Effect of the carbonization and activation processes on the adsorption capacity of rice husk activated carbon

Vietnam Journal of Science and Technology 55 (4) (2017) 485-493 DOI: 10.15625/2525-2518/55/4/9216 EFFECT OF THE CARBONIZATION AND ACTIVATION PROCESSES ON THE ADSORPTION CAPACITY OF RICE HUSK ACTIVATED CARBON Hoa Thai Ma1,2, Van Thi Thanh Ho3, Nguyen Bao Pham2, Dat Chi Nguyen1, Kieu Thi Diem Vo4, Hung Cam Ly3, Tuan Dinh Phan3, * 1Ho Chi Minh City University of Technology - Vietnam National University, 268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City 2Tra Vinh University, 126 Nguyen Thien Thanh Street, Ward 5, Tra Vinh City 3Ho Chi Minh City University of Natural Resources and Environment, 236B Le Van Sy street, District Tan Binh, Ho Chi Minh City 4Dong Nai Technology University, Nguyen Khuyen Street, Ward Trang Dai, Bien Hoa city *Email: pdtuan@hcmunre.edu.vn Received: 3 January 2017; Accepted for publication: 28 June 2017 ABSTRACT In this study, rice husk was used as a precursor to prepare activated carbon using steam as a physical activation agent. Steam can be used to activate for almost all raw materials. This activation method was followed of the initial carbonization step. The study also investigated the effects of preparation parameters on the adsorption capacity of iodine (I) and methylene blue (MB). These parameters included the range of temperature and time in the carbonization and activation. The initial carbonization, done at temperatures up to 500°C in 60 min., was a highly exothermic process where the temperature was strictly controlled. The creation of the internal surface was done during the activation step with steam at temperatures 800 °C in 30 min., for which the BET (Brunauer, Emmett and Teller) specific surface area was up to 710.8 m2/g. Besides, the I and MB adsorption capacity of the activated carbon were the best that reached 866.0 ± 6.5 and 217.9 ± 1.0 mg/g, respectively, while the I and MB adsorption capacity of carbonization step only reached 311.7 ± 3.4 and 28.7 ± 0.8 mg/g, respectively. The entire synthetic procedure was simple, environmental-friendly and economical-effectively. The application prospect of the activated carbon prepared in this work was much more promising due to its high adsorptive capacity. Keywords: rice husk, carbonization, activation, activated carbon and adsorption capacity. 1. INTRODUCTION Activated carbon with a large adsorption capacity is the carbonaceous material highly needed for various industrial purifications and waste water treatments [1]. The demand for Hoa Thai Ma, et al. 486 activated carbon (AC) is increasing rapidly with enhanced awareness about environment protection. However, the price of AC is very expensive due to the fact that most of the commercial AC products are derived generally from costly natural materials such as wood or coal. Therefore, the exploration for a cheap and easily available precursor for AC production is of great importance. There is a growing interest in the production of AC using cheap agricultural and industrial wastes [2]. Several wastes including coffee husks, rice husks, walnut shells, cotton stalks, coconut husks, and corn cobs have been investigating as the activated carbon precursors and are still receiving renewed attention because of their cheap prices and abundant resources. Many of the waste-based AC also enabled comparable or even better performance than the commercial AC in many applications. Consequently, the conversion of cheap biomass wastes into value- added AC not only opens a low-cost path for AC production but also provides an efficient way to reduce the environmental pollution caused by disposing agricultural and industrial wastes [3]. Rice is an important staple food for 3.5 billion people on the world. According to Food and Agriculture Organization of the United Nations (FAO) in 2015, the yield of paddy on the world reached 749.1 million tones and it will tend to increase in the following years. According to statistics rice husk (RH), a by-product from the milling process, made up about 20 % the weight of paddy. Thus, the annual generated RH is about 150 million tons. In Vietnam, it was about approximately 8.94 million tons, equivalent 5.96 % of the world's RH. Currently, the amount of husk is still underused in a reasonable manner especially in developing countries, including Vietnam. Most RH is burned or dumped directly into canals causing environmental pollution. According to