Mechanism of uranium(vi) sorption on α-aminophosphonate sorbents: multimodal spectroscopy and computational study

Carbon-free nuclear energy meets growing energy demand; uranium recycling enhances sustainability, economic, and environmental benefits. Herein, efficient three α-aminophosphonates-based sorbents were previously synthesized via a one-pot method using distinct amine precursors (aniline, O-phenylenediamine, anthranilic acid), yielding S-H, S-NH₂ aminated, and S-COOH carboxylated, respectively enhanced aminophosphonate. Elemental analysis confirms three α-aminophosphonate sorbents (S-H, S-COOH, S-NH₂) with amine-dependent structures. Optimal U(vi) sorption was observed at pH 4.0, 25 ± 1 °C, and 90 min contact time, with Langmuir-derived capacities (q _m) of 1.312, 0.762, and 0.601 mmol U per g for S-H, S-NH₂, and S-COOH, respectively. Multimodal characterization combining FTIR, XPS, and SEM-EDX with Density Functional Theory (DFT) simulations elucidated structure-property relationships and binding mechanisms via integrated experimental/computational analysis. FTIR analysis of uranyl-loaded sorbents (S-H-U, S-NH₂-U, S-COOH-U) revealed inner-sphere U(vi) complexation via nitrogen (>NH/-NH₂) and oxygen (P[double bond, length as m-dash]O, P-O-Ph) ligands, modulated to probe coordination environments and redox behavior. XPS revealed ligand-dependent redox selectivity: S-H-U retained 46.30% U(vi), whereas S-NH₂-U and S-COOH-U preferentially stabilized U(iv) (61.44-86.69%), underscoring tunable uranium speciation. Enamine-imine tautomerism at bridging >NH sites dictated U(vi) coordination geometry. SEM-EDX analysis correlated enhanced U(iv) sorption with nanoscale/hierarchical surface roughness, while post-sorption morphological changes confirmed active-site saturation and morphology-governed sorption. DFT simulations validated experimental spectra, revealing U(vi) coordination geometries and energetics, where deprotonation states and functional group chemistry governed binding thermodynamics and stability. This study pioneers molecular-level design criteria for α-aminophosphonate sorbents through structure-property relationships connecting tailored functional group engineering (e.g., >NH, P[double bond, length as m-dash]O, -COOH) and surface-texture to optimize U(vi) binding energetics.