Biochar for Environmental Remediation

Abstract

The environment is deteriorating rapidly, and it is essential to restore it as soon as possible. Biochar is a carbon-rich pyrolysis result of various organic waste feedstocks that has generated widespread attention due to its wide range of applications for removing pollutants and restoring the environment. Biochar is a recalcitrant, stable organic carbon molecule formed when biomass is heated to temperatures ranging from 300°C to 1000°C under low (ideally zero) oxygen concentrations. The raw organic feedstocks include agricultural waste, forestry waste, sewage sludge, wood chips, manure, and municipal solid waste, etc. Pyrolysis, gasification, and hydrothermal carbonization are the most frequent processes for producing biochar due to their moderate operating conditions. Slow pyrolysis is the most often used method among them. Biochar has been utilised for soil remediation and enhancement, carbon sequestration, organic solid waste composting, water and wastewater decontamination, catalyst and activator, electrode materials, and electrode modification and has significant potential in a range of engineering applications, some of which are still unclear and under investigation due to its highly varied and adjustable surface chemistry. The goal of this chapter is to look into the prospective applications of biochar as a material for environmental remediation.

1. Introduction

Biochar (biomass-derived char) is a versatile renewable source and is gaining popularity due to its diverse raw material sources, high porosity, large surface area, surface functional groups, and high treatment efficacy for a variety of contaminants [1]. Biochar is produced from three types of materials (plant residue, sewage sludge, and animal litter) that are pyrolyzed with little or no oxygen (typically below 1000°C) [2]. Biochar production not only deals with waste, but also benefit from waste, for example, pyrolysis of sewage sludge can reduce pollutants and turn it into a valuable resource [3]. Therefore, it is a great way to make biochar out of solid waste. Because of its unique properties, biochar has sparked widespread concern about its potential for use in the environment [4]. As indicated by the increase in the number of published publications regarding biochar in the last 10 years, it has gotten a lot of attention (Figure 1). Biochar’s main technique for removing contaminants and remediating the environment is sorption. And, biochar’s sorption capacity is directly related to its physiochemical features, such as surface area, pore size distribution, functional groups, and cation exchange capacity, which vary depending on the preparation conditions [4]. Like, biochar produced at high temperatures has a larger surface area and carbon content than biochar produced at lower temperatures, due to the rising micro-pore volume caused by the elimination of volatile organic molecules at high temperatures [4]. The yields of biochar, on the other hand, decreases as the temperature goes up [6]. Therefore, in terms of biochar yields and adsorption capacity, an ideal synthesis method is required. To increase its physiochemical characteristics, biochar can further be modified with different chemicals like acids, alkalis, oxidizing agents, and ions for various environmental processes [7]. Due to its own properties such as large surface area, recalcitrance, and catalysis, biochar has been widely used in environmental applications such as soil remediation, carbon sequestration, water treatment, and wastewater treatment. In addition, biochar’s application for energy and as an agricultural amendment is not a new concept. Biochar has also found its application in climate change mitigation and as a renewable energy source [8]. Biochar’s use in engineering applications has received far less attention, despite the fact that economic estimates for biochar production for direct agricultural use have been poor for some time [9]. To that aim, a summary of our current understanding of biochar’s potential for use in a variety of environmental remediation applications, as well as emerging obstacles and prospects for biochar usage in environmental remediation, is discussed below.

Figure 1.

The number of articles published in recent 10 years. (Source: [5]).

