Optimized Biochar from Chicken Manure via Hydrothermal Activation and Catalytic HTC: Properties and CO₂ Reduction Potential

Abstract

Chicken manure (CM) is a nutrient-rich but environmentally problematic biomass that requires sustainable management. This study applied a three-step process consisting of hydrothermal activation (ZnCl₂ or H₃PO₄), catalytic hydrothermal carbonization (HCl or FeCl₃), and low-temperature pyrolysis (250 °C) to develop an energy-efficient method for producing biochar. The resulting biochars were systematically analyzed for their physicochemical properties, heavy metal content, and carbon sequestration potential, and compared with conventional pyrolysis-based biochars. Among the tested samples, the biochar produced via H₃PO₄ activation and HCl-catalyzed HTC [P-HTC(HCl)] exhibited the most favorable characteristics, including the highest carbon content (59.5 wt.%) and the lowest H/C ratio (0.65). As a result, it achieved the highest total potential carbon (TPC, 158.8 g_carbon/kg_biochar) and CO₂ reduction potential (CRP, 465.9 g_CO2-eq/kg_biochar), attributed to the strong dehydration and decarboxylation reactions and effective inorganic removal induced by Brønsted acid action. In contrast, conventional pyrolysis biochars showed significantly higher concentrations of heavy metals—up to 633 mg/kg of Cu and 2331 mg/kg of Zn—due to thermal concentration effects, whereas P-HTC(HCl) biochar presented a more balanced and environmentally acceptable heavy metal profile. In conclusion, the proposed low-temperature hydrothermal-assisted process demonstrates great potential for producing high-performance biochar from chicken manure with enhanced environmental safety and carbon storage efficiency.

1. Introduction

Chicken manure (CM), a byproduct of the poultry industry, is generated in millions of tons annually. If not properly managed, its high organic content and nutrient overload can lead to greenhouse gas (GHG) emissions, water pollution, and soil contamination [1,2,3]. Among various valorization strategies for CM, biochar production has gained attention due to its potential for carbon sequestration. While biochar is generally known to provide approximately 20% carbon sequestration efficiency, the application of CM-derived biochar has been suggested to further enhance this effect [4,5,6,7].

Pyrolysis is the most conventional method for biochar production; however, it requires high temperatures (350–700 °C) and a pre-drying step, making it energy-intensive when applied to feedstocks with high moisture content (40–60 wt.%) such as CM [8,9,10,11]. To address this limitation, HTC has been proposed as a pre-treatment method, eliminating the need for drying and reducing energy consumption by approximately 40–70% [12,13]. HTC operates at moderate temperatures (180–280 °C), where decarboxylation, dehydration, and hydrolysis facilitate the aromatization of hydrocarbons, resulting in a densely carbonized structure and enhanced porosity in the final product [14,15].

Recently, various HTC-based integrated processes have been actively studied to enhance the physicochemical properties and functionality of biochar. These processes include sequential carbonization (HTC–pyrolysis), catalytic HTC, and hydrothermal activation. Shi et al. applied a sequential carbonization process to agricultural digestate and reported that the carbon content of the resulting biochar increased up to 59.86 wt.%. They also found that while standalone pyrolysis required up to 1557.6 MJ/ton of energy, the integrated HTC–pyrolysis process consumed only 960.6 MJ/ton [16]. This demonstrates not only the improved properties of the resulting biochar but also the practical feasibility of the integrated approach. Such findings support the notion that sequential carbonization processes can be a promising strategy, offering higher carbonization efficiency and milder operational conditions compared to single-step pyrolysis.

Meanwhile, Ipiales et al. reported that the application of HCl in the HTC process promoted the hydrolysis and decarboxylation of pig manure, resulting in a 47.9% increase in the carbon content of the hydrochar, reaching up to 57.7 wt.% compared to non-catalytic conditions [17]. In contrast, Tran et al. added FeCl₃ during HTC, which enhanced the aromatization of lignin and unsaturated hydrocarbons, leading to a 16.72 wt.% increase in carbon content and approximately a 35% improvement in heating value [18]. These results indicate that HCl and FeCl₃, as representative HTC catalysts with Brønsted and Lewis acid characteristics respectively, can reinforce carbonization reactions by facilitating the hydrolysis, dehydration, decarboxylation, and aromatization of lignocellulosic components [19].

Chatir et al. demonstrated that HTC of nut shells, combined with H₃PO₄ activation, promoted mesopore formation and carbonization, yielding a biochar with a specific surface area of 1880 m²/g and a carbon content of 90.08 wt.% [20]. Additionally, Allwar et al. investigated hydrothermal activation with ZnCl₂ (80 °C) and subsequent HTC (200 °C), reporting improved carbonization and porosity, leading to enhanced phenol adsorption capacity [21].

In hydrothermal environments, ZnCl₂ is known to be effective in promoting dehydration-based carbonization, while H₃PO₄ contributes to structural stabilization and the formation of aromatic structures. Both activators facilitate the development of carbon-retentive structures even under hydrothermal conditions. Such integrated processes have been shown to generate strong synergistic effects, significantly enhancing the properties of the resulting biochar.

These synergistic interactions also improve process efficiency, allowing for the relaxation of reaction conditions such as temperature and residence time. Therefore, investigating the synergistic effects between multiple processes is essential for the commercialization of these technologies. However, research on this promising approach for biochar production remains limited. Accordingly, this study proposes a novel biochar production strategy combining hydrothermal activation with sequential carbonization. The effects of hydrothermal activators (ZnCl₂ or H₃PO₄) and HTC catalysts (HCl or FeCl₃) on the characteristics of the resulting biochars were systematically investigated. In particular, considering the multi-step nature of the process, the final pyrolysis was conducted at a low temperature (250 °C), which is the threshold for initiating carbonization. The physicochemical properties and carbon sequestration potential of the produced biochars were then compared with those produced via conventional methods.

