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.