The removal of cationic dyes from aqueous solutions by adsorption onto pistachio hull waste

The removal of cationic dyes from aqueous solutions by adsorption onto pistachio hull waste

chemical engineering research and design 8 9 ( 2 0 1 1 ) 2182–2189 Contents lists available at ScienceDirect Chemical Engineering Research and Desig...

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chemical engineering research and design 8 9 ( 2 0 1 1 ) 2182–2189

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The removal of cationic dyes from aqueous solutions by adsorption onto pistachio hull waste Gholamreza Moussavi ∗ , Rasoul Khosravi Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modarres University, Tehran, Iran

a b s t r a c t The efficacy of pistachio hull powder (PHP) prepared from agricultural waste was investigated in this study as a novel adsorbent for the elimination of dye molecules from contaminated streams. Removal of methylene blue (MB) as a cationic model dye by PHP from aqueous solution was studied under different experimental conditions. The selected parameters were solution pH (2–10), PHP dosage (0.5–3 g/L), MB concentrations (100–400 mg/L), contact time (1–70), and solution temperature (20–50 ◦ C). The experimental results indicated that the maximum MB removal could be attained at a solution pH of 8. The dosage of PHP was also found to be an important variable influencing the MB removal percentage. The removal efficiency of MB improved from 94.6 to 99.7% at 70 min contact time when the MB concentration was decreased from 300 to 100 mg/L at a pH and PHP dosage of 8 and 1.5 g/L, respectively. The kinetic analysis showed that the pseudo-second-order model had the best fit to the experimental data. The Langmuir equation provided the best fit for the experimental data of the equilibrium adsorption of MB onto PHP at different temperatures. In addition, the maximum adsorption capacity increased from 389 to 602 mg/g when the temperature was increased from 20 to 50 ◦ C. The thermodynamic evaluation of MB adsorption on PHP revealed that the adsorption phenomenon under the selected conditions was a spontaneous physical process. Accordingly, pistachio hull waste was shown to be a very efficient and low-cost adsorbent, and a promising alternative for eliminating dyes from industrial wastewaters. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Cationic dye; Methylene blue; Waste materials; Adsorption; Pistachio hull



Dyes are synthetic organic compounds that are increasingly being produced and used as colorants in many industries worldwide, including textile, plastic, paper, etc. (Crini, 2006; Wu and Tseng, 2008). The wastewater generated by the processes of these industries usually contains up to around 10% of used dye (Forgacs et al., 2004). Most of the dyes are toxic and carcinogenic compounds; they are also recalcitrant and thus stable in the receiving environment, posing a serious threat to human and environmental health (Crini, 2008). Accordingly, to protect humans and the receiving ecosystem from contamination, the dyes must be eliminated from the dye-contained wastewaters before being released into the environment. Because dye molecules are resistant to biodegradation (Ravi Kumar et al., 1998), biological processes are not useful or

efficient methods for the removal of dyes from effluents. Although different chemical advanced oxidation processes (AOPs) have been shown to be efficient for the degradation of several classes of dyes, they are expensive and economically non-attractive. Adsorption is one of the promising alternative techniques used for the removal of dyes from water and wastewater (Gupta and Suhas, 2009), and activated carbon is the most widely used adsorbent (Wu et al., 2005). However, the production of activated carbon is complex and expensive, making this technology economically nonefficient. Accordingly, the critical challenge for applying the adsorption method to dye removal is finding a low-cost adsorbent that is profoundly available with high removal capacity so adsorption can successfully compete with other dye removal techniques. This is the driving force behind further studies attempting to find an efficient low-cost adsorbent. Waste

Corresponding author. Tel.: +98 21 82883827; fax: +98 21 82883825. E-mail address: [email protected] (G. Moussavi). Received 9 October 2010; Accepted 25 November 2010 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.11.024

chemical engineering research and design 8 9 ( 2 0 1 1 ) 2182–2189


–CO groups on its surface (Moussavi and Barikbin, 2010). Distilled water was used for the preparation of dye solutions. All other chemicals were of analytical grade and purchased from Merck.

