Preparation of Activated Carbon Using the Copyrolysis of Agricultural and Municipal Solid Wastes at a Low Carbonization Temperature

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1 2011 International Conference on Biology, Environment and Chemistry IPCBEE vol.24 (2011) (2011)IACSIT Press, Singapoore Preparation of Activated Carbon Using the Copyrolysis of Agricultural and Municipal Solid Wastes at a Low Carbonization Temperature AlOthman, Z. A. +, Habila, M.A.and Ali, R. Department of Chemistry, Collage of Science, King Saud University, Riyadh 11451, Saudi Arabia Abstract. Activated carbon was prepared by the copyrolysis of mixed solid wastes (biomass, cartons and polystyrene) at low carbonization temperatures. The effects of carbonization temperature (200 C, 300 C and 400 C) and chemical activation using different concentrations of a ZnCl 2 solution (0.0, 0.5, 1.0 and 2 M) on the yield and adsorption capacity of activated carbon were investigated. The results show that the activated carbon yield is significantly dependent on both the carbonization temperature and the ZnCl 2 concentration. The yield increased as the ZnCl 2 concentration increased from 0.0 to 2 M and decreased as the carbonization temperature increased from 200 C to 400 C. In conclusion, treating solid wastes with a low carbonization temperature (200 C) followed by chemical activation with 2 M ZnCl 2 yielded AC with a high adsorption efficiency. Keywords: Copyrolysis, Municipal solid waste, Chemical activation, Activated carbon, Adsorption. 1. Introduction Activated carbon (AC) is a carbonaceous material that can be prepared by the pyrolysis of many inexpensive materials that have a high carbon content and low inorganic content. On a commercial scale, activated carbon is produced by the pyrolysis and activation of high-cost starting materials, such as wood, petroleum and coal, making it expensive and unjustified as a method of pollution control [1, 2]. To reduce the production cost, the utilization of renewable and less expensive precursors for the preparation of activated carbon is attracting the interest of researchers all over the world. The precursors of interest are primarily industrial and agricultural byproducts and forest wastes, such as coconut shell [3], sugar beet bagasse [4], rice straw [5], bamboo [6], rattan sawdust [7], molasses [8], rubber wood sawdust [9], oil palm fiber [10], waste apricot [11], and coconut husk [12]. Waste plastics and tires, which are organic materials, can also be converted into activated carbon. Several experimental studies [13-18] have reported the production of char and activated carbon from waste tires. The potential of these products as possible adsorbents of various pollutants has been assessed and found to be very great. The disposed solid waste (SW) may be used as precursor in the production of a low cost adsorbent to treat wastewater contaminated with heavy metals and refractory compounds [19]. The conversion of locally available solid wastes such as biomass, waste cartons, waste news papers, plastics and industrial byproducts into activated carbon for wastewater treatment would improve the economic value by providing an alternative to costly activated carbon [20, 21]. Pyrolysis is one of the ways to take advantage of the energetic and organic value of these waste materials. Many authors have studied the pyrolysis of biomass and plastic waste and have demonstrated that it is a suitable method of waste processing [22-26]. Williams and Williams [25] and Fortuna et al. [27] studied the pyrolysis of waste tires and obtained an end product consisting of around 33% solid residue, 35% oil, 12% scrap and 20% gas. The typical slow pyrolysis method of carbonization was used to produce char [28 32]. Some scientists [33] pyrolyzed mixtures of plastics and pine residues to study the effect of the experimental conditions on product composition and yield. The goals of this study were to prepare activated carbon from 67

