Mesoporous activated carbon from wood sawdust by K2CO3 activation using microwave heating

Bioresource Technology 111 (2012) 425–432

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Mesoporous activated carbon from wood sawdust by K2CO3 activation using microwave heating 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 25 August 2011 Received in revised form 21 January 2012 Accepted 24 January 2012 Available online 2 February 2012 Keywords: Activated carbon Adsorption Methylene blue Microwave Wood sawdust

a b s t r a c t Wood sawdust was converted into a high-quality activated carbon (WSAC) via microwave-induced K2CO3 activation. The operational variables including chemical impregnation ratio, microwave power and irradiation time on the carbon yield and adsorption capability were identified. The surface physical characteristics of WSAC were examined by pore structural analysis, scanning electron microscopy and nitrogen adsorption isotherms. The adsorptive behavior of WSAC was quantified using methylene blue as model dye compound. The best conditions resulted in activated carbon with a monolayer adsorption capacity of 423.17 mg/g and carbon yield of 80.75%. The BET surface area, Langmuir surface area and total pore volume were corresponded to 1496.05 m2/g, 2245.53 m2/g and 0.864 cm3/g, respectively. The findings support the potential to prepare high surface area and mesoporous activated carbon from wood sawdust by microwave assisted chemical activation. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Neobalanocarpus heimii, locally known as Chengal, is a unique tree species belonging to the order Malvales and family of Dipterocarpaceae. The tree is typically 60–65 m tall and has a diameter of 5.33 m (Ashton, 1969). Chengal trees are dominant in mixed dipterocarp tropical rainforests. The wood is heavy, pale-yellow and moderately lustrous, with an air-dry density of 915–980 kg/m3. Chengal is an important natural resource primarily used for the production of particleboards, fiberboards, furniture and packing materials. It plays a key role in the manufacture of household commodities, and is suitable for all forms of architectural moldings and heavy constructions, particularly boat-buildings, bridges, railway sleepers, power line posts and rubber coagulating tanks, where great strength and durability are required (Iwata et al., 2000). Processing of the wood generates lignocellulosic biomass, in the form of wood sawdust, off-cuts and chips, which can amount to 20% of the total input mass (Gan et al., 2004). These residues are disposed of by on-site combustion deposition in landfills. In an effort to upgrade this abundantly available biomass, microwave irradiation for preparation of activated carbon from Chengal sawdust via K2CO3 activation was explored. The significant influences of microwave power, radiation time and chemical impregnation ratio on the carbon yield and adsorption capacity were investigated systematically. Textural, functional and surface characteriza-

tion of the prepared adsorbent was performed and adsorption equilibrium, isotherms and kinetics were outlined. 2. Methods 2.1. Adsorbate Methylene blue (MB), a monovalent cationic dye with the molecular structure C16H18N3SCl, was selected as the model adsorbate in the present study. A stock solution of 1000 mg/L was prepared by dissolving an appropriate quantity of MB in double distilled water and then diluted to the desired concentrations. 2.2. Preparation of activated carbon Locally obtained Chengal wood sawdust (WS) was thoroughly washed, air-dried and sieved to obtain particles of 1–2 mm in diameter. The carbonization process was carried out by loading 500 g of dried wood particles into a tubular furnace and heating to a carbonization temperature of 700 °C under purified N2 flow (150 cm3/min) (Foo and Hameed, 2012a). The char produced was mixed with K2CO3 pellets with different impregnation ratio (IR), defined as:

IR ¼ ⇑ Corresponding author. Tel.: +60 45996422; fax: +60 45941013. E-mail address: [email protected] (B.H. Hameed). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.01.141

wK2 CO3 wchar

ð1Þ

where wK2 CO3 and wchar is the dry weight of K2CO3 pellets (g) and char (g).

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Microwave heating was conducted in a 2.45 GHz commercial microwave oven. The oven has a power controller to select different power levels and a timer for various exposure times at a set microwave power level. Activation was performed in a reactor fixed in the chamber of the microwave oven. Nitrogen gas at a pre-set flow rate of 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 hydrochloric acid and rinsed repeatedly with hot and cold distilled water until the filtrate reached neutral pH. The yield was defined as the weight of activated carbon per weight of char utilized for activation (Foo and Hameed, 2012a). 2.3. Adsorption equilibrium studies Batch adsorption experiments were undertaken in a set of 250mL Erlenmeyer flasks containing 0.20 g of adsorbent and 200 mL of solutions containing 50–500 mg/L of the dye. The mixture was agitated in a thermostatic orbital shaker at 30 °C with an agitation speed of 120 rpm. The dye concentration in the supernatant was determined using a double beam UV–Vis spectrophotometer (UV-1601 Shimadzu, Japan) at 668 nm. Each experiment was carried out in triplicates under identical conditions and an average value was determined. MB uptake at equilibrium, qe (mg/g), was calculated by:

