Cysteine-grafted MOF-808(Ce) for Pb²⁺ and Cd²⁺ removal: A mechanistic study integrating adsorption models and DFT...

The efficient removal of toxic metals such as lead (Pb²⁺) and cadmium (Cd²⁺) from the aqueous environments remains a critical environmental challenge. Herein, we report the first development of a cysteine-functionalized cerium-based MOF-808(Ce) via a post-synthetic modification strategy, integrating multiple metal-binding functional groups (-SH, -NH₂, and -COOH) within a robust MOF architecture to achieve enhanced heavy-metal capture. In this study, a cysteine-functionalized cerium-based metal-organic framework (Cys-MOF-808(Ce)) was synthesized via post-synthetic modification of MOF-808(Ce) to enhance its affinity towards heavy metals through the introduction of thiol (-SH), amino (-NH₂), and carboxyl (-COOH) functional groups. The physicochemical properties of the material were characterized using XRD, FTIR, FESEM, TEM, and XPS, confirming successful functionalization without compromising its structural integrity. Batch adsorption was performed to evaluate the adsorption performance of Cys-MOF-808(Ce), which exhibited remarkable maximum adsorption capacities of 720 ± 92 mg/g for Pb²⁺ and 257 ± 20 mg/g for Cd²⁺, respectively. Additionally, the adsorption isotherm and kinetic models were best fit to the Freundlich and pseudo-second-order models, indicating that the adsorption process occurs on a heterogeneous surface and is predominantly chemisorption-driven. Furthermore, the material demonstrated higher affinity for Pb²⁺ and Cd²⁺ compared to other coexisting ions and showed decent regenerability. Density functional theory (DFT) calculations further elucidated the adsorption mechanism by revealing a strong interaction between metal ions and functional groups. These results highlight the potential of Cys-MOF-808(Ce) as a promising adsorbent for real-world applications.