Aragonite Sand And Phosphate Adsorption

[Miami Reef]

Yes, you are right. Measure phosphate before and after mixing calcium carbonate with fresh saltwater. If it releasing phosphate, It could take a couple hours to overnight to reach a detectable concentration.

In addition to stirring or shaking, the sample you test must be filtered. For powdered calcium carbonate you might try the syringe filter that comes with the Hanna low nitrate range kit, otherwise, buy a few 0.45 micron syringe filters (cellulose or nylon will work)

Just to make sure, can I use these? They seem to be 0.22um and hydrophilic ?

LabZhang 24pcs Syringe Filter,Syringe Lab Filters,Hydrophilic Nylon Membrane 25mm Diameter 0.22um Pore Size,Non Sterile Filtration,Green(nylon-25mm 0.22um) https://a.co/d/47ucWJR

 

[Dan_P]

Just to make sure, can I use these? They seem to be 0.22um and hydrophilic ?

LabZhang 24pcs Syringe Filter,Syringe Lab Filters,Hydrophilic Nylon Membrane 25mm Diameter 0.22um Pore Size,Non Sterile Filtration,Green(nylon-25mm 0.22um) https://a.co/d/47ucWJR

Nylon is OK if you are not filtering acidic solutions.

This is what I use at the moment - 0.22 micron was not available when I last ordered.

 

[Dan_P]

I don't know that is true. If the phosphate is bound to aragonite, calcite, dolomite, etc. and then new structure is is added on top, then the phosphate can be trapped, but without new growth, I have never, ever heard of the bind being non reversible. I can tell you that sand in my tanks can bind and unbind based on water equilibrium. Were they taking about new growth trapping something underneath?

amorphous calcium phosphate to hydroxy- or fluoroapatite is what I have come across as candidates

 

[Garf]

Think I’m gonna get a Hanna, which one do folks suggest?. Checked some sand from a 30 month undisturbed sandbed to see if it leaked anything thus;

1) Obtained a sample from approximately 1.5 inch depth
2) Rinsed the sample (2.5 grammes ish) twice in 0ppm new saltwater.
3) Added 10mls new saltwater
4) left the sample covered in a wide container for 12hrs, but gently stirred it every 3 ish hours.
5) Filtered and tested the sample.

The result was about 0.1ppm (salifert). This just so happens to be the concentration of the running tank. However this tank has been From zero to 1ppm back down to 0.1 in its lifetime.
pH of the saltwater removed from the same location of the sand was 7.9. Overlying pH of the tank 8.2.

 

[Miami Reef]

Think I’m gonna get a Hanna, which one do folks suggest?. Checked some sand from a 30 month undisturbed sandbed to see if it leaked anything thus;

1) Obtained a sample from approximately 1.5 inch depth
2) Rinsed the sample (2.5 grammes ish) twice in 0ppm new saltwater.
3) Added 10mls new saltwater
4) left the sample covered in a wide container for 12hrs, but gently stirred it every 3 ish hours.
5) Filtered and tested the sample.

The result was about 0.1ppm (salifert). This just so happens to be the concentration of the running tank. However this tank has been From zero to 1ppm back down to 0.1 in its lifetime.
pH of the saltwater removed from the same location of the sand was 7.9.

It depends. In general, I like the Hanna Phosphate ULR because it has a better accuracy than the low range phosphate, and it reads in ppm and doesn’t have the 3 min shut-off engineering fail on the phosphorous checker.

 

[Miami Reef]

There are 3 types:

Phosphate Low Range (ppm)

Range	0.00 to 2.50 ppm
Resolution	0.01 ppm
Accuracy @ 25°C/77°F	±0.04 ppm ±4% of reading

Phosphate ULR (ppm)

Range	0.00 to 0.90 ppm
Resolution	0.01 ppm
Accuracy @ 25°C/77°F	±0.02 ppm ±5% of reading

Phosphorus ULR (ppb)

Range	0 to 200 ppb
Resolution	1 ppb
Accuracy @ 25°C/77°F	±5 ppb ±5% of reading

 

[jda]

Any hannah is fine. In the low range, one does the math to convert the phosphorous ppb into phosphate ppm. Otherwise, the same thing, IMO. The ppb checker is more granular, if you want to do more precise kind of things.

