Results from my home study phosphate binding kinetics

[JonasRoman]

I thought so, but it might have adressed the concerns about residual PO₄ others mentioned.

I don't know much about how these stones form, but if PO₄ accumulates throughout the whole formation of the rock, it might be really hard to get fully rid of it. Dissolving the surface layer of the rock with acid would just expose the underlying layer with just as much phosphate. You would basically need a solution that is capable of releasing bound phosphate without slowly dissolving the rock.

I can still imagine that the residual phosphate in your experiment was negligible and probably didn't exceed the expected error of the used measuring device, but I understand why people are having some concerns about residual PO₄.

I remember reading that you boiled the stone (sounds dangerous btw), and that you did your best to get rid of organics. I was just asking because the measured results might look differently in a reef tank with bacterial activity.

Good thoughts

 

[Dan_P]

I remember reading that you boiled the stone (sounds dangerous btw),

Why does boiling a stone sound dangerous?

 

[EnterName]

I remember reading that you boiled the stone (sounds dangerous btw),

Why does boiling a stone sound dangerous?

If water is trapped inside the rock the heat expansion and vaporization of water can cause it to "explode" and shoot sharp rock fragments through the air.

Not much of a concern if you are absolutely sure there is no water trapped inside, but still an unsettling thought.

 

[areefer01]

Why does boiling a stone sound dangerous?

On the surface it probably is not. My thought as I read it is that it was more to do with the term "cooking rock" and each hobbyist definition of what that term means. I am guessing it probably has a lot of misinterpretation and caution as a result and social media posts over the years.

Maybe bleach or muriatic acid is more widely recommended these days if one is reusing or using dry? I don't know.

 

[JonasRoman]

Some have talked about that it could be PO4 deep inside not released with my first cleaning acidifaction. I do not think thats a crucial issue, becasue IF that is so, then that part is definitely not released in the second acidification either and definitely never released into a tank. The PO4 of interest is only those po4 that is incuded in the kinetics and dynamics, not what maybe can be released if you melt down the whole stone.

My test shows whats going on on surface and some beneath that, and that I still think mimics a tank situation. If nothing is released during soaking in RO water, we can be sure that nothing is released when soak in marine tank water, when coming to hydroxyapatit. So I still say that this easy study gives evidence for what I say in the saummary::-)

Next test now is to quantify how much PO4 the stone can bind, and most of interest, long term binding of insoluble apatit. The short term binding goes back and forth according to the equlibrium, but also that could be nice to quantify so we know the amplitude of that player so to speak.

Will maybe start such a test aswell, and try measure how much PO4 is consumed in the surrounding water. I will again boil the stone and use reference ATI water to avoid any organics so we do not have any bacterial activity that may absorb PO4.

/Jonas

 

[Dan_P]

Some have talked about that it could be PO4 deep inside not released with my first cleaning acidifaction. I do not think thats a crucial issue, becasue IF that is so, then that part is definitely not released in the second acidification either and definitely never released into a tank. The PO4 of interest is only those po4 that is incuded in the kinetics and dynamics, not what maybe can be released if you melt down the whole stone.

My test shows whats going on on surface and some beneath that, and that I still think mimics a tank situation. If nothing is released during soaking in RO water, we can be sure that nothing is released when soak in marine tank water, when coming to hydroxyapatit. So I still say that this easy study gives evidence for what I say in the saummary::-)

Next test now is to quantify how much PO4 the stone can bind, and most of interest, long term binding of insoluble apatit. The short term binding goes back and forth according to the equlibrium, but also that could be nice to quantify so we know the amplitude of that player so to speak.

Will maybe start such a test aswell, and try measure how much PO4 is consumed in the surrounding water. I will again boil the stone and use reference ATI water to avoid any organics so we do not have any bacterial activity that may absorb PO4.

/Jonas

I still think you are still being too easy on yourself.

One thing you have not demonstrated is the amount of phosphate removed from the saltwater medium equals the amount adsorbed to the rock, or what portion adsorbed to the rock desorbs after six hours. If you are right about irreversible binding, the mass balance will indicate how much becomes permanently bond to the mineral.

Also, when conducting the desorption study, I would avoid the water rinse and just plunge the wet rick in 0 ppm saltwater, taking frequent measures to catch any quick desorption kinetics that occur before the 0-6 desorption process occurs. There might be more learn

Can you switch to aragonite instead of limestone (calcite)? It would be consistent with the polymorph that is in aquaria. The adsorption kinetics and thermodynamics of the two polymorphs are a bit different.

