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Zhao, Z., Ibrahim, M. M., Wang, X., Xing, S., Heiling, M., Hood-Nowotny, R., Tong, C., and Mao, Y. (2019). "Properties of biochar derived from spent mushroom substrates," BioRes. 14(3), 5254-5277.

Abstract

Spent mushroom substrates, Tremella fuciformis (Tf), Flammulina velutipes (Fv), and Lentinula edodes (Le), were used to produce biochar at different temperatures (300 °C, 400 °C, 500 °C, 600 °C, and 700 °C). Elemental compositions and surface properties of derived biochar were determined. The yield and volatile matter (VM) of the biochars decreased as the pyrolysis temperature increased with Le300 having the highest yield (47.4%). The highest VM was obtained in Tf300 (79.6%). The biochars were alkaline, with Fv700 having the highest pH (11.6). Pyrolysis temperature and feedstock influenced nutrient composition of biochars and highest values were obtained in: Tf300 (N=2.07%), Fv700 (P=12.0 g/kg), Le700 (K=21.9 g/kg), Fv600 (CEC=32.3 cmol/kg), Fv700 (Ash=33.4%) and Le700 (C=58.6%). Heavy metals in the Fv biochar were highest but within their tolerable limits. Fourier transform infrared spectra showed various functional groups on the biochar surfaces with C-O being dominant (except on Le biochar). X-ray diffraction revealed that SiO2 and CaCO3 were present on biochar surfaces. The Fv biochars had the largest surface area with Fv400 having the highest value (210.6 m2g-1) while Le400 had the highest average pore diameter (159.7 Å). These properties render the biochars suitable as soil amendment and in environmental remediation.


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Properties of Biochar Derived from Spent Mushroom Substrates

Zhuang Zhao,a,b Muhammed Mustapha Ibrahim,a,b,c Xiaodan Wang,a,b Shihe Xing,a,b Maria Heiling,d Rebecca Hood-Nowotny,e Chenxiao Tong,a,f and Yanling Mao a,b,f *

Spent mushroom substrates, Tremella fuciformis (Tf), Flammulina velutipes (Fv), and Lentinula edodes (Le), were used to produce biochar at different temperatures (300 °C, 400 °C, 500 °C, 600 °C, and 700 °C). Elemental compositions and surface properties of derived biochar were determined. The yield and volatile matter (VM) of the biochars decreased as the pyrolysis temperature increased with Le300 having the highest yield (47.4%). The highest VM was obtained in Tf300 (79.6%). The biochars were alkaline, with Fv700 having the highest pH (11.6). Pyrolysis temperature and feedstock influenced nutrient composition of biochars and highest values were obtained in: Tf300 (N=2.07%), Fv700 (P=12.0 g/kg), Le700 (K=21.9 g/kg), Fv600 (CEC=32.3 cmol/kg), Fv700 (Ash=33.4%) and Le700 (C=58.6%). Heavy metals in the Fv biochar were highest but within their tolerable limits. Fourier transform infrared spectra showed various functional groups on the biochar surfaces with C-O being dominant (except on Le biochar). X-ray diffraction revealed that SiO2 and CaCO3 were present on biochar surfaces. The Fv biochars had the largest surface area with Fv400 having the highest value (210.6 m2g-1) while Le400 had the highest average pore diameter (159.7 Å). These properties render the biochars suitable as soil amendment and in environmental remediation.

Keywords: Biochar; Spent mushroom substrates; pyrolysis; environmental remediation

Contact information: a: College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou, 350002, Fujian Province, China; b: Key Research Laboratory of Soil Ecosystem Health and Regulation in Fujian Provincial University, Fuzhou, 350002, Fujian Province, China; c: Department of Soil Science, University of Agriculture, Makurdi, Nigeria; d: Soil and Water Management and Crop Nutrition Laboratory, Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, International Atomic Energy Agency (IAEA), Vienna, Austria; e: Department of Chemical Ecology and Ecosystem Research, University of Vienna, Vienna, Austria; f: Fujian Colleges and Universities Engineering Research Institute of Conservation and Utilization of Natural Bioresources, College of Forestry, Fujian Agriculture and Forestry University, Fuzhou, 350002, Fujian Province, China;

* Corresponding author: fafum@126.com

INTRODUCTION

Agricultural wastes, such as plant residues, are increasingly being recognized as important renewable feedstocks as a result of their carbon-rich composition (Bais-Moleman et al. 2018). China has a massive mushroom industry that accounts for approximately 75% of the annual global mushroom production and generates substantial amounts of spent mushroom substrate (SMS) (Mao et al. 2018). Gutian county of the Fujian province is one of the main production areas, which is often referred to as “the capital of Chinese edible fungus.”

