Advancements in Sustainable Biochar Production from Waste: Pathways for Renewable Energy Generation and Environmental Remediation

Abstract

In response to significant environmental challenges, biochar has garnered attention for its applications across diverse fields. Characterized by high carbon content resulting from the thermal degradation of biomass, biochar offers a sustainable strategy for waste valorization and environmental remediation. This paper offers a comprehensive overview of biochar production from residual biomass, emphasizing feedstock selection, conversion pathways, material properties, and application potential. Key production techniques, including pyrolysis, gasification, and hydrothermal carbonization, are critically evaluated based on operational conditions, energy efficiency, product yield, and environmental implications. The functional performance of biochar is further discussed in the context of soil enhancement, wastewater treatment, renewable energy generation, and catalytic processes, such as biohydrogen production. By transforming waste into value-added products, biochar technology supports circular economy principles and promotes resource recovery. Ongoing research aimed at optimizing production processes and understanding application-specific mechanisms is crucial to fully realizing the environmental potential of biochar.

1. Introduction

As a result of the considerable growth in technological innovations, the world is rapidly advancing across various domains, leading to swift industrial and urban development. This development is essential to ensure improved living conditions and fulfill the requirements of forthcoming generations. Despite the positive aspects of global progress, the environmental cost is significant due to the enormous production of waste. Globally, an annual production of 7–9 billion tons of waste occurs, including municipal, industrial, and hazardous waste [1]. These waste materials, as highlighted by Ustohalova [2], have adverse effects on various ecosystems, including soil, water, and air, ultimately impacting human life and health in diverse ways. This issue has become a significant concern for both human health and the environment [3]. Addressing this challenge is imperative as the world increasingly moves toward sustainability. Consequently, there is a growing emphasis on implementing a cost-effective and sustainable solution. In this context, the conversion of waste into biochar and its application has garnered attention, especially for removing contaminants. Biochar, with robust resistance to decomposition, is characterized by a carbonaceous and porous structure, owning a significant degree of aromatization. It is produced by subjecting biomass obtained from plants or animals to thermal degradation under high temperatures with limited oxygen supply, resulting in exceptional physical and chemical properties [4]. Recent data show that biochar’s elemental composition, density, porosity, and pH are strongly influenced by the feedstock and production process. These characteristics, in turn, impact the applicability of biochar for diverse uses. In the realm of water and wastewater treatment, biochar is employed to eliminate both organic and inorganic pollutants [5]. In agriculture, it finds application in enhancing soil quality, reducing nutrient degradation rates, and overall improving soil characteristics [6]. Additionally, owing to its high carbon content, biochar can serve as a fuel for power generation [7]. Biomass, in general, is recognized as a promising source of a wide range of chemicals, renewable energy, and minerals [8]. Agricultural residues, crop byproducts, activated sludge, algae biomass, animal waste, and energy crops are the primary biomass feedstock sources [9]. Various approaches, including physical, biochemical, and thermochemical techniques, can be applied to convert biomass into valuable end products. Producing biochar involves employing diverse thermal breakdown techniques such as pyrolysis, torrefaction, gasification, and hydrothermal carbonation [10]. While pristine biochar has demonstrated considerable potential in environmental applications, its effectiveness is often limited by characteristics such as low surface area, limited active sites, and variable adsorption capacities. These constraints underscore the growing importance of biochar modification. Techniques such as physical activation, chemical treatment, and surface functionalization have been extensively investigated to enhance its structural and functional attributes, enabling improved pollutant removal and application-specific performance [11]. This review aims to provide a comprehensive and integrative overview of biochar production from residual biomass, emphasizing sustainable resource utilization, production techniques, and environmental applications. The primary objective is to bridge the gap between waste valorization and practical implementation, demonstrating how biochar can contribute meaningfully to environmental sustainability. The paper critically examines various biomass feedstocks, thermal conversion techniques, and the influence of production parameters on the quality and functionality of the resulting biochar. Importantly, this review distinguishes itself from the existing literature by offering a multidimensional analysis of the entire biochar life cycle—from waste-derived feedstock selection and production methods to post-treatment modifications and cross-sectoral applications. Special emphasis is placed on the relationship between production conditions and biochar performance in diverse environmental sectors. By synthesizing recent advances and highlighting research gaps, this work provides a strategic perspective that supports targeted optimization and scalability of biochar for real-world environmental solutions.

2. Feedstock for Biochar Production

The production of biochar offers a sustainable approach to managing solid waste, responsibly using a range of biomass feedstocks, including both lignocellulosic and non-lignocellulosic sources. Comprising plant-derived materials, lignocellulosic biomass stands as a rich bioresource, encompassing agricultural waste (e.g., rice straw [12], wheat straw [13], corn stovers [14], rice husk [15], coconut shell [16], sugarcane bagasse [17]), energy crops (e.g., elephant grass [18], switchgrass [19], miscanthus [20]), and forest residue (e.g., trees [21], bark [22], logging residues [23], sawdust [24], wood chips [25]). Non-lignocellulosic biomass includes animal waste (e.g., cattle manure [26], chicken litter [27], bones [28]), municipal solid waste (e.g., food waste such as potato peel [29], orange peel [30], banana peel [31], groundnut [32], cassava peel [33], tea leaves [34], etc.), recyclable waste (e.g., plastics [35], cardboard [36]), aquatic plants and marine materials (e.g., algae [37], coral reef [38], kelp [39]), and sewage sludge [40]. Animal and municipal waste, constituting non-lignocellulosic biomass, poses a greater environmental hazard with elevated concentrations of heavy metals and heteroatoms such as sulfur, phosphorus, and nitrogen [41]. A comprehensive assessment of the feedstock, particularly non-lignocellulosic biomass, is imperative prior to its incorporation into biochar production. This examination ensures that the feedstock does not contain excessive amounts of heavy metals, which could potentially transfer to the end product [9]. The use of lignocellulosic biomass has attracted considerable attention due to its renewable nature, accessibility, and straightforwardness. Forests and agriculture stand out as the main providers of lignocellulosic biomass. The properties of biochar are shaped by the specific composition and type of the lignocellulosic feedstock used in its creation [42]. The total carbon content in the resultant biochar is contingent upon the attributes of the thermochemical degradation procedure, lignocellulosic material, and numerous other factors [43]. Essential constituents like cellulose, hemicellulose, and lignin are present in lignocellulosic biomass. Cellulose, located in the plant cell wall and occurring in its pure form in cotton fibers, consists of a six-carbon ring sugar, specifically β–D-glucopyranosyl [44,45]. Offering structural support to the plant cell, cellulose derives its linear structure from the dehydration of glucose. The intertwining of crystalline and non-crystalline phases leads to the creation of microfibrils, where the three hydroxyl groups of the pyranose ring play a role in establishing a crystalline structure. This imparts stability and mechanical strength to cellulose [43]. Functioning as a connection between cellulose and lignin, hemicelluloses are polysaccharides characterized by multiple branches [46]. Unlike cellulose, hemicelluloses exhibit an amorphous, non-crystalline structure, with their composition and arrangement varying depending on the specific type of lignocellulosic material [47]. Hemicelluloses contain monomers, such as galactose, arabinose, mannose, and glucose, which have a lower degree of polymerization than cellulose. Generally, hemicelluloses consist of 50–200 monomers [48]. Lignin, characterized by its aromatic, branched, amorphous, and heterogeneously cross-linked polymer nature, forms robust connections with cellulose and hemicellulose polymers [49]. Located mainly in the external fibers, lignin plays a vital role in upholding structural integrity, making up roughly 40% of lignocellulosic biomass. It serves as a binding agent within the cell wall, connecting cellulose and hemicellulose, with documented lignin content of approximately 25% in hardwoods and 33% in softwoods [49,50]. Apart from the chemical constituents, biomass comprises inorganic elements and extractives, including saponins, lipids, resins, alkaloids, gums, terpenes, sugars, proteins, and more. The considerable focus of scientists, production industries, and agricultural communities on the extensive research and development activities related to biochar has raised significant concerns. These concerns include potential adverse effects such as heightened competition for agricultural land, increased food prices, threats to biodiversity, and elevated greenhouse gas emissions [51]. A holistic approach to overcoming land-related limitations necessitates the consideration of various socioeconomic and socioecological factors. Policymakers hold a pivotal role in advancing the transition to biomass conversion, supporting sustainable biochar production, and enabling the expected carbon sequestration. Prior to establishing a biochar production unit, a thorough evaluation of all potential feedstock supply sources is imperative, with an emphasis on sustainability perspectives from local communities and businesses [52]. Bringing about this goal necessitates considerable endeavors to highlight the significance of sustainable practices, emphasizing the requirement for a stringent framework of criteria when harnessing the potential of accessible biomass feedstocks for biochar production.

3. Biochar Production Methods

Biomass contains diverse forms of energy that can be exploited using appropriate technologies. The careful selection of suitable conversion technologies is crucial to maximizing the extraction of energy and value-added chemicals from biomass [53]. The primary thermochemical processes for biomass conversion into valuable products, including biochar, biofuel, and syngas, are pyrolysis, gasification, hydrothermal carbonization, and torrefaction (Figure 1). At elevated temperatures and pressure, in the absence of substantial oxygen, biomass decomposes into biochar [54]. To clearly illustrate the diversity of biochar production technologies and support the selection of appropriate methods based on specific objectives, a comparative overview of the principal thermochemical and hydrothermal techniques is provided in Supplementary Table S1. This table outlines each method’s key process parameters, advantages, limitations, and typical application scenarios, offering a comprehensive understanding of their practical relevance and performance.

3.1. Pyrolysis

The pyrolysis process involves the thermal breakdown of biomass into smaller components within an oxygen-limited or inert environment, following defined temperatures and operating conditions. This method offers a notable advantage in transforming biomass into a solid-carbon-rich residue called biochar, along with non-condensable gaseous components (syngas) and condensable liquid (bio-oil) [55]. The amount of biochar produced is contingent upon factors such as the pyrolysis temperature, heating rate, residence time, and the particular feedstock employed. Studies suggest that higher pyrolysis temperatures are associated with a reduction in the yield of biochar [55]. Moreover, a higher heating rate is linked to a decline in the production of biochar, according to research findings [56]. The pyrolysis process involves three distinct pathways in the direct conversion of biomass: the production of char, depolymerization, and fragmentation [44]. Enhanced thermal stability of the residue is achieved through intermolecular and intramolecular rearrangement events, promoting the generation of char. This entails the formation and subsequent combination of benzene rings to create an aromatic polycyclic structure [57]. Typically, these rearrangement reactions lead to the release of gases or liquids. Depolymerization involves breaking down polymer bonds, followed by stabilizing processes that yield trimer, dimer, and monomer units. These volatile compounds condense under ambient temperatures and are present in the liquid fraction. Fragmentation occurs when both monomer and polymer links break, producing diverse organic liquids and gases that can condense at surrounding temperatures [58]. Gases and liquids produced directly during conversion are unstable at pyrolysis temperatures and may undergo side reactions, including cracking or recombination, given sufficient residence time [59]. Cracking reactions entail the breakdown of volatile chemicals into molecules of lower molecular weight, while recombination involves the combination of volatile compounds to create larger molecules with reduced volatility at pyrolysis temperatures [60]. Recombination reactions additionally yield secondary char, and the primary char produced can function as a catalyst in subsequent reactions. Pyrolysis is categorized into fast and slow pyrolysis based on the rate of heating and reaction temperature. Fast pyrolysis involves introducing biomass or feedstock into the reactor only when it reaches the desired temperature, with a residence time lasting several seconds [61]. Fast pyrolysis, while advantageous in certain aspects, has a notable limitation in terms of lower biochar production. The process involves specific conditions, including a temperature surpassing 500 °C and a heating rate exceeding 300 °C min⁻¹ within an oxygen-limited environment. The distribution of products in fast pyrolysis typically comprises around 60% bio-oil, 20% biochar, and 20% syngas. Unlike fast pyrolysis, slow pyrolysis adopts a different approach by introducing the feedstock into the reactor at the commencement of the pyrolysis process, allowing for a more extended residence time spanning several hours. The conditions for slow pyrolysis encompass temperature ranges of 300–600 °C and a moderate heating rate of 5–7 °C min⁻¹, ultimately leading to an increased biochar yield. Given the higher biochar production, slow pyrolysis is favored over its faster counterpart [62]. Recent technological advancements have introduced new approaches to biochar production, namely, flash, vacuum, and microwave pyrolysis. Flash pyrolysis, which is characterized by temperatures ranging from 900 to 1200 °C, a heating rate exceeding 1000 °C, and a residence time of less than a minute, has emerged. Nevertheless, the industrial viability of this method is constrained by its high temperature range and rapid heating rate [63]. Vacuum pyrolysis, as the name implies, takes place in a vacuum or under extremely low pressure conditions. The pressure typically falls within the range of 0.05 to 0.20 MPa, while the temperature varies from 450 to 600 °C. This technique proves effective in eliminating vapors, yielding biochar of high quality [64]. Microwave pyrolysis, a recent technological advancement in contrast to other conversion techniques, relies on dielectric heating. This approach enables the attainment of comparable characteristics at even lower temperatures, with heat energy transferred without direct contact between the heat source and the mixture. The recommended parameters for biochar production using microwave pyrolysis are typically identified as a microwave power of 400 W, a temperature of 450 °C, and a heating rate ranging between 4 and 6 °C min⁻¹. Furthermore, biochar produced through microwave pyrolysis demonstrates an increased calorific value when contrasted with alternative pyrolysis methods [55].

