Ion exchange induced removal of Pb(II) by MOF-derived magnetic inorganic sorbents

Nanoporous adsorbents of ZnO/ZnFe₂O₄/C were synthesized by using a metal organic framework (Fe^III-modified MOF-5) as both the precursor and the self-sacrificing template. The adsorption properties of ZnO/ZnFe₂O₄/C toward Pb(II) ions were investigated, including the pH effect, adsorption equilibrium and adsorption kinetics. The adsorption isotherms and kinetics were well described by using the Langmuir isotherm model and pseudo-second-order model, respectively. The MOF-derived inorganic adsorbents exhibited high absorption performance with a maximum adsorption capacity of 344.83 mg g^-1. X-ray powder diffraction and high-resolution X-ray photoelectron spectroscopy suggest that Zn(II) was substituted by a significant portion of Pb(II) on the surface of ZnO nanocrystals. Microscopic observations also demonstrate the effect of Pb(II) ions on ZnO crystals as reflected by the considerably reduced average particle size and defective outer layer. Quantitative measurement of the released Zn(II) ions and the adsorbed Pb(II) ions indicated a nearly linear relationship (R² = 0.977). Moreover, Pb-containing ZnO/ZnFe₂O₄/C adsorbents are strongly magnetic allowing their separation from the water environment by an external magnet.