Preparation and Adsorption Studies of High Specific Surface Area Activated Carbons Obtained from the Chemical Activation of Jute Stick

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1 761 Preparation and Adsorption Studies of High Specific Surface Area Activated Carbons Obtained from the Chemical Activation of Jute Stick Mohammad Asadullah *, Muhammad A. Rahman, Mohammad A. Motin and Mohammad B. Sultan Department of Applied Chemistry and Chemical Technology, University of Rajshahi, Rajshahi 6205, Bangladesh. (Received 30 October 2006; revised form accepted 28 December 2006) ABSTRACT: High specific surface area activated carbon has been produced from the chemical activation of jute stick using H 3 as an activating agent at different temperatures in the range K. The BET surface area of the activated carbon was measured by nitrogen adsorption methods. The maximum specific surface area was found to be 1734 m 2 /g. The liquid-phase adsorption of iodine and Methylene Blue dye was studied. The maximum iodine number was found to be 1234 mg/g. The adsorption of Methylene Blue dye from dilute solutions proceeded via monolayer coverage which could be described by the Langmuir adsorption isotherm. However, multilayer adsorption was achieved when the equilibrium concentration of the dye solution was increased. The vapour-phase adsorption of CCl 4 at 303 K was also very high (1200 mg/g), which was much higher than that of a commercial activated carbon (330 mg/g). INTRODUCTION Highly porous carbon materials with exceptionally high specific surface areas are termed activated carbons. The extensively developed internal pore structures of activated carbons may be sub-divided into micropores (radii, r < 1 nm), mesopores (r = 1 25 nm) and macropores (r > 25 nm). Micropores are especially responsible for the large surface area of activated carbons, as typically determined by nitrogen adsorption (Menendez et al. 1999; Deng et al. 1997). Their microporosities and large surface areas make activated carbons exceptionally good adsorbents. In the adsorption of micro-molecules, the macropores are used as entrances to the activated carbon, the mesopores for transportation and the micropores are where adsorption largely takes place. Thus, the surface area and pore volume are two important parameters which control the adsorption performance of activated carbons (Mangun et al. 1998). The pore volume controls the size of the molecules that can be adsorbed, while the surface area controls the amount of material which can be adsorbed. However, the surface area is the major factor in the practical application of activated carbons as adsorbents. The mechanical strength of activated carbons is also an important factor in their regeneration and recycling. Due to their unique adsorptive capacities and high surface areas, activated carbons have found several important uses. These include solution purification, the removal of tastes and odours from domestic and industrial water supplies, the purification of vegetable and animal fats and oils, alcoholic beverages, chemicals and pharmaceuticals, and wastewater treatment. Gas purification * Author to whom all correspondence should be addressed. asad@ru.ac.bd; asadullah8666@yahoo.com (Mohammad Asadullah).

2 762 M. Asadullah et al./adsorption Science & Technology Vol. 24 No and solvent recovery from the liquid phase are also important industrial usages. Activated carbons provide very efficient catalyst supports in the field of heterogeneous catalysis (Li et al. 2001). Many organic compounds such as chlorinated solvents (Abe et al. 2001), gasoline, pesticides, etc. can be adsorbed by activated carbons. They are also effective for the removal of chlorine (Mancl and Sailus 1995) and moderately effective for removal of some heavy metals (Kobya et al. 2005). They are also used for liquid-phase adsorption or decolourizations (Rahman et al. 2005) of usually light, fluffy powders produced from low-density material such as sawdust or peat. However, gas-phase adsorption processes require hard, dense granular activated carbons produced from high-density raw materials such as bamboo, coconut shells, palm kernel shells, coal or coke (Yang 1997). Wide varieties of carbonaceous materials such as agricultural by-products (Srinivasakannan and Bakar 2004; Daud et al. 2000), coals (Pis et al. 1998), synthetic materials (Hayashi et al. 2005), etc. can be used as precursors for the production of activated carbons. Different activation techniques, such as physical and chemical activation, may also be employed. In physical activation, the carbonaceous materials are first pyrolyzed in an inert atmosphere at ca. 773 K to produce a char. After this process, the char is activated corrosively using reagents such as steam or carbon dioxide (Sanchez et al. 2001) at temperatures above 1073 K. In the pyrolysis step, the volatile organic fraction of the carbonaceous materials is removed, leaving the solid char behind which reacts with steam or carbon dioxide in the activation step to produce pores. Since only the carbon portion of the precursor is transformed in this process, the final yield is low (ca. 20%) depending on the temperature and activation time employed (Sanchez et al. 2001). However, in chemical activation, various chemicals such as H 3, ZnCl 2, KOH, K 2 CO 3, etc. may be employed as activating agents. These usually remove water molecules at elevated temperatures (ca. 773 K) from the hydrogen and oxygen of the different functional groups of the precursor materials, leaving a mainly carbon skeleton behind. Some of the carbon atoms also become unsaturated during this process (Mattson and Mark 1971). In this case only a very small fraction of the carbon is removed by corrosive reaction, so that the yield of final product is high (ca. 50%). Due to the variation of raw materials and activation techniques used in the production of activated carbons, those available exhibit markedly different adsorption characteristics. Thus, in selecting an activated carbon, it is important to have a clear understanding of both the adsorptive and physical characteristics of the material in order to optimize the performance capabilities. Although many studies have been reported in the literature relating to the preparation and characterization of activated carbons prepared from agricultural wastes, no reference has been made to the use of jute stick for such preparations. Jute stick is abundantly available in various countries of the world such as Bangladesh, India, etc. Hence, in this paper, the interesting results obtained regarding the preparation and adsorption studies of high surface area activated carbons prepared from jute stick are reported. EXPERIMENTAL Materials Jute sticks were used as the precursor for the preparation of the activated carbons studied. Prior to activation, the jute sticks were crushed to mm particle size and then repeatedly washed with distilled water in order to remove dust and other inorganic impurities. Following this procedure, the resulting particles were oven-dried for 12 h at 383 K to reduce their moisture content. We have previously reported the characterization of jute sticks by proximate and ultimate

