Sustainable Immobilization of Cadmium, Lead, and Arsenic in Contaminated Soils Using Iron–Phosphorus–Thiol-Functionalized Trachycarpus fortunei Hydrochar

Abstract

Simultaneously stabilizing cadmium, lead, and arsenic in contaminated soils is challenging due to their significant differences in physical and chemical properties. This study developed a composite material by modifying hydrochar with iron (Fe), phosphorus (P), and sulfur (S) to address this issue. The iron–phosphorus–thiol-modified Trachycarpus fortunei hydrochar (H-PAL-Fe2-P-T) effectively stabilized these metals. Experimental results showed that the H-PAL-Fe2-P-T achieved over 90% stabilization for DTPA-extracted cadmium, lead, and arsenic. Characterization by XRD, SEM, and FTIR revealed structural and functional changes in the hydrochar. Column leaching tests simulating acid rain showed that the composite material maintained stable stabilization effects, with the fluctuations in the stabilization rates remaining below 20%. Additionally, the composite-modified hydrochar enhanced the stabilization of water-soluble, DTPA-extracted, and TCLP-extracted heavy metals in soil, demonstrating good stability and durability for long-term use. These findings suggest that Fe-, P-, and S-modified hydrochar is a promising and sustainable approach for the remediation of soils contaminated with cadmium, lead, and arsenic.

1. Introduction

Hydrothermal carbon (HTC) is a carbon-rich material produced via hydrothermal carbonization of biomass using water as a solvent at 150–375 °C under self-generated pressure [1]. Unlike pyrolytic biochar, HTC allows high-moisture feedstocks, eliminating dehydration. Its process emits no gases, reducing the environmental impact [2]. The lower preparation temperature decreases energy consumption and increases the carbon yield. HTC is mildly acidic, rich in oxygen-containing functional groups, and contains soluble organics. These properties enhance its adsorption capacity and reactivity, making it suitable for environmental applications. Additionally, HTC contributes to a sustainable organic carbon pool, helping reduce atmospheric greenhouse gases and mitigate climate change [3]. Its unique characteristics make HTC a promising material for waste valorization and carbon sequestration.

Recent studies have highlighted the eco-friendly potential of hydrothermal carbon (HTC) for heavy metal adsorption, offering a sustainable alternative to conventional chemical-intensive remediation methods [4,5,6]. The primary adsorption mechanisms can be summarized as follows: surface adsorption, where heavy metal ions are adsorbed onto the surface of HTC, typically occurring on HTC with a large specific surface area and high surface energy, which strongly attracts heavy metal ions [7]; chemical precipitation or chelation, where heavy metal ions react with the mineral components of HTC (e.g., phosphates, carbonates) to form precipitates or chelate with specific modified functional groups in HTC [8]; ion exchange, where heavy metal ions exchange with oxygen-containing functional groups (such as hydroxyl and carboxyl groups) on the surface of HTC; and cation–π interactions, where heavy metal ions interact with the π electrons in conjugated structures (such as C≡C, C=C, or aromatic rings) present in HTC. The extent of the aromaticity in HTC is a key factor influencing cation–π interactions, making this interaction more pronounced in HTC with a higher aromatic content [9,10,11].

HTC is a non-toxic, eco-friendly material with low sulfur content, excellent hydrothermal stability, and significant environmental benefits [12]. As research into HTC deepens, various sustainable modification techniques have been explored to enhance its functionality while minimizing ecological footprints [13]. In particular, for the remediation of soils contaminated with cadmium, lead, and arsenic, researchers have applied modifications such as iron-based, phosphorus-based, and thiol-based treatments. By combining three types of modifications or adjusting the order of addition of different modified biochars, they have successfully developed stabilization materials that combine high-temperature pyrolytic biochar with calcium-based and silicon-based inorganic components for the stabilization of cadmium, lead, and arsenic. This approach not only immobilizes heavy metals but also repurposes biomass waste into value-added remediation agents, reducing the dependency on non-renewable resources. These studies provide novel, sustainable pathways for the application of HTC in environmental remediation.

2. Materials and Methods

2.1. Hydrothermal Carbon (HTC) Preparation

Crushed rice straw, palm leaves, pine wood, and bamboo were selected as the experimental materials. These materials (200 g) were placed in a high-pressure reactor (2 L) and maintained at 180 °C for 6 h. After the reaction, the samples were dried and ground through a 60-mesh sieve to obtain hydrothermal carbon (yield of 27%). The cooled hydrothermal carbon was then further ground and sieved through a 200-mesh sieve. Pyrolytic carbon was used with the same experimental materials, placed in a vacuum tube furnace, filled with nitrogen for half an hour and kept at 650 °C for 2.5 h, carbonized and cooled to room temperature and ground through a 60-mesh sieve to obtain high-temperature pyrolytic biochar.

