Computational Chemistry Insights into Pollutant Behavior During Coal Gangue Utilization

Abstract

Coal serves as the primary energy source for China, with production anticipated to reach 4.76 billion tons in 2024. However, the mining process generates a significant amount of gangue, with approximately 800 million tons produced in 2023 alone. Currently, China faces substantial gangue stockpiles, characterized by a low comprehensive utilization rate that fails to meet the country’s ecological and environmental protection requirements. The environmental challenges posed by the treatment and disposal of gangue are becoming increasingly severe. This review employs bibliometric analysis and theoretical perspectives to examine the latest advancements in gangue utilization, specifically focusing on the application of computational chemistry to elucidate the structural features and interaction mechanisms of coal gangue, and to collate how these insights have been leveraged in the literature to inform its potential utilization routes. The aim is to promote the effective resource utilization of this material, and key topics discussed include evaluating the risks of spontaneous combustion associated with gangue, understanding the mechanisms governing heavy metal migration, and modifying coal byproducts to enhance both economic viability and environmental sustainability. The case studies presented in this article offer valuable insights into the gangue conversion process, contributing to the development of more efficient and eco-friendly methods. By proposing a theoretical framework, this review will support ongoing initiatives aimed at the sustainable management and utilization of coal gangue, emphasizing the critical need for continued research and development in this vital area. This review uniquely combines bibliometric analysis with computational chemistry to identify new trends and gaps in coal waste utilization, providing a roadmap for future research.

1. Introduction

The current low-carbon energy transition is at a pivotal moment within the framework of carbon peaking and carbon neutrality goals. Energy activities are the leading source of global CO₂ emissions, with coal contributing a substantial portion [1]. Coal gangue, a byproduct of coal mining and washing, is generated in large quantities. Its effective resource utilization is essential for environmental protection and resource conservation as shown in Figure 1 [2]. The accumulation of coal gangue presents challenges such as secondary contamination and environmental degradation, which necessitate further research into its effects [3]. There is a growing interest in the recycling of coal gangue for various applications, including construction materials [4,5,6], energy generation [7,8], and soil improvement [9,10,11], all of which can help mitigate its environmental impact [12]. Despite its potential, the current utilization rate of coal gangue remains low, primarily due to considerable barriers to effective recycling, including the need for pre-treatment methods [13]. Furthermore, the management of coal gangue is complicated by its inherent properties, such as alkalinity and HM content, which pose risks during transportation and storage [14]. There is a notable lack of comprehensive strategies for the global management of coal gangue, highlighting the urgent need for innovative approaches and policies to address its disposal and utilization effectively.

Studies have shown that coal gangue contains a certain amount of useful components, such as coal, kaolin, bauxite, etc. [15]. These components can be extracted through advanced separation and conversion technologies and used to produce a variety of high-value-added products. At present, a variety of ways to utilize coal gangue resources have been proposed, such as preparing calcined kaolin, power generation, ceramic microbeads, fiber materials, and silica aerogels, as well as innovative applications such as extracting alumina, white carbon black, producing fertilizers and soil conditioners [16,17,18]. Most recently, research on the fine separation and efficient conversion of abundant silicon, aluminum, iron, and rare metal elements in gangue has gradually become popular [19,20]. However, although these solutions held great potential for coal gangue utilization, they are often limited by problems such as difficulty in exploring reaction mechanisms, low technical maturity, and secondary environmental pollution. To substantially realize the multi-dimensional and high-value utilization of gangue, it is necessary to accurately explain the physical and chemical reactions and corresponding mechanisms in the process of gangue resource utilization.

Computational chemistry methods such as molecular simulations, density-functional theory (DFT), and ab initio approaches are crucial for analyzing the behavior of molecules during the reaction and the transformation of forms before and after the reaction, which is essential for addressing challenges related to coal utilization and environmental impact. Molecular simulations, including molecular dynamics and Monte Carlo simulations, are employed to model the behavior of gases like CO₂ within coal gangue structures under varying temperature and pressure conditions [21]. In addition, molecular dynamics simulations have been employed to study the adsorption characteristics of water molecules in coal gasification fine slag, elucidating the impact of pore structures on flotation efficiency [22]. The combination of molecular dynamics simulations with flotation techniques has established a theoretical foundation for improving the separation processes and resource recovery from coal waste [20]. Quantum chemical calculations are known for their accuracy and efficiency, offering significant advantages in the establishment of molecular models, the prediction and analysis of chemical reactivity, the simulation of reaction pathways, and the forecasting of potential excited and transition states [23,24]. Quantum chemistry research makes a significant contribution to our understanding of complex coal systems and their chemical behaviors, especially in processes such as gasification and liquefaction. The application of quantum chemistry has uncovered fundamental attributes of coal, including its electronic properties and reactivity, which are crucial for optimizing resource utilization [16,25]. The advent of GPU-enabled reactive molecular dynamics (ReaxFF MD) has facilitated the simulation of large-scale coal models, yielding insights into coal pyrolysis mechanisms and product profiles. These approaches not only enhance our comprehension of coal chemistry but also pave the way for innovative solutions to optimize resource extraction and waste management within the coal industry.

This review employs bibliometric analysis to first map emergent research hotspots in coal gangue utilization and then couples these findings with computational chemistry perspectives, principally DFT, to dissect the key structural motifs and interaction mechanisms that dictate reactivity. We also address the challenges and opportunities associated with coal gangue utilization, highlighting the necessity of a synergistic approach that integrates theoretical predictions with experimental validation. The review concludes with recommendations for future research, emphasizing the development of sustainable and economically viable pathways for coal gangue utilization, informed by the insights derived from bibliometric analysis and computational chemistry. This integrated approach advances the science and technology of coal gangue resource utilization, contributing to a circular economy and furthering sustainable development goals.

