INTRODUCTION
Biochar is a material produced from the thermochemical conversion process of biomass in a limited oxygen environment (Khan et al., 2015). Biochar has varied physical and chemical characteristics and roles (Spokas, 2010). These are determined by materials’ physical and chemical properties and pyrolysis conditions during production (Ogawa, Okimori, & Takahashi, 2006; Zimmerman, Gao, & Ahn, 2011). Information about the optimum temperature for each biomass from agricultural waste as biochar raw material is still limited. Both raw material and pyrolysis temperature affect the formation of surface area, pore shape, pore size, pore volume, functional group number, and the chemical properties of biochar produced (Sohi, Krull, Lopez- Capel, & Bol, 2010). Raw materials with high cellulose and lignin content contribute very well to biochar’s
chemical and physical properties. Agricultural waste biomass has been studied along with its benefits as biochar materials, including wheat stalk and hickory waste (Jindo et al., 2012; Zhang et al., 2015), wood debris, corn stalks, and macadamia, rice straw (Masulili, Utomo, & Syechfani, 2010), tankos, and bengkalis peat (Indrawati, Ma’as, Utami, & Hanuddin, 2017), canola straw, corn, soybeans, and peanuts (Yuan, Xu, & Zhang, 2011), wheat straw, corn straw and peanut shells (Gai et al., 2014), oil palm biomass (Kong, Loh, Bachmann, Rahim, & Salimon, 2014) oat hull (Gonzales et al., 2015), pea pod, cauliflower leaf, and orange peel wastes (Stella Mary et al., 2016), and rice husk and sugarcane bagasse wastes (Nwajiaku et al., 2018).
Biochar is resistant to weathering and rich in carbon and thus can be used as a soil ameliorant (Lehmann & Joseph, 2015; Sohi, 2012). This material has potential as an adsorbent for polycyclic aromatic ARTICLE INFO
Keywords:
Agricultural waste Biochar
Lignocellulosic Pore structure Article History:
Received: August 8, 2021 Accepted: August 23, 2022
*) Corresponding author:
E-mail: [email protected]
ABSTRACT
Biochar quality is influenced by the type of its raw material and pyrolysis temperature. Nevertheless, the quality criteria of biochar as a nutrient carrier remain unanswered. This study aimed to find the chemical properties, micromorphology, and optimum pyrolysis temperature from various agricultural wastes to obtain good biochar as a nutrient carrier.
This experiment was conducted at three level temperatures: 400, 500, and 600°C, and the raw materials were coconut shells, oil palm shells, and corn stalks. The chemical and physical properties of biochar were:
pH-H2O, OC, CEC, total N, P, K, Mg, Ca, and Na, ash, functional groups, amorphous carbon, morphology, and SSA. The results show that the coconut shells and oil palm shells biochars contained high levels of N-total and the chain-C aromatic, and the pore structure was solid and regular. Corn stalks biochar containing ash is high, and C-aromatic is low and fragile. Increased temperature of pyrolysis produced well-crystallized minerals. It is concluded that 500°C is the optimum temperature for oil palm shells pyrolysis resulting in biochar with the highest C-aromatic structure and arrangement of pores which are strong, regular and uniform, and high stability, but the nutrient content was low.
ISSN: 0126-0537Accredited First Grade by Ministry of Research, Technology and Higher Education of The Republic of Indonesia, Decree No: 30/E/KPT/2018
Cite this as: Mansyur, N. I., Hanudin, E., Purwanto, B. H., & Utami, S. N. H. (2022). Chemical properties and micromorphology of biochars resulted from pyrolysis of agricultural waste at different temperature. AGRIVITA Journal of Agricultural Science, 44(3), 431-446. http://doi.org/10.17503/agrivita.v41i0.3085
Chemical Properties and Micromorphology of Biochars Resulted from Pyrolysis of Agricultural Waste at Different Temperature
Nur Indah Mansyur1), Eko Hanudin2*), Benito Heru Purwanto2) and Sri Nuryani Hidayah Utami2)
1) Department of Agrotechnology, Faculty of Agriculture, Universitas Borneo Tarakan, North Kalimantan, Indonesia
2) Department of Soil Science, Faculty of Agriculture, Universitas Gadjah Mada, Yogyakarta, Indonesia
hydrocarbons (PAH), potentially toxic elements (PTEs), and pesticides (Bolan, Mahimairaja, Kunhikrishnan, Seshadri, & Thangarajan, 2015; Waqas et al., 2015). In addition, it reduces CO2 emissions and N2O (Ameloot et al., 2013; Bruun et al., 2011), increases inorganic N retention, and reduces loss in the form of NH3 (Zhao, Cao, Mašek, & Zimmerman, 2013). Biochar also plays a significant role in improving soil fertility, including increasing soil pH, increasing nutrient retention, decreasing the solubility of Al and Fe, and reducing greenhouse gas emissions (Gai et al. 2014; Masulili, Utomo, & Syechfani, 2010; Wang, Dai, Yang, & Luo, 2017; Yuan, Xu, & Zhang, 2011).
