Biodegradation of selected organic compounds

• Hydrocarbons• Halogenated aliphatic compounds• Halogenated aromatic compounds

Hydrocarbons

• Over 2 billion metric tons of petroleum are produced every year worldwide and large amounts of these products end up polluting the environments.

• These include: Low-level routine discharges (urban runoff, effluents oil treatment of roads, etc) account for over 90% of the total petroleum hydrocarbon discharges.

• Accidents such as tanker disasters, pipeline breaks, and well blowouts account for less than 10% of these discharges.

• Hydrocarbon in crude petroleum are classified as alkanes, cycloalkanes, aromatics, polycyclic aromatics, asphatines, and resins.

• Alkenes are generally not encountered in crude oil but may be present in small quantity in refined petroleum products.

Alkanes

The most biodegradable of the petroleum hydrocarbons.

However, in the C5 to C10 range, alkanes may be inhibitory to many hydrocarbon degraders because as solvents they disrupt lipid membranes. In the C20 to C40 range (waxes), their low solubility interferes with their biodegradation.

Alkanes--Degradation pathway:

The monooxygenase enzyme attacks the terminal methyl group to form an alcohol. The alcohol is oxidized further to an aldehyde and then to a fatty acid. The fatty acid is degraded further by -oxidation of the aliphatic chain.

Alkanes

In general, the degradation products are less volatile than the parent compounds. In some cases, the parent alkanes are highly volatile and may be removed from soil by stripping under aerobic conditions.

Alkenes

Location of the unsaturated linkage is a factor. 1-alkenes are more degradable than alkenes with an internal double bond.

Two general pathways: either the double bond is oxidized or the saturated chain end is oxidized.

Cycloalkanes

Less degradable than the straight chain alkanes but more degradable than the polycyclic aromatics. Part of the decreased degradability is due to decreased solubility.

Alkyl-substituted cycloalkanes are more readily degraded than nonsubstituted hydrocarbons, and cyloalkanes with long-chain side groups are more easily degraded than those with methyl or ethyl groups.

Cycloalkanes

Cycloalkanes are usually degraded by oxidase attack to produce a cyclic alcohol which is dehydrogenated to a ketone.

Aromatics

Aromatics

Biodegradation involves two steps: activation of the ring, and ring cleavage

Activation is achieved by enzymes known as oxygenases.

Acids produced are readily utilized by microorganisms for cell synthesis and energy.

Polycyclic aromatic hydrocarbons

Produced during high temperature industrial operations such as petroleum refining, coke production, and wood preservation.

In general, PAHs with increase molecular weight and number of ring structures have decreased solubility and volatility.

PAHs are degraded one ring at a time

Asphaltines and Resins

High molecular weight compounds containing N, S, and O.

They are usually recalcitrant to biodegradation.

Cometabolism may be significant in degrading these compounds.

Halogenated aliphatic compounds

• Industrially important ones include: chlorinated and brominated alkanes and alkenes in C1 to C3 range.

• Chlorinated ethanes and ethenes are commonly used as cleaning solvents and in dry-cleaning operations and semiconductor manufacturing.

• Brominated compounds are used as pesticides (e.g. ethylene dibromide or EDB, dibromochloropropane or DBCP) and halogenated methanes (CHCl3, CHCl2Br, CHClBr2, and CHBr3) are formed during the disinfection of water.

• The greater the number of halogens in the molecule, the less biodegradable the compound will be in aerobic systems and the more degradable it will be in anaerobic systems.

• The biodegradation rate is also dependent on the type of halogen in the compound. In general, Br>Cl>F.

Degradation pathways

Substitutions

Oxidations

Reductions.

Halogenated aromatic compounds

• Include: solvent, lubricants, pesticide, plasticizers, PCBs (insulators in electrical transformers and capacitors), and pantachlorophenol (a wood preservative)

• The more halogen substituents the compound has, the more likely it is to undergo reductive dehalogenation in reducing environments.

Degradation pathways

Oxidations and ring cleavage

Reductive dehalogenagtion