The total amount of SMS amounts to 30 million tons per year, and in the case of Gutian county, there were 1.02 billion bags equivalent to approximately 0.37 million tons. The current disposal strategies of SMS include burning, land spreading, burying, composting with animal manure, or landfilling (Phan and Sabaratnam 2012).

Spent mushroom substrates are used as raw materials to produce value-added products such as biogas, bulk enzymes, and organic fertilizer by bioconversion; they can also be used as animal feed supplements (Lim et al. 2013). Several methods of re-use have been reported, including the use as biological pesticides (Hossain et al. 2013), fuel and energy materials (Yadav and Samadder 2018), organic fertilizers (Paula et al. 2017), and other uses.

However, the authors posit that a more effective way of reusing this agricultural waste is through its conversion to biochar via pyrolysis. Pyrolysis is one of the most widely used thermo-chemical conversion technologies and refers to the thermal decomposition of organic components in biomass in an inert gas atmosphere with oxygen limited conditions (Kung et al. 2015). It produces a carbon-rich material with an abundant pore structure, which is termed biochar when it is specifically produced for soil amendment purposes (Chen et al. 2016).

The application of biochar has attracted increased attention because of its potential in global warming mitigation, soil fertility improvement, pollution remediation, and agricultural waste recycling. Research has shown that biochar plays an important role in improving soil structure (Song et al. 2018), promoting plant growth (Subedi et al. 2016), fixing heavy metals (Beiyuan et al. 2017; Wang et al. 2017; Yoo et al. 2018), reducing greenhouse gas emissions (Mechler et al. 2018), reducing nutrient losses (Kerré et al. 2017; Song et al. 2018), and as a carrier for bacterial inoculants (Egamberdieva et al. 2017, 2018).

Evident from the increasing volume of scientific literature, the use of biochar has created considerable interest in recent years. The majority of the biochar studies have concentrated on crop straws (Bian et al. 2018; Kang et al. 2018), wood (Yargicoglu et al. 2015; Chen et al. 2016; Vecstaudza et al. 2018; Wang et al. 2018), manure (Gascó et al. 2018; Xiao et al. 2018; Zhou et al. 2019), and municipal sludge (Wang et al. 2017; Kim et al. 2018). However, there are few, if any, comprehensive studies on the production and characterization of biochar made from different SMS and their applications based on the observed properties.

Pyrolysis temperature is the major factor that influences the properties of biochar and these properties vary depending on feedstock types (Lehmann et al. 2011). To address this research gap, this study was conducted to investigate the biophysical properties of three types of SMS raw materials and the derived biochar obtained through pyrolysis under different temperatures.

Nutrient composition, structure, and surface characteristics of the biochar samples were studied to obtain the most suitable uses for the derived biochar based on feedstock and pyrolysis temperature. This information will give an insight into the effective and appropriate use of these substrates, as well as providing a preliminary and theoretical basis for the application of the derived biochar into the environment.

EXPERIMENTAL

Materials

Biochar preparation

The biomass feedstocks were three types of spent mushroom substrates, viz. Tremella fuciformis (Tf) and Lentinula edodes (Le), which were collected from Gutian City, Fujian Province, China, and Flammulina velutipes (Fv) that was collected from the Juncao Center of Fujian Agriculture and Forestry University (Fuzhou, China). The feedstock materials were separately homogenized and broken into small pieces using a universal crusher (WN-200, Guangzhou Xulang Machinery Equipment, Guangzhou, China) to approximately 1 cm-3, and subsequently air-dried for a week to reduce the moisture content to below 10%. A designed convenient biomass carbonizer (SSBP-50004, Biomass Technology Co. Ltd, Jiangsu, China.) was used for slow pyrolysis, and a constant stream of compressed nitrogen (N2) was fed into the reactor to remove O2 at a flow rate of 3000 NmL/min-1 for 2 min before the heating process began to create an oxygen-limited environment. The feedstock was fed into the carbonizer and pyrolyzed at a heating rate of 10 ℃ min-1 up to 300 ℃, 400 ℃, 500 ℃, 600 ℃, and 700 ℃ ± 10 ℃ for 2 h to carbonize the samples. After cooling to ambient temperature, the prepared biochar was ground using a universal crusher (WN-200, Guangzhou Xulang Machinery Equipment, Guangzhou, China) and passed through a 0.149 mm sieve for subsequent physicochemical analyses.

