The physico-chemical conditions for the precipitation of phosphate with calcium

The most important factors controlling the chemical speciation of calcium and inorganic phosphate in fresh waters and waste waters are discussed. Solubility diagrams are used to illustrate the range of compositions that are predicted to be stable with respect to the formation of key calcium phosphate minerals and calcite. These include calcium hydroxyapatite, octacalcium phosphate, tricalcium phosphate and dicalcium phosphate dihydrate. The coprecipitation of phosphate with calcite is also considered. The solution speciation, calculated from information on the formation of calcium phosphate ion-pairs ata given temperature, enables ion activities to be determined and the ion-activity product for the appropriate mineral to be compared with the thermodynamic solubility product. Examples of such calculations are presented for different types of water and some conclusions are drawn concerning future research developments.