Environmental Engineering Reference
In-Depth Information
To produce Mg
x
Cu
1-x
Fe
2
O
4
(0 < X < 1), an amount of Mg(NO
3
)
2
, Cu(NO
3
)
2
and
Fe(NO
3
)
3
salts with a molar ratio of X:(1-X):2 were first dissolved in 200 mL
deoxygenated water with vigorous mechanic stirring, and the subsequent procedures
followed those of MgFe
2
O
4
synthesis. The reaction is delineated as follows (Hu et al.,
2007a):
XMgCl
2
+ (1-X)CuCl
2
+ 2FeCl
3
+ 8NaOH Mg
X
Cu
1-X
Fe
2
O
4
(s) + 8NaCl + 4H
2
O
(Eq. 9.3)
Depending on the preparation conditions, such as temperature and pH, MnFe
2
O
4
may
contain Mn(III) or Fe(II) ions in addition to Mn(II) and Fe(III). Specifically, the
MnFe
2
O
4
particles produced at lower temperature (100
o
C) are composed of solely Mn(II)
and Fe(III) (Bonsdorf et al., 1998). The Mn(II) from MnFe
2
O
4
can be easily oxidized by
adsorbed Cr(VI), hence the regeneration of MnFe
2
O
4
using NaOH may become
ineffective. To avoid any possible redox reactions between Cr(VI) and adsorbent
nanoparticles, the surface of pure MnFe
2
O
4
should be modified using an oxidizing agent
until external Mn(II) oxidizes to the highest valence. Mn(II) can be fully oxidized to
Mn(IV) by heating Mn(II) in a strong basic solution with a continuous supply of oxygen.
Subsequently, the treated MnFe
2
O
4
were added into 200 mL of 2 M NaOH solution
while bubbling with the condensed air, and stirred. The mixture was then heated in 100
o
C water bath for 2 h. The surface-modified MnFe
2
O
4
nanoparticles were separated via
magnetic field and washed with ultrapure water. This washing procedure was repeated
three times and the modified MnFe
2
O
4
nanoparticles were obtained after freeze-drying.
The modifying reaction is suggested as follows:
MnFe
2
O
4
+ 2NaOH + 2O
2
MnO
2
+ Fe
2
O
3
+ Na
2
Fe
2
O
4
+ H
2
O (Eq. 9.4)
9.3.3 Surface-Coating and Metal-Doping of Magnetic Nanoparticles
To enhance the adsorption capacity of magnetic nanoparticles for removal of
Cr(VI) and to achieve the recovery of the adsorbent nanoparticles used, two techniques
have been developed involving surface-coating and metal-doping for adsorbent
modification. The -Fe
2
O
3
nanoparticles synthesized from the sol-gel method were
chosen for the modification. The principles of choosing coated materials or doped metal
are that: (a) the enhanced adsorption is expected due to the increased surface area or
improved surface properties after modification; (b) the modification methods are
comparatively simple without using more reagents; (c) the other important parameters of
the original adsorbent would not be impaired significantly, e.g., magnetic properties,
adsorption rate, etc.; and (d) the modified nanoparticles are still stable under the
experimental conditions.
Search WWH ::
Custom Search