3.2. Torrefaction

Torrefaction, a variant of pyrolysis carried out at moderate temperatures, entails the gradual heating of materials to temperatures falling within the range of 200 to 300 °C, with a rate of increase not exceeding 11 °C s⁻¹, all conducted in the absence of air under atmospheric pressure [65]. Torrefaction, designed to eliminate surplus water and volatiles, entails a partial breakdown of biopolymers, such as cellulose, hemicelluloses, and lignin, resulting in the release of organic volatiles. The primary output of torrefaction is the generation of solid materials, with a notable absence of liquids or gases [42]. The European Commission suggests that biochar is ideally characterized by an oxygen-to-carbon (O/C) ratio below 0.4. Nevertheless, torrefied material often presents a heightened O/C ratio, disqualifying it from being categorized as biochar. Torrefied biomass displays properties that lie intermediate between original biomass feedstocks and char. Torrefaction is frequently employed as a pre-treatment technique to diminish moisture content in biomass, boost its density to minimize transportation expenses, and improve the biomass’s calorific value [66]. In addition, torrefaction enhances the hydrophobicity, grindability, and biodegradability of biomass materials in contrast to unaltered biomass sources. Torrefied biomass exhibits the capability for prolonged storage without deterioration. Typically, torrefaction yields around 70–80% of mass and 80–90% of energy [67].

3.3. Hydrothermal Carbonization

Most biomass materials have elevated moisture content, often reaching up to 95 wt%. Biomass exceeding 30% moisture content necessitates drying before entering the pyrolysis process [68]. Hydrothermal carbonization involves applying pressure and heat in the presence of water to convert biomass into carbonaceous biofuel. This approach presents a potentially beneficial solution for converting moist biomass into biofuels, eliminating the requirement for energy-intensive drying [69]. Functioning as both a solvent and a reactive agent, water plays a crucial role in the hydrothermal carbonization (HTC) process. In this method, solid biomass, along with liquid, is heated at low temperatures (<250 °C) in a sealed chamber under autogenous pressure, leading to the creation of primary solids referred to as hydrochar [70]. Hydrothermal liquefaction (HTL) operates within the temperature range of 250 to 350 °C, primarily converting biomass fuel into a liquid product. In contrast, hydrothermal gasification (HTG) occurs at higher than the critical pressure and temperature, closely approximating that of water (22.1 MPa and 374 °C), predominantly transforming biomass into a gaseous medium (CO₂, CH₄, CO, and H₂). Notably, hydrochar specifications typically exhibit a higher H/C ratio compared to biochar specifications [71,72].

3.4. Gasification

The process of gasification involves the thermochemical conversion of carbon-based materials using a gasification agent, which can be steam, oxygen, or air. This can occur under ambient conditions or at elevated pressures and temperatures exceeding 750 °C. The result of this process includes the formation of oxygen-deficient states and biochar. When air serves as the oxidizing medium, the resulting gas comprises approximately 85% syngas, consisting of N₂, CH₄, CO₂, CO, H₂, and C₂H₂. Notably, the steam gasification mode has been observed to yield higher H₂ with a superior heating value [73]. The gasification procedure generally involves four consecutive phases: drying, pyrolysis, partial oxidation, and reduction [74]. Gasification is commonly classified into three primary types based on gas–solid contact modes: entrained flow, fluidized bed, and fixed bed. While this method is primarily focused on generating syngas, it results in a relatively low char output [44]. The biochar yields achieved in these conditions are not substantial enough to deem gasification as a feasible technology for biochar production. Similarly, burning is an unsuitable method for biochar production, as optimal combustion conditions would lead to an inconsequential biochar yield. The main limitation of this approach is the minimal production of biochar as a byproduct and the release of greenhouse gases [73]. Additionally, the operational conditions of gasification vary to optimize energy yield from various carbonaceous feedstocks. Ensuring effective control of the operational parameters in gasification is essential for optimizing its performance [75]. Overall, pyrolysis emerges as the most efficient, sustainable, and preferred method for biochar production.

4. Biochar Characteristics

Produced through clean technology, biochar is defined by the European Biochar Certificate as a heterogeneous compound rich in minerals and aromatic carbon, resulting from the regulated pyrolysis of biomass obtained through sustainable harvesting [76]. Any application not demanding swift mineralization to CO₂ can utilize biochar, making it suitable for functions such as soil improvement. When comparing biochar to other carbonaceous compounds, such as charcoal and char, recognizing this distinction is essential [10]. For biochar production, it is imperative that the biomass utilized is both renewable and sustainable. Biochar has diverse applications, including wastewater treatment, soil management, and renewable energy production. The primary composition of plant biomass is cellular lignocellulosic material, constituting the non-starch fibrous fraction of plant materials. As highlighted earlier, the three main components of lignocellulosic biomass are lignin, hemicellulose, and cellulose [76]. The plant cell wall’s primary structural component is cellulose. Following cellulose, hemicellulose and lignin are the second and third most abundant polymers in lignocellulosic biomass, respectively [77]. In the pyrolysis process, cellulose and hemicellulose undergo faster decomposition than lignin within a narrower temperature range, influencing the physicochemical properties of the resulting biochar [76]. Furthermore, biomass comprises both organic extractives and inorganic chemicals. The inorganic chemicals, accounting for less than 10% of the biomass weight, undergo conversion into ash during the pyrolysis process. The term “organic extractives” denotes non-structural biomass components that can be extracted using nonpolar or polar solutions. These encompass starches, resins, gums, simple sugars, terpenes, proteins, waxes, and fatty acids, along with saponins, mucilages, glycosides, pectins, phenolics, and alkaloids [78]. Moreover, the presence of considerable amounts of free and bound water in biomass significantly impacts the characteristics of biochar, with the properties being heavily influenced by both the pyrolysis conditions and the raw materials applied in its manufacturing [79]. The extent of biomass devolatilization is largely determined by the pyrolysis temperature. At approximately 160 °C, the initial expulsion involves free and bound water. Subsequently, the thermal degradation of biomass commences with the volatilization of extractives at 220 °C [80]. Within the temperature range of 220 to 315 °C, hemicellulose, being a relatively unstable polymer, undergoes degradation. Cellulose, with its high polymerization, displays enhanced thermal stability and decomposes between 315 and 400 °C. The pyrolysis of lignin presents a more complex challenge due to its wide-ranging temperature gradient during decomposition, spanning from 160 to 900 °C [81]. Various quality attributes are typically assessed in biochar, including electrical conductivity, cation exchange capacity (CEC), elemental composition, porosity, water-holding capacity, polycyclic aromatic hydrocarbons (PAHs), water and ash content, pH level, heavy metal content, nutritional composition, surface area, volatile compound content, carbon stability, bulk density, and carbon content [82] (Figure 2).

Parameters affecting the biochar characteristics are as follows.

The quality of biochar is contingent upon the pyrolysis methodology and the biomass origin. Key pyrolysis parameters, including temperature, residence time, and heating rate, invariably affect the properties of biochar. The temperature at which the pyrolysis of biochar occurs plays a pivotal role in determining its characteristics [83]. It is essential to recognize that the properties of biochar can differ depending on the raw material used in its production, with specific feedstocks exhibiting greater efficacy as soil amendments. When evaluating the application of biochar in agriculture, certain quality parameters take precedence. Surface area, pore volume, bulk density, water-holding capacity, ash content, volatile chemical concentration, and pH are critical quality characteristics that significantly impact crop productivity [84]. In activities associated with soil fertility and carbon sequestration improvement, the stability of carbon holds paramount importance. Key quality indicators for promoting soil fertility include the nutrient content and surface area [85]. The molar ratio of hydrogen to carbon (H/C) is a foundational parameter for biochar characterization, reflecting the level of carbonization and stability in the material. H/C ratios surpassing 0.7 indicate subpar biochar quality and constraints in the pyrolysis process. The molar ratio of oxygen to carbon (O/C) plays a crucial role in distinguishing biochar from other carbonization products. An O/C ratio exceeding 0.4 indicates reduced stability in biochar [86]. The molar ratios of O/C and H/C in lignocellulosic biomass are typically around 0.7 and 1.5, respectively. Pyrolysis results in the devolatilization of biomass and the concentration of the solid fraction with carbon. In this process, oxygen and hydrogen are selectively emitted, resulting in decreased H/C and O/C ratios as biomass transforms into biochar. These ratios are pivotal in determining the aromaticity and maturity of the biochar [87].

5. Techniques for the Modification of Biochar

5.1. Physical Activation

The process of physical activation, alternatively termed thermal activation, entails heating the acquired biochar within the temperature range of 700 to 900 °C in the presence of CO₂, steam, or air. Within this high-temperature oxidative setting, pores are induced through auto-gasification, leading to the reduction in volatiles or gases and the development of a porous carbon material. As this process unfolds, the pores, initially saturated with carbon volatiles, expand, facilitating diffusion between neighboring pores. These occurrences notably augment the specific surface area and encourage the extensive creation of micropores, accompanied by a reduced concentration of mesopores. The fundamental concept behind physical modification involves the passage of elevated-temperature steam through the pores of biochar, aiming to augment the surface area [88]. Typically, this approach is easy to implement, is cost effective, and does not require the incorporation of costly chemicals throughout the modification procedure. Nevertheless, the sorption effectiveness of biochar modified through physical means is generally lower than that of biochar subjected to chemical modification. Ball milling, gas filling, steam activation, and other techniques are commonly employed for physical modification. Steam activation, performed under anaerobic conditions within the temperature range of 400 to 800 °C, stimulates the formation of macropores and mesopores on the biochar surface as volatile components decompose. This process elevates the cation exchange capacity of biochar and enhances its sorption capabilities [89]. Biochar activated by steam presents an augmented specific surface area, demonstrating a 55% higher sorption capacity in comparison to conventional biochar [90]. The gas-filling process entails the introduction of gases such as CO₂, N₂, and others during the pyrolysis of biochar. This increases the surface area, enhances the microporous structure, and improves the sorption performance. Experiments revealed that biochar produced under CO₂ conditions displayed twice the total pore volume and surface area in comparison to biochar produced under N₂ conditions [91]. Ball milling involves the incorporation of biochar with ball milling technology to mechanically reduce the size of solid particles to ultrafine dimensions [92]. This method enhances both the external and internal surface areas of biochar, revealing its graphite structure and increasing the sorption capacity by 6.47 times [93].

