Utilizing Ca3(PO4)2 and calcium superphosphate fertilizer for the passage of heavy metals in coal gangue: Mechanical insights and efficiency evaluation

Hualin Zhang; Wenlong Bai; Shaojing Tian; Mengfei Zhao; Youming Yang; Xiaoliang Jiang; Tinggang Li

doi:10.4491/eer.2024.322

Environ Eng Res > Volume 30(4); 2025 > Article

Zhang, Bai, Tian, Zhao, Yang, Jiang, and Li: Utilizing Ca3(PO4)2 and calcium superphosphate fertilizer for the passage of heavy metals in coal gangue: Mechanical insights and efficiency evaluation

Research

Environmental Engineering Research 2025; 30(4): 240322.

Published online: December 27, 2024

DOI: https://doi.org/10.4491/eer.2024.322

Utilizing Ca₃(PO₄)₂ and calcium superphosphate fertilizer for the passage of heavy metals in coal gangue: Mechanical insights and efficiency evaluation

Hualin Zhang^1,^2,³, Wenlong Bai⁴, Shaojing Tian², Mengfei Zhao^1,^2,³, Youming Yang¹, Xiaoliang Jiang^2,^†

, Tinggang Li^1,^2,^3,^5,^6,^†

¹School of Metallurgical engineering, Jiangxi University of Science and Technology, Ganzhou 341099, China

²Key Laboratory of Rare Earth, Jiangxi Province Key Laboratory of Cleaner Production of Rare Earths, Jiangxi Institute of Rare Earths, Ganjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341003, China

³CAS Key Laboratory of Green Process and Engineering, Innovation Academy for Green Manufacture, National Engineering Research Center of Green Recycling for Strategic Metal Resources, Beijing Engineering Research Centre of Process Pollution Control, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

⁴Inner Mongolia Dongyuan Environmental Protection Technology Co., Ltd., Ordos 014300, China

⁵State Key Laboratory of Biochemical Engineering, Key Laboratory of Biopharmaceutical Preparation and Delivery, Chinese Academy of Sciences, Beijing 100190, China

⁶University of Chinese Academy of Sciences, Beijing 100049, China

^†Corresponding author: E-mail: xljiang@gia.cas.cn (X.L.J.), tgli@ipe.ac.cn (T.G.L.), Tel: +86-07972130689 (X.L.J.), +86-10-82545052 (T.G.L.), Fax: +86-07972130689 (X.L.J.), +86-10-82545052 (T.G.L.), ORCID: 0000-0002-5196-8391 (X.L.J.), 0000-0003-2780-639X (T.G.L.)

Received May 26, 2024 Revised August 26, 2024 Accepted December 25, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Coal gangue is a typical industrial waste, which will bring environmental challenges and resource depletion when piled up in large quantities, and heavy metal pollution is a prominent problem. In this study, Ca₃(PO₄)₂ and calcium superphosphate fertilizer (CSF) were utilized to passivate coal gangue at different pollution levels, and its passivation mechanism was discussed. The results show that both Ca₃(PO₄)₂ and CSF can effectively passivate coal gangue and effectively reduce moderate or high levels of coal gangue pollution to a slight or clean level. The passivation effect of CSF is slightly better than that of Ca₃(PO₄)₂. Through characterization of coal gangue before and after passivation and simulation with Visual MINTEQ and PHREEQC software, it is found that Ca₃(PO₄)₂ can provide PO₃²⁻, while CSF releases HPO₃⁻, which is ionized to generate more PO₃²⁻, and finally forms insoluble or slightly soluble phosphate with heavy metals. Leaching experiments show that the treated samples contain obviously more stable heavy metal components, and the leaching risk is lower than that of untreated coal gangue. In summary, this method proves its effectiveness in fixing heavy metals in coal gangue and provides an effective method for the harmless treatment of coal gangue.

