Preparation of Soybean Straw Carbon by Microwave Method

Zhong Xu Haerbin University of Commerce

Haerbin, China xuzh@hrbcu.edu.cn

Zhuo Wang Haerbin University of Commerce

Haerbin, China wangzhuo0027@163.com

Abstract—Technology of preparing activated carbon from soybean stalks with ZnCl2 assisted by microwave irradiation was studied. The effects of zinc and soybean straw proportion, soaking time, microwave power and microwave heating time on the yield and adsorption properties of methylene blue were measured and analyzed in order to optimize these operation conditions. It turned out that the proportion of ZnCl2 and soybean straw was 3:1, the microwave power was 600w�the time of microwave heating was 17min; the time of activation was 24 hours. Adsorptive value of the methylene blue is 6.3mL/0.1g, BET surface area was 983.1m2/g and its average diameter was 2.397nm, total pore volume was 0.5892mL/g.

Keywords-soybean stalks�microwave�activated carbon

I. INTRODUCTION Activated carbon is a kind of absorbent of high

performance and an indispensable product in people’s daily life which is widely applied in product manufacture, medical treatment, military and environmental protect etc[1]. Currently, the per capitademand on activated carbon in some western developed countries has reached 300 to 400 g/a for environmental protection. It can be estimated that activated carbon has a broad prospect in the environmental protection industry. The research and development of activated carbon with a high specific surface area has become the hot spot of studies by scientific workers along with the development of environmental protection, electronic, medicine, chemistry and military industries [2-4].

China is an agricultural power with crop sown area ranking the first in the world, and stalk, the agricultural waste, is the most by-products in the agricultural production. China has utilized agricultural by-products as the fuel, feed and fertilizer since long before. The utilization of these by-products has gradually decreased owing to the development of rural economy and productivity and the extensive use of chemical fertilizers and pesticide. It is badly in need of solution on how to dispose and use these by-products. Many reports home and abroad discussed the resource utilization of the corn and wheat stalk [5-7], while few are talking about the comprehensive utilization of soybean stalk [8]. This article aims to produce activated carbon with the agricultural waste of soybean stalk as the material, ZnCl2 as the activating agent and microwave as the energy and study the influencing factors and best technical conditions, in order to provide a new method for the resource regeneration and sustainable use of soybean stalks.

II. EXPERIMENT

2.1 Materials and Reagents

Soybean stalk, Heilongjiang 93 Grease Factory; ZnCl2, analytically pure, Harbin Chemical Reagent Factory; HCl, analytically pure, Harbin Xinda Chemical Factory; KH2PO4, analytically pure, Tianjin No.3 Chemical Reagent Factory; Na2HPO4, analytically pure, Tianjin No.3 Chemical Reagent Factory; CuSO4, analytically pure, Harbin Chemical Reagent Factory; methylene blue, analytically pure, Tianjin Tianxin Fine Chemistry Development Center.

2.2 Instrument

Microwave oven, Haier Group; JA2003 electronic balance, Beijing Sartorius Instrument Company; DHG-9123A electric thermostat blast drying oven, Shanghai Yiheng Scientific Instrument Co., Ltd.; 721E visible complementary filter, Shanghai Spectrum Instruments Co., Ltd.; thermostat water-bath, Harbin Analytical Instrumentation Factory; Autosorb-1-c specific surface area tester, US Quantachrome Company.

2.3 Test Method

2.3.1 Method of Activated Carbon Preparation

Mix the soybean stalk with ZnCl2, the activating agent, stir and fully immerse. Heat the pretreated materials in the heating oven and complete the carbonization and activation by changing the power and time of microwave for pyrolysis, when the fiber of materials will be decomposed to be activated carbon. Cool the product after carbonization, leach the activating agent and obtain the product of activated carbon after certain treatment on experimental product.

2.3.2 Test on the Influence Factors for the Preparation of Activated Carbon

Conduct the test on the influence factors that have comparatively large influence on the performance and production of activated carbon based on the evaluating indicator of the adsorptive value of methylene blue, including the immersion ratio (the immersion ratio of the activating agent to soybean stalk of 2:1, 3:1, 4:1 and 5:1), immersion time (12h, 24, 36h and 48h), heating power of microwave oven (500w, 600w, 700w and 800w) and heating time (14min,

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16min, 18min and 20 min).

