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Wang, X., Ibrahim, M., Tong, C., Hu, K., Xing, S., and Mao, Y. (2020). "Influence of pyrolysis conditions on the properties and Pb2+ and Cd2+ adsorption potential of tobacco stem biochar," BioRes. 15(2), 4026-4051.

Abstract

Converting biomass into biochar is a smart recycling strategy. Biochar was produced from tobacco stems at temperatures of 400 °C, 500 °C, 600 °C, and 700 °C and holding times of 1.5 h, 2 h, 2.5 h, and 3 h. Its properties and adsorption capacities for Pb2+ and Cd2+ were evaluated. While the yield decreased, pH, phosphorus, potassium, ash, and surface area increased with increasing pyrolysis temperature and holding time. Nitrogen, volatile matter, and pore diameter decreased as the temperature increased, with an irregular effect of the holding time. A peak C content (652 g/kg) was recorded at 600 °C (2 h). The highest values obtained for the N, P, and K content were 25.6 g/kg (400 °C and 2 h), 7.82 g/kg and 168 g/kg (600 °C and 3 h), respectively. The heavy metal contents were within tolerable limits. The highest surface and micropore areas of 50.6 and 57.1 m2g-1, respectively, were obtained at 700 °C (3 h). The biochar had a wide range of aliphatic and aromatic C functional groups. The highest adsorption percentages of Pb2+ and Cd2+ (44.5 % and 38.3 %, respectively) by biochar produced at 700 °C (3 h) signified its suitability for heavy metal adsorption. These properties made the biochar a suitable soil amendment.


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Influence of Pyrolysis Conditions on the Properties and Pb2+ and Cd2+ Adsorption Potential of Tobacco Stem Biochar

Xiaopeng Wang,a,b Muhammed Mustapha Ibrahim,a,b,d Chenxiao Tong,a,b Kun Hu,a,b Shihe Xing,a,b and Yanling Mao a,b,c,*

Converting biomass into biochar is a smart recycling strategy. Biochar was produced from tobacco stems at temperatures of 400 °C, 500 °C, 600 °C, and 700 °C and holding times of 1.5 h, 2 h, 2.5 h, and 3 h. Its properties and adsorption capacities for Pb2+ and Cd2+ were evaluated. While the yield decreased, pH, phosphorus, potassium, ash, and surface area increased with increasing pyrolysis temperature and holding time. Nitrogen, volatile matter, and pore diameter decreased as the temperature increased, with an irregular effect of the holding time. A peak C content (652 g/kg) was recorded at 600 °C (2 h). The highest values obtained for the N, P, and K content were 25.6 g/kg (400 °C and 2 h), 7.82 g/kg and 168 g/kg (600 °C and 3 h), respectively. The heavy metal contents were within tolerable limits. The highest surface and micropore areas of 50.6 and 57.1 m2g-1, respectively, were obtained at 700 °C (3 h). The biochar had a wide range of aliphatic and aromatic C functional groups. The highest adsorption percentages of Pb2+ and Cd2+ (44.5 % and 38.3 %, respectively) by biochar produced at 700 °C (3 h) signified its suitability for heavy metal adsorption. These properties made the biochar a suitable soil amendment.

Keywords: Tobacco stem; Recycling; Biochar; Heavy metals; Adsorption

Contact information: a: College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou, Fujian Province 350002 China; b: Key Research Laboratory of Soil Ecosystem Health and Regulation in Fujian Provincial University, Fuzhou, Fujian Province 350002 China; c: Fujian Colleges and Universities Engineering Research Institute of Conservation and Utilization of Natural Bioresources, College of Forestry, Fujian Agriculture and Forestry University, Fuzhou, Fujian Province 350002 China; d: Department of Soil Science, University of Agriculture, Makurdi 972211 Nigeria

* Corresponding author: fafum@126.com

INTRODUCTION

Agricultural activity in China produces a large quantity, variety, and distribution of biomass resources. As a widely grown cash crop in China, tobacco has a unique economic value (Zhang 2002). The total area of the annual tobacco crops grown in China is reported to be approximately 1 million hectares, with an annual output of 2.36 million tons of tobacco leaves (Sun et al. 2016). When comparing the ratio of the tobacco stem (rod) to the tobacco leaves produced at harvest, the annual tobacco stem output is higher. The efficient use of tobacco as biomass has not been fully explored, as it is considered residue after harvest, as well as having a hard texture. After the tobacco leaves are harvested, the tobacco stem becomes a source of waste. They are either discarded as a solid waste or locally incinerated, therefore contributing to air pollution (Wang 2007).

