Hydrothermal synthesis of hydroxyapatite nanocrystals from calcium-rich limestone sludge waste: Preparation, characterization, and application for Pb2+ adsorption in aqueous solution

This paper reports on the recycling of the Ca2+ ion-rich filtrate of limestone sludge for use as a raw material in the hydrothermal synthesis of hydroxyapatite (HAp) applicable to the removal of Pb2+ ions from aqueous solutions. Samples of HAp (eco-HAp) were prepared using a range of Ca/P molar ratios (Ca/P MRs) (1, 1.67, 2.33 and 3) and hydrothermal temperatures (393-453 K). Fourier transform infrared spectroscopy revealed the presence of O-H groups and PO43-(nu 1) groups (indicative of HAp). Transmission electron microscopy revealed the presence of elliptical surface microstructures. Solid-state nuclear magnetic resonance (NMR) spectroscopy revealed single crystal sites of inorganic phosphorus (31P) and hydroxyls (1H). The adsorption capacity of ecoHAp samples reached 476.19 mg/g within 20 min. The results of adsorption kinetics were characteristic of pseudo-second-order kinetics. Langmuir adsorption isotherms revealed that the Pb2+ adsorption capacity of ecoHAp was 277.77 mg/g, which can be attributed to a synergistic mechanism involving ion exchange, chemical precipitation, surface complexation, and dissolution/precipitation. Taken together, these results demonstrate the potential of eco-HAp as an adsorbent material with broad industrial applicability for the removal of heavy metals from aqueous solutions.