Experiment: phosphate in rocks

[VintageReefer]

Another place where this matters is that folks like to post that your substrate or rocks might leach (or leech to some) po4 which is why algae might be out of hand. This cannot happen unless you lower the water concentration of po4. The correct terms, or description, might help somebody spend or focus on the right things to fix an algae issue and thinking that rocks leaching/leeching po4 when they already have 0.40 (or whatever) level of po4 is not really all that helpful.

Yes I agree to that. Otherwise they may believe that their p levels will constantly increase as something - sand/rock - continues to leach. This is incorrect

Rather, what they notice is as they take steps to reduce p, it remains the same, or it goes down temporarily and then returns. This is the leaching/equilibrium effect in action. And to remedy it, will take time and continuous effort.

 

[mizimmer90]

Salinity and temperature matter. RODI did not do anything that I could measure. Water alone does not remove much... you need media or chemicals to really make a dent. I would be shocked if a 100% water change with 0.50 water got down to even 0.49 once everything was back to equilibrium. You can make pretty big dents in the bound phosphate with large doses of LC, but I would not do this in a tank with fish or corals.

Hmm, It's possible that PO4 would be more soluble with the ions present in the water but I thought it was pretty soluble to begin with. Maybe the salinity has more of a kinetic effect, facilitating exchange? To be clear I'm not doubting your experience but trying to think about the mechanisms!

Temperature can definitively affect the equilibrated concentrations as well as the rate to reach those concentrations though.

Also, by 0.5 and 0.49, are these the concentrations in the water column after a 100% WC? If so, that would indicate that 0.5 ppm was removed from the rocks and doing this over and over would keep removing that amount. Or are you suggesting that the concentration on the rocks is so high that removing 0.5 ppm is actually negligible? If the later, I agree that addition of something to continually remove phosphate from the WC to shift the equilibrium would be more efficient!

 

[jda]

I am saying that water changes are not going to do you much good. Surface area or of the aragonite is too variable to come up with an equation, but I doubt that you could get out 1% with a 100% water change... this is just me guessing.

 

[mizimmer90]

Surface area or of the aragonite is too variable to come up with an equation

No worries, we can still model it with the surface area as a floating variable

I doubt that you could get out 1% with a 100% water change... this is just me guessing.

interesting! That would mean that the concentration bound to the rocks is very high and/or the equilibrium drastically favors the rocks (neither would surprise me).

I'm trying to look up some papers on the subject and found this one where the abstract looks relevant (looking at sorption dynamics of phosphate in carbonate aquifers off of Biscayne bay), but alas it's behind a pay wall that I don't have access to :`(

Rapid and Intense Phosphate Desorption Kinetics When Saltwater Intrudes into Carbonate Rock - Estuaries and Coasts

It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock...

link.springer.com

 

[VintageReefer]

Water changes will lower phosphate temporarily. Things will balance back out. But, they will not repeat this process forever.

A turf scrubber is constantly removing phosphate from the water and absorbing it in the algae it grows. When you dispose of the algae, the phosphate is permanently removed from the ecosystem

As the water columns phosphate is lowered, it allows the phosphate to be released from the rock. Which then raises the water level p back to original values. Remember when testing water you are testing only the water. You have no idea how much p is bound to the rock. But looking at your problem algae rocks, you can see how much of your rock has p issues. Example - see my photo

The rock typically in problem scenarios is the source. But it is not infinite. But it is large due to all the crevices and surface area.

The scrubber constantly removes p from the water, the rock continues to replenish the p in the water. The cycle runs 18-20 hours a day. And eventually. After several weeks or months….if the scrubber is appropriately sized and is of good design, the rock will have nothing left to unbind. And then…you will have made some real progress in getting the situation under control and your waters numbers will start to go down, excess algae will diminish also. And from then forward, any new phosphate will be absorbed by the corals and the scrubber instead of the rock.

 

[NanJ]

Salinity and temperature matter. RODI did not do anything that I could measure. Water alone does not remove much... you need media or chemicals to really make a dent. I would be shocked if a 100% water change with 0.50 water got down to even 0.49 once everything was back to equilibrium. You can make pretty big dents in the bound phosphate with large doses of LC, but I would not do this in a tank with fish or corals.

This thread certainly explains the difficulty in decreasing phosphate in a tank that has long had less than optimal levels. In our 300+g tank we do about 20% water change a month and the phosphate level may go down +/- 15% after the change but they always go back up. Now I understand why.

 

[VintageReefer]

This thread certainly explains the difficulty in decreasing phosphate in a tank that has long had less than optimal levels. In our 300+g tank we do about 20% water change a month and the phosphate level may go down +/- 15% after the change but they always go back up. Now I understand why.

Your water change is (a good thing) a temporary fix. A scrubber will be a constant ally working day and night for you filtering out that phosphate.

My system has had the scrubber long enough where my rocks are cleaned, my corals get the phosphate they need, the scrubber gets the excess, and I have a bare minimum of display algae. I no longer need to do water changes or use other means for algae control.

 

[TWYOUNG]

To "test" the theory of phosphates leaching out of rock, I put a rock from the DT into a bucket of saltwater taken from the auto-water change reservoir tank.

