Sorption of basic dye from aqueous solution by pomelo (Citrus grandis) peel in a batch system

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Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 78–84

Sorption of basic dye from aqueous solution by pomelo (Citrus grandis) peel in a batch system B.H. Hameed ∗ , D.K. Mahmoud, A.L. Ahmad School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Received 27 April 2007; received in revised form 10 July 2007; accepted 22 August 2007 Available online 25 August 2007

Abstract A new, low-cost, locally available sorbent, pomelo (Citrus grandis) peel (PP), was tested for its ability to remove basic dye (methylene blue) from aqueous solutions. Adsorption equilibrium and kinetics of methylene blue (MB) from aqueous on PP were studied in a batch process. The equilibrium data were analyzed using the Langmuir, Freundlich, and Temkin isotherm models. Sorption equilibrium studies demonstrated that the biosorption followed Langmuir isotherm model. The monolayer adsorption capacity was 344.83 mg/g at 30 ◦ C. Kinetic analyses were conducted using pseudo-first-, second-order and intraparticle diffusion models. It was found that the sorption kinetics of MB on PP obeyed pseudo-second-order sorption kinetics. The results in this study indicated that pomelo peel was an attractive candidate for removing MB from aqueous solutions. © 2007 Elsevier B.V. All rights reserved. Keywords: Pomelo peel; Methylene blue; Adsorption isotherm; Equilibrium; Kinetics

1. Introduction Textile industry is a growing industry in Malaysia. Effluents from the dyeing and finishing processes in this industry are highly colored, and their discharge into rivers makes water unfit for domestic, agricultural and industrial purposes. Since a very small amount of dyes in water is highly visible and can be carcinogens and toxic to aquatic life in water [1–3], it is important to remove these pollutants from the wastewaters before their final disposal. Dyes can be effectively removed by adsorption process. Currently, the focus of the research is to exploit the use of biomass and other low-cost materials as potential adsorbents such as orange peel [4], banana and orange peels [5], kohlrabi peel [6], peanut hull [7], hazelnut shells [8], apple pomace and wheat straw [9], shale oil ash [10], palm ash [11], Posidonia oceanica (L.) fibres [12] and sand [13]. Pomelo (Citrus grandis), the largest of citrus fruits, belongs to the family Rutaceae. The pomelo is native to southeastern Asia and all of Malaysia. In Malaysia, pomelo is widely grown

∗

Corresponding author. Tel.: +60 4 599 6422; fax: +60 4 594 1013. E-mail address: [email protected] (B.H. Hameed).

0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.08.033

in the state of Perak, Kedah, Melaka, Kelantan and Johor. It is the largest citrus fruit, growing as large as 30 cm in diameter and weighing as much as 10 kg; the rind is very thick but soft and easy to peel away. The thick peel of the pomelo is sometimes used to prepare marmelades and sweet candies. Due to the high consumption of pomelo, massive amounts of the peels (as waste products) are disposed, causing a severe problem in the community. In the interest of the environment, we utilized this agricultural waste as a low-cost sorbent to remove basic dye from aqueous solutions. Therefore, the objective of this study was to explore the feasibility of using pomelo peel (PP), an agricultural waste, for removal of methylene blue from aqueous solutions. 2. Materials and methods 2.1. Sorbate Basic dye used in this study was methylene blue (MB) purchased from Sigma–Aldrich. The MB was chosen in this study because of its known strong adsorption onto solids. MB has a molecular weight of 373.9 g/mol, which corresponds to methylene blue hydrochloride with three groups of water. The maximum absorption wavelength of this dye is 668 nm. The structure of MB is shown as follows:

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2.5. Batch kinetic studies Kinetic experiments were identical to those of equilibrium tests. The aqueous samples were taken at preset time intervals and the concentrations of MB were similarly measured. All the kinetic experiments were carried out at pH 7. The amount of sorption at time t, qt (mg/g), was calculated by

