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.