Soil contamination by heavy metals and organic pollutants has become a major environmental concern, stemming from industrialization, mining, urbanization, and excessive agrochemical use. These contaminants pose serious risks to human health by entering the food chain. Conventional soil remediation techniques are often costly and unsustainable, necessitating alternative solutions. A review by Ramijur Rahman and Kulendra Nath Das, published in the Journal of Soil, Environment & Agroecology, explores the potential of biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More as a soil detoxifier.

Biochar has garnered attention for its detoxification capabilities. Its high surface area, porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More, cation exchange capacity (CEC), and negative surface charge allow it to remove pollutants through adsorption, electrostatic interactions, and redox reactions. Beyond contaminant removal, biochar also enhances the physical, chemical, and biological properties of soil. The effectiveness of biochar in soil remediation is influenced by factors such as feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More type, pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More temperature, and application methods.

Physically, biochar is a fine-grained, porous material with a high surface area, which boosts its capacity to adsorb ions. Chemically, it is alkaline due to its high inorganic and ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content, with a pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More ranging from 6.5 to 9 depending on the pyrolysis temperature. Its high CEC allows it to develop both positive and negative surface charges, reducing the leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More of both cationic and anionic contaminants. Biochar’s high proportion of stable aromatic carbon structures contributes to its stability and sorption capacity. Biologically, biochar’s porous structure supports microbial colonization, enhancing nutrient cycling and soil enzyme activities.

Biochar improves soil health by increasing water holding capacityWater holding capacity is the amount of water that soil can retain. Biochar can significantly increase the water holding capacity of soil, improving its ability to withstand drought conditions and support plant growth.   More, porosity, structure, and aeration, creating an optimal habitat for microbial populations. This porous structure reduces runoff and enhances moisture infiltration, increasing available soil water. Its base cation composition (Na+, K+, Mg2+, and Ca2+) and ash content increase soil pH, especially in acidic conditions, through the dissolution of carbonates and hydroxides. Negatively charged functional groups on the biochar surface adsorb cations, boosting the soil’s CEC and immobilizing metal ions, which reduces their bioavailability. Biochar is also rich in essential nutrients (Ca, Mg, K, P, and N), which vary with feedstock and can reduce nutrient leaching. It stimulates root nodule formation and enhances nitrogen immobilization, promoting plant growth.

The detoxification mechanisms of biochar include adsorption, complexation, precipitation, ion exchange, electrostatic interaction, and redox potential. High-temperature pyrolysis tends to produce biochar with larger pore volume and surface area, enhancing van der Waals adsorption of heavy metal ions. Biochar increases soil pH and CEC, strengthening electrostatic attraction between soil particles and metal ions, thereby reducing their mobility and bioavailability. Its alkaline nature promotes hydrolysis of heavy metals, forming stable metal hydroxide precipitates. Biochar also introduces phosphates, carbonates, and sulfates, which react with metal ions to form stable precipitates. The aromatic structure of biochar enhances its stability and adsorption of non-polar organic pollutants like pesticides, polycyclic aromatic hydrocarbons (PAHs), and persistent organic pollutants (POPs). Biochar also supports microbial populations, enhancing bacterial diversity and increasing beneficial microbial groups, which are crucial for degrading contaminants and organic matter.

Biochar has proven effective in immobilizing heavy metals and reducing their bioavailability and plant uptake. For instance, 5% chicken dung biochar reduced copper (Cu) uptake in Oenothera picensis plants by 45.5% while increasing shoot and root biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More by 3.5 and 3.1 times, respectively. Sulfur-amended rice husk biochar increased Hg2+ adsorption by 73% and reduced freely available Hg by up to 99.3%. Sewage sludge biochar (SSB) lowered methylmercury (Me-Hg) and total Hg (THg) accumulation in rice tissues by 73.4% and 81.9%, respectively. Casuarina-derived biochar at 4% reduced various heavy metals in plant roots, including lead (Pb) by 85.0% and cadmium (Cd) by 25.7%. Cotton straw biochar reduced Cd accumulation in plant organs by 65-87%. Biochar also strongly inhibits the bioaccumulation of PAHs in plants, with straw biochar reducing PAH concentration by 70.3% in Salix viminalis.

Despite its potential, concerns remain regarding biochar’s long-term stability, aging effects, and potential release of toxic compounds like PAHs and volatile organic compounds (VOCs). The pyrolysis process can generate dioxins and heavy metals, posing risks to plant health and soil microorganisms. VOC content in biochar can range from 0.34 to 16000 μg/g depending on feedstock and pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More, and high levels can stifle plant germination and disrupt nutrient cycling. PAHs in biochar, formed during pyrolysis, can diffuse into the environment and harm microbial populations. Animal manure and sewage sludge biochars tend to be more enriched with heavy metals than plant-based biochars.

Strategies to mitigate biochar toxicity include using uncontaminated biomass (e.g., wood and agricultural residues), optimizing pyrolysis conditions at higher temperatures (500-700°C) to reduce PAH and VOC content, and post-treatment methods like washing and composting to further reduce contaminants. Improved industrial production and regulatory guidelines for PAHs, VOCs, and heavy metals in prepared biochar are also crucial for safe agricultural use.

Limitations of biochar include variable efficiency based on feedstock and pyrolysis, potential nutrient deficiency due to adsorption of nitrogen and phosphorus, and indirect impacts on soil microbial communities that may disrupt ecological balance. Biochar is also less effective at removing highly soluble pollutants like nitrates and certain pesticides, and its efficiency can be influenced by pH, ionic strength, and soil type. High production and application costs can also limit its large-scale implementation.

Future research needs to address the long-term effectiveness of biochar in field conditions, thoroughly examine environmental risks like leaching of toxic compounds, and develop advanced strategies such as biochar composites, functionalization, and microbial integration to enhance detoxification efficiency. Accurate application strategies and regulatory frameworks are also necessary to ensure safe and effective use.

Source: Rahman, R., & Das, K. N. (2025). The Potential of Biochar as a Soil Detoxifier: A Review. Journal of Soil, Environment & Agroecology, 5(1), 1-18.