However, this competence takes place in all AOTs involving HO
• in carbonated
solutions. Changes due to mineralization processes affect reaction rates and attention
should be paid to this in almost all AOTs (Gogate and Pandit 2004a).
In UV/H 2 O 2 processes, the rate of degradation of compounds depends on the
concentration of H 2 O 2 . Generally, the rate increases with H 2 O 2 concentration up to
an optimum value; at higher concentrations, an inhibitory effect has been found
(Lopez et al. 2000; López Cisneros et al. 2002). When HO
• concentrations are high,
recombination of these radicals is a competitive reaction, which regenerates H 2 O 2
(reverse of reaction 7.97); other competitive reactions occur according to reactions in
Eqs. 7.100, 7.101, 7.102 and 7.103, from which reactions 7.100 and 7.103 consume
HO
• and inhibit oxidation, while HO 2
• radicals are much less reactive than HO
•
(Baxendale and Wilson 1957).
HO
•
þ H 2 O 2 ! HO 2
•
þ H 2 O
ð7:100Þ
HO 2
•
þ H 2 O 2 ! HO
•
þ H 2 O þ O 2
ð7:101Þ
2HO 2
•
! H 2 O 2 þ O 2
ð7:102Þ
HO 2
•
þ HO
•
! H 2 O þ O 2
ð7:103Þ
Figure 7.4 shows a scheme for the reactions occurring in UV/H 2 O 2 processes
(Legrini et al. 1993).
In order to avoid this drawback, the optimal H 2 O 2 concentration must be determined through treatability tests, as an excess can retard the degradation; this depends
on the concentration and chemical nature of the pollutants.
When low-pressure mercury vapor lamps are used, a high H 2 O 2 concentration has
to be added to produce enough HO
•
, and the process is less effective. This limitation
can be overcome using high-intensity UV lamps.
The advantages of the UV/H 2 O 2 process are the following: H 2 O 2 is a commercially accessible oxidant, it is thermally stable, and can be easily stored on site, It
presents an infinite solubility in water, and produces 2 HO
• per each H 2 O 2 molecule,
i.e., it is an effective source of HO
•
. In contrast with processes using ozone, there are
no mass transfer problems because the UV/H 2 O 2 system is homogeneous, the capital
investment is low and the operation is simple. However, as the cross-section
absorption at 254 nm of H 2 O 2 is low, high concentrations of the reagent should be
used, with a continuous monitoring of the concentration during the process in order
to avoid its depletion. The method is not efficient for waters exhibiting high
absorbance at λ < 300 nm or containing compounds that can compete with the
generation of HO
•
, requiring a high amount of H 2 O 2 in those cases.
UV/H 2 O 2 is one of the oldest AOPs. It is useful for the treatment of industrial
effluents, especially from dye industry (López Cisneros et al. 2002; Ince et al. 1997).
Organochlorinated aliphatics, aromatics, phenols and chlorinated phenols, pesticides, and pharmaceuticals are among the compounds that can be degraded (Legrini
et al. 1993; Gogate and Pandit 2004a; Lopez et al. 2003).
7 Introduction to Oxidative Technologies for Water Treatment
149
• in carbonated
solutions. Changes due to mineralization processes affect reaction rates and attention
should be paid to this in almost all AOTs (Gogate and Pandit 2004a).
In UV/H 2 O 2 processes, the rate of degradation of compounds depends on the
concentration of H 2 O 2 . Generally, the rate increases with H 2 O 2 concentration up to
an optimum value; at higher concentrations, an inhibitory effect has been found
(Lopez et al. 2000; López Cisneros et al. 2002). When HO
• concentrations are high,
recombination of these radicals is a competitive reaction, which regenerates H 2 O 2
(reverse of reaction 7.97); other competitive reactions occur according to reactions in
Eqs. 7.100, 7.101, 7.102 and 7.103, from which reactions 7.100 and 7.103 consume
HO
• and inhibit oxidation, while HO 2
• radicals are much less reactive than HO
•
(Baxendale and Wilson 1957).
HO
•
þ H 2 O 2 ! HO 2
•
þ H 2 O
ð7:100Þ
HO 2
•
þ H 2 O 2 ! HO
•
þ H 2 O þ O 2
ð7:101Þ
2HO 2
•
! H 2 O 2 þ O 2
ð7:102Þ
HO 2
•
þ HO
•
! H 2 O þ O 2
ð7:103Þ
Figure 7.4 shows a scheme for the reactions occurring in UV/H 2 O 2 processes
(Legrini et al. 1993).
In order to avoid this drawback, the optimal H 2 O 2 concentration must be determined through treatability tests, as an excess can retard the degradation; this depends
on the concentration and chemical nature of the pollutants.
When low-pressure mercury vapor lamps are used, a high H 2 O 2 concentration has
to be added to produce enough HO
•
, and the process is less effective. This limitation
can be overcome using high-intensity UV lamps.
The advantages of the UV/H 2 O 2 process are the following: H 2 O 2 is a commercially accessible oxidant, it is thermally stable, and can be easily stored on site, It
presents an infinite solubility in water, and produces 2 HO
• per each H 2 O 2 molecule,
i.e., it is an effective source of HO
•
. In contrast with processes using ozone, there are
no mass transfer problems because the UV/H 2 O 2 system is homogeneous, the capital
investment is low and the operation is simple. However, as the cross-section
absorption at 254 nm of H 2 O 2 is low, high concentrations of the reagent should be
used, with a continuous monitoring of the concentration during the process in order
to avoid its depletion. The method is not efficient for waters exhibiting high
absorbance at λ < 300 nm or containing compounds that can compete with the
generation of HO
•
, requiring a high amount of H 2 O 2 in those cases.
UV/H 2 O 2 is one of the oldest AOPs. It is useful for the treatment of industrial
effluents, especially from dye industry (López Cisneros et al. 2002; Ince et al. 1997).
Organochlorinated aliphatics, aromatics, phenols and chlorinated phenols, pesticides, and pharmaceuticals are among the compounds that can be degraded (Legrini
et al. 1993; Gogate and Pandit 2004a; Lopez et al. 2003).
7 Introduction to Oxidative Technologies for Water Treatment
149