Factors affecting the carbon yield and adsorption capability of the mangosteen peel activated carbon prepared by microwave assisted K2CO3 activation

Chemical Engineering Journal 180 (2012) 66–74

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Factors affecting the carbon yield and adsorption capability of the mangosteen peel activated carbon prepared by microwave assisted K2 CO3 activation K.Y. Foo, B.H. Hameed ∗ School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

a r t i c l e

i n f o

Article history: Received 11 July 2011 Received in revised form 28 October 2011 Accepted 2 November 2011 Keywords: Activated carbon Adsorption Mangosteen peel Methylene blue Microwave

a b s t r a c t This study presents the preparation of activated carbon from mangosteen peel via microwave assisted K2 CO3 activation. The operational parameters including chemical impregnation ratio, microwave power and irradiation time on the carbon yield and adsorption capability of the mangosteen peel derived activated carbon (MPAC) were investigated. The virgin characteristics of the prepared MPAC were examined by pore structural analysis, Scanning Electron Microscopy, Fourier transform infrared spectroscopy, nitrogen adsorption isotherm, elemental analysis, surface acidity/basicity and zeta potential measurement. The adsorptive property of MPAC was tested using methylene blue as dye model compound. The best conditions resulted in MPAC with a maximum monolayer adsorption capacity of 379.63 mg/g and carbon yield of 80.95%, respectively. Equilibrium data were favorably described by Langmuir isotherm, while adsorption kinetics was best fitted to the pseudo-second-order model. The findings support the feasibility of mangosteen peel derived activated carbon as a promising and economic adsorbent. © 2011 Elsevier B.V. All rights reserved.

1. Introduction By nature, mangosteen (Garcinia mangostana), colloquially known as “the queen of fruits”, is a tropical seasonal plant species belonging to the members of family Clusiaceae and genus of Garcinia. Mangosteen trees thrive well in a warm and humid climate, ideally in the temperature range from 25 to 30 ◦ C, with a height from 7 to 25 m. The fruit, capped by the prominent calyx at the stem end and with 4–8 triangular, flat remnants of stigma in a rosette at the apex, is round, dark to red-purple and smooth externally. The inner arils are tangy, juicy, sweet and distinctly acid in flavor [1]. Mangosteen fruit is primarily eaten fresh and available as food complements in desserts, salads, fruit cocktail, jam, juice combinations or can food processing industries [2]. Nevertheless, its wide scale implementations by the food manufacturing industries are deteriorated by the massive generation of peel and stem waste. For each kg of mangosteen harvested, approximately 0.6 kg of mangosteen peels (MPs) can be obtained [3]. According to the statistical data reported by the Ministry of Agricultural and Agro-Based Industry Malaysia, the local production of mangosteen in 2010 was projected at 29,520 MT/yr, translating to approximately 17,712 MT of mangosteen peels as the by-products [4]. In the formal practice, some quantity of these residues is used as boiler fuel, where major portion is discarded by open burning.

∗ Corresponding author. Tel.: +60 45996422; fax: +60 45941013. E-mail address: [email protected] (B.H. Hameed). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.11.002

Although MPs have been reputed to have anti-inflammatory properties and traditional used as a remedy for chronic diarrhea, thrush, urinary tract disorders, and applied externally as astringent lotions on eczema, skin infections and wounds [5–7], there are relatively limited studies in this field. Therefore, it is necessary to find a rapid route towards upgrading of the available biomass from the food processing plants. To the best of our knowledge, no study has been reported on the preparation of activated carbon from MP via microwave-induced activation. In this sense, the present work is aimed at evaluating the viability of microwave irradiation for preparation of activated carbon from MP via K2 CO3 activation. Methylene blue, the most commonly used substance in the dying process was chosen as the model adsorbate in this study, due to its potential risk towards the environmental pollution and ecosystems. The significant influences of chemical impregnation ratio, microwave power and radiation time on the carbon yield and adsorption capacity were investigated systematically. Textural, functional and surface chemistry of the prepared adsorbent was performed. Moreover, the adsorption equilibrium, isotherms, kinetics and thermodynamics were elucidated. 2. Materials and methods 2.1. Adsorbate Methylene blue (CI: 52015; chemical formula: C16 H18 ClN3 S; molecular weight: 319.86 g/mol, maximum wavelength: 668 nm), a cationic pollutant difficult to be degraded in natural environment was selected as the adsorbate in this study (Fig. 1). A standard stock

