Question

Is groundwater quality affected by PCBs? state reasons

Answer 1

PCB are a class of man-made organic chemicals. Each PCB molecule contains two phenyl rings. A phenyl ring is a ring of 6 carbon atoms to which hydrogen atoms are attached. In PCBs, chlorine atoms replace some of these hydrogen atoms.

Chlorine atoms may be present at some or all of the 10 possible positions which are numbered 2–6 on one ring, and 2'–6' on the other ring (see figures below). In total, 209 different PCBs can be formed. These different combinations are called congeners, each having a specific number of chlorine atoms located at specific positions.

One example below shows a PCB with five chlorine atoms, two chlorine atoms at the 3 and 5 positions on one ring and three chlorine atoms at the 2’, 4’ and 6’ positions on the other ring. The other example shows a PCB with chlorine atoms at the 2 position on one ring, and the 2’ position on the other.

2’,3,4’,5,6’-pentachlorobiphenyl (PCB 121)

2,2’-dichlorobiphenyl (PCB 4)

Dioxins Study

Two different systems are used for naming PCBs. In the IUPAC system used in the examples above, the numbers at the beginning of the name specify the sites where chlorines are attached to the phenyl rings. Another system assigns a separate number, from 1 to 209, to each of the 209 specific PCB congeners.

PCBs which have the same number of chlorine atoms attached but at different positions, are referred to as isomers.

The two rings in a PCB molecule can rotate around the bond connecting them. The shape of the molecule is further influenced by the repulsion between nearby chlorine atoms so that the rings of a specific PCB will either lie approximately in the same plane (called coplanar) or in different, more perpendicular planes (termed non-planar). Those PCBs which have both rings lying in the same plane are considered to be most toxic, based on combined health effects considerations. They are also referred to as "dioxin-like".

Physical properties of PCBs:

• PCBs persist in the environment because they are highly unreactive and are largely resistant to breakdown by acids, bases and heat
• Generally, PCBs are not very soluble in water, but readily soluble in fats. This solubility in fat explains why PCBs can build up in animal fat and along the food chain The degree of solubility is largely dependent on the number and position of chlorine atoms in the PCB molecule.

Although in general PCBs do not easily evaporate, especially those with more chlorine atoms, PCB evaporation does occur, and can account for significant amounts of PCB transport