The accumulation of agricultural wastes and their disposal via burning constitutes an environmental problem of global concern (Sobati et al. 2016). The burning of tobacco crop residue is a key source of air-borne carbonaceous aerosols, which is highly hazardous to the health of both humans and the ecosystem in China (Zhang et al. 2013). Options for disposing of the increasing quantity of these residues poses a serious challenge for agriculture in China. It is therefore of utmost importance to find a suitable alternative recycling method for tobacco stems, which may have potential as a high value-added product. Research on the utilization of hard agricultural waste, e.g., straw and woody biomass, primarily focuses on the manufacturing of fuel, the extraction of raw chemical materials, and papermaking (Yan et al. 2008).

The conversion of agricultural wastes into biochar has been proposed as an effective means of handling agricultural wastes (Kuzyakov et al. 2014). Biochar refers to the carbon-rich organic material obtained when biomass is heated to temperatures greater than 250 °C under oxygen-limited conditions (Lehmann and Joseph 2015). The application of biochar in soil has shown great potential in terms of crop production, capturing greenhouse gases, and improving soil properties (Kang 2018). The porous structure and high carbon content of biochar influence its physical and chemical properties, e.g., its high stability and strong adsorption (O’Laughlin and McElligott 2009). Therefore, the incorporation of biochar into the soil can also produce a number of important ecological benefits: increasing the soil fertility and improving agricultural production (Anna and Patryk 2015; Subedi et al. 2016); as a bacterial inoculant carrier (Egamberdiva et al. 2018); capturing greenhouse gases (Mechler et al. 2018); increasing the soil carbon stocks; and reducing the risk of contaminants and heavy metals in the soil (Beiyuan et al. 2017; Wang et al. 2017; Yoo et al. 2018), in addition to several other applications.

Unlike other types of anthropogenic-induced pollutants, heavy metals are non-degradable and can therefore only be removed from the environment via remediation (Ahmad et al. 2018). Biochar has been widely used as an adsorbent for the removal of contaminants due to the numerous functional groups present on its surface (e.g. alkyl, hydroxyl, carbonyl, carboxyl, alkyne, amide, etc.) and developed pore structure (Ahmad et al. 2018; Li et al. 2019). However, sorption of ions is a complex characteristic of biochar that is difficult to predict (Pignatello et al. 2017), due to the varying properties of biochar prepared using different feedstock and production conditions. Therefore evaluating the properties and sorption potentials of biochar produced from tobacco stems will be useful to provide its alternative uses in environmental management.

The pyrolysis conditions, i.e., the temperature and holding time, and the nature of feedstock biomass are important factors that determine the properties of the derived biochar and can therefore influence its environmental application (Sun et al. 2014). However, detailed information on the properties of tobacco stem biochar and how it can influence its application has not received sufficient research. This poses a major research gap in the exploration of the usage of agricultural waste products as biochar. Its conversion into biochar and exploring its unique properties will pave way for determining any potential alternative uses. For this study, tobacco stems were collected and used to produce biochar under different conditions (temperatures and holding times), and the nutritional and elemental composition, structure and surface characteristics, and Pb2+ and Cd2+ adsorption potential of each biochar was studied.

EXPERIMENTAL

Materials

Preparation of biochar

The tobacco stem (variety Cuibi No. 1) was obtained from the Fujian Tobacco Agricultural Science Research Institute (Fujian, China). The stems were air-dried, crushed, and passed through a 1 mm sieve. Before pyrolysis, the sample was placed in an oven at 70 °C for 24 h to reduce the moisture content to approximately 7%. After weighing the biomass, slow pyrolysis was carried out using a biomass carbonizer (SSBP-50004, Biomass Technology Co. Ltd, Jiangsu, China). Nitrogen gas was introduced for 5 min to remove the internal oxygen prior to pyrolysis. The feedstock was pyrolyzed at a heating rate of 10 °C/min at temperatures of 400 °C, 500 °C, 600 °C, and 700 °C, and for each of the carbonization temperatures a holding time of 1.5 h, 2 h, 2.5 h, and 3 h was used. The carbonized sample was left to cool at room temperature, taken out, and then ground through a 0.149 mm sieve for subsequent characterization.

Methods

Biochar characterization

The biochar yield was estimated under a dry ash free (daf) basis with the following relationship, as shown in Eq. 1 and Eq. 2,

Ybiochar; daf = 100 x (Ybiochar – A) / (100 – M – A) (1)

Ybiocharad = 100 x Mbiochar Mbiomass (2)

where Mbiochar (wt%) represents the weight of the biochar, Ybiochar; ad (wt%) represents the air-dried biochar yield, Mbiomass represents the weight of the biomass, and M and A (wt%) represent the moisture and ash content of the biomass, respectively.

Using ASTM E1755-01 (2015), the ash content was determined by the mass loss after the combustion of the dry biochar samples in an open crucible placed in a muffle furnace for 4 h at 700 °C. The determination of the total amount of volatile matter was performed by measuring the weight loss before and after the combustion of 1 g of biochar in a crucible at 950 °C (Li et al. 2018).

