Environmental Engineering Reference
In-Depth Information
continuously for 10 minutes. 0.2 mL of Fe(CO)
5
was then injected using a syringe into
10 mL of octyl ether and mixed with 1.28 g of oleic acid in the two-necked bottle at 100
o
C. The resulting mixture was heated to reflux in the silicone oil and kept at that
temperature for 1 h. 0.34 g of dehydrated (CH
3
)
3
NO was injected using a syringe after
cooling the resulting solution to room temperature. The mixture was then heated to 130
o
C under a nitrogen environment and maintained at this temperature for 2 h. The
temperature was slowly increased to reflux and kept constant for 1 h. After the solution
was cooled to room temperature, ethanol was added to yield a black precipitate which
could be separated via an external magnetic field. The resulting powders of several
nanometers were re-dispersed in hydrocarbon solvents such as hexane, octane, and
toluene. By varying the ratio of Fe(CO)
5
and oleic acid, particles with various sizes
ranging from 4 nm to 30 nm could be obtained. Since the -Fe
2
O
3
nanoparticles
synthesized using this method would easily aggregate in the aqueous solution without
any surface treatment, they were pretreated before the adsorption experiments. Some
methods were applied to handle the -Fe
2
O
3
nanoparticles involving strong base washing
and ethanol washing before Cr(VI) adsorption. For a strong base washing method, the -
Fe
2
O
3
nanoparticles were added into 2 M NaOH solution and heated in 100
o
C water
bath for 1 h. For ethanol washing, 0.1 L of 99.9% ethanol was used for the nanoparticle
washing. After washing and freeze-drying, the -Fe
2
O
3
nanoparticles were harvested.
9.3.2 Synthesis of MeFe
2
O
4
Nanoparticles
All the MeFe
2
O
4
nanoparticles were synthesized using the chemical co-
precipitation method. First, 200 mL of ultrapure water was deoxygenated by bubbling
with nitrogen gas for 30 minutes, and the desired amount of Me(NO
3
)
2
(Me = Mn, Co,
Cu, Mg, Zn, Ni) and Fe(NO
3
)
3
salts with a molar ratio of 1:2 were successively
dissolved in deoxygenated water with vigorous mechanic stirring. Under the protection
of nitrogen gas, the mixture was heated to 70
o
C in a water bath and then 2 M NaOH
was added dropwise into the above solution until pH 11. To ensure complete growth of
the nanoparticle crystals, the reaction was kept at 70
o
C for 2 h. After that, the stirrer was
turned off with magnetic particles gradually settling. The precipitate was isolated under
an external magnetic field and the supernatant was decanted. To obtain the pure and
neutral products, synthesized materials were rinsed three times with ultrapure water and
the rinse solution was discarded. The CoFe
2
O
4
and MnFe
2
O
4
nanoparticles could be
produced after freeze-drying. To synthesize MgFe
2
O
4
, ZnFe
2
O
4
, CuFe
2
O
4
and NiFe
2
O
4
nanoparticles, subsequent calcination at 400
o
C for 2 h was necessary to assure complete
crystallization. The reaction is expressed as follows (Hu et al., 2007a):
2FeCl
3
+ MeCl
2
+ 8NaOH MeFe
2
O
4
(s)
+ 8NaCl + 4H
2
O (Eq. 9.2)
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