Phosphate leaching basics

[LadyTang2]

There are lots of threads on phos leaching but I dont understand the chemical basics of why this happens and was hoping for an explation of why it happens and the most common reasons (under what conditions) phos goes into rock and the most common reason phos leaves or leaches rock.

Lastly, when I first buy rocks is it common for them to be high in phos and why, and whats the best method to reduce? Thanks

 

[Randy Holmes-Farley]

The reason it binds (and can unbind) is that it has an affinity to calcium carbonate surfaces, just like it does to phosphate binders like GFO or aluminum oxide. It gets onto the surface of calcium carbonate in place of carbonate, as calcium phosphate.

It binds an amount directly related to the concentration in the water. the more in the water, the more binds. The less in the water, the less binds.


I discuss it in the contact of binding to fresh CaCO3 surfaces made in the tank by precipitation here:

Phosphate in the Reef Aquarium

The phosphorus atom is one of living matter's basic building blocks. It is present in every living creature and in every reef aquarium's water. Unfortunately, it is often present in excess in reef aquaria, and that excess has the potential to cause at least two substantial problems for...

www.reefedition.com

One possible mechanism could be through calcium phosphate precipitation, as outlined above. A second mechanism for potential phosphate reduction when using high pH additives is the binding of phosphate to calcium carbonate surfaces. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding occurring at lower and higher pH values. Habib Sekha (owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many aquarists find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way this can be true is if the excess calcium and alkalinity, which such additions typically add to the aquarium, are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock). It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels; phosphate binds to these growing surfaces and becomes part of the solid precipitate.

If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite all external phosphate sources having been cut off. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may become buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 is somehow dissolved. Additionally, if these crystals are in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and skimmed out of the aquarium.

If phosphate binds to calcium carbonate surfaces to a significant extent in reef aquaria, then this mechanism may be attained with other high pH additive systems (such as some of the two-part additives, including Recipe #1 of my DIY system). However, this potential precipitation of phosphate on growing calcium carbonate surfaces will not be as readily attained with low pH systems, such as those using calcium carbonate/carbon dioxide reactors or those where the pH is low due to excessive atmospheric carbon dioxide, because the low pH inhibits the precipitation of excess calcium and alkalinity as calcium carbonate, as well as inhibiting the binding of phosphate to calcium carbonate.

 

[LadyTang2]

The reason it binds (and can unbind) is that it has an affinity to calcium carbonate surfaces, just like it does to phosphate binders like GFO or aluminum oxide. It gets onto the surface of calcium carbonate in place of carbonate, as calcium phosphate.

It binds an amount directly related to the concentration in the water. the more in the water, the more binds. The less in the water, the less binds.


I discuss it in the contact of binding to fresh CaCO3 surfaces made in the tank by precipitation here:

Phosphate in the Reef Aquarium

The phosphorus atom is one of living matter's basic building blocks. It is present in every living creature and in every reef aquarium's water. Unfortunately, it is often present in excess in reef aquaria, and that excess has the potential to cause at least two substantial problems for...

www.reefedition.com

One possible mechanism could be through calcium phosphate precipitation, as outlined above. A second mechanism for potential phosphate reduction when using high pH additives is the binding of phosphate to calcium carbonate surfaces. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding occurring at lower and higher pH values. Habib Sekha (owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many aquarists find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way this can be true is if the excess calcium and alkalinity, which such additions typically add to the aquarium, are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock). It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels; phosphate binds to these growing surfaces and becomes part of the solid precipitate.

If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite all external phosphate sources having been cut off. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may become buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 is somehow dissolved. Additionally, if these crystals are in the water column, e.g., if they form at the local area where limewater hits the aquarium water, then they may become coated with organics and skimmed out of the aquarium.

If phosphate binds to calcium carbonate surfaces to a significant extent in reef aquaria, then this mechanism may be attained with other high pH additive systems (such as some of the two-part additives, including Recipe #1 of my DIY system). However, this potential precipitation of phosphate on growing calcium carbonate surfaces will not be as readily attained with low pH systems, such as those using calcium carbonate/carbon dioxide reactors or those where the pH is low due to excessive atmospheric carbon dioxide, because the low pH inhibits the precipitation of excess calcium and alkalinity as calcium carbonate, as well as inhibiting the binding of phosphate to calcium carbonate.

Awesome Randy thanks, one more question, does new dry rock you buy typically have high phos in it already or little and it pulls out of water when first put in? If high, how do you lower it, curing?