2. Materials and Methods

2.1. Materials

Chicken manure (CM) was collected from a livestock farm located in Gurey-gun, Jeollanam-do, Republic of Korea. The physicochemical properties of the CM feedstock are presented in

Table 1. Physicochemical properties of biochar produced with activators (ZnCl₂ and H₃PO₄).

Sample	FC (wt.%)	VM (wt.%)	Ash (wt.%)	FR	C (wt.%)	H (wt.%)	N (wt.%)	O (wt.%)	S (wt.%)	H/C	O/C	SSA (m2/g)	HHV (MJ/kg)	Yield (wt.%)
CM	10.1 (0.5)	69.8 (1.5)	20.1 (0.3)	0.14 (0.01)	40.4 (0.3)	5.1 (0.1)	3.3 (0.1)	30.1 (0.5)	1.0 (0.1)	1.51 (0.05)	0.56 (0.01)	0.70	15.90 (0.16)	-
Control	27.1 (1.1)	31.3 (0.5)	41.6 (1.0)	0.87 (0.09)	39.4 (0.1)	2.7 (0.1)	4.2 (0.2)	11.7 (0.4)	0.4 (0.1)	0.82 (0.02)	0.22 (0.02)	4.18	14.44 (0.11)	47.3 (2.1)
Zn-HTC	40.5 (1.6)	32.0 (0.4)	27.5 (0.6)	1.27 (0.03)	47.7 (0.1)	3.1 (0.1)	4.2 (0.1)	17.2 (0.4)	0.3 (0.1)	0.78 (0.01)	0.27 (0.02)	8.79	17.47 (0.14)	43.6 (1.8)
P-HTC	36.6 (1.6)	34.1 (0.6)	29.3 (0.5)	1.07 (0.01)	45.6 (0.2)	3.0 (0.1)	4.7 (0.1)	16.8 (0.4)	0.6 (0.1)	0.79 (0.01)	0.28 (0.01)	7.91	16.66 (0.19)	36.1 (1.2)

.

Chemical reagents used in this study were of reagent grade and obtained from Merck (Darmstadt, Germany). Zinc chloride (ZnCl₂, ≥98%) and phosphoric acid (H₃PO₄, ≥85%) were used for hydrothermal activation, while hydrochloric acid (HCl, ≥37%) and ferric chloride (FeCl₃, 97%) were employed as catalysts for HTC. Additionally, nitric acid (HNO₃, 70%) was used for post-treatment washing after ZnCl₂ activation.

2.2. Experimental Procedure

The overall experimental procedure is illustrated in Figure 1. For hydrothermal activation, two separate batches of CM (500 g each) were individually mixed with 10 wt.% aqueous solutions of ZnCl₂ and H₃PO₄, respectively, maintaining a solid-to-solution ratio of 1:1. Each mixture was then heated at 80 °C for 4 h to ensure proper impregnation, a condition previously identified to enhance porosity [21]. After activation, solids were separated by vacuum filtration. The ZnCl₂-treated solid was washed three times with 10 wt.% nitric acid (HNO₃), while the H₃PO₄-treated solid was rinsed with hot water to remove impurities and residual chemicals [21,22].

Following activation, HTC was conducted in a 3 L batch reactor. Each activated mixture was mixed with distilled water in a precise ratio to achieve a solid-to-water ratio of 1:4. Subsequently, 1.2 M solutions of HCl and FeCl₃ were individually added as HTC catalysts.

The reactor was heated to 250 °C at a rate of 5 °C/min under optimized carbonization conditions and maintained for 4 h [23,24,25]. It was then cooled down to 80 °C, followed by vacuum filtration. The solid product was then dried at 105 °C for 12 h. Depending on the applied activator, activated products produced through non-catalytic HTC were labeled as “Zn-HTC” and “P-HTC”. Additionally, to evaluate the effect of activator addition, a product produced without any activators was labeled as “Control”.

Following this process, the dried solid products were subjected to pyrolysis in a 15 L muffle furnace, where they were heated at a rate of 10 °C/min under a nitrogen (N₂) flow of 100 mL/min until they reached 250 °C and maintained at this temperature for 1 h. This pyrolysis temperature was selected based on previous studies as the minimum requirement for sufficient volatilization and enhanced carbonization [26]. The biochars produced under different conditions were labeled as “Zn-HTC(HCl)”, “Zn-HTC(Fe)”, “P-HTC(HCl)”, and “P-HTC(Fe)” according to the applied hydrothermal activators and HTC catalysts.

For comparison with the conventional biochar production method, additional biochars were produced by first drying CM at 105 °C for 12 h before subjecting it to pyrolysis. The pyrolysis was conducted at 250 °C, 500 °C, and 750 °C for 1 h under a nitrogen (N₂) atmosphere with a flow rate of 100 mL/min, with the temperature increased at a rate of 10 °C/min. The resulting biochars were labeled as “Py-250”, “Py-500”, and “Py-750”, respectively.

2.3. Characterization

The produced biochar was analyzed for its mass yield, proximate and elemental composition, higher heating value (HHV), specific surface area (SSA), heavy metal content, and CO₂ reduction potential (CRP).

