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Effects of pyrolysis temperature on the physicochemical properties of empty fruit bunch and rice husk biochars
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Introduction
Biochar is a carbon-rich byproduct that is produced when bio- mass is heated through the process of pyrolysis in an oxygen- depleted environment (Kookana et al., 2011). It can be produced from all sorts of biomass residues under various pyrolytic condi- tions, and thus it can have varying chemical and physical proper- ties. Biochar contains organic material with a composition varying from barely pyrolysed lignin at low temperatures to a highly carbonized material at high temperature and coke that has been formed by reaction of volatile components (Sharma et al., 2004). The pyrolysis process generally results in the formation of gaseous products rich in CO2, some solid residues, and liquid products. Pyrolysis may lead to the formation of carbon-rich char that is highly resistant to decomposition (Thies and Rilig, 2009).
The pyrolysis process greatly affects biochar properties and its potential usage in the field of agriculture and environment.
According to Maschio et al. (1992), pyrolysis processes for the production of biochar can be divided into three classes depending on the pyrolysis temperature range: conventional pyrolysis or carbonization (550–950°C), fast pyrolysis (850–1250°C), and flash pyrolysis (1050–1300°C). In addition, Keiluweit et al.
(2010) had suggested four categories of biochar based on their molecular structures upon pyrolysis: transition, amorphous, com- posite, and turbostratic. Transition chars are dominated by pre- cursor plant materials with evidence of depolymerization of
lignin and cellulose. Amorphous chars which have lost most of the crystalline character associated with cellulose comprised a random amorphous mix of heat-altered molecules and aromatic poly condensates. Composite chars consist of poorly ordered gra- phene stacks embedded in amorphous phases and turbostratic chars are dominated by disordered graphite domains.
Porosity and loss of surface area of the biochar are dependent on various factors namely the pyrolysis highest treatment tem- perature, heating rate, pressure, retention time, and ash content.
At high treatment temperature, the micropores of biochars and the walls between the adjacent pores are destroyed and expanded
Effects of pyrolysis temperature on the physicochemical properties of empty fruit bunch and rice husk biochars
N Claoston
1, AW Samsuri
1, MH Ahmad Husni
1and MS Mohd Amran
2,3Abstract
Biochar has received great attention recently due to its potential to improve soil fertility and immobilize contaminants as well as serving as a way of carbon sequestration and therefore a possible carbon sink. In this work, a series of biochars were produced from empty fruit bunch (EFB) and rice husk (RH) by slow pyrolysis at different temperatures (350, 500, and 650°C) and their physicochemical properties were analysed. The results indicate that porosity, ash content, electrical conductivity (EC), and pH value of both EFB and RH biochars were increased with temperature; however, yield, cation exchange capacity (CEC), and H, C, and N content were decreased with increasing pyrolysis temperature. The Fourier transform IR spectra were similar for both RH and EFB biochars but the functional groups were more distinct in the EFB biochar spectra. There were reductions in the amount of functional groups as pyrolysis temperature increased especially for the EFB biochar. However, total acidity of the functional groups increased with pyrolysis temperature for both biochars.
Keywords
Agricultural wastes, biochar, empty fruit bunch, pyrolysis, rice husk
1 Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia, Selangor Darul Ehsan, Malaysia
2 Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor Darul Ehsan, Malaysia
3 Materials Processing and Technology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, Selangor Darul Ehsan, Malaysia
Corresponding author:
Samsuri A Wahid, Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia, Selangor Darul Ehsan, Serdang, 43400, Malaysia.
Email: [email protected]
Original Article
332 Waste Management & Research 32(4) causing enlargement and swelling of pores inside the biochar. In
addition, high treatment temperature will produce biochar with a high bulk density and mechanical strength.
Empty fruit bunch (EFB) is a major agricultural byproduct in oil palm mills. Omar et al. (2011) reported that oil palm fresh fruit bunches consist of 21% oil, 7% palm kernel, 14% mesocarp fibre, 7% palm kernel shell, and 23% EFB by weight. In average, the nutrient content of EFB on a dryweight basis are 0.8% N, 0.1% P, 2.5% K and 0.2% Mg, and it is composed of 81.4% holo- cellulose and 17.6% lignin (Law et al., 2007). Due to high cellu- lose and lignin content, the EFB has a low commercial value and therefore its disposal is a problem to oil palm mills due to its large quantity. Fresh EFB is usually burnt and the ash is recycled into the plantation as fertilizer because it may return the mineral nutrients and organic matter to the soil and therefore help to maintain soil fertility (Salates et al., 2004).
