Experimental Study on the Oxidation Reaction of Coal-Pyrite and Mineral-Pyrite with the Participation of Fe(III) and Bacteria under Acidic Conditions

Abstract

As one of the crucial factors contributing to coal spontaneous combustion, the oxidation of pyrite is a complex process involving multiple reactions, particularly in the presence of oxidants (Fe³⁺ and O₂) and bacteria. However, experimental results based on mineral-pyrite are not entirely applicable to coal-pyrite due to their differences in formation environments and compositions. This study selected two types of coal-pyrite and one type of mineral-pyrite as research to conduct oxidation experiments with the participation of oxidant (Fe³⁺) and bacteria (Acidithiobacillus ferrooxidans), respectively, to obtain the following conclusions. Under natural conditions, the chemical oxidation rate of pyrite is slow, but the addition of oxidant Fe³⁺ and bacteria can significantly accelerate the oxidation rate. The promotion effect of oxidant Fe³⁺ on the oxidation reaction is stronger than that of bacteria. Under the same conditions, the oxidation rate of coal-pyrite samples is slightly higher than that of mineral-pyrite, due to the relatively higher impurities content, poorer crystal structure, and humic acid in the coal seams. Additionally, different compositions of coal-pyrite samples can lead to various oxidation degrees under different conditions. Therefore, the oxidation process and mechanism of pyrite in coal seams are complex and affected by many factors, which need further study to prevent coal spontaneous combustion accurately and effectively.

1. Introduction

Coal spontaneous combustion is one of the major disasters in coal mining and storage, which can not only burn up coal resources and cause huge economic losses but also produce harmful gases and seriously pollute the environment [1,2]. It mostly occurs in areas of coal accumulation and air leakage and is affected by the accumulation environment (climate, air connectivity, moisture content) and coal properties (carbonization degree, particle size, pyrite content), etc. [3,4]. Among them, pyrite oxidation is one of the main factors contributing to coal spontaneous combustion [3].

As the most widely distributed sulfide mineral in the crustal [5], pyrite is usually found in coal seams in various forms, such as nodular, lenticular, layered, fissure filling, and aggregated massive [6]. Pyrite has the characteristic of self-thermal oxidation, producing ferric sulfate and ferrous sulfate, which makes the coal porosity and increases the contact area with oxygen. These oxidation decomposition products are more oxygen-absorbing than coal and can transfer the adsorbed oxygen to coal particles, causing coal oxidation [7]. Moreover, pyrite has a smaller specific heat capacity, and its temperature increment is three times larger than that of coal when absorbing the same amount of heat, which accelerates the heating of the coal seam [5,7].

Pyrite can be dissolved and oxidized simultaneously in an acidic solution. The initial step is the adsorption of H₂O and O₂ on the surface of pyrite, which is a prerequisite for the oxidation reaction to occur [8]. Then, Fe²⁺ in the mineral lattice can be dissolved and oxidized to Fe³⁺, and the oxidation intermediates of the sulfur element are further oxidized to SO₄²⁻. Besides, SO₄²⁻ is also the main anion in acid mine drainage (AMD) [9], which can enhance the mobility of some heavy metals and enter the stratum and pollute the environment with the flow of AMD [10]. However, it is important to note that the oxidation reaction of pyrite is relatively slow, especially the oxidation of Fe²⁺ to Fe³⁺, which is also a decisive factor influencing the oxidation rate of pyrite [11]. Several factors influence the oxidation rate of pyrite, including oxidant (Fe³⁺ or O₂), pH, redox potential (E_h), etc. [12].

Under the condition that the solution pH is smaller than 3.0, Fe³⁺ is the dominant oxidant of pyrite and the pyrite oxidation rate initiated by Fe³⁺ is 3–4 orders of magnitude higher than that caused by O₂ [13]. Isotope labeling experiments have demonstrated that the oxygen atoms in the oxidation product of SO₄²⁻ come from H₂O rather than O₂ [14,15]. The oxidation rate of Fe²⁺ in the reaction system is positively correlated with O₂ concentration but negatively correlated with pH value. The increase in pH value will transform Fe³⁺ into Fe(OH)₃, which covers the surface of pyrite to inhibit further oxidation [16]. Besides, Boon et al. [17] found that pyrite is challenging to oxidize when the E_h of the reaction system is below 0.30 V. Sun et al. [18] suggested that the pyrite oxidation rate increases linearly with the increase of E_h between 0.70 V and 0.90 V.

In addition, some bacteria can also participate in the oxidation process of pyrite in coal seams and acid mine water. These bacteria, such as the iron-and-sulfur-oxidizing bacteria Acidithiobacillus ferrooxidans, Acidithiobacillus ferridurans, and Acidithiobacillus ferrivorans; the sulfur-oxidizing bacteria Acidithiobacillus caldus, Acidithiobacillus thiooxidans, and Acidithiobacillus albertensis [19]; and the iron-oxidizing bacteria Leptospirillum ferrooxidans, etc., can adsorb on the mineral surface or dissociate in the reaction system, and then secrete various compounds to accelerate the oxidation of iron, sulfur, and other inorganic components [20]. The pyrite oxidation rate by microorganisms is several to even hundreds of times higher than pure chemical oxidation [21], and forms secondary minerals and elements through different oxidation pathways from purely chemical reaction processes [22]. Among them, Acidithiobacillus ferrooxidans is considered the most important and representative microorganism in the coal mine acidic environment [23].

In summary, the oxidation process of pyrite is very complex with the participation of oxidants (Fe³⁺ and O₂) and bacteria in the coal seams. However, most of the previous studies have used mineral-pyrite for simulation experiments of pyrite oxidation [5,11,14,24]. There are some differences in the formation environments and compositions between coal pyrite and mineral pyrite, which may lead to incomplete applicability of experimental results based on mineral-pyrite to coal-pyrite. Therefore, this study selected two types of coal-pyrite and one type of mineral pyrite as the research objects and carried out the oxidation experiments with the participation of oxidant (Fe³⁺) and bacteria (Acidithiobacillus ferrooxidans), respectively. The difference in the reaction process between coal pyrite and mineral pyrite and the influencing factors were analyzed to help understand the oxidation characteristics of pyrite in coal seams and prevent coal spontaneous combustion.

