Reef Chemistry Question of the Day #114 GFO and CaCO3 precipitation

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #114

Many aquarists using GFO (granular ferric oxide) as a phosphate binder have reported significant precipitation of carbonates on the solid GFO and elsewhere in the system. Such precipitation can, for example, be a contributing factor in the caking of such materials and can coat other surfaces in the aquarium. This precipitation can also contribute to a detectable drop in alkalinity, and possibly pH, as it removes carbonate from the water column.

Dissolution of these precipitates with acid, accompanied by bubbling, indicates that these deposits are carbonates and are most likely calcium carbonate because it is supersaturated in most reef aquaria (and in the ocean).

Which of the following is the least likely explanation for the precipitation of calcium carbonate when using GFO?

A. Removal of nearly all of the phosphate from the water in the vicinity of the GFO may accelerate the precipitation of calcium carbonate.
B. Addition of soluble iron to the water in the vicinity of the GFO may accelerate the precipitation of calcium carbonate.
C. Removal of nearly all of the strontium from the water in the vicinity of the GFO may accelerate the precipitation of calcium carbonate.
D. Removal of many organics from the water in the vicinity of the GFO may accelerate the precipitation of calcium carbonate.

Good luck!








































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[beaslbob]

none of the above.

 

[chefjpaul]

C.
Wouldn't the removal of strontium do the opposite as its a calcium builder?

 

[fuzzone99]

B
I can't imagine the mechanism where additional Fe would cause precip of CaCO3?

 

[Randy Holmes-Farley]

And the answer is...C. Removal of nearly all of the strontium from the water in the vicinity of the GFO may accelerate the precipitation of calcium carbonate

Removing (or adding) strontium will have relatively little effect on the solubility of calcium carbonate in seawater.

Phosphate can get onto the growing calcium carbonate crystal surface and can reduce the rate of crystal growth. So removing phosphate can speed precipitation of calcium carbonate. Organics act the same way, so removing them with GFO may accelerate precipitation.

Soluble iron seems to be able to nucleate the precipitation of calcium carbonate, at least in fresh water and likely in seawater.

Happy Reefing!

 

[chefjpaul]

Removing (or adding) strontium will have relatively little effect on the solubility of calcium carbonate in seawater.

So, to understand this fully. Why wouldn't strontium have an effect since it's a close relative to calcium as a skeleton/ bone building chemical?

I guess what I'm asking is what the major difference vs calcium that would have little effect? And why you don't really advise worrying / testing for.
(Besides it's in our artificial salts).

Would it be appropriate to ask if we took out the gfo scenario and the relationship of the two with a high calcification, like harder sand? Does the level of strontium affect this?

Sorry if this doesn't sound right, just love to learn the ocean chemistry.

 

[Randy Holmes-Farley]

If you have calcium carbonate exactly at saturation (the maximum amount that can dissolve) and then add some strontium chloride, it will have very little effect on the solubility of the calcium carbonate.

Instead of asking why it wouldn't, a better question is why would it?

The solubility of salts is, to a first approximation, independent of other salts. So, for example, the solubility of sodium chloride is not impacted much by the presence of potassium nitrate. Solubility of something like sodium chloride is determined by the product of the sodium concentration times the chloride concentration.

For any of the things in this question to impact precipitation, one needs something unusual to happen. In some of these cases those things are ions getting onto the growing calcium carbonate crystal and messing it up. The amount if strontium is so low in seawater that it doesn't tend to do that much. Magnesium does, but there is hundreds of times more of it present, and the mixed crystals with calcium tend to be less soluble. I don't think happens with strontium.