Phosphate Absorption Rates in Aragonite

[jda]

I was chatting with @mcarroll the other day about phosphate binding to aragonite.

I have seen many instances where phosphate bound live rock and sand requires massive amounts of media and water changes to remove all of the phosphate. Two instances:
1). Local purchased 120lbs of Pukani live rock which took about 6 gallons of GFO to get the container water phosphate down to 2-3 PPB. This was after an acid bath.
2). 10 year old FOWLR with about 300 lbs of Marshall Island rock (fantastic rock) that had P in the tank about 1.6-1.8. Based on #1 above, used aluminum based media on this rock and it still took about fifteen pounds of it (almost a whole bucket).

I am wondering if anybody has any kind of a formula for how much aragonite can bind out of the water column at a certain level. ...like 10 lbs of pure aragonite can absorb 200 ppm of phosphate from the water column at a final "equilibrium" level of .1 in the tank water? (I just made that up)

If there is not any available, I was going to run an experiment, but need help...

0). Make sure that my fresh saltwater mix is at zero phosphate with Hannah Ultra Low. I have a recipe with IO, Muratic Acid and Dowflake that gets to 1.026, 7.2 and 425
1). Wash and soak 5 lbs of Carib Sea Large Size ARM in Fresh Saltwater for a few days - test with Hannah Ultra Low to see if it is truly phosphate free. If not, then use GFO to get it phosphate free.
2). Mix a solution of X.0 PPM of phosphate into 5G of heated water - no idea about how much to add
3). Put the 5lbs of now phosphate free aragonite in a reactor on the tank
4). Measure every day the water phosphate level with Hannah Regular or ULR checker until it stops moving.
5). Evaluate
6). Maybe add more phosphate to the tank and see if the final stopping/equilibrium point in linear or exponential.

Would this tell us how much 5lbs of aragonite can bind from the water column and where the end "equilibrium" level will be? ...or did I mess this up bigtime?

My hypothesis is that something like 2.0 PPM of water based phosphate with this kind of media will result in a final tank reading of about 2-3 PPB... but this is just a wild guess.

I do understand that not all structure has the same surface area and ability bind the same, but I am just looking for something rough.

I do need some clarification:
1). Heard before that phosphate binding/unbinding is more effective at higher temps, so going to run test at 78 like most reefs. True/False on this one?
2). What level should I start the fresh saltwater? I was thinking 2.0, but that might be low.
3). What kind of phosphate should I buy? Any idea of a rough amount to get to the level in #2?
4). Was going to use saltwater since I have no idea if the binding rates are different in fresh. Does salinity matter? I was planning on being at 1.026 anyway.

 

[Randy Holmes-Farley]

The problem is knowing the surface area. Knowing that, we could calculate from literature data how much might bind at a given phosphate concentration. Without it, an experiment is needed, and then will only apply to that exact material.

Higher temp gives more binding. I just read that paper today,

pH is important.

Yes, salinity matters,

The concentration to start with will necessarily be a function of the water volume to aragonite surface area. The first test will necessarily be a guess, and something readily measured seems reasonable. Say, 0.5 ppm or 0.2 ppm.

Source of phosphate won’t matter much, except the small pH effect.

 

[jda]

Sodium Triphosphate OK? Seems cheap, pure and available. Would 5 pounds be enough for this? I have no idea how much to add without trial and error... or finding a calculator.

 

[Randy Holmes-Farley]

Sodium Triphosphate OK? Seems cheap, pure and available. Would 5 pounds be enough for this? I have no idea how much to add without trial and error... or finding a calculator.

5 pounds is way, way more than enough lol

2 ppm (~2 mg/L) in 100 liters of water takes only 0.2 grams of phosphate, so 1 g TSP is enough.

 

[mcarroll]

I still don't grok the equations, but this article (full text!) seems to explain it:
https://s3.amazonaws.com/academia.e...=Adsorption_and_desorption_of_phosphate_o.pdf

The part I do get is that 80% of whatever is bound to aragonite was able to de-sorb within 24 hours. It's a fast process, not a slow one.

