5. Results and Discussion
The results of dye decolorization in nine experimental categories for azo dye of ARB and
non–azo dye of MB are given in Tables 3 and 4, respectively. The time profiles of dye
decolorization in nine experimental categories are shown in Figs. 1–3 for both dyes.
Representative results of simulations of cavitation bubble dynamics are given in Fig. 4 for a 5
m air bubble, which shows time histories of radius of the bubble, temperature and pressure
inside the bubble, extent of solvent vaporization in the bubble, the micro–convection and shock
(or acoustic) waves generated by the bubble. The summary of the simulation results is presented
in Table 5. It could be seen that the smaller bubble of 5 m (representing unsaturated liquid
medium) undergoes more intense collapse, with higher peaks of temperature and pressure
attained during collapse, than the 10 m bubble (representing saturated medium). This
essentially means that the sonochemical effect of radical production is higher in unsaturated
medium, as represented by mole fractions of •OH and •O radicals for 5 m bubble, which are
one order of magnitude higher than those for 10 m bubble. Quite interestingly, the physical
effect of micro–turbulence shows an inverse trend. In this case, the magnitude of micro–
convection velocity generated by the 10 m bubble is greater than 5 m. Nonetheless, the
pressure amplitude of the acoustic waves generated by 5 m bubble is higher than the 10 m
bubble.
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It is generally accepted that sonochemical effect (i.e. attack of •OH radicals generated by
transient cavitation) contributes to degradation of organic pollutant. However, in the context of
present study, in which dye decolorization is used as model reaction, both physical and chemical
effects of cavitation can be of relevance. Saharan et al. [69] have demonstrated that high shear
generated by micro–convection and acoustic waves can break the chromophores of the dye
molecules, leading to decolorization. This, of course, is the initial step in decolorization, and
does not lead to complete mineralization. The complete mineralization is achieved by reactions
induced by oxidizing radicals.
In Figs. 1–3, the extent of dye decolorization in the first five minutes of treatment, and
complete treatment of 60 min has been given separately. As seen from the dye decolorization
profiles given in Figs. 1–3, a major fraction of total decolorization (in 60 min of treatment) was
achieved in first 30 min itself; and hence, the pseudo 1st order kinetic constant for dye
decolorization has been determined using decolorization data for this period only. Some distinct
features of dye decolorization that can be perceived from data presented in Tables 3–4 and the
profiles shown in Figs. 1–3 are as follows:
(1) Trends in extent of decolorization with different experimental categories are similar for both
dyes, although actual quantitative values of decolorization are different for ARB and MB dyes.
The discrepancy of dissimilar extent of degradation is linked to the differences in chemical
structure and the prevalent chemical mechanism of degradation of the two dyes. However, the
qualitative similarity in trends in extent of decolorization is highlights resemblance of the
physical mechanism of degradation of the two dyes.
(2) Among the individual AOPs, sonolysis gives least decolorization for both dyes. For the other
two AOPs, photo–ferrioxalate and Fenton–like systems, significant decolorization is observed in
60 min treatment. Almost total decolorization of MB is achieved in 60 min of treatment with
Fenton–like system. The least decolorization for sonolysis can be explained as follows:
The radicals formed inside the cavitation bubble during transient collapse get released
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into the bulk medium with fragmentation of the bubble at the instance of maximum
compression. However, these radicals are extremely unstable and do not diffuse into the liquid
medium from the point of bubble collapse. This puts a strong limitation on the interaction
between the radicals and the molecules – which could further lead to hydroxylation/ oxidation
reactions. Only those dye molecules that are in close vicinity of the bubble interface are likely to
intercept the radicals released from the bubble. Due to rather small concentration of the dye in
the reaction mixture (20 ppm), the probabilities of presence of dye molecules at the bubble
interface, and the interception of radicals by them are small, which leads to low extent of dye
decolorization. Moreover, occurrence of transient cavitation events is a highly sporadic
phenomenon in space as well as time domain. As stated earlier, cavitation phenomenon induced
by the gas nuclei present in the liquid medium, which are contributed by freely suspended
bubbles and gas pockets trapped in the crevices of the solid boundaries in reaction system. The
volumetric distribution of these nuclei in the reaction mixture is not likely to be uniform. As a
consequence, production of oxidizing radicals through sonolysis is also not uniform
phenomenon, which puts a severe limit on the yield (or extent of dye decolorization, in the
present context) of the sonolysis process.
