Aragonite Sand And Phosphate Adsorption

[Dan_P]

Aragonite Sand And Phosphate Adsorption. Alkalinity And pH Observations

I ran two experiments using the same sand. The experiments consisted of exposing the sand to phosphate in Instant Ocean followed by a series of Instant Ocean “washes” until no more phosphate was detected and then exposing the sand again to phosphate and then washing out the phosphate. The first exposure was 15 minutes and last was for 16 hours. Results are summarized in the table below just after each adsorption phase of the experiment. I added a control in the second experiment to determine whether pH and alkalinity readings were related to phosphate adsorption or just drifting for other reasons.

The pH changes might be random, up or down, but alkalinity is consistently increasing, though within the margin of error and more tellingly, increasing with the same magnitude in the control. Repeating this experiment with fresh sand might be interesting because first time adsorptions tend to be somewhat larger then subsequent adsorptions.

 

[Garf]

First full week of low level dosing (0.2 ppm per day) on sand that’s already been exposed to 22ppm phos (6 litres water, 2kgs calcite sand), I’m getting a regular reading of 0.2 ish ppm (salifert), so in my book it must be consuming around 0.2ppm per day.

 

[Dan_P]

First full week of low level dosing (0.2 ppm per day) on sand that’s already been exposed to 22ppm phos (6 litres water, 2kgs calcite sand), I’m getting a regular reading of 0.2 ish ppm (salifert), so in my book it must be consuming around 0.2ppm per day.

This could also be confirmation of my calculation that the sand is not yet saturated with phosphate.

For those reading this for the first time, 22 ppm sounds like a big number. The mg of phosphate consumed by this much sand in so little water is a small portion of its capacity. It could go on for a long time consume 0.2 ppm of phosphate, but persistence pays. Also, only a fraction of the phosphate appears to be able to come off again. I am in the midst of figuring out what that fraction is. 400 packets of Hanna PO4 reagent down and starting on the new 300 packet supply today.

The good news is that adsorption is fast and the speed of adsorption is primarily governed by phosphate diffusion through the sand. Stirring would definitely speed things up but very boring to do

 

[Dan_P]

Aragonite Sand And Phosphate Adsorption. Evidence For Irreversible Binding.

I have enough observations now to suggest that a portion of phosphate that binds to aragonite sand does not come off. I am in the process of clarifying this phenomenon. The plot below shows some of the evidence for this irreversible binding.

I will help clarify the complicated plot. The green dashed line is the adsorption curve for aragonite sand. The almost hidden yellow line is the desorption curve created by desorbing the phosphate from the sand used to generate the adsorption data at each green dot. As mentioned before, the fact that the yellow dots lie above the green line suggests the sand is not releasing all the phosphate. To confirm this, sand was exposed to ~2 ppm PO4 for 24 hours and then exposed repeatedly to fresh Instant Ocean until no more PO4 desorbed or leached from the sand. At that point, the cycle was repeated, the sand was exposed to ~2 ppm PO4 again and then desorbed, two more times. This was done under strong and weak mechanical stirring. Starting at the “1st Adsorption” point on the graph and moving left, you can see that the amount of desorbed PO4 is low, i.e., above the green line. The next two cycles zig-zag upwards to the ”3rd Desorption”.

Where does the zig-zag line stop rising? Does the concentration of PO4 to which the sand is exposed affect the zig-zag line? Is this phenomenon important below 0.2 ppm PO4 where most of our aquaria live?

 

[Garf]

Did this again, same process but have waited 96hrs to test, results show slightly over 0.03 on salient, certainly lower than 0.1ppm.

Once again, 2.5grms ish of old sand,I replaced the water with fresh saltwater, 20 mls, occasional stir, waited 10 days since last test, same result, somewhere between 0.03 and 0.1ppm (salifert). In summary, this is the third de-adsorption, 10mls (0.25ppm), 20mls (0.03-0.1ppm), 20mls (0.03-0.1ppm).

 

[Dan_P]

Once again, 2.5grms ish of old sand,I replaced the water with fresh saltwater, 20 mls, occasional stir, waited 10 days since last test, same result, somewhere between 0.03 and 0.1ppm (salifert). In summary, this is the third de-adsorption, 10mls (0.25ppm), 20mls (0.03-0.1ppm), 20mls (0.03-0.1ppm).

