36 Degradation of Pesticides

Meenakshi Nandal

Objectives:

• To study about pesticide classification
• To about Pesticide Degradation Mechanisms
• To know about the factors affecting retention of pesticide in soil.

Introduction

Despite increasing competency and legislation restriction of pesticides, insecticides and fungicides (with decreasing applied amounts), environment is still contaminated. To mollify agricultural and environmental concerns, the fate of pesticides is to be understood in a better way, to ascertain the exposure and inevitably the impact on the target and non-target organisms. The continuous pesticide applications increase soil percolation rate through direct application or by foliage wash-off and a preponderant role of soil as a buffering agent is seen. The main bio-physico-chemical processes that sway the path for the fate of pesticides in soils are volatilization and retention that could lead to a temporary or permanent accumulation of pesticides in soils. They deduce the pesticide level in the soil solution and affect the pesticide percolation in ground or surface waters along with eco-toxicological aspects to soil organisms. The soil gets heavily contaminated, posing a threat to soil microbes and increase the burden on soil interfering with elements cycles and entering food chains. Pesticides are highly diverse in their chemical structures and reactivity and are also affected by the pedoclimatic conditions (soil temperature and water content) as this, influence the retention and degradation of pesticides. So, widespread use and release of these toxic pesticides cause environmental pollution and reduction in soil fertility, making it an issue of growing global concern. To combat these issues, sustainable agriculture is being considered as a priority issue in the 21st century with special attention to the most essential areas, environment protection and conservation of resources to be practiced through remediation of contaminated soils and aquatic bodies.

36.2 Classification of Pesticides

36.2.1 Based on their use

There is a large variety of pesticides designed to kill specific pests – those most widely used are listed below:

Insecticides (for controlling insects) such as carbamates, organophosphates and organochlorines. The insect repellents such as diethyltoluamide (DEET) and citronella (of natural origin) also includes in this category.

Herbicides used to control weeds (e.g. paraquat, glyphosate and propanil).

Fungicides used to control fungi: They are called wood preservatives so, applied to wood,

Rodenticides (to control mice, rats, moles and other rodents).

Fumigants At room temperature exist in gaseous or vapour form (examples cyanide, aluminium phosphate and methyl bromide) extremely toxic and cause rapid environmental dissemination and absorption in human or animal

Other pesticides include miticides (to control moths), algaecides (to control algae), and acaricides (to control ticks).

36.2.2 Based on their chemical nature

There are many classes of pesticides based on chemical nature. Major pesticide classes are given below:

Organochlorine pesticides: It consists of carbon atoms, chlorine, hydrogen and occasionally oxygen. They are soluble in lipids so, accumulate in fatty tissue of animals, are transferred through the food chain; toxic to a variety of animals, long-term persistent. Example: DDT, aldrin, lindane, chlordane, mirex.

Organophosphate pesticides: The organophosphate pesticides can be aliphatic, cyclic and heterocyclic possessing central phosphorus atom in the molecule. They are soluble in organic solvents and also in water. They infiltrate reaching the groundwater but are less persistent than chlorinated hydrocarbons. They are absorbed by plants, transferred to leaves and stems, which are then supplied to leaf-eating insects as feed. In insects they affect the central nervous system.

Example: Malathion, methyl parathion, diazinon

Organotin pesticides: Presence of tin as a central atom of the molecule.

Organosulphur pesticides: They have a sulfur central atom in the molecule, very toxic to mites or insects

Carbamates: The chemical structure is based on a plant alkaloid Physostigma venenosum. Carbamate acid derivatives kill a limited spectrum of insects, but are highly toxic to vertebrates with relatively low persistence. Example: Sevin, carbaryl

Thiocarbamates: They differ from carbamates in their molecular structure, containing an-S-group in its composition.

Pyrethroids: The compounds are similar to the synthetic pyrethrins (alkaloids obtained from petals of Chysanthemum cinerariefolium). They affect the nervous system; are less persistent than other pesticides; are the safest in terms of their use, some are used as household insecticides. Example: Pyrethrins.

