Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2002
Publication Date: January 20, 2003
Citation: Zheng, W., Papiernik, S.K., Guo, M., Yates, S.R. 2003. Remediation of methyl iodide in aqueous solution and amended soils with thiourea. Journal of Environmental Science and Technology. 38:1188-1194. Interpretive Summary: Methyl iodide (MeI) has been increasingly recognized as a promising alternative to replace methyl bromide, which is scheduled for phaseout in industrialized countries in 2005. Even so, MeI emissions may contribute to environmental contamination. A large number of management practices have been proposed to control fumigant emissions, including: tarping, application of organic wastes, application of nucleophilic compounds, and biologically-based remediation technologies. Our recent studies have revealed that several sulfur-based agrochemicals, may accelerate the degradation of halogenated fumigants in soil. For example, nitrification inhibitors (NI) are used to minimize fertilizer N loss by limiting the rate of formation of nitrate from ammonium. This study explored the potential of a NI, thiourea, to abiotically transform the fumigant MeI in aqueous solution and soil. This involved an investigation of MeI hydrolysis and its nucleophilic transformation in the presence of a thiourea buffer solution at pH ranging from 4 to 12. The effects of temperature, moisture content and soil texture on degradation of MeI in thiourea-amended soil were systematically investigated. The information obtained on fumigant degradation in amended soil will be useful in developing a reactive surface barrier approach to reduce fumigant emissions from the soil surface.
Technical Abstract: Methyl iodide (MeI) is considered a very promising fumigant alternative to methyl bromide for controlling soil-borne pests. Because atmospheric emission of highly volatile fumigants contributes to air pollution, feasible strategies to reduce emissions are urgently needed. In this study, thiourea (a nitrification inhibitor) was shown to accelerate the degradation of MeI in soil and water. In aqueous solution, the reaction between MeI and thiourea was independent of pH, although the rate of MeI hydrolysis increased in alkaline solution. Substantial increases in the rate of MeI dissipation were observed in thiourea-amended soils. Transformation of MeI by thiourea in aqueous solution was single chemical reaction process, while MeI degradation in thiourea-amended soil apparently involved a catalytic mechanism. The electrostatic interaction of the thiuronium ion with the surfaces of soil particles may lower the energy requirements for reaction with MeI, resulting in an enhancement of the degradation rate. The soil half-life for MeI was reduced from >300 h for unamended soils to only a few hours in soil or sand amended with thiourea at a 2:1 molar ratio (thiourea:MeI). The MeI transformation rate in thiourea-amended soil increased with increasing soil temperature and decreasing soil moisture. Therefore, spraying thiourea on the soil surface to form a "reactive surface barrier" may be an effective and innovative strategy for controlling fumigant emissions to the atmosphere and for improving environmental protection. The applicability and benefits of this technique to reduce soil emissions needs further evaluation in the field.