The pH of the samples was determined by weighing 0.5 g of biochar and placing it into a centrifuge tube. Then, 10 mL of distilled water was added (at a ratio of 1:20, by w:v) and shaken at 150 rpm for 24 h at room temperature (Jindo et al. 2014). The pH was measured using a pH meter (PHS-3E, INESA Scientific Instrument Co., Ltd., Shanghai, China).

The carbon (C) and nitrogen (N) contents of the biochar were determined via an elemental analyzer (VarioMax; Elementar, Lagenselbold, Germany). The total phosphorus (P) and potassium (K) content were determined using the APHA standard 4500-P (1992). The total P concentration was measured using the vanadium molybdenum yellow colorimetric method and the total K was measured using a flame atomic spectroscopy (FP640, AOPU Analytical Instruments, Shanghai, China). The available P was determined using the Olsen [sodium bicarbonate (NaHCO3)] extracting solution method. The extract was analyzed for P colorimetrically. Available K was measured by weighing 1 g of biochar into a 50 mL Erlenmeyer flask, and 25 mL of 1 mol·L-1 NH4OAc solution was added and shook at 25 oC for 30 min, filtered, and measured using flame atomic spectroscopy. The alkaline N was determined using the alkaline solution diffusion method. Briefly, 2 g of biochar was weighed into the outer chamber of a diffusion dish, and 2 mL of boric acid indicator was added into the inner chamber of the dish. The dish was covered with a frosted glass and held using glycerin. Two mL of 1 mol L-1 NaOH was added to the outer chamber through the frosted glass gap and immediately covered tightly and held in place with rubber bands. The dish was incubated at 40 ℃ for 24 h and thereafter the solution in the inner chamber was titrated with 0.005 mol·L-1 H2SO4. The titre value was used to estimate the alkaline N content.

The heavy metal concentration was determined by weighing 0.5 g of the biochar sample and adding it into 30 mL polytetrafluoroethylene crucible, and then 1 to 2 drops of ultrapure water were added. Five mL of HNO3:HClO4 (in a ratio of 1:1, by v:v) and 5 mL of HF were added and left to stand overnight. The samples were then heated for 1 h at 100 ℃ and then the temperature was raised to 250 ℃ until the samples were devoid of color. After cooling, the samples were filtered in a 25 mL volumetric flask with ultrapure water, and the heavy metal content (cadmium (Cd), lead (Pb), copper (Cu), zinc (Zn), and nickel (Ni)) was determined via inductively coupled plasma mass spectrometry (ICP-MS) (NexlON 300X, Perkin Elmer, Waltham, MA, USA).

Fourier transform infrared (FTIR) spectroscopy was performed to determine the functional groups on the biochar surface. The biochar sample and KBr were crushed together in a ratio of 1:100 in an agate mortar after drying overnight in an oven at 80 °C. The greyish mixture was pressed into a delicate transparent sheet and each FTIR spectra was obtained via laser scanning with an FTIR spectrometer (vertex 70, Bruker, Billerica, MA) using a resolution of 4 cm−1 at wavenumbers ranging between 500 cm−1 and 4000 cm−1.

For the determination of the specific surface area and the pore size distribution of the biochar sample, 0.1 g of biochar was weighed and degassed for 10 h at 105 °C to remove the substances adsorbed by the surface of the biochar sample. A multipoint Brunauer – Emmet -Teller (BET) device (Trister II 3020, Micromeritics Instrument Corp., Shanghai, China) was used to measure the specific surface area. The sample interface was obtained by scanning biochar samples via an electron microscope (NovaTM NanoSEM 230, FEI Company, Hillsboro, OR, USA).

Heavy metal adsorption

The heavy metal adsorption study was performed as described by Zou et al. (2018), and Pb(NO3)and Cd(NO3)2 were used to prepare two solutions that each had a mass concentration of 50 mg L-1. The pH of the solution was adjusted to 5.5 using 0.1 mol L-1 HCl and 0.1 mol L-1 NaOH. For each solution, 30 mL was poured into a triangular flask, and 0.05 g of the biochar sample was weighed and added into the flask. The mixtures that contained the biochar and Pb(NO3)or Cd(NO3)2, were shaken at 150 r/min for 2 h at a constant temperature shaker of 25 °C. It was then filtered and the filtrate was analyzed via an atomic absorption spectrometer coupled to a mass spectrometer ((ICP-MS) (NexlON 300X, Perkin Elmer, Waltham, MA) to determine the concentration of Pb2+ and Cd2+ in the respective solutions, both before and after the adsorption reaction. The adsorption of these heavy metals by the biochar was calculated according to Eq. 3,

Qt = (Co-Ct)V/M (3)

where Qt (mg g-1) is the amount adsorbed by the biochar, Co (mg L-1) is the initial mass concentration of the Pb2+ and Cd2+ solutions, Ct (mg L-1) is the mass concentration of the Pb2+ and Cd2+ in the solution after 2 h of adsorption, V (mL) is the volume of the solution L, and M (g) is the mass of the biochar.

