Removal of herbicide diuron and thermal degradation products under Catalytic Wet Air Oxidation conditions

The Catalytic Wet Air Oxidation (CWAO) of diuron (N-(3,4-dichlorophenyl)-N,N-dimethylurea), a herbicide widely used in agriculture and belonging to the phenylurea family, has been investigated in aqueous solution in the presence of a Ru/TiO2 catalyst at 140–180 °C and 5 MPa total air pressure. Diuron and Total Organic Carbon (TOC) have been analyzed. Some reaction products were identified by LC–ES/MS, and the amount of the inorganic ions and organic products generated during the process have been measured. Thermal degradation is the main initial process yielding mainly 3,4-dichloroaniline (DCA) and dimethylamine (DMA). Further oxidations lead to ring opening and smaller organic molecules from DCA, but the DMA fragment was more difficult to eliminate. Reactions between DCA and carboxylic acids conducted to the formation of condensation products. On the other hand, dechlorination was fast and complete. The work points out that CWAO over Ru supported catalysts may not a viable technique for degradation of diluted aqueous solutions of diuron. The mineralization is incomplete compared to other Advanced Oxidation Processes, and a possible leaching of the metal by the amines formed by thermal degradation of diuron may be critical.