STUDIES ON DECAY PROFILES. OF QUINALPHOS AND THIRAM PESTICIDES

Abstract

Pesticides are among the few toxic materials deliberately disbursed into the environment to preserve the agricultural produce. Population is growing explosively and as a consequence of this the need for more food is increasing steadily and simultaneously. The farmers are getting aware of the benefits that can be harvested by the application of agrochemicals without understanding the implications of the enhanced rate of usage of these chemicals. This intrinsically dangerous practice is being promoted in a setting without technical and human resources to control it properly. Large volumes of pesticides are used in developed countries and their use in developing countries is on a fast increase. This relentless use of pesticides results in irrevocable havoc on the health and environment of a country. The issue of pesticides and their use is actually quite complex, involving sometimes the conflicting interests of more crop and environmental safety. The topic is important as all of us are affected in some way or the other by their presence in our environment. Genetic damage, reproductive problems and possible links to cancer are just some of the risks associated with the use of pesticides. Sometimes the metabolites are more toxic than the parent compound. Lower rate of degradation of pesticides results in bioaccumulation in organisms which causes various diseases. Once the pesticides are released into the environment, they residually impair the quality of water, soil and air depending on their persistence. The degradation of the pesticides and the types of metabolites formed involve complex mechanism and depend upon the chemical nature of pesticide, type of medium and the environmental conditions. The medium can be air, water, soil, plant and animal. In order to keep a proper track of the cycle of these materials a careful study on their persistence in different parts of ecosystem under different environmental conditions is required. The pesticides can be classified on the basis of target use like, fungicide, insecticide, and herbicide or their chemical composition. The most acceptable classification includes the following five major categories based on their chemical composition- 1. Organochlorine pesticides 2. Organophosphate pesticides 3. Carbamate pesticides 4. Inorganic pesticides 5. Botanical and biopesticides Among the pesticides, organophosphates and carbamates 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. Market and field survey shows that nowadays quinalphos and thiram are the most commonly used pesticides. The indiscriminate use of these pesticides inagriculture, forestry and public health leaves considerable amount of residues and their metabolites in the environment. A survey of literature reveals that scanty reports are available on the decay profiles of the above mentioned pesticides in water and soil under laboratory conditions. The effects of various parameters affecting the decay and the identification of metabolites have not been systematically investigated. However, the field data on the persistence profiles under different field conditions are scarce. Inadequate studies are documented on the degradation of these pesticides in Wistar albino rats. In view of the above premise it was planned to investigate the decay of quinalphos and thiram under controlled laboratory and field conditions and identify the metabolites formed. It was also important to look into the effect of biological conditions on the decay pattern of quinalphos in Wistar albino rats. For the sake of clarity and convenience in presentation the work embodied in the thesis has been divided into the following five chapters 1. General introduction II. Experimental methodology III. Decay profiles and metabolic pathways of quinalphos in water, soil and plants IV. Decay profiles and metabolic pathways of thiram in water, soil and plants V. In vitro and in vivo studies on the degradation of quinalphos in rats Chapter I deals with the role of the pesticides and their classification. The problem of pesticides pollution is highlighted. The parameters affecting the decay of the pesticides are discussed. Finally the aims and objectives of the present study are defined. The relevant literature on different aspects has been included in the respective chapters. Chapter II details out the optimum operating conditions developed for the analysis of quinalphos and thiram. Both have been analysed using RP- HPLC with a UV detector. This is (iv) followed by a description on the extraction procedures adopted in the present investigations. Different procedures were explored to develop a method for each pesticide and the percentage recovery was noted. Ethyl acetate is found to be a good extractant for the recovery of quinalphos from water and soil. QuEChERS (quick, easy, cheap, effective, rugged and safe) method using acetonitrile with slight modification has been employed for the quantitative recovery of quinalphos from plants. For the best recovery of quinalphos from biological fluids, methanol is employed. The quantitative extraction of thiram from water, soil and plants is carried out using ethyl acetate, acetonitrile and dichloromethane, respectively. For studies on Wistar albino rats the animals have been divided into different groups and the details are discussed in the present chapter. Details of digestion methods for simulated gastric and intestinal phases have also been explained in the chapter. Chapter III describes the decay profiles and metabolic pathways of quinalphos pesticide in water and soil in laboratory conditions and in plants under field conditions. Effect of temperature, pH and organic content (humic acid) on the degradation of quinalphos in water is discussed. Results of the decay profiles of the pesticide in three different types of soil are also illustrated. The degradation was monitored for nearly four half lives. In all the cases the decay is exponential in nature. In the case of field study, pesticide solution of appropriate concentration was spotted on the different parts of radish and tomato plants, namely leaf, fruit and root and the spotted parts were harvested for the analysis at various time intervals. The pesticide from spotted part of plant was extracted and analysed using HPLC. For the identification of metabolites of quinalphos GC-MS was used. The results indicate that the different metabolites are formed by de-estrification, hydrolysis and oxidation. On the basis of metabolites identified in different matrices metabolic pathways of quinalphos have been proposed. Chapter IV presents results of the effect of temperature, pH and organic content (humic acid) on the decay of thiram in water. The findings about the decay profiles of the pesticides in three different types of soil are also included. The samples were spiked with a known amount of the pesticide, extracted at different time intervals and analysed using HPLC. The degradation, in all the cases, was monitored for nearly four half lives and the decay follows first order kinetics. The degradation rate in water is found to increase with, the increase in the temperature, pH and humic acid content. A similar effect of pH is observed in the soil. The (v) chapter IV also incorporates the degradation study of thiram on/in radish leaf, radish root and tomato fruit in field conditions. Pesticide solution of appropriate concentration was spotted on the desired parts of the plant which were then harvested for the analysis at various time intervals. The pesticide was extracted from the spotted part of the plant and analysed using HPLC as described in chapter II. Finally metabolic pathways of thiram in different matrices were presented by identifying the individual metabolites using LC-MS. The results indicate that the different metabolites are formed by N-dealkylation, hydrolysis, oxidation. However, different metabolites are observed with the change in the matrix or its characteristics. Chapter V describes the in vitro and in vivo studies of quinalphos in albino rats. The degradation of quinalphos in simulated gastric and intestinal phases was studied. The metabolic intermediates of quinalphos in simulated gastric and intestinal phases as well as. in blood serum and urine were identified at different time intervals after dosing the animals. All the samples were lyophilized, extracted and analysed by HPLC/GC-MS. In simulated in vitro studies some isomerisation derivatives have been identified which are missing in the blood and urine of the treated animals. The thesis concludes with a brief discussion on the findings of the present investigations. The proposed study will be helpful in assessing the safety aspects of use of these commonly used pesticides. The decay profiles give an idea as to when it is less harmful to consume the food articles sprayed with them. The systematic studies carried out on the degradation of these two pesticides would give a better insight in understanding the role of various environmental parameters in choosing this period. By identifying the different metabolites it may be possible to comment on the toxicity of the metabolites as compared to the parent pesticide. In addition to above, the studies carried out on albino rats are significantly important to indicate the behaviour of ingested pesticide in human systems. The results may be helpful to forensic scientists investigating cases of homicide and suicide.