Converting Agroforestry Biowaste into Stable Near-Natural Chars via Hydrothermal Humification and Pyrolysis for Immobilizing Plasticizer

Abstract

To ensure agricultural safety and ecological security, it is crucial to effectively immobilize emerging organic pollutants, such as plasticizers, to prevent their migration in various environmental matrices. However, the ideal immobilization agent with the advantages of being environmentally friendly is very rare. In this study, low-cost and stable near-natural immobilization agents, char-derived artificial humic acids, CHAs, were proposed and prepared via hydrothermal humification (180 °C) and pyrolysis (300, 500, or 700 °C) of agroforestry biowaste. The resulting CHAs exhibit high purity (composed primarily of C (67.28–81.35%), O (6.65–21.64%), H (1.40–5.28%), and N (0.36–0.58%)) with remarkably low ash content (5.43–10.02%). Characterization revealed a compact structure with a limited porosity with small surface area (0.27–0.32 m² g⁻¹) and pore volume (2.99–3.43 × 10⁻⁴ cm³ g⁻¹). Notably, high-temperature pyrolysis induced consumption of oxygen-containing functional groups while promoting aromatic structure formation. The sorption behavior of diethyl phthalate, a representative plasticizer, on CHAs was well described by both Langmuir isotherm and pseudo-second-order kinetic models. The CHAs exhibited remarkable sorption performance for diethyl phthalate, with a maximum sorption capacity reaching 3345 mg kg⁻¹ as determined by the Langmuir model. The sorption of diethyl phthalate onto CHAs is mainly multi-layer sorption dominated by physical processes, mainly including pore filling, partitioning, hydrogen bonding, and π–π stacking. Mean sorption energies ranging from 2.56 to 4.99 × 10⁻³ kJ mol⁻¹ indicate the predominance of physical sorption mechanisms. This study developed a method to convert the liquid by-product produced during hydrothermal humification of biowaste into stable near-natural and carbon-rich char materials, and the proposed materials show great promise in immobilizing pollutants from various environmental matrices.

1. Introduction

Phthalic acid esters (PAEs), widely employed as plasticizers to enhance the durability and flexibility of plastics, are typically incorporated into plastic products at concentrations of 20–40% by weight [1,2]. These compounds are ubiquitous in personal care products, consumer goods, and agricultural and industrial materials. However, since PAEs are not chemically bonded to the plastic matrix, they can readily leach out and migrate into the surrounding environment [3]. Especially in agricultural production, the use of greenhouse plastics and plastic films has led to extremely serious pollution of soil PAEs, reaching levels of mg/kg. The bioaccumulation and biomagnification characteristics of PAEs are a significant threat to food security. Among PAEs, diethyl phthalate (DEP) is one of the most commonly used components. Notably, DEP has been demonstrated to exhibit significant toxicity to human health, particularly affecting the reproductive system and fetal development, as evidenced by multiple toxicological studies [4,5]. Due to its weak molecular interactions with plastics, DEP easily leaches into environmental media and enters organisms through direct contact, pharmaceutical exposure, dietary intake, and other pathways [6]. This widespread dispersion poses significant risks to ecosystems and human health. Therefore, effective strategies to immobilize DEP and minimize its migration in various environmental matrices are critically needed, especially in agricultural production.

Humic acids are complex mixtures of organic compounds naturally present in aquatic and terrestrial environments, where they play a crucial role in element cycling and the migration and transformation of pollutants [7]. These macromolecules contain diverse functional groups, such as N- and O-containing moieties, which enable hydrogen bonding with organic pollutants. Additionally, their hydrophobic components facilitate the partitioning of organic contaminants, while aromatic benzene rings contribute to pollutant immobilization through π–π stacking interactions [8]. Due to these properties, humic acids can effectively immobilize DEP in environmental systems. Naturally occurring humic acids derive from the decomposition of animal, plant, and microbial residues, forming through microbial humification processes predominantly in soils and sediments. Notably, high-pressure hydrothermal environments induced by crustal movements can also accelerate the formation of humic acids. However, the natural abundance of humic acids is limited, and their extraction from soils or coal is challenging, restricting their large-scale application for remediating DEP-contaminated environments.

