Characterization of Cu2+ adsorption for eco-hydroxyapatite derived from limestone sludge via hydrothermal synthesis.

This study developed a novel approach to the hydrothermal synthesis of eco-hydroxyapatite (eco-HPA) from recycled limestone sludge for the removal of Cu²⁺ from aqueous solutions. The resulting eco-HPA was characterized using X-ray fluorescence, thermal field emission scanning electron microscope, X-ray powder diffraction analysis, and Brunauer–Emmett–Teller surface area measurements. Under a hydrothermal temperature of 120 °C and Ca/P molar ratio of 1, this study obtained the nitrogen adsorption isotherm and desorption curves indicative of hydroxyapatite. Based on the IUPAC classification system, these are the type-IV adsorption isotherm curves of a mesoporous structure with an H3-type hysteresis loop. The same sample achieved an adsorption capacity of 210 mg/g. The adsorption kinetics closely fit the pseudo-second-order kinetic model (R² = 0.9990–1.000). Adsorption isotherms presented the strongest correlation with the Langmuir model (R² = 0.97–0.99), exceeding that of the Freundlich model (R² = 0.91–0.98). Taken together, these results demonstrate that the proposed eco-HAP is a viable low-cost environmentally friendly adsorbent with broad applicability for the removal of heavy metals from aqueous solutions. [ABSTRACT FROM AUTHOR]