Data processing and statistical analysis

All data obtained were subjected to analysis of variance (ANOVA) using SPSS (version 20.0, IBM, Armonk, NY) software. The means were separated using the least significant difference (LSD) at the 5% level of probability (p-value was less than 0.05). Omnic (version 8.0, ThermoFisher, Waltham, MA) software was used to analyze the FTIR spectra of the biochar samples, while Origin (version 9.0, Originlab Co., Wellesley, MA, USA) software was used to process the FTIR figures.

RESULTS AND DISCUSSION

Physicochemical Properties of the Biochar Samples

The results in Table 1 showed that biochar yield decreased as the temperature increased. The yield decreased by 26.7% when the carbonization temperature was increased from 400 °C to 700 °C. At the same carbonization temperatures, the total biochar yields gradually decreased as the holding time increased (as shown in Table 2). No statistical difference (p<0.05) was observed in the yield change with a 2.5 h to 3 h holding time among all temperatures, except at 700 °C, which led to a reduction in the total yield. The highest yield (30.9%) was observed at 400 °C with a 1.5 h holding time, while the lowest yield (16.0%) was observed at 700 °C with a 3 h holding time (Table 2). This indicated that a lower holding time at a lower temperature resulted in a higher yield. The decrease in total biochar yield as the pyrolysis temperature and holding time were increased could be attributed to the breakdown of the basic primary structure of the biochar during pyrolysis. A decrease in the total biochar yield as the temperature increased had also been reported in oak, pine, sugarcane, and peanut shell biochar (Zhang et al. 2015); spent mushroom substrates biochar (Sarfraz et al. 2019; Zhao et al. 2019); pig manure (Gasco et al. 2018); and straw and lignosulfonate (Zhang et al. 2015). Previous reports had also shown that feedstock was partially combusted at lower pyrolysis temperatures, which resulted in a higher yield; while a higher temperature resulted in the complete combustion of the biomass (Angin 2013; Ghanim et al. 2016) as well as increased gasification (Colantoni et al. 2016; Li and Chen 2018), which resulted in a decrease in the total yield.

Investigating the effects of the pyrolysis temperature on the properties of the biochar showed that all the biochar samples were alkaline under all tested conditions, regardless of pyrolysis temperature and holding time (Tables 1 and 2). The alkaline properties of biochar had been previously been established by Mechler et al. (2018). Although the pH increased as the pyrolysis temperature was increased, the trend for the change in pH with respect to the holding time showed an irregular pattern, as there was variation among the evaluated holding times. However, the highest pH value for each temperature variation was observed with a holding time of 3 h, with the exception of 400 °C, where the highest pH value (9.7) was observed with a holding time of 2.5 h (Table 2). The interactive effect of the temperature and holding time yielded pH values that ranged from 9.6 to 10.1. The highest pH value (10.1) was obtained at 700 °C with a holding time of 3 h. The biochar produced at a lower carbonization temperature had a lower pH, because at this temperature, there was major amount of moisture loss, in addition to little volatilization of the organic constituents. However, as the pyrolysis temperature increased, there was an increase in the number of organic components in the raw material that volatilized as the proportion of the inorganic mineral component also increased, which resulted in a higher pH (Novak 2009). Similarly, a high pH at the peak pyrolysis temperature has been attributed to an increase in the ash content. The large number of cations, e.g., calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na), contained in the ash are directly correlated to the high biochar pH (Cao et al. 2009). This result showed that a higher pyrolysis temperature and maximum holding time (3 h) increased the pH of the biochar. Zhao et al. (2019) reported that the highest pH value among different spent mushroom substrates was obtained at a peak temperature of 700 °C. An increase in the biochar pH as the pyrolysis temperature was increased has been reported for crop residue-derived biochar (Keiluweit et al. 2010; Yuan et al. 2011; Mukherjee and Zimmerman 2013), sugarcane straw biochar (Melo et al. 2013), and poultry litter biochar (Song and Guo 2012). Therefore, the derived biochar can be used successfully as a soil amendment, especially for acidic soils, when produced under the aforementioned conditions (700 °C and a 3 h holding time).

