Environmental Engineering Reference
In-Depth Information
Reducing environment
Oxidizing environment
Activation energy
Activation energy
A
mmoniu
m
Nitrate
Net energy yield
Net energy yield
Nitrate
Ammonium
Going with potential energy
Going with potential energy
Activation energy
Activation energy
A
mmoniu
m
Nitrate
Net energy cost
Net energy cost
Ammonium
Nitrate
Going against potential energy
Going against potential energy
FIGURE 11.5
Representative Gibb's free energy diagrams for ammonium and nitrate ions
in oxidizing and reducing environments. The activation energy is the energy required to move
from a state of high to low potential energy, and the energy yield is what is released when
the transformation occurs. The activation energy plus the energy yield are required to ac-
complish a transformation from low to high potential energy.
energy than nitrate under oxidizing conditions. The activation energy of
this conversion is too high for the reaction to proceed at significant rates
under normal conditions found in natural aquatic environments.
Dissolved oxygen gas (O
2
) is a major determinant of redox because it
has a tremendous affinity for electrons. When O
2
is present, redox values
are high—generally in excess of 200 mV. This high redox potential signals
an oxidizing environment that allows only specific chemical reactions to
proceed without a net input of energy.
Iron concentrations in a dimictic lake are an example of how O
2
con-
centrations regulate redox potential to control the concentration of a chem-
ical in the aquatic environment (Fig. 11.7). In this case, ferrous iron (Fe
2
),
the reduced form of iron, is soluble, but ferric iron (Fe
3
), the oxidized form
of iron, forms an insoluble precipitate in water. Ferrous iron converts read-
ily to ferric iron in the presence of O
2
(i.e., the activation energy for the re-
action is low) and then forms insoluble precipitates that settle with other
sediments. When the lake is mixed fully and in contact with the atmosphere,
iron concentrations are low throughout the water column. As O
2
is depleted
from the deeper stratified layers, the dissolved iron concentration increases
as ferrous iron diffuses out of the sediment. In this case, low redox poten-
tial and high dissolved iron concentrations are correlated closely.
Organisms can promote chemical reactions that would not otherwise
occur by lowering the activation energy. This promotion is accomplished
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