5.2. Chemical Modification

Chemical modification often involves expensive and complex processes using harmful reagents. Widely adopted techniques encompass the manipulation of organic compounds, bases, acids, various carbon-containing structures, and graphene [94]. A commonly employed method for modification involves acid treatment, which typically consists of submerging the biochar material in potent acids such as citric acid, oxalic acid, phosphoric acid, nitric acid, sulfuric acid, hydrochloric acid, and others [95]. Through acid modification, acidic functional groups are incorporated into the surface of biochar, resulting in heightened surface acidity, modifications to the porous structure, and enhanced adsorption of heavy metals via complexation and ion exchange [96]. For instance, modification with H₂SO₄ substantially reduces the volatile matter and ash content in biochar while simultaneously augmenting the fixed carbon content [97]. Treating biochar with an HNO₃/H₂SO₄ solution (1:1) significantly increases the prevalence of carboxyl functional groups on the biochar surface, effectively improving the adsorption of Cd(II) [98]. Modification with H₃PO₄ results in the breakdown of functional groups in biochar, leading to the formation of a disordered carbon structure and the creation of abundant surface micropores [99]. Modification using strong bases like NaOH, KOH, or ammonia water involves immersing the biochar material in the solution [100,101]. The modification with a base can concentrate organics, dissolve ash, surface oxygen content, and adjust biochar pH [102]. Through NaOH modification, alkali-soluble carbon is eliminated from the surface of biochar, resulting in an augmentation of surface area and hydrophobicity [103]. Modification with Ca(OH)₂ improves ion exchange performance while decreasing the complexation of functional groups on the surface of biochar [104]. The modification with KOH brings about changes in certain microporous structures, converting them into mesopores/macropores and clearing obstructed pores and inorganic substances [105]. Biochar modified with metals and oxides enhances its adsorption capacity and introduces oxygen-containing functional groups through mechanisms such as surface complexes, electrostatic attraction, ion exchange, and cation-π bonds [106]. Modification with KMnO₄ loads the surface of biochar with MnO₂ [107]. Modification with ZrOCl₂·8(H₂O) enhances the presence of hydroxide groups and zirconia on the surface of biochar, resulting in a high specific surface area with a porous structure [108]. Biochar modified with organic compounds employs functional groups to establish robust bonds between the biochar surface and pollutants, thereby enhancing its pollutant adsorption capacity [88]. The introduction of sulfhydryl functional groups onto biochar through modifications with Na₂S·9H₂O, NaHSO₄·H₂O, thioglycolic acid, and N,N-dimethylformamide leads to a reduction in pore volume and specific surface area (SSA) [109].

5.3. Biological Modification

The activation of biochar using biological methods, such as bacterial conversion or anaerobic digestion from biologically pre-treated biomass feedstocks, proves to be a feasible technique. Anaerobic digestion, a process utilizing anaerobic bacteria to transform organic matter into biogas, generates a residue that can be effectively utilized in the production of biochar through pyrolysis [110,111]. The biochar derived from both pyrolysis and anaerobic digestion demonstrates enhanced hydrophobicity, cation exchange capacity (CEC), anion exchange capacity (AEC), surface area, pH, and a more negative surface charge when compared to the original biochar. These improvements are linked to changes in the redox potential and pH values of the biomass feedstock during the digestion process [112]. The enhanced cation exchange capacity (CEC) and anion exchange capacity (AEC) imply the capability of biologically activated biochar to capture both negatively and positively charged ions from water, rendering it proficient as an ion exchanger. Previous studies have demonstrated that anaerobic digestion augments the adsorption capacity of biochar for PO₄³⁻ and heavy metals [111,113,114], positioning it as a viable option for remediating environmental pollutants [112]. The activation of biochar is deemed crucial for its application in soil treatment. This activation process, which can naturally transpire over months to years in soils, results in biochar improving soil water retention and nutrient sorption, thereby rendering it more available to plants. The acceleration of the natural activation process in the soil can be achieved by blending biochar with manure or compost. Non-activated biochar, produced at low temperatures, exhibits a diminished surface area and adsorption capacity when contrasted with biologically activated biochar. Biochar that undergoes activation usually possesses a surface area within the range of 200 to 1000 m⁻². kg⁻¹, whereas non-activated biochar may exhibit a surface area as low as 10 m⁻². kg⁻¹ [115,116]. Explorations into the biological modification of biochar derived from wood and activated sludge have delved into enhancements in surface functional groups, surface charge, and porosity, contributing to improved microbial metabolism. Moreover, biochar activated through bioleaching with Acidithiobacillus ferrooxidans has demonstrated greater efficacy in adsorbing heavy metals [117,118]. While these inquiries establish the foundation for eliminating Pb using microorganisms and charcoal, the formation of Pb minerals and the interactions of microorganisms on the biochar surface remain undisclosed. Chen et al. [119] conducted an investigation into the augmentation of charcoal immobilization of Pb²⁺ in a solution using phosphate-solubilizing bacteria (Enterobacter sp.). The amalgamation of biochar and phosphate-solubilizing bacteria, sourced from materials such as rice husk or sludge, effectively eliminated Pb²⁺ from an organic medium, achieving removal percentages of 24.11% and 60.85%, respectively [119]. Utilizing biochar as a carrier for functional microorganisms in this amalgamation introduces a distinctive and effective strategy for the removal of heavy metals [120,121]. The collaborative effect of biochar and microorganisms not only tackles concerns associated with nutrient deficiencies and competition that may affect the effectiveness of bioremediation but also surmounts the hurdle of the comparatively lower efficiency of bioremediation methods. Qi et al. [122] discovered in their research that five varieties of biochar heightened microbial metabolic activity and elevated the quality of the soil environment. In a separate study, the introduction of biochar loaded with a combination of bacteria, created through physical adsorption and sodium alginate embedding, displayed more effective immobilization of Cd and U in comparison to the application of biochar alone. The method of physical adsorption exhibited enhanced effectiveness in immobilizing U and Cd over a relatively brief duration. This modification not only stimulated the growth of celery but also diminished the accumulation of U and Cd within the plant. The strategy of employing a combination of biochar and functional bacteria for addressing heavy metal contamination in agricultural soil demonstrated efficacy, attributed to the brief remediation period and heightened efficiency in physical adsorption [122]. The integration of biochar and bioremediation employing functional fungal or bacterial strains is regarded as a valuable and emerging method for the extended revitalization of polluted soil [119,123]. In research conducted by Tu et al. [124], maize straw-derived biochar, equipped with heavy metal-tolerant bacteria (Pseudomonas sp. NT-2), was administered to soil contaminated with a mixture of Cd and Cu. The objective was to demonstrate the influence of microbial inoculation on the stabilization of Cd/Cu in the soil. The utilization of biochar loaded with NT-2 proved successful in diminishing the lability and bioavailability of both Cd and Cu, particularly at elevated application rates. Moreover, the introduction of NT-2 loaded biochar was observed to enhance soil enzymatic activity in the contaminated soil.

5.4. Modification Through Doping or Co-Doping

The regulation of electronic properties in biochar through the introduction of non-metal heteroatoms has been recognized as a means to ultimately improve its catalytic capacity [125]. Modifying the electronic structure of biochar through nitrogen doping is a prevalent technique, leading to the enhancement of active sites. The adsorption properties and electron transfer of biochar benefit from its surface alkalinity by activating adjacent sp² carbon atoms [126]. The introduction of nitrogen through doping can result in three distinct types of nitrogen bond configurations, namely, pyrrolic N, pyridinic N, and graphitic N. Furthermore, biochar can be doped with other heteroatoms, like sulfur (S), in addition to nitrogen. Research into co-doping with both metals and non-metals has revealed that this combination improves the catalytic performance of biochar in adsorbing contaminants compared to the effects of individual metal or non-metal doping [127]. For instance, enhanced efficiency in contaminant degradation was observed in a catalyst formed through co-doping with copper and nitrogen, denoted as Cu-N/biochar, as opposed to the impact of individual doping [128]. An alternative and promising method consists of blending biochar with diverse nanometallic hydroxides or oxides, leading to the creation of catalysts based on biochar that exhibit high efficiency [125]. Different types of nanometallic oxides/hydroxides have the potential to influence the functional properties of nanoparticles and physicochemical traits, such as catalysis and ferromagnetism. Introducing metal into biochar is deemed advantageous, as it has the potential to boost catalytic capabilities and prevent the leaching of metal ions [126]. Composite materials involving transition metals and biochar, including composites like Mn-biochar, Cu-biochar, Co-biochar, and Fe-biochar, have been created and applied. Among these, catalysts based on Fe are gaining recognition owing to their advantageous features of low toxicity, high activity, and recyclability [129]. Copper (Cu) has attracted interest among transition metals due to its affordability and sustainability. Opting for Cu-biochar materials is viewed as a more sustainable alternative compared to frequently utilized transition metals, like cobalt (Co). Furthermore, manganese (Mn) is acknowledged for its lower toxicity compared to other transition metals, rendering Mn composites efficient catalysts for environmental remediation. Limited studies have been conducted on catalysts based on manganese (Mn) in contrast to the research available on iron (Fe), cobalt (Co), and copper (Cu) catalysts [130]. For a more in-depth comparative evaluation of the biochar modification methods discussed, a detailed summary table (Table S2) is provided in the Supplementary Material. This table systematically compares the key attributes of physical, chemical, biological, and composite modification techniques, highlighting their respective advantages, limitations, and typical applications.

6. Applications of Biochar

6.1. Water and Wastewater Treatment

Biochar has emerged as a promising solution for addressing water pollution caused by both conventional and emerging pollutants. In recent years, research initiatives driven by global concerns about water pollution and challenges associated with the generation and discharge of significant volumes of industrial effluents [131,132] have explored effective and cost-efficient methods. This coincides with the increased interest in producing biochar as a valuable material derived from the pyrolysis-based biomass valorization process. This section explores various sustainability aspects [133] associated with using biochar for treating polluted water.

6.1.1. Removal of Nutrients

Examining nutrient removal through adsorption [134,135], biochar (BC) has been explored as an adsorbent. The adsorption capacity of biochar is significantly shaped by its porosity, specific surface area, shape, and surface chemistry. Various studies have investigated the impact of functional groups on the BC surface on nutrient adsorption. The determination of functional groups on the BC surface is crucial and is influenced by process conditions during pyrolysis, especially temperature, and feedstock properties [136]. In a study by Banik et al. [137], various parameters were investigated for their impact on the resulting BC’s cation exchange capacity (CEC), anion exchange capacity (AEC), point of zero salt effect (PZSE), and zero net charge (PZNC). Biochar prepared at 500 °C exhibited a low zero net charge, point of zero salt effect, and anion exchange capacity but high cation exchange capacity, mainly due to negative charges on the BC surface from phenolate functional groups and carboxylate. Another study [138] indicated that increasing pyrolysis temperature could result in biochar with low cation exchange capacity and high point of zero salt effect, zero net charge, and anion exchange capacity, attributed to a positive surface charge from non-hydrolyzable bridging oxonium (oxygen heterocycles) groups on the biochar surface. Biochar with AlCl₃ pre-treatment showed high zero net charge, point of zero salt effect, and anion exchange capacity due to variably charged aluminol groups on the biochar surface. Biochar with positive surface charges exhibited a high capacity for adsorbing cationic contaminants [139,140]. However, additional factors, like feedstock type, contribute to biochar properties, including elemental composition, ash content, specific surface area (SSA), CEC, pH, and yield [141]. Treatment efficiency is influenced by operating conditions. For example, Fidel et al. [142] illustrated that the adsorption capacity of biochar, produced from corn stover and red oak at three pyrolysis temperatures (600, 500, and 400 °C), relies on pH and electrostatic interactions for nitrate and ammonium sorption. Recent research has investigated the effectiveness of biochar products for the simultaneous removal of different nutrients from contaminated effluents. Li et al. [143] successfully accomplished the simultaneous extraction of nutrients, such as ammonium and phosphate, alongside specific organic compounds from swine effluents by employing magnesium oxide (MgO) impregnated biochar (BC). The research showcased peak adsorption capacities of 247 mg/g for humate, 22 mg/g for ammonium, and 398 mg/g for phosphate. Further investigations into the simultaneous elimination of nutrients from aqueous environments [144,145] underscored the adaptability of BC in tackling a diverse range of nutrients.

