Is High-Entropy Strategy Beneficial for Iodine Capture? A Comparative Study of ZIF-8 Derivatives from Monometallic to Bimetallic and to Polymetallic Systems.

Efficient capture of radioactive iodine isotopes from nuclear waste is crucial for environmental protection and human health. In this study, a high-entropy strategy was introduced into the zeolitic imidazolate framework-8 (ZIF-8) system to overcome the inherent limitations of conventional monometallic ZIF-8 and to elucidate entropy-regulated iodine capture behavior. Using a solvothermal method assisted by triethylamine (TEA), a series of ZIF-8 derivatives with entropy levels ranging from low to high were successfully synthesized to explore the relationship between entropy level and iodine adsorption performance. Furthermore, given that Cd incorporation has been reported to enhance the structural flexibility of ZIF-8, the influence of different Zn/Cd ratios was investigated. Characterization results showed that although TEA enhanced the doping efficiency of poorly coordinating metal ions, it simultaneously reduced crystallinity and BET surface area, leading to pore blockage and a substantial decline in iodine adsorption performance. Iodine adsorption did not show a positive correlation with increasing compositional complexity. The synergistic effects of multimetal nodes did not improve the iodine uptake capacity of ZIF-8, and the high-entropy ZnCdCoMgNi-ZIF-8 exhibited the lowest uptake capacity (1.10 g⋅g-1). In contrast, Cd doping increased the iodine uptake capacity to 2.80 g·g-1 in phase-pure Cd-ZIF-8, corresponding to a 2.2-fold enhancement over conventional ZIF-8. Moreover, a structural phase transition was induced during iodine adsorption in Cd-rich systems. This work provides important insights into the design of high-performance iodine capture materials and highlights the potential of cadmium-modified ZIF-8 derivatives as promising adsorbents.