oxidation states of iron occur not only as free ions Fe
2+ and Fe
3+ but also as many
hydroxo- and other complexes (sulfate, chloride, etc.). The activities of these ions are
then significantly different from the analytical total concentrations (often almost
negligible) of the valence forms and therefore the Eh calculation is incorrect.
This behavior can be documented using a simple example: in a solution at
pH ¼ 4.88 and Eh ¼ 481.8 mV, there is 20 μmol/L of dissolved iron (1.12 mg/L).
Analytically, the concentrations of dissolved iron in a divalent form of 17.4 μmol/L
(87.2% of the total concentration of dissolved iron) and in a trivalent form of
2.4 μmol/L were determined. The redox potential calculated from these total concentrations using the relations (Eq. 1.12) and (Eq. 1.9) gives Eh ¼ 720 mV, a
significantly different value from the input value of 481.1 mV. This also shows an
oxidizing environment, while the actual redox potential corresponds to anoxic or
mildly reducing conditions (Fig. 1.1). Under the given conditions, both cations
undergo a complexation reaction with the hydroxyl anion and form
hydroxocomplexes. While free Fe
2+ ions are prevalent, constituting 99.63% of its
total Fe
+II content (Fe(OH)
+ being in the minority), for a trivalent iron
hydroxocomplex, Fe(OH)
2+ constitutes 92.96% of its overall content and free Fe
3+
ions contribute only negligibly to the total content (less than 0.01%). If we use the
free concentrations of the participating substances calculated by the speciation
model, we obtain the true value of 481.8 mV for the redox potential.
1.6 Geochemical Processes in Water
The main geochemical parameters and the stability of the environment are determined by pH and the oxidative reduction potential of Eh. The stability of pH in
natural waters is determined by the carbonate system and exchange reactions of clay
minerals. The acidity of the natural environment is determined by the CO 2 cycle and
alkalinity by dissolving and weathering limestone and silicate rocks. In the case of
Eh, the oxidation capacity reservoir is not only the oxygen itself (in the atmosphere
and dissolved in the water), but also oxidized substances (e.g., nitrates). Reduction
capacity is determined primarily by dead organic matter and also by reduced substances (e.g., sulfides). Putting aside the crucial redox reactions of biogenic processes (photosynthesis vs. respiration and decomposition), it is possible to describe
the most important redox actions in exogenous processes by the following reactions:
Oxygen reduction/decomposition of water.
½O 2 þ 2e
À
þ 2H
þ
$ H 2 O
ð1:13Þ
Weathering/crystallization of pyrite.
FeS 2 þ 8H 2 O $ Fe
2þ
þ 2SO
2À
4 þ 14e
À
þ 16H
þ
ð1:14Þ
Oxidation/reduction of nitrogen.
10
M. Černík and J. Zeman
2+ and Fe
3+ but also as many
hydroxo- and other complexes (sulfate, chloride, etc.). The activities of these ions are
then significantly different from the analytical total concentrations (often almost
negligible) of the valence forms and therefore the Eh calculation is incorrect.
This behavior can be documented using a simple example: in a solution at
pH ¼ 4.88 and Eh ¼ 481.8 mV, there is 20 μmol/L of dissolved iron (1.12 mg/L).
Analytically, the concentrations of dissolved iron in a divalent form of 17.4 μmol/L
(87.2% of the total concentration of dissolved iron) and in a trivalent form of
2.4 μmol/L were determined. The redox potential calculated from these total concentrations using the relations (Eq. 1.12) and (Eq. 1.9) gives Eh ¼ 720 mV, a
significantly different value from the input value of 481.1 mV. This also shows an
oxidizing environment, while the actual redox potential corresponds to anoxic or
mildly reducing conditions (Fig. 1.1). Under the given conditions, both cations
undergo a complexation reaction with the hydroxyl anion and form
hydroxocomplexes. While free Fe
2+ ions are prevalent, constituting 99.63% of its
total Fe
+II content (Fe(OH)
+ being in the minority), for a trivalent iron
hydroxocomplex, Fe(OH)
2+ constitutes 92.96% of its overall content and free Fe
3+
ions contribute only negligibly to the total content (less than 0.01%). If we use the
free concentrations of the participating substances calculated by the speciation
model, we obtain the true value of 481.8 mV for the redox potential.
1.6 Geochemical Processes in Water
The main geochemical parameters and the stability of the environment are determined by pH and the oxidative reduction potential of Eh. The stability of pH in
natural waters is determined by the carbonate system and exchange reactions of clay
minerals. The acidity of the natural environment is determined by the CO 2 cycle and
alkalinity by dissolving and weathering limestone and silicate rocks. In the case of
Eh, the oxidation capacity reservoir is not only the oxygen itself (in the atmosphere
and dissolved in the water), but also oxidized substances (e.g., nitrates). Reduction
capacity is determined primarily by dead organic matter and also by reduced substances (e.g., sulfides). Putting aside the crucial redox reactions of biogenic processes (photosynthesis vs. respiration and decomposition), it is possible to describe
the most important redox actions in exogenous processes by the following reactions:
Oxygen reduction/decomposition of water.
½O 2 þ 2e
À
þ 2H
þ
$ H 2 O
ð1:13Þ
Weathering/crystallization of pyrite.
FeS 2 þ 8H 2 O $ Fe
2þ
þ 2SO
2À
4 þ 14e
À
þ 16H
þ
ð1:14Þ
Oxidation/reduction of nitrogen.
10
M. Černík and J. Zeman