Adsorption of gaseous radioactive iodine using bismuth-incorporated mesoporous silica

Abstract

Radioiodine $(^{129}I)$ generated by nuclear fission reaction is volatilized as gaseous forms when spent nuclear fuel is reprocessed. The very long half-life $(1.57 × 10^7 y)$ and high environmental mobility have made the man-agement of $^{129}I$ a challenging issue. Currently, silver-exchanged zeolites (AgX or AgZ) are widely used to capture $^{129}I_2$ (g). However, iodine physisorption within zeolite poses a serious problem related to long-term disposal of $^{129}I$. Furthermore, silver-based sorbents cannot be the ultimate solution given that high price and toxicity of silver. In chapter 3, new adsorbents based on bismuth were investigated for the capture of $^{129}I$ in off-gas produced from spent fuel reprocessing. Porous bulky materials were synthesized with polyvinyl alcohol (PVA) as a sacrificial template. Major findings showed that the iodine trapping capacity of as-synthesized samples could reach 1.9-fold that of commercial silver-exchanged zeolite (AgX). The thermodynamic stability of the reaction products explains the high removal efficiency of iodine. It was also found that the pore volume of each sample was closely related to the ratio of the reaction products. In chapter 4, bismuth-embedded SBA-15 mesoporous silica was firstly applied for iodine capture and stor-age. SBA-15 was functionalized with thiol (-SH) groups, followed by bismuth adsorption with Bi-S bonding, which was thermally treated to form Bi2S3 within SBA-15. The bismuth-embedded SBA-15s demonstrated high iodine loading capacities (up to 540 mg-I/g-sorbent), which benefitted from high surface area and porosity of SBA-15 as well as the formation of thermodynamically stable BiI3 compound. Iodine physisorption was effectively suppressed due to the large pores present in SBA-15, resulting in chemisorption as a main mechanism for iodine confinement. Furthermore, a chemically durable iodine-bearing material was made with a facile post-sorption process, during which the iodine-incorporated phase was changed from BiI3 to chemically durable $Bi_5O_7I$. Thus, the results showed that both efficient capture and stabilization of $^{129}I$ would be possible with the bismuth-embedded SBA-15, in contrast to other sorbents mainly focused on iodine capture. In chapter 5, stabilization of bismuth-embedded SBA-15 that captured iodine gas was studied by fabrica-tion of monolithic waste forms. The iodine containing waste was mixed with $Bi_2O_3$ (a stabilizing additive), and low-temperature sintering glass, followed by pelletizing and sintering process to produce glass composite materials. Iodine volatility during sintering process was significantly affected by the ratio of $Bi_2O_3$ and the glass composition. It was confirmed that $BiI_3$, the main iodine phase within bismuth-embedded SBA-15, was effectively transformed to the mixed phases of $Bi_5O_7I$ and BiOI. The initial leaching rates of iodine from the glass composite waste forms ranged $10^{-3}-10^{-2} g/m^2$ day, showing stability of the iodine phases encapsulated by glassy networks. It was also observed that common groundwater anions (e.g., chloride, carbonate, sulfite, and fluoride) elevate iodine leaching rate by anion exchange reactions. The results of this dissertation suggest that glass composite waste form of bismuth-embedded SBA-15 could be a candidate material for stable storage of $^{129}I$.