The Infamous Dioxin
Some of the more severe pollution problems associated with pesticides have
come from their manufacture. One of the more notorious of these was dioxin, known
chemically as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the structure of which is
shown below:
2,3,7,8-Tetrachlorodibenzo-
p-dioxin (commonly called dioxin)Cl
Cl
O
C
C
C
C
C
C O C
C
C
C
C
C
Cl
Cl
H
H
H
H
Dioxin was produced as a byproduct of the manufacture of 2,4,5-trichlorophenoxyacetic
acid, 2,4,5-T, the infamous “Agent Orange” used to defoliate jungles in the Vietnam
war. Dioxin is essentially insoluble in water, melts at 305 ̊C (very high for an organic
compound), is chemically stable up to 700 ̊C, and does not undergo biodegradation well.
Although it is remarkably toxic to some animals (especially guinea pigs) it is not extremely
toxic to humans. It has no uses, but is generated as a byproduct of the manufacture
of some chemicals and during the incineration of chlorine-containing plastics, such as
polyvinyl chloride. It was a highly undesirable impurity from the synthesis of 2,4,5-T
herbicide mentioned above.
Because of its extremely low water solubility, dioxin is not a common water
pollutant. However, it can accumulate in sediments. It gained notoriety as a hazardous
waste in the early 1970s when dioxin-contaminated wastes were mixed with waste oil
and sprayed for dust abatement on roads, horse arenas, and other areas in the state of
Missouri. Horses and birds were killed, and the entire town of Times Beach, Missouri,
was contaminated with the tainted oil. The U.S. Environmental Protection Agency bought
out the whole town in March 1983, at a cost of $33 million. Soil from the town along
with other dioxin-contaminated soil was subsequently incinerated at a total estimated
cost of about $80 million.
7.15. Polychlorinated Biphenyls (PCBs)
Polychlorinated biphenyls (PCBs), discussed in Section 5.4 and shown in Figure
5.5, consist of a class of 209 compounds made from subsituting from 1 to 10 chlorine
atoms for H atoms on biphenyl. PCBs are notable for their extreme chemical and thermal
stability, resistance to biodegradation, low vapor pressure, and high dielectric constants.
They even survive ordinary combustion processes and are dispersed to the atmosphere
with stack gases. Until their manufacture and use were banned by the Toxic Substances
Control Act of 1976, they were widely used as coolant-insulation fluids in transformers
Chap. 7. Water, The Ultimate Green Solvent 183