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What is persistence?

Persistence in general is used for (mainly organic) materials, to point to the fact that such a material is not readily biodegradable by bacteria, fungi or other natural means. In itself that is not a negative or positive property, that is only a question of purpose of the material and its toxicity.
If you need materials with a short life cycle, it can be benificial to use something that is readily compostable, like biodegradable bags for kitchen waste. But we think it is not such a good idea to use biodegradable waste water pipes, they will not last for long with such an amount of bacteria in the water... Glass, concrete, bricks, PVC,... all are persistent, non-toxic materials, for which the persistence is a positive property during their usefull life.
The positive possibility of biodegrading for some materials like wood, is a negative property when in use, because you have to protect the wood against too early biodegradation. That means that some protection must be applied (pesticides, paints), which can make it difficult to impossible to have biodegration at the end-of-life. Most of pesticides, fungicides and other protecting means are used on biodegradable materials, not on persistent materials.
Of course, there are harmfull materials that are persistent and toxic and liable to bio-accumulate, because they are fat-soluble. The emissions of that kind of materials must be reduced to a minimum. Some organochlorines like PCB's and dioxin belong to that class of persistent organic pollutants (POP's), but also non chlorinated materials like PAH's and nitro-PAH's are POP's. Also some pesticides like DDT, hexachlorphene and wood preservatives like creosote (contains lots of PAH's) and PCP were very persistent and toxic. Most of these are (to be) replaced by less toxic and/or fast bio-degrading pesticides.
But almost all organochlorines are not persistent and/or not toxic and/or don't bio-accumulate.

While there is a tendency that organochlorines are more persistent with higher chlorination, there is a very wide variation in persistence between organochlorines, depending on medium and circumstances. The same variation can be seen for non-chlorinated materials.


Persistence in air

A lot of chemicals, even dioxins, are rather fast degraded by UV-light. This kind of light has a high energy content and can destroy all kinds of molecules. The stronger the bond, the higher the energy content of the UV-light must be to destroy the bond. That is the reason why CFC's with rather strong bonds are persistent at earth level and only at higher level, where stronger UV-light occurs in the stratosphere, will release their chlorine atoms, which can destroy ozone at that level.
Other chemical reactions, ultimately leading to complete destruction into basic materials like water, hydrochloric acid and carbon dioxyde also occur by the reaction of volatile molecules with oxygen or ozone in the air.
In general it can be said that the higher the degree of chlorination is, the more persistent to destruction it normally will be. E.g. HCFC's, containing some hydrogen, have a much lower ozone depletion potential, because they are broken down more rapidely - before the bulk reaches the ozone layer - than CFC's, which have only chlorine and fluorine atoms bounded to carbon.

The half life time is the timespan in which half the amount of material is destroyed.

Half life time in air (in sunlight)
vinylchloride (VCM):2 days 
dioxins:12 daysyes, dioxins are completely destroyed by UV-light!
dichloroethane:4 weeks 
chloroform:23 weeks 
methylchloride:75 weeks99% natural [40], accounts for 20% of the ozone depletion potential [8]!
tetrachlorinecarbon> 2000 weeks95% natural [40], a potent ozone depleting chemical.
Source: Eurochlor publication "Halogenated Organic Compounds in the environment", COC 94/2 [37].


Persistence in water

Here too, there is a wide variation in persistence of chlorinated organics.
Different types of bacteria are specialised in different materials that they use as foodstuff or that they attack while they eat their 'normal' food. Some of them exchange chlorine in a molecule for oxygen and hydrogen (hydroxyl), others make vinylchloride from chlorinated solvents. Recently one has found a new type of bacteria that completely breaks down chlorinated solvents into more elementary materials.

More than 97% of VCM chlorinated manufacturing waste is destroyed in less than three days in the biological waste water treatment of the factory where we are working [38]. That is comparable to more than 95% of hydrocarbons of refinaries in similar conditions.

On the other hand PAH's, dioxins, PCB's and DDT need more than 20 years to be reduced to halve by bacteria, when these materials are discharged from land or precipitate from air into rivers and are absorbed by particulates which accumulate in the river mud [39].


Persistence in soil

Soil is a hard medium to biodegrade chlorinated (and non-chlorinated) materials. Even in deposits, only a part of biodegradable materials is really biodegrading. It largely depends of the type of soil and of the availability of air for what will happen. Further it can be very important if the material is above the groundwater level or not.
Here too there are large differences in half life time, depending of the type of material and circumstances. Chlorinated phenols are halved in a few days, while on the other hand dioxins were found in 60 million years old clay layers in Arkansas (USA), with apparently no human influence.

Half life time in soils
several chlorinated alkanes:weeksshort-chained chlorinated hydrocarbons in heavely contaminated soil disappeared completely within 20 years by natural breakdown [39]
DDT/DDE:20 years 
Dioxin: millions of years?  



Nature as well as all human activities produce persistent materials. If these are non-toxic, there is no problem, only a lot of benefits for the use of these materials. Examples are rock, glass, brick, concrete and PVC.
Nature as well as human activities produce persistent organic pollutants (POP's), these are toxic materials that don't break down rapidely and above a certain level can be harmfull to life and ecosystems. Examples are dioxins, PCB's, PAH's and nitro-PAH's. These are mainly produced by all kinds of incineration and combustion, including traffic and natural forest fires. See for a ratio of quantities, the emission of POP's by seaships. Also some older largely applied highly chlorinated pesticides like DDT and hexachlorophene and wood preservatives like creosote (PAH's) and PCP were/are too persistent for that purpose.


There is no reason to treat chlorinated persistent materials different from non-chlorinated. For non-toxic ones, there are a lot of benefits. For toxic ones, especially if they are liable to bio-accumulate, they must be avoided as much as possible, no matter if they contain chlorine or not. But there is no reason at all to accuse chlorine chemistry in general to be the only origin of persistent organic pollutants (POP's) in the environment.


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Created: May 26, 1998.

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