From biowaste to biochar: sustainable pathways for waste valorization, environmental benefits, and practical uses

1 Introduction

Global generation of biowaste has increased enormously due to rapid urbanization, industrial growth, population expansion, and intensified agricultural activities. Biomass waste is produced in massive quantities across the world and includes agricultural residues, forestry waste, food waste, livestock manure, municipal organic waste, and industrial biowaste. Agricultural and forestry residues such as rice husks, wheat straw, sawdust, tree branches, and animal manure are among the most abundant biodegradable wastes because of their high availability and continuous generation from farming and wood-processing industries. A large accumulation of these wastes occupies valuable land resources and creates severe environmental pollution through uncontrolled decomposition, open burning, and landfill disposal. These practices release greenhouse gases and toxic pollutants that negatively affect ecosystems and human health (Chia et al., 2020; Guo et al., 2024). Food waste has emerged as another major component of the global biowaste crisis. Nearly one-third of the food produced for human consumption is wasted without being utilized. India faces serious challenges in food waste management due to rapid modernization, urban expansion, and population growth. Poor handling of food waste contributes to environmental contamination and increases risks associated with public health and sanitation (Sahoo et al., 2023). Conventional waste disposal methods, such as landfilling, generate methane-rich emissions, leachate formation, and toxic compounds that intensify climate change and environmental degradation. Agriculture alone contributes nearly 10% of global greenhouse gas emissions because of industrial farming systems and inefficient resource management (Behera et al., 2026).

Scientific interest has therefore shifted toward sustainable waste valorization strategies that can convert biowaste into valuable bioenergy and bioproducts. Modern circular economy approaches emphasize efficient utilization of organic waste streams for producing biofuels, biomaterials, and carbon-rich products while minimizing environmental burdens. Advanced technologies, including anaerobic digestion, composting, hydroponic integration, and microbial biorefineries, have shown significant potential for reducing waste generation, recovering nutrients, capturing CO₂, and generating renewable energy. Sustainable management of biowaste is therefore essential for environmental protection, climate resilience, resource recovery, and development of a circular bioeconomy (Behera et al., 2026; Jain et al., 2022). Sustainable waste valorization has become an essential strategy for addressing the increasing burden of solid and organic waste generated from rapid population growth, urbanization, industrialization, and modern consumption patterns. Plastics, paper, metals, food residues, and agricultural biomass are discarded in enormous quantities after their useful life. Conventional waste management practices such as landfilling and direct combustion generate toxic chemicals, greenhouse gases, and leachates that severely affect environmental quality and human health. Scientific attention has therefore shifted toward the waste-to-wealth concept in which discarded waste materials are transformed into secondary resources, bioenergy, and value-added products. This approach not only minimizes environmental pollution but also conserves natural resources and improves economic sustainability (Tejaswini et al., 2022).

Circular bioeconomy principles strongly support sustainable waste valorization by promoting continuous reuse, recycling, and regeneration of biological resources within closed-loop systems. Food waste alone accounts for nearly one-third of global food production loss, corresponding to approximately 1.6 billion tons annually and causing economic losses of nearly USD 1.2 trillion (Pant et al., 2026). Food waste contains large amounts of biodegradable carbon compounds that can be converted into renewable products such as biofuels, biohydrogen, renewable natural gas, enzymes, organic acids, fertilizers, biodegradable plastics, and biochar through thermochemical, biochemical, and chemical conversion pathways. Biomass residues and waste CO₂ are also emerging as sustainable feedstocks for green chemical production and carbon-neutral manufacturing systems (Sheldon, 2024). Resource recovery strategies play a central role in achieving sustainable development by extracting energy, nutrients, and valuable molecules from waste streams. Technologies such as pyrolysis, gasification, anaerobic digestion, microbial fermentation, hydrothermal processing, and composting have demonstrated strong potential for recovering renewable energy and industrially useful materials from biomass waste (Konyannik and Lavie, 2025). Waste-to-energy systems further contribute to environmental sustainability by reducing fossil fuel dependence and lowering greenhouse gas emissions. Modern approaches integrating artificial intelligence, machine learning, and Internet of Things technologies are improving process optimization and industrial scalability of waste valorization systems (Pant et al., 2026). Sustainable waste valorization directly supports the United Nations Sustainable Development Goals, particularly SDG 11 for sustainable cities, SDG 12 for responsible consumption and production, and SDG 14 for environmental protection and resource conservation (Tejaswini et al., 2022). Circular resource management, therefore, represents a practical pathway toward climate resilience, energy security, ecological restoration, and sustainable economic development.

Biochar has emerged as an important carbon-based material because of its environmental, agricultural, and industrial significance. Biochar is a carbon-rich porous substance produced through thermochemical conversion of biomass under oxygen-limited conditions, mainly by pyrolysis. Organic feedstocks such as agricultural residues, wood chips, sawdust, manure, crop residues, and food waste are commonly utilized for biochar production. Historical evidence of biochar application can be traced to the ancient Amazonian “Terra Preta” soils, where indigenous populations intentionally incorporated charred organic matter into infertile soils to improve soil fertility and long-term agricultural productivity. Terra Preta soils contained stable carbon compounds and showed remarkable nutrient retention and microbial activity compared with surrounding soils. This historical practice demonstrated the long-term stability and beneficial properties of carbonized biomass materials in soil systems (Varkolu et al., 2025; Yu et al., 2023). Biochar possesses a highly porous structure with extensive surface area and abundant surface functional groups, including hydroxyl (-OH), carboxyl (-COOH), ketonic carbon (C=O), and phenolic groups that contribute to its multifunctional behavior (Shabir et al., 2023; Zhu et al., 2023). Pyrolysis temperature, feedstock composition, residence time, and heating conditions strongly influence the physicochemical properties of biochar. Porous architecture and large pore size distribution enhance water retention capacity, nutrient adsorption, ion exchange, and microbial colonization in soil systems. Biochar also exhibits high cation exchange capacity and prolonged carbon storage ability, which support soil organic matter stabilization and climate change mitigation. Certain biochars can absorb water up to 400% of their weight because of their hydrophilic pore networks and irregular particle structures (Adhikari et al., 2023; Pandian et al., 2024).

Multifunctional properties make biochar highly attractive for sustainable environmental management. Biochar improves soil structure, nutrient cycling, microbial diversity, and crop productivity while reducing nutrient leaching and greenhouse gas emissions. Biochar also acts as an efficient adsorbent for removing heavy metals, dyes, pharmaceuticals, and other organic and inorganic contaminants from wastewater because of its negatively charged surface and functionalized porous matrix (Tadesse et al., 2025). Biochar further contributes to renewable energy systems through biogas purification and carbon sequestration applications by adsorbing CO₂, moisture, and H2S. Scientific interest in biochar, therefore, continues to increase because of its ability to simultaneously support sustainable agriculture, environmental remediation, resource recovery, and circular bioeconomy development (Zhu et al., 2023). This review comprehensively discusses the conversion of biowaste into biochar and its significance in sustainable waste valorization. Biowaste feedstocks, including agricultural residues, forestry waste, food waste, and organic industrial waste, are critically examined. Biochar production pathways with special emphasis on pyrolysis technologies and reaction mechanisms are systematically explained. Physicochemical characteristics such as porosity, surface area, elemental composition, and functional groups are discussed in relation to biochar performance. Environmental and industrial applications, including soil improvement, wastewater treatment, and carbon sequestration, are highlighted. Future research directions for advanced and sustainable biochar development are finally presented.

2 Biowaste as feedstock for biochar production

2.1 Classification of biowaste

Biowaste can be classified into agricultural waste, forestry waste, kitchen waste, animal-derived waste, municipal waste, and industrial waste. A pictorial representation of overall biowaste for the biochar production is given in Figure 1.