ðC o  C e ÞV qe ¼ W

ð2Þ

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, carbon (WSAC) 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 HCl or NaOH. The pH was measured using a pH meter (Ecoscan, EUTECH Instruments, Singapore). 2.4. Adsorption isotherm 2.4.1. Freundlich isotherm Freundlich isotherm (Freundlich, 1906) is a model describing non-ideal, reversible and multilayer adsorption, with non-uniform distribution of adsorption heat and affinity over the heterogeneous surface. The Freundlich isotherm was derived as:

qe ¼

K F C e1=2

ð3Þ

where KF (mg/g) (L/mg)1/n and 1/n are the Freundlich adsorption constant and a measure of adsorption intensity, respectively. 2.4.2. Langmuir isotherm Langmuir isotherm (Langmuir, 1916) assumes monolayer adsorption can only occur at a finite number of localized sites that are identical and equivalent. The mathematical expression of Langmuir isotherm is defined as:

qe ¼

Q 0K LCe 1 þ K LCe

ð4Þ

Its derivation is characterized by a uniform distribution of binding energy. The Temkin isotherm has been used in the form of:

qe ¼ B ln ðACe Þ

ð5Þ

where B = RT/b, with b (J/mol), A (L/g), R (8.314 J/mol K) and T (K) are the Temkin constant related to heat of sorption, equilibrium binding constant, gas constant and absolute temperature, respectively. 2.4.4. Redlich–Peterson isotherm The Redlich–Peterson isotherm (Redlich and Peterson, 1959) is a hybrid isotherm featuring both Langmuir and Freundlich isotherms, which incorporates three parameters into an empirical equation given by:

qe ¼

K R Ce 1 þ aR C ge

ð6Þ

where KR (L/g) and aR (1/mg) are Redlich–Peterson isotherm constants, and g is the isotherm exponent. The model can be applied either in homogeneous or heterogeneous systems. It approaches the Freundlich isotherm model at high concentration and in accordance with the low concentration limit of the ideal Langmuir condition. 2.4.5. Hill isotherm The Hill equation (Hill, 1910) was postulated to describe the binding of different species onto homogeneous substrates defined as:

qe ¼

qsH C ne H

ð7Þ

K D þ C ne H

where KD, nH, and qsH (mg/L) are Hill isotherm constant, Hill cooperativity coefficient of the binding interaction and Hill isotherm saturation uptake. The model assumes that adsorption is a cooperative phenomenon by which ligand binding at one site of the macromolecule may influence different binding sites on the same macromolecule. 2.4.6. Toth isotherm The Toth isotherm model (Toth, 1971) is another empirical equation developed to improve Langmuir isotherm fittings. It is useful in describing heterogeneous adsorption systems, and satisfies both low and high-end boundary of concentrations:

qe ¼

K TCe ðaT þ C e Þ1=2

ð8Þ

where KT (mg/g), aT (L/mg) and t are the Toth isotherm constants. Its correlation presupposes an asymmetrical quasi-Gaussian energy distribution where most binding sites have an adsorption energy lower than the peak value. The validity of the models was verified by root-mean-square deviation (RMSD), the commonly used statistical tool measuring the predictive power of a model derived as:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n P ðqexp  qP Þ2 RMSD ¼

i¼1

n1

ð9Þ

where Q0 (mg/g) and KL (L/g) are Langmuir constants related to adsorption capacity and energy of adsorption, respectively.

where qexp (mg/g) and qp (mg/g) are the experimental and theoretical adsorption capacity, respectively.

2.4.3. Temkin isotherm The Temkin isotherm (Tempkin and Pyzhev, 1940) assumes the heat of adsorption (function of temperature) of all molecules in the layer decreases linearly rather than logarithmically with coverage.

2.5. Adsorption kinetics For interpretation of kinetic experimental data, the aqueous samples were withdrawn at different time intervals and the

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concentrations of MB were measured. The amount of adsorption at time t, qt (mg/g), was calculated by:

qt ¼

ðC o  C t ÞV W

ð10Þ

where Ct (mg/L) is the liquid-phase concentration of dye at time, t. The pseudo-first-order equation (Langergren and Svenska, 1898) is defined as:

dq ¼ k1 ðqe  qÞ dt

ð11Þ

where k1 (1/h) is the adsorption rate constant. Contrary to pseudofirst-order equation, pseudo second-order equation (Ho, 1995) predicts the behavior over the whole range of adsorption and represented by:

dq ¼ k2 ðqe  qÞ2 dt

ð12Þ

where k2 (g/mg h) is the adsorption rate constant of pseudo secondorder equation. The Elovich kinetic equation (Aharoni and Tompkins, 1970) is one of the most useful models describing a chemisorption process and is given by:

dqt ¼ aebqt dt

ð13Þ

where a (mg/g h) is the initial sorption rate and b (g/mg) is related to the extent of surface coverage and activation energy for chemisorption. The value of (1/b) is indicative of the available number of sites for adsorption while (1/b) ln(ab) is the adsorption quantity when ln t = 0. The suitability of the kinetic model to describe the adsorption process was further ascertained by the normalized standard deviation, Dq (%) given by:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P ½ðqexp  qcal Þ=qexp 2 Dq ¼ 100 n1

ð14Þ

where n is the number of data points, qexp (mg/g) and qcal (mg/g) are the experimental and calculated adsorption capacity, respectively.

