PO4 Precipitation

[gman0526]

I've seen many times people advising to use kalk to precipitate PO4 when this is a problem. My take on it is that you're not removing, but storing for later problems.

My questions are... Is there any possible way of making this work? Could you make PO4 precipitate, so it might be removed by good skimming? Are PO4 molecules skimmable?

Assuming they could be removed by skimming... does it make more sense to use such a "fix" in a BB where there's no sustrate that can pontentially trap PO4? Or could it be used in a system with any kind of sustrate provided there's enough flow to keep everything in suspension?

My chem is lackluster to say the least so your insight would be greatly appreciated.

 

[wrightme43]

Hey Boomer, Or Nikki the link Master. Help!!!!!!!!
Great question.

 

[Curtswearing]

I've seen many times people advising to use kalk to precipitate PO4 when this is a problem. My take on it is that you're not removing, but storing for later problems.

The amount of precipitation is limited at best. I'm HIGHLY guilty of spreading the myth that this is a good way to solve phosphate problems. You are quite correct. Any amount that is precipitated is STILL IN THE SYSTEM.

My questions are... Is there any possible way of making this work? Could you make PO4 precipitate, so it might be removed by good skimming? Are PO4 molecules skimmable?

Orthophosphates are inorganic. Skimming works by Van der Wall's forces and highly depends on hydrophobicity. Most organics are hydrophobic but most inorganics are not. Skimming won't take the PO4 out directly as a result. That's why many people with SPS are running their skimmate wet. The bubbles aren't binding them like organics. However, they are leaving much like a small, continual waterchange. Detritus in the water column is also leaving in the same manner before being allowed to degrade. RHF suggests that there is a possibility that inorganic P can be chemically removed by a two step process. Basically PO4 adsorbs to Calcium or Magnesium to become Calcium phosphate or Magnesium phosphate. Then bacteria (which are organic) coat it, and then because the bacteria is hydrophobic, it can be skimmed out. I tend to agree with him but I don't know for sure. I've watched Major NSW Elements disappear in softy tanks in excess of what I would consider needed for coralline algae and can't explain it any other way.

Phosphate might be incorporated into certain inorganic particulates, like CaCO3/MgCO3, which could be skimmed if coated with organics. Of course, calcium and magnesium in these particulates are also removed.

Protein Skimming: How it works by RHF

Assuming they could be removed by skimming... does it make more sense to use such a "fix" in a BB where there's no sustrate that can pontentially trap PO4? Or could it be used in a system with any kind of sustrate provided there's enough flow to keep everything in suspension?

My attitude is use kalk what it is good for. Kalk maintains Calcium and alkalinity in a balanced manner and elevates pH during the night when pH naturally drops.

 

[gman0526]

TY Curt. Randy pretty much replied the same thing to my question on RC. You did go into a lot of more detail that really helped.

"Maybe, or maybe it is already partially skimmed out as the fine calcium phosphate particulates are coated with organics and become skimmable." RHF

Now I gotta go read some more and be back with more questions later.

 

[Curtswearing]

I'm not curious like a cat and feel no need to look. NOT!!!

 

[MikeS]

I've always heard that kalk helps pecipitate PO4 that has accumulated on the rocks and substrate back into the water column, thus making it easier to be removed by skimming and/or phosphate reducing media....not sure of the mechanics behind this, however....

MikeS

 

[NaH2O]

Then bacteria (which are organic) coat it, and then because the bacteria is hydrophobic, it can be skimmed out.

I definately don't disagree here, however, what if the organically coated particles don't make their way to the skimmer? What if they are allowed to settle? Do you think bacteria would aid in re-releasing the PO4 back into the water?

 

[gman0526]

Likewise, phosphate can precipitate onto the surface of calcium carbonate, such as onto live rock and sand. The absorption of phosphate from seawater onto aragonite is pH dependent, with the maximum binding taking place around pH 8.4 and with less binding at lower and higher pH values. If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can make it difficult to completely remove excess phosphate from a tank that has experienced very high phosphate levels, and may permit algae to continue to thrive despite cutting off all external phosphate sources. In such cases, removal of the substrate may even be required.

The relationship of calcium carbonate to the phosphate cycle has been studied by Frank Millero in the Florida Bay ecosystem (click here for Millero's studies). If aragonite crystals are growing, as they often are in some parts of our systems, then I’d expect some of this phosphate to get buried and locked into the aragonite crystals.


http://www.advancedaquarist.com/issues/sept2002/chem.htm

From RHF's article in Advanced Aquarist.

 

[Boomer]

I think it was well answered other than this part


Likewise, phosphate can precipitate onto the surface of calcium carbonate, such as onto live rock and sand


The relationship of calcium carbonate to the phosphate cycle has been studied by Frank Millero in the Florida Bay ecosystem (click here for Millero's studies).

I will argue this will not really happen. You need a fresh surface and in the ocean there is basically no such thing, as everything is coated with bacteria, about the same thing for tanks, except when you are starting out fresh

Basically PO4 can precipitate as Calcium phosphate or Magnesium phosphate.

Then bacteria (which are organic) coat it, and then because the bacteria is hydrophobic, it can be skimmed out.

I really do not buy this 100 %. That is if it is "light" enough and makes it to the skimmer. IMHO it will just fall to the bottom for the most part, where it can now go back in solution.

People seem to have lost the original intent of kalk removing PO4. It purpose was not in tank use but to add the kalk to tap water to remove PO4. The PO4 falls to the bottom and you then siphon off the top PO4 free make-up water

Craig talks all about it here;

http://web.archive.org/web/20030618...twork.com/fish2/aqfm/1999/aug/bio/default.asp

 

[reedman]

People seem to have lost the original intent of kalk removing PO4. It purpose was not in tank use but to add the kalk to tap water to remove PO4. The PO4 falls to the bottom and you then siphon off the top PO4 free

Great post Boomer. This has definitely been lost over time.

 

[gman0526]

...I really do not buy this 100 %. That is if it is "light" enough and makes it to the skimmer. IMHO it will just fall to the bottom for the most part, where it can now go back in solution.

People seem to have lost the original intent of kalk removing PO4. It purpose was not in tank use but to add the kalk to tap water to remove PO4. The PO4 falls to the bottom and you then siphon off the top PO4 free

That's exactly the way I looked at it, that's why I included a question relating to flow in the tank being important/needed for this to conceivably work.

The PO4/tap water/kalk treatment makes a whole lot of sense. TY Great post!!