Preparation of Activated Carbons from Oreganum Stalks by Chemical

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Energy & Fuels 2006, 20, 2636-2641

Preparation of Activated Carbons from Oreganum Stalks by Chemical Activation Serkan Timur,† I. Cem Kantarli,† Erdinc Ikizoglu,‡ and Jale Yanik*,† Faculty of Science, Department of Chemistry, Ege UniVersity, 35100 BornoVa, Izmir, Turkey, and Faculty of Engineering, Department of Bio-Engineering, Ege UniVersity, 35100 BornoVa, Izmir, Turkey ReceiVed May 17, 2006. ReVised Manuscript ReceiVed July 11, 2006

In this study, the activated carbons were prepared from Oreganum stalks by chemical activation with phosphoric acide and zinc chloride. The influence of process variables such as impregnation ratio and impregnation time on the surface area of activated carbons was studied to optimize these parameters. The most important parameter was found to be the impregnation ratio. Under optimum conditions, for the activating agents phosphoric acid and zinc chloride, activated carbons with a surface area of 719 m2 g-1 and 944 m2 g-1 were obtained, respectively. Zinc chloride produced activated carbon with a higher micropore volume compared to that produced by phosphoric acid activation. The aqueous adsorption tests showed that the activated carbons had a notable adsorption capacity for both methylene blue and phenol. The adsorption capacity of activated carbon prepared with zinc chloride was more than that of activated carbon prepared using phosphoric acid. In addition, 73-86% of activating agent was recovered and reused in impregnation step.

1. Introducton Activated carbon can be prepared from a wide range of carbonaceous raw materials inculding peat, lignite, wood, and various agricultural byproducts. Activated carbons are commercially produced by either physical or chemical activation method. The physical activation method involves carbonization of the raw material and the subsequent activation at high temperature in a carbon dioxide or steam atmosphere. The chemical activation method involves the carbonization of the raw material previously impregnated with a chemical agent such as zinc chloride, phosphoric acid, potassium hydroxide, etc. The advantage of chemical activation over physical activation is that it can be performed in only one step and at a relatively low temperature.1,2 In addition, chemical activation produces a much higher yield than the physical activation and leads to a decrease in the mineral matter content.3 However, there also are some disadvantages of the chemical activation process, such as the corrosiveness of the process and the washing steps. A large number of agricultural byproducts, such as coconut shells,4-6 palm-kernel shells, wood chips,7 sawdust,8,9 corn cobs,2 seeds,10 etc., have been successfully converted into activated carbons by chemical activation. The qualities and characteristics of * Corresponding author. Tel.: +90 232 3884000-2386. Fax: +90 232 3888264. E-mail: [email protected]. † Department of Chemistry, Ege University. ‡ Department of Bio-Engineering, Ege University. (1) Ahmadpour, A.; Do, D. D. Carbon 1997, 35 (12), 1723-32. (2) Tsai, W. T.; Chang, C.; Lee, S. L. Bioresour. Technol. 1998, 64, 211-17. (3) Lillo-Rodenas, M. A.; Cazorla-Amoros, D.; Linares-Solano, A. Carbon 2003, 41, 267-75. (4) Laine, J.; Yunes, S. Carbon 1992, 30 (4), 601-4. (5) Laine, J.; Calafat, A.; Labady, M. Carbon 1989, 27, 191-5. (6) Hu, Z.; Srinivasan, M. P. Microporous Mesoporous Mater. 1999, 27, 11-8. (7) Jagtoyen, M.; Derbyshire, F. Carbon 1993, 31 (7), 1185-92. (8) Ferraz, M. C. A. Fuel 1998, 67, 1237-41 (9) Srinivasakannan, C.; Abu Bakar, M. Z. Biomass Bioenergy 2004, 27, 89-96. (10) Gergova, K.; Petrov, N.; Eser, S. Carbon 1994, 32, 693-702.