427

2.7. Characterization of WSAC The pore structural characteristics of WSAC prepared under best conditions (high adsorption uptake and carbon yield) were determined by nitrogen adsorption at 77 K using an automatic volumetric adsorption analyzer (Micromeritics ASAP-2020). Prior to analysis, the sample was degassed for 2 h under vacuum at 573 K. The sample was then transferred to the analysis system where it was cooled in liquid nitrogen. The specific surface area (SBET) was calculated by the Brunauer–Emmett–Teller (BET) equation; the total pore volume (VT) was evaluated by converting the adsorption volume of nitrogen at relative pressure 0.95 to equivalent liquid volume of the adsorbate, while the micropore volume, micropore surface area and external surface area were obtained using the t-plot method. The surface morphologies were examined using a scanning electron microscope (Zeiss Supra 35VP). Chemical characterization of functional groups was detected using pressed potassium bromide (KBr) pellets containing 5% of carbon sample by Fourier transform infrared spectrometry (FTIR-100, Shimadzu) in the scanning range of 4000–400 cm1. The content of C, H, N, O and S in the ultimate analysis was performed using an Elemental Analyzer (Perkin-Elmer-2400 CHNS/O analyzer). The pore structure can be described in terms of fractal geometry, Ds, an index of the roughness or irregularity of the solid surfaces. In the present work, the fractal dimension was determined according to the Frenkel–Halsey–Hill (FHH) model proposed as:

  DS 3 q P0 ¼ K ln q0 p

ð18Þ

where q (cm3/g STP) is the amount of N2 adsorbed at equilibrium pressure P(mmHg), q0 (cm3/g STP) is the amount adsorbed filling micropore volume, P0 (mmHg) is the saturated pressure, DS is the fractal dimension and K is the FHH constant. A perfectly smooth surface has the fractional dimension, Ds = 2, the dimension of a Euclidean surface, whereas a highly rough and irregular surface has the fractional dimension, Ds of 3 (Gregg and Sing, 1982). 2.8. Surface acidity/basicity and zeta potential measurement (pHpzc)

2.6. Adsorption mechanism To examine the mechanism of the adsorption process, the Weber and Morris intraparticle diffusion model (Weber and Morris, 1963) was applied to analyze the kinetic results:

qt ¼ kpi t 0:5 þ C i

ð15Þ 0.5

where kpi (mg/g h ) is the diffusion rate constant and Ci gives an idea about the thickness of the boundary layer. If pore diffusion is the rate limiting step, then the plot qt versus t0.5 gives a straight line with the slope, kpi and the intercept, Ci. Intraparticle diffusion is the sole rate limiting step if the plot passes through the origin. To differentiate between the pore and film diffusion step involved in the adsorption process, the kinetic result was further analyzed using the Boyd model (Boyd et al., 1947) expressed as:

Bt ¼ 0:4977  ln ð1  FÞ

ð16Þ

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

F¼

qt qe

ð17Þ

If the plot Bt versus t passes through the origin, pore diffusion is the rate-limiting step. Conversely, the adsorption process is film diffusion controlled.

The surface acidity was estimated by mixing 0.20 g of WSAC with 25 cm3 of 0.05 M NaOH solution in a closed flask, and agitating for 48 h at room temperature. The suspension was decanted, and the remaining NaOH was titrated with 0.05 M HCl. The surface basicity was measured by titration with 0.05 M NaOH after incubation 0.20 g of WSAC with 0.05 M HCl. The determination of pHpzc was conducted by adjusting the pH of 50 cm3 0.01 M NaCl solution to a value between 2 and 12. 0.15 g of WSAC 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 activated carbon 3.1.1. Effect of chemical impregnation ratio Effect of chemical impregnation ratio (IR) on the carbon yield and adsorption uptake of MB was evaluated at the microwave input power of 360 W and irradiation time of 5 min (Fig. 1a). Augmenting IR from 0.25 to 1.25 showed an increase in carbon yield from 75.84% to 85.70%. Beyond 1.25, a further increase in IR illustrated a gradual decrease in carbon yield. Similarly, increasing IR from 0.25 to 1.25 indicated an increase in adsorption uptake from 27.18 to 232.47 mg/g, and then a steady decrease. The best adsorption

K.Y. Foo, B.H. Hameed / Bioresource Technology 111 (2012) 425–432

Yield (%)

(a)

86

260

84

220

82

180

80

140

78

100 Yield

76

60

Adsorption uptake (mg/g)

428

Adsorption uptake 74

20 0

0.5

1

1.5

2

2.5

Impregnation ratio

(b) 100

250 90 Yield (%)

200 80

150 100 Yield

70

Adsorption uptake

50

60 0

200

400

600

800

Adsorption uptake (mg/g)

300

0 1000

Power level (W)

(c) 84

300 Yield (%)

80 280 76

260 240

Yield 72

Adsorption uptake

Adsorption uptake (mg/g)

320

220 68

200 3

4

5

6

7

8

9

Radiation time (minutes)

