Pyrazolate-based MOFs with open Zn2+ sites for highly effective and rapid adsorption of iodine in water†

Capturing radioactive iodine from water is a critical challenge in nuclear waste management and environmental conservation, necessitating the development of superior adsorbents. In this work, we introduce pyrazolate-based metal–organic frameworks (MOFs) as highly efficient adsorbents for iodine capture from aqueous solutions. We synthesized two novel Zn-triazolate MOFs, [Zn₃(HTPPA)₂(CH₃COO)₂] (DZU-109) and [Zn₇(OH)₂(TPPA)₄(H₂O)] (DZU-110), utilizing the tris(4-(1H-pyrazol-4-yl)phenyl)amine (H₃TPPA) ligand and Zn²⁺ ions, employing a modulator-induced synthesis approach. SCXRD analysis revealed structural diversities in the two MOFs, attributed to the conformational flexibility of the triazolate ligand and the distinct coordination modes of Zn-building units. In a static I₃⁻ (I₂/KI in water) adsorption setup, DZU-110 outperformed DZU-109, achieving an adsorption capacity of 2.31 g g⁻¹ and a rate of 3.08 × 10⁻² g mg⁻¹ min⁻¹, establishing a new benchmark among pristine MOFs in iodine capture from water. DZU-110 also displayed remarkable removal efficiency and adsorption capacity under dynamic flow-through conditions. Mechanistic investigations indicate that the synergistic action of pyrazolate rings and open Zn²⁺ sites in DZU-110 is pivotal for I₃⁻ adsorption, enabling various interactions such as I⋯Zn, I⋯π, I⋯H and I⋯N between the framework and iodine species. This study not only offers a valuable strategy for the synthesis and structural modulation of pyrazolate-based MOFs but also underscores the potential of these materials as promising candidates for iodine adsorption in water purification processes.