Pyrolysis of Hydrothermal Sewage Sludge and Food Waste Digestate for Heavy Metals Stabilization and Ecological Risk Reduction

Abstract

The application of municipal sewage sludge is often limited by concerns over heavy metal (HM) safety. This study explored the reduction of HM content in hydrothermal sewage sludge (HTS) through co-pyrolysis with food waste digestate (FD), aiming to lower ecological risks in the produced biochar. Results indicated that FD addition effectively lowered HM concentrations in biochar, mainly via dilution effect. Moreover, increased pyrolysis temperatures and FD addition promoted the stabilization of Cr, Ni, Cu, Zn, As, Cd, and Pb. Notably, a 50% FD mix significantly increased the proportion of HMs in the residual fraction of Ni (75.66%), Cu (71.66%), Zn (98.13%), and Cd (58.14%) compared to solo pyrolysis at 700 °C. Consequently, the potential ecological risk index significantly dropped from 47.86 to 26.29. Biochar created under optimal conditions (700 °C with a 50% FD ratio) showcased improved application prospects due to reduced bioavailability, thus diminishing HM-related ecological dangers.

1. Introduction

China’s rapid urbanization and industrialization have significantly increased sewage sludge production, with a growth rate surpassing 13% in recent years [1]. By 2020, the annual sewage volume exceeded 7.34 × 10¹⁰ t/a (ton/annual), resulting in a treatment demand for approximately 7.29 × 10⁷ t/a of sludge post-initial treatment [2]. Traditional disposal methods like ocean dumping, agricultural application, and landfilling are declining due to heightened environmental consciousness and stricter safety regulations [3]. Challenges in sludge dewatering and the presence of contaminants such as heavy metals (HMs), polycyclic aromatic compounds, and persistent organic pollutants impede its treatment and reuse [4,5]. Hydrothermal pretreatment has been effective for enhanced dewatering, removal of harmful substances, and sludge volume reduction [6,7,8,9]. Nonetheless, the residues from hydrothermal processes present recycling difficulties and potential risks of HM pollution. Consequently, it is crucial to focus on the stabilization of heavy metals within hydrothermal sewage sludge (HTS) residues.

The management of anaerobic food waste digestate (FD) is an equally pressing environmental issue. By the close of 2020, over 14.43 million tons of FD were produced. Although anaerobic digestion substantially reduces its organic content, this process leads to land quality degradation and restricts its commercial viability [10]. The considerable quantity, elevated moisture levels, and harmful constituents within FD raise significant environmental and water resource issues, necessitating efficient treatment strategies [11].

Recent developments in thermal technologies, particularly pyrolysis, have opened new pathways for energy recovery and efficient waste management [12]. Pyrolysis effectively minimizes solid waste volume and neutralizes hazardous pollutants, transforming HMs in sewage sludge into a more stable state. The co-pyrolysis process, involving biomass waste like FD, has demonstrated potential in securing HMs within sludge. This is due to the dilution effect from the lower HM concentration in biomass waste and the enhanced binding properties provided by functional groups, surface area, and pore structure of biomass [13,14,15]. Moreover, calcium oxide (CaO), serving as an alkaline catalyst, has shown to interact more effectively with HMs, fostering their stabilization. CaO also plays a role in capturing chloride released during pyrolysis, thereby mitigating environmental pollution [16,17,18,19,20]. Notably, FD not only shares the characteristics of biomass waste but is also rich in metal oxides, predominantly CaO, positioning it as an excellent additive for sludge pyrolysis [21]. Consequently, the co-pyrolysis of HTS and FD enables the simultaneous treatment of both wastes using the same equipment. The high organic matter and oxide content in FD contribute to increased gas production during pyrolysis, which enhances the pore structure and specific surface area of the resulting biochar. This process not only improves biochar quality but also stabilizes HMs, thereby reducing environmental hazards. The produced biochar can be widely used in fields such as adsorption and soil improver according to its properties, thus achieving integrated waste utilization [22,23,24].

