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汞元素赋存之地理环境模型(07)  

2017-11-10 08:21:45|  分类: 不归类文章 |  标签: |举报 |字号 订阅

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Methylmercury Formation and Adsorption in Streams Impacted by Mine Drainage

Streams impacted by mercury mine drainage have elevated levels of mercury, methylmercury, and anions such as sulfate, low pH, and streambeds that are coated by iron oxyhydroxides. Both sulfate- and iron-reducing bacteria (Marvin-DiPasquale and others, 2000) are important methylators of mercury in this high iron and sulfate environment, and stream waters may contain up to several ng/L methylmercury (Rytuba, 1997). As mercury mine drainage mixes with oxygenated stream waters, dissolved iron (II) is oxidized to iron (III) and precipitated as iron oxyhydroxide. High concentration of dissolved iron (up to 8800 ppm) results in iron oxyhydroxide being the most abundant particulate phase present in streams impacted by mine drainage. Aluminum hydroxide and other aluminum silicate phases are also important adsorption substrates. Adsorption of mercury and methylmercury onto iron oxyhydroxide and other aluminum phases is an important process that controls the concentration of mercury species in streams impacted by mercury mine drainage. As a result, most of the mercury species present in streams impacted by mine drainage are present in the particulate phases and the concentration of dissolved mercury species concentrations is very low (Rytuba, 2000). Over the pH range of 3.2 to 7.1, the concentration of mercury in iron oxyhydroxide may be as much as three orders of magnitude greater than that in the coexisting stream water and be as high 220 μg/g. Above pH 7.1, mercury adsorption is less effective and macroscopic experiments have demonstrated that the sorptive behavior of mercury (II) onto goethite exhibits a gradual decrease with increasing pH after reaching a maximum adsorption at neutral pH (Barrow and Cox, 1992). Over the pH range 3.2 to 7.1, methylmercury is also strongly adsorbed. Methylmercury concentration is as much as two orders of magnitude greater than that in the coexisting water, up to 105 ng/g. In waters with high sulfate concentration, the negatively charged sulfate complex, CH3HgSO4 - predominates at low pH (Sanz and others, 1999). It is more effectively adsorbed by iron oxyhydroxide in low pH stream waters than is the neutral methylmercury hydroxide species that is the predominant species in alkaline waters. The distribution coefficient, Kd in L/kg (fig. 7), for both mercury and methylmercury with respect to iron oxyhydroxide significantly decreases at pH above 7.1. Thus, the total amount of mercury and methylmercury present in mine drainage impacted streams primarily reflects the amount of mercury and methylmercury-enriched iron and aluminum phases present as particles in the water column. The concentration of dissolved mercury species is very low as a consequence of effective adsorption.

汞元素赋存之地理环境模型(07) - 不在眉头愁 - 吉建斌的网易博客

Figure 7. Distribution coefficient, log Kd in L/kg, for mercury and methylmercury with respect to iron oxyhydroxide versus pH (Rytuba, 2000).

Environmental Impact of Mercury Mine Drainage

Mine drainage provides a favorable environment for methylation of mercury because the high concentration of sulfate typically present in these low pH waters is microbially reduced and ionic mercury (Hg2+) is methylated as part of a cometabolic process carried out by sulfate- reducing bacteria. In watersheds impacted by mercury mine wastes and drainage, mercury and methylmercury adsorbed onto iron precipitates, clays, and organic particles accumulate in the streambed during the dry season when water flow is low. A relatively small amount of mercury and methylmercury is transported downstream into larger aquatic systems during this period. During the wet season when stream flows are high, mercury- and methylmercury-enriched sediment is removed from the bed load of the stream and distributed downstream into wetlands, lakes, and reservoirs. The process of sediment redistribution from watersheds impacted by mercury mine drainage and mercury-enriched mine tailings provides a seasonal supply of mercury into aquatic systems where microbial methylation causes methylmercury to enter the trophic levels of the aquatic food web. Methylmercury is biomagnified as it passes upward through the food web to higher organisms, such as fish. Because historic mercury mine sites and active hot springs (or combinations of the two) typically generate the highest concentrations of iron-rich mercury-bearing precipitates, these sites tend to be a primary sources for new mercury entering the cycle.

CLIMATE

In wet climates oxidation and dissolution of pyrite and marcasite increase the possibility of acid- mine drainage and enhance the release of mercury from mine wastes. Wet climates have a higher biologic productivity and as a result dissolved organic carbon concentrations are higher, thus increasing the potential for methylation of mercury by sulfate-reducing bacteria. In dry, high temperature climates, soil-gas emission of elemental mercury is important, and this process may increase the area affected by elevated mercury if adjacent plant communities are present to uptake mercury in their leaves.

REFERENCES CITED

汞元素赋存之地理环境模型(07) - 不在眉头愁 - 吉建斌的网易博客 
汞元素赋存之地理环境模型(07) - 不在眉头愁 - 吉建斌的网易博客 
汞元素赋存之地理环境模型(07) - 不在眉头愁 - 吉建斌的网易博客 
汞元素赋存之地理环境模型(07) - 不在眉头愁 - 吉建斌的网易博客

 

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