Introduction to Biochar

Biochar, also known as biomass charcoal, is a class of stable, highly aromatic, porous solid material with high organic carbon content produced by thermal cracking of biomass feedstock at high temperatures under anoxic or anaerobic conditions [17-19]. Biochar can be prepared from a wide range of raw materials, and currently the common raw materials for its preparation are agricultural wastes such as straw and husk, organic wastes such as municipal sludge and garbage, animal bones and feces, etc. [20-23]. As a suitable soil amendment material, biochar exhibits many advantages. Biochar has been proved by a number of studies to be characterized by high specific surface area, porosity, high pH, and strong solidification of heavy metals in soil [24]. It has been found that biochar has a stable carbon structure, which can exist in the soil for hundreds or even thousands of years without being degraded, and has a strong carbon sequestration effect [25], thus contributing to the mitigation of the greenhouse effect, and playing a role in increasing the soil carbon pool storage. Structure determines nature, as biochar has a relatively large specific surface area, rich porous structure, and a large number of functional groups on its surface, such as carboxylic acid, carbonyl, and hydroxyl, etc., which creates a strong adsorption capacity for heavy metals and other organic pollutants, and thus reduces the environmental risk of some of the pollutants. Biochar applied to the soil can provide a rich source of carbon to the soil [26], which in turn increases the soil organic matter content. Biochar application can improve the microstructure of the soil, as well as increase the abundance of soil microorganisms and significantly increase the activity of soil enzymes [27-28], which can help to enhance the yield of crops.

The physicochemical properties of biochar determine its application potential, and the selection of biomass feedstock, preparation method, pyrolysis temperature and time are the key to determine its physicochemical properties [29]. Therefore, the differences in biomass feedstock and preparation conditions have certain effects on the structure and performance of biochar. Meanwhile, the research in recent years mainly focuses on the adsorption effect of single biomass raw material biochar on single heavy metal solution under different preparation conditions, while there are fewer researches on the remediation of tailings with composite heavy metal contamination by different raw material biochar.

Preparation of Biochar and Influencing Factors

Biochar preparation methods mainly include pyrolysis, hydrothermal carbonization and microwave carbonization [30]. The physical and chemical properties of biochar, such as specific surface area, pore structure, type and number of functional groups, are closely related to its preparation method. Compared with pyrolysis, hydrothermal carbonization does not require drying and has a higher biochar yield [31]. Biochar prepared by hydrothermal and microwave carbonization methods contains high concentrations of organic matter and is not actually considered as a soil remediation material [32], thus pyrolysis is the most commonly used method for biochar preparation. Pyrolysis is the heating of biomass feedstock at high temperatures (300-1000ºC) under anaerobic or low-oxygen conditions, which causes chemical changes in the feedstock due to the increase in temperature and the cleavage of macromolecules into small molecules, resulting in the production of biomass oils, biochar, and combustible gases [33]. The physicochemical properties of biochar determine its potential for application as an amendment to improve the quality of soil environment. Although there are many factors affecting the physicochemical properties of biochar, biochar is produced by pyrolysis of biomass feedstocks, so biomass feedstocks and pyrolysis temperature are important factors affecting the physical and chemical properties of biochar [34-35].

Various kinds of solid waste biomass are commonly used as raw materials for the preparation of biochar, mainly including straw, animal manure, food waste, fruit peels and sludge. Biochar prepared by pyrolysis can be divided into fast pyrolysis process and slow pyrolysis process according to the different heating rates [36], and the biochar prepared by the two pyrolysis processes has different physicochemical properties [32]. Some researchers believe that the pyrolysis temperature can affect the specific surface area of biochar, thus affecting its adsorption capacity for heavy metal Cd [37]. And this difference in pyrolysis temperature leads to different specific surface area, pore structure, EC value, pH value, ash content and N and P content of biochar [38-40], which further affects the remediation effect of biochar on soil environment as a soil amendment. Numerous studies have shown that different pyrolysis temperatures and feedstocks affect the ash content, carbon content, aromaticity and pH of biochar, which in turn affects the remediation of heavy metal pollutants by biochar [41]. The composition, structure, and physicochemical properties of biochar are mainly influenced by the biomass feedstock and pyrolysis conditions. In addition, factors such as pyrolysis time, atmosphere conditions, and rate of temperature increase also directly affect the basic physicochemical properties of biochar [42]. Table 1 compares the differences in physicochemical properties of biochar prepared from different biomass feedstocks and different pyrolysis conditions.