Methods

Feedstock and biochar characterization

Moisture and ash content of feedstock and biochar samples were determined by the ASTM E871-82 and ASTM E1755-01standards, respectively. Briefly, the moisture content of the feedstock was estimated by measuring the weight loss after drying the fresh samples at 105 °C for 24 h. Ash contents were determined by the mass loss after burning the dried samples in an open crucible in a muffle furnace for 4 h at 700 °C. To remove the effect of moisture and ash, the biochar yields were expressed into a dry ash free (daf) basis as follows,

where Mbiochar (wt%) represents the weight of biochar, Ybiochar; ad (wt%) represents the air-dried basis yields of biochar, Mbiomass represents the weight of the biomass, while M and A(wt%) are the moisture and ash contents of biomass, respectively. The volatile matter was determined by measuring the weight difference before and after the combustion of about 1 g of biochar in a crucible at 950 oC (Li et al. 2018).

The pH values of samples were measured with a pH meter (PHS-3E; INESA Scientific Instrument Co., Ltd., Shanghai, China) using a 1:5, sample to deionized water (Gaskin et al. 2010), after stirring for 1 h at constant temperature (25 ℃). The contents of C, N, and S were determined by an elemental analyzer (VarioMax; Elementar, Lagenselbold, Germany). The P and K contents of the samples were measured using the APHA Standard Method 4500-P. Total P concentration was measured by colorimetric analysis and total K with flame atomic spectroscopy (FP640; AOPU Analytical Instruments, Shanghai, China). The contents of heavy metals (Cd, Pb, Cr, Cu, Zn, Mn, and Ni) were measured by an inductively coupled plasma-mass spectrometer (ICP-MS) (NexlON 300X; Perkin Elmer, Waltham, MA, USA).

Cation exchange capacity (CEC) of the biochar samples was measured using the 1M ammonium acetate method (Gai et al. 2014). This was done by taking 0.50 g sample and leaching it with 100 mL deionized water five times to reduce the interference of soluble salts. Then, the sample was leached with 100 mL 1 mol/L-1 sodium acetate (pH 8.2) five times to ensure that the exchange sites were saturated with sodium ions. Thereafter, the sample was leached with 100 mL ethyl alcohol five times to remove the excess sodium ions. Finally, the sample was leached with 100 mL 1 mol -1 ammonium acetate (pH 7) five times, and the sodium ion concentration of the reserved leachate above was measured by a flame photometer (FP640; AOPU Analytical Instruments, Shanghai, China). The cation exchange capacity (CEC) was calculated by the sodium ion concentration.

The surface areas of the samples were measured using multipoint Brunauer – Emmet -Teller (BET) (Trister II 3020; Micromeritics Instrument Corp., Shanghai, China) after degassing for 10 h at 105 ℃ to remove the substance adsorbed on the surface of biochar. The interface of the samples was observed by scanning, using an electron microscope (SEM) (NovaTMNanoSEM 230; FEI Company, Hillsboro, OR, USA) with micrographs generated by topographic contrast. The samples were coated with a thin layer of gold because a conductive material was a prerequisite to generate the images. Image resolution used was high vacuum mode with magnifications of 1000, 20000, and 50000 at 1.0 nm with an acceleration voltage of 15 kV, and an energy resolution of 132 eV.

Fourier transform infrared (FTIR) spectroscopy was used to identify the chemical functional groups on the biochar sample surfaces. All the biochar samples were oven-dried at 80 °C for a period of 24 h before the FTIR analyses. Subsequently, the samples were crushed with spectroscopic grade KBr at a weight ratio of 0.5% (KBr/biochar), and then the mixture was pressed into transparent sheets. The observable FTIR spectra was obtained using a FTIR spectrometer (Nicolet iS5; Thermo Fisher Scientific, Waltham, MA, USA) scanned in the range of 400 cm−1 to 4,000 cm−1 at a resolution of 4.0 cm−1.

