GFO vs iron nail

[takayan]

I read Randy's article which is Iron Oxide Hydroxide (GFO) Phosphate Binders.
Some of Japanese aquarists put some iron nail which is without rust prevention in their sump. Of course, the iron nail will be rusted because it is not with rust prevention. Their purpose is to reduce the phosphate level. Actually, some of the aquarits succeeded to reduce the phosphate level below 0.1ppm. (It might be less because most of them do not have good phosphate test kit.)

As Randy mentioned, GFO is Iron oxide hydroxide, but how does an iron nail reduce the phosphate level? Is it the same chemical reaction?
GFO: 2FeO(OH) -> Fe2O3 + H2O
Iron nail: Fe3+ + 3 H2O ⇌ Fe(OH)3 + 3 H+

Is Fe(OH)3 rust from the iron nail?

There are a lot of rusts on the surface of the iron nail. Eventually, it will be precipitation in their sump. (They actually remove the precipitation.)

[Randy Holmes-Farley]

Yes, you can form some iron oxide on the nail surface as nails rust. The concerns are; the purity of the metal in the iron nail (which you may well not want other metals that may be in cheap nails) and in the surface area on the nails, which is far less than on commercial GFO. I do not know the exact values, but you might need 100 pounds of nails (or more) to match the surface area in a cup of GFO (which can have quite a high surface area for binding phosphate).

[takayan]

Randy,
I noticed the nail surface is much less than GFO, but they only put 20 nails. Even though, they can maintain under 0.1ppm phosphate level. It may be because the rust is keep rusting and precipitating in the sump, so the nail always can have fresh Fe+++ surface to the water.

Only what I donot understand is the following thing.
Iron nail has precipitation of rust in the bottom of the sump, but GFO just bind the phosphate without no rust precipitation.

[Randy Holmes-Farley]

As the nails rust, the rust may flake off and be replace by some new surface area. So there may be some turnover, although it also may end up deposited somewhere in the tank if it really does come off (and that wouldn't be my preference by may be OK).

The GFO already is rust, clumped into a particle that is easy to retain and remove. that's really what the big inventive part of GFO is for water purification.

In any case, the mechanism of making some iron phosphate/oxide/hydroxide will be the mechanism. :)

[takayan]

Is the following chemical equation correct? Which one is the rust chemical form? Is it FeO(OH)?

Iron nail:
Fe+++ + 3OH- -> Fe(OH)3 -> FeO(OH) + H2O -> Fe2O3 + H2O

GFO
FeO(OH) -> Fe2O3 + H2O

[Randy Holmes-Farley]

An iron nail starts as Fe metal (Fe without any +), and gets oxidized to Fe+++ and converted to the oxide/hydroxide at about the same time.

I am not sure of the manufacturing process for GFO, nor exactly what portion might be converted to Fe2O3 in the tank.

[takayan]

Thank you, Randy!

[Randy Holmes-Farley]

You're welcome.

Happy Reefing. :)