Precipitation of calcite in the presence of inorganic phosphate

The precipitation of calcite from Ca(HCO3)2 solutions was studied over a wide range of dissolved inorganic phosphorus concentrations (0–500 μmol dm−3 KH2PO4) and a dissolved calcium concentration of 2.5 mmol dm−3 at 10 °C. At low phosphorus concentrations (<20 μmol dm−3), calcite growth was retarded and phosphate ions were coprecipitated according to theory and gave a phosphate surface density of coprecipitated phosphate of 0.150 μmol m−2. As the phosphorus concentration was increased further, calcite growth ceased and a separate calcium phosphate phase was formed. In these experiments, CO32− ion concentrations were <190 μmol dm−3. The calcite seed played no part in the nucleation of the calcium phosphate phase as the growth rates were found to be independent of the amount of calcite seed material used in the reactions. The boundaries for the homogeneous nucleation of calcium phosphate were investigated by simple solution mixing of Ca(HCO3)2 and KH2PO4 over a wide range of supersaturations and quantified in terms of HAP, tricalcium phosphate (TCP) and octacalcium phosphate (OCP) saturation indices.