Phosphate Absorption Rates in Aragonite

[Randy Holmes-Farley Verified Community Expert]

The more you add, the more will bind!

 

[jda]

In almost 24 hours, phosphate in the water column is at 19 ppb, or .058 ppm. ...so we dropped from 3.6 ppm to .058 in about 22 hours. I will test again tomorrow.

This is faster than I thought that it would be.

 

[Randy Holmes-Farley Verified Community Expert]

Also shows the amount that can come back off is quite substantial.

 

[jda]

Also shows the amount that can come back off is quite substantial.

I know that for sure from just my own experiences. Some people doubt the amount, which is why I wanted to try this out and at least put some scale to it if exact numbers were not possible.

Dr - does Aluminum Oxide release phosphate like aragonite or GFO if the water column has a lower concentration, or the bind more permanent?

 

[Randy Holmes-Farley Verified Community Expert]

GFO and aluminum oxide and calcium carbonate are all reversible . [emoji3]

 

[jda]

Phosphate the next day was 21 ppb... so probably within the same range of a hobby-grade test. Looks like the aragonite absorbed all that it could within a day... this time.

I put 2 teaspoons of powder in and this time I got 21.7 ppm of phosphate.. this is a long way off from 3.6 last time. I did not believe it, tested again and got 22.0. I did dry add the stuff to a container and shook it up, which I did not do before. ...I wonder if it settled. Rookie mistake not mixing the stuff. I am going to test it again in the morning just to see if mixing for longer period of time matters at all.

 

[Randy Holmes-Farley Verified Community Expert]

You mean you added dry sodium phosphate? I agree its better to dissolve it first, but it should dissolve OK in the water with the rock. The concern might be localized precipitation of calcium phosphate or magnesium phosphate on or near the solid grains.

 

[jda]

I did take the reactor full of aragonite off while I let the powder mix.

 

[jda]

Two days with the aragonite on and we are at 50ppb on the Hannah Ultra low.

Running total: 25.3ppm total in water column nets .1533 ppm left after aragonite absorption.

Adding another 2tsp later today.

 

[jda]

Water P was 22.1 after addition of more powder. At 57 ppb the next day.

I am starting to get some film algae, so I think that I am near-done with this experiment. I did put a towel over the tank to cut the light, but I do want to run for another week to see if the aragonite will further absorb deeper down.

Total: 57.4 ppm in water column to .1748 ppm after aragonite was introduced.

FWIW - there is something different about the test kits... the Normal Hanna tests out a .71 ppm, but the ultra-low is at .1748. I guess that could be expected on a hobby-grade test kit near the low end of one of them, but still worth noting.

 

[jda]

Final measurement was 52 ppb in the water. .1594ppm.

57.4 total ppm added yields an absorption of 359x at a ratio of about 5 gallons per 1 pound of Florida-based aragonite. We also know that the aragonite is not likely saturated. Is there anything else to learn from?

 

[Randy Holmes-Farley Verified Community Expert]

Nice experiment. It gives a ballpark estimate of phosphate that might bind per pound of rock with the residual phosphate in the water being 50 ppb or less. It’s a very large amount!

 

[Turbo's Aquatics Verified Sponsor]

Final measurement was 52 ppb in the water. .1594ppm.

57.4 total ppm added yields an absorption of 359x at a ratio of about 5 gallons per 1 pound of Florida-based aragonite. We also know that the aragonite is not likely saturated. Is there anything else to learn from?

Nice experiment. It gives a ballpark estimate of phosphate that might bind per pound of rock with the residual phosphate in the water being 50 ppb or less. It’s a very large amount!

Wow...I'll have to read the rest of this thread sometime soon!

 

[madweazl]

Water P was 22.1 after addition of more powder. At 57 ppb the next day.

I am starting to get some film algae, so I think that I am near-done with this experiment. I did put a towel over the tank to cut the light, but I do want to run for another week to see if the aragonite will further absorb deeper down.