2.1.1 Agricultural waste

Agricultural waste represents one of the largest sources of global biowaste generated from farming and agro-processing activities. Rice husk is abundantly produced during rice milling and contains high silica and lignocellulosic content that make it suitable for biochar and energy production (Jose et al., 2025). Wheat straw is another major agricultural residue generated after harvesting and is rich in cellulose, hemicellulose, and lignin. Open-field burning of wheat straw causes severe air pollution and greenhouse gas emissions. Sugarcane bagasse is produced in large quantities from sugar industries and serves as an important biomass feedstock because of its high carbon content and calorific value. Corn stover includes leaves, stalks, and cobs remaining after corn harvesting. Corn stover possesses strong potential for thermochemical conversion into biochar, biofuels, and renewable energy products. Coconut shell is a hard lignocellulosic waste generated from coconut processing industries. Coconut shells contain high fixed carbon and low ash content, which supports the production of high-quality porous biochar and activated carbon. Agricultural residues, therefore, provide renewable and inexpensive feedstocks for sustainable waste valorization, resource recovery, and circular bioeconomy development (Kapoor et al., 2020).

2.1.2 Forestry waste

Forestry waste is an important lignocellulosic biomass generated from wood processing and forest management activities. Wood chips are commonly produced during timber cutting and furniture manufacturing processes. Wood chips contain high cellulose and lignin content that support biochar and bioenergy production. Bark is another major forestry residue generated during wood peeling and processing operations. Bark possesses high carbon content and several bioactive compounds useful for thermochemical conversion. Sawdust is a fine particulate waste formed during the sawing and polishing of wood materials. Sawdust exhibits excellent suitability for pyrolysis because of its low moisture content, uniform particle size, and high carbon-rich composition (Rahmana Putra et al., 2024).

2.1.3 Food and kitchen waste

Food and kitchen waste represent a major fraction of biodegradable municipal waste generated from households, restaurants, and food industries. Fruit peels are rich in cellulose, pectin, sugars, and bioactive compounds that support biochar and bioenergy production. Vegetable waste contains high organic matter and moisture content suitable for composting and thermochemical conversion processes. Spent coffee grounds are generated in large quantities from domestic and commercial coffee consumption activities. Spent coffee grounds possess high carbon and lipid content that enhances their suitability for biofuel and porous biochar production. Food and kitchen waste, therefore, provide sustainable feedstocks for waste valorization, renewable energy generation, and circular bioeconomy development (Dari et al., 2025; Esteban-Lustres et al., 2022).

2.1.4 Animal-derived waste

Animal-derived waste is an important organic biomass generated from livestock and poultry farming activities. Poultry litter consists of manure, feathers, bedding materials, and feed residues produced in poultry production systems. Poultry litter contains high nitrogen, phosphorus, and organic carbon that support biochar and biogas production. Cow dung is abundantly generated from dairy farms and rural agricultural systems. Cow dung possesses rich microbial and organic matter content suitable for anaerobic digestion, composting, and thermochemical conversion. Pig manure is another nutrient-rich livestock waste containing high levels of organic compounds and minerals. Pig manure can be effectively converted into renewable energy, organic fertilizers, and carbon-rich biochar through sustainable waste valorization technologies (Rafsan et al., 2024; Zhang et al., 2024).

2.1.5 Municipal and industrial organic waste

Municipal and industrial organic waste constitutes a significant source of biodegradable biomass generated from urban and manufacturing activities. Sewage sludge is produced during wastewater treatment processes and contains high organic matter, nutrients, microorganisms, and residual contaminants. Sewage sludge can be converted into biochar and bioenergy through thermochemical treatment methods. Paper sludge is generated from pulp and paper industries during paper processing and recycling operations (Cao et al., 2023). Paper sludge contains cellulose-rich fibrous materials suitable for carbon-rich biochar production. Brewery waste is produced from beer manufacturing industries and mainly includes spent grains, yeast residues, and organic sediments. Brewery waste possesses high moisture and nutrient content that support anaerobic digestion, composting, and renewable energy generation through sustainable waste valorization technologies (Chattaraj et al., 2024).

2.2 Chemical composition of biomass

Biomass mainly consists of cellulose, hemicellulose, and lignin that collectively determine the physicochemical properties and performance of biochar (Figure 2). Cellulose is the most abundant structural polysaccharide present in plant biomass and is composed of glucose units linked through β-1,4-glycosidic bonds (Nasriani and Nasiri Ghiri, 2026). Cellulose generally decomposes within the temperature range of 300 °C–380 °C and contributes to the formation of porous carbon structures through dehydration, decarboxylation, and aromatization reactions during pyrolysis. Biomass rich in cellulose often produces biochar with high porosity, large surface area, and abundant oxygen-containing functional groups that improve adsorption performance (Gholizadeh et al., 2024).

Hemicellulose is an amorphous branched polysaccharide composed of sugars such as xylose, mannose, arabinose, and glucuronic acid (Nasriani and Nasiri Ghiri, 2026). Hemicellulose decomposes at lower temperatures between 200 °C and 350 °C and releases large amounts of volatile gases and organic acids during thermal degradation. High hemicellulose content generally decreases biochar yield because of extensive volatile release. Hemicellulose-rich biomass, however, promotes the development of pore networks and enhances surface area formation in biochar (Escalante et al., 2022).

Lignin is a complex aromatic biopolymer formed from p-hydroxyphenyl, guaiacyl, and syringyl units interconnected through ether and carbon-carbon bonds. Lignin decomposes slowly over a broad temperature range extending up to 600 °C–900 °C because of its highly cross-linked aromatic structure. Lignin-rich biomass produces biochar with higher fixed carbon content, greater aromaticity, and superior thermal stability. High lignin concentration also improves long-term carbon sequestration potential because aromatic carbon structures resist microbial degradation and oxidation (Dadhich et al., 2025).

Ash content represents the inorganic mineral fraction of biomass and includes compounds containing potassium, calcium, magnesium, silica, and phosphorus. High ash content can reduce pore development and surface area because minerals catalyze cracking and secondary reactions during pyrolysis (Puri et al., 2024). Biomass with excessive ash often produces biochar with lower adsorption capacity and reduced carbon stability. Volatile matter consists of thermally degradable organic compounds released as gases and vapors during pyrolysis. High volatile matter generally increases liquid and gas formation while reducing solid biochar yield (Afshar and Mofatteh, 2024). Fixed carbon refers to the stable carbon fraction remaining after removal of moisture and volatile components. High fixed carbon content enhances biochar stability, carbon retention, and adsorption efficiency because stable aromatic carbon networks support micropore formation and surface functionality. Biomass composition therefore strongly governs biochar yield, porosity, stability, and environmental application potential (Li and Tasnady, 2023).

2.3 Advantages and limitations of different feedstocks

Agricultural residues, forestry biomass, food waste, and animal-derived waste are widely explored as feedstocks for biochar production because of their abundance and renewable nature. A summarize major characteristics of six categories of biowaste feedstocks, for their feedstock composition, availability, moisture content, ash generation, contaminant risks, suitability for biochar production, energy efficiency, and potential application are given in Table 1. Agricultural residues such as rice husk, wheat straw, and corn stover are inexpensive and readily available in farming regions. Rice husk contains high silica and ash content that can improve thermal resistance but may reduce pore development and adsorption efficiency of biochar because silica particles block active surface sites (Wan et al., 2020). Wheat straw and corn stover possess high cellulose and hemicellulose fractions that support rapid thermal degradation and enhanced pore formation during pyrolysis. High volatile matter in these residues, however, decreases biochar yield because large amounts of gaseous products are released during heating (Hassan and Strezov, 2025).

TABLE 1. Comparative characteristics of major biowaste feedstocks for biochar production: composition, availability, cost, ash generation, contaminant risks, energy efficiency, and potential applications (Alkharabsheh et al., 2021; Maj et al., 2025; Pradhan et al., 2024; Ravindiran et al., 2024; Singh et al., 2023; Tomczyk et al., 2020).

