Effects of NO₂ aging on bismuth nanoparticles and bismuth-loaded silica xerogels for iodine capture

The capture of long-lived radioactive iodine (¹²⁹I) from oxidizing off-gasses produced from reprocessing used nuclear fuel is paramount to human health and environmental safety. Bismuth has been investigated as a viable iodine getter but the phase stability of bismuth-based sorbents in an oxidizing environment have not yet been researched. In the current work, bismuth nanoparticle-based sorbents, as free particles (Bi-NPs) and embedded within silica xerogel monoliths made with a porogen (TEO-5), were exposed to I_2(g) before and after aging in 1 v/v% NO₂ at 150 °C. For unaged sorbents, BiI₃ was the dominant phase after iodine capture with 8-30 mass% BiOI present due to native Bi₂O₃ on the surface of the unaged nanoparticles. After 3 h of aging, 82 mass% of the Bi-NPs was converted to Bi₂O₃ with only a small amount of iodine captured as BiOI (18 mass%). After aging TEO-5 for 3 h, iodine was captured as both BiI₃ (26 %) and BiOI (74 %) and no Bi₂O₃ was detected.". Additionally, bismuth lining the micrometer-scale pores in the TEO-5 led to enhanced iodine capture. In a subsequent exposure of the sorbents to NO₂ (secondary aging), all BiI₃ converted to BiOI. Thus, direct capture of iodine as BiOI is desired (over BiI₃) to minimize loss of iodine after capture.