Environmental Engineering Reference
In-Depth Information
Fenton-like or so-called “advanced Fenton reactions.” Hence, the rate of
decolorization/mineralization is limited by the rate of mass transfer of sol-
utes to the catalyst surface and the concentration of H
2
O
2
, which under-
goes decomposition at the metal surface:
Fe (s) + H
2
O
2
Fe
+
+ OH
-
+ HO (7.28)
Note that the advanced Fenton process is further effective when car-
ried out in the presence of adsorbents such as granular activated carbon to
generate additional adsorption/reaction sites while utilizing the mechani-
cal effects of ultrasound for cleaning the surface of the adsorbent and the
catalyst to prevent accumulation of the oxidation/reduction byproducts
[85,86].
Another important catalyst to integrate with ultrasound is exfoliated
graphite, which in the presence of peroxide has been found to exhibit an
outstanding activity in mineralization of fourteen different azo dyes at short
frequency ultrasound [60]. The synergy was explained by the likely rupture
of the graphite shield via hydrodynamic sheer forces and the subsequent
decreases and increases in its particle size and surface area, respectively.
The role of monosulfate and persulfate anions in the presence of ultra-
sound has already been discussed, but will once again be highlighted in
heterogeneous media. It has been shown that persulfate is easily activated
by heat, cavitation, hydrogen peroxide or catalysts such as Co (II), Fe (II),
Ag (I) to generate sulfate radicals and HO , both of which are reactive with
textile dyes to produce dye-OH adducts and oxidized dye intermediates as:
S
2
O
8
2-
+ )))
SO
4
2-
+ SO
4
-
(7.29)
SO
4
-
+ H
2
O SO
4
2-
+ HO + H (7.30)
S
2
O
8
2-
+ M
n+
SO
4
2-
+ SO
4
-
(7.31)
Dye + SO
4
-
+ HO Dye
-
+ Dye-OH adduct + SO
4
2-
(7.32)
Accordingly, 90-97% decolorization and more than 60% mineralization
of reactive azo dyes have been reported for Co/US-activated persulfate sys-
tem, and complete decolorization with 73% mineralization reported for
Fe/US-activated persulfate [87].
Last but not least, manganese dioxide at acidic pH was found to exhibit
considerable activity in decolorization and mineralization of synthetic azo
dyes by ultrasound via the presence of very active reaction sites on the
mineral surface and the solid-liquid interface for redox reactions [26]. The
results were attributed to: (i) reduced diameter and increased surface area
of the mineral via cavitation effects, (ii) enhanced heterogeneous reactions
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