Forestry waste such as sawdust, bark, and wood chips demonstrates high lignin content that contribute to greater biochar yield and improved structural stability. Sawdust-derived biochar often exhibits high aromaticity and carbon retention because lignin decomposes slowly under pyrolytic conditions. Forestry biomass generally produces low-ash biochar with higher surface area and superior adsorption performance compared with crop residues. Transportation and collection of forestry waste from dispersed forest regions, however, increase operational costs and limit economic feasibility in some areas (Virmond et al., 2021).

Food and kitchen waste provide low-cost and continuously available biomass for sustainable waste valorization. Fruit peels, vegetable residues, and spent coffee grounds contain high organic carbon and biodegradable compounds suitable for biochar and bioenergy production. High moisture content and rapid microbial decomposition of food waste, however, require pre-drying and stabilization before thermochemical processing. Food waste may also contain salts, oils, and contaminants that affect pyrolysis efficiency and biochar quality (Rai et al., 2025).

Animal-derived wastes such as poultry litter, cow dung, and pig manure possess high nitrogen, phosphorus, and mineral content that improve nutrient recovery potential. Poultry litter-derived biochar demonstrates a strong adsorption capacity for heavy metals because mineral ash contributes to ion exchange activity. Animal wastes, however, generate biochar with high ash content and lower fixed carbon levels. Animal-derived feedstocks may also contain antibiotics, pathogens, and toxic metals that pose environmental risks if pyrolysis conditions are not properly controlled. Feedstock composition, therefore, strongly influences production cost, energy efficiency, ash generation, contaminant behavior, and physicochemical quality of biochar (Hadroug et al., 2025).

3 Biochar production technologies

Thermochemical conversion is an effective approach for transforming biomass into solid, liquid, and gaseous energy products through controlled thermal degradation processes. Biomass feedstocks rich in cellulose, hemicellulose, and lignin are converted into biochar, bio-oil, syngas, and renewable fuels using technologies such as pyrolysis, gasification, hydrothermal carbonization, and torrefaction. Pyrolysis is the most widely applied thermochemical method for biochar production because it operates under oxygen-limited conditions and efficiently converts lignocellulosic biomass into carbon-rich materials. Fast pyrolysis performed at 450 °C–600 °C with rapid heating rates commonly produces 60–75 wt.% bio-oil, 10–20 wt.% biochar, and 10–25 wt.% syngas. Reactor configuration, temperature, heating rate, and biomass composition strongly influence conversion efficiency and product distribution (Ali et al., 2026; Amalina et al., 2022).

3.1 Pyrolysis

It has evolved from a simple charcoal production method into an advanced thermochemical process for converting biomass into valuable energy products. There is growing interest in biomass pyrolysis because it can transform various lignocellulosic materials into biochar, bio-oil, and syngas while offering a cleaner alternative to incineration and landfilling. Biomass pyrolysis occurs in the absence of oxygen at temperatures between 300 °C and 700 °C, where heat breaks down complex organic compounds into smaller molecules. The process efficiency and product distribution depend on factors such as temperature, heating rate, residence time, and reactor atmosphere. Pyrolysis is especially preferred for bio-oil production due to its high yield and simple reactor design, although bio-oil often requires upgrading to improve stability and energy content. There are also pretreatment methods, such as torrefaction and hydrothermal carbonization, that improve biomass quality and enhance pyrolysis performance, energy recovery, and environmental sustainability (Ungureanu et al., 2025; Wang and Wu, 2023. Table 2 represents a comparison table for slow, intermediate, fast, flash, microwave-assisted, and vacuum pyrolysis.

TABLE 2. Compares slow, intermediate, fast, flash, microwave-assisted, and vacuum pyrolysis. The table summarizes key operating parameters including temperature, heating rate, residence time, product distribution, major advantages, and limitations.

3.1.1 Slow pyrolysis

It is a thermochemical conversion process mainly designed to maximize biochar production through low temperatures, slow heating rates, and long residence times. Slow pyrolysis generally operates between 300 °C and 550 °C with heating rates of 0.1 °C–1 °C/s, allowing biomass to decompose gradually and favoring secondary reactions that enhance char formation. There is a strong interest in this process because it can handle different biomass types, including agricultural residues, woody materials, and organic wastes with relatively high moisture content. The prolonged retention of solids and vapors inside the reactor promotes the production of carbon-rich biochar, while smaller amounts of bio-oil and syngas are generated (Jamil et al., 2024). Slow pyrolysis has been used since the early 20th century for producing charcoal and valuable chemicals such as methanol and acetic acid. Modern technologies have improved the process through better temperature control, feedstock pretreatment, and emission management systems, resulting in greater efficiency and lower environmental impact. The process can be performed in several reactor types, including fixed-bed reactors, rotary kilns, and batch systems, depending on feedstock characteristics and production goals (Jamil et al., 2024).

Biochar produced from slow pyrolysis has attracted attention because of its multiple environmental and industrial applications. It improves soil fertility by enhancing water retention, nutrient availability, and microbial activity, while also reducing nutrient loss and dependence on chemical fertilizers. Its stable carbon structure supports long-term carbon sequestration, making slow pyrolysis an important carbon-negative technology for climate change mitigation. Biochar is also used in activated carbon production, metallurgy, renewable solid fuels, and soil remediation (Ronsse et al., 2013). The process also faces challenges such as long processing times, lower liquid fuel yields, and economic dependence on heat recovery and product valorization strategies. Despite these limitations, slow pyrolysis remains an important and sustainable biomass conversion technology due to its environmental benefits, waste valorization potential, and contribution to renewable energy and circular economy systems (Okolie et al., 2022).

3.1.2 Intermediate pyrolysis

It is a thermochemical conversion process that operates between slow and fast pyrolysis, providing a balanced production of biochar, bio-oil, and syngas. Intermediate pyrolysis generally works at temperatures of 450 °C–700 °C with moderate heating rates and biomass residence times ranging from a few seconds to about 40 s. There is increasing interest in this process because it combines operational flexibility with efficient utilization of all product streams, making it suitable for integrated biorefineries and circular economy applications (Sakhiya et al., 2020). Intermediate pyrolysis produces around 40–50 wt% liquid products, together with solid char and combustible gases. The resulting bio-oil usually contains lower tar and water content than oils produced from fast pyrolysis, improving its thermal stability and suitability for direct heating or fuel upgrading. At the same time, the generated syngas is often richer in methane and hydrogen, which increases its value for combustion and combined heat and power systems. The biochar formed during the process is dry, porous, and carbon-rich, making it useful as a soil amendment, biofertilizer, adsorbent material, or feedstock for gasification (Kazawadi et al., 2021).

The process can handle a wide variety of biomass feedstocks, including agricultural residues, woody materials, food-processing wastes, and agro-industrial by-products such as olive pomace, grape marc, wheat straw, and brewers’ spent grain. One of its major advantages is the ability to process biomass with different particle sizes and moisture contents without extensive preprocessing. This lowers energy consumption and supports decentralized applications in rural or agro-industrial regions where biomass resources are abundant (Polipalli et al., 2025). Several reactor systems are used in intermediate pyrolysis, including screw reactors, rotary kilns, and fluidized bed reactors. Screw-based systems are especially attractive because of their compact design and continuous operation. Despite its advantages, the technology still faces challenges related to reactor fouling, tar accumulation, heat integration, and bio-oil upgrading. Nevertheless, intermediate pyrolysis remains a promising approach for sustainable biomass conversion because it supports waste valorization, renewable energy production, carbon sequestration, and reduced greenhouse gas emissions (Moser et al., 2023).