Total: 57.4 ppm in water column to .1748 ppm after aragonite was introduced.

FWIW - there is something different about the test kits... the Normal Hanna tests out a .71 ppm, but the ultra-low is at .1748. I guess that could be expected on a hobby-grade test kit near the low end of one of them, but still worth noting.

Wow, that is considerably higher absorption (and quicker) than I had imagined.

Lasse noted some differences between the low range (713) and ULR (736) in a post a while back as well. It caught my eye because the Triton results I've had always read about 33% of my ULR results. I think he'd mentioned that the 713 was closer to Triton results (for whatever that is worth when it comes to these kits).

 

[chipmunkofdoom2]

Final measurement was 52 ppb in the water. .1594ppm.

57.4 total ppm added yields an absorption of 359x at a ratio of about 5 gallons per 1 pound of Florida-based aragonite. We also know that the aragonite is not likely saturated. Is there anything else to learn from?

Thanks a lot for taking the time to do this. The amount and rate at which aragonite can bind phosphates is stunning. This certainly offers some insight into the issues I've been experiencing with my live rock, and it also has some really interesting implications for the long-term management of reef tanks and phosphates.

 

[jda]

I think that the two test kits are within a margin of error for a hobby-grade tool. Remember one tests phosphorus and the other phosphate, so you have to do the division/multiplication by 3.066. The difference in the triton is probably a type difference.

 

[mcarroll]

it also has some really interesting implications for the long-term management of reef tanks and phosphates.

A good experiment probably always creates more questions than it answers.

For one thing, I think it calls even further into question our concept of "ultra low nutrient reefs".

For some more.... What happens to this sorption and desorption rates when the experiment is carried on long enough for the aragonite to develop a biofilm? How about long enough to grow a mature periphyton layer? How significant could this source of PO4 be on a mature reef where bare rock might be minimal?

I don't even know much about it....I'm sure someone else would have more and better questions after an experiment like this.

 

[ZaneTer]

It may be possible to use it as a phosphate buffer to stabilize tanks that experience fluctuations, perhaps leading to a much more stable environment?
A good question could be how much do you have to add before it stabilizes at 0.05ppm?

 

[jda]

Aragonite is a very fast and effective phosphate buffer... and it does it whether you want to or not.

Rock that I have gotten from the pacific has very little phosphate bound in it. How could it have much? ...the water has such a low level. There is some, but it is hard to tell if that was truly in the rock or if was absorbed during the curing process with the organic die-off. In any case, the still will maintain a 1 to 5 ppb tank quite easily.

What this shows me is that people who want to not have phosphate problems later need to change water and do maintenance early even when their test kits do not show high levels. The intake to the tank is likely the same in the early and late stages, but the aragonite binding will hide it from the early. Most do not know that this is happening. A 20% water change early is just as important as a 20% water change in a few years. It might even be more important to change water early if the binding rate is exponential like Dr. H-F suggested.

You could probably figure out a ratio for your tank. 57 ppm in the water was .16 at a ratio of 1 pound of aragonite per 5 gallons of water. It would be rough, but all that we do it rough.

 

[Randy Holmes-Farley Verified Community Expert]

It may be possible to use it as a phosphate buffer to stabilize tanks that experience fluctuations, perhaps leading to a much more stable environment?
A good question could be how much do you have to add before it stabilizes at 0.05ppm?

The answer will depend strongly on the exact surface area of the material used, which varies tremendously between different types of rocks, sand, etc.

Using jda's exact material, and based on my expectation of the shape of typical phosphate adsorption curves (see below for an example from one of my materials, an organic polymer, not aragonite; the binding starts up fast as the phosphate concentration is raised, then begins to level off), the amount bound to his material at at 0.05 ppm will be a bit more than 0.05/0.16 ppm x 57 ppm = 18 ppm.

So I think his material would bind somewhat more than 18 ppm at 0.05 ppm.