2. Experiments and Methods

2.1. Sample Information and Preparation

The coal-pyrite samples used in this study were sampled from the coal seam at the Xiongxin Coalmine and Jiashun Coalmine in the Xingyi coalfield, Guizhou province, China, as shown in Figure 1. The coal seams belong to the Upper Permian coal-bearing series and are of the coastal tidal flat-lagoon sedimentary facies [25,26].

The coal seams were formed in tidal flats adjacent to deltas, and they have a higher sulfur content due to the influence of river and ocean tides [25,26]. The sulfur content of the coal samples is shown in

Table 1. Sulfur content in coal samples, wt%.

Coal Mine	Sample ID	St.d	Sp.d		So.d		Ss.d		Classification
			S0	P	S0	P	S0	P
Jiashun	C1	5.7	3.9	68.8	1.7	29.0	0.1	2.3	High sulfur
Xiongxin	C2	5.3	4.7	89.1	0.5	9.6	0.07	1.3	(St.d > 3.0)

Note: S_t.d is the total sulfur, %; S_p.d is the pyritic sulfur, %; S_o.d is the organic sulfur, %; S_s.d is the sulfate sulfur, %; S₀ is the content of sulfur in the coal, %; and P is the proportion of total sulfur, %.

. According to the China national standard (GB/T 15224.2-2021), the total sulfur of the two coal samples is higher than 3%, which categorizes them as high-sulfur coal. Pyrite is the main form of sulfur in these coal samples, accounting for 68.76% and 89.06%, respectively.

The coal-pyrite samples need to be screened and extracted from the coal body to obtain the coal-pyrite samples. The coal body was crushed to find the pyrite enrichment profile and site. The pyrite particles were carefully picked out and any impurities were scraped out using tweezers. In addition, a high-purity mineral-pyrite sample collected from the Yaogangxian metal mine in Hunan Province, China, was used as experimental control in this study, which is close to the Guizhou Province and contains pyrite veins with good ore-forming conditions (Figure 1). The sulfur isotopes analysis results of different sulfides in Yaogangxian metal mine show that the source of sulfur in the deposit is single and mainly from magma [27]. Both the coal-pyrite and mineral-pyrite samples were then crushed to the same size (75 μm) to minimize the size effect of the reaction [28]. Subsequently, the samples underwent ultrasonic treatment using deionized water, cleaned with anhydrous ethanol, vacuum dried at 50 °C for 24 h, and were sealed to prevent oxidation.

2.2. Experimental Setup

2.2.1. Oxidation Reaction with Fe³⁺

To simulate the acidic mine water environment in coal seams, the experimental conditions of the pyrite oxidation reaction are set as pH of 2.0 and temperature of 25.0 °C. Firstly, an experimental solution with 6 mmol/L of Fe³⁺ was prepared using ferric chloride hexahydrate (FeCl₃·6H₂O) and ultra-pure water. The pH of the solution was adjusted to 2.0 by adding sodium chloride (HCl, with a concentration of 0.5 mol/L) and sodium hydroxide (NaOH, with a concentration of 0.1 mol/L). Then, the solution was injected into a 900 mL reaction container and placed on a magnetic agitator, with corresponding measuring instruments (conductivity instrument, pH meter, and thermometer) connected inside the container. When the solution temperature reached 25.0 °C, the prepared pyrite mineral particles (2.000 ± 0.002 g) were poured into the reaction container and the rotational speed was set at 700 r/min to make the pyrite particles float. The oxidation experiment lasted for 120 min, collected a reaction solution of 5 mL every 30 min, and recorded the measuring date during each collection.

2.2.2. Oxidation Reaction with Bacteria

The Acidithiobacillus ferrooxidans (type 23270, from American Type Culture Collection (ATCC)) was selected for the oxidation experiment of pyrite with bacteria. Firstly, a Fe-free 9 K medium of 200 mL was injected into a conical bottle, which was then sterilized at a high temperature and inoculated with a 10% bacterial solution of 20 mL. The bacteria were activated and cultured for three days at a temperature of 30 °C.

To perform the oxidation experiments on pyrite minerals using bacterial leaching, it was necessary to account for the clustering of the bacteria. Firstly, the glass fiber was filled at the bottom of the connected glass tube with an inner diameter of 50 mm and a height of 500 mm. Then 20 g of reagent-grade quartz sand with a particle size of 0.5–1 mm was spread on the glass fiber layer, followed by a mixture of 30 g of quartz sand and 5 g of the prepared pyrite. Next, a 500 mL solution with a pH of 2 was prepared using ultra-pure water and dilute sulfuric acid, and added 10 mL of the activated bacteria solution to it. This solution was proportioned to leach the pyrite sample circularly to prevent bacteria clustering. The leaching experiment lasted for 40 days, and 5 mL of the solution was quantitatively extracted every 5 days for component testing. At the same time, another leaching experiment of the acid solution without bacteria was carried out on mineral-pyrite, as a blank control.

2.3. Analysis Method

2.3.1. Morphology Observation

The morphology and composition of pyrite samples were observed and measured using the FEI Quanta™ 250 scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) at the Advanced Analysis and Computation Center (AACC) of China University of Mining & Technology (CUMT). And the flat and uniform regions on the surface of the pyrites were selected for the EDS analysis. The SEM-EDS was operated in high vacuum mode with a working distance of 14–20 mm, a beam voltage of 30 kV, an aperture of 6 μm, and a spot size of 3.5–5.0 μm.

