Biodegradation of selected organic compounds

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Biodegradation of selected organic compounds
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• Hydrocarbons
• Halogenated aliphatic compounds
• Halogenated aromatic compounds

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• 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.

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• 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.

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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.
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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 b-oxidation of the
aliphatic chain.
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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.
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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.
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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.
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Cycloalkanes Cycloalkanes are usually degraded
by oxidase attack to produce a cyclic alcohol
which is dehydrogenated to a ketone.
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Aromatics
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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.
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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
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Asphaltines and Resins High molecular weight
compounds containing N, S, and O. They are
usually recalcitrant to biodegradation. Cometab
olism may be significant in degrading these
compounds.
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• 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.

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• 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,
  BrgtClgtF.

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Degradation pathways Substitutions
Oxidations Reductions.
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• 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.

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Degradation pathways Oxidations and ring
cleavage Reductive dehalogenagtion