In-depth study of the mechanism of heavy metal trapping on the surface of hydroxyapatite

In the present study, the efficiency and selectivity and the role of surface groups of hydroxyapatite in the Cr(III) trapping, present in solution as single species or in co-presence of Ni(II) and Pb(II), have been investigated. Transmission electron microscopy (TEM) analysis and synchrotron radiation X-ray powder diffraction (XRPD) measurements provided comprehensive descriptions of the HAP surface and structure. The surface chemical properties, density and strength of the acid and base sites of HAP, have been determined by 2-phenylethylamine and benzoic acid probe adsorption, respectively. The amphoteric properties of HAP turned over to only acid property in water solution. The sorption capacity of HAP has been batch-tested in simulated polluted waters containing Cr(III) at different initial concentrations (from ca. 0.3 to ca.6 mM) and pH values (from 4 to 9). Competitive adsorption phenomena in multi-metal systems (Cr(III) with Ni(II) and Pb(II)) have also been studied. An appropriate set of analytical techniques, XRPD, TEM, STEM-EDX, N-2 adsorption-desorption analyses, FT-IR spectroscopy, have been used to characterize the metal loaded hydroxyapatite samples. Different mechanisms of metal uptake were active depending on the nature of the metal species and pH value of solutions: surface complexation, precipitation, ion-exchange, or dissolution-precipitation.