The carbon content of the tobacco stem biomass was 312 g/kg (Table 3). However, as observed in Table 1, the carbon content of the biochar was enriched after pyrolysis, which ranged from 577 g kg-1 at a pyrolysis temperature of 400 °C up to 606 g kg-1 at a pyrolysis temperature of 600 °C, and subsequently decreased to 599 g kg-1 at a pyrolysis temperature of 700 °C. This occurs since pyrolysis is a process of carbon enrichment. Observations also showed that the maximum C content was obtained with a holding time of 2 h for the various temperatures evaluated (Table 2). The C content values obtained at a holding time of 2 h were 644.6 g kg-1, 645.2 g/kg, 651.7 g kg-1, and 647.8 g kg-1 for 400 °C, 500 °C, 600 °C, and 700 °C, respectively (as shown in Table 2). The maximum C content of the biochar (651.7 g kg-1) was obtained at 600 °C. The C content values obtained for the biochar samples were in the range of values obtained for the bamboo biochar in the study by Chen et al. (2016), the various crop residues, i.e., wheat straw, maize straw, rice straw and rice husk, in the study by Bian et al. (2016), and the biochar from other feedstocks in the study by Sun et al. (2014). At all tested pyrolysis temperatures, the C content increased at a holding time of 1.5 h to 2 h and decreased at a holding time of 3 h. This could be attributed to the fact that as the temperature increased and the biochar was held for an increased period of time, more organic matter was converted into ash and an amorphous form of carbon (CO2). The initial increase in the C content of the biochar samples as the temperature increased indicated that the degree of carbonization of the tobacco stem increased, which subsequently stabilized at the peak temperature. A decrease in the total biochar C content at the peak temperature had been previously been reported and was attributed to the loss of C-containing compounds at a higher pyrolysis temperature (Bergeron et al. 2013; Han et al. 2016; Wang et al. 2016; Zhao et al. 2019). During pyrolysis, the C-H and C-O bonds found in the biomass are broken, and the hydrogen (H) and oxygen (O) previously bound to the carbon are lost, in the form of gas or steam, and consequently the C content is increased (Mašek et al. 2013). Therefore, a high carbonization temperature (600 °C) and a moderate holding time of 2 h are suitable for the production of biochar that has a high carbon output requirement. The high C content of this biochar sample made it useful for the C sequestration of excess carbon from the atmosphere.

The ash content of the biochar increased with as the temperature increased (Table 1).

Table 1. The Main Effect of Pyrolysis Temperature on the Biochar Properties

Table 2. The Interaction Effect of Temperature and Holding Time on the Biochar Properties

Evaluating the effects of the total holding time on the ash content of the biochar samples showed that the ash content increased as the temperature and holding time increased, and was highest (34.9%) at a temperature of 700 °C and a holding time of 3 h (Table 2).

During the carbonization of the tobacco stem, a portion of the constituent internal mineral elements were converted into ash. A similar increase in the ash proportional content as the temperature increased had been reported in corn stalk and sawdust by Liu et al. (2014). Keiluweit et al. (2010) reported that biochar produced from herbaceous grasses had an ash content greater than 20% at a temperature of at least 400 °C and was statistically higher than the pine wood biochar (less than 4%). Ranges observed in this study were similar to those reported by Bian et al. (2016) for wheat straw, rice straw, maize straw, and rice husks. In addition, the total volatile matter (VM) content of the biochar decreased as the pyrolysis temperature was increased (Table 1). Similarly, it was observed that the VM content of the biochar decreased as the holding time increased. However, a slight increase in the VM content was observed at a pyrolysis temperature of 400 °C from 36.8% with a holding time of 2.5 h to 36.8% with a holding time of 3 h. The highest VM content (43.9%) was observed with a pyrolysis temperature of 400 °C and a holding time of 1.5 h, while the lowest VM content (21.4%) was obtained with a pyrolysis temperature of 600 °C and a holding time of 3 h (Table 2). A decrease in the VM content with an increase in the pyrolysis temperature had been previously documented by Li et al. (2018) for switchgrass, water oak, and biosolids. In addition, it was reported by Cantrell et al. (2012) that a greater amount of VM was removed while ash and fixed carbon were enriched at higher pyrolysis temperatures. The presence of cellulose and hemicellulose at lower temperatures had been linked to a higher amount of VM at lower temperatures (Jindo et al. 2014). At a pyrolysis temperature of 600 °C to 700 °C, there was no statistical difference in the VM content (as shown in Table 1), which indicated that most of the VM likely decomposed at a pyrolysis temperature of 600 °C. Consequently, no statistical decrease in the VM content was observed at pyrolysis temperatures greater than 600 °C (Li et al. 2018; Zhao et al. 2019).