2.3.1. Mass Yield

The mass yield of biochar was calculated using Equation (1):

$$\mathrm{Mass}\mathrm{yield}(\mathrm{wt}.\%)={\displaystyle \frac{\mathrm{Dried}\mathrm{biochar}\left(\mathrm{g}\right)}{\mathrm{Dried}\mathrm{chicken}\mathrm{manure}\left(\mathrm{g}\right)}}\times 100(\mathrm{wt}.\%)$$

(1)

2.3.2. Proximate and Elemental Analysis

Proximate analysis was conducted according to ASTM D1762-84 [27] to determine volatile matter (VM) and ash content. The fixed carbon (FC) content was calculated using Equation (2):

FC (wt.%) = 100 (wt.%) − Ash (wt.%) − VM (wt.%)

(2)

The fuel ratio (FR), an indicator of biochar stability, was calculated based on proximate analysis using Equation (3):

$$\mathrm{Fuel}\mathrm{ratio}\left(\mathrm{FR}\right)={\displaystyle \frac{\mathrm{Fixed}\mathrm{carbon}\left(\mathrm{FC}\right)(\mathrm{wt}.\%)}{\mathrm{Volatile}\mathrm{matter}\left(\mathrm{VM}\right)(\mathrm{wt}.\%)}}$$

(3)

Elemental composition was determined using a TurSpec Elemental Analyzer (LECO Co., St. Joseph, MI, USA) to measure carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents. The oxygen (O) content was computed using Equation (4):

O (wt.%) = 100 (wt.%) − (C + H + N + S + Ash) (wt.%)

(4)

Based on the elemental composition, including ash, the higher heating value (HHV) was estimated using Equation (5) [28]:

HHV (MJ/kg) = 0.3491·C + 1.033·H + 0.1005·S − 0.0151·N − 0.103·O − 0.0211·Ash

(5)

2.3.3. Specific Surface Area (SSA)

The specific surface area (SSA) of biochar was analyzed using BET analysis based on N₂ adsorption–desorption isotherms. The analysis was conducted using a porosimetry analyzer (Autosorb IQ, Quantachrome, Thane, India).

2.3.4. Heavy Metal Content

The concentrations of heavy metals (Cd, As, Zn, Cu, Pb, Ni, and Cr) in biochar were determined using ICP-OES (Optima 8300, PerkinElmer, Waltham, MA, USA), following the ASTM D6349-21 standard [29].

2.3.5. CO₂ Reduction Potential (CRP)

Mean residence time (MRT), BC₊₁₀₀ (carbon remaining after 100 years), total potential carbon (TPC), and CO₂ reduction potential (CRP) are key indicators for assessing the CO₂ reduction capacity of biochar. These indicators were calculated using a previously established method detailed in Equations (6)–(9) [30]. MRT and BC₊₁₀₀, which evaluate the stability of biochar, were calculated using Equations (6) and (7), respectively.

MRT (years) = 4501 × exp(−3.2(H/C))

(6)

BC₊₁₀₀ (wt.%) = 1.05 − 0.616(H/C)

(7)

The TPC, representing the carbon sequestration potential of biochar in soil, was calculated using Equation (8), and the CRP, indicating the CO₂ reduction potential of biochar, was calculated using Equation (9).

TPC (g_carbon/kg_biochar) = Mass yield (wt.%) × FC (wt.%)

(8)

CRP (g_CO2-eq/kg_biochar) = TPC × (80/100) × (44/12)

(9)

3. Results and Discussion

3.1. Effect of Hydrothermal Activators on Biochar

Table 1 presents the proximate and elemental analysis results of biochar produced from chicken manure (CM) using different hydrothermal activators. When biochar is applied to soil, fixed carbon plays a crucial role in long-term carbon sequestration [31]. A higher fixed carbon content generally indicates a greater carbon sequestration potential, and the fuel ratio (FC/VM) is used to compare the relative stability of biochar in soil.

The raw CM exhibited a fixed carbon content of 10.1 wt.% and a volatile matter content of 69.8 wt.%, resulting in a low fuel ratio of 0.14, which aligns with the fuel characteristics of typical poultry manure. Such low fixed carbon content suggests that the biochar derived from untreated CM may decompose more readily in soil over time, limiting its effectiveness for long-term carbon sequestration.

In contrast, biochars produced through pre-activation and sequential carbonization showed significant changes in their characteristics. Zn-HTC and P-HTC samples exhibited fixed carbon contents of 40.5 wt.% and 36.6 wt.%, respectively, with improved fuel ratios of 1.27 and 1.07. This indicates that both ZnCl₂ and H₃PO₄ effectively enhanced the fixed carbon content, although their influence on pyrolysis behavior varied. Zn activation promoted dehydration reactions, increasing the carbon density, while the strong Lewis acid properties of ZnCl₂ modified the chemical structures of lignin, cellulose, and hemicellulose, facilitating deoxygenation (DO) even at lower temperatures [32]. On the other hand, H₃PO₄ activation induced phosphorylation, reinforcing carbon–carbon bonding through interactions with polyphenols and lignin, thereby increasing the carbon content [33].

Consequently, hydrothermal activation using ZnCl₂ and H₃PO₄ promotes the aromatization and stabilization of carbonaceous structures even at low temperatures through a combination of mechanisms: (i) acid-catalyzed depolymerization and dehydration of biopolymers, (ii) esterification and cross-linking reactions induced by the activators, and (iii) the breakdown of complex fiber structures such as hemicellulose, cellulose, and lignin. These reactions collectively facilitate the removal of hydrogen and oxygen elements in the form of H₂O, CO₂, and other volatiles during carbonization, ultimately yielding biochar with a higher fixed carbon content [34].

As expected, the increase in fixed carbon density due to chemical activation was accompanied by a reduction in mass yield. The mass yields of biochars produced using ZnCl₂ and H₃PO₄ were 43.6 wt.% and 36.1 wt.%, respectively, significantly lower than that of the Control sample. The lower yield of H₃PO₄-activated biochar can be attributed to the strong acidic nature of phosphoric acid, which enhances biomass degradation.

