Study on Pyrolysis Behavior of Avermectin Mycelial Residues and Characterization of Obtained Gas, Liquid, and Biochar

Abstract

The proper disposal of antibiotic mycelial residue (AMR) is a critical concern due to the spread of antibiotics and environmental pollution. Pyrolysis emerges as a promising technology for AMR treatment. In this study, we investigated the effect of pyrolysis temperature on the thermal decomposition behavior and product characteristics of avermectin (AV) mycelial residues. Various characterization techniques were employed to analyze thoroughly the compositions and yields of the obtained gas, liquid, and biochar products. The results indicated that most of the organic matter such as protein, carbohydrate, and aliphatic compounds in AV mycelial residues decomposed intensely at 322 °C and tended to end at 700 °C, with a total weight loss of up to 72.6 wt%. As the pyrolysis temperature increased, the biochar yield decreased from 32.81 wt% to 26.39 wt% because of the enhanced degradation of volatiles and secondary reactions of the formed aromatic rings. Accordingly, more gas components were formed with the gas yield increased from 9.76 wt% to 15.42 wt%. For bio-oil, the contents were maintained in the range of 57.43–60.13 wt%. CO and CO₂ dominated the gas components with a high total content of almost 62.37–97.54 vol%. At the same time, abundant acids, esters (42.99–48.85%), and nitrogen-containing compounds (32.14–38.70%) such as nitriles, amides, and nitrogenous heterocyclic compounds were detected for the obtained bio-oil. As for the obtained biochars, particle accumulation and irregular pores were presented on their bulk surface, which was primarily composed of calcium oxalate (CaC₂O₄) and calcium carbonate (CaCO₃). This work can provide theoretical insights for the harmless disposal and resource recovery for AMR, contributing significantly to the field of solid waste reuse and management.

1. Introduction

Nowadays, China has become the largest producer and consumer of antibiotics in the world, with annual production of 248,000 tons, accounting for more than 70% of the global market [1,2]. During the process of antibiotic fermentation, antibiotic mycelial residues (AMRs) are produced, mainly composed of residual antibiotics, fermentation metabolites, and abundant organic substrates such as protein, carbohydrate, crude fiber, lipid, and nutrient salts [3]. Normally, about 8–10 tons of AMRs are generated for production of 1 ton of antibiotics, and more than 2 million tons of AMRs are produced annually in China [4]. If AMR is not treated properly, serious environmental problems, such as water pollution, odors occurrence and particularly resistant gene development can be caused [5,6]. Therefore, realizing safe and proper disposal of AMR (listed as hazardous waste in China since 2008) has great significance for the human and natural microbial systems and is becoming a pressing problem around the world [7].

Currently, AMRs are primarily disposed of via incineration and safe landfill, which are costly and can no longer meet the environmentally friendly requirements owing to the potential secondary pollution by release of toxic substances [8]. On the other hand, antibiotic residues contain not only trace antibiotics, but also contain a high content of nutrient substances with similar carbon content, C/H ratio, and C/O ratio, as typical biomass resource [9,10]. Therefore, development of novel treatment method for AMR aimed not only to eradicate antibiotic pollution but also to enhance the resources utilization and nutrient recovery is very important [11]. Pyrolysis can transform bacterial residue into various high-value products (biochar, bio-oil, and gas) and has attracted extensive attention from researchers [12,13,14]. Additionally, the potential residual antibiotics and resistance genes in the AMR can also be degraded at high temperatures [15,16]. Wang et al. [17] studied the pyrolysis of penicillin fermentation residue to produce biochar and found that the β-lactam resistance genes can be completely destroyed during pyrolysis and the prepared biochar exhibited high removal efficiency of 93.32% for penicillin in aqueous solution, with the maximum adsorption capacity of 44.05 mg·g⁻¹. Xie et al. [18] prepared valuable biochar from pyrolysis of pleuromutilin at different temperatures and heating rates. The resulting biochar exhibited excellent fuel properties (18.1–19.8 MJ/kg), stability against degradation in soil, and low potential ecological risk with low heavy metal (Cu, Zn, and Pb) leaching concentrations. Wang et al. [19] investigated the probable pyrolysis mechanisms and product distributions of penicillin fermentation residue. The results showed that the derived bio-oil contained a high fraction of nitrogenated compounds, oxygenated species, and a few hydrocarbons, exhibiting the potential to become a biofuel in the future. Inspired by the fact that AMR contains high level of nitrogen, Zhou et al. [20] and Hao et al. [21] successfully synthesized nitrogen-doped porous carbon materials from penicillin mycelial residues and used them as anode material for Li-ion batteries and efficient catalysts for peroxymonosulfate activation, respectively. In this respect, various AMRs can be safely disposed of and converted into valuable biochar or bio-oil products under suitable pyrolysis conditions.