2. Bibliometric Analysis of Coal Gangue

The methodology of bibliometric analysis applied to coal gangue research entails systematic data collection and analysis, which unveils trends in publication, research focus, and the environmental implications of coal gangue utilization. Lin et al. [26] provided a macroscopic trend map (country, journal, keyword) in the comprehensive utilization of coal gangue, while our work delved into the detailed mechanisms and linked several key hotspots with specific computational chemistry methods.

Data were sourced from the “Web of Science” Core Collection, covering the period from 2000 to 2024. A search for the term “coal gangue” resulted in 2998 documents, predominantly articles, which accounted for 82.09% of the total, followed by meeting papers at 13.81% and review articles at 2.43%. The combination of “coal gangue” and “utilization” yielded 579 documents, where articles made up the majority at 94.30%, with conference papers comprising 8.12% and review articles 5.87%. Other document types represented a negligible fraction. As illustrated in Figure 2, there has been a significant increase in the number of published articles on coal gangue. Notably, the annual publication volume related to coal gangue utilization reflects a sustained and growing interest in discovering innovative and fundamental solutions for the effective management and utilization of this waste material.

CiteSpace was also employed for visual analysis and to map scientific knowledge. By clustering keywords, keywords with close relationships among the many keywords are grouped into one category for scholars to analyze and study. CiteSpace offers three algorithms: Latent Semantic Indexing (LSI), Log-Likelihood Ratio (LLR), and Mutual Information (MI). In this work, the LSI algorithm was utilized. Running CiteSpace resulted in keyword clustering; the modularity value Q (Modularity Q) is greater than 0.3, and the average silhouette value S (Weighted Mean Silhouette) is greater than 0.7 [27]. By analyzing these co-occurrence networks of key terms, researchers can gain insights into the knowledge structure and research frontiers of the coal gangue research field, identify the evolution of research hotspots across different years, and obtain the current research hotspots and development trends, as well as understand the interdisciplinary and cross-disciplinary integration in the field of coal gangue utilization.

As illustrated in Figure 3, Figures S1 and S2, the scientific knowledge maps illustrate the co-occurrence relationships and developmental trends of key terms in coal gangue research. The chart in Figure 3 employs color coding to distinctly identify 13 core research themes, each marked by a unique color and number, thereby facilitating easy recognition and analysis. The size of the nodes reflects the frequency of key terms in the literature, while the lines between nodes indicate co-occurrence relationships, with the thickness of the lines underscoring the strength of these connections. For instance, “coal gangue” as a central theme is represented by pink nodes that persist along the timeline. Furthermore, the chart reveals dynamic changes in research focus. Earlier research priorities such as “spontaneous combustion,” “heavy metals,” and “kaolinite” are depicted as larger nodes, indicating their enduring significance in the field [28]. The structural changes during calcination, such as dehydroxylation and sintering, significantly enhance the reactivity of coal gangue, making it suitable for use in construction materials [29,30]. Calcination desiliconization is conducted to synthesize molecular sieve products to realize high-value resource utilization of coal gangue [31].

As illustrated in Figure 4, the timeline on the map extends from 2000 to 2024, providing insights into the evolving research interests and advancements in the utilization of coal gangue. The terms range from “co-combustion” to “comprehensive utilization,” reflecting a multifaceted approach to managing and utilizing coal gangue. Notably, the inclusion of terms such as “mechanical properties” and “compressive strength” highlights the focus on the material characteristics of coal gangue and its potential applications in construction. The prominence of “ecological restoration” indicates a rising awareness of the environmental impacts associated with coal gangue, as well as the development of strategies to address these issues. Additionally, the mention of “solid waste” and “fly ash” situates coal gangue within the broader context of waste management, emphasizing its potential role in contributing to sustainable solutions.

These trends highlight the innovative approaches to repurpose coal gangue and related byproducts for sustainable applications, addressing both environmental concerns and resource efficiency. Coal gangue is activated to highly active Si-Al species for the synthesis of fluidized catalytic cracking (FCC) catalysts [32]. Coal gangue is being utilized in the production of composite cementitious materials, enhancing waste utilization and providing thermal insulation properties [33,34]. Magnetic catalysts derived from coal gangue have exhibited remarkable catalytic properties for the removal of pollutants, highlighting their potential applications in environmental cleanup [35,36,37,38]. Coal gangue has been successfully synthesized into sodalite, which demonstrates effective adsorption capabilities for HM ions and dyes [39]. The adsorption processes of Cd²⁺ ions and methylene blue by sodalite follow the Langmuir model and pseudo-second-order kinetics, indicating a strong interaction between the adsorbate and adsorbent [40].

In particular, the process of “co-combustion” and “roasting activation” involves complex chemical reactions and pathways that are crucial for efficient coal resource utilization. The relationship between the chemical structure and the thermal reactivity is essential for predicting behaviors of “co-combustion” and “roasting activation”. For example, ReaxFF molecular dynamics simulations provide a powerful tool for investigating the kinetics of coal pyrolysis at the atomic level, allowing for the detailed analysis of reaction mechanisms and product formation [41]. Large-scale molecular models, such as those for Illinois no. 6 coal, have been utilized to study structural modifications and the effects of heteroatoms like sulfur on reaction kinetics during pyrolysis [42]. The integration of simulation results with experimental data enhances the understanding of coal thermal chemistry and aids in the development of more efficient coal utilization strategies [43]. In delving into the theoretical research of coal gangue utilization, the application of computational chemistry is particularly crucial. The insights from computational chemistry encompass an in-depth analysis of chemical structure and reactivity, while also addressing environmental concerns associated with coal gangue utilization, ensuring sustainable development in this field.

