Transformation of pesticides in the environment

1. Transformation of Pesticides in the
Environment
Dr. Neethu Narayanan
Agricultural Chemicals
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2. Transformation of pesticides
• Once released into the environment, pesticides are subject to processes
of abiotic and biotic transformation, often inducing the formation of
stable compounds
• Most important mechanisms for detoxifying pesticides in soils
• They differ in
– Environmental behaviour (persistence, mobility, degradability)
– (Eco)toxicological profile from the starting molecule
• Generally pesticide transformation products will have a lower toxicity to
biota than the parent compound
• However, in some instances a transformation product may be more toxic
Eg: Transformation of Aldrin to Dieldrin
Aldrin: Rat acute oral LD50 = 67mg/kg
Dieldrin: Rat acute oral LD50 = 37mg/kg
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3. Pesticide
transformation
Abiotic
Chemical
Oxidation Reduction Hydrolysis
Photochemical
Direct Indirect
Biotic
Microbial
Classification of transformation of pesticides in the environment
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4. An overview of transformations of pesticides in the
environment
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5. Oxidation
• Oxidation of pesticides is a reaction process whereby the dissolved
oxygen in the environment reacts with pesticides
• Oxidation process can also be achieved by singlet oxygen, ozone,
hydrogen peroxide or hydroxy radicals
• Hydroxy radical (.OH) are the primary agents that bring about
chemical oxidation of pesticides in water or atmosphere
• The radical can be formed from either the pesticides or from other
molecules in the environment
• Eg: p,p’-DDT undergoes both oxidation and reduction in soil
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6. • Reduction of pesticides is a chemical reaction in which the substrate
(pesticide) undergoes a reduction in oxidation state
• The reducing agents in the environment are usually +H
• Eg: Malathion undergoes a reduction reaction in acidic aquatic
environment which proceed by the substitution of one of the ethyl
group with +H resulting into the formation of two functional isomeric
molecules of malathion monoacid at the end of one half life. However,
malathion diacid would be the product at extended reaction time
Reduction
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7. • Hydrolysis is a pH dependent reaction in which pesticides react with
water (i.e. Hydrogen ion and hydroxy ion)
• Hydrolysis is one of the most common reactions that most pesticides
undergo in the environment
• Most organophosphates and carbamates have particularly shown to
be highly responsive to hydrolysis reaction under alkaline condition
• A pesticide that is very soluble in water will tend not to accumulate in
soil or biota because of its stronger polar nature and it will degrade via
hydrolysis which is the reaction that is favoured in water
• Eg: Hydrolysis of atrazine in water
Hydrolysis
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8. Photolysis/ Phototransformation
• Chemical decomposition induced by light or other radiant energy
• Involves the breakdown of organic pesticides by direct or indirect
energy from sunlight
• Photolytic reactions occur near the surface of the ground (in the top
few hundredths of an inch) or near water surfaces, where light can
penetrate
• Soil photolysis is a measure of the degradation rate of pesticides in
surface soil due to light decomposition
• Water photolysis is a measure of the degradation of pesticides in
surface water due to light decomposition
• The absorption maxima of 240–310 nm in UV region is mandatory to
exhibit photodegradation of OPs
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9. Photolysis of parathion
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10. Direct photodegradation
• Most pesticides show UV–Vis absorption bands at relatively short UV
wavelengths
• Since sunlight reaching the Earth’s surface (mainly UV-A, with varying
amounts of UV-B) contains only a very small amount of short wave length
UV radiation, the direct photodegradation of pesticides by sunlight is
expected to be, in general, of only limited importance
• Based on the absorption of light by a molecule
• This can then transfer energy from its excited state to the pesticides, that
can then undergo various reactions as following direct photodegradation
• Advantage: Possibility of using light of wavelengths shorter than those
corresponding to the absorption band of pesticides
Indirect photodegradation
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11. Microbial degradation
• The transformation process that results when soil microorganisms
(bacteria and fungi) either partially or completely metabolize (break down)
a pesticide
• The rate of microbial degradation of pesticides increases in soil which is
subjected to one or more previous applications of the same pesticide or
another pesticide with a similar chemical structure. This phenomenon is
known as accelerated or enhanced degradation
• When exposed to pesticides, microorganisms of the receptor ecosystems
can be eliminated (sensitive strains), unaffected (indifferent strains) or can
be selected (user strains)
• This last class is particularly interesting as it includes microorganisms
able to biotransform or degrade the contaminant
• These shifts in microbial communities can lead to environmental
perturbation and to an alteration of the ecosystem functioning
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12. Pesticides can follow different pathways depending upon the
microorganisms present
Eg: Microbial degradation of 2, 4-D
Pathway a : Bacteria Flavobacterina and Arthrobacter Sp.
