Pesticide Degradation in Water - Pesticides: Evaluation of Environmental Pollution - page 90

Oxidation reactions occur frequently in the soil and are extremely important in the
transformation pathway. Sulfur-containing herbicides are often rapidly oxidized to sulfox-
ide and afterward more slowly to sulfones. Sulfoxidation can occur in soil and water medi-
ated chemically or biologically (Ankumah et al. 1995; Bronstert et al. 2002; Turgut 2007).
This oxidation is so rapid and complete that sulfoxides are often the compounds found
in soil, shortly after application of the parent sulfide compound. Furthermore, in some
cases, sulfoxides and sulfones are suspected to have the pesticidal activity (Campbell and
Penner 1985).
Not only degradative reactions but also rearrangements in the molecule can alter the
toxicity of active substances. For example, isomers of malathion and fenitrothion are sev-
eral orders of magnitude more active inhibitors of acetylcholinesterase than the parent
compound (Ryu et al. 1991).
Organophosphate and organosulfur insecticides commonly have degradation products
with insecticidal activity, often of greater potency than the parent compounds. Sulfoxide
and sulfone of sulfide insecticides are usually quite active on a spectrum of pests similar to
the parent compounds. Formation of aldicarb sulfoxide and sulfone is an example of this.
This sulfoxidation increases the acetylcholinesterase-inhibiting capacity of the organo-
phosphate compounds (Miles 1991). Even though few insecticides have previously been
found to extensively impact the groundwater, the potential for persistent and mobile deg-
radation products to impact the groundwater may be significant.
Organophosphorus insecticides have traditionally been designed to undergo transfor-
mation reactions that activate them to more potent forms in organisms or environment.
The thione moiety (P=S) is oxidized to the oxon form (e.g., malathion to maloxon). This
is often three orders of magnitude more potent as an inhibitor of acetylcholinesterase. In
most cases, this toxic form is considerably less stable in the environment. Ethylene bisdi-
thiocarbamate (EBDC) and daminozide are two fungicides whose degradation products,
ethylene thiourea (ETU) and dimethylhydrazine (UDMH), are known to be carcinogenic
(Kobayashi 1992).
Some degradation products of pesticides have shown toxicity to aquatic organisms.
Herbicides such as alachlor, metolachlor, and diuron showed that the toxicity to the bacteria,
Vibrio fischeri, was enhanced upon degradation (Osano et al. 2002; Tixier et al. 2000, 2001).
Degradation of acifluorfen shows toxicity to Daphnia magna after 36 h of irradiation, where
98% of acifluorfen is degraded (Scrano et al. 2002). On the contrary, other studies showed that
herbicides and their degradation products cannot be considered a risk to the environment.
This is the case of some sulfonylurea herbicides, in which neither the active substance nor
the metabolites are toxic to D. magna and V. fischeri (Martins et al. 2001; Vulliet et al. 2004).
Major degradation products of some herbicides also have herbicidal activity against
target and/or nontarget pests. Some herbicide degradation products are of significance
in crop protection by being effective against the target pest. It has been demonstrated
that the formation of the sulfoxide by-product of thiocarbamate herbicides, such as butyl-
ate, increased the herbicidal activity (Tuxhorn et al. 1986). On the contrary, some can be
responsible for inadequate weed control by inducing rapid degradation of their parent
compounds.
The herbicidal activity of carbamothiate herbicide sulfoxides has been previously
reported (Warner and Morrow 2007). In soils treated with butylate, herbicide residues
of the parent compound were not detected in significant amounts within a few weeks
after application. However, good control of weeds was observed in these fields. The good
performance of this herbicide, despite its lack of persistence, was probably due to the
by-products formed. Another degradation product that is effective in controlling target