POLYCHLORINATED BIPHENYLS PCBs

1
POLYCHLORINATED BIPHENYLS (PCBs)
2
PCBs

• Chlorination of biphenyl can lead to the
  replacement of from 1 to 10 hydrogen atoms by
  chlorine.
• The general chemical formula C 12 H 10-n Cl n
  
• Relative molecular mass ranges from 188 for
  monochlorobiphenyl to 494 for C 12 Cl 10

3
PCBs

• PCBs do not occur naturally in the environment.
  They are a formed as the result of a commercial
  manufacturing process.
• Depending upon the reaction conditions the degree
  of chlorination can vary between 21 and 68 with
  the yield always being a mixture of different
  isomers and congeners.
• There are a total of 209 different chemical
  compounds which are possible but only about 130
  are likely to occur in commercial products or
  mixtures.

4
PCBs

• the phenyl rings of a PCB molecule are not
  constrained through ring fusion. Therefore, the
  molecule has relatively unconstrained rotational
  freedom.
• Chlorine atoms at the ortho (2,2' 6,6')
  positions introduce constraints on rotational
  freedom.
• PCBs with ortho substitutions are generally
  referred to in the biological literature as
  "planar" or "coplanar" PCBs, while all other PCBs
  are referred to as nonplanar or noncoplanar PCBs.

5
PCBs

• Manufacturers ID System
• A 4 digit code.
• Aroclor 1260 - the number 12 refers to it
  being a biphenyl and the 60 refers to the age of
  chlorination (by weight).
• Table 1
• Impurities known to be present in commercial PCBs
  include chlorinated dibenzofurans and chlorinated
  naphalenes.

6
Uses for PCBs

• PCBs are fire resistant with high flash points
  (170 - 380 C), they possess very low electrical
  conductivity and have an extremely high
  resistance to thermal breakdown.
• Over 50 used in capacitors and transformers, 30
  in plasticizers, 12 in hydraulic fluids and
  lubricants.
• The high resistance to thermal breakdown made
  them useful as cooling liquids in electrical
  equipment.

7
Uses for PCBs

• The production of PCBs decreased sharply after
  1970 (Monsanto Chemical Company) by voluntary
  limitation of sales.
• PCBs are now only used in closed systems such as
  capacitors and transformers.

8
Release Into The Environment

• At least 600,000 tons of PCBs have been released
  into the environment in North America. No
  estimate world wide.
• PCBs are dispersed into the environment through
  atmospheric transfer and on a regional scale
  following release into bodies of water. PCBs
  have a world-wide distribution.
• The atmospheric transfer of PCBs occurs following
  incineration. Since PCBs have a high flash point
  they require very high temperatures for
  destruction.
• PCBs that are not destroyed during burning adsorb
  onto particles that are released into the air.
• Much of the PCB contamination that enters the
  aquatic habitat comes from atmospheric fallout
  (mainly wet deposition by rain).

9
Environment

• PCBs are not very water soluble. Log K(ow)
  values range from 4.46 to 8.18. Most of the PCBs
  entering aquatic environments partitions out into
  the sediment.
• Sewage treatment appears to remove PCBs from
  waste water concentrating them into the sludge.
  The sludge is often discharged onto open land
  contaminating the soil.
• PCBs are mobilized in the soil or in landfills
  but the rates at which this occurs are not known.

10
Thermal Breakdown

• PCBs must be burned at temperature 800 - 1000 C
  to decrease conversion to other toxic products.
• PCBs can be converted to polychlorinated
  dibenzofurans (up to 10 conversion) at
  temperatures conversion to PCDDs and PCDFs is 250 380 C.

11
Bioaccumulation and Elimination

• Very high lipid solubility, they accumulate in
  all organisms.
• Very slow rate of metabolism and elimination.
• Accumulate preferentially in fat-rich tissues.
• In microorganisms the uptake is rapid with BCF of
  3200 - 7000 on wet weight basis
• In invertebrates the uptake is also rapid with
  BCF of 3 to 4 for Nereis (a sediment dwelling
  worm), uptake is from sediment.
• BCFs in oysters (from water) 1200 - 48000 , 1800
  for clams, 800 for Nereis.

