Environmental Engineering Reference
In-Depth Information
The sea loor moves at a rate of several centimeters per year. The past upheaval formed
the mountains consisting of marine sediments containing carbonates. The formation of
high mountains by limestone is a typical example of the fate of carbonates formed by coc-
colith and foraminifers. Sedimentary rock and soil contain varying amounts of carbonate,
and over long time periods, the carbonate fractions in rock and soil may be subjected to
metamorphism.
12.3.2.2 Coral Reef and Shells in Coastal Areas
Coral reefs are well-known sources of carbonates. At the beach, shells constitute sources
of carbonates. The carbonates of coral reefs and shells are widely distributed as a result of
past mass movements, and may have undergone metamorphism—usually in the form of
dissolution and recrystallization.
12.3.2.3 Microbially Induced Carbonates
There are microbial processes that can produce carbonates. In chemical reactions with
nitrogen (organic matter), carbonate can be produced as follows (Castanier et al., 1999):
Organic matter + Ca
2+
→ CaCO
3
↓ (aerobic)
(12.2)
Organic matter + Ca
2+
+ NO
3
→ CaCO
3
↓ (anaerobic)
(12.3)
Organic matter + Ca
2+
+ urease or uric acid → CaCO
3
↓ (aerobic),
(12.4)
where the organic matter can be degraded by the proper (aerobic or anaerobic) microbes.
In a sulfur cycle, carbonate will be produced as follows:
In the case of reduction of
SO
2−
2
+
2
+
2
−
−
OrganicmatterCa
+
+ →+ + →
O
O
CO
HS
CaC
O
3
↓
,
(12.5)
4
3
3
2
where the organic matter can be degraded by anaerobic microbes. If hydrogen sulide
(H
2
S) exists under aerobic conditions, the following reaction occurs.
+→
+
2
−
HS
2
OH+SO
2
(12.6)
2
2
4
Thus, sulfuric acid (H
2
SO
4
) is obtained. In this case, the pH decreases and no carbonate
will be produced.
A good example of a form of artiicial diagenesis is beachrocks, which are cemented by
carbonates. According to Vieira and De Ros (2006), beachrocks are sediments cemented
with carbonate in tropical coastal areas. There are, however, other views on the mecha-
nisms involved in the formation of beachrocks. Neumeier (1999), for example, indicates
that the binder for beachrock is Mg-calcite or aragonite. Regardless of the type or types of
mechanism involved in the formation of beachrocks, there are certain necessary conditions
for the formation of beachrocks. Neumeier (1999) and Khadkikar and Rajshekhar (2003)
have pointed out that microbes play an important role in the formation of beachrocks—a
fact that was experimentally conirmed by Neumeier (1999).
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