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

Abstract

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 (PO, 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, PO, –COOH) and surface-texture to optimize U(VI) binding energetics.