Methyl Iodide Adsorption on
Reduced Silver- Functionalized Silica Aerogel: How Temperature Impacts
the Adsorption?
Siqi Tang, Seungrag Choi, and Lawrence L. Tavlarides*
Department of Biomedical and Chemical Engineering, Syracuse University,
329 Link Hall, Syracuse, NY 13244, USA
Abstract
To understand the effect of temperature to the adsorption, 104 ppbv and
1044 ppbv methyl iodide (CH3I) adsorptions on reduced
silver-functionalized silica aerogel (Ag0-Aerogel) at
100, 150 and 200 ℃ were performed. In the experiments, a significantly
high uptake rate (3 – 4 times higher than that at 100 and 150 ℃) was
observed for the 104 ppbv adsorption at 200 ℃. To explain such behavior,
a potential reaction pathway was proposed and multiple physical analyses
including nitrogen titration, x-ray photoelectron spectroscopy (XPS) and
scanning electron microscopy (SEM) were performed. Based on the results,
the contributing factors appear to be the formation of different Ag-I
components induced by temperature, higher silver site availability,
decreasing diffusion limitation, and increasing reaction rate described
by the Arrhenius relationship.
Topical Heading: Separations: Materials, Devices and Processes
Keywords: Adsorption, Iodine Removal, Separation, Silver
Aerogel
Introduction
Since 1951, when the first nuclear power plant was built in Idaho, the
nuclear waste treatment arose as a crucial difficulty and environmental
concern.1 During the nuclear fission and the
successive aqueous reprocessing of the radioactive waste,129I was released in both organic and inorganic forms
to off-gas streams, along with 85Kr,14C, 3H etc.2-4According to Jubin and Strachan4, the radioactive
iodine (both inorganic and organic forms) mainly exists in dissolve
off-gas (DOG) and vessel off-gas (VOG), which contain 5-10 ppmv and
20-30 ppbv respectively. Unlike I2, it is more difficult
to study the organic iodides because of their additional alkyl groups
and the low concentrations. Multiple forms of organic iodides (from
CH3I to C12H25I) were
observed, and among these organic iodides, CH3I and
C12H25I were reported to be the two most
abundant ones.5-10
To remove the radioactive iodine, multiple adsorbents were developed and
studied including reduced silver-containing materials such as silver
functionalized silica aerogel
(Ag0-Aerogel)11, hydrogen reduced
silver exchanged mordenite
(Ag0Z)12,13, silver nitrate
impregnated alumina (AgA)14; bismuth-containing
materials such as
Bi-Bi2O3-TiO2-C15,
bismuth-decorated electrospinning carbon nanofiber16;
and other organic materials such as graphene
aerogel17, nitrogen-rich covalent organic framework
(SCU-COF)18 and metal organic frameworks
(MOF)19. Among these materials, the silver-containing
ones are the most matured and commonly used. To provide references and
information for the design and future industrial application of
Ag0-Aerogel, the adsorption experiments of 104 ppbv
and 1044 ppbv CH3I on Ag0-Aerogel at
100, 150 and 200 ℃ were conducted. During the experiments, an abnormally
high uptake rate was observed for 104 ppbv CH3I
adsorption at 200 ℃, and multiple physical analyses including nitrogen
titration, scanning electron microscopy (SEM) and x-ray photoelectron
spectroscopy (XPS) were performed. Based on the data collected and
theoretical analyses, the factors impacting the uptake rate appear to
be, unusual reactions and products, difference in silver site
availability, change of diffusion limitation and impact of temperature
to reaction rate described by the Arrhenius relationship.
Method
Continuous Flow Adsorption
System
The organic iodide adsorption experiments were performed using a
continuous flow adsorption system. The system includes the
Dynacalibrator (VICI, Model 450 and 500), adsorption column, furnace,
microbalance and the corresponding data acquisition system.