Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 86

The influence of different concentrations of HA in the photodegradation of alloxydim
has been studied by Sevilla-Morán and coworkers (2010a). Photolysis of this cyclohexane-
dione herbicide, in the presence of different concentrations of HA (1-20 mg/L), showed
that increasing concentrations of HA decrease the photolysis rate of this herbicide, indicat-
ing that it absorbed most of the emitted photons, thereby slowing down the direct pho-
tochemical reaction of alloxydim. On the contrary, Sakkas and coworkers (2002a) studied
the photodegradation of the fungicide chlorothalonil in the presence of HA and FA under
simulated solar irradiation. Results showed that, for both substances, an increased concen-
tration increased the photodegradation rate of chlorothalonil.
In addition to DOM, there are other water constituents that influence the photolysis
of pesticides. The most prominent examples are nitrate (NO 3 − ), nitrite (NO 2 − ), and iron
ions (Fe(II) and Fe(III)). NO 3 − concentrations in the environment range between 1 and
30 mg/L, whereas NO 2 − concentrations are lower (less than 1 mg/L) (Somasundaram
and Coats 1991b; Wilson and Mabury 2000). In many freshwaters, photolysis of NO 3 − and
NO 2 − appears to be the major source of OH•. In this sense, pesticides such as metolachlor
(Dimou et al. 2005), carboxin (DellaGreca et al. 2004), and diuron (Shankar et al. 2007)
showed an increase in the degradation rates. However, not always does the presence of
nitrate accelerate the degradation of pesticides; and no effect was observed in the photo-
degradation of other pesticides (Andreozzi et al. 2003; Chaabane et al. 2007; Sevilla-Morán
et al. 2008, 2010a). The irradiating power available in solution can be reduced when the
pesticide itself strongly absorbs in the same UV range, diminishing the amount of radia-
tion absorbed by nitrate ions.
The chemical transformation of pesticides is mediated by different types of reactions
such as oxidation, hydrolysis, reduction, elimination, cyclation, and rearrangements.
Bavcon and coworkers (2007) studied the aqueous degradation of different organophos-
phorus compounds. Oxidation of chlorpyrifos to chlorpyrifos-oxon was the main deg-
radation route of the pesticide (Figure 4.3). The oxidative desulfuration is very common
in compounds that present a P=S group. Oxidation pathways are extremely important
transformation reactions.
The sulfoxidation of sulfides to sulfoxide and sulfone (sulfoxidation) is often rapid
and one of the most important degradation pathways in pesticides containing a sulfur
atom (Somasundaram and Coats 1991b). Pesticides such as aldicarb, carboxin, disulfoton,
fenamiphos (Miles 1991), and clethodim (Sevilla-Morán et al. 2010a) are known to degrade
to their corresponding sulfoxide and/or sulfone in environmental waters. Photocyclization
after HCl elimination was one of the main reaction pathways of the photolysis of propi-
conazole in pure water at 254 nm (Vialaton et al. 2001). Detomaso and coworkers (2005a,b)
identified up to seven photoproducts during high-pressure UV degradation of carbofu-
ran in aqueous solution, mainly resulting from photo-Fries rearrangement, hydroxylation

Cl

Cl

O

S

C 2 H 5 O
C 2 H 5 O

C 2 H 5 O

P O Cl

P O Cl

C 2 H 5 O

N

N

Cl

Cl

Chlorpyrifos Chlorpyrifos-oxon

FIGURE 4.3
Main photodegradation products of chlorpyrifos.