Effects of steam activation on the pore structure and surface chemistry of activated carbon derived from bamboo waste

Applied Surface Science 315 (2014) 279–286

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Effects of steam activation on the pore structure and surface chemistry of activated carbon derived from bamboo waste Yan-Juan Zhang a,b , Zhen-Jiao Xing b , Zheng-Kang Duan a , Meng Li b , Yin Wang b,∗ a b

School of Chemical Engineering, Xiangtan University, Xiangtan 411105, China Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 36102, China

a r t i c l e

i n f o

Article history: Received 19 April 2014 Received in revised form 19 July 2014 Accepted 21 July 2014 Available online 30 July 2014 Keywords: Activated carbon Bamboo Textural property Surface chemistry Adsorption

a b s t r a c t The effects of steam activation on the pore structure evolution and surface chemistry of activated carbon (AC) obtained from bamboo waste were investigated. Nitrogen adsorption–desorption isotherms revealed that higher steam activation temperatures and/or times promoted the creation of new micropores and widened the existing micropores, consequently decreasing the surface area and total pore volume. Optimum conditions included an activation temperature of 850 ◦ C, activation time of 120 min, and steam flush generated from deionized water of 0.2 cm3 min−1 . Under these conditions, AC with a BET surface area of 1210 m2 g−1 and total pore volume of 0.542 cm−3 g−1 was obtained. Changes in surface chemistry were determined through Boehm titration, pH measurement, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Results revealed the presence of a large number of basic groups on the surface of the pyrolyzed char and AC. Steam activation did not affect the species of oxygen-containing groups but changed the contents of these species when compared with pyrolyzed char. Scanning electron microscopy was used to observe the surface morphology of the products. AC obtained under optimum conditions showed a monolayer adsorption capacity of 330 mg g−1 for methylene blue (MB), which demonstrates its excellent potential for MB adsorption applications. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Adsorption is a superior approach for removing heavy metals, organic matter, and dyes from wastewaters [1,2]. Activated carbons (ACs) are well-known adsorbents with excellent adsorption abilities for various pollutants. There is a general agreement that microporous ACs are favorable in small molecule adsorption, while mesoporous ACs are very suitable in large molecule adsorption [3]. Suitable textural properties, including a large surface area and proper pore size distribution, are also required for ACs to perform well in a particular application [4]. Besides, the surface chemistry of ACs, especially their surface oxygen-containing groups, has been proven to have significant impacts on the selectivity of ACs serving as absorbents [5]. Therefore, ACs with identical textural properties but produced from different methods may show very different adsorption capacities because of differences in their surface chemistry [6]. The unique adsorption capacity of ACs is influenced by their textural characteristics and surface chemistry. As such,

∗ Corresponding author. Postal address: Tel.: +86 592 6190787; fax: +86 592 6190787. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.apsusc.2014.07.126 0169-4332/© 2014 Elsevier B.V. All rights reserved.

investigations of the textual characteristics and surface chemistry of ACs has attracted considerable attention. Organic wastes, such as palm shell, coconut shell [7], grape seed [8], rice straw [2] and corn cob [9], are often used to produce ACs because the use of these wastes reduces production costs and contributes to disposal problems. Bamboo, a tropical plant, is widely distributed in China and has been largely used as a construction material for scaffoldings. Conversion of bamboo scaffolding waste into ACs has attracted tremendous research attention [10]. Several research works have recently been published on producing bamboo-based ACs. Hameed et al. [11], for example, prepared AC from bamboo by physicochemical activation using potassium hydroxide and carbon dioxide as activating agents. González et al. [12,13] used bamboo to prepare ACs by steam activation and potassium hydroxide activation. Liu et al. [14] prepared bamboo-based ACs by microwave-induced phosphoric acid activation. Ip et al. [15] produced large-surface area bamboo-derived AC by phosphoric activation. Mui et al. [16] prepared a series of bamboo-based ACs using HCl, HNO3 , and H2 SO4 as activating agents. Physical activation, especially steam activation, conforms to the need for cleaner production, and the product can be widely used in many fields, such as in pharmaceutical and food industries. Therefore, in production, physical activation is more favorable than chemical activation.

