CHLORINE AND PERSISTENCE
FACTS AND FIGURES
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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) |
| material |
time |
remarks |
|
|
|
| vinylchloride (VCM): |
2 days |
|
| dioxins: |
12 days |
yes, dioxins are completely destroyed by UV-light! |
| dichloroethane: |
4 weeks |
|
| chloroform: |
23 weeks |
|
| methylchloride: |
75 weeks |
99% natural [40], accounts for 20% of
the ozone depletion potential [8]! |
| tetrachlorinecarbon |
> 2000 weeks |
95% 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 |
| material |
time |
remarks |
|
|
|
| chlorophenols: |
days |
|
| several chlorinated alkanes: |
weeks |
short-chained chlorinated hydrocarbons in heavely contaminated soil
disappeared completely within 20 years by natural breakdown [39] |
| DDT/DDE: |
20 years |
|
| Dioxin: |
millions of years? |
|
THE ALTERNATIVES
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.
CONCLUSION
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.
You are at level two of the Chlorophiles pages
Created: May 26, 1998.
Last update: September 8, 2001
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