Perozone
® (or peroxone), where both direct and indirect ozone oxidation of organic
compounds take place (Fischbacher et al. 2018).
As known, H 2 O 2 is a weak acid, a powerful oxidant (Table 7.1), and an unstable
compound, which disproportionates with a maximal rate at the pH of its pK a :
H 2 O 2 ⇆HO 2
À þ H
þ pK a ¼ 11:6
ð7:16Þ
H 2 O 2 þ 2e À þ 2H
þ
! 2H 2 O
ð7:17Þ
H 2 O 2 ! H 2 O þ ½ O 2
ð7:18Þ
H 2 O 2 þ HO 2
À ! H 2 O þ O 2 þ HO À
ð7:19Þ
The redox potential increases with the decrease of pH (Neyens and Baeyens
2003). In alkaline medium, H 2 O 2 will react with HO
– to produce perhydroxyl ions:
H 2 O 2 þ HO À ! HO 2
À þ H 2 O
ð7:20Þ
Therefore, an acidic medium is more favorable to using H 2 O 2 processes for water
treatment. H 2 O 2 has been widely used for removing low levels of pollutants from
wastewaters (chlorine, nitrites, sulfites, hypochlorites, etc.) and it is also used as a
disinfectant (Neyens and Baeyens 2003). However, when low H 2 O 2 concentrations
are used (to avoid increasing costs), reaction rates are low, and this makes its use
ineffective in treating high levels of refractory pollutants, such as highly chlorinated
aromatic compounds and some inorganic compounds (e.g., cyanides).
O 3 decomposition is initiated by H 2 O 2 through an electron-transfer reaction
producing HO
• (Eq. 7.21) (Huang et al. 1993); alternatively, this reaction can be
interpreted as the activation of H 2 O 2 by ozone (Gurol and Akata 1996):
O 3 þ H 2 O 2 ! HO
•
þ O 2 þ HO 2
•
ð7:21Þ
The major effects of combining O 3 and H 2 O 2 result from the increase of the
oxidation efficiency by conversion of O 3 to HO
• and the improvement of O 3 transfer
from the gaseous to the liquid phase (Boczkaj and Fernandes 2017; Roche and
Prados 1995). In basic medium, H 2 O 2 will generate HO 2
– , which can promote the
decomposition of O 3 to HO
• with higher effectiveness.
The process can treat organic pollutants at very low concentrations (μg/L), at an
optimal O 3 /H 2 O 2 molar ratio ffi 2:1.
Although the process is expensive, it is fast and effective in decomposing
organochlorinated compounds (trichloroethylene (TCE), tetrachloroethylene, etc.).
It is very convenient in the posttreatment of water submitted to disinfection treatments using chlorine or chlorine dioxide because it can decompose potential
byproducts such as THM or related compounds. This approach can be applied to
degradation of pesticides (Beltrán et al. 1994a, b).
7 Introduction to Oxidative Technologies for Water Treatment
127
® (or peroxone), where both direct and indirect ozone oxidation of organic
compounds take place (Fischbacher et al. 2018).
As known, H 2 O 2 is a weak acid, a powerful oxidant (Table 7.1), and an unstable
compound, which disproportionates with a maximal rate at the pH of its pK a :
H 2 O 2 ⇆HO 2
À þ H
þ pK a ¼ 11:6
ð7:16Þ
H 2 O 2 þ 2e À þ 2H
þ
! 2H 2 O
ð7:17Þ
H 2 O 2 ! H 2 O þ ½ O 2
ð7:18Þ
H 2 O 2 þ HO 2
À ! H 2 O þ O 2 þ HO À
ð7:19Þ
The redox potential increases with the decrease of pH (Neyens and Baeyens
2003). In alkaline medium, H 2 O 2 will react with HO
– to produce perhydroxyl ions:
H 2 O 2 þ HO À ! HO 2
À þ H 2 O
ð7:20Þ
Therefore, an acidic medium is more favorable to using H 2 O 2 processes for water
treatment. H 2 O 2 has been widely used for removing low levels of pollutants from
wastewaters (chlorine, nitrites, sulfites, hypochlorites, etc.) and it is also used as a
disinfectant (Neyens and Baeyens 2003). However, when low H 2 O 2 concentrations
are used (to avoid increasing costs), reaction rates are low, and this makes its use
ineffective in treating high levels of refractory pollutants, such as highly chlorinated
aromatic compounds and some inorganic compounds (e.g., cyanides).
O 3 decomposition is initiated by H 2 O 2 through an electron-transfer reaction
producing HO
• (Eq. 7.21) (Huang et al. 1993); alternatively, this reaction can be
interpreted as the activation of H 2 O 2 by ozone (Gurol and Akata 1996):
O 3 þ H 2 O 2 ! HO
•
þ O 2 þ HO 2
•
ð7:21Þ
The major effects of combining O 3 and H 2 O 2 result from the increase of the
oxidation efficiency by conversion of O 3 to HO
• and the improvement of O 3 transfer
from the gaseous to the liquid phase (Boczkaj and Fernandes 2017; Roche and
Prados 1995). In basic medium, H 2 O 2 will generate HO 2
– , which can promote the
decomposition of O 3 to HO
• with higher effectiveness.
The process can treat organic pollutants at very low concentrations (μg/L), at an
optimal O 3 /H 2 O 2 molar ratio ffi 2:1.
Although the process is expensive, it is fast and effective in decomposing
organochlorinated compounds (trichloroethylene (TCE), tetrachloroethylene, etc.).
It is very convenient in the posttreatment of water submitted to disinfection treatments using chlorine or chlorine dioxide because it can decompose potential
byproducts such as THM or related compounds. This approach can be applied to
degradation of pesticides (Beltrán et al. 1994a, b).
7 Introduction to Oxidative Technologies for Water Treatment
127