I have the phosphorous ultra low. It does 0 to 200 ppb. The only time that I need more than 200 ppb (or about .6 ppm) is if I get a bunch of rock from a tank shut down and it needs work to get it functional again. In this case, 200 is no different than 10,000 since 1 is the goal.

 

[sixty_reefer]

OK thanks for the clarification.

In principle, the answer is no. The scale and ratios I chose to work with do not affect the equilibrium distribution of phosphate between sand and water.

A higher ratio of water to sand would give a different equilibrium distribution of phosphate than a lower ratio, but both scenarios lie on the absorption curve I posted. The higher ratio scenario has a larger amount of phosphate to distribute between the water and sand. It would result in more phosphate in the sand, but also more in solution.

Thank you it does make sense for the absorbing side of the experiment, would it be the same scenario on the desorption process? if you were to double the ratio of water volume to sand on the sample would the phosphates maintain X ppm or half X ppm.
I believe most of us would expect it to half the residual although there’s a chance that for equilibrium the X could be maintained and flattened the curve as more volume is added.

 

[Garf]

In other news I added 3ppm phos (to bring it to 4ppm ish) to my other test last week (2 kgs sand , 6 litres water). Tested today and it’s barely detectable, certainly below the 0.03 colour change on salifert which is not what has happened previously, it normally sticks at 0.1ish. So that’s 22ppm so far, I’ll be setting up the doser in a bit.

 

[Dan_P]

Think I’m gonna get a Hanna, which one do folks suggest?.

Which Hanna to buy depends on the lowest concentration you want to measure and the range of concentrations. The Hanna ultra low range (ULR) model measures in the 0 - 0.05 ppm a bit more reliably than the low range (LR) model. The LR model has a maximum measurement of 2.5 ppm where as the ULR model tops out at 0.6 ppm. @Rick Mathew can add more background to this topic

Checked some sand from a 30 month undisturbed sandbed to see if it leaked anything thus;

Cool experiment. I want to do this to compare to your results. If you are interested in comparing PO4 in sand beds, I added information to help standardize the method

1) Obtained a sample from approximately 1.5 inch depth

This will be tricky to standardize. Ideally we would grab a core sample of the sand bed and then cut out a slice of the core around 1.5 cm. Until, then we we will do our best to scoop out sand at comparable depths

2) Rinsed the sample (2.5 grammes ish) twice in 0ppm new saltwater.

In my sand studies, phosphate is rapidly released in 0 ppm water. It is possible that you rinsed off 10-25% of the phosphate from the sand grain surface in each rinse.

I suggest that we take a level scoop of known volume (we must know the sand volume to compare each others results) and use something adsorbent like a tissue to wick away the excess water in the sand bed sample while it is in the scoop by touching the edge of the tissue to the sand. We will have to live with any detritus in the sand sample.

If you are a detritus hater, we could try quickly rinsing with a minimum amount of tank water and then wicking off the excess. It would be fun and useful to compare the phosphate measurements for sand treated with your original sand sample wash, my suggested no wash and the quick was with aquarium water.

I measure my samples with little chemical scoops. The red Salifert scoop (0.05 mL) delivers ~80 mg of sugar sand. The white Red Sea scoop (0.15 mL) delivers ~220 mg of sugar sand.

3) Added 10mls new saltwater

Good. A known amount of water for the desorption run. I have been able to get a reading with ratios of 12 mL water to 200 mg sand. More sand is better when you need a higher concentration to measure

4) left the sample covered in a wide container for 12hrs, but gently stirred it every 3 ish hours.

Good. Stirring is important. Time is important. 24 hours might be slightly better in ensure equilibrium.

5) Filtered and tested the sample.

Excellent. Very critical. Even a small amount of “dust” will throw off the reading

The result was about 0.1ppm (salifert). This just so happens to be the concentration of the running tank. However this tank has been From zero to 1ppm back down to 0.1 in its lifetime.
pH of the saltwater removed from the same location of the sand was 7.9. Overlying pH of the tank 8.2.

I calculate 1 micro gram was desorbed from 2.5 g of sand, or 0.4 micro gram PO4 per gram of sand @1.05 micro molar PO4…this seems like a reasonable number but we would need to know the phosphate concentration surrounding the sand when sampled to say any more about the number. The 0.4 value suggests it was surrounded by a higher phosphate concentration than 0.1 ppm of the aquarium water. This is reasonable though. Pore water can have a higher concentration of phosphate, ammonia, nitrate, etc.