Srinivassan et al. (2015) divide the role of biochar into 5 (five) groups, i.e., 1) soil ameliorants:
high CEC leading to high nutrient uptake and high pH and ash content functioning as liming agent; 2) bio-energy: high calorific value (potential fuel); 3) carbon sequestration: high carbon content and a low ratio of aromatic H/C and aromatic O/C (stable); 4) biocomposites: low ash content, high carbon content and high specific surface area functioning as filler material; and 5) remediation material: high specific surface area (adsorbent).
The study conducted by Gai et al. (2014) on twelve biochar made from wheat straw, corn straw, and peanut shells at pyrolysis temperatures of 400, 500, 600, and 700°C showed that an increase in temperature from 400 to 700°C decreased the content of N, H and O and increased pH, ash content, and carbon content. Likewise, Zhang et al. (2015) show that the NMR spectrum from biochar produced at higher pyrolysis temperatures, 400 and 600°C, changed the form of labile carbon (aliphatic) to C-aromatic structure, increased CEC, and reduced CO2 emissions.
This study aims to examine the effect of pyrolysis temperature on the chemical characteristics and micromorphology of biochar produced from raw materials with different lignocellulosic compositions.
MATERIALS AND METHODS Raw Materials Collection
This research was conducted from September 2018 to April 2019, using two factors arranged in a factorial completely randomized design. The first
factor was the raw materials of biochar consisting of corn stalks, coconut shells, and oil palm shells, while the second factor was pyrolysis temperature composed of 400, 500, and 600°C. Biochar was made at the Energy and Heat Transfer Laboratory, PAU, Universitas Gadjah Mada, Yogyakarta. Each raw material is cleaned of adhering garbage, dried in the sun for 3 days, and cut into pieces to produce a uniform size. Then the raw materials are put into the pyrolisator with a capacity of 20 kg. Furthermore, the pyrolysis process is carried out at a predetermined temperature for 4 hours.
Chemical Composition Characterization of Raw Materials and Biochar
The chemical compositions of the raw materials analyzed were: lignin, cellulose, and hemicellulose (%, modification of chlorous acid) (Browning ,1967) (Table 1). The chemical properties of raw materials and biochar observed were: moisture content (%, Gravimetry); pH H2O (1: 5, pH meter); organic carbon (dry combustion); CEC (NH4Cl saturation);
total N (Kjeldahl); complete P, K, Mg, Ca, Na (wet combustion (HNO3 and HClO4)); ash content (dry oxidation/combustion at 550 to 600°C) (Sluiter et al., 2008) and proximate analysis (Browning, 1967;
Zaher, Buffiere, Steyer, & Chen, 2009). Physical character analysis included: functional groups (FTIR, SHIMADZU PRESTIGE 21, Resolution: 16) (Gómez- Serrano, Piriz-Almeida, Durán-Valle, & Pastor- Villegas, 1999); amorphous carbon (XRD, RIGAKU MULTIFLEX 2 KW); pore surface (Scanning Electron Magnetic (SEM), JEOL-JSM-6510LV, Resolution:
3.0 nm (30kV) - 4.0 nm (30kV); biochar surface, pore volume and pore size (Quanta Chrome/Gas Nitrogen:
Quantachrome Nova Win2 Instrument Version 2.2).
