Capture of Organic Iodides from Vessel Off-Gas Streams

Abstract

Silver-functionalized silica aerogel (Ag 0 -aerogel) is considered a promising porous adsorbent for the capture of iodine from nuclear fuel reprocessing off-gas. The Ag 0 -aerogel, as mentioned in Part I, experiences a steady loss of capacity when exposed to dry air, humid air, NO, and NO2 in the off-gas. This phenomenon is known as aging and hinders the capture performance of both I2 and organic iodides. From previous studies, NO2 exhibited the strongest contribution to sample aging whilst its underlying governing processes remain unclear.Therefore, we exposed Ag 0 -aerogel samples to 2% NO2 in dry air at 150 ºC for up to 1 month, followed by I2 loading. Samples were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, Xray photoelectron spectroscopy, and X-ray absorption spectroscopy. Our results elucidated the precise Ag species in Ag 0 -aerogel that adsorb I2: (i) Ag 0 nanoparticles, (ii) Ag thiolates (Ag-S-r) covering the nanoparticles and the aerogel backbone, and (iii) Ag-S complexes including amorphous Ag2S. We unraveled the two predominant underlying processes of aging in NO2: (i) Ag-S-r is oxidized by NO2 forming silver sulfonate (Ag-SO3-r) which is oxidized further to silver sulfate (Ag2SO4) molecules, (ii) Ag2SO4 molecules then migrate from the pores to the aerogel surface and aggregate yielding Ag2SO4 particles that do not adsorb I2. Plausible reaction pathways and aggregation mechanisms are explored. These findings may be used to guide the discovery of more advanced materials for iodine capture and the development of accurate predictive models.