The quantity of biogas residues is continually increasing and causing a great threat to the environmental protection and sustainable development of bio-natural gas engineering. This study concentrated on evaluating the feasibility of biochar production using various biogas residues. The feedstock of the anaerobic digestion was sewage sludge (SR), spent mushroom compost (MR), wheat straw (STR) and distillers’ grains (DL). Biogas residue collected from the above biogas plants was applied to prepare biochars at different temperatures (300°C, 500°C and 700°C). The results showed that the biochar yield ranged from 45.13–66.42% at 300°C, 34.13–51.50% at 500°C, and 30.33–47.06% at 700°C. The C content in STR- and DL-biochar increased under higher temperatures; this was due to the loss of –OH functional surface groups as a result of dehydration and the decomposition of nitrogen-functional groups. With increasing temperature, the P and K contents, pH, specific surface area (SSA) and pore volume increased. The functional groups of the STR-biochar decreased significantly, and the aromatic compounds were decomposed. The biochar (except SR-biochar) structures were more stable under high temperature. The morphological structure of DL-biochar at 700°C was the most stable. The SSA at 700°C increased 11.98- to 33.43-fold compared with that at 300°C. The total pore volume at 700°C increased 2.22 to 10.25 times that at 300°C. Overall, based on the principal component analysis, the biochar derived from wheat straw biogas residue exhibited the best performance regarding its physicochemical properties.
Research Article
Biochars prepared from biogas residues: Temperature is a crucial factor that determines their physicochemical properties
https://doi.org/10.21203/rs.3.rs-1511736/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 22 Aug, 2022
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Biogas residue
Biochar
Pyrolysis temperature
Yield
Physicochemical properties
In China, large amounts of crop straw (~ 650 million tons) and municipal sludge (~ 80 million tons) are generated annually (Chadwick et al. 2015; Ning et al. 2021). Different types of waste, such as animal manure, sewage sludge, and agricultural waste, are used efficiently via anaerobic digestion, an environmentally friendly method of energy production (Magdalena et al. 2021; Wang et al. 2021). However, the production of biogas is accompanied by many by-products such as biogas residues. Returning these biogas residues to the field to increase soil fertility, enhance water holding capacities, and change microbial community structures is the main method of utilizing biogas residues at present (Ning et al. 2021) because of its richness in C, N, P, K, and other nutrients (Tambone et al. 2010). However, some studies indicated that the odour emission, pathogens, toxic organic compounds, phytotoxicity, and heavy metals of biogas residues might cause biogas residues to have a high potential of threatening the soil environment and harming soil bacteria (Nkoa 2014; Zheng et al. 2021). Therefore, the treatment and utilization methods of biogas residue still need to be considered.
Regarding environmental issues, the utilization of biochar may offer an amendment, and it has been a catalyst for recent global enthusiasm (Novak et al. 2016). Converting biogas residue into biochar can sequester carbon and retain most mineral nutrients (Campos et al. 2020). Biochar is a renewable resource and a predominantly stable black carbon that is produced under the partial or total absence of oxygen environment with a pyrolysis temperature range of 400–700°C (Irfan et al. 2016; Xing et al. 2018). This causes biochar to contain amounts of aromatic carbon that are so stable that they can protect against microbial degradation (Liu et al. 2019). Furthermore, as biochar structures have a high porosity, they can adsorb many extractable humic-like and fluvic-like substances (Lin et al. 2012), and the functional groups on the surface of biochar can effectively chelate heavy metals and other ions (Sun et al. 2014). The higher surface area of biochars makes them useful adsorbents for water, NH4+ and other contaminants (Magdalena et al. 2021; Liu et al. 2016). Thus, biogas residue biochar has better application potential in soil amendments than that of biogas residue. However, the pyrolysis technology and product characteristics of different types of biogas residue biochar have not been systematically studied. What kind of biogas residue is more beneficial as a soil amendment remains to be confirmed.
Pyrolysis temperature and feedstock are generally considered the main factors when designing a specialized biochar for use in soil amendments (Zhao et al. 2013; Muhammad et al. 2020). Studies have shown that pyrolysis temperature has significant impacts on the properties of biochar (Irfan et al. 2016; Xing et al. 2018). The pH of biochar increases with the biochar pyrolysis temperature, and > 80% labile fraction carbon is transformed into recalcitrant aromatic carbon at 400–600°C (Zhang et al. 2017). Most of the pore volume, porosity, and BET surface area of biochar build under pyrolysis temperatures, and pores develop via the discharge of volatile organics at high temperatures (Kavitha et al. 2018). However, a high pyrolysis temperature (> 500°C) also promotes the partial decomposition of the organic compounds in biomass into small and highly reactive free radicals, which can form more stable polycyclic aromatic hydrocarbons (PAHs) through superheated synthesis (Qiu et al. 2015; Brown et al. 2006). Feedstock types are another important factor that affects the properties of biochar (Antal et al. 2003; Reza et al. 2020). The BET surface area, pore volume, and distribution of elements in biochars are generally determined by the organic structure of the raw material itself. Thermal treatment to produce biochars or prepare modified biochars from straw, pig manure, and chicken manure biogas residues is a very effective tool for biowaste management, and biochar obtained specifically from sewage sludge has the highest excess of fulvic acids, 93–96 mg g-1 (Zheng et al. 2018; Magdalena et al. 2021; Wang et al. 2021). Therefore, when converting biogas residue into biochar, it is necessary to understand the physicochemical properties of the carbonization products of various biogas residues at different pyrolysis temperatures. This will help us choose suitable biochar products according to local soil conditions.
