Aragonite Sand And Phosphate Adsorption

[Dan_P]

Yeah, let's go with that. Cheers

1 g TSP dodecahydrate (MW 380) in 100 mL RODI is 2500 ppm PO4. Dilute 4.8 mL of this solution to 20 mL RODI to make it less concentrated so as to avoid forming a precipitate when adding it to 6 L of saltwater. Stir the 6 L while adding this diluted solution. The PO4 should be 2 ppm.

 

[sixty_reefer]

1 g TSP dodecahydrate (MW 380) in 100 mL RODI is 2500 ppm PO4. Dilute 4.8 mL of this solution to 20 mL RODI to make it less concentrated so as to avoid forming a precipitate when adding it to 6 L of saltwater. Stir the 6 L while adding this diluted solution. The PO4 should be 2 ppm.

Wouldn’t it be more advisable to have a lower ppm concentration, for a better accuracy with the test kit?

 

[Garf]

Wouldn’t it be more advisable to have a lower ppm concentration, for a better accuracy with the test kit?

For what it’s worth, the ELOS test kit (with Salifert Alkalinity vial) reads the solution at 2ppm (diluted by 50% because the maximum on the test is 1ppm), and the Salifert reads it at 1 ppm which is quite obviously wrong (I even bought a new fresh kit, which still reads 1 ppm ), lol. This confirms my belief that Salifert has always been reading low, and now I’ve got a 2 ppm standard ( thanks Dan ), that has been confirmed.

 

[Randy Holmes-Farley Verified Community Expert]

Wouldn’t it be more advisable to have a lower ppm concentration, for a better accuracy with the test kit?

Not if the sand binds most or all of it.

 

[Dan_P]

Wouldn’t it be more advisable to have a lower ppm concentration, for a better accuracy with the test kit?

Since we are taking the difference of the initial and final PO4 reading, we need to have the difference much greater than the standard deviation of the measurement. The propagation of error makes the variation in the difference greater than in either measurement. By using 2 ppm, we should see roughly a 1 ppm drop.

 

[Dan_P]

What I did with GFO was add phosphate to GFO in suspension in salt water several times (maybe over 1-2 weeks) to attain a stable phosphate reading of 0.1 ppm in the fluid. Drained off the liquid, added back fresh water, and measured the phosphate in the water over time.

Clearly, phosphate was desorbing there, and by quantitatively tracking the phosphate, I could start at the bottom right of your graph 1, move up it (although I never went nearly as high as you did), then moved back down the curve durign desorption.

Attaining the same result from both directions is very strong evidence that the process has attained equilibrium and is certainly reversible.

i had intended to compare different brands for binding, but it became too lengthy of an experiment. I only looked at Rowaphos (that I think was claiming irreversibility at the time) and Salifert.

I recast the adsorption data (plot 1} and corresponding desorption data (plot2) into the plot below which shows what happens to the amount of PO4 adsorbed to the same sample of sand after 24 hours adsorbing PO4 and 96 hours after desorbing PO4. Only 4 samples were involved in this study. As you correctly pointed out, the sand is not equilibrating. In fact, almost a constant amount of PO4 desorbs from the sand regardless of content. Wild, right?

I am thinking about cycling the same sample of sand several times through adsorption-desorption to observe what happens to its ability to adsorb PO4.

 

[Garf]

I recast the adsorption data (plot 1} and corresponding desorption data (plot2) into the plot below which shows what happens to the amount of PO4 adsorbed to the same sample of sand after 24 hours adsorbing PO4 and 96 hours after desorbing PO4. Only 4 samples were involved in this study. As you correctly pointed out, the sand is not equilibrating. In fact, almost a constant amount of PO4 desorbs from the sand regardless of content. Wild, right?

I am thinking about cycling the same sample of sand several times through adsorption-desorption to observe what happens to its ability to adsorb PO4.

After 24 hrs by Salifert, nothin has really gone down, still reading near 1 ppm, I’m thinking perhaps the colour change above 1 ppm is just too subtle.

Yesterday and today;

The perceived colour change I think is primarily lighting source with Salifert.

However, ELOS has definately detected a change (still 50% dilution), so reading approx 0.5 to 0.75 ppm, down from 2 ppm.

I’m gonna let it run, see if it falls further.