The elemental composition of the biochar was influenced by both the temperature and the holding time (Tables 4 and 5). A pyrolysis temperature of 400 °C led to an N content (22.8 g kg−1) that was higher than the feedstock N content (20.5 g kg−1) (Tables 3 and 4). The values in Table 4 showed that the N content in the biochar decreased as the pyrolysis temperature increased. In addition, the alkaline N content increased as the pyrolysis temperature increased, although a slight decrease was observed at a pyrolysis temperature of 500 °C. However, there was an increasing trend for the alkaline N content as the total holding time was increased at a pyrolysis temperature of 500 °C and 600 °C, while it slightly decreased at a holding time of 3 h and a pyrolysis temperature of 400 °C (Table 5). The highest value of this fraction (51.1 mg/kg) was obtained at a pyrolysis temperature of 600 °C and a holding time of 3 h. However, variation in the holding time yielded an initial increase in the N content with a holding time of 1.5 h to 2 h, the N content value decreased with a holding time of 2.5 h, and, slightly increased with a holding time of 3h (Table 5). A maximum N content was observed with a holding time of 2 h across all tested pyrolysis temperatures. The highest N content (25.6 g kg−1) was obtained with a pyrolysis temperature of 400 °C and a holding time of 2 h. Previous studies have shown that the N content of biochar decreases with an increase in carbonization temperature or time (Yin 2014), because the NH4+-N, NO3-N, and N-containing volatile components in the biomass are lost at high temperatures (Xiao et al. 2018). Similar to the author’s observations, Zhao et al. (2015) showed that when the pyrolysis temperature was increased from 300 °C to 600 °C, the N content of the biochar, which was derived from apple twigs, first increased and then decreased, and obtained the highest N content value at a pyrolysis temperature of 400 °C.

As shown in Table 4, there was an increased amount of enrichment of the total P and K as the pyrolysis temperature increased, when compared to the total P and K content in the feedstock, 1.3 g kg−1 and 65.6 g kg−1, respectively (Table 3). Similarly, the amount of effective P and K increased as the pyrolysis temperature increased (Table 4). An increase in P and K with an increase in pyrolysis temperature had been previously reported by Ahmad et al. (2017). The total P value increased by 59.6 % from an average of 2.2% at a pyrolysis temperature of 400 °C to an average of 5.47% at a pyrolysis temperature of 700 °C. The data in Table 5 showed that the P-value increased consistently as the holding time increased from 1.5 h to 3 h across all tested pyrolytic temperatures. However, an irregular pattern emerged for the amount of effective P at a pyrolysis temperature of 500 °C and 700 °C. The highest P concentration (7.82 g kg−1) was obtained with a pyrolysis temperature of 600 °C and a holding time of 3 h. The amount of effective P was observed to follow a similar trend to the total P content in terms of the temperature and holding time, although its highest value was obtained with a pyrolysis temperature of 700 °C and a holding time of 3 h. The total potassium content of the biochar increased as the temperature increased from 400 °C to 600 °C and then decreased at a pyrolysis temperature of 700 °C (Table 4). In addition, Table 4 showed an increase in the K content as the holding time increased from 500 °C and 600 °C. However, at a pyrolysis temperature of 400 °C, there was a slight decrease in the K content at a holding time of 2.5 h (117.5 g kg−1) from a holding time of 2 h (119.3 g kg−1). This was also observed at a pyrolysis temperature of 700 °C, where the K content values decreased from 134.9 g/kg at a holding time of 2.5 h to 131.5 g kg−1 at a holding time of 3 h. The highest K content (167.8 g kg−1) was observed with a pyrolysis temperature of 600 °C and a holding time of 3 h (as shown in Table 5). The K content values of the tobacco stem biochar samples ranged from 88.4 g kg−1 to 167.8 g kg−1. Previous studies had reported a wide range of N, P, and K concentrations in biochars, although plant-derived biochars contained a relatively lower amount of nutrient elements than those derived from manure and food wastes (Cantrell et al. 2012). In a meta-analysis by Chan and Xu (2009), the reported concentrations of total N contents that ranged from 1.8 g kg−1 to 56.4 g kg−1, total P contents that ranged from 2.7 g kg−1 to 480 g kg−1, and total K contents that ranged from 1.0 g kg−1 to 58 g kg−1. In addition, a wide range of values had been obtained in various crop residues. The total N content ranged from 4.3 g kg−1 (coconut shell biochar) to 47.8 g kg−1 (cotton stalk biochar). The total P content of the various biochars ranged from less than 0.1 g kg−1 for palm shell and coconut shell biochars to 4.2 g kg−1 for olive pomace biochar, while the total K content ranged from 0.6 g/kg for palm shell biochar to 60 g/kg for wheat straw biochar (Windeatt et al. 2014). In crop residue biochars (wheat straw, rice straw, maize straw, and rice husk), the N content ranged from 8.47 g kg−1 to 16.1 g kg−1, the P content ranged from 1.67 g kg−1 to 4.43 g kg−1, and the K content ranged from 12.9 g kg−1 to 32.0 g kg−1 (Bian et al. 2016). Similar ranges had been reported in spent mushroom substrate biochars (Sarfraz et al. 2019; Zhao et al. 2019). A meta-analysis by Ippolito et al. (2015) reported an average total concentration of 0.9 g kg−1 to 32.8 g kg−1 for N, an average total concentration of 0.32 g kg−1 to 60.8 g kg−1 for P, and an average total concentration of 0.7 g kg−1 to 116 g kg−1 for K, for a wide range of biochar materials. The range of the values obtained from this study showed that the total N, P, and K content values found in the tobacco stem biochar were, on average, higher than most ranges reported in other studies. This indicated that the tobacco plant took up higher nutrients from the soil. Therefore, this biochar will be useful as a fertilizer either by itself or as part of a biochar compound fertilizer, which can be re-applied into the soil after harvest to return back most of the soil nutrients taken up, so to maximize its benefit.