Statistical Analysis
The chemical properties of biochar, including pH, organic carbon, fixed carbon, cation exchange capacity (CEC), total N, P, K, Ca, Mg, Na, and ash content, were analyzed with the Analysis of Variance (F test) at 5% using SAS Version 9.3. Meanwhile, FTIR, XRD, SEM, and SAA data were interpreted according to Boresi & Schmidt (2003), Gai et al. (2014), Peng, Ye, Wang, Zhou, & Sun (2011), Yuan, Xu, & Zhang (2011), and Zielińska, Oleszczuk, Charmas, Skubiszewska- Zięba, & Pasieczna-Patkowska (2015).
Table 1. Cellulose, hemicellulose and lignin contents of several biochar raw materials
Component Unit Type of raw materials
Coconut shell Oil palm shell Corn stalk
Cellulose % 38.19 38.18 39.8
Hemicellulose % 32.65 30.27 27.96
Lignin % 33.17 37.58 26.68
Table 2. Effects of raw material and pyrolysis temperature on the chemical properties of biochars
Type of raw materials
T (oC)
Chemical characterization pH (1:5)CECTotal PTotal KTotal CaTotal NaTotal Mg
Moisture content Volatil material
Ash contentC-Fixed H2O(cmol/kg)--- (g/kg) ------ (%) --- Coconut shell
6.3421.45 0.20 2.00 0.50 2.20 0.50 13.70 63.00 0.7222.59 4009.68 bc6.07 d3.80 d7.10 c0.47 a863 c0.53 a0.23 d33.75 c1.21 i64.82 bc 5009.86 bc6.66 d2.33 d7.00 c0.73 a880 c0.60 a0.25 cd23.83 f1.41 h74.51 ab 60010.24 ab6.07 d3.57 d8.40 c0.43 a8.73 c0.50 a0.32 ab13.18 i1.79 g84.72 a Oil palm shell
5.514.520.40 1.40 4.10 1.90 0.50 14.7663.162.1620.00 4007.45 e3.76 d1.90 d5.13 c0.39 a5.84 c0.37 a0.22 d36.99 b3.98 f58.82 c 5008.71 d3.36 d3.47 d3.67 c8.37 a4.90 c0.93 a0.26 bcd21.29 e4.85 e73.60 ab 60010.67 a4.55 d3.07 d3.47 c10.37a4.53 c1.00 a0.31 abc12.76 h5.15 d81.78 a Corn stalk
5.7763.16 3.10 25.80 0.90 22.70 1.20 17.6656.677.4318.24 4009.39 cd44.02 a11.17 c28.50 b4.33 a25.17 b0.93 a0.32 a37.63 a14.15 c47.90 e 50010.00 abc33.97 b14.5 b43.77 a3.43 a39.43 a1.00 a0.25 cd24.52 d18.68 b56.55 c 60010.09 abc15.96 c21.57 a45.03 a3.77 a41.00 a1.07 a0.25 cd17.03 g22.07 a60.65 c Remarks: Means followed by the same lowcase letters in the same column are not significantly different according to DMRT at 5%
RESULTS AND DISCUSSION
Chemical Composition Characterization of Raw Materials and Biochar
The quality of biochar is strongly influenced by the lignocellulosic component of the raw material and its pyrolysis temperature. The cellulose contents of the three raw materials are relatively the same, ranging from 38.18 to 39.8% (Table 1). Meanwhile, the hemicellulose content of coconut shell (32.65%) is higher than that of oil palm shell (30.27%), followed by that of corn stalk (27.96%). On the other hand, the lignin content of oil palm shell (37.58%) is the highest, followed by that of coconut shell (33.17%) and corn stalk (26.68%). Plant biomass contains around 90% lignocellulose, 40-60% cellulose, 15- 30% hemicellulose, and 10-25% lignin (Gupta
& Verma, 2015; Wang, Dai, Yang, & Luo, 2017), depending on the type of biomass (Zhao, Qiao, Cao,
& Shao, 2017). Under the same pyrolysis conditions, biomass that contains a lot of lignin will produce a lot of stable aromatic carbon compounds, while biomass that includes a lot of cellulose and hemicellulose will produce ash and alkaline cations (Boresi & Schmidt, 2003; Lehmann & Joseph, 2015). Therefore, oil palm shells are more resistant to overhaul than coconut shells and corn stalks.