Hence, this study was conducted to characterize four types of residues including sludge residue (SR), mushroom biogas residue (MR), straw residue (STR) and distillery lees (DL) biomass-derived biochar under different pyrolysis temperatures including 300, 500 and 700°C. Chemical, physical, and carbon analyses were carried out to describe the actual compositions. The internal fractures, phase transitions, and porosity present in the biochar surface, functional groups and chemical composition were verified via TGA and FT-IR analysis. The results of this study will help to design biogas residue biochar for applications in soil amendment, environmental improvement, and other industrial fields. The specific objectives of this study were to produce and judge the physical, chemical, elemental and nutrient compositions that could be used as predictors to predict their suitability in wider applications.
Biogas residue collection and biochar preparation
Four different biogas residues, the solid fraction of digestate, were collected from four different biogas plants that are located in Qingdao, Shandong Province, China. The main feedbacks of the four biogas reactors were sewage sludge (SR), spent mushroom compost (MR), wheat straw (STR) and distillers’ grains (DL). Their operation temperatures were between 35 and 37°C with a 40-day retention time. The biogas residue samples were then mixed, washed with deionized water, oven-dried (105°C, 24 h) and then prepared with sizes between 0.5-1.0 cm.
The detailed progress of biochar production was as follows: 1) pyrolysis temperatures of 300°C, 500°C and 700°C; 2) heating rate of 5°C min− 1; 3) residence time of 2 h; and 4) inert gas of N2 (99.999%) with a 0.5 dm3 min− 1 flow rate. Thereafter, the prepared biochar was crushed and sieved to < 0.25 mm.
Biochar yield
The percentage of different biochar yields (Y) was calculated using the equation as follows:
where Y is the mass yield of biochar, %; m (biochar) is the mass of biochar, kg; and m (raw) is the mass of raw biomass, kg.
The produced biochar from different biogas residues is denoted by sludge residue (SR), mushroom biogas residue (MR), straw residue (STR) and distillery lees (DL). The different temperatures for each produced biochar are denoted by subscripts (B300, B500 and B700).
Analysis of chemical properties
The pH and EC of the suspensions were measured by a digital pH and EC metre (FE38-FiveEasyPlus™, Mettler-Toledo, Switzerland) at a 1:10 (w/v) ratio. The elemental amounts of carbon (C), nitrogen (N) and sulfur (S) were determined using an element analyser (Vario EL cube, Germany). Phosphorus (P) and potassium (K) were determined by a sulfuric acid digestion spectrophotometer. During the determination of dissolved organic carbon (DOC) content, biochar and deionized water were stirred under a magnetic stirrer at a certain ratio (w:v = 1:10) at 400 rpm for 1 hour and then filtered with a 0.45 µm filter membrane. The total organic carbon (TOC) content of the biochar was determined by a Multi N/C-3100 (Jena Analytik, Germany).
BET surface area analysis
The BET surface area of different biochar samples was obtained using the Brunauer–Emmett–Teller method from the nitrogen adsorption-desorption isotherms at 77°K using a Micromeritics ASAP-2420 (Micromeritics Instruments, Norcross, GA). Biochar samples weighing between 0.2 and 0.5 g were vacuum degassed at 300°C for 14 h prior to analysis. BET surface area output was obtained through the Micromeritics ASAP-2420 automated software using the BET (Brunauer, Emmettand Teller) equation. The total pore volumes (TPV) of the biochar samples were estimated by converting the adsorbed quantity (g) into liquid N2-volume at atmospheric pressure close to saturation. Micropore volume (MPV, < 2.0 nm pore diameter) along with their surface area were calculated following the t-plot technique, and we plotted the amounts adsorbed on the porous biochar against the respective multilayer thickness (Hung et al., 2017). The average pore diameter (APD) of different biochar samples was calculated as [4 × Vt/SBET], in which Vt denoted the total pore volume and SBET denoted the BET surface area. Here, we assumed that all the pores were cylindrical, straight and not interconnected.
Fourier transform infra-red and thermogravimetric analysis
Biochar was characterized by FTIR. The spectra were obtained in an FTIR spectrophotometer using an attenuated total reflectance (ATR) module (Shimadzu, IRaffinity-1, Japan) with nitrogen purge. FTIR measurements were recorded in the range of 400–4000 cm− 1 at 2.0 resolution, and interferograms of 1024 scans were averaged for the spectrum.
