Abstract
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.