In the subsequent sections of this paper, we will elaborate on the close connection between coal gangue utilization and computational chemistry. By exploring the simulation of coal gangue’s spontaneous combustion risk, the mechanism of HM migration, and the modification of coal byproducts, we can comprehensively enhance the efficiency and safety of coal and its byproducts management. This collection of research themes not only enriches our theoretical understanding of coal gangue utilization but also provides a solid scientific foundation for achieving sustainable management of coal gangue.

3. Theoretical Insights into Key Hotspots Identified by Bibliometric Analysis

Guided by the bibliometric findings presented in Section 2, this review concentrates on three pivotal research hotspots: the spontaneous combustion mechanism of coal gangue, heavy metal migration and solid-phase control, and surface modification/activation for enhanced utilization. It critically examines molecular simulations and quantum chemical studies published in the past years, distilling key computational models, dominant reaction mechanisms, and their quantitative implications for resource-oriented pathways. The resulting theoretical analysis is intended to furnish a systematic reference for subsequent experimental validation and multi-scale technological integration.

3.1. Spontaneous Combustion Risk of Coal Gangue

The spontaneous combustion of coal gangue piles results in a compromised structure and diminished stability, consequently leading to significant disasters such as landslides and collapses. These events pose serious threats to both the surrounding environment and the safety of personnel. The phenomenon of spontaneous combustion in coal gangue has emerged as a prominent area of research due to its recurrent nature and the considerable hazards it entails. This process involves a complex interplay of physicochemical changes, influenced by a range of factors categorized as internal and external. Internal factors pertain to the physical properties and the inherent combustibility tendencies of the coal gangue. In contrast, the principal external factor is the permeability of the gangue piles, which governs the accessibility of oxygen to the coal gangue and plays a crucial role in facilitating spontaneous combustion [44,45,46].

Extensive studies were conducted on permeability and diffusion characteristics using quantum chemistry and molecular simulation methods [47]. According to the coal–oxygen complex principle [48], the spontaneous combustion of dispersed coal gangue is a process of oxygen adsorption by coal gangue. Coal gangue is a porous material with a large specific surface area and many active sites on its surface, which have a strong adsorption capacity for oxygen. As oxygen diffuses toward the surface particles of the gangue, it is continuously absorbed, leading to the release of heat through physicochemical reactions. This heat accumulation can ultimately trigger the spontaneous combustion of coal gangue piles [49]. To mitigate the risk of spontaneous combustion in coal gangue hills, Wang et al. [50] investigated the adsorption, diffusion, and seepage behaviors of oxygen within the slit structures of coal gangue. Molecular dynamics (MD), grand canonical Monte Carlo (GCMC) simulations, and finite-element analyses were employed to quantify the influence of temperature and pressure on O₂ behavior at the pore scale (Figure 5). The findings reveal that increased pressure enhances oxygen adsorption and decreases the diffusion coefficient, while elevated temperatures have the opposite effect. At 10 MPa, the O₂–gangue system exhibits the largest variation in total energy, underscoring pressure as the dominant thermodynamic driver for both adsorption and diffusion. Below 423.15 K, physisorption prevails; the concomitant exposure of kaolinite surfaces amplifies O₂ uptake, thereby accelerating oxidation and increasing the probability of self-ignition. The oxygen percolation process was further resolved into four sequential stages governed by (i) slit-channel geometry, (ii) gas diffusivity, and (iii) the intrinsic oxygen consumption rate of the gangue. Molecular simulations using Materials Studio and COMSOL software further elucidate the adsorption–diffusion behavior and microscopic pore seepage characteristics of coal gangue. Collectively, the findings indicate that pressure regulation is more effective than temperature control in suppressing spontaneous combustion of coal gangue, thereby providing a theoretical basis for proactive prevention and management strategies.

GCMC and MD simulations are also widely used to study the adsorption and diffusion behavior of gases in coal. Huang et al. [51] focused on the adsorption and diffusion mechanisms of CO₂ and CH₄ during carbon dioxide-enhanced coalbed methane (CO₂-ECBM) recovery in high-volatile bituminous coal, whereas Gao et al. [21] examined the kinetics of the same process within anthracite pores. Consistent with thermodynamic expectations, both studies reported a preferential adsorption of CO₂ over CH₄, indicating that CO₂ exhibits a higher affinity for the coal matrix. Moreover, the diffusion coefficients of CO₂ and CH₄ decrease with the increase in pressure and increase with the rise in temperature. Wang et al. [52] conducted GCMC and MD simulations to investigate the CO₂ and N₂ adsorption behaviors of three bituminous coal molecular models—LL (C₁₈₆H₁₅₃NO₆), PDS-1 (C₂₀₇H₁₆₅NO₁₁), and XZ (C₁₈₁H₁₆₃NO₆)—representing a systematic gradient in volatile matter content. The results show that the adsorption capacity and rate of CO₂ by bituminous coal molecules are both greater than those of N₂. The ultimate adsorption capacity of low-volatile bituminous coal is the largest and decreases with the increase in volatile matter. The adsorption heat of both gases is less than 40 kJ/mol, which means they are not sensitive to changes in pressure and temperature and are more likely to be adsorbed near oxygen-containing functional groups and benzene rings. The diffusion capacity of CO₂ and N₂ in coal is greater than that of CO₂, as CO₂ has a stronger adsorption capacity. The application of molecular simulation methods to explore the influence of different coals on the gas adsorption law has positive significance for injecting inert gas for fire prevention and extinguishing in coal mine goaf areas.

