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Dye adsorption characteristic of ultrasound pre-treated pomelo peel
Journal of Environmental Chemical Engineering 6 (2018) 3502–3509
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Dye adsorption characteristic of ultrasound pre-treated pomelo peel Suk Khe Low, Mei Ching Tan
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T
Department of Chemical and Petroleum Engineering, Faculty of Engineering, Technology and Built Environment, UCSI University, 56000 Cheras, Kuala Lumpur, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasound Pomelo peel Methylene blue dye Adsorbent Isotherm model
Pomelo peels were pre-treated with ultrasound at 30, 60 and 90% amplitude to enhance its ability to remove methylene blue from aqueous solution. The adsorption capacity of the peels at different solution pH (2–10), dye concentration (50–250 mg/L), adsorbent dose (0.05–0.25 g) and contact time (0.5–5 h) was investigated and the equilibrium isotherms were studied. Ultrasound pre-treated pomelo peel took shorter time to reach higher saturation limit of adsorption capacity than the non-treated pomelo peel. Pomelo peel treated at 30% ultrasound amplitude had highest adsorption capacity, which increased with solution pH, dye concentration and adsorbent dose, respectively reached maximum increment of 5%, 9.9% and 9.3% compared to the non-treated pomelo peel. The adsorption isotherm data fitted best by the Dubinin-Radushkevich model with the highest correlation coefficient in the range of 0.9424–0.9951, compared to Langmuir, Freundlich and Temkin models.
1. Introduction Dyes are heavily used by textile, leather, and paper industries among other industries to color their products. Concurrently, substantial amount of water is used too. At the end of the process, considerable amount of colored wastewater is discharged into the river streams, typically without any pre-treatment. The color presence in water as a visible pollutant affects public perception on the water quality. Concurrently, the organic compounds and toxic substances contained in the wastewater affect the aquatic ecosystem and human health. Hence, it is necessary to remove the dye from waste streams before being discharged into public water sources. While treatment processes such as cation exchange membranes, micellar enhanced ultrafiltration, electrochemical degradation, photocatalytic degradation, sonochemical degradation, and integrated chemical-biological degradation can be used for dye removal from wastewater [1], adsorption is still considered as the most economic and an easy process [2]. Numerous works have been published on using agro-waste materials as biosorbent of dyes in water, namely jackfruit peel [3], papaya seeds [4], grapefruit peel [5], garlic peel [6], orange peel [7,8], yellow passion fruit peel [9], lemon peel [10], pomegranate peel [11], lychee peel [12], banana peel [13], and pomelo peel [14,15]. Typically, the thick peel of pomelo, which accounted for approximately half of its total fruit weight are peeled off and discarded as waste [16]. Thus, the present work aimed to reduce the biological resource wastes by further exploring this agro-waste’s potential as a biosorbent. Methylene blue is a hydrophilic basic dye with cationic properties originating from
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Corresponding author. E-mail address: [email protected] (M.C. Tan).
https://doi.org/10.1016/j.jece.2018.05.013 Received 4 January 2018; Received in revised form 12 April 2018; Accepted 6 May 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.
positively charged nitrogen and sulfur atoms [17]. Methylene blue, as the most commonly used dye in various industries, was selected in the present work as a model compound of dye to be removed from aqueous solutions, due to its known strong adsorption onto solids [18,19]. According to Zhang et al. [2], mesopores or micropores could trap the methylene blue molecules through the impregnation process, while macropores promote mass transfer process. In order to enhance the efficiency of dye removal, dye adsorption process can be performed in ultrasound environment simultaneously [20,21]. The results showed remarkable improvement on dye removal efficiency. It was suggested to be due to the high-pressure shock waves and high-speed microjects during ultrasound cavitation [20], which decreased the diffusion resistance and eased the insertion of dye into the adsorbent [22]. However, the application of the simultaneous ultrasound may not be practical for industrial scale due to the big volume of wastewater need to be treated. To possibly overcome such constraint and to improve the dye removal efficiency, the pomelo peel is first treated with ultrasound prior to the adsorption process in the present study. 2. Materials and methods 2.1. Adsorbate A strong adsorption cationic dye of methylene blue [14,23] was obtained from Friendemann Schmidt Chemicals. A stock solution with concentration of 250 mg/L was prepared by dissolving 0.25 g of
Journal of Environmental Chemical Engineering 6 (2018) 3502–3509
S.K. Low, M.C. Tan
solution when equilibrium was reached at the end of contact time by filtration. The concentrations of dye solution at each pH were then measured using UV–vis spectrophotometer.
methylene blue powder into 1000 mL distilled water. It was then further diluted into different concentrations of 50, 100, 150 and 200 mg/L. 2.2. Adsorbent
2.6. Analytical methods Pomelo peel wastes were thoroughly washed with distilled water to remove the dirt particles prior to cutting into small pieces of 2–3 cm. A 250 mL beaker containing 12 g of pomelo peels with 100 mL of distilled water was sonicated using ultrasound probe system (UP200S, Hielscher Ultrasonics GmbH, Germany) equipped with a 2 mm diameter micro tip (24 kHz, 600 W/cm2) located inside an anolyte compartment. The tip was submerged at the center of the mixture. As different samples, the peels were sonicated with ultrasound at 30, 60 and 90% amplitude for 30 min. Pomelo peels without ultrasound treatment were considered as a control sample. Ultrasound treated and non-treated pomelo peels were dried in a hot air oven (Carbolite Gero, AX series, United Kingdom) at 100 °C for 5 h to reach constant weight. The dried samples were grinded into 1–2 mm particle size by using grinder (MX-800S, Panasonic, Malaysia) and stored in an airtight container.
2.6.1. UV–vis spectrophotometer The filtrated dye solution was loaded into a cuvette upon the removal of adsorbent from the dye solution. The dye concentration was then measured using UV–vis spectrophotometer by monitoring its absorbance at wavelength 480 nm. The concentration value was then determined from the calibration curve generated by using the standard methylene blue solution with the known concentration. 2.6.2. Scanning electron microscope (SEM) The surface morphology of pomelo powder with and without ultrasound treatments were examined using scanning electron microscope (JSM 6400, Jeol, Japan) to generate the SEM images. All the samples were coated with 600 nm of gold layer.
2.3. Batch kinetic and equilibrium studies
2.6.3. Analysis of porosity An analysis of pores size distribution on surface of pomelo powder was conducted on the constructed images obtained from SEM analysis by using ImageJ software. The number of pores at size ranged from 1 to 20 μm diameter was analyzed at 640 × 382 pixels. A histogram of the pores size distribution was plotted by using Statistical Package for the Social Sciences (SPSS 16.0) software (Chichago, IL, USA).
