Phosphonate-driven inner-sphere chemisorption of samarium(III) by a tris-phosphorylated Schiff-Mannich adsorbent: Selective recovery from spent SmCo magnet leachate and quantified circular Sm ₂ O ₃ production

The sustainable recovery of rare earth elements (REEs) from secondary resources is essential as demand rises and primary ores face economic and environmental constraints. Here, a tris-phosphorylated Schiff-base ligand (PTREN) is rationally constructed as a molecular adsorbent for highly selective uptake of Sm3+ from spent SmCo magnet leachates. The three phosphonic acid groups create a pre-organized, multidentate O-donor pocket tailored for inner-sphere Sm3+ coordination. PTREN was thoroughly characterized by FTIR, NMR, XPS, TGA, BET, and MALDI-TOF, confirming its discrete structure and stable phosphonate functionality. Batch experiments show fast kinetics (equilibrium in 45 min) and a high Langmuir capacity of 469.4 mg g−1 at pH 5.0. Isotherm, kinetic, and thermodynamic analyses collectively indicate monolayer chemisorption, a spontaneous and endothermic process, and increased interfacial disorder upon binding. Sm3+ is efficiently desorbed with 0.5 M HNO3 and converted to high-purity Sm2O3 nanoparticles, closing the material loop. PTREN maintains over 85% of its initial capacity after nine adsorption – desorption cycles, evidencing excellent structural robustness and regenerability. This work introduces a tunable phosphonate-based molecular platform that couples selective Sm recovery from complex waste streams with direct generation of value-added oxide products, advancing circular REE recycling. A full list of abbreviations and acronyms used throughout this manuscript is provided in Table S-1 of the Supporting Information.