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Review

The Role of Modified Biochar for the Remediation of Coal Mining-Impacted Contaminated Soil: A Review

by
Subhash Chandra
1,*,
Isha Medha
2,3 and
Ashwani Kumar Tiwari
4,*
1
Department of Civil Engineering, GITAM School of Technology, Gandhi Institute of Technology and Management University, Visakhapatnam 530045, India
2
Department of Mining Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
3
Department of Civil Engineering, Vignan’s Institute of Information Technology, Duvvada, Visakhapatnam 530049, India
4
School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(5), 3973; https://doi.org/10.3390/su15053973
Submission received: 30 December 2022 / Revised: 13 February 2023 / Accepted: 15 February 2023 / Published: 22 February 2023
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Abstract

:
Land degradation and the release of contaminants such as heavy metals into the environment due to mining activities is a concerning issue worldwide. The bioaccumulation of heavy metals in the environmental matrix can severely damage flora and fauna and negatively impact human health. The poor physicochemical properties of mine spoil generated through mining operations make restoration of such contaminated and degraded lands challenging. In recent years, an exponential growth in the development and applications of biochar and its composites for the remediation of heavy metal-polluted environmental matrices such as soil and water has been observed. The literature review found that 95 review papers were published in the last five years reviewing the utility of biochar for heavy metals removal from the aqueous environment. However, no paper was published focusing on the application of biochar and its composites for the remediation of heavy metal-contaminated coal mine soil. The objective of the present review is to critically review the impact of mining activities on the environment and the role of biochar and its composites in the remediation of heavy metal-contaminated mine soil. This review presented a detailed discussion and sufficient data on the impact of mining practices in India on the environment. In addition, it critically discussed the methods of the production of biochar from various wastes and methods of modifying the pristine biochar to develop functionalized biochar composites. The detailed mechanism through which biochar and its composites remove and immobilize the heavy metals in the soil was discussed. The efficacy of biochar for the remediation of contaminated mine soil was also critically evaluated using various case studies and data from previously published articles. Thus, the major conclusion drawn from the review is that the application of various functionalized biochar composites could effectively manage and remediate heavy metal-contaminated mine soil.

1. Introduction

The release of pollutants, particularly heavy metals, into the environment has become ubiquitous due to industrialization and urbanization. In the last decade, rapid growth in the worldwide population has created a massive demand for supply chain systems and energy requirements. To cater to the increased demands, particularly the energy requirement, which has almost doubled, most developing countries rely on coal-based thermal power plants to meet their energy demand [1]. Globally, coal production is around 7.90 billion tonnes (BTs) [2], of which China is the leading producer, followed by India and the USA. Recent energy data estimated that coal produces approximately 40% of the world’s electricity. It has also been forecasted to be the most demanded fuel to fulfill energy needs over the next three decades [2]. India is the world’s seventh-largest economy and the second most populated country [3]. The continuous increase in the population and economy has increased India’s energy demand per capita. In India, coal is the primary energy source to meet the continuously increasing energy demands. India has a total of 306.60 BTs of coal reserves [2]. During the financial year 2020–2021, India produced around 756.50 million tonnes of coal, which is forecasted to grow by one billion tonnes by 2022 [3]. During the last decade, coal production in India has increased by 32% to meet growing demands [4].
The production of coal and other minerals to fulfill the supply chain can only be done through mineral mining, such as coal mining, bauxite mining, and iron ore mining. Mining activities are often associated with changes in the landscape, deforestation, migration of humans and animals, loss of soil, acid mine drainage, and release of heavy metals into the environmental matrix [5]. India has 319.02 billion tonnes of coal reserves. It was reported that India produced around 778.19 million tonnes of coal in the year 2021–2022, which was 8.67% higher than last year’s production figures ([6]). Coal production through mining involves the removal of the green cover and scavenging massive amounts of rock materials. The rock materials are dumped over a specified location of the active mining area [7]. The mine spoils in the form of dumps also pose a serious threat to the environment, as the mine spoil is loaded with heavy metals and mostly comprise a coarser fraction [8]. Coal mine spoils are rich in pyrite, often leading to acid mine drainage. Acid mine drainage is responsible for the leaching of heavy metals from the dump into the water resources due to the lowering of pH of the aqueous soil solution [9]. The release of heavy metals through the leaching process into the environment poses a serious threat to human beings, flora, and fauna. Therefore, it is essential to restore such contaminated sites with the most common approach of revegetation [10].
Various methods have been practiced and suggested for restoring such contaminated and degraded mine spoils lying over the overburden (OB) dumps, such as phytoremediation and application of mulch, lime, biosolids, and fly ash as soil conditioners ([11,12,13]). However, the application of the techniques mentioned above has certain limitations. For example, fly ash application in the soil is associated with the risk of heavy metals contamination [14]. Biosolids applications in the soil may cause contamination of soil with organic pollutants (pesticides and herbicides) and can increase the bioavailability of some heavy metals [15]. The lime and manure application is only suitable for acidic soils and can increase CO2 emissions from the soil [16]. Other technologies are also available for soil remediation and pollutant removal, such as adsorption, biodegradation, and in situ chemical oxidation (ISCO), which can substantially retain and degrade the pollutants in the soil [17]. In this context, biochar has emerged as a wonderful material that can be used to improve the soil structure, immobilize heavy metals through adsorption and oxidation/reduction processes, and support vegetation growth [18,19,20]. Biochar is a porous carbon material derived from the thermochemical conversion of biomass waste under anoxic conditions in the temperature range of 400–700 °C for a duration varying between 1 and 3 h [21,22]. Biochar can be produced from a variety of raw materials, such as corn straw, rice straw, hardwood waste, food waste, and agricultural waste, by pyrolyzing them at the temperature mentioned above [23,24,25,26]. Recently, a ball-milling technique has been developed to produce very fine or nano-sized biochar particles for their application in the efficient removal of pollutants from the environmental matrix [23,27,28]. Biochar is known for its high porosity, functional groups, aromatic character, presence of inorganic elements, and high specific surface, making it a suitable material to be used as a soil ameliorant in contaminated soil [29,30]. For example, Turan [31] reported the application of olive pulp-derived biochar in combination with calcite in Ni-contaminated soil to reduce its mobility within the soil and in crop biomass based on a pot-culture study. In another study, Turan [32] reported the combined application of hazelnut husk-derived biochar and arbuscular mycorrhizal fungi that have substantially reduced the bioavailable and DTPA-extractable fraction Ni in the soil besides improving the soil enzymatic activity. In addition, the chemical characteristics of the biochar can be changed by doping its surface with other inorganic elements, acid or alkaline treatment, and the development of composites [33,34]. The modification methods of biochar can be classified into five types: acid, alkali, steam, metals, and amination modifications [21]. The acid modification removes the impurities from pristine biochar and introduces acidic functional groups [21]. The alkali and steam modifications increase the surface area of pristine biochar and pore structure [21]. Metal impregnation is done to improve the adsorption capacity and catalytic performance of biochar [21]. Thus, the modified biochars are highly efficient in retaining nutrient loss and reducing the availability of heavy metals in the soil [35,36]. He et al. [37] reported the development of chitosan and alginite biochar from rice straw waste to be used as soil conditioner. Results indicated that the application of the chitosan and alginite-modified biochar substantially reduced the soil acidification and the availability of Cd2+ ions in the aqueous soil solution [37]. Li et al. [38] reported the development of MgO flake-modified rice husk biochar to immobilize heavy metals in the soil. Results of the study indicated that the MgO-modified biochar showed high adsorption capacity for the adsorption of Cd (II) (104.68 mg kg−1), Cu (II) (173.22 mg kg−1), Zn (II) (104.38 mg kg−1), and Cr (VI) (47.07 mg kg−1) in the aqueous solution and reduced the exchangeable fractions of these heavy metals by 9.60% in the soil [38]. Li et al. [39] reported the development of Mg/Al-modified biochar (MAB) through the one-pot pyrolysis method for immobilizing heavy metals and compared its efficiency with pristine biochar. Results indicated that compared with the pristine biochar, the application of the MAB in the soil increased the removal efficiency of As, Pb, and Cd from −62%, 17%, and 5% to 52%, 100%, and 66%, respectively [39]. Gholami and Rahimi [40] reported the synthesis of CH4N2S-modified potato peel biochar for application in the soil to remove heavy metals. Results of the study showed that the application of modified biochar at a rate of 8% (w/w) significantly reduced the soluble and exchangeable fractions of Cd, Zn, and Cu in the soil to 102.97, 94.40, and 76.18 mg kg−1, respectively, compared with the application of pristine biochar [40]. In addition, the application of modified potato–biochar composite also improved the soil pH, cation exchange capacity, and organic carbon content in the soil [40]. Yang et al. [41] reported the development of pig carcass-derived phosphorous-rich biochar and plant-derived Fe-modified biochar and their application for immobilizing Cd and Pb in the soil. It was found that the application of P-rich biochar showed better immobilization of Pb2+ in the soil under redox conditions than Fe-modified biochar, whereas both P-rich and Fe-modified biochars showed lower immobilization of Cd2+ ions compared with pristine biochar under pre-maintained redox conditions (Eh ≤ −100 mV) [41]. Song et al. [42] reported the development and application of biochar-supported nano-zero valent iron (nZVI) composite in the multimetal-contaminated soil. Findings from the study demonstrated that the application of (nZVI)–biochar composite substantially immobilized the heavy metals (Cu, Zn, As, Cd, and Pb) and improved the pH, CEC, and enzymatic activities in the soil [42].
The motivation behind writing this review article is that during the last five years (2018–2022), a total of 481 research articles have been published, as shown in Figure 1 (sourced from Scopus, Google Scholar, and Web of Science). The articles were searched based on the keywords related to biochar production, biochar for soil amendment, and biochar for remediation of contaminated soil. Out of a total of 481 publications, only 95 review articles were published on the same topic. However, not a single article has been published highlighting the environmental impacts of mining practices in India and the role of modified biochar and biochar composites in the remediation of such heavy metal-contaminated mine soils. The main objective of the present review article is to critically evaluate the impact of the mining practices in India and abroad on the environment and how it can lead to the damage of sensitive ecosystems, apart from deteriorating the soil and water quality. In addition, this article also discussed the production methods of biochar and the physical and chemical methods to produce modified biochar and biochar composites. Finally, the role and efficiency of the modified biochar and biochar composites for the remediation of heavy metal-contaminated mine soil and the underlying mechanism were also evaluated. The detailed structure of the manuscript and the interrelation of various topics discussed in this manuscript can be understood with the help of a diagram presented in Figure 2.