K.Y. Foo, B.H. Hameed / Chemical Engineering Journal 180 (2012) 66–74

67

2.4. Characterization of MPAC

Fig. 1. Molecular structure of methylene blue dye.

solution of 1000 mg/L was prepared and suitably diluted to the required initial concentrations. Deionized water supplied by USF ELGA water treatment system was used to prepare all the reagents and solutions. 2.2. Preparation of activated carbon Mangosteen peel (MP), a by-product collected from the local food processing factory, was the precursor used in the present study. The raw precursor was washed exhaustively with deionized water to remove adhering dirt from the surface. Dried MP was cut, crushed, and screened to a particle size of 1–2 mm. The carbonization process was carried out by loading 500 g of dried precursor into a modified muffle furnace, under N2 gas flow (150 cm3 /min) and heated up to a carbonization temperature of 700 ◦ C, at the heating rate of 10 ◦ C/min. The char produced was mixed with K2 CO3 pellets with different impregnation ratio (IR), defined as: IR =

wK2 CO3 wchar

(1)

where wK2 CO3 and wchar are the dry weight of K2 CO3 pellets (g) and char (g). Microwave heating was conducted in a 2.45 GHz commercial microwave oven with suitable modifications. The oven has a power controller to select different power levels and a timer for various exposure times at a set microwave power level. The reaction was performed in a reactor fixed in the chamber of microwave oven. Nitrogen gas at a pre-set flow rate (300 cm3 /min) was used to purge air in the reactor before the start of microwave heating and it continued to flow during the activation stage. The resultant activated carbon was washed with 0.1 M of hydrochloric acid and rinsed repeatedly with hot and cold distilled water until the filtrate reached to neutral pH. The yield is defined as the weight of activated carbon per weight of char utilized for activation. 2.3. Adsorption equilibrium studies The batch adsorption experiments were undertaken in a set of 250 mL Erlenmeyer flasks containing 0.20 g of adsorbent and 200 mL of dye solutions within the initial concentration range of 50–500 mg/L. The mixture was agitated at 120 rpm and 30 ◦ C until the equilibrium was reached. The concentration of methylene blue (MB) dye solution was determined using a double beam UV–Vis spectrophotometer (UV-1601 Shimadzu, Japan) at 668 nm. MB uptake at equilibrium, qe (mg/g), was calculated by: (Co − Ce )V qe = W

(2)

where C0 and Ce (mg/L) are the liquid-phase concentrations of dye at initial and equilibrium, respectively. V (L) is the volume of the solution, and W (g) is the mass of adsorbent used. The effect of pH on dye removal was examined by varying the pH from 2 to 12, with initial dye concentration of 500 mg/L, MPAC dosage of 0.20 g/200 mL and adsorption temperature of 30 ◦ C. The initial pH of the dye solution was adjusted by addition of 0.10 M solution of HCl or NaOH.

