Valorization of oyster shell waste into hydroxyapatite for removal of Cr(VI) and As(III) from water

The valorization of oyster shell waste into functional materials provides a sustainable pathway for water treatment within a circular economy framework. In this study, hydroxyapatite (HAp) was synthesized from oyster shell waste and applied for the removal of Cr(VI) and As(III) from aqueous solutions. The synthesized HAp exhibited a predominantly mesoporous structure (88.94%), a specific surface area of 6.94 m 2 /g, and aggregated particles of 2–3 μm, with Ca–P phases and functional groups (CO 3 2- , OH - , PO 4 3- ) contributing to adsorption. Batch experiments indicated that removal efficiency was strongly influenced by pH, adsorbent dosage, contact time, and initial concentration. Under optimal conditions (90 min, 0.05 g, 10 mg/L), the maximum adsorption capacities reached 139.37 mg/g for Cr(VI) (92% removal) and 17.18 mg/g for As(III) (43.8% removal), demonstrating clear selectivity toward Cr(VI). Adsorption behavior followed the Freundlich isotherm and Elovich kinetic model, indicating heterogeneous and multi-step interactions. The higher adsorption capacity observed for Cr(VI) in the single-solute systems investigated in this study may be attributed to the favorable interaction of chromate species with reactive sites on the HAp surface, whereas As(III), which predominantly exists as neutral H 3 AsO 3 under near-neutral conditions, exhibited lower adsorption efficiency. These observations highlight the influence of contaminant speciation on adsorption performance. Overall, oyster shell-derived HAp represents a low-cost and effective adsorbent for the removal of Cr(VI) and As(III) from aqueous solutions.