Plant origin Products: They are derived directly from plants. Not chemically synthesized.

Biological: Only the Bacillus thuringiensis (Bt) and its subspecies, Viruses, microorganisms or their metabolic products are used. They are applied against forest pests and crops, particularly against butterflies and caterpillars. Example: Dispel, foray, thuricide.

Dinitrophenols: They are recognized by the presence of two nitro groups (NO2) bonded to a phenol ring.

Urea derivatives: The compounds include the urea bound to aromatic compounds.

Copper: Inorganic compounds of copper.

Diverse composition: Triazines, talimides, carboxyamide, trichloroacetic and trichloropicolinic acids derivatives, guanidines and naphthoquinones.

36.3 Pesticide Retention in Soil:

The retention of pesticides in soils mainly occurs due to the adsorption in solute (aqueous phase) is retained to a solid adsorbent surface (soil). The various processes possible for pesticide adsorption due to polar and ionizable groups present in soil are hydrogen bindings, polar interactions, Vander Waals dispersion forces ion exchanges, interactions with metallic cations, charge transfers and hydrophobic effects. Another process desorption which is reverse and is inversely related to adsorption. The various methods used for measurement of retention have been described below:

1. a) Batch experiments– Generally done by soil suspensions that retain the pesticides in accordance with OECD 106 guidelines. By this method the degradation or the adsorption of the pesticide on the surface of flask has to be determined. The quantity of pesticide retained in the soil is calculated as the difference between initial concentration of pesticide in solution and supernatant concentration that has been centrifuged. So, smaller the value of concentrationhigher is the amount of pesticide adsorbed per unit mass of soil

However, this process has a drawback as dispersion of soil structure occurs due to intensive shaking of soil-pesticide causing higher sorption sites availability leading to overestimation of pesticide adsorption.

b) Centrifugation – The sample treated with pesticide is incubated and centrifuged for collection of soil solution proceeded by direct analysis for pesticide concentration.

c) Filters– This method was developed majorly for undisturbed soils. In this method 3-5mm thick soil layer is developed by soil remolding at controlled water content. With help of glass microfiber filters, soil solution volume and dissolved pesticide concentration is measured on soil surface.

d) Soil columns – Under dynamic conditions water flow is applied through rainfall simulation or pressure head control on a soil column with pesticide application. Determination of sorption coefficients can be done by inverse modeling of the elution curves and retardation factor calculation. Vertical distribution of the pesticide residues in the soil core could be estimated by such column experiments when leachate recovery is incomplete.

e) Lysimeters – A cylinder with undisturbed soil block placed in an inert container with a permeable bottom to drainage water or leachate. To represent crop practices under field climatic conditions this method is known for its efficacy to monitor mass fluxes of water and chemicals as well as their distribution, metabolites and transformation rates. The outdoor lysimeter studies show better results due to natural environmental- field conditions with minimum disturbance in the soil structure but has a major drawback of variations caused by uncontrolled pedoclimatic conditions.

36.4 Pesticide Degradation in Soil

The Pesticides present in soil degrade in the environment through physical, biological and chemical processes. The biodegradation of pesticides is influenced by soil properties such as pH, structure, moisture, temperature, organic matter content and the soil micro biota microbes.

Microbial degradation and microbial detoxification has been reported as the most promising and cost-effective method for the organic pollutants removal, detoxification of pesticides and remediation of soil in the environment. The biotic transformation of pesticides is mediated by the micro biota, while abiotic conversion occurs through chemical and photochemical processes and reactions. The structure and the environment conditions help in specific degradation reactions for a particular pesticide. The redox gradients in soils, sediments and aquifers occasionally are used to deduce the type of transformations that can occur. In a similar way, the photochemical transformations require sunlight that is available only on the surface layers of lakes or rivers, plant or sub soil layers. Also, accumulation of biodegradation transformation intermediates occurs when the enzymes reaction rate decreases, affecting the production of the intermediates.