activated carbons depend on the properties of the starting materials as well as the activation methods and processes.11 Oregano is an aromatic and medical plant. Turkey is one of the world’s largest suppliers of oregano. Oregano grows wild in the Mediterranean and is harvested by locals. The part of the plant used is the leaves. The main constituents of the leaves are the essential oils and monoterpene hydrocarbons. On the other hand, Oreganum stalks are abundant agricultural wastes from the harvest of Oreganum, and their accumulation around fields creates big problems. They are disposed of by burning on the field, which results in air pollution and a decrease in soil quality. The objective of the present work was to investigate the feasibility of producing activated carbons from agricultural waste Oreganum stalks. The activated carbons were prepared by chemical activation of Oreganum stalk with phosphoric acid and zinc chloride. Moreover, recovery of chemical reagent was also investigated. Also, Oreganum stalks as raw precursors are used for the first time in the present investigation for the preparation of activated carbon. 2. Experimental Section 2.1. Biomass. The stalks of Oreganum onites L supplied from a field were air-dried for several days and ground below 1 mm. Some properties of this material are given in Table 1. 2.2. Production of Activated Carbon. Air-dried biomass (75 g) was mixed in a beaker with 250 mL of H3PO4 or ZnCl2 solution with varying concentrations. H3PO4 or ZnCl2 solution was used in impregnation ratios of 0.4:1, 0.6:1, 0.8:1, 1:1, and 1.2:1 of weight of impregnation reagent/weight of biomass (referred to as 40, 60, 80, 100, and 120 wt % loading). The slurry was then dried in a moisture oven at 105 °C for a predetermined time. To explore the effect of the impregnation time, the impregnated sample with 40% loading was dried in a moisture oven at 110 °C for different periods (between 2 and 24 h). (11) Guo, J.; Lua, C. J. Porous Mater. 2000, 7, 491-7.

10.1021/ef060219k CCC: $33.50 © 2006 American Chemical Society Published on Web 09/12/2006

ActiVated Carbons from Oreganum Stalks

Energy & Fuels, Vol. 20, No. 6, 2006 2637

3. Results and Discusion

Table 1. Properties of Biomass proximate analysis (as received, wt %) moisture 9.0 ash 4.0 volatile matter 73.0 fixed carbon 23.0 ultimate analysis (dry, %) C H N S Oa a

45.89 6.32 1.65 0.25 45.89

Calculated from difference.

The impregnated biomass samples were pyrolyzed in a fixedbed design and stainless steel reactor (L, 210 mm; Ø, 60 mm). which was placed in an electrical heating furnace. The reactor was heated at a heating rate of 5 °C min-1 to 600 °C and held at this temperature for 2 h. The reactor was continuously purged with nitrogen at a flow rate of 25 mL min-1. After pyrolysis, the furnace was cooled to room temperature in a nitrogen gas stream overnight. The char was boiled with 200 mL of 10 % HCl solution for 60 min, filtered in a vacuum flask, and washed with hot water and finally with cold water to remove the chloride ions and other inorganics. After washing, activated carbon samples were dried at 110 °C for 24 h and weighed (m2). The yield of activated carbon from chemical activation was calculated by

% yield ) (m2/M) × 100 where M ) initial mass of biomass. For comparison, the char obtained from Oreganum stalk without chemical activator was also subjected to washing with acid and water. In a series of experiments, first 250 mL of filtrate was collected and its chemical content was determined to reuse as the impregnation solution. The amounts of phosphate and zinc in filtrates were determined by the gravimetric method, by precipitation as magnesium ammonium phosphate and as zinc pyrophosphate. 2.3. Characterization of Oreganum Stalks and Activated Carbons. The elemental analysis (C, H, N, and S) of biomass was determined with an elemetal analyzer (LECO CHNS 932). Proximate analyses were carried out by using the following standard methods: ASTM D3173 (moisture), ASTM D3175 (volatile matter), and ASTM D3174 (ash). The BET (BrunauerEmmett-Teller) surface area measurements were obtained from nitrogen adsorption isotherms at 77 K using a Micrometrics FlowSorb II-2300 surface area analyzer. An automatic equipment (TriStar 3000) was used to obtain the nitrogen adsorption isotherms. The micropore volumes and the external area were calculated by the t-plot approach. The scanning electron micrograph (SEM) analyses were recorded by using JEOL-FEG. 2.4. Aqueous Adsorption Characteristics. Selected activated carbons were investigated for their aqueous adsorption characteristics using phenol and methylene blue. The methylene blue and phenol adsorption isotherms were carried out using a batch equilibration technique in a 250 mL conical flask at room temperature. Each flask was filled with 100 mL of methylene blue (MB) or phenol at a known concentration (ranging between 100 and 400 mg L-1 for MB and 25-200 mg L-1for phenol) and 0.1 g of activated carbon. The flask was then shaken for a determined equilibrium time (24 and 4 h for MB and phenol, respectively) and filtered through Whatman No. 1 filter papers. The filtrate was analyzed for adsorbate concentration using the UV spectrophotometer at 665 nm (MB) and 269 nm (phenol).

