Evaluation of the intermediates generated during the degradation of Diuron and Linuron herbicides by the photo-Fenton reaction

Freshwater polluted with herbicides is a problem of general concern since these compounds are commonly toxic and non-biodegradable. An innovative technology for the elimination of such compounds is the coupling between an Advanced Oxidation Process (AOP) and a biological treatment. The success of this coupled methodology depends on the biodegradable nature of the by-products originated at the end of the chemical stage. The present paper is based on the analysis of the intermediates generated during the chemical oxidation of Diuron and Linuron herbicides using three different doses of photo-Fenton reactants. Among the three effluents obtained after the chemical pre-treatment, only the most oxidised is completely biodegradable. Several analytical methods: reverse phase ultra pressure liquid chromatography UPLC(RP)/MS, ionic chromatography IC and gas chromatography/MS have been used to elucidate the degradation mechanism. Beyond conventional separation methods, hydrophilic interaction chromatography HILIC coupled with mass spectrometry has been necessary to identify small polar compounds at the end of oxidation process. The first steps of the degradation mechanism have been ascertained. Furthermore, different by-products have been found at the end of the chemical process when different reactant doses were used. These differences have been based on the presence of urea derivates (methylurea, 1,1-dimethylurea) and unidentified chlorinated compounds.