Arsenic contamination in paddy soils threatens food safety because flooded conditions can mobilize the toxic element and make it available for uptake by rice. A new study suggests that biochar coated with chitosan, a natural material derived from chitin, could help trap arsenic in soil and prevent it from reaching the edible grain.
Researchers found that fresh chitosan-functionalized biochar reduced plant-available arsenic in contaminated soil by 21.1% and lowered arsenic concentrations in rice grains by 43.1% compared with untreated soil. Even after six months of natural aging, the modified biochar continued to reduce arsenic accumulation in roots and grains while supporting healthier root development.
"Our findings show that modifying biochar with chitosan can do more than simply capture arsenic in soil. It can also influence how arsenic is distributed within the rice plant and strengthen the plant's ability to tolerate contamination," said Hailong Wang, a corresponding author of the study. "The results also highlight why the natural aging of remediation materials must be considered when evaluating their long-term performance."
The study, published in the journal Biochar, compared four materials: fresh unmodified biochar, fresh chitosan-functionalized biochar, naturally aged unmodified biochar, and naturally aged chitosan-functionalized biochar. The aged materials were buried in soil under outdoor conditions for six months before being recovered and tested.
The researchers then added the materials to arsenic-contaminated soil and grew rice for 130 days under flooded greenhouse conditions. They examined soil chemistry, arsenic availability, rice growth, root structure, plant stress responses, and the distribution of arsenic within plant cells.
Fresh chitosan-functionalized biochar reduced arsenic concentrations in rice roots by 48.1%, while the naturally aged version produced a 37.1% reduction. Grain arsenic declined by 43.1% with the fresh material and 32.8% with the aged material.
By contrast, fresh and aged unmodified biochar increased the amount of arsenic available in soil. The researchers attributed this partly to their alkaline properties, which may increase soil surface charge and promote the release of arsenic from iron and aluminum minerals. The finding indicates that applying conventional biochar alone may not always reduce arsenic risks.
At the cellular level, chitosan-functionalized biochar redirected arsenic toward the cell walls of rice roots and leaves. This kept more arsenic away from soluble cellular fluids and sensitive organelles, where it could interfere with photosynthesis, respiration, and other essential processes.
Micro-X-ray fluorescence imaging also revealed a close spatial association between arsenic and iron, particularly in the aged chitosan-functionalized treatment. The results suggest that iron-rich coatings on rice roots and iron compounds accumulated on biochar surfaces may act as barriers that immobilize arsenic before it enters plant cells.
Although natural aging weakened some of the material's initial arsenic-binding capacity, the aged chitosan-functionalized biochar still promoted root length, surface area, diameter, and root tip formation. It also altered plant stress compounds and root-released organic acids, suggesting that its benefits extended beyond direct contaminant adsorption.
The researchers caution that the experiment was conducted in pots using a controlled greenhouse system and a relatively high biochar application rate. Longer field trials across different soils, climates, rice varieties, and management systems will be needed to assess durability, application costs, environmental safety, and practical scalability.
The findings provide a foundation for developing biochar amendments that remain effective after exposure to real soil conditions, potentially supporting safer rice production in arsenic-contaminated regions.
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Journal Reference: Li, M., Li, J., Xiong, H. et al. Chitosan-functionalized biochar modulates arsenic speciation, distribution, and stress tolerance during rice growth in contaminated paddy systems: role of natural aging. Biochar 8, 128 (2026).
https://doi.org/10.1007/s42773-026-00644-6
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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.
