Mechanism and In Situ Prevention of Oxidation in Coal Gangue Piles: A Review Aiming to Reduce Acid Pollution

Abstract

The acid pollution produced from coal gangue piles is a global environmental problem. Terminal technologies, such as neutralization, precipitation, adsorption, ion exchange, membrane technology, biological treatment, and electrochemistry, have been developed for acid mine drainage (AMD) treatment. These technologies for treating pollutants with low concentrations over a long period of time in coal gangue piles appear to be costly and unsustainable. Conversely, in situ remediation appears to be more cost-effective and material-efficient, but it is a challenge that coal producing countries need to solve urgently. The primary prerequisite for preventing acidic pollutants is to clarify the oxidation mechanisms of coal gangue, which can be summarized as four aspects: pyrite oxidation, microbial action, low-temperature oxidation of coal, and free radical action. The two key factors of oxidation are pyrite and coal, and the four necessary conditions are water, oxygen, microorganisms, and free radicals. The current in situ remediation technologies mainly focus on one or more of the four necessary conditions, forming mixed co-disposal, coverage barriers, passivation coatings, bactericides, coal oxidation inhibitors, microorganisms, plants, and so on. It is necessary to scientifically and systematically carry out in situ remediation coupled with various technologies based on oxidation mechanisms when carrying out large-scale restoration and treatment of acidic coal gangue piles.

1. Introduction

As a primary energy source, the status of coal as the main energy source and its promoting effect on regional economic development will not change significantly in the short term. A large amount of coal gangue is generated with the development and utilization of coal [1]. The comprehensive utilization of coal gangue is limited due to its difficult transportation, and the discharged coal gangue is stored outdoors for a long time, forming coal gangue piles [2]. The open-air stacking of coal gangue not only seriously damages the terrain, landscape, farmland, and visual effects, but it also causes safety hazards such as landslides, collapses, and mudslides in some mining areas due to its loose structure and improper stacking methods [3].

Acidic coal gangue piles have the most serious impact on the ecological environment and human health. As shown in Figure 1, acidic coal gangue undergoes an oxidation reaction in the air, releasing heat and producing atmospheric pollutants. Under unfavorable thermal conductivity conditions, the temperature of the coal gangue pile rises until spontaneous combustion occurs [4], seriously affecting the personal safety and respiratory health of surrounding residents. Under rain leaching, coal gangue piles are prone to form acid mine drainage (AMD) containing high concentrations of sulfate and accompanied by a large amount of harmful heavy metals and toxic non-metals [2], which pollutes the surrounding soil, surface water, and groundwater through convection, leaching, dispersion, diffusion, and other effects [5], seriously endangering the surrounding ecological environment.

The treatment of acid pollution generated by coal gangue piles has received widespread attention worldwide [6,7]. Generated AMD can be reduced by pollution treatment technologies, such as neutralization [8], precipitation [9], adsorption [10], ion exchange [11], membrane technology [12], biological treatment [13], electrochemistry [14], and artificial filtering arrays [15], to reduce the negative impacts on the receiving water and ecosystems. Pollution treatment technologies can handle AMD with different properties flexibly and unrestrainedly, but even for AMD that can be treated, the treatment technologies must continue for decades or even hundreds of years until pollutants are no longer generated. In the process of the continuous leaching of pollutants, it is not only necessary to supply a chemical reagent, energy, and manpower continuously [16] but also to dispose of a large amount of hazardous sludge generated by the addition of alkaline reagents, such as lime [17]. Therefore, pollution treatment technologies are expensive and unsustainable in the long run. Moreover, pollution treatment, which could produce by-products and secondary pollution, mostly involve the transfer of pollutants rather than reducing them fundamentally.

Given the limitations of terminal treatment technology, by inhibiting the oxidation process of coal gangue at the source, controlling the generation of acid pollutants becomes more cost-effective and material-efficient. Most governments, companies, and researchers have believed that pollution control technology to prevent coal gangue oxidation is a better solution than treating emitted pollutants [18]. However, in situ remediation technology for acid pollution in coal gangue piles has not been systematically reported yet. Most overviews on in situ remediation measures for acidic pollution in coal gangue piles are based on the mechanism of pyrite oxidation [19,20]. An overview of in situ remediation measures responding to other more comprehensive and complex oxidation mechanisms, such as microbial action, the low-temperature oxidation of coal, free radical action, and so on, is lacking. Therefore, acid pollution of coal gangue is taken as the research object in this paper, and in situ remediation measures are systematically summarized based on the review of the oxidation mechanism of coal gangue, providing a scientific theoretical basis for the remediation of acidic coal gangue piles.

2. Oxidation Mechanism of Coal Gangue

As previous scholars have mentioned, understanding oxidation not only requires understanding the role of oxidation but also understanding how oxidation works [21], which is historic for promoting disciplinary development and engineering technology innovation. Since the spontaneous combustion of coal gangue piles has attracted widespread attention, many scholars have proposed different oxidation mechanisms, which can be summarized as pyrite oxidation, bacterial action, the low-temperature oxidation of coal, and free radical action.

2.1. Pyrite Oxidation

Pyrite oxidation leading to acid production in coal gangue is widely recognized currently, because pyrite is the most typical and abundant mineral in coal gangue, and the acid production process in coal gangue could be better revealed by the oxidation reaction of pyrite [22].

