Date of Award

Winter 12-22-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical and Chemical Engineering

Advisor(s)

Tavlarides, Lawrence L.

Keywords

Aging Mechanisms, Iodine Adsorption, Reaction Kinetics, Silver Particles, Silver-Exchanged Mordenite, Silver-Functionalized Silica Aerogel

Subject Categories

Chemical Engineering | Engineering

Abstract

Iodine-129 (129I) is one of the volatile radioactive species in the off-gas streams generated from the spent nuclear fuel reprocessing system, and it is essential to remove 129I vapor because 129I has a long half-life (1.6 x 107 years) and can be fatal to the human body if released to the environment. Radioactive iodine vapor can be effectively captured by silver containing adsorbents such as silver exchanged mordenite (Ag0Z) and silver functionalized silica aerogel (Ag0-aerogel) used in this study. However, the aging impact of off-gas streams on the adsorbents is one of major issues in the removal of radioactive iodine vapor during off-gas treatment. Previous studies have shown the remarkable performances of the Ag0Z and Ag0-aerogel for iodine adsorption, but have shown that the iodine loading capacity on both silver containing adsorbents decrease over time due to the chemical and physical changes to the adsorbents when exposed to off-gas streams. However, aging mechanisms of the processes caused by the off-gas streams on both the Ag0Z and A0-aerogel were not clear, so the mechanism and kinetics in the aging processes were evaluated in this study. In this study, the aging impacts of off-gas streams on the kinetics of iodine capture on both Ag0Z and Ag0-aerogel were investigated. The adsorbents were aged in a continuous-flow aging system using dry air, humid air, 1% NO, 2% NO2 and a gas mixture consisting of all the gas streams under different aging conditions, such as aging temperature, time and water concentration, and iodine adsorption experiments on the aged adsorbents were conducted in a continuous-flow adsorption system. Results showed that humid air, 1% NO and 2% NO2 have negative impacts on the performance of Ag0Z and Ag0-aerogel; especially, 2% NO2 has the most severe impact on the iodine loading capacity of both adsorbents. In addition, it was found that iodine loading capacity of the aged Ag0Z and Ag0-aerogel decreased as aging time and temperatures increased. In addition, characterization studies were conducted with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD) to better understand the mechanisms of the aging processes on Ag0Z and Ag0-aerogel by observing the chemical and physical changes through aging processes. Kinetic models were developed and evaluated to simulate the aging processes in different gases. Reversible and irreversible reaction models were suggested to describe the reaction of the aging components with the silver adsorbent pellets.

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Open Access

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