Analysis of H 2 O 2 formation versus initial dye concentration has been largely used
(Guzman-Duque et al. 2011; Ferkous et al. 2015b; Boutamine et al. 2017). H 2 O 2 is
formed at the bubble surface via self-recombination of
Á
OH (Kanthale et al. 2008;
Merouani et al. 2010b). Note that H 2 O 2 may also be formed via
HO 2
Á + HO 2
Á
! H 2 O 2 + O 2 but the rate constant of this reaction k ¼ 8.3 Â 10
5 M
À1 s
À1
is too low than that of
Á OH +
Á OH ! H 2 O 2 , i.e., k ¼ 5.5 Â 10
9 M
À1 s
À1 (Merouani
et al. 2014a). At lower dye concentration, H 2 O 2 accumulated at higher rate in the
solution, but, when the initial dye concentration increased, H 2 O 2 formation rate
decreased progressively until reaching plateau (Ferkous et al. 2015b; Boutamine
et al. 2017). These outcomes reflect that the flux of the dye molecules toward the
interfacial region increased at elevated concentration provoking efficient scavenging
of
Á OH.
Therefore, the degradation rate of the dye will be then much higher at higher
pollutant concentration. At very high concentration, the pyrolysis mechanism can
also be take place as the temperature of the bubble-solution interface is too high, i.e.,
~ 1900 K (Flint and Suslick 1991; Suslick et al. 1999).
5.5 Reaction Zone and Oxidation Pathways for Textile Dyes
Identification of the degradation by-products during sonolysis may give idea about
the degradation mechanism and the reaction zone (Thompson and Doraiswamy
1999; Adewuyi 2001; Mason and Lorimer 2002). For example, the presence of
hydroxylation products confirms the
Á OH radial pathway, whereas the formation of
pyrolytic products such as methane and acetylene may reflect the pyrolysis mechanism inside the bubble (Barbier and Petrier 1996; Francony and Pétrier 1996; Pétrier
and Francony 1997; Petrier et al. 1998, 2007; Pétrier 2015). However, for the case of
dyes, the determination of such by products is very difficult due to the big and
complex molecular structures of dyes. Other techniques for probing the reaction
zone and the oxidation mechanism during the sonochemical degradation of dyes
have been recently reported.
5.5.1 Radical Probe Technique
For this way, the strategy is to use radical scavengers for different regions. Alcohols
were generally used for the gas-phase reactions. Alcohols are volatile compounds
that easily penetrate into the bubble and degraded through a free-radical pyrolysis
mechanism in which alcohols can react with
Á OH and, therefore, limit the concentration of radicals at the bubble-solution interface. Thus, alcohols trials may confirm
(i) the
Á OH attack of the dye molecules at the outside of the bubble and (ii) can give
evidence of the non-pyrolytic reaction of dyes in the acoustic cavity.
166
S. Merouani and O. Hamdaoui
(Guzman-Duque et al. 2011; Ferkous et al. 2015b; Boutamine et al. 2017). H 2 O 2 is
formed at the bubble surface via self-recombination of
Á
OH (Kanthale et al. 2008;
Merouani et al. 2010b). Note that H 2 O 2 may also be formed via
HO 2
Á + HO 2
Á
! H 2 O 2 + O 2 but the rate constant of this reaction k ¼ 8.3 Â 10
5 M
À1 s
À1
is too low than that of
Á OH +
Á OH ! H 2 O 2 , i.e., k ¼ 5.5 Â 10
9 M
À1 s
À1 (Merouani
et al. 2014a). At lower dye concentration, H 2 O 2 accumulated at higher rate in the
solution, but, when the initial dye concentration increased, H 2 O 2 formation rate
decreased progressively until reaching plateau (Ferkous et al. 2015b; Boutamine
et al. 2017). These outcomes reflect that the flux of the dye molecules toward the
interfacial region increased at elevated concentration provoking efficient scavenging
of
Á OH.
Therefore, the degradation rate of the dye will be then much higher at higher
pollutant concentration. At very high concentration, the pyrolysis mechanism can
also be take place as the temperature of the bubble-solution interface is too high, i.e.,
~ 1900 K (Flint and Suslick 1991; Suslick et al. 1999).
5.5 Reaction Zone and Oxidation Pathways for Textile Dyes
Identification of the degradation by-products during sonolysis may give idea about
the degradation mechanism and the reaction zone (Thompson and Doraiswamy
1999; Adewuyi 2001; Mason and Lorimer 2002). For example, the presence of
hydroxylation products confirms the
Á OH radial pathway, whereas the formation of
pyrolytic products such as methane and acetylene may reflect the pyrolysis mechanism inside the bubble (Barbier and Petrier 1996; Francony and Pétrier 1996; Pétrier
and Francony 1997; Petrier et al. 1998, 2007; Pétrier 2015). However, for the case of
dyes, the determination of such by products is very difficult due to the big and
complex molecular structures of dyes. Other techniques for probing the reaction
zone and the oxidation mechanism during the sonochemical degradation of dyes
have been recently reported.
5.5.1 Radical Probe Technique
For this way, the strategy is to use radical scavengers for different regions. Alcohols
were generally used for the gas-phase reactions. Alcohols are volatile compounds
that easily penetrate into the bubble and degraded through a free-radical pyrolysis
mechanism in which alcohols can react with
Á OH and, therefore, limit the concentration of radicals at the bubble-solution interface. Thus, alcohols trials may confirm
(i) the
Á OH attack of the dye molecules at the outside of the bubble and (ii) can give
evidence of the non-pyrolytic reaction of dyes in the acoustic cavity.
166
S. Merouani and O. Hamdaoui