Biochar is a carbon-rich and stable material produced through the pyrolysis of biomass under oxygen-limited or anoxic conditions. As a green resource, biochar shows great promise in agriculture, environmental remediation, waste management, and climate change mitigation. Its performance and application effectiveness are largely determined by its chemical and physical properties. The following sections discuss the key properties of biochar and their functional mechanisms.
Biochar is made up of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and more. Carbon (C) is typically dominant (50%–95%), serving as the core of its stability. The elemental composition is significantly affected by biomass type and pyrolysis temperature. For example, woody biomass-derived biochar generally has a higher carbon content and greater stability, whereas herbaceous or manure-derived biochar contains more ash and nutrients.
The H/C ratio is a critical indicator of the degree of carbonization in biochar. It reflects the proportion of hydrogen to carbon atoms as well as the degree of aromaticity of the carbon skeleton. With increasing pyrolysis temperature, the H/C ratio usually decreases. A lower H/C ratio indicates a higher proportion of aromatic rings, higher carbonization degree, greater structural stability, and stronger resistance to microbial degradation.
The specific surface area and pore structure of biochar are key features for its adsorption and water retention capabilities. The larger the specific surface area, the stronger the adsorption capacity of biochar, making it widely applicable in pollutant removal, fertilizer carrier applications, and water retention.
Biochar surfaces contain a variety of functional groups, such as carboxyl (–COOH), phenolic hydroxyl (–OH), and aldehyde (–CHO) groups. The presence of these functional groups influences the chemical reactivity, hydrophilicity, and adsorption capacity of biochar.
The types and quantities of these functional groups depend on factors such as feedstock type, pyrolysis temperature, and duration. Consequently, different biochars may vary in their functions and effects.
The adsorption capacity of biochar is a key chemical property. It supports its wide use in environmental protection and pollution control.
The pH value of biochar is generally alkaline (pH 7–12). This is mainly because the pyrolysis process produces alkaline substances such as Ca, Na, and K.
The porous structure of biochar allows it to retain and gradually release water, enhancing soil water-holding capacity.
The nutrient content of biochar affects its ability to support plant growth and soil fertility. Biochar can contain varying levels of macronutrients like NPK. It may also include trace amounts of micronutrients such as Cu, Mn, and Zn. All are essential for plant health.
The interaction between biochar and soil microorganisms determines how effectively nutrients are retained, organic matter is decomposed, and soil fertility is maintained.
Cation exchange capacity (CEC) measures the ability of biochar to retain and exchange positively charged ions such as calcium, magnesium, and potassium. Biochar can enhance the CEC of soil, increasing nutrient retention (preventing leaching) and providing plants with a reliable nutrient supply over time.
The key characteristics of biochar, such as the hydrogen-to-carbon ratio, pH value, specific surface area, pore structure, stability, and mineral composition, determine its effectiveness in different fields. With ongoing research, biochar, as a sustainable resource, has demonstrated great potential in agriculture, environmental protection, and climate change mitigation. In the future, enhancing biochar’s performance and expanding its applications will rely on adjusting pyrolysis conditions, optimizing feedstock selection, and improving post-treatment technologies.