2. Degraded and Contaminated Soils in Mining Areas

The mined-out degraded soils are artificial habitats developed through the deposition of overburdened materials during the mining process. These degraded soils experience a wide range of problems with establishing vegetation. The physicochemical properties of such soil inhibit vegetation growth due to the lack of nutrients and the presence of heavy metals [43]. The physicochemical properties, nutrient conditions, and contamination levels of coal mine-degraded soil are discussed in the subsequent sections.

2.1. Physicochemical Properties

Mining activities are associated with severe land and soil productivity damage. Mine soil is developed anthropogenically due to changes in the landscape after post-mining. The soil in the mined-out land is typically acidic, low in organic matter and nutrients, and has a high content of heavy metals [44]. The mine soil reportedly has poor structure, high bulk density, and a coarser texture. The high bulk density and poor structure of mine soil can be linked to heavy machinery during mining operations and the intrusion of rock fragments within the soil during mining that results in soil aggregation and low water-holding capacity [45]. The typical characteristics of mine soil are given in Table 1.

2.2. Impact on Soil Ecosystem

Ecology has two components, viz., organisms and environment, and combinedly, they are called the ecosystem. The ecosystem has two components: abiotic (includes all living organisms) and abiotic component (includes all non-living parts of the ecosystem) [12]. Ecosystem stability is another crucial component within the soil. A stable ecosystem maintains the species richness and nutrient pool dynamics within the soil structure. An ecosystem is considered stable if it maintains the species richness, structure, and functioning from year to year [12]. Mine soil is often considered to have poor ecosystem stability due to the loss of vegetation, nutrients, and frequent mining operations. Topsoil removal during mining operations disturbs the microbial activities and prevailing vegetation, which makes it hard to re-establish the vegetation cover [58]. The impacts of mining activities on the ecosystem can be visualized in Figure 3. The microorganisms and their microhabitats play an essential role in maintaining vegetation growth over degraded mine soil through nutrient recycling, humus formation, and mineralization [12]. Assessing microbial biomass gives information about the growth of the overall microbial community in the soil [45]. The mine soil respiration, cellulose decomposition, and soil enzymes are the most commonly measured parameters to estimate the growth of the microbial community in the soil. The soil enzymes are another critical parameter to evaluate the quality of the soil ecosystem within the soil matrix. The intercellular and extracellular microorganisms secrete the soil enzymes and perform essential biochemical functions to promote organic matter formation and nutrient recycling within the soil [45]. Mined-out land lacks active soil enzymatic activities within the soil matrix and, thus, has a meager nutrient recycling rate and poor soil ecosystem [59]. Previous studies have demonstrated that enzymatic activities in the soil could be used as indices for monitoring the degraded mine soil reclamation success [45,59].