 

[Randy Holmes-Farley Verified Community Expert]

I recast the adsorption data (plot 1} and corresponding desorption data (plot2) into the plot below which shows what happens to the amount of PO4 adsorbed to the same sample of sand after 24 hours adsorbing PO4 and 96 hours after desorbing PO4. Only 4 samples were involved in this study. As you correctly pointed out, the sand is not equilibrating. In fact, almost a constant amount of PO4 desorbs from the sand regardless of content. Wild, right?

I am thinking about cycling the same sample of sand several times through adsorption-desorption to observe what happens to its ability to adsorb PO4.

The fact that absorption and desorption do not match means it has not reached equilibrium in one or both tests.

Chemically, I do not think it is logical to think that the phosphate cannot desorb given enough time, but that time may be very long, possibly for exposed surfaces, but most easily understood for buried particles.

An analogous situation is that seawater is actually above saturation with respect to calcium carbonate formation, but the time from to actually grow crystals from seawater is quite long.

 

[Dan_P]

The fact that absorption and desorption do not match means it has not reached equilibrium in one or both tests.

Chemically, I do not think it is logical to think that the phosphate cannot desorb given enough time, but that time may be very long, possibly for exposed surfaces, but most easily understood for buried particles.

An analogous situation is that seawater is actually above saturation with respect to calcium carbonate formation, but the time from to actually grow crystals from seawater is quite long.

OK, I follow what you are saying. Thanks.

Is there a possibility PO4 adsorption is not a equilibrium process? Is something else going on?

 

[Dan_P]

After 24 hrs by Salifert, nothin has really gone down, still reading near 1 ppm, I’m thinking perhaps the colour change above 1 ppm is just too subtle.

Yesterday and today;

The perceived colour change I think is primarily lighting source with Salifert.

However, ELOS has definately detected a change (still 50% dilution), so reading approx 0.5 to 0.75 ppm, down from 2 ppm.

I’m gonna let it run, see if it falls further.

Hey, this sounds like it’s going according to plan, despite the test kit FUBAR.

 

[Randy Holmes-Farley Verified Community Expert]

OK, I follow what you are saying. Thanks.

Is there a possibility PO4 adsorption is not a equilibrium process? Is something else going on?

Simple phosphate binding to a mineral surface is an equilibrium process but exactly how long it takes to reach equilibrium, I'm not sure. Not too long though.

But if you somehow precipitate something else on top of the coated particles, such as calcium carbonate, it might be a nonequilibrium process in the time frame you are interestesd in.

This has some good data:

https://d1wqtxts1xzle7.cloudfront.net/34146197/Calcite-P_AqGeoch2001-with-cover-page-v2.pdf?Expires=1667514127&Signature=Q9q7QLH7OGoyZK~NYtNTOJDHT21J-zhN41w0ENnnmOGNke5qUfr~pVotgCzbm98gv2YrhbNR4WYqL3P2pByhzdCjPTfhG40GgMM32GS-973DuOhneDG5DnwYwIWiWlNDKHeqdXhUG0MkCRZbswRCJHfcj9ELHV0C4vW3Bl29sSnHgysNA0gBj3N1D98LQ2qfm~3A66gYkza7N4jrmu4bD8Axe2TQSsLscJZ7P1D3NH9IwD7EgOKslZrgd25l7jQeSqa8OQctAGnMw1lVOISReF7R2r9zPOpvWpRUViw8z~XuNuEH6gitT9BeNC4oYEt8V5-QeZEPI53NZffScbI9XQ__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA

Figure 1 shows that over a few hours, calcium carbonate is precipitating onto fresh aragonite surfaces.

Figure 3 shows mostly rapid adsorption of phosphate onto aragonite from seawater

"The results of our adsorption and desorption kinetics study indicate that calcium carbonate can act as both a source and sink of phosphate in natural waters. The rapid adsorption and desorption of phosphate is similar to the chemisorption reported by Stumm and Leckie (1970) in their phosphate kinetics studies."

"Our kinetic measurements show that the rates of adsorption and desorption are quite fast (30 min). Up to 80% of the adsorbed phosphate is released from calcium carbonate over one day. The amount of PO4 left on the CaCO3 is close to the equilibrium adsorption."

 

[Dan_P]

Simple phosphate binding to a mineral surface is an equilibrium process but exactly how long it takes to reach equilibrium, I'm not sure. Not too long though.

But if you somehow precipitate something else on top of the coated particles, such as calcium carbonate, it might be a nonequilibrium process in the time frame you are interestesd in.