The batch adsorption studies were conducted in a 250 mL stoppered glass Erlenmeyer flask by adding 0.20 g of adsorbent into 100 mL of dye solution at different initial concentrations (50, 100, 150, 200 and 250 mg/L) without pH adjustment at 30 °C. The flask was placed on a magnetic stirrer plate and stirred at 150 rpm for 5 h until equilibrium was reached. For batch kinetic studies, the concentration of dye solution was continuously measured at 30 min intervals. The adsorbent was first removed from the dye solution by using filter paper before the concentration of dye solution was analyzed using UV–vis spectrophotometer (Hitachi U-2900 Spectrophotometer, Japan) at wavelength 480 nm. The amount of adsorption at time t, qt (mg/g) was calculated by using Eq. (1).
qt =
(Co − Ct ) V W
2.7. Statistical analysis Curve fitting was carried out using solver function in Microsoft Excel which adopted the Generalised Reduced Gradient (GRG2) nonlinear optimization code to determine the isotherm constants in nonlinear form of Langmuir model, Freundlich model, Temkin model, and Dubinin-Radushkevich model [24,25]. The best fitted line was generated through the goal setting of minimum sum of square errors (SSE) and total corrected sum of squares (SST) in obtaining all the constants SSE value with goodness of fit, R2 = 1 − SST [25,26]. The error bars in the graphs are standard deviation of means of duplicate samples.
(1)
where Co and Ct (mg/L) are the initial dye concentration and concentration at time t respectively, V is the volume of solution (L) and W is the mass of dry adsorbent (g) used. For batch equilibrium studies, the amount of adsorption when the equilibrium is reached was measured. The amount of equilibrium adsorption, qe (mg/g) was calculated by using Eq. (2),
qe =
(Co − Ce ) V W
3. Results and discussion 3.1. Effect of initial concentration and contact time
(2)
Agriculture by-product normally consists of lignin and cellulose as major constituents having polar structure for the adsorption of dyes [27]. Adsorption capacity of adsorbent with and without ultrasound treatment was investigated at different initial concentration of methylene blue (50–250 mg/L) as shown in Fig. 1. The dye uptake by the adsorbent with and without ultrasound treatment was rapid for the first 30 min. This was due to high number of vacant anionic sites at the initial stage. These vacant sites decreased with time which leads to saturation or equilibrium state. Ahmad [28] reported that the rate of crystal violet dye adsorption was fast at the first 60 min due to high availability of adsorption sites at the initial stage. The vacancy of adsorption sites was then decreased with time and caused the adsorption rate to slow down in later stage until it reaches saturation. The nontreated adsorbent reached equilibrium at the first hour for 50 mg/L initial concentration, while it took 4 h to reach equilibrium for methylene blue with 100–250 mg/L initial concentration. Adsorbent took longer time to reach saturation as the dye concentration increased. Comparatively higher concentration of methylene blue contained higher amount of cations in the dye solution. Thus, the cations took longer time to compete among each other to bind on the anionic sites of adsorbent surface. Adsorbent treated with 30% and 60% ultrasound
where Ce is the concentration at equilibrium. 2.4. Effect of adsorbent dose Different amount of adsorbent (0.05, 0.1, 0.15, 0.20 and 0.25 g) was added to 100 mL of dye solution with initial concentration of 150 mg/L in a 250 mL stoppered glass Erlenmeyer flasks at temperature of 30 °C. The solution was stirred with magnetic stirred plate at 150 rpm for 4 h of contact time to reach the equilibrium. The adsorbent was then filtered out from the dye solution and the dye concentration was then measured using UV–vis spectrophotometer. 2.5. Effect of solution pH 0.2 g of adsorbent was added to each 100 mL dye solution with initial concentration of 150 mg/L at different initial pH (2, 4, 6, 8 and 10). The dye solution pH was adjusted using 0.1 mol/L HCl or 0.1 mol/L NaOH before the adsorption experiment was carried out. The dye solution containing adsorbent was stirred with magnetic stirrer plate at 150 rpm and 30 °C for 4 h. The adsorbent was separated from the dye 3503
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Fig. 1. Effect of initial concentration and contact time on methylene blue for adsorbent (a) without ultrasound treatment, and adsorbent with ultrasound treated at (b) 30%, (c) 60% and (d) 90% amplitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
amplitude did not show any obvious changes compared with the control. The effect of ultrasound decreased as the amount of adsorbent increased. The dye removal of ultrasound treated adsorbent at 30% amplitude showed the highest increment of 39% at 0.05 g of adsorbent, while the lowest increment of 9% at 0.25 g adsorbent, respectively when compared with the control. The increase of adsorbent ability was due to the increase of adsorption sites on adsorbent surfaces for methylene blue binding [3].
amplitude took 1.5 to 2 h to reach the equilibrium for methylene blue with 100–200 mg/L initial concentration, and 3.5 h for 250 mg/L initial concentration. Fig. 1a indicated that equilibrium adsorption of non-treated adsorbent increased from 15.50 to 94.88 mg/L when the initial concentration increased from 50 to 250 mg/L. The results showed that ultrasound treated adsorbent has higher adsorption capacity compared with non-treated adsorbent. Dye uptake by adsorbent at lower initial concentration of methylene blue (50–150 mg/L) increased as the ultrasound amplitude increased from 30% to 90%. At higher initial concentration (200–250 mg/L), the adsorption capacity of adsorbent decreased when the ultrasound amplitude applied increased. Adsorbent treated at 30% ultrasound amplitude shows the highest adsorption capacity with 4% and 10% higher than non-treated adsorbent, while 6% and 10% higher than absorbent treated at 90% ultrasound amplitude, respectively at 200 mg/L and 250 mg/L initial concentration of methylene blue. Ultrasound treated adsorbent reached equilibrium state in shorter time than non-treated adsorbent, yet with higher adsorption capacity. It is suggested that ultrasound cavitation effect has created more pores on the adsorbent surface, which contribute to generate more anionic adsorption sites for the binding purpose with methylene blue cations. Therefore, it speeds up the adsorption process of methylene blue on adsorbent with high amount of adsorption.