Fig. 1. Effects of (a) chemical impregnation ratio (preparation conditions: microwave power = 360 W; radiation time = 5 min), (b) microwave power (preparation conditions: chemical impregnation ratio = 1.25; radiation time = 5 min) and (c) radiation time (preparation conditions: chemical impregnation ratio = 1.25; microwave power = 600 W) on the carbon yield and adsorption capacity.

surface caused blockage of the pores leading to a dramatic decrease in accessible area. Additionally, a further increase in impregnation ratio would intensify a vigorous activation reaction, which leads to carbon burn off and transition of micropores–mesopores into macropores lowering the carbon yield and adsorption uptake. 3.1.2. Effect of microwave power Effect of microwave power on the adsorption uptake and carbon yield was investigated at the IR of 1.25 and irradiation time of 5 min (Fig. 1b). As suggested by the result, under low microwave power levels of 90 and 180 W, the pore structure was not adequately developed, and the adsorption uptake and carbon yield remained almost unchanged, indicating no continual reaction between char and activating agent. Increasing microwave power from 180 to 600 W resulted in a drastic increase in adsorption uptake, possibly ascribed to the combined effect of internal and volumetric heating responsible for the expansion of the carbon structure and creation of high porosity and a larger surface area. However, at a high radiation power of 800 W, the adsorbed microwave energy exceeded a certain level which led the plentiful energy to cause excessive destruction of pore structures, and a progressive decrease in adsorption uptake and carbon yield. The weight loss of carbon increased proportional to the microwave power level, mainly due to the fierce reaction at higher thermal radiation which intensified devolatilization, dehydration and decomposition (Foo and Hameed, 2012b). 3.1.3. Effects of radiation time Microwave radiation time is a decisive factor affecting the adsorption uptake and carbon yield. Effect of microwave radiation time was conducted at an IR of 1.25 and a microwave input power of 600 W. Fig. 1c reveals that prolonged radiation time increased adsorption uptake from 204.98 to 299.44 mg/g. The phenomenon implied that prolonging exposure promotes an acceleration of temperature, which in turn increases reaction rates, thus developing the porosity of the pore network. However, as the radiation time arrives at its optimum value, absorption and reflection of energy tends to balance and WSAC achieves its maximum adsorption uptake. As activation proceeded, temperature increased dramatically and led to opening of micropores and mesopores which resulted in enlargement of the average diameter. Moreover, exorbitant temperatures might produce local hotspots, leading to external ablation, shrinkage and collapse of the carbon framework, reducing accessibility of active sites (Foo and Hameed, 2012c). Higher pyrolytic temperature promoted C–K2CO3, C–K2O and C– CO2 reactions facilitating breaking of C–O–C and C–C bonds thus decreasing carbon yield (Adinata et al., 2007). Therefore, the optimal combination for improvement of carbon yield and adsorption performance was inferred at the IR of 1.25, microwave input power of 600 W and activation time of 6 min. 3.2. Textural and surface characterization

uptake and carbon yield of WSAC was obtained at a K2CO3/char ratio of 1.25. It was presumed that K2CO3 activation involved the reduction of K2CO3 under inert condition to form K, K2O, CO and CO2 (Mckee, 1983). The potassium compound formed during the activation step would diffuse into the internal structure of char matrix and widen existing pores. Therefore, by increasing the ratio of K2CO3 to char, the activation process would play a key role in pore formation. The pore width was successively broadened and new micropores–mesopores were formed in the original pore walls, giving a sustaining increase in BET surface area and pore volume. Correspondingly, the adsorption uptake was further enhanced. Beyond the optimum value, excess K2CO3 and metallic potassium left in the carbon

The nitrogen adsorption–desorption curve provides qualitative information on the adsorption mechanism and porous structure of carbonaceous materials. The nitrogen adsorption isotherm analysis (Fig. 2a) demonstrated that the isotherm resembles an intermediate between type I and type II isotherms, in accordance with the International Union of Pure and Applied Chemistry (IUPAC) classification. This adsorption behavior suggests a combination of microporous–mesoporous structure. The desorption branch presents a hysteresis loop at high relative pressures, indicative of the presence of mesoporosity. Detailed characteristics of the porosities of char and WSAC are listed in Table 1. Mesopores of the WSAC accounted for about

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(a) Quantity Adsorbed (cm³/g STP)

580 550 520 490 460 430 400 0

0.2

0.4

0.6

0.8

1

Relative pressure (P/Po)

(b) Incremental pore volume (cm3/g.Å)

0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000 15

20

25

30

35

40

45

Pore width (Å)

Fig. 2. Nitrogen adsorption–desorption curve (a) and pore size distribution (b) of WSAC.

Table 1 Porosity structure and fractional dimension of the WS derived char and WSAC. Properties

Char

WSAC

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) Fractional dimension (DS)

159.48 111.92 47.56 237.82 0.102 0.059 0.043 23.64 2.71

1496.05 892.79 603.26 2245.53 0.864 0.470 0.394 23.06 2.84

46% of the total pore volume, with a well developed porous structure. A comparison of the key parameters of activated carbon with the carbonized char shows a significant improvement in BET surface area, Langmuir surface area and total pore volume, implying pore development and widening of the existing pores during the microwave irradiation stage.