The pyrite in coal gangue has strong reducibility and is prone to reactions at low temperatures, whether in the gas–solid phase or the gas–liquid–solid phase (Figure 2). In the gas–solid phase, pyrite is oxidized to generate Fe₂O₃ and elemental sulfur (Equation (1)), and SO₂ is released in the presence of sufficient oxygen (Equation (2)). In the gas–liquid–solid phase, an important reaction of pyrite oxidation is formed (Equation (3)), where pyrite is oxidized to form dissolved Fe²⁺, SO₄²⁻, and H⁺, leading to an increase in the total dissolved solids and the acidity of the water. If the surrounding environment is fully oxidized, which depends on the O₂ concentration, pH value, and bacterial activity, bacteria use oxygen and hydrogen ions to oxidize most of the ferrous metal to trivalent iron (Equation (4)) [23]. Trivalent iron precipitates in the form of Fe(OH)₃ and jarosite (Equation (5)), when the pH value is between 2.3 and 3.5, reducing the pH value and leaving a small amount of Fe³⁺ in the solution. The portion of trivalent iron produced by reaction (4) that did not precipitate through Equation (5) may be used to oxidize additional pyrite (Equation (6)) [24]. All the above reactions are oxidation reactions, accompanied by abundant exothermic phenomena, which leads to the accumulation of heat and an increase in temperature inside the coal gangue piles with poor thermal conductivity, further accelerating the oxidation of pyrite.

4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂

(1)

4FeS₂ + 3O₂ → 2Fe₂O₃ + 8S

(2)

2FeS₂ + 7O₂ + 2H₂O → 2Fe²⁺ + 4SO₄²⁻ + 4H⁺

(3)

4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

(4)

Fe³⁺ + 3H₂O → Fe(OH)₃(s) + 3H⁺

(5)

FeS₂(s) + 14Fe³⁺ + 8H₂O → 15Fe²⁺ + 2SO₄²⁻ + 16H⁺

(6)

The reasons for the reaction of pyrite may be as follows: (1) The crystal structure of FeS₂ contains S⁻, which is more reactive than S²⁻ [25]. (2) During the process of crystal growth, pyrite is prone to forming lattice defects [26], which reduces the symmetry but increases the diffusion coefficient and reaction activity. (3) In the structure of pyrite, there are usually some dissociation surfaces with high surface activity and unstable structure. The pyrite at the dissociation surface is prone to fracture and dissociation [27], exposing Fe atoms with insufficient coordination and spin polarity, which can react with surrounding paramagnetic O₂. (4) As the dissociation surface occurs, the crystal lattices plane is cut off, which generates a large number of dangling bonds easily, causing an increase in the surface energy, increasing the surface activity of pyrite, and making it easier to react with the surrounding O₂ [28,29].

2.2. Microbial Action

Authigenic and autotrophic bacteria in coal gangue grow in acidic pore water, where it is difficult for other ordinary microorganisms to survive. They synthesize cell tissue by taking up CO₂ and O₂ in the air and other trace elements in the water, promoting the oxidation of sulfur, iron, and other components in minerals. Simultaneously, they obtain metabolic energy and play a key role in catalytic oxidation, self-heating latency, and the weathering of minerals in the low-temperature oxidation of coal gangue (

Table 1. The key role of bacteria in coal gangue oxidation.

Role	Mechanization	Crucial Microbial
Catalysis	Catalyzing chemical oxidation or participating in biological oxidation.	IOB, T. t, T. a, Sulfobacillus, Thermoacido philic archaebacteria
Self-heating	(1) Releasing biochemical heat, accompanied by microbial growth. (2) Accelerating heat release of chemical reaction through catalytic action.	T. f, Psendomonas
Mineral weathering	(1) Chemical dissolution through organic acid, produced by mycelia. (2) Mechanical decomposition by mycelia, increasing specific surface area.	Fungus, Actinomycetes

Note: IOB = iron-oxidizing bacteria, containing Thiobacillus ferrooxidans (T. f), Psendomonas, Leptothrix, Crenothrix, Leptospirillum, etc.; T. t = T. thermophilic; T. a = T. acidophilus.

) [30].

In coal gangue, the reactions involving microorganisms mainly include the following types: (1) Initially, pyrite is oxidized by oxygen to produce hydrogen ions without the catalytic effect of microorganisms. At this time, the pH value is greater than 4.0 with the slow chemical oxidation reaction. (2) As the oxidation reaction proceeds and the environmental acidity increases, microorganisms such as T. f and T. t, which attach to the surface of pyrite, begin to grow and participate in the oxidation reaction system, oxidizing the sulfur in FeS₂ to SO₄²⁻ through the enzymolysis function. (3) As SO₄²⁻ dissolves from the lattice, the chemical reaction rate of the Fe²⁺ to Fe³⁺ conversion (Equation (4)) could by increase about six orders of magnitude when mediated by IOB, compared to reactions without microbial involvement [31]. (4) The strong oxidizing Fe³⁺ generated by bacterial catalysis further promotes the oxidation of pyrite to generate Fe²⁺ and S (Equations (6) and (7)), where S can be oxidized as energy by bacteria, generating H₂SO₄ (Equation (8)). (5) Furthermore, aerobic bacteria, such as Pseudomonas, play a catalytic role in the oxidation of organic sulfur (Equation (9)).

