A phase diagram for Ca- and Fe-bound phosphate has been calculated based on two criteria:
the adsorption of ortho-phosphate onto iron hydroxide
the solubility product of apatite as measured in two hard water rivers.
The phase diagram explains why ortho-phosphate (o-PO4) in sediments, containing high concentrations of CaCO3 (about 10–30%), is bound onto the iron hydroxide rather than onto the CaCO3. The concentration of the iron hydroxide has an effect on the equilibrium between the two phosphates, whilst the concentration of CaCO3 has no effect whatsoever.
calcium bound phosphate iron bound phosphate phase diagram