1. Introduction
The removal of volatile organic compounds (VOCs) from the air is quite important in industry. Typical VOCs emitted by processes used in the chemical and petroleum industries contain aromatics, ethers, aldehydes, halogenated compounds, and so on [
1]. Toluene is among the most important VOCs emitted by various industrial processes. It has been widely used as a representative solvent in the manufacture of paints, inks, rubbers, adhesives, and various other chemical substances. Toluene is emitted to the atmosphere via vaporization, leakage, and in exhausts. The emission of gaseous toluene is a public health concern because toluene is neurotoxic, and exposure can cause headaches, dizziness, mental depression, cognitive dysfunction, and many other symptoms [
2,
3].
Many methods have been proposed for use in the removal of VOCs from the air, including adsorption, catalytic decomposition, absorption, photocatalytic oxidation, thermal oxidation, and bioremediation. Among them, adsorption is widely applied. It is considered to be a superior VOC removal method because of its low energy consumption, cost effectiveness, and high capacity for VOC removal, even at low VOC concentrations [
4,
5]. Various adsorbents have been used for this purpose; in particular, activated carbon is commonly used. Activated carbon is one of the most important forms of industrially used carbon. It has a very high specific surface area and a well-developed surface pore structure, which make it suitable for use as an adsorbent for the removal of gaseous pollutants [
6].
Activated carbon can be prepared from various plant-derived biomasses. Coconut shell and sawdust are the main raw materials used in commercial production. Other natural substances have also been used, for example, sugarcane bagasse [
7], straws [
8], rice husks [
9], corncob [
10], spent coffee grounds [
11], nutshells [
12], and many other agricultural wastes [
13]. In Japan, buckwheat (
Fagopyrum esculentum Moench) is frequently consumed as an ingredient in foods such as noodles, crackers, and beverages. Buckwheat hull is generated during the production of buckwheat flour. It was once used to stuff pillows, but this practice is now less common because of concerns regarding allergic effects and vermin. Buckwheat hull has thus become an agricultural waste. Therefore, applications that effectively utilize buckwheat hull are strongly desired. Although buckwheat hull is expected to be a good raw material for activated carbon production, this possibility has not undergone much progress recently. Kuwamura and Takahashi reported that a carbonaceous product was obtained by the thermal decomposition of buckwheat hull at 300–550 °C [
14]. In this example, however, the only activation method used was heating. Other activation methods, such as steam and chemical activation, were not employed. The activation of the prepared activated carbon using chemicals is expected to significantly increase its adsorption capacity.
In the present study, activated carbons were prepared from buckwheat hull to produce adsorbents for the removal of gaseous toluene. To introduce the pores needed for toluene adsorption, chemical activation with potassium carbonate (K2CO3) was applied. The main aim of this work was to evaluate the effect of the K2CO3 chemical activation of adsorbents derived from buckwheat hull on their gaseous toluene removal capacity. The toluene removal capacities of the adsorbents prepared with and without K2CO3 activation were compared. The effect of K2CO3 activation on toluene adsorption was investigated through I2 adsorption, methylene blue adsorption, scanning electron microscope (SEM) observation, and toluene adsorption.
K
2CO
3 chemical activation is known to introduce pores and increase the specific surface area of adsorbents [
15]. Typical chemical activating agents for the preparation of activated carbon include K
2CO
3, ZnCl
2, H
3PO
4, NaOH, and KOH. Among them, K
2CO
3 is the least harmful and the least corrosive; these are advantageous characteristics for its industrial applications in activated carbon production. Despite these advantages, there are a few examples of the preparation of the activated carbon from buckwheat hull by chemical activation using K
2CO
3. In the present case, the K
2CO
3-activated adsorbent adsorbed much more toluene than the unactivated adsorbent. The enhanced toluene removal capacity was explained by the differences in specific surface areas and mesopore structures between the unactivated and K
2CO
3-activated adsorbents. These results showed that chemical activation via K
2CO
3 was effective in improving the toluene adsorption capacity of the buckwheat hull-derived carbon adsorbent.
