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Title: DEGRADATION AND PHASE-PARTITION OF METHYL IODIDE IN SOIL

Author
item GAN, JIANYING - U.C. RIVERSIDE
item Yates, Scott

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Methyl iodide (CH3I) has been proposed as a direct replacement for methyl bromide (CH3Br) in soil fumigation because of its broad spectrum of activity and its insignificant contribution to ozone depletion. However, at present, no information exists for assessing its behavior and safety in the environment. In this study, we compared CH3I and CH3Br for their transformation and phase partitioning in soil, and characterized processes that affect the dissipation of CH3I from water. In moist soil, the adsorption coefficient Kd of CH3I is greater, and the Henry's Law constant KH is smaller, than that of CH3Br. In the same soil, CH3I was about twice as persistent as CH3Br, and the persistence decreased with increasing soil organic matter content. Chemical reactions, likely nucleophilic substitutions on soil organic matter, were identified as the predominant pathway that CH3I and CH3Br were degraded under the concentrations studied. In water, when exposed to an artificial UV source at 254 nm, CH3I was rapidly degraded to I2 and I , with the ratio of I2 to I about 2.5 under the studied conditions. Under outdoor conditions, rapid volatilization and photohydrolysis led to fast dissipation of CH3I from water, with a t1/2 of 26 h. It is postulated that only in confined environments, e.g., aquifers and covered wells, will CH3I persist in water for a significant time because of its slow hydrolysis. When the water is opened to the atmosphere, CH3I will dissipate rapidly from water and therefore should not become a recalcitrant surface water contaminant.

Technical Abstract: Methyl iodide was recently proposed as a direct replacement for methyl bromide in soil fumigation based on extensive laboratory and field-plot trials. Unlike methyl bromide, methyl iodide is considered safe for the stratospheric ozone. At present, no information exists for evaluating its behavior and safety in the environment. In this study, we compared methyl iodide and methyl bromide for their persistence and distribution in soil, and investigated processes that affect the dissipation of methyl iodide from water. In moist soil, adsorption of methyl iodide was stronger than that of methyl bromide. In the same soil, methyl iodide was about twice as persistent as methyl bromide, and the persistence decreased with increasing soil organic matter content. Chemical reactions, likely nucleophilic substitutions on soil organic matter, were identified as the predominant pathway that both methyl iodide and methyl bromide were degraded under the concentrations studied. In water, when exposed to an artificial UV source, methyl iodide was rapidly degraded to iodine and iodide. Under outdoor conditions, rapid volatilization and photohydrolysis led to fast dissipation of methyl iodide from water, with a half-life of 26 h. It is postulated that only in confined environments, e.g., aquifer and covered wells, will methyl iodide persist in water for a significant time because of its slow hydrolysis. When the wter is opened to the atmosphere, methyl iodide will dissipate rapidly from water and therefore should not become a recalcitrant surface water contaminant.