Excellent! I want to do some figuring. What was the phosphate concentration of the water that the old sand came from. Thanks

 

[Garf]

Excellent! I want to do some figuring. What was the phosphate concentration of the water that the old sand came from. Thanks

Tank had reached about 1ppm, months before but when I took the sample it was 0.1, or so I thought. When you commented previously that you thought it was higher, I retested and got consecutive 0.25 readings (Salifert).

 

[Dan_P]

Tank had reached about 1ppm, months before but when I took the sample it was 0.1, or so I thought. When you commented previously that you thought it was higher, I retested and got consecutive 0.25 readings (Salifert).

Your aquarium sand seems to be behaving like my clean sand in the tests.

From your information, the rate of phosphate desorbed is 0.0015-0.0031 mg per gram of sand. The adsorption line predicts 0.013 mg/g adsorbed at 0.25 ppm phosphate which can desorb ~25% of that total or 0.0032 mg per gram of sand.

I imagine your sand will need 1-3 more treatments before no more detectable PO4. I found that desorption takes 15 minutes if the flask is constantly swirled. You could maybe end this experiment in 45 minutes plus testing time

 

[Garf]

You could maybe end this experiment in 45 minutes plus testing time

Lol, youngsters. Don’t you think there’s maybe a slow desorption thing possible, where irreversible adsorption is mitigated by long term desorption? (Makes sense to me, at least)
I’ve a more pressing question at this point. Why don’t folk use a cheap sand for phosphate removal?

 

[jda]

Nearly nobody on the board even knows what phosphates truly are, what they do and that even bind to aragonite. Most think that they are coral "food" since the parrots online have told them so.

Mined calcite or dolomite could also remove lots of phosphate too...

 

[Garf]

Nearly nobody on the board even knows what phosphates truly are, what they do and that even bind to aragonite. Most think that they are coral "food" since the parrots online have told them so.

Mined calcite or dolomite could also remove lots of phosphate too...

I Certainly wouldn’t buy GFO, if I were concerned of high phosphate, which I’m not, within reason.

 

[Dan_P]

Lol, youngsters.

Thank you

Don’t you think there’s maybe a slow desorption thing possible, where irreversible adsorption is mitigated by long term desorption? (Makes sense to me, at least)

I did not find such a mechanism. If it exists, it was below detection limit in my experiments.

I’ve a more pressing question at this point. Why don’t folk use a cheap sand for phosphate removal?

@taricha mentioned a similar idea when he saw the data. Worth looking into.

 

[jda]

Also, I forgot... there is no BRS video about how to remove po4 with aragonite, so it will be fake news to much of the crowd.

If somebody wanted to have a fun experiment, see how much $1 of aragonite sand vs $1 of GFO could remove in a high concentration water sample.

Personally, I would use something that was from the ocean and has been crushed. I have long suspected that "smooth" sand has had many pores and structure washed away from erosion. This is one reason that I never cared much for southdown or other sugar sand - it also never reacted to vinegar like other sands did. Crushed sand and coral substrate is also cheaper.

 

[Garf]

If somebody wanted to have a fun experiment, see how much $1 of aragonite sand vs $1 of GFO could remove in a high concentration water sample.

I’ve decided to revive my “bag o’ sand in the sump”, on my 60gallon cube, directly downstream of my kalk dosing line. I previously noted that this configuration appeared to make kalk less potent, so I will see if it happens again.Squeezed 1.85kgs (presumed calcite) into a mesh bag, flattened onto an egg crate shelf to allow flow above and below. Rinsed in RODI, to minimise fines (slight haze visible in tank).
It occurs to me this sand should be somewhat subject to regeneration if rinsed in RODI enough, even if my sand is only £1 / Kg. I’ll probably keep track of progress on my build thread.

 

[taricha]

I’ve a more pressing question at this point. Why don’t folk use a cheap sand for phosphate removal?

@taricha mentioned a similar idea when he saw the data. Worth looking into.

My recollection is that occasionally someone would say "aragonite ought to be a great cheap alternative to GFO" ...but nobody ever reported back "eureka! it worked!"

