Cadmium fixation by synthetic hydroxyapatite in aqueous solution--thermal behaviour.

This study deals with the mechanism of the cadmium uptake by synthetic hydroxyapatite (HA: Ca10(PO4)6(OH)2) in aqueous solution. The rate of cadmium fixation by hydroxyapatite was investigated at 10 and 50 degrees C using batch experiments. Inductively coupled plasma atomic emission spectrometry, X-ray diffraction, FT-IR spectroscopy and electron microscopy were used to characterize the starting HA and the samples. The thermal behaviour of the powders was determined with the help of three thermoanalytical techniques (TGA, DTA, and MS) and temperature programmed X-ray diffraction. Cadmium immobilization kinetics can be divided into two steps: substitution of Ca2+ ions by Cd2+ in the HA lattice at the particle's surface, followed by their incorporation into the hydroxyapatite bulk. This results in the formation of an apatite solid solution, which is very important because in this way decontamination and storage can be performed with the same material.