The pore structural characteristics of MPAC prepared under optimum preparation conditions were determined by nitrogen adsorption at 77 K using an automatic Micromeritics ASAP-2020 volumetric adsorption analyzer. Prior to analysis, the sample was degassed for 2 h under vacuum at 573 K. The sample was transferred to the analysis system where it was cooled in liquid nitrogen. A 21-point analysis was carried out at 77 K to obtain the nitrogen adsorption isotherm. The SBET was calculated by the BET equation, micropore volume, micropore surface area and external surface area were obtained using the t-plot method. Chemical characterization of functional groups was detected by Fourier transform infrared spectrometer (FTIR-100, Shimadzu) in the scanning range of 4000–400 cm−1 . The surface morphology was examined using a scanning electron microscope (Zeiss Supra 35 VP) and elemental analysis was performed using an Elemental Analyzer (Perkin-Elmer-2400). 2.5. Surface acidity/basicity and zeta potential measurement (pHpzc ) The surface acidity was estimated by mixing 0.20 g of MPAC with 25 cm3 of 0.05 M NaOH solution in a closed flask, and agitated for 48 h at room temperature. The suspension was then decanted, and the remaining NaOH was determined by titration with 0.05 M HCl solution. The surface basicity was obtained by a similar procedure, where 0.05 M HCl solution was contacted with 0.20 g of MPAC and the titration solution was 0.05 M NaOH. The determination of pHpzc was conducted by adjusting pH of 50 cm3 0.01 M NaCl solution to a value between 2 and 12. 0.15 g of MPAC was added and the final pH was measured after 48 h under agitation. The pHpzc is the point where pHinitial − pHfinal = 0. 3. Results and discussion 3.1. Preparation of MPAC 3.1.1. Effect of chemical impregnation ratio Effect of chemical impregnation ratio (IR) on the carbon yield and adsorption equilibrium of MB was investigated at the microwave input power of 360 W and irradiation time of 4 min (Fig. 2a). It can be observed that augmenting IR from 0.25 to 1.25 showed an enhancement of carbon yield from 76.03 to 88.10%. Beyond the value, subsequent increase in IR illustrated a gradual decrease in carbon yield. Similarly, increasing IR from 0.25 to 1.25 indicated an increase of adsorption uptake from 150.30 to 217.13 mg/g, and then it steadily decreased. It was presumed that K2 CO3 activation involved the reduction of K2 CO3 under inert condition to form K, K2 O, CO and CO2 . The potassium compound formed during the activation step would diffuse into the internal structure of char matrix, widens the existing pores and creates new porosities, in according with the reactions [8,9]: K2 CO3 + 2C → 2K + 3CO

(3)

K2 CO3 → K2 O + CO2

(4)

K2 O + 2C → 2K + CO

(5)

Therefore, by increasing the ratio of K2 CO3 /char, the activation process would be strengthened. Correspondingly, the adsorption uptake was further enhanced. Beyond the optimum value, the excess of K2 CO3 and metallic potassium left in the carbon surface caused blocking of the pores leading to a dramatic decrease of accessible area. Additionally, the pores would be widened lowering the adsorption uptake and carbon yield. Therefore, the IR was

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Fig. 2. Effects of (a) chemical impregnation ratio (preparation conditions: microwave power = 360 W; radiation time = 4 min), (b) microwave power (preparation conditions: chemical impregnation ratio = 1.25; radiation time = 4 min) and (c) radiation time (preparation conditions: chemical impregnation ratio = 1.25; microwave power = 600 W) on the carbon yield and adsorption capacity.

proposed at 1.25 for effective activation at minimum consumption of activating agent.

3.1.2. Effect of microwave power Effect of microwave power on the adsorption uptake and carbon yield was evaluated at the IR of 1.25 and irradiation time of 4 min (Fig. 2b). Under low microwave power of 90 and 180 W, the adsorption uptake and carbon yield remained almost unchanged, indicating no continual reaction between the char and activating agent. Enhancing microwave power from 180 to 600 W showed a drastically increase in adsorption uptake, possibly ascribed to the combined effect of internal and volumetric heating responsible for the expansion of carbon structure and creation of high porosity and larger surface area. However, at high radiation power of 800 W, over gasification might occur with detrimental impact to cause carbon burn and destruction of the pore structures, thus the adsorption uptake and yield were progressively decreased. Meanwhile, the weight loss of carbon was found to be increased proportional to the rising in microwave power level, mainly attributed to the degradation reaction at higher thermal radiation which intensified rapid volatilization, dehydration and decomposition.

