(258f) Adsorption of Organic Iodides from Vessel Off-Gas (VOG) Streams on Silver-Containing Adsorbents

¹²⁹I is generated in the Vessel Off-Gas (VOG) streams in the organic form such as CH₃I, C₂H₅I, ..., and C₁₂H₂₅I. Due to the radioactive nature, the organic iodides must be removed before the streams can be released to the atmosphere. However, comparing with other Off-Gas streams, organic iodides concentration in VOG is extremely low (less than 100 ppbv). Therefore, more specific studies on the adsorption process at low concentrations are required. Multiple materials have been studied in this area including reduced silver exchanged mordenite (Ag⁰Z), silver functionalized silica aerogel (Ag⁰-Aerogel) and silver nitrate impregnated alumina (AgA). The most efficient adsorbent was preliminarily determined by conducting single-layer low concentration adsorption experiments. Because of the relatively high adsorption rate and resistance to the aging effect, Ag⁰-Aerogel was considered as the superior material among Ag⁰Z, Ag⁰-Aerogel, and AgA.

Furthermore, to understand the adsorption kinetics, 113, 266, 1130 and 10400 ppbv CH₃I adsorption experiments on Ag⁰-Aerogel at 150 ^oC were performed. A core-shrinking process was observed, and the corresponding shrinking core model was applied to determine the reaction rate constant and CH₃I pore diffusivity in Ag⁰-Aerogel. A constant pore diffusivity was calculated to be approximately 4.59 ± 0.10 ×10^-4 cm²/s. The determined reaction rate constants were changing with CH₃I concentration, indicating a possible non-1^st order reaction. Therefore, the shrinking core model was modified and we found the reaction order is approximately 1.37. To understand the rate-determine step, the contributions of multiple physical and chemical processes in the adsorption were calculated. The results show that the adsorption process is overall controlled by pore diffusion, but at low concentration, the reaction between CH₃I and silver particles is more significant and the adsorption process can be reduced to a surface reaction at low concentrations in a relatively short period (in at least one year for 113 ppbv compared with equilibrium time of more than 50 years).