The XRD patterns, i.e., mineralogical characterization of the biochar samples were obtained using an X-ray diffractometer (Ultima IV; Rigaku Corporation, Wilmington, MA, USA). The samples were scanned in the range of 5° to 85° at 40 kV and 40 mA and at a speed of 6° min-1. Jade 6.0 software (Jade Software Corporation, Christchurch, New Zealand) was used to remove the background radiation of the XRD results.

Data processing and statistical analysis

Collected data was subjected to analysis of variance (ANOVA) using SPSS 20.0 software (IBM Corp, Armonk, NY, USA). The means were separated using the least significant difference (LSD) at a 5% level of probability (P < 0.05). Origin 9.0 (OriginLab, Northampton, MA, USA) and Jade 6.0 software (Jade Software Corporation, Christchurch, New Zealand) were used to process the figures for FTIR and XRD, respectively.

RESULTS AND DISCUSSION

Physicochemical Properties of Feedstock Biomass and Biochar

The physicochemical properties of the feedstock biomass and biochar samples are shown in Tables 1 and 3, respectively. As shown in Table 1, feedstock compositions had lower values compared to the derived biochar as previously reported (Bian et al. 2018; Wang et al. 2018). The results in Table 3 show that the biochar yield decreased with increased pyrolysis temperature as earlier reported (Zhang et al. 2015; Gascó et al. 2018). Temperature is known to be the major factor influencing biochar mass yield (Weber and Quicker 2018). In addition, the rate of weakening and disappearance of peaks of functional groups present on the biochar surfaces at higher temperature such as –OH and C-H groups is consistent with a significant mass loss (Jin et al. 2016). All of the biochar had the highest yields at 300 ℃. The Le had a relatively high production yield compared to the other feedstocks with a peak value of 47.4% at 300 ℃. The Fv biochar had the least yield ranging from 33.7% at 300 ℃ to 30.3% at 700 ℃. The C=C stretching vibrations identified in the Tf and Le biochars produced at low temperatures 300 °C could be one of the factors that increased its initial resistance to degradation resulting to higher yields at this temperature compared to the Fv biochars. An early stabilization of yield was observed in the Tf and Fv biochar where the yield ranged from 41.0% to 38.6% and from 33.7% to 30.3%, respectively, from 300 ℃ to 700 ℃. The stabilization of biochar yield suggested that the major carbonization was completed between this temperature range. Dunnigan et al. (2018) observed higher yields between 400 ℃ and 550 ℃ using rice husk as feedstock. Contrary to this trend, stabilization of yield was observed at elevated temperatures of 600 ℃ to 800 ℃ using switch-grass, water oak, biosolid (Li and Chen 2018), and agricultural residues (Jindo et al. 2014). The variation in yield with respect to the biochar could have been a result of the different feedstocks used. The decrease in yield of biochar with increasing temperature as observed in this study has also been reported using Rhodes grass and palm fronds (Jouiad et al. 2015), cellulosic and lignocellulosic biomass (Peng et al.2011; Hmid et al. 2014), and animal manures (Cely et al. 2015). Biochar yield decline may have been attributed to the primary decomposition of the biomass and a possible secondary decomposition of the produced biochar during the pyrolysis process at higher temperatures. The decrease in yield of biochar with increasing pyrolysis temperature was attributed to increased gasification (Colantoni et al. 2016). At a high pyrolysis temperature, there is an intensified carbonization of the biomass through rapid dehydrogenation, gasification, and condensation, resulting in the reduction of solid biochar produced (Li and Chen 2018). Generally, more organic matter decomposes as the temperature increases, thereby resulting in the feedstock materials with lower yields at higher pyrolysis temperatures (Ghanim et al. 2016; Li et al. 2016).

