Transformation of Chloropicrin and 1,3-Dichloropropene by Metam Sodium in a Combined Application of Fumigants

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 2 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 S(N)2 nucleophilic substitution process. The nucleophilic reaction of cis-1,3-D with metam sodium was significantly faster than that of the trans isomer and was correlated with a lower reaction activation energy for the cis isomer in the transition state. Combining Telone C-35 (65% 1,3-D and 35% chloropicrin) and metam sodium in solution might yield some nucleophilic sulfur species, which played 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.