Geology Reference
In-Depth Information
Box 4.7 Oxidation and reduction ('redox') reactions:
Eh
In forming stable compounds with other elements, iron
may adopt one of two oxidation states. A
ferrous
atom has
donated or shared
two
of its electrons in forming bonds
with other atoms (Chapter 7) and is said to be
di
valent, or
in oxidation state II. Silicate minerals containing ferrous
ions are commonly grey or green in colour (e.g. olivine,
Box 9.10). Alternatively an iron atom may commit
three
electrons to bonding, forming a
tri
valent species (
ferric
iron, oxidation state III). Many compounds of ferric iron
have a red or orange colour (e.g. rust). These states of
iron can be symbolized Fe(II) and Fe(III). This notation rec-
ognizes that iron in these oxidation states may not
neces-
sarily
take the form of ions Fe
2+
and Fe
3+
: this depends on
the type of bonding involved, as explained in Chapter 7.
Pure metallic iron is said to be in oxidation state 0, writ-
ten Fe(0). Oxidation states of metals are discussed in
more detail in Chapter 9.
A reaction that causes iron to
increase
the number of its
electrons committed to bonds with other atoms is called
an oxidation reaction, as it increases the oxidation state
of iron. An example is the weathering of the iron-rich oli-
vine fayalite (a ferrous compound):
Oxidation of an ion (e.g. oxidizing Fe
2+
to Fe
3+
) equates to
the
removal
of one or more electrons.
Reduction is the opposite process, for example when
an oxidized element receives back some or all of its bond-
ing electrons, illustrated by the formation of native iron
when a basaltic lava comes into contact with bituminous
shale:
2
FeMgSiO
+ →+
C
2
Fe
2
MgSiO O
+
(4.7.2)
4
3
2
'
olivine carbon ative yroxenee as
component ontent
'
'
'
'
'
iron
:
component
()
inmelt
:
of shaleFe
inm
elt
Fe I
()
Reaction 4.7.2 also illustrates an extremely important
geochemical property of organic matter (containing C, H
and O) and elemental carbon: their capacity to act as a
natural
reducing agent
in a wide variety of geological
circumstances.
Oxidation and reduction ('redox') reactions are impor-
tant for many other elements that exist in nature in
more than one oxidation state. Carbon, for example,
can exist as the pure element [C(0) in diamond or
graphite], or combined with oxygen ('oxidized carbon',
C(IV) as in calcium carbonate, CaCO
3
). In organic com-
pounds, however, it is combined with hydrogen, an ele-
ment of lower electronegativity (Figure 6.3), and in this
2
Fe SiOOHO
+
+
→
2
Fe O
+
2
H SiO
(4.7.1)
2
4
2
2
23
2
4
olivine
air
water ematite issoll ved
ferric
(
formof
oxide
)
SiO
2
examples of
Eh
-pH diagrams are given in the classic
topic by Garrels and Christ (1965) and in a recent com-
pilation by the Geological Survey of Japan (2005).
rising in the Himalayas and Tibet - the Ganges and
Brahmaputra - discharge into the Indian Ocean.
Population pressures mean that surface water supplies
are often too polluted microbiologically to provide safe
drinking water (see Box 4.8). Groundwater develop-
ment for water supply purposes has therefore been
actively encouraged by government and international
agencies as a means of reducing the incidence of water-
borne diseases. Millions of wells have been drilled,
mostly into alluvial aquifers 10-60 m deep, although in
the south, where these shallow aquifers are too saline
for human consumption, deeper boreholes abstract
from aquifers at depths between 100 and 150 m.
However, drawing water from these deeper aquifers -
in fluvial sediments rich in organic matter - introduces
Case study - arsenic in Bangladesh groundwater
and drinking water
Arsenic is a ubiquitous
(trace)
element found in the atmos-
phere, soils and rocks, natural waters and organisms. …
Most environmental arsenic problems are the result of
mobilisation under natural conditions
(Smedley and
Kinniburgh, 2002)
Bangladesh is a small, densely populated country
located on a flood-prone delta where two major rivers
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