Aragonite Sand And Phosphate Adsorption

[Dan_P]

Im just wondering if some phosphates are just being trapped in between the sand grains instead of being inside the sand grain. And I’m wondering if agitation of the sand will increase the overhaul phosphates.

Do you have any ideas about the trapping mechanism? Could PO4 be dissolved in high concentration in the water between the sand grain or in the stagnant boundary layer around the sand Grain? Are there other solid materials in an aquarium that reside between the sand grain and adsorb PO4?

 

[Randy Holmes-Farley Verified Community Expert]

what if they were to agitate the sand in they’re current equilibrium? Would that not increase phosphates?

No, it would not. Agitation does not break ionic bonds or change the equilibrium.

 

[Randy Holmes-Farley Verified Community Expert]

I’m gonna confuse things further, lol. The inorganic phosphate bound to the calcium carbonate can reduce iron in the water column, correct? I’m sure I’ve read Dino’s produce siderophores that can liberate this iron. Is this key to Dino proliferation? I’ve had a hard week of nights so be easy with me :)

Many metals do bind to aragonite and calcite surfaces.

Does having any phosphate already there make iron binding more extensive and more available after it binds? Possibly. I've never seen such a coadsorption study.

 

[Dan_P]

Many metals do bind to aragonite and calcite surfaces.

Does having any phosphate already there make iron binding more extensive and more available after it binds? Possibly. I've never seen such a coadsorption study.

@Garf got me interested in this. Since I have the equipment set up, I will take a look at iron adsorption when I finish PO4 adsorption.

 

[Randy Holmes-Farley Verified Community Expert]

@Garf got me interested in this. Since I have the equipment set up, I will take a look at iron adsorption when I finish PO4 adsorption.

It’s a complicated question because iron bound to organics will be much less prone to bind.

 

[Dan_P]

It’s a complicated question because iron bound to organics will be much less prone to bind.

I will be generating some data for discussion but will not be spending much time in this rabbit hole :)

 

[Garf]

Another 3ppm added this week, same outcome. It’s reliably reducing from 2ppm to around 0.5ppm.

 

[Dan_P]

Another 3ppm added this week, same outcome. It’s reliably reducing from 2ppm to around 0.5ppm.

This is making sense to me now that I have seen the small scale results. I was about to post them but they were so different from how we think of the aragonite “sponge” I decided to repeat the measurements a couple different ways. Adsorb on!

 

[Garf]

For anyone struggling through the tedium of my posts, same again this week. Added another 3ppm. That’s 13ppm in 6 litres of water with 2 kgs of sand. Dropping to around 0.5ppm, still.

 

[Dan_P]

For anyone struggling through the tedium of my posts, same again this week. Added another 3ppm. That’s 13ppm in 6 litres of water with 2 kgs of sand. Dropping to around 0.5ppm, still.

I’m watchin’, I’m watchin’.

I calculate that your sand on average has taken up 0.13 micromoles per gram. That is roughly 13% of the total that I found on small scale experiments with Caribsea sand for the 2-3 ppm range. Millero reports about 10X that for 200 mesh aragonite. I am going to grind aragonite sand to 200 mesh and see if it meets this capacity. That would support the notion that surface area determines how much PO4 can be bound by calcium carbonate.

 

[Garf]

I’m watchin’, I’m watchin’.

I calculate that your sand on average has taken up 0.13 micromoles per gram. That is roughly 13% of the total that I found on small scale experiments with Caribsea sand for the 2-3 ppm range. Millero reports about 10X that for 200 mesh aragonite. I am going to grind aragonite sand to 200 mesh and see if it meets this capacity. That would support the notion that surface area determines how much PO4 can be bound by calcium carbonate.

13%, that much? 12 months to go then, lol

 

[Dan_P]

13%, that much? 12 months to go then, lol

Fear not. I will be like a faithful dog, lying at your feet, waiting for the next morsel of data to drop :)

 

[Randy Holmes-Farley Verified Community Expert]

I'd personally be more interested in stopping dosing more and seeing if it goes down further than the 0.5 ppm through slow binding

At 0.5 ppm, we are pushing the limits of what one would typically want or expect in a reef tank. But if the goal is too see how much binds in equilibrium with 0.5 ppm phosphate in the water, then carry on.