2.3. Heavy Metals Pollution and Health Impacts

Metals such as lead (Pb), zinc (Zn), chromium (Cr), cadmium (Cd), mercury (Hg), copper (Cu), and nickel (Ni) having densities greater than 5 g/cm3 are categorized as heavy metals. The presence of heavy metals in the soil in quantities greater than the permissible limit leads to soil pollution. The heavy metals in the soil may intrude into the food chain and can profoundly impact the environment and human health. The toxicity and the chances of intrusion of heavy metals from the soil into the food chain can be measured using the geoaccumulation index (Igeo), which is used to evaluate the level of heavy metal contamination and their mobility in the soil [61]. The geoaccumulation index can be calculated using the following formula reported by [62].
Igeo = log2 (Cn/1.5Bn)
Cn is the heavy metal concentration measured in the mine soil and Bn is the background concentration of the respective heavy metal in the mining area. The calculated value of Igeo index can be evaluated using Table 2.
The presence of heavy metals in the environmental matrix is associated with several health issues and ecological disturbances [64,65]. The impact of heavy metals on the environment and human health can be understood with the help of Figure 4 and Table 3, respectively.
The soil in mining areas around the world is contaminated with heavy metals such as Cd, Cr, Cy, Ni, Zn, and Pb. The spatial distribution of the heavy metals varies from place to place and depends upon the soil’s geology, rock formation, and physicochemical properties. The distribution of heavy metals in coal mining areas worldwide is presented in Table 4.

3. Remediation of Heavy Metal-Contaminated Soil

3.1. Phytoremediation

As per the World Health Organization (WHO) and the Environmental Protection Agency (EPA), heavy metals contamination in the soil and water resources is one of the most significant issues. There are various methods reported for the remediation of such contaminated sites. Among the reported methods, phytoremediation is an easy, eco-friendly, and sustainable approach to remediate heavy metal-contaminated sites [73]. The word phytoremediation was derived from the Greek word “Phyto”, which means plants, and the Latin word “remedium”, meaning to correct or remove [74]. Phytoremediation is an in situ remediation technique that utilizes particular types of plants called hyperaccumulators to extract heavy metals present in the soil and, thus, reduce their mobility [74]. The phytoremediation process can further be sub-divided into various processes, namely, phytoextraction, phytostabilization, phytodegradation, phytovolatilization, Rhizodegradation, and Rhizofiltration, which integratively contributes to the remediation process [74,75]. The details of the various types of phytoremediation processes and the mechanism of phytoremediation can be understood with the help of Figure 5 and Figure 6, respectively.
Various factors affect the effectiveness of the phytoremediation process. This includes the plant species, growth rate, selectivity of metals, plant immunity, and species’ growing conditions [76]. In addition, geographical factors such as climatic conditions, temperature, and humidity also affect the phytoremediation potential of any plant species [77]. In addition, the plant species’ immunity and root structure significantly affect the soil’s phytoremediation [77,78]. The details of different varieties of plant species used for the phytoremediation of heavy metal-contaminated soils are presented in Table 5.

3.2. Bioremediation

Bioremediation is a biological process that utilizes microbes (aerobic and anaerobic bacteria and fungi) to immobilize heavy metals in the soil. The main principle of the bioremediation process is to degrade and convert the pollutants to their less toxic form [87]. In the bioremediation process, microbes play an essential role. Two types of microbes participate in the remediation process: aerobic and anaerobic. The aerobic microbes consist of bacteria such as Pseudomonas, Acinetobacter, Sphingomonas, Nocardia, Flavobacterium, Rhodococcus, and Mycobacterium, which are capable of degrading and transforming the heavy metals in the soil [88]. The effectiveness of the bioremediation process is controlled by various factors such as the concentration and chemical nature of the pollutant, physiochemical characteristics of the environment (pH, temperature, oxygen, and nutrients), and the accessibility of the pollutant to the microbes [87]. The biotic factors that significantly affect the bioremediation process are the microbial cell enzymatic activities, mutation, gene transfer, microbial biomass production, and the microbes’ population size [89]. The abiotic factors that affect the bioremediation process are soil temperature, pH, moisture, soil structure, water solubility, nutrient availability, redox potential, pollutants concentration, and toxicity level [89,90]. The bioremediation technique is frequently used for the remediation of heavy metal-contaminated soils. The mechanism through which microbes remediate the heavy metal-contaminated soils is presented in Figure 7. Briefly, the heavy metals in the soil remain in their ionic form. Electrophilic heavy metals have the vacancy of electrons in their outer orbit. When such electrophilic heavy metals come into contact with microbes applied in the soil for bioremediation, the electron donor components present inside the microbes’ cells, such as sugars or fatty acids, transfer the electrons to the electrophilic heavy metals. Thus, the heavy metals are reduced to non-toxic and stable forms by accepting the electrons. The details of the various types of microbes used for the remediation of heavy metal-contaminated soil are presented in Table 6.

4. Biochar and Its Composites for Soil Remediation

Biochar is a porous carbon material produced through the pyrolysis of various biomass wastes [102]. The biochar can be produced through various pyrolysis processes, such as slow pyrolysis, fast pyrolysis, gasification, and hydrothermal carbonization, as shown in Figure 8.
Slow pyrolysis consists of prolonged biomass heating in the presence of insufficient oxygen over a temperature range of 300–800 °C [103,104]. Slow pyrolysis is primarily marked by a low heating rate (5–7 °C/min) and long solid and vapor residence time (usually longer than 1 h) [104]. The primary product of slow pyrolysis is biochar and a small quantity of bio-oil and syngas. Biochar yield through slow pyrolysis of biomass waste varies from 35–45%, and bio-oil and syngas gas yields vary from 25–35% and 20–30%, respectively [105,106,107,108]. Alternatively, the biochar can be produced by the fast pyrolysis process, which is characterized by high heating temperature, high heating rate, and short retention time. The biomass waste is typically heated at a temperature higher than 500 °C at a high heating rate (≥300 °C min−1) and heated at the final temperature for a few seconds (<2 s) [109]. The primary product of fast pyrolysis is reportedly the bio-oil having a yield of more than 60%, followed by syngas at 10–20%, and biochar having a yield of approximately 10% [108,109,110]. Another method is the gasification process, which is typically carried out at a very high temperature (>700 °C). During this process, heat transfer occurs from the bulk to the biomass particles leading to the evaporation of water followed by volatiles [108]. The primary product of the fast pyrolysis process is the syngas (>90%), followed by the generation of a small quantity of biochar (<5%) [111]. Hydrothermal carbonization (HTC) was developed to process wet biomass waste into carbon material called hydrochar. The advantage of HTC over other pyrolysis processes is that it can process wet biomass waste to hydrochar [112]. The HTC process is performed in the temperature range of 180–250° at an autogenic pressure (0.5–2 Mpa) for 8–10 h in a closed reactor or kiln [108,111]. The primary outcomes of the HTC process are the hydrochar having a yield of up to 80% and the production of dissolved carbon slurry having a yield of up to 20%. Unlike biochar, hydrochar is rich in nutrients and has a hydrophilic nature, as well as a low O/C ratio [108].

