Sustainable manufacture and application of biochar to improve soil properties and remediate soil contaminated with organic impurities: a systematic review

Biochar production and application have become increasingly popular in the past 15 years. Biochar, derived from diverse biomass types, offers a rich carbon source created through thermal combustion. Biochar production primarily depends on pyrolysis conditions and feedstock type. This review focuses on the multifaceted aspects of biochar, encompassing hydrothermal carbonization, gasification, and pyrolysis temperatures in biochar production and its role in bioeconomy and soil remediation. Biochar has yielded valuable insights, notably in decreasing nutrient leaching, curbing greenhouse gas (GHG) emissions, reducing the bioavailability of environmental pollutants, sequestering carbon (C) in soils, and enhancing agricultural productivity. Consequently, it has emerged as a valuable commodity for the bioeconomy, which involves harnessing bioresources through bioengineering to create economically valuable products. As a marketable output, biochar finds application in energy, diverse biochar-based product manufacturing, and the agricultural sector. Thus, biochar production not only enhances soil quality but also unlocks additional revenue streams. This review underscores the critical role of feedstock selection and pyrolysis conditions in optimizing biochar production. Furthermore, it highlights biochar as a sustainable and effective tool for improving various soil types and remediating soil contamination caused by organic impurities, including persistent organic compounds and antibiotics.

1 Introduction

Biochar is a carbon-rich, porous substance with a unique structure comprising carbon aromatic compounds, ample functional groups, high cation exchange capacity, negative surface charges, neutral to alkaline pH, and high specific surface area. As a soil conditioner, biochar significantly impacts soil quality by altering biological and physiochemical soil traits (Murtaza et al., 2021a; Murtaza et al., 2021b), which can increase soil quality and crop yields (Chen et al., 2022; Chen et al., 2023). The effectiveness of biochar depends on the feedstock used, production mechanism, crop type, and soil type (Issaka et al., 2022). Biochar is produced from various biomass sources, including paper mill byproducts, animal waste, and crop residues, offering a sustainable means of converting these residues into valuable products (Singh et al., 2020).

Biochar production can range from small-scale to large-scale operations using pyrolysis methods, a thermochemical process that transforms feedstock into bio-oil, biochar, and syngas within a temperature range of 300°C–750 °C. Gasification and pyrolysis are thermochemical processes that convert biomass into solid biochar (Kamali et al., 2022). These processes include fast and slow pyrolysis, with each method dependent on heating and residence time rates. Fast pyrolysis yields more liquids and oils, while slow pyrolysis generates more syngas. Slow pyrolysis also accelerates biochar formation more efficiently than gasification and fast pyrolysis (Mandal et al., 2021). Fast pyrolysis occurs at high temperatures with a brief residence time of approximately one second. The key distinction between fast and slow pyrolysis lies in their product yields. The biochar production process commences with feedstock drying, during which particles are heated to release volatile organic compounds (VOC) (Qian et al., 2023), such as hydrogen, methane, CO, CO₂, and condensable compounds like methanol and acetic acid. Gas-phase polymerization and cracking reactions transform the overall product composition (Lu et al., 2022).

Gasification enhances the surface area, aromaticity, and porosity of biochar. In gasification, feedstock is converted into a gaseous form, leading to a limited amount of oxidizing agents (water and oxygen) at a higher temperature (Tauqeer et al., 2021). During biochar formation, the solid carbon product is the main result, with the release of volatile components and water evaporation contributing to the increase in stable C content in the solid. Polymerization of organic substances in gases and vapors results in a more compact char structure (Anae et al., 2021). Table 1 summarizes the reaction mechanisms involved in biochar production, and Table 2 presents information on the influence of pyrolysis temperature and feedstock on various biochar elemental compositions. Biochar typically consists of carbon and ash, with its elemental composition and properties varying based on the pyrolysis temperature and feedstock material. The structure of biochar primarily comprises carbon and minerals with varying pore sizes. Micropores contribute to the extensive surface area and sorption capacity of biochar, while mesopores facilitate liquid–solid sorption mechanisms, and macropores play a crucial role in soil structure, quality, aeration, hydrology, and root movement (Yin et al., 2021). The size and arrangement of these pores depend on the specific pyrolysis temperature and feedstock constituents used during production. Scanning electron microscopy (SEM) provides insight into the pore size distribution and morphology in biochar derived from different sources, as illustrated in Figure 1.

TABLE 1

TABLE 1. Details of the methods in biochar preparation.

TABLE 2

TABLE 2. Pyrolysis temperatures and feedstocks on various biochar elemental compositions.

FIGURE 1

FIGURE 1. Effect of porosity and surface of biochar of various feedstock (A) Sugar beet tailing (Li et al., 2022) (B) Ricc hull (Rashid et al., 2020) (C) Wood-chips (Samoraj et al., 2022) (D) Fruit bunches (Liang et al., 2021). (E) Human waste (Vanapalli et al., 2021) (F) Wood bark (Yin et al., 2021) (G) Dairy residue (Lehmann et al., 2011) (H) Poultry waste (Zahed et al., 2021).

Shaheen et al. (2022) defined biochar as a porous, solid carbon substance produced through the thermochemical transformation of organic substances in an oxygen-depleted environment, resulting in ideal chemical and physical characteristics for long-lasting and safe carbon sequestration in the environment. However, biochar occurs in some types of charcoal but is obtained under fixed moderated circumstances, which allows carbon to be most stable and transformed to useable output (Rashid et al., 2020). The micropores in biochar enhance microbial activity and enable the sorption of dissolved organic substances, facilitating the remediation of organic contaminants in soil (Li et al., 2022). Biochar holds significant potential for deactivating pesticides in soil, retaining soil quality and fertility, and enhancing abiotic biodegradation. Consequently, biochar enhances phytoremediation techniques for removing various soil pollutants, complementing other remediation techniques. This review discusses i) the key physiochemical characteristics influenced by various soil amendments to acidic or alkaline soils and their preparation conditions, ii) the remediation potential of biochar in soils contaminated with organic pollutants and other environmental benefits such as gas remediation and greenhouse gas (GHG) mitigation, iii) the economic benefits of biochar, and iv) future research directions on biochar.

2 Significance of feedstock selection and pyrolysis conditions for biochar production

Biomass/feedstock is a complex organic/inorganic or biological solid material derived from plants and other living organisms (Reyhanitabar et al., 2020). It encompasses different types of residue and litter, including industrial waste, papermill residues, animal manure, and poultry litter (Behnam and Firouzi, 2022). Biomass/feedstock can be broadly categorized into woody and non-woody feedstock. Woody feedstock typically comprises waste from trees and forestry (Singh et al., 2022), characterized by low voidage, high bulk density, high calorific value, and low ash and moisture contents (Huang et al., 2021). Non-woody feedstock consists of industrial solid waste, urban waste, animal manure, and agricultural residues (Singh et al., 2022), characterized by high voidage, high ash and moisture contents, low bulk density, and low calorific value (Mukherjee et al., 2022). The moisture content within biomass significantly influences biochar production (Al-Rabaiai et al., 2022), with high moisture contents elevating the energy required to reach carbonization temperature, which hinders biochar production (Mukherjee et al., 2022). Lower moisture contents typically favor biochar production due to the significant reduction in heat energy and time required for carbonization, making the process more economically feasible than pyrolysis using feedstock with higher moisture contents (Behnam and Firouzi, 2022). Different moisture/water content levels in feedstock lead to biochar production with diverse physicochemical attributes (Huang et al., 2021). For instance, the moisture/water contents of softwood and hardwood bark significantly influence the surface chemistry of the resulting biochars (Xu et al., 2022). A low moisture content in maple bark results in a more graphite-like and polyaromatic char surface, possibly due to prolonged effective pyrolysis after water evaporation (Wani et al., 2022).

Biochar production comprises three main stages: i) pre-pyrolysis, ii) main-pyrolysis, and iii) the formation of carbonaceous soil products (Balmuk et al., 2023). The first stage (100°C–200°C) involves the evaporation of light volatiles and moisture. This moisture evaporation causes bond breakages and forms hydroperoxide, -CO, and -COOH groups (Dhar et al., 2022). The second stage (200°C–500°C) focuses on removing volatiles and decomposing cellulose and hemicelluloses at an accelerated rate (Balmuk et al., 2023). The third and final stage (>500 °C) involves the degradation of organic matter and lignin with strong chemical bonds (Greenough et al., 2021). The carbonization temperature strongly correlates with changes in the biochar’s physicochemical attributes and structure (Kalina et al., 2022). Table 3 presents data on these relationships. The pyrolysis temperature significantly affects the physicochemical characteristics of biochar, including functional groups, pH, and surface area, and its role as a soil amendment (Wani et al., 2022). Higher pyrolytic temperatures enhance volatile matter, pH, and carbonized fractions and reduce surface functional groups and cation exchange capacity (CEC).

