Degradation of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 55

Sulfonyl urea on chemical degradation forms sulfonamides and heterocyclic amines
(Sarmah and Sabadie 2002).

3.3 Photodegradation
Photodegradation is the breakdown of pesticides by sunlight. Photodegradation can
destroy pesticides on foliage, on the surface of the soil, in water, and even in the air. The
photodegradation of pesticides is influenced by the intensity of sunlight, properties of the
application site, exposure time, properties of the pesticides, pH of medium, water depth,
and presence of other commonly occurring ions.
Photochemical reactions play a key role in the environmental degradation of pes-
ticides that contain organic chromophore or metal-organic complexes capable of
absorbing light energy directly. Pesticides having no chromophore group undergo
photosensitive reactions. Photodegradation of the pesticides in soils is increased in the
presence of soil organic matter such as fulvic acid and humic acid, dyes such as rose
Bengal, pigments such as chlorophyll and xanthone, secondary plant metabolites such
as riboflavin, tyrosine, etc. Surfactants used in various pesticide formulations act as
photosensitizers.
During a photochemical reaction, homolytic bond cleavage with the formation of a
free radical occurs. During a photodegradation reaction of pesticides, free radicals ˙CH 3 ,
˙R, RO˙, ROO˙, NO˙, ˙OH, ˙NO 3 , etc. are formed, which require energy of approximately
400 kJ/mole (of ~300 nm wavelength). Photo-oxidation is one of the most prominent means
of photodegradation initiated by various reactive species such as singlet oxygen, free radi-
cals, organic hydroxyperoxidase, hydrogen peroxidase, etc.
Direct photolysis reaction (Equations 3.1 and 3.2) occurs as

Pesticide . Pesticide → (3.1)

Pesticide .

→ Photoproducts (3.2)

(Pesticide = Pesticide in ground state and pesticide˙ = Pesticide in excited state) (Equation 3.3)

Pesticide .

+ → X Photoproducts (3.3)

where X is solvent or other molecules in solution.
Indirect photochemical reaction (Equations 3.4 and 3.5) occurs as

2 . OH

H O hv

2 + →

(3.4)

2

. OH Pesticide

+ → Photoproducts (3.5)