5.6.4 Dissolved Gases
Argon, air, nitrogen, and sometimes CO 2 are the most used gases for studying the
effect of saturation gases on the efficiency of dye removal using the sonochemical
process (Joseph et al. 1999; Okitsu et al. 2005; Minero et al. 2008; Guzman-Duque
et al. 2011; Ferkous et al. 2015b; Taamallah et al. 2016; Chadi et al. 2018a). Overall,
the degradation rate of dyes decreased in the order Ar < air < N 2 , whereas CO 2
suppress completely the degradation process (Joseph et al. 1999; Okitsu et al. 2005;
Minero et al. 2008; Guzman-Duque et al. 2011; Ferkous et al. 2015b; Taamallah
et al. 2016; Chadi et al. 2018a).
Dissolved gases influence the sonochemical activity via several ways: (i) more
nucleation sites can be created when using a gas of high solubility, i.e., higher
number of bubble could be produced in this situation, and (ii) a gas of greater
polytropic index γ ¼ c p /c v and lower thermal conductivity produces higher collapsing temperatures within the bubble, and this promotes the chemical activity of the
cavity. Nevertheless, Okitsu et al. (2006) have reported a new aspect of saturation
gases toward the bubble temperature. Measurements of this last for different saturation gases of identical γ, and diverse thermal conductivities have given the same
temperature values. The authors have concluded that the thermal conductivity has
not a significant influence on the bubble temperature and the gases effect could be
closely related to the number of bubbles.
Hence, the best degradation performance obtained under argon atmosphere is due
to its higher γ and solubility x than other gases, i.e., γ Ar ¼ 1.66 compared to 1.4 for
air and N 2 and x Ar ¼ 2.748 Â 10
À5 mol/mol compared to x N2 ¼ 1.276 Â 10
À5 mol/
mol and x air ¼ 1.524 Â 10
À5 mol/mol. However, the difference in impacts of air and
N 2 on the dye removal was attributed to the direct involvement of these gases in the
combustion reactions inside the bubble, since the two gases have an identical γ and a
marginal difference in their solubilities. Computational analysis revealed that the
single bubble yield decreased as the concentration of N 2 increased in the bubble,
mainly due to the huge consumption of
Á OH by NO that resulted from reactions
N 2 + O Ð NO+N and NO 2 + M Ð O + NO+M (Merouani et al. 2014c). For the case
of CO 2 , its effect was attributed to the too high solubility of CO 2 in water (46 times
much higher than air), which could provoke a suppression of the inertial cavitation
responsible for all chemical effects of ultrasound in aqueous solutions (Chadi et al.
2018a).
5.6.5 Initial Dye Concentration
In general, the concentration of synthetic dyes in textile effluents is variable. This
parameter has been extensively studied by researchers (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;
172
S. Merouani and O. Hamdaoui
Argon, air, nitrogen, and sometimes CO 2 are the most used gases for studying the
effect of saturation gases on the efficiency of dye removal using the sonochemical
process (Joseph et al. 1999; Okitsu et al. 2005; Minero et al. 2008; Guzman-Duque
et al. 2011; Ferkous et al. 2015b; Taamallah et al. 2016; Chadi et al. 2018a). Overall,
the degradation rate of dyes decreased in the order Ar < air < N 2 , whereas CO 2
suppress completely the degradation process (Joseph et al. 1999; Okitsu et al. 2005;
Minero et al. 2008; Guzman-Duque et al. 2011; Ferkous et al. 2015b; Taamallah
et al. 2016; Chadi et al. 2018a).
Dissolved gases influence the sonochemical activity via several ways: (i) more
nucleation sites can be created when using a gas of high solubility, i.e., higher
number of bubble could be produced in this situation, and (ii) a gas of greater
polytropic index γ ¼ c p /c v and lower thermal conductivity produces higher collapsing temperatures within the bubble, and this promotes the chemical activity of the
cavity. Nevertheless, Okitsu et al. (2006) have reported a new aspect of saturation
gases toward the bubble temperature. Measurements of this last for different saturation gases of identical γ, and diverse thermal conductivities have given the same
temperature values. The authors have concluded that the thermal conductivity has
not a significant influence on the bubble temperature and the gases effect could be
closely related to the number of bubbles.
Hence, the best degradation performance obtained under argon atmosphere is due
to its higher γ and solubility x than other gases, i.e., γ Ar ¼ 1.66 compared to 1.4 for
air and N 2 and x Ar ¼ 2.748 Â 10
À5 mol/mol compared to x N2 ¼ 1.276 Â 10
À5 mol/
mol and x air ¼ 1.524 Â 10
À5 mol/mol. However, the difference in impacts of air and
N 2 on the dye removal was attributed to the direct involvement of these gases in the
combustion reactions inside the bubble, since the two gases have an identical γ and a
marginal difference in their solubilities. Computational analysis revealed that the
single bubble yield decreased as the concentration of N 2 increased in the bubble,
mainly due to the huge consumption of
Á OH by NO that resulted from reactions
N 2 + O Ð NO+N and NO 2 + M Ð O + NO+M (Merouani et al. 2014c). For the case
of CO 2 , its effect was attributed to the too high solubility of CO 2 in water (46 times
much higher than air), which could provoke a suppression of the inertial cavitation
responsible for all chemical effects of ultrasound in aqueous solutions (Chadi et al.
2018a).
5.6.5 Initial Dye Concentration
In general, the concentration of synthetic dyes in textile effluents is variable. This
parameter has been extensively studied by researchers (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;
172
S. Merouani and O. Hamdaoui