 

[Rick Mathew]

Think I’m gonna get a Hanna, which one do folks suggest?. Checked some sand from a 30 month undisturbed sandbed to see if it leaked anything thus;

1) Obtained a sample from approximately 1.5 inch depth
2) Rinsed the sample (2.5 grammes ish) twice in 0ppm new saltwater.
3) Added 10mls new saltwater
4) left the sample covered in a wide container for 12hrs, but gently stirred it every 3 ish hours.
5) Filtered and tested the sample.

The result was about 0.1ppm (salifert). This just so happens to be the concentration of the running tank. However this tank has been From zero to 1ppm back down to 0.1 in its lifetime.
pH of the saltwater removed from the same location of the sand was 7.9. Overlying pH of the tank 8.2.

I think @jda nailed it...The ULR Checkers both the Phosphate and Phosphorous will do the job...Your desired testing range would define your decision...I have evaluated both and their performance is equal to if not better than Hanna reports.

 

[jda]

I have probably said this before, but the exact numbers are really not that important, IMO - close works here. The important thing to note is that aragonite can bind a LOT of phosphate and that the binding is exponential as the water level increases. Having a Hannah is good for just regular hobby use anyway, but without being able to quantify surface area, I found it a fools errand to try and come up with formula for all to use. I just tell people that I got like 50+ ppm of po4 down to .16 ppm water level in 10g of water with a pound of aragonite calcium reactor chunks in a reactor with active flow (if I remembered all of that right).

My guess is that even with the most accurate testing, any results probably will be expressed as a range since surface area is so important and impossible to determine. Aragonite source can also be a huge variable with terrestrial being different than fresh ocean-based. This can sill be VERY helpful even with all of the unknowns.

 

[Dan_P]

I just tell people that I got like 50+ ppm of po4 down to .16 ppm water level in 10g of water with a pound of aragonite calcium reactor chunks in a reactor with active flow (if I remembered all of that right).

One explanation for theses results is the irreversible binding of phosphate. It is unlikely to have this much reversibly bound phosphate in equilibrium with 0.1 ppm PO4 in seawater unless the rubble used in the experiment had the surface area of GAC which seems like an impossibility or at least an unverified phenomenon.

 

[Dan_P]

Thank you it does make sense for the absorbing side of the experiment, would it be the same scenario on the desorption process? if you were to double the ratio of water volume to sand on the sample would the phosphates maintain X ppm or half X ppm.
I believe most of us would expect it to half the residual although there’s a chance that for equilibrium the X could be maintained and flattened the curve as more volume is added.

Increasing the amount of water for desorption would result in more PO4 desorbing to reach an equilibrium.

 

[Miami Reef]

@Dan_P

I did the test. I let the calcium carbonate mix with my magnetic stirrer for less than a day. The initial phosphate reading was 0.00 and the reading after filtering the water from the mixture was 0.01ppm which is within the claimed accuracy of the checker.

So, it didn’t really add any phosphates.

 

[Dan_P]

@Dan_P

I did the test. I let the calcium carbonate mix with my magnetic stirrer for less than a day. The initial phosphate reading was 0.00 and the reading after filtering the water from the mixture was 0.01ppm which is within the claimed accuracy of the checker.

So, it didn’t really add any phosphates.

Nice follow up experiment.

 

[Garf]

If you are a detritus hater

I wasn’t until I probed this little sandbed in the sump. I bought a little wet/dry vacuum, now I have no sandbed. I’m a little surprised how it was apparently depressing pH.

 

[Dan_P]

I wasn’t until I probed this little sandbed in the sump. I bought a little wet/dry vacuum, now I have no sandbed. I’m a little surprised how it was apparently depressing pH.

You are observing an increase in the pH of your system since removing?

 

[Garf]

You are observing an increase in the pH of your system since removing?

Yes, it’s only been about 9 hrs but my CO2 bubbly scrubber thing has barely run (been on bypass) with all windows and doors shut, solid @8.4pH. Guess I’ll know more tonight, the lights have only been off a few hours. I calibrated the probe, just because it was quite unexpected.

 

[Dan_P]

Yes, it’s only been about 9 hrs but my CO2 bubbly scrubber thing has barely run (been on bypass) with all windows and doors shut, solid @8.4pH. Guess I’ll know more tonight, the lights have only been off a few hours. I calibrated the probe, just because it was quite unexpected.

Very interesting!