Coal is the second-most-significant global energy source, contributing 27.1% to the main energy generation [1]. At the beginning of the 18th Century, coal became the main energy source, eventually leading to the Industrial Revolution. From the late Eighteenth Century onwards, the techniques of underground mining were developed, mainly in Britain [2]. Despite its positive impact on human development, coal mining has had adverse effects on the environment. Its activities have resulted in significant waste generation (tailings) and long-term environmental destruction, such as polluting the water systems, soil erosion, mine subsidence, ecosystem and biodiversity destruction, and land surface disruption [3]. Furthermore, mining has resulted in significant societal costs, including fatal accidents, health risks, and community displacement [4].

Historic mining practices have caused various hazards to the environment and humans. Some of those traditional practices are continued today in many parts of the world. In industrialized cultures, mining and reclamation laws have mitigated the degradation by stronger environmental requirements and requiring reclamation once mining has ended. Current mining and reclamation standards regulate site preparation, blasting, road building, overburden removal and placement, water control and maintenance, topsoil replacement, and vegetation establishment. Together, these practices greatly reduce the potential for land and water degradation during and after mining [5]. The largest problem in many countries is reclaiming sites that were abandoned and thus remain exposed to continued erosion and degradation of the surrounding environment. This problem often occurs where funding is unavailable for reclaiming abandoned lands or where laws have not been passed to deal with these abandoned sites.

Coal mining can be classified into two types: surface and underground mining. Large quantities of waste are generated in either case due to the coalification process and other impacts associated with mining and processing technologies [6]. Based on the geological and mining circumstances, a total of 0.4–0.7 t of waste is produced for every tonne of coal that is mined [7]. Compared to underground mines, opencast (surface) coal mines have higher environmental impacts on the soil, water, and air, whereas underground mining entails the risk of land subsidence and groundwater impacts below the land [8]. Heavy metals from dumps of spoil or refuse can be mobilized by surface runoff, which can leach into the subsurface soil or into nearby water resources [9]. The disturbed areas in coal mining regions can, therefore, be environmentally compromised, exposing organisms in the food chain to pollutants by direct contact, along with the inhalation of toxic metals, which can translate into human health effects [10].

Soil contamination studies conducted in coal mining areas have shown that mining and the associated operations release toxic elements into the soil, which then affect nearby riparian environments, surface water, and farmland ^[11][12][11,12]. The elevated levels of heavy metals in materials left on the surface pose a potential risk to the environment and human health, because of their bioaccumulation, toxicity, and release to organisms in the environment ^[13][14][13,14]. This elevated metal pollution may be absorbed by plants and crops, which leads to inferior growth and plant tissue contamination [15]. Polycyclic aromatic hydrocarbons (PAHs) are the primary organic toxic components found in coal mining areas [16]. The emissions from the combustion of small coal particles and coal gangue (also called coal refuse or coal waste) are the primary sources of PAHs in coal mine districts. Depositional environments and coal rank are two primary factors that determine the PAH content in coal gangues [17]. Excessive deposition of coal gangue on the surface can lead to spontaneous combustion under the right conditions, resulting in atmospheric pollution. At the same time, leaching due to precipitation events will result in heavy metals and PAHs contained in the gauge being released into the soil and surface water [18].

The acidic environment and high levels of heavy metals at coal mine sites lead to a decrease in microbial abundance and diversity. In addition, coexisting abiotic parameters including dissolved oxygen, organic carbon, temperature, and ionic composition also contribute to the composition of microorganisms that survive in such an environment ^[19][150]. AMD-tolerant microorganisms include archaea, bacteria, and even certain eukaryotes, such as algae and fungi ^[20][151]. The adverse environmental conditions in soil and water at coal mine sites restrict the survival of bacteria to a few phylogenetic groups such as Thiobacillus spp. and Methanogen spp. ^[21][152]. The first microorganism that has been successfully separated from the acidic environment is Thiobacillus ferrooxidans ^[22][153]. The acid and metal tolerance of Rhodococcus spp., Acinetobacter spp., Enterobacter spp., Staphylococcus spp., Klebsiella spp., and Bacillus spp., which were obtained from coal mines and eventually cultured, have been reported ^[23][24][154,155]. According to Shylla et al. ^[25][156], indigenous bacteria present in soils with elevated levels of heavy metals could potentially be considered viable options for the bioremediation of heavy metals in polluted areas. Due to the fact that these bacteria are native to the system, it is possible that their ongoing interaction with the conditions of low pH and AMD enabled them to develop resistance. The differences in their ability to tolerate metals may be due to variations of in vitro mechanisms or cell wall composition ^[26][157].