12
Bioconcentration Example

• In Lake Ontario Water 0.05 ppb, sediment 150
  ppb, plankton 1880 ppb, catfish 11,580 ppb,
  herring gull 3,3530 ppb

13
Toxic Effects of PCBs

• Lab animal data shows reproductive effects,
  developmental effects.
• Cancers were observed in rats. Mink and monkey
  are the most sensitive species.
• The most common sublethal effect of PCBs in all
  organisms is an increase in Phase I enzyme
  activity (enzyme induction). Many PCBs bind to
  the Ah receptor.
• Physiological functions controlled by steroid
  hormones may be altered (growth, molting,
  reproduction).
• Certain PCBs (the conformationally restricted
  ones), and hydroxylated metabolites, may also
  bind to the estrogen receptor and have estrogenic
  activity.

14
POLYCHLORINATED DIBENZO-P-DIOXINS (PCDDs) AND
POLYCHLORINATED DIBENZOFURANS (PCDFs)

• These compounds are formed as by-products in
  several chemical manufacturing processes.
• They may be contaminants in PCB mixtures or in
  other chemical mixtures.
• Herbicide 2,4,5-T was made from chlorophenol
  compounds and was contaminated with 2,3,7,8-TCDD
  (tetrachlorodibenzo-p-dioxin). Agent Orange.

15
Sources of Compounds

• Pulp and paper mills produce PCDDs and PCDFs and
  they are released into the water near the mills.
• Any manufacturing process that uses carbon and
  chlorine can produce these compounds as side
  products.
• Also found in flue gases from municipal
  incinerators. In cigarette smoke, automobile and
  diesel truck emissions, charcoal-broiled steaks,
  and fireplaces.

16
Toxic Effects

• Phase I enzyme induction, reproductive
  impairment, weight loss, immune suppression,
  hormonal alterations.
• Dioxins have been shown to be carcinogenic in
  rats and mice.
• TCDD and TCDF have antiestrogenic and
  antiandrogenic effects. They do not bind to
  steroid hormone receptors (they bind to Ah
  receptors) but the Ah receptor is thought in some
  way to influence the antiestrogenic effects.
• TCDD can decrease GnRH action on the pituitary.

17
POLYBROMINATED BIPHENYLS (PBBs)

• Theoretically there can be 209 different forms
  (congeners) of PBBs depending upon the number and
  distribution of Br.
• PBBs do not occur naturally. They are produced
  commercially and the existence of any of the 209
  congeners is possible in any commercial mixture.
• Most research on PBBs has been conducted with a
  commercial product named Firemaster, which
  accounts for most of the manufactured products
  and most of the environmental contamination.

18
Uses and Characteristics

• Primarily as a flame retardant in plastic
  products and cable coatings.
• PBBs show high degree of chemical stability They
  do degrade rapidly with UV irradiation however.
• PBBs are only slightly soluble in water and
  solubility decreases with increasing bromination.
• Log K(ow) ranges from 4.59 to 8.58
• Particulate matter influences solubility with
  PBBs 200x more soluble in landfill leachate than
  is distilled water. Leachate can also contain
  organic solvents.
• Manufacture in US ceased in 1977.

19
Sources and Toxicity

• The principal route of PBBs into the aquatic
  environment is from industrial waste streams into
  receiving waters.
• Because PBBs are poorly soluble in water they are
  primarily found in sediments of polluted lakes
  and rivers. Up to 77mg/kg near Michigan Chemical
  Corp plant on Pine River in Michigan. 6.2mg/kg
  one-half mile downstream, and 0.1mg/kg 24 and 29
  miles downstream.
• Toxicity is similar to PCBs