88
E. Pelizzetti . P. Calza
hydrophobic organic materials called marine humus; these materials have chemical
properties that suggest they are formed in situ by auto-oxidative reactions of plankton-derived organic compounds.
3.3.2
Indirect Photoreactions
Indirect photoprocesses are common and important because they can alter molecules
resistant to direct photolysis, such as transparent species chromophores. There is evidence that dissolved substances in natural waters can photosensitize a variety of reactions; these photosensitized reactions are sometimes the dominant pathway for
photochemical transformation, especially when direct photolysis is negligible.
In indirect photolysis, a reaction is initiated through light absorption by a chromophore other than the substrate itself. If the chromophore is regenerated, it plays a
photocatalytic role; this occurs in energy transfer processes and in some cyclic redox
reactions. If the chromophore changes irreversibly, it has undergone direct photolysis
itself, which simultaneously causes indirect photolysis of other substances presented.
Various studies (Miller and Zepp 1979) have demonstrated that the photolysis rate
of xenobiotics can be altered by the dissolved and suspended matter in the aquatic
environment. The photolysis rate is effected by both physical and photochemical factors. Physical effects include light attenuation or scattering by natural substances.
Changes in photochemistry occur through photosensitization by dissolved humic
substances and other natural substances. Although semiconductor powder like Ti0 2
promotes rapid photocatalysed reactions of xenobiotics, some studies have shown that
natural suspended sediments do not mediate such photoreactions. A few qualitative
studies suggest that certain xenobiotics are more photolabile when absorbed to algae
than when dissolved in distilled water (Zepp 1980).
In this chapter we will consider only the processes involving indirect photoreactions. In Fig. 3.2, the possible absorbers the photoreactant and the main factors that
influence the photoprocesses are summarized. The most common light absorbers
present in water are dissolved organic matter (DOM), semiconductors (SC) and inorganic anions, such as NO;. These absorbers, through photo-absorption, originate active species (B); the more common are the generation of radicals, such as hydroxyl radical and peroxy radical, and of oxidants, such as hydrogen peroxide and singlet oxygen.
The sources and sinks of those species will be discussed below. These species, reacting with an organic compound, give origin to a photo-transformed compound. The
characteristics and concentration of the dissolved organic matter, concentration of dissolved oxygen and the quantity of the activated species photogene rated influence the
rate of this process.
The known indirect photoreactions involve initial excitation of chromophore followed by energy transfer, by transfer of electrons or hydrogen atoms to or from other
system components. The energy transfer reaction is the first recognized photoprocess
in natural waters (Joussot-Dubien and Kadiri 1970) and has been widely studied; it
almost represents the largest quantum yield as well. A significant fraction of ultraviolet sunlight absorbed in natural waters excites organic chromophores, particularly at
wavelengths below 450 nm. Even processes with low efficiencies originating from these
chromophores may be important because of these high excitation rates. Electronic
E. Pelizzetti . P. Calza
hydrophobic organic materials called marine humus; these materials have chemical
properties that suggest they are formed in situ by auto-oxidative reactions of plankton-derived organic compounds.
3.3.2
Indirect Photoreactions
Indirect photoprocesses are common and important because they can alter molecules
resistant to direct photolysis, such as transparent species chromophores. There is evidence that dissolved substances in natural waters can photosensitize a variety of reactions; these photosensitized reactions are sometimes the dominant pathway for
photochemical transformation, especially when direct photolysis is negligible.
In indirect photolysis, a reaction is initiated through light absorption by a chromophore other than the substrate itself. If the chromophore is regenerated, it plays a
photocatalytic role; this occurs in energy transfer processes and in some cyclic redox
reactions. If the chromophore changes irreversibly, it has undergone direct photolysis
itself, which simultaneously causes indirect photolysis of other substances presented.
Various studies (Miller and Zepp 1979) have demonstrated that the photolysis rate
of xenobiotics can be altered by the dissolved and suspended matter in the aquatic
environment. The photolysis rate is effected by both physical and photochemical factors. Physical effects include light attenuation or scattering by natural substances.
Changes in photochemistry occur through photosensitization by dissolved humic
substances and other natural substances. Although semiconductor powder like Ti0 2
promotes rapid photocatalysed reactions of xenobiotics, some studies have shown that
natural suspended sediments do not mediate such photoreactions. A few qualitative
studies suggest that certain xenobiotics are more photolabile when absorbed to algae
than when dissolved in distilled water (Zepp 1980).
In this chapter we will consider only the processes involving indirect photoreactions. In Fig. 3.2, the possible absorbers the photoreactant and the main factors that
influence the photoprocesses are summarized. The most common light absorbers
present in water are dissolved organic matter (DOM), semiconductors (SC) and inorganic anions, such as NO;. These absorbers, through photo-absorption, originate active species (B); the more common are the generation of radicals, such as hydroxyl radical and peroxy radical, and of oxidants, such as hydrogen peroxide and singlet oxygen.
The sources and sinks of those species will be discussed below. These species, reacting with an organic compound, give origin to a photo-transformed compound. The
characteristics and concentration of the dissolved organic matter, concentration of dissolved oxygen and the quantity of the activated species photogene rated influence the
rate of this process.
The known indirect photoreactions involve initial excitation of chromophore followed by energy transfer, by transfer of electrons or hydrogen atoms to or from other
system components. The energy transfer reaction is the first recognized photoprocess
in natural waters (Joussot-Dubien and Kadiri 1970) and has been widely studied; it
almost represents the largest quantum yield as well. A significant fraction of ultraviolet sunlight absorbed in natural waters excites organic chromophores, particularly at
wavelengths below 450 nm. Even processes with low efficiencies originating from these
chromophores may be important because of these high excitation rates. Electronic