Environmental Engineering Reference
In-Depth Information
For example, the efficiency of sono-Fenton process for the decolorization
and degradation of dyestuff at short frequency ultrasound was found to
increase sharply with increasing solution temperature (T) and power den-
sity (
P
) up to some threshold (T = 40 C, P = 50 WL
-1
pH = 3.0), above
which it declined [84]. The result was attributed to enhanced solubility and
mass transfer of ferrous ions at the given pH, and to the excess of energy
that provided additional Fe-O
2
H
2+
and HOO radicals. The deceleration of
the reactions at more extreme conditions of T and
P
is the result of an
increase of the equilibrium vapor pressure, which led to a higher vapor
concentration in the bubbles or a lower collapse temperature. A simplified
reaction scheme that describes the sono-Fenton process in the presence of
a dye molecule is outlined in Equations 7.13-7.16.
Fe
2+
+ H
2
O
2
+ H
+
Fe
3+
+ HO + H
2
O
(7.22)
Fe
3+
+ H
2
O
2
Fe-O
2
H
2+
+ H
+
(7.23)
Fe-O
2
H
2+
+ ))) Fe
2+
+ HOO (7.24)
Dye + HO [Dye-OH adduct]* Oxidized dye + CO
2
+ H
2
O (7.25)
A costly means of enhancing the efficiency of sono-Fenton reaction is
the immersion of a light source into the reactor (sonophoto-Fenton) to
provide excess OH radicals via photolytic decomposition of Fe-OOH com-
plex and H
2
O
2
:
[Fe (OOH)
2+
] + hυ Fe
2+
+ HO (7.26)
H
2
O
2
+ hυ 2HO (7.27)
As such, the degradation of C.I. Reactive Violet 2 was found to be
10-times faster than that by the classical photo-Fenton reaction under
equivalent conditions, and the addition of a surfactant rendered an initial
rapid mineralization via the reductive cleavage of the surfactant to provide
excess radical species [64].
7.3.4 Sonocatalysis
The basic principle of sonocatalysis is diffusion and sorption of soluble sol-
utes onto a solid surface or its interface followed by a sequence of hetero-
geneous chemical reactions on the active sites. It is important at this point
to distinguish three kinds of solid media that are of value in sonocatalytic
processes: (i) those which originally have no reactive sites, but are partially
covered with them upon the formation and collapse of cavity bubbles on
their surface; (ii) those which originally contain reactive metals on their
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