I'm moving this to a new thread. Many have observed algae die-offs in
tanks treated with Seachem Flourish Excel and I want to float (so to
speak) a hypothesis as to why it might be algicidal. One of the
components in Flourish Excel converts iron from it's ferric 3+ state
back to its more soluble ferrous 2+ state.
Iron is not generally toxic by itself, but it functions as an oxidizing
and reducing agent in biological systems, including fishtanks. My
hypothesis is that reduced iron is generating toxic oxygen species in
iron-rich tanks treated with Flourish Excel. The chemistry here is from
this review.
http://www.gci.ac.uk/lab/reviews/pwrev.html
Iron tends to react with dissolved oxygen in water. This reaction is
predominantly to the right as ferric iron tends to fall out of solution
and superoxide is rare under normal tank conditions.
Fe2+ + 02 -- Fe3+ + 02·- (superoxide radical)
There is also a relatively rare reaction of ferrous iron and water to
create hydroxyl radicals. Hydroxyl radicals are very reactive and will
tend to rip ·H off of the next organic molecule they encounter, be it
DOC, fish, plant, or algae.
Fe2+ + OH- -- Fe3+ ·OH (hydroxyl radical)
So superoxide radicals form an equilibrium in water and hang around for
a while at pH 7. Superoxide is somewhat toxic and I'd be surprised if
algae and bacteria can handle large amounts.
O2·- + H+ -- HO2·
Three reactions take place to convert superoxide into hydrogen peroxide.
Now we're getting somewhere. Algae definately don't like peroxide.
2 HO2· -- H2O2 + 02
HO2· + O2·- + H+ -- H2O2 + O2
2 O2·- + 2 H+ -- H2O2 + O2.
Peroxide can react again by the Fenton Reaction. More toxic hydroxyl
radicals are generated in this pathway.
Fe2+ + H2O2 - Fe3+ + OH- + ·OH
So, my guess is that the the ferrous iron created by the iron reducing
reagent in Flourish Excel is creating peroxide, superoxide, and hydroxyl
radicals in the tanks (in unknown proportions). The former two are more
toxic to lower life forms than fish and plants because fish and plants
have enzymes to detoxify them. Hydroxyl radicals are highly reactive
and will damage anything they encounter. Most probably just react with
DOC in the water column. As long as the Excel reducing agent keeps the
tank's iron reduced, reactive oxygen species will continue being
generated. The amount will depend at a minimum upon on the tank's iron
levels, pH, and DOC.
Please, anyone who's good with iron chemistry see if this makes sense.
--
__ Elaine T __
__' http://eethomp.com/fish.html '__