Biochar-DOM composites help lock cadmium in soil

The study found that the molecular weight of DOM strongly shaped remediation performance: biochar composites carrying high-molecular-weight DOM showed stronger Cd²⁺ adsorption, raised soil pH, reduced Cd bioavailability, and limited Cd uptake by Chinese cabbage. By shifting Cd from more mobile forms into more stable residual fractions, these composites offer a promising route for designing targeted soil amendments that can reduce heavy metal risks in food crops while supporting safer agricultural production.

Cadmium contamination is a persistent challenge in farmland because Cd is highly toxic, mobile under certain soil conditions, and readily absorbed by crops, creating potential risks to food safety and human health. Biochar is widely studied as a soil amendment because of its porous structure, aromatic carbon framework, and surface functional groups, while organic fertilizers can improve soil quality and crop productivity. However, organic fertilizer-derived DOM can either bind metals or increase their mobility, depending on its composition. The role of specific DOM molecular-weight fractions in biochar-assisted Cd immobilization has therefore remained unclear, making it necessary to examine how different DOM fractions influence Cd stabilization in contaminated soils.

A study (DOI:10.48130/aee-0026-0008) published in Agricultural Ecology and Environment on 06 May 2026 by Zhongzhen Liu’s & Yanhong Wang’s team, Guangdong Academy of Agricultural Sciences, shows that high-molecular-weight DOM-loaded biochar composites most effectively immobilize Cd by enhancing adsorption, increasing soil pH, transforming Cd into more stable fractions, and reducing crop Cd accumulation.

The researchers first produced biochar from pomelo branches and prepared two modified biochars through water washing and acid washing. DOM was extracted from a commercial bio-organic fertilizer and separated by ultrafiltration into three molecular-weight fractions: <3 kDa, 3–10 kDa, and >10 kDa. These fractions were then loaded onto the modified biochars to form six biochar-DOM composites. The team evaluated Cd²⁺ adsorption using isothermal adsorption experiments and applied Langmuir, Freundlich, and Temkin models to analyze adsorption behavior. Fourier-transform infrared spectroscopy (FTIR) was used to examine changes in functional groups before and after Cd adsorption. The adsorption results showed that DOM loading improved Cd²⁺ adsorption compared with biochar alone, and the enhancement generally increased with DOM molecular weight. The maximum adsorption capacity reached 84.25 mg·g⁻¹ in the acid-washed biochar loaded with >10 kDa DOM. Mechanistic analysis suggested that Cd immobilization mainly involved π-electron coordination with aromatic structures and complexation with oxygen-containing functional groups. High-molecular-weight DOM appeared especially effective because of its greater aromaticity and stronger ability to form stable Cd-associated structures. The researchers then conducted a 90-day soil incubation experiment using Cd-contaminated farmland soil collected near a mining area in Guangxi, China. All biochar-DOM composites increased soil pH and reduced available Cd. After 90 days, available Cd decreased by 70.70%–73.99%, while residual Cd increased by 28.82%–123.77%, indicating a shift from mobile to stable Cd forms. In pot experiments with Chinese cabbage, the composites also reduced soil available Cd and limited Cd movement into plants. The high-molecular-weight DOM treatments were particularly effective, reducing Cd in cabbage shoots by up to 74.46% and lowering plant enrichment factors and translocation factors, although the highest-molecular-weight treatments also showed some growth inhibition.

Overall, the study demonstrates that the molecular weight of organic fertilizer-derived DOM is a key factor controlling the performance of biochar-based Cd remediation materials. By combining biochar with selected DOM fractions, especially high-molecular-weight DOM, it may be possible to design more precise soil amendments for Cd-contaminated farmland. The authors note that further work should validate these findings through long-term field trials, explore DOM from diverse organic sources, and assess cost-effectiveness before large-scale agricultural application.

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References

DOI

10.48130/aee-0026-0008

Original Source URL

https://doi.org/10.48130/aee-0026-0008

Funding Information

This study was funded by the Young Scientists Fund of the National Natural Science Foundation of China (Grant Nos 42307343, 42207316), Guangxi Key Natural Science Foundation (Guangdong-Guangxi Joint Fund Project) (Grant No. 2022GXNSFDA080008), and the Provincial Science and Technology Project of Guangdong (Key Assignment of Rural Science and Technology Commissioner) (Grant No. KTP20240938).

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Agricultural Ecology and Environment (e-ISSN 3070-0639) is a multidisciplinary platform for communicating advances in fundamental and applied research on the agroecological environment, focusing on the interactions between agroecosystems and the environment. It is dedicated to advancing the understanding of the complex interactions between agricultural practices and ecological systems. The journal aims to provide a comprehensive and cutting-edge forum for researchers, practitioners, policymakers, and stakeholders from diverse fields such as agronomy, ecology, environmental science, soil science, and sustainable development.