2.3.2. Mineral Content and Forms of Sulfur

A D8 Advance X-ray diffractometer (XRD) was used to analyze the mineral compositions of the pyrite samples at the AACC of CUMT, with a Cu X-ray source at 40 kV and 30 mA. The tested angle ranged from approximately 3° to 70° and the diffractograms were analyzed by the Jade 6.5 software. Besides, the chemical valence state and relative content of pyrite samples were analyzed using an Escalab 250Xi X-ray photoelectron spectroscopy (XPS). The acquisition parameters of XPS survey scans: the acquisition time is 128 s; the electron gun source type is Al Kα, the beam spot is 650 kbps; the step size is 1.0 eV; the energy step number is 1361.

2.3.3. Ion Content in the Solution

The ion content of the oxidation reaction solution was measured after filtration. A Dionex-500 ion chromatograph was used to test the content of SO₄²⁻ produced by the oxidation of pyrite according to the China national standard (GBT5750.5-2006) at the AACC of CUMT. And the contents of Fe²⁺ and Fe³⁺ were analyzed using a 721-type spectrometer according to the Geological and Mineral Industry Standards of China (DZ/T 0064.23-2021).

3. Experimental Results

3.1. Occurrence and Structure of Pyrite in Coal

3.1.1. Sample Morphology

The macroscopic morphology of mineral-pyrite and coal-pyrite samples were shown in Figure 2. The mineral pyrite presents a regular cubic and pentagonal dodecahedral crystalline shape, characterized by a light brass-yellow hue and high purity, besides some fractured granular morphologies. The original forms of coal-pyrite samples in the coal body comprise aggregated massive pyrite and layered pyrite. Coal-pyrite 1 (C1) is yellow-black and composed of medium-coarse equiaxed crystal grains, while coal-pyrite 2 (C2) is a copper-yellow color, dense layered, with a fine-microcrystalline structure.

The double pycnometer method was utilized to determine the densities of pyrite samples, showing that C1 and C2 possess densities of 3.56 g/cm³ and 3.63 g/cm³, respectively, while mineral pyrite displays a density of 4.92 g/cm³. Additionally, the XRD analysis results showed typical diffraction peaks of pyrite minerals in Figure 3, and there are rare diffraction peaks of impurity minerals. It means that the selected pyrite samples possess higher purity and are suitable for the study.

3.1.2. Microscopic Morphology and Composition

The microscopic morphology and composition of pyrite in the coal body were observed and shown in Figure 4 using the SEM-EDS method. These pyrites manifest different shapes, which can be divided into framboidal, fissure-filled, aggregated massive, euhedral crystalline, caviar-like, fine-grained agglomerated pyrites.

Among them, framboidal pyrites are aggregates of pyrite spheres (Figure 4a) that commonly occur in groups. Filled pyrite is the most prevalent type observed in coals and typically appears in banded (Figure 4b), dendritic, or reticulated patterns. The surface of the agglomerated massive pyrite presents pores and contains organic matter or minerals (Figure 4b), and some are formed by the bonding of fine-grained pyrite particles (Figure 4f). The intact crystal facets of euhedral crystalline pyrites exhibit triangular, square, rhombic, pentagonal, hexagonal, etc. (Figure 4c,g), and are often associated with clay minerals. Caviar-like pyrite has a crystal shape that is densely piled up in groups (Figure 4d) or intermittently distributed along layers in a beaded form. The shape of fine-grained agglomerated pyrite (Figure 4e) appears irregular with uneven edges, semi-colloidal, having a porous surface, and is filled with clay minerals.

The fresh pyrite section displays strong chemical activity and is susceptible to oxidization. Pyrite in coal bodies can be mixed with clay minerals and organic matter, which further affects its properties. The element contents on the pyrite surface were tested by the EDS method, as shown in

Table 2. Element content on the coal-pyrite samples/mole fraction %.

Sample ID	Test Site	Morphology	Element Proportion %						N(S)/N(Fe)
			O	S	Fe	Al	Si	C
C1	1	Framboidal	45.6	23.2	21.7	0.63	0.55		1.07
	2	Fissure-filled	7.3	30.0	16.2			46.5	1.85
	3	Aggregated massive		53.2	38.2	4.5	4.1		1.39
	4	Euhedral crystalline		66.6	33.4				1.99
	5	Caviar-like		53.9	46.1				1.17
C2	6	Fine-grained agglomerated	39.0	30.6	27.9	1.21	1.04	0.25	1.10
	7	Aggregated massive	29.8	21.4	13.9	0.21	0.17	34.5	1.54
	8	Euhedral crystalline	13.1	55.1	30.1	0.81	0.79		1.83

Note: N(S)/N(Fe) is the number ratio of sulfur atoms to iron atoms.

. Except for euhedral crystalline and caviar-like pyrite, impurity elements such as carbon and silicon elements were observed in other forms of pyrite. Notably, framboidal, fine-grained agglomerated, and aggregated massive pyrites have a high content of oxygen element, indicating that these pyrites may have been oxidized or mixed with clay minerals.

The atomic ratio of sulfur to iron (N(S)/N(Fe)) in pyrite is an indicator of the lattice filling saturation, and lower values suggest more defects in the crystal and greater chemical reactivity of the pyrite samples. As shown in Table 2, except for euhedral crystalline and fissure-filled pyrite, other forms of pyrite generally exhibit low ratio values, especially fine-grained agglomerated and framboidal pyrite. Similarly, the framboidal pyrite found in the Carboniferous coal-bearing rocks from the Czech part of the Upper Silesian Coal basin also showed a low N(S)/N(Fe) value of 1.02 [29]. Euhedral crystalline pyrite usually forms in the early diagenesis stage, and the fissure-filled pyrite results from hydrothermal fluid precipitation infiltrated into fractures during the epigenetic stage. Both of them form under sufficient time and material supply conditions. However, other forms of pyrite are relatively depleted in sulfur and mixed with organic matter and other minerals in coal seams, leading to their incomplete crystal forms (Figure 4).

