Environmental Engineering Reference
In-Depth Information
Table 7.1 Magnetite and maghemite physical and magnetic characteristics
Characteristics
Magnetite
Maghemite
Chemical formula
Fe
3
O
4
γ
-Fe
2
O
3
Color
Black
Brown-red
Density (g/cm
3
)
5.18
4.87
Melting temperature (
C)
1,583-1,597
-
Dourness
5.5
5
Magnetism type
Ferromagnetic
Ferromagnetic
Curie temperature (K)
858
820-986
Saturation magnetization (emu/g)
92-100
60-80
Crystallization system
Cubic
Cubic or tetragonal
Structural type
Inverse spinel
Spinel with flows
Network parameter (nm)
a
¼
0.8396
a
¼
0.83474 (cubic)
a
¼
0.8347,
c
¼
2.501 (tetragonal)
The complete precipitation of the magnetite should be produced at pH values
from 8 to 14, in a non-oxidative media, since magnetite is sensitive to oxidation
resulting in maghemite according to the following reaction:
2Fe
3
O
4
þ
=
2
O
2
!
3
Fe
2
O
3
ð
7
:
2
Þ
1
γ
The precipitation process consists of two steps [
10
,
11
,
58
-
61
]:
• Nucleation, when the species concentration reaches supersaturation;
• Slow increase of the nucleus.
7.2.2.2 Sol-Gel Method
This method consists of hydrolysis and condensation of some precursors in solution
that results in obtaining a sol of nanometric particles. Organic condensation leads to
the formation of a tridimensional network of metal oxides, called wet gel. Since this
reaction occurs at room temperature, thermic treatments are required in order to
obtain the final crystalline structure [
62
,
63
].
Gamarra et al. [
64
] precipitated iron oxyhydroxide (FeOOH) in water in the
presence of a surfactant, after which they reduced the Fe
3+
at Fe
2+
partially, by a
light drying in N
2
atmosphere, obtaining at the end magnetite particles.
This method has certain advantages like [
65
]:
• Depending on experimental conditions materials with pre-established structure
can be obtained;
• The possibility of obtaining some amorphous pure phases and the possibility of
obtaining single dispersed particles with a good dimension control;
• The possibility of encapsulating of the iron oxide in different matrices,
maintaining the properties and stability of the particles.