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However, only one report used steam as an activating agent among the works described in this study, and no report has yet been published on the influence of activation parameters on the evolution of textural characteristics of bamboo-based AC produced by steam activation. ACs prepared from various carbonaceous precursors via steam activation has been investigated by many researchers, however, when the effect of steam on the evolution of porosity is analyzed, some of the results appear to be contradictory. Guillermo et al. [17] have reported that steam activation can produce ACs with narrower and more extensive micropores than CO2 activation, whereas Eomán et al. [18] have concluded that the carbons activated by steam present wider pore size distribution and more obvious macropores. In order to produce bamboo-based AC with desired pore size via steam activation, the knowledge on the evolution of porosity under different processing parameters is of great importance. X-ray photoelectron spectroscopy (XPS) is widely known as a surface-sensitive quantitative spectroscopic technique that can measure the elemental composition and chemical states of elements present in a material. However, no XPS information on AC derived from bamboo by steam activation can be found in the literature. In view of the above facts, the main purpose of the present study is to bring plain understanding to the evolution of surface area and porosity of AC produced from bamboo waste with varying steam activation parameters as well as changes in surface chemistry from the bamboo precursor to the pyrolyzed char and to AC systematically. Textural properties were explored by N2 adsorption–desorption. Surface chemistry was analyzed using Boehm titration, pH measurement, Fourier-transform infrared spectroscopy (FTIR), and XPS analyses. In addition, the surface morphologies of the materials were analyzed using scanning electron microscopy (SEM). The cationic dye methylene blue (MB) was chosen as a model compound to evaluate the adsorption capacity of the AC prepared under optimum conditions. 2. Material and methods 2.1. Materials Waste bamboo materials from a construction site in Xiamen were used to prepare ACs. The materials were dried in an air oven at 50 ◦ C for 8 h to remove most of the moisture and then chipped to small pieces of about 20 mm × 10 mm × 5 mm. The average proximate analyses of the bamboo precursor were measured as 71.7, 26.6 and 1.7% for volatile matters, fixed carbon and ash on dry basis, respectively. The average ultimate analyses were 47.1, 6.5, 45.8 and 0.6% for C, H, O and N contents, respectively. 2.2. Preparation of activated carbon This work adopts the established two-step method for preparing AC. Both pyrolysis and activation were carried out in a quartz tube reactor (450 mm height × 50 mm inner diameter) placed in a vertical tube furnace. For pyrolysis, about 100 g of bamboo chips was placed on the air distributor of the reactor. The furnace temperature was then increased and held at 450 ◦ C for 60 min. About 15 g of the resulting char was then activated at 550–850 ◦ C for 60–150 min under a steam flush of 0.2 cm3 min−1 generated from deionized water in a heated tube. The activating molar flow rate per unit weight of char was calculated to be 0.045 mol g−1 h−1 . The same heating rate (i.e., 5 ◦ C min−1 ) was used in the pyrolysis and activation process. Nitrogen, as the protecting gas, was flushed into the reactor at a rate of 500 mL min−1 throughout pyrolysis and activation. The resulting ACs were stored in desiccators for further analysis and