Thermal characterization of biochar was carried out by thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG). Non-isothermal TG analysis was performed using a Pyris 1 TGA. The analyses were carried out under a 20 cm3 min− 1 N2 flow at a heating rate of 10°C min− 1 from 30°C to 800°C. Approximately 1.5 mg of sample was used for each experiment. The mass loss (TG) and derivative curves (DTG) of the samples were represented as a function of temperature so that the pyrolysis temperatures could be selected.
Scanning electron microscopy and mineral fraction analysis
The surface morphology of the biochar was observed by scanning electron microscopy (SEM) (Hitachi, SU-70, Japan) at 500× magnification. SEM mapping analysis (EDS Aztec X-Max 80, Oxford Instruments, Britain) was used to identify metals in the samples with atomic numbers in the range from oxygen to molybdenum (O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Fe, Mo) and to measure these element contents.
Crystallographic structure analysis
The existence and crystalline form of structures present in residues from biogas production and biochar samples was checked using an Empyrean (PAN alytical, 2012) X-ray diffractometer. XRD measurements were conducted using standard powder diffraction procedures.
Statistical analysis
Principal component analysis (PCA) and clustering of variables were carried out using CANOCO5 (CANOCO, Microcomputer Power Inc., Ithaca, NY, USA). PCA was employed for factor mapping and clustering of variables and quality of representation of the variables in a given principal component.
Effect of pyrolysis temperature on biochar yield
Depending on the temperature, the yield ranged from 45.13–66.42% at 300°C, from 34.13–51.50% at 500°C, and from 30.33–47.06% at 700°C. The pyrolysis yield was SR > MR > DL > STR, which decreased gradually with increasing temperature. There were 23.1% and 30.7% declines in yield at 500°C and 700°C, respectively, compared with that at 300°C. Figure 1a and 1b further indicate that the quantity of yield loss increased with increasing temperature during pyrolysis. The decreasing rate of biochar yield varied from 0.0377 to 0.048%/°C with increasing temperature. This was a general phenomenon (Zhang et al. 2020; Swapna et al. 2021) in which biochar yields decreased consistently with increasing temperature. An important reason was that the vigorous release of volatile matter formed more aromatic compounds and thermal breakdown of biomass at 600°C (Zhang et al. 2017; Swapna et al. 2021).
At higher temperatures, the loss of chemically bound moisture, decomposition of organic substances and dehydration of hydroxyl groups of the thermal cellulose structure could also be caused by the formation of aromatic structures, and the reduction in yield might be due to the loss of large amounts of CO2, CO, H2O, and H2 (McHenry et al. 2009). The biochar yield and reduction trend in biochar yield that were observed agreed with the findings of Hernandez-Mena et al. (2014) and Swapna et al. (2021). The inorganic composition in biomass accumulated with unstable compound degradation (as shown in SEM–EDS) and produced more biochar at higher pyrolysis temperatures (Palamanit et al. 2019; Ahmed et al. 2020).
Chemical characterization of biochar
Elemental composition
Differences in element composition were observed between the biochars depending on the type of pyrolyzed materials. The carbon (C) content in all the biochars ranged from 19.05–58.57%. The average C content was DLBs (54.77%) > STRBs (48.19%) > MRBs (35.62%) > SRBs (24.23%), as shown in Table 1. The content of C in SRBs decreased with high temperature from 27.60–19.05%. In the MRBs, no significant difference in C content was observed with temperature.
However, in DLBs and STRBs, the C content increased with temperature. The sulfur (S) contents in the SRB, MRB, and STRB treatments at 500°C and the DLB treatment at 700°C were lower than those in the other treatments. A slight or no increase in C content during pyrolysis is explained by the simultaneous high content of mineral components or mass losses in a particular biochar (Al-Wabel et al. 2013; Stefaniuk et al. 2015). This indicated that the C content was greatly affected by temperature and biochar raw materials. The nitrogen (N) content in all biochars and S content in DLBs decreased with temperature. There were 81.3%, 50.3%, 38.4% and 61.3% losses in N of SRB700, MRB700, STRB700 and DLB700 compared with that of the biochars at 300°C. This is also explained by the changes in functional groups (FTIR) and the removal of nitrogen in the form of NOx and NH3 during the generation of waste gases (Stefaniuk et al. 2015).
