production of activated carbon from acacia arabica sawdust20121221

Production of Activated Carbon from Acacia Arabica Sawdust - 01-24-2013by admin - International Journals and Conference of management, statistics and social sciences -http://www.hup.edu.pk/hujournal

Production of Activated Carbon from Acacia Arabica Sawdust

by admin - Thursday, January 24, 2013

http://www.hup.edu.pk/hujournal/?p=224

Production of Activated Carbon from Acacia Arabica Sawdust

Moinuddin Ghauria*, Muhammad Tahira, Tauqeer Abbasa

a Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore,Pakistan

* Corresponding Author E-mail: [email protected] Tel: +92-42-111-001-007

Abstract

In this article a technique to produce activated carbon from acacia arabica sawdust is described. Thesawdust was first pyrolysed in a tube furnace under nitrogen gas atmosphere at a temperature range of350 to 450 ?C. The pyrolysed samples were further treated with zinc chloride (ZnCl2) to activate thecarbon produced in the pyrolysis step. The samples of the formed activated carbon were used to carry outthe adsorption study using Congo red as a model adsorbate dye. It was found that the most active carbonwas produced when the pyrolysis temperature was 700 ?C for 30 minutes and ZnCl2 to carbon mass ratiowas 2.5: 1(ZnCl2: Carbon). The iodine value of this sample was found to be 905 mg/g sample. It was alsofound that the activated carbon produced using this technique can remove Congo red dye very efficientlyfrom aqueous solution by adsorbing it. The adsorption data obtained in this study were also used to fitdifferent adsorption kinetics and equilibrium models published in the literature.

Keywords: Adsorption, Activated Carbon, Kinetics, Isotherm, Saw dust, Equilibrium.

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1. Introduction

Wastewater from industries such as textile, paper, leather tanning, rubber, plastic and cosmetic is oftenpolluted by dyes [1-4]. Discharging of wastewater containing dyes into the environment even in smallamount has a deleterious effect on aquatic life, food web and can lead to allergic dermatitis, skinirritation, cancer and mutation. This is due to the presence of a large number of contaminants like toxicorganic residues, acids, bases and inorganic contaminants [5-7]. The removal of the spent dyes fromwastewater is a major problem due to their chemical stability. Dyes are difficult to remove byconventional effluent treatment methods such as coagulation, ozonation, chemical oxidation, membranefiltration, solvent extraction, precipitation, and microbiological techniques etc. [8-11]. The separation byadsorption using low cost adsorbent is considered to be a better option due to its low cost, easyavailability of adsorbents, simplicity of design, high efficiency, ease of operation and biodegradability ofmany adsorbents [12-15].Activated carbon as an adsorbent is widely used because of its high adsorptionabilities for a large number of chemicals [16-18]. The activated carbon is quite expensive material, and itscost prevents it to be used in a large number of separation applications. There are a number of studiesgoing on to find economical process to prepare activated carbon. Many low cost materials such as coirpith carbon, fly ash, coir dust, sawdust, palm shells, biomass fiber waste, rice husk, coffee grounds andrubber wood saw dust etc. [19-28]; have been applied over the period of time with varying degree ofsuccesses for the removal of dyes from aqueous solutions. The present study is another effort to produceeconomically viable activated carbon.

Activated carbon can be produced either by physical activation or chemical activation technique [29]. Thephysical activation technique consists of carbonization of the precursor material followed by gasificationof the resulting char in steam or carbon dioxide. The chemical activation technique is performed bycarbonizing the raw material that has been impregnated with chemical reagent (e.g., ZnCl2, H3PO4 and/orKOH). The chemical activation technique has been the subject of many studies in the past years as itpresents several advantages over the physical activation technique. The chemical reagents promote theformation of cross-links, reduce volatile loss, and enhance the yield and increase the surface area of theresulted product [30, 31]. ZnCl2 is one of the most widely used chemical activating agents for thepreparation of activated carbon [32].

The purpose of present work was to test the possibility of activated carbon formation from acacia arabica(kikar) wood sawdust which is abundantly available as waste material in the furniture industry ofPakistan. For the preparation of activated carbon, experimentation schedule involved the variation of zincchloride concentration, carbonization temperature and times in order to determine its influence on thecharacteristics of the activated carbon material formed.

2. Experimental

2.1 Material and Chemical Reagents

Acacia arabica (kikar) wood sawdust, waste of the furniture industry was used for the production ofactivated carbon and it was collected from a local workshop. Zinc chloride, iodine, Congo red dye and allother chemicals used in this study were reagent grade and were obtained from Merck (Germany).

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2.2 Preparation of Activated Carbon