characterization. The percent yields after pyrolysis or activation were obtained by determining weight differences. Activation burnoff was calculated as: wc − wa Burn − off (%) = (1) wc Where wc and wa are the masses of carbons before and after activation, respectively, on a dry-ash-free basis. The nomenclature used for pyrolyzed char and ACs obtained in this study are as follows: C denotes char, followed by the pyrolysis temperature and time, i.e., C450-60 refers to char pyrolyzed at 450 ◦ C for 60 min. Similarly, AC is followed by the activation temperature and time, i.e., AC850-60 refers to AC activated at 850 ◦ C for 60 min. 2.3. Characterization of materials Ultimate analysis of carbon, hydrogen, and nitrogen in the materials was performed using a Vario MAX elemental analyzer. The oxygen content was obtained by difference of carbon, hydrogen and nitrogen. Textural properties were determined using N2 adsorption–desorption isotherms at −196 ◦ C with a Micromeritic ASAP 2020. Prior to measurement, the sample was degassed under vacuum at 350 ◦ C for 4 h. The surface area was measured using the Brunauer–Emmet–Teller (BET) equation obtained from the nitrogen adsorption data. Total pore volume (Vtot ) was calculated from the nitrogen adsorbed at a relative pressure of 0.99. The micropore volume (Vmic ) and pore size distribution were calculated by nonlocal density functional theory (NLDFT) model for carbon slit-shaped pores from the measured isotherms. The median pore width of slit-shaped micropores (Dp ) was estimated by the Horvath–Kawazoe (HK) method [19]. Boehm titration was performed to determine total acidic and basic groups. Total acidic and basic groups were neutralized with solutions of 0.05 mol L−1 NaOH and 0.1 mol L−1 HCl, respectively. pH was measured according to the Chinese national standards GB/T 12496.7–1999. FTIR spectra of the samples were obtained using a NicoletTM iSTM10 FTIR spectrometer. Oven-dried samples were mixed with potassium bromide in an agate mortar, and the resulting mixtures were pressed into pellets for analysis. The spectra were recorded from 4000 cm−1 to 400 cm−1 , and 16 scans were taken with a resolution of 4 cm−1 . XPS analysis was carried out at normal emission using a Kratos AXIS ULRADLD spectrometer equipped with a monochromatic Al K ␣ x-ray source (1486.6 eV). The pass energy was set to 160 eV for the survey spectrum and 20 eV for the high-resolution spectrum. The base pressure of the analysis chamber was about 2 × 10−9 torr. The binding energy was calibrated at 284.8 eV for the C1s level. The energy steps of the survey and high-resolution spectra were 1 and 0.01 eV, respectively. All samples were analyzed under identical conditions. Micromorphological characteristics of the materials were determined by SEM using a Hitachi S-4800. 2.4. MB adsorption Around 0.1 g of AC was added to a series of 25 mL of MB solution in 100 mL Erlenmeyer flasks with initial concentrations ranging from 150 mg L−1 to 1500 mg L−1 . The MB solutions were prepared without pH adjustment. The flasks were shaken at 25 ◦ C for 24 h until equilibrium was achieved. The concentration of MB remaining in solution was measured using a UV-1100 spectrophotometer at 665 nm. The equilibrium adsorption quantity (qe [mg g−1 ]) was cal(C −C )V culated as follows:(2)qe = 0 M e

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Table 1 Yield, ultimate analysis, and textural characteristics of chars pyrolyzed at 450 ◦ C and for 60 min. Ultimate analysis (wt. %)

Yield (wt. %)

C

H

N

Oa

78.7

3.4

1.2

16.7

a

29.4

Textural characteristics SBET (m2 g−1 )

Vtot (cm−3 g−1 )

Vmic (cm−3 g−1 )

Vmic /Vtot (%)

Dp (nm)

363

0.15

0.13

86.7

0.93

Obtained by difference.

Where C0 (mg L−1 ) and Ce (mg L−1 ) are the initial and equilibrium concentrations of the MB solutions, respectively; V (L) refers to the volume of solution; and M (g) is the mass of the AC used. 3. Results and discussion 3.1. Pyrolysis Table 1 shows the ultimate analyses, textural characteristics, and yields of the char produced at pyrolysis temperature of 450 ◦ C and pyrolysis time of 60 min. The sample obtained was mostly rich in carbon followed by oxygen, which indicates that pyrolysis enriches the carbon content by eliminating non-carbon species [20]. The BET specific surface area and total volume of the pyrolyzed bamboo char were 363 m2 g−1 and 0.150 cm−3 g−1 , respectively, which are comparable with the values of chars produced from other lignocellulosic biomass types [21]. This large surface area obtained renders the char substantially easier to activate because of the greater availability of surface on which reactions may take place, especially in the case where reaction between the activating agent and solid carbon is the controlling step of the reaction rate [7]. The char yield was about 30%, similar to the char yield obtained from pyrolysis of a lignocellulosic precursor.

surface area increment was very small, considering that the pyrolyzed char before activation had a surface area of 363 m2 g−1 . This change may be explained by the fact that at low temperatures, steam is highly likely to remove disorganized carbons resulting from deposition and decomposition of tar first and then promote the development of new pores [24]. The pore structure is not adequately developed at low temperatures but becomes better as the temperature increases. At temperatures over 650 ◦ C, the surface area and total pore volume remarkably increased. Endothermic reactions between carbon and steam are favored under elevated temperatures, which produce well-developed carbons. Similar trends in total pore volume were observed, as shown in Table 2. A slight increase in total pore volume at low temperatures and a distinct increase in volume at 850 ◦ C were observed. Table 2 shows that while the ACs produced were mainly microporous, the micropore volume ratio (Vmic /Vtot ) decreased from 89.0% to 84.6% with increasing the activation temperature. This change suggests that the development of new micropores is accompanied by the widening of existing pores, which transforms part of the micropores into mesopores. The median pore width of

