N-Acetyl-L-cysteine-grafted metal-organic framework for heavy metal removal from hazardous waste

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.