Organic carbon content
The TOC content reflects the carbon sequestration capacity of different types of biochar to a certain extent. In our study (Table 1), the TOC content of biochar from different biogas residues varied slightly at 3 g kg− 1. The TOC content of SRB and DLB was the highest at 500°C, but in the MRB and STRB, it was highest at 300°C. This showed that high-temperature pyrolysis (~ 700°C) was not conducive to the carbon sequestration of the four biogas residue biochars. The biochar DOC played a critical role in the transport of contaminants in the soil and was a very important parameter for assessing the soil organic carbon dynamics. The DOC content of MRBs was the highest from 2.69–3.05 g kg− 1, while the DOC content of the other three biogas residue biochars was no more than 3.00 g kg− 1. The DOC content of SRBs increased with increasing pyrolysis temperature. However, in the other types of biochar, the DOC was lowest at 500°C. This consequence was different from the study of Das et al. (2021), who considered that the DOC of biochar decreased significantly with increasing pyrolysis temperature and that they were low at 600°C, followed by 500°C and high at 400°C. This was related to the types of biogas residues. Mushroom residue contains many proteins, cellulose and amino acids (Li et al. 2020), which easily decompose to form liable organic carbon (e.g., DOC). Although sludge also contains many carbohydrates, proteins, fats and other substances, the proportion of conversion to DOC may be reduced due to the wrapping effect of clay minerals (Aggarwal and Hakovirta 2021). Studies also showed that feedstocks with a high lignin content produced lower amounts of DOC, mainly because they can better form solid biochar than liquid bio-oil, which may be an important reason why straw residue and distillery lees have lower DOC contents than those of mushroom residues (Das et al. 2021). Therefore, because biochar exhibits a high stable organic carbon content or a high dissolved organic carbon content, it can be utilized to improve the soil according to the current obstacles involved in soil.
EC, pH, and zeta potential
The investigated biochars were characterized by a wide range of salinities from 4.93 to 19.84 mS cm− 1 (Table 1). Significant differences were observed. In SRBs, the highest EC was found at 700°C. In MRBs and DLBs, the EC was under 500°C and 700°C and was higher than that at 300°C, respectively, in which the maximum EC values were found at the temperature of 500°C. This may be due to the amount of soluble salts and volatilization of heavy metals (e.g., Zn) with low melting points in raw materials (Wang et al. 2018). A decrease in EC with increasing pyrolysis temperature was recorded in the STRBs. In addition, EC values increased with increasing pyrolysis temperature, resulting from the loss of volatile material and from the concentration of the elements that increased the salinity (Cantrell et al. 2012). Our study indicated that EC differed with the types of biogas residue.
The biochars were characterized by a pH ranged from 7.05 to 10.30 (Table 1). Pyrolysis resulted in an increase in the pH of the biochars produced. With increasing pyrolysis temperatures, the value of this parameter was also found to increase. Under a 500°C pyrolysis temperature, the value of the pH was highest in STRBs.
The increasing pH value was mainly due to the decomposition and volatilization of nitrogen and sulfur compounds at 700°C (Table 1) and the high content of alkali elements (Mg and Ca in particular) at high temperatures of 500°C and 700°C, which was confirmed by further investigation using SEM–EDS. Some studies also indicated that the increase in pH could have been caused by the concentration of basic inorganic components in the biochars and elimination of organic materials (Al-Wabel et al. 2013). Acidic functional groups such as carboxylic acid from the biomass matrix were eliminated and led to an increased concentration of the basic functional groups; hence, the derived biochars were found to be alkaline. Moreover, calcium, magnesium and silican increased with pyrolysis temperature from 500 to 700°C, which contributed to the alkaline nature of the biochars (Yuan et al. 2011).
Physical characterization of biochar
The BET surface area (BET), total pore volume (TPV) and average pore diameter (APD) of the biochar samples are presented in Table 2. The BET of biochar was found to be 2.44 m2 g− 1-6.63 m2 g− 1 at 300°C and 500°C and greatly increased at 700°C. Biochar was obtained with a higher surface area at a pyrolysis temperature of 600°C than at 400°C and 500°C (Swapna et al. 2021). The highest BET value was observed in the STRB, i.e., 112.85 m2 g− 1 at a temperature of 700°C, which was 26.3 times higher than that in STRB300. The TPV increased with increasing temperature. Straw biogas residue contains more lignin, cellulose and hemicelluloses. The high aromaticity via the thermal decomposition of lignocelluloses and the volatilization of inorganic minerals was useful for generating pores (Hung et al. 2017).
The BET surface area and TPV of both biomass-derived biochars increased with temperature from 300 to 700°C, mainly due to thermal degradation of biomass organic matter at higher temperatures (see supporting information). It released more volatile matter and pore-blocking substances from the biomass matrix, hence creating more pores and channel-like structures in the biochar samples.
Biochar that was prepared at high temperature contained many micropores and mesopores, and the pore diameter was mainly distributed between 1–2 nm (see supporting information). The mean pore diameter of all the biochar samples decreased from 1.350 to 1.176 nm with increasing pyrolysis temperature except for the STRBs. This was due to the evolution of micropores in the biochar samples with high temperature. These results were in agreement with the results obtained by Zhu et al. (2018), Wu et al. (2019) and Tomczyk et al. (2020). The pore diameter increased with temperature in the STRBs. This indicated that more pores were evaluated at high temperature because of the decomposition of lignin, cellulose and hemicellulose in straw biogas residue.
