Re: Aquatic Plants Digest V6 #101

[Roger Miller <roger at spinn_net>]

On Monday 31 March 2003 04:38, Paul Sears wrote:

> 	There have been discussions of this before.  As far as I know,
> plants have no problem using ferric iron.  In fact, NaFeEDTA (Fe III) is
> used as an iron supplement in agriculture.  I seem to recall Dave
> Huebert confirng this quite a while ago.  The problem with iron III
> in an aquarium is that if it isn't chelated, the Fe+++ ion is very
> readily hydrolysed, giving the insoluble oxohyroxide, which ends up in
> the filter.  If it _is_ chelated, it doesn't matter which oxidation
> state it's in.  If plants used ferric iron only, I can't see that much
> would grow anywhere - ever.

Light breaks down ferric-edta (and other ferric chelates) through a reaction 
called photoreduction.  Photoreduction is one of the major pathways by which 
iron can get wrestled out of a strong complex like ferric-edta.  When 
ferric-edta is broken down by photoreduction the chelating ligand is oxidized 
and the iron is reduced and released into the water as ferrous iron, not as 
ferric iron.

The ferrous iron will remain in solution for a while until it is taken up, 
oxidized back to ferric iron and/or recomplexed possibly by another chelating 
ligand and possibly by hydroxide or other common ligands in the water.


Roger Miller
Yeah, yeah.  I'm still here.