article · Environment International
This study examines how various inorganic mercury compounds influence the generation of toxic methylmercury in paddy soils and its subsequent uptake across different parts of rice plants. Researchers tested soil contaminated with mercury chloride, dissolved organic matter-bound mercury, nano-particulate cinnabar, and two crystalline forms of mercury sulphide at two pollution concentrations against an uncontaminated control. Soil treated with mercury chloride produced the highest amounts of methylmercury and resulted in the greatest accumulation across rice roots, stalks, leaves, and grains. Other mercury forms, including sulphide and organic complexes, also mobilised and transformed into methylmercury to varying extents. Brown rice grown in soil with mercury chloride accumulated methylmercury at concentrations exceeding safe human health risk indices, demonstrating that specific geochemical forms of mercury directly dictate food contamination risks in agricultural settings.
Methylmercury is a potent neurotoxin that can accumulate in staple food crops such as rice. Understanding how different chemical forms of mercury behave in agricultural soils helps environmental managers accurately evaluate contamination risks. This knowledge allows public health bodies to identify which types of industrial or natural mercury pollution present the most immediate threat to the human food chain.
This research provides baseline geochemical data that can inform environmental testing services, soil remediation companies, and agricultural regulatory bodies evaluating contaminated farmland. While the findings improve environmental risk assessments and monitoring frameworks for mercury-laden sites, the study is early-stage fundamental research and does not present a market-ready product, technology, or mitigation tool.
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The formation of neurotoxic methylmercury (MeHg) in paddy fields and its accumulation by rice plants is of high environmental concern. The contribution of different geochemical mercury (Hg) pools in paddy soils to MeHg production and its accumulation by rice seedlings is not well-studied up to now. Therefore, we investigated the impact of different inorganic Hg forms, including HgCl<sub>2</sub>, nano-particulated HgS (nano-HgS), Hg bound with dissolved organic matter (Hg-DOM), β-HgS, and α-HgS, at levels of 5 mg Hg/kg soil and 50 mg Hg/kg soil, on the production of MeHg in the soil during rice growing season. Further, we studied the uptake of MeHg by the roots, stalks, leaves, and grains of rice in the tillering, panicle formation, and ripening growth stages, and compared these treatments to a non-polluted soil (control). MeHg contents in HgCl<sub>2</sub> polluted soil were the highest, and were 13.5 times and 36.1 times higher than control in 5 and 50 mg/kg Hg treatments, respectively. MeHg contents in α-HgS, β-HgS, nano-HgS, and Hg-DOM polluted soil were 3.9, 2.6, 2.4, and 1.7 times, and 4.4, 15.1, 6.7, and 10.9 times higher than control in 5 and 50 mg/kg Hg treatments, respectively, suggesting the mobilization and methylation of these Hg complexes. The ratio of MeHg to total Hg in the pore water (indication of methylation potential) in HgCl<sub>2</sub> and β-HgS treatments were higher than in Hg-DOM, α-HgS, and nano-HgS treatments. HgCl<sub>2</sub> treatment resulted in significantly higher MeHg contents in the root, stalk, leaf, and brown rice than nano-HgS, Hg-DOM, β-HgS, and α-HgS treatments both in 5 and 50 mg/kg Hg polluted soils. Rice grain in HgCl<sub>2</sub> treatment showed a potential hazard to human health, as indicated by high health risk index (HRI > 1) of MeHg. Current results improve our understanding of MeHg production in soil polluted with different Hg forms, and the assessment of human health risks from consumption of MeHg-laden rice grain at Hg polluted sites with different Hg forms in soils.
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DOI: 10.1016/j.envint.2019.04.068
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