2.2. Sorbent Pomelo peels (PP) were collected from nearby market as solid waste. The collected materials were then washed with distilled water for several times to remove all the dirt particles. The washed materials were cut into small pieces (1–2 cm) and dried in a hot air oven at 60 ◦ C for 48 h. Then, ground and finally screened to obtain a particle size range of 0.5–1 mm and stored in plastic bottle for further use. No other chemical or physical treatments were used prior to adsorption experiments. The surface functional groups of PP were detected by Fourier transform infrared (FTIR) spectroscope (FTIR-2000, PerkinElmer). The spectra were recorded from 4000 to 400 cm−1 . The spectrum displayed the following bands: 3411.07 cm−1 (–OH stretching vibrations), 2930.01 cm−1 (O–H stretching vibration), 1739.36 cm−1 (C O stretching vibration), 1637.56 cm−1 (C C stretching vibration), 1384.42 cm−1 (salts of carboxylic acids), 1054.48 cm−1 (C–O–H). 2.3. Equilibrium studies Batch sorption studies were carried out by adding a fixed amount of sorbent (0.20 g) into 250 mL Erlenmeyer flasks containing 200 mL of different initial concentrations (50, 100, 200, 300, 400, and 500 mg/L) of dye solution and pH 7. The flasks were agitated in an isothermal shaker at 100 rpm and 30 ◦ C for 5.15 h until equilibrium was reached. Aqueous samples were taken from the solutions and the concentrations were analyzed. At time t = 0 and at equilibrium, the dye concentrations were measured by a double beam UV–vis spectrophotometer (Shimadzu, Model UV 1601, Japan) at 668 nm. The amount of equilibrium adsorption, qe (mg/g), was calculated by qe =

(C0 − Ce )V W

(1)

qt =

(C0 − Ct )V W

(2)

where Ct (mg/L) is the liquid-phase concentrations of dye at any time. 3. Results and discussion 3.1. Effect of solution pH on dye uptake Effects of pH on sorption of basic dye (methylene blue) have been studied by many researchers, and the results indicated that pH of solution could significantly influence biosorption process [12,14–16]. The effect of pH on dye biosorption by the PP was studied first because the initial solution pH can significantly influence biosorption of dye. To study the influence of pH on the sorption capacity of PP, the experiments were performed at 100 mg/L initial dye concentration with 0.2 g sorbent mass at 30 ◦ C for 5.15 h equilibrium time. Fig. 1 shows the variation in amount adsorbed at equilibrium, qe (mg/g) at different solution pH. As shown, the equilibrium sorption capacity was minimum at pH 2 (4.96 mg/g), increased up to 6 and remained nearly constant (47.75–46.36 mg/g) over the initial pH ranges of 6–10. Lower adsorption of MB at acidic pH is probably due to the presence of excess H+ ions competing with the cation groups on the dye for adsorption sites. As surface charge density decreases with an increase in the solution pH, the electrostatic repulsion between the positively charged dye (MB) and the surface of the PP is lowered, which may result in an increase in the rate of adsorption. A similar result was reported for the adsorption of methylene blue onto jute fibre carbon [17]. At higher solution pH, the PP may get negatively charged, which enhances the positively charged dye cations through electrostatic forces of attraction.

where C0 and Ce (mg/L) are the liquid-phase concentrations of dye at initial and equilibrium, respectively. V the volume of the solution (L) and W is the mass of dry sorbent used (g). 2.4. Effect of solution pH The effect of initial solution pH was determined by agitating 0.2 g of PP sorbent and 200 mL of dye solution of initial basic dye concentration 100 mg/L using water-bath shaker (30 ◦ C) at different solution pH ranging from 2 to 10. Agitation was provided for 5.15 h contact time which is sufficient to reach equilibrium with a constant agitation speed of 100 rpm. The pH was adjusted by adding a few drops of diluted 1.0N NaOH or 1.0N HCl before each experiment. The pH was measured by using a pH meter (Ecoscan, EUTECH Instruments, Singapore).

Fig. 1. Effect of initial pH on the equilibrium sorption capacity of PP (C0 = 100 mg/L, temperature 30 ◦ C, stirring rate 100 rpm and W = 0.2 g).