Increased pyrolysis temperature caused changes in the physical characteristic and chemical composition of biochar produced (Table 2). Various raw materials and pyrolysis temperatures significantly affected the moisture content, ash content, volatile materials, fixed carbon, CEC, total K, and Na. The biochar of corn stalks had a high value of total P, total Na, total K, moisture content and ash content that were higher than those of the biochar of coconut shells and oil palm shells. Meanwhile, the three ingredients’ Ca and Mg content were not significantly different. This is related to the content of Ca and Mg in the raw materials, which were relatively low.
The corn stalks contained more hemicellulose and cellulose, so the formed biochar produced more ash, volatile materials, and alkaline cations. However, the increase in pyrolysis temperature increased the mineral content of biochar, especially Ca, Na, and K (Taherymoosavi, Joseph, Pace, & Munroe, 2018), P, and Mg (Zhang et al., 2015). According to Campos et al. (2020) that high pyrolysis temperature (600
°C) resulted in the greatest pH and C content in the biochars. In general, elemental composition and ash content were dependent on the type of organic waste used as feedstock.
The CEC value of corn stalks biochar was higher than that of oil palm shells and coconut shells biochars. Meanwhile, the CEC values of oil palm and coconut shells were not significantly different. Corn straw biochar has a higher pore space than coconut shell biochar, and palm shell biochar, with a large surface area, provides more significant space for nutrient adsorption and cations exchange. The CEC value of corn stalks biochar significantly decreased with the increasing temperature. In contrast, the temperature did not considerably affect the CEC value of oil palm shells and coconut shells biochars.
The decrease in CEC value was caused by chemical structure changes in cellulose, hemicellulose, lignin, and other compounds as functional group sources.
Agegnehu, Srivastava, & Bird (2017) state that cellulose and hemicellulose in corn waste contain a C-aliphatic (O-Alkyl) compound thus, an increase in temperature up to 400°C will form a phenolic functional group (Chen et al., 2018), pyrolysis temperature of up to 700°C could reduce the CEC of corn stalks biochar (Yang et al., 2018).
The ash content of biochar made from corn stalks was significantly higher than other biochar, and it also increased considerably with the increasing temperature. The high mineral content indicates the high ash content in biochar and alkaline cations, especially P, K, Ca, Na, and Mg (Taherymoosavi, Joseph, Pace, & Munroe, 2018;
Zhang et al., 2015). This can also be attributed to the content of lignocellulosic compounds in the raw materials. Biochar produced from hemicellulose-rich biomass contains higher ash and less carbon-bound carbon than lignin-rich biomass (Jiang et al., 2016).
All biochar in this study was alkaline with a pH between 7.45-10.67. In the three raw materials, pH increased significantly with the increasing pyrolysis temperature, and at a temperature of 600°C, the pH value was almost the same. This study follows the research of Gai et al. (2014) and Hossain, Strezov, Chan, Ziolkowski, & Nelson (2011), stating that at the increased temperature of 700°C, the pH of corn stalks biochar could reach 10.4. According to Taherymoosavi, Joseph, Pace, & Munroe (2018), a high pH value originates from a high concentration of base cations in biochar and increases with the increasing pyrolysis temperature.
All types of biochar made at a temperature of 600°C had a high content of fixed carbon. The selected carbon content depends on the raw material and ash content (Enders, Hanley, Whitman, Joseph, & Lehmann, 2012). Lignin contains higher
carbon and lowers oxygen content than cellulose and hemicellulose (Chen et al., 2018). During the pyrolysis process, higher temperature supports the formation of fixed carbon (Mayakaduwa et al., 2016). Fixed carbon increased at 600°C and 700°C (Nwajiaku et al., 2018). Fixed carbon is often regarded as an index of biochar resistance to the environment, functioning as the primary source of soil carbon retention. Fixed carbon or aromatic carbon contained in biochar is a carbon sink that plays a significant role in controlling the leaching process of nutrients and controlling carbon emissions. The results of Indrawati, Ma’as, Utami,
& Hanuddin (2017) showed that raw materials containing high levels of hemicellulose and high lignin.
