Valorization of vanadium industry waste into calcium-rich hydroxyapatite for ultrahigh-capacity lead remediation: A multimechanistic approach to lead sequestration

Lead contamination poses a severe threat to ecosystems and human health; however, conventional remediation technologies are limited by inefficiency, high costs, and secondary pollution risks. Herein, present a sustainable strategy to transform vanadium industry waste—neutralization slag (Ns) into calcium-rich hydroxyapatite (Ns-HAP) for ultrahigh-capacity lead sequestration. Leveraging the abundant calcium sulfate matrix in Ns, Ns-HAP was synthesized via phosphate modification and hydrothermal crystallization, achieving a Ca/P ratio of 1.90 to enhance ion-exchange sites and reduce phosphorus consumption. Multi-technique characterization (XRD, SEM-EDS, XPS, Raman) revealed that Pb²⁺ immobilization by Ns-HAP operates via synergistic mechanisms: rapid surface complexation with hydroxyl/phosphate groups, ion exchange (Ca²⁺ and Pb²⁺), and covalent Pb–O–P bonding via lattice substitution, culminating in the formation of stable hydroxypyromorphite (Pb₁₀(PO₄)₆(OH)₂). The adsorbent exhibited an exceptional maximum Langmuir capacity of 2165.12 mg·g⁻¹, surpassing most reported hydroxyapatite-based materials, while maintaining a relatively high Pb²⁺ selectivity in multi-ion systems (Cd²⁺, Ni²⁺). Thermodynamic analyses confirmed spontaneous (ΔG° = −9.88 kJ·mol⁻¹) and endothermic (ΔH° = 50.92 kJ·mol⁻¹) chemisorption, with kinetics governed by reaction-controlled lattice diffusion. Crucially, Ns-HAP demonstrated robust performance in high-concentration wastewater (6000 mg·L⁻¹ Pb²⁺), achieving >99 % removal efficiency at an optimal dosage of 4 g·L⁻¹. This work establishes a closed-loop paradigm for valorizing industrial waste into high-value adsorbents while elucidating mechanistic insights into Ns-HAP for advancing heavy metal remediation technologies aligned with circular economy principles.