3.1.3 Fast pyrolysis

It is an advanced thermochemical conversion process mainly developed to maximize bio-oil production from biomass through rapid heating and short reaction times. Fast pyrolysis generally operates at temperatures between 400 °C and 600 °C, although wider temperature ranges may also be used depending on the desired products. There is strong interest in this technology because it can convert biomass into large quantities of renewable liquid fuel while also producing biochar and syngas as valuable by-products (Rasaq et al., 2021). Fast pyrolysis relies on extremely high heating rates and very short vapor residence times, usually less than a few seconds, to minimize secondary cracking reactions and preserve condensable vapors. Under optimized conditions, bio-oil yields can reach 50–75 wt%, while smaller amounts of char and non-condensable gases are produced. To achieve efficient heat transfer, biomass feedstocks are usually dried to low moisture content and ground into fine particles before processing. Woody residues, agricultural wastes, energy crops, and some industrial by-products are commonly used as feedstocks (Kan et al., 2016).

Several reactor systems are applied in fast pyrolysis, including bubbling and circulating fluidized beds, auger reactors, rotating cone reactors, and ablative systems. Among these, fluidized bed reactors are widely preferred because they provide excellent temperature control, rapid heat transfer, and scalability for industrial applications. The complete process also includes feedstock preparation, vapor–solid separation, bio-oil condensation, and heat recovery systems to improve energy efficiency (Jalalifar et al., 2020). The produced bio-oil is a dark combustible liquid with moderate heating value, but it often contains significant amounts of water and oxygenated compounds. These properties reduce its stability, increase acidity, and limit direct use as a transportation fuel. Therefore, upgrading methods such as catalytic hydrotreatment and emulsification are commonly applied to improve fuel quality. Biochar generated during the process can still be used for soil improvement, carbon sequestration, and activated carbon production. Fast pyrolysis offers important environmental benefits by reducing biomass waste, lowering greenhouse gas emissions, and supporting renewable energy production. However, challenges remain in feedstock preparation, bio-oil stability, storage, and overall process costs, which continue to drive research and technological improvements (Ungureanu et al., 2025).

3.1.4 Flash pyrolysis

Flash pyrolysis is an advanced thermochemical conversion process designed to maximize the production of bio-oil and syngas from biomass while minimizing solid residue formation. The process operates at extremely high temperatures, generally between 600 °C and 1,400 °C, with very rapid heating rates exceeding 1,000 °C/s and very short vapor residence times of less than 0.5–2 s. Such operating conditions enable the rapid decomposition of lignocellulosic biomass components, including cellulose, hemicellulose, and lignin, into volatile compounds that are quickly removed and condensed to prevent secondary cracking reactions (Wang et al., 2019). Finely pulverized biomass with particle sizes typically below 2 mm and a moisture content lower than 10% is essential for efficient heat transfer and uniform thermal conversion. Common feedstocks include wood powders, agricultural residues, and energy crops. Flash pyrolysis generally produces a mixture of gases, liquid bio-oil, and biochar. Under optimized conditions, bio-oil yields may reach 60–80 wt%, although high gas production is also characteristic of the process. The resulting bio-oil contains significant oxygen and water content, making it chemically unstable, acidic, and corrosive. Additional upgrading processes, such as catalytic hydrogenation, are therefore required to improve fuel quality and stability (Kan et al., 2016).

Various reactor systems have been developed for flash pyrolysis, including fluidized bed, entrained flow, cyclonic, and auger screw reactors. Fluidized bed reactors are particularly favored because of their excellent heat transfer efficiency and scalability. Advanced systems often integrate inert gas circulation, staged condensation, and catalytic vapor upgrading to improve product recovery and quality. Flash pyrolysis offers several advantages, including ultra-fast processing, high syngas generation, and efficient biomass utilization for renewable fuel and chemical production. However, high energy demand, costly feedstock preparation, reactor complexity, and bio-oil upgrading challenges remain significant limitations. Despite these challenges, flash pyrolysis continues to show strong potential for sustainable energy generation and biomass waste management (Ungureanu et al., 2025).

3.1.5 Microwave-assisted pyrolysis

Microwave-assisted pyrolysis is an emerging thermochemical conversion technology that uses microwave energy to convert biomass into bio-oil, syngas, and biochar. Unlike conventional pyrolysis, where heat is transferred from the surface to the interior through conduction and convection, microwave heating generates heat directly within the biomass through electromagnetic energy interaction with molecular dipoles. This internal and volumetric heating mechanism enables rapid and uniform temperature distribution, resulting in faster and more efficient biomass decomposition (Fernández and Menéndez, 2011). Microwave-assisted pyrolysis has attracted considerable attention because of its high heating efficiency, precise process control, rapid start-up and shutdown, and lower environmental impact. The process can handle wet biomass without extensive pre-drying, making it advantageous for treating sewage sludge, agricultural residues, wood waste, and oily sludge. Feedstock properties such as moisture content, particle size, lignocellulosic composition, and heating value significantly influence product distribution. Different biomass materials produce varying yields of liquid, gas, and char under similar operating conditions (Al-Qahtani, 2023). Process performance is strongly affected by temperature, heating rate, residence time, and sweep gas flow rate. Lower temperatures with short residence times generally favor liquid bio-oil production, whereas higher temperatures promote gas formation because of secondary cracking reactions. Increased gas residence time and higher inert gas flow rates often enhance syngas production while reducing liquid and char yields. Temperature also influences biochar characteristics, including pore structure and surface area (Chhiti et al., 2012).

Biomass materials usually exhibit poor microwave absorption; therefore, microwave absorbers such as graphite, silicon carbide, biochar, and metal oxides are often added to improve heating efficiency. These additives enhance reaction temperatures, modify product distribution, and improve syngas quality. Microwave pyrolysis has also demonstrated the ability to produce bio-oils with lower polycyclic aromatic hydrocarbon content and biochars with high surface areas, often reaching 450–800 m²/g (Syed et al., 2023). Several advantages make microwave-assisted pyrolysis attractive for sustainable energy production, including rapid and selective heating, efficient energy transfer, improved product quality, and reduced pollutant formation. However, the technology also faces challenges such as high electricity demand, expensive microwave systems, and sensitivity to feedstock characteristics and operating conditions. Despite these limitations, microwave-assisted pyrolysis remains a promising approach for biomass valorization and renewable fuel generation (Islam et al., 2025).

3.1.6 Vacuum pyrolysis

Vacuum pyrolysis is a thermochemical conversion process carried out under reduced pressure and slow pyrolysis conditions, characterized by low heating rates and long residence times. The process promotes the formation of liquid bio-oil and solid biochar while operating at comparatively lower temperatures, thereby reducing overall energy consumption. Reduced pressure suppresses oxidation reactions and minimizes secondary cracking, resulting in improved bio-oil quality and enhanced product stability (Ungureanu et al., 2025). Vacuum pyrolysis can process a wide variety of lignocellulosic feedstocks, including forestry residues, agricultural wastes, sewage sludge, and industrial biomass, without requiring extensive pretreatment. Larger biomass particles can also be utilized, which lowers feedstock preparation costs. Typical reactor systems consist of a heated vacuum chamber equipped with conveyor mechanisms and mechanical agitation to ensure uniform heat transfer and efficient thermal decomposition. External burners or molten salt-based inductive heating systems are often employed to maintain controlled operating temperatures (Yadav et al., 2016).

Bio-oil obtained from vacuum pyrolysis generally contains valuable polycyclic organic compounds and exhibits a heating value between 22.4 and 40 MJ/kg, with product yields ranging from 35 to 50 wt%. The produced biochar retains high carbon content and possesses significant porosity and surface area, making it suitable for applications such as soil amendment, carbon sequestration, adsorption, and solid fuel production. Studies on biomass such as Napier grass and cassava stem waste have demonstrated that vacuum pyrolysis-derived biochar improves soil fertility, enhances water retention, and shows strong adsorption capacity for pollutants. These characteristics make vacuum pyrolysis a promising technology for sustainable biomass valorization and waste management (Foong et al., 2020).