Annually, approximately 80 million tonnes of rice husk are produced worldwide and about 97% of them are produced by developing countries. According to Noor Syuhadah and Rohasliney (2012), 408,000 metric tonnes of RH are produced in Malaysia each year. The RH contains approximately 75–90%
organic matter such as lignin, cellulose, and hemicelluloses, and the remainders are adsorbed water, silica, alkaline minerals, and other trace elements (Kumar et al., 2012). In many countries, RH is burned in the open air and their ashes are scattered back to the rice field as fertilizers (Kondoh et al., 2005).
The EFB and RH biochars have potentials to be used in agri- culture as a soil amendment and they can also be used to mitigate environmental problems. Hence, this study was undertaken to determine the effects of pyrolysis temperature on the physico- chemical characteristics of biochars derived from empty fruit bunches of oil palm and rice husk.
Materials and methods
Biomass sources and pyrolysis process
EFB samples were collected from Seri Ulu Langat Palm Oil Mill, Ulu Langat and RH samples were collected from BERNAS Rice Factory, Sekinchan. Samples were oven dried at 105°C until con- stant weight was achieved, and then ground to <2 mm. The setup of biochar production was according to Leng et al. (2011).
Samples were placed into ceramic crucibles with fitting lids and subjected to pyrolysis at a defined temperature (350, 500, and 650°C) for 2 h in large chamber muffle furnace (Type 62700;
Thermo Scientific Barnstead/Thermolyne, USA). The resulting biochars were designated RH350, RH500, and RH650 for RH;
and EFB350, EFB500, and EFB650 for EFB. All biochars were then ground to pass a 1-mm sieve and kept at room temperature prior to analysis. The yield of biochar was calculated as follows:
Biochar yield
mass of biochar g oven dry mass of raw material g
(%) ( )
(
=
))
×100% (1)
Characterization of EFB and RH biochar
pH of biochars was measured according to Savova et al. (2001).
About 4.0 g biochar was mixed with 100 mL water in a conical flask. The flask was covered with a watch glass and boiled for 5 min. Then, the mixture was left to cool and the supernatant was decanted. The supernatant was left to cool at room temperature and the pH was determined using a 827 pH Lab (Metrohm, USA). For measurement of EC, biochar was soaked with deion- ized water at a solid/water ratio of 1:5 and agitated for 24 h and the EC was measured using a CON 700 EC meter (Eutech Instruments, USA).
Ash content of biochars was determined by a dry combustion method. In brief, about 5.0 g of biochar was heated at 500°C for 8 h (Song and Guo, 2012). The crucible was then cooled to room temperature and reweighed. The percentage ash content was then calculated as:
Ash content weight of ash g dry mass of biochar g
(%) ( )
( ) %
= ×100 (2)
The cation exchange capacity (CEC) was measured based on the method of Song and Guo (2012). About 0.50 g biochar was saturated with 40 mL 1 M ammonium acetate in a 50-mL Falcon tube. The mixture was then shaken on end-to-end for overnight.
The mixture was filtered using a vacuum pump and then another 40 mL fresh ammonium acetate was added. The filtrate was col- lected. Then three parts of 30 mL isopropanol were added into the vacuum pump and decanted. The remaining biochar was leached with four portions of 50 mL 1 M KCl and the leachate was col- lected. The extracted NH4+ was determined using an auto- analyser (QuikChem 8000 Series FIA+ System; Lachat Instruments, USA) and the exchangeable cations in the biochar were measured in the NH4OAc extract by atomic absorption spectrometry (AAnalyst 400; PerkinElmer, USA).
Total nutrients in the biochar were extracted by adding 5 mL concentrated HNO3 to 0.20 g biochar in a digestion vessel. The vessel was placed in a microwave digestion system (ETHOS 1;
Milestones, Italy) and heated at 200°C for 30 min at 1-kW power level. After cooling to room temperature, the digested solution was filtered into a 100-mL volumetric flask and brought to vol- ume with Millipore Direct-Q water. The nutrients were deter- mined using a Optima 8300 ICP-OES spectrometer (PerkinElmer, USA). The composition of C, H, N, and S was determined using a TruSpec CHN analyser (Leco, USA).
