Preparation and Characterization of Activated Charcoal as an Adsorbent

Transcription

1 1 J. Surface Sci. Technol., Vol 22, No. 3-4, pp , Indian Society for Surface Science and Technology, India. Preparation and Characterization of Activated Charcoal as an Adsorbent M. A. RAHMAN, M. ASADULLAH*, M. M. HAQUE, M. A. MOTIN, M. B. SULTAN and M. A. K. AZAD Department of Applied Chemistry and Chemical Technology University of Rajshahi, Bangladesh Abstract Charcoal was prepared from coconut shell and activated at three temperatures 300, 350 and 400 C in constant flow of air or nitrogen. Adsorption of oxalic acid and maleic acid from their aqueous solution on charcoal was studied. Oxalic acid gave Langmuir type of adsorption isotherm indicating monolayer formation but maleic acid gave sigmoid shaped isotherm, BET (Brunauer, Emmett and Teller) type II isotherm. These contrasting results are explained as follows: activated charcoal, like graphite has both polar and basal sites and the polar heads of the carboxylic groups in oxalic and maleic acids are chemisorbed on the polar sites of activated charcoal but in addition to this the unsaturated hydrocarbon chain in maleic acid opens up to form weak bonds with the basal sites of the activated charcoal. It has been observed that the amount of adsorption of both oxalic and maleic acid increases with the increase of temperature of activation. Keywords : Active carbon, coconut shell, surface area, adsorption. INTRODUCTION Charcoal, known as an excellent adsorbent, is well known for its wide applications. Its main use today is in the treatment of solutions (or effluents) for the removal of noxious constituents or colouring matters by sorption. Recovery of solvents from gases, removal of colouring matters from aqueous and other solutions as well as removal of odour and the more recent use of active carbons in ionic change process, are only a few of the examples where the unit operation of adsorption has been employed. Naturally occurring earth s (fuller s earth, clays and other inert earth s) has been utilized in a number of occasions [1]. In other cases, products were made *Author for correspondence : asad@ru.ac.bd; asadullah8666@yahoo.com

2 134 Rahman et al. (silica gel, magnesia, alumina, carbons, etc) to help certain other specific purposes [2]. Charcoal is presently used as a raw material for active carbon. Active carbon in its present efficient form is a relatively new comer in the field, particularly in its employment on a commercial scale for a number of purposes, for example, gas-phase application [3], liquid-phase application [4,5], application as molecular sieving [6], application as shape-selective catalyst as well as catalyst support [7,8], chromatographic applications [9] and ion-exchange application [10] are only a few to mention. Application of active carbon for effluent treatment [11] and pollution control [12] as well as energy conservation [13] can never be overemphasized to day. But the word, active carbon, has been rather vaguely defined so far. Charcoals of the activated type are available for various adsorption operations, but each particular sample seems to posses a sort of specificity which makes it useful for a definite purpose only. Adsorption capacity of activated carbon depends on the magnitude of the internal surface, the distribution of pore size and shape. The surface chemistry of activated carbon can be modified by a number of ways [4]. Surface chemistry of activated carbon refers to chemically bonded elements of activated carbon which can originate in the starting material or incorporate during activation or subsequent chemical treatment [14]. Specially the surface chemistry of activated carbon is determined decisively by the type, quantity, concentration and bonding of hetero atoms (oxygen specially) which are believed to form functional groups such as hydroxyl or carboxyl groups [15]. Surface chemistry of active carbon can be also modified by means of microwave-induced treatments to transform an acidic carbon into basic carbon with relatively low oxygen content. Surface of active carbon may be modified in such a way that, like zeolite, activated carbon exhibit shape selectivity [16]. From the foregoing discussion, it is evident that a carbon that might give the best value for citric acid purification may have little effect on cotton seed oil [17]. Granular activated carbons which are mechanically strong, relatively dense and highly active are required for industrial gas and vapour adsorption [18]; for this purpose finely pulvarised, highly porous decolourizing carbons are practically worthless. It is noteworthy that two varieties of active carbons may prove differently in decolourizing sugar solutions of two different sources [19]. Effluents from various industries (e.g., pulp, paper, leather, pharmaceutical and other chemical process industries) contains organic compounds in appreciable amounts. The aliphatic organic compounds are ultimately oxidized to carboxylic acid and most of them are pungent in smell and toxic. Thus, the adsorption of acid on the activated carbon is important. Therefore, two dibasic acids, oxalic acid and maleic acid, have been chosen as adsorbates for this study.