In chemical activation, the yield and properties of activated carbon depend on the impregnation conditions, such as impregnation ratio and time, as well as carbonization conditions, such as activation temperature, soaking time, activation atmosphere, and heating rate. All these process variables vary with the carbon precursor and the activating agent. In earlier studies, activation temperature and impregnation rate received more attention. The effect of activation temperature on the characteristics of the activated carbon was deeply investigated. By increasing the temperature, an increase in the mesopore volume corresponded to a decrease in microporosity. After a certain temperature, surface area decreased as the temperature increased, which led to contraction of the carbons’ porous structures.12,13 The highest surface areas obtained by H3PO4 and ZnCl2 activation were generally found for activation temperatures lower than 600 °C. In most of the studies, the temperature of 500 °C was chosen as optimum for selected lignocellulosic material and impregnation ratio.1,2,12,14-18 Ahmadpour and Do1 activated macadamia nutsell with 100 wt % ZnCl2 at temperatures of 500, 600, and 700 °C. The surface area decreased from 1718 m2 g-1 to 1300 m2 g-1 with an increase in temperature. Similarly, in the activation of corn cob with ZnCl2 at 100 wt % of impregnation ratio, the surface areas decreased from 911 m2 g-1 to 786 m2 g-1 as the temperature increased in the range of 500-700 °C.2 In contrast, it was found that the maximum surface areas were obtained at the carbonization temperature of 600 °C in the preparation of activated carbon from lignin by using H3PO4 and ZnCl2.19 On the other hand, the influence of carbonization temperature on the surface areas of activated carbons was found to be dependent on the impregnation ratio and activating agent.14 Activation time (soaking time) is another critical parameter that affects the quality of activate carbon. Srinivasakannan and Abu Bakar9 studied the production of activated carbon from rubber wood sawdust using H3PO4 at 400 and 500 °C with a variation of the activation time. For the impregnation ratio of 1.5, an increase in iodine number was observed with an increase in activation time up to 1 h for activation temperatures of 400 °C, with a maximum iodine number of 810. However, at 500 °C, the iodine number was found to reach a maximum of 1096 at 45 min of activation time and to decrease with further increases in activation time. Girgis et al.20 activated the sugar cane bagasse with 50 wt % H3PO4 at 500 °C and observed a decrease in surface area from 1105 to 973 and 780 m2 g-1 with increasing soaking time from 1 to 3 and 10 h. Similarly, SuarezGarcia et al.16 observed a decrease in surface area with increasing soaking time from 1 to 4 and 8 h. They also mentioned that increasing the activation time has an effect qualitatively similar to that of increasing temperature. (12) Jagtoyen, M.; Derbyshire, F. Carbon 1998, 36 (7-8), 1085-97. (13) Tsai, W. T.; Chang, C. Y.; Lee, S. L.; Wang, S. Y. J. Therm. Anal. Calorim. 2001, 63, 351-7. (14) Williams, P. T.; Reed, A. R. J. Anal. Appl. Pyrolysis 2004, 71, 971-86. (15) Molina-Sabio, M.; Rodriguez-Reinoso, F.; Caturla, F.; Selles, M. J. Carbon 1995, 33 (8), 1105-13. (16) Suarez-Garcia, F.; Martinez-Alanso, A.; Tascon, J. M. D. J. Anal. Appl. Pyrolysis 2002, 63, 283-301. (17) Vernersson, T.; Bonelli, P. R.; Cerrella, E. G.; Cukierman, A. L. Bioresour. Technol. 2002, 83, 95-104. (18) Castro, J. B.; Bonelli, P. R.; Cerrella, E. G.; Cukierman, A. L. Ind. Eng. Chem. Res. 2000, 39, 4166-72. (19) Hayashi, J.; Kazehaya, A.; Muroyama, K.; Watkinson, A. P. Carbon 2000, 38, 1873-8. (20) Girgis, B. S.; Khalil, L. B.; Tawfik, T. A. M. J. Chem. Technol. Biotechnol. 1994, 61, 87-92.