2.2. Hydrothermal Carbon Activation Modification

A 1 mol/L KOH solution was mixed with hydrothermal carbon in a certain ratio (2:1) and subjected to shaking and soaking at room temperature for 1 h [14]. The mixture was then subjected to pyrolysis at 300 °C for 1 h in a tube furnace and subsequently dried to obtain the activated hydrothermal carbon (H-PAL). The activated biochar was added to 0.2 mol/L Fe(NO₃)₃ solution, and 7.20 g of urea was added and mixed thoroughly, and then slowly heated to 95 °C in a constant-temperature water bath and held for 4 h, then washed and dried, and then placed into a vacuum tube furnace at 300 °C for calcination for 1 h to obtain the modified hydrothermal carbon (H-PAL-Fe1). The activated hydrothermal carbon was then mixed with a 0.5 mol/L Fe²⁺/Fe³⁺ solution and stirred thoroughly for 30 min. The pH of the system was adjusted to 10 using a 5 mol/L NaOH solution, followed by stirring for an additional 30 min. The pH was monitored and re-adjusted to 10 if necessary. The mixture was filtered and dried, followed by calcination in a vacuum tube furnace at 300 °C for 1 h to obtain iron-modified hydrothermal carbon (H-PAL-Fe2). Subsequently, 20 g of iron-modified hydrothermal carbon was mixed with 12 g of hydroxyapatite in a beaker, followed by the addition of distilled water and high-temperature magnetic stirring for 1 h. After filtration and drying, the material was washed several times with deionized water until the pH became neutral, then dried at 80 °C to obtain phosphorus-modified hydrothermal carbon (H-PAL-P). Finally, a mixture of 3-mercaptopropyltrimethoxysilane, ethanol, and water in a certain ratio (1:8:0.5) was prepared, and phosphorus-modified hydrothermal carbon was added to the mixture and stirred at room temperature for 6 h. Any excess thiol groups were removed by ethanol washing, and the product was dried at 80 °C and ground, yielding the activated thiol-modified material hydrothermal carbon (H-PAL-T).

Based on the preliminary experiments, (H-PAL-Fe2), (H-PAL-P), and (H-PAL-T) were selected for composite modification, resulting in five types of activated composite-modified hydrochar: activated iron–phosphorus (H-PAL-Fe2-P), activated phosphorus–iron (H-PAL-P-Fe2), activated iron–thiol (H-PAL-Fe2-T), activated iron–phosphorus–thiol (H-PAL-Fe2-P-T), and activated phosphorus–iron–thiol (H-PAL-P-Fe2-T). The composite-modified pyrolytic carbon (P-PAL-Fe2-P-T) was modified in the same way as (H-PAL-Fe2-P-T).

2.3. Batch Adsorption Experiment

The composite-modified hydrothermal carbon was mixed with soil and thoroughly homogenized, then placed in a constant-temperature shaker for 24 h of agitation at 25 °C and 180 r/min. After filtration, the supernatant was collected for chemical analysis to evaluate its removal efficiency for cadmium, lead, and arsenic. The composite-modified hydrothermal carbons with the best removal performance were selected and added to the soil at a 5% ratio. The mixture was thoroughly homogenized, then ultrapure water was added to control the soil moisture content at 50%. The treated soil samples were placed in a constant-temperature shaker and agitated at 25 °C and 180 r/min for 3 days. Afterward, the samples were dried and ground through 20 mesh. The samples were subjected to extraction with ultrapure water, and the cadmium, lead, and arsenic concentrations were measured.

2.4. Instrumental Analysis

The aqueous and solid phases were separated by centrifugation, and the supernatant was filtered through a 0.22 μm membrane. The concentrations of cadmium, lead, and arsenic in the supernatant were determined using ICP-MS (Perkin Elmer, Elan DRC-e, Waltham, MA, USA). The adsorption concentration of the target ions on the biochar was calculated based on the mass difference. The statistical significance of the correlation was analyzed using an analysis of variance (ANOVA) in OriginPro 9.