The presence of inorganic sulfur and organic sulfur in pyrite samples was analyzed and shown in Figure 5 through the XPS method.

Table 3. Statistics of sulfur content and types based on XPS%.

Sample	Sulfate Sulfur (Ss,d)	Organic Sulfur (So,d)			Pyrite Sulfur (Sp,d)	Other Sulfides
		Sulphoxides	Thiophenes	Total
M1	2.41	1.16	1.97	3.13	93.08	1.37
C1	14.90	0.00	4.61	4.61	76.90	3.59
C2	16.54	2.09	1.84	3.93	76.71	2.82

quantitatively analyzes each component and demonstrates that the proportion of pyrite sulfur amounts to over 76% in these samples. Sulphoxides and thiophenes are the main forms of organic sulfur, and the content of organic sulfur in coal-pyrite is higher than that in mineral-pyrite, indicating a close association between coal-pyrite and organic matter. Besides, the sulfate contents of coal-pyrite samples are higher than that of the mineral-pyrite sample.

3.2. Experimental Results of Pyrite Oxidation

3.2.1. Pyrite Oxidation with Fe³⁺

In the inorganic oxidation experiment of pyrite, Fe³⁺ was used as the oxidant. Figure 6 illustrates the changes in pH value and the concentration of each component (Fe³⁺, Fe²⁺, and SO₄²⁻) in the solution during the reaction process. As the reaction progressed, the pH value and the content of Fe³⁺ concentration gradually decreased (Figure 6a,b), indicating that pyrite oxidation involves acid production and oxidant consumption. Moreover, the content of Fe²⁺ and SO₄²⁻ as oxidation products also increased (Figure 6c,d). In the initial stage of the reaction (0–60 min), the content of each component changes rapidly and then tends to become stable.

The consumption of Fe³⁺ as an oxidant to form Fe²⁺ plays a crucial role in determining the oxidation reaction rate of pyrite. The concentration ratio of Fe²⁺ to Fe³⁺ serves as regarded as the oxidation-reduction couples, and the E_h of this reaction system can be calculated according to the Nernst equation:

$$E_h=E_0-\frac{\mathrm{RT}}{\mathrm{F}}\ln \frac{\left[{\mathrm{Fe}}^{2+}\right]}{\left[{\mathrm{Fe}}^{3+}\right]}$$

(1)

where, E₀ is the standard electrode potential, E₀ = 770 mV, R is Avogadro constant, 8.314 J·K⁻¹·mol⁻¹; F is Faraday constant, 96,485.33 C·mol⁻¹, T is the reaction temperature, 298.15 K.

The higher E_h of oxidant in the reaction system has stronger oxidizability. According to the concentration ratio of Fe²⁺ to Fe³⁺ (Figure 7a), the E_h of the reaction system was calculated and measured, as shown in Figure 7b,c. In the initial reaction stage (0–90 min), the significant increase of Fe²⁺ concentration led to a rapid decrease in E_h value. Subsequently, the E_h value exhibited a slow decreasing trend (Figure 7b). Furthermore, the calculated E_h values of coal-pyrite samples were slightly lower than that of mineral-pyrite. This is because, in the condition of the same oxidation-reduction couples [Fe²⁺/Fe³⁺], the fast increased concentration of Fe²⁺ in the reaction system of coal-pyrite samples decreases the calculated E_h values at the 30th minute. In addition, the measured E_h values of each reaction system were higher than their calculated values. This could be attributed to the dissolution of other soluble iron-bearing minerals that can be oxidized more easily at the beginning, which increased the measured E_h value and made it higher than the calculated values. Nonetheless, their impact on the reaction system gradually reduced as the reaction proceeded. The measured E_h values of each pyrite sample were similar in the later stage.

3.2.2. Pyrite Oxidation with Bacteria

Similarly, in the pyrite biological oxidation experiment carried out with Acidithiobacillus ferrooxidans under acidic conditions, changes in pH and the concentration of components (Fe²⁺, Fe³⁺, and SO₄²⁻) in the solution were measured and shown in Figure 8. Comparison of the pH and component concentration in the reaction solution of mineral-pyrite (M1) without bacteria revealed that the addition of Acidithiobacillus ferrooxidans significantly improves the degree of pyrite oxidation. Moreover, the increase of product (SO₄²⁻) in the mineral-pyrite (M1) with bacteria was 13-fold higher than that in M1 without bacteria. Due to the relatively small number of microorganisms, the composition in the reaction system of each pyrite sample changed slowly during the initial stage (0–10 days), and then, the rate of change of each ion concentration increased.

In the reaction system of the pyrite-with bacteria, the pH value gradually decreased, while it initially increased and then decreased in the reaction system of M1-without bacteria. This may be related to the consumption of H⁺ by the dissolution of pyrite and its impurities, followed by the slow and weak production of H⁺ during the pyrite oxidation. The concentration of the redox products (Fe²⁺ and SO₄²⁻) continually increased throughout the reaction. While the Fe³⁺ content was found to increase initially and then decrease, which might be due to the Fe³⁺ formed in the initial stage participating in the inorganic oxidation reaction of pyrite as an oxidant. Furthermore, the variation range for each component in the coal-pyrite reaction system was slightly higher than that of mineral-pyrite.

