Solidification and stabilization of silver mordenite used to control radioiodine emissions from Hanford’s Waste Treatment Plant

Adsorption of iodine on hydrogen‐reduced silver‐exchanged mordenite: Experiments and modeling

“…Figure a displays the isotherms of total adsorption of iodine with contributions by both physisorption and chemisorption. An average total iodine loading of 13.5 wt % was achieved at the optimal temperature of 423 K. This optimal adsorption temperature agreed with previous studies . The linear isotherms show a slight slope, which is mostly due to the increase of physisorbed iodine as the iodine concentration in the gas stream increased.…”

“…The optimal reduction conditions of Ag 0 Z (12.0 wt % Ag) used in this study were found to be at 673 K and for 24 h. Based on the equilibrium data from single‐layer pellet adsorption experiments, the adsorption of molecular iodine on Ag 0 Z is mostly chemisorption through the Ag‐I reaction. The optimal adsorption temperature was found to be 423 K, which confirmed the conclusions in previous studies . An average maximum iodine loading of 12.3 wt % by chemisorption (13.5 wt % by both chemisorption and physisorption) was achieved at 423 K, and the corresponding Ag utilization efficiency is 88%.…”

“…Mordenite is high in silicon‐aluminum ratio (typically 5:1), and therefore is thermally stable at high temperatures and resistant to acidic off‐gas streams. Even though studies have shown that there exist other chemical forms in addition to the major form (AgI) when iodine is adsorbed into Ag zeolites, the stronger Ag–I chemical bond makes AgZ preferred over adsorbents that adsorb iodine molecules physically.…”

“…Studies also have been done on the formation of AgI and long‐term storage of iodine loaded Ag 0 Z . For instance, Chapman et al reported that the silver‐iodine reaction occurred at the surface of the Ag 0 Z crystals mainly formed γ‐AgI (in a cubic zincblende type structure) while that at the inside of the crystals formed α‐AgI (in a face‐centered cubic structure), which is different to the polymorphic forms of commercial bulk AgI.…”

“…AgZ was treated in a hydrogen stream at temperatures between 423 and 773 K for a period of time from hours to days . Thomas et al achieved Ag utilization efficiency of 72.3% with Ag 0 Z (Ag content: ∼20 wt %) reduced at 773 K for 24 h. The same reduction temperature of 773 K was also used by Scheele et al In comparison, Ag 0 Z (Ag content: 9.5 wt %) used in the study at the U.S. Oak Ridge National Laboratory was reduced at 543 K for 10 days yielding a 7.2 wt % iodine loading (64.5% Ag utilization efficiency) . The Si/Al molar ratio in these AgZ adsorbents was not reported but is expected to be in the range of ∼5:1.…”