Fig. 1 – The structure of methylene blue dye. materials have recently been viewed as potential low-cost adsorbents, and many reports have been published showing their ability to adsorb various contaminants including dyes (e.g., Wu and Tseng, 2008; Gupta and Suhas, 2009; Rafatullah et al., 2010). Pistachio hull, an agriculture waste, is proposed as a no-cost and profoundly accessible potential dye adsorbent. To evaluate the adsorption potential of pistachio hull wastes for removing dyes from wastewater, methylene blue (MB) was selected as a model cationic dye; several waste materials have been studied using MB as the model dye. Rafatullah et al. (2010) recently reviewed the works published on MB removal by adsorption onto different low-cost adsorbents and reported a wide range of adsorption capacity from 2 to 915 mg/g, which is comparable to that of commercial activated carbon. Among the waste materials tested over the past decade, some materials like papaya seeds (Hameed, 2009a) and grass waste (Hameed, 2009b) have shown considerable adsorption capacity. However, the applications of these materials are limited practically because the adsorbent is not available in great enough amounts. In the other words, despite having a significant capacity for dye adsorption, most of these materials are not produced in a central location in one country; therefore, they are not available in sufficient bulk to be commercialized for full scale application. Indeed, a candidate material that can act an alternative to activated carbon must both have a high adsorption capacity and be profoundly available in bulk at no or low cost. In this context, the pistachio hull, an agricultural waste, was proposed to be investigated for its potential to adsorb MB. Pistachio hulls are a waste generated in pistachio peeling factories and are accessible in bulk at no cost. Therefore, it is very interesting to test the capability of pistachio hull waste to eliminate dyes from wastewater. The main purpose of the present study was to explore the capability of pistachio hull waste to remove a basic model dye, MB, from liquid media under different experimental conditions. The effects of main parameters, i.e., solution pH, mass of adsorbent, dye concentration, contact time, and solution temperature, were studied for MB removal. The kinetics, isothermics and thermodynamics of MB adsorption under optimal experimental conditions were also evaluated.


Materials and methods



MB, a model of cationic dyes with a molecular formula of C16 H18 N3 ClS, was purchased from a local market. The structure of MB is shown in Fig. 1. The MB used in the present study has a molecular weight of 319.85 with its maximum absorbance at a wavelength of 665 nm. A powder made from pistachio hulls, hereafter referred to as pistachio hull powder (PHP), was used as an adsorbent. The characteristics of PHP have been given elsewhere (Moussavi and Barikbin, 2010). In summary, PHP is a micropore adsorbent with a BET surface area of 1.04 m2 /g and containing –OH, –CH, C O, and

2.2. Procedure of adsorption and equilibrium experiments The ability of PHP to adsorb MB was tested at different conditions (Table 1) using a series of batch tests in a shakerincubator instrument (IKA® KS4000i). For each test, 100 mL of dye solution with the desired composition (pH and dye concentration as given in Table 1), was poured into an Erlenmeyer flask, the given amount of PHP was added, the temperature of the incubator was adjusted to the desired level, and the instrument was used to stir the mixture at 100 rpm for a preset time. When mixing was completed, the suspension was filtered through a 0.2 ␮m fiberglass filter; the filtrate was then analyzed for its maximum absorbance at a wavelength of 665 nm. The concentration of MB was calculated from the calibration curve prepared by measuring the absorbance of the serial dilutions of dye at 665 nm, and then plotting the concentration versus absorbance. Five flasks were used to evaluate the MB adsorption equilibrium on PHP; 100 mL of 400 mg/L MB solution with optimum pH of 8 (see Section 3.1) was poured into each of them and a different mass of PHP (0.05, 0.1, 0.15, 0.2, 0.25, and 0.3 g) was then added to flasks and put on the shaker for mixing for 12 h. We assumed the mixtures would reach equilibrium. At the end of the shaking time, the residual concentration of MB was measured in the filtrate (as described above) and the adsorption capacity (mg/g) was calculated as the mass of MB adsorbed onto PHP divided by the amount of PHP added to the flask. The adsorption equilibrium of MB onto PHP was examined at various temperatures between 20 and 50 ◦ C under conditions given in Table 1. To ensure the repeatability of the data, all tests were conducted in duplicate and the mean of values was used for data analysis.


