Physical and chemical properties and adsorption type of activated carbon prepared from plum kernels by NaOH activation
Introduction
Activated carbon is a widely used adsorbent in the treatment of wastewater and drinking water. Adsorption type and capacity are primarily based on the physical properties of the pores, namely the specific surface area, the pore size and distribution. Measurement of adsorption and desorption of N2 at 77 K and scanning electron microscope (SEM) observation provided important pore characteristics; then the adsorption type and capacity of the activated carbon could be evaluated. The data of adsorption and desorption isotherm of N2 at 77 K are the most investigated physical property of activated carbon. This measurement has been used in 89% related to activated carbon prepared from raw plant material in 2005. These data provide not only specific area (Sp), total pore volume (Vpore), and pore size distribution but are also used to calculate micropore volume and exterior surface area (Sext). Studying the shapes of the isotherm curve and hysteresis can be used to infer distribution, shape and structure of the pores inside activated carbon. This measurement is a prerequisite. Differences in surface structure features of activated carbon prepared from various raw materials, activation methods and conditions can be investigated with SEM observation. Surface features of activated carbon prepared under various conditions were observed in the previous studies [1], [2]. The results were used in inferring the activation reaction mechanism.
Factors affecting adsorption type are pore structure, surface chemistry, and adsorbent properties. Among them functional groups and chemical compositions play important roles in the adsorption mechanism and capacity. Measurements of elemental analysis and temperature programmed desorption can be employed for understanding chemical properties of activated carbon. Chemical compositions of activated carbon were measured with elemental analysis by some investigators [3], [4], [5]. Literature reveals little about relationships between the chemical compositions of activated carbon and its preparation conditions as well as activation mechanisms. The changes in the chemical compositions of the material during the activation process cannot be completely described. Thus, investigation of the chemical compositions of activated carbon with elemental analysis is important. Temperature programmed desorption (TPD) can be utilized to determine the strength of the surface active groups, quantity, and type of activated carbon. Stones prepared activated carbons by steam and CO2 activation [6]. Carbons were desorbed with TPD and the functional groups (carbonyl, carboxylic, phenolic, etc.) were investigated. Wu et al. [7] prepared activated carbon and measured it with TPD. It was found that KOH-activated carbon released more CO2 than steam-activated carbon did. TPD measurement is getting more important in the field of activated carbon research.
Recently, NaOH activation, which reduces chemical activation cost and environmental load, has gradually been drawing more attention from researchers [8], [9], [10], [11], [12], [13]; but studies have been limited to coal as raw material. Activated carbon prepared from plant material is higher of O/C and H/C ratios and its physical properties, chemical properties, and adsorption type are significantly different from those prepared from coal. Good effects of activated carbon prepared from hard plum kernels were found in the previous studies [14], [15]. Thus, in this study, high surface area activated carbon was prepared from plum kernels by NaOH activation, having a lower environmental load. The physical properties (adsorption and desorption isotherm of N2 at 77 K and scanning electron microscope) and the chemical properties (elemental analysis and temperature programmed desorption) of activated carbon were measured. Adsorption on dyes and phenol was used to investigate adsorption type. In order to understand the NaOH activation mechanism of raw plant material, the optimum dosage of NaOH activation had to be evaluated.
Section snippets
NaOH activation
In this study, the plum kernels were pretreated at 110 °C for more than 24 h until dry and then put into a high temperature oven, which was kept oxygen-starved by introducing nitrogen gas. The oven was kept at 450 °C for 2 h to carbonize the plum kernels. The carbonization yield of plum kernels was 37.0 wt.%.
The char was ground and sieved and the part with a particle size between 0.833 and 1.65 mm was activated. The activation was done by dissolving some NaOH in water to which the char was added,
Physical properties of activated carbon
Before designing and using the adsorption process, the pore structure of the adsorbent must be done [18], [22]. Fig. 1 shows the curves of the 77 K N2 isotherm adsorption/desorption of the activated carbon obtained at six different values of the NaOH/char ratio in this study. Values of NaOH/char ratio of stage 1 activated carbon were from 0 to 1 (empty circles in the figure); those of stage 2 were from 2 to 4 (solid circles in the figure). Fig. 1 shows that the adsorption volume of stage 1
Conclusions
Activated carbon prepared from the char of plum kernels using NaOH activation were studied. Microporous activated carbon with a surface area from 1478 to 1887 m2 g−1 were obtained at a NaOH/char ratio from 2 to 4. According to the pore characteristics and SEM observation, it was inferred that there were two kinds of activation mechanisms: surface pyrolysis (NaOH/char ratio from 0 to 1) of stage 1 activated carbon; chemical etching and swelling (NaOH/char ratio from 2 to 4) of stage 2 activated
Acknowledgment
Financial support for this work by the National Science Council, Taiwan, under Grant NSC95-2221-E-239-022 is gratefully acknowledged.
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