There Is Phosphate In Aragonite Sand

[tmz]

I can't tell if this thread is making me less confused about nutrient dispersal, or more so

I don't exactly know where the thread is going or what the hypothesis is. So it's easy to be confused by it .

In this , perhaps over simplistic statement of my understanding, ,nutrients including inorganic phosphate will diffuse from areas of high concentration to areas of lower concentrations.

Higher inorganic phosphate concentrations are likely where acidification from organic degradation contibutes to dissolution and/ or weakening surface bonding as well as from organic degradation itself as occurs in interstitial water in the substrate or pores of rock.

The speed at which the diffusion occurs is influenced by exposure to water with lower concentrations . This exposure to water at lower concentrations is less in more stagnant areas such as in pores of rock or sand or interstitial water in substrate than it is in the open water of the aquarium. These more stagnant areas can become even more so as organics or precipitants clog things up . Thus, it's not surprising the water from those areas would have a higher concentration of inorganic phosphate than the open water of the aquarium at a given point in time.

Hope that helps.

If this is inaccurate I trust Sherminator will help clear it up.

 

[Dan_P]

I think all naturally occuring aragonite will contain some inorganic phosphate sunk in the crystal matrix. It doesn't mean it will leach out or affect the aquarium without dissolution of the argonite in acidic conditions . There may also be some bound to surfaces which will equilibrate with the water.

I found Toonen's Advance Aquarist article online in Aquaculture

An experimental comparison of sediment-based biological filtration designs for recirculating aquarium systems (Robert J. Toonen, Christopher B. Wee)

In both versions of the article, phosphate in the water column for coarse sand was reported to be 0.1 ppm higher than that for the fine sand which was close to none detected. This was a very strong experimental response.

What I discovered in the Aquaculture article was that the coarse sediment in the test was Florida crushed coral and the fine sediment was South Down play sand. If play sand is what I think it is, a silica based sand, then the large difference in phosphate levels between crushed coral versus silica sand is of interest to our discussion. The source of the phosphate, desorption, dissolution or organophodphate, who knows.

 

[tmz]

The "Southdown" white sand marketed at that time as white play sand (Southdown Tropical Play Sand (mean particle diameter ~0.2 mm), used in the study was aragonite not silica sand as I recall. It hasn't been available through home improvement stores for years though. New aragonite sand is thought to offer fresh nucleation sites and more adsorbtion sites. The greater surface area afforded by smaller grains or argonite vs the larger particles of coarser crushed coral could have something to do with the lower PO4 readings with the finer sediment.

From the study:

"By the end of the experiment, pH was significantly higher in aquaria with fine (7.98 ± 0.01 SE) than coarse (7.91 ± 0.01 SE) sediments (df = 1, F = 10.31, p < 0.01). Alkalinity was significantly higher in tanks with fine (2.36 meq / L ± 0.08 SE) than with coarse (1.80 meq / L ± 0.08 SE) sediments (df = 1, F = 23.21, p < 0.001), and tanks with plenums (2.20 meq / L ± 0.08 SE) than without plenums (1.99 meq / L ± 0.08 SE) (df = 1, F = 4.86, p < 0.05). In contrast, phosphate ended up significantly higher in aquaria with coarse (0.32 mg / L ± 0.01 SE) than fine (< 0.01 mg / L ± 0.01 SE) sediments (df = 1, F = 211.37, p < 0.001). No other source variables or interaction terms were significant for final pH, phosphate, or alkalinity values. "

The increased alk could in part reflect carbonate contributions via dissolution . Silica sand wouldn't contribute alkalinity.

 

[shermanator]

The "Southdown" white sand marketed at that time as white play sand and used in the study was aragonite not silica sand as I recall.

That is consistent with what I have found on the interwebs from that time.

New aragonite sand is thought to offer fresh nucleation sites and more adsorbtion sites. The greater surface area afforded by smaller grains or argonite vs the larger particles of coarser crushed coral could have something to do with the lower PO4 readings with the finer sediment.

This explanation certainly makes the most sense. Greater surface area = less phosphate in the water column.

Unfortunately, most Advanced Aquarist articles don't have an experimental section, so we don't know exactly what was done. For example, did they wash the sand?

 

[shermanator]

I don't exactly know where the thread is going or what the hypothesis is. So it's easy to be confused by it .