Pesticide degradation can occur by various mechanisms; significant role played by physical, chemical, and biological agents in insecticide, herbicide and fungicide transformations. Many soil applied pesticides are degraded more rapidly following repeated application at the same site. The Transformation processes are oxidation, reduction, conjugation, hydrolysis, isomerization, hydration and cyclization. The molecules of these are degraded to various resultant products that generally show lesser bioactivity as compared to parent pesticide, but with exceptional cases metabolites showing greater bioactivity have also been observed. There are structural changes that alter the physical, chemical properties and significance of the degradation products.

The introduction of “environmental activation” concept is done mainly to explain significance of the pesticide transformation into a degradation product in the environment as a result of its environmental toxicology or chemistry.

As the pesticides begin to synthesize and purify they begin to degrade. The process of formulation can initiate decomposition of active ingredients at a minor rate. The breakdown may also occur due to harsh environmental conditions while the products are stored and shipped. The pesticide product may also degrade due to chemical interactions among parent molecules, water or other pesticides when prepared as a tank mix. They are attacked by detoxification enzymes in an organism on target and non-target sites. A part of pesticide applied always remains in environment as residues in soil, water, and air that are subject to transformation by organism uptake and moving to a different location.

Chemically pesticides can be classified in various ways, but majorly they are grouped considering their chemical composition to develop a correlation between structures, toxic activity

and degradation mechanisms in a uniform and scientific way. Although pesticides are benign in regulating the growth of pests but depending on the toxicity of pesticides and degree of sensitivity of organisms, their uncontrolled and indiscriminate usage cause adverse effect to human health, life forms, to the ecosystems and its increased quantity in soils and waters could cause entry to the food chains called biomagnification. Also, pesticides can damage living organisms as they are rapidly soluble in fatty layer and bioaccumulation in non-target organisms. Some of the environment persistent and least biodegradable pesticides (like organochlorines), in many countries their use is still on rise despite of their banning.

36.5 Processes of Pesticide Degradation

The combination of physical, chemical and biological processes generally affects the breakdown of a pesticide chemical, over a time period. The pattern of chemical use, physical and chemical structure is the factors that affect the agents of transformation. Light and heat are the two primary physical agents implicated in the process of degradation. The Photolysis of pesticide residues has significant impact on vegetation, in water, soil surface and in the atmosphere· Photolysis and thermal decomposition of the chemicals is directly affected by sunlight and degradation is accompanied by photo-degradative reactions. In cold/freezing conditions suspension or microencapsulation are the major processes of pesticide degradation. The main factor responsible for breakdown of pesticides in solution is water along with pH extremes. Even minor variance in pH can initiate rapid decomposition of pH sensitive compounds. Various oxidation products are generated from molecular oxygen and its other reactive forms (e.g., ozone, superoxide, peroxides). Various processes involved in pesticide degradation significant degradation occurs by biological agents such as microorganisms (bacteria, fungi, actinomycetes) depending on their prevalence in environment and on the proneness of pesticides collection in the media. The three major degradation strategies exhibited by microbes are:

1. Co-metabolism– the process of biotransformation of a pesticide molecule coinciding with the normal metabolic functions of the microbial processes of life i.e. growth, reproduction and dispersal.
2. Catabolism– the process of utilization of an organic molecule as an energy source or nutrient. In some cases carbon or nitrogen is used as a sole source of energy causing enhanced microbial disintegration. The relative ease of hydrolysis, the nutritional value of the hydrolysis products and the toxicity in soil are the major factors affecting the resistance of a chemical to EMD.

3. Enzymatic– The enzymes are the digestion substrates e.g., phosphates and amidases and may persist in the soil even after death of the parent microbial cells. The extracellular enzymes can aid the degradation process with capability of biochemical catalysis. Among other biological degradation agents like plants, invertebrates and vertebrates latter possess the most advanced enzymatic arsenal for biotransformation of xenobiotics and also highest rates of detoxification and elimination.

Biodegradation

There are three phases of enzymatic reactions that results into biodegradation of pesticides in living beings:

Phase 1– functional group reactions (oxidation, reduction and hydrolysis) of the parent compound by involvement of additional functional groups such as OH, NH2, SH and COOH resulting in formation of physically and biologically modified metabolites.