Table 1. Overview of the utilization of biochar (BC) for the removal of nutrients from aqueous solutions.

Feedstock Type	Pyrolysis Temperature, °C	Nutrient Type	Initial Concentration	Adsorption Capacity	References
Rice Husk	500	NH4+	3000 mg N/L	44.04 mg N/L	[147]
Oak Sawdust	650	NH4+	450 mg N/L	32.7 mg N/L	[148]
Municipal Waste	650	NH4+	1000 mg N/L	128.3 mg N/L	[149]
Cassava Peel	500	NH4+	200 mg N/L	23.67 mg N/L	[150]
Peanut Shell	500	NH4+	60 mg N/L	3.83 mg N/L	[151]
Wheat Straw	550	NH4+	40 mg N/L	2.08 mg N/L	[152]
Corn Cob	400	NH4+	100 mg N/L	1.09 mg N/L	[153]
Oak Wood	450	PO43−	400 mg P/L	5.5 mg P/L	[149]
Municipal Waste	650	PO43−	400 mg P/L	14.3 mg P/L	[149]
Peanut Shell	700	PO43−	5 mg P/L	0.613	[154]
Orange Peel	700	PO43−	2.4 mg P/L	83.3%	[155]
CaCao Shell	300–350	PO43−	0.1–50 mg P/L	1 mg P/L	[156]
Brown Marine Macroalgae	480	PO43−	200 mg P/L	7 mg P/L	[157]
PressCake	650	PO43−	400 mg P/L	30 mg P/L	[149]
Mixed Hardwood	300	PO43−	24 mg P/L	0.48 mg P/L	[158]

provides an overview of BC applications for nutrient removal from aqueous media. Recent studies have explored the creation of biochar-based electrodes for the absorption of inorganic compounds from water. In their study, Yao et al. [146] found that nitrogen doping in biochar derived from loofah sponge (applied as the cathode) not only enhanced the adsorption capacity for bromate but also promoted electron transfer. This led to a notably effective electrocatalytic removal of nutrients.

6.1.2. Heavy Metal Removal

The growing focus on using biochar (BC) to remove heavy metals from contaminated water streams is prompted by the hazardous nature of heavy metals and their detrimental effects on both human health and the ecosystem [159,160]. In addressing toxic water-containing heavy metals (HMs), conventional treatment approaches have demonstrated inefficiency [161,162]. This has led to recent advancements in the application of biochar for HM removal, proving to be a more effective and economically viable alternative than activated carbon. Notably, BC exhibits enhanced efficacy, particularly in the removal of heavy metals, such as chromium and zinc [131,163]. Various types of BC, both unmodified and modified, have shown differing performances in the removal of HMs from polluted water, as summarized in

Table 2. Overview of the use of biochar (BC) in removing heavy metals from aqueous solutions.

Metals	Biochar Feedstock Type	Pyrolysis Temperature, °C	Biochar Dose, g/L	Adsorption Capacity, mg/g	References
Pb2+	Cow Manure	700	-	149.3	[172]
	Banana Peels	600	2.5	247.1	[173]
	Ragweed	450	-	358.7	[174]
	Gingko Leaf	800	1	138.9	[175]
Cu2+	Loofah	700	0.5	54.68	[176]
	Banana Peels	600	2.5	75.99	[173]
	Ginkgo Leaf	800	1	59.9	[175]
Cd2+	Ragweed	450	-	139	[174]
	Peanut Husk	-	40	28.99	[177]
	Green Waste	600	2	6.72	[178]
Cr6+	Rice Husk	450–500	1	435.7	[179]
	Loofah	700	0.5	30.14	[176]
Cr3+	Sewage Sludge	300	4	-	[180]
Zn2+	Rice Husk	550	12.5	3.8	[181]
As5+	Papermill Sludge	720	1	22.8	[182]
AS3+	Pine Wood	400	10	1.78	[183]

. The efficiency of HM removal is influenced by the BC’s origin, modifications, and experimental conditions. The pyrolysis temperature, in particular, plays a critical role in determining the adsorptive capability of BC for HMs. For instance, Burton et al. [164] studied Perilla leaf-derived biochars (BCs) produced at temperatures of 300 and 700 °C, exhibiting varying effectiveness, with the BC prepared at 700 °C showing superior performance, especially in the removal of As(III) in contrast to As(V). This outcome was attributed to the increased specific surface area and surface aromaticity of the biochar prepared at 700 °C, favoring the adsorption of As compared to the BC prepared at 300 °C. In certain instances, alterations to the BC structure may have negative impacts on its characteristics, such as a decrease in specific surface area (SSA). However, these modifications can improve the suitability of BC for certain applications. Recent investigations have indicated a notable enhancement in the BC’s capacity to adsorb heavy metals with the inclusion of clay [165]. This improvement is attributed to the substantial ion exchange capability of clay-derived materials, like montmorillonite, for diverse cations. The focus on creating magnetic biochar composites has increased in the past few years, driven by their superior effectiveness in eliminating both organic and inorganic pollutants when compared to non-magnetic BC [166]. Materials with magnetic properties, specifically those incorporating Fe²⁺ and Fe³⁺, demonstrate improved adsorption capacities, particularly for heavy metals such as Cu(II), Cd(II), Pb(II), Zn(II), and others. The incorporation of magnetic components into rice straw before pyrolysis, as shown by Tan et al. [167], led to the generation of hematite (γ-Fe₂O₃). This process preserved the BC’s initial functional groups and enhanced its capacity for cadmium adsorption. The heavy metal adsorption is also facilitated by the γ-Fe₂O₃ generated during the pyrolysis process. Under CO₂ pyrolysis conditions, the BCs displayed increased adsorption capacities for heavy metals. This enhancement can be attributed to the emergence of oxygen-containing functional groups, including CO₃^2– and PO₄^3– [168,169]. For instance, these groups have the ability to adsorb Pb²⁺ [169] by creating pyromorphite (Pyro) [Pb₅(PO₄)₃X (X = Cl, F, OH)], which exhibits extremely low solubility [170]. The presence of hydrogenated and oxygenated groups, when interacting with CO₂, contributes to the augmentation of the specific surface area in biochar prepared under a carbon dioxide medium. This increase, in turn, improves its ability to adsorb heavy metals [169]. Further investigation is required to deepen our understanding of the sorption mechanisms implicated in introducing magnetic elements into the biochar structure or under different pyrolysis conditions. Industrial effluents commonly contain various environmental contaminants, and the removal of a broad spectrum of contaminants is a pivotal factor for the overall effectiveness of any treatment technology. Recent studies have indicated the concurrent removal of heavy metals, prompting ongoing research efforts to develop highly efficient BC capable of simultaneously extracting various heavy metals from actual effluents [166,171].

6.1.3. Organic Compounds

The focus has been on improving the effectiveness of biochar (BC) in treating water contaminated with diverse organic substances, including antibiotics, aromatics, pesticides, phenolics, halogenated hydrocarbons, and dyes [184]. For eliminating organic pollutants, researchers have explored both adsorption and degradation mechanisms, with a focus on biochar (BC) and its modified forms. The predominant mechanism for purifying water contaminated with organic compounds involves adsorption, defined by intermolecular forces such as covalent bonding, coulombic attraction, π-interaction, dipole-induced interaction, dipole–dipole interaction, H bonding, and hydrophobic interaction [135]. Crucial factors influencing the adsorption capacity of biochar (BC) include parameters such as specific surface area (SSA), ash content, porosity, and the nature of functional groups present on its surface. Elevated pyrolysis temperatures typically result in heightened microporosity, SSA, hydrophobicity, and organic carbon content of BC, all of which promote the sequestration of organic contaminants [185,186]. A study examining the adsorption of crystal violet (CV) dye by Wathukarage et al. [187] revealed that BC produced at 700 °C demonstrated the most substantial adsorption rate, reaching 125.5 mg/g at pH 8. Modified BC also displayed markedly superior efficiencies compared to its pristine counterpart. Faheem et al. [188] developed amino-grafted biochar (AMBC) with a remarkable monolayer adsorption capacity for an anionic dye, reaching 89.3 mg/g. Another study by Sewu et al. [189] demonstrated the efficacy of biochar (BC) derived from Korean cabbage waste in the adsorption of cationic dyes, particularly showing high efficiency for crystal violet but not for Congo red. The control of dye adsorption onto biochar (BC) has been effectively regulated by the functional groups and ash content. Enhancing biochar activation has proven to be a successful strategy for improving its efficacy in adsorbing organic compounds. In the activation process, NaOH [190] and NaCl [191] were employed to enhance biochar for the adsorption of organic compounds from contaminated water. Substantial efforts have been invested in increasing the specific surface area of biochars (BCs) through the manipulation of pyrolysis conditions. Pyrolysis in a carbon dioxide atmosphere has been demonstrated to augment the porosity of BC, attributed to the reaction between CO₂ and hydrogenated and oxygenated groups, influencing overall porosity [170]. Exploration has taken place regarding the post-thermal activation of BC under carbon dioxide, aiming to enhance the specific surface area for improved organic compound adsorption [192]. However, there is a necessity for economic and environmental evaluations of these approaches to facilitate their broader commercialization. Enhancing the adsorption capacity of BC through nitrogen doping proves effective by augmenting the presence of N-containing functional groups, thereby facilitating the adsorption of contaminants. Additionally, nitrogen doping can stimulate π-electron polarization and generate π-electron-rich sites on the biochar surface, contributing to the adsorption of pollutants, particularly aromatic compounds [193]. Activation agents like urea, serving as nitrogen precursors, have been recognized for their role in enhancing the porosity of biochar, ultimately elevating its adsorption capacity [194]. Employing porogens, such as MgCl₂, has proven to be an effective strategy in augmenting the specific surface area and porosity of biochar. MgCl₂ functions as a porogen by enhancing the liberation of volatile compounds during the decomposition and dehydration of biomass, leading to the formation of a porous biochar structure [195]. This sustainable method augments the porosity of BC, generating nanostructured materials within the biochar structure to facilitate catalytic reactions for the decomposition of adsorbed organic compounds [196]. Although adsorption proves effective in eliminating pollutants from an aqueous phase, the potential transfer of adsorbed pollutants to the solid phase can give rise to secondary environmental concerns. Hence, there has been a focus on devising systems that efficiently degrade pollutants. Research has been directed towards BC-based photocatalysts designed for visible light irradiation (VLI), aiming to catalyze the photocatalytic degradation of contaminated water. Zhai et al. [197] introduced a composite, BC@CoFe₂O₄/Ag₃PO₄, designed for the photocatalytic degradation of bisphenol A under visible light irradiation (VLI) with a wavelength of 420 nm. Furthermore, BC has been utilized in conjunction with catalytic materials such as TiO2 (supported by Salvinia molesta biochar) [198], Ag [199], and ZnO [200] to tackle organic pollutants. Altered forms of BC are also effective in facilitating the breakdown of organic pollutants via Fenton and Fenton-like reactions [188]. Incorporating nano-zero-valent iron (nZVI) into the biochar framework has demonstrated improved efficacy in degrading dyes. Achieving complete degradation of the RY145 dye at a concentration of 0.5 g/L modified biochar through Fenton reactions highlights the enhanced efficiency [201]. The amalgamation of BC with nZVI expedites electron transfer from nZVI to the contaminant, resulting in swift decomposition facilitated by the elevated electrical conductivity of BC [202]. While certain oxidation agents boast high redox potentials, their optimal efficiency in degrading resistant compounds [193,194] may necessitate preactivation, a role efficiently fulfilled by BC. Activated peroxymonosulfate (PMS) was employed for the degradation of triclosan (TCS) in water, utilizing sludge-derived BC (SBC) [203]. Under optimized conditions of BC dosage = 1.0 g/L, pH = 7.2, and PMS concentration = 0.8 mM at 25 °C, the porous structure of SBC (specific surface area = 157 m²/g) efficiently activated PMS. The initiation of PMS by SBC resulted in the production of singlet oxygen, sulfate radicals, and hydroxyl radicals, thereby contributing to the breakdown of pollutants. Additionally, hydroxyl radicals formed bonds with the biochar surface, facilitating the degradation of previously adsorbed pollutants. Nguyen et al. [204] developed iron-modified biochar using spent coffee grounds (SCGs) to oxidatively eliminate tetracycline in the presence of persulfate (PS). Effective activation of persulfate (PS) was observed, especially at high PS dosage, low initial pollutant concentration, low pH, and with the inclusion of applied biochar (BC). Metallic compounds in BC can activate sulfate-based chemicals, and targeted modifications have been made to improve BC for these applications. A composite consisting of biochar (BC) supported with Co₃O₄ was created for the activation of peroxymonosulfate (PMS), aiming to enhance the degradation of chloramphenicols at a concentration of 30 mg/L. This composite demonstrated a considerable enhancement in contaminant degradation compared to the Co₃O₄/PMS system alone. The biochar played a crucial role in facilitating electron transfer between cobalt (Co) and hydrogen sulfate ions (HSO₅⁻), thereby accelerating the Co³⁺/Co²⁺ redox cycle [205]. For a comprehensive overview of BC efficiencies in the removal of various organic pollutants, refer to

Table 3. Overview of biochar (BC) applications in eliminating organic contaminants.