Raw materials’ chemical composition influences biochar’s physical and chemical characteristics. For example, biochar made from corn stalks contains higher Na, Ca, P, K, and ash content than biochar from coconut shells and palm shells. Significant structural changes occur during the pyrolysis process at higher temperatures up to 600°C, producing high ash content and containing several basic cations such as K, Ca, and Na. These alkaline and alkaline metal oxide forms cause the pH of biochar to increase. Nwajiaku et al. (2018) found that biochar total nitrogen decreased with increasing pyrolysis temperature while ash content, pH, EC, total carbon, extractable Ca, Mg, Na, available, and phosphorus were increased.
The pyrolysis temperature also significantly affects converting the composition and chemical structure from raw material to biochar. Increasing the temperature to 600°C during the production process of biochar from corn stalks will produce a lot of ash material, while palm shell and coconut shell biochar will produce charcoal. During the pyrolysis process, the raw material containing a lot of lignin and cellulose is more strongly converted into charcoal rather than ash with the increase in temperature. The chemical properties of biochar depend on the raw material’s composition and the pyrolysis temperature (Zimmerman, Gao, &
Ahn, 2011). Stella Mary et al. (2016) showed that in the pyrolysis process (300°C), the orange peel and cauliflower leaf waste containing high lignin contribute to higher char formation, while pea pod with low hemicelluloses and cellulose content contribute to ash formation.
SEM Image, X- RD pattern, and IR Spectra Morphology and Porosity of Biochar
Based on the results of SEM analysis, it can be seen that the increase in pyrolysis temperature causes a change or destruction of the structure of biochar produced from coconut shells, oil palm shells, and corn stalks (Fig. 1, Fig. 2, and Fig. 3 ). Peng, Ye, Wang, Zhou, & Sun (2011) revealed that along with the increasing temperature, biochemical particles became smaller and supported the evidence that there was a reduction (destruction) of the structure of the biochar of coconut shells and corn stalks at increased pyrolisis to 600°C.
The morphological changes in the surface of biochar made from coconut and oil palm shells were very clear. Changes in the surface morphology of coconut shells biochar showed the development of pores, the structure of which was regular and uniform, seen at 400°C. However, the increasing temperature of 500°C to 600°C caused damage to pores structures (irregular, not uniform, brittle, and easily broken) (Fig. 1). biochar pores developed well at 500°C, and the structure became more regular and uniform, forming cylinders even though they had not been formed completely. Meanwhile, in biochar made at 600°C, the pores were well developed, regular, and uniform but fragile and easily broken compared to the structure of oil palm shells biochar made at 500°C. This is evidenced by the presence of more broken fragments in the biochar made at a temperature of 600°C (Fig. 2). Fig. 3 shows that the biochar pores of the corn stalk are increasingly irregular and brittle so that broken fragments are formed with the increasing pyrolysis temperatures up to 600°C. Differences in the biochar pore structure resistance can be related to the composition of raw materials of lignin, cellulose and hemicellulose.
Ahmad et al. (2013) correlated the morphological differences in biochar with the composition of lignin, cellulose and hemicelluloses compounds in raw materials. Furthermore, Sharma et al. (2014) state that an increase in temperature above 400°C can reduce the biochar surface area caused by the collapse of the micropore wall, blocking the biochar sorption site. Gai et al. (2014) state that the pore structure of biochar changes after pyrolysis, and showed that corn waste biochar contained microparticles and micropores. Its irregular structures able collapse of the pore wall caused by increased temperatures.
