What is HCH?

Chemistry

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Lindane, the gamma isomer of the hexachlorocyclohexane (HCH, C6H6Cl6) is an organochlorine compound.  Just like the rest of the HCH isomers, it is a synthesised molecule that does not exist in nature. T. van der Linden discovered it in 1943, thus deriving its name from its discoverer.

The lindane is generated due to the reaction of benzene with free chlorine using ultraviolet light. This way we obtain what we call “technical HCH”, that it is afterwards distillated with methanol with the objective of isolating the gamma isomer.

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Another product of the distillation process is a dense non-aqueous phase liquid (DNAPL), a phase that is denser than water and at the same time, insoluble in it.

HCH is an organic compound which is persistent in the environment and the organisms, and bioacumulable, being therefore capable to be transmitted along the food chain. It can also be transported by air and water.

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Use as a pesticide

During the second half of the XX century, the lindane was one of the most used pesticides worldwide. Its use was widely accepted in agriculture and for the treatment of parasites in animals and humans. The insecticide properties of the lindane are limited to the gamma isomer of the HCH, however, the rest of the isomers have been also included in the list of toxic products for health and the environment.

At the process of lindane manufacturing – and after distillation to separate it from the rest of the isomers with a lower insecticide capacity – a high quantity of residues was generated: around 85-90% of the product.

There are many locations all over the world with this type of contaminant residues stored. As for today, there are still no technical and economical solutions that are feasible in a short term. The definitive solutions depend mainly on the efforts and wise choices done in coordinated processes of R+D+i.

The compounds of the different HCH isomers, as well as the byproducts of the manufacturing process (benzene, chlorobenzenes…) and intermediate degradation products of these organochlorine families (chloro-cyclohexanes, chloro-fenols …) are included in the Stockholm Convention and catalogued as priority or preferential substances.

PRIORITARY SUBSTANCES

  • Benzene                           10 – 50 mg/l
  • HCH                                   0,02 – 0,04 mg/l
  • Pentachlorobenzene       0,007 – NA mg/l
  • Pentachlorofenol              0,4 – 1 mg/l
  • Triclorobenzenes            0,4 – NA mg/l

PREFERENTIAL SUBSTANCES

  • Chlorobenzene                  20 mg/l
  • Dichlorobenzene              20 mg/l
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Historical use in Aragon

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There were four locations of lindane manufacturing in Spain: two of them in the Basque Country, another one in Galicia, and the last one in Huesca at the old INQUINOSA factory, whose facilities still remain in ruins.

The coincidence of several factors, made the lindane production waste to be stored in Sardas and Bailin landfills. These landfills, together with the factory itself, constitute one of the major environmental problems in Aragon and Spain.

This sum of factors places the Government of Aragon at the front of an enormous challenge with multiple aspects: technical, economical, social and political.

"Grasshoper effect"

The so called “global distillation effect”  (or “grasshopper effect”) determines that the persistent organic pollutants, as well as the organochlorides and others, find their way through the atmosphere from tropical and warm regions towards higher latitudes, where they condense and impregnate vegetation, soil and water.

That is why these contaminants have been found in the trees and the ice of the Artic and sub-Artic regions, or in the mountain lakes that were thought to be spotless. As an example, the PCB levels (among them the HCH) in the mother’s milk of the inuit womens of the Artic, multiplies by twenty the OMS maximum allowed values. (1)

Due to the change of temperature of the air through its transport, the majority of the molecules of mobile contaminants suffer several cycles of evaporation and condensation, as they are migrating gradually towards the areas of colder climates. (2)

This means that there is a planetary movement from the tropics towards the Artic and a local deep movement from the valleys towards the mountains. (3)

1- Riechmann, Jorge. “Cuidar la Tierra. Políticas agrarias y alimentarias sostenibles para entrar en el siglo XXI”. Ed. Icaria, 2003, pág 165.

2 – Baird, Colin. “Química ambiental”. Ed. Reverté, 2001, pág 37

3 – Introducción al conocimiento y prevención de los Contaminantes Orgánicos Persistentes. Ministerio de Agricultura, Alimentación y Medio Ambiente. 2012.