Capture of iodine and organic iodides using silica zeolites and the semiconductor behaviour of iodine in a silica zeolite

Abstract

During the reprocessing of spent nuclear fuel rods, a highly moist off-gas mixture containing various volatile radioactive species, such as iodine (I₂), organic iodides and nitric acid, is produced. Efforts have been made to devise materials, which can effectively capture radioactive iodine (I₂) and organic iodides from the off-gas mixture without being damaged by moisture, nitric acid, and I₂. In the investigation described herein, we observed that all-silica zeolites, such as silicalite-1 and Si-BEA, are stable in 5 M nitric acid and adsorb I₂, CH₃I, and CH₃CH₂I from highly acidic off-gas mixture to much greater extents than activated carbon does. In particular, the hydrophobicity-intensified silicalite-1 performs best. We further found that I₂ forms a unique semiconducting three-dimensional supramolecular network within the silicalite-1 channels. The conductivity of the fully I₂ loaded silicalite-1 is observed to be ca. 10⁴ S m⁻¹, which is ca. 10⁸-fold higher than that of solid I₂.