oxide electrodes. The integration of techniques resulted in full decolorization with a
minute concentration of Cl¯, which was reduced by 75% as a result of UV irradiation
(Riera-Torres and Gutiérrez 2010). Another solution has been proposed as the
replacement of traditional one-cell reactor with a double-section electrolytic cell
partitioned with anion-exchange membrane. So-called dual-electrode oxidation
reactor allows simultaneous oxidation at both electrodes, thereby reducing the
energy demand by multiple folds (Raghu et al. 2009).
Ozonation in an Alkaline Medium
Most of the organic compounds can be degraded and decolorized with ozone, mainly
because oxidation potential of 2.07 V allows ozone to break the unsaturated bonds in
complex hydrocarbons and chromophores constituting humic and colored substances (Alaton et al. 2002). Ozone works in one of the two ways known as direct
and indirect mechanism; the former involves the action of molecular ozone, and the
latter corresponds to the action of free radicals generated by the decomposition of
ozone in aqueous solution (Shu and Huang 1995).
Ozone was used to decolorize Sirius blue SBRR, a direct dye, in a batch bubble
column reactor. A high inlet gas flow at highly basic pH resulted in a large amount of
bubble and complete color removal in less than 30 min. Toxicity tests performed on
the dye decolorization breakdown products revealed that breakdown products are
nontoxic (Turhan and Turgut 2009). The residual dyes from the tanning industry
were also successfully decolorized by ozone. A high pH value and low dye concentrations were recommended for faster processing (Srinivasan et al. 2009).
Ozonation of 19 and Acid Red 27 at different salt concentrations and pH
values has been studied. The process yielded 98% color removal in half an hour
with 45 and 20% mineralization of dyes 19 and Acid Red 27, respectively (Silva
et al. 2009). Reactive Blue 19 was also decolorized and degraded with ozone in a
cylindrical batch reactor. With a concentration of 800 mg/L dye solution and
duration of 1.5 h, 55% removal of chemical oxygen demand and 17% of total
organic carbon reduction were achieved. The addition of other oxidative agents
was suggested to cope with the challenge of incomplete degradation and partial
mineralization (Tehrani-Bagha et al. 2010).
Anionic sulfonated azo dye and a suspected carcinogen, Reactive Orange 16, was
decolorized and degraded via ozonation in a bubble column reactor. At pH value of
12, about 70% decolorization and 100% degradation were achieved within 8-min
duration (Turhan and Ozturkcan 2013). Another study reported over 98% decolorization efficiency of azo dye Reactive Red 120 at a contact time of half an hour by
ozonation method, emphasizing the suitability of ozonation for dyeing effluents and
wastewater treatment (Abidin et al. 2015).
The performance of three different processes involving O 3 , namely, ozonation,
O 3 /H 2 O 2 , and O 3 /activated carbon, was compared against Acid Blue 92 dye. First
two processes worked best in basic conditions; however, the third one caused the
maximum degradation at a low pH. Open reflux analysis revealed the chemical
632
R. S. Ashraf et al.
minute concentration of Cl¯, which was reduced by 75% as a result of UV irradiation
(Riera-Torres and Gutiérrez 2010). Another solution has been proposed as the
replacement of traditional one-cell reactor with a double-section electrolytic cell
partitioned with anion-exchange membrane. So-called dual-electrode oxidation
reactor allows simultaneous oxidation at both electrodes, thereby reducing the
energy demand by multiple folds (Raghu et al. 2009).
Ozonation in an Alkaline Medium
Most of the organic compounds can be degraded and decolorized with ozone, mainly
because oxidation potential of 2.07 V allows ozone to break the unsaturated bonds in
complex hydrocarbons and chromophores constituting humic and colored substances (Alaton et al. 2002). Ozone works in one of the two ways known as direct
and indirect mechanism; the former involves the action of molecular ozone, and the
latter corresponds to the action of free radicals generated by the decomposition of
ozone in aqueous solution (Shu and Huang 1995).
Ozone was used to decolorize Sirius blue SBRR, a direct dye, in a batch bubble
column reactor. A high inlet gas flow at highly basic pH resulted in a large amount of
bubble and complete color removal in less than 30 min. Toxicity tests performed on
the dye decolorization breakdown products revealed that breakdown products are
nontoxic (Turhan and Turgut 2009). The residual dyes from the tanning industry
were also successfully decolorized by ozone. A high pH value and low dye concentrations were recommended for faster processing (Srinivasan et al. 2009).
Ozonation of 19 and Acid Red 27 at different salt concentrations and pH
values has been studied. The process yielded 98% color removal in half an hour
with 45 and 20% mineralization of dyes 19 and Acid Red 27, respectively (Silva
et al. 2009). Reactive Blue 19 was also decolorized and degraded with ozone in a
cylindrical batch reactor. With a concentration of 800 mg/L dye solution and
duration of 1.5 h, 55% removal of chemical oxygen demand and 17% of total
organic carbon reduction were achieved. The addition of other oxidative agents
was suggested to cope with the challenge of incomplete degradation and partial
mineralization (Tehrani-Bagha et al. 2010).
Anionic sulfonated azo dye and a suspected carcinogen, Reactive Orange 16, was
decolorized and degraded via ozonation in a bubble column reactor. At pH value of
12, about 70% decolorization and 100% degradation were achieved within 8-min
duration (Turhan and Ozturkcan 2013). Another study reported over 98% decolorization efficiency of azo dye Reactive Red 120 at a contact time of half an hour by
ozonation method, emphasizing the suitability of ozonation for dyeing effluents and
wastewater treatment (Abidin et al. 2015).
The performance of three different processes involving O 3 , namely, ozonation,
O 3 /H 2 O 2 , and O 3 /activated carbon, was compared against Acid Blue 92 dye. First
two processes worked best in basic conditions; however, the third one caused the
maximum degradation at a low pH. Open reflux analysis revealed the chemical
632
R. S. Ashraf et al.