Aragonite Sand And Phosphate Adsorption

[Randy Holmes-Farley]

Aragonite Sand And Phosphate Adsorption. Sand Grain Size Study

The data I reported above raised some questions that needed addressing, especially, “why were my results so different from published data”. I have new data that provides a plausible answer: surface area.

@Randy Holmes-Farley has made clear that surface area is an important factor in determining the phosphate adsorbing capacity of calcium carbonate. Taking the hint, I ground my aragonite sand in a mortar and pestle, and than with a 200 mesh sieve, isolated sand grains that were comparable to those used by Millero, et al. The plot below (with its hobby friendly units) shows Millero’s results (green), and my results for sugar sand (orange), and ground and sieved sugar sand (blue). The match between Millero’s results and mine for ground sand is not perfect but close. A possible reason for the curve shape difference is mineral content. Millero used pure aragonite whereas I used aragonite sand which is likely a mixture of calcite and aragonite. In fact, my results resembles Millero‘s results for calcite. By the way, the idea of adding powdered calcium carbonate to remove phosphate from the water (but maybe not from the aquarium) gets a boost in credibility from this data, though we are likely adding the lower binding capacity mineral calcite.

Interesting. How much do you think the surface area per g increased?

I'm wondering if any of the effect was to expose fresh surface area rather than the increase in surface area per g. Maybe the old sand had something on the surfaces that was inhibiting binding (such as organics).

 

[Dan_P]

Interesting. How much do you think the surface area per g increased?

I'm wondering if any of the effect was to expose fresh surface area rather than the increase in surface area per g. Maybe the old sand had something on the surfaces that was inhibiting binding (such as organics).

If we use internet data, going from sand to silt is approximately a 10-20x increase in surface area. The new adsorption data is consistent with increased surface area.

I did nothing that might address the idea about adsorption differences between clean vs “dirty” sand grain surfaces. To your point about the presence of organic surface coatings, I stained new sand with methylene blue and examined the grains. There were blue stains on some of the grains that resembled algae and bacteria growth patterns that I have observed on aquarium sand grains. Maybe a quick acid wash would be a way to clean a calcium carbonate surface without drastically changing the surface area, but if the adsorption rate did not change, one could argue that I did not remove the organic coating I will keep surface area fouling when I consider phosphate desorption rates and nuisance algae growth.

 

[Dan_P]

Aragonite Sand And Phosphate Adsorption. Adsorption-Desorption Study.

In this study, two approaches were taken to observe phosphate adsorption by aragonite sand. The single exposure is how Millero performed the experiment. A known amount of sand is mixed with a phosphate solution for 24 hours and the decrease in phosphate concentration determined. Fresh sand was used for every concentration of phosphate studied. In the multiple exposure study, I used the same sand sample for all phosphate concentrations studied. This was accomplished using the single exposure approach but instead of throwing away the sample, the spent phosphate solution was removed and the excess solution in the sand was wicked away with a paper towel before adding the next higher concentration of phosphate. In both approaches a desorption experiment was performed at each concentration by removing the depleted phosphate solution, wicking away the medium from the sand, adding Instant Ocean (0 ppm PO4) and mixing for 24 hours before measuring the phosphate concentration. Desorption observations required replicate adsorption samples to be run for the multiple exposure approach.

The graph below shows similar behavior for single (green) and multiple (blue) exposure approaches. The highest concentration in the multiple exposure experiment seems to have been near the saturation point of the sand.

Desorption rates of phosphate in both experiments were similar but less than expected, leaving more bound phosphate on the sand than would be expected based on the absorption curves. Because the 24 phosphate measurements did not differ from the 48 h measurements, it is assumed that all sand samples studied were at equilibrium. The literature on phosphate adsorption to calcium carbonate suggests that phosphate initially binds by chemisorption and is then converted to an insoluble calcium phosphate mineral. This might account for the less than expected amount of phosphate desorption and has implications for the amount of phosphate that can be released from aragonite surfaces.

