Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 4(9), 34-40, September (2014) Res. J. Chem. Sci. International Science Congress Association 34 Adsorption of Benzene on Activated Carbon from Agricultural Waste MaterialsAkpa J.G.* and Nmegbu C.G.J. Department of Chemical/Petrochemical Engineering, Rivers State University of Science and Technology, Port-Harcourt, Rivers State, NIGERIA Department of Petroleum Engineering, Rivers State University of Science and Technology, Port-Harcourt, Rivers State, NIGERIA Available online at: www.isca.in, www.isca.me Received 22nd August 2014, revised 9th September 2014, accepted 15th September 2014Abstract Activated carbon was produced from three carbon rich agricultural waste materials; bamboo, coconut and palm kernel shells by pyrolysis and chemical activation (using tri oxo nitrate (v) acid (HNO)). The activated carbons were characterized and used as adsorbent for the removal of benzene from aqueous solution. The effects of various parameters such as particle size, adsorbent dosage and initial benzene concentration on the adsorption efficiency of the activated carbons were investigated. Benzene adsorption decreased with increase in particle size of activated carbon, decrease was minimal with activated carbon produced from bamboo. Benzene adsorption increased with increase in adsorbent dosage. Increase in initial benzene concentration resulted in increased adsorption using activated carbon from coconut and palm kernel shells; a decrease in adsorption using activated carbon from bamboo. Keywords: Activated carbon, benzene adsorption, chemical activation.Introduction Active carbon adsorption is a process where the molecules of the contaminant are transferred from a dissolved state in the aqueous phase to the surface of active carbon. Carbonaceous adsorbents have been found to be useful for the adsorption of different substances from gaseous and liquid phases due to their good kinetic properties and high adsorption capacities. In most applications, activated carbon is used for the removal of impurities from fluids, vapours or gases. Activated carbon is also used in gas purification, decaffeination, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and respirators, filters in compressed air and many other applications. The use of activated carbon for purification purposes (removal of unwanted components from streams) has been performed severally; removal of heavy metal ions such as copper, zinc and chromium and mercury, in waste water treatment for purification, decolourization and the removal of toxic organics and heavy metal ions. Activated carbon is produced by carbonization and activation(enlargement of the pore structure, increases the internal surface area and makes it more accessible) of a “carbon rich material”. Carbonization is the process of converting a carbon-rich material to pure carbon through pyrolysis – heating the carbon-rich material in a furnace in the absence of oxygen at extreme temperatures (600 - 900C). The carbon produced is then activated to make it adsorbent. This can occur in one of two ways: physical or chemical treatment. In physical treatment, the carbonized material is exposed to oxidizing atmospheres (oxygen or steam) and heated between 600 - 1200°C. In chemical treatment, it could be a one step process where carbonization and chemical activation occur simultaneously thus: prior to carbonization, a bath of a strong dehydration agent such as an acid, strong base or a salt is prepared and the carbon rich material submerged. The bath is then heated to temperatures of 450 – 900 °C or the two step process where carbonization and chemical activation occur separately. Chemical Activation is often the preferred method10 due to its shorter production time, lower production temperatures and higher purity of activated carbon produced. Typical raw materials used for the production of activated carbon have high carbon but low inorganic contents like wood, lignite, peat and coal11; contain a minimum amount of organic material, have a long storage life, are hard enough to maintain their properties under usage conditions, may be obtained at a low cost, and are capable of producing high-quality activated product when processed12. The use of industrial waste, agricultural by-products13 and residual wastes in the production of activated carbon abounds: paper mill sludge and peach stones, bagasses fly ash- a sugar industry waste14, macadamia nutshell15, rice husk16, date stones17, apricot stones18, olive seed waste19, oil palm wastes20, palm kernel fibre21, bagasse and rice husk and bamboo7,10. The processing of these wastes for the manufacture of activated carbon has reduced the problems of disposal and management of these wastes and by products. This work investigates the use of activated carbon for the absorption of benzene. Benzene is a natural constituent of crude Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(9), 34-40, September (2014) Res. J. Chem. Sci. International Science Congress Association 35 oil and gasoline (and therefore motor vehicle exhaust). It is also an important industrial solvent and a precursor in the manufacture of some industrial chemicals including drugs, plastics, synthetic fibbers, lubricants, dyes, detergents, explosives and insecticides. Trace amounts of Benzene also result whenever carbon rich materials undergo incomplete combustion. It is a component of combustion products produced by the burning of PVC (polyvinyl chloride). These processes cause the release of benzene into