Removal of CH₃I in Gas by Modified Hierarchical Molecular Sieve Materials

During the dissolution process of spent nuclear fuel, significant amounts of high-temperature and high-humidity radioactive off-gas are generated. These gases not only contain substantial radioactive substances, such as radioactive iodine, but may also include nitrogen oxides, rendering the gases acidic. The acidic nature of these off-gases complicates their treatment and induces corrosion in adsorbent materials, potentially diminishing their adsorption efficiency and reducing their service life. The performance of hydrophobically modified hierarchical porous silver-loaded zeolites for methyl iodide (CH₃I) removal was explored in this study. Their effectiveness under high-temperature and high-humidity conditions was specifically examined. To create a hierarchical porous structure, ZSM-5 zeolite was first chemically treated with hydrofluoric acid (HF). This etching process significantly increased the specific surface area of the zeolite, and created additional pore sites and active centers, thereby enhancing its adsorption capacity for targeted substances. Silver ions were then introduced into the etched ZSM-5 zeolite using an impregnation technique. To further improve its adsorption performance in high-humidity environments, the silver-loaded zeolite was modified with phenyltriethoxysilane to impart hydrophobic properties. The incorporation of hydrophobic groups effectively minimizes moisture interference, thereby improving the stability and durability of the zeolite in humid conditions. This process yields a series of zeolite materials with adjustable hydrophobic characteristics. The adsorption performance of these materials was systematically evaluated using a gas adsorption apparatus, with a focus on their ability to remove CH₃I from simulated spent nuclear fuel dissolution off-gases. The results reveal that HF treatment and silver loading significantly increase the specific surface area and pore sites of the zeolite, enabling it to accommodate more silver ions and achieve a notably higher adsorption capacity compared to untreated ZSM-5 zeolite. Furthermore, hydrophobically modified zeolites exhibite superior CH₃I removal performance relative to unmodified zeolites, demonstrating that hydrophobic modification enhances moisture resistance and significantly improves CH₃I removal efficiency. Moreover, it is found that retaining a certain amount of fluoride ions could improve the adsorption capacity of silver-loaded zeolite by charge transfer with iodide ions during the adsorption process, which results in the presence of I⁻ in the form of \mathrmI_3^- or \mathrmI_5^- , and improves the binding capacity of silver and iodine. This study highlights that optimized hydrophobically modified silver-loaded zeolites offer significant potential for the filtration of radioactive iodine-containing gases. These findings present a promising new pathway for the development of advanced adsorbent materials for spent nuclear fuel dissolution off-gas treatment systems.