 

[JonasRoman]

One thing you have not demonstrated is the amount of phosphate removed from the saltwater medium equals the amount adsorbed to the rock, or what portion adsorbed to the rock desorbs after six hours. If you are right about irreversible binding,

correct, and reason was that I had too much water and too high po4 versus stone mass, to be sure that my test method would have sensitivity enough, as the precision and resolution is lower for a po4 tests with values that high as 0.4 ppm, as I had in the "soaking" water.

I have just started a new test, and will use just 0.1 ppm po4, and less water, to detect how much PO4 "vanish" from the water during different time periods.

I checked up concerning if my preparation with acid would have removed all hydryapatite, and yes, at that low pH I used to clean the stone, all hydroxyapatite is removed. If we have something left inside the core its not interesting as that will in that case not be released later either. Again, the test is to see how the dynamic works in a normal situation, thus in a tank.

My believe tis that when we start a tank with dead clean stone, the surface and beneath that is not for sure saturated with hydroxyapatite, and that is what we see, the first months of a newly set up tank, it absorbs po4, then quite suddenly, it stop absorb po4, and po4 raise more linear to the input.
Then we have the fast back and forth equlibrium, the loose bound bridging, that is onging all time, and is not so interesting according to my opinion. Becasue that part is more like a buffert, or a damper, of the Po4, maybe explaining that you can see a raise or opposite also in a mature tank, not related to po4 import, but exlained by loose bound Po4 that is more related to pH and conc of Po4 in water column. For instance, lest say you make a water change, you lower the PO4 immediately by X, but next day, its back to same Po4. This is tha bridged Po4 that explains this.

Again, for me its more of interest what happens in long term, and if stone has build up significant amount of hydroxyapatit over lets say 4 months, that is never released back.


Jonas

 

[Dan_P]

correct, and reason was that I had too much water and too high po4 versus stone mass, to be sure that my test method would have sensitivity enough, as the precision and resolution is lower for a po4 tests with values that high as 0.4 ppm, as I had in the "soaking" water.

I have just started a new test, and will use just 0.1 ppm po4, and less water, to detect how much PO4 "vanish" from the water during different time periods.

I checked up concerning if my preparation with acid would have removed all hydryapatite, and yes, at that low pH I used to clean the stone, all hydroxyapatite is removed. If we have something left inside the core its not interesting as that will in that case not be released later either. Again, the test is to see how the dynamic works in a normal situation, thus in a tank.

My believe tis that when we start a tank with dead clean stone, the surface and beneath that is not for sure saturated with hydroxyapatite, and that is what we see, the first months of a newly set up tank, it absorbs po4, then quite suddenly, it stop absorb po4, and po4 raise more linear to the input.
Then we have the fast back and forth equlibrium, the loose bound bridging, that is onging all time, and is not so interesting according to my opinion. Becasue that part is more like a buffert, or a damper, of the Po4, maybe explaining that you can see a raise or opposite also in a mature tank, not related to po4 import, but exlained by loose bound Po4 that is more related to pH and conc of Po4 in water column. For instance, lest say you make a water change, you lower the PO4 immediately by X, but next day, its back to same Po4. This is tha bridged Po4 that explains this.

Again, for me its more of interest what happens in long term, and if stone has build up significant amount of hydroxyapatit over lets say 4 months, that is never released back.


Jonas

It might make more sense to work with sand because there is far more surface areas than rock. Sand dominates the the adsorption-desorption thermodynamics in an aquarium.

 

[JonasRoman]

It might make more sense to work with sand because there is far more surface areas than rock. Sand dominates the the adsorption-desorption thermodynamics in an aquarium

Agree, but at the moment I stick to stone:-). It says what it says, then you can always extrapolate to situations with more CaCO3 in the tank.

Jonas

 

[Dan_P]

Agree, but at the moment I stick to stone:-). It says what it says, then you can always extrapolate to situations with more CaCO3 in the tank.

Jonas

The extrapolation could difficult and will raise further debate, but since this is not a life-or-death issue, go for it. Looking forwards to what you observe.

