State Key Laboratory of Pollution Control and Resources Reuse, School of the Environment, Nanjing University, Nanjing, 210023, PR China.
Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, PR China.
Environ Pollut. 2016 Jun;213:232-239. doi: 10.1016/j.envpol.2016.02.021. Epub 2016 Feb 20.
There is increasing interest in understanding factors controlling methylmercury (MeHg) production in mercury-contaminated rice paddy soil. Sulfate has been reported to affect MeHg biogeochemistry under anoxic conditions, and recent studies revealed that selenium (Se) could evidently reduce MeHg production in paddy soil. However, the controls of sulfate and Se on net MeHg production in paddy soil under fluctuating redox conditions remain largely unknown. Microcosm experiments were conducted to explore the effects of sulfate and Se on net MeHg production in rice paddy soil. Soil was added with 0-960 mg/kg sulfate, in the presence or absence of 3.0 mg/kg selenium (selenite or selenate), and incubated under anoxic (40 days) or suboxic conditions (5 days), simulating fluctuating redox conditions in rice paddy field. Sulfate addition moderately affected soil MeHg concentrations under anoxic conditions, while reoxidation resulted in evidently higher (18-40%) MeHg levels in sulfate amended soils than the control. The observed changes in net MeHg production were related to dynamics of sulfate and iron. However, Se could inhibit sulfate-mediated MeHg production in the soils: Se addition largely reduced net MeHg production in the soils (23-86%, compared to the control), despite of sulfate addition. Similarly, results of the pot experiments (i.e., rice cultivation in amended soils) indicated that soil MeHg levels were rather comparable in Se-amended soils during rice growth period, irrespective of added sulfate doses. The more important role of Se than sulfate in controlling MeHg production was explained by the formation of HgSe nanoparticles irrespective of the presence of sulfate, confirmed by TEM-EDX and XANES analysis. Our findings regarding the effects of sulfate and Se on net MeHg production in rice paddy soil together with the mechanistic explanation of the processes advance our understanding of MeHg dynamics and risk in soil-rice systems.
人们越来越关注了解控制汞污染稻田中甲基汞（MeHg）生成的因素。据报道，硫酸盐会影响缺氧条件下的 MeHg 生物地球化学，最近的研究表明，硒（Se）可以明显减少稻田中 MeHg 的生成。然而，在波动的氧化还原条件下，硫酸盐和 Se 对稻田中净 MeHg 生成的控制作用仍知之甚少。本研究采用微宇宙实验，探讨了硫酸盐和 Se 对水稻土中净 MeHg 生成的影响。在添加 0-960mg/kg 硫酸盐的条件下，研究了添加或不添加 3.0mg/kg 硒（亚硒酸钠或硒酸钠）对缺氧（40 天）或亚缺氧（5 天）条件下净 MeHg 生成的影响，模拟了稻田中波动的氧化还原条件。硫酸盐添加适度影响缺氧条件下的土壤 MeHg 浓度，而再氧化导致添加硫酸盐的土壤中 MeHg 水平明显升高（比对照高 18-40%）。净 MeHg 生成的变化与硫酸盐和铁的动态有关。然而，Se 可以抑制土壤中硫酸盐介导的 MeHg 生成：尽管添加了硫酸盐，但 Se 的添加大大减少了土壤中的净 MeHg 生成（与对照相比减少了 23-86%）。盆栽实验（即在添加土壤中种植水稻）的结果也表明，在水稻生长期间，添加 Se 的土壤中 MeHg 水平相当可比，而与添加的硫酸盐剂量无关。TEM-EDX 和 XANES 分析证实，无论硫酸盐的存在与否，HgSe 纳米颗粒的形成都可以解释 Se 在控制 MeHg 生成中的作用比硫酸盐更重要。本研究结果阐明了硫酸盐和 Se 对水稻土中净 MeHg 生成的影响及其作用机制，有助于加深我们对土壤-水稻系统中 MeHg 动态和风险的理解。
