Identification of diuron radical oxidation by-products
During drinking-water production, pesticides can be modified under the action of powerful oxidizing agents such as ozone. However, the mineralization process is rarely complete and it is therefore important to know both the nature and the concentration of intermediate by-products. The main goal of this work was to identify the Diuron oxidation reaction by-products in order to explain the reaction mechanisms and determine the efficiency factors of proposed treatments to destroy these substances in drinking-water. Trials were carried out in a continuous "bubbling column" reactor type operating in an up-flow mode. This Pyrex reactor was one meter high with a one liter volume. The Diuron concentration in test solutions was fixed at either 0.01 mg L-1, which is the maximum value found in natural water, and 5 mg L-1 to facilitate the identification of the reaction by-products. Test solutions were prepared from a standard solution by dilution into ultrapure water (Total Organic Carbon=TOC < 0.1 mg L-1) buffered with phosphate at pH=7. The following experimental conditions were used for the 0.01 mg L-1 and 5 mg L-1 test solutions, respectively: O3 dose=2 and 6 mg L-1 ; H2O2 /O3 molar ratio=0.35; contact times=8 and 30 min. Hypotheses on the nature of the Diuron oxidation reaction by-products were based on previous experiments carried out on Isoproturon and Metoxuron (Allemane 1994; Mansour 1992). To identify the reaction products, we performed a liquid phase-solid phase extraction on C18 -grafted silica cartridges. Diuron byproducts were identified by gas chromatography-mass spectrometer (GC-MS), after a derivatization reaction with butyl-iodide. Results show that during the oxidation reaction the aromatic cycle is preserved in three by-products: N-(3,4 dichlorophenyl)-N-(methyl)-urea (DCPMU), N-(3,4 dichlorophenyl)-urea (DCPU) and 3,4 dichloroaniline (DCA). These compounds were found in Diuron solutions treated with ozone or with the ozone-hydrogen peroxide couple. They were identified by comparing their chromatograms with those of the pure isolated substances (retention times and mass spectra). Products were quantified by High Performance Liquid Chromatography (HPLC). The Diuron transformation percentages were found to be 80% with ozone and 90% with the O3 /H2O2 couple under extreme experimental conditions i.e. Co =5 mg L-1, [O3]=6 mg L-1 and a 30 minutes reaction time. The TOC measurements show that under these conditions the Diuron mineralization process reaches 50%. A mass balance showed that DCPMU was one of the main oxidation reaction residual by-products with amounts corresponding to 5-7% of the initial quantity of Diuron.