Hexachlorobenzene
Definition of hexachlorobenzene (HCB):
Hexachlorobenzene is a white crystalline solid which doesn't occur naturally. Hexachlorobenzene was widely used as a pesticide until 1965. It was also used to make fireworks, ammunition, and synthetic rubber. [1]
This is the common definition for hexachlorobenzene (HCB), other definitions can be discussed in the article
|
Notes
Due to environmental concerns, the use of HCB in such applications has now virtually ceased in Europe and the US, although it may still be in use in some other parts of the world. However releases still occur and prolong the presence of HCB in the environment. It is formed as a by-product during the manufacture of chemicals used as solvents, other chlorine-containing compounds, and pesticides. Small amounts of hexachlorobenzene can also be produced during combustion processes such as burning of city wastes. European emissions in 1997 were 100 kg in waste waters and 4 kg into the atmosphere. World wide emissions in 1995 still ranged between 12.000 and 95.000 kg. [2]
Hexachlorobenzene is among the most persistent environmental pollutants because of its chemical stability and resistance to degradation. It can both be released into the atmosphere and into waste waters. In the atmosphere of temperate regions it takes 1,94 years to half it's concentration, in atmosphere of polar regions even up to 6,28 years. This high persistence makes long-range global transport very likely. From the atmosphere HCB can washed out by rainfall or snowfall, or removed via dry deposition. In water HCB has a very low solubility of only 6 µg/l, and most of it is adsorbed to suspended particulate matter and sediments. It is also very persistent in water and soils and it can take between 2,7 and 6 years to half its aquatic or soil concentration. Although HCB is moderately volatile, it's strong adsorption to particles causes it to remain present in the marine environment.
Hexachlorobenzene shows very high bioaccumulation rates. Lichens have been reported to bioaccumulate HCB concentrations by direct adsorption, 17 million times higher than those in the surrounding environment, Fishes 21.900 times higher. Through biomagnification it poses primary a threat to high trophic level marine mammals or sea birds.