Recently, hydrothermal humification (operated at 180–300 °C) has been proven to be a reliable method for converting lignocellulosic biomass—such as wheat straw, corn stalk, tea residue, and sugarcane bagasse—into artificial humic acids [9,10]. Moreover, a recent study demonstrated that the yield of artificial humic acids from the hydrothermal carbonization of corn straw can reach 28.28% [11]. Compared to the production of artificial humic acid through biological fermentation, hydrothermal humification requires extremely less time. The high conversion efficiency of biowaste into artificial humic acids makes large-scale environmental remediation applications feasible. However, artificial humic acids are susceptible to microbial degradation, which could lead to the re-release of immobilized pollutants. Therefore, enhancing their stability is crucial not only for long-term environmental applications but also for carbon sequestration. Pyrolysis, a well-established thermal conversion technology, transforms organic materials into stable chars under oxygen-limited conditions [12]. Cheng’s research group has developed chars derived from simple biowastes, including invasive plant biomass and marine macroalgae [13,14]. These chars exhibit stable physicochemical properties, consistent with those produced from other feedstocks [15,16]. Furthermore, the chars demonstrated significant sorption capacity for DEP. Unlike agroforestry biomass, the original structure of artificial humic acids differs significantly, leading to distinct pyrolysis products. Furthermore, converting artificial humic acids into chars can enhance their environmental stability, mitigating rapid degradation and enabling long-term immobilization of organic pollutants such as DEP [17]. The advantage is that artificial humic acids are similar to natural humic acids that are widely distributed in various environments. Through pyrolysis of artificial humic acids, the structure and surface properties of the produced chars are also similar to natural materials, which benefits the achievement of near-natural remediation. Nevertheless, the potential of char materials derived from artificial humic acids for pollutant immobilization remains underexplored.

In this study, typical agroforestry biowaste, bamboo waste, was employed as a precursor to fabricate novel char materials derived from artificial humic acids (CHAs) for the first time, through an integrated process of hydrothermal humification followed by pyrolysis. To investigate the influence of pyrolysis temperature, CHAs were produced at 300 °C, 500 °C, and 700 °C. The physicochemical properties of CHAs, including pore structure, elemental composition, graphite-like structures, and surface functional groups, were systematically characterized. Additionally, the sorption kinetics and isotherms of DEP onto CHAs were analyzed to elucidate the sorption behavior and underlying mechanisms. This study provides a cost-effective and stable near-natural immobilization agent derived from biowaste for the remediation of organic pollutant-contaminated environments.

2. Materials and Methods

2.1. Materials and Reagents

Bamboo waste, collected from the Huangshan Experimental Station of the Nanjing Institute of Environmental Sciences, was used as the raw material for CHA preparation. CaCl₂ (≥96%), KOH (≥85%), HCl (36–38%), and ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). DEP (99.5% purity) was acquired from Dr. Ehrenstorfer GmbH (Augsburg, Germany), while HPLC-grade methanol was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

2.2. Preparation of CHAs

CHAs were prepared in April 2023. The preparation process of CHAs is illustrated in Figure 1. Bamboo samples were first washed with ultrapure water (Milli-Q system, Hanlong Testing Equipment Co., Xuzhou, China), oven-dried at 110 °C, mechanically ground, and sieved through an 80-mesh screen. For hydrothermal humification, 16 g of bamboo powder was thoroughly mixed with 50 mL deionized water and 4 g KOH in a 100 mL high-pressure reactor. The sealed reactor was heated at 180 °C for 4 h in a forced-air oven. The resulting solid–liquid mixture was centrifuged (8000 rpm, 5 min), with the solid residue repeatedly washed with alkaline solution (0.1 mol L⁻¹ NaOH). The liquid fraction was acidified to pH 1 using HCl and allowed to settle for 24 h. Subsequent centrifugation (8000 rpm, 5 min) yielded artificial humic acids, which was washed to neutrality, dried, and ground. For pyrolysis, 8 g of artificial humic acids was placed in a muffle furnace (TL1200, Boyuntong Instrument Co., Nanjing, China) under a N₂ atmosphere (100 mL min⁻¹ flow rate). The temperature was raised at 10 °C min⁻¹ to target temperatures (300, 500, or 700 °C), maintained for 2 h, then cooled to room temperature. The resulting chars were thoroughly rinsed with distilled water, vacuum-filtered, dried at 80 °C, and stored in sealed glass containers as 300CHA (67.5% yield), 500CHA (42.5% yield), and 700CHA (37.5% yield).