Phase 2 – conjugation process resulting in metabolite formation by linking of activated metabolite with cell constituents that are excreted or sequestered by organisms.

Phase 3 – oligomerization process among several parent compound or conjugation reactions between them resulting in formation of high-molecular weight compounds that are incorporated and stabilized within the cells. The hydrophilic property of metabolites increases with decrease in mobility and toxicity as the various phase proceeds except in the case of polymers that are insoluble.

Transformation

There are three basic types of reactions occurring: degradation, synthetic and rearrangements. The organic pesticides undergo various reactions before complete degradation and the biological activity is altered by just one or two frequent transformation generally by four fold detoxification strategies. To facilitate elimination of the xenobiotic molecule it is transformed into a more water-soluble and polar molecule like oxygen or oxygen containing moieties like sugars, phosphate, sulfates and amino acids secondly, the functional groups are altered in an attempt to render the less toxic molecule (e.g., amines and sulfhydryls). In the third approach breakdown to decrease the toxic impact of the chemical element into two or more fragments occurs. Additionally, the component parts of the molecule after the breakdown process are used as nutrients for microbial catabolism.

Figure 36.1: Various processes involved in pesticide degradation

Source: www.intechopen.com

36.6 Factors controlling the retention of pesticides in soils: The physico-chemical properties

of pesticides as described below are controlling their retention in soils.

Surface, volume, and branching: The adsorption of pesticides is directly proportional to volume, solubility and degree of branching which encodes the intermolecular accessibility and is correlated to the surface area (Mamy & Barriuso, 2005; Sabljic et al., 1995).

Electronic structure: The electronic structure of pesticide is determined by nature of atoms and functional groups that govern the soil-pesticide interaction. Adsorption is governed by molecular substitutions; spatial arrangement and it influences the reactions between functional groups.

Ionization: The charge of chemical determines ionization that depends on pH of soil, pKa or pKb of molecule in case of weak acids. Generally, the negatively charged surfaces like clays, hydroxides, humic substances and oxides show strong cationic absorption while anions have high absorption in soils with positive charges e.g. tropical soils.

Hydrophilic/hydrophobic balance: The adsorption of pesticides increases with their hydrophobicity and decreases when their increases in water solubility because of their higher affinity for the water phases. However, it may also depend on the hydrophilic/hydrophobic balances of the soil adsorbent.

36.7 Examples of pesticide biodegradation

Structural changes or absolute degradation of pesticide is being carried out by microorganisms which chemically and physically have the ability to interact with substances. Bacteria, fungi, and actinomycetes among the microbial population plays important role in pesticide degradation. Pesticides and other xenobiotics get bio transformed by fungi while introducing minor structural changes to the molecule, depicting it harmless. Moreover applying enzymes to transform or degrade pesticides is an excellent treatment technique for removal of these toxic chemicals from polluted environments. Pesticide may be more effectively degrade by enzyme- catalyzed reaction than existing chemical methods.

Phosphotriesterase belongs to Pseudomonas diminuta MG shows high catalytic activity towards organophosphate pesticides. Flavobacterium ATCC 27551 contains the opd gene that encodes a PTE (Phosphotriesterase enzyme) (Latifi et al. 2012). These enzymes specifically hydrolyze phosphoester bonds, such as P–O, P–F, P–NC, and P–S, and the hydrolysis mechanism involves a water molecule at the phosphorus center.

Fig. 36.2 Parathion degradation pathway; A) and B) Aerobic pathways of parathion degradation. C) Parathion degradation under anaerobic conditions

Source: L. Ortiz-Hernández et al. 

Based on the metabolic products formed, the degradation pathway for chlorpyrifos by the strain Cladosporium cladosporioides was proposed by (Gao et al. 2012). Specifically, the fungus is capable of using chlorpyrifos as the sole carbon source, it is first metabolized by hydrolysis to produce 3,5,6-trichloro-2-pyridinol (TCP) and diethylthiophosphoric acid (DETP). The hydrolysed product TCP was further transformed by breakage of ring, resulting in its complete detoxification.

