Treatment of Off-Gas Emissions: Kinetics of Silver Mordenite Catalyzed Methyl Iodide Decomposition

Abstract

Methyl iodide, a common methylation intermediate used in the production of pesticides and pharmaceuticals, is characterized by both acute and chronic toxicity. In the nuclear industry, some of the radio-iodine release occurs in the form of methyl iodide, exacerbating the hazard posed by the chemical. Sorbents containing silver, particularly silver mordenites, have been preferentially investigated for capturing radio-iodine emissions including those of methyl iodide. Literature reports suggest that methyl iodide is catalytically decomposed by silver mordenite into diatomic iodine and hydrocarbons/hydrocarbon derivatives. Continuous column studies utilizing silver mordenite pellets have experienced difficulties obtaining consistent results within a reasonable time when dealing with air streams containing organoiodides at low (∼100 ppb) concentrations. An alternate approach based on utilizing a thin coating of reduced silver mordenite on an inert honeycomb cordierite matrix is described in the present study. This dynamic sorption study was conducted at 170°C with air streams containing methyl iodide at 110-140 ppb concentrations in absence of moisture. The reduction in the silver mordenite mass needed to conduct the investigations resulted in reduced runtimes and successful quantification of the catalytic decomposition of methyl iodide. Kinetics of this decomposition was determined by varying gas residence time and monitoring the methyl iodide conversion. Data analysis using the integral method revealed the reaction rate had a likely first order dependence on methyl iodide concentration with a rate constant of 0.1202 L/g sorbent/s at the operating temperature. Control experiments revealed that sodium mordenite did not exhibit a similar catalytic decomposition behavior.