2. Biochar mechanisms for contaminants removal

Biochar’s function mostly refers to its ability to uptake (e.g., sorption) other substances. The sorption of biochar can be divided into two categories, chemical sorption and physical sorption. Moreover, in term of biochar’s interaction with other substances, there are three types of interactions: sorption, catalysis, and redox as shown in Figure 2. Sorption is a major environmental process that has a major impact on pollutant biogeochemistry. In sorption, the surface properties of biochar, which includes surface functional groups (carboxyl, carbonyl, phenolic–OH, ester, aliphatic, aromatic, hydroxyl, amino, and azyl groups), surface charges, and free radicals, are important for the behaviour of the interface between biochar and organic and inorganic pollutants, as it provides important sites for sorption and catalytic degradation of pollutants. These functional groups can form hydrogen bonds with other substances, As a result, Biochar can adsorb a variety of pollutants, including organic compounds, metals, nutrients, gases, and microbes [11, 12]. Moreover, the removal of some contaminants are also achieved by partitioning, electrostatic interaction, and pore-filling between biochar and pollutants and depends largely on biochar and pollutant characteristics [5]. Biochar also aids in the transformation of abiotic contaminants through various methods such as free radicals mediated transformation. Free radicals on the surface of biochar can react with chemicals like hydrogen peroxide and persulfate and promote the breakdown of organic pollutants [13]. Apart from that, biochar surfaces contain a variety of catalytic sites, such as quinone and phenolic functional groups, as well as persistent free radicals (PFRs), they enable biochar-mediated pollutants transformation [14]. For example, surface functional groups like quinones, convert sulphide into polysulfides, which accelerates the breakdown of azo dyes by increasing electron transport [14]. PFRs on the surface of biochar have a high reactivity and act as a catalyst in pollutant breakdown [13]. Also, the dissolved fractions in biochar, which are primarily composed of aliphatic and aromatic with quinone-like structures, has been tested and found to enhance the photochemical transformation of many organic pollutants by generating reactive intermediates or reactive oxygen species (ROS) [15]. Surface redox active moieties are the main contributors to the redox of biochar even though there are only a handful of relevant reports in publication so far. The surface redox-active moieties in biochar can directly react with pollutants via non-radical pathways, as well as activate some oxidants to form reactive radicals like OH and SO₄. For example, OH generated from the activation of H₂O₂ in biochar reduces about 20% of p-nitrophenol (PNP); however, about 80% of PNP is degraded by directly interacting with reactive sites, most likely the hydroquinone in biochar. Therefore, biochar not only enhances the degradation or transformation of pollutants by facilitating the transfer of electrons as a catalyst, but it can also directly react with pollutants, which will have a significant influence on the environmental behaviour of contaminant [16]. Apart from that, In terms of element composition, the major elements that make up the matrix of biochar are C, H, O, and N, while other elements like Si, P, and S have varying mass percentages in different biochars and play a special or even major role in sorption of various other specific pollutants. For example the sorption of Pb and Al on biochar is attributed to coprecipitation with P and Si in the biochar as Pb₅ (PO₄)₃ (OH) and KAlSi₃O₈, respectively. An overview of metal ion precipitation and coprecipitation is shown in Figure 2. Ion exchange is another crucial phenomenon in the sorption of some heavy metals by biochar [11]. Furthermore, in biochar, there are two different phases: organic and inorganic. By raising pyrolysis temperatures, which results in increased surface area, pore volume, and aromaticity, sorption mechanisms evolved from partitioning-dominant to adsorption-dominant, and sorption components developed from polar-selective to porosity-selective [4, 17]. Furthermore, due to the movement of the organic components from aliphatic to aromatic, the sorption rate shows a transitional process: from fast to slow, then back to fast. In terms of inorganic components, it was discovered that ash has a catalytic effect on the formation of biochar with more orderly graphitic structures during the pyrolysis process; additionally, deashing after pyrolysis increases hydrophobic sorption sites, favouring the sorption of hydrophobic organic contaminants [10]. Therefore, the surface structure, functional groups and surface area and mechanisms of these functional groups are observed in the removal of pollutants.

Figure 2.

Biochar remediation mechanisms. (Source: [10]).

