Dosing Iron to reduce Phosphates?

[Randy Holmes-Farley Verified Community Expert]

I’m dosing 10x the amount of Fe that I started with, and I see “zero reduction” in PO4 at this point in time, and everything is exactly the same. No changes.

How much iron is that? It is probably far too small to directly detect iron phosphate precipitation. Folks using that method dose large amounts.

 

[dschuffert]

This is an interesting thread. I have dosed Fe for coral color. I didn't realize there could also be an impact to Phosphate reduction.

 

[Reefahholic]

How much iron is that? It is probably far too small to directly detect iron phosphate precipitation. Folks using that method dose large amounts.

I’ll have to look at it. I don’t think it’s a lot. A small to moderate amount for a 105/G maybe.

 

[Randy Holmes-Farley Verified Community Expert]

This is an interesting thread. I have dosed Fe for coral color. I didn't realize there could also be an impact to Phosphate reduction.

You need to dose large amount to have a useful effect. Far more then ordinary iron supplementation.

 

[dschuffert]

You need to dose large amount to have a useful effect. Far more then ordinary iron supplementation.

True if the gap is large. I am dosing 4ML daily (Red Sea Fe) in 900L, keeping me around the 12 ppb range.

 

[Randy Holmes-Farley Verified Community Expert]

True if the gap is large. I am dosing 4ML daily (Red Sea Fe) in 900L, keeping me around the 12 ppb range.

If you dosed a full 12 ppb every day, even if every iron atom dosed precipitated as ferric phosphate, you’d only remove 20 ppb of phosphate.

 

[dschuffert]

If you dosed a full 12 ppb every day, even if every iron atom dosed precipitated as ferric phosphate, you’d only remove 20 ppb of phosphate.

Good point. I am not dosing Fe to lower phosphate. I commented on the post that I dose Fe for coral color and was not aware of any impact on phosphate. I am learning about the phosphate discussion and just learned a little more from you, thanks!

 

[Levinson]

Dosing iron to bind phosphate by creating iron phosphate precipitate sounds very interesting.
I dose iron solution made from chelated iron tablet (ferrous bisglyncinate chelate) for my algae turf scrubber (thanks to @Randy Holmes-Farley for the recipe). I only dose a little occasionally but if it can be used to reduce phosphate in the water by dosing more, I'm interested to learn more. I have some questions if I may ask.

Q1.
Is there a risk of the precipitates, forming small particles and harming the animals in the tank like how it is often considered a risk for lanthanum dosing?

Q2.
I was wondering if dosing iron into an anaerobic environment would make it more effective or if I would be better off dosing it in the open?
I read iron can react with phosphate more efficiently in an anaerobic environment. It was something about how iron III can be reduced to iron II which has higher affinity for phosphate. I don't understand what it all means but it sounded like iron dosing to create phosphate precipitate would work better if done in low or no oxygen environment. Does anyone know if this is the case?
I have an anaerobic chamber (Donnovan's nitrate destroyer, I think it only becomes anaerobic when carbon is dosed into it though I'm not certain) and I can dose iron into it if it can be more effective.

Q3.
Would the dosed iron also react to, or bind with silicate (like how GFO would)? Form precipitates maybe?
I think I have much silicate in the water though I have no way of testing for it. I was wondering if it would make the iron dosing to reduce phosphate less effective or reduce the level of silicate in the water.
I've put in some ceramic siporax media in the DND on purpose so when vinegar is dosed, it would release silicate into the water (only happens when vinegar is dosed, the low pH of vinegar seems to be the cause rather than the anaerobic condition). I wanted the silicate for the sponges and diatoms. I had no way of measuring the level of silicate but have tested by removing the siporax media temporarily (replaced with other media) and the sponge growth stopped and it started to grow again after a few days the siporax media was put back in.

 

[Randy Holmes-Farley Verified Community Expert]

A1. I do not know if there is a particulate risk to fish from precipitation iron phosphate, and I'm also not even sure if it does form suspended particulates when it does precipitate. (FWIW, the lanthanum issue has also not been proven to be particulates, IMO).

A2. Ferrous phosphate is more soluble than ferric phosphate. Not sure where the anaerobic idea comes from, but it is not correct.

A3. Dosed iron may bind some silicate, but I think the effect will be pretty small at the levels of iron and silicate we encounter in reef tanks.

 

[Levinson]

A1. I do not know if there is a particulate risk to fish from precipitation iron phosphate, and I'm also not even sure if it does form suspended particulates when it does precipitate. (FWIW, the lanthanum issue has also not been proven to be particulates, IMO).

A2. Ferrous phosphate is more soluble than ferric phosphate. Not sure where the anaerobic idea comes from, but it is not correct.

