Accelerated Degradation of Methyl Iodide by Agrochemicals

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 (t(1/2)) of MeI was approximately 108 d, while the transformation of MeI in aqueous solutions containing selected agrochemicals was more rapid, with t(1/2) < 100 d (t(1/2) < 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 t(1/2) of MeI was <20 h in thiourea- and allylthiourea-amended soil, considerably less than that in unamended soil (t(1/2) > 300 h).

Remediation of Methyl Iodide in Aqueous Solution and Soils Amended with Thiourea

“…Our recent explorations revealed that some sulfur-based agrochemicals, such as thiourea, which has been proposed for use as a nitrification inhibitor (27)(28)(29), might accelerate the degradation of halogenated fumigants in soil (18). Nitrification inhibitors are a group of agrochemicals used to minimize fertilizer N loss by limiting the rate of formation of nitrate from ammonium.…”

Methyl Iodide

“…The end products N-methylaniline (84%), N,N dimethylaniline (13%), and residual MeI (3%) were identified by gas chromatography-mass spectrometry (GC-MS) after 25 days of incubation. Similarly, Zheng et al (2003) noted that MeI half-life in water was reduced from 108 days in the presence of 1.0 mM concentrations of a range of agricultural fertilizers and nitrification inhibitors. Most significant reductions were observed for sodium diethyldithiocarbamate and ammonium diethyldithiocarbamate, with half-lives of 0.13 and 0.14 days, respectively.…”

“…The authors therefore suggested that the nondegraded plant residues of the potting mix may not have been very effective in inducing MeI degradation. Zheng et al (2003) examined the effects of a range of agricultural fertilizers (oxamide, calcium cyanamide, and urea), and nitrification inhibitors (dicyandiamide, thiourea, 1-allyl-2-thiourea, ammonium thiosulfate, sodium diethyldithiocarbamate, and ammonium diethyldithiocarbamate) on the degradation of MeI in soil. Compared to control soil (half-life of 309 h), they observed that MeI half-life was reduced to between 16 and 227 h across all treatments.…”

“…The authors attributed the effectiveness of ammonium thiosulfate to its existence (as the thiosulfate ion) in the aqueous soil phase where reaction with the fumigant takes place. ATS is unstable in soil, because the thiosulfate ion can be easily oxidized to sulfate (which is not reactive with MeI); therefore, Zheng et al (2003) suggested that in order to impact emissions from soil, ATS application should be carried out concurrently with fumigant application.…”

“…Transformation of MeI has been shown to follow first-order kinetics (Gan and Yates, 1996;Zheng et al, 2003Zheng et al, , 2004. According to Gan and Yates (1996) …”