Studies of the dissipation of p,p'-ddt in Mtwapa, Kilifi district, of the coast province, Kenya

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.