3.2. Activation Activation has a crucial function in developing porosity and creating an ordered structure [22]. To deduce textural properties and determine the influences of activation variables (e.g., activation temperature and activation time) clearly in this study, nitrogen adsorption isotherms of all AC samples were utilized. 3.2.1. Effects of activation temperature Fig. 1a displays the nitrogen adsorption–desorption isotherms of the ACs prepared at different activation temperatures. The shapes of the N2 adsorption-desorption isotherms of the ACs indicate changes in pore size [21]. The curve shapes of the ACs prepared at temperatures below 750 ◦ C show a typical type I isotherm, which indicates the presence of predominantly microporous structures. AC prepared at 750 ◦ C presented an adsorption isotherm with a discernible slope at high pressure, which implies a wide distribution of micropores. The small adsorption–desorption hysteresis loop observed which associated with capillary condensation occurring in mesopores shows the emergence of mesopores [23]. The isotherm of AC prepared at 850 ◦ C suggests an increasing number of mesopores. According to the IUPAC definition, pores within porous solids may be classified into three groups, namely: micropores (size less than 2 nm), mesopores (size between 2 nm and 50 nm), and macropores (larger than 50 nm). The pore size distribution displayed in Fig. 1b shows that carbons activated at lower temperature are predominantly microporous but mesopores gradually developed with increasing activation temperature. The surface area and pore structure parameters of ACs obtained under different activation temperatures are presented in Table 2. The surface area clearly increased with increasing activation temperature. At an activation temperature of 550 ◦ C, however, the

Fig. 1. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of activated carbons prepared at various activation temperatures (550–850 ◦ C) and an activation time of 60 min.

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Table 2 Effects of activation temperature and time on the surface area, porosity, yield and burn-off of carbons. Activation varies a

Temp. ( C) 550 650 750 850 Time b (min) 80 100 120 150 a b

SBET (m2 g−1 )

Vtot (cm3 g−1 )

Vmic (cm3 g−1 )

Vmic /Vtot (%)

Dp (nm)

Yield (wt.%)

Burn-off (wt.%)

459 612 773 870

0.192 0.256 0.329 0.375

0.171 0.225 0.280 0.317

89.0 87.7 85.0 84.6

0.77 0.84 0.93 0.93

89.6 76.0 64.7 51.7

11.0 25.7 36.7 50.1

995 1022 1210 1149

0.434 0.448 0.542 0.519

0.364 0.373 0.447 0.426

84.0 83.2 82.5 82.0

0.94 0.94 0.96 1.07

49.5 44.9 33.8 21.5

53.0 57.6 68.2 79.8

◦

Activation was conducted under the same activation time of 60 min. Activation was conducted under the same activation temperature of 850 ◦ C.

micropores (Dp ) also increased with increasing activation temperature because of pore-widening.

3.2.2. Effects of activation time Fig. 2a shows the nitrogen adsorption–desorption isotherms of ACs prepared at 850 ◦ C and activation times of 80–150 min. The presence of a large hysteresis loop in all isotherms indicates the notable contribution of mesopores to the porosity of the ACs, which can be confirmed by the pore size distribution shown in Fig. 2b. The type of all hysteresis loops are ascribed to Type H4 loop, which is associated with narrow slit-like pores [23]. The volume of adsorbed nitrogen decreased as the activation time increased to 150 min; this phenomenon will be discussed more thoroughly in the following section.

Fig. 2. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of activated carbons prepared at 850 ◦ C and various activation times (80–150 min).

