Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2002
Publication Date: 1/20/2003
Citation: Zheng, W., Papiernik, S.K., Guo, M., Yates, S.R. 2003. Accelerated degradation of methyl iodide by agrochemicals. Journal of Agricultural and Food Chemistry. 51:673-679.
Interpretive Summary: Methyl bromide (MeBr) has been used extensively to control plant pathogens. Because of its stratospheric ozone depletion potential, MeBr has been scheduled for elimination developed countries by the year 2005, leading to an intensive search for alternatives. Methyl iodide (MeI) is often referred to as the "drop-in replacement" because its fate and transport characteristics and effectiveness as a biocide are similar to MeBr. MeI does not cause significant stratospheric ozone depletion but excessive MeI emission into air may be hazardous environmental and public health. To minimize the negative impact of fumigants on the environment, new management strategies have been proposed to reduce emissions. In this study, we evaluated the effects of several agrochemicals (fertilizers and nitrification inhibitors) on MeI degradation. Information on the rate of MeI degradation by these agrochemicals will be useful in developing effective management practices to prevent atmospheric emissions and remediate soils contaminated by pesticides.
Technical Abstract: The fumigant methyl iodide (MeI, iodomethane) is considered a promising alternative to methyl bromide (MeBr) for soil-borne pest control in high-cash-value crops. However, the high vapor pressure of MeI results in emissions of a significant proportion of the applied mass into the ambient air, and this may lead to pollution of the environment. Integrating the application of certain agrochemicals with soil fumigation provides a novel approach to reduce excessive fumigant emissions. This study investigated the potential for several agrochemicals that are commonly used in farming operations, including fertilizers and nitrification inhibitors, to transform MeI in aqueous solution. The pseudo-first-order hydrolysis half-life ( t1/2) of MeI was ~108 d, while the transformation of MeI in aqueous solutions containing selected agrochemicals was more rapid, with t1/2 < 100 d (t1/2 < 0.5 d in some solutions containing nitrification inhibitors). The influence of these agrochemicals on the rate of MeI degradation in soil was also determined. Adsorption to soil apparently reduced the availability of some nitrification inhibitors in the soil aqueous phase and lowered the degradation rate in soil. In contrast, addition of the nitrification inhibitors thiourea and allylthiourea to soil significantly accelerated the degradation of MeI, possibly due to soil surface catalysis. The t1/2 of MeI was <20 h in thiourea- and allylthiourea-amended soil, considerably less than that in unamended soil ( t1/2 > 300 h).