BIODEGRADATION OF PCBs FROM ENVIRONMENT

1. BIODEGRADATION OF
PCBs
HARITHA T NAIR
16ENVA10
II MSc EVS

2. PCB
 Polychlorinated Biphenyls
 Family of compounds synthesised by the catalytic
chlorination of biphenyls
 These contains specific levels of chlorine.
 Its general formula is C12H(10-n)Cln , where ‘n’
can vary from 1 to 10.
 It is widely used in industrial application from
1929.

3. Properties
 Extremely thermostable (up to 350 ͦ c)
 Resistant to oxidation, acids and bases.
 Have excellent electrical insulating and dielectric
characteristics.
 Physical properties: liquids, low vapour pressure, low
water solubility, excellent dielectric properties
 Chemical properties: stability to oxidation, flame
resistance, relative inertness

4. Risk of PCBs to environment and
health
 A significant reason why PCBs pose a risk to people is that
they are bio accumulated in food chain.
 The hydrophobic nature of the compound causes the PCBs to
be attracted to lipids or fats, resulting in the accumulation of
chemicals in living cells.
 This causes the concentration of PCBs to increase as they
move from simple aquatic forms(fish) to humans through
ingestion

5.  If humans are exposed to the PCBs the chemical
remains in their system.
 Repeated exposures lead to accumulation of the
compound and can result in toxic effect.
 PCBs are known to be carcinogenic.
 It also have other significant ecological and
human health effects including neurotoxicity,
reproductive and developmental toxicity, immune
system suppression, endocrine disruption etc.

6. Biodegradation of PCBs
 In early days PCBs are considered as non-
biodegradable.
 Now it have been shown biodegradable via
two distinct microbially mediated
mechanisms:
 Aerobic biodegradation
 Anaerobic Dechlorination

7. Aerobic Biodegradation
 Aerobic bacterial biodegradation of PCBs is
widely known and has been studied
extensively.
 Numerous microorganisms have been
isolated that can aerobically degrade a wide
variety of PCBs, although the more lightly
congeners are preferentially degraded.

8.  These organisms attack PCBs via the 2,3-
dioxygenase pathway, converting PCB
congeners to their corresponding
chlorobenzoic acids.
 Then these chlorobenzoic acids can be
readily degraded by indigenous bacteria,
producing carbon dioxide, water, chloride
and biomass.

10. Anaerobic Dechlorination
 Anaerobic bacteria attack more highly chlorinated
PCB congeners through reductive dechlorination.
 Here the organisms utilize PCBs as an electron
acceptor, with addition of the carbon-chlorine
bond, chlorine loss and hydrogen abstraction from
an unknown species.

11.  This microbial process affects the
preferential removal of meta and para
chlorines, thus converting highly
chlorinated PCB congener to lower
chlorinated, ortho-substituted congeners.

12. Reductive dechlorination catalyzed by
anaerobic microorganisms:
Ar-Cl + e + R-H Ar-H + Cl + R
(H2O) (HO)
(H2) (H)
e Cl
Ar-Cl Ar-Cl Ar Ar-H + R

13. Two- step combined
anaerobic/aerobic process
 Here initial anaerobic treatment converts
highly chlorinated PCBs to lightly
chlorinated derivatives.
 Subsequent aerobic treatment destroys the
remaining material