Bimetallic Coordination in Two-Dimensional Metal-Organic Framework Nanosheets Enables Highly Efficient Removal of Heavy Metal Lead (II)

Two-dimensional (2D) metal-organic frameworks (MOFs) have emerged as intriguing 2D materials because of their specific features of 2D morphology and designable skeletons, which have elicited great interest in environment remediation. In this work, 2D MOF nanosheets are fabricated via a mixed-solvent solvothermal method, and a regulation strategy of metal inorganic clusters on MOFs is used to construct two different 2D MOFs with monometallic and bimetallic coordination, that is, Ni-MOF and Ni/Cd-MOF. Binary metal coordination renders more crystal defects and vacancies in the framework; thus, compared to monometallic Ni-MOF, bimetallic Ni/Cd-MOF exhibits fewer layers (4 similar to 5 layers), higher specific surface area, larger pore size, and higher surface electronegativity, which leads to its excellent adsorption removal for Pb2+, with higher adsorption rate and affinity, and superior adsorption capacity (950.61 mg/g, almost twice as high as that of monometallic Ni-MOF). Besides, the adsorption mechanism further confirmed that the carboxyl groups (-COO-) from organic linker on 2D MOFs serve as the main binding sites for Pb2+ coordination, and bimetallic Ni/Cd-MOF has more active -COO- sites for Pb2+ capture. Thus, the bimetallic Ni/Cd-MOF regulated by heterogeneous metal atoms shows promising application for highly efficient adsorption of heavy metal ions.

Automated summary

Two different two-dimensional metal-organic frameworks, Ni-MOF and Ni/Cd-MOF, were synthesized, with the latter showing superior adsorption performance for heavy metal ions such as Pb2+ due to its higher specific surface area, larger pore size, and more active binding sites. The study highlights the potential application of bimetallic Ni/Cd-MOF in the efficient removal of heavy metal ions.