 

[Randy Holmes-Farley]

Awesome Randy thanks, one more question, does new dry rock you buy typically have high phos in it already or little and it pulls out of water when first put in? If high, how do you lower it, curing?

Either one depending on its origin and history.

 

[LadyTang2]

Either one depending on its origin and history.

so calcium phosphate binds into and becomes part of the structure that's predominantly calcium carbonate?

And this happens on rock and sand surfaces, what about as part of the actual calcium carb in coral skeleton, there too?

I could see how phos would then be released when the ca carb structure is dissolving but not if its static, why would it release if theres no increase or decrease in ca carb mass? Is it the outermost portion of ca phos that can dissolve since that layer has yet to be covered and protected by an outerlayer of ca carb?

 

[Randy Holmes-Farley]

so calcium phosphate binds into and becomes part of the structure that's predominantly calcium carbonate?

And this happens on rock and sand surfaces, what about as part of the actual calcium carb in coral skeleton, there too?

I could see how phos would then be released when the ca carb structure is dissolving but not if its static, why would it release if theres no increase or decrease in ca carb mass? Is it the outermost portion of ca phos that can dissolve since that layer has yet to be covered and protected by an outerlayer of ca carb?

Phosphate binds to a surface that is exposing calcium ions, forming a type of calcium phosphate in situ. It can just come back off again, leaving the calcium and releasing the phosphate. That's a big oversimplification, but it is the net process that involves lots of things attaching and detaching from the surface all the time.

 

[LadyTang2]

Phosphate binds to a surface that is exposing calcium ions, forming a type of calcium phosphate in situ. It can just come back off again, leaving the calcium and releasing the phosphate. That's a big oversimplification, but it is the net process that involves lots of things attaching and detaching from the surface all the time.

So when there is more ionic Ca rather than Cacarb, this is when more phos can bind that free ca ion?

Do free Ca ions have an affinity for the cacarb rock, they settle on rock then phos can bind in place of carb? Or is it more that when the Cacarb is dissolving there's free Ca ions on surface? (if so static part confuses me)

 

[Randy Holmes-Farley]

So when there is more ionic Ca rather than Cacarb, this is when more phos can bind that free ca ion?

Do free Ca ions have an affinity for the cacarb rock, they settle on rock then phos can bind in place of carb? Or is it more that when the Cacarb is dissolving there's free Ca ions on surface? (if so static part confuses me)

At the risk of oversimplifying, there is both Ca++ and CO3-- at the surface. Both are coming off and going back on fast and constantly. Once the surface gets clogged with other stuff (like organics), there is somewhat less affinity for phosphate.

 

[chicago]

nice write up.. Randy.. thoughts... on this.. so just refilled my calcium reactor with ARM coarse media.. of course the package says.. no phosphate. but as we know not true.. I just tested with a hanna and it is showing effluent at 20 ml per min has .15 phosphate. Thoughts... Concerned that to will be a significant amount to keep adding to the system.. 600 gallon system.. Thoughts...

 

[Randy Holmes-Farley]

nice write up.. Randy.. thoughts... on this.. so just refilled my calcium reactor with ARM coarse media.. of course the package says.. no phosphate. but as we know not true.. I just tested with a hanna and it is showing effluent at 20 ml per min has .15 phosphate. Thoughts... Concerned that to will be a significant amount to keep adding to the system.. 600 gallon system.. Thoughts...

Not likely very significant compared to foods.

Take the daily volume through the reactor (7.6 gallons per day) and divide by the total system volume (600 gallons). That is the dilution factor. 0.15 x 7.6/600 = 0.002 ppm phosphate added per day.

Foods will add something like 0.02 to 0.2 ppm phosphate per day.

 

[chicago]

Thanks randy... just to recap.. the tank water is testing at .02 The effluent is testing at .15. Thank you again

 

[Randy Holmes-Farley]

Thanks randy... just to recap.. the tank water is testing at .02 The effluent is testing at .15. Thank you again

OK, so the boost from the reactor is even smaller and foods are predominating the phosphate additions.

 

[Tankkeepers]

Easy way to explain it is this the water and rocks sand will balance so if it's higher in the water it'll bind to the rocks or sand if the rocks sand are higher it'll leach into the water

 

[Tankkeepers]

But its a balancing act as with most things they want to be at the same level

 

[chicago]

thanks all..