Main Physical and Chemical Properties of Biochar

Biochar is a carbon-rich organic material obtained by pyrolyzing biomass under low or no oxygen conditions. The physicochemical properties of biochar mainly include pore structure, specific surface area, ash content, pH value, and the type and number of surface functional groups. A large number of studies in recent years have confirmed that biochar usually has a large specific surface area, high porosity, high pH, and a large amount of nitrogen, phosphorus, potassium and other elements. Studies have shown that biochar has a better sustained carbon sequestration capacity, and the carbon it contains can be sequestered for a long period of time, which can reduce the emission of greenhouse gases [50]. The unique pore structure of biochar can be effectively returned to the field to reclaim the soil, and the nutrients it contains can improve the soil nutrient status [51]. In the process of biochar pyrolysis preparation, the pore structure of the raw material is mostly preserved intact, and thus the prepared biochar generally has a larger specific surface area and a greater number of surface functional groups [52]. Biochar has a relatively stable porous aromatic carbon skeleton and rich functional groups within it, which to a certain extent determines its strong adsorption capacity and stabilizes heavy metals and other pollutants in the soil [53]. In addition, biochar has high carbon content and strong stability, and can exist in the soil for hundreds of years.

Different biomass feedstocks as well as different preparation processes can lead to differences in the physicochemical properties of the produced biochar. Biochar is usually alkaline due to the reduction of acidic functional groups during pyrolysis at high temperatures [54]. Vaughn et al. [55] found that the pH and K content of rice straw biochar were higher than that of wood biochar; another study [56] found that legume straw charcoal was better than non-legume straw charcoal in terms of the improvement effect of acidic red soils, which may be related to the fact that legume straw biochar has a higher pH than that of non-legume straw biochar. This may be related to the higher pH value of legume straw biochar than non-legume straw biochar. In addition, pyrolysis temperature also affects the pore volume and specific surface area of biochar, etc. Park et al. [57] found that the specific surface area of sesame straw biochar increased from 46.9 to 289.2 m²/g, and the total pore volume increased from 0.0716 to 0.1433 cm³/g as the pyrolysis temperature was increased from 500 to 600ºC. The porous nature of the biochar, its large specific surface area, and the abundance of functional groups on the surface (-OH) were also found to be a significant factor. functional groups (-OH, -COOH,-C=O, etc.) on the surface of biochar make it have strong adsorption capacity for heavy metals [58].Deng et al.. [59] found that the surface functional group of biochar, -C=C-, plays an important role in the adsorption of Cd, whereas -C=O plays a major role in the adsorption of Cu. Therefore, the physicochemical properties of biochar determine its potential for remediation of soil heavy metal pollution, and it is a low economic cost, ecologically safe, wide source of raw materials, and renewable material for remediation of soil pollution; and at the same time, it can be applied to the removal of heavy metals and organic pollutants from water bodies by preparing biochar adsorbents [60].

Soil Amendment by Biochar

The ash in biochar contains mineral elements Ca, Mg, etc. and their oxides or carbonates, etc., thus biochar is generally alkaline and can be used for acidic soil improvement. The main reason why biochar can be used for soil amelioration is that biochar can increase the pH, soil ion exchange capacity and microbial activity of acidic soils [50]. Nitrogen, phosphorus, potassium and other elements contained in biochar can provide nutrients directly to the soil, which in turn improves the quality of the soil environment. Guo Xiongfei et al. [61] investigated the effect of applying biochar on soil nutrients through potting experiments, and the results showed a significant increase in soil total nitrogen and phosphorus content. Wang Guijun et al. [62] added biochar to different degrees of saline soils and found that the quick-acting nitrogen content of the soil decreased, and quick-acting phosphorus and effective potassium content increased. And some studies also found that the effect of biochar on soil nitrogen levels before and after its application did not change significantly [63]. Soil nutrient cycling and fertility improvement cannot be separated from the role of microbial communities, and Warnock et al. [64] suggested that biochar can indirectly affect plant-mycorrhizal interactions by changing the physicochemical properties of soil. It has been found [65, 66] that biochar affects the microbial-driven cycling and morphological transformation of mineral elements such as C, N, and P in the soil, which in turn improves soil fertility. The porous structure and surface properties of biochar can provide attachment sites and living space for microorganisms, which in turn can promote microbial communities to improve soil fertility.