2.5. Adsorption Kinetics and Isotherms

2.5.1. Adsorption Kinetics

The pseudo-first- and -second-order models were adopted to evaluate the kinetic behavior and elucidate the mechanism of cadmium, lead, and arsenic adsorption. The non-linear forms of the two model equations are generally expressed as shown in Equations (1) and (2).

$$q_e=q_e(1-e^{-k_{1}t})$$

(1)

$$q_t={\displaystyle \frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}}$$

(2)

where q_e (mg/g) and q_t (mg/g) are the amount of cadmium, lead, and arsenic adsorbed per unit mass of adsorbent at equilibrium and at contact time t, respectively. k₁ (min⁻¹) and k₂ (g/mg/min) represent the kinetic rate constants of the pseudo-first-order and pseudo-second-order adsorption kinetic models, respectively.

2.5.2. Adsorption Isotherms

Two classical isotherm models, Langmuir and Freundlich, were used to analyze the obtained experimental data. The linear forms of the two typical isotherm models are expressed as shown in Equations (3) and (4).

$$q_e={\displaystyle \frac{q_{m}b{C}_e}{1+b{C}_e}}$$

(3)

$$q_e=k{C}_e^{1/n}$$

(4)

where q_e (mg/g) and q_max (mg/g) are the equilibrium adsorption capacities and the equilibrium maximum adsorption capacities of cadmium, lead, and arsenic adsorption into the adsorbent, respectively. C_e (mg/L) represents the equilibrium concentrations of cadmium, lead, and arsenic and b (L/mg) is the Langmuir model equilibrium constant associated with the interaction energy of the adsorbents and cadmium, lead, and arsenic. k ((mg/g)(L/mg)^1/n) and n are the Freundlich model equilibrium constant associated with the adsorption capacity and adsorption intensity, respectively.

3. Results and Discussion

3.1. Characterization of Biochar

The XRD results shown in Figure 1a indicate that the H-PAL-Fe2-P-T contains quartz, magnetite, and a calcium phosphate–calcium nitrate mixture, suggesting successful iron modification that resulted in the formation of magnetite attached to the biochar. In the phosphorus modification, hydroxyapatite formed calcium phosphate, which acts as a precursor to hydroxyapatite and can generate hydroxyapatite during hydrolysis, thus stabilizing the heavy metals in the soil [15].

The scanning electron microscopy (SEM) images in Figure 1b reveal that the surface of the H-PAL-Fe2-P-T is rough and exhibits a blocky and flaky structure. Elemental analysis shows the presence of O, C, Fe, Ca, P, Si, and S, with mass percentages of 41.36%, 32.77%, 14.52%, 5.17%, 1.74%, and 1.68%, respectively. The rough surface and unique structure enhance the material’s contact with the target pollutants, while the Fe, Ca, and P elements in the material are capable of binding to heavy metal ions.

Fourier transform infrared (FTIR) spectroscopy analysis (Figure 1d) reveals a broad peak at 3354.97 cm⁻¹ corresponding to the stretching vibration of the associated hydroxyl groups (-OH), primarily derived from the carbohydrates in the biomass. A peak at 2923.89 cm⁻¹ corresponds to the stretching vibration of -OH in the carboxylates, while the peaks at 1606.27 and 1416.80 cm⁻¹ correspond to the antisymmetric and symmetric stretching vibrations of carboxylates, respectively. The peaks at 1246.77 and 1032.85 cm⁻¹ correspond to the stretching vibrations of C–O–C in ethers, and the peaks at 602.66 and 561.34 cm⁻¹ are associated with the stretching vibrations of aliphatic ketones in α-positions with and without substitution.

According to the BET analysis, the specific surface area of the unmodified hydrothermal carbon was 10.839 m²/g, while that of the H-PAL-Fe2-P-T hydrothermal carbon increased to 38.452 m²/g, indicating that the modification process significantly increased the surface area. The increase in surface area enhances the material’s contact with heavy metal pollutants, promoting surface adsorption [16].

The cation exchange capacity (CEC) of the stabilized H-PAL-Fe2-P-T was found to be 3.22 cmol/kg, which is higher than that of the experimental soil (0.65 cmol/kg), indicating that the H-PAL-Fe2-P-T hydrothermal carbon exhibits excellent adsorption potential for heavy metal ions.

3.2. Stabilization Effects of Biochar on Heavy Metals

3.2.1. Stabilization Effects of Hydrochars from Different Raw Materials on Cadmium, Lead, and Arsenic

Crushed rice straw, palm leaves, pine wood, and bamboo were used as raw materials and placed in a high-pressure reactor, where they were treated at 180 °C for 6 h. After drying, the materials were ground and sieved through a 60-mesh screen to obtain hydrochar. The prepared unmodified hydrochar was then reacted with soil, thoroughly mixed, and incubated in a constant-temperature shaker for 24 h at 25 °C with a shaking speed of 180 r/min. Crushed rice straw hydrothermal charcoal (H-CRU), palm leaf hydrothermal charcoal (H-PAL), pine wood hydrothermal charcoal (H-PIN), and bamboo hydrothermal charcoal (H-BAM) were obtained, respectively. After filtration, the supernatant was collected for chemical analysis to evaluate its effectiveness in removing cadmium, lead, and arsenic. The results in Figure 2a indicate that the hydrochar derived from palm leaves exhibited relatively better performance in removing these contaminants.

