Download HAp synthesis – processes on Ca(OH)2 grain surface. (a) (b) (c)

EuAsC2S-12/S1-OP26
HAp synthesis – processes on Ca(OH)2 grain surface.
I. Kreicbergs, I. Dreyer, L. Berzina-Cimdina, S. Bulina
Institute of General Chemical Engineering, Riga Technical University, Azenes Str. 14/24, Riga, Latvia,
e-mail: [email protected]
Calcium hydroxyapatite (HAp) is similar to mineral components of bone and teeth. There is a
growing demand for high, repeatable quality HAp which is used as a bone tissue replacement
material.
Process scale up with resulting large scale synthesis without side products encouraged thinking
about reaction mechanism. The aim of this study is to research processes on the surface of
calcium hydroxide particles, because synthesis is carried out in heterogenic environment.
HAp was synthesized by precipitation method. Simplified reaction stoichiometric equation:
Ca(OH)2 [solid] + H3PO4[liquid] → Ca5(PO4)3OH[solid] + 9H2O
Calcium hydroxide suspension was heated to 45 oC and then phosphoric acid was slowly added,
endpoint of adding was determined by suspension pH.
Processes during synthesis were investigated using freeze drying method. The mixture was
sampled at different time intervals, quenched with liquid nitrogen, and freeze-dried. The dried
samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy
(FT-IR) and scanning electron microscopy (SEM). FT-IR and XRD analysis show that calcium
hydroxide slowly disappeared from reaction mixture but in meanwhile HAp signal intensity
grew. SEM microphotography shows changes of particle morphology during synthesis.
(a)
2µm
(b)
2µm
(c)
2µm
SEM microphotographs of the powders at various time intervals - (a) 0 min, pure Ca(OH)2, (b)
15 min and (c) 30 min.
Diffusion on the Ca(OH)2 surface is main driving force of mass transfer in the reaction. In our
case diffusion is important to provide reaction between phosphates and calcium ions. This
reaction gives space in solution for next calcium ions from calcium hydroxide; it means place for
future reactions. Fast addition of phosphoric acid leaded to formation of practically insoluble
apatitic coverage on calcium hydroxide and reaction rate was slowed down. If synthesis is
continued with phosphoric acid adding in high flow rate, despite to HAp solubility increase in
acidic environment, Ca(OH)2 surface remains insoluble. As result, added acid doesn’t react,
mixture pH becomes acidic and H3PO4 addition is stopped. This leads to appearance of unwanted
impurities in the product – tricalcium phosphate and calcium hydroxide.
EuAsC2S-12/S1-OP26
Acknowledgment: This work was supported by a grant from the national research program of the Republic of
Latvia V7709.