In a recent review published in Next Materials, J.I. Mnyango and colleagues delve into the promising role 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 in sustainable wastewater treatment, particularly for removing synthetic dyes. The article highlights biochar’s favorable physicochemical properties, such as large 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, and functional groups, which enable effective dye adsorption. Traditional treatment materials like activated carbonActivated carbon is a form of carbon that has been processed to create a vast network of tiny pores, increasing its surface area significantly. This extensive surface area makes activated carbon exceptionally effective at trapping and holding impurities, like a molecular sponge. It is commonly More are often hindered by high costs and energy demands, making biochar an appealing, sustainable, and economically viable alternative.

Synthetic dyes, prevalent in industries such as textiles, packaging, and cosmetics, pose significant environmental and health risks due to their complex structures, toxicity, and resistance to biodegradation. The textile industry alone contributes to 60-70% of dye-laden effluents discharged into aquatic environments. Biochar, produced from agro-wastes, forestry residues, and municipal sludge through 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, offers a compelling solution. Its production typically involves heating 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 at 300 to 1000 °C in oxygen-limited environments, resulting in a carbonaceous material with properties ideal for binding a wide range of dye molecules.

The effectiveness of biochar in dye removal is governed by a combination of mechanisms, including electrostatic interactions, hydrogen bonding,π−π stacking, and pore filling. For instance, biochars derived from algae and municipal waste primarily use electrostatic interactions to remove cationic dyes like methylene blue, sometimes amplified by alkaline or physical activation that increases negative surface charge. Notably, Opuntia ficus-indica biochar activated with NaOH achieved an impressive 1341.0 mg/g adsorption capacity for Malachite green through ion exchange and electrostatic attraction. In contrast, materials dominated by single or less interacting mechanisms, such as municipal waste biochar (7.2 mg/g) and hickory chip biochar (9.2 mg/g), typically show more moderate capacities. This highlights that maximizing adsorption capacity requires a balance of functional groups and structural features that facilitate multiple simultaneous interactions.

Factors influencing dye removal efficiency are categorized into pre-process and process conditions. Physical treatments like grinding and sieving increase surface area and contact efficiency. Thermal pre-treatments, including pyrolysis, carbonization, and hydrothermal processing, significantly affect biochar’s adsorption capabilities by influencing its surface area and porosity. Chemical modifications using acidic, alkaline, or oxidizing agents further enhance porosity, reactivity, and selectivity towards specific pollutants. Kinetic studies consistently show that dye adsorption onto biochar predominantly follows pseudo-second-order kinetics, indicating that chemisorption, involving chemical interactions and electron exchange between dye molecules and biochar’s functional groups, is the rate-limiting step. This holds true across diverse biochar sources and dye types, including acid orange 7, methylene blue, and rhodamine B.

Economically, biochar presents a competitive edge over conventional adsorbents like activated carbon, primarily due to the abundance of its raw materials and significantly lower manufacturing and modification costs. Operating costs for biochar can range from $0.05 to $0.50 per gram, compared to activated carbon which often costs 5-10 times more. This cost advantage is further pronounced when biochar is sourced from waste streams, which also helps reduce disposal costs and supports regional economies. Modified biochars, while sometimes more expensive to produce, can offer enhanced performance, often outperforming standard activated carbon at a lower overall cost. For instance, aminated biochars can be four times more effective at adsorbing certain dyes while costing only 12.5% of activated carbon. This makes biochar a scalable and affordable solution, particularly for developing regions with limited wastewater treatment budgets.

The sustainable “biochar loop” involves using agricultural and food wastes to produce biochar for dye removal, which is then repurposed as dye-laden biochar for secondary uses like soil conditioning or integration into construction materials (e.g., bricks and concrete). This circular model promotes waste valorization and carbon sequestration, aligning with United Nations Sustainable Development Goals. Although challenges remain regarding long-term environmental behavior and regulatory acceptance, ongoing research aims to address these issues, solidifying biochar’s potential as a robust and eco-feasible solution for industrial dye-saturated wastewater treatment.

Source: Mnyango, J. I., Nyoni, B., Phiri, C., Fouda-Mbanga, B. G., Amusat, S. O., Maringa, A., … & Hlangothi, S. P. (2025). Sustainable wastewater treatment: Mechanistic, environmental, and economic insights into biochar for synthetic dye removal. Next Materials, 9, 100974.