3.2 Gasification

Gasification is a thermochemical conversion process that transforms carbonaceous materials such as biomass into combustible gaseous products under oxygen-limited conditions at temperatures generally above 750 °C. The process uses gasifying agents such as air, oxygen, or steam to convert biomass into syngas, which mainly consists of hydrogen, carbon monoxide, carbon dioxide, methane, and small amounts of other gases. Steam gasification is particularly effective for increasing hydrogen production and improving the heating value of syngas (Nidheesh et al., 2021). Gasification proceeds through four major stages: drying, pyrolysis, partial oxidation, and reduction. During these stages, moisture is removed, volatile compounds are released, and solid carbon reacts with the gasifying medium to produce combustible gases. Depending on the mode of gas–solid interaction, gasifiers are commonly classified as fixed-bed, fluidized-bed, or entrained-flow systems. Each design offers different advantages in terms of efficiency, feedstock flexibility, and syngas quality (Yaashikaa et al., 2020).

Biomass gasification has gained significant attention as a sustainable alternative to fossil fuels because it enables the production of renewable energy, hydrogen, and second-generation biofuels. Operating conditions such as temperature, pressure, catalysts, and equivalence ratio strongly influence the quality and composition of syngas. Higher temperatures and catalytic gasification generally reduce tar formation and enhance hydrogen yield (Mohanty et al., 2025). Although gasification produces small amounts of biochar, its primary objective is energy generation through syngas production. Limited biochar yield and greenhouse gas emissions remain key challenges. Nevertheless, gasification continues to be an important technology for renewable power generation and biomass utilization.

3.3 Torrefaction

Torrefaction is a mild thermal pretreatment process used to improve the fuel properties of biomass before further thermochemical conversion. The process is carried out at temperatures between 200 °C and 300 °C under inert or low-oxygen conditions with relatively low heating rates. During torrefaction, moisture and volatile compounds are removed, while partial decomposition of hemicellulose, cellulose, and lignin occurs. This treatment produces a solid material with properties intermediate between raw biomass and char (Manyà et al., 2020). Torrefied biomass exhibits enhanced energy density, hydrophobicity, grindability, and storage stability compared to untreated biomass. Reduced moisture and oxygen contents increase its calorific value from approximately 18–19 MJ/kg to about 20–24 MJ/kg. The removal of hydroxyl groups during the process makes the material water resistant, allowing long-term storage without significant biodegradation. Torrefaction generally achieves a mass yield of 70%–80% and an energy yield of 80%–90%, making it an energy-efficient pretreatment technology (Enaime et al., 2020; Tumuluru et al., 2021).

Torrefaction also improves the performance of subsequent processes such as pyrolysis, gasification, and combustion by increasing syngas yield, reducing tar formation, and stabilizing reactor operation. The process produces mainly solid products, with smaller amounts of gases such as CO and CO₂. Various reactor systems, including fixed-bed, rotary drum, microwave, fluidized-bed, and moving-bed reactors, are used for torrefaction. Moving-bed reactors are particularly attractive because they provide uniform product quality and easier scale-up. Despite its advantages, large-scale commercialization of torrefaction remains challenging due to reactor design complexity, product quality control, and economic considerations. However, integration with existing biomass processing systems can improve its commercial feasibility and sustainability (Tumuluru et al., 2021).

3.4 Hydrothermal carbonization

Hydrothermal carbonization (HTC) is a wet thermochemical conversion process developed for biomass with high moisture content. It operates under moderate temperatures of about 180 °C–250 °C and pressures of 2–4 MPa in the presence of water. Unlike conventional thermochemical methods that require prior drying, HTC efficiently converts wet biomass such as sewage sludge, food waste, algae, and municipal solid waste directly into a carbon-rich solid known as hydrochar. Water acts both as a solvent and a reactive medium during the process, promoting the transformation of biomass under autogenous pressure in a closed reactor (Karthik et al., 2021). Hydrochar produced through HTC possesses high energy density and structural characteristics similar to lignite coal. Compared with conventional biochar, hydrochar generally exhibits a better H/C ratio and enhanced fuel properties, making it suitable for energy generation and soil amendment applications. The porous structure and surface chemistry of hydrochar also support applications in carbon sequestration and heavy metal adsorption (Liu et al., 2021).

HTC offers significant environmental and economic advantages because it eliminates energy-intensive drying steps and enables efficient utilization of wet biomass resources. Related hydrothermal processes include hydrothermal liquefaction and hydrothermal gasification, which mainly produce liquid and gaseous fuels, respectively. Despite its strong industrial potential, challenges related to process water treatment and organic contaminant emissions still require further research and optimization (Ponnusamy et al., 2020).

4 Reaction mechanisms during pyrolysis

Lignocellulosic biomass mainly consists of cellulose, hemicellulose, and lignin. Cellulose provides structural strength, hemicellulose supports flexibility, while lignin enhances rigidity, resistance, and thermal stability in plant cell walls.

4.1 Mechanism of cellulose pyrolysis

Cellulose pyrolysis is an important stage in the thermochemical conversion of lignocellulosic biomass into fuels and valuable chemicals. Biomass mainly consists of cellulose, hemicellulose, and lignin, with cellulose accounting for nearly 40%–50% of the total composition. Cellulose is a linear polysaccharide formed by glucose units connected through β-1,4-glycosidic bonds. These bonds become unstable at high temperatures and break during pyrolysis, leading to the formation of different products such as acids, alcohols, gases, char, and anhydrosugars. Among these products, levoglucosan and furans are mainly generated through cleavage of glycosidic bonds (Hu and Gholizadeh, 2019).

Several reactions occur simultaneously during cellulose pyrolysis, including dehydration, depolymerization, decarboxylation, fragmentation, and isomerization. Pyrolytic decomposition generally follows two reaction pathways. Low temperatures and slow heating rates promote dehydration, producing active cellulose or anhydrocellulose. Higher temperatures and rapid heating rates favor depolymerization, resulting mainly in levoglucosan and tar formation, along with gases and char. Char formation occurs through condensation reactions, whereas depolymerization involves the breakdown of polymer chains into smaller compounds (Figure 3). Understanding these mechanisms is essential for improving reactor design, optimizing bio-oil yield, and advancing commercial biomass conversion technologies (Yang et al., 2020a).

4.2 Mechanism of hemicellulose pyrolysis

Hemicellulose pyrolysis is a complex thermochemical process influenced by its amorphous and highly branched structure. Hemicellulose surrounds cellulose fibers and consists of different saccharides such as xylose, mannose, arabinose, galactose, and glucose. Unlike cellulose, which possesses a crystalline structure, hemicellulose decomposes more easily because its amorphous chains require less energy to break. Thermal degradation of hemicellulose mainly occurs within 220 °C–315 °C, while the maximum weight loss is generally observed near 310 °C. During this stage, significant formation of CO₂ and char takes place (Zadeh et al., 2020). Pyrolysis begins with the dissociation of side chains and cleavage of glycosidic bonds, followed by opening of the xylopyranose rings. Studies on xylan, a major hemicellulose component, indicate that depolymerization and ring-opening reactions start around 200 °C. Increasing temperature intensifies decarbonylation, dehydration, and fragmentation reactions, leading to the production of furfural, anhydrosugars, low molecular weight compounds, CO, CO₂, CH₄, and condensable vapors. Further heating between 450 °C and 600 °C promotes dehydrogenation and polycondensation, producing fused-ring aromatic char structures along with higher H₂ and CH₄ yields (Figure 4) (Yang et al., 2007; Yang et al., 2020b).

