degree of purity required by law or by the final use of the water (e.g., drinking water,
irrigation, recreational activities). In addition, some of the contaminants containing,
e.g., aromatic, sulfur, nitrogen, oxygen, double-bond groups, or high molecular
compounds are difficult to be transformed by these technologies. Groundwater and
soils contaminated with organic compounds such as solvents are often difficult to
remediate. The standard technology for groundwater treatment, known as pumpand-treat remediation, is expensive and takes years to complete. Moreover, the
contaminants held in low permeability zones are often difficult to access by standard
pump-and-treat systems, thus prolonging the time required for remediation and
increasing the cost of the remediation. Conventional technologies such as separation
by gravity, centrifugation, coagulation, flotation, adsorption, biological methods,
filtration methods, thermal oxidation, etc., generate byproducts like sludge, syrups,
or salts that need further treatment or that can be expensive and dangerous to handle.
Furthermore, some types of effluents cannot be treated directly by biological technologies because of the presence of high levels of recalcitrant compounds, like
aromatic hydrocarbons.
In those cases, chemical oxidation technologies are efficient processes to treat
systems containing these recalcitrant pollutants. Chemical oxidation has been largely
applied to wastewater treatment by means of oxidants such as hydrogen peroxide,
potassium permanganate, ozone, and combinations of ozone with other oxidants.
These systems have the potential to offer rapid (weeks to months) removal of the
contaminants. Other more powerful processes efficient at treating water polluted
with very resistant contaminants are the so-called advanced oxidation technologies
or processes (AOTs, AOPs). They have yielded very good results in industrialized
countries and are beginning to be employed in developing regions (Legrini et al.
1993; Huang et al. 1993; US EPA 1998; Calgon Carbon Corporation 1996; Bolton
and Cater 1994; Domènech et al. 2004; Litter 2005; Oppenländer 2003; Wang and
Xu 2012; Boczkaj and Fernandes 2017; Tijani et al. 2014; Stefan 2018a).
AOTs concept was initially established by Glaze (1987) and Glaze et al. (1987,
1992), who defined AOTs as processes involving generation and use of powerful
transitory species, principally the hydroxyl radical (HO
•
), able to change in the
structure of chemical species. These species can be generated photochemically
(including the use of solar light) or by other forms of energy, and have enough
potential to oxidize organic matter (OM). The most important oxidant is HO
•
, which
is the most energetic species after fluorine (Table 7.1) and can attack virtually all
organic compounds. HO
• reacts 10
6
–10
12 times more rapidly than alternative oxidants. Some AOTs (heterogeneous photocatalysis, radiolysis, etc.), can also produce
reducing species, allowing the transformation of pollutants that are difficult to be
oxidized, such as some metal ions or halogenated compounds. However, reduction
technologies will not be dealt with in this chapter.
HO
• are able to react by hydrogen abstraction from aliphatic carbon atoms
(Eq. 7.1), electrophilic addition to double bonds or aromatic rings (Eq. 7.2), and
electron transfer reactions (Eq. 7.3). HO
• can also oxidize metals or metalloids such
as As(III). Another active oxygen species like the superoxide radical, O 2
•– and its
conjugate acid form, the hydroperoxyl (also known as perhydroxyl) radical, HO 2
•
,
are also produced in many AOTs, but they are by far less active than HO
• .
120
M. I. Litter
irrigation, recreational activities). In addition, some of the contaminants containing,
e.g., aromatic, sulfur, nitrogen, oxygen, double-bond groups, or high molecular
compounds are difficult to be transformed by these technologies. Groundwater and
soils contaminated with organic compounds such as solvents are often difficult to
remediate. The standard technology for groundwater treatment, known as pumpand-treat remediation, is expensive and takes years to complete. Moreover, the
contaminants held in low permeability zones are often difficult to access by standard
pump-and-treat systems, thus prolonging the time required for remediation and
increasing the cost of the remediation. Conventional technologies such as separation
by gravity, centrifugation, coagulation, flotation, adsorption, biological methods,
filtration methods, thermal oxidation, etc., generate byproducts like sludge, syrups,
or salts that need further treatment or that can be expensive and dangerous to handle.
Furthermore, some types of effluents cannot be treated directly by biological technologies because of the presence of high levels of recalcitrant compounds, like
aromatic hydrocarbons.
In those cases, chemical oxidation technologies are efficient processes to treat
systems containing these recalcitrant pollutants. Chemical oxidation has been largely
applied to wastewater treatment by means of oxidants such as hydrogen peroxide,
potassium permanganate, ozone, and combinations of ozone with other oxidants.
These systems have the potential to offer rapid (weeks to months) removal of the
contaminants. Other more powerful processes efficient at treating water polluted
with very resistant contaminants are the so-called advanced oxidation technologies
or processes (AOTs, AOPs). They have yielded very good results in industrialized
countries and are beginning to be employed in developing regions (Legrini et al.
1993; Huang et al. 1993; US EPA 1998; Calgon Carbon Corporation 1996; Bolton
and Cater 1994; Domènech et al. 2004; Litter 2005; Oppenländer 2003; Wang and
Xu 2012; Boczkaj and Fernandes 2017; Tijani et al. 2014; Stefan 2018a).
AOTs concept was initially established by Glaze (1987) and Glaze et al. (1987,
1992), who defined AOTs as processes involving generation and use of powerful
transitory species, principally the hydroxyl radical (HO
•
), able to change in the
structure of chemical species. These species can be generated photochemically
(including the use of solar light) or by other forms of energy, and have enough
potential to oxidize organic matter (OM). The most important oxidant is HO
•
, which
is the most energetic species after fluorine (Table 7.1) and can attack virtually all
organic compounds. HO
• reacts 10
6
–10
12 times more rapidly than alternative oxidants. Some AOTs (heterogeneous photocatalysis, radiolysis, etc.), can also produce
reducing species, allowing the transformation of pollutants that are difficult to be
oxidized, such as some metal ions or halogenated compounds. However, reduction
technologies will not be dealt with in this chapter.
HO
• are able to react by hydrogen abstraction from aliphatic carbon atoms
(Eq. 7.1), electrophilic addition to double bonds or aromatic rings (Eq. 7.2), and
electron transfer reactions (Eq. 7.3). HO
• can also oxidize metals or metalloids such
as As(III). Another active oxygen species like the superoxide radical, O 2
•– and its
conjugate acid form, the hydroperoxyl (also known as perhydroxyl) radical, HO 2
•
,
are also produced in many AOTs, but they are by far less active than HO
• .
120
M. I. Litter