Geology Reference
In-Depth Information
complexation characteristics. Adsorption processes cannot be ignored in
biogeochemical cycling. Particles tend to have a much shorter residence
time in the water column than do dissolved constituents. Scavenging of
trace components by falling particles accelerates deposition to the
sediment sink.
Several elements in seawater may undergo alkylation via either chemical
or biological mechanisms.
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Type I mechanisms involve methyl radical or
carbonium ion transfer and no formal change in the oxidation state of the
acceptor element. The incoming methyl group may be derived for exam-
ple from methylcobalamin coenzyme, S-adenosylmethionine, betaine or
iodomethane. Elements involved in Type I mechanisms include Pb, Tl, Se
and Hg. Other reaction sequences involve the oxidation of the methylated
element. The methyl source can be a carbanion from methylcobalamin
coenzyme. Oxidative addition from iodomethane and enzymatic reactions
has also been suggested. Some elements that can undergo such methyl-
ation processes are As, Sb, Ge, Sn and S. Methylation can enhance the
toxicity of some elements, especially for Pb and Hg. The environmental
mobility can also be affected. Methylation in the surface waters can
enhance volatility and so favour evasion from the sea, as observed for S,
Se and Hg. Methylation within the sediments may facilitate transfer back
into overlying waters.
Elements may exhibit multiple oxidation states in seawater. Redox
processes can be modelled in an analogous manner to the ion pairing
and complexation outlined previously. The information is often pre-
sented graphically in the form of a predominance area diagram, which is
a plot of pe vs. pH showing the major species present for the designated
conditions. Although a single oxidation state might be anticipated from
equilibrium considerations, there are several ways in which multiple
oxidation states might arise. Biological activity can produce non-equi-
librium species, as evident in the alkylated metals discussed above.
Whereas Mn(IV) and Cu(II) might be expected by thermodynamic
reasoning, photochemical processes in the surface waters can lead to
the formation of significant amounts of Mn(II) and Cu(I). Fe(III) is the
favoured redox state of Fe in seawater, but it is relatively insoluble and
exists predominantly in a colloidal phase. Photochemical reduction to
Fe(II), which only slowly oxidises to Fe(III), might act as a very
important mechanism rendering Fe bioavailable to marine organisms.
Goldberg
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has presented impressive information (given that seawater
concentrations are as low as 1.5 pg L
1
for Ir and 2 pg L
1
for Ru) on
the speciation, including redox state, of platinum group metals as a
means of interpreting distributions in seawater and marine sediments. Pt
and Pd are stabilised in seawater as tetrachloro-divalent anions. Their
