7.3 Photochemical Technologies
7.3.1 Use of Light Irradiation for Water Purification
Usually, light appreciably increases the reaction rate of AOTs in comparison with
the same technology in the absence of illumination. As source of light, high-pressure
mercury or xenon arc lamps, with good emission in the near-UV range, can be used.
Some applications require short-UV irradiation, and in this case, cheap and easily
available germicide lamps can be used. Operating costs are reduced because of a
lower power consumption to generate HO
• compared with other rather more expensive AOPs. If solar light can be used, a consequent saving of electrical power will be
produced, with safer industrial installations. As light will be totally directed to the
system, the photochemical industrial equipment can be more compact, and smaller
tanks will be employed. The use of light also increases the flexibility of the system,
allowing the use of a variety of oxidants and operability conditions. Another
advantage of the photochemical technologies is that no drastic changes are needed
in the effluents as, for instance, in alkaline ozonation. It is worthwhile to point out,
however, that the light-mediated AOPs, especially homogeneous processes, are not
adequate for treating mixtures of substances of high absorbance, or containing high
amounts of solids in suspension, because the quantum efficiency decreases by loss of
light, by dispersion, and/or by competitive light absorption.
Although the use of direct irradiation with UVC light can lead to the transformation of the molecules, direct photolysis is generally not useful for the treatment of
pollutants. Direct photolysis is important only for compounds that react very slowly
with HO
• or do not react at all, as nitrophenols, NO 2
– , halogenated compounds
(Burrows et al. 2002), trihalomethanes (THM), chloromethanes, chloroethanes,
chlorinated aromatics, and chlorinated phenols. The use of 254-nm irradiation is
well documented in the literature (Legrini et al. 1993; Calgon Carbon Corporation
1996), and it is effective to discolor textile dyes at low concentrations; when direct
photolysis is compared with other processes such as 254-nm UV/TiO 2 and combined
TiO 2 photocatalysis/activated carbon, it was demonstrated that, at low dye concentrations (5–10 mg/L), the photolytic treatment is 2–3 times faster than the other
processes for color removal (Gomes da Silva and Faria 2003).
Irradiation with KrCl excimer lamps (222 nm) is used for chlorinated aliphatics as
CCl 4 or 1,1,1-trichloroethane because the rupture of the C–Cl bond takes place at
210–230 nm. Generally, the technology is combined with other conventional
methods. Limitations of the process are: (i) low efficiency, (ii) application only to
compounds absorbing at 200–300 nm, (iii) only one target compound can be treated
with reasonably good results. The mechanism and products of UV radiation decomposition have been described for important pollutants such as DDT, lindane, PCP,
TNT, and atrazine (Golimowski and Golimowska 1996 and references therein).
In many cases, direct photolysis may be favored in the presence of oxygen and
substances that can act as photosensitizers. Sensitizers (Sens) are compounds that
absorb visible light and are excited to a higher energy state from which an energy
7 Introduction to Oxidative Technologies for Water Treatment
145
7.3.1 Use of Light Irradiation for Water Purification
Usually, light appreciably increases the reaction rate of AOTs in comparison with
the same technology in the absence of illumination. As source of light, high-pressure
mercury or xenon arc lamps, with good emission in the near-UV range, can be used.
Some applications require short-UV irradiation, and in this case, cheap and easily
available germicide lamps can be used. Operating costs are reduced because of a
lower power consumption to generate HO
• compared with other rather more expensive AOPs. If solar light can be used, a consequent saving of electrical power will be
produced, with safer industrial installations. As light will be totally directed to the
system, the photochemical industrial equipment can be more compact, and smaller
tanks will be employed. The use of light also increases the flexibility of the system,
allowing the use of a variety of oxidants and operability conditions. Another
advantage of the photochemical technologies is that no drastic changes are needed
in the effluents as, for instance, in alkaline ozonation. It is worthwhile to point out,
however, that the light-mediated AOPs, especially homogeneous processes, are not
adequate for treating mixtures of substances of high absorbance, or containing high
amounts of solids in suspension, because the quantum efficiency decreases by loss of
light, by dispersion, and/or by competitive light absorption.
Although the use of direct irradiation with UVC light can lead to the transformation of the molecules, direct photolysis is generally not useful for the treatment of
pollutants. Direct photolysis is important only for compounds that react very slowly
with HO
• or do not react at all, as nitrophenols, NO 2
– , halogenated compounds
(Burrows et al. 2002), trihalomethanes (THM), chloromethanes, chloroethanes,
chlorinated aromatics, and chlorinated phenols. The use of 254-nm irradiation is
well documented in the literature (Legrini et al. 1993; Calgon Carbon Corporation
1996), and it is effective to discolor textile dyes at low concentrations; when direct
photolysis is compared with other processes such as 254-nm UV/TiO 2 and combined
TiO 2 photocatalysis/activated carbon, it was demonstrated that, at low dye concentrations (5–10 mg/L), the photolytic treatment is 2–3 times faster than the other
processes for color removal (Gomes da Silva and Faria 2003).
Irradiation with KrCl excimer lamps (222 nm) is used for chlorinated aliphatics as
CCl 4 or 1,1,1-trichloroethane because the rupture of the C–Cl bond takes place at
210–230 nm. Generally, the technology is combined with other conventional
methods. Limitations of the process are: (i) low efficiency, (ii) application only to
compounds absorbing at 200–300 nm, (iii) only one target compound can be treated
with reasonably good results. The mechanism and products of UV radiation decomposition have been described for important pollutants such as DDT, lindane, PCP,
TNT, and atrazine (Golimowski and Golimowska 1996 and references therein).
In many cases, direct photolysis may be favored in the presence of oxygen and
substances that can act as photosensitizers. Sensitizers (Sens) are compounds that
absorb visible light and are excited to a higher energy state from which an energy
7 Introduction to Oxidative Technologies for Water Treatment
145