3.2.2. Stabilization Effects of Hydrochars Modified by Different Methods on Cadmium, Lead, and Arsenic

After KOH activation, the hydrochar was subjected to different modifications: iron modification 1 (H-PAL-Fe1), iron modification 2 (H-PAL-Fe2), phosphorus modification (H-PAL-P), and thiol modification (H-PAL-T). The stabilization effects of the five hydrochars prepared through activation and modification on cadmium, lead, and arsenic are shown in Figure 2b. The results indicate that the H-PAL-Fe2 treatment achieved stabilization rates of 99.19% and 96.79% for lead and arsenic, respectively, while the stabilization rate for cadmium was only 45.10%. The H-PAL-Fe1 treatment resulted in stabilization rates of 96.95%, 89.78%, and 51.82% for cadmium, lead, and arsenic, respectively. The H-PAL-P treatment achieved stabilization rates of 71.32%, 99.94%, and 97.21% for cadmium, lead, and arsenic, respectively. Different modifications of the KOH-activated hydrochar exhibited varying stabilization efficiencies for cadmium, lead, and arsenic, with H-PAL-Fe2 and H-PAL-P showing the highest effectiveness for lead and arsenic, while cadmium stabilization remained relatively low across the treatments.

3.2.3. Stabilization Effect of Composite-Modified Hydrochars on Cadmium, Lead, and Arsenic

Different activation methods have a significant impact on the adsorption capacity of hydrochar [17]. The H-PAL-Fe2, H-PAL-P, and H-PAL-T hydrochars were selected for composite modification, resulting in five composite-modified hydrochars: H-PAL-Fe2-P, H-PAL-P-Fe, H-PAL-Fe2-T, H-PAL-Fe2-P-T, and H-PAL-P-Fe-T. The stabilization effects of the five composite-modified hydrochars on cadmium, lead, and arsenic are shown in Figure 2c. The results indicate that the H-PAL-Fe2-P- and H-PAL-Fe2-P-T-modified hydrochars exhibited the best overall stabilization effects for heavy metals. The stabilization rates of the H-PAL-Fe2-P-modified hydrochar for cadmium, lead, and arsenic were 91.62%, 99.96%, and 99.01%, respectively, while the stabilization rates of the H-PAL-Fe2-P-T-modified hydrochar were 93.85%, 99.96%, and 98.34%, respectively.

The two hydrochars with the best stabilization effects, the H-PAL-Fe2-P and H-PAL-Fe2-P-T hydrochars, were added to the soil for cultivation. After water extraction, the concentrations of cadmium, lead, and arsenic were analyzed, and their stabilization effects are shown in Figure 2d. The results suggest that the activated iron–phosphorus–thiol-modified hydrochar exhibited superior overall stabilization effects for heavy metals, with stabilization rates of 67.68%, 98.97%, and 49.38% for cadmium, lead, and arsenic, respectively. After treatment, the concentrations of cadmium, lead, and arsenic in the soil leachate met the Class IV standards of the Groundwater Quality Standards (GB/T 14848-2017) [18,19]. Based on these results, the H-PAL-Fe2-P-T hydrochar was selected as the target material for subsequent experiments.

The variation in the stabilization rates of soil heavy metals during simulated freeze–thaw erosion is shown in Figure 2e. The results indicate that after a simulated three-year freeze–thaw test, the stabilization rates of cadmium, arsenic, and lead decreased by 3.01%, 0.74%, and 2.96%, respectively, compared to the initial freeze–thaw state. After simulated freeze–thaw erosion, the stabilization rates for cadmium, arsenic, and lead remained stable, with fluctuations of less than 5%. This demonstrates that H-PAL-Fe2-P-T, as a stabilizing agent, effectively reduces the bioavailable content of heavy metals in the soil, mitigates their bio-toxicity, and maintains the long-term effectiveness of heavy metal stabilization. The composite modification of hydrochar significantly enhanced the heavy metal stabilization, with H-PAL-Fe2-P-T showing the best performance in soil applications, maintaining long-term stabilization effectiveness even after simulated freeze–thaw erosion.

