Agriculture Reference
In-Depth Information
then the overall reaction is
6H
+
−−−→
2[soil—]
−
+
4Fe
2
+
+
+
+
CO
2
+
9H
2
O
(
3
.
44
)
In Reaction (3.44), for each mol of Fe reduced the surface negative charge
increases by 0
.
5mol
c
and 1.5mol of H
+
are consumed.
Roth
et al
. (1969) found increases in surface negative charge equivalent to
10-60% of the initial charge for a range of soils and soil clays. The change
could be attributed quantitatively to the removal of the positively charged oxide
coatings and was reversed by re-oxidizing the samples. Changes in charge with
reductive dissolution of oxides have been demonstrated using chemical reduc-
ing agents (Roth
et al
., 1969) and microbial reducing agents (Bloomfield, 1951;
Ottow, 1973; Lovley, 1991), and under field conditions (Favre
et al
., 2002).
Dissolution and reduction of crystalline Fe(III) minerals is accelerated by chela-
tion with carboxylate ligands in the presence of Fe(II) (Zinder
et al
., 1986; Blesa
et al
., 1987; Phillips
et al
., 1993; Kostka and Luther, 1994). Therefore as soil
reduction proceeds and carboxylates formed in oxidation of organic matter accu-
mulate in solution together with Fe
2
+
, dissolution and reduction of crystalline
Fe(III) will commence. Dissolution of oxyhydroxide coatings will therefore lag
behind the initial reduction of Fe(III).
2[soil—2Fe(OH)
2
.
5
]
CH
2
O
Reduction of Structural Fe
. There may also be changes in charge due to reduc-
tion of structural Fe(III). Virtually all soil clay minerals contain some iron in
their crystal structures and reduction of this structural Fe by chemical or micro-
bial reducing agents, with the iron remaining octahedrally coordinated in the clay
structure, is well documented (Stucki, 1988; Stucki
et al
., 1997). The extent of
reduction, whether by microbes or chemical reducting agents, can be as much
as 90% of the octahedral Fe(III) in a few days (Kostka
et al
., 1999). The rate
is enhanced by the presence of organic chelating agents that commonly occur in
sediments and flooded soil solutions, and under such conditions Fe(III) reduction
may lead to partial dissolution of the clay (Kostka
et al
., 1999).
As structural Fe(III) is reduced, the negative charge on the clay will increase.
It is found experimentally that the increase in negative charge is not directly
equivalent to the amount of Fe(III) reduced, and the more reduced the clay is the
smaller is the change in charge. An example is shown in Figure 3.9. The mech-
anism behind this is uncertain but involves dehydroxylation of the clay structure
during reduction and sorption of metal cations from the solution (Stucki
et al
.,
1997; Drits and Manceau, 2000). The extent of dehydroxylation and sorption
varies with the extent of reduction, hence the change is nonlinearly related to the
amount of Fe reduced. For example, for a nontronite:
M[Si
7
Al]Fe
(
III
)
4
O
20
(
OH
)
4
+
m
M
+
+
n
H
+
+
p
e
−
−−−→
M
1
+
m
[Si
7
Al]Fe
(
III
)
4
−
p
Fe
(
III
)
p
O
20
(
OH
)
4
−
n
+
n
H
2
O
(
3
.
45
)
