Jian LIANG,Qin CAI,Bo LI,Changyan WU,Hanlin LIU,Jing CHENG,Jianqi ZHOU,Song BAI

School of Chemical Engineering,Guizhou Institute of Technology,Guiyang 550003,China

Activated carbon is a common adsorption material with stable chemical properties.It is characterized by resistance to acid,alkali and high temperature,rich porous structure and huge surface area.Activated carbon is an important multifunctional adsorption material,and it is widely used in pharmaceuticals,food,environmentalprotection,chemical production,agricultural sciences and other fields[1-6].With the expansion of application field and rapid development of industry,the demand for activated carbon is increasing.The raw materials for preparation of activated carbon have attracted growing attention from researchers.In particular,the preparation of activated carbon from agricultural waste has become currently one of the research hotspots[7-10].China is a large producer of glutinous rice.In recent years,with the continuous development of deep processing products of glutinous rice,the cultivation area of glutinous rice is also increased continuously.However,large amounts of glutinous rice straw are produced.The activated carbon prepared from glutinous rice straw has advantages of wide material source,high carbon content and low ash content.Therefore, glutinous rice straw is an excellent raw material for preparation of activated carbon[11-12].The preparation of activated carbon from glutinous rice straw not only improves comprehensive utilization value of glutinous rice straw and provides a new raw material for preparation of activated carbon,but also avoids environmental pollution caused by blind burning of glutinous rice straw.In this study,using glutinous rice straw as raw material and potassium hydroxide as activator,high-performance activated carbon was prepared,providing certain reference for comprehensive utilization of glutinous rice straw and preparation of high-performance acti-vated carbon.

Material and Methods

Test material

The test material was waste glutinous rice straw.The used equipment and instruments included high-resolution field emission scanning electron microscope(ΣIGMA,Carl Zeiss,Germany), electronicthermostatic drier box(GZXGF-9023A,Shanghai Hengxi Industry Co.,Ltd.,China),water bath oscillator(SHA-CD,Changzhou Aohua Instrument Co.,Ltd.,China),boxtype resistance furnace(SRJX-2.5-13,Tianjin Taisite Instrument Co.,Ltd.,China),standard sieve (Shanghai Xinzheng Instrument Co.,Ltd.,China)andhigh precision pH meter(PHS-3,ShanghaiLeici InstrumentFactory,China).The used reagents included hydrochloric acid,potassium hydroxide and sodium hydroxide.They were all of analytical grade,and were produced by the Sinopharm Chemical Reagent Co.,Ltd.

Test methods

Testing of adsorption performance of activated carbonThe adsorption value of iodine by activated carbon was determined in accordance with the"Test methods of wooden activated carbon-Determination of iodine number"(GB/T12496.8-1999),and of methylene blue was determined in accordance with the"Test methods of wooden activated carbon-Determination of methylene blue adsorption"(GB/T12496.10-1999).

Preparation of activated carbonA certain amount of waste glutinous rice straw was collected.It was washed,dried by natural wind and then dried in an oven at 100℃.Subsequently,the glutinous rice straw was smashed,and carbonized at 350-450℃for 20 min under the protection of nitrogen gas.The obtained ash was cooled naturally,ground,and passed through a 20-mesh sieve.And then,the experiments were conducted in a tubular resistance furnace under conditions of different activator concentrations,different activation times and different activation temperatures.After cooled to room temperature,the obtained intermediate products were dissolved in hydrochloric acid (0.2 mol/L)and stirred for 30 min.The solutions were then adjusted to neutral state with hot distilled water,and finally dried to constant weights at 100℃.Thus the targetactivated carbon adsorbent products were prepared.

Results and Analysis

Screening of activator

Parallel test was conducted for screening out the optimum activator.Under the same carbonization temperature,activation temperature,activation time and activator concentration,the activated carbon was prepared from glutinous rice straw using sodium hydroxide and potassium hydroxide as the activator,respectively.The adsorption values of iodine by the prepared activated carbon were prepared(Table 1).