FeS₂ + 2Fe³⁺ → 3Fe²⁻ + 2S

(7)

2S + 3O₂ + 2H₂O →2H₂SO₄

(8)

Sulfur-containing organic compounds + O₂ →···→ CO₂ + H₂O + H₂SO₄

(9)

2.3. Low-Temperature Oxidation of Coal

In addition to being black mudstone, coal gangue usually carries 10% to 25% of low-grade coal that was not separated during the washing process [32]. Coal in gangue, as a natural active substance (especially silk coal and vitrinite), will slowly undergo physical adsorption, chemical adsorption, and a chemical reaction with oxygen at low temperatures (below 80–100 °C), while releasing heat [33].

Firstly, oxygen is physically adsorbed on the surface of coal and diffuses into the pores of the coal at a normal temperature and pressure (Equation (10)). Secondly, oxygen is chemically adsorbed onto active groups (Equation (11)), forming intermediate carbon oxygen complexes after oxidation (Equation (12)). Thirdly, as the temperature increases, it will cause the decomposition of unstable oxygen-containing intermediate groups (Equations (13) and (14)) [34,35].

Kinetic studies have shown that the reactions (11)–(13) are relatively exothermic [36], reflecting that the dominant reactions of coal in gangue include chemical adsorption on the coal surface, the generation of intermediate complexes, and the decomposition of unstable oxygen-containing intermediate groups.

Coal(s) + O₂(g) → Coal-O_{2(Physisorbed)}

(10)

Coal-O_{2(physisorbed)} → Coal-O_{(Chemisorbed)}

(11)

Coal-O_{(Chemisorbed)} → Unstable intermediates

(12)

Unstable intermediates → CO₂, H₂O (main reaction)

(13)

Unstable intermediates^# → CO, C_xH_y, H₂, SO_x, NO_x (side reactions)

(14)

2.4. Free Radical Action

Free radicals are crucial in the oxidation of coal gangue, whether in the system of pyrite or in the low-temperature oxidation of coal [37].

Hydroxyl groups are the most important free radicals in the oxidation of pyrite. As shown in Figure 3, there are four main pathways for the production of hydroxyl groups: (1) Under a neutral environment, oxygen is activated by the structural-state Fe (II) on the surface of pyrite (Process 1) or dissolved Fe²⁺ (Process 2) to produce hydroxyl groups [38]. (2) Under an acidic environment, O₂ is activated by the structural-state Fe (II) to produce H₂O₂ (Process 3), which then reacts with the dissolved Fe²⁺ through the Fenton reaction to produce hydroxyl groups or Fe (IV), or through H₂O₂ thermal decomposition to produce hydroxyl groups [39]. (3) When H₂O₂ acts as an oxidant, pyrite could be oxidized by it to form dissolved Fe³⁺ (Process 4). Fe³⁺ is reduced by pyrite to form dissolved Fe²⁺ (Process 5), which then reacts with H₂O₂ via the Fenton reaction to produce hydroxyl groups [40,41]. Synchronously, metal impurity ions on the surface of pyrite can catalyze the decomposition of H₂O₂ to generate hydroxyl groups. (4) Under an anaerobic environment, water molecules may be oxidized by Fe (III) on sulfur defect sites on the surface of pyrite to produce adsorbed hydroxyl groups (Process 6) [42]. The hydroxyl groups generated by the above steps will further oxidize pyrite.

The free radical reaction continues throughout the whole process of coal oxidation and plays a key role in the low-temperature oxidation of coal gangue. The main structural units of organic macromolecular coal are composed of active structures, such as condensed aromatic nuclei, which are interconnected through methylene bonds, ether bonds, and aldehyde bonds. In addition to the main structure, there are also many reactive functional groups in coal that act as intermediate products during the oxidation process [43,44]. During the process of mining, washing, transportation and storing, the condensed aromatic nuclei of coal generate a large number of cracks under the action of shear stress, resulting in the fracture of the coal molecular chain. The essence of molecular chain breakage is the breaking of covalent bonds, resulting in the generation of a large number of free radicals (hydroxyl, methyl, methylene, etc.) [45], the activity of which is almost unaffected by the number of aromatic rings [46], showing strong activity and easily reacting with oxygen.

From the carbon free radicals and methylene reacting with oxygen to generate peroxyl radicals, which are the key intermediate products in the free radical chain reaction, to the generation of CO, CO₂, and CxHy by aliphatic free radicals, aldehydes, and carboxylic acids, and the generation of water by hydroxyls, the free radical reaction system of coal oxidation is jointly constituted (Figure 4) [33,47,48,49,50,51,52]. The free radical reaction system of coal has a self-heating latent effect in the early stage of oxidation in coal gangue piles, and it will trigger more oxidation reactions with increases in temperature [34].

