Capture of Elemental and Organic Iodine from Dilute Gas Streams by Silver-exchanged Mordenite

Abstract

The treatment of off-gas streams arising from reprocessing of used nuclear fuel (UNF) is an area of active study by the U.S. Department of Energy. Such off-gas streams contain volatile fission products, including long-lived 129I. Although 129I is released into the off-gas at multiple points within the chemical reprocessing flowsheet, previous research has focused on removal from the dissolver off-gas stream (DOG). The DOG is expected to contain up to 98% of iodine in UNF at ppm levels within the stream. Other off-gas streams will also contain iodine but at substantially lower concentrations. Recent work has shown that compliance with U.S. regulations will likely require capture of iodine from these dilute streams in addition to capture from DOG.In particular, the vessel off-gas (VOG) stream is expected to contain 1-3% of the total iodine inventory at ppb concentrations. A review of literature also indicates that the speciation of iodine in the VOG stream will differ from that of the DOG, with the DOG containing primarily I2 and the VOG containing a mixture of I2 and organic iodine species.Silver-exchanged mordenite (AgZ) has been identified for use in the removal of iodine from off-gas streams. It is an effective capture material for I2 at the concentrations expected in the DOG, but little is known about its performance in gas streams that may contain both I2 and organic iodides at very dilute concentrations.The experiments to be described were designed to separately characterize the adsorption of I2 and methyl iodide on AgZ through extended duration testing. Simulated vessel off-gases containing low levels of either I2 or methyl iodide were contacted with AgZ sorbent beds for up to four months. Through the use of sorbent beds in series and varied sampling times, key parameters such as adsorption rate, decontamination factor, and performance over time could be determined for the capture of each species by AgZ.This paper will discuss the literature relating to the speciation of iodine in the VOG and will discuss the results of adsorption experiments. An examination of the difference between capture of I2 and organic iodides will be conducted, and the impacts of those results on future iodine removal work will be considered.