TABLE 3

TABLE 3. Effects of diverse ranges of pyrolysis temperatures and feedstocks on biochar properties.

3 Biochar as a soil amendment for remediation of contaminated soil

Biochar addition accelerates the bioremediation process for organic compounds by promoting microbial activity that degrades saturated hydrocarbons in biochar-amended soils. Certain metalloids, such as boron, arsenic, and silicon, can not be removed from the soil but can be transformed, typically from higher to lower concentrations (Yang et al., 2021). When remediating metalloids, specific considerations must be taken into account, including the adsorption of metalloids by plants and bioenergy crops in contaminated cultivated land and feedstock and the conversion of metalloids into less harmful products (Azeem et al., 2022; Rizwan et al., 2024). Biochar properties conducive to soil remediation include a large surface area, high degradation resistance, and negative charges (Mazarji et al., 2021). A source refers to the site of the contaminant, while a receptor refers to the site that induces damage. The pathway is the route through which contaminants migrate from the source to the receptor. Contaminants that move from the source to the receptor in sufficient quantities to cause harm are considered pollutants (Murtaza et al., 2022). One of the most common methods to address polluted soil is to remove the receptor or the source (Zahed et al., 2021). However, these methods can be costly and impractical, especially in the case of widespread soil contamination. Contaminants can also transfer from sources to receptors through dissolution into solution via various mechanisms (Wang L et al., 2021). Biochar intervenes in the source route–receptor relationship by adsorbing contaminants on its surface and reducing their concentrations in soil solutions (Xu et al., 2021). Successful remediation occurs when biochar permanently eliminates contaminants from the soil solution, eliminating the pathway to receptors. After adsorption onto biochar surfaces, contaminants no longer pose a risk of harm. The large surface area of biochar plays a crucial role in its ability to remove metal ions and organic pollutants (Natasha et al., 2022).

Figure 2 illustrates the remediation of pollutants (inorganic and organic) through biochar in soil based on the source–pathway–receptor relationship, with Stage 1 showing a contaminated site in the absence of biochar and Stage 2 showing a pollution-free site after biochar application. Biochar derived from various biomasses exhibits robust adsorption abilities for various organic pollutants and pesticides (Table 4), which can surpass the natural soil organic matter by a factor ranging from 10 to 98. Various studies have reported significant reductions in organic contaminants in biochar-amended soils (Van Nguyen et al., 2022). According to Murtaza et al. (2022), the processes involved primarily include surface partitioning and adsorption. The adsorption of organic pollutants onto biochar occurs repeatedly due to partitioning (in uncarbonized fraction) in lower-temperature biochar and surface adsorption (in carbonized fraction) in higher-temperature biochar. Increasing the pyrolysis temperature enhances carbonization, increasing the biochar-specific surface area (SSA) by decreasing amorphous organic materials. Consequently, biochar targets the bioavailable fraction of organic pollutants. While this can be beneficial in reducing biopesticide residues in crops, it may also reduce the efficacy of biopesticides, necessitating higher application rates of these substances (Sun et al., 2021). The effectiveness of biochar on herbicides depends on the herbicide’s specific molecular properties and mode of action (Muhammad et al., 2020). Understanding the positive effects of biochar on pesticide remediation and its impact on pesticide effectiveness is crucial for pollutant remediation objectives and composite queries.

FIGURE 2

FIGURE 2. Biochar remediation of contaminants in the soil by a source pathway-receptor connection: Biochar application for soil remediation (Van Nguyen et al., 2022).

TABLE 4

TABLE 4. Quantitative impact of biochar on various soil parameters.

Biochar application to soil can increase negative charges on the soil surface by enhancing CEC and reducing zeta potential (Wei et al., 2023), stimulating electrostatic attraction between the soil and positively charged heavy metals. Due to the presence of various functional groups on the biochar surface, such as OH and COO, biochar forms complexes with heavy metals, reducing their bioavailability (Rizwan et al., 2020a; Rizwan et al., 2020b; Qiu et al., 2022). As a result, essential plant nutrients can become immobilized. While this can be advantageous in nutrient-rich conditions, it may be detrimental in nutrient-poor soils, leading to nutrient deficiencies. Pollutants and nutrients on the biochar surface help balance the immobilization of pollutants and nutrient deficiency in contaminated soil (Cara et al., 2022). Heavy metal precipitation, including copper, zinc, cadmium, and lead, can also increase soil pH, decreasing mobilization (Van Nguyen et al., 2022). Figure 3 illustrates different processes in biochar-amended soils, including improvements in biochar preparation in Malaysia, as presented at an international seminar on biochar (Pan et al., 2021).

FIGURE 3

FIGURE 3. Figure presenting several mechanisms in biochar soil bio-remediation.

4 Biochar impacts on soil characteristics

The impact of biochar on soil characteristics depends on soil type and biochar composition, as illustrated in Figure 4, with quantitative effects detailed in Table 4.

FIGURE 4

FIGURE 4. Effect of biochar additions on soil properties.

4.1 Soil physical characteristics

The modification of soil traits depends on soil type, biochar properties, and biochar application rate (Kayiranga et al., 2023). Biochar addition to soil affects soil wettability, stability, aggregation, water retention, and water infiltration, contributing to mitigating drought, reducing nutrient losses, combating erosion, and improving groundwater quality.

4.1.1 Soil bulk density

Soil bulk density is an important physical trait of soil closely related to soil compaction (Xiao et al., 2023). Low soil bulk density improves soil structure, decreases soil compaction, and facilitates nutrient release and retention (Mazarji et al., 2023). Studies have shown that biochar application decreases soil bulk density compared to control samples. For instance, 25 g kg^–1 biochar application reduced the soil bulk density of silt soil from 1.52 to 1.30 g cm^–3 (Sun et al., 2022), with similar findings reported by Mazarji et al. (2022). Soil bulk density decreases with biochar incorporation by stimulating microbial activity and fungal growth, improving soil agglomeration, and increasing hyphae and root development (El-Naggar et al., 2021). Moreover, biochar addition to soil enhances total soil porosity (Gautam et al., 2021). The reduction in soil bulk density due to biochar application varies with soil type, biochar type, particle size, and application rate.

4.1.2 Soil porosity

Soil pores provide oxygen and space for soil organisms and influence water utilization, storage, and transformation. Biochar particle size, pore distribution, and connectivity significantly affect soil pore structure (Burachevskaya et al., 2021). Sandy soil, which has poor water retention ability and large pores, experiences improved permeability and porosity after biochar addition (Gouma et al., 2022). Biochar application alters soil pore size distribution, shifting to smaller pore sizes that enhance crop growth (Baiamonte et al., 2019). For instance, adding biochar to medium- and coarse-textured soils altered soil porosity by 4–9 µm and 20 μm, increasing crop yields by 10% and 12%, respectively (Lu et al., 2023). Biochar addition to frozen soil enhanced soil porosity and pore content >0.30 mm and increased the structural stability index (Gorovtsov et al., 2020). Boguta et al. (2019) reported that adding straw biochar enhanced mesopore and macropore numbers in clay soils, and soil particles combined with biochar to form stable agglomerates. Thus, biochar addition increases soil porosity and distribution, enhances air and water circulation, improves water retention, and increases soil compaction, fertility, and productivity.

4.1.3 Soil aggregation

Biochar significantly influences soil aggregation (Xiao et al., 2023). Applying biochar derived from peanut hulls and corn straw at a rate of 7.80 t ha^–1 enhanced the proportion of soil macroaggregates. However, the impact of biochar on aggregate stability depends on soil texture (Baiamonte et al., 2019). In one study, biochar-amended silt-loam soils exhibited increased aggregate stability, whereas sandy loam soils exhibited no substantial effect (Chen et al., 2023). In contrast, Gouma et al. (2022) reported reduced aggregate stability with biochar addition. The interaction between biochar and SOC, minerals, and microorganisms influences the impact of biochar on soil aggregation and its stability (Sun et al., 2022). Biochar provides a habitat for soil microbes, sheltering them from predators and desiccation. These microbes release polysaccharides that enhance soil aggregation (Kayiranga et al., 2023).

