Adsorption performance of hydroxyapatite with different crystalline and porous structure towards metal ions in multicomponent solution

Hydroxyapatite (HA) is a well-known and widely used efficient adsorbent for toxic metal ion and radionuclide removal from wastewater. However, the relationship between the structural parameters of HA and its adsorption characteristics (capacity, selectivity, kinetic parameters) requires in-depth study. In this study, HA with different crystalline and porous structures was obtained using crystallisation inhibitors of different natures, and their adsorption properties in a multicomponent solution (Cd2+, Co2+, Cu2+, Fe3+, Ni2+, Pb2+ and Zn2+) was investigated. Using low doses of inhibitors (Mg2+ ions and hydroxyethylenediphosphonic acid) had a positive effect on HA adsorption characteristics, due to their low degree of crystallinity (amorphous calcium phosphate) and a well-developed porous structure (ABET up to 200-240m(2) g(-1)). The adsorption capacity of the HA samples in relation to the Cd2+, Cu2+, Fe3+, Pb2+, Zn2+ ions and their simultaneous presence in the solution varied from 0.13 to 1.28 mmol g(-1). The achieved residual concentration of Pb2+, Zn2+ and Cu2+ ions was below the maximum concentration limit, and the ions of Fe3+ were almost equal to zero. The highest adsorption of Co2+ and Ni2+ ions was observed for HA obtained in the presence of Mg2+. HA prepared with HEPD has been shown to have the highest efficiency towards Cd2+, Cu2+, Fe3+, Pb2+, Zn2+ ions. The adsorption kinetic data fitted well with the pseudo-second-order model. It was shown that, depending on the composition and structural characteristics of the prepared HA, ion-exchange and dissolution-precipitation were the two main mechanisms in their interaction with metal ions.