Indigenous strains of Thiobacillus and Bacillus that are tolerant to metals have been discovered in mining environments. These strains have been proven to assist in the bio-reduction of toxic metals, including vanadium. This suggests that they could be a favorable method for metal bio-remediation ^[27][158]. For example, metal-resistant Bacillus spp. as described by Gupta et al. ^[28][159] can thrive in soils with Fe concentrations of 400–550 mg/kg. Furthermore, Upadhyay et al. ^[29][160] found that Bacillus spp. taken from polluted soils in coal mine areas exhibited tolerance to Cr (VI) and showed an efficient reduction of Cr (VI) to Cr (III). Several explanations have been provided regarding the mechanism underlying this conversion. It may happen when chromate functions as the final electron acceptor to obtain energy, or when bacteria produce waste products, or when multiple enzymes interact with Cr(VI) to mitigate its toxicity by turning it into Cr(III). Syed and Chinthala ^[30][161] found that B. subtilis, B. cereus, and B. licheniformis were effective at reducing the content of Pb in soils by 86%, 87%, and 78%, respectively. A study on bacteria in mine soils polluted with metals found that Bacillus was the most-predominant species identified. These organisms demonstrated notable levels of tolerance, as evidenced by their ability to endure the highest levels of 207–414 mg/kg Pb and 65–196 mg/kg Zn ^[31][162]. In this study, producing metal-chelating substances, including siderophores and organic acids, has been crucial in facilitating metal detoxification and enhancing metal tolerance in bacteria. Singh and Tiwary ^[32][163] conducted a study at the Chirimiri coal mines in India and discovered that the Pseudomonas stutzeri bacteria collected from the mines exhibited the ability to degrade high concentrations of pyrene and phenanthrene in polluted soil. In this study, several dioxygenase enzymes, namely protocatechuate, catechol, and gentisic acid, were produced by Pseudomonas stutzeri bacteria in order to degrade PAHs.

Despite all the reported advantages of this remediation strategy, Kurniawan et al. ^[33][164] claim that, since heavy metals are not effectively separated from the treated medium using bioaugmentation alone, this method cannot be used to treat heavy-metal-polluted soil in real-world settings at large scales. The majority of studies that have documented the effective removal of heavy metals from contaminated soil were primarily conducted in laboratory settings, where environmental conditions are carefully regulated.

The cultivation of hyperaccumulator plants, which are either naturally occurring or created as a result of genetic modification, is one of phytoremediation strategies for removing metals from the soil and accumulating them in plant biomass ^[34][165]. It has the potential to not only strengthen the ecological environment of the mining region, but also facilitate the recovery of vegetation ^[35][166]. Phytoremediation is a comprehensive remediation strategy that encompasses a range of techniques, including rhizodegradation, rhizofiltration, phytoevaporation, phytostabilization, and phytoextraction, all of which are mediated by microbial interactions with roots and soil [13]. In addition, it has economic benefits and aesthetic values, both of which can result in a wide range of social advantages ^[36][37][167,168]. As an illustration, Pandey and Bajpai ^[38][169] suggested in their study that abandoned mining sites have the potential to be converted into public spaces, such as parks, in order to offer ecosystem services and products. Additionally, this finding demonstrates the various advantages of phytoremediation for promoting sustainable ecosystem development ^[39][170]. Some concern has been expressed about the disposal of the biomass of hyperaccumulator plants, but there is evidence that metals bound into plant biomass can be stabilized and less available for release over time.

In coal gangue sites that experienced a decade of phytoremediation with single/mixed elm/poplar, Bai et al. ^[40][171] found that the soil contents of Th, U, Bi, Co, Ni, Pb, and Cu were considerably decreased. In their study, Feng et al. ^[41][172] applied herbaceous plants for performing vegetation restoration efforts in coal mines located in Hulunbuir (Inner Mongolia). The findings of this study indicated that the restoration of vegetation has had a positive impact on the overall restoration of the ecosystem within the studied area. According to a report by Ameh and Aina ^[42][73], the only plants that exhibited high potential as phytostabilizers of Cd in a coal mine soil in Nigeria contaminated with toxic metals were Fuirena umbrellata and Selaginella myosurus. Furthermore, Hyptis Suaveolens exhibited hyperaccumulation potential for Cu (>1000 mg/kg accumulation). Furthermore, they concluded that it is possible for native plants to naturally eliminate toxic elements from soils that have been contaminated with metal pollutants.