The oxygen compounds on the carbon surface material, which contain acidic and basic groups, were verified by the Boehm titration method (Kalijadis et al., 2011). About 0.20 g biochar was soaked with 20 mL base solution (0.1 M NaOH, 0.1 M Na2CO3, or 0.05 M NaHCO3) and shaken for 24 h. The mixture was filtered and 10 mL filtrate was pipetted into a conical flask, followed by addition of 15 mL 0.1 M HCl. The mixture was back titrated with 0.1M NaOH to endpoints using phenolphthalein as the indicator.
Specific surface area and total pore volume of both biochars were obtained using the Brunauer–Emmett–Teller) technique using Autosorb-1 surface area analyser (Quantochrome Instruments, USA). Morphology of EFB and RH biochar were determined by scanning electron microscopy (JSM-6400; JEOL, Japan) and the functional groups in the biochars were determined by Fourier transform infrared (FTIR) spectrometry (Spectrum 100; PerkinElmer, USA).
Results and discussion Physical properties
Biochar yield. Table 1 shows the biochar yield at different pyrol- ysis temperatures. The yield of biochar was decreased as the pyrolysis temperature was increased from 350–650°C. The high- est RH biochar yield was 50.67% (at 350°C) and the lowest was 29.02% (at 650°C), and for EFB biochar, the highest yield obtained was 37.57% (at 350°C) and the lowest was 20.93% (at 650°C). A large yield decrease was observed between 350 and 500 °C for both biochars but the difference in yield between 500 and 650 °C was small.
The pyrolysis temperature range of 350–650°C was chosen because our initial study had shown that at 300°C not all the feed- stock were converted to biochar and that above 800°C the bio- char yield was reduced due to decomposition of volatile fractions as well as intermediate melt in the biochar structure (Lua et al., 2004). The temperature of 800°C is the highest treatment tem- perature that increases the biomass decomposition and decrease the potential for biochar formation. Demirbas (2004) reported that the yield of biochar depended on destructive reaction of cel- lulose and polymerization process of biochars. Besides that, a decrease in yield with increasing temperature can be due to either greater primary decomposition of EFB and RH or secondary decomposition of the biochar residue at higher temperatures (Horne and Williams, 1996). As the pyrolysis temperature increases, the final solid residue initially decreases, as a result of the competition between charring and devolatilization reactions, of which the latter is more favoured (Di Blasi et al., 1999).
BET surface area. In the pyrolysis temperature range studied (350–650°C), the BET surface areas of the resulting biochars increased with increasing temperature. The BET surface areas at 350, 500, and 650°C were for RH biochars were 32.70, 230.91, and 261.72 m2 g–1, respectively, and for EFB biochars were 11.76, 15.42, and 28.20 m2 g–1, respectively. The values were
higher at 500 and 650°C possibly due to the rigorous reactions at this temperature resulting in the creation of mesoporous pores in the biochars. In general, the increase in surface area at a high pyrolysis temperature is due to the removal of volatile material resulting in increased micropore volume (Ahmad et al., 2012). At all pyrolysis temperatures, the RH surface areas were higher than those for EFB.
As the pyrolysis temperature was increased, increasingly greater amounts of volatile matters were discharged progressively during pyrolysis, thus resulting in the development of some new porosity and, hence, progressively increasing the surface area of the biochars. Arenas and Chejne (2004) stated that the outcome of the highest treatment temperature is the development of porous structure in the char and destruction of the walls of the pores. As observed in surface area of the biochars (Figures 1d and 2d), the structures of the biochar become more ordered and the pores become wider at treatment temperatures above 500°C (Angin, 2013); hence, more macropores were formed at pyrolysis tem- peratures above 500°C (Figures 1c and d and 2c and d).