Avermectin belongs to a series of 16-membered macrocyclic polyketides derived from Streptomyces avermitilis. These compounds exhibit extensive applications in agriculture, veterinary medicine, and medicine due to their minimal harm to hosts and their effectiveness against a wide range of nematodes, mites, and insects [22]. According to statistics, the annual production of avermectin in China reached more than 7000–8000 tons, accounting for almost 80% of the worldwide market [23]. However, few studies have examined the pyrolysis behavior and production distribution of avermectin (AV) mycelial residues, which are principal for their safe disposal and resource utilization. Arising from these backgrounds, this study aimed to investigate the influence of pyrolysis temperature on the decomposition process of AV mycelial residues. The compositions and yields of obtained gas, liquid, and biochar products were comprehensively characterized by various techniques. Particularly, the dependence of the microscopic morphology, crystallographic composition, porous structure, and functional groups of the resulting biochar on pyrolysis temperature were discussed in detail.

2. Materials and Methods

2.1. Material and Methods

In this study, the tested avermectin mycelial residue was obtained from the avermectin production process in a pharmaceutical factory in Shandong Province, China. After being dried to constant weight in an oven at 105 °C for 24 h, the sample was ground, sieved into a particle size of 80 mesh, and placed in desiccators for further utilization.

Table 1. Proximate and ultimate analyses of AV material.

Proximate Analysis (wt%) a				Ultimate Analysis (wt%)					H/C Ratio	O/C Ratio	HHV (MJ/kg)
Moisture	Ash	Volatiles	Fixed Carbon	C	H	O b	N	S
4.41	8.70	75.77	11.12	45.32	6.26	31.95	6.76	0.61	1.66	0.56	19.46

^a: Air-dried base. ^b: Determined by difference.

gives the proximate, ultimate and component analysis results of AV. Briefly, the proximate analysis of the AV is 4.41 wt% moisture, 75.77 wt% volatiles, 11.12 wt% fixed carbon, and 8.70 wt% ash. The ultimate analysis showed that the AV contained 45.32 wt% C, 6.26 wt% H, 33.99 wt% O (calculated by difference), 6.76 wt% N, and 0.61 wt% S, respectively. The higher heating value (HHV) for AV mycelial residue was calculated to be 19.46 MJ/kg using the following equation [18]:

HHV = 0.3491C + 1.1783H + 0.1005 S − 0.1034O − 0.0151N − 0.0211ash

(1)

2.2. Experimental Apparatus and Procedure

Pyrolysis experiments were performed in a quartz tube fixed-bed reactor with a 25 mm inner diameter and a reaction zone with a height of 1000 mm, as shown in Figure 1. For each experiment, approximately 6 g of AV was dispersed onto quartz wool above the grid of a quartz tube inside the pyrolysis reactor. With N₂ continuously purged into the reactor system at a flow rate of 50 mL·min⁻¹, the reactor was heated from ambient temperature to 400–700 °C with a heating rate of 10 °C·min⁻¹. A temperature interval of 50 °C was chosen to investigate the influence of pyrolysis temperature on the decomposition behavior and product characteristics of AV material. After maintaining the target temperature for 30 min, the heating was stopped, and the device was naturally cooled down to room temperature under continuously introduced N₂ gas to prevent the pyrolysis products from being oxidized. The generated volatiles passed through a condensing system consisting of three connected empty Erlenmeyer flasks cooled by ice–salt baths and collected as gas and bio-oil products. The solid residues on the quartz wool after pyrolysis were collected as biochar and weighed to determine its yield. As soon as the reactor started to be heated, the non-condensed gases were vented to the gas sample bag for further gas chromatography (GC) analysis. The condensable part in the flasks and quartz bottles was weighed before and after the pyrolysis experiment to calculate its mass fraction.

