Environmental Engineering Reference
In-Depth Information
9.3
Laboratory Production of Magnetic Nanoparticles
9.3.1 Synthesis of -Fe
2
O
3
Nanoparticles
To get the -Fe
2
O
3
nanoparticles, the Fe
3
O
4
nanogel was first produced using the
sol-gel method. Fe
3
O
4
nanoparticles were prepared by adding the appropriate amounts of
Fe(II) salt and Fe(III) salt in an alkaline solution to form Fe
3
O
4
, precipitates from the
solution:
2Fe
3+
+ Fe
2+
+ 8NH
3
+ 4H
2
O Fe
3
O
4
(s)
+ 8NH
4
+
(Eq. 9.1)
The procedures are as follows: first, 200 mL of ultrapure water (resistivity of 17.8 M-
cm) was deoxygenated by bubbling with nitrogen gas for 30 minutes. Then 10.4 g
FeCl
3
·6H
2
O or 4.0 g Fe
2
(SO
4
)
3
and 4.0 g FeCl
2
·4H
2
O salts were dissolved in the
deoxygenated water with mechanical stirring. 1.5 M NH
4
OH solution was added
dropwise with N
2
gas bubbling. The precipitate was initially brown in color and then
turned to black. When the pH reached 8.0, the stirrer was turned off, and the Fe
3
O
4
particles settled gradually. The black precipitate was isolated by an external magnetic
field of 3000 G with the supernatant decanted. To obtain the pure and neutral products,
synthesized Fe
3
O
4
particles were rinsed with ultrapure water three times. Finally, Fe
3
O
4
nanogel was obtained by adding 1 mL of 25% tetramethylammonium hydroxide into the
Fe
3
O
4
precipitate. This ionic surfactant can produce electrostatic repulsion in an aqueous
medium and keep Fe
3
O
4
nanoparticles from aggregating.
The Fe
3
O
4
nanogel was then freeze-dried and dispersed in 99% octyl ether with
condensed air bubbling. The mixture was heated to 250
o
C and maintained at this
temperature for 2 h. Octyl ether could provide a high temperature atmosphere due to its
high boiling point (290
o
C). Finally, reddish brown -Fe
2
O
3
nanoparticles were collected
via the external magnetic field after ethanol-washing and freeze-drying.
To study the effect of particle size on adsorption, it is necessary to synthesize
monodispersed -Fe
2
O
3
nanoparticles with various dimensions. Since the sol-gel method
cannot produce monodispersed nanoparticles with diameters smaller than 10 nm, the
organic decomposition method was alternatively applied (Hyeon et al., 2001). The
organic solution-phase decomposition of the iron precursor at high temperatures
produces uniform -Fe
2
O
3
nanoparticles with their sizes controlled by regulating the
ratio of reagents and the reaction time during the synthesis process. The procedure is
described below.
The schematic diagram and corresponding photo of the setup for nanoparticle
synthesis are shown in Figures 9.1. First, oxygen in this system was purged out by a
vacuum pump (Air cadet 7530-50, Cole-Parmer, USA) with the nitrogen gas adding
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