2638 Energy & Fuels, Vol. 20, No. 6, 2006

In the case of high impregnation ratios, surface areas sharply decreased with increase in the carbonization time depending on the carbon precursors.17,18 Thus, in the activation process with 200 wt % H3PO4 loading at 500 °C, for sugar cane bagasse, maximum BET surface (1132 m2 g-1) was attained for a carbonization time of 1 h. Prolongation of carbonization time to 3 h considerably decreased the surface area (675 m2 g-1).18 Under the same conditions, activated carbon from Arundo canes having the maximum surface area (1309 m2 g-1) was obtained for 0.5 h. Prolonged carbonization time (1 h) gave the activated carbon having the surface area of 1114 m2 g-1.17 In contrast, in our case, the carbonization temperature of 600 °C was found as optimum for a carbonization time of 2 h in the preparation of activated carbon from Oreganum stalks. In view of the above explanations, it can be expected that results different from those obtained by other workers may be obtained. Carbonization temperature and time were chosen according to preliminary experiments by using 40% activating agent loading. Other process variables such as impregnation ratio and time and the type of activation agent have received more attention, and their potential applications in the water treatment industry have also been investigated. 3.1. Influnce of Impregnation Time on Surface Area of Activated Carbon. Predrying of impregnated materials plays a critical role in determining the characteristics of the produced activated carbon.21 The hydration of precursor is facilitated, and the swelling of the interior channels of the botanic structure allows for better access of the reactant to the interior of the particle.22 It is well-known that ZnCl2 and H3PO4 are able to catalyze dehydration reactions. During the hold of impregnated biomass at 110 °C, the dehydration of the lignocellulosic material can occur. The study concerning the investigations on the possible mechanisms of phosphoric acid activation of hardwoods showed that there was measurable contraction at temperatures as low as 50 °C, consistent with the action of the acid in lowering the temperature threshold for dehydration and weight loss.12 The contraction sharply increased to about 18% for white oak and 28% for yellow poplar at 100-150 °C. Dehydration is not the only means of some mechanism as for acid-catalyzed dehydration of alcohols. Jagtoyen and Derbyshire showed that there is a very significant release of methane, CO2, and CO beginning at ∼100 °C in the pyrolysis of white oak in the presence of H3PO4.12 In activation of cotton stalks by H3PO4,21 the predried (overnight at 110 °C) impregnated material produced an activated carbon having higher adsorption characteristics and more developed microporosity than the activated carbon for undried impregnated material. A similar result has been reported by Ahmadpour and Do in the chemical activation of macadamia nutshell.1 In literature, the predrying process was mostly carried out at 110 °C overnight. The other ways, such as drying at atmospheric temperature overnight and keeping at 110 °C to dryness, have also been used. In all chemical activation studies, impregnation time has not received attention. The effect of impregnation time on the BET surface area of activated carbons prepared with the impregnation ratio of 40% for 2-8 h is shown in Table 2. Regardless of the type of impregnating agent, increasing the impregnation time from 2 to 4 h increased surface area of activated carbon obtained by (21) Girgis, B. S.; Ishak, M. F. Mater. Lett. 1999, 39, 107-14. (22) Molina-Sabio, M.; Rodriguez-Reinoso, F. Colloids Surf., A 2004, 241, 15-25.

