DECHLORINATION OF CHLOROPICRIN AND 1,3-DICHLOROPROPENE BY HYDROGEN SULFIDE SPECIES: REDOX AND NUCLEOPHILIC SUBSTITUTION REACTIONS

Authors: ZHENG, WEI - UNIVERSITY OF CALIFORNIA; Yates, Scott; Schneider, Sharon; GUO, MINGXIN - DELAWARE STATE UNIVERSITY; GAN, JIANYING - UNIVERSITY OF CALIFORNIA

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2006
Publication Date: 2/18/2006
Citation: Zheng, W., Yates, S.R., Papiernik, S.K., Guo, M., Gan, J. 2006. Dechlorination of chloropicrin and 1,3-dichloropropene by hydrogen sulfide species: redox and nucleophilic substitution reactions. Journal of Agricultural and Food Chemistry. 54(6):2280-2287.

Interpretive Summary: Fumigant chemicals are widely used in agricultural production to control soil-borne pests and pathogens. After soil treatment, fumigants dissipate rapidly through volatilization and degradation. Even so, fumigants such as 1,3-dichloropropene (1,3-D) and chloropicrin (CP), have been detected in the atmosphere and groundwater in many states. It has been proposed that ammonium thiosulfate be used to rapidly degrade halogenated fumigants, at the soil surface, and reduce atmospheric volatilization of fumigants. However, there are other S-containing compounds that generate hydrogen sulfide species that could be used to degrade soil fumigants. Therefore, the importance of this degradation pathway deserves study. The rate of 1,3-D dissipation increased with increasing total hydrogen sulfide concentration at a constant pH. Transformation of 1,3-D was more rapid at high pH, suggesting that the reactivity of hydrogen sulfide species in the experimental system stems primarily from HS-. The information obtained from this study could be used to develop new emission reduction methods that are efficacious and environmentally safe. A potential problem that needs further investigation is the relatively low smell threshold values and potential environmental persistence of organic sulfur products yielded by the reaction.

Technical Abstract: The chlorinated fumigants chloropicrin and 1,3-dichloropropene (1,3-D) are extensively used in agricultural production for the control of soil-borne pests. Reaction of these two fumigants with hydrogen sulfide species (H2S and HS ) was examined in well-defined anoxic aqueous solutions. Chloropicrin underwent an extremely rapid redox reaction in the hydrogen sulfide solution. Transformation products indicated reductive dechlorination of chloropicrin by hydrogen sulfide species to produce dichloro- and chloronitromethane. The transformation of chloropicrin in hydrogen sulfide solution significantly increased with increasing pH, indicating that H2S is less reactive toward chloropicrin than HS is. For both 1,3-D isomers, kinetics and transformation products analysis revealed that the reaction between 1,3-D and hydrogen sulfide species is an SN2 nucleophilic substitution process, in which the chlorine at C3 of 1,3-D is substituted by the sulfur nucleophile to form corresponding mercaptans. The 50% disappearance time (DT50) of 1,3-D decreased with increasing hydrogen sulfide species concentration at a constant pH. Transformation of 1,3-D was more rapid at high pH, suggesting that the reactivity of hydrogen sulfide species in the experimental system stems primarily from HS-. Because of the relatively low smell threshold values and potential environmental persistence of organic sulfur products yielded by the reaction of 1,3-D and HS-, the effects of reduced sulfide species should be considered in the development of alternative fumigation practices, especially in the integrated application of sulfur-containing fertilizers.