2.3. Feedstock and Pyrolysis Product Characterization

The proximate analysis of AV was carried out according to the Chinese National Standard (GB/T 28731-2012) using thermogravimetric analyzer SDTGA5000 (Sundy, Changsha, China), and the elemental analysis of AV (C, H, N, and S) was measured by an elemental analyzer (Vario Macro Cube, Elementar, Frankfurt, Germany) according to Chinese National Standard (GB/T 30733-2014) [24]. The O content was calculated by the following equation [11]:

O = 100 − Ash (dried base) − C − H − N − S

(2)

The thermal degradation of AV feedstock was investigated using a thermogravimetric analyzer (STA409PC, NETZSCH, Selb, Germany). The pyrolysis gas compositions and yields were measured by GC (Agilent 7890A, Santa Clara, CA, USA) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Nitrogen percentage was used as an internal standard to quantify the produced gas yield. The gas chromatography/mass spectrometer with an HP-5 MS capillary column (GC/MS/7890A-5975C, Agilent, Santa Clara, CA, USA) was used to analyze the liquid composition. The detailed operation conditions can be found in our previous work [24]. Usually, the peak area of the GC/MS chromatogram is considered a good approximation for quantitative analyses of various compounds in liquid product. The surface functional groups of AV and pyrolysis biochar were characterized by FT-IR (Nicolet iS20, Thermo Fisher Scientific, Waltham, MA, USA) in the wavelength range of 400–4000 cm⁻¹. The crystalline phase of AV and pyrolysis biochar were analyzed using X-ray powder diffraction measurements (XRD, D/max-IIIAX, Rigaku, Tokyo, Japan) with Cu Ka radiation. The scan range of 2θ was 10–90°, with a scanning velocity of 0.02·s⁻¹. Scanning electron microscope (SEM, SUPR 55, ZEISS, Oberkochen, Germany) was used to analyze the morphologies and nanostructures of the AV material and derived biochar. To improve the electroconductivity, the samples were subjected to a gold-coating process prior to SEM analysis.

3. Results and Discussion

3.1. TG Analysis of AV Material

The thermal decomposition of AV mycelial residues was investigated by TG–DTG analysis, and the results are shown in Figure 2. According to the inflection points shown in the TG and DTG curves, the pyrolysis of AV can be approximately divided into five stages. The first stage started from room temperature to 165 °C, mainly caused by the evaporation of water and light volatiles in the mycelial residues [25]. In the second stage, most of the organic matter such as carbohydrate, protein, and aliphatic compounds decomposed at a temperature interval from 165 °C to 431 °C, with a sharp weight loss of 52.95% [24]. As a result, the DTG curve presented quite a high weight loss rate, corresponding to two strong peaks at 267 °C and 322 °C, as well as a shoulder peak at 367 °C, respectively [26]. Moreover, aromatic species can also be formed in this stage due to thermal decomposition of aliphatic molecules. With further increase of pyrolysis temperature to the third stage of 431–520 °C, the thermal decomposition of more stable residual avermectin mycelial bacteria together with secondary reactions of these formed aromatic rings such as polymerization and condensation occurred, leading to a 6.92% weight loss and formation of biochar [19]. For the fourth stage at 520 °C to 669 °C, the carbonization process of mycelial residues proceeded with a small weight loss of 3.83% in the TG curve and a smooth weight loss rate observed in the DTG curve. The final stage ended above 830 °C and donated 1.23% weight loss, which is closely associated with the decomposition of inorganic substances (such as carbonate) and the release of inorganic species like potassium [27]. According to the result of TG analysis, a total weight loss of up to 72.6 wt% occurred for the AV mycelial residues during the whole pyrolysis process. The weight of residue was almost stable at 26.4% when the pyrolysis temperature increased to 700 °C, which was mainly assigned to the mixture of fixed carbon and ash [28]. Therefore, the pyrolysis temperature range of 400–700 °C was chosen for further investigation of product compositions and distributions during the AV pyrolysis process.