Herein, in the present study, we creatively proposed a cost-effective, efficient, and low-toxicity synergistic pyrolytic treatment of HTS and FD to immobilize HMs. The European Community Bureau of Reference three-step sequential extraction method and the potential ecological risk assessment index were chosen as the evaluation method for the safety of HMs in the product. The selection of pyrolysis temperatures (300 °C, 500 °C, and 700 °C) in our study was based on an extensive literature review and the known efficacy of these temperatures in optimizing the immobilization of HMs in biochar [25,26,27]. The research focused on how pyrolysis temperature and the proportion of FD influence the migration and transformation of common HMs such as Cr, Ni, Cu, Zn, As, Cd, and Pb. The study aimed to (1) confirm the combined effect of FD addition on the natural stabilization of HMs in biochar; (2) uncover the impact and underlying mechanisms of pyrolysis temperature and FD ratio on the structural transformation of HMs; (3) assess the safety of HMs and the potential ecological threats posed by the resulting biochar.

2. Materials and Methods

2.1. Materials

The FD employed in the co-pyrolysis was sourced from an anaerobic reactor processing food waste. The liquid and solid components of the sewage sludge were derived from a pilot-scale facility in Tong’an, Fujian Province, China, specializing in sludge dewatering via hydrothermal pretreatment at 180 °C for 30 min. For an in-depth understanding of the sewage sludge treatment process at this plant, please consult the prior research conducted by Li et al. [28]. Both the HTS and FD underwent drying at 105 °C for 24 h to remove moisture, after which they were ground into 0.45 μm powder for further analysis and pyrolysis experiments.

Table 1. Basic parameters of HTS and FD.

Sample	Proximate Analysis (wt.%)			Elemental Analysis (wt.%)
	Ash	VM a	FC b	N	C	H	S	O	H/C	N/C
HTS	57.54	40.13	2.33	2.68	22.21	7.75	1.91	7.91	4.19	0.10
FD	48.12	46.53	5.35	2.06	19.86	4.81	1.13	24.02	2.91	0.09

^a Volatile matter; ^b fixed carbon.

provides the basic parameters of HTS and FD.

2.2. Co-Pyrolysis for Biochar Generation

The pyrolysis experiments were carried out in a fixed-bed quartz reactor, which was charged with 30 g of varying ratios of HTS or HTS-FD mixtures. Throughout the pyrolysis, an anaerobic atmosphere was sustained by a nitrogen flow of 15 mL/min. The reactor was uniformly heated to the target temperature over 30 min, with an electric heating jacket providing a rate of 10 °C/min. After the heating phase, the reactor was allowed to cool naturally to below 50 °C. The biochar produced was then carefully placed in a glass desiccator for further characterization and analysis.

The proximate analysis, which encompasses the determination of ash content, volatile matter, and fixed carbon, was executed following the guidelines of the national standard GB/T 28731-2012 [29]. The ultimate analysis, assessing the elemental composition, was conducted using an element analyzer (Vario MACRO, Frankfurt, Germany). For a comprehensive visual representation of the pyrolysis apparatus, refer to Figure 1, and the corresponding experiment conditions are displayed in

Table 2. Description of experimental conditions.

Temperature	Feedstocks	Naming of Biochar Samples
300 °C	HTS + 0% FD	B300-0, B500-0, B700-0
500 °C	HTS + 20% FD	B300-20, B500-20, B700-20
700 °C	HTS + 50% FD	B300-50, B500-50, B700-50

.

2.3. Heavy Metals Analysis

The quantification of HMs in both the initial materials and the produced biochar was carried out through the acid digestion technique. A 0.10 g sample of the dry material was placed into a digestion tube, to which 5 mL of HNO₃, 4 mL of HClO₄, and 4 mL of HF (all of high purity) were added in sequence. The graphite digester (model FTS25-20, Leiboterry, Tianjin, China) was set to increase the temperature at a rate of 20 °C/min up to 120 °C, maintain this temperature for 20 min, then elevate it to 190 °C at a rate of 10 °C/min and hold for 4 h. Afterward, the acids evaporated at 160 °C over 6 h, and the tube was allowed to cool to ambient temperature. The digested solution was then filtered and diluted as needed for the analysis of Cr, Ni, Cu, Zn, As, Cd, and Pb using inductively coupled plasma mass spectrometry (ICP-MS, model Agilent 7500cx, Santa Clara, CA, USA).

