C.I. Reactive Red 141, the degradation rate was null for 20 kHz and increased in the
order 1145 kHz > 861 kHz > 577 kHz. Rayaroth et al. (2015) have examined the
degradation of Coomassie Brilliant Blue upon 200, 350, 620, and 1000 kHz. They
indicated that 350 kHz was the optimum frequency for the degradation of the dye.
In the study on sonochemical degradation of azo dye Reactive Black 5 done by
Vajnhandl and Marechel (2007), 20 kHz low-frequency system and high-frequency
(279 and 817 kHz) system were used. The greatest decolorization rate of Reactive
Black 5 was observed at 817 kHz. In the study done by Ma et al. (2006) on the
decomposition of acid orange 7, the degradation rates were higher at a frequency of
1 MHz than at a frequency of 20 kHz. In another study, degradation of rhodamine B
and orange II was investigated by Inoue et al. (2006) using ultrasound at frequencies
of 118, 224, 404, and 651 kHz at actual power of 28–29 W. The lower decolorization
degree was achieved at the lowest frequency, for both azo dyes.
Therefore, all given works agreed with the fact that higher frequency ultrasound,
i.e., more than 100 kHz, is more effective for the destruction of textile dyes. Besides,
an optimal frequency in the interval 200–800 kHz is highly probable for the best
degradation performance.
Recently, detailed numerical simulations of frequency effect on the single bubble
yield and the number of active collapsing bubbles have been conducted by Merouani
et al. (Merouani et al. 2014a, c). The authors found that the chemical bubble yield,
i.e., hydroxyl radical production, decreased substantially with increasing frequency
up to 1000 kHz, whereas the number of bubbles increased monotonically with
increasing frequency in nearly the same range of frequency. Therefore, the existence
of an optimum frequency for the sonochemical activity can be highlighted by the
concurrence between the two factors; the number of bubbles controls the overall
sonochemical generation of radicals when the frequency increased up to the optimum frequency, while the single bubble yield imposes its effect when the frequency
continued to increase above the optimal frequency. However, it should be noted that
the optimum value of frequency was dependent to other operational parameters such
as power and liquid temperature, as demonstrated by Mark et al. (1998).
5.6.2 Ultrasonic Power
Most reported results agreed with the fact that the degradation of textile dyes
increased with increasing ultrasonic power when operating above the cavitation
threshold (Rehorek et al. 2004; Wang et al. 2008; Merouani et al. 2010c; GuzmanDuque et al. 2011; Dalhatou et al. 2015; Ferkous et al. 2015a; Rayaroth et al. 2015;
Fassi and Petrier 2016; Taamallah et al. 2016; Boutamine et al. 2017; Merouani and
Hamdaoui 2017). The beneficial effect of increasing power can be explained via the
impact of this parameter on the single bubble yield and the number of bubbles. A
computational analysis of this issue is available in refs. (Ferkous et al. 2015a;
Merouani et al. 2015).
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S. Merouani and O. Hamdaoui
order 1145 kHz > 861 kHz > 577 kHz. Rayaroth et al. (2015) have examined the
degradation of Coomassie Brilliant Blue upon 200, 350, 620, and 1000 kHz. They
indicated that 350 kHz was the optimum frequency for the degradation of the dye.
In the study on sonochemical degradation of azo dye Reactive Black 5 done by
Vajnhandl and Marechel (2007), 20 kHz low-frequency system and high-frequency
(279 and 817 kHz) system were used. The greatest decolorization rate of Reactive
Black 5 was observed at 817 kHz. In the study done by Ma et al. (2006) on the
decomposition of acid orange 7, the degradation rates were higher at a frequency of
1 MHz than at a frequency of 20 kHz. In another study, degradation of rhodamine B
and orange II was investigated by Inoue et al. (2006) using ultrasound at frequencies
of 118, 224, 404, and 651 kHz at actual power of 28–29 W. The lower decolorization
degree was achieved at the lowest frequency, for both azo dyes.
Therefore, all given works agreed with the fact that higher frequency ultrasound,
i.e., more than 100 kHz, is more effective for the destruction of textile dyes. Besides,
an optimal frequency in the interval 200–800 kHz is highly probable for the best
degradation performance.
Recently, detailed numerical simulations of frequency effect on the single bubble
yield and the number of active collapsing bubbles have been conducted by Merouani
et al. (Merouani et al. 2014a, c). The authors found that the chemical bubble yield,
i.e., hydroxyl radical production, decreased substantially with increasing frequency
up to 1000 kHz, whereas the number of bubbles increased monotonically with
increasing frequency in nearly the same range of frequency. Therefore, the existence
of an optimum frequency for the sonochemical activity can be highlighted by the
concurrence between the two factors; the number of bubbles controls the overall
sonochemical generation of radicals when the frequency increased up to the optimum frequency, while the single bubble yield imposes its effect when the frequency
continued to increase above the optimal frequency. However, it should be noted that
the optimum value of frequency was dependent to other operational parameters such
as power and liquid temperature, as demonstrated by Mark et al. (1998).
5.6.2 Ultrasonic Power
Most reported results agreed with the fact that the degradation of textile dyes
increased with increasing ultrasonic power when operating above the cavitation
threshold (Rehorek et al. 2004; Wang et al. 2008; Merouani et al. 2010c; GuzmanDuque et al. 2011; Dalhatou et al. 2015; Ferkous et al. 2015a; Rayaroth et al. 2015;
Fassi and Petrier 2016; Taamallah et al. 2016; Boutamine et al. 2017; Merouani and
Hamdaoui 2017). The beneficial effect of increasing power can be explained via the
impact of this parameter on the single bubble yield and the number of bubbles. A
computational analysis of this issue is available in refs. (Ferkous et al. 2015a;
Merouani et al. 2015).
170
S. Merouani and O. Hamdaoui