Influence of elevated temperature and oxygen on the capture of radioactive iodine by silver functionalized silica aerogel

Abstract

Reprocessing is considered a competent strategy for spent nuclear fuel management, yet radioiodine (¹²⁹I) is emitted in reprocessing off-gas as a hazardous byproduct. Silver functionalized silica aerogel (Ag⁰-aerogel), a promising iodine capture material, experiences a reduction in its capacity after prolonged exposure to off-gas components at elevated temperatures, a phenomenon termed as aging. To fully understand this process, we isolated the contribution of each aging factor, exposing Ag⁰-aerogel samples to N₂ and dry air gas streams, respectively, at 150 °C for different time periods. Aged samples were loaded with I₂ to examine the capacity change and comprehensively characterized to investigate the evolution of their properties. Results show that temperature alone did not alter Ag⁰-aerogel's capacity but triggered Ag⁰ nanoparticles sintering and generated organic sulfur species. The presence of O₂ reduced the capacity by ∼20 %, causing (i) formation of silver sulfide (Ag₂S) crystals and (ii) oxidation of Ag-thiolate (Ag-S-r) to Ag sulfonate (Ag-SO₃-r). Given that Ag₂S readily adsorbs I₂, the formation of Ag-SO₃-r is the major inhibitor for iodine adsorption. This hypothesis was supported by density functional theory (DFT) simulations. These findings unraveled key mechanisms of Ag⁰-aerogel aging, which are useful in the development of materials that withstand realistic spent-nuclear-fuel-reprocessing off-gas conditions.