Organic Contaminants	Biochar Feedstock Type	Pyrolysis Temperature	Removal Efficiency %	References
Polyaromatic Hydrocarbons	Rice straw	600	58.8	[206]
Metalaxyl	Wood	450	70.1	[207]
Tiamethoxan	Sugarcane filter cake	380	70	[208]
Ibuprofen	Cotton gin waste	700	50	[209]
Diclofenac	Rice hull	350	97	[210]
Sulfamethoxasole	Bagasse	600	83.3	[211]
Sulfamethoxasole	Yeast	900	98.97	[212]
Norfloxacin	Wheat straw	400	88.1	[213]
Sulfadiazine	Corn straw	350	74	[214]
Tetracycline	Rice straw	500	97	[215]
Oxytetracycline	Sugarcane bagasse	450	86	[216]

.

6.2. Soil Amendment

Introducing biochar (BC) into soil can lead to diverse modifications in soil characteristics, including changes in chemical and physical properties, pollution levels, soil fertility, and microbial activity [217,218]. The nature and magnitude of these effects are intricately linked to the specific characteristics of the biochar employed.

6.2.1. Soil Properties

The addition of biochar can induce a variety of effects on the physical, chemical, and biological aspects of soil. With its porous structure influenced by factors such as feedstock and pyrolysis conditions, the incorporation of biochar generally leads to a reduction in bulk density, attributed to its low density, thereby enhancing soil porosity, aggregation properties, and water-holding capacity [219]. The substantial specific surface area [220] of biochar has the potential to improve both water-holding capacity (WHC) and aggregation. Recent studies emphasize that changes in soil WHC, resulting from biochar incorporation, are associated with the concentration and particle size of biochar [221]. Vilas-boas et al. [222] conducted studies showing an increase in water-holding capacity (WHC) from 210 g/kg to 345 g/kg after a three-week incubation of sandy soil with biochar derived from biological sludge at a concentration of 10%. Biochar incorporation may alter soil hydrophobicity, and whether biochar exhibits hydrophobic or hydrophilic characteristics is influenced by the presence of various surface functional groups. The hydrophilicity or hydrophobicity of biochar is influenced by groups such as alkyl aliphatic (CH) and hydroxyl (OH) [223]. Using biochar for the adsorption or degradation of hydrophobic organic compounds has the potential to alleviate soil water repellency [224], leading to improved soil water content and aeration [225], positively impacting soil fertility. Additionally, the alkaline properties of biochar can raise soil pH, a significant adjustment for acidic soils, particularly in mining environments, thereby contributing to enhanced fertility [226].

Table 4. Impacts of adding biochar to specific acidic soils.

Soil Type	Biochar Feedstock Type	Pyrolysis Temperature, °C	Biochar Properties	Impact of Incorporating Biochar into the Soil	References
Ultisol	Rice straw	350	pH = 9.94Total C = 48.71%O/TOC = 0.29	Biochar had a more significant impact on phosphorus availability, soil chemical properties, microbial activity, and crop yields. It improved soil fertility by increasing available P, altering pH, and promoting beneficial microbial functional genes.	[244]
	Chicken manure	400	pH = 9.97, EC = 5.03 (mS cm−1), total C = 28.8%, CEC = 95.7 (cmolc kg−1)	The addition of biochar increased soil pH and enhanced phosphorus (P) availability by raising water-extractable and labile P while reducing Fe- and Al-bound P fractions. Furthermore, it significantly improved maize growth, dry matter yield, and chlorophyll content.	[245]
	Peanut straw	400–500	pH = 10.54, alkalinity = 505 (cmolkg−1), total C = 44%, C/N = 15	The application of biochar increased soil pH and reduced the concentrations of total Al, monomeric Al, and monomeric inorganic Al, resulting in higher canola seed and straw yields.	[246]
Latosol	Coconut shell	600		A significant rise in the phosphorus (P), potassium (K), and magnesium (Mg) levels in the modified soil.	[247]
Latosol	Filter cake	550	pH = 8.50, CE = 0.323 (mS cm−1), total C = 287.9 (g kg −1)	Biochar application improved soil chemical and microbiological properties, with the 1% dose enhancing pH, CEC, organic carbon, microbial biomass, and Ca content. Higher doses (4%) increased N and Mg levels, but 1% biochar and mineral fertilization showed the most significant overall benefits for soil quality.	[248]
Alfisol	Pigeon pea	350–500	pH = 8.52, EC = 2.16 g (dS m−1), total C = 69%	Biochar (BC) improved soil health by enhancing pH, carbon content, and nutrient cycling, which contributed to a 7–25% increase in onion growth, yield, and quality.	[249]
	Canola stalk	650	pH = 11.0, TOC = 327 (g kg−1)	Biochar application in soil increased total P, Olsen P, and H2O-extractable P while enhancing Al-P, Fe-P, and Ca-P availability. Moreover, the positive correlation between biochar pH and soil P availability highlights its role in improving nutrient dynamics.	[250]
	Rice straw	450	pH = 10.63, TOC = 518.93 (g kg−1)	Biochar application enhances soil structure by increasing porosity, microporosity, and water-holding capacity, thereby improving plant-available water and soil resilience.	[251]

outlines the documented impacts of incorporating biochar into specific acidic soils like ultisols, latosols, and alfisols. The remarkable carbon content of biochar (comprising ≥50% w/w of soil) plays a role in increasing soil organic carbon (SOC) [227]. In-depth analyses, as demonstrated by Oduor et al. [228], confirm that biochar incorporation leads to improvements in soil parameters, including a reduction in soil bulk density (averaging approximately 7.5%), increased aggregate stability (averaging about 8%), enhanced available water capacity (AWC, averaging around 15%), and elevated soil conductivity (averaging about 25%). The impact of biochar on soil properties is influenced by the size of its particles. Importantly, the introduction of biochar provides a substantial benefit by enhancing the soil’s capacity to store moisture. Abujabhah et al. [229] applied biochar, derived from Acacia green waste through pyrolysis at 550 °C for 30–40 min, at a rate of 47 tons per hectare. This application resulted in a significant increase in the organic content of the soil, fostering crop production. Furthermore, the presence of functional groups within the chemical composition of biochar can increase soil pH, cation exchange capacity, and nutrient levels, contributing to improved soil fertility and countering the depletion caused by intensified agricultural practices [222,230]. Careful examination of the ratio of biochar to soil addition is essential, as it stands as one of the most critical parameters influencing its properties before considering large-scale application in soils. Głąb et al. [231] employed two varieties of straw (winter wheat and Miscanthus) to produce biochar at 300 °C for the improvement of loamy sand soils. The researchers incorporated different proportions (0.5%, 1%, 2%, and 4% w/w) of biochar into the soil, demonstrating significant improvements in available water content (AWC), porosity, and bulk density, particularly with a maximum addition of 4%. Senbayram et al. [232] noted enhancements in several soil properties, such as water-holding capacity, electrical conductivity, and pH, with the incorporation of varying compositions of biochar (8%, 4%, and 2% w/w) produced from corn straw carbonized at 280 °C for two days. Nevertheless, an 8% incorporation of biochar led to negative impacts on soil respiration rates. Consequently, the ratio of biochar to soil emerges as a pivotal sustainability parameter, necessitating meticulous examination before implementing extensive applications in soils. It is essential to recognize that the ideal ratio is directly contingent on the properties of biochar, including carbon content, heavy metals (HMs), and minerals. The existing literature offers support for the notion that modifying the surface of pristine biochars can result in unique properties tailored for particular applications. For example, Upamali et al. [180] discovered that porosity in biochar can be improved through modifications involving acid or alkaline treatments [233,234]. Moreover, customizing biochar for particular purposes can be achieved by introducing surfactants, impregnating it with mineral sorbents, and incorporating magnetic agents, such as zero-valent iron, which is recognized for reducing water-leachable heavy metals (HMs) in biochar structures [235]. The enhancement of biochar’s specific surface area and porosity, crucial for its effectiveness in the immobilization and adsorption of pollutants, like Cd, in the soil, has been achieved through acid or alkaline modifications. To facilitate soil improvement, it is essential to incorporate chemical elements such as Fe, Al, and Ca into the biochar matrix for subsequent transfer into the soil environment [236]. To introduce these elements into the biochar structure, a method involves incubating it with specific chemicals. For example, Yang et al. [237] conducted a three-month incubation of biochar derived from walnut shells (pyrolyzed at 500 °C under N₂) with kaolinite, CaCl₂, AlCl₃, and FeCl₃, representing soil minerals. The analysis of the resultant materials indicated the presence of substances, like AlCl₃·6H₂O and Fe₈O₈(OH)₈Cl_1.35, either on the surface or within the pores of the biochar. The identification of organometallic compounds, such as Fe−O−C, was also documented. These results indicate the ability of biochar to adsorb minerals from the soil, underscoring its efficacy as a sorbent. The feedstock influences the variety and concentration of mineral elements present in biochar. For example, biochar derived from chicken manure exhibits elevated concentrations of essential minerals, like calcium (Ca) and potassium (K) [238], playing a pivotal role in assessing soil quality for optimal crop production [239]. Nevertheless, specific biochar varieties, such as those derived from oil palm empty fruit bunches [240], might not possess these particular elements. Under such circumstances, the utilization of inorganic fertilizers in conjunction with biochar presents a feasible approach to supplying the essential elements to the soil [241]. As an example, biochars derived from woody sources exhibit high carbon content, whereas those obtained from sludge and manure possess relatively elevated nitrogen (N) and phosphorus (P) contents. Biochars obtained from manure sources might also supply substantial quantities of calcium (Ca). Despite the majority of studies exploring the impact of biochar addition on soil physical attributes being conducted in laboratory settings, successful field trials have exhibited changes in soil properties. Examining biochar characteristics, such as particle size, Obia et al. [242] explored the impact on soil physical properties, encompassing parameters like bulk density and soil aggregate stability. The findings led the authors to assert that the particle size of biochar significantly influences the characteristics of loamy soils, particularly in terms of reducing soil density. Examining the field effects of biochar derived from herbaceous biomass through slow pyrolysis at two distinct peak temperatures (400 and 600 °C), Jeffery et al. [243] aimed to influence the hydrological attributes of sandy soil in the Netherlands. In one experimental set, biochars (BCs) were applied at a rate of 10 t/ha, whereas in another set of trials, the BC produced at 400 °C was employed for soil treatment at rates ranging from 1 to 50 t/ha. No significant effects of biochar (BC) addition on soil parameters such as conductivity, aggregate stability, and water retention were observed in any of these experiments. The authors contended that the biochar samples exhibited significant hydrophobicity, hindering water infiltration into the biochar structure. As a result, the addition of biochar did not impact water retention. Based on the literature review, it can be inferred that for the widespread application of biochar (BC), it is essential to identify the specific soil properties that need modification. Subsequently, the focus should be on investigating biochar types with the desired characteristics for the intended soil modification. Moreover, conducting brief, small-scale experiments is necessary to determine the optimal ratio of biochar to soil, aiming for optimal outcomes while adhering to sustainability criteria and mitigating potential ecological consequences.