Conversely, biochar produced without an activator (Control) exhibited a significantly lower fixed carbon content (27.1 wt.%) and fuel ratio (0.87) than its activated counterparts. This clearly demonstrates that ZnCl₂ activation promoted dehydration reactions, while H₃PO₄ activation influenced fixed carbon formation through phosphorylation.

Elemental analysis results further support these findings. The Zn-HTC sample exhibited the highest carbon content (40.5 wt.%), followed by P-HTC (36.6 wt.%) > CM (40.4 wt.%) > Control (39.4 wt.%). This trend aligns with the increase in fixed carbon content, confirming that chemical activation with ZnCl₂ and H₃PO₄ contributed to carbon retention.

The H/C and O/C ratios, which indicate the degree of carbonization, demonstrated a stable carbonization process across all samples. The Zn-HTC sample exhibited the lowest H/C ratio (0.78), whereas the Control sample had the lowest O/C ratio (0.22). These results suggest that, compared to the Control sample (H: 2.7 wt.%, O: 11.7 wt.%), Zn-HTC (H: 3.1 wt.%, O: 17.2 wt.%) and P-HTC (H: 3.0 wt.%, O: 16.8 wt.%) contained relatively higher hydrogen and oxygen contents. This phenomenon can be attributed to the limited activation reaction rate at a low pyrolysis temperature (250 °C).

Zhang et al. (2024) reported that Zn activation facilitates dehydrogenation and deoxygenation during pyrolysis by promoting aromatization and removing functional groups such as methyl (−CH₃) and hydroxyl (−OH) [35].

However, at temperatures below 300 °C, residual hydrogen and oxygen may remain, thereby diminishing these effects. Similarly, Huang et al. suggested that at lower temperatures, some oxygen-containing functional groups released through phosphorylation reactions could be retained in the carbon matrix, suppressing the dehydrogenation process [36]. Accordingly, due to the low activation efficiency of ZnCl₂ and H₃PO₄ at reduced temperatures, the resulting biochars exhibited limited BET surface areas. All biochars showed a moderate increase in porosity, with Zn-HTC exhibiting the highest BET surface area (8.79 m²/g), followed by P-HTC (7.91 m²/g) and the Control (4.18 m²/g). ZnCl₂ is known to promote dehydrogenation and deoxygenation, allowing Zn to penetrate the carbon layers and act as a “pore-drilling” agent. This facilitates the formation of a porous structure by creating pathways for volatile compounds to escape through the carbon matrix [34]. Similarly, in the case of H₃PO₄, the enhanced phosphorylation and dehydration reactions help alleviate biomass structure shrinkage. The formation of polyphosphate intermediates temporarily fills voids in the carbon structure, which are later removed during washing, leaving behind mesopores [34]. Nevertheless, the BET surface areas of all samples remained below 10 m²/g, indicating that the porosity achieved was insufficient to significantly enhance the functional properties of the biochar. This limitation is likely due to the reduced reactivity of ZnCl₂ and H₃PO₄ under low-temperature conditions, as described earlier.

The higher heating value (HHV) ranged from 14 to 18 MJ/kg across all samples, indicating that the energy density of biochar did not significantly increase compared to raw CM. The Control sample exhibited an HHV of 14.44 MJ/kg, which was lower than that of CM (15.90 MJ/kg), due to an increase in ash content (41.6 wt.%), which resulted from the loss of combustible components (FC+VM) during carbonization.

Similarly, the HHV values of Zn-HTC (17.47 MJ/kg) and P-HTC (16.66 MJ/kg) were not significantly higher, as their ash contents (Zn-HTC: 27.5 wt.%, P-HTC: 29.3 wt.%) remained elevated compared to CM. These findings indicate that, for organic waste materials with high ash content, improving energy density requires not only the formation of a carbon-rich structure but also controlling the ash behavior.

Considering the physicochemical changes observed in Table 1, it is evident that biochar with a high carbon content, increased energy density, and enhanced porosity requires pyrolysis at a sufficiently high temperature [11]. However, this study focuses on optimizing biochar production at a low pyrolysis temperature (250 °C) by integrating hydrothermal activation and catalytic HTC to minimize energy consumption.

The next section investigates how HTC catalysts influence pyrolysis efficiency, particularly in overcoming limitations associated with low-temperature pyrolysis, such as reduced activation efficiency and lower carbonization degree.

3.2. Effect of HTC Catalysts on Biochar

Table 2. Physicochemical properties of biochar produced with HTC Ccatalysts (HCl and FeCl₃).

Sample	FC (wt.%)	VM (wt.%)	Ash (wt.%)	FR	C (wt.%)	H (wt.%)	N (wt.%)	O (wt.%)	S (wt.%)	H/C	O/C	SSA (m2/g)	HHV (MJ/kg)	Yield (wt.%)
Zn-HTC(HCl)	46.1 (0.5)	37.2 (0.3)	16.7 (0.1)	1.24 (0.01)	56.2 (0.3)	3.4 (0.2)	3.8 (0.1)	19.6 (0.4)	0.3 (0.1)	0.73 (0.01)	0.26 (0.01)	33.19	20.73 (0.21)	31.0 (0.3)
Zn-HTC(Fe)	43.3 (1.1)	35.8 (0.4)	20.9 (0.8)	1.21 (0.02)	54.0 (0.2)	3.0 (0.2)	4.1 (0.1)	17.7 (0.3)	0.3 (0.1)	0.67 (0.02)	0.25 (0.01)	24.61	19.65 (0.10)	23.3 (0.8)
P-HTC(HCl)	47.7 (0.5)	36.5 (0.2)	15.8 (0.1)	1.31 (0.04)	59.5 (0.2)	3.2 (0.1)	3.7 (0.1)	17.4 (0.5)	0.4 (0.1)	0.65 (0.01)	0.22 (0.01)	19.00	21.94 (0.11)	33.3 (0.2)
P-HTC(Fe)	45.7 (0.6)	35.4 (0.6)	18.9 (0.3)	1.29 (0.01)	56.8 (0.1)	3.4 (0.1)	3.6 (0.1)	16.9 (0.4)	0.4 (0.1)	0.72 (0.01)	0.22 (0.01)	14.26	21.19 (0.13)	25.0 (1.0)