Table 2 further shows the surface area and pore structure parameters of ACs produced under different activation times. A clear increase in surface area and pore volume can be seen, and maximum values of 1210 m2 g−1 and 0.542 cm3 g−1 , respectively, were achieved at 120 min. Thereafter, further increases in activation time to 150 min caused a slight decrease in both surface area and pore volume. These findings are attributed to activation during the initial stage, which facilitates the opening of rudimentary pores formed during pyrolysis and development of new pores [25]. After a long period of activation, pore-widening becomes a dominant effect whereas pore-deepening and new pore formation becomes minor activities. Thus, more meso- and macropores are evolved and the BET surface area and pore volume decrease with increasing activation time. This decrease in pore volume, thus, causes reductions in adsorbed nitrogen volume. The micropore volume ratio (Vmic /Vtot ) decreased from 84.0% to 82.0% and Dp increased from 0.94 nm to 1.07 nm (Table 2) with increasing activation time, which further confirms that steam activation causes obvious pore-widening effects. Similar effects of activation time on the evolution of surface area and pore structure of ACs have been observed in other ACs produced from other precursors [26,27]. AC yields decreased from 89.6% to 21.5% with increasing activation temperature and activation time (Table 2). This decrease is caused by a combination of continuous volatile matter devolatilization and carbon-steam reaction [28]. The main reason for the decrease in carbon yields at low temperatures is devolatilization of volatile matter [9], while the main reason for the decrease in carbon yields at high temperatures is carbon gasification [10]. The decrease in yield was accompanied by an increase in AC burn-off, which is caused by the burn-off of fixed carbon in the AC [29]. The carbon burn-off (from 11.0% to 79.8%) in this study was close to the result obtained by Jaime et al. [30], who used Mucuna mutisiana seeds to prepare AC with steam. 3.2.3. Effects on surface chemistry The surface chemistries of ACs are derived from the various hetero-atoms on the surfaces of the materials. These hetero-atoms include oxygen, nitrogen, sulfur, etc., among others, which either originate from the raw material or are incorporated into the carbon surface during activation or subsequent chemical modification of the materials [31]. Several researchers have focused on oxygen because the presence of oxygen-containing surface groups determines the acid-base characteristics of a material and affects the electrochemical properties of carbons [32], thereby influencing their adsorption capacities as adsorbents. The species and abundance of hetero-atoms and surface oxygen-containing groups rely on the raw material and preparation method used [33]. Table 3 shows results of the ultimate analysis of C450-60 and AC850-120. Both samples predominantly consisted of carbon and contents of hetero-atoms, such as hydrogen, oxygen and nitrogen,

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Table 3 Results of elementary analysis, Boehm titration, and pH for pyrolyzed char and activated carbon. Sample

C450-60 AC850-120 a

Total acidic groups (␮eq g−1 )

Ultimate analysis (wt.%) a

C

H

O

78.62 81.18

3.38 1.31

16.85 16.73

Total basic groups (␮eq g−1 )

pH

1030 1080

9.62 ± 0.02 9.97 ± 0.03

N 1.15 0.78

121 50

Obtained by difference

decreased after activation, which indicates decreases of functional groups containing these atoms. The pH and total acidic and basic groups of C450-60 and AC850120 are also summarized in Table 3. Both acidic and basic groups were present on the surface of the pyrolyzed char and AC. However, the content of total basic groups was substantially higher than that of total acidic groups. Steam activation slightly decreases the content of acidic groups but slightly increases that of basic groups, which therefore increases pH. FTIR is frequently used to analyze the surface chemistry of carbons [28,34,35] as it can provide useful information on the chemical nature of a material. Fig. 3 presents the FTIR spectra of bamboo, C450-60, and AC850-120. The bamboo precursor exhibited more spectral bands than the char and AC. Noticeable differences, which mainly concern band intensities, were also observed between the pyrolyzed char and AC. The intense band at about 3450 cm−1 seen in all samples was ascribed to the O H stretching vibrations of hydrogen-bonded hydroxyl groups (free and phenol) [36]. Adsorbed water that had not been fully removed after oven-drying of the samples could participate in the formation of hydrogen bonds. The two weak adsorption bands at 2920 and 2850 cm−1 in the spectra of char and AC were attributed to C H asymmetric stretching vibrations and C H symmetric stretching vibrations, respectively, which indicates the presence of methyl and methylene groups [35]. In the bamboo precursor, these two bands merged to form a wide band at 2914 cm−1 . The band at about 1734 cm−1 in the spectrum of the bamboo precursor was assigned to stretching vibrations of the C O group in ketones, aldehydes, lactones, and carboxylic groups [37]. The band corresponding to C O stretching vibrations in the FTIR spectra of the char and AC was located at about 1630 cm−1 . Low-frequency values for C O stretching vibrations were caused by conjugation of C O groups and C C bonds [38]. The presence of aromatic rings was proven by bands at about 1560 cm−1 , which are related to ring vibrations in large condensed aromatic carbon skeletons [5]. The sharp band at about 1384 cm−1 in all samples was assigned to the bending vibrations of CH3 groups. According to Serrano et al. [35], bands seen1249–1050 cm−1 in the spectrum of the bamboo