In Australia, successful Se extraction (48% and 28%) was achieved from phytoremediated post-mining coal wastes area by Brassica juncea ^[43][173]. Further Se extraction may have been accomplished in this study; however, B. juncea crops should be harvested immediately after they reach maturity. If the biomass of B. juncea is not harvested and dried, it has a tendency to break into chip-like particles that become easily dispersed by wind, potentially leading to the dispersal of accumulated heavy metals. Matanzas et al. ^[44][174] found that herbaceous species including Lotus hispidus and Medicago lupulina have the ability to translocate As and Pb in the polluted soils of a brownfield site from Spain. Consequently, these two plants demonstrate their capacity to serve as bioindicators for the presence of As and/or Pb pollution. Moreover, they could potentially function as phytoextractors or accumulators under varying conditions, such as when there is a higher concentration of potentially toxic elements in the soil compared to the study site. In a more-recent study at this site, Fernández-Braña et al. ^[45][175] found that Buddleja davidii, Betula celtiberica, and Acer pseudoplatanus were effective for phytostabilization in regions with elevated pollutant concentrations. However, these plant species were only suitable for phytoextraction in soils with low-to-moderate pollution levels.

Desai et al. ^[46][121] found that using Alnus glutinosa and Betula pendula for forestation was successful in remediating metals (Mn, Pb, Zn, Cu, and Cd) on moderately polluted lands resulting from opencast coal mining in South Wales. Additionally, this study examined soil measurements taken from various points along a 14-year forestation chronosequence. The results consistently showed that, as the age of the tree plantation increased, the level of soil metal contaminants decreased. This finding provides further evidence that forestation has a positive impact on reducing soil metal loadings. After 25 years of phytoremediation by the Eucalyptus hybrid tree in the Jharia coalfield (India), Bandyopadhyay et al. ^[47][176] found a considerable reduction in the soil content of Cu, Zn, and Pb. They suggested the application of metal-tolerant woody trees, specifically the Eucalyptus hybrid, with high biomass enhanced phytoremediation of coal mine overburden dumps contaminated with metals. These trees have the ability to accumulate considerable quantities of metals in their tissues and decrease metal contents in the soil. Niu et al. ^[48][90] reported that particular plant species, such as Weigela hortensis and Ligustrum lucidum, were found to have a significant potential for reducing the content of Pb, Ni, and Cr in soil from a reclaimed coal mining area in China. According to Mellem et al. ^[49][177], Amaranthus dubius is capable of hyperaccumulating As in regions associated with coal mining, but has limited capacity for the bioaccumulation of Ni, Cu, Pb, Hg, and Cr. This plant can extract and transport As to its aerial parts and is able to tolerate high levels of this metal.

Phytoremediation is a strategy that has various advantages and disadvantages. It is important to take these into account when considering the application of this strategy. Although cost-effectiveness is a favorable aspect, the duration required to observe the outcomes may be prolonged. It is imperative that the concentration of pollutants and the presence of other toxins should not exceed the tolerance thresholds of the plant species under consideration for utilization. The process of identifying plants with the capability to remediate multiple contaminants concurrently is a challenging operation. It is necessary to consider the limitations and potential for these plants to become part of the food web when implementing this strategy ^[50][178].

Biochar is an appropriate substance for environmental applications, particularly in the remediation of contaminated soils, due to its low production cost and availability ^[51][179]. It is typically alkaline and possesses a significant surface area along with various active functional carbon groups capable of binding numerous cations ^[52][180]. Thus, biochar has the ability to reduce the availability, leachability, and mobility of toxic elements in polluted soils. Additionally, it can also decrease the absorption of these elements by plants ^[53][181]. Furthermore, biochar exhibited significant efficacy in mitigating the levels of PAHs in soil. Biochar generated at temperatures exceeding 400 °C indicated a notable propensity for PAHs owing to surface oxidation–reduction reactions. In addition, they show a higher level of aromatic clusters in a condensed form and possess a high degree of porosity characterized by the presence of well-developed nano- and micro-pores, which enable the sorption of low-molecular-weight PAHs ^[54][182].

