Degradation kinetics of DDT in tropical soils: A proposed multi-phase zero order kinetic model that takes into account evaporation, hydrolysis, photolysis, microbial degradation and adsorption by soil particulates.

Abstract

The Food and Agriculture Organization (FAO) and International Atomic Energy Agency (IAEA) commissioned a 5-year international programme during 1989 to 1993 to study the behaviour of DDT in 10 tropical countries: India, Indonesia, Pakistani, China, Philippines, Egypt, Tanzania, Kenya, Panama and Brazil. The traditional approach of plotting the residual pesticide concentration as a function of time was employed to treat the data in order to arrive at the half-life of the pesticide in the specific environment. However, most of the curves obtained were not first order, as the dissipation was observed to be biphasic, prompting the authors to suggest that the dissipation could best be characterized in terms of a multi-phase dissipation model. The aim of the present work was to derive a possible multi-phase dissipation kinetic model to account for the observed poly-phasic nature of the data from the FAO/IAEA study. Data are presented showing that when the loss in the pesticide concentration, rather than the residual pesticide concentration, is plotted as a function of time, the curve can be resolved into biphasic or triphasic linear curves, which are interpreted in terms of pseudo zero order rate equations derived taking into account dissipation of the pesticide via evaporation, hydrolysis, photolysis, microbial degradation and adsorption by soil and colloidal particles. The predictions of the zero order dissipation kinetic models when compared to the first order rate law are discussed.Capsule:The degradation of DDT in tropical soils is characterized in terms of multi-phase zero order kinetics.