3.3. Mechanism of Heavy Metal Stabilization

3.3.1. Changes in the Metal Speciation of Polluted Soils

BCR sequential extraction was performed on the soil before and after remediation to analyze the changes in the concentration distribution of different forms of cadmium, lead, and arsenic, as shown in Figure 3. The results indicate that the addition of H-PAL-Fe2-P-T hydrochar promotes the transformation of cadmium from weak acid extractable to reducible and residual forms, lead from weak acid extractable, oxidizable, and reducible forms to residual form, and arsenic from residual to oxidizable, reducible, and weak acid extractable forms. Notably, H-PAL-Fe2-P-T hydrochar may have an activation effect on arsenic. Overall, the interaction between the material and the soil facilitates the transformation of heavy metals, such as cadmium and lead, into more chemically stable forms, which is consistent with the findings of Gao et al. [20,21]. However, no similar stabilization effect was observed for arsenic.

3.3.2. Adsorption Kinetics

Figure 3a–c show the adsorption kinetics of the H-PAL-Fe2-P-T hydrochar for single contaminants, while Figure 3d–f display the adsorption kinetics for cadmium, lead, and arsenic in a mixed solution. The H-PAL-Fe2-P-T hydrochar exhibits high porosity and surface area, and its adsorption of Cd²⁺, Pb²⁺, and As³⁺ reaches equilibrium rapidly, achieving equilibrium at 3 h, 3 h, and 40 h, respectively. The slower adsorption phase is primarily due to the diffusion of cadmium, lead, and arsenic into the micropores, where they adsorb into the internal surfaces, with the diffusion process slowing down the adsorption kinetics due to the influence of multiple factors. Fitting the data with pseudo-first-order and pseudo-second-order kinetic models, it was found that for single contaminants, the pseudo-second-order model fits better for cadmium, lead, and arsenic, indicating that the adsorption of these three heavy metals is predominantly governed by chemical adsorption. For the adsorption kinetics in the mixed solution containing cadmium, lead, and arsenic, the pseudo-second-order model also fits better for arsenic, suggesting that its adsorption in the presence of other heavy metals is primarily a chemical adsorption process. However, for cadmium and lead, the pseudo-first-order model fits better, indicating that their adsorption in the mixed solution is mainly a physical adsorption process. The adsorption rate and affinity order are lead > arsenic > cadmium.

The analysis of heavy metal speciation in polluted soil after treatment shows that the H-PAL-Fe2-P-T hydrochar promotes the transformation of cadmium and lead into more stable chemical forms. The characterization results indicate that the material has a rough surface with blocky and flaky structures, containing quartz, magnetite, and a mixture of dicalcium phosphate and sodium nitrate. The surface is rich in functional groups such as hydroxyls, carboxylates, and ethers, which can interact with heavy metal ions in soil through adsorption, complexation, precipitation, and co-precipitation mechanisms. The adsorption experiments demonstrate that the material primarily adsorbs arsenic through chemical adsorption. For cadmium, the adsorption is mainly chemical in a single cadmium solution, while in a mixed solution of cadmium, lead, and arsenic, the adsorption of cadmium is dominated by physical adsorption. Similarly, for lead, the adsorption is mainly chemical in a single lead solution, but in a mixed solution, the adsorption of lead is dominated by physical adsorption. The H-PAL-Fe2-P-T hydrochar exhibits rapid and efficient heavy metal adsorption, with chemical adsorption dominating for single contaminants and physical adsorption prevailing for cadmium and lead in mixed solutions, while also promoting the stabilization of heavy metals in soil through adsorption, complexation, and precipitation mechanisms.

3.4. Optimization of Cadmium–Arsenic–Lead Stabilization Composite Materials

The P-PAL-Fe2-P-T exhibited stabilization rates of 99.68%, 99.93%, and 90.05% for cadmium, lead, and arsenic, respectively, in the soil TCLP leachate. However, in the soil incubation experiments, the stabilization rates were 90.24%, 98.93%, and 57.32% for cadmium, lead, and arsenic, respectively. The H-PAL-Fe2-P-T hydrochar demonstrated stabilization rates of 93.85%, 99.96%, and 98.34% for cadmium, lead, and arsenic in the TCLP leachate, but the stabilization rates in the soil incubation experiment were 67.68%, 98.97%, and 49.38%, respectively. Therefore, to optimize the application effects of these two materials in soil, a blend optimization study was conducted.

