toxicological profile for malathion - Agency for Toxic Substances and ...

MALATHION 191
6. POTENTIAL FOR HUMAN EXPOSURE
include malaoxon, inorganic phosphate, thiophosphate, monomethyl phosphate, dimethyl phosphate, and
diethyl maleate (Laveglia and Dahm 1977).
Konrad et al. (1969) observed a more rapid initial degradation of malathion in sterile soils than in an
inoculated aqueous system in which malathion did not undergo biodegradation until after a 7-day lag
period, indicating that actual biodegradation of the compound requires acclimation by the microbial
population. In a study on the degradation of malathion in sterile and nonsterile soils, these researchers
also found that the degradation rate of malathion was more rapid in soils that exhibited more rapid initial
adsorption of the compound (Konrad et al. 1969). The study authors concluded that the degradation of
malathion in soil is directly related to the adsorption of the compound to the soil surfaces, which serves to
catalyze the degradation process and allows for almost immediate degradation of the compound. Based
on the results of the study, the researchers also concluded that (direct) biodegradation by soil
microorganisms does not play an important role in malathion degradation in soils (Konrad et al. 1969).
Other researchers have also concluded that the degradation of malathion in soil is mainly due to
exoenzymes in some soils, and is a combination of microbial metabolism; exoenzyme activity,
particularly in the organic matter fraction of the soil; and hydrolysis (Gibson and Burns 1977).
In a study on the cometabolism of malathion in soil, Merkel and Perry (1977) found that the presence of
certain cosubstrates (alkanes and 1-alkenes) increased the rate of malathion biodegradation in soil from a
tobacco field and sediment from an estuary of the Neuse River in North Carolina. Compared with a
control (unamended system), the addition of 1-heptadecene and n-heptadecane each increased the rate
(2–3 times) of metabolically produced 14 CO2 in the soil system; acetate, succinate, pyruvate, and citrate
did not effectively serve as cosubstrates for the oxidation of malathion in either of the soils studied.
Data on the photodegradation of malathion on soil were scarce in the available literature. In a
photodegradation study of malathion exposed to natural sunlight and UV irradiation (maximum
wavelength of 360 nm) as thin films on glass, the compound was relatively stable to sunlight and
exhibited only slight degradation (16%) by 25 hours under UV light; none of the degradates were present
at >0.01% (Chukwudebe et al. 1989). In the EPA RED for malathion, a photodegradation on soil half-life
of 173 days was reported for a pH 6.5 sandy loam soil (EPA 2000a). Based on these limited data,
photodegradation on soil is not likely to be a significant fate process for malathion in the environment.