Greater adsorption and desorption of N2 by biochars were observed at higher temperatures. The specific surface area and pore volume of biochar prepared at high temperature are large (confirmed by Table 2). It is clear from Fig. 2 that temperature significantly affects the structure of micros. Relative pressure (P/Po) < 0.7, N2 adsorption and desorption curves overlapped, which indicated that the micros were airtight holes closed at one end, especially in SRB300 and SRB500 (Zhang 2017). At higher relative pressures (P/Po > 0.7), the N2 adsorption and desorption curves were obviously separated, indicating that open cylinder holes were present in the biochars, especially in MRB300, MRB500, DLB300, and DLB500. However, at a relative pressure < 0.1, the adsorption curves tended to the Y axis, indicating that slit apertures existed in SRB700 and STRB700. Moreover, under SRB700 and MRB700, the inflection point in desorption curves indicated that open holes and “inkstand shape” holes were present in these biochars (Zhang 2015). The isotherms of SRB700, MRB700 and STRB700 did not close at low pressure due to the capillary condensation phenomenon that occurs at low pressure, as some micropores and minor mesopores exit (Xia et al. 2016).
FTIR spectroscopy analysis
FTIR analysis was used to verify the effect of pyrolysis temperature on the surface functional groups of biochars. In SRBs, MRBs and STRBs, the functional groups of -CH2 (aliphatic carbon compounds) disappeared at 700°C. A similar phenomenon was observed at peaks of 1576–1630 cm− 1 (aromatic compounds). This indicated that aliphatic carbon compounds decomposed at 700°C. In straw biochars, the functional groups of STRB500 were more abundant than those in STRB300 and STRB700. The strength of the functional groups in SRB500 and MRB500 decreased sharply compared with the biochars at temperatures of 300 and 700°C. The functional group structure of DLBs is relatively stable. The strength of the functional groups of biochar prepared at high temperature is weakened, but the type of functional groups does not change. Various bands in the spectra represented vibrations of functional groups in biochars, e.g., -OH (3000–3690 cm− 1) in alcohol groups present in all biochars, as shown in Table 3.
The detected peaks in the range of wavenumbers from 2960 to 2850 cm− 1 were attributed to the C-H bond stretching in aliphatic formation, which is an indication of the presence of cellulose, hemicellulose, and lignin in the precursor. Peaks from 1610 to 1590 cm− 1 indicate C = C stretching in hemicelluloses. In our study, aliphatic formation and hemicelluloses decomposed at 700°C in MRB and STRB. The peaks in the area of 1480 − 1410 cm− 1 are attributed to stretching of C-H deformation in cellulose and hemicellulose, which is stable in SRBs, MRBs and STRBs. The significant peaks that occurred in the wavenumber range of 1120 − 1050 cm− 1 indicated C-O stretching in cellulose and hemicelluloses, and this differed with the types of residues. The C = C stretching of alkene vinylidene (peaks between 895 and 880 cm− 1) and Si-O (1038 cm− 1) groups were found in SRBs.
The results indicated that the pyrolysis temperature had a significant impact on the functional groups of biochars. In all biochars, several peaks disappeared or weakened during the process of high-temperature pyrolysis. Except for straw residue biochars, other biochars had many functional groups with large strengths and abundant types at low temperature (see supporting information). The biochars derived had high volatile matter at a lower pyrolysis temperature of 300°C, mainly due to the presence of functional groups of C-O and C-H and incomplete carbonization. These results were in agreement with the findings of Muigai et al. (2021).
Morphological characterization
Great differences between these biochar samples were observed in the SEM images (Fig. 3, upper right). Overall, the pictures showed that the porosity increased from volatiles escaping during thermochemical degradation with increasing pyrolysis temperature. At low temperatures (300°C), the pores were not sufficiently developed in biochars. Biochar images at 500 × magnification showed the major characteristics of the physical structure of biochars; however, no obvious pores were found under higher temperatures. This indicated that biochar at high temperature had more micropores and mesopores (similar to the supporting information and Table 2). However, at 700°C, the SRBs were flocculated, while the structure in the biochars of MRBs, STRBs and DLBs became more ordered, which was in agreement with the study of Claoston et al. (2014). The figures show that the structure of biochar retained the major characteristics of the physical structure of the original feedstock.
The EDS spectrum on the surface of the biochars (Fig. 3, left side) identified the major elements (including C, O, P, Ca, Si, Mg, Fe, and Al). The types of metal elements in SRBs and MRBs were abundant. The relative percentages of Mg, Si, Ca and Fe in SRBs increased gradually with high temperature. In the MRBs, the Fe percentage decreased, while the Mg and Si percentages increased gradually with increasing temperature. The Mg and Ca contents of STRB500 were higher than those at 300°C and 700°C. This could also explain why STRB500 had the highest pH (as shown in Table 1). Except for C, O, K and P, only Si was detected in the DLBs, and the silicon content decreased with increasing temperature. From the SEM–EDS observation, it still suggested that the STRB-500 had potential for the use of high pH in soil acidification improvement.
