Preparation and characterization of functionalized hybrid hydroxyapatite from phosphorite and its potential application to Pb²⁺ remediation

Calcium hydroxyapatite–aspartate (CaHAp–Asp) and calcium hydroxyapatite–glutamate (CaHAp–Glu) hybrid materials were prepared by the dissolution of Tunisian Phosphate Rocks in concentrated nitric acid and re-precipitation in basic solution in the presence of aspartic and glutamic acids . Chemical analyses, X-ray powder diffraction, infrared spectroscopy, ¹³C MAS-NMR spectroscopy, thermogravimetry and scanning electron microscopy were used to characterize the materials and also the organic–inorganic interfaces. All techniques converge to the formation of hybrid organic–inorganic compounds composed of hydroxyapatite solids with organic anions incorporated at the surface. XRPD and SEM observations showed that not only the crystallinity significantly was affected by the presence of organic anions, but also the structural and morphological properties. The presence of organic anions has been confirmed by IR and ¹³C MS-NMR spectroscopies, and their amount has been evaluated by TG and chemical analyses. The three CaHAp, CaHAp–Asp and CaHAp–Glu materials were evaluated for Pb²⁺ sorption from aqueous solutions. Sorption isotherms have been analyzed using Langmuir and Freundlich models showing that the Pb²⁺ sorption capacity is increased for CaHAp–Asp and CaHAp–Glu. The comparison of the two models indicates a single active site corresponding to a homogeneous interface between Pb²⁺ aqueous adsorbate and the solid adsorbent surface.