How does salt water help curing rock get rid of phosphates?

texdoc77

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I already asked this question in the main reef forum and got lots of great answers, but not exactly what I was looking for. If you want to peruse that thread here it is: https://www.reef2reef.com/threads/why-do-you-circulate-dry-rock-in-salt-water.694505/

What I would really like to know is what it is chemically about curing rock in salt water (or lanthanum chloride) that removes the organics from the rock. We established in that thread that the main worrisome organics were phosphate and silicates and that circulating dry rock in saltwater is helpful.

This may be stretching the chemistry part of this forum a bit, but I thought I would give it a try.

Thanks in advance,

SC
 

brandon429

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no intended distraction, but a specific add on chem question:

Im under the impression this is a bacterially-mediated event, extracting phosphate from dry rocks. no liberation of po4 from the matrix without acidic liberation, digestion, from respiring organisms is what I thought

how else is a matrix of caco3 going to release po4 without acid help let me know pls.


if its bacterially mediated, then curing in saltwater might cure for those bacteria. a bacterial film is going to form on either fw or sw submerged rocks, perhaps selected strains dont matter at the time of curing?

awaiting clarification by Randy and crew how saltwater itself is ideal for curing vs freshwater.
 

brandon429

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also possible: their website / marco rocks says nothing bad is in the rocks needing cure

so, its possible the whole hobby is doing crazy stuff like they do in cycling threads. I guess if there are logs of verified pre and post cure po4 measures with this rock, that'll decide.
 

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What I would really like to know is what it is chemically about curing rock in salt water (or lanthanum chloride) that removes the organics from the rock. We established in that thread that the main worrisome organics were phosphate and silicates and that circulating dry rock in saltwater is helpful.

Curing the rock is just "exposing" it's pores, to release die off, phosphates and other goodies, before it is going to be cycled. Acid may "expose" the pores faster, but both do the same.

That's my take on curing with saltwater or acid.
 
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texdoc77

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Sorry to bring this back up but I never really got a good answer yesterday and kept plugging away. Many of you were correct in saying that the organics are removed better in salt water. I figured this, but wanted to know why. A few ventured guesses and I will not quote the wrong ones, but in order for all to have better information going forward here is the main issue - and, no, it does not have anything to do with bacteria, sorry.

It's a little thing called the Stetschenow constant. This is basically a chemical reaction that causes organics to dissolve better in NaCl than without. The constant is different for each organic compound, salinity and temperature. This means that organics will unbind from the rock they are on and enter the salt solution where they can be taken out, as many of you suggested. The how is a little more complicated and I will quote the Research article which originally appeared in Marine Environmental Research in Dec 1997, Vol 44, Issue 4, pgs 429-444, by Xie, et al.

"The hydrophobicity of organic solutes is therefore increased by this factor (stetschenow constant), as is the air-water partition coefficient, implying an increased partitioning from aqueous solution into air, organic carbon and lipid phases. The effect must be quantified when comparing the behavior of organic contaminants in freshwater and marine conditions."

So now I know, and so do you.
 

brandon429

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that implies curable targets are part of floc that is stuck to the outside, how does that mechanism account for cleaving caco3 complex to release po4 though? that may not even be required, the site says rocks dont require curing but something sure does drive aquarists to do it. still a neat mechanism had not heard of that. if adhered floc is the reserve of what we cure that sounds very neat above

Ive often wondered even in ideal curing conditions, how do we know its deep enough shedding etc, perhaps test levels show differences in nitrate or po4 after curing?
 
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texdoc77

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that implies curable targets are part of floc that is stuck to the outside, how does that mechanism account for cleaving caco3 complex to release po4 though? that may not even be required, the site says rocks dont require curing but something sure does drive aquarists to do it. still a neat mechanism had not heard of that. if adhered floc is the reserve of what we cure that sounds very neat above

Ive often wondered even in ideal curing conditions, how do we know its deep enough shedding etc, perhaps test levels show differences in nitrate or po4 after curing?
The kind of rock we use is very porous. Those pores on a molecular level are relatively large. No CaCo3 cleaving necessary. But it does take longer the further in you go because it takes a while to saturate all those pores with salt water. Basically everything is on the outside.
 
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Basically everything is on the outside.

