The Mercury Cycle
|Mercury in the environment is constantly cycled and recycled through
a biogeochemical cycle. The cycle has six major steps:
- Degassing of mercury from rock, soils, and surface waters,
or emissions from volcanoes and from human activities.
- Movement in gaseous form through the atmosphere.
- Deposition of mercury on land and surface waters.
- Conversion of the element into insoluble mercury sulfide.
- Precipitation or bioconversion into more volatile or soluble
forms such as methylmercury.
- Reentry into the atmosphere or bioaccumulation in food chains.
Mercury cycles in the environment as a result of natural (ex:
geothermal activity) and anthropogenic (human) activities. The
primary anthropogenic sources are: fossil fuel combustion and
smelting activities. Both these natural and human activities release
elemental mercury vapor (Hg0) into the atmosphere. Once in the
atmosphere, the mercury vapor can circulate for up to a year,
and hence become widely dispersed. The elemental mercury vapor
can then undergo a photochemical oxidation to become inorganic
mercury that can combine with water vapors and travel back to
the Earth’s surface as rain. This ‘mercury-water’
is deposited in soils and bodies of water. Once in soil, the mercury
accumulates until a physical event causes it to be released again.
(See forest fire research below) In water, inorganic mercury can
be converted into insoluble mercury sulfide which settles out
of the water and into the sediment, or it can be converted by
bacteria that process sulfate into methylmercury. The conversion
of inorganic mercury to methylmercury is important for two reasons:
- Methylmercury is much more toxic than inorganic mercury.
- Organisms require a long time to eliminate methylmercury,
which leads to bioaccumulation.
Now the methylmercury-processing bacteria may be consumed by
the next higher organism up the food chain, or the bacteria may
release the methylmercury into the water where it can adsorb (stick)
to plankton, which can also be consumed by the next higher organism
up the food chain. This pattern continues as small fish/organisms
get eaten by progressively bigger and bigger fish until the fish
are finally eaten by humans or other animals. Alternatively, both
elemental mercury and organic (methyl) mercury can vaporize and
re-enter the atmosphere and cycle through the environment.
New research in this area