POLYCHLORINATED BIPHENYLS (PCB's) - Environmental Implications

PHYSICO-CHEMICAL CHARACTERISTICS OF PCB’S

PCB’s are the mixture of aromatic chemicals, manufactured by the chlorination of biphenyls in the presence of a suitable catalyst. The chemical formula of PCB’s can be presented as C12H10-nCln, where `n’ is a number of chlorine atoms within the range of 1-10. The relative molecular mass of PCB compounds depend on the degree of substitution. Monochlorobiphenyl has a relative molecular mass of 188, while completely chlorinated biphenyl [C12Cl10] has relative molecular mass of 494. Some physical and chemical characteristics of PCB’s are presented in Table 3.

Table  3  Physico-chemical properties of selected Aroclors

(Polychlorinated Biphenyls)

Aroclor

Compound

Water solubility (mg/l) 25 0C

Vapour Pressure (torr) 25 0C

Density (g/cm3) 25 0C

Appearance

Boiling point

(0C)

at 750 torr

1016

0.42

4.0 x 10-4

1.33

Clear oil

325-356

1221

0.59

6.7 x 10-3

1.15

Clear oil

275-320

1232

0.45

4.1 x 10-3

1.24

Clear oil

290-325

1242

0.24

4.1 x 10-3

1.35

Clear oil

325-366

1248

0.054

4.9 x 10-4

1.41

Clear oil

340-375

1254

0.021

7.7 x 10-5

1.50

Light yellow viscous oil

365-390

1260

0.0027

4.0 x 10-5

1.58

Light yellow sticky resin

385-420

                                Source: IARC (1978), WHO/EURO (1987).

All congeners of PCB’s are lipophilic and have very low water solubility. Solubility of PCB’s in water and in organic solvents, such as lipids, greatly influences their transport and persistence in the environment and this may be the reason that these compounds easily enter the food chain and accumulate in the fatty tissues. Solubility of PCB’s in water generally decreases with increase in the degree of chlorination. Individual chlorobiphenyls vary in their water solubility from about 6 ppm for monochlorobiphenyl to as low as 0.007 ppm for octachlorobiphenyls. Solubilities of PCB’s are also influenced by the environmental conditions. The aqueous phases in the environment generally contain dissolved organic substance, which probably increase the concentration of PCB’s in the solution. Conversely, sorption of PCB’s on soil and sediment surfaces in the aquatic environment help in decreasing their solution concentration.

PCB compounds have very low vapor pressure, which like their solubility in water, decrease with increased chlorination. In environmental samples where PCB’s are sorbed on soil or sediment surfaces, the rate of vaporization of PCB’s is greatly reduced. The vaporization rate of PCB’s depend upon the sorption surface, thus the vaporization from soil and sediment surface is comparatively less than its vaporization from aqueous solution from where it is anomalously high because of low vapor pressure and high molecular weight. Commercial PCB’s mixtures are light to dark yellow in colour. They do not crystallize, even at low temperatures, but turn into solid resins. PCB’s are fire resistant with rather high flash points. They form vapours heavier than air, but these do not form any explosive mixtures with air. PCB compounds have very low electrical conductivity, high thermal conductivity and extremely high resistance to thermal breakdown. Because of these physical properties they are extensively used as cooling liquids in electrical equipments. PCB compounds are chemically stable under normal conditions, however when heated, vapours of other toxic compounds such as polychlorinated dibenzofurans (PCDFs) can be produced.

 PCB’s are practically insoluble in water, whereas they dissolve easily in hydrocarbons, fats and other organic compounds and they are readily absorbed by fatty tissues. All congeners of PCB’s are lipophilic and have a very low water solubility that’s the reason, they easily enter the food chain and accumulate in fatty tissues.