Adsorption of Methyl Iodide on Reduced Silver-Functionalized Silica Aerogel: Kinetics and Modeling

Siqi Tang, Seungrag Choi, Yue Nan^* and Lawrence L. Tavlarides

Department of Biomedical and Chemical Engineering, Syracuse University, 329 Link Hall, Syracuse, NY 13244, USA

The low concentration methyl iodides (CH₃I) adsorption process on reduced silver-functionalized silica aerogel (Ag⁰-Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH₃I pore diffusivity was calculated, the CH₃I-Ag⁰-Aerogel reaction was identified as a 1.37 order reaction instead of first order reaction, and the n^th order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (<100 ppbv), the CH₃I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.

Separations: Materials, Devices and Processes

Adsorption, iodine removal, separation, silver aerogel, shrinking core model

Nuclear power has been widely used since the 20^thcentury for its low emission rate of air pollutants.^1-3 However, multiple radioactive isotopes, including radioactive Iodine (¹²⁹I), are produced in the uranium fission.^4,5 During aqueous reprocessing of the nuclear waste, ⁸⁵Kr, ¹⁴C,¹²⁹I, and ³H are released to the off-gas streams. The off-gas streams include dissolver off-gas (DOG), cell off-gas (COG), waste off-gas (WOG) and vessel off-gas (VOG).⁶ According to Bruffey et al.^6,7, approximately 95% - 98% of iodine is contained in DOG, remaining iodine (in I₂ and organic iodides form) exists in VOG, and VOG flow rate is 100× higher than that of DOG. Therefore, unlike the ppm level in DOG, the iodine concentration in VOG decreases to below 100 ppbv. Although only trace amount of iodine exists in VOG, the off-gas cannot be emitted directly into atmosphere, and the emissions are governed by 10 CFR 20, 40 CFR 61 and 40 CFR 190.^8-10 The composition of the organic iodides existing in VOG varies from methyl iodide (CH­₃I) to iodododecane (C₁₂H₂₅I) and among 12 different organic iodides, CH₃I and C₁₂H₂₅I were reported to be the two most abundant components.^6,11,12

Multiple silver containing materials, including macroreticular resins, silver impregnated alumina (AgA), silver exchanged faujasite (AgX), hydrogen-reduced silver exchanged mordenite (Ag⁰Z) and reduced silver-functionalized silica aerogel (Ag⁰-Aerogel), have been developed and studied for I₂ and organic iodides adsorption.^4,13-18 The reason for selecting silver-containing materials over traditional liquid scrubbing methods is concluded to be stronger Ag-I bond and solid form, therefore, higher removal efficiency and lower operation cost.^4,19-22 Among these silver-containing materials, Ag⁰Z and Ag⁰-Aerogel have been studied continuously in US national laboratories and universities for their high iodine removal efficiency and relatively greater resistance to aging caused by potential contaminants in VOG and DOG (NO_x, water vapor, air).^12,17,23 Nan et.al^16,19 have conducted the single layer adsorption experiments of I₂on Ag⁰Z and reported approximately 12 wt% I₂ adsorption capacity at 423K. Deep-bed adsorption of I₂ and CH₃I on Ag⁰Z at both ppb and ppm level have been studied by Jubin et al.^14,18, Bruffey et al.^6,24 and Soelberg and Watson^25,26, indicating the adsorption rate of CH­₃I is lower than that of I₂.

Deep-bed Ag⁰-Aerogel adsorption experiments of CH_­3I and I₂ under the VOG conditions were conducted in multiple US national laboratories. Strachan et al.²⁷ performed 4.2 ppmv I₂ adsorption experiments at 150 ℃, concluding that fresh Ag⁰-Aerogel was able to remove I₂ for at least 99.99% efficiency and observing that Ag⁰-Aerogel changed from black to a brown, earth-tone color during the adsorption experiment. Soelberg and Watson^28,29 conducted I₂ adsorption on Ag⁰Z and Ag⁰-Aerogel at 150 ℃ with NO_x and H₂O present, with I₂ concentration ranging from 2 to 370 ppmv. Their results indicated, at similar conditions, Ag⁰-Aerogel obtained higher resistance to NO_x than Ag⁰Z did and, therefore, showed higher removal efficiency in deep-bed adsorption experiments. In addition, deep-bed CH₃I adsorption studies have also been conducted by Bruffey and Jubin⁷, Jubin et al.¹⁴and Soelberg and Watson¹⁷. Ag⁰-Aerogel showed an adequate capability to adsorb CH₃I at various conditions such as ppb or ppm level concentrations and with or without the presence of NO_x. The penetration depth of CH₃I in the column was measured to be 3-7 cm, depending on the CH₃I and NO_x concentration; and the decontamination factor (DF), defined as inlet concentration of desired adsorbed species (eg. I₂ and CH₃I) over outlet concentration, can reach approximately 1000.

