Selective recovery of rare earth elements from mine wastewater by Feco bimetallic metal-organic framework

The increasing demand for rare earth elements (REEs) and their potential environmental hazards have made the selective recovery of REEs from acidic mine wastewater a critical task. In this study, a bimetallic FeCo metal-organic framework (MOF) was synthesized via a facile room-temperature stirring method, and its adsorption performance toward 13 REEs and competing metal ions (CM ions) including K, Ca, Mg, Mn, and Al was systematically evaluated in acid mine drainage (pH 5.5). Batch adsorption experiments at 303 K demonstrated that 1 g·L^-1 FeCo MOF achieved REEs adsorption efficiencies ranging from 5.3 ± 0.2% to 99.9 ± 0.1% within 5 h, whereas potassium exhibited a low adsorption efficiency of only (3.7 ± 0.2%). The total adsorption capacity of FeCo MOF for 13 rare earth elements in real acid mine drainage reached 27.07 mg·g^-1, with Langmuir monolayer capacities of 11.69, 8.36, and 5.53 mg·g^-1 for Y, Nd, and La, respectively. The distribution coefficients (K_d) of REEs were substantially higher than those of CM ions, with K_d(Tb) reaching 5.0 × 10⁴ mL·g^-1 (conservative lower-bound estimate) compared to K_d(K) below 40 mL·g^-1. Moreover,the used adsorbent could be efficiently regenerated by 0.1 M sodium citrate solution, maintaining more than 48.0% of its original adsorption capacity after five consecutive adsorption-desorption cycles. Multiple characterization techniques, including scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, Brunauer-Emmett-Teller (BET), zeta potential, and X-ray photoelectron spectroscopy (XPS), were employed to reveal the structural and surface chemical properties of the as-prepared FeCo MOF adsorbent. These characterizations confirmed that REEs were mainly bound to -COO^- and μ-OH groups in the robust MOF structure, without obvious redox reactions or framework collapse. Therefore, the bimetallic FeCo MOF exhibits promising selectivity and stability in authentic mine wastewater, showing potential as a low-energy-consumption adsorbent for selective recovery of critical REEs from complex mine effluents, particularly for Tb enrichment.