Transcript Degradation of Organic Biocides

Degradation of organic biocides
What happens to biocides when they enter the environment?
Two related aspects:
1) Chemical stability (mechanism & rate it degrades)
2) Mobility (mechanism and rate of transport)
Rapid degradation  mobility less important
Fast transport is fast  different degradation mechanisms may
operate as the pesticide moves to a new environment
Degradation products may have biocidal properties – in some
cases enhanced ones
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Degradation of Organic Biocides
Chemical Stability
The breaking up of an organic species into (ultimately) simple
inorganic species (e.g., CO2, H2O) is called mineralisation
Several steps in mineralisation – intermediates with different
toxicities & chemical reactivity
Degradation products usually less toxic
(E.g., DDT  DDE)
Removal of chlorine from orgaanochlorine molecules nearly
always has a detoxifying effect
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Degradation of Organic Biocides
Consider photolytic reactions and chemical transformation
(hydrolysis, oxidation, and reduction)
Photolytic reactions
Need sunlight! Reactions occur during day time, the chemical
is either in the gas phase, atmospheric aerosol, in surface waters
or on the surface of plants or soils
Molecules must absorb solar radiation of enough energy to
break bonds, and the quantum yield for decomposition must be
significant compared to yields for other deactivation pathways
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Photolytic Reactions
Solar spectrum at earth’s surface
cuts off at 285 nm
No extensive photodegradation of
alkanes (do not absorb > 285nm)
Limited degradation of naphthalene:
absorbs strongly at 286 and 312
nm, but the energy required to
break aromatic C-C or C-H bond
is greater than that taken up from
solar radiation
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Photolytic Reactions
However, we would predict that the soil fungicide fenaminosulf is
susceptible to photolysis:
- ability of the azo group to absorb light
- relatively weak C-N bonds
Fenaminosulf
Azo absorbs at 340nm → 351 kJ mol-1
C-N bond energy ~ 305 kJ mol-1
These predictions are observed in practice
Called direct photolysis
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Photolytic Reactions
Indirect photolysis
Another molecule (the sensitiser) is radiatively excited
- if sensitiser is long lived it can transfer energy to another
molecule in the solution
Therefore without absorbing radiation directly, a receptor
molecule can be activated to take part in subsequent chemical
reactions
Rotenone
Rotenone can absorb sunlight and transfer additional energy to
aldrin leading to aldrin’s chemical degradation
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Non-Photolytic Reactions
Non-photolytic reactions may be either biotic or abiotic:
Abiotic: degradation via chemical reactions but not mediated by
microorganisms
Biotic: microorganisms degrade the biocide as a primary
substrate from which they derive energy
Consider the following types of reactions:
hydrolysis
oxidation
reduction
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Hydrolysis
Hydrolysis - nucleophilic reaction where water reacts with a
substrate molecule to replace a portion (leaving group) of the
molecule with OH
RX + H2O → ROH + HX
This type of reaction proceeds either by purely chemical or
microbiological mechanisms
Consider hydrolysis of different functional groups
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Hydrolysis
Ethers, esters and thioesters (C=S replaces C=O) undergo
hydrolysis:
Hydrolysis of 2,4-dichlorophenoxyacetic acid (2,4-D), a widely
used herbicide:
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Hydrolysis
Amides are hydrolysed to an acid and an amine:
Metolachlor, an insecticide, undergoes this type of hydrolysis
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Hydrolysis
Nitriles are hydrolysed to an amide and a carboxylic acid:
The herbicide ioxynil undergoes this type of hydrolysis
process
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Oxidation
Oxidation produces the final mineralised product
Need an oxidant
Nature of the oxidant depends on environmental circumstance
→ OH, H2O2, O3, O(1D) are all powerful oxidants
Under anaerobic conditions, NO3- and SO42- act as weak oxidants
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Oxidation
Some types of oxidation are:
• Alkanes and aliphatic substituents
RCH3 → RCH2OH → RCHO → RCOOH
• Oxidation of alkenes also produces alcohols and carboxylic
acids
• Aromatics are resistant to oxidation
– rate and extent strongly influenced by the nature of the substituents on
the molecule
– Cl & NO2 stabilise the molecule relative to other groups
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Oxidation
Consider the oxidation of benzene:
• Initial formation of an epoxide which is subsequently
converted to the diol with rearomatisation of the benzene ring
• Further oxidation can lead to ring fission with the production
of dicarboxylic acid
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Reduction
Reduction can occur in anoxic groundwater and flooded soils
1)
Dehalogenation:
E.g., Reduction of DDT results in the formation of DDD
(dichlorodiphenyl-dichloroethane)
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Reduction
2)
Vicinal dehalogenation:
E.g., Reduction of lindane
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