3.3. Effect of solution pH on dye adsorption Fig. 3 indicated that the adsorption of methylene blue was the lowest at pH 2. At pH 4, the adsorption of methylene blue by all samples increased drastically about 6 times higher. Dye removal of non-treated adsorbent was nearly constant from pH 4 to 10. Fig. 3 showed a decrease in adsorption capacity as the amplitude application increased from 30% to 90%. However, as the pH increased from pH 2 to pH 10, the adsorbents treated with higher amplitude of 60% and 90% show remarkable increases in adsorption capacity. All the ultrasound treated adsorbents have higher adsorption capacity than the control at pH 10, where the highest was achieved by 30% of ultrasound amplitude with 87.75% of dye removal. Dye removal increased with pH due to the increase of negative charges on adsorbent surface [29], which created high electrostatic forces to attract the cationic dye for binding [6]. However, the adsorption processes seemed saturated with the increase in solution pH as observed in non-treated adsorbent from pH 4 to pH 10. This could be due to the limitation of adsorption sites that available on the adsorbent surface, where the cationic dyes competes with each other. Ultrasound enhances the adsorption capacity by generating more porous structures on the adsorbent surface which contributes more anionic adsorption sites for cationic dye. Hence, with the high electrostatic force generated
3.2. Effect of adsorbent dose on dye adsorption Fig. 2 shows the effect of different doses of ultrasound treated and non-treated adsorbent on dye adsorption. The dye uptake by adsorbent increased as adsorbent doses increased. The adsorption start to remain nearly constant when the amount of adsorbent increased up to 0.2 g. Dye removed by ultrasound treated adsorbents at 30% and 60% amplitude were higher than non-treated adsorbent at any doses, while 90% 3504
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Fig. 2. Batch equilibrium adsorption capacity on methylene blue by different dosage of non-treated adsorbent (■), and ultrasound treated adsorbent at 30% ( ), 60% ( ) and 90% ( ) amplitude, and respective percentage of dye removal ( ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.5. Porosity of pomelo peel powder
at high solution pH [14], ultrasound application at high amplitude can enhance the dye adsorption.
Fig. 5(a)–(d) respectively show the overall total number of pores at different pore diameter for non-treated pomelo peel, and pomelo peel treated at 30%, 60% and 90% ultrasound amplitude. The result indicates that ultrasound pre-treatment has increased the number of pores on the non-treated pomelo peel, from 228 pores with 2.03 μm of pore diameter to the highest number of 793 pores with 1.23 μm of pore diameter when it was pre-treated with ultrasound at 30% amplitude. However, the increases of ultrasound amplitude from 30% to 60% and 90% cause the pores number to reduce about 18% and 59% respectively with increasing pore size. The non-treated pomelo peel was categorized with macroporous structure with its pore diameter in the range of 2–20 μm, while ultrasound treated pomelo peel was in mesoporous structure with pore diameter less than 2 μm [31,32]. In previous study of methyl orange removal by porous carbon, Hazhen et al. [33] found that the porous carbon derived from pomelo peel consists of numerous micropores, and small amount of mesopores and macropores.
3.4. Surface morphology (SEM) Fig. 4 showed the surface morphology of the samples characterized using SEM. The surface of original untreated pomelo peel powder is relatively smooth without any pore as shown in Fig. 4(a) compared with ultrasound treated pomelo peel powder. Fig. 4(b)–(d) showed pomelo peel powder treated with 30%, 60% and 90% ultrasound amplitude respectively. Average size of the porous structure is obviously formed on powder surface when treated with 30% ultrasound amplitude. The formation of porous structure is due to the hydrodynamic cavitation effect of ultrasound. It enhances the adsorption efficiency of the adsorbent through the active site created, which provides larger surface area for the dye component to attach with. The cavitation effect at 60% ultrasound amplitude starts to cause surface rupture with less pore exposed compared to 30% amplitude as illustrated in Fig. 4(c). This suggested that higher ultrasound amplitude relative to higher power caused over hydromechanical shear forces on the powder surface and lead to surface rupture which proceed to form bigger pores as indicated in Fig. 4(d) when ultrasound amplitude increased to 90%. Bigger pore size per unit volume has less specific area as compared to high porosity in smaller size per unit volume. Du Plessis [30] observed an inverse relationship between pore size and specific surface area in the study of nanoporous silicon.
3.6. Isotherm analysis The isotherm data of dye concentration during equilibrium was used to describe the interaction of adsorbate and adsorbent by using isotherm models of Langmuir, Freundlich, Temkin and DubininRadushkevich in non-linear form. Langmuir model was applicable on many monolayer adsorption processes where it assumes that adsorption
Fig. 3. Batch equilibrium adsorption capacity on methylene blue at different pH by non-treated adsorbent (■), and ultrasound treated adsorbent at 30% ( ), 60% ( ) and 90% ( ) amplitude, and respective percentage of dye removal ( ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. SEM micrograph of pomelo peel (a) before, and after ultrasound treated at (b) 30%, (c) 60% and (d) 90% amplitude (magnification: ×1000).
Fig. 5. Pore diameter distribution of pomelo peel (a) without ultrasound pre-treatment, and with ultrasound pre-treatment at (b) 30, (c) 60 and (d) 90% amplitudes.
