Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2003
Publication Date: 11/15/2003
Citation: Lin, C.H., Lerch, R.N., Garrett, H.E., George, M.F. 2003. Degradation of isoxaflutole balance herbicide by hypochlorite in tap water. Journal of Agriculture and Food Chemistry. 51:8011-8014.
Interpretive Summary: Chlorine has been widely employed for the disinfection of drinking water. When chlorine gas reacts with water it forms hypochlorite (OCl-), which may react with organic compounds in water, including herbicides and their metabolites (i.e., breakdown products). Balance (isoxaflutole) is a new corn herbicide introduced to the U.S. market in 1998. Balance is a very unstable compound without herbicidal activity, and it is rapidly converted in water to a di-ketonitrile (DKN) metabolite, which is the actual herbicide compound. The DKN metabolite will itself degrade to a non-herbicidal benzoic acid (BA) metabolite. Balance was conditionally registered by the U.S. EPA because of concerns associated with surface and ground water contamination by DKN and BA. Our preliminary experiments indicated that the neither Balance nor DKN were stable in tap water containing hypochlorite. The implications of this finding were two-fold: 1) does chlorination effectively remove DKN from drinking water supplies?; and 2) would tank solutions prepared with chlorinated tap water result in significant deactivation of the herbicide? We performed a series of experiments which confirmed that hypochlorite was, in fact, the agent responsible for degrading DKN in water, that the BA metabolite was the primary product, and that DKN, at parts per billion levels, completely disappeared in chlorinated tap water within 48 hours. In agronomic settings, highly concentrated tank solutions of Balance may be prepared with chlorinated tap water since <5% of the herbicide was converted to BA, and therefore, herbicide efficacy would not be significantly reduced. This research benefits public drinking water suppliers and farmers by showing that chlorination can remove DKN from drinking water supplies without compromising herbicide efficacy when chlorinated tap water is used for preparing tank mixes of Balance.
Technical Abstract: Chlorine has been widely employed for the disinfection of drinking water. Additionally, it has the capacity to oxidize many organic compounds in water. Isoxaflutole (Balance; IXF) belongs to a new class of isoxazole herbicides. Isoxaflutole has a very short soil half-life and rapidly degrades to a stable and phytotoxic metabolite, diketonitrile (DKN). Further degradation of DKN produces a nonbiologically active benzoic acid (BA) metabolite. In experiments using high performance liquid chromatography-UV spectroscopy (HPLC-UV) and HPLC tandem mass spectrometry (HPLC-MS/MS), DKN was found to rapidly react with hypochlorite in tap water, yielding the benzoic acid metabolite as the major end product. One mg/L of hypochlorite residue in tap water was able to completely oxidize up to 1600 µg/L (4.45 µmole/L) of DKN. In tap water, the disappearance of IXF became much more rapid than in DI water. As soon as the IXF was hydrolyzed to DKN, the DKN quickly reacts with the OCl- to form nonphytotoxic BA. As a result, the herbicide solutions prepared with tap water at 500 µg/L will no longer possess any herbicidal activity after 48 hours of storage. However, in agronomic settings, highly concentrated tank solutions (600 to 800 mg/L) may be prepared with tap water since the conversion of IXF to BA would represent <5% of the herbicide, and therefore, any impact on the herbicide efficacy would be negligible. Results of this study show that current chlorination disinfection completely eliminates the phytotoxic form of this new herbicide, DKN, from drinking water supplies, yet farmers can use chlorinated tap water without significant loss of efficacy.