Combined experimental and theoretical studies on iodine capture of Zr-based metal-organic frameworks: Effect of N-functionalization and adsorption mechanism

The potential leakage of nuclear waste, especially radioiodine, is a major safety concerning issue around the world. To remove radioiodine from nuclear waste efficiently, there is an urgent demand for adsorbents that possess both high stability and strong adsorption affinity for environmental remediation. Herein, two Zr-based metal-organic frameworks (Zr-MOFs) and their N-functionalized analogues have been synthesized and researched for iodine adsorption in both vapours and solutions. It was found that Zr-MOFs with N-enriched ligands (e.g., pyridine and amino) exhibited the faster iodine adsorption rate and the higher iodine uptake amount (e.g., reaching adsorption equilibrium within 4 hours with the removal rate of above 85% for iodine solution adsorption) than their unfunctionalized counterparts (UiO-66 and UiO-67). The critical role played by N-enriched groups in enhancing iodine adsorption has been revealed through versatile model fittings, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy characterizations, as well as density functional theory (DFT) calculations. Compared to those in amino-group, the N-atoms in pyridine-groups showed a deeper affinity towards iodine molecules. Remarkably, the N-enriched UiOs adsorbents also exhibited good recyclability, especially UiO-66-PYDC and UiO-67-NH2 could maintain the removal efficiency of 89.05% and 85.49% after four adsorption-desorption recycling tests. With the strong iodine uptake affinity and outstanding regeneration performance, this work has systematically investigated the impact of N-functionalization on the enhanced performance for iodine capture by using the N-enriched UiO MOFs as promising adsorbents, providing an insightful guideline into the physical chemistry of adsorption mechanism behind the radioiodine capture.