3.2. Product Yields and Composition Analysis of Pyrolysis Gas and Bio-Oil

Typically, the obtained products from biomass pyrolysis include gas, liquid, and biochar, and the liquid consists of aqueous phase and bio-oil. Figure 3a shows the product yields from the pyrolysis of AV at different temperatures. Obviously, the biochar yields decreased from 32.81 wt% to 26.39 wt% upon raising the temperature from 400 to 700 °C, which may be because the increase of temperature promoted the degradation reactions of volatiles including decarboxylation, decarbonylation, dehydration, as well as condensation and cyclization, converting into more tar and gases (CO₂, CH₄, H₂, etc.) [29]. As the major pyrolysis product, the content of bio-oil was calculated to be more than 57.43% and reached its maximum value of 60.13 wt% at 500 °C. This is related to the fact that most of the organic matter such as protein, carbohydrate, and aliphatic compounds rapidly decomposed below 520 °C as was indicated in the TG analysis [30]. On further increase of temperature to 600 and 700 °C, secondary reactions of the formed aromatic rings such as cracking, polymerization, and condensation occurred [31], resulting in a slight decrease of bio-oil yield to 58.19 wt%. Correspondingly, the pyrolysis gas yield was enhanced from 9.76 wt% to 15.42 wt%. In addition, some aromatic components underwent recombination or polymerization to form soot or coke, which was also the main reason for the slow decrease in biochar yield at high temperatures [32].

The volumetric fractions of the main gaseous components (N₂ carrier gas free) obtained from AV pyrolysis at different temperatures are depicted in Figure 3b. Obviously, the pyrolysis temperature has a significant influence on gas distribution. At a lower pyrolysis temperature of 400 °C, CO₂ and CO derived from the decarboxylation reactions of protein and saccharides were the dominant gas compositions with high contents of 70.56 vol% and 26.98 vol%, respectively. As the pyrolysis temperature increased, the carboxyl groups tended to undergo condensation reactions with hydroxyl or amino groups, resulting in the decrease of CO₂ and CO concentration from 70.56 and 26.98 vol% to 37.77 and 20.66 vol%, respectively [33]. The increased CO concentration at 700 °C might be attributed to the progressive secondary decarbonylation reactions [31]. Conversely, the H₂ concentration increased greatly from 4.03 vol% to 23.49 vol% due to the enhanced cracking and dehydrogenation of organic matter at higher pyrolysis temperature, as well as additional dehydration condensation of aromatic compounds in the biochar [34]. As for CH₄, it was detected for the first time at 450 °C, with the content increased from 7.98 to 11.68 vol% initially and then decreased slightly to 9.67 vol% at 700 °C. This might be related to the enhanced cracking and dehydrogenation by forming more H₂ [11]. Moreover, a small fraction (2.46–6.26 vol%) of C_nH_m resulting from the cracking and cleavage of aliphatic side chains during the carbonation process were also detected for all the experiments, which mainly contained C₂H₄, C₂H₆, and C₃H₈ hydrocarbons.