2. Materials and Methods
Adsorbents were prepared from buckwheat hull obtained in Abira Town, Hokkaido, Japan. The buckwheat hull was ground and sifted to obtain a powder with particle sizes of 1 mm to 180 µm. The hull powder (10 g) was added to an aqueous potassium carbonate (K2CO3, Junsei Chemicals, Tokyo, Japan) solution consisting of 10 g of K2CO3 and 15 mL of water. The mixture was then stored in a desiccator for 24 h under reduced pressure to impregnate the hull powder with K2CO3 and vaporize the water. After impregnation, a pasty mixture was obtained. The mixture was further dried at 110 °C for 24 h. The dried mixture was ground with a mortar and pestle and heated in a tubular furnace (FT-01 VAC-WM, Full-tech, Yao, Japan) in nitrogen flow under reduced pressure. The heating temperature and time were 800 °C and 2 h, respectively. The obtained carbonaceous product was boiled in water for 1 h to remove potassium-containing products and unreacted K2CO3. Next, the carbonaceous product was filtered and dried at 110 °C until it was completely dry. An adsorbent was also prepared without K2CO3 impregnation via the simple thermal decomposition of buckwheat hull powder under the same conditions. The prepared adsorbents were characterized by iodine and methylene blue adsorption tests, X-ray diffractometry measurement, scanning electron microscopy (SEM), and Fourier transform infrared absorption (FT-IR) spectroscopy.
The buckwheat hull powders that were prepared with and without K
2CO
3 impregnation were both converted to black carbonaceous solids via heating. However, the product yields differed somewhat. The calculated yields (amount of product/amount of buckwheat hull powder) were 26 wt% and 23 wt% for the unactivated and K
2CO
3-impregnated hull powders, respectively. The low yield of activated carbon is common. For example, the yield of activated carbons obtained through gas activation has been reported to be about 10% [
16]. Muroyama et al. reported that the yield of activated carbons obtained from bean curd via K
2CO
3 activation was about 5–20% depending on the heating temperature used [
17]. These low yields are due to the gasification of carbon with various reactants, such as H
2O, O
2, CO
2, K
2O, K
2CO
3, and so on. When the hull powder was carbonized in the presence of K
2CO
3, the reduction of K
2CO
3 via carbon occurred, and K
2CO
3 and carbon reacted to generate metallic potassium, CO, and CO
2 [
18]. Consequently, some of the carbonaceous product was lost. Burning was observed when the K
2CO
3-impregnated product and the inner wall of the quartz tube used for carbonization were in contact with water. This behavior indicates the formation of metallic potassium through the reduction of K
2CO
3 via carbon.
Figure 1 shows photographs of 1 g samples of the unactivated and K
2CO
3-activated adsorbents. Their bulk densities are clearly different. This difference suggests that K
2CO
3 activation effectively introduces pores into adsorbents. This significant lowering of the bulk density via K
2CO
3 activation is caused by the thermal decomposition of K
2CO
3 itself and/or the reduction of K
2CO
3 via carbon. Both of these reactions generate CO and CO
2, and they effectively create pores, as shown later in the paper. This pore formation results in the decrease in apparent bulk density. The apparent bulk densities of the adsorbents were roughly estimated from the weight (1 g) to volume ratios, which were calculated from the height and diameter of the particle layers in the test tubes. The densities were estimated to be 0.5 g/cm
3 and 0.1 g/cm
3 for the unactivated and K
2CO
3-activated adsorbents, respectively.