In this , perhaps over simplistic statement of my understanding, ,nutrients including inorganic phosphate will diffuse from areas of high concentration to areas of lower concentrations.

Higher inorganic phosphate concentrations are likely where acidification from organic degradation contibutes to dissolution and/ or weakening surface bonding as well as from organic degradation itself as occurs in interstitial water in the substrate or pores of rock.

The speed at which the diffusion occurs is influenced by exposure to water with lower concentrations . This exposure to water at lower concentrations is less in more stagnant areas such as in pores of rock or sand or interstitial water in substrate than it is in the open water of the aquarium. These more stagnant areas can become even more so as organics or precipitants clog things up . Thus, it's not surprising the water from those areas would have a higher concentration of inorganic phosphate than the open water of the aquarium at a given point in time.

I agree with all of this. Usually the simplest explanation is the correct one.

 

[tmz]

did they wash the sand?

They say it was autoclaved.

 

[Dan_P]

The "Southdown" white sand marketed at that time as white play sand (Southdown Tropical Play Sand (mean particle diameter ~0.2 mm), used in the study was aragonite not silica sand as I recall. It hasn't been available through home improvement stores for years though. New aragonite sand is thought to offer fresh nucleation sites and more adsorbtion sites. The greater surface area afforded by smaller grains or argonite vs the larger particles of coarser crushed coral could have something to do with the lower PO4 readings with the finer sediment.

From the study:

"By the end of the experiment, pH was significantly higher in aquaria with fine (7.98 ± 0.01 SE) than coarse (7.91 ± 0.01 SE) sediments (df = 1, F = 10.31, p < 0.01). Alkalinity was significantly higher in tanks with fine (2.36 meq / L ± 0.08 SE) than with coarse (1.80 meq / L ± 0.08 SE) sediments (df = 1, F = 23.21, p < 0.001), and tanks with plenums (2.20 meq / L ± 0.08 SE) than without plenums (1.99 meq / L ± 0.08 SE) (df = 1, F = 4.86, p < 0.05). In contrast, phosphate ended up significantly higher in aquaria with coarse (0.32 mg / L ± 0.01 SE) than fine (< 0.01 mg / L ± 0.01 SE) sediments (df = 1, F = 211.37, p < 0.001). No other source variables or interaction terms were significant for final pH, phosphate, or alkalinity values. "

The increased alk could in part reflect carbonate contributions via dissolution . Silica sand wouldn't contribute alkalinity.

Makes sense. A marine sediment study observed lower phosphates available when finer aragonite sand was present. Thanks!

 

[rgulrich]

@Dan_P - does this add any interesting facets to your research?
http://www.aoml.noaa.gov/flbay/millero1.html

Cheers,
Ray

 

[tmz]

Makes sense. A marine sediment study observed lower phosphates available when finer aragonite sand was present. Thanks!

You are welcome.

 

[Dan_P]

The phosphate bound on (and in) calcium carbonate is always from the water column. As the rock is forming, if phosphate is present it will be incorporated within the rock. Of course in nature it's abnormal to have high phosphate. To get the phosphate out of a rock formed in high phosphate, would be impossible, you would have to dissolve the entire rock.

In the case of rock for our aquaria, phosphate is absorbed onto water-accessible surfaces to make calcium phosphate. So when people "cook" rocks on the timescales used it's to remove the calcium phosphate on the water-accessible surface.



The K-sp for calcium carbonate is much higher than the K-sp for calcium phosphate (10^-9 vs 10^-33). But there is much more calcium in solution. You could do the math pretty easily, but it's well known that phsophate leaches from rock that was previously in high PO4 water (when put in low PO4 water).

Here is how solubility products relate to solubilities. Unless the salt have the same ratios of ions, you cannot compare Ksp's. Results are for the Ksp's I currently have in my spreadsheet.

CaCO3 Ksp: 3.8x10^-9 = Solubility: 6.2x10^-5 M or 0.15 ppm Ca++ (!)
Ca3(PO4)2 Ksp: 1.0x10^-32 = Solubility: 1.6x10^-07 M or 0.03 ppm PO4--- (!)

Calcium carbonate is about 400 times more soluble than calcium phosphate. In the presence of 400 ppm Ca++, the solubilities drop pretty drastically.

