> Some time ago I wrote that EDTA would be one of the largest
> problems in German tap water industry. I was asked for dates -
> here they are.
> I have a report of the tap water industry of the Lake Bodensee
> area.
The area that you mention here is the same one that has several very
large chemical plants in addition to some of the largest molecular
biology labs in the world. Being a molecular biologist I know that we use
a great deal of EDTA in many of our buffers etc. It is rather useful for
a number of purposes. It is also becoming the standard addition in
micronutrient fertilisers for gardening and agricultural use. It makes
the formulations more stable to oxidisation, which is particularly true
of aquatic systems.
> The water contence of the river waters lays
between > 0.6. and 21 (!!) mg/cubic meter. The latter date is more as ten
> as much as any of the other complex builders.
This means that one teaspoon of EDTA would be dissolved in roughly 5,000
cubic meters of water or about 5megalitres.
>
> Although aquarism uses only low concentrations of EDTA,
> it is a certain part of the EDTA pollution of our waters. As
> to my other emails on this matter this is just one point why
> EDTA shouldn't be used.
Here we have a difference of opinion. There are a large number of
variables that you have not mentioned about the way that EDTA is bad. I
have not heard of a way in which EDTA is toxic other than by chelation
of metals. This can have some very drastic consequences, however I think
that some clarification is in order.
In water that contains EDTA it does not exist free, it binds to any
available metal ion. Where the water has an excess amount of
metal ions it will all be bound and thus not free to bind to new ions.
Where there is a vast excess of metal ions it will bind to the ions that
it is attracted to most strongly first ie Ca, Mg, Cu, Zn and especially
the heavier metals. Now in the very hard water that you have (ie >200ppm)
EDTA at a concentration of .021ppm will be almost totally bound to Ca in
the water.It also means that less than one thousandth of any particular
ion will be chelated with EDTA.
> > Don't forget that EDTA is absolutely useless if you want to
> remove chlorine or other compounds which had been added
> to desinfect the water.
Absolutlely true, EDTA will not bind anionic or neutral species at all.
Iron precipitates rapidly in water without chelation. It has a half life
in the order of hours and it is added to iron containing fertalisers to
make the solution more stable and hence the time that the iron is
available to the plants. The alternative is to add much more non chelated
iron to have the same effect and I can assure you that the increased iron
level in the water would be of far greater concern that a fraction of a
ppm of EDTA.
There is another problem Harro that may make things a bit harder for you.
The only alternatives to EDTA are compounds that are even more stable
than EDTA. The reason why EDTA has taked off in use is that is is a much
better agent than the traditional Ammonia, citric acid, oxalic acid
(toxic directly) and tartaric acid. I agree that finding it in the
waterways is of concern, but I would like to know how it is really being
toxic if it is at all before attempting to blanket ban the use of the
chemical.
Feel free to email me directly if you would like to have a more private
discussion.
Peter Hughes