Iodine Capture Using Zr-Based Metal-Organic Frameworks (Zr-MOFs): Adsorption Performance and Mechanism

The effective capture of radioiodine, produced or released from nuclear-related activities, is of paramount importance for the sustainable development of nuclear energy. Here, a series of zirconium-based metal-organic frameworks (Zr-MOFs), with a Zr₆(μ₃-O)₄(μ₃-OH)₄ cluster and various carboxylate linkers, were investigated for the capture of volatile iodine. Their adsorption kinetics and recyclability were investigated in dry and humid environments. The structural change of Zr-MOFs during iodine trapping was studied using powder X-ray diffraction and pore structure measurements. Experimental spectra (Raman and X-ray photoelectron spectroscopy) and density functional theory (DFT) calculations for the linkers and Zr clusters were performed to understand the trapping mechanism of the framework. When interacting with iodine molecules, MOF-808, NU-1000, and UiO-66, with highly connected and/or rigid linkers, have better structural stability than UiO-67 and MOF-867, which have flexible linkers with less connectivity. Particularly, MOF-808, with a rigid and tritopic benzenetricarboxylate linker, has the highest iodine adsorption capacity (2.18 g/g, 80 °C), as well as the largest pore volume after iodine elution. In contrast, UiO-67, with long linear ditopic linkers, exhibits the weakest stability and lowest adsorption capacity (0.53 g/g, 80 °C) because of its most serious collapse of pore structures. After incorporating with strong electron-donating imidazole/pyridine ligands, both the stability and adsorption capacity of MOF-808/NU-1000 decrease. DFT calculations verify that the N-heterocycle groups could enhance the affinity toward iodine by strong charge transfer. DFT calculations also suggest that the terminal -OH in MOF-808 has a strong affinity toward iodine (-54 kJ/mol I₂) and water (-63 kJ/mol H₂O) and a weak affinity toward NO₂ (-27 kJ/mol NO₂). With high adsorption capacity and excellent stability, MOF-808 shows great potential for the sustainable removal of radioiodine.