Geoscience Reference
In-Depth Information
mangrove swamps, producers and destroyers of sulphides coexist. But,
in the main, sulphides accumulate in large quantities.
Looked at in detail, matters are slightly more complex than stated
above. Many kinds of iron oxide, listed in Table 12.9, may form. It should
not be surprising then that iron sulphides answer to the formulae FeS,
FeS
2
or Fe
3
S
4
, according to authors.
Formation of different kinds of sulphides is related to differences in
pH. Also, they interact. For example, mackinawite can be oxidized to
marcasite or to pyrite. Sulphur, S
2
, may appear transiently.
Table 12.9
Iron sulphides found in mangrove swamps (Bloomfield and
Coulter 1973).
Formula
Crystal system
Name
Fe
(1 -
x
)
S
with 0 <
x
< 0.126
Hexagonal or monoclinic
Pyrrhotite
(
Fe, Ni
)
9
S
8
Tetragonal
Mackinawite
Fe
3
S
4
Cubic
Griegite
Fe
3
S
4
Hexagonal or monoclinic
Smythite
FeS
2
Cubic
Pyrite
FeS
2
Orthorhombic
Marcasite
When mangrove swamps are dried out by polderization and drainage for
making them cultivable, the sulphides are again oxidized to sulphates.
The reaction, complex in details, may be summarized as follows
(Le Brusq
et al
. 1987):
Oxidation of sulphides
FeS
2
(
solid
) +
15
2
O
2
+
5
___
__
2
H
2
O
Æ
FeOOH
(
solid
) + 2
SO
4
2 -
+ 4
H
+
(1)
Ferric oxyhydroxides are precipitated. At the same time, sulphuric
acid is obtained in solution, leading to considerable acidification. The
pH drops to 2, especially when the sediments are not calcareous. It is
observed that this kind of reaction takes place not only in mangrove
swamps but also in certain mine spoils leading to severe pollution.
Bacteria such as
Thiobacillus ferrooxidans
and
Thiobacillus thiooxidans
are
involved. But their action is complex. Firstly, they do not attack all iron
sulphides. Secondly, they participate in more or less varied intermediate
reactions.
When acidity is high, the oxyhydroxides are not stable and are
transformed as follows:
FeOOH
+
K
+
+ 2
SO
4
2 -
+ 3
H
+
Æ
KFe
3
(
SO
4
)
2
(
OH
)
6
(2)