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the interaction of adsorbent-adsorbate, by assuming that the heat of adsorption of all molecules in the layer decreases linearly rather than logarithmic with the coverage of adsorbent surface [35,38]. The nonlinear form of Temkin model is shown in Eq. (6),
occur energetically equivalent at specific homogenous sites on adsorbent surface [34] with finite capacity [35]. The non-linear Langmuir isotherm is represented by Eq. (3),
qe =
qm K a Ce 1 + K a Ce
(3)
qe =
where Ce is the equilibrium concentration (mg/L), qe is the amount of adsorbate adsorbed per unit mass of adsorbate (mg/g), qm and Ka are the Langmuir constants denoted the maximum sorption capacity (mg/ g) and energy of the adsorption (L/mg) respectively. The magnitude of qm showed that monolayer saturation capacity increased with decreasing ultrasound amplitude and reached maximum value of 7041.52 mg/g at 30% amplitude, while the value of Ka increased with an increase in amplitude, indicates a high adsorption affinity [36]. The essential characteristics of Langmuir isotherm can be expressed in terms of separation factor, RL that defined by Eq. (4),
RL =
1 1 + K a Co
where Ka is the Langmuir constant and Co is the highest initial concentration of adsorbate (mg/L). The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0) [3,35]. The separation factor of methylene blue was found in the range of 0.2817–0.9302, indicating favorability of the adsorption process between the test amplitudes. Freundlich isotherm is applicable on both homogeneous and heterogeneous surface of adsorbent [35], which describes the heterogeneity of the surface, assuming that the adsorption occurs at sites with different energy levels of adsorption. The non-linear form of Freundlich model is given as Eq. (5),
qe = qs exp(−BD εD2 ) ⎜
E=
Langmuir
Freundlich
Temkin
DubininRadushkevich
Ultrasound Amplitude (%) 30
60
90
qm (mg/g) Ka (L/mg) RL R2 KF (L/g) 1/n R2 A (L/g) b R2 qs (mg/g)
1883.88 0.0009 0.8163 0.9166 1.0360 1.1153 0.9248 6.0508 2798.74 0.9198 113.14
7041.52 0.0003 0.9302 0.8004 0.5191 1.3844 0.9249 7.6801 2471.44 0.8043 157.79
2708.19 0.0006 0.8696 0.9282 0.7644 1.2018 0.9734 6.1894 2764.04 0.9352 112.23
246.10 0.0102 0.2817 0.9681 1.4800 1.0157 0.8731 6.2469 2750.16 0.9027 93.27
BD (mol2/J2) E (J/mol) R2
0.000111 67.25 0.9951
0.000139 59.94 0.9620
0.000113 66.54 0.9424
0.000046 104.53 0.9478
(8)
1 2BD
(9)
Mean adsorption energy, E per molecule of adsorbate is usually applied to differentiate between physical and chemical adsorption [43]. The adsorption process is considered as chemical ion exchange if the mean adsorption energy is between 8 and 16 kJ/mol, while it is physical adsorption if the mean adsorption energy is below 8 kJ/mol [39,42,44]. Table 1 shows the adsorption energy, E in range of 59.94–104.53 J/mol, indicated the methylene blue adsorption onto pomelo peel is a physical adsorption process. As shown in Table 1 and illustrated in Fig. 6, highest correlation coefficient, R2 was derived by fitting the experimental data into Dubinin-Radushkevich isotherm model (0.9424 ≤ R2 ≤ 0.9951) compared to Langmuir isotherm model (0.8004 ≤ R2 ≤ 0.9681), Freundlich isotherm model (0.8731 ≤ R2 ≤ 0.9734) and Temkin isotherm model (0.8043 ≤ R2 ≤ 0.9352). This concludes that Duninin-Radushkevich isotherm model was most suitable for the experimental data to describe the adsorption phenomena, suggested that the characteristic sorption curve of this study is related to the porous structure of the sorbent [41], due to ultrasound cavitation effect.
Table 1 Isotherm parameters for the removal of methylene blue from aqueous solution by pomelo peel without and with ultrasound treatment at different amplitudes.
0
⎟
where R is gas constant (8.314 J/mol/K), T is the absolute temperature (K), while qs is the Dubinin-Radushkevich constant related to adsorption capacity (mg/g), BD is the activity coefficient related to mean sorption energy (mol2/J2), and εD is the Polanyi potential [41,42]. The mean adsorption energy, E is calculated from Eq. (9),
(5)
Parameters
(7)
1⎞ εD = RT ln ⎛1 + C e⎠ ⎝
where KF and n are the Freundlich constants respectively represent the adsorption capacity of the adsorbent (mg/g (L/mg)1/n) and adsorption intensity of the adsorbent. The value 1/n ranges between 0 and 1 measure the adsorption intensity or surface heterogeneity. The adsorption process become more heterogeneous as 1/n gets closer to zero, whereas 1/n below unity indicate chemisorption process and 1/n above one represent the process of cooperative adsorption [38]. The adsorption of methylene blue onto ultrasound treated and untreated pomelo peel was a cooperative process with Freundlich constant 1/n above one as shown in Table 1. Temkin isotherm is applicable for adsorption on heterogeneous solid adsorbent surface and liquid adsorbate, which explicitly consider
Isotherm
(6)
where R is gas constant (8.314 J/mol/K), T is the absolute temperature (K), while A and b are Temkin constants that related to the maximum binding energy (L/g) and heat of adsorption (J/mol) respectively [35]. The maximum binding energy, A of ultrasound treated pomelo peel was higher than the non-treated pomelo peel. The value of A was highest at lowest ultrasound amplitude, with maximum value of 7.6801 L/mg at 30% amplitude. This shows that the physical binding capacity between methylene blue and pomelo peel increased in the adsorption process at lower application of ultrasound amplitude compared to higher ultrasound amplitude and the non-treated pomelo peel [39]. The adsorption energy in Temkin model, b is positive for methylene blue adsorption from the aqueous solution showing that the adsorption is exothermic [36]. Dubinin-Radushkevich isotherm describes the adsorption process following a pore filling mechanism, which generally applied on both homogeneous and heterogeneous surfaces [37,40]. The non-linear form of Dubinin-Radushkevich model is illustrated as Eqs. (7) and (8),
(4)
qe = KF Ce1/ n
RT ln(ACe ) b
4. Conclusion Ultrasound pre-treatment on pomelo peel has increased its adsorption capacity to remove methylene blue from aqueous solution at increasing solution pH (2–10), adsorbent dose (0.05–0.25 g) and dye concentration (50–250 mg/L) in short time. The well-fitting of equilibrium adsorption isotherm data in Dubinin-Radushkevich model supported that adsorption of methylene blue on pomelo peel surface following the pore filling mechanism. The application of ultrasound as pre-treatment on adsorbent can thus be beneficial in dye removal 3507
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Fig. 6. Non-linear fitting of (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) Dubinin-Radushkevich isotherm models for methylene blue dye adsorption onto pomelo peel without and with ultrasound treatment at different amplitudes.
process. It appears as a more practical way to apply the ultrasound pretreated adsorbent as a ready product to remove the dye pigment from wastewater compared to the application of ultrasound simultaneously in adsorption process.
