MIL-101(Fe) Metal–Organic Framework Nanoparticles Functionalized with Amino Groups for Cr(VI) Capture

Capture of heavy metal ions from aqueous solutions is a critical but challenging step in environmental treatment. In this work, NH2-MIL-101(Fe) nanoparticles were synthesized by simple solvothermal methods and applied to capture Cr(VI) anions from aqueous solutions. The results revealed that NH2-MIL-101(Fe) exhibited a high capture capacity (375.2 mg g–1) and an ultrafast capture rate (4 min reaching equilibrium). The competitive effects of coexisting ions such as Cl–, HCO3–, PO43–, Cd2+, Ca2+, and Cu2+ on Cr(VI) are very weak, suggesting that the adsorption selectivity significantly enhanced by loading amino groups on MIL-101(Fe). Density functional theory (DFT) calculation was performed to better understand the adsorption mechanism of Cr(VI), showing that the amino groups could increase the adsorption energy of Cr(VI) for enhancing the adsorption capacity. Based on powder X-ray diffraction, X-ray photoelectron spectroscopy, and DFT calculations, the Cr(VI) capture mechanism by NH2-MIL-101(Fe) nanoparticles was a combination of coordination between Fe and Cr(VI), electrostatic interaction between Cr2O72– and the protonated amino group, and reduction of Cr(VI) to Cr(III).