According to the proximate analysis result, AV has a high volatile matter content (75.77 wt%), which tended to form more bio-oil under pyrolysis. Figure 4 shows the main compounds and their relative proportions (% area) detected in bio-oil, particularly nitrogen-containing compounds, analyzed by GC/MS from AV pyrolysis at different temperatures. The chromatograms of the obtained bio-oil samples are shown in Figure S1, and the main compounds detected are listed in Table S1. Clearly, the bio-oils are quite complex mixtures, thus the identified compounds were classified into five groups for analysis, namely, nitrogen-containing compounds, acid and ester compounds, long-chain aliphatic compounds, monoaromatic hydrocarbons, and phenolic compounds. According to Figure 4a, the bio-oils obtained at different pyrolysis temperatures have similar components but slightly different proportions. The nitrogen-containing compounds and acids and esters were the dominant components of the bio-oil, which accounted for 32.14–38.70% and 42.99–48.85% of the quantified proportions, respectively. These results suggested that the pyrolysis process was mainly the decomposition of protein, lipids, and saccharides, similar to that reported in the literature [31,35]. In detail, the detected acids and esters are primarily acids, particularly octadecenoic acid, octadecadienoic acid, and n-hexadecanoic acid, which together account for almost 80% of the total quantified acid proportion. During the carbonization process, the hydrocarbons such as aliphatic and aromatic compounds were produced from the thermal degradation of long chain fatty acids together with the secondary reactions of oxygen-containing compounds [36]. As the pyrolysis temperature increased, the proportions of hydrocarbons especially long-chain aliphatic compounds increased from 7.69% to 14.47%, indicating that higher temperature is beneficial for cracking, decarboxylation, glycosidic bond cleavage, inner ring C–O group fragmentation, and C–C bond breakage processes [30]. The small fraction (3.1–4.96%) of phenolics detected in the bio-oil could be derived from decomposition of lignin [37].

As for the nitrogen-containing compounds, including nitrile and acylamide compounds as well as many kinds of nitrogenous heterocyclic compounds, these were present in the bio-oil. Their relative contents are shown in Figure 4b. Among these species, nitrile and acylamide compounds occupied a high proportion under various pyrolysis conditions. In particular, pentadecanenitrile and oleanitrile accounted for 20.91–25.67% and 57.50–63.95% of the quantified nitrile compounds. Regarding acylamide compounds, hexadecanamide (25.47–32.73%), 9-octadecenamide, andoctadecanamide (49.48–53.02%) were conspicuous components. Typically, acylamide compounds are derived from reactions between amino radical intermediates formed during the cyclization of amino acids and acetic/carboxylic acids at lower pyrolysis temperatures [38]. With increasing temperatures above 550 °C, dehydration reaction of these amides would occur, resulting in the formation of nitrile [39]. As a result, the fraction of amides decreased gradually from 15.35% to 11.73%, while the content of nitriles decreased from 14.71% to 12.01% at low temperature and then increased to 14.99% above 550 °C. Regarding the evolution of nitrogenous heterocyclic compounds, the relative content reached the maximum value of 12.36% at 450 °C, and then decreased to 5.42% with increasing temperature. This is mainly related to the release of pyridinic-N and pyrrolic-N in the char and cleavage of tryptophan [12,38]. In summary, the bio-oil derived from AV pyrolysis is complex in composition, containing a large number of oxygen- and nitrogen-containing compounds. Therefore, further denitrification, deoxygenation, or refinement are needed for bio-oil before being applied in various fields.

3.3. Characterization of Pyrolysis Biochar

Figure 5 showed the surface morphology of natural AV and the biochar samples derived at different pyrolysis temperatures. Evidently, the AV feedstock exhibited a lamellar overlapping structure with a smooth surface [28]. After pyrolysis, the lamellar structure disappeared and microscopic structures such as particle accumulation and pores were formed on the bulk surface, mainly attributed to devolatilization during pyrolysis. From Figure 5b–e, it can be observed that similar morphologies were presented for the obtained biochar samples. For biochar obtained at 400 °C, an interlaced grid pore structure with 2–3 μm can be seen, which was filled with agglomerated particles. With the increase of pyrolysis temperatures to 500 and 700 °C, the smooth surface of AV was severely damaged by gas release, and more fragmentation, pore structure, and particle agglomerations were produced [40]. In particular, the pore size was smaller and denser, and the particle shapes became more regular and uniform at higher temperature. This result suggested that a higher pyrolysis temperature was beneficial for pore formation due to the complete cracking of the organic matter.

