maxima at λ ¼ 248 nm and at λ ¼ 254 nm, and after irradiation at λ ¼ 254 nm, HO
•
and SO 4
•– are generated through the homolytic cleavage of the peroxo bond (Karpel
Vel Leitner 2018; Brienza and Katsoyiannis 2017). Quantum yield of SO 4
•– formation is much larger than that of HO
• formation from H 2 O 2 (Karpel Vel Leitner 2018).
SO 4
•– is transformed into HO
• in strong alkaline media. PMS is activated to produce
both SO 4
•– and HO
• radicals, when the peroxide bond (–O–O–) is homolytically
cleaved, as reported in the following equations (Brienza and Katsoyiannis 2017):
HSO 5
À þ hν ! SO 4
• À þ HO
•
ð7:127Þ
HSO 5
À þ M
nþ
! SO 4
• À þ M
ðnþ1Þþ
þ OH À
ð7:73Þ
HSO 5
À þ M
nþ
! M
ðnþ1Þþ
þ HO
•
ð7:128Þ
The following order of efficiency has been reported: UV/PDS > UV/PMS > UV/
H 2 O 2 , supported by the absorbance of the oxidants at 254 nm and the higher
photosensitivity. For catalyzed UV/M/Ox technologies, tested for several metals,
the order was: UV/Fe(III)/H 2 O 2 > UV/Fe(II)/H 2 O 2 > UV/Co(II)/PMS > UV/Ag(I)/
PDS (Anipsitakis and Dionysiou 2004b).
7.3.12 Heterogeneous Photocatalysis
Heterogeneous photocatalysis is a process based either on direct or indirect absorption of visible or UV radiant energy by a solid, normally a wide bandgap semiconductor. In the interfacial region between the excited solid and the solution,
destruction or removal of contaminants takes place, with no chemical change of
the catalyst. Several reviews can be found about this technology (e.g., Legrini et al.
1993; US EPA 1998; Calgon Carbon Corporation 1996; Litter 2005; Fujishima and
Zhang 2006; Lee and Park 2013; Ohtani 2010).
When a semiconductor particle is excited by light of energy higher than that of the
bandgap, electron–hole pairs are created, and electrons and holes migrate to the
surface where they react with adsorbed species, acceptors (A) or donors
(D) (Fig. 7.5) (Mills and Le Hunte 1997). Electron–hole pairs that cannot separate
and/or react with surface species, recombine with energy dissipation. Various
materials are candidates to act as photocatalysts as, for example, TiO 2 , ZnO, CdS,
iron oxides, WO 3 , ZnS, etc. These materials are economically available, and many of
them participate in chemical processes in the nature. Besides, most of these materials
can be excited with light of wavelength in the range of the solar spectrum
(λ > 310 nm); this increases the interest of a possible use of sunlight. So far, the
most investigated photocatalysts are metallic oxides, particularly TiO 2 ; this
158
M. I. Litter
•
and SO 4
•– are generated through the homolytic cleavage of the peroxo bond (Karpel
Vel Leitner 2018; Brienza and Katsoyiannis 2017). Quantum yield of SO 4
•– formation is much larger than that of HO
• formation from H 2 O 2 (Karpel Vel Leitner 2018).
SO 4
•– is transformed into HO
• in strong alkaline media. PMS is activated to produce
both SO 4
•– and HO
• radicals, when the peroxide bond (–O–O–) is homolytically
cleaved, as reported in the following equations (Brienza and Katsoyiannis 2017):
HSO 5
À þ hν ! SO 4
• À þ HO
•
ð7:127Þ
HSO 5
À þ M
nþ
! SO 4
• À þ M
ðnþ1Þþ
þ OH À
ð7:73Þ
HSO 5
À þ M
nþ
! M
ðnþ1Þþ
þ HO
•
ð7:128Þ
The following order of efficiency has been reported: UV/PDS > UV/PMS > UV/
H 2 O 2 , supported by the absorbance of the oxidants at 254 nm and the higher
photosensitivity. For catalyzed UV/M/Ox technologies, tested for several metals,
the order was: UV/Fe(III)/H 2 O 2 > UV/Fe(II)/H 2 O 2 > UV/Co(II)/PMS > UV/Ag(I)/
PDS (Anipsitakis and Dionysiou 2004b).
7.3.12 Heterogeneous Photocatalysis
Heterogeneous photocatalysis is a process based either on direct or indirect absorption of visible or UV radiant energy by a solid, normally a wide bandgap semiconductor. In the interfacial region between the excited solid and the solution,
destruction or removal of contaminants takes place, with no chemical change of
the catalyst. Several reviews can be found about this technology (e.g., Legrini et al.
1993; US EPA 1998; Calgon Carbon Corporation 1996; Litter 2005; Fujishima and
Zhang 2006; Lee and Park 2013; Ohtani 2010).
When a semiconductor particle is excited by light of energy higher than that of the
bandgap, electron–hole pairs are created, and electrons and holes migrate to the
surface where they react with adsorbed species, acceptors (A) or donors
(D) (Fig. 7.5) (Mills and Le Hunte 1997). Electron–hole pairs that cannot separate
and/or react with surface species, recombine with energy dissipation. Various
materials are candidates to act as photocatalysts as, for example, TiO 2 , ZnO, CdS,
iron oxides, WO 3 , ZnS, etc. These materials are economically available, and many of
them participate in chemical processes in the nature. Besides, most of these materials
can be excited with light of wavelength in the range of the solar spectrum
(λ > 310 nm); this increases the interest of a possible use of sunlight. So far, the
most investigated photocatalysts are metallic oxides, particularly TiO 2 ; this
158
M. I. Litter