A new study shows that thiol-modified biochar can maintain long-term mercury immobilization in contaminated soils, even when repeated drying and rewetting mimic heatwave-accelerated weathering.
As heatwaves become more frequent, soils are increasingly exposed to rapid dry and wet cycles. These shifts can speed up mineral weathering and may reactivate toxic pollutants such as mercury, a persistent contaminant that can move through soil, enter food chains, and threaten ecosystems and human health. A new study published in Biochar reports that thiol-modified biochar, a sulfur-rich engineered carbon material, can help stabilize mercury in soil under these challenging conditions.
The research team investigated mercury-contaminated soils that had been treated with thiol-modified biochar, or TMB, and then subjected them to 30 simulated dry and wet cycles. The experiment was designed to represent soil weathering under increasingly common heatwave-related moisture fluctuations. The researchers examined mercury leaching, mercury bioavailability, mercury species transformation, soil mineral changes, organic matter release, acid rain resistance, and microbial community shifts.
"Our results show that thiol-modified biochar does more than simply bind mercury on its own surface," said corresponding author Yao Huang. "It also reshapes the soil environment, promotes mineral weathering, and helps redistribute mercury into less available and more stable forms. This gives us a clearer explanation of why the remediation effect can persist under repeated climate-related disturbances."
The study found that TMB strongly reduced mercury mobility under acid rain conditions. In soils treated with 0.4% TMB, leachable total mercury decreased by 82.3% after 30 dry and wet cycles, compared with only 7.1% in untreated control soil. TMB also lowered bioavailable mercury, which is the fraction more likely to be taken up by organisms or released into the environment.
A key mechanism was the TMB-driven transformation of soil minerals. The material promoted the dissolution of calcium carbonate, increased soil pH, and made soil particles more negatively charged, all of which favored mercury precipitation and adsorption. The team also found that TMB facilitated the conversion of iron and aluminum oxides into hydroxylated forms, such as FeO·OH and Al(OH)3, which provide stronger binding sites for mercury.
Importantly, TMB shifted mercury away from more bioavailable forms. In the 0.4% TMB treatment, the combined proportion of exchangeable and carbonate-bound mercury decreased by 89.7% compared with the control after 30 cycles. At the same time, mercury was redistributed toward oxide-bound and organic matter-bound fractions, which are less available and generally more stable in soil.
The material also reduced methylmercury accumulation. Methylmercury is a highly toxic form of mercury that can accumulate in organisms. In the 0.4% TMB treatment, total methylmercury stayed far below the control and declined during the dry and wet cycling process, suggesting that TMB may help suppress net methylmercury formation.
To further test durability, the researchers used continuous simulated acid rain leaching equivalent to long-term rainfall exposure. The 0.4% TMB treatment reduced total mercury release by 92.9% before cycling and 87.4% after 30 cycles, demonstrating strong persistence of mercury immobilization even after repeated weathering stress.
The study also revealed changes in the soil microbiome. TMB increased the relative abundance of groups such as Bacillales and Gemmatimonadales and improved microbial richness and evenness, suggesting that a "functional material and microorganism" system may contribute to sustained mercury stabilization.
Together, the findings provide a scientific basis for using thiol-modified biochar in large-scale remediation of mercury-contaminated soils, especially in regions facing frequent heatwaves, acid rain, and intensified dry and wet cycles.
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Journal Reference: Wang, Z., Zhang, L., Hu, H. et al. Redistribution of soil mercury species mediated by thiolated biochar under dry–wet cycles. Biochar 8, 90 (2026).
https://doi.org/10.1007/s42773-026-00608-w
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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.
