Chemistry Reference
In-Depth Information
(Jensen and Jernelöv, 1967; 1969; Jernelöv, 1968). The
methylmercury formed is rapidly taken up by living
organisms in the aquatic environment, and, by deg-
radation, gases of (CH
3
)
2
Hg are formed and released
into the air (Jensen and Jernelöv, 1968). Dimethylmer-
cury may be decomposed in the atmosphere by acidic
rainwater to monomethylmercury compounds and
thereby reenter the aquatic environment (Jensen and
Jernelöv, 1972). It may eventually be demethylated,
thereby completing the cycle. Little is known about
the quantitative aspects of these cycles. The local load
of MeHg may be considerably increased by industrial
release of mercuric compounds. This has been shown
for mercury release from chemical factories—paper-
pulp factories and alkaline-chlorine factories (Swedish
Expert Group, 1971) and the Almaden mercury-min-
ing region in Spain (Gray
et al
., 2004). These investi-
gators found that mine waste contained total mercury
in the range of 160-34,000
approximately 100 metric tons per year for the period
1996-2005. The uses have changed dramatically over
time. In the mid-70s, of the mercury produced yearly
for commercial purposes, 25% was consumed by the
chloralkali industry, 20% used in electrical equip-
ment, 15% in paints, 10% in measurement and control
systems (e.g., thermometers and sphygmomanom-
eters), 5% in agriculture, 3% in dentistry, and 2% in
laboratories. The remaining 20% was divided among
military uses, such as detonators, mercury-containing
catalysts, preservatives in the paper-pulp industry,
pharmaceuticals, cosmetic preparations, and others
(Korringa and Hagel, 1974; Nriagu, 1979). The cyto-
toxic properties of mercury compounds have resulted
in their widespread use as germicides and fungicides
in pharmaceuticals, plastics, paints, and other prod-
ucts. Methylmercury and ethylmercury have been
extensively used in seed treatment. Most industrial
countries have banned this use, and the production of
alkylmercury compounds has decreased. Dimethylm-
ercury is used by chemists as a reagent. Phenylmer-
cury acetate has been extensively used as a fungicide
and algaecide in paints, plastics, and other products.
Methoxyethylmercury compounds have replaced the
short-chain alkylmercury compounds in seed treat-
ment. Under the conditions in which they are used,
both types of compounds are unstable and slowly
release inorganic mercury.
g/g and that methylmer-
cury varied from <0.2-3100 ng/g. They also reported
variations in the methylation rates at various sites and
that stream water near the Alamaden mine contained
total mercury in concentrations up to 13,000 ng/L and
methylmercury up to 30 ng/L. Stream sediments also
contained total mercury concentrations up to 2300
µ
µ
g/
g and methylmercury concentrations of up to 82
g/g.
These data suggest a dynamic relationship between
the stream sediments and the water column.
More recent studies (Mehrotra and Sedlak, 2005)
have shown decreased mercury methylation rates
by factors of 2.1-6.6 after iron amendment of anoxic
wetlands sediment slurries with 30 mmol/L Fe(II).
Hence the chemical nature of local environments may
have a marked effect on the environmental mercury
methylation cycle. In addition, the transalkylation of
mercury (Hg
2+
) from tetraethyl lead to form ethyl-
mercury at a tetraethyl lead-contaminated site in
Germany (Hempel
et al
., 2000) was reported, indicat-
ing that other processes and forms of alkyl mercurials
should be considered in evaluating the environmental
cycling of mercury.
µ
5 ENVIRONMENTAL LEVELS
AND EXPOSURES
5.1 General Environment
5.1.1 Food and Daily Intake
Alkylmercury, formed in the bottom sediment
of the ocean and in freshwater systems, is enriched
to a high degree in the aquatic food chain, with the
highest levels occurring in predatory fi sh. From the
aquatic environment, methylmercury becomes incor-
porated in the terrestrial environment by species
feeding on aquatic organisms. However, enrichment
has not been seen to the same extent in the terrestrial
food chain. Large tuna fi sh, exceeding 60 kg, have
levels up to 1 mg/kg of methylmercury in their mus-
cles (Peterson
et al
., 1973), whereas terrestrial animals
rarely have levels in muscles exceeding 50
4.2 Uses
According to the USGS (2005), only approxi-
mately 20 metric tons of mercury are now consumed
domestically in the United States. Recent estimates
indicate that of this amount approximately 50% is
used for chlorine-caustic soda manufacture and the
remaining 50% for dental amalgams, mercury elec-
tronic switches, and fl uorescent lamps and other
electronic devices. This represents a marked decline
in consumption, because USGS (2005) has estimated
that the chlorine-caustic soda industry alone used
µ
g/kg and
generally average 20
g/kg (Swedish Expert Group,
1971). In areas with polluted water, levels of methyl-
mercury in fi sh fl esh may exceed 10 mg/kg, with a
tendency to increasing levels with increasing size and
age of fi sh. Another determining factor for the meth-
ylmercury content in fi sh is pH. A positive association
µ
