Chemistry Reference
In-Depth Information
Table 3.9
Residence times of some major species in
seawater
Table 3.10
Cation exchange capacities of some sorptive
materials
Ion
Residence time (years)
Cation exchange capacity
K
+
7
¥
10
6
Material
(milliequivalents per 100 g)
Na
+
6.
8
¥
10
7
Li
+
2.3
¥
10
6
Kaolinite
3-15
Mg
2
+
1.2
¥
10
7
Illite
10-40
Ca
2
+
1
¥
10
6
Chlorite
20-50
Sr
2
+
4
¥
10
6
Montmorillonite
8
0-120
2
¥
10
2
Fe(OH)
3
Fe(OH)
3
(freshly precipitated)
10-25
1
¥
10
2
Al(OH)
3
Silicic acid (amorphous)
11-34
Cl
-
1.3
¥
10
8
Humic acids
170-590
Br
-
1
¥
10
8
SO
2
-
1.2
¥
10
7
Source: Ref. 23.
HCO
-
9
¥
10
4
Source: Refs 10 and 22.
chemical composition. This has a significant impact
on the substantial loads of suspended sediments
carried by rivers and the speciation of dissolved
cations. The position of this mixing zone shows sea-
sonal variation due to non-uniform river flux.
The pH of river water will vary depending upon
the geology over which the river flows, but it is
normally below that of seawater (pH 8) and the im-
portance of OH
-
ions for the water chemistry
increases down the estuary. Increasing Cl
-
concen-
tration causes the formation of chloro-complexes of
metals such as Cd, Hg and Zn. Metals such as Mn
and Zn, which in river water may be complexed by
humic materials, may become displaced in the com-
plexes by Ca and Mg as the latter metals increase in
concentration as the river meets the sea.
The most significant chemistry taking place in the
estuary, however, concerns particulate dissolution,
precipitation and deposition. River particulates are
kept in suspension by the mutual repulsion of their
surface negative charges. As ionic strength increases,
these charges become neutralised by cations, leading
to coagulation or flocculation with organic materials
followed by deposition. Other particulate processes
include cation exchange and the precipitation of Fe
and Mn oxyhydroxides.
Estuaries are areas of high biological activity and
can capture and trap dissolved nutrients. When
the fresh and brackish water organisms meet
stronger saline conditions they die and rupture,
releasing the nutrients to create localised areas of
high concentration.
280 mgkg
-1
at the surface, where it is in equilibrium
with atmospheric oxygen, to 20 mgkg
-1
at 500 m
depth as the oxygen is used in chemical and bio-
chemical oxidation. In deeper waters, which origi-
nate at the poles where oxidation reactions are less
prevalent, the oxygen level is higher, rising to 150 mg
kg
-1
at 4000 m depth. The dissolved oxygen concen-
tration determines the redox potential of the water.
Oxidising conditions normally prevail. However,
where large quantities of organic material reach the
deep ocean, oxygen depletion can lead to reducing
conditions in which, for example, NO
3
-
is reduced to
NO
2
-
. In exceptional circumstances some enclosed
seas such as the Black Sea and some Norwegian
fiords can become anoxic at depth.
Surface chemistry also plays a significant role in
biogeochemical cycling, particularly of metal ions.
Exchange of material from solution to suspended
particulates followed by sedimentation provides an
effective scavenging mechanism. The cation ex-
change capacities of some sorptive materials are
given in Table 3.10.
4.2 Chemistry of estuaries
The ion composition of freshwater principally com-
prises Ca
2+
and HCO
3
-
, whereas seawater is dom-
inated by Na
+
and Cl
-
. Moreover, the total ionic
concentration of river water is 500 times less than
seawater. Estuaries are mixing zones between the
two, with sharp gradients in ionic strength and
