Environmental Fate and Transport of Pesticides - Pesticides: Evaluation of Environmental Pollution - page 32

Cordeiro et al. (1986) proposed that the oxidative breakdown of phosphonates involves
coordination with a metal oxide via a free-radical pathway.
Hydrolysis : Hydrolysis is a reaction between pesticide and water molecules involving
catalysis by proton, hydroxide, or inorganic ions such as phosphate ion in the aquatic
environment.
Hydrolysis of pesticides in water is generally believed to follow first-order kinetics,
where the rate of pesticide degradation, −d[P]/dt, is proportional to the pesticide concen-
tration, [P], and k obs is the hydrolysis rate constant (Equation 2.1).

− [ ]

[ ]

d P dt = k P (2.1)

obs

Hydrolysis reactivity depends on the chemical structure and functional group(s) of a
pesticide molecule, but it also varies within the same class. For example, organophospho-
rus pesticides are prone to alkaline hydrolysis, whereas acidic hydrolysis is more common
for some phosphorodithioates (Davisson et al. 2005). Hydrolysis is also temperature and
pH dependent (Auld and Vallee 1971). Ruzicka et al. (1967) studied the hydrolysis rate
of organophosphorus pesticides, and they found that, at higher temperatures, the hydro-
lysis kinetics rate constant is much larger than at lower temperatures. In field applica-
tion, the residence time of pesticides can significantly vary with temperature (Aharonson
et al. 1987). pH also controls the hydrolysis of pesticides. Normally, the pH-rate profile of
pesticide hydrolysis is a U- or V-shaped curve (Figure 2.2). As mentioned above, certain
pesticides may undergo alkaline or acidic hydrolysis, in which a pH >7 or <7 will prompt
the chemical degradation of these pesticides, respectively.
In some cases, metal ions have the ability to catalyze the hydrolysis of pesticides.
Mortland and Raman (1967) showed that Cu (II) can catalyze some organic phosphate
pesticides that are normally considered relatively stable.
Photolysis : Photolysis or photodegradation becomes a significant degradation path-
way when there are high levels of UV radiation. The process begins when the pesticide
molecule receives energy and gets excited, after which the molecule either breaks up or
forms less stable bonds that can easily break up later. Pesticide molecules can utilize photo
energy in two ways: direct, in which the pesticide receives UV light within the spectrum of
sunlight (<300 nm), or indirect, in which the energy is transmitted from other compounds
that absorb the photo energy. There have been many studies focusing on the different

Acid-catalyzed Base-catalyzed

Neutral

pH

FIGURE 2.2
pH-Rate profile of pesticide hydrolysis.