Precipitation of calcium phosphate from simulated milk ultrafiltrate solutions

The present work deals with the influence of pH and temperature on the spontaneous precipitation of calcium phosphate from simulated milk ultrafiltrate (SMUF) solutions. The pH range investigated is 5.7-7.0, and the temperature varied from 55 to 75 degrees C. It was found that the precipitates were prisms of hydroxyapatite (HAP) with low crystallinity preceded by amorphous calcium phosphate (ACP). Crystallinity improved with solution aging. Microscopic particles in the range of 200 nm were obtained at relatively high supersaturation (pH = 6.8), whereas at relatively low supersaturation (pH = 6.0), aggregates in the range of 1 mu m were formed. Moreover, it was found that although the SMUF solution is also supersaturated with respect to magnesium phosphate, no magnesium salt was identified in the precipitates, thus precluding the coprecipitation of magnesium phosphate. At the same temperature, the supersaturation was regulated by adjusting the solution pH. Kinetics study of precipitation showed a parabolic dependence of the (initial) rates on the solution supersaturation, suggesting a surface diffusion-controlled mechanism with activation energy equal to 96 kJ mol(-1). The rates of precipitation were significantly reduced in the presence of citrate ions due to the decrease of the solution supersaturation caused by complexation of citrates with Ca2+ ions. Additionally, a decreasing effect of citrates on the precipitation rate resulted from the adsorption of these species on the formed crystals, blocking some active sites of crystal growth and diminishing the constant of precipitation rate.