Robust EDTA-Functionalized MIL-101(Cr)-NH₂ for Affinity-Enhanced Rare-Earth Ion Capture: Mechanistic Evaluation and Density Functional Theory Insights

Sustainable extraction of rare-earth elements (REEs) from wastewater streams presents a critical technological challenge in resource recovery. This work developed a chelating metal-organic framework composite through ethylenediaminetetraacetic acid (EDTA) functionalization of MIL-101(Cr)-NH₂, demonstrating superior rare-earth ion affinity across light to heavy lanthanides (La³⁺, Ce³⁺, Gd³⁺, Dy³⁺). Under optimized aqueous conditions (pH ≈ 5.0, 298 K), maximum Gd³⁺ uptakes reached 89.18 mg g^-1. Structural and morphological analyses by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and Brunauer-Emmett-Teller confirmed that EDTA modification did not compromise the crystalline integrity; instead, it introduced carboxyl and amino groups that are essential for binding. The composite exhibited competitive adsorption performance, surpassing many reported adsorbents in capacity, while showing encouraging selectivity trends within single-ion tests. Equilibrium and kinetic analyses revealed surface-localized chemisorption mechanisms through coordination bonding, as evidenced by compliance with the Langmuir isotherm and pseudo-second-order models. Thermodynamic profiling confirmed spontaneous adsorption driven by favorable entropy changes, overcoming endothermic energy barriers through an enhanced system disorder. Furthermore, MIL-101(Cr)-NH-EDTA preserved its structural integrity over multiple regeneration cycles, underlining its practical reusability. Mechanistic insights, supported by density functional theory calculations, revealed significant electron transfer from EDTA carboxyl groups to the REEs, corroborating the strong coordinate bonding. Although selectivity was evaluated in single-ion systems, these results highlight EDTA-grafted MIL-101(Cr) as a promising platform for future multi-ion and real-water studies on sustainable REE capture.