Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 87

of the benzene ring, oxidation, cleavage of a carbamate group, and hydrolysis of the ether
group.
The aquatic photochemistry of trifluralin in natural water (Dimou et al. 2004b) was
investigated under simulated solar irradiation (Suntest apparatus). Photodecomposition of
trifluralin generally involves three major routes: oxidative dealkylation of propylamines,
cyclization, and reduction of nitro groups.
Consequently, information about photodegradation of pesticides is necessary to estimate
their persistence and to identify the factors that influence their behavior in the environment.
Furthermore, it is important to investigate what these compounds degrade into, the persis-
tence of the by-products relative to the parent compounds, and whether the degradation
products retain the activity of the active substance to cause a toxicological effect on nontarget
organisms in aqueous systems.
4.3 Environmental Relevance of Transformation Products
4.3.1 Properties Affecting the Fate of Transformation Products
Many pesticides applied to the field are completely degraded or mineralized into innoc-
uous compounds. However, different transformation products are formed before the
pesticide is complete degraded. Sometimes, one or two transformations in its molecular
structure are enough to modify its properties. In fact, for some pesticides, biological activ-
ity and/or environmental contamination attributed to the parent compound can be due
to the degradation products.
Historically, some of the most serious concerns about the safety of pesticides have been
raised from its transformation products which can cause detrimental side effects. The major-
ity of the environmental concerns about 1,1,1-trichloro-2,2-bis( p -chlorophenyl)ethane (DDT)
were attributable to one of its breakdown products, 1,1-dichloro-2,2-bis( p -chlorophenyl)
ethylene (DDE). This compound lacked insecticidal activity, but it was extremely persistent,
mobile in the environment, and bioaccumulative.
Different studies confirm that many degradation products are more mobile and some
others are more persistent than their respective parent compound (Boxall et al. 2004; Coats
1993; Khan and Saidak 1981; Richards and Baker 1993).
Some physicochemical properties of transformation products are characteristic of their
own molecule, such as water solubility, vapor pressure, volatility, and water-octanol
partition coefficient. However, other parameters such as half-life in soil and water com-
partments depend not only on the chemical structure of the molecule but also on the envi-
ronmental conditions. However, calculation of these properties is sometimes difficult due
to the absence of analytical standards.
As mentioned before, the different abiotic/biotic reactions that take place in the environ-
ment modify the physicochemical properties of the parent molecule. Biotic transformation
processes generally produce transformation products that are more polar and water-soluble
than the parent compound. Organisms transform oxidized xenobiotics to more soluble and
polar molecules to facilitate their elimination from their organism. Hence, the resulting trans-
port behavior of metabolites may be different.
Most of the oxidative reactions (hydroxylation, sulfoxidation, dealkylation, etc.) and
hydrolysis contribute, also to some degree, to the increase in polarity and hence, water
solubility of the molecule. Therefore, the new xenobiotics are more mobile in the soil.