Conclusion
The aging processes on Ag0Z in dry air and humid air as off-gas streams from the spent nuclear fuel reprocessing system were studied. Iodine loading capacity of Ag0Z after being aged in gas streams including dry air and humid air (d.p. -40oC, -15 oC, and +15oC) at 100 oC, 150oC, and 200 oC for up to 6 months were obtained. Iodine loading capacity of Ag0Z aged in dry air and humid air (d.p. -15 oC) at 150oC for 1 month decreased by roughly 35% and 50%, respectively, which indicate that humid air has more adverse impact on the performance of the Ag0Z. Besides, aging temperatures and water vapor concentrations are key factors which have negative influences on the Ag0Z; iodine loading capacity was decreased with increasing aging temperatures and water vapor concentrations.
The aging mechanisms of Ag0Z in dry air and humid air were suggested with SEM, XRD, and XPS analyses. It was found that the mechanism includes the following steps; (1) oxidation of Ag0 to Ag+, (2) migration of Ag+ from the surface to the pores/channels of mordenite crystals, (3) binding of Ag+ within the zeolite framework by replacing the H, and (4) binding of the generated H+ with O2- or HO-producing H2O. In addition, given that similar reactions in both dry air and humid air and faster aging process occur in humid air than dry air, the kinetics of the overall aging process were most likely controlled by the oxidation reaction of Ag0.
Furthermore, the Pseudo reaction model with the oxidation reaction of Ag0 from dry air and humid air aging processes is capable to describe the kinetics of deactivation on Ag0Z. The results of iodine loading capacity and model fitting reveal that the Ag0Z lost a significant iodine capacity when being exposed to dry air and humid air. While the kinetics of the aged Ag0Z in dry air and humid air were suggested in this study, future work should be involved not only in further understanding of the aging processes in gas mixture of dry air and humid air, but in ascertaining alternative adsorbents which are tolerant to dry air and humid air off-gas streams.