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Polym. 132 (2015) 245–251. [33] H. Li, Z. Sun, L. Zhang, Y. Tian, G. Cui, S. Yan, A cost-effective porous carbon derived from pomelo peel for the removal of methyl orange from aqueous solution, Colloid Surf. A 489 (2016) 191–199. [34] Ü. Geçgel, G. Özcan, G.Ç. Gürpınar, Removal of methylene blue from aqueous solution by activated carbon prepared from pea shells (Pisum sativum), J. Chem. (2013) (2012). [35] S.A. Chaudhry, T.A. Khan, I. Ali, Adsorptive removal of Pb (II) and Zn (II) from water onto manganese oxide-coated sand: isotherm, thermodynamic and kinetic studies, Egypt. J. Basic Appl. Sci. 3 (2016) 287–300. [36] P. Sampranpiboon, P. Charnkeitkong, X. Feng, Equilibrium isotherm models for adsorption of zinc (II) ion from aqueous solution on pulp waste, WSEAS Trans. Environ. Dev. 10 (2014) 35–47. [37] K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isotherm systems, Chem. Eng. J. 156 (2010) 2–10. [38] A.O. Dada, A.P. Olalekan, A.M. Olatunya, O. Dada, Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms studies of equilibrium sorption of Zn2+ unto phosphoric acid modified rice husk, J. Appl. Chem. 3 (2012) 38–45. [39] H.S. Shin, J.H. Kim, Isotherm, kinetic and thermodynamic characteristics of adsorption of paclitaxel onto Diaion HP-20, Process Biochem. 51 (2016) 917–924. [40] X. Chen, Modeling of experimental adsorption isotherm data, Information 6 (2015) 14–22. [41] K. Vijayaraghavan, T.V.N. Padmesh, K. Palanivelu, M. Velan, Biosorption of nickel (II) ions onto Sargassum wightii: application of two-parameter and three-parameter isotherm models, J. Hazard. Mater. 133 (2006) 304–308. [42] S.N. Milmile, J.V. Pande, S. Karmakar, A. Bansiwal, T. Chakrabarti, R.B. Biniwale, Equilibrium isotherm and kinetic modeling of the adsorption of nitrates by anion exchange Indion NSSR resin, Desalination 276 (2011) 38–44. [43] A.O. Dada, A.P. Olalekan, A.M. Olatunya, O. Dada, Langmuir, Freundlich Temkin and Dubinin-Radushkevich isotherm studies of equilibrium sorption of Zn2+ unto phosporic acid modified rice husk, IOSR J. Appl. Chem. 3 (2012) 38–45. [44] M.B. Ibrahim, S. Sani, Comparative isotherms studies on adsorptive removal of Congo red from wastewater by watermelon rinds and neem-tree leaves, Open J. Phys. Chem. 4 (2014) 139.
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Dye adsorption characteristic of ultrasound pre-treated pomelo peel Suk Khe Low, Mei Ching Tan
⁎
T
Department of Chemical and Petroleum Engineering, Faculty of Engineering, Technology and Built Environment, UCSI University, 56000 Cheras, Kuala Lumpur, Malaysia
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasound Pomelo peel Methylene blue dye Adsorbent Isotherm model
Pomelo peels were pre-treated with ultrasound at 30, 60 and 90% amplitude to enhance its ability to remove methylene blue from aqueous solution. The adsorption capacity of the peels at different solution pH (2–10), dye concentration (50–250 mg/L), adsorbent dose (0.05–0.25 g) and contact time (0.5–5 h) was investigated and the equilibrium isotherms were studied. Ultrasound pre-treated pomelo peel took shorter time to reach higher saturation limit of adsorption capacity than the non-treated pomelo peel. Pomelo peel treated at 30% ultrasound amplitude had highest adsorption capacity, which increased with solution pH, dye concentration and adsorbent dose, respectively reached maximum increment of 5%, 9.9% and 9.3% compared to the non-treated pomelo peel. The adsorption isotherm data fitted best by the Dubinin-Radushkevich model with the highest correlation coefficient in the range of 0.9424–0.9951, compared to Langmuir, Freundlich and Temkin models.
1. Introduction Dyes are heavily used by textile, leather, and paper industries among other industries to color their products. Concurrently, substantial amount of water is used too. At the end of the process, considerable amount of colored wastewater is discharged into the river streams, typically without any pre-treatment. The color presence in water as a visible pollutant affects public perception on the water quality. Concurrently, the organic compounds and toxic substances contained in the wastewater affect the aquatic ecosystem and human health. Hence, it is necessary to remove the dye from waste streams before being discharged into public water sources. While treatment processes such as cation exchange membranes, micellar enhanced ultrafiltration, electrochemical degradation, photocatalytic degradation, sonochemical degradation, and integrated chemical-biological degradation can be used for dye removal from wastewater [1], adsorption is still considered as the most economic and an easy process [2]. Numerous works have been published on using agro-waste materials as biosorbent of dyes in water, namely jackfruit peel [3], papaya seeds [4], grapefruit peel [5], garlic peel [6], orange peel [7,8], yellow passion fruit peel [9], lemon peel [10], pomegranate peel [11], lychee peel [12], banana peel [13], and pomelo peel [14,15]. Typically, the thick peel of pomelo, which accounted for approximately half of its total fruit weight are peeled off and discarded as waste [16]. Thus, the present work aimed to reduce the biological resource wastes by further exploring this agro-waste’s potential as a biosorbent. Methylene blue is a hydrophilic basic dye with cationic properties originating from
⁎
Corresponding author. E-mail address: [email protected] (M.C. Tan).
https://doi.org/10.1016/j.jece.2018.05.013 Received 4 January 2018; Received in revised form 12 April 2018; Accepted 6 May 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.
positively charged nitrogen and sulfur atoms [17]. Methylene blue, as the most commonly used dye in various industries, was selected in the present work as a model compound of dye to be removed from aqueous solutions, due to its known strong adsorption onto solids [18,19]. According to Zhang et al. [2], mesopores or micropores could trap the methylene blue molecules through the impregnation process, while macropores promote mass transfer process. In order to enhance the efficiency of dye removal, dye adsorption process can be performed in ultrasound environment simultaneously [20,21]. The results showed remarkable improvement on dye removal efficiency. It was suggested to be due to the high-pressure shock waves and high-speed microjects during ultrasound cavitation [20], which decreased the diffusion resistance and eased the insertion of dye into the adsorbent [22]. However, the application of the simultaneous ultrasound may not be practical for industrial scale due to the big volume of wastewater need to be treated. To possibly overcome such constraint and to improve the dye removal efficiency, the pomelo peel is first treated with ultrasound prior to the adsorption process in the present study. 2. Materials and methods 2.1. Adsorbate A strong adsorption cationic dye of methylene blue [14,23] was obtained from Friendemann Schmidt Chemicals. A stock solution with concentration of 250 mg/L was prepared by dissolving 0.25 g of
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solution when equilibrium was reached at the end of contact time by filtration. The concentrations of dye solution at each pH were then measured using UV–vis spectrophotometer.
methylene blue powder into 1000 mL distilled water. It was then further diluted into different concentrations of 50, 100, 150 and 200 mg/L. 2.2. Adsorbent
2.6. Analytical methods Pomelo peel wastes were thoroughly washed with distilled water to remove the dirt particles prior to cutting into small pieces of 2–3 cm. A 250 mL beaker containing 12 g of pomelo peels with 100 mL of distilled water was sonicated using ultrasound probe system (UP200S, Hielscher Ultrasonics GmbH, Germany) equipped with a 2 mm diameter micro tip (24 kHz, 600 W/cm2) located inside an anolyte compartment. The tip was submerged at the center of the mixture. As different samples, the peels were sonicated with ultrasound at 30, 60 and 90% amplitude for 30 min. Pomelo peels without ultrasound treatment were considered as a control sample. Ultrasound treated and non-treated pomelo peels were dried in a hot air oven (Carbolite Gero, AX series, United Kingdom) at 100 °C for 5 h to reach constant weight. The dried samples were grinded into 1–2 mm particle size by using grinder (MX-800S, Panasonic, Malaysia) and stored in an airtight container.
