Synergistic Immobilization of Lead and Cadmium and Enhanced Phosphorus Utilization in Contaminated Soil Using Phosphorus-Modified Hydrochar and Zeolite

Abstract

Phosphorus (P)-modified hydrochar can immobilize lead (Pb) and cadmium (Cd) in contaminated soils, but improving P utilization while mitigating P release remains challenging. In this study, banana peel hydrochar (BPH200) was synthesized via phosphoric acid modification at 200 °C, and a coal gangue-derived zeolite (ZL) was prepared. A 100-day soil incubation experiment was conducted to evaluate the individual (1% BPH200 and 2% BPH200) and combined (1% BPH200 + 1% ZL) effects on Pb/Cd immobilization and P release risk. The immobilization performance, expressed as the reduction in toxicity characteristic leachingprocedure (TCLP)-extractable Pb and Cd, followed the order: 1% BPH200 + 1% ZL (78.61% for Pb, 67.61% for Cd) > 2% BPH200 (54.93% for Pb, 54.71% for Cd) > 1% BPH200 (27.41% for Pb, 21.13% for Cd). The P utilization efficiency, defined as the proportion of BPH200-derived P that contributed to Pb/Cd immobilization and to the increase in soil available P, showed a similar trend: 1% BPH200 + 1% ZL (73.29%) > 1% BPH200 (55.12%) > 2% BPH200 (50.22%). Temporal analysis revealed that soil Olsen-P in all treatments peaked sharply on the 10th day and then declined, with the 2% BPH200 treatment causing excessive Olsen-P levels (more than 20 mg/kg) throughout the incubation, indicating a persistent risk of P release. In contrast, co-application with ZL mitigated this risk while maintaining effective metal immobilization. These results demonstrate that ZL synergistically enhances P utilization for metal immobilization and reduces P mobility, offering a more sustainable remediation strategy for Pb- and Cd-contaminated soils.

1. Introduction

The acceleration of global economic growth has intensified the release of lead (Pb) and cadmium (Cd) into the environment through activities, such as mining and smelting. Compared with mining, smelting emitted more Pb and Cd, resulting in significant soil contamination. Anthropogenic activities such as landfills and industrial emissions lead to toxic metal accumulation in soils, posing risks to ecosystems and public health. The coexistence of these two heavy metals (HMs) in soil has garnered significant attention due to their carcinogenicity and high toxicity. Through crop uptake in co-contaminated soil, Pb and Cd enter the food chain, posing substantial risks to human and animal health. Human and animal exposure to Pb and Cd through dietary intake has been linked to detrimental effects on the cardiovascular, neurological, and reproductive systems, including depression and cancer. ,

In this context, the environmental risk associated with Pb and Cd contamination is strongly governed not only by their total concentrations but also by their bioavailability and mobility in the soil systems. Previous studies have highlighted that bioavailable metal fractions largely determine the ecological toxicity and plant uptake behavior. The complex interactions of diverse factors such as pH, organic matter, and competing cations highlight the difficulty of assessing metal bioavailability in different environmental matrices. Furthermore, the migration of HMs from contaminated soils into surrounding water environments further increases environmental and human health risks, emphasizing the need for sustainable remediation strategies capable of limiting metal mobility and secondary pollution.

Concurrently, the management of nutrient pollution, particularly phosphorus (P), presents a significant environmental dilemma. The widespread application of inorganic P fertilizers has become a standard agricultural practice to maintain soil fertility and ensure crop productivity. Between 1961 and 2019, the total global consumption of P fertilizers increased by 34.50 million tons, contributing to the acceleration of water eutrophication. China alone is responsible for approximately one-third of the global P fertilizer use. Excessive application of chemical P fertilizers poses a considerable threat to water quality and ecosystem stability. In response, there is an urgent need to develop and adopt novel green fertilizers that enhance the P utilization efficiency and mitigate P-induced environmental risks, thereby supporting the sustainability of agricultural systems. When P-based materials are used for soil remediation, the same risk of P release remains a critical concern.

In situ immobilization using P-based amendments has emerged as a highly promising and cost-effective strategy for HM-contaminated soils. , This technique employs amendments that decrease the bioavailability and mobility of HMs through adsorption, precipitation, or complexation reactions. P-based materials have demonstrated significant efficacy in immobilizing Pb and Cd by forming highly stable minerals, such as pyromorphite and cadmium phosphates. , For instance, Huang and co-workers reported that phosphoric acid-modified peanut biochar decreased soil DTPA-Cd by 40.44–47.26%. Meta-analyses confirm that combining biochar with phosphate rocks or zeolites effectively converts exchangeable Cd and Pb to stable residual forms. , Similarly, aluminosilicates (e.g., synthetic zeolites) are also widely used because of their high cation exchange capacity (CEC) and porous structure, which enable effective adsorption of diverse HMs, including Pb and Cd.

However, a critical research gap remains at the junction of this remediation approach. Although effective for immobilizing specific HMs like Pb and Cd, the application of soluble P-based materials (e.g., chemical fertilizers) poses a risk of secondary pollution from the leaching of excess soluble P. , For instance, a recent meta-analysis on phosphorus-containing materials reported that phosphate fertilizer application elevated soil Olsen-P by two- to threefold above background levels for extended periods, with the effect remaining significant after 60 days of incubation. , Such an unintended consequence could accelerate eutrophication in adjacent water bodies, effectively substituting one environmental issue for another. Consequently, developing an integrated remediation strategy that ensures the simultaneous immobilization of these two HMs and control of P release is essential for advancing sustainable environmental management.

Although various P-modified hydrochars/biochars have been reported for HM immobilization, most studies focus on single amendments without addressing the dual challenge of maximizing metal retention while minimizing the risk of P release. To tackle this challenge, we propose a combined amendment incorporating P-modified hydrochar and synthetic zeolite for application in contaminated soils. Derived from the hydrothermal carbonization of biomass, hydrochar serves as a sustainable, porous carbon substrate with a high contaminant affinity, which is further specialized for Pb and Cd immobilization through P modification. Synthetic zeolite acts complementarily, offering substantial CEC that adsorbs HMs and potentially functions as a P sink to mitigate their release. , We hypothesize that a synergistic interaction between these two components will not only improve the concurrent immobilization of Pb and Cd but also effectively sequester P, thereby minimizing its environmental leaching.