3. Environmental remediation by biochar

3.1 Soil remediation and amelioration

Biochar can be used to clean up soil pollution caused by organic contaminants and heavy metals. Soil remediation using biochar is mostly accomplished by sorption and the mechanisms involved are surface complexation, hydrogen binding, electrostatic attractions, acid-base interactions, and π–π interactions as shown in Figure 3. For example, biochar produced from Carya tomentosa (a tree in the Juglandaceae or walnut family) and Pecan (Carya illinoinensis) (the tree is cultivated for its seed in the southern United States) can adsorb Clomazone and Bispyribac sodium (herbicides used in agriculture) in soil, and effectively reduce the leaching of clomazone and bispyribac sodium. Similarly, sawdust-derived biochar and wheat straw-derived biochar, on adding to the soil, significant reduces the polycyclic aromatic hydrocarbons (PAHs) [13]. Table 1 shows how adding biochar to soil can help remove several forms of organic contaminants. However, there are several factors such as the types of feedstock, the applied dose, the targeted pollutants, and their concentrations all affect the removal of organic pollutants in soil by biochar. Biochar has the potential to absorb heavy metal ions as well in soil. The heavy metal adsorption mechanism on biochar includes surface complexation, precipitation, cation exchange, chemical reduction, and electrostatic attraction [29]. For example, the adsorption of Pb, Cd, Cr, Cu, and Zn by sesame straw-derived biochar demonstrates varied adsorption capacities for each among them. Pb absorption is the highest in biochar among the metals. Furthermore, when the metals are present together, Cd adsorbed on by sesame biochar is easily replaced by other metal ions. And water hyacinth-derived biochar can adsorb around 90% of As (V) whereas rice straw-derived biochar is able to adsorb Zn²⁺ [30, 31]. Adsorption of antibiotics like sulfamethazine on biochar increases and subsequently decreases with pH, which affects the surface charge of both biochar and sulfamethazine, and the sorption processes evolve from electron donor–acceptor interaction to negative charge-assisted H-bond. And, metal ion adsorption occurs on the biochar surface’s proton-active carboxyl and phenolic hydroxyl functional groups, and adsorption increased with pH in the range of pH 7. Apart from that, ion exchange and cation bonding are also found responsible for the sorption of K⁺ and Cd²⁺ by [32]. The types of feedstock and experimental conditions have a big impact on the removal efficiencies. A number of parameters affect the adsorption capacity of biochar, including pH, surface functional groups, porosity, surface charge, and mineral composition. Therefore, when biochar is used as a remediation method, optimization of various parameters should be done based on the targeted organic contaminants. Table 2 summarizes the removal of heavy metals from soil by biochar. Tables show how different biochars remove organic pollutants and heavy metals at varying rates. As shown in the Table 2, the types of biochar used and heavy metals are so different, it is difficult to compare them [46, 47]. Because different biochars have distinct physiochemical properties, they have varying adsorption capacities for inorganic and organic contaminants. As a result, selecting the right feedstock is more significant for removing impurities than adjusting the pyrolysis temperature or changing the surface characteristics of biochar [19]. Additionally, modification of biochar is another option for increasing the removal capability of heavy metals. Apart from the removal of organic contaminants and heavy metal from soil, biochar can neutralize acidic soil, boost cation exchange capacity, and improve soil fertility, for example, the acidity of soil can be enhanced by 2 units after 1 month of treatment with soy bean stover-derived biochar and oak-derived biochar. Moreover, the cation exchange capacity can be increased significantly with 5% biochar. As a result, it aided maize growth and with 3% biochar [13]. The addition of biochar made from bamboo also enhances maize production and growth [8]. The addition of biochar to soil improves soil fertility due to the following reasons: (1) increased water retention capacity (2) increased soil aggregate stability; (3) reduction of soil compaction; and (4) decreased soil bulk density and increased porosity. The aforementioned factors may encourage root growth, boosting crop growth and yields even more. However, based on varied soil and feedstock, the most important reason for improving soil fertility needs to be investigated further.

Figure 3.

Biochar mechanisms in soil for contaminants removal. (Source: [18]).