3 Preparation and Characterization of Activated Charcoal as an Adsorbent 135 Raw materials for the active carbon manufactures are usually wood, coconut shells, peat and carbon [20, 21]. More recently activated char and activated carbon are prepared from biomass containing lignocellulose [22]. These generally contain hydrogen, oxygen and other extraneous matters. The major portion of the extraneous substances is ordinarily driven off by heat. The main purpose of the heat treatment (carbonization) is to reduce the proportion of extraneous materials, at the same time a certain degree of activity results. For this work charcoal was prepared by combustion of grey hard coconut shell and the process of combustion and activation was performed in presence of air or nitrogen. Performance of the prepared activated charcoal as an adsorbent was tested using oxalic acid and maleic acid as adsorbates. EXPERIMENTAL Materials : Coconut shell was collected from Local market; Oxalic acid (99.5%), Maleic acid (99.5%), Sodium hydroxide (97.5%) and Hydrochloric acid (37%) (sp. gr. 1.17) were procured from the BDH Chemicals Ltd. Methods : Preparation of Charcoal from Coconut shell Two processes of preparation of charcoal in a muffle furnace were followed, in one the combustion was carried out in constant flow of air and in the other in a constant flow of nitrogen. Preparation of Charcoal in Presence of Air Firstly, the gray hard coconut shell was broken up into small pieces. The pieces were washed with water, dried in an oven and transferred to the furnace. The coconut shell pieces were burnt distinctively in the furnace for an hour fixing the temperature at 300, 350 and 400 C. Charcoal thus produced was withdrawn from the furnace, cooled, washed with tap water and dried in an oven at 110 C and ground in a morter by means of a pestle applying moderate pressure. They were sieved through mesh sieve and then activated at 300, 350 and 400 C in a furnace for 2 hours in a constant flow of air. After cooling in a desiccator the activated charcoal was ready for use. Preparation of Charcoal in Presence of Nitrogen In this process a constant flow of nitrogen instead of air was maintained for burning at 300, 350 and 400 C and activating at 300, 350 and 400 C. Determination of Equilibrium Adsorption of Oxalic Acid and Maleic Acid from Solution on Activated Charcoal Solutions of different concentrations of oxalic acid and maleic acid were prepared (approximately 0.2N, 0.4N, 0.6N, 0.8N, 1N and 1.2N solutions). 50 ml each of these solutions were pipetted out into 100 ml volumetric flasks and 0.1g of freshly activated charcoal after cooling in a desiccator was immersed into the contents in each of the volumetric flask. The content of the flask was shaken occasionally and for attainment of equilibrium kept standing for 48 hours.

4 136 Rahman et al. The mixtures were finally filtered to separate charcoal. The filtrates obtained were titrated with standard NaOH solution using phenolphthalein as an indicator. The amounts of oxalic or maleic acid adsorbed were determined from the difference of concentrations before and after adsorption. Adsorption isotherms were constructed by plotting the amount of oxalic (or maleic) acid adsorbed per 0.1g of adsorbent against its equilibrium concentration (concentration after adsorption). RESULTS AND DISCUSSION Adsorption of Oxalic Acid on Activated Charcoal from Aqueous Solution Adsorption isotherms of oxalic acid adsorbed on charcoal activated in presence of air and nitrogen are presented in Fig. 1 and 2, respectively. The equilibrium amounts of acid adsorbed on activated charcoal sharply increased initially and gradually settle to a maximum value with increase of concentration of oxalic acid indicating that the activated charcoal is very easily saturated with oxalic acid molecules at low concentration. This resulted in a Langmuir type (BET type-i) of adsorption isotherm indicating the formation of monolayer. Amount adsorbed / g Concentration of oxalic acid / normality Fig. 1. Oxalic acid adsorbed from aqueous solution on 0.1g charcoal activated in presence of air.

5 Preparation and Characterization of Activated Charcoal as an Adsorbent 137 Amount adsorbed / g Concentration of oxalic acid / normality Fig. 2. Oxalic acid adsorbed from aqueous solution on 0.1g charcoal activated in presence of nitrogen. Adsorption of Maleic Acid on Activated Charcoal from Aqueous Solution Adsorption isotherms of maleic acid adsorbed on charcoal activated in presence of air and nitrogen are presented in Fig. 3 and 4. The equilibrium amount of maleic acid adsorbed on activated charcoal sharply increases initially then tends to settle but, unlike oxalic acid, the amount of maleic acid adsorbed went on increasing as the concentration of maleic acid solution was increased further giving a sigmoid shaped (BET type-ii) adsorption isotherm. Both oxalic and maleic acids are dibasic acid but it is interesting to note that they give two distinct types of isotherms the BET type- I and BET type-ii, respectively. A probable explanation of this difference may be as follows. Activated charcoal produced under the condition applied in this work contains two specific types of adsorption sites. One of them is the polar site due to the presence of a number of oxygenated functional groups such as COOH, -OH and = CO. [23] and the other one is the basal sites as found in graphite [24]. Oxalic acid the first member of dibasic carboxylic acid contains only the polar heads of carboxyl groups and chemisorbed only on the polar sites of activated charcoal. But when maleic acid is brought in contact to charcoal, like oxalic acid, the polar heads of the carboxyl groups readily get adsorbed on the polar site of

6 138 Rahman et al. Fig. 3. Maleic acid adsorbed from aqueous solution on 0.1g charcoal activated in presence of air. Amount adsorbed / g Aamount adsorbed / g Concentration of maleic acid / normality Concentration of oxalic acid / normality Fig. 4. Maleic acid adsorbed from aqueous solution on 0.1g charcoal activated in presence of nitrogen.

