Green valorization of waste bone into a nano-hydroxyapatite adsorbent for Pb(ii) removal: synthesis, physicochemical...

Lead (Pb(ii)) exposure poses significant health risks to various organ systems, including the hematological, renal, neurological, and cardiovascular systems. In this study, nano-hydroxyapatite (nano-HAp) was prepared via an ultrasonication-assisted hydrothermal method to remove Pb(ii) from an aqueous solution. The nano-HAp adsorbent was characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area, and point of zero charge (pH_pzc) analyses. The effects of various factors, including pH (2-8), initial Pb(ii) concentration (40-200 mg L^-1), adsorbent dose (0.01-0.07 g L^-1), and contact time (30-130 min), were investigated, with the residual Pb(ii) concentrations measured via atomic absorption spectroscopy (AAS). The highest removal efficiency of 99.84% was achieved at an optimum pH of 5, an initial Pb(ii) concentration of 40 mg L^-1, an adsorbent dose of 0.03 g L^-1, and a contact time of 110 min. Based on the coefficients of determination (R ²), the pseudo-second-order kinetic (R ² = 0.9912) and Langmuir isotherm (R ² = 0.9993) models best described the adsorption process. The maximum monolayer adsorption capacity was determined to be 133.33 mg g^-1. Thermodynamic analysis confirmed that the process was spontaneous and endothermic, as indicated by positive ΔH and negative ΔG values. Furthermore, reusability studies demonestrated that the nano-HAp adsorbent maintained a removal efficiency above 86% after five successive cycles. Therefore, the prepared nano-HAp adsorbent represents a promising, eco-friendly adsorbent for Pb(ii) removal.