6.2.2. Dynamics of Microbial Communities

The quality of soil is significantly impacted by microbial activity. Incorporating BC has been demonstrated to enhance soil quality, influencing the conditions for soil microorganisms and their metabolic processes. In barren Oxisol soil, Yu et al. [252] observed an increase in the complexity of microbial networks due to the introduction of BC. Investigating the influence of biochar incorporation on soil microbial activity and the release of soil-borne greenhouse gases, Senbayram et al. [233] conducted a study. Analyzing biochars (BCs) obtained from diverse sources like cotton stalks, pistachio shells, corn cobs, and olive mills, the researchers incorporated them into two distinct soil types: alkaline clay soil and acidic sandy soil. The microbial community in the acidic soil exhibited heightened activity, and there was an elevation in greenhouse gas emissions (N₂O and CO₂) following the application of biochar derived from olive mills. The enrichment of specific species within the soil bacterial community occurred as a result of incorporating biochar modified with nutrients, primarily attributed to the immobilization of heavy metals. The research conducted by Wu et al. [253] showcased that biochars modified with sulfur and sulfur–iron contributed to a reduction in exchangeable Cd within the soil, consequently influencing shifts in soil microbial communities. The adjustments in soil properties induced by the addition of biochar, encompassing variations in total nitrogen (TN), soil organic carbon (SOC), cation exchange capacity (CEC), water-holding capacity (WHC), pH, and others, establish more conducive environments for microbial communities. Moreover, biochar offers a refuge and ecological habitat for microbial communities, promoting their growth and maturation [254]. The provision of nutrients from biochar to microbial communities supports their development. The findings of Kolton et al. [255] indicated that the existence of biochar in the rhizosphere enhances bacterial diversity, resulting in elevated utilization of carbohydrates and phenolic compounds by plants. The adjustment of soil nitrogen content, particularly induced by the introduction of biochar, can lead to alterations in soil microbial communities. Utilizing biochar derived from corn straw, prepared at 500 °C for 1.5 h in a nitrogen environment, Xu et al. [256] incorporated varying percentages (2%, 4%, and 8% w/w) into the soil along with urea (250 kg N/ha) for soil improvement. The findings indicated that incorporating biochar led to a decrease in nitrogen leaching from the soil by approximately 20%. This enhancement, along with improvements in soil characteristics, like water-holding capacity (WHC), electrical conductivity, and pH, resulted in an augmentation of soil microbial diversity. Yet, the degree of changes in microbial communities is intricately tied to the attributes of biochar, encompassing porosity, specific surface area, nutrient composition, pH, and carbon content. Gul et al. [257] carried out an examination of the impact of biochar on soil microbial characteristics, emphasizing the influence of biochar production conditions and the selection of feedstock materials. Biochar produced through slow pyrolysis within the temperature range of 300–600 °C demonstrated enhanced physicochemical and microbial attributes in the soil, persisting even after a brief 90-day incubation in field applications. The research underscored the significance of the feedstock type, noting that biochar derived from crop residues or manure had a more pronounced effect on microbial abundance when contrasted with materials sourced from wood. Over an extended period, biochar obtained from woody sources demonstrated favorable outcomes on microbial populations. Despite these beneficial outcomes, additional research is necessary to enhance our comprehension of the impact of microbial communities in diverse soil types and to evaluate whether these modifications positively contribute to overall environmental quality from a sustainability standpoint. Additionally, it was suggested that biochars obtained from crop residues or manure exerted a more pronounced influence on microbial abundance in contrast to those originating from wood-based materials. Significantly, biochar generated from woody materials demonstrated beneficial effects on the microbial population over a comparatively prolonged duration. Anticipating a prolonged residence time in the soil, potentially spanning hundreds of years [258], biochar (BC) is likely to sustain the alterations in soil microbial activity. In the context of sustainability, there is a continual requirement for a more in-depth comprehension of how microbial communities are influenced across diverse soil types. Furthermore, investigations are necessary to ascertain whether these alterations can positively contribute to the overall quality of the environment [259,260].

6.2.3. Fertility of the Soil and the Growth of Plants

The primary goal of introducing biochar (BC) into the soil is to enhance soil fertility, leading to increased crop yield [84,261]. The determination of local land use patterns relies significantly on the state of soil fertility. In a comprehensive five-year field trial in China, Jin et al. [262] examined the impact of applying BC on soil fertility and crop yield. The findings indicated that biochar application resulted in increased soil pH and enhanced nutrient availability. Improvements in hydraulic properties, enzymatic activities, nitrogen content, and total organic carbon (linked to increased NO₃⁻-N and NH₄⁺-N) were observed, although the sustainability of some of these effects decreased over time. Moreover, soil fertility and crop yield are notably influenced by factors such as salt stress and drought. The current literature indicates an increasing focus on investigating the capacity of BC to address these challenges, especially in areas with limited water resources. Deshani et al. [263] conducted a literature review on the role of BC in alleviating the adverse effects of drought on crop yield. They argued that the addition of BC provides a feasible solution for boosting crop yield by improving photosynthesis in soils treated with BC. The authors suggested that biochar could modulate the absorption of potassium (K) and sodium (Na) under conditions of salt stress, thereby supporting plant growth. The occurrence of drought stress may result in challenges, including oxidative stress and a reduction in the enzymatic activities associated with antioxidants. Abbas et al. [264] found that introducing biochar (BC) at concentrations of 3.0% and 5.0% (w/w) notably mitigated drought stress in wheat plants at the age of 45 days. This led to significant enhancements in parameters such as chlorophyll content, spike length, and plant height. Similarly, Rizwan et al. [265] demonstrated that the incorporation of biochar, particularly in the presence of moderate drought conditions, enhanced plant growth. The addition of BC resulted in a decrease in the soil bioavailability of cadmium (Cd), though it led to elevated soil electrical conductivity and pH compared to the control samples. Rock erosion and anthropogenic activities were recognized as major contributors to the release of cadmium (Cd) into the soil, significantly hindering plant growth [266,267]. Different biochars (BCs) have been investigated for their potential in soil improvement with a focus on heavy metal (HM) remediation. Lal et al. [266] studied the application of rice straw-derived biochar (BC) to soils containing cadmium (Cd) and noted a substantial decrease in Cd concentrations across different plant organs, including grains, shoots, and roots. The application of biochar to the soil led to a reduction in cadmium (Cd) levels and promoted plant growth. However, despite the various benefits associated with soil amendment using biochar, including improved plant growth and carbon sequestration, there is a limited body of research on the environmental impact of biochar-treated soil in the surrounding ecosystem. Liu et al. [268] investigated the influence of incorporating wheat straw-derived biochar, produced through slow pyrolysis at 500 °C, at concentrations ranging from 1% to 5% (w/w) on dissolved organic matter (DOM) within a cropland Entisol soil type [269]. The results showed a 59% increase in leaching from soils treated with biochar compared to the control. Therefore, there is a need for more comprehensive studies to delve into the long-term impacts of biochar incorporation on the surrounding environment, with the goal of establishing a sustainable approach to biochar application in soil management.

6.2.4. Soil Decontamination

Human activities, including metal mining, the use of pesticides and fertilizers, the combustion of fossil fuels, and the release of household and industrial effluents containing diverse organic and inorganic compounds, are the primary contributors to soil pollution. The presence of contaminants in the soil poses significant environmental challenges, leading to a deterioration in soil quality for agricultural purposes and causing ecological imbalances. Persistent pollutants, like microplastics and heavy metals, contribute to long-term environmental problems due to their extended residence in the soil.

Adsorption of Heavy Metals

Biochar plays a crucial role in eliminating soil contaminants through adsorption onto its active surface sites [270]. Lin et al. [271] investigated Fe-Mn-modified biochar composites (FMBCs) with different specific surface areas, addressing arsenic pollution in paddy soils. The study revealed that biochar, whether modified or not, enhanced soil characteristics, affecting acidity, redox properties, and biological processes. Manganese oxide and iron inclusion in biochar composition improved its capacity to immobilize arsenic, influencing its speciation. The unexpected efficacy of non-modified biochars highlights the importance of manganese in the oxidation process of heavy metals, mitigating environmental impact. Studies on heavy metal mobility are limited, but Beiyuan et al. [272] explored arsenic and lead migration in contaminated soils using pine sawdust-derived biomass. Different pyrolysis temperatures resulted in varied biochar effects on element mobility and phytoavailability. The combination of biochar with compost demonstrates promise in remediating heavy metal-polluted soils. Liang et al. [273] observed reduced cadmium and zinc levels, increased copper release, and elevated soil pH. The interaction between biochar and compost was crucial, affecting hydrogen ion binding and overall soil fertility. Biochar’s potential in stabilizing heavy metals was evident in studies like Shen et al. [274], where biochar reduced the mobility of copper and lead in soil washing residue. Minimal biochar additions, as suggested by Bashir et al. [275], proved effective in immobilizing chromium and cadmium. While biochar shows promise, the pyrolysis conditions significantly impact its composition, especially heavy metal content. Lu et al. [276] demonstrated that pyrolysis temperature determines heavy metal presence, with temperatures exceeding 500 °C leading to increased heavy metal proportions. However, high pyrolysis temperatures, surpassing 800 °C, may lead to biochars with elevated heavy metal levels but low ecological risks [277]. Stainless steel reactor erosion can introduce contamination, making biochar unsuitable for soil applications [278].

Removal of Organic Pollutants

The utilization of biochar plays a crucial role in removing organic pollutants from soil, especially herbicides, fungicides, pesticides (e.g., pyrimethanil, chlorpyrifos, simazine, carbofuran, atrazine), drugs/antibiotics (acetaminophen, sulfamethoxazole, ibuprofen, tyrosine, sulfamethazine, tetracycline), volatile organic compounds (trichloroethylene, benzene, furan), industrial chemicals, including polycyclic aromatic hydrocarbons (p-nitrotoluene, m-dinitrobenzene, pyrene, polychlorinated biphenyl, naphthalene, catechol), and cationic aromatic dyes (methyl-violet, rhodanine, methylene blue) [279]. Biochar demonstrates efficacy in addressing diverse waste stream compounds, including inhibitory substances in biomass breakdown (such as phenolic compounds and furfural), toxic organic compounds within landfill leachate, and estrogen compounds in animal manure and sewage [280]. The removal of organic contaminants relies on the interplay between these pollutants and various biochar characteristics at the interfaces. The predominant techniques for eliminating organic contaminants involve chemisorption, characterized by electrophilic interactions, and physisorption. These mechanisms leverage hydrophobic interactions, repulsion or electrostatic attraction through hydrogen bonding, pore diffusion, and π-π electron donor–acceptor interactions, notably with functional groups like carboxylic acids, diols, and alcohols in biochar [281]. Various mechanisms play a role in the interaction between biochar and organic contaminants. These include partitioning, chemical transformation (involving reductive reactions or electrical conductivity), and mineralization facilitated by a diverse range of microorganisms located within the micropores and on the surface of biochar [282]. The interactions are significantly influenced by factors such as the type of feedstock, pH, ratios of pollutants to biochar, and pyrolysis temperature. Elevated pyrolysis temperatures lead to increased microporosity and surface area in biochar, enhancing its efficiency in removing nonpolar organic contaminants. Biochar generated at lower temperatures does not possess these characteristics [283]. Pyrolysis temperatures exceeding 500 °C result in heightened aromaticity, diminished polarity, and acidity in biochar, which reduces hydrogen- and oxygen-containing functional groups. This decrease accelerates hydrophobic interactions. Conversely, biochar produced at temperatures below 500 °C might possess an increased presence of O- and H-containing functional groups, thereby augmenting its attraction to polar organic compounds [284]. For example, specific interactions with biochar can efficiently eliminate polar insecticide and herbicide compounds. The potential of biochar to reduce the bioavailability of organic contaminants in soil has been demonstrated, thereby limiting their uptake by plants and microorganisms [281]. This has noteworthy ramifications for the restoration of soil tainted with diverse organic compounds, encompassing agrochemicals.

