| dc.description.abstract | The rate of disappearance of the insecticide, p,p'-DDT,
[l,l,1-trichloro-2,2'-(4-chlorophenyl)ethane], in Mtwapa,
Kilifi District, Coast Province, Kenya, was carried out using
nuclear techniques. A plot was prepared by digging, and
removing all weeds and stones. PVC cylinders (length: 15 em;
diameter: 10.4 cm), were driven into the soil and 14C-p,p'-DDT
was applied to the soil contained in the cylinders at rates of
3.78 ppm ( total radioactivity, 5.88 pCi). Sampling was done
weekly for the first month, biweekly for the second and third
month, monthly for the fourth month and bimonthly for the rest
of the nine month study period. The amount of 14C-p,p'-DDT
remaining was monitored by a Liquid Scintillation Counter
(LSC), for the solvent extractable 14C-p,p'-DDT equivalents
(residues) and by a Biological Material Oxidiser (BMO), for
the bound (unextractable 14C-p,p'-DDT equivalents (residues).
The dissipation half-life (time taken for 50 % of ~he
test compound to disappear) for the parent compound 14_p,p'_
DDT, was found to be 196.9 days assuming a 1st-order rate
process. The p,p'-DDT was lost either by volatilisation or by
conversion to p,p'-DDE. The dissipation curve indicated a
biphasic pattern, i.e., an initial rapid rate of disappearance
from 0-56 days, and a slower rate from 56-292 days. Bound
residues rose to 5.7 % of the applied 14-C-p,p'-DDT showing a
low rate of binding in this type of soil.
The degradation products were investigated using
Electron-capture Gas Chromatography (GC-EC), a TLC Linear
Analyser (TLC-LA), and a combined Gas Chromatograph/Mass
Spectrometer. The initial analysis on GC-EC revealed the
presence of three major compounds. These were identified as
the parent compound p,p'-DDT, and the possible metabolites
p,p'-DDE [l,l-dichloro-2,2'-(4-chlorophenyl)ethene, and p,p'-
DDD [1,1-dichloro-2,2'-(4-chlorophenyl)ethane].Identification
was by comparison of their retention times with those of DDT,
DDE, and DDD standards. However, analysis of the same sample
solutions on the TLC-LA indicated the presence of only two
components. To confirm the identity of components of the
sample solution, structural determination was carried out
using GC/MS. This revealed the presence of the parent compound
DDT, with the main metabolite being DDE. The structure of the
compound formerly identified as DDD (its retention time was
similar to that of DDD) was that of 2,4- dimethyl-decane. Its
presence in the sample solution was attributed to
contamination, and ~ot to the degradation of DDT. | en |
