transfer occurs, the excess of energy being transferred to other molecules present in
the system (Faust et al. 1999). In this sense, some dyes like Rose Bengal (RB),
phthalocyanines, or methylene blue promote singlet oxygen (
1 O 2 ) formation in
excellent quantum yields (Wilkinson et al. 1993); singlet oxygen is a powerful
oxidant able to attack OM and microorganisms (US EPA 1998; García 1994;
Meng et al. 2017):
Sens þ hν !
1 Sensà !
3 Sens
ð7:90Þ
3 Sens þ
3 O 2 ! Sens þ
1 O 2
ð7:91Þ
1 O 2 þ A ! AO 2
ð7:92Þ
For water purification, the efficiency is strongly dependent on the production rate
of
1 O 2 in the aqueous solution. This process has not been commercialized yet; one of
the main restrictions is the requirement of removing the dye from the water after the
treatment. For this reason, attempts of immobilization to different supports have
been made, but this process leads to a decrease in the efficiency of
1 O 2 production
(Schaap et al. 1975). More research is needed to be done in order to improve this
technology on account of the potential the system has for effective disinfection of
drinking water.
7.3.2 Vacuum-Ultraviolet Photolysis of Water
In this process, light of wavelengths lower than the UVC (<190 nm) is used,
corresponding to the vacuum-ultraviolet range (VUV) (Legrini et al. 1993; Litter
2005; Oppenländer 2003). Xe excimer lamps (λ exc ¼ 172 nm) are the most used light
sources. Excitation with these energies produces the homolytic breakage of chemical
bonds, degrading OM in condensed and gaseous phases. Fluorinated and chlorinated
hydrocarbons can be converted to dehalogenated compounds (Legrini et al. 1993;
US EPA 1998). However, the application of direct VUV is limited, and VUV water
photolysis (Eq. 7.93) is the most important use for degradation of pollutants.
H 2 O þ hν ! HO
•
þ H
•
ð7:93Þ
This way, HO
• and H
• are generated in situ, without the addition of external
agents. As water has high-absorption cross-section, the total incident VUV radiation
is absorbed within a very narrow layer around the lamp axle (Lopez et al. 2000;
Alapi et al. 2018; Zoschke et al. 2014). Quantum yield of reaction 7.93 varies
between 0.33 at 185 nm and 0.72 at 147 nm (Heit et al. 1998). The process produces
also aqueous electrons, which are strong reductants, with a lower quantum yield
(0.05), almost independent of the wavelength at 160–190 nm (Lopez et al. 2000).
146
M. I. Litter
the system (Faust et al. 1999). In this sense, some dyes like Rose Bengal (RB),
phthalocyanines, or methylene blue promote singlet oxygen (
1 O 2 ) formation in
excellent quantum yields (Wilkinson et al. 1993); singlet oxygen is a powerful
oxidant able to attack OM and microorganisms (US EPA 1998; García 1994;
Meng et al. 2017):
Sens þ hν !
1 Sensà !
3 Sens
ð7:90Þ
3 Sens þ
3 O 2 ! Sens þ
1 O 2
ð7:91Þ
1 O 2 þ A ! AO 2
ð7:92Þ
For water purification, the efficiency is strongly dependent on the production rate
of
1 O 2 in the aqueous solution. This process has not been commercialized yet; one of
the main restrictions is the requirement of removing the dye from the water after the
treatment. For this reason, attempts of immobilization to different supports have
been made, but this process leads to a decrease in the efficiency of
1 O 2 production
(Schaap et al. 1975). More research is needed to be done in order to improve this
technology on account of the potential the system has for effective disinfection of
drinking water.
7.3.2 Vacuum-Ultraviolet Photolysis of Water
In this process, light of wavelengths lower than the UVC (<190 nm) is used,
corresponding to the vacuum-ultraviolet range (VUV) (Legrini et al. 1993; Litter
2005; Oppenländer 2003). Xe excimer lamps (λ exc ¼ 172 nm) are the most used light
sources. Excitation with these energies produces the homolytic breakage of chemical
bonds, degrading OM in condensed and gaseous phases. Fluorinated and chlorinated
hydrocarbons can be converted to dehalogenated compounds (Legrini et al. 1993;
US EPA 1998). However, the application of direct VUV is limited, and VUV water
photolysis (Eq. 7.93) is the most important use for degradation of pollutants.
H 2 O þ hν ! HO
•
þ H
•
ð7:93Þ
This way, HO
• and H
• are generated in situ, without the addition of external
agents. As water has high-absorption cross-section, the total incident VUV radiation
is absorbed within a very narrow layer around the lamp axle (Lopez et al. 2000;
Alapi et al. 2018; Zoschke et al. 2014). Quantum yield of reaction 7.93 varies
between 0.33 at 185 nm and 0.72 at 147 nm (Heit et al. 1998). The process produces
also aqueous electrons, which are strong reductants, with a lower quantum yield
(0.05), almost independent of the wavelength at 160–190 nm (Lopez et al. 2000).
146
M. I. Litter