I feel like my main question is about the equation. From their usage I think we could compute the amount of PO4 that can be adsorbed on a given surface area.

 

[mcarroll]

The problem is knowing the surface area.

Let's say 500 cm2.

For the acro skeleton (image a)in this pic:

(That's just a large estimation from the selection of acropora surface area measurements in an article I found. "Coral surface area quantification–evaluation of established techniques by comparison with computer tomography")

 

[Randy Holmes-Farley]

I still don't grok the equations, but this article (full text!) seems to explain it:
https://s3.amazonaws.com/academia.e...=Adsorption_and_desorption_of_phosphate_o.pdf

The part I do get is that 80% of whatever is bound to aragonite was able to de-sorb within 24 hours. It's a fast process, not a slow one.

I feel like my main question is about the equation. From their usage I think we could compute the amount of PO4 that can be adsorbed on a given surface area.

The equilibrium is fast from an exposed surface, but not from a deep pore or down in a sand bed where the diffusion is slow.

 

[Randy Holmes-Farley]

Let's say 500 cm2.

For the acro skeleton (image a)in this pic:

(That's just a large estimation from the selection of acropora surface area measurements in an article I found. "Coral surface area quantification–evaluation of established techniques by comparison with computer tomography")

I think you’d be spinning your wheels to try to guess the available surface area of live rock or sand without measurements. The microscopic roughness is very important.

 

[mcarroll]

The equilibrium is fast from an exposed surface, but not from a deep pore or down in a sand bed where the diffusion is slow.

Even considering that, it's hard to imagine this being the sole effect reponsible the LARGE amounts of leaching that are apparaently being reported.

Wouldn't the attenuated speed of desorpotion from the interiors eventually give way to a MUCH SMALLER po4 level as it fell into line with the rate of consumption?

With the bulk of desorbing from "available surfaces" being 80% done in 24 hours, this falling in line seems likely to me.

I think you’d be spinning your wheels to try to guess the available surface area of live rock or sand without measurements. The microscopic roughness is very important.

Those appear to be the best estimates available (the number and skeleton photo are from a journal article) and that specific measure I quoted accounted for the most interior skeleton space of of any of the models they used.

So I wonder what the calculation would show on 500 cm2?

(If there's a better estimate that comes with a pic I'm happy for that to be used instead....but this is the best I could find, it's pretty recent (2009) and I'd really like to know the outcome of that formula.)

 

[Randy Holmes-Farley]

I’m sorry, I’m not understanding what the confusion or uncertainty is??? You don’t believe much can absorb onto live rock? Or that it would equilibrate faster then people see?

 

[Randy Holmes-Farley]

Fwiw, sand can have a surface area of tens to hundreds of cm2 per gram, depending on particle size, so 50 pounds of sand could possibly have hundreds of thousands to millions of cm2.

 

[Randy Holmes-Farley]

I think the 500 cm2 number you are showing is estimated coral tissue surface area, not the skeleton surface area.

 

[Randy Holmes-Farley]

FWIW, BRS tested the total surface area of some rock types and got about 0.5 m2/g for Pukani. That’s 5,000 cm2 per gram. A hundred pounds of rock has 227 million cm2. Not all of that is likely accessible by phosphate, but a lot of it will be since phosphate is pretty small.

https://www.bulkreefsupply.com/video/view/Which-rock-has-the-most-surface-area/

 

[jda]

I agree that it can release most of what it is going to release pretty quickly. I notice phosphate levels in lower concentration water rise over about 48 hours, but with lots of the areas covered in coral, coralline or other micro/macro fauna, this seems reasonable with the varying degrees of flow getting out of the rock. There can probably be some local concentrations that are higher than the tank water which will stop release until lower concentration water gets localized by washing out the higher concentration water. I think that the 24 hours on clean aragonite vs 48ish hours in a real tank are pretty close.

Where we read it differently is that it appears that you are expecting the aragonite to release 80% of the total amount of phosphate back into the water, regardless of concentration in the rock. I have experienced nothing like this... it will release all that it will will release rather quickly, but then it will stop when the water hits a certain concentration. It will not ever release any more until that water level is dropped, even if the aragonite still has massive amounts of phosphate in it.