The ash content of the biochar gradually increased with the increasing temperature. Ash content was found to range from 20.6% in Le biochar at 300 ℃ to 33.4% in Fv biochar at 700 ℃, indicating that the highest ash content was derived at 700 ℃ in the Fv biochar. Although higher ash contents were observed at 500 ℃ and 600 ℃ in the Tf and Le biochar, suggesting that other factors apart from temperature could also affect the ash composition of biochar. Ash content of biochar has been previously reported to increase with rising pyrolysis temperature, and it varied among different biochar depending on the type of feedstock (Cely et al. 2015; Bian et al. 2016). The biochar had variable ash contents, suggesting the ash composition was influenced by the nature of feedstocks. Xiao et al. (2018) also reported that ash contents along with specific inorganic compositions in the biochar generally increased with rising pyrolysis temperature. A positive correlation was observed between the biochar yield and both the feedstock and biochar ash content (Windeatt et al. 2014).

Among the feedstocks investigated, Tf had the highest volatile matter (VM) content (84.2%), followed by Fv (80.0%) and Le (72.0%) (Table 1). A decrease in VM content in biochar as pyrolysis temperature increased was observed in the biochars (Table 3). Such decrease in VM with temperature has been reported by Li et al. (2018) on switch grass, water oak, and biosolids. This was attributed to the fact that more VM was removed while ash and fixed carbon were retained at higher pyrolysis temperatures (Cantrell et al. 2012). However, the Le biochar had a lower VM compared to Tf and Fv, which may have resulted to its higher yield observed in Le300. The high amount of VM content of the biochars at lower temperatures could be due to the presence of cellulose and hemicellulose (Jindo et al. 2014). There was no significant difference in the VM content at 600 °C and 700 °C suggesting that majority of the VM could have been decomposed at 600 °C. Subsequently, at temperatures above 600 °C, no significant decrease in VM was observed, as also reported by Li et al. (2018).

Table 1. Physicochemical Properties of Feedstock Biomass

Means with similar letters are statistically similar. Those with different letters are significantly different (P < 0.05)

As shown in Table 1, the pH values of the feedstock ranged between 4.2 to 6.1, indicating that they were strongly to moderately acidic, with Le possessing the statistically lowest pH. However, pH of the all the biochar samples ranged from 8.2 to 11.6 (Table 3). Alkaline properties of biochar have previously been reported (Cely et al. 2015; Mechler et al. 2018). Differences in pH were observed for the different biomass types. However, the Tf biochar had higher pH values across the various temperatures than other biochar produced at similar pyrolysis conditions, and the data showed that pH values increased as the temperature increased, indicating that the pyrolysis temperature had a profound effect on the pH of the derived biochar.

Table 2. Heavy Metal Contents of Feedstock Biomass

Means with similar letters are statistically similar. Those with different letters are significantly different (P < 0.05)

Table 3. Physicochemical Properties of Biochar

Means with similar letters are statistically similar. Those with different letters are significantly different (P < 0.05) for individual biochar

The highest pH was recorded at 700 ℃ in the Fv biochar. Generally, high pH values are a result of an accumulation of high alkali metals (Ca, Mg, K, and Na) due to the thermal degradation of the organic fraction in biomass during the pyrolysis process (Kim et al. 2018). In addition, at higher pyrolysis temperatures, the amount of carboxyl groups in the resulting biochar are reduced and/or the acidic groups have become deprotonated to the conjugate bases, resulting in a more alkaline pH of the biochar in suspension (Ronsse et al. 2013). The rise in pH at higher pyrolysis conditions could also be attributed to the relative increase in ash content in the biochar. Biochar produced at higher pyrolysis temperatures are therefore expected to have more advantages in agriculture as they can improve of soil health because the alkalinity of biochar (Molnar et al. 2016) which could create a preferable environment for the soil microbial communities that function in C and N cycling (Jaafar et al. 2015). Therefore, pyrolysis temperature, nature and composition of feedstock can be determining factors to produce biochar with specific uses such as soil amendment.

