Environmental Engineering Reference
In-Depth Information
6.5 Sonophotocatalysis
Harada [12] showed the possibility of achieving the effect of a hybrid sonochemical and
photocatalytic reaction simultaneously where water was hardly decomposed to H
2
and O
2
by photocatalysis or sonolysis alone. Furthermore, in order to decompose water, he used a
powdered TiO
2
photocatalyst suspended in distilled water and simultaneously irradiated
the water sample by light and ultrasound. This sonophotocatalytic reaction was shown to
be effective in the decomposition of water to H
2
and O
2
[13].
Separately, the work carried out by Kavitha and Palanisamy [14] showed the acceler-
ated sonophotocatalytic degradation of tracing dye under visible light using dye sensitized
TiO
2
activated by ultrasound was very effective. The effect of sonolysis, photocatalysis,
and sonophotocatalysis under visible light showed the inluence on the degradation rates
by varying the initial substrate concentration, pH, and catalyst loading and to ascertain the
synergistic effect on the degradation techniques.
Ultrasonic activation was shown to contribute to the degradation through cavitation and
leading to the splitting of H
2
O
2
produced by both photocatalysis and sonolysis, combined.
This results in the formation of oxidative species, such as singlet oxygen O
2
and superoxide
O
2
−
radicals in the presence of oxygen. The increase in the amount of reactive radical spe-
cies, which induce faster oxidation of the substrate and degradation of intermediates, and
also the deaggregation of the photocatalyst are responsible for the synergy observed under
soniication. A comparative study of photocatalysis and sonophotocatalysis using TiO
2
by
Hombikat [15] using UV and ZnO was also reported.
The degradation of chitosan (polysaccharide) by means of ultrasound irradiation and its
combination with heterogeneous TiO
2
was investigated [16]. Emphasis was given on the
effect of additives on degradation rate constants. Ultrasound irradiation (24 kHz) was pro-
vided by a sonicator, while an ultraviolet source of 16 W was used for UV irradiation. The
extent of sonolytic degradation increased with increasing ultrasound power (in the range
30-90 W), while the presence of TiO
2
in the dark generally had little effect on degradation.
On the other hand, TiO
2
sonophotocatalysis led to complete chitosan degradation in
60 min with increasing catalyst loading. TiO
2
sonophotocatalysis was always faster than
the respective individual processes owing to the enhanced formation of reactive radicals as
well as the possible ultrasound-induced increase of the active surface area of the catalyst.
The degraded chitosan were characterized by x-ray diffraction, gel permeation chroma-
tography, and Fourier transform infrared spectroscopy, and the average molecular weight
of ultrasonicated chitosan was determined by measurements of the relative viscosity of
samples. The results show that the total degree of deacetylation (DD) of chitosan did not
change after degradation, and the decrease of molecular weight led to transformation of
the crystal structure. A negative order for the dependence of the reaction rate on total molar
concentration of chitosan solution within the degradation process was suggested [17].
6.6 Sonoluminescence
The sonoluminescence effect can occur when a sound wave of suficient intensity induces
a gaseous cavity within a liquid only to collapse quickly, and the effect was irst discovered
at the University of Cologne, Germany, in 1934 as a result of work on sonar technology by