3.1.3. Effect of radiation time Microwave radiation time is another key factor affecting the adsorption uptake and carbon yield. Effect of microwave radiation time was conducted at the IR of 1.25 and microwave input power of 600 W. From Fig. 2c, it is clearly revealed that prolonging radiation time exhibited an enhancing of adsorption uptake from 151.81 mg/g to 298.32 mg/g. Obviously, prolonging time exposure promotes an acceleration of energy, which in turn increases the

reaction rates, thus developed the porosity and rudimentary of the pore structure. A slight drop was observed at 6 min. The phenomenon implied that temperature has risen dramatically as activation proceeded, entailed the opening of the micropores and mesopores enlarging the average diameter. Besides, further heat treatment might produce local hotspots, leading to the ablation and shrinkage of the carbon internal channels reducing the accessibility of the pore structure. Meanwhile, higher pyrolytic temperature promotes C–K2 CO3 , C–K2 O and C–CO2 reactions facilitating elimination and breaking of the C–O–C and C–C bonds thus decreased the carbon yield [10]. 3.2. Characterization of MPAC The porous structure examination of the samples can be clearly seen from the SEM photographs, depicted in Fig. 3. It can be clearly seen that the surface texture of char is dense, compact, uneven, undulating and covered by deposited tarry substances (Fig. 3a). However, MPAC displays a well-pronounced porosity, with homogeneous pores distributed around the surface (Fig. 3b). Comparison of the surface morphology verifies substantial changes occurred due to microwave irradiation. Nitrogen adsorption–desorption curve provides qualitative information on the adsorption mechanism and porous structure of the carbonaceous materials (Fig. 4). The isotherm resembles a combination of type I and type II isotherms, in accordance with the IUPAC classification [11]. This adsorption behavior exhibits a combination of microporous–mesoporous structure. The surface physical parameters obtained from the N2 adsorption isotherms were summarized in Table 1. From the data, it is evident that the BET surface area, Langmuir surface area and total pore volume of

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69

Table 2 Elemental analysis of the MP derived char and MPAC. Char (wt%)

MPAC (wt%)

Carbon Oxygen Hydrogen Nitrogen Sulphur

62.66 28.83 2.56 5.79 0.16

78.45 16.62 1.41 3.37 0.15

MPAC were greatly improved, indicating pore development during the activation stage. Meanwhile, the mesopores of MPAC accounts about 45% of the total pore volume, with a well-developed porous structure. The chemical composition of char and MPAC was listed in Table 2. The content of oxygen, hydrogen and nitrogen decreased significantly after microwave treatment but the carbon level indicated an opposite trend. This might be due to the removal of volatiles compounds and elimination of oxygen containing groups under microwave irradiation leaving a high purity carbon. Whereas, the sulphur containing groups in the carbonized char were thermally stable. The FTIR spectrum (Fig. 5) of char indicates the peaks located at 3436–3247, 2361, 2343, 2010, 1648, 1420, 1276, 1053, 934 and 806 cm−1 , identical to the presence of N–H, C C (alkynes), –COOH, C N, in-plane O–H (hydroxyl), –CH2 (alkyl), C–O–C (ester, ether and phenol), C–O (anhydrides), out-of-plane O–H and out-of-plane C–H derivatives. Meanwhile, the surface chemistry of MPAC illustrated some shifts of adsorption bands, at 3436–3247, 2361, 2343, 1993, 1642, 1420, 1276 and 1053 cm−1 , corresponds to the N–H, C C (alkynes), –COOH, C N, in-plane O–H (hydroxyl), –CH2 (alkyl), C–O–C (ester, ether and phenol) and C–O (anhydrides) functionalities. The surface acidity and basicity is an important criterion interpreting the surface chemistry of the carbon adsorbents [12]. MSAC showed an acidic property, with a surface acidity of 2.63 mmol/g and 1.08 mmol/g as surface basicity. From the result, it is well established there were greater amounts of oxygen-containing groups (mainly carboxylic, anhydrides, lactones and phenols) than oxygenfree Lewis sites, carbonyls, pyrone and chromene type structures at the edge of the carbon layers. The surface chemistry of MPAC was further justified by determination of zero point of charge (pHZPC ), the point at which the net charge of adsorbent is zero. In the present work, the pHZPC was found to be 6.25.

Fig. 3. SEM micrograph (500×) of the MP derived char (a) and MPAC (b).

