INCREASING ADSORPTION OF ACTIVATED CARBON FROM PALM OIL SHELL FOR ADSORB H2S FROM BIOGAS PRODUCTION BY IMPREGNATION

Biogas is the combustible gas produced through a bi ological process, known as anaerobic digestion which is the process operated at low-temperature an d without air. Biogas consists of 55-80% CH 4, 2045% CO2 with trace amount of H2S and other impurities. Common H 2S removal technologies from biogas fall into one of adsorption on a solid such as iron oxide based materials, activated carbon or impregnated activated carbon. Conventionally, activ ated carbon is produced from biomass residues and agricultural residues such as palm oil shell which promising approach for the production of cheap. It is so due to the palm oil shell carries a large amount of carbon content which it is the main composition of activated carbon. Therefore, it is usable as raw material for producing impregnated activated carbo n and used as adsorbents. The aim of this study is a prod uce the activated carbon from palm oil shells by chemical activation using ZnCl 2 and optimal conditions after impregnated them with NaOH, KI and K2CO3 for H2S absorption from biogas product. In this research, production of activated carbon involved three stages; (i) carbonization of raw mat erial in an inert atmosphere which was carbonized i n a muffle furnace at 600°C for 1 h; (ii) secondly ac tivation of char product from the first stages at f ixed bed reactor (stainless steel with 54.1 mm internal di meter and 320 mm length) which was studied to observe the effect of char product: Chemical agent ratio (ZnCl2, 1:1 to 1:3), which there are activated at 700°C activation temperature for 2 h; and (iii) finally alkali impregnated activated carbon which were immersed 1:3 ratio in 500 mL of 1 N NaOH, KI a nd K2CO3 solutions and stirred for 30 min. The result showed that the surface area and the pore vo lume increased progressively with increasing the char product: Chemical agent ratio. The maximum sur face area total pore volume of activated carbon product was 532.31 m /g and 0.2973 cc g −1 was obtained on ACZn13. While, the maximum surface rea and the pore volume of modified impregnated activat ed carbon was 741.71 m /g and 0.4210 cc g −1 was obtained on K2CO3-AC. The results of the experiment demonstrated tha t with regard to adsorption of H2S on K2CO3-AC which it had more than on KI-AC and NaOH was 3. 26 and 8.8%, respectively. The conclusion showed that surface area, total pore vol ume and micropore volume were increasing with increased the char product: Chemical agent ratio (1 :1 to 1:3). ACZn13 has good chemical and physical Wasan Phooratsamee t al. / American Journal of Environmental Sciences 10 ( 5): 431-445, 2014 432 Science Publications AJES properties such as chemical content and surface are a, which showed that the highest surface area (532. 21 m/g) and total pore volume (0.2973 cc g ). Therefore, K2CO3-AC impregnated activated carbon has a high surface area and showed be an efficient adsorbent f or removal of H2S from biogas product. Moreover, evaluating the content of H 2S adsorption was suggested.


