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THE APPLICATION OF CALCIUM PHOSPHATE PRECIPITATION CHEMISTRY TO PHOSPHORUS RECOVERY: THE INFLUENCE OF ORGANIC LIGANDS

Jacqueline A.M. van der Houwen* (jacv@nhm.ac.uk) and Eugenia Valsami-Jones (evj@nhm.ac.uk)

Department of Mineralogy, The Natural History Museum, Cromwell Road, SW7 5BD, London, UK

Abstract

Current knowledge of calcium phosphate precipitation chemistry is brought together and compared with the needs for understanding application in processes for recovering phosphates from wastewaters. The effects of the high concentrations of organic species present in such waters appears as a key factor requiring further research and laboratory experiments in this area are described. The supersaturation required for precipitation of calcium phosphate at 25ºC, pH 7, 0.1 M ionic strength and near-stoichiometric (for hydroxylapatite) calcium to phosphate molar ratio was determined under homogeneous precipitation conditions. The experiments were carried out in air. The phase precipitated at the critical concentration was allowed to grow using a constant composition method. The influence of organic ligands on the precipitation was investigated using two small molecular weight organic ligands, acetate and citrate, present at a concentration of 10-3 M. The precipitate was studied using X-ray diffraction. Good reproducibility of the experiments, which were carried out in triplicate, was observed.

The study assessed the supersaturation degree necessary for precipitation of hydroxylapatite to be 10.28, assuming a solubility constant of log K=-57.13. The required supersaturation was not affected by the presence of acetate. However, citrate was found to increase the degree of supersaturation to 11.07.  It is proposed that this is due to binding of citrate on the active sites of newly formed nuclei, thereby inhibiting precipitation. All experiments showed formation of a poorly crystalline hydroxylapatite.

Keywords: citrate, homogeneous precipitation, hydroxylapatite, supersaturation

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