| dc.description.abstract | The dissipation and degradation of 14C-malathion {S-l, 2- bis (ethoxy carbonyl) ethyl 0, 0-
dimethyl phosphorodithioate} and T'Csdimethoate {O, O-dimethyl S-methyl carbamoyl
methyl phosphorodithioate} from the soil and the garden pea (Pisum sativum) have been
studied by use of radioisotope tracer techniques. Field experiments were conducted on the
dissipation and degradation of 14C-malathion in Nairobi and Kisii areas.
Greenhouse experiments were conducted at the University of Bayreuth in Germany, where
the dissipation and degradation of "Cvdimethoate and 14C-malathion were conducted with
the potted garden pea plants and the soil. Weather conditions close to those prevailing in the
Kenyan situation were simulated. "Cvlabelling on the malathion molecule was done at the
two methoxy groups in one set of experiments and at the 2
nd
, 3
rd carbons in the diethyl
malonate of the malathion molecule in another set of experiments. Only the two carbons on
the methoxy groups of dimethoate were labeled. The extractable residues of the pesticides
were quantified by liquid scintillation counting while the bound residues were first
combusted by a biological materials oxidiser to 14C02
, which was trapped by a cocktail
before counting. Metabolites from the degradation of malathion and dimethoate were
identified by TLC, autoradiography and confirmed by GC-MS.
In greenhouse studies, the dissipation and degradation studies on the garden pea, show that
"Cvmalathion dissipated fast from the foliar surface of the garden pea. There was only 5.7%
of the initial pesticide dose on the foliar surface of the garden pea as surface (dislodgeable)
residues aft\ 16 days. The extractable residues initially increased in the plant cells due to
penetration t~ugh the cuticle of the plant from the surface, reaching a maximum level
after 3 days and then started decreasing. Sixteen days after inoculation, 9% was present as
extractable residues. The non-extractable (bound) residues were only 4.2 % of the initial
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dose and 16.7% was translocated from the treated leaf surface to the rest of the plant. A high
proportion of the pesticide (17%) was lost through evapotranspiration due to the high rate of
translocation of the malathion residues. The dissipation of the total residues of malathion
from the pea plant was exponential and a half-life value as determined by first order kinetics
model, was 11 days. The metabolites, 0, O-dimethyl phosphorodithioate and diethyl
mercaptosuccinate which result from chemical hydrolysis of malathion, were detected both
in the soil and plant samples. Malaoxon, an oxygen analogue, was only detected in the soil.
"Cvdimethoate was comparatively more persistent than "Cvmalatbion in the pea plant.
There were 21%, 19.4% and 5% of dislodgeable, extractable and bound residues of
dimethoate, respectively recovered from the pea plant. Only 4.6% of applied 14C-dimethoate
was translocated. The half-life value of the total residues of dimethoate in the pea plant, as
determined from first order kinetics model, was 29 days. Dimethoxon, the oxygen analogue
of dimethoate, was detected both on the surface of the leaves and inside the pea plant.
The dissipation rate of 14C-malathion was lower from the soil than from the garden pea plant
in the greenhouse. The extractable residues in the soil dissipated quickly, with only 1.5% of
the initial pesticide dose remaining 32 days after inoculation. The bound residues constituted
a higher proportion of the total residues in the soil. The bound residues rapidly built up in
the soil up to 42% after 8 days. Thereafter, the bound residue levels decrease until there was
only 20.6% remaining in the soil after 32 days. The decrease in the bound residues has been
attributed to th~iOdegradation by microorganisms to 14C02.The total residues of malathion
dissipated from the soil with a half-life value of 17 days. The uptake of 14C-malathion from
the soil by the garden pea was low with 2.9% of the pesticide being taken up.
14C-dimethoate was more persistent than "Cunalathion in the soil. Only the extractable
residues of dimethoate were degraded to CO2 while the bound residues remained at a
v
maximum level of 41.3% after 32 days. Due to the high solubility of dimethoate in water, its
uptake from the soil was higher (4.1 %) than malathion (2.9%) uptake by the garden pea. The
soil bound more pesticide residues than the pea plant as expected.
Under field conditions, malathion dissipated slightly faster during the long rain season (tll2 =
36.7 days) than in the short rain season (t1l2= 41 days) in Kisii. In both seasons, the bound
residue levels decreased due to biodegradation to CO2 after 21 and 7 days for the short and
long rains seasons respectively. In Nairobi soil, malathion dissipated more slowly with a
half-life value of 770 days. The difference between these rates may be attributed to the
prevailing weather conditions at the two sites. However, the dissipation patterns of
malathion in Kisii and Nairobi areas, separately, correlated well with the prevailing weather
conditions and soil characteristics.
The adsorption and desorption patterns of malathion and dimethoate conformed to the
Freundlich isotherm equation, which assumes a multi-layer type of heterogeneous
adsorption. Malathion, being more lipophilic, was adsorbed to the Kenyan soil with higher
organic matter. | en_US |
