Single and Hybrid Applications of Ultrasound for Decolorization and Degradation of Textile Dye Residuals in Water - Green Chemistry for Dyes Removal from Wastewater - page 285

via passivation of the outer oxide layer by the micro-jets, (iii) enhanced
dissolution of the catalyst and replenishment of its surface with fresh min-
eral via the presence of US-generated H 2 O 2 .
7.3.5 Sonoelectrocatalysis
The superiority of US-assisted electrocatalysis over standard electrochemi-
cal oxidation processes is based on the presence of high energy microjets,
which render intense agitation of the liquid and continuous degassing on
the electrode surface [109,110]. Agitation facilitates ion transport across
the electrode double layer and reduces the rate of ion depletion in the dif-
fusion layer, while degassing at the electrode surface prevents the accu-
mulation of gas bubbles or the formation of a current barrier [96]. The
efficacy of the method has been tested on a variety of synthetic dyes (e.g.,
methylene blue, reactive brilliant red X-3B, Rhodamine B, methyl orange)
and dyebaths and it has been reported that the process is highly sensitive
to the applied potential, the acoustic power and frequency, and to the solu-
tion pH [69,96-98]. As such, decolorization was appreciable only at E 4V
and within a power range of 20-60 W [61]. Note also that the process was
found more effective at short frequency irradiation, which allows more
intense microjets and thus a higher degree of agitation.
As in most hybrid processes, sonoelectrocatalysis has been found more
cost-effective than simple electrochemical oxidation, as demonstrated
for C.I. Reactive Blue 19, whose decomposition was enhanced by 50% (to
reach 81% mineralization) and energy consumption reduced by 38-50%
via integrating a conventional electrochemical oxidation process with low-
frequency ultrasound [96].
7.4 Conclusions
Treatment of textile dyeing wastewater and process streams has long been
a major concern of environmental engineers, scientists and entrepreneurs
not only due to the complexity of the effluent and the strict regulations
on its discharge, but also to the socioeconomic and political dimensions
of the problem. The aim of this chapter was to present an overview of the
nature of textile dyeing mill wastewater and its treatability by ultrasonic
irradiation and/or a variety of hybrid processes that utilize ultrasound. The
discussion has focused particularly on the degradation and mineraliza-
tion of reactive azo dyes, which due to their high color fastness are vastly