A3. Dosed iron may bind some silicate, but I think the effect will be pretty small at the levels of iron and silicate we encounter in reef tanks.

Thank you for the answer!

 

[Randy Holmes-Farley Verified Community Expert]

Thank you for the answer!

You’re welcome,

Happy Reefing!

 

[Levinson]

You’re welcome,

Happy Reefing!

It seems it is the opposite of what I have commented in Q2. I am not confident if I have understood this correctly but it seems Ferrous iron will oxidize to Ferric iron with high oxygen level and ferric iron will strongly bind with phosphate and precipitate, forming ferric phosphate, removing some phosphate in the water column (assuming there is enough phosphate in the water).
In low oxygen environment, the Ferrous iron will oxidize slower making it more bio available for macro algae and such but less effective for binding dissolved phosphate.
So from what I understand, it seems dosing into a high oxygen environment would be better for binding phosphate but not for macro algae since it the iron would be less bio available and the opposite is the case for dosing into a low oxygen environment.
Also, the chelation may slow down oxidation, keeping the iron in the ferrous form for longer making it more bio available. Which got me thinking if I used non chelated iron supplement, (such as ferrous gluconate or ferrous bisglyncinate) or oxidize the iron solution to some extent in advance on purpose to turn it into ferric iron, would it be more effective for reducing dissolved phosphate?

 

[Lasse]

I read iron can react with phosphate more efficiently in an anaerobic environment. It was something about how iron III can be reduced to iron II which has higher affinity for phosphate. I don't understand what it all means but it sounded like iron dosing to create phosphate precipitate would work better if done in low or no oxygen environment. Does anyone know if this is the case?

It seems it is the opposite of what I have commented in Q2. I am not confident if I have understood this correctly but it seems Ferrous iron will oxidize to Ferric iron with high oxygen level and ferric iron will strongly bind with phosphate and precipitate, forming ferric phosphate, removing some phosphate in the water column (assuming there is enough phosphate in the water).
In low oxygen environment, the Ferrous iron will oxidize slower making it more bio available for macro algae and such but less effective for binding dissolved phosphate.
So from what I understand, it seems dosing into a high oxygen environment would be better for binding phosphate but not for macro algae since it the iron would be less bio available and the opposite is the case for dosing into a low oxygen environment.
Also, the chelation may slow down oxidation, keeping the iron in the ferrous form for longer making it more bio available. Which got me thinking if I used non chelated iron supplement, (such as ferrous gluconate or ferrous bisglyncinate) or oxidize the iron solution to some extent in advance on purpose to turn it into ferric iron, would it be more effective for reducing dissolved phosphate?

I think your second statement is the right - especially when hydrogen sulphide is produced in the anaerobic environment, Phosphate release in the fresh water lakes (and the Baltic) is always connected with anaerobic sediment and hydrogen sulphide production. If you adjust your denitrator to have a NO3 concentration around 1-2 mg/L NO3 in the outlet - you will not have so much H2S production inside and limit the release of PO4 from its metal contra-parts.

NO3 in the water favour the type of heterotrophic bacteria that use NO3 instead of O2 in an anaerobic environment and supress the heterotrophic bacteria that use sulphur and sulphur containing substance instead of O2 in a anaerobic environment.

Note - this bacteria is not the same that are active in a sulphur based denitrator - those are autotropic bacteria.

Sincerely Lasse

 

[Randy Holmes-Farley Verified Community Expert]

It seems it is the opposite of what I have commented in Q2. I am not confident if I have understood this correctly but it seems Ferrous iron will oxidize to Ferric iron with high oxygen level and ferric iron will strongly bind with phosphate and precipitate, forming ferric phosphate, removing some phosphate in the water column (assuming there is enough phosphate in the water).
In low oxygen environment, the Ferrous iron will oxidize slower making it more bio available for macro algae and such but less effective for binding dissolved phosphate.
So from what I understand, it seems dosing into a high oxygen environment would be better for binding phosphate but not for macro algae since it the iron would be less bio available and the opposite is the case for dosing into a low oxygen environment.
Also, the chelation may slow down oxidation, keeping the iron in the ferrous form for longer making it more bio available. Which got me thinking if I used non chelated iron supplement, (such as ferrous gluconate or ferrous bisglyncinate) or oxidize the iron solution to some extent in advance on purpose to turn it into ferric iron, would it be more effective for reducing dissolved phosphate?

The bioavailability of ferrous vs ferric iron is a very complicated topic that I’ve addressed a few times. There’s no simple answer of what form of iron dosed results in the most bioavailability. Many organisms can convert the two forms, and solubility limits and other factors impact bioavailability.

Fortunately, we can easily dose so much more than natural levels that exactly what dosed form gives the highest bioavailability is not important.