The specific objectives of this study are to (1) synthesize and characterize BPH and ZL, (2) assess the individual and combined effectiveness of these two amendments in immobilizing Pb and Cd in a contaminated soil through a soil incubation experiment, and (3) analyze the influence of ZL on P utilization efficiency in the BPH200 via P balance analysis. This research aims to provide an innovative and environmentally sustainable strategy for the coordinated remediation of co-contaminated environments, contributing to both ecosystem safety and sustainability.

2. Materials and Methods

2.1. Preparation of BPHs and ZL

Banana peels were collected from a supermarket in Taiyuan city, Shanxi province, China. To remove surface impurities, these peels were washed several times with distilled water, cut into small square pieces (approximately 10 mm × 10 mm), and then soaked in a 42.50% H₃PO₄ solution at a mass/volume ratio of 1:10. After stirring evenly, the mixture was transformed into a Teflon-lined stainless steel autoclave. The autoclave was sealed and placed in an oven, then heated to the target temperature (120, 160, 200, or 240 °C) at a rate of 5 °C for 2 h under autogenous pressure. The resulting solid–liquid mixture was then vacuum-filtered and washed repeatedly with distilled water (approximately 5–6 times) until the filtrate reached neutral pH. The washed product was dried at 90 °C for 12 h, ground in an agate mortar, and passed through a 100-mesh sieve to obtain a uniform powder. These P-modified banana peel hydrochars (BPHs) prepared at different temperatures are referred to as BPH120, BPH160, BPH200, and BPH240. Details about the preparation process of the BPHs are presented in Figure S1. The pH of the pristine hydrochar (PH) prepared at 200 °C without H₃PO₄ modification was determined.

The raw coal gangue was obtained from a coal washing plant in the suburbs of Taiyuan city, Shanxi province, China (38°11″N, 111°47″E). Briefly, it was milled and then passed through a 150-mesh sieve. The coal gangue powder was mixed with NaOH at a mass ratio of 1:1.25 and calcined at 850 °C for 2 h. The resulting fused mixture was then ground, dissolved in deionized water, and subjected to hydrothermal treatment at 90 °C for 12 h. The synthesized zeolite was vacuum-filtered and washed with deionized water, then dried at 105 °C for 6 h, and finally ground to pass through a 200-mesh sieve. Details about the synthetic process are shown in Figure S2.

2.2. Characterization of BPHs and ZL

The pH value, electrical conductivity (EC), CEC, and specific surface area (SSA) of the PH, BPH200 and ZL were measured to characterize the properties of these amendments. Specifically, pH was determined at a solid-to-water ratio of 1:20 (w/v) after stirring for 30 min. EC was measured at a ratio of 1:10 (w/v). CEC was analyzed using the modified compulsive exchange method. SSA was obtained by N₂ adsorption–desorption isotherms at 77 K using the Brunauer–Emmet–Teller method.

X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) were used to determine the chemical composition, crystal phase species, micromorphology, and surface functional groups of these amendments. Specifically, XRF analysis was conducted under vacuum to determine the elemental composition. The Rh-target X-ray tube was operated at a voltage of 50 kV and a current of 50 mA. Pressed powder pellets were employed for all XRF analyses. XRD patterns were recorded with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA, with 2θ ranging from 5 to 60° at a scan rate of 8°/min. FT-IR spectra were collected in the range of 400–4000 cm^–1 using the KBr pellet method (sample/KBr = 1:100, w/w). SEM micrographs were acquired at an accelerating voltage of 15 kV. Details about the characterization method and measurement technology of the P species are provided in the Supporting Information (Amendment characterization and P species in BPH200 and PH).

2.3. Soil Incubation Experiment

Soil contaminated with both Pb and Cd was collected from the 0 to ∼20 cm surface layer of farmland soil near a coal-mining area (37°12′–38°15′N, 112°49′–113°53′E) in Taiyuan city, China. After air-drying, these soil samples were ground and then passed through a 2-mm sieve. The selected soil properties were as follows: sand, 61%; silt, 23%; clay, 16%; pH, 7.65; EC, 0.23 dS/m; soil organic matter (SOM), 0.61%; CEC, 11.77 cmol/kg; available P (Olsen-P), 4.42 mg/kg; total Pb, 537.35 mg/kg; and total Cd, 4.71 mg/kg. According to GB 15618–2018, “Soil environmental qualityRisk control standard for soil contamination of agricultural land,” the total contents of Pb and Cd are 3.16 and 7.83 times greater, respectively, than the risk control values (170 and 0.60 mg/kg, respectively, for soil pH > 7.50) in China. Thus, this soil was heavily contaminated by Pb and Cd.

The immobilization experiment of Pb and Cd was conducted in plastic pots (polyethylene material, 24 cm in height and 18 cm in diameter). Two kilograms of contaminated soil samples were added to each pot. On the basis of the reports of Hussain et al. and Boostani et al., which identified 1–2% (w/w) as an effective and cost-efficient range for Pb/Cd immobilization in soil, four treatments (control, 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL) were arranged with three replicates. The combined treatment was designed with a total addition of 2% (w/w), identical to the high single treatment, enabling a direct comparison between single and mixed amendments at equal mass input. The percentage refers to the mass ratio of the amendment to that of the soil. The amendments were thoroughly mixed with the soil to ensure homogeneity. Soil moisture was maintained at 70% of the field water-holding capacity throughout the incubation period. At 10, 50, and 100 days of incubation, the entire soil in each pot was manually homogenized, and then, a representative sample of approximately 10 g was collected from at least five random points within the pot and combined into one composite sample. The collected samples were air-dried and sieved (2 and 0.15 mm) to determine soil pH, Olsen-P, and the bioavailability of Pb and Cd. After 100 days of incubation, the chemical fractions of P, Pb, and Cd in the soil were analyzed.

At 10, 50, and 100 days of incubation, a sample was collected from each pot to measure the soil pH value, Olsen-P, and Pb and Cd bioavailability following air-drying and sieving (2 and 0.15 mm). After 100 days of incubation, the chemical fractions of P, Pb, and Cd were determined.