4.1.4 Soil water-holding capacity

Soil moisture content is a critical factor in soil systems and is influenced by precipitation rates and soil texture (Hussain et al., 2020). Biochar’s high surface area and porosity reduce soil water permeability resistance, enhance soil water-holding capacity, alter water residence time (Wani et al., 2020), and change the flow path in soil (Gorovtsov et al., 2020). Biochar addition can enhance soil water-holding capacity in the field, with a more pronounced effect in sandy soils than clay soils due to sandy soil’s poor water retention ability (Bolan et al., 2022). Shafiq et al. (2023) reported that soil bulk density decreased after biochar addition (sandy soil > loamy soil), while soil water holding capacity increased. Sandy soil with 5% and 25% biochar addition retained 250% and 360% more water than control samples, respectively (Blanco-Canqui, 2021). However, the relationship between biochar application rate and water-holding capacity remains unclear. Thus, biochar can be an environmentally friendly tool for improving sandy soils in humid regions, with its effectiveness adjusted according to the biochar:soil ratio.

4.2 Soil chemical characteristics

4.2.1 Soil pH

Biochar addition significantly influences soil pH (Bolan et al., 2022). Biochar pH typically ranges from 4 to 12, and its alkaline nature directly influences soil pH. Biochar can modulate soil pH and enhance base saturation. When added to soils, biochar can exchange with Al³⁺ and H⁺ ions in water, decreasing these ion concentrations in the soil (Ramezanzadeh et al., 2023). The presence of metal carbonates, hydroxides, and oxides in biochar increases soil pH, as negatively charged phenolic, hydroxyl, and carboxyl groups in biochar bind H⁺ ions from the soil solution, decreasing H⁺ ion activity and increasing soil pH (Patwa et al., 2021). However, there are cases where biochar addition can decrease alkaline soil pH, attributed to acid production during biochar oxidation (Khan et al., 2022). Na₂CO₃ and NaHCO₃ contents in alkaline soils can convert CaCO₃ and Ca (HCO₃)₂, decreasing soil pH (Blanco-Canqui, 2021). Studies have reported that acidic contents produced from soil organic matter decomposition decrease alkaline soil pH (Hussain et al., 2020). Kocsis et al. (2022) reported that biochar application increased calcareous soil pH. Similarly, biochar derived from steam activation and slow pyrolysis decreased calcareous soil pH by 0.2–0.5 units (Ji et al., 2022). Even at high biochar application rates, the soil’s buffering capacity prevented changes in the soil reaction (Zuo et al., 2022). Incorporating poultry manure-derived biochar increased alkaline soil pH, decreasing crop nutrient availability (Chen et al., 2023). In contrast to acidic soil, relatively few studies have investigated the effect of biochar on alkaline soil pH. Biochar addition to acidic soil increased soil pH to varying degrees: 4.30–4.60 (Osman et al., 2022), 4.80–6.30 (Albert et al., 2021), and 4.60–4.90 (Yadav and Bag, 2023). Thus, biochar addition can positively impact acidic and alkaline soils, depending on the specific soil conditions.

4.2.2 Cation exchange capacity

The CEC measures a soil’s capacity to retain, absorb, and exchange cations (Hossain et al., 2020). Increasing the number of soil cation exchange sites can enhance soil CEC. Soils with higher CEC are more likely to adsorb cations such as Mg²⁺, Ca²⁺, K⁺, and NH⁴⁺, which can improve nutrient utilization and decrease nutrient loss in soil (Egamberdieva et al., 2022). With its acidic aromatic carbon on the surface, biochar can create numerous functional groups, such as–COOH and–OH, which enhance the adsorption capacity for soil cations and, thus, CEC (Da Silva Mendes et al., 2021). Several studies have shown that biochar addition to soils enhances the total soil charge and CEC by approximately 25%–45% compared to control samples. Even small amounts of applied biochar can greatly increase soil nutrient and alkaline cation contents (Cui et al., 2022). Biochar addition to alkaline or acidic soils enhances soil CEC due to the high number of anions on the surface (Egamberdieva et al., 2022). However, biochar addition in highly organic soils may not increase soil CEC significantly, as soils with high organic matter already have a high CEC (Elkhlifi et al., 2021).

4.2.3 Electrical conductivity and soil organic carbon

Biochar addition can increase the electrical conductivity of soil, primarily due to the release of soluble components (mineral and organic) when biochar reacts with water (Da Silva Mendes et al., 2021). For instance, 100 t ha^–1 biochar enhanced soil electrical conductivity 15-fold (Farid et al., 2022). While biochar can enhance electrical conductivity in acidic soils (Abagandura et al., 2022), it may decrease it in salt-stressed soils due to physical entrapment of salts in biochar pores (Rombola et al., 2022). Biochar has been observed to alleviate salt stress effects. Under salinity stress, electrical conductivity decreased with increasing biochar application rates (Lee et al., 2022).

Biochar application can increase soil organic carbon content due to its high carbon content of recalcitrant nature (Adeniyi et al., 2022). Gross et al. (2022) reported 30%–40% higher soil organic carbon contents with varying biochar application rates. The increase in soil organic carbon with biochar addition is attributed to the cumulative effects of carbon from biochar, microbial activity, rhizosphere decomposition, and root exudates (Qianqian et al., 2022). For example, corn-derived biochar with lower volatile matter and higher ash content reduced carbon mineralization in clay soils (Bolan et al., 2022).

4.2.4 Nutrient retention

Biochar’s heterogeneous structure and surface properties, including basic, acidic, hydrophobic, and hydrophilic characteristics, enhance its ability to adsorb soil solution constituents, affecting fertilizer retention. Biochar can enhance nutrient retention through sorption processes. For example, bamboo-derived biochar produced at 900 °C adsorbed 1.3 mg g^–1 nitrate (Enaime et al., 2020), while biochar derived from pepperwood and Arachis hulls at 600 °C decreased the total amounts of PO₄^3–, NH₄⁺, and NO₃⁻ in leachates by 39.1%, 14.4%, and 34.3% and 20.6%, 34.7%, and 34.0% respectively. Wood-derived biochar at 400 °C adsorbed approximately 250–430 mg g^–1 phosphorus (Chen et al., 2023), and biochar derived from Spartina spartinae at 350 °C adsorbed approximately 0.5 mmol g^–1 potassium. Thus, biochar can be used to decrease nutrient leaching in soil. Biochar can also enhance soil fertility by mitigating gaseous nitrogen losses (Teodoro et al., 2020). Ramadan and Abd-Elsalam (2020) reported that N₂O emissions decreased by 70% after biochar application. Increased nutrient retention after biochar addition is also likely related to reduced bulk density, enhanced water storage capacity and porosity, and increased biological activities (Medha et al., 2021). The effectiveness of nutrient retention by biochar depends on application rate, pyrolysis temperature, and feedstock type.

4.3 Soil biological attributes

Soil biological attributes, including microbial biomass and enzymatic activity, are vital soil health components that can be affected by biochar addition.

4.3.1 Microbial biomass

Biochar can directly and indirectly affect soil microbial populations (Mulabagal et al., 2022). Due to its higher aromatic hydrocarbon structure, biochar provides a conducive environment for soil microorganisms such as fungi, bacteria, and algae, which benefit from the nutrients and habitat created by biochar (Aboughaly and Fattah, 2023).

Biochar can alter the structural composition of soil microbes. Biochar surfaces contain partially soluble nitrogen (N) and carbon (C) sources suitable for microbial activities (Abukari et al., 2020). Biochar’s porous structure and high specific surface area (SSA) can retain nutrients and water, providing habitats for microbes (Manirakiza et al., 2019). Soil microorganisms can convert biochar into humus carbon, stimulating humus carbon development (Pathy et al., 2020).

While the increased amount of organic matter in soil increases microbial growth, biochar addition significantly reduces soil NO₃⁻-N content (Mulabagal et al., 2022). Some studies have reported increases in soil microbial populations following biochar addition (Aboughaly and Fattah, 2023), while others have observed no significant change (Bamdad et al., 2022) or even a reduction (Saleem et al., 2022) in microbial populations. The effects of biochar on soil microorganisms are complex and depend on factors such as the experimental setting, biochar type, soil texture, fertility level, nutrient management, and land use patterns.

