Environmental Engineering Reference
In-Depth Information
option for such biologically persistent wastewater is the use of the advanced technolo-
gies based on chemical oxidation called advanced oxidation processes (AOPs), which
are widely recognized as highly efficient treatments for recalcitrant wastewater (Pera-
Titus et al., 2004). These methods rely on the formation of highly reactive chemical
species which degrade even the most recalcitrant molecules into biodegradable com-
pounds. Although reacting systems vary (Comninellis et al., 2008), all of them are
characterized by the production of hydroxyl radicals (OH), which are able to oxidize
and mineralize almost any organic molecule, yielding CO
2
and inorganic ions. They
are also non-selective, which is a useful attribute for wastewater treatment and solution
of pollution problems. The versatility of the AOPs is also enhanced by the fact that
hydroxyl radicals may be produced in different ways, facilitating compliance with the
specific treatment requirements. Methods based on UV, H
2
O
2
/UV, O
3
/UV and other
combinations use photolysis of H
2
O
2
and ozone to produce the hydroxyl radicals, but
generation of UV radiation by lamps and ozone production are expensive. So future
applications of these processes could be improved through the use of catalysis and
solar energy. Therefore, research is focusing more and more on those AOPs which can
be driven by solar irradiation. Of special interest is photo-Fenton, which is based on
addition of H
2
O
2
to dissolved iron salts and irradiation with UV-VIS light, because
sunlight can be used for it (Pignatello et al., 2006).
12.2 SOLAR PHOTO-FENTON
Fenton and Fenton-like processes are probably among the advanced oxidation pro-
cesses most applied in the treatment of industrial wastewater (Suty et al., 2004). The
first proposals for wastewater treatment applications were reported in the 1960s.
Yet it was not until the early 1990s that the first studies on the application of the
photo-Fenton process for the treatment of wastewater were published by the groups
of Pignatello, Lipcznska-Kochany, Kiwi, Pulgarín and Bauer (Pignatello et al., 2006).
Much of the literature on photo-Fenton includes the possibility of driving the pro-
cess with solar radiation because it seems to be the most suitable of all AOPs for
being driven by sunlight, because soluble iron-hydroxyl and especially iron-organic
acid complexes even absorb part of the visible light spectrum (Figure 12.2.1), not only
ultraviolet radiation (Malato et al., 2009).
Hydrogen peroxide is decomposed to water and oxygen in the presence of iron
ions in the Fenton reaction in aqueous solutions, Equation 12.2.1, as first reported
by H.J.H. Fenton (Fenton, 1894). Mixtures of ferrous iron and hydrogen peroxide
are called Fenton reagents. Equations 12.2.1-12.2.3 show the basic reactions in the
absence of other interfering ions and organic substances. Regeneration of ferrous iron
from ferric iron by Equations 12.2.2 and 12.2.3 is the rate limiting step in the catalytic
iron cycle, if iron is added in small amounts.
Fe
2
+
+
Fe
3
+
+
OH
−
+
OH
•
H
2
O
2
→
(12.2.1)
Fe
3
+
+
HO
2
→
Fe
2
+
+
H
+
O
2
+
(12.2.2)
Fe
3
+
+
O
•
2
Fe
2
+
+
→
O
2
(12.2.3)
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