6. Conclusion
In the present study, an attempt is made to gain insight into physical mechanism of
binary and ternary hybrid AOPs with combination of sonolysis, Fenton–like system and photo–
ferrioxalate system. The decolorization profiles of the two model dyes, viz. Acid Red B (azo
dye) and Methylene Blue (non–azo dye), essentially reveal same trends in all experimental
categories. This result points at similarities in the physical mechanism of decolorization/
degradation of the two dyes despite significant dissimilarity in the chemical structure. Among
the individual AOPs, sonolysis is revealed to have least decolorization efficiency, which is
attributed to highly sporadic nature of transient cavitation events and low probability of
interception of the radicals generated from transient cavitation with dye molecules. Even for an
unsaturated medium, in which the cavitation bubble undergoes more intense collapse,
contribution of sonolysis to decolorization is interior compared to other two systems. The
contribution of ultrasound and cavitation in hybrid AOP is, thus, mostly of physical nature.
Coupling of sonolysis with either Fenton – like system or photo–ferrioxalate system gives
20
enhanced decolorization due to positive energy.
However, coupling of all three AOPs gives a negative synergy due to interaction of the
chemical mechanisms of Fenton–like and photo–ferrioxalate system. Dissolved oxygen in the
reaction system is also revealed to have a crucial role. For unsaturated reaction systems with
depleted dissolved oxygen, the pathway of photo–ferrioxalate is further hindered resulting in
reduced in decolorization efficiency.