  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.