INTRODUCTION
Thailand has been well recognized as one of the top exporters of agricultural biotechnology with traditional energy sources. The amount of waste from agriculture has increased due to rapid industrial development in recent years. Agricultural industries and livestock farms generally produce several organic wastes and wastewaters, which greatly affect air and water pollution (Sitthikhankaew et al., 2014). Focus on environmental problems from industrial waste, palm oil shells from factory wastes agriculture in Thailand. Palm oil shells from oil palm occurs carbon primarily in large quantities. Utilization of agricultural wastes to produce activated carbon is an important approach in air pollution control strategy. Converting these wastes into value added product such as activated carbon could solve environmental problems such as accumulation of agricultural waste, air pollution and water pollution (Nor et al., 2013). The products of palm oil mill are to plant oil palm in Thailand can be produced 1.7×10 6 tons/year. Palm oil extraction process had palm oil shell as by product. Considering the amount of substances from the image processing palm oil palm shell were left over from production of about 9% by weight, or to enter a scope with a bunch of tons into the production process. Palm oil shell occurs from extraction process about 153,000 tons/year of general machinery, appliances, palm fiber and shell as fuel to power the boiler. The heat loss from the burning of fibers was less than 5 MJ kg −1 . The heat loss from palm oil shell (dry) were about 17 MJ kg −1 . Heat from the burning palm shell was higher than the needs of the plant. Most people prefer to use the fibers in burning more fuel. Cause the accumulation of shells in the palm oil mill up. Choice of using palm shell to use is the production of activated carbon. Presently, low cost palm oil residues are considered promising adsorbents for adsorption applications. Besides, it is cheaper and readily available materials.
Activated carbon is one of the most widely used adsorbents because of its high adsorptive capacity; therefore, it has been widely used as adsorbent and in catalysis or separation processes. As results, the demand for activated carbon is increasing (Saka, 2012).
An Activated Carbon (AC) adsorbent is usually dominated by micropores (width<2 nm) whereas its specific volume of mesopores (width 2-50 nm) is less than 0.2 Cm 3−1 g. Highly mesoporous AC powders have industrial relevance, such as for desulfurization, adsorption of large molecules, catalyst supports and high capacitance-carbon electrodes (Sun and Hong, 2011). Production of activated carbon involved two stages; (i) carbonization of raw material in an inert atmosphere and (ii) secondly activation if the carbonized char. The product is categorized in laboratory scale into two: Which is, on includes the synchronous process of carbonization and activation (one-stage process), the other is asynchronous process in which a separate carbonization process is included prior to the activation process (two-stage process). The carbonization involves thermal decomposition of the carbonaceous material in an inert environment to eliminate volatile components to produce fixed carbon with limited or undeveloped pores. Activation enlarges the pores created during carbonization together with formation of new pore structures. Activation is achieved via physical or chemical means: Physical activation involves oxidation of char using steam, air, oxygen or carbon dioxide at high temperature (800-1100°C), chemical activation involves impregnation of chemicals (such as ZnCl 2 , NaOH, HNO 3 , KI, Na 2 CO 3 , K 2 CO 3 ) and then pyrolysis at 500-800°C (Nasri et al., 2014).
Hydrogen sulfide is one of the major environmental pollutants having its source in natural and anthropogenic activities. One of the methods of its removal is adsorption. Various material have been extensively investigated either as H 2 S adsorbents or adsorbent' supports. High distribution of catalytic enters promoting oxidation and ability to retain water is considered as important factors promoting an enhancement in the adsorption capacity of various media. While water is the pore system contributes to dissolution of H 2 S, the basic surface promotes it dissociation to HS-ions, which then are oxidized either to elemental sulfur or sulfur oxides/oxysulfur acids. Even though zinc oxide exhibits good capacity at elevated temperatures at room temperatures Science Publications AJES activated carbons are widely used in desulfurization processes (Seredych and Bandosz, 2011). Biogas is the combustible gas produced through a biological process, known as anaerobic digestion, which is the process, operated at low-temperature and without air. Biogas consists of 55-80% CH 4 , 20-45% CO 2 with trace amount of H 2 S and other impurities (Pipatmanomai et al., 2009). Biogas can be utilized for generation of electrical power and heat production, generally; biogas (as well as natural gas and synthesis gas from coal gasification) contained hydrogen sulfide (H 2 S) that was highly toxic for human body and corrosive for machine. The content of H 2 S in biogas was about 0-1% and H 2 S must be removed from fuel gases prior utilization (Sitthikhankaew et al., 2011). Common H 2 S removal technologies for H 2 S removal from biogas fall into one of the following (Pipatmanomai et al., 2009): (i) Absorption into a liquid either water or caustic solution; (ii) adsorption on a solid such as iron oxide based materials, activated carbon or impregnated activated carbon and (iii) biological conversion by which sulfur compounds are converted into elemental sulfur by sulfide oxidizing microorganisms with addition of air/oxygen. Each technology has its own advantages and disadvantages.
In this study, H 2 S removal by NaOH, KI and K 2 CO 3 impregnated activated carbon adsorption was selected and compared to commercial activated carbon. The aim of the study is to produce activated carbon from palm oil shell by chemical activation ZnCl 2 .The aim of this study to achieving the following: (i) Production of activated carbon by chemical activation with ZnCl 2 using palm oil shell as a raw material, (ii) impregnation activated carbon with NaOH, KI and K 2 CO 3 asimpregnation agent, (iii) analysis of chemical and physical properties for activated carbon product is performed by ultimate analysis, proximate analysis, FTIR, XRD, SEM and surface area and (iv) Hydrogen Sulfide (H 2 S) adsorption.

