Ask A Scientist Chemistry Archive

 
name         Candy
status       student
grade        9-12
location     N/A

Question -   When a degradable plastic decays, the starch or cellulose 
backbone may break down completely into carbon dioxide, water, and 
organic matter.
What happens to the fluorine, chlorine, or heteroatom groups on the 
polymer chains as backbone decays?
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If we are talking about synthetic biodegradable polymers, then this 
usually means that some industrial, non-biodegradable polymer is mixed 
with organic biodegradable such as starch or cellulose. In this case, 
bacteria will digest the biodegradable part, and the non-biodegradable 
part will be broken down into small powders. These kinds of 
"biodegradable" polymers are really an industry trick to gain the mark of 
biodegradability for the purpose of selling to consumers. But really, the 
only degradable part is the one eaten by the bacteria. The by-products of 
the degradation then is the by-product of bacterial digestion plus the 
original non-biodegradable -still polymeric- component that is now in 
powder form.

True biodegradability (all of the polymer material being biodegraded) has 
been achieved by allowing bacteria to produce the polymers. Essentially 
such bacteria (e. colli) naturally produce polymers for energy storage 
(like we do fat). The polymers are harvested from the bacteria and the 
rule of thumb is: "If some organism will produce it, then some organism 
will eat it." Thus, these biodegradable polymers are completely digested 
by bacteria, and the by-products are whatever the bacteria releases as 
by-products.

Greg (Roberto Gregorius)
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The answer to this is complex because it depends upon the type of polymer.
Fluor-polymers tend to be remain stable unless heated to high temperature.
Chloro-polymers can undergo a number of decay reactions, but a common one
is the photochemical induced elimination of HCl. Amide functional
polymers, like Nylon, tend to be rather stable but when they do decompose,
the major products are usually the di-amine and di-carboxylic acids by
reaction with water. This is the reverse of the reaction of formation of
the polymer. These are just "typical" decay reactions however. In "real
life" the decay reactions are much more numerous and complicated.

Vince Calder
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Fluorine, chlorine, and bromine content substantially slow the biodegradation
of whatever plastic they are in.   In the long run some bacterium might
slowly digest them along with large amounts of usable food.
A starch-like backbone is such usable food.
When a bacterium dies, or expels halogenated waste,
water exposure and oxidation may extract them as acids: HF, HCl, HBr.
These are water-soluble, and quickly neutralized to salts: NaF, NaCl, NaBr.
These in turn can be expected to travel to the ocean and be trapped there.

Phosphorus and sulfur in the plastics are lively parts of biochemistry
and can be easy points of attack for digestion (certainly true of detergents).
A few heavy metals may be detrimental, but eventually they get handled by 
some species.
I've heard that gold-veins in the earth were residues from geo-bacteria
incidentally collecting the atoms as they fed on something in seeping 
groundwater.
Bismuth is not a problem, and even lead is not as toxic to bacteria as it 
is to us.
Silver and copper are more toxic to most bacteria.

Jim Swenson
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