Results from my home study phosphate binding kinetics

[EnterName]

I just started two experimental aquaria in which I didn’t want phosphate adsorption by the sand be confounded with biological uptake. On a small scale in a beaker I determined how much phosphate my sand adsorbs when the solution was at my preferred phosphate concentration of 0.1 ppm. With this information I made a large batch for the two aquaria.

Okay, so the idea of pre-saturating sand isn't that bad after all :D

 

[JonasRoman]

I can repeat the experiment with acid-treated sand if you want. You added 100mL of 0.2M HCl to 500mL resulting in approx. 600mL 0.03M HCl. This is very dilute and I only have 37% HCl at hand right now, so I will prepare a 0.2M HCl solution first and then dilute it the same way you did.

1. For how long should I keep the sand in the dilute HCl bath?
2. Do you want me to create vessels with the previous 18ppb and 50ppb P levels again or without any additional phosphate? (This will be difficult, as the reagent I used was admittetly old, but I will try)
3. I will use "fresh" sand again instead of re-using the sand that has been sitting in the test vessels.

Just to make sure I'm not missing something here's the math I did to get from a 37% solution to 0.2M:

• HCl has a molar mass of 36.46g/mol.
• the density of 37% HCl is approx. 1.1837kg/L = 1183.7g/L (at 20°C/68°F).
• 37% HCl is approx. 438g/L HCl in solution: 37% HCl ⋅ 1183.7g/L HCl ≈ 438g/L HCl.
• 37% HCl is approx 12.01M: 438g/L HCl / 36.46g/mol ≈ 12.01mol/L.
• to create 100mL of 0.2M HCl I can therefore use approx. 16.7mL of the 37% HCl solution for 1L 0.2M HCl: 0.2M/12.01M ≈ 0.0167.

Nice. Yes the math seems correct. Approx 17 ml 38% Hcl in 1000ml RO will give 0.2 M Hcl sol.

I soaked my stone in Hcl for a few hours.

Will be very interesting to see when you after that repeat the test, if sand now will absorb po4 better:-). Our theory is that it should. Hope so;-)

Jonas

 

[EnterName]

I can repeat the experiment with acid-treated sand if you want. You added 100mL of 0.2M HCl to 500mL resulting in approx. 600mL 0.03M HCl. This is very dilute and I only have 37% HCl at hand right now, so I will prepare a 0.2M HCl solution first and then dilute it the same way you did.

1. For how long should I keep the sand in the dilute HCl bath?
2. Do you want me to create vessels with the previous 18ppb and 50ppb P levels again or without any additional phosphate? (This will be difficult, as the reagent I used was admittetly old, but I will try)
3. I will use "fresh" sand again instead of re-using the sand that has been sitting in the test vessels.

Just to make sure I'm not missing something here's the math I did to get from a 37% solution to 0.2M:

• HCl has a molar mass of 36.46g/mol.
• the density of 37% HCl is approx. 1.1837kg/L = 1183.7g/L (at 20°C/68°F).
• 37% HCl is approx. 438g/L HCl in solution: 37% HCl ⋅ 1183.7g/L HCl ≈ 438g/L HCl.
• 37% HCl is approx 12.01M: 438g/L HCl / 36.46g/mol ≈ 12.01mol/L.
• to create 100mL of 0.2M HCl I can therefore use approx. 16.7mL of the 37% HCl solution for 1L 0.2M HCl: 0.2M/12.01M ≈ 0.0167.

Nice. Yes the math seems correct. Approx 17 ml 38% Hcl in 1000ml RO will give 0.2 M Hcl sol.

I soaked my stone in Hcl for a few hours.

Will be very interesting to see when you after that repeat the test, if sand now will absorb po4 better:-). Our theory is that it should. Hope so;-)

Jonas

Sand has been soaking in the HCl solution for a few hours (I took the sand from the 18ppb P water, because it already lost a bit of its initial phosphate to the water making the treatment a bit more effective.)

It's sitting in 79ppb P (≈ 0.24ppm PO4) water right now, which is kind of unfortunate as 50ppb would be better comparable to my initial experiment, but if you are just interested in the overall binding kinetics of acid-treated substrate it should be fine.

I use sterile 0.22μm PES syringe filters to clean up the samples prior to measurement and will report back as soon as I have the required data for 48-ih hours or so.

 

[EnterName]

@JonasRoman

After 18 hours the phosphorus level has not really changed. The initial measurement was 79ppb and we are now at 83ppb. The pH of my initial experiment and the current one is somewhere between 6 and 7.
The good thing: This means I sufficiently rinsed the sand after the acid treatment to not make a difference.
The bad thing: This pH doesn't really encourage phosphate precipitation.

Do you think it would make sense to increase pH to around 8 with sodium hydroxide (NaOH)?

 

[JonasRoman]

@JonasRoman

After 18 hours the phosphorus level has not really changed. The initial measurement was 79ppb and we are now at 83ppb. The pH of my initial experiment and the current one is somewhere between 6 and 7.
The good thing: This means I sufficiently rinsed the sand after the acid treatment to not make a difference.
The bad thing: This pH doesn't really encourage phosphate precipitation.

Do you think it would make sense to increase pH to around 8 with sodium hydroxide (NaOH)?

Yes, definitely, we wants to mimic the natural situatio where we assume that "clean" sand absorb PO4, but that is very dependent on pH. So I would try raise pH to approx 8.3 to mimic the tank situation. Can be hard to calculate how much is needed (Randy can for sure), so maybe just add a very small dose, measure pH and increase in small steps unitl you reach 8.3. I would also encourage to circulate water from now and then to further facilitate the process, but "shake" the bottles from time to time (even better with a very gentle small powerhead but maybe difficult to achieve.

 

[JonasRoman]

As a comment on this meantime we wait for "EnterName" finaly results.
When I used calciumcarbonate reactors, fed by Co2, I tested the outlet water in PO4. As expected that water was skyhigh in PO4, telling us that the sand has previosly bounded PO4, BUT this PO4 was only released when pH was dropped to approx 6 ish (as we have in a reactor). Same substrate in marine water with normal pH did not release its po4. And this was so called special aragonite that was claimed to be low of PO4, but it wasnt. That doesnt surprise me, as I think most of our clean sand is taken from a biological system from the beginning at some point (or?).

Jonas

 

[JonasRoman]

@EnterName one question (maybe you have mentioned): Is the water a true marine water? I think that also is important to het decired Precipitation of CaPO4, as possibility of Precipitation is a combination of concentration of Ca, CO3, PO4 and pH. Even if we want precip on surface and core, I think water content of the precipitants play a role.

 

[EnterName]

@EnterName one question (maybe you have mentioned): Is the water a true marine water? I think that also is important to het decired Precipitation of CaPO4, as possibility of Precipitation is a combination of concentration of Ca, CO3, PO4 and pH. Even if we want precip on surface and core, I think water content of the precipitants play a role.

I used Nyos Pure (ICP results) and RO/DI (< 0.1µS/cm). This means we are "only" lacking Rubidium, Arsenic, Beryllium, Chrome, Cobald, Copper, Selenium, Zinc and Tin.

I think I didn't do the best job with adjusting the pH so I will finally order a new pH probe and repeat the process with RedSea blue bucket or so.

 

[SantaMonica]

On a next test, you might make P the variable to test the flow rate into and out of rock, using reef levels of P and pH.