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Journal of Wood Science

Official Journal of the Japan Wood Research Society

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Japanese cedar (Cryptomeria japonica) ash as a natural activating agent for preparing activated carbon

Journal of Wood Science volume 61, pages 316–325 (2015)Cite this article

Abstract

Japanese cedar (Cryptomeria japonica) ash was used as a natural activating agent and also a precursor to prepare activated carbon (AC) using various ash to water ratios (0.20, 0.25, and 0.33 wt%), activation temperatures (750, 850, and 950 °C), and activation durations (0.5, 1.0, and 1.5 h). The yields, iodine value, and methylene blue adsorption value of Japanese cedar AC were in the ranges of 27.3–30.7 %, 953.5–1150.9, and 695.1–1368.0 mg/g. As the ash to water ratio increased, the BET specific surface area and total pore volume of the AC increased. Japanese cedar AC prepared using an ash to water ratio of 0.33 wt%, an activation temperature of 850 °C, and an activation duration of 1.0 h exhibited the highest BET specific surface area (1430 m2/g) and total pore volume (0.74 cm3/g). The nitrogen adsorption–desorption isotherms of the Japanese cedar AC were classified as type IV, indicating the presence of microporous and mesoporous structures, according to the Bruauer, Deming, Deming, and Teller classification. Therefore, Japanese cedar ash with a suitable ash to water ratio can serve as an excellent natural activating agent for preparing AC because a high BET specific surface area can be obtained.

Introduction

Activated carbon (AC) is generally regarded as a porous material that has a large specific surface area, a high adsorption capacity, chemical stability, and the ability to regenerate through desorption. Therefore, AC is widely used as an adsorbent for separating and purifying gaseous or aqueous solution systems as a catalyst supporter, or for recovering precious metals [1–3]. Two methods are commonly used for preparing AC: physical and chemical activation. Physical activation consists of two different steps, i.e. carbonization and subsequent partial gasification of the raw material (precursor) with steam, air or CO2, usually at high temperature (800–1000 °C) [1–3]. In chemical activation, carbonization and activation proceed simultaneously and at a much lower temperature (450–700 °C) in the presence of an activating agent. The activating agents are dehydrating agents, which influence the pyrolytic decomposition of the precursor and inhibit tar formation [1, 2]. Furthermore, many types of activating agents exist, such as H3PO4, K2CO3, ZnCl2, and KOH [4–8]. These agents are generally strong acids or alkalis, or substances containing alkali metals. Among the chemical activating agents, ZnCl2 and H3PO4 are typically used for preparing AC [4, 5] through the corrosion, dissolution, or dehydration of precursors; moreover several agents oxidize precursors. Chemical activation requires a lower activation temperature and a shorter activation duration than does physical activation, and can be used to obtain an excellent pore structure and high carbon yield [2]. In the United States, most AC (40–50 %) is prepared using chemical activation, but the environmental pollution, eutrophication, equipment corrosion, and residual activating agent in the AC remain serious problems [9, 10]. Therefore, developing another suitable activating agent for industrially preparing AC is critical, and natural materials are considered to be excellent choices for this purpose.

In recent years, because of the rising price of petroleum and natural gas, and the threat of increasing greenhouse gases, wood pellets have attracted considerable attention [11]. Wood pellets are used in large-scale applications, such as industrial heating systems, coal- powered boilers, and heat gasification [12], or in small-scale operations involving district area and household heating systems [13–15]. The wood pellet market is increasing substantially, and more than 442 pellet plants exist in the world, with a total production capacity of 14 million tons a year [14, 16]. Generally, wood pellets are used for energy production, but the large amount of wood pellet ash generated during combustion goes to landfills [17–19] or is used in farmland or forest fertilization and soil improvement because of its nutrient and trace element content, and acid neutralization capacity [18, 20–29]. However, wood ash does not improve tree growth or biomass production because of its low nitrogen content, and causes biomass to decrease slightly [21, 23–25]; furthermore, it is strongly alkaline (pH 13.3) and contains several metal salts, oxides, hydroxides, and carbonates [28, 30]. According to Marsh and Rodríguez-Reinoso [31], the mechanism for preparing AC involves alkali salt activation, in which the oxygen atoms of hydroxide and potassium or sodium carbonate gasify carbon atoms to produce CO gas, thereby creating porosity. This gasification is catalyzed by the alkali metal salt. The reduction of an alkali metal salt produces potassium or sodium metal atoms, which are intercalated into the carbon structure, thereby expanding the carbon matrix [4, 5, 7, 32–34].