Heavy metal remediation mechanisms in biochar involve precipitation, cation exchange, electrostatic attraction, reduction–oxidation, and physical adsorption. Biochar has been noticed to possess an interesting characteristic, whereby it can have an impact on the behavior of metals. The porous structures found in biochar have the capability to facilitate the transformation of metals into stable forms ^[55][183]. The -COOR and -OH functional groups in biochar surfaces contribute to the sorption of many heavy metals, making them unavailable ^[56][184]. According to studies, a competitive condition arises among metal ions in their interaction with functional groups on biochar surfaces due to the cationic nature of most metals. This leads to an enhanced immobilization of potentially hazardous elements in polluted soil. In general, the application of biochar treatment results in a significant increase in the levels of reducible and oxidizable heavy metals ^[57][185]. Biochar contains graphene moieties that serve as sites for both redox and adsorption reactions, resulting in a high affinity for metal ions and the capacity to transfer electrons to adsorbed reactants ^[56][184].

Reclamation principles are generally similar across different mine environments, despite differences in pollution type and level. The general acceptance of biochar for coal mine reclamation has been limited by lacking knowledge regarding its advantages, its accessibility at reclamation areas, the techniques required for its application, its cost, and the long-term effects of its field implementation. In addition, the study of biochar for the purpose of coal mine reclamation is a novel field of study that needs patience to assess its long-term effects when used in degraded areas ^[58][186].

The impact of applying chemical fertilizer and Eucalyptus wood biochar simultaneously to remediate Co-, Ni-, Zn-, and Cr-polluted soils from an operational coal mine dump in India was evaluated by Chandra et al. ^[59][187]. This study revealed that enhancing the mixing ratio of biochar from 0.5 to 5% (w/w) resulted in a considerable reduction of extractable heavy metal concentrations in the soil. The study conducted by Mujtaba Munir et al. ^[60][188] assessed the potential synergistic impacts of hydrothermally treated coal (HTC), raw coal (RC), and biochar (BC) on the accumulation, transformation, speciation, and immobilization of Pb, Cr, and Cd in soil polluted by the Huainan coalfield in Anhui, China. The findings showed that the co-application of BC-2% and BC-HTC amendments proved to be more efficient in mitigating Cd, Cr, and Pb contents in comparison to the singular application of RC or HTC amendments. This was achieved by increasing the organic carbon content and pH in the soil. Additionally, the application of BC-2% and BC-HTC amendments resulted in a respective increase of 1.5 and 2.5 units in soil pH. This led to the reduction of Pb, Cr, and Cd to more-stabilized forms in the soil.

Using soil from farms in the Huainan coal mine district, Dai et al. ^[61][189] conducted a laboratory experiment to investigate the effects of biochar amendments on the bioavailability and speciation of heavy metals. In this study, the concentrations of Cd, As, Zn, and Cu in polluted soil decreased by 42%, 7%, 51%, and 57%, respectively, when rice–straw-derived biochar was added to the soil. In a study conducted by Jain et al. ^[62][190], Lemongrass (Cymbopogon ciatratus) -derived biochar was applied as a soil amendment for spoil samples taken from coal deposits characterized by elevated levels of sulfur. According to the results, the application of biochar had a positive impact on the Palmarosa plant’s metal tolerance index, increasing it by 54%. Additionally, it led to a reduction in acid generation from acidic mine waste. In an earlier study, Jain et al. ^[63][191] assessed the impact of Lemongrass-derived biochar application on the heavy metal contents of Bacopa monnieri plants growing in acidic coal mine spoil. In this study, using biochar in acidic mine spoil led to reduced levels of heavy metals; Pb decreased by 93%, Fe by 50%, Cu by 42%, Cr by 65%, and Al by 60%. After the application of algal-derived biochar on coal mine stockpiles, Roberts et al. ^[64][192] found that the concentrations of Cr, Ni, and Zn were reduced by 49%, 2%, and 55%, respectively. In this study, remediated soils with this biochar had lower or, in some cases, equal contents of metals compared to soil without biochar remediation.