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Title: Studies on the Mechanisms of Methyl Iodide Adsorption and Iodine Retention on Silver-Mordenite

Abstract

Silver-containing mordenite (MOR) is a longstanding benchmark for radioiodine capture, reacting with molecular iodine (I 2) to form AgI. However the mechanisms for organoiodine capture are not well understood. Here we investigate the capture of methyl iodide from complex mixed gas streams by combining chemical analysis of the effluent gas stream with in depth characterization of the recovered sorbent.

Authors:
 [1];  [2]
  1. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
  2. Idaho National Lab. (INL), Idaho Falls, ID (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE), Fuel Cycle Technologies (NE-5)
OSTI Identifier:
1171585
Report Number(s):
SAND2014-17740R
537547
DOE Contract Number:
AC04-94AL85000
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Nenoff, Tina Maria, and Soelberg, Nick. Studies on the Mechanisms of Methyl Iodide Adsorption and Iodine Retention on Silver-Mordenite. United States: N. p., 2014. Web. doi:10.2172/1171585.
Nenoff, Tina Maria, & Soelberg, Nick. Studies on the Mechanisms of Methyl Iodide Adsorption and Iodine Retention on Silver-Mordenite. United States. doi:10.2172/1171585.
Nenoff, Tina Maria, and Soelberg, Nick. Mon . "Studies on the Mechanisms of Methyl Iodide Adsorption and Iodine Retention on Silver-Mordenite". United States. doi:10.2172/1171585. https://www.osti.gov/servlets/purl/1171585.
@article{osti_1171585,
title = {Studies on the Mechanisms of Methyl Iodide Adsorption and Iodine Retention on Silver-Mordenite},
author = {Nenoff, Tina Maria and Soelberg, Nick},
abstractNote = {Silver-containing mordenite (MOR) is a longstanding benchmark for radioiodine capture, reacting with molecular iodine (I2) to form AgI. However the mechanisms for organoiodine capture are not well understood. Here we investigate the capture of methyl iodide from complex mixed gas streams by combining chemical analysis of the effluent gas stream with in depth characterization of the recovered sorbent.},
doi = {10.2172/1171585},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Mon Sep 01 00:00:00 EDT 2014},
month = {Mon Sep 01 00:00:00 EDT 2014}
}

Technical Report:

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  • The adsorption of methyl iodide onto hydrogen-reduced silver-exchange mordenite was studied. The removal of organic iodides from off-gas streams is an important step in controlling the release of radioactive iodine to the environment during the treatment of radioactive wastes or the processing of some irradiated materials. Nine well accepted mass transfer models were evaluated for their ability to adequately explain the observed CH{sub 3}I uptake behavior onto the Ag-Z. Linear and multidimensional regression techniques were utilized in the estimation of the diffusion constants and other model parameters which then permitted the selection of an appropriate mass transfer model. To date,more » only bulk loading data exist for the adsorption of CH{sub 3}I onto Ag-Z. Hence this is believed to be the first study to quantify the controlling mass transfer mechanisms of this process. It can be concluded from the analysis of the experimental data obtained by the single-pellet type experiments and for the process conditions used in this study that the overall mass transfer rate associated with the adsorption of CH{sub 3}I onto Ag-Z is affected by both micropore and macropore diffusion. The macropore diffusion rate was significantly faster than the micropore diffusion, resulting in a two-step adsorption behavior which was adequately modeled by a bimodal pore distribution model. The micropore diffusivity was determined to be on the order of 2 {times} 10{sup {minus}14} cm{sup 2}/s. The system was also shown to be isothermal under all conditions of this study. Two other conclusions were also obtained. First, the gas film resistance to mass transfer for the 1/16 and 1/8-in.-diam Ag-Z pellets can be ignored under the conditions used in this study. Finally, it was shown that by decreasing the water vapor content of the feed gas, the chemical reaction rate appeared to become the initial rate-limiting factor for the mass transfer. 75 refs.« less
  • An experimental laboratory program was conducted to develop economical solid adsorbents for the retention of krypton from a dissolver off-gas stream. The study indicates that a solid adsorbent system is feasible and competitive with other developing systems which utilize fluorocarbon absorption nd cryogenic distillation. This technology may have potential applications not only in nuclear fuel reprocessing plants, but also in nuclear reactors and in environmental monitoring. Of the 13 prospective adsorbents evaluated with respect to adsorption capacity and cost, the commercially available hydrogen mordenite was the most cost-effective material at subambient temperatures (-40/sup 0/ to -80/sup 0/C). Silver mordenite hasmore » a higher capacity for krypton retention, but is 50 times more expensive than hydrogen mordenite.« less
  • At the Pacific Northwest Laboratory (PNL), we performed two sets of experiments to determine the effects of pertinent operational parameters and gas compositions on organic radioiodine (in particular methyl iodide (CH/sub 3/I)) capture by silver mordenite (AgZ). In the first set of experiments, we studied the effects of (1) hydrogen pretreatment of AgZ, (2) change in particle size of AgZ, and (3) the presence of water in the gas phase. In the second set of experiments we evaluated on a semi-quantitative basis the effects of nitric oxide (NO), nitrogen dioxide (NO/sub 2/), superficial face velocity, and temperature on CH/sub 3/Imore » capture by reduced (hydrogen pretreated) silver mordenite (Ag/sup 0/Z). These studies have shown that AgZ, especially Ag/sup 0/Z, is an effective trap for CH/sub 3/I. However, its effectiveness varies with changes in operational parameters and is affected by other gases found in the process off gas of a nuclear reprocessing plant. Optimum trappig efficiency was achieved with Ag/sup 0/Z rather than AgZ, 20-40 mesh Ag/sup 0/Z rather than 0.16 cm extrudate, moisture in the gas stream, higher temperatures up to 200/sup 0/C, absence of NO, and lower superficial face velocities down to 3.75 m/min. Additionally, CH/sub 3/I can be converted to elemental iodine (I/sub 2/) in the presence of NO or NO/sub 2/ by controlling the operational parameters. Since I/sub 2/ is easier to trap than organic iodides, this should improve trapping efficiency,.« less
  • The aim of the iodine program at PNL is to develop technologies that will provide safe and effective removal and packaging of /sup 129/I for interim storage and shipping and for permanent isolation. Iodine recovery and fixation processes, and the evaluation of fixation compounds and materials are included in the program. Iodine-containing compounds have been evaluated on the basis of chemical and thermal stability, solubility, and cost and availability. Other materials examined were silver mordenite and faujasite, AC-6120, and a synthetic iodine-containing sodalite. Work on silver-containing absorbents includes study of the effects of operating parameters on the removal of organicmore » iodides by reduced silver mordenite, on the recycle of silver mordenite, and on the general mechanisms of iodine fixation on metal zeolites. We have shown that normally only a small amount of the total silver capacity is utilized in forming stable silver-iodine compounds. The remainder of the iodine absorbed is held in a less stable complex and can be removed by heating. None of the forms examined, with the possible exception of silver-iodide-containing materials, have combined stability-leach rate characteristics compatible with time periods of more than a few thousand years. Criteria for storage or disposal need to be established before the results of the chemical and thermal evaluations can be applied. In this regard, some effort has recently been directed at the criteria question. 18 figures, 2 tables.« less