View Full Version : Kalk and Phosphate Precipitation
sanababit
05/03/2008, 11:00 AM
I have read about people using kalk to keep up with cal and alk, but they also state that it precipitates phosphate, which in turn keeps it really low and not have that green film on glass every day, is this true???, does it really help keep the glass clean on our fish boxes??, please enlighten me people
I ask cause i use phosban or phospure and still get that green film every day, some diatom blooms and cyano every so often and i blame it on phosphate, nitrate is ~2ppm
sana
Boomer
05/03/2008, 11:30 AM
but they also state that it precipitates phosphate
This is more or less a claim with nothing to back it up, which has continued over the last 20 years. Think of it has a urban myth ;) Who ever said it keeps the glass clean is talking 200 % pure nonsense. Thousands of people use kalk and still have stuff on the glass and algae problems and you are proof of it :D
Aquarist007
05/03/2008, 11:43 AM
<a href=showthread.php?s=&postid=12462515#post12462515 target=_blank>Originally posted</a> by sanababit
I have read about people using kalk to keep up with cal and alk, but they also state that it precipitates phosphate, which in turn keeps it really low and not have that green film on glass every day, is this true???, does it really help keep the glass clean on our fish boxes??, please enlighten me people
I ask cause i use phosban or phospure and still get that green film every day, some diatom blooms and cyano every so often and i blame it on phosphate, nitrate is ~2ppm
sana
phosban can only work if you have the source of importing phosphates in check
how is your flow in your tank--its should be between 20-40 times your tank vol in gph
once a week take a turkey baster and lightly baste the substrate and reef--this will get phosphates and nitrates back into the water column where they can be filtered off. Combined with excellent flow rates this probably help your occasion algae and cyano blooms
Billybeau1
05/03/2008, 12:06 PM
Here is an excerpt from one of Randy's fine articles about the subject.
"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."
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