Timur et al. Table 2. Variances in BET Surface Area of Activated Carbons with Impregnation Time BET surface area, m2 g-1 impregnation time, h

H3PO4

ZnCl2

2 4 8 12 16 24

375 497 498 504 502 505

480 598 598 601 602 601

Table 3. Variances in the Yield and Some Properties of Activated Carbons with the Concentration of Impregnation Agent activating agent

reagent concentration, wt %

yield, wt %

ash, wt %

BET surface area, m2 g-1

40 60 80 100 120 40 60 80 100 120

42 37 37 36 35 36 36 34 36 36 35

5.0 6.2 6.3 6.5 7.0 7.3 1.0 1.0 1.0 1.2 1.1

138 497 597 624 719 730 598 868 894 944 960

thermal H3PO4

ZnCl2

Table 4. BET Surface Areas of Activated Carbons Prepared Using Washing Liquids chemical content of impregnation solution, wt %

activated carbon type


AC1 AC2 AC3

719 635 590

AC1 AC2 AC3

944 901 855

PO4 28.80 23.24 17.01 Zn 14.40 10.80 9.22

chemical activation. However, for impregnation times longer than 4 h, the surface area remained almost constant. The impregnation time of 4 h was determined to be optimum and was used as the impregnation time for all experiments. 3.2. Influence of Impregnation Ratio. Table 3 summarizes the effects of impregnation agent loading on the yield, the BET surface area, and the ash content of the activated carbon. Impregnation with ZnCl2 or H3PO4 decreased the yield of resulting carbon as a result of promoted gasification of the char in this study, which is in agreement with the observations of other workers.13,21,23 Tsai et al. suggested that, in the temperature ranges of 400-600 °C, some aromatic condensation reactions also take place among the adjacent molecules, which result in the evolution of gaseous products from the hydroaromatic structure of carbonized char.13 Girgis and Ishak also, in activation of cotton stalks with H3PO4, observed that impregnation with phosphoric acid increased the total weight loss as a result of promoted gasification of the carbonaceous material.21 It has been reported that, for higher concentrations of zinc chloride, above 20 wt %, the yield of activated carbon decreased.23 In contrast to the above-discussed studies, it has been also observed that the high conversion of the precursor to carbon can be obtained as a result of the inhibition of tar production by chemical agents, hence enhancing the carbon yield.7,12,15,16,24-29 (23) Hu, Z.; Srinivasan, M. P. Microporous Mesoporous Mater. 2001, 43, 267-75. (24) Philip, C. A.; Girgis, B. S. J. Chem. Technol. Biotechnol. 1996, 67, 248-54.



Table 5. Characteristics of Activated Carbons Prepared from Oreganum Stalks with Chemical Activation