Data analysis

The effect of the experimental operational parameters of pH, dye concentration, PHP dose, mixing time, and temperature on MB removal from the dye solution was analyzed by plotting the fraction of MB remaining in the solution versus the investigated variable. After finding the optimum conditions of the tested variables where the residual dye in the solution was minimal (maximum dye removal), the kinetics, equilibrium, and thermodynamics of MB adsorption onto PHP were evaluated. To assess the adsorption kinetics, the results of the effect of MB concentration and contact time were analyzed using pseudo-first-order and pseudo-second-order models (Gómez et al., 2007), as well as the intraparticle diffusion model (Weber and Morris, 1963). The linear form of these models for boundary conditions of q = 0 at t = 0 and qt = qe at t = te are presented in “Equations and nomenclature” section. The Langmuir, Freundlich, and Dubinin–Radushkevich models were used to evaluate the MB adsorption equilibrium for PHP. The linear forms of these equations (Hameed, 2009c) are given in “Equations and nomenclature” section. The thermodynamics of MB adsorption were assessed based on the data of the designated experiments (Table 1) using the changes of Gibbs free energy of adsorption (G◦ ), standard enthalpy change (H◦ ) and stan-


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Table 1 – Experimental runs and conditions. Experiment


Effect of pH Effect of PHP dose and time Effect of MB concentration and time Equilibrium of adsorption Effect of temperature

Solution pH

PHP dose (g/L)

MB concentration (mg/L)

Temperature (◦ C)

Contact time (min)

2–10 8 8 8 8

1 0.5–2 1.5 0.5–3 0.5

100 100 100–300 400 400

20 20 20 20–50 20–50

10 1–70 1–70 720 720

where S stands for adsorption sites on the surface of PHP. The optimum pH at which maximum MB adsorption attained was 8, and so the rest of the experiments were conducted at this optimum pH. This finding agrees with other researchers who have also reported maximum MB adsorption on different adsorbents at an alkaline pH (e.g., Al-Ghouti et al., ˘ 2006; Almeida et al., 2009; Hameed, 2009a; Dogan et al., 2007).


Fig. 2 – The profile of MB removal as a function of solution pH (conditions: MB concentration = 100 mg/L, pH = 2–10, PHP dosage = 1 g/L, and contact time = 10 min). dard entropy change (S◦ ) (Nie et al., 2007; Bayramoglu et al., 2009).


Results and discussion


Effect of solution pH

The effect of solution pH was studied for MB removal by PHP in a range of 2–10 under the conditions given in Table 1. The mean of the duplicated experimental results is plotted in Fig. 2, indicating that pH considerably affected MB adsorption, particularly under acidic conditions. A similar trend was also reported for MB adsorption from aqueous solution as a function of pH by other researchers, including yellow passion fruit peels (Pavan et al., 2008), papaya seeds (Hameed, 2009a) and jackfruit peel (Hameed, 2009c). As can be seen in Fig. 2, the degree of MB adsorption onto PHP increased from around 59.2% to a maximum of 99.6% when the solution pH was increased from 2 to 8. The reason that PHP behaved differently in adsorbing MB at different solution pHs can be explained by considering the pHzpc of the adsorbent as well as molecular nature of MB (cationic molecule). The pHzpc of PHP particles’ surface is 5, meaning that the adsorbent’s surface was positively charged at solution pHs below 5. This causes competition between protons and MB formed cations for adsorption locations (Hamdaoui, 2006; Almeida et al., 2009) as well as the repulsion of cationic MB molecules, resulting in the reduction of dye adsorption. The lower the pH goes below pHzpc, the greater the density of positive ions on the surface of PHP will be which in turn allows for less adsorption. This is confirmed by low MB removal at a strongly acidic pH of 2 (Fig. 2). When solution pH increases above pHzpc (5), a negative charge is present on the surface of PHP, causing better MB cations adsorption through the electrostatic attraction phenomenon shown by the simplified reaction as follows: −