The detailed organic iodides-Ag reaction mechanism was not conclusive. Scheele et al.³⁰ proposed several possible reactions between Ag and CH₃I in Ag⁰Z:

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In these proposed reactions, organic compounds are generated in gas form and only iodine is captured by silver. The generated CH₃OCH₃ (dimethyl ether) and CH₃OH (methanol) were observed by Soelberg and Watson²⁵ and the desorption of C₂H₆ in similar reactions was suggested by Zhou et al³¹. With the presence of NO_x in the gas stream, other organic compounds such as CH₃NO₂ (nitromethane) and C₃H₉NO (3-amino-1-propanol) were also detected.²⁶ Assuming similar reactions happen between Ag⁰-Aerogel and CH₃I and only iodine is left in Ag⁰-Aerogel pellets, the maximum iodine capture capacity for I₂ should be the same as that for CH₃I.

Limited by the nature of deep-bed adsorption experiments, the mass of deep-bed adsorption column cannot be measured continuously and most of the curves were composed by discrete data points instead of real-time data. Therefore, as complements to the deep-bed adsorption experiments, single-layer CH₃I adsorption experiments on Ag⁰-Aerogel were conducted in the present work. One of the purposes of performing the single-layer adsorption experiments is determining the pore diffusivity and reaction rate constant and these parameters can be related and applied for column adsorption modeling.^32-34

In the single-layer adsorption experiments, Ag⁰-Aerogel was placed in a tray connected to a microbalance, enabling real-time and high-precision measurement of the mass change. The CH₃I concentrations selected were 113, 266, 1130 and 10400 ppbv; temperature was 150 ℃, same as the experiments described above; and gas flow rate was set to be 500 sccm in order to satisfy 1.1 m/s superficial gas velocity suggested by Nan et al.¹⁹ The adsorption kinetic data at various CH₃I concentrations were obtained and used to evaluate the pore diffusivity and reaction rate of CH₃I in Ag⁰-Aerogel. To explain the inconsistency of reaction rate constants at different concentrations, an n^thorder shrinking core model was applied and the modeling results were used to improve the predictions of adsorption behavior at various CH₃I concentrations.

Reduced silver-functionalized silica aerogel (Ag⁰-Aerogel) was obtained from the Pacific North National Laboratory (PNNL) in 2017. It was first developed there for gaseous iodine capturing in nuclear waste treatment.^35,36 The Ag⁰-Aerogel pellets are primarily black; the shape and size are arbitrary and some of the pellets contain certain yellow areas which may be due to unevenly coated silver in the manufacturing process. The particle radius is approximately 0.1 cm. Reported by Jubin et al.¹⁴, the bulk density of the pellet is 0.54 g/cm³ and 0.62 g/cm³ for ‘as-received material’ and ‘post-run material’ respectively. For the modeling work to be discussed in the following sections, an average value of 0.58 g/cm³ was used. The maximum iodine loading capacity reported ranged from 33 to 47 wt%^27,29,37. To achieve accurate modeling results, the iodine loading capacity was measured to be approximately 37 wt% using the continuous flow adsorption system.

The continuous flow adsorption system was used for iodine and water vapor adsorption on multiple materials previously by Nan et al.^19,38,39and Lin et al.^40,41 and has been modified for performing the adsorption of CH₃I on Ag⁰-Aerogel experiments. Figure 1 shows the schematic diagram of the modified adsorption system.