Iron Oxide (II) vs Iron Oxide (III). GFO?

NatureHold

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Wanted to throw this question up in the forums as I caught myself researching but didnt really find an answer.

Point 1. So we use GFO (ferric/iron oxide) to remove PO4 from water, and the only prophylactic prior to its use is a good rinse to get rid of dust.

Point 2. I see a number of posts on the forum everytime corals retract, fish act weird, etc, etc, people always say to check your equipment for rust which is also an iron oxide.

I was hoping someone with a chemistry background could provide insight as to why GFO is generally recommended for use, but rust can be "problematic" in a tank.

Thanks,
Nature.
 

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It is probably not the iron that would be causing issues from rust, but rather other metals that can be particularly harmful at elevated levels. According to Randy, Iron is depleted pretty rapidly in a reef tank, which makes sense given that it is used in pretty much every photosynthetic thing in the tank (I do not know how it is used though). Here is his read on iron https://reefs.com/magazine/chemistry-and-the-aquarium-iron-in-a-reef-tank/
 
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NatureHold

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Pump parts are steel and not iron. When they corrode they release a host of other metals.
I agree. I was more thinking specifically along the lines of a magnet. Maybe in a frag rack or powerhead. From what I've read, most cheaper magnets have some type of iron in them.
 
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It is probably not the iron that would be causing issues from rust, but rather other metals that can be particularly harmful at elevated levels. According to Randy, Iron is depleted pretty rapidly in a reef tank, which makes sense given that it is used in pretty much every photosynthetic thing in the tank (I do not know how it is used though). Here is his read on iron https://reefs.com/magazine/chemistry-and-the-aquarium-iron-in-a-reef-tank/
Good read. Great info. I didnt know that iron was a necessity for photosynthesis.

However, I am presenting this question more on the contamination side. I.E. a form of iron oxide (not FE, but FE2O2 & FE2O3) that is not a natural part of your reef tank, and may be detrimental to the health of the coral and fish.
 

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Good read. Great info. I didnt know that iron was a necessity for photosynthesis.

However, I am presenting this question more on the contamination side. I.E. a form of iron oxide (not FE, but FE2O2 & FE2O3) that is not a natural part of your reef tank, and may be detrimental to the health of the coral and fish.


I'll tag @Randy Holmes-Farley
 

Randy Holmes-Farley

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I would not be concerned about pure iron oxide in a reef tank. Many folks, myself included, dosed iron regularly.

Pure iron is expensive and piles of rusted metal are not pure iron oxide.

I'd also add GFO is specially made to expose a high surface area to the water for treating drinking water (mostly to remove arsenic). It is not just random chunks of iron oxide.

Many chemical forms of iron oxide can be used to bind arsenic (and phosphate). This article compares the attributes of them. Table 1 has a summary:

 

Randy Holmes-Farley

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and may be detrimental to the health of the coral and fish.

How do you conclude that? The "may" is resolved by the fact that it has been very widely used without issues in most cases. Excessively low phosphate seems the biggest concern.

I'd also dispute the not natural part.
 
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NatureHold

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I would not be concerned about pure iron oxide in a reef tank. Many folks, myself included, dosed iron regularly.

Pure iron is expensive and piles of rusted metal are not pure iron oxide.

I'd also add GFO is specially made to expose a high surface area to the water for treating drinking water (mostly to remove arsenic). It is not just random chunks of iron oxide.

Many chemical forms of iron oxide can be used to bind arsenic (and phosphate). This article compares the attributes of them. Table 1 has a summary:


So is it fair to say when people freak out about rusted magnets or pump parts in the tank, the iron oxide is not so much the concern, it is the other chemicals that may have been produced in the rust formation?
 

Randy Holmes-Farley

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So is it fair to say when people freak out about rusted magnets or pump parts in the tank, the iron oxide is not so much the concern, it is the other chemicals that may have been produced in the rust formation?

Definitely.

Iron depletes very fast from reef tanks, by precipitation and by consumption. It is unusual (but not unheard of) to see any iron by ICP in a reef tank, even when dosing and using GFO.

Here's a copy of the iron section of an article of mine on ICP testing of my tank (I was using GFO at the time):

Iron (Fe). The natural iron level varies a lot with depth, but surface seawater may have only 0.006 µg/L. The Triton LOD = 0.3 µg/L. I dose iron, and when I dose it I boost iron to roughly 1-2 µg/L, which would be detectable. This sample was taken more than a week after the last iron dosing, and none was detected as it gets depleted in the meanwhile. I’ve not yet seen a Triton test result for a real aquarium sample that had detectable iron, but that doesn’t mean these tanks are necessarily deficient. Iron is also a case where the form is critical, and ICP cannot distinguish form. Binding to organic matter, for example, can alter the bioavailability of iron.
 
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NatureHold

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Definitely.

Iron depletes very fast from reef tanks, by precipitation and by consumption. It is unusual (but not unheard of) to see any iron by ICP in a reef tank, even when dosing and using GFO.

Here's a copy of the iron section of an article of mine on ICP testing of my tank (I was using GFO at the time):

Iron (Fe). The natural iron level varies a lot with depth, but surface seawater may have only 0.006 µg/L. The Triton LOD = 0.3 µg/L. I dose iron, and when I dose it I boost iron to roughly 1-2 µg/L, which would be detectable. This sample was taken more than a week after the last iron dosing, and none was detected as it gets depleted in the meanwhile. I’ve not yet seen a Triton test result for a real aquarium sample that had detectable iron, but that doesn’t mean these tanks are necessarily deficient. Iron is also a case where the form is critical, and ICP cannot distinguish form. Binding to organic matter, for example, can alter the bioavailability of iron.
Thank you! That answers my question. I didn't understand why GFO would be used if Iron Oxide was potentially problematic. But in conclusion, it is the other chemicals produced with rust formation that are considered problematic. Thanks Randy!
 

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