3. In Situ Prevention of Oxidation

3.1. Mixed Co-Disposal

Mixed co-disposal is the process of mixing coal gangue with alkaline materials to increase the buffering capacity for acidic substances. The following four mechanisms could be considered as the mechanisms for controlling oxidation: (1) The supply of oxidants is limited by the precipitation of Fe³⁺. (2) The activity of oxidative microorganisms (A. f, A. t, etc.), which are high in acidic conditions, are inhibited by the increase in the pH value. (3) A coating with extremely low permeability is formed on the surface of sulfide minerals, reducing the reaction surface area. Commonly used alkaline materials include limestone, quicklime, sodium carbonate, sodium bicarbonate, phosphate minerals (such as phospholime Ca₅(PO₄)₃F), etc. In addition, alkaline by-products with a high neutralization potential generated by coal-fired power plants, pulp mills, and steel mills are more cost-effective for neutralizing the acidity of coal gangue piles [53], such as fly ash, mesa lime, green liquor dregs, argon oxygen decarbonization (AOD) slags, red gypsum, sugar foam, biomass combustion ash, cement kiln ash, desulfurization gypsum, red mud bauxite, etc. [18,54,55]. Most leachates from mixtures of waste rock with alkaline additives have neutral characteristics and low concentrations of As, Fe, Cu, Pb, and Zn. However, the concentrations of S and Al in the mixture leachates are high. Significantly, the leachate of AOD slags contains a high pH value and high Cr levels [18]. Red gypsum, sugar foam, and biomass combustion ash can increase the pH value to 5~8, respectively, and significantly reduce the leaching rates of Zn and Cu. It is worth noting that the combination of the above three products can reduce the migration rate of As [54]. When the mass ratio of clay salt coal gangue to red mud was 12:1, using the alternate one-layer stacking method, the pH value of the leaching solution was within the range of drinking-water quality for a long time and the leaching solution had the smallest impact on the hardness of groundwater [55].

However, in order to carry out mixed co-disposal operations effectively, it is necessary to use the correct the stoichiometric balance between acid production and acid consumption. For example, a lack of acid-consuming substances will not be able to neutralize acidic leachate, while an excess may form strong alkaline leachate (pH > 10), thereby increasing the solubility of hazardous metals, such as Al, Cu, Ni, Pb, and Zn [56]. In addition, it should be pointed out that mixing alkaline materials with sulfide tailings in tailings ponds or waste rock yards is a laborious task, and the problem of dealing with large and dangerous sludge should be given special consideration [57].

3.2. Coverage Barrier

The oxidation mechanism of coal gangue reveals the significant effect of oxygen and water in the oxidation process. Therefore, the necessary measure to inhibit the oxidation of coal gangue is to reduce the infiltration of oxygen and water. A relatively economical technique is providing a coverage barrier, because the oxidation reaction inside the gangue piles can be maintained for a long time at a stable and low level if all the stored heat can be dissipated while the seal is maintained [58].

The main mechanisms of inhibiting oxidation include the following aspects: (1) The overlying barrier, which maintains high saturation, acts as a capillary oxygen barrier for the underlying coal gangue, minimizing oxygen penetration by taking advantage of the low diffusion efficiency (1.9 × 10⁻⁹ m²/s) and low solubility (8.6 g/cm⁻³ at 25 °C) of oxygen in water (Figure 5a) [59,60]. (2) The underlying gangue maintains a low oxygen content the through oxygen consumption reaction in the overlying barrier (Figure 5b) [61]. (3) The overlying barrier, containing alkaline materials, neutralizes the acidic substances migrating upward through capillary motion, controlling the oxidation rate of the underlying gangue (Figure 5c) [62]. (4) The low permeability of the overlying barrier is used to achieve a balance flux between water storage and release, reducing the net infiltration flux of water (Figure 5d). (5) The increased reaction surface area of coal gangue caused by erosion can be avoided by adding a coverage barrier.

Commonly used covering materials mainly include an inactive combination of fine slag and natural minerals, low-sulfur tailings, clay or ash mixed with AMD treatment sludge or natural soil, industrial alkaline waste, organic materials, etc. [58,63,64]. However, it is difficult to keep single-structured materials in a highly saturated state because the performance of the covering materials is affected by key parameters, such as the material height, initial moisture, saturated moisture, performance control, and construction quality. Additionally, ensuring their integrity under the alternating effects of settlement, shrinkage, drying, precipitation, fire, and root invasion is another difficulty, which poses great challenges to their sustainability. The performance of most single-structured materials exhibit prominent degradation within a certain period of time, which may not be efficient enough to prevent AMD generation, particularly where the water table is deeper than 2 m below the surface of tailings, resulting in a decreased pH value and an increased heavy metal concentration of the leachate [65]. The above problems could be solved properly by scientific theoretical methods. The material property could be optimized by closely combining the soil–water characteristic curve [66], the hydraulic conductivity function [67], the capillary barrier concept [68], and the relationship between saturation and oxygen diffusion [59], and the performance in engineering applications could be predicted by using numerical simulation [68] and monitoring using multiparameter sensors (suction, water content, water content, climate, and water volumes) [64]. Anderson et al. [64] used a five-layer structure, with each layer 0.3m thick, to suppress acid production from coal gangue oxidation. From the bottom to the top, the layers were mixed garbage, compacted ash (capillary barrier), compacted clay, compacted ash (capillary barrier), and organic soil. After one year of continuous monitoring, the moisture content of the clay layer remained 100% saturated, corresponding to negligible oxygen diffusion into the waste. The covered yard significantly reduced the infiltration water by about 85% compared to the uncovered yard within one year. The oxygen and water barrier function of the covering layer directly inhibited the oxidation of coal gangue, resulting in a pH of nearly 7 (pH < 2 without a covering) and a conductivity of nearly 4 mS/cm (pH > 40 mS/cm without a covering) in the leachate. The environmental temperature associated with the degree of oxidation reaction was approximately 20 °C (approximately 38 °C without a covering). And there was no performance degradation phenomenon, showing good sustainability. Thomas et al. used numerical hydrogeochemistry to evaluate the effectiveness of three different structures of cover layers over a period of 10 years. The results showed that the three-layer cover layer with a capillary barrier effect was more effective in reducing oxygen flux and AMD generation than the single-layer structure [59].

