Environmental Engineering Reference
In-Depth Information
O
2
e-
O
2
-
•
UV light
H
2
O
h+
OH•
H+
FigUre 17.2
Schematic of general photocatalysis activity under UV irradiation.
nanosized Al
2
O
3
is one of the most widely studied nanosized metal oxides, aside from nanosized Fe
2
O
3
[39]. pacheco et al.
[11] showed that nanosized Al
2
O
3
was capable of removing five different metal ions, including pb, cr, cu, cd, and Hg, from
aqueous solutions. However, the adsorption of pb was quite low (50%), whereas the adsorption of other cations exceeded 90%.
Meanwhile, Jang et al. [12] studied As(V) adsorption using highly ordered nanostructured SiO
2
media impregnated with metal
oxides. SbA-15 was impregnated with three types of metal oxides, namely, Al
2
O
3
, zinc oxide (ZnO), and Fe
2
O
3
. Aluminum-
impregnated SbA-15 showed higher As(V) adsorption capacity and faster adsorption rate than activated alumina. This result
was due to the large number of active sites created by the dispersion of Al
2
O
3
in the mesopore SiO
2
structures. The homoge-
neous and open pore structure of aluminum-impregnated SbA-15 also contributed to the transport of arsenate species, thereby
increasing adsorption performance. Önnby et al. [40] demonstrated that nanosized Al
2
O
3
was successfully incorporated into
crygels, resulting in higher adsorption capacity and faster kinetics for the treatment of arsenic-contaminated waters, compared
with nanosized Al
2
O
3
. However, the As removal efficiency of the aforementioned nanoadsorbent was reduced due to the
existence of co-ions in water. The adsorption of phosphate ions (pO
4
3−
), nO
3
−
, and sulfate ions (SO
4
2−
) by the nanoadsorbent
was 69, 9, and 6%, respectively.
17.2.2
Water remediation by nanophotocatalysts via photocatalytic degradation
photocatalytic degradation is one of the advanced oxidation processes (AOps) using ultraviolet (UV) light as an energy source
with a suspended powder or an immobilized form of semiconductor. AOps are chemical remediation methods that involve the
use of semiconductors [41], where active hydroxyl radicals (OH•) species are generated by a high-energy source and serve as
the primary oxidant to induce chemical reduction or oxidation reaction [42]. photocatalytic degradation possesses several
advantages compared to other AOps such as Fenton and photo-Fenton catalytic reactions and hydrogen peroxide (H
2
O
2
)/UV
processes. The advantages included the ability to operate under ambient conditions, the use of inexpensive and nontoxic
photocatalysts, as well as atmospheric air as oxidant [43].
photocatalytic degradation can be categorized as either homogeneous or heterogeneous [44]. Homogeneous photocatalytic
degradation involves the dissolution of catalysts and oxidants, such as H
2
O
2
or ozone, by direct UV photolysis of the solution.
Meanwhile, heterogeneous photocatalytic degradation, also called semiconductor photocatalytic degradation, involves the use of
a heterogeneous semiconductor using oxygen (O
2
) as an electron scavenger and UV light to remove organic impurities [45]. The
basic mechanism of heterogeneous photocatalytic degradation involves the migration of electrons (e−) and holes (h+) to the sur-
face of the photocatalyst through UV light irradiation. The photogenerated electrons and holes react with water (H
2
O) and O
2
,
which is adsorbed on the surface of the photocatalyst, to produce reactive oxygen species, such as oxygen radicals (O
2
−
⋅) and OH⋅
[46]. Figure 17.2 shows the schematic of the general photocatalytic activities for photocatalysis under UV irradiation.
17.2.2.1 Titanium Dioxide Nanoparticles
TiO
2
or titania nanoparticles are the most frequently used semiconductor for pho-
tocatalytic degradation of contaminated water, with good UV scattering effect [14]. The anatase crystalline phase of TiO
2
has
been assumed to be more efficient than rutile and brookite TiO
2
because anatase TiO
2
have smaller particles (<50 nm), a larger
surface area, and a higher degree of crystallinity, which favors photocatalytic degradation [47]. In addition, anatase TiO
2
also
contains high concentrations of surface hydroxyl groups (OH) [48]. TiO
2
nanoparticles are inexpensive, nonselective, nontoxic,
insoluble in water, photostable, and highly photoactive with a wide band-gap energy (3.2 eV), and can completely mineralize
