The precipitation of calcium phosphates has been studied extensively in the past 30 years with the main focus on biological processes as, for example, bone and tooth mineralisation. In the last two decades further interest in calcium phosphate precipitation has arisen from its potential as a mechanism of phosphorus removal from wastewater (1,2), although few studies have considered the underpinning chemistry. There is currently a particular need for research on calcium phosphate precipitation in organic rich systems that simulate wastewater environments. A small number of studies exist, which have assessed the effect of organic ligands on the precipitation of dicalcium phosphate dihydrate (DCPD) (3,4), octacalcium phosphate (OCP) (5) and the inhibition of hydroxylapatite (HAP) precipitation (6). All these studies were carried out as seeded growth experiments (heterogeneous nucleation) and all reported inhibition of precipitation kinetics by the presence of the organic ligands. Adsorption of the ligands on active growth sites of the seeding material was given as the cause for this inhibition. The influence of citrate on calcium phosphate precipitation onto OCP seeding material has been studied (7). This study concluded that phospho-citrate complexes formed on active growth sites inhibited precipitation kinetics.
The study of calcium phosphates is further challenged by uncertainty over the first phase to precipitate. Previous research suggested the formation of an amorphous calcium phosphate preceding the formation (or transformation) of a more crystalline calcium phosphate (8). Which specific crystalline calcium phosphate forms will depend mostly on pH and kinetics (9). The phase predicted to be stable is DCPD (CaHPO4*2H2O) at acidic pH around 5, OCP (Ca4H(PO4)3*2.5H2O) at pH around 6 and HAP (Ca5(PO4)3OH) at pH of 7 and above (10).
Previous studies into homogeneous nucleation proceeded by mixing of calcium and phosphate solutions at a desired supersaturation degree and leaving the nuclei enough time to grow into crystals whilst keeping the pH constant (11). A disadvantage of this method is that the solution composition will change continuously (i.e. calcium and phosphate concentrations fall), as soon as precipitation starts. This, in turn, may result in variation in the nature (mineralogy and/or crystallinity) of the precipitate. Homogeneous nucleation has been criticised for giving non-reproducible results due to chance nucleation onto foreign particles in the solution (12). The aim of the present study was to elucidate the chemical principles of calcium phosphate precipitation at neutral pH, particularly in the presence of organic ligands. The critical supersaturation degree for homogeneous precipitation of calcium phosphate at pH of 7 was determined. The small molecular weight organic ligands