Enhanced removal of toxic hexavalent chromium from aqueous solution by magnetic Zr-MOF@polypyrrole: performance and mechanism

Abstract

Magnetic Zr-based metal organic framework (UiO-66) @Polypyrrole (magnetic UiO-66@Ppy) was prepared to eliminate Cr(VI) from water. SEM and TEM results clearly revealed that the magnetic UiO-66@Ppy was a core-double-shell structure with the core of Fe₃O₄, inner shell UiO-66, and outer shell Ppy. The introduction of zirconium MOFs UiO-66 effectively prevented the agglomeration of polypyrrole and provided more available adsorption sites, the surface area increased from 9.57 m²/g (Ppy) to 10.57 m²/g (Fe₃O₄@Ppy) and 52.49 m²/g (magnetic UiO-66@Ppy). The magnetic UiO-66@Ppy possessed a high adsorption capacity of 259.1 mg/g in removing Cr(VI) from water. Adsorption kinetics followed the pseudo-second-order model. The removal of Cr(VI) involved the following mechanisms: (1) electrostatic attraction and ions exchange, the HCrO₄^- was adsorbed on the surface of magnetic UiO-66@Ppy by the protonated N(PpyN⁺) and Cl^-; (2) reduction, Cr(VI) was reduced to Cr(III) by the reductive functional group(-NH-); (3) chelation, Cr(III) was immobilized on adsorbent by amine groups.

Keywords: Adsorption; Cr(VI); Magnetic UiO-66; Mechanism; Ppy.