order to achieve the degradation target and it can be applied to any other AOP that
uses UV irradiation, but it does not give explanations about the mechanism or details
of the process. It is a more fundamental concept than the previous parameters and, if
properly reported, it could be used to compare different systems based on reports and
papers from different years and laboratories, regardless of the energy costs in a given
year or the different experimental setups used.
7.3.4 Photoinduced Ozonation (UV/O 3 )
UV irradiation of O 3 in water produces H 2 O 2 quantitatively:
O 3 þ hν þ H 2 O ! H 2 O 2 þ O 2
ð7:104Þ
The generated H 2 O 2 is photolyzed (Eq. 7.97), generating HO
• radicals, and also
reacts with the excess of O 3 , according to Eq. 7.21. This method might be considered
in principle only as an expensive form of generating H 2 O 2 and then HO
•
. Indeed, the
process is a combination of UV/H 2 O 2 and O 3 /H 2 O 2 , but the advantage is that O 3 has
a higher absorption coefficient than H 2 O 2 (ε 254 ¼ 3,300 1/(M cm)), and such method
can be used to treat water with high UV absorption background. The efficiency is
higher than that of O 3 or direct UV, and the reactor does not need to be in quartz
because UVB light (280–315 nm) can be used. If λ < 310 nm is used, photolysis of
O 3 takes place, generating additional HO
• and other oxidants, with the subsequent
increase of the efficiency (Peyton and Glaze 1988):
O 3 þ hν ! O 2
1
Δg
À
Á þ O
1 D
À Á
ð7:105Þ
O
1 D
À Á þ H 2 O ! 2HO
•
ð7:106Þ
Generally, an increase of the O 3 concentration increases the degradation rate of
the pollutant, as demonstrated for the case of atrazine. Although direct ozonation can
contribute, 87% of the oxidation process proceeds, in this case, through the radical
pathway (Beltrán et al. 1994a).
In contrast with the results in the absence of light, alkaline pH reduces the reaction
rate, as it has been observed in the case of 2,6-DNT degradation. The decrease on the
rate is due to the dissociation of HO
• in the less active oxygen anion radical (Eq. 7.107)
and to the lower solubility of O 3 at high pH (Beltrán et al. 1998).
HO
•
! O
• À
þ H
þ
ð7:107Þ
Although ozonation is improved under UV light, it was found that the use of high
initial concentrations of O 3 (1000 mg/L) (without irradiation) was more effective
than the UV/O 3 combination to treat formulated pesticides like atrazine, alachlor,
carbofuran, etc., because of the presence of large amounts of HO
• scavengers in the
formulations (Chiron et al. 2000).
7 Introduction to Oxidative Technologies for Water Treatment
151
uses UV irradiation, but it does not give explanations about the mechanism or details
of the process. It is a more fundamental concept than the previous parameters and, if
properly reported, it could be used to compare different systems based on reports and
papers from different years and laboratories, regardless of the energy costs in a given
year or the different experimental setups used.
7.3.4 Photoinduced Ozonation (UV/O 3 )
UV irradiation of O 3 in water produces H 2 O 2 quantitatively:
O 3 þ hν þ H 2 O ! H 2 O 2 þ O 2
ð7:104Þ
The generated H 2 O 2 is photolyzed (Eq. 7.97), generating HO
• radicals, and also
reacts with the excess of O 3 , according to Eq. 7.21. This method might be considered
in principle only as an expensive form of generating H 2 O 2 and then HO
•
. Indeed, the
process is a combination of UV/H 2 O 2 and O 3 /H 2 O 2 , but the advantage is that O 3 has
a higher absorption coefficient than H 2 O 2 (ε 254 ¼ 3,300 1/(M cm)), and such method
can be used to treat water with high UV absorption background. The efficiency is
higher than that of O 3 or direct UV, and the reactor does not need to be in quartz
because UVB light (280–315 nm) can be used. If λ < 310 nm is used, photolysis of
O 3 takes place, generating additional HO
• and other oxidants, with the subsequent
increase of the efficiency (Peyton and Glaze 1988):
O 3 þ hν ! O 2
1
Δg
À
Á þ O
1 D
À Á
ð7:105Þ
O
1 D
À Á þ H 2 O ! 2HO
•
ð7:106Þ
Generally, an increase of the O 3 concentration increases the degradation rate of
the pollutant, as demonstrated for the case of atrazine. Although direct ozonation can
contribute, 87% of the oxidation process proceeds, in this case, through the radical
pathway (Beltrán et al. 1994a).
In contrast with the results in the absence of light, alkaline pH reduces the reaction
rate, as it has been observed in the case of 2,6-DNT degradation. The decrease on the
rate is due to the dissociation of HO
• in the less active oxygen anion radical (Eq. 7.107)
and to the lower solubility of O 3 at high pH (Beltrán et al. 1998).
HO
•
! O
• À
þ H
þ
ð7:107Þ
Although ozonation is improved under UV light, it was found that the use of high
initial concentrations of O 3 (1000 mg/L) (without irradiation) was more effective
than the UV/O 3 combination to treat formulated pesticides like atrazine, alachlor,
carbofuran, etc., because of the presence of large amounts of HO
• scavengers in the
formulations (Chiron et al. 2000).
7 Introduction to Oxidative Technologies for Water Treatment
151