The differences in permeability of various gases are not only dependent on the diffusion properties of the gases themselves but also on the structure and type of the permeable material. Utilizing the Wuhai mining area as a reference point, Wang et al. [53] quantified the permeation behavior of five gas species in coal and coal gangue models under variable environmental conditions through grand canonical Monte Carlo (GCMC) simulations coupled with molecular dynamics (MD). The coal model featured a complex molecular structure of bituminous coal, incorporating cycloalkanes and methyl groups, while the kaolinite model represented the coal gangue. A 1nm slit model was created, and ten molecules each of SO₂, NO₂, O₂, N₂, and O₃ were introduced into both the coal molecular model and the kaolinite model, thereby forming a material–gas system, as depicted in Figure 6. Adsorption simulations conducted under varying pressures using a gas permeation model demonstrated that, at 298K, an increase in pressure significantly influences gas permeation through the coal gangue slit model. Based on the calculations of continuous isodensity and potential energy of five gas molecules O₂, N₂, SO₂, NO₂, and O₃ in the coal model and the coal gangue slit model at the temperature of 298 K, the blue areas in Figure 6 represent the low-energy regions of gas molecule adsorption, while the red areas represent the high-energy regions of gas adsorption. The larger the display area and the darker the blue zone, the more adsorption sites there are and the lower the energy required, making it easier for gas molecules to adsorb. Conversely, the smaller the displayed area and the darker the red zone, the less likely the gas molecules are to be adsorbed. By analyzing the isosteric heat of adsorption in conjunction with potential energy and the isodensity surface distribution area, it was observed that five types of gas molecules display a consistent adsorption capacity pattern in both coal and coal gangue, ranked as SO₂ > O₂ > O₃ > N₂ > NO₂. The energy distribution at adsorption equilibrium further confirmed these findings, indicating that the adsorption capacity for SO₂ surpasses that of the other gas molecules. Additionally, when examining the effect of temperature on the permeation coefficient, the diffusion coefficient, solubility coefficient, permeation coefficient, and isosteric heat of adsorption were assessed across different temperatures. The results revealed that while SO₂ molecules exhibit the weakest diffusion capability, they possess the highest solubility, reinforcing the established pattern. Increasing the temperature aids in the permeation and diffusion of gases, with the rate of permeation being contingent upon the physicochemical properties of the model’s surface, and the adsorption of most gas molecules is primarily physical. In the study of the effects between gas molecules, the gas molecules in the coal model were divided into three groups for comparative study: O₂ + N₂ (ambient air), SO₂ + NO₂ + O₃ (pollutant gases), and O₂ + N₂ + SO₂ + NO₂ + O₃ (mixed gases). It was discovered that the permeation and diffusion of gas molecules in coal and coal gangue are influenced by their mixed components. In a mixed gas system, there is a synergistic effect on the diffusion of gas molecules and a hindering effect on adsorption. Moreover, the more components there are in the mixed gas, the stronger the permeation effect of each gas molecule.

Current investigations of coal gangue are predominantly limited to macroscopic experiments or isolated simulations of kaolinite; consequently, the coupled influence of organic carbon and kaolinite on gas adsorption and spontaneous combustion remains insufficiently characterized. Zhang et al. [54] offer a comprehensive analysis of CO gas dynamics within coal gangue slits, highlighting the critical role of coal’s carbon content in gas adsorption and diffusion processes. As shown in Figure 7, a novel kaolinite–coal–kaolinite (KCK) model was constructed by integrating the Hongqingliang (HQL) coal macromolecular framework with the Bish kaolinite structure to accurately represent the fracture architecture of coal gangue. Molecular dynamics (MD), reactive force-field (ReaxFF), grand canonical Monte Carlo (GCMC), and conventional Monte Carlo (MC) simulations were integrated to probe gas adsorption at the atomic scale. The Freundlich isotherm was subsequently employed to quantify adsorptive capacity; the kaolinite–coal–kaolinite (KCK) model exhibited a CO adsorption capacity 110.4–132.9% higher than that of the reference tri-kaolinite (TriK) framework. The simulation results indicate that CO production is intricately linked to the decomposition of ether and phenol within the organic structure of coal, with reduced oxygen levels accelerating CO generation. The KCK model’s carbon layer provides additional adsorption sites, leading to a higher adsorption capacity and a more accurate reflection of gas dynamics within the gangue. The study also finds that the diffusion coefficient of CO in the KCK model is lower than in the TriK model, with temperature playing a significant role in influencing CO diffusion rates. By integrating these theoretical calculations, the research provides a robust framework for understanding the adsorption and diffusion of CO in coal gangue. The findings are pivotal for managing the risks associated with spontaneous combustion in coal waste piles and for developing strategies to enhance coal mine safety and waste management practices. The comprehensive approach of this study, combining theoretical models with practical implications, contributes to the field’s knowledge base and informs future research directions in coal gangue management. While DFT and MD studies excel in characterizing initial adsorption, a critical gap persists in modeling large-scale pile combustion dynamics. The integrated model is needed currently to bridge quantum-scale reactions to macro-scale heat/mass transfer, hindering risk prediction capabilities.