the atmosphere, polluting the environment and causing health problems. Acute occupational exposure to benzene may cause narcosis (headache, dizziness, drowsiness, and loss of consciousness); while chronic exposure can cause cancer in humans, reduce the production of both red and white blood cells from bone marrow, resulting in anemia and damage of the immune system22. Hence it is necessary to reduce the benzene concentration in these processes/wastes and the environment to minimal levels that are not injurious to human health. One way of achieving this is by adsorbing benzene from these streams. The suitability of activated carbon for the absorption of benzene is therefore investigated. In this study, activated carbon was produced from three agricultural waste materials: coconut shell, bamboo and palm kernel shell and used to evaluate their adsorption rate of benzene (removal capacity of benzene from the aqueous phase). The effects of the particle size, the quantity (dose) of the adsorbent and initial concentration of the benzene on the adsorption capacity and efficiency of these activated carbons were also investigated. Material and Methods Materials: The agricultural waste materials – bamboo was obtained from used scaffold at a construction site at the Rivers State University of Science and Technology, while coconut and palm kernel shells were obtained from Ataba Town, a coastal town in Andoni Local Government area, Rivers State in the Niger Delta region of Nigeria. The various chemical reagents used in this experiment were analytical grades and purchased at a chemical shop at the industrial chemical section of Mile 3 market in Port-Harcourt, Rivers State. The pyrolysis reaction was performed using the “pyrolysis set up” in the reaction kinetics laboratory of the Department of Chemical/Petrochemical Engineering, Rivers State University of Science and Technology, Port-Harcourt, Rivers State, Nigeria. The activation of the carbon, characterization of the produced activated carbons and batch adsorption experiments were also performed at the reaction kinetics laboratory. Methods: The production of activated carbon from bamboo, coconut and palm kernel shells was performed as follows: Preparation of Samples: The (carbon rich) raw materials: bamboo was cut to 2cm long with a cutting machine while coconut and palm kernel shells were size reduced using a crushing machine. The raw materials were washed with distilled water and sun dried. Carbonization Process: A measured weight of each raw material was introduced into the reactor and pyrolized at 300°C - 350°C in the absence of air for two hours thirty minutes20. The distillates formed during the pyrolysis were collected by the receiver connected to the condenser to avoid air pollution, while the char materials were cooled at room temperature before discharging into containers. Chemical Activation: The carbonated materials (carbon) from bamboo, coconut and palm kernel shells were crushed into powder using a crusher and sieved with a 1.88 mm sieve to obtain a uniform particle size. Measured weights of each carbonated materials were soaked in nitric acid of known concentration in a crucible. Each sample solution was stirred until the mixture turned to paste. The pastes were heated in a muffle furnace at 800C for 2hrs in the absence of air to increase the surface area of the samples for adsorption purposes23. The activated carbons were cooled to room temperature, washed with distilled water until their pH approximates 7 (no change in colour when tested with a red litmus paper) indicating no trace of nitric acid, dried for 6hrs and stored in an air-tight nylon. Characterization of Produced Activated Carbon: The physiochemical properties of the activated carbons such as the oil yield, Bulk Density, Porosity, moisture content, ash content and pH were determined using standard procedures. All results were the average of duplicate analysis. Yield of Carbon: The char material cooled after carbonization was weighed and used to obtain the percentage yield of carbon using the formula:  \n \r  (1) Determination of Bulk Density: Clean dry empty 10ml centrifuge tube was weighed (W), the centrifuge tube was filled with the different activated carbon samples (tap tube as it is filled until sample fills to the mark) and weighed (W). The difference in the weights gives the weight of the activated carbon powder in the bottle. The bulk density of the activated carbon powder was calculated using the equation: !"#$%&'()*+,-+ /,-*--+ (2) Porosity Determination: The porosity of the activated carbon was obtained using the formula: 1 %&'2,-3,-45-*--+6 (3) The pore volumes of the activated carbons were obtained using the formula: 1 7"89-:;*=5)+*;*= (4) Therefore the porosity is: Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(9), 34-40, September (2014) Res. J. Chem. Sci. International Science Congress Association 36 1 %&'9-:;*=5)+*;*=.06;萰43,-6 (5) Determination of Moisture Content: The moisture content was determined using the ASTM D2867-09 method24. Clean empty crucible was oven dried at 110C, cooled in a desiccator and weighed (W). Known weights of each powdered air dried activated carbon powder was put into the crucible, weighed (W) and dried in an oven maintained at about 110C; the drying activated carbon was constantly reweighed at 30 minutes interval until the weight of the carbon became constant. The crucible was taken out, cooled in a desiccator and weighed again (W). The loss in weight of the activated carbon is used to calculate