2 mixed wastes (biomass, waste cartons and plastics) by chemical activation with ZnCl 2, to investigate the effect of carbonization temperature and concentration of the impregnating agent on the surface properties of the AC, and to find the optimum conditions for making AC with well-developed porosity. The adsorptive properties of the prepared AC samples were also evaluated using a model compound, Methylene Blue (MB), which is a cationic dye that dissociates into Methylene Blue cations and chloride ions in aqueous solutions [34]. 2. Experimental 2.1. Materials The precursors used in this study were trunks of palm trees, waste cartons and plastics collected from a municipal solid waste station in Riyadh, Saudi Arabia. The proximate and ultimate analysis of the precursor materials was carried out according to ASTM standard techniques, and the results are given in Table 1. Table 1. Ultimate and proximate analysis of the precursor materials Activated carbon preparation In this study, the three-stage process [35] was used for the preparation of activated carbon from mixed wastes. In this process, the precursors are carbonized, impregnated, and then activated for a specific period of time. The carbon yield of the each sample was calculated using Eq. (1). Yield (%) = W 1 /W o 100 (1) where, W 1 is the dry weight (g) of the final activated carbon and W o is the dry weight (g) of the precursor material Characterization of Activated carbon The ph of the prepared AC samples was measured using a Metrohm model 744 ph meter. The bulk or apparent density was determined by a standard procedure in which a known volume of the gently tapped AC samples was weighed in a graduated cylinder. The apparent density was calculated as the ratio between the weight and the known volume of the closely packed sample. For scanning electron microscope (SEM) analysis, samples were mounted on an aluminum stub, coated with a thin layer of gold and then examined using Jeol (JSM-6380 LA) Japan. Fourier transformation infrared (FTIR) spectra of samples were recorded using a spectrophotometer (Thermo Scientific USA). Elemental analysis was performed using a Perkin Elmer Series II CHN analyzer. The adsorption capacity of the AC samples was evaluated using methylene blue (MB) as an adsorbate using eq. 2. Qe = (C 0 -Ce) * V/ M (2) where, Qe is the adsorption capacity (mg/g), C0 is the initial concentration of methylene blue, Ce is the equilibrium concentration of methylene blue, V is the volume of the solution (L), and M is the mass of the adsorbent (g). 3. Results and discussion 3.1. Thermal analysis of raw materials The thermal properties of the raw materials (palm stems, cartons and polystyrene) were investigated by TGA and DSC. Figure 1a, 1b and 1c shows the TGA-DSC curves of raw palm stems, cartons and polystyrene, respectively. The TGA of the raw materials was carried out at a heating rate of 10 C/min with a temperature range of 25 C to 1000 C. For the palm stems and cartons, the weight loss at the beginning of the process (100 C) was due to evaporation of free moisture content. As the temperature was increased further, the water bound in the lignocellulosic material also evaporated. A major weight loss occurs in the 68

3 temperature range of 200 to 500 C that is attributed to the release of volatile matter. No further change in weight was observed above 500 C. In the case of polystyrene, no weight loss occurred below 350 C because it had no moisture content. A sharp decrease in weight occurred between 400 and 500 C because of the release of volatile matter, and above 500 C no further weight loss was observed. Therefore, 500 C was selected as the activation temperature in this study. a b c Figures 1a, 1b and 1c shows the TGA-DSC curves of raw palm stems, cartons and polystyrene, respectively Activated carbon yield The overall yields of activated carbon samples obtained at different carbonization temperatures and by impregnation with different concentrations of ZnCl 2 are listed in Table 2. It is clear that the yield was significantly dependent on both the carbonization temperature and the ZnCl 2 concentration. The yield increased as the ZnCl 2 concentration increased from 0.0 to 2 M and decreased with increasing carbonization temperature from 200 C to 400 C. The higher yield of activated carbon may be due to the inhibition of the formation of volatile matter by ZnCl 2. The inhibiting effect of ZnCl 2 on volatile matter increases with higher concentration, thereby increasing the carbon yield. The ZnCl 2 acts as a Lewis acid, enhancing the aromatic condensation reactions by facilitating the evolution of molecular hydrogen from the hydroaromatic structure of the precursor and thereby leaving some active sites on adjacent molecules that can undergo aromatization (polymerization) reactions. As a result of these reactions, the volatile molecules are stabilized, and the carbon yield is increased [36, 37]. It is evident from Table 2 that the carbon yield decreased as the carbonization temperature increased. The increase in carbonization temperature accelerated the dehydration and elimination reactions, resulting in an increase in evolution of volatile matter. Table 2. Yield (wt. %), ph and density of carbon samples obtained from mixed wastes by carbonization at different temperatures and concentrations of ZnCl Ultimate and proximate composition It is clear from Table 3 that the AC samples obtained from the carbonization of mixed wastes at different temperatures and ZnCl 2 concentrations showed a reduction in volatile matter and ash content as the concentration (impregnation ratio) of ZnCl 2 was increased from 0.0 to 2.0 M. This may have been caused by the inhibition of excessive burn-off as the concentration ZnCl 2 increased, resulting in a higher yield of carbon and fixed carbon and a lower content of volatile matter and ash. Temperature had a reverse effect: increasing the temperature from 200 to 400 C promoted excessive burn-off, lowering the yield and increasing the ash content. The moisture content of the prepared AC samples increased with increasing ZnCl 2 concentration; this is due to the high affinity of ZnCl 2 for water adsorption. Table 3. Proximate and ultimate analysis of carbon samples obtained from mixed wastes by carbonization at different temperatures and ZnCl 2 concentrations. 69