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3.2. Effect of and initial dye concentration and contact time The effect of initial MB concentration on sorption by PP was investigated in the range of 50–500 mg/L of the initial dye concentration at sorbent concentration of 0.20 g, temperature of 30 ◦ C and initial pH 7. The variation of the uptake values with contact time were depicted in Fig. 2. It is clear that sorption of MB on PP depends on its initial concentration. An increase in the initial dye concentration led to an increase in the amount of dye adsorbed on PP. This may be attributed to an increase in the driving force of the concentration gradient with the increase in the initial dye concentration. The amount of MB adsorbed increased from 24.86 to 135.43 mg/g as the concentration was increased from 50 to 500 mg/L. Apparently, the initial concentration plays an important role in affecting the sorption capacity of dye onto PP. Tan et al. [18] reported similar result for the adsorption of methylene blue on oil palm fibre activated carbon. The sorption of MB on PP was also studied as a function of contact time in order to find out the equilibrium time for maximum adsorption. The results show that the contact time needed for MB solutions with initial concentrations of 50–100 mg/L to reach equilibrium was 3.45 h. For MB solutions with initial concentrations of 300–500 mg/L, equilibrium time of 4.31 h was required. As shown in Fig. 2, the sorption process at different concentrations is rapid in the initial stages (45 min) and gradually decreases with the progress of adsorption until the equilibrium is reached. These changes in dye uptake may be due to the fact that, initially, all sorbent sites were vacant and the solute concentration was high. After that period, only a very low increase in the dye uptake was observed because there are few active sites on the surface of PP. Data on the sorption kinetics of MB by various adsorbents have shown a wide range of adsorption rates. For example, Ncibi et al. [12] have studied methylene blue biosorption by P. oceanica (L.) fibres and reported 10 min equilibrium adsorption time. Senthilkumaar et al. [19] reported the adsorption rate of crystal violet (basic dye) on phosphoric and sulphuric acid activated carbons (PAAC and SAAC) at different time intervals and different initial dye concentrations and found the equilibrium time

Fig. 2. Effect of initial concentration and contact time on the uptake of MB at 30 ◦ C (pH 7 and W = 0.2 g).

was 50, 80, 120 and 180 min for PAAC and 120, 150, 180 and 220 min for SAAC for the initial concentration, 10, 20, 30 and 40 mg/L, respectively. 3.3. Equilibrium modelling The isotherm data were obtained by dye concentration measurement after sorbent/adsorbate contact periods equal to the equilibrium times. Out of several isotherm equations developed to describe adsorption isotherm relationships, three models were applied for the equilibrium data of PP: the Langmuir, Freundlich and Temkin isotherms. Langmuir adsorption model [20] assumes that adsorption occurs at specific homogeneous adsorption sites within the adsorbent and intermolecular forces decrease rapidly with the distance from the adsorption surface. The model further based on the assumption that all the adsorption sites are energetically identical and adsorption occurs on a structurally homogeneous adsorbent. Langmuir model which has been successfully applied to many sorption processes is qe =

Qo bCe 1 + bCe

(3)

The linear form of Langmuir isotherm is expressed as 1 1 1 = + qe Qo bQo Ce

(4)

where qe is amount of dye sorbed at equilibrium per unit mass of sorbent (mg/g) and Ce is the equilibrium concentration of dye in solution (mg/L). The constant Qo signifies the maximum sorption capacity (mg/g) and b is related with the energy of the adsoprtion (L/mg). A plot of 1/qe versus 1/Ce (Fig. 3) yields a straight line with slope 1/bQo and intercept 1/Qo . The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL that is given by Eq. (5) [21]: RL =

1 1 + bC0

(5)

where C0 is the highest initial concentration of sorbate (mg/L), and b (L/mg) is the Langmuir constant. The value 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

Fig. 3. Langmuir isotherm for MB sorption onto PP at 30 ◦ C.

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Table 1 Langmuir, Freundlich and Temkin model constants and correlation coefficients for sorption of MB onto PP Isotherm

Constants

Langmuir Qo (mg/g) b (L/mg) R2

344.83 0.0028 0.99

Freundlich KF n R2

Fig. 4. Freundlich isotherm for MB sorption onto PP at 30 ◦ C.