Keywords: Coal gangue, Heavy metal, Passivation, Software simulation

Graphical Abstract

Keywords: Coal gangue, Heavy metal, Passivation, Software simulation

Introduction

Coal gangue, a type of rock containing over 50% dry ash content, is generated during the coal mining and washing processes. The gangue sources include coal roadway gangue, washery gangue, rock roadway gangue, spontaneous combustion gangue, hand-picked gangue, and stripping gangue [1]. With abundant sources, it constitutes approximately 10–25% of the total coal production, making it the largest solid waste in the coal industry and one of the largest industrial solid wastes in terms of land area, contributing to more than 20% of the national industrial solid waste. In recent years, heavy metal pollution in coal gangue has been widely concerned [2]. Consequently, the safe treatment of heavy metals in coal gangue has emerged as a pressing issue in the field of environmental science. Coal gangue is a byproduct of coal mining and processing, which is rich in minerals and useful elements such as iron, aluminum, and sulfur, making it a valuable raw material for industrial production [3]. In 2023, Zhang prepared functional materials by mixing coal gangue and steel slag according to the mass ratio of 2:1. The sulfur fixation efficiency of total sulfur gas, H₂S and SO₂ was 67.87%, 94.73% and 66.91%, indicating that coal gangue is a better material matrix [4]. Through appropriate technological means, valuable elements can be extracted from coal gangue, facilitating resource reuse and recycling [5–6]. In 2022, Chen studied the occurrence state and enhanced leaching mechanism of key metals (lithium, niobium and rare earth elements) in coal gangue, providing theoretical basis and technical support for the extraction of key metals in coal gangue [7]. This not only reduces the demand for natural resources but also stimulates the development of local industries, leading to new economic growth opportunities. In 2024, Liu synthesized low-silica zeolite X by two-step activation method, which has a good effect on capturing CO₂, and provides a promising solution for the high value-added utilization of coal gangue and has the potential for large-scale production [8]. However, the existence of harmful substances in coal gangue, especially heavy metals, poses a serious threat to microorganisms, the environment, and human health [9–13]. When exposed to atmospheric, aquatic, and soil environments, the heavy metal elements in coal gangue may undergo dissolution, migration, and transformation, resulting in environmental pollution [14–16] and potential health risks for humans [16], such as neurological damage, cancer, and immune system disorders [18]. This risk is particularly heightened for populations residing near coal mines, as prolonged exposure to heavy metal pollutants in coal gangue increases their health risks.

Therefore, harmless utilization of coal gangue as a resource is very importance. Researchers are actively developing and optimizing control technologies and strategies to solve heavy metal pollution in coal gangue, aiming at minimizing the release and migration of heavy metal elements [16,19]. These governance technologies encompass physical methods, chemical methods, and biological methods. Nevertheless, the governance of heavy metal pollution in coal gangue presents ongoing challenges and complexities. Typically, there are two approaches to decreasing the bioavailability of heavy metals in coal gangue. One method involves the removal of heavy metals by leaching agent, but this method can cause environmental damage and potential secondary pollution. Another method is to passivate heavy metals, which effectively reduces their biological effectiveness and reduces the possibility of their migration. The degree of heavy metal hazard depends not only on the heavy metal content but also on their forms. Heavy metal forms can be categorized into four types, with exchangeable, reducible, and oxidizable forms collectively referred to as the effective or mobile state, while the remainder exist in the residual form within coal gangue. Heavy metals in the residual state are less susceptible to migration and are the most stable among the four forms. The core of passivation is to transform the movable state in the sample into the immobile state, including from exchangeable state, reducible state, and oxidizable state to the residual state. The commonly used single passivating agents fall into three main categories: inorganic passivating agents, organic passivating agents, and novel material composite passivating agents. Phosphorus-containing materials offer multiple advantages in passivating heavy metals in coal gangue [20]. These include efficient transformation of heavy metals into stable phosphates to reduce their bioavailability, environmental with low risks to ecosystems and human health, enhancement of soil quality and plant growth, and long-term benefits by limiting heavy metal release. Their multifunctionality, such as improving soil structure, and cost-effectiveness, particularly when utilizing inexpensive or recycled materials, make them effective and sustainable choices [21]. There was a review of research that summarized the research progress on the remediation of heavy metal pollution by phosphorus-containing materials and explained the importance of phosphorus-containing materials in the treatment of heavy metal pollution [22]. Liu explored the humification process and passivation mechanism of heavy metals in the pig manure composting process by using calcium magnesium phosphate fertilizer. The results showed that calcium magnesium phosphate fertilizer had the greatest passivation effect on Zn [23]. In 2024, we used CaO and calcite to passivate different levels of heavy metal pollution, which also yielded positive outcomes. The average passivation efficiencies of CaO on Pb²⁺, Zn²⁺ and Cu²⁺ were 37.4%, 37.8% and 25.5%, respectively, and the average passivation efficiencies of calcite on Pb²⁺, Zn²⁺ and Cu²⁺ were 53.8%, 43.5% and 34.4%, respectively [24]. CaO and calcite have exhibited good passivation effects on heavy metals, but there is still room for improvement. Scholars usually use phosphorus-containing passivators to passivate heavy metals in soil, but there are few reports on the passivation of heavy metals in waste minerals. Phosphorus-based compounds show great potential in passivating heavy metals in soil because of their unique chemical properties [25]. Studies have shown that the addition of phosphorus can change the chemical forms of heavy metals and reduce their bioavailability [26, 27].

The aim of this study was to investigate the passivation effects of Ca₃(PO₄)₂ and calcium superphosphate fertilizer (CSF) on heavy metals in coal gangue, and to elucidate the specific chemical interactions and processes that contribute to the immobilization of these metals. Our focus was on understanding how these phosphorus-based compounds react with heavy metals to form stable, less soluble complexes, thereby reducing their environmental mobility and bioavailability. In this study, Ca₃(PO₄)₂ and CSF were used as passivators, and systematic passivation experiments were carried out on coal gangue with different pollution levels. We not only optimized the experimental method to ensure the controllability of experimental conditions, but also combined with material characterization, such as X-ray diffraction (XRD) and scanning electron microscope (SEM), to reveal the mineralogical changes and microstructure characteristics in the passivation process. In addition, this study also evaluated the risk of secondary leaching of heavy metals in coal gangue by simulating acid rain, aiming to provide practical methods for the restoration of coal gangue. The primary components of this study include the following steps: The BCR sequential extraction method is used to analyze the various forms and proportions of heavy metals in coal gangue, facilitating an assessment of the ecological environmental risk posed by heavy metals in coal gangue; additionally, passivation treatments are implemented on coal gangue at different pollution levels using Ca₃(PO₄)₂ and CSF. We utilized Visual MINTEQ and PHREEQC to simulate the passivation process, thereby revealing the passivation mechanisms of Ca₃(PO₄)₂ and CSF on heavy metals in coal gangue. Finally, acid rain leaching experiments were performed on passivated coal gangue to compare the ecological environmental risk before and after passivation with Ca₃(PO₄)₂ and CSF.