Morphology of biochar through SEM. The SEM images of EFB and RH biochars Figures 1 and 2, respectively, show a great difference between the samples. The porous structures of each resulting biochar were clearly seen in the SEM microo- graphs, exposing a variety of shapes in the micropores, macro- pores, and mesopores. The biochars of RH350 and EFB350 were filled with tissue that was not devolatilized; hence the pores were not fully developed. However, at 500°C the mor- phology of the biochar became honeycomb-like with cylindri- cal holes interconnected by some large holes. The micrographs of both biochars show many pores formed over the surface and they are arranged in an orderly fashion. As can be seen from the surface of the EFB biochar, a regular pattern of small per- pendicular blocks were formed. However, at 650°C the regular pattern of these blocks was destroyed. Similar observations were made for the RH biochar. According to Guo and Lua (1998), properly arranged pore structures of biochars possess high BET surface area and adsorptive capacity.
Cracks and shrinkages could be observed on the surface of the EFB650 and RH650 biochars due to high temperature. Biochars pyrolysed at 650°C had highly porous, hollow, spherical parti- cles, and well-arranged structures (Figures 1b and 2b). The struc- tures seemed to be fragile because of their thin walls. With increasing pyrolysis temperature, the structure in the biochars became more ordered because the number of micropores decreased while the number of macropores increased.
Table 1. Physical properties of rice husk and empty fruit bunch biochars produced at different pyrolysis temperatures.
Physical property Rice husk Empty fruit bunch
350°C 500°C 650°C 350°C 500°C 650°C
Yield (%) 50.67 34.63 29.02 37.57 24.07 20.93
Ash content (%) 52.87 61.33 72.54 77.06 78.13 86.28
BET surface area (m2 g–1) 32.70 230.91 261.72 11.76 15.42 28.20
BET: Brunauer–Emmett–Teller.
334 Waste Management & Research 32(4)
Ash content. Ash contents of the biochars produced at different pyrolysis temperatures are presented in Table 1. The results showed that biochar ash content increased with increasing pyrol- ysis temperature. This was to be expected because increased devolatilization during pyrolysis resulted in a char with a high amount of carbon. The percentage of ash content was 52.87, 61.33, and 72.54% for RH350, RH500, and RH650 and for
EFB350, EFB500, and EFB650 was 77.06, 78.13, and 86.28%, respectively. In general, the increased ash content is due to the reduction in the content of other elements during pyrolysis. Ele- ments such as C, H, N, O, and S are volatilized during heating while the inorganic salts such as quartz and calcite are not fully volatilized. According to Tsai et al. (2012), increase in ash con- tent in the biochar with increase in pyrolysis temperature is Figure 1. SEM images of empty fruit bunch biochar produced at different pyrolysis temperatures.
EFB350, EFB500. EFB650: empty fruit bunch biochar produced at 350, 500, and 650°C, respectively.
Figure 2. SEM images of rice husk biochar produced at different pyrolysis temperatures.
EH350, EH500. EH650: rice husk biochar produced at 350, 500, and 650°C, respectively.
because of progressive concentration of minerals and destructive volatilization of lignocellulosic matters.
Functional group determination by FTIR. Figure 3 shows the FTIR spectra for the RH and EFB biochars produced under dif- ferent pyrolysis temperatures. The band assignments for both biochars are summarized in Table 2, and the data show that the biochars contained the following functional groups: carbonyl, carboxyl, hydroxyl, ether group, aromatic C=N bond, and N–H3
bond.
In the EFB biochar, the broad band close to wave number 3300 cm–1 (EFB350 and EFB500) is recognized as due to the stretching of hydrogen-bonded hydroxyl groups, indicating the presence of phenols and alcohols (Cantrell et al., 2012; Pütün et al., 2005). Throughout the increasing pyrolysis temperature, the stretching band of hydrogen-bonded hydroxyl groups started to slowly diminish (Figure 3). This was expected due to higher mass loss during thermal decomposition and gas product evolu- tion. The C–H stretch vibration between 3010 and 3040 cm–1, which indicates the existence of alkene groups, was observed in EFB pyrolysed at 650°C and RH pyrolysed at all temperatures.
However, the –CH2 group (2915 to 2935 cm–1) was identified in the spectrum of EFB350. Stretching vibration of the C≡C group (2190–2260 cm–1) was observed in EFB350. The secondary N–H bend (1550 to 1650 cm–1) was present in RH biochar at all tem- peratures and EFB650. The band at 1560 cm–1 (EFB350) and 1563 cm–1 (EFB500) are attributed to carboxylate functional
group. However, methyl C–H asymmetric band such as –CH3 were only observed in RH biochar and not in EFB.
