Chemisorption of Sm( III ) on a trisphosphorylated Schiff‐base adsorbent: Statistically verified kinetic, isotherm, and thermodynamic route to closed‐loop Sm 2 O 3 recovery from magnet leachates

Abstract A novel trisphosphorylated Schiff‐base adsorbent (PTREN) was developed and statistically validated for targeted samarium(III) recovery from leachates of spent SmCo permanent magnets. Structural and physicochemical characterization via Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), matrix‐assisted laser desorption/ionization–time‐of‐flight–mass spectrometry (MALDI‐TOF‐MS), thermogravimetric and derivative thermogravimetric analysis (TGA/DTG), x‐ray diffraction (XRD), scanning electron microscopy (SEM), and N 2 ‐sorption confirmed successful phosphonate functionalization (Brunauer‐Emmett‐Teller BET), thermal stability up to approximately 430°C, and a hierarchical microporous–mesoporous architecture with a BET surface area of 68.47 m 2 g −1 . Batch adsorption studies demonstrated pronounced pH sensitivity, with maximum Sm(III) removal at pH 5.0—proximate to the point of zero charge—and rapid equilibrium within 45 min using 0.05 g adsorbent, yielding a maximum uptake capacity of 456.4 mg g −1 . Kinetic modelling confirmed pseudo‐second‐order behaviour ( R 2 = 0.9938), supported by Elovich and intraparticle diffusion analyses, indicating chemisorption dominance with multi‐stage mass‐transfer contributions. Equilibrium data conformed to the Langmuir isotherm ( R 2 = 0.9948; q max = 454.55 mg g −1 ; R L = 0.011–0.060), while Dubinin–Radushkevich analysis yielded a mean adsorption energy of 28.87 kJ mol −1 , corroborating inner‐sphere SmOP coordination. Thermodynamic parameters indicated a spontaneous (negative Δ G °), endothermic (Δ H ° = +9.097 kJ mol −1 ), and entropy‐driven (Δ S ° = +40.152 J mol −1 K −1 ) process. Response‐surface methodology and ANOVA yielded a robust quadratic predictive model ( R 2 = 0.9874; CV% = 2.85%), identifying initial Sm(III) concentration and adsorbent dosage as dominant variables, with a non‐significant lack‐of‐fit. Stripping with 0.5 mol L −1 HNO 3 enabled crystalline Sm 2 O 3 recovery, while PTREN retained above 85% capacity over nine regeneration cycles, confirming its viability for closed‐loop, industrially scalable samarium recovery.