article · Environment International
Biochar amendments are often proposed to reduce mercury accumulation in rice crops, yet their performance under fluctuating redox conditions has raised concerns. In laboratory microcosm experiments testing soil under dynamic redox potential, adding rice hull biochar broadened the redox range and promoted reduction-oxidation reactions. Rather than sequestering contaminants, the biochar increased the mobilisation of dissolved total mercury through the reductive dissolution of iron and manganese (hydr)oxides. Consequently, the availability of mercury rose, leading to increased methylmercury formation. The amendment also altered soil organic matter chemistry, with specific organic components such as lipids and alkylaromatics correlating positively with methylmercury concentrations. These results reveal unexpected environmental risks, demonstrating that biochar application under dynamic soil redox conditions can inadvertently enhance toxic mercury release and methylation instead of mitigating exposure risks.
Biochar is widely promoted as a soil additive to lower toxic metal uptake in staple crops. This research shows that in changing water and redox conditions typical of rice paddies, biochar can trigger the opposite effect by accelerating the production of harmful methylmercury. Agricultural managers must consider these biogeochemical risks before deploying biochar to protect food safety.
This laboratory-based microcosm study informs agricultural soil management and remediation practices. The findings primarily serve environmental regulators, soil amendment manufacturers, and agronomic advisors designing heavy metal mitigation programmes for paddy fields. Operating at an early research stage, the work highlights that commercial biochar deployments require rigorous risk assessments and field testing under variable hydrological and redox regimes before being marketed safely for mercury mitigation.
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Biochar amendment to paddy soils was promising to mitigate mercury (Hg) accumulation in rice; thus, it was applied to reduce human Hg exposure via rice consumption. However, how biochar affects Hg mobilization and MeHg formation in soil under changed redox potential (E<sub>h</sub>) conditions remained unknown. Here, we explored the change of dissolved total Hg (DTHg) and dissolved MeHg (DMeHg), and their controlling biogeochemical factors in a soil with(out) biochar amendment under changing E<sub>h</sub> conditions using biogeochemical microcosm. Biochar amendment resulted in a widen E<sub>h</sub> range (-300 to 400 mV) compared to the control (-250 to 350 mV), demonstrating that biochar promoted reduction-oxidization reactions in soil. Biochar amendment enhanced Hg mobilization by mediating reductive dissolution of Fe/Mn (hydr)oxides. Thus, the increased Hg availability promoted MeHg formation in the soils. Biochar amendment changed the soil organic matter (SOM) composition. Positive correlations between the relative abundance of LIPID (lipids, alkanes/alkenes), ALKYL (alkylaromatics), and suberin and MeHg concentrations indicate that these SOM groups might be related to MeHg formation. Biochar enhanced the releasing and methylation of Hg by promoting the mobilization of Fe(oxyhydr)oxides and alternation of carbon chemistry under dynamic E<sub>h</sub> conditions. There is an unexpected environmental risk associated with biochar application to paddy soils under dynamic E<sub>h</sub> condition, and one should be aware this risk when applying biochar aiming to minimize human Hg exposure health risks via rice consumption.
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DOI: 10.1016/j.envint.2022.107484
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