Environmental Engineering Reference
In-Depth Information
-800
-360
80
520
960
1400
O
2
- Reduction
Denitrification
Mn (IV) oxide to Mn (II)
NO
3
-
Reduction
Reductions
Fe (III) oxide to Fe (II)
Reduction organic material
SO
4
2-
Reduction
CH
4
Fermentation
N
2
to NH
4
+
H
2
Formation
Oxidat. org. mat.
Sulfide to SO
4
2-
Oxidations
Oxidation of Fe (II)
NH
4
+
to NO
3
-
O
xidation of Mn
(II)
N
2
to NO
3
-
O
2
- Formation
-800
-360
80
520
960
1400
E
H
(mV)
FIGURE 11.6
Microbe-mediated chemical transformations plotted to show energy yield as
the difference between the tail and the head of the arrow and redox potential required to
complete transformation (redrawn from W. Stumm, and J. J. Morgan,
Aquatic Chemistry: An
Introduction Emphasizing Chemical Equilibria in Natural Waters
. Copyright © 1981 John
Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.).
with enzymes that lower activation energy and catalyze the reactions. In
the previous example, in which ammonium is stable in aquatic habitats
containing O
2
, microorganisms can lower the activation energy required to
oxidize ammonium to nitrate. This reaction releases energy because nitrate
has a lower potential energy than ammonium in the presence of O
2
. The
bacteria can direct this energy toward cellular growth. The process is called
nitrification and will be discussed in greater detail in Chapter 13.
Organisms can also drive chemical reactions against potential energy
(create more energetic chemical compounds). Such reactions require more
input of potential energy than is stored in the products. Photosynthesis is
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