3.4.1. Study on the Proportions of Cadmium-Arsenic-Lead Stabilization Composite Materials

Several environmentally friendly reagents that are effective for heavy metals, including cadmium, lead, and arsenic, were selected: sodium dihydrogen phosphate (F1), sodium phosphate (F2), potassium aluminum sulfate (F3), polyaluminum chloride (F4), polyferric sulfate (F5), zeolite powder (F6), steel slag (F7), gypsum (F8), FeCl₃ (F9) (with an addition rate of 2%), TMT102 (F10), TMT15 (F11) (liquid additives at 2 mL), red mud (F12), and sodium humate (F13) (at 5% addition rate). Composite reagent 1 (F14) consists of 1 g gypsum + 1 g FeCl₃ + 3 mL TMT102, and composite reagent 2 (F15) consists of 1 g gypsum + 1 g FeCl₃ + 3 mL TMT15. Fifty grams of test soil was placed in 100 mL polyethylene bottles, and the reagents were added in specified proportions. After thorough mixing, 50 mL of distilled water was added, and the mixture was shaken (24 h on the first day, followed by 4 h of shaking each day). After 7 days of incubation, the samples were air-dried, ground, and the concentrations of water-soluble cadmium, lead, and arsenic, as well as their available forms in the soil, were measured.

The above-mentioned effective reagents were mixed with the prepared H-PAL-Fe2-P-T to form composite stabilization agents. The names and formulations of the composite stabilization agents are listed in

Table 1. Naming and formulation of the composite stabilization agents.

Reagent Naming	Stabilization Agent
ST-1	1 g polyferric sulfate + 2.5 g H-PAL-Fe2-P-T hydrochar + 2 mL TMT102
ST-2	2 g polyferric sulfate + 2.5 g H-PAL-Fe2-P-T hydrochar + 3 mL TMT102
ST-4	1 g polyferric sulfate + 2.5 g H-PAL-Fe2-P-T + 2 mL TMT + 0.5 g sodium sulfide
ST-5	1 g polyferric sulfate + 2.5 g H-PAL-Fe2-P-T + 2 mL TMT + 1 g zeolite
T-6	1 g polyferric sulfate + 3 mL TMT + 2 g zeolite
T-7	1 g polyferric sulfate + 2 mL TMT102 + 1 g CaO
T-8	1 g zeolite + 0.5 g sodium sulfide + 1 g ferric chloride + 1 g CaO
T-9	2 g polyferric sulfate + 3 mL TMT + 0.5 g sodium sulfide

. Fifty grams of test soil was placed in 100 mL polyethylene bottles, and the reagents were added according to the formulations in the table. After mixing thoroughly, 50 mL of distilled water was added, and the mixture was shaken (24 h on the first day, followed by 2 h of shaking each day). After 7 days of static incubation, the samples were air-dried, ground, and the concentrations of cadmium, lead, and arsenic in the water-soluble, DTPA-extracted, TCLP-extracted, and CaCl₂-extracted solutions were measured.

The group that achieved the best stabilization effects for cadmium, arsenic, and lead in the previous experiments was selected as the target cadmium–arsenic–lead stabilization composite material. To verify the effectiveness of this composite material in stabilizing heavy metals in soils other than the test soil, it was applied to experimental soils collected from the Hengyang Shuikoushan Lead-Zinc Smelting Plant. After sufficient reaction, the effective content of cadmium, arsenic, and lead in the soil was measured and compared with untreated soils. The stabilization rates of cadmium, arsenic, and lead in the experimental soils from the Hengyang Shuikoushan Lead-Zinc Smelting Plant were then calculated.

3.4.2. Stabilization Effect of Cadmium–Arsenic–Lead Stabilization Composite Materials

The stabilization effects of various stabilization agents on the water-soluble cadmium, lead, and arsenic in soil are shown in Figure 4a. The results indicate that the agents with the best stabilization effects for cadmium are sodium phosphate, TMT102, sodium humate, TMT15, and red mud, with the stabilization rates from highest to lowest as follows: sodium phosphate > TMT102 > sodium humate > TMT15 > red mud. For lead, red mud shows the best stabilization effect. For arsenic, the agents with the best stabilization effects are FeCl3, sodium phosphate, polyaluminum sulfate, zeolite powder, and gypsum, with the stabilization rates from highest to lowest as follows: FeCl₃ > sodium phosphate > polyaluminum sulfate > zeolite powder > gypsum. The stabilization effects of these agents on the effective forms of cadmium, lead, and arsenic in soil are shown in Figure 4b. The results indicate that the agents with the best stabilization effects for cadmium are TMT102, TMT15, sodium humate, and gypsum, with the stabilization rates from highest to lowest as follows: TMT102 > TMT15 > sodium humate > gypsum. For lead, the agents with the best stabilization effects are sodium dihydrogen phosphate, sodium phosphate, TMT102, sodium humate, and TMT15, with the stabilization rates from highest to lowest as follows: sodium dihydrogen phosphate > sodium phosphate > TMT102 > sodium humate > TMT15. For arsenic, the agents with the best stabilization effects are polyferric sulfate and FeCl₃, with the stabilization rates from highest to lowest as follows: polyferric sulfate > FeCl₃.