CaCO3 Ksp: 3.8x10^-9 = Solubility: 3.8x10^-11 M
Ca3(PO4)2 Ksp: 1.0x10^-32 = Solubility: 5x10^-14 M

At lower pH's, both salts would exhibit higher solubilities. These numbers make it clear that both salts are supersaturated in seawater. The situation is comparable to a rubberband stretched to its bracking point. The fact that we don't have precipitate all over our aquariums is amazing.

When I finish my calcium phosphate solubility experiments, I will compare the results to the above calculations.

 

[Dan_P]

@Dan_P - does this add any interesting facets to your research?
http://www.aoml.noaa.gov/flbay/millero1.html

Cheers,
Ray

Ray, I read this very interesting reference already,but thanks much for helping out! Dan

 

[GreshamH]

Just on a biological note: silica sand can offer its own troubles: diatoms use silica.

SiO2 also is used in glass aquariums :headwally: Very little goes into solution, and diatoms are in fact beneficial to marine aquaria. Out of control diatom growth isn't however. I've used silica sand in numerous aquariums, never had much of an issue.

 

[shermanator]

Calcium carbonate is about 400 times more soluble than calcium phosphate. In the presence of 400 ppm Ca++, the solubilities drop pretty drastically.

Don't forget to include the carbonate concentration of saltwater in your calculations.

These numbers make it clear that both salts are supersaturated in seawater. The situation is comparable to a rubberband stretched to its bracking point. The fact that we don't have precipitate all over our aquariums is amazing.

This is outside my area of expertise, but having 20,000 ppm chloride (and many other ions in high concentration) could make the K-sp values meaningless.

 

[Dan_P]

Don't forget to include the carbonate concentration of saltwater in your calculations.



This is outside my area of expertise, but having 20,000 ppm chloride (and many other ions in high concentration) could make the K-sp values meaningless.

Good catch! Thanks.

 

[dkeller_nc]

BTW, for those interested in reading the whole Millero article, here's a link to the full text.

 

[dkeller_nc]

This is outside my area of expertise, but having 20,000 ppm chloride (and many other ions in high concentration) could make the K-sp values meaningless.

This is indeed the case, not only due to the high ionic concentrations in solution, but also the presence of a huge variety of organics, many of which have strong kinetic effects on dissolution, and possibly on the final equilibria as well.

But I would suggest that Ksp calculations/comparisons aren't totally useless, even though many if not most of the constants in the literature are determined in high-purity water rather complex matrices - they do at least give one a several-orders-of-magnitude assessment of what should and shouldn't be very soluble in seawater.

 

[dkeller_nc]

Unfortunately, most Advanced Aquarist articles don't have an experimental section, so we don't know exactly what was done. For example, did they wash the sand?

I read both of these articles closely; unfortunately, not much can be concluded about sequestration/processing of phosphate in the substrates (the authors primary purpose was comparing plenum/dsb systems on nitrogen processing) because of the experimental conditions. Both substrates (the crushed coral and the fine aragonite) received a generous dose of liquefied squid to establish initial bacterial colonization (and therefore an initial dose of phosphate), and the authors didn't assay the substrates for initial phosphate concentration before autoclaving/cycling.

Since the initial condition is unknown, it's hard to say whether the differences in final free phosphate concentrations in the tank water between the aragonite and crushed coral were because of plenum/no plenum/dsb and/or differences in bacterial action, or simply that the crushed coral was already saturated with phosphate, and so could not adsorb the tank water's load.

 

[Dan_P]

This is indeed the case, not only due to the high ionic concentrations in solution, but also the presence of a huge variety of organics, many of which have strong kinetic effects on dissolution, and possibly on the final equilibria as well.

But I would suggest that Ksp calculations/comparisons aren't totally useless, even though many if not most of the constants in the literature are determined in high-purity water rather complex matrices - they do at least give one a several-orders-of-magnitude assessment of what should and shouldn't be very soluble in seawater.

A possible fine point but one worth clarifying. Dissolution versus preciptiation/crystallization. Crystallization is a kinetic process and is the "thing" that is often studied to understand the deposition of phosphate. Supersaturation of calcium phosphate, the condition where solutes are present at much higher levels than the solubility calculated from the Ksp, is the mystery being solved. The presence of ions and organic molecules can interfer with crystallization.