The crystallographic structures of AV before and after pyrolysis were analyzed by XRD (Figure 5f). It could be observed that several inorganic compounds containing essential elements (Ca and P) were the major crystalline phases in both AV and its derived biochar. This is because calcium salts are commonly added to fermentation liquor to improve the metabolic function of antibiotic mycelium [41]. For AV feedstock, the diffraction peaks belonging to calcium oxalate hydrate (CaC₂O₄ (H₂O)_n, PDF#17-0541) were observed, with the crystallinity calculated to be 10.18% by Segal’s method [42]. After pyrolysis at 400–700 °C for 2 h under N₂ atmosphere, dehydration and degradation of CaC₂O₄ (H₂O)_n occurred, resulting in the formation of calcium oxalate (CaC₂O₄, PDF#18-0297) and calcium carbonate (CaCO₃, PDF#05-0586) (CaC₂O₄ (H₂O)_n → CaC₂O₄ + n H₂O, CaC₂O₄ → CaCO₃ + CO) [18]. Noticeably, the peak intensity of CaCO₃ became weaker as the temperature increased to 600 °C and 700 °C, which was attributed to the decomposition of carbonate with CO₂ release. Meanwhile, partial calcium salts reacted with phosphorus species at high pyrolysis temperatures and formed new Ca–P complexes such as hydroxyapatite (Ca₅(PO₄)₃(OH), PDF#09-0432), indicating that Ca can be helpful for phosphorus fixation [31,41].

The main element contents of natural AV and biochars obtained at different pyrolysis temperatures were determined by EDS and the results are presented in mass percentage, as listed in

Table 2. EDS data of AV and biochars obtained at different pyrolysis temperatures.

Samples	Elements (wt%)
	C	O	P	Ca
AV	28.43 ± 0.23	62.93 ± 0.06	1.85 ± 0.17	6.78 ± 0.13
400 °C	31.14 ± 0.13	47.79 ± 0.29	5.80 ± 0.11	15.27 ± 0.10
500 °C	22.52 ± 0.19	36.28 ± 0.24	7.74 ± 0.20	33.28 ± 0.15
600 °C	17.60 ± 0.16	39.64 ± 0.11	9.43 ± 0.19	33.33 ± 0.08
700 °C	21.87 ± 0.26	37.40 ± 0.14	9.03 ± 0.09	31.71 ± 0.12

. The results indicated that the contents of C, O, Ca, and P for natural AV were 28.43 wt%, 62.93 wt%, 6.78 wt%, and 1.85 wt%, respectively. After pyrolysis, similar element species but with different proportions were detected in the resulting biochar samples. At a lower pyrolysis temperature of 400 °C, higher C content (31.14%) and O content (47.79%) were detected. This can be attributed to the incomplete decomposition of protein, carbohydrate, and aliphatic compounds, resulting in the formation of large quantities of labile matter with C and O elements. As the temperature increased to 600 °C, thermal decomposition of the more stable organic matter and deoxygenation and dehydration reactions occurred, with the C and O contents decreased to 17.6% and 38.64%, respectively. A further slight increase in C content (21.87%) and a decrease in O content (37.4%) at 700 °C were mainly caused by the enhanced polymerization, aromaticity, and carbonaceous reaction of biochar at high temperatures [43]. For Ca element, its content exhibited an increasing trend from 15.27% to 34.73%, followed by a slight decrease to 31.31%. As illustrated in the XRD, this can be related to the formation of CaCO₃ from degradation of CaC₂O₄ (H₂O)_n and CaC₂O₄ at low temperature and its decomposition into CO₂ above 600 °C. Additionally, the similar variation trend of P content with that of Ca confirmed the formation of Ca–P complexes in the form of Ca₅(PO₄)₃(OH). This result is consistent with that of the XRD discussed above.