The Taiwanese government has strongly encouraged afforestation, using species such as Cunninghamia lanceolate, Cryptomeria japonica, and Taiwania cryptomerioides. The plantation growing stock is now over 30 years old, and has not been subjected to thinning, particularly Japanese cedar, which has currently reached 23,000 ha with approximately 4 million m3 of wood. Implementing massive thinning is essential to obtain optimal tree growth, because thinning produces logs with a small diameter utilization and generates approximately 3.6 × 105 m3 of woody biomass residues [35]. Japanese cedar is a suitable precursor for preparing charcoal [35, 36], bio-charcoal [37] and a potential material for producing wood pellets [19, 38, 39]. In addition, Japanese cedar wood is suitable for preparing AC because of its high carbon content and low cost [2, 8, 40–42]. Extensive research has been conducted to investigate the applications of wood pellet ash in farmland and forest fertilization or soil improvement [18, 20–29]; however, new applications are rare. In this study, Japanese cedar ash was used as a natural activating agent to prepare AC from the thinning wood of Japanese cedar and also as a precursor under diverse conditions; specifically, the activation temperature, activation duration, and ash to water ratios. The yield, iodine value, methylene blue adsorption value, BET specific surface area, and porosity characteristics of the AC were investigated. The results can be used as a reference for using the ash of wood pellets after combustion, and can also be used as an example of an activation method for preparing AC to implement the sustainable reuse of resources.

Materials and methods

Materials

A Japanese cedar (Cryptomeria japonica D. Don) tree, used to obtain the precursor material, was obtained from the Experimental Forest at the College of Bio-Resources and Agriculture, National Taiwan University in Nantou County, Taiwan. The tree was approximately 40 years old, and the average specific gravity was approximately 0.41. The tree was debarked, crushed and sieved to obtain particles with a size between 0.38 and 0.83 mm [43]. To obtain wood ash, the thinned wood log (with a dimension of approximately 20–30 cm) was stacked on an iron grid and burned. The wood ash was then collected to be used as a natural activating agent, and sieved to remove dust before preparing the AC.

Preparing wood ash–water mixture with different ratio

Japanese cedar wood ash and distilled water were mixed using various weight proportions, boiled for 10 min, and filtered to obtain various ash to water ratios (0.20, 0.25, and 0.33 wt%). The sample codes were JC-0.20, JC-0.25, and JC-0.33. The pH value of the wood ash–water mixture was tested using a pH meter (Suntex TS-1).

Preparation of activated carbon

The precursor and wood ash–water mixture were mixed, impregnated for 24 h, and filtered. The mixture was heated in an oven at 105 °C for 24 h. The weight gains of the precursors JC-0.20, JC-0.25, and JC-0.33 were approximately 7.6, 17.2, and 23.2 %, respectively. The resulting mixture and the sample in the control group (without wood ash–water mixture impregnation) were loaded in a crucible, which was placed inside an upright high-temperature activation furnace (inner diameter, 26 cm; inner height, 40 cm). Each sample was heated under a nitrogen flow (200 mL/min) for carbonization at a rate of 10 °C/min, and maintained at the desired temperature for 0.5, 1.0, and 1.5 h. The activation temperature was set at 750, 850, and 950 °C [4, 5, 34]. The sample code was Japanese cedar wood ash (JC)–ash to water ratio–activation temperature–activation duration. After cooling the furnace to room temperature under the nitrogen flow [4, 5, 7, 34], the AC was washed with a 1 N HCl solution, stirred for 30 min to leach out the activating agent, and washed repeatedly with distilled water at 90 °C to remove excess acid until the pH value of the washed solution was approximately 6–7 [44, 45]. Subsequently, the AC was dried in the oven at 105 °C for 24 h.

Basic properties of Japanese cedar wood

Japanese cedar particles were washed with distilled water to remove dust, dried for 24 h at 105 °C to reduce the moisture content, and grounded to obtain fine particles with sizes less than 0.25 mm.

The carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S) contents of the Japanese cedar particles were determined using an elemental analyzer (Elemental Vario CHNS/O Analyzer, EA, Germany).

