Tailoring hydroxyapatite morphology via the effect of divalent cations on the hydrolysis of α-TCP: Oriented crystal growth towards the application in water treatment

Abstract

The hydrothermal synthesis method to produce calcium hydroxyapatite (HA) of controlled morphology without the use of organic additives is developed in this study. The influence of smaller (Mg2+, Mn2+) and larger (Sr2+, Ba2+) ions on the hydrolysis of α-TCP and crystal growth of HA particles was investigated. It was revealed that both the hydrothermal synthesis conditions (temperature, time) and the nature and concentration of the foreign ion affected the phase purity and morphology of the final products. Of all the ions investigated, Mn2+ had the highest, and Sr2+ had the lowest inhibiting effect on the hydrolysis process of α-TCP. Under certain synthesis conditions, we were able to obtain samples consisting of plate-like or rod-like particles only. The dye adsorption test revealed that the adsorption of plate-like particles for cationic dyes was more efficient, possibly, due to the higher surface area (SBET). Upon normalizing the test results to SBET, it became apparent that the electrostatic attraction between HA particles and dye molecules also impacted the adsorption efficiency. Rod-like particles had a higher ratio of exposed (a,m)-planes and hence lower zeta potential, resulting in increased adsorption of the cationic dye.