Experiment

2.1. Reagents and Instruments

The primary reagents used were coal gangue sourced from Inner Mongolia Ordos, domestically produced analytical pure Ca₃(PO₄)₂, superior purity HCl and HNO₃ purchased from Westlake Scientific Co., Ltd., and CSF acquired from Henan Xinxiang. Double-distilled water was used in the laboratory. The primary instruments used were a BSA224S electronic analytical balance (Sartorius, Germany), a PQ9000 inductively coupled plasma optical emission spectrometer (ICP–OES) (Jena, Germany), a DHG-9140A electric blast drying oven (Zhongyi Guoke Technology, China), a FE28-Standard pH meter/ORP meter (Mettler Toledo, Switzerland), a 600Y grinder (Yongkang City Boliu Hardware, China), a SHZ82 oscillator (Changzhou Zhiborui Instrument), a WX-8000 microwave digestion system (Yiyao Technology, China), a H2500R high-speed refrigerated centrifuge (Hunan Xiangyi, China), a SZ-93 double-distilled water apparatus (Shanghai Yinzhe, China), a JSM-IT800 high-resolution field emission scanning electron microscope (JEOL, Japan), and a D8 Advance X-ray powder diffraction instrument (Bruker, Germany).

2.2. Passivation Experiment

For the passivation experiment, we accurately weighed out precise amounts of coal gangue with different levels of pollution, with each sample weighing 20 grams. These weighed samples were transferred into individual epoxy resin (EP) tubes, which are suitable for containing the samples during the mixing process. Subsequently, we introduced predetermined amounts of Ca₃(PO₄)₂ and CSF into the coal gangue samples. The mass ratios used for the introduction were 20:1 and 40:1, respectively, for Ca₃(PO₄)₂ and CSF to the coal gangue. To each EP tube containing the coal gangue and the passivating agents, we added 20 milliliters of deionized water to facilitate the mixing process. The EP tubes were then placed in a room-temperature oscillator, which provided a consistent mixing environment. The samples were oscillated at a speed of 200 revolutions per minute (r/min) for a duration of 5 consecutive days, ensuring thorough interaction between the coal gangue, the passivating agents, and water. Following the oscillation period, the samples were removed from the oscillator and directly dried in a temperature-controlled drying oven set at 105 degrees Celsius. Once the samples were adequately dried, we conducted a heavy metal BCR (Community Bureau of Reference) detection analysis [28]. Moreover, to further characterize the samples post-passivation, we performed X-ray diffraction (XRD) scanning using a Cu Kα target source. The XRD analysis was conducted under the following parameters: an operating voltage of 30 kilovolts (kV), a current of 10 milliamperes (mA), and a scanning range from 5° to 90° with a step size of 0.02° and a counting time of 0.7 seconds per step. These parameters ensured a comprehensive scan that provided detailed information on the crystalline phases present in the samples.

2.3. Simulated Acid Rain Dissolution Experiment

Simulated rainwater was prepared using sulfuric acid, nitric acid, and deionized water, maintaining a controlled volume ratio of SO₄²⁻ to NO₃⁻ of 3:1. The pH of the rainwater was adjusted to 2.5 and 4.5 using NaOH and HCl, respectively. An SO₄²⁻ concentration of 4.0 mg/L was simulated for the acid rain. The passivated coal gangue samples were dried and ground. For the simulated acid rain experiments, 10 mL of simulated acid rain was added at different pH values, the mixture was oscillated at 200 r/min for 24 h, the samples were filtered through a 0.45 μm microporous membrane, and the heavy metal content was analyzed via ICP–OES. Fig. 1 showed the whole experimental flow chart, including the steps of crushing coal gangue, adding passivator, passivation process, sampling analysis, etc. Through this method, the passivation effect and passivation mechanism can be explored.

2.4. Data Processing

The potential ecological risk assessment method is a commonly used approach for risk assessment [29]. Using this method to evaluate heavy metals in coal gangue, the expression is shown in Eq. (1):

(1)

C_{f} = \frac{C_{i}}{C_{n}} = \frac{F_{1} + F_{2} + F_{3}}{F_{4}}

where C_f is the potential toxic element pollution coefficient; C_i is the potential mobile component (labile fraction) of potential toxic elements; C_n is the stable component of potential toxic elements; and F1–F4 represent the weak acid extractable, reducible, oxidizable, and residual fractions, respectively. For C_f<1, the pollution level is considered uncontaminated; for 1≤C_f<3, the pollution level is considered slight contamination; for 3≤C_f<6, the pollution level is considered moderate contamination; for 6≤C_f<9, the pollution level is considered high contamination; and for C_f≥9, the pollution level is considered very high contamination.

