| Mercury is a highly toxic metal with global concern, and mercury(Hg) mining areas are considered as hotspots for Hg research. Previous studies have shown that rice can bioaccumulate methylmercury(MeHg) in Wanshan Hg mining area(WMMA), eastern Guizhou Province and rice is the main pathway of methylmercury exposure for local residents. Meanwhile, selenium(Se) is an associated element in WMMA, which may affect Hg biogeochemical cycling. Therefore, it is necessary to evaluate the effects of Se on transportation and transformation of Hg in the paddy field. This study contains two parts: field surveys and laboratory simulation experiments. Rice samples were collected in WMMA to measure total mercury(THg), MeHg and total Se contents. It is aimed to analyze the relationship between Hg and Se concentrations in rice grain and to investigate the effects of selenium on THg and MeHg bioaccumulation in rice grain. For laboratory simulation experiments, hydroponic cultivation of rice seedlings were carried out by adding Se in the solution,which is designed to explore the impacts of selenium on the absorption, transportation, and transformation of MeHg in rice plants. It can advise bioremediation in Hg contaminated areas.1) The geometric mean of THg contents in rice from WMMA was 24.0 ng·g-1(with a range of 2.40~401 ng·g-1), and 51% of the samples exceeded national limit(20 ng·g-1). The geometric mean of MeHg was 8.97 ng·g-1(with a range of 1.10~45.1 ng·g-1), and the average of pertentages of THg as MeHg in rice samples was 44.8±23.9%. THg concentrations in rice samples showed significant spatial distribution and it demonstrated a gradient at different sites with the distance to the pollution sources in the two catchments. The results suggested that Hg mining activities are important Hg pollution sources to local environment. The geometric mean of total Se contents in rice samples was 124.3 ng·g-1(with a range of 21.45~1991 ng·g-1), which was significantly higher than national average level(40~70 ng·g-1). It may be resulted from high Se contents in local soils.2) There is no significant correlation between total Se and THg contents in rice samples collected from WMMA. When Se and MeHg contents in rice were at low levels, no significant correlation was observed between them; however, when Se or MeHg concentrations were at high levels, a significant inverse correlation was found between them. It showed that selenium can significantly inhibit the absorption and enrichment of MeHg in rice samples.3) The simulation experiments showed that selenium had significant influences on the MeHg contents in rice seeds during rice growing period. Within a certain range of concentrations, Se(VI)can effectively inhibit the absorption of MeHg in rice; when Se(VI)concentrations exceed this range, it played a catalytic role and the inhibition effects were different at differentSe( VI) concentrations. While the same concentrations of Se(IV)were able to promote absorption of MeHg in rice.4) Rice roots had high ability to bioaccumulate MeHg, but the transportation to aerial part was relatively difficult. During the rice growing period, Se had significant impacts on MeHg diatribution between roots and stems in rice(P<0.05). The root absorption index of MeHg in rice roots increased with rice growing and ranged from 2.54 to 122.02. The transportation index in rice decreased with rice growing and ranged from 0.22 to 0.74. Se(VI)addtion within a certain threshold can reduce the accumulation of MeHg in roots and the transportation of MeHg from roots to stems and leaves in rice, but Se(IV)treatment will promote the absorption of MeHg in rice roots.5) By a estimation, it concluded that Se(VI)will inhibit bioaccumulation of MeHg in rice grain and Se(IV)might promote the enrichment of MeHg in rice grain at the mature time. |
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