Bonechar-hydroxyapatite composite for hydroxychloroquine adsorption: A statistical optimization and mechanistic insights

Hydroxychloroquine (HDC) is a persistent emerging contaminant whose detection in aquatic systems has increased significantly following its widespread use. This study investigates the adsorption of HDC onto pulverized bovine bone char (CP), an industrial by-product, compared to a composite modified with MnFe₂O₄ and graphene oxide (CP-MG). Response surface methodology (RSM) determined the optimal operating conditions as 0.5 g L −1 adsorbent dosage, 10 mg L −1 initial HDC concentration, and natural pH. Contrary to expectations, CP demonstrated significantly superior adsorption capacity (q max = 52 mg g −1 ) compared to the modified composite (CP-MG5, q max = 36 mg g −1 ) at 318 K. To explain this counterintuitive result, an in-depth mechanistic investigation was conducted. Adsorption on both materials was rapid (equilibrium in 120 min) and followed the pseudo-second-order kinetic and Sips isotherm models. Thermodynamic studies and spectroscopic characterization revealed that the process is governed predominantly by hydrogen bonding and π–π interactions rather than electrostatic attraction. The findings suggest that the functionalization process, while adding specific surface area, inadvertently caused physical blockage and chemical masking of the intrinsic hydroxyapatite active sites essential for HDC uptake. Furthermore, CP proved highly effective in complex aqueous matrices (removal >90%) and maintained stability over five regeneration cycles. This study provides crucial insights, demonstrating that the unmodified industrial by-product is a more effective, economical, and sustainable solution for HDC removal than complex modified composites.