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
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).
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.
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)
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).