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Title: X-ray Absorption Spectroscopy Investigation of Iodine Capture by Silver-Exchanged Mordenite

Abstract

Capture of radioactive iodine is a significant consideration during reprocessing of spent nuclear fuel and disposal of legacy wastes. While silver-exchanged mordenite (AgZ) is widely regarded as a benchmark material for assessing iodine adsorption performance, previous research efforts have largely focused on bulk material properties rather than the underpinning molecular interactions that achieve effective iodine capture. As a result, the fundamental understanding necessary to identify and mitigate deactivation pathways for the recycle of AgZ is not available. In this paper, we applied X-ray Absorption Fine Structure (XAFS) spectroscopy to investigate AgZ following activation, adsorption of iodine, regeneration, and recycle, observing no appreciable degradation in performance due to the highly controlled conditions under which the AgZ was maintained. Fits of the extended XAFS (EXAFS) data reveal complete formation of Ag 0 nanoparticles upon treatment with H 2, and confirm the formation of α-AgI within the mordenite channels in addition to surface γ/β-AgI nanoparticles following iodine exposure. Analysis of the nanoparticle size and fractional composition of α-AgI to γ/β-AgI supports ripening of surface nanoparticles as a function of recycle. Finally, this work provides a foundation for future investigation of AgZ deactivation under conditions relevant to spent nuclear fuel reprocessing.

Authors:
ORCiD logo [1];  [2];  [2]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Chemical Sciences Division
  2. Syracuse Univ., NY (United States). Dept. of Biomedical and Chemical Engineering
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States); Syracuse Univ., NY (United States)
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Nuclear Energy (NE), Nuclear Reactor Technologies (NE-7). Nuclear Energy University Program (NEUP); USDOE Office of Science (SC)
OSTI Identifier:
1376447
Grant/Contract Number:
AC05-00OR22725; AC02-06CH11357; NE0008275
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Industrial and Engineering Chemistry Research
Additional Journal Information:
Journal Volume: 56; Journal Issue: 16; Journal ID: ISSN 0888-5885
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS

Citation Formats

Abney, Carter W., Nan, Yue, and Tavlarides, Lawrence L. X-ray Absorption Spectroscopy Investigation of Iodine Capture by Silver-Exchanged Mordenite. United States: N. p., 2017. Web. doi:10.1021/acs.iecr.7b00233.
Abney, Carter W., Nan, Yue, & Tavlarides, Lawrence L. X-ray Absorption Spectroscopy Investigation of Iodine Capture by Silver-Exchanged Mordenite. United States. doi:10.1021/acs.iecr.7b00233.
Abney, Carter W., Nan, Yue, and Tavlarides, Lawrence L. Wed . "X-ray Absorption Spectroscopy Investigation of Iodine Capture by Silver-Exchanged Mordenite". United States. doi:10.1021/acs.iecr.7b00233. https://www.osti.gov/servlets/purl/1376447.
@article{osti_1376447,
title = {X-ray Absorption Spectroscopy Investigation of Iodine Capture by Silver-Exchanged Mordenite},
author = {Abney, Carter W. and Nan, Yue and Tavlarides, Lawrence L.},
abstractNote = {Capture of radioactive iodine is a significant consideration during reprocessing of spent nuclear fuel and disposal of legacy wastes. While silver-exchanged mordenite (AgZ) is widely regarded as a benchmark material for assessing iodine adsorption performance, previous research efforts have largely focused on bulk material properties rather than the underpinning molecular interactions that achieve effective iodine capture. As a result, the fundamental understanding necessary to identify and mitigate deactivation pathways for the recycle of AgZ is not available. In this paper, we applied X-ray Absorption Fine Structure (XAFS) spectroscopy to investigate AgZ following activation, adsorption of iodine, regeneration, and recycle, observing no appreciable degradation in performance due to the highly controlled conditions under which the AgZ was maintained. Fits of the extended XAFS (EXAFS) data reveal complete formation of Ag0 nanoparticles upon treatment with H2, and confirm the formation of α-AgI within the mordenite channels in addition to surface γ/β-AgI nanoparticles following iodine exposure. Analysis of the nanoparticle size and fractional composition of α-AgI to γ/β-AgI supports ripening of surface nanoparticles as a function of recycle. Finally, this work provides a foundation for future investigation of AgZ deactivation under conditions relevant to spent nuclear fuel reprocessing.},
doi = {10.1021/acs.iecr.7b00233},
journal = {Industrial and Engineering Chemistry Research},
number = 16,
volume = 56,
place = {United States},
year = {Wed Mar 29 00:00:00 EDT 2017},
month = {Wed Mar 29 00:00:00 EDT 2017}
}