Dan

 

[BoaConservationist]

Theres definitely a superior way to go about dosing, you have to think ionic-ally the way that RO water works etc, things will bind at different rates, and so much as mixing too fast can actually create heat /friction and cause loss/ precip etc. Going to give this a thorough read but the TLDR is we lose like what, 50%? :)

 

[JonasRoman]

when I consider this together with my other tests, and my experience how PO4 behave in a newly setup tank with dead lime stone, my believe is that in long term the stone bind much more than 50% as hydroxyapatite. I think the stone binds up much more PO4 as insoluble hydroxapatite than the loose bound, in long term. It make sense when I see how mayn tanks behave the first period: The consume po4 that just "dissappear", and after like 6 month or so, the consumption stops and po4 raise more according to what you add. This absorbed Po4 during these 6 months I believe is hydroxyapatite, thus is never leaking back. That also fits with at least my experience, that I will never get any uncontrollable raise in PO4 after this "saturation" period, but more logic rasie in Po4 linear to what I add. My tests was just a few days, and then its was a ratio of 50/50 versus hard bound and loose bound, but as I said, that ratio I believe is something else if you wait longer. This would be interesting to try to show in a long term trial.

Jonas

 

[Dan_P]

when I consider this together with my other tests, and my experience how PO4 behave in a newly setup tank with dead lime stone, my believe is that in long term the stone bind much more than 50% as hydroxyapatite. I think the stone binds up much more PO4 as insoluble hydroxapatite than the loose bound, in long term. It make sense when I see how mayn tanks behave the first period: The consume po4 that just "dissappear", and after like 6 month or so, the consumption stops and po4 raise more according to what you add. This absorbed Po4 during these 6 months I believe is hydroxyapatite, thus is never leaking back. That also fits with at least my experience, that I will never get any uncontrollable raise in PO4 after this "saturation" period, but more logic rasie in Po4 linear to what I add. My tests was just a few days, and then its was a ratio of 50/50 versus hard bound and loose bound, but as I said, that ratio I believe is something else if you wait longer. This would be interesting to try to show in a long term trial.

Jonas

Algae can consume much more phosphate than calcium carbonate. Understanding the consumption trends of phosphate in new aquaria probably cannot be understood by considering phosphate adsorption kinetics alone.

 

[JonasRoman]

Algae can consume much more phosphate than calcium carbonate. Understanding the consumption trends of phosphate in new aquaria probably cannot be understood by considering phosphate adsorption kinetics alone.

Yes definitely algue plays a role. That's why I tried to isolate what only CaCo3 do with po4

 

[EnterName]

After stumbling upon a comment from Randy about how new sand could drain phosphate levels, I'm currently trying to figure out a smart (and fast) way to saturate dry sand with phosphate, so it won't cause a huge drop (or spike) when moving tanks (I will have to move tanks myself soon).

One concern is how much the phosphate may drop when adding the new sand. [...]
I expect all will be fine, chemistry-wise, though corals are often quite disgruntled by such a move.

As this thread is related to phosphate binding kinetics, I thought I would ask around for ideas on how to properly do this.

In theory I could just put the sand in containers, fill them with water of the desired phosphate concentration and keep adding phosphate until levels remain where I want them. However, this is a slow process that requires a lot of phosphate tests, manual adjustment, and could cause possibly unwanted bacteria or microalgae growth.

I wondered if it is possible to approximate the sand's phosphate binding behavior when comparing two or more vessels with different initial phosphate levels over time, so I prepared two vessels with 250g dry sand, and 500mL of freshly mixed saltwater respectively. For one vessel the used 500mL were additionally spiked with monosodium phosphate dihydrate (NaH₂PO₄ ⋅ 2 H₂O).

The sand was rinsed thoroughly with RO/DI before use to avoid cloudy water that might affect the measurements. You can see the results over time down below:

(Phosphorus levels over 48h)
​

• The area around the measurement values indicates the HI736's expected measurement error of ± 5ppb ± 5% of the reading.​
• After every test the vessels were shaken thoroughly to reduce local differences in phosphate levels for a while.​
• Both vessels are pretty much air-tight and were stored in a closed cabinet to reduce growth of photosynthetic life.​
• The dip at 12h for the 50ppb phosphate vessel was also observed in a previous experiment which I messed up too much to provide as proper data source, but I'm relatively certain the dip is not a measurement error.​
• The 24h spike was tested twice to ensure it is valid as well.​
• In hindsight, I should have added a third vessel without sand as control, but before I repeat the whole procedure again, I wanted to hear everyone's thoughts and ideas.​
• Edit: The pH in those vessels appeared to be rather low (6 - 7) which doesn't really encourage phosphate precipitation. If I repeat the experiment I would need to increase pH accordingly.​
• I used Carib Sea Special Grade for this experiment.​

From an initial 32ppb difference, both samples are now at a 27ppb difference and the phosphate levels are actually higher than initially. I would have hoped for both samples to meet somewhere in the middle between their initial phosphate levels, but apparently the whole thing is a bit more complex than initially thought or might take longer than expected.

Sometimes it might be worth sharing "failed" experiments, so I thought I add what I've got so far even if it isn't worth much.

