Phospate leaching from sand/rock

[Reef_Hobbyist]

Hi Randy,


In my tank I am going to use a dry rock called Pukani. It is the calcium carbonate coral skeletons mined from a dead coral reef no longer submerged in the ocean. It is also claimed to leach phosphates into the tank for quite some time (even years).

One group of folks, including BRS that sells Pukani, are adamant the phosphate comes from breakdown of organic material hidden in the unseen inner cavities of the rock - NOT from the rock itself.

Others feel phosphate it is being released from the rock and they soak it in acid which they claim eats away at the outer skin of the rock (where the phosphates are bound).

I have been curing my Pukani in large saltwater bins in the garage for over 2 months and I still see phosphate levels build back up after each 100% water change (although it seems to be diminishing over time).

I was originally thinking it was due to organic breakdown but after reading your article Phosphate In The Reef Aquarium: By Randy Holmes-Farley I am beginning to wonder if perhaps both groups are right and that it might be a combination?

In your article, you talked about the binding of phosphate to calcium carbonate surfaces in a higher pH environment (maximized at around pH 8.4) and how both aragonite sand and coral “skeletons” can act as a sink/reservoir for phosphate (assuming the pH remains at these higher levels). This allows algae to thrive despite cutting off all other external sources of phosphate.

I assume you mean the algae is feeding off this phosphate and if so, how is it released given the same higher pH environment maintained in most reef tanks?

Does pH play a significant factor here in terms of phosphate binding/releasing and if so, at what levels would we expect to see it shift one way or the other?

 

[Randy Holmes-Farley Verified Community Expert]

At all pH values, there is an on/off equilibrium between phosphate in the water and phosphate on calcium carbonate surfaces. That equilibrium will generally be attained fairly fast (hours, probably) for a flat surface. Some pH values have more bound than others, but I wouldn't worry much about pH unless you are going all the way to dissolving calcium carbonate with acid (see below).

Many types of calcium carbonate rock, however, are porous, and it can take a considerable time for phosphate down inside of pores to escape (or alternatively, for phosphate to get in from outside the rock.

Finally, phosphate need not be just on the surface but might actually be part of the rock, especially if it was exposed to elevated phosphate when the rock was still being made (such as in an aquarium, which may not apply here). Thus as any of the rock dissolves, more phosphate will be released. This part is hard to deal with.

According to BRS, the Pukani comes out of the ocean (at some point) and has organic matter. Organics deep inside of it will take a vary long time to degrade, and like phosphate already in pores, a long time to come out. Removal of organics can be accelerated with bleach (which breaks down the organics), and to some extent with acid which will dissolve the outerlayer of calcium carbonate out from under the organics.

Pukani does have a long term phosphate issue for whatever reason (not sure which or both).

IMO, a bleach then acid treatment is the best plan, but whether both are needed, I cannot be sure. Folks who have done both seem satisfied.

Alternatively, you can just deal with the phosphate once in the tank using binders such as GFO or aluminum oxide.

 

[Randy Holmes-Farley Verified Community Expert]

FWIW, you might also consider a lanthanum treatment of the water it is in now to drop the phosphate very low and accelerate the release process (of inorganic phosphate) from the rock. Once the phosphate rises int he rinse water, you are not removing it nearly as fast as if it is very low (again, for inorganic phosphate only, it won't impact the organic matter decay).

 

[Reef_Hobbyist]

FWIW, you might also consider a lanthanum treatment of the water it is in now to drop the phosphate very low and accelerate the release process (of inorganic phosphate) from the rock. Once the phosphate rises int he rinse water, you are not removing it nearly as fast as if it is very low (again, for inorganic phosphate only, it won't impact the organic matter decay).

Thanks, and yes I have seen a lot of methods used to curing the rock which seem to be effective. So are you in agreement that the rock can have phosphates bound to it and they can release (leach) into the water (in addition to organic breakdown)?

I am not too concerned with the Pukani, I am more interested in this process of phosphate binding to the calcium carbonate surfaces and releasing back into the water. I am curious about the equilibrium you spoke of.

At all pH values, there is an on/off equilibrium between phosphate in the water and phosphate on calcium carbonate surfaces. That equilibrium will generally be attained fairly fast (hours, probably) for a flat surface. Some pH values have more bound than others, but I wouldn't worry much about pH unless you are going all the way to dissolving calcium carbonate with acid (see below).​

So are you saying it is not a lower pH that is causing the phosphate to release from the rock, it is because the saltwater has less phospate in it than the rock and it is moving towards equilibrium? What would this process look like in our typical reef tanks? Would you please elaborate on that?

