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Carbonate hydroxyapatite (CHAP) was synthesized from domestic hen egg shells. The obtained CHAP was characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy and investigated as metal adsorption for Pb 2+ from aqueous solutions. The effect of various parameters on the adsorption process such as contact time, solution pH, and temperature was studied to optimize the conditions for maximum adsorption. The results showed that the removal efficiency of Pb 2+ by carbonate hydroxyapatite calcined at 600°C (CHAPF) reached 99.78 %, with an initial Pb 2+ concentration of 200 mg · L -1 , pH ) 3, and a solid/liquid ratio of 1 g · L -1 . The equilibrium removal process of lead ions by CHAPF foam at pH ) 3 was well described by the Langmuir isotherm model, with a maximum adsorption capacity of 500 mg · g -1 at (25 and 35)°C. The removal mechanism of Pb 2+ by the CHAPF varies, depending on the initial concentration of lead in the aqueous solution: the dissolution of CHAPF and precipitation of hydropyromorphite (Pb 10 (PO 4 ) 6 (OH) 2 ) is dominant at low concentration [(20 to 200) mg · L -1 ], and the adsorption mechanism of Pb 2+ on the CHAPF surface and ion exchange reaction between Ca 2+ of hydroxyapatite and Pb 2+ in aqueous solution is dominant at high concentration [(500 to 700) mg · L -1 ]. The thermodynamics of the immobilization process indicates an exothermic sorption process of Pb 2+ .

International audienceIn order to reinforce the clinical applications of hydroxyapatite (HAP) sol–gel coatings deposited onto 316 L stainless steel, we suggest the introduction of an intermediate thin layer of titania (TiO2) on the substrate. The titania sub-layer is introduced in order to improve both the corrosion resistance and the mechanical properties of the HAP/316 L stainless steel coated system. The two coatings, HAP and TiO2, were studied separately and afterwards, compared with the bi-layered coating. A film without any cracks is obtained under the optimum conditions in terms of annealing temperature, dipping rate and aging effect. Microstructural, morphological and profilometry analysis revealed the non-stoichiometric carbonated porous nature of the hydroxyapatite coatings, which were obtained after annealing at 500 °C during 60 min in the atmosphere. The obtained TiO2 coatings exhibit a dense and uniform surface. Addition of TiO2 as sub-layer of the HAP coating tends to increase the homogeneity and the crystallinity rate as compared to the HAP one.The mechanical properties, i.e. hardness and elastic modulus, are determined by means of nanoindentation experiments and the adhesion between the coating and substrate is estimated by scratch tests. The corrosion behavior is evaluated by potentiodynamic cyclic voltammetry tests. As a main result, the values of the elastic modulus and hardness, respectively of 30 GPa and 2.5 GPa, are relatively high for the HAP–TiO2 bilayer coating. This result allows the use of such coated material as a replacement material for hard tissues. The adhesion strength increased from 2925 mN up to 6430 mN after the addition of the TiO2 intermediate film. According to the Tafel's analysis, the 316 L stainless steel specimens coated with both HAP and titania layers (ECorr = − 234 mV, lCorr = 0.089 μA cm− 2) present a better resistance than the HAP-coated specimens (ECorr = − 460 mV, lCorr = 0.860 μA cm− 2)

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Titania-Hydroxyapatite (TiO/HAP) reinforced coatings are proposed to enhance the bioactivity and corrosion resistance of 316L stainless steel (316L SS). Herein, spin- and dip-coating sol-gel processes were investigated to construct two kinds of coatings: TiO/HAP composite and TiO/HAP bilayer. Physicochemical characterization highlighted the bioactivity response of the TiO/HAP composite once incubated in physiological conditions for 7days whereas the TiO/HAP bilayer showed instability and dissolution. Biological analysis revealed a failure in human stem cells adhesion on TiO/HAP bilayer whereas on TiO/HAP composite the presence of polygonal shaped cells, possessing good behaviour attested a good biocompatibility of the composite coating. Finally, TiO/HAP composite with hardness up to 0.6GPa and elastic modulus up to 18GPa, showed an increased corrosion resistance of 316L SS. In conclusion, the user-friendly sol-gel processes led to bioactive TiO/HAP composite buildup suitable for biomedical applications.