There Is Phosphate In Aragonite Sand

[tmz]

Setting aside the calcium phosphate equilbration from surface bound phosphate and aragonite dissolution ,I think organics will settle in substrate or on substrate or aragonite rock including pores and degrade via bacterial activity over time until they are completely refractory. This may result in a higher concentration of soluble reactive phosphate including inorganic phosphate in the substrate as diffusion catches up with it.

The amount of inorganic phosphate in the crystal matrix of coral sand or aragonite rock can stay sunk there for millenia or longer ,absent dissolution. It is of no consequence in reef tanks, imo. How much is actually in a coral sand or live rock depends on the phosphate used by the calcifying organisms that made them .The amount of phosphate sunk via biotic precipitation would depend on the availability of phosphate at the time it was made and the organisms ability to incorporate it in it's skeletal structure.

 

[shermanator]

The ratio of phosphate on the surface to the bulk water is determined by the equilibrium constant but is mediate through chemical diffusion. It takes time to equilibrate because of diffusion. Even an exposed surface has a boundary layer that slows the migration of the phosphate from surface to bulk water. When that surface is buried in a pore, it takes a very long time.

Diffusion of ions in a solution is a very fast process (K ~ 10^8). Compare that to a K-sp on the order of 10^-33. I'd say diffusion isn't an issue.

Given the Ksp, I don't think cakcium phosphate dissolves to a detectable amount in saltwater.

If you don't trust the math, do the experiment and you'll see detectable phosphate starting from calcium phosphate. When you are thinking about this stuff, don't forget that saltwater has carbonate and the formation of calcium carbonate (starting from calcium phosphate) is important to consider.

 

[shermanator]

Setting aside the calcium phosphate equilbration from surface bound phosphate and aragonite dissolution ,I think organics will settle in substrate or on substrate or aragonite rock including pores and degrade via bacterial activity over time until they are completely refractory. This may result in a higher concentration of soluble reactive phosphate including inorganic phosphate in the substrate as diffusion catches up with it.

The amount of inorganic phosphate in the crystal matrix of coral sand or aragonite rock can stay sunk there for millenia or longer ,absent dissolution. It is of no consequence in reef tanks, imo. How much is actually in a coral sand or live rock depends on the phosphate used by the calcifying organisms that made them .The amount of phosphate sunk via biotic precipitation would depend on the availability of phosphate at the time it was made and the organisms ability to incorporate it in it's skeletal structure.

Agreed with all of this 100%.

 

[Dan_P]

Diffusion of ions in a solution is a very fast process (K ~ 10^8). Compare that to a K-sp on the order of 10^-33. I'd say diffusion isn't an issue.

You are right and wrong. Diffusion through pores and sediment is a very slow process. I proved it to myself with methlene blue diffusion through sand. 2 mm in three hours! I suspect phosphate ion is not much faster.

Also, you are mixing up rate constants and equilibrium constants. There is little connection between the end state and how quickly the system reaches the end state.

If you don't trust the math, do the experiment and you'll see detectable phosphate starting from calcium phosphate. When you are thinking about this stuff, don't forget that saltwater has carbonate and the formation of calcium carbonate (starting from calcium phosphate) is important to consider.

Calcium carbonate is more soluble than calcium phosphate. Not sure I follow the logic here.

 

[shermanator]

You are right and wrong. Diffusion through pores and sediment is a very slow process. I proved it to myself with methlene blue diffusion through sand. 2 mm in three hours! I suspect phosphate ion is not much faster.

You had said that methylene blue "sticks" to sand so I'm not sure how you can measure the diffusion of that.

Also, you are mixing up rate constants and equilibrium constants. There is little connection between the end state and how quickly the system reaches the end state.

You said diffusion was an issue and my Point is that diffusion is fast for ions. The dissolution of ions is slow but then diffusion is rapid.

Calcium carbonate is more soluble than calcium phosphate. Not sure I follow the logic here.

It's more soluble in water with no calcium or carbonate ions. Do the math and see if that is still true in saltwater with high calcium and carbonate. My point is that it's more complex than looking at solubility constants.

 

[tmz]

Two variables beyond solubility constants, I'd consider in assessing variations in water taken from the substrate vs the tank water are :

Fluid dynamics : the rate diffusion from one area to the other depends on water volume and exchange rate. Slow flow areas in pores or substrate limit the rate of diffusion vs the open water of the tank. Even natural reefs rely on upwelling from advective currents to move nutrients along with higher levels at depths than at the surface for PO4 and dissolved nitrogen as nitrate for example.