3.2.3. Oxidation Reaction Rate

Based on the experimental conditions, the SO₄²⁻ in the inorganic oxidation reaction system with Fe³⁺ is produced by the oxidation of pyrite. Therefore, the concentration change rate of SO₄²⁻ can be used to represent the pyrite oxidation rate. Similarly, in the biological oxidation reaction system with bacteria, since the addition of sulfuric acid to adjust the pH value, the concentration change rate of iron ions (Fe²⁺ and Fe³⁺) was selected to characterize the pyrite oxidation rate. The atomic ratio of Fe to S in pyrite is 2:1, and the pyrite oxidation rate (R) can be expressed as follows:

$$R=-\frac{\mathrm{d}\left[pyrite\right]}{\mathrm{d}t}=\frac{\mathrm{d}\left[{\mathrm{SO}}_4^{2-}\right]}{2\mathrm{d}t}=\frac{\mathrm{d}\left(\left[{\mathrm{Fe}}^{2+}\right]+\left[{\mathrm{Fe}}^{3+}\right]\right)}{\mathrm{d}t}$$

(2)

where, [pyrite], [SO₄²⁻], [Fe²⁺], and [Fe³⁺] are the concentration changes of pyrite, SO₄²⁻, Fe²⁺, and Fe³⁺ in the reaction system, respectively; t is the reaction time.

The variation trend of oxidation rates of these pyrite samples under Fe³⁺ and bacteria conditions is shown in Figure 9. The oxidation rate of pyrite under the Fe³⁺ condition was significantly higher than that under the bacteria condition. In addition, the oxidation rate decreased gradually with reaction time.

In the M1-without bacteria system, the oxidation rate of pyrite was initially very slow but increased noticeably until the 20th day. However, under the influence of bacteria, the oxidation rate of pyrite increased first and then decreased with reaction time. This means that the initial oxidation rate was slower, and the oxidation rate of pyrite gradually increased as oxidizing bacteria reproduced. Subsequently, as pyrite was gradually oxidized completely, the oxidation rate of pyrite decreased accordingly.

4. Discussion

4.1. Mechanism of the Pyrite Oxidation Reaction

4.1.1. Oxidation Mechanism under the Fe³⁺

Pyrite contains Fe and S with valences of +2 and −1, respectively, which are both unstable [30]. The active center (Fe²⁺) on the pyrite surface can continuously adsorb O₂ and H₂O, undergo redox reactions, and release heat [24]. A schematic diagram presented the cyclic oxidation exothermic reaction process of pyrite, as shown in Figure 10.

The initial step in the oxidation process of pyrite involves the absorption of O₂ and H₂O on its surface, particularly at regions with high surface energy such as lattice defects, corners, and edges of the crystal. This led to the generation of H⁺, Fe²⁺, and SO₄²⁻. The chemical reaction formula is as follows:

$${\mathrm{FeS}}_2+{\mathrm{H}}_2\mathrm{O}+3.5{\mathrm{O}}_{2(\mathrm{a}\mathrm{q})}\to {\mathrm{Fe}}^{2+}{+2\mathrm{SO}}_4{}^{2-}+2{\mathrm{H}}^{+}+{\mathrm{Q}}_1$$

(3)

Under sufficient oxygen condition, FeSO₄ will be further oxidized to form Fe₂(SO₄)₃ according to the following reaction:

$${\mathrm{Fe}}^{2+}+{0.25\mathrm{O}}_2{}_{\left(\mathrm{aq}\right)}{+\mathrm{H}}^{+}\to {\mathrm{Fe}}^{3+}+0.5{\mathrm{H}}_2\mathrm{O}+{\mathrm{Q}}_2$$

(4)

The chemical equilibrium equation for this reaction is as follows:

$${\lg [\mathrm{Fe}}^{2+}]={\lg [\mathrm{Fe}}^{3+}]-0.25\lg [{\mathrm{O}}_2]+\mathrm{pH}-\lg K$$

(5)

In this experiment, the reaction solution is under sealed conditions with an initial Fe³⁺ concentration of 6 mmol/L and a pH of 2.0. The saturated concentration of dissolved oxygen is 8.25 mg/L at 25.0 °C. According to the equilibrium parameters of this reaction (

Table 4. Chemical reaction parameters during the oxidation of pyrite.

Chemical Equation	Temperature (°C)	ΔH (kJ)	ΔS (J/K)	ΔG (kJ)	K	Lg(K)
Equation (3)	25	−1285.507	−563.038	−1117.637	6.632 × 10195	195.822
Equation (4)	25	−99.673	−62.924	−80.912	1.502 × 1014	14.177
Equation (6)	25	96.507	−22.294	103.154	8.441 × 10−19	−18.074
Equation (7)	25	109.910	317.894	15.130	2.234 × 10−3	−2.651

Where ΔG is the Gibbs free energy change; ΔH is the enthalpy change; ΔS is the entropy change; K is the reaction equilibrium constant.

), the concentration of Fe²⁺ that may be oxidized is calculated to be 3.15 × 10⁻¹⁴ mol/L. Previous research has shown that Fe²⁺ concentration dominates the oxidation of pyrite when pH < 4 [31]. Therefore, it can be considered that Fe²⁺ is stable under these experimental conditions.

The added Fe³⁺ in the experimental solution may be hydrolyzed, as shown in Equation (6):

$${\mathrm{Fe}}^{3+}{+3\mathrm{H}}_2\mathrm{O}\to {\mathrm{Fe}\left(\mathrm{OH}\right)}_3{+3\mathrm{H}}^{+}+{\mathrm{Q}}_3$$

(6)

At the same time, Fe³⁺ in the system will oxidize pyrite as an oxidant, producing a large amount of Fe²⁺, according to Equation (7), to form an “oxidation-reduction-reoxidation” reaction cycle of pyrite.

$${\mathrm{FeS}}_2+14{\mathrm{Fe}}^{3+}+8{\mathrm{H}}_2\mathrm{O}\to 15{\mathrm{Fe}}^{2+}+2{\mathrm{SO}}_4{}^{2-}+16{\mathrm{H}}^{+}+{\mathrm{Q}}_4$$

(7)

In summary, the cyclic oxidation process of pyrite under oxygen-rich conditions can be expressed by Equations (3), (4) and (7), with the final products of Fe³⁺ and SO₄²⁻. Among them, the reaction rate of Equation (4) is relatively slow and controls the oxidation rate of pyrite.