3 High Specific Surface Area Activated Carbons from the Chemical Activation of Jute Stick 763 analyses (Asadullah et al. 2004). Ultimate analysis showed that the volatile fraction, fixed carbon and ash content of the jute sticks amounted to wt%, wt% and 0.62 wt%, respectively. In addition, ultimate analysis showed that jute sticks contained wt% C, 6.02 wt% H, wt% O, 0.19 wt% N, 0.05 wt% Cl and 0.05 wt% S. Hence, jute sticks have a high carbon and low ash content which make them a suitable precursor for the preparation of activated carbons. The carbon content of the jute sticks was higher than that of bagasse and much higher than that of rice straw (Asadullah et al. 2004). Phosphoric acid (E-Merk, India, 99%) was used as an activating agent. Iodine (99.5%, Loba Chemicals, India), potassium iodide (99.9%, Loba Chemicals, India), potassium thiosulphate (99.9%, Loba Chemicals, India), potassium dichromate (99.5%, Loba Chemicals, India), activated carbon [Laboratory Reagent, Thomas Baker (Chemicals) Ltd.] and Methylene Blue dye (99.9%, Aldrich) were also used in this investigation. Preparation of activated carbons For the preparation of the activated carbons studied, ca. 20 g of jute stick particles was mixed with 33 g of 60% phosphoric acid solution and allowed to soak in the solution for 12 h. The amount and concentration of H 3 solution was adjusted to maintain a H 3 /jute stick ratio of 1:1. Carbonization of jute stick was undertaken in a stainless steel reactor with a height of 25 cm and an internal diameter (i.d.) of 5 cm as shown in Figure 1. The reactor was aligned horizontally, the extension pipe of 10 mm diameter connected at the conical end of the reactor allowing a nitrogen gas flow of 200 ml/min inside the reactor. A second extension pipe connected to the other end of the reactor acted as a gas outlet. A thermocouple was inserted in the thermo well set into the reactor with the other part of the thermocouple being connected to a temperature controller. A second thermocouple was inserted in between the inside wall of the furnace and the outside wall of the reactor. After loading the requisite amount of jute stick particles, the reactor was closed and sealed with a stainless steel cap. To ensure complete sealing, two thin stainless steel gaskets were placed inside the interface between the cap and reactor and tightened with 8 screws by means of an angel key. The reactor temperature was maintained at 473 K for 15 min and then was increased to different values within the range K to optimize the activation process. Both carbonization and activation were initiated by heating the sample at a rate of 283 K/min from room temperature (ca. 303 K) up to the desired temperature. Furnace N 2 gas Thermocouple Thermocouple Jute stick particles Reactor Gas out Figure 1. Diagrammatic representation of the design of the activation reactor system employed in the present work.