The stabilization effects of the composite stabilization agents with different compositions on the TCLP-extracted cadmium, lead, and arsenic in soil are shown in Figure 4c. The results indicate that the composite agents have good stabilization effects on cadmium and lead but show no significant stabilization effect on arsenic. This may be due to the specific forms of arsenic present in the experimental soil, as TCLP is typically used for extracting heavy metals such as lead, cadmium, copper, and zinc. Comparisons show that the stabilization effects of heavy metals in the groups without added H-PAL-Fe2-P-T hydrochar (T-5 to T-9) are much lower than those of the groups with added materials. Among them, the stabilization rates of cadmium and lead in the ST-3 group are 87.65% and 97.61%, respectively. This indicates that the addition of composite-modified H-PAL-Fe2-P-T hydrochar enhances the stabilization effect of the composite stabilization agents on cadmium and lead. This study shows that various stabilization agents effectively reduce water-soluble cadmium, lead, and arsenic in soil, with the composite agents, especially those including H-PAL-Fe2-P-T hydrochar, significantly enhancing the stabilization for cadmium and lead but showing limited effects on arsenic.

The stabilization effects of the composite stabilization agents with different compositions on the water-soluble cadmium, lead, and arsenic in soil are shown in Figure 4d. The results indicate that all the groups exhibit good stabilization effects for arsenic but show no significant stabilization effects for cadmium and lead, which may be due to the relatively low water-soluble content of cadmium and lead in the experimental soil. The groups with stabilization rates for arsenic above 90% include ST-2, T-6, and T-9, with stabilization rates of 90.64%, 97.48%, and 94.02%, respectively.

The stabilization effects of the composite stabilization agents with different compositions on DTPA-extracted cadmium, lead, and arsenic in soil are shown in Figure 4e. The results indicate that the comprehensive stabilization effects for cadmium, lead, and arsenic are best in the ST-2, T-6, and T-9 groups. Specifically, the ST-2 group shows stabilization rates of 87.93% for cadmium, 99.77% for lead, and 91.41% for arsenic; the T-6 group shows stabilization rates of 95.74% for cadmium, 99.60% for lead, and 94.07% for arsenic; and the T-9 group shows stabilization rates of 93.75% for cadmium, 99.83% for lead, and 97.64% for arsenic. The results suggest that the addition of H-PAL-Fe2-P-T hydrochar improves the stabilization effects of heavy metals in soil, and the addition of zeolite in the T-6 group significantly enhances the stabilization effects of the composite stabilization agents.

The stabilization effects of the composite stabilization agents with different compositions on CaCl₂-extracted cadmium, lead, and arsenic in soil are shown in Figure 4f. The results indicate that, except for the T-9 group, the stabilization rates for cadmium, lead, and arsenic in all the groups are below 90%, indicating that the composite stabilization agents are not very effective for the removal of CaCl₂-extracted cadmium, lead, and arsenic from soil.

To find a composite stabilization agent that achieves a stabilization rate of over 90% for cadmium, lead, and arsenic in soil, five additional groups were added based on the previous experimental results. The specific compositions and names of these groups are shown in Figure 4g. Fifty grams of experimental soil was placed in a 100 mL polyethylene bottle, and the agents were added according to the compositions, followed by mixing and the addition of 50 mL of distilled water. The mixture was shaken (24 h on the first day, then 2 h daily) and left to stand for seven days before sampling, air-drying, and grinding. The cadmium, lead, and arsenic contents in the DTPA extract of the soil were measured, and the stabilization rates of each composite stabilization agent for cadmium, lead, and arsenic were calculated. This study demonstrates that the composite stabilization agents, especially those with H-PAL-Fe2-P-T hydrochar and zeolite, significantly enhance the stabilization of cadmium, lead, and arsenic in soil, with stabilization rates above 90% for all three heavy metals in specific composite groups, particularly in DTPA-extracted soil (

Table 2. Formulation and naming of the additional composite stabilization agents used in the study.