4.3.2 Soil enzyme activity

Biochar application can also influence soil enzyme activities. The interactions between biochar, enzymes, and substrates strongly affect soil enzyme activities (Sormo et al., 2021). Biochar can either limit or promote enzymatic activity based on the sorption of enzymes and substrates to different functional groups in the biochar (Krahn et al., 2023). Several studies have reported the positive impact of biochar on soil enzyme activities (Bamdad et al., 2022). Mulabagal et al. (2022) reported that biochar enhanced enzyme activities through increased organic substrates such as SOC, nitrogen pools, and microbial biomass carbon (Saleem et al., 2022). Rice husk-derived biochar at 12 t ha^–1 significantly improved alkaline phosphatase, invertase, catalase, and urease activities (Krahn et al., 2023). In another study, soil invertase activity positively correlated with SOC, available phosphorus, and nitrogen (Bamdad et al., 2022). Pathy et al. (2020) reported increased phosphatase activity with biochar application, indicating an increased fraction of bioavailable P (Shikha et al., 2023). Aboughaly and Fattah (2023) showed that almond-derived biochar positively impacted urease and dehydrogenase activities. Baskar et al. (2022) found no significant relationship between SOC and enzyme activity. In another study, invertase activity positively correlated with NO₃⁻-N and NH₄⁺-N contents, whereas urease activity positively correlated with available phosphorus and nitrogen, attributed to stimulated root growth exuding these enzymes (Winchell et al., 2021). Dehydrogenase activity in soils incubated with biochar produced at 700 °C decreased by 48%, whereas biochar at 350 °C enhanced dehydrogenase activity by 70% (Sormo et al., 2023). Irrespective of soil type, biochar application increased alkaline phosphomonoesterase activity but inhibited acidic phosphomonoesterase activity (Qianqian et al., 2022). Lade (2023) reported that biochar treatment in alfisols increased phosphomonoesterase activity (alkaline and acidic). Biochar application to vermicompost from sewage sludge decreased heavy metal bioavailability and toxicity, enhancing earthworm growth and reproduction (Lu et al., 2023).

5 Elimination of organic pollutants from soil

Biochar has gained recognition as an effective tool for eliminating various organic contaminants from soils, including pesticides, herbicides, fungicides, insecticides, industrial chemicals, aromatic dyes, VOC, and drugs/antibiotics (Dong et al., 2023), that can pose significant threats to the environment and human health.

Biochar enhances the adsorption and degradation of organic contaminants in soil. For example, the pesticide carbofuran decreased due to degradation and adsorption onto the surface of biochar particles during carbonization, increasing porosity and pesticide sorption (Lima et al., 2021). The sorption of pesticides onto biochar surfaces is likely related to the quality of phenolic and carboxylic functional groups (Panahi et al., 2020). Therefore, optimizing the quantity of biochar added to soil is crucial to enhance contaminant adsorption (Xiang et al., 2021). The processes involved in the removal of organic pollutants primarily revolve around physisorption and chemisorption mechanisms, such as repulsion/electrostatic attraction through π-π electron donor–acceptor interactions, pore diffusion, hydrophobic interactions, H bonding, and electrophilic interactions, especially with functional groups such as alcohols and carboxylic acids (Velusamy et al., 2021). Additional processes include chemical conversion and partitioning, ultimately leading to the mineralization of bonded pollutants through biodegradation (Gonzalez-Hourcade et al., 2022).

Several factors influence the interaction between biochar and organic pollutants, including the ratio of pollutants to biochar, feedstock type, pyrolysis temperature, and soil pH. Higher pyrolysis temperatures produce biochar with increased microporosity and surface area, making it particularly effective in removing non-polar carbon-based contaminants (Clurman et al., 2020). The physical properties of biochar, such as particle size and surface area, affect its sorption capabilities. Biochar with smaller particle sizes has higher SSA and, thus, better sorption effects, leading to shorter timeframes for removing contaminants (Lima et al., 2021). Moreover, soil conditions and specific biochar attributes contribute to pollutant degradation and adsorption. For instance, pesticide adsorption is more likely at lower pH (Panahi et al., 2020).

Biochar produced at temperatures exceeding 500 °C tends to have low acidity, reduced polarity, and increased aromaticity, which can degrade hydrogen- and oxygen-comprising functional groups, accelerating hydrophobic relations. Conversely, biochar produced at temperatures below 500 °C pyrolysis temperature tends to retain more hydrogen- and oxygen-bearing functional groups, making it more attractive to polar organic compounds (Tang et al., 2022). This temperature-dependent variation in biochar properties has implications for its effectiveness in removing pollutants from soil. For example, the removal of polar herbicide and insecticide compounds results from particular polar interactions and the mechanism by which various carbon-based pollutants are removed from soil using specific biochar types and their associated processes (Delgado-Moreno et al., 2021). Biochar can reduce the bioavailability of carbon-based pollutants in soil, limiting their uptake by microbes and plants (Grimm et al., 2022). Table 5 presents additional details on eliminating persistent carbon-based contaminants in soil using biochar.

TABLE 5

TABLE 5. Biochar’s ability to eliminate some specific contaminants.

5.1 Mechanisms of soil remediation by biochar

Previous studies have revealed that biochar is effective in remediating contaminated soils (Tables 5, 6), particularly those polluted with persistent organic pollutants, petroleum hydrocarbons, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, antibiotics, herbicides, and pesticides (Zheng et al., 2022; Zhao et al., 2023). After biochar is incorporated into polluted soil with thorough mixing, it rapidly interacts with organic pollutants and soil microbes. Organic pollutants are stabilized on the surface and within biochar pores and may be subsequently decomposed by soil microbes (Haider et al., 2022). Biochar’s abundant functional groups and porous surfaces can adsorb various organic contaminants through several mechanisms (Duwiejuah et al., 2020). As organic contaminants are adsorbed by biochar, their levels in soil water decrease, and their bio-accessibility to soil microbes diminishes. Furthermore, biochar improves soil quality and productivity by enhancing soil biological and physiochemical attributes (Qi et al., 2022; Zhao et al., 2023). Moreover, biochar introduces a substantial amount of biodegradable organic carbon (OC) as a microbial substrate while also supplying and retaining mineral nutrients (e.g., S, Mg, Ca, K, P, and N) (Yaashikaa et al., 2020). Consequently, biochar-treated soils exhibit improved microbial community structures and increased microbial activity, including microbial biomass, enzyme activity, and respiration rates (Qi et al., 2022; Rashid et al., 2022). This enhanced microbial activity leads to the microbial mineralization of various organic contaminants in soil.

TABLE 6

TABLE 6. Elimination of persistent organic pollutants in soil by biochars.

Biochar alleviates/stabilizes organic pollutants through various physical/chemical sorption mechanisms (Figure 5), including London forces and electrostatic interactions between polar and non-polar molecules (Wang et al., 2023). Despite being relatively weak, London forces enhance the surface area of molecules and become more dominant as biochar particle size decreases (Chen K et al., 2022; Wu et al., 2023). Biochar contains mineral elements and functional groups (e.g., N, H, O, and C) (Table 7) that allow it to form H bonds with polar organic molecules (Figure 5). Zeng et al. (2022) reported that numerous functional groups in biochar become protonated at pH < pH_ZNC and dissociated at pH > pH_ZNC, resulting in biochar with positive and negative charges, respectively. The biochar’s surface charge facilitates the electrical attraction of ionized organic contaminants and polar organic molecules with counter-charges such as antibiotics, hormones, and pesticides. As conventionally denoted, this electrostatic interaction is usually stronger than H-bonding and dispersion force interactions (Zeghioud et al., 2022). Intensified electrostatic interactions may contribute to inner-sphere adsorption (specific surface interaction), in which ionized organic molecules react chemically with surface functional biochar groups (Zhen et al., 2023). Moreover, the aromatic carbon in biochar, which possesses excess π-electrons, can interact with electron-deficient organic compounds, particularly those containing Br, Cl, P, S, N, and O, via π-π electron donor–acceptor interactions (Hao et al., 2021).

FIGURE 5

FIGURE 5. Main mechanisms through which biochar removes organic contaminants in soil.

TABLE 7

TABLE 7. Influences of pyrolysis process and feedstock on biochar attributes.

