Environmental Engineering Reference
In-Depth Information
Elemental metallic mercury volatilizes on exposure to air, especially if heated,
and in that state poses a distinct hazard. Mercury spills and the mercury from
broken thermometers and barometers must be meticulously cleaned and the space
ventilated and isolated until the mercury vapor level is no longer detectable by
a “mercury sniffer” or similar device. Metallic mercury should never be incin-
erated; toxic gases would be released. Mercury should normally be stored and
handled in an airtight enclosure with extreme care. Laboratory use must be care-
fully controlled and monitored. Certain compounds of mercury may be absorbed
through the skin, gastrointestinal tract, and respiratory system (up to 98 percent),
although elemental mercury and inorganic mercury compounds are not absorbed
to any great extent through the digestive tract because they do not remain in the
body.
Mercury is ubiquitous in the environment. The sources are both natural
and manmade. Natural sources are leachings, erosion, and volatilization from
mercury-containing geologic formations. Carbonaceous shales average 400 to
500 ppb Hg, up to 0.8 ppm in soil. Manmade sources are waste discharges
from chlor-alkali and paper pulp manufacturing plants, mining and extraction of
mercury from cinnabar, chemical manufacture and formation, the manufacture
of scientific instruments, mercury seals and controls, treated seeds, combustion
of fossil fuels, atmospheric deposition, and surface runoff. The mercury ends up
in lakes, streams, tidal water, and the bottom mud and sludge deposits.
Microorganisms and macroorganisms in water and bottom deposits can
transform metallic mercury, inorganic divalent mercury, phenylmercury, and
alkoxyalkylmercury into methylmercury. The methylmercury thus formed
and perhaps other types, in addition to that discharged in wastewaters, are
assimilated and accumulated by aquatic and marine life such as plankton, small
fish, and large fish. Alkaline waters tend to favor production of the more volatile
dimethylmercury, but acid waters are believed to favor retention of the dimethyl
form in the bottom deposits. Under anaerobic conditions, the inorganic mercury
ions are precipitated to insoluble mercury sulfide in the presence of hydrogen
sulfide. The process of methylation will continue as long as organisms are
present and have access to mercury. It is a very slow process, but exposure of
bottom sediment such as at low tide permits aerobic action causing methylation
of the inorganic mercury.
The form of mercury in fish has been found to be practically all methylmer-
cury, and there are indications that a significant part of the mercury found in
eggs and meat is in the form of methylmercury.
The concentration of mercury in fish and other aquatic animals and in wildlife
is not unusual. Examination of preserved fish collected in 1927 and 1939 from
Lake Ontario and Lake Champlain in New York has shown concentrations up to
1.3 ppm mercury (wet basis). Fish from remote ponds, lakes, and reservoirs have
Seven to 8 percent from food and 15 percent or less from water ( Guidelines for Drinking Water
Quality , Vol. 2, World Health Organization, Geneva, 1984, p. 122).
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