Hamdaoui and Merouani 2017b). All results agreed with the following behavior:
when the initial concentration of pollutant is increased, the removal efficiency
decreased, but the initial degradation rate increased rapidly at low dye concentration
and then tends to a limit value at too high values of dye concentrations. Correspondingly, the production rate of H 2 O 2 followed the inverse effect; it is higher in pure
water, but it decreased progressively with increasing initial dye concentration up to
reaching a minimum value.
These results were attributed to the displacement of the reaction zone with respect
to the initial dye concentration (Okitsu et al. 2005, 2015; Merouani et al. 2010c;
Guzman-Duque et al. 2011; Moumeni et al. 2012; Dalhatou et al. 2015; Ferkous
et al. 2015b; Taamallah et al. 2016; Boutamine et al. 2017; Hamdaoui and Merouani
2017b). The bulk solution is the preferable degradation zone at lower dye concentration. Therefore, the great portion of
Á
OH could recombine at the bubble interface
to form H 2 O 2 at higher rates, and only a lower quantity of
Á
OH may reach the bulk of
the solution to react with the dye molecules. Increasing the dye concentration, the
reaction zone could be progressively moved to the bubble-solution interface where
an elevated concentration of radicals is located. In this case, the fraction of
Á OH
trapped by the dye molecules increased, leading to higher degradation rate and lower
formation rate of hydrogen peroxide. At too high dye concentration, bubble surface
could be completely occupied with dye molecules, and at this point any increase in
dye concentration could not have any effect on the initial degradation rate (Merouani
et al. 2010c; Moumeni et al. 2012).
5.6.6 pH
The solution pH of textile effluents is very variable. The solution pH can modify the
physical properties of dyes and their reactivity toward the sonochemical treatment
(Ince and Tezcanli-Güyer 2004; Wang et al. 2008; Ghodbane and Hamdaoui 2009a;
Merouani et al. 2010c; Dalhatou et al. 2015; Ferkous et al. 2015a, b; Rayaroth et al.
2015; Taamallah et al. 2016; Fassi and Petrier 2016; Boutamine et al. 2017;
Merouani and Hamdaoui 2017).
Practically, all results reported that acidic conditions favor higher degradation rate
than neutral pHs (~7–8). Researchers have attributed these findings to the less
ionization state of most dyes in acidic conditions, which increases their hydrophobic
character. The factor that precise the ionization state is the pK a , which is mostly not
available for synthetic dyes. However, in the majority of cases, the percentage of the
ionized form of dyes increases with pH rise. Therefore, more hydrophilic forms may
be present in the solution at higher pH values, thereby allowing degradation to occur
in the liquid bulk where the concentration of hydroxyl radicals is low.
On the other hand, increasing pH in the basic medium have shown two possible
tendencies: (i) a continuous decrease of the degradation rate (Dalhatou et al. 2015;
Hamdaoui and Merouani 2017b) and (ii) a re-increase of the degradation rate (Wang
et al. 2008; Ghodbane and Hamdaoui 2009a; Merouani et al. 2010c). Dalhatou et al.
5 Sonochemical Treatment of Textile Wastewater
173
when the initial concentration of pollutant is increased, the removal efficiency
decreased, but the initial degradation rate increased rapidly at low dye concentration
and then tends to a limit value at too high values of dye concentrations. Correspondingly, the production rate of H 2 O 2 followed the inverse effect; it is higher in pure
water, but it decreased progressively with increasing initial dye concentration up to
reaching a minimum value.
These results were attributed to the displacement of the reaction zone with respect
to the initial dye concentration (Okitsu et al. 2005, 2015; Merouani et al. 2010c;
Guzman-Duque et al. 2011; Moumeni et al. 2012; Dalhatou et al. 2015; Ferkous
et al. 2015b; Taamallah et al. 2016; Boutamine et al. 2017; Hamdaoui and Merouani
2017b). The bulk solution is the preferable degradation zone at lower dye concentration. Therefore, the great portion of
Á
OH could recombine at the bubble interface
to form H 2 O 2 at higher rates, and only a lower quantity of
Á
OH may reach the bulk of
the solution to react with the dye molecules. Increasing the dye concentration, the
reaction zone could be progressively moved to the bubble-solution interface where
an elevated concentration of radicals is located. In this case, the fraction of
Á OH
trapped by the dye molecules increased, leading to higher degradation rate and lower
formation rate of hydrogen peroxide. At too high dye concentration, bubble surface
could be completely occupied with dye molecules, and at this point any increase in
dye concentration could not have any effect on the initial degradation rate (Merouani
et al. 2010c; Moumeni et al. 2012).
5.6.6 pH
The solution pH of textile effluents is very variable. The solution pH can modify the
physical properties of dyes and their reactivity toward the sonochemical treatment
(Ince and Tezcanli-Güyer 2004; Wang et al. 2008; Ghodbane and Hamdaoui 2009a;
Merouani et al. 2010c; Dalhatou et al. 2015; Ferkous et al. 2015a, b; Rayaroth et al.
2015; Taamallah et al. 2016; Fassi and Petrier 2016; Boutamine et al. 2017;
Merouani and Hamdaoui 2017).
Practically, all results reported that acidic conditions favor higher degradation rate
than neutral pHs (~7–8). Researchers have attributed these findings to the less
ionization state of most dyes in acidic conditions, which increases their hydrophobic
character. The factor that precise the ionization state is the pK a , which is mostly not
available for synthetic dyes. However, in the majority of cases, the percentage of the
ionized form of dyes increases with pH rise. Therefore, more hydrophilic forms may
be present in the solution at higher pH values, thereby allowing degradation to occur
in the liquid bulk where the concentration of hydroxyl radicals is low.
On the other hand, increasing pH in the basic medium have shown two possible
tendencies: (i) a continuous decrease of the degradation rate (Dalhatou et al. 2015;
Hamdaoui and Merouani 2017b) and (ii) a re-increase of the degradation rate (Wang
et al. 2008; Ghodbane and Hamdaoui 2009a; Merouani et al. 2010c). Dalhatou et al.
5 Sonochemical Treatment of Textile Wastewater
173