BIODEGRADATION AND BIOTRANSFORMATIONS OF ORGANIC COMPOUNDS

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BIODEGRADATION AND BIOTRANSFORMATIONS OF ORGANIC
COMPOUNDS
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Bioavailability
Not accessible
Accessible
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Biodegradation

• Aerobic and anaerobic degradation
• Reduces aqueous concentrations of contaminant
• Reduction of contaminant mass
• Most significant process resulting in reduction
  of contaminant mass in a system

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Fundamentals of Biodegradation

• All organics are biodegradable, BUT
  biodegradation requires specific conditions
• There is no Superbug - not Volkswagon
• Contaminants must be bioavailable
• Biodegradation rate and extent is controlled by a
  limiting factor

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Requirements for Microbial Growth(limiting
factors for biodegradation)
Carbon/Energy Source
Environmental Conditions (Temp, pH, Eh)
Nutrients (N, P)
Trace elements
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Aerobic vs Anaerobic Biodegradation (A matter of
terminal electron acceptor)

• If oxygen is the terminal electron acceptor, the
  process is called aerobic biodegradation
• All other biological degradation processes are
  classified as anaerobic biodegradation
• In most cases, bacteria can only use one terminal
  electron acceptor
• Facultative aerobes use oxygen, but can switch to
  nitrate in the absence of oxygen

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Microorganisms

• Obligate aerobes - Microorganisms for which the
  presence of oxygen is essential. Oxygen is the
  only electron acceptor that these species can
  employ.
• Facultative anaerobes - Can use oxygen if it is
  available but are able to switch to alternate
  electron acceptors when oxygen is depleted.
• Obligate anaerobes - Use alternate electron
  acceptors exclusively. Oxygen is toxic.

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Electron Exchange
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Heterotrophic Metabolism And Electron Acceptor
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Electron Acceptor Zone Formation
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Metabolism and Oxidation

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Methanotrophs, example of cometabolism

• Use methane as the primary substrate, but
  cometabolize chlorinated solvent compounds.
• They oxidize methane to methanol using methane
  monooxygenase (MMO).
• MMO is non-specific, and cometabolizes
  trichloroethene (TCE) to TCE epoxide
• This eventually degrades to CO2, Cl- and H2O.

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Biodegradation and Biotransformation

• Conversion of contaminants to mineralized (e.g.
  CO2, H2O, and salts) end-products via biological
  mechanisms
• Biotransformation refers to a biological process
  where the end-products are not minerals (e.g.,
  transforming TCE to DCE)
• Involves the process of extracting energy from
  organic chemicals via oxidation of the organic
  chemicals

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Oxidation and Extraction of Energy

• Oxidation of organic matter provides energy for
  living organisms because such reactions are
  thermodynamically favored
• ¼ CH2O ½ O2 ? ¼ CO2 ¼ H2O
• ?G? -119.98 kJ/electron equivalent
• Microorganisms employ catalyzing enzymes to
  surmount kinetic barriers.
• Enzymes function by forming a complex with the
  reactants, bringing them in close contact.

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Biotransformation of Organic Substances

• If carbon is in oxidized form, biotranformation
  by reduction is more important
• If carbon is reduced, biotranformation by
  oxidation is more efficient

perchloroethene (PCE) C(II)
carbon tetrachloride (CT) C(IV)
vinyl chloride C(-I)
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Enzymes

• Oxygenase - An enzyme that catalyzes oxidation
  reactions.
• Reductase - An enzyme that catalyzes reduction
  reactions.

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Dependence on Redox Condition













1. Highly biodegradable
2. Moderately biodegradable
3. Slow biodegradation
4. Not biodegraded
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Biodegradation of Chlorinated Organics

• More resistant to biodegradation than aromatic
  hydrocarbons.
• Bacteria cannot use most of these compounds as a
  substrate.
• Most biodegradation occurs via cometabolism.
• Cometabolism is slower than heterotrophic
  metabolism and requires the presence of suitable
  primary substrates.

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Dehalogenation

• Dehalogenation refers to the process of stripping
  halogens (generally Chlorine) from an organic
  molecule
• Dehalogenation is generally an anaerobic process,
  and is often referred to as reductive
  dechlorination
• RCl 2e H gt RH Cl
• Can occur via dehalorespiration or cometabolism
• Some rare cases show cometabolic dechlorination
  in an aerobic environment

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Dehalogenation of PCE

• PCE (perchloroethylene or tetrachloroethylene)
• TCE (trichloroethylene)
• DCE (cis-, trans-, and 1,1-dichloroethylene
• VC (vinyl chloride)

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Anaerobic Transformation

• Reductive dechlorination - Most common anaerobic
  process. A cometabolic process in which the
  solvent is reduced by the replacement of a
  chlorine atom with a hydrogen atom.
• Compounds that contain more than 1 Cl atom
  dechlorinate in a series of steps, each involving
  loss of a single Cl atom.
• Carbon tetrachloride degrades to chloroform via
  reductive dechlorination. The latter is more
  resistant and accumulates.
• PCE and TCE degrade with cis-1,2-DCE and vinyl
  chloride as intermediates.
• VC is highly mobile and toxic.

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Anaerobic biotransformation pathways of
chlorinated aliphatic hydrocarbons (From Vogel et
al., 1987). a abiotic pathway.
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Reductive dechlorination of TCE under anaerobic
conditions (from Vogel, 1994).
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Oxygen Utilization of Substrates

• Benzene C6H6 7.5O2 gt 6CO2 3H2O
• Stoichiometric ratio (F) of oxygen to benzene
• Each mg/L of benzene consumes 3.07 mg/L of O2

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Stoichiometry

• Electron Donor to Electron acceptor ratios
• Hydrocarbon requirements for electron acceptor
  are well defined
• Electron donor requirements for dechlorination
  are poorly defined
• Cometabolic processes are not predictable
• Each Electron Donor/Electron Acceptor pair has a
  unique stoichiometric ratio