Environmental Engineering Reference
In-Depth Information
In the heterogeneous phase, the physical steps in addition to chemical
changes take place on the surface of the catalyst at the active sites where
mass tranfer limited adsorption of reactant molecules occurs [75,81]. Also
heterogeneous solid catalysts can support Fenton-like reactions over a
wide range of pH values, because the iron (III) species in solid catalysts is
immobilized within the structure and the catalyst can maintain its ability
to generate hydroxyl radicals from H
2
O
2
. And the catalyst can easily be
recovered after the reaction. A wide range of solid materials, such as transi-
tion metal exchanged zeolites, and iron oxide minerals have been reported
in the literature as heterogeneous catalysts for the oxidative removal of
dyes through Fenton oxidation [69,82].
The efficiency of the treatment can be considerably increased when
ultraviolet light is simultaneously irradiated in the Fenton process, the so-
called photo-Fenton's process [83,84]. Under irradiation, ferric ion com-
plexes produce extra OH radicals and the regeneration of Fe (II), which
will further react with more H
2
O
2
molecules in Fenton reaction.
Some studies performed on Fenton's Reagent oxidation of dye pollut-
ants are illustrated in Table 3.4.
3.2.1.2.1 Factors Influencing the Fenton's Reagent Oxidation Efficiency
The oxidation efficiency of the Fenton process depends on several vari-
ables, namely pH, iron (Fe
2+
) concentration, H
2
O
2
concentration, and tem-
perature for a given wastewater.
pH
Fenton oxidation is known as a highly pH-dependent process since pH
plays an important role in the mechanism of OH production in the
Fenton's reaction [62]. The efficiency of the Fenton reaction system is
mainly a function of pH. It directly affects the generation of hydroxyl radi-
cals and thus the oxidation efficiency. The optimum pH recommended is
between pH = 3 and pH = 6.
For the homogeneous Fenton reaction, the solution pH drastically influ-
ences the efficiency, whereas this effect in heterogeneous Fenton reactions
is generally only moderate. In heterogeneous Fenton systems, the pH can
potentially affect surface adsorption of the organic contaminant, the com-
plex formation of H
2
O
2
with active sites, and changes in iron (II, III) recy-
cling due to the pH-dependent oxidation [85].
A change in pH of the solution involves a variation of the concentration
of Fe
2+
, and therefore the rate of production of OH radicals responsible
for oxidation dyes is restricted [86].
For pH values less than 3, the reaction between hydrogen peroxide and
Fe
2+
is affected, leading to a reduction in hydroxyl radical production, and
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