Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2004
Publication Date: 5/20/2004
Citation: Zheng, W., Yates, S.R., Guo, M., Papiernik, S.K., Kim, J. 2004. Transformation of chloropicrin and 1,3-dichloropropene by metam sodium in a combined application of fumigants. Journal of Agricultural and Food Chemistry.52:3002-3009. Interpretive Summary: Methyl bromide (MeBr) has been used extensively for pre-plant fumigation. However, it has been scheduled for phase-out because of its potential to deplete stratospheric ozone. Potentially, there are enormous adverse economic impacts from the methyl bromide ban and this has triggered a feverish search for alternatives. Only a few registered soil fumigants remain: chloropicrin, 1,3-dichloropropene (1,3-D), metam sodium and dazomet, however, none can be considered an complete replacement for MeBr. Therefore combinations of fumigants are being considered to broaden the spectrum of pest control. This has led to the use of multi-fumigant formulations, such as commercial mixtures of 1,3-D with 17 and 35% chloropicrin. However, the environmental impacts of a multi-fumigant formulation need to be determined and assessed prior to widespread adoption and use. This has led us to conduct research to: characterize the transformation of chloropicrin and 1,3-D in the presence of metam sodium in both aquatic and soil systems, study the cause of chemical incompatibility when these fumigants are applied simultaneously with metam sodium, and propose a sequential strategy for multiple fumigant application. The information obtained from these experiments is needed to develop optimal multi-fumigant application systems, and accurately assess and predict the environmental fate of fumigants to avoid and mitigate the risk to human and environmental health associated with fumigant application.
Technical Abstract: Combined application of fumigants is a potential strategy to replace methyl bromide in the control of soil-borne pests. Unfortunately, abiotic and biotic interactions among fumigants restrict some combined application approaches. In this study, the kinetics and mechanisms of reaction between metam sodium (sodium methyldithiocarbamate) and the halogenated fumigants chloropicrin (trichloronitromethane) and 1,3-dichloropropene (1,3-D) were investigated in aqueous solution. For chloropicrin, an extremely rapid oxidation-reduction process occurred in the presence of metam sodium. The second-order rate constant for the reaction between chloropicrin and metam sodium was approximately two orders of magnitude greater than that for the reaction between 1,3-D isomers and metam sodium. Transformation of 1,3-D by metam sodium was associated with an aliphatic SN2 nucleophilic substitution process. The nucleophilic reaction of cis-1,3-D with metam sodium was significantly faster than that of trans-isomer, and was correlated with a lower reaction activation energy for the cis isomer in the transition state. Combining a fumigant mixture (65% 1,3-D and 35% chloropicrin) and metam sodium in solution might yield some nucleophilic sulfur species, which could play an important role in the dissipation of 1,3-D. The incompatibility of chloropicrin and 1,3-D with metam sodium was also examined in soil under different application scenarios. Simultaneous application of metam sodium with chloropicrin or 1,3-D accelerated the transformation of the two halogenated fumigants, reducing their availability in soil. A sequential strategy for multiple fumigants was developed, which could be applied without the loss of active ingredient that occurs due to the reaction between fumigants. The proposed methodology may enhance pest control while maintaining environmental protection.