Elemental composition analysis of feedstock biomass and derived biochar samples indicated that biomasses were typically lower in nutrient composition compared to the derived biochar (Tables 1 and 3). Pyrolysis temperatures had large impacts on the biochar’s elemental composition during the pyrolysis process. One main goal of biochar production is the change in chemical composition compared to that of raw biomass, more importantly the increase in carbon content (Weber and Quicker 2018). In general, when the temperature rose from 300 °C to the peak temperature of 700 ℃, carbon contents initially increased and subsequently decreased at 600 ℃ for all of the feedstocks except Le, where there was a uniform increase with increasing temperature. The highest C content was observed at 500 °C for Fv and Tf biochar, with Fv having the higher value of 58.61% (Table 3). The C contents in the biochar samples were in the range of values obtained using other feedstocks in previous reports ( Sun et al. 2014; Chen et al. 2016). However, Case et al. (2015) reported C content of 72.3% in biochar derived from hardwood trees as feedstock at 400 ℃. This suggests that feedstock materials determine the C composition of derived biochar. The sharp decline of C contents in all the biochar at the peak temperature of 700 ℃ could be attributed to the loss of C-containing compounds during pyrolysis at elevated temperatures (Han et al. 2016; Wang et al. 2016). Similar temperature effects on carbon contents have previously been observed (Bergeron et al. 2013). The relatively high C content of these biochar makes them suitable for C sequestration of excess carbon from the atmosphere (Herath et al. 2015).

Nitrogen content decreased as pyrolysis temperature increased across all the biochars evaluated. However, there was an observed increase in the N content of Le biochar from 600 ℃ to 700 ℃. The Tf biochar contained relatively higher N contents (1.80% to 2.07%) compared to the others and could be of greater benefit when used as N fertilizer. Although compared to its feedstock that contained 2.05% N, the Tf biochar contained its highest N content of 2.07% at 300 ℃. However, N content in biochar derived from 400 ℃ to 700 ℃ (Table 3) for the Tf biochar were lower than that of the feedstock (Table 1), suggesting that the Tf biochar had a high quantity of volatile N-containing compounds that volatilized at higher pyrolysis temperatures. The nature of the feedstock material also had an influence on the N content, as the Le biochar made from hardwood feedstock had a lower N content. The highest N content was obtained at 300 ℃, indicating that the pyrolysis temperature had a significant effect on the N contents. The decrease in biochar N content was probably due to volatilization of N-containing compounds associated with the decomposition of amino acids as the temperature increased (Xiao et al. 2018). Therefore, the ratio of C:N increased in all of the biochar samples except for Le biochar at 700 ℃.

Similarly, sulfur contents decreased with increasing temperature, but this was not consistent across all the biochar as the Fv400 increased to 1.60 % from 1.49 % in Fv300 before declining. The observed decrease in sulfur may be due to the decomposition of organic sulfur during pyrolysis (Nanda et al. 2014). There was an increase in potassium and phosphorus contents as the pyrolysis temperature increased, although an irregular pattern emerged. Increase in these elements with an increase in pyrolytic temperature has also been observed (Ahmad et al. 2017). Such increases in these non-volatile elements have been previously reported (Colantoni et al. 2016). There were no clear trends of cation exchange capacity (CEC) with increasing temperature, but values declined at the peak temperature of 700 ℃, with Fv600 having the highest CEC (32.29 cmol kg-1). The higher CEC value of this biochar indicated its stronger ability to hold essential nutrients as well as greater its resistance against soil acidification (Mukherjee et al. 2011). A similar observation was made using switchgrass, water oak, and biosolids where CEC declined at the peak temperature of 800 ℃ (Li and Chen 2018). The CEC is a measure of the ability of materials to adsorb cations such as Ca2+, Mg2+, or K+. Gascóet al. (2018) used pig manure feedstocks and observed the highest CEC values at 600 ℃.

Table 4. Heavy Metal Contents of Biochar

Moreover, Cely et al. (2015) showed that the CEC of biochar depends on the pyrolysis temperature and raw material properties. The greater CEC of the biochar could have been a result of a higher charge density per unit surface area, the formation of carboxyl groups, a more porous structure, or a combined effect of the three factors (Sun et al. 2014). Therefore, the biochars produced at 600 ℃ and below will be of greater advantage as soil amendments in agronomy based on the CEC. Generally, the biochar materials used herein were generally high in nutrient composition compared to their feedstock materials. These properties render the derived biochar of high agronomic value (Wang et al. 2015; Domingues et al. 2017) and can thus be used as soil amendments, especially in combination with organic and inorganic fertilizers. But this factor was outside the scope of this study and such research is suggested to be undertaken.