2.3. Characterization of CHAs

CHAs were characterized in June 2023. The physicochemical properties of CHAs were comprehensively characterized using multiple analytical techniques. Elemental composition (C, H, O, and N) was determined using a Vario EL III elemental analyzer (Elementar, Langenselbold, Germany). Ash content was determined by heating CHAs at 800 °C for 4 h under air atmosphere [18,19]. Surface morphology and elemental mapping were examined by scanning electron microscopy (SEM) (Regulus 8100, Hitachi, Chiyoda, Japan) equipped with an Ultima Max 170 energy-dispersive X-ray spectrometer (EDX) (Oxford Instruments, Oxford, UK). Textural properties were evaluated through N₂ adsorption–desorption measurements using an ASAP 2460 analyzer (Micromeritics, Norcross, GA, USA). Crystalline phases were identified by X-ray diffraction (XRD) (Smartlab 9 kW, Rigaku, Akishima-shi, Japan), while structural ordering was assessed via Raman spectroscopy (InVia, Renishaw, Kingswood, UK). Surface functionality was investigated by Fourier transform infrared spectroscopy (FTIR) (Nicolet IS50, Thermo Fisher Scientific, Waltham, MA, USA). Chemical state analysis was performed using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) to determine surface elemental composition and bonding configurations.

2.4. Sorption of DEP onto CHAs

Sorption experiments were conducted from June to July 2023. First, a 100 g L⁻¹ DEP stock solution was prepared by dissolving 1.00 g DEP in HPLC-grade methanol. For ionic strength control, a 0.01 mol L⁻¹ CaCl₂ solution was prepared by dissolving 2.22 g CaCl₂ in 2000 mL deionized water. The working solution (5 mg L⁻¹ DEP) was obtained by diluting the stock solution with the CaCl₂ solution and stored under refrigeration in light-protected containers. For kinetic studies, 20 mL aliquots of the 5 mg L⁻¹ DEP solution were added to 20 mL amber glass vials containing 30 mg of each CHAs (300CHA, 500CHA, or 700CHA), followed by agitation at 200 rpm and 25 °C. Samples were collected at predetermined intervals (0.5–96 h) in triplicate, with 2 mL aliquots centrifuged at 10,000 rpm for 10 min at 25 °C prior to HPLC analysis of the supernatant. For isotherm studies, DEP solutions with concentrations ranging from 2 to 70 mg L⁻¹ were prepared by diluting the stock solution with 0.01 mol L⁻¹ CaCl₂. After 72 h of equilibration under identical conditions to the kinetic studies, samples were processed and analyzed following the same centrifugation and HPLC protocols, with triplicate measurements at each concentration point.

2.5. High-Performance Liquid Chromatography Analysis

The DEP concentration was quantified using HPLC (Ultimate 3000, Thermo Fisher Scientific, USA) equipped with a C18 reverse-phase column (5 μm, 4.6 mm × 250 mm) and a UV-Vis detector. Chromatographic separation was achieved under optimized conditions: mobile phase consisting of methanol and water (80:20, v/v) at a flow rate of 0.8 mL min⁻¹, column temperature maintained at 35 °C, and injection volume of 10 μL. Detection was performed at 235 nm, which provided optimal sensitivity for DEP quantification.

