Mixed-Ligand Strategy for the Creation of Hierarchical Porous ZIF-8 for Enhanced Adsorption of Copper Ions

The adsorption of heavy metals using metal-organic framework-based adsorption technology has been pointed out as a promising technique for the removal of these toxic elements from water. However, their adsorption capacity needs to be enhanced. Thus, the current work reports the effect of using a mixed-ligand strategy on the MOF framework and its effect on the removal of copper ions from water by adding terephthalic acid (BDC) linker to the ZIF-8precursors (2-methylimidazole (mI) and Zn²⁺) under solvothermal synthesis, leading to the formation of a hierarchical microporous mesoporous MOF, named Zn-mI-BDC, which was characterized by SEM, EDX, XRD, TGA, BET, and FTIR. As a result, all of these techniques revealed that the addition of a controlled amount of BDC did not alter the crystallinity of ZIF-8, resulting in the creation of a pore size of 4.2 nm. The new hierarchical porous MOF was tested for the adsorption of copper and exhibited an enhanced adsorption capacity compared to pristine ZIF-8 and many other standard adsorbents. The adsorption isotherm matched well with the Langmuir isotherm model, suggesting that the adsorption process chemisorption had a dominant role in the adsorption of Cu²⁺ species. Therefore, the current work is considered as an important step toward the use of a mixed-ligand strategy in enhancing the adsorption capacity of heavy metals using MOF materials.