Crystallographic structures
XRD analysis was used to determine the presence of different crystalline materials that influence biochar properties for its applications. The peak value of biochar was scattered under high temperature (see supporting information). The peak at 22.6°2θ was a crystallographic plane of cellulose (Swapna et al., 2021), which was observed in DLBs from 300°C to 700°C. This implied that the crystalline structure of cellulose was stable in DLBs. Except for the DLBs, the peak quantity of other biochars was greater at 500°C. The peak of SRB500 occurred at 26.7°2θ. The main peak values of MRB500 mainly appeared at 26.7°2θ, 28.0°2θ, 28.5°2θ, 29.48° and 31.04°2θ, 40.6°2θ (KCl). The main peak values of STRB500 were mainly observed at 26.7°2θ, 28.5°2θ, and 40.6°2θ (KCl). This indicated that there were aromatic structures and fat structures observed in biochars at a temperature of 500°C. The peaks at 28.57° and 40.65° indicated the crystalline phase of sylvite (KCl) in biochars prepared from MR and STR at 500°C. The presence of calcite was verified by the peaks at 29.48° and 31.04°, which were observed in MRB500. These findings were in agreement with the results reported by Pariyar et al. (2020). The intensity of the peaks gradually decreased with temperature from 500 to 700°C. This implied that the crystalline structure of biochar was lost under high temperature (Clemente et al. 2018). K 2θ peaks were observed under MRB500 and STRB500. Except for the DLBs, similar structures of some inorganic minerals, such as SiO2 and Al/Si oxides SiO2 (2θ = 21.22°, 26.44°, 28.32°, 28.5° and 39.45°), were observed in other biochars (Zhou et al. 2017; Santhosh et al. 2020). The characteristic peaks of Fe were 20.1°2θ, 21.3°2θ and 22.19°2θ. The characteristic Fe peaks were observed in SRB500 and SRB700, and the signals strengthened with increasing temperature. This phenomenon was in agreement with the SEM–EDS results (as shown in Fig. 3)
Thermogravimetric analysis
The pyrolytic behaviour and thermal resistance of biochar were analysed via thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). Sample rates of weight loss were used as a function of time and temperature, which were recorded for the biochar products obtained at different peak temperatures. Different components of biomass and biochar were deconvoluted via derivative thermogravimetry (Ajaykannan et al. 2016). Three major degradation stages, including stage 1, 0-200°C; stage 2, 200–500°C; and stage 3, 500–800°C, were revealed for the decomposition of biochars with their weight losses (Fig. 4). In the first stage, < 10% weight loss occurred mainly because of the elimination of water, but the weight losses of STRB700 and SRB500 were 12% and 15%, respectively. In the second stage, the weight loss was dramatic, mainly due to the removal of volatile matter by the degradation of hemicellulose and cellulose.
The weight of SRB500 obviously dropped, and the residue was approximately 49%; the other biochar residue was more than 67%. After 200°C, the weight of STRB700 did not change significantly. Finally, in the third stage, the weight loss was generally due to the decomposition of lignin (Naik et al. 2010). Therefore, lignin decomposition occurred in SRBs in the third stage. The structure of biochar prepared at high temperature was more stable except for SRBs. Recalcitrant constitutions and crystal structures formed easily at higher pyrolysis temperatures (Zhang et al. 2020). The biochar prepared at high temperature had no obvious weight loss and exhibited a strong thermal stability. The lignin and hemicellulose of biochar prepared at low temperature did not decompose. It was still rich in volatile and heat-resistant substances. The biochar prepared at low temperature showed obvious weight loss at 450°C, mainly due to lignin decomposition. The decomposition process of hemicellulose was observed in the biochars prepared at 500°C (Fig. 4).
Principal component analysis
We classify the quantifiable indices in this paper into the following categories: chemical properties, physical properties, and functional group composition. Principal component analysis (PCA) was used to cluster various indicators and describe the correlation between them. Positive correlated variables are grouped together, while negative correlated variables are positioned on opposite sides of the plot origin. Figure 5a shows that among themselves, K, TOC, DOC, P, S and N were highly positively correlated and formed one cluster. Similarly, pH and EC formed the second cluster, zeta potential formed the third cluster, and C formed the fourth cluster. Figure 5b further confirmed that the type of biogas residue played a more important role than that of temperature in the impact on the chemical properties of biogas residue biochar. This was because the sample points with the same biogas residue type and different pyrolysis temperatures had the shortest distance and could gather better. Figure 5c shows that APD, MPV, and TPV were highly positively correlated and formed one cluster. Similarly, BET formed the second cluster, and TG formed the third cluster. Figure 5d further confirmed that the pyrolysis temperature played a more important role than that of the type of biogas residue in the impact on the physical properties of biogas residue biochar. This was because the sample points with the same pyrolysis temperature had the shortest distance and could gather better. Moreover, the biochar samples at 300°C and 500°C had more similar physical characteristics. Figure 5e shows that the C = C stretching of alkene vinylidene, Si-O, polymeric -CH2 aromatic rings, and C-O stretching in cellulose and hemicellulose formed one cluster. Similarly, C-H deformation in hemicellulose and cellulose formed the second cluster; C = C stretching in hemicelluloses formed the third cluster, C-H bond stretching in aliphatic formation formed the fourth cluster, and H-O in alcohol groups formed the fifth cluster. Relatively speaking, biochar samples more easily aggregate due to the type of biogas residue, and the biochar functional groups at 300°C and 500°C pyrolysis temperatures are more similar (Fig. 5f). This indicated that the chemical properties of biogas residue biochar were more likely to be determined by biogas residue type, the physical properties were more likely to be affected by pyrolysis temperature, and the characteristics of functional groups were the result of the joint action of biogas residue type and pyrolysis temperature. The change in chemical composition largely depends on the chemical composition of the raw material itself. A study by Das et al. (2021) also showed that with increasing pyrolysis temperature, the BET surface area, MPV and TPV of biochar increased significantly due to faster lignin decomposition and the rapid discharge of gases such as CH4 and H2. The change in functional groups is partly determined by the chemical composition of the raw material itself, and the other part is that the original functional groups are cracked and reorganized at high temperature to form new functional groups (Qiu et al. 2015; Brown et al. 2006).
