STUDIES ON THE DECAY OF SOME PESTICIDES AND IDENTIFICATION OF THEIR METABOLITES

Abstract

Pesticides are among the few toxic materials deliberately disbursed into the environment to preserve the agricultural produce. The growth of population is making increasing demands on the natural resources thus resulting into a large scale use of the pesticides. The lack of proper training to the farmers sometimes leads to their indiscreet use and subsequent proliferation of these biological poisons into the environment. A problem inherent to pesticide application technology is the drift or disposal of the pesticides from the point of application. Only 10 to 15% of the applied pesticides actually reach the target with the remaining 85 to 90% dispersed off the target to air, soil and water. Awareness on the deposition of the pesticides outside the target area and the potential health effects due to these chemicals, nevertheless,is growing among the general public and the regulatory agencies. The widespread occurrence of pesticide residues in the different agricultural commodities in developing nations like India has been accepted as an unrefutable fact. In order to keep a proper track of the cycle of these materials a careful study on the decay profiles and formation of metabolites in various segments of the ecosystem is the need of the hour. Although some of the pesticides have been banned due to their toxicity and tendency to bioaccumulate in the living organisms their use still continues owing to their effectiveness and economic reasons. But before restricting the use of certain pesticides, it is imperative to follow their persistence behaviour and toxicity of the metabolites. The decay of the pesticides takes place through complex mechanisms. The persistence and the type of the metabolites formed, to a large extent, depend

upon nature of the medium and the environmental conditions. The medium can be water, soil or plant and the environmental conditions may include temperature,humidity,wind velocity etc. Ina particular type of environmental segment there can be large variation which cannot be controlled. Even in the simplest case like water there are variations in different physico-chemical parameters. Therefore large scale variations may be observed in the data on the decay profiles. For any meaningful interpretation of the results it becomes important to generate a database on decay patterns of the pesticides under controlled conditions. It remains a fact that the laboratory results cannot be necessarily extrapolated to the field. Thus a correct approach to the problem is to conduct the laboratory and field studies simultaneously. With the awareness in pesticides pollution, voluminous literature has accumulated on residue analysis which essentially involves the identification and quantification of the pesticides on different food products. Studies have been conducted on the kinetics of decay of some of the pesticides under different laboratory conditions. Also substantial information exists on the photodegradation and subsequent identification of the metabolites. However, the field data on the persistence profiles of the pesticides under different meteorological conditions are scarce. Out of the different categories of the pesticides the organochlorines have received maximum attention with regard to the above mentioned studies. Among the pesticides, organophosphorus compounds enjoy a favoured position due to their relatively fast decomposition and low accumulation in the biological food chain. The economic considerations also dictate their use particularly, in the developing countries. A market survey revealed that out of the organophosphorus pesticides, malathion, methylparathion and dimethoate (v) are the most commonly used ones. Most of the organochlorines are banned on an international level but some of them are still in use in India. Endosulfan, a pesticide of organochlorine group with cyclodiene moiety, is being used in India liberally. A survey of literature reveals that studies have been conducted on the decay profiles of the above mentioned organophosphorus compounds in soil and water under laboratory conditions but the effect of various parameters affecting the decay and the identification of metabolites have not been systematically investigated. There are some results available on the decomposition and the metabolic pathways of endosulfan. However, no field data are available on the decay profiles of the said pesticides under different environmental conditions. In view of the above premise it was planned to investigate the decay of malathion and methylparathion under controlled laboratory and environmental conditions and identify the metabolites formed. It was also important to look into the effect of various variables on the decay rate of endosulfan in water and soil. For the sake of clarity and convenience in presentation the work embodied in the thesis has been divided into the following five chapters:- I. General Introduction. II. Experimental Methodology. II1(a). Degradation of Malathion and Methylparathion in Water and Soil Under Laboratory Conditions. 111(b). Degradation of Endosulfan in Water and Soil Under Laboratory Conditions. IV. Degradation of Malathion and Methylparathion in Plant and Soil Under

Field Conditions. V. Identification of Metabolites of Malathion and Methylparathion. Chapter I deals with the role of the pesticides and their classification. The problem of pesticide pollution is highlighted. The parameters affecting the decay of the pesticides and the different metabolites formed are discussed. Finally the aims and objectives of the present study are defined. The relevant literature on the different aspects has been included in the respective chapters. Chapter II details out the optimum operating conditions developed for the analysis of malathion, methylparathion and endosulfan. The malathion and methylparathion were analysed by using RP-HPLC and endosulfan by employing GLC with an electron capture detector. This is followed by a description on the extraction procedures adopted in the present investigations. Different procedures were explored to develope a method for each pesticide and the percentage recovery was noted. Methanol-water extraction proved to be most efficient for the plant and soil samples and methylene chloride for water samples. Methanol- water extractsfrom the plant and the soil were partitioned into methylene chloride, suitably cleaned and finally analysed by the appropriate chromatographic technique. The details of the equipments and the reagents used are also given in this chapter. Chapter III presents results of the effect of temperature, pH and organic content (humic acid) on the decay of malathion,methylparathion and endosulfan in water.The findings about the decay profiles of the pesticides in three different (vii) types of soil are also included . The samples were spiked with a known amount of the pesticide, extracted at different time intervals and analysed. The degradation was monitored for four weeks and in all the cases the decay is exponential in nature. The degradation rate in water was found to increase with the increase in the temperature and change in the pH from acidic to alkaline region. A similar effect of pH was observed in the soil. The presence of humic acid decreases the half-life of the organophosphates but increases that of endosulfan. The degradation of endosulfan is distinctly slower in soil than in water. Chapter IV incorporates the degradation study of malathion and methylparathion in radish and carrot and the adjoining soil in three different seasons namely winter, summer and postmonsoon. Pesticide formulations of appropriate concentration were sprayed and the plants were harvested for the analysis at various time intervals. The pesticide from the whole plant was extracted and analysed using the procedure described in chapter II. The decay in summer showed two distinct profiles, initially it was faster and subsequently it slowed down. The decay in winter and postmonsoon followed the usual profile of pseudo first-order kinetics. The rate of decay follows the sequence winter ~ postmonsoon < summer. Chapter Vdescribes the possible metabolic pathways of organophosphorus pesticides in radish,water and soil. The metabolites were identified by GC-MS in samples employed for the decay studies. The results indicate that the different metabolites are formed by de-esterification,hydrolysis, oxidation and/or reduction. In water the cleavage occurs at C-S or P-S bond of malathion whereas hydroxylation of phenyl moiety occurs in methylparathion. In soil and radish both the pesticides follow different routes. (viii) It can be concluded that degradation of pesticides follow a first order kinetics. The laboratory data are more or less similer to field results in winter and postmonsoon. However, in summer two different profiles are observed. The study on the identification of metabolites of malathion and methylparathion indicates that they are formed as a results of hydrolysis, oxidation reduction and/or de-estrification. At some stage during the decay cycle the oxons of malathion and methylparathion are formed which are more toxic than their parent compounds. At the end some important findings of the present work have been summarized.