Proximate analysis was conducted according to Chinese National Standard (CNS) 10821 [46], CNS 10822 [47], CNS 10823 [48], and CNS 10824 [49]. The results regarding moisture, ash, volatile matter, and fixed carbon content were subsequently obtained.

Thermogravimetric analyses (TGA) were conducted using a thermogravimetric analyzer (Perkin Elmer Pyris1) to determine the pyrolysis behavior of both the Japanese cedar wood and the wood impregnated by wood ash–water mixture at various ash to water ratios. A sample weighing approximately 5.0–10.0 mg was placed on the tray of the analyzer and heated at a temperature range of 25–850 °C, with a heating rate of 10 °C/min, under a nitrogen flow at a rate of 100 mL/min.

Characterization of wood ash

The pH value of Japanese cedar wood ash was determined according to CNS 697 (1965) [50]. A sample weighing 10 mg was placed on a flask equipped with a boiler-reflux condenser, and 30 mL of distilled water was added to the flask and boiled for 5 min. Finally, the supernatant liquid was determined using a pH meter after cooling the mixture to room temperature.

Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was conducted using an ULTIMA 2000 (Horiba Jobin–Yvon) to determine the content of Si, Al, Na, K, Mg, Ca, Ti, and Fe of wood ash. Briefly, ash and co-solvent (3 mg of lithium carbonate and 10 mg of boric acid) were weighed and mixed thoroughly. The sample was transferred into a platinum crucible to be burnt at 1000 °C, and approximately 9.09 % of nitric acid was added. Finally, the content of trace elements was determined by conducting ICP-OES after ultra-sonication. The wood ash–water mixtures were also measured using ICP-OES after the mixture was diluted with ultra-pure water to a 1 % concentration.

Characterization of activated carbon

The yield of AC (based on a dry basis) was calculated using the following equation:

$$ {\text{Yield}}, \, \% = {\text{activated carbon, g}}/{\text{Japanese cedar wood}},{\text{ g }} \times 100. $$

Iodine value was determined according to JIS K 1474 (1991) [51]. The activated carbon particle size was 40 mesh to 60 mesh, and the equation for determining the iodine adsorption value was I = (10 − K × f) × 12.69 × 5/M; (I iodine adsorption value (mg/g); K volume of sodium thiosulfate solution used for titration (mL); f ratio of 0.1 N sodium thiosulfate solution to 0.1 N iodine solution; M absolute dry weight of the sample).

To determine the methylene blue (MB) adsorption value, AC (1 mg) was added to a 25 mL aqueous solution containing 1 g/L of MB, and shaken at room temperature (30 °C). When the aqueous solution became colorless, MB was added into the flask repeatedly to assure the equilibrium adsorption of MB. After filtration, the concentration of residual MB was determined using a UV–vis spectrophotometer (CECIL, CE3041) at a wavelength of 664 nm. This test method was based on that used by Wu et al. [52].

The potassium (K) ions of AC (without acid washing) were determined by applying X-ray fluorescence (XRF) using an XGT-1000WR X-ray fluorescent analyzer (HORIBA).

The pore structure characteristics of the various samples were measured by nitrogen adsorption–desorption isotherms at 77 K using a Micromeritics ASAP 2020 at a relative pressure (P/P 0) ranging from 10−2 to 1. BET specific surface area (S BET) was determined using the standard BET equation, applied to a relative pressure range from 0.06 to 0.2. The t-plot and Barret-Joyner-Halenda (BJH) [53] analysis software were used to calculate the external surface area (S ex), micropore volume (V mi), and pore size distribution. The total pore volume (V tot) was estimated as the liquid volume of nitrogen at a high relative pressure of 0.98, and the average pore diameter (D) was calculated using the equation (4V tot/S BET) × 103 [54].

In addition, the results expressing mean and standard deviation were statistically analyzed by applying Duncan’s multiple range tests at a 5 % significance level, using the Statistical Package for Social Science (SPSS 16.0) software.

Results and discussion

Elemental and proximate analysis

The results of elemental analysis are shown in Table 1. The carbon content of Japanese cedar wood was about 48.96 %. Lin et al. [55] reported that the average carbon content of nine types of conifers was 48.21 %, and Japanese cedar exhibited the highest carbon content (49.03 %). The results of the proximate analysis of Japanese cedar wood are also shown in Table 1. Compared with other woody materials, such as birch, Chinese fir, and bamboo [42, 56, 57], the fixed carbon content of which was between 12.0 and 14.98 %, and the ash content was approximately 0.2–1.22 %, Japanese cedar wood exhibited a high fixed carbon content (16.63 %) and a low ash content (0.22 %). These results suggest that Japanese cedar wood is a suitable precursor for preparing AC.