The passivation efficiency η is expressed as shown in Eq. (2):

(2)

η = \frac{Effective state before passivation - Effective state after passivation}{Effective state before passivation} \times 100 %

All the figures were generated using Origin 2021. The passivation mechanism was simulated using Visual MINTEQ 3.1 and PHREEQC 3.7.3.15968.

Results and Discussion

3.1. Heavy Metal Content and Speciation in Original Coal Gangue Samples

BCR analysis was performed on coal gangue samples at different pollution levels, revealing the predominant presence of heavy metals such as Pb²⁺, Zn²⁺, and Cu²⁺ and a small amount of Cr³⁺. Fig. S1 showed the stacked column chart of coal gangue accumulation at the different pollution levels. Approximately 50% of Pb²⁺ were present in the weakly acid-soluble form, 38% in the reducible form, and the remaining 12% in the residual form, with no oxidizable form present. The content of Zn²⁺ in coal gangue at different pollution levels also differed, with approximately 70% in the weakly acid-soluble form, 18% in the reducible form, and 12% in the residual form, with no oxidizable form. The content of Cu²⁺ in coal gangue at different pollution levels varies significantly, with the weak acid-soluble form ranging from 10% to 30%, the reducible form accounting for approximately 20%, the oxidizable form ranging from 10% to 30%, and the remaining approximately 10% in the residual form. Fig. S1(c) indicated that the pollution levels of Pb²⁺ and Zn²⁺ in coal gangue vary considerably at different pollution levels However, they can be categorized as at least moderately polluting. In contrast, Cu²⁺ demonstrates high levels of pollution in coal gangue. Overall, the sum of the weakly acid-soluble, reducible, and oxidizable forms of Pb²⁺, Zn²⁺, and Cu²⁺ is relatively high, while the residual form is relatively low, indicating a greater proportion of mobile heavy metals, which may cause varying degrees of environmental pollution. The weak acid extractable (F1) in coal gangue typically represents a form that is more readily mobile and bioavailable, whereas the residual (F4) indicates a more stable state, with the general order of mobility being acid extractable (F1) > reducible (F2) >oxidizable (F3) > residual (F4). Before and after passivation, the proportion of F1 to F3 fractions decreased while F4 increased, indicating that passivation treatment facilitated the transformation of metals into more stable forms [30,31].

3.2. Characterization of the Coal Gangue Materials Before and After Passivation

SEM and XRD were employed to characterize the materials before and after passivation, as illustrated in Fig. 2. Fig. 2(a) clearly showed that the original state of the coal gangue mostly exhibited a flaky morphology, a porous structure, an uneven surface, and a relatively clean appearance. In Figs. 2(b, c), passivation results in a more compact overall structure of coal gangue, characterized by an increased number of grooves on the surface and numerous small particles attached. This side view provides evidence that the passivator has successfully adhered to the surface of coal gangue or formed precipitation after passivation. In Fig. 2(d), there was a slight overall difference between the original state and coal gangue before and after passivation, with SiO₂ (PDF#46-1045) and Al₂O₃ (PDF#10-0173) being the primary components, along with a small amount of kaolinite. After Ca₃(PO₄)₂ passivation, there are fewer insoluble substances, such as Zn(OH)₂ (PDF#41-1359) and Cu(OH)₂ (PDF#35-0505), while after CSF passivation, there are insoluble substances, such as Zn₃(PO₄)₂ (PDF#37-0465) and Cu₅(PO₄)₂(OH)₄ (PDF#37-0449) confirming the reaction and combination of the passivating agent with coal gangue. It is speculated that the reaction shown in Eq. (3–4) may occur, where M represents Pb²⁺, Zn²⁺ or Cu²⁺. However, because the overall proportion of heavy metals in coal gangue is relatively low, the characteristic peaks is not prominent in the XRD spectra. The combination of SEM and XRD provides mutual corroboration, confirming that the reaction between coal gangue and the passivating agent results in the formation of insoluble precipitates and the achievement of passivation.

(3)

M^{2 +} + 2 {OH}^{-} = M {(OH)}_{2}

(4)

