Engineering linker-defects of MIL-101 series metal organic frameworks for boosted Yb(III) adsorption

Developing the highly efficient adsorbents for improved recovery of rare earth elements from industrial wastewater is significant. Therefore, two defective MIL-101 series, named as MIL-101(Cr)-D and MIL-101(Fe)-D, were fabricated by introducing benzoic acid as the second ligand to coordinate with metal clusters. The controlled samples were characterized by XRD, ATR-IR, TGA, EPR, and BET tests, verifying the presence of oxygen-vacancy sites and the integrity of crystalline structure. In addition, the adsorption kinetics, the adsorption isotherms, the influences of pH and different REE ions on adsorption performance, and the cycling stability of defective and normal MIL-101s were carefully discussed and compared. The results showed that the adsorption capacities of MIL-101(Fe)-D10 and MIL-101(Cr)-D20 were 27.88 and 37.45 mg·g−1 respectively for Yb(III) capture at 30 °C and pH = 6, were much higher than those of the corresponding normal MIL-101. Furthermore, the adsorption mechanisms of two defective MIL-101s have been analyzed by EDS, mapping and XPS, confirming the surface complexation interactions between Yb(III) and O-vacancy sites originated from Cr-O and Fe-O clusters. Meanwhile, the excellent reusability of MIL-101(Cr)-D10 was validated by the successive twelve adsorption recycles and XRD analysis. Hence, this work supplies the feasible and facile strategy for manufacturing the missing-linker defective MOFs, endowing these adsorbents with the enhanced adsorption performance towards Yb(III) from aqueous solution.