Biochar sorbs non-ionic organic molecules through partitioning or surface adsorption. Smaller pore sizes in biochar have higher surface energy and tend to sorb organic contaminants in micropores first (Viggi et al., 2022). At low surface coverage (such as equilibrium solute amount to solute water solubility ratio ≤0.2), non-linear competitive adsorption of organic solutes into micropores in biochar’s carbonized porous surface is dominant; at higher surface coverage (i.e., equilibrium solute amount/water solubility >0.2), the adsorption shifts progressively to linear, non-competitive partition into biochar’s un-carbonized carbon moiety (Jing et al., 2022a). Pore filling is another mechanism by which biochar adsorbs organic pollutants (Figure 5). Studies have shown that the amount/degree of surface adsorption is correlated with biochar surface porosity, surface area, and aromaticity (Sanchez-Hernandez, 2021), while pore-filling adsorption relies on biochar ash and OC contents (Yi et al., 2021).

Moreover, the biochar surface may contain polyvalent metal elements, e.g., Mg, Ca, Al, and Fe (Zhang et al., 2023), and ionized or polar organic molecules can form complexes with these metal ions, leading to surface deposition. Kinetic sorption of various organic compounds onto biochars follows a pseudo-second-order pattern, suggesting the significance of surface precipitation and chemisorption (Figure 4) in biochar–organic material interactions (Viggi et al., 2022). Du et al. (2023) concluded that methyl violet sorption by two crop residue-derived biochars mainly occurred through surface precipitation and electrostatic attraction.

Biochar surface properties such as hydrophobicity, aromaticity, pore volume and size, polarity, and surface area influence its interactions with organic molecules (Sima and Jiang, 2020). Generally, biochars produced at high pyrolytic temperatures have greater surface area, hydrophobicity and aromaticity, and lower surface polarity than those produced at low temperatures due to the loss of hydrogen and oxygen-comprising functional groups (Pidlisnyuk et al., 2021). Vu and Mulligan (2023) reported that increasing pyrolysis temperatures from 200°C to 600 °C progressively increased the surface area of rice straw-derived biochar from 7.20 to 194 m² g⁻¹ and its sorption capacity for insecticide in the soil. The mineral ash content of biochar, particularly manure-derived biochar, can block micropores and modify its polarity, hydrophobicity, and SSA (Hao et al., 2021). Fagnano et al. (2020) reported that there was an increase in the SSA biochar derived from pig manure at 350°C and 700°C from 23.80 to 32.60 m² g⁻¹ and from 66.90 to 22.0 m² g⁻¹, respectively, but CEC level reduced from 97.0 to 113 cmol_c kg⁻¹ and from 5.60 to 8.70 cmol_c kg⁻¹, respectively. Consequently, the sorption capacities for pesticides atrazine and carbaryl of pig manure-derived biochars significantly increased (Zeghioud et al., 2022). Different feedstock-derived biochars have diverse adsorption capabilities and primary mechanisms for alleviating soil organic pollutants. Biochar mainly adsorbs hydrophobic and non-polar organic pollutants (e.g., petroleum hydrocarbons and PAHs) through the hydrophobic effect, partitioning, and pore filling. In contrast, biochar adsorbs ionized and polar organic contaminants, including antibiotics and pesticides, through surface precipitation, specific interactions, electrostatic attraction, and H bonding (Figure 5).

5.2 Efficiency variations

The efficacy of biochar in mitigating organic contaminants and facilitating their removal from soil varies with pollutant type, soil type, biochar source and type, particle size, and application rate (Table 6). For instance, biochars derived from rice hulls, soybean straw, peanut straw, and canola straw pyrolyzed at 350 °C exhibited varying sorption capabilities for methyl violet removal from soil (Beljin et al., 2023). In another study, a 5% application rate of biochars derived from wheat straw, coconut shell, and willow decreased the freely dissolved and bio-accessible fractions of PAHs in soils collected from different industrial contaminated sites by 30%–88% compared to control soils (Zhu et al., 2023). It is worth noting that biochars can contain PAHs but at relatively low and ecologically insignificant levels (∼6 0 mg kg⁻¹) (Dai et al., 2022).

The efficiency of biochar treatment can also vary with soil type. Clay (OC = 2.4%, pH = 5.80) and sandy loam (OC = 0.55%, pH = 5.70) soils treated with 5% biochar derived from sugarcane (CEC 114 cmol_c kg⁻¹, pH 8.70; surface area 59 m² g⁻¹) had enhanced 17α-ethinylestradiol sorption and decreased microbial mineralization (Wang H et al., 2021). Moreover, the sandy loam soil had a notably higher adsorption efficiency for 17α-ethinylestradiol than clay, and biochar decreased the adsorption coefficient of the sandy loam soil but improved it in the clay soil (Wang L et al., 2021). In a study involving a phenanthrene-contaminated acidic soil (pH 3.20), the addition of 1% biochar (SSA = 3.50 m² g⁻¹; N = 2.1%) derived from burn nettle enhanced phenanthrene degradation by 40% (Tian et al., 2022). The biochar primarily acted to stabilize/alleviate metals and did not significantly adsorb the phenanthrene. However, the biochar improved soil quality and stimulated microbial activity. Valentina et al. (2021) reported that 2% rice biochar application to petroleum-contaminated soil improved microbial activity, increasing petroleum hydrocarbon biodegradation by 20%. Moreover, the biochar had a higher affinity for hydrophilic, polar organic contaminants than hydrophobic, non-polar organic molecules.

Nguyen et al. (2023) investigated the effect of biochar on biodegradation and herbicide (simazine) leaching in different soil types. Two particle size fractions of wood-derived biochar were examined: fine (<2 mm) and coarse (2–10 mm). For 0.5% and 5% biochar applications, both fractions inhibited simazine biodegradation and decreased its leaching in clay loam (OC = 3.50%, pH = 6.20) and loamy sand (OC = 1%, pH = 4.80) soils, with more notable impacts at the higher addition rate, in the clay loam soil, and for the fine particle size (Kou et al., 2023). Biochar particle size affects the uniformity of biochar distribution in soil, affecting its remediation efficiency (Nguyen et al., 2023).

Incorporating biochar into the soil through mechanical mixing achieved more uniform biochar distribution than shovel or hand mixing, increasing polychlorinated biphenyl retention in the soil by 2.80% and decreasing the bioaccumulation of polychlorinated biphenyls in pumpkin roots by 50% (Keerthanan et al., 2021).

Thus, biochars produced at higher pyrolysis temperatures from plant and wood wastes generally have higher aromaticity levels, porosity, and SSA than manure-derived biochars, making them preferable for sorbing organic pollutants. Over time, biochar enhances mineralization and ultimately removes organic pollutants from soils by increasing soil microbial activities (Tian et al., 2022).

6 Nutrient release from biochar

Numerous studies have shown that biochar affects nutrient availability, with the potential as a moderate-release fertilizer in soils. Nutrient release from biochar depends on its desorption characteristics, with several factors affecting nutrient desorption from biochar (Issaka et al., 2022). For example, Zhang et al. (2022) reported that the NH₄⁺ desorption ratio from Betula-derived biochar increased from 17% to 30% when the pyrolysis temperature decreased from 500 °C to 300°C, indicating that lower pyrolysis temperatures can increase nutrient desorption from biochar.

Brtnicky et al. (2021) studied P desorption from black or regur soil, reporting that the average desorbed P increased from 35% to 40% when the biochar addition rate increased from 0% to 10%. Approximately 70% of the P adsorbed by biochar was released at higher P loadings. These findings indicate that increasing the P loadings and biochar addition ratio can enhance P desorption from biochar. Furthermore, different feedstock types used to produce biochar can have varying effects on nutrient desorption. For instance, biochar derived from cacao shell desorbed 1,485 mg g^–1 phosphate, while biochar derived from corn cobs desorbed 170 mg g^–1 phosphate (Razzaq et al., 2022). Thus, the desorption properties of biochar depend on factors such as biochar application rate, feedstock type, and preparation temperature. Based on the research on desorption characteristics, it is possible to select different types of biochar for specific soil nutrient management purposes. Biochar can be applied to maintain various soil nutrients in the same soil or preferentially in different soils to optimize nutrient supply. This flexibility makes biochar a valuable tool for sustainable soil management and improving nutrient availability in agricultural soils.

