The Calcium Phosphate−Calcium Carbonate System: Growth of Octacalcium Phosphate on Calcium Carbonates

The kinetics of nucleation and crystal growth of octacalcium phosphate [Ca8(PO4)6H2·5H2O, OCP] from calcium phosphate supersaturated solutions inoculated with seed crystals was investigated at 37 °C, pH 7.40, at conditions of constant solution supersaturation. Stable calcium phosphate solutions, supersaturated with respect to OCP and hydroxyapatite [Ca5(PO4)3OH, HAP] were inoculated with calcium carbonate-based bone cement powder consisting of mixed aragonite and calcite crystals and with well-characterized calcite seed crystals. On all substrates tested, OCP nucleated followed by crystal growth of the nuclei formed past the lapse of induction times, inversely proportional to the solution supersaturation. From the dependence of the induction time on the solution supersaturation with respect to OCP, a value of 10 mJ m−2 was calculated for the nucleating phase. The rates of OCP crystal growth on the carbonate substrates showed linear dependence on the solution supersaturation that in combination with the independence from the fluid dynamics in the reactor suggested a surface diffusion-controlled mechanism. Moreover, the independence of the crystallization rates on the amount of the inoculating seed crystals suggested that nucleation and growth took place exclusively on the crystalline substrates. The transient calcium phosphate phase, OCP, was stabilized in our experiments, and it was the only phase growing at constant driving force, despite the fact the solutions were supersaturated with respect to HAP as well

Crystal growth of aragonite in the presence of phosphate

The crystal growth of aragonite was investigated at pH 7.8, 37 °C and constant solution supersaturation from aragonite-seeded supersaturated solutions. The effect of the presence of orthophosphate ions in the supersaturated solution on the kinetics of crystallization of aragonite was investigated over the range of orthophosphate concentrations of 0.25 μM–1 mM. In the presence of orthophosphate in the range of 0.25 μM–8 μM, the crystal growth rate of aragonite decreased with increasing phosphate concentration. At orthophosphate concentration levels exceeding 2 μM, induction times were measured and were found to increase with orthophosphate concentration. At orthophosphate concentration levels > 8 μM, the crystal growth of aragonite was inhibited, suggesting the blockage of the active growth sites by the adsorption of orthophosphate ions. Adsorption was confirmed by the investigation of orthophosphate uptake on aragonite, which was: i) found to depend on the equilibrium concentration of orthophosphate in aqueous solutions saturated with respect to aragonite; ii) not influenced by the ionic strength of the electrolyte up to 0.15 M NaCl, showing that electrostatic interactions between orthophosphate and CaCO3 did not play a significant role in this concentration range. Adsorption data of orthophosphate on the aragonite crystals gave satisfactory fit to the Langmuir adsorption model and was confirmed by XPS analysis

Determination of Soluble Calcium Variations in Saline Precipitation in an Experimental Scale Solar Distiller

El objetivo de la investigación fue estudiar la precipitación salina a temperatura constante de 75 °C en un destilador solar experimental. Se utilizó agua subterránea con alto contenido de calcio, bicarbonatos y sulfatos solubles como afluente. Una de las variables más importantes en la operatividad y productividad de agua potable con esta tecnología es la precipitación salina en la base del destilador. Esta ocurre debido a las características fisicoquímicas del agua subterránea típica de las zonas desérticas del norte del estado de Chihuahua. Las variaciones en la concentración de calcio soluble del afluente se midieron cada hora con un espectrofotómetro de absorción atómica y se calculó la cantidad total precipitada de calcio. Con la técnica de difracción de rayos X y microscopía electrónica de barrido se analizó el precipitado. Se determinó que la reacción de la precipitación se estabilizó a la tercera hora. La concentración de calcio aumentó durante la cuarta y la quinta hora, lo que indicó que el calcio soluble remanente no reaccionó. Se concluye que la reacción de precipitación salina no es isotérmica, sino que está impulsada por gradientes de temperatura. En el análisis de la identificación de las fases cristalinas del precipitado en las incrustaciones se encontró CaCO3 en forma de calcita y CaSO4 como anhidrita y basanita.The objective of the research was to study salt precipitation at a constant temperature of 75 °C in an experimental solar still. Groundwater with high content of calcium, bicarbonates and soluble sulfates was used as an input water. One of the most important variables in the operability and productivity of drinking water with this technology is the saline precipitation at the base of the distiller. This occurs due to the physicochemical characteristics of the groundwater typical of the desert areas of the north of the state of Chihuahua. Variations in the soluble calcium concentration of the input water were measured every hour with an atomic absorption spectrophotometer and the precipitated amount was calculated. The precipitate was analyzed using the X-ray diffraction and scanning electron microscopy technique. It was determined that the precipitation reaction occurred between the first and second hours. The calcium concentration increased during the fourth and fifth hours, indicating that the remaining soluble calcium did not react. It concludes that the salt precipitation reaction is not isothermal but is driven by temperature gradients. The analysis of the identification of the crystalline phases of the precipitate in the scales found CaCO3 in the form of calcite and CaSO4 as anhydrite and basanite

Calcium Carbonate Crystallization on a Microalgal Matrix: The Effects of Heavy Metal Presence

Microalgae and other microorganisms often play a significant role in the transportation of heavy metal ions in the environment, while at the same time they are closely related to the formation of minerals in aquatic systems, especially with the formation of calcium carbonate (CaCO3). The biomineralization of calcite was investigated in aqueous solutions, supersaturated with respect to calcium carbonate (7.94 calcite Acutodesmus obliquus (A. obliquus). The presence of metals tested in the supersaturated solutions, at concentration levels below the threshold of precipitation of the respective hydroxides, reduced the rate of calcium carbonate precipitation by 40 to 90% depending on the solution supersaturation. The presence of A. obliquus culture increased the rates of calcium carbonate precipitation by 80%. The presence of the test metals inhibited the growth of A. obliquus, especially the presence of Cd. The uptake of the test metals on calcite fitted Langmuir adsorption isotherm. Cadmium uptake onto calcite reached 85% of the total amount in the solutions. Charged ion pairs of test metals play an important role in their activity with respect to calcium carbonate precipitation and algal growth