The spectrum of each biochars were characterized by the inor- ganic functional groups, showing that the peaks at around 1400 and 1600 cm–1 could be the presence of O–H or C–O stretching vibration of phenol and C=O stretching of aromatic rings, respec- tively (Yao et al., 2011). A nitro-compound stretching spectrum was found at about 1377 cm–1 in EFB350 and at 1366 cm–1 in EFB500. The formation of tertiary amine C–N stretches at 1158 cm–1 and primary amine C–N stretches at 1025 cm–1 were observed in EFB350. Secondary amine C–N stretches were pre- sent in all RH biochars, at 1167 cm–1 in RH350 and RH500, and at 1170 cm–1 in RH650. Aromatic ethers in the form of aryl–O stretch were found in EFB650 (1247 cm–1), RH350 (1244 cm–1), RH500 (1245 cm–1), and RH650 (1249 cm–1).
Based on the FTIR spectra for EFB biochar, pyrolysis at lower temperatures resulted in dehydration, beginning of bond break- age, and transformational products (Cantrell et al., 2012). Both of EFB and RH biochars exhibited FTIR spectral behaviour attrib- uted to the presence of aromatic C=C stretching and C=O stretch- ing of conjugated quinines and ketones (1600 cm–1), O–H bending of phenols (1375 cm–1), and symmetric C–O stretching (1100 cm–1). However, the symmetric (~1050 cm–1) and asymmetric (~1150 cm–1) stretching of C–O bonds in RH biochar began to disappear as the pyrolysis temperature increased, which suggests degradation and depolymerization of cellulose, hemicelluloses Figure 3. FTIR spectra peaks of rice husk and empty fruit bunch biochars.
336 Waste Management & Research 32(4)
and lignin (Cantrell et al., 2012). Instead, intensities of O–H and C=C vibrations decreased with temperature which may suggest that phenolic compounds in lignin had been degraded (Souza et al., 2009). At 350 and 500°C, FTIR spectra of EFB biochar revealed a decrease in H-bonded hydroxyl groups. This was attributed to the acceleration of dehydration reaction in biomass as increasing pyrolysis temperature (Chen et al., 2012). There were also amide groups in both EFB and RH biochars, as indi- cated by the high N concentrations (Table 4). Bilba and Ouensanga (1996) mentioned that peaks around 1700 cm–1 indicate the exist- ence of holocellulose and lignin. The modifications of biochar structure start at a pyrolysis temperature of 200°C, which is the temperature where hemicelluloses begin to degrade, and the mod- ifications increase with temperature.
Biochars from EFB and RH showed a small peak at 1091 cm–1, which could be due to aliphatic ether C–O or alcohol C–O stretching. Secondary amines of C–N stretching peaks are more apparent other than the other two bands. The peak could also be an indication of asymmetric Si–O–Si stretching, which could also be attributed to the high Si content of both biochars (Qian and Chen, 2013). The presence of bands below 600 cm–1 may be due to the stretching variation of both inorganic compounds such as KCl and CaCl2 (Hossain et al., 2011; Tsai et al., 2012).
Chemical properties of biochar
pH. pH increased with temperature for both RH and EFB bio- chars (Table 4). This is because minerals begin to separate from the organic matrix when ashes are formed at temperatures above 350°C (Tsai et al., 2012). This result is in accordance with the increase in ash content as temperature increased (Table 1). The increasing pH with increasing pyrolysis temperature is due to the fact that minerals that are contained in the RH and EFB biochar
begin to separate from the organic matrix, and separation of the organic compound (carbon) and inorganic compounds such as alkali metal salts (ash) has been observed (Ahmad et al., 2012;
Al-Wabel et al., 2013; Tsai et al., 2012).
Electrical conductivity. EC increased with temperature for both biochars. EC was 0.52, 0.92, and 1.11 mS cm–1 for RH350, RH500 and RH650 and 2.87, 5.36, and 6.75 mS cm–1 for EFB350, EFB500 and EFB650, respectively. The increase in EC with increasing temperature were consistent with the increase in ash content. This is due to the loss of volatile mat- ters, resulting in concentration of elements in the ash fraction (Cantrell et al., 2012). Furthermore, higher mineral ash in bio- char probably has higher electrical conductivity especially those that have high K+ ion content, due to the mobility of the K+ ion (Joseph et al., 2007).
