Exploring on the optimal preparation conditions of activated carbon produced from solid waste produced from sugar industry and Chinese medicine factory
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The waste biomass produced from sugar industry and Chinese medicine factory such as bagasse, reed root residue, pueraria residue and liquorice residue were selected as the raw material for the preparation of activated carbon with zinc chloride as activator. With the same activation time, the influence of temperature and impregnation ratio on the preparation of activated carbon was investigated and the reasonable preparation conditions of activated carbon were monitored and analyzed. The obtained activated carbon samples were characterized by scanning electron microscopy, Brunauer–Emmett–Teller, methylene blue adsorption, thermogravimetric analysis and nitrogen adsorption–desorption. Analysis from the experimental data, bagasse, reed root residue, pueraria residue are suitable for preparing activated carbon. For bagasse, the optimum preparation condition was 700 °C and the impregnation ratio was 1:1, the adsorption capacity of methylene blue reached 246.83 mg/g at the moment. For reed root residue, the optimum preparation condition was 600 °C and the impregnation ratio was 1:2, the adsorption capacity of methylene blue reached 268.07 mg/g at the moment. For Pueraria residue, the optimum preparation condition was 700 °C and the impregnation ratio was 1:2. The adsorption capacity of methylene blue reached 297.33 mg/g at the moment.
KeywordsActivated carbon Waste biomass Nitrogen adsorption–desorption Optimize
Activated carbon can be made from almost any organic material which are rich in carbon, such as coal , wood , and nut shells [3, 4]. These carbon-containing materials are converted into activated carbon by pyrolysis in an activation furnace. In recent years, more and more people choose to use biomass waste as activated carbon raw materials. The development of sugar industry and Chinese medicine industry has brought about a lot of biomass solid wastes, such as bagasse, reed root residue, pueraria residue, liquorice residue and other low-cost low-ash lignin materials. At present, these kinds of waste residue are used as agricultural fertilizers, biogas materials and animal food, with low utilization rate and high carbon emissions. Furthermore, due to the large amount of waste residue and easy fermentation, most of it has not been effectively reused, resulting in a lot of pollution and waste.
Activated carbon is a general term for carbon materials with developed pore structure, large specific surface area, abundant surface chemical groups and strong specific adsorption ability . In isolation from the air,the organic material (shell, coal, wood, etc.) was heated to reduce the non-carbon components (carbonization) and reacted with the activator (activation), eroding the surface to produce a microporous structure .
Activation process is a microscopic process that the surface erosion of carbide occurs in the form of pitting. Therefore, carbonization could make the surface of activated carbon produce a large number of fine pores. Activated carbon is a very effective adsorbent, with the dual characteristics of physical adsorption and chemical adsorption, it could be selected through adsorption of various substances in the gas phase, liquid phase, to achieve decolorization purification, disinfection and deodorization and decontamination purification purposes. Almost all applications of activated carbon are based on its porous structure and surface chemical properties [1, 7].
The surface of activated carbon is rich in pore structure even a small amount of activated carbon has a huge surface area. The surface area of each gram of activated carbon can reach to 500–1500 m2 . The adsorption performance of activated carbon depends not only on the physical (pore) structure of activated carbon, but also on the chemical structure of the surface of activated carbon. During the preparation of activated carbon, unsaturated chemical bonds were formed in the carbonization stage, which could react with heterocyclic atoms such as oxygen, hydrogen, nitrogen and sulfur to form different surface groups, thus affecting the adsorption performance of activated carbon.