2.4. Characterization and Analysis of Soil Properties

The soil pH was determined in a slurry with a soil-to-water ratio of 1:1.25 using a pH meter (Leici pH-3C, Shanghai, China). The bioavailability and immobilization rate of Pb and Cd in soil were assessed using the toxicity characteristic leaching procedure (TCLP) according to USEPA Method 1311. Briefly, a 2 g soil sample was mixed with 40 mL of TCLP extraction fluid, shaken at 30 r/min for 18 h, and then filtered through a 0.45-μm pore size membrane filter (Millipore, mixed cellulose ester). The concentrations of these two HMs in the filtrates were measured via atomic absorption spectrometry (AA-1800C, Macylab Instruments Inc., Shanghai, China). Four Pb and Cd chemical fractions in the soil, including acid-soluble, reducible, oxidizable, and residual fractions, were extracted via the Bureau Community of Reference (BCR) sequential extraction procedure. The detailed steps of this sequential extraction are listed in the Supporting Information (sequential extraction procedure).

The Olsen-P in the soil was extracted with a NaHCO₃ solution (0.50 mol/L, pH 8.50) and detected with a spectrophotometer (UV-6100, Shanghai, China) via the molybdenum blue method. The P chemical fractions in the BPHs and soil included water-soluble P (H₂O-P), NaHCO₃ extracted inorganic P (NaHCO₃-P_i), NaHCO₃ extracted organic P (NaHCO₃-P_o), NaOH extracted inorganic P (NaOH-P_i), NaOH extracted organic P (NaOH-P_o), HCl extracted P (HCl-P), and residual P (residual-P). The detailed sequential fractionation procedure for these P fractions in hydrochar/soil is presented in the Supporting Information (P chemical fractions in BPHs and soil).

2.5. Statistical Analysis and Quality Control

For each treatment, the average values of the experimental data (n = 3) were recorded to guarantee accuracy and reliability. All statistical analyses were performed using SPSS Statistics 25.0 (IBM, USA). Data are presented as mean ± standard deviation of three replicates (n = 3). Homogeneity of variances was verified by Levene’s test (P > 0.05). One-way analysis of variance was conducted to compare treatment means among the four groups (control, 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL). When a significant difference was detected (P < 0.05), Tukey’s honestly significant difference post-hoc test was applied for pairwise comparisons. The certified soil standard reference material (GBW07498) was used to control quality. The recovery rates of Pb and Cd were 95.70 to ∼103.70%. Graphs were generated via Origin 2023. It is acknowledged that three replicates represent the statistical minimum; future studies with more replicates and field trials are needed to enhance generalizability. The immobilization rate (IR) for Pb and Cd was calculated using the following equation:

$$\mathrm{IR}(\%)=\left(1-\frac{C_{\mathrm{TCLP.am}}}{C_{\mathrm{TCLP.ck}}}\right)\times 100\%$$

where C _TCLP,am is the concentration of Pb or Cd extracted by the TCLP method from the amendment-treated soil, and C _TCLP,ck is the corresponding concentration from the control soil.

3. Results and Discussion

3.1. Properties of Coal Gangue-Based Zeolite (ZL)

The chemical components of coal gangue and ZL are presented in Table S1. Silicon dioxide and aluminum oxide are the main components of coal gangue. Typically, the optimal molar ratio of silicon dioxide and aluminum oxide in coal gangue for the synthesis of NaX-type zeolite was 2.90:1. The corresponding molar ratio (3:1) in the coal gangue used in this study was close to this optimal molar ratio. Compared with the SSA value (27.17 m²/g) of coal gangue, the higher SSA value (167.43 m²/g) of synthetic zeolite suggested a strong adsorption and desorption capacity for HMs and phosphate radicals. The SEM image [Figure (a)] revealed a rough and dense microsurface of coal gangue, whereas the ZL surface was smooth and uniform [Figure (b)]. The XRD pattern of the coal gangue [Figure (c)] revealed that kaolin and quartz were the main minerals present in the coal gangue. These two components can typically be activated by solid NaOH after calcination at high temperature, after which gels are produced. After that, nucleation and crystal growth are promoted in water under alkaline hydrothermal conditions. The XRD pattern of the final product indicated that the phase of coal gangue disappeared and that the phase (2θ = 6.20, 10.11, 11.72, 15.50, 23.41, 26.82, and 31.22°) of NaX zeolite (JCPDS card: 12-0228) was newly formed. In addition, synthetic zeolite presented the same surface functional groups as commercial-grade zeolite, such as Si–O/Ai-O, Si–O–Si/Si–O–Al, SiO₄/AlO₄, and −OH [Figure (d)]. In short, NaX-type zeolite was successfully prepared from coal gangue.

1.

(a) SEM image of coal gangue. (b) SEM image of ZL. (c) XRD patterns of coal gangue and ZL. (d) FT-IR spectra of ZL and commercial-grade zeolite. ZL: coal gangue-based zeolite.

3.2. P Chemical Fractions in BPHs and Properties of BPH200

The concentrations of the P chemical fractions in the BPHs prepared at different temperatures are shown in Figure . As the temperature increased from 120 to 240 °C, the concentration of total P increased from 2210 to 2504 mg/kg due to the aquathermolysis of H₃PO₄ and banana peels. This reaction caused the accumulation of P in the modified hydrochar. This trend is consistent with a meta-analysis of 278 data sets showing that hydrothermal carbonization increased solid-phase total P by 67.9% on average, with greater enhancement above 200 °C, and with studies on wetland biomass and algae (Chlorella vulgaris and Microcystis sp). biomass. ,

2.

P chemical fractions in sequential extracts of BPHs. BPH120, BPH160, BPH200, and BPH240: H₃PO₄-modified hydrochars prepared from banana peels at 120, 160, 200, and 240 °C, respectively. Different lowercase letters indicate significant differences (P < 0.05) in the same fraction between the H₃PO₄-modified hydrochars prepared at different temperatures.