2.6.1. UV–vis spectrophotometer The filtrated dye solution was loaded into a cuvette upon the removal of adsorbent from the dye solution. The dye concentration was then measured using UV–vis spectrophotometer by monitoring its absorbance at wavelength 480 nm. The concentration value was then determined from the calibration curve generated by using the standard methylene blue solution with the known concentration. 2.6.2. Scanning electron microscope (SEM) The surface morphology of pomelo powder with and without ultrasound treatments were examined using scanning electron microscope (JSM 6400, Jeol, Japan) to generate the SEM images. All the samples were coated with 600 nm of gold layer.
2.3. Batch kinetic and equilibrium studies
2.6.3. Analysis of porosity An analysis of pores size distribution on surface of pomelo powder was conducted on the constructed images obtained from SEM analysis by using ImageJ software. The number of pores at size ranged from 1 to 20 μm diameter was analyzed at 640 × 382 pixels. A histogram of the pores size distribution was plotted by using Statistical Package for the Social Sciences (SPSS 16.0) software (Chichago, IL, USA).
The batch adsorption studies were conducted in a 250 mL stoppered glass Erlenmeyer flask by adding 0.20 g of adsorbent into 100 mL of dye solution at different initial concentrations (50, 100, 150, 200 and 250 mg/L) without pH adjustment at 30 °C. The flask was placed on a magnetic stirrer plate and stirred at 150 rpm for 5 h until equilibrium was reached. For batch kinetic studies, the concentration of dye solution was continuously measured at 30 min intervals. The adsorbent was first removed from the dye solution by using filter paper before the concentration of dye solution was analyzed using UV–vis spectrophotometer (Hitachi U-2900 Spectrophotometer, Japan) at wavelength 480 nm. The amount of adsorption at time t, qt (mg/g) was calculated by using Eq. (1).
qt =
(Co − Ct ) V W
2.7. Statistical analysis Curve fitting was carried out using solver function in Microsoft Excel which adopted the Generalised Reduced Gradient (GRG2) nonlinear optimization code to determine the isotherm constants in nonlinear form of Langmuir model, Freundlich model, Temkin model, and Dubinin-Radushkevich model [24,25]. The best fitted line was generated through the goal setting of minimum sum of square errors (SSE) and total corrected sum of squares (SST) in obtaining all the constants SSE value with goodness of fit, R2 = 1 − SST [25,26]. The error bars in the graphs are standard deviation of means of duplicate samples.
(1)
where Co and Ct (mg/L) are the initial dye concentration and concentration at time t respectively, V is the volume of solution (L) and W is the mass of dry adsorbent (g) used. For batch equilibrium studies, the amount of adsorption when the equilibrium is reached was measured. The amount of equilibrium adsorption, qe (mg/g) was calculated by using Eq. (2),
qe =
(Co − Ce ) V W
3. Results and discussion 3.1. Effect of initial concentration and contact time
(2)
Agriculture by-product normally consists of lignin and cellulose as major constituents having polar structure for the adsorption of dyes [27]. Adsorption capacity of adsorbent with and without ultrasound treatment was investigated at different initial concentration of methylene blue (50–250 mg/L) as shown in Fig. 1. The dye uptake by the adsorbent with and without ultrasound treatment was rapid for the first 30 min. This was due to high number of vacant anionic sites at the initial stage. These vacant sites decreased with time which leads to saturation or equilibrium state. Ahmad [28] reported that the rate of crystal violet dye adsorption was fast at the first 60 min due to high availability of adsorption sites at the initial stage. The vacancy of adsorption sites was then decreased with time and caused the adsorption rate to slow down in later stage until it reaches saturation. The nontreated adsorbent reached equilibrium at the first hour for 50 mg/L initial concentration, while it took 4 h to reach equilibrium for methylene blue with 100–250 mg/L initial concentration. Adsorbent took longer time to reach saturation as the dye concentration increased. Comparatively higher concentration of methylene blue contained higher amount of cations in the dye solution. Thus, the cations took longer time to compete among each other to bind on the anionic sites of adsorbent surface. Adsorbent treated with 30% and 60% ultrasound
where Ce is the concentration at equilibrium. 2.4. Effect of adsorbent dose Different amount of adsorbent (0.05, 0.1, 0.15, 0.20 and 0.25 g) was added to 100 mL of dye solution with initial concentration of 150 mg/L in a 250 mL stoppered glass Erlenmeyer flasks at temperature of 30 °C. The solution was stirred with magnetic stirred plate at 150 rpm for 4 h of contact time to reach the equilibrium. The adsorbent was then filtered out from the dye solution and the dye concentration was then measured using UV–vis spectrophotometer. 2.5. Effect of solution pH 0.2 g of adsorbent was added to each 100 mL dye solution with initial concentration of 150 mg/L at different initial pH (2, 4, 6, 8 and 10). The dye solution pH was adjusted using 0.1 mol/L HCl or 0.1 mol/L NaOH before the adsorption experiment was carried out. The dye solution containing adsorbent was stirred with magnetic stirrer plate at 150 rpm and 30 °C for 4 h. The adsorbent was separated from the dye 3503
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Fig. 1. Effect of initial concentration and contact time on methylene blue for adsorbent (a) without ultrasound treatment, and adsorbent with ultrasound treated at (b) 30%, (c) 60% and (d) 90% amplitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
amplitude did not show any obvious changes compared with the control. The effect of ultrasound decreased as the amount of adsorbent increased. The dye removal of ultrasound treated adsorbent at 30% amplitude showed the highest increment of 39% at 0.05 g of adsorbent, while the lowest increment of 9% at 0.25 g adsorbent, respectively when compared with the control. The increase of adsorbent ability was due to the increase of adsorption sites on adsorbent surfaces for methylene blue binding [3].
amplitude took 1.5 to 2 h to reach the equilibrium for methylene blue with 100–200 mg/L initial concentration, and 3.5 h for 250 mg/L initial concentration. Fig. 1a indicated that equilibrium adsorption of non-treated adsorbent increased from 15.50 to 94.88 mg/L when the initial concentration increased from 50 to 250 mg/L. The results showed that ultrasound treated adsorbent has higher adsorption capacity compared with non-treated adsorbent. Dye uptake by adsorbent at lower initial concentration of methylene blue (50–150 mg/L) increased as the ultrasound amplitude increased from 30% to 90%. At higher initial concentration (200–250 mg/L), the adsorption capacity of adsorbent decreased when the ultrasound amplitude applied increased. Adsorbent treated at 30% ultrasound amplitude shows the highest adsorption capacity with 4% and 10% higher than non-treated adsorbent, while 6% and 10% higher than absorbent treated at 90% ultrasound amplitude, respectively at 200 mg/L and 250 mg/L initial concentration of methylene blue. Ultrasound treated adsorbent reached equilibrium state in shorter time than non-treated adsorbent, yet with higher adsorption capacity. It is suggested that ultrasound cavitation effect has created more pores on the adsorbent surface, which contribute to generate more anionic adsorption sites for the binding purpose with methylene blue cations. Therefore, it speeds up the adsorption process of methylene blue on adsorbent with high amount of adsorption.
