Environmental Engineering Reference
In-Depth Information
Carbon-based TiO
2
composites have attracted much attention and become a very active field of research due to their unique
properties. A combination of TiO
2
and AC could create many active sites for photocatalytic degradation. AC not only increases
the surface area but also modifies the acid-base properties and the uv spectrum of TiO
2
[19b]. Promising results were also
obtained with carbon nanotubes (CNTs)-TiO
2
hybrids, tested for the photodegradation of some organic compounds [38]. The
photoactivity of the CNTs-TiO
2
is generally higher than that of TiO
2
and TiO
2
-AC samples. it is considered that photo-induced
charge transfer occurs in the electronic interaction between the carbon layers or walls of the multiwalled carbon nanotube
(mWCNT) and TiO
2
. The electrons excited in the conduction band of TiO
2
may migrate into the nanocylinder of the CNTs and
lead to a reduced electron-hole pair recombination and thus to an increase in photon efficiency. O
2
adsorbed on the surface of
the CNTs may accept the electron and form the
·
OH radical, which oxidizes the adsorbed dye directly on the surface [19b].
24.3
catalytic ozoNatioN
24.3.1
introduction
Ozonation is an attractive and increasingly important method for the degradation of water and wastewater organic pollutants.
unfortunately, total mineralization of organic pollutants cannot be achieved by this process. Ozonation of organic compounds
usually leads to the formation of aldehydes and carboxylic acids, which cannot be degraded with ozone and may produce more
toxic and resistant by-products [39]. Additionally, ozonation rate is relatively slow and this reaction is strongly pH-dependent
and occurs faster with an increase of pH [40]. it has been demonstrated that ozonation of organic compounds can effectively be
improved in the presence of a catalyst. in particular, solid metal oxides are more useful in catalytic ozonation than ionized
metals, as ionized metals are considered toxic substances in water [41]. These processes are also very often characterized by a
lower usage of ozone when compared with ozonation alone. in contrast to ozonation, catalytic ozonation allows for the effective
formation of hydroxyl radicals even at a low pH. The ozone efficiency in heterogeneous catalytic ozonation is higher than that
found in homogeneous catalytic ozonation. The catalytic ozonation can proceed by direct molecular ozone reactions and by an
indirect pathway forming OH radicals after ozone decomposition.
24.3.2
heterogeneous catalytic ozonation
A short overview of the literature concerning catalytic ozonation indicates that different or contradictory results are published
in the literature. The controversies in this field are due to the unknown mechanisms of catalytic processes. Some authors suggest
that hydroxyl radicals are the active species responsible for oxidation reactions, while others indicate that catalytic oxidation
proceeds without hydroxyl radical formation. in the first case, it is proposed that catalysts cause ozone decomposition leading
to hydroxyl radical formation. Another important issue is the adsorption of ozone and/or organics on the surface of the catalyst,
as the usage of catalysts in aqueous solutions will lead to competition between water, ozone, and organic compounds for
catalytic active sites [40].
The catalytic ozonation process is significantly influenced by key factors such as temperature and pH. The pH is the main
factor for both ozone stability and catalyst surface properties such as the surface charge of the particles, which influences the
adsorption capacity of the catalyst. TiO
2
P25 is the most commonly tested catalyst for the ozonation of organic pollutants in
water. The main characteristics influencing the TiO
2
activity are surface area, crystalline phase, particle size, and the aggregate
size in suspension [3]. Owing to the very strong van der Waals interactions, TiO
2
nanoparticles are almost inclined to form
aggregates. The pH and surface charges of the solution mainly determine the stability of TiO
2
nanoparticles in aqueous solution
[42]. While agglomeration has been shown to be an important factor for the catalyst's activity, its measurement is difficult. An
activity decrease could be expected by bringing the surface close to neutrality.
in an investigation on catalytic ozonation of nitrobenzene in the presence of nano-TiO
2
, it was found that only the nanometer
rutile TiO
2
is catalytically active
.
Adsorption of nitrobenzene was found to play an important role and catalytic oxidation pro-
ceeds via hydroxyl radical formation. High mineralization of ozonation by-products takes place in the presence of the nano-
meter rutile TiO
2
[43].
Nanosized TiO
2
supported on Zeolite was prepared by Wang et al. and good catalytic performance of TiO
2
/Zeolite was
achieved in catalytic ozonation [9m].
rodríguez et al. synthesized TiO
2
-supported nickel nanoparticles. The presence of two phases (NiO/Ni) in the Ni/TiO
2
slightly improved the conversion of 2,4-dichlorophenoxyacetic acid compared with TiO
2
[44].
recently, photocatalytic ozonation with TiO
2
(O
3
/uv/TiO
2
) has received enormous attention for the oxidation of organic
pollutants. Jing et al. prepared an anatase TiO
2
photocatalyst by hydrothermal method. TiO
2
displayed more photocatalytic