Table 1 Elemental and proximate analyses of Japanese cedar wood
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Thermogravimetric analysis

Thermal analysis (TGA/DTA) of the Japanese cedar wood was performed to examine its decomposition characteristics. Three stages were observed in the TGA curve for Japanese cedar wood (without activation with wood ash): 45–250, 250–380, and 380–850 °C (Fig. 1), representing dehydration, devolatilization, and the consolidation of the char structure, respectively [10, 58]. A sharp weight loss was observed over 250 °C. In the second stage, the specimen registered a sharp weight loss of approximately 60.4 % at the temperature range of 250–380 °C, in which the carbonization process initiated and in which hemicellulose and cellulose fractions were mainly decomposed [3]. The weight losses gradually decreased as the temperature increased from 380 to 850 °C in the final stage. When the decomposition of the chemical components was devolatilized, the weight of the specimen decreased by approximately 81.3 % at the temperature range of 25–850 °C. The curves obtained through the TGA and differential thermogravimetric (DTG) analysis showed that for the three samples impregnated with the wood ash–water mixture of JC-0.20, JC-0.25, and JC-0.33, the peaks of the pyrolysis temperature were 242, 236, and 234 °C, and the corresponding weight losses were 38.96, 36.08, and 46.01 %, respectively. The thermal decomposition behavior changed, and the pyrolysis temperature of the samples decreased with as the ash to water ratio increased. In addition, when the temperature increased to 564, 577, and 540 °C, the weight loss of JC-0.20, JC-0.25, and JC-0.33 gradually increased, respectively. Hayashi et al. [4] prepared AC from various nutshells by chemical activation with K2CO3 at temperatures above 627 °C; the carbon in the shell chars was consumed by reducing K2CO3 to carbon monoxide, resulting in additional weight loss. Therefore, we deduced that weight loss occurred at temperatures over 540 °C. The Japanese cedar char was consumed through the reduction of K ions in wood ash, resulting in additional weight loss.

Fig. 1
figure 1

TGA and DTG curves for Japanese cedar wood and three samples activated with Japanese cedar ash. Legends: JC-0.00, JC-0.20, JC-0.25, JC-0.33. TGA thermogravimetric analyses, DTG differential thermogravimetric, JC Japanese cedar wood ash

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Analysis of the activating agent

The pH value of Japanese cedar wood ash was 12.86 (Table 2). Sano et al. [19] reported that the pH value of the ash from Japanese cedar wood pellets used for fuel was 13.2, measured using a leaching test (ash to water ratios = 0.1 wt%). Although the test methods differed, the results were similar. The results of Table 2 also revealed that the pH value of the wood ash–water mixture was between 13.68 and 14.03, and increased when the ash to water ratio increased. In addition, the results of the elemental analysis of Japanese cedar ash and the wood ash–water mixture, obtained using ICP-OES are also given in Table 2. K, Ca, and Mg were the predominant elements in the wood ash. The predominant compounds in the wood ash were K2O (34.3 %), CaO (34.3 %), MgO (5.75 %), and SiO2 (3.86 %). These results were the same as the results obtained using ash from Japanese cedar wood pellets used for fuel. In Sano et al. [19], K (160 g/kg) and Ca (180 g/kg) were the predominant elements, followed by Si (40 g/kg) and Mg (34 g/kg). In addition, the highest concentration of K was observed in the wood ash–water mixture, which was between 47832.4 and 72509.0 ppm, and was also related to the ash to water ratio. However, K2O dissolved in the mixture of wood ash–water more easily than CaO did because the predominant compound in K2O was water-soluble potassium (approximately 90 %). Thus, the concentration of Ca was between 2.9 and 4.8 ppm, which was lower than that of K.