4 Data from the ultimate analyses of the prepared AC samples are presented in Table 2. As can be seen, the carbon content (fixed carbon and elemental carbon) increased with ZnCl 2 concentration and temperature. The value of elemental carbon was consistent with that of fixed carbon, and for each sample the fixed carbon was lower than that of the elemental carbon. The influence of temperature and ZnCl 2 on the heteroatoms is as follows: both hydrogen and oxygen content decreased whereas the nitrogen content increased Fourier transformation infrared analysis (FTIR) The FTIR spectra showed that the functional groups of the AC samples obtained by activation with ZnCl 2 differed significantly from those of the char obtained at different carbonization temperatures. The spectra of the char prepared at different carbonization temperatures display bands at 3650 and 3629 cm -1, arising from the O-H stretching vibration in alcohols; 3447 and 3428 cm -1, arising from the N-H stretching vibration in primary amines; 2928 cm -1, arising from the CH stretch of a methylene; and 1700cm -1, arising from the C=O stretching vibration in carboxylic acid. There are strong bands around 1600, 1585 and cm -1, due to C=C stretching in an aromatic ring, and peaks around 1450 cm -1, indicating the presence of pyrones and aromatic groups. The peaks around 1317 and 1211 cm -1 show the presence of C-N stretching in aromatic tertiary amines. The peak around 1056 cm -1 indicates the presence of C-O stretching in primary alcohol. The peaks around 914 and cm -1 indicate C-H out-of-plane bending in an aromatic ring Methylene Blue adsorption Capacity The adsorption capacity of the prepared AC samples was studied by the uptake of MB, a cationic dye that has fairly large molecules. It is clear from Figure 2 that the carbon samples treated with a 0 M solution of ZnCl 2 adsorbed a very small amount of MB. For the carbons prepared by impregnation with ZnCl2, the adsorption of MB increased with increasing ZnCl 2 concentration from 0.5 to 2 M. Scanning electron microscope images (Figure 3 a, b, c, and d) for carbon activated with 0.0, 0.5, 1.0 and 2 M ZnCl 2 after carbonization at 200 C for 2 h confirm that chemical activation with ZnCl 2 promotes porosity in carbon. Greater carbon porosity makes it more accessible to the bulky molecules of MB, thereby raising its removal efficiency. Carbonization temperature also had functional groups also give insight to the adsorption capacity of activated carbon. a considerable effect on the adsorption capacity of the prepared AC samples. The adsorption capacity of the carbon samples increased with increasing carbonization temperature from 200 C to 400 C; increasing the carbonization temperature widens the pores, thereby increasing the porosity accessible to MB. The adsorption efficiency of MB for carbon produced at 200 C for 2 h and activated with 2 M ZnCl 2 was high and similar to that of carbon produced at 400 C for 2 h. Qe (mg/g) 400 o C 300 o C 200 o C ZnCl 2 / Activated carbon ratio Figure (2) Effect of the ZnCl 2 / Activated carbon ratio on the MB adsorption capacity of carbon produced at 200 C, 300 C and 400 C for 2 h. 70

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