The extracted HMs were divided into four fractions: F1, the acid-soluble fraction; F2, the reducible fraction; F3, the oxidizable fraction; and F4, the residual fraction. The bioavailability of these fractions decreases sequentially from F1 to F4. The F1 fraction is the weakly acidic extractable fraction, typically adsorbed on particle surfaces or associated with carbonates, and is sensitive to changes in water ion composition, leading to sorption and desorption. The F2 fraction is mainly linked with iron and manganese oxides and is less stable under anoxic or anaerobic conditions. The F3 fraction includes HMs bound to organic matter. The F4 fraction is the most stable, consisting of HMs integrated with silicate minerals, crystalline iron, and magnesium oxides, which are difficult to mobilize or utilize [30]. For a detailed extraction procedure of HMs, please refer to the previous study [21]. The leachates from the F1, F2, F3, and F4 fractions were subjected to ICP-MS testing.

The risk assessment code (RAC) serves as a tool to evaluate the level of HM contamination in raw materials and biochar by analyzing the percentage of the F1 fraction for each HM [31]. The risk assessment is categorized into five levels: “No Risk” (LR) for values between 1 and 10%, “Medium Risk” (MR) for values between 10 and 30%, “High Risk” (HR) for values between 30 and 50%, and “Very High Risk” (VHR) for values exceeding 50%.

The potential risk levels for HMs in the environmental samples are evaluated using the potential ecological risk index (RI), as described by Hakanson [32]. The corresponding equations for RI are provided as follows:

C_f = W_s/W_n

(1)

E_r = T_f × C_f

(2)

RI = ΣE_r

(3)

where W_s is the total F1, F2, and F3 fractions of HMs in the samples; W_n is the F4 fraction; T_f is the HM biotoxicity response factor, and the magnitude of response factors for the eight typical HMs are Cd (30) > As (10) > Ni (6) > Cu (5) = Pb (5) > Cr (2) > Zn (1); E_r is the HMs single potential ecological risk factor; and RI is the HMs potential ecological risk index. The relationships between potential risk assessment indicators and contamination levels are shown in Table S1 in the Electronic Supplementary Information (ESI).

3. Results

3.1. Heavy Metals Concentration

Figure 2 and Table S2 present the total concentrations of Cr, Ni, Cu, Zn, As, Cd, and Pb in feedstock and biochar samples, which were prepared under various conditions. Among the seven evaluated HMs, the total concentrations in the HTS samples were ranked in descending order, as follows: Zn > Cu > Cr > Pb > Ni > As > Cd. In the HTS samples, Zn and Cu exhibited relatively high concentrations at 4137.98 and 3856.72 mg/kg, respectively, while As and Cd had lower concentrations at 28.76 and 1.94 mg/kg, respectively. In contrast, FD exhibited significantly lower concentrations of all HMs compared to HTS. Remarkably, the concentrations of Cu and Zn in HTS, which were notably high, were reduced to just 84.63 mg/kg for Cu and 273.69 mg/kg for Zn in FD. This substantial reduction indicates that incorporating FD as an additive offers a promising approach to mitigate HM concentrations during co-pyrolysis with HTS [33,34,35]. The concentrations of all HMs, with the exception of Cd, were observed to escalate as the pyrolysis temperature increased. This trend can primarily be ascribed to the relatively minor weight loss experienced by HMs in comparison to organic matter, which consequently leads to an elevated accumulation of these metals in the resulting biochar [31,36]. The concentration of Cd displayed an enrichment pattern at temperatures of 300 °C and 500 °C yet exhibited a decline at 700 °C. This behavior is likely due to the thermal reduction of Cd oxides into gaseous forms (Cd volatilization) at elevated temperatures, a process that is supported by the chemical reactions denoted as (4) and (5), in agreement with the studies of Kistler et al. and Trinkel et al. [37,38].

CdCO₃ → CdO + CO₂

(4)

CdO + C_char → Cd_(g) + CO

(5)

3.2. Heavy Metals Speciation Distribution

The detailed analysis reveals that the total concentration and chemical fraction of HMs in HTS and its corresponding biochar are pivotal determinants that significantly impact environmental risk assessments [39]. To evaluate the immobilization efficacy of HMs in HTS biochar, the chemical speciation distributions of HMs were meticulously analyzed across varying pyrolysis temperatures and FD addition ratios. These detailed investigations are depicted in Figure 3.