 

[jda]

As far as rinsing new sand with tap, I am mostly of the mind of "who cares" as long as you have low amounts of copper and heavy metals. You want some phosphate to bind to the sand anyway so that the tank is not growth limited on the low side in the beginning - a buffer, if you will. However, I live in an area where the tap water is pretty good. If you get some phosphate bound from the tap, then the sand will allow a trace buffer in the tank sooner.

If you are ok on the heavy metals, then using tap is probably a drop in the ocean compared to the stump removers and phosphate additives that people use thinking that po4 is coral food.

Keep in mind that I am not of the belief that more than a trace of po4 in the tank is necessary... like 1-3 ppb for me works great. If I start a new tank with fresh ocean sand, I can read zero for a month or two before I get to 1-3 ppb when the sand has enough to be a buffer (in a good way). I know that both zero and 1-3 ppb is within the test error range for Hannah, but the zero happens at the right time and so does the 1-3. When real ocean live rock and live sand was used more commonly, I feel that the 1-3 month guidance for keeping more sensitive corals had at least a little part to do with allowing them to stop binding the water clean of po4. Nearly all ocean rock and sand that I have ever gotten was devoid of po4, as you would expect from the environment where it came from.

I might have posted this before, but live rock in tubs can release P for a long time. I imagine that binding can also take a long time. I can get the tub at 0 ppb for a few days after a few months of LC dosage and a few weeks later read 10 ppb again.

 

[Garf]

After spending weeks understanding PO4 adsorption in the 0.2 ppm and above range, it dawned on me that our aquaria water usually sit at <0.1 ppm

I’ve decided to utilise a spare dosing pump and add phosphate throughout the day to get rid of the boom and bust of phosphate availability. I’ll probably add about 0.2ppm a day to start, and see what happens. I’ll see if I can maintain between 0.1 and 0.25ppm, which seems realistic.

Edit - something I did think about was pH and binding ability. 8.3pH seems to maximise it, I wonder if bacterial activity in a tank substantially lower the pH in a bed therefore reducing binding.

 

[Dan_P]

I might have posted this before, but live rock in tubs can release P for a long time. I imagine that binding can also take a long time. I can get the tub at 0 ppb for a few days after a few months of LC dosage and a few weeks later read 10 ppb again.

Is that ppb PO4?

 

[Dan_P]

I’ve decided to utilise a spare dosing pump and add phosphate throughout the day to get rid of the boom and bust of phosphate availability. I’ll probably add about 0.2ppm a day to start, and see what happens. I’ll see if I can maintain between 0.1 and 0.25ppm, which seems realistic.

Edit - something I did think about was pH and binding ability. 8.3pH seems to maximise it, I wonder if bacterial activity in a tank substantially lower the pH in a bed therefore reducing binding.

Maybe lowering sand bed pH by bacterial activity results in a release of PO4, maybe not enough to detect once it becomes diluted in the aquarium water, but a significant amount at the sand bed surface where nuisance algae lurk.

 

[jda]

I only have a Hannah Ultra Low Phosphorous checker. All in ppb.

Just conjecture, but imagine that if aragonite is dissolved in the sandbed and the local concentration increases, then the nearby solid aragonite would bind it before it gets far.

 

[Miami Reef]

I haven‘t had a chance to read through the whole thread, but what do you think the effects of phosphates would be when dosing calcium carbonate powder?

They use it as a water clarity treatment, noted in this thread: https://www.reef2reef.com/threads/extreme-water-clarity-and-cyano-eradication-made-easy.916381/

 

[Dan_P]

I only have a Hannah Ultra Low Phosphorous checker. All in ppb.

Just conjecture, but imagine that if aragonite is dissolved in the sandbed and the local concentration increases, then the nearby solid aragonite would bind it before it gets far.

Thanks, I should have guessed it was a Hanna low range.

Yeah, the surrounding sand would bind the pH desorbed phosphate until surrounding water with a lower phosphate concentration seeps in and the phosphate desorbs. Some of this now more phosphate concentrated water seeps to other aragonite sand grains where it is re-adsorbed but also some eventually seeps to the top of the sand bed and out. I have no idea how long such a process takes.