The I2 adsorption test was performed as follows. First, a 0.05 mol/L iodine (I2) aqueous solution containing potassium iodide (KI) was prepared. KI was used to make I2 soluble. The molar ratio of I2 and KI in the solution was 1:3. Each prepared adsorbent (0.1 g) was weighed and added separately to 50 mL of the aqueous I2 solution. The mixtures were stirred at 25 °C for 5 h in the dark. They were then centrifuged to separate the adsorbent from the solution, and the I2 concentration of the supernatant was determined via redox titration using a 0.1 mol/L aqueous sodium thiosulfate (Na2S2O3) solution. The amount of adsorbed I2 was determined from the difference in the measured I2 concentrations before and after adsorption. The detection limit of I2 amount was expected to be 1.27 mg under the experimental condition; 0.01 mL (the minimum scale) of the Na2S2O3 solution corresponded to 1.27 mg of I2.
The methylene blue adsorption test was performed using a 0.40 mmol/L aqueous methylene blue solution. This solution (20 mL) was mixed with an appropriate amount of the unactivated or K2CO3-activated adsorbent. The mixture was stirred for 2 h at 25 °C and then centrifuged to separate the adsorbent from the solution. The visible light absorbance of the supernatant at 650 nm was measured using a UV–visible spectrophotometer (V-630, JASCO, Tokyo, Japan). The amount of adsorbed methylene blue was determined using a calibration curve.
Powder X-ray diffraction (XRD) patterns of the prepared adsorbents were recorded with an X-ray diffractometer (RINT-2000, Rigaku, Tokyo, Japan) using CuKα radiation and a scan rate of 2°/min. SEM images (VE-8800, Keyence, Osaka, Japan) and FT-IR spectra (FT/IR-6600, JASCO, Tokyo, Japan) were recorded. The adsorbents were tableted using potassium bromide before the FT-IR measurements.
Toluene adsorption tests were conducted using a Tedlar
® bag (AAK-5, GL Science, Tokyo, Japan). The tests were conducted at ambient temperature (15–16 °C). An adsorbent (0.2 g) was placed in a corner of the bag, and the corner was partitioned with a clip. Then, toluene and air were mixed in the rest of the bag to make a gaseous air–toluene mixture with a toluene concentration of 200–240 ppm. After the toluene was vaporized, the initial toluene concentration was measured using a gas detector tube (No. 122, GASTEC, Kanagawa, Japan). The clip used for partitioning was then removed to enable adsorption. The change in toluene concentration was observed in the same manner for 120 min or 24 h.
Figure 2 shows the experimental procedure for the toluene adsorption test. The gas detector tube indicates toluene concentration by coloration with I
2 generated through the reduction of I
2O
5 via toluene. The stated detection limit was 1 ppm, and the stated coefficients of variation of measured concentration were 5% (10–100 ppm) and 10% (100–300 ppm).
4. Conclusions
The toluene adsorption capabilities of unactivated and K2CO3-activated carbonaceous adsorbents derived from buckwheat hull were compared. The K2CO3-activated adsorbent adsorbed much more toluene than the unactivated adsorbent. The enhanced toluene removal capacity was explained by the differences in specific surface areas and mesopore structures between the unactivated and K2CO3-activated adsorbents. These results showed that chemical activation via K2CO3 was effective in improving the toluene adsorption capacity of the buckwheat hull-derived carbon adsorbent. In the present case, the K2CO3-activated adsorbent was able to reduce the relatively high initial toluene concentration (220 ppm) to almost zero after 24 h. Although the maximum adsorption capacity was not precisely determined in these experiments, an impressive change in the adsorption capacity via K2CO3 activation was confirmed. This is an important finding with regard to the utilization of buckwheat hull as a raw material for activated carbon.
In the present case, the main adsorption mechanism of toluene is the simple physisorption of toluene molecules on the carbon surface; no chemical reactions between toluene and the adsorbent surface occurred in this case. Therefore, adsorbates other than toluene, such as benzene, xylene, and water vapor, may also be competitively adsorbed on the adsorbent prepared in this work. In addition, temperature also affects the adsorption behavior of toluene. The selectivity of toluene adsorption, the effect of humidity, and the effect of temperature on the adsorption capacity of toluene are subjects that remain in need of further study.