7 Other environmental benefits

7.1 Biochar application for gas remediation

Biochar is highly effective in the remediation of toxic components from gases. Various types of biochar, including those derived from pig manure, poultry manure, hardwood chips, camphor, sludge, bamboo, and rice hulls, have demonstrated efficient removal (>90%) of hydrogen sulfide (H₂S) from biogas, with sorption capacities ranging from 100 to 380 mg H₂S g^–1 biochar (Das et al., 2020). The amount of H₂S removal is influenced by biochar moisture content, chemical bonding, and surface area with functional groups. H₂S reacts with the alkali-based biochar surface through ionic interactions with–COOH and–OH functional groups in water and oxygen to form Na₂SO₄ and K₂SO₄ (Gao et al., 2022). Biochars derived from soybean straw and peanut shells prepared at high pyrolysis temperature (700 °C) were more effective for trichloroethylene removal than biochars produced at low pyrolysis temperatures (300 °C) due to their increased surface area and hydrophobicity and decreased polarity (Ambika et al., 2022).

7.2 Migration of greenhouse gases (GHGs)

GHG emissions are a significant driver of climate change, with CO₂ emissions contributing to over 70% of these emissions (Atilano-Camino et al., 2022). Reducing CO₂ pollutants from agricultural soil is crucial for mitigating climate change. Biochar can reduce CH₄ and N₂O emissions and enhance soil C sequestration (Romero et al., 2021). However, its effects on soil GHG emissions can differ depending on the feedstock type, pyrolysis temperature, and water contents (Hua et al., 2021). While there are numerous studies on the effects of biochar on soil CO₂ emissions, the outcomes are not always significant due to variations in methodologies and research materials. Reducing GHG emissions through biochar application is a multifaceted process and is becoming better understood through systematic research (Ginebra et al., 2022). One study reported that adding biochar to soil enhances the activity of microbes involved in reducing N₂O to N (Sarauer et al., 2019). However, the alkaline nature of biochar can also enhance the activity of N₂O-decreasing organisms. In cases where soil pH is increased, the disadvantages of biochar may lead to reduced soil emissions and acidity (Cui et al., 2021). Biochar provides ample sites for N and N₂O removal due to its high SSA, decreasing the release of these gases from the soil environment (Levesque et al., 2020).

8 Different properties of biochar play key roles in environmental remediation

The physiochemical characteristics of biochars affect their adsorption capabilities. For example, enhanced acidic (e.g., hydroxyl and carboxyl) functional groups in biochar increase NH₄⁺ sorption (Danesh et al., 2022). Biochar has numerous functional groups, high stability, and SSA, influenced by the pyrolysis process and feedstock type (Lopes and Astruc, 2021). Various feedstocks, including poultry manure, organic waste, wood chips, and plant residues, can be used to prepare biochar. Pyrolysis temperatures typically range from 250 °C to over 750 °C (Shan et al., 2020). Table 7 presents the effects of the pyrolysis process and feedstock type on biochar properties.

8.1 Cation exchange capacity

The CEC reflects the ability of biochar to adsorb cations, such as Ca²⁺ and NH₄⁺, which are essential for plants. A high biochar CEC can reduce nutrient losses from soil leaching (Gabhane et al., 2020). CEC values of biochar vary based on factors such as feedstock type and pyrolysis temperature. For example, the CEC of biochar produced from Spartina increased from 7 to 45.4 cmo1_c kg^–1 and then decreased to 31 cmo1_c kg^–1 as the pyrolysis temperature increased from 250°C to 600 °C. Oatmeal pine biochars prepared at 200, 300, 350, and 600 °C had CEC values of 16.5, 16.9, 23.8^, and 2.1 cmo1_c kg^–1, respectively (Patwardhan et al., 2022). In addition, sugarcane biochar had a CEC that increased from 6.45 cmo1_c kg^–1 (at 250 °C) to 8.99 cmo1_c kg^–1 (at 500 °C) and then decreased to 5.10 cmo1_c kg^–1 (at 600 °C) (Kamarudin et al., 2022). These findings suggest that biochars prepared at higher temperatures tend to have lower CEC values (Danesh et al., 2022). The reduction in CEC in biochar produced at higher temperatures is attributed to aromatization and the loss of functional groups (hydroxyl and carbonyl) (Kang et al., 2022). One study reported that biochar prepared at temperatures of up to 480 °C retained carboxyl and phenolic acid groups (acidic-oxygenated functional groups) (Li et al., 2022). Lu et al. (2022) demonstrated that biochar CEC depends on its surface distribution and oxygen-enrich functionalities, with negative charge sites on biochar surfaces accredited to phenolate and carboxylate functional groups (Kamali et al., 2022). Some studies have reported that biochars with higher SSA (produced at temperatures ≥600 °C) have enhanced CEC due to increased surface microporosity (Shaheen et al., 2022) despite the loss of volatile matter (Samoraj et al., 2022). Biochar CEC also depends on feedstock type. For instance, pig manure-derived biochar had a lower CEC (32 cmo1_c kg^–1) than chicken manure biochar (81 cmo1_c kg^–1) produced at 500 °C (Yaro et al., 2023). However, paper mill-derived biochar had a markedly lower CEC (9–18 cmo1_c kg^–1) than sugarcane-derived biochar (112 cmo1_c kg^–1) (Chai et al., 2021). This variation was attributed to higher ash contents in feedstock, resulting in biochar with greater CEC values (Behl et al., 2020).

8.2 Surface chemical composition (surface functional groups)

The biochar preparation temperature significantly influences the surface functional groups of biochar, which affect the sorption properties of biochar (Bao et al., 2022). Pyrolysis temperatures ranging from 350°C to 650 °C rearrange and break chemical bonds in the feedstock, creating various functional groups (e.g., pyrrole, pyridone, pyridine, pyrone, ether, phenol, anhydride, chromene, quinine, lactol, lactone, and carboxyl) (Zhou et al., 2021). Generally, high pyrolysis temperatures decrease O and H contents and the molar H/C ratio, with a notable reduction in polar functional groups such as C-O and–OH (Fang et al., 2021). Fourier-transform infrared (FTIR) analysis of biochar often reveals the dominance of functional groups containing oxygenated hydrocarbons, reflecting the carbohydrate structure of hemicelluloses and cellulose (Kumari et al., 2023). For example, sawdust-derived biochar had a broad band between 3,000 cm⁻¹ and 3,600 cm⁻¹, with a smaller band between 2,700 cm⁻¹ and 3,000 cm⁻¹ (Vanapalli et al., 2021). The band centered at 3,339 cm⁻¹ was accredited to the presence of phenolic and alcoholic functional groups (Yang et al., 2021), while the band at 2,907 cm⁻¹ corresponded to alkyl C-H stretching. Another band arising at 1,600 cm⁻¹ was associated with aromatic C–O and C–C stretching of conjugated quinones and ketones (Azeem et al., 2022), a band at 1,735 cm⁻¹ was related to C=O stretching of esters, aldehydes, and ketones (Zahed et al., 2021), and a band arising at 1,238 cm⁻¹ was attributed to the presence of C–O–C groups and phenolic, aryl ethers linked with lignin (Wang H et al., 2021).

Biochars produced at high temperatures (e.g., 600°C and 700 °C) exhibit hydrophobic natures with well-ordered carbon layers (Xu et al., 2021). However, they contain lower amounts of O- and H-comprising functional groups due to the deoxygenation and dehydration of biomass during pyrolysis (Natasha et al., 2022). These surface functional groups can function as electron acceptors and donors, creating co-existing zones with attributes ranging from hydrophilic to hydrophobic and acidic to basic (Van Nguyen et al., 2022). Consequently, biochar produced at high temperatures may have a lower ion exchange capability (Wang L et al., 2021). In contrast, biochar produced at lower temperatures (300°C and 400 °C) exhibits a more diverse organic nature due to cellulose and aliphatic-based structures. These biochars tend to have well-organized carbon layers and a higher content of surface functional groups than those produced at higher temperatures (Sun et al., 2021).