The preparation of activated carbon can be divided into three types: chemical activation method, physical activation method and physico-chemical activation method. Chemical activation method is to prepare activated carbon through the process of carbonization, activation, chemical recovery, rinsing, drying and so on. Reagents such as phosphoric acid , zinc chloride , potassium hydroxide , sodium hydroxide , sulfuric acid , potassium carbonate , polyphosphoric acid and phosphate ester  could be used as activators. These chemicals all contribute to the activation of the raw material, although different chemical reactions occurred, some had erosion, hydrolysis or dehydration of the raw materials, and some had oxidation. Zinc chloride is one of the most widely used activator in the preparation of activated carbon by chemical activation method . Impregnation with ZnCl2 causes the degradation of cellulose and dehydration during carbonization. These processes led to carbonization and aromatization of the carbon skeleton and thus form a preliminary pore structure. After the removal of zinc chloride with water, the pore structure is formed. In addition, some scholars believe that zinc chloride forms the skeleton of new carbon deposition during carbonization, and when it is washed away, the surface of carbon is exposed, forming the inner surface of activated carbon with adsorption . Therefore, the chemical activation by zinc chloride (ZnCl2) improves the pore development in the carbon structure.
The most important characteristics of activated carbon is the specific surface area and adsorption ability which are determined by the preparation parameters of activated carbon, such as carbonization temperature, carbonization time and chemical impregnation ratio, putting influence on the pore development and surface characteristics of the prepared activated carbon. The impregnation ratio could determine the distribution of specific surface area and pore diameter, and carbonization time and temperature are important preparation variables for obtaining activated carbon  (Namasivayam and Kadirvelu 1997).
In this work, activated carbon was produced from 4 kinds of biomass solid waste (bagasse (BR), reed root residue (RR), pueraria residue (PR), liquorice residue (LR)) aiming to explore the reasonable conditions for preparing activated carbon, where Taguchi orthogonal experiment  and response surface method  were used to optimize the preparation conditions of ACs. The activated carbon samples were characterized by Scanning Electron Microscope (SEM), Brunauer–Emmett–Teller (BET), Thermogravimetric Analysis (TGA), nitrogen adsorption desorption and methylene blue adsorption.
Materials and methods
Different types of biomass solid wastes were collected including bagasse (BR) which provided by GuangXi COFCO JiangZhong Sugar CO., LTD., and reed root residue (RR), pueraria residue (PR), liquorice residue (LR) which were provided by GuangDong LianFeng Chinese medicine decoction pieces co. LTD. After being fully washed with distilled water and dried at 100 °C for 24 h, the waste residues were crushed and sieved to the size below 0.500 cm. Methylene blue (AR) and ZnCl2 (AR) were supplied by Fengchuan Chemical Reagent Co., Ltd., Tianjin, China. Nitrogen and air were supplied by Liufang Gases Chemical Reagent Co., Ltd., Tianjin, China. Deionized water was prepared by the laboratory.
Preparation of XACs
XACs were prepared under the conditions which included impregnation ratio (ZnCl2/Corncob, w/w), at 1:0.5, carbonization temperature 800 °C, and carbonization time 1.0 h. In each experiment, 3.0 g of waste residues were thoroughly mixed with 30 mL of ZnCl2 solution of different concentrations . The mixtures were placed at ambient temperature for 12 h and then dried at 100 °C for 12 h getting the samples impregnated. Sequently, the impregnated samples were placed into Nickel containers which have been numbered. Then the sample-laden Nickel containers were put into a 316 stainless steel tubular reactor (SK3-5-12-6 Energy-saving vacuum tube furnace, HangZhou ZhuoChi Instrument co., LTD.) and heated at a rate of 10 °C min−1 to the preset temperature in the presence of N2. After being held at the presented temperature for 1 h, the AC samples were cooled and washed with 1.0 mol L−1 of hydrochloric acid solution at 50 °C for 30 min to remove the metal ions and the ash content. Then, the AC samples were washed with hot deionized water repeatedly until the pH of filtrate reached 7. The washed AC samples were dried at 80 °C in vacuum for 12 h and ground/sieved to the size ranging from 0.180 to 0.425 mm. The final AC samples were labelled as XAC, where X stands for the kind of waste residues.
Optimization of preparation conditions for XAC
Factors and levels of RSM
Experiment conditions of XAC preparation
The pyrolysis behavior of the waste residues was investigated using a thermogravimetric analyzer  (TG209F3, NETZSCH, GER).