400

Quantity Adsorbed (cm³/g STP)

Element

375

350

325

3.3. Effects of initial concentration, contact time and solution pH on the adsorption equilibrium

300

Generally, adsorption uptake capacity and dye removal efficiency increased with prolonging the contact time. The curve adsorption uptake capacity, qt as a function of time, t in the initial concentration range of 50–500 mg/L was displayed in Fig. 6. Initially, the amount of dye adsorbed onto the carbon surface increased rapidly, and with a lapse of time, the process slowed down and reached a plateau. The initial concentration provides an essential driving force to overcome the mass transfer resistance between the aqueous phase and the solid medium [13]. In the present study, the adsorption equilibrium, qe increased from 50.49 to 384.18 mg/g with an increase in initial concentration from 50 to 500 mg/L. Conversely, there was a reverse relationship between the equilibrium concentrations with the initial MB concentrations. The equilibrium concentration, Ce obtained at 50, 100, 150, 200, 300, 400 and 500 mg/L was 0.12, 0.31, 1.22, 3.85, 34.11 and 120.34 mg/L, respectively, indicating high percent removal of MB even at high initial concentrations.

0

0.2

0.4

0.6

0.8

1

Relative Pressure (P/Po) Fig. 4. Nitrogen adsorption–desorption curve of MPAC. Table 1 Porosity structure of the MP derived char and MPAC. Properties

Char

MPAC

BET surface area (m2 /g) Micropore surface area (m2 /g) External surface area (m2 /g) Langmuir surface area (m2 /g) Total pore volume (cm3 /g) Micropore volume (cm3 /g) Mesopore volume (cm3 /g) Average pore size (Å)

9.73 5.12 4.61 16.62 0.012 0.002 0.010 28.56

1098.75 626.16 472.59 1662.52 0.611 0.334 0.277 22.25

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22.0

Char 21.0 2010

20.0

2361 1648 2343

1276 806

%T

19.0 934

1420

18.0

MPAC

3436 3247

17.0 1053

1993 2361

16.0

1642

2343

1276 1420

3436 3247

15.0

14.0 4000

1053

3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800

600

400

Wave number cm-1 Fig. 5. FTIR spectra of the MP derived char and MPAC.

Solution pH affects adsorption by regulating the adsorbents surface charge as well as degree of ionization of adsorbates present in the solution [14]. The adsorption behavior of MB over a broad pH range of 2–12 was shown in Fig. 7. It was found that increasing solution pH serves to increase the adsorption capacity, with a significant enhancement as the pH increased from 6 to 8. This can be attributed to the protonation of MB in the acidic medium, and presence of excess H+ ions competing with dye cations for the adsorption sites. At higher solution pH, MPAC may get negatively charged and the formation of electric double layer changes its polarity, consequently the dye uptake increases [15]. Effect of pH can be described on the basis of zero point of charge (pHZPC ). The experimental determination of pHZPC of MPAC was identified to be 6.25. Activated carbon adsorbent will react as a positive surface when solution pH < pHZPC , and as a negative surface when solution pH > pHZPC . Therefore, for pH values above 6.25, the negative charge density of MPAC increased which favors the adsorption of cationic dye.

appropriate correlation for the equilibrium curves. The equation parameters of these equilibrium models reveal the sorption mechanism, surface properties and affinity of the carbonaceous adsorbent. Due to the inherent bias resulting from linearization, alternative isotherm parameter sets were determined by non-linear regression [16]. For non-linear regression, a trial and error procedure, which is applicable to computer operation, was developed to determine the isotherm parameters by maximizing the respective coefficient of determination between experimental data and the isotherms. This provides a mathematically rigorous method for determining isotherm parameters using the original form of isotherm equation. Thus, in this study the nonlinear Langmuir [17], Freundlich [18], Temkin [19] and Dubinin–Radushkevich [20] isotherm models were established to analyze the equilibrium data: qe =

Q0 KL Ce 1 + KL Ce

(6)

1/n

3.4. Adsorption isotherm

qe = KF Ce

(7)

qe = B ln(ACe )

(8)

In the endeavor to explore novel adsorbents in accessing an ideal adsorption system, it is important to establish the most

400 390

400

380

qt (mg/g)

400 mg/L 300 mg/L

200

200 mg/L

qe (mg/g)

500 mg/L

300

370 360 350

100 mg/L

340

50 mg/L

100

330 320

0 0

5

10

15

20

25

30

0

2

4

6

8 pH

10

12

14

Time (h) Fig. 6. Adsorption equilibrium of the adsorption of MB onto MPAC at 30 ◦ C.