 

[JonasRoman]

After stumbling upon a comment from Randy about how new sand could drain phosphate levels, I'm currently trying to figure out a smart (and fast) way to saturate dry sand with phosphate, so it won't cause a huge drop (or spike) when moving tanks (I will have to move tanks myself soon).


As this thread is related to phosphate binding kinetics, I thought I would ask around for ideas on how to properly do this.

In theory I could just put the sand in containers, fill them with water of the desired phosphate concentration and keep adding phosphate until levels remain where I want them. However, this is a slow process that requires a lot of phosphate tests, manual adjustment, and could cause possibly unwanted bacteria or microalgae growth.

I wondered if it is possible to approximate the sand's phosphate binding behavior when comparing two or more vessels with different initial phosphate levels over time, so I prepared two vessels with 250g dry sand, and 500mL of freshly mixed saltwater respectively. For one vessel the used 500mL were additionally spiked with monosodium phosphate dihydrate (NaH₂PO₄ ⋅ 2 H₂O).

The sand was rinsed thoroughly with RO/DI before use to avoid cloudy water that might affect the measurements. You can see the results over time down below:

(Phosphorus levels over 48h)
​

• The area around the measurement values indicates the HI736's expected measurement error of ± 5ppb ± 5% of the reading.​
• After every test the vessels were shaken thoroughly to reduce local differences in phosphate levels for a while.​
• Both vessels are pretty much air-tight and were stored in a closed cabinet to reduce growth of photosynthetic life.​
• The dip at 12h for the 50ppb phosphate vessel was also observed in a previous experiment which I messed up too much to provide as proper data source, but I'm relatively certain the dip is not a measurement error.​
• The 24h spike was tested twice to ensure it is valid as well.​
• In hindsight, I should have added a third vessel without sand as control, but before I repeat the whole procedure again, I wanted to hear everyone's thoughts and ideas.​
• I used Carib Sea Special Grade for this experiment.​

From an initial 32ppb difference, both samples are now at a 27ppb difference and the phosphate levels are actually higher than initially. I would have hoped for both samples to meet somewhere in the middle between their initial phosphate levels, but apparently the whole thing is a bit more complex than initially thought or might take longer than expected.

Sometimes it might be worth sharing "failed" experiments, so I thought I add what I've got so far even if it isn't worth much.

Interesting, so the experiment did the opposite what we expected. One reason, and most logic, is that the sand was not clean from po4. In my tests I prepared the lime stone in very low pH (Hcl) to leak out the hard and loose bounded po4. If the sand was taken from ocean or any living system I suppose it has bound some po4 over time. One question is, what was pH approx in those 2 buckets? Also, maybe you shouldnt shake the bottles, becasue then you get a lot of insoluble lime stone particles in the sample, and that could give false high readings as the measuring process may dissolute them as we acidify the sample when we do Po4 tests normaly. I would instead take water from the buckets and let them pass a microfilter. And also prepare the sand prior to this with acidifaction, and then heavily rinsing, so we are sure the sand is not satured from beginning.

Jonas

 

[EnterName]

One reason, and most logic, is that the sand was not clean from po4. In my tests I prepared the lime stone in very low pH (Hcl) to leak out the hard and loose bounded po4. If the sand was taken from ocean or any living system I suppose it has bound some po4 over time.

My goal was to find out how much additional PO₄ I would need, to reach a saturation point at which phosphate levels wouldn't be reduced once I changed tanks and substrate. That's why I didn't bother with the acid treatment. I blindly assumed the sand would have a low phosphate saturation and therefore I expected having to increase its saturation to reach a point at which water parameters would remain stable.

I understand that this procedure might make the data less comparable to your initial study, but I'm not sure the sand only contains phosphate in its upper layer. It might be possible that no matter how much you dissolve with acid, the next layer just has the same phosphate saturation. Of course this is something that would require testing.

I shook the vessels after taking the samples for the measurements, so the particles had 6 - 12 hours to settle between each test. The water was crystal clear before each measurement. I will still use a sterile filter next time just to reduce the chance of dissolution due to pH changes during the testing procedure. Unfortunately I had not recorded the pH over time as the only pH probe I have available right now is the GHL probe in my tank. My hand-held probe didn't operate properly anymore and went to the trash recently. I will get a new one, but still have to decide which one is actually reliable and can be easily stored in KCl without much hassle and worrying too much about evaporation.

 

[JonasRoman]

My goal was to find out how much additional PO₄ I would need, to reach a saturation point at which phosphate levels wouldn't be reduced once I changed tanks and substrate. That's why I didn't bother with the acid treatment. I blindly assumed the sand would have a low phosphate saturation and therefore I expected having to increase its saturation to reach a point at which water parameters would remain stable.

