Carbon Science Vol. 5, No. 2 June 2004 pp. 75-80 Validation of Adsorption Efficiency of Activated Carbons through Surface Morphological Characterization Using Scanning Electron Microscopy Technique Ruchi Malik*, Manisha Mukherjee**, Aditya Swami**, Dilip S. Ramteke *** and Rajkamal Sarin*** National Environmental Engineering Research Institute Nehru Marg, Nagpur - 440 020, India Project Assistant, **Senior Research Fellow, ***Scientists e-mail: ds_ramteke@rediffmail.com (Received February 6, 2004; Accepted April 30, 2004) Abstract The studies on activated carbon prepared from walnut shell and groundnut shell were undertaken to ascertain the effect of initial state of precursor and activation process on the development of porosity in the resulting activated carbon. Walnut shell based carbon shows the presence of cellular pores while Groundnut shell based carbon shows fibrillar pore structure. The adsorption parameters, characterization of product and scanning electron microscopic studies carried out showed the presence of mainly Micro, Meso and Macro porosity in carbon prepared from Walnut shell while mainly micro porosity was observed in Groundnut shell based activated carbon. An interrelationship between the adsorption efficiency and porosity in terms of quality control parameters, for before and after activation, was validated through the scanning electron microscopic data. Keywords : Activated carbon, Porosity, Adsorption efficiency, Quality control parameters, SEM 1. Introduction Activated carbon is one of the most important types of adsorbents widely used as a tool in environmental protection in various industrial applications. Each application requires activated carbon of specific characteristics such as surface area, pore size and pores distribution and adsorption characteristics. The selection of a suitable activated carbon is an integral part of the design of a carbon treatment plant. A primary characterization of the product before its application in adsorption process shows the primary sign of versatility in a convinced manner. The adsorptive properties of any activated carbon is highly dependent upon active surface sites incorporating functional groups, specific surface area, particle size, pore volume, pretreatment if any, etc. These factors have remarkable effects on the adsorption capacity and hence play important role in examining the suitability of the adsorbent for a system [1]. The properties of activated carbons are determined to a large extent by nature of precursor and pyrolysing conditions [2, 3]. Although the surface area is said to be the blue print of the quality of the activated carbon, McCreary and Snoeynik [4] found that surface area only do not provide the effectiveness of the activated carbon for a particular use. They observed that activated carbons prepared from the same raw material may have different surface area depending on the type of activation, raw material and the method of estimation. So, it is inferred that the total surface area does not only necessarily mirror the utility of activated carbon. In actual sense, the adsorption capacity of any activated carbon depends not only on the available surface area, but also on the surface morphology of the activated carbon. The crystalline structure, fractures, surface edges, nature and distribution of pores regulate the adsorption efficiency. Nature of raw material and its method of activation are mostly affecting the pores development and distribution. Moreover, the quality control parameters like Phenol value, Iodine number and Methylene blue number play key role to assess the adsorption efficiency of activated carbon for its application in terms of taste and odour removal, cross linkage between the carbon molecules and the colour removal capacity respectively. An attempt has been made to correlate the quality control parameters to surface morphology and porosity of activated carbon. Activated carbons from Walnut shell and Groundnut shell have been prepared and studied thoroughly after activation and evaluated for its adsorption efficiency through physical a nd adsorption characterization as well as morphological characterization using Scanning Electron Microscopy Technique.
76 R. Malik et al. / Carbon Science Vol. 5, No. 2 (2004) 75-80 2. Experimental 2.1. Raw Materials Agricultural solid wastes i.e. Walnut and Groundnut shells were used as precursor for preparation of activated carbon. The walnut shell waste was processed into small pieces of definite shape and dried for dehydration at 120 o C for further use of carbonization. Whereas groundnut shell was properly washed to remove any mud debris etc., dried and then grinded upto particle size of 2 mm for further process of carbonization. 2.2. Process for Activated Carbon Development The waste materials were carbonized in the electrical conventional heating reactor by two stages carbonization process known as low temperature carbonisation (LTC) and high temperature carbonisation (HTC) in the range of 250~600 o C and 600~1000 o C respectively. The materials were placed in closed stainless steel vessels by maintaining inert conditions and pyrolysis was carried out at 400 o C for 30 minutes followed by next stage to develop the pore size structure so that an accessible internal surface could be created. This was achieved by impregnating the carbonized char with chemical agent i.e. ZnCl 2 for 24 hrs. The impregnated material was dried at 100 o C and subsequently carbonized at higher temperature under optimized conditions. During activation process, the carbonisation conditions like Char-ZnCl 2 Ratio, Activation time and temperature were optimized for better pore size development and checked for quality carbon parameters. The activated product was treated with (1+1) HCl for the removal of excess of chemical agent which can be recovered by evaporation. The acid washed product was thoroughly washed with hot distilled water to remove acidity and chlorides. The product was finally dried and sieved to get particular particle size. The general schematic diagram for preparation of activated carbon has been given in Fig. 1. 2.3. Characterization of the Product The physical characteristics of the activated carbons were determined by estimating their particle size [5], bulk density [6] and surface area (BET N 2 ) using instrument (Micromeritics, ASAP-2000). 2.4. Proximate & Ultimate Analysis This was carried out by using the IS-1350 part 1, 1984 method [7]. It comprises moisture, ash volatile matter and fixed carbon content in activated carbon and expressed in terms of percentage by weight. Ultimate analysis was done by using CHNS analyzer (Carlos Erva, model no. 1108, Italy). It reveals the elementary composition of the product and expressed in terms of percentage by weight of element viz. carbon, hydrogen, nitrogen and sulphur. 2.5. Iodine Number It is the measurement of the porosity of an activated carbon by adsorption of the iodine from solution. It was measured by contacting a single sample of carbon with an Fig. 1. Schematic Presentation for preparation of Activated Carbons from Walnut Shell and Groundnut Shell.