In what regard or maybe the better question is what do you mean by everything is on the outside. If we are talking about dry rock it is a bit more complicated than this. If everything was on the outside then it wouldn't take so long to fully mature. This can be months if not years. Depends of course on the quality of the rock and hobbyist effort and patience.
 
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texdoc77

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what do you mean by everything is on the outside.

The organics that I and other hobbyists are concerned with are on the surfaces of the rock, whether that means the outermost surfaces or innermost surfaces. All of those surfaces are available to the saltwater, some just more accessible than others. If there were organics not accessible by salt water then they are not dissolvable and would thus cause no threat to the tank.
 
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The organics that I and other hobbyists are concerned with are on the surfaces of the rock, whether that means the outermost surfaces or innermost surfaces. All of those surfaces are available to the saltwater, some just more accessible than others. If there were organics not accessible by salt water then they are not dissolvable and would thus cause no threat to the tank.

I believe this will vary would it not? Macro Rock is aragonitic I thought or made mostly of it rather than cement based. Compare that to dry Pukani which is 100% natural oceanic. Thus different organics. Not sure which one is worse or if it matters but one of the more common concerns when using dry rock is the prolonged leaching of phosphates in the water. Prolong I mean years because it isn't limited to the surface.

There are videos out there, BRS comes to mind, regarding dry rock preparation and cleaning to combat this. I'm not one to use acid baths but this was one of the more commonly recommended ways to not only clean the rock but reduce phosphates. I concern with this would be the reduction of rock volume which was as high as 25% depending on the duration of the acid bath. On the other hand results are the best.

Interesting read never the less. Thanks. Personally speaking I will never use dry rock again in any of my tanks. While I'm happy 19 or so months later it was the wrong choice when upgrading / merging tanks.
 

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My comment - in 6 months - dry rock is 'wet rock'. Now - not all 'dry rock' is equivalent - if you're talking about rock from a cheap supplier thats using basically cement - it may be different. I agree with @saf1 that using acid reduces a lot of rock. It makes me think though - about the OP

you asked several questions - curing in salt water - and then parenthetically lanthanum. @Randy Holmes-Farley since he is a worlds expert on lanthanum chloride would be the go to person here - but - I think each individual group of rock is different. The way to tell is measuring phosphate - if phosphate is leaching - you will see it. If not - you don't need to go through any process. If your rock has no PO4 - you dont need to do anything. I think many of the protocols that are suggested tend to go after the 'worst case scenario' -i.e. you NEED to leave your tank fallow for xxx days - you NEED to cycle for xxx weeks, you cant add an anemone until XXX months, etc etc. Every tank is different IMHO - and testing your water (and knowing the source of your rock) will give you the answer.

In short - some rock will release PO4 and Silicates. Many do not
 
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I believe this will vary would it not? Macro Rock is aragonitic I thought or made mostly of it rather than cement based. Compare that to dry Pukani which is 100% natural oceanic. Thus different organics. Not sure which one is worse or if it matters but one of the more common concerns when using dry rock is the prolonged leaching of phosphates in the water. Prolong I mean years because it isn't limited to the surface.

There are videos out there, BRS comes to mind, regarding dry rock preparation and cleaning to combat this. I'm not one to use acid baths but this was one of the more commonly recommended ways to not only clean the rock but reduce phosphates. I concern with this would be the reduction of rock volume which was as high as 25% depending on the duration of the acid bath. On the other hand results are the best.

Interesting read never the less. Thanks. Personally speaking I will never use dry rock again in any of my tanks. While I'm happy 19 or so months later it was the wrong choice when upgrading / merging tanks.

Yeah, MacroRocks are apparently from reefs when the earth was younger so on some level they are 100% oceanic too, right LOL. You know the idea of whether the phosphate is coming from the surface or "trapped" within the matrix of the rock is, I think, very interesting. There is no question that there is quite a bit on the surface, but whether or not trapped phosphates are released over time from the matrix of the rock is another question entirely. Based on the fact that acid can reduce the volume of the rock by 25% would speak to the fact that the rock can be dissolved in certain solvents, but does this happen in salt water or not? I would also postulate that the routine tank maintenance that is employed to deal with the organics that are produced with feeding and waste would also hopefully deal with any long term leaching. But of course this is the rub, right? I have seen many postulate that leaching rocks are the reason for consistently high phosphate readings despite conservative fish population, disciplined feeding and appropriate water changes. I have seen enough tanks out there started by dry and wet rock without high phosphate and algae issues to believe it can be done either way. I guess the problem is too many variables and setting up a tank with ideal conditions and replicating this process to isolate an issue is likely not very feasible.