3.2. Mechanisms of HM Migration in Coal Gangue

Coal gangue is enriched with various HM ions. During prolonged stockpiling, these ions can be progressively mobilized and transported into adjacent soils and groundwater, leading to their accumulation in mining districts and subsequent exceedances in downstream surface waters [55,56]. The attendant risk of diffuse secondary contamination threatens local ecosystems and undermines prospects for the sustainable valorization of coal gangue. Consequently, elucidating the environmental fate and ecological impacts of HM release from coal gangue is imperative for its safe reuse.

Traditional extraction methods provide empirical measurements of HM speciation, without quantifying binding affinities, thereby constraining the reliability of long-term risk assessments. Dong et al. [23] investigated the release and migration behaviors of six HMs (Cd, As, Pb, Ni, Cu, and Cr) using a combined experimental and computational approach, as shown in Figure 8. It was found that they exhibited significant differences in speciation distribution and binding energy but underwent similar release behaviors. Coal gangue is predominantly composed of kaolinite, α-quartz, and anatase. Sequential extraction shows that heavy metals (HMs) are primarily sequestered within stable silicate phases and Fe–Mn oxides. Toxicity characteristic leaching procedure (TCLP) analysis classifies Cu, Cr, Ni, and Pb as highly toxic; consequently, continuous monitoring and containment are warranted. Meanwhile, the initial concentration of HMs in CG plays a dominant role in short-term release and risk, while the presence state also significantly impacts migration and long-term risk. To elucidate the migration pathways of heavy metals (HMs) in coal gangue, periodic density-functional theory (DFT) calculations were performed to quantify binding energies (E_b) and preferred adsorption sites on the principal mineral constituents—kaolinite, α-quartz, and anatase. Structural models of each mineral were constructed, and the six target HM cations were placed at distinct surface positions to evaluate E_b. Across all surfaces, Cr³⁺ and As³⁺ exhibited the highest E_b, followed by Cu²⁺ and Ni²⁺, whereas Pb²⁺ and Cd²⁺ displayed the weakest interaction. On the kaolinite (0 0 1) surface, coordination to surface hydroxyl groups dominates HM binding, whereas inner-surface hydroxyls contribute negligibly. For α-quartz (0 0 1), electrostatic attraction between HMs and the bridging oxygen atoms of SiO₂ tetrahedra is the primary interaction. Adsorption on the anatase (0 0 1) surface is governed by a combination of van der Waals forces and electrostatic interactions. In addition, silicate frameworks can immobilize HMs through lattice incorporation and precipitation, further enhancing stability. Theoretical predictions corroborate sequential extraction data: HMs exhibiting the highest E_b (51.95, 46.36, 43.22, and 35.90 eV) display the lowest proportion of readily mobilized fractions (2%).

Coal gangue is rich in kaolinite, and kaolinite, as a common mineral adsorbent, is effective in reducing the hazards of HM ions [57,58]. Zhao et al. [59] conducted a theoretical study on the adsorption of HMs Cd, Cu, Hg, and Ni(II) on the kaolinite (0 0 1) surface using DFT calculations. The adsorption of Cd(II), Cu(II), Ni(II), and Hg(II) on the kaolinite (0 0 1) surface was systematically examined across coverages θ = 0.11–1.0 ML (Figure 9). Cd(II) preferentially occupies two-fold bridge sites, followed by one-fold top sites; Cu(II) and Ni(II) favor the top sites, whereas Hg(II) adopts three-fold hollow positions. Adsorption energies (Eads) increase monotonically with θ for Cd, Cu, and Hg, indicating stabilizing lateral interactions and island formation. Conversely, Eads for Ni(II) decrease at higher θ, consistent with repulsive Ni–Ni interactions. The intrinsic affinity sequence is Ni(II) > Cu(II) > Cd(II) > Hg(II). Partial density of states (PDOS) analyses reveal pronounced charge redistribution, confirming the formation of both ionic and covalent bonds between the adsorbates and surface oxygen atoms. The findings provide a deeper understanding of the microscopic adsorption mechanisms of HM atoms on the kaolinite surface and contribute to the development and optimization of kaolinite as an adsorbent for environmental remediation applications.