presents the key physicochemical properties of biochar produced through hydrothermal activation with ZnCl₂ and H₃PO₄, followed by HTC with different catalysts. HCl and FeCl₃ are known to enhance carbonization, energy properties, and porosity during HTC due to their unique chemical properties [37,38]. However, research on the synergistic effects of these catalysts with hydrothermal activators (ZnCl₂, H₃PO₄) remains extremely limited. Thus, this study investigates how the physicochemical characteristics of biochar change when different HTC catalysts (HCl, FeCl₃) are applied based on the selected hydrothermal activators.

Overall, regardless of the type of hydrothermal activator used, the addition of a catalyst significantly improved fixed carbon content, fuel ratio, total carbon content, H/C and O/C ratios, as well as BET surface area. Notably, HCl exhibited a greater enhancement in biochar properties than FeCl₃. For instance, Zn-HTC(HCl) recorded a fixed carbon content of 46.1 wt.% and a total carbon content of 56.2 wt.%, whereas Zn-HTC(Fe) exhibited slightly lower values at 43.3 wt.% and 54.0 wt.%, respectively. However, the H/C and O/C ratios for Zn-HTC(Fe) were 0.67 and 0.25, respectively, indicating a more stable carbon structure [39].

Both ZnCl₂ and FeCl₃ function as Lewis acids, which facilitate strong dehydration effects during activation and HTC, potentially contributing to the enhanced carbon stability of Zn-HTC(Fe) [40,41]. However, in terms of BET surface area, Zn-HTC(HCl) exhibited the highest value (33.19 m²/g) among all produced biochars, which can be attributed to the synergistic effects between Zn activation and HCl catalysis. This enhancement is likely due to the removal of residual ZnO and Zn(OH)₂ formed on the biochar surface during Zn activation, which were later dissolved by HCl during HTC, leading to the formation of mesopores [42,43]. These results confirm that the synergistic effect between Zn activation and HCl catalysis contributed not only to a more stabilized carbon structure but also to improved textural properties of the biochar.

Similarly, the synergistic effects of H₃PO₄ activation and HTC catalysts (HCl, FeCl₃) followed a comparable trend. As observed with Zn activation, HCl consistently outperformed FeCl₃ in enhancing biochar properties. P-HTC(HCl) exhibited the highest fixed carbon content (47.7 wt.%) and total carbon content (59.5 wt.%), while also recording the highest HHV (21.94 MJ/kg). In terms of BET surface area, P-HTC(HCl) showed a higher porosity (19.00 m²/g) compared to P-HTC(Fe), further confirming that the synergy between H₃PO₄ activation and HCl catalysis contributed more significantly to improving biochar characteristics than FeCl₃.

Furthermore, the H/C and O/C ratios of P-HTC(HCl) were 0.65 and 0.22, respectively, indicating the highest degree of carbonization among all samples. This suggests that Brønsted acids (e.g., HCl) play a more dominant role than Lewis acids (e.g., FeCl₃) during HTC. Specifically, Lewis acids (FeCl₃) promote lignin decomposition, enhancing hydrocarbon aromatization and improving carbonization, whereas Brønsted acids (HCl) contribute more significantly to the degradation of lower molecular weight fiber components such as cellulose and hemicellulose. Since livestock manure, including CM, is primarily composed of cellulose and hemicellulose, HCl proves to be a more effective catalyst than FeCl₃ in HTC [44,45].

Ultimately, strong acids like HCl rapidly hydrolyze the glycosidic bonds in cellulose and hemicellulose, releasing monosaccharides that subsequently undergo dehydration, decarboxylation, and aromatization reactions—eventually forming condensed carbonaceous solids [19]. In contrast, FeCl₃ facilitates the formation of condensed carbon structures through stepwise hydrolysis reactions (FeCl₃ + 2H₂O → FeOCl·H₂O + 2HCl↑; FeOCl·H₂O → FeOOH + HCl↑), during which the in situ generation of HCl enhances the catalytic effects [46]. Therefore, direct application of HCl is likely to promote carbon condensation more effectively than FeCl₃, which generates HCl indirectly. In addition, the mass yield of biochar treated with HCl was lower than that treated with FeCl₃, indicating a more extensive conversion of fibrous components into condensed carbon structures under HCl catalysis.

These observations further support that the choice of hydrothermal activator and HTC catalyst plays a critical role in determining the carbon density, stability, and porosity of the resulting biochar. Specifically, the combination of ZnCl₂ and HCl primarily facilitated the development of porous surfaces, while the pairing of H₃PO₄ and HCl promoted the formation of a denser and more stable carbon structure.

3.3. Comparison with Conventional Biochar Production

3.3.1. Comparison of Physicochemical Properties of Biochar

Table 3. Physicochemical properties of biochar produced by various methods.