Pathway b : Fungus Aspergillus niger
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13. Aerobic
• Normally transformed into carbon dioxide and water
• Process of microbial degradation that takes place in the root zone of
soils and surface water
Anaerobic
• Microorganism degradation may produce additional end products
such as methane
• Microorganisms using anaerobic metabolism for breaking down
pesticides are typical in: waterlogged soils in terrestrial systems,
bottom sediments of ponds, lakes, and rivers, groundwater
Types of microbial metabolism
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14. Microorganism Pesticide
Place of
isolation Reference
Bacteria
Ochrobactrum sp. Methyl parathion Soil Qiu et al., 2006
Arthrobacter sp. Endosulfan Soil Weir et al., 2006
Sphingomonas spp. Isoproturon Soil
Bending &
Rodríguez, 2007
Burkholderia sp. Fenitrothion Soil Hong et al., 2007
Sphingomonas sp. Chlorpyrifos Wastewater Li et al., 2007
Enterobacter spp. Chlorpyrifos Soil Singh et al., 2004
Acinetobacter radioresistens Methyl parathion Liu et al., 2007
Ochrobactrum sp.,
Castellaniella sp., Variovorax
sp., Pseudomonas sp.,
Psychrobacter sp.
Igepal CO-210
Igepal CO-520
Sewage
sludge
DiGioia et al., 2008
Pseudomonas
frederiksbergensis
Dimetoate,
Malathion Soil
Al-Qurainy &
Abdel-Megeed, 2009
Organisms isolated that degrade pesticides (Laura and Enrique, 2011)
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15. Microorganism Pesticide
Place of
isolation Reference
Bacillus pumilus Chlorpyrifos Soil Anwar et al., 2009
Bacillus sp. Mesotrione Soil Battison et al., 2009
Serratia liquefaciens, Serratia
marcescens, Pseudomonas
sp.
Diazinon Soil Cycón et al., 2009
Enterobacter aerogenes
Bifenthrin
Fenpropathrin
Cypermetrine
Sewage
sludge Liao et al., 2009
Pseudomonas putida,
Burkholderia gladioli. Prophenofos Soil Malghani et al., 2009
Stenotrophomonas sp. DDT Soil Mwangi et al., 2010
Providencia stuartii Chlorpyrifos Soil Rani et al., 2009
Pseudomonas putida Propiconazole
Tea
rhizosphere Sarkar et al., 2009
Micrococcus sp. Diuron
Diuron
storage Sharma et al., 2010
Sphingobium sp.
Methyl parathion
Fenpropathrin
Sewage
sludge Yuanfan et al., 2010
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16. Microorganism Pesticide
Place of
isolation Reference
Fungus
Aspergillus niger Endosulfan Soil
Bhalerao & Puranik,
2007
Ganoderma australe Lindane
Pinus pinea
stump Rigas et al., 2007
Trichosporon sp. Chlorpyrifos
Sewage
sludge Xu et al., 2007
Verticillium sp. Chlorpyrifos Soil Fang et al., 2008
T. versicolor (R26) Atrazine Soil
Bastos & Magan,
2009
Aspergillus sydowii, Bionectria
sp., Penicillium miczynskii,
Trichoderma sp.
DDD
Marine
sponge
Ortega et al. 2010
Algae
Chlorophyceae sp.,
Scenedesmus spp.,
Chlamydomonas sp.,
Stichococcus sp.,
Chlorella sp.,
Cyanobacteria
Nostoc spp.
