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

An activated carbon was prepared from pea shells and used for the removal of methylene blue (MB) from aqueous solutions. e in�uence of various factors such as adsorbent concentration, initial dye concentration, temperature, contact time, pH, and surfactant was studied.e experimental data were analyzed by the Langmuir and Freundlichmodels of adsorption.e adsorption isothermwas found to follow the Langmuirmodel.emonolayer sorption capacity of activated carbon prepared frompea shell for MB was found to be 246.91 mg g at 25 C. �wo simpli�ed kinetic models including pseudo-�rst-order and pseudo-second-order equation were selected to follow the adsorption processes. Kinetic studies showed that the adsorption followed pseudo-secondorder kinetic model. Various thermodynamic parameters such as ΔHH, ΔSS, and ΔGG were evaluated. e 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.


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].erefore 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][5][6][7][8][9][10].
ere are two basic processes to activate carbon materials, physical and chemical.e 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, ZnCl 2 [11], H 3 PO 4 [12], HCl [13], and H 2 SO 4 [14] or with basic reagents KOH [14], K 2 CO 3 [15], NaOH [12], and Na 2 CO 3 [16].
However there are no reports of obtaining active carbon from pea shells by chemical and physical activation methods [37].erefore, in this paper, the adsorption equilibrium and kinetics of methylene blue on activated carbon from pea shells were studied.e 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.e thermodynamics of the adsorption were also evaluated.) supplied by Merck was used as an adsorbate and was not puri�ed 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 (ZnCl 2 ) and the anionic surfactant sodium dodecylsulfate (SDS) were obtained from Merck.e 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 puri�cation.

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.e washed shell materials were then dried in sunlight for two weeks.Chemical activation of the pea shells was carried out with ZnCl 2 solution.10 g of dried pea shells were well mixed, by stirring, with 100 mL of an aqueous solution that contained 20 g of ZnCl 2 .e mixture was heated at 80 ∘ C for 1 h.Aer heating, the slurry was kept in an oven at 100 ∘ C for 24 h.e resulting ZnCl 2 treated sample was then subjected to carbonization and activation process in a programmable electrical furnace (Severn Furnaces Ltd.) at 500 ∘ C for 1 h.Aer 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.e resulting product was kept in a desiccator for further use.

Adsorption Experiments.
Adsorption equilibrium and kinetics were determined by the batch method.Adsorption isotherms were performed in a series of 250 mL conical �asks 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.e �asks 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 Roto�x 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.e percentage removal of dye from solution was calculated by the following: where  0 and   (mg L −1 ) are the initial dye concentration and concentration at time , respectively.e procedures of kinetic experiments were basically identical to those of equilibrium tests.e 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. e amount of adsorption at time ,   (mg g −1 ) was calculated by

Effect of Contact Time and Initial Concentration of Dye.
e adsorption data for the removal of MB versus contact time at different concentrations are presented in Figure 1.e dye removal percentage increased with the increase in contact time and remained constant aer 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%.is 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.e dynamic equilibrium was in�uenced by the initial concentration.e 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 ).

Effect of Adsorbent Concentration
. e adsorption of methylene blue was investigated by taking the following values as constant: 250 mg L −1 dye 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%.is 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. of contact time, a temperature of 25 ∘ C, and a 150 rpm stirring speed.e pH of the dye solution varied between 2.0 and 11.5, as it can be seen in Figure 3. e pH zpc (pH of zero net proton charge) is de�ned here as the pH value at which the net surface charge is equal to zero.e point of zero charge (pH zpc ) of active carbon that was prepared from pea shells was found to be 5.86.At high pH values (pH > pH zpc ), 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 < pH zpc ), 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 pH zpc 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.e fact that, in an acidic medium the initial pH value of adsorption �rstly increases and then reaches an initial value by decreasing can be accepted as an indication of the previous situation.

Effect of
3.4.Effect of Temperature.e 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.e 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].e 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.

