Engineering vanadium extraction residue into Mn-functionalized hydroxyapatite precursor for enhanced antibiotic removal: Molecular-level insights into a waste-to-resource strategy

The discharge of pharmaceutical wastewater containing antibiotics and the accumulation of metallurgical solid waste represent two pressing environmental challenges. This study presents a novel “waste-to-resource” solution by molecularly engineering the neutralization slag from vanadium extraction into a Mn-functionalized hydroxyapatite precursor (Mn-HAP-VNR) for enhanced antibiotic removal. Systematic optimization of hydrothermal synthesis identified a condition (pH 7, 0.7 M (NH 4 ) 2 HPO 4 , Ca/Mn=2:1), yielding Mn-HAP-VNR with exceptional Langmuir maximum adsorption capacities of 322.58 mg·g -1 for norfloxacin (NOR) and 214.79 mg·g -1 for tetracycline (TC), significantly outperforming the undoped material and many reported adsorbents. Comprehensive characterization confirmed the successful isomorphous substitution of Ca 2+ by Mn 2+ within the HAP lattice. The introduced Mn 2+ sites were identified as key active sites; based on experimental measurements, Mn doping enhanced the adsorption capacity by over 213% for TC and 628% for NOR compared to undoped HAP. Adsorption isotherms and kinetics were best described by the Langmuir and pseudo-second-order models, respectively, indicating monolayer chemisorption as the dominant mechanism. In-depth spectroscopic analyses (FTIR and Raman) deciphered the distinct chemisorption pathways: NOR primarily coordinates via a bidentate complex between its deprotonated carboxylate group and surface Mn 2+ /Ca 2+ sites, whereas TC engages through synergistic interactions involving its phenolic hydroxyl and ketone carbonyl groups. Thermodynamic studies corroborated the endothermic and spontaneous nature of the process. This work not only demonstrates the superior performance of a waste-derived adsorbent but also elucidates the molecular-level engineering mechanism, establishing a viable closed-loop strategy for the valorization of industrial residue into a high-performance, tailor-made material for targeted water decontamination.