Organics: degradation in low flow areas lead to higher localized concentrations which can outpace diffusion rates in those areas.

 

[Dan_P]

You had said that methylene blue "sticks" to sand so I'm not sure how you can measure the diffusion of that.

Sorry about the confusion. I have silica sand and that is what I did the dye study with. Aragonite binds methylene blue.

You said diffusion was an issue and my Point is that diffusion is fast for ions. The dissolution of ions is slow but then diffusion is rapid.

Wow, this point is getting all balled up Forget about diffusion from the crystal surface. Free ions in fine sand or in sub-millimeter pores in live rock where there is no advection take hours to move short distances. The methylene blue migration in fine silica sand is a visual demonstration of that principle.

When a crystal dissolves, if the immediate vicinity of the crystal face reaches saturation, dissolution stops. So, in a pore where diffusion away from the surface is very slow, saturation occurs, stopping the dissolution of even a readily soluble substance.

It's more soluble in water with no calcium or carbonate ions. Do the math and see if that is still true in saltwater with high calcium and carbonate. My point is that it's more complex than looking at solubility constants.

What is complex and confusing is the supersaturation of calcium phosphate in seawater. It is way over the solubility limit. The reason for this is due solely to the kinetics of crystallization. It takes a long time if ever to relieve this supersaturation. I don't know if the explanation has been finalized, but it is as you say complex. Forming seed crystals and crystal face contamination by other ions take calcium phosphate crystal growth makes the story more than the Ksp.

Dissolution is more straightforward. If the solution is saturated, there will be no net dissolution. I say it that way because the crystal surface is not inert. It does dissolve but redeposits again.

I will report back on my observation of the dissolution of calcium phosphate in seawater. I will have a good laugh if it dissolves.

Dan

 

[tmz]

Even an exposed surface has a boundary layer that slows the migration of the phosphate from surface to bulk water. When that surface is buried in a pore, it takes a very long time.

I'm not clear on what you mean by a "boundry layer".

 

[bertoni]

"Boundary layer" is a concept from fluid mechanics. The Wikipedia article is a little complex, but the point is that bulk flow slows near stationary objects due to viscosity.

 

[tmz]

Thanks Jonathan.

Not sure that would effect diffusion of PO4 significantly though; somewhat I suppose given a lesser volume of water to work with .

 

[dkeller_nc]

Dan - I will first congratulate you for your intellectual curiosity and willingness to put some effort into experimentation. Many of the protagonists/antagonists of arguments on Reef Central and other reefkeeping sites rely solely on published scientific papers that, because of the author's intent and experimental conditions, have almost no applicability to the argument. Yet these same papers are still cited as "proof".

With respect to your initial experiment, you're correct that there's a basic problem. Phosphorus can exist in many forms and be incorporated into many different molecules. Broadly, you can separate these into inorganic forms (like the orthophosphate ion) and organic forms, where phosphate is incorporated into a bewildering variety of organic compounds (examples include phospholipids, proteins, ATP/ADP, and DNA, and a boatload of other molecules).

That's not an issue if you're asking the question of "how much total phosphorus is in this sample of substrate, whether inorganic or organic", you simply break down the organic forms into inorganic simple phosphates by means of reacting the entire sample with an oxidizing strong acid (nitric is typically used), and quantitate the total amount of phosphorus.

If, however, your goal is to determine how much phosphate is in/on the actual matrix of the substrate in the form of bound/precipitated inorganic phosphorus but excluding phosphorus contributed by organic forms, including whole bacteria, algae, films of absorbed biomolecules, etc..., you have a much more difficult problem. One way of doing this would be to very slightly ablate the outer surface of the substrate particles with a weak concentration of acid in conjunction with copious washing to remove any biofilms/absorbed films of biomolecules, then dissolve the remaining washed substrate in strong acid and quantitate the phosphorus. That's still going to be imprecise, since you're never going to be sure of exactly how much outer surface of the particles you removed, and what amount of inorganic/precipitated phosphate that it contained. But it would at least get you in the ballpark if done carefully.

A few procedural comments about your initial experiment. You're correct that you need to test the hydrochloric acid that you used for its phosphate content, as well as the baking soda. Strong mineral acids will readily become contaminated from materials and equipment used in the production, transportation and storage processes. The industrial/technical grade of hydrochloric acid readily available to hobbyists from pool supply and home stores are often contaminated, at least at a low level, with a variety of metals and other compounds. Really pure mineral acids can be purchased as analytical reagents, but they are quite expensive.