For Cr, the predominant form in HTS was the F4 fraction, constituting 77%, with the F3 fraction comprising 22.04%. This distribution is consistent with the results reported by Li et al. in their study on hydrothermal sludge [40]. This indicates that Cr predominantly exists in oxidizable and residual forms, which are less bioavailable. With the increase in pyrolysis temperature, a greater proportion of the F3 fraction transformed into the F4 fraction. Specifically, the F4 fraction increased to 90.58% at 300 °C, 94.96% at 500 °C, and 98.40% at 700 °C. This shift is linked to the non-volatile character of Cr, which tends to bind more readily to carbon matrices under high-temperature conditions. High temperatures facilitate chemical bonding and complexation reactions between HMs and functional groups present on the biochar surface, such as carboxyl, hydroxyl, and phenolic groups [41]. These interactions enhance the stability and retention of heavy metals within the biochar. Additionally, during pyrolysis, the organic components of FD undergo thermal decomposition, leading to the formation of biochar. This process involves the breaking of chemical bonds in organic matter, resulting in the release of volatile compounds and the formation of a carbon-rich solid residue. The high temperatures facilitate the conversion of HMs from their more volatile and mobile forms to more stable forms that are incorporated into the carbon matrix of the biochar, which further promoted the complexation of Cr with it to a more stable F4 fraction [42]. During pyrolysis, high temperatures and the reducing environment can facilitate the conversion of Cr (VI) to Cr (III). This reduction is favored by the presence of organic matter and carbon, which act as reducing agents. The overall reaction can be represented as follows: Cr(VI) + Reducing Agent → Cr(III).

For Ni, the primary forms in HTS were the F1 and F3 fractions, representing 54.63% and 28.38%, respectively. Despite pyrolysis contributing to the stabilization of HMs, the F1 fraction of Ni does not show a significant reduction, which can be attributed to the robust thermal stability of nickel compounds [43]. The application of FD during pyrolysis markedly improves the safety profile of Ni, as evidenced by the substantial enhancements at FD addition ratios of 20% and 50%. Notably, at a pyrolysis temperature of 700 °C, the F4 fraction of Ni increased from 55.14% to 74.29% with a 20% FD addition and further to 75.66% with a 50% FD addition. This improvement is likely due to the solidification reaction between CaO present in FD and Ni occurring during pyrolysis, which results in the formation of stable silicate compounds [44].

NiO + CaO + 4SiO → NiCaSi₄O₁₀

(6)

For Cu, the F4 and F3 fractions accounted for 34.03% and 61.44%, respectively. The pyrolysis temperature and the ratio of FD addition significantly elevated the percentages of both F3 and F4 fractions, suggesting a reduced environmental risk. Remarkably, a 50% FD addition proved to be more effective under pyrolysis conditions ranging from 300 °C to 700 °C, maintaining the F4 and F3 fractions at approximately 70% and 30%, respectively. This shift from the F3 to the F4 fraction is attributed to the breakdown of organic Cu compounds, leading to the formation of CuO, as well as metal-phosphate and metal-silicate complexes within the mineral and lattice structures [45,46]. The extensive decomposition of organic matter is a precursor to transformation processes. Nonetheless, elevated temperatures may result in a diminished yield of copper cysteine-like substances (Cu(I)-S), owing to the concurrent oxidation and desulfurization that occur during pyrolysis [47]. The incorporation of FD not only augmented the F4 fraction but also fostered the development of aromatic structures. It enhanced the specific surface area, thereby aiding in the entrapment of released HMs. This process is facilitated by the swelling structure of FD and the chelation with functional groups and inorganic substances [34,48].

For Zn in HTS, the F1 fraction initially constituted a considerable 28.62%. However, post-pyrolysis at temperatures of 300 °C, 500 °C, and 700 °C, the F1 fraction markedly decreased to 4.53%, 2.80%, and 1.61%, respectively, indicating a notable enhancement in safety. Conversely, a portion of the F4 fraction transitioned to the F3 fraction at 500 °C, diverging from the trend observed at 300 °C. This pattern aligns with the findings of numerous other studies [49], suggesting that Zn is a moderately volatile HM readily released at that temperature. Furthermore, the enhanced stabilization of Zn at 700 °C is likely due to the formation of Zn compounds that possess higher binding energies, such as ZnO, ZnS, and Zn halides (mainly including ZnF₂, ZnCl₂, and ZnBr₂) [30]. The incorporation of FD notably enhanced the F4 fraction in biochar, particularly at 700 °C. The F4 fraction rose to 97.21% with a 20% FD addition and to 98.13% with a 50% FD addition, respectively. This increase is ascribed to the formation of thermally stable metal compounds, primarily from the chlorides and oxides (chiefly CaO) present in FD [50].