Table 2 pH values and elemental analysis of Japanese cedar wood ash and Japanese cedar wood ash–water mixture
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Characterization of activated carbon

As shown in Table 3, the yields of the sample in the control group (without wood ash–water mixture impregnation) ranged from 25.1 to 25.7 % for Japanese cedar char prepared at carbonization temperatures of 750, 850, and 950 °C and a carbonization duration of 1.0 h. The results revealed that the difference in the carbon yield was not significant (5 %), in accordance with Duncan’s multiple range tests. In addition, the iodine value and MB value ranged from 156.0 to 337.0 mg/g and from 0.6 to 13.5 mg/g, respectively. The highest iodine value (337.0 mg/g) was obtained at a carbonization temperature of 850 °C, and the difference in the iodine value of the chars was significant. Moreover, the AC yields ranged from 27.3 to 30.7 % when using ash activation. Guo and Lua [33] prepared AC from palm shell using potassium hydroxide impregnation at various stages. The impregnation of the raw palm shell was achieved by replacing a fraction of the hydrogen ions with potassium (K) ions, and forming cross-linked complexes. The presence of potassium hydroxide in the interior of the precursor restricted the formation of tar and other liquids such as acetic acid and methanol, which are produced during the carbonization process, resulting in a relatively high yield compared with that produced by the process without impregnation. Therefore, the AC produced by wood ash activation produced higher yields than those of the sample in the control group (Table 3). We deduced that the presence of K ions (Tables 2, 3) derived from the Japanese cedar wood ash in the interior of the precursor slowed down the formation of tar and other liquids by forming cross-links, thus resulting in an increased yield. However, the activation temperature and reaction duration strongly influenced the yield. The results of Table 3 obtained that the iodine value and MB values of the AC prepared using wood ash activation ranged from 953.5 to 1150.9 mg/g and from 695.1 to 1368.0 mg/g, respectively; these values were higher than those of the sample in the control group (which registered iodine and MB values of 156.0–203.0 and 0.6–13.5 mg/g, respectively). The iodine value (953.5–1121.4 mg/g) and MB value (695.1–1108.0 mg/g) increased with either the activation temperature (750–950 °C) or ash to water ratio (0.20–0.33 wt%), respectively. JC-0.33-850-0.5 exhibited the highest iodine value (1150.9 mg/g), and JC-0.33-950-1.0 exhibited the highest MB value (1368.0 mg/g). The iodine value denotes the amount of micropores, whereas the MB value can be used to estimate the capability of AC for adsorbing large organic molecules [59], and indicates the amount of mesopores in the AC [3]. Commercial ACs produce iodine values normally ranging from 600 to 1000 mg/g [52]. Therefore, the iodine values of the ACs prepared using wood ash activation were similar to the commercial value.

Table 3 Yield, iodine value, methylene blue adsorption value, and K ions content of ACs prepared under various conditions
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K ions content

The K ions content in the Japanese cedar char and AC (without acid washing) was determined using XRF. Table 3 shows that the K ions content in Japanese cedar char was between 0.40 % and 1.74 %. The K ions content increased with as the activation temperature increased, and the difference in the K ions content of the sample in the control group (without impregnated with ash–water mixture) was significant (5 %). However, the K ions content in ACs prepared using wood ash activation ranged from 6.10 to 13.10 %, and was higher than that of the sample in the control group. When using an activation temperature of 850 °C with a duration time of 1.0 h, the K ions content increased as the ash to water ratio and iodine value increased (Table 3). This occurred because the oxygen atoms of the alkali metal salt in wood ash gasify carbon atoms to produce CO gas and produced porosity. Thus, the alkali metal salt of wood ash may play a role as a catalyzer [31]. Hayashi et al. [4] studied that ACs were prepared from various nutshells by activating K2CO3, and the carbon in the shell chars was gasified through the reduction of K2CO3 to form carbon monoxide. The specific surface area and pore volume of the shell AC increased at activation temperatures higher than 627 °C. Table 3 also shows that at an ash to water ratio below 0.33 wt% and activation temperatures from 750 to 850 °C, the K ions content decreased slightly from 12.87 to 12.60 %. A previous study reported that AC is derived from lignin by activating K2CO3. The results of the recovery ratios of K2CO3 over the temperature range of 500– 900 °C were also presented. The recovery ratio decreased slightly with an increase in temperature, indicating that K2CO3 is reduced by carbon in inert atmospheres [32]. However, in this study, the amount of K ions decreased dramatically to 6.10 % at an activation temperature of 950 °C. The melting point of K2CO3 was 891 °C [4], and temperatures greater than 900 °C resulted in evaporation. Thus, we deduced that the content of K ions decreased dramatically at the activation temperature of 950 °C; some of the K ions were reduced by carbon and the rest were vaporized.