6.3. Renewable Energy Production

Applications of biochar sourced from biomass extend across the realm of renewable energy generation. Its utility spans acting as an electrode within microbial fuel cells (MFCs) and functioning as a catalyst for the synthesis of biodiesel and hydrogen.

6.3.1. Utilization of Biomaterials in Microbial Fuel Cells (MFCs) to Produce Bioelectricity

Amidst challenges related to energy shortage and environmental contamination, the utilization of microbial fuel cells (MFCs) emerges as a hopeful solution. A typical MFC consists of distinct anodic and cathodic compartments separated by a proton exchange membrane. It employs microorganisms as catalysts to transform chemical energy into electrical energy. Microbial metabolism of organic material is integral to the process, resulting in the generation of electrons and protons [285]. Under anaerobic conditions, a sustained current is produced as electrons traverse various pathways from the anode to the cathode [286]. Various microorganisms, including but not limited to Clostridium butyricum, Rhodospirillum, Arenibacter palladensis, and Shewanella oneidensis MR-1, have been investigated for their potential in MFC applications [287]. The production of electricity in MFCs is influenced by factors such as the type of substrate, the rate of electron transfer, electrode efficiency, proton transfer rate, external operating conditions, oxygen reduction rate, and circuit resistance [288]. Numerous studies have examined different electrode materials, and their physical and chemical characteristics play a role in microbial adhesion, electrode resistance, and the rate of electron transfer [289]. Despite progress, obstacles such as limited current output and elevated expenses impede the widespread application of MFCs on a large scale. Research indicates that a substantial portion of the operational expenses of MFCs, ranging from 20% to 50%, can be traced back to electrode materials, frequently derived from non-renewable sources [288]. Electrode materials play a pivotal role in MFC function. A variety of materials, such as stainless steel, gold, titanium, copper, and nickel, have been investigated. Nevertheless, these metals frequently necessitate extra surface alterations to facilitate the production of microbial biofilms and electron transfer, with a concurrent rise in their global demand and associated expenses [290]. Carbon-based materials, such as carbon nanotubes and graphene, have attracted interest because of their biocompatibility, high conductivity, expansive surface area, and relatively economical nature [291]. Despite their benefits, the manufacturing process of these materials requires costly equipment and chemicals. Conversely, biochar, a carbon material obtained from biomass, is gaining recognition as a sustainable and economical substitute for electrode manufacturing [292]. The production of biochar can utilize regionally abundant raw materials, like agricultural and forestry residues, sludge, and crop waste, thereby minimizing expenses linked to the acquisition, transportation, and storage of feedstock [293]. Biochar-based electrodes are regarded as cost effective when contrasted with other commercially available alternatives. As illustrated by Cao et al. [294], the creation of a budget-friendly N/Fe co-doped carbon (N/Fe-C) electrode involved the direct carbonization of waste adsorbent. The estimated material cost ranged from 0.03 to 0.08 USD g⁻¹, demonstrating a notable reduction compared to commercial Pt electrodes and establishing it as a more economical choice.

Creation of an Anodic Electrode

Various biomass sources have been explored for formulating anodes in microbial fuel cells (MFCs). The characteristics of the electrode depend on factors like surface properties, available surface area, and pore size, as outlined in

Table 5. Application of biochar derived from biomass for the construction of electrodes in microbial fuel cell systems.

Electrode	Biochar Feedstock Type	Pyrolysis Conditions for Biochar Preparation		Power Density	References
		Temperature, °C	Time (h)
Anode	Sewage Sludge	900	2 h	1069 mW/m2	[286]
	Microalgal	900	1 h	12.86 W/m3	[305]
	Corn Straw	900	1 h	8.89 W/m3	[306]
	Waste Food	1000	1 h	-	[297]
	Deoiled Azolla Biomass	600	3 h	-	[293]
Cathode	Banana Peels	900	2 h	528.2 mW/m2	[307]
	Lotus Leaves	900	2 h	511.5 mW/m2	[308]
	Eggplant	800	1 h	667 mW/m2	[309]
	Pamelo Peel	1000	1.5 h	799 mW/m2	[310]
	Olive Mill Waste	800	45 min	271 mW/m2	[311]

. As an illustration, Hemalatha et al. [293] utilized the pyrolysis of deoiled Azolla pinnata biomass at 600 °C to create an anode, resulting in an MFC setup with a voltage of 382 mV and a 65.6% decrease in chemical oxygen demand (COD). Carbonaceous materials, including biochar, employed in electrode preparation, can be obtained through methods like gasification, hydrothermal carbonization, and pyrolysis of biomass. Carbon from biomass absorbs carbon dioxide captured during photosynthesis, while heteroatoms such as nitrogen, phosphorus, and sulfur originate from enzymes involved in photosynthesis [295]. During pyrolysis, the removal of water yields a porous carbon framework, and heteroatoms function as inherent dopants, contributing to exceptional electrical conductivity. Zhang et al. [296] conducted the pyrolysis of bread, resulting in a three-dimensional carbon foam co-doped with phosphorus, sulfur, and nitrogen (NPS-CFs). This material exhibited a significant surface area of 295.07 m² g⁻¹ and a considerable content of nitrogen, phosphorus, and sulfur. In a study by Huggins et al. [288], biochar was produced from wood biomass sourced from milling residue and forestry residue, employing a pyrolysis technique at 1000 °C for a duration of 1 h. The biochar derived from milling residue and forestry residue, characterized by pore sizes of 29.4 and 37.6 Å and surface areas of 469.9 and 428.6 cm² g⁻¹, respectively, demonstrated power outputs of 457 and 532 mW m⁻² when employed as electrodes. Significantly, the manufactured electrodes exhibited a cost that was 90% lower than that of commercial granular activated carbon and graphite granules. In addition to electricity generation, the use of microbial fuel cells (MFCs) extends to wastewater treatment, offering the benefit of reduced sludge generation compared to traditional anaerobic digestion processes. Huggins et al. [297] designed an MFC using cathode and anode electrodes crafted from biochar obtained through the pyrolysis of food waste. This MFC exhibited notable reductions, including a 95% decrease in chemical oxygen demand (COD) and the removal of 73% of ammonia and 88% of phosphorus. Furthermore, the MFC achieved a power generation of up to 6 W m⁻³. The characteristics of the electrodes and their interaction with bacteria, as evidenced by biofilm formation, are pivotal factors influencing electricity production in MFCs. The process of biofilm formation encompasses multiple stages, including bacterial attachment to the electrode, the development of multiple and single layers, the establishment of a polymeric scaffold, and the maturation of the biofilm’s three-dimensional structure. The adherence of bacteria is influenced by the characteristics of the electrode, including hydrophilicity/hydrophobicity, microtexture, and functional groups on the surface [298]. During the initial phases, MFCs demonstrate limited electrocatalytic activity and current generation. With the maturation of the biofilm, there is a subsequent enhancement in efficiency [299]. The mature biofilm contributes to enhanced mass transfer, directly impacting the electron transfer between the biofilm and the electrode, thereby influencing the anode’s current response [300]. External resistance plays a pivotal role in regulating and promoting the formation of the biofilm [301]. In a study by Zhang et al. [302], the influence of varying external resistances on biofilm development was explored. The research revealed that lower resistances (10 and 50 Ω) led to superior biofilm formation accompanied by increased current output. This phenomenon was attributed to the abundant voids within the biofilm, facilitating the efficient exchange of substrate and buffer, as well as the removal of byproducts. The effective formation of a biofilm is contingent upon the pore size of the anode, a critical consideration. Microorganisms typically fall within the size range of 1 to 2 µm, and numerous studies have emphasized that pores ranging from 2 to 10 µm are conducive to the colonization of bacterial cells. Carbon electrodes sourced from wild mushroom (CEWM), corn stem (CECS), and king mushroom (CEKM) were fabricated by Karthikeyan et al. [303], who investigated their pore sizes and biofilm formation. Upon conducting field-emission scanning electron microscopy (FESEM) analysis, it was observed that CECS, CEWM, and CEKM, displayed pore sizes of 2–7 µm, 75–200 µm, and 10–120 µm, respectively. The investigation established that pores of around 10 µm are essential for the facile infiltration of microbes and the generation of a biofilm ranging from 10 to 100 µm. Nevertheless, this pore size remains insufficient for effective biofilm formation. As the biofilm obstructs the pores, the flow of nutrients into the electrode and the elimination of metabolites from its interior are hindered. This impedes biofilm expansion and leads to the demise of microbes within the pore. An anode featuring a pore size within the range of 75–200 µm permits substantial colonization without encountering issues of clogging [304].

Creation of the Cathodic Electrode

The high cost of platinum and its alloys, frequently employed in the fabrication of cathode electrodes, poses a constraint to their widespread application in microbial fuel cells (MFCs). Similar to anode electrodes, cathode electrodes can be fashioned from biochar obtained from plant biomass, as outlined in Table 5. The air cathode comprises a gas diffusion layer (GDL), a catalyst layer (CL), and support. The CL facilitates the diffusion of ions and oxygen, the hydrophobic GDL prevents electrolyte leakage, and the support contributes to electron conduction [311]. The formation of the CL involves applying a mixture of a catalyst and binder to the reverse side of the carbon cloth. Various substances, including polyvinylidene fluoride (PVDF), Nafion, and polydimethylsiloxane (PDMS), serve as binders to enhance oxygen transfer. Bose et al. [312] manufactured a cathode by combining activated carbon derived from sugarcane residue with a PVDF binder, directly applied onto a stainless steel mesh. Its application in an MFC intended for wastewater treatment yielded a current density of 0.40 mA m⁻² and a power density of 110 mW m⁻² [312]. The efficiency of oxygen reduction is contingent on cathode characteristics. In a study by Watson et al. [313], diverse activated carbons sourced from various biomass origins (phenolic resin, hardwood, coconut shell, coal, and peat) were examined in MFCs. It was observed that materials derived from hardwood, possessing the highest count of acidic functional groups, demonstrated subpar MFC efficiency (630 mW m⁻²) [313]. Their conclusion was that relying solely on surface area as an indicator of cathode performance is insufficient, and the oxygen reduction potential of the cathode is determined by the presence of a significant number of acidic functional groups.