Table 2 summarizes a comparison of porosity structure of activated carbons prepared from different types of wood sawdust. WSAC prepared in this study demonstrated a well-developed porous structure with a higher BET surface area of 1496.05 m2/g and total pore volume of 0.864 m3/g, respectively as compared to the literature. The pore size distribution is a model of solid internal structure representing the complex void spaces within the real solid. According to the classification of IUPAC-pore dimensions, the pores of adsorbents are grouped into micropore (d < 2 nm), mesopore (d = 2–50 nm) and macropore (d > 50 nm). The pore size distribution of WSAC was ascertained by the Density Functional Theory (DFT) model. The sharpest peak occurred at pore diameters between 2 and 4 nm, with an average pore size of 23.06 Å (Fig. 2b) which shows that a vast majority of the pores fell into the mesopore range. Besides, it is noticeable that the fractal dimension obtained for WSAC was slightly higher compared to char. The variation of fractal dimension gives an idea of the evolution of surface roughness associated with microwave heating and chemical activation which created the cross-linking between neighboring elementary crystallites. The surface morphology of the char and WSAC was examined using scanning electron micrographs with a magnification of 500, as depicted in Supplementary Fig. 1. The surface of the primary char was constricted, comprising mainly macro and mesopores without a deeper pore structure. Conversely, WSAC displayed a well pronounced porosity, with a series of irregular cavities distributed over the surface. Comparison of the surface morphology verified substantial changes occasioned by microwave irradiation. 3.3. Elemental and functional characterization The results of the elemental analysis of char and WSAC are listed in Supplementary Table 1. Microwave treatment led to an increase in carbon content but caused a drastic decrease in oxygen content, resulting in an increase in the C/O ratio from 1.30 to 2.78. This is due to the partial decomposition of volatiles compounds and degradation of organic substances under microwave irradiation leaving a high purity carbon. The content of hydrogen and nitrogen was slightly decreased, but the sulphur content in the carbonized char was thermally stable. Representative FTIR spectra of char and WSAC are presented in Supplementary Fig. 2. The band located at 3233 cm1 is related to the N–H groups, and the signal at 2362 cm1 is assigned to the C–C stretch of alkynes. Similarly, the presence of –COOH structures shows a region between 2346/2343 cm1 while intensive peaks at 1993, 1645–1498, 1420, 1276, 1053, 828 and 809 cm1 were corresponded to the C–N, in-plane O–H (hydroxyl), –CH2 (alkyl), C–O–C (ester, ether and phenol), C–O (anhydrides), out-of-plane N–O and out-of-plane C–H derivatives. For WSAC, the elimination of peaks at 2362, 1645 and 828 cm1 were indicative of the decomposition of functional groups and release of volatiles matters by microwave irradiation.

Table 2 Comparison porosity structure of the activated carbons prepared from different wood sawdust. Wood sawdust

Activation time (minutes)

Activating agent

BET surface area (m2/g)

Total pore volume (cm3/g)

Pore structure

Reference

Chengal Rattan Havea braziliansis Pinewood Rubber wood Oat sawdust

6 120 120 120 60 120

K2CO3 KOH-CO2 K2CO3 CO2 CO2 K2CO3

1496.05 1083.00 686.30 352.00 683.63 1240.00

0.864 0.644 0.290 0.194 0.468 0.768

Mesoporous Mesoporous Microporous Microporous Microporous Microporous

Present study Hameed et al. (2007) Krishnan et al. (2010) Nowicki and Pietrzak (2010) Taer et al. (2010) Zhang et al. (2011)

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Surface acidity and basicity are important criteria for interpreting the surface chemistry of carbon adsorbents. Analysis of the data presented in Supplementary Table 1 shows that WSAC exhibited an acidic behavior, with surface acidity of 2.93 and 1.23 mmol/ g as surface basicity. Thus, greater amounts of oxygen-containing groups (mainly carboxylic, anhydrides, lactones and phenols) than oxygen-free Lewis sites, carbonyls, pyrone and chromene type structures were present at the edge of the carbon layers. The zero point of charge (pHZPC), an index of the propensity of the surface charge as a function of pH of WSAC was found to be 6.80. 3.4. Effects of initial concentration, contact time and solution pH on the adsorption equilibrium The curve adsorption uptake, qt as a function of time, t at the initial concentrations of 50–500 mg/L is presented in Fig. 3a. Adsorption uptake and dye removal efficiency increased with prolonged contact time. The adsorption process increased sharply at the initial stage (Fig. 3a) indicating availability of readily accessible sites. The process gradually slowed down as the equilibrium approached. In the present study, the adsorption equilibrium, qe increased from 50.48 to 425.30 mg/g with an increase in initial concentration from 50 to 500 mg/L, mainly ascribed to the higher concentration gradient which acts as a driving force for the adsorption process. The time profile of dye uptake is a single, smooth and continuous curve leading to saturation, suggesting possible monolayer coverage of dye onto the surface of WSAC. Solution pH affects adsorption by regulating the adsorbents surface charge as well as degree of ionization of adsorbates present in the solution (Foo and Hameed, 2011a). The effect of pH was con-