Depending on the difference in raw materials, the biochars exhibited different properties. The C content of distiller grain biochar and straw biochar increased with temperature, while the nitrogen content also decreased with increasing preparation temperature. Except for the distiller's grain biochar and straw biochar, the P, K, pH and conductivity increased with temperature. The accumulation of Ca and Mg ions and the decomposition and loss of silicate lead to an increase in pH value. Therefore, the pH value of biochar was determined by the content of calcium, magnesium and silicon in the raw materials. The average yield of biochar of each material was sludge > mushroom > straw > distiller's lees and decreased gradually with preparation temperature because of more material loss at high temperatures. Pyrolysis effectively resulted in the disappearance of the main functional groups (e.g., -OH, C = C, C-O). Cellulose, hemicelluloses in STRB and alkene vinylidene in SRB decomposed at high pyrolysis temperatures, generating more pores. Greater adsorption and desorption of N2 by biochars were observed at 700°C because of the greater surface area and pore volume. Therefore, biochars from straw residue, mushroom biogas residue and sludge residue at 700°C may be used to produce slow-release nitrogen fertilizer. Biochar prepared by STR at 700°C might be a better biogas residue choice for biochar preparation.
Acknowledgements Not applicable.
Data Availability Statement The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions PC:Experiment conduction, data process and draft writing; SS: data process and draft writing; WS: Experiment conduction and biochar characterization; JD: draft review and editing; JY: draft review; XZ: Conceptualization, methodology, funding and supervision.
Funding This work was supported by National Natural Science Foundation of China (31901195), Shandong Provincial Natural Science Foundation (ZR2019BD062, ZR2021QD036), Agricultural Science and Technology Innovation Program of Chinese Academy of Agricultural Sciences (ASTIP-TRIC-ZD01), and the Science Foundation for Young Scholars of Tobacco Research Institute of Chinese Academy of Agricultural Sciences (2019B04, 2020B01).
Availability to data and materials The datasets generated during the current study are available from the corresponding author on reasonable request.
Ethics approval and consent to participate This work does not contain any studies with human participants or animals. All authors provided informed consent to participate in this study.
Consent to publish The authors transfer the publication rights to the journal and guarantee that this contribution is original. All the authors mutually agree for its publication.
Competing interest The authors declare no competing interests.
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Table 1 Main properties of biochars produced from sludge residue (SR), mushroom biogas residue (MR), straw residue (STR) and distillery lees (DL) with different pyrolysis temperatures of 300℃, 500℃ and 700℃
|
Biochar |
N (%) |
C (%) |
S (%) |
P (%) |
K (%) |
TOC (g kg-1) |
DOC (g kg-1) |
EC (mS cm−1) |
pH |
Zeta Potential (mV) |
|
SRB300 |
3.85 |
27.60 |
3.03 |
2.35 |
0.15 |
2.31 |
1.94 |
6.50 |
7.05 |
-34.97 |
|
SRB500 |
4.05 |
26.05 |
2.40 |
3.84 |
0.27 |
5.45 |
2.80 |
4.93 |
7.69 |
-19.20 |
|
SRB700 |
0.72 |
19.05 |
4.01 |
3.95 |
0.38 |
3.23 |
2.93 |
10.94 |
9.29 |
-18.07 |
|
MRB300 |
2.86 |
36.25 |
0.40 |
1.61 |
2.84 |
3.62 |
3.00 |
10.72 |
7.88 |
-20.07 |
|
MRB500 |
2.12 |
35.14 |
0.37 |
2.13 |
3.61 |
3.18 |
2.69 |
17.13 |
10.08 |
-19.87 |
|
MRB700 |
1.42 |
35.46 |
0.42 |
2.39 |
3.98 |
2.61 |
3.05 |
17.03 |
10.18 |
-21.33 |
|
STRB300 |
1.90 |
46.68 |
0.25 |
0.16 |
0.73 |
3.47 |
2.32 |
19.84 |
7.81 |
-27.77 |
|
STRB500 |
1.53 |
47.57 |
0.22 |
0.93 |
3.39 |
2.97 |
2.00 |
17.39 |
10.30 |
-29.40 |
|
STRB700 |
1.17 |
50.33 |
0.27 |
0.81 |
3.93 |
3.21 |
2.15 |
14.04 |
10.08 |
-20.63 |
|
DLB300 |
2.25 |
50.86 |
0.22 |
0.21 |
0.40 |
2.62 |
2.64 |
8.56 |
7.68 |
-26.27 |
|
DLB500 |
2.28 |
54.87 |
0.20 |
0.62 |
1.30 |
3.30 |
1.95 |
11.89 |
9.29 |
-27.57 |
|
DLB700 |
0.87 |
58.57 |
0.10 |
0.66 |
1.31 |
3.18 |
2.52 |
11.38 |
9.63 |
-27.87 |
pH: in H2O at a 1:10 (w/v) ratio; EC: electrical conductivity; C, N, S, P, K: total content with element analysis; TOC: total organic carbon; DOC: dissolved organic carbon; SR: sludge residue; MR: mushroom biogas residue; STR: straw residue; DL: distilled lees; B300: the pyrolysis temperature of biochar is 300 °C; B500: the pyrolysis temperature of biochar is 500 °C; B700: the pyrolysis temperature of biochar is 700 °C.
