Environmental Engineering Reference
In-Depth Information
or another species of similar reactivity such as sulfate radical anion ( SO - 4 ),
which can destroy even the recalcitrant pollutants [14].
The oxidation potential of some important oxidizing agents are sum-
marized in Table 3.1 [15].
The hydroxyl radical has the second highest oxidizing potential and is
the most preferred choice for use in oxidation processes. The hydroxyl rad-
icals are highly reactive species that initiate a sequence of reactions result-
ing in destruction and removal of organic pollutants.
In general, AOPs involve two stages of oxidation:
Stage 1: the formation of strong oxidants (mainly hydroxyl
radicals)
Stage 2: the reaction of these oxidants with organic contami-
nants in water
The AOPs have common principles in terms of the participation of
hydroxyl radicals that are assumed to be operative during the reaction.
Due to the instability of OH radical, it has to be generated continuously
in situ through chemical or photochemical reactions [16].
The technologies that can be classified as AOPs fall under two general
categories in terms of whether UV source is used in the process. Table 3.2
shows the classification of AOPs [17].
In recent years, the research found in literature has shown that toxic and
refractory organics in wastewater could be destroyed by most AOPs, such
as ozonation, photocatalytic degradation, Fenton's reagent (H 2 O 2 /Fe 2+ ),
photo-Fenton, and wet air oxidation, and have proven to be very effective
for dye-containing effluents [18-23]. Various combinations of these AOPs
may also provide efficient removal of dyes from aqueous media.
Of the many AOPs developed, four have garnered the most study and
use in scientific research: ozonation, Fenton's reagent oxidation, catalytic
wet air oxidation and photocatalytic oxidation. Consequently, this chapter
Table 3.1 Relative oxidation power of some oxidizing species.
Name of oxidant
Oxidation potential (eV)
Fluorine
3.06
Hydroxyl radical
2.80
Sulfate
2.60
Ozone
2.07
Hydrogen peroxide
1.77
Chlorine
1.36
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