The process of detoxification involves abiotic or biotic transformation of pesticides into relatively harmless substances, it has often been observed that toxicity of substances depends on its chemical structure and a slight change in which may result in radical changes in its properties.
Loss of toxicity naturally follows if active groups are detached and in many cases a little change in structure of the compound detoxifies it completely. Substances possessing highly branched structure, increased substitution, aromiatic compounds, cycloparaffins etc. are degraded with difficulty and hence their persistence. Degradation of pesticides in environment involves:
1. Abiotic transformation:
Abiotic transformation includes mainly photolysis, hydrolysis, cleavage of various types of bonds, adsorption by humid material and other colloids and formation of stable complexes within the medium. Solar radiations, presence of moisture, temperature, the chemical nature of the holmic material and other substances present in the medium etc. play an important role in abiotic transformation of poisonous substances in the environment.
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2. Biotic transformation:
Biotic transformation involves biochemical reactions brought about by enzymatic machinery of living organisms. Most of the pesticides are lipophilic compounds which though readily absorbed by living system cannot be excreted in an unchanged form.
They have to be acted upon by the enzyme systems in such, a way as to convert them into hydrophilic ones so that they could be excreted out. The process of bio-transformation, however, does not always result in detoxification. In certain cases products much more toxic than parent compounds are also formed. Important biochemical reactions which occur in a living system bring about a variety of transformations. These include the following:
(a) Degradation reactions:
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These reactions involve oxidation, reduction, hydrolysis, cleavage of important bonds etc. which result in displacement of important functional groups of the pesticide molecules by groups like -OH, -COOH, NH2 etc. Many pesticides loss their toxicity in the process.
(b) Conjugation reactions:
Conjugation reactions which usually follow degradation reactions involve synthetic reactions in which the pesticide molecules, now carrying groups acquired during the earlier reaction, are conjugated to other molecules within the living system to form highly ionized water soluble substances. These are quickly excreted from the living system.
Although all living organisms are capable of bio-degradation, tiny microbes such as bacteria, algae, fungi etc. play a very important role in decomposition of pesticides in the environment. They are able to perform diverse bio-chemical reactions at a much faster rate per unit body weight as compared to higher plants and animals while the diversity in their species composition ensures that a wide variety of pollutants shall be subjected to the degradation activity. Usually a number of species have to act, each performing its own specific task to bring about the complete decomposition of pesticide molecules. Therefore, rapid decomposition of pesticides in the environment depends upon:
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1. The presence of suitable microbes capable of performing the biochemical reactions involved.
2. Establishment of contact between the substrate and microbial species involved and induction in some cases of the enzymatic machinery required for the purpose.
3. Availability of favourable conditions of environment such as pH, temperature, moisture, nutrients etc. under which microbes may grow and perform their activity.
In most of the living organisms a battery of enzymes is located on endoplasmic reticulum which is a network of lipoprotein membranes within a cell. Microbial decomposition is brought about mainly by these enzymes. In higher animals including man the cells of the liver carry most of the function of degradation, though other cells are also capable of performing a certain degree of bio-degradation activity.