Agriculture Reference
In-Depth Information
may be complicated by the presence of other redox species with which Fe reacts,
such as sulfide, and by the slow kinetics of redox and precipitation reactions and
the need for microbial mediation. Thus for example siderite is rarely found in
soils though in many cases it is the thermodynamically favoured phase as shown
in Figure 4.2. These points are discussed further in later sections.
Because of the sensitivity of pe to pH it is often convenient to compare pe
o
values 'corrected' to pH 7 and termed pe
o
∗
,where:
1
n
log
(
Red
)
(
Ox
)
−
7
m
pe
o
∗
=
pe
o
−
(
4
.
22
)
n
As discussed earlier, the concentration-dependent term in Equation (4.22) will
often be small in comparison with the pH term and can be ignored. For couples
in which the concentration term is more important, such as Fe(OH)
3
-Fe
2
+
,pe
o
∗
values can be calculated for representative concentrations.
Table 4.2 gives pe
o
∗
values for important redox couples in natural systems
arranged in order of decreasing pe
o
∗
with strong oxidants at the top and strong
reductants at the bottom. From such a table it is possible to infer which couples
will react when present together and which will have the oxidizing role and which
the reducing role. The table shows for example that Fe(OH)
3
can readily oxidize
organic matter 'CH
2
O' to form CO
2
and Fe
2
+
but it cannot oxidize N
2
to NO
3
−
.
However note that the pe
o
∗
value for the Fe(OH)
3
-Fe
2
+
couple is sensitive to
the value of
(
Fe
2
+
)
.
4.1.5
ENERGETICS OF REACTIONS MEDIATED BY MICROBES
Most redox reactions
in vitro
reach equilibrium only extremely slowly with half
times of the order of months or years, even though they may be highly favoured
thermodynamically. This is illustrated by the persistence of N
2
in oxic systems
even though its oxidation to NO
3
−
is strongly favoured (Table 4.1). However,
microbes in soil and water are capable of catalysing particular reactions from
which they obtain energy for metabolism. The half times of such microbially
catalysed reactions are of the order of hours or days.
The amounts of energy consumed or produced in redox reactions, and hence
the efficiency with which they can be exploited by microbes, can be calculated
from thermodynamic data. This gives surprisingly good insights into the dynam-
ics of microbial communities in natural systems without detailed knowledge of
the biochemical and physiological pathways involved. For example, the sequence
of reduction reactions that occur in submerged soils following exclusion of O
2
matches the order of decreasing free energy change for the corresponding redox
reactions. Note that organisms cannot carry out
gross
reactions that are thermo-
dynamically impossible: they do not oxidize substrates or reduce oxidants
per se
,
but merely catalyse the process by mediating the electron transfers occurring. The
energy produced or consumed in a given redox reaction is calculated as follows.