As can be seen in Table 4, the biochar generated from 500 °C to 600 °C contained higher concentrations of metals as compared to those produced at 300 °C and 400 °C. Higher pyrolysis temperatures resulted in biochar with higher micronutrients content. Kim et al. (2018) observed a similar trend using sludge, rice straw, and spent coffee ground biochar from 550 °C and 700 °C and attributed this phenomenon to the higher degree of thermal degradation of the organic mass faction in samples pyrolyzed at a higher temperature. The Zn content did not follow this trend in the Fv biochar, as the highest value of 432.8 mg kg-1 was observed at 300 °C. There was also an irregular effect of pyrolysis temperature on the Cd and Pb contents in all of the biochar. The Ni content was drastically reduced at the peak temperature of 700 °C in all the biochar samples. This may have been attributed to Ni containing compounds being degraded at relatively high temperatures. Generally, the highest concentration of heavy metals was obtained in the Fv feedstock and derived biochar (Tables 2 and 4). This illustrated its importance of the immobilization of heavy metals and can be a useful trait in bioremediation. Detoxification of heavy metal ions, to variable extents, can be achieved by introducing biochar into the soil as they can form specific complexes with these metals (Han et al. 2013). It has been demonstrated that biochar has an excellent ability to immobilize metals and organic contaminants from the aqueous phase and soil (Beiyuan et al. 2017; Wang et al. 2017; Yoo et al. 2018). However, the bioavailability of these metals in the biochar should be further studied to ensure their safe use. The EBC (2017) gave the acceptable range of heavy metals in biochar on a dry matter basis as stated: Pb < 150 mg kg-1; Cd < 1.5 mg kg-1; Cu < 100 mg kg-1; Ni < 50 mg kg-1; Zn < 400 mg kg-1 and Cr < 90 mg kg-1. The results (Table 4) show that all the heavy metals in the obtained biochar were within safe limits except Zn, which was a bit higher than the threshold given for the metal in biochar in the Fv biochar as well as Ni in Le600 biochar. Zinc content ranged from 173.32 mg kg-1 at 300 °C in the Le biochar to 432.84 mg kg-1 at 300 °C in the Fv biochar. The content of Ni in Le600 was 51.44 mg kg-1 which was above the threshold of 50 mg kg-1 given. However, this does not call for serious concern as the amounts of biochar used in agriculture are relatively low compared to those of compost and manure. Therefore, toxic accumulation of heavy metals could practically be ruled out, even when thresholds are higher (EBC 2017) and as such, these biochar materials could be useful soil amendments without any environmental concern.

Microstructure and Surface Functional Analysis of Biochar

The SEM images of biochar taken under different magnification times at 400 oC to investigate their surface structures are shown in Figs. 3 to 5. It was obvious from the images that the surface morphology of the biochars varied and can be largely attributed to the nature of feedstocks. As shown on Fig. 5, SEM images of the Le400 biochar even at a lower magnification time (x20000) compared to Tf400 and Fv400 (x50000) in Figs. 3 and 4 respectively showed higher pores and relatively lower surface area as also reported on Table 5 and Fig. 1. The images also indicated the presence of porous structure on the surfaces, but Fig. 3 and 4 showed that some pores were filled by volatile matter, which decreased the average pore diameter (Table 5 and Fig. 1). The presence of volatiles in the pore structure of biochar on SEM images has been reported in rice husk and elm sawdust biochar (Wang et al. 2014).

The pore distribution (Table 5, Fig 1) revealed a porous structure of all the biochar, with little variation between the biomass feedstock types, except for the Le biochar that showed higher average pore diameter ranging from 37.6 Å at 600 ℃ to 159.7 Å at 400 ℃ (Fig 1).

Table 5. BET Surface Area and Porosity of Biochar

Fig. 1. Surface area and porosity of the biochar studied

The highest pore diameter of 159.69 Å was observed in the Le biochar at 400 ℃ and was drastically reduced at higher temperatures (600 ℃ to 700 ℃). This could have been attributed to the nature of the feedstock, as thermal decomposition of the raw biomass did not fully occur until higher temperatures. The higher average pore diameter of this biochar at 400 ℃ and 500 ℃ will result to a smaller surface area and might limit the adsorption capacity of this biochar in the soil. The Tf and Fv biochar had their highest pore diameter at 600 ℃. During pyrolysis, there is an increase in porosity of the resulting biochar due to the release of volatiles and chemical reactions that occur between the volatiles, minerals, and inorganic compounds that exist in the biomass (Bian et al. 2016). The BET surface area of the biochar samples in this study ranged from 3.09 m2 g-1 in the Le biochar at 400 ℃ to 210.57 m2 g-1 in the Fv biochar at 400 ℃ (Table 5 and Fig 1).