Fig. 7. Effect of pH on the adsorption of MB onto MPAC (initial concentration = 500 mg/L; temperature = at 30 ◦ C).

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71

Table 3 Isotherm parameters for the adsorption of MB onto MPAC at 30, 40 and 50 ◦ C. Temperature (◦ C)

Isotherms

Constants Qo (mg/g)

KL (L/mg)

R2

1.04 1.02 1.01

0.996 0.999 0.999

n

KF (mg/g) (L/mg)1/n

R2

175.28 167.92 153.98

0.849 0.816 0.808

A (L/g)

B

R2

50.35 46.55 40.30

0.948 0.917 0.906

qs (mg/g)

E (J/mol)

R2

341.01 331.66 303.70

2124.30 1990.47 1965.70

0.902 0.937 0.942

Langmuir

30 40 50

379.63 357.14 328.18

Isotherms

Temperature (◦ C)

Constants

Freundlich

30 40 50

5.41 5.73 6.10

Isotherms

Temperature (◦ C)

Constants

Temkin

30 40 50

35.21 36.86 42.70

Isotherms

Temperature (◦ C)

Constants

Dubinin–Radushkevich

qe = qs exp(−kad ε2 )

30 40 50

(9)

where Q0 (mg/g) and KL (L/g) are Langmuir constants related to adsorption capacity and free energy of adsorption, respectively, and KF (mg/g) (L/mg)1/n and 1/n are the Freundlich adsorption constant, and a measure of the adsorption intensity. B = RT/b, where b, A, R and T are the Temkin constant related to heat of sorption (J/mol), equilibrium binding constant (L/g), gas constant (8.314 J/mol K) and absolute temperature (K). Meanwhile, qs (mg/g) is denoted as the theoretical isotherm saturation capacity, and Polanyi potential, ε can be correlated as:



ε = RT ln 1 +

1 Ce



(10)

The constant BDR gives the mean free energy, E (J/mol) of sorption per molecule of the adsorbate when it is transferred to the surface of solid from infinity in the solution, and can be computed by the relationship:



E=





1

from the char surface widening the porosity in the original carbon network. Moreover, microwave heating (internal and volumetric heating) has assisted the penetration of activating agent within the char matrix, which created a more orderly porous structure by opening of previously inaccessible pores and formation of new pores [32]. 3.5. Adsorption kinetics Adsorption kinetic describes the controlling mechanism of adsorption processes which in turn governs mass transfer and the equilibrium time [33]. The experimental data of MB adsorption onto MPAC at different time intervals were examined with pseudofirst-order and pseudo second-order models, using the plots of ln(qe − qt ) against t and t/qt versus t, respectively. The first-order rate expression of Lagergren [34] based on solid capacity is derived in the form of: ln

(11)

2BDR

The detailed parameters of different forms of isotherm equations at the temperatures 30, 40 and 50 ◦ C were listed in Table 3. It was observed that the equilibrium data was getting valid for Langmuir isotherm model, while Freundlich, Temkin and Dubinin–Radushkevich isotherms do not much well represent the experimental adsorption data. The applicability of Langmuir isotherm model suggests that the adsorption takes place on homogeneous sites within the adsorption site; with each molecule possess constant enthalpies and sorption activation energy. The results also demonstrate no interaction and transmigration of dyes in the plane of the neighboring surface. A comparison of the adsorption capacity of MB with the literature [21–31], under optimum activation conditions was summarized in Table 4. It can be concluded that the adsorption capacity of the activated carbon prepared in this work was comparative with the previous studies. The activation time due to present work is much shorter owing to the thermal efficiency of microwave heating system. This irradiation has promoted the release of volatiles

 q e



qe − qt

=

k1 t 2.303

(12)

where qe and qt (mg/g) are the amounts of adsorbate adsorbed at equilibrium and at any time, t (h), respectively and k1 (1/h) is the adsorption rate constant. Contrary to other model, pseudosecond-order equation [35] predicts the behavior over the whole time of adsorption, with chemisorption being the rate controlling step given by: 1 1 + k2 t = qe (qe − qt )

