed the endothermic nature of the adsorption process. The positive value of ΔS° shows the increasing randomness of the solid-solution and adsorption medium interface during the adsorption.Table 4
Values of thermodynamics parameters for MB adsorption on activated carbon prepared from pea shell.
Temperature (K)
Δ
G
° (k J mol−1)
Δ
H
° (k J mol−1)
Δ
S
° (k J mol−1 K−1)
R
2
298
−28.40
14.57
0.147
0.9891
308
−30.46
318
−32.12
328
−33.73Figure 10
Plot oflnK versus 1/T for the enthalpy change of the adsorption process.


## 4. Conclusions

The adsorption of MB from aqueous solution onto activated carbon prepared from pea shells has been studied. The removal of methylene blue from wastewater is primarily controlled by the dye concentration in the wastewater, the adsorbent concentration, the pH of the wastewater, and the temperature of the adsorption process. The percentage of dye removal was found to increase with an increase in solution temperature, contact time, and adsorbent concentration and found to decrease with an increase in initial dye concentration. The percentage of dye removal increased with increases and decreases in pH and the maximum adsorption was obtained at pH 2.0 and 11.5. It was found that the sorption of the methylene blue was increased by the presence of anionic surfactant, and on the other hand was decreased by the presence of cationic and nonionic surfactant. Equilibrium data were fitted to Langmuir and Freundlich isotherms and the equilibrium data were best described by the Langmuir isotherm model, with maximum adsorption capacity of 246.91 mg g−1 at 25°C. The kinetics of the adsorption process was found to follow the pseudo-second-order kinetic model. By second-order model to study the mechanism of adsorption, calculated qe value agreed well with the qe experimental values. The thermodynamic parameters indicate that the adsorption process is endothermic and spontaneous in nature. As a result of this study, activated carbon prepared from pea shells can be used as an effective adsorbent for the removal of methylene blue from wastewater.



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*Source: 614083-2012-10-10.xml*