3.3. Passivation Coating

The oxidation reaction of sulfur-containing minerals can be effectively inhibited through the construction of passivation coating to inhibit surface oxidation, surface dissolution, surface adsorption, and other chemical reactions [69]. Studies have shown that the surface of pyrite can be passivated under the condition of pH > 6, and the oxidation rate can be remarkably reduced by about 95% [70]. Passivators can be divided into inorganic reagents, microorganisms, organic reagents, and organic-inorganic reagents [71,72,73,74].

The inorganic passivation system of the surface of pyrite is composed of carbonate (limestone) as a passivator to form an iron oxide coating [75], silicate (Na₂SiO₃) as a passivator to form a stable silicate–iron (oxygen) hydroxide coating [71,76], pyrite oxidized by H₂O₂ combined with sodium acetate (NaOAc) to maintain a pH value of about 6 to form an Fe (III) precipitation coating [77], silicate (H₄SiO₄) as a passivator to form a ferric silicate coating, phosphate as a passivator to form a ferric phosphate coating [78], and Ti⁴⁺/Al³⁺/Si⁴⁺ hydroxide generated under alkaline conditions.

Microbial passivation inhibits oxidation by forming a complex biofilm on pyrite, which is mainly composed of facultative anaerobes, which interact with aerobic bacteria (A. f or A. t) and obligate anaerobes (sulfate-reducing desulfosporosinus) to maintain an oxygen-free microenvironment around the pyrite (Figure 6a). Additionally, the formed biofilm has a certain hydrophobicity, which prevents contact between the pyrite and H₂O. In essence, microbial passivation is a biological source treatment (BST) [79], which is described in detail in Section 3.6.

There is a wide range of organic passivators, whose basic principle is to produce Me-O-Si bonds on the surface of the pyrite matrix and form Si-O-Si cross-linked networks to produce a passivation effect, including 8-hydroxyquinonline [80], triethylenetetramine (TETA) [81], sodium triethylenetetramine-bisdithiocarbamate (DTC-TETA) [82], phospholipid (total iron (TI) reduced by 90% and SO₄²⁻ reduced by 95% in 3 yr) [83], organosilane [84,85], etc. Currently, there are many studies on organosilane, including γ-aminopropyltrimethoxysilane (APS) (TI reduced by 49.4%), vinyltrimethoxysilane (VTMS) (pyrite oxidation (PO) reduced by 71.4%), γ-mercaptopropyltrimethoxysilane (PropS-SH) (PO reduced by 89.2%), n-propyltrimethoxysilane (NPS) (TI reduced by 96% in 70 h), tetraethylorthosilicate (TEOS) (TI reduced by 59% in 70 h) and polysiloxane (low surface tension and excellent film-forming performance) [72,86].

There are three common problems when a passivator is used alone: (1) The metal precipitates formed by inorganic passivators are crystals, and their regular arrangement leads to gaps, which limits their efficiency as a coating. (2) When the concentration of organosilane is low (e.g., the volume concentration of PropS-SH is less than 10%), the coating surface is prone to microcracks or small defects, and it is not cost-effective to add high doses of organosilane to increase the thickness of coating. (3) Traditional passivators have no selectivity to minerals, leading to their rapid depletion in systems mixed with complex minerals.

The limited oxidation inhibition rate of crystalline coatings formed by an inorganic passivator is solved by jointly using multiple inorganic passivators to form an amorphous metal precipitate, which has shown higher inhibition efficiency in studies [71]. As shown in Figure 6b, when carbonate and silicate coexist, the silicate structurally combines with iron precipitates formed by carbonate to form amorphous ferric (oxygen) hydroxide, which is compact and even, leading to pyrite surface passivation. In addition, at a pH value > 4, the transformation of amorphous ferric (oxygen) hydroxide to crystalline iron could be inhibited with the presence of silica, forming an amorphous Fe³⁺-iron hydroxide-silica barrier layer [87].

An effective measure to prevent the microcracks generated by organosilane is to introduce nanoparticles into the coatings to improve the performance of the coating by repairing the cracks [88,89]. A typical example is the formation of PropS-SH/SiO₂ nanocomposites by embedding SiO₂ nanoparticles into the PropS-SH (Figure 6c). The hydroxyl groups on the surface of SiO₂ nanoparticles easily form covalent bonds with the active silanol (SiOH) groups of PropS-SH, making the SiO₂ nanoparticles closely bind to the PropS-SH coating matrix [90]. The effect of PropS-SH/SiO₂ nanocomposite coatings with a different content of SiO₂ nanoparticles on pyrite oxidation inhibition was evaluated through electrochemical measurements and chemical leaching testing by Liu et al. The results showed that the proper amount of SiO₂ nanoparticles significantly improved the oxidation inhibition rate of PropS-SH, which increased from 53.3% to 81.1% with a 2 wt% of SiO₂ nanoparticles combined with a 3% (v/v) PropS-SH solution [91].

