Cr(VI) Reduction and Immobilization by Core-Double-Shell Structured Magnetic Polydopamine@Zeolitic Idazolate Frameworks-8 Microspheres

A well-defined core-double-shell structured magnetic polydopamine@zeolitic idazolate frameworks-8 (MP@ZIF-8) hydrid microsphere consisting of the core of magnetic Fe₃O₄ nanoparticles, the inner shell of a polydopamine layer, and the outer shell of a porous ZIF-8 nanocrystal was prepared through a facile and green approach to achieve synergistic reduction and adsorptive removal of Cr(VI). The microsphere property was characterized methodically. The batch adsorption experiments showed that the MP@ZIF-8 exhibited high efficiency in the Cr(VI) removal from aqueous solutions, affording Cr(VI) removal capacity of 136.56 mg g^-1, surpassing pristine MP (92.27 mg g^-1). The pseudo-second-order model fitted the Cr(VI) removal kinetics well. Cr(VI) removal on the MP@ZIF-8 relied highly on pH values. More significantly, with the reduction of nitrogen atom group on ZIF-8 and PDA, Cr(VI) was easily converted into low toxicity Cr(III) and then immobilized on the MP@ZIF-8. Thus, the hybrid microspheres provided excellent adsorptive activity in treating Cr-contaminated wastewater.