Currently, biochar is mainly applied to improve acidic soils, improve fertility-poor soils and remediate heavy metal polluted soils [51]. Biochar surface is rich in oxygen-containing functional groups, which can provide more exchange sites and contribute to the increase of soil cation exchange, thus promoting the sequestration of nutrients in the soil [67]. Zhang Wen et al. [68] applied 20 t/hm² of charcoal biochar to the soil and found that the soil cation exchange value increased from 5.7 cmol/kg to 5.9 cmol/kg. Biochar application can reduce the accumulation of soil heavy metals in plant tissues. Khan et al. [69] demonstrated that application of biochar extracted from manzanita significantly increased spinach biomass and reduced Ni accumulation in spinach tissues. In addition, the soil improvement by biochar and the remediation of soil heavy metal pollution need to consider the amount of biochar applied, and the addition of the appropriate amount of biochar can often achieve a better soil improvement effect. At the same time, the porous structure of biochar can provide a certain habitat for soil microorganisms to help their populations to reproduce, and at the same time, it can also provide a place for enzymatic reactions, and achieve the effect of promoting and slowing down the enzymatic reactions [70], which to a certain extent can affect the activity of soil enzymes.

The main properties of biochar determine its potential for soil improvement. The main properties of biochar for soil improvement are summarized in Fig. 1.

Adsorption Mechanism of Soil Heavy Metals by Biochar

Heavy metals exist in soil in the form of exchangeable heavy metals, carbonate-bound heavy metals, iron and manganese oxide-bound heavy metals, organic matter-bound heavy metals, and residual heavy metals [71]. Biochar can effectively adsorb and passivate heavy metals in soil, and its passivation effect is closely related to the unique physicochemical properties of biochar itself, and the mechanism of passivation of soil heavy metals by biochar made from pyrolysis of different biomass raw materials is also different [48]. The mechanism of biochar on soil heavy metals mainly includes the following aspects: electrostatic effect, ion exchange, complexation effect, precipitation and physical adsorption, etc., and its interaction mechanism is shown in Fig. 2 [50].

Electrostatic Effects

The surface of biochar is rich in oxygen-containing functional groups, which can generate a certain amount of negative charges. These negative charges attach to the surface of biochar and attract positively charged metal ions through electrostatic action, thus realizing the adsorption of heavy metal cations in the soil [67], thus achieving the purpose of remediation of soil heavy metal pollution. The electrostatic interaction between biochar and heavy metal ions is closely related to the pH of the soil environment and the zero-charge point of biochar [72-73]. When the pH of the soil environment increases, the negative charge carried by the soil colloid increases to a certain extent, and more heavy metal cations are adsorbed through electrostatic action [74-75]. When the soil pH is greater than the zero-charge point of biochar, the surface of biochar is negatively charged and can be electrostatically adsorbed with heavy metal cations; conversely, biochar is electrostatically adsorbed with heavy metal anions [76]. In addition, the biochar formed from the pyrolysis of biomass has a large specific surface area and a developed pore structure, which increases its contact area with heavy metal ions and thus improves the electrostatic adsorption of heavy metal ions by biochar. The electrostatic adsorption of biochar with heavy metal ions was enhanced with the increase of the initial concentration of heavy metals [77].

Fig. 1.

Main properties and role of biochar.

Fig. 2.

Mechanisms of action of biochar on soil heavy metals.

Physical Adsorption

The porous structure and large specific surface area of biochar can enable heavy metal ions to be adsorbed on its surface or diffuse into the micropores, thus realizing the physical adsorption of heavy metal ions by biochar. It was found that silica fertilizer (Na₂SiO₃·9H₂O) was able to reduce the content of exchangeable Pb in soil through physical adsorption [87], and rice straw biochar containing Si was more significant in reducing the Pb content in soil [88]. Porosity and specific surface area are important factors affecting the physical adsorption of biochar. Generally, biochar prepared under high temperature conditions has larger porosity and specific surface area, and its ability to adsorb heavy metals is also larger [89]. Ji Haiyang et al. [90] used silk quilt waste as raw material to prepare biochar by pyrolysis at 300, 500, and 700ºC, respectively, and investigated its adsorption of Cd²⁺, and the results showed that the specific surface area of the biochar prepared at 700ºC as well as the amount of adsorbed Cd²⁺ were the largest, which were 37.8 m²/g and 91.07 m²/g, respectively.