Validation of Adsorption Efficiency of Activated Carbons through Surface Morphological Characterization 77 iodine solution (0.1 N) and extrapolating to 0.02 N by an assumed isotherm slope [5]. 2.6. Methylene Blue Number It is correlated with ability of activated carbon to adsorb colour and high molecular weight substances. It was measured by extent of adsorption of milligrams of Methylene blue adsorbed by one gram of carbon in equilibrium with a solution of Methylene blue having a concentration of 1.0 mg/l [5]. 2.7. Phenol Value It's defined as the amount of carbon required to reduce the phenol concentration in a litre of water from 0.1 to 0.01 parts per million. So this was determined by estimating the dose of the activated carbon for which the residual concentration of phenol was 0.01 parts per million [6]. 2.8. Instrumentation Scanning electron microscopy (SEM) is one of the most versatile instruments available for the examination and analysis of the micro structural characterization of solid objects [8]. Surface Morphology of Walnut shell and Groundnut shell based activated carbon was observed through scanning using Electron Probe Micro Analyzer Model JEOL-JXA 840A made by JEOL, Japan. For char and activated carbon samples, the foremost requirement is that they must be moisture free. The pulverized sample was mounted onto a substrate with a conductive adhesive. Coating with a thin film of conducting material is the primary requisite for all non-conducting specimens to be examined in SEM or EPMA. In the present study, conducting material coating on specimen was done with gold metal by vacuum evaporation to get uniform thickness of specimen during analysis, regardless of the specimen topography. 3. Results and Discussion 3.1. Proximate and Ultimate Characteristics Proximate analysis in terms of Ash, moisture, volatile matter and fixed carbon of both the carbons indicate marginal variation specially in the volatile matter and fixed carbon. Less percentage of volatile matter represents the completion of the carbonization process leaving more fixed carbon (aromatized carbon) in the carbonaceous sample and it reflects on the total carbon present in the product. Both the carbons are showing considerable total carbon content. Sulphur and nitrogen content was found to be more in the GSPAC i.e. groundnut shell based activated carbon because of its leguminous plant origin as compared to WSGAC i.e. walnut shell based activated carbon. The results are depicted in Table 1. Table 1. Comparative Proximate and Ultimate Analysis of Activated Carbons Sr. No. Parameters Value (%) WSGAC GSPAC Proximate Analysis 1 Moisture 8.0 9.0 2 Ash 5.5 8.2 3 Volatile Matter 15.9 11.4 4 Fixed Carbon 70.6 71.4 Ultimate Analysis 1 Carbon 79.2 75.8 2 Hydrogen 4.20 2.95 3 Nitrogen 0.16 1.53 4 Sulphur 0.54 0.32 3.2. Physical Characteristics Results of the physical characteristics of activated carbons are given in Table 2. Bulk density of GSPAC is lower as compared to WSGAC because of its powdered nature. Percentage loss on ignition is higher for WSGAC indicating its high carbon content. Surface area is very high in case of GSPAC due to its small particle size, which shows the possibility of high adsorptive nature. 3.3. Effect of Chemical Activation on Pore Size Development The amount and distribution of pores of activated carbon play a key role in determining adsorption efficiency. The physical and chemical nature of activated carbon is dependent on the type of material and the mode of activation. This is mostly due to the influence that raw material and activation have on pore size and its distribution. Activation process plays major role in porosity development, which is largely responsible for the extent of surface area and adsorptive capacity of carbon. ZnCl 2 causes hydrolysis reactions, weakens the precursor structure and release of volatile matter takes place producing microporous structure Table 2. Comparative physical characteristics of Activated Carbons Sr. No. Parameters WSGAC GSPAC 1 Bulk Density (g/cm 3 ) 0.64 0.23 2 Particle Size (mm) 0.80 0.075 3 Surface area (m 2 /g) Char Carbon 430 750 550 1200 4 Loss on Ignition 95 91.8 WSGAC: Walnut Shell Based Granular Activated Carbon GSPAC: Groundnut Shell Based Powdered Activated Carbon
78 R. Malik et al. / Carbon Science Vol. 5, No. 2 (2004) 75-80 Plate 1. SEM Micrograph of Walnut Shell based Char showing Surface Morphology before Activation. Plate 2. SEM Micrograph of Walnut Shell based Granular Activated Carbon showing Pore Developments after Activation. in carbon [9]. This fact can be quite clear from the SEM studies on chars and activated carbons of both samples. Plates 1 and 3 show SEM micrographs of Walnut shell and Groundnut shell based chars. Hard woody material gives cross-interconnected pores while softer woody material gives fibrillar structure [10]. As the nature of precursor and method of activation, both have strong influence on porous structure and adsorption capacity of resulting activated carbon. SEM micrographs of chars at 400oC obtained from walnut shell & groundnut shell showed the presence of longitudinal fibers in case of groundnut shell. Whereas walnut shell char showed the cellular pore structure. Plates 2 and 4 show SEM micrographs of the activated carbons. As seen from these micrographs, chemical activation with ZnCl2 results in widening up of pores being corrosive and dehydrating in nature. In hard wood, the pores widen much more as compared to soft wood during pyrolysis. Chars of both the nutshell wastes are showing rudimentary pores. Whereas on activation of char, there is development of micro, meso and macro pores in Plate 3. SEM Micrograph of Groundnut Shell based Char showing Fibrillar Structure before Activation. Plate 4. SEM Micrograph of Groundnut Shell based Powdered Activated Carbon showing Pore Developments after Activation. walnut shell and micro pores are clearly showing up inside macro pores. Groundnut shell based carbon, being powdered in nature, showing the presence of micro pores. 3.4. Surface Area Development The results of surface area of char & carbons are given in Table 2. Surface area of both the activated carbons are more as compared to char, this is because of high porosity development of carbons due to activation as compared to char. The availability of the surface area is dependent on the pores developed during activation. Surface area of GSPAC is very high as compared to WSGAC because of small particle size of GSPAC as surface area is also dependant on particle size of adsorbent, smaller the particle size, more is the surface area of activated carbon. Although the surface area varies with both the source of raw material and the temperature used for activation process, but the surface area is said to be the blue print of the quality of the activated carbon, and generally, larger the surface area of activated
Validation of Adsorption Efficiency of Activated Carbons through Surface Morphological Characterization 79 carbon, higher the adsorption capacity. 3.5. Adsorption Properties The adsorption characteristics are denoted as the quality control parameters for any activated carbon. Quality control parameters are generally expressed in terms of iodine number, methylene blue number and phenol number. Adsorption of Methylene Blue, iodine and phenol has been used for a long time as a tool for the evaluation of the adsorption properties as far as the physical structure of activated carbon is concerned. Adsorptive properties are directly linked with the porosity of activated carbon as the highly porous carbons can adsorb relatively large quantities of adsorbate [11]. The comparative adsorption properties for walnut shell and groundnut shell based chars and activated carbons are given in Figs. 2-4. Iodine values for chars and carbons are shown in Fig. 2 for both materials. Iodine value of both the carbons are more as compared to char. High iodine values for carbons denote the presence of highly porous structure and intermolecular cross linkage, which is developed due to activation of char at high temperature. Iodine value also gives an idea of capacity of any activated carbon to adsorb low molecular weight substances. Iodine value for GSPAC is more which could be because of its high micro porosity. Methylene blue number is correlated with the capability of Fig. 4. Comparative Phenol Value Developed on Activation for Walnut shell and Groundnut shell based Activated Carbons. activated carbon to adsorb higher molecular weight substances whereas phenol number indicates the taste and odour efficiency of activated carbon. Both the carbons are having more Methylene blue and phenol value as compared to char as shown in Figs. 3 and 4. The high molecular weight substances need high surface area and more number of macro pores for adsorption on to any activated carbon which reflects here also for more M.B. number for activated carbon than chars because of porosity development during activation as proved by SEM Micrographs. Methylene blue number for WSGAC is more than GSPAC because of presence of more Macro pores developed in case of WSGAC. High porosity of carbons along with the surface oxygenated groups developed during the process of activation is also proved by high phenol values of activated carbon. 4. Conclusion Fig. 2. Comparative lodine Value of Activated Carbon before and after Activation. Adsorption capacity and porosity of any activated carbon depends largely upon Pyrolysis activation conditions, morphology & physical state of precursor i.e. hard or soft wood. Activation process plays the major role in porosity development of any type of raw material as proved from the results of SEM micrographs and adsorption properties of chars and activated carbons. Development of pore type is dependant upon quality of precursor as there is presence of Macro, Meso and micro pores in WSGAC because of it's woody nature whereas GSPAC is found to be microporous as initial state of precursor is soft wood type. So, it can be concluded that the adsorptive capacity and porosity development of any activated carbon are interlinked as the values of iodine, Methylene blue & phenol number also support this fact. Acknowledgement Fig. 3. Comparative Methylene Blue Value of Activated Carbons. The authors are thankful to CSIR (India) for granting fellowship and Director, NEERI, Nagpur for providing facilities to carryout this research work and kind permission for this
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