Dissolution is genrally not the process that is being studied. Sparingly soluble salts can take a long time to reach equilibrium, but the amount of dissolved solid is not due to interferring with the process of crystal species leaving the matrix by something. The notion is that the equilibrium is shifted. As noted, common ions have a large influence on solubilty and ionic strength can have an effect as well.

So, the upshot is that there is a limit of how high a concentration of phosphate can be obtained by dissolving calcium phosphate, but by separately adding phosphate and calcium, higher levels can be obtained. I am trying to get some numbers for this discussion.

 

[Dan_P]

I read both of these articles closely; unfortunately, not much can be concluded about sequestration/processing of phosphate in the substrates (the authors primary purpose was comparing plenum/dsb systems on nitrogen processing) because of the experimental conditions. Both substrates (the crushed coral and the fine aragonite) received a generous dose of liquefied squid to establish initial bacterial colonization (and therefore an initial dose of phosphate), and the authors didn't assay the substrates for initial phosphate concentration before autoclaving/cycling.

Since the initial condition is unknown, it's hard to say whether the differences in final free phosphate concentrations in the tank water between the aragonite and crushed coral were because of plenum/no plenum/dsb and/or differences in bacterial action, or simply that the crushed coral was already saturated with phosphate, and so could not adsorb the tank water's load.

As you pointed out earlier, the larger surface area of the fine aragonite is the simple explanation for there being less phosphate with finer sand. The higher alkalinity in the finer sand case should have worked against phosphate adsorption, but I am not sure it was enough to overpower the surface area affect. More to ponder.

 

[tmz]

Silica sand:

This:

"....In the first experiment I took 3 cups of sand, and suspended it in 3 gallons of freshly made Instant Ocean salt mix that initially contained less than 0.8 mM of silica (0.05 ppm SiO₂). After 48 hours of gentle stirring with a powerhead (the water was stirring, but not the sand), the silica concentration had risen to 17 mM (1.0 ppm SiO₂).
I then rinsed the same sand 5 times with 1 gallon RO/DI water (1 minute each time), discarded the contents, and then ran the same stirring experiment with 2 new gallons of Instant Ocean salt mix. In 48 hours the silica concentration had again risen, this time to 15 mM (0.92 ppm SiO₂). Then I let it sit unstirred for another 96 hours, and the concentration had risen more, to 23 mM (1.4 ppm SiO₂).
In a different experiment, I took about 45 pounds of sand, and added 2 gallons of Instant Ocean salt mix. I let this mixture sit for 7 days, with once a day mixing with my hands for about 30 seconds. At then end of this test, the concentration was 90 mM (5.4 ppm SiO₂).
It has been suggested that the amount of silica coming from calcerous sand might actually be as high or higher than that from silica sand. To test this hypothesis, I repeated the small-scale experiments above on a calcium carbonate sand from Home Depot (Southdown). In this case, there was some soluble silica released after the first 48 h, but only 1.6 mM (0.1 ppm SiO₂), or about a factor of 10 lower than the silica sand. In a long-term test, the concentration had only risen to 5 mM (0.3 ppm SiO₂) in 14 days with once a day stirring.
From these experiments, I conclude that:

1. The “silica” play sand that I purchased from Home Depot can substantially raise the dissolved silica concentration in seawater.
2. The dissolvable portion of the silica sand cannot be completely removed by several rinses with either fresh or salt water, although it may be decreased somewhat by that process.
3. Southdown calcium carbonate sand (likely aragonite) can release soluble silica, but about ten fold less than the “silica” sand.

Is it OK to use silica sand? Probably. Many people do so. I also believe that not all “silica “ sands will be the same for the reasons described above relating to processing of the sand and the nature of the mineral inclusions present. So the fact that many people successfully use some (or many) types of silica sand does not necessarily imply that all people can use any type of “silica” sand without a problem....."


The above is from this article by Randy Holmes-Farley:


http://www.advancedaquarist.com/2003/1/aafeature1


Silca sand is also reported by some users :


to be more likely to scratch glass than aragonite during normal maintenance ,


be difficult on sand sifting organisms including some gobies given it's relatively sharp edges.


It doesn't not offer the buffering provided by argonite sand.



Diatoms are benficial at reasonable levels,imo.


Soluble silicate has benefits for them and some other desirable organisms like sponges and mollusks.


GFO removes silicate .BTW.