Reference	Organic pollutants	Removal efficiency	Feedstock
[19]	Dibutyl phthalate	87.5%	Bamboo
[20]	Phenanthrene	100%	Conifer
[21]	Imidacloprid	—	Rice-straw
[3]	Diethyl phthalate	90%	Bamboo
[22]	Carbaryl	71.8%	Pig manure
[23]	Tylosin	66%	Hardwood
[24]	Acetamiprid	52.3%	Eucalyptus spp.
[25]	Atrazine	>66%	Dairy Manure 450
[26]	Pentachlorophenol	96.2%	Rice-straw
[27]	Chlorpyrifos	34%	Gossypium spp.
[28]	Terbuthylazine	>88%	Sawdust

Table 1.

Adsorption of organic pollutants in soil by biochar.

Reference	Heavy metal	Removal efficiency	Feedstock
[33]	Cd^2+	80%	Eucalyptus wood
	Pb^2+	93.7%
	Zn^2+	97.1%
	Cu^2+	99.8%
	Cd^2+	90%	Poultry litter
	Pb^2+	99.8%
	Zn^2+	99.3%
	Cu^2+	99.9%
[34]	Pb^2+	55.9%	Sewage sludge
	Zn^2+	51.2%
[35]	Cd^2+	56%	Bamboo
[36]	Pb^2+	—	Pine cone
[37]	Cd^2+	97.1%	Rice straw
[38]	Cd^2+	>99%	Tree bark
[39]	Cu^2+	>99%	Pine bark
[40]	Ni^2+	93%	Woody biomass, Gliricidia sepium
[41]	Zn^2+	54%	Sugar cane straw
[42]	Ni^2+	99.5%	Deinking paper sludge
[43]	Pb^2+	90%	Soybean stover
[44]	Pb^2+	93.5%	Chicken manure
[45]	Cd^2+	93.6%	Wheat straw

Table 2.

Heavy metal stabilization in soil by biochar.

3.2 Carbon sequestration

The process of storing carbon in soil organic matter and thereby removing carbon dioxide from the atmosphere is known as carbon sequestration. As part of attempts to establish climate resilient agriculture practices, the idea of using biochar to trap carbon in the soil has gotten a lot of attention in recent years. Biochar (biological charcoal) is a carbon sink that absorbs carbon from the atmosphere and stores it on agricultural grounds. Biochar is biologically inert, allowing it to retain fixed carbon in the soil for years to millennia while also absorbing net carbon from the atmosphere [20]. In addition, agriculture fixes 30 gigatons of carbon per year, but 30 gigatons of carbon return to the atmosphere as the plants die, resulting in no net change. When Biochar is combined with compost, soil, and plants, it recovers and stores a significant amount of carbon in the ground, resulting in a continuous and significant reduction in atmospheric greenhouse gas (GHG) levels. In recent years, climate change has sparked an increased interest in lowering carbon dioxide emissions into the atmosphere. Soil, being a major carbon sink, plays a critical role in the global carbon cycle, which has a direct impact on climate change. Carbon sequestration has offered as a strategy to reduce carbon dioxide emissions. Biochar has a great resistance to biodegradation due to its extremely condensed aromatic structure. As a result, biochar is thought to have a positive impact on soil carbon sequestration. Many investigations have been carried out to determine the impact of biochar on soil for carbon sequestration. However, due to the variability in carbon dioxide emissions, no consistent result can be presented. For example, adding carbon from fire to soil increased soil organic carbon turnover. However, adding biochar made of wood sawdust to soil inhibited carbon mineralization, resulting in more carbon sequestration. The mineralization of soil organic matter after the addition of biochar is shown to be higher in low-fertility soils than in high-fertility soils [21]. Carbon mineralization is also higher in soils with low organic carbon concentration than in soils with high organic carbon content. Also, the application of biochar to soil has found an increase in the rate of organic matter decomposition. This so-called “priming effect” affects carbon sequestration efforts since increased microbial activity might lead to breakdown rates exceeding carbon input rates. While the exact mechanism causing this impact has yet to be determined, it could be due to the increase of microbial activity as bacteria consume the carbon and nitrogen in biochar. However, the carbon in biochar can be separated into two types: liable and recalcitrant carbon. When biochar is introduced to the soil, soil microbes may quickly consume available carbon, resulting in an increase in carbon mineralization at first. This explains why adding biochar to soil accelerates carbon mineralization. Moreover, recalcitrant carbon content in biochar is significantly higher than labile carbon concentration. In soil, recalcitrant carbon can persist for a long time. As a result, the carbon input generated by biochar is more than the carbon outflow induced by relevant carbon mineralization. And, shorter pyrolysis times and higher pyrolysis temperatures, according to recent research [4], result in more recalcitrant biochar (i.e., it persists for longer periods in the soil). However, these pyrolysis conditions yields less biochar per unit feedstock, there are trade-offs. The effect of biochar addition on carbon sequestration is largely unknown in general. The priming impact varies depending on the feedstock and pyrolysis conditions, suggesting that the relationship between biochar’s effect and feedstock type must be investigated further. The inherent properties of biochar, as determined by feedstock and pyrolysis conditions, interact with environmental factors like precipitation and temperature to determine how long biochar carbon is held in the soil. Soil texture, as is typically the case, plays an important influence in the stability of biochar carbon. Biochar interacts with soil particles to stabilize itself in the soil.