4 764 M. Asadullah et al./adsorption Science & Technology Vol. 24 No After activation had been completed, the reactor was cooled under the flow of nitrogen and opened. The resulting activated products were washed repeatedly with hot distilled water in order to remove H 3. This process was continued until the ph of the resulting solution attained a value of 6.5, when the sample was finally washed with 0.5% NaOH solution to effect complete neutralization. Any residual NaOH was washed out via a further washing with distilled water. The final samples were dried at 383 K for 24 h and stored in desiccators. From the total weight loss of the jute sticks induced by activation, it was possible to calculate the yield and activation burn-off, respectively, using the following equations: Y (%) = M/M Y (%) = (M 0 M)/M where Y and Y are the yields of activated carbon and activation burn-off, respectively, M (g) is the mass of activated carbon obtained and M 0 is initial mass of jute stick employed on a dry basis. Characterization of activated carbons The activated carbons were characterized via measurements of the Brunauer Emmett Teller (BET) surface area, iodine number, carbon tetrachloride adsorption and Methylene Blue dye adsorption. The BET surface area was determined by nitrogen adsorption at 77 K using a Micromeritics Gemini ASAP 2010 instrument. For such measurements, ca. 0.1 g of sample was taken in a glass sample tube and placed inside the instrument, thereby allowing the surface area to be obtained automatically. For iodine number determinations, ca. 0.2 g of activated carbon was placed in a 250 ml conical flask containing 20 ml of 0.05 M iodine (UNI-Chem) solution. The flask was then stored in the dark for ca. 1 h, during which time the iodine was adsorbed onto the surface of the activated carbon. The solid mass was then separated from the solution by centrifugation, following which 5 ml of the supernatant solution was pipetted out and titrated with 0.05 N sodium thiosulphate solution, thereby allowing the remainder of the iodine in the system to be measured. The amount of iodine adsorbed was calculated from the difference between the amount of iodine in the initial solution and in the supernatant. Finally, the iodine number was calculated as the number of mg iodine adsorbed per g activated carbon. The adsorption capacity of carbon tetrachloride was measured as follows. A known amount (ca. 0.2 g) of activated carbon was weighed out onto a watch glass and placed inside a small desiccator. The desiccator was then placed in a thermostatic bath at 303 K. A second watch glass was filled with carbon tetrachloride and placed inside the desiccator. The pressure inside the desiccator was reduced slightly by means of a vacuum pump, thereby allowing the carbon tetrachloride to boil off. Thus, the desiccator was suddenly filled with carbon tetrachloride vapour which was adsorbed by the activated carbon. This process was allowed to continue for a sufficient time to enable an equilibrium state to be attained. The percentage adsorption of carbon tetrachloride was calculated from the equation: {(amount of carbon tetrachloride adsorbed)/(the amount of activated carbon)} 100. The solution-phase dye adsorption capacity was investigated at 303 K using Methylene Blue dye (E-Merck, Germany) as the adsorbate. Equilibrium adsorption isotherms were determined by mixing 0.1 g activated carbon with 10 ml dye solution in sealed bottles, employing dye solutions with concentrations within the range g/ml. Adsorption was allowed to proceed for ca. 48 h to allow equilibrium conditions to be attained. The residual dye concentrations after each adsorption test were measured using an ANA-75 UV vis spectrophotometer at a λ max value of 626 nm. All experiments were carried out in duplicate.

5 High Specific Surface Area Activated Carbons from the Chemical Activation of Jute Stick 765 RESULTS AND DISCUSSION Preparation of activated carbons The char derived from the semi-carbonization of jute stick at 473 K for 15 min was activated at different temperatures ranging from 673 K to 973 K. Both semi-carbonization and activation were performed under a nitrogen atmosphere in the same stainless steel reactor. After semicarbonization, the solid was found to have been blackened and converted into a sticky dry char. The weight loss during this period was within the range of 20 23%. During the first stage of activation at 473 K, constituents of jute stick such as cellulose, hemicellulose and lignin are degraded to form a plastic mass, which is subsequently transformed into a sticky dry char. The phosphoric acid present in the system was dehydrated during this stage and converted into pyrophosphoric acid (Sax and Lewis 1987). The decrease in mass during activation is called the activation burn-off which varies depending upon the activation temperature employed. Figure 2 depicts the activation burn-off and yields of activated carbons obtained at different temperatures. The yield of activated carbon decreased sharply to ca. 50% as the activation burn-off increased with increasing temperature up to 748 K. Activation burn-off primarily occurred through reaction between the activating agent and the hydrogen and oxygen atoms of the functional groups of the char to generate steam. This steam subsequently took part in the reforming reaction of char to produce carbon monoxide and hydrogen. This resulted in the generation of new pores on the outer surface of the char, with an increase in pore size occurring in the micro-pore surface. Such reactions are completely temperature-dependent, i.e. the higher the reaction temperature the faster the reforming reaction. However, in our experiments, it was found that the activation burn-off increased slightly even when the temperature was increased above 748 K. This was because some of the reactive species present in the system reacted within this temperature range Yield of activated carbon (%) Activation burn-off (%) Temperature (K) Figure 2. Influence of temperature on the yield of activated carbon and activation burn-off.