Materials
Palm Oil Shell (POS) was used as staring material in the preparation of activated. POS, AS Received from Suratthani provice, southen Thailand, in successive steps were first air-dried, crused and sized, the fraction of particle sizes between 2-2.8 mm being chosen. The proximate, ultimate and component analyses of Pam Oil Shell (POS) are shown in Table  1. The commercial activated carbon (Commercial grade; code No.CGC-11A) was supplied from C. Gigantic Carbon Co. Ltd. Zinc Chloride (ZnCl 2 ), potassium carbonate (K 2 CO 3 ), sodium hydroxide (NaOH), potassium iodine (KI) were dissolved in distilled water to prepare a saturated solution.

Experimental Method
The experiments methods used in this study include the following steps: (i) Adsorbent preparation, (ii) impregnated activated carbons and (iii) adsorption of Hydrogen Sulfide (H 2 S) by modified activated carbon. Figure 1 shows flow chart of experiments.

Adsorbent Preparations
The preparations of adsorbents were divided into two steps: Carbonization and activation process;

Carbonization Process
Carbonization process was the first step of the experiment. Palm Oil Shell (POS) was dried in a hot air oven at 110°C for 24 h to remove any surface moisture. They were then ground and sieved to a size range of 2-2.8 mm. In the carbonization step, a stainless steel tube reactor of length 320 mm and internal diameter of 54.1 mm. filled with 40 g of palm oil shell was carbonized in a muffle furnace at 600°C for 1 h under the flow of N 2 gas at 150 Cm min 3−1 . The heating rate was controlled at 10°C min −1 , which the char products were allowed to cool to room temperature. Figure 2 shows the schematic diagram of the experimental set-up.

Activation Process
Activated carbon produced by chemical activation with zinc chloride (ZnCl 2 ) under conditions of activation 1:1-1:3 of char ratio: ZnCl 2 (w/w %), activation temperature 500°C for 2 h (as shown in Table 2), which the chemical activation was poured to a beaker containing 0.1 M (mol/dm 3 ) HCl and stirred for 10 min. Finally, the activated carbon sample was washed with hot water and water until pH of the washing solution reached 6-7 and dried in an oven at 105°C for 3 h.

Modified Activated Carbon
Preparation of impregnated activated carbon; three types of alkali-impregnated activated carbons were prepared by the same procedure but with various impregnation solutions. The activated carbons were immersed 1:3 ratio in 500 mL of 1 N NaOH, KI and K 2 CO 3 solutions and stirred for 30 min. The immerged activated carbons were then filtered at from the impregnation solutions and dried in an oven at 120°C for 60 h. The activated carbons immersed in NaOH, KI and K 2 CO 3 solution were noted as NaOH-AC, KI-AC, K 2 CO 3 -AC, respectively.

H 2 S Adsorption by Modified Activated Carbon
The H 2 S adsorption using small column test equipment was conducted on the adsorbents of the activated carbon product. Adsorbent (10g) of the impregnated activated carbon was placed in a PVC column 5 Cm diameter, using biogas product (Saitawee et al., 2014) as test gas with the total flow rate 15 Cm 3 min −1 . Biogas detector was used to analyze the concentration of H 2 Sin the inlet and outlet gases, as shown Fig. 3.

Characterization of Palm Oil Shells
The chemical composition of palm oil shell was determined by CHNS analysis.

Characterization of the Carbonized Carbon and Activated Carbon
Moisture and dry matter content, an amount of 1.0 g each have carbonized sample was placed in a clean silica crucible that was previously dried in desiccators and weighed. They were then dried in an air-circulated oven at 105°C for 3 h after which they were cooled in desiccators and weighed. Results were expressed as percentage of the initial weight carbonized samples.