Reaction pathways of hemicellulose are strongly influenced by functional groups such as acetyl, methoxy, and carboxyl substituents. Computational studies using density functional theory have shown that ring-opening reactions possess lower activation energies than ring-contraction or elimination reactions, making them kinetically favorable. The presence of functional groups also accelerates decomposition by lowering reaction barriers. Investigations in hydrogen plasma further revealed that active hydrogen radicals enhance dehydrogenation reactions and alter product distribution (Dai et al., 2021). Advanced analytical techniques, including TG-FTIR, Py-GC/MS, in situ spectroscopy, and quantum chemical calculations, have significantly improved the understanding of hemicellulose pyrolysis. Such mechanistic insights are essential for optimizing biomass conversion, improving bio-oil quality, and designing efficient thermochemical reactors for sustainable biofuel and chemical production (Wang et al., 2017).

4.3 Mechanism of lignin pyrolysis

Lignin is a highly branched natural polymer composed of phenylpropane units linked mainly through carbon–carbon and ether bonds. Approximately 20–30 wt.% of wood consists of lignin, making it an abundant renewable source for aromatic hydrocarbons and biofuel production. Pyrolysis is considered one of the most effective thermochemical methods for converting lignin into valuable chemicals, bio-oil, gases, and bio-char. The complexity of lignin structure, however, makes its pyrolysis behavior highly dependent on lignin origin, extraction method, and functional group distribution (Zadeh et al., 2020).

Pyrolysis of lignin proceeds through a series of radical reactions, bond cleavages, rearrangements, and eliminations. Initial degradation generally occurs between 160 °C and 330 °C, where lignin linkages begin to break, releasing light gases such as CO and CO₂. Simultaneously, polymerization reactions may occur, producing larger molecular fragments. As temperature increases from 200 °C to 330 °C, cracking reactions dominate and generate monomers, dimers, and trimers in bio-oil. Structural softening of lignin also occurs during this stage, leading to changes in surface morphology and functional groups. Above 330 °C, intense pyrolysis takes place with extensive decomposition of oxygen-containing groups and formation of lighter aromatic compounds (Figure 5) (Li et al., 2020). Depolymerization mainly occurs through homolytic cleavage and concerted decomposition pathways. Model compounds such as diphenyl ether and guaiacol are commonly used to study these mechanisms because they represent major lignin linkages and subunits. Radical intermediates, including phenoxy, phenyl, and hydroxyphenoxy radicals, are formed during decomposition. Subsequent recombination and rearrangement reactions influence final product selectivity. Temperature, pressure, inorganic species, and degree of substitution significantly affect radical stability and the distribution of pyrolysis products (Leng et al., 2022).

5 Physicochemical properties of biochar

Biochar possesses diverse physicochemical properties that determine its environmental performance, adsorption behavior, nutrient retention, and long-term stability. Feedstock composition and pyrolysis temperature strongly influence these properties because thermal degradation changes the carbon structure, pore system, mineral composition, and surface chemistry of biochar (Wang et al., 2015). Biochar produced from woody biomass generally contains higher fixed carbon and larger surface area, whereas grass and manure-derived biochars contain greater ash and mineral fractions. Pyrolysis at elevated temperatures between 600 °C and 900 °C promotes carbonization and aromatic condensation that increase biochar stability and recalcitrance (Zhang et al., 2015; Zhao et al., 2013).

Surface area and porosity are among the most important characteristics of biochar because they control adsorption capacity, ion exchange, and water retention. High-temperature biochars develop extensive microporous structures due to progressive removal of volatile compounds and decomposition of cellulose and hemicellulose. Micropores smaller than 1 nm become more abundant with increasing pyrolysis temperature and improve the adsorption of gases and dissolved contaminants (Fu et al., 2011; Suliman et al., 2016). Biochars with highly interconnected pores exhibit superior fluid transport and contaminant diffusion because pore connectivity facilitates the movement of water and ions through the carbon matrix. Douglas fir and pine-derived biochars possess larger pore volumes and higher water uptake because of greater intrinsic porosity. Freeze–thaw cycling can alter pore structure and particle morphology by reducing grain size and increasing intraporosity, particularly in plant-derived biochars (Suliman et al., 2016).

Elemental composition significantly affects biochar functionality and environmental applications. Carbon content generally increases with pyrolysis temperature, whereas hydrogen, oxygen, nitrogen, and sulfur contents decline because dehydration and decarboxylation reactions remove volatile molecules such as CO₂, CO, CH4, and H2O (He et al., 2018). Low H/C and O/C ratios indicate a highly aromatic and stable carbon structure with enhanced resistance to microbial degradation. Biochars produced above 500 °C often possess O/C ratios between 0.02 and 0.05, which are favorable for long-term carbon sequestration (Usman et al., 2015). Fixed carbon concentration increases with temperature due to progressive elimination of labile organic compounds. High aromaticity also improves thermal stability and persistence in soil systems.

Surface functional groups contribute significantly to cation exchange, hydrophilicity, nutrient retention, and metal binding. Carbonyl, hydroxyl, carboxyl, and phenolic groups are abundant in low-temperature biochars because incomplete thermal decomposition preserves oxygen-containing functionalities. Fourier-transform infrared spectroscopy studies reveal the gradual disappearance of hydroxyl and carbonyl groups with increasing pyrolysis severity due to condensation reactions and aromatic restructuring. Surface oxygenated groups regulate electrostatic attraction and surface complexation mechanisms during the adsorption of arsenic and lead ions (Wang et al., 2015). Carboxyl and phenolic groups also influence interaction with nutrients and metals because these acidic functional groups contribute to cation exchange capacity and polarity. Acidic and oxidative modifications can further enhance surface functional groups and increase adsorption performance (Ghorbani et al., 2024).

Biochar pH and electrical conductivity are highly dependent on ash composition and pyrolysis conditions. Alkaline minerals such as K, Ca, Mg, and Na become concentrated during pyrolysis because organic matter decomposes while mineral phases remain in the char matrix. High-temperature biochars therefore exhibit elevated pH and electrical conductivity values due to increased ash alkalinity (Suliman et al., 2016). Poultry litter and turkey litter biochars contain exceptionally high ash and electrical conductivity because manure feedstocks possess abundant inorganic nutrients. High pH biochars can neutralize acidic soils and improve nutrient availability in agricultural systems (Biederman and Harpole, 2013).

Adsorption capacity is closely linked to surface area, pore structure, mineral composition, and functional groups. Biochars produced at elevated temperatures generally demonstrate greater adsorption efficiency because higher porosity and aromatic carbon networks provide more active sorption sites. Electrostatic interaction, precipitation, ion exchange, and surface complexation contribute to the adsorption of Pb(II), As(V), and other contaminants. Acidic and H₂O₂ modifications can increase specific surface area by more than 50% and significantly improve pollutant removal efficiency (Ghorbani et al., 2024).

Hydrophobicity and water holding capacity are additional important properties that affect soil applications. Low-temperature biochars often exhibit hydrophobic surfaces that reduce water uptake because residual volatile compounds coat pore surfaces. High-temperature biochars become more hydrophilic as aromatic condensation removes waxes and volatile organics (Wang et al., 2015). Porous biochars can store water up to 400 wt.% depending on particle size distribution, pore geometry, and hydrophilic surface chemistry. Biochar addition therefore improves soil hydrology, water retention, and microbial activity in agricultural ecosystems (Biederman and Harpole, 2013).

Physicochemical properties ultimately determine the multifunctional applications of biochar in environmental remediation, soil amendment, and carbon sequestration. Feedstock type, pyrolysis temperature, and post-treatment modifications collectively regulate carbon stability, porosity, pH, cation exchange capacity, and adsorption efficiency. Optimizing these parameters enables production of engineered biochars tailored for specific agricultural, industrial, and environmental applications (Khan et al., 2026).

6 Application of biochars

Biochar, produced from biomass through thermochemical conversion under limited oxygen, supports circular economy and carbon-neutral strategies. Life cycle assessment studies indicate that, despite variations in feedstocks and production technologies, biochar consistently demonstrates strong potential for carbon sequestration, resource efficiency, climate change mitigation, and long-term environmental and economic sustainability (Carvalho et al., 2022).