3. Results and Discussion

3.1. Specific Properties of CHAs

The pyrolytic yields of CHAs exhibited significant temperature dependence, with 300CHA demonstrating the highest yield (67.5%), while yields at higher temperatures (500–700 °C) decreased substantially to 37.5–42.5%. Compared to rice husk char (25.5–33%), corncob char (20.2–34.2%), and pine char (26–58%) produced under similar pyrolysis conditions, the CHAs derived from artificial humic acids demonstrated a higher yield [20]. Elemental analysis revealed that C (67.28–81.35%), O (6.65–21.64%), H (1.40–5.28%), and N (0.36–0.58%) constituted the primary elemental composition of CHAs (

Table 1. Elemental analysis of artificial humic acids and CHAs.

Materials	Yield (%)	Elementary Composition (%)				Ash (%)	Atomic Ratio (%)
		C	H	N	O		O/C	H/C	(O + N)/C
Artificial humic acids	--	57.42	5.77	0.31	34.09	2.07	0.45	1.21	0.45
300CHA	67.50	67.28	5.28	0.36	21.64	5.43	0.24	0.94	0.25
500CHA	42.50	76.72	2.96	0.47	10.02	9.83	0.10	0.46	0.10
700CHA	37.50	81.35	1.40	0.58	6.65	10.02	0.06	0.21	0.07

). A distinct thermal transformation trend was observed with increasing pyrolysis temperature: C and N contents increased progressively, while O and H contents decreased markedly. This pattern can be attributed to thermal decomposition processes including dehydration, decarboxylation, and functional group reorganization [21]. Notably, the synthesized CHAs exhibited remarkably low ash content (5.43–10.02%), representing a significant departure from conventional char materials, such as marine macroalgae-derived chars (33.64–52.71%), aquatic invasive plant-derived chars (24.7–34.1%), lignosulfonate-derived chars (45.61–59.69%), and rice straw-derived chars (10.82–17.6%) [13,14,22,23]. This distinctive characteristic can be attributed to the unique processing conditions for precursors: (1) Alkaline hydrothermal humification promotes rapid biomass decomposition, facilitating the release and subsequent removal of inherent metal elements and ash components through dissolution in the alkaline medium. (2) The subsequent acid precipitation step maintains metal salts in ionic form, enabling their effective elimination via filtration. This two-stage purification process—combining alkaline dissolution and acid precipitation—effectively minimizes ash content in the final CHA product.

The proposed method demonstrates significant potential for converting heavy-metal-rich solid wastes (e.g., municipal sludge and industrial sludge) into low-ash carbonaceous materials. The combined alkaline hydrothermal and acid precipitation treatment effectively removes metallic constituents while preserving the carbon matrix, offering a promising approach for value-added recycling of contaminated waste streams. The atomic ratios provided further insight into structural evolution. The H/C ratio declined from 0.94 to 0.21, indicating progressive aromatization, while the O/C ratio reduction (from 0.24 to 0.06) suggested enhanced surface hydrophobicity at elevated temperatures [24]. Notably, the polarity index ((N + O)/C) decreased 3.6-fold across the temperature range (300–700 °C), reflecting substantial loss of polar functional groups. Comparative analysis showed 300CHA retained significantly higher O content (2.16× and 3.60× greater than 500CHA and 700CHA, respectively) and polarity index (3.6× higher than 700CHA), confirming better preservation of oxygen-rich structures.

Interestingly, the textural properties exhibited minimal variation, with consistently low specific surface areas, 0.274 m² g⁻¹ for 300CHA, 0.219 m² g⁻¹ for 500CHA, and 0.320 m² g⁻¹ for 700CHA. The specific surface area of the CHAs was comparable to that of their precursor material, artificial humic acids (0.185 m² g⁻¹). Compared with reported chars, including wood chars (184 m² g⁻¹), crop waste chars (98.2 m² g⁻¹), and manure/biosolid chars (52.2 m² g⁻¹), the CHAs exhibited a significantly less developed porous structure [25]. This observation suggests that the applied pyrolysis conditions (particularly the rapid nitrogen flow rate and heating rate) may have inhibited pore development. The minimal pore volumes (3.43 × 10⁻⁴ cm³ g⁻¹ for 300CHA, 4.01 × 10⁻⁴ cm³ g⁻¹ for 500CHA, 2.99 × 10⁻⁴ cm³ g⁻¹ for 700CHA) further support this interpretation, likely resulting from either dense molecular packing or micropore occlusion by pyrolytic products. From a material characterization standpoint, the remarkably low ash content of CHAs, while advantageous for environmental compatibility, may potentially compromise their pore development during thermal processing. This phenomenon occurs because the mineral components typically present in conventional chars (which are largely removed in our preparation method) normally serve as natural activating agents during pyrolysis, catalyzing pore formation through various mechanisms including: (1) template effects, (2) catalytic gasification, and (3) structural rearrangement [12,26]. Consequently, the absence of these mineral constituents in CHAs likely contributes to their observed underdeveloped pore architecture, suggesting an important trade-off between material purity and porosity development that warrants further investigation for optimization purposes.