Fig. 3. FTIR spectra of (a) the bamboo precursor, (b) C450-60, and (c) AC850-120.

precursor may be assigned to C O stretching vibrations of different functional groups. The band at 1249 cm−1 may be related to esters and epoxides as well as acyclic C O C groups conjugated with C C. The shoulder bands located at 1157 cm−1 were attributed to C O stretching vibrations in a six-membered ring ether structure. The small shoulder band at 1114 cm−1 and the intense band at 1050 cm−1 were assigned to the C–O stretching vibrations of secondary and primary C OH, respectively. Moreover, the intense band at 1050 cm−1 observed in the spectrum of the bamboo precursor weakened in the spectra of the char and AC, which implies a decrease in the primary C OH structure. The broad band between 1200 and 990 cm−1 in the spectra of the char and AC replaced the weak shoulder in the spectrum of the bamboo precursor, which indicates an increase in ether structures after pyrolysis or activation. The weak bands at about 900 and 664 cm−1 correspond to the out-of-plane bending vibrations of C H and O H, respectively. To obtain more insights into changes in the surface chemistry of the char and AC during activation, C450-60 and AC850-120 were investigated by XPS. Fig. 4 shows the XPS survey spectra of C450-60 and AC850120. Both spectra showed distinct C1s and O1s peaks with binding energies of ca. 285 and 532 eV, respectively, which illustrates that the samples mainly consist of carbon and oxygen. Bands with low intensities were observed at around 398 and 102 eV and are attributed to N1s and Si2p, respectively. The relative contents of carbon and oxygen were determined from the XPS survey spectra, and the atomic ratios of O1s/C1s were calculated to be 0.18 and 0.15 for the char and AC, respectively. This finding indicates a decrease in the total amount of oxygenated groups on the carbon surface during activation. The results of the XPS survey spectra are consistent with the ultimate analysis results. The high-resolution C1s and O1s spectra were curve-fitted using mixed Gaussian-Lorentzian functions by XPS Peak 4.1 after background subtraction according to Shirley. Fig. 5 shows the typical high-resolution XP spectra of the materials together with the fitted C1s and O1s peaks. The results of C1s and O1s curve-fitting are summarized in Table 4. Findings indicate that sp3 bulk carbon (peak 2) decreased while the graphitic carbon (peak 1) significantly increased and became

Fig. 4. XPS survey spectra of (a) C450-60 and (b) AC850-120.

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Fig. 5. High-resolution XPS spectra for C1s of (a) C450-60 and (b) AC 850-120. High-resolution XPS spectra for O1s of (c) C450-60 and (d) AC 850-120.

the main component after activation. Activation exhibited no effect on the distribution of carbon species bonded to oxygen-containing groups but the intensity of the peak varied, as demonstrated by peaks 3, 4, and 5 in Fig. 5a and b. After activation, the relative contents of alcohols and ethers (peak 3) increased, but the relative contents of carbonyls, quinones, and ketones, (peak 4) as well as carboxyl and ester groups (peak 5) decreased. These results are in accordance with the O1s findings, in which the contents of ether structures in esters and anhydrides, as well as oxygen atoms in hydroxyl groups (peak II), increased. Compared with the previous FTIR analysis, increases in peak 3 and peak II after activation were attributed to increases in ether-type structures. Results thus far indicate that activation facilitates the extension of graphic regions by removing sp3 bulk carbon, and the main oxygen-containing groups in pyrolyzed char are carbonyl and quinones and that the main oxygen-containing groups in ACs are ethers.