Cation exchange capacity. CEC of both biochars decreased with increasing temperature. CEC was 10.80, 9.39, and 8.35 cmol+kg–1 for RH350, RH500, and RH650 and 21.50, 14.60, and 12.03 cmol+kg–1 for EFB350, EFB500 and EFB650, respec- tively. The decrease of CEC throughout the increasing pyrolysis temperature is due to the oxidation of the aromatic C and for- mation of carboxyl groups in biochar (Liang et al., 2006). Avail- ability of other compound groups, such as phenolic, hydroxyl, carbonyl, and quinone also influences the CEC of biochar. An increase in total acidity with increasing O/C ratio was due to the occurrence of free –OH bonds, which enhanced the depolymer- ization process of lignocelluloses and thermal oxidation and generated a greater BET surface area of biochar (Table 1). It can be observed from the FTIR spectra (Figure 3) that free –OH bonds decreased at higher pyrolysis temperatures. McBeath and Smermik (2009) reported that subsequent aromatization of C in the ash of biochars produced at high pyrolysis temperatures may have reduced the CEC.
Table 2. Functional groups observed in the FTIR spectra of rice husk and empty fruit bunch biochars produced at different pyrolysis temperatures.
Wave number
(cm–1) Rice husk Empty fruit bunch Vibration characteristics
350°C 500°C 650°C 350°C 500°C 650°C
3400–3200 – – – + + – O–H stretching
3040–3010 + + + + – – C–H stretching
2935–2915 – – – + – – C–H stretching
2260–2190 – – – + – – C≡C stretching
1740–1700 + + + + + + C=O stretching
1650–1550 + + + – – + N–H bend
1610–1550 – – – + + – Carboxylic acid
1470–1430 + + + – – – Methyl C–H asymmetric
1380–1350 – – – + + – Nitro compound stretching
1270–1230 + + + – – + Aryl-O stretching
1210–1150 – – – + – – C–N stretch of tertiary amine
1190–1130 + + + – – – Secondary amine of C–N stretch
1090–1020 – + + + – + Primary amine of C–N stretch
~1150a + + + + + – Tertiary alcohol, C–O stretch
~1050a + + – + – – Primary alcohol, C–O stretch
1000–625 + + + – – – Substituent of aromatic rings
aApproximate centre of range for the group frequency.
Elemental composition. The results for elemental analysis of EFB and RH raw biomass are shown in Table 3. It was obvious that the EFB and RH biomass comprise a large percentage of oxygen followed by carbon. The elemental analyses of RH and EFB biochar prepared at various pyrolysis temperatures are shown in Table 4. As the temperature increased, oxygen content increased while C, H, N, and S decreased. The decrease in C content with temperature was due to declining degree of carbon- ization. However, the fluctuation value of O and H elements were due to breaking of weaker bonds in biochar structure, and highly carbonaceous materials yielded with increased temperature (Imam and Capareda, 2012). The atomic ratio of H/C decreased while O/C was increased with increasing temperature, suggesting the presence of dehydrogenation and demethanation process of the biochars at high temperatures (Sharma et al. 2004). However, at lower temperatures where the H/C ratio was higher than O/C ratio, the processes involved were dehydration, decarboxylation, and decarbonylation, thus producing CO2, CO, and water. This is because the char becomes gradually more aromatic and carbona- ceous with increasing temperature.
The elements Al, Ca, Mg, Na, P, and K were increased with increasing pyrolysis temperature but there was no clear pattern
for Fe and Si. The variability of the nutrients in the biochars with increasing temperature is due to their volatility and effect of pyrolysis temperature on both composition and chemical struc- ture of the biochar. Besides, the concentration of nutrients in the biochar also depend on the process of partial defractionation and/
or devolatilization of these nutrients at elevated temperatures (Hossain et al., 2011).
Oxygen surface group: Boehm titration method. The Boehm titration method determines surface functional groups such as carboxylic (–COOH), lactone (C=O), and phenolic (–OH) groups. Table 5 shows the results of surface functional groups of RH and EFB biochars pyrolysed at different temperatures. For RH biochar, total acidity was 0.554, 0.668, and 0.854 meq g–1 for RH350, RH500, and RH650 and 0.435, 0.727, and 0.885 meq g–1 for EFB350, EFB500 and EFB650, respectively. Hydroxyl, car- boxyl, carbonyl, ether, and lactone groups are oxygen-containing functional groups formed in biochar during pyrolysis and they can be used to determine the sorptive capacity of biochar for ionic compounds (Dai and Antal, 1999). Both RH and EFB bio- chars contained high amounts of lactone and carboxylic acid.
