Agriculture Reference
In-Depth Information
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.0
0.5
1.0
1.5
2.0
Amount of Fe(III) reduced (mmol g
−
1
)
Figure 3.9
Relation between surface charge and reduction of structural Fe in a diocta-
hedral smectite. Points are experimental data; lines are theoretical relations discussed in
the text (Drits and Manceau, 2000). Reproduced by permission of Clay Minerals Society
where M is a sorbed cation and
m, n
and
p
are coefficients. The solid line in
Figure 3.9 shows the fit to Equation (3.45) and the dotted line shows the expected
relationship if only dehydroxylation occurs. If the generic reaction is simplified to
[clay—Fe
(
III
)
OH]
+
n
H
+
+
e
−
−−−→
[clay—Fe
(
II
)
OH
1
−
n
]
(
1
−
n)
−
+
n
H
2
O
(
3
.
46
)
then using the upper value
n
=
0
.
75 (Figure 3.9), the complete reaction with
simultaneous oxidation of organic matter (as for Reaction 3.44) is:
4[clay—Fe
(
III
)
OH]
+
CH
2
O
−−−→
4[clay-Fe
(
II
)
OH
0
.
25
]
0
.
25
−
+
H
+
+
CO
2
+
H
2
O
(3.47)
In Reaction (3.47), for each mol of Fe reduced the surface negative charge
increases by 0
.
25 mol
c
and 0.25mol of H
+
are released.
For moderate reduction the changes are completely reversible but they are
progressively less so with more thorough reduction (Stucki
et al
., 1984; Komadel
et al
., 1995; Gates
et al
., 1996). There are concomitant changes in the clay's
physical and chemical properties, including its surface area, swelling behaviour,
and capacity to sorb cations.
Changes in pH-dependent Charge
. Changes in pH with soil reduction will cause
changes in the charges on inorganic -OH functional groups and organic mat-
ter. From Equation (3.41), the increase due to organic functional groups will
be approximately 0
.
5mmol
c
g
−
1
organic C per unit pH increase, or, for a soil
with 1% organic C, 5 mmol
c
kg
−
1
soil. This is small compared with the changes
due to dissolution of oxide coatings and reduction of structural Fe, which are of
the order of several tens of mmol
c
kg
−
1
soil. But it may be important in highly