In the coal or coal gangue power generation, HM ions are prone to enrichment in fine particulate matter and escape from pollution control devices into the atmosphere, constituting a significant source of air pollution [60]. Therefore, it is essential to explore the migration issues of HM ions during the combustion process, and kaolinite is commonly added as an ion adsorbent. The computational chemistry strategies provide sustainable management solutions by employing quantum chemistry to explore the mechanisms of interaction between coal gangue and heavy metal species. Cheng et al. [61] employed an integrated experimental–theoretical approach to elucidate the adsorption behavior and mechanistic origins of Pb- and Cd-containing vapors on Si/Al-based sorbents. Adsorbate species—Pb, PbCl, PbCl₂, Cd, CdCl, and CdCl₂—were modeled on the metakaolinite (0 0 1) surface, and adsorption energies together with charge-density-difference analyses were evaluated. Chemisorption was identified as the dominant interaction mode, with Pb-bearing species exhibiting consistently higher adsorption energies than their Cd counterparts. Surface-exposed oxygen atoms and under-coordinated aluminum centers act as primary active sites: Pb/Cd atoms coordinate to surface O sites, while Cl atoms interact strongly with unsaturated Al atoms, thereby facilitating the retention of Pb and Cd chlorides by metakaolinite. Through the application of frontier orbital theory and DFT calculations, Li et al. [62] elucidated the adsorption characteristics and active sites involved in the capture of semivolatile heavy metal elements during coal combustion. The computational models revealed that metakaolin, a thermal transformation product of kaolinite, exhibits stronger adsorption activity than kaolinite itself, with PbO being more readily adsorbed than PbCl₂ onto the kaolinite (DeOH 001) surface. The dehydroxylation process of kaolinite and the subsequent changes in aluminum coordination numbers have been shown to significantly influence the adsorption activity, highlighting the importance of understanding the thermal conversion processes of coal gangue minerals. Yang et al. [63] conducted a comprehensive study on the efficient capture and characteristics of HMs (Pb, Cd, Cr, and Zn) emanating from coal combustion using modified kaolinite as illustrated in Figure 10. They employed a combination of experimental methods and DFT simulations to explore the adsorption behaviors and mechanisms. Utilizing Frontier Molecular Orbital (FMO) analyses, they elucidated the adsorption of HM oxides and chlorides onto original kaolin (OKA), metakaolin (MKA), θ-Al₂O₃, and α-Al₂O₃. Competitive adsorption was observed between oxides and chlorides, with oxides showing a significant advantage. Specifically, for OKA and MKA, the Al-O-Si-H and Al-O-Si surfaces were identified as the most favorable for adsorption. However, it was noted that CdO and CdCl₂ showed minimal adsorption on the Si-O surface of OKA, highlighting the selectivity of the adsorbent surfaces towards different HM species. These theoretical findings offer valuable guidance for the development of modified kaolinite additives designed to enhance the capture of HMs in high-temperature furnaces, thereby contributing to the sustainable management of coal gangue and the mitigation of environmental pollution from heavy metal emissions.

Unburned carbon in fly ash from power generation is also considered a promising adsorbent [64], and theoretical insights from computational chemistry offer strategic advantages in the utilization of this byproduct from coal combustion. Yan et al. [65] conducted a comprehensive density-functional theory (DFT) study of Hg⁰ adsorption on defective carbonaceous surfaces to identify the active sites governing mercury capture. The study systematically modeled various defective structures of carbonaceous surfaces, including those with zigzag and armchair edges, to evaluate their adsorption potential. The DFT calculations revealed that defective carbonaceous surfaces exhibit enhanced chemisorption properties compared to non-defective ones, with significant improvements in adsorption energy observed at surface defects. As shown in Figure 11, the electron localization function (ELF) and electron density difference analyses elucidate the nature of interactions between Hg⁰ and the carbon surface, underscoring the role of vacancies as primary adsorption sites. Furthermore, the study delves into the influence of oxygen-containing functional groups on the adsorption process, finding that semiquinone groups on certain defective surfaces promote chemisorption of Hg⁰. This theoretical work not only advances the understanding of mercury capture mechanisms on carbon materials but also paves the way for the development of cost-effective and environmentally benign strategies for mercury removal from coal combustion flue gas, thereby contributing to the sustainable management of coal gangue.

Hua et al. [66] systematically studied the adsorption process of PbCl₂ on the surface of unburned carbon with different defects and found that the defect adsorption site was the best site for PbCl₂ adsorption. In the investigation of oxidized biochar (OBC) derived from weathered coal as a sorbent for Pb(II), Xu et al. [67] determined that the carboxyl (-COOH) group possesses the lowest adsorption energy for Pb(II) and forms markedly more stable chemical bonds than hydroxyl (-OH), carbonyl (-C=O), or ether (-C-O-C-) moieties. These oxygen-containing functional groups enhance Pb(II) uptake by simultaneously promoting electrostatic attraction and enabling the formation of covalent bonds. Xie et al. [68] probed the Pb(II)–biochar interaction at the molecular level. ZnCl₂-activated biochar exhibited a polyaromatic scaffold (C₄₇H₂₃NO₁₂) with an extensive microporous network and a high density of oxygenated surface groups. Adsorption preferentially occurs at electron-rich oxygen sites, where Pb²⁺ is stabilized by cooperative weak ionic bonding with aldehydic moieties, van der Waals interactions with aromatic rings, and cation–π effects.

Alkaline metals are ubiquitously present across various types of coal, with particularly high concentrations found in certain coals known as high-alkali coals [69,70]. High-alkali coals are commonly blended with Si/Al-rich coals to suppress ash-related fouling and slagging during combustion [71,72]. Kaolinite and its dehydroxylated analogue, metakaolin, have proven highly effective in scavenging alkali metal vapors [73,74,75]. Consequently, kaolinite faces simultaneous exposure to both alkali metals and heavy metals (HMs), mandating an improved mechanistic understanding of competitive adsorption and the rational design of strategies that favor selective HM capture. Luo et al. [76] quantified the influence of alkali metals (Na and Ca) on the adsorption of Cu, Zn, Pb, and Cd by kaolinite during coal combustion. Density-functional theory (DFT) calculations reveal that NaCl and Na₂O suppress HM uptake on the metakaolin (0 0 1) surface, whereas the CaO-modified surface sustains high adsorption capacities. Charge-density analysis identifies Ca as the preferential anchoring site for oxygen atoms of oxidized HM species, thereby stabilizing the adsorbate.

