Valorization of waste oyster shells via thermal and acid activation for Congo red dye adsorption from aqueous media

A scalable route to valorize waste oyster shells into an effective adsorbent for Congo red removal is reported. Sequential thermal calcination (500 °C) and H₃PO₄ activation convert the CaCO₃ matrix into Ca-phosphate-rich surfaces (XRD, FTIR) bearing abundant -OH/PO₄ groups. Despite a moderate BET area (MOS: 22.15 m² g^-1), the modified oyster shell achieves rapid uptake (>80-90% removal within 10 min; near-complete by 60 min), broad pH tolerance with optimal performance below pH_pzc ≈ 8.06, and high capacity (q _max = 50.89 mg g^-1). Kinetics follow a pseudo-second-order model (R ² = 0.994; k ₂ = 0.0127 g mg^-1 min^-1) and equilibrium data fit both Freundlich (R ² = 0.997; K _F = 37.19; n = 2.97) and Langmuir (K _L = 4.21 L mg^-1) models, indicating chemisorptive affinity on an energetically heterogeneous surface. MOS is durable and regenerable (∼95% removal after five cycles). Density functional theory calculations corroborate strong dye-phosphate site interactions. The combined thermal-acid treatment thus yields a low-cost, reusable adsorbent suitable for practical dye-laden wastewater treatment.