Environ. Sci. Technol. 2006, 40, 3842-3847
Simulated Solar Light Irradiation of Mesotrione in Natural Waters ALEXANDRA TER HALLE AND CLAIRE RICHARD* Laboratoire de Photochimie Mole´culaire et Macromole´culaire, UMR number 6505 CNRS-Universite´ Blaise Pascal, 63177 Aubie`re Cedex, France
FIGURE 1. Mesotrione metabolism in plants, mammals, and soils.
Photolysis is expected to be a major degradation process for pollutants in surface waters. We report here the first photodegradation study on mesotrione, a new triketone herbicide for use in maize. In a first step, we investigated the direct photolysis of mesotrione at 365 nm from a kinetic and analytical point of view. Mesotrione sensitizes its own oxidation through singlet oxygen formation and sensitizes the oxidation of H-donors through electron or H-atom transfer. In a second step, irradiation experiments were performed under conditions prevalent in the aqueous environment. Mesotrione in submicromolar concentrations was exposed to simulated sunlight, in addition to Suwannee River natural organic matter and/or nitrates. Suwannee River natural organic matter sensitizes the oxidation of mesotrione through the intermediacy of singlet oxygen, and the rate of mesotrione transformation is significantly enhanced for Suwannee River natural organic matter concentrations equal to or above 10 mg/L. Nitrates played a negligible role in SRNOM solutions.
Introduction Important crop products, like corn and wheat, heavily depend on the use of pesticides. In corn production, new herbicides were recently introduced since the interdiction of atrazine. Mesotrione is frequently used as a replacement since its registration in 2000 in EU and in 2001 in the U.S. Mesotrione belongs to a new class of triketone derivatives introduced by Zeneca Ag Products (now Syngenta) (1). The interest in mesotrione is caused by the recent increase in its use in crops. The ecological fate of this compound in the environment is not completely elucidated. Two main metabolites, MNBA (4-(methylsulfonyl)-2-nitrobenzoic acid) and AMBA (4-(methylsulfonyl)-2-aminobenzoic acid), were identified (Figure 1) in soils and plant and mammal metabolisms (2). But, to our knowledge, no photodegradation kinetic study of aqueous mesotrione has been reported until now. In natural waters, contaminants can undergo direct and/ or indirect phototransformation. Because of the variety of species present in natural waters, there is a wide array of indirect or photosensitized degradation pathways (3, 4). Dissolved organic matter (DOM) in surface waters is in concentrations typically ranging from 1 to 30 mg/L, and DOM is considered to be the principal source of reactive species. The main reactive transients produced upon irradiation of DOM are singlet oxygen (5), radical species (6), and excited triplet states (7-10). Depending upon the studied contaminant, the relative contribution of these reactive species may * Corresponding author phone: +33 (0)473407142; fax: +33 (0)473407700; e-mail:
[email protected]. 3842
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 12, 2006
vary significantly (11-13). Nitrate, present in concentrations up to several hundreds of micromoles, could also be considered an important component in surface waters. Under irradiation, nitrate is a source of hydroxyl radicals, a nonselective and highly reactive intermediate capable of degrading all types of organic contaminants (14). However, its contribution is often small because it is mostly trapped by other constituents of natural waters such as DOM or carbonate ions. The steady state concentration of hydroxyl radicals could be simply estimated from the DOC concentration of the water (15). This paper covers different aspects of aqueous mesotrione photochemistry. At first, we investigated the direct photolysis of mesotrione (0.1 mM, pH 6.5) at 365 nm. The addition of various H-donors allowed us to clarify an original aspect of mesotrione photochemistry: mesotrione being able to photosensitize the degradation of H-donor derivatives such as alcoholic or phenolic compounds. The effect of Suwannee River natural organic matter (SRNOM) on mesotrione (0.1 µM) under simulated solar light was then studied; an important photosensitization effect was demonstrated. This effect was further investigated using quenchers to identify the reactive species involved. Finally, experiments with both nitrate and Suwannee River natural organic matter in concentrations ranging within the natural ones allowed us to delineate the role of these water constituents in the photochemistry of mesotrione.
Experimental Procedures Chemicals and Materials. All chemicals were reagent grade and used as received. 4-Nitroanisole (97%), 2,4,6-trimethylphenol (97%), sodium azide (99%), isoprene, and furfuryl alcohol (FFA) (99%) were purchased from Aldrich; pyridine (99%) was from Lancaster; phenol was from Prolabo; coniferyl alcohol (97%) and sodium nitrate (>99%) were from Fluka; and 2-propanol (extra pure), Bengal Rose B, and mesotrione were from Riedel de Ha¨en. Suwannee River natural organic matter (SRNOM) isolated by reverse osmosis (1R101N) was received from the International Humic Substances Society. Experiments were performed in water purified by a Millipore milli-Q device (Millipore RQ, resistivity 18 ΜΩ cm, DOC