O-P O-Z


Langmuir surface area, m2 g-1

external surface area, m2 g-1

micropore area, m2 g-1

micropore volume, cm3 g-1

percentage of micropore

719 944

1081 1425

424 531

294 413

0.15 0.22

40.8 43.8

Although, for both ZnCl2 and H3PO4, the concentration of impregnation agent has no effect on the yield of activated carbon, it has a significant effect on the surface area of the activated carbons from Oreganum stalks. The surface areas increased with an increase in the concentration of activation agent. A similar tendency has been described for other precursors.12,13,16,21,30 In the literature, the pore structure of activated carbons was also found to be related to the concentration of the impregnation agents. Several researchers reported that activated carbons obtained at low impregnation ratios were essentially microporous and that, as the amount of impregnation agent increases, the activated carbons become predominantly mesoporous.12,15,21,22,30,31 By considering the effect of the concentration of the impregnation solution on both the BET surface area and the porosity of activated carbon, we concluded that an impregnation ratio of 100% can be taken as the optimum value for the Oreganum stalks. The surface areas of the activated carbons produced using phosphoric acid were somewhat lower than those produced using zinc chloride. The reason may be linked to the presence of phosphates within the carbon. It is known that removing the chemicals in the carbonized sample by washing yields porosity in the carbon structure.32 The metal salts and water-soluble components are easily removed in the washing stage for the activated carbon obtained with zinc chloride, and this is due to the high solubility of zinc salts (ZnO, ZnCl2) left in the carbon mass.1,2 In addition, the fact that the ash content in activated carbon obtained with zinc chloride impregnation was lower than that in char obtained without chemical impregnation confirms that the inorganic constituent of the raw material combined with zinc chloride gives soluble components. However, in the case of activation with phosphoric acid, phosphate and polyphosphate species are incorporated to the carbon matrix, through C-O-P bonds.33 Because of the bonding of the phosphorus to the carbon structure, all the phosphorus would not be removed with washing.14 Comparison may be made of the surface areas found for Oreganum stalk used in this study with more traditionally used lignocellulosic materials. Activated carbons having a surface area ranging between 1267 and 1070 m2 g-1 were produced from several kind of nutshells by chemical actvation with H3PO4 at 450 °C followed by activation with air at 300 °C.34 On (25) Suarez-Garcia, F.; Martinez-Alanso, A.; Tascon, J. M. D. J. Anal. Appl. Pyrolysis 2002, 62, 93-109. (26) Teng, H.; Yeh, T. S. Ind. Eng. Chem. Res. 1998, 37, 58-65. (27) Namasivayan, C.; Kadirvelu, K. Bioresour. Technol. 1997, 62, 1237. (28) Girgis, B. S.; El-Hendawy Abdel-Nasser, A. Microporous Mesoporous Mater. 2002, 52 (2), 105-17. (29) Suarez-Garcia, F.; Martinez-Alanso, A.; Tascon, J. M. D. Polym. Degrad. Stab. 2002, 75 (2), 375-83. (30) Baquero, M. C.; Giraldo, L.; Moreno, J. C.; Suarez-Garcia, F.; Martinez-Alonso, A.; Tascon, J. M. D. J. Anal. Appl. Pyrol. 2003, 70, 77984. (31) Suarez-Garcia, F.; Martinez-Alanso, A.; Tascon, J. M. D. Carbon 2001, 39 (7), 1111-5. (32) Cuturla, F.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Carbon 1991, 29 (7), 999-1007. (33) Moreno-Castilla, C.; Carrasco-Marin, F.; Lopez-Ramon, M. V.; Alverez-Merino, M. A. Carbon 2001, 39, 1415-20.

the other hand, the activated carbon obtained from macadamia nutshell using ZnCl2 in a 1:1 impregnation ratio at 500 °C had a surface area of 1718 m2 g-1.12 In the preparation of activated carbon from lignin using ZnCl2 and H3PO4, with activation at a impregnation ratio of 1:1 and 600 °C carbonization temperature, the surface areas were around 1000 m2 g-1 and 1300 m2 g-1, respectively.19 Generally, the activated carbons having the surface area ranging from 1032 to 1333 m2 g-1 have been obtained from several kinds of lignocellulosic precursors using H3PO4 at the impregnation ratios between 1 and 3 at the carbonization temperatures of 450-500 °C.21,31,35-37 3.3. Recovery of the Activating Agents. Since the spent activating agents led to environmental pollution, the recovery of the activating agents was investigated. For this purpose, a series of experiments was carried out. In these group experiments, the filtrate from the washing with acid and distillated water of activated carbon was used as activating agent solution. Thus, the biomass was impregnated with the first 250 mL of filtrate from the washing of activated carbon (AC1) obtained with 100 wt % fresh activating agent (H3PO4 and ZnCl2). Then the slurry was pyrolyzed. The resulting activated carbon (AC2) was demineralized with acid and water. The first 250 mL of filtrate was impregnated on the biomass, and the slurry was pyrolyzed. Then the resulting activated carbon (AC3) was washed with acid and water. The BET surfaces areas of activated carbons are given in Table 4. The concentrations of phosphate and zinc chloride in impregnation solutions are also presented in the Table 4. Between 73 and 86% of used activating agent could be recovered in 250 mL of filtrate, independent of the type of activating agent. In the case of H3PO4, impregnation ratios were approximately 80 and 60% for the preparation of AC2 and AC3, respectively. In the case of ZnCl2, AC2 and AC3 were obtained with impregnation ratios of 75 and 64%, respectively. From the results in Tables 3 and 4, it is clearly seen that, for both H3PO4 and ZnCl2, the BET surface areas of activated carbon (AC2 and AC3) prepared with filtrates are similar to those of activated carbons prepared with the fresh impregnation solutions at the same concentrations. The fact that the chemicals used in the production of activated carbon can be recovered is important from the point of view of environment and process economy. It is clear that the impregnation solutions can be prepared by mixing of filtrate and fresh impregnation agent. 3.4. Surface Properties and Adsorption Isotherms of Activated Carbons. Some texture parameters of the activated carbons having the highest surface area from different activating agents are shown in Table 5. The activated carbons obtained with 100 wt % ZnCl2 and H3PO4 were designated as O-Z and O-P, respectively. It is known that the microporosity of the activated carbon depends on the precursor and the activation conditions.1 The (34) Toles, C. A.; Marshall, W. E.; Johns, M. M. J. Chem. Technol. Biotechnol. 1998, 72, 255-63. (35) Girgis, B. S.; Yunis, S. S.; Soliman, A. M. Mater. Lett. 2002, 57, 164-72. (36) Ahmedna, M.; Marshall, W. E.; Rao, R. M. Bioresour. Technol. 2000, 71, 113-23. (37) Dastgheib, S. A.; Rockstraw, D. A. Carbon 2001, 39, 1849-55.

