Geology Reference
In-Depth Information
Fractional contribution Expression Value for
½
Cl
¼
0
:
559mmol L
-
1
½
Hg
2
þ
:
½
Hg
2
þ
T
7
:
3
10
15
D
½
HgCl
þ
:
½
Hg
2
þ
T
D
K1
½
Cl
2
:
2
10
8
D
b
2
½
Cl
2
½
HgCl
2
:
½
Hg
2
þ
T
3
:
7
10
2
ð
4%
Þ
D
b
3
½
Cl
3
½
HgCl
3
:
½
Hg
2
þ
T
1
:
5
10
1
ð
15%
Þ
½
HgCl
2
-
4
D
b
4
½
Cl
4
:
½
Hg
2
þ
T
8
:
2
10
1
ð
82%
Þ
Thus, at the high chloride concentration found in seawater, the
mercury speciation is dominated by the tetra- and tri-chloro species.
Note (1) If the total mercury content for a given seawater sample were
known, the concentration of each species is readily calculated. That is
[Hg
21
]
¼
f
HG
21
[Hg
21
]T;
(2) This is a simplified example that has not considered other species
(bromides, hydroxides, etc.) that might be important in seawater.
The speciation of constituents in solution can be calculated if the
individual ion activities and stability constants are known. This infor-
mation is relatively well known with respect to the major constituents in
seawater, but not for all trace elements. Some important confounding
variables create considerable difficulties in speciation modelling. Firstly,
it is assumed that equilibrium is achieved, meaning that neither biolog-
ical interference nor kinetic hindrance prevents this state. Secondly,
seawater contains appreciable amounts (at least in relation to the trace
metals) of organic matter. However, the composition of the organic
matrix, the number of available binding sites, and the appropriate
stability constants are poorly known. Nevertheless, speciation models
can include estimates of these parameters. Organic material can form
chelates with relatively high-stability constants and dramatically de-
crease the free ion activity of both necessary and toxic trace elements.
Organisms may make use of such chemistry, producing compounds
either to sequester metals in limited supply (e.g., siderophores to com-
plex Fe) or to detoxify contaminants (e.g., metallothionein to chelate
Cd, Hg, etc.). Thirdly, surface adsorption of dissolved species onto
colloids or suspended particles may remove them from solution. As with
organic matter, an exact understanding of the complexation character-
istics of the suspended particles is not available, but approximations can
also be incorporated into speciation models.
Elements may be present in a variety of phases other than in true
solution. Colloidal formation is particularly important for elements such
as Fe and Mn, which produce amorphous oxyhydroxides with very great
