Aragonite Sand And Phosphate Adsorption

[Dan_P]

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?

Spookily close to the adsorption rate that I observed with my sand, about 1/30th of Millero’s results. Love it.

In principle, if you re-expose the sand to 2 ppm PO4, it should bind more PO4, but the equilibrium PO4 concentration will be higher than 0.25. Eventually, repeated 2 ppm exposures will end up with the equilibrium PO4 concentration getting stuck at 2 ppm. I think with repeated exposures, the sand would adsorb a maximum of 1/30th as much as Millero’s aragonite at 2 ppm, about 4 times as much as in this experiment.

I think one more dose would confirm that the sand is performing at 1/30th of Millero’s aragonite but still following the published adsorption curve, just divided by ~30. If you have the energy, go for it.

 

[Garf]

Spookily close to the adsorption rate that I observed with my sand, about 1/30th of Millero’s results. Love it.

In principle, if you re-expose the sand to 2 ppm PO4, it should bind more PO4, but the equilibrium PO4 concentration will be higher than 0.25. Eventually, repeated 2 ppm exposures will end up with the equilibrium PO4 concentration getting stuck at 2 ppm. I think with repeated exposures, the sand would adsorb a maximum of 1/30th as much as Millero’s aragonite at 2 ppm, about 4 times as much as in this experiment.

I think one more dose would confirm that the sand is performing at 1/30th of Millero’s aragonite but still following the published adsorption curve, just divided by ~30. If you have the energy, go for it.

Dosed upto 2ppm again today (from just under 0.25 I reckon (Salifert and ELOS)). This time I’m not going to stir the sand as I’m curious how an actual in situ sand bed would react. I’m assuming that the sand will eventually adsorb till about 0.5ppm this time? Cheers

 

[Dan_P]

Dosed upto 2ppm again today (from just under 0.25 I reckon (Salifert and ELOS)). This time I’m not going to stir the sand as I’m curious how an actual in situ sand bed would react. I’m assuming that the sand will eventually adsorb till about 0.5ppm this time? Cheers

Looking forwards to results!

 

[Garf]

Looking forwards to results!

Interestingly after 48 hrs, Salifert is reporting 0.5ppm, which is the same as previous.

 

[Dan_P]

Interestingly after 48 hrs, Salifert is reporting 0.5ppm, which is the same as previous

I believe this time the sand is unstirred, right?

I expect the sand will keep adsorbing PO4 with each new 2 ppm PO4 exposure, though less each time, until it adsorbs the amount that is the maximum amount at 2 ppm, i.e., when it reaches eauilibrium.

 

[Randy Holmes-Farley]

I believe this time the sand is unstirred, right?

I expect the sand will keep adsorbing PO4 with each new 2 ppm PO4 exposure, though less each time, until it adsorbs the amount that is the maximum amount at 2 ppm, i.e., when it reaches eauilibrium.

My expectation as well, with the sand on the bottom of an unstirred layer only very slowly getting phosphate that manages to move deeper down. That is a very slow process if there is no bulk flow through the sand (like a plenum might help produce).

 

[Garf]

My expectation as well, with the sand on the bottom of an unstirred layer only very slowly getting phosphate that manages to move deeper down. That is a very slow process if there is no bulk flow through the sand (like a plenum might help produce).

Tested today and it appears to be stuck at 0.5ppm ish (Salifert). I'm in no rush though so I'll let it run for a few more days as it is, see if it drops a little further. I'll knock something up to improve flow through the sand when this batch stops absorbing (could be another 10 days or so, I suppose).

 

[Garf]

This sand keeps on sucking it up. Added another 1.25ppm Thursday and tested again today and added another 1ppm. So far the sand has adsorbed 4ppm in 6 litres of water with 2 kgs of sand (allowing for 2ppm now in solution).

 

[Dan_P]

This sand keeps on sucking it up. Added another 1.25ppm Thursday and tested again today and added another 1ppm. So far the sand has adsorbed 4ppm in 6 litres of water with 2 kgs of sand (allowing for 2ppm now in solution).

Great!

I am bringing up the rear with lab results. I am repeat dosing sand in Instant Ocean thst is mechanically stirred. There won’t be any questions about flow or reaching equilibrium. I look forwards to comparing my small scale results to your large scale data.

When you are finished, will you add 0 ppm saltwater to the sand to see how much PO4 comes off the sand?

 

[Garf]

will you add 0 ppm saltwater to the sand to see how much PO4 comes off the sand?

Certainly will.

 

[Garf]

Another 2 ppm dosed this week (1 ppm, twice). Even checked the level immediately after dosing to see if it was indeed increasing, and indeed, it was. This adsorption does seem to be a lot more than I imagined, that’s 18ppm, weight of sand compared to weight of water. There’s no evident bacterial growth in the container, but it has now been running 4 weeks, so I guess there is some.

Earlier on this year, I played around with a mesh bag of this sand in the aquarium and it did appear to have an effect on phosphate.