2640 Energy & Fuels, Vol. 20, No. 6, 2006

Timur et al.

Figure 1. Nitrogen adsorption isotherms of activated carbons.

Figure 3. Methylene blue adsorption isotherms.

Figure 4. Phenol adsorption isotherms.

Figure 2. SEM images of activated carbons: (a) O-P and (b) O-Z.

micropore volumes of activated carbons prepared by ZnCl2 are higher than that of activated carbon obtained using H3PO4. Figure 1 shows the N2 adsorption isotherms on the activated carbons. The isotherm of activated carbon obtained by impregnation with ZnCl2 exhibits a type 1b isotherm, which is typical for materials with wide micropores.38 However, in the activated carbon prepared by impregnation with H3PO4, the increase in nitrogen adsorption is sustained throughout the entire pressure range and the isotherms take a shape resembling a combination of types I and II, showing a combination of microporosity and mesoporosity. This observation is in agreement with the results of other studies. Molina-Sabio and Rodriguez-Reinoso, in the preparation (38) Suarez-Garcia, F.; Martinez-Alonso, A.; Tascon, J. M. D. Carbon 2004, 42, 1419-26.

of activated carbons from peach stones with H3PO4 and ZnCl2, observed that ZnCl2 developed both wide micropores and low mesopores, whereas H3PO4 developed large mesopores and even macropores.22 It was also demonstrated that ZnCl2 was suitable for the production of activated carbons, which are essentially microporous.39 On the other hand, Guo and Lua prepared the activated carbons from oil-palm stones impregnated with ZnCl2, H3PO4, or KOH. The degree of microporosities was in the following order: ZnCl2 > H3PO4 > KOH.11 Scanning electron micrographs (SEMs) of the external structures of activated carbons are given in Figure 2. It can be seen from the micrographs that the activated carbons prepared from Oreganum stalks both with H3PO4 and ZnCl2 have cavities on their external surface. 3.5. Aqueous Adsorption Characteristics. Aqueous adsorption tests were conducted on selected activated carbons with the aim of assessing potential applications in the water-treatment industry. The two adsorbate compounds were used in this study cover a range of molecular sizes, which makes them useful for the investigation of adsorption in pores of different dimensions. Phenol is preferentially adsorbed in small- and medium-sized micropores, while methylene blue is mainly adsorbed in medium- and large-sized micropores.40 Methylene blue and phenol adsorption isotherms for the activated carbons prepared by ZnCl2 and H3PO4 impregnation are shown in Figure 3 and Figure 4, respectively. The adsorption capacity of activated carbon prepared by ZnCl2 impregnation (39) Rodriguez-Reinoso, F.; Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; Vol. 21, Chapter 1. (40) San Miguel, G.; Fowler, G. D.; Sollars, C. J. Carbon 2003, 41, 1009-16.