M^{2 +} + {CO}_{3}^{2 -} = {MCO}_{3}

3.3. Passivation Effect Analysis

3.3.1. Analysis of the passivation effect of Ca₃(PO₄)₂

Ca₃(PO₄)₂ was added to coal gangue at various pollution levels in a specified ratio, and the passivation results are illustrated in Fig. 3. In the passivation experiment, the weak acid-soluble and reducible states of Pb²⁺, Zn²⁺, and Cu²⁺ persist in the final coal gangue, possibly influenced by various complex factors, with some forms potentially enduring. The passivation process may require a certain amount of time and other conditions, especially for forms that are challenging to convert due to limitations imposed by kinetics and other factors. This may result in the continued presence of some weak acid-soluble and reducible states. Different minerals in coal gangue may have different responses to passivation. Some minerals may be passivated more easily, while others may resist the passivation process, resulting in incomplete transformations. The effectiveness of the passivation reaction may be affected by the extent of contact between the reagent and coal gangue. Different sections of coal gangue may experience inadequate contact with the reagent, leading to uneven transformation effects. The excessive addition of Ca₃(PO₄)₂ may lead to environmental and cost-related concerns. The phosphate-based passivator used in this study has undergone strict environmental risk assessment at the recommended dosage. The complex of phosphate and heavy metals formed in passivation process has been proved to be highly stable, which has been verified by simulated acid rain leaching experiments. The passivator and method adopted in this study will not have adverse effects on the environment. On the contrary, it provides an effective means to reduce the environmental risk of heavy metals in coal gangue, while maintaining ecological balance and water health. Therefore, in this study, Ca₃(PO₄)₂ was introduced to coal gangue at mass ratios of 20:1 and 40:1. In comparison to the original state of coal gangue (Fig. S1), the addition of Ca₃(PO₄)₂ leads to a significant decrease in the proportions of weak acid-soluble, reducible, and oxidizable states, along with a marked increase in the residual state. The weak acid-soluble state of Pb²⁺ decreased from 50% to approximately 1%, the reducible state increased slightly from 38% to approximately 50%, and the proportion of the residual state increased from 12% to approximately 50%. The average passivation rate of Pb²⁺ in the Ca₃(PO₄)₂ system was calculated to be 39.2%.

3.3.2. Analysis of the passivation effect of CSF

CSF is a prevalent mineral, and its primary constituent is calcium hydrogen carbonate. Introducing calcium superphosphate into coal gangue at different pollution levels in specific proportions led to the passivation outcomes illustrated in Fig. 4. Compared with the original state of coal gangue (Fig. S1), the proportion of soluble, reducible and oxidizable state of weak acid decreased significantly with the addition of calcium superphosphate, while the residual state increased significantly, indicating a passivation effect similar to that of Ca₃(PO₄)₂. In Fig. 4(a), the weakly acidic soluble state of Pb²⁺ decreased from 50% to 0%, the reducible state decreased to approximately 12%, and the proportion of the residual state increased to approximately 85%. The passivation sequence of the different forms of Pb²⁺ in the calcium superphosphate system is acid extractable>reducible. The calculated average passivation rate for Pb²⁺ in the calcium superphosphate system is 66.4%. The weakly acidic soluble state of Zn²⁺ decreased to approximately 20% after 70%, the reducible state decreased to approximately 3% after 18%, and the proportion of the residual state increased to approximately 80% after 12%. The passivation sequence of the different forms of Zn²⁺ in the calcium superphosphate system is acid extractable>reducible. The calculated average passivation rate for Zn²⁺ in the calcium superphosphate system was 56.0%. The weakly acidic soluble state of Cu²⁺ decreased to approximately 10%–20% from 10%–40%, the reducible state remained relatively constant, the oxidizable state decreased to approximately 0%–20% from 10%–30%, and the proportion of the residual state increased to approximately 40%–50% from 10%. The passivation sequence of the different forms of Cu²⁺ in the calcium superphosphate system was acid extractable>oxidizable>reducible. The calculated average passivation rate for Cu²⁺ in the calcium superphosphate system was 36.5%. The passivation rates of Pb²⁺, Zn²⁺, and Cu²⁺ in coal gangue with different pollution levels using calcium superphosphate are greater than those in the Ca₃(PO₄)₂ passivation system, as evidenced. Table 1 provides a summary of recent heavy metal passivation outcomes, indicating diverse passivation rates for different heavy metal substrates using various passivation materials. Passivating heavy metals in coal gangue with Ca₃(PO₄)₂ and calcium superphosphate has obvious advantage, and the passivation rate is relatively high. Fig. 4(c) shows the C_f diagram at a ratio of 40:1. Following passivation with calcium superphosphate, Pb²⁺, Zn²⁺, and Cu²⁺ exhibit substantial changes in C_f values. Pb²⁺ transforms from moderate or high pollution to nonpolluted, Zn²⁺ pollution levels shift from moderate or high pollution to nonpolluted or slightly polluted, and Cu²⁺ pollution levels transition from high pollution to slight pollution, resulting in lower risk levels and achieving passivation effects.

