268
the ferric iron complexes. The inner filter effects refer to the competitive adsorption of photons by other light-absorbing species in the water.
Even if the photo-Fenton has higher photoactivity than the heterogeneous photocatalysis, its feasible operation is largely dependent on several water quality parameters. In the photo-Fenton reaction, the formation of the highly photoactive iron
complexes is highly dependent on the water pH and ion content [75]. It was reported
that pH 2.8 was the frequent optimum pH for photo-Fenton reaction [42, 85]. This
is owing to the fact that at such low pH 2.8, the precipitation does not take place and
further promotes the presence of dominant iron species of [Fe(OH)]
2+
in water. Such
a low optimum pH 2.8, however, is not cost effective for operation as it requires high
chemical costs for pH rectification. The presence of different ions such as carbonate
(CO 3
2−
), phosphate (PO 4
3−
), sulfate (SO 4
2−
), and chlorine (Cl
−
) also affects the iron
equilibrium in water. These ions have the potential to raise the water pH and effectively lower the photo-Fenton reaction rate. Both CO 3
2−
and PO 4
3−
have a double
detrimental effect on the reaction, as they precipitate the iron as well as scavenge the
OH
−
radicals. A higher pH of 4.0–5.0 was determined to be sufficient to sustain the
photo-Fenton reaction with 2–6 mM of iron for the initiation of the treatment [107].
To date, the maximal iron loading reported was 450 mg/L [249, 307].
Although H 2 O 2 may be generated via the TiO 2 photocatalysis (Eq. 13.11), its
relative amount in the system may be inadequate to drive the Fenton reaction. Many
researchers have reported the addition of H 2 O 2 in enhancing both the photo-Fenton
and TiO 2 photocatalysis reactions. The H 2 O 2 can inhibit the recombination of electron/hole pair while further providing additional OH
−
radicals through the following
mechanisms:
H O e
HO HO
2 2 + →
+
−
⋅
−
(13.16)
O H O
O HO HO
2
2 2
2
⋅−
⋅
−
+
→
+
+
(13.17)
This combined TiO 2 photocatalysis and photo-dark-Fenton reaction is particularly useful for the disinfection process [81, 210, 211]. The addition of H 2 O 2 to the
photocatalysis and dark-Fenton system results in a residual disinfection to avoid
microbial regrowth. Rincón and Pulgarin [277] performed trials with TiO 2 photocatalysis and photo-Fenton reaction for the disinfection of water contaminated with
Escherichia coli. They found that the bacterial inactivation rate was higher than the
photocatalysis alone and the decrease in bacterial number continued in dark conditions without significant regrowth within the following 60 h. However, it was found
that such residual disinfection effect was highly dependent on the light intensity
used during the irradiation period, as well as the relative concentrations of Fe
3+
and
H 2 O 2 . Further addition of H 2 O 2 was found to decrease the overall reaction rate in
several studies, owing to the formation of less penetrative HO 2
⋅
radicals, as
described by Eq. (13.18):
HO H O
HO H O
⋅
⋅
+
→
+
2 2
2
2
(13.18)
13 Wastewater
the ferric iron complexes. The inner filter effects refer to the competitive adsorption of photons by other light-absorbing species in the water.
Even if the photo-Fenton has higher photoactivity than the heterogeneous photocatalysis, its feasible operation is largely dependent on several water quality parameters. In the photo-Fenton reaction, the formation of the highly photoactive iron
complexes is highly dependent on the water pH and ion content [75]. It was reported
that pH 2.8 was the frequent optimum pH for photo-Fenton reaction [42, 85]. This
is owing to the fact that at such low pH 2.8, the precipitation does not take place and
further promotes the presence of dominant iron species of [Fe(OH)]
2+
in water. Such
a low optimum pH 2.8, however, is not cost effective for operation as it requires high
chemical costs for pH rectification. The presence of different ions such as carbonate
(CO 3
2−
), phosphate (PO 4
3−
), sulfate (SO 4
2−
), and chlorine (Cl
−
) also affects the iron
equilibrium in water. These ions have the potential to raise the water pH and effectively lower the photo-Fenton reaction rate. Both CO 3
2−
and PO 4
3−
have a double
detrimental effect on the reaction, as they precipitate the iron as well as scavenge the
OH
−
radicals. A higher pH of 4.0–5.0 was determined to be sufficient to sustain the
photo-Fenton reaction with 2–6 mM of iron for the initiation of the treatment [107].
To date, the maximal iron loading reported was 450 mg/L [249, 307].
Although H 2 O 2 may be generated via the TiO 2 photocatalysis (Eq. 13.11), its
relative amount in the system may be inadequate to drive the Fenton reaction. Many
researchers have reported the addition of H 2 O 2 in enhancing both the photo-Fenton
and TiO 2 photocatalysis reactions. The H 2 O 2 can inhibit the recombination of electron/hole pair while further providing additional OH
−
radicals through the following
mechanisms:
H O e
HO HO
2 2 + →
+
−
⋅
−
(13.16)
O H O
O HO HO
2
2 2
2
⋅−
⋅
−
+
→
+
+
(13.17)
This combined TiO 2 photocatalysis and photo-dark-Fenton reaction is particularly useful for the disinfection process [81, 210, 211]. The addition of H 2 O 2 to the
photocatalysis and dark-Fenton system results in a residual disinfection to avoid
microbial regrowth. Rincón and Pulgarin [277] performed trials with TiO 2 photocatalysis and photo-Fenton reaction for the disinfection of water contaminated with
Escherichia coli. They found that the bacterial inactivation rate was higher than the
photocatalysis alone and the decrease in bacterial number continued in dark conditions without significant regrowth within the following 60 h. However, it was found
that such residual disinfection effect was highly dependent on the light intensity
used during the irradiation period, as well as the relative concentrations of Fe
3+
and
H 2 O 2 . Further addition of H 2 O 2 was found to decrease the overall reaction rate in
several studies, owing to the formation of less penetrative HO 2
⋅
radicals, as
described by Eq. (13.18):
HO H O
HO H O
⋅
⋅
+
→
+
2 2
2
2
(13.18)
13 Wastewater