Sorption kinetics and diffusion of cadmium in calcium hydroxyapatites

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