|Lerch, Robert - Bob|
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2007
Publication Date: 3/7/2007
Citation: Lerch, R.N., Lin, C.H., Leigh, N.D. 2007. Reaction Pathways of the Diketonitrile Degradate of Isoxaflutole with Hypochlorite in Water. Journal of Agricultural and Food Chemistry. 55(5):1893-1899. Interpretive Summary: Herbicides are used to control weeds on farmer's fields to improve crop yields. Some of the herbicide sprayed on fields will be transported by runoff to surface waters, such as streams, lakes, and reservoirs, which serve as public water supply sources. Water treatment facilities routinely use chlorine to remove bacteria and reduce levels of dissolved organic matter typically present in untreated water. When herbicides are present in the untreated water, they can also react with the chlorine used for water treatment. On the one hand, this is a good thing since the chlorine can reduce the concentrations of the herbicides present in drinking water. On the other hand, breakdown of herbicides by chlorine may result in the formation of other harmful chemicals that will then be present in the finished drinking water. In this research, we studied the breakdown of the new corn herbicide, isoxaflutole (Balance), by chlorine. Balance is an unusual herbicide in that it must react with water in the soil to form a breakdown product, DKN, which is the actual herbicide. DKN has been shown to be present in surface waters of the major corn-growing states, and it has also been shown to rapidly react with chlorine. Therefore, we wanted to study in detail the reaction of DKN with chlorine to better understand if DKN will be completely broken down during the water treatment process, and to see if other potentially harmful products were formed that would then be present in drinking water supplies. Our results showed that DKN will be completely broken down by chlorine during the water treatment process, but two potentially harmful products will be formed: cyclopropane-carboxylic acid (CPCA) and dichloroacetonitrile (DCAN). However, it appears that the levels of CPCA and DCAN formed will be below those reported to cause toxic effects to humans or animals. This research benefits the general public by identifying two potentially harmful products and showing that they were present at sub-toxic levels and that DKN will not be present in drinking water supplies that use chlorination.
Technical Abstract: Isoxaflutole (IXF; Balance(TM)) belongs to the new class of isoxazole herbicides. Isoxaflutole has a very short half-life in soil and rapidly degrades to a stable and phytotoxic degradate, diketonitrile (DKN). DKN was previously discovered to rapidly react with hypochlorite (OCl-) in tap water, yielding the benzoic acid (BA) degradate as a major end product, but the complete reaction pathway and mechanism have not been elucidated. Thus, the objectives of this work were to: 1) determine the stoichiometry of the reaction between DKN and OCl-; 2) identify end products in addition to BA; and 3) propose a complete pathway and reaction mechanism for oxidation of DKN by OCl-. Stoichiometry of the reaction showed a molar ratio of OCl-/DKN of 2. In addition, two previously uncharacterized chlorinated intermediates were identified under conditions in which OCl- was limiting. The proposed chemical structure of a chlorinated benzoyl intermediate was inferred from a series of HPLC/MS and HPLC/MS/MS experiments and the use of mass spectral simulation software. A chlorinated ketone intermediate was also identified using ion trap GC/MS. Two additional end products were also identified: cyclopropanecarboxylic acid (CPCA) and dichloroacetonitrile (DCAN). These two products are known human toxins, but they will likely be present in drinking water at concentrations below those causing toxic effects. Based on the reaction stoichiometry, the structure of the chlorinated intermediates and the identification of the end products, two reaction pathways are proposed. Both pathways involve a two-step nucleophilic attack and oxidation of the diketone structure of DKN, leading to formation of BA, DCAN, and CPCA.