Phosphates precipitating at pH 8.5

rrome

New member
Hi all. I have been reading about phosphates at wetwebmedia.com. One of their suggested ways to reduce phosphates is to : "Raising pH like with kalkwasser... to 8.4-8.5 to precipitate out the phosphates for good."

Now, I have very high phosphates (off scale with salifert test kit) but I wanted to see if there is some merrit to this. I took a water sample (about 40 mL), starting pH was 8.15 and added 1 drop of 1.0 M NaOH. the pH raised to about 8.75. Maybe there were a couple of small crystals formed, but no major precipitation. I tested the water and the phosphates are at about the same level. I tested the water and the phosphates are at about the same level. I them lowered the pH back to 8.08 by adding 0.02 M HCl, retested and got the same results.

Questions:

1) Has anyone done this and have it work in a tank?

2) What is the insoluble phosphate species that is supposed to form?

3) Wouldn't this be a reversible process?

4) Is the precipitation supposed to be a slow process (ie hours?)

The saifert test kit is not supposed to work very well if the alkalinity is higher than 7 meq/L, I guess because you acidify the solution while doing the test. my alkalinity is 4.46 meq/L.

The pH after adding the first reagent ( <3.5 m HCl) is 1.4

I am repeating the experiment raising the pH to 8.54 with 0.1 M NaOH and letting the sample equilibrate 8 and 24 hours. I personally plan to use Phosban to reduce the level.s

Thanks!
 
Last edited:

Randy Holmes-Farley

Reef Chemist
Premium Member
Precipitation of phosphate, such as with limewater is a very complicated issue. It may, for example, only happen when the pH is locally in the 9+ range where the limewater hits the tank water, not throughout the tank where the pH is much lower.

I discuss such issues in these articles:


Phosphorus: Algaeââ"šÂ¬Ã¢"žÂ¢s Best Friend
http://www.advancedaquarist.com/issues/sept2002/chem.htm

from it:

"How to Export Phosphate

So now that we know where phosphate comes from, and how much, we can proceed to ask where it goes and how to maximize those export processes. Certainly, some phosphorus goes into the bodies of growing organisms, including bacteria, algae, corals, and fish. Some of these organisms stay permanently in the tank, and others may be removed by harvesting of algae, skimming of small organisms, and even pruning of corals.

A less frequently discussed mechanism for phosphate reduction may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef tanks will be supersaturated with respect to this material, as the equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. As with CaCO3, the precipitation of Ca3(PO4)2in seawater may be limited by kinetic factors more than equilibrium factors, so it is impossible to say how much might precipitate under reef tank conditions (without, of course, somehow determining it experimentally). This precipitation may be especially likely where calcium and high pH additives (like limewater) enter the tank water. The locally high pH converts much of the HPO4-- to PO4---. Combined with the locally high calcium, the locally high PO4--- may push the supersaturation of Ca3(PO4)2 to unstable levels, causing precipitation.

Likewise, phosphate can precipitate onto the surface of calcium carbonate, such as onto live rock and sand. The absorption of phosphate from seawater onto aragonite is pH dependent, with the maximum binding taking place around pH 8.4 and with less binding at lower and higher pH values. 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 make it difficult to completely remove excess phosphate from a tank that has experienced very high phosphate levels, and may permit algae to continue to thrive despite cutting off all external phosphate sources. In such cases, removal of the substrate may even be required.

The relationship of calcium carbonate to the phosphate cycle has been studied by Frank Millero in the Florida Bay ecosystem (click here for Millero's studies). If aragonite crystals are growing, as they often are in some parts of our systems, then Iââ"šÂ¬Ã¢"žÂ¢d expect some of this phosphate to get buried and locked into the aragonite crystals.

A side effect of the adsorption of phosphate onto aragonite may well be the reported impact of phosphate on the calcification of corals. The presence of phosphate may inhibit the formation of calcium carbonate crystals via surface adsorption, and this effect may very well be the factor that inhibits calcification of corals at high phosphate levels.

Many reef keepers accept the concept that limewater addition reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and published them in Aquarium Frontiers. While many may not care what the mechanism is, knowing it would help to understand the limits to this method, and how it might best be employed.

Habib Sekha (the owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef tanks. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef tanks will exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many find that calcium and alkalinity levels are stable over long periods with just that scenario. The only way that can be true is if such additions typically dump excess calcium and alkalinity into the tank that is subsequently removed by precipitation of calcium carbonate (such as on heaters).

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 true, this mechanism may be attained with other high pH additive systems (like some of the two-part additives such as the original B-ionic) if enough is added. However, it will not be as readily attained with low pH systems, such as calcium carbonate/carbon dioxide reactors because the low pH inhibits the precipitation of excess calcium and alkalinity."


and


What Your Grandmother Never Told You About Lime
http://reefkeeping.com/issues/2005-01/rhf/index.htm

from it:


http://www.reefkeeping.com/issues/2005-01/rhf/index.php#15


What Else Does Limewater Do In An Aquarium? Reduce Phosphate

Many reefkeepers accept the concept that adding limewater reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and has published them in the old Aquarium Frontiers. While many aquarists may not care what the mechanism is, knowing it would help to understand the limits of this method, and how it might best be employed.

Habib Sekha (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 will exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way that can be true is if the excess calcium and alkalinity that such additions typically dump into the aquarium are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock, etc.).

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. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding at lower and higher pH values. 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 having cut off all external phosphate sources. In such extreme cases, removal of the substrate may even be required.

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

An alternative mechanism for phosphate reduction via limewater may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef aquaria will be supersaturated with this material, as the equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. The supersaturation of calcium phosphate will be even higher in the high pH/high calcium fluid present where limewater enters reef aquaria. The locally high pH converts much of the HPO4-- to PO4---, and it is the concentration of PO4--- that ultimately determines supersaturation. That high supersaturation may tip the balance to precipitation of calcium phosphate, just as too much limewater all at once can tip the balance to precipitation of calcium carbonate. As with CaCO3, the precipitation of Ca3(PO4)2 in seawater may be limited more by kinetic factors than by equilibrium factors, so it is impossible to say how much might precipitate under reef tank conditions (without, of course, somehow determining it experimentally).

As with the precipitation of CaCO3 containing some phosphate, if these calcium phosphate crystals are in the water column (e.g., if they form at the local area where limewater hits the tank water), then they may become coated with organics and be skimmed out of the aquarium.
 

rrome

New member
Ah! I see.

Yes, I took the pH 8.54 sample and added 1.0 m NaOH to it. I can see some ppt where the NaOh hits the liquid, so I think you are correct..

I took the whole sample to pH 9+ (9.6) and I will see... I need time to read your article :)

I personally would not mess with this approach on a live tank, seeing that there are other, safer methods.
 
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