Despite the low total concentration of Cd, the prevalence of the F1 and F2 fractions in HTS was 27.70% and 40.68%, respectively. This predominance suggests potential negative environmental implications, as these fractions are more bioavailable and can be readily absorbed by living organisms. Consequently, this could lead to accumulation and pose detrimental effects on both the environment and human health [11]. Therefore, it is imperative to take into account both the total content of HMs and their specific fractions in HTS when evaluating potential risks. As the pyrolysis temperature and the FD addition ratio increase, there is a corresponding gradual reduction in the percentages of the F1 and F2 fractions. Notably, these fractions were reduced to only 27.7% at a pyrolysis temperature of 700 °C with a 50% FD addition. Furthermore, as detailed in Section 3.1, the total concentration of Cd also diminished significantly throughout this process, which is indicative of a considerable enhancement in safety.

The F4 fraction of As was the primary form found in the HTS, and its proportion incrementally rose with the increase in pyrolysis temperature, achieving 92.79% at 700 °C. The integration of FD further elevated the F4 fraction. This enhancement is likely due to the abundant presence of CaO in FD, which acts to inhibit the volatilization of As. Moreover, CaO has the potential to react with arsenic compounds such as As₂O₃, As₂S₃, and NaAsO₂, leading to the formation of a thermally stable compound, (Ca(AsO₂)₂) [51]. The F4 fraction of Pb consistently represented a high percentage in both the HTS and the biochar samples. This suggests that there is limited scope for further treatment enhancement, a conclusion that is in agreement with the outcomes of other research studies [48,52].

3.3. Risk Analysis of Heavy Metals

Table 3. Risk assessment codes of HMs in HTS and biochar.

Samples	Cr	Ni	Cu	Zn	As	Cd	Pb
HTS	0.11/NR	28.38/MR	4.47/LR	28.62/MR	5.37/LR	27.70/MR	0.64/NR
B300-0	0.02/NR	26.33/MR	3.19/LR	4.53/LR	4.14/LR	21.55/MR	0.23/NR
B300-20	0.01/NR	23.58/MR	3.12/LR	3.34/LR	4.28/LR	20.99/MR	0.13/NR
B300-50	0.01/NR	19.1/MR	0.86/NR	1.20/LR	3.79/LR	19.44/MR	0.13/NR
B500-0	0.01/NR	20.68/MR	2.69/LR	2.80/LR	3.74/LR	17.40/MR	0.14/NR
B500-20	0.01/NR	17.57/MR	2.81/LR	1.40/LR	3.77/LR	18.92/MR	0.12/NR
B500-50	0.01/NR	10.67/MR	0.72/NR	1.09/LR	3.13/LR	14.36/MR	0.10/NR
B700-0	0.01/NR	14.22/MR	1.73/LR	1.61/LR	2.12/LR	16.73/MR	0.12/NR
B700-20	0.00/NR	7.08/LR	0.55/NR	0.78/NR	1.59/LR	12.98/MR	0.09/NR
B700-50	0.00/NR	5.31/LR	0.43/NR	0.22/NR	1.84/LR	13.99/MR	0.10/NR

delineates the risk assessment codes (RACs) for the presence of Cr, Ni, Cu, Zn, As, Cd, and Pb in the HTS and biochar samples under various conditions. The risk levels for Cr and Pb were consistently categorized as “No Risk” in both HTS and all biochar samples. Additionally, Cu and Zn, which were initially deemed “Low Risk” and “Medium Risk”, respectively, in HTS, were reclassified to “No Risk” in the biochar samples produced via FD-assisted pyrolysis at 700 °C. As for As, it maintained a “Low Risk” classification throughout the process, regardless of FD addition, with a notable decrease in the RAC value post-treatment. Ni in HTS, subjected to pyrolysis alone, retained a “Low Risk” status even at 700 °C; the introduction of FD further mitigated the risk to “No Risk”. In the case of Cd, the RAC value was reduced from 27.7 to 13.99 following FD-assisted pyrolysis at 700 °C, despite the initial “Medium Risk” classification. Overall, these results underscore the efficacy of FD-assisted high-temperature pyrolysis in diminishing the environmental hazards posed by heavy metals in sewage sludge.

