Abstract
Iodine radioisotopes released during nuclear fuel reprocessing must be removed from the off-gas stream before discharge. One promising material for iodine capture is reduced silver mordenite (AgZ). Nevertheless, the adsorbent's capacity will degrade, or age, over time when the material is exposed to other off-gas constituents. Though the overall impact of aging is known, the underlying physical and chemical processes are not. To examine these processes, AgZ samples were prepared and aged in 2% NO2 in dry air and in 1% NO in N2 gas streams at 150 °C for up to six months. Aged samples were then characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray absorption spectroscopy. These techniques show that aging involves two overarching processes: (i) oxidation of the silver nanoparticles present in AgZ and (ii) migration of oxidized silver into the mordenite's inner network. Silver on the nanoparticle's surface is oxidized through adsorption of O2, NO, and NO2. Raman spectroscopy and X-ray absorption spectroscopy indicate that nitrates are the primary products of this adsorption. Most of these nitrates migrate into the interior of the mordenite and exchange at framework binding sites, returning silver to its unreduced state (AgZ). The remaining nitrates exist at a persistent concentration without aggregating into bulk-phase AgNO3. X-ray absorption spectroscopy results further indicate that iodine adsorption occurs on not just AgZ but also on AgZ and a portion of the nitrates in the system. AgZ adsorbs a sizable quantity of iodine early in the aging process, but its capacity drops rapidly over time. For well-aged samples, nitrates are responsible for up to 95% of mordenite's iodine capacity. These results have enhanced our understanding of the aging process in silver mordenite and are expected to guide the development of superior adsorbents for the capture of radioactive iodine from reprocessing off-gas.
Original language | English |
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Pages (from-to) | 49680-49693 |
Number of pages | 14 |
Journal | ACS Applied Materials and Interfaces |
Volume | 12 |
Issue number | 44 |
DOIs | |
State | Published - Nov 4 2020 |
Funding
This research was supported by the Nuclear Energy University Program, Office of Nuclear Energy, U.S. Department of Energy, and used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science user facility operated for the DOE Office of Science by the Argonne National Laboratory under contract no. DE-AC02-06CH11357. The beamline used in this study (10-ID-B) is supported by the Materials Research Collaborative Access Team (MRCAT). MRCAT operations are supported by the Department of Energy and the MRCAT member institutions.
Keywords
- adsorption
- nuclear fuel reprocessing
- off-gas iodine
- separations
- silver mordenite