Agriculture Reference
In-Depth Information
dissociates to HS
−
(
p
K
=
7
.
02
)
. For the reduction of SO
4
2
−
the net reaction is
1
4
'CH
2
O'
+
1
8
SO
4
2
−
+
1
8
H
+
=
1
4
CO
2
(
g
)
+
1
8
HS
−
=−
20
.
5kJmol
−
1
at pH 7. H
2
SandHS
−
are also produced in the
hydrolysis of the S-containing amino acids.
The HS
−
formed further dissociates to S
2
−
(
p
K
=
13
.
9
)
. However in most
submerged soils the concentration of Fe
2
+
in the soil solution is sufficient that
virtually all S
2
−
is precipitated as amorphous ferrous sulfide and very small
concentrations of H
2
SandHS
−
remain in solution. The relations between the
SO
4
2
−
-HS
−
and Fe(OH)
3
-Fe
2
+
systems at neutral pH are shown in Figure 4.12.
Amorphous ferrous sulfide may gradually crystallize as mackinawite (FeS). Under
some circumstances pyrite is then formed, e.g. FeS(s)
+
S(s)
→
FeS
2
(s), leading
to potential acid sulfate soils (Section 7.3).
There may be a cycling of S compounds of different oxidation state between
anaerobic and aerobic zones in the soil, such as at the soil — floodwater interface.
In reduced lake and marine sediments this leads to accumulation of insoluble
sulfides as SO
4
2
−
carried into the sediment from the water above is immobilized.
Such deposits function as sinks for heavy metals. Plants absorb S through their
rootsasSO
4
2
−
;H
2
S is toxic to them. Therefore HS
−
must be oxidized to SO
4
2
−
in the rhizosphere before it is absorbed.
Figure 4.12 shows changes in the concentration of SO
4
2
−
in the soil solution
following submergence of a range of soils. In neutral and alkaline soils con-
centrations of SO
4
2
−
greater than 10 mM may decrease to 0 within 6 weeks of
submergence. In acid soils the concentration of SO
4
2
−
in solution may initially
increase following submergence and then slowly decline over several months.
and
G
o
FeS
FeCO
3
Fe(OH)
3
0
−
2
H
2
S
+
HS
−
SO
4
2
−
−
4
−
6
−
8
Fe
2
+
−
10
−
12
−
8
−
6
−
4
−
202468 0
pe
Figure 4.12
7,
C
T
(total carbonate carbon)
=
5mM and [SO
4
2
−
]
+
[H
2
S(aq)]
+
[HS
−
]
=
1 mM (modified
from Stumm and Morgan, 1996). Reproduced by permission of Wiley, New York
Concentration - pe diagram for FeS, FeCO
3
and Fe(OH)
3
at pH
=
