Sorption kinetics and diffusion of cadmium in calcium hydroxyapatites

The kinetics of Cd sorption from aqueous solution on a synthetic calcium hydroxyapatite was investigated at 18, 28 and 75 degrees C in batch experiments, using also X-ray diffraction, electron microscopy and nuclear microprobe analysis. Cadmium is incorporated into the hydroxyapatite structure via diffusion and substitution for calcium ions, but exchange equilibrium is not achieved even after 12 days of contact at 75 degrees C and sorption is only partly reversible. Values of diffusion coefficients were estimated from X-ray line broadening and sorption kinetics. The fact that Cd is incorporated into the bulk of the apatite with partial reversibility is important in the context of the safe storage of used sorbent material after decontamination of water polluted by cadmium.