SORPTION OF ZN-2+ AND CD-2+ ON HYDROXYAPATITE SURFACE

This study examines the mechanisms and kinetics of Zn2+ and Cd2+ sorption onto hydroxyaPatite surfaces. The concentrations of Zn2+ and Cd2+ in the study range from 0 to 2.5 mmol/L. Although the sorption data follow Langmuir isotherms, a detailed examination reveals that surface complexation and coprecipitation are the most important mechanisms with possibly ion exchange and solid diffusion also contributing to the overall sorption process. pH-controlled experiments point to significant deprotonation of hydroxyapatite surface and sorption by metal complexation with surface functional groups such as =POH. The major metal surface species are likely to be =POZn+ and =POCd+. The coprecipitation of Zn2+ and Cd2+ with Ca to hydroxyapatite is implied by significant changes in Ca and phosphate concentrations during the metal sorption processes. Cd2+ coprecipitation appears to be more significant than Zn2+ coprecipitation. Desorption data for the metals suggest that Zn2+ is held more strongly than Cd2+ on the mineral surfaces. The observation is mainly interpreted as a result of redissolution of the coprecipitates because of more Cd coprecipitate than Zn coprecipitate. Solid diffusion of the metals into the mineral interior may also contribute to the desorption results because Zn diffusion is promoted by a much smaller size and ionic radius of Zn2+.