Adsorption of molecular iodine and alkyl iodides from spent-nuclear-fuel-reprocessing off-gas using reduced silver mordenite

Radioactive iodine is a hazardous byproduct of spent-nuclear-fuel reprocessing that must be removed from the off-gas stream before it can be discharged. Reduced silver mordenite (Ag⁰Z) is currently the baseline material for iodine capture in the U.S. Although the performance characteristics and capture mechanisms of I₂ and CH₃I have been established, similar investigations into long-chain organic iodides have yet to be performed. In this study, thin beds of Ag⁰Z were loaded with I₂, CH₃I, C₄H₉I, and C₁₂H₂₅I (5 ppm to 50 ppm) carried in a dry air stream at 150 °C. The maximum iodine capacity was 105 ± 5 mg I/g Ag⁰Z for all species. Saturated Ag⁰Z samples were characterized using scanning electron microscopy, X-ray fluorescence, X-ray photoelectron spectroscopy, diffuse reflectance UV–visible spectroscopy, pair distribution function analysis, and thermogravimetric analysis. Near-complete Ag utilization and similar physical/chemical properties were observed for all samples. Through a comparison with previous studies and an investigation of aged Ag⁰Z, we propose that iodine species react with Ag⁺ at exchange sites, forming α-AgI within the mordenite channels, and with surface Ag⁰ nanoparticles, yielding β-/γ-AgI. The available Ag sites in the interior (Ag⁺) and exterior (Ag⁰) of mordenite determine the adsorption capacity since α-AgI formation is limited by the total pore volume. Potential iodine uptake routes were summarized for aging and non-aging environments. A scalable predictive model was implemented for deep-bed iodine removal, and predictions were in good agreement with experimental data. Sensitivity analysis suggests that iodine uptake kinetics is governed by pore diffusion.