water-soluble gases. Oxygen, which is actually a bi-radical, is often employed as a
cheap electron-pair acceptor for oxidation of undesirable inorganic water components like Fe
2+ or Mn
2+ ions. However, molecular oxygen oxidative capabilities
towards organic pollutants are very limited. This feature is called spin restriction of
oxygen. For the reaction with molecular oxygen, both electrons need to be of the
same spin, which is the condition biomolecules or organic pollutants rarely satisfy
(Halliwell and Gutteridge 1984). Dissolved ClO 2 has been researched for a growing
number of water treatment applications (Aieta and Berg 1986; Huber et al. 2005),
which will be further discussed in Sect. 9.2.
Some oxidants such as H 2 O 2 , O 3 , or S 2 O 8
2À are precursors of a number of other
free radicals. The umbrella term reactive oxygen species (ROS) also encompasses
non-radical yet reactive oxygen moieties, e.g., singlet states of oxygen (Halliwell
2006). The formation of these short-living radical species is driven by specific
reaction conditions among which the temperature and the pH play the key role by
influencing the kinetics of each of the simultaneously running reactions. Another
important parameter is the reactor exposure to light, especially to the light of shorter
wavelengths (UV) (see also Chap. 10, which focuses on UV water treatment). UV
light mechanism of action is multimodal. Besides direct organic compound photolysis, ROS are generated under oxygen saturated or oversaturated water conditions.
In addition, UV is often used to initiate ROS production in advanced oxidation
processes (AOPs) for drinking water supply since UV can provide the right portion
of energy for symmetrical bond cleavage. Hydroxyl (
•
OH) and sulfate radicals
(SO 4
•–
) are species that are directly responsible for transformation of organic
pollutants and for water disinfection. This chapter focuses on describing the conditions for radical species generation, on radical reactivity with the target pollutants
and radical scavenging by other water components, and on disinfection by-products
(DBP). Some of the processes tend to be more employed in drinking water supply
(chlorine and ozone), other find its application first in wastewater treatment and
groundwater remediation (Fenton reagent and peroxydisulfate).
9.2 Chlorine Species
Chlorine species are traditionally used as drinking water disinfectants, which make
water supply safe from microbiological point of view. Presence of organic compounds is generally unfavorable in the treated water (Ramseier 2010), as will be
explained further. Therefore, chlorination is not a suitable technology for pollutant
degradation.
Most usually, chlorine gas (Cl 2 ) or sodium hypochlorite (NaOCl) is dosed. In the
USA, monochloramine (NH 2 Cl) is still in use as residual disinfectant for water
distribution systems. High natural organic matter (NOM) and bromide (Br
À
) content
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P. Hrabák and S. Wacławek
cheap electron-pair acceptor for oxidation of undesirable inorganic water components like Fe
2+ or Mn
2+ ions. However, molecular oxygen oxidative capabilities
towards organic pollutants are very limited. This feature is called spin restriction of
oxygen. For the reaction with molecular oxygen, both electrons need to be of the
same spin, which is the condition biomolecules or organic pollutants rarely satisfy
(Halliwell and Gutteridge 1984). Dissolved ClO 2 has been researched for a growing
number of water treatment applications (Aieta and Berg 1986; Huber et al. 2005),
which will be further discussed in Sect. 9.2.
Some oxidants such as H 2 O 2 , O 3 , or S 2 O 8
2À are precursors of a number of other
free radicals. The umbrella term reactive oxygen species (ROS) also encompasses
non-radical yet reactive oxygen moieties, e.g., singlet states of oxygen (Halliwell
2006). The formation of these short-living radical species is driven by specific
reaction conditions among which the temperature and the pH play the key role by
influencing the kinetics of each of the simultaneously running reactions. Another
important parameter is the reactor exposure to light, especially to the light of shorter
wavelengths (UV) (see also Chap. 10, which focuses on UV water treatment). UV
light mechanism of action is multimodal. Besides direct organic compound photolysis, ROS are generated under oxygen saturated or oversaturated water conditions.
In addition, UV is often used to initiate ROS production in advanced oxidation
processes (AOPs) for drinking water supply since UV can provide the right portion
of energy for symmetrical bond cleavage. Hydroxyl (
•
OH) and sulfate radicals
(SO 4
•–
) are species that are directly responsible for transformation of organic
pollutants and for water disinfection. This chapter focuses on describing the conditions for radical species generation, on radical reactivity with the target pollutants
and radical scavenging by other water components, and on disinfection by-products
(DBP). Some of the processes tend to be more employed in drinking water supply
(chlorine and ozone), other find its application first in wastewater treatment and
groundwater remediation (Fenton reagent and peroxydisulfate).
9.2 Chlorine Species
Chlorine species are traditionally used as drinking water disinfectants, which make
water supply safe from microbiological point of view. Presence of organic compounds is generally unfavorable in the treated water (Ramseier 2010), as will be
explained further. Therefore, chlorination is not a suitable technology for pollutant
degradation.
Most usually, chlorine gas (Cl 2 ) or sodium hypochlorite (NaOCl) is dosed. In the
USA, monochloramine (NH 2 Cl) is still in use as residual disinfectant for water
distribution systems. High natural organic matter (NOM) and bromide (Br
À
) content
204
P. Hrabák and S. Wacławek