(a) 500

400

500 mg/L 400 mg/L

qt (mg/g)

300

300 mg/L 200 mg/L

200

100 mg/L 50 mg/L

100

0 0

5

10

15

20

25

30

Time (h)

(b) 500

qe (mg/g)

460

420

380

340 0

2

4

6

8

10

12

14

pH

Fig. 3. Effects of initial concentration and contact time (a) and solution pH (b) on the adsorption equilibrium of MB onto WSAC at 30 °C.

ducted by varying the pH of dye solutions from 2 to 12 with an initial concentration of 500 mg/L, as shown in Fig. 3b. An increase in pH exerted a significant enhancement of the adsorption capacity of MB from 373.64 to 483.36 mg/g. Lower adsorption at strong acidic pH is due to the protonation of MB and high mobility of H3O+ ions competing with dye cations for the adsorption sites. At higher pH, WSAC may become negatively charged and the formation of electric double layer changes its polarity, consequently dye uptake increases (Foo and Hameed, 2011b). The effect of pH can be described on the basis of zero point of charge (pHZPC), the point at which the net charge of adsorbent is zero. The pHZPC of WSAC was 6.80. At a solution pH lower than pHZPC, activated carbon will react as a positive surface and as a negative surface when the solution pH is higher than pHZPC. Therefore, for pH values above 6.80, the negative charge density of WSAC increased and favored adsorption of the cationic dye. 3.5. Adsorption isotherm The adsorption isotherm describes the interaction between the adsorbates and the carbonaceous adsorbents. Adsorption equilibrium is established when an adsorbate-containing phase has been contacted with the adsorbent for sufficient time, and the adsorbate concentration in the bulk solution is in a dynamic balance with the interface concentration. Typically, the mathematical correlation is accessed by linear regression analysis: however, recent studies have suggested a tendency of linearized models to alter regression results and to be inappropriate for predicting the goodness of fit for a particular set of conditions, resulting in improper conclusions (Foo and Hameed, 2010). Thus, in the present study, the alternative isotherm parameter sets were determined by non-linear regression. This provides a mathematically rigorous method for determining isotherm parameters using the original form of isotherm equation. A comparison between the experimental data points and the theoretical isotherms plot is displayed in Fig. 4. The detailed parameters of these different forms of isotherm equations are listed in Table 3. The Langmuir isotherm model was satisfactory in describing the adsorption equilibrium, as the lowest RMSD values and R2 were higher than 0.99. The applicability of the Langmuir isotherm model suggests that the adsorption process occurred at a monolayer with each molecule having equal enthalpies and activation energy. The results also demonstrate no interaction and transmigration of dyes in the plane of the neighboring surface. The result was justified by the exponent values of the Redlich–Peterson, 4. Hill and Toth isotherm models, g (0.978), nH (0.922) and t (1.030) which approximated to unity, resulted to the original Langmuir equation. Table 4 lists the maximum monolayer adsorption capacities of various activated carbons derived from different precursors. The activated carbon prepared in this study showed a comparatively high adsorption capacity of 423.17 mg/g. Thus, it is noteworthy that considerable changes in the surface properties were achieved within a short time, which may be attributable to the distinct mechanism of microwave heating. This irradiation promoted the release of volatiles from the char surface widening the porosity in the original carbon network. Moreover, microwave heating (internal and volumetric heating) has assisted the penetration of the K2CO3 within the char matrix, and created a more orderly porous structure by opening of previously inaccessibly pores and formation of new pores (Foo and Hameed, 2009). 3.6. Adsorption kinetics Adsorption kinetic provides an invaluable insight into the controlling mechanism of an adsorption process which in turn gov-

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500

400

Experimental

qe (mg/g)

300

Freundlich Langmuir Temkin Hill

200

Redlich–Peterson Toth 100

0 0

10

20

30

40

50

60

70

80

Ce (mg/L) Fig. 4. Adsorption isotherm for the adsorption of MB onto WSAC at 30 °C.

Table 3 Isotherm parameters for the adsorption of MB onto WSAC at 30 °C. Isotherms Langmuir Freundlich Temkin Hill Redlich– Peterson Toth

Constants Qo (mg/g) 423.17 n 4.90 A (L/g) 24.86 qsH (mg/L) 430.62 aR (1/mg)g 1.22 aT (L/mg) 0.82