AJES
Ash content, crucibles were heated in a furnace at 500°C, then cooled in desiccators and weighed. The oven-dried samples from the previous section were placed in the crucible and transferred into the muffle furnace at 900°C for 3 h. The crucibles/content were allowed to cool in desiccators and then weighed to obtain the weight of the ash. The ash content was expresses as percentage of the oven dry weight.
The chemical composition of product was determined by CHNS/O analyzer (PE2400 Series II) and Fourier Transform Infrared (FTIR) using a diffuse reflectance infrared fourier transform spectrometer. A total of 256 scans were recorded per spectrum over the range 4000-650 cm −1 at a resolution of 4 cm −1 . To improve the reflectivity of the samples, the samples were diluted with KBr is ratio of KBr: Sample, 20:1. The FTIR spectra of the adsorbents were obtained under 50 mL helium flows at 25ºC. A spectrum of KBr recorded in He served as background.
The surface area of samples was analyzed using nitrogen adsorption at 77 K in a Quanta chrome Autos orb automated gas sorption system.
Scanning Electron Microscope (SEM) images of activated carbon produced from palm oil shell by ZnCl 2 activation and K 2 CO 3 , KI, NaOH impregnation was recode by using JSM-6400.

Characterization of Inlet and Outlet Gas from H 2 S Adsorption Test
The inlet and outlet gas from H 2 S adsorption test were analyzed the chemical composition by using Portable gas analyzer (BIOGAS 5000).

Effect of Activated Ratio on Activated Carbon Product
The results of the proximate analysis of palm oil shell indicated in Table 1. The results are compared with previous research on palm oil shell. The fixed carbon, volatiles, moisture and ash content for POS are 10.39, 87.78, 3.86 and 1.83 respectively. The high volatile matter (87.78%) and moderate amount of fixed carbon (10.39%) is a typical property of agricultural residues. The results from the proximate analysis are within the range of literature review (Nasri et al., 2014). The high volatile contents make it more reactive and therefore aid during the carbonization process in creating more pores on the carbonaceous material. The ultimate analysis of POS indicated a carbon, hydrogen and nitrogen content of 47.26, 6.18 and 0.94%, respectively (Table 1). These values are similar to those previously determined (Nasri et al., 2014). The low nitrogen content from the analysis indicates that POS is environmental friendly. The preparation of the activated carbons from palm oil shell was carried out at carbonized in a muffle furnace at 600°C for 1 h and activation temperature 500°C for 2 h for ZnCl 2 are given Table 2.
The chemical content of activated carbon is shown in Table 3. The carbon content was found to be between 72.42 and 74.81 wt %. The result showed that the ratio of char product: Activating agent used for zinc chloride has effect on the chemical content of activated carbon, which the carbon content in the activated carbon increased with increasing the char product: ZnCl 2 ratio. Furthermore, the carbon content to obtain the maximum was increased with the increasing of ZnCl 2 , which it is ACZn13. The ultimate analysis of AC, ACZn13, ACZn12 and ACZn11 indicated a carbon, hydrogen and nitrogen content of 72.70, 1.12 and 0.60% (AC), 74.81, 0.76 and 0.28% (ACZn13), 72.44, 0.87 and 0.42% (ACZn12), 72.42, 0.88 and 0.65% (ACZn11), respectively.

Comparison of Three Impregnated Agent for Activated Carbon Production on Chemical Content
These studies were performed to compare the impregnated activated carbon from three agents (K 2 CO 3 , KI and NaOH) and the results are given in Table 4, which activated carbon was produced from char product: ZnCl 2 at 1:3 on 500ºC and 2 h. Due to this conditions had the highest carbon content. It is to note that the average carbon, hydrogen and nitrogen content for impregnated activated carbon of the three impregnate agents was not the same, varied in the range 73.31-78.60, 0.79-0.49 and 0.71-0.87%, respectively. According to Table 4, the K 2 CO 3 -AC gave the highest average carbon content, followed with NaOH-AC and KI-AC.