My experience has been that it will release to an equilibrium level, not amount. When the water level of PO4 is lowered, it will release a bit more up to just about where it was before.

I want to try and figure out what that level is... like if you have 100 lbs of pukani, .5 in the tank then the sand&rock would need 2.5 pounds of GFO to get it to .01. ...or something like this. BTW - this is not far off from what I have experienced.

Dr - off topic, but does aluminum based phosphate binder release into lower concentration water, or are the binds permanent? I have always wondered this.

 

[jda]

I have RO and fresh salt mix at 0 phosphate. There was 1 BBP in my salt mix, so I had to put the reactor on it for a bit, but the levels in water come down very quickly with nothing in there to release. The brute that I used cured some live rock a while back and I am sure that some is still in there - this is no issue for my reef since it has very little phosphate, but it could get some organics into this test with horrible control.

The reactor that I am using gets used about once a month with a bit of GAC. I am sure that it has organics and bacteria in it even after a normal hobbyist wash. ...just another data point for the lack of true control if people want to get picky. If it matters, I used aluminum to lower the 1PPB to 0PPB in the fresh mix.

I got out 32 ounce Big Gulp of large sized arm. ...a little less than a pound. I will soak it in the 0 PPB saltwater and check again on Sunday. I want to make sure that this is phosphate free stuff. If it is not, it could take a few weeks for it all to work out.

 

[jda]

Here is the sweet setup that I mustered. This is probably how the do it at NASA, Harvard Med or any other awesome lab.

That is 345.3 grams of Large Grade ARM soaking in the Phosban 550 on the left. I cannot get the huge Ehiem in there, so it will just have to see what happens at room temp. I will swirl it every time that I walk by over the next few days.

That Phosban 550 on the right was just filled up to de-phosphate a whole bunch of rock that I have been accumulating for a 800ish gallon build later this year. I used it on this fresh salt to take it from 1 PPB to 0 PPB - just took 20 minutes, or so. The rock that I am using it on since is used, nasty and bound all up with phosphate. I have it in the 225G in the photo below. This is about the tenth treatment this size (sorry, I don't know the actual count), and I have gotten the phosphates down to .58 on my Hannah Ultra low. The last treatment this size started at .61, which was too high for the ultra-low to read. When I first put this on, the water will read 0 PPB by the morning. Sometime later tomorrow, the concentration in the water will start to rise. By Sunday night, they will be all the way back up to about .55 (guessing). I will end up using a whole bucket of Seachem Phosguard for this... which is still a bargain to reuse live rock, so of which is very nice from the Marshalls and will be fantastic once the microfauna returns and it is phosphate free.

Sweet setup:


225G full of phosphate bound live rock:

 

[mcarroll]

The problem is knowing the surface area. Knowing that, we could calculate from literature data how much might bind at a given phosphate concentration.

This is actually what I was talking about.

You mentioned the problem is knowing the surface area. So I found an example surface area that you could use for the calculation.

 

[Randy Holmes-Farley]

This is actually what I was talking about.

You mentioned the problem is knowing the surface area. So I found an example surface area that you could use for the calculation.

But that was tissue surface area. You need calcium carbonate surface area, which I showed some rough data for and is far higher.

 

[Randy Holmes-Farley]

For one particular type of natural calcium carbonate sediment, Millero shows the max absorption is about 10 umole/g

https://link.springer.com/article/10.1023/A:1011344117092

Taking that value, 50 pounds of such sediment would bind 227,000 micromoles (22,700,000 ug, or 22,700 mg) of phosphate.

If that all desorbed into 100 L of seawater, it would give a concentration of 227 ppm.

 

[jda]

Here is where my theory comes into play. I think that if you put 50 lbs of that sediment into 100L of water and then added 227 ppm of phosphate, I think that it would absorb most if it and the water would have a very low concentration left relative to the start.

I think that it would end up being somewhere around 1.0 (just guessing) in the water. If you put in just enough GFO to treat 1.0 PPM in the water column, you would have to treat 227 times to remove it all.

Does this make sense?