Remediation of methyl iodide in aqueous solution and soils amended with thiourea

Methyl iodide (MeI) is considered a very promising fumigant alternative to methyl bromide (MeBr) 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 by a single chemical reaction process, while MeI degradation in thiourea-amended soil apparently involved a catalytic mechanism. The electron delocalization between the thiourea molecule and the surfaces of soil particles is energetically favorable and would increase the nucleophilic reactivity of the thiono group toward MeI, resulting in an enhancement of the dissipation 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.