Organic mercury compounds, volatility

Many bacteria are resistant to inorganic and organic mercury compounds. Mercuric reductase (MerA) is a key enzyme in the mercury detoxification pathway. MerA catalyzes the NADPH-dependent reduction of Hg to its volatile, uncharged, elemental state (Hg ). MerA is a cytosolic protein that is homologous to GR, but also has a short C-terminal extension and a long N-terminal extension not found in GR. MerA contains three pairs of cysteines one in the C-terminal extension, one in the N-terminal extension, and one in the GR-like region of the protein. The N-terminal domain binds one molecule of mercury and delivers it to the catalytic core of the protein, made up of the GR-like portion and the C-terminal extension, where it is reduced. The disulfide from... 

Many mercury compounds are labile and easily decomposed by light, heat, and reducing agents. In the presence of organic compounds of weak reducing activity, such as amines (qv), aldehydes (qv), and ketones (qv), compounds of lower oxidation state and mercury metal are often formed. Only a few mercury compounds, eg, mercuric bromide/77< 5 7-/7, mercurous chloride, mercuric s A ide[1344-48-5] and mercurous iodide [15385-57-6] are volatile and capable of purification by sublimation. This innate lack of stabiUty in mercury compounds makes the recovery of mercury from various wastes that accumulate with the production of compounds of economic and commercial importance relatively easy (see Recycling). 

Mercury may be present in air in different chemical states such as the elemental form (as a vapour or adsorbed on particular matter) or in the form of volatile mercury compounds (mercury chloride, methyl-mercuric chloride, and dimethyl mercury). Although elemental mercury is only one of the mercury forms which is not as toxic as its organic or ionic forms, analytical determination of elemental mercury is of special importance. Such analysis is used not only for determination of elemental mercury in environment, but also as a method for determination of other forms of mercury after reductive treatment. 

This element is a chalcophile, and in unweathered rocks is most commonly found as the mineral cinnabar (HgS). In soil environments, the cationic form, is most common, as the reduced oxidation state (+1) has a limited stability range. Reduction to the metallic elemental form, H, is easily achieved in soils by both biological and chemical reactions. Elemental mercury is somewhat volatile, and the vapor is extremely toxic to organisms. Under anaerobic conditions at least, soil microbes methylate mercury, forming volatile organomercury compounds that are bioavail-able and present a health hazard. At the same time, however, anaerobic conditions can convert Hg into the exceedingly insoluble sulhde, HgS. Some of the more important transformations possible for mercury in soil are summarized in Figure 9.9. 

Microbial processes can also detoxify mercury ions and organic compounds by reducing the mercury to the elemental form, which is volatile (86). This certainly reduces the environmental impact of compounds such as methylmercury, however, such a bioprocess would have to include a mercury capture system before it could be exploited on a large scale with pubHc support. 

Automated analyzers may be used for continuous monitoring of ambient poUutants and EPA has developed continuous procedures (23) as alternatives to the referenced methods. Eor source sampling, EPA has specified extractive sampling trains and analytical methods for poUutants such as SO2 and SO [7446-11-9] sulfuric acid [7664-93-9] mists, NO, mercury [7439-97-6], beryUium [7440-41-7], vinyl chloride, and VOCs (volatile organic compounds). Some EPA New Source Performance Standards requite continuous monitors on specified sources. 

Applicable pollutants for this equipment are Particulate Matter (PM), including particulate matter less than or equal to 10 micrometers (/im) in aerodynamic diameter (PM,q), particulate matter less than or equal to 2.5 /im in aerodynamic diameter (PMj,), and hazardous air pollutants (HAPs) that are in particulate form, such as most metals (mercury is the notable exception, as a significant portion of emissions are in the form of elemental vapor). Wet ESPs are often used to control acid mists and can provide incidental control of volatile organic compounds. 

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