Environmental Engineering Reference
In-Depth Information
7.2.3
Addition of Chemical Reagents
The most commonly added chemical reagents to enhance sonochemical
degradation of textile effluents are hydrogen peroxide (H
2
O
2
), ferrous/
ferric ions (Fe
2+
/Fe
3+
), carbon tetrachloride (CCl
4
) and surface active
compounds. The major contribution of H
2
O
2
was specified in the pre-
vious section as dissociation to produce HOO radicals [62], but it has
a more vital role in the presence of Fe
2+
/Fe
3+
via initiation of Fenton
reactions (to be discussed in the next section on hybrid processes). The
effect of CCl
4
arises from its high volatility (pi
i
= 91 mm Hg/20 C), i.e.,
the ability to diffuse to the gaseous cavity bubbles to undergo molecular
fragmentation and release reactive chlorine radicals, hypochlorous acid
and hydrogen chloride. Moreover, CCl
4
reacts strongly with hydrogen
in the interface and the bubble interior, reducing the probability of its
combination with HO (k
CCl4-H
= 3.8 10
7
M s
-1
, k
)o)0t
= 4 10
10
M s
-1
)
[63]. This is why it is considerably effective under low-frequency ultra-
sound, at which the rate of HO ejection to the bulk solution is low
[28,56]. Surfactants such as linear alkyl sulfonates (LAS) are also unique
with their potential to migrate to the cavity bubble, where they undergo
pyrolysis during the collapse phase to release reactive organic radicals
as CH [64].
Despite their reactivity with OH, the presence of anions such as car-
bonate, nitrate, bromide and iodide may also accelerate decolorization
reactions, provided that they are maintained at a proper dose. For example,
it was found that rates of degradation of C.I. Acid Blue 40 and C.I. Reactive
Red 141 gradually increased with increasing concentrations of CO
3
-2
up
to a plateau above which they started to decline [28,44,58]. Acceleration
at moderate doses of carbonate ions is due to the formation of carbon-
ate radicals (CO
3
-
) and also the fact that although these radicals are less
reactive with organic compounds than HO , their recombination is less
likely. This and their larger stability than HO make them more mobile and
transferrable to the bulk solution, where they react with many hydrophilic
compounds including synthetic dyes [46,48].
Finally, the addition of peroxymonosulfate (HSO
5
-
) and persulfate
(S
2
O
8
2-
) has been found to be remarkably effective not only for decolor-
ization but also for the mineralization of synthetic dyebaths containing
radical scavenging reagents [65]. This effect was attributed to ultrasonic
activation of the additives to generate SO
4
-
and HO , which at pH 8.6
were equally powerful as oxidizing agents.
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