I think Dan's data does a good job of explaining why - to get an impressive result you probably need to...
1) grind the sand to really fine particles
and
2) have a desorb part of the cycle where you let your sand equalize with zero PO4 water, then connect it back to your system.

 

[Aqua Man]

Nearly nobody on the board even knows what phosphates truly are, what they do and that even bind to aragonite. Most think that they are coral "food" since the parrots online have told them so.

I thought that the algae inside the coral was benefiting from PO4 and indirectly feeding coral this way? Or helped keep the coral healthy?

 

[jda]

Just equalizing once with clean water probably won't get you where you need to go. You likely will need a constant stream of RO/DI to get enough po4 out of the media before it is ready to use again. You can do this with GFO or Al Oxide too. Some tried this with GFO and it was better to toss it and start again.

Phosphorous is a building block of life. po4 is just one form. All organic tissue needs it in small quantity and it is poison in larger quantities. It has no energy and is not a food. More is not better, like people think of foods. For algae inside of the coral, the host can recycle building blocks for the symbionts and can gather some for them. For the host, they need to get new building blocks commensurate with growth and what they cannot recycle - less to maintain and more to grow. Just a trace of po4 is fine - something in the range of .005-.01 ppm is not growth limiting at all. Lasse and I have talked a lot about recycling and stuff if you want to search for this - he is good about posting research and myself not so much. Anything more than a trace of po4 can actually slow down growth of true coral - it is a well understood and studied fact that higher po4 levels can inhibit calcification. This happens in my tanks alone where just coralline will grow SO fast when my po4 is between 1-3 ppb and it will be cut in half, or less, if I let the po4 get to 5 or 10 ppb. The coralline will still grow at levels much higher, just if you have never see it grow at flank, then you have no idea.. it can cover grates on my power heads in a few months. Macro algae (chaeto for me) also grow significantly faster when po4 is lower - another cruel joke from nature. A bacteriologist on here once sent me some good research that most corals get their po4 (nitrogen too - which reminds me that people don't usually understand nitrate either) from bacteria and small organisms that they catch in their slime coat and not so much from the water column - surface area is often more key to collecting building blocks than what we can measure in the water column. Slime coat absorption is efficient whereas polyp catching usually is not, depending on the 'coral" where true coral (hard) don't do well with polyp catching all the way to NPS (which are not truly coral) do very well. This will happen enough on it's own in mature and diverse tanks. Lastly, nobody knows which kind of phosphorous most inhabitants in our tanks use, and there are likely many kinds... and we only test for the one kind which I cannot even remember if it is organic or inorganic po4. I am sure that I messed some of this up and there are many chemistry and biology lessons and classes needed to understand it all, but in a nutshell it will work to remember that it is not a food and to have a trace, but not too much coming from different sources.

Once I get ready for taxes, I can put some po4 into some water and see if some crushed coral or GFO in a reactor will remove more from the water column. If somebody is going to use aragonite for po4 reduction, then a reactor test is probably best. As can be seen in my tests a few years ago, active use in a reactor can remove a lot of po4 from the water column.

 

[Dan_P]

My recollection is that occasionally someone would say "aragonite ought to be a great cheap alternative to GFO" ...but nobody ever reported back "eureka! it worked!"

I think Dan's data does a good job of explaining why - to get an impressive result you probably need to...
1) grind the sand to really fine particles
and
2) have a desorb part of the cycle where you let your sand equalize with zero PO4 water, then connect it back to your system.

I wonder if a dilute acid wash would speed up the regeneration process.

 

[Garf]

I wonder if a dilute acid wash would speed up the regeneration process.

Does dilute acid mess around with phosphate tests? Say testing the sample before and after acidification.

Edit - to clarify I’m not talking about regenerating sand, more obtaining total adsorbed phosphate concentrations.

 

[taricha]

Does dilute acid mess around with phosphate tests? Say testing the sample before and after acidification.

Edit - to clarify I’m not talking about regenerating sand, more obtaining total adsorbed phosphate concentrations.

yeah. if I acidified, I would ph-correct back to near neutral before trying the hanna PO4 test.
I don't remember how far away from neutral to sw ph you can get away with.