Effect of PHP dose

The effect of PHP dose on MB adsorption was performed in a range of 0.5–2 g/L under the conditions specified in Table 1. The results are shown in Fig. 3 in terms of MB removal as a function of contact time at various PHP doses. Fig. 3 makes two important points. First, for all levels of PHP doses, the removal percentage improved with the increase of contact time, with the highest rate at the initial times. This can be attributed to free adsorption sites available at the initial phases of the test (Kavitha and Namasivayam, 2007), as well as to a higher mass transfer rate at initial contact times where a higher concentration of MB is available to be adsorbed; thus, the rate of adsorption was greater. The second point in Fig. 3 is the increase of MB removal with the increase of PHP dose, resulting in a shorter treatment time. According to Fig. 3, around 81.6, 91.4, 96.8, and 99.1% of 100 mg/L MB was removed in the first minute of contact time. The degree of removal reached a maximum of 92 and 97.5% after 70 min when in contact with doses of 0.5 and 1 g/L, respectively, whereas 99.7 and 99.9% MB removal efficiencies were achieved for 70 min contact time in the presence of 1.5 and 2 g/L PHP, respectively. The enhancement of MB removal as a function of PHP dose is due to the presence of a greater surface for adsorption and thus more available sites (Hameed, 2009c) for MB molecules to be adsorbed (Eq. (1)), leading to the uptake of more molecules

Fig. 3 – The influence of PHP dosage on MB removal as a function of mixing time (conditions: MB concentration = 100 mg/L, pH = 8, PHP mass = 0.05–0.2 g, and mixing time = 1–70 min).

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Fig. 4 – The influence of MB concentration on MB removal by PHP as a function of mixing time (conditions: MB concentration = 100–300 mg/L, pH = 8, PHP mass = 0.15 g, and mixing time = 1–70 min). from solution in high doses for a similar contact time. This resulted in a greater percentage of MB removal. Our findings are in agreement with those reported previously for adsorption of MB onto papaya seeds (Hameed, 2009a) and jackfruit peels (Hameed, 2009c), although we had a different removal percentage, which was due to use of different operational conditions. From an engineering perspective, because PHP is profoundly available at no cost, a higher dosage should be used to reduce the size, operation and maintenance (mixing chamber), and thereby the cost of the treatment system.


Effect of initial MB concentration

The dye in the effluent of different industries may have different concentrations, which raises the question of how different dye concentrations influence the performance of PHP for removing MB. In this work, the adsorption of 100, 200 and 300 mg/L MB concentrations was investigated for PHP versus a contact time of up to 70 min under the conditions given in Table 1. The means of the duplicate data are depicted in Fig. 4, indicating a considerable influence of initial MB concentration on its removal. Based on data illustrated in Fig. 4, the degree of MB removal was 96.8, 87.8, and 73.9% for the first minute of contact for initial dye concentrations of 100, 200 and 300 mg/L, respectively. By increasing the contact time to 70 min, we obtained 99.7, 98, and 94.6% removal for MB at the tested concentrations, respectively. Therefore, the increase of initial MB concentration resulted in a reduction of its adsorption percentage due to limited adsorption sites. The reduction of removal percentage of MB with the increase of its initial concentration can be explained as follows: because the mass of PHP is constant for all three concentrations, the MB molecules must compete for sites onto which they can adsorb. In fact, when the MB concentration is higher, the ratio of MB to PHP is greater, and therefore the percentage of MB removed is lower. Nevertheless, the experimental adsorption capacity of MB at the selected conditions increased from 66.5 to 189 mg MB/g PHP (data not shown) when the concentration of dye was increased from 100 to 300 mg/L. This can be explained by the enhancement of mass transfer rates due to a higher MB gradient concentration at a higher initial dye concentration, subsequently causing the uptake of more MB molecules and thereby increasing the mass ratio of dye to PHP (i.e., adsorption capacity).


Fig. 5 – Pseudo-second-order plots of adsorption of different concentrations (100–300 mg/L) of MB onto PHP.