Fig. 1. Scanning electron microscopy (SEM) of biochars made from coconut shells pyrolyzed at temperature of (a) 400, (b) 500, and (c) 600°C for 4 hours
Fig. 2. Scanning electron microscopy (SEM) of biochars made from oil palm shells pyrolyzed at temperature of (a) 400, (b) 500, and (c) 600°C for 4 hour
Fig. 3. Scanning electron microscopy (SEM) of biochars made from corn stalks pyrolyzed at temperature of (a) 400, (b) 500, and (c) 600°C for 4 hours
Based on the appearance of surface morphology, all biochar had various pore sizes, i.e., micropore (15.3-19.2 Å), meso pore (21.6-471.5 Å), and macro pore (508.8-1113.3 Å). The pore size range refers to pore type criteria according to Smisek and Cerry (1970): macro pore> 500 Å, meso pore 20-500 Å, and micropore <20 Å (Boresi
& Schmidt, 2003). The surface becomes rough with varying pore sizes, which is very important as a place to bind materials. Increasing pyrolysis temperature causes the biochar pores to be formed with irregular structures resulting in a large specific surface area, well-developed pore structure, and uniform pore size distribution (Sun, Wan, & Luo, 2013).
In general, the surface area of the corn stalk biochar was higher than other biochar, while coconut shell biochar was higher than oil palm shell biochar. The surface area of biochar made from corn stalks, coconut shells, and oil palm shells was 14.75-23.66, 5.60-14.36, and 4.65-13.03 m2/g, respectively (Table 3). The increase in pyrolysis temperature caused an increase in the surface area of biochar made from coconut shells and oil palm shells, while the opposite occurred in corn stalk biochar. The difference in biochar surface area was influenced by the content of raw materials of cellulose, hemicellulose, and lignin (Ahmad et al., 2013; Mimmo, Panzacchi, Baratieri, Davies, &
Tonon, 2014). The increased surface area provides more significant adsorption space for nutrients and water retention. Increased surface area at 600°C is related to the volatilization of volatile organic matter (Agrafioti, Bouras, Kalderis, & Diamadopoulos, 2013; Gao et al., 2014).
Biochar made from oil palm shells at a temperature of 500°C can potentially be used as nitrogen fertilizer coatings. Although it has a lower surface area, pore volume, and pore size than other biochar, the morphological appearance shows that biochar made from oil palm at 500°C forms a stable structure with a strong pore bulkhead and forms a uniform surface pore. Other biochars have a higher surface area but irregular and fragile construction, starting to widen and damage pores, which cause the pore structure to fall apart.. Arrangements and neatly arranged pore shapes will increase the role of biochar in improving soil fertility (Lehmann &
Joseph, 2015). Usevičiūtė & Baltrėnaitė-Gedienė (2021) say that internal pores affecting water holding capacity biochar. Around 54% of average pore size predicted wettable and 77% of biochar water holding capacity be affected its ash content. Biochars pores adsorb water on its surface and internal pores as well as retention of water among its particles due to capillary forces.
Table 3. Values of several biochars variables (specific surface area, total pore volume, and pore size average) of BET method
Biochar Temperature Variables observed
Specific surface area (m2/g) Total pore volume (cc/g) Pore size average (Å) Coconut shell
400°C 5.60 0.011 40.9
500°C 7.74 0.012 30.0
600°C 14.36 0.022 30.3
Oil palm shell
400°C 4.65 0.010 44.3
500°C 5.33 0.010 36.3
600°C 13.03 0.020 30.8
Corn stalk
400°C 23.66 0.038 32.6
500°C 16.74 0.035 42.7
600°C 14.75 0.029 39.3
Fig. 4. X-ray diffraction of biochars produced from (a) coconut shells, (b) oil palm shells, and (c) corn stalk pyrolyzed at temperature of 400, 500, and 600°C for 4 hours
Fig. 5. Infrared Spectrograph of biochars produced from (a) coconut shells, (b) oil palm shells, and (c) corn stalk pyrolyzed at temperature of 400, 500, and 600°C for 4 hours
Identification of Mineral Composition Using X-RDThe level of mineral crystallization differed between biochar. The results of X-RD analysis of biochar made from coconut shells, oil palm shells, and corn stalks are presented in Fig. 4. According to Fig. 4(a), in coconut shells, biochar is made at temperatures of 400, 500, and 600°C. Sylvite is found, indicated by the peak of 3.15 and 2.22, and calcite at the peak of 3.04. Heating to 600°C was able to form quartz minerals, as characterized by the presence of a height of 4.24. The sharp rise of 3.15 shows that sylvite is crystallized well. Sylvite was crystallized well in coconut shells biochar and did not change with the increasing temperature.
