Environmental Engineering Reference
In-Depth Information
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.
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In fish, most mercury is in the form of methylmercury, and there are indications
that a significant part of the mercury found in eggs and meat is also 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
shown 0.05 to 0.7 ppm or more mercury, with the larger and older fish showing
the higher concentration.
In view of the potential hazards involved, steps have been taken to provide
standards or guidelines for mercury. The maximum allowable concentration for
8-hour occupational exposure has been set at 0.05 mg metallic vapor and inor-
ganic compounds of mercury per cubic meter of air. For organic mercury the
threshold limit is 0.01 mg/m
3
of air. The suggested limit for fish is 0.5 ppm; for
shellfish, it is 0.2 ppm. The primary standard for drinking water is 0.002 mg/1 (2
ppb) as total mercury. A standard of 0.05 ppm has been suggested for food.
A maximum ADI of 0.03 mg for a 70-kg (154-1b.) man would provide a safety
factor of 10. If fish containing 0.5 ppm mercury were eaten daily, the limit of
0.03 mg would be reached by the daily consumption of 60 g (about 2 oz.) of fish.
The safe levels would be 2
µ
g/100 ml for whole blood and 6 ppm for hair.
There is no evidence to show that the mercury in the current daily dietary
intake has caused any harm, although this does not rule out possible nondetectable
effects on brain cells or other tissues. The general population should probably
not eat more than one freshwater-fish meal per week, with special concern for
pregnant women.
Since mercury comes from manmade and natural sources, every effort must
be made to eliminate mercury discharges into the environment.
Habashi has summarized techniques for the removal of mercury at metallur-
gical plants in the United States, Europe, and Japan.
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The author reports that
“the removal and recovery of traces of mercury from SO
2
gases or from sulfuric
acid has been proved to be technically and economically feasible.” Insofar as
water supply is concerned, approximately 98 percent inorganic mercury may be
removed by coagulation and settling at a pH of 9.5 followed by filtration through
a granular activated carbon filter.
Methemoglobinemia
The presence of more than 45 mg/1 nitrates (10 mg/1 as N), the standard for
drinking water, appears to be the cause of methemoglobinemia, or “blue baby”