3.4. Kinetic of MB adsorption reaction onto PHP at optimum pH and PHP concentration To analyze the kinetics of adsorption of different concentrations of MB onto PHP, the experimental data from phase 3 of the study (see section 3.1) was fitted with pseudo-firstand second-order kinetics (Eqs. (2) and (3)). The fitted linear regression plots showed that the experimental data had its best fit with the pseudo-second-order model for all three investigated concentrations with higher determination coefficients (R2 > 0.999) than those of the pseudo-first-order model. Therefore, it is found that the rate of MB adsorption onto PHP is of pseudo-second-order, suggesting that the adsorption of MB onto PHP is influenced by both the dye and the PHP concentrations under the investigated conditions (Abramian and El-Rassy, 2009). The fitted linear regression plots of pseudo-second-order model are shown in Fig. 5. The values of pseudo-second-order adsorption constants (k2 ) for MB concentrations of 100, 200 and 300 mg/L were 0.250, 0.024, and 0.005 mg/(g min), respectively, showing a considerable decrease of k2 with the increase of Mb concentration. This trend supports the conclusion that the mass transfer was likely the limiting step of adsorption, and that the rate of MB mass transfer to PHP improved with increased initial concentrations. In addition, the experimental adsorption capacity (qe,exp ) values were very close to the model-calculated adsorption capacity (qe,cal ) data (not shown), verifying the high correlation of adsorption to the pseudo-second-order model. The adsorption of organic compounds onto an adsorbent involves the following consecutive steps: bulk solution transport, external (film) resistance to transport, internal (pore) transport, and adsorption. The slowest step limits the adsorption rate. The literature indicates that the first and last steps do not usually limit the adsorption process under experimental conditions (Abramian and El-Rassy, 2009). It is therefore presumed that either film resistance (boundary layer diffusion) or diffusion into the adsorbent particles (intraparticle diffusion) is the rate-limiting step. Intraparticle diffusion was analyzed using Eq. (4). The plots of intraparticle diffusion of MB onto the PHP at the initial concentrations of 100, 200 and 300 mg/L are illustrated in Fig. 6a, b and c, respectively. Referring to Fig. 6, two distinct regions are observed for three concentrations, showing that different adsorption mechanisms are involved at different interval


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Table 2 – Isotherm modeling of MB adsorption onto PHP. Equilibrium temperature (◦ C)

Isotherm model 20

Langmuir 0.096 KL 389 qmax 0.991 R2 0.03 RL Freundlich 112.3 KF n 4.2 0.955 R2 Dubinin–Radushkevich 0.02 KDR √ 5.0 E = 1/ 2KDR 0.855 R2

Fig. 6 – Plots of intraparticle diffusion model of adsorption of different concentrations (a) 100 mg/L, (b) 200 mg/L, and (c) 300 mg/L of MB onto PHP.

˘ contact times (Dogan et al., 2007). The first section of plots indicates that boundary layer diffusion probably limited MB adsorption (Dizge et al., 2008). The second section shows the occurrence of intraparticle diffusion as the adsorption limiting step (Hameed, 2009d; Cheung et al., 2007). Fig. 6 also illustrates that, for higher MB concentrations, the contribution of boundary layer diffusion decreases, whereas the time period during which the intraparticle diffusion is the adsorption rate limiting factor is longer. Another important point found in Fig. 6 is the increase of slope of both sections of plots with the increase of the MB concentration, showing that the influence of the rate-limiting step is more pronounced for higher dye concentrations. Similar kinetic behavior has also been reported for adsorption of MB onto other low-cost adsorbents such as papaya seeds (Hameed, 2009a), modified wheat straw (Han et al., 2010), and rejected tea (Nasuha et al., 2010).


Isotherm of MB adsorption onto PHP

Isotherms correlate the equilibrium adsorption data with different mathematical models to describe the behavior of