BET specific surface area and porosity

The S BET and pore characteristics of ACs from Japanese cedar wood ash activated under various conditions are shown in Table 4. The S BET, S ex and V tot increased as the ash to water ratio increased at an activation temperature of 850 °C. JC-0.33-850-1.0 exhibited the highest values, with a S BET, S ex, and V tot of 1430, 707 m2/g, and 0.74 cm3/g. The optimal S BET and porosity can be obtained using an activation duration of 1.0 h. However, with an activation temperature of 950 °C (JC-0.33-950-1.0), the S BET decreased from 1430 to 1345 m2/g, the S ex decreased from 707 to 631 m2/g, and the V tot decreased from 0.74 to 0.68 cm2/g. We deduced that an increase in the activation temperature resulted in a collapse of the internal walls of the micropores or the combination of micropores, creating large pores, and thus decreasing the S BET and V tot of the AC. The activation temperature and ash to water ratio significantly affected the S BET and porosity of AC. An AC with S BET that ranged from approximately 850 to 1430 m2/g was prepared by activation with Japanese cedar wood ash. Such high surface areas were similar to commercial AC values, which range from 500 to 2000 m2/g [60]. Furthermore, as shown in Tables 2, 3, and 4, the K ions content substantially increased as the ash to water ratio increased, which produced a high iodine value, MB value, S BET, S ex, and V tot of AC. In other words, the development of micropores is due to the intercalation of K ions in the carbon structure [7]. Besides, Adinata et al. [34] used palm shells to prepare AC using K2CO3. The K ions may intercalate and expand the inter-layers of adjacent hexagonal network planes composed of carbon atoms, thereby increasing pore formation. In this study, the pore structure of AC was formed by activating Japanese cedar wood ash. The potassium compound formed during the activation step diffused into the internal structure of the char matrix, widened the existing pores and created new porosities. The activation was conducted according to the reaction [4, 5, 61–63] shown in the following equation:

$$ {\text{K}}_{2} {\text{O}} + 2{\text{C }} \to 2 {\text{K}} + {\text{CO}} $$

Therefore, we deduced that the development of porosity was catalyzed by the alkali metal salt of wood ash; in addition, the activation process was strengthened by increasing the ash to water ratio. Pore formation was thus enhanced because the K ions contained in the ash intercalated and enlarged the carbon matrix.

Table 4 Porosity of AC prepared under various conditions
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Nitrogen adsorption–desorption isotherms and pore size distribution

Figure 2 shows the nitrogen adsorption–desorption isotherms of the ACs prepared using three activation temperatures (750, 850, and 950 °C), an ash to water ratio of 0.33 wt%, and an activation duration of 1.0 h. When the relative pressure P/P 0 value was near 0, the quantity of N2 adsorbed at 750, 850, and 950 °C was 212, 294, and 279 cm3/g. The quantity of N2 adsorbed by JC-0.33-750-1.0 exhibited no considerable differences when the P/P 0 value ranged from the minimal to maximal value. The AC obtained was defined as type I, in accordance with the Brunauer, Deming, Deming, and Teller (BDDT) classification based on the type of adsorption isotherm [64]. The major uptake occurred at low relative pressures (less than 0.1), and the predominant microporous materials exhibited a narrow pore size distribution (Fig. 3). The isotherms of JC-0.33-850-1.0 and JC-0.33-950-1.0 exhibited a well-defined plateau and hysteresis loop, and both the hysteresis loop and adsorption “tail” were obvious at a high relative pressure (P/P 0). According to the BDDT classification, these ACs were type IV, indicating the micro-mesopore content of the adsorbents. The pore size distribution of the ACs is shown in Fig. 3. As the activation temperature increased to 850 °C, the amount of micropores and the mesopores increased drastically, indicating the creation of a microporous structure and the growth of micropores to mesopores. The creation of mesopores is known to be enhanced by an increase in the activation temperature. Activation temperatures of 850 and 950 °C resulted in an obvious peak with a pore diameter of 4.1 nm (Fig. 3), proving that ACs produced at activation temperatures above 850 °C contained both microporous and mesoporous structures.

