Degradation and phase partition of methyl iodide in soil

Methyl iodide (CH3I) was recently proposed as a direct replacement for methyl bromide (CH3Br) in soil fumigation, but no information exists on 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 K-d of CH3I is greater, and the Henry's law constant K-H 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 through which CH3I and CH3Br were degraded under the concentrations studied. In water, CH3I degraded to I-2 and I- under 254-nm UV irradiation and dissipated rapidly (t(1/2) = 26 h) through volatilization and photohydrolysis under outdoor conditions.