6.1 Carbon sequestration

Carbon sequestration through biochar has emerged as a promising strategy for mitigating climate change while improving environmental sustainability. Biochar is produced through the pyrolysis of biomass residues, and its stable carbon structure enables long-term storage of atmospheric carbon in soils. Large-scale studies on olive tree pruning residues in Spain demonstrated that optimized biochar production at 650 °C with a 15-min residence time maximized biochar stability, yield, and carbon retention. A pyrolysis plant processing 6.5 tonnes of biomass per hour was found to be energy self-sufficient and capable of permanently removing around 2.68 tCO₂e per tonne of biochar produced. Annual carbon removal potential reached approximately 24,450 tCO₂e, highlighting the feasibility of industrial-scale deployment. Economic analysis further revealed that biochar systems can be profitable under current carbon pricing and biochar market conditions (Fawzy et al., 2022).

Country-level assessments also indicate substantial sequestration potential. Conversion of crop residues into biochar in China showed that more than 920 kg CO₂e could be sequestered from every tonne of biomass, with national sequestration potential estimated at 0.50 Pg CO₂e annually (Yang Q. et al., 2021). Environmental benefits extend beyond carbon storage, including reductions in soil and water acidification and protection of aquatic ecosystems. Potassium-enriched biochar has also shown enhanced sequestration efficiency, increasing carbon storage potential by up to 45% while improving nutrient content for agricultural use (Mašek et al., 2019). Field experiments further confirmed that biochar amendments reduce greenhouse gas emissions, increase soil organic carbon, and improve crop productivity. Application rates around 30 t ha⁻¹ significantly lowered CO₂, CH₄, and N₂O emissions while enhancing soil carbon sequestration and corn yields. Biochar, therefore, represents an effective and sustainable negative-emission technology with strong environmental and economic potential (Yang W. et al., 2020).

Biochar contributes to carbon sequestration primarily through the conversion of labile biomass carbon into highly stable aromatic carbon structures during pyrolysis. Pyrolysis promotes dehydration, decarboxylation, and condensation reactions that transform cellulose, hemicellulose, and lignin into polyaromatic carbon networks with low H/C and O/C ratios. These condensed carbon structures resist microbial decomposition and remain stable in soil for decades to centuries. Biochar also enhances soil organic carbon stabilization by promoting organo-mineral associations and physical protection of carbon within soil aggregates. Furthermore, biochar reduces carbon losses through decreased mineralization of native soil organic matter and improved nutrient retention that supports greater plant biomass production and subsequent carbon inputs into soil. These mechanisms collectively contribute to long-term atmospheric CO₂ removal and climate change mitigation (Li and Tasnady, 2023; Yaashikaa et al., 2020).

6.2 Mitigation of greenhouse gas

Greenhouse gas (GHG) emissions from agricultural activities pose a major challenge to global food security and environmental sustainability. Rising concentrations of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) negatively affect crop productivity and accelerate climate change. Biochar has emerged as a promising soil amendment for mitigating these emissions while simultaneously improving soil health and agricultural resilience. Extensive research conducted across different agroecosystems highlights the significant role of biochar in reducing GHG emissions through physical, chemical, and biological mechanisms (Adekiya et al., 2026).

Meta-analytical evidence from numerous studies conducted between 2000 and 2024 demonstrates that biochar significantly alters soil microbial processes linked to GHG production. Application of biochar increased the abundance of amoA and nosZ functional genes while reducing nirS gene abundance, resulting in a notable decline in N₂O emissions. Improved soil aeration, enhanced oxygen diffusion, and pH buffering created favorable conditions for nitrification over denitrification, thereby suppressing N₂O formation. Reduced CH₄ emissions were associated with inhibition of methanogenic microorganisms and stimulation of methane oxidation processes. Stable carbon present in biochar also promoted long-term carbon sequestration, contributing to lower atmospheric CO₂ concentrations (Bai et al., 2025).

Improvement in soil properties further strengthened the mitigation potential of biochar. Increased microbial biomass carbon, nitrogen, and phosphorus enhanced nutrient cycling and microbial diversity. Better soil structure, reduced bulk density, and improved moisture retention created healthier conditions for plant growth and soil microbial activity. Emission reductions were particularly effective when biochar and nitrogen fertilizer were applied at moderate rates. Climatic and regional conditions also influenced biochar performance, with stronger CH₄ mitigation observed in warmer southern paddy systems and greater N₂O reduction in cooler northern regions affected by freeze–thaw cycles (Al-Ismaily et al., 2026; Zhang et al., 2025).

Biochar mitigates greenhouse gas emissions through several interconnected mechanisms. Biochar improves soil aeration and oxygen diffusion because of its porous structure, thereby suppressing anaerobic conditions that favor methane-producing methanogenic microorganisms. Biochar surfaces also provide habitats for methane-oxidizing bacteria that convert CH₄ into CO₂. Reduction of N₂O emissions occurs through changes in nitrogen cycling pathways. Biochar increases soil pH and stimulates populations of microorganisms carrying the nosZ gene, which enhances the conversion of N₂O to harmless N₂ gas during denitrification. Biochar further adsorbs ammonium (NH₄ ⁺) and nitrate (NO₃ ⁻), reducing nitrogen losses and limiting substrates available for N₂O formation. Stable carbon storage within biochar additionally lowers net CO₂ emissions by preventing rapid oxidation of biomass carbon (Adekiya et al., 2026; Al-Ismaily et al., 2026; Ngaba et al., 2026).

6.3 Improve microbial growth in soil

Biochar application plays an important role in improving microbial growth and soil quality, especially in contaminated and low-organic-matter soils. Its porous structure enhances the retention of macro-, micro-, and nano-plastics, which supports the remediation of polluted soils and sediments. Biochar also promotes the activity of beneficial microorganisms such as Nevskia, Ralstonia, Gemmatimonas, and Lysobacter, which help degrade polycyclic aromatic hydrocarbons and phthalate esters. Enhanced microbial activity contributes to the removal of organic pollutants and improves overall soil health (Kapoor et al., 2024; Yang Y. et al., 2021).

Biochar properties, including pore structure, surface charge, pH, and mineral composition, vary according to feedstock type and pyrolysis temperature. These characteristics strongly influence nutrient availability and microbial composition in soils and sediments. Studies on calcareous soils showed that corn residue biochar produced at 200 °C significantly increased soil organic carbon, nitrogen, phosphorus, potassium, microbial biomass carbon, and enzyme activities such as catalase and dehydrogenase. Improved nutrient availability created favorable conditions for microbial respiration and biological activity (Karimi et al., 2020).

Biochar also serves as an effective carrier for plant growth-promoting bacteria by providing suitable pore spaces, moisture retention, and a stable habitat for microbial colonization. Its physical characteristics, including a large surface area and water-filled pores, improve the survival and activity of beneficial microbes in soil environments. Studies have shown that biochar produced from pine wood at higher pyrolysis temperatures effectively maintains bacterial populations without affecting important microbial functions such as auxin synthesis and ACC deaminase activity. Careful selection of feedstock type and pyrolysis conditions is therefore essential for enhancing microbial growth, improving soil fertility, and reducing potential phytotoxic effects in agricultural ecosystems (Hale et al., 2015; Ruzickova et al., 2021).

Biochar improves soil quality through physical, chemical, and biological mechanisms. Biochar porosity decreases soil bulk density and increases water-holding capacity, thereby improving root growth and drought tolerance. Biochar surfaces contain oxygen-containing functional groups such as carboxyl (-COOH), hydroxyl (-OH), and carbonyl (-C=O), which increase cation exchange capacity and nutrient retention. Biochar also acts as a reservoir for nitrogen, phosphorus, potassium, calcium, and magnesium, reducing nutrient leaching. Biochar pores provide protected microhabitats for beneficial microorganisms, including rhizobacteria, mycorrhizal fungi, and nutrient-cycling microbes. Enhanced microbial colonization stimulates enzymatic activities involved in carbon, nitrogen, and phosphorus cycling. These combined effects improve soil fertility, nutrient use efficiency, microbial diversity, and crop productivity (Nepal et al., 2023; Zhu Z. et al., 2025).