3.098 1.475 0.93

Temkin A (l/g) B R2

value of RL for adsorption of MB onto PP was 0.417. This value showed that the sorption behaviour of PP was favorable for the dye (RL < 1). The Freundlich isotherm [22] is an empirical equation employed to describe heterogeneous systems. The Freundlich equation is expressed as

and can be linearized as

qe = KF Ce1/n

qe = B ln A + B ln Ce

(6)

where KF and n are the Freundlich constants with n giving an indication of how favorable the adsorption process is and KF (mg/g (L/mg)1/n ) is the adsorption capacity of the sorbent. The magnitude of the exponent, 1/n, gives an indication of the favorability of adsorption. Values of n > 1 represent favorable adsorption condition [23–25]. Eq. (6) may be written in the logarithmic form as   1 ln qe = ln KF + (7) ln Ce n Values of KF and n are calculated from the intercept and slope of the plot (Fig. 4). The Temkin equation [26] suggests a linear decrease of sorption energy as the degree of completion of the sorptional centres of an adsorbent is increased. The heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbent–adsorbate interactions. The adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The Temkin isotherm has been generally applied in the following form: qe =

RT ln(ACe ) b

Fig. 5. Temkin isotherm for MB sorption onto PP at 30 ◦ C.

0.061 47.106 0.97

(9)

where B = RT/b, b is the Temkin constant related to heat of sorption (J/mol); A the Temkin isotherm constant (L/g), R the gas constant (8.314 J/(mol K)) and T is the absolute temperature (K). Therefore, by plotting qe versus ln Ce enables one to determine the constants A and b as shown in Fig. 5. The constants A and B are listed in Table 1. The isotherm constants for all the isotherms studied, and the correlation coefficient, R2 with the experimental data are listed in Table 1. In view of the values of linear regression coefficients in Table 1, the Langmuir model exhibited better fit (R2 = 0.99) to the sorption data of MB than the Freundlich and Temkin models at 30 ◦ C and in the studied initial concentration range. Ofomaja [27] also observed that the isotherm data for sorption of methylene blue uptake on to palm kernel fibre were described by the Langmuir equation. The monolayer adsorption capacity was found to be 344.83 mg/g at 30 ◦ C. The fact that the Langmuir isotherm fits the experimental data very well may be due to homogeneous distribution of active sites onto PP surface. The maximum sorption capacity (Qo ) of the PP was compared with other reported data as shown in Table 2. The performance of the PP is seen to be considerably better than these other sorbents.

(8) Table 2 Comparison of adsorption capacities of various adsorbents for cationic dyes Dyes

Adsorbents

Qo (mg/g)

T (◦ C)

References

Basic blue 3 Methylene blue Methylene blue Methylene blue Rhodamine-B Rhodamine-B Neutral red Basic violet 1, Basic violet 10, Basic green 4, Astrazon yellow 7GL

Pomelo peel Palm kernel fibre Banana peel Orange peel Banana peel Orange peel Kohlrabi peel Sugarcane dust Sugarcane dust Sugarcane dust Wheat bran

344.83 217.96 20.80 18.60 20.60 14.30 112.36 50.40 13.90 20.60 69.06

30 26 30 30 30 30 20 ± 2 25 25 25 30

This work [27] [5] [5] [5] [5] [6] [28] [28] [28] [29]

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Fig. 6. Pseudo-first-order kinetics for sorption of MB on PP at 30 ◦ C. Fig. 7. Pseudo-second-order kinetics for sorption of MB on PP at 30 ◦ C.

3.4. Adsorption kinetics

The pseudo-second-order equation based on equilibrium adsorption [25] is expressed as

Lagergren proposed a method for adsorption analysis which is the pseudo-first-order kinetic equation [30] in the form:

1 1 t = + t qt k2 qe2 qe

dqt = k1 (qe − qt ) dt

where k2 (g/mg h) is the rate constant of second-order adsorption. k2 can be determined experimentally from the slope and intercept of plot t/qt versus t (Fig. 7). The calculated R2 for pseudo-second-order kinetic model are shown in Table 3. Table 3 lists the results of rate constant studies for different initial dye concentrations by the pseudo-first- and second-order models. The experimental data showed a good compliance with the pseudo-second-order equation and the correlation coefficients for the linear plots were 0.99 for all the experimental data. These suggested that the pseudo-second-order adsorption mechanism was predominant and that the overall rate of the MB sorption process appeared to be controlled by chemical process involving valency forces through sharing or exchange of electrons between sorbent and sorbate [32]. Similar kinetics were also observed for the removal of methylene blue from aqueous solution using raw P. oceanica (L.) fibres, a marine lignocellulosic biomass [12], adsorption of MB on activated carbon derived from bamboo [33] and oil palm fibre [18]. For the pseudosecond-order model in Table 3, in general, the rate constant decreases with an increasing of initial dye concentration. Since neither the pseudo-first-order nor the second-order model can identify the diffusion mechanism, the kinetic results were further analyzed by the intraparticle diffusion mode to elucidate the diffusion mechanism, which model is expressed as