The morphological structures of XACs were observed using a field-emission scanning electron microscope (SEM) (Nanosem 430, FEI, USA).
The textural characteristics of XACs were determined by physical N2 adsorption–desorption at 77 K using an auto-adsorption system (Autosorb-iQ2-MP, Quantachrome, USA). The specific surface area (SBET) was calculated through N2 adsorption isotherm using Brunauer–Emmett–Teller (BET) equation. The external surface area (Sext) was determined using t-plot method. The microporous specific surface area (Smic) was calculated through the difference between SBET and Smic. The total pore volume (Vt) was defined as the liquid N2 volume adsorbed at the relative pressure of 0.99.
Adsorption value of methylene blue of each XAC samples was determined according to the standard of “Test methods of wooden activated carbon Determination of methylene blue adsorption” formulated by CSBTS (state bureau of quality and technical supervision), China .
Results and discussion
Pyrolysis behavior of different waste residue
The pyrolysis of these 4 waste residues could be divided into 3 stages according to the weight loss (Fig. 1a). The 1st stage occurred at 25–250 °C during which the weight loss was less than 10%. The weight loss could be attributed to the moisture removal. Most of the weight loss happened in the 2nd stage which occurred at 250–400 °C. The weight losses were 60% for pueraria residue, 58% for reed root residue, 60% for liquorice residue, and 58% for bagasse. The corresponding endothermal peaks for this stage (Fig. 1b) of these 4 waste residues were all in the range of 300–350 °C which could be attributed to the pyrolysis of biomass components such as cellulose, hemicellulose and lignin. There was no separation peak of pueraria at this stage, which indicated that pueraria mainly contained lignin, while the remaining bagasse, licorice, and pueraria contained more hemicellulose [21, 22, 23]. Temperature over 400 °C corresponded to the 3rd stage with small weight loss which may be attributed to the slow gasification of small number of intermediate products of pyrolysis.
Figure 1 indicates that most carbonization of the four waste residues was completed below 600 °C. Figure 1a also shows the AC yield at 800 °C according to the residue weight fraction. It indicated that the AC yield sequence of 4 waste residues was RR > LR > PR > BR, among which RR yield was the highest, about 28.04%, while BR yield was the lowest, about 18.62%.
Characterization of XACs
The pore diameter distribution (Fig. 2b) of PAC and RAC samples showed an obvious peak distribution in the range of mesoporous and microporous (< 0.6 nm) which provided high specific surface area . It indicated that there were almost no micropores in LAC, and the pore size distribution was mainly mesoporous and microporous, while the pore diameter distribution of LAC showed different characteristics that the pore diameter was larger than 0.6 nm. The pore size distribution of BAC was similar to LAC, but according to BJH analysis, BAC had a distribution peak at 0.6 nm, indicating that there were mesoporous holes of about 0.6 nm in BAC.
Textural properties of XACs
Specific surface area (m2 g−1)
Pore volume (cm3 g−1)
By comparison with specific surface area, pore volume and pore diameter distribution, the following rules were obtained: PAC > BAC > LAC. Among the 4 XACs, the RAC exhibited the largest specific surface area, pore volume and pore size distribution, thus may have better adsorption properties.
SEM (Fig. 2c) shows that RAC and PAC have uniform pore structure with small pore diameter, BAC has slightly larger pore diameter, and the structure of LAC is loose and almost no regular pore structure, which was caused by different reactivity and distribution of different components.
Effect of factors on activated carbon prepared
AC yield and methylene blue absorption of XACs
Carbonization temperature (°C)
AC yield (%)
Methylene adsorption (mg/g)
The influence of impregnation ratio
The influence of impregnation ratio on AC yield.