3.3.3. Passivation mechanism analysis

The passivation mechanism was simulated using Visual MINTEQ 3.1. Visual MINTEQ software is widely used to simulate the balance of ions and minerals in an environmental water equilibrium solution or water body [36,37]. In the initial experiments investigating the passivation of heavy metals in coal gangue using calcium superphosphate, the measured average pH before passivation was 7.5, increasing to an average of 7.8 after passivation, with no notable temperature changes. Hence, the simulation covered pH values from 7.0 to 8.0 in 0.1 intervals, and the results are depicted in Fig. 5. As shown in Fig. 5(a), at pH 7.5, Pb existed primarily in the form of Pb²⁺ and PbHPO₄. At pH 8.0, the proportion of PbH₂PO⁴⁺ decreased, while the proportion of PbHPO₄ significantly increased. Hence, it can be inferred that the ionic reaction equation for the passivation process of Pb²⁺ is shown in Eq. (5–7). In Fig. 5(b), at pH 7.5, Zn is mainly present in the form of ZnHPO₄, accounting for nearly 100%. Similarly, in Fig. 5(c), at pH 7.5, Cu is predominantly in the form of CuHPO₄, constituting almost 100%. The simulation results from Visual MINTEQ software indicate the provision of PO₄³⁻ or H₂PO₄²⁻ by heavy metal ions and calcium superphosphate. These ions react in solution to produce HPO₄²⁻, which combines with heavy metal ions to form insoluble or slightly soluble precipitates. This is in accordance with the results obtained from SEM and XRD, providing additional confirmation of the passivation mechanism.

(5)

{PbH}_{2} {PO}_{4}^{+} + {OH}^{-} = {PbHPO}_{4} + H_{2} O

(6)

{PO}_{4}^{3 -} + H_{2} O = {HPO}_{4}^{2 -} + {OH}^{-}

(7)

H_{2} {PO}_{4}^{-} + {OH}^{-} = {HPO}_{4}^{2 -} + H_{2} O

In the experiments involving the passivation of coal gangue with calcium superphosphate, the pH simulation indicated that passivation essentially involved the interaction of heavy metal ions or metal hydroxides with HPO₄²⁻, resulting in the formation of stable phosphates or complexes rich in PO₄³⁻ or HPO₄²⁻. Consequently, heavy metals transition from a mobile state to a residual state. In coal gangue, the heavy metals Pb²⁺, Zn²⁺, and Cu²⁺ predominantly exist in weakly acidic soluble forms. The PO₄³⁻ or HPO₄²⁻ in the passivation agent combines with Pb²⁺, Zn²⁺, and Cu²⁺ heavy metals, forming insoluble or sparingly soluble phosphates. Weakly acidic soluble metal ions exhibit higher reactivity in the environment because they can react with surrounding substances, including other ions, molecules, and compounds. In coal gangue, a small fraction of the heavy metals Pb²⁺, Zn²⁺, and Cu²⁺ exist in a reducible state. The HPO₄²⁻ in the passivation agent may undergo redox reactions with these heavy metal ions. The competitive adsorption capacity of heavy metal ions depends on factors such as the ion's primary hydrolysis constant, the ion radius, electronegativity, and the ability to form covalent bonds with adsorption sites. The general order of competitive strength is: Pb²⁺>Cu²⁺>Zn²⁺, indicating that Pb²⁺ is more likely to enter the interior and surface functional groups of the passivation agent, leading to passivation reactions [38].

