Environmental Engineering Reference
In-Depth Information
are formed in the bulk solution, which also helps in the degradation of the contaminant.
The performance of the system depends on the electrodes and electrolytes used and the
current applied. To increase conductivity, NaCl is often used as a supporting electrolyte.
However, use of NaCl leads to the formation of organochlorine compounds. The introduc-
tion of dissolved iron can enhance the eficiency by reducing H
2
O
2
to hydroxyl radicals,
thus incorporating the beneits of Fenton's reaction. Boron-doped diamond is becoming
more popular as the anode.
25.4.5 Sonolysis
This method involves the use of high-intensity ultrasound waves at high frequencies,
which can produce cavitation. The cavitation bubbles act as microreactors and produce
hydroxyl radicals. More volatile compounds get degraded faster since they are usually
present at the liquid-gas interface due to sonochemical reactions. The presence of iron
will increase the performance since H
2
O
2
is produced during the sonochemical reactions.
25.4.6 Photocatalysis
Photocatalysis comes under AOPs in which oxidative species are produced under a light
source. It can be divided into homogeneous and heterogeneous photocatalysis depending
on the phase of the photocatalyst and the reactant. Among these, heterogeneous photoca-
talysis is more commonly employed.
25.4.7 Heterogeneous Photocatalysis
When the photocatalyst and the reactant are not in the same phase, the reaction is known
as heterogeneous photocatalysis. Semiconductors such as TiO
2
, CdS, and ZnO belong to
this category. Semiconductors have a illed valence band and an empty conduction band.
When a semiconductor surface is irradiated by a light source, electrons from the valence
band absorb the energy of photons, get excited, and are shifted to the conduction band,
leaving a hole in the valence band. Thus electron-hole formation occurs. In the absence of
suitable electron or hole acceptors, the electrons and holes are recombined. The excitation
of electrons occurs only if the energy of the photons (
h
ν
) is greater than or equal to the
band-gap energy (
E
G
). The electrons and holes formed react with water to produce highly
oxidizing species such as hydroxyl radicals and superoxide radicals. These radicals react
with the pollutants or pathogens in water to degrade it. The reactions continue to produce
various intermediates, and inal products are formed. In most of the cases, it is completely
mineralized to CO
2
and water. Among various semiconductors, TiO
2
has proven to be the
most suitable one for photocatalysis applications.
25.5 TiO
2
as a Photocatalyst
TiO
2
is photocatalytically active, biologically and chemically inert, does not undergo photo-
corrosion and chemical corrosion, and it is inexpensive. UV/TiO
2
has been used to remove
a wide range of pollutants in the liquid and the gas phase over the past several decades. In
the case of titanium dioxide, the band-gap energy is 3.2 eV and it needs UV radiation for