The oil palm shells biochar made at a temperature of 400°C formed calcite minerals as indicated by peaks of 3.03, 2.49, 2.28, 1.92, 1.87 and 3.84Å, quartz by the height of 3.33Å and Sylvite by the height of 2.21Å (Fig. 4(b)). At increased temperatures up to 500 and 600°C, sylvite was no longer found. Heating up to 600°C formed dolomite minerals indicated by a peak of 2.84Å, and CaCO3 was well crystallized with a peak intensity at 3.03Å.
This is consistent with the Ca content in oil palm shells biochar, increasing pyrolysis temperature.
During pyrolysis hapen decrease of functional groups such as -OH, C=O, –CONH– and C=C with increasing pyrolysis temperature. However, aromatic and hetero-aromatic structures were more dominan, and exist CaCO3, SiO2 and MgCO3 in biochars (Aktar et al. 2022). Biochars produced at 600°C with a high pH of around 11.5 affected mineral CaCO3 aragonite or calcite and increased with the increasing temperature (Gai et al., 2014).
In the corn stalk biochar made at a temperature of 400°C, four types of minerals were found, i.e.
sylvite, calcite, quartz, and dolomite. At increased temperatures, up to 500°C were not found quartz.
At increased temperatures up to 600°C were found, sylvite and dolomite only. A sharp peak of 3.15 was found at the temperature of 400, 500, and 600°C (Fig. 4(c)), indicating sylvite is well crystallized, while peaks of 2.84 at 600°C indicate dolomite is crystallized well. According to Yuan, Xu, & Zhang (2011), in corn straw biochar, quartz crystals and sylvites were well crystallized. Furthermore, Wu et al.
(2012) stated that biochar’s production temperature would affect ash’s composition and the presence of dolomite, calcite, quartz, and potassium chloride as the dominant minerals in biochar produced at
600°C. The crystallized minerals in the corn stalk biochar were sylvite at 400, 500 and 600°C and dolomite at 600°C.
Functional Group Components of Biochar The pyrolysis temperature directly affects the functional groups in all types of biochar, and higher temperature causes some functional groups to disappear. Fig. 5 shows C-H aromatic bending vibrations from raw material and biochar of coconut shells, oil palm shells, and corn stalks appearing in the range of 971.82-910.40, 871.82-879.54, and 871.82-894.97 per cm, respectively. These peaks appeared at biochar at a temperature of 400, 500, and 600°C. Stretching vibrations of aliphatic C-O-C (C-O ether bond) and alcohol -OH of the three biochars appeared in the range of peaks of 1033- 1265.3, 1056.99-1265.3, and 1056.99-1265.3 per cm. The intensity decreased at a temperature of 400°C and was lost when the temperature increased up to 500°C.
Stretching vibrations of C-H alkane bonds in a coconut shell, and palm shells biochar appeared in the range of 1373.22-1435.04 and 1381.03-1427.32 per cm, consecutively. The intensity decreased with the increasing temperature of pyrolysis. The stretching vibration of the C-H methylene bond in the corn raw material appeared at the peak of 1327.03 per cm. In comparison, the bending vibration of the CH2-bond appeared at peak 1381.03-1427.32 per cm,which is the higher the temperature, the lower the intensity. Likewise, the peak of 2862.36-2924.09 per cmshows the stretching vibration of the C-H methyl and C-H methylene bonds. When the temperature was increased up to 600°C, the peak disappeared.
Stretching vibrations of aromatic C=C and amines N-H from the three biochar appeared in the peak range of 1527.62-1589.34, 1581.63, and 1496.76- 1566.20 per cm. This peak was found in biochar at temperatures 400, 500, and 600°C. The vibration of H-bonds from phenol (-OH) in biochar made from coconut shells, oil palm shells, and corn stalks appeared in the peak range of 3410.15-3448.72, 3410.15-3425.58, and 3410.15-3425.58 per cm, respectively. The intensity decreased with the increasing pyrolysis temperature (Sastrohamidjojo, 2018).