3.3. Effect of solution pH on dye adsorption Fig. 3 indicated that the adsorption of methylene blue was the lowest at pH 2. At pH 4, the adsorption of methylene blue by all samples increased drastically about 6 times higher. Dye removal of non-treated adsorbent was nearly constant from pH 4 to 10. Fig. 3 showed a decrease in adsorption capacity as the amplitude application increased from 30% to 90%. However, as the pH increased from pH 2 to pH 10, the adsorbents treated with higher amplitude of 60% and 90% show remarkable increases in adsorption capacity. All the ultrasound treated adsorbents have higher adsorption capacity than the control at pH 10, where the highest was achieved by 30% of ultrasound amplitude with 87.75% of dye removal. Dye removal increased with pH due to the increase of negative charges on adsorbent surface [29], which created high electrostatic forces to attract the cationic dye for binding [6]. However, the adsorption processes seemed saturated with the increase in solution pH as observed in non-treated adsorbent from pH 4 to pH 10. This could be due to the limitation of adsorption sites that available on the adsorbent surface, where the cationic dyes competes with each other. Ultrasound enhances the adsorption capacity by generating more porous structures on the adsorbent surface which contributes more anionic adsorption sites for cationic dye. Hence, with the high electrostatic force generated
3.2. Effect of adsorbent dose on dye adsorption Fig. 2 shows the effect of different doses of ultrasound treated and non-treated adsorbent on dye adsorption. The dye uptake by adsorbent increased as adsorbent doses increased. The adsorption start to remain nearly constant when the amount of adsorbent increased up to 0.2 g. Dye removed by ultrasound treated adsorbents at 30% and 60% amplitude were higher than non-treated adsorbent at any doses, while 90% 3504
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Fig. 2. Batch equilibrium adsorption capacity on methylene blue by different dosage of non-treated adsorbent (■), and ultrasound treated adsorbent at 30% ( ), 60% ( ) and 90% ( ) amplitude, and respective percentage of dye removal ( ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.5. Porosity of pomelo peel powder
at high solution pH [14], ultrasound application at high amplitude can enhance the dye adsorption.
Fig. 5(a)–(d) respectively show the overall total number of pores at different pore diameter for non-treated pomelo peel, and pomelo peel treated at 30%, 60% and 90% ultrasound amplitude. The result indicates that ultrasound pre-treatment has increased the number of pores on the non-treated pomelo peel, from 228 pores with 2.03 μm of pore diameter to the highest number of 793 pores with 1.23 μm of pore diameter when it was pre-treated with ultrasound at 30% amplitude. However, the increases of ultrasound amplitude from 30% to 60% and 90% cause the pores number to reduce about 18% and 59% respectively with increasing pore size. The non-treated pomelo peel was categorized with macroporous structure with its pore diameter in the range of 2–20 μm, while ultrasound treated pomelo peel was in mesoporous structure with pore diameter less than 2 μm [31,32]. In previous study of methyl orange removal by porous carbon, Hazhen et al. [33] found that the porous carbon derived from pomelo peel consists of numerous micropores, and small amount of mesopores and macropores.
3.4. Surface morphology (SEM) Fig. 4 showed the surface morphology of the samples characterized using SEM. The surface of original untreated pomelo peel powder is relatively smooth without any pore as shown in Fig. 4(a) compared with ultrasound treated pomelo peel powder. Fig. 4(b)–(d) showed pomelo peel powder treated with 30%, 60% and 90% ultrasound amplitude respectively. Average size of the porous structure is obviously formed on powder surface when treated with 30% ultrasound amplitude. The formation of porous structure is due to the hydrodynamic cavitation effect of ultrasound. It enhances the adsorption efficiency of the adsorbent through the active site created, which provides larger surface area for the dye component to attach with. The cavitation effect at 60% ultrasound amplitude starts to cause surface rupture with less pore exposed compared to 30% amplitude as illustrated in Fig. 4(c). This suggested that higher ultrasound amplitude relative to higher power caused over hydromechanical shear forces on the powder surface and lead to surface rupture which proceed to form bigger pores as indicated in Fig. 4(d) when ultrasound amplitude increased to 90%. Bigger pore size per unit volume has less specific area as compared to high porosity in smaller size per unit volume. Du Plessis [30] observed an inverse relationship between pore size and specific surface area in the study of nanoporous silicon.
3.6. Isotherm analysis The isotherm data of dye concentration during equilibrium was used to describe the interaction of adsorbate and adsorbent by using isotherm models of Langmuir, Freundlich, Temkin and DubininRadushkevich in non-linear form. Langmuir model was applicable on many monolayer adsorption processes where it assumes that adsorption
Fig. 3. Batch equilibrium adsorption capacity on methylene blue at different pH by non-treated adsorbent (■), and ultrasound treated adsorbent at 30% ( ), 60% ( ) and 90% ( ) amplitude, and respective percentage of dye removal ( ). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. SEM micrograph of pomelo peel (a) before, and after ultrasound treated at (b) 30%, (c) 60% and (d) 90% amplitude (magnification: ×1000).
Fig. 5. Pore diameter distribution of pomelo peel (a) without ultrasound pre-treatment, and with ultrasound pre-treatment at (b) 30, (c) 60 and (d) 90% amplitudes.