Sample	FC (wt.%)	VM (wt.%)	Ash (wt.%)	FR	C (wt.%)	H (wt.%)	N (wt.%)	O (wt.%)	S (wt.%)	H/C	O/C	SSA (m2/g)	HHV (MJ/kg)	Yield (wt.%)
Zn-HTC(HCl)	46.1 (0.5)	37.2 (0.3)	16.7 (0.1)	1.24 (0.01)	56.2 (0.3)	3.4 (0.2)	3.8 (0.1)	19.6 (0.4)	0.3 (0.1)	0.73 (0.01)	0.26 (0.01)	33.19	20.73 (0.21)	31.0 (0.3)
P-HTC(HCl)	47.7 (0.5)	36.5 (0.2)	15.8 (0.1)	1.31 (0.04)	59.5 (0.2)	3.2 (0.1)	3.7 (0.1)	17.4 (0.5)	0.4 (0.1)	0.65 (0.01)	0.22 (0.01)	19.00	21.94 (0.11)	33.3 (0.2)
Py-250	19.1 (0.2)	50.1 (0.6)	30.8 (0.2)	0.38 (0.01)	40.0 (0.3)	3.7 (0.1)	4.1 (0.2)	21.1 (0.2)	0.3 (0.1)	1.11 (0.02)	0.40 (0.01)	1.77	14.93 (0.35)	71.2 (2.6)
Py-500	32.9 (0.4)	21.2 (0.1)	45.9 (0.3)	1.55 (0.03)	40.1 (0.3)	1.7 (0.1)	2.8 (0.1)	9.4 (0.2)	0.1 (0.1)	0.51 (0.01)	0.18 (0.01)	14.73	13.79 (0.19)	42.7 (3.3)
Py-750	17.0 (0.2)	20.2 (0.1)	62.8 (0.5)	0.84 (0.01)	28.2 (0.3)	0.4 (0.1)	1.9 (0.1)	6.4 (0.1)	0.3 (0.1)	0.17 (0.01)	0.17 (0.01)	119.85	8.28 (0.09)	31.6 (2.1)

presents the key physicochemical properties of biochar produced using optimized activator–catalyst combinations compared to biochar produced through conventional pyrolysis at different temperatures. In conventional biochar production, higher pyrolysis temperatures lead to increased volatilization of organic components, resulting in lower volatile matter, H/C, and O/C ratios in Py-750. The mass yield decreased from 71.2 wt.% at 250 °C to 31.6 wt.% at 750 °C.

However, high-temperature pyrolysis also maximizes the loss of combustible matter, leading to a significant increase in ash content. As the pyrolysis temperature increased, the ash content of biochar increased from 30.8 wt.% at 250 °C to 45.9 wt.% at 500 °C and 62.8 wt.% at 750 °C. In contrast, Zn-HTC(HCl) and P-HTC(HCl) maintained significantly lower ash contents of 16.7 wt.% and 15.8 wt.%, respectively, due to the strong acidic nature of the activators and HCl catalyst, which enhanced the solubility of inorganic components, causing partial removal through the process water [17,47].

On the other hand, the rapid increase in ash content in conventional biochar production directly impacts carbon content and energy properties. The total carbon content decreased from 40.0 wt.% at 250 °C to 28.2 wt.% at 750 °C, while the higher heating value (HHV) declined from 14.93 MJ/kg to 8.28 MJ/kg. In contrast, Zn-HTC(HCl) and P-HTC(HCl) exhibited significantly higher total carbon contents of 56.2 wt.% and 59.5 wt.%, respectively, with HHV values of 20.73 MJ/kg and 21.94 MJ/kg, demonstrating superior carbon retention compared to conventional biochar.

Figure 2a illustrates the triangular (ternary) diagram representing fixed carbon, volatile matter, and ash content of CM and the produced biochars. CM exhibited a high volatile matter content and was positioned within the biomass region. Conventional biochars were located outside the solid fuel category, shifting further as pyrolysis temperature increased. This shift is attributed to the thermal concentration effect, where volatile matter is removed while residual mineral components become more concentrated in the solid phase, leading to a rapid increase in ash content [48,49].

Conversely, biochars produced with hydrothermal activation and HCl catalysis exhibited fuel maturity levels similar to lignite, as the strong acidic nature of HCl effectively removed ash and minimized the thermal concentration effect during pyrolysis. This finding suggests that for ash-rich feedstocks such as CM, conventional biochar production may significantly reduce their applicability as fuel and carbon materials due to excessive ash accumulation.

While conventional biochars experienced a rapid loss of organic components at high temperatures, their H/C and O/C ratios clearly decreased, indicating a higher degree of carbonization. Figure 2b presents a Van Krevelen diagram based on the H/C and O/C values of CM and the produced biochars. Similar to the ternary diagram (Figure 2a), CM was positioned within the biomass region. However, unlike the ternary diagram, conventional biochars exhibited a clear transition in fuel maturity: Py-250 was classified as lignite/peat, Py-500 as bituminous coal, and Py-750 as anthracite. This shift reflects enhanced devolatilization at higher temperatures, promoting the formation of more carbon-dense solid fuels.

In contrast, Zn-HTC(HCl) and P-HTC(HCl) were classified as lignite in the ternary diagram, but as bituminous coal in the Van Krevelen diagram. Compared to Py-250, which was produced under the same pyrolysis temperature (250 °C), these biochars exhibited significantly higher fuel maturity, suggesting enhanced carbonization efficiency despite the lower pyrolysis temperature.

Although the Van Krevelen diagram is commonly used to evaluate biochar carbonization, incorporating the ternary diagram in studies involving high-ash biomass and char can provide more precise insights into carbon sequestration and CO₂ reduction efficiency [26].

The BET surface area analysis further highlights the differences between conventional and hydrothermally activated biochars. Py-250 and Py-500 exhibited surface areas of 1.77 m²/g and 14.73 m²/g, respectively, which were lower than those of Zn-HTC(HCl) and P-HTC(HCl). However, Py-750 exhibited a significantly higher BET surface area (119.85 m²/g). Given its high ash content (62.8 wt.%), this increase in surface area is likely attributed to the formation of micropores via inorganic phase sintering, rather than structural rearrangement of the carbon matrix [50].

