Selenite capture by MIL-101 (Fe) through FeOSe bonds at free coordination Fe sites

Selective immobilization of SeO₃^2- is highly desired for the remediation of Se-contaminated water. Thus, the irreversible sorption of SeO₃^2- ions by adsorbents through unique coordination bonds with high affinity is needed. Herein, we demonstrated that Fe-based metal-organic framework (MOF) (Fe-MIL-101) with free coordination sites (FCSs) enabled selective and irreversible capture of SeO₃^2- ions from aqueous solution with fast kinetics and a high uptake capacity of 183.7 mg∙g^-1, owing to large MOF apertures and substantial numbers of FCSs as capture sites through forming Fe-O-Se bonds. Meanwhile, Fe-MIL-101 maintained excellent performance in a broad pH range (4-11) and high selectivity for SeO₃^2- ions in the presence of excessive competitive anions (e.g., CO₃^2-, PO₄^3-). Density functional theory (DFT) calculation, extended X-ray absorption fine structure (EXAFS), and Mössbauer fittings confirmed that the capture on Fe-MIL-101 was through the Fe-O-Se coordination bonds between FCSs and SeO₃^2-. Moreover, Fe-MIL-101 could effectively remove SeO₃^2- in simulated natural water and sewage by overcoming the influence of co-existing ions and organic matters. This study highlights new opportunities for the design of MOF-based materials for removing toxic and radioactive anions with irreversibility and high selectivity from natural and waste water.