Generally, H₂O-P and NaHCO₃-P are considered labile P, which is easily taken up by plants. NaOH-P mainly refers to moderately labile P. According to the literature, this P fraction primarily contains Fe/Al-bound P and organic P with low recalcitrance. , Owing to the easy transformation from Fe/Al-bound P to labile P and the weak binding of this organic P to soil minerals, NaOH-P can also serve as a potential P resource for plant growth. , In contrast, HCl-P and residual-P are deemed sparingly labile P, which are stable and insoluble forms, with HCl-P mainly corresponding to Ca-bound P (e.g., apatite) and residual-P referring to permanently sealed P that is largely unavailable to plants. In this study, the H₂O-P contents in BPH120, BPH160, BPH200, and BPH240 were 743, 697, 671, and 633 mg/kg total P, respectively. The NaHCO₃-P contents were 359, 389, 430, and 410 mg/kg total P, and the NaOH-P contents were 281, 313, 352, and 346 mg/kg total P, respectively (Figure ). It is possible that the decrease in H₂O-P with increasing temperature from 120 to 240 °C was due to the transformation of solution P species into less soluble forms through interactions with organic matter and the formation of metal-phosphate complexes during hydrothermal carbonization. Dai and co-workers reported that hydrothermal carbonization of manure significantly reduced H₂O-P by more than 80%, attributing this to the formation of apatite P and the precipitation of P with multivalent cations. In addition, as the temperature rose from 120 to 200 °C, the contents of NaHCO₃-P and NaOH-P significantly increased (Figure ). This could be explained by the fact that acid preparation conditions are expected to promote the formation of NaHCO₃-P and NaOH-P. However, a further increase in temperature to 240 °C resulted in slightly lower amounts of NaHCO₃-P and NaOH-P compared to those obtained at 200 °C. The reason was that the cross-linking reaction took place between H₃PO₄ and fresh banana peels, and P–O–C and C–O–PO₃ were produced at high temperatures. Their overproduction not only reduced the content of exchangeable P but also prevented P from being bound to Fe/Al and the surface of the SOM. Furthermore, the sum of H₂O-P, NaHCO₃-P, and NaOH-P in BPH200 was higher (1453 mg/kg) than that (1383, 1402, and 1389 mg/kg, respectively) in BPH120, BPH160, and BPH240. Unlike H₂O-P and NaHCO₃-P, which are used directly by plants, since NaOH-P can act as a buffer for exchangeable P in soil, its marked increase in BPH200 effectively compensated for the decline in water-soluble and exchangeable P fractions. This suggests that BPH200 may release more Olsen-P into the soil, thereby supporting medium-term plant growth requirements. Accordingly, BPH200 was selected as the most suitable candidate for further investigation in P release and HM immobilization.

Notably, compared with BPH120, the concentration of HCl-P in BPH160, BPH200, and BPH240 increased dramatically by 44.21, 98.56, and 140.48%, respectively (P < 0.05). Similar trends could also be found in previous studies. High temperatures (200–240 °C) promote the thermal dehydration of H₃PO₄, leading to its condensation into oligomeric and polymeric P species. , Studies have explained that, with increasing temperature, H₃PO₄ first condenses to pyrophosphoric acid and then forms metaphosphoric acid and other polyphosphoric acids. , This transformation was confirmed by the ³¹P nuclear magnetic resonance (NMR) spectra of BPH200, which revealed the presence of both orthophosphate and metaphosphate (Figure S3). Ca-phosphates are usually the dominant component in biochar/hydrochar, especially for raw materials with high Ca contents.

Because H₃PO₄ and NaOH were used in the BPH200 and ZL preparations, respectively, BPH200 presented an acidic pH (4.42), whereas ZL presented an alkaline pH (8.10) (Table S3). As the study of Chen and co-workers suggested, because of the high degree of dehydration and carbonization, the percentages of oxygen (O) and hydrogen (H) in BPH200 decreased, and the proportion of carbon (C) increased. The results revealed higher values of C/O (2.98), C/(O+N) (2.72), and C/H (9.91) in BPH200 than those (2.45, 2.39, and 8.19, respectively) in PH without H₃PO₄ modification. The relatively high C/O and C/(O+N) values indicate the low hydrophilicity and relatively low polarity index of the material. In parallel, the high C/H ratio indicates a high degree of aromaticity, confirming that H₃PO₄ modification enhanced the degree of carbonization. In addition, the content of total P (2470 mg/kg) in BPH200 was much greater than that (190 mg/kg) in PH, which confirmed that an enormous amount of P was embedded in BPH200.

SEM images of PH and BPH200 are shown in Figure . The surface of PH without H₃PO₄ modification was smooth and thick [Figure (a)], whereas BPH200 exhibited a rough surface with many pores [Figure (b)]. This means that BPH200 contained more adsorption sites than did PH. The formation of this porous structure could be attributed to the impregnation of H₃PO₄ into the cellulose and lignin matrix of the banana peels. At elevated temperatures, the catalytic dehydration reaction between H₃PO₄ and these biopolymers likely generated water vapor, creating pores during its release. , Some organic substances also reacted with H₃PO₄, leading to their decomposition and the formation of carbon dioxide and water vapor. Consequently, a porous structure was formed. The XRD patterns revealed a broader and stronger peak approximately 22° in BPH200 than in PH [Figure (c)], which can be explained by the disordered graphite amorphous phase structure in BPH200 and PH. Owing to proton catalysis. BPH200 contained more amorphous carbon than did PH. Moreover, there were more functional groups (i.e., −OH, −COOH, P–O–C, -PO– 3, and −CH₂) on the surface of BPH200 than on the surface of PH [Figure (d)]. These groups can complex with HMs in soil and transform metals into stable fractions.

3.

(a) SEM image of PH. (b) SEM image of BPH200. (c) XRD patterns of PH and BPH200. (d) FT-IR spectra of PH and BPH200. PH: pristine hydrochar derived from banana peels without H₃PO₄ modification at 200 °C; BPH200: hydrochar prepared from banana peels with H₃PO₄ modification at 200 °C.

3.3. Soil pH, Olsen-P, and TCLP Extracted Pb and Cd in Soil

Adding BPH200 alone or in combination with ZL significantly (P < 0.05) reduced the weakly alkaline soil pH [Figure (a)]. Because the pH value of BPH200 was acidic, the BPH200 addition decreased the soil pH. Conversely, since the pH value of ZL was alkaline, the extent of the pH decrease in the 1% BPH200 + 1% ZL treatment was less than that in the 1% BPH200 and 2% BPH200 treatments.

4.

(a) Soil pH, (b) bioavailability of Pb, (c) bioavailability of Cd, and (d) Olsen-P in the different treated soils. Different letters above the column represented a significant difference (P < 0.05) at the same incubation time among control and different treatments.

The ability of TCLP to extract Pb and Cd could reflect their bioavailability. The highest TCLP-extractable concentrations of Pb and Cd (91.20–93.67 and 0.71–0.73 mg/kg, respectively) were found in the control. The 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments remarkably (P < 0.05) decreased the TCLP extracted concentrations of Pb and Cd by 27.41% and 21.13%, 54.93% and 54.71%, 78.61% and 67.61%, respectively [Figure (b) and (c)]. The results indicated that the immobilization rates of Pb and Cd by the co-application of these two remediation agents were superior to those by the mono-application of BPH200 at the same dosage.