6.4 Waste water treatment

Wastewater treatment is becoming increasingly important because of the continuous release of toxic chemicals, antibiotic residues, dyes, and biological contaminants into aquatic environments. Conventional technologies such as disinfection, advanced oxidation, and activated carbon treatment are effective, but their high operational costs limit large-scale use in developing regions. Biochar-based treatment has emerged as a sustainable and economical alternative due to its strong adsorption capacity, porous structure, and reusability. Sewage-sludge biochar produced through pyrolysis demonstrated a DNA adsorption capacity of 1 mg g^-1, which was significantly higher than that of iron-oxide-coated sand with 0.2 mg g^-1. Research also reported removal efficiencies of 85% for antibiotic resistance genes and 97% for total environmental DNA using sewage-sludge biochar (Calderón-Franco et al., 2021).

Microbial immobilization technology combined with biochar further improves wastewater remediation by enhancing microbial activity, stability, and pollutant degradation. Rice straw biochar, nitrogen-doped biochar, and date palm biochar have shown promising performance in removing heavy metals, organic pollutants, antibiotics, and phenol from wastewater (Foong et al., 2022; Kasera et al., 2022). Date palm biochar prepared at 600 °C achieved 64% phenol removal with an adsorption capacity of 15.93 mg g^-1. Modified biochars, including acid-treated and magnetic biochars, also improve adsorption efficiency. Despite these advantages, challenges related to regeneration, large-scale application, and secondary pollution still require further investigation for sustainable wastewater management (Fseha et al., 2023).

Biochar removes contaminants from wastewater through adsorption, ion exchange, pore filling, electrostatic attraction, surface complexation, and precipitation mechanisms. Large surface area and well-developed pore networks facilitate physical adsorption of dyes, pharmaceuticals, pesticides, and organic pollutants. Oxygen-containing functional groups such as hydroxyl, carboxyl, and phenolic groups form complexes with heavy metal ions including Pb²⁺, Cd²⁺, Cu²⁺, and Cr⁶⁺. Mineral components present in biochar can promote precipitation reactions that immobilize toxic metals. Electrostatic interactions between charged biochar surfaces and dissolved pollutants further enhance removal efficiency. Modified biochars containing iron oxides, magnetic nanoparticles, or nitrogen functional groups exhibit improved adsorption capacity through additional catalytic and redox reactions. Biochar also serves as a support matrix for biofilm development, which enhances microbial degradation of organic contaminants and antibiotics (Haider et al., 2022; Qin et al., 2026).

7 Environmental risks and safety considerations of biochar

Biochar produced from contaminated feedstocks such as sewage sludge, animal manure, municipal biosolids, and industrial organic wastes may contain undesirable contaminants that could limit its environmental application. Heavy metals, including Cd, Pb, Cr, Ni, Cu, Zn, and As, can become concentrated during pyrolysis because organic matter is volatilized while inorganic elements remain in the solid fraction. Although pyrolysis often reduces metal mobility through mineral stabilization, excessive concentrations may still pose ecological and human health risks after soil application (Oni et al., 2019; Tian et al., 2025). Table 3 represents the risk assessment and quality control framework for biochar produced from potentially contaminated feedstocks.

TABLE 3. Risk assessment and quality control framework for biochar produced from potentially contaminated feedstocks.

Persistent organic pollutants (POPs), polycyclic aromatic hydrocarbons (PAHs), dioxins, pharmaceuticals, and antibiotic residues can occur in biochar depending on the raw material used and the pyrolysis conditions. Low-temperature or incomplete pyrolysis may increase the formation and retention of harmful compounds such as PAHs. Biochar produced from animal manure may also contain leftover veterinary antibiotics and high salt levels, which can negatively affect soil microorganisms and plant growth if used excessively. Livestock manure often contains heavy metals like copper, zinc, and cadmium because of feed additives and intensive farming practices. If not managed properly, these metals can accumulate in soil and harm ecosystems. Pyrolysis offers an effective solution by reducing pollutants, recovering energy, recycling nutrients, and stabilizing heavy metals. It can also treat wastewater sludge contaminated with POPs, achieving high removal efficiencies while producing useful products such as biochar, bio-oil, and biogas. Proper control and regulation make pyrolysis a sustainable waste management option (Vera-Loor et al., 2026; Zhao et al., 2026).

Biochar produced from sewage sludge and waste biomass is widely used for environmental remediation, wastewater treatment, and carbon sequestration because of its porous structure, high surface area, and strong pollutant adsorption capacity. However, its environmental safety depends on feedstock quality and production conditions. A key concern is the accumulation of heavy metals in sludge-based biochar, which can pose risks to soil and water systems. Studies show that co-pyrolysis of sewage sludge with biomass helps stabilize heavy metals, reducing their mobility and improving biochar properties such as surface area and adsorption performance (Embaye et al., 2025).

Another important issue is the formation of polycyclic aromatic hydrocarbons (PAHs) during pyrolysis. High levels of toxic PAHs are mainly associated with poor reactor design, uneven heating, and vapor condensation rather than feedstock type alone. Proper process control and reactor design can significantly reduce PAH contamination and improve biochar safety (Buss et al., 2022). Comprehensive risk assessment combining chemical characterization, contaminant leaching tests, ecotoxicological evaluation, and long-term field monitoring is necessary before large-scale deployment. Such measures can ensure that biochar contributes to sustainable waste valorization, environmental remediation, and carbon sequestration without creating secondary environmental pollution.

8 Conclusion and future perspectives

Biochar has emerged as a promising strategy for achieving carbon neutrality and supporting sustainable environmental management through carbon sequestration, soil improvement, waste valorization, and circular economy practices. Its diverse physicochemical properties enhance soil structure, water retention, nutrient availability, and crop productivity while also reducing greenhouse gas emissions and promoting long-term carbon storage. Biochar production from agro-forest and industrial residues further supports waste-to-energy approaches and aligns with global sustainable development goals. In addition, its applications in wastewater treatment and environmental remediation highlight its growing industrial significance. Economic opportunities through carbon trading systems, subsidies, and reduced dependence on fertilizers and irrigation inputs may further encourage large-scale adoption of biochar technologies.

However, biochar cannot be considered a universal solution for all climate and agricultural challenges. Several studies have reported adverse effects associated with certain feedstocks and production conditions, including the accumulation of heavy metals, polycyclic aromatic hydrocarbons, volatile organic compounds, and persistent free radicals. Negative impacts on soil microorganisms, earthworms, and crop growth under specific conditions also indicate the need for cautious application. Variations in biochar performance across soil types, climatic regions, and agricultural systems demonstrate that its effectiveness is highly site-specific and influenced by feedstock composition, pyrolysis conditions, aging behavior, and soil interactions.

Future research should therefore focus on long-term field-scale investigations, eco-toxicological risk assessment, and the optimization of biochar production technologies for specific applications. Advanced approaches such as co-pyrolysis, microwave-assisted modification, machine learning, life cycle assessment, and techno-economic analysis can improve biochar efficiency and sustainability. Greater collaboration among researchers, policymakers, industries, and farmers is essential to promote awareness, establish transparent carbon policies, and strengthen international cooperation. Such integrated efforts will support the development of safe, economically viable, and high-performance biochar systems for achieving carbon neutrality and sustainable environmental development.

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Footnotes

Edited by: Lakhan Kumar, Maulana Azad National Institute of Technology, India

Reviewed by: Vishal Rajput, Swami Rama Himalayan University, India

Van Anh Thai, Hutech University, Vietnam