7 Preparation and Characterization of Activated Charcoal as an Adsorbent 139 charcoal but as the concentration of maleic acid solution was increased the unsaturated hydrocarbon chain of maleic acid starts getting adsorbed on the basal sites through weak bonding of the carbon atom in the unsaturated methylene groups. Thus a BET type-ii adsorption isotherm results indicating multiplayer type of adsorption. Langmuir equation, applies to large number of adsorption systems where dilute solutions are involved, but a number of interesting cases of Sigmoid or BET type-ii isotherms have been reported by Hanzen et al. [25], who found that for a number of higher acids and alcohols (four or more carbon atoms) adsorbed on various carbons from aqueous solution, the isotherms showed no saturation effect but rather the general sigmoid shape characteristic of multilayer adsorption. Effect of Activation Temperature in Presence of Air and Nitrogen on Adsorption Activation of charcoal was performed at three different temperatures, 300, 350 and 400 C. Experimental data from Fig. 1 to 4 clearly demonstrate that adsorption of both oxalic acid and maleic acid on the activated charcoal increased with increase of the activation temperature in the constant flow of both air and nitrogen. It is seen from these figures that the adsorption of both the acids is greater on the charcoal activated in nitrogen atmosphere than on that activated in air. CONCLUSION Although both oxalic acid and maleic acid are dibasic acids they give distinctly two different types of adsorption isotherms (Langmuir type and BET type-ii) on activated charcoal prepared from coconut shell. REFERENCES 1. J. W. Hosterman and S. H. Patterson, Bentonite and fuller s earth resources of the United States : U.S. Geological Survey Bulletin, 1522, 45 (1992). 2. R. K. Iler, The Chemistry of Silica : Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, John Wiley and Sons Publisher, E. D. Dimotakis and M. P. Cal, J. Economy, M. J. Rood, S. M. Larson, Environ. Sci. Technol., 29, 1876 (1995). 4. S. K. W. Sing, Carbon, 27, 5 (1989). 5. L. R. Radovic, I. F. Silva, J. I. Ume, J. A. Menendez, C. A. Leony Leon. A. W. Scaroni, Carbon, 35, 1339 (1997). 6. Y. J. Eguchi, Japan Petrol Inst., 13, 105 (1970). 7. L. R. Radovic and F. Rodriguez-Reinoso, In : Thrower P. A., editor. Chemistry and Physics of Carbon, Vol. 25, New York : Marcel Dekker, 1996, p 243.

8 140 Rahman et al. 8. L. Zhong-ru, F. Yi-lu, J. Ming, H. Tian-duo, L. Tao and X. Ya-ning, J. Catalysis, 199, 155 (2001). 9. R. Leboda, A. Lodyga. and A. Gierak, Mater. Chem. and Phys., 51, 216 (1997). 10. J. Choma, W. Burakiewiez-Mortka, M. Jaronice, Z. Li and K. Klinik, J. Colloid Interface Sci., 214, 438 (1999). 11. P. C. Vandevivere, R. Bianchi, W. Verstraete, Review : J. Chem. Tech. Biotech.72, 289 (1999). 12. R. A. Corbitt, Air Quality Control. In : Corbitt R. A. (ed), Standard Handbook of Environmental Engineering. McGraw-Hill, New York, pp 4115 (1990). 13. A. C. Dillon, K. M Jones, T. A. Bekkedahl, C. H. Kiang, D. S. Bethune and M. J. Heben, Nature 386, 377 (1997). 14. R. T. Yang, Gas Separation by Adsorption Processes, Imperial College Press, pp 364 (1997). 15. V. A. Garten et al., Aust. J. Chem., 10, 195 (1957). 16. Y. Kawabuchi, S. K. Kawano and I. Mochida, Carbon, 34, 711 (1996) 17. G. M. Roy, Activated Carbon Applications in the Food and Pharmaceutical Industries, CRC Press, pp 193 (1995). 18. E. B. Sansone and L. A. Jonas, Am. Ind. Hyg. Assoc. J., 42, 688 (1981). 19. Handbook of Sugar Refining: A Manual for the Design and Operation of Sugar Refining Facilities, Chung Chi Chou (Ed). John Wiley and Sons Publisher, pp 768 (2000) 20. J. J. Pis, T. A. Centeno, M. Muhamud, A. B. Fuertes, J. B. P. J. A. Pajaves, R. C. Bansal, Fuel Processing Technol., 47, 119 (1996). 21. V. Gomes-Serrano, J. Pastor-Villegas, A. Perez-Florindo, C. Duran-Valle, C. Valenzuela-Calahorro, J. Anal. Appl. Pyro., 36, 71 (1996). 22. C. Srinivasakannan, M. Z. A. Bakar, Biomass Bioenergy, 27, 89 (2004). 23. R. Leboda, A. Lodyga and A. Gierak, Mater. Chem. Phy. 51, 216 (1997). 24. A. J. Groszek and R. E. Witheridge, ASLE Transaction, 14, 245 (1974). 25. R. S. Hanzen, Y. Fv. and F. E. Barell, J. Phys. Colloid Chem. 53, 709 (1949).