These computational studies not only enhance our understanding of the complex interactions between alkaline metals and HMs on kaolinite surfaces but also provide a theoretical foundation for developing strategies to mitigate the environmental impact of HM emissions from coal combustion. The integration of experimental data with DFT simulations offers a robust approach to guide the sustainable management of coal gangue by elucidating structure–property relationships that inform the selection of conditions for HM adsorption and retention, thereby supporting the development of cleaner coal utilization technologies. With respect to heavy metal migration and solid-phase control, the literature reveals a pronounced pivot from single-metal adsorption studies toward multi-component competitive complexation models incorporating Al-(oxyhydr)oxide surface terminations. However, current DFT-derived complexation energies are overwhelmingly reported for idealized (hydr)oxide slabs, leaving the influence of nanoporous carbon matrices, ubiquitous in weathered gangue, largely unexplored. We pinpoint this omission as an opportunity for reliable risk assessment and stabilization design from computational chemistry.

3.3. Modified Coal Gangue with Enhanced Activity

To enhance the value of coal gangue and promote resource utilization, various modification methods have been extensively studied as shown in Figure 12, including mechanochemical modification [77], acid or alkali treatment [78], surface organic modification [79], calcination [80], hydrothermal modification [81], and composite modification. These methods aim to alter the physicochemical properties of coal gangue, thereby improving its reactivity and stability and expanding its potential applications across various fields. Computational chemistry offers theoretical support and predictive tools for the modification of coal gangue by simulating and calculating the electronic structure of molecules and materials. Furthermore, the application of computational chemistry is also reflected in the assessment of thermodynamic and kinetic stability during the coal gangue modification process, which is crucial for designing and developing novel modification methods.

Calcination modification refers to the process of transforming the low-surface-activity kaolinite in coal gangue into highly active metakaolin through high-temperature calcination. This process alters the porosity and crystal structure within the coal gangue. The extent of calcination modification is primarily influenced by the calcination temperature and duration. Variations in these two key factors lead to different phase transformations in the kaolinite within the coal gangue, resulting in performance disparities in the calcined coal gangue. High-temperature calcination can remove carbon components from the coal gangue and hydroxyl groups from kaolinite, leading to changes in the Si-O and Al-O tetrahedral structures, significantly enhancing the activation level of the coal gangue [82]. After reasonable treatment, it can be transformed into beneficial catalysts and electrode materials [83]. The various adsorption sites on the surface can participate in interfacial reactions to construct materials with high electrochemical activity [84,85]. Sun et al. [86] prepared a coal gangue-derived MoO₂–coal gangue electrocatalyst, which serves as the cathode for lithium–oxygen batteries (LOBs) as shown in Figure 13. The LOB assembled with the MoO₂–coal gangue cathode exhibits excellent rate performance and long-lasting cycling stability. The reason is that the activated coal gangue with an amorphous flake-layer structure acts as a protective layer, optimizing the structural stability of MoO₂–coal gangue, providing ample space for product deposition, and uniformly loading N-doped MoO₂ nanoparticles and coal gangue, which are conducive to optimizing catalytic performance and serving as substrates to ensure structural stability. To clarify the intrinsic properties of MoO₂–coal gangue composites and to assess how N-doped MoO₂ influences the formation pathway of LiO₂ intermediates, periodic density-functional theory (DFT) calculations were conducted. Amorphous SiO₂ present on the gangue surface exhibits semiconducting behavior with a computed band gap of 1.68 eV, whereas N–MoO₂@SiO₂ displays a metallic character and markedly enhanced electronic conductivity. The N-doped MoO₂ nanoparticles accelerate charge transport and increase electrocatalytic activity. Adsorption of LiO₂ is pivotal for Li₂O₂ formation/decomposition and overall Li–O₂ battery (LOB) performance; N–MoO₂@SiO₂ affords an adsorption energy of −2.84 eV, substantially exceeding that of pristine SiO₂ (−1.78 eV). This stronger LiO₂ anchoring promotes LiO₂ oxidation and improves both oxygen reduction (ORR) and evolution (OER) kinetics during cycling. The N–MoO₂ nanoparticles thus enhance electronic conductivity and LiO₂ adsorption, while the amorphous SiO₂ substrate stabilizes the composite architecture. This work provides a new route for valorizing mining solid waste as electrocatalysts for energy storage systems and advances the sustainable utilization of coal gangue.

Coal gangue primarily consists of kaolinite, quartz, and other minor minerals such as illite and calcite, which affect its reactivity [87]. The presence of high alumina content in coal gangue is linked to the transformation of kaolinite into metakaolinite, which is crucial for enhancing chemical reactivity [88]. Different types of coal gangue exhibit varying mineral compositions, which can influence their physical properties and reactivity [89]. Kaolinite has attracted widespread attention due to its excellent surface properties, such as regular contours, abundant adsorption sites, and rich hydroxyl groups. Zhang et al. [90] explored the utilization of coal gangue for the activation of potassium monopersulfate (PMS) to degrade tetracycline hydrochloride (TC). The research underscores the pivotal role of CG’s surface-bonded and structural hydroxyl groups in facilitating the adsorption of PMS and the generation of active species, which are crucial for TC degradation. DFT results indicate that peroxymonosulfate (PMS) exhibits the highest adsorption affinity for coal gangue-derived carbon (CG) among the tested oxidants—sodium persulfate (PS), potassium persulfate (PDS), and hydrogen peroxide (H₂O₂). Fukui-function analyses further identify the preferential electrophilic, nucleophilic, and radical attack sites on tetracycline (TC) molecules. CG activates PMS, PS, PDS, and H₂O₂ for TC degradation without external energy input, underscoring the broad applicability of the CG/PMS system in pollutant remediation. In addition, Dong et al. [91] presented a comprehensive study on the photocatalytic potential of coal gangue (CG) for antibiotic degradation, in which theoretical insights gained from DFT and quantitative structure–activity relationship (QSAR) models highlight the significance of superoxide anion (^⁎O²⁻) and electron–hole (e⁻-h⁺) pairs in TC degradation. The research further proposes that oxygen vacancies on CG’s surface and the presence of hydroxyl groups contribute to the enhanced production of ^⁎O²⁻ and facilitate electron–hole separation, respectively. These works contribute to the sustainable management of coal gangue by transforming a waste material into a catalyst for advanced oxidation processes (AOPs).