The key to solve the lack of selectivity of passivators is to introduce redox-sensitive organic compounds to carry and transfer passivation materials in the process of coating, causing insoluble metal ions specifically precipitated on the surface of pyrite or arsenopyrite through electrochemical reactions, namely carrier-microencapsulation (CME) technology. For instance (Figure 6d), stable metal (or metal-like) organic complexes (such as [Si(cat)₃]²⁻ and [Ti(cat)₃]²⁻) were formed by low-solubility metal (or metal-like) ions (Ti⁴⁺ [92], Si⁴⁺ [93], Al³⁺ [94], and Fe³⁺ [95]) and carriers (organic ligands, such as catechol and 1,2-dihydroxybenzene) in the aqueous phase. Afterward, such complexes are electrochemically oxidized and decomposed on the surface of sulfide minerals, releasing metal (or metal-like) ions, which precipitate in the form of oxides or hydroxides. The precipitation adheres to the pyrite and directly reduces its oxidation degree by protecting the cathode position from the effects of oxidants, such as dissolved O₂ (DO) and Fe³⁺ [96,97]. Moreover, dissociative catechol may indirectly inhibit the oxidation of pyrite by consuming DO and forming stable complexes with Fe³⁺ [98,99].

3.4. Bactericides

Acidophilic iron-oxidizing and sulfur-oxidizing bacteria (e.g., T. f and T. t) can significantly promote pyrite oxidation. Studies have shown that up to 80% of AMD may be affected by oxidizing bacteria [100,101], which means that the use of bactericides to inhibit the growth of specific microorganisms could effectively inhibit the oxidation of acid gangue. The existing bactericides can be divided into inorganic and organic bactericides.

Inorganic bactericides include heavy metal bactericide and photocatalytic bactericide, which are depicted in Figure 7. Heavy metal bactericide is synthesized by ion exchange or the adsorption of metal (Ag, Cu, or Zn) and its compounds with inorganic porous carrier minerals (zeolite, glass, apatite, calcium phosphate, zirconium phosphate, etc.) [102,103]. Heavy metal bactericide causes a relatively high concentration of heavy metal cations outside the microbial membrane, changing the normal polarization state inside and outside the biomembrane, and causing a new ion concentration difference (ICD), thus hindering or damaging the transport of substances needed by cells to maintain physiology (such as the transport of sugar and amino acids under the driving of a Na⁺/K⁺ pump (Process 1)). Some metal ions can also enter the microorganism cells, inactivating most enzymes, so that the biochemical reactions catalyzed by enzymes cannot be carried out normally. They can also combine with nucleic acid, destroying the cell’s ability to divide and reproduce (Process 2). On the basis of heavy metal bactericides, nano bactericide materials with better antibacterial effects have been developed. For one thing, nano-bactericides have a lower minimum bactericidal concentration than conventional bactericides; for example, the minimum bactericidal concentration of silver-loaded bactericide at the nano level is about one-fourth of that at the micron level. For another thing, nano-bactericides have a larger surface area and stronger adsorption effect on microorganisms due to the nano-scale of the carrier. Photocatalytic bactericides are composed of N-type semiconductor metal oxides, such as TiO₂, ZnO, CdS, WO₃, SnO₂, and Fe₂O₃ [104,105,106]. Photocatalytic bactericides mainly rely on a strong oxidation free radical (·OH) activated by light energy to play the bactericidal role (Process 3), which can indiscriminately oxidize and degrade all organic matter, including penetrating the cell membrane and damaging the membrane structure. In addition to the advantages of heat resistance, durability, continuity, and safety of inorganic heavy metal bactericides, photocatalytic bactericides also have the characteristics of instant effect, no secondary pollution, a wide antibacterial spectrum, and semi-permanence. Moreover, a nano-TiO₂ bactericide, with a better bacteriostatic effect, has been developed on the basis of photocatalytic bactericides. The above inorganic bactericide has not been effectively applied in coal gangue piles, which is worth further study.

There are various types of organic bactericides (

Table 2. Descriptions and representative substances of organic bactericides.

Description	Representative Substances
Organometallics	Zn-Pentachlorophenate
Organic halide	Na-Pentachlorophenate
Alcohols, phenols, and ethers	Ethanol, p-nitrophenol, and ethylene glycol-methyl ether
Aldehydes, ketones, and quinones	Glutaraldehyde, o-hydroxycyclopentenedione, and spergon
Acids and salts	Sorbic acid
Esters	Dimethyl fumarate
Nitriles	Chlorothalonil
Guanidines	Chlorhexidine
Organo-nitro compounds	Furacilin
Organic phosphorus and organic arsenics	Lauric arsine
Heterocycles	/

), which have complex pathways to produce the bacteriostatic effect, mainly through acting on the cell wall, cell membrane, biochemical reaction enzymes, genetic material, and other systems, resulting in damage to the cell structure (concavation, atrophy, distortion, or splitting decomposition) and efflux of the proteins, plasma, lipids, and nucleic acid [107,108]. Organic bactericides have the advantages of strong bactericidal properties and superior immediate effects. Meanwhile, they also have the disadvantages of poor heat resistance, easy migration, and possible microbial drug resistance. The organic bactericides possessing low toxicity and biodegradability applied to coal gangue piles are listed in

Table 3. Organic bactericides used in coal gangue piles.