3.3.2. Comparison of Heavy Metal Distribution in Biochar

Table 4. Heavy metals content in biochar produced by various methods.

Sample	As (mg/kg)	Cd (mg/kg)	Pb (mg/kg)	Cr (mg/kg)	Cu (mg/kg)	Ni (mg/kg)	Zn (mg/kg)
Global Standard 1	13	1.5	120	90	100	50	400
CM	0.51 (0.01)	<0.10	<1.5	5.4 (0.6)	248 (4)	7.0 (0.4)	306 (12)
Zn-HTC(HCl)	0.88 (0.01)	<0.10	<1.5	6.2 (0.5)	222 (2)	6.3 (0.5)	12569 (132)
P-HTC(HCl)	0.98 (0.01)	<0.10	2.3 (0.1)	59.7 (0.9)	143 (6)	40.3 (1.2)	71 (6)
Py-250	1.43 (0.02)	<0.10	1.8 (0.1)	17.9 (0.8)	395 (9)	12.9 (0.9)	661 (16)
Py-500	7.51 (0.01)	<0.10	2.9 (0.1)	66.9 (1.6)	466 (8)	45.6 (1.1)	2191 (56)
Py-750	9.42 (0.01)	<0.10	3.2 (0.1)	97.4 (2.1)	633 (9)	55.0 (1.7)	2331 (38)

¹ In accordance with EBC-Agro and WBC-Premium class.

presents the concentrations of major heavy metals in the produced biochars. CM exhibited relatively stable heavy metal concentrations, but Cu and Zn levels were notably high at 248 mg/kg and 306 mg/kg, respectively. This can be attributed to feed additives and manure management practices [51].

Biochars produced through hydrothermal activation and catalytic HTC exhibited significant differences in heavy metal behavior depending on the treatment conditions. In particular, Zn-HTC(HCl) exhibited balanced heavy metal concentrations, except for Zn, which increased drastically to 12,567 mg/kg. This corresponds to approximately 31.4 times the international biochar standards (EBC-Agro and WBC-Premium Class, Zn: 400 mg/kg), making it unsuitable for soil amendments and environmental energy applications [52,53].

This result can be primarily attributed to the residual Zn from ZnCl₂ activation. However, since ZnCl₂ is commonly used as an activator, the excessive Zn retention can also be explained by Zn fixation during the production process. In conventional Zn activation, biochar is typically heat-treated at over 600 °C, whereas the biochars in this study underwent low-temperature activation (80 °C), followed by HTC and pyrolysis at 250 °C. This multi-step low-temperature treatment, despite including acid washing, may have promoted selective diffusion of Zn²⁺ ions into the biochar matrix, leading to strong Zn fixation [54].

In contrast, P-HTC(HCl) exhibited well-balanced heavy metal concentrations, including Zn, with a notable reduction compared to the original CM feedstock (Cu: 143 mg/kg; Zn: 71.4 mg/kg). This can be attributed to the strong Brønsted acid action of H₃PO₄ activation and HCl-catalyzed HTC, which effectively removed most heavy metals [55,56]. These results are consistent with the low ash content of P-HTC(HCl) (15.8 wt.%), indicating efficient mineral leaching during the production process.

Meanwhile, biochars produced through conventional pyrolysis exhibited increased heavy metal concentrations as pyrolysis temperature increased, which can be explained by the thermal concentration effect, where higher carbonization efficiency leads to excessive ash accumulation, concentrating inorganic components [57,58]. Notably, even in Py-250 (produced at the lowest temperature), Cu and Zn concentrations already exceeded international standards, reaching 395 mg/kg and 661 mg/kg, respectively.

At 750 °C (Py-750), severe thermal concentration effects resulted in extremely high Cu and Zn concentrations of 633 mg/kg and 2,331 mg/kg, respectively. Additionally, Cr and Ni also exceeded regulatory thresholds. These results indicate that when biochar is produced from heavy metal-contaminated feedstocks such as CM, thermal concentration effects must be considered, necessitating careful selection of production methods to mitigate heavy metal accumulation.

3.3.3. Comparison of Carbon Reduction Effects on Biochar

Table 5. Carbon reduction effects in biochar produced by various methods.

Sample	H/C	Yield (wt.%)	FC (wt.%)	MRT (years)	BC+100 (wt.%)	TPC (gcarbon/kgbiochar)	CRP (gCO2-eq/kgbiochar)	Ref.
Zn-HTC(HCl)	0.73 (0.01)	31.0 (0.3)	46.1 (0.5)	441 (34)	60.3 (6.0)	142.9 (14.3)	419.2 (22.0)	This study
P-HTC(HCl)	0.65 (0.01)	33.3 (0.2)	47.7 (0.5)	571 (37)	65.2 (6.6)	158.8 (15.9)	465.9 (26.3)
Py-250	1.11 (0.02)	71.2 (2.6)	19.1 (0.2)	129 (26)	36.6 (4.7)	136.0 (11.7)	398.9 (29.1)
Py-500	0.51 (0.01)	42.7 (3.3)	32.9 (0.4)	884 (92)	73.7 (9.2)	140.5 (10.3)	412.1 (28.4)
Py-750	0.17 (0.01)	31.6 (2.1)	17.0 (0.2)	2611 (161)	94.5 (9.4)	53.7 (5.8)	157.6 (16.0)
CM Biochar-300	0.95	68.8	19.2	215	46.5	132.1	387.5	[55]
CM Biochar-550	0.61	37.2	26.8	639	67.4	99.7	292.4	[56]
CM Biochar-600	0.28	35.6	37.5	1837	87.8	133.5	391.6	[57]

presents the calculated CO₂ reduction potential (CRP) and related parameters based on the physicochemical properties of the produced biochars. Additionally, a comparison with previous studies on CM biochar production was conducted to validate that the conventional biochar samples in this study were properly produced [59,60]. According to Venkatesh et al. the total potential carbon (TPC) and CRP of biochar can be estimated using Equations (8) and (9), both of which are primarily determined by the H/C ratio, mass yield, and fixed carbon content of the biochar [30].