SEM analysis revealed significant temperature-dependent morphological evolution of CHAs (Figure 2). The microstructural transition was particularly evident when comparing 300CHA/500CHA with 700CHA: while lower-temperature chars (300–500 °C) displayed heterogeneous surfaces with irregular pore networks and polydisperse particles, 700CHA exhibited a more uniform morphology characterized by laminar structures with diminished pore apertures. This morphological transformation suggests a three-stage thermal restructuring process: (1) initial pore formation at 300 °C, (2) pore network development at 500 °C, and (3) structural collapse at 700 °C leading to particle fragmentation and pore occlusion [27]. This observed discrepancy likely stems from fundamental structural and compositional differences between artificial humic acids and lignocellulosic biomass (Figure 2). EDX spectral mapping confirmed the predominance of C and O as primary surface elements in artificial humic acids and CHAs, with distribution patterns that corroborate the bulk elemental analysis results. The EDX analysis further confirmed the effective removal of numerous impurities through the combined hydrothermal humification and pyrolysis treatment process.

XRD analysis provided critical insights into the structural evolution of CHAs (Figure 3). The 300CHA spectrum exhibited a distinct diffraction peak at 10–18° corresponding to the cellulose d₁₀₁ crystal plane, reflecting its origin from bamboo-derived hydrothermal products containing residual cellulose and hemicellulose components. This low-temperature char also showed characteristic peaks of crystalline cellulose and amorphous carbon. As pyrolysis temperature increased to 500–700 °C, the cellulose-related peaks diminished significantly due to hemicellulose decomposition and structural rearrangement. Concurrently, broad diffraction features emerged at 20–45°, attributable to the (002) plane of developing graphitic microcrystals [28]. Compared to artificial humic acids, the pyrolysis process facilitated the formation of graphite-like structures (Figure 3). These observations demonstrate two concurrent transformation pathways: (1) progressive degradation of lignocellulosic structures and (2) thermal-induced graphitization of artificial humic acids, leading to the formation of graphite-like structures [29].

Raman spectroscopy was employed to investigate the graphitic structural evolution of CHAs across different pyrolysis temperatures (Figure 4). Distinct D (disordered carbon) and G (graphitic carbon) bands emerged at 500 °C and 700 °C, located at approximately 1350 cm⁻¹ and 1580 cm⁻¹, respectively. The D band reflects structural defects and edge disorders in carbon matrices, while the G band corresponds to the in-plane vibration of sp²-hybridized carbon atoms, indicative of graphitic ordering [30]. Notably, while pristine artificial humic acids showed no Raman-active features, the appearance of a G band at 300CHA confirmed the initiation of structural reorganization. Comparative analysis revealed that 300CHA maintained structural similarities to the original artificial humic acids, exhibiting higher crystallinity in non-aromatic domains (Figure 4b). The spectral deconvolution demonstrated that 500CHA and 700CHA exhibited more intense D bands relative to G bands (Figure 4c,d), suggesting a predominant amorphous carbon character. This observation correlates well with the morphological features observed by SEM. Quantitative evaluation of graphitization degree through the I_G/I_D ratio revealed 500CHA (1.63) > 700CHA (1.25), indicating that excessive pyrolysis temperature (700 °C) introduced additional structural defects rather than enhancing graphitic perfection, likely due to thermal-induced bond scission and lattice distortion [31].