Conclusions
Pyrolysis temperature has great impact on the physicochemical properties of biochars derived from EFB and RH. The results show that yield, CEC, and total C, H, and N were decreased with increasing pyrolysis temperature. However, BET surface area, EC, ash content, and micronutrient concentrations increased with the increase in pyrolysis temperature. The results also indicated that biochars produced at 300°C are acidic in nature whereas
Table 4. The chemical properties of rice husk and empty fruit bunch biochars produced at different pyrolysis temperatures.
Chemical property Rice husk Empty fruit bunch
350°C 500°C 650°C 350°C 500°C 650°C
pH 6.66 7.99 8.88 8.31 9.89 10.29
EC (mS cm–1) 0.52 0.92 1.11 2.87 5.36 6.75
CEC (cmol+kg–1) 10.80 9.39 8.35 21.50 14.60 12.03
Elemental analysis (wt%)
C 47.99 44.64 42.95 61.96 65.88 64.53
N 0.73 0.77 0.53 1.08 0.87 0.72
S 0.03 0.02 0.02 0.02 0.01 0.02
H 5.27 3.96 2.11 7.67 4.23 3.10
Oa 46.01 50.63 54.41 29.29 29.02 31.65
H/C molar ratio 1.32 1.06 0.59 1.49 0.77 0.58
O/C molar ratio 0.72 0.85 0.95 0.35 0.33 0.37
Al 0.011 0.018 0.019 0.045 0.065 0.060
Ca 0.124 0.154 0.189 0.381 0.544 0.561
Fe 0.039 0.154 0.049 0.096 0.304 0.150
Mg 0.095 0.135 0.175 0.260 0.396 0.415
Si 0.014 0.040 0.015 0.033 0.036 0.017
Na 0.061 0.066 0.079 0.046 0.056 0.058
P 0.074 0.142 0.196 0.104 0.151 0.190
K 0.735 1.120 1.195 4.175 5.600 6.550
aEstimated by difference: %O = 100% – (%C + %H + %N).
Table 3. Elemental analysis of rice husk and empty fruit bunch biomasses.
Biomass C H N Oa S
Rice husk 38.8 5.3 0.42 55.48 0.02
Empty fruit bunch 44.8 6.0 0.42 48.88 1.06
aEstimated by difference: %O = 100% – (%C + %H + %N).
338 Waste Management & Research 32(4)
those produced at 650°C are alkaline. The FTIR spectra of EFB and RH biochars revealed some variations and also similarities in their functional groups. At low pyrolysis temperature, bands of carboxyl, aromatic bonds, hydroxyl, carboxyl, amine, and other aromatic groups were clearly developed. However, as tempera- ture increased, some of the peaks were diminished. Formation and disappearance of functional groups with increasing pyrolysis temperature were more distinct in EFB biochar than in RH bio- char. In addition, with increasing temperature, the morphology and structure of biochars show distinct shapes and form. Different forms of cracks and hollows, which shows the porosity, can be observed on the surface and the tip of each of the biochars, due to differential decomposition of hemicelluloses, cellulose, and lignin at different temperatures.
Declaration of conflicting interests
The authors declare that there is no conflict of interest.
Funding
This work was supported by the Universiti Putra Malaysia under the Fundamental Research Grant Scheme (07-12-10-1037FR).
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Table 5. Acid functional groups in rice husk and empty fruit bunch biochars produced at different pyrolysis temperatures.
Parameter Rice husk Empty fruit bunch
350°C 500°C 650°C 350°C 500°C 650°C
Total acidity (meq g–1) 0.554 0.668 0.854 0.435 0.727 0.885
Strong acid (meq g–1)a 0.154 0.235 0.318 0.085 0.263 0.316
Moderate acid (meq g–1)b 0.302 0.319 0.370 0.282 0.350 0.351
Weak acid (meq g–1)c 0.098 0.114 0.166 0.068 0.114 0.218
aMainly carboxylic acid.
bHydrolysis products of lactones.
cMainly phenols.
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