Chemistry Reference
In-Depth Information
most metals transported to estuaries are precipitated
because of fl occulation of dissolved and suspended
organic matter. Also, the fraction of hydrated metal
ions, which is often the most bioavailable, is reduced.
To ease speciation calculation, computer software have
been developed, which on the basis of experimentally
determined stability constants and chemical description
of natural waters (concentrations of Cl
−
, CO
3
−−
, PO
4
−−−
,
SO
4
−−
, Na
+
, Ca
++
, etc.), calculate metal speciation. Exam-
ples are CHESS (Santore and Driscoll, 1995), MINEQL+
(Schecher and McAvoy, 1995), and MINTEQA2 (Brown
and Allison, 1987). Also, interactions between metals
and natural organic matter (NOM) have been integrated
in speciation modeling programs as WHAM (Tipping,
1994) and NICA (Benedetti
et al
., 1995), the latter model
incorporating multiple binding sites and competition
between cations. Because of the extensive heterogeneity
of NOM, this kind of modeling represents an extensive
task. Also, all such programs assume equilibrium and
cannot predict speciation in dynamic systems.
For a number of surface-active metals, these proc-
esses result in low concentrations in the upper part of
the water column, because the metals are being bound
to particle surfaces and transported downward. It is
characteristic for these metals (e.g., cadmium, cop-
per, nickel, zinc) that their depth profi le is similar to
the depth profi le of the major nutrients (Bruland, 1980;
Bruland
et al
., 1978).
The metals are removed from the upper layers of the
water column by a biologically mediated, downwards
transport process (Fowler
et al
., 1987). The higher the
affi nity of a specifi c metal for binding to biological
material is (e.g., refl ected in concentration factors for
the metals in zooplankton fecal pellets), the faster the
metal is removed from the water column, leading to
short residence time for the metal in the ocean (Cherry
et al
., 1978; Higgo
et al
., 1977). Concentrations in the
seawater and residence times for metals (and also
nonmetallic elements) in the ocean show a positive
correlation (Whitfi eld, 1981).
The partitioning coeffi cient (K
y
) for a metal (Y) is
defi ned as the concentration of the metal in seawater
and the average concentration in the crust of the earth
(K
y
= [Y]
seawater
/[Y]
crust
). Metals with a high K
y
do thus
occur at a high concentration in the seawater relative
to the average concentration in the Earth's crust. For
elements, an electronegativity function, Q
yo
, can be
defi ned, such that Q
oy
= (x
y
− x
o
)
2
and x
o
and x
y
are the
electronegativities of oxygen and the element Y, respec-
tively. For elements whose concentration in the seawater
is controlled by solid-phase chemistry, there is a nega-
tive correlation between the partitioning coeffi cient (K
y
)
and the electronegativity function, Q
yo
, illustrating that
metals with a high affi nity for binding to O
−
-dominated
surfaces are removed faster from the ocean water than
metals with a low affi nity for binding (Whitfi eld, 1982).
2.3 Metal Transport in the Ocean
When metal-carrying particles hit the surface of the
oceans, the metals may—depending on the charac-
ter of their binding to the particles and the chemical
properties of the individual metals—stay bound to the
particles or dissolve in the seawater.
The upper hundreds of meters of the oceans are the
habitat of fi lter-feeding zooplankton organisms that
obtain their food by fi ltering particles (mainly unicel-
lular algae) from the water. Because only a minor frac-
tion of the metal content of the food is taken up into
the organisms during the passage of the gut (for many
metals considerably less than 10%), the main fraction
of the original metal content of the unicellular algae is
excreted in the fecal pellets of the zooplankton organ-
isms. A considerable part of the pelagic fi lter feeders
are crustaceans (copepods, euphausiids, etc.), and the
fecal pellets of these organisms are lined by a thin chi-
tin membrane. Whereas unicellular algae and fi ne clay
particles sink very slowly down through the water
column (~2 m per day), crustacean fecal pellets have
much higher sinking rates, approximately 400 m/day
(Fowler and Small, 1972; Komar
et al
., 1981; Small
et al
.,
1979). This means that the activity of the fi lter feeders
transfers the particle-bound metal in the upper layers
of the water column to a form in which it is trans-
ported downward in the ocean. On their way down
through the water column, the fecal pellets may be
eaten several times, and the original content of organic
material, nutrients, and metals is gradually liberated
to the water masses, but some of the content reaches
the ocean bed.
2.4 Transport of Metals in Freshwater
and Estuaries
Metals are continuously added to freshwater sys-
tems because of the erosion caused by rainwater. The
erosion in the drainage areas of river systems typically
is in the order of 5 cm/1000 years, and the rivers of the
world each year transport approximately 24 billion
tons of material to the oceans. Human activities such as
ploughing, forest destruction, etc. are estimated to have
increased the amount of material transported by the
rivers by a factor 2-3 (Salomons and Förstner, 1984).
The metal contents and chemical composition of
freshwater in different river systems vary strongly
with the geological characteristics of the drainage
areas. Whereas the chemical composition of seawater
(regarding inorganic content) shows little variation,