This has some good data:

https://d1wqtxts1xzle7.cloudfront.net/34146197/Calcite-P_AqGeoch2001-with-cover-page-v2.pdf?Expires=1667514127&Signature=Q9q7QLH7OGoyZK~NYtNTOJDHT21J-zhN41w0ENnnmOGNke5qUfr~pVotgCzbm98gv2YrhbNR4WYqL3P2pByhzdCjPTfhG40GgMM32GS-973DuOhneDG5DnwYwIWiWlNDKHeqdXhUG0MkCRZbswRCJHfcj9ELHV0C4vW3Bl29sSnHgysNA0gBj3N1D98LQ2qfm~3A66gYkza7N4jrmu4bD8Axe2TQSsLscJZ7P1D3NH9IwD7EgOKslZrgd25l7jQeSqa8OQctAGnMw1lVOISReF7R2r9zPOpvWpRUViw8z~XuNuEH6gitT9BeNC4oYEt8V5-QeZEPI53NZffScbI9XQ__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA

Figure 1 shows that over a few hours, calcium carbonate is precipitating onto fresh aragonite surfaces.

Figure 3 shows mostly rapid adsorption of phosphate onto aragonite from seawater

"The results of our adsorption and desorption kinetics study indicate that calcium carbonate can act as both a source and sink of phosphate in natural waters. The rapid adsorption and desorption of phosphate is similar to the chemisorption reported by Stumm and Leckie (1970) in their phosphate kinetics studies."

"Our kinetic measurements show that the rates of adsorption and desorption are quite fast (30 min). Up to 80% of the adsorbed phosphate is released from calcium carbonate over one day. The amount of PO4 left on the CaCO3 is close to the equilibrium adsorption."

I am going to repeat a 24 h adsorption measurement with much higher mixing, suspending the sand. Poor mixing seems like the only factor that could be giving much lower 24 h adsorption than Millero’s.

 

[Garf]

Another 24 hrs and my sand continues to bind (appears to be a 50% ish drop every 24 hrs). Salifert is now reading 0.5 ppm, along with ELOS.

 

[Dan_P]

Another 24 hrs and my sand continues to bind (appears to be a 50% ish drop every 24 hrs). Salifert is now reading 0.5 ppm, along with ELOS.

Sounds good. The rate of binding is likely affected by how much the sand-water mixture is being stirred. The drop might continue for some time in your system.

 

[Garf]

Sounds good. The rate of binding is likely affected by how much the sand-water mixture is being stirred. The drop might continue for some time in your system.

So it appears, halved again today to 0.25 ish.

 

[Dan_P]

So it appears, halved again today to 0.25 ish.

Has this sand been washed to remove dust? I discovered the “fines” could adsorb PO4 10-20 times better than the sand. A little dust can go a long way in binding PO4

 

[Garf]

Has this sand been washed to remove dust? I discovered the “fines” could adsorb PO4 10-20 times better than the sand. A little dust can go a long way in binding PO4

It was initially yes, in fresh saltwater, but I wasn’t at all thorough, just got the vaste majority out. I think my “gentle” stirring may be producing some as it does seem to clear a little slower.

Edit - I could knock up a reactor (to avoid sand molestation) and re run the test if you could see any advantage to that.

 

[Dan_P]

It was initially yes, in fresh saltwater, but I wasn’t at all thorough, just got the vaste majority out. I think my “gentle” stirring may be producing some as it does seem to clear a little slower.

Edit - I could knock up a reactor (to avoid sand molestation) and re run the test if you could see any advantage to that.

Let’s maybe wait for your final results before deciding on making you do more work. But it is an interesting idea and parallel’s JDA’s experiment where he used lumps of araginite in a reactor. I am slowly talking myself into saying yeah do it but let’s wait for data.

 

[Garf]

Let’s maybe wait for your final results before deciding on making you do more work. But it is an interesting idea and parallel’s JDA’s experiment where he used lumps of araginite in a reactor. I am slowly talking myself into saying yeah do it but let’s wait for data.

Adsorption appears to have stopped at 0.25ppm, been the same for the last three days (Salifert and ELOS). Should I redose another 2ppm to see if it can suck up more?

 

[Randy Holmes-Farley Verified Community Expert]

Adsorption appears to have stopped at 0.25ppm, been the same for the last three days (Salifert and ELOS). Should I redose another 2ppm to see if it can suck up more?

IMO, if the goal is understanding the amount that is bound at a reef relevant concentration, I would not think going higher is needed. More will bind as the equilibrium concentration rises.