Chemistry Reference
In-Depth Information
8.2.3 e
n v i r of n m e n T a l
f
a T e
of f
o
r g a n o m e r c u r y
As noted earlier, diverse forms of organomercury are released into the environment
as a consequence of human activity. Methyl mercury presents a particular case. As
a product of the chemical industry, it may be released directly into the environment,
or it may be synthesized in the environment from inorganic mercury which, in turn,
is released into the environment as a consequence of both natural processes (e.g.,
weathering of minerals) and human activity (mining, factory effluents, etc.).
The environmental cycling of methyl mercury is summarized in Figure 8.3.
Dimethyl mercury, being highly volatile, tends to move into the atmosphere following
its generation in sediments; once there, it can be converted back into elemental mer-
cury by the action of UV light. Some dimethyl mercury is taken up by fish and
transformed into a methyl cysteine conjugate, which is excreted. However, the most
important species of methyl mercury in aquatic and terrestrial food chains is CH
3
Hg
+
, which exists in various states of combination with S- groups of proteins and
peptides, and with inorganic ions such as chloride. Total methyl mercury of tissues,
sediments, etc., is determined by chemical analysis, but the state of combination is
not usually known. Some free forms of methyl mercury, for example, CH
3
HgCl, are
highly lipophilic and undergo bioaccumulation and bioconcentration with progres-
sion along food chains in similar fashion to lipophilic polychlorinated compounds.
In a report from the U.S. EPA (1980), fish contained between 10,000 and 100,000
times the concentration of methyl mercury present in ambient water. In a study of
methyl mercury in fish from different oceans, higher levels were reported in pred-
ators than in nonpredators (see Table 8.2). Taken overall, these data suggest that
predators have some four- to eightfold higher levels of methyl mercury than do non-
predators, and it appears that there is marked bioaccumulation with transfer from
prey to predator.
In a laboratory study (Borg et al. 1970), bioaccumulation of methyl mercury was
studied in the goshawk (
Accipiter gentilis
). The details are shown in Table 8.3 below.
Thus, chickens bioaccumulated methyl mercury to about twice the level in their
food, whereas goshawks bioaccumulated methyl mercury to about four times the level
present in the chicken upon which they were fed. The period of exposure was similar
C
2
H
6
(CH
3
)
2
Hg
Atmosphere
Hg
Water
Hg
2+
Bacteria
Bacteria
Sediment
CH
3
Hg
+
(CH
3
)
2
Hg
Hg
HgS
fIgure 8.3
Environmental fate of methyl mercury (adapted from Crosby 1998).
Search WWH ::
Custom Search