Chemistry Reference
In-Depth Information
1. Temperature
phase of lakes and it has, likewise, been shown that
freshwater algae are capable of degrading methylmer-
cury.
The higher temperature, the higher methylation.
2. Dissolved organic material
a. If the dissolved organic material can be used
as an energy source by the microorgan-
isms, the rate of metabolism in these may be
increased, which also leads to an increase in
the methylation rate.
b. If the dissolved organic material binds
Hg(II)—which is the substrate in the meth-
ylation process—the rate of methylation may
decrease with increasing concentration of
organic material.
3. pH
8.10.2.3 Net Methylation
The amount of methylmercury present in a specifi c
ecosystem will, therefore, not only depend on the rate
by which methylmercury is synthesized, but rather the
equilibrium between methylation and demethylation
processes.
The total concentration of mercury in seawater
is typically ranging from 1-5 ng/L. The fraction that
methylmercury constitutes of the total mercury con-
tent in the seawater may vary somewhat, but it is gen-
erally in the order of 1%; in freshwater, the fraction of
methylmercury is typically somewhat higher.
The net methylation rate generally increases when
the pH value decreases. The rate of both methyla-
tion and demethylation may increase with decreas-
ing pH, but because the rate of methylation most
often increases more than the rate of demethyla-
tion, the net methylation rate will increase.
8.10.2.4 Reduction of Mercury
In the aquatic environment, Hg
++
may be reduced
to elemental mercury that can evaporate to the atmos-
phere. The reduction of Hg
++
proceeds both abiotically
and biotically—the latter probably predominantly in
connection with enzymatic activity on the surfaces of
unicellular algae (Poulain
et al
., 2004).
4. The concentration of bioavailable Hg
++
:
The larger the Hg
++
concentration, the higher rate
of methylation.
8.10.2.5 Oxidation of Elemental Mercury
Elemental mercury is relatively volatile, and it evap-
orates from both soil surfaces and the aquatic envi-
ronment to the atmosphere, where the mercury has a
residence time of approximately 1 year. The elemental
mercury may be oxidized to Hg
++
in the atmosphere
with the participation of ozone, and the divalent mer-
cury is predominantly bound to particles in the atmos-
phere (Morel
et al
., 1998).
5. Complex binding ligands (both organic and
inorganic, see Figure 1):
The higher amount of ligands, the lower rate of
methylation.
Either methylmercury (CH
3
-Hg
+
) or dimethylm-
ercury (CH
3
-Hg-CH
3
) is formed in the methylation
process. At neutral and low pH, methylmercury (as
CH
3
-Hg
+
Cl
−
) is the most stable of the two compounds,
whereas dimethylmercury is most stable in an alkaline
environment. The pH value in seawater is approxi-
mately 8.2 (and thereby higher than in most freshwater
systems), and in seawater, the formation of dimethyl-
mercury plays a role, although it is of less importance
than the formation of methylmercury in most freshwa-
ter systems. Dimethylmercury is fairly volatile, and it
may evaporate to the atmosphere. Dimethylmercury is
generally found at lower concentrations in the upper
layers of the oceans than in the deeper, and this may be
because of the evaporation to the atmosphere.
8.10.3 The Global Mercury Flux
Mercury exists in nature in equilibrium between the
elemental mercury, Hg(II) salts, and organic mercury
compounds, especially methylmercury, and the overall
fl ux of mercury between soil, atmosphere, and oceans
is especially related to mercury's shift in oxidation
state between elemental and divalent mercury.
The evaporation of mercury to the atmosphere is
larger from soil than from the oceans, and because the
fraction of land as well as industrial activity is larg-
est in the northern hemisphere, higher mercury con-
centrations are generally found in the atmosphere of
the northern hemisphere than in the atmosphere of
the southern hemisphere (Fitzgerald and Mason, 1997;
Mason
et al
., 1994).
The emissions of mercury from industrialized areas
are decreasing in Europe and North America but
increasing in South East Asia (UNEP, 2002).
8.10.2.2 Demethylation
Certain types of bacteria are capable of demethylat-
ing methylmercury. The demethylation may take place
in both aerobic and anaerobic bacteria, but the most
quantitatively important demethylation seems to take
place in the aerobic environment. The demethylation
has been demonstrated both in the sediment and water
