Removal of organic pollutants by lanthanide-doped MIL-53 (Fe) metal–organic frameworks: Effect of dopant type in magnetite precursor

A series of undoped and lanthanide doped MIL-53 (Fe)/Ln-Fe3O4 (Ln = La, Nd, or Gd) metal-organic frameworks (MOFs) were prepared by the solvothermal method. All prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) measurements, scanning electron microscopy (SEM), and thermal analysis. XRD and FT-IR results ascertain the successful MOF formation for all prepared samples. MIL-53 (Fe)/La–Fe3O4 has the smallest particle size of 8.6 nm, the largest BET surface area of 54.2 m2/g, and the highest porosity. Undoped and different lanthanide doped MIL-53 (Fe) were employed as sorbents for the removal of methylene blue (MB) dye from aqueous solutions to examine the doping benefit and the effect of the dopant size on the sorption performance. Doping causes MOFs to act as pH-independent sorbents, which make it applicable at any condition. Adsorption follows pseudo-second-order kinetic model, and doped sorbents attain equilibrium faster. Langmuir isotherms are followed, except for MIL-53 (Fe)/La–Fe3O4. The adsorption capacity increases with increasing the dopant ion size, 100.5 mg/g for La-doped MOF, which is about four times higher than that of undoped MOF. The adsorption mechanism involves chemical interactions between Lewis acid in magnetite MIL-53(Fe) series and Lewis base in MB.