Description	Bactericide	Effective Dosage (mg/L)	Fe oxidation Inhibition Rate (%)	Price (CNY/kg)	Reference
Preservatives	Kathon	30	75–83.50	12	[108,109]
	SBZ	30	75.89	12
	Triclosan	16	75–83.50	160
Anionic surfactants	SDS	10–30	75.69–82.83	14.6	[110]
Cationic surfactants	CTAB	5	80.84	200	[111]
Low-molecular-weight organic acids	Formic acid	9.20–11.68	65–100	5	[112]
	Acetic acid	48	95	5
	Propionic acid	59.20	100	6

, which includes the effective dosage, the Fe oxidation inhibition rate, and the price of preservatives (Kathon, sodium benzoate (SBZ), and. triclosan), anionic surfactants (sodium dodecyl sulfate (SDS)), cationic surfactants (cetyltrimethylammonium bromide (CTAB)), and low-molecular-weight organic acids (formic, acetic, and propionic acids).

3.5. Coal Oxidation Inhibitors

As described in Section 2.3, the low-temperature oxidation process of coal plays an important role in the pre-stage heat storage of coal gangue piles. Therefore, oxidation inhibitors that can inhibit the oxidation of coal are crucial for in situ restoration. According to their properties, oxidation inhibitors can be divided into inorganic and organic inhibitors, and their mechanisms are mainly water absorption and free radical reaction blocking. The classification, representative substances, and mechanisms of action of coal oxidation inhibitors are shown in

Table 4. Descriptions and inhibition mechanisms of coal oxidation inhibitors.

Description	Name	Mechanization	Reference
Inorganic inhibitors	MgCl2	Water absorption.	[118]
	CaCl2
	NaCl
	Na3PO4	Influencing the pathway of hydroxyl decomposition, promoting its conversion to ether, and improving thermal stability.	[114]
Organic inhibitors	HTY	(1) Reacting with highly active free radicals, such as ·OH and ·OOH, to produce lowly active polyphenol free radicals, cutting off the chain reaction of free radicals. (2) Two hydroxyl groups on the HTY molecule interacting to form hydrogen bonds (O18-H19 is the hydrogen bond donor), and the resulting o-diphenol structure clearing hydroxyl radicals through the H transfer mechanism.	[119,120]
	SOD	ROO· is rapidly quenched at room temperature by disproportionation (giving electrons to generate H2O2), terminating the chain propagation reaction, and restraining the generation of free radicals, such as ·OOH, CHO·, and ·OH.	[121,122,123]
	CAT	Rapidly neutralizing superoxide free radicals and decomposing hydrogen peroxide into harmless molecules.	[121]
	Cu-SOD	Forming cyclization reaction and eliminating ROO·.	[121,122,123]
	Zn-SOD
	Mn-CAT
	Polyethylene glycol	Inhibiting the formation of surface functional groups during oxidation.	[113]
	Tea polyphenol	Inhibiting peroxide free radicals.	[124]
	Acrylic acid plus ascorbic acid	Preventing free radical chain reaction.	[125]
	Diphenylamine	/	[126]
	Tetramethyl piperidine	Binding with free radicals to form inactive substances.	[117]

Note: HTY = Hydroxytyrosol (3,4-Didperoxylphenylethanol, DPE, C₈H₁₀O₃); SOD = Superoxide dismutase; CAT = Catalase.

, among which the inorganic inhibitors mainly include CaCl₂, NaCl, and Na₃PO₄. MgCl₂, CaCl₂, and NaCl have a short duration and minimal efficiency [113], but the inhibition rate of Na₃PO₄ on coal oxidation can reach 50% at 265 °C [114]. The organic inhibitors mainly include HTY [115], SOD [116], CAT, polyethylene glycol (coal oxidation reduced by 75%~83%) [113], tea polyphenol, acrylic acid plus ascorbic acid, diphenylamine, and tetramethyl piperidine (coal oxidation reduced by 73.08%) [117]. Moreover, taking the economic cost into account, industrial wastes, such as gypsum and fly ash, are considered to have excellent performance in inhibiting the oxidation of coal [118].

3.6. Microorganisms

Biological source treatment (BST) technology, which prevents AMD by stimulating the growth of reducing bacteria in situ, is considered more promising. In recent decades, the most widely studied and representative microorganisms have been iron-reducing bacteria (IRB) and sulfate-reducing bacteria (SRB) [127,128,129,130].

IRB and SRB both take organic carbon as the carbon source, decompose and mineralize the organic carbon to produce electrons, and transfer the electrons to the extracellular space through direct contact, electron shuttle, and conductive appendages [131]. High-valent irons, which are considered strong oxidizing substances, are reduced as electron acceptors by IRB [132], inhibiting the oxidation in coal gangue piles. SRB uses sulfate as an electron acceptor to conduct SRB reduction metabolism, consuming weak alkaline substances and producing strong alkali. SRB can adjust the pH value by neutralizing acid production, and it inhibits the growth of oxidizing bacteria simultaneously though changes in the environment.

