Geoscience Reference
In-Depth Information
Because the oxidation of organic matter is a bacterially mediated process, oxi-
dation rate is a function of water temperature. Temperature correction of the rate
can be made using an Arrhenius type of equation. Temperature correction coefficients
(
θ
) range from 1.02 to 1.09. A commonly used value for this coefficient is 1.047.
45
4.1.4.1.4 Oxidation of Inorganic Matter
Under aerobic conditions, most common metals and some nonmetallic elements,
like sulfur, are thermodynamically stable in their highest oxidation states. Ferric iron
(Fe
3+
) is more stable than ferrous iron (Fe
2+
), and similarly, sulfate is more stable
than sulfide. A series of chemical reductions occurs when the oxygen is consumed.
All of these reactions tend to reverse if oxygen is reintroduced.
23
4.1.4.1.4.1 Sulfide Oxidation
Sulfide is produced in anaerobic sediment as a result of organic matter oxidation,
in which sulfate is used as the electron acceptor. A portion of the sulfide precipitates
as FeS
(S)
, while the remaining dissolved sulfide diffuses into the aerobic zone,
where it is oxidized back to sulfate. Dissolved oxygen is consumed during this
last step.
27,74
H
2
S
+
2O
2
→
H
2
SO
4
(4.10)
The particulate sulfide (FeS
(S)
) can also be mixed into the aerobic zone, where it can
be oxidized by oxygen to Fe
2
O
3(S)
. A portion of the FeS
(S)
is buried by sedimentation.
74
9
4
1
2
FeS
+
O
2
+
H
2
O
→
Fe
2
O
3
+
H
2
SO
4
(4.11)
4.1.4.1.4.2 Iron Oxidation
Fe
2+
compounds are more soluble than Fe
3+
compounds, and thus exist in the low
mg l
−1
range in sediment pore waters. As a consequence, Fe
2+
can diffuse to the oxic
layer of the sediment, and via the loss of one electron, can be oxidized to Fe
3+
by
the oxygen present there.
74
Fe
2+
→
Fe
3+
+
e
−
(4.12)
Half-reaction for oxygen is
O
2
+
4H
+
+
4e
−
→
2H
2
O
(4.13)
followed by the precipitation of iron oxyhydroxide
Fe
3+
+
2H
2
O
→
FeOOH
(S)
+
3H
+
(4.14)
to yield the overall redox reaction
Fe
2+
+
1
4
O
2
+
H
2
O
3
2
→
FeOOH
(S)
+
2H
+
(4.15)
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