This is exactly why I gave up trying to compare phosphate binding to some GFO brands. The experiment just takes a long time. lol

 

[Garf]

I'd personally be more interested in stopping dosing more and seeing if it goes down further than the 0.5 ppm through slow binding

Yep, I can do that. How long do you want me to leave it?

 

[Randy Holmes-Farley Verified Community Expert]

Yep, I can do that. How long do you want me to leave it?

I'd wait a few days, retest the water, and if it hasn't dropped, wait more and do a final test.

If it seems to drop a bit then keep watching as long as it does. :)

 

[Dan_P]

I'd personally be more interested in stopping dosing more and seeing if it goes down further than the 0.5 ppm through slow binding

At 0.5 ppm, we are pushing the limits of what one would typically want or expect in a reef tank. But if the goal is too see how much binds in equilibrium with 0.5 ppm phosphate in the water, then carry on.

This is exactly why I gave up trying to compare phosphate binding to some GFO brands. The experiment just takes a long time. lol

I am finding that mechanically stirred sugar sand stops detectable PO4 uptake within 24 hours, and possibly within 6 hours. Attempting vigorous stirring with a magnetic stirrer attrits the sand grains, producing silt and screwing up adsorption observations.

I am in the process of confirming the adsorption rate at several PO4 concentrations as we speak. I am also going to look at desorption kinetics. And I am repeating Millero’s study with <200 mesh ground aragonite sand rather than the synthesized aragonite In his study.

 

[Randy Holmes-Farley Verified Community Expert]

I am finding that mechanically stirred sugar sand stops detectable PO4 uptake within 24 hours, and possibly within 6 hours. Attempting vigorous stirring with a magnetic stirrer attrits the sand grains, producing silt and screwing up adsorption observations.

I am in the process of confirming the adsorption rate at several PO4 concentrations as we speak. I am also going to look at desorption kinetics. And I am repeating Millero’s study with <200 mesh ground aragonite sand rather than the synthesized aragonite In his study.

Sounds great!

 

[Garf]

I'd wait a few days, retest the water, and if it hasn't dropped, wait more and do a final test.

If it seems to drop a bit then keep watching as long as it does. :)

Well, it’s been dropping for the last week of no phosphate additions. Down to around 0.1ppm today. I will give it another week.

 

[Dan_P]

Well, it’s been dropping for the last week of no phosphate additions. Down to around 0.1ppm today. I will give it another week.

A couple technical notes. Calcium carbonate might never reach an equilibrium but adsorption does slow down to a very boring rate.

For a well mixed pile of sand - think sand storm in a beaker - about 70% of the amount bound in a twenty four hour period is on the sand by one hour. The other 30% binds in 23 hours. Adsorption continues after that.

For your system, the sand is not near saturation and will continue to bind phosphate. You are about 10% or so of your target.

For your less than well stirred system, phosphate adsorption is slowed by diffusion through the sand. Where a well stirred system nears completion at twenty four hours, an unstirred systems could take weeks.

 

[brandon429]

Dan whats your opinion on this data repeat pattern I'm seeing

tap water is supposed to be variable/high in phosphates and impurities

we have so many uncountable rip clean threads all using tap and I give you my word nobody has ever pm'd me or skewered the thread one time in all these years about us ruining their substrate with tap water...causing it to become fertilizer for plants that it wasn't before. rip cleans have about 90% rate testimony for helpful prevention of algae and sandbed matted invaders, a 10% neutral/no change/same frequency of invasion report, and a zero pct report rate of a tap water rinse on sandbed for 3 hours making things worse.


is it contact time? things added into a reef tank are sinked and staying, maybe three hours doesn't transmit much phosphate for the bind into the matrix?

perhaps we do give em a lil phosphate for the uptake but the actual impact of it changes, chemically along the way before plant or moneran mat uptake at a higher rate

perhaps we give them phosphate but the net benefit of ripping out all the leaking organic stores outweighs it

some detail of exposing sands to high phosphate water/ three hour increments on average isn't really amounting to much - impact. its all +