Characterization of Impregnate Activated Carbon
Alkali carbonate, alkali hydroxides and alkali iodine (specifically K 2 CO 3 , NaOH and KI) are generally used as impregnated agents for the activated carbon production because of the valuable properties of materials produced by this activator. In particular, porosity development by alkali activation is associated with a well-established gasification mechanism. Sitthikhankaew et al. (2014), studies the alkali Science Publications AJES impregnated activated carbon and found that impregnated activated carbon had higher surface area than commercial grade impregnated activated carbon. Table 5 is presented BET surface area and total pore volume of commercial grade activated carbon, activated carbon and impregnated activated carbon product. It can be seen that the ACZn13 BET surface area and total pore volume had higher than ACZn12 and ACZn11, which it is 532.31, 515.12 and 515.12 m 2 /g and 0.3112, 0.2973 and 0.2899 cc/g, respectively.
Comparison of N 2 adsorption experiment, the surface properties of impregnated activated carbon can be calculated as presented in Table 5. It was found that K 2 CO 3 -AC has BET surface area of 743.71 m 2 /g, total pore volume of and 0.4210 cc/g, which there was more than commercial standard, KI-AC has BET surface area of 627.02 m 2 /g, total pore volume of and 0.3536 cc/g, while NaOH-AC has BET surface area of 551.04 m 2 /g, total pore volume of and 0.3112 cc/g. This improvement is due to the chemical agent (especially alkali carbonate) of activated carbon during impregnated, which eventually increases the porosity of the samples. Okman et al., (2014) produced activated carbon from agricultural by chemical activated carbon with K 2 CO 3 and KOH, it was found reported that K 2 CO 3 as activated had higher surface area. Our results are in good agreement with these reports.
The FTIR spectra, which were obtained from the impregnated activated carbon, kept under 50 mL/min He flow, for all the impregnated activated carbon are given in Fig. 4 and Table 6. It is convenient to investigate the The spectral zone 3872.54-3564.52 cm −1 Range Displays Bands Mostly Attributable To hydroxyl species. The position of the band is characteristic of stretching vibration of hydroxyl compounds from carboxyl's, phenols or alcohols. This band diminishes with the K2CO3 and NaOH impregnated activated carbon (Fig 4b and d). There are five main bands in the spectral regional 1800-800 cm −1 confirming the presence of surface oxygen groups: (i) A band of1739.85-1729.82 cm −1 characteristic of C = O stretching vibration in carboxylic groups, lactones and anhydrides which occurred in impregnated activated carbon (K 2 CO 3 -AC, KI-AC and NaOH-AC), (ii) a band of 1652.14 cm −1 attributed to C = C stretching vibration in aromatic rings of quinone and keto-enol groups which occurred only in K 2 CO 3 -AC show that the extent of aromatization is very high, (iii) a band of 1365.57-1351.32 Cm −1 which occurred KI-AC and NaOH-AC may be attributed to C-H deformation vibration in CH 2 or C(CH 3 ) 3 groups, (iv) a band of 1216.47-1202.92 cm −1 is the C-O-C stretching vibration in lactones and ethers which occurred in only KI-AC and (v) a band of 1058.55-1044.20 cm −1 attributed to C-OH stretching vibration. These results were comparable with the work done by (Tan et al., 2008) (9) which reported that more acidic groups such as carboxylic and ether were produced by acid treatment on a modified oil palm shell activated carbon.
Surface morphology of impregnated activated carbon was then performed using a Scanning Electron Microscope (SEM), as shown in Fig. 5. It can be observed that the activated carbons contain pores of different sizes which are a common characteristic of activated carbon as can seen in the other hand (Sitthikhankaew et al., 2014). The micrographs that the external surface of impregnated activated carbon have cracks, crevices and some grains in various sizes in large hole. It can conclude from SEM images taken during operated as a carbonization temperature of 600ºC that porous structure was formed because of most of the organic volatiles were evolved, leaving behind the ruptured surface of activated carbon with a small number of pores. These results are agreement with in the other hand (Saka, 2012). In addition, SEM images of K 2 CO 3 -AC showed spongy-like structures with many of small and large holes more than KI-AC and NaOH.

H 2 S adsorption
The H 2 S adsorption of commercial activated carbon and impregnated activated carbon were investigated at ambient temperature. Table 7 gives the summary of H 2 S content from biogas before adsorption and H 2 S content after adsorption with impregnated activated carbon for AC, K 2 CO 3 -AC, KI-AC and NaOH-AC. There is increase in adsorption and there is lower H 2 S content after adsorption at 26.75 ppm, 21.71 ppm and 20.5 ppm with impregnated in NaOH, KI and K 2 CO 3 , respectively. One can see that K 2 CO 3 -AC exhibited a higher H 2 S adsorption than KI-AC, AC and NaOH-AC due to development of more porosity in the impregnated activated carbon which it had both far larger surface area and total pore volume than comercial activated carbon and other impregnated activated carbon.   Table 3. Characteristics of palm oil shell, standard activated carbon and activated carbon product (AC, ACZn11, ACZn12 and ACZn13) Proximate analysis dry basis, wt (%)