Compared with the control, the soil Olsen-P in the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments remarkably (P < 0.05) increased by 3.37-, 5.37-, and 3.54-fold, respectively [Figure (d)]. The content of soil Olsen-P is usually 10–20 mg/kg, which can satisfy the requirements for most plant growth. Nevertheless, the concentration of soil Olsen-P was out of this range in the 2% BPH200 treatment [Figure (d)], especially at 10 and 50 days. With increasing incubation time, soil Olsen-P clearly (P < 0.05) decreased. For example, soil Olsen-P at 50 days decreased by 11.04, 21.50, and 17.08% compared with that at 10 days in the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments, respectively. Moreover, there was no significant (P > 0.05) difference in the content of soil Olsen-P between the 1% BPH200 + 1% ZL treatment and the 1% BPH200 treatment. This finding suggests that the combined use of zeolites does not necessarily lower soil P availability. Some studies have demonstrated that P availability is primarily governed by CaCO₃ content and pH, while the influence of added clay minerals such as zeolite on soil P dynamics is often negligible. Zhang and co-workers confirmed that zeolite addition to calcareous soils increases the P adsorption capacity, indicating that zeolite may act as a P sink rather than a P activator in such soils.

In addition to the overall immobilization, the temporal variations of Pb, Cd, and P availability provide critical insights into the remediation dynamics (Figure ). For the control, TCLP-extractable Pb and Cd remained relatively stable over the 100-day incubation. Notably, compared with the 1% BPH200 and 2% BPH200 treatments, the 1% BPH200 + 1% ZL treatment showed the fastest and most continuous decline in both Pb and Cd bioavailability, reaching the lowest values on the 100th day. This temporal pattern suggests that the co-application of BPH200 and ZL not only enhances the overall immobilization but also accelerates stabilization kinetics, likely due to the synergistic effects of precipitation and adsorption. Regarding P availability, soil Olsen-P in all treatments peaked sharply on the 10th day, then declined on the 50th day and slightly further on the 10th day. The initial rise reflects immediate labile P release from BPH200, while the subsequent decline is attributed to P immobilization via adsorption and precipitation with HM ions. Among the treatments, 2% BPH200 consistently showed the highest Olsen-P at all time points, exceeding the plant requirement range (10–20 mg/kg), indicating a persistent P leaching risk. However, the 1% BPH200 + 1% ZL treatment exhibited a more moderate peak on the 10th day and a less pronounced decline thereafter, resulting in values on the 100th day similar to those in the 1% BPH200 treatment. This suggests that ZL acts as a P buffer, reducing the initial soluble P burst and moderating its release over time.

3.4. Chemical Fraction of P, Pb, and Cd in Soil

The concentrations (proportions) of acid-soluble Pb, reducible Pb, oxidizable Pb, and residual Pb in the soil were 118.19 mg/kg (22.01%), 117.44 mg/kg (21.87%), 135.79 mg/kg (25.10%), and 166.09 mg/kg (30.93%), respectively, in the control. The concentrations of the corresponding Cd fractions were 0.77 mg/kg (16.52%), 0.58 mg/kg (12.43%), 1.16 mg/kg (24.62%), and 2.18 mg/kg (46.43%) (Figure and Table S4). Overall, compared with the control, the 1% BPH200 + 1% ZL treatment resulted in a greater decrease in acid-soluble Pb and Cd by 12.00 and 11.61%, respectively, than did the 1% BPH200 (5.74 and 4.41%, respectively) and 2% BPH200 (10.24 and 9.78%, respectively) treatments. These three treatments had little effect on the reducible and oxidizable Pb in the soil, while only a slight (P > 0.05) decrease in the reducible Cd content of 1.53–4.10% was found. The amount of oxidizable Cd slightly (P > 0.05) increased by 1.97–5.50%. Moreover, the residual Pb fraction significantly (P < 0.05) increased by 9.86 and 13.48% in the 2% BPH200 and 1% BPH200 + 1% ZL treatments, respectively, whereas a slight (P > 0.05) increase of 5.68% was found in the 1% BPH200 treatment. For the Cd chemical fractions, the 2% BPH200 and 1% BPH200 + 1% ZL treatments dramatically (P < 0.05) increased the residual Cd fraction by 9.09 and 10.09%, respectively, whereas the 1% BPH200 treatment slightly (P > 0.05) improved this Cd fraction by 3.97%, The comparison results suggested that the 1% BPH200 + 1% ZL treatment was the most effective for immobilizing these two HMs in soil, possibly by converting the acid-soluble fraction to the residual fraction.

5.

Effect of adding BPH200 alone or in combination with ZL on the relative proportion of the (a) Pb and (b) Cd fractions in the soil.

Several mechanisms may explain the higher immobilization rate observed in the 1% BPH200 + 1% ZL treatment compared to the 2% BPH200 treatment. (1) A large amount of metal-P precipitations likely occurred between Cd/Pb and BPH200, forming minerals such as Pb₃(PO₄)₂OH, Cd­(PO₃)₂, and Cd₃(PO₄)₂OH, as reported in our previous study. These precipitates formed on the BPH200 surface block its surface pores, limiting further release of P-containing species. Zeolite, with the high SSA and CEC, adsorbs both HM ions and these precipitates. , The extent of pore blocking decreased. This, in turn, may facilitate the release of more P-containing particles (i.e., PO₄ ^3– and H₂PO₄ ^3–) from the internal pores of BPH200 via diffusion, thereby enhancing precipitation. (2) ZL could simultaneously adsorb soluble Pb/Cd and P-containing particles in the soil, promoting the formation of metal-P precipitation on the surface of ZL. As a result, more P-containing particles might be continuously released from the inner pores of BPH200, further contributing to precipitation. In addition to this coprecipitation mechanism, other processes such as complexation, ion exchange, metal−π electron interactions, and physical adsorption between Pb/Cd and BPH200/ZL may also have contributed to the immobilization of these two HMs. Besides, synergistic interactions among soil properties also enhance Pb and Cd retention. Recent studies have found that initial metal concentration exhibits strong synergy with both CEC and clay content. Higher CEC provides abundant exchange sites; when combined with elevated initial metal concentration, the increased concentration gradient facilitates more efficient ion exchange, thereby improving metal immobilization. Clay minerals increase SSA and site accessibility, a synergy that is particularly pronounced under high initial metal concentration. In this study, the tested soil had moderate clay (16%) and CEC (11.77 cmol/kg). The application of BPH200 and ZL likely enhanced local exchangeable sites and altered the soil matrix, promoting synergistic capture of Pb and Cd. The superior immobilization efficiency of the 1% BPH200 + 1% ZL treatment may be partly attributed to such synergy between the amendments and the soil CEC and clay fractions.