Using PHREEQC to simulate the passivation mechanism. PHREEQC is a hydro geochemical simulation software package, which can simulate the speciation distribution of heavy metals [39]. As shown in Fig. 6, the forms of Pb²⁺, Zn²⁺, and Cu²⁺ were simulated at concentrations ranging from 1 to 10000 mg/L under different pH values. The Fig. 6 demonstrated that at pH levels of 2 to 3 and ion concentrations ranging from 1 to 100 mg/L, Pb²⁺, Zn²⁺, and Cu²⁺ are present as ions. With increasing ion concentration, Pb²⁺ initially binds with PO₄³⁻ to form Pb₃(PO₄)₂, subsequently, Cu²⁺ binds with PO₄³⁻ to generate Cu₃(PO₄)₂, whereas Zn²⁺ continues as an ion. At approximately pH 4 to 6, with increasing ion concentrations, Pb²⁺, Zn²⁺, and Cu²⁺ all react with PO₄³⁻, resulting in the formation of Pb₃(PO₄)₂, Zn₃(PO₄)₂, and Cu₃(PO₄)₂. At a pH of about 7 to 8, Pb²⁺ and Zn²⁺ not only bind with PO₄³⁻ but also with OH⁻, leading to the formation of Pb(OH)₂ and Zn(OH)₂. With further increases in pH, Pb²⁺, Zn²⁺, and Cu²⁺ all react with PO₄³⁻ and OH⁻, yielding Pb₃(PO₄)₂, Pb(OH)₂, Zn₃(PO₄)₂, Zn(OH)₂, Cu₃(PO₄)₂, and Cu(OH)₂. The red circles in the figure represent the passivation area in this study. In this area, the predominant forms of Pb²⁺, Zn²⁺, and Cu²⁺ are Pb₃(PO₄)₂, Pb(OH)₂, Zn₃(PO₄)₂, Zn(OH)₂, Cu₃(PO₄)₂, and Cu(OH)₂, respectively. This finding aligns with the results from the SEM, XRD, and Visual MINTEQ simulations, which indicated that after passivation, Pb²⁺, Zn²⁺, and Cu²⁺ were effectively passivated into insoluble or sparingly soluble precipitates, which formed different minerals with other functional groups and achieved passivation effects. Ca₃(PO₄)₂ releases PO₄³⁻ ions in aqueous solutions, whereas CSF generates HPO₄²⁻ ions during dissolution. These phosphate ions react swiftly with heavy metal ions like Pb²⁺, Zn²⁺, and Cu²⁺ present in coal gangue, resulting in the formation of phosphate precipitates. The formation of these precipitates markedly reduces the solubility of heavy metals, consequently decreasing their bioavailability and environmental mobility. Both alkaline conditions and carbonates can passivate heavy metals [40]; it has been established that carbonate coprecipitation is an effective method for treating heavy metals, and the resulting precipitates are relatively stable [41,42]. Simulations conducted with Visual MINTEQ and PHREEQC software further validate this passivation process. Simulations indicate that, in the presence of passivators, the precipitates formed from the reaction between heavy metal ions and phosphate ions demonstrate high stability under varying pH conditions. This indicates that passivation treatment can effectively stabilize heavy metals and prevent their release into the environment, even amidst changing environmental conditions [43,44]. At the microscopic level, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses reveal changes in the surface morphology and crystal structure of coal gangue after passivation treatment, including the formation of new phosphate minerals, which further confirms the chemical binding of passivators with heavy metals in the coal gangue. The passivation mechanism identified in this study includes chemical precipitation reactions and surface adsorption, as illustrated in Fig. 7. We specially selected Ca₃(PO₄)₂ and CSF as passivators to treat coal gangue containing heavy metals. This choice is based on several key factors. Firstly, as a naturally occurring mineral, Ca₃(PO₄)₂ has been proved to react effectively with various heavy metal ions to form insoluble phosphate, thus reducing the bioavailability and environmental mobility of these metals. Secondly, CSF, as a widely used fertilizer, not only provides phosphorus, an important nutrient element for plant growth, but also makes it a cost-effective passivator choice because of its universality in agriculture. After passivation treatment, heavy metals in coal gangue are transformed into insoluble phosphate forms, thus reducing the risk of their migration from solid waste to the surrounding environment. This process not only helps to reduce the pollution of soil and water, but also provides a scientific basis for the safe disposal and utilization of coal gangue, and helps to formulate relevant policies and management measures to reduce the potential impact of heavy metal pollution on the health of residents near the mining area. The formation of insoluble phosphate minerals effectively reduces the environmental risk associated with heavy metals in coal gangue. This discovery offers a novel strategy for the environmental management of coal gangue and the control of heavy metal contamination.

3.4. Simulated Acid Rain Dissolution Experiment

There is spontaneous combustion of coal gangue, which produces toxic and harmful gases under incomplete combustion. Other air exposed to rain or humidity is prone to form acid rain, which has a significant impact on the existence of heavy metals in coal gangue. Hence, leaching tests were conducted to simulate acid rain at different pH levels on passivated coal gangue. Acid rain has a significant effect on the chemical forms and stability of heavy metals in coal gangue because of its acidic characteristics. Acid rain can promote the dissolution and migration of heavy metals, thus increasing the risk of heavy metals entering the environment and ecosystem. In order to simulate this environmental condition and evaluate the stability of coal gangue after passivation treatment, we chose acid rain as the medium of leaching experiment. The standard GB/T34230-2017 outlines the "Test Methods for Leaching of Coal and Coal Gangue," which involves using pure water for leaching coal gangue. In our laboratory, initial experiments with pure water showed no detection of heavy metal ions for different pollution levels or passivation systems. Consequently, simulated acid rain was adopted for leaching coal gangue. Fig. S2 depicted the leaching of passivated coal gangue with simulated acid rain before and after passivation for the different pollution levels. The graph clearly indicates that higher pollution levels lead to increased leaching of heavy metals by simulated acid rain, with the order of leached amounts being Zn²⁺>Pb²⁺>Cu²⁺, all falling below the standard values. The leaching amounts of Ca₃(PO₄)₂ and calcium superphosphate fertilizer after passivation are noticeably lower than those before passivation, providing evidence that passivation effectively hinders the mobility of heavy metals. Experimental results from simulated acid rain leaching confirm that passivation can effectively decrease the mobility of heavy metal ions, offering an effective approach for preventing harm from coal gangue, particularly the stabilization of heavy metal pollution.

Conclusion

Investigating the passivation of coal gangue polluted by various heavy metals using Ca₃(PO₄)₂ and CSF, we draw the following conclusions:

