Degradation and Phase Partition of Methyl Iodide in Soil

Methyl iodide (CH 3 I) was recently proposed as a direct replacement for methyl bromide (CH 3 Br) in soil fumigation, but no information exists on its behavior and safety in the environment. In this study, we compared CH 3 I and CH 3 Br for their transformation and phase partitioning in soil and characterized processes that affect the dissipation of CH 3 I from water. In moist soil, the adsorption coefficient K d of CH 3 I is greater, and the Henry's law constant K H is smaller, than that of CH 3 Br. In the same soil, CH 3 I was about twice as persistent as CH 3 Br, 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 CH 3 I and CH 3 Br were degraded under the concentrations studied. In water, CH 3 I 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.