KL (L/mg) 1.03 KF (mg/g).(L/mg)1/n 193.65 B 61.99 KD 1.03 KR (L/g) 477.96 KT (mg/g) 385.19

nH 0.922 g 0.978 t 1.03

R2 0.998 R2 0.834 R2 0.948 R2 0.998 R2 0.998 R2 0.998

RMSD 2.89 RMSD 23.19 RMSD 12.94 RMSD 2.27 RMSD 1.88 RMSD 1.88

erns the residence time of adsorbates at the solid–liquid interface. The experimental data on MB adsorption onto WSAC at different time intervals were examined by pseudo-first-order, pseudo second-order and Elovich kinetic models, using the plots ln (qeqt) against t, t/qt versus t and qt against ln t, respectively. The corresponding results are tabulated in Supplementary Table 2. It was evident that the Lagergren and Elovich kinetic models fit well at the initial stage and thereafter deviate from theory: however, the experimental data showed good agreement with the pseudo-second-order kinetic model, with the lowest normalized standard deviation, Dq values which ranged between 0.81% and 5.71%. The correlation coefficient values for the second-order kinetic model were higher than Lagergren and Elovich kinetic

models for all MB concentrations. This suggested that the adsorption system follows the pseudo-second-order model, based on the assumption that the rate-limiting step may be chemisorption, which involves valency forces through electrons sharing between the hydrophilic edge sites of WSAC and dye cations. 3.7. Adsorption mechanism The transportation of adsorbate from the solution phase to the surface of adsorbent particles is controlled by film diffusion, pore diffusion, surface diffusion, adsorption into the interior surface of adsorbent or a combination of one or more steps. In Supplementary Fig. 3a, the first region corresponded to instantaneous adsorption, representing the mass transfer of adsorbate molecules from the bulk solution to the adsorbent surface. The second region reflected the gradual adsorption stage where intraparticle diffusion was the rate-limiting step. The third region was the final equilibrium stage where intraparticle diffusion started to slow down due to extremely low adsorbate concentrations left in the solutions. The third region did not exist for initial concentrations lower than 200 mg/L (Supplementary Fig. 3a). Besides, the linear plots of the second and third stages did not pass through the origin, indicating that intraparticle diffusion was not the only rate-determining step. As illustrated in Supplementary Fig. 3b, the linear curves did not pass through the origin, and the points were scattered around the plots. This finding demonstrates that the adsorption of MB onto WSAC was mainly governed by a film diffusion-controlled mechanism.

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

Activation method

Chengal sawdust Microwave heating Cotton stalk Microwave heating Piassava fibers Conventional heating Rice straw Conventional heating Coffee press cake Conventional heating Durian peel Conventional heating Norit SA3 (Commercial grade powdered AC) Nuchar WWH (Commercial grade granular AC)

Activating agent

Activation time (min)

Monolayer adsorption capacity (mg/g)

Reference

K2CO3 KOH ZnCl2 (NH4)2HPO4 N2 CO2

6 10 180 120 60 60 – –

462.10 294.12 276.40 129.50 14.90 284.00 91.00 21.50

Present study Deng et al. (2010) Avelar et al. (2010) Gao et al. (2011) Nunes et al. (2009) Nuithitikul et al. (2010) Yener et al. (2008) Yener et al. (2008)

432

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4. Conclusion This study highlighted the feasibility of wood sawdust as a promising precursor for the manufacture of activated carbon with a noticeable decolorization capacity. Microwave heating shortened the processing period, and produced a high-quality activated carbon by opening of previously inaccessibly pores and creation of new pores, presumably due to the interior and volumetric heating of microwave irradiation. The findings indicate a new way of producing activated carbon and represent saving of cost and energy. Future studies should innovate this batch preparation process into a continuous preparation system. Acknowledgements The authors acknowledge the financial support provided by Universiti Sains Malaysia under the Research University (RU) Scheme (Project No. 1001/PJKIMIA/814072) and RU-PRGS grant scheme (Project 465 No. 8043030).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2012.01.141. References Adinata, D., Wan Daud, W.M.A., Aroua, M.K., 2007. Preparation and characterization of activated carbon from palm shell by chemical activation with K2CO3. Bioresour. Technol. 98, 145–149. Aharoni, C., Tompkins, F.C., 1970. In: Eley, D.D., Pines, H., Weisz, P.B. (Eds.), Adv. Catal. Related Subj. 21, Academic Press, New York. Ashton, P.S., 1969. Speciation among tropical forest trees: some deductions in the light of recent evidence. Biol. J. Linn. Soc. 1, 155–196. Avelar, F.F., Bianchi, M.L., Gonçalves, M., da Mota, E.G., 2010. The use of piassava fibers (Attalea funifera) in the preparation of activated carbon. Bioresour. Technol. 101, 4639–4645. Boyd, G.E., Adamson, A.W., Myers Jr., L.S., 1947. The exchange adsorption of ions from aqueous solutions by organic zeolites, II: kinetics. J. Am. Chem. Soc. 69, 2836–2848. Deng, H., Li, G.X., Yang, H.B., Tang, J.P., Tang, J.Y., 2010. Preparation of activated carbons from cotton stalk by microwave assisted KOH and K2CO3 activation. Chem. Eng. J. 163, 373–381. Foo, K.Y., Hameed, B.H., 2009. Recent developments in the preparation and regeneration of activated carbons by microwaves. Adv. Colloid Interface Sci. 149, 19–27. Foo, K.Y., Hameed, B.H., 2010. Insights into the modeling of adsorption isotherm systems. Chem. Eng. J. 156, 2–10. Foo, K.Y., Hameed, B.H., 2011a. Utilization of rice husks as a feedstock for preparation of activated carbon by microwave induced KOH and K2CO3 activation. Bioresour. Technol. 102, 9814–9817. Foo, K.Y., Hameed, B.H., 2011b. Preparation and characterization of activated carbon from sunflower seed oil residue via microwave assisted K2CO3 activation. Bioresour. Technol. 102, 9794–9799.