I understand that this procedure might make the data less comparable to your initial study, but I'm not sure the sand only contains phosphate in its upper layer. It might be possible that no matter how much you dissolve with acid, the next layer just has the same phosphate saturation. Of course this is something that would require testing.

I shook the vessels after taking the samples for the measurements, so the particles had 6 - 12 hours to settle between each test. The water was crystal clear before each measurement. I will still use a sterile filter next time just to reduce the chance of dissolution due to pH changes during the testing procedure. Unfortunately I had not recorded the pH over time as the only pH probe I have available right now is the GHL probe in my tank. My hand-held probe didn't operate properly anymore and went to the trash recently. I will get a new one, but still have to decide which one is actually reliable and can be easily stored in KCl without much hassle and worrying too much about evaporation.

Ah, thanks for clarifying.
So obviously your sand was saturated.
I do not think that's strange as we don't know "history " of the sand. Even new sand may be collected in first place from ocean ?
It would now be even more interesting to redo your test after you have soaked the sand in Hcl solution. I think as long as you have Hcl enough it will penetrate into also the deeper layers. I guess you can carefully turn around the sand and few times.
The evidence of acid enough is the formation of gas that is CO2. Then I think you definitely will dissolve the bounded po4.
And then rinse very carefully and redo the test. Would be very very exciting if you now the second time could show that po4 is consumed by the sand.

Many thanks for your efforts and that you share this with us. Nice work mate

Jonas

 

[Dan_P]

After stumbling upon a comment from Randy about how new sand could drain phosphate levels, I'm currently trying to figure out a smart (and fast) way to saturate dry sand with phosphate, so it won't cause a huge drop (or spike) when moving tanks (I will have to move tanks myself soon).


As this thread is related to phosphate binding kinetics, I thought I would ask around for ideas on how to properly do this.

In theory I could just put the sand in containers, fill them with water of the desired phosphate concentration and keep adding phosphate until levels remain where I want them. However, this is a slow process that requires a lot of phosphate tests, manual adjustment, and could cause possibly unwanted bacteria or microalgae growth.

I wondered if it is possible to approximate the sand's phosphate binding behavior when comparing two or more vessels with different initial phosphate levels over time, so I prepared two vessels with 250g dry sand, and 500mL of freshly mixed saltwater respectively. For one vessel the used 500mL were additionally spiked with monosodium phosphate dihydrate (NaH₂PO₄ ⋅ 2 H₂O).

The sand was rinsed thoroughly with RO/DI before use to avoid cloudy water that might affect the measurements. You can see the results over time down below:

(Phosphorus levels over 48h)
​

• The area around the measurement values indicates the HI736's expected measurement error of ± 5ppb ± 5% of the reading.​
• After every test the vessels were shaken thoroughly to reduce local differences in phosphate levels for a while.​
• Both vessels are pretty much air-tight and were stored in a closed cabinet to reduce growth of photosynthetic life.​
• The dip at 12h for the 50ppb phosphate vessel was also observed in a previous experiment which I messed up too much to provide as proper data source, but I'm relatively certain the dip is not a measurement error.​
• The 24h spike was tested twice to ensure it is valid as well.​
• In hindsight, I should have added a third vessel without sand as control, but before I repeat the whole procedure again, I wanted to hear everyone's thoughts and ideas.​
• I used Carib Sea Special Grade for this experiment.​

From an initial 32ppb difference, both samples are now at a 27ppb difference and the phosphate levels are actually higher than initially. I would have hoped for both samples to meet somewhere in the middle between their initial phosphate levels, but apparently the whole thing is a bit more complex than initially thought or might take longer than expected.

Sometimes it might be worth sharing "failed" experiments, so I thought I add what I've got so far even if it isn't worth much.

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.

 

[EnterName]

Ah, thanks for clarifying.
So obviously your sand was saturated.
I do not think that's strange as we don't know "history " of the sand. Even new sand may be collected in first place from ocean ?
It would now be even more interesting to redo your test after you have soaked the sand in Hcl solution. I think as long as you have Hcl enough it will penetrate into also the deeper layers. I guess you can carefully turn around the sand and few times.
The evidence of acid enough is the formation of gas that is CO2. Then I think you definitely will dissolve the bounded po4.
And then rinse very carefully and redo the test. Would be very very exciting if you now the second time could show that po4 is consumed by the sand.

Many thanks for your efforts and that you share this with us. Nice work mate

Jonas

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.