4.1. Various Synthesis Methods of Biochar Composites

Pristine biochar is a beneficial soil-ameliorating material owing to specific physicochemical properties such as high porosity, functional groups, alkaline pH, and inorganic elements. However, the pristine biochar has certain limitations, such as hydrophobicity, low specific surface area, and variations in acidic and basic functional groups that further limit its application areas. Various methods have recently been developed to modify biochar’s physicochemical properties and develop metals or organics-doped biochar composites. The modifications in the biochar’s physicochemical properties enable its application in various areas, such as for heavy metals remediation, water treatment, energy storage, and application as a slow-release fertilizer in the soil. The various methods for the modifications in the properties of biochar are graphically shown in Figure 9 and discussed in detail below.
The changes in the properties of biochar can be done either by modifying the treatment methods (pre- and post-treatments) or by doping the biochar’s structure with some foreign inorganic or organic elements and by surface modification methods. The surface modifications can be done by grafting the surface of biochar with carboxylic acid, amine, and amide functional groups to modify their surface chemical properties, such as an increase in hydrophilic character or hydrophobic micro and macro pores [113].

4.1.1. Modifications in the Treatment Methods

(a)
Pre-treatment methods
The pre-treatment methods have been developed to impregnate the biomass waste with organic or inorganic elements through natural diffusion or bioconcentration processes by keeping the biomass waste in contact with the foreign element through solvent treatment. Various biomass wastes were reportedly pre-treated with AlCl3, ZnCl2, MgCl2, and KMnO4-based metal solvent for some time before pyrolyzing the biomass. Such pre-treatment of the biomass waste resulted in the impregnation of these nanoparticles within the biochar’s structure and the formation of Al2O2, CaO, MgO, and ZnO nanocrystals [114,115]. Thus, the intrusion of foreign nanoparticles within the pristine biochar’s structure significantly changes its physicochemical properties for its wide applications. Lyu et al. [116] reported the development of nano zerovalent iron–biochar composite (nZVI/BC) through various methods, such as hydrothermal carbonization, chemical precipitation, and ball milling, for the remediation of contaminated soil. Chen et al. [117] reported the development of an iron–biochar composite through the pre-treatment of barley grass with FeCl3 followed by pyrolysis at 350 °C for the remediation of Cr-contaminated soil. Yu et al. [118] reported the development of MgO-modified biochar through the pre-treatment of corn straw biomass with KMnO4 (0.079 mol/L) for 24 h, followed by pyrolysis at 600 °C for 0.5 h for its application for the adsorptive removal of As from contaminated red soil. In addition, biomass waste can also be pre-treated with acid and alkaline solutions to change the functional groups and specific surface area. Acid pre-treatment of biomass waste before pyrolysis introduces acidic functional groups in the biochar along with a reduction in the mineral matter content.
In contrast, the alkaline pre-treatment increases the specific surface area of the biochar along with the enrichment of oxygen-containing functional groups [34]. Pap et al. [119] reported the synthesis of acid (50% H2SO4) fruit waste biomass-derived functionalized biochar to remove Pb and Cr heavy metals from the aqueous solution. Chen et al. [120] reported the application of alkali pre-treated biochar to remove Thorium (IV) from the aqueous solution, which was developed through the pre-treatment of duckweed biomass with 0.1 M NaOH followed by pyrolysis. Thus, it can be deduced that the modified or pre-treatment process can modify the properties of biochar as required to improve the removal of contaminants from the environmental matrix.
(b)
Post-treatment methods
Post-treatment methods are different from pre-treatment methods, which leads to modifications in the biochar’s physicochemical properties that also differs from the biochar produced through pre-treatment methods. Post-treatment refers to the process of modification of biochar properties after the pyrolysis of biomass waste. Post-treatment can be done using impregnation, evaporation, heating, and co-precipitation [34]. Dong et al. [121] reported the synthesis of magnetic biochar through the co-precipitation of biochar particles with FeCl3 and FeCl2 in an alkaline solution and its application for the remediation of contaminated soil. Imran-Shaukat et al. [122] reported the post-treatment of palm shell-derived biochar with 20% NaOH followed by acid treatment (HCl) to remove heavy metals from an aqueous solution. Samsuri et al. [123] reported the application of Fe (III)-coated biochar for the removal of As (III) and As (V) from the aqueous solution, which was developed by impregnation of biochar particles with Fe ions in a solution containing FeCl3 salt for 24 h. Lyu et al. [124] reported the post-treatment of biochar with 0.063 moles of FeSO4 aqueous solution to produce FeS-coated biochar particles to remove hexavalent chromium from the aqueous solution.

4.1.2. Co-Doped Biochar Composites

Co-doped mineral biochar is developed using chemical and mechanical methods such as co-precipitation, ball milling, and chemical doping, as shown in Figure 10. Co-doped or mineral biochar has a certain advantage over pristine or pre-treated biochar because the presence of mineral or co-doped elements significantly affects the performance of biochar through the change in its physicochemical properties. Co-doped biochars are applied in soil remediation, soil fertility improvement, heavy metals removal, and wastewater treatment [33]. For example, montmorillonite, a natural clay material, co-doped biochar significantly improved the metals and cationic retention within its interlayers through electrostatic attraction and cation exchange processes [125,126]. Li et al. [127] reported the development of Mg/Al-layered double hydroxide biochar by pre-treating the biomass waste with 75 mmol L−1 of AlCl3 and by post-treating with 0.12 mol L−1 CH4N2O and 40 mmol L−1 MgSO4·7H2O followed by hydrothermal carbonization at 160 °C for 8 h. The application of Mg/Al-layered double hydroxide composite significantly increased Cd (II) and Cu (II) adsorptive removal from the aqueous solution. Wang et al. [19] reported the development of a clay–biochar composite derived through the co-pyrolysis of various clay minerals (Kaolin, Vermiculite, and Attapulgite) with rice husk biochar at 500 °C for 90 min. The study results showed that the application of mineral–biochar composite significantly improved the physicochemical properties of the contaminated soil and reduced the phytoavailability of the heavy metals in the soil [19]. Zhang et al. [128] reported the development of the EDTA and Chitosan bi-functionalized bamboo waste-derived magnetic biochar composite, which was produced through co-precipitation and chemical modification methods to remove Cd and Zn from the aqueous solution. Ma et al. [129] reported the synthesis of microorganism-coated biochar composite known as Bacillus TZ5-coated biochar composite to immobilize Cd in the contaminated soil. Tu et al. [130] reported the development and application of a maize straw-derived NT-2-loaded biochar composite for Cd and Cu immobilization in the soil. Yin et al. [131] reported the application of rice straw-derived Fe–biochar composite synthesized through the co-precipitation method followed by the pyrolysis of biomass waste for the immobilization of Cd and As in the soil. Yu et al. [119] reported the development and application of corn straw-derived MnO-doped biochar composite to immobilize As in the soil and reduce their phytoavailability to Indian rice (Oryza sativa L.) cultivated in such contaminated soil. Apart from that, the biochar can also be simultaneously co-doped with metals and non-metallic elements, such as sulfur and iron co-doped functionalized biochar developed for the efficient stabilization of Cd and Pb in the contaminated soil [132].