6 766 M. Asadullah et al./adsorption Science & Technology Vol. 24 No BET surface area (m 2 /g) Temperature (K) Figure 3. Influence of temperature on the BET surface area of activated carbon. Effect of activation temperature on the characteristics of the activated carbons produced The properties of activated carbon are directly related to its surface area and pore size, which in turn are dependent on the activation temperatures employed. Figure 3 shows the effect of different activation temperatures on the BET surface areas of the activated carbons generated in the present work. It will be seen that the BET surface area increased with increasing activation temperature to attain a maximum value at 748 K after which the surface area decreased. The numbers of micropores generated contributed to the BET surface area, with a higher value leading to a higher BET surface area. A low activation temperature (673 K) resulted in incomplete burn-off which contributed to the lower number of micropores and, hence, a lower surface area (1355 m 2 /g). The maximum surface area (1734 m 2 /g) was achieved at 748 K. The decrease in the BET surface area beyond the optimum activation temperature was attributed to an increase in pore size due to the collapse of smaller pores. The number of micropores as well as mesopores increased as the temperature was increased up to the optimum activation temperature. However, beyond the optimum activation temperature, only the mesopore volume increased at the expense of the micropore volume, thereby reducing the surface area (Vernersson et al. 2002). Adsorption of iodine from the liquid phase The maximum sorption capacity of an activated carbon from the solution phase is a very important parameter from a practical point of view. Figure 4 depicts the iodine sorption ability at 303 K of activated carbons produced at different activation temperatures. It will be seen from the figure that the variation in the iodine sorption capacity with activation temperature was almost the same as that of the BET surface area. However, the maximum capacity towards iodine sorption was different. Whereas this maximum was achieved by the activated carbon generated at an activation temperature of 773 K, that exhibiting the maximum BET surface area was generated by thermal treatment at 748 K. Figure 5 shows the BET surface area and iodine sorption capacity as a function of activation burn-off. The mesopore structure best suited for iodine sorption was achieved at 54% burn-off. Higher activation burn-offs beyond that necessary for maximum iodine sorption resulted

7 High Specific Surface Area Activated Carbons from the Chemical Activation of Jute Stick Iodine number (mg/g) Temperature (K) Figure 4. Influence of temperature on the iodine number of activated carbon BET surface area (m 2 /g) Iodine number (mg/g) Activation burn-off (%) Figure 5. Influence of activation burn-off on the BET surface area and iodine number of activated carbons. in the destruction of the mesopore structure and the formation of a macroporous carbon. This, in turn, led to a decrease in both the BET surface area and the iodine sorption capacity. Adsorption of carbon tetrachloride from the vapour phase Figure 6 shows the influence of time on the adsorption of carbon tetrachloride vapour onto an experimental sample produced by activation at 748 K and a commercial sample activated at 303 K. It is seen that both the experimental and commercial activated carbons adsorbed increasing amounts of carbon tetrachloride vapour with increasing adsorption time. However, the experimental sample adsorbed a much higher amount of carbon tetrachloride vapour (1200 mg/g)

8 768 M. Asadullah et al./adsorption Science & Technology Vol. 24 No CCl 4 adsorption capacity (g/g) Adsorption time (min) Figure 6. Carbon tetrachloride adsorption isotherm on the ( ) commercial sample and ( ) experimental sample. than the commercial sample (310 mg/g). Adsorption equilibrium was attained within 30 min for the commercial activated carbon whereas this took 60 min for the experimental activated carbon. This finding for the experimental sample is important from an application point of view. Adsorption isotherm of Methylene Blue dye Methylene Blue dye is one of the standard materials used to characterize activated carbons by adsorption from the solution phase. The equilibrium adsorption of Methylene Blue at different initial dye concentrations has been used in the present study to evaluate the adsorption capacity of the experimental activated carbon produced at 748 K. The results obtained are summarized in C Table 1 and depicted as a plot of versus C in Figure 7, where C is the equilibrium concentration x/ m C and x/m is the amount of dye adsorbed/g activated carbon. The magnitude of increased sharply x/ m initially as the equilibrium concentration increased, but changed at a particular concentration when only a slow increase was subsequently observed. This result indicates the existence of a Langmuir type of adsorption involving the formation of monolayer up to the turning point. The subsequent slow rise in the equilibrium adsorption may be attributed to multilayer adsorption. The adsorption capacity of the activated carbon towards Methylene Blue dye was 399 mg/g. Although the Langmuir isotherm has been applied to a large number of adsorption systems involving dilute solutions, a number of interesting cases of sigmoid or BET type IV isotherms have been reported. This occurred for a number of higher acid and alcohols (four or more carbon atoms) adsorbed from aqueous solution onto various activated carbons. In these cases, the isotherms showed no saturation effect but rather the general sigmoid shape characteristic of multilayer adsorption.

