Chapter 2: Literature Review
2.8 Effect of Process Parameters on the Properties of Activated Carbons
Depending on the nature of the precursor and by adjustment of the process parameters, different pore sizes can be obtained. The specific surface areas and porosities of AC are greatly affected by the precursors of carbonaceous materials and methods of preparation. The adsorption capacity and the adsorption rate of an AC are directly associated with the specific surface areas and the pore size distribution of the AC. In general, the larger the specific surface area, the greater the adsorption capacity. However, for the adsorption of larger molecules, the adsorption capacity and the adsorption rate are largely dependent on the mesoporous (and macroporous) volumes [78].
2.8.1 Activating agents
Nowicki et al. [79] studied the physical activation of cherry stone-derived biochars produced at pyrolysis temperature of 500 °C and 800 °C. The physical activation of the biochars were carried out at a temperature of 800 °C under a stream of CO2. The authors reported a surface area of 367 m2/g and
361 m2/g for the activated carbons produced from 500 °C-biochar and 800 °C-biochar respectively. The
authors asserted that the physical activation of the biochars does not permit a substantial development of the porous structure of the activated carbons.
Similarly, Sun and Jang [80] investigated the physical activation of rubber-seed shell using steam. The precursor was pyrolyzed to produce biochar at a temperature range of 450 – 650 °C prior to activation. The activation was carried out in a flow of vapor steam at a temperature range of 800 – 900
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°C. The authors reported an increase in the creation of mesopores and decrease in the micropores as the activation temperature increased. At high temperature, the micropores were enlarged and the walls between the pores collapsed and formed mesopores.
For chemical activation, activating agents can be alkaline or acidic in nature. For the physical activation, the activating agents are steam or CO2. The effect of the activating agent on the properties
of the activated carbon are evident on its surface area, porosity and surface chemistry. Bazan-Wozniak et al. [81] investigated the effect of physical and chemical activating agents on the surface area, porosity and surface functional groups of activated carbon produced from pistachio nutshells and its biochar. The biochars were produced at a temperature of 500 and 700 ºC. The physical activating agent used was CO2 with a flow rate of 0.250 L/min and the chemical activating agents used were H3PO4 and
K2CO3. The authors reported the highest surface area of 1204 m2/g for activated carbons obtained from
the physical activation of pistachio nutshell-biochar at an activation temperature of 900 ºC. The activated carbons obtained from the direct impregnation of the precursor with K2CO3 was also reported
to have a high surface area of 1093 m2/g.
Among the chemical activating agents, ZnCl2 and H3PO4 are commonly used for activation
purpose for lignocellulosic materials, whereas compounds such as potassium hydroxide (KOH) are used for the activation of coal precursors or chars. When compared to zinc chloride, phosphoric acid is preferred because of the environmental disadvantages associated with zinc chloride which includes problems of corrosion, inefficient chemical recovery and the carbons obtained using zinc chloride cannot be used in pharmaceutical and food industries as they may contaminate the product [82, 83]. It also gives higher yield of activated carbon and has non-toxic properties [84]. However, the use of ZnCl2 havebeen reported to produce higher surface area and more microporous structure [85] while
H3PO4 is effective in producing the mesopores, and resulting in higher pore volumes and diameter.
The use of KOH as activating agents has been found to be effective in production of activated carbon with large microporosity and narrow pore size distribution but its yield is lower than carbon activated with zinc chloride or phosphoric acid, and at high temperature, i.e. ± 650 0C, the carbon
content is lower than the fixed carbon in the initial precursor. The presence of metallic potassium will intercalate to the carbon matrix [82, 86, 87].
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2.8.2 Effect of impregnation ratio
Impregnation ratio represents the mass ratio of the activating agent to precursor or vice versa. The impregnation ratio ensures that the chemical is able to penetrate totally into the precursor and react with the components (i.e cellulose, hemicellulose and lignin for lignocellulosic precursor). The effect of the impregnation ratio on the porosity of the resulting product is evident from the fact that the volume of salt (obtained from the activation agent) in the carbonized material is equal to the volume of pores, after the salt has been extracted.
Patnuko and Pavasant [88] investigated the effect of impregnation ratio on the production of activated carbon from Eucalyptus camaldulensis Dehn bark using phosphoric acid. It was reported that activated carbons with better adsorption capacity were obtained with impregnation ratio of 1:1 while a reduction in the adsorption capacity of the carbon at higher impregnation ratio was observed. Also, a higher BET surface area of 1239 m2/g was obtained at the impregnation ratio of 1:1. In a similar
research, Kalderis et al. [89] investigated the production of activated carbon from bagasse and rice husk using zinc chloride, sodium hydroxide and phosphoric acid with impregnation ratio 0.25:1, 0.5:1, 0.75:1 and 1:1 (i.e. ratio of activating agent to precursor). BET surface areas obtained with ZnCl2 as activating
agent and impregnation ratio of 1:1 for rice husk and 0.75:1 for bagasse were reported to be 750 m2/g
and 674 m2/g respectively.
2.8.3 Effect of carbonization temperature
Juejun et al. [90] conducted a physical activation of coconut shell-derived biochars (produced at a carbonization temperature range of 250 – 750 °C) using CO2 at a temperature of 850 °C for 60 and
120 min. The authors reported that the porosity of the activated carbon decreased with increase in the carbonization temperature. The char produced at the lowest carbonization temperature of 250 °C and activated at 850 °C for 120 min gave the highest BET surface area and pore volume of 1056 m2/g and
0.533 cm3/g respectively. A comparison of the work done by Nowicki et al. [79] on physical activation
of cherry stone-derived activated carbon showed that pyrolysis of the cherry stones at temperature of 500 °C and 800 °C respectively before activation using CO2 produced activated carbons with surface
areas of 367 and 361 m2/g respectively. Meanwhile, the chemical activation of the cherry stone-derived
biochars (pyrolysis temperature of 500 °C and 800 °C) using KOH as chemical agents produced activated carbons with surface areas of 1324 m2/g and 1173 m2/g respectively. In both activation cases,
the biochars produced at pyrolysis temperature of 800 °C resulted in activated carbons with low surface area.
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Furthermore, Bazan-Wozniak et al. [81] reported a surface area of 1204 m2/g for activated
carbon produced from physical activation of biochar obtained at a carbonization temperature of 700 °C. The activated carbon from biochar obtained at a carbonization temperature of 500 °C under similar activating condition was reported to have a surface area of 277 m2/g. The authors attributed the
significant difference in the surface areas of the activated carbon to the carbonization temperature used. When the biochars were chemically activated, the biochar obtained at a carbonization temperature of 700 °C gave an activated carbon with a surface area of 530 m2/g, which is not significantly different
when compared to the activated carbon from biochar obtained at a carbonization temperature of 500 °C (513 m2/g). The authors explained that the negligible difference in the chemically obtained activated
carbons may due to the fact that carbon structure of the biochars is ordered enough so that the activating agent used (K2CO3) or the impregnation ratio used (2:1) were insufficient for effective development of
the porous structure.