Single and Hybrid Applications of Ultrasound for Decolorization and Degradation of Textile Dye Residuals in Water - Green Chemistry for Dyes Removal from Wastewater - page 268

Figure 7.2 Potential reaction sites during sonolysis of an azo dye [35].
The chemical structure and nature of o-substituents also impact the
rate of decolorization by ultrasound. For example, a simple molecule
with a low molecular weight and no more than one or two substituents
has a higher decolorization rate constant than a complex one with sev-
eral aromatic rings and a larger number of o-substituents. Moreover, a
high pKa, low SO 3 - content, and high preference of the hydrazone iso-
mer over the azo form are factors that facilitate the decay of color by
ultrasound [41,42,45]. High pKa and low SO 3 - are believed to decrease
the anionicity of the molecule, which in turn facilitates its approach
to the negatively charged cavity bubbles; while the hydrazone form is
associated with internal hydrogen bonding and reactivity with H 2 O 2 /
HO 2 - pair.
Figure 7.3 shows chemical structures and tautomerization of two dis-
similarly structured azo-naphtol dyes, both with an o-hydroxyl substituent
about the azo bond to indicate the preference of the hydrazone tautomer.
The apparent decolorization rate constants as determined under equiva-
lent ambient/ultrasonic conditions were 4.90 10 -4 and 2.35 10 -4 min -1 ,
respectively [40]. The main reason for the lower reactivity of the second
dye is its more complex structure (618 g mol -1 ) and the presence of a
second o-substituent (SO 3 - ), which weakens the internal H-bonding via
steric effects and competition for H-abstraction from the HO -adduct.
Moreover, delocalization of the C-O bond electrons of the carboxyl amine
group (attached to the naphthol-ring) stabilizes the molecule, thus further
decelerating the rate of bleaching.