9 High Specific Surface Area Activated Carbons from the Chemical Activation of Jute Stick 769 TABLE 1. Methylene Blue Dye Adsorption at Various Initial Concentrations onto an Activated Carbon at 303 K Amount of Equilibrium Amt. of dye 10 3 x/m 10 4 C activated carbon, conc., C adsorbed, x/ m m (10 2 g) (10 5 mol/l) x (10 3 g) (mol/l) C x/m (10 2 mol/l) Equilibrium conc., C (10 3 mol/l) Figure 7. Langmuir adsorption isotherm for Methylene Blue dye adsorption onto activated carbon. CONCLUSIONS Activated carbon with a high specific surface area (1734 m 2 /g) was produced by the chemical activation of jute stick at different temperatures using H 3 as the activating agent. The maximum adsorption capacity was achieved at ca. 50% activation burn-off. The much higher amounts of CCl 4 adsorbed from the vapour phase onto the experimental sample than onto

10 770 M. Asadullah et al./adsorption Science & Technology Vol. 24 No a commercial sample suggests the possible application of the activated carbons studied for the vapour-phase separation of organic contaminants from many industrial products. The efficient adsorption of Methylene Blue dye from dilute aqueous solutions indicated that this activated carbon could also be used for effluent treatment. ACKNOWLEDGEMENTS This research was supported financially by the Ministry of Science and Information and Communication Technology under the project Pilot Plant Project on the Production of Liquid Fuel from Biomass No. MOSICT-SAP /INT-13. REFERENCES Abe, I., Fukuhara, T., Maruyama, J., Tatsumoto, H. and Iwasaki, S. (2001) Carbon 39, Asadullah, M., Miyazawa, T., lto, S.I., Kunimori, K., Yamada, M. and Tomishige, K. (2004) Appl. Catal. A 267, 95. Daud, W.M.A.W., Ali, W.S.W. and Sulaiman, M.Z. (2000) Carbon 38, Deng, X., Yue, Y. and Gao, Z. (1997) J. Colloid Interface Sci. 192, 475. Hayashi, J., Yamamoto, N., Horikawa, T., Muroyama, K. and Gomes, V.G.J. (2005) J. Colloid Interface Sci. 281, 437. Kobya, M., Demirbas, E., Senturk, E. and Ince, M. (2005) Bioresource Technol. 96, Li, Z.-R., Fu, Y., Jiang, M., Hu, T., Liu, T. and Xie, Y. (2001) J. Catal. 199, 155. Mancl, W.L.K. and Sailus, M. (1995) Home Water Treatment, Northeast Regional Agricultural Engineering Service Cooperative Extension, Ithaca, NY, U.S.A. Mangun, C.L., Daley, M.A., Braatz, R.D. and Economy, J. (1998) Carbon 36, 123. Mattson, J.S. and Mark, H.B. (1971) Activated Carbon, Marcel Dekker, New York. Menendez, J.A., Menendez, E.M., Iglesias, M.J., Garcia, A. and Pis, J.J. (1999) Carbon 37, Pis, J.J., Mahamud, M., Pajares, J.A., Parra, J.B. and Bansal, R.C. (1998) Fuel Process. Technol. 57, 149. Rahman, I.A., Saad, B., Shaidan, S. and Rizal, E.S.S. (2005) Bioresource Technol. 96, Sanchez, A.R., Elguezabal, A.A. and Saenz, L.D.L.T. (2001) Carbon 39, Sax, N.J. and Lewis, R.J. (1987) Hawley s Condensed Chemical Director, Van Nostrand Reinhold, New York. Srinivasakannan, C. and Bakar, M.Z.A. (2004) Biomass Bioenergy 27, 89. Vernersson, T., Bonelli, P.R., Carrella, E.G. and Cukierman, A.L. (2002) Bioresource Technol. 83, 95. Yang, R.T. (1997) Gas Separation by Adsorption Processes, Imperial College Press, London.