(10)

where qe and qt are the amounts of MB adsorbed at equilibrium and at time t in mg/g, respectively, and k1 is the pseudo-first-order rate constant (h−1 ). The integration of Eq. (10) with the initial condition, qt = 0 at t = 0 leads to log(qe − qt ) = log qe −

k1 t 2.303

(11)

A linear plot of log(qe − qt ) against time allows one to obtain the rate constant (Fig. 6). If the plots were found to be linear with good correlation coefficient, indicating that Lagergren’s equation is appropriate to MB sorption on PP. So, the adsorption process is a pseudo-first-order process [30,31]. The rate constants, predicted equilibrium uptakes and the corresponding correlation coefficients for all concentrations tested have been calculated and summarized in Table 3. Correlation coefficients were found to be above 0.88, but the calculated qe is not equal to experimental qe , suggesting the adsorption of MB on PP is not likely to be a pseudo-first-order for the initial concentrations examined.

(12)

Table 3 Comparison of the pseudo-first-order, pseudo-second-order sorption rate constants and calculated and experimental qe values obtained at different initial MB concentrations Initial conc. (mg/L)

qe, exp (mg/g)

Pseudo-first-order kinetic model k1

50 100 200 300 400 500

24.86 47.79 94.21 124.89 132.39 135.43

(h−1 )

0.773 0.899 0.886 1.374 1.283 1.078

Pseudo-second-order kinetic model

qe, cal (mg/g)

R2

k2 (g/(mg h))

qe, cal (mg/g)

R2

20.08 47.65 91.62 190.37 172.66 97.41

0.94 0.92 0.96 0.94 0.88 0.97

0.054 0.025 0.011 0.010 0.012 0.020

27.78 54.35 109.89 144.93 147.06 144.93

0.99 0.99 0.99 0.99 0.99 0.99

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used without prior treatment before use, with great potential in removal of basic dye, and needs not be regenerated after being loaded with dye and can be disposed off by burning. References

Fig. 8. Intraparticle diffusion plots for removal of MB at 30 ◦ C.

Table 4 Intraparticle diffusion constants for different initial MB concentrations at 30 ◦ C Initial conc. (mg/L)

qe, cal (mg/g)

kid (mg/(g h1/2 ))

C

R2

50 100 200 300 400 500

25.64 49.84 99.66 135.05 140.87 142.27

7.12 14.74 32.29 42.06 39.24 30.11

9.48 16.39 26.38 39.58 51.79 73.91

0.99 0.98 0.95 0.92 0.94 0.92

qt = kid t 1/2 + C

(13)

where C is the intercept and kid is the intraparticle diffusion rate constant (mg/(g h1/2 )), which can be evaluated from the slope of the linear plot of qt versus t1/2 [34]. According to this model, the plot of uptake, qt , versus the square root of time, t1/2 (Fig. 8), should be linear if intraparticle diffusion is involved in the adsorption system and if these lines pass through the origin, then intraparticle diffusion is the rate-controlling step. The intercepts (C) and the intraparticle rate constant values calculated from the slopes of the linear portions of the plots of Fig. 8 were presented in Table 4. The deviation of straight lines from the origin (Fig. 8) may be due to difference in rate of mass transfer in the initial and final stages of sorption. Further, such deviation of straight line from the origin indicates that the pore diffusion is not the sole rate-controlling step. 4. Conclusions The present investigation showed that the pomelo peel is a promising sorbent for the removal of MB from aqueous solutions. The removal of basic blue by sorption on PP was found to be rapid at the initial period of contact time and then slows down with increasing contact time. The sorption was dependent on solution pH and initial MB concentration. Sorption of MB onto PP followed the Langmuir isotherm model. The monolayer adsorption capacity of PP was 344.83 mg/g at 30 ◦ C. The higher adsorption capacity of PP (344.83 mg/g) shows that this PP could be used as sorbent for the removal of MB. The sorption kinetics was well described by the pseudo-second-order kinetic model equation. The pomelo peel, an agricultural waste, is available in large quantity in Malaysia. The sorbent was successfully

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