Table 3. Elemental Composition in the Tobacco Stem Feedstock

Table 4. The Main Effect of Pyrolysis Temperature on the Elemental Composition in the Biochar Samples

Investigating the effects of the pyrolysis temperature on the heavy metal content indicated that the concentrations of Ni and Cd in the biochar increased after the biomass underwent pyrolysis (Table 7). The amount of Ni and Cd found in tobacco stems was lower in comparison to the other heavy metals found (Table 6). However, the amount of Zn found in the biochar was reduced after undergoing pyrolysis, and the amount of Cu decreased at all pyrolysis temperatures, excluding a pyrolysis temperature of 700 °C, in which the amount of Cu increased. An irregular trend occurred for the Pb concentration as the pyrolysis temperature increased. The highest heavy metal concentrations were observed at the peak temperature (700 °C), except for Cd, which was highest at a pyrolysis temperature 400 °C. The effects of the holding time on the heavy metals concentration in the biochar were shown in Table 8, and the Ni concentration ranged from 1.4 mg kg−1 at a pyrolysis temperature of 400 °C and a holding time of 1.5 h to 13.3 mg kg−1 at a pyrolysis temperature of 500 °C and a holding time of 2 h. The Cu concentration ranged from 4.4 mg kg−1 at a pyrolysis temperature of 400 °C and a holding time of 1.5 h to 20.9 mg kg−1 at a pyrolysis temperature of 500 °C and a holding time of 2 h. The concentration values for Zn ranged from 14.5 mg kg−1 at a pyrolysis temperature of 600 °C and a holding time of 3 h to 104.4 mg kg−1 at a pyrolysis temperature of 700 °C and a holding time of 2 h.

Table 5. The Interaction Effect of Temperature and Holding Time on the Elemental Composition in the Biochar Samples

Table 6. The Heavy Metal Concentrations in the Tobacco Stems

The Cd concentration ranged from 0.2 mg kg−1 at a pyrolysis temperature of 500 °C and a holding time of 1.5 h to 1.7 mg kg−1 at a pyrolysis temperature of 600 °C and a holding time of 3 h. In addition, the Pb concentration ranged from 1.1 mg kg−1 at a pyrolysis temperature of 500 °C and a holding time of 1.5 h to 7.9 mg kg−1 at a pyrolysis temperature of 500 °C and a holding time of 2.5 h.

Table 7. The Main Effect of Pyrolysis Temperature on the Concentrations of Heavy Metals in the Biochar Samples

Table 8. The Interaction Effect of Temperature and Holding time on the Concentrations of Heavy Metals in the Biochar Samples

The European Biochar Foundation (EBF) (2017) stated a safe range for the following heavy metal concentrations in biochar: the Pb concentration should be less than 150 mg kg−1; the Cd concentration should be less than 1.5 mg kg−1; the Cu concentration should be less than 100 mg kg−1; the Ni concentration should be less than 50 mg kg−1; and the Zn concentration should be less than 400 mg kg−1. According to the values given above, the Cd concentration (1.71 mg/kg) at a pyrolysis temperature of 600 °C and a holding time of 3 h was higher than the given threshold (less than 1.5 mg kg−1). However, the other heavy metal concentrations were well below the thresholds required for the safe usage of the biochar in the environment, and therefore, could be applied to the soil without environmental concerns. Previous studies have shown that the concentration of heavy metals can be greater in the biochar than in the feedstock (Sarfraz et al. 2019; Zhao et al. 2019). However, biochar is able to effectively bind and immobilize heavy metals for a long period of time, but the length of time is yet to be determined (EBF 2017). Since the amount of biochar used for agricultural purposes is relatively low, any toxic accumulation could be ruled out, particularly when the concentration thresholds are slightly higher than stated by the EBF.

Microstructure and Surface Properties of the Biochar Samples

The surface morphology of the biochar samples at 400 ℃, 500 ℃, 600 ℃, and 700 ℃ (all at a holding time of 2 h) were determined via scanning electron microscopy (SEM) images, which indicated their porous structure (as shown in Fig. 6a through Fig. 6d). Uzun et al. (2010) reported that during pyrolysis, the porosity of the feedstock increased, due to the discharge of the volatile compounds and the chemical reactions that occurred between the volatile compounds, minerals, and inorganic compounds present in the feedstock, which was similar to the observations the authors noticed in the SEM images. At higher temperatures (600 ℃ and 700 ℃), there was an increase in the surface area of the biochar samples. An increased surface area implied a greater number of porous structures within the biochar (Inyang et al. 2010; Kloss et al. 2014).