However, numerous uncertainties remain about the efficiency of biochar in carbon sequestration. It is also crucial to investigate the link between pyrolysis conditions and biochar’s carbon sequestration ability. While biochar contains a lot of carbon, it is unclear how long that carbon will stay in the soil after it has been applied. In terms of boosting soil carbon reserves and combating climate change, biochar remains a hot topic. Many uncertainties remain, however, before definitive conclusions can be drawn about what conditions allow biochar to contribute positively to soil carbon sequestration.

3.3 In organic solid waste composting

The constant increase in solid waste seems to have a negative impact on human society’s long-term development, which has raised numerous concerns. Organic waste accounts for around half of all solid waste generated. The ability to effectively treat organic solid waste is critical for successful solid waste disposal. Composting has received a lot of attention as a waste treatment method because of its benefits, such as low cost. Composting is a biological process that takes place. Organic matter from raw materials is exposed to biological breakdown during the process. Biochar has a direct influence on microbes, which has an impact on composting. Many researches have been carried out to see how biochar affects the composting of organic waste. The following are the effects of biochar on microorganisms during the composition of organic solid waste: (1) providing a habitat for microorganisms; (2) providing ideal growing conditions for microorganisms; (3) enriching the microbial diversity. It is documented that biochar addition accelerated the decomposition of organic solid waste due to the favorable effect of biochar addition on composting. Table 3 shows the impact of adding biochar to the composting process. In general, adding biochar to compost has a good impact on the process. The priming effect, on the other hand, can be overlooked in low-fertility, alkaline, temperate soil. The type of soil affects the performance of biochar in compositing [22]. Furthermore, the types and doses of biochar, as well as the soil types, have a significant impact on the composting of organic solid waste. As a result, a biochar application strategy should be developed depending on the characteristics of organic solid waste composting and soil. Furthermore, it was discovered that bacterial consortiums combined with biochar can stimulate microbial activity to accelerate degradation, increase bacterial community richness, and change the specific selection of bacteria, providing a method for effectively improving microbial activity and enhancing organic solid waste degradation.

