revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 406
5. POTENTIAL FOR HUMAN EXPOSURE
Vandal et al. 1991). The major pathways for transformation of mercury and various mercury compounds in
air, water, and soil are shown in Figure 5-6.
5.3.2.1 Air
The primary form of atmospheric mercury, metallic mercury vapor (Hg 0 ), is oxidized by ozone to other
forms (e.g., Hg +2 ) and is removed from the atmosphere by precipitation (Brosset and Lord 1991). The
oxidation/reduction of mercury with dissolved ozone, hydrogen peroxide, hypochlorite entities, or
organoperoxy compounds or radicals may also occur in the atmosphere (Schroeder et al. 1991). The overall
residence time of elemental mercury in the atmosphere has been estimated to be 6 days to 2 years, although
in clouds, a fast oxidation reaction on the order of hours may occur between elemental mercury and ozone.
Some mercury compounds, such as mercuric sulfide, are quite stable in the atmosphere as a result of their
binding to particles in the aerosol phase (Lindqvist 1991b). Other mercury compounds, such as mercuric
hydroxide (Hg[OH] 2 ), which may be found in the aqueous phase of the atmosphere (e.g., rain), are rapidly
reduced to monovalent mercury in sunlight (Munthe and McElroy 1992). The main atmospheric
transformation process for organomercurials appears to be photolysis (EPA 1984b; Johnson and Bramen
1974; Williston 1968).
5.3.2.2 Water
The most important transformation process in the environmental fate of mercury in surface waters is
biotransformation. Photolysis of organomercurials may also occur in surface waters, but the significance of
this process in relation to biotransformation is not clear (Callahan et al. 1979).
Any form of mercury entering surface waters can be microbially converted to methylmercuric ions, given
favorable conditions. Sulfur-reducing bacteria are responsible for most of the mercury methylation in the
environment (Gilmour and Henry 1991), with anaerobic conditions favoring their activity (Regnell and
Tunlid 1991). Yeasts, such as Candida albicans and Saccharomyces cerevisiae, whose growth is favored
by low pH conditions, are able to methylate mercury and are also able to reduce ionic mercury to elemental
mercury (Yannai et al. 1991). Methyl cobalamine compounds produced by bacterial synthesis appear to be
involved in the nonenzymatic methylation of inorganic mercury ions (Regnell and Tunlid 1991). The rate
of methylmercury formation by this process is largely determined by the concentration of methyl
cobalamine compounds, inorganic mercuric ions, and the oxygen concentration of the water, with the rate