3. FORMS and TRANSFORMATIONS in the ENVIRONMENT

Several physical, chemical, and biological reactions influence the behaviour of polychlorinated biphenyls in the environment. The discussion in the following sections outlines the transformations of PCBs caused by these reactions.

3.1 Physico-chemical transformations

PCBs are non-polar compounds. Their non-polar nature makes them only slightly soluble in water. In general, the water solubility of a PCB compound decreases as the degree of chlorination increases (Table 5). However, there are some exceptions to this rule; for instance, decachlorobiphenyl is about twice as soluble as 2,2',3,3',4,4',5,5'-octachlorobiphenyl. Within a group of chlorobiphenyls containing the same number of chlorine atoms, the solubility depends on the positions of the chlorine atoms on the biphenyl ring.

The solubility of PCBs is also influenced by the environment as these compounds or preparations show a strong affinity for sediment and organic fractions. Sorption of PCBs on suspended and bottom sediment in an aqueous environment would result in lower concentrations of PCBs in water. Sorption on the dissolved organic fraction, on the other hand, will probably enhance the concentration of PCBs in water (Sawhney, 1986). PCBs have been shown to adsorb relatively rapidly onto plastic and glass containers (Hutzinger et al., 1974).

Owing to their low solubility's in water, PCBs are often associated with the solid fraction (e.g., particulate matter, sediments) of the aquatic and terrestrial environments. The sorption reactions of PCBs in aquatic and terrestrial systems play an important role in determining their fate and transport in the environment. In general, sorption of PCBs increases with the degree of chlorination (Haque and Schmedding, 1979), the surface area (Hiraizumi et al., 1979), and the organic carbon content of the sorbents (Karickhoff et al., 1979; Weber et al., 1983).

PCBs exhibit low vapour pressure which decreases with increased chlorination. The factors which influence the vaporisation of PCBs include surface area of the sorbent, organic matter content, clay type, and pH (Sawhney, 1986; Moore and Walker, 1991).

Haque et al. (1974) noted that about 60% of Aroclor 1254 sorbed by Ottawa sand was lost by vaporisation in a 4-week period, while no significant loss occurred from Woodburn soil in the same period.

PCBs are chemically inert compounds; as a result they are resistant to chemical degradation reactions in the environment. However, photochemical breakdown of PCBs has been noted by several investigators. In studying the photolysis of hexachlorobiphenyl in methanol, Andersson et al. (1973) identified a series of ortho-methoxy PCBs and methoxy-substituted chlorodibenzofurans among the 80 compounds formed. The photolysis of 2,5-dichlorobiphenyl and 2,2',5,5'-tetrachlorobiphenyl in aqueous suspension yielded low quantities (~0.2%) of 2-chlorodibenzofuran; also, the higher molecular weight polychlorinated dibenzofurans could be the primary photochemical products, undergoing photo-reduction in water. In aqueous environments, photochemical degradation is limited to the uppermost layers of the water column.

The biphenyl molecule exhibits two absorption maxima; the main band at a wavelength of 202 nm and the k band at a wavelength of 242 nm. Chlorine substitution on the biphenyl molecule produces a shift on the k band; the shift is much greater for the para -substituted than for the meta -substituted biphenyls. For more highly substituted chlorinated biphenyls, both the main and k bands are shifted towards the visible region with increasing chlorination. This implies that higher chlorinated biphenyls photo-degrade faster than lower chlorinated biphenyls (Moore and Ramamoorthy, 1984).

3.2 Biological transformations

A considerable variety of biota are capable of metabolising lower chlorinated biphenyls (LCBPs) of up to 6 Cl atoms into polar metabolites. The metabolic degradation of PCB in animal tissues (e.g., rats, birds, cows, and fish) is, therefore, characterised by the disappearance or reduction in the concentration of LCBPs. The breakdown of PCBs may yield hydroxylated products (with or without the arene oxide intermediary) which may differ with species (Moore and Ramamoorthy, 1984).

Isomerization and dechlorination reactions have been implicated in the metabolism of higher chlorinated biphenyls (HCBPs) (McKinney, 1976; Hutzinger et al., 1974). However, the identification of potentially toxic dibenzofuran structures in some metabolites, and lower ratios of PCBs to polychlorinated dibenzofurans (PCDFs) in liver tissue have caused concern regarding the metabolic formation and accumulation of PCDFs in the liver (Kuratsune et al., 1976).

Micro-organisms are assumed to play a major role in the breakdown of environmental chemicals. Studies have shown that mono-, di-, and trichlorobiphenyls are significantly biodegraded and volatilised, whereas PCBs with 5 Cl atoms tend to sorb to suspended particulates and sediments, and resist biodegradation (Clark, 1979; Tulp et al., 1978). Among commercial mixtures, Aroclors 1221 and 1232 showed significant biodegradation (Tabak et al., 1981), whereas Aroclor 1248 and 1260 showed virtually no biologically induced breakdown at concentrations of 5 mg/L or 10 mg/L. The availability of C-H bonds in PCB determines the extent of hydroxylation and, in turn, biodegradation.

A faster degradation rate has been reported for commercial PCB mixtures than their single components. It has also been shown that HCBPs such as Aroclor 1254 exhibit enhanced degradation in the presence of LCBPs such as Aroclor 1221 (the process of co-metabolism) (Baxter et al., 1975). Emulsification of PCB mixtures by sodium lignosulfonate greatly enhances the microbial degradation of PCB mixtures from Aroclor 1221 (lower chlorinated) to Aroclor 1254 (higher chlorinated). This is due to the increase in the surface area of the substrate, which is the limiting factor in the biodegradation process.