the report of Trinh et al. [4], rice husk is carbonized at 450 ÷ 520 oC, after that the product of this process is activated by over heating at above 700 oC in 1.0 to 2.0 hours. The mass ratio of steam and raw material is in the range 0.09 ÷ 0.3. The surface area of obtained activated carbon is 276.68 m2/g. They are fundamental steps to build a manufacturing technological process of domestic rice husk activated carbon production [4]. Burning RH gives 14–20 % ash which contains 80–95 % silica and minor amounts of metallic elements [5]. RH is little commercial value and not useful to feed either humans or animals because it contains high content of silicon dioxide [6] which is dissolved in hot concentrated alkali [7]. The activation of AC includes of physical and chemical methods. By physical activation, AC is obtained after two steps: the first is carbonization, which is the pyrolysis of carbonaceous materials at high temperature (500 - 1000 oC) in an inert atmosphere to eliminate maximum amounts of oxygen and hydrogen elements; the second is the thermal activation at the same temperature in the presence of an oxidizing gas such as steam, carbon dioxide or both [8]. Chemical activation involves impregnation of raw materials with such chemicals as H3PO4, KOH, NaOH, or ZnCl2, etc. These chemicals are dehydrating agents that can influence the pyrolytic decomposition and retard the formation of tars during the carbonization process, thereby increasing the yield of AC [9]. In general, AC is produced by carbonizing the raw materials prior to steam activation. The carbonization process leaves an imprint effect on the final product and the careful selection of carbonization process is enriching the carbon content and creating an initial porosity in the char. The activation process further develops the porosity and creates some ordering of the structure to generate a highly porous solid as final product [10]. Effect of the carbonization and activation process on the adsorption capacity of rice husk 487 The alkali treatment of raw material can greatly increase the BET surface area and total pore volume of the produced AC [11, 12]. It is highly necessary to advance the alkali treatment technology for AC or to develop a utilization technology for the generated waste solution. The aim of this study is to explore the temperature and time conditions of carbonization and activation process of producing AC from RH with the raw material treatment by alkaline. That promises to leverage agricultural by-product to produce AC using for environmental treatment. 2. MATERIAL AND METHODS 2.1. Raw material RH from a rice mill at Tra Vinh province was used as raw material. Then it was washed thoroughly with distilled water to remove soil and dust, then dried at 105 oC in an oven for 24 hours. 2.2. Experiment procedures The dried rice husk was impregnated with 2M NaOH solution with the ratio of RH: NaOH = 1:8 and stirred at least 1 hour at 100 oC. After the basis solution was drained, the RH was washed with distilled water until the filtrate was neutral, and then dried at 105 oC for 24 hours. For approximately 40 g of the dried rice husks were placed in a quartz reactor which was inserted into a vertical electric heating furnace. The whole system was purged nitrogen gas to remove the oxygen. Then the reactor was heated at 400 – 550 oC in the presence of nitrogen flow with a heating rate of 10 oC/min. and was held at this temperature for 1 hour. There after the temperature was raised to 750 – 850 oC. After the activation temperature was reached, the gas was switched to steam and sample was kept under this condition for 30 – 90 min. before it was finally cooled in nitrogen flow (shown in Fig.1.). Figure 1. Diagram of the carbonization and activation process. (Note: 1: Nitrogen gas tank, 2: water bottle, 3: reactor, 4: quartz reaction vessel, 5: temperature sensor, 6: temperature control, 7: ice, 8: acetone solution, 9: tank insulation). 1 3 4 5 6 2 7 9 8 Hoa Thai Ma, et al. 488 The carbonization and activated carbon product was analyzed with I and MB adsorption capacity by GB-T 12496.8-1999 and GB-T 12496.10-1999 standard. Scanning electron micrographs of samples was also observed. The recovery of AC was calculated by the formula: The recovery of AC (%) *100AC RH w w = here wAC is weight of AC; wRH is weight of RH. 2.3. Determination of the adsorption capacity The iodine adsorption capacity was determined by GB-T 12496.8-1999 standard. The methylene blue adsorption capacity was determined GB-T 12496.10-1999 standard. The I and MB adsorption capacity was calculated by the following equation:
           where wi is the initial mass of iodine or methylene blue before adsorption process and wt is the mass of iodine or methylene blue after adsorption process (mg), wM is the mass of sample (g). All of determinations were performed in triplicate. 3. RESULTS AND DISCUSSION 3.1. Effect of the carbonization temperature The ranges of temperature from 400 to 550 oC in 60 min. were conducted which are causing the I and MB adsorption capacity of char varied from 46.2 to 311.7 and 16.6 to 28.7 mg/g, respectively. A maximal I and MB adsorption capacity reached 311.7 ± 3.4 mg/g and 28.7 ± 0.8 mg/g, respectively which was observed at 500 oC in 60 min. as shown in Fig. 2. Figure 2. The effect of the carbonization temperature on the I and MB adsorption capacity. 109.4 113.6 311.7 46.2 21.4 27.2 28.7 16.6 0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400 450 500 550 A d so rp ti o n c a p a ci ty ( m g /g ) Temperature (oC) The iodine adsortion capacity (mg/g) The methylene blue capacity (mg/g) Effect of the carbonization and activation process on the adsorption capacity of rice husk 489 This was not only because the main reaction of carbonization was actively proceeded with the increase of carbonization temperature, but also because RH showed the reduction of adsorption characteristics due to the contraction and sintering of char at high temperatures [13] (above 500 oC). Therefore, the temperature condition at 500 oC was chosen to produce char which was used as feedstock for activation process to increase the efficiency of carbonization process. 3.2. Effect of the carbonization time Figure 3. The effect of carbonization time on the I and MB adsorption capacity. The carbonization time had a significant effect on the efficiency of carbonization process. The ranges of the time from 30 to 90 min. were conducted which caused the change of I and MB adsorption capacity of char from 58.7 to 311.7 mg/g and 16.6 to 28.7 mg/g, respectively. The maximal I and MB adsorption capacity values reached 311.7 mg/g and 28.7 mg/g, respectively which was observed at 500 oC in 60 min. as shown in Fig.3. It was explained as follows: when reaction has started for 60 min., the pores were sintered during the increase of thermal exposure time, thus decreased the adsorption characteristics. This result was similar to the observations by other researchers [14]. Therefore, the carbonization condition at 500 oC and 60 min. was chosen to produce char which was used as feedstock of activation process. 3.3. Effect of activation temperature The activation of char was the most important stage of the production process of AC, since this stage increased porosity and specific surface area of the product. In general, there were three types of pores to be developed in the solid, that were micropore (< 2 nm), mesopore (2 ÷ 20 nm) and macropore (> 50 nm). The size of the formed pores would effect the porosity, the total surface area available for adsorption and most importantly, it determined the size of molecules that could diffuse into the pore of solid. Thus development of an appropriate pore structure was necessary if the AC was produced for a particular application [10]. The experiments were conducted with superheated-steam at 750, 800 and 850 oC. The AC product was tested for the I and MB adsorption capacity. The results were shown in Fig. 4. It was pointed out that at 800 oC, the sample possessed the largest adsorption capacity, clearly indicating that the pore volume 58.7 311.7 98.2 5.8 28.7 19.1 0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 30 60 90 A d so rp ti o n c a p a ci ty ( m g /g ) Time (min.) The iodine adsorption capacity (mg/g) The methylene blue capacity (mg/g) Hoa Thai Ma, et al. 490 mostly increased under this activation condition. Comparatively, as the activation temperature was lower than 800 oC, the char was only partially activated [11], leading to undeveloped porosity and hence lower surface area. Figure 4. The effect of activation temperature on the I and MB adsorption capacity. Similarly, however, when the activation temperature was higher than 800 oC, violent gasification reactions might cause a part of the micropore structure to be destroyed by collapsing or combining together, resulting in the reduction of micro-porosity and accordingly the increase of mesopore