8.3 pH

The pH of biochar is typically alkaline, with values ranging from 7 to 10, but variations in pH can occur due to pyrolysis temperature and feedstock type (Muhammad et al., 2020). For instance, biochars derived from soybean, peanut, and corn straw at 300 °C had pH values of 7.70, 8.60, and 9.40, respectively, while corn straw derived-biochar was acidic (Wei et al., 2023). Woody biomass-derived biochars had average pH values two units lower than non-woody biomass-derived biochars under similar carbonization conditions (Qiu et al., 2022). Biochars derived from non-woody biomass sources had higher pH values (around three units), attributed to salts like calcium and potassium chlorides and carbonates in the ash (Cara et al., 2022). Sugarcane-derived biochar had a lower pH (8.5) than rice husk-derived biochar (9.0), possibly due to its lower ash content (19.10% vs. 40%) (Kayiranga et al., 2023). The increase in pH values could be due to the concentration of non-pyrolyzed inorganic components and the decomposition of the organic matrix (Xiao et al., 2023). Biochar pH is likely associated with its cellulose, hemicellulose, and lignin contents and oxygen-based functional groups (Sun et al., 2022). El-Naggar et al. (2021) stated that the -O- and -COO groups and carbonate content in biochar were responsible for their alkaline nature. Gouma et al. (2022) reported that biochar pH positively correlated with oxygen content (R² = 0.7), consistent with findings that oxygen-containing functionalities, including quinine, diketone, chromene, and pyrone, are responsible for the basicity of biochar (Kumari et al., 2023). The carbonization process during biochar production can lead to the formation of oxygen-rich functionalities (Burachevskaya et al., 2021), and this process is associated with the condensation reactions of aliphatic elements and the dehydration of feedstock (Bandara et al., 2020). Moreover, rice straw and rice bran-derived biochar pH values were negatively correlated with anomeric (O-C-O) and aliphatic O-alkylated carbons but positively correlated with aromatic C-O and fused ring aromatic structures (Zheng et al., 2022). Thus, numerous carboxylic groups in produced biochars that decrease during carbonization and acidic groups that turn into deprotonated to the conjugated bases result in a more alkaline pH of biochars (Venkatesh et al., 2022).

Biochar application can increase soil pH by increasing cation retention (e.g., K⁺, Mg²⁺, and Ca²⁺) (Armah et al., 2022). Biochars prepared at higher pyrolysis temperatures have greater pH due to the release of alkali salts from organic biomass substances (Kamarudin et al., 2022). For example, the pH value of maize straw biochar increased from 8 to 11.5 when the preparation temperature increased from 350°C to 550 °C (Kamali et al., 2022). Similarly, pig waste biochar prepared at 500°C and 700 °C had pH values of 8.55–10.53, respectively (Vijayaraghavan, 2019). Thus, biochar with a high pH and CEC can potentially preserve K⁺ and NH₄⁺ fertilizers and enhance their effectiveness when applied to soil (Varjani et al., 2019).

8.4 Specific surface area

Specific surface area is an essential biochar attribute influenced by pyrolysis conditions and feedstock/biomass type (Balajii and Niju, 2019). A high SSA helps biochar to adsorb substances, such as organic compounds and heavy metals (Atilano-Camino et al., 2022). Different feedstocks and pyrolysis temperatures can result in variations in SSA. For example, vine pruning and orange pomace-derived biochars with low ash contents had lower SSA values (8.10 and 1.20 m² g^–1, respectively) (Jing et al., 2022b), possibly indicating less porous biochar (Albalasmeh et al., 2020). Enhanced biochar porosity is attributed to lignin decomposition, rapid release of CH₄ and H₂, and reactions involving aromatic condensation as the pyrolysis temperature increases (Rizwan et al., 2020a; Amusat et al., 2021). For example, sugarcane-derived biochar had a larger SSA and pore size (253 and 0.1 m² g^–1, respectively) than coconut shell-derived biochar (25.80 and 0.1 m² g^–1, respectively) (Velusamy et al., 2021). Similarly, sugarcane-derived biochar had a higher SSA and larger pore size (185 and 0.1 m² g^–1, respectively) than rice husk-derived biochar (157 and 0.1 m² g^–1, respectively). (Gonzalez-Hourcade et al., 2022), reflecting differences in thermal degradation, cellulose, and lignin content (Sun et al., 2021). Biochar derived from peanut shells and pine cones had larger SSA values (2.0 and 1.8 m² g^–1, respectively) than biochar derived from corn stalks and cake (0.8 and 0.5 m² g^–1, respectively) (Tang et al., 2022), likely due to the large quantity of lignin in the peanut shells and pine cones (Kumar et al., 2023). Biochar derived from apricot stones, grapes, and cobnuts shells had higher SSA values (11.60, 14.5, and 14.70 m² g^–1, respectively), pore volumes (0.2, 0.2, and 0.1 cm g^–1, respectively), and porosity values (all 0.1%) than their respective biomasses (SSA: 10.60, 5.80, and 10.50 m² g^–1, respectively; pore volume: all 0.1 cm g^–1; and porosity: all 0.1%) (Jeyasubramanian et al., 2021), attributed to varying lignin and cellulose degradation (Goswami et al., 2022).

Increasing the pyrolysis temperature can enhance micropore formation and SSA in biochar. For example, the SSA of sugarcane-derived biochar increased from 0.55 to 13.2 m² g^–1 as the pyrolysis temperature increased from 300°C to 700 °C (Kumari et al., 2023). Similarly, soybean-derived biochar prepared at 650 °C had an SSA of 419 m² g^–1, much higher than that prepared at 400 °C (Danesh et al., 2022).

The choice of feedstock also plays a role in determining SSA. Biochar prepared from cocopeat had a lower SSA (13.7 m² g^–1) compared to that from bagasse (202 m² g^–1), attributed to the volatiles released from the biomass during pyrolysis (70% and 87%, respectively) (Kamarudin et al., 2022). The release of volatile substances, particularly those produced from hemicellulose and cellulose during pyrolysis, can enhance the formation of a vascular bundle structure in biochar and improve its pore structure and SSA (Armah et al., 2022). Reducing the volatile content within Euphorbia mammillaris and corncob biochar increased the surface area by 60.7–193.1 m² g^–1 (Bao et al., 2022).

8.5 Biochar stability

Biochar stability helps determine its carbon sequestration ability (Nzediegwu et al., 2021). The pyrolysis temperature used in the carbonization process has been considered evidence of biochar stability (Nzediegwu et al., 2022). This proximate analysis is a traditional method used to assess the nature of the fixed carbon, ash, charcoal, moisture, and volatile matter in biochar. However, it requires high temperatures (around 900°C–950°C for volatile matter and 700°C–800°C for ash determination) for an extended period (Janu et al., 2021), which can overestimate carbon content and underestimate ash content (Da et al., 2022). Techniques for evaluating the stability of biochars can be divided into three classes: i) quantification of carbon structures, particularly those with aromatic characteristics as they are considered relatively stable and less prone to degradation; ii) quantification of stable carbon using thermochemical/thermal degradation and chemical oxidation; and iii) biochar incubation in soil and carbon mineralization modeling (Fu et al., 2019). Biochar stability is associated with the carbon structure within the biochar, which contains amorphous and crystalline phases (Bakshi et al., 2020; Islam et al., 2021).

Biochar stability can vary depending on the feedstock type and pyrolysis temperature (Petersen et al., 2023). Higher pyrolysis temperatures generally lead to more stable biochar (Gopal et al., 2020). For instance, increasing the pyrolysis temperature from 300°C to 600 °C significantly improved the stability of sugarcane-derived biochar (Gluckler et al., 2021). In contrast, biochar prepared at lower pyrolysis temperatures is more susceptible to degradation (Mai et al., 2019).

8.6 Elemental composition

The elemental composition of biochar is a function of pyrolysis temperature and feedstock species (Almutairi et al., 2023). Higher pyrolysis temperatures typically increase the carbon and ash contents of biochar (Muigai et al., 2021). Moreover, different feedstock species contribute varying elemental amounts to the resulting biochar. For instance, chicken waste-derived biochar has higher Mg, Ca, K, P, and N contents than woodchip biochar, while woodchip biochar has a higher total carbon content (Choudhary et al., 2019; Rodriguez et al., 2021).

The nitrogen content of biochar can also be influenced by pyrolysis temperature, with lignocellulosic-rich biochar showing a moderate increase as the pyrolysis temperature increased, while biochar derived from sewage sludge and animal manure followed a downward trend (Stylianou et al., 2020). Pyrolysis conditions, including residence time and temperature, can affect the accumulation and release of specific elements. For example, a longer residence time (about 60 min) and higher temperature accumulated K and P, released Si, Mg, and Ca, and retained S, Mn, and Fe (Hassan et al., 2020).