3.2. Sorption Kinetics of DEP onto CHAs

The sorption kinetics of DEP onto CHAs were systematically analyzed using four models: pseudo-first-order, pseudo-second-order, Elovich, and intra-particle diffusion models (Figure 5,

Table 2. Intra-particle diffusion model for the sorption of DEP onto artificial humic acids and CHAs.

Materials	ki1	ci1	R2	ki2	ci2	R2	ki3	ci3	R2
Artificial humic acids	778.84	−34.35	0.958	140.24	1089.98	0.924	−0.14	1898.78	0.835
300CHA	150.17	12.76	0.944	31.69	0.69	0.884	11.31	5.65	0.847
500CHA	75.37	11.41	0.944	11.85	1.47	0.931	5.60	5.60	0.851
700CHA	52.13	7.06	0.955	8.83	8.83	0.907	0.23	5.10	0.888

). The intra-particle diffusion model revealed a three-stage sorption process: (1) rapid surface sorption, where DEP molecules quickly attach to external surface sites; (2) gradual intra-particle diffusion, characterized by slower pore penetration with non-zero intercept values (c = 0.69, 1.47, and 8.83 for 300CHA, 500CHA, and 700CHA, respectively), indicating that while pore diffusion occurs, it is not the sole rate-limiting step; and (3) final equilibrium, establishing dynamic sorption–desorption balance [32]. Like artificial humic acids and most reported chars, organic pollutant sorption follows the three aforementioned steps (Figure S1) [13,33]. Notably, the second stage contributed significantly to total sorption capacity, particularly for 300CHA. Despite the limited porosity of CHAs, significant intraparticle diffusion sorption was observed. This internal sorption process enhances the long-term immobilization capacity of CHAs, making them particularly effective for controlling contaminant migration in environmental applications.

Obviously, the time needed in stage 2 is much higher than in other stages, indicating that intra-particle diffusion is a rate-limited step. Kinetic modeling demonstrated a superior fit with the pseudo-second-order model (R² = 0.946–0.986) compared to the pseudo-first-order model (R² = 0.903–0.957), with calculated q_e values closely matching experimental measurements. This suggests that chemical-like sorption mechanisms (e.g., π–π interactions, hydrogen bonding) likely govern the sorption rate, which is consistent with artificial humic acids and reported char materials such as mesoporous cellulose char and corn straw char (

Table 3. Pseudo-first-order, pseudo-second-order, and Elovich models for the sorption of DEP onto artificial humic acids and CHAs.

Materials	Pseudo-First-Order			Pseudo-Second-Order			Elovich
	kf	qfe	R2	ks	qs	R2	a	b	R2
Artificial humic acids	0.45	1774.43	0.980	1.63	1904.66	0.994	5658.36	0.0037	0.981
300CHA	0.18	268.30	0.903	0.01	291.38	0.946	5.19 × 102	1.93 × 10−3	0.984
500CHA	0.99	152.74	0.944	0.00	160.50	0.980	1.13 × 104	8.86 × 10−5	0.989
700CHA	0.62	117.82	0.957	0.01	123.08	0.986	3.00 × 103	3.34 × 10−4	0.987

) (Figure S1) [34,35,36]. The sorption rate constants followed the order 300CHA (0.012) > 700CHA (0.007) > 500CHA (0.002), consistent with 300CHA’s superior DEP sorption (Figure 5b). Further supporting chemical-like sorption, the Elovich model (Figure 5c) yielded excellent fits (R² = 0.984–0.989), confirming heterogeneous sorption sites on CHA surfaces. These findings collectively indicate that DEP sorption involves both physical pore diffusion and chemical-like interactions, with surface chemistry playing a predominant role in sorption rate.

3.3. Sorption Isotherm of DEP onto CHAs

The sorption behavior of DEP onto CHAs was evaluated using Langmuir, Freundlich, and Dubinin–Radushkevich (D-R) isotherm models (Figure 6,

Table 4. Langmuir and Freundlich models for the sorption of DEP onto artificial humic acids and CHAs.