(3) Almost similar extent of dye decolorization in categories 2 and 4, in which individual AOP
of photo–ferrioxalate with mechanical stirring and hybrid AOP of sono–photo–ferrioxalate was
applied, indicates that contribution of chemical effect of sonolysis in the hybrid AOPs trivial.
Most of the decolorization is achieved via the photo–ferrioxalate route, with ultrasound and
cavitation contributing only in terms of physical effect of generating convection (or stirring) in
the medium. This result is consistent for decolorization of both ARB and MB dye.
(4) Similar arguments also hold for categories 3 and 5, in which individual Fenton–like and the
hybrid sono–Fenton–like systems have been applied. The extent of decolorization in these
categories is almost similar, confirming only physical role of ultrasound and cavitation in the
hybrid AOP. Again, this result is consistent for both ARB and MB dyes.
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(5) Trivial role of sonolysis in the hybrid AOP is also evident from the extent of dye
decolorization in categories 6 & 7 and 8 & 9. Application of sonolysis in addition to the Fenton–
like system and photo–ferrioxalate results in minuscule rise in decolorization. The extent of
decolorization obtained with binary AOP of Fenton–like + photo–ferrioxalate system is almost
similar as for the ternary AOP of sonolysis + Fenton–like + photo–ferrioxalate system. This
result is consistent for reaction systems with saturated and unsaturated media. As seen from the
results of simulation of cavitation bubble dynamics, the intensity of transient cavitation increases
with unsaturation. Despite this, the contribution of chemical effect of sonolysis to the overall
decolorization is trivial, for the reasons explained earlier.
(6) The relative contributions of Fenton–like and photo–ferrioxalate pathways in the binary
hybrid AOPs applied in categories 6 and 8 can also be determined on the basis of chemical
mechanism of the individual AOPs given in section 2. In these categories, H2O2 is added in
excess, while oxalate addition is in stoichiometry, with respect to the Fe3+. In such situation,
Fe3+ is competitively consumed by H2O2 and 2
2 4 C O . Reaction of Fe3+ with H2O2 generates Fe2+,
which can further react with H2O2 through regular Fenton reactions to produce •OH radicals.
Thus, the pathways of photo–ferrioxalate system and Fenton–like system hinder each other,
which results in negative or adverse synergy between these systems. Although extent of
decolorization of both dyes with individual Fenton–like and photo–ferrioxalate systems is
greater than 50%, the total decolorization with both AOPs applied together is less than 100%.
This indicates negative synergy in the binary AOP. For both dyes, the extent of decolorization is
almost similar for saturated medium as well as unsaturated medium, in which the dissolved
oxygen content of the medium is lowered. The chemical mechanism of the Fenton–like AOP
does not involve dissolved oxygen. In the chemical mechanism of photo–ferrioxalate pathway,
however, combination of oxalate radical (produced from photo decomposition of 3
2 2 4 3 Fe (C O ) )
with dissolved oxygen produces radical species of 2 O and •OH that contribute to decolorization
18
/ degradation of dye. Therefore, the extent of decolorization through photo–ferrioxalate pathway
is expected to reduce with depletion of dissolved oxygen in unsaturated medium.
If the decolorization in categories 6 and 8 are compared for both dyes, we find negligible
change in total decolorization for both dyes, although the kinetic constant for the MB dye
reduces slightly with unsaturation. These results essentially indicate that contribution of the
photo–ferrioxalate pathway to the overall decolorization is relatively lesser than the Fenton–like
pathway.
Synergy of the hybrid AOPs: A useful tool to assess interactions among individual chemical
pathways of different AOPs in a hybrid system on a gross level is to determine the synergy
among the individual AOPs. The synergy could be defined on the basis of kinetic constants of
decolorization, or on the basis of total decolorization achieved in hybrid system as compared to
the individual systems. In the present case, we have used the former approach, and have defined
synergy as:
kineti