Coal gangue contaminated by various heavy metals was passivated using Ca₃(PO₄)₂ and CSF. Post-passivation, weak acid soluble, oxidizable, reducible, and residual states were extracted from the coal gangue via the BCR method. The average passivation rates of Ca₃(PO₄)₂ for Pb²⁺, Zn²⁺, and Cu²⁺ are 39.2%, 54.3%, and 30.9%, respectively, while those of calcium superphosphate fertilizer are 66.4%, 56.0%, and 36.5%, respectively, showing slightly superior passivation effectiveness compared to that of Ca₃(PO₄)₂.
Evaluating the coal gangue before and after passivation, it was found that the C_f value of the coal gangue changed post-passivation. Passivation with Ca₃(PO₄)₂ and CSF could transform heavily and moderately polluted coal gangue into lightly polluted or pollution-free, demonstrating effective passivation capabilities. Effectively reducing moderate or high level pollution of coal gangue to a slight or clean level of pollution.
Software simulations using Visual MINTEQ and PHREEQC were conducted to explore the passivation mechanism. Visual MINTEQ revealed that heavy metal ions could combine with PO₄³⁻ and H₂PO₄⁻ supplied by Ca₃(PO₄)₂ and CSF to form insoluble precipitates or minerals. The simulation results from PHREEQC corroborated those of Visual MINTEQ.
The passivated coal gangue, subjected to leaching by simulated acid rain, demonstrated greater stability compared to its pre-passivation state. This confirmed that passivation effectively impeded the migration of heavy metals, thereby achieving the detoxification of heavy metals in coal gangue. Ca₃(PO₄)₂ and CSF can harmlessly treat coal gangue polluted with heavy metals to varying degrees, paving the way for resource-based treatment of coal gangue.

Supplementary Information

eer-2024-322-supplementary.pdf

Acknowledgements

This research was supported by the Science and Technology Major Program of Ordos City of China (2022EEDSKJZDZX014-1), Technological Innovation Guidance Program of Jiangxi Province (20212BDH81029), Rare Earth Industry Fund (IAGM2020DB06) and the Key Research Program of the Chinese Academy of Sciences (ZDRW-CN-2021-3-3).

Notes

Conflict-of-Interests Statements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

Z.H.L. (PhD student) carried out all the experiments and data management and wrote the manuscript. B.W.L. (Deputy general manager) developed the methodology and conceptualization. T.S.J. (PhD student) helped with writing and methodology. Z.M.F. (PhD student) helped in editing, review, and conceptualization. Y.Y.M. (Professor) provided methodology and conceptualization. J.X.L. (Research Assistant) provided resources, project administration, writing, and editing. L.T.G. (Professor) provided validation, conceptualization, resources, project administration and helped with writing, editing.

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Fig. 1

Overall experimental flowchart.

Fig. 2

SEM and XRD spectra of moderately polluted gangue before and after passivation; (a) SEM image of the original sample of moderately polluted gangue, (b) SEM image of Ca₃(PO₄)₂ passivation, (c) SEM image of CSF passivation, and (d) XRD spectra of moderately polluted gangue before and after Ca₃(PO₄)₂ and CSF passivation.

Fig. 3

Ca₃(PO₄)₂ passivation test results for heavy metals from different types of coal gangue pollution: (a) stacking chart of heavy metal detection after Ca₃(PO₄)₂ passivation, (b) percentage map of heavy metal detection after Ca₃(PO₄)₂ passivation, (c) comparison diagram of the C_f values of heavy metals in different contaminated coal gangue samples, and the red dotted line in Figure (c) represents the corresponding grade of the pollution coefficient.

Fig. 4

CSF passivation test results for heavy metals in different types of coal gangue pollution: (a) stacking chart of heavy metal detection after CSF passivation, (b) percentage map of heavy metal detection after CSF passivation, (c) comparison diagram of the C_f values of heavy metals in different contaminated coal gangue samples, and the red dotted line in Figure (c) represents the corresponding grade of the pollution coefficient.

Fig. 5

Pb²⁺, Zn²⁺, and Cu²⁺ in the simulated Ca₃(PO₄)₂ passivation gangue at 25 °C under different pH conditions; (a), (b), and (c) show the simulated spectra of Pb²⁺, Zn²⁺, and Cu²⁺, respectively.

Fig. 6

PHREEQC simulations of the morphological changes in Pb²⁺, Zn²⁺ and Cu²⁺ at different pH values and concentrations.

Fig. 7

Schematic diagram of passivation mechanism of heavy metals in coal gangue by Ca₃(PO₄)₂ and CSF.

Table 1

Passivation effect of different passivators on heavy metals in different substrates in recent years.

Passivator	Passivated matrix	Proportion of passivator (mass ratio)	Passivating heavy metal species	Average passivation rate	References
Phosphate rock and calcium superphosphate	sludge	5%, 3%	Pb, Zn, Cu	15%, 10%, 35%	[32]
Calcium magnesium phosphate fertilizer	pig manure	10%	Zn	13.85%	[33]
Bone Meal	Sewage Sludge	2%~15%	Pb, Cr, Zn	28%, 3%, 5%	[34]
Superphosphate	pig manure	18%	As, Cu, Mn, Zn, Fe	40%, 3%, 1%, 1%, 30%	[35]
CaO	coal gangue	2.5%, 5%	Pb, Zn, Cu	37.4%, 37.8%, 25.5%	[24] Preliminary work
Calcite	coal gangue	2.5%, 5%	Pb, Zn, Cu	53.8%, 43.5%, 34.4%	[24] Preliminary work
Ca₃(PO₄)₂	coal gangue	2.5%, 5%	Pb, Zn, Cu	39.2%, 54.3%, 30.9%	This study
CSF	coal gangue	2.5%, 5%	Pb, Zn, Cu	66.4%, 56.0%, 36.5%	This study