Anyway more food for thought for me. Thanks for the discussion.
 

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I already asked this question in the main reef forum and got lots of great answers, but not exactly what I was looking for. If you want to peruse that thread here it is: https://www.reef2reef.com/threads/why-do-you-circulate-dry-rock-in-salt-water.694505/

What I would really like to know is what it is chemically about curing rock in salt water (or lanthanum chloride) that removes the organics from the rock. We established in that thread that the main worrisome organics were phosphate and silicates and that circulating dry rock in saltwater is helpful.

This may be stretching the chemistry part of this forum a bit, but I thought I would give it a try.

Thanks in advance,

SC

Your question is rather confusing as I’m not sure you understand what organic means. There are no organic silicates in seawater, and the phosphate that desorbs and binds to lanthanum is inorganic.

In any case, if there is inorganic phosphate on the rock, it is in equilibrium with phosphate in the water. If you remove the phosphate in the water, by changing water or precipitating it with lanthanum or other binders, more comes off the rock to replace it.

Organics of all types, including some that may contain phosphate may be partly broken down by various processes and released from the rock. Not all will be removed, but the longer you wait the more will be removed (unless bacterial growth, etc) keeps adding them back.
 

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Yeah, MacroRocks are apparently from reefs when the earth was younger so on some level they are 100% oceanic too, right LOL. You know the idea of whether the phosphate is coming from the surface or "trapped" within the matrix of the rock is, I think, very interesting. There is no question that there is quite a bit on the surface, but whether or not trapped phosphates are released over time from the matrix of the rock is another question entirely. Based on the fact that acid can reduce the volume of the rock by 25% would speak to the fact that the rock can be dissolved in certain solvents, but does this happen in salt water or not? I would also postulate that the routine tank maintenance that is employed to deal with the organics that are produced with feeding and waste would also hopefully deal with any long term leaching. But of course this is the rub, right? I have seen many postulate that leaching rocks are the reason for consistently high phosphate readings despite conservative fish population, disciplined feeding and appropriate water changes. I have seen enough tanks out there started by dry and wet rock without high phosphate and algae issues to believe it can be done either way. I guess the problem is too many variables and setting up a tank with ideal conditions and replicating this process to isolate an issue is likely not very feasible.

Anyway more food for thought for me. Thanks for the discussion.

Phosphate that is not exposed at the rock surface will not be released. Phosphate at the surface, which includes the surfaces inside pores that connect to the bulk water, is in equilibrium with phosphate in the water, and will come off or go on depending on the relative concentrations.

acid etching dissolves the surface calcium carbonate, as well as phosphate that was on the old surface and phosphate that became exposed at the new surface.
 
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texdoc77

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Your question is rather confusing as I’m not sure you understand what organic means. There are no organic silicates in seawater, and the phosphate that desorbs and binds to lanthanum is inorganic.

In any case, if there is inorganic phosphate on the rock, it is in equilibrium with phosphate in the water. If you remove the phosphate in the water, by changing water or precipitating it with lanthanum or other binders, more comes off the rock to replace it.

Organics of all types, including some that may contain phosphate may be partly broken down by various processes and released from the rock. Not all will be removed, but the longer you wait the more will be removed (unless bacterial growth, etc) keeps adding them back.
Sorry, I don't mean to be confusing, but I feel like this is a confusing topic with lots of misinformation out there. Right now I'm trying to pull phosphates off my MacroRocks I just bought. I have it sitting in just RODI water and saw where it is recommended to put it in NaCl water. The Stentschenow constant does indicate certain increased solubility in salt water and I suspect now this is why it is recommended to bathe rock in salt water. Is this correct? You mentioned the phosphate being in equilibrium between the water column and the rock.

As far as the term organic is concerned, I was misusing it, and I know better. My o-chem teacher was not great and it was a long time ago, but still no excuse, please don't tell him.
 

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