Abstract
Off – gas stream from used nuclear fuel recycling operations comprises of various contaminants such as 3H, 14C, 85Kr, 131I, and 129I. Radioactive iodine, 129I, is of significant concern due to its tendency to form hard-to-capture toxic volatile organic compounds such as methyl iodide, half-life in excess of 15 million years, and tendency to accumulate in human thyroid glands. It also has adverse impacts on the environment due to its toxic nature. Organic iodides can be hazardous even at ppb level concentrations, and hence the capture of 129I is extremely important. Various methods such as absorption and adsorption have been employed in the past to capture volatile iodine species. Recent studies have been focusing on the adsorption of iodine species using solid porous adsorbents, and the current work involves the capture of organic iodine species on mordenites. The off-gas environment was simulated using ppb level concentration of iodine species and the dynamic adsorption experiments were conducted to determine the efficiency of mordenites as adsorbents. Three different mordenites - sodium mordenite, partially exchanged silver mordenite, and fully exchanged silver mordenite were used as adsorbents. The silver mordenites were synthesized by ion exchange between silver nitrate solution and sodium mordenite and subjected to structural and compositional analysis. Adsorption-desorption cycles at different temperatures and using different materials of construction for the column were conducted and the effect of silver content on the adsorption capacity of sorbents was determined.
The sorption columns exhibited behavior atypical of pure adsorption with no breakthrough and exhaustion of the adsorbents. It was hypothesized columns were behaving as reactors operating at a steady-state. Kinetic analysis of the system was conducted incorporating the effects of diffusion and mass transfer. The analysis indicates that a second-order reaction was the rate-controlling mechanism for methyl iodide removal.
These research findings could give good insights about the behavior of volatile organic iodine species at lower concentrations and their effective capture.