Environmental Engineering Reference
In-Depth Information
17.7.8.2 Magnetic Nanoparticles
One of the dificult challenges of nanomaterials in water treatment is related to their sepa-
ration from water. Separation of the nanosized adsorbents from media is quite complicated
in practice. Magnetic nanosized sorbents can, however, overcome the separation issue and
seem to be promising owing to ease of their separation. Application of an external mag-
netic ield for the recapture of the nanoparticles is a low-cost and effective option. The
magnetic properties of undecorated nanoparticles could be affected by forming aggre-
gates because of their high chemical activity, possibility of oxidation in air, loss of mag-
netism, dipole-dipole attraction, and agglomeration. Therefore, mounting a passive layer
through postsynthetic surface modiication is often practiced.
Chang et al. (2006) studied two effective types of superparamagnetic nanoscale adsor-
bents of bayerite/SiO
2
/Fe
3
O
4
via three sequential steps: (i) chemical precipitation of Fe
3
O
4
,
(ii) coating of SiO
2
on Fe
3
O
4
using an acidifying method, and (iii) further coating of bayerite
(Al(OH)
3
) on SiO
2
/Fe
3
O
4
-adopting sol-gel (MASG) or homogeneous precipitation (MAHP)
methods for the elimination of luoride from water and comparing the adsorption poten-
tial of the prepared sorbents with that of commercial AA (CA). The Langmuir isotherm
describes the equilibrium of luoride adsorption on CA, MASG, and MAHP materials. The
results showed that the luoride adsorption capacity of CA could be improved when the
initial pH value was 3.5, which was the approximate pK value of the dissociation constant
of hydroluoric acid. Among these, MASG was establish to be the most effective adsorbent
with the adsorption capacity of 38 g/kg, and it competed with CA at an even higher pH
value such as 6.0 vs. 3.5. Zhao et al. (2010) combined the advantages of Al(OH)
3
and mag-
netic nanoparticles to develop nanosized adsorbents with high surface area, high afinity
toward luoride, and good magnetic separation, to develop a new type of magnetic luoride
adsorbent. This nanosized adsorbent was prepared by using hydrous aluminum oxide
embedded with Fe
3
O
4
nanoparticles (Fe
3
O
4
@Al(OH)
3
NPs). This was applied to remove
luoride from aqueous solution. Fe
3
O
4
@Al(OH)
3
NPs exhibited effective adsorption of luo-
ride when the pH was between 5.0 and 7.0, which was attributed to the electrostatic attrac-
tion between the positively charged Fe
3
O
4
@Al(OH)
3
surface and luoride. The adsorption
capacity estimated by the Langmuir model was 88.48 mg/g at pH 6.5. Fluoride removal
in the presence of the anion was on the order of
PO
− − − − −
< < ≈ ≈
. Fluoride
adsorption was claimed to be driven by both electrostatic attraction and surface complex-
ation, which was conirmed by XPS data and experimental results.
Chai et al. (2013) synthesized a novel adsorbent of sulfate-doped Fe
3
O
4
/A l
2
O
3
nanopar-
ticles with magnetic separability for luoride removal from drinking water. The luoride
adsorption isotherm was well described by the Elovich model. Fluoride adsorption capac-
ity by a two-site Langmuir model was 70.4 mg/g at pH 7.0. This nano-adsorbent performed
well over a wide pH range of 4-10, and the luoride removal eficiencies reached up to 90%
and 70% throughout the pH range of 4-10 with initial luoride concentrations of 10 and
50 mg/L, respectively. The observed sulfate-luoride displacement and decreased sulfur
content on the adsorbent surface reveal that the anion-exchange process was an impor-
tant mechanism for luoride adsorption by the sulfate-doped Fe
3
O
4
/A l
2
O
3
nanoparticles.
With the exception of
PO
3−
, other coexisting anions (
NO
3
−
, Cl
−
, and
SO
2−
) did not evidently
inhibit luoride removal by the nanoparticles. Findings of this study demonstrate the
potential utility of the material.
Maliyekkal et al. (2010) synthesized nanoparticles of MgO by self-propagated combus-
tion of the magnesium nitrate trapped in cellulose ibers to enhance the adsorption capac-
ity of MgO for luoride removal. Characterization studies revealed that the synthesized
3
2
SO
Br
NO
Cl
4
4
3