DISCUSSION
Activated carbon produced from palm oil shell to be suitable for the removal of air pollutants. In this research, activated carbon prepared from char product of palm oil shell using ZnCl 2 activation (1:1 to 1:3 ratio of char product: ZnCl 2 ) were studied. It was found that the highest chemical content and surface area occurred in the ACZn13. One can see that the surface are incresed with increasing the ratio of char product: ZnCl 2 which it can be assumed that a large amount of activated agent expand the macro-pore entrance or open the empty surface area of the activated carbon. In the impreganated process, both chemical content, surface area, FITR band and SEM image indicate that impregnated activated carbon behavior depends on the alkaline impregnate agent, which there were K 2 CO 3 , KI and NaOH.
When chemical impregnated with K 2 CO 3 was used, higher than of chemical content due to be release of volatile products as a result of intenifying dehydration and elimination reaction; it also indicated that activated carbon was gasified by K 2 CO 3 . On other hand studied the gasification of graphite by alkali metal and found that Science Publications AJES K 2 CO 3 was redued in an inert atmosphere by carbon whereby activated carbon reacted with K 2 CO 3 and then the later was removed during gasification. Therefore, the surface area and pore volume highest (Adinata et al., 2007).
The H 2 S adsorption performed of comercial activated carbon and impregnated activated carbon under ambient temperature were tested and the results are presented in Table 7. For impregnated activated carbon, after H 2 S adsorption, the concentration of H 2 S reached almost 20.5 ppm of K 2 CO 3 -AC, 21.75 ppm of KI-AC and 26.75 ppm of NaOH-AC. While the concentration of H 2 S reached almost 24 ppm of comercial activated carbon. In this results showed that K 2 CO 3 -AC had slighly higher H 2 S adsprprion than KI-AC and NaOH-AC were 3.26% and 8.8%, respectively. A reaction of H 2 S adsorption occurs along with physical adsorption which there are two main reaction: The first, H 2 S treanferred from the bulk stream into the pore or surface of the activated carbon; and the second H 2 S adsorbed on activated carbon from which H 2 S (g) and H 2 S (ads) correspond to H 2 S in gas and/or adsorbed phases (as can seen in Equation 1 (Sitthikhankaew et al., 2014): Science Publications AJES chemical content and surface area, which showed that the highest surface area (532.21 m 2 /g) and total pore volume (0.2973 cc/g).
For modified activated carbon, it was found that the impregnation with K 2 CO 3 results in the lower concentration of hydrosulfide (H 2 S) (20.5 ppm) and enhances the oxidation of H 2 S (Xiao et al., 2008) which is in high concentration of hydrosulfide ion (HS −1 ). It had H 2 S adsorption more than commercial activated carbon (8%). Factors affecting adsorption types are pore structure, surface chemistry and adsorbent properties. Among them funtional groups and chemical compositions play important roles in the adsorption mechanism and capacity. Moreover, alkali impreganated activated carbon (K 2 CO 3 -AC and KI-AC) had H 2 S adsorption more than commercial activated carbon while NaOH-AC had H 2 S adsorption lower than commercial activated carbon about 4% which could possibly be due to the less vapor water content effect of carbon surface (lower that other alkaline impregnated activated carbon) (Sitthikhankaew et al., 2014). Therefore, K 2 CO 3 -AC impregnated activated carbon in this study has these special characteristics and can be recommended as economical and environmentally friendly activated carbon.
Accordingly, in this case the activated carbon produced from palm oil shell are widely used as adsorbents of gases including removing H 2 S compound, especially those which contitute an environmental hazard but there are limitation in the amount of biogas, which it was operated in laburatory scale effect to had operated small scale in H 2 S adsorption. Therefore, in the further research, other alkalin impregnation, other ratio of impregnated ratios on activated carbon and the other method such as physical activated carbon for modified activated carbon will be used in studies similar to those described in this research. Especially, other gases adsorption such as CO 2 will be operate on modified activated carbon in the next research which use to industrial scale and evaluating the performace capacity of adsorption were suggest.

ACKNOWLEDGMENT
The gratefully acknowledge the provision of Scientific and Technological Research Equipment, Chulalongkorn University researchers gratefully acknowledge the provision of Scientific and Technological Research Equipment, Chulalongkorn University and King's Mongkut University of Technology Thonburi.