Abstract
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–(ν1) 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 eco-HAp 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.
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