Modified biochar locks up arsenic and cadmium

By combining sulfur species, ferrihydrite nanoparticles, and rice-straw biochar into one multifunctional material, the team created reactive surfaces capable of capturing both cationic Cd(II) and anionic As(III). The material achieved markedly higher adsorption capacities than pristine biochar, ferrihydrite, or ferrihydrite-modified biochar alone. In soil incubation tests, SFB also reduced the bioavailable fractions of Cd and As and shifted them into more stable residual forms.

Cadmium and arsenic pollution has become a major challenge for agricultural soil management, especially in paddy systems affected by mining, smelting, fertilizer use, and contaminated irrigation. These toxic metal(loid)s can move from soil into crops and then enter the food chain, creating ecological and health risks. Current remediation strategies often struggle because Cd and As behave very differently in soil: Cd mainly exists as a positively charged ion, whereas arsenic often occurs as mobile oxyanions whose behavior depends strongly on pH and redox conditions. Conventional amendments may capture one contaminant well but perform poorly against the other. Due to these problems, multifunctional materials are needed to stabilize both Cd and As under complex soil conditions.

A study (DOI:10.48130/aee-0026-0002) published in Agricultural Ecology and Environment on 28 February 2026 by Dan Liu’s team, Zhejiang A&F University, reports that sulfur-ferrihydrite-modified biochar can capture Cd(II) and As(III) through complementary precipitation, oxidation, ion-exchange, and surface-complexation mechanisms.

The researchers first produced rice-straw biochar by slow pyrolysis at 550 °C under oxygen-limited conditions. Ferrihydrite was then synthesized and loaded onto the biochar surface, followed by sulfur modification using an alkaline carbon disulfide reaction system. The final SFB material showed a rough, porous surface enriched with iron, sulfur, oxygen-containing groups, and Fe–S–C interfacial domains. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and related analyses confirmed that sulfur and ferrihydrite were distributed across the biochar surface and created multiple reactive binding sites. The team compared SFB with pristine biochar (BC), ferrihydrite (FH), and ferrihydrite-modified biochar (FB) in batch adsorption experiments. The adsorption behavior was strongly affected by pH, dosage, contact time, and initial contaminant concentration. SFB showed the strongest overall performance, with maximum adsorption capacities of 76.69 mg g⁻¹ for Cd and 8.28 mg g⁻¹ for As. Kinetic analysis showed that adsorption followed pseudo-second-order behavior, indicating that chemisorption played a major role. Isotherm analysis suggested that Cd adsorption was better described by the Langmuir model, while As adsorption was better fitted by the Freundlich model, reflecting differences in their binding behavior. Mechanistic analyses showed that Cd(II) was immobilized mainly through ion exchange, surface coordination, and the formation of CdS and FeS precipitates. In contrast, As(III) was partly oxidized to As(V) and then strongly retained through inner-sphere complexation with Fe–O functional groups. Soil incubation experiments further demonstrated the practical remediation potential of SFB. Compared with untreated control soil, SFB reduced bioavailable Cd by 32.59%–41.26% and bioavailable As by 50.06%–64.06%, while increasing their residual, less mobile fractions.

Overall, this study shows that coupling sulfur and ferrihydrite within a biochar matrix can overcome the difficulty of treating Cd/As co-contamination. Rather than relying on a single adsorption pathway, SFB works through a coordinated Fe–S–C interface that captures both contaminants in more stable forms. The findings support SFB as a promising soil amendment for agricultural land affected by mixed metal(loid) pollution, while also pointing to the need for longer-term studies on sulfur transformation, redox stability, leaching risk, and field-scale performance.

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References

DOI

10.48130/aee-0026-0002

Original Source URL

https://doi.org/10.48130/aee-0026-0002

Funding Information

This study was funded by the Natural Science Foundation of China (Grant Nos 32271532, 42407016), Zhejiang Provincial Natural Science Foundation of China (Grant No. MS26D010008), and Research and Development Fund of Zhejiang A&F University (Grant No. 2020LFR052).

About Agricultural Ecology and Environment

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.