Environmental Engineering Reference
In-Depth Information
Table 13.3.1
Main chemical reactions involved in the dark Fenton and
photo-Fenton processes; k values in every case represents
rate constant.
(1) Fe
2
+
+
O
2
→
O
•
2
+
Fe
3
+
=
1.15M
−
1
s
−
1
k
(2) H
2
O
2
↔
HO
2
+
H
+
=
1.26
×
10
−
2
M
−
1
s
−
1
k
(3) Fe
2
+
+
H
2
O
2
←
Fe
3
+
+
HO
•
+
HO
−
=
55M
−
1
s
−
1
k
(4) Fe(OH)
2
+
+
hv
→
Fe
2
+
+
HO
•
=
2
×
10
−
3
M
−
1
s
−
1
k
(5) H
2
O
2
+
hv
→
2HO
•
φ
HO
·
=
0, 98, 254 nm
(6) HO
•
+
H
2
O
2
→
HO
2
+
H
2
O
=
2.7
×
10
7
M
−
1
s
−
1
k
(7) Fe
2
+
+
HO
•
→
Fe
3
+
+
HO
−
=
2.7
×
10
7
M
−
1
s
−
1
k
past include V-trough and L-shaped collectors, as shown in Figure 13.2.1. The perfor-
mance of other non-imaging geometries have been found quite close to those found for
CPCs, due mainly to a more uniform photon distribution in the photorreactor rather
than radiation intensity reaching the receiver (Brito et al., 2012).
13.3 SOLAR HOMOGENOUS PHOTOCATALYSIS
The Fenton process is one of the most widely used AOPs for water and wastewa-
ter treatment. Fenton's reaction uses hydrogen peroxide and ferrous salt to generate
hydroxyl radicals (HO), chemical species possessing inherent properties that enable
them to mineralize dissolved organic pollutants into CO
2
, water, and mineral acids
(Bandala et al., 2007). When this process is driven by ultraviolet (UV) radiation, visi-
ble light, or both it is known as the photo-Fenton process. The photo-Fenton process
possesses several advantages over the dark Fenton reaction, mainly an increased reac-
tion rate and the possibility of using a cheap, clean, and widely distributed energy
source: solar radiation. Table 13.3.1 depicts the main chemical reactions involved in
both dark Fenton and photo-Fenton processes along with reaction rate constants for
each reaction.
In agreement with the information in Table 13.3.1, it is worthy to note Equations
13.3.3 and 13.3.4 since they are the main processes involved in the solar photo-Fenton
reaction. Equation 13.3.3 shows the actual decomposition of hydrogen peroxide
catalyzed by ferrous salt. Hydrogen peroxide decomposition catalyzed by Fe
2
+
gen-
erates one hydroxyl ion and one hydroxyl radical. Hydroxyl radicals are the most
important specie generated during the reaction since it may react with organic mat-
ter to oxidize it. In the case of dark Fenton reactions, ferrous salt may be considered
the limiting reagent anytime the reaction described in Equation 13.3.3 stops once all
the available ferrous iron has been oxidized to ferric iron independent of the amount
of hydrogen peroxide added to the reaction. The advantage of including radiative
energy in the process is shown in Equation 13.3.4. In the case of photo-assisted
Fenton reactions, radiation of a certain wavelength (usually UV and part of the visi-
ble radiation) may generate the so-called photo-reduction of Fe
3
+
to Fe
2
+
depicted in
Equation 13.3.4 plus one mole of hydroxyl radicals. Photo-reduced iron can partici-
pate again in the decomposition of hydrogen peroxide as in Equation 13.3.3, and the
process may keep occurring if enough hydrogen peroxide is provided to the reaction
mixture. As stated earlier, the useful wavelength range for carrying out the catalytic
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