6.3.2. Incorporating Biochar into the Production of Biodiesel

Given the substantial content of long-chain fatty acids (C14–C20), biodiesel stands out as a promising energy source for conventional engines [314]. Biodiesel production encompasses the transesterification of oils sourced from diverse outlets, including algal oil, waste animal fats, or plant oils. Catalysts, essential to this procedure, are categorized as either homogeneous or heterogeneous. Biochar, identified as a heterogeneous catalyst, has demonstrated efficacy in catalyzing both transesterification and esterification reactions [315]. Free fatty acids (FFAs) are commonly found in waste cooking oil (WCO) and can influence the transesterification process. The application of alkali catalysts may lead to the formation of soap, while acid-catalyzed reactions tend to exhibit sluggish kinetics [316]. Heterogeneous catalysts possess the ability to conduct both transesterification and esterification reactions. The esterification of free fatty acids (FFAs), coupled with alcohol dehydration, can lead to water production. This water then reacts with triacylglycerol (TG), resulting in the generation of additional FFAs through TG hydrolysis [317]. A significant portion of the research in this domain has concentrated on biochar modified with either alkali or acid. The porous nature of biochar enables convenient access for reactants to reach active sites, thereby promoting transesterification. Additionally, the hydrophobic surface of biochar contributes to the removal of water, an undesirable byproduct generated during catalytic activity [318]. Biochar treated with acid is commonly manufactured through a sulfonation procedure using H₂SO₄ or -SO₃ vapors. This introduces -SO₃H groups to the surface of the biochar, serving as a catalyst. Conversely, basic biochar includes alkaline sites, such as oxides of Na, K, or Ca, formed through the calcination of minerals in biomass or introduced through the impregnation of a precursor with K, Na, or Ca [318]. In the course of the heating procedure, the reactants exist in distinct phases, with alcohols in the gaseous phase and lipids in the liquid phase. The porous structure of biochar increases the collision frequency of these reactants under ambient pressure. The interaction of reactants in two-phase colloids within multiple pores simulates the catalytic effect, effectively lowering the activation energy of transesterification [318]. Dehkhoda and Ellis [319] employed sulfonated biochar to carry out the concurrent transesterification and esterification of oleic acids and canola oil in the production of biodiesel. The process resulted in a 48% yield of fatty acid methyl ester (FAME). Maize residue biochar was prepared by Jung et al. [320], who subsequently conducted transesterification of coconut oil at 380 °C. This process yielded an 87% production of fatty acids ethyl ester (FAEE). Various biochars, including Japanese wood, maize residue, rice husk, mixed wood, pinecone, and bamboo, were examined by Lee et al. [321] in pseudo-catalytic transesterifications of waste cooking oil (WCO). The study reported a maximum yield of fatty acid methyl ester (FAME) at 90% using maize biochar. Their conclusion highlighted that the process is influenced by the lignin content, whereas holo cellulose does not exert a significant effect [321]. Mustapha et al. conducted an economic evaluation of converting lipids from Scenedesmus sp. into biodiesel, employing biochar derived from algae with extracted lipids as a catalyst. The findings revealed a reduction in the per-unit production cost of microalgal biodiesel, decreasing from USD 2.03 per kg to a range of USD 1.70–$1.74 per kg when compared to the use of homogeneous catalysts [322].

6.3.3. Generation of Biohydrogen

Biochar has found application in diverse processes for the production of biohydrogen, encompassing anaerobic digestion, methane steam reforming, and water splitting, as elaborated in the following sections.

The Use of Biochar as a Catalyst in the Water-Splitting Process to Generate Hydrogen

The eco-friendly water-splitting method for hydrogen production encounters challenges in efficiency due to the elevated overpotential observed in the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reactions (HERs) occurring at the cathode [323]. Research has explored electrocatalysts utilizing precious metals and their oxides, such as ruthenium dioxide (RuO₂) and iridium dioxide (IrO₂), for hydrogen production. However, their practical application is limited by their instability and high cost in an alkaline medium. Carbon-based materials, gaining interest as electrocatalysts due to their cost effectiveness and commendable electrical conductivity, often undergo doping with transition metal compounds to reduce overpotential and enhance hydrogen production. This process results in the formation of additional active sites that facilitate efficient electron transfer. Biomass, inherently rich in alkali and alkali earth metals, contributes to the activation of carbon and the creation of a porous structure through the effects of ionic migration at elevated temperatures [323]. The CCW-x nanocomposites, developed by Yang et al. using watermelon peel-derived biochar and the integration of cobalt nanoparticles, demonstrated superior hydrogen evolution reaction (HER) efficiency compared to the extensively studied Pt/C electrocatalyst [324]. Catalysts that operate effectively in acidic environments are of particular interest, aligning with current proton exchange membranes and exhibiting reduced ohmic loss [324]. Electrolysis under acidic conditions requires electrocatalysts based on precious metals, such as Pt and Ru. Investigations have been conducted on diverse transition metal phosphides, nitrides, and carbides to address this challenge. Molybdenum carbide (Mo₂C) derived from biochar has garnered recent attention for its stability and structure reminiscent of Pt. However, the electrocatalytic efficiency of Mo₂C suffers from a limited number of exposed catalytic sites and the formation of robust Mo-H bonds. The integration of a sizable surface area within a porous structure can offer an abundance of catalytic sites [325]. Guo et al. developed a three-dimensional porous nanostructure composed of Mo₂C/C, originating from biochar coactivated with nitrogen and potassium (NKAB). The Mo₂C/NKAB electrocatalyst exhibited enhanced hydrogen evolution reaction (HER) efficiency, achieving a lower overpotential (161 and 144 mV) to attain 10 mA.cm⁻², coupled with a reduced Tafel slope of 57 and 53 mV.dec⁻¹ [326]. Humagain et al. synthesized porous Mo₂C through magnesiothermic reduction reactions, employing biomass derived from birch forestry residue. The Mo₂C catalysts exhibited an overpotential within the range of 35 to 60 mV to achieve current densities of −10 and −100 mA.cm⁻² in 0.50 M H₂SO₄, demonstrating sustained operational stability for more than 100 h [327].

The Use of Biochar as a Catalyst in the Process of Methane Steam Reforming for Hydrogen Production

The primary component of natural gas, methane, can be generated through the anaerobic digestion of various organic waste materials. Its elevated hydrogen–carbon ratio (4:1) makes methane advantageous for steam reforming, leading to increased hydrogen production and reduced COx generation [328]. Gaining attention as a means of obtaining pure hydrogen is the thermocatalytic decomposition (TCD) of methane. Non-catalytic thermocatalytic decomposition (TCD) procedures require high temperatures, approximately around 1200 °C, to initiate the reaction and achieve notable rates of hydrogen evolution (HER). To enhance methane transformation at lower temperatures, various metallic catalysts such as copper, iron, and nickel have been employed. However, these catalysts often experience a rapid decline in activity. Another challenge faced in thermocatalytic decomposition (TCD) processes involves the presence of sulfur within natural gas, which is detrimental to the catalysts. Carbonaceous materials, valued for their enhanced stability and resilience to sulfur, offer a viable alternative. The addition of small amounts of metals to carbon materials enhances their effectiveness by creating active sites with high energy levels within the structure of non-crystalline carbon. These active sites have an affinity for methane molecules, thereby enhancing the overall conversion process [328]. In the context of methane thermocatalytic decomposition (TCD), Harun et al. utilized activated biochar doped with ruthenium (Ru-AB) as a catalyst. The result was a methane conversion rate of 51% at 800 °C after a reaction time of 60 h [328]. Examining various compositions of nickel catalysts supported on activated carbon, Prasad et al. investigated Ni10, Ni20, Ni30, and Ni40 for methane thermocatalytic decomposition (TCD). Among them, the Ni30 catalyst demonstrated a hydrogen production rate of 1.62 L. h⁻¹ [329]. Biochar, derived from biosolids recovered from wastewater treatment plants, was created by Patel et al. and employed in the thermocatalytic decomposition (TCD) of methane. The study reported a conversion rate of 65.2% [330].

The Incorporation of Biochar in Anaerobic Digestion for the Production of Hydrogen

Commonly used for managing biowaste, the anaerobic digestion (AD) process results in bioenergy production. Numerous studies indicate that incorporating biochar during the AD process enhances hydrogen yield, particularly in the short lag phase. Although the precise mechanism by which biochar boosts H₂ production is not fully understood, there is a suggestion that biochar contributes to biofilm formation, stabilizes pH levels, and augments the production of volatile fatty acids (VFAs) [331]. Biochar contains minerals that offer vital nutrients to microbes, fostering their growth and contributing to both enzyme synthesis and activity. During a two-phase anaerobic digestion of liquid carbohydrates, Sunyoto et al. introduced biochar derived from pine dust, resulting in enhanced methane yield (10.0%) and hydrogen yield (31.0%), along with shortened lag phases of 36.0% and 41.0%, respectively [332]. Biochar obtained from calcium lignosulphonate was employed by Zhao et al. to augment hydrogen production, revealing a 50.9% increase in comparison to the control [333]. When cultivating a co-culture of Escherichia coli and Enterobacter aerogenes for hydrogen production from municipal solid waste, the incorporation of biochar led to a hydrogen yield of 96.63 mL g⁻¹ and reduced the lag phase from 12.5 h to 8.1 h [334]. Examining the influence of rice straw biochar on fermentation, Li et al. observed a rise of 118.5% in H₂ production during ethanol fermentation and a 79.6% increase in butyrate-based fermentation [335]. Playing a pivotal role, biochar buffers pH, improves porosity, and reduces the redox potential, thereby facilitating cell immobilization and elevating hydrogen production. In waste biomass hydrolysates, inhibitors like phenols, heavy metals, ammonia, and biomass-derived substances such as vanillin, hydroxymethylfurfural (HMF), and furfural are commonly present. Because of their compact size, these inhibitors can permeate cell membranes, resulting in harm to internal structures, changes in cell morphology, and the inhibition of RNA and protein synthesis [336]. Inhibiting glycolytic enzymes and ATP regeneration systems, weak acids contribute to energy depletion and diminished microbial growth [337]. Although microbes might eventually adapt to inhibitors, this acclimatization process is time intensive and can have adverse effects on productivity. The incorporation of biochar, however, proves advantageous in alleviating the toxic impacts of inhibitors and aiding in their removal. As an illustration, Lin et al. examined how furfural influenced the dark fermentation of glucose by E. aerogenes. They noted that decreased concentrations of furfural at 5 mM led to an increase in H₂ production, whereas higher concentrations (30 mM) led to a significant reduction in H₂ production [338]. Utilizing torrefied biomass, Doddapaneni et al. successfully eliminated furfural from torrefaction condensate, achieving a 60% removal of furfural. The adsorption followed a kinetic model of pseudo-second order. The anaerobic digestion of treated torrefaction condensate resulted in a decreased lag phase for methane production, reducing it from 25 days to 15 days [339]. The research findings emphasize the capacity of biochar to eliminate a range of inhibitors from hydrolysates, thereby enhancing the anaerobic digestion process and resulting in improved hydrogen production.

7. Conclusions and Future Directions

The conversion of common organic wastes into biochar offers an environmentally sustainable solution for waste treatment. The practical feasibility of biochar is supported by its straightforward preparation process and the use of affordable feedstock. The thermochemical conversion processes of biomass lead to significant variations in both the yield and quality of biochar, mainly influenced by changes in heating rate, reaction temperature, and oxygen availability. In general, the production of biochar decreases with an increase in the quantity of available oxygen or heating rate. Biochar demonstrates versatility across industries, serving as a cost-effective adsorbent in environmental applications and contributing to rural economic development. Recent research highlights several promising application trends. As an electrode material, modified biochar offers high surface area and electrical conductivity, making it a promising candidate for bioelectricity generation applications, especially in microbial fuel cells, offering energy recovery from waste. As a catalyst, both pristine and modified forms of biochar facilitate pollutant degradation and redox reactions, showing great potential in biohydrogen production by supporting key reactions in microbial and chemical processes. As a filter, its porous structure and surface chemistry enable effective water and wastewater treatment. As a soil conditioner, biochar improves fertility, water retention, and microbial activity, contributing to agricultural productivity and carbon sequestration. However, to fully understand and optimize biochar’s diverse applications, further investigation is needed to correlate biochar properties with observed outcomes across different sectors. A comprehensive investigation into the variations in properties and impacts of different biochar compositions is crucial, given the lack of standardization in the field. Despite its versatility, challenges persist due to the variability in biochar properties and the absence of standardized production protocols. Most findings to date are based on lab-scale studies, with limited field validation. Future work should focus on integrating production methods, such as combining hydrothermal carbonization (HTC) with pyrolysis, to enhance carbon retention and tailor biochar properties for specific applications. Establishing application-specific standards for quality control will help ensure consistency and reliability across various uses. Additionally, conducting field-scale trials and life cycle assessments is essential to evaluate the long-term performance, sustainability, and real-world impact of biochar in diverse environments. Furthermore, exploring the relationship between biochar properties and its performance in different applications will guide targeted modifications to optimize functionality. Ongoing research aimed at utilizing, modifying, and advancing biochar has the potential to contribute to zero-waste development and improve cost effectiveness in environmental management and sustainability. Advancing these areas will enable the scalable deployment of biochar across environmental, agricultural, and energy systems, strengthening its role in circular economy strategies and contributing to climate change mitigation. Furthermore, establishing a standardized framework is critical to ensure the robust and sustainable growth of the biochar industry.