Bioremediation Of Pesticide Contaminated Soil

 

Bioremediation Of Pesticide Contaminated Soil

Abstract

The global population is increasing rapidly and there is a lot of pressure on the agricultural sector to feed this large population. The agriculture sector is taking every measure to ensure that it meets the ever increasing demand for agricultural products. To ensure crop safety several steps are taken by individual farmers and corporations. One of them is to use pesticide to protect the crop from pests. Pesticides are chemicals used to control pests and maximize the yield and quality of crops. They are used to kill, repel and prevent pests as they are made up of various toxic chemicals. Hence the global usage of pesticides has increased exponentially. They are classified into different categories on the basis of their origin and the type of pest they target. Most commonly used pesticides are insecticides, herbicides  and fungicides.

 

Moreover  these chemicals show adverse effects on the environment, such as reduced population of insect pollinators and a threat to endangered species and habitat of birds when uses in excess. The excessive use of these chemicals and their persistence in the environment causes serious problems namely pollution of soil, water and to a lower extent air causing harmful effects to the ecosystem and along the food chain. some studies demonstrated that soil is an environmental sink for pesticides and acts as a secondary pollution source for environmental compartments such as ground water and surface water also it will make some  changes in soil microbiological and biochemical process. The removal efficiencies of these processes depend on the type of pollutant and the chemical and physical conditions of soil. The toxicity of pesticides depends on the electronic properties and the structure of the molecule, dosage and exposure times.  So pesticides are now major pollutant and their proper disposal and degradation  are matters of great concern. The pesticide can be degraded  and transformed into one or more metabolites through photochemical, chemical, and microbiological processes.

 

Introduction

Bioremediation has become a common method for removing pesticide residues from soil and thus reducing their toxicity.  chemical and physical pesticide remediation is largely discouraged because of unsustainably in terms of application difficulties, high cost and poor environment safety. Compared to sophisticated engineering based, expensive and toxic methods, bioremediation is an inexpensive and eco friendly approach. Bioremediation consist of the use of organisms, mainly microorganisms and plants to facilitate transformation and degradation of environmental pollution. The removal can be achieved by exploiting the microbial activity of microorganism. The microorganisms  will transform pesticides into less complex compounds, carbon dioxide, water, oxides or mineral salts, which can be used as carbon, mineral and energy source. Microorganism from diverse taxonomic groups such as bacteria, fungi, and algae are known to effectively metabolize pesticides and alter their chemical structures facilitating their degradation and making the contaminated environment free from harmful pesticides residues.

 

The pesticides are classified into different categories based on their chemical classes, functional groups, mode of action, and toxicity. Chemical and biopesticides are classified by their origin. The pesticides like organochlorines, organophosphate, carbomates , pyrethrin and pyrethroids are classified by their chemical composition. Some pesticides are classified by the role that they play and the type of pests that they attack. Insecticides, herbicides, rodenticides, bactericides and fungicides are comes this category.

BIOREMEDIATION TECHNIQUES

Bioremediation techniques can be divided into two categories depending on where the remediation treatment is done, namely 1. In situ, 2.ex situ

1.In the in situ technique the treatment will be done in the contaminated site and usually , the process is aerobic. The main in situ techniques are natural attenuation, bioaugmentation, biostimulation, bioventing and biosparging. 2. in the ex situ methods the contaminated  soil is removed from polluted sites and transported to other places for treatment. Bioreactors, composting, landforming and biopiles are ex situ treatments.

Process of biodegradation

During biodegradation process pesticides are transformed into degradation products which use the pollutant compound as nutrients for their metabolic activity. enzymes such as hydrolases, peroxidases and oxygenases  are plays an important role in biotransformation mechanism. The pesticide degradation process are divided into three phases. In first phase the pesticides are transformed into more water soluble and less toxic products through oxidation and hydrolysis reaction. In phase two the phase one products  are converted  into sugars and amino acids which have higher water solubility and lower toxicity. In phase three the conversion of the phase 2 metabolites into less toxic secondary  conjugates .

Bacterial degradation

Bacteria mediated pesticide bioremediation is a major approach for the decontamination of pesticide polluted sites. Several bacterial strains are identified as capable of degrading the pesticides present in the soil. Several bacterial genera such as Pseudomonas, Bacillus, Actinobacter, Acinetobacter, Burkholderia are effective for pesticide bioremediation of diverse chemical classes. For example Novosphingobium sp for the bioremediation of carbofuran and degradation of 2,4-D herbicide using bioaugumentation approach. And widely used endosulfan removed by applying Pseudomonas sp and DDT by Chryseobacterium sp. The interesting factor here is most of the potential bacterial strains are isolated from soil. So it is indicating the cost effectiveness of the bacterial bioremediation  of pesticides. Microbial consortium isolated from biomixture is efficient in pesticide bioremediation in liquid medium. Identified bacteria from microbial consortium support the fact that pesticide bioremediation in bio beds involved a diversity of microorganism, being bacteria part of them. Each bacterium has a specificity , each bacterial strain can’t degrade every pesticide. For example the experiment type, reaction condition and  result will be vary based on the pesticide going to degrade and the bacterial organism used for biodegradation. for example Acinetobacter used for bioremediation of Diazinon and here batch trial experiment are used and the result is 88.27% of degradation. And also they can’t perform in every environment. Their role, development, and adaptation will be depend on some operative condition such as temeperature,Ph, water content, and type of pollution.  The major problem in bacterial bioremediation is, during the degradation process  the metabolites of those pesticides can form additional problems in environment. Thus they are more difficult to remove than the original compound. But few microorganism can degrade both the pesticide and their metabolites. Example: Ochabactrum sp.

