Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 89

Water-octanol partition coefficient (K ow ) is characteristic of the lipophilicity of the mol-
ecule and indicates that it may accumulate in living organisms. In terms of polarity, log K ow
above 4-5 is specific for nonpolar compounds, whereas log K ow below 1-1.5 corresponds to
polar compounds. Between these two values, compounds are classified as moderately polar.
Other important factors are the vapor pressure of the transformation product and the
Henry law constant (H), which allow prediction of the by-product volatilization. It is gener-
ally considered that compounds with values below 10 −5 Pa m 3 /mol have little tendency to
volatilize. The vapor pressure of pentachlorocyclohexene (PCCH), a metabolite of lindane,
is 14 times greater than that of lindane (Cliath and Spencer 1972). Therefore, a majority of
the lindane volatized from soil would be in the form of PCCH.
The soil sorption coefficient (K oc ) represents the compound distribution between the
soil and liquid phases of the soil. This parameter has a great environmental relevance for
assessing the potential leaching of pesticides and their transformation products in soil.
This coefficient is normalized as a function of the organic carbon content, which plays a
very important role for the nonionized compounds at natural pH in soils. The higher is
the K oc , the more sorbed is the molecule. It is considered that compounds with K oc values
below 50 are considered to be highly mobile; values of 150-500 signify moderately mobile,
and above 2000, slightly mobile compounds. Half-life is another factor to predict the fate
of metabolites in soil and water. As mentioned before, there is not a single half-life for a
transformation product, and the measurements depend on the environmental conditions.
However, laboratory studies permit to carry out experiments under controlled conditions.
Recent studies have shown that some of the degradation products of the pesticides from
different chemical classes (carbamates, triazines, organophosphorus compounds, and sul-
fonylureas) exhibit half-lives in soil higher than those of their degradates, thus being more
persistent than their corresponding parent compounds (Boxall et al. 2004). Half-life and
K oc are related to the GUS factor (Gustafson 1989), which is used to assess the leaching
potential of a compound.
4.3.2 Biological Activity of Transformation Products
Except for propesticides designed to be converted after application to a new compound
with pesticidal activity, the activity of transformation products on target pests is generally
significantly lower than that of the parent compound. However, in some cases, the degra-
dation products can be more toxic (Sinclair and Boxall 2003). Furthermore, evidence shows
that for some pesticides, the pesticidal activity attributed to the parent compound is partly
due to the products formed (Bresnahan et al. 2004; Tuxhorn et al. 1986). In some cases, pes-
ticides are formed as degradation products of other pesticides. For instance, chlorthiamid,
a benzonitrile herbicide, is the parent compound and the precursor of dichlobenil, which
is a degradation product formed in soil and which is also a herbicide. Boxall and cowork-
ers (2004) showed that among different classes of pesticides and their transformation prod-
ucts, 41% of the transformation products were less toxic than the parent compounds, and
39% had a toxicity level similar to that of their parents, but 20% were over 3 times more
toxic, and 9% were over 10 times more toxic than their parent compounds. In general, the
biggest increases in toxicity were observed for parent compounds that had a low toxicity.
Four hypotheses tried to explain these results: (a) the active moiety of the parent com-
pound is still present in the degradation product; (b) the transformation product is the
active component of a propesticide; (c) the bioconcentration factor of the degradation prod-
uct is greater than that of the parent; and (d) the transformation results in a compound
with a more potent mode of action than that of the parent.