Journal Article:
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  • The adsorption process of iodine, a major volatile radionuclide in the off-gas streams of spent nuclear fuel reprocessing, on hydrogen-reduced silver-exchanged mordenite (Ag 0Z) was studied at the micro-scale. The gas-solid mass transfer and reaction involved in the adsorption process were investigated and evaluated with appropriate models. Optimal conditions for reducing the silver-exchanged mordenite (AgZ) in a hydrogen stream were determined. Kinetic and equilibrium data of iodine adsorption on Ag 0Z were obtained by performing single-layer adsorption experiments with experimental systems of high precision at 373–473 K over various iodine concentrations. Results indicate approximately 91% to 97% of the iodinemore » adsorption was through the silver-iodine reaction. The effect of temperature on the iodine loading capacity of Ag 0Z was discussed. In conclusion, the Shrinking Core model describes the data well, and the primary rate controlling mechanisms were macro-pore diffusion and silver-iodine reaction. © 2016 American Institute of Chemical Engineers AIChE J, 2016« less
  • No abstract prepared.
  • No abstract prepared.
  • Volatile gas emissions from a nuclear fuel recycle facility in the United States are governed by several key regulations, including 10 CFR 20, 40 CFR 61, and 40 CFR 190. Under 40 CFR 190, the total quantity of iodine that may be released to the environment from the entire fuel cycle is limited to 5 millicuries of I-129 per gigawatt-year of electrical energy produced by the fuel cycle. With a reasonable engineering margin, an iodine decontamination factor (DF) of approximately 1000 will be required for the complete fuel cycle. Off-gas treatment in a fuel reprocessing plant must address several gasmore » streams containing iodine, among a number of volatile radionuclides. Past research and developmental activities identified silver-exchanged mordenite (AgZ) as a very promising sorbent based on its acid resistance, relatively high iodine and methyl iodide capacity, and high achievable DF. Recent studies at ORNL have focused on the impacts of long-term exposure to simulated off-gas streams (aging) and pretreatment on the iodine adsorption performance of hydrogen-reduced silver-exchanged mordenite (Ag{sup 0}Z). Experiments were conducted to determine the effects of long-term exposure to both dry and moist air on the iodine sorption capacity of Ag{sup 0}Z. The data indicates that aging reduces the capacity of Ag{sup 0}Z, which must be accounted for to prevent degradation of DF. Because of its high acid resistance, a AgZ sorbent has been selected specifically for application in treating off-gas streams containing iodine. While extensive tests have been conducted in the United States on a form of this sorbent, the specific material previously tested is no longer commercially available and similar materials are currently being evaluated. As part of this evaluation, tests were conducted to determine the iodine sorption properties of this replacement media and the effects of long-term (up to 6 months) exposure to simulated off-gas streams. The ultimate goal is to develop an understanding of the fundamental phenomena that controls aging for this material and other zeolites that could be considered for use in off-gas treatment in the future. The trends in the study results indicate that the amount of elemental silver observed by XRD increases from 0.3 wt% in vendor-supplied AgZ to approximately 5 wt% by reducing the material with hydrogen. The study also concluded that aging decreases the quantity of elemental silver in the material. After 2 months of aging, the Ag{sup 0} content of an experimental sample was reduced from 5 wt% to about 1.3 wt%. The form into which the elemental silver is converted during aging was not determined. Experimental tests have been initiated to study how aging of Ag{sup 0}Z impacts iodine loading on the zeolite. Loading tests with un-aged Ag{sup 0}Z resulted in an 81% silver utilization. The loading capacity of iodine on Ag{sup 0}Z was reduced with aging in dry air. Material aged for 6 months in dry air had a 40% reduction in iodine loading capacity. Under moist-air aging conditions, a significant decrease in the rate and total loading (∼45% of theoretical) of iodine uptake can be observed beginning with the shortest aging period (i.e., after 1 month) when compared with the loading curve using Ag{sup 0}Z with no aging. Increasing exposure time to the humid air used to age the Ag{sup 0}Z beyond 1 month resulted in a slight additional reduction in capacity to about 35% of theoretical at 2 months. Virtually identical capacity was observed with 4 months of aging. Compared to the non-aged material, the 1 month dry-air aged Ag{sup 0}Z shows about a 35% reduction (approximate) in iodine loading capacity and the 6 month dry-air aged Ag{sup 0}Z shows about a 50% reduction. These studies generated several questions that will be addressed in future tests. They include the following: Is there indeed degradation over time (in storage) in the iodine adsorption performance of Ag{sup 0}Z? Once reduced, how should the Ag{sup 0}Z be stored- under a hydrogen atmosphere, an inert atmosphere, a desiccant, or some other method or combination of methods? Does Ag{sup 0}Z have a 'shelf life' that must be considered after receipt from a vendor and before use in a plant? Also, how should a column of Ag{sup 0}Z be stored offline in a processing plant (parallel column) arrangement? Future tests will also include increasingly challenging aging environments such as acid vapor conditions. (authors)« less