Effect of Surfactants.
As surfactants are typically present in wastewater, an effect of their presence on dye adsorption was studied.e effect of surfactants on MB adsorption is shown in Figure 5. 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.is 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.
3.6.Adsorption Isotherms.e 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.e Langmuir model assumes that the adsorptions occur at speci�c homogeneous sites on the adsorbent and is used successfully in many monolayer adsorption processes [40].e Freundlich model endorses the heterogeneity of the surface and assumes that the adsorption occurs at sites with different energy levels of adsorption [41].e applicability of the isotherm equation is compared by judging the correlation coefficients,  2 .e linear form of Langmuir isotherm equation is given as where   is the equilibrium concentration of the dye solution (mg L −1 ),   is the amount of dye adsorbed per unit mass of adsorbent (mg g −1 ), and   (mg g −1 ) and  (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   /  versus   for the adsorption of MB onto activated carbon at different temperatures give a straight line of slope (1/  ) and intercept (1/  ).e essential characteristics of Langmuir isotherm can be expressed in terms of dimensionless equilibrium parameter (  ) [42].e parameter is de�ned by where  is the Langmuir constant and   is the highest dye concentration (mg L −1 ).e value of   indicates the type of the isotherm to be either unfavorable (  > 1), linear (  = 1), favorable (0 <   < 1), or irreversible (  = 0).e logarithmic form Freundlich equation is given by the following: where   (mg g −1 ) (L mg −1 ) 1/ and 1/ are Freundlich constants related to adsorption capacity and adsorption intensity of the adsorbent, respectively.eir values were obtained from the intercepts (ln   ) and slope (1/) of linear plots of ln   versus ln   at different temperatures (Figure 7).e slope 1/ 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/ below one indicates a normal Langmuir isotherm while 1/ above one is indicative of cooperative adsorption [44].
Table 1 shows the values of the parameters of the two isotherms and related correlation coefficients at different temperatures.e comparison of correlation coefficients ( 2 ) of the linearized form of both equations indicates that the Langmuir model yields a better �t for the experimental equilibrium adsorption data than the Freundlich model.With the maximum adsorption capacity,   values increased with increasing temperature.Its maximum value was determined T 1: e related parameters for the adsorption of MB on activated carbon prepared from pea shell at different temperatures.as 270 mg g −1 at 55 ∘ C. In the present study, the values of   were in the range of 0-1.is again con�rmed 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/, obtained for the present systems indicates favorable adsorption, as it lies between 0 and 1.Table 2 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.T 3: Pseudo-�rst-order and pseudo-second-order models for the adsorption on activated carbon prepared from pea shell.3.7.Adsorption Kinetics.e kinetic adsorption data were processed to understand the dynamics of adsorption process in terms of the rate constant order.us, the kinetic of MB adsorption onto activated carbon prepared from pea shells was analy�ed using pseudo-�rst-order and pseudo-secondorder kinetic models.e rate constant of adsorption is determined from the pseudo-�rst-order equation given by Langergren and Svenska [45]:

Kinetic models and its parameters
where   and   are the amounts of MB adsorbed (mg g −1 ) at equilibrium and at time t (min), respectively, and  1 (min −1 ) is the adsorption rate constant.A plot of log (  −   ) versus  should give a linear relationship, from which  1 and   can be determined from the slope and intercept of the plot, respectively (Figure 8).e pseudo-second-order kinetic model [46] is expressed as where ℎ is the initial adsorption rate (mg g −1 min −1 ) and  2 (g mg −1 min −1 ) is rate constant of second-order adsorption.
If the second-order kinetics is applicable, then the plot of /  versus  should show a linear relationship.Values of  2 and equilibrium adsorption capacity   were calculated from the intercept and slope of the plots /  versus  (Figure 9).Table 3 lists the results of the rate constant studies for di�erent initial dye concentrations by the pseudo-�rst-order and second-order models.e correlation coefficient,  2 for the pseudo-second-order adsorption model has a high value, and its calculated equilibrium adsorption capacity,  cal , is consistent with experimental data,   .Hence it can be T 4: Values of thermodynamics parameters for MB adsorption on activated carbon prepared from pea shell.assumed that the adsorption of MB follows the pseudosecond-order kinetic model.

ermodynamic Studies. ermodynamic parameters
including Gibbs free energy change (Δ ∘ ), enthalpy change (Δ ∘ ), and entropy change (Δ ∘ ) were calculated from the following: where  is the universal gas constant (8.314J mol −1 K −1 ),  is the absolute temperature (Kelvin), and  is the equilibrium constant.�ince Langmuir adsorption isotherm is the best �t, the value of  was used in place of  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 ln  versus 1/ should be a straight line as shown in Figure 10.e values of Δ ∘ and Δ ∘ were calculated from the slope and intercept of van't Hoff plote, respectively.e thermodynamic parameters associated with the adsorption of MB onto activated carbon prepared from pea shells are listed in Table 4. Negative values of Δ ∘ 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 14.57k J mol −1 and 0.147 k J mol −1 K −1 , respectively.e positive value of Δ ∘ 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. is further con�rmed the endothermic nature of the adsorption process.e positive value of Δ ∘ shows the increasing randomness of the solid-solution and adsorption medium interface during the adsorption.

Conclusions
e adsorption of MB from aqueous solution onto activated carbon prepared from pea shells has been studied.e 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.e 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.e 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 �tted 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. e kinetics of the adsorption process was found to follow the pseudosecond-order kinetic model.By second-order model to study the mechanism of adsorption, calculated   value agreed well with the   experimental values.e 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.

− 1 F 1 :F 2 :
Solution pH.e 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 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). of adsorbent (g L −1 ) Effect of adsorbent concentration on MB adsorption ( 0 = 250 mg L −1 , pH = 6.85, temp.= 25 ∘ C, and contact time = 180 min).

F 6 :F 7 :
Langmuir isotherm plots for MB adsorption at different temperatures.Freundlich isotherm plots for MB adsorption at different temperatures.