Biochar is widely recognized as a promising material for improving soil health, cleaning contaminated water, and storing carbon. But scientists are increasingly focusing on a less visible feature inside biochar that may determine both its benefits and risks: persistent free radicals.
A new study led by researchers at Dalian University of Technology reveals how a common chemical treatment using phosphoric acid fundamentally changes how these free radicals form and behave. The findings offer a pathway to design safer and more effective biochar for environmental applications.
Persistent free radicals are long-lived, highly reactive species that can last from hours to months. They play a dual role. On one hand, they can help break down pollutants and support redox reactions. On the other hand, they may pose environmental and health risks due to their stability and reactivity.
“Controlling persistent free radicals is essential if we want to fully unlock the potential of biochar while minimizing its risks,” said the study’s corresponding author. “Our work shows that phosphoric acid can be used as a powerful tool to regulate these radicals in a predictable way.”
The researchers systematically investigated how phosphoric acid affects free radicals during biochar production under different temperatures and conditions. They found a striking temperature-dependent effect.
At lower temperatures below 500 degrees Celsius, phosphoric acid significantly increased the formation of free radicals. It does this by accelerating the breakdown of biomass and promoting chemical reactions that generate radical precursors.
However, at higher temperatures above 500 degrees Celsius, the effect reverses. Instead of promoting radicals, phosphoric acid helps reduce them. This happens because it enhances the rearrangement of carbon structures and facilitates reactions that neutralize existing radicals.
This “switching” behavior provides a practical way to tune biochar properties depending on the desired application.
The study also showed that phosphoric acid shifts the type of radicals present. It favors carbon-centered radicals over oxygen-centered ones, which are typically less stable. This transformation occurs because the chemical treatment promotes dehydration and structural changes in the carbon matrix.
In addition to temperature, the team found that the composition of the original biomass plays an important role. Materials rich in certain elements such as metals or cellulose influence how radicals form and respond to chemical modification. Different feedstocks can therefore lead to very different outcomes, even under similar processing conditions.
The researchers also identified optimal preparation conditions. Moderate phosphoric acid dosage and controlled temperatures can balance performance and cost while effectively regulating radical content.
Overall, the study provides new mechanistic insights into how biochar chemistry can be engineered at the molecular level.
“These findings give us a clearer roadmap for designing biochar materials tailored for specific environmental uses,” the researchers noted. “By controlling free radicals, we can enhance beneficial reactions while reducing potential risks.”
As biochar continues to gain attention as a climate and environmental solution, understanding and controlling its hidden chemistry will be key to its safe and effective deployment.
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Journal Reference: Gao, Y., Gao, Y. & Li, A. Effect mechanism of H3PO4 on the formation and transformation of persistent free radicals in biochar. Biochar 7, 28 (2025).
https://doi.org/10.1007/s42773-024-00405-3
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.
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