The baking soda will probably not contain much phosphorus if it was produced by the Solvay process. However, there are significant amounts of naturally-occurring deposits of sodium bicarbonate and sodium carbonate (that can be converted to sodium bicarbonate) that are mined on the order of a few million tons a year; these natural sources would potentially contain significant phosphorus (phosphate) concentration as a contaminant.

 

[dkeller_nc]

BTW, folks, there's a bit of imprecision being used with respect to the term "diffusion". The diffusion that shermanator refers to is the scientific/engineering definition of the term - it is dispersion of a molecular entity away from a source of concentration in solid or liquid of another substance solely as a result of molecular motion.

Obviously, convective flow can also move concentrations of a substance away from a source. But this isn't "diffusion" in the strictest sense of the definition.

From the standpoint of dissolution from a solid material into a liquid, all of the mass transfer from the surface of the solid is by diffusion because of the presence of the boundary layer that Jonathan referred to. It turns out that at a microscopic level, there is no such thing as convective flow of a liquid or a gas right at the surface of a solid. However, (and intuitively), convective flow of a fluid past a solid surface that's dissolving makes a very large difference in the rate of mass transfer from the solid; this occurs because the thickness of the boundary layer around the solid is dependent on this bulk convective flow - high flow means thinner boundary layers; low flow means thicker boundary layers.

We now return you to your regularly scheduled broadcast after this nerdy chemical engineering interlude...

 

[Dan_P]

Dan - I will first congratulate you for your intellectual curiosity and willingness to put some effort into experimentation. Many of the protagonists/antagonists of arguments on Reef Central and other reefkeeping sites rely solely on published scientific papers that, because of the author's intent and experimental conditions, have almost no applicability to the argument. Yet these same papers are still cited as "proof".

With respect to your initial experiment, you're correct that there's a basic problem. Phosphorus can exist in many forms and be incorporated into many different molecules. Broadly, you can separate these into inorganic forms (like the orthophosphate ion) and organic forms, where phosphate is incorporated into a bewildering variety of organic compounds (examples include phospholipids, proteins, ATP/ADP, and DNA, and a boatload of other molecules).

That's not an issue if you're asking the question of "how much total phosphorus is in this sample of substrate, whether inorganic or organic", you simply break down the organic forms into inorganic simple phosphates by means of reacting the entire sample with an oxidizing strong acid (nitric is typically used), and quantitate the total amount of phosphorus.

If, however, your goal is to determine how much phosphate is in/on the actual matrix of the substrate in the form of bound/precipitated inorganic phosphorus but excluding phosphorus contributed by organic forms, including whole bacteria, algae, films of absorbed biomolecules, etc..., you have a much more difficult problem. One way of doing this would be to very slightly ablate the outer surface of the substrate particles with a weak concentration of acid in conjunction with copious washing to remove any biofilms/absorbed films of biomolecules, then dissolve the remaining washed substrate in strong acid and quantitate the phosphorus. That's still going to be imprecise, since you're never going to be sure of exactly how much outer surface of the particles you removed, and what amount of inorganic/precipitated phosphate that it contained. But it would at least get you in the ballpark if done carefully.

A few procedural comments about your initial experiment. You're correct that you need to test the hydrochloric acid that you used for its phosphate content, as well as the baking soda. Strong mineral acids will readily become contaminated from materials and equipment used in the production, transportation and storage processes. The industrial/technical grade of hydrochloric acid readily available to hobbyists from pool supply and home stores are often contaminated, at least at a low level, with a variety of metals and other compounds. Really pure mineral acids can be purchased as analytical reagents, but they are quite expensive.

The baking soda will probably not contain much phosphorus if it was produced by the Solvay process. However, there are significant amounts of naturally-occurring deposits of sodium bicarbonate and sodium carbonate (that can be converted to sodium bicarbonate) that are mined on the order of a few million tons a year; these natural sources would potentially contain significant phosphorus (phosphate) concentration as a contaminant.

Once again good points and appreciate the suggestions.

 

[tmz]

BTW, folks, there's a bit of imprecision being used with respect to the term "diffusion". The diffusion that shermanator refers to is the scientific/engineering definition of the term - it is dispersion of a molecular entity away from a source of concentration in solid or liquid of another substance solely as a result of molecular motion.

I agree with that.

Still not clear about the effect of boundry layers and it's meaning in the context of this discussion though.