4.2. Effect of Biochar and Its Composites on Soil Physicochemical Properties

The application of biochar into the soil is acknowledged to have benefits through the improvements in the soil’s physical, chemical, and biological properties, as shown in Figure 11 [133]. Biochar is characterized as having a porous structure, functional groups, nutrients, and other inorganic elements, as shown in Figure 12. The porous structure of biochar helps to increase soil porosity and reduce its bulk density. Blanco-Canqui [134] reported that the application of biochar increased the soil porosity by 14 to 64% while reducing the soil bulk density by 3 to 31%. Further, the presence of functional groups and inorganic elements in the biochar structure helps it to retain nutrients within the soil and promote overall fertility [57]. However, pristine biochar has certain limitations regarding functionalized functional groups, inorganic elements, and high specific surface area, which are the key factors in retaining soil moisture and nutrients and promoting overall soil growth. In this context, the development of functionalized biochar and biochar composites has opened up a way to cater to such issues. Functionalized biochars are characterized by having a high specific surface area and the presence of functionalized metallic groups on their surface [135,136,137,138]. Lyu et al. [117] reported that applying iron–biochar composite in the soil improved soil aggregation. This is because the iron oxide-coated surface of the biochar promoted the co-precipitation of dissolved organic carbon in the aqueous soil solution, and the organometallic interaction enhanced the cementing properties within the soil structure. Zhang et al. [139] reported that the application of Mg/Fe-layered double hydroxide–biochar composite in the soil enhanced the retention of NH+ and NO3– ions through hydrogen bonding and anion exchange processes, respectively. Chen et al. [140] reported that the montmorillonite–biochar composite increased the retention of NH4+ and PO42– ions through cation exchange and electrostatic attraction processes, which further regulated the controlled release of the nutrients in the soil. Chandra et al. [36] reported that the application of potassium–iron–biochar composite in the soil reduced the leaching of NH4+ and PO42– ions and increased the plant availability of the nutrients in the soil. In another study, Shen et al. [141] reported that the application of rice husk-derived MgCO3–biochar composite in the soil significantly increased the soil’s water retention capacity and the retention of PO42– ions through chemisorption. Thus, biochar composites have advantages over pristine biochar; however, both biochar and its composites can be used as an effective soil ameliorant.

4.3. Effect on Heavy Metals

The presence of heavy metals in soil and water resources is known to have a deleterious effect on the environment and human health. The various types of heavy metals found in contaminated mine soils are Pb, Cr, As, Zn, Cd, Cu, Hg, and Ni [57,143]. In the contaminated soil, the heavy metals are highly mobilized due to acidic pH and lack of organic matter and, thus, may cause the leaching of heavy metals into the water resources [144]. Various methods of immobilizing heavy metals in such contaminated soil include physical remediation, vitrify technology, chemical leaching, chemical fixation, electrokinetic methods, and phytoremediation [145]. Among the above-reported methods, the phytoremediation technique was well appreciated due to its high efficiency and low application cost [146]. However, with the phytoremediation technique, some limitations have been observed, such as the accumulation of heavy metals in the above-ground biomass, low growth rate of hyperaccumulators in the degraded and contaminated soil, and a prolonged process to achieve complete success [147]. The amendment of such contaminated soil with biochar as a soil ameliorant supports vegetation growth and assists in immobilizing heavy metals within the aqueous soil solution [148]. The various mechanisms through which biochar immobilizes or adsorbs the heavy metals in the aqueous soil solution are the electrostatic attraction, ion exchange, surface complexation, and precipitation processes [148,149]. The detailed mechanism of removing heavy metals in the aqueous solution using biochar can be understood with the help of the diagram presented in Figure 13. Although pristine biochar is a promising adsorbent for the removal of heavy metals, it also has certain limitations that restrict its application to certain types of heavy metals, such as Cr (VI), As, V, etc. This is due to having a negative charge on the biochar’s surface [150,151]. In recent years, efforts have been made to develop various types of biochar composites to address the problems associated with pristine biochar. The modified biochar has the advantages of having functionalized organic groups, high specific surface area, improved pore structure, an abundance of active sites, and co-doped inorganic elements [152,153]. Liu et al. [154] reported the development of a magnetic macroporous biochar for the remediation and removal of Cd (II) and As (V) from the contaminated soil. Zhang et al. [155] reported the development of a corncob-derived multifunctional carboxymethyl cellulose sodium-encapsulated phosphorous-enriched biochar composite for the removal of Pb (II), Cd (II), and Ni (II) from the aqueous solution. Sajjadi et al. [156] reported the synthesis of a double-layered magnetized and silica-functionalized biochar composite. They observed that the application of biochar composite significantly improved the removal of Ni (II) and Pb (II) by 139 and 38%, respectively, compared with the pristine biochar. Chen et al. [118] reported the development and application of iron–biochar composite for the remediation of Cr-contaminated soil. The results indicated that the amendment of Cr-contaminated soil with Fe–biochar composite immobilized the Cr ions within the soil and substantially reduced their leaching into the groundwater. In addition, the recently developed biochar-supported microbial composite is an exciting development in biochar-based composites due to having a high potential for the remediation of heavy metals polluted soils. In this regard, Qu et al. [157] reported the development of bone char-supported biochemical composite carrying phosphate solubilizing bacteria, iron sulfide, and carboxymethyl cellulose on its surface for the remediation of Pb-contaminated soil. The major chemistry for the removal of heavy metals using biochar composites are surface complexation with the acidic or basic functional groups, co-precipitation with inorganic doped elements, oxidation or reduction by accepting electrons and protons, electrostatic attraction between oppositely charged heavy metals and biochar’s surface, and π–π electron interaction between heavy metal ions and π-electrons [158]. The detailed mechanism of the chemistry behind the higher removal efficiency of heavy metals by biochar composites can be understood by referring to Figure 14. Further, the biochar was reported to have shown excellent heavy metals removal efficiency and remediation processes when combined with the phytoremediation technique [57,159]. Gong et al. [160] reported that the application of biochar varying from 100 to 5000 mg kg−1 in the soil improved the translocation of Cd from soil to biomass and, thus, increased the efficiency of the phytoextraction capacity of the hyperaccumulators. Rathika et al. [161] studies the synergistic effect of biochar with EDTA (chelating agent) in the heavy metal-contaminated soil. It was found that the co-application of biochar with EDTA in combination with the growth a Brassica juncea in the Pb-contaminated soil significantly improved the phytoextraction of Pb from the soil. It was reported that the application of biochar enhanced the plant growth by providing nutrient support and reducing the leaching of heavy metals from the soil, which, in turn, increased the phytoextraction of heavy metals by the hyperaccumulators [162]. The details of the various types of biochar reported to assist in the phytoremediation of heavy metals from the soil is given in Table 7.