In addition, according to Table 4, Equations (3) and (4) can occur spontaneously at room temperature (ΔG < 0), and both of them are exothermic reactions (ΔH < 0), particularly Equation (3). The Gibbs free energy change calculation (ΔG = ΔH−T × ΔS) indicates that Equation (7) is only spontaneously carried out at an ambient temperature of 345.74 K (72.59 °C). It means that Equation (7) can easily occur when the continuous occurrence of Equations (3) and (4) results in a temperature increase within the system, leading to the cyclic oxidation exothermic reaction of pyrite.

4.1.2. Oxidation Mechanism under the Bacteria

The pyrite oxidation by bacteria is a combined process including direct contact and indirect contact [32]. In direct contact oxidation, microorganisms adsorb on the mineral surface, dissolve and oxidize pyrite to form Fe²⁺ and SO₄²⁻ directly through the metabolites or biological enzymes [33]. In indirect contact oxidation, microorganisms increase the rate of Fe²⁺ oxidation to Fe³⁺ without directly touching the pyrite surface. The mechanism of the pyrite oxidation by microbial was summarized based on previous research [34,35,36], which is shown in Figure 11. Firstly, microorganisms grow rapidly by oxidizing Fe²⁺ in the solution, and the formed Fe³⁺ can further oxidize sulfur on the surface of pyrite, leading to a weak dissolution pit [36,37]. This process lasts for a relatively long time (Figure 11a). For instance, the concentrations of Fe³⁺ and SO₄²⁻ began to increase rapidly on the 10th–15th day in this study (Figure 9b).

Subsequently, microorganisms continuously attached to the pyrite surface and formed microbial films, leading to the appearance of numerous dissolution pits and secondary precipitation appeared on the surface [38]. With the synergistic oxidation and erosion of pyrite by microorganisms and Fe³⁺, a large number of erosion pits are formed on the surface and develop deep (Figure 11b). However, since the pyrite samples used in this study were crushed into particles, only a part of the pyrite surfaces was observed to be blurred by SEM method, as shown in Figure 12. Energy dispersive (EDS) analysis results showed that the pyrite surface had high contents of oxygen and carbon elements, which may be related to the biological oxidation of pyrite. Previous studies have found two types of dissolution pits formed on the surface of pyrite after being oxidized by microorganisms: cell morphology and mineral morphology [36,37]. The former is formed by microbial metabolism directly eroding the pyrite surface, while the latter is the chemical oxidation product of pyrite by Fe³⁺ and appears in a regular hexagon.

In addition, previous studies [36,39] have shown that during the oxidation process of pyrite by microorganisms, the persulfate ions (S₂²⁻) are oxidized first to form thiosulfate (S₂O₃²⁻), which then be oxidized to form SO₄²⁻ by Fe³⁺, as shown in Equations (8) and (9). However, under acidic conditions, S₂O₃²⁻ can undergo disproportionation to produce native sulfur (S₀) and SO₂ (Equation (10)). The formed SO₂ dissolves in water and is further oxidized to form SO₄²⁻ (Equations (11) and (12)). Therefore, the oxidized surface of pyrite under the action of microorganisms may produce sulfur-rich layers of different forms of sulfur, as shown in Figure 11b. Li et al. [36] detected multiple sulfur-containing components on the pyrite surface using the XPS method after microbial oxidation, such as S₂²⁻/S²⁻, S_n²⁻, S₀, S₂O₃²⁻, SO₃²⁻ and SO₄²⁻, etc.

$${\mathrm{FeS}}_2+6{\mathrm{Fe}}^{3+}+3{\mathrm{H}}_2\mathrm{O}\to 7{\mathrm{Fe}}^{2+}+{\mathrm{S}}_2{\mathrm{O}}_3{}^{2-}+6{\mathrm{H}}^{+}$$

(8)

$${\mathrm{S}}_2{\mathrm{O}}_3{}^{2-}+8{\mathrm{Fe}}^{3+}+5{\mathrm{H}}_2\mathrm{O}\to 8{\mathrm{Fe}}^{2+}+2{\mathrm{SO}}_4{}^{2-}+8{\mathrm{H}}^{+}$$

(9)

$${\mathrm{S}}_2{\mathrm{O}}_3{}^{2-}+2{\mathrm{H}}^{3+}\to \mathrm{S}\downarrow +{\mathrm{SO}}_2\uparrow +{\mathrm{H}}_2\mathrm{O}$$

(10)

$${\mathrm{SO}}_2+{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{H}}^{+}{+\mathrm{SO}}_3^{2-}$$

(11)

$${2\mathrm{SO}}_3^{2-}+{\mathrm{O}}_2\to {2\mathrm{SO}}_4^{2-}$$

(12)

4.2. Differences between Coal-Pyrite and Mineral-Pyrite

This study conducted oxidation experiments on mineral-pyrite and coal-pyrite under the addition of Fe³⁺ and microorganisms, respectively. The results showed that under the same conditions, the change in each component in the reaction system of coal-pyrite was higher than that of mineral-pyrite (Figure 6 and Figure 8). Moreover, the oxidation rate of coal-pyrite was also higher than that of mineral-pyrite in the initial reaction (Figure 9). As shown in Figure 2 and Figure 4, coal-pyrite samples displayed high heterogeneity, with different mineral composition and crystal structure compared to mineral-pyrite. In addition, the organic matter in the coal body can also affect the oxidation process of pyrite.

4.2.1. Mineral Impurities

The final products of pyrite oxidation are Fe²⁺/Fe³⁺ and SO₄²⁻. As mentioned in Section 3.2.3, SO₄²⁻ in the reaction system with Fe³⁺ was produced by the pyrite oxidation, while the iron ions (Fe²⁺ and Fe³⁺) were formed by the pyrite oxidation in the reaction system with bacteria. After subtracting the ion content change in the original reaction solution, the increase of iron ion and SO₄²⁻ caused by pyrite oxidation under the action of Fe³⁺ and bacteria were calculated respectively, and presented in

Table 5. The concentration increase of iron ion and SO₄²⁻ caused by pyrite oxidation, mmol/L.

