Cadmium Uptake by Hydroxyapatite Synthesized in Different Conditions and Submitted to Thermal Treatment

This paper intends to evaluate the uptake of cadmium ions from aqueous solution by 21 hydroxyapatite samples which have been synthesized in different conditions. It has been determined that the variation on the hydroxyapatite sorption capacity is neither related to sample solubility nor to hydroxyapatite Ca/P molar ratio. Cd2+ sorption is controlled by sample BET surface area, which shows a direct dependence on the hydroxyapatite crystallite dimensions. The hydroxyapatite pore distribution presented modes at 1000 and 60 000 Å, corresponding to intracrystallite voids and voids between the agglomerate of these crystallites, respectively. Pores belonging to the former mode immobilize the major part of Cd2+. The influence of sample thermal treatment on Cd2+ sorption efficiency has been studied using hydroxyapatite samples calcined at temperatures ranging from 500 to 1140 °C. Similarly to nonthermally treat samples, the Cd2+ sorption on calcined hydroxyapatite could be described by Langmuir isotherms. The results showed that the maximum sorption capacity decreased from 0.631 mmol g-1 for the noncalcined sample to 0.150 mmol g-1 for the one calcined at 900 °C. This drop in the sorption capacity could also be explained by a reduction in its specific surface area, which is induced by the increase of the crystal size.