0.059 516 0.971 0.04

0.046 541 0.970 0.05

0.048 602 0.957 0.05

92.9 3.1 0.934

77.7 2.7 0.940

81.6 2.6 0.910

0.04 3.5 0.818

0.08 4.0 0.851

0.04 3.5 0.611

the adsorption process; an optimized design of an adsorption system provides valuable information (Özer and Dursun, 2007). Accordingly, we evaluated the fitness of the equilibrium data obtained from the experiments at different temperatures (Table 1) with the Langmuir (Eq. (5)), Freundlich (Eq. (6)), and Dubinin–Radushkevich (Eq. (7)) models. The best fitted model was selected based on the determination coefficient (R2 ). The data from the isotherm evaluation is summarized in Table 2. Based on the information in Table 2, the R2 of the Langmuir isotherm model for all tested temperatures was higher than the other fitted models, showing that the experimental equilibrium data was better explained by the Langmuir equation. This finding supports the assumption that the MB is adsorbed as a homogeneous monolayer onto PHP particles sites and has a free-energy change for all adsorption sites (Crittenden et al., 2005). This result agrees with the findings of other researchers (e.g., Hameed et al., 2009), who used the Langmuir model for describing the adsorption of MB onto different waste materials. The fitted Langmuir model estimated that the maximum MB adsorption capacity increased from 389 to 602 mg/g PHP with the increase of the solution temperature from 20 to 50 ◦ C, close to the experimental equilibrium adsorption capacity, which confirmed the precise fit of the model with the experimental data. A comparison of the maximum adsorption capacity of PHP found for MB adsorption to that of other waste-based adsorbents including jackfruit peels (Hameed, 2009b), pineapple stems (Hameed et al., 2009), and rejected tea (Nasuha et al., 2010) revealed the PHP had a greater adsorption capacity for MB than most of those previously reported waste-based adsorbents. In addition to having high adsorption capacity, the main practical advantage of PHP is that a large amount can be simply prepared from profoundly available waste materials; this is not the case for most of the previously mentioned adsorbents. Therefore, PHP has proven to be a very efficient and emerging low-cost adsorbent for the removal of MB from polluted streams. The favorability of MB adsorption onto PHP was further analyzed using a dimensionless parameter (RL = 1/(1 + KL Ci )) derived from the Langmuir equation where Ci is the initial concentration of the target dye under adsorption. The adsorption process can be defined as irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1) in terms of RL (Sivaraj et al., 2001). The calculated values of RL for adsorption of MB at temperatures between 20 and 50 ◦ C fall between 0 and 1 (Table 2); therefore, the adsorption process of MB onto


chemical engineering research and design 8 9 ( 2 0 1 1 ) 2182–2189

Table 3 – Thermodynamic parameters of MB adsorption onto PHP. Parameter

Temperature (K)

G◦ (kJ/mol) S◦ (kJ/(mol K)) H◦ (kJ/mol)

Fig. 7 – The influence of solution temperature on removal capacity and percent of MB by PHP (conditions: MB concentration = 400 mg/L, pH = 8, PHP mass = 0.05 g, and mixing time = 720 min).

PHP was favorable. Further, the value of the constant n in the Freundlich model, included in Table 2, was greater than unity, which also verifies that PHP is an appropriate adsorbent for the removal of cationic dyes. Referring to Table 2 for information given about the D–R isotherm model, the free energy (E) of MB adsorption onto PHP was in the range of 3.5–5 kJ/mol for solution temperatures between 20 and 50 ◦ C. In the D–R isotherm, the value of E indicates the mechanism by which adsorption takes place. A value of E below 8 kJ/mol indicates physical adsorption, and a value between 8 and 16 kJ/mol indicates chemical adsorption (Ada et al., 2009). Therefore, the removal of MB dye by PHP in the present work was likely dominated by physicosorption.

3.6. Effect of temperature and thermodynamic of MB adsorption onto PHP The effect of solution temperature on equilibrium adsorption of MB onto PHP was assessed for four temperatures of 20, 30, 40, and 50 ◦ C under the conditions specified in Table 1. Fig. 7 depicts the effect of temperature on removal of MB by PHP in terms of removal efficiency and adsorption capacity. Based on data plotted in Fig. 7, the increase of solution temperature from 20 to 50 ◦ C resulted in a large increase of the equilibrium removal percentage of MB from 48 to 68% under the selected experimental conditions. For the same situation, adsorption capacity also improved from 384 to 585 mg-MB/gPHP. It can be therefore inferred from these findings that MB adsorption onto PHP is an endothermic phenomenon. The positive influence of temperature on the observed adsorption can be related to the enhancement of the diffusion rate of MB through the boundary layer due to decrease of solution viscosity as well as mobility of dye molecules with the increase of temperature, and within internal pores of the PHP particles (Özcan et al., 2005; Nasuha et al., 2010). In other words, considering a fixed concentration of MB and PHP, a greater number of dye molecules were transferred from the bulk solution onto adsorbent particles, resulting in both an increase of removal percentage and adsorption capacity. Similar effects were observed by other researchers for temperature on the removal of MB, although onto different adsorbents (e.g., Almeida et al., 2009; Nasuha et al., 2010; Sánchez-Martín et al., 2010).