Reference	Feedstock	Applied dose	Performance
[48]	Peanut shell	0.75% biochar and 0.75% compost (w %)	Increase the growth of sesbania, seashore mallow, and overall biomass.
[49]	Rice husk	24 g compost + 16 g biochar in 400 g soil	Reduce the availability of Cd and Zn and enhance the availability of Cu by increasing total organic carbon and water-extract organic carbon.
[50]	Acacia	2 t ha^−1 biochar, 10 t ha^−1 compost and 92 kg N ha^−1	Improve the grain yields and N uptake
[51]	Acacia green waste	47 t ha^−1 biochar and 10 t ha^−1 compost	Enhance macroporosity and bioturbation; increase microbial abundance; modify microbial structure
[52]	Logs	2.5 t ha^−1 biochar and 25 t ha^−1 compost	Increase soil organic carbon, nutritional status, and water content, as well as maize output.
[53]	Hardwood, coniferous wood	8 t ha^−1 biochar and 55 t ha^−1 compost	Vine growing on low-fertility, alkaline, temperate soil has no immediate commercial value.
[54]	Wood	0.3 kg compost and 0.27 kg biochar	Increase the oxygen intake by accelerating the humification of sludge organics.
[55]	Beech wood	100 mg/kg biochar and 100 mg/kg compost	Increase plant height, total organic carbon, and total nitrogen content; decrease ammonium content
[56]	Quercus serrate	10% biochar and 90% compost	Change the microbial community structure; Increase
[57]	Hardwood coniferous wood	8 t ha^−1 biochar and 63 t ha^−1 compost	Increase microbial number and activity while having no influence on the amount of copper available.

Table 3.

Impact of adding biochar to the composting process.

3.4 Decontamination of water and wastewater

Many studies have demonstrated that biochar may adsorb contaminants from water and wastewater, including both organic and inorganic pollutants. Antibiotics, for example, are becoming common organic contaminants in the environment. Sludge-derived biochar has been shown to be a cost-effective and reusable adsorbent for the elimination of antibacterial drugs. Table 4 shows how biochar can remove organic pollutants from water via adsorption [68, 69].

Reference	Feedstock	Removal efficiency	Organic pollutant
[58]	Chicken manure	100%	Microcystin-LR
[59]	Sewage sludge	26%-60%	Tetracycline
[60]	Corn stalks	97.62%	Norfloxacin
[61]	Pinus radiata sawdust	100%	Sulfamethoxazole
[58]	Mangosteen peel	80%	Methylene blue
[3]	Cool Planet LLC	<6%	Ibuprofen
	Organic Farms LLC	<10%	Sulfamethoxazole
	Corncob	—	Bisphenol A
[62]	Waste Douglas fir	100%	salicylic acid
[63]	Corn straw	100%	Atrazine
[64]	Wood	20%-30%	Sulfamethoxazole
[65]	Rice-husk	~90%	Tetracycline
[66]	Buffalo-weed	88.47%	Trichloroethylene
[67]	Soybean Stalk	99.5%	Phenanthrene

Table 4.

Organic pollutant removal by biochar in waste.