Table 6. Parameters of the Langmiur and Freundlich Adsorption Models of MB Langmuir model

Table 7. Parameters of the Langmiur and Freundlich Adsorption Models of Phenol

Freundlich model

Langmuir model

Freundlich model

activated carbon

SM, mg g-1

KL, L mg-1

R2

1/n

KF

R2

activated carbon

SM, mg g-1

KL, L mg-1

R2

1/n

KF

R2

O-Z O-P

333.3 285.7

0.697 0.686

0.99 0.99

0.115 0.080

210.1 196.2

0.99 0.99

O-Z O-P

92.59 94.34

0.040 0.010

0.98 0.90

0.404 0.610

11.8 2.8

0.99 0.98

(O-Z) was more than that of activated carbon prepared by H3PO4 (O-P). The shape of the experimental isotherms reflects that the MB uptake on the carbon surfaces is monolayer. The isothermal equilibrium data were processed by employing Langmuir and Freundlich isotherm equations. The Langmuir model is expressed as

Ce/qe ) 1/KLSM + Ce/SM where SM (mg g-1) is the maximum amount of adsorption corresponding to complete monolayer coverage on the surface, qe (mg g-1) is the amount of MB or phenol adsorbed, KL (l g-1) is the Langmuir constant, and Ce is the equilibrium liquid-phase concentrations of MB or phenol solution. The freundlich model is given as

Qe ) KF Ce1/n where KF is the Freundlich constant and roughly an indicator of the adsorption capacity. 1/n gives an indication of the favorability of adsorption. Heterogeneity becomes more prevalent as 1/n gets closer to zero. It is worth mentioning that the Freundlich model is considered to be suitable for highly heterogeneous surfaces. The Freundlich isotherm indicates that significant adsorption takes place at low concentrations, but the increase in the amount adsorbed with concentration becomes less significant at higher concentration.41 On the other hand, the Langmuir model is used for homogeneous surfaces and demonstrates monolayer coverage of the adsorbate at the outer surface of the adsorbent. Table 6 shows the Langmiur and Freundlich parameters obtained by fitting the MB adsorption on activated carbons. Both equations were found to fit the data well (R2 > 0.99). The Langmuir capacity (SM) of O-Z was more than that of O-P. This shows that O-P contains wider pores than O-Z. With respect to the Langmuir model, a large KL value implies strong bonding of the MB to the adsorbent. For phenol adsorption, O-P showed a closer fit to the Freundlich model, whereas the adsorption data for the O-Z was wellfitted with both models. O-P had the Freundlich constant (KF) approximately 1/4 of those of O-Z. As a result of the less-

developed microporous structure of O-P, its adsorption capacity for phenol was significantly lower than that of O-Z (Figure 4). Moreover, the n values shown in Table 7 being greater than unity indicates that phenol is favorably adsorbed42 by both O-Z and O-P. 4. Conclusions In this study, the activated carbons were prepared from Oreganum stalks by the chemical activation method. Phosphoric acid and zinc chloride were used as the activating agent. The chemical-to-biomass ratio of 100% (by mass) and the impregnation time of 2 h were found as the optimum conditions for the production of high-surface-area activated carbons from lignocellulosic material by chemical activation. The surface areas of the activated carbons prepared under optimum conditions were 719 m2 g-1 and 944 m2 g-1 for phosphoric acid and zinc chloride, respectively. The micropore volume of activated carbon prepared by ZnCl2 is higher than that of activated carbon obtained using H3PO4. In addition, the used activating agent could be recovered in 73-86% and was reused in the impregnation of biomass. The aqueous adsorption tests showed that the activated carbons from Oreganum stalks had a notable adsorption capacity for both MB and phenol. The adsorption capacity of activated carbon prepared with ZnCl2 was more than that of activated carbon obtained using H3PO4. In conclusion, the present investigation showed that “Oreganum stalks” can be effectively used as a raw material for the preparation of activated carbon by chemical activation. Cost analysis for the preparation of activated carbons has not been carried out, but because the agricultural waste used is abundant in Turkey, the carbon cost is expected to be economical. Acknowledgment. We would like to thank to Ege University for financial support under the contract 2004/Fen/057. It is pleasure to thank Dr. Yunus Onal for analysis support. EF060219K (41) Teng, H.; Hsieh, C. T. Ind. Eng. Chem. Res. 1998, 37 (8), 361824. (42) Otero, M.; Rozada, F.; Calvo, L. F.; Garcia, A. I.; Moran, A. Biochem. Eng. J. 2003, 15, 59-68.

Preparation of Activated Carbons from Oreganum Stalks by Chemical

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