Single and Hybrid Applications of Ultrasound for Decolorization and Degradation of Textile Dye Residuals in Water - Green Chemistry for Dyes Removal from Wastewater - page 271

Consequently, the degradation of textile dyes by high-frequency ultra-
sound is much more effective, however, with the hindrance caused by the
“threshold frequency,” above which sonochemical effects fade away. As
such, the effect of frequency on the rate of decolorization of thiazine dyes
is reported as k 22.8kHz < k 490kHz < k 127kHz , highlighting the impact of “cavita-
tion threshold” [34].
On the other hand, at equivalent power densities, the collapse tem-
perature under short frequency ultrasound is always higher than that of
high frequency (4558 K at 20 kHz, 2458 K at 500 kHz) [54] implying that
the degradation of hydrophilic solutes such as textile dyes is not directly
related to the temperature. As such, a more significant parameter affect-
ing the efficiency of hydrophilic substrate degradation by ultrasound is
the incidence of efficient bubble collapse and its duration that dictate the
probability of radical combination reactions. This explains why these sol-
utes react faster at high frequency ultrasound despite the milder collapse
conditions, and why the bubbles are very small and short-lived (transient
cavitation), undergoing a larger number of oscillations with a more fre-
quent occurrence of efficient bubble collapse (25-fold) that promotes free
radical transfer to the bulk solution. Moreover, our experience has shown
that the effective frequency is not only a matter of bubble dynamics and the
solubility of the dye, but also that of the solution matrix, which controls the
rate of production/depletion of OH radicals.
The range of frequencies tested on the degradation of azo and a variety
of other dyes is listed in Table 7.4 with additional information on the oper-
ation parameters such as pH, the sparge gas and the acoustic power. Note
that the applied pH in general (except for C.I. Basic Violet) lies within a
highly acidic to closely neutral range, because most of the investigated dyes
at acidic conditions are protonated with a slight enrichment in hydropho-
bicity, which lowers their resistance to diffuse to the negatively-charged
cavity bubbles, or the interfacial region. Hence, acidification is a positive
encouragement to the mass transfer of highly soluble dye molecules from
the bulk solution to the gas-liquid interface. Note that acidity is not only
maintained by the addition of strong acids to the solution, but also by air
bubbling to form reactive nitrogen species and HNO 3 . In fact, the injection
of air or another gas is also essential to enhance the number of nucleation
sites and the violence of collapse. The use of inert gases for this purpose is
very popular due their high pyrolytic gas ratios γ (Cp/Cv), while bubbling
of the solution with a mixture of two gases (e.g., air/O 2 , Ar/O 2 ) in a well-
selected molar ratio is also very common due to a variety of additional
benefits [55].