 

[Dan_P]

Another 2 ppm dosed this week (1 ppm, twice). Even checked the level immediately after dosing to see if it was indeed increasing, and indeed, it was. This adsorption does seem to be a lot more than I imagined, that’s 18ppm, weight of sand compared to weight of water. There’s no evident bacterial growth in the container, but it has now been running 4 weeks, so I guess there is some.

Earlier on this year, I played around with a mesh bag of this sand in the aquarium and it did appear to have an effect on phosphate.

We will have to some figurin’ when additions are stopped. While there seems to be a large adsorption of phosphate, we will need to converted it to mg PO4 to g of sand.

How much bacteria grow will be determined by the amount of organic carbon and nitrogen there is. There is undoubtedly some growth but I don’t expect it be using up much PO4.

I am still mechanically stirring sand in Instant Ocean and adding PO4. The results confirm that CaribSea sand PO4 adsorption is about 1/10 of the reported value in the literature, likely a result of the larger particle size and smaller surface area. I continue to expect rock adsorption of PO4 to be much less. I ran one desorption test, and the expected amount of PO4 came off, ~20% of the adsorption total. A subsequent desorption run produced no detectable PO4. Where is the other 80%? Part of the answer is that the amount was below the detection limit of the Hanna Checker, and maybe that is the entire reason. More work needed.

 

[sixty_reefer]

Hello, I’ve been following the tread for some time and as usual great work from all involved. I just got a couple questions regarding the uptake of phosphates.

1. Are the phosphates being absorbed by the sand

2. Are the phosphates being absorbed by the sand

3. Is it a mix of absorbing and trap phosphates

I ask this as before I had the opportunity to test my sanded and come to a conclusion that it may be a mix of absorbing, trapped and bacterial consumption.

is there a way to determine this? In my observation agitating a certain amount of sand with a fresh salt mix was enough to add the phosphates back into the water column. Making me believe that some of the phosphates were trapped in the sandbed area instead of being absorbed by the sand?

is it possible for @Dan_P and @Garf to aggressively agitate the sand in the experimental vessel and retest phosphates to observe if any phosphates get released into the water column that way?

 

[Randy Holmes-Farley]

Hello, I’ve been following the tread for some time and as usual great work from all involved. I just got a couple questions regarding the uptake of phosphates.

1. Are the phosphates being absorbed by the sand

2. Are the phosphates being absorbed by the sand

3. Is it a mix of absorbing and trap phosphates

I ask this as before I had the opportunity to test my sanded and come to a conclusion that it may be a mix of absorbing, trapped and bacterial consumption.

is there a way to determine this? In my observation agitating a certain amount of sand with a fresh salt mix was enough to add the phosphates back into the water column. Making me believe that some of the phosphates were trapped in the sandbed area instead of being absorbed by the sand?

is it possible for @Dan_P and @Garf to aggressively agitate the sand in the experimental vessel and retest phosphates to observe if any phosphates get released into the water column that way?

It is direct chemical binding of phosphate to the calcium carbonate surfaces. There is no trapping of phosphate between sand grains in an adsorption experiment such as those described here.

Agitation will only accelerate the access of phosphate to buried surfaces. It does not ever knock phosphate off the surfaces.

Of course, in a desorption experiment (load up the surfaces with phosphate and replace the water with zero phosphate water), then phosphate will be coming off to atain a new equilibrium, and will be higher between grains until it has a chance to get out, and agitation can speed that release.

 

[Dan_P]

Hello, I’ve been following the tread for some time and as usual great work from all involved. I just got a couple questions regarding the uptake of phosphates.

1. Are the phosphates being absorbed by the sand

2. Are the phosphates being absorbed by the sand

3. Is it a mix of absorbing and trap phosphates

I ask this as before I had the opportunity to test my sanded and come to a conclusion that it may be a mix of absorbing, trapped and bacterial consumption.

is there a way to determine this? In my observation agitating a certain amount of sand with a fresh salt mix was enough to add the phosphates back into the water column. Making me believe that some of the phosphates were trapped in the sandbed area instead of being absorbed by the sand?

is it possible for @Dan_P and @Garf to aggressively agitate the sand in the experimental vessel and retest phosphates to observe if any phosphates get released into the water column that way?

To help me understand the question better, would you explain what “trapping phosphate” means?

 

[Garf]

To help me understand the question better, would you explain what “trapping phosphate” means?

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

 

[sixty_reefer]

Of course, in a desorption experiment (load up the surfaces with phosphate and replace the water with zero phosphate water), then phosphate will be coming off to atain a new equilibrium, and will be higher between grains until it has a chance to get out, and agitation can speed that release.

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

 

[sixty_reefer]

To help me understand the question better, would you explain what “trapping phosphate” means?

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.

 

[sixty_reefer]

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

I also read something like this regarding dinoflagellates

 

[Dan_P]

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

It would definitely increase the rate of adsorption in the @Garf experiment.

I have upgraded my mixing to mechanical stirring which keeps the sand bed churning. This results in faster equilibration but the equilibrium adsorption is still about 1/10 of that found by Millero. Surface area adequately explains the difference. I will have a report with the latest results in a couple weeks.