Pb(II) Adsorption on Amine-Grafted Fe₃O₄@MCM-41: Fast Uptake and O‑Anchored Binding Revealed by DFT

Fe₃O₄@MCM-41-NH₂ core-shell particles synthesized via coprecipitation and CTAB-templated silica coating with postgrafted APTES demonstrate ordered mesoporosity and magnetic separability. Characterization by XRD, FT-IR, TEM, N₂ sorption, and VSM confirms BET surface area of 175 m²·g^-1 and strong ferrimagnetism. Under batch conditions (pH 6.0, 25 °C, dose = 0.90 g·L^-1), Pb-(II) removal reached 94.8% in 60 min (q _e = 21.1 mg·g^-1), with kinetics following a pseudo-second-order model (k ₂ = 0.21 mg^-1·g·min^-1). Equilibrium data were described by Langmuir and Freundlich models; residual-focused metrics indicate mild site heterogeneity. Cluster-DFT calculations (r²SCAN-3c/B3LYP-D3/def2) reveal dominant O-anchored inner-sphere Pb-O complexes on the aminated silica surface in aqueous medium, thermodynamically favored over Pb-N coordination, with amine groups providing secondary environmental-tuning functionality. Multicycle reusability demonstrates robust performance. Overall, Fe₃O₄@MCM-41-NH₂ provides fast Pb-(II) uptake, mechanistic clarity via DFT, and practical magnetic recovery with sustained multicycle performance, positioning it as a promising regenerable adsorbent for sustainable wastewater remediation.