	Stabilization Agent
+ST-1	1 g polyferric sulfate + 2.5 g H-PAL-Fe2-P-T hydrochar + 3 mL TMT102
+ST-2	2 g polyferric sulfate + 2.5 g H-PAL-Fe2-P-T hydrochar + 3 mL TMT102
+ST-3	2 g polyferric sulfate + 3 g H-PAL-Fe2-P-T hydrochar + 3 mL TMT102
+ST-4	2 g polyferric sulfate + 3 g H-PAL-Fe2-P-T hydrochar + 4 mL TMT102
+ST-5	2 g polyferric sulfate + 5 g H-PAL-Fe2-P-T hydrochar + 6 mL TMT102

).

The stabilization effects of the added composite stabilization agent groups on cadmium, lead, and arsenic in soil are shown in Figure 4h. The results indicate that all the groups exhibit good stabilization effects on cadmium, lead, and arsenic. Specifically, the +ST-5 group achieves stabilization rates of over 90% for all three metals, with a stabilization rate of 93.33% for cadmium, 99.68% for lead, and 99.58% for arsenic. Additionally, the ratios of H-PAL-Fe2-P-T in the composite stabilization agents are altered, and soil cultivation remediation experiments were conducted to measure the DTPA extractable, TCLP extractable, and water-soluble cadmium, lead, and arsenic contents before and after stabilization (

Table 3. Formulation and naming of the additional composite stabilization agents used in the study.

Reagent Naming	Stabilization Agent
H-PAL-Fe2-P-T	2 g Polymeric Ferric Sulfate + 4 g H-PAL-Fe2-P-T Hydrochar + 3 mL TMT102

).

The results indicate that, after adjusting the ratios and dosages, the H-PAL-Fe2-P-T composite-optimized agent can increase the stabilization rates of water-soluble cadmium, arsenic, and lead in soil to over 90%. Furthermore, the composite-optimized agents maintain stabilization rates for TCLP-extracted lead and DTPA-extracted lead in soil above 95%, with the stabilization rates for DTPA-extracted arsenic exceeding 88%. The stabilization rate for TCLP-extracted cadmium remains above 79%.

The sampling site is located in an acid rain region, where the distribution and geochemical behavior of soil heavy metals may change during acid rain infiltration [22,23]. To assess the impact of continuous acid rain infiltration on the stabilization capacity of the stabilization agents for arsenic, lead, and cadmium in soil, column experiments were conducted to simulate the changes in the stabilization rates of soil heavy metals during acid rain leaching (see Figure 5). The results show that, after a simulated three-year acid rain leaching test, the stabilization rates for H-PAL-Fe2-P-T composite agent decreases by 12.02%, 3.46%, and 9.01%, respectively. This indicates that the H-PAL-Fe2-P-T composite-optimized agents exhibit good long-term effectiveness in stabilizing cadmium, arsenic, and lead, with the stabilization rate fluctuations controlled within 20%. The H-PAL-Fe2-P-T composite stabilization agents, particularly the +ST-5 group, demonstrate over 90% stabilization for cadmium, lead, and arsenic in soil, with long-term effectiveness maintained even after simulated acid rain leaching, showing minimal fluctuations in the stabilization rates.

4. Conclusions

This study demonstrates the remarkable efficacy of composite stabilization agents formulated with hydrochar (H-PAL-Fe2-P-T) for immobilizing cadmium, lead, and arsenic in contaminated soils. The optimized +ST-5 formulation, comprising 2 g polyferric sulfate, 5 g H-PAL-Fe2-P-T hydrochar, and 6 mL TMT102, achieved stabilization rates exceeding 90% for DTPA-extractable heavy metals. Furthermore, the composite agents incorporating H-PAL-Fe2-P-T effectively enhanced the stabilization rates of water-soluble cadmium, lead, and arsenic to above 90%, highlighting their dual functionality in addressing both bioavailable and soluble metal fractions. Long-term stability assessments under simulated acid rain leaching over three years revealed minimal fluctuations (<20%) in the stabilization performance, underscoring the durability of these materials under environmentally relevant conditions. Field validation at the Hengyang Shuikoushan Lead-Zinc Smelting Plant further verified their practical effectiveness, achieving stabilization rates of 95.1% for lead and 84.17% for arsenic, demonstrating their feasibility for real-world soil remediation. Overall, these findings suggest that biochar-based composite agents provide an effective and sustainable solution for heavy metal stabilization in contaminated soils. Future studies could focus on optimizing the formulation ratios, exploring additional metal contaminants, and assessing the long-term field performance under varying environmental conditions.