The fate of Pb and Cd in contaminated soil is also governed by hydrodynamic processes, particularly convection and diffusion, which control the transport of dissolved and bound HMs from soil to groundwater. For instance, Wu and co-workers found that, at the soil–pore water interface, diffusion played a more significant role than convection in the migration of various pollutants, including HMs. Conversely, at the soil–perched water interface, convection dominated the transport of certain metals such as Ni and Pb. Moreover, a threshold or saturation effect was observed between convection intensity and pollutant concentrations in groundwater, beyond which the rate of HM release accelerated.

In comparison with other functionalized gangue-derived zeolites, Zhu and co-workers recently developed an amino-modified coal gangue-based NaX zeolite (NH₂-NaX) with high Pb/Cd adsorption capacities (214.1 and 256.7 mg/g) and effective immobilization in soil. Nevertheless, under simulated acid rain (pH 5.0) leaching, NH₂-NaX exhibited relatively high early-stage metal release and increased exchangeable fractions post-leaching. In this study, the unmodified ZL, especially when combined with BPH200, consistently reduced TCLP-extractable Pb and Cd over 100 days [Figure (b) and (c)], demonstrating stronger acid-rain resistance. Although amino grafting enhances the theoretical adsorption capacity, the grafted functional groups are susceptible to hydrolysis or protonation under low-pH conditions, thereby compromising the medium-term stability. In contrast, the unmodified ZL primarily immobilizes metals via ion exchange and physical adsorption, mechanisms that exhibit greater robustness in acidic environments. Future development of gangue-derived zeolites may therefore benefit from hybrid modification strategies, such as cross-linking or the application of inorganic coatings, to synergistically achieve high adsorption capacity and enhanced acid resistance.

It should be noted that the BCR sequential extraction method may systematically overestimate the mobility risks of HMs in alkaline soils. Recent studies have demonstrated that, in alkaline soils, the acid-soluble fraction can increase due to the formation of carbonate precipitates, and the reducible and oxidizable fractions may also accumulate. , Nevertheless, these forms are environmentally stable. Consequently, risk indices based solely on BCR fractions tend to overestimate the true mobility of Pb and Cd in alkaline soils. The soil used in this study had an initial pH of 7.65 (weakly alkaline). Although the addition of acidic BPH200 slightly decreased the soil pH, the overall pH remained neutral to weakly alkaline throughout the incubation period. Therefore, because of the reduction in acid-soluble Pb/Cd and the increase in residual fractions, the immobilization efficiencies reported herein likely represent a conservative estimate of the actual risk reduction.

Compared with the control, the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments markedly increased (P < 0.05) H₂O-P by 28.77, 49.42, and 17.40%, respectively (Table S5). In the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments, NaHCO₃-P_i increased by 10.64, 23.51, and 9.17%, respectively, while NaHCO₃-P_o showed slight (P > 0.05) enhancements of 7.57, 16.75, and 7.87%. For NaOH-P_i and NaOH-P_o, no significant difference (P > 0.05) was detected among these three treated soils. For HCl-P, increases of 5.37, 8.89, and 5.73% were observed in the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments, respectively, and the residual-P fraction dramatically (P < 0.05) improved by 8.51, 17.52, and 19.27%, respectively. In addition, compared with the control, the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments increased the labile P (H₂O-P + NaHCO₃-P_i + NaHCO₃-P_o) by 10.78, 22.71, and 9.27%, respectively, with corresponding increases of 2.13, 4.51, and 6.10% for moderately labile P (NaOH-P_i + NaOH-P_o), along with improvements of 8.27, 16.85, and 18.26% in the stable P (HCl-P + residual-P). Therefore, the 1% BPH200 + 1% ZL soils presented greater stable P and lower labile P than the other two soils did, suggesting a low risk of P release after co-application of BPH200 and ZL.

After 100 days of incubation, based on the data in Table S5, the effects of BPH200 applied alone or in combination with ZL on the relative proportions of P chemical fractions in the soil were calculated and shown in Figure S4. Figure shows the results of the correlation analysis among the chemical fractions of P, Pb, and Cd in the soil (number of data pairs is 12). Due to the appearance of orthophosphate and metaphosphate in BPH200 (Figure S3), acid-soluble/reducible Pb and Cd may be immobilized through mechanisms such as precipitation and adsorption onto the P-containing functional groups. The significant negative correlation between residual P and acid-soluble/reducible Pb and Cd, with r values of from −0.93 to ∼−0.97 and from −0.95 to ∼−0.96, respectively, is consistent with the proposed coprecipitation mechanism, although such statistical association does not constitute direct evidence. While the formation of metal-phosphate precipitates was a plausible pathway, adsorption onto the P-modified hydrochar surface could also contribute to the observed retention. In the soil environment, Pb and Cd in the presence of P can be immobilized mainly as pyromorphite and phosphor-cadmium mineral. The significant (P < 0.05) positive correlation between residual-P and residual Pb/Cd further confirmed this coprecipitation mechanism. However, unlike previous studies concerning the remarkably negative correlation between acid-soluble Pb/Cd and H₂O-P and NaHCO₃-P in soil, in this study, acid-soluble Pb/Cd had no significant (P > 0.05) correlations with H₂O-P, NaHCO₃-P_i, or NaHCO₃-P_o (Figure ). The discrepancy may be attributed to the presence of ZL in the combined treatment, which likely altered the fate of P released from BPH200. Recent studies have demonstrated that zeolite materials can adsorb soluble phosphate species through electrostatic attraction and ligand exchange mechanisms, , effectively sequestering P in forms not immediately extractable by NaHCO₃. However, in the present study, the zeolite-amended treatment (1% BPH200 + 1% ZL) consistently exhibited Olsen-P concentrations greater than or equal to those without zeolite (1% BPH200) throughout the incubation period [Figure (d)]. This indicates that although zeolite can adsorb phosphate, it did not reduce soil P availability. Furthermore, P in the presence of zeolite may rapidly bind with HMs (i.e., Pb, Cd, and Cu) to form stable precipitates, thereby decoupling the direct relationship between labile P and available metals. Zeolite can retain soluble phosphate via electrostatic attraction and ligand exchange, as supported by recent studies. − The retained P is subsequently available for reaction with Pb²⁺ and Cd²⁺ diffusing in the soil solution, leading to localized supersaturation and enhanced precipitation of stable metal phosphates. This interpretation is supported by the strong negative correlation observed between acid-soluble Pb/Cd and NaOH-P. The corresponding r values ranged from −0.94 to ∼−0.99 (Figure ), suggesting that P involved in metal immobilization was primarily derived from moderately labile fractions rather than the most labile pools. Additionally, reducible Pb and Cd mainly refer to iron (Fe)–manganese (Mn) oxide-bound Pb and Cd. This fraction exists in finely dispersed particles and is formed by anion adsorption or coprecipitation because of the large SSA of active Fe–Mn oxides. The slight decrease in soil pH also contributed to the decomposition of this fraction. Therefore, Pb/Cd combines with inorganic P or P-containing organic matter in soil and transforms into more stable forms. It should be emphasized that the proposed coprecipitation mechanisms are inferred from indirect evidence. Further studies employing direct spectroscopic techniques (e.g., X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, extended X-ray absorption fine structure, and transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy mapping) are required. These surface-sensitive analyses are essential to unambiguously identify the speciation, coordination environment, and spatial distribution of Pb, Cd, and P on the amendment surfaces.