volume [15]. Other similar explanations, the gradually enhanced reaction between steam and carbon resulted in more developed pore structures. When the activation temperature was over 800 °C, the reactions between steam and carbon started to be diffusion- controlled and thus caused a nonhomogeneous reaction in the particle, resulting in the reactions mainly on their external surfaces that had little contribution to the pore generation. Furthermore, the reaction between steam and carbon became too violent to widen the pores and cause some micropores to be converted into mesopores and macropores, decreasing the specific surface area and total pore volume. Beside the influence of the increasing temperature, the ash formation also increased, resulting in a decrease of surface area. Pore widening and pore collapse could also occur and resulted in a decrease of surface area [16]. That was demonstrated by the fact that the AC activated at 850 °C had a smaller micropore volume than that activated at 800 °C. That meant the AC activated at 850 °C had a smaller micropore volume than that activated at 800 °C [3]. This led to the best adsorption capacity of I and MB at 800 oC as 706.9 mg/g and 204.5 mg/g, respectively, as shown in Fig. 4. However, at the condition, the recovery of AC was slightly lower than that activated at 750 oC as shown in Table 1. It might be due to the reaction between steam and carbon, resulting in the carbon loss at higher treatment temperature and longer residence time [17]. Table 1. Effect of activation conditions on the recovery of AC. Temperature (oC) Time (min.) 750 800 850 20 30 60 90 Recovery of AC, % 18.7±0.2 17.0±0.4 5.2±0.1 43.0±0.5 23.8±0.3 17.0±0.2 3.0±0.4 600.9 706.9 640.6 110.9 204.5 148.8 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 750 800 850A d so rp ti o n c a p a ci ty ( m g /g ) Temperature (oC) The Iodine adsorption capacity (mg/g) Effect of the carbonization and activation process on the adsorption capacity of rice husk 491 3.4. Effect of activation time Figure 5. The effect of activation time on the iodine and methylene blue adsorption capacity. DA Micropore Volume = 0.001 cc/g DA Micropore Volume = 0.471 cc/g Pore Radius (mode)= 9.800e + 00 Å Pore Radius (mode)= 8.700e+00 Å Figure 6. SEM micrographs of the AC treated at 500 oC/60 min. (a), and of the char treated at 800oC/30 min. (b). According to Fig. 5. there were the significant effects of the activation time on the I and MB adsorption capacity. At 800 oC in 30 min. the I and MB adsorption capacity of AC reached the highest values as 866.0 ± 6.5 and 217.9 ± 1.0 mg/g, respectively. Those values were much higher than 311.7 ± 3.4 and 28.7 ± 0.8 mg/g, respectively, received by carbonization step. Besides, the specific surface area by BET method of 710.8 m2/g and the micropore volume of 0.471 cc/g have figured out a sharp increase in comparison with the char micropore of 0.001 cc/g. The results were in good agreement with the earlier reports [16, 18]. It could be explained that during early stage of the activation process, the development of mesopores was less significant but it rapidly increased thereafter. The continuous development of mesopores and the expense of micropores were the typical characteristics of steam activation [19]. Besides, the high I and MB adsorption capacity indicated the high degree of activation (typical range 500 ÷ 1200 mg/g) and the relationship with the micropore content. At higher temperature and longer activation time, the adsorption decreased. That might be due to a partly reduction of micropores. Besides, the recovery percentage of AC had the tendency to decrease with the increase of 854.5 866.0 706.9 667.7 200.3 217.9 204.5 160.4 0.0 200.0 400.0 600.0 800.0 1,000.0 20 30 60 90 A d so rp ti o n c a p a ci ty (m g /g ) Time (min.) The Iodine adsorption capacity (mg/g) (a) (b) Hoa Thai Ma, et al. 492 temperature or retention time as shown in Table 1. From these results, it could be inferred that a significant amount of micropores was formed in the AC sample and it was generally agreed among investigators [20] that the adsorption capacity of AC largely depended on the amount of micropores presented in the solid. AC adsorption capacity was proportional to the quantity of the micropore volume in the material. Fig.6. showed that the activation significantly increased with the amount of AC micropores (a) in comparison with that of char (b). 4. 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