In addition, the mechanism of action of biochar on soil heavy metals is often not a single action, but a combination of multiple actions. Jiang et al. [91] conducted soil cultivation experiments using rice straw biochar, and the results showed that the adsorption of soil Pb(II) by rice straw biochar gradually increased with the increase of the addition amount, and it was concluded that Pb(II) was adsorbed and immobilized on the surface of biochar mainly through ion exchange and complexation with oxygen-containing functional groups on the surface of biochar. Ding et al. [92] investigated the effect of adsorption effect and mechanism of sugarcane bagasse biochar made at different pyrolysis temperatures on Pb. The experimental results showed that the adsorption mechanism of Pb by biochar made at low pyrolysis temperatures included complexation with functional groups, cation exchange and precipitation, whereas the biochar made at high pyrolysis temperatures mainly involved precipitation and intra-granular diffusion. Wang Hong et al. [93] found that the adsorption rate of water hyacinth charcoal on Zn and Pb in soil gradually increased with the increase of pyrolysis temperature and biochar addition, and that ion exchange and complexation were the main mechanisms of biochar remediation of soil heavy metal pollution. Table 2 summarizes the effect and mechanism of adsorption of soil heavy metals by biochar from different raw materials.

In summary, the solidification and stabilization mechanisms of biochar for different heavy metals are somewhat different; the passivation mechanisms of biochar produced under different biomass sources or pyrolysis conditions are also different due to the differences in their physicochemical properties. At the same time, the remediation of heavy metals in soil by biochar is often characterized by the co-existence of multiple mechanisms of action.

Hazards of Soil Heavy Metal Pollution

Due to natural or man-made reasons, heavy metal ions enter the soil environment and accumulate to a certain extent, exceeding the self-purification capacity of the soil, making the quality of the soil environment deteriorate, that is, causing soil pollution. Heavy metal elements that pollute the soil environment mainly include mercury, cadmium, lead and copper and other biotoxic elements. Heavy metals in soil are difficult to degrade, hidden, long-term accumulation, irreversible and highly toxic [100]. One of the major characteristics of soil heavy metal pollution is that it is difficult to degrade and more difficult to remove, and the vast majority of heavy metal substances can be decomposed by microorganisms such as bacteria and fungi in the soil by utilizing them, unlike ordinary organic matter. In addition, heavy metals in soil can be absorbed by crops and plants, and enter the animal body or human body through the food chain enrichment pathway, thus directly or indirectly causing harm to animal and human health; at the same time, heavy metals in soil can also be toxic to plants, and their accumulation in plants will cause changes in metabolic pathways, growth retardation, and yield reduction [101]. Heavy metals will continue to accumulate in the soil after entering the soil, changing the physicochemical properties of the soil and affecting the structure, abundance and diversity of the soil microbial community, thus affecting the stability of the whole soil ecology [102-103]. Some heavy metal elements (e.g., Zn, Cu, Mn, etc.) are necessary for plants and animals to carry out their life activities, but they can be harmful in excess of appropriate amounts [104]. Different heavy metal elements cause different hazards, such as Cd reduces the photosynthetic efficiency of green plants and inhibits further plant growth [104]; Pb has an obvious toxic effect on the root system of plants, resulting in thinning of roots, black color, and rotting of the root crown [105]. Soil heavy metal pollution can also seriously affect human health. Chronic Cu poisoning will cause vomiting, memory loss, etc. [105]; Pb will have more obvious damage to the nervous system of the brain as well as cardiovascular and cerebral vessels, etc. [106]. Fig. 3 represents the main impacts caused by heavy metal contamination of soil.

Therefore, the harm of soil heavy metal pollution should not be underestimated. The prevention and remediation of soil heavy metal pollution is of great significance for ecological environmental protection, human health protection and sustainable social development.

