Sulfur and iron influence the transformation and accumulation of mercury and methylmercury in the soil-rice system

Much attention is directed to the accumulation of mercury and methylmercury (MeHg) in rice grown on Hg-contaminated paddy fields, since they pose a risk to the health of both people and wildlife. Ultimately, measures to control the accumulation of Hg and MeHg in rice should become a key focus of research on this topic. The objective of this study is to test whether Hg and MeHg accumulation are reduced in polished rice when iron and sulfur are added together to the paddy fields during the cultivation of rice. In this experimental study, rice plants were grown in pots amended with sulfur and iron. Rice paddy soil contaminated by mercury chloride in the sewage water used for irrigation was simulated in this experiment by Hg-contaminated soil. The total mercury (THg) content added to the soil as mercury chloride reached 120 mg kg(-1) as the weight of the dry soil. Two levels of iron (0 and 200 mg kg(-1) as FeCl2) as well as sodium sulfate and cysteine (Cys) were added via watering. Two 30-day rice seedlings were transplanted into a root-bag (20 cm long, 12 cm wide), filled with fine quartz sand and cultured in a plastic barrel filled with 4.5 kg of waterlogged soil. When the rice plants matured, samples were taken. The addition of iron and sulfur to the Hg-contaminated paddy soil increased the dry weight of rice grains. The THg and MeHg mainly concentrated in the upper soil layer, and the concentrations of MeHg in the treatments with iron and sulfur were especially higher than that of the control. The addition of iron, sulfate and iron, and cysteine and iron decreased THg concentrations in polished rice by 17.7, 38.3, and 21.3%, respectively. The addition of iron, sulfate and iron, or cysteine and iron decreased the MeHg concentration up to 29.9, 36.4, and 48.2% in polished rice, respectively. Thus, we infer that the coupling of sulfur and iron (II) plays an important role in decreasing the accumulation of MeHg in polished rice. The addition of iron and sulfur together decreased the concentration of THg and MeHg in polished rice. These results can help control concentration levels of MeHg in rice; however, further study of the mechanism of the interaction of sulfur and iron in Hg-contaminated paddy soil should be conducted.