Unseeded precipitation of calcium and magnesium phosphates from modified seawater solutions

Homogeneous (unseeded) nucleation and precipitation kinetics of Ca and Mg p hosphates from modified seawater solutions with variable pH, Mg2+, HCO3-, F -, and organic acid concentrations were investigated at 20 degrees C. The i nduction period of nucleation and the composition of precipitated solid pha ses were determined as a function of saturation state (6-2000 with respect to octacalcium phosphate (OCP)), Mg2+/Ca2+ activity ratio (0-10) and soluti on pH (7.2-9.1). The addition of Mg2+, HCO3-, F-, polycarboxylic, aromatic, and amino acids along with variations in pH has a weak effect on the nucle ation kinetics of Ca-Mg phosphates in seawater solutions with a 35% salinit y. Mg-bearing solutions produce an amorphous precipitate with a Ca:Mg:P mol ar ratio varying from 6.5: 1 : 5 to 2.6: 1:2.5 depending on (a(Mg2+)/a(Ca2)) ratio. The precipitation of this amorphous phase in Mg-bearing solutions corresponds to the formation of a metastable hydrous Ca-Mg phosphate ((Ca, Mg)(4)H(PO4)(3).xH(2)O) as a precursor of apatite which normally precipitat es in Mg-free solutions. Spontaneous inorganic precipitation of Ca phosphat e in most natural aquatic systems is kinetically impossible. (C) 1999 Elsev ier Science B.V. All rights reserved.