5. Conclusions and Future Prospects

The following conclusions can be drawn from the above analysis and discussions regarding the impact of mining on the ecosystem and environment, methods of remediation of heavy metal-contaminated soil, methods of production of biochar and its modification methods, and the role of the biochar and its composites for the remediation of such contaminated soils:
(i)
The coal mining activities destroy the landscape, soil quality, and ecosystem in the impacted area to such an extent that restoring such degraded lands to their original state becomes difficult.
(ii)
The degraded landscape triggers the release of heavy metals such as Cr, Cd, As, Mn, Co, Ni, and Zn from the contaminated mine soil into various surface and groundwater resources, which may have significant health impacts on humans and animals.
(iii)
Phytoremediation is the most common and low-cost method to restore such degraded and contaminated mine soil by promoting the growth of metal-tolerant species in such soil. However, the poor physiochemical properties of the mine soil further limit the growth of hyperaccumulators in the mine soil.
(iv)
The surface properties of pristine biochar can be modified by pre- and post-treatment methods. The acid- or alkali-treated biochar shows the change in the abundance and characteristics of functional groups and surface porosity. The biochar can also be combined with other minerals through thermal and chemical treatment methods to produce biochar composites having enhanced surface properties compared with the pristine biochar.
(v)
The application of biochar to the contaminated and degraded soil improves the overall soil physicochemical properties and promotes the immobilization of heavy metals within the soil matrix, thus, promoting the phytoremediation of the soil. Precipitation, surface complexation, electrostatic attraction, and ion exchange are the dominant methods through which biochar adsorbs and immobilizes heavy metals in the soil matrix.
(vi)
Remediating mine soils contaminated with heavy metals such as As, Cr, and Cd using pristine biochar is challenging. The application of modified biochar or biochar composites enhances the remediation process. The modified biochar’s surface properties can be changed through the modification process to remove a particular class of heavy metals with anionic characteristics in the aqueous soil solution.
The modified biochar and biochar-based composites can have multiple applications other than the soil remediation process. The modified biochar can be used as an effective adsorbent to remove emerging pollutants of concern from wastewater, as their surface properties can be modified to promote the adsorption of such pollutants. In addition, modified biochar can be used as electrode material in electrochemical cells designed to electrochemically remove heavy metals, ions, and organic pollutants from wastewater.

Author Contributions

S.C.: Conceptualization and writing original draft. I.M.: Data collection, validation, and conceptualization. A.K.T.: Heavy metals and soil data collection and Data validation. All authors have read and agreed to the published version of the manuscript.

Funding

The research received no external funding.

Acknowledgments

The authors would like to thank the Sustainability Journal for giving us an opportunity to publish in this esteemed journal. The authors would also like to extend their sincere thanks to the anonymous reviewers for providing their valuable comments during the peer review process to improve the overall quality of the manuscript.

Conflicts of Interest

The authors have no conflict of interest to declare.