The surface functionalities of AV and derived biochar were characterized by FT-IR spectra and are displayed in Figure 6. Obviously, there were abundant functional groups observed in the AV feedstock. The strongest absorption peak near 3430 cm⁻¹ corresponded to the stretching vibration of –OH on the hydroxyl and carboxyl groups, indicating the presence of alcohol, phenol, and carboxylic acid [44]. The absorption spectra at 3283 cm⁻¹ typified the stretching vibration of N–H attributed to the presence of the amino group. The three weak absorption peaks located at 2850, 2925, and 2960 cm⁻¹ were typically derived from stretching vibrations of –CH₂ and –CH₃, confirming the existence of aliphatic C–H bonds [45]. The absorption peaks at 1647 and 1531 cm⁻¹ corresponded to the stretching vibration of amine I band and amine II band [41], respectively, while the peaks at 1450–1380 cm⁻¹ typified the in-plane bending vibration of the C–H bond. The absorption peaks in the range of 1050–1241 cm⁻¹, arising from the stretching vibration of C–O and C–N, indicated the presence of aliphatic compounds and aliphatic amines in the AV material [46]. The small absorption peak at 518 cm⁻¹ may be related to the stretching vibration of –S–S– bonds [47]. After pyrolysis, the surface functionalities of biochar samples changed greatly and only a few functional groups were present. The decrease in the intensity of O–H vibration may be caused by the dehydration reaction. The C–H, C=O, and C–O vibration intensities also decreased due to the decomposition of carbohydrates and proteins to aromatics with the release of CH₄, CO₂, CO, and other gases. These N-containing groups were gradually converted into nitrogen-containing species in the gas, liquid, and biochar during carbonation process. Moreover, some new peaks assigned to in-plane asymmetric C–O stretching and out-of-plane bending of carbonates were present at 1436 cm⁻¹ and 895 cm⁻¹ after pyrolysis at 400–600 °C, which disappeared with further increase of temperature to 700 °C. According to the XRD results, this was mainly related to the decomposition of inorganic matter (carbonate) at high temperatures. In contrast, the vibration intensity of PO₄³⁻ at 1025 cm⁻¹ showed an increasing trend, revealing the stabilization of inorganic phosphorus species in the biochar [31].

4. Conclusions

In this study, a systematic investigation into the effect of pyrolysis temperature on resulting gas–liquid–solid product compositions and distributions of AV mycelial residues was conducted. The TG results showed that significant decomposition of organic matter occurred starting at 322 °C, with decomposition mostly concluded by 700 °C. With increasing pyrolysis temperature, the degradation of volatiles and secondary reactions of the formed aromatic rings were enhanced. As a result, the biochar yield decreased from 32.81 wt% to 26.39 wt%, while the gas yield increased from 9.76 wt% to 15.42 wt%. The contents of bio-oil were maintained in the range of 57.43–60.13 wt%. The CO and CO₂ derived from the decarboxylation reactions of proteins and saccharides were the dominant gas compositions with a total high content of almost 62.37–97.54 vol%. As for the obtained bio-oil, nitrogen-containing compounds such as nitriles, amides, and nitrogenous heterocyclic compounds, as well as acids and esters accounted for 32.14–38.70% and 42.99–48.85% of the quantified proportions, respectively. In particular, the acylamide species are formed at lower pyrolysis temperatures and converted into nitrile compounds via dehydration with increasing temperature. Different from the lamellar overlapping structure of AV feedstock, particle accumulation and pores were present on the bulk surface of biochar, primarily composed of calcium oxalate (CaC₂O₄) and calcium carbonate (CaCO₃). The calcium salts were especially helpful to phosphorus fixation at higher pyrolysis temperatures in the form of Ca₅(PO₄)₃(OH). This work can provide theoretical reference for harmless disposal and resource recovery for AMR, which is of significant interest to the field of solid waste reuse and management.