Application of Biochar in the Remediation of Heavy Metal Pollution in Tailings Soil

Potential of Biochar for Remediation of Heavy Metals in Tailings Soils

While mining and utilization of mines bring social and economic benefits, they also produce certain environmental pollution problems. As the tailings produced by mining contain heavy metals such as copper, lead, cadmium, etc., the disposal of tailings in stockpiles adversely affects the soil ecosystem [107]. For the soil around the mining area, the random accumulation and discharge of large quantities of waste rock, waste water, and sludge generated by industrial activities without effective treatment can lead to the migration of large quantities of heavy metals to the surrounding areas of the mining area [108]. At the same time, due to the specificity of mining activities and the complexity of the composition of the associated mineral metal elements [8], the heavy metal pollution of the tailings waste area and the surrounding soils is often accompanied by a variety of heavy metal composite pollution. In recent years, soil heavy metal pollution remediation technology is constantly developing, forming physical remediation, chemical remediation, biological remediation and joint remediation methods. The management measures of tailings mainly include engineering remediation, chemical remediation and bioremediation [109], of which chemical remediation mainly includes techniques such as drenching, solidification and stabilization. Stabilization is a technology that stabilizes the tailings by adding organic or inorganic amendments such as biochar and lime to the tailings soil in order to reduce the concentration of heavy metals in the tailings through surface complexation, precipitation, ion exchange, and adsorption, thus reducing the mobility of heavy metals in the tailings [110]. Due to the special characteristics of the soil environment around the mining area, the use of in situ passivation technology to manage soil heavy metal pollution is simple, fast, and low-cost, which is suitable for the large-scale remediation of tailings soil. And the raw material of biochar has a wide source, which can be applied to the in situ passivation remediation of heavy metals in tailings soil.

Fig. 3.

Main hazards of heavy metal pollution of soil.

Biochar is a promising material for remediation of soil heavy metal pollution due to its developed pore structure, large specific surface area and abundant oxygen-containing functional groups, which makes it a good application prospect in the in situ remediation of heavy metals in soil and the development of highly efficient heavy metal adsorbent materials. Biochar can effectively reduce the biological activity and toxicity of heavy metals in contaminated soil [111, 112], and has the role of remediation of heavy metal contaminated soil, thus reducing the environmental risk of soil heavy metals. The potential application of biochar in tailings heavy metal contaminated soils is enumerated in Fig. 4. Currently, studies [13-15] have shown that biochar has a better application prospect in metal mine contaminated soil and site remediation, etc.

Remediation of Heavy Metals in Tailings Soil by Biochar

In recent years, some researchers have explored the use of biochar for the remediation of heavy metals in tailings soil, and found that biochar has good remediation effect on soil heavy metal pollution. Fig. 5 represents the process of adsorption and removal of heavy metals from tailings soil by biochar. Table 3 compares the remediation effect of different biochar on heavy metals in mine soil. The study showed that surface modification treatment of biochar can realize the improvement and enhancement of biochar performance, which can help to improve the effect of biochar on the remediation of soil heavy metal pollution. Zhang Zezhan et al. [113] investigated the immobilization of Pb and Zn in tailings contaminated soil by preparing phosphate-modified biochar, and the results showed that the adsorption of heavy metals Pb and Zn by KH₂PO₄ modified biochar increased significantly. It was also found by [114] that the application of Hibiscus sabdariffa core biochar, sludge biochar and chicken manure biochar could directly immobilize heavy metal contamination in tailings and help to improve the stability of the tailings microbial community.