Materials	Langmuir				Freundlich
	kl	Qm	Rl	R2	kf	1/n	R2
Artificial humic acids	1.77 × 10−2	26,529.73	0.37	0.993	606.29	1.235	0.989
300CHA	2.62 × 10−2	3344.67	0.24	0.992	215.58	1.828	0.977
500CHA	2.83 × 10−2	2855.11	0.23	0.990	190.40	1.826	0.976
700CHA	2.97 × 10−2	2478.55	0.23	0.990	176.34	1.873	0.974

). The equilibrium sorption capacity increased progressively with rising DEP concentration for all CHAs. The Langmuir model (R² = 0.9902–0.9920) demonstrated superior fitting compared to the Freundlich model (R² = 0.9739–0.9774), suggesting monolayer sorption predominates in the sorption process [37]. For DEP, the sorption behavior of CHAs closely resembled that of artificial humic acids, with the isotherm data exhibiting an excellent fit to the Langmuir model (Figure S2, Table 4). These findings align with previous reports on antibiotic sorption by pristine char materials [33]. The Freundlich intensity parameters (n) followed the order 700CHA (1.873) > 300CHA (1.828) > 500CHA (1.826), indicating favorable sorption, with 300CHA and 500CHA exhibiting the strongest affinity for DEP on its heterogeneous surface [16]. The maximum sorption capacities determined from the Langmuir model were 3344.67, 2855.11, and 2478.55 mg kg⁻¹ for 300CHA, 500CHA, and 700CHA, respectively, with 300CHA showing the highest sorption capacity. The CHAs demonstrated sorption capacity comparable to natural materials, including humic acids and clay organo-mineral complexes [38]. While CHAs may not demonstrate superior sorption capacity compared to conventional chars reported in the literature, they possess unique environmental advantages due to their humic acid-derived nature [13,39,40]. Unlike typical chars, CHAs maintain the fundamental pore structure of artificial humic acid precursors through controlled carbonization. These near-natural characteristics position CHAs as particularly advantageous for ecological restoration applications where environmental impact minimization is paramount, rather than maximum sorption performance. D-R model analysis (R² = 0.976–0.985) revealed mean sorption energies (E) of 4.71 × 10⁻³, 4.99 × 10⁻³, and 2.56 × 10⁻³ kJ mol⁻¹ for 300CHA, 500CHA, and 700CHA, respectively (

Table 5. Dubinin–Radushkevich models for the sorption of DEP onto artificial humic acids and CHAs.

Materials	E	β	LnqD-R	R2
Artificial humic acids	3.19 × 10−3	5.08 × 10−6	8.87	0.940
300CHA	4.71 × 10−3	1.11 × 10−5	7.53	0.976
500CHA	4.99 × 10−3	1.24 × 10−5	7.42	0.985
700CHA	2.56 × 10−3	3.28 × 10−6	7.30	0.981

). All E values were significantly below 8 kJ mol⁻¹, confirming physical sorption mechanisms (e.g., pore filling, partitioning) dominate the sorption process [41,42,43]. The partition–adsorption model can be applied to analyze the sorption mechanism. At elevated DEP concentrations, the partitioning process was observed to contribute to the overall sorption, albeit with a relatively minor contribution. This limited partitioning effect can be primarily attributed to the abundant oxygen-containing functional groups present on the CHAs’ surfaces, which preferentially facilitate adsorption-dominated interactions. To effectively immobilize organic pollutants in heavily contaminated environments, carbonaceous materials should be engineered with tailored polarity matching that of the target contaminants. By precisely tuning both surface and bulk polarity characteristics of the sorbent to align with pollutant properties, optimal sorption performance can be achieved through enhanced affinity interactions. This polarity-matching approach represents a promising strategy for developing selective remediation materials for specific pollution scenarios.

