Influence of synthesis method of nano-hydroxyapatite-based materials on cadmium sorption processes

Hydroxyapatite is a member of apatite mineral family, with a high stability and flexibility of the apatitic structure, which allows the substitution of Ca2+ from its structure with other metals. This makes it an ideal material for the disposal of long-term contaminants because of its high sorption capacity for heavy metals. The synthesis parameters variation to obtain materials with specific physical-chemical properties in function of the application field is a necessary step in process optimization. The goal of this paper was to prepare hydroxyapatite-based materials with increased sorption capacity for cadmium retaining from aqueous solutions. The materials were characterized with X-ray diffractometer, transmission electron microscopy and the average particle size was also determined. The influence of synthesis method (co-precipitation and sol-gel), silica/silicon doping, granulometry, initial cadmium concentration and temperature was studied. pH and calcium ion concentration were monitored during sorption and compared to values obtained during dissolution. The kinetic data were fitted to pseudo-first order, pseudo-second order and intraparticle diffusion models. The sorption process follows pseudo-second-order kinetics with a contribution of intraparticle diffusion. The sol-gel prepared materials follow a different reaction mechanism than those prepared by co-precipitation method.