Chuong Mary, Phan Kongkea, Irgum Knut, Skyllberg Ulf, Björn Erik
Department of Chemistry, Umeå University, Umeå SE-901 87, Sweden; Graduate School of Science, Royal University of Phnom Penh, Phnom Penh 12150, Cambodia.
Faculty of Science and Technology, International University, Phnom Penh 120801, Cambodia.
J Hazard Mater. 2025 Aug 15;494:138560. doi: 10.1016/j.jhazmat.2025.138560. Epub 2025 May 9.
Methylmercury (MeHg) can form through the microbial transformation of divalent inorganic mercury (Hg). However, it remains unknown whether the total concentration of Hg is a main controlling factor for this methylation process in paddy soils unaffected by local Hg point sources. Here we study the occurrence and controlling factors for MeHg levels in non-contaminated rice paddy soil in Cambodia using 164 soil and 100 overlying water samples from different provinces in wet and dry seasons. Paddy soils were characterized with respect to particle size classes, nutrients, and biogeochemical parameters expected to influence Hg processes. Total mercury (THg) and MeHg concentrations in the soils were not related to geographical location or sampling season but to soil physical and chemical properties. We observed significant positive relationships between the concentrations of divalent inorganic Hg (Hg) and MeHg, suggesting that the concentration of Hg is the main factor determining the net formation of MeHg in non-contaminated rice paddy soils. The %MeHg of THg was used as a proxy of the potential for MeHg formation and was significantly, and inversely, correlated with the redox conditions of the soils, as approximated by the oxidation state of sulfur. The study elucidates critical factors driving MeHg levels in rice paddy soil, enhances the understanding of the MeHg formation process and provides a refined basis for soil quality regulation regarding Hg. The results suggest that reducing Hg inputs to paddies will be effective to lower MeHg concentrations in the soil, ultimately reducing its presence in rice grains.
甲基汞（MeHg）可通过二价无机汞（Hg）的微生物转化形成。然而，在不受当地汞点源影响的稻田土壤中，汞的总浓度是否是这种甲基化过程的主要控制因素仍不清楚。在此，我们利用柬埔寨不同省份在雨季和旱季采集的164份土壤和100份上覆水样，研究了未受污染稻田土壤中甲基汞含量的分布及其控制因素。对稻田土壤的粒径分级、养分以及预期会影响汞过程的生物地球化学参数进行了表征。土壤中的总汞（THg）和甲基汞浓度与地理位置或采样季节无关，而是与土壤物理和化学性质有关。我们观察到二价无机汞（Hg）浓度与甲基汞浓度之间存在显著的正相关关系，这表明汞浓度是决定未受污染稻田土壤中甲基汞净形成的主要因素。甲基汞占总汞的百分比（%MeHg）被用作甲基汞形成潜力的指标，并且与土壤的氧化还原条件显著负相关，土壤氧化还原条件由硫的氧化态近似表示。该研究阐明了驱动稻田土壤中甲基汞含量的关键因素，增进了对甲基汞形成过程的理解，并为汞相关的土壤质量调控提供了更精确的依据。结果表明，减少汞向稻田的输入将有效地降低土壤中的甲基汞浓度，最终减少其在稻米中的含量。
