1 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

2 Degradation of Organic Biocides Chemical Stability The breaking up of an organic species into (ultimately) simple inorganic species (e.g., CO 2, H 2 O) 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

3 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

4 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

5 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 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

6 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

7 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

8 Hydrolysis Hydrolysis - nucleophilic reaction where water reacts with a substrate molecule to replace a portion (leaving group) of the molecule with OH RX + H 2 O → ROH + HX This type of reaction proceeds either by purely chemical or microbiological mechanisms Consider hydrolysis of different functional groups

9 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:

10 Hydrolysis Amides are hydrolysed to an acid and an amine: Metolachlor, an insecticide, undergoes this type of hydrolysis

11 Hydrolysis Nitriles are hydrolysed to an amide and a carboxylic acid: The herbicide ioxynil undergoes this type of hydrolysis process

12 Oxidation Oxidation produces the final mineralised product Need an oxidant Nature of the oxidant depends on environmental circumstance → OH, H 2 O 2, O 3, O( 1 D) are all powerful oxidants Under anaerobic conditions, NO 3 - and SO 4 2- act as weak oxidants

13 Oxidation Some types of oxidation are: Alkanes and aliphatic substituents RCH 3 → RCH 2 OH → 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 & NO 2 stabilise the molecule relative to other groups

14 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

15 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)

16 Reduction 2)Vicinal dehalogenation: E.g., Reduction of lindane