Reaction Conditions	Reaction Time
	0 min		30 min		60 min		90 min		120 min
	Iron Ion	SO42−	Iron Ion	SO42−	Iron Ion	SO42−	Iron Ion	SO42−	Iron Ion	SO42−
Ml- with Fe3+	0	0	0.068	0.09	0.173	0.225	0.224	0.259	0.245	0.294
C1- with Fe3+	0	0	0.128	0.11	0.21	0.24	0.246	0.279	0.266	0.309
C2- with Fe3+	0	0	0.085	0.14	0.205	0.241	0.257	0.312	0.291	0.335
Reaction conditions	0 day		5 day		10 day		15 day		20 day
	Iron ion	SO42−	Iron ion	SO42−	Iron ion	SO42−	Iron ion	SO42−	Iron ion	SO42−
M1-without bacteria	0	0	0	0.002	0.001	0.002	0.001	0.002	0.001	0.003
M1-with bacteria	0	0	0.019	0.024	0.034	0.039	0.064	0.088	0.103	0.116
C1-with bacteria	0	0	0.016	0.037	0.043	0.098	0.101	0.166	0.133	0.212
C2-with bacteria	0	0	0.021	0.017	0.031	0.063	0.087	0.113	0.119	0.193
Reaction conditions	25 day		30 day		35 day		40 day
	Iron ion	SO42−	Iron ion	SO42−	Iron ion	SO42−	Iron ion	SO42−
M1-without bacteria	0.002	0.005	0.011	0.016	0.012	0.017	0.018	0.026
M1-with bacteria	0.126	0.204	0.141	0.225	0.149	0.248	0.166	0.282
C1-with bacteria	0.141	0.249	0.160	0.259	0.170	0.291	0.182	0.307
C2-with bacteria	0.137	0.203	0.150	0.225	0.165	0.276	0.174	0.293

. However, the concentration increase of iron ion and SO₄²⁻ did not meet the ratio of 1:2, indicating that there are other mineral impurities involved in the reaction besides pyrite even though these pyrite samples have been purified.

A concentration difference parameter (D) was used to characterize the reaction degree of mineral impurities, defined as the concentration difference between the theoretical increase value and the actual increase value of a certain component:

$$\mathrm{D}=\left[Measured value\right]-\left[Calculated values\right]$$

(13)

Since the SO₄²⁻ in the Fe³⁺ reaction system only came from pyrite, the theoretical concentration increase value of iron ion was calculated according to the concentration increment of SO₄²⁻, and then obtained the concentration difference parameter (D_Fe) of iron ion. Similarly, calculated the concentration difference parameter (D_S) of SO₄²⁻ in the bacterial reaction system. The calculation results were shown in Figure 13.

It can be seen in Figure 13a that the concentration difference parameter of ions ion (D_Fe) is a positive value and the measured increase value is slightly higher than the calculated value. This suggests that some iron-containing minerals were dissolved in the reaction system. Except for pyrite, the common iron-containing minerals in coal seams include siderite (FeCO₃), ankerite (Ca(Mg, Fe)(CO₃)₂), etc. These minerals are more easily dissolved in acid solutions, resulting in a higher Fe²⁺ concentration increase and reaction rate in the coal-pyrite reaction system than in the mineral-pyrite system in the initial stage (Figure 6c and Figure 9a). At the same time, the pH value of the reaction solution of M1-without bacteria slightly increased in the early stage, as shown in Figure 8a, which also reflected the consumption of H⁺ by the dissolution of impurity minerals. Coal-pyrite samples have relatively higher impurity content than mineral-pyrite, and the growth rate of Fe ion content in the early stage is faster (Figure 6c). As the reaction progressed, the D_Fe increased slowly, indicating that the influence of mineral impurities on the reaction gradually reduces.

The pyrite oxidation by microorganisms is a relatively complex process (Figure 11). During this process, sulfur in pyrite can be oxidized into various forms, resulting in a negative value of Ds (Figure 13b) and the relatively deficient content of SO₄²⁻ in the initial stage of the reaction. As the reaction progressed, other forms of sulfur are eventually oxidized to SO₄²⁻, and the absolute value of Ds decreases. However, the actual increase value of SO₄²⁻ was always smaller than the calculated value, indicating a greater increase of iron ions in the reaction system.

4.2.2. Crystal Structure

The formation process of pyrite involves two evolution modes, direct precipitation and comprehensive genesis [40]. During this process, impurity elements can enter the lattice by lattice substitution and isomorphism. For instance, Co, Ni, and Cu can replace Fe atoms, and As, Se and Te can replace S atoms [41]. Alternatively, impurity elements can enter the crystal gap in the form of mechanical mixtures, such as Au, Ag, Pb, and Zn [42], resulting in crystal defects of pyrite minerals. Murphy et al. [43] confirmed that crystal defects can cause pyrite crystals to break and dissociate easily, and the under-coordinated Fe atoms in the fracture boundary region will react with the surrounding paramagnetic materials, such as O₂. In the meanwhile, the impurity elements in the lattice can catalyze the oxidation of pyrite. Liu et al. [44] have found that the morphology and distribution of the impurity elements in pyrite crystal can significantly affect the corresponding oxidation activity of pyrite. Due to the differences in genesis and sedimentary environment, there are obvious differences in crystal structure between coal-pyrite and mineral-pyrite. Coal-pyrite samples have lower lattice filling saturation (Table 2), and their densities are also much smaller than mineral-pyrite, indicating that the internal defects of coal pyrite are more noticeable [45] and make it easier to be oxidized (Figure 9).