3.3. Analysis of morphology SEM was used to study changes in morphology in the materials during pyrolysis and activation. Fig. 6 shows a 1000× magnified section of the (a) bamboo precursor, (b) C450-60, and (c) AC850-120. The bamboo precursor clearly showed the canal structure associated with cellulose fibers, which is an excellent texture for AC production as the activating agent could easily diffuse into the carbon micropores and develop a large surface area [37]. During pyrolysis most of the non-carbon elements were removed with the basic skeleton of the polymer remaining, which produced a char retaining the shape of the precursor and formed the initial porosity of the char. Fig. 6c shows that the pore walls of the AC became significantly thinner compared with that of the char, which indicates that subsequent steam gasification occurs on the surfaces of the canals. Fig. 6d shows a 5000× magnified SEM photo of the AC powder. The micrograph shows that considerable porosity is present in

Table 4 Deconvolution results of the C1s and O1s regions of XPS spectra of C450-60 and AC850-120. Region

Peak

Position (eV)

Assignment [29,40,41]

C1s

1 2 3 4 5 I II

284.7 285 285.8 287.2 289.3 531.2 532.5

III

533.9

Graphitic carbons Sp3 bulk carbons on the edge of the graphene sheets carbon in alcohol and ether carbon in carbonyl, quinones and ketones carbon in carboxyl and ester groups Carbonyl, oxygen in quinones Non-carbonyl oxygen (ether structures) in esters and anhydrides, oxygen atoms in hydroxyl groups Oxygen atoms in carboxyl groups ( COOH and COOR)

O1s

Relative content (%) C450-60

AC850-120

35.6 39 11.2 11 3.2 38.2 27.0

50.4 21.7 17 8 2.9 26.7 41.1

34.7

32.0

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Fig. 6. SEM microphotographs of (a) the bamboo precursor (1000 × ), (b) C450-60(1000 × ), (c) AC850-120(1000 × ), and (d) AC850-120 powder (5000 × ).

AC in the form of an interconnected network, which accounts for the high BET surface area and pore volume of the material.

Freundlich isotherm: ln qe = ln KF +

3.4. Methylene blue adsorption The adsorption isotherm of MB was obtained to evaluate the liquid adsorption capacity of AC prepared under optimum conditions (AC850-120). Fig. 7 shows the adsorption isotherm of MB at 25 ◦ C on AC850-120. The equilibrium data were further fitted to the linear forms of the Langmuir and Freundlich isotherm models presented below. Langmuir isotherm: Ce 1 = + qe Q0 b

1 Q0

Ce

(3)

1 ln Ce n

(4)

where Ce (mg L−1 ) is the equilibrium MB concentration; qe (mg L−1 ) refers to the mass of adsorbate adsorbed per unit mass of adsorbent; Qo and b are Langmuir constants related to adsorption capacity and affinity, respectively; and KF and n are Freundlich constants related to adsorption capacity and intensity, respectively. The coefficients (R2 ) for Langmuir and Freundlich isotherm models are 0.999 and 0.957, respectively which indicated that the Langmuir isotherm model was more suitable for describing the adsorption of MB onto the AC sample than the Freundlich model, and also confirmed the monolayer adsorption of MB on AC. The value of the dimensionless equilibrium parameter (RL ) was 0.003, which demonstrates that the conditions used in this study are favorable for MB adsorption onto AC [11]. The maximum monolayer adsorption capacity of MB on the bamboo-based AC obtained by steam activation was 330 mg g−1 , which is larger than the capacity of most ACs prepared from other lignocellulosic materials [39] and comparable with bamboo-based carbon produced by physiochemical activation [11]. 4. Conclusions

Fig. 7. Adsorption isotherm of MB onto AC850-120 (conditions: adsorbent dose = 4 g/L; temperature = 25 ◦ C; contact time = 24 h; stirring rate = 160 rpm; pH: without adjusted).

