Adsorption of some metal ions from wastewater using a novel composite of recycled chitosan and hydroxyapatite

In this study, an environmentally sustainable nanocomposite of recycled chitosan (CT) from shrimp waste and hydroxyapatite (HAp) was successfully synthesized and characterized. The adsorption performance of the prepared (HAp@CT) composite and its pristine components toward (Mn²⁺), (Fe³⁺), and (Pb²⁺) ions from aqueous solutions were systematically investigated. The structural and physicochemical properties were analyzed using Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), and Thermogravimetric Analysis (TGA) to confirm the functional group interactions, crystalline structure, and thermal stability. Transmission Electron Microscopy (TEM) revealed nanoscale CT particles (~ 5.31 nm) and highly crystalline HAp with well-defined lattice fringes. The measured interplanar spacings (1.603 and 1.714 nm) correspond to characteristic planes of the hexagonal HAp crystal system, confirming its ordered atomic arrangement. The batch adsorption was studied by optimizing the adsorption parameters. The maximum adsorption efficiency, were 160.20, 172.70, and 229.02 mg/g for Mn²⁺ using HAp, CT, and HAp@CT, respectively, for Pb²⁺ were 216.23, 201.28, and 179.33 mg/g, while for Fe³⁺ were 190.02, 200.31, and 206.92 mg/g, respectively, these values were obtained at 12.0 mg/L of initial concentration, 0.05 g/L of adsorbent dose, 120 min of contact time, and pH = 5.0–6.0, respectively. Overall, the results demonstrate that the HAp@CT nanocomposite exhibits competitive adsorption performance, particularly for Mn²⁺ and Fe³⁺ ions, while maintaining enhanced thermal stability inherited from the HAp component. These findings highlight the potential of sustainable biopolymer–inorganic hybrid nanocomposites for environmentally friendly water treatment applications and provide a foundation for further optimization and scale-up studies.