To clarify,I think the volume of water available for molecular movement is a factor; isn't it? For example in an area where water exchange was 1 ml per minute vs 1 gallon per minute ,the later would support higher rates of molecular difussion ;wouldn't it? I think advection and other means of water movement moves more water through in effect enlarging the pool of water in which molecular diffusion occurs.

 

[Dan_P]

BTW, folks, there's a bit of imprecision being used with respect to the term "diffusion". The diffusion that shermanator refers to is the scientific/engineering definition of the term - it is dispersion of a molecular entity away from a source of concentration in solid or liquid of another substance solely as a result of molecular motion.

I agree with that.

Still not clear about the effect of boundry layers and it's meaning in the context of this discussion though.

To clarify,I think the volume of water available for molecular movement is a factor; isn't it? For example in an area where water exchange was 1 ml per minute vs 1 gallon per minute ,the later would support higher rates of molecular difussion ;wouldn't it? I think advection and other means of water movement moves more water through in effect enlarging the pool of water in which molecular diffusion occurs.

In thinking about a boundary layer, it is in the context of a stationary surface and fluid flow. Slow fluid flow results in a thicker boundary layer, that is, a stagnant fluid layer over the surface. Speed up the fluid flow and this stagnant layer thins. Since molecules move only by diffusion in the boundary layer, the thicker the layer, the longer it takes to move from the bulk water to the surface or surface to the bulk. Boundary layers also resist heat flow.

In a sand bed or inside live rock there is physical situation we typically don't take into account: there is no flow, no advection. This results in a huge stagnant layer. There are very long distances for molecules to travel through sand and live rock pores to reach the bulk water. Increasing fluid flow over sand and live rocks helps in that it reduces the concentration of the diffusing species at the mouth of a very long tunnel of the pore, but does little else to speed up diffusion deep within.

Vacuuming sand may be working not because it is removing visible detritus but it is greatly speeding up the removal of dissolved and particulate organic material that typically take ages to diffuse through and out of the sand bed. If detritus is reducing diffusion rates in sand beds, its removal is an additional benefit. Unfortunately, live rock cannot be vacuumed.

 

[shermanator]

In thinking about a boundary layer, it is in the context of a stationary surface and fluid flow. Slow fluid flow results in a thicker boundary layer, that is, a stagnant fluid layer over the surface. Speed up the fluid flow and this stagnant layer thins. Since molecules move only by diffusion in the boundary layer, the thicker the layer, the longer it takes to move from the bulk water to the surface or surface to the bulk.

Diffusion doesn't require flow (note: advection does, but the principle of a high concentration equilibrating across a tank is diffusion). I generally don't like to cite Wikipedia, but go check out the diffusion article. If there is local high concentration of phosphate it will diffuse at a rate of roughly 10^8 M-1 s-1.

Regardless, I think we can end this (academic) debate at this point. Do you have access to calcium phosphate? I probably have some in my lab (I'm a chemistry professor) or I can borrow some from a colleague.

 

[CStrickland]

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

 

[Dan_P]

Diffusion doesn't require flow (note: advection does, but the principle of a high concentration equilibrating across a tank is diffusion). I generally don't like to cite Wikipedia, but go check out the diffusion article. If there is local high concentration of phosphate it will diffuse at a rate of roughly 10^8 M-1 s-1.

Regardless, I think we can end this (academic) debate at this point. Do you have access to calcium phosphate? I probably have some in my lab (I'm a chemistry professor) or I can borrow some from a colleague.

Thanks for the offer, but I won't take advantage of it just yet. I found some calcium phosphate on eBay and it should be here this week.

I will try to compare the Wiki diffusion rate to what is being quoted for diffusion rates in sediment...hopefully!

 

[Dan_P]

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

These forum discussions really don't progress our knowledge about the hobby, but I do find the debates useful for the challenges, critiques and ideas. This forum could in principle answer some tough questions if we had the time, energy and discipline to become citizen scientists and to collect and pool our data.

 

[Dan_P]

Thanks for the offer, but I won't take advantage of it just yet. I found some calcium phosphate on eBay and it should be here this week.

I will try to compare the Wiki diffusion rate to what is being quoted for diffusion rates in sediment...hopefully!

Here is a neat calculator which approximates distance traveled by various molecules in water at 25 C.

http://www.physiologyweb.com/calculators/diffusion_time_calculator.html

While the coefficient of diffusion is not given for phosphate, oxygen and calcium are. These chemical species do not diffuse in water very quickly. I agree we can end the academic discussion until we have some data to look over.