Environmental Engineering Reference
In-Depth Information
Fig. 7.1 Inversed spinel
structure of magnetite [
7
]
Fig. 7.2 Behavior of the
magnetite particle
according to the acid or
basic pH [
11
]
surface of the crystals. The center of the crystals is also oxidized, by the diffusion of
the Fe
2+
ions at the surface were converted to Fe
3+
. The speed at which oxidation
occurs is determined by the diffusion speed of the Fe
2+
ions and by the distance to
the surface of the particle. This is why the particles that are bigger remain unaf-
fected by the oxidation phenomena, while the small particles can be oxidized even
at room temperature.
At temperature higher than 300
C, magnetite is oxidized at hematite (
-Fe
2
O
3
)
[
10
]. This is antiferromagnetic, as a consequence this conversion can damage if is
used in specific applications.
The chemistry of the surface and its proprieties are especially important for
specific applications. The iron atoms from the surface of the magnetite which are
not bound by the oxygen act as Lewis acids, and coordinate the molecules that can
donate a pair of electrons. In aqueous systems, these atoms coordinate the water
molecules that dissociate quickly resulting in surface functionalized magnetite with
hydroxyl groupings like Fe-OH. In this way the chemistry of the magnetite particles
is strongly dependent on the pH values; at low pH values the surface of the
magnetite particles is charged positively, and at high pH values it is charged
negatively (Fig.
7.2
). The hydroxyl groupings formed at the surface of the magne-
tite have an amphoteric character.
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