2.3.3 Measurement of the Specific Surface Area and Structural Parameters

Specific surface area, specific pore volume and pore diameter and distribution are important macrostructure parameters to describe the absorbent, based on which we can obtain the information on the microstructure of the absorbent. This is exactly an important property of the absorbent. The specific surface area and pore structure features are measured by the Autosorb-1-c type full automatic specific surface area and porosity analyzer produced by US Quantachrome Company based on the static nitrogen adsorption. Its basic principle is to vacuumize the sample, observe the absorption and desorption of nitrogen and calculate the specific surface area and porosity according to the data model.

III. RESULTS AND DISCUSSION

3.1 Experimental Results

The effect of the immersion ratio of ZnCl2 to soybean stalk on the absorption and production rate of methylene blue is shown in Fig. 1. The production rate gradually decreases along with the increase of the ratio, and the absorption of methylene blue raises firstly and then declines. ZnCl2 is polar compound that absorbs microwave. In a lower immersion ratio, the pyrolysis is incomplete owing to the less absorption of microwave and the lower activating temperature. The activating temperature raises with the increase of comtent of ZnCl2, so the reaction is comparatively complete. However, too high reaction temperature may lead to the ignition loss of activated carbon generated and thus reduce the production rate.

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Figure 1.The effect of zinc and soaking soybean straw proportion on

methylene blue adsorption and yield of influence

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Figure 2. The effect of immersion time on methylene blue adsorption

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See the effect of immersion time on methylene blue adsorption and yield in Fig. 2. The production rate slightly improve along with the increase of immersion time, and meanwhile the methylene blue absorption raises firstly and later declines. It is mainly because that the ZnCl2 solution cannot fully wet and activate the fiber materials in too short immersion time, so the reaction of carbonization and activation cannot fully completed and then the methylene blue absorption is comparatively low; when the immersion time is properly increased, the solution fully wets the material and thus the material can be completed carbonized and activated with higher methylene blue absorption

See the effect of microwave power on methylene blue absorption and yield in Fig. 3. The production gradually reduces along with the increase of microwave power, while the methylene absorption raises firstly and then decreases. It is mainly because that too low microwave power leads to a lower reaction temperature, where the activated carbon cannot be fully carbonized and activated, resulting in low methylene blue absorption. When the microwave power reaches a certain value, the mateiral can be fully carbonized and activated to be a sound activated carbon product, so the absorption is better. However, when the power is too high resulting in too high temperature, some carbon in the material will be lost upon ignition and large amount of ash content will be remained at the surface of the activated carbon. The ash content will be washed away by the washing water in the follow-up treatment of the experiment. So the methylene blue absorption decreases and the production significantly declines

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Figure 3. The effect of microwave power on methylene blue adsorption and yield of influence

See the effect of the heating time of microwave on the

methylene blue absorption and yield in Fig. 4. The methylene blue absorption and yield presents as cruves along with the increase of heating time and respectively reach the peak at the 16min and 18min. It is mainly because that too short time of microwave cannot completely activate and form a good pore structure of activated carbon. Therefore the methylene blue absorption and yield will be lower; but too long time will lead to over temperature that changes the pore structure cause the ignition loss, thus reducing the production.

The computer program can obtain the specific surface area of 983.1m2/g and overall pore volume of 0.5892mL/g based on the figures, with a mean pore diameter of 2.397nm.

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IV. CONCLUSION The activated carbon is prepared with soybean stalk as

materials and ZnCl2 as activating agent based on microwave method. The best conditions for the activated carbon of soybean stalk based on microwave method with the methylene blue adsorption as the major indicator include an immersion ratio of 3:1, heat time of 17min, immersion time of 24h and microwave power of 600w. The methylene blue adsorption of the soybean stalk carbon is 6.3mL/0.1g. The specific surface area and pore structure parameter can be measured by the specific surface area analyzer respectively. The obtained specific surface area of 983.1m2/g, mean pore diameter of 2.397nm and overall pore volume of 0.5892mL/g.

Figure4. The effect of microwave heating time on methylene blue adsorption capacity and the yield was influenced

Figure5 Activated carbon adsorption and take off attached line

Relative Pressure,P/Po

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cr

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3.2 Specific Surface Area And Features of Pore Structure of the Sample

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/ccg

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Figure6 Pore distribution curve

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