The increase in pyrolysis temperature causes a change in the spectral properties of biochar, which is indicated by the appearance of a typical adsorption spectrum at any increase in
temperature (Peng, Ye, Wang, Zhou, & Sun, 2011).
It is further said that some of the emerging peaks show the functional group types found in biochar, in which a height of 3400 per cm is the stretching of O-H, an elevation of 2900 per cm is aliphatic C-H, and a peak of 800-1600 per cm is C-H, C=C, C=O (aromatic). In general, the increase in temperature caused a decrease in the intensity of the adsorption spectrum at the peak of 3400 and 2900 per cm, meaning that there was a reduction in O, H, and C-H aliphatic bonds, but intensified at the height of 1400 per cm indicating an increase in C-aromatic (Yuan, Xu, & Zhang, 2011).
The infrared spectral interpretation shows that biochar producing a lot of charcoal has a dominant aromatic functional group rather than the aliphatic functional group. For example, in biochar made from coconut and oil palm shells at increased pyrolysis temperature up to 500°C, aliphatic groups were converted to produce many aromatic functional groups (Zhang et al., 2015). Meanwhile, rising temperatures up to 600°C decreased the number of aromatic functional groups. According to Indrawati, Ma’as, Utami, & Hanuddin (2017), the pyrolysis temperature and the duration of pyrolysis affected the number and type of functional groups. According to Fu et al. (2019), biochars from food wastes mix at 500°C have specific surface is high (18 m2/g), andshowed higher polarity and aromaticity than that of biochars at 300°C.
Aliphatic functional group conversion resulted in fewer aromatic functional groups in the corn stalk biochar. Active groups in biochar produced from oil palm shells and coconut shells at 500°C were dominated by aromatic C-H and -OH and vibration bonds of methyl C-H and methylene C-H. Still, the intensity of functional groups was higher in palm shells biochar than in coconut shells biochar. The intensity of each functional group in palm shell biochar was 23.82, 17.74 and 15.15, while that of coconut shell biochar was 19.18, 9.26, and 10.73. With high aromatic and phenolic functional groups, oil palm shells biochar has higher stability than coconut shells biochar. Lehmann & Joseph (2015) stated that stable biochar had a chemical structure resistant to weathering and recalcitrance, so the average residence time reached hundreds to thousands of years. The biochar forms an aromatic C ring with type H bonds, carboxylic surfaces, or phenolic groups (Sun, Wan, & Luo, 2013). According
to Hassan et al. (2020) that with increasing HTT, biochar pH, surface area, pore size, ash content, hydrophobicity and O/C vs. H/C (ratios that denote stability) increased, whereas, hydrophilicity, yield of biochar, O/C, and H/C decreased.
CONCLUSION
The chemical and micromorphology properties of the biochars are strongly influenced by the composition of the raw material compounds and the pyrolysis temperature. For example, biochars from raw materials containing high cellulose and lignin produce high bound carbon and low ash and are not easily destroyed. On the other hand, biochar from raw materials containing low cellulose and lignin has low bound carbon content and high ash and is easily crushed.
During the pyrolysis process, an increase in the pyrolysis temperature causes a decrease in mass by the dehydration reaction and decarboxylation of oxygen-containing functional groups, decreases volatile matter, and increases the ash and carbon content. The carbonation process at high temperatures (> 400°C) reduces the H and O content, results in a high C in aromatic structure, and is a significant constituent of biochar.
The increase in pyrolysis temperature also causes biochar pores which are arranged with irregular fold structures and turn rough. Biochar produced from raw materials containing high lignin and cellulose at a pyrolysis temperature of 500°C has a well- organized and stable structure, uniform pore size, and low surface area. However, the pore distribution is dominated by micropores and meso pores. In addition, the biochar surface contains negatively charged functional groups that play a role in nutrient retention.
ACKNOWLEDGEMENT
The authors would like to express gratitude to the Directorate of Research and Community Service, the Directorate General of Research and Development Strengthening, the Ministry of Research, Technology and Higher Education who have funded this research through Doctoral Dissertation Grant with Research Contracts, Numbers: 062/SP2H/LT/DRPM/2017, and Research Directorate of UGM.
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