Fungal degradation

Fungi  can be better degraders than bacteria due to their characteristics such as specific bioactivity, growth morphology and high resistance even at high concentration of pollutant.  Certain fungi and algae are considered as potential weapons for the degradation of pesticides in soil. Aspergillus sp, Boletus edulis, Laccaria sp are most commonly used fungal species used in bioremediation of pesticide contaminated soil. Fungi use organic material (pesticides) as their carbon sources and their long mycilial network cover significant  rhizosperic area to biodegrade various pesticides. Same as bacterial degradation Each fungi has a specificity , each fungal strain can’t degrade every pesticide. For example the experiment type, reaction condition , and  result will be vary based on the pesticide going to degrade and the bacterial organism used for biodegradation. for example Aspergillus sp used for bioremediation of Endosulfan and here 12 day lab inoculation are used and the result is 100% of degradation. Using various types of microbes in the consortium can symbiotically assist in the complete degradation of many pesticides. However the enzymatic catalysis of pesticides by fungal enzymes is a significant role in bioremediation mechanism.

Enzymatic pesticide degradation

Bacteria and fungi secrete various functional enzymes during the biodegradation approach. This can be effective in the bioremediation of pesticides and their toxic metabolites. They produce various bio catalytic enzymes. monoxygenases, oxidoreductases, dehalogenases, dioxygenases are effective  bio catalytic enzymes for the bioremediation of various pesticides. For example  monooxygenases derived from Arthrobacter sp are effective in bioremediation of alderin, endosulfan and malathion. And also Laccases are an important class of enzymes that can be derived from many bacterial and fungal species. Their unique structure assist the catalysis of various toxic pollutant such as humic acid, xenobiotics, polycyclic aromatic hydrocarbons, pesticides and poly phenol compounds. The main metabolic reactions are oxidation, hydrolysis reduction and conjugation. Oxidation is the first step of degradation of pesticides, consist of the transfer of an electron from reductants to oxidants. Oxygenase and laccase enzymes may be involved in this reaction. Then hydrolysis permits the cleavage of bonds of the substrate by adding hydrogen or hydroxyl groups from water molecules. The pesticide molecule are thus divided into smaller chain compounds than the original ones. Typical enzymes involved in the hydrolysis pathways are lipase, esterase and cellulase. The next process is reduction which permits the transformation through reductive enzymes. Then the final process is conjugation it involves the addition of exogenous or endogenous natural compounds to facilitate the mineralization of pesticide. The green catalytic degradation of pesticides and their toxic metabolites is preferable to microbial degradation because of its simplicity in application and prompt effectiveness. Thus enzymatic biodegradation is emerging as a white biotechnology toolkit for the degradation of pesticides contaminated soil.

Use of earthworms in the bioremediation of pesticide contaminated soil

Earthworms are used to degrade the pesticide either directly through the release of pesticide detoxifying enzymes in their gastrointestinal tract, or indirectly through stimulation and dispersion of microorganisms that are capable of degrading pesticides. However earthworm mediated degradation of pesticides is limited to the luminal microenvironment of the gastrointestinal tract and to the soil under their influence. Therefore not all earthworm species will have a similar impact on pesticide degradation, which will depend on the feeding habits and microhabitats of the three ecological groups of earthworms. Theoretically endogeic and aneic earthworm will have a greater impact on pesticide degradation than epigeic earthworms, because the former consume greater amounts of soil and display more intense burrowing activity than the latter. They stimulate microbial proliferation, therefore increasing the biodegradation capacity of contaminated soil. Earthworm activity can increase pesticide bioaccessibility to microbial degradation. Although the mutualistic relationship between earthworms and symbiont microorganisms is key in the degradation of pesticide residues, earthworm actively also creates external hot spots of microbial activity in the casts, middens and burrow walls. For example the endogeic species Aporrectodea caliginosa stimulated the proliferation of alphaprobacteria which actively degraded the herbicide 2-methyl-4-chlorophenoxyacetic acid in the casts and burrow walls, but apparently not in the gastrointestinal content. The most important thing is earthworms are sensitive to pesticide is a key point in the efficacy of bioremediation

Conclusion

The bioremediation of pesticides and other hazardous synthetic agrochemicals using potential microorganisms and their functional enzymes is a promising sustainable approach for the reclamation of contaminated sites and farmlands, and also earthworm can degrade and decrease the concentration and toxicity of pesticide residues in soil. Bioremediation is comparatively cost effective, eco friendly and non destructive compared to physical and chemical remediation methods. However the microbial and enzymatic bioremediation of pesticides and their toxic metabolites has exhibited several limitations such as optimal growing conditions, inconsistent performance, slow adaptation with the adverse environment and specificity to pesticides. In many countries significant land area is contaminated with pesticides that are of concern. Soil quality is very important  for agricultural production, and improving  productivity is a high priority because of the need for food, feed and other products. The regulation on the use of pesticides is strictly connected to their environmental impacts. Now a days  every country  adopt regulations to restrict the consumption of pesticides, prohibit the most harmful ones and define the admissible concentration in the soil.