Environmental Engineering Reference
In-Depth Information
the necessary conditions to initiate their activity, they can act on any given type
of waste.
The ultimate products of biodegradation by microorganisms are inorganic com-
pounds (CO
2
and H
2
O). This kind of a complete conversion process is called
mineralization
. The following questions regarding mineralization become important:
(a) Why do microorganisms transform compounds? (b) What mechanisms do they
employ in breaking down complex molecules? (c) How do the microorganisms affect
chemical transformation rates?. The first question can be answered from thermody-
namic principles. The second question needs principles from biochemistry, which we
will not pursue here.The third question concerns chemical kinetics, which is the focus
of this section.
Thermodynamics provides clues to the energy of microbial processes. Consider
the oxidation of glucose:
C
6
H
12
O
6
+
8O
2
−→
6CO
2
+
12H
2
O
+
2870 kJ.
(6.230)
This is called an
exergonic
reaction. For every 1 mol of glucose consumed, an organ-
ism can obtain 2870 kJ of energy.
Endergonic
reactions, on the other hand, require
organisms to consume energy from the surroundings for conversion of chemicals.
Microorganisms (living cells) couple exergonic and endergonic reactions to accom-
plish the goal of lowering the free energy. This is done using intermediate chemicals
that can store energy released during an exergonic process, and transfer the stored
energy to the site where an endergonic process takes place. This is the basis of liv-
ing cell metabolism. Table 6.16 gives the magnitudes of
G
0
for some common
Δ
microbial-mediated environmental processes.
To accomplish the types of processes discussed above, living cells make use
of intermediates such as adenosine triphosphate (ATP) and guanosine triphosphate
(GTP). By far,ATP is the most important and is the universal transfer agent of chemi-
cal energy-yielding and energy-requiring reactions. ATP formation will store energy,
whereasitshydrolysiswillreleaseenergy.Organismsthathaveadenosinediphosphate
(ADP) will first convert it toATP by the addition of a phosphate group, which requires
energy to be consumed.This is called
phosphorylation
. Subsequent hydrolysis ofATP
toADP releases the stored energy (
G
0
Δ
=−
30 kJ/mol). During a redox process, the
TABLE 6.16
A Sampling of Microbially Mediated Environmental Redox Processes
G
0
(
w
)
(kJ/mol)
Reaction
−
Δ
Fermentation:
4
CH
2
O
+
4
H
2
O
4
CO
2
+
2
H
2
(
g
)
1.1
Aerobic respiration:
4
CH
2
O
+
2
O
2
4
CO
2
(
g
)
+
4
H
2
O
119
Nitrogen fixation:
6
N
2
(
g
)
+
1
3
H
+
+
1
4
CH
2
O
1
3
NH
4
1
4
CO
2
+
14.3
Carbon fixation:
4
CO
2
(
g
)
+
1
2
H
2
O
1
4
CH
2
O
1
4
O
2
(
g
)
+
−
119
Note:
CH
2
O represents 1/6C
6
H
12
O
6
(glucose).
Δ
G
σ
(w)
was defined in Section 5.8 as the difference
in
pe
0
(w)
between the oxidant and reductant.
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