POLYCHLORINATED BIPHENYLS (PCBs)

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

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.

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.

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.

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.

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.

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).

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.

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 < 1000 C. Optimum temp for conversion to PCDDs and PCDFs is 250 – 380 C.

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.

Bioconcentration Example

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

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.

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.

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.

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.

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.

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.

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