(13)

where k2 is the pseudo-second-order rate constant (g/mg min). The corresponding results were shown in Table 5. The suitability of the kinetic model to describe the adsorption process was further validated by the normalized standard deviation, Äq (%) defined as:



q = 100

[(qexp − qcal )/qexp ] n−1

2

(14)

where n is the number of data points, qexp (mg/g) and qcal (mg/g) are the experimental and calculated adsorption capacity, respectively. Pseudo-second-order kinetic model yielded the best fit, with the

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Table 4 Comparison adsorption capacities of various activated carbons for MB. Precursor

Activation method

Activating agent

Activation time (min)

Adsorption capacity (mg/g)

Reference

Mangosteen peel Coffee press cake Bamboo Cotton stalk Spent catalysts of vinyl synthesis Pine wood powder Posidonia oceanica (L.) dead leaves Rice straw Walnut shell Durian peel Sludge Norit SA3 (Commercial grade powdered AC) Nuchar WWH (Commercial grade granular AC)

Microwave heating Microwave heating Microwave heating Microwave heating Microwave heating Microwave heating Conventional heating Conventional heating Conventional heating Conventional heating Conventional heating

K2 CO3 – H3 PO4 KOH Steam ZnCl2 ZnCl2 (NH4 )2 HPO4 ZnCl2 CO2 Steam

5 6 10 10 40 10 120 120 60 60 40 – –

379.63 0.14 286.10 294.12 285.00 200.00 285.70 129.50 315.00 284.00 263.16 91.00 21.50

Present study [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [31]

500 kp1

400

kp2

kp3 500 mg/L 400 mg/L

300

qt (mg/g)

lowest Äq values which ranged between 0.08 and 13.67% for initial MB concentrations ranging from 50 to 500 mg/L. Besides, the correlation coefficient values for the second-order kinetic model were almost equal to unity for all initial MB concentrations. This suggested that the overall rate of the adsorption process was controlled by chemisorption which involved valency forces through sharing or exchange of electrons between the adsorbent and adsorbate.

300 mg/L 200 mg/L 100 mg/L

200

50 mg/L

3.6. Adsorption mechanism 100

To examine the practical applications of adsorption system, the Weber and Morris intraparticle diffusion model [36], derived from the Fick’s second diffusion law was applied to analyze the kinetic data: qt = kpi t 0.5 + Ci

0 0

1

2

3

(15)

Bt = −0.4977 − ln(1 − F)

5

6

3

(mg/g h0.5 )

2.5 2

500 mg/L 400 mg/L 300 mg/L

1.5

200 mg/L

Bt

is the diffusion rate constant and Ci gives an where kpi idea about the thickness of the boundary layer. If intraparticle diffusion occurs, then qt versus t0.5 will be linear, and intraparticle diffusion is the sole rate limiting step if the plot passes through the origin. Refer to the intraparticle diffusion plot as depicted in Fig. 8a, the first, sharper portion is attributed to the diffusion of adsorbate through solution to the external surface of adsorbent (external surface adsorption). The second portion describes the gradual layer adsorption stage, where intraparticle diffusion is rate limiting and the third portion is the final equilibrium stage where intraparticle diffusion starts to slow down due to extremely low adsorbate concentrations left in the solution. From Fig. 8a, it is revealed that the third region did not exist for initial concentrations lower than 200 mg/L, as the equilibrium has been attained within the first 60 min. Moreover, the linear plots of the second and third region did not pass through the origin, mainly ascribed to the deviation in mass transfer rate in the initial and final stages of adsorption, which indicated that intraparticle diffusion was not the only rate limiting step [37]. The kinetic data were further analyzed using the Boyd model [38] expressed as:

4

t0.5 (h0.5) (a)

100 mg/L

1

50 mg/L

0.5 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Time (h) (b) Fig. 8. Plots of intraparticle diffusion (a) and Boyd (b) models for the adsorption of MB onto MPAC at 30 ◦ C.

