Fe-modified hydroxyapatite with regulated surface properties for enhanced Mn²⁺ adsorption: Structural evolution and...

Hydroxyapatite (HAp) has garnered considerable attention as an adsorbent for heavy metal removal owing to its favorable surface reactivity and ion-binding capability. Nevertheless, its adsorption performance is strongly governed by surface chemical environments and structural characteristics. In this study, pristine hydroxyapatite (P-HAp), Fe-modified hydroxyapatite (Fe-HAp), and Ti-modified hydroxyapatite (Ti-HAp) were synthesized and systematically evaluated for Mn²⁺ removal from aqueous solutions. Batch adsorption experiments revealed that metal modification altered the adsorption behavior of the HAp-based materials. The pH-dependent adsorption results showed that Fe-HAp exhibited the highest Mn²⁺ uptake among the three materials under optimal conditions (pH 5), with adsorption capacities of 13.25, 16.44, and 11.55 mg g^-1 for P-HAp, Fe-HAp, and Ti-HAp, respectively. For P-HAp, the Langmuir model provided a better fit. For Fe-HAp and Ti-HAp, the Freundlich model gave higher fitting accuracy at most temperatures, implying that heterogeneous adsorption regions and sites with diverse binding affinities collectively promoted Mn²⁺ uptake following metal modification. Kinetic analysis revealed that the pseudo-second-order model generally offered a superior fit for all three materials, indicating that surface chemical interactions were central to the adsorption process. Structural and surface characterization confirmed that Fe modification regulated the pore structure and surface chemical environment while preserving the characteristic hydroxyapatite framework. BET analysis demonstrated improved pore characteristics of Fe-HAp, and XPS analysis suggested the involvement of oxygen-containing and phosphate-related surface species during Mn²⁺ adsorption. Furthermore, DFT calculations indicated that Fe incorporation altered the electronic structure and frontier orbital features of HAp, which may facilitate coordination interactions between Mn²⁺ and surface oxygen-containing species. Overall, this study demonstrates that metal modification constitutes an effective strategy for tailoring the adsorption properties of HAp-based materials. Fe incorporation improves the adsorption behavior of HAp by modifying structural features and surface chemical environments, thereby boosting Mn²⁺ removal through synergistic interactions dominated by surface coordination.