6.

Pearson correlation analysis of the chemical fractions of P, Pb, and Cd in the soil at 100 days of incubation (number of data pairs is 12). The intensities of red and blue represent the strengths of the positive and negative correlations, respectively, and the correlation coefficients are denoted by the r value.

In addition to the interactions with BPH200 and ZL, the immobilization of Pb and Cd in soil is also influenced by the native iron (Fe), manganese (Mn), and aluminum (Al) oxides in the natural soil matrix. Specifically, Mn oxides consistently enhance Pb and Cd immobilization due to their low point of zero charge and abundant defect sites, exhibiting a near-monotonic positive contribution. Fe oxides show a hump-shaped effect: at low to moderate contents, they promote Pb and Cd adsorption, but at higher levels, the contribution diminishes or becomes negative, likely due to particle aggregation, crystallization, and surface coating. In contrast, Al oxides generally suppress Pb and Cd immobilization by masking reactive sites on Fe and Mn oxides and inducing surface passivation. Moreover, synergistic interactions occur between Mn and Fe oxides at high loadings, whereas Fe and Al oxides exhibit antagonistic effects. The addition of BPH200 introduces substantial amounts of P and enhances the formation of metal phosphate precipitates. However, the coexisting native Fe and Mn oxides may compete for the same P species or provide additional sorption sites, potentially altering the distribution of P among the different fractions. Zeolite, with its high CEC, may also interact with these oxides by adsorbing or releasing cations (e.g., Ca²⁺ and Al³⁺), thereby influencing the surface charge and reactivity of native oxides.

Besides, the effective solubility product constant (K _sp) of chloropyromorphite in normal soil (∼10^–30) is far higher than its theoretical minimum (∼10^–84); a similar trend holds for cadmium phosphates. This thermodynamically explains why applying soluble P alone often fails to simultaneously achieve high heavy metal immobilization and low P availability in calcareous soils. , In this study, coapplying BPH200 with ZL overcame this barrier. ZL possibly serves as a reaction interface that locally concentrates Pb²⁺/Cd²⁺ and phosphate ions released from BPH200, thereby overcoming the thermodynamic barrier of elevated K _sp. Meanwhile, the competitive sorption of phosphate by Ca²⁺, Mg²⁺, Al³⁺, and Fe³⁺ in the bulk soil solution is partially bypassed, as the precipitation occurs preferentially on or near the ZL surface.

3.5. Estimated P Utilization Efficiency in BPH200

In this study, the P derived from BPH200 was mainly allocated to two pathways: (1) immobilization of Pb and Cd in the soil and (2) increase of soil Olsen-P content. The P utilization efficiency of BPH200 was then estimated as the proportion of the total P input that was allocated to these two pools. Given that in calcareous soils like the one used in this study, the concentration of other multivalent cations (e.g., Ca, Mg, Al, and Fe) is higher than that of Pb and Cd, part of the added soluble P is expected to be primarily immobilized through reactions with these more abundant cations, forming insoluble Ca–P, Al–P, or Fe–P minerals. Therefore, the present estimation of P allocated only for Pb and Cd immobilization may overestimate the P utilization efficiency for these target metals. Because the soil was heavily polluted by Pb and Cd, the amount of P consumed by immobilization of other HMs was assumed to be negligible. According to the precipitation mechanism discussed in Section , it was assumed that the increase in the residual Pb fraction was attributed to Pb₃(PO₄)₂, and the increase in the residue Cd fraction contributed to Cd₃(PO₄)₂ and Cd­(PO₃)₂. On the basis of the chemical formula of Pb₃(PO₄)₂ and the atomic masses of Pb and P, immobilizing 10 mg of Pb consumes approximately 1 mg of P. The increasing concentrations of the residual Pb fraction in the 1% BPH200, 2% BPH200, and 1% BPH200 + 1% ZL treatments were 30.50, 52.95, and 67.02 mg/kg, respectively. Based on ³¹P NMR analysis, the mass ratio of ortho-P (88.50%) to meta-P (11.49%) in BPH200 was 7.7:1 (Figure S3). Using the chemical formulas of Cd₃(PO₄)₂ and Cd­(PO₃)₂, the amount of P consumed for Cd immobilization was also calculated. The soil Olsen-P content increased from 4.42 mg/kg (control) to 14.90, 23.74, and 15.63 mg/kg, respectively [Figure (d)]. The detailed accounting of P distribution among Pb immobilization, Cd immobilization, and Olsen-P increase is summarized in Table .

