Agriculture Reference
In-Depth Information
dm
−
3
dm
3
s
−
1
,
with
all
concentrations
in
mol
suspension).
Adsorption
is
described by
[Fe
2
+
]
ad
/
[Fe
2
+
]
=
K
[Fe(III)]
/
[H
+
]
(
4.38a
)
10
−
14
.
3
. Other metal
oxidation reactions catalysed by sorption onto oxide surfaces are described in
Section 7.3.
where [Fe(III)] is the concentration of Fe(OH)
3
and
K
=
Soil
A similar catalysis occurs on soil surfaces. Ahmad and Nye (1990) and Kirk
and Solivas (1994) studied the kinetics of Fe
2
+
oxidation in soil suspensions by
measuring changes in extractable Fe
2
+
in the whole soil and in solution during
oxidation at constant [O
2
]. They found that 75 % of the initial Fe
2
+
was oxidized
rapidly
(t
1
/
2
≈
8days
)
.Inthe
soils studied, the pH fell from near neutral to less than 5 over the course of the fast
reaction. Measurements of the fast reaction at constant pH (Figure 4.16) showed
that the oxidation of adsorbed Fe
2
+
was much faster than solution Fe
2
+
,and
that the adsorbed Fe
2
+
was oxidized at a rate that was nearly independent of pH.
Figure 4.16 shows that the overall rate of oxidation is more dependent on the con-
centration of sorbed Fe
2
+
(
[Fe
2
+
]
S
)
than the concentration in solution
(
[Fe
2
+
]
L
)
.
Thus, although at pH 6.5 [Fe
2
+
]
L
drops to one-tenth of its initial value within 1 h,
d[Fe
2
+
]
/
d
t
does not decrease to nearly the same extent. The figure also shows that
oxidation of sorbed Fe
2
+
, indicated by the slopes of the lines in Figure 4.16(c),
is surprisingly little influenced by pH and roughly follows first-order kinetics.
The overall rate equation at constant pH is therefore
2h
)
and the remainder only very slowly
(t
1
/
2
≈
−
d[Fe
2
+
]
/
d
t
=
Rk
L
[O
2
]
L
[Fe
2
+
]
L
+
k
S
[O
2
]
L
[Fe
2
+
]
S
(
4
.
39
)
where
k
L
and
k
S
are the rate constants for the reactions in the solution and
solid and
R
the solution to solid ratio. Values of
k
S
, calculated from the data in
Figure 4.16(c) range from 0
.
19 mol
−
1
dm
3
s
−
1
at pH 6.5 to 0
.
15 mol
−
1
dm
3
s
−
1
at pH 5.
Initially, most of the readily oxidizable Fe(II) is sorbed on the soil exchange
complex. As the soil is oxidized, the Fe(OH)
3
formed provides fresh sorption
sites, as well as possibly blocking some of the original sites. Ahmad and Nye
(1990) estimated the importance of the freshly formed Fe(OH)
3
in sorbing Fe
2
+
compared with the original soil exchange complex, and found that the importance
of the freshly formed Fe(OH)
3
was much greater at higher pH, consistent with
the expected greater pH-dependence of sorption on Fe(OH)
3
surfaces.
They also found that the oxidation of Fe
2
+
when sorbed on a mixture of soil
exchange and Fe(OH)
3
sites was much slower than on Fe(OH)
3
in the absence
of soil, described by Equations (4.38) and (4.38a).
