Construction of two-dimensional zeolitic-imidazolate frameworks for enhanced Pb(II) and Cd(II) removal in industrial wastewater

Abstract

Zeolitic imidazole-like frameworks (ZIFs) were widely used for removing heavy metal ions in industrial wastewater. However, ZIFs were featured with three-dimensional configuration that always limited by lower structure opening, which were unfavorable for the interaction with heavy metal ions. Herein, a novel two-dimensional (2D) ZIFs material was developed via bottom-up approach using polyvinyl pyrrolidone (PVP) as regulating agent. The PVP molecules tended to reduce the surface energy and promoted the stacking along the direction of (002) lattice plane to achieve the low-dimensional construction. The specific surface area of the 2D ZIFs material was increased from 35.86 m²/g to 163.07 m²/g. Benefiting from the large specific surface area and highly opened active sites, the maximal adsorption capacity of Pb(II) and Cd(II) ions was up to 571.97 and 257.81 mg/g, 1.88 and 1.33 times of conventional ZIFs bulks, and the adsorption kinetics was determined by intra-particle diffusion phase, improved by 18.3 % and 39.6 %, respectively. In fact, the hydrated hydroxyls on benzimidazole skeleton and Zn center were main active sites, coordinated selectively with Pb(II) and Cd(II) ions to form -C=N-O-Pb and -C=N-O-Cd, and Zn-O-Pb and Zn-O-Cd complex, respectively. In addition, the 2D ZIFs exhibited excellent performance both in consecutive Cd(II) and Pb(II) ions adsorption-desorption cyclic tests and practical industrial wastewater treatment, which not only promoted the practical application of ZIFs materials in adsorption technology, but also could be integrated into other water reuse technologies for various heavy metal involved industrial wastewater treatment.