Chemistry Reference
In-Depth Information
mainly a mixture of oxidation states III and IV (Burdige,
1993); the oxidized form of manganese is often referred
to as MnO
2
. Mn
++
dominates under reduced conditions
in the environment. Mn
++
is the dominating species and
MnCl
+
, MnSO
4
0
, and MnCO
3
0
also play a role (Mantoura
et al
., 1978).
Concentrations of dissolved manganese in the oceans
are typically higher (~1 nmol/L) in the upper part of the
water column than in the deep waters (0.25 nmol/L)
(Landing
et al
., 1995), probably because of the impact
of atmospheric deposition of manganese transported
from the terrestrial environment (Statham
et al
., 1998).
Manganese concentrations in coastal waters are gener-
ally higher (~one order of magnitude) than concentra-
tions in the surface waters of the oceans (Hydes and
Kremling, 1993). Sixty-nine percent of the manganese
transported in the rivets of the world are precipitated
in the estuaries (Salomons and Förstner, 1984).
Anthropogenic emission of manganese into the
atmosphere is lower than the natural fl ux with a mobi-
lization factor of 0.53 (Lantzy and Mackenzie, 1979).
Manganese plays an important role in oxidation-
reduction processes in marine sediments (Burdige,
1993). Under oxic conditions, the manganese present
in the upper few millimeters of the sediment is present
in the oxidized form, but if the bottom waters (e.g.,
below thermo- or haloklines) are depleted in oxygen
because of eutrophication, manganese is reduced and
Mn
++
leaks out of the sediment to the bottom waters
where concentrations approaching 1 mg/L may be
reached (Kremling, 1983). Mn
++
dominates below an
oxygen saturation of approximately 16% (Gerringa,
1991), and the reoxidation of Mn
++
after an incident of
oxygen depletion occurs fairly slowly (in the order of
weeks) (Dehairs
et al
., 1989).
Increased concentrations of manganese have been
found in benthic, marine organisms in areas affected
by oxygen depletion (Baden
et al
., 1994; Baden and
Neil, 2003; Eriksson and Baden, 1998), but the highest
concentrations (approximately 1 mg/L) reached in the
seawater are hardly toxic to marine organisms.
until the early 1970s, approximately three quarters of
the global use of approximately 10,000 tons use was
released to nature.
Ecotoxicological and toxicological problems with
the uses and releases of mercury were revealed both
in the terrestrial and the aquatic environment during
the 1960s—mainly in Swedish and Japanese investi-
gations. The use of alkylmercury compounds as seed
dressers resulted in elevated levels of mercury in ter-
restrial wildlife in agricultural areas—especially avian
top predators such as hawks, owls, and harriers were
affected (Berg
et al
., 1966; Johnels
et al
., 1979). Releases
from industrial uses in the chloroalkali and paper
industries led to contamination of the aquatic envi-
ronment, the top predators again showing the highest
degree of contamination (Johnels
et al
., 1967). Dis-
charges to the Minamata Bay in Japan caused severe
mercury poisoning in the local population after con-
sumption of fi sh and shellfi sh from the bay (further
described in Chapter 33).
Today, the discharge of mercury from industry has
been considerably reduced, but the human mobiliza-
tion of mercury during the last century has increased
the emission of mercury to the atmosphere with
approximately a factor of 3 (UNEP, 2002). An average
sediment enrichment factor of 8.7 has been reported for
lakes and coastal areas (Salomons and Förstner, 1984).
8.10.2 The Transformation of Mercury in Nature
In nature, mercury may be transformed between
elemental mercury, divalent mercury, and methylmer-
cury, and these transformations play a vital role both in
the global cycling of mercury and for the accumulation
and adverse effects of mercury in organisms.
8.10.2.1 Methylation
Inorganic mercury may in nature be transformed to
methylmercury by both abiotic and biotic processes,
and the biological transformation is believed to be the
more important. Most of the knowledge on methyla-
tion processes has been obtained from studies in fresh-
water systems.
Several different types of microorganisms are capa-
ble of methylating Hg(II), and the methylation takes
place especially at the interface between oxic and
anoxic zones in nature—typically in the upper parts of
sediments where the oxygen gradient is sharp. Sulfate
reducing and methanogenic bacteria are capable of
methylating inorganic mercury, and in some environ-
ments the quantitatively most important methylation
is carried out by these organisms.
The degree of methylation in aquatic ecosystems
depends on a number of different factors:
8.10 Mercury
8.10.1 Background Concentrations, Uses, and
Emissions
Mercury concentrations in the oceans range between
0.7 and 1.1 pmol/L with no apparent concentration
gradients between surface and deeper waters (Dalziel,
1995).
In modern times, mercury has been used for many
different purposes (chloroalkali industry, seed dress-
ers, antimicrobial agent in the paper industry, etc.), and
