because all parameters of Eq. 5.2 are sensitive to experimental conditions. Table 5.1
provides some estimated values of N under different experimental frequencies.
Overall, it was found that billions of bubbles can be formed and increasing frequency
of ultrasonication engenders a substantial increase in the number of bubbles.
5.4 Sono-oxidation of Textile Dyes
Significant amount of works on the degradation of dyes by ultrasound has been
published, particularly, in the last two decades. Table 5.2 reports the most significant
studies published between 2000 and 2019 for a variety of synthetic dyes. The
significant results of each study were included together with experimental conditions
and type of the reactor. In all these cases, consequences of using sonochemical
treatment are nearly the same: dyes were effectively degraded, and at least partial
mineralization was achieved.
During the treatment, the dye concentration decays exponentially with time until
total bleaching, suggesting first-order reaction law. The lower TOC and COD
removals under ultrasound were attributed to the formation of highly hydrophilic
by-products, which have no tendency to accumulate at the reactive bubble-solution
interface (Torres et al. 2007; Guzman-Duque et al. 2011; Boutamine et al. 2017;
Hamdaoui and Merouani 2017b). However, these compounds may be readily biodegradable, since biodegradability analysis demonstrated that the ratio BOD 5 /COD
was superior than 0.4 (Guzman-Duque et al. 2011).
On the other hand, the degradation rate was strongly sensitive to the operation
parameters. Generally, best degradation performances were obtained at higher levels
of delivered power (or intensity), solution temperature, and lower solution pH,
although strong basic pH provided higher conversion rates for some cases (Rehorek
et al. 2004; Ghodbane and Hamdaoui 2009a; Merouani et al. 2010c; Dalhatou et al.
2015). Additionally, higher frequency ultrasound (higher than 100 kHz) was most
effective for dye degradation than low frequencies. It seems also that there is an
optimum frequency, between 200 and 800 kHz, for the degradation of textile dyes
(Eren and Ince 2010; Dükkanci et al. 2012; Ferkous et al. 2015b; Rayaroth et al.
2015).
Dyes are generally highly water-soluble substrate of lower vapor pressure and
Henry’s law constants (Taamallah et al. 2016). They are classified as nonvolatile
compounds which likely degraded via reaction with hydroxyl radicals at the exterior
of the collapsing bubble (Ince and Tezcanli-Güyer 2004; Okitsu et al. 2005, 2015;
Merouani et al. 2016). However, the local reaction zone at which degradation
occurred may be moved between the bulk solution and the interfacial area,
depending on several experimental parameters. For instance, experimental evidence
showed that when the concentration of the dye is low, the bulk of the solution is the
preferential reaction zone. However, the reaction zone shifts toward the bubblesolution interface by progressive increase of the initial pollutant concentration in the
solution (Okitsu et al. 2005; Chiha et al. 2010; Chadi et al. 2018b).
5 Sonochemical Treatment of Textile Wastewater
155
provides some estimated values of N under different experimental frequencies.
Overall, it was found that billions of bubbles can be formed and increasing frequency
of ultrasonication engenders a substantial increase in the number of bubbles.
5.4 Sono-oxidation of Textile Dyes
Significant amount of works on the degradation of dyes by ultrasound has been
published, particularly, in the last two decades. Table 5.2 reports the most significant
studies published between 2000 and 2019 for a variety of synthetic dyes. The
significant results of each study were included together with experimental conditions
and type of the reactor. In all these cases, consequences of using sonochemical
treatment are nearly the same: dyes were effectively degraded, and at least partial
mineralization was achieved.
During the treatment, the dye concentration decays exponentially with time until
total bleaching, suggesting first-order reaction law. The lower TOC and COD
removals under ultrasound were attributed to the formation of highly hydrophilic
by-products, which have no tendency to accumulate at the reactive bubble-solution
interface (Torres et al. 2007; Guzman-Duque et al. 2011; Boutamine et al. 2017;
Hamdaoui and Merouani 2017b). However, these compounds may be readily biodegradable, since biodegradability analysis demonstrated that the ratio BOD 5 /COD
was superior than 0.4 (Guzman-Duque et al. 2011).
On the other hand, the degradation rate was strongly sensitive to the operation
parameters. Generally, best degradation performances were obtained at higher levels
of delivered power (or intensity), solution temperature, and lower solution pH,
although strong basic pH provided higher conversion rates for some cases (Rehorek
et al. 2004; Ghodbane and Hamdaoui 2009a; Merouani et al. 2010c; Dalhatou et al.
2015). Additionally, higher frequency ultrasound (higher than 100 kHz) was most
effective for dye degradation than low frequencies. It seems also that there is an
optimum frequency, between 200 and 800 kHz, for the degradation of textile dyes
(Eren and Ince 2010; Dükkanci et al. 2012; Ferkous et al. 2015b; Rayaroth et al.
2015).
Dyes are generally highly water-soluble substrate of lower vapor pressure and
Henry’s law constants (Taamallah et al. 2016). They are classified as nonvolatile
compounds which likely degraded via reaction with hydroxyl radicals at the exterior
of the collapsing bubble (Ince and Tezcanli-Güyer 2004; Okitsu et al. 2005, 2015;
Merouani et al. 2016). However, the local reaction zone at which degradation
occurred may be moved between the bulk solution and the interfacial area,
depending on several experimental parameters. For instance, experimental evidence
showed that when the concentration of the dye is low, the bulk of the solution is the
preferential reaction zone. However, the reaction zone shifts toward the bubblesolution interface by progressive increase of the initial pollutant concentration in the
solution (Okitsu et al. 2005; Chiha et al. 2010; Chadi et al. 2018b).
5 Sonochemical Treatment of Textile Wastewater
155