Foo, K.Y., Hameed, B.H., 2012a. Microwave-assisted preparation and adsorption performance of activated carbon from biodiesel industry solid reside: influence of operational parameters. Bioresour. Technol. 103, 398–404. Foo, K.Y., Hameed, B.H., 2012b. Preparation, characterization and evaluation of adsorptive properties of orange peel based activated carbon via microwave induced K2CO3 activation. Bioresour. Technol. 104, 679–686. Foo, K.Y., Hameed, B.H., 2012c. Factors affecting the carbon yield and adsorption capability of the mangosteen peel activated carbon prepared by microwave assisted K2CO3 activation. Chem. Eng. J. 180, 66–74. Freundlich, H.M.F., 1906. Over the adsorption in solution. J. Phys. Chem. 57, 385– 471. Gan, Q., Allen, S.J., Matthews, R., 2004. Activation of waste MDF sawdust charcoal and its reactive dye adsorption characteristics. Waste Manage. 24, 841–848. Gao, P., Liu, Z.H., Xue, G., Han, B., Zhou, M.H., 2011. Preparation and characterization of activated carbon produced from rice straw by (NH4)2HPO4 activation. Bioresour. Technol. 102, 3645–3648. Gregg, S.J., Sing, K.S., 1982. Adsorption Surface Area and Porosity. Academic Press, London. Hameed, B.H., Ahmad, A.L., Latiff, K.N.A., 2007. Adsorption of basic dye (methylene blue) onto activated carbon prepared from rattan sawdust. Dyes Pigm. 75, 143– 149. Hill, A.V., 1910. The possible effects of the aggregation of the molecules of haemoglobin on its dissociation curves. J. Physiol. (London) 40, iv–vii. Ho, Y.S., 1995. Adsorption of heavy metals from waste streams by peat, Ph.D. Thesis, University of Birmingham, Birmingham, UK. Iwata, H., Konuma, A., Tsumura, Y., 2000. Development of microsatellite markers in the tropical tree Neobalanocarpus heimii (Dipterocarpaceae). Mol. Ecol. 9, 1661–1686. Krishnan, K.A., Sreejalekshmi, K.G., Varghese, S., 2010. Adsorptive retention of citric acid onto activated carbon prepared from Havea braziliansis sawdust: kinetic and isotherm overview. Desalination 257, 46–52. Langergren, S., Svenska, B.K., 1898. Zur theorie der sogenannten adsorption geloester stoffe. Veternskapsakad Handlingar 24, 1–39. Langmuir, I., 1916. The constitution and fundamental properties of solids and liquids. J. Am. Chem. Soc. 38, 2221–2295. McKee, D.W., 1983. Mechanisms of the alkali metal catalyzed gasification of carbon. Fuel 62, 170–175. Nowicki, P., Pietrzak, R., 2010. Carbonaceous adsorbents prepared by physical activation of pine sawdust and their application for removal of NO2 in dry and wet conditions. Bioresour. Technol. 101, 5802–5807. Nuithitikul, K., Srikhun, S., Hirunpraditkoon, S., 2010. Influences of pyrolysis condition and acid treatment on properties of durian peel-based activated carbon. Bioresour. Technol. 101, 426–429. Nunes, A.A., Franca, A.S., Oliveira, L.S., 2009. Activated carbons from waste biomass: an alternative use for biodiesel production solid residues. Bioresour. Technol. 100, 1786–1792. Redlich, O., Peterson, D.L., 1959. A useful adsorption isotherm. J. Phys. Chem. 63, 1024–1026. Taer, E., Deraman, M., Talib, I.A., Umar, A.A., Oyama, M., Yunus, R.M., 2010. Physical, electrochemical and supercapacitive properties of activated carbon pellets from pre-carbonized rubber wood sawdust by CO2 activation. Curr. Appl. Phys. 10, 1071–1075. Tempkin, M.I., Pyzhev, V., 1940. Kinetics of ammonia synthesis on promoted iron catalyst. Acta Phys. Chim. USSR 12, 327–356. Toth, J., 1971. State equations of the solid gas interface layer. Acta Chem. Acad. Hung. 69, 311–317. Weber, W.J., Morris, J.C., 1963. Kinetics of adsorption on carbon from solution. J. Sanitary Eng. Div. Proc. Am. Soc. Civil Eng. 89, 31–59. Yener, J., Kopac, T., Dogu, G., Dogu, T., 2008. Dynamic analysis of sorption of methylene blue dye on granular and powdered activated carbon. Chem. Eng. J. 144, 400–406. Zhang, J.B., Zhong, Z.P., Zhao, J.X., Yang, M., Li, W.L., Zhang, H.Y., 2011. Study on the preparation of activated carbon for direct carbon fuel cell with oak sawdust. Can. J. Chem. Eng. 9999, 1–7.