Adsorption of organic Iodides on Reduced Silver Functionalized Silica Aerogel

In the nuclear waste reprocessing, radioactive iodine is released in both organic and inorganic forms into off-gas streams. Due to its properties of long half-life and accumulation in human bodies, the radioactive iodine is required to be removed by the Environmental Protection Agency (EPA) and Nuclear Regulatory Commission (NRC). In this presented work, the removal of organic iodides is studied particularly. Unlike that of inorganic iodine, the organic iodides were not well studied because of the low concentrations and the corresponding technical difficulties. Most of the studies on the organic iodides are semi-quantified, focusing on the performances of the adsorption columns of certain combinations of conditions and the quantified single-layer adsorptions were rarely reported. To provide the information and results supporting the adsorption columns design, single layer continuous-flow organic iodides adsorptions using silver-containing adsorbents were performed. The solid adsorption method was developed in replacement of the liquid scrubbing strategy for its low operational cost and simplicity of design. The most commonly used adsorbents include reduced silver exchanged mordenite (Ag0Z) and silver nitrate impregnated alumina (AgA), and they have been applied in multiple waste reprocessing plants around the world. In the 2010s, a novel silver-containing material, reduced silver functionalized silica aerogel (Ag0-Aerogel), was developed at the Pacific Northwest National Laboratory (PNNL), and has been considered as an outstanding material for its high silver content and adsorption rate. The efficiencies of the three materials were evaluated and the Ag0-Aerogel was identified to be the optimum adsorbent among Ag0-Aerogel, Ag0Z and AgA. Therefore, the CH3I adsorptions on Ag0-Aerogel were performed at various concentrations and temperatures. The data were analyzed using multiple models and the parameters were determined. The results indicated that the CH3I adsorption on Ag0-Aerogel is a surface reaction at the specified conditions and the adsorption rate increases with the adsorption temperature. Additionally, adsorptions of other iodoalkanes (C3H7I, C6H13I, C8H17I and C12H25I) were performed and the corresponding dependencies on temperatures, concentrations and the length of carbon chain were determined. The C6H13I and C12H25I adsorptions are likely to be zero-order adsorptions and the temperature dependencies may vary at different conditions. Moreover, the adsorption rates of C3H7I and C8H17I are higher than expected, accordingly, further studies are suggested. Using the parameters determined, the column adsorption modeling of organic iodides was conducted and the modeling results were comparable to the literature works. The model was also applied to predict the breakthrough of the column, and the outcomes indicate that a column of at least 15-20 cm is required to remove the organic iodides of up to approximately 100 ppbv. Based on the results of organic iodides adsorptions and the modeling works, the potential research objectives were recommended and the properties of the next generation materials and adsorption systems were also suggested