The pores found in the biochar samples could be divided into micropores (less than 0.8 nm), small pores (0.8 nm to 2 nm), mesopores (2 nm to 50 nm), and macropores (greater than 50 nm) (Chia et al. 2015). The range of values obtained showed that all the pores found in the biochar samples were mesopores (Fig. 1b). The surface area increased as the pyrolysis temperature increased from 400 ℃ to 700 ℃ (Fig. 1a), while the average pore diameter decreased as the pyrolysis temperature increased (as shown in Fig. 1b). However, when the holding time was varied, the biochar samples produced with a holding time of 1.5 h had macropores at all tested temperatures, i.e., 400 °C (53.5 Å), 500 °C (52.5 Å), and 600 °C (52.5 Å). The improved porosity of the biochar samples had been reported by Joseph et al. (2010) to play an important role in soil aeration and moisture retention when applied as a soil amendment. However, after pyrolysis, there was a decrease in the pore diameter of the biochar samples as the temperature increased (as shown in Fig. 1b). In addition, the effects of variation in the total holding time on the pore diameter were shown in Table 9. Consequently, the surface and micropore areas of the biochar samples increased as the temperature and holding time increased (Fig. 1 and Table 9). The highest surface area of the biochar samples (50.6 m2 g-1) and micropore areas (57.1 m2 g-1) were obtained with a pyrolysis temperature of 700 °C and a holding time of 3 h. The decrease in the average pore size of the biochar samples as the temperature increased could be attributed to the gradual development of mesopores on the surface of the biochar samples and the appearance of micropores, or the smaller pores becoming clogged. An increase in the surface area and micropore diameter as the pyrolysis temperature increased had been previously reported for rice husk and cotton straw biochar (Jia et al. 2018), crop residues (Bian et al. 2016), and hardwood (Sun et al. 2014; Domingues et al. 2017). Surface area is a key factor that regulates the ability of a material to adsorb chemical compounds, and a larger surface area implies a greater number of porous structures within the biochar (Inyang et al. 2010; Kloss et al. 2014). The application of the experimental biochar to soil could enhance its interaction with the plant roots, soil microorganisms, and minerals to promote soil properties, therefore improving the soil and plant productivity (Joseph et al. 2010; Liu et al. 2013). The tobacco biochar produced with a pyrolysis temperature of 700 °C and a holding time of 3 h would be ideal for nutrient retention as a slow-release fertilizer and could be useful for adsorption of contaminants in soil or water.

Fig. 1. Main effect of pyrolysis temperature on the BET surface area and porosity of the biochar samples. Error bars indicate standard error of the mean (n=3)

Fourier Transform Infrared Spectrometry Analysis of the Biochar Samples

Figures 2 through 5 show the FTIR peaks appearing from 400 cm-1 to 4000 cm-1 and showed the change in the chemical bonds as the pyrolysis temperature was increased with various holding times. The weak O-H group present in all of the biochar samples that was found between 3400 cm-1 and 3700 cm-1 could be the result of acid and/or alcohol structures (Wu et al. 2012). The adsorption peaks that represented the C=O and C=C stretching, and the aromatic C-H appeared between 1400 cm-1 and 1600 cm-1. The aromatic structures (C=C) found in the FTIR spectra at holding times of 1.5 h and 3 h and the C-H bonds found in all the holding times were gradually lost as the pyrolysis temperature increased from 400 °C to 700 °C.

Table 9. Interaction Effect of Temperature and Holding Time on the BET Surface Area and Porosity of the Biochar Samples

Fig. 2. FTIR spectra of biochar samples retained for 1.5 h at different temperatures

Fig. 3. FTIR spectra of biochar samples retained for 2 h at different temperatures

Fig. 4. FTIR spectra of biochar samples retained for 2.5 h at different temperatures

Fig. 5. FTIR spectra of biochar samples retained for 3 h at different temperatures

The disappearance of these functional groups at higher temperatures resulted in an increased mass loss and thus, a reduced biochar yield (Zhao et al. 2019). The weakening of the peaks as the pyrolysis temperature increased had been previously reported by Jin et al. (2016). The CO2 peak was present with a high intensity for all the biochar samples, regardless of temperature and holding time. The aliphatic and aromatic C compounds observed in the biochar samples signified their high C content. Adsorption peaks that represent various functional groups are often observed in biochar (Domingues et al. 2017). The presence of reactive O-containing functional groups on the surface of the biochar samples, as well as carboxylic or hydroxylic groups, showed its ability to interact with polar organic compounds (Brodowsky et al. 2005). Generally, the cation exchange capacity of the soil amended with biochar can be improved, due to the presence of oxidized functional groups on the surface of the biochar (Liang et al. 2006). Therefore, the tobacco stem biochar samples with oxygen functional groups could be suitable for soil amendment, in order to improve the physical property and chemical properties of the soil.