Environmental Engineering Reference
In-Depth Information
cases, of only limited extent. Direct photolysis can occur if the considered pol-
lutant absorbs light. However, the ability to undergo chemical changes after
the absorption of photons is an intrinsic property of the molecule and can
vary drastically among compounds. Direct irradiation will lead to the promo-
tion of the molecules to their excited singlet states, which may then through
intersystem crossing produce triplet states. Such excited states can then un-
dergo, among other processes: (i) homolysis, (ii) heterolysis, and (iii) pho-
toionization (Fig. 1). The reactions occurring as a consequence of direct light
absorption by the pollutant could be fragmentation, isomerization, hydro-
gen atom abstraction, intramolecular rearrangement, dimerization-addition
reactions, cyclization, and electron transfer reactions.
2.2
Sensitized
/
Indirect Photolysis
Photosensitized photodegradation is based on the absorption of light by
a chemical substance other than the pollutant, usually naturally occurring
substances (NOS) of the aquatic environment. This may then transfer en-
ergy from its excited state to the micropollutants, which can undergo different
degradation processes as shown in Fig. 1. Photosensitized or indirect pho-
tolytic reactions are thought to proceed due to the presence of chemical
transients such as hydroxyl (OH), alkyl peroxy (ROO), hydroperoxyl (HO 2 )
and carbonate (CO 3 - ) radicals, superoxide ion (O 2 - ), singlet oxygen ( 1 O 2 ),
hydrogen peroxide (H 2 O 2 ), aquated electrons (e aq ), and colored dissolved or-
ganic matter (cDOM) in its excited triplet state. Waters containing sufficiently
high metal ion concentration (through ligand-to-metal charge transfer re-
action and photo-Fenton chemistry) and semiconductor particles (through
electron-hole pair formation) have been also shown to sensitize the photo-
transformation of organic contaminants in aqueous solutions [15-17].
Previous research has shown that the hydroxyl radical (produced through
the photolysis of nitrate, nitrite, and cDOM) plays a significant role in the
transformation of organic contaminants in natural waters due to its reactiv-
ity and non-selectiveness [18-20]. In addition carbonate radicals, generated
from the reaction of OH with either carbonate or bicarbonate ions, react
rapidly with electron-rich compounds and have been shown to play a sig-
nificant role in limiting the persistence of pesticides [21, 22]. Moreover, alkyl
peroxy radicals, produced through the reaction of groundstate oxygen with
excited cDOM chromophores, and singlet oxygen formed upon the absorp-
tion of sunlight by cDOM and subsequent energy transfer to ground-state
oxygen ( 3 O 2 ) [23, 24] may also promote the photodegradation of the mi-
cropollutant. Short-lived triplet states of cDOM ( 3 cDOM )maycontribute
significantly to the decomposition of micropollutants through electron ab-
straction, hydrogen transfer, or both. Also, aquated electron e aq ,ahighly
reactive and strongly reducing species, has previously been reported to be
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