 

[Randy Holmes-Farley Verified Community Expert]

So are you saying it is not a lower pH that is causing the phosphate to release from the rock, it is because the saltwater has less phospate in it than the rock and it is moving towards equilibrium? What would this process look like in our typical reef tanks? Would you please elaborate on that?

Yes. pH will have an impact on binding, but it is not as if lower pH suddenly releases something that otherwise would have stayed put (unless the ph gets low enough to begin to dissolve the calcium carbonate, which it might inside pores and such, but not typically on the surface). .

 

[Reef_Hobbyist]

So we should expect that over time in our typical reef tank, since we add a lot of phosphate through feeding, calcium carbonate surfaces will act as a phosphate sink? And when we use methods to remove phosphate from the water, that phosphate moves from the rock back to the water to reach equilibrium?

 

[Randy Holmes-Farley Verified Community Expert]

So we should expect that over time in our typical reef tank, since we add a lot of phosphate through feeding, calcium carbonate surfaces will act as a phosphate sink? And when we use methods to remove phosphate from the water, that phosphate moves from the rock back to the water to reach equilibrium?

Yes, all in response to the concentration of phosphate in the water. More phosphate in the water drives more phosphate onto the rock, and less phosphate in the water allows it to come back off again.

 

[Reef_Hobbyist]

That is very interesting. One more factoid to add to the list of P in the aquarium.

Thanks for being patient with all my questions.

 

[Randy Holmes-Farley Verified Community Expert]

You're welcome any time.

This sort of discussion is why the forum is here.

 

[Dimorb]

Is there a way to calculate the amount of phospate that could be stored inside live rock?

Im having trouble with my rock/phospate.

My phospate are not moving from 55 ppb even if I exchange the rowaphos. The value on the water coming out from the filter is 30 ppb and the flow 52 gallons/hour.

So why isnt the overall value in the aquarium going down?

When i started I had 101 ppb and it quickly moved down to 73 and after that 55 but not it has stopped. Dosent matter if I change the media. Getting the same output from the filter and value in the aquarium isnt changing.

 

[Dimorb]

You're welcome any time.

This sort of discussion is why the forum is here.

If you do have the time, please give me your take on this problem.

Is there a way to calculate the amount of phospate that could be stored inside live rock?

Im having trouble with my rock/phospate.

My phospate are not moving from 55 ppb even if I exchange the rowaphos. The value on the water coming out from the filter is 30 ppb and the flow 52 gallons/hour.

So why isnt the overall value in the aquarium going down?

When i started I had 101 ppb and it quickly moved down to 73 and after that 55 but not it has stopped. Dosent matter if I change the media. Getting the same output from the filter and value in the aquarium isnt changing.

 

[Randy Holmes-Farley Verified Community Expert]

No, trying to calculate it is too difficult. You’d have to have an independent measure of the available surface area of the rock and sand.

 

[Xanka]

Hi Randy,

What do you mean by ‘bleach’ is this NaOCl? Can you also use NaOH to remove organics?

 

[Randy Holmes-Farley Verified Community Expert]

Hi Randy,

What do you mean by ‘bleach’ is this NaOCl? Can you also use NaOH to remove organics?

Bleaching is usually accomplished with NaOCl, yes, but other strong oxidizers can also bleach things and break down organic matter. Sodium hydroxide at room temp is not likely to adequately remove organics.

 

[Xanka]

One question. How long do you rinse the rock after this treatment, before you can use it in the tank?

 

[Clownfishy]

Just so I understand this, I have always had a zero reading of phosphate and low Nitrate (4ppm) but have algae. Therefore, the rock could be full of phosphate which is leaching into the water but is immediately taken up by the algae? If this is true and I use GFO, does the GFO beat the algae to the phosphate or the other way around?

 

[cracker]

I notice where algae seems to grow in the same places on certain rocks. I think this is where the algae finds nutrients to grow . That's where the Po4 is leaching. That's my theory at least. It took me 11 months to leach out excess Po4 from some old Haitian rock. In a darkened barrel with regular water changes.

 

[SashimiTurtle]

I had issues with uncured pukani in my tank... didn't read up on it before hand. I could use all the GFO wanted, and the rocks would be covered in algae or cyano, and sometimes both. I ended up replacing it all with cycled and coralline encrusted rock from a running tank because ofnthe problems I was having. I definitely agree with everyone who does the bleach and acid plus aging in a tub for several months prior to use... the amount of phosphates bound up in that type of rock is insane.

 

[Scottsquatch]

Is this also for the reef saver type rock? Like the kind sold by Marco rock? Or is this for the pukani with all the crevices where organic matter acumulates?