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

Abstract

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 nth 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.

Topical Heading

Separations: Materials, Devices and Processes

Keywords

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

Introduction

Nuclear power has been widely used since the 20thcentury 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 (NOx, water vapor, air).12,17,23 Nan et.al16,19 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 Watson25,26, indicating the adsorption rate of CH­3I is lower than that of I2.
Deep-bed Ag0-Aerogel adsorption experiments of CH­3I 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 Watson28,29 conducted I2 adsorption on Ag0Z and Ag0-Aerogel at 150 ℃ with NOx and H2O present, with I2 concentration ranging from 2 to 370 ppmv. Their results indicated, at similar conditions, Ag0-Aerogel obtained higher resistance to NOx 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 NOx. The penetration depth of CH3I in the column was measured to be 3-7 cm, depending on the CH3I and NOx 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:
.
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 NOx 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 nthorder shrinking core model was applied and the modeling results were used to improve the predictions of adsorption behavior at various CH3I concentrations.

Materials and Methods

Silver-Functionalized Silica Aerogel

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.

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.