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Figure 1. Total numbers of articles published in the peer-reviewed journals related to the keywords biochar, remediation, heavy metals, and contamination (articles were sourced from various research databases: Scopus, web of Science, and google scholar).
Figure 1. Total numbers of articles published in the peer-reviewed journals related to the keywords biochar, remediation, heavy metals, and contamination (articles were sourced from various research databases: Scopus, web of Science, and google scholar).
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Figure 2. Overall structure chart of the review highlighting various topics covered and their interrelation.
Figure 2. Overall structure chart of the review highlighting various topics covered and their interrelation.
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Figure 3. Impact of mining activities on the ecosystem and environment (reproduced with permission from [60] License no. 5406921447438).
Figure 3. Impact of mining activities on the ecosystem and environment (reproduced with permission from [60] License no. 5406921447438).
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Figure 4. Impact of heavy metals on the environment and ecosystem.
Figure 4. Impact of heavy metals on the environment and ecosystem.
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Figure 5. Processes of phytoremediation of heavy metals in the soil.
Figure 5. Processes of phytoremediation of heavy metals in the soil.
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Figure 6. Mechanism of phytoremediation of heavy metals in the soil.
Figure 6. Mechanism of phytoremediation of heavy metals in the soil.
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Figure 7. Mechanism of the bioremediation of heavy metals by the microbes present in the soil.
Figure 7. Mechanism of the bioremediation of heavy metals by the microbes present in the soil.
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Figure 8. Various pyrolysis processes to produce biochar, bio-oil, and wood vinegar as the product through the pyrolysis of various biomass wastes.
Figure 8. Various pyrolysis processes to produce biochar, bio-oil, and wood vinegar as the product through the pyrolysis of various biomass wastes.
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Figure 9. Various methods for the modifications in the properties of biochar (reproduced with permission from [34], License number: 5425161366000 obtained on 10 November 2022).
Figure 9. Various methods for the modifications in the properties of biochar (reproduced with permission from [34], License number: 5425161366000 obtained on 10 November 2022).
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Figure 10. Co-doping of biochar with various metals and minerals to produce biochar composites for their applications in various areas (reproduced from [33] under open access Creative Common Attributions Non-commercial License).
Figure 10. Co-doping of biochar with various metals and minerals to produce biochar composites for their applications in various areas (reproduced from [33] under open access Creative Common Attributions Non-commercial License).
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Figure 11. Effect of biochar application on soil physicochemical and biological properties, as well as on plant growth (reproduced with permission from [142] under License number: 5430160064883).
Figure 11. Effect of biochar application on soil physicochemical and biological properties, as well as on plant growth (reproduced with permission from [142] under License number: 5430160064883).
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Figure 12. Chemical structure of biochar (reproduced with permission from Zhang et al. [142] under License number: 5430160064883).
Figure 12. Chemical structure of biochar (reproduced with permission from Zhang et al. [142] under License number: 5430160064883).
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Figure 13. Mechanism of removal of heavy metals from soil using biochar (reproduced with permission from [148] under Creative Commons Attribution License).
Figure 13. Mechanism of removal of heavy metals from soil using biochar (reproduced with permission from [148] under Creative Commons Attribution License).
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Figure 14. Mechanism of heavy metals adsorption or removal using various functionalized biochar composites (reproduced with permission from [158] under license number: 5431750879544).
Figure 14. Mechanism of heavy metals adsorption or removal using various functionalized biochar composites (reproduced with permission from [158] under license number: 5431750879544).
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Table
Table 1. Typical physicochemical characteristics of mine soil.
Table 1. Typical physicochemical characteristics of mine soil.
S. No.Study AreaTexture ClassBulk Density (Mg/m3)Moisture Content %pHOrganic Matter (%)Available N (mg kg−1)Available P (mg kg−1)Exchangeble K (mg kg−1)CEC (cmol/100 g Soil)References
1Viswakarma opencast mines Jharia coal fieldSandy loam1.166.566.531.6782.078.7626.9814.88[46]
2Daliuta Mining area, Shanxi Province, Northwest ChinaSandy loam1.609.075.673.06----[47]
3Mining city, Anhui Province, China-1.37-7.922.150.7218.9653.82-[48]
4Pingshuo mine, China-1.62-8.701.78206.2552-[49]
5Silesian Upland in Southern Poland-- 5.70--14.90212.10-[50]
6East-central Ningxia Province, Northwest China--7.588.043.203.245.80158.70-[51]
7Antaibao opencast coal mine, Pingshuo, Shanxi ProvinceSandy loam1.356.458.00--4.91151.30-[52]
8Cartagena-La Union mining district, SE Spain---7.401.64----[53]
9Latiand Sambarata open coal mine sites, Berau Regency, East KalimantanProvince-1.65-4.500.87-2.40103-[54]
10Mahakam Sumber Jaya open-pit mines, East Kalimantan Province, IndonesiaSilty loam1.8319.093.271.42-5.02296.4014.69[55]
11Pingshuo Mine, ChinaSandy loam1.42-8.121.04-4.7382.63-[56]
12Bastacolla, Jharia Coalfield, Dhanbad, Jharkhand, IndiaSandy loam-13.185.861.03194.050.7095.886.16[57]
Table
Table 2. Standards for the interpretation of geoaccumulation index values [62,63].
Table 2. Standards for the interpretation of geoaccumulation index values [62,63].
ClassIgeo ValuesPollution Level in the Soil
0<0Practically uncontaminated
10–1Uncontaminated to moderately contaminated
21–2Moderately contaminated
32–3Moderately to heavily contaminated
43–4Heavily contaminated
54–5Heavily to extremely contaminated
6>5Extremely contaminated
Table
Table 3. Effect of heavy metals on human health (Source: [66]).
Table 3. Effect of heavy metals on human health (Source: [66]).
S. No.Heavy MetalTarget OrgansHealth Impacts
1Arsenic (As)Pulmonary, Nervous System, SkinPerforation of Nasal Septum, Respiratory Cancer, Peripheral Neuropathy, Dermatomes, Skin Cancer
2Cadmium (Cd)Renal, Skeletal, PulmonaryProteinuria, Glucosuria, Osteomalacia, Aminoaciduria, Emphysema
3Chromium (Cr)PulmonaryUlcer, Perforation of Nasal Septum, Respiratory Cancer
4Manganese (Mn)Nervous SystemCentral And Peripheral Neuropathies
5Lead (Pb)Nervous System, Hematopoietic System, RenalEncephalopathy, Peripheral Neuropathy, Central Nervous Disorders, Anemia
6Nickel (Ni)Pulmonary, SkinCancer, Dermatitis