In addition, the use of biochar combined with phytoremediation and other methods to repair the tailings soil, to a certain extent, can reduce the effectiveness of heavy metals in the mining area and other soils, so as to achieve the enhancement of the effect of soil heavy metal management.Jun et al. [122] applied different amounts of lychee twig biochar to heavy metal contaminated soils in the mining area and promoted the growth of sunflower plants, of which the 5% biochar addition was the best, and the heavy metal concentration in the soil decreased significantly. Pandey et al. [123] applied 3% biochar to heavy metal contaminated soil and planted wheat at the same time, which showed that compared with the control group without biochar, the toxicity of heavy metals in soil and the toxicity of heavy metal enrichment in plants were reduced by biochar treatment, and it could promote the growth of wheat. Meanwhile, some researchers have used various raw materials to prepare biochar and explored its remediation effect on soil heavy metals in mining areas. Tao Ling et al. [124] investigated the control effect of prepared sludge-biotite co-pyrolysis biochar on heavy metals in contaminated soils in mining areas, and the results showed that after the addition of this biochar for the passivation of heavy metals in contaminated soils in mining areas, the content of Cu, Cd, Ni, Zn, and Cr in the soil was kept at a low risk level. In recent years, as a very potential, environmentally friendly, carbon sequestration function, and a wide source of raw materials for the remediation of soil heavy metal pollution, biochar has received more attention and research, and its application to the remediation of heavy metal pollution in tailings soil has a broader prospect.

Fig. 4.

Potential of biochar for heavy metal remediation of tailings soils.

Fig. 5.

Adsorption of heavy metals from tailings soil by biochar.

Table 3.

Remediation effect of different biochar on heavy metals in tailings soil.

Raw material for biochar	Objects of restoration	Effects of the restoration	Mechanisms of action of biochar on soil heavy metals	References
Pig manure, fruit	Heavy metals in contaminated soil from agricultural land in mining areas	Pig manure biochar was better than fruit biochar in terms of remediation, and the remediation was better as the amount of both types of biochar added increased.	Adsorption	[115]
Soybean straw	Cu, Zn, Pb and Cd in soil contaminated by lead and zinc tailings	3% biochar can inhibit the decrease of soil pH caused by the pollution of lead and zinc tailings; biochar can significantly reduce the content of heavy metals in the roots of polluted soil cabbages, and 3% biochar can prevent the migration and enrichment of Cu, Zn, Pb, and Cd from the polluted soil of lead and zinc tailings to the aboveground part of cabbages.	Passivation	[116]
Rabbit excrement	Soil from a mining site in Spain	It was shown that biochar prepared at 450°C and 600°C applied with oilseed rape was able to reduce the content of As, Cu, Co, Cr, Se and Pb in the soil; the addition of biochar resulted in a decrease in the transfer factor of Co, Cr, Cd, Cu, Ni, Zn, Pb, and As in the soil, and the accumulation of these metals within the roots of the plants. In all cases, the addition of biochar increased biomass production.	Adsorption	[117]
Ginkgo biloba	Contaminated soil from a lead-zinc mine in Panzhihua, Sichuan Province	NaOH and nitrogen-doped modified treatment of Ginkgo biloba biochar were used for soil cultivation experiments, and the results showed that both modified biochars could achieve the stabilization of the active state Pb, Cd, and Cu in the soil, and at the same time, they could increase the pH and nutrient element content of the soil, in which nitrogen-doped modified biochars had a better effect of enhancing the nitrogen in the soil, and the NaOH-modified biochars had a greater increase of the phosphorus and potassium content in the soil.	Ion exchange, precipitation, complexation, adsorption	[118]
Rice straw, corn stalks	Soil of a lead-zinc mining area in Kaili, Guizhou	The addition of rice straw biochar and corn stover biochar for the release of heavy metals from the soil of lead and zinc mining area under acid rain leaching conditions showed that different biochar had different effects on the leaching of Pb, Zn, Cd and As. The addition of corn stover biochar significantly reduced the leaching of heavy metals Pb from the soil of the mining area, and its cumulative effluent was reduced by 49.11%, but it had a significant The rice straw biochar accelerated the release of Pb from the soil, and had no obvious effect on Zn and Cd, and both biochars had an activating effect on soil As.	Functional group complexation, ion exchange, precipitation	[119]
Corn stalks	Soil of the main mining area of Baiyun’ebo	The addition of biochar for indoor mining soil leaching experiments showed that biochar could alleviate the acidity of the soil to a certain extent and reduce the risk of heavy metal migration to the lower soil, and that biochar had the best passivation effect on Pb.	Ion exchange, complexation, adsorption, precipitation	[120]
Black pine, red willow	Soil from a mine in Idaho, USA (contaminated with Cd, Cu, Pb and Zn)	The application of both types of biochar significantly increased soil pH, while being able to reduce the bioavailability of Cd, Cu, Pb and Zn in mining soils.	Precipitating effect	[121]