1. Estimated P Consumption for Pb/Cd Immobilization and Olsen-P Increase in Different Treatments.

parameter	1% BPH200	2% BPH200	1% BPH200 + 1% ZL
P for Pb immobilization (mg)	6.10	10.59	13.40
P for Cd immobilization (mg)	0.17	0.35	0.39
P for Olsen-P increase (mg)	20.96	38.64	22.42
total estimated P consumed (mg)	27.23	49.61	36.21
total P added (mg)	49.40	98.80	49.40
estimated P utilization efficiency (%)	55.12	50.22	73.29

As an estimation, the mass proportion (55.12%) of P utilization in the 1% BPH200 treatment was close to the percentage (58.82%) of the sum of H₂O-P, NaHCO₃-P, and NaOH-P fractions in the total P in BPH200 (Figure and Table S2). The H₂O-P and NaHCO₃-P fractions are the liable P pools that can be more easily taken up by plants than HCl-P and residual-P. Pb and Cd immobilization and the rapid increase in soil Olsen-P are mainly due to the input of BPH200, which has a high content of Olsen-P. NaOH-P refers to Fe/Al oxides and humic substances, which are moderately labile P. This P fraction can transform into labile NaHCO₃-P by mineralization. For the 2% BPH200 treatment, the P utilization efficiency decreased to 50.22%, although the total P in the soil improved twofold compared with that in the 1% BPH200 treatment. The results indicated that a high dosage of BPH200 added individually could cause a high P release risk, which was not a wise choice for Pb- and Cd-contaminated soil remediation. However, after the 1% BPH200 + 1% ZL treatment, the estimated P utilization efficiency (73.29%) in the 1% BPH200 treatment was close to the percentage (78.94%) of the sum of the H₂O-P, NaHCO₃-P, NaOH-P, and HCl-P fractions in the total P in BPH200 (Figure and Table S2). The HCl-P fraction is relatively stable P. This fraction mainly includes Ca-bound P that cannot be directly ingested by plants. However, with the help of ZL, Ca-bound P may transform to Olsen-P via an ion exchange mechanism. Therefore, adding BPH200 along with ZL to Pb- and Cd-contaminated soil could increase the P utilization efficiency in BPH200 and decrease the P release risk. Moreover, the co-application of BPH200 and ZL exhibited a stronger immobilization effect on Pb and Cd in soil than did the monoapplication of BPH200, producing an excellent “1 + 1 > 2” effect.

This research revealed that H₃PO₄-modified hydrochar applied alone presented a relatively high risk of P release in the soil environment. However, the combined application of P-modified hydrochar and ZL could avoid the high P release risk caused by a high dosage of P-modified hydrochar alone while strengthening Pb and Cd immobilization in the soil. The P utilization efficiency for P-modified hydrochar improved, and on the basis of soil P fraction data, the combined treatment may lower the risk of P release. The inference of reduced P mobility upon the combined treatment is based solely on indirect evidence (higher stable P and lower labile P fractions), which suggests only a lower P release potential under static incubation. This does not constitute direct proof under real-world hydrological conditions; field-scale lysimeters are needed to validate actual P mobilization reduction. Besides, further studies should focus on carefully evaluating P release risk, the optimum dosage ratio of P-modified hydrochar and zeolite according to soil contamination level, and should include simulated rainfall runoff tests and packed column leaching experiments to directly quantify P and metal leaching. In addition, to refine the estimation, future work should include the possible reactions of P with other metal species (e.g., Zn, Cu, Fe, and Al). Moreover, a deeper understanding of these competitive interactions will also be crucial for optimizing the BPH200 + ZL dosage ratio according to specific soil conditions.

4. Conclusions

This study compared the potential of adding BPH200 alone and ZL to simultaneously immobilize Pb and Cd in the soil and increase the amount of plant-available P in BPH200. Compared with the 1%BPH200 (27.41% and 21.13%, respectively) and 2% BPH200 (54.93% and 54.71%, respectively) treatments, the 1%BPH200 + 1%ZL treatment presented the highest Pb and Cd immobilization rates (78.61% and 67.61%, respectively). The comparison results are consistent with ZL may enhance the coprecipitation mechanism between Pb/Cd and P in the soil following BPH200 addition within a certain period. Furthermore, although ZL addition did not significantly alter soil Olsen-P concentrations, it was associated with improved P utilization efficiency within BPH200 from 55.12% (1% BPH200) and 50.22% (2% BPH200) to 73.29% (1% BPH200 + 1% ZL). Unlike conventional P-based materials that often pose a risk of excess P release, the combined amendment shows high HM immobilization while suggesting a lower potential for P release risk in BPH200 in soil. This dual benefit, derived from the mechanistic synergy between BPH200 and ZL, represents a potential advantage over single-amendment strategies. Therefore, this study indicates the positive role of the co-application of BPH200 and ZL in Pb and Cd immobilization in co-contaminated soil without increasing the risk of P release. Nevertheless, this study did not include plant uptake, phytotoxicity, agronomic validation, crop yield, or economic evaluation; thus, its practical agricultural relevance remains unproven. Future studies should examine this combined amendment across a spectrum of soil types to assess its robustness, and further plant growth experiments are warranted to evaluate metal uptake, biomass production, and P bioaccumulation. Moreover, field trials and life-cycle cost analysis are required before large-scale application. In addition, because dynamic leaching and variable pH/redox or wet–dry cycles were not assessed, future column and redox-controlled incubation studies are needed to validate the stability and environmental safety of the combined amendment.

The data supporting the findings of this study are available from the corresponding author upon reasonable request, subject to field confidentiality restrictions.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c03989.

• Chemical reagents used in this study; Amendment characterization methods (pH, CEC, SSA, elemental analysis, XRF, XRD, SEM, FT-IR); ³¹P nuclear magnetic resonance (NMR) analysis of P species in BPH200 and PH; BCR sequential extraction procedure for Pb and Cd fractions in soil; Sequential fractionation procedure for P chemical fractions in BPHs and soil; Chemical composition of coal gangue and synthetic NaX zeolite; P chemical fraction distribution in BPHs prepared at different temperatures; Physicochemical properties of PH, BPH200, and ZL; Pb and Cd fractions in different treated soils; Effects of different treatments on soil P chemical fractions (PDF)

Qilong Ge: writing original draft, methodology, validation, and visualization; Chunjuan Dong: conceptualization, investigation, and writing – reviewing and editing preparation; Guoying Wang: resources, software, and validation; Jing Zhang: supervision, formal analysis, and data curation; Rui Hou: visualization, project administration, and writing – reviewing and editing preparation.

The authors declare no competing financial interest.