The adsorption of pollutants by biochar in water depends on the physiochemical characteristics of targeted pollutants and the types of biochar. For example, the sawdust-derived biochar can remove entirely 20.3 mg/l of sulfamethoxazole while wood-derived biochar demonstrates substantially lower removal effectiveness of sulfamethoxazole (20–30%). For biochar obtained from organic farm, it demonstrates the lowest removal effectiveness of sulfamethoxazole (<6%) [23]. Varying pyrolysis temperatures led in different tetracycline removal efficiencies for biochar generated with rice husk [24]. The removal efficiency of tetracycline ranged from 26% to 60% when the pyrolysis temperature was 800°C and the initial concentration of tetracycline was 200 mg/l. When the pyrolysis temperature was 500°C and the initial tetracycline concentration was 5 mg/l, the removal efficiency was around 90%. It is therefore, established that pyrolysis temperature had important effect on the adsorption capacity of biochar. Other parameters such as pyrolysis time, in addition to pyrolysis temperature, can influence the physiochemical characteristics of biochar, which in turn affects the adsorption capacity of biochar. Heavy metal contamination is a major problem that requires immediate attention. Heavy metals can be removed from the aquatic environment using adsorption as well. Biochar’s ability to remove heavy metal ions is listed in Table 5 [80]. The removal of heavy metals by biochar is dependent on the types of heavy metals and the types of feedstock, similar to the removal of organic pollutants by biochar. Biochar has a lower removal capacity for Cd²⁺ and As⁵⁺ than other heavy metals like Pb²⁺ and Zn²⁺ among the major heavy metals [25]. Biochar produced from corn straw, for example, had a different Cu²⁺ adsorption capability like 0.1 g/l of biochar can remove 1 mM of Cu²⁺ when the pyrolysis temperature is set at 800°C. And, when the pyrolysis temperature is set to 400°C, 20 g/l biochar can remove 20 mg/l Cu²⁺ [26]^. Similarly, biochar produced from water hyacinths shows different adsorption capacities for Cd²⁺ and Pb^2+, demonstrating that biochar adsorption capability varies depending on the targeted heavy metals. Zhang et al. [27] discovered that biochar prepared at high temperatures was effective in removing Cr (VI). A recent study found that sludge-derived biochar may successfully remove ammonium by monolayer chemical adsorption [59], implying that competition adsorption occurred when biochar was utilised as adsorbents for the removal of heavy metals and organic pollutants in the presence of ammonium. It should be highlighted that the adsorption capacity of the functional groups-modified biochar is clearly improved by the functional groups. The amino-modified biochar, for example, significantly increases the adsorption of Cu (II) due to strong complexation [60]. Moreover, biochar can enrich microorganisms, which can aid in the removal of organic matter, in addition to adsorption. Luo et al. [48] discovered that the proportion of Archaea was significantly greater in the presence of fruitwood-derived biochar, which relieved the stress of ammonia and acids on the microbes, raising microbial activity even more. Lu et al. [35] discovered a similar phenomenon as well. When using biochar for water and wastewater treatment, it’s important to keep in mind that it can be recycled and reused. Based on the foregoing findings, biochar performs well in batch experiments in removing the contaminants of concern. However, various contaminants coexist in water and wastewater. Competitive adsorption may occur, resulting in results that differ from those obtained in the laboratory. In addition, the adsorption of contaminants by biochar may be affected by actual flow conditions. As a result, more research should be done in the lab to imitate the real-world condition and study the efficacy of biochar in the removal of contaminants.

Reference	Feedstock	Removal efficiency	Heavy metal
[70]	Corn straws	97.7%	Cu^2+
[33]	Rape straw	100%	Cd^2+
[49]	Sawdust and swine manure	100%	Pb^2+
[71]	Mangosteen peel	80%	Cd^2+
[72]	Corn straw	99.24%	Cd^2+
[73]	Celery	97.7%	Pb^2+
[74]	Scots pine	~23%	Cd^2+
[75]	Water hyacinths	~60%	Cd^2+
[75]	Sugar cane bagasse	~80%	Pb^2+
[76]	Macroalga	~80%	Cu^2+
[77]	Wheat straw	100%	Cd^2+
[78]	Hickory wood	95.9%	Cd^2+
[79]	Pinewood	~35%	As^5+
[3]	Rice husk	~100%	Cr^6+
[3]	Anaerobic digested sludge	26%	Ni^2+

Table 5.

Heavy metal uptake by biochar in water.

3.5 Building sector

Biochar is a good building material for insulating buildings and managing humidity because of its low thermal conductivity and capacity to absorb water. Biochar, together with cement mortar clay and lime, can be used with sand in a 1: 1 ratio. As a result, the plaster made using this technology has excellent insulation and breathing capabilities, allowing it to sustain humidity levels of 45–70% in both summer and winter. This prevents dry air, which can cause respiratory problems and allergies, as well as moisture caused by air condensing on the outer walls, which can lead to mould growth [27].

4. Future research

The capacity to carefully adjust the structure and chemistry of biochar at nanoscale (nm) scales allows certain aspects of the biochar to be altered to target certain environmental engineering solutions, comparable to the proposed “designer biochar” for agricultural uses. It is crucial to remember, however, that once in the field; biochar characteristics do not remain constant over time. Even at ambient temperatures, ageing, oxidation, and microbial degradation can modify surface functional groups and chemistry, affecting sorption characteristics. The list of biochar’s potential engineering applications is continually growing. Due to its unique magnetic properties, magnetic biochar opens the door to facilitating removal of various contaminants from soil or other media. This broadens the scope of biochar’s possible use in environmental remediation.

