Synergistic enhancement of hydroxyapatite-phosphoric acid-activated metakaolin-based geopolymers for sustainable and efficient removal of toxic metal ions in wastewater treatment: An experimental study supported by density functional theory insights

Toxic metal contamination poses a critical threat to aquatic environments, demanding the development of efficient and cost-effective remediation strategies. Hydroxyapatite (HAP) is a promising sorbent due to its tunable surface chemistry and low production cost; however, its powdered form is impractical for large-scale wastewater treatment. In this work, we report the first synthesis of HAP-containing composites based on acid-activated metakaolin geopolymers (PAGP). The influence of HAP incorporation and varying phosphoric acid (PA) concentrations (10 %, 15 %, and 20 %) on the structural, textural, thermal, and mechanical properties of the composites was investigated. The optimized material, HAP-15PAGP, displayed superior performance compared to pristine PAGP, including an increased specific surface area (66 m²/g), adequate compressive strength (1.96 MPa), and enhanced adsorption capacity. A clear structure performance correlation was established, linking the material properties of the composites to their capacity for the simultaneous removal of Pb(II), Cu(II), Ni(II), and Cd(II). Regeneration studies demonstrated stable cation removal efficiency and reusability over six consecutive adsorption–desorption cycles, underscoring the material’s practical potential for sustainable wastewater treatment. Complementary density functional theory (DFT) calculations and non-covalent interaction (NCI) analyses revealed a synergistic effect between HAP and PAGP, attributed to electron donor–acceptor complementarity and an enhanced dipole moment in PAGP. These features collectively improved the binding affinity of the composite toward metal ions. Batch adsorption experiments confirmed the high efficiency and rapid kinetics of Pb(II), Ni(II), Cu(II), and Cd(II) removal from synthetic wastewater, highlighting the potential of HAP-15PAGP as a robust and sustainable adsorbent for water treatment applications.