Fishbone-Derived Hydroxyapatite from Distinct Species: Effect of Calcination and pH on Heavy Metal Adsorption from Water

In this study, hydroxyapatite (HAp) was obtained from fishbones of four species: gilt-head bream (Sparus aurata), salmon (Salmo salar), hake (Merluccius merluccius), and megrim (Lepidorhombus boscii). Batch adsorption experiments were performed with Cr3+, Ni2+, and Zn2+ ions under different pH conditions (natural, 3, and 11) and contact times (6 and 72 h), which is innovative in this study and allows a unified comparison across species and thermal treatment (non-calcined vs. calcined). Results indicated that non-calcinated materials were particularly effective for Ni2+ and Zn2+ removal at natural and acidic pH, whereas calcinated samples were more suitable for Cr3+ adsorption under alkaline conditions. Given the precipitation of its insoluble hydroxide under alkaline conditions, zinc removal was limited to natural and acidic pH. Among the tested precursors, megrim and hake-derived (non-calcined) HAp exhibited the highest performance, achieving up to 99.99% removal efficiency at 6 h of contact time and 20 °C. The analysis of the used adsorbents confirmed metal incorporation into the HAp lattice with minimal crystallographic disruption. These findings demonstrate the potential of fishbone-derived HAp as an efficient and low-cost adsorbent for heavy metal removal from aqueous systems, while simultaneously contributing to the valorization of fishery waste.