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Re: Aquatic Plants Digest V3 #204


Mark Fisher asked:

>      Does anyone know the solubility of hydroxyapatite, and how
>      pH-sensitive is this reaction?  If it only occurs at high pH, then it
>      isn't really feasible for aquarium use.  Would the addition of Ca be
>      as effective as a phosphate-absorbing resin?

Under favorable conditions, hydroxylapatite is essentially insoluble.  I
turned around a calculation (perhaps a little too quickly?) and found that
at pH 8 with 4 degrees of general hardness (all as calcium)
hydroxylapatite solubility would allow something like 0.6 part per
trillion (not a typo) of HPO4 in solution.  With 8 degrees of general
hardess this drops to about 0.2 parts per trillion.

If you want to check my numbers, for the reaction:

Ca5(PO4)3(OH) + 4 H+ = H2O + 3 HPO4-2 + 5 Ca+2
Log(K) = -3.421

Below a pH of 7, H2PO4 rather than HPO4 is the primary phosphate species.

All that aside, I wasn't aware that hydroxylapatite played a role in water
treatment.  Someone might set me straight on that.  Under natural
conditions apatite (usually fluorapatite rather than hydroxylapatite)
forms mostly from replacement of carbonates, not from direct
precipitation.  I suspect there may be kinetic problems with the direct
precipitation of apatite.

Were I to pick a calcium phosphate salt that might precipitate directly
from solution I'd probably look to something simpler, like CaHPO4.2H20
which is actually fairly soluble (316 mg/l in cool water according to my
CRC handbook).  Otherwise I'd look for other mechanisms, like
coprecipitation or adsorption and sedimentation for the decrease in
phosphorus concentrations during water treatment.

Roger Miller