303 −0.93

293 −0.33

0.06 17.25

In order to explain and confirm the mechanism of MB adsorption onto PHP, the thermodynamics of adsorption were evaluated using G◦ , H◦ , and S◦ , given by Eqs. (8) and (9). As seen in Table 3, the value of G◦ for all tested temperatures was calculated to be negative, which suggests that the adsorption of MB onto PHP is spontaneous and indicates that PHP has a high affinity for the adsorption of MB from solution under experimental conditions (Crini, 2008). Values of G◦ between −20 and 0 kJ/mol indicate a physical adsorption process (Almeida et al., 2009); thus, the results of thermodynamic investigation reconfirmed the hypothesis of physicosorption of MB onto PHP. Furthermore, the values of H◦ and S◦ in the present experiment were 17.25 kJ/mol and 0.06 kJ/(mol K), respectively. A positive value of H◦ proves the adsorption phenomenon is endothermic (Cheung et al., 2007). Also, the positive value of S◦ indicates increased randomness at the interface of PHP-solution during the adsorption process (Jain and Jayaram, 2010).



The present work introduced the agricultural waste of pistachio hulls, a new low-cost waste material, as an adsorbent for dye removal from wastewater. The prepared powder, PHP, appeared to be an adsorbent with a relatively high specific surface area that was capable of efficiently removing high concentrations of cationic dyes at mild pH conditions. The maximum adsorption capacity calculated from the Langmuir isotherm, the best fit model to the experimental data, was between 389 and 602 mg/g for temperatures between 20 and 50 ◦ C, respectively. A thermodynamic evaluation confirmed that the adsorption of MB onto the prepared powder was dominated by spontaneous physical sorption and that the adsorbent had a high affinity for adsorption of the tested dye molecules. Therefore, we conclude that pistachio hulls produced as an agriculture waste material are a viable and very promising alternative adsorbent for color removal from industrial wastewater.

Equations and nomenclature Pseudo-first-order equation :

ln(qe − qt ) = ln qe − k1 t


t 1 t = + qt qe k2 q2e


Pseudo-second-order equation :

Intraparticle diffusion model : Langmuir :

Freundlich :

qt = kid t0.5 + C

1 Ce Ce = + qe KL qmax qmax ln qe = ln KF +

1 ln Ce n

(4) (5)



chemical engineering research and design 8 9 ( 2 0 1 1 ) 2182–2189

Dubinin–Radushkevich(D–R) :

ln qe = ln qm − KDR ε2


Changes of Gibbs free energy :

G◦ = −RT ln K


Standard enthalpy change : Standard entropy change :

ln K = −

H◦ S◦ + RT R

G◦ = H◦ − T S◦

(9) (10)

C0 Ct Ce E kid k1 k2 kL KDR KF M n qt qe

initial MB concentrations (mg/L) MB concentrations at time t of reaction (mg/L) MB concentrations at equilibrium time (mg/L) mean of adsorption free energy (kJ/mol) constant of intraparticle diffusion (mg/(g min0.5 )) pseudo-first order rate constant (1/min) pseudo-second order rate constant (mg/(g min)) Langmuir constant (L/mg) D–R constant (mol2 /kJ2 ) Freundlich adsorbent capacity (mg/g(L/mg)1/n ) mass of PHP added to the solution (g) the reciprocal of reaction order adsorption capacity at time t (mg/g) adsorption capacity at equilibrium conditions (mgMB /gPHP ) qmax maximum adsorption capacity (mg/g) R gas constant (8.314 J/(mol K)) T absolute temperature (K) V volume of the MB solution (L) ε Polanyi potential (J/mol) K = Cae /Ce the thermodynamic equilibrium constant G◦ changes of Gibbs free energy

Acknowledgement The authors acknowledge the Tarbiat Modares University for financial and technical support during this study.

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