 

[sixty_reefer]

@jda I agree that larger systems don’t require as many phosphates to be available although is worth pointing out that there is a difference between larger volumes of water in comparison to smaller volumes of water as dilution will play a big part.
For example 3ppb in your 240 gallons and 3ppb in my 25 gallons makes your tank having 9.6 times more phosphates available in comparison to mine. We often overlook dilution wend talking of nutrients and smaller tanks will often require to replenish those at a more often rate in comparison to a larger volume of water, the same applies to other nutrients like calcium and alkalinity were they need to be added more regularly to keep a desired residual in check.
in my calculations assuming that both systems have a residual of 3 ppb, your tank would have 2.724 total phosphates and mine would have 0.282 total phosphates available for organisms to use at any given time.
For me to have the same phosphates as you I would have to set my residual to 0.028 ppm to match your 0.003ppm if that makes sense.
If we were to calculate 1ppb in the sea in comparison to one of the larger systems on R2R the number would still be astronomical.
Not sure if @Dan_P will be touching on that during the experiment.

 

[Randy Holmes-Farley]

Aragonite Sand And Phosphate Adsorption. Adsorption-Desorption Study.

In this study, two approaches were taken to observe phosphate adsorption by aragonite sand. The single exposure is how Millero performed the experiment. A known amount of sand is mixed with a phosphate solution for 24 hours and the decrease in phosphate concentration determined. Fresh sand was used for every concentration of phosphate studied. In the multiple exposure study, I used the same sand sample for all phosphate concentrations studied. This was accomplished using the single exposure approach but instead of throwing away the sample, the spent phosphate solution was removed and the excess solution in the sand was wicked away with a paper towel before adding the next higher concentration of phosphate. In both approaches a desorption experiment was performed at each concentration by removing the depleted phosphate solution, wicking away the medium from the sand, adding Instant Ocean (0 ppm PO4) and mixing for 24 hours before measuring the phosphate concentration. Desorption observations required replicate adsorption samples to be run for the multiple exposure approach.

The graph below shows similar behavior for single (green) and multiple (blue) exposure approaches. The highest concentration in the multiple exposure experiment seems to have been near the saturation point of the sand.

Desorption rates of phosphate in both experiments were similar but less than expected, leaving more bound phosphate on the sand than would be expected based on the absorption curves. Because the 24 phosphate measurements did not differ from the 48 h measurements, it is assumed that all sand samples studied were at equilibrium. The literature on phosphate adsorption to calcium carbonate suggests that phosphate initially binds by chemisorption and is then converted to an insoluble calcium phosphate mineral. This might account for the less than expected amount of phosphate desorption and has implications for the amount of phosphate that can be released from aragonite surfaces.

Thanks for the info!

 

[Dan_P]

Not sure if @Dan_P will be touching on that during the experiment.

Would you clarify. I don’t see where experimentation would connect to this idea.

 

[sixty_reefer]

Would you clarify. I don’t see where experimentation would connect to this idea.

If we were to scale your 40ml of water and 3 grams of sand to gallon and pound of sand we would get 1 pound of sand for 1.6 gallons of water and garf would be 1 pound of sand to 0,36 gallon.

In my mind the fact that larger volume of water will hold more phosphates in comparison to smaller volumes of water, would the ratio of sand to water used in this experiment affect the results once applied to a reef tank?
for example the accepted general rule for live rock is normally 1-1/2 pound per gallon of water and half pound of live rock will have a smaller surface than half pound of sand.
im just asking if the ratio of sand to water chosen would affect real life scenarios as less volume of water was chosen to conduct the experiment.
And by seeing that different volume of water can affect the real phosphate absorbing idea and that most of us don’t really use 1pound of sand per 1.6 gallons of water in our systems would that affect the results?

 

[Miami Reef]

@Randy Holmes-Farley @Garf @Dan_P

Yesterday I dosed calcium carbonate powder to see the effects in my tank. I noticed today that phosphates seem higher than usual the following day, but I cannot say if the powder caused it because I scraped all the algae from the back wall and changed my activated carbon.