the moisture content (Xo) of the activated carbon from the relationship: //  (6) Determination of ash content: The ash content of the activated carbons was determined according to the ASTM D2866-94 method25. Clean empty crucible was oven dried at 110C, cooled in a desiccator and weighed (W). Known weights of each powdered air dried activated carbon powder was put the crucible, weighed (W) and transferred into a preheated muffle furnace set at a temperature of 800C. The furnace was left on for one hour after which the crucible and its content was transferred to a desiccator and allowed to cool. The crucible and its content were reweighed (W). The percentage ash content of each activated carbon sample (dry basis) was obtained from the equation: B%CDEF  //  (7) Where: IJK = Weight of ash (grams). Determination of pH: 1 g of each produced activated carbon was weighed and dissolved in 3 ml of de-ionized water. The mixture was heated and stirred for 3 minutes to ensure proper dilution of the sample. The solution was filtered and out and its pH was determined using a digital pH meter. Liquid Phase Adsorption of Benzene: Batch experiments were performed to study the adsorption capability of the activated carbons produced for benzene in solution. 2g of each activated carbon sample was weighed into different 500ml conical flasks. 20ml of benzene was measured from the original mixture and poured into each 50ml conical flask and agitated at a stirring speed of 200 rpm to ensure intimate contact of the adsorbent and the solute in the solution. Each conical flask was observed for 20mins and filtered with a filter paper. A portion of the filtrate from each flask was poured into a cuvette and the concentration of benzene in the solution was measured using a spectrophotometer. Effect of Process Parameters on Adsorption Rate: Batch experiments were performed to study the effects of certain process parameters such as particle size, adsorbent dose and initial benzene concentration on the adsorption process. Particle Size: Various particle sizes of the produced activated carbons were used to conduct experiments to determine percentage removal of benzene at an initial concentration of 4ppm. The three different activated carbons produced were sieved into different particle sizes of 800 µm, 600µm, 300µm, 150µm and 75µm. 0.25gram of each particle size was weighed into a beaker. 25ml of benzene stock solution (4ppm) was measured into the beaker containing each activated carbon, stirred and agitated for 20mins. The solution was filtered using a filter paper and a conical flask. The filtrate was collected and the concentration of benzene in the solution was measured using a spectrophotometer. Adsorbent (Carbon) Dosage: The effect of activation carbon dose (quantity in weight) on the extent of solute adsorbed was investigated by varying the dose of 600 µm particle size activated carbon from 0.2 to 1.0g under the selected initial solute mass concentration (1ml of benzene in 250ml of distilled water, 4ppm) (concentration of benzene in solution). Different weight (0.2g, 0.4g, 0.6g, 0.8g and 1.0g) of each sample was weighed into a beaker. 20ml of the stock solution (4ppm) was measured into the beaker containing the sample, stirred and agitated for 20mins. The solution was filtered using a filter paper and a conical flask. The filtrate was collected and the concentration of benzene in the solution was measured using a spectrophotometer. Initial Concentration of Solute: Two initial concentration of stock solution (1ml of benzene in 250ml of distilled water (4ppm) and 2.5ml in 250ml (10ppm)) were prepared. 0.25gram of 600 µm particle size of each activated carbon was weighed into three different beakers. 20ml of each initial concentration of stock solution was measured into each beaker, stirred and agitated for 20mins. The solutions were filtered using a filter paper into a conical flask. The filtrates were collected and the concentrations of benzene in the solution were measured using a spectrophotometer. Results and DiscussionCharacterization of Activated Carbon: The results of some physiochemical properties of the activated carbons produced from the three different raw materials are presented in table 1. These properties affect the efficiency of the activated carbon in diverse ways, such as: The ash content26 which is the inorganic, inert, amorphous and unusable part present in the activated carbon reduces the overall activity of activated carbon; while the porosity classified by the size of the diameter of the pores, describes the number of pores present in a sample enhances adsorption capacity of the adsorbent27. The overall effect of these properties is a combination of the individual effects. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(9), 34-40, September (2014) Res. J. Chem. Sci. International Science Congress Association 37 Adsorption of Benzene using Activated Carbon: Effect of activated carbon Particle size on amount of benzene adsorbed: The effect of activated carbon particle size on the removal rate of benzene (% benzene adsorbed) is shown in figure 1. Figure 1 reveals that an increase in the particle size of the activated carbon results in an average corresponding decrease in the amount of benzene adsorbed for all activated carbon produced; however, the effect of particle size on the activated carbon from bamboo is very minimal. The finer (smaller) the particles size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption; also the smaller the particle size distribution, the smaller the pore spaces and the activated carbon particles tends to retain more benzene molecules. In contrast, a lager particle size distribution implies larger pore spaces incapable of retaining benzene molecules seeping through28. These trends are show in figure-1. Effect of Adsorbent dosage on amount of benzene adsorbed: The effect of activated carbon dose on the extent of benzene adsorption is shown in figure 2. Figure 2 shows that the amount of benzene adsorbed (removal efficiency) by the activated carbon from bamboo, coconut and palm kernel increases as the dose (quantity of activated carbon) increases. An increase in adsorbent dose increases the number of active sites available to the solute (benzene) for adsorption, thus increasing the rate of adsorption29. Table-1 Physiochemical Analysis of Produced activated carbons S/N Activated carbon source Parameter Yield (%) Bulk Density (g/dm) Porosity Moisture Content Ash Content (%) pH 1. Bamboo 50 0.475 0.71 2.44 8.27 6.64 2. Coconut Shell 51 0.55 0.82 5.18 6.13 6.34 3. Palm Kernel Shell 50.5 0.65 0.96 2.35 6.44 6.1 Figure-1 Variation of amount of benzene adsorbed with particle size 204060801001200100200300400500600700800900Amount of benzene adsorbed (%)Particle size (µm) Bamboo Coconut shell Palm Kernel Research Journal of Chemical Sciences ____ _ Vol. 4(9), 34-40, September (2014) International Science Congress Association However, the amount of benzene adsorbed using activated carbon from bamboo increased, reached a maximum value, decreased and became constant, this may be due to the establishment of equilibrium between the benzene adsorbed to the adsorbent and those remaining unabsorbed in the solution. The minimum amount of adsorbent corresponding to the maximum adsorption (the op timum dose) in the present study for bamboo is 0.42 g. Variation o f amount of benzene adsorbed with adsorbent dosage Histogram of amount of benzene adsorbed by adsorbent for different concentrations of Benzene 10203040506070809010000.2Amount of Benzene absorbed (%) Bamboo 0.020.040.060.080.10.120.140.160.18 Bamboo Amount of benzene adsorbed 1ml in 250ml (4ppm) _ _____________________________________________ _ International Science Congress Association However, the amount of benzene adsorbed using activated carbon from bamboo increased, reached a maximum value, decreased and became constant, this may be due to the establishment of equilibrium between the benzene adsorbed to the adsorbent and those remaining unabsorbed in the solution. The minimum amount of adsorbent corresponding to the timum dose) in the present study The amount of benzene adsorbed using the activated carbon from coconut and palm kernel increased continuously with increase in dose. No optimum dose was observed for activated carbon from coconut and palm kernel shells. Effect of initial benzene concentration on amount of benzene adsorbed: The effect of initial benzene concentration on the removal of benzene by the activated carbon from the different sources is shown in figure 3. Figure-2 f amount of benzene adsorbed with adsorbent dosage Figure-3 Histogram of amount of benzene adsorbed by adsorbent for different concentrations of Benzene 0.40.60.81Adsorbent Dosage Bamboo Coconut Palm Kernel Bamboo Coconut shellPalm kernel shell 1ml in 250ml (4ppm) 2.5ml in 250ml (10ppm) _ ________ ISSN 2231-606X Res. J. Chem. Sci. 38 The amount of benzene adsorbed using the activated carbon from coconut and palm kernel increased continuously with increase in dose. No optimum dose was observed for activated carbon from coconut and palm kernel shells. Effect of initial benzene concentration on amount of benzene The effect of initial benzene concentration on the removal of benzene by the activated carbon from the different Histogram of amount of benzene adsorbed by adsorbent for different concentrations of Benzene 1.2 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(9), 34-40, September (2014) Res. J. Chem. Sci. International Science Congress Association 39 The amount of benzene removed decreased with increase in initial benzene concentration using activated carbon produced from bamboo; that is the higher the initial concentration the lower the adsorption rate. This is due to the fact that the available sites are insufficient to adsorb proportionately the increased benzene in the initial aqueous solution. The active sites thus become saturated with adsorbed benzene27, thus decreasing the amount of benzene subsequently adsorbed onto the activated carbon. However, the amount of benzene removed increased with increase in initial benzene concentration using activated carbon produced from coconut and palm kernel shells. The initial concentration provides the necessary driving force to overcome the mass transfer resistance of benzene between the aqueous and the solid phases26. Hence an increase in initial benzene concentration will increase the mass transfer of benzene and ultimately enhance the adsorption rate of benzene by the activated carbon. ConclusionActivated carbon was produced in the laboratory from three local agricultural waste materials; bamboo, coconut shell and palm kernel shell. The materials were carbonized at a temperature of 300 – 400C and activated with tri oxo nitrate (v) acid. The produced activated carbons were characterized (determination of physico-chemical properties such as: percentage yield, bulk density, porosity, ash content and pH) and used for adsorption of benzene to ascertain their efficiencies. 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