Civil Engineering Reference
In-Depth Information
in the degradation of organic pollutants as well as inorganic compounds,
from nitrogen oxides to complex metal salts, in gas and liquid phase.
Several works on photocatalysis focus on the removal of organic pollut-
ants from wastewaters, specially referring to dyes, which are toxic to micro-
organisms and aquatic life: as reported by Konstantinou and Albanis (2004),
up to 20% of dyes used in manufacturing processes are dispersed in waste-
waters, therefore effi cient processes that avoid their release into natural
water sources are vital to preserve the ecosystem. Several reviews can be
found on TiO
2
effectiveness in dye degradation: we therefore suggest the
reader refers to specifi c literature for further information (e.g., Akpan and
Hameed, 2009; Han
et al.
, 2009).
However, in recent years the aspect of removing gas phase organic com-
pounds has also attracted attention. Extensive studies have been carried
out on the removal of outdoor and indoor pollutants, and more specifi cally
volatile organic compounds (VOCs), by TiO
2
nanoparticles under UV illu-
mination. These particles are often integrated in air purifi cation devices,
which are commercially available from several companies. VOCs are often
responsible for malodorous air in buildings. They derive from several
sources, such as cooking, cleaning products, furniture, etc.; in outdoor envi-
ronments, they are usually part of combustion gases, industrial exhausts,
cigarette smoke (Fujishima
et al.
, 2007). A long list of experimental studies
is available in the scientifi c literature on this topic as well.
Photocatalysts are not only used for breaking down large volumes of
soilage, they are also capable of destroying it as it accumulates, e.g., to
prevent cigarette smoke residue stains, or unpleasant odours due to the
presence of VOCs, of the order of 10 ppb by volume. At these concentra-
tions, TiO
2
should be able to decompose such compounds even with scarce
UV light (even as low as 1
W/cm
2
according to some authors). Bright UV
lamps are also used in city and highway tunnels to reduce pollution released
by traffi c, degrading both VOCs and nitrogen oxides, NO
x
, as described by
Demeestere
et al.
(2008), Toma
et al.
(2004), and many others.
μ
13.3.2 Self-cleaning
Besides photocatalytic applications of TiO
2
, another fascinating phenome-
non arises from UV irradiation, that is, the alteration of TiO
2
wettability
and formation of a highly hydrophilic surface state: this behaviour is defi ned
as photoinduced superhydrophilicity (Wang
et al.
, 1997), and involves the
reduction of Ti
4+
to Ti
3+
by electrons and simultaneous hole trapping at
lattice sites. This reduces the bond strength between reduced titanium and
the closest oxygen, which is then removed when another water molecule
arrives in contact with the surface and adsorbs on it. This creates a highly
hydroxylated surface layer, which is responsible for hydrogen bonds with
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