Surface functionalization of mineral-based composite catalysts derived from coal gangue offers a promising route for sustainable pollutant degradation. Li et al. [92] examined the interfacial coupling between TiO₂ and kaolinite and the influence of surface-bound hydrogen, using a CH₃COOH-functionalized TiO₂/kaolinite model (Figure 14a). Both the pristine and CH₃COOH-functionalized composites exhibit a direct band gap, which facilitates enhanced charge carrier mobility and accelerates reaction kinetics. The introduction of hydrogen altered the orbital constitution, increasing the proportion of d and f orbitals, which is favorable for electron transition or migration. Zhang et al. [93] focused on the hybrid catalysts (CN-CGs) derived from coal gangue and graphitic carbon nitride (g-C₃N₄), as shown in Figure 14b, which are designed to activate peroxymonosulfate (PMS) for the degradation of bisphenol A (BPA). The introduction of double-nitrogen defects, cyano groups, and nitrogen vacancies into the g-C₃N₄ framework, facilitated by the hydroxyl groups of CG, results in a narrowed band-gap energy and enhanced catalytic activity. By leveraging the properties of kaolinite and integrating it with FeO_x/MnO_y, this formed FeMn@kaolinite, a catalyst with enhanced activity for peroxymonosulfate (PMS) activation, as shown in Figure 14c [94]. Zhai et al. [95] employ DFT calculations to elucidate the mechanism behind PMS activation and the degradation pathways of polyvinyl alcohol (PVA), which reveals the role of oxygen vacancies in facilitating electron transfer and the interaction between PVA and the catalyst. The computational chemistry perspective provides insights into the sustainable utilization of coal gangue by transforming it into a functional material that can effectively degrade pollutants, thereby offering an environmentally friendly solution for waste management and contamination control.

Alkaline metal-catalyzed gasification has emerged as a novel approach for the clean and efficient utilization of coal gangue and biomass [96]. Theoretically, alkali activation is feasible for any material composed of silicon and aluminum [97]. Miao et al. [98] conducted a comprehensive study combining experimental and theoretical analyses to investigate the effects of Na₂CO₃ and Na₂SO₄ catalysts on the catalytic gasification reactivity and mineral structure of coal gangue. To precisely elucidate the differences between Na₂SO₄ and Na₂CO₃ on the structure of coal gangue, DFT calculations were employed to determine the bond lengths and Bader charges when kaolinite interacts with alkali metals. The interaction of coal gangue with Na₂SO₄ results in longer Al-O bond lengths of 1.97Å and 1.89Å compared to 1.76Å with Na₂CO₃, indicating that Na₂SO₄ promotes the depolymerization of the aluminum–oxygen framework, which facilitates aluminum extraction. This structural modification makes the gasification ash structure with added Na₂SO₄ more favorable for aluminum removal. Catalytic gasification theory posits that alkali metals, particularly Na, accelerate the process via the formation of C–O–Na ensembles at carbon edge sites. Electron density is transferred from the negatively charged edge carbon of the char to Na⁺, weakening C–C and C–O bonds and thereby lowering the activation energy for gasification. This insight highlights the role of Na₂SO₄ in enhancing gasification reactivity and aluminum extraction, underscoring its potential in the valorization of coal gangue. Activation strategies for value-added utilization have transitioned from simple alkaline roasting to mechanochemically driven defect engineering coupled with heteroatom doping. Meanwhile, there is the absence of validated structure–activity relationships that connect quantified defect densities (e.g., EPR-derived •O⁻/g) with catalytic or adsorptive performance, thereby preventing rational scale-up. Computational simulations elucidate the mechanisms by which selected modification agents alter the surface reactivity of coal gangue under defined process conditions, providing mechanistic insight to inform subsequent experimental work.

4. Conclusions and Prospects

Overall, this article provides a comprehensive review of the application of current theoretical calculation research on the utilization of coal gangue. Various computational methods and models, including DFT and MD, have been employed to investigate the properties and reactivity of coal gangue, providing a more nuanced understanding of the chemical transformations involved in its utilization, facilitating the development of more efficient and environmentally friendly processes. Future research should combine materials science, environmental science, and computational chemistry, leveraging advances in high-performance computing and machine learning to improve coal gangue modeling and utilization strategies. These insights will advance sustainable management practices, support cleaner coal technologies, and contribute to environmental protection and resource optimization in the coal industry. The theoretical insights gained from computational chemistry strategies are pivotal for advancing our understanding of coal gangue utilization and for devising sustainable management practices. The application of computational chemistry in coal gangue modification not only enhances its utilization value but also promotes the further development and refinement of coal gangue resource utilization technologies. With the continuous advancement of computational chemistry methods, their application in coal gangue catalytic activation holds broad prospects and is expected to provide new ideas and solutions for the high-value utilization of coal gangue. These efforts will clarify the underlying mechanisms for cleaner coal utilization and provide a mechanistic basis for environmental protection and resource recovery in the coal industry.