This study demonstrated that bamboo waste may be used to prepare microporous AC via pyrolysis and steam activation. The surface area evolution, porosity, yield, and burn-off of AC were significantly affected by the activation conditions. In general, higher steam activation temperatures and/or times facilitated the formation of new micropores but widened existing micropores. Considering the evident pore-widening effect caused by steam activation, prolonged activation eventually led to reductions in BET surface area and total pore volume. Optimum conditions for preparing AC from char include activation temperature of 850 ◦ C,

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activation time of 120 min, and steam flush of 0.2 cm3 min −1 generated from deionized water. Subsequent pyrolysis at 450 ◦ C for 60 min is recommended. Steam activation, promotes the extension of graphic regions by removing sp3 bulk carbon. As to the oxygen-containing groups, similar oxygen-containing groups were observed between the pyrolyzed char and AC, which indicates that activation does not affect the species of oxygen-containing groups but changes the amounts of these species in the products. The main oxygen-containing groups in pyrolyzed char were carbonyl and quinones; in AC, the main oxygen-containing groups were ethers. The basic nature of the AC indicates its potential application in which basic groups are desired such as in adsorption of cationic dyes and heavy metals. The SEM photographs demonstrated pore development in the char during activation. The MB adsorption experiment revealed the immense potential of AC derived from bamboo waste by steam activation for use in MB removal from aqueous solutions. Acknowledgments Financial supports for this work by National Natured Science Foundation (No.51176197), Main Project of Chinese Academy Sciences (KZZD-EW-16) and Xiamen Science & Technology Major Program (No. 3502Z20131018) are gratefully acknowledged. References [1] S. Sato, K. Yoshihara, K. Moriyama, M. Machida, H. Tatsumoto, Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution, Appl. Surf. Sci. 253 (2007) 8554–8559. [2] L. Lin, S.-R. Zhai, Z.-Y. Xiao, Y. Song, Q.-D. An, X.-W. Song, Dye adsorption of mesoporous activated carbons produced from NaOH-pretreated rice husks, Bioresource Technol. 136 (2013) 437–443. [3] C.M. Srinivasakannan, A.B. Zailani, Production of activated carbon from rubber wood sawdust, Biomass Bioenerg. 27 (2004) 89–96. [4] A. Klijanienko, E. Lorenc-Grabowska, G. Gryglewicz, Development of mesoporosity during phosphoric acid activation of wood in steam atmosphere, Bioresource Technol. 99 (2008) 7208–7214. [5] P. Burg, P. Fydrych, D. Cagniant, G. Nanse, J. Bimer, A. Jankowska, The characterization of nitrogen-enriched activated carbons by IR, XPS and LSER methods, Carbon 40 (2002) 1521–1531. [6] S. Timur, I.C. Kantarli, S. Onenc, J. Yanik, Characterization and application of activated carbon produced from oak cups pulp, J. Anal. Appl. Pyrol. 89 (2010) 129–136. [7] W.M.A.W. Daud, W.S.W. Ali, Comparison on pore development of activated carbon produced from palm shell and coconut shell, Bioresource Technol. 93 (2004) 63–69. [8] I. Okman, S. Karagöz, T. Tay, M. Erdem, Activated carbons from grape seeds by chemical activation with potassium carbonate and potassium hydroxide, Appl. Surf. Sci. 293 (2014) 138–142. [9] N.V. Sych, S.I. Trofymenko, O.I. Poddubnaya, M.M. Tsyba, V.I. Sapsay, D.O. Klymchuk, A.M. Puziy, Porous structure and surface chemistry of phosphoric acid activated carbon from corncob, Appl. Surf. Sci. 261 (2012) 75–82. [10] K.K.H. Choy, J.P. Barford, G. McKay, Production of activated carbon from bamboo scaffolding waste—process design, evaluation and sensitivity analysis, Chem. Eng. J. 109 (2005) 147–165. [11] B.H. Hameed, A.T.M. Din, A.L. Ahmad, Adsorption of methylene blue onto bamboo-based activated carbon: kinetics and equilibrium studies, J. Hazard. Mater. 141 (2007) 819–825. [12] P.G. González, Y.B. Pliego-Cuervo, Physicochemical and microtextural characterization of activated carbons produced from water steam activation of three bamboo species, J. Anal. Appl. Pyrol. 99 (2013) 32–39. [13] P. González-García, T.A. Centeno, E. Urones-Garrote, D. Ávila-Brande, L.C. OteroDíaz, Microstructure and surface properties of lignocellulosic-based activated carbons, Appl. Surf. Sci. 265 (2013) 731–737. [14] Q.-S. Liu, T. Zheng, P. Wang, L. Guo, Preparation and characterization of activated carbon from bamboo by microwave-induced phosphoric acid activation, Ind. Crop. Prod. 31 (2010) 233–238.

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