Mechanistic study of potentially toxic element removal from water by waste putty powder-derived hydroxyapatite

Waste putty powder-derived hydroxyapatite (P-HAP) was synthesized via a chemical precipitation method using waste putty powder as the calcium source. The adsorption performance and mechanisms of P-HAP for the removal of potentially toxic elements, including Cd(ii), Pb(ii), and Cr(vi), from aqueous solutions were investigated. The underlying adsorption mechanisms were elucidated using XRD, SEM, EDS, and FT-IR analyses. The results showed that: ① under a P-HAP dosage of 1.6 g L^-1 and a temperature of 25 °C, the adsorption capacity of P-HAP for Pb(ii) reached 62.50 mg g^-1, with a removal efficiency of 100%, at an initial Pb(ii) concentration of 100 mg L^-1 and pH 4. Under an initial Cd(ii) concentration of 100 mg L^-1 and pH 10, the adsorption capacity for Cd(ii) was 40.63 mg g^-1, with a removal efficiency of 65.00%. For Cr(vi), at an initial concentration of 10 mg L^-1 and pH 4, the adsorption capacity was 1.43 mg g^-1, corresponding to a removal efficiency of 22.94%. ② The adsorption behavior of the three potentially toxic elements (PTEs) was better described by the Langmuir isotherm and pseudo-second-order kinetic models. The adsorption of Pb(ii) and Cd(ii) onto P-HAP primarily occurred through dissolution-precipitation, surface complexation, and ion exchange, whereas the adsorption of Cr(vi) was dominated by surface complexation. In addition, electrostatic attraction has a synergistic effect on the adsorption process.