Is there a test/experiment you can suggest to me to see if the calcium carbonate powder releases phosphates in fresh saltwater?

I can test the water before and after, but should the powder be stirred?

 

[jda]

Larger tanks likely have a proportionally larger amount of things that need building blocks, like po4. Some things are much more prevalent in larger tanks, like coralline algae with much more glass surface area, for example. I imagine that the rock surface area is more than 9x too.

@Dan_P - I doubt that you could saturate local concentrations of aragonite to where it would need to unbind up-the-chain too much. I have no evidence, but phosphate becoming free after aragonite dissolves seems like a rounding error to me, in relation to the same tank/system. Of course, sand that is a deep reservoir from another tank could make the po4 rise in my tank, but it would not need to dissolve to do this... just regular old unbinding would do the trick.

 

[Dan_P]

im just asking if the ratio of sand to water chosen would affect real life scenarios as less volume of water was chosen to conduct the experiment.

OK thanks for the clarification.

In principle, the answer is no. The scale and ratios I chose to work with do not affect the equilibrium distribution of phosphate between sand and water.

A higher ratio of water to sand would give a different equilibrium distribution of phosphate than a lower ratio, but both scenarios lie on the absorption curve I posted. The higher ratio scenario has a larger amount of phosphate to distribute between the water and sand. It would result in more phosphate in the sand, but also more in solution.

 

[Dan_P]

Larger tanks likely have a proportionally larger amount of things that need building blocks, like po4. Some things are much more prevalent in larger tanks, like coralline algae with much more glass surface area, for example. I imagine that the rock surface area is more than 9x too.

@Dan_P - I doubt that you could saturate local concentrations of aragonite to where it would need to unbind up-the-chain too much. I have no evidence, but phosphate becoming free after aragonite dissolves seems like a rounding error to me, in relation to the same tank/system. Of course, sand that is a deep reservoir from another tank could make the po4 rise in my tank, but it would not need to dissolve to do this... just regular old unbinding would do the trick.

I like your rounding error idea.

I came across something interesting, possibly. Adsorbed phosphate seems to become irreversibly bound to sand and the amount of phosphate that can bind and come off again seems to decline with repeated exposures of sand to phosphate. Until I can wrap my mind around this, I will follow your lead and call it a rounding error

 

[jda]

I don't know that is true. If the phosphate is bound to aragonite, calcite, dolomite, etc. and then new structure is is added on top, then the phosphate can be trapped, but without new growth, I have never, ever heard of the bind being non reversible. I can tell you that sand in my tanks can bind and unbind based on water equilibrium. Were they taking about new growth trapping something underneath?

I just don't think that calcium carbonate buffering is enough of a thing to worry about... just happens in such small amounts in most tanks. In tanks with pH issues or with super low carbonate levels, then probably more... but phosphate from dissolving aragonite is probably the 100th thing to worry about if a tank has a massive amount of CaCO3 buffering.

Edit: po4 can get "trapped" in GFO when biofilm covers the grains. The biofilm can also keep GFO from absorbing. It is an access issue. Perhaps the trapping in sand is because the water cannot reach the pure aragonite structure? Just guessing here.

 

[Dan_P]

@Randy Holmes-Farley @Garf @Dan_P

Yesterday I dosed calcium carbonate powder to see the effects in my tank. I noticed today that phosphates seem higher than usual the following day, but I cannot say if the powder caused it because I scraped all the algae from the back wall and changed my activated carbon.

Is there a test/experiment you can suggest to me to see if the calcium carbonate powder releases phosphates in fresh saltwater?

I can test the water before and after, but should the powder be stirred?

Yes, you are right. Measure phosphate before and after mixing calcium carbonate with fresh saltwater. If it releasing phosphate, It could take a couple hours to overnight to reach a detectable concentration.

In addition to stirring or shaking, the sample you test must be filtered. For powdered calcium carbonate you might try the syringe filter that comes with the Hanna low nitrate range kit, otherwise, buy a few 0.45 micron syringe filters (cellulose or nylon will work)