An Introduction to Environmental Chemistry

276 Chapter Seven

PCBs

x = 1 to 5

and/or

y = 1 to 5

Clx Cly

Cl

Cl Cl

Cl

Cl Cl

DDT HCH

Cl

Cl

C Cl

C

Cl

Cl

PCDFs

x = 1 to 5

and/or

y = 1 to 5

Clx Cly

O

PCDDs

x = 1 to 5

and/or

y = 1 to 5

Clx Cly

O

O

Fig. 7.25Chemical structures of the contrasting persistent organic pollutants (POPs), polychlorinated
biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and furans (PCDDs/PCDFs), p,p¢-dichlorodiphenyl
trichloroethane (DDT) and hexachlorocyclohexane (HCH). Symbols x and y indicate the possible number
of chlorines attached to the ring structures.

1 Multiple volatilization–transport–condensation events: ‘leapfrogging’

2 Single volatilization–transport–condensation events

Condense

Condense

Volatilize Equatorial source region

Temperate source region

Polar

region

2

Volatilize

1

Fig. 7.26Global distillation of persistent organic pollutants.