There was no specific effect of temperature on the surface area as there was an irregular pattern observed across the biochar types. However, the surface area was dependent on the nature of feedstock because at the same temperatures with other feedstocks, Fv biochar had higher surface areas. Although, lower surface areas at low temperatures using pine wood, wheat straw, green waste, and dried alga has been observed (Ronsse et al. 2013). When the pyrolysis temperature was further increased, the BET surface area either reduced or decreased in all the biochar types, which was likely due to restructuring taking place in the biochar or due to the onset of ash melting at higher temperatures. When comparing the different biochar, Fv offers the highest potential of surface area at 400 ℃ as all other biochar types had a BET specific surface below an average of 50 m² g-1. The Fv biochar with the highest surface area also had the least ash content while the Le with its highest ash content at 600 ℃ had the least surface area at the same temperature. It has been reported that the higher amount of inorganics (i.e., ash content) in the biochar negatively correlate with specific surface area in the produced biochar (Li and Chen 2018). This is possibly explained by fusion of ash filling up pores in the biochar, thereby decreasing accessible surface area in the other biochar. While the Le biochar had its highest surface area at 600 ℃, the Fv and Tf biochar had their peak values at 400 ℃ and at 500 ℃, respectively. The Le biochar was observed to have relatively smaller surface area compared to the other biochar. Differences in feedstock materials have been suggested as the main reason for the differences in surface area and micropore distribution of biochar (Zhang et al. 2018). The majority of pores observed on the biochar samples in this study were mesopores (Table 5 and Figs. 3 to 5). Because the surface areas of Fv biochar ranged from 148.3 mg-1 to 210.6 mg-1 and was much higher than other samples analyzed (Table 5 and Fig. 1), when added to the soil can improve plant root growth, soil microorganism abundance, soil mineral nutrients, and influence other soil properties (Song et al. 2018). Surface area is an important index that they can significantly influence a material’s adsorption capacity. A larger surface area resulted in more porous structures within biochar (Windeatt et al. 2014), which suggested that Fv biochar may be particularly useful as a soil amendment for water treatment or environmental remediation. Although, further pot and field trials would be required to confirm this.

Fourier Transform Infrared Analysis

The infrared spectra of the biochar revealed the transformation of their complex chemical bond structures at elevated pyrolysis temperatures (Fig. 2). The FTIR peaks appeared between 400 cm-1 and 4000 cm-1 in all biochar (Fig. 2).

Fig. 2. FTIR spectra of the biochar studied

The disappearance of the -OH group as the pyrolysis temperature increased to 400 °C and above in the Tf and Fv biochar indicated that the organic -OH was very unstable at elevated temperatures. In addition, increasing temperature also had an effect on the functional groups present in the biochar by reducing them, especially the -OH functional groups of phenols, ethers, and alcohol (Sardella et al. 2015). Unstable functional groups, such as O-H (near 3400 cm-1), were slightly detected at a high temperature (> 600 °C). In all but Le, the C-O stretching vibration appeared at around 1400 cm-1. Compared to Le, the Fv and Tf biochar exhibited stronger peak intensities for aromatic C-O. Similarly, the -OH groups and C-H groups were weak functional groups, which disappeared for the biochar produced at high pyrolysis temperatures (> 400 °C). The cleavage of -OH and C-H groups contributed to significant mass loss during thermal decomposition and gasification, resulting in decreased biochar yield at high pyrolysis temperatures. Weakening of the peak with the increase in the pyrolysis temperature is often expected (Jin et al. 2016). When the pyrolysis temperature was raised to 600 °C, almost no aliphatic functional groups could be found in the biochar. Instead, aliphatic structures were reformed into aromatic structures, resulting in the increased presence of phenolic functional groups and ethers. The C=O stretching vibration in the carboxyl group in the Le biochar appeared at 1400 cm-1 and 1600 cm-1. Adsorption peaks that are representatives of different functional groups were observed (Yuan et al. 2015; Domingues et al. 2017).

Fig. 3. SEM images of Tf400 biochar at different magnification times

Fig. 4. SEM images of Fv400 biochar at different magnification times