3.4. Sorption Mechanisms

Kinetic analysis using pseudo-second-order and intra-particle diffusion models revealed that DEP sorption onto CHAs follows a multistep process, with both intra-particle diffusion and chemisorption acting as rate-limiting steps [44]. SEM and Raman characterization confirmed that 300CHA maintained the original artificial humic acid structure with abundant oxygen-containing functional groups, in agreement with FTIR observations. Isotherm modeling demonstrated the coexistence of physical sorption mechanisms, with Langmuir and D-R isotherm fitting showing that 700CHA exhibited significantly higher k_i values, indicating pronounced pore-filling effects at elevated pyrolysis temperatures [45]. FTIR analysis (Figure 7) demonstrated that 300CHA preserved the characteristic functional groups of artificial humic acids, showing a broad absorption band at 3415 cm⁻¹ (O-H stretching) and distinct peaks at 1514/1692 cm⁻¹ (C=C/C=O stretching), 1214 cm cm⁻¹ (C-O stretching), and 832 cm cm⁻¹ (aromatics). O-containing functional groups can facilitate hydrogen bonding with DEP, and C=C can facilitate π–π stacking with DEP [46]. In contrast, higher pyrolysis temperatures led to progressive degradation of oxygen-containing groups, as evidenced by altered peaks of O-H stretching (3438–3429 cm cm⁻¹) and aromatic C=C/C=O vibrations (1591–1609 cm cm⁻¹). The significant peak shifts observed after DEP sorption, particularly for 300CHA, confirmed the dominant roles of hydrogen bonding and π–π interactions in the sorption process [47].

The high-resolution XPS analysis demonstrated that the synthesized CHAs exhibit superior purity compared to conventional chars, with their chemical composition predominantly consisting of C and oxygen (O) elements. This enhanced purity is attributed to the effective removal of mineral impurities during the hydrothermal humification and pyrolysis processes, as evidenced by the minimal detection of other elemental components in the survey spectra. To elucidate the sorption mechanism of DEP onto 300CHA, we conducted high-resolution XPS analysis of C 1s and O 1s spectra before and after sorption (Figure 8). The surface elemental composition revealed the predominant presence of C, N, and O in 300CHA. Deconvolution of the C 1s spectrum identified four characteristic peaks at 284.5 eV (C=C), 284.93 eV (C-C), 286.2 eV (C-O), and 288.5 eV (O=C-O) [48]. The percentage of C-O decreased by 12.76% after the sorption of DEP, while C=C and C-C increased by 9.10% and 3.67%, suggesting that C-O is involved in the chemical-like sorption process [49]. The O 1s spectrum exhibited three distinct peaks at 531.9 eV (C=O), 533.10 eV (C-O), and 533.51 eV (COOH). Following DEP sorption, significant reductions in COOH and C-O components were observed, providing direct evidence for hydrogen bonding interactions between DEP and the surface functional groups of CHAs [50].

4. Conclusions

In summary, this study developed an innovative two-step approach combining hydrothermal humification and pyrolysis to convert agroforestry biowaste into cost-effective, stable near-natural char materials for DEP immobilization. The synthesized carbonized humic acids, CHAs, primarily consisted of C, O, H, and N elements, exhibiting significantly lower ash content (5.43–10.02%) than conventional char materials. While the current CHAs show limited pore structure development—an aspect requiring future optimization—We found that high-temperature pyrolysis (700 °C) facilitates graphite-like structure formation, whereas low-temperature treatment (300 °C) effectively preserves abundant surface functional groups. The sorption process was well fitted by both Langmuir isotherm (R² = 0.990–0.992) and pseudo-second-order kinetic (R² = 0.946–0.986) models. The sorption capacity is comparable to that of natural materials (e.g., humic acids), reaching 3345 mg kg⁻¹, and the mechanisms were identified as pore filling, partitioning, hydrogen bonding, and π–π stacking interactions, with hydrogen bonding playing a particularly crucial role in enhancing sorption capacity. The development of optimized theoretical models is needed to better characterize sorption mechanisms at liquid–solid interfaces. This work presents artificial humic acids as a novel precursor for char material preparation, offering multiple environmental benefits including biowaste utilization, carbon sequestration, and cost-effective pollution remediation. Future research should focus on evaluating the multiple factors, including dosage, pH, temperature, and ionic strength, on the immobilization of pollutants, assessing practical performance, immobilization stability, environmental compatibility, and life cycle, as well as optimizing production and application parameters to further enhance the material’s effectiveness.