Single and Hybrid Applications of Ultrasound for Decolorization and Degradation of Textile Dye Residuals in Water - Green Chemistry for Dyes Removal from Wastewater - page 270

specifically provides an expression that relates the efficacy of decoloriza-
tion to the applied power:

064 .

P
V

(7.15)

E 75 9 .

where E is the decolorization efficiency and P/V is the specific power.
Another significant operation parameter is the frequency, but there is
such a wide range of application that it is not possible to define a specific
frequency that provides maximum decolorization or mineralization of tex-
tile dyes [29,34,35,48]. In practice, three ranges of frequencies are avail-
able for three distinct uses of ultrasound: (i) high frequency or diagnostic
ultrasound (2-10 MHz), (ii) low frequency or conventional “power ultra-
sound” (20-100 kHz), and medium frequency, or “sonochemical-effects”
ultrasound (300-1000 kHz) [33]. It is this latter range, where chemical
reactions of particularly hydrophilic compounds are uniquely catalyzed
through unstable and short-lived cavitation recognized with effective
oscilations, that leads to more frequent collapse events, allowing the ejec-
tion of some radical species to the bulk solution.
Hence, selection of the right frquency is of utmost significance to obtain
good reaction yields, and this is possible by considering the solubility
and/or volatility of the target contaminants. For example, if the target is
hydrophobic with a high vapor pressure, it easily diffuses to the gas-liquid
interface and the gaseous bubble interior to undergo oxidative and ther-
mal decomposition, respectively. The most suitable frequency range for
the reaction of such compounds is 20-100 kHz, which is characterized by
long-lived “stable” cavities that generate very extreme temperatures dur-
ing their violent collapse. In contrast, hydrophilic compounds, particu-
larly those at very low concentrations, tend to remain in the bulk liquid,
where they undergo sonochemical degradation only if there is sufficient
OH and/or other reactive species in solution. The condition is satisfied at
the 200-800 kHz range or under “unstable” and short-lived cavitation that
allows the ejection of some radicals to the bulk liquid [49].
Owing to the high solubility of textile dyes, power ultrasound (20-100
kHz) has been found ineffective for their degradation unless applied in
the pulse mode or in the presence of a volatile reagent that produced
additional radicals via pyrolysis [28,29,50]. The inefficiency is due to the
nature of long-lived and large-resonate cavitation bubbles (1.0 × 10 −5 s;
170 μm), which contain massive quantities of water vapor that reduce the
theoretical values of their collapse temperatures (4000-5000 K) [51-53].