(16)

Table 5 Kinetic models parameters for the adsorption of MB onto MPAC at different initial MB concentrations at 30 ◦ C. C0 (mg/L)

50 100 200 300 400 500

qe, exp (mg/g)

50.49 101.33 204.23 278.51 366.86 384.18

Pseudo-first-order

Pseudo-second-order

k1 (1/h)

qe, calc (mg/g)

R2

q (%)

k2 (g/mg min)

qe, calc (mg/g)

R2

q (%)

3.308 1.400 1.580 0.936 0.838 0.672

63.16 49.40 73.00 180.78 255.67 271.67

0.955 0.857 0.799 0.844 0.896 0.862

25.08 51.25 64.26 35.09 30.31 29.28

0.095 0.096 0.096 0.020 0.010 0.011

55.05 101.79 200.95 278.72 343.86 331.66

0.999 0.999 1.000 1.000 0.999 0.997

9.03 0.54 1.60 0.08 6.27 13.67

K.Y. Foo, B.H. Hameed / Chemical Engineering Journal 180 (2012) 66–74 Table 6 Thermodynamic parameters for the adsorption of MB onto MPAC. H◦ (kJ/mol)

−1.240

S◦ (J/mol K)

3.770

(Project No. 1001/PJKIMIA/814072) and RU-PRGS grant scheme (Project 465 No. 8043030).

G◦ (kJ/mol) 303 K

313 K

323 K

−0.097

−0.063

−0.021

which Bt is the mathematical function of F and F represents the fraction of solute adsorbed at time, t (h), given by: F=

qt qe

(17)

Pore diffusion is the rate-limiting step if the plot Bt versus t passes through the origin. Conversely, the adsorption process is film diffusion controlled. As illustrated from the curve as shown in Fig. 8b, the linear curves did not pass through the origin, and the points were scattered around the plots, thus ascertained that the adsorption of MB onto MPAC was governed by film diffusion controlled mechanism. 3.7. Thermodynamic modeling The concept of thermodynamic assumes that in an isolated system where energy cannot be gained or lost, the entropy change is the driving force. Thus, in the present study, thermodynamic parameters, Gibbs free energy (G), enthalpy (H) and entropy (S) change were determined using the equations [39,40] below: ln KL =

S H − R RT

G = −RT ln KL

73

(18) (19)

where R (8.314 J/mol K) is the universal gas constant, T (K) is the absolute solution temperature and KL (L/mg) is the Langmuir isotherm constant. The values of H and S were estimated from the slope and intercept of the van’t Hoff plot of ln KL versus 1/T. The calculated values were presented in Table 6. Positive S showed the affinity of MPAC for MB and increasing randomness at the solid–solution interface with some structural changes in the adsorbates and adsorbents during the adsorption process. Negative G indicated the feasibility and spontaneous nature of adsorption with high preference of MB onto MPAC while negative value of H represents exothermic nature of the adsorption interaction. Increasing temperature leads to the decrease of dye adsorption due to the enhancement of desorption step in the sorption mechanism. This phenomenon might due to weakening of adsorptive forces between the active sites of MPAC and the dye species, and also between the adjacent dye molecules on the adsorbed phase [41]. 4. Conclusion It appears that mangosteen peel may serve as a potential suitable biomass source for the manufacture of activated carbon with a well-developed porous texture. This study has attempted a new path for multi-purpose utilization of the biomass waste of mangosteen peel. MPAC prepared by microwave assisted K2 CO3 activation attained maximum BET surface area as 1098.75 m2 /g, total pore volume as 0.611 cm3 /g and a high contribution of mesopores as 45% at IR of 1.25, microwave power of 600 W and irradiation time of 5 min. Equilibrium data could be favorably described by the Langmuir isotherm and pseudo-second-order models. Acknowledgements The authors acknowledge the financial support provided by Universiti Sains Malaysia under the Research University (RU) Scheme

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