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Carbon&Iron...again
I hope this time our discussion of this subject will lead to some
more definitive answers. Last time I got involved in this topic (about a
year ago, I guess), I asked a lot of questions that never were answered to
my satisfaction. At the time, I was sceptical of statements that carbon
would remove trace elements, and I also had not seen a theory that would
explain how such removal would take place. The experiential evidence I
received in response convinced me that the removal was indeed taking
place. What I would call reliable sources (including K. Randall), told me
there was lots of evidence in both the terrestrial and aquatic plant world
showing loss of iron in the presence of carbon. Then others explained to
me the mechanism involved: Carbon doesn't remove the iron itself, but
rather the chelating agent which holds the iron in a usable form, or
prevent the iron from oxidizing so quickly that plants presumably get
little out of it. So, chelating agents like EDTA, which are indeed
organic comlexes could in theory be removed by activated carbon, since
carbon does indeed remove organics. I still want to know at what rate,
and in fact would like to see the chemistry involved which would show that
carbon removes those specific organics such as EDTA.
I also pointed out that while iron in the water column would
theoretically be adsorbable by carbon, iron in the substrate might be a
different story. Presumably a certain percentge ends up in the water
column, but it seemed to me at the time that plants pulling nutrients out
of the substrate would grab a hold or trace elements like iron fast enough
that they would satisfy their requirement before the iron could end up in
the water vcolumn, where it _might_ disappear faster than it could be used
by the plants. (sorry for the run-on sentence there ;-)). And it seemed
to me that there were other factors involved, such as the role humic
acids, such as would come from peat, bog wood etc, would play.
Many people also pointed out that only fresh carbon was really a
concern, and that adding trace elements every day to the water column
would most likely offset any adsorption/oxidizing of the EDTA/iron. It
seems to me that there is somewhat unchartered territory here.
By the time this particular round of the iron/carbon discussion
came to a close (back in June, '96, I believe), Jim Layton of Aquarium
Pharmaceuticals was commencing experiments on the effects of activated
carbon on chelated Cu, Fe and so on. He said he would publish the data on
the NG ( and presumably here) when he was done. I imagine we will have a
while to wait, since he made it clear he wasn't going to publish
half-finished data.
Hope this discussion proves as fruitful as most of the others here
do,
Andrew