Facile preparation of magnetic Zr-MOF for adsorption of Pb(II) and Cr(VI) from water: Adsorption characteristics and mechanisms

A novel magnetic Zr-MOF named Ni0.6Fe2.4O4-UiO-66-PEI was synthesized by modifying UiO-66-NH2 with Ni0.6Fe2.4O4 and polyethyleneimine and its sorption characteristics for two representing heavy metals Pb(II) and Cr(VI) in water were examined. This material showed good thermal stability and was sufficiently magnetic to allow effective solid-liquid separation. The material had a maximum adsorption capacity of 273.2 mg/g for Pb(II) at pH 4.0 and 428.6 mg/g for Cr(VI) at pH 3.0, as obtained from the model fitting by the Hill equation. The sorption kinetics for both heavy metals on the material was fast, with adsorption equilibrium established in 60 min and 30 min for Pb(II) and Cr(VI), respectively. Model fitting of the kinetic data showed that Pb(II) and Cr(VI) sorption on Ni0.6Fe2.4O4-UiO-66-PEI followed the pseudo-second-order model. This material had good selectivity for competing ions, and still hold fairly good binding capacities after 5 times of regeneration. Adsorption mechanisms for Pb(II) and Cr(VI) by Ni0.6Fe2.4O4-UiO-66-PEI were proposed by analyzing FTIR, XPS and the electrokinetic property of the material. Specifically, Pb(II) adsorption on Ni0.6Fe2.4O4-UiO-66-PEI could be attributed to chelation of Pb(II) with the imine/amine functional groups in the material’s matrix, while Cr(VI) adsorption might stem from a combination of electrostatic attraction, chelation and redox reaction. It seems that Ni0.6Fe2.4O4-UiO-66-PEI could be used as a green and environmental-friendly material for heavy metals removal, particularly from acidic water streams.