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the interaction of adsorbent-adsorbate, by assuming that the heat of adsorption of all molecules in the layer decreases linearly rather than logarithmic with the coverage of adsorbent surface [35,38]. The nonlinear form of Temkin model is shown in Eq. (6),
occur energetically equivalent at specific homogenous sites on adsorbent surface [34] with finite capacity [35]. The non-linear Langmuir isotherm is represented by Eq. (3),
qe =
qm K a Ce 1 + K a Ce
(3)
qe =
where Ce is the equilibrium concentration (mg/L), qe is the amount of adsorbate adsorbed per unit mass of adsorbate (mg/g), qm and Ka are the Langmuir constants denoted the maximum sorption capacity (mg/ g) and energy of the adsorption (L/mg) respectively. The magnitude of qm showed that monolayer saturation capacity increased with decreasing ultrasound amplitude and reached maximum value of 7041.52 mg/g at 30% amplitude, while the value of Ka increased with an increase in amplitude, indicates a high adsorption affinity [36]. The essential characteristics of Langmuir isotherm can be expressed in terms of separation factor, RL that defined by Eq. (4),
RL =
1 1 + K a Co
where Ka is the Langmuir constant and Co is the highest initial concentration of adsorbate (mg/L). The value of RL indicates the shape of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0) [3,35]. The separation factor of methylene blue was found in the range of 0.2817–0.9302, indicating favorability of the adsorption process between the test amplitudes. Freundlich isotherm is applicable on both homogeneous and heterogeneous surface of adsorbent [35], which describes the heterogeneity of the surface, assuming that the adsorption occurs at sites with different energy levels of adsorption. The non-linear form of Freundlich model is given as Eq. (5),
qe = qs exp(−BD εD2 ) ⎜
E=
Langmuir
Freundlich
Temkin
DubininRadushkevich
Ultrasound Amplitude (%) 30
60
90
qm (mg/g) Ka (L/mg) RL R2 KF (L/g) 1/n R2 A (L/g) b R2 qs (mg/g)
1883.88 0.0009 0.8163 0.9166 1.0360 1.1153 0.9248 6.0508 2798.74 0.9198 113.14
7041.52 0.0003 0.9302 0.8004 0.5191 1.3844 0.9249 7.6801 2471.44 0.8043 157.79
2708.19 0.0006 0.8696 0.9282 0.7644 1.2018 0.9734 6.1894 2764.04 0.9352 112.23
246.10 0.0102 0.2817 0.9681 1.4800 1.0157 0.8731 6.2469 2750.16 0.9027 93.27
BD (mol2/J2) E (J/mol) R2
0.000111 67.25 0.9951
0.000139 59.94 0.9620
0.000113 66.54 0.9424
0.000046 104.53 0.9478
(8)
1 2BD
(9)
Mean adsorption energy, E per molecule of adsorbate is usually applied to differentiate between physical and chemical adsorption [43]. The adsorption process is considered as chemical ion exchange if the mean adsorption energy is between 8 and 16 kJ/mol, while it is physical adsorption if the mean adsorption energy is below 8 kJ/mol [39,42,44]. Table 1 shows the adsorption energy, E in range of 59.94–104.53 J/mol, indicated the methylene blue adsorption onto pomelo peel is a physical adsorption process. As shown in Table 1 and illustrated in Fig. 6, highest correlation coefficient, R2 was derived by fitting the experimental data into Dubinin-Radushkevich isotherm model (0.9424 ≤ R2 ≤ 0.9951) compared to Langmuir isotherm model (0.8004 ≤ R2 ≤ 0.9681), Freundlich isotherm model (0.8731 ≤ R2 ≤ 0.9734) and Temkin isotherm model (0.8043 ≤ R2 ≤ 0.9352). This concludes that Duninin-Radushkevich isotherm model was most suitable for the experimental data to describe the adsorption phenomena, suggested that the characteristic sorption curve of this study is related to the porous structure of the sorbent [41], due to ultrasound cavitation effect.
Table 1 Isotherm parameters for the removal of methylene blue from aqueous solution by pomelo peel without and with ultrasound treatment at different amplitudes.
0
⎟
where R is gas constant (8.314 J/mol/K), T is the absolute temperature (K), while qs is the Dubinin-Radushkevich constant related to adsorption capacity (mg/g), BD is the activity coefficient related to mean sorption energy (mol2/J2), and εD is the Polanyi potential [41,42]. The mean adsorption energy, E is calculated from Eq. (9),
(5)
Parameters
(7)
1⎞ εD = RT ln ⎛1 + C e⎠ ⎝
where KF and n are the Freundlich constants respectively represent the adsorption capacity of the adsorbent (mg/g (L/mg)1/n) and adsorption intensity of the adsorbent. The value 1/n ranges between 0 and 1 measure the adsorption intensity or surface heterogeneity. The adsorption process become more heterogeneous as 1/n gets closer to zero, whereas 1/n below unity indicate chemisorption process and 1/n above one represent the process of cooperative adsorption [38]. The adsorption of methylene blue onto ultrasound treated and untreated pomelo peel was a cooperative process with Freundlich constant 1/n above one as shown in Table 1. Temkin isotherm is applicable for adsorption on heterogeneous solid adsorbent surface and liquid adsorbate, which explicitly consider
Isotherm
(6)
where R is gas constant (8.314 J/mol/K), T is the absolute temperature (K), while A and b are Temkin constants that related to the maximum binding energy (L/g) and heat of adsorption (J/mol) respectively [35]. The maximum binding energy, A of ultrasound treated pomelo peel was higher than the non-treated pomelo peel. The value of A was highest at lowest ultrasound amplitude, with maximum value of 7.6801 L/mg at 30% amplitude. This shows that the physical binding capacity between methylene blue and pomelo peel increased in the adsorption process at lower application of ultrasound amplitude compared to higher ultrasound amplitude and the non-treated pomelo peel [39]. The adsorption energy in Temkin model, b is positive for methylene blue adsorption from the aqueous solution showing that the adsorption is exothermic [36]. Dubinin-Radushkevich isotherm describes the adsorption process following a pore filling mechanism, which generally applied on both homogeneous and heterogeneous surfaces [37,40]. The non-linear form of Dubinin-Radushkevich model is illustrated as Eqs. (7) and (8),
(4)
qe = KF Ce1/ n
RT ln(ACe ) b
4. Conclusion Ultrasound pre-treatment on pomelo peel has increased its adsorption capacity to remove methylene blue from aqueous solution at increasing solution pH (2–10), adsorbent dose (0.05–0.25 g) and dye concentration (50–250 mg/L) in short time. The well-fitting of equilibrium adsorption isotherm data in Dubinin-Radushkevich model supported that adsorption of methylene blue on pomelo peel surface following the pore filling mechanism. The application of ultrasound as pre-treatment on adsorbent can thus be beneficial in dye removal 3507
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Fig. 6. Non-linear fitting of (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) Dubinin-Radushkevich isotherm models for methylene blue dye adsorption onto pomelo peel without and with ultrasound treatment at different amplitudes.
process. It appears as a more practical way to apply the ultrasound pretreated adsorbent as a ready product to remove the dye pigment from wastewater compared to the application of ultrasound simultaneously in adsorption process.
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