Page 400 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials as Adsorbents 383

stress. This hypothesis could explain the desorption hysteresis, which
increases as the size decreases.

Iron oxide–doped TiO 2 nanoparticles
for the degradation of organic pollutants
Recently, photocatalytic oxidation with TiO nanoparticles (6–20 nm)
2
has been investigated as a promising water-treatment process. When
irradiated with UV light, TiO nanoparticles can adsorb and degrade
2
a wide variety of environmental organic pollutants (Bianco Prevot and
Pramauro, 1999; Bianco Prevot et al., 1999a; Bianco Prevot et al.,
1999b; Pramauro et al., 1998). For instance, the strong affinity between
the surface of TiO 2 nanoparticles for arsenic organic species
(monomethylarsonic [MMA] and dimethylarsinic [DMA] acids) were
shown by EXAFS (Jing et al., 2005). Results show that both MMA and
DMA are covalently bounded to the surface of nanoparticles through
bidentate (As MMA -Ti 3.32 Å) and monodentate (As DMA -Ti 3.37 Å)
inner sphere complexes, respectively.
In treating organic pollutants in water, there are some problems that
arise from using TiO nanoparticles. Two limiting factors are the low effi-
2
ciency of the utilization of visible light and the recombination between
the photogenerated electrons and holes, even if the kinetics of degra-
dation of organic pollutants are increased in presence of TiO nanopar-
2
ticles. It has been reported that doping TiO nanoparticles with ions
2
2 2 3 3
(Zn , Mn , Al , K , Fe . . . ) at 0.1–0.5 percent may significantly
increase the photocatalytic activity (Bessekhouad et al., 2004; Choi et al.,
1994; Liu et al., 2005a). The doped ions act as charge separators of the
photoinduced electron–hole pair and enhanced interfacial charge trans-
fers. For instance, Liu et al., (2004) have studied the photocatalytic
activity of TiO nanoparticles doped with ZnFe O (20 nm). The ZnFe O 4
2
2
4
2
doping strongly enhanced the photocatalytic activity of TiO nanopar-
2
ticles and improved the degradation of chlorinated pesticide (Liu
et al., 2004).
The main problem is that the separation of the nano-photocatalyst
from the polluted aqueous media is difficult and TiO suspensions are
2
not easily regenerated. To get around these difficulties, iron oxide
nanoparticles (Fe O , CoFe O , or (Ba,S,Pb)Fe O ) have recently been
3
19
12
2
3
4
coated with TiO 2 to synthesize magnetic photocatalytic nanoparticles
(Fu et al., 2006). In such core-shell nano-structures (20–100 nm), the
magnetic core is useful for the recovery of nano-photocatalyst from the
treated water stream by applying an external magnetic field while a
outer shell is used to destroy organic contaminants in wastewaters
TiO 2
(Fu et al., 2006). The topic of photocatalytic degradation of organic con-
taminants is discussed in detail in Chapter 5.