(263a) Adsorption of Organic Iodides from Vessel Off-Gas (VOG) Streams on Silver-Containing Adsorbents

During the nuclear fission process, radioactive organic iodides are generated in the gas form including CH₃I, C₂H₅I, ..., C₁₂H₂₅I. For environmental protection purpose, silver-containing materials such as reduced silver exchanged mordenite (Ag⁰Z), silver functionalized silica aerogel (Ag⁰-Aerogel) and silver nitrate impregnated alumina (AgA) are used to adsorb these contaminants in the vessel off-gas stream (VOG). In the presented work, the organic iodide (CH₃I) adsorption at extremely low concentrations (ppb levels) was studied.

By performing the adsorption of 104 and 1044 ppbv CH₃I at 150℃ on Ag⁰Z, Ag⁰-Aerogel and AgA, the Ag⁰-Aerogel has been identified as the most effective adsorbent for its high adsorption rate and strong aging resistance. Therefore, further studies including the temperature effect and alkyl group effect, were conducted using Ag⁰-Aerogel.

The 104, and 1044 ppbv CH₃I adsorption experiments at 100, 150 and 200 ℃ on Ag⁰-Aerogel were conducted, and an abnormal increase of uptake rate was observed at 104 ppb, 200 ℃ (200% - 300% higher than that at 150 ℃). To explain this behavior, firstly, a step-wise potential reaction pathway was proposed. The model includes the cleavage of CH₃I, binding of the free radicals, and the diffusion of the gas form product. Since diffusion limitation of the product is considered in this pathway, it can be further used to explain the abnormal behavior. Secondly, the porosities of the Ag⁰-Aerogel at different drying conditions (100, 150 and 200 ℃) were measured using the nitrogen titration method. The results indicate that as the temperature decreases, the increasing in-pellet water concentration decreases the pore surface area and may decrease the availability of the silver sites during the adsorption. Thirdly, physical analyses including scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDX) and x-ray photoelectron spectroscopy (XPS) were performed. The spectra show that an additional Ag-I compound is formed at 104 ppb, 200 ℃, which may also lead to the significant increase in the uptake rate.

After reviewing the potential explanations, the significances of the factors influencing the uptake rate appear to be (from the most to the least), formation of an additional Ag-I compound, temperature effect introduced by the Arrhenius relationship, the decrease of the diffusion limitation and the increase of the silver site availability.