As mentioned earlier, the oxidation reaction of pyrite occurs on the crystal surface, and a larger contact area between pyrite crystal and O₂/H₂O leads to a faster reaction rate. The complex genesis results in various forms of coal-pyrite (Figure 4). For instance, framboidal and caviar-like pyrite have mineral morphologies similar to spherical or spherical mass, providing much larger specific surface areas than the mineral-pyrite that is dominated by regular cube and pentagonal dodecahedron. Moreover, the lattice filling saturations of these two pyrites are relatively low, with the N(S)/N(Fe) ratios of 1.07 and 1.17, respectively (Table 2), which further improves the oxidation reaction rate. In addition, coal seams commonly contain a significant amount of fine-grained euhedral crystalline pyrite with diameters less than 60 μm [6], which provides a larger specific surface area for the oxidation reaction.

4.2.3. Organic Matter

Compared to mineral-pyrite, the formation of coal-pyrite is more obviously affected by organic matter. Coal seams contain a large number of humic acids that are rich in active groups, such as carboxyl and hydroxyl functional groups [46], which have strong capabilities of adsorption, complexation, and ion exchange [47]. Humic acid has a complicated effect on the oxidation of pyrite. On the one hand, the adsorption of humic acid on the pyrite surface can slow down or even prevent the interface reaction between Fe³⁺/O₂ and pyrite [43]. On the other hand, humic acid can promote the production of hydroxyl radical of pyrite and enhance the oxidation ability [48], although some scholars believe that humic acid cannot change the oxidation mechanism of pyrite [49]. Moreover, humic acid and organic matter in the coal seams can provide carbon nutrients for microorganisms, which leads to the proliferation of microbes, and their metabolites and metabolic activities can also destroy the humic acid layer on the pyrite surface [50]. The coal-pyrite samples used in this study have been broken and purified before the oxidation experiment, so the residual humic acid can hardly form a dense barrier layer on the surface. Therefore, it is considered that residual humic acid is beneficial to the oxidation of coal-pyrite in this study.

In addition, coal-pyrite contains a higher content of organic sulfur than mineral-pyrite (Table 3), which mostly exists in the form of sulphoxides and thiophenes. Previous studies have shown that sulfide sulfur can be oxidized to sulfoxide at low temperatures and further oxidized to form sulfone [51]. The oxidation of organic sulfur by microorganisms promotes the enrichment of sulfur on the mineral surface, but it is difficult to transform into SO₄²⁻ [52]. Therefore, the concentration change of SO₄²⁻ under the bacteria action comes from the oxidation of pyrite (Figure 8d).

In summary, the differences in mineral impurity composition, crystal structure, and organic components between coal-pyrite and mineral-pyrite lead to a high oxidation rate of coal pyrite in this experiment. Besides, probably due to the different compositions and environments, the oxidation rate of the C2 sample is higher than that of the C1 sample in the Fe³⁺ condition, while the C1 sample has a higher oxidation reaction degree than the C2 samples under the action of microorganisms. Therefore, further research is necessary to study the oxidation mechanism of pyrite in coal seams under the influence of multiple factors.

4.3. Inspiration to Prevent Coal Spontaneous Combustion

The oxidation of pyrite plays a crucial role in the occurrence of coal spontaneous combustion [53]. The natural chemical oxidation rate of pyrite is generally slow and uniform, such as the reaction of M1-without bacteria in this study (Figure 9b), but the oxidation rate can be greatly increased under the participation of Fe³⁺ and bacteria conditions. Some studies have shown that the oxidation rate of pyrite in acidic water with microorganisms can be increased by 10⁶ times, which is 10–20 times faster than that of the inorganic chemistry reaction under Fe³⁺ condition [54]. Furthermore, the oxidation rate of coal-pyrite is higher than that of mineral-pyrite under the influence of multiple factors.

Therefore, preventing the catalytic effect of Fe³⁺ and bacteria on pyrite oxidation is an important method to avoid coal spontaneous combustion. According to the reaction process of pyrite oxidation and exotherm (Figure 10), the accumulation of moisture, temperature, and heat will form a reaction cycle of “oxidation-reduction-reoxidation” of pyrite. Therefore, it is essential to monitor the temperature and humidity of the coal accumulation environment and ensure that the air remains calm [55]. In addition, the bio-oxidation of pyrite by microorganisms cannot be ignored. Antibiotics and surfactants can be sprayed regularly on tailings and gangue to sterilize and prevent microorganisms from contact with pyrites, such as Triclosan, Kathon (isothiazolinones), and sodium dodecyl sulfate (SDS) [56].

5. Conclusions

In this study, two kinds of coal-pyrite and one kind of mineral-pyrite samples were used to conduct the oxidation experiments under the participation of Fe³⁺ and thiobacillus ferrooxidans in acid solutions. Through analyzing the changes in pH and each component in the reaction system, the following conclusions were drawn:

(1)

The natural chemical oxidation rate of pyrite is slow and uniform. The participation of oxidant Fe³⁺ and bacteria can significantly increase the oxidation rate of pyrite, although their action process and mechanism are different. In this study, bacteria had a lower impact on the pyrite oxidation than the oxidant.

(2)

Under the same conditions, the oxidation degree and rate of coal-pyrite samples were slightly higher than those of mineral-pyrite. The relatively higher impurities content and more incomplete crystal structure of coal-pyrite samples can help to improve the oxidation reaction rate. Moreover, the organic matter in coal has a close relationship with coal-pyrite and affects the oxidation process of coal-pyrite.

(3)

Compared to mineral-pyrite, the oxidation process and mechanism of coal-pyrite are more complex and affected by many factors. The various compositions of coal-pyrite samples can also lead to different oxidation processes. It is necessary to carry out an in-depth study to understand the oxidation mechanism of coal-pyrite under the influence of multiple factors and take corresponding measures to prevent coal spontaneous combustion accordingly.