Over the past century, industrial development has led to severe environmental problems, including the contamination of wastewater (such as from rivers and seas) with dissolved heavy metals (e.g., Pb2+ and Cd2+), which pose serious health risks. To remove these heavy metals, heterogeneous solid-state adsorbents, such as metal-organic frameworks (MOFs) have attracted significant attention due to their porous extended structures, strong binding sites, ease of separation from water, and non-toxic, stable nature. In this work, we functionalized MOF-808 with N-acetyl-L-cysteine (AcC), which contains soft-base –SH groups capable of binding soft-acid metal ions (Pb2+ and Cd2+), via grafting onto the Zr6 metal nodes. Approximately 83% of the Zr6 nodes in MOF-808 were successfully functionalized with AcC, yielding AcC@MOF-808. This modification enabled strong interactions between the AcC groups and heavy metals, consistent with hard-soft acid-base (HSAB) theory, while maintaining the crystallinity and availability of free –SH sites. As a result, AcC@MOF-808 demonstrated highly efficient capture of Pb2+ and Cd2+, facilitated by the thiol groups along with the aliphatic and acetyl functionalities that enhance adsorption efficiency, as confirmed by crystallographic, spectroscopic, and microscopic analyses. The maximum adsorption capacities for Pb2+ and Cd2+ were 231 mg g−1 and 109 mg g−1. AcC@MOF-808 achieved 99% removal of both Pb2+ and Cd2+ from 10 ppm trace metal solutions, exhibiting particularly rapid uptake (within seven minutes). The corresponding rate constant (k) values were 6.6×103 g mg−1 min−1 for Pb2+ and 1.1×103 g mg−1 min−1 for Cd2+, and adsorption kinetics that followed a pseudo-second order model. Furthermore, AcC@MOF-808 showed high selectivity toward Pb2+ and Cd2+ over other divalent metal ions, including both alkali and transition metals, demonstrating its strong potential for heavy metal removal from real wastewater systems.
Seong et al. (Mon,) studied this question.