7Tin (Sn)Nervous System, PulmonaryCentral Nervous System Disorders, Visual Defects, EEG Changes, Pneumoconiosis
Table
Table 4. Distributions of heavy metals in the soil in the mining areas around the world.
Table 4. Distributions of heavy metals in the soil in the mining areas around the world.
S. No.Study AreaHeavy Metals (mg/kg)Reference
CuPbZnCrNiCd
1Shizishan Mining Area7967139N.A.330.71[48]
2All Opencast Coal Mining Pits, China28.6028.2677.9463.8627.910.24[66]
3Lakhra Coalfield, Province of Sindh, Pakistann. a.2.38n. a.n. a.n. a.3.46[66]
4Rohini Opencast Mining Area, North Karnpura, Jharkhand, Indian. a.6.57n. a.15.29n. a.1.40[67]
5Raniganj Coal Mining, West Bengal, India677256.6893.7851.7811.4n. a.[68]
6Coal Mining Region, Tai’an City, Shandong Province, China26.5527.6266.6820.6629.610.20[69]
7Jharia Coalfield, Jharkhand, India11.3611.4319.9023.4011.380.80[70]
8Barapukaria Mining Area, Bangladesh31.66n. a.101.9682.3756.54n. a.[71]
9Wuhai Coal Mining Area, China19.602855.2061.7024.700.16[72]
Table
Table 5. Plant species used for the phytoremediation of heavy metal-contaminated soil.
Table 5. Plant species used for the phytoremediation of heavy metal-contaminated soil.
S. No.Type of ContaminationPlant Species UsedInitial Concentration
(mg kg−1)		BCFshootType of StudyReference
1Pb and Cr contaminated soilParthenium hysterophorus
Cannabis sativa		Pb = 14.54
Cr = 4.48		Parthenium (Pb) = 1.01
(Cr) = 0.58
Cannabis
(Pb) = 1.03
(Cr) = 0.54		Field study[79]
2Cu, Zn, and Cd contaminated soilPhyllostachys praecoxCu = 195
Zn = 2980
Cd = 14.5		Cu = 0.18
Zn = 0.33
Cd = 0.26		Field study[80]
3Cd contaminated soilMedicago sativa
Bidens Pilosa		Cd = 0.0312.90Pot-culture experiment (40 days)[81]
4Multi metals contaminated soilRhazya strictaCd = 50
Pb = 10
Cu = 10
Zn = 10		Cd = 1.48
Pb = 0.36
Cu = 0.52
Zn = 1.46		Pot-culture experiment (3 months)[82]
5Multi metals contaminated soilSwitchgrassZn = 68.1
Cr = 35.9
Pb = 16.9
Ni = 27		-Pot-culture experiment [83]
6Multi metals contaminated soilWheat CropCr = 70.2
Cu = 339
Zn = 202
Pb = 156
Ni = 113		Cr = 8.2
Cu = 2.2
Zn = 2.6
Pb = 2.8
Ni = 2		Pot-culture experiment (10 weeks)[84]
7Multi metals contaminated soilRobina pseudoacaciaCu = 3166.7
Cd = 3.66
Pb = 137.06		Cu = 0.044
Pb = 0.170
Cd = 1.358		Field study[85]
8Anshan Mining, Lioning Province, ChinaSoybeanZn = 149.5
Pb = 42.1
Cd = 0.8		-Field study (105 days)[86]
Table
Table 6. Microbes used for the bioremediation of heavy metals in the soil.
Table 6. Microbes used for the bioremediation of heavy metals in the soil.
S. No.Type of Microbes UsedSite DescriptionHeavy Metals RemediatedReference
1M. Circinelloides
T. asperellum
Mortierella sp.		Metal-contaminated mine tailings, Anshan Mining Group, Lioning Province, ChinaZn, Cu, Pb, and Cd[86]
2Bacillus Idriensis strains
B. subtilis BR151		Neighborhood of Lead and Zinc smelters, Northern FranceCd[91]
3Perenniporia subtephropora
Cerrena aurantiopora
Aspergillus niger MH541017
Aspergillus fumigatus		MSW Landfill, Jinjang Utara, Kuala LumpurNi, Pb, and Zn[92]
4 Rhizobium -legume Heavy metal-contaminated soil Cd, Cu, and Pb[93]
5Agrocybe AegeritaArtificially contaminated soilNi and Cd[94]
6Eisenia fetida
Cylindrospermum stagnale		Artificially contaminated soilCd [95]
7Agrococcus
Streptomyces
Microbacterium		Chromium-contaminated site in Henan ProvinceCr (VI)[96]
8Sporosarcina pasteuriiArtificially contaminated soilPb[97]
9Acaulospora melleaVicinity of a Pb and Zn smelter located in Dongtang Town, Shaoguan City, Guangdong Province, ChinaPb and Zn[98]
10Phanerochaete chrysosporiumAbandoned mining area in Hechi City, Guangxi Province, ChinaAs, Cd, Cr, V, Sn, and Zn[99]
11Bacillus thuringiensis HM—311Contaminated farmland soil in Jiangning District, Nanjing, Jiangsu ProvincePb and Cd[100]
12SRB 1 (Clostridium, Desulfosporosinus, and Desulfovibrio genera)Huilong nonferrous metal smelter, Guangxi Province, ChinaAs[101]
Table
Table 7. Biochar used as a soil amendment in combination with plant species for the phytoremediation of mine soil.
Table 7. Biochar used as a soil amendment in combination with plant species for the phytoremediation of mine soil.
S. No.Feedstock UsedPyrolysis ConditionsMine Soil TypeHeavy Metals PresentPlant Species UsedInferencesReferences
1Orchard pruning residuesSlow pyrolysis at 500 °CLa Mina Monaca Mine site, SpainCd, Cu, Pb, As, and ZnZea MaysBiochar amendment significantly reduced the mobility of heavy metals in the aqueous soil medium[163]
2Rice strawn. a.Lanping Pb–Zn mines, Yunnan province, ChinaCd and PbMaizeBiochar application decreased the availability of both total and plant-available contents of Pb and Cd in the soil[164]
3Sewage sludgeSlow pyrolysis at 500 °CContaminated soil, Sergipe experimental station, Northeast BrazilCuCorn seedsBiochar was applied at the rates of 30 t ha−1 and 60 t ha−1 reduced the exchangeable fraction of Cu by 96.2 and 57.5%, respectively[165]
4Hardwood wasteSlow pyrolysis at 500 °C for one hourDegraded mined-out land Cr, Cu, Zn, Pb, and MnOryza sativaBiochar applied at a rate of 3% (w/w) reduced the available contents of Cr, Cu, Zn, Pb, and Mn by 99.1, 71.7, 61.7, 36.4, and 47.9%, respectively[166]
5Chicken manure and Oat hullSlow pyrolysis at 500 °C and 300 °C, respectivelyVentanas Cu smelter, National Copper Corporation of Chile (CODELCO), Puchuncaví, Valley of Central ChileCuOenothera picensisApplications of biochars at the rates of 1 and 5% reduced the exchangeable and carbonate-bounded fractions of Cu in the contaminated soil[167]
6Eucalyptus woodSlow pyrolysis at 400 and 600 °C for 150 minCoal mine, Bastacolla area, Dhanbad, Jharkhand, IndiaCr, Zn, Ni, and CoAccacia AuriculiformisApplication of different temperatures biochar applied at the rates of 0.5, 1, 2, 3, and 5% (w/w) significantly reduced the concentrations of total and exchangeable heavy metals in the soil[57]
7FishboneSlow pyrolysis at 600 °CPolluted soil
located in an area near a gold mine in the governorate of Mahd ad-
Dahab, Saudi Arabia		Pb, Cu, Zn, and CdYecora RojoFishbine biochar application at a rate of 30 g kg−1 reduced the mobility of Pb, Cu, Zn, and Cd by 43.0, 66.2, 55.6, and 33.8%, respectively[168]
8Rice straw
Coconut shell
Sludge		Slow pyrolysis at 500 °CMount Manao abandoned lead-zinc mine areas in Chenzhou city, Hunan Province, ChinaCd, Cu, Pb, and Zn-Application of rice straw, coconut shell, and sludge-derived biochar at the rates of 0.5, 2.5, and 5% improved the soil’s physicochemical properties and reduced the mobility of heavy metals[169]
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Chandra, S.; Medha, I.; Tiwari, A.K. The Role of Modified Biochar for the Remediation of Coal Mining-Impacted Contaminated Soil: A Review. Sustainability 2023, 15, 3973. https://doi.org/10.3390/su15053973

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Chandra S, Medha I, Tiwari AK. The Role of Modified Biochar for the Remediation of Coal Mining-Impacted Contaminated Soil: A Review. Sustainability. 2023; 15(5):3973. https://doi.org/10.3390/su15053973

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Chandra, Subhash, Isha Medha, and Ashwani Kumar Tiwari. 2023. "The Role of Modified Biochar for the Remediation of Coal Mining-Impacted Contaminated Soil: A Review" Sustainability 15, no. 5: 3973. https://doi.org/10.3390/su15053973

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