Chemistry Reference
In-Depth Information
Cl
m
Cl
n
Fig. 4 General molecular structures of PCBs
PCBs were found for the first time in the environment by Soren Jensen, who
found these contaminants in pike from Swedish waters [
4
]. After few years of
research, the adverse effects of these compounds for organisms were identified and
led to restrictions in their production and use. Finally (late 1970s), they were
banned in most industrialised countries [
24
,
25
]. The position of the chlorine
atoms in the molecule influences the toxicity properties of the PCBs. They are
more toxic when the
ortho
positions of the molecule are not substituted, like in the
case of PCB-77 (3,3
0
,4,4
0
-tetraCB), PCB-126 (3,3
0
,4,4
0
5-pentaCB) and PCB-169
(3,3
0
,4,4
0
,5,5
0
-hexaCB). In these conditions, the two phenyl groups can rotate freely,
and the molecule is stabilised as a flat structure (planar or coplanar PCBs) due to the
interaction between the
orbitals of both biphenyls.
Mono-ortho
and
di-ortho
PCBs (with the 2 and 2
0
positions substituted) are less toxic [
25
] than the
unsubstituted
ortho
isomers. Reported biological effects on terrestrial animals
exposed to PCBs involve liver damage, dermal disorders (chloracne), reproductive
toxicity, thymic anthropy, body weight loss, immunotoxicity, teratogenicity and
induction of several cytochrome P-450 (enzyme) isosystems [
25
]. Although the use
of these products is prohibited for many years now, they still cause toxic effects at
trace levels, as it has been shown in a study on Dutch school children [
12
].
PCBs are discharged into the environment by leaking from hydraulic systems,
diffusion from coatings and during production, waste incineration or waste dis-
posal. Atmospheric PCBs are predominately presented in the gas phase. However,
significant fractions of the more chlorinated congeners are also presented in the
particulate phase [
26
].
p
2.2 Polycyclic Aromatic Hydrocarbons
PAHs are constituted of C and H atoms in the form of fused benzene rings. The
atmospheric PAH mixtures are essentially generated in incomplete combustion
processes involving fossil fuels or any other organic materials, such as wood.
These sources are numerous (Fig.
5
). At present, residential emissions from wood
and coal combustion for domestic heating and vehicular exhaust emissions are
important sources. The composition of the PAH mixture depends on the precursor
organic material and combustion conditions [
27
,
28
]. Diagenetic processes may
also produce PAHs, but their significance as sources for the atmosphere is small.
Most PAHs enter into the environment by atmospheric emissions. They may
therefore be atmospherically transported over long distances, especially when they
are bound to aerosols. In this way, they have become ubiquitous contaminants
reaching remote areas [
29
]. There is a direct relationship between pyrolytic PAH
