Environmental Engineering Reference
In-Depth Information
reported that Cu
2+
and Pb
2+
sorption to HFO was unaf ected by dif erent
ionic strength from 0.005 to 0.5 M NaClO
4
, or by change in the nature of
the background electrolyte from NaClO
4
to a complex artii cial seawater
mixture.
Another important magnetic nanomaterial is nanosized magnetite.
Chemical co-precipitation has been widely used to prepare magnetite
nanoparticles by adding alkaline carbonate into solution containing Fe
2+
and Fe
3+
in a molar ratio of 1:2 [25, 26]. It was found that the particle
size was reduced when a surfactant (such as oleic acid) was used during
the preparation. Two methods were reported to prevent the change of
the ratio caused by air oxidation. One is to conduct the reaction under
an inert environment with nitrogen gas. Another is to set the initial Fe
3+
:
Fe
2+
molar ratio less than 2:1 so that at er the oxidation of Fe
2+
to Fe
3+
, the
ratio approaches 2:1 [27, 28]. In addition, nano-Fe
3
O
4
will be oxidized to
nano-γ-Fe
2
O
3
. h e size of the resultant Fe
3
O
4
or γ-Fe
2
O
3
hydrosols were
considerably smaller than that reported in the literature, and no surfactants
are needed to stabilize the sols. A decrease of specii c saturation magneti-
zation value was observed when the nanoparticles were coated with oleic
acid. Scanning electron micrograph showed that the prepared Fe3O4 nan-
oparticle sol had an average diameter of 8.5 ± 1.3 nm, where needlelike
γ-Fe
2
O
3
nanoparticles with lengths of 20-50 nm and widths of 4-6 nm
are visible [29]. Hu
et al.
investigted Cr(VI) removal by nano maghemite
and found that the equilibrium period was independent of initial Cr(VI)
concentration and the adsorptive capacity increased when pH decreased
[30]. Nano maghemite has emerged as being highly selective for Cr(VI)
from water. Negligible competition was observed from many coexisting
ions. h e adsorption capacity of nano maghemite for Cr(VI) (19.2 mg/g) is
higher than that of diatomite (11.55 mg/g) [31], anatase (14.56 mg/g) [32],
commercial activated carbon (15.47 mg/g) [33], and beech sawdust (16.13
mg/g) [34]. Based on X-ray dif raction (XRD), X-ray photoelectron spec-
troscopy (XPS), and Raman spectroscopic techniques, it could be deduced
that no chemical redox reaction occurred during Cr(VI) retention, which
also hints at the stability of nanoscale γ-Fe
2
O
3
. h e adsorption mech-
anism of Cr(VI) onto γ-Fe
2
O
3
is suggested to be electrostatic attraction,
particularly at a relatively low pH.
9.2.2
Nano Manganese Oxides (NMnOs)
In the past decades, NMnOs have been exploited for sorption of cationic or
anionic pollutants from natural waters, such as heavy metal ions, arsenate
and phosphate, due to their polymorphic structures and higher specii c
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