Environmental Engineering Reference
In-Depth Information
Figure 7.6
Estimated efficiencies of TiO
2
and zero-valent Cu for catalyzing
sonochemical (20 kHz) degradation of C.I. Direct Yellow 9 at pH 6.9. The solid lines
display the quadratic relations between moles of dye removed per mass of particle added,
and “EF” is the efficiency factor defined as the linear slope of the fitted curve.
negatively charged moieties and the solid surface (note that the solution
pH originally at 6.9 decreased readily to around 5.0 via oxidation); (ii) the
larger surface area of the nanoparticles and the likely formation of e-h pairs
on their massive surfaces upon the extreme conditions of non-symmetrical
collapse; (iii) the wide wavelength range of single-bubble sonolumines-
cence (200-500 nm), at which the semiconductor surface is likely excited,
as in photocatalytic processes [88].
The effectiveness of US/TiO
2
for dye degradation may further be
improved by the use of composite nano-semiconductors such as TiO
2
/ZnO
to enrich the active sorption sites and the number of reactive species [89].
For example, a unique composite as Er
3+
: YAlO
3
/TiO
2
-ZnO was found to
significantly improve the degradation of organic dyes under ultrasound
via direct decomposition of dye molecules on the surface of the catalyst,
separation of the electron-hole pair and the up-conversion luminescence
of the Er
3+
: YAlO
3
composite by ultrasonic cavitation [90,101].
The use of zero-valent iron (ZVI) together with ultrasonic irradiation
also has a special role owing to the unique activity of the catalyst surface
and its vicinity, where dye molecules are exposed to reductive and oxidative
decomposition while the metal surface is continuously cleaned by hydro-
dynamic sheer forces of ultrasound [94,100]. The reactivity of the surface
arising from corrosion of the metal (at acidic pH) leads to heterogeneous
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