SRB, which achieved success in polluted sites within two years [133], has been a proven microorganism for the remediation of acid coal gangue piles. After SRB treatment, the pH value of leaching water tends to be stable in neutral, while the concentration of Fe and SO₄²⁻ can reach the pollution-free level in 7–14 days. Moreover, SRB has a good removal effect for heavy metals, with the removal rates of Cd, Cu, Zn, and other metals greater than 99%, and the removal rate of Ni greater than 87%. The research on IRB has mainly focused on the regulation of the synergy between IRB and T. f, constructing the new dynamic balance between Fe²⁺ and Fe³⁺ in pore water and iron among coal gangue minerals, and inhibiting the acid production in coal gangue piles [132].

3.7. Plants

Planting technology is considered to be a key link in the pollution control and ecological restoration of coal gangue piles. The construction of a topsoil layer combined with vegetation planting, which is constructed on a coverage barrier, can not only prevent dust, beautify the landscape, and provide a carbon sink, but also the growth and reproduction of plants can inhibit the oxidation and acid production of coal gangue through the following three mechanisms: (1) After the construction of the topsoil layer, pioneering herbaceous plants can quickly establish a herbaceous cover, which is an aerobic layer to reduce the diffusion of oxygen to the deep layers by consuming oxygen. Therefore, the oxidation of sulfides and the reproduction of aerobic microorganisms will be slowed down or even stopped by the respiration of the plant roots and the decomposition of organic matter in the topsoil layer [134]. (2) Atmospheric precipitation is consumed by plant transpiration and topsoil evaporation, maintaining the specific water content of the topsoil layer, so that the oxygen and water isolation effect of the coverage barrier is given full play. (3) Plants and the topsoil system can inhibit the erosion and prolong the service life of the coverage barrier (the plant roots do not penetrate the coverage barrier), thereby slowing down the weathering of coal gangue and reducing the production of porous materials, inhibiting oxidation [135].

4. Summary and Prospect

Acid production resulting from the oxidation of coal gangue piles presents a significant global ecological and environmental challenge, with severe adverse impacts on the living environment and human health in mining areas. Currently, there are well-established and large-scale technologies available for treating AMD, including neutralization, precipitation, adsorption, ion exchange, membrane technology, biological treatment, and electrochemical technology. While these technologies offer flexibility and a high load capacity, their application to long-term pollutants with low concentration found in coal gangue piles might be costly and unsustainable. Therefore, in situ remediation represents a more cost-efficient and resource-effective approach, which is imperative for major coal-producing countries worldwide.

The fundamental prerequisite for preventing and controlling acid pollutants at the source lies in acquiring a profound understanding of the oxidation mechanism of coal gangue piles. Over the course of several decades, research on the oxidation mechanism of coal gangue has been abundant, encompassing pyrite oxidation, microbial action, a low-temperature oxidation of coal, and free radical action. Pyrite and low-grade coal are pivotal components in the oxidation reaction within coal gangue. Meanwhile, water, oxygen, microorganisms, and free radicals constitute essential conditions for the oxidation process. In terms of pyrite, in oxidation reactions specifically initiated by water and oxygen, Fe²⁺, Fe³⁺, and ·OH serve as key elements in cyclic pyrite oxidation reactions, and T. f and T. t primarily act as microbial catalysts to significantly enhance the rate of pyrite oxidation. Coal predominantly contributes to self-heating incubation stages within coal gangue, while carbon free radicals, methylene groups, and oxygen initiate the oxidative reactions in coal. Peroxide free radicals represent critical intermediates during the oxidation process of coal, while aliphatic free radicals, aldehydes, and carboxylic acids participate extensively throughout most reaction processes. Chemical adsorption, the generation of intermediate complexes, and the decomposition of unstable oxygen-containing intermediates in coal oxidation contribute substantially to heat production during the early self-heating incubation stages.

Based on the oxidation mechanism of coal gangue, the current in situ remediation technologies, which encompass mixed co-disposal, coverage barriers, passivation, bactericides, coal oxidation inhibitors, microorganisms, and plants, primarily focus on four essential conditions of oxidation reactions (oxygen, water, microorganisms, and free radical). Currently, coverage barriers and plant technologies are widely employed at demonstration sites. The application of mixed co-disposal technology is limited to historical residual piles due to the need for pre-planning before storing coal gangue. Environmental safety concerns regarding passivation, bactericides, and coal oxidation inhibitor technologies require further consideration due to the addition of reagents into gangue piles. Microbial technology has not yet been widely used due to its difficulties in on-site proliferation, effects control, and ensuring its sustainability and stability.

During large-scale remediation in coal gangue piles, it is crucial to identify key links based on oxidative mechanism. Limitations associated with single approaches could be overcome by optimizing and coupling various remediation technologies according to the local conditions based on key links, avoiding a simplistic superposition of techniques. Finally and prospectively, based on successful experience, it is essential to systematically establish an in situ restoration technology model for the application and innovation of restoration technology in acidic coal gangue piles.