Effects of pH and Ca/P molar ratio on the quantity and crystalline structure of calcium phosphates obtained from aqueous solutions

Milk and dairy products contain large amounts of calcium phosphate salts that can precipitate. The chemical composition and the crystalline structure of the calcium phosphate precipitates that are formed in dairy industry depend on the physico-chemical conditions, particularly, pH and mineral composition. The objective of this study was to determine, using mineral solutions, the effects of pH and of the concentrations of calcium and phosphate on the quantity and crystalline structure of calcium phosphate precipitates. Experiments were carried out at 20 C with 20.00 mmol.L-1 phosphate and three Ca/P molar ratios (1.00, 1.50 and 2.00). The initial pH (5.50, 6.70, 7.50, 8.50 and 9.50) were drifting or kept constant during a reaction time of 3 h. After filtration of the suspensions, the mineral compositions of filtrates were quantified. The lyophilized precipitates were characterized using X-ray diffraction, infrared spectroscopy and scanning electron microscopy techniques. At drifting pH (final pH values were between 4.6 and 6.0), the mineral analyses showed that the Ca/P ratio did not influence the amounts of precipitated calcium and phosphate. The analyses of precipitates revealed the formation of brushite as the main crystalline phase. At constant pH, the mineral analyses showed that the Ca/P ratio strongly influenced the precipitation efficiency of calcium phosphate. The analyses of precipitates revealed the formation of poorly crystallized calcium-deficient apatites. A decrease of crystallinity with an increase in initial pH was observed. In conclusion, pH can be a key factor to control the quantity and crystalline structure of calcium phosphates obtained by precipitation. This factor should be considered for the recovery of calcium phosphates from dairy co-products. pH is also important in the fouling phenomena of membranes and heat exchangers caused by calcium phosphate precipitation.