As shown in Table 1,under the activation temperature of 550℃,the adsorption performance of activated carbon was better when potassium hydroxide was used as the activator;under the activation temperature of 600℃,the adsorption performance of activated carbon was better when sodium hydroxide was used as the activator.Based on the comprehensive comparison of several parallel factors,it was concluded that the adsorption performance of prepared activated carbon was better under the conditions of potassium hydroxide concentration of 2 mol/L,carbonization temperature of 400℃,activation temperature of 550℃and activation time of 40 min.

Determination of optimum technological conditions for preparation of activated carbon from glutinous rice straw

In order to optimize the technological conditions for preparing activat-ed carbon,the L9(34)orthogonal experiment was conducted (Table 2,Table 3).Potassium hydroxide was selected as the activator.Total four factors were arranged,including activation time,activation temperature,carbonization temperature and activator concentration,and each factor had three levels.

Table 1 Activator screening test table

Table 2 Factors and their levels affecting performance of activated carbon

Table 3 Results of L9(34)orthogonal experiment

Table 4 Detection results of quality indicators of prepared activated carbon from glutinous rice straw

Table 3 showed that the ranges of the four factors ranked ascarbonization temperature(C)＞activation time(A)＞activator concentration (D)＞activation temperature(B),indicating that in terms of effect on preparation of activated carbon from glutinous rice straw,the four factors ranked as C＞A＞D＞B.Table 3 also showed thatk3A＞k2A＞k1A;k3B＞k2B＞k1B;k3C＞k1C＞k2C;k2D＞k3D＞k1D;the adsorption value of iodine was highest by activated carbon prepared under conditions of A3B3C3D2.There are some differences in adsorption performance of activated carbon among different technological conditions.In the activation process,potassium hydroxide plays a role of catalytic dehydration.The hydrogen and oxygen in raw material were converted into water and released in the form of vapor,thus the porous structure was gradually formed.However,when the use amount of potassium hydroxide was too low,the activation temperature and time were insufficient,leading to insufficient activation of raw material by potassium hydroxide,and further leading to more compact structure of prepared activated carbon.Under the condition of same activator concentration,the higher the activation temperature was and the longer the activation time was,the more sufficient the activation reaction was.A large amount of pore structure was generated through continuous decomposition of carbon material.However,with the extension of activation time,carbon structure would be eroded excessively.Subsidence would occur between the channels,and some micropores were eroded into medium pores,and even channels.The destruction of the pore structure would reduce the adsorption capacity of the product.Considering the comprehensive effect of various factors on adsorption performance of activated carbon,the optimum technological conditions for preparation of activated carbon from glutinous rice straw were determined as follows:activator concentration of 2 mol/L,activation time of 60 min,activation temperature of 600℃and carbonization temperature of 450℃.The adsorption values of methylene blue and iodine by activated carbon prepared under the optimum conditions were 10.21 ml/0.1 g and 920.74 mg/g,respectively,which were all higher than relevant national standards(GBT13803.2-1999,Wooden activated carbon for water purification).

Determination of product quality

According to GBT13803.2-1999,the quality indicators of activated carbon from glutinous rice straw were analyzed.As shown in Table 4,the quality indicators of activated carbon from glutinous rice straw all reached the grade two of national standard about wooden activated carbon for water purification. Therefore, the prepared activated carbon from glutinous rice straw has certain application value.

Conclusions and Discussion

It is feasible to produce activated carbon with glutinous rice straw as raw material and potassium hydroxide as activator.The optimum technological conditions were as follows:activator concentration of 2 mol/L,activation time of 60 min,activation temperature of 600℃,and carbonization temperature of 450℃.The adsorption values of methylene blue and iodine by activated carbon prepared under the optimum conditions were 10.21 ml/0.1 g and 920.74 mg/g,respectively.

Under laboratory conditions,the quality indicators of activated carbon prepared from glutinous rice straw all reached the grade two of national standard about wooden activated carbon for water purification.Therefore,the activated carbon prepared from glutinous rice straw has certain application value.This study will provide a new way for comprehensive utilization of glutinous rice straw.

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