5. Environmental concern of biochar

Along with the widespread use of biochar, it may have some disadvantages which may lead to harmful impact on the environment. When using bio-char in the environment, one of the most crucial aspects to consider is stability. The carbon structure makes up the majority of biochar. Biochar stability refers to the stability of the carbon structure in general. Aromaticity and the degree of aromatic condensation in biochar are markers of its carbon structure. Biochar stability must be considered because different biochars have varying physiochemical properties. Due to the instability of biochar, Huang et al. [28] observed the potential dissolution of organic matter from biochar in the complexation of heavy metals, implying that dissolved organic matter from biochar can be discovered in solution. Furthermore, the aromaticity, stability, and resistivity of the dissolved organic matter may be high. When biochar is used in the treatment of water and wastewater, the carbon content of the water body may rise due to the release of carbon from the biochar. Furthermore, biochar, particularly sludge-derived biochar, includes heavy metals, which may leach out during the water and wastewater treatment process, resulting in heavy metal contamination. When biochar is used as a catalyst support, the catalyst’s stability tends to deteriorate after a few uses. One reason for the lower catalyst stability could be charcoal structural degradation. As a result, biochar stability is also linked to water and wastewater treatment quality. In conclusion, the stability of biochar has a significant impact on its environmental applicability. As a result, more research is needed in the future to determine the stability of biochar. Because pyrolysis conditions can change carbon content and structure however, research into the relationship between biochar stability and pyrolysis conditions is important. Biochar’s possible toxicity on microorganisms should be considered in addition to its stability. Biochar increases the enzymatic activities of soil microorganisms at low doses, according to Gong et al. [75], demonstrating that low doses of biochar had no toxicity on the bacteria. Dong et al. [79] shown that Fe₃O₄-modified bamboo biochar has a low cytotoxicity potential. In contrast, high doses of tobacco stem-derived biochar exhibited cytotoxic and genotoxic effects in epithelial cells through promoting ROS production. As previously stated, biochar has a wide range of physical and chemical properties. More research into the potential toxicity of biochar to the environment is needed to support its effective application. Fish, algae, water fleas, and luminous bacteria can all be used to conduct toxicity tests.

6. Conclusions and remarks

This chapter provided an overview of biochar application and its interaction with other substances, focusing on its use in environmental remediation. Firstly, the raw material especially waste materials used for biochar production offers a treatment option for wastes that contributes to environmental sustainability. Furthermore, biochar’s practical applicability is aided by its low-cost feedstock and simple preparation technique. Biochar has the ability to remediate, improve soil, and mitigate climate change, all of which contribute to environmental sustainability. However, the primary explanation for the increase in soil fertility remained unknown, and the work on the impact of biochar on carbon sequestration needs to be conducted and understood. Composting organic waste using biochar can help promote biological decomposition of organic waste. However, different doses of biochar were required for various organic wastes and biochar kinds. As a result, a biochar application strategy should be developed depending on the characteristics of organic solid waste composting and soil. Biochar can be employed as absorbents in the decontamination of water and wastewater, but its adsorption capacity and stability must be improved. Biochar can activate persulfate, which can be used to remove hazardous organic pollutants from water and wastewater, however the relationship between biochar structure and persulfate activation needs to be studied further to figure out how it works. In conclusion, biochar has a bright future in improving environmental sustainability. The majority of bio-char research is currently being done in laboratories. Biochar’s environmental impact has yet to be fully understood. Furthermore, the real world is more complex than the laboratory, resulting in ambiguity about biochar’s environmental impact. More in situ tests are needed to determine the true impact of biochar on the environment, such as environmental microorganisms, before it is used on a broad basis. Furthermore, the preparation conditions of biochar for industrial use must be enhanced depending on the various environmental reasons.