Table 2 Surface pore structure characteristics of biochar from different biogas residues at three pyrolysis temperatures
|
Biochar |
BET (m2 g-1) |
TPV (cm3 g-1) |
MPV (cm3 g-1) |
APD (nm) |
|
SRB300 |
2.493 |
0.009 |
0.0010 |
1.196 |
|
SRB500 |
3.122 |
0.009 |
0.0021 |
1.191 |
|
SRB700 |
29.874 |
0.033 |
0.0237 |
1.183 |
|
MRB300 |
2.102 |
0.009 |
0.0011 |
1.191 |
|
MRB500 |
6.631 |
0.025 |
0.0036 |
1.190 |
|
MRB700 |
70.271 |
0.073 |
0.0553 |
1.176 |
|
STRB300 |
4.291 |
0.008 |
0.0034 |
1.191 |
|
STRB500 |
3.185 |
0.014 |
0.0010 |
1.193 |
|
STRB700 |
112.846 |
0.082 |
0.1126 |
1.350 |
|
DLB300 |
2.442 |
0.009 |
0.0016 |
1.195 |
|
DLB500 |
4.452 |
0.013 |
0.0029 |
1.189 |
|
DLB700 |
5.531 |
0.020 |
0.0028 |
1.191 |
BET: BET surface area; TPV: total pore volume; MPV: micropore volume; APD: average pore diameter. SR: sludge residue; MR: mushroom biogas residue; STR: straw residue; DL: distillery lees; B300: the pyrolysis temperature of biochar is 300℃; B500: the pyrolysis temperature of biochar is 500℃; B700: the pyrolysis temperature of biochar is 700℃.
Table 3 Functional groups of Fourier Transform Infrared (FTIR) spectroscopy for different biogas residue biochars with three pyrolysis temperatures.
Functional group |
SRB300 |
MRB300 |
STRB300 |
DLB300 |
SRB500 |
MRB500 |
STRB500 |
DLB500 |
SRB700 |
MRB700 |
STRB700 |
DLB700 |
|
H-O in Alcohol groups |
3380 |
3390 |
3402 |
3416 |
3406 |
3433 |
3374 |
3431 |
3402 |
3413 |
3415 |
3433 |
|
C–H bond stretching in aliphatic formation |
2922 |
2925 |
2926 |
2924 |
2921 |
2923 |
2923 |
2922 |
||||
|
C=C stretching in hemicelluloses |
1630 |
1604 |
1607 |
1614 |
1619 |
1622 |
1579 |
1585 |
1619 |
1576 |
||
|
C–H deformation in hemicellulose and cellulose |
1433 |
1436 |
1435 |
1423 |
1425 |
1383 |
1480 |
1428 |
1498 |
|||
|
C–O stretching in cellulose and hemicelluloses |
|
1095 |
1099 |
1098 |
1081 |
1099 |
1115 |
1093 |
1095 |
|||
|
Si-O |
1033 |
1030 |
1034 |
1037 |
1036 |
|||||||
|
C=C stretching of alkene vinylidene |
873 |
874 |
876 |
|||||||||
|
polymeric -CH2/aromatic rings |
794 |
795 |
797 |
797 |
|
799 |
796 |
800 |
|
794 |
|
800 |
Reference from Reza et al. (2020) and Zhang et al. (2016). SR: sludge residue; MR: mushroom biogas residue; STR: straw residue; DL: distillery lees; B300: the pyrolysis temperature of biochar is 300 ℃; B500: the pyrolysis temperature of biochar is 500 ℃; B700: the pyrolysis temperature of biochar is 700 ℃.