Based on the BCR chemical speciation analysis of HMs, the C_f, E_r, and RI were calculated for HTS and derived biochar samples (

Table 4. Ecological risk assessment of the HMs in HTS and derived biochar.

Sample	Cf							Er
	Cr	Ni	Cu	Zn	As	Cd	Pb	Cr	Ni	Cu	Zn	As	Cd	Pb
HTS	0.29	6.30	1.94	0.76	0.21	3.68	0.04	0.57	37.80	9.69	0.76	2.09	110.5	0.19
B300-0	0.10	3.42	1.10	0.26	0.13	2.89	0.02	0.21	20.49	5.48	0.26	1.29	86.60	0.09
B300-20	0.09	2.30	0.99	0.24	0.14	2.37	0.02	0.19	13.79	4.94	0.24	1.39	71.09	0.08
B300-50	0.03	1.88	0.48	0.11	0.14	1.78	0.02	0.06	11.29	2.39	0.11	1.42	53.34	0.09
B500-0	0.05	2.81	1.27	0.31	0.15	1.46	0.01	0.11	16.87	6.35	0.31	1.49	43.82	0.07
B500-20	0.05	1.56	0.83	0.21	0.14	1.30	0.01	0.10	9.37	4.16	0.21	1.37	38.96	0.07
B500-50	0.02	1.26	0.44	0.15	0.11	0.89	0.01	0.04	7.54	2.18	0.15	1.13	26.59	0.07
B700-0	0.02	0.81	1.17	0.18	0.08	1.20	0.01	0.03	4.88	5.86	0.18	0.78	36.06	0.07
B700-20	0.01	0.35	0.52	0.03	0.06	0.87	0.01	0.01	2.08	2.59	0.03	0.64	26.16	0.07
B700-50	0.01	0.32	0.40	0.02	0.07	0.72	0.01	0.02	1.93	1.98	0.02	0.69	21.60	0.06

and Figure 4). The impact of pyrolysis temperature and FD addition ratio on HMs reduction varied for different HMs, resulting in the difference in the variation of E_r values observed in the biochar.

In the HTS, the ecological risk (E_r) values for all HMs except Cd were categorized as low risk. Ni was on the cusp of the threshold with an E_r value of 37.80, just under the limit of 40. However, Cd presented a significant concern in HTS, with an E_r value of 110.45, surpassing the moderate-risk threshold of 80. Following pyrolysis at 300 °C and 500 °C, the E_r values for Cd remained at considerable (86.60) and moderate (43.82) risk levels, respectively. Yet, the biochar produced at 700 °C demonstrated a low risk for HMs, although the E_r value for Cd was still relatively high, nearing the threshold.

This highlights the critical role of FD-assisted pyrolysis in mitigating the ecological risks posed by HMs. Notably, incorporating FD during pyrolysis at 500 °C significantly lowered the E_r values, with 20% and 50% FD additions reducing them to 38.96 and 26.59, respectively. At 700 °C, these values decreased further to 26.16 and 21.60 with 20% and 50% FD additions, respectively. The risk index (RI) also reflected the high risk associated with Cd, contributing over 68% in both HTS and derived biochar, indicating a substantial potential ecological risk. However, this risk can be reduced to a low level when the pyrolysis temperature exceeds 500 °C. A better outcome was observed with a 50% FD addition at 700 °C-assisted pyrolysis, which brought the RI value down to 26.29.

4. Conclusions

FD, besides being a solid waste, is rich in calcium oxide, which serves as a potential alkali catalyst. This study leveraged this characteristic by using FD as an additive in co-pyrolysis with HTS to enhance biochar stability and mitigate heavy metal risks. The effects of three pyrolysis temperatures (300 °C, 500 °C, and 700 °C) and varying FD additive ratios were examined in detail. Results demonstrated that FD significantly reduced the concentrations of Cr, Ni, Cu, Zn, As, Cd, and Pb in HTS, especially Cu and Zn, through a dilution effect. Higher pyrolysis temperatures also promoted heavy metal stabilization, and the inclusion of FD notably increased the residual fraction of Ni and Cu. Moreover, the ecological risk index analysis indicated that biochar produced with a 50% FD blend at 700 °C posed minimal ecological risk. This research highlights the co-management of HTS and FD, emphasizing the environmental safety and practical applications of the resulting biochar. Future work will further explore the use of B700-50 as an adsorbent material, construction material, and soil conditioner, with findings to be reported later.