-- AWC saltwater phosphate read 0.09 ppm with Hanna Checker. (Pretty high for "plain" saltwater.....need to figure that part out.)
-- About 3/4 gallon AWC saltwater in a bucket.
-- Added a rock from the DT that was about 5-6" in diameter.
-- Phosphates 4 days later read 0.38 ppm.
-- Phosphates 4 days later read 0.54 ppm

This was not a very controlled experiment but I think it does show phosphates leach out of rocks. We are considering an algae scrubber and I wanted to verify the information I was given that rocks are a reservoir for phosphates. I assume that the pH of the water in the bucket changed over the week and impacted the phosphates leaching out of the rock. Like I said, it wasn't a controlled experiment!

The phosphates in our tank consistently run 0.35-0.40 ppm and we have to scrub patches of hair algae on the rocks each week to stay ahead of the algae. We are hoping an algae scrubber will help "get our hands out of the tank".

Shared this information just to add to the knowledge base.

As you state this experiment is definitely lacking controls. I'm curious what the phosphate level was in your DT. I assume it must be very high if the rock you removed, and presumably was at equilibrium with the tank water, has so much phosphate to release after placed in 0.09 water. Isn't it perhaps possible the rock experienced die off after being placed in a bucket and likely in the dark? These decaying organics could then be driving up nutrient levels in the bucket.

 

[Randy Holmes-Farley]

As you state this experiment is definitely lacking controls. I'm curious what the phosphate level was in your DT. I assume it must be very high if the rock you removed, and presumably was at equilibrium with the tank water, has so much phosphate to release after placed in 0.09 water. Isn't it perhaps possible the rock experienced die off after being placed in a bucket and likely in the dark? These decaying organics could then be driving up nutrient levels in the bucket.

Other folks experiments would indicate it need not be that high to release large amounts. One experiment described in post 13 of this thread had 50 ppm of phosphate bound at only 0.16 ppm in the water remaining.

 

[DCR]

The total amount of PO4 in the aquarium water is very small even at high concentrations. Even a concentration of 0.5 ppm in 100 gallons of water is less than 2 mg. Spread that out over 100 lbs. of rock and sand and it represents a little more than 0.0004% of the total weight of the rock and sand. The rock and sand can probably bind a lot more PO4 than that.

 

[NanJ]

As you state this experiment is definitely lacking controls. I'm curious what the phosphate level was in your DT. I assume it must be very high if the rock you removed, and presumably was at equilibrium with the tank water, has so much phosphate to release after placed in 0.09 water. Isn't it perhaps possible the rock experienced die off after being placed in a bucket and likely in the dark? These decaying organics could then be driving up nutrient levels in the bucket.

It was more a “proof of concept” than an experiment, I guess. The phosphates in our tank run 0.35-0.40 ppm using the Hanna Checker.

 

[Markdickenschwanz]

Well, that is discouraging for my plan!

As I understand algae scrubbers, the light supply has to be pretty powerful to outcompete algae in the tank. Because algae scrubbers are so expensive and sometimes due to space limitations, people often undersize them for the amount of load (phosphate-saturated rocks, fish feeding/waste) they are up against . I am not saying that is your situation, just relating what I have learned.

Lots of online tips on utube in how to build a cheap scrubber

 

[TWYOUNG]

Other folks experiments would indicate it need not be that high to release large amounts. One experiment described in post 13 of this thread had 50 ppm of phosphate bound at only 0.16 ppm in the water remaining.

I guess my question would be if her DT water had no higher phosphate levels than the bucket water, why wouldn't the rock already have released that phosphate in an attempt to reach equilibrium with the tank?

 

[Randy Holmes-Farley]

I guess my question would be if her DT water had no higher phosphate levels than the bucket water, why wouldn't the rock already have released that phosphate in an attempt to reach equilibrium with the tank?

Rocks will release some phosphate into low phosphate water. The amount will depend on many factors, but in general, it may take a huge amount of water to strips rocks this way.

For example, taking jda’s experiment showing 50 ppm worth bound at 0.16 ppm, it would take hundreds of water changes, each reaching a value like 0.16 ppm, to take out most of it.

 

[Tritie]

I did an experiment and more than 50 ppm of po4 bound to .16, or so in half of a Phosban reactor of aragonite in a 10g tank. The aragonite can hold a lot. It also can come back off.

There is no actual evidence that a ATS algae can outcompete algae in a tank under any conditions where a reef would survive. I know that a few members seem to think so, but it does not happen. The ATS is just another place where algae can grow uninterrupted (no consumers) and is easy to harvest - no more. If you don't have enough consumers in your display tank, then it becomes an ATS or refuge too.

@jda Can you clarify this a bit? Are you saying you had 50ppm of phos in the water, and after adding the aragonite (I'm assuming sand??) You found only 0.16ppm in the water meaning the aragonite bound 49.84ppm of phos?

 

[Randy Holmes-Farley]

@jda Can you clarify this a bit? Are you saying you had 50ppm of phos in the water, and after adding the aragonite (I'm assuming sand??) You found only 0.16ppm in the water meaning the aragonite bound 49.84ppm of phos?

Yes. It means he added the equivalent of 50 ppm of phosphate, and equilibrated with only 0.16 ppm left in the water.
Of course that number is critically dependent on the water volume and the calcium carbonate amount and surface area. It is not a number one should memorize as being useful. Double the aquarium volume in the same experiment and the number drops by half.

The point is, it can be a lot!