| Activated carbon with well-developed porous structure and large adsorption capacity is a well known material used for water treatment, gas filter, super capacitor, catalytic carrier, and so forth. Activated carbon can be produced, theoretically, from any carbonaceous material rich in elemental carbon. As the increasing of energy crisis and environmental problem, waste biomass as a kind of low cost and abundant resource has attracted much attention all over the world. Using waste biomass as raw material of activated carbon production, can not only reduce the cost of manufacture, but also solve the environmental pollution caused by the direct burning and improper dumping of waste biomass. In this study, three waste biomass materials, i.e., Chinese fir sawdust, walnut shell and herb residues, were chosen as raw material. Microporous and mesoporous activated carbons were prepared by ZnCl2 vacuum chemical activation which was the first time used inland. The results show that all of the three biomass materials are suitable precursors for activated carbon preparation.The activated carbons prepared by vacuum chemical activation and traditional chemical activation were compared in detail. The activated carbon prepared under vacuum exhibited higher values of the BET surface area and total pore volume than those of the activated carbon obtained under atmosphere. The advantages of vacuum condition have been explained. The effect of system pressure on the pore structure of activated carbon was detected, and the optimal system pressure was found to be 30 kPa. It is interesting to note that the porosity of activated carbon is closely related to the system pressure. As the increase of system pressure, the microporosity decreases and mesoporosity increases gradually.Activation temperature and impregnation ratio are the most important parameters in the preparation of activated carbon by zinc chloride activation. The effects of these two parameters on the pore structure and adsorption capacity of activated carbon were investigated in this study. The results show that the optimal activation temperature for vacuum zinc chloride activation is 450℃, which is lower than that for atmospheric activation. Furthermore, the adsorption behavior of methylene blue on the obtained activated carbon was investigated. The practical adsorption efficiency of the activated carbon was also evaluated.In order to obtain mesoporous activated carbons with high specific surface area, the authors pretreated the raw material with 0.5 mol/L NaOH on the basis of vacuum chemical activation. The results show that both of the vacuum condition and pretreatment of raw material are beneficial to improve the specific surface area and pore volume of activated carbon. When raw material was pretreated by base, and activated at 450℃with an impregnation ratio of 2.5, the obtained activated carbon has a BET surface area of 1952 m2/g, a total pore volume of 1.479 cm3/g, and a mesoporosity of 98%.Furthermore, a Doehlert matrix was used to optimize the experimental conditions for preparation of activated carbon from herb residues by vacuum chemical activation. Adsorption of methylene blue and iodine was adopted as an indicator of the adsorption capacity of activated carbon. In appropriate experimental regions, activation temperature and impregnation ratio, the most influential factors in activated carbon preparation, were optimized. The results showed that, for all the responses, the effects of activation temperature were greater than impregnation ratio, and both of the two factors have little effects on total yield of activated carbon. The optimal activated carbon was obtained under the activation temperature of 474℃and impregnation ratio of 1.225. The adsorption values of methylene blue and iodine of the optimal activated carbon were 316 mg/g and 994 mg/g, respectively, which show good agreement with those calculated from the model. Compare to commercial activated carbon, the activated carbon prepared in this study have higher adsorption efficiency. |
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