A  number   of   bacteria   capable   of   degrading   carbofuran   from   the   environment   such   as

Pseudomonas, Flavobacterium, Achromobacterium, Sphingomonas, Arthrobacter. Carbofuran extensively used in agriculture is one of the pesticides belonging to the N-methylcarbamate class.

It has been classified as highly hazardous and exhibits high mammalian toxicity. Carbofuran was degraded by Sphingomonas sp.  first to carbofuran phenol than to 2-hydroxy-3-(3-methylpropan-

2-ol) phenol (Kim et al.,2004) and Arthrobacter sp. (De Schrijver & De Mot, 1999).

Fig. 36.4 Biodegradation of carbofuran by Sphingomonas sp. Source: Kim et al.,2004

Summary:

We studied the pesticide effect on soil and classification of pesticides based on their use and structure.

Breakdown of pesticide in the soil can take place by chemical, biological processes and also by physical factors leading to very complex process.

The decomposition in the absence of living organisms is activated by any of the methods thermal, photochemical, radiochemical, electrochemical and also by the interaction with the soil.

We studied the mechanism involved in pesticide degradation

We studied the agents affecting the pesticide degradation mechanisms. We studied the major degradation strategies exhibited by microbes

The pesticides degradation takes place in most cases on or near to the soil surface in the presence of oxygen ,light and microorganisms for this reason the residence time of theagent in this active layer is a determinant factor.

Risk assessment of pesticides requires information on the toxicological and ecotoxicological properties of these compounds as well as on their levels in food and environmental compartments.

References:

• Abo-Amer A.E. (2012). Characterization of a strain of Pseudomonas putida isolated from agricultural soil that degrades cadusafos (an organophosphorus pesticide).World J. Microbiol. Biotechnol. 28, 805-814.
• Chen S., Liu C., Peng C., Peng C., Liu H., Hu M. and Zhong G. (2012). Biodegradation of chlorpyrifos and its hydrolysis product 3,5,6-trichloro-2-pyridinol by a new fungal strain Cladosporium cladosporioides Hu-01. PLoS ONE 7, 1-12
• De Schrijver, A. & De Mot, R. (1999). Degradation of Pesticides by Actinomycetes, Critical Review in Microbiololy, Vol. 25, No. 2, pp. 85-119, ISSN 1040-841X
• Franzmann, P.D., Zappia, L.R., Tilbury, A.L., Patterson, B.M., Davis, G.B., Mandelbaum, R.T. 2000. Bioaugmentation of Atrazine and Fenamiphos Impacted Groundwater: Laboratory Evaluation, Bioremediation Journal, 24(3), 48-68.
• Gao Y., Chen S., Hu M., Hu Q., Luo J. and Li Y. (2012). Purification and Characterization of a Novel Chlorpyrifos Hydrolase from Cladosporium cladosporioides Hu-01. PLoS ONE 7, 1-8.
• Gao Y., Chen S., Hu M., Hu Q., Luo J. and Li Y. (2012). Purification and Characterization of a Novel Chlorpyrifos Hydrolase from Cladosporium cladosporioides Hu-01. PLoS ONE 7, 1-8.
• Gupta, R. C. 2006. Toxicology of Organophosphate & Carbamates Compounds, Academic Press.
• Johnsen, R.E. 1986. DDT Metabolism in Microbial Systems, Pesticide Reviews, 61, 1-28.
• Kim, I.S.; Ryu, J.Y.; Hur, H.G.; Gu, M.B.; Kim, S.D. & Shim, J.H. (2004). Sphingomonas sp.
• Latifi A.M., Khodi S., Mirzaei M., Miresmaeili M. and Babavalian H. (2012). Isolation and characterization of five chlorpyrifos degrading bacteria. Afr. J. Biotechnol.11, 3140-3146.
• Strain SB5 Degrades Carbofuran to a New Metabolite by Hydrolysis at the Furanyl Ring, Journal of Agricultural and Food Chemistry, Vol. 52, No. 8, (April 2004), 2309- 2314, ISSN 0021-8561.