Fenamiphos Soil Cáceres et al., 2008
Anabaena sp. Fenamiphos Water Cáceres et al., 2008
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17. Mechanisms of microbial degradation
• Mineralization
– Conversion of a pesticide to inorganic compounds such as CO₂,
H₂O and inorganic ions
– Support microbial growth using the pesticide as carbon and
energy source
• Cometabolism
– A microbial population growing on one compound may
fortuitously transform a contaminating chemical
– does not directly provides carbon and energy for microbial growth
during degradation process
– Also called co- oxidation or gratuitous or fortuitous metabolism
– Usually the primary substrate induces production of enzymes that
alter the molecular structure of pesticides
– This transformation may result in minor modification of the
molecule or may lead to incomplete or even complete degradation
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18. Major microbial degradation reactions
• Hydrolysis : major critical reaction in pesticide degradation
Eg: Carbofuran to carbofuran phenol
• S oxydation: S Sulfoxide Sulfone
Eg: Aldicarb to aldicarb sulfoxide then to aldicarb sulfone
• Epoxidation: Addition of oxygen to double bond
Epoxides are recalcitrant to microbial attack
Persist in the environment for a long time
Eg: Heptachlor to heptachlor epoxide
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19. • Dehalogenation:
Reductive dehalogenation: replacement of halogens by H
Eg: DDT to DDD
Hydrolytic dehalogentaion: replacement of halogens by OH
• Nitro reduction: Reduction of NO₂ to NH₂
Eg: Parathion to amino parathion
• Replacement of S with O
Eg: Parathion to paraoxon
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20. Factors affecting microbial degradation
• Chemical structure and concentration of compound being degraded
• Type and number of organisms present
• Soil water content:
 Waterlogged conditions in combination with high nutrients
promote anaerobic microbes
 Extremely dry conditions limit microbial conditions
• Acidity: Acidity depresses the growth of bacteria than fungi
• Nutrients: Addition of exogenous nutrients increase microbes
• Temperature :
 Higher temperature doesn’t promote microbial growth
 Certain microorganisms are well adapted to colder climates
 Microbial degradation decreases dramatically below 10°C and ceases
to operate at temperatures below 5°C in all environments
• Faster degradation in tropical regions than temperate regions
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21. Main environmental degradation routes for top 10 pesticide
classes (Fenner et al., 2013)
Pesticide class
Major representative
active
substance and structural
motif
Major use
category
Percent
of global
pesticide
use
Main
environmental
degradation
route
Dithiocarbamates
Mancozeb
Fungicides 7.1 Acid-catalyzed
hydrolysis
Organophosphates
Chlorpyrifos
Insecticides 6.7 Microbial
transformation
(oxidation and
hydrolysis)
Phenoxy
alkanoic acids
2,4-D
Herbicides 4.7 Microbial
transformation
(oxidative
dealkylation and
aromatic ring
cleavage)
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22. Pesticide class
Major representative
active
substance and structural
motif
Major use
category
Percent
of global
pesticide
use
Main
environmental
degradation
route
Amides
S- Metalachlor
Herbicides 4.2 Microbial
transformation
(hydrolysis and
glutathione
coupling)
Bipyridyls
Diquat
Herbicides 3.2 Only very slowly
biotransformed
due to strong
sorption to soil
matrix
Triazines
Tributhylazine
Herbicides 2.3 Microbial
transformation
(oxidative
dealkylation and
hydrolysis)
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23. Pesticide class
Major representative
active
substance and structural
motif
Major use
category
Percent
of global
pesticide
use
Main
environmental
degradation
route
Triazoles, diazoles
Propiconazole
Fungicides 2 Slow microbial
transformation
(oxidation);
phototransformation
of
specific
representatives
Carbamates
Pirimicarb
Insecticides/
Herbicides
2 Ready microbial or
base-catalyzed
transformation
(hydrolysis of ester
bond);
phototransformation
of specific
representatives
Urea derivatives
Isoproturon
Herbicides 1.7 Microbial
transformation
(oxidative
dealkylation and
hydrolysis
Pyrethroids
Cypermethrin
Insecticides 1.3 Microbial
transformation
(hydrolysis,
oxidation);
phototransformation
(direct and indirect)
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24. Classification of compounds based on persistence
Low Persistence
(T1/2 < 30 days)
Moderate
Persistence
(T1/2 30 - 100
days)
High Persistence
(T1/2 > 100day)
Aldicarb Aldrin Bromacil
Captan Atrazine Chlordane
Dalapon Carbaryl Lindane
Dicamba Carbofuran Paraquat
Malathion Diazinon Picloram
Methyl Parathion Endrin Trifluralin
Permethrin Parathion DDT
2, 4-D Glyphosate ------
2, 4, 5-T Heptachlor ------
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