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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 BiocidesConsider 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 ReactionsHowever, 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

These predictions are observed in practice

Called direct photolysis

Fenaminosulf

Azo absorbs at 340nm → 351 kJ mol-1

C-N bond energy ~ 305 kJ mol-1

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Photolytic ReactionsIndirect 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 ReactionsNon-photolytic reactions may be either biotic or abiotic:Abiotic: degradation via chemical reactions but not mediated by

microorganismsBiotic: microorganisms degrade the biocide as a primary

substrate from which they derive energy

Consider the following types of reactions:hydrolysisoxidationreduction

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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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OxidationOxidation 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 SO4

2- act as weak oxidants

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OxidationSome 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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ReductionReduction 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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Reduction2) Vicinal dehalogenation:

E.g., Reduction of lindane