Fig. 2
figure 2

Nitrogen adsorption–desorption isotherms at 77 K for ACs prepared using various activation temperatures and an ash to water ratio of 0.33 wt%. Legends: JC-0.33-750-1.0, JC-0.33-850-1.0, JC-0.33-950-1.0. ACs activated carbons, JC Japanese cedar wood ash

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Fig. 3
figure 3

Pore size distribution of ACs prepared using various activation temperatures and an ash to water ratio of 0.33 wt%. Legends: JC-0.33-750-1.0, JC-0.33-850-1.0, JC-0.33-950-1.0. ACs activated carbons, JC Japanese cedar wood ash

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To determine the effect of the ash to water ratio on porosity, ACs were prepared using an activation temperature of 850 °C and an activation duration of 1.0 h. The N2 adsorption–desorption isotherms and pore size distributions of the ACs prepared using various ash to water ratios are shown in Figs. 4 and 5. All the ACs exhibited the hysteresis loop. As the ash to water ratio increased, the knee of the isotherm became increasingly rounded, indicating that both the N2 adsorption and the V tot increased (Table 4). The pore structure of the produced ACs mainly consisted of micropores when the ash to water ratio was below 0.25 wt% (Fig. 5; Table 4). The ratio of micropore volume to total pore volume (V mi/V tot) was between 57.8 and 66.0 %. The average pore diameter (D) was between 1.96 nm and 2.00 nm. However, when the ash to water ratio increased to 0.33 wt%, the V mi/V tot decreased to 44.1 %, and the D increased to 2.08 nm, demonstrating the formation of a microporous structure and the widening of micropores to mesopores. According to the prior statement, K ions function as catalysts during char gasification and are intercalated into the carbon matrix, resulting in the increase and enlargement of pores through merging, which clearly reflects the considerable effects of the ash to water ratio on the pore structure of AC.

Fig. 4
figure 4

Nitrogen adsorption–desorption isotherms at 77 K for ACs prepared using various ash to water ratios and an activation temperature of 850 °C. Legends: JC-0.20-850-1.0, JC-0.25-850-1.0, JC-0.33-850-1.0. ACs activated carbons, JC Japanese cedar wood ash

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Fig. 5
figure 5

Pore size distribution of ACs prepared using various ash to water ratios and an activation temperature of 850 °C. Legends: JC-0.20-850-1.0, JC-0.25-850-1.0, JC-0.33-850-1.0. ACs activated carbons, JC Japanese cedar wood ash

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Conclusions

The thinning wood of Japanese cedar was used as the precursor to prepare AC, using Japanese cedar ash as the natural activating agent. The pH value of Japanese cedar ash was 12.86, and its CaO, K2O, and MgO contents were 34.3, 34.3, and 5.75 %. Japanese cedar wood exhibited a high fixed carbon content of 16.63 % and a low ash content of 0.22 %, indicating that this precursor was suitable for preparing AC. The yield (27.3–30.7 %), iodine value (1055.9–1106.4 mg/g), and MB value (695.1–1108.0 mg/g) of the AC increased as the ash to water ratio increased. An increase in the ash to water ratio also resulted in an increase in the S BET, S ex, and V tot of the AC. The AC prepared using an ash to water ratio of 0.33 wt%, an activation temperature of 850 °C and an activation duration of 1.0 h exhibited the highest S BET (1430 m2/g), S ex (707 m2/g), and V tot (0.74 cm3/g), and its nitrogen adsorption–desorption isotherm was classified as type IV, indicating the presence of micro- and mesoporous structures. Hence, the results indicated that Japanese cedar wood ash with a suitable ash to water ratio can serve as a natural activating agent for preparing AC.

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Acknowledgments

Fund for this study was provided by the Experimental Forest of National Taiwan University (100 A05).

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Authors and Affiliations

  1. Ph. D Program of Agriculture Science, College of Agriculture, National Chiayi University, 300 University Rd., Chiayi, 60004, Taiwan

    Chia-Wen Peng

  2. Experimental Forest, College of Bio-Resources and Agriculture, National Taiwan University, Nantou, Taiwan

    Chia-Wen Peng

  3. Department of Wood Based Materials and Design, National Chiayi University, NCYU, 300 University Rd., Chiayi, 60004, Taiwan

    Han Chien Lin

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Peng, CW., Lin, H.C. Japanese cedar (Cryptomeria japonica) ash as a natural activating agent for preparing activated carbon. J Wood Sci 61, 316–325 (2015). https://doi.org/10.1007/s10086-015-1463-1

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