Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 88

Reduction reactions are characterized in environments with low oxygen concentra-
tions, low pH, and anaerobic microorganisms. These reactions are less commonly
observed and generally give rise to products with lower polarity.
Pesticides with reactive moieties in their structures are known to give rise to degra-
dation products of environmental concern. For example, sulfide pesticides undergo
oxidation of the sulfur atom and parent compound is converted into products that are
more water-soluble and thus more mobile in soil. Thus, sulfoxidation is one of the most
important pesticide degradation pathways since the sulfoxide product has a much higher
groundwater contamination potential (Bavcon Kralj and Trebše 2007; Lacorte et al. 1995;
Miles 1991). In soil, sulfide pesticides are often rapidly oxidized to sulfoxides, while sul-
fones are formed more slowly (Miles 1991) and for this reason, sulfoxides are the major
compounds found in soil. This oxidation is so rapid and complete that sulfoxides are often
the dominant species found in soil, shortly after application of the parent compound. In
most cases, sulfoxide and sulfone also have pesticidal activity. Some sulfur pesticides
known to suffer this reaction are aldicarb, malathion, parathion, methomyl, fenamiphos,
and methiocarb (Barceló et al. 1996; Dabestani et al. 2000; García de Llasera and Bernal-
González 2001; Priddle et al. 1992; Wang and Hoffman 1991). Recent studies indicate that
about one third of the degradation products derived from a range of pesticide types have
an organic carbon absorption coefficient (K oc ) of at least one order of magnitude lower
than that of the corresponding parent compound. Thus, these transformation products
may be more likely to be transported to surface waters and groundwater.
Nowadays, great efforts are being made to predict the movement of pesticides and trans-
formation products through the soil to the groundwater. However, physicochemical data
of the degradation products are not always available, and different methods are used to
estimate the unknown parameters. Quantitative structure-activity relationships (QSAR)
permit to calculate the physicochemical properties of a molecule from its chemical struc-
ture, such as K oc , Henry´s law constant, and persistence. This data would be used in models
for estimating the exposure or for assessing the ecotoxicity (Cronin and Livingstone 2004).
There are several basic physicochemical properties of the transformation products that
allow partly assessing their behavior and fate in the environment. Water solubility indi-
cates the tendency of a by-product to be removed from soil, for example, by runoff, and
to reach the surface water. This parameter cannot by itself be used to predict its mobility
in groundwater. The processes of degradation of pesticides generally increase the water
solubility and the polarity of the compounds and, as a consequence, favor their mobility
in groundwater. The increase in solubility is caused by the loss of carbon, the incorpora-
tion of oxygen, and the addition of carboxylic acid functional groups. For every carbon
atom that is removed, the water solubility increases two or three times. For example, atra-
zine solubility is 33 mg/L; deethylatrazine solubility, which implies the loss of two carbon
atoms, is 670 mg/L; and deisopropylatrazine solubility, which has lost three carbon atoms,
is 3200 mg/L (Mills and Thurman 1994). Addition of oxygen in the molecule of aldicarb, by
sulfoxidation, gives rise to sulfoxide and sulfone metabolites that are 55 and 1.4 times more
soluble, respectively, than the parent aldicarb (Somasundaram and Coats 1991b). Increases
in water solubility is dramatic when the degradation product is a carboxylic acid, which
can occur when there is an alkyl chain, a ketone group, or an aldehyde group that can be
degraded (Hornsby et al. 1996).
Water-soluble products are frequently the primary metabolites formed by biological
and chemical oxidations and hydrolysis in the soil. Those polar molecules demonstrate a
greater tendency to leach into and to be dissolved in the runoff water, as do compounds
with low binding constants.