MRT (mean residence time) and BC₊₁₀₀ (the percentage of carbon remaining after 100 years) serve as key indicators of biochar stability, primarily determined by the hydrogen-to-carbon (H/C) ratio. These values reflect the potential retention period of biochar in soil and its long-term carbon sequestration efficiency. In general, conventional biochars exhibited higher stability, with Py-750 showing the highest MRT (2611 years) and BC₊₁₀₀ (94.5 wt.%), making it the most stable biochar produced in this study. Conversely, Py-250 exhibited the lowest stability, with an MRT of 129 years and BC₊₁₀₀ of 36.6 wt.%, indicating that carbonization was insufficient at lower pyrolysis temperatures.

In contrast, biochars produced via hydrothermal activation and catalytic HTC showed considerably improved stability compared to Py-250. Zn-HTC(HCl) exhibited an MRT of 411 years and BC₊₁₀₀ of 60.3 wt.%, significantly higher than Py-250 despite being produced at the same pyrolysis temperature. Similarly, P-HTC(HCl) exhibited even higher stability, with an MRT of 571 years and BC₊₁₀₀ of 65.2 wt.%. This is attributed to its lower H/C ratio, indicating a more stable carbon structure. Notably, the stability of P-HTC(HCl) was higher than Py-250 but lower than Py-500, suggesting that its stability is comparable to conventional biochar produced at around 420 °C. This finding implies that the combination of H₃PO₄ activation and HCl catalysis not only enables effective carbonization at lower pyrolysis temperatures but also enhances CO₂ reduction potential.

TPC represents the total amount of carbon sequestered in biochar, while CRP quantifies the CO₂ reduction effect in terms of CO₂-equivalent emissions. Unlike MRT and BC₊₁₀₀, which evaluate biochar stability based solely on organic components, TPC and CRP provide a broader assessment, incorporating the overall characteristics of the produced biochar. Among the conventional biochars, higher pyrolysis temperatures generally improved stability, but TPC and CRP peaked at Py-500. Py-500 exhibited the highest TPC (140.5 g_carbon/kg_biochar) and CRP (412.1 g_CO2-eq/kg_biochar). However, TPC and CRP values decreased sharply in Py-750, with TPC dropping to 56.7 g_carbon/kg_biochar and CRP to 157.6 g_CO2-eq/kg_biochar, significantly lower than even Py-250. Interestingly, Py-250 exhibited a TPC of 136.0 g_carbon/kg_biochar and CRP of 398.9 g_CO2-eq/kg_biochar, showing only a minor difference from Py-500. This result can be attributed to the high mass yield (71.2 wt.%) of Py-250, which partially compensated for its lower fixed carbon content.

On the other hand, Zn-HTC(HCl) and P-HTC(HCl) exhibited higher TPC and CRP values than conventional biochars. Despite their relatively lower mass yields, these biochars demonstrated greater carbon sequestration capacity due to their denser carbon structures and enhanced stability. Notably, P-HTC(HCl) exhibited the highest TPC (158.8 g_carbon/kg_biochar) and CRP (465.9 g g_CO2-eq/kg_biochar), making it the most effective biochar for CO₂ reduction among all samples.

These results indicate that despite being produced at a lower pyrolysis temperature, H₃PO₄ activation and HCl catalysis facilitated the formation of a highly stable and carbon-rich structure, significantly enhancing carbon sequestration potential.

4. Conclusions

This study employed a three-step process—hydrothermal activation (ZnCl₂ or H₃PO₄), catalytic hydrothermal carbonization (HCl or FeCl₃), and low-temperature pyrolysis (250 °C)—to produce biochar from chicken manure (CM), and compared its properties with those of conventionally pyrolyzed biochars. The activators enhanced the degree of carbonization and structural properties through dehydration and phosphorylation reactions. Among the catalysts, HCl promoted hydrolysis, decarboxylation, and aromatization, resulting in the highest carbon density. Specifically, the ZnCl₂–HCl combination contributed to increased porosity, while the H₃PO₄–HCl combination facilitated the formation of a stable, condensed carbon structure. As a result, Zn-HTC(HCl) exhibited the highest BET surface area (33.19 m²/g), and P-HTC(HCl) showed the highest carbon content (59.5 wt.%) and the lowest H/C ratio (0.65). Due to this condensed carbon structure, P-HTC(HCl) also demonstrated the best carbon sequestration performance, recording the highest values for both total potential carbon (TPC, 158.8 g_carbon/kg_biochar) and CO₂ reduction potential (CRP, 465.9 g_CO2-eq/kg_biochar). In terms of heavy metal distribution, P-HTC(HCl) exhibited the most balanced profile, attributed to the strong acidity of both the activator and catalyst, which facilitated the removal of inorganic components. In contrast, Zn-HTC(HCl) and conventional biochars showed excessive Zn retention or thermal concentration, limiting their environmental applicability. In conclusion, the combination of H₃PO₄ activation and HCl catalysis proved to be an effective strategy for promoting the formation of a condensed carbon structure in biochar, thereby enhancing its carbon sequestration potential.