Similarly, increased pyrolysis temperature and reaction time removed some unstable constituents (comprising O and H elements) of feedstock due to decarboxylation, dehydration, and deoxygenation reactions, resulting in VOC losses and decreased H/C and O/C ratios (Venkatesh et al., 2022), contributing to a more stable graphite structure, improved aromatic structure, and greater carbonization level in the biochar (Balajii and Niju, 2019). For instance, ∼400°C–500°C pyrolysis temperatures resulted in O/C ratios ranked as follows: Oryza sativa straw > O. sativa husk > apple tree > oak tree (Albalasmeh et al., 2020). Thus, wood-derived biochar (oak tree and apple tree), with higher lignin contents and slower mineralization rates, had lower O/C ratios than herb-derived biochar (rice husk and rice straw) and sewage sludge-derived biochar and thus greater stability (Jing et al., 2022a).

9 Importance of biochar for bioeconomy

Biochar plays a crucial role in the bioeconomy, which involves the sustainable use of bioresources to create economically valuable bioproducts (Rex et al., 2023). Biomass (feedstock), as the primary bioresource, is transformed into various bioproducts, with biochar being one of the most significant (Han et al., 2021). The bioeconomy encompasses activities such as awareness promotion, marketing, commercialization, and production, all of which contribute to job creation, both indirectly and directly (Sri Shalini et al., 2021). In the context of the bioeconomy, the safety, quantity, and quality of bioproducts, including biochar, affect its success. Efficient biochar production has economic and agronomic advantages. For instance, using biochar in crop cultivation can increase yields and generate additional income, enhancing economic stability (Seow et al., 2022). The global energy demand continues to grow with the increasing global population (Hamidzadeh et al., 2023), making sustainable energy sources essential. Feedstock is one such energy source that can be converted using thermochemical, physical, biochemical, and mechanical methods. Thermochemical transformation is particularly effective, breaking chemical bonds in carbon-based substances to produce syngas, bio-oil, and biochar. Biochar, in particular, is gaining prominence due to its sustainability benefits, financial advantages, and growing demand in the energy and environmental sectors (Kwon et al., 2020).

While developed nations have progressed significantly in advancing their bioeconomies, many developing nations need to raise awareness and increase bioproduct production (Amalina et al., 2022). As a profitable and commercially viable material, biochar finds applications in energy, manufacturing, and agriculture. Biochar production can help improve soil quality, providing opportunities for additional profit. Using forest biomass-derived biochar as a soil amendment in agriculture can significantly impact the economy, especially in forest and agricultural sectors, and stimulate related industries, such as biochar equipment manufacturing and carbon sequestration (Anastopoulos et al., 2019). Studies have shown that CO₂ sequestration remittance could enhance biochar efficiency (Kwon et al., 2020). Biochar’s potential application for enriching soils and enhancing crop production appears promising (Sieradzka et al., 2022), as it can improve water and soil retention, decrease irrigation and fertilizer costs, and rejuvenate depleted soils (Ghodake et al., 2021).

Feedstock pyrolysis produces two key byproducts: oil and syngas, both of which have applications in renewable energy and fuel (Schmidt et al., 2021). Syngas can be used as an input in the pyrolysis process and as a heating source in drying methods. Oil can be used as a standalone product, but emissions can be heavy in black carbon and particulates. Therefore, it is essential to refine oil into biodiesel for use as fuel in transportation (Song et al., 2021).

10 Economic significance of biochar

The economic significance of biochar production is influenced by various factors, with transportation playing a significant role. Campion et al. (2023) conducted an economic analysis of biochar production and transportation costs. They assessed the feasibility of mobile pyrolysis facilities by analyzing two types of biochar in three states with frequent transportation needs. Their analysis included shipping logistics based on GIS data and used a Monte Carlo financial simulation model. They found that the net present value of biochar production often increases when the mobile pyrolysis facility is located closer to the biomass source (Allohverdi et al., 2021). Figure 6 illustrates the economic benefits of biochar utilization.

FIGURE 6

FIGURE 6. Economic profits of biochars derived from various feedstocks. Source: sugar beet tailing (Ramanayaka et al., 2020) Rice shell (Scaria et al., 2022) wood-chips (Gouma et al., 2022) fruit bunches (Qiu et al., 2022) Human manure (Mazarji et al., 2022) wood barks (Xiao et al., 2023) dairy residue (Rex et al., 2023) poultry waste (Han et al., 2021).

Biochar production has garnered attention due to its potential in the environmental and energy sectors. Seow et al. (2022) presented economic assumptions for biochar production in Selangor, Malaysia, estimating annual costs at US$ 532 and total revenue from biochar sales at US$ 8,012 (Table 8). Consequently, the net value of biochar production demonstrated positive economic feasibility for biochar (Seow et al., 2022). The cost-effectiveness of biochar production also depends on factors such as selling price. Tang et al. (2022) identified a break-even selling price of approximately US$ 220 per ton for biochar produced at 300 °C and approximately US$ 280 per ton for biochar produced at 450 °C. Dong et al. (2023) reported promising net margins for biochar production using agricultural residues such as horse, cattle, and livestock manure, with net margins of US$ 69 and US$ 162 for high and low revenue scenarios of CO₂, respectively. Thus, the economic viability of biochar production depends on various factors, including the costs associated with biomass collection, storage, transportation, carbonization, application, and processing (Jing et al., 2022b). Research suggests that the net margin of biochar production can be enhanced by using low-cost biomass sources and adopting efficient processing technologies (Venkatesh et al., 2022).

TABLE 8

TABLE 8. Economic investigation of biochar production in three states of Malaysia (Rashid et al., 2022).

11 Conclusion

Biochar is a versatile product derived from various feedstock materials and pyrolyzed under different conditions, resulting in a wide range of chemical and physical properties. These properties make biochar a valuable tool for improving soil quality, enhancing carbon sequestration, mitigating soil erosion, promoting photosynthesis, reducing urban heat island effects, and decreasing GHG emissions. One of its key benefits is its ability to reduce the mobility and bioavailability of inorganic and organic contaminants in soils.

The specific pyrolysis conditions and the feedstock type strongly influence the quality and quantity of biochar. Therefore, it is crucial to consider these factors when using biochar in soil improvement and remediation efforts. Understanding how specific biochars immobilize contaminants over time as their adsorption sites interact with native soil organic matter and competing contaminants is essential for effective long-term application.

Biochar plays a significant role in the bioeconomy concept, which focuses on the sustainable exploitation and exploration of bioresources to create valuable products using biotechnology. As a commercially viable bio-product (biogas and bio-oil) with applications in energy, various industries, and agriculture, biochar production can benefit soils and provide opportunities for additional income. Furthermore, biochar offers advantages such as ease of transport, cost-effectiveness compared to traditional fertilizers, and long-lasting effects on soil improvement. Introducing strategies related to CO₂ sequestration costs can further incentivize the use of biochar in various applications.

12 Future outlook

The future outlook for biochar holds significant promise and potential for further advances in environmental and agricultural applications. Several important areas of research and exploration should be considered to harness these benefits, including:

1. Risk assessment and mitigation: As biochar is produced from various feedstocks, some of which may be contaminated, it is crucial to conduct further studies to assess the associated risks.

2. In-depth characterization: Systematic research is needed to investigate the relationships between biomass types, carbonization conditions, and the physiochemical attributes of biochar, including biochar stability and structure and lignin, cellulose, and hemicellulose contents. Moreover, novel statistical analysis techniques, such as machine learning, could be adapted to help predict and optimize biochar properties for specific uses.

3. Innovative carbonization techniques: Experiments are needed to investigate the feasibility of combining novel carbonization techniques with modification approaches to generate more efficient biochars. These methods include co-pyrolysis and microwave-assisted modifications to develop modified biochars with enhanced properties, such as higher surface areas and reduced environmental risks.

4. Field validation: Most research on biochar effects has been conducted under controlled laboratory conditions; thus, validating these findings under real-world field conditions is essential. Field experiments should encompass diverse soil types and agro-climatic zones to better understand how biochar performs under varying environmental conditions.

Author contributions

MR: Original draft. GM: Original draft. FZ: Review and editing. AM: Review and editing. RI: Review and editing. ZA: Review and editing. SI: Review and editing. IK: Review and editing. TL: Review and editing. JC: Review and editing. MZ: Review and editing. KHMS: Resources and supervision. LL: Resources and supervision. HL: Resources and supervision.

Funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This work was supported by the Science and Technology Innovation Program of Hunan Province (2021RC4005) and the Open Project of Xiangjiang Laboratory (No. 22XJ03003).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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