This phenomenon is due to the fact that zinc chloride has a protective effect on the carbon skeleton when the impregnation ratio is relatively low, and because of the short activation time, the burning loss of activated carbon is difficult to occur. At the same time, in the activation process, zinc chloride has the function of catalytic dehydroxy and dehydration, which causes the hydrogen and oxygen in the raw materials to be expelled as water vapor. In addition, this process can inhibit the production of tar, avoid clogging pores, thus producing XACs with porous structure. However, if the impregnation ratio is raised to a certain value, the AC yield will not continue to increase due to the limited carbon content in the raw material itself, and excessive zinc chloride will enhance the dehydration, which may cause the decline of the AC yield when the impregnation ratio reaches 1:2.
The influence of impregnation ratio on adsorption capacity of methylene blue.
Analyzed by SEM.
The morphological structures of XACs were observed using a field-emission scanning electron microscope (SEM)  (Nanosem 430, FEI, USA).
The SEM of BACs prepared at different carbonization temperatures and impregnation ratios is shown in Fig. 5b. The influence of impregnation ratio was discussed at the condition of same material and different carbonization temperatures.
As for the impregnation ratio, the SEM pictures show clearly that an appropriate increase in temperature and impregnation ratio is conducive to the formation of pore structure, but an excessive temperature and impregnation ratio will lead to excessive erosion and pyrolysis of activated carbon structure.
The influence of carbonization temperature
The influence of carbonization temperature on AC yield.
It also can be seen from the figure, with the increase of carbonization temperature, the AC yield of XACs at each impregnation ratio showed a downward trend. When the carbonization temperature was from 600 to 700 °C, the temperature change had little effect on the AC yield, while the temperature reached 800 °C, the AC yield curve of all the impregnation ratio decreased significantly.
The influence of carbonization temperature on adsorption capacity of methylene blue.
In general, the higher the activation temperature was, the more complete the volatilization of the residual volatiles, the more developed the microporous structure, and the greater the specific surface area and adsorption activity were. Some samples such as LAC followed the above rules, that the increase of activation temperature promoted the formation of porous structure of activated carbon and the increase in adsorption of methylene blue.
Analyzed by SEM.
The morphological structures of XACs were observed using a field-emission scanning electron microscope (SEM) (Nanosem 430, FEI, USA).
SEM of XACs with different carbonization temperature with impregnation ratio at 1:1.
The activated carbon samples prepared by the raw material BR, PR, RR and LR are characterized by TG analysis, BET and SEM. TG analysis predicted that the AC yield sequence of 4 waste residues was RR > LR > PR > BR, while the experimental data after activation of zinc chloride did not follow this rule. This suggests that the activator has different effects on each waste residue. Nitrogen adsorption–desorption data and pore size distribution measured by BET all indicate that BAC, RAC and PAC samples have microporous and mesoporous structures and may have good adsorption performance which have been confirmed by experimental data of the adsorption capacity of methylene blue and SEM pictures. Therefore, BR, PR and RR are suitable for preparing activated carbon.
According to the data of the yield of AC and adsorption capacity of methylene blue, the suitable preparation conditions of the four XACs were analyzed from the aspects of impregnation ratio and carbonization temperature.
Due to its different structure and composition, the optimal conditions for the preparation of activated carbon from these waste residues have different characteristics.
For bagasse, the optimum preparation condition was 700 °C and the impregnation ratio was 1:1, the adsorption capacity of methylene blue reached 246.83 mg/g, AC yield was 40.98% of the moment.
For reed root residue, the optimum preparation condition was 600 °C and the impregnation ratio was 1:2, the adsorption capacity of methylene blue reached 268.07 mg/g, AC yield was 34.3% of the moment.
For Pueraria residue, the optimum preparation condition was 700 °C and the impregnation ratio was 1:2. The adsorption capacity of methylene blue reached 297.33 mg/g, AC yield was 34.1% of the moment.
In general, with the increase of carbonization temperature, the carbon yield of BAC, PAC and RAC decrease. BAC, PAC and RAC can achieve better carbon yield when the impregnation ratio is 1:1, but the impregnation ratio needs to be further increased to 1:2 to reach the maximum adsorption capacity of methylene blue.
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