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Acacia arabica (kikar) wood sawdust was collected from the local furniture industry. Moisture, volatilematter, and ash contents of the raw material were determined according to the standard test methodsASTM D 2867-96, ASTM D 5832-96, and ASTM D 2866-96, respectively. The proximate analysis of theraw material is shown in Table 1. The saw dust was divided into a number of small samples ofapproximately 50 grams. The samples were pyrolysed in a tube furnace at a temperature range between350 to 400oC in a nitrogen atmosphere for 3 to 4 hours. The carbonized samples were then ground to aparticle size range between 0.295 to 0.150 mm. The post grinding chemical activation of was carried outwith zinc chloride. In one experiment, 10g of the ground sample was taken in an Erlenmeyer flask andwas stirrer at 100 rpm with a solution containing 25g of ZnCl2 in 200 ml of distilled water held at 80oC ina constant temperature water bath. For all the experiments, the chemical ratio (ZnCl2 to precursor ratio)was varied from 1.25 to 5.0 in this study. The slurries containing ZnCl2 and precursor were dried in anoven at 110oC for 24 hours. The dried samples were placed in a stainless-steel tubular reactor (L= 45.72cm, i.d= 3.81cm) and heated (10oC/min) to the final carbonization temperature under a nitrogen flow rateof 100 ml/min. Figure 1 shows the schematic diagram of the tubular reactor. The samples were kept at thefinal temperature for different carbonization times ranging between 30 to 120 minutes after which theywere allowed to cool under the flow of nitrogen gas. The carbonized products were washed with 250ml of0.2N HCl solutions by stirring at 80oC for 30 minute followed by filtration. The filtered products werefurther washed with distilled water several times by stirring and filtration cycles until the pH of thewashed water and the carbon mixture was higher than 6. The final washed products were then dried at110oC for 24 hours in an oven to give the activated carbon. In all the experiments pre carbonizing history,heating and nitrogen flow rates (100ml/min) were kept constant. The carbon activation experiments werecarried out at different carbonization temperatures ranging from 450 to 800oC.

Figure 1: Schematic structure of tubular reactor used for activation process

Table 1: Proximate analysis of acacia arabica (kikar) wood sawdust

Component % ageMoisture 3%

Volatile matter 15%Ash 1%

Fixed carbon 81%

2 .3 Characterization of Activated Carbon

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The adsorption capacities of an activated carbon sample depend on the number of pores, their size andsize distribution. These properties are measured by adsorbing some standards adsorbents on the activatedcarbon. The commonly used adsorbates are iodine, methylene blue, phenol and carbon tetra chloride. Theresults of these adsorption indices give a technical estimate of the adsorption capability of the activatedcarbon; however these adsorbents do not provide detailed information of the nature of the pore size andits distribution. To establish the pore size and its distribution BET method is used. Iodine adsorption isanother common method to measure the surface area of the activated carbon [33]. In the present work,later technique was employed for the surface characterization of the formed activated carbon. The iodinenumber (mg of iodine adsorbed per g of active carbon) was determined according to ASTM D 4607-86test method.

3. Results and Discussion

3.1 Characterization of Activated Carbon

The effects of ZnCl2 to precursor ratios, activation temperatures and the activation durations on the iodinenumber were studied. The following sections describe the details of these variables on the iodine number.

3.1.1 Effect of Chemical Ratio on Iodine Number

The effect of different chemical ratios (ZnCl2 mass to precursor mass ratio) on the experimentally foundvalue of the iodine number is shown in Figure 2. It indicates that initially the iodine number increases asthe chemical ratio is increased and shows a maximum iodine number (670) at a chemical ratio of 2.5.Thereafter the iodine number decreases as the chemical ratio is increased. This trend of the iodine numberas a function of the chemical ratio indicates, that the activation carried out by varying the ZnCl2

concentration strongly influence the development of the porous texture. The decrease in the iodinenumber at higher chemical ratios may be due to the formation of multi-layers of activating agent at higherchemical ratio. Similar results were reported for the preparation and characterization of activated carbonfrom impregnation pitch by ZnCl2 [32].

Figure 2: Effect of ZnCl2 to carbon ratio on iodine number (Slurry mixing time 3 hr, temperature 450oC,carbonization time 60 min).

Figure 3: Effect of carbonization temperature on iodine number, (carbonization time 60 min, chemicalratio 2.5).

3.1.2 Effect of Activation Temperature on Iodine Number

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The effect of carbonization temperature on the iodine member is shown in Figure 3. It shows that initiallythe iodine number increases as a function of carbonization temperature. The maximum iodine number905 was observed at a carbonization temperature of 700oC. A further increase in the carbonizationtemperature lowers the iodine number. This trend is due to the fact that at temperature around 450oCpyrolysis reaction just commences and starts forming active sites on the carbon being formed. As thetemperature is increased, the pyrolysis reaction becomes faster and more active sites on the carbon areformed. As the temperature is further increased beyond 700oC, the active surface area decreases, thismight be due to the sintering effect at higher temperatures causing shrinkage of char, and realignment ofthe carbon structure which results in reducing the iodine number. Similar trends were observed whenactivated carbon was produced from Terminalia Arjuna nuts for the removal of chromium (VI) fromdilute aqueous solution [31].

Figure 4: Effect of carbonization time on iodine number (carbonization temperature 700oC, chemical ratio2.5).

3.1.3 Effect of Activation Time on Iodine Number

The variation of the iodine number with varying carbonization time from 30 to 120 minutes at atemperature of 700oC and chemical ratio of 2.5 is shown in Figure 4. The iodine number first increaseswith the carbonization time and reached at its maximum value of 905 for 60 minute duration andthereafter it starts decreasing. At first the iodine number increases because the pyrolysis reaction hasstarted and completes in about 60 minutes. The decrease in the iodine number after 60 minute may be dueto the reasons that some of the pores being sealed off as a result of sintering at excessive time duration. Itmeans that activation time is a critical factor in order to get the optimum surface area of the activatedcarbon formed by this route. When ZnCl2 was used activating agent to prepare activated carbon frombituminous coal, a similar trend was observed [29].

4. Conclusions

The activated carbon prepared from acacia arabica (kikar) wood sawdust attained a maximum value of theiodine number of 905 mg per gram of the